Fischer Tropsch synthesis on supported cobalt based ... · PDF fileFischer-Tropsch Syn thesis...

Fischer-Tropsch Syn thesis

on Supported Cobalt- Based Catalysts:

Influence of Various Preparation Methods and Supports

on Catalyst Activity and Chain Growth Probability

Thesis

for obtaining the degree of

Doktor der Naturwissenschaften (Dr. rer. nat.)

of the Faculty of Chemistry

Ruhr-Universität Bochum

submitted by

Diplom-Chemiker

Martin Kraum

Bochum 1999

II

Submitted at : 04. October 1999

Examination at : 05. November 1999

Board of examiners:

Chairman : Prof. Dr. H. Sander

Referee : Prof. Dr. M. Baerns

Co-Referee : Prof. Dr. M. Muhler

3rd Examiner : Prof. Dr. H. Sander

III

Für meine Eltern

Ich Danke Euch Für Alles !

Nicht weil es schwer ist wagen wir es nicht,

sondern weil wir es nicht wagen,

ist es schwer.

Seneca

IV

Some parts of the work here described were done between January 1996 and June

1996 in the laboratories of the chair "Technische Chemie" at Ruhr-Universität Bo-

chum. From July 1996 to December 1998 the work was carried on at the Institut für

Angewandte Chemie Berlin-Adlershof e.V. in fulfilment of the requirements for the

Ph.D. degree.

I wish to express my deep gratitude to my advisor, Professor Dr. M. Baerns, for his

encouragement and support, and for his willingness to entrust so much of the devel-

opment of the project to my judgement.

Further, I wish to appreciate Dr. O.V. Buyevskaja for her everlasting readiness for

help, for lots of fruitful discussions and for the very good co-operation.

I thank Dr. N. Steinfeldt for his kinetic studies on Fischer-Tropsch synthesis and Dr.

J.P. Müller for his development of the plasma-induced preparation technique for co-

balt catalysts. For their interpretation of TPR, XRD and XPS results, I have to thank

Dres. H. Bernd, M. Schneider and M. Pohl.

Many thanks to Mrs. R. Dambowsky for the ICP-OES analysis and to Mrs. S. Evert

and Mrs. K. Struwe for their support by carrying out the TPR measurements.

Furthermore, I thank all members of the Institut für Angewandte Chemie for the good

atmosphere and co-operation that was very helpful for the success of this thesis. A

special thanks goes to Mr. E. Ostrowski and his crew for technical support.

This work was supported by the European Commission (Contract no.: JOF3-CT95-

0016). The contract was administrated by Ruhr-Universität Bochum; the experimental

work was conducted at Institut für Angewandte Chemie Berlin-Adlershof e.V..

V

Abstract

1. Objective

As state-of-the-art in FISCHER-TROPSCH synthesis cobalt catalyst (Co-Ref) supported

on titania a chain growth probability α of 0.91 and a turn-over-frequency; i.e. the mol

of converted carbon monoxide divided by the mol of active metal per second), of

17.7·10-3 s-1 was reported. (Treac = 200 °C, ptot = 20 bar, H2:CO ratio of 2:11). The goal

of the present thesis was the development of a cobalt catalyst which reveals a higher

activity, i.e., an improvement of the turn-over-frequency (TOF), with an equally high

chain growth of ≈ 0.90.

2. Methods

Catalysts

The activity of cobalt catalysts used in Fischer-Tropsch synthesis is closely related to

the accessible cobalt surface area which depends on cobalt dispersion (DCo). Cobalt

dispersion can be affected by the preparation procedure. To increase Co dispersion

various preparation techniques were applied: incipient wetness-, precipitation-,

spreading and plasma- induced techniques. Furthermore, it was known from lit-

erature that the type of applied cobalt precursor influences dispersion and hence ac-

tivity and selectivity.. Based on this knowledge, six different cobalt precursors (cobalt

-acetate, -oxalate, -(II) + (III) acetyl acetonate, -EDTA and -hydroxide) were used in-

stead of the usually applied cobalt nitrate. Additional, three more catalysts were pre-

pared supported on CeO2, ZrO2 and TiO2 (pure rutile type) as alternate support mate-

rial to titania (Degussa P25; mixture of rutile and anatase) as it is known also that the

support influences the cobalt dispersion.

Characterisation

It was anticipated that the preparation technique and the kind of cobalt precursor in-

fluence the bulk and surface composition of the catalyst, the reducibility as well and

the dispersion of cobalt on the support. These possible effects were studied by

means of XRD, XPS, TPR and CO-pulsing over the catalyst. The total quantity of co-

balt of the catalyst sample was determined by ICP-OES. The interaction of the reac-

tants (H2 + CO) with the catalytic surface was examined by DRIFT and a transient

adsorption technique.

Catalytic Evaluation

The catalytic tests were carried out in a fixed-bed reactor at pre-set reaction condi-

tions: Treac = 200 °C, ptot = 20 bar, H2:CO:N2 = 12:6:2 bar, GHSVSTP = 1200 h-1. This

1 E. Iglesia, S.L. Soled, R.A. Fiato, J. Catal. 137 (1992) 212

VI

procedure allows an easy comparison of the obtained catalytic data (XCO, TOF, α)with the state-of-the-art cobalt catalyst (Co-Ref). Furthermore, the Fischer-Tropsch

reaction was carried out in a slurry phase to study the influence of the various reac-

tion conditions on carbon monoxide conversion and product distribution. The rate of

carbon monoxide consumption and methane formation was determined for a se-

lected catalyst in a gradientless recycle reactor (Berty- type).

3. Results

The catalytic results obtained at standard conditions in a fixed-bed reactor are given

in Table A1. An improvement of TOF was obtained on all catalysts. The most active

catalyst sample was SPR-OXA: XCO = 32.3 % and α = 0.81 as compared to Co-Ref

(XCO = 14.7 %, α = 0.83). The data in Table A1 show that TOF is affected by the ap-

plied cobalt precursor, the preparation technique and added promoter. Furthermore,

the kind of support material influenced the catalytic performance as well. On ceria

supported catalyst an improvement of XCO = 23.7 % was achieved in comparison to

Co-Ref.

Table A1: Overview of carbon monoxide conversion, selectivity towards C5+ fraction,

α−value, TOF and TOFnom of some selected catalysts applied in the present thesis

(average for steady-state conditions, Treac = 200 °C, ptot = 20 bar, H2:CO:N2 = 12:6:2,

GHSV = 1200 h-1)

Catalyst Cobalt precursor XCO

[%]

SC5+

[wt%]

α

[-]

DCored i

[%]

TOF j

[103s-1]

TOFnom k

[-]

Co-Ref a nitrate 14.7 80.0 0.83 6.1 17 1.0

IW-OXA-NH3 a,b oxalate 12.8 75.9 0.81 5.4 18 1.1

PR-EDTA-Ru c,d EDTA 15.8 81.5 0.84 6.5 26 1.5

IW-ACAC3 a acetyl acetonate 23.6 67.9 0.71 9.8 43 2.5

IW-ACAC3-Ru a,d acetyl acetonate 29.3 80.4 0.80 8.5 42 2.5

SPR-OXA e

oxalate 32.3 83.0 0.81 7.7 61 3.9

IWB-NIT f nitrate 10.5 79.3 0.77 4.2 11 0.66

IWC-NIT g nitrate 23.7 80.3 0.81 7.7 19 1.12

IWZ-NIT h nitrate 9.7 61.0 0.68 4.2 19 1.14a prepared by impregnation b NH3 was used as solvent for impregnation instead of H2O c prepared by

precipitation d catalyst doped with Ru e prepared by spreading f supported on titania (rutile-type) g sup-

ported on ceria h supported on zirconia i DCored = mol CO adsorbed on catalyst / mol Co in metallic

state j TOF = nCO · XCO / 100 · nCo k TOFnom = TOFcat / TOFCo-Ref

VII

Catalyst IW-ACAC3 examined in a slurry reactor showed that a change of the re-

action variables has a positive effect on the formation of high boiling hydrocarbons.

Under slurry-conditions, an increase of the chain growth probability to 0.86 was

achieved in comparison to an α-value of 0.71 obtained in a fixed-bed reactor.

In kinetic studies an apparent Eapp of 103 kJ/mol was estimated.

4.Conclusions

From the catalytic results it can be concluded that the kind of cobalt precursor influ-

ences catalyst activity as expressed the carbon monoxide conversion in the following

order:

Co hydroxide (XCO = 2.3 %) < Co (II) acetyl acetonate < Co EDTA < Co nitrate <

Co (III) acetyl acetonate< Co acetate < Co oxalate (XCO = 32.4 %)

Some of the catalysts were doped with ruthenium. On all catalyst an increasing XCO

and α-value was obtained that can be related to a stabilising effect of Ru on cobalt

surface particles.

The catalytic performance depends also on the preparation technique; a catalyst

prepared ex cobalt oxalate by means of the incipient- wetness technique showed a

threefold lower carbon monoxide conversion (XCO = 12.8 %) than to the catalyst pre-

pared by the spreading technique (XCO = 32.4 %). Furthermore, it could be proven

that with increasing cobalt dispersion activity (carbon- monoxide conversion) is in-

creased also. This effect can be assigned to the increasing number of accessible ac-

tive cobalt species.

The three different supports used for Co affected catalyst activity:

ZrO2 (XCO =9.7 %) < TiO2 (rutile) < TiO2 (Degussa P25) < CeO2 (XCO = 23.4 %)

From the catalytic tests carried out in slurry reactor it can be concluded that this re-

actor type suits to the Fischer-Tropsch synthesis better than fixed bed reactor: under

slurry conditions a shift to high boiling hydrocarbons was observed; furthermore,

slurry- reactor operation allows a better control of the temperature.

VIII

Zusammenfassung

1. Zielsetzung

Der bisher erfolgreichste Fischer-Tropsch Katalysator, Kobalt (12 wt%) geträgert auf

Titandioxid (Co-Ref), erreichte eine Reaktionslaufzahl (TOF) von 17,7·10-3 s-1 und

eine Kettenwachstumswahrscheinlichkeit von 0,91 bei einer Reaktionstemperatur

von 200 °C, einem Gesamtdruck von 20 bar und einem H2:CO Verhältnis von 2:1.

Das Ziel der vorliegenden Arbeit war die Entwicklung eines Kobalt-Katalysators der

eine höhere TOF, verbunden mit einer hohen Kettenwachstumswahrscheinlichkeit

erzielt.

2. Methodik

Katalysatoren

Die Aktivität der Fischer-Tropsch Katalysatoren kann mit der Anzahl frei zugängli-

cher, reduzierter Kobaltatome korreliert werden. Die Verteilung der Kobaltatome auf

dem Trägermaterial, d.h. die Kobaltdispersion (DCo), kann sowohl durch die Präpara-

tionsmethode als auch durch die Art der eingesetzten Kobalt-Precursoren beeinflußt

werden. Daher wurden vier verschieden Herstellungsmethoden angewandt: Nasse

Imprägnierung, Fällung, Spreitung und Plasma gestützte Verfahren. Als Kobalt-Pre-

cursoren wurden Kobaltacetat, -oxalat, -(II) + -(III) acetylacetonat, -EDTA und -hydro-

xid eingesetzt. Weiterhin wurden drei weitere Trägermaterialien (CeO2, ZrO2 und

TiO2-Rutil) neben Titandioxid (Typ: Degussa P25; Mischung aus Rutil und Anatas)

verwandt, um den Trägereinfluß auf Aktivität und Kettenwachstumswahrscheinlich-

keit zu untersuchen.

Charakterisierung

Die angewandten Präparationsmethoden und Kobalt-Precursoren können die Pha-

sen- und Oberflächenzusammensetzung, die Reduzierbarkeit und die Kobaltdisper-

sion beeinflussen. Daher wurden die Kontakte durch XRD, XPS, TPR und CO-Puls

Messungen charakterisiert. Der Gesamtgehalt an Kobalt wurde für jeden Katalysator

durch ICP-OES analysiert. Die Reaktant-Katalysator Wechselwirkungen wurden

durch TAP- und DRIFT- Messungen untersucht.

Katalytische Messungen

Die Austestung der Katalysatoren erfolgte zunächst in einem Festbett-Reaktor unter

folgenden Bedingungen: Treac = 200 °C, ptot = 20 bar, H2:CO:N2 = 12:6:2 bar,

GHSVSTP = 1200 h-1. Die Bedingungen wurde für alle Katalysatoren gleich gewählt,

IX

um einen schnellen Vergleich der zu ermittelten Größen XCO, TOF und α mit dem

Referenzkatalysator Co-Ref zu erlauben. Der Einfluß veränderter Reaktionsbedin-

gungen auf CO-Umsatz und Produktbildung wurde in einem Slurry-Reaktor unter-

sucht. Einführende kinetische Messungen zur Bestimmung der CO-Verbrauchs- und

Methanbildungsgeschwindigkeit wurden in einem gradientenfreien Kreislaufreaktor

(Berty-Reaktor) durchgeführt.

3. Ergebnisse

In Tabelle A1 sind die Ergebnisse der katalytischen Evaluierung an einigen ausge-

wählten Katalysatoren aufgelistet. Der Katalysator SPR-OXA war der aktivste aller

vermessenen Kontakte und erreichte einen CO-Umsatz von 32,3 % mit einer dazu-

gehörigen Kettenwachstumswahrscheinlichkeit von 0,81 im Vergleich zum Co-Ref

Katalysator (XCO = 14,7; α = 0,83).

Tabelle A1: Auflistung des CO-Umsatzes, der Selektivität zur C5+ Fraktion, α−Werte,

TOF und TOFnom einiger, ausgewählter Katalysatoren (Mittelwerte für stationäre Be-

dingungen, Reaktionsbedingungen: Treac = 200 °C, ptot = 20 bar, H2:CO:N2 = 12:6:2,

GHSV = 1200 h-1)

Katalysator Kobalt Precursor XCO

[%]

SC5+

[wt%]

α

[-]

DCored i

[%]

TOF j

[103s-1]

TOFnom k

[-]

Co-Ref a Nitrat 14,7 80,0 0,83 6,1 17 1,0

IW-OXA-NH3 a,b Oxalat 12,8 75,9 0,81 5,4 18 1,1

PR-EDTA-Ru c,d EDTA 15,8 81,5 0,84 6,5 26 1,5

IW-ACAC3 a Acetylacetonat 23,6 67,9 0,71 9,8 43 2,5

IW-ACAC3-Ru a,d Acetylacetonat 29,3 80,4 0,80 8,5 42 2,5

SPR-OXA e

Oxalat 32,3 83,0 0,81 7,7 61 3,9

IWB-NIT f Nitrat 10,5 79,3 0,77 4,2 11 0,66

IWC-NIT g Nitrat 23,7 80,3 0,81 7,7 19 1,12

IWZ-NIT h Nitrat 9,7 61,0 0,68 4,2 19 1,14

a nasse Imprägnierung b NH3 wurde als Lösungsmittel eingesetzt anstatt H2O c Fällung d Katalysator

promotiert mit Ru e Spreitung f Trägermaterial: TiO2-Rutil g Trägermaterial: CeO2 h Trägermaterial:

ZrO2 i DCored = mol CO absorbiert am Katalysator / mol Co im metallischen Zustand j TOF = nCO · XCO /

100 · nCo k TOFnom = TOFcat / TOFCo-Ref

X

Die Zugabe von Ru zu den Katalysatoren wirkte sich auf die Aktivität und Selektivität

aus. Im Falle des Katalysators IW-ACAC3 führte der Promotor zu einer Steigerung

von XCO von 23,6 auf 29,3 % und von α von 0,71 auf 0,80 (siehe Tabelle A1). De-

sweiteren konnte ein Trägereinfluß nachgewiesen werden, da auf dem Ceroxid

geträgertem Kontakt ein CO-Umsatz von 23,7 % bestimmt worden ist.

Der IW-ACAC3 Katalysator wurde neben dem Festbett-Reaktor auch unter Slurry-

Bedingungen getestet. Die geänderten Bedingungen wirkten sich positiv auf die

Kettenwachstumswahrscheinlichkeit aus. Der α-Wert stieg von 0,71 auf 0,86 im Ver-

gleich zum Festbett-Reaktor.

Während der Messungen im Berty-Reaktor konnte die Aktivierungsenergie der Fi-

scher-Tropsch Synthese zu 103 kJ/mol bestimmt werden.

4. Schlußfolgerung

Aus den zuvor beschriebenen Ergebnisse konnte ein Einfluß des Kobalt-Precursor

auf den CO-Umsatz festgestellt werden:

Co-Hydroxid (XCO = 2.3 %) < Co (II) Acetylacetonat < Co-EDTA < Co-Nitrat <

Co (III) Acetylacetonat < Co-Acetat < Co-Oxalat (XCO = 32.4 %)

Der Mechanismus, wie die verschieden Precursoren mit dem Trägermaterial wech-

selwirken, konnte nicht aufgeklärt werden.

Weiterhin wirkte sich die Zugabe von Ruthenium als Promotor in allen Fällen positiv

auf die Katalysatorleistung aus. An allen promotierten Kontakten konnte ein Steige-

rung des α-Wertes festgestellt werden. Dieses Ergebnis kann auf einen stabilisieren-

den Effekt von Ru auf das an der Oberfläche lokalisierten Kobalt erklärt werden.

Die Präparationsmethode beeinflußt gleichfalls die Katalysatorleistung wie es an

Kontakten, hergestellt aus Kobaltoxalat, nachgewiesen werden konnte. Der Kataly-

sator, welcher durch nasse Imprägnierung präpariert wurde, zeige eine um den Fak-

tor 3 geringen CO-Umsatz im Vergleich zu dem durch Spreiten hergestellten Kataly-

sator (XCO IW-OXA = 12,8 %, XCO SPR-OXA = 32,4 %).

Desweiteren konnte nachgewiesen werden, daß mit ansteigender Kobaltdispersion

ein Anstieg des CO-Umsatzes einher geht. Dieses Ergebnis kann mit der erhöhten

Zahl an aktiven Zentren begründet werden.

XI

Vier verschiedene Trägermaterialien wurde für die nasse Imprägnierung mit Kobalt-

nitrat eingesetzt: Auch in diesem Fall konnte ein Einfluß auf die Aktivität der Kataly-

satoren festgestellt werden. Die Trägermaterialien können in folgender, aufsteigen-

der Reihenfolge angeordnet werden:

ZrO2 (XCO =9.7 %) < TiO2 (Rutil) < TiO2 (Degussa P25) < CeO2 (XCO = 23.4 %)

Aus den Ergebnissen die im Slurry-Reaktor bestimmt worden sind kann gefolgert

werden, daß die Reaktionsführung der FTS in diesem Reaktortyp begünstigt ist. Dies

äußerte sich einerseits in vermehrten Bildung hochsiedender Kohlenwasserstoffe als

auch in der verbesserten Reaktionskontrolle. Unter Slurry-Bedingungen konnte in

kürzerer Zeit, verglichen zu Festbettmessungen, stationäre Bedingungen erreicht

werden.

XII

Table of Content

1 INTRODUCTION ............................................................................................1

2 STATE OF THE ART .....................................................................................4

2.1. A SHORT HISTORY OF FISCHER-TROPSCH SYNTHESIS.........................................4

2.2. REACTION PATHWAYS AND THERMODYNAMICS OF FTS........................................5

2.2.1. REACTION PATHWAYS .......................................................................................5

2.2.2. THERMODYNAMICS ............................................................................................6

2.3. REACTION MECHANISMS OF FISCHER-TROPSCH SYNTHESIS.................................8

2.3.1. CARBIDE MECHANISM........................................................................................8

2.3.2. HYDROXY-CARBENE MECHANISM .......................................................................9

2.3.3. CO INSERTION MECHANISM ...............................................................................9

2.3.4. MECHANISM FOR CO CATALYSTS SUPPORTED ON MNO .......................................9

2.3.5. ALKENYL MECHANISM......................................................................................10

2.3.6. SUMMARY OF THE VARIOUS REACTION MECHANISM ...........................................11

2.4. SCHULZ-FLORY DISTRIBUTION..........................................................................12

2.5. FT - KINETICS.................................................................................................13

2.6. FTS ON COBALT CATALYSTS ...........................................................................14

2.6.1. SUPPORTED COBALT CATALYSTS .....................................................................14

2.6.2. EFFECT OF SUPPORT MATERIALS ON THE PERFORMANCE OF COBALT BASED

CATALYSTS.....................................................................................................21

2.6.3. EFFECT OF PROMOTERS ON THE PERFORMANCE OF COBALT BASED CATALYSTS .21

2.6.4. EFFECT OF COBALT PRECURSOR .....................................................................26

2.6.5. EFFECT OF PREPARATION TECHNIQUE ..............................................................27

2.6.6. CONCLUSION FROM PREVIOUS WORK ...............................................................28

3 OBJECTIVES AND METHODS ...................................................................30

3.1. OBJECTIVES ...................................................................................................30

3.2. METHODS.......................................................................................................31

3.2.1. PREPARATION OF CATALYSTS ..........................................................................31

3.2.2. CHARACTERISATION OF CATALYSTS..................................................................32

3.2.3. CATALYTIC EVALUATION ..................................................................................33

3.2.4. KINETIC EVALUATION.......................................................................................33

XIII

4 EXPERIMENTAL..........................................................................................34

4.1. CHARACTERISATION OF CATALYSTS..................................................................34

4.1.1. XRD- INVESTIGATION......................................................................................34

4.1.2. TPR- EXPERIMENTS........................................................................................34

4.1.3. TPO- AND TPD- EXPERIMENTS .......................................................................34

4.1.4. CO-PULSE EXPERIMENTS ................................................................................35

4.1.5. XPS MEASUREMENTS .....................................................................................35

4.1.6. PSEUDO IN-SITU XPS MEASUREMENTS.............................................................36

4.1.7. ICP- OES......................................................................................................36

4.1.8. TEM- MEASUREMENTS ...................................................................................36

4.1.9. DRIFT- MEASUREMENTS ................................................................................37

4.1.10. TAP- REACTOR- SYSTEM................................................................................37

4.2. PREPARATION OF CATALYSTS ..........................................................................37

4.2.1. SUPPORT PRETREATMENT ...............................................................................38

4.2.2. CATALYST PRECURSOR TREATMENT.................................................................38

4.2.3. OVERVIEW OF AL PREPARED CATALYST ............................................................38

4.2.4. INCIPIENT WETNESS TECHNIQUE ......................................................................38

4.2.5. SPREADING OF COBALT PRECURSORS..............................................................40

4.2.6. PRECIPITATION................................................................................................41

4.2.7. PLASMA INDUCED PREPARATION ......................................................................42

4.3. CATALYTIC TESTING ........................................................................................43

4.3.1. FIXED-BED REACTOR.......................................................................................46

4.3.2. SLURRY REACTOR ..........................................................................................46

4.3.3. BERTY REACTOR.............................................................................................47

4.3.4. ANALYSIS OF PRODUCTS .................................................................................47

4.4. DETERMINATION OF XCO, S(CN), α, TOF AND TONNOM......................................48

4.5. KINETIC EXPERIMENTS ....................................................................................49

5 RESULTS AND DISCUSSION .....................................................................51

5.1. CHARACTERISATION OF CATALYSTS..................................................................51

5.1.1. CHARACTERISATION OF REFERENCE CATALYSTS...............................................51

5.1.2. CHARACTERISATION OF IMPREGNATED CATALYSTS............................................62

5.1.3. CHARACTERISATION OF COBALT BASED CATALYSTS SUPPORTED ON CERIA,

ZIRCONIA AND TITANIA (RUTILE TYPE) ...............................................................68

5.1.4. CHARACTERISATION OF SPREADED CATALYSTS .................................................71

5.1.5. CHARACTERISATION OF PRECIPITATED CATALYST ..............................................74

XIV

5.1.6. CHARACTERISATION OF CATALYST APPLYING PLASMA INDUCED PREPARATION ....77

5.1.7. DISCUSSION OF CHARACTERISATION RESULTS ..................................................81

5.2. CATALYTIC EVALUATION ..................................................................................87

5.2.1. REFERENCE CATALYST....................................................................................87

5.2.2. CATALYTIC EVALUATION OF IMPREGNATED CATALYST ........................................89

5.2.3. CATALYTIC EVALUATION OF IMPREGNATED CATALYSTS SUPPORTED ON CERIA,

ZIRCONIA AND TITANIA (RUTILE TYPE) ...............................................................92

5.2.4. CATALYTIC EVALUATION OF CATALYSTS PREPARED BY SPREADING.....................93

5.2.5. CATALYTIC EVALUATION OF PRECIPITATED CATALYSTS ......................................95

5.2.6. CATALYTIC EVALUATION OF PLASMA PREPARED CATALYST.................................97

5.2.7. DISCUSSION OF CATALYTIC RESULTS................................................................98

5.2.8. DISCUSSION OF CATALYTIC RESULTS FOR THE NEW CATALYSTS.......................100

5.3. SLURRY REACTOR OPERATION.......................................................................106

5.3.1. CATALYTIC EVALUATION ................................................................................106

5.3.2. DISCUSSION OF CATALYTIC EVALUATION.........................................................107

5.4. EVALUATION OF FTS-KINETIC ........................................................................109

5.4.1. RESULTS OF KINETIC STUDIES .......................................................................109

5.4.2. CO CONSUMPTION RATE ...............................................................................111

5.4.3. FORMATION RATE OF METHANE .....................................................................116

5.4.4. ESTIMATION OF EACT ......................................................................................117

5.4.5. DISCUSSION OF KINETIC DATA........................................................................117

6 CONCLUSIONS .........................................................................................119

7 LITERATURE.............................................................................................122

APPENDIX .................................................................................................132

XV

Index of Symbols and Acronyms

Symbols

∆G° Gibbs energy [kJ/mol]

∆Had heat of adsorption [kJ / mol]

DCo cobalt dispersion [%]

DCored cobalt dispersion [%]

Eact activation energy [kJ / mol]

GHSV gas hourly space velocity [h-1]

k rate constant

K adsorption constant

ptot operation pressure [bar]

ra rate of chain termination

rc rate of chain growth

S selectivity [%]

t.o.s. time on stream [h]

TOF turn-over-frequency [s-1]

Treac reaction temperature [°C]

wt% weight percentage [%]

wt% weight percentage [%]

XCO carbon monoxide conversion [%]

Y yield [-]

α chain growth probability [-]

XVI

Acronyms

DRIFT Diffuse Reflectance Infrared Fourier Transmission

FID Flame Ionisation Detector

FTS Fischer-Tropsch Synthesis

ICP-OES Inductively Coupled Plasma-Optical Emission Spectrometry

TAP Temporal Analysis of Products reactor

TCD Thermal Conductivity Detector

TEM Transmission Electron Microscopy

TPD Temperature Programmed Desorption

TPO Temperature Programmed Oxidation

TPR Temperature Programmed Reduction

XRD X-Ray Diffraction

XPS X-ray Photoelectron Spectroscopy

1 Introduction 1

1 Introduction

The endeavour to find an alternative to crude oil for the production of chemical raw

materials and motor fuels syngas chemistry (CO + H2) has been an interesting sub-

ject of development and research since the early 30ties. The syngas chemistry offers

many routes to fuels and chemicals through hydrogenation of carbon monoxide to al-

kanes, alkenes and oxygen containing products [1]. The hydrogenation of carbon

monoxide is best known as FISCHER-TROPSCH Synthesis (FTS) which involves a step-

wise hydrocarbon chain growth described by the SCHULZ-FLORY distribution [2,3]. The

direct production of gasoline and diesel oil via FTS is from today’s standpoint not

economic because the direct refining of crude oil is much cheaper. Therefore much

work had been carried out either to increase the amount of C2-C4 olefins which are

an important feedstock for the chemical industries or the selectivities towards C18+ al-

kanes (high-boiling waxy hydrocarbons) which may be converted by hydrocracking to

any desired alkane fraction [4,5]. This point is getting more important since in the

field of partial oxidation of methane to syngas very promising results were obtained,

so that natural gas could be directly converted at its source. This will minimise the in-

vestment cost in comparison to gas pipelines [6]. At present, FTS is utilised in indus-

trial scale at SASOL (Fe-catalyst) in South Africa and by SHELL (modified Co-catalyst)

in Malaysia in the production of middle distillates [7,8]. Another pathway for produc-

ing gasoline starting from syngas was the MTG process announced by MOBIL. The

process is based on the production of methanol from syngas which was converted toaromatic gasoline and short-chained olefins. [9].

The exothermic FTS is heterogeneously catalysed by Group VII metal. Within this

group the specific activity is decreasing in the following order [10]:

Ru > Fe > Co > Rh > Pd > Pt >Ir

Despite the lower specific activity in comparison to Ru and Fe much emphasis is

presently put on Co-based catalysts due to their high chain growth probability (α) as

well as the low activity towards the watergas-shift reaction. One reason for the low

activity is the low cobalt dispersion on supported catalysts in connection with the

small fraction of accessible cobalt on which the Fischer-Tropsch reaction is taking

place.

Furthermore, the present FT catalyst technology suffers from limitations in catalyst

selectivity and deactivation beside the ability of the catalyst to withstand the long-

term adverse effects of poising, carbon deposition and water vapour.

The activity of cobalt catalysts needs, however, further improvement. Variation of the

Co-precursor and support material is being considered as a means to reach this aim.

1 Introduction 2

Various studies have been performed on the influence of support materials such as

titania [11-15], silica [16], alumina [17,18], zirconia and ceria [19-21] on the activity of

Co catalysts.

Besides cobalt nitrate which is usually applied as precursor the Co-EDTA complex

[20], cobalt carbonyls [21] as well as Co-acetate [22,23] were used for preparing

alumina- and silica-supported catalysts. Furthermore, the dependence of catalyst

performance on preparation variables (e.g.; temperature, pH-value of the cobalt

containing solution, solvent) [24-28] was examined by many groups.

At the present state of research activities a Co/TiO2 catalyst studied by IGLESIA et al.

[11] set the standard. This catalyst was prepared by impregnation of titania with an

aqueous solution of Co(NO3)2. On this catalyst a carbon monoxide conversion of

60 % with a corresponding C5+ selectivity of 90.1 % was obtained at a reaction tem-

perature of 200 °C, a total pressure of 20 bar and a feed ratio of H2/CO = 21.

The main objective of the present thesis was the development of a catalyst that dis-

tinguishes itself by a higher carbon monoxide conversion and α-value than the latter

described catalyst invented by IGLESIA (later called as reference catalyst).

It is assumed that high cobalt dispersion (DCo) can be correlated with a high carbon

monoxide conversion (XCO). To reach this aim, the effect of different cobalt precur-

sors on cobalt dispersion, surface and bulk compositions as well as reducibility were

studied for titania-supported catalysts. For preparing these catalysts various prepara-

tion methods were used. The incipient wetness impregnation technique and precipi-

tation from an aqueous solution in the presence of TiO2 were applied. Furthermore,

the applicability of the spreading technique and plasma- induced catalyst preparation

were examined. Plasma- induced preparation technique is described by decomposi-

tion of cobalt precursor by means of oxygen plasma in contrast to the conventionally

applied oxidation (or decomposition) in an oven at high temperature (above 200 °C).

The idea is that the lower temperature of decomposition will prevent the formation of

large cobalt clusters, which should led to high cobalt dispersion. As reported by

NONNEMAN and PONEC [29] and VANNICE [30,31] the support material influenced

catalytic activity of cobalt catalysts. Therefore, three further support materials, i.e.,

ceria, zirconia and titania (rutile type), beside titania(Degussa P25, mixture of ana-

tase and rutile) were applied.

The influence of the preparation procedure and support material on the state of the

Co particles, i.e. on cristallinity, phase composition, reducibility and cobalt dispersion

were characterised by means of XRD, XPS, TPR and CO-pulse examinations.

1 The applied GHSV or adjusted flow rate of the feed gas was not mentioned within the article

1 Introduction 3

The catalytic evaluation took place at fixed reaction conditions of Treac = 200 °C,

ptot = 20 bar, pH2:pCO:pN2 = 12:6:2 bar and a GHSV of 1200 h-1 inside a fixed-bed

reactor. These conditions were chosen in order to allow an easy assessment of the

newly prepared catalyst samples in comparison to the reference catalyst; the refer-

ence catalyst was evaluated at the beginning of the studies under the same condi-

tions. The products formed were analysed applying a gas chromatograph that al-

lowed the determination of all permanent gases (on-line analysis) and hydrocarbons

up to C50 (C1-C6 on-line analysis; C7+ = off-line analysis). Additionally, one selected

catalyst (IW-ACAC3 catalyst) was transferred to slurry-operation in order to examine

if an improvement in the catalyst activity, i.e., XCO, and/or the chain growth is

achieved by this mode of operation.

Within a gradientless recycle reactor (BERTY-type) the influence of the partial pres-

sures of hydrogen and carbon monoxide on the rate of carbon monoxide consump-

tion and rate of product formation was examined to determine kinetic parameters of

the reaction. Therefore, pH2 was varied in a range of 4 to 12 bar and pCO in a range

of 1 to 6 bar, respectively.

2 State of the Art 4

2 State of the Art

2.1. A SHORT HISTORY OF FISCHER-TROPSCH SYNTHESIS

The development of the Fischer-Tropsch Synthesis is closely related to the first hy-

drogenation reaction of carbon monoxide to methane over nickel and cobalt catalysts

found by SABATIER and SENDERS in the year 1902 [32]. 1923 FRANTZ FISCHER and

HANS TROPSCH obtained Synthol, a mixture of oxygenated hydrocarbons, olefins and

paraffins, on alkalised iron catalysts. Starting from Synthol a way for the production of

liquid hydrocarbons alternative to coal hydrogenation was found [2,33]. During the

years 1933 to 1936 pilot plant tests were performed at RUHRCHEMIE AG applying co-

balt catalysts that resulted in the first commercial plant in Germany in 1937. This

multi-stage process under atmospheric pressure lead to a production of 200.000 t/a

of motor fuels [34]. During world war II the medium-pressure process and a more ef-

ficient gas-recycle reactor was introduced; furthermore, the cobalt catalyst was re-

placed by an improved iron catalyst. A pressure range from 15 to 25 bar and tem-

peratures between 230 °C and 310 °C were applied in the medium-pressure process.

All these efforts resulted in a yearly production of 600.000 t of products consisting of

46 % of gasoline, 23 % of diesel oil, 3 % of lubricating oil and 28 % of waxes at the

end of 1944 in Germany [35]. The last German FTS-plants managed by SCHERING

AG and KRUPP-TREIBSTOFFWERKE were shut down in the 1950ties due to low profit-

ability.

SASOL I was the first commercial, successfully operating plant after the war located

in South Africa and it is still in operation. Two types of reactors were installed: a fixed-

bed reactor with syngas recycling developed by RUHRCHEMIE as well as an entrained-

solids reactor worked out by KELLOGG. The fixed-bed reactor had an inner diameter

of 3 m and 2052 pipes with a length of 12 m were installed in which 35 t of catalyst

was placed. The pipes were surrounded with water in order to allow an intensive

cooling in the form of evaporation heat. The FTS was carried out within a tempera-

ture range from 220 to 235 °C and an operating pressure of 25 bar. The entrained-

solids reactor has a height of 36 m and contained a deposit zone with a diameter of

5 m and a reaction zone with a diameter of 2.2 m. The 130 t of catalyst was circu-

lated by 300.000 m3/h gas and the reaction was performed in a range from 320-

340 °C and a operation pressure between 20 – 30 bar. In both units iron catalysts

were employed. In the early 80ties SASOL II and III were put in operation [36-38].

During the period of 1953 to 1972 the interest in a further improvement of FT tech-

nology tailed off due to a low oil price with the exception of South Africa.

However, the OPEC oil embargo in 1973 led to a new enthusiasm in the scientific re-

search for producing motor fuels beside crude oil as source feedstock. Since the


early 1970s, THE RESEARCH OF SHELL has been involved in syngas chemistry with

special focus on routes to convert natural gas into easily transportable liquid hydro-

carbons. The so-called Shell Middle Distillate Synthesis Process (SMDS) consists of

three stages: the syngas manufacturing, the heavy paraffin synthesis and the heavy

paraffin conversion. The heavy paraffins were produced by Fischer-Tropsch synthe-

sis on cobalt catalyst. The synthesis was carried out in a fixed-bed reactor similarly

designed as the reactor for SASOL I by RUHRCHEMIE. The reaction was carried out at

temperatures between 215 to 235 °C and an operating pressure of 25 bar. The pro-

duced paraffins were converted by hydroisomerization and hydrocracking to kero-

sene and gas oil. The first SMDS plant came on stream in Bintulu, Malaysia in 1992

[8,39]

MOBIL announced a new process for converting methanol to aromatic gasoline and

short-chained olefins in the year 1976 (MTG – process) as an alternative to the FTS

and commercialised the process 1985 in New Zealand [40].

A recent incentive is partly related to the research activity in the field of partial oxida-

tion of methane to syngas. Therefore, many groups are working on the improvement

of Fischer-Tropsch catalysts in order to obtain catalysts whit a high specific activity

and chain growth probability which allow an economical, industrial process of liquid

fuels and chemical raw products.

2.2. REACTION PATHWAYS AND THERMODYNAMICS OF FTS

2.2.1. REACTION PATHWAYS

The Fischer-Tropsch Synthesis is a complex network of primary and secondary reac-

tions and can be compared to a polymerisation reaction in which the surface mono-

mer -CH2- is formed from carbon monoxide. Based on the monomer -CH2- alkanes,

alkenes and oxygenated hydrocarbons were formed. The formation of the -CH2-

group (2.2), methane (2.6) and the Boudouard reaction (2.8) can be considered as

primary reaction steps; all others are secondary ones.

The relative velocity of each reaction step depends strongly on catalyst type and cho-

sen reaction conditions. The stoichiometry can be derived from two basic reactions

[41-43]:

][35 kJ -172.3C)(227HOH nHCH 1)(2nnCO R222nn2 =°+→++ + (2.1)

[35] kJ 165.0- C)(227HOH -)(-CH H 2 CO R222 =°∆+→+ (2.2)

The reaction account to equation (2.1) and (2.2), describing the FT reaction, is best

catalysed using cobalt catalysts. The watergas-shift reaction (2.3), a negligible path-

way on cobalt catalysts, increases in importance for secondary reactions on iron-


based catalysts.

]35[kJ 39.8- )C227(HH CO OH CO R222 =°∆+↔+ (2.3)

Two further reactions for the hydrocarbon synthesis can be described as follows:

]35[kJ 5.244-)C227(HCO 2 -)(-CH H CO 3 R222 =°∆+→+ (2.4)

]35[kJ 12.2- )C227(H OH 2 -)(-CH H 3 CO R2222 =°∆+→+ (2.5)

These main reaction pathways may be accompanied by the following side-reactions:

]35[ kJ 214.8- )C227( H OH CH H 3 CO R242 =°∆+→+ (2.6)

]35[ kJ 254.1 -)C227( H CO CH H 2 CO 2 R242 =°∆+→+ (2.7)

]35[kJ 134.0- )C227(HCO C CO 2 R2 =°∆+↔ (2.8)

The formation of methane [Eqs. (2.6)-(2.7)] and the decomposition of CO to elemen-

tary carbon and carbon monoxide (so-called BOUDOUARD reaction) are undesired re-

action pathways. Especially the deposited elementary carbon will block the active

sites of the catalyst leading to its deactivation.

The formation of alcohols and aldehydes is also possible and is described as follows:

]35[kJ 369.9- )C227(H OH 1)-(n OHHC H 2n CO n R212nn2 =°∆+→+ + (2.9)

]35[kJ 213.0- )C227( HOH CHOHCHCO R21n2n2 )1n2( 1)(n =°∆→ ++++ + (2.10)

2.2.2. THERMODYNAMICS

The thermodynamic probability of the formation of individual products (Pi) can be de-

rived from calculation of the coupled reaction equilibrium under the assumption that

the selected individual reaction (i.e. the formation of Pi) occurs independently of one

another [41]. Calculations carried out based on the above mentioned equations (2.1-

2.9), have shown that the formation of methane is favoured in a temperature region

from 50 to 350 °C. In Fig. 2.1 the change of the GIBBS free energy for equations (2.1),

(2.5), (2.8) and (2.9) or more precisely for the formation of methane, alkanes (C2,

C20), ethane, methanal and alcohols (C1, C2) are given. It is obvious that the forma-

tion of methanal and methanol is not favoured because they have positive ∆G° val-

ues at reaction temperatures above 200 °C.

Within the group of paraffins the formation probability decreases with increasing car-

bon number; for the olefinic hydrocarbons an inversely proportional trend was ob-

tained. An increasing reaction temperature leads to a shift of the product spectrum

towards the olefinic and oxygenated hydrocarbons with a simultaneous decrease of

alkane formation. The formation probability of higher hydrocarbons can be improved

by increasing the total reaction pressure. A syngas mixture with a high H2/CO ratio


favours alkane formation; by decreasing the H2/CO ratio more alkenes are formed in

comparison to paraffins. To sum up these findings the probability of formation of

products is decreasing in the following order:

methane > alkanes > alkenes > oxygenates

It should be mentioned that this order of formation probability is based on thermody-

namical calculations without paying attention to a kinetical control. The kinetic limita-

tions, which may determine a catalytic system, can have consequences on the order

of formation probability.

Based on the illustrated course of ∆G° the influence of temperature on each single

reaction step of the reaction equilibrium can be seen. A lower reaction temperature

leads to a shift of the thermodynamical equilibrium towards the products, i.e., the

formation of hydrocarbons and oxygenates is preferred. A similar statement can be

made for the influence of the reaction pressure on the product formation since the

reaction is accompanied by a volume contraction; a higher ptot has a positive influ-

ence on the thermodynamic equilibrium (∆G°) especially for the long-chained hydro-

carbons.

The real product distribution of the FTS deviates considerably from the calculated

thermodynamic data as reported by many researchers, e.g., R.B. ANDERSON [36] as

well as PICHLER and SCHULZ [44]. These findings can be explained by a kinetically

controlled reaction pathway; the chosen catalytic system as well as the reaction con-

ditions shows a great influence on the formation probability of products.

The thermodynamic calculations were carried out under the assumption that all indi-

vidual reactions were independent. Indeed, there is no syngas composition which

expresses the stoichiometric starting point of all possible FT reactions (please refer to

equations (2.1) to (2.9)), e.g., for the formation of methane a H2/CO ratio of 2:1 and

for ethene at ratio of 4:1 is necessary. This leads to the conclusion that the product

distribution is strongly influenced by the composition of the applied syngas.

All individual reaction steps are exothermic. Apart from the methane formation all FT

reactions are thermodynamically improbable above a reaction temperature of 450 °C,

so that an increase in reaction temperature favours the formation of methane at the

expense of long chained hydrocarbons.


Fig. 2.1: Gibbs free energy as a function of reaction temperature for various FT-prod-

ucts [36].

2.3. REACTION MECHANISMS OF FISCHER-TROPSCH SYNTHESIS

Up to now the reaction mechanism of FTS is not completely understood and is

therefore still a topic in the work of some research groups. In the following the sup-

posed mechanistic pathways are described; if not especially mentioned the intro-

duced mechanistic ideas can be applied to all types of catalytic systems, i.e. iron, co-

balt or nickel based catalysts. This chapter will close with a summary and discussion

of the introduced mechanisms in order to point out which mechanisms are accept-

able from today’s view.

2.3.1. CARBIDE MECHANISM

The carbide mechanism was first supposed by FISCHER and TROPSCH [45]. The initial

step is the dissociative adsorption of carbon monoxide on the catalyst surface under

formation of a carbide that consecutively reacts to a M-CHx species. The insertion of

one of two neighbouring CHx species into the metal-carbon bond of the other species

leads to the formation of a higher hydrocarbon. The chain growth is interrupted by the

desorption of the hydrocarbon. However, this reaction pathway does not explain the

formation of oxygenated hydrocarbons (alcohols and aldehydes) which are by-

products of the FTS. P. BILOEN et al. [46,47] pointed out that the predominant propa-


gation of normal, oxygen-free hydrocarbons can be described by the following reac-

tion step:

M 2HC CH-M HC-M yx1mxym +→+ ++ (2.11)

Where x = 0-3, often x = 2 and y = 2m+1

2.3.2. HYDROXY-CARBENE MECHANISM

Based on experimental results KUMMER et al. [48] supposed a hydroxy-carbene as

main intermediate and as starting point of chain propagation. The M-CHOH species

is formed by the partial hydrogenation of undissociated adsorbed carbon monoxide.

Due to condensation of two hydroxy-carbenes by elimination of water a carbon-car-

bon bound formation occurs. This model explains the formation of both oxygenates

and hydrocarbons and was supported by several research groups [49-56].

2.3.3. CO INSERTION MECHANISM

The main intermediate in the CO insertion mechanism proposed by PICHLER and

SCHULZ [57] is a metal carbonyl, which is partially reduced by adsorbed hydrogen to a

metal-alkyl species (see Fig. 2.2). The resulting alkyl-intermediate can then undergo

a variety of reactions to form acids, aldehydes, alcohols and hydrocarbons. The

propagation of chain growth can be explained by the insertion of an associatively ad-

sorbed carbon monoxide into the metal-alkyl bond. The formation of higher hydro-

carbons was terminated by the desorption of products from the catalysts surface. For

more detailed information see a review of V. Ponec [52,56], A.T. BELL [58] and M.A.

VANNICE [59].

Fig. 2.2: CO-Insertion Mechanism [60]

2.3.4. MECHANISM FOR CO CATALYSTS SUPPORTED ON MNO

The type of catalytic material influences significantly the reaction pathways because

of the different interactions of the metal surface with the reactants. In the following a

mechanism for a supported cobalt catalyst (Co/MnO) is introduced [60]. On cobalt


catalysts, four different intermediates are considered to be the key-species for chain

growth, i.e., formyl (I), hydroxy-carbene (II), carbene (III) and methyl (IV) as pre-

sented in Fig. 2.3. As shown before most of the proposed mechanisms deal with only

one C1 intermediate as starting point for chain growth. Only one C1-species is too

specific or electronically unreasonable in order to explain all possible products that

can be formed during FTS. It is likely that the coupling of C1 intermediates is con-

trolled by the electronic interaction of electrophilic or nucleophilic carbon. The formyl

and hydroxy-carbene can account for the electrophilic species and the carbene and

methyl for the nucleophilic species.

As shown in Fig. 2.3 the chain propagation would then involve pairing of (I) or (II) with

(III) and (IV). These reactions would lead to a hydroxyethyl intermediate (1) or to a

bridged complex (2) that can be converted to (1) by additional hydrogen. The hy-

droxyethyl intermediate would react further via reduction and formation of water to

either an ethylidene complex (3) or a metal ethyl group (4). These two intermediates

can participate in the propagation of chain growth or desorb as ethene or ethane.

Fig. 2.3: Supposed mechanism for Co/MnO Catalysts [60]

2.3.5. ALKENYL MECHANISM

The alkenyl mechanism introduced by MAITLIS and co-workers is based on experi-

ments performed on ruthenium catalysts supported on silica [61-63]. The chain initia-

tion step would be the formation of a surface vinylic species formed by coupling of

two neighbouring methyne- and methylene species (see Fig. 2.4). The chain growth

could be initiated by a reaction of a methylene group with the vinylic species. After


isomerization another methylene group can insert into the double bond, which can be

expressed as chain propagation. Chain growth is terminated by the reaction of the

molecule with a surface hydride.

Fig. 2.4: The alkenyl mechanism for stepwise polymerisation of methylene in the FT-

reaction [61]

2.3.6. SUMMARY OF THE VARIOUS REACTION MECHANISM

The mechanisms proposed may be divided into three main classes:

• metal-carbide mechanism

• condensation mechanism

• CO-insertion mechanism

The metal-carbide mechanism is from today's view tolerable, because the formation

of oxygen containing hydrocarbons cannot be explained and subsequent studies

have failed to support the carbide theory since the end of 60ties. However, the de-

velopment of surface analytical instrumentation derived that carbon was present on

the catalyst surface but not a significant quantity of oxygen; this finding revived the

carbide mechanism. The carbide mechanism is now believed to be restricted to a

surface metal carbide structure in contrast to bulk carbide as supposed by FISCHER

and TROPSCH [45] as investigated by DAVIS et al. [64].

The hydroxy-carbene mechanism (condensation mechanism) suffers the experi-


mental support and is nowadays in not discussed in general anymore [65].

The CO-insertion mechanism was supported by many experimental findings like:

- the CO chemisorption at the active metal goes along with similar binding lengths

and energies like the metal carbonyls [66].

- experiments with 14C label olefins showed that CO can be replaced by olefins as

reactant for chain initiation and propagation [67].

- the chemisorption of H2, CO and ethane led to carboxylat structures via a CO-in-

sertion [68]

The latest supposed reaction mechanisms by KIENNEMANN [60] and MAITLIS [61] are

acknowledged and will be further studied.

From the presented facts it becomes clear that at the presented state the "real"

mechanism was not found yet, but the advanced carbide-, CO-insertion-, and alkenyl

mechanism are the most probable ones. Revealing the FT-mechanism will be a topic

in future studies.

2.4. SCHULZ-FLORY DISTRIBUTION

The molecular distribution of FT-products is very similar to that obtained during po-

lymerisation and oligomerisation processes. The statistical description of this distri-

bution was revealed by SCHULZ [69] and FLORY [70] and applied to FTS by ANDERSON

[71,72].

The rate of chain growth is designated as rc and of chain termination as ra, which de-

scribes both the β-hydride elimination and the terminal reduction steps on the catalyst

surface, the chain growth probability α can be defined as follows:

ac

c

rrr+

=α (2.12)

Under the assumption that both the reaction rates ra and rc are independent from the

chain length of the formed hydrocarbons α is constant and describes the product dis-

tribution unequivocally for a specific catalyst and fixed reaction conditions.

The weight percentage wn for the hydrocarbon Cn decreased in accordance with a

geometric row and was described by the Schulz-Flory distribution as:

n2n n)(lnw α⋅⋅α= (2.13)

With n = number of carbon atoms in the respective hydrocarbon.


For practical applications this formula is mostly used in the logarithmic form:

α⋅+α= logn)(ln logn

wlog 2n (2.14)

When the logarithm of the quotient of wn and n is plotted versus n, the chain growth

probability α can be derived from the slope of the straight line.

The chain growth probability is a temperature dependent value that decreases with

increasing reaction temperature. Only a weak dependence of α on the partial pres-

sure of CO exists as shown by BAERNS and BUB [73,74]:

-0.3a ,Pk1

1aCO

≈+

=α (2.15)

2.5. FT - KINETICS

Based on the hydroxy-carbene mechanism (section 2.3.2) the following rate expres-

sion was proposed [75,76]:

OHCO

COHHCO

2

2

2 KCC

CkCr

+=− +

(2.16)

while for the CO-insertion- and carbide-mechanism the following rate law was sug-

gested [77]:

COHOH

CO2H

HCO CC'KC

CC'kr

22

2

2 +=− +

(2.17)

(with k,k’ = rate constants, K, K’= adsorption constants, C = concentration, r = reaction rate)

At low carbon monoxide conversion, i.e., at small concentrations of the produced

water, the following equation can be used as suggested by BAERNS et al. [78] and

KÖNIG and GAUBE [79].

22 HHCO kCr =− + (2.18)

From equation (2.18) it may be assumed that the rate determining step is influenced

by hydrogen concentration. Nevertheless, hydrogen is involved in each reaction step

as shown in section 2.2.1, hence the identification of the rate determining step is still

a question within the kinetic studies in many groups.

It should be mentioned that, like the reaction mechanism, various kinetics based on

different approaches are currently discussed. The kinetic equations given above are

formal descriptions of the FT-reaction for the application in reaction engineering.


2.6. FTS ON COBALT CATALYSTS

Iron-, nickel-, ruthenium- and cobalt- based materials are the classic Fischer-Tropsch

catalysts, but on each type of catalyst a different product distribution is obtained. On

Fe- catalysts, low-boiling hydrocarbons and alcohols are the predominant products

besides a great tendency towards the water-gas shift reaction. High-boiling hydro-

carbons can be obtained on nickel catalysts in addition to oxygenated carbon prod-

ucts. On ruthenium, the formation of polyethylene was found by PILCHER [80].

In the present work cobalt catalysts were the subject of interest. The advantage of

cobalt catalysts are the absence of the watergas-shift reaction, the favoured forma-

tion of high-boiling, unbranched hydrocarbons (waxes), barley oxygenated by-prod-

ucts and, finally, the good availability and the relatively low price.

In the following studies carried out on supported cobalt catalysts starting from the

1990ties are reviewed. A detailed summary of the earlier studies was given by

LINDNER [81], GANTZ [82], MALESSA [83], JACOBS [84] and KLEINE [85].

It is generally accepted that several parameters affect the performance of cobalt

catalysts for FTS, i. e., the catalyst support, the nature and amount of added promot-

ers as well as the cobalt dispersion. The latter is, in turn, affected by the former pa-

rameters together with the preparation method applied. The literature data con-

cerning these subtle influences are discussed below; the main conclusions that can

be drawn from these data concerning the present work will be summarised in chapter

2.6.6.

2.6.1. SUPPORTED COBALT CATALYSTS

The characterisation results and catalyst evaluation obtained on cobalt catalysts

supported on alumina, silica, titania, zeolites and niobia are presented.

Cobalt Supported on Al 203

HILMEN et al. [86] derived from TPR-measurements that the reduction of unsupported

pure cobalt oxide proceeds in two-steps as identified by 2 TPR maximum located at

352 and 384 °C. The stoichiometry of each reduction step is described by equations

(2.18) and (2.19):

OHCoO 3HOCo 2243 +→+ (2.19)

OH Co H CoO 20

2 +→+ (2.20)

The alumina- supported cobalt catalysts with a cobalt loading of 20 wt%, prepared

starting from cobalt nitrate by the incipient wetness technique, showed a different re-

duction performance. The TPR-plot consisted of four maximum. The first, at 277 °C


was assigned to the reduction of remaining cobalt nitrate on the support. At 362 °C

one broad peak was detected; at this temperature the reduction of cobalt oxide took

place. The single peak indicates broad a broad distribution of the particle size of the

oxide particles on the surface, because the reduction of Co3+ to Co2+ could not be re-

solved. The shoulder in the temperature region between 497 and 797 °C was as-

signed to the reduction of highly dispersed amorphous surface cobalt oxide. Finally,

at 927 °C cobalt aluminate (CoAl2O4) was reduced. This spinel was formed by a dif-

fusion of cobalt ions into the alumina lattice during the calcination process. These

findings for alumina supported cobalt catalysts were supported by other groups

[87,88].

The influence of the duration of hydrogen treatment on the degree of reduction was

examined by SEWELL et al. [89] on a catalyst with a cobalt content of 9 wt%, prepared

from cobalt nitrate as cobalt precursor. The catalyst sample was reduced at 500 °C.

After 1 h of reduction the degree of reduction amounted to 8 % and increased to

29 % after 5 h and to 40 % after 20 h. Additionally, the reduction temperature was

varied in a range between 300 to 600 °C with a fixed reduction time of one hour. As

expected, the amount of reduced cobalt metal increased from 1 % (300 °C) to 27 %

(600 °C). Complementary studies were carried out by ZSOLDOS and co-workers [90].

They examined the influence of reduction time (1 h to 4 h) on the degree of reduc-

tion. They found that depending on the reduction time zerovalent cobalt phases of

two different structures were obtained. Short reduction times lead to the formation of

very small Co0 particles, while long–term reduction resulted into the formation of

large, bulk-like crystallites.

The effect of various calcination temperatures on the catalytic activity of a catalyst

with 20 wt% cobalt loading was the subject of a work carried out by BELAMBE et al.

[91]. The amount of reduced cobalt decreased from 77 % to 59 % with increasing

calcination temperature from 200 to 400 °C as derived from hydrogen chemisorption

measurements; on the other hand a decrease in carbon monoxide conversion from

11.1 to 5.1 % was associated with an increase in calcination temperature as derived

from the catalytic data. However, the intrinsic activity, i.e. TOF1 was not touched by

this effect and amounted to ≈ 300 s-1.

The influence of cobalt loading in a range from 1.5 to 30 wt% on the reducibility and

catalytic performance was examined by WANG and CHEN [92]. The authors found that

the maximum reduction temperature decreases conversely proportional to the metal

loading; the same dependency was found for the degree of reduction. The catalytic

1 TOF (1/s) was calculated based on total hydrogen chemisorption; reaction conditions: H2/CO = 10,

P = 1 atm, Treac = 220 °C


activity was tested under atmospheric pressure at a temperature of 250 °C and a

GHSV of 1800 h-1. On the catalyst loaded with 1.5 wt% the lowest activity was meas-

ured. This was explained by a nearly complete transformation of Co into inactive

CoAl2O4. As shown in Fig. 2.5 the estimated rate constant went through a maximum

located at 12 wt% of cobalt. On the other hand, the authors pointed out that the se-

lectivity towards methane decreased with increasing cobalt loading. S(CH4) dropped

from 71 % (1.5 wt% Co) to 39 % (30 wt% Co) coinciding with an increase in C5+ frac-

tion. TUNG et al. [93] found a similar TPR-behaviour for different metal loadings.

0 5 10 15 20 25 300,00

0,02

0,04

0,06

0,08

0,10

RC

O =

µm

ole

/ g

co

ba

lt s

wt% Co

Fig. 2.5 Dependence of specific activity on cobalt metal loading derived from catalytic

tests (Treac = 250 °C, ptot =1 bar, GHSV = 1800 h-1) [92].

Cobalt Supported on SiO 2

The metal-support interactions on silica are very similar to those reported for alumina

supported catalysts. Several groups showed that the reduction of the supported co-

balt oxide consists of two steps; firstly the stepwise reduction of Co3O4 toCo0 via

CoO between 300 to 450 °C and secondly an additional hydrogen consumption

above 600 °C during TPR was observed due to reduction of Co2SiO4 [94-100].

LAPSZEWICZ and co-workers were interested in the influence of the pore size of the

applied silica on the catalytic performance [101,102]. As shown in Tab. 2.1 the cobalt

dispersion (derived from H2-adsorption) is decreasing in reverse order with the pore


diameter; in contrast to this results remained DCo derived from CO adsorption con-

stant for all pore diameters. The most likely explanation of this phenomenon is varia-

tion in the adsorption stoichiometry of carbon monoxide adsorption on different size

cobalt crystallites. The carbon monoxide conversion as well the methane selectivity

decreased in the same order as DCo. From the results it can be concluded that a link

between the catalyst porosity and the intrinsic properties of the active sites exists, in

turn have an effect on the product distribution in the Fischer-Tropsch synthesis.

Tab. 2.1: Overview of carbon monoxide conversion, methane selectivities, hydrogen

and carbon monoxide uptake and cobalt dispersion obtained on supported cobalt

catalysts in dependence of the average pore diameter of silica [101].

Co

[wt%]

pore size

[nm]

XCOa)

[%]

S(CH4)

[%]

BET

[m2/g]

H2-uptake

[µm/g]

CO-uptake

[µm/g]

DCob)

[%]

10.9 4 19.2 19.5 530.6 88.6 41.0 9.6

10.4 6 14.5 11.9 375.9 67.0 43.3 7.6

10.2 10 9.4 5.8 260.1 52.8 42.1 6.1

12.6 20 8.6 2.9 121.9 33.6 42.6 3.1

a) reaction conditions: Treac = 202 °C, p = 20 bar, H2/CO = 2.5, inlet gas flow = 180 ml/min, 5 g catalyst

suspended in 80 g octacosane (slurry reactor)b) derived from hydrogen adsorption

Cobalt Supported on TiO 2

IGLESIA et al. [11,103-105] reported a carbon monoxide conversion of 60 % at a reac-

tion temperature of 200 °C, a total pressure of 20 bar and a H2/CO ratio of 2 over a

Co/TiO2 catalyst; unfortunately the authors missed to denote the applied gas hourly

space velocity (GHSV). This catalyst was prepared by use of the incipient wetness

technique in aqueous solution. Cobalt nitrate was employed as cobalt precursor and

supported on titania (Degussa P25). The support was pretreated for 14 h at 600 °C

under ambient conditions before impregnation. Afterwards the catalyst precursor was

oxidised and then reduced at 400 °C for 4 h. The amount of cobalt after reduction

was determined to 12 wt%.

Based on the results obtained on catalysts in a wide range of cobalt dispersion (1.2

to 6.5 %) the FT rate per total Co content is linearly increasing with dispersion (Co-

time yield). On the other hand the authors reported that the TOF (or site time yield as

defined by IGLESIA) is constant over the whole examined region, independent of the

magnitude of dispersion and support material. However, a shift to higher hydrocar-

bons (as expressed in SC5+) correlated with an increasing DCo could be noticed. Un-


fortunately the authors did neither mention how the different dispersions were ob-

tained (i.e., pretreatment, preparation procedure etc.) nor the adjusted conversion or

GHSV (see Tab. 2.2).

DRIFT studies were performed by MOTHEBE et al. [106] on a Co/TiO2 catalyst. The

data obtained indicate that the extent of cobalt reduction on the support surface re-

sulted in a shift of the position of ν(CO) adsorption bands towards higher wavenum-

bers; furthermore, a ν(CO) band at 2060 cm-1 indicated the presence of a CO group

attached to partially oxidised cobalt atoms. Additionally, it could be shown that car-

bon monoxide is able to readily reduce oxidised cobalt already at room temperature.

Therefore, it is probable that cobalt oxide is transformed to Co0 under FT conditions,

too. By addition of Ru a different DRIFT spectrum, which can be ascribed to Ru-Co

interaction, occurs in comparison to the undoped catalyst [107].

Tab. 2.2: Overview of Co-time yields (Co-TY), Site-time yields (Site-TY) and selec-tivities toward methane and C5+ obtained on titania supported cobalt catalysts withdifferent cobalt dispersion [11]

Co [wt%] DCo [%] Co-TYa [104s-1]c Site-TYb [103s-1]c SCH4 [%] SC5+ [%]

11.6 1.2 2.8 23.1 8.1 81.5

11.9 2.2 5.0 22.7 6.8 84.5

10.5 2.9 7.5 25.9 n.m. 83.0

11.6 3.0 8.9 29.6 7.0 82.5

12.1 5.3 11.4 21.5 n.m. n.m.

11.8 6.5 15.3 23.5 5.4 90.1

a Co-TY = (converted CO / metal s); metal = total amount of cobaltb Site-TY = (converted CO / surface metal s); surface metal = total amount of reduced cobalt as de-

rived from hydrogen chemisorptionc 473 K, 2000 kPa, H2 / CO = 2.1, XCO = 50-63%


Cobalt Supported on Zeolites

Zeolites, especially ZSM-5, are a preferred support when targeting to gasoline pro-

duction, because:

· its pore structure assists shape selectivities

· the acidic surface supports reactions like oligomerisation, cracking and aromati-

sation

· it is insensitive against cooking

· it is stable under FT conditions [108]

JONG and CHENG used ZSM-5 zeolites as support for cobalt based catalyst [109].

They prepared catalyst by incorporation of cobalt in the zeolite synthesis gel as well

as by precipitation/impregnation of cobalt oxide on the zeolites; in both cases highly

dispersed cobalt particles were obtained. The chemical nature of the obtained cobalt

particles is different as revealed by TPR-examinations. The main cobalt species for

the co-precipitated catalysts was substituted cobalt within the ZSM-5 framework;

some additional extra-framework Co3O4 existed only in small amounts. For the im-

pregnated catalyst the predominant species was cobalt silicate assemblies attached

to the ZSM-5 framework. These assemblies were difficult to reduce so that tem-

peratures above 720 °C were necessary to obtain metallic cobalt; the zeolites frame

work was, however, hardly affected by these high temperatures. The low carbon

monoxide conversions which were reported by the authors (about 4 %) can be ex-

plained by the low reducibility of these species.

In a more detailed study the ZSM-system was examined by S. BESSEL [110-112] with

paying attention to the channel size. Therefore different ZSM-zeolites, namely, ZSM-

5, ZSM-11, ZSM-12 and ZSM-34 were applied. From this work it was concluded that

the kind of ZSM applied influenced mainly the activity of the catalytic system (Tab.

2.3). Moreover, the activity could be linked to the channel size of the zeolite. The

highest CO conversion was obtained over Co supported on ZSM-12 which consists

of a 12 membered ring system, followed by the 10 membered rings of ZSM-5 and

ZSM-11, while on the 8 membered rings of ZSM-34 the lowest carbon monoxide

conversion was obtained. No significant differences occurred in methane selectivity

and α-value. This result was explained by the absence of any electronic cobalt-

support effect. The increase in CO-conversion was ascribed to a higher dispersion of

cobalt species, i.e., an increasing formation of smaller cobalt crystals with increasing

dimensions of the zeolite channels.


Tab. 2.3: CO-Conversion, methane selectivity and α-value for various ZSM-catalysts

(H2:CO = 2:1, 240 °C, 20 bar, GHSV = 1000 h-1) [110]

Catalyst XCO [%] S(CH4) [%] α [−]

ZSM-5 60 21 0.81

ZSM-11 61 20 0.82

ZSM-12 79 19 0.79

ZSM-34 45 18 0.82

Cobalt Supported on Nb 2O5

Niobia was applied as catalytic support by SCHMAL and co-workers [113]. TPR-ex-

periments resulted in the well-known, two-peaked spectrum structure as previously

reported for alumina and silica supported catalysts. The catalytic tests were carried

out at a preset reaction temperature of 220 °C, a H2/CO ratio of 1.4 and total pres-

sure between 0.1 and 3 MPa. With the increasing pressure a shift to higher hydro-

carbons was expect but an unusual deviation form the Schulz-Flory distribution was

observed (Tab. 2.4): the range C13-18 increased whereas C5-12 decreased. This result

was ascribed by the authors to the formation and propagation of chains on two types

of sites or chain growth can be attributed to the increasing readsorption probability of

products at higher pressure. Finally, the authors assumed that a strong cobalt-

support interaction took place at the high pressure experiments. The reducing feed

gas might penetrate into gaps between the Co-NbOx interface, thereby allowing the

formation of Cox-NbOy on the support surface. This modification should lead to an

altered distribution between Co2+ and Co3+ species and the appearance of the new

species, which increases sharply the selectivity within the C13-18 fraction.

Tab. 2.4: Overview over the obtained selectivities on Co/Nb2O5 catalysts at different

reaction pressures [113].

MPa Selectivity (wt%)*

C1-4 C5-12 C13-18 C19-26 C27+

0.1 4.11 8.69 18.24 40.70 4.25

3.0 5.24 4.14 41.03 36.80 1.50

*(220 °C, H2:CO=1.43)


2.6.2. EFFECT OF SUPPORT MATERIALS ON THE PERFORMANCE OF COBALT

BASED CATALYSTS

REUEL and BARTHOLOMEV studied the influence of support and Co dispersion on ac-

tivity and selectivity of supported cobalt systems. Co/Al2O3, Co/SiO2 and Co/TiO2

catalysts with varying metal content (3 and 10 wt%) were prepared from cobalt nitrate

applying the incipient wetness technique. [114,115]. In the course of this work they

established that the TOFCO is a function of support, dispersion, metal loading and

preparation. By catalytic tests at atmospheric pressure and 225°C it was found that

the activity decreases in the following order: Co/TiO2 > Co/SiO2 > Co/Al2O3 > Co/C >

Co/MgO (all 3 wt% Co). On all catalysts no complete reduction could be obtained;

the catalyst loaded with 3 wt% Co was reduced to 14% only, the 10 wt% catalyst to

47%. Within one set of catalysts the specific activity in CO hydrogenation decreased

with increasing dispersion. The product distribution was also influenced by the above

mentioned parameters. The formation of hydrocarbons can be correlated with dis-

persion and extent of reduction, i.e., low boiling hydrocarbons and a higher CO2/H2O

ratio was observed on catalysts having a higher dispersion and a lower extent of re-

duction. In a later study [116] the researchers concluded that the dispersion effect

can be minimised if only highly reduced catalysts (>90%) are examined. Additionally

it should be noted that the reported results were obtained on systems with different

metal loadings.

A survey of the catalytic data reported above is given in Tab. 2.5. One can conclude

that the applied support influences the reaction performance as well as the product

distribution. For cobalt catalysts (10 wt%) supported on Al2O3, C, MgO and TiO2,

which were evaluated under comparable reactions conditions, the reported TOF-val-

ues vary between 2.2 and 38⋅10-3 s-1. Furthermore, the extent of reduction and the

dispersion was also affected.

2.6.3. EFFECT OF PROMOTERS ON THE PERFORMANCE OF COBALT BASED

CATALYSTS

Lanthana Promoted Systems

The promoter influence of La2O3 on a cobalt catalyst (20 wt%) prepared from cobalt

nitrate applying the impregnation technique was examined within a La/Co atomic ra-

tio from 0.0 to 0.75 by HADDAD et al. [117-119]. At La/Co ratios below 0.5 the amount

of reducible Co increased from 30 to 50 % with increasing La content; no effect on

the reducibility of cobalt silicates was observed. Above a La/Co ratio of 0.5 the

Co2SiO4 species became easier to reduce. The presence of La appeared to moder-

ate the formation of strong Co-support interactions leading to better reducibility of the

Co oxide phase and to a large number of exposed Co0 atoms. From catalytic tests it


was derived that the presence of La did not influence the overall activity because the

estimated TOF* values were of the same order of magnitude independent of the cho-

sen La/Co ratio. On the other hand, the α-value increased from the undoped catalyst

(α = 0.57) to the doped one (α = 0.71, La/Co ratio of 0.75). The influence of La as a

promoter for alumina- supported FTS catalysts on their catalytic performance was

examined by VADA et al. [120].

The addition of La with a ratio of La/Co = 0.1 led to a decreasing value of XCO in

comparison to the unpromoted catalyst; on the other hand the formation of high-

boiling hydrocarbons was favoured as expressed by a higher α-value.

On the catalysts described above an increasing chain growth probability was noticed.

These findings can be explained by the presence of La. It is proposed that La3+ may

enhance olefin readsorption near the La3+-Co interface which is responsible for the

higher chain growth probability. ADACHI and FUJIMOTO supported these findings [121].

Ruthenium Promoted Systems

The promoter influence on carbon monoxide conversion and C5+ selectivity on a

Co/TiO2 catalyst was examined by IGLESIA [12]. In this case Ruthenium was added to

the previously described catalyst. The research group obtained a 3 times higher

turnover rate in comparison to the undoped catalyst; additionally, SC5+ increased from

84.5 to 91.1 %. These findings can be explained by a structural promotion effect of

Ru [13], i.e., the promoter acts stabilising on the catalysts phase composition and on

the previously applied cobalt oxide. This structural promotion resulted in a lower re-

duction temperature and in preventing the agglomeration of CoOx species on the

support material. Furthermore, an electronical promotion was observed as shown by

the higher TOF values obtained. It seems that the intimate contact of ruthenium and

cobalt increases the number of exposed metal atoms that are involved in the Fischer-

Tropsch reaction.

* TOF calculated based on hydrogen chemisorption; reaction conditions: Treac = 220 °C, p = 1 atm, H2/

CO = 2:1

2 State of the A

rt23

Tab. 2.5: O

verview of physico-chem

ical properties, reaction conditions and catalytic

results for various supportsreference

[148]

[114.115]

[114.115]

[114.115]

[136]

[114,115]

[114,115]

[92]

[92]

[104]

[111]

[111]

[111]

[92]

α

n.m.

n.m.

n.m.

n.m.

0.79

n.m.

n.m.

n.m.

n.m.

0.94

0.81

0.81

0.78

n.m.

S(C5+)

[wt%]

16.1

35.7

16

6.2

38.2

42.2

53.7

34

9

84.5

n.m.

n.m.

n.m.

14

S(C1)

[wt%]

52.3

32

53

55

8.3

29

16

13

49

7

22

21

26

42

TOF

[103s-1]

45

12

2.2

0.13

n.m.

7.5

38

n.m.

70.9

17.7

n.m.

n.m.

n.m.

6.07

XCO

[%]

9.7

5.6

4.7

7.4

44.1

7.5

5.6

5

n.m.

48.7

32.4

59.6

33.9

n.m.

GHSV

[h-1]

n.m.

500

150

200

268

500

900

1500

1800

n.m.

1000

1000

1000

1800

p

[bar]

1

1

1

1

5.4

1

1

n.m.

1

20

20

20

20

1

H2:CO

2

2

2

2

1

2

2

1.2

2

2.05

2

2

2

2

Treac

[°C]

260

200

200

300

268

200

200

210

250

200

240

240

240

250

Red.

[%]

89

34

47

13

n.m.

92

47

n.m.

34

n.m.

38

100

52

64

DCo

[%]

3.8

9.9

36

1.9

n.m.

10

4.5

n.m.

7.6

2.2

22

6

18

6.3

Precur-

sor

nitrate

nitrate

nitrate

nitrate

nitrate

nitrate

nitrate

Car-

bonate

nitrate

nitrate

nitrate

nitrate

nitrate

nitrate

Support

Nb2O5

Al2O3

C

MgO

MnO

SiO2

TiO2

CeO2

Al2O3

TiO2

Y

ZSM-5

Mordenite

Al2O3

Co

[wt%]

5

10

10

10

10

10

10

11

12

12

7.7

7.8

8.8

20


Furthermore, Ru facilitated the reduction of cobalt catalyst supported on silica as de-

rived from TPR experiments and an increase of the number of exposed cobalt metal

atoms on the catalyst surface was noticed. The authors concluded that Ru acts as a

structural promoter for Co by increasing the reducibility and dispersion of cobalt.

Similar results were obtained by the use of Pt as promoter by SCHANKE et al.

[122,123] and KAPOOR et al. [124,125] as well as for alkali promotion as shown by

BLEKKAN et al. [126].Starting from cobalt carbonyls JÄÄSKELÄNIEN added Ru as pro-

moter to the catalyst precursor [127,128]. On these catalysts higher site-time yields in

comparison to undoped, impregnated catalysts was achieved; on the other hand this

type of catalysts deactivates within a time of 10 h; a Co-Ru/SiO2 catalysts lost about

40% in activity.

On alumina supported cobalt catalysts the chain growth probability stayed nearly

constant by promoting with ruthenium as shown by KOGELBAUER and co-workers

[129], but an increase of carbon monoxide conversion was observed.

Cobalt Catalysts Promoted with Metals

ALI et al. introduced Zr as a promoter for cobalt- based catalysts supported on silica

[130] with a weight content of Zr in the range of 0.7 to 8.5 wt% applying the kneading

technique, i.e., the mechanical mixture of previously prepared Co/SiO2 catalyst with

ZrO2, and the incipient wetness procedure. The addition of zirconium led in both

cases to an increase in activity and chain growth probability. It appeared that the ef-

fect of Zr promotion depends on the method of preparation and on the Zr/Co ratio.

IGLESIA et al. studied Re as a promoter for a Co/TiO2 catalyst; the addition of 0.8 wt%

Re led to an increase of cobalt dispersion from 2.2 to 5.3 % without any effect on

FTS turnover rate, apparently by forming Re oxide species that anchor CoOx clusters

and avoid sintering during the catalyst oxidation procedure [103].

MgO was used as promoter by NIEMELÄ and KRAUSE [131]; they obtained an increase

in activity due to the added magnesia as well as a decrease in selectivity towards

methane and carbon dioxide. This shift in the product distribution was caused by a

chemical modification of the active cobalt sites; the way how magnesia interacts with

surface cobalt is still a point of interest. Table 2.5 gives an overview over the catalytic

data obtained; it is visible that no direct link between dispersion and carbon monox-

ide conversion exists.


Tab. 2.6: Cobalt dispersion, degree of reduction and carbon monoxide conversion

obtained on cobalt catalysts promoted with MgO (ptot = 0.5MPa, Treac = 235 °C,

GHSV = 2600 h-1) [131].

Catalyst Dispersion [%] Reduction [%] XCO [%/mgmet]

a)

Co5 wt% 5.5 54 7

MgO2.5wt%-Co5wt% 5.1 55 15

MgO10wt%-Co5wt% 12.0 35 12

MgO2.5wt%-Co10wt% 9.7 41 13

a) XCO= Co[mg]xD[%]/100xR[%]/100

The addition of thoria to a 6 wt% Co/SiO2 catalyst led to an increase in XCO from

21.2 % to 29.2 % as reported by HO [132, 133]. The presence of Th did not influence

the extent of reduction but the activity of the catalyst for CO disproportionation and

for the formation of new CO adsorption modes, i.e., subcarbonyl-like species

Co(CO)x and multi-fold (Co)xCO were detected by FTIR measurements. By changing

the solvent for the impregnation (ethanol instead of water) a further increase in car-

bon monoxide conversion was observed. These findings were explained by a more

extensive Co-ThO2 interface formed due to a better dispersed Th phase.

The effect of different iron loadings (0.1 %, 1 %, 5 %, 10 %) on the catalytic perform-

ance of a 10 wt% Co/TiO2 catalyst was examined by DUVENHAGE and COVILLE [134].

Increasing the amount of iron led to a decrease in carbon monoxide conversion as

well as in TOF. Furthermore, a higher yield of middle distillate products at the ex-

pense of high-boiling hydrocarbons (waxes) was noticed; with an iron loading of

5 wt% the water-gas-shift reaction was no longer negligible. As expected, a de-

creasing paraffin to olefin ratio as well a raise in oxygenated by-products was ob-

served.

COLLEY et al. applied manganese oxide as support and examined the promotional

effect of potassium and chromium on catalyst activity [135,136]. The incorporation of

Cr (2 wt%) shifted the product distribution to higher hydrocarbons (C16+ fraction). An

increase in the amount of Cr (4-25%) resulted in no further increase of the selectivity

towards the high-boiling hydrocarbons. Likewise the activity remained the same;

these findings was explained by only a little electronic influence of the promoter on

the active cobalt which achieved its maximum at 2 wt% of Cr. XPS and XRD exami-

nations revealed no changes in the bulk- and surface structure with increasing chro-

mium loadings. The addition of potassium (0.01 – 1.0 %) was observed to decrease

the hydrogenation ability of the catalysts but led to an increase of the α-value (α =


0.79 for Co/MnO, α=0.86 for Co-K(0.01%)/MnO) accompanied by a favoured forma-

tion of long-chained alcohols.

In another investigation the promotional effect of added Zr, Th and Cr to Co/ZSM-5

was studied [137]. In all cases the addition of the transition metals resulted in an in-

crease in carbon monoxide conversion together with a shift towards high-boiling hy-

drocarbons. All promoters applied resulted in an increased cobalt dispersion that was

linked to the increase in activity. On the catalyst 75Co:5Cr:100ZSM-5 a conversion of

72 % and a C2+ selectivity of 83 % was obtained under the following reaction condi-

tions: H2:CO = 2:1, Treac = 240 °C, ptot = 20 bar, GHSV = 1000 h-1

2.6.4. EFFECT OF COBALT PRECURSOR

VAN DE LOOSDRECHT et al. [20] introduced Cobalt-EDTA and cobalt citrate as cobalt

precursors for alumina supported catalyst. They prepared catalysts with a cobalt

content of 2.5 wt% which resulted in smaller particle sizes in comparison to catalysts

starting from cobalt nitrate on which the particle size was in a range of 73 Å. The av-

erage particle size amounted to 7 Å for Co/Al2O3/EDTA and to 17 Å for

Co/Al2O3/citrate as derived from XPS examinations. Unfortunately, the change of co-

balt precursor did not result in an increase of carbon monoxide conversion. This fact

was explained by the very small particles that can be involved more easily into metal-

support interactions than the comparatively large cobalt particles obtained from the

nitrate precursor. TPR examinations proved this assumption, because on the catalyst

prepared from organic cobalt precursors a lower amount of metal cobalt is available

for reduction as derived from hydrogen-consumption measurements. However, the

alternative cobalt precursors lead to a reduced methane formation as well as to an

increased α-value.

Cobalt carbonyls, i.e., Co2(CO)8 and Co4(CO)12, were used instead of cobalt nitrate

for the preparation of silica supported catalyst by NIEMELÄ et al. [138]. The change of

cobalt precursor resulted in a significant improvement of cobalt dispersion as shown

by hydrogen adsorption measurements. A DCo of 80 % and 32 % was obtained for

catalysts made from dioctacarbonyl and Co4(CO)12, respectively. On the conven-

tionally prepared catalyst, starting from cobalt nitrate, the dispersion amounted to

7.1 %. As expected, the carbonyl precursors led to smaller cobalt particles (about 2.3

nm). The catalytic performance was accessed in a pulse reactor. For the catalyst with

the highest DCo a different product distribution was found in comparison to the other

catalysts. On this catalyst methane formation was preferred, no CO2 and only small

amounts of water were detected. In the authors’ opinion the product distribution can-

not only be related to the particle size but also to the extent of reduction. In fixed-bed

measurements it was found that although the carbonyl derived catalysts had a ten-

fold higher dispersion no improvement of XCO could be obtained [139]. For more de-


tailed remarks please refer to a review about supported metal carbonyls by PHILLIPS

and DUMESIC [140].

Cobalt acetate was introduced as a novel cobalt precursor by MATSUZAKI and co-

workers [141,142] for silica supported catalysts. This kind of cobalt precursor leads

neither to an improvement of XCO nor chain growth probability in comparison to the

classical catalysts.

2.6.5. EFFECT OF PREPARATION TECHNIQUE

The ALE technique (Atomic Layer Epitaxy) was employed by BACKMANN et al. [143]

as an alternative way for catalyst preparation beside the classical techniques like im-

pregnation and precipitation. By use of this technique a step-up in cobalt dispersion

was achieved at the expense of reducibility; in comparison to the impregnated cata-

lysts (84 %) only 4 % of the cobalt available was reduced after 8 h of reduction in hy-

drogen at 400 °C. The ALE technique consisted of the following steps. First the SiO2

support was heated at 450 °C for 3 h under a nitrogen atmosphere. The silica was

cooled down to 180 °C and a nitrogen feed loaded with cobalt acetyl acetonate was

passed through the silica bed until the support was saturated. After a purge period

with pure nitrogen the cobalt precursor was decomposed in synthetic air at 450 °C for

4 h. This procedure was repeated until the desired amount of cobalt was deposited

on the support.

BARBIER [144] used the so-called ammonia method for preparing stable and well-

dispersed Co/SiO2 catalyst. This method consists of adding ammonia to a cobalt ni-

trate solution in order to obtain Co amine ions that can react with the silica before

calcination. Unfortunately, the authors lack a comparison with catalysts prepared in

the conventional manner as well as catalytic data, therefore it is difficult to judge the

feasibility of this method.

The influence of different preparation methods on ZSM-5 catalysts was subject of a

study from SHAMSI et al. [145]. Catalysts were prepared by:

1) direct decomposition of C5H5Co(CO)2 on ZSM-5

2) impregnation of ZSM-5 with cobalt nitrate

3) physically mixing Co3O4 with ZSM-5

Two different metal loadings of 3 and 9 wt % were adjusted and the catalytic per-

formance of these catalysts was assessed under the following reaction conditions:

H2/CO = 1, 280 °C, 21 bar and a WHSV of 0.77 h-1. For both metal loadings the

catalysts prepared by method 1) showed the highest XCO which amounted to 7.0 and

13.2 %, respectively. The methane selectivity increased from 17.7 % to 26.5 % for

the catalysts prepared from a mechanical mixture.


Similar results on the influence of the preparation method were reported for Co/Y

catalysts by CHUNG et al. [146], although the activity of these catalysts were very low

(XCO = 3 %).

The influence of different preparation techniques on Co/Nb2O5 catalysts was exam-

ined by SCHMAL et al. [147,148]. They applied the incipient wetness technique (A)

and two different ways of precipitation (B) + (C). The catalytic tests pointed out a shift

towards heavy-weight hydrocarbons in the following order: (C) > (A) > (B). Due to the

different preparation techniques the acidity of the catalysts decreased in the same

way as derived from TPD-experiments. The doping of the catalysts with Rh lead to an

increased α-value [149, 150]

2.6.6. CONCLUSION FROM PREVIOUS WORK

The main objective of this thesis was the development of new, highly active catalysts.

From the described literature data the following conclusions can be drawn:

Support Influence

The support material influenced dramatically the performance of cobalt catalysts, i.e.,

the carbon monoxide conversion and the chain growth probability (α-value).

The catalyst activity decreased in the following order:

TiO2 > Al2O3 > SiO2 > C > MnO > MgO

It is surprising that titania as support is not exhaustivly examined up to now; only a

few studies were carried out as reported by IGLESIA [103], REUL and BARTHOLOMEV

[115]. Based on that titania should be applied for the catalyst development.

Influence of Cobalt Precursor

The influence of cobalt precursor applied for catalysts preparation on the catalytic

performance and physical properties was intensively examined up to now. In Tab. 2.7

the data obtained by LOOSDRECHT [88] and NIMELÄ [131] are shown. It is obvious that

the precursor chosen lead to a dramatically change of the particle size of the formed

surface cobalt species. For the alumina supported catalysts, the cobalt particles are

about ten times smaller when alternative precursors like cobalt carbonyls in compari-

son to cobalt nitrate are applied. This change in cluster size does not affect the car-

bon monoxide conversion but the product distribution was changed. The formation of

methane was suppressed and high-boiling hydrocarbons were favoured which was

expressed by an increase in chain growth probability. The same effects occur on the

silica catalysts as well.

Based on that conclusion other cobalt precursors as cobalt nitrate, e.g., cobalt EDTA,

cobalt acetyl acetonate and cobalt oxalate, should be applied. The cobalt precursor


variation should point out if the same positive influence on cobalt dispersion as ob-

served for alumina and silica based catalysts can be transferred to Co/TiO2 samples.

Tab. 2.7: Overview of particle size, cobalt dispersion and catalytic results (carbon

monoxide conversion, selectivities and α) of catalysts in dependence of applied co-

balt precursor

Co

[wt%]

Precursor Sup-

port

particle

size

[Å]

DCo

[%]

XCO

[%]

SCH4

[%]

SC3+

[%]

α

[-]

source

2.5 nitrate Al2O3 73 n.m. 6.2a 54 31 0.46 [88]

2.5 citrate Al2O3 17 n.m. 1.3 a 56 9 0.49 [88]

2.5 EDTA Al2O3 7 n.m. 1.4 a 57 12 0.49 [88]

5.0 nitrate Al2O3 155 n.m. 12.0 a 56 31 0.47 [88]

5.0 citrate Al2O3 39 n.m. 3.8 a 36 44 0.69 [88]

4.9 nitrate SiO2 140 7.1 12.8b 60.1 n.m. 0.74 [151]

4.6 Co2(CO)8 SiO2 23 80 9.0 b 55.3 n.m. 0.78 [139]

4.7 Co4(CO)12 SiO2 34 32 7.8 b 60.2 n.m. 0.73 [139]

a Treac = 250 °C, H2:CO=2 (30ml/min), preac = 1 barb Treac = 235 °C, GHSV = 6500 h-1, H2:CO=2, preac = 5 bar

Influence of Promoter

A added promoter, like Ru or Na, resulted in a more stable surface cobalt which in

turn resulted in a higher carbon monoxide conversion as well as α. Therefore, the

new catalyst samples that achieve an improvement of TOF in comparison to the Co-

Ref reference should be doped with ruthenium or sodium, respectively.

Influence of Preparation Technique

As previously shown by the studies of BARBIER [144], JONG [109] and ALI [130] the

applied preparation technique did influence the cobalt dispersion as well as the cata-

lyst activity and product distribution. These effects can be attributed to a lowering of

reduction temperature, formation of different surface cobalt species and dismissing

SMSI effects. The mechanisms that are the basis for the noticed effects are not re-

vealed up to now and are still a matter of interest. Therefore, different preparation

techniques, e.g., incipient wetness, spreading and precipitation will be applied during

catalyst development.

3 Objectives and Methods 30

3 Objectives and Methods

The objectives of this thesis are presented together with the methods applied to

reach these goals. A detailed description of the experimental set-up is provided in

chapter 4.

3.1. OBJECTIVES

The main objective of this work was to reach a higher activity, i.e. an improvement of

the turn over frequency (TOF) as well and an increase of the α-value in comparison

to the state-of-the-art catalyst (XCO = 60 %, SC5+ = 90.1 %, α = 0.94) invented by

IGLESIA1 [11].

To achieve this aim, the work focussed on the following items:

• Variation of preparation procedure

• Variation of cobalt precursor

• Variation of support material

The preparation conditions, i.e. kind of solvent, drying procedure and reduction con-

ditions, influence the cobalt deposition and cobalt dispersion in a positive manner as

shown by many workgroups. A high cobalt dispersion resulted in a high carbon mon-

oxide conversion because of the greater number of accessible cobalt. The prepara-

tion procedure influences the kind of cobalt surface species formed on the support

surface. Therefore in this thesis the variation of the preparation procedure on cobalt

based catalyst support on titania were carried out in order to influence the reducibility

and cobalt cluster size. Additionally different methods, namely incipient wetness, pre-

cipitation, mechanical mixing with subsequent spreading and plasma induced prepa-

ration were applied in order to study their effect on the physico-chemical and catalytic

properties of the resulting catalyst.

The cobalt dispersion and the kind of surface cobalt as well were affected by different

cobalt precursors. One reason for these findings is the different molecule size of the

alternative precursor in comparison to cobalt nitrate that might prevent the migration

of cobalt atoms to large-scale cobalt clusters. Within this work eight different cobalt

precursors were applied in order to examine their influence on DCo in comparison to

the usually applied cobalt precursor cobalt nitrate. Additionally the promotional effect

of Ru and alkali on carbon monoxide conversion and chain growth probability was

studied .

1 The GHSV or flow rate for the feed gas was not mentioned within the article


Up to now alumina and silica was the mostly used support material for Fischer-

Tropsch catalysts. Based on the results of IGLESIA et al. [1] titania was considered as

a promising support material. Beside the good mechanical properties and its low

price, titania interacts in many cases with the supported active phases This perform-

ance determines the unique catalytic properties of the latter [152]. This is the reason

for giving a special attention to this system. Three additional catalysts were prepared

using different supports (CeO2, ZrO2 and Bayer-TiO2) in order to examine the support

influence on the catalytic performance.

3.2. METHODS

3.2.1. PREPARATION OF CATALYSTS

Most catalysts were prepared by the incipient wetness technique because it is a very

simple, nevertheless effective and reproducible method. Some catalysts were pre-

pared by precursor variation in order to examine the influence of the precursor influ-

ence on the catalytic performance in comparison to the reference catalyst Co-Ref

(prepared from cobalt nitrate); in more detail cobalt (II) acetyl acetonate (IW-ACAC2

catalyst), cobalt (III) acetyl acetonate (IW-ACAC3 catalyst), cobalt oxa late (IW-OXA

catalyst) and cobalt acetate (IW-ACE catalyst) were applied. The influence of the ap-

plied solvent was examined at two catalysts; instead of water a ammonia solution

(IW-OXA-NH3 catalyst) as well as acetone (IW-NIT-AC catalyst) prepared from co-

balt nit rate) was used.

The spr eading technique is based on the mechanism of the chemical transport reac-

tion. Under the chosen conditions the spreading of the cobalt precursors was as-

sumed, as formerly reported for vanadia [152-156], which should lead to higher co-

balt dispersions; 4 different catalysts (SPR-OXA, SPR-NIT, SPR-CoTiO3 and SPR-

Co3O4) were prepared by this method.

The precipitation of cobalt as hydroxide by variation of the pH-value (PR-8 and PR-

12 catalysts) and EDTA complex (PR-EDTA catalyst) was used in order to test if a

prevention of the sorption of supports pore system with the cobalt precursor led to a

higher cobalt dispersion.

Beside the conventional preparation methods the usefulness of plasma-inducedpreparation techniques were tested in order to obtain an increase of cobalt dispersionand a controlled deposition of cobalt onto the chosen support. Therefore three dif-ferent techniques were applied, namely plasma induced decomposition, plasmasputtering and plasma activation [158]. For the plasma induced decomposition tech-nique catalysts prepared by incipient wetness technique were applied. The coatedcatalyst precursor was then decomposed by means of an oxygen-plasma. It was as-


sumed that the moderate decomposition conditions (low temperature ≈ 60°C, pres-

sure ≈ 0.1 bar) prevent the migration of surface cobalt. Another possibility for the ap-

plication of the plasma technique was plasma sputtering; pure cobalt was used ascathode. An argon plasma sputtered cobalt atoms from the cathode that precipitatedon the support material sited above the anode. It was supposed that this procedurewould lead to thin cobalt layer on the support surface. Furthermore cobalt acetylacetonate was added to the support as a powder. This mixture was continuouslystirred by use of a vibrational membrane and the cobalt precursor was decomposedwithin an oxygen plasma-field, so that cobalt atoms adsorbed on the support.

3.2.2. CHARACTERISATION OF CATALYSTS

The catalyst bulk structure was examined by means of x-ray diffraction (XRD) to in-

vestigate the influence of different cobalt precursors on the formation of cobalt oxide

species, i.e. of CoO and Co3O4, respectively. Furthermore, the existence of cobalt ti-

tanate could be established. Additionally, the bulk composition was determined by

the use of ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry).

The reduction behaviour was studied by the use of Temperature Programmed Re-

duction (TPR). Based on TPR experiments more detailed information can be gath-

ered on the structure of the surface cobalt. It is well known that small particles are

more difficult to reduce than the larger ones. Likewise, the extent of reduction and the

reduction temperature are key points in the assessment of the feasibility of the newly

prepared catalyst as it is known that a low reduction temperature can be assigned to

easily accessible surface cobalt. The adsorption behaviour of the reference catalyst

was examined by Temperature Programmed Desorption (TPD) of CO in order to in-

vestigate the surface processes. The oxidation behaviour was asses by Temperature

Programmed Oxygenation (TPO), also. Cobalt dispersion was determined by the use

of CO-pulse measurements under the assumption that bridge and double bonding of

carbon monoxide can be neglected. Then, the percentage of accessible cobalt can

be directly derived from the amount of adsorbed carbon monoxide.

The surface composition and the oxidation state of the supported elements of the

catalysts were examined by means of X-ray Photoelectron Spectroscopy (XPS).

The application of the Temporal Analysis of Products (TAP) reactor system allowed a

principal insight on the interaction of H2 and CO on the surfaces of the catalysts. In

addition to these experiments DRIFT and pseudo in-situ XPS were carried out to ex-

amine the nature of the adsorbed species.


3.2.3. CATALYTIC EVALUATION

Fixed-bed-reactor: The reference catalysts Co-Ref and Co/Ru-Ref were tested under

fixed-conditions (H2:CO = 2, ptot = 20 bar, GHSV = 1200 h-1). The obtained carbon

monoxide conversion (XCO) and chain growth probability (α-value) obtained on the

reference catalysts was set as standard so that the derived data of the prepared

catalyst (as mentioned within section 3.2.1) samples were set in comparison. This

allows an easy assessment if an improved catalyst was prepared.

Slurry reactor: The catalyst IW-ACAC2 was transferred into a slurry reactor with the

aim to check the product distribution and activity by changing reaction variables and

conditions.

3.2.4. KINETIC EVALUATION

The rate of CO consumption and methane formation was obtained applying a Berty-

type gradientless recycle reactor. The advantage of this type of reactor is that the

rate constant can be directly estimated from the consumption of carbon monoxide

under the assumption that only differential amounts of the reactant is involved in the

catalytic reaction, i.e., low conversion levels only. The experiments were carried out

on IW-ACAC3 catalyst by varying reaction temperature, partial pressure of CO and

H2 in the feed-gas and residence time.

4 Experimental 34

4 Experimental

The applied characterisation techniques and experimental conditions are described

in detail. Afterwards the preparation recipes of the new catalytic materials are given.

Furthermore, specifications of the employed catalytic reactors, the analytical system

as well as the evaluation of the catalytic results are presented. Used chemicals, their

suppliers and purity are listed in the attachment.

4.1. CHARACTERISATION OF CATALYSTS

4.1.1. XRD- INVESTIGATION

The XRD-analysis was carried out by transmission powder diffractomery (Stoe STA-

DIP). A CuKα1 radiation was used and the 2 θ range from 10 to 70° was examined.

The crystalline phases were identified by use of the JCPS data bank.

4.1.2. TPR- EXPERIMENTS

TPR (Temperature Programmed Reduction, Oxidation and Desorption) experiments

were performed in a DSC-apparatus (Setaram). The gaseous compounds were ana-lysed by a quadrupole mass spectrometer.

The catalyst was pretreated in a flow of oxygen before TPR-examination (1 h at400 °C; flow rate: 30 ml/min). After the pretreatment, the catalyst bed was cooleddown to room temperature and heated up linearly to 400 °C with a ramp rate of 10K/min; afterwards the temperature was kept constant at 400°C for 30 min. A flow ofdiluted hydrogen (5 % H2 in He) was passed over the catalyst bed during TPR analy-sis.

Another set of TPR experiments was carried out using a power-input controlled oven(Eurotherm 4472) which allowed linear heating up to 400 °C; the effluent concen-tration and hence, the hydrogen consumption was measured by means of a thermalconductivity detector (TCD, educt gas: 5 % H2 in Ar; flow rate: 45 ml/min).

4.1.3. TPO- AND TPD- EXPERIMENTS

TPO and TPD experiments (Temperature Programmed Oxidation and Desorption)

were performed in a DSC-apparatus (Setaram). The gaseous compounds were ana-lysed by a quadrupole mass spectrometer.

The sample was reduced in a flow of hydrogen (400 °C, 2 h, 30 ml/min) before ex-perimentation (for TPO as well as TPD). For TPO-experiments a flow of diluted oxy-gen (5 % O2 in He) was used. For TPD-examinations the catalyst surface was firstcovered with carbon monoxide (5 % CO in He). Subsequently, a flow of helium was

4 Experimental 35

passed through the reactor. The flow rate for all gas mixtures mentioned above was30 ml/min.

After pretreatment the catalyst bed was cooled down to room temperature andheated up linearly to 600 °C with a ramp rate of 20 K/min.

4.1.4. CO-PULSE EXPERIMENTS

Cobalt dispersion was determined from the amount of CO that remained on the sur-face during subsequent pulsing of CO over the reduced catalyst. A defined amount of

catalyst (pressed, crushed and sieved to a fraction of 255-350 µm) was filled into a

quartz tube and incorporated in a temperature controlled oven. The reactor outletwas connected to a Thermal Conductivity Detector (TCD). The catalyst was reduced

in a flow of hydrogen by 400 °C for 2 h (flow rate H2: 45 ml/min). Afterwards the sam-ple was purged with He at 400 °C for one hour and finally cooled down to room tem-perature. Carbon monoxide was pulsed at 23 °C over the reduced catalyst until theintensities of the CO pulses as measured by TCD remained constant.

For determining Co dispersion from CO pulse experiments it was assumed that:

· one carbon monoxide molecule adsorbs on one cobalt site (i.e., no bridgedbonding was accounted for)

· the formation of any cobalt carbonyls can be neglected

The cobalt dispersion was calculated based on the total amount of cobalt on thecatalyst as derived by ICP measurements.

100% Co Mol

CO MolD

avaibletotally

catalyst on adsorbedCo ⋅= (4.1)

Additionally the cobalt dispersion was determined based on the amount of reducedcobalt as derived from TPR-examinations:

100% Co Mol

CO Mol redD

)0(Co state reducded the in avaible

catalyst on adsorbedCo ⋅= (4.2)

4.1.5. XPS MEASUREMENTS

For determining the surface concentration of cobalt and titania, the previously oxi-

dised catalysts were studied by XPS (Fisons ESCALAB 220i-XL). The measurements

were performed applying an AlKα (1486.3 eV) X-ray source. The pressure in the

analysis chamber was below 10-8 mbar. Before taking the XP spectra in the analysis

chamber, it was possible to pretreat the sample under different atmospheres inside a

reaction chamber at elevated temperature. A transfer of the sample from the reaction

chamber to the analysis chamber without contact to air was also possible.

4 Experimental 36

The assignment of the measured binding energies to different species and oxidationstate was made by use of published reference data [159]. The correction of peak lo-cation was made applying the Ti 2p3/2 signal at 458.8 eV for Ti. The experimentswere carried out under charge compensation (Flood Gun).

An examination of the Ru-Region was not possible, due to the low amount of Ru pre-sent in the samples (0.1 wt%) as well as to the always present background noise ofcarbon which overlaps the Ru-region.

4.1.6. PSEUDO IN-SITU XPS MEASUREMENTS

Pseudo in-situ XPS measurement is defined as a examination of a sample within areaction chamber attached to the XPS apparatus. The sample can be transferred tothe analysis chamber without any contact to air and can be examined directly afterthe reaction was stopped.

First the fresh, unreduced reference catalyst was examined. Afterwards the catalystwas oxidised in a flow of oxygen (30 ml/min) at a temperature of 400 °C for 3h andthen reduced in a flow of hydrogen at 400 °C for 3h in the reaction chamber of thespectrometer. The chamber was evacuated, the sample was transferred into theanalysis chamber without contact to ambient air for XPS-spectra recording. After-wards a flow of H2:CO with a ratio of 10:1 was passed over the catalyst outside theanalysis chamber. An excess of H2 was used to avoid the formation of higher hydro-carbons that would soil the reaction chamber. Fischer-Tropsch reaction took place for12 h at a reaction temperature of 200 °C at a pressure of 18 bar. After that time onstream the reaction chamber was evacuated again and the catalyst was character-ised.

4.1.7. ICP- OES

The catalyst was dissolved in a mixture of HNO3 and HF (ratio: 1:5) and treated sub-sequently in a microwave oven (8 bar operating pressure) in order to ensure a com-plete dissolution of the sample. Subsequently the sample was analysed in the ICP-apparatus (Perkin-Elmer: Optima 3000 XL).

4.1.8. TEM- MEASUREMENTS

The TEM examinations were carried out in a Hitachi H-8100 electron microscope; theenergy of the electron beam was 100 keV. A resolution of 0.5 nm and a magnificationof 3105 was used. For the element analysis energy dispersive x-ray analysis (EDX)was applied.

4 Experimental 37

4.1.9. DRIFT- MEASUREMENTS

The cobalt catalyst was reduced in a flow of hydrogen for 3 h at a temperature of400 °C. There, a background spectrum of the fresh catalyst was taken as reference.A gas mixture (5% CO, 10% H2, 85% N2) was passed through the catalyst bed. Thereaction temperature was varied in a range between 180 °C to 220 °C. The totalpressure amounted to 1 bar. The time difference between each spectrum amountedto 1 min. The reference spectrum was subtracted from every DRIFT-spectrum in or-der to obtain a difference spectra.

4.1.10. TAP- REACTOR- SYSTEM

The pulse experiments applying the TAP-reactor system (Temporal Analysis ofProducts) were carried out under vacuum conditions (110-8 mbar) and the pulse size

was approximately 21014 molecules/pulse.

All catalysts were reduced in-situ in a flow of hydrogen at 400 °C. Afterwards, the re-actor was evacuated; the pulse experiments were carried out at a reaction tem-perature of 200 °C and mixtures of H2/He (1:1) and CO/Ne (1:1) were used. Thesample was reduced in a flow of hydrogen at 400°C for 3 h under atmospheric pres-sure. CO/Ne or H2/Ne (21014 molecules per pulse) were pulsed over the catalyst bedunder variation of temperature (180-220°C) and products were detected by a quad-rupole mass spectrometer.

Heat of Adsorption

The adsorption enthalpy was determined by means of the TAP reactor system. As-

suming that CO diffusion was in the range of Knudsen-Diffusion, the heat of adsorp-

tion for CO on the doped catalyst can be derived by equitation (4.3) as described by

GLEAVES et al. [160].

0,d

0,aadad

i

iCO

k

kln

RT

H

RT

EE

t

ttln +

∆−=

−=

− (4.3)

The term tCo describes the mean residence time of CO in the reactor system. ti cor-responds to the mean residence time of an inert gas molecule. By plotting ln(tco-ti)/ti)vs. 1/T (the so-called ARRHENIUS-plot), the heat of adsorption of CO on Co/Ru-Refcatalyst can be determined.

4.2. PREPARATION OF CATALYSTS

The preparation of the catalysts is described. The goal was to obtain supportedcatalysts containing 12 wt% of cobalt and in the case of the doped catalysts 0.1 wt%of Ru (or alkali, respectively).

4 Experimental 38

4.2.1. SUPPORT PRETREATMENT

Titania (Degussa P25) was calcined at 560°C for 16 h under ambient conditions be-fore exposure to the cobalt precursor. This pretreatment of titania leads to an enrich-ment of the rutile fraction (> 70 % rutile and < 30 % anatase), which is the more ac-tive modification of titania as had been shown by IGLESIA’S work [161].

The other supports (ceria, zirconia, and Bayer-titania) were calcined under ambient

conditions at 560°C for 12 h.

4.2.2. CATALYST PRECURSOR TREATMENT

After coating of the support with a cobalt precursor applying incipient wetness-,spreading- and precipitation techniques all catalyst precursors were decomposedand transformed into cobalt oxide in a flow of oxygen at 400°C for 4 h (heating line-arly from room temperature to 400°C; ramp rate 6 K/min; flow rate: 30 ml/min). Thedecomposition of the nitrogen containing precursors led to NO2 and H2O and of thehydrocarbon containing precursors to CO2 and H2O respectively.

4.2.3. OVERVIEW OF AL PREPARED CATALYST

In the following table 4.1 all prepared catalyst are listed sorted by the applied prepa-ration technique along with the used support material and if indicated a change of thestandard preparation technique.

4.2.4. INCIPIENT WETNESS TECHNIQUE

Preparation of Catalyst: Co-Ref, IW-ACE and IW-OXA

Co nitrate (0.33 mmol/ml, Co-Ref), cobalt acetate (0.01 mmol/ml, IW-ACE), or cobaltoxalate (0.01 mmol/ml), respectively, was dissolved in deionized water. The solutionof cobalt precursor was added to the support until visible wetness was obtained; thematerial was subsequently dried at 130 °C for 4 h. This procedure was repeated untilapprox. 12 wt% of cobalt was deposited on the support.

Variation of Cobalt Precursor: IW-ACAC2 and IW-ACAC3

Co(II)-acetyl acetonate (0.4 mmol/ml, IW-ACAC2) and Co(III)-acetyl acetonate (0.25

mmol/ml, IW-ACAC3) was dissolved in CH2Cl2. After each impregnation step the

catalyst precursors were dried in a rotary evaporator (70 mbar, 70 °C bath tempera-

ture). The impregnation steps for IW-ACAC2 and IW-ACAC3 were repeated two and

three times, respectively.

4 Experimental 39

Tab. 4.1: Overview of all prepared catalysts, their names applied cobalt precursorsand support materials

Catalyst Technique Precursor

Co

Support Remarks

Co-Ref impregnation nitrate TiO2

Co/Ru-Ref impregnation nitrate TiO2 promoter: Ru

IW-ACAC2 impregnation (II) acetyl acetonate TiO2 solvent: CH2Cl2

IW-ACAC3 impregnation (III) acetyl acetonate TiO2 solvent: CH2Cl2

IW-ACAC3-Ru impregnation (III) acetyl acetonate TiO2 solvent: CH2Cl2

promoter: Ru

IW-NIT-AC impregnation nitrate TiO2 solvent: acetone

IW-NIT-Step impregnation nitrate TiO2 stepwise decomposition

IW-OXA impregnation oxalate TiO2

IW-OXA-NH3 impregnation oxalate TiO2 solvent: NH3

IW-ACE impregnation acetate TiO2

IWB-NIT impregnation nitrate TiO2-r pure rutile

IWC-NIT impregnation nitrate CeO2

IWZ-NIT impregnation nitrate ZrO2

SPR-NIT spreading nitrate TiO2

SPR-OXA spreading oxalate TiO2

SPR-OXA-RU spreading oxalate TiO2

SPR-Co3O4 spreading Co3O4 TiO2

SPR-CoTiO3 spreading CoTiO3 TiO2

PR-8 precipitation Co(OH)2 TiO2

PR-12 precipitation Co(OH)2 TiO2

PR-12-Na precipitation Co(OH)2 TiO2 promoter: Na

PR-EDTA precipitation Co-EDTA TiO2

PR-EDTA-Ru precipitation Co-EDTA TiO2 promoter: Ru

PD-NIT plasma nitrate TiO2 decomposition

PD-ACE plasma acetate TiO2 decomposition

PD-ACAC2 plasma (II) acetyl acetonate TiO2 decomposition

PS-Co plasma Co TiO2 sputtered

PL-100W plasma Co TiO2 sputtered

Pl-150W plasma Co TiO2 sputtered

PL-AT plasma (III) acetyl acetonate TiO2 decomposition

PL-AP plasma (III) acetyl acetonate TiO2 decomposition

4 Experimental 40

Doping of IW-ACAC3 and Co-Ref with Ruthenium: IW-ACAC3-Ru and Co/Ru-Ref

The above described IW-ACAC3 as well as Co-Ref catalyst was doped with ruthe-

nium. Ru(III) chloride (1.38⋅10-5 mol/gprecursor) was dissolved in CH2Cl2 and added to

the uncalciend catalyst precursor used. The impregnated catalyst was dried at 70 °Cfor 2 h in a rotary evaporator as well.

Variation of Solvent: IW-NIT-AC and IW-OXA-NH3

The procedure described above was modified by change of solvent for the cobaltprecursor solution. Acetone (IW-NIT) or ammonia (IW-OXA) was used instead ofwater, otherwise the procedure was carried out in the same manner. Acetone wasevaporated at a pressure of 70 mbar at 50 °C (bath temperature) in a rotary evapo-rator. IW-OXA-NH3 was dried in an oven at 130 °C for 2 h.

Variation of Decomposition Procedure: IW-NIT-Step

Another impregnated cobalt catalyst was prepared based on cobalt nitrate. Thepreparation procedure was varied in the way that the applied cobalt nitrate was de-composed in a flow of pure oxygen after each impregnation step instead of drying,only. This procedure was repeated five times and the loading was increased from2.4 wt% (1. step) over 7.4 wt% (3. step) until the desired amount of approx. 12 wt%(5. Step) was achieved.

Variation of Support Material: IWC-NIT, IWZ-NIT and IWB-NIT

CeO2, ZrO2 and Bayer-titania (pure rutile type) were chosen as supports instead oftitania [162,163,164,165]. The supports were impregnated with an aqueous solutionof Co(NO3)2 (0.33 mmol/ml) until visible wetness and dried at 110 °C. This procedurewas repeated until the desired Co loading of 12 wt% was reached.

4.2.5. SPREADING OF COBALT PRECURSORS

Spreading of Cobalt Oxalate: SPR-OXA

Catalyst SPR-OXA was prepared from cobalt oxalate (3.12 g) and titania (10.00 g) bymechanical mixing. The mixture was heated from room temperature to 250°C with aramp-rate of 10 K/min under an inert atmosphere (N2). After 3 h at 250 °C the tem-perature was raised to 400°C; the decomposition of cobalt oxalate begins at 343 °C[166].

Doping of SPR-OXA with Ruthenium: SPR-OXA-Ru

The above described SPR-OXA catalyst was doped with Ru after spreading of the

precursor. Ru(III) chloride (1.38⋅10-5 mol/gprecursor) was dissolved in CH2Cl2, added to

the catalyst sample and dried at 80 °C for 2 h in an oven.

4 Experimental 41

Spreading of Cobalt Oxide and Cobalt Titanate: SPR-Co 3O4 and SPR-CoTiO 3

The catalysts SPR-Co3O4 and SPR-CoTiO3 were prepared from cobalt oxide and co-balt titanate as cobalt precursors. The calcined support material titania (10.00 g foreach catalyst) was mechanically mixed with cobalt oxide (5.12 g) and cobalt titanate(3.29 g), respectively. The mixture was heated from room temperature to 400 °C witha ramp-rate of 10 K/min under an inert atmosphere (N2), the final temperature washeld for 12 h.

4.2.6. PRECIPITATION

Precipitation of Co(OH) 2 : PR-8 and PR-12

For preparing PR-8 catalyst, Co(NO3)2 was dissolved (0.33 mmol/ml) under stirring indeionized water containing titania (1.1 mmol/ml) at 60 °C; then a saturated NaHCO3

(1 mmol/ml) solution was dropped into the slurry. The pH-value was adjusted withaqueous HNO3 until pH=8 was achieved. After two hours of stirring the solid catalystprecursor, containing cobalt hydroxide precipitated on titania was separated by filtra-tion from the solution; washed five times with water (each time: 3ml (H2O)/g catalyst)and then dried in air at 130 °C for 12 h.

In order to examine the influence of the pH-value on the cobalt deposition a catalystwas prepared by precipitation of Co(OH)2 on the support material at a pH-value of 12(PR-12).

Doping of Precipitated Co(OH) 2: PR-12-Na

The catalyst PR-12 was doped with 0.1 wt% Na as a citrate, since alkaline com-pounds are known to suppress cobalt migration during calcination [167].

Precipitation of Cobalt-EDTA-complex: PR-EDTA

To prepare Co-EDTA catalyst, Co(NO3)2 (0.33 mmol/ml) was dissolved in distilledwater to which titania was added and heated to 60 °C. After 1 h of vigorous stirringthe necessary amount of 0.03 mol EDTA was added and the pH-value was adjustedto 9 with an aqueous 25 vol% -NH3-solution. The water was evaporated and the re-sulting solid material was subsequently dried under ambient pressure at 130 °C overnight.

Doping of PR-EDTA with Ruthenium: PR-EDTA-Ru

After drying of the precipitated complex (see above) ruthenium chloride, dissolved inCH2Cl2, was added to the catalyst which was subsequently dried in a rotary evapo-rator (40 hPa, bath temperature: 70 °C) consequently.

4 Experimental 42

4.2.7. PLASMA INDUCED PREPARATION

Plasma Decomposition: PD-NIT, PD-ACE, PD-ACAC2

Three different kinds of cobalt precursors, cobalt acetyl acetonate, cobalt acetate and

cobalt oxalate were used for preparing the catalyst precursors applying the incipient

wetness technique. After coating of the support (TiO2) and subsequent drying at

110 °C, the impregnated support was treated in an oxygen-plasma. The plasma was

generated by a micro-wave source with a power of 120 W. The decomposition took

place at approx. 60 °C. A scheme of the apparatus is given in Fig. 4.1.

Plasma Sputtering: PS-Co

Titania (pressed and sieved into a fraction of 250-355 µm) was placed into the

plasma apparatus. Solid cobalt metal was used as cathode. Small amounts of cobaltwere scraped off by the atomic, ionic and radical oxygen species within the plasmafield. These cobalt species then precipitated on the support (see Fig. 4.1).

impregnated catalyst

anode

kathode

fitting

glas cylinder

rotor

pla

sma

filed

Fig. 4.1: Scheme of the apparatus applied for plasma decomposition

Vibrational Plasma Sputtering: PL-100W and PL-150W

Two catalysts were prepared by the means of a novel plasma preparation technique.

The support was sieved into a fine fraction of < 125 µm; in this case titania was used

but every other support material may also be coated by the described method.

4 Experimental 43

This fine powder was placed on a membrane stimulated by a frequency generator, sothat the powder was mixed and turned around by the vibrations (seeFig. 4.2). Themembrane itself was fixed within the plasma field that allows an even coating of thesupport material. As cathode a cobalt cylinder was used. The cobalt was sputteredby an argon-plasma under a pressure of 65 mbar. Two catalysts were prepared ap-plying a plasma power of 100W [PL-100W] and 150W [PL-150W], respectively.

Plasma Activation: PL-AT and PL-AP

For plasma activation, the cobalt precursor (cobalt (III) acetyl acetonate) was mixed

with the sieved support instead of applying impregnated precursor as described for

the plasma decomposition technique. The cobalt precursor was then decomposed in

an oxygen-plasma under the conditions described above. For the catalyst PL-AT a

tablet of cobalt precursor was used, for the catalyst PL-AP cobalt acetyl acetonate

was added as a powder (see Fig. 4.2).

anode

kathode

pla

sma

filed

membrane

catalyst precursor

Fig. 4.2: Plasma apparatus for vibrational and activation technique.

4.3. CATALYTIC TESTING

The feed gas was supplied by three PC-controlled mass-flow-controllers (MFC 1 toMFC 3), which provided carbon monoxide, hydrogen and nitrogen up to 50 bar totalpressure. A flow sheet of the testing equipment is shown in (see Fig. 4.3). Behind themass-flow controller’s three valves (V1 to V3) were mounted which were attached tothe personal computer and shut down in case of a malfunction of the equipment. The

4 Experimental 44

gaseous streams were passed through high-grade steel tubes (type: V4A) with anouter diameter of 6 mm. The feed gas passed a safety valve (SV1); it was purifiedand mixed in an Oxisorb- and a Hydrosorb-cartridge (PF1 and PF2). The Oxisorb-cartridge removed eventually existing oxygen and the feed gas was dried by the Hy-drosorb-cartridge. Behind another safety valve (SV2), a 3-way-valve (3V4) was in-stalled to lead the^23456 789ijo gas mixture through the catalytic reactor or over abypass-tube.

The fixed-bed reactor could be replaced by either a slurry or a Berty-type reactor.The temperature was monitored by axial movable thermocouples (type: PT-100) overthe whole reactor length (TC2 and TC3). A maximum axial temperature gradient of3°C was measured under steady-state conditions; thus, the reactor was nearly iso-thermal. The reactor was heated applying a heating tape; its electrical power inputwas controlled by a personal computer and the temperature was monitored via ther-mocouple TC4.

After the reactor a 3-way valve (3V5) led the stream either to the condenser C1 or toC2. The condenser C1 had a volume of 1.5l, allowing catalytic tests overnight orweekend. The pressure of the gaseous mixture was released by a back pressureregulator PC1 and led to a Siemens gas chromatograph (see below) under atmos-pheric pressure for product analysis; the pre-pressure of the analyse-gas was moni-tored by PI3 in order to guarantee a constant flow through the GC. The pre-pressurewas adjusted by two valves V7 and V8. The analysed gas was then passed to theexhaust.

The pressure within the testing-rig was registered by two pressure gauges mountedbefore the reactor (PI1) and after the condensation line (PI2). This construction al-lowed a rapid detection of a blockage within the reactor, condensers or tubes.

All the tubes between the safety valve SV2 and the pressure regulator PC1 were

heated by means of heating tapes; the temperature of the lines was kept constant at

200 °C during the catalytic tests.

4 Experimental 45

Fig. 4.3: Flow-diagram of the Fischer-Tropsch testing-rig

4 Experimental 46

4.3.1. FIXED-BED REACTOR

The tubular fixed-bed was made of high-grade steel (V4A) with a length of 8 cm and

an inner diameter of 1.5 cm. The reactor was heated with a heating tape; the electri-

cal power input was controlled by a personal computer. 8 g of catalyst, diluted by in-

ert material in a ratio of catalyst to inert material = 4:1, was placed into the fixed-bed

reactor. Cooling of the reactor was not necessary because of its large surface-to-vol-

ume ratio, the small amount of catalyst and the relatively low catalytic activity.

The catalysts were tested at a reaction pressure of 20 bar and a space velocity of

1200 h-1. The feed gas consisted of hydrogen and carbon monoxide at a ratio of 2:1,

nitrogen was added as internal standard (p(H2):p(CO):p(N2) = 12:6:2). The catalysts

were reduced in a flow of hydrogen for 4 h at 400 °C before reaction start-up.

Fig. 4.4: Scheme of the fixed-bed reactor.

4.3.2. SLURRY REACTOR

The slurry-reactor (Autoclave Engineers, 0.3 l) was applied as an alternative to the

fixed-bed reactor in the testing rig [168,169]. The impeller was driven by a magnetic

drive. The reactor was heated by a heating tape with temperature control by a per-

sonal computer; the temperature inside the slurry was monitored by a thermocouple.

As liquid phase tetracosan was used and the reaction was carried out in a semi-con-

tinuous (gas continuous, liquid batch) manner. To the molten wax, 10 g of previously

reduced IW-ACAC3 catalyst (particle size < 50 µm) was added. The reaction pres-

sure was adjusted to 20 bar and a H2 to CO ratio of 2:1 by a GHSV of 1200 h-1 was

used.

4 Experimental 47

4.3.3. BERTY REACTOR

A Berty Reactor with internal recycle (Autoclave Engineers) was used for kineticmeasurements[170]. A scheme of the reactor is given in Fig.4.5. The reactor waselectronically heated. One thermocouple was placed below the catalyst bed and an-other one was above the catalyst in the upper part of the reactor. The temperaturegradient between both thermocouples was 1 K during all experiments. The reactiontemperature was adjusted to 202 and 218 °C, respectively.

The internal impeller was driven by a magnetic drive with a speed of 1500 rpm in or-der to guarantee a gradientless reaction.

As catalyst IW-ACAC3 (10g, fraction 500 – 1000 µm) was used. Before the catalytic

tests the sample was reduced in-situ at 400 °C with pure hydrogen (100 ml/min). Theratio of H2:CO was varied between 1.3 and 5.4; furthermore the GHSV was variedbetween 1200 and 2000 h-1.

Fig. 4.5: Scheme of the Berty- type reactor [171].

4.3.4. ANALYSIS OF PRODUCTS

A Siemens gaschromatograph (SiCHROMAT 2) was applied for FT product analysisequipped with a Thermal Conductivity Detector (TCD) and two Flame Ionisation De-tector’s (FID). The products obtained during FTS were analysed on-line as well asoff-line. By on-line analysis carbon monoxide, hydrogen, nitrogen, carbon dioxide andC1 to C6 hydrocarbons were separated by means of two capillary columns; a GS-Q(later on Al2O3-column) and a Molsieve 5a with a length of 30 m and a inner diameter

4 Experimental 48

of 0.53 mm was used. The permanent gases were detected by an TCD, for the hy-drocarbons a FID was applied.

Condensed high boiling hydrocarbons were dissolved in CS2 and analysed off-line.The solution was mixed with iso-propanol as internal standard and directly injected tothe column (DB-1 capillary column; length: 60 m; i.d. 0.32 mm; later on a CP-PONA:length 50 m; i.d. 0.23 mm), which allows to separate the hydrocarbons up to C50. Thetemperature programs applied during on-line and off-line analysis are given in Tab.4.2.

Tab. 4.2: Temperature program applied for on-line and off-line analysis

on-line Analysis off-line Analysis

Temp.

Start [°C]

Temp.

End [°C]

Ramp

[K/min]

Duration

[min]

Temp.

Start [°C]

Temp.

End [°C]

Ramp

[K/min]

Duration

[min]

50 50 0 8 60 270 10 21

50 180 20 6.5 270 270 0 159

180 180 0 32.5

4.4. DETERMINATION OF XCO, S(CN), α, TOF AND TONNOM

The carbon monoxide conversion, XCO, can be expressed as follows:

100% n

nnX

inletCO

outletCO

inletCO

CO •−= •

••(4.2)

where: inletCOn•

= molar flow entering the reactor

outletCOn•

= molar flow leaving the reactor

The yield (YCi) is defined as the amount of the desired hydrocarbon fraction (Ci)

formed during the reaction, taking the stoichiometry factors (ν) into consideration, i.e.

in the case of Ci = propane the value for ν(C3) amounted = 1 and for ν(CCO) = 3.

100% n

nY

Ci

CO

CO

CiCi •

νν

⋅=Α

•

•(4.3)

The selectivity towards Ci that corresponds to the fractional amount of Ci formed from

4 Experimental 49

CO converted can be calculated according to:

)CO(XY

S CiCi = (4.4)

Later the selectivity is given in wt%. These values can be derived by dividing SCi withthe molecular weight of Ci related to the sum of all products formed.

The chain growth probability was derived from the slope of the straight line whichwas obtained by plotting the weight concentration of Ci divided by the carbon number

n against n (see section 2.4). The described α – value is based on the hydrocarbonswith a carbon number >10.

In order to allow an easier comparison of the obtained data's the turn-over-frequencyas well as a normalised TOF is given later in the discussion of the catalytic results.The TOFnom is the quotient of the derived TOF for the tested catalyst (TOFcat) and theTOF obtained for the reference catalyst Co-Ref. This number allows an easy com-parison if the newly prepared catalyst got a better performance than the referencecatalyst; all numbers greater then 1 indicate a higher catalytic performance than onCo-Ref. The equations used for the calculation of the TOF values are given in (4.5)and (4.6).

][s n 100

X(CO) n TOF 1-

Co

CO

⋅⋅=

•(4.5)

with nCo = Co atoms as derived from CO-pulse experiments

Ref-Co

catnom TOF

TOF TOF = (4.6)

4.5. KINETIC EXPERIMENTS

Experiments were performed in a gradientless reactor (Berty-type). The partial pres-sure of nitrogen was used as internal standard to guarantee an accurate calculationof the molar flow of carbon monoxide; for detailed descriptions please refer to theappendix.

The advantage of the gradientless recycle reactor is that the rate of carbon monoxideconsumption can be derived directly out of the quotient of mole stream of carbon

monoxide at the reactor inlet ( Α

•

COn ) and outlet ( Ω

•

COn ). The CO consumption is de-

fined as:

4 Experimental 50

−=

Ω•

Α•

CO

CO

CO

n

n1X (4.7)

The rate of CO consumption related to the mass of catalyst was calculated as fol-lows:

mXnR

cat

COCOCO

⋅=−$

(4.8)

The rate of formation of compound k was determined under the assumption that theconcentration of [nk] is zero with the start of the reaction and no consecutive reac-tions with nk took place by:

mnR k

k

=

The number of moles of H2O could be estimated from the oxygen balance.

nnn H2OCOCO

$ += (4.9)

As CO2 formation was not observed in all experiments, carbon dioxide was not ac-counted for in C-and O- balance.

Furthermore, it was assumed that oxygenated hydrocarbons were not formed during

the reaction. The higher hydrocarbons were not taken into account in calculation of

partial pressures, since the number of moles of these products were low especially at

low conversion.

5 Results and Discussion 51

5 Results and Discussion

The characterisation results obtained for all catalysts are described. First, for com-

parison the data of characterisation studies on the reference catalysts (Co-Ref and

Co/Ru-Ref) will be shown. Thereafter the physico-chemical findings obtained on the

prepared samples will be presented arranged by the applied preparation technique.

The characterisation results of all catalysts will be discussed afterwards. Next, the

catalytic results arranged in the above described order are given and discussed. The

chapter closes with the presentation and discussion of the results obtained during

slurry tests and kinetic measurements.

5.1. CHARACTERISATION OF CATALYSTS

5.1.1. CHARACTERISATION OF REFERENCE CATALYSTS

XRD

No difference could be observed in the XRD pattern of the doped and undoped cata-

lyst. Three phases were detected: Co3O4 [172] and titanium dioxide in its two modifi-

cations anatase [173] and rutile [174]. The ratio of the latter two phases amounted to

3.4 and was not influenced by added ruthenium. Other possible phases like TiOX,

Co3O2, CoO, CoTiO2 and Co2TiO4 were not detected (see Fig. 5.1 ).

10 20 30 40 50 60 70

(o)(o)

Intensity / a.u.

(o)

(o)

2 θ

Fig. 5.1: XRD pattern of oxidised cobalt reference catalysts Co/Ru-Ref obtainedwithin a 2θ range from 10° to 70°. (o) =Co3O4


TPR

The reduction of the doped (Co/Ru-Ref) and undoped (Co-Ref) catalyst was investi-

gated by means of TPR within a temperature range from 25 to 500°C (reducing

agent: H2 (5%) in He). The TPR plots showed in both cases two maximum (see Fig.

5.2). The two peaks are located at 328 °C and 395 °C for the undoped and at 200 °C

and 340 °C for the doped catalyst, respectively.

TPO

Only one maximum appeared during TPO on both reduced samples (see Fig. 5.3)

which can be assigned to the re-oxidation of Co0 to cobalt oxide (Co3O4). The pres-

ence of Ru also influenced the oxidation temperature that decreased from 303 °C for

the Co-Ref catalyst to 280 °C for the doped catalyst (Co/Ru-Ref).

TPD

In the TPD-profile (see Fig. 5.4) of the doped catalyst two maximum at 130 °C and

227 °C and a broad shoulder at 340 °C were detected which correspond to three dif-

ferent adsorption states of carbon monoxide. For the catalyst Co-Ref only one de-

sorption maximum was detected at a temperature of 107 °C. At a temperature of

240°C the desorption of CO2 was observed on both catalysts.

100 200 300 400 500

395°C

II328°C

I

340°C

II200°C

I

Co/Ru-Ref

Co-Ref

H2-Consumption / a.u.

T / °C

Fig. 5.2: TPR-profiles of doped (Co/Ru-Ref) and undoped (Co-Ref) cobalt referencecatalyst (ramp rate: 10K/min, flow rate: 30 ml/min, reducing agent: H2 (5%) in He )


100 200 300 400 500

H2-Consumption / a.u.

Co/Ru-Ref

Co-Ref

303 °C

280 °C

T / °C

Fig. 5.3: TPO-profiles of doped (Co/Ru-Ref) and undoped (Co-Ref) reference catalyst(ramp rate: 20 K/min, flow rate: 30 ml/min, oxidising agent: O2 (5%) in He)

100 200 300 400 500

241°C

237°C

Co/Ru-Ref

Co-Ref

Co/Ru-Ref

Co-Ref

T / °C

100 200 300 400 500

340°C

227°C130°C

107°C

CO - desorption / a.u.CO 2-desortion / a.u.

T / °C

a) b)

Fig. 5.4: TPD-profiles of a) CO2 and b) CO desorption for reference catalysts Co-Refand Co/Ru-Ref (ramp rate: 20 K/min, flow rate: 30 ml/min, surface covered with CObefore experimentation)


Pseudo in-situ XPS Examination

Pseudo in-situ XPS measurements were made in order to follow the changes in the

surface composition after oxidation in O2, reduction in H2 and reaction with a syngas

feed (i.e. the sample can be transferred between the reaction and analysis chamber

without any contact to air; that allows a so-called pseudo in-situ XPS examination di-

rectly after oxidation, reduction and reaction, respectively).

In Fig. 5.5 the XP-spectra of the carbon region are plotted (binding energies between

275 to 300 eV). The fresh catalyst, i.e., an unused sample right after the de-

composition of the catalysts precursor, showed a carbon peak at 285.0 eV and a not

identified peak at 292.3 eV which might be a surface carbon species which combine

with oxygen from the catalysts structure. On the reduced catalyst no distinct peaks

are visible with the exception of a very broad shoulder, which indicates that most of

the carbon was removed from the catalysts surface due to preceding oxygen treat-

ment. The catalyst used within the XPS- apparatus shows two peaks in the carbon

region at 283.6 eV and 279.7 eV (see Fig. 5.5), the latter may be assigned to a car-

bide carbon species.

In the spectra of the fresh catalyst the presence of both Co0 and Co3O4 was ob-

served as shown in Fig. 5.6 [159]. The amount of Co3O4 decreased after reduction as

indicated by a significant decrease in the intensity of the shake-up satellite at

786.7 eV. The examination of the pseudo in-situ reduced catalyst (Flow of hydrogen

under atmospheric pressure; 400 °C, 3 h) pointed out, that not all available cobalt

was reduced to its metallic state because still Co3O4 was present. After the Fischer-

Tropsch reaction took place only Co0 was detected. The Co3O4 was totally reduced.

Within the Ti- region (450 to 470 eV) no change during oxidation, reduction and FT

reaction was recognised; that indicates that the support material was not affected

during the FTS.

Dispersion of Co

The cobalt dispersion amounted to 1.70 % for Co-Ref and to 1.65 % for the doped

catalyst. Therefore, doping the catalyst with ruthenium had no significant effect on

the cobalt dispersion. IGLESIA et al. determined a Co dispersion of 2.3 % [12].


300 295 290 285 280 275

292.3 eV

285.0 eV

283.6 eV 279.5 eVreference catalyst afterFT reaction

reference catalyst afteroxidation and reduction

fresh catalyst

Intensity / a.u.

binding energy (eV)

Fig. 5.5: XP-spectra measured in the carbon region on the fresh, reduced and used

(FTS within the reaction chamber) reference catalyst Co-Ref.

810 800 790 780 770

802.3 eV

795.0 eV

786.7 eV

779.8 eV

781.0 eV796.1 eV

777.6 eV792.5 eVreference catalyst after

FT reaction

reference catalyst afteroxidation and reduction

fresh catalyst

Intensity / a.u.

binding energy (eV)

Fig. 5.6: XP-spectra measured in the Co region on the fresh, reduced and used (FTS

within the reaction chamber) reference catalyst Co-Ref.


TEM-Results

The TEM patterns show that different, distinguishable regions on the sample exist.

Small spots are located as separate spheres on the support material. These spheres

consisted mainly of cobalt as revealed by the elementary analysis applying EDX

(Tab. 5.1).

Tab. 5.1: Results of elementary analysis of Co/Ru-Ref catalysts determined by EDX

examinations

Component Sample Substrate Sphere

[wt%] Aa) [wt%] Ba) [wt%] Ca) [wt%] Da) [wt%]

Ti 53.5 22.4 49.4 13.4 24.1

O 36.3 75.7 50.3 2.9 32.7

Co 9.1 1.8 0.02 78.2 40.5

Ru 0.9 0.0 0.1 5.3 2.6

a) refer to Fig. 5.7

Fig. 5.7: TEM photography of Co/Ru-Ref (resolution 0.5 nm, magnification 3·105)

DRIFT Adsorption Spectra

The DRIFT-spectra measured during exposure of a gaseous mixture of CO, H2 and

N2 to Co/Ru-Ref catalyst under atmospheric pressure at 200°C indicated the forma-

tion of an alkene-species (band at 862 cm-1, Fig. 5.8), beside gaseous carbon dioxide


(band at 1542 cm-1) already after one minute of reaction. This result was to be ex-

pected by suggesting the previously reported mechanistic models in which an alkene

species plays a key-role in the chain propagation. After a short period of time (6 min.)

bands of adsorbed alkane-species (2783 cm-1) in addition to adsorbed CO

(2029 cm-1) were also visible. An evidence for bridge-bonded carbon monoxide was

not detected. The assignment of the resonance frequencies are in good agreement

with results obtained by FREDRIKSEN et al. [175] and Price et al. [176].

Tab. 5.2: Assignment of the bands from in-situ DRIFT experiments over Co/Ru-Ref

catalyst (reaction conditions: 5% CO, 10% H2, 85% N2; ptot = 1 bar; Treac = 200 °C)

Species cm -1 Species cm -1

ν (C-H), alkane 3079 ν (CO) 2118

ν (C-H), alkane 2935 (CO2), gas phase 1542

ν (C-H), alkane 2783 ν (CH), alkene 1444

(CO2), gas phase 2182 ν (RC=CH2) 862

2400 2000 1600 1200 800

2182 cm-12029 cm-1

862 cm-1

1542 cm-11444 cm-1

61 min

36 min

18 min

6 min

1 min.

0 min.

cm-1

2118 cm-1

Fig. 5.8: DRIFT- spectra of adsorbed reaction intermediates on Co/Ru-Ref catalyst,

exposed to a gaseous mixture of CO/H2/N2 (p = 0.05/0.1/0.85 bar) at 200 °C.


TAP-investigation

• Heat of Adsorption of carbon monoxide

An ARRHENIUS- plot of 1/T versus ln(tCO-tNe/tNe) for the Co/Ru-Ref catalyst is given in

Fig. 5.9. Within the examined temperature region from 455 °C to 485 °C a value of

∆Had for CO was determined to be -43 kJ/mol.

• H2 and CO surface interactions

In the TAP-reactor system two different catalysts were examined, namely the doped

and undoped (Co-Ref and Co/Ru-Ref) reference catalysts. First the adsorption of hy-

drogen was investigated in order to find out the role of the promoter on the adsorption

behaviour. For this purpose H2 was pulsed over quartz, the doped and undoped ref-

erence catalyst. Adsorption of H2 occurred on the undoped and doped reference

catalyst, as derived from the broad response signal in comparison to the quartz re-

sponse signal (Fig. 5.10). Adsorption of H2 was stronger over the Co/Ru-Ref catalyst

in comparison to the undoped catalyst as can be concluded from the broader shape

of the response signal in case of the doped catalyst.

0,00208 0,00212 0,00216 0,00220

-0,4

-0,2

0,0

0,2

0,4

0,6

∆Had = -43kJ / mol

ln (

t CO-t

Ne /

t N

e )

1/T [K]

Fig. 5.9: ARRHENIUS plot of 1/T versus ln(tCO-tNe/tNe) for the determination of the heat

of adsorption of CO on Co/Ru-Ref.


0,0 0,1 0,2 0,3 0,4 0,50,0

0,2

0,4

0,6

0,8

1,0

QuartzCo-Ref

Co/Ru-Ref

Intensity / a.u.

t / s

Fig. 5.10: Response signal of H2 after pulsing over quartz, doped and undoped refer-

ence catalyst (feed: H2/He = 1/1, pulse size: 2·1014 molecules per pulse, temperature:

200 °C, preac = 1·10-8 mbar)

The interaction of CO with the catalyst surface was examined for the reduced, un-

doped catalyst at 200 °C. In Fig. 5.11 the response signals of CO are plotted which

expressed the adsorption capacity of the catalysts surface. After the first pulse only

4.6 % of the pulsed CO was detected at the reactor outlet as derived from the peak

area of the response signal. The area of the response signals increased with the

number of pulses. After the 10th pulse 31 % of the pulsed CO molecules was de-

tected at the reactor outlet. These results indicate a saturation of the catalyst surface

with CO. From the pulse experiments the cobalt dispersion can be estimated; a DCo

of 0.62 % was obtained. This value is about 3 times smaller in comparison to the re-

sult obtained from CO pulse experiments at room temperature and atmosperic pres-

sure.

In addition, CO was pulsed again over the pre-saturated catalyst (saturated with 100

pulses on CO), but not the whole amount of pulsed CO reached the reactor outlet.

This loss might be due to a reaction or decomposition of CO on the catalyst surface.

It was found that besides the adsorption of CO its reaction to CO2 took place during

CO pulsing. This CO2 was formed by the BOUDOUARD-reaction and therefore, carbon

deposits must have formed on the surface. Another result is that CO2 desorped faster

from the catalyst surface than CO. These data supported the results obtained during

the DRIFT-measurements on which gas-phase carbon dioxide was observed, also.


Directly after the CO-pretreatment hydrogen was pulsed over the Co/Ru-Ref catalyst

and the formation of methane species could be detected (see Fig. 5.12). This meth-

ane must have formed by a reaction of H2 with the carbonaceous species deposited

on the surface during the CO pretreatment.

0,0 0,2 0,4 0,6 0,8 1,00,0

0,2

0,4

0,6

0,8

1,0

1,2

10.th Pulse (area = 31.3 %)

5.th Pulse (area = 14,2 %)

2.nd Pulse (area = 8.9 %)

1.st Pulse (area = 4.6 %)

Intensity / a.u.

t / s

Fig. 5.11: Response signals of CO pulsed over the freshly reduced undoped catalyst

Co-Ref (feed: CO/Ne = 1/1, pulse size: 2·1014 molecules per pulse, temperature:

200 °C, preac = 1·10-8 mbar).

No increase in CH4 formation was observed when pre-adsorbed CO (live-time = 100-

200 ms) was present on the surface, as was shown by pulsing CO and H2 sequen-

tially.

When H2 was pulsed over the catalyst after a night break, the yield of methane was

higher compared to the above described experiment. After additional pulsing of CO

the yield of CH4 decreased again (see Fig. 5.13). Therefore, it may be concluded that

freshly adsorbed CO covers the carbon deposits active in methane formation.


0,0 0,2 0,4 0,6 0,8 1,0

CH4

H2

normalized intensity

t / s

Fig. 5.12: Formation of methane during pulsing of H2 over the CO saturated (100

pulses) catalysts surface (feed: H2/He = 1/1, pulse size: 2·1014 molecules per pulse,

temperature: 200 °C, preac = 1·10-8 mbar).

0,0 0,2 0,4 0,6 0,8 1,00,0

0,5

1,0

1,5

2,0

2,5

3,0

CH4 ( after additional CO pulsing)

CH4 ( directly after night break)

Intensity / a.u.

t / s

Fig. 5.13: Formation of methane during pulsing H2 over the surface containing carbon

from CO decomposition (feed: H2/He = 1/1, pulse size: 2·1014 molecules per pulse,

temperature: 200 °C, preac = 1·10-8 mbar).


Summary of Characterisation Results Obtained on Reference Catalysts

As shown by TPR the added promoter Ru influenced the reduction behaviour of the

reference catalyst as expressed in a lower reduction temperature (340 °C) in com-

parison to the undoped catalyst Co-Ref (395 °C). Additionally, the temperature of re-

oxidation (280 °C instead of 303 °C as derived from TPO) as well as the desorption

behaviour of pre-adsorbed CO is affected (3 desorption peak instead of 2 as derived

from TPD) by the presence of ruthenium. As shown TAP experiments a stronger in-

teraction of CO with Co/Ru-Ref compared to Co-Ref was noticed as expressed in a

longer mean residence time of pulsed CO.

The bulk compositions (XRD) as well as the cobalt dispersion (CO-pulse) was not in-

fluenced by promoter addition; on both catalysts the same results were obtained.

From DRIFT studies the formation of alkene species at the beginning of the FT reac-

tion was derived; with progressing reaction time alkanes were formed. Furthermore,

the presence of bridged bounded carbon monoxide on the catalyst could excluded.

5.1.2. CHARACTERISATION OF IMPREGNATED CATALYSTS

The characterisation results obtained on the impregnated catalysts, namely IW-

ACAC3, IW-ACAC3-Ru, IW-ACAC2, IW-NIT-Step, IW-NIT-AC, IW-OXA, IW-OXA-

NH3, and IW-ACE (for explanation of the applied catalysts name refer to section

4.2.3) are described.

XRD

All impregnated catalysts were examined by means of XRD in their oxidised state.

On all catalysts crystalline Co3O4 was observed independent from the type of applied

precursor. The interference peaks of cobalt oxide on the catalyst were not very

sharp.

Only for IW-ACAC3 catalyst a CoTiO3 phase was observed in addition to Co3O4; it

have been formed during the calcination procedure. The formation of CoTiO3 by the

use of cobalt(III) acetyl acetonate as precursor has not been reported in the literature

yet. The addition of Ru to the IW-ACAC3 catalyst obviously influenced the formation

of crystalline phases during the decomposition process of the cobalt precursor; for

IW-ACAC3-Ru, only Co3O4 was detected; no peaks corresponding to CoTiO3 phase

were present (see Fig. 5.14).


20 30 40 50 60 70

(o)

(o)

(o)

(o)

(o)

(o) ****

Co-ACAC3-Ru

Co-ACAC3

Inte

nsity

/

a.u

.

2 θ

Fig. 5.14: XRD- pattern of the oxidised doped and undoped Co-ACAC3 catalyst ob-

tained within a 2θ range from 20° to 70° (∗ = CoTiO3, (o) = Co3O4)

In order to examine the influence of the decomposition procedure on the catalyst

phase composition IW-NIT-Step catalyst was prepared. On this catalyst the impreg-

nated cobalt precursor was decomposed in a flow of oxygen after each impregnation

step. In Fig. 5.15 the XRD-spectra of each decomposition step is given in comparison

to the catalyst Co-Ref. It is obvious that during the first step Co3O4 (Fig. 5.15, 1st

step) is formed. Although after each decomposition step the total amount of cobalt on

the catalyst increased (from 4.4 wt% to 12 wt%), the amount of crystalline cobalt

Co3O4 did not increase in the same manner as expressed in the intensity of the peak

at 2θ = 37°. From the difference of the 5th decomposition step minus the 1st decom-

position step could be derived that most of the cobalt oxide existed in the amorphous

state; if all applied cobalt nitrate was transformed to crystalline cobalt oxide the inten-

sity should have been al least 2times higher.

TPR

Under reaction conditions most of the cobalt exists preferentially in its reduced, i.e.,

its metallic state (Co0), especially at the low conversion rates reported in this thesis.

Therefore, the reducibility of the cobalt-oxide catalyst precursors was studied by

TPR.


During a blanktest carried out with the pure support material no hydrogen consump-

tion was measured; therefore it could be concluded that all TPR peaks are for the re-

duction of cobalt oxide species. For titania-supported catalysts made from organic or

complex cobalt precursors (IW-ACE, IW-OXA and IW-ACAC3) only one reduction

maximum was detected, although the presence of cobalt oxide was ascertained by

XRD-analysis for all catalysts. For Co3O4 two reduction maximum were expected

since on the reference catalyst Co-Ref two maximum were detected at 328 °C and

395 °C, respectively (see chapter 5.1.1). The catalysts reduction behaviour differed

by the location of the peak maximum in the temperature range from 332 °C (IW-ACE)

over 383 °C (IW-OXA) to 397 °C (IW-ACAC3).

20 30 40 50 60 70

In

ten

sity

/ a

.u.

Co-Ref

3rd Step

5th Step

5th - 1st. Step

1st Step

2 θ

Fig. 5.15: XRD-spectra of oxidised IW-NIT-STEP catalyst obtained within a 2θ range

from 20° to 70°. Decomposition of cobalt precursor after each impregnation step

As presented in Fig. 5.16 the TPR-plot of catalyst IW-OXA-NH3 consisted of 2 re-

duction peaks which were located at 297 °C and 365 °C. On catalyst IW-OXA an

about 18 K higher temperature was necessary in order to reduce the cobalt oxide. In

the case that IW-ACAC3 was doped with Ru the reduction temperature decreased

about 76 K to 319 °C (see Fig. 5.17). Further, on IW-ACAC3-Ru catalyst two distinct

maximum in contrast to IW-ACAC3 was revealed; the first occurred at 183 °C and the

second one at 319 °C.


The dependence of reduction temperature on metal loading is shown in Fig. 5.18 for

the catalyst prepared by stepwise decomposition after each impregnation step. It's

obvious that with an increasing cobalt content from 2.4 wt% to 12 wt% on IW-NIT-

Step catalyst, the reduction temperature increases from 146 °C to 384 °C. On the

other hand, the decomposition of the cobalt nitrate after each impregnation step

caused only one reduction maximum in comparison to Co-Ref.

0 10 20 30 40 50 60

297 °C

383 °C

IW-OXA

H 2

- co

nsum

ptio

n /

a.

u.

time on stream / min

0

100

200

300

400

365 °C IW-OXA-NH3

T / °C

Fig. 5.16: TPR-plot of IW-OXA and IW-OXA-NH3 catalysts (ramp rate: 10 K/min, re-ducing agent: H2 (5%) in He, flow: 30 ml/min)

XPS –, ICP- and CO-pulse examinations

In Tab. 5.3 the values of the Co/Ti ratios of the new samples prepared by means of

the incipient wetness technique are presented. No change of binding energy within

the titan region (binding energies from 450 eV to 470 eV) was observed. Therefore,

the formation of mixed surface oxides (CoTiOx) can be excluded with the exception of

IW-ACAC3, otherwise a shift in the Ti signal must have been detected. For the sam-

ples prepared by applying complex precursors instead of nitrate, an increase of sur-

face cobalt content was observed. The ratio of Co/Ti rises from 0.39 (Co-Ref) via

0.44 (IW-ACE) and 0.53 (IW-OXA) to 0.78 (IW-ACAC3). Additionally it should be

mentioned that the presence of Ru led to a decrease of the Co/Ti ratio from 0.78 to

0.58 for the IW-ACAC3-Ru catalyst.


100 200 300 400 500

IW-ACAC3397°C

319°C

183 °C

IW-ACAC3-Ru

H 2 -

con

sum

ptio

n /

a.

u.


0

100

200

300

400

T /

°C

Fig. 5.17: TPR-Profile of doped and undoped IW-ACAC3 catalyst (ramp rate: 10

K/min, reducing agent: H2 (5%) in He, flow: 30 ml/min)

0 20 40 60

Co-Ref

391 °C

320 °C

H2-

cons

umpt

ion

/

a.u.


0

100

200

300

400

384 °C

370 °C

146 °C93 °C

5th Step 12 wt.% Co

3rd Step 7.2 wt.% Co

1st Step 2.4 wt.% Co

T / °C

Fig. 5.18: Dependence of reduction temperature on metal loading on catalyst IW-NIT-

Step (ramp rate: 10 K/min, reducing agent: H2 (5%) in He, flow: 30 ml/min))


The variation of the preparation procedure when using cobalt nitrate did not effect the

cobalt surface ratio. The obtained Co/Ti values were between 0.39 (Co-Ref) and 0.41

(IW-NIT-Step).

The total cobalt content of the prepared catalysts was determined by ICP-OES (see

Tab. 5.3) The total Co content ranges from 10.4 to 12.1 %. On the reference catalyst

a DCo of 1.6 % and a DCored of 6.1 % was obtained. An increase in cobalt dispersion

up to 1.8 % (DCored = 7.8 %) was achieved on the catalyst IW-ACE and IW-NIT-AC.

On the catalysts IW-ACAC3-Ru and IW-NIT-Step the value of DCo amounted to of

1.6 % but the DCored was to 8.5 and 6.3 %, respectively. Furthermore, it should be

mentioned that Ru addition to IW-ACAC3 resulted in an 31 % higher dispersion in

comparison to the undoped sample. For the other catalysts the dispersion was be-

tween 1.0 and 1.4 %.

Tab. 5.3: Overview of Co/Ti ratio, bulk composition, cobalt dispersion, degree of re-

duction and DCored of impregnated catalysts

Catalyst Co/Ti

(XPS)

Co [wt%]

(ICP)

DCo [%]a)

(CO-pulse)

Red. [%]

(TPR)

DCored [%]b)

(CO-pulse)

Co-Ref 0.39 12.0 1.6 73.7 6.1

IW-ACAC2 0.40 11.1 1.2 76.0 5.0

IW-ACAC3 0.78 11.7 1.1 88.7 9.8

IW-ACAC3-Ru 0.58 10.4 1.6 81.8 8.5

IW-ACE 0.44 11.0 1.8 76.9 7.8

IW-NIT-AC 0.41 12.1 1.8 76.9 7.8

IW-NIT-Step 0.40 10.7 1.6 74.6 6.3

IW-OXA 0.44 10.3 1.0 56.5 2.3

IW-OXA-NH3 0.54 11.8 1.4 74.0 5.4

a) DCo = mol CO pulsed / mol Co on catalyst b) DCored = mol CO pulsed / mol Co in metallic state

Summary of Characterisation Results Obtained on Impregnated Catalysts

Cobalt oxide (Co3O4) was present on all catalysts as derived from XRD. Only in the

case of IW-ACAC3 an additional crystalline phase, namely CoTiO3, was observed; by

doping of IW-ACAC3 with Ru the formation of a cobalt titanate was not observed

anymore.

On catalysts prepared from organic and complex precursors only one peak for the

reduction of cobalt oxide was obtained (TPR). The use of different solvents starting


from the same cobalt precursor resulted in a lower reduction temperature in compari-

son to catalysts prepared from an aqueous cobalt precursor solutions (TPR).

An raise in cobalt dispersion (DCored) was obtained applying cobalt acetate (DCored =

7.8 %) and (III) acetyl acetonate (DCored = 9.8 %) instead of cobalt nitrate (DCored =

6.1 %) for impregnation as derived from CO-pulse experiments; further an enrich-

ment of surface cobalt (described by Co/Ti ratio) was obtained (XPS).

5.1.3. CHARACTERISATION OF COBALT BASED CATALYSTS SUPPORTED ON

CERIA, ZIRCONIA AND TITANIA (RUTILE TYPE)

XRD

When ceria was used as support no crystalline cobalt oxide neither Co3O4 nor CoO

beside the interference pattern of crystalline CeO2 was detected by XRD examina-

tions (see Fig. 5.19). On zirconia supported catalyst, crystalline cobalt oxide Co3O4

was formed during precursor decomposition and the peaks were very narrow. For

IWB-NIT catalyst, only titania in its rutile modification and crystalline Co3O4 were pre-

sent.

10 20 30 40 50 60 70

IWC-NIT

IWB-NIT

IWZ-NIT

(O)(O)(O)(O)(O) (O)

Inte

nsi

ty /

a.u

.

2 θ

Fig. 5.19: XRD pattern obtained within a 2θ range from 20° to 70° of oxidised cata-

lysts prepared ex cobalt nitrate supported on ceria (IWC-NIT), titania-rutile type (IWB-

NIT) and zirconia (IWZ-NIT); (O) = Co3O4


TPR-results

The TPR-profiles of cobalt supported on ceria and zirconia were similar to that ob-

tained for the reference catalyst. The profile was composed of one shoulder and one

maximum. As plotted in Fig. 5.20, the shoulder for the catalyst IWC-NIT was more

distinctive than for the IWZ-NIT catalyst. The TPR-profile indicates that amorphous

cobalt oxide must be present on ceria (IWC-NIT) because of the well defined re-

duction peaks for Co3O4. Furthermore, the catalyst IWC-NIT was more easily to re-

duce; the reduction peak maximum was located at 365 °C. For the catalyst supported

on zirconia a temperature of 395 °C was required to obtain a reduced catalyst.

The cobalt catalyst IWB-NIT which was supported on Bayer-titania (rutile type)

showed only one reduction maximum which was located at 265 °C. This temperature

was about 130 °C lower than on the reference catalyst Co-Ref as described in

section 5.1.1.

20 40 60

331°C392°C

380°C

T

/ °

C


H 2

- co

nsum

ptio

n /

a.

u.

IWZ-NIT

0

100

200

300

400314°C

251°C

IWB-NIT

IWC-NIT

Fig. 5.20: TPR-profiles of catalysts supported on various materials (ramp rate: 10

K/min, reducing agent: H2 (5%) in He, flow: 30 ml/min)

XPS, ICP and CO-pulse examinations

As shown in Tab. 5.4 on all supports an enrichment of cobalt on the samples surface

was detected in comparison to the Co-Ref catalyst (Co/Ti = 0.39). The highest

Co/support ratio was observed for the zirconia supported catalysts (0.59). As shown


by means of ICP examinations in all cases the desired amount of approx. 12 wt%

cobalt was achieved.

An improvement of cobalt dispersion was achieved on the IWC-NIT catalyst (see

Tab. 5.4). On this sample the DCO and DCored amounted to 2.4 % and 7.7 %, respec-

tively. The IWB-catalyst (pure rutile type of titania) showed a little higher DCO of 1.9 %

in comparison to Co-Ref catalyst prepared on Degussa P25 in which a mixture of ru-

tile and anatase was present. However, the DCored value derived was to 4.2 % in

comparison to 6.1 % for the reference catalyst due to the low amount of reduced co-

balt. On the zirconia supported IWZ-NIT catalyst only a cobalt dispersion value of

1.0 % was obtained.

Tab. 5.4: Overview of Co/support ratio, cobalt content, cobalt dispersion, degree of

reduction and DCored for cobalt catalysts supported on different carriers.

Catalyst Co/Sup ratio

(XPS)

Co [wt%]

(ICP)

DCo [%]

(CO-pulse)

Red. [%]

(TPR)

DCored [%]

(CO-pulse)

Co-Ref 0.39 12.0 1.7 73.7 6.1

IWB-NIT 0.49 11.8 1.9 54.7 4.2

IWC-NIT 0.52 12.5 2.4 68.8 7.7

IWZ-NIT 0.59 12.0 1.0 76.1 4.2


Summary of Characterisation Results Obtained on Cobalt Based Catalyst Sup-ported on Ceria, Titania (rutile) and Zirconia

Co3O4 was present on all catalysts with the exception of IWC-NIT (XRD). Therefore,

amorphous cobalt oxide must be formed on IWC-NIT during calcination because

from TPR studies it can be concluded that Co3O4 was on the catalyst surface. The

temperature, necessary for the reduction of cobalt oxide, was lower on all catalyst in

comparison to Co-Ref.

Only on IWC-NIT an improvement of DCored (7.7 %) was obtained compared to Co-

Ref (6.1 %).


5.1.4. CHARACTERISATION OF SPREADED CATALYSTS

The results obtained on the catalysts SPR-Co3O4, SPR-CoTiO3, SPR-OXA and SPR-

OXA-Ru are given afterwards (for explanation of the applied catalyst name refers to

section 4.2.3).

XRD

The XRD-pattern of the SPR-CoTiO3 and SPR-Co3O4 catalyst is plotted in Fig. 5.21.

Titania was present in both its modifications i.e., rutile and anatase in both cases. On

the catalysts prepared from Co3O4 and CoTiO3 no other crystalline phase beside

those of the applied precursors was observed; that means that no phase transforma-

tion of Co3O4 to CoTiO3 or reverse, occurred.

On the SPR-OXA catalyst as well as on SPR-OXA-Ru no phenomena worth men-

tioning occurred. Only titania and cobalt oxide (Co3O4) were present. However, the

addition of Ru did not effect the phase composition of the catalyst.

TPR

As presented in Fig. 5.22 the TPR-profiles of pure Co3O4 are given along with the

supported cobalt oxide (SPR-Co3O4) as well as pure CoTiO3 beside SPR-CoTiO3.

The TPR- pattern of the pure cobalt oxide consists of two maximum located at

346 °C and 384 °C in contrast to the SPR-Co3O4 catalyst which consists of only one

reduction peak (378 °C). Furthermore the reduction maximum of the latter was about

6 K lower in comparison to pure cobalt oxide. An opposite behaviour was observed

for SPR-CoTiO3; for the supported cobalt titanate a 17 K higher reduction tempera-

ture was necessary.

The reducibility of SPR-OXA catalyst is illustrated in Fig. 5.23 in comparison to the

doped catalysts SPR-OXA-Ru. In the spectrum of the SPR-OXA only one reduction

peak located at 384 °C exists. For the doped catalyst, two peaks were detected. The

presence of ruthenium leads to a decreasing reduction temperature, the major re-

duction peak was shifted to 352 °C. A small reduction peak was visible at 203 °C.


20 30 40 50 60 70

CoTiO3CoTiO3

CoTiO3CoTiO3

Co3O4

Co3O4

Co3O4

SPR-Co3O4

SPR-CoTiO3In

ten

sity

/

a

.u.

2 θ

Fig. 5.21: XRD-spectra of oxidised SPR-CoTiO3 and SPR-Co3O4 catalysts obtainedwithin a 2θ range from 20° to 70°.

100 200 300 400 500

384°C346°C

407°C

378°C

390°C

SPR-CoTiO3

pure Co3O4

pureCoTiO3

SPR-Co3O4

H 2

- c

onsu

mpt

ion

/

a.u.

T / °C

Fig. 5.22: TPR-profiles of pure Co3O4 and CoTiO3 and spreaded catalysts SPR-

Co3O4 and SPR-CoTiO3 (ramp rate: 10 K/min, reducing agent: H2 (5%) in He).


0 10 20 30 40 50 60

SPR-OXA-Ru

SPR-OXA

H2-

cons

umpt

ion

/

a.u.


0

100

200

300

400352 °C

384 °C

203 °C

Fig. 5.23: TPR-plot of doped and undoped SPR-OXA catalyst (ramp rate: 10 K/min,

reducing agent: H2 (5%) in He, flow: 30 ml/min)

XPS-, ICP-OES-and CO-pulse measurements

On the catalysts starting from pure Co3O4 or CoTiO3 a decreasing surface content of

cobalt was observed (see Tab. 5.5).The Co/Ti ratio amounted to 0.07 (SPR-CoTiO3)

and 0.25 (SPR-Co3O4), respectively. On the other side the use of cobalt oxalate led

to an increasing Co/Ti ratio (0.53) in comparison to Co-Ref catalyst (0.39); this find-

ing was similar to the catalysts prepared from organic cobalt precursors by means of

the impregnation technique. On the doped catalyst SPR-OXA-Ru an increasing Co/Ti

ratio of 0.53 to 0.56 compared to SPR-OXA was observed.

An increase of cobalt dispersion on the catalysts prepared by the spreading method

was not observed. On SPR-Co3O4 and SPR-CoTiO3 the lowest DCo of all prepared

catalyst was obtained with DCo values of 0.3 % and 0.2 %. On SPR-OXA the cobalt

dispersion was to 1.0 % and a slight increase to 1.2 % was observed after the addi-

tion of ruthenium as promoter. However, that was no improvement in comparison to

the catalyst CO-Ref (1.6 %). But when the cobalt dispersion was calculated based on

the amount of reduced cobalt a raise in DCored to 7.7 % (SPR-OXA) and 8.3 % (SPR-

OXA-Ru) compared to the reference catalyst (Co-Ref) was obtained.


Tab. 5.5: Overview of Co/Ti ratio, cobalt content, cobalt dispersion, degree of reduc-

tion and DCored for catalysts prepared by spreading of cobalt precursor

Catalyst Co/Ti

(XPS)

Co/Ti

(ICP)

Co [wt%]

(ICP)

DCo [%]a)

(CO-pulse)

Red. [%]

(TPR)

DCored [%]b)

(CO-pulse)

Co-Ref 0.39 0.16 12.0 1.6 73.7 6.4

SPR-Co3O4 0.25 0.14 10.0 0.3 93.6 4.7

SPR-CoTiO3 0.07 0.14 10.3 0.2 80.0 1.0

SPR-OXA 0.53 0.16 11.5 1.0 87.1 7.7

SPR-OXA-Ru 0.56 0.16 11.5 1.2 85.5 8.3


Summary of Characterisation Results Obtained on Catalysts Prepared bySpreading

XRD examinations revealed no special feature than the expected pattern of CO3O4

and CoTiO3 beside titania.

In the case of SPR-OXA the addition of Ru influenced the reduction behaviour of the

catalyst; a 32 K lower reduction temperature was observed on SPR-OXA-Ru com-

pared to SPR-OXA. Additionally, a second reduction maximum at 203 °C was ob-

tained (TPR). Furthermore, an improved cobalt dispersion of 7.7 % (SPR-OXA) and

8.3 % (SPR-OXA-Ru) in comparison to 6.1 % (Co-Ref) was achieved (CO-pulse).

5.1.5. CHARACTERISATION OF PRECIPITATED CATALYST

Five catalysts were prepared applying the precipitation method, namely PR-8, PR-12,

PR-12-Na, PR-EDTA, and PR-EDTA-Ru. The data obtained during characterisation

measurements are given afterwards (for explanation of the applied catalyst name

refers to section 4.2.3).

XRD

As an example for the phase composition of the precipitated catalysts the XRD

spectra of catalyst PR-12 is shown in Fig. 5.24. Besides titania in its both modifica-

tions crystalline cobalt oxide (Co3O4) was present. In comparison to catalyst prepared

applying the incipient wetness technique with cobalt nitrate as source material the

XRD-peaks are broader, which indicates the presence of large cobalt clusters on the

catalyst surface. The catalysts, PR-EDTA, PR-8, PR-12-Na and PR-12-K were also

examined, but no peculiarities were observed; so that these XRD-plots are not given.


20 30 40 50 60 70

(O)

(O)

(O)

PR-12

Inte

nsi

ty

/

a.u

.

2 θ

Fig. 5.24: XRD-profile of the oxidised, precipitated catalyst PR-12 catalysts obtained

within a 2θ range from 20° to 70°; (o) = Co3O4

TPR

In Fig. 5.25 the TPR-profile for the preticipated catalyst PR-12 along with that of the

catalyst PR-EDTA are plotted. Two shoulders at 348 °C and 400 °C (the shoulder

was obtained after 8 min within the isothermal zone at the end of the TPR run) and

one maximum at 400°C can be detected for PR-12. By doping the catalyst with

sodium no significant influence on the reduction behaviour was observed. The

slightly lower reduction peaks at 346 °C and 397 °C were within the margins of

errors. The change of the pH-value to 8 shows in contrast to PR-12 catalyst a

reduction maximum located 13 K lower. Further was no additional shoulder within the

isothermal zone detected (please refer to Tab. 5.6 for TPR-results). On catalyst PR-

EDTA only one reduction maximum was observed. This is in accordence with results

obtained for other catalyst (e.g. IW-ACE, IW-ACAC2, IW-ACAC3) prepared from

organic cobalt precursors. For the doped catalyst PR-EDTA-Ru, the reduction

maximum was at 307 °C.


10 20 30 40 50 60

400°C

400°C8 min. isothermal

348°CPR-12

H2-

upta

ke /

a.u.

time on stream / min.

0

100

200

300

400

311°CPR-EDTA

T / °C

Fig. 5.25: TPR-Profile of PR-12 und PR-EDTA catalyst as a function of time onstream (ramp rate: 10 K/min, reducing agent: H2 (5%) in He, flow: 30 ml/min)

Tab. 5.6: Overview of resolved reduction maximum for precipitated catalysts as de-

rived from TPR experiments (temperature range: 25- 400 °C, ramp rate: 10 K/min,

reducing agent: H2 (5%) in He, flow: 30 ml/min)

Catalyst Reduction maximum [°C]

PR-8 337(m), 384(m)

PR-12 346(s), 400 (m), 400(si)

PR-12-Na 346(s) 397 (m), 400 (si)

PR-EDTA 311(m)

PR-EDTA-Ru 307(m)

(m) = maximum, (s) = shoulder, (si) = shoulder within the isothermal zone

XPS, ICP and CO-pulse-measurements

An overview of surface and bulk composition is given in Tab. 5.7 for the precipitated

catalysts. Based on results obtained from precipitation at pH 8 or 12 it can be ascer-

tained that an increase of the pH-value leads to an increase of the cobalt amount (ex-

pressed as Co/Ti) from 0.57 and 0.83 on the catalyst surface. The use of Na as pro-

moter resulted in a significant decrease of the Co/Ti surface ratio from 0.83 to 0.67.


The same effect was noticed by doping of PR-EDTA catalyst with ruthenium but in

that case the difference between the two samples is minimal and amounted to 0.58

(PR-EDTA) and 0.53 (PR-EDTA-Ru), respectively.

Tab. 5.7: Overview of Co/Ti ratio, bulk composition, cobalt dispersion, degree of re-

duction and DCored of precipitated catalysts

Catalyst Co/Ti

(XPS)

Co [wt%]

(ICP)

DCo [%]a)

(CO-pulse)

Red. [%]

(TPR)

DCored [%]b)

(CO-pulse)

Co-Ref 0.39 12.0 1.7 73.7 6.1

PR-12 0.83 10.4 1.7 75.7 7.0

PR-12Na 0.67 10.7 0.2 93.5 3.1

PR-8 0.57 11.1 1.3 71.1 4.5

PR-EDTA 0.58 10.3 1.3 79.3 6.3

PR-EDTA-Ru 0.53 11.2 1.2 81.5 6.5


As reported previously the chosen pH value influenced the Co/Ti ratio. In the same

manner DCo was effected; the cobalt dispersion increased from 1.3 % (PR-8) to 1.7 %

(PR-12) and was in the same magnitude than determined for the Co-Ref catalyst

(please refer to Tab. 5.7). The addition of Na and Ru, respectively, resulted in a de-

creasing cobalt dispersion. Similar as reported for XPS results this effect was more

strongly marked on the PR-12 catalyst.

Summary of Characterisation Results Obtained on Precipitated Catalysts

On the precipitated catalysts was the only crystalline phase beside titania in its both

modification; but the broad interference peaks indicate that larger cobalt clusters than

on impregnated catalysts were formed (XRD).

The necessary reduction temperature was for all catalysts prepared from Co(OH)2

higher than on Co-Ref. On PR-EDTA catalysts an about 70 K lower reduction tem-

perature was achieved compared to Co-Ref; An increase in DCo was noticed also.

5.1.6. CHARACTERISATION OF CATALYST APPLYING PLASMA INDUCED

PREPARATION

The plasma technique was applied in order to introduce a novel preparation tech-

nique to the FTS. The catalysts PD-Nit, PD-ACAC2, PD-ACE were prepared by the


decomposition of previously impregnated catalyst within an oxygen plasma. Cobalt

metal was sputtered for PS-Co catalyst. PL-AT, PL-PP, PL-100W and PL-150W

catalysts were prepared by a decomposition of a mechanical mixture of titania and

cobalt acetyl acetonate. The physico-chemical properties for the catalysts are de-

scribed afterwards (for explanation of the applied catalyst name refer to section

4.2.3).

XRD

XRD measurements on PD-ACAC2, PD-ACE and PD-Nit resolved only small

amounts of Co3O4 beside species of cobalt precursor as can be derived out of the in-

tensity of the cobalt oxide reference peaks; the amount of crystalline cobalt was to

only 0.7 % related to the area of the interference peaks of titania.

From the XRD examination the presence of cobalt oxide (see Fig. 5.26) could not be

proved on the PS-Co catalyst because the specific interference maximum were not

visible. An other point of interest is that the typical distribution of titania phases, e.g.;

rutile and anatase, had changed; on the plasma catalyst rutile is the only detectable

phase. For the catalysts PL-100W, PL-150W, PL-AT and PL-PP not worth mention-

ing occurrences were detected; neither a change within the rutile – anatase ratio nor

the absent of Co3O4 as crystalline phase.

10 20 30 40 50 60 70

(O)

(O)(O)(O)

(O)

(O)(O)

PS-Co catalyst

= expected Co3O

4 signals

Inte

nts

ity /

a.u

.

2 θ

Fig. 5.26: XRD-spectra of the plasma prepared catalyst PS-Co obtained within a 2θrange from 20° to 70°; (o) = Co3O4


TPR

All catalysts of the PD series showed reduction peaks known for transformation of

Co3O4 to CoO (between 310 °C and 320 °C) and for CoO to Co0 (between 380 °C

and 400 °C). The catalyst prepared from cobalt nitrate (PD-NIT) showed an addi-

tional peak at 230 °C. The catalysts prepared from organic cobalt precursors showed

additional peaks at 250 °C (PD-ACE) and 230 °C (PD-ACAC) also.

The TPR profiles of PL-100W and PL-150W catalyst are presented in Fig. 5.27. This

plot consists of one maximum at 238 °C and a adjacent shoulder at 345 °C. On PL-

150W a maximum at 243 °C was obtained but the baseline was not reached and until

the end of the TPR run a hydrogen consumption was visible. The TPR plots for PL-

AT and PL-PP showed only one reduction maximum located at 379 °C and 376 °C,

respectively.

Tab. 5.8: Overview of resolved reduction maximum for plasma- induced catalysts asderived from TPR experiments (temperature range: 25- 400 °C, ramp rate: 10 K/min,reducing agent: H2 (5%) in He, flow: 30 ml/min)

Catalyst Reduction maximum [°C]

PD-ACAC 230 (m), 314 (s), 392 (m)

PD-ACE 250 (m), 310 (m), 399 (m)

PD-NIT 230 (m), 317 (s), 386 (m)

PL-100 W 238 (m), 345(s)

PL-150W 243 (s), 320 - 400(si)

PL-AT 379 (m)

PL-PP 376 (m)

(m) = maximum, (s) = shoulder, (si) = shoulder within the isothermal zone


10 20 30 40 50 60

PL-150W

H2-

cons

umpt

ion


0

100

200

300

400

T / °C

238 °C

243 °C

PL-100W

Fig. 5.27: TPR-Profile of PL-100W and PL-150W (ramp rate: 10 K/min, reducing

agent: H2 (5%) in He, flow: 30 ml/min)

ICP examination and CO-pulse experiments

The change of preparation procedure effected on the one hand the amount of loaded

cobalt on the support, on the other hand the cobalt dispersion. First, the cobalt con-

tent was more than 5 times higher comparing to PL-100W with PL-AT. Furthermore,

an increasing DCo from 0.4 % to 1.8 % was achieved and this was similar to the ref-

erence catalyst (see Tab. 5.9).

Tab. 5.9: Overview of bulk composition, cobalt dispersion, degree of reduction and

DCored obtained for plasma catalysts

catalyst Co [wt%]

(ICP)

DCo [%]

(CO-pulse)

Red. [%]

(TPR)

DCored [%]

(CO-pulse)

Co-Ref 12.0 1.7 73.7 6.1

PL-100W 1.0 0.4 16.2 1.2

PL-150W 1.2 0.3 33.2 0.9

PL-AT 5.5 1.8 71.1 7.0

PL-PP 6.5 0.6 75.7 3.1



5.1.7. DISCUSSION OF CHARACTERISATION RESULTS

First, the characterisation results obtained for the reference catalysts Co-Ref and

Co/Ru-Ref will be discussed; subsequently the findings on the new catalysts ar-

ranged by the applied preparation technique will be commented.

Reference Catalyst

Bulk composition of reference catalysts

As shown by the XRD-results the phase composition of the reference catalyst (Co-

Ref) and the doped catalyst (Co/Ru-Ref) was identical. However, a change of phase

composition was not expected because of the low amount of added Ru. Neverthe-

less, the formation of CoRuO4 cannot completely be excluded because such small

quantities might be below the detection limit.

Reducibility of reference catalysts

The TPR profiles of the doped and undoped catalysts consist of two separate peaks.

For both samples, the maximum can be assigned to the stepwise reduction of Co3O4

(maximum 1) via CoO (maximum 2) to Co0 as described by stoichiometric formulae

(5.1) and (5.2). This result agrees with TPR-examinations reported in literature [5].

OH CoO 3 H OCo 2243 +→+ (5.1)

OH 3 Co 3 H 3 CoO 3 20

2 +→+ (5.2)

As revealed by the TPR-results (see Fig. 5.2 in section 5.1.1) the addition of small

amounts of Ruthenium to the cobalt reference catalyst resulted in a decreasing re-

duction temperature of 55 K in comparison to Co-Ref (reduction maximum at

395 °C). This may be explained by interactions between cobalt and ruthenium in-

duced by the higher mobility of Ru oxides as well as by the formation of Co-Ru ox-

ides [12]. This assumption is supported by the fact that Co2RuO4 forms a spinel

isostructural to Co3O4 [177]. On the other hand one can assume that in the presence

of Ru the reduction of Co is accelerated by a spill-over process, i.e., hydrogen ad-

sorbed on Ru has a higher adsorption enthalpy than on cobalt. The adsorbed hydro-

gen (Ru-H2) dissociate and lead to activated hydrogen (H-Ru-H) which then can dif-

fuse into the neighbouring cobalt oxide [178]. This assumption is supported by TAP

data on hydrogen interaction with the surface of Co-Ref and Co/Ru-Ref catalyst. On

the Ru-doped reference catalyst (Co/Ru-Ref) a longer mean residence time in com-

parison to the undoped catalyst was derived; this finding can be correlated with a

stronger interaction of Co/Ru-Ref catalyst with hydrogen than on Co-Ref catalyst or

in turn a stronger adsorption of H2 Co/Ru-Ref was due to the promoter. However, the

obtained TAP data were not sufficient to prove the latter assumption entirely; further


studies will be necessary.

Oxidation behaviour of reference catalysts

The observed decrease in re-oxidation temperature (as derived by TPO experiments

given in section 5.1.1, Fig. 5.3) for the doped catalyst Co/Ru-Ref (280 °C) in com-

parison to the undoped catalyst Co-Ref (303 °C) might also be due to an oxygen ac-

tivation over Ru, since on Ru oxygen spill-over was reported, also [178].

Adsorption behaviour of CO on reference catalysts

For TPD experiments the reduced catalysts surface was covered with carbon mon-

oxide and in the case of Co/Ru-Ref catalyst 3 desorption peaks (130 °C, 227 °C,

340 °C) and one desorption peak (107 °C) for Co-Ref was obtained as shown before

in Fig. 5.4. Although the existence of bridged bonded carbon monoxide beside a

linearly one is conceivable only linearly adsorbed carbon monoxide was present on

the catalysts surface as shown by DRIFT (see Fig. 5.8). Additionally, this finding jus-

tifies the applied CO-pulse method for the determination of cobalt dispersion DCo and

DCored because each adsorbed CO molecule can be counted to one cobalt atom.

One explanation for the peaks at 130 °C and 227 °C is that two different adsorption

sites are present on the catalyst. The first one can be assigned to oxidised cobalt and

the second one to reduced cobalt. This assumption is in good agreement with results

obtained by CHOI et al. [179]. The observed shoulders at 340 °C on Co/Ru-Ref cata-

lyst and at 410 °C for the undoped catalysts originate from surface carbon species

which combine with oxygen from the catalysts structure, instead from residual gases

such as CO and CO2 at elevated temperature. It can be assumed that this carbon

species was formed during the BOUDOUARD reaction. Another hint for the BOUDOUARD

reaction was given by TAP data. During the pulsing of CO over the reduced refer-

ence catalysts CO2 was found.

Surface composition of reference catalysts

As shown by XPS oxidation and reduction of the reference catalyst does not affect

the binding energies of TiO2 and Co3O4 within the Ti- and Co- region, or more precise

the distance between the 2p1/2 and the 2p

3/2 photoemission lines, for the reference

catalyst (see Fig. 5.6 in section 5.1.1. Pseudo in-situ XPS Examination). Therefore, it

can be concluded that no accountable formation of bimetallic mixed oxides took

place, as was reported to occur on supports like SiO2 [180] and Al2O3 [181]. During

pseudo in-situ XPS measurements the formation of a carbide species was observed.

Probably this CoC2 was formed in the Fischer-Tropsch reaction. The presence of co-

balt carbide was reported by CHAUMETTE et al. [182] on a Co/TiO2 catalyst. This result

indicates that under high pressure condition cobalt carbide might be involved in the

Fischer-Tropsch synthesis. The carbide mechanism can be seen as a parallel reac-


tion pathway beside the CO-insertion mechanism.

According to TEM studies an accumulation of cobalt within separated spheres were

detected (see Fig. 5.7). This result may explain the small cobalt-dispersion and sup-

port the studies of PUSKAS et al. [183] carried out on promoted cobalt catalysts in

which a sphere formation was correlated with a low cobalt dispersion. From the low

amount of Co found in the substrate by EDX it can be concluded that only a small

amount of cobalt migrated into the support material; this explains also the enrichment

of the catalysts surface as derived by XPS and expressed by the increasing Co/Ti ra-

tio.

Catalysts Prepared by Incipient Wetness Technique

Bulk and surface composition of impregnated catalysts

As revealed by XRD the formation of a cobalt titanate (CoTiO3) was observed on IW-

ACAC3 catalyst (see Fig. 5.14 in section 5.1.2). In addition the highest surface Co/Ti

ratio of all impregnated catalysts was obtained on IW-ACAC3 catalyst (Co/Ti = 0.78)

in comparison to Co-Ref catalyst (Co/Ti = 0.39) as derived by XPS (see Tab. 5.3).

Further, it should be mentioned that this enrichment of surface cobalt was observed

for all organic cobalt precursors in comparison to a catalyst made from inorganic

salts like cobalt nitrate. This effect is significant and can be related to the applied

precursor; comparable results were obtained according to a study by NIEMELÄ et al.

[25]. However, the mechanism of how the cobalt precursor interacts with the support

material is not solved up to now.

One can assume that a high Co/Ti ratio should also lead to a high cobalt dispersion

but the opposite is the case with the exception of IW-ACE. As derived from TEM ex-

periments for Co-Ref catalyst the cobalt oxide particles are arranged as spheres on

the support. During XPS experiments only uppermost located species will be de-

tected (5 to 10 atomic layers); therefore a high Co/Ti ratio indicates that compara-

tively large particles are present which in turn resulted in a low cobalt dispersion.

Similar conclusions were drawn by NIEMANTSVERDRIET [184] and LINDNER and PAPP

[185]. A reason for the high cobalt dispersion on IW-ACE might be the sorption of

Co2+ ions on titania in an acetic solution according to NICHOLSON [186], which is not

the case for other cobalt salts.

Reducibility of impregnated catalysts

For the organic cobalt precursors as well as for the stepwise prepared catalysts ex

cobalt nitrate (IW-NIT-Step) only one TPR peak was resolved within a temperature

range from 332 °C for IW-ACE catalyst to 397 °C for IW-ACAC3 catalyst (see Fig.

5.16 and Fig. 5.17). One explanation could be that only CoO was on the supports

surface. But this assumption is not supported by the XRD results in which exclusive


Co3O4 was detected. Therefore, another explanation can be given following the

studies carried out by WANG and CHEN [92]. In this work only one broad peak was

obtained during the reduction of pure Co3O4. The authors assigned it to the stepwise

reduction of cobalt oxide over Co3+ → Co2+→ Co0 and the broad peak results from an

overlap of both reduction steps.

Furthermore the reduction behaviour of the undoped and promoted IW-ACAC3 cata-

lyst should be discussed; on IW-ACAC3 catalyst only one reduction maximun located

at 397 °C in contrast to IW-ACAC3 catalyst on which two peaks at 183 °C and 319 °C

were obtained (see Fig. 5.17). The decreasing reduction temperature can be as-

cribed to the presence of Ru only and could be assigned with an easier reduction of

cobalt in the vicinity of Ru. This result led to the conclusion that bimetallic effects that

can certainly be ascribed to the close contact of the two metals affect the reduction

process. Similarly, a decreasing reduction temperature after doping with ruthenium

was also observed by thermogravimetric measurements carried out by IGLESIA et al.

[12,187] on a cobalt catalyst made from cobalt nitrate.

Catalysts Prepared by Incipient Wetness on Ceria, Zirconia and Titania (rutile)

Bulk and surface composition of cobalt catalysts supported on Ceria, Zirconia and

Titania (rutile)

The XRD results for the IWC-NIT (Fig. 5.20 in section 5.1.3) catalyst indicated that no

crystalline cobalt oxide was present on the supports surface because no interference

peaks according to Co3O4 were detected. On the other hand the presence of cobalt

oxide could be derived from TPR studies; therefore the cobalt oxide must be present

in an amorphous state. It seems that strong interactions between the support

material (ceria) and the cobalt nitrate supressed the formation of crystalline cobalt

oxide or reduced it to such small amounts that they were not detectable by XRD.

These interactions cannot be excluded for zirconia and titania (rutile-typ) supported

catalysts but it can be assumed that on these catalysts the SMSI effects was not that

marked. On the other hand the high cobalt dispersion of the IWC-NIT catalyst

(DCo = 2.4 %) in comparison to Co-Ref catalyst (DCo = 1.6 %) can be correlated with

the amorphous surface cobalt. Within an amorphous oxide no near order is effective,

i.e., the cobalt particles were not arranged in a well-defined crystalline structure.

Therefore, a great number of defect sites should be present which in turn affect the

number of accesible cobalt [188].

Reducibility of cobalt catalyst supported on Ceria, Zirconia and Titania (rutile)

The lower reduction temperature of IWB-NIT catalyst (265 °C) in comparison to Co-

Ref catalyst (395 °C) can be explained by reduced SMSI effect [189] due to the

different phase composition (see Fig. 5.20). A similar support effect on dispersion


and reduction behaviour was reported by BARTHOLOMEW et al. [190] and LAPIDUS et

al. [191] on a nickel catalyst and by PONEC and NONNEMAN [192] on a Rh catalyst,

respectively.

Catalysts Prepared by Spreading of Cobalt Precursors

Bulk and surface composition of catalyst by spreading of cobalt precursor

The catalysts prepared from pure Co3O4 and CoTiO3 showed no exceptional obser-

vations by XRD; no transformation of Co3O4 to CoTiO3 and reversed was noticed

(see Fig. 5.21 in section 5.1.4). Further, it should be mentioned that no well dis-

persed cobalt on the catalysts could be achieved; on SPR-Co3O4 the value of DCo

amount to 0.2 and for SPR-CoTiO3 to 0.3. This might be due to a neglectable migra-

tion tendency of the pure phases. The use of cobalt oxalate resulted in a better dis-

persed system as derived by CO-pulse for SPR-OXA and was to DCo = 1.0 (see Tab.

5.5). An explanation might be that cobalt oxalate melt at a temperature of 263 °C so

that the melting can fill the supports pore system [166].The addition of Ru leads to a

small increase in cobalt dispersion.

Reducibility of catalysts prepared by spreading of cobalt precursor

In the case of SPR-CO3O4 the necessary reduction temperature decreased from

384 °C to 378 °C compared with pure Co3O4 (see Fig. 5.22). This effect was previ-

ously reported for Co/SiO2 catalysts [86] and may be subscribed to a greater surface

area of the supported cobalt oxide than for pure Co3O4. An explanation for the 17 K

higher reduction temperature (407 °C) on SPR-CoTiO3 in comparison to pure cobalt

titanate was not found, yet; further examination will be necessary.

The addition of Ru to SPR-OXA catalyst led to a similar decrease in reduction tem-

perature as reported for IW-ACAC3-Ru before catalysts, instead of one maximum at

384 °C two reduction peaks were obtained at 203 °C and 352 °C; this finding can

also related to the spill-over as well as bimetallic effect.

Catalysts Prepared by Precipitation

Bulk and surface composition of catalysts prepared by precipitation

From XRD studies no special features were obtained.

The observed high Co/Ti ratio of 0.83 on PR-12 can be explained by a distinct

hydroxylation of the support. It can be assumed that the precipitated cobalt precursor

was surrounded by Ti-OH groups which explains the high amount of surface cobalt,

because the migration into the supports lattice was extremly hindered; on the other

hand the hydroxylated titania species were responsible for a suppression of cobalt

clusters aggregation which leads to the high cobalt dispersion in comparison to the


PR-8 catalyst.

As derived from ICP-OES in no case the desired amount of 12 wt% on cobalt was

achieved, instead the Co contentent amounted from 10.1 to 11.0 wt%. This finding

can be explained by an incomplete precipitation of cobalt ions onto the support (see

Tab. 5.7 in section 5.1.5).

Reducibility of catalysts prepared by precipitation

The TPR-profiles for the cobalt hydroxide based catalyst showed additional peaks

within the isothermal region (400 °C) at the end of the TPR run up to now were not

noticed for the other new catalyst (see Tab. 5.6 and Fig. 5.25 in section 5.1.5). For

catalyst PR-12 the shoulder at 348 °C and the maximum at 400 °C could be related

to the reduction of Co3O4 to Co0. The second shoulder within the isothermal zone can

be assigned to the reduction of not totally decomposed Co(OH)2. The reduction of

cobalt hydroxide can be described as follows:

OH 2 Co H )OH(Co 20

22 +→+ (5.4)

Catalyst Prepared by Plasma- Induced Techniques

Surface composition for catalysts prepared by plasma- induced techniques

The catalyst prepared by sputtering of cobalt metal onto the support did not lead to

the desired improvement in DCo nor to the desired amount of cobalt (only 0.2 wt%) on

the support. This result led to the conclusion that the sputtered plasma technique has

to be modified in a way that a longer covering time or a higher plasma power should

be used.

The catalysts PL-AT and PL-PP (see Tab. 5.9 in section 5.1.6) which were mingled

with cobalt (III) acetyl acetonate and then decomposed by an oxygen plasma

achieved an acceptable metal loading (5.5 wt% for PL-AT and 6.5 wt% for PL-PP)

cobalt dispersion (DCo(PL-AT) = 1.8 %, DCo(PL-PP) = 0.6). Up to now no comparable

preparation methods were reported in the literature and it can be assumed that due

to the mixing of support and precursor an optimal layer thickness was received. This

layer was thin enough in order to guarantee a total decomposition of the precursor

and also thick enough to obtain the highest cobalt loading of all plasma prepared

catalyst

Reducibility of catalysts prepared by plasma- induced techniques

On the TPR – spectra of catalysts from the PD (decomposition of cobalt precursors

within an oxygen plasma) series more than the two typical reduction peaks for the

stepwise reduction of Co3+ via Co2+ (between 310 to 320 °C) to Co0 (between 380 to


400 °C) were detected (see Tab. 5.8). During TPR studies on PD-NIT catalyst an ad-

ditional peak at 230 °C was obtained which can be assigned to the decomposition of

NO3 to NO2. It may be assumed that only the first layer of the impregnated precursor

was decomposed and so the expected goal to obtain a well dispersed cobalt surface

layer was not reached. This result leads to the conclusion that the plasma-decom-

position is not useful for preparation of FTS catalysts in the case of impregnated

samples, because the oxygen plasma did not penetrate the whole coating of the co-

balt precursor

5.2. CATALYTIC EVALUATION

The catalytic results obtained during FTS tests in a fixed-bed reactor on the differ-

ently prepared catalyst samples are reported. The data are arranged by the applied

preparation technique.

5.2.1. REFERENCE CATALYST

The results on syngas conversion and product distribution for the reference catalyst

Co-Ref are listed in Tab. 5.10 as a function of time on stream. After an initial period

of 18 h the CO conversion amounted to about 7 % and reached a nearly constant

value of approximately 14 to 15 % after 70 to 140 h on stream. While CO conversion

increased the selectivity to methane dropped from approx. 15 % to a nearly constant

value of 10 %. Concomitantly, the selectivity towards the C5+ fraction increased to

79 % under pseudo steady–state conditions. The chain growth probability amounted

to 0.84 after the initial start-up procedure. A typical Schulz-Flory plot in for estimation

of the α-value is given in Fig. 5.28 acting for all catalysts.

For the Ru-doped catalyst, a very similar reaction performance was observed. After

24 h t.o.s. a carbon monoxide conversion of 8.1 % was obtained and raised over

12.6 % to 16.3 %. Later, constant XCO of approx. 8 % was observed for over 40 h.

The methane selectivity behaved in the same way and decreased with a drop in XCO

to 10 % under steady-state conditions. Although the reaction temperature was raised

to 210 °C after 136 h t.o.s. the prior XCO of 16 % could not be obtained anymore; this

indicates catalyst deactivation. As shown in Tab. 5.11 and in contrast to the unpro-

moted catalyst the formation of higher-hydrocarbons was favoured. This trend was

also expressed in a higher α-value of 0.90.

Summary of catalytic results obtained on reference catalysts

The level of carbon monoxide conversion was on both catalysts in same order of

magnitude about 15 %. The addition of Ru to the reference catalyst Co-Ref resulted

in an increasing α-value from 0.84 to 0.91 on coast of long-term stability. Co/Ru-Ref


catalyst deactivated after 102 h t.o.s. compared to Co-Ref; on the later XCO stayed

constant over the whole run.

0 5 10 15 20 25 30-4

-3

-2

-1

0

1

2

3

a = 0.84

ln (

mn/

n )

n

Fig. 5.28: Schulz-Flory plot for Co-Ref catalyst

Tab. 5.10: XCO, α and selectivities of hydrocarbons depended on t.o.s. obtained for

Co-Ref catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2: CO: N2 = 12:6:2 bar)

t.o.s. (h) 18 51 74 102 138

T. cat. bed (°C) 200 200 200 200 200

XCO(%) 6.8 12.5 14.7 14.3 15.1

α C10+ 0.82 0.84 0.84 0.84 0.83

Selectivities (wt%)

C1 14.7 10.1 9.6 9.5 10.2

C1-C4 22.6 20.2 20.5 18.7 20.7

C5-C9 13.6 12.8 11.9 12.5 14.6

C10-C13 17.1 19.5 20.3 21.4 22.1

C14-C21 20.0 23.5 24.0 23.6 20.9

C22-C24 12.7 10.9 8.1 8.7 8.9

C25+ 12.9 13.5 15.1 14.9 13.5

C5+ 76.3 80.2 79.4 81.3 80.0

C-Balance(%) 98.8 100.9 99.7 98.2 101.0



Co/Ru-Ref catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2: CO: N2 = 12:6:2 bar)

T.o.s. (h) 24 46 102 136 163

T. cat. bed (°C) 200 200 200 210 210

XCO (%) 8.1 12.6 16.3 7.9 8.7

α C10+ 0.86 0.89 0.91 0.90 0.90

Selectivities (wt%)

C1 12.1 9.7 11.3 10.5 9.8

C1-C4 20.4 15.3 16.5 17.1 15.3

C5-C9 16.9 10.7 14.1 16.3 14.9

C10-C13 13.7 12.1 10.9 12.7 11.0

C14-C21 28.0 22.3 26.8 25.2 22.8

C22-C24 13.2 10.9 6.9 7.2 8.1

C25+ 8.8 28.7 24.8 21.5 27.9

C5+ 79.6 84.7 83.5 82.9 84.7

C-Balance(%) 99.3 100.6 101.4 99.9 100.1

5.2.2. CATALYTIC EVALUATION OF IMPREGNATED CATALYST

The catalytic data of catalysts prepared by impregnation are described, results on

IW-NIT-AC and IW-ACAC3 are given in more detail. An overview of the important

catalytic parameters (XCO, SC5+, TOF, α) obtained on the other impregnated catalysts

is given at the end of this paragraph in the summary in comparison to the reference

catalyst (Tab. 5.14).

The catalyst IW-NIT-AC was prepared ex cobalt nitrate by using acetone as solvent

instead of water. As given in Tab. 5.12 after 24 h a carbon monoxide conversion of

10.6 % was obtained which raises up to 19.7 after 60 h time on stream. This conver-

sion level stayed constant for a period of 68 h and dropped then to 8.2 %. The rate of

conversion during steady-state operation was higher than on the catalysts Co-Ref.

Unfortunately the selectivity towards methane was always higher then 10 % and

amounted in the average to 14.5 %. The selectivity towards the C14-21 was all the time

in the range of 25 % and the C25+ hydrocarbons were to 9.2 % in average before the

drop in conversion took place. The α-value was calculated to 0.82. The catalyst pre-

pared by stepwise decomposition of cobalt nitrate IW-NIT-Step achieved a higher

selectivity towards the high boiling hydrocarbons (SC5+ = 83.2 %; α = 0.90) on cost of

activity since a carbon monoxide conversion of only 16.8 % was achieved.



IW-NIT-AC catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2: CO: N2 = 12:6:2 bar

t.o.s. (h) 24 60 93 128 152

T. cat. bed (°C) 200 200 200 200 200

XCO(%) 10.6 19.7 19.2 20.2 8.2

α C10+ 0.80 0.82 0.82 0.82 0.81

Selectivities (wt%)

C1 16.2 12.8 11.7 12.6 19.2

C1-C4 28.2 22.6 23.6 22.9 29.8

C5-C9 14.7 12.5 11.9 10.7 16.2

C10-C13 16.3 19.4 19.2 21.6 20.9

C14-C21 21.4 26.9 25.8 26.3 21.7

C22-C24 10.9 10.1 9.7 9.5 8.9

C25+ 8.5 8.7 9.8 10.0 2.7

C5+ 71.9 77.4 76.3 78.1 70.4

C-Balance(%) 102.3 100.3 100.9 98.6 99.7

The IW-ACAC3 catalysts reached its highest activity (XCO = 30.3 %) after 57 h time

on stream (see Tab. 5.13). Afterwards it dropped to 23.9 % after 93 h t.o.s. and this

degree of conversion stayed constant until the end of the experiment (136 h t.o.s.).

During the steady-state period the methane selectivity was to 16.7 % and the chain

growth probability amounted to 0.71. This low value was due to the low selectivity

towards the C14+ fraction, moreover only a SC5+ of approximately 66 % was reached

in average. The addition of Ru to this catalyst influenced the activity and the chain

growth probability; in both cases an increase was noticed; the XCO was assessed to

29.3 % and the α-value was to 0.80. Furthermore, no deactivation of the catalyst was

observed within the whole experiment (145 h t.o.s.). For the catalyst IW-ACAC2 (see

Tab. 5.14) an α-value of 0.84 was determined for a carbon monoxide conversion of

7.3 %. Additionally, no deactivation of the catalyst was noticed. The SCH4 was in an

order of 14.7 % and the selectivity towards the higher hydrocarbons (SC5+) was

around 75.5 %. The catalyst IW-ACE reached steady state-conditions after a short

start period of 40 h carbon monoxide was converted with XCO = 26.4 % during the

whole experiment without deactivation at the end of the run (total time on stream

115 h). The formation of high boiling hydrocarbons was not specially favoured and

the selectivity to C5+ amounted to 66.9 % with a corresponding α-value of 0.74.


Tab. 5.13 : XCO, α and selectivities of hydrocarbons depended on t.o.s. obtained for

IW-ACAC3 catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2: CO: N2 = 12:6:2 bar)

t.o.s. (h) 19 57 93 111 129

T. cat. bed (°C) 200 200 200 200 200

XCO(%) 12.5 30.3 23.9 24.6 22.5

α C10+ 0.68 0.71 0.71 0.71 0.71

Selectivities (wt%)

C1 22.9 17.6 15.3 19.8 15.0

C1-C4 41.6 35.4 32.0 36.3 33.3

C5-C9 18.9 16.3 15.2 14.3 16.9

C10-C13 21.4 18.7 19.8 18.0 18.2

C14-C21 14.3 16.5 17.4 15.2 16.7

C22-C24 3.8 10.2 11.0 9.8 8.2

C25+ 1.2 4.6 4.9 6.0 6.5

C5+ 58.4 65.0 67.9 63.2 66.8

C-Balance(%) 102.3 100.3 100.9 97.9 99.8

On the IW-OXA catalyst, only a very small carbon monoxide conversion of 5.2 % was

measured which was obtained after 23 h t.o.s. and did not change until the end of the

run. The methane selectivity was always higher than 16 % that led to a low SC5+ of

58.3 %. By changing the solvent during the impregnation procedure (IW-OXA-NH3

catalyst) a change in the activity level was observed; the average XCO amounted to

12.8 %, which was constant during 43 h. At the end of catalysts evaluation a drop in

conversion to 6.9 % was noticed. An α-value of 0.81 was derived from the Schulz-

Flory-Plot with a corresponding S5+ of 75.9 %.

Summary of catalytic results obtained on impregnated catalysts

In comparison to Co-Ref catalyst an significant improvement of carbon monoxide

conversion and TOF was achieved on IW-ACAC3 (XCO = 23.6 %, TOFnom = 2.5) and

IW-ACE (XCO = 26.4 %, TOFnom = 1.8) catalysts. In the case of IW-ACAC3 the doping

of catalyst with ruthenium resulted in an increasing value of XCO = 29.3 % and α =

0.80 (see Tab. 5.14).

The applied precursor cobalt (II) acetyl acetonate and oxalate failed because neither

a raise in carbon monoxide conversion (or TOF) nor chain growth probability was

achieved. Further, no improvement of the α-value was noticed with the exception of

IW-NIT-Step; on this catalyst the chain growth probability amounted to 0.90 com-

pared with Co-Ref (α = 0.83).


Tab. 5.14: Overview of carbon monoxide conversion, selectivity towards C5+ fraction,

α−value and TOF values for impregnated catalyst compared to Co-Ref (average for

steady-state conditions, Treac = 200 °C, GHSV = 1200 h-1, H2: CO: N2 = 12:6:2 bar,

ptot=20 bar)

Catalyst XCO [%] SC5+ [wt%] α [-] TOF [103s-1] a) TOFnom[-] b)

Co-Ref 14.7 80.0 0.83 17 1.0

IW-ACAC2 7.3 75.3 0.79 13 0.7

IW-ACAC3 23.6 67.9 0.71 43 2.5

IW-ACAC3-Ru 29.3 80.4 0.80 42 2.5

IW-ACE 26.4 66.9 0.74 32 1.8

IW-NIT-AC 19.7 77.2 0.83 21 1.3

IW-NIT-Step 16.8 83.2 0.90 23 1.3

IW-OXA 5.2 58.3 0.68 12 0.7

IW-OXA-NH3 12.8 75.9 0.81 18 1.1

a) TOF = nCO · XCO / 100 · nCo b) TOFnom = TOFcat / TOFCo-Ref

5.2.3. CATALYTIC EVALUATION OF IMPREGNATED CATALYSTS SUPPORTED ON

CERIA, ZIRCONIA AND TITANIA (RUTILE TYPE)

The reaction performance of the IWC-NIT catalyst was difficult to access because its

conversion did not a achieve a constant level. After 24 h on stream a CO conversion

of 17.3 % was obtained which dropped to 14.2 % after 72 h. After a steady-state pe-

riod of 36 hours with a XCO of 23.7 % the activity finally decreased to 19.5 %. It is

worth mentioning that during the steady-state period a lower methane selectivity of

8.8 % was noticed then in the time with a lower activity. The determined chain growth

probability was to 0.81. On the catalyst IWZ-NIT only a low conversion of 9.4 % was

achieved combined with an undesirable methane formation of 24.2 % over a period

of 121 h. Due to the high SCH4 only a low α-value of 0.68 was reached. The formation

of hydrocarbons >15 was observed in only small amounts. After a short start time of

16 h on IWB-NIT catalyst reached a XCO of 10.5 % which retain during 80 h. A drop in

conversion to 8.5 % was noticed after 100 h on stream. The methane selectivity

amounted to 12.3 % and SC5+ was 79.3 % with a corresponding chain growth prob-

ability of 0.78. For a detailed overview of the obtained product distribution refer to the

appendix.


Summary of catalytic results obtained on impregnated catalysts

The TOFnom values of IWC-NIT and IWZ-Nit are in the same order of magnitude of ≈1 as derived for Co-Ref catalyst (see Tab. 5.15); on IWB-NIT a lower TOFnom of 0.6

was achieved.

The carbon monoxide conversion varied between 9.7 to 23.4 %. Therefore the sup-

ports can be lined up in the following ascending order according to the obtained car-

bon monoxide conversion:

ZrO2 (XCO=9.7 %) < TiO2 (rutile) < TiO2 (Degussa P25) < CeO2 (XCO = 23.4 %)


α−value and TOF values for impregnated catalyst supported on titania (rutile type),

ceria and zirconia compared to Co-Ref (average for steady-state conditions, Treac =

200 °C, GHSV = 1200 h-1, H2: CO: N2 = 12:6:2 bar, ptot = 20 bar)

Catalyst XCO [%] SC5+ [wt%] α [-] TOF [103s-1] a) TOFnom [-] b)

Co-Ref 14.7 80.0 0.83 17 1.0

IWB-NIT 10.5 79.3 0.77 11 0.6

IWC-NIT 23.7 80.3 0.81 19 1.1

IWZ-NIT 9.7 61.0 0.68 19 1.1


5.2.4. CATALYTIC EVALUATION OF CATALYSTS PREPARED BY SPREADING

The average carbon monoxide conversion achieved on the SPR-Co3O4 catalyst

amounted to 10 %. With the exception of a short-term drop to 8.4 % after 109 h time

on stream this conversion level stayed constant during the whole run (134 h on

stream). The methane selectivity was at a high level of ≈ 16 % without any changes

during the experiment. A selectivity towards the C5+ fraction of 75.9 % was observed

with a corresponding α-value of 0.78. On SPR-CoTiO3 a very low value for XCO of

2.1 % was determined; this conversion was unchanged during the steady-state pe-

riod for 95 h. On this catalyst a high tendency to methane formation (21.5 %) was

also observed. The main products were within the C1-9 fraction. Therefore, a low SC5+

selectivity of 60.3 % and an α of 0.64 resulted.

Catalyst SPR-OXA was one of the most active ones. After 45 h of operation the car-

bon monoxide conversion increased from 26.3 % to 34.7 %. Under steady-state con-

ditions XCO was 32.3 % and only a slight drop to 29.6 % was observed at the end of


the experiment (163 h t.o.s.). SCH4 follows the same trend in an inverse way; from an

initial value of 17.9 % it decreases to 15.3 % and then remained at 11.0 % under

steady-state conditions. In the same way as the methane selectivity decreases the

SC25+ increases from 5.3 % to 12.3 %. As a SC5+ of 83.0 and a corresponding chain

growth probability of 0.81 was obtained in average (see Tab. 5.17). On the doped

catalyst SPR-OXA-Ru a slightly lower carbon monoxide conversion of 31.3 % in

comparison to SPR-OXA was determined (32.4 %). In contrast to SPR-OXA this

conversion level was constant over 114 h. A low methane selectivity (9.6 %), and a

respective C1-4 selectivity of 17.9 % and an α-value of 0.85 was observed.

Tab. 5.16: : XCO, α and selectivities of hydrocarbons depended on t.o.s. obtained for

IW-ACAC3 catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2: CO: N2 = 12:6:2 bar)

t.o.s. (h) 28 52 76 104 128

T. cat. bed (°C) 200 200 201 201 200

XCO(%) 30.8 31.4 31.0 31.6 32.1

αC10+ 0.85 0.85 0.84 0.85 0.85

Selectivities (wt%)

C1 9.3 9.6 9.4 9.6 10.0

C1-C4 17.9 18.4 17.7 17.7 19.0

C5-C9 12.1 12.6 12.4 12.5 12.7

C10-C13 21.3 21.7 22.1 21.4 21.1

C14-C21 24.1 23.6 24.0 23.6 23.9

C22-C24 9.1 10.0 9.3 8.7 9.6

C25+ 15.5 13.7 15.5 13.9 13.7

C5+ 82.1 81.6 82.3 82.3 81.0

C-Balance (%) 102.0 100.5 99.4 100.1 99.4

Summary of catalytic results obtained on catalysts prepared by spreading

The catalyst SPR-OXA was the most active one as tested in the present study; a

value of XCO = 32.3 % and α = 0.81 in comparison to Co-Ref catalyst was obtained.

The catalyst prepared from pure cobalt oxide and cobalt titanate failed because no

improve in XCO as well as α was obtained.



α−value and TOF values for catalyst prepared by spreading of cobalt precursors

compared to Co-Ref (average for steady-state conditions, Treac = 200 °C, GHSV =

1200 h-1, H2: CO: N2 = 12:6:2 bar, ptot = 20 bar).

Catalyst XCO [%] SC5+ [wt%] α [-] TOF [103 s-1] TOFnom [-]

Co-Ref 14.7 80.0 0.83 17 1.0

SPR-Co3O4 10.0 76.0 0.78 8 0.4

SPR-CoTiO3 2.1 60.3 0.65 2 0.1

SPR-OXA 32.3 83.0 0.81 60 3.5

SPR-OXA-Ru 31.3 81.8 0.83 50 2.9


5.2.5. CATALYTIC EVALUATION OF PRECIPITATED CATALYSTS

The catalyst PR-8 and PR-12 as well as the doped one (PR-12-Na) obtained by pre-

cipitation resulted in an approx. carbon monoxide conversion of 2 % only after 60 h

time. Because of that poor activity performance these catalysts were not considered

any further.

The CO conversion determined for catalyst PR-EDTA is in the same range as re-

ported for the reference catalyst (refer to Tab. 5.19). After 80 h on stream an average

value for XCO of approximately 14% was reached. Deactivation of the catalyst as ex-

pressed by a decreasing carbon monoxide conversion as detected after 153 h on

stream. During the start period the methane selectivity was 17.3 % and decreased to

10.1 % under steady-state operation. The total selectivity of the SC5+ fraction was

81.0 % and the chain growth probability amounted to 0.83. After the addition of Ru to

the PR-EDTA catalyst a small increase of XCO to 15.8 % (see Tab. 5.18) occurred.

Furthermore the Ru-promoter led to a more stable catalytic performance; only a short

start period of 12 h was necessary and no change in activity during the whole run

was observed. Methane selectivity was around 10 % and a high C5+ selectivity of

81.3 % was noticed (α-value = 0.85).



PR-EDTA-Ru catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2: CO: N2 = 12:6:2 bar)

t.o.s. (h) 24 50 74 98 134

T. cat. bed (°C) 200 200 200 200 200

XCO(%) 16.0 15.5 15.8 16.0 15.6

αC10+ 0.85 0.85 0.84 0.85 0.85

Selectivities (wt%)

C1 11.3 10.0 9.6 9.8 10.3

C2-C4 17.7 18.3 19.6 18.7 19.0

C5-C9 13.7 12.9 12.5 12.9 12.5

C10-C13 20.4 21.4 21.3 21.4 21.5

C14-C21 25.1 24.3 23.7 23.4 23.3

C22-C24 10.0 10.2 10.1 10.3 10.6

C25+ 13.1 12.9 12.9 13.3 13.1

C5+ 82.3 81.7 80.4 81.3 81.0

C-Balance (%) 99.6 102.4 100.8 101.3 99.5

Summary of catalytic results obtained on precipitated catalysts

The use of the cobalt EDTA precursor resulted in a slight increase of TOFnom to 1.5.

On the doped PR-EDTA catalyst an increasing a-value from 0.83 to 0.84 was ob-

tained compared to Co-Ref (see Tab. 5.19). The catalysts prepared starting from

Co(OH)2 were unsuitable to the FT reaction because only low carbon monoxide con-

version levels were achieved.


α−value and TOF values for precipitated catalyst compared to Co-Ref (average for

steady-state conditions, Treac = 200 °C, GHSV = 1200 h-1, H2: CO: N2 = 12:6:2 bar, ptot

= 20 bar)

Catalyst XCO[%] SC5+[wt%] α [-] TOF [103 s-1] a) TOFnom [-] b)

Co-Ref 14.7 80.0 0.83 17 1.0

PR-EDTA 14.4 81.0 0.83 26 1.5

PR-EDTA-Ru 15.8 81.4 0.84 27 1.5



5.2.6. CATALYTIC EVALUATION OF PLASMA PREPARED CATALYST

No catalytic data for the catalysts of the PD as well as the PL series are presented

because only low conversions of about 1 % were obtained. Within this range of con-

version no reasonable determination of product distribution was possible.

On catalyst PL-AT a carbon monoxide conversion of 41.6 % was obtained, which

was constant for more than 40 h. An unexpected low methane selectivity of 1.3 %

was achieved. Furthermore, the C1-C4 selectivity was to 5.0 % only. The total amount

of the high boiling hydrocarbon fraction was 94.8 % connected with an α-value of

0.91. For the catalyst PL-PP the value of XCO was 31.7 % in average combined with

a methane selectivity of approx. 6 % (see Tab. 5.20). A selectivity towards heavy-

weight hydrocarbons (SC5+) of 67.6 % was obtained. The determination of α was not

possible because the catalyst was assessed for 32 h only. Therefore, the required

amount of liquid hydrocarbons that would guarantee an accurate detection of all the

formed products, could not be collected.

A straight comparison of the obtained TOF data's on PL-PP and PL-AT catalysts with

Co-Ref was not possible due to a change in the reaction conditions; instead of the

standard reaction conditions a reaction temperature of 237 °C and a GHSV of

1500 h-1 was applied. Therefore the IW-ACAC3 catalyst is invoked for the assess-

ment of the catalytic values of the plasma samples.

Tab. 5.20: Reaction conditions, XCO, α and TOF values and selectivities towards hy-

drocarbons for the catalysts PL-AT and PL-PP.

Catalyst IW-ACAC3 PL-PP PL-AT

T cat. bed. [°C] 235 237 237

GHSV [h-1] 1500 1500 1500

XCO [%] 34.0 31.7 41.6

αC10+ [-] 0.88 n.d. 0.91

TOF [s-1] 0,0008 0,001 0,0023

TOFiw [-] 1.0 1.2 2.6

Selectivities [%]

C1 4.7 5.9 1.3

C1-C4 13.0 32.3 5.0

C5+ 86.8 67.6 94.8

a) TOF = nCO · XCO / 100 · nCo b) TOFiw = TOFcat / TOFIW-ACAC3


5.2.7. DISCUSSION OF CATALYTIC RESULTS

The catalytic results obtained on the reference catalysts will be discussed followed by

comments on the catalytic data of the new prepared catalyst will be given.

Reference Catalyst

As shown in section 5.1.7 the presence of Ruthenium influenced the reduction and

oxidation performance, the strength of interaction of the feed gases with the catalytic

surface and the surface concentration of cobalt. For all doped catalysts these find-

ings should have repercussions on the catalytic data. As presented before the pro-

moter influenced the reaction performance in two ways: firstly it led to an increase in

chain growth probability, secondly a higher maximum carbon monoxide conversion

was obtained at the cost of long-time stability, however.

It is well known that an important step in chain propagation is the readsorption of

ethene (or other α-olefins) as mentioned by LAHTINEN and SOMORJAI [193] as well as

by IGLESIA [194]. The observed strong adsorption of CO and hydrogen on the pro-

moted catalyst Co/Ru-Ref may be assumed to the olefins also, especially due to a

higher adsorption enthalpy of olefins in comparison to CO and H2 [195]. This as-

sumption was underlined by the observed shift within the C5+ hydrocarbon fraction as

derived during the catalytic assessment.

After 102 h time on stream a drop in conversion was noticed for Co/Ru-Ref. The

common explanations for deactivation like sintering, catalyst poisoning and losses of

metal or promoter can nearly be excluded because the calcination temperature for

the catalyst was 200 °C higher than the reaction temperature; catalyst poising due to

oxidation was avoided by an oxygen-trap; also the formation of cobalt – and/or ruthe-

nium carbonyls was not reported in literature so far. Therefore, it is most probably

due to a filling of the pores of the support with long-chain hydrocarbons in conjunc-

tion with a blocking of active sites on the catalyst surface due to undesorped, still

growing, hydrocarbons. As the pores fill with condensed liquids, more and more of

the diffusion occurs through the liquids, and diffusion in the gas phase may entirely

cease. When the reagents need to diffuse through FT liquids, diffusion does not ap-

pear to favour hydrogen enrichment in the liquids. Thus, changing from gasphase

diffusion to diffusion through liquids should lead to a decreased reaction rate and de-

creased methanation. PUSKAS and co-workers argued in the same manner for a

Co/Mg Fischer-Tropsch catalyst [196,197].

Additionally, another point should be discussed. As this thesis was funded by the

European commission, the catalytic performance of the reference catalysts Co/Ru-

Ref was examined by all project partners.

As shown in Tab. 5.21 a great deviation within the obtained carbon monoxide con-


versions in the different laboratories exist; XCO vary between 16.3 % (RUB/ACA4) and

67.7 % (ENI). For explanation of this finding the reaction conditions in all labs should

be described. At ENI1 laboratory FT reaction was examined at a reaction temperature

of 231 °C, a GHSV of 1200 h-1 and an operation pressure of 20 bar; the feed gas

consisted of hydrogen and carbon monoxide in a ratio of 2:1 (13.3:6.6 bar). In the lab

of IFP2 the reference catalyst was studied at a temperature of 220 °C, a space veloc-

ity of 1500 h-1, at a total pressure of 20 bar and a syngas with a H2:CO ratio of

2:1(13.3:6.6 bar) was applied. At RUB/ACA the evaluation of catalyst was carried out

at a temperature of 200 °C, a GHSV of 1200 h-1 and a total pressure of 20 bar; to the

feed gas nitrogen was added, used as internal standard for the analysis of products,

and the ratio of H2:CO was adjusted to 2:1 (pH2:pCO:pN2 = 12:6:2). A total pressure of

17 bar and a reaction temperature of 200 °C was used at SINTEF3; the applied

space velocity was to 1400 h-1. Based on that, the deviation of XCO can be ascribed

to the applied reaction conditions, because the reaction temperature (as described by

the ARRHENIUS equation) as well as the space velocity influence the degree of carbon

monoxide conversion. Furthermore, BESSEL found that the presence of N2 leads to a

decrease in CO-conversion in comparison to a feed gas without nitrogen [198].

The established selectivities towards the C5+ fraction and the respective α-values are

in reasonable agreement.

Tab. 5.21: Overview of the reaction conditions (Tbed, GHSV, ptot and feed composi-

tion) applied as well as the catalytic data (XCO, SC5+ and α) obtained on Co/Ru-Ref

catalyst in the different labs

Lab. Tbed

[°C]

GHSV

[h-1]

Ptot

[bar]

feed H2:CO

[bar]

co-feed

N2

[bar]

XCO

[%]

SC5+

[wt%]

α

[-]

ENI1 231 1200 20 13.3:6.6 0 67.7 88.7 0.91 [199]

IFP2 220 1500 20 13.3:6.6 0 30.2 80.1 0.90 [200]

SINTEF3 200 1400 17 n.m. n.m. 23.0 91.0 n.d. [201]

RUB/ACA4 200 1200 20 12:6 2 16.3 83.5 0.91 [202]

1 Eniricerche 2 Institut Francais du Petrole 3 Stiftelsen for Industriell og Teknisk Forskning4 Ruhr-Universität Bochum / Institut für Angewandte Chemie Berlin-Adlershof e.V.


5.2.8. DISCUSSION OF CATALYTIC RESULTS FOR THE NEW CATALYSTS

First, the obtained catalytic data of the new catalysts will be compared with the refer-

ence catalyst. Following the influences of cobalt oxide precursor, the Ru promoter,

the preparation conditions and the resulting cobalt dispersion on the catalyst activity

and selectivity is discussed.

Comparison of Catalytic Data of the New Prepared Catalyst with Co-Ref

Tab. 5.22: Overview of carbon monoxide conversion, selectivity towards C5+ fraction,α−value and TOF values of all tested catalyst compared to Co-Ref (average forsteady-state conditions, Treac = 200 °C, GHSV = 1200 h-1, H2: CO: N2 = 12:6:2 bar, ptot

= 20 bar)

Catalyst XCO [%] SC5+ [wt%] α [-] TOF [103s-1] a) TOFnom[-] b)

Co-Ref 14.7 80.0 0.83 17 1.0

SPR-CoTiO3 2.1 60.3 0.65 2 0.1

IWB-NIT 10.5 79.3 0.77 11 0.6

IW-OXA 5.2 58.3 0.68 12 0.7

IW-ACAC2 7.3 75.3 0.79 13 0.7

IWZ-NIT 9.7 61.0 0.68 19 1.1

IW-OXA-NH3 12.8 75.9 0.81 18 1.1

IWC-NIT 23.7 80.3 0.81 19 1.1

IW-NIT-Step 16.8 83.2 0.90 23 1.3

IW-NIT-AC 19.7 77.2 0.83 21 1.3

PR-EDTA 14.4 81.0 0.83 26 1.5

PR-EDTA-Ru 15.8 81.4 0.84 27 1.5

IW-ACE 26.4 66.9 0.74 32 1.8

IW-ACAC3 23.6 67.9 0.71 43 2.5

IW-ACAC3-Ru 29.3 80.4 0.80 42 2.5

SPR-OXA-Ru 31.3 81.8 0.83 50 2.9

SPR-OXA 32.3 83.0 0.81 60 3.5



In Tab. 5.22 the catalytic results obtained on the new prepared catalysts in compari-

son to the reference catalyst is given. The catalyst prepared by spreading of cobalt

oxalate over titania (SPR-OXA) achieved the highest improvement of turn-over-fre-

quency that was 3.5 times higher than on Co-Ref catalyst. The addition of ruthenium

to the catalyst SPR-OXA led to a slight decrease in XCO; but on this catalyst an α-

value of 0.83 was observed which is in the same order of magnitude as for Co-Ref

catalyst.

The catalysts IW-ACAC3 and IW-ACAC3-Ru can also count to the better catalysts.

since the TOF was improved by a factor of 2.5 compared to the reference catalyst.

Furthermore, a slight increase in activity was obtained on PR-EDTA and PR-EDTA-

Ru; a TOFnom of 1.5 was estimated.

All other catalyst achieved a TOF value nearly in the same order of magnitude as on

Co-Ref; i.e. IW-NIT-AC, IW-NIT-Step, IWZ-NIT, IWC-NIT and IW-OXA-NH3, or even

worse like IW-OXA, IWB-NIT and SPR-CoTiO3.

Comparison of Catalytic Data with Literature

Since cobalt FT catalysts supported on titania was not intensively studied up to now

only a few catalytic data obtained by IGLESIA et al. [104] and REUL and BARTHOLOMEW

[114] can be cited (see Tab. 5.23). In addition some data obtained for catalysts sup-

ported on silica and alumina is given.

Tab. 5.23: Comparison of literature data with Co-Ref and SPR-OXA catalyst

Co

[wt%]

Support DCored

[%]

XCO

[%]

SCH4

[%]

SC5+

[%]

α

[-]

TOF

[103 s-1]

1) 20.0 Al2O3 6.3 n.m. 42 14.0 n.m. 6.0 d) [92]

2) 10.0 SiO2 10.0 7.5 29 42.2 n.m. 7.5 a) [114]

3) 10.0 TiO2 4.5 5.6 16 53.7 n.m. 38 b) [114]

4) 12.0 TiO2 2.2 48.7 7.0 84.5 0.94 17.7 c) [104]

5) 12.0 TiO2 6.1 14.7 9.6 80.0 0.83 17e) Co-Ref

6) 11.5 TiO2 7.7 32.3 10.0 83.0 0.81 60e) SPR-OXAa) Treac = 200 °C, H2:CO = 2, ptot = 1 bar, GHSV = 500 h-1

b) Treac = 200 °C, H2:CO = 2, ptot = 1 bar, GHSV = 900 h-1

c) Treac = 200 °C, H2:CO = 2.1, ptot = 20 bar, GHSV = n.m.d) Treac = 250 °C, H2:CO = 2, ptot = 20 bar, GHSV = 1800 h-1

e) Treac = 200 °C, H2:CO = 2, ptot = 20 bar, GHSV = 1200 h-1


Catalyst 4) is the “reference catalyst” as described by IGLESIA [11]. From the good

agreement of the obtained TOF, SCH4 and SCH5+ data can be concluded that the re-

production of this catalyst was successful; hence the applied GHSV was not men-

tioned by Iglesia a discussion of the deviation within the value of XCO is not possible.

Further, it can be seen that titania is a very suitable support material for the FTS

aiming on waxes because the SC5+ is higher than on silica 1) catalyst (14.2 %) and

alumina 2) catalyst (42.2 %). Within this row of presented results on catalyst SPR-

OXA the highest TOF was obtained, followed by catalyst 3).

Precursor Influence

For impregnation 5 different cobalt precursors, namely cobalt nitrate, cobalt (III) ace-

tyl acetonate, cobalt (II) acetyl acetonate, cobalt oxalate and cobalt acetate was ap-

plied (see Tab. 5.14 in section 5.2.2) and the carbon monoxide conversion varied

between 5.2 % (IW-OXA) and 23.6 % (IW-ACAC3). Therefore, an influence of cobalt

precursor on the catalytic performance can be assumed.

By use of cobalt acetate or cobalt (III) acetyl acetonate an improvement of catalyst

activity, expressed by the turn-over-frequency, by a factor of 1.8 and 2.5 was ob-

tained. In the case of IW-ACAC3 a new phase, CoTiO3, was present which might in-

fluence the catalytic performance in a affirmative way. Therefore, catalytic tests were

carried out on a catalyst prepared starting from pure cobalt titanate (SPR-CoTiO3) but

the obtained value of XCO was only 2 %. Based on that it may be assumed that

CoTiO3 play a role in stabilising the cobalt metal layer on the catalysts surface of IW-

ACAC3 catalyst. Further, the higher activity of IW-ACAC3 catalyst can be correlated

with the achieved high cobalt dispersion (DCored = 9.8 %)

Two reasons for the higher activity of IW-ACE might be assumed: the reported sorp-

tion of Co2+ ions into the support and the higher DCo [186].

The kind of precursor also affected the selectivity of the catalysts. In the case of IW-

OXA (XCO = 5.2 %) and IW-ACAC2 (XCO = 7.3 %) the selectivity towards the C5+-

fraction increased from 58.5 to 75.3 %. On the catalysts Co-Ref (XCO = 14.7%) and

IW-NIT-Step (XCO = 19.7) the SC5+ raised from 80.0 % to 83.2 %.

The above described findings are in accordance with results reported by other

groups [88,131] who examined Co-EDTA and Co-citrate supported on alumina; in

this study different values of XCO were observed applying the same preparation tech-

nique also. However, the mechanism of how the precursor influenced the reaction

performance is not revealed up to now; further studies should be carried out.

Promoter Influence

The Ru addition to the catalyst prepared ex cobalt (III) acetyl acetonate lead to higher

values of XCO and α compared to IW-ACAC3; the carbon monoxide conversion in-


creased from 23.6 % (IW-ACAC3) to 29.3 % (IW-ACAC3-Ru) and α raised from 0.71

to 0.80 . This effect can be related to bimetallic effects which can certainly be as-

cribed to the close contact of the two metals which should lead to the formation of

stable Co-Ru oxides [2]. In addition LATHTINEN and SOMORJAI found a shift to higher

hydrocarbons due to the presence of a promoter [203]; the promoter inhibited the

deposition of carbonous species on the surface during the reaction and the main

promotion effect is due to an increased resistively towards graphite formation. This

graphite blocks the adsorption sites for hydrogen, carbon monoxide as well as α-ole-

fins.

SPR-OXA catalysts achieve a fourfold higher TOF than the reference catalyst with a

slightly lower chain growth probability of 0.81 instead of 0.83. The latter α−value can

be reached by adding Ru as promoter to the catalysts on the coast of activity. On

PR-EDTA catalysts a TOFnom to 1.5 was observed. Furthermore, a higher formation

tendency towards the high-boiling hydrocarbons was noticed when Ru was added to

the catalyst (PR-EDTA = 0.83, PR-EDTA-Ru = 0.84). Finally it could be said that the

addition of Ru led to more active and stable samples.

Solvent Influence

For the catalysts IW-OXA and IW-NIT the use of an other solvent instead of water in

the preparation procedure led to an increased carbon monoxide conversion. For IW-

NIT-AC acetone was applied as solvent and a XCO of 19.7 % compared to 14.7 %

(Co-Ref) was obtained. For the preparation of IW-OXA-NH3 an ammonia solution of

cobalt oxalate was used and the carbon monoxide conversion increased from 5.2 %

(IW-OXA) to 12.8 %.

These results may be explained by a lower surface tension of acetone and NH3(aq.)

in comparison to water. The lower surface tension is probably responsible for a better

expansion of the cobalt precursor solution on the support. This explanation was sup-

ported by higher cobalt dispersion on IW-NIT-AC (6.3 % instead of 6.1 %) and IW-

OXA-NH3 (5.4 % instead of 2.3 %). Furthermore, acetone and NH3 might result in the

formation of a Con+ complex that hindered the aggregation of the cobalt species on

the support. This interpretation of the obtained results is supported by work carried

out by HO [132,133]. Thus, the solvent influences the cobalt dispersion that in turn

has an effect on the cobalt conversion.


Support Influence

The catalysts IWC-NIT, IWZ-NIT and IWB-NIT were prepared by means of the incipi-

ent wetness technique ex cobalt nitrate. As derived from catalyst evaluation the ac-

tivity, i.e. carbon monoxide conversion, was different on each catalyst. Therefore the

supports can be lined up in the following ascending order of XCO :

ZrO2 (9.7 %) < TiO2–rutile (10.5 %) < TiO2 (Degussa P25) (14.7 %) < CeO2 ( 23.4 %)

This effect can be attributed to a change in the power of a the SMSI effect on the dif-

ferent sup port materials. The SMSI effect can be described as follows: electrons

from the cobalt crystallites were withdrawn by the support leading to a more metallic

performance of the cobalt species; This state is known to have a high specific activity

and selectivity for high-molecular-weight hydrocarbons in carbon monoxide hydro-

genation as previous reported by VANNICE [204]. BARTHOLOMEV et al. came to a com-

parable conclusion on nickel catalysts supported on alumina, silica and titania [190].

The pure rutile support showed a disadvantageous effect on the catalyst activity. This

result is in good agreement with a catalytic test carried out on a catalyst with an

amount of 16 % of rutile. On this catalyst a lower carbon monoxide conversion was

observed [199]. From this result the conclusion can be drawn that an optimum com-

position of both the phases, rutile and anatase, is necessary in order to get a catalyst

which reached a high CO conversion.

Dispersion Influence

In Fig. 5.29 the carbon monoxide conversion of all new catalysts is plotted versus the

cobalt dispersion (DCored). It is obvious that a dependence of carbon monoxide con-

version on cobalt dispersion exists because an increase in XCO goes along with a

raise in DCored. Since DCored expressed the amount of accessible cobalt an increas-

ing number of active sites can be ascribed to an increasing X(CO). The obtained in-

fluence of cobalt dispersion on XCO was also reported by REUEL and BARTHOLOMEV

[114]. However, there are some exceptions, namely IW-ACAC3 and IW-NIT-AC; on

this catalysts a Dcored of 9.8 % and 7.7 % and according values of XCO to 23.6 % and

19.7 % were obtained; in comparison to catalysts with a similar cobalt dispersion like

IWC-NIT (DCored = 7.7 %; XCO = 23.7 %) the obtained XCO is significant lower.


0 2 4 6 8 10 120

10

20

30

40X

CO

/

%

DCo

red / %

Fig. 5.29: Dependence of carbon monoxide conversion (XCO) on cobalt dispersion(DCored). ( = examined catalysts)

Catalysts prepared by Plasma- induced technique

The improvement in activity of the PL-PP in comparison to IW-ACAC3 catalyst is

minimal and goes along with a higher selectivity towards the undesirable methane

and C2-C4 hydrocarbon fraction (see Tab. 5.20).

A unique low SCH4 of 1.3 achieved on the PL-AT catalyst can not be explained up to

now and such a low amount of methane was not reported in literature so far. In order

to ensure the obtained data the catalyst was prepared again and the results of the

reproduced samples PP-REP1 and PP-REP2 is given in Tab. 5.24 [205]. First, on the

latter samples a higher amount of cobalt (8.1 wt% and 12.0 wt%) and a lower cobalt

dispersion was noticed (0.3 % and 0.7 %) in comparison to PL-PP (Co = 5.5 wt% and

DCo = 1.8 %). Beside the deviation within the physico-chemical properties the cata-

lytic data differ also. The carbon monoxide conversion was for both catalysts about

30 % with an according methane selectivity of approx. 6 %. It became clear that the

results obtained on PL-PP were not reached in any point nor are the results obtained

on PL-REP1 and PL-REP2 even close. One reason might be, that for the reproduc-

tion preparation uncalcined titania (Degussa P25) was applied. However, the calci-

nation of titania is very important because during the pretreatment the desired ratio of

anatase/rutile (30/70) was set. Further, the variation of the total cobalt content as well

as dispersion is responsible for the lower activity; the latter was discussed above. It


seems that the catalysts prepared by plasma- induced techniques are hard to re-

produce; therefore, further studies on this technique should be carry out until a recipe

was found which lead to “same” catalyst all the time because that is essential for in-

dustrial application.

Tab. 5.24: Overview of cobalt content, cobalt dispersion, XCO, α and selectivities to-wards hydrocarbons for the catalyst PL-PP compared to the catalysts PP-REP1 andPP-REP2 (Treac = 238 °C, ptot = 20 bar, H2:CO:N2 = 12:6:2, GHSV = 1500 h-1)

Catalyst Co[wt%] DCo [wt%] XCO [%] SC1 [%] SC1-C4 [%] SC5+ [%]

PL-PP 5.5 1.8 41.6 1.3 5.0 94.8

PP-REP1 8.1 0.3 30.5 6.2 22.8 77.2

PP-REP2 12.0 0.7 31.7 5.9 32.3 32.3

Non active catalysts

The catalysts prepared by precipitation cobalt hydroxide on titania were not active

towards the Fischer-Tropsch reaction, although a cobalt dispersion (DCo) higher than

1 % was determined. It seems probable that the effect, the hydroxylated titania which

was responsible for the high surface cobalt concentration (Co/Ti ratio), influenced the

catalytic performance in a manner that the unremoveable Ti-OH species block active

sites on the catalysts; that prevent the adsorption and/or dissociation of H2 and CO.

The catalyst PS-Co prepared by plasma- induced sputtering of cobalt was also not

active in FT reaction. The only reason for this result is the low amount of cobalt be-

cause only 0.5 wt% was determined by ICP-OES.

5.3. SLURRY REACTOR OPERATION

Performance of catalyst IW-ACAC3 was examined additionally in slurry-reactor op-

eration. Therefore, tetracosan was used as fluid phase because it was know from an

examination carried out by GORMELY et al. [206] and HUANG and co-workers [207]

that in waxes with a relative low average carbon number (< C30) a better long-term

stability (i.e., no deactivation) and solubility of synthesis gas was granted.

5.3.1. CATALYTIC EVALUATION

The first run was carried out at a reaction temperature of 231 °C, 10 g of previously

reduced catalysts and a feed gas composition of H2:CO:N2 = 12:6:2 was used (see

Fig. 5.30). Already after 3 h the catalyst reached steady state conditions and keeps

the conversion level of 19.3 % for 30 h time on stream. After that period a small drop


in conversion to 18.9 % was observed. The methane selectivity was to 4.1 % and no

worth mentioning change in the formation tendency was observed. However, SC2 was

very low and amounted to 2 % in the average. The overall C5+ selectivity was 85.8 %

corresponding to an α-value of 0.86. A rise of reaction temperature to 250 °C caused

an increasing XCO to 32.6 % that dropped to 29.8 % at the end of the experiment

(60 h time on stream). The selectivity to methane increased slightly to approx. 5 %.

The formation of high boiling hydrocarbons was not strongly effected by the higher

conversion level (please refer to Fig. 5.31) and amounted to 83.6 % (α-value = 0.86).

5.3.2. DISCUSSION OF CATALYTIC EVALUATION

Start-up behaviour

A very short start period was noticed for both reaction temperatures; steady-state

condition was reached after 3 h (231 °C) or 2 h (250 °C), respectively. This observa-

tion can be explained by the fact that the catalyst pore system was filled with waxes

from the beginning due to the liquid phase operations. During the fixed-bed examina-

tions these pore system was filled up by products formed during the reaction. During

this filling procedure the transportation conditions within the pore changes the whole

time until the pore was completely filled. Thus, effected the carbon monoxide conver-

sion and was responsible for the long start time necessary for fixed-bed evaluation.

0 10 20 30 40 500

5

10

15

20

X(C

O)

/ %

t.o.s. / h

2

3

4

5

6

Sel

ectiv

ities

(wt%

)

Fig. 5.30: Course of carbon monoxide conversion (x) and selectivity towards towards

C1(), C2(z), C3(), C4(), C5(), C6(+) in dependence on time on stream ; (Treac =

231 °C, ptot = 20 bar, H2:CO:N2 = 12:6:2 bar, GHSV = 1200 h-1)


0 10 20 30 40 500

5

10

15

20

25

30

35X

(CO

) /

%

t.o.s. / h

0

1

2

3

4

5

6

7

Se

lect

ivity

(w

t%)

Fig. 5.31: Course of carbon monoxide conversion (x) and selectivity towards C1(),

C2(z), C3(), C4(), C5(), C6(+) in dependence on time on stream ; (Treac = 250 °C,

ptot = 20 bar,H2:CO:N2 = 12:6:2 bar, GHSV = 1200 h-1)

Activity and product distribution

In Tab. 5.25 an overview of the obtained catalytic core values of IW-ACAC3 under

fixed-bed and slurry conditions is given in comparison to the Co-Ref catalyst. First, in

all cases an improved carbon monoxide conversion as well as a higher TOFnom is no-

ticed. A higher reaction temperature under slurry operation is necessary in order to

reach a similar XCO of approx. 20 % by a space velocity of 1200 h-1. This can be ex-

plained by the fact that no all of the syngas is dissolved within the liquid phase, i.e. a

part of the syngas pass through the reaction vessel without any contact to the cata-

lyst. Furthermore, is the diffusion through the liquid phase a higher transport limita-

tion than the phase transfer between gas/solid that was the case under fixed-bed

conditions [208]. On the other hand the presence of the liquid phase favour the for-

mation of high-boiling hydrocarbons which was expressed by the higher SC5+ and α-

value for the slurry phase catalyst.

Finally, it can be concluded that slurry phase operation is the more suitable reactor

type because of its short start times, a higher tendency towards the C5+ fraction as

well as the easier temperature control [209-211].


Tab. 5.25: Overview of reaction conditions and obtained XCO, SC5+, α-value and turn-

over-frequency catalysts for IW-ACAC3 catalyst compared to Co-Ref.

catalyst reactor

type

GHSV

[h-1]

Treac

[°C]

XCO

[%]

SC5+

[wt%]

α

[-]

TOF

[103 s-1]

TOFnom

[-]

Co-Ref fixed-bed 1200 200 14.7 80.0 0.83 17 1.0

IW-ACAC3 fixed-bed 1200 200 23.6 67.9 0.71 43 2.5

IW-ACAC3 slurry 1200 231 19.3 85.8 0.86 30 2.1

IW-ACAC3 slurry 1200 250 32.6 83.6 0.86 60 3.6

5.4. EVALUATION OF FTS-KINETIC

Catalyst IW-ACAC3 was applied for the kinetic studies as for the slurry examinations.

Within these experiments the catalyst deactivation performance, the CO consumption

rate and the methane formation rate were examined.

5.4.1. RESULTS OF KINETIC STUDIES

Catalyst Stability

The rate of CO consumption as a function of time-on-stream (600 h) was given in

Fig. 5.34. During the first 225 h the reaction temperature was kept constant at

202 °C; no change in catalyst activity was observed and the rate of CO consumption

amounted to 6.47x10-6 mol/minxgcat. Later, the reaction temperature was increased to

218 °C in consequence the rate of carbon monoxide conversion increased to 2.7x10-5

mol/minxgcat. After 360 h t.o.s. the reaction temperature was dropped to 202 C again;

a further increase of the conversion rate to 11.0x10-6 mol/minxgcat was noticed. At the

end of the experiment the conversion rate of carbon monoxide dropped, however, to

9x10-6 mol/minxgcat.

The rate of methane formation can be correlated with the rate of CO consumption as

plotted in Fig. 5.33 in that way that an increasing RCO resulted in an increasing RCH4

and reverse. In detail stayed RCH4 constant for the first 195 h and amounted to ap-

prox. 100x10-8 mol/minxgcat. Later it increased over 130 to 260x10-8 mol/minxgcat after

450 h time on stream. At the end of the experiment the rate of methane formation

dropped to 170x10-8 mol/minxgcat.

The summarised results of experimental kinetic studies for catalyst IW-ACAC3 at 202

and 218°C are shown in Tab. 5.26 and Tab. 5.27. The tables contain values of car-

bon monoxide conversion, rates of CO consumption, formation rates of C1 to C5 al-

kanes and C2 to C5 α- olefins in dependence on partial pressures in the reactor.


Fig. 5.32 Rate of CO consumption as a function of time on stream (pH2 = 12, pCO = 2,

Treac = 200 and 218°C;see also Tab. 5.26 and Tab. 5.27)

Fig. 5.33: Rate of methane formation as a function of time on stream (pH2 = 12, pCO =

2, Treac = 200 and 218°C; see also Tab. 5.26 and Tab. 5.27)


5.4.2. CO CONSUMPTION RATE

The partial pressure of CO be varied between 2.2 and 5.9 at fixed pH2 in order to ex-

amine its influence on the rate of consumption. As listed in Tab. 5.26 at a reaction

temperature of 202°C a dependence of partial pressure of carbon monoxide on RCO

was noticed. At a constant pH2 of 12 bar the partial pressure of carbon monoxide was

adjusted between 2.1 and 5.6 bar. The highest RCO of 10.7x10-6 mol/minxgcat was

achieved at a H2:CO ratio of 2.

When pH2 was increased to 13.8 bar by variation of pCO between 2.2 to 4.4 bar the

highest RCO of 12.0x10-6 mol/minxgcat was obtained by a H2:CO ratio of 3. As plotted

in Fig. 5.34 the rate of CO consumption raised by increasing the partial pressure of

CO, i.e., the reducing of the H2:CO ratio. A further decrease of pH2 to 10 bar had the

same effect . The highest RCO was obtained going along with the lowest H2:CO ratio

of 1.2 and amounted to 12.0x10-6 mol/minxgcat.

When the reaction temperature was increased from 200 °C to 218°C, linked with a

higher carbon monoxide conversion (between 20-40 % instead of 5-15 %) the influ-

ence of the partial pressure on RCO could almost be dismissed (please refer to Fig.

5.35). At a pH2 of 13.4 bar and selected H2:CO ratios of 7.8 (XCO= 29 %), 5.3 (XCO=

23 %) and 3.1 (XCO= 13.3 %) the determined rate of CO consumption amounted to

2.6, 2.8 and 2.6x10-6 mol/minxgcat. A similar performance was reported for pH2 of 11.2

and 8.7. Within the variation of pCO between 1.9 to 6.7 the RCO amounted to approx.

2 and 1.8x10-6 mol/minxgcat, respectively.


0 1 2 3 4 5 6 7 8 9 100,0

5,0x10-5

1,0x10-5

1,5x10-5

2,0x10-5

PCO reactor (bar)

-RCO (mol min-1 g-1cat)

Fig. 5.34: Rates of CO consumption in dependence on CO partial pressure in the re-

actor at T = 202°C and ptot = 20 bar; z : pH2 = 13.8 bar, : pH2 = 11.9 bar ¿ : pH2 =

9.8 bar.

0 1 2 3 4 5 6 7 8 9 100,0

1,0x10-5

2,0x10-5

3,0x10-5

4,0x10-5

5,0x10-5

PCO reactor (bar)

-RCO (mol min-1 g-1cat)

Fig. 5.35: Rates of CO consumption in dependence on CO partial pressure in the re-

actor at T = 218°C andz : pH2 = 13.8 bar, : pH2 = 11.9 bar ¿ : pH2 = 8.7 bar.


Tab. 5.26 :Feed gas flow rates, rates of CO consumption, formation rates of CH4, C2 to C5 alkanes and α-olefins in dependence on H2

and CO partial pressures in the reactor at T = 202°C and ptot = 20 bar (Ri (STP) in mol/min*g-1cat )

run flow/ml/min

pH2

/bar

PCO

/barCOconv./%

-rCO

/10-6RCH4

/10-8

RC2H6

/10-8

RC2H4

/10-8

RC3H8

/10-8

RC3H6

/10-8

RC4H10

/10-8

RC4H8

/10-8

RC5H12

/10-8

RC5H10

/10-8

1* 200 12 2,2 11,0 6,8 106 8,9 0 7,35 5,7 4,9 4,3 3,8 1,22 200 11,9 2,8 7,5 6,3 88 6,7 0,8 5,3 5,9 3,9 3,9 3,5 1,43 200 12 4 4,0 4,5 62 4,8 1,3 3,6 5,6 2,8 4,5 2,4 1,44 200 11,9 4,9 6,4 7,8 52 3,9 1,45 3,0 5,4 2,25 2,7 1,9 1,45 200 11,8 5,9 7,5 10,7 47 3,2 1,8 2,4 5,3 2 3,7 1,7 1,56* 200 11,9 2,1 11 6,5 100 8,5 0 8,0 4,0 5,3 4 4,2 17 332 13,8 2,2 8,2 7,6 138 9 0 8,1 5,9 5,8 7,4 4,8 1,48 332 13,8 2,8 7,6 9,3 105 7,6 0 6,5 6,5 4,7 3,5 4 1,59 332 13,6 4,4 7,1 12,0 72 4,6 0,6 3,7 6,1 2,7 8,2 2,4 1,710 120 10 2,2 12,0 4,0 80 6 0 6,0 3,4 3,8 3 2,7 0,711 120 9,8 2,8 10,8 4,8 67 5,4 0 5,1 3,9 3,4 3,9 2,7 0,812 120 9,8 4,9 8,1 5,9 39 3 0,9 2,6 3,9 2 3,7 1,9 0,813 120 9,8 5,9 7,15 6,7 36 2,7 0,9 2,1 3,9 1,7 3 1,4 114 120 9,6 7,8 10,8 12,5 26 2 1,1 1,6 3,7 1,3 3,2 1 115* 200 12 2,1 11,7 6,6 110 8,1 0 7,8 4,7 5,3 4 4 122* 200 12 2,1 12,5 7,0 130 7,4 0 8,5 3,0 6 3 5 0,726 332 14 3,9 5,6 9,5 110 6,7 0 6,8 5,5 5,4 9,4 5,3 1,827 200 11,8 3,8 8,0 7,5 100 6 0 6,3 4,8 4,8 6 4,1 1,228 120 9,8 3,8 9,8 6,5 80 5 0 5,4 4,0 4,0 3,4 3,4 0,929 120 9,6 6,9 9,0 9,0 47 3 0,9 2,6 4,4 2,1 2,5 1,9 1,330* 200 11,9 2,0 20 11 260 12 0 15 2,7 10 1,2 8 0,637* 200 12 2,1 16 9 170 8,3 0 10 3 7,7 1,8 7,3 0,8


Tab. 5.27:Feed gas flow rates, rates of CO consumption, formation rates of CH4, C2 to C5 alkanes and α-olefins in dependence on H2

and CO partial pressures in the reactor at T = 218°C and ptot = 20 bar (Ri (STP) in mol/min*g-1cat )

run flow

/ml/min

pH2

/bar

PCO

/bar

CO conv.

/%

-rCO

/10-5

RCH4

/10-8

RC2H6

/10-8

RC2H4

/10-8

RC3H8

/10-8

RC3H6

/10-8

RC4H10

/10-8

RC4H8

/10-8

RC5H12

/10-8

RC5H10

/10-8

17 200 11,2 1,5 48,0 2,7 750 64 0 53 2,2 34 1,3 25 0,9

18 200 11,3 2,4 28 2 460 6 0,6 4,7 5,3 3,4 3,6 3 1,3

19 200 11,3 3,5 20 2 330 4,6 1,3 3,3 5,3 2,5 4,2 2,2 1,3

20 200 11,4 4,7 14,4 1,8 220 3,5 1,5 2,7 5,1 2,1 2,5 1,7 1,3

21 200 11,4 5,7 14,5 2 200 3,1 1,7 2,4 4,9 1,8 3,4 1,5 1,4

23 332 13,4 1,7 29 2,6 1000 50 0 47 3,5 30 6 21 0

24 332 13,4 2,5 23 2,8 880 44 0 44 5,4 29 8 23 1,3

25 332 13,4 4,2 13,3 2,6 430 23 0 25 10 19 11 15 2,5

32 120 7,8 1,9 48 2 600 37 0 39 5,2 46 3,3 34 1,2

33 120 8,6 4,5 21 1,6 180 11 0,8 13 7 9,6 2,8 8,5 1,6

34 120 8,9 5,7 16,5 1,5 130 7,7 0,9 8 6,7 6 3,7 6 1,7

35 120 8,7 7,6 19 2,1 100 6,9 1,2 6,2 7,7 4,9 4,8 4,6 2,2

36 120 8,9 6,7 16 1,7 120 6,9 1,0 6,4 7,2 4,9 4,2 4,5 2,0


5.4.3. FORMATION RATE OF METHANE

In contrast to RCO the rate of methane formation was strongly influenced by the cho-

sen feed-gas composition. Here the increase of carbon monoxide partial pressure

caused a strong inhibition of the methane formation rate both at 202°C and 218°C as

shown in Fig. 5.36 and Fig. 5.37. As one example the RCH4 decreased from 1000 to

430x10-8 mol/minxgcat by change the pH2:pCO from 13.4:1.7 to 13.4:4.2. Additionally is

should be mentioned that the methane formation rate at a reaction temperature of

218°C is about ten times higher then at 202°C.

With increase of carbon monoxide partial pressures in the reactor the differences in

the formation rate of methane between various hydrogen partial pressures at the

same carbon monoxide partial pressure are smaller.

0 1 2 3 4 5 6 7 80,0

5,0x10-7

1,0x10-6

1,5x10-6

pCO reactor (bar)

RCH4 (mol / min-1 ·g-1cat)

Fig. 5.36 Formation rate of methane at 202°C and ptot = 20 bar in dependence of

partial pressure on CO; λ : pH2 = 13.8 bar, ν : pH2 = 11.9 bar σ : pH2 = 9.8 bar.


1 2 3 4 5 6 7 80,0

2,0x10-6

4,0x10-6

6,0x10-6

8,0x10-6

1,0x10-5

pCO reactor(bar)

RCH4 (mol/ min-1 · g-1cat)

Fig. 5.37: Formation rate of methane at 218°C and ptot = 20 bar in dependence of

partial pressure of CO; λ : pH2 = 13.4 bar, ν : pH2 = 11.3 bar σ : pH2 = 8.7 bar.

5.4.4. ESTIMATION OF EACT

In Fig. 5.38 a ARRHENIUS plot the temperature dependence of the rate of (CH2) for-

mation is given. Within this graph results achieved during experiments in the Berty-

reactor as well as in the slurry reactor (see section 5.3) are presented. When the ln

RCO was plotted versus 1/T from the ascent of the line the activation energy of the

IW-ACAC3 catalyst can be determined (pH2:pCO = 12:6; Treac = 200-250 °C); the Eact

was derived to 103 (±9) kJ/mol.

5.4.5. DISCUSSION OF KINETIC DATA

Influence of feed-gas composition on the R CO and RCH4

The influence of the partial pressure of each compound of the syngas on the rate of

carbon monoxide consumption was reported by WOJCIECHOWSKI [212], also. From

the obtained set of data it can be concluded that the feed gas composition can used

as a parameter to adjust a desired product distribution, i.e., the lower the H2:CO ratio

the higher the α-value because of a depressed tendency towards methane formation.

A surplus of hydrogen will favour the direct formation of methane and block the From

the obtained set of data it can be concluded that the feed gas composition can be

used as a parameter to adjust a desired product distribution, i.e., the lower the H2:CO

ratio the higher the α-value because of a depressed tendency towards methane for-


mation. A surplus of hydrogen will favour the direct formation of methane and block

the adsorption sites for carbon monoxide from there the chain growth is inhibited.

Activation Energy

The derived Eact of 103 kJ/mol is in good correspondence with values cited in litera-

ture. WITHERS et al. [213] reported an activation energy of 97 kJ/mol for Co/SiO2

catalyst and DECKWER and co-worker [214] a value of 109 kJ/mol on a iron catalyst,

respectively. Further it can be concluded that the catalyst shows in both reactor types

the same activity.

0,00190 0,00195 0,00200 0,00205 0,00210 0,00215

-5,0

-4,5

-4,0

-3,5

-3,0

-2,5

-2,0

berty reactor

slurry reactor

ln R

CO (

g(C

H2)h

-1 g

cat-1

1 / T

Fig. 5.38: Arrhenius plot of temperature dependence of the rate of CO consumption

for IW-ACAC3 catalyst (pH2 = 12 bar, pCO = 6 bar/; Treac = 200-250 °C)

6 Conclusions 119

6 Conclusions

The goal of the present study was to prepare catalysts which achieved an increase in

activity (expressed as turn over frequency) in comparison to the state-of-the-art

catalyst introduced by IGLESIA et al. [11] and to elucidate the effect of cobalt precur-

sor, applied preparation technique and cobalt dispersion on catalyst activity and se-

lectivity. The catalysts were applied mainly in fixed-bed reactor but partly also in

slurry-reactor operation. Furthermore, the kinetics for CO consumption and CH4 for-

mation as a means of reactor design and performance prediction were determined.

Compared to the state-of-the-art Fischer-Tropsch catalyst (TOF = 17.7·10-3 s-1, TOF-

non = 1, α = 0.91 achieved at Treac = of 200 °C, ptot = 20 bar, H2:CO = 2:1) in the pres-

ent study for catalysts SPR-OXA (TOFnom = 3.9), IW-ACAC3 (TOFnom = 2.5) and PR-

EDTA (TOFnom = 1.55) a significant improvement of the catalytic activity was ob-

tained.

From IGLESIA'S point of view a further improvement of the catalytic performance of

cobalt based catalyst was not possible because he postulated that the catalysts ac-

tivity is independent on cobalt dispersion, kind of applied support and preparation

technique. However, within the present study the following dependencies were found:

• Carbon monoxide conversion depend on the type of precursor: Within the present

study seven different precursors were applied namely cobalt nitrate, cobalt ace-

tate, cobalt (II) + (III) acetyl acetonate, cobalt EDTA, cobalt hydroxide and cobalt

oxalate. From the catalytic tests an influence of the precursor on carbon monox-

ide conversion in the following ascending order from XCO = 2.3 % (cobalt hydrox-

ide) to XCO = 32.3 % (cobalt oxalate) could be derived:

Co(OH)2 < ACAC2 < EDTA < NIT < ACAC3 < ACE < OXA

However, the mechanism how the different cobalt precursors interact with the

support material was not revealed up to now.

• Catalytic activity and chain growth probability is dependent on preparation tech-

nique: Starting from cobalt oxalate two different preparation techniques, incipient

wetness and spreading, were applied. The spreaded catalyst SPR-OXA achieved

a threefold increase on carbon monoxide conversion (XCO = 32.4 %) in compari-

son to the impregnated catalyst IW-OXA-NH3 (XCO = 12.8 %).

• Carbon monoxide conversion is dependent on preparation conditions: When in-

stead of water an other suitable solvent was used in all examined cases a positive

effect on XCO and chain growth propagation was noticed. By staring from cobalt

nitrate as precursor the use of acetone (IW-NIT-AC catalyst) instead of water lead

to a raise in carbon monoxide conversion from 14.7 to 19.7 %. For the impreg-

6 Conclusions 120

nated samples based on cobalt oxalate an increase of the value of XCO from 5.2

(IW-OXA) to 12.8 % (IW-OXA-NH3) was noticed combined with a higher α-value

of 0.81 by using a NH3-solution. This effect can be explained by a lower surface

tension that may affect the expansion of the cobalt precursor solution on the sup-

port; that in turn has an effect on cobalt dispersion.

• Activity depends on the applied support material: Three different supports were

loaded with cobalt nitrate by means of the incipient wetness technique. The car-

bon monoxide conversion varied between 9.7 to 23.4 %. Therefore the supports

can be lined up in the following ascending order :

ZrO2 (XCO=9.7 %) < TiO2 (rutile) < TiO2 (Degussa P25) < CeO2 (XCO = 23.4 %)

One reason for this finding may be a different strength of precursor-support inter-

action which affected the size of the cobalt cluster as well as cobalt dispersion.

• Ru as promoter led to a better catalytic performance: The above mentioned im-

proved catalyst was doped with ruthenium in order to achieve a further improve-

ment. On all catalysts an increasing XCO and α-value was obtained. That was to a

stabilising effect of Ru on surface cobalt.

• Carbon monoxide conversion is depended on cobalt dispersion: It could be shown

that cobalt dispersion had an effect on activity (XCO); in most cases with an in-

creasing cobalt dispersion a rise in carbon monoxide conversion could be corre-

lated with the exception of IW-ACAC3 and IW-NIT-AC. This effect can be as-

signed to the increasing number of accessible active cobalt species.

To sum it up it can be said that the improvement of the turn-over-frequency cannot

be assigned entirely to one of the reported influences; furthermore, it is supposed

that a combination of the above findings lead to an improved catalyst.

From the catalytic tests carried out in a slurry reactor can be concluded that this re-

actor type suits the Fischer-Tropsch synthesis better than a fixed-bed reactor. On the

one hand under slurry conditions a shift to high boiling hydrocarbons was observed;

further allows the slurry operation a better control of reaction conditions, i.e., tem-

perature control.

In kinetic studies the rates of carbon monoxide conversion and of methane formation

was determined in dependence on reaction temperature and gas composition. At a

reaction temperature of 218 °C the rate of carbon monoxide consumption is nearly in-

dependent from the adjusted H2:CO ratio. At a pH2 of 13.4 bar and selected H2:CO

ratios of 7.8 (XCO = 29 %), 5.3 (XCO = 23 %) and 3.1 (XCO = 13.3 %) the determined

rate of CO consumption amounted to 2.6, 2.8 and 2.6x10-6 mol/minxgcat, respectively.

From the set of data obtained for the rate of methane formation it can be concluded

6 Conclusions 121

that the feed gas composition can be used as a parameter to adjust a desired prod-

uct distribution, i.e., the lower the H2:CO ratio the higher the α-value because of a de-

pressed tendency towards methane formation. An excess of hydrogen will favours

the direct formation of methane and blocks the adsorption sites for carbon monoxide

from there the chain growth is inhibited. An apparent Eact to 103 kJ/mol for the cata-

lyst IW-ACAC3 was derived.

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[196] I. Puskas, Catal. Lett. 22 (1993) 283

[197] I. Puskas, B.L. Meyers, J.B. Hall, Symp. Chem. Charac. Sup. Met. Catal. 206th

national Meeting, ACS, Chicago (1993) 905

[198] S. Bessel, in: H.E. Curry-Hyde, R.F. Howe (eds.),“Natural Gas Conversion II”

Elsevier Amsterdam (1994) 483

[199] M. Clerici, “ 1st periodic report of the ParSEC-Project”, unpublished results

(1996)

[200] P. Chaumette, B. Rebours, J. Lynch; “1st periodic report of the ParSEC-

Project”, unpublished results (1996)

[201] E. A. Blekkan, “2nd periodic report of the ParSEC-Project”, unpublished results

(1996)

[202] M. Kraum, M. Baerns, “ Mid-term report of the ParSEC-Project”, unpublished

results (1997)

[203] J. Lahtinen, G.A. Somorjai, J. Molec. Catal. A:Chemical 130 (1998) 255

[204] M. A. Vannice, J. Catal. 50 (1997) 228

[205] personal note send by Dr. N. Steinfeldt (02.04.1999); Institut für Angewandte

Chemie Berlin-Adlershof .e.V.

[206] R.J. Gormley, M.F. Zarochak, P.W. Deffenbaugh, K.R.P.M. Rao, Appl. CatalA:General 161 (1997) 263

[207] S.H. Huang, H.M. Liu, F.N. Tsai, K.C. Chao, Ind. Eng. Chem. Res. 27 (1988)

162

[208] A. Zaidi, Y. Louisi, M. Ralek, W.D. Deckwer, Chem.-Ing.-Tech. 50(8) (1978)

628

7 Literature 132

[209] W.D. Deckwer, Y. Louisi, A. Zaidi, M. Ralek, Ind. Eng. Chem. Process Div. 19

(1980) 699

[210] H.W. Pennline, M. Zarochak, R.E. Tischer, R.R. Schehl, Appl. Catal. 21 (1986)

313

[211] S. Bhattacharjee, J.W. Tierney, Y.T. Shah, Ind. Eng. Process Des. Dev. 25

(1986) 117

[212] B.W. Wojciechowski, Catal. Rev.-Sci. Eng. 39(4) (1988) 629

[213] H.P. Withers, Jr., K.F. Eliezer, J.W. Mitchell, Ind. Eng. Chem. Res. 29 (1990),

1807

[214] W.D. Deckwer, Y. Serpemen, M. Ralek, B. Schimdt; Ind. Eng. Chem. Process

Des. Dev. 21 (1982) 222

Appendix 132

Appendix

Table of Content

A.1. CATALYTIC EVALUATION OF CATALYSTS...........................................133

A.2. APPLIED CHEMICALS..............................................................................139

A.3. CALIBRATION FOR PRODUCT ANALYSIS.............................................140

A.4. LEBENSLAUF............................................................................................141

Appendix 133

A.1. CATALYTIC EVALUATION OF CATALYSTS

Afterwards the results obtained during catalytic test on new prepared catalysts are

given.

Tab. A.1: CO-Conversion and selectivitys towards the C1 to C25+ fraction for the IW-

ACAC2 catalyst (GHSV = 1200h-1, ptot =20bar, H2:CO:N2=12:6:2)

t.o.s. (h) 36 71 95 143 179

T. cat. bed (°C) 200 200 200 210 210

XCO(%) 5.2 6.9 10.3 7.2 7.8

α C10+ 0.82 0.84 0.84 0.83 0.84

Selectivities (wt%)

C1 12.8 15.3 13.5 17.8 14.1

C1-C4 23.8 24.2 24.7 26.4 23.5

C5-C9 16.3 15.4 16.8 14.7 15.9

C10-C13 15.2 13.1 13.5 15.3 13.6

C14-C21 39.8 34.9 27.0 29.7 30.7

C22-C24 10.0 9.2 12.1 8.8 11.6

C25+ 4.8 3.2 5.9 5.1 4.7

C5+ 76.2 75.8 75.3 73.6 76.5

C-Balance(%) 101.2 100.3 99.8 100.2 100.1

Appendix 134


ACE catalyst (GHSV=1200h-1, ptot=20bar, H2:CO:N2=12:6:2)

t.o.s. (h) 19 45 73 91 115

T. cat. bed (°C) 200 200 200 200 200

XCO(%) 15.7 27.1 26.2 26.7 25.8

α C10+ 0.69 0.73 0.74 0.74 0.73

Selectivities (wt%)

C1 17.4 12.6 13.1 12.7 13.6

C1-C4 43.4 34.1 31.8 32.5 34.1

C5-C9 19.6 22.3 21.7 20.9 20.1

C10-C13 20.8 23.0 25.2 26.0 26.9

C14-C21 10.7 12.5 12.9 12.7 10.6

C22-C24 3.7 3.1 2.9 3.3 2.7

C25+ 1.9 5.2 5.5 4.6 4.1

C5+ 56.7 66.1 68.2 67.5 65.8

C-Balance(%) 99.8 99.8 101.2 99.6 100.6


ACAC3-Ru catalyst (GHSV=1200h-1, ptot=20bar, H2:CO:N2=12:6:2)

t.o.s. (h) 24 48 76 100 144

T. cat. bed (°C) 200 200 200 200 198

XCO(%) 30.4 31.6 32.6 32.0 30.9

α C10+ 0.80 0.80 0.80 0.80 0.80

Selectivities (wt%)

C1 10.1 9.2 9.0 9.3 9.1

C2-C4 18.6 19.7 19.6 19.1 19.6

C5-C9 12.6 11.3 10.9 11.4 11.2

C10-C13 20.9 16.6 17.1 17.0 17.1

C14-C21 23.5 22.9 23.4 23.6 23.0

C22-C24 11.1 13.8 13.2 13.9 13.6

C25+ 13.3 15.7 16.2 15.0 15.1

C5+ 81.4 80.3 80.4 80.9 80.4

C-Balance (%) 97.8 101.4 100.0 99.5 98.9

Appendix 135

Tab. A.4: CO-conversion and selectivitys towards the C1 to C25+ fraction for the IWC-

NIT catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2:CO:N2 = 12:6:2)

t.o.s (h) 24 72 116 140 168

T. cat. bed (°C) 200 200 202 200 200

XCO (%) 17.3 14.2 23.8 23.6 19.5

α C10+ 0.79 0.80 0.81 0.81 0.79

Selectivities (wt %)

C1 12.5 11.9 8.7 9.1 12.3

C1-C4 23.6 20.8 19.8 19.5 24.4

C5-C9 17.3 16.2 15.4 16.0 19.2

C10-C13 20.0 15.3 17.4 16.5 19.2

C14-C22 26.4 30.6 32.7 33.2 24.7

C23-C24 2.1 6.8 2.5 3.1 2.9

C25+ 10.6 10.3 12.4 11.7 9.5

C5+ 76.4 79.2 80.2 80.5 75.6

Cbal (%) 99.6 100.3 100.1 99.7 100.0

Tab. A.5: CO-conversion and selectivitys towards the C1 to C25+ fraction for the IWZ-


T.o.s (h) 32 56 104 121 169

T. cat. bed (°C) 200 200 200 200 200

XCO (%) 8.9 10.5 9.7 8.5 7.3

α C10+ 0.68 0.69 0.68 0.53 0.50


C1 23.9 22.5 24.9 25.8 24.7

C1-C4 42.0 36.3 38.5 42.7 44.6

C5-C9 20.3 18.4 19.2 22.3 23.1

C10-C13 17.2 15.4 13.1 16.4 13.6

C14-C22 13.4 14.6 13.8 12.5 13.4

C23-C24 4.2 10.7 10.9 2.7 1.9

C25+ 3.2 4.3 4.5 3.2 2.9

C5+ 58.0 63.7 61.5 57.3 55.4

Cbal (%) 100.1 98.6 99.8 96.4 93.8

Appendix 136

Tab. A.6: CO-conversion and selectivitys towards the C1 to C25+ fraction for the IWB-


t.o.s (h) 24 39 76 100 123

T. cat. bed (°C) 200 200 200 200 200

XCO (%) 10.4 10.2 10.9 8.9 8.3

α C10+ 0.78 0.78 0.77 0.78 0.77


C1 12.3 10.8 13.1 11.4 9.9

C1-C4 21.6 18.0 22.7 19.7 20.1

C5-C9 14.6 11.5 12.9 13.5 12.8

C10-C13 16.8 16.3 17.5 15.9 16.2

C14-C22 31.5 34.7 33.8 33.4 31.9

C23-C24 10.1 12.2 6.9 13.1 12.8

C25+ 5.6 7.3 6.2 4.4 6.2

C5+ 78.6 82.0 77.3 80.3 79.9

Cbal (%) 96.8 101.2 97.1 101.0 100.8

Tab. A.7: CO-Conversion and selectivitys towards the C1 to C25+ fraction for the SPR-

Co3O4 catalyst (GHSV=1200h-1, ptot=20bar, H2:CO:N2=12:6:2)

t.o.s. (h) 30 54 78 109 133

T. cat. bed (°C) 200 200 200 200 200

XCO(%) 10.0 11.3 10.2 8.4 10.0

α C10+ 0.78 0.77 0.78 0.77 0.78

Selectivities (wt%)

C1 16.4 15.9 16.4 16.0 16.3

C1-C4 24.7 23.2 24.1 24.2 23.7

C5-C9 13.6 14.0 13.7 14.1 14.3

C10-C13 17.1 16.7 17.0 16.6 17.0

C14-C21 19.0 19.6 19.3 20.3 19.9

C22-C24 12.7 12.9 12.9 11.5 12.0

C25+ 12.9 13.6 13.0 13.3 13.1

C5+ 75.3 76.8 75.9 75.8 76.3

C-Balance (%) 101.2 99.1 99.4 100.9 99.8

Appendix 137

Tab. A.8: CO-Conversion and selectivitys towards the C1 to C25+ fraction for the SPR-

CoTiO3 catalyst (GHSV=1200h-1, ptot=20bar, H2:CO:N2=12:6:2)

T.o.s. (h) 27 61 95 117 154

T. cat. bed (°C) 200 200 201 200 200

XCO(%) 5.9 6.3 6.1 4.3 3.7

α C10+ 0.65 0.64 0.65 0.64 0.63

Selectivities (wt%)

C1 21.7 20.4 21.3 21.6 22.7

C2-C4 39.7 38.6 39.1 38.7 39.3

C5-C9 19.6 19.4 20.0 19.7 19.2

C10-C13 20.5 21.0 20.3 21.3 21.1

C14-C21 12.1 12.7 13.0 12.5 12.8

C22-C24 4.7 4.9 4.4 4.8 5.1

C25+ 3.4 3.4 3.2 3.0 2.5

C5+ 60.3 61.4 60.9 61.3 60.7

C-Balance (%) 98.3 100.3 99.4 100.8 99.1

Tab. A.9: CO-conversion and selectivity’s towards the C1 to C25+ fraction for the SPR-

OXA catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2:CO:N2 = 12:6:2)

T.o.s (h) 17 45 93 120 163

T. cat. bed (°C) 200 201 200 200 200

XCO (%) 26.3 34.7 32.4 32.3 29.6

α C10+ 0.73 0.79 0.81 0.81 0.79


C1 17.9 15.3 10.9 11.2 14.7

C1-C4 26.3 22.7 17.6 16.4 23.5

C5-C9 15.3 17.3 18.9 19.4 17.2

C10-C13 12.6 14.3 20.9 22.6 13.7

C14-C22 37.5 31.1 25.8 24.6 31.7

C23-C24 3.2 4.2 4.7 4.5 3.9

C25+ 5.3 10.4 12.1 12.5 9.8

C5+ 73.7 77.3 82.4 83.6 76.5

Cbal (%) 101.2 99.3 101.4 100.9 99.6

Appendix 138

Tab. A.10: CO-conversion and selectivitys towards the C1 to C25+ fraction for the

IW-EDTA catalyst (GHSV = 1200 h-1, ptot = 20 bar, H2:CO:N2 = 12:6:2)

t.o.s (h) 23 47 102 126 161

T. cat. bed (°C) 200 198 200 200 200

XCO (%) 8.7 12.7 14.2 14.7 10.9

α C10+ 0.79 0.81 0.83 0.83 0.81


C1 17.3 12.8 9.7 10.5 11.8

C1-C4 26.5 18.1 14.6 16.3 18.9

C5-C9 23.5 12.7 14.0 15.6 19.2

C10-C13 15.3 12.8 14.3 15.8 10.8

C14-C22 23.3 39.3 35.6 32.0 23.7

C22-C24 4.3 5.9 6.8 7.2 6.5

C25+ 7.1 11.2 14.7 13.1 10.9

C5+ 75.4 78.5 80.8 81.2 79.3

Cbal (%) 99.7 100.2 98.6 100.1 100.1

Appendix 139

A.2. APPLIED CHEMICALS

Tab.A.11: Applied chemicals, supplier and their purity

Chemical Supplier purity

Bayer-titania Bayer p.a.

Carbon monoxide Messer-Griesheim 3.7

ceria Alfa 99+%

CH2Cl2 Merck p.a.

cobalt (II) acetylacetonate Merck 99+%

cobalt (II) acetylacetonate Merck 99+%

cobalt acetate Alfa 95+%

cobalt nitrate Merck 99+%

cobalt oxalate Alfa 95+%

cobalt oxalate Alfa 99+%

cobalt oxide Alfa 99+%

cobalt titanate Alfa 95+%

CS2 Merck p.a.

EDTA (Triplex III) Merck p.a.

Helium Messer-Griesheim 5.0

Hydrogen Messer Griesheim 5.0

NH3(aq) 25% Merck p.a.

Nitogen Messer-Griesheim 5.0

O2 (2% in Argon) Messer-Griesheim 5.0

Oxygen Messer-Griesheim 5.0

potassium citrate Alfa 95+%

ruthenium (III) chloride Alfa 95+%

sodium citrate Alfa 95+%

synthetic air Messer-Griesheim 5.0

titania P25 Degussa p.a.

zirconia Alfa 99+%

Appendix 140

A.3. CALIBRATION FOR PRODUCT ANALYSIS

The analysis of FT products was divided in an on-line and an off-line analysis. The

permanent gases and C1-C6 hydrocarbons were analysed on-line. The C7+

hydrocarbons were condensed and analysed off-line. The used equations for the

calculation of the quantitative amount of each product is given afterwards.[1-5]

on-line Analysis

For determination of the permanent gases and C1 to C6 hydrocarbons nitrogen was

used as internal standard.

i2

i22N C areaN % Vol

C % VolN area (i)f

⋅⋅=

The calculation of mass percentage of the individual compounds were derived by out

of the standardisation of the total peak areas after division with the calibration factors:

∑=

(i)f / i area(i)f / i area

mN2

N2i

off-line Analysis

The higher hydrocarbons were solved in CS2 and as standard iso-propanol (iso) was

added. For determination foff(i) a calibration mixture of C6 to C50 hydrocarbons

supplied by Supleco was used. of each hydrocarbon fraction can be derived from the

following equation:

iso aera(i)f m i area

m offisoi

⋅⋅=

Carbon balance

The carbon balance was derived out of the difference of the number of carbon at the

reactor inlet minus the number of carbon not converted and the number of carbon

part of the FT-products.

[1] W.A. Dietz, J. of G.C.; February 1967

[2] H.Y. Tong, F.W. Karasek, Anal. Chem., 56 (1984) 2124

[3] J. H. Marsman, B.B. Breman, A. A. C. M. Beenackers; J. High. Res. Chrom.,16 (1993) 141

[4] R. R. Anderson, C.M. White, J. High. Res. Chrom., 17 (1994) 245

[5] R.A. Dictor, A.T. Bell, Ind. Eng. Chem. Funda., 23 (1984)252

Appendix 141

A.4. LEBENSLAUF

Name: Martin Kraum

Geburtstag: 21.10.1969

Geburtsort: Recklinghausen

Schulbildung: 1976 - 1980 Grundschule in Recklinghausen

1980 – 1989 Gymnasium Petrinum zu Recklinghausen

Abschluß: Allgemeine Hochschulreife

Studium: WS 1989 – 1995 Chemie – Studium an der Ruhr-Universität

Bochum

Schwerpunkt: Technische Chemie

Diplomarbeit zum Thema: “Selektive partielle Oxidation von

Propan zu Acrylsäure“ unter der Betreuung von Herrn Prof. Dr.

M. Baerns

Diplom: 25.01.1995

Beginn der Arbeit

zur Dissertation:

Januar 1996 am Lehrstuhl für Technische Chemie an der Ruhr-

Universität Bochum

Juli 1996 Fortführung der Arbeit am Institut für Angewandte

Chemie Berlin-Adlershof e.V. unter der Betreuung von Herrn

Prof. Dr. M. Baerns

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