lect2-sp14

ChE 511 - Catalysis Prof. Dr. Işık Önal

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PROMOTERS

CATALYST

Function:Chemical Activity

Types:• Metals• Semiconductor Oxides

and Sulfides• Insulator Oxides

and Sulfides

High Surface AreaPorosityMechanical PropertiesStabilityDual Functional ActivityModification of Active ComponentTypes:

• High M. P. Oxides• Clays• Carbon

Function: on support

•Structural

•Activity inhibition

•Activity promotion

on active component

•Morphology

•Poisoning

•Electronic

Catalyst Components


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Al2O3, SiO2, MgOSiO2-Al2O3

Zeolites

PolymerizationIsomerizationCrackingDehydration

InsulatorsCarbonium ions

Oxides

NiO, ZnO, CuO

Cr2O3, MoS2

Selective HydrogenationHydrogenolysisOxidation

SemiconductorsRedox

Oxides and

Sulfides

Fe, Ni, PtPd, Cu, Ag

HydrogenationHydrogenolysisOxidation

Conductors Redox

Metals

ExamplesReactionsConductivity/Reaction Type

Class

Classification of Active Components


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CLASSIFICATION OF SOLID CATALYSTS ACCORDING TO THEIR ELECTRICAL CONDUCTIVITY

Stoichiometric oxides: Al2O3, SiO2, B2O3, MgO, etc.

10-9-10-20Insulators

Metalloids: Si, Ge, etc.Non-stoichiometric oxides and sulfides:

ZnO, Cu2O, NiO, ZnS, etc.

103-10-9Semiconductors

Numerous metals and alloys106-104Metals

EXAMPLESRANGE OFCONDUCTIVITY

(Ω-1 cm-1)

CLASS


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Metals Dispersed Low Loading: Pt/Al2O3, Ru/SiO2

High Loading: Ni/Al2O3, Co/kieselguhrPorous Raney Ni, Co, etc.

Fe-Al2O3-K2OBulk Pt, Ag gauze

Multimetallic Dispersed (Pt-Re, Ni-Cu, Pt-Au)/Al2O3, etc.Clusters, Alloys

Oxides Single Al2O3, Cr2O3, V2O5

Dual, co-gels SiO2-Al2O3, TiO2-Al2O3

Complex Ba TiO3, CuCr2O4, BiMoO6

Dispersed NiO/Al2O3, MoO3/Al2O3

Cemented NiO-CaAl2O4

Sulfides Dispersed MoS2/Al2O3, WS2/Al2O3

Type State Examples

Types of Catalytic Materials


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Acids Dual, co-gels SiO2-Al2O3

Crystalline ZeolitesNatural clays MordenitePromoted acids Super Acids, SbF4, BF, Supported Halides

Bases Dispersed CaO, MgO, K2O, Na2O

Other Chlorides TiCl3-AlCl3Compounds Carbides Ni3C

Nitrides Fe2NBorides Ni3BSilicides TiSiPhosphides NiP

Other forms Molten salts ZnCl2, Na2CO3

Anchored Homogeneous CatalystsAnchored Enzymes

Type State Examples

Types of Catalytic Materials (Con’t.)


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METALS: (>70% of known catalytic reactions involve metals)Overlapping electronic energy bands promote electron transfer withadsorbing molecules.

Redox or charge transfer reactions are found.

Systematic variation with atomic electron configurations explains trendsin adsorption and catalysis.

Similarities in group within the periodic table are rationalized.

Orbital considerations, such as type, occupancy and symmetry mayexplain differences in metals or crystal planes.

SEMICONDUCTING OXIDES AND SULFIDES:• A very large class.• More complicated surface configurations.• Greater geometric complexity

More selective redox reactionse.g. partial oxidation

hydrodesulfurizationdenitrogenation


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Li

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

Ra

Sc

Y

La

Ti

Zr

Hf

V

Nb

Ta

Cr

Mo

W

Mn

Tc

Re

Fe

Ru

Os

Co

Rh

Ir

Ni

Pd

Pt

Cu

Ag

Au

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

IA IIA

IIIB IVB VB VIB VIIB VIIIB IB

TRANSITION “d” METALS

most successful

RARE EARTH “f” METALS – Widely used as oxide promoters or supports

“s” METALS

Primarily used as promoters

11028

2726

4343

4343

sdCu

sdNi

sdCo

sdFe


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PROMOTERS

CATALYST

Function:Chemical Activity

Types:• Metals• Semiconductor Oxides

and Sulfides• Insulator Oxides

and Sulfides

High Surface AreaPorosityMechanical PropertiesStabilityDual Functional ActivityModification of Active ComponentTypes:

• High M. P. Oxides• Clays• Carbon

Function: on support

•Structural

•Activity inhibition

•Activity promotion

on active component

•Morphology

•Poisoning

•Electronic

Catalyst Components


9

Supports function as stable surfaces over which the active component is dispersed in such a way that sintering is reduced.

High Area Supports Commonly Used

-Al2O3

SiO2

C (activated)

Diatomaceous clays

SiO2-Al2O3


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Promoter: A third agent which when added, often in small amounts, results in desirable activity, selectivity or stability effects.

EXAMPLES OF PROMOTERS IN MAJOR PROCESSESPromoters are designed to assist either the support or the active component.

Decreases Cu sinteringZnOCu-ZnO-Al2O3 Low T Shift

Active Component: FeAl2O3-K2OFe-(Al2O3-K2O)

Increases C removalKNi/ceramic supportsSteam reforming

Increases hydrogenolysis and sintering.Increases MoO3 dispersion

Ni, CoP, B

MoO3/Al2O3 Hydrotreating

Decreases hydrogenolysis and sinteringRePt/Al2O3Catalytic Reforming

increases acidity and thermal stabilityincreases hydrogenation

Rare earth IonsPd

Zeolites Cracking catalyst

Increases CO oxidationPtSiO2-Al2O3Cracking catalyst and matrix

Retards sintering of active componentMgOPreferred phase for support: γ-Al2O3 High surface areaAcidityForms solid solutions

Improves thermal stability; poisons coking sites;increases acidity

SiO2, ZrO2, P, K2O (potash), HCl

γ-Al2O3 (both support and catalyst)

FUNCTIONPROMOTERCATALYST


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14

Adsorption too weak

Adsorption too strong

Cat

alyt

ic a

ctiv

ity

Parameter measuring strength of adsorption


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16


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Sabatier Catalytic Cycle of N2O Decomposition

N2O + Fe3+

N2O + FeO3+

O2 + Fe3+

FeO3++ N2


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N2O decomposition on [FeO]1+-ZSM-5

55 2 ZSMOFeOONZSMFeO

+


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N2O decomposition on [OFeO]1+-ZSM-5

55 22 ZSMOFeOONZSMOFeO

+


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O2 desorption from [O2FeO]1+-ZSM-5

22 55 OZSMOFeZSMOFeO

+


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-200

-150

-100

-50

0

50

100

Reaction Coordinate

Rela

tive

Ener

gy,k

J/m

ol

[FeO]+1

[OFeO]+1

[FeO2O]+1

[FeO]+1 + O2

-28.9

113.2

-100.7

-154.8

-122.7

176.8

32.15

A summary energy (including ZPE correction)


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24


25


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27


28


29


30


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C-C=C-C

HMethyl Cyclopropane

Butene ButaneC-C-C-C

Metal

Support

A bifunctional catalyst such as platinum on Al2O3 facilitates the isomerization of methylcyclopropane to 2-butene as well as the

hydrogenation of 2-butene to butane (after Boudart)


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CATALYTIC ACTIVITY

Rate = k f(ci)k = A`exp(-E`/RT)

WhereA`= Pre-exponential factorE` = Apparent Activation Energy

Turnover Frequency=TOF=))(.(

.TimeSitesActiveofNoproductaofmoleculesofNo

for a fixed set of reaction conditions (temperature, pressure orconcentration, and reactant ratio)

or

)(1 1 sdtdn

STOF Where; S= No. of Active Sites

Lower Limit for TOF for Heterogeneous catalysis:S=A= Total No. Of Exposed Surface Catalyst AtomsAlso,

)(1 12 scmdtdn

ARaterealA Where; A= Measurable Surface Area


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Magnitude of Turnover Frequencies

For most heterogeneous catalytic reactions involving smallmolecules in the temperature range 100 – 500oC and lowpressures (up to a few bars):

TOF between 10-2 and 102 s-1

Compare these values with well-known enzymatic reactions:

103 for chymotrypsin104 for urease and acetylcholinesterase

107 for catalase


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AMMONIA SYNTHESIS CATALYSTFe-Al2O3-K2O

Historically it was thought that

Al2O3 prevents Fe sintering upon reduction

K poisons acid sites inadvertently introduced by Al2O3

Modern research now indicates,

Structural promotion Al2O3 promotes Fe by stabilizing [111]surface planes which are 500 times more active than other planes.

K doubles the promotion by being an electronic modifier by donatingelectrons from the ionization state. These improve -bonding of nitrogen leadingto faster dissociation and higher rates. Similar effects have been reported forhydrogenation of carbon monoxide on Ni.


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DIFFERENCES IN CRYSTAL PLANES AMMONIA SYNTHESIS

Plane Activity

[100]

[110]

[111]

21

1

440


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-30 -

-20 -

-10 -

0 -

10 -

20 -

30 -

40 -50 -

E hom

oECat

Ads

orpt

ion

Surf

ace

Rea

ctio

n

Des

orpt

ion

13103xmogeneoushratecatalyticrate

223

221 HN

aa HN 3 aNH3

HOMOGENEOUS REACTION

3NH

REACTION →

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32

2

2

2

)()()(

)(

22

NHNHNHHNH

NHHNH

NHHN

NNHH

a

aaa

aaa

aa

a

a

a

RATE DETERMINING STEP(only EACT=12 kcal/mol)

ADSORPTION(CHEMISORPTION)

SURFACE RECOMBINATION

REACTIONS

DESORPTION


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20

40

60

80

100

0100 200 300 400 500 600 7000

3/23

22

322

NHNHNH

EQU

ILIB

RIU

M C

ON

VER

SIO

N, %


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LONG LIFE

CATALYST PELLET

FLUID FLOW•Flow distribution

•Low pressure drop

•Mechanical Strength

HIGH ACTIVITY•Chemical Activity

•High Specific Active Surface

•Porous Pellet

STABILITYResistant to Sintering

PoisoningFouling

Engineering features of the catalyst

lect2-sp14

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