Eindhoven University of Technology

MASTER

Low-Energy Ion Scattering and spinal surfaces in catalysis

Reijne, S.

Award date:1994

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t(Î) Eindhoven University of Technology

Faculty of Physics

Low-Energy Ion Scattering and

Spinel Surfaces in Catalysis

S. REUNE FEBRUARY 1994

Solid State Physics Division

Physics of Surfaces and Interfaces

Master Thesis

Mentors:

ee ir. J.-P. Jacobs -prof. dr. H.H. Brongersma Schuit katalyse instituut

Abstract

Low-Energy Ion Scattering and Spinel Surfaces in Catalysis

Abstract

Low-Energy Ion Scattering is used todetermine the composition of the uppermost atomie

layer of solid surfaces. Besides the top layer sensitivity easy quantification by calibration

makes this technique very well suited for application in catalytic research.

The influence of the chemical environment and surface roughness on the LEIS intensity

from aluminum and nickel is investigated in alloys and oxides. It was found that in the

alloys and oxides the signal was independent of the matrix which will allow quantification by calibration. In NiAl{ 110}, however, strong evidence of a matrix-induced decrease of

the neutralization for the aluminum was observed. The obtained knowledge is used in the investigation of catalytically active spinels.

Different ferrites were prepared; F~04, ZnF~04 and MgFe20 4, also a- and y-F~03 were included. Catalytic activity, LEIS and XRD show that the octahedral sites are

preferentially exposed on these spinel surfaces.

Furthermore, the influence of the preparation metbod and addition of Sn and Ca on the

surface structure of zinc aluminates, a support material in catalysis, were studied. Ca was

deposited on the spinel surface, while Sn induced a restructuring of the surface. Although large differences in the structure of the support material were found, the influence on the activity in isobutane dehydrogenation of the Pt supported zinc aluminates was small.

1

Preface

Preface

One can only see what one observes, and one observes only things which

are a/ready in the mind.

Alphonse Bertillon

I can only hope this quote is not always valid, but I would like to use this preface to

thank all people of the surface and interface group at the faculty of physics at the

Eindhoven University of Technology for the pleasant time during the last year of my

study. Special thanks to Jean-Paul Jacobs, my coach, for his enthusiastic aid and

cooperation during the project. I should not forget Hidde Brongersma, not only for the last

year, but also for giving me the opportunity to do a project in Japan in April 1992. I would also like to thank Hans Dalderop and Gerard Wijers for their technica! assistance during this period. Furthermore thanks to everybody who had any attribution to this work. Finally l'd like to dedicate this work to my parents, for their support during my study.

Eindhoven, february 1994

Stef Reijne.

11

Contents

Abstract ........................................................ i

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Chapter 1

Introduetion

Chapter 2

3

Theoretica! aspectsof Low-Energy Ion Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Basic principles of LEIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Quantification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 The differential cross section . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.2 The ion fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Chapter 3

Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2 The NODUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2.1 The UHV system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.2 The ion souree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.3 The cylindrical mirror analyser . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.4 Detection of the ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.2.5 Charging of samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3 Other characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.3.1 BET surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3.2 Catalytic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3.3 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.4 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Chapter 4 Quantification of the composition of alloy and oxide surfaces using Low-Energy Ion

Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.3 Results and discussion ................................... 21

4.3.1 Alloys ......................................... 22

4.3.2 Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1

4.3.3 Surface roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3.4 Surface composition of powders . . . . . . . . . . . . . . . . . . . . . . . 28

4.4 Conclusions .......................................... 28

4.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Chapter 5 The surface structure of catalytically active spinels, Perrites . . . . . . . . . . . . . . . . . . . 31

5.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2.1 Catalyst preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2.2 Characterization of the catalysts . . . . . . . . . . . . . . . . . . . . . . . 34

5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.4 Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Chapter 6 The influence of the preparation metbod on the surface structure of ZnA120 4 ••••••• 41

6.1 Introduetion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6.2.1 Catalyst preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6.3 Results and discussion ................................... 45

6.3.1 Elemental composition and compounds .................. 45

6.3.2 Surface area and porosity ............................ 46

6.3.3 Surface analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

6.3.4 Catalytic test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Chapter 7

Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

2

I ntroduction

Chapter 1

Introduetion

The sixties gave us The Stones, The Beatles, the sexual revolution and Ultra High Vacuum (UHV). The continuing availability of improved UHV equipment has allowed

surface science to develop in the last decades. Most surface science techniques are based

on beam analysis. In particular, Low-Energy Ion Scattering (LEIS) is a technique where

gas ions are directedontoa target with an energy from 0.5 up to 10 keV. The collisionsof

projectiles and target atoms can easily be described by classica! mechanics, and give

straightforward information about the targets' surface. Despite its' unique capability of

prohing only the outermost atomie layer, quantification of the surface with LEIS still

causes many problems. The influence of physical shielding and neutralization are not

clear. Recently the influence of the chemica! environment of the target atom on the

collision (matrix effects), originally thought to be negligible, has been examined again.

Can any influence be detected and predicted? In chapter 4 strong evidence for matrix

effects found in a NiAl alloy and the influence of surface roughness on the LEIS signal

are described.

Surface science is of great importance to get a better understanding of semiconductors,

polymer coating, as well heterogeneons catalysis. The investigations on the catalysts are

carried out within the scope of catalytic research of the Schuit lnstitute of Catalysis at the

Eindhoven University of Technology. The chemica! reactions, in heterogeneons catalysis,

run on the surface of a catalyst The activity and selectivity depend on the properties of

the catalyst, morphology, defects, strengthand coordination of cation-anion honds. A high surface area is required for a high reaction rate. Highly porous powders can often fulfil

this requirement. In this study, powders with a spinel structure are investigated. Do

complex structures as spinels expose preferenrial surface planes? lf so, can this surface be

affected by different preparation? In chapter 5 recent ideas about spinel surfaces are corroborated, and in chapter 6 the influence of different preparation methods on zinc

aluminates is described. The spinels were investigated using different analysis techniques.

In the next chapter the basic concepts of low-energy ion scattering are described. In chapter 3 an experimentalset-up which enables surface characterization by LEIS and other methods are briefly discussed. The chapters with experimental results (4,5 and 6) are or will be submitted for pubHeation and may therefore possibly overlap.

3

Theoretica/ aspects of LEIS

Chapter 2

Theoretica! aspectsof Low-Energy Ion Scattering

2.1 Introduetion

Low-energy ion scattering (LEIS) or ion scattering spectroscopy (ISS) is one of the most

surface sensitive analysis techniques known. It's unique sensitivity for the outermost layer

of the surface cao provide important additional infonnation to more common techniques

such as XPS, AES, SIMS etc. The technique cao be applied to the study of both single crystals and conducting materials, while more complex materials, such as polymers and insuiaring powders, cao be analysed when a correct experimental set-up is chosen. The

basics and some background of LEIS is described in this chapter.

2.2 Basic principles of LEIS

In low-energy ion scattering, inert gas ions with energies in the range of 0.5 to 10 keV are

directed onto a target When the ion hits the sutface, several processes occur. The ion will

be scattered by the atoms in the target sutface. The ion cao either penetrate into the bulk, or, when the impact is close enough to a surface atom, be backscattered. The backscattered partiele cao either be a neutra!, metastable or an ion. In LEIS only the scattered ions, evolving mainly from collisions with atoms in the topmost atomie layer, are selected and

detected. Diffraction effects are oot relevant in LEIS, since the lattice parameter (10-10m) is much

larger than the De Broglie wavelength (10-12m). The distance of ciosest approach is in the

order of 10·11m, thus the scattering process cao be considered a two body collision

between the projectile and a target atom. Furthennore the thennal motion of the target

atoms is small compared to the velocity of the projectile, hence the interaction time oo-16s) is much shorter than the elastic vibration time of the target atoms (10-13s), so the

interaction effectively takes place with thennally displaced atoms at rest. Durlog the

collision the target atom cao be considered as a free atom. Thus, in a first approximation,

it is sufficient to describe the scattering kinematics in tenns of two body collisions using

classical mechanics, see figure 2.1 for a schematic representation. However, this

approximation does oot include the neutralization behaviour that may result in different

scattering events. Using the classicallaws of conservation of energy and momentum, one

can derive an equation where the final energy E1 of a projectile after backscattering, only depends on the primary energy E; of the projectile before the collision, the scattering angle

e and the masses of the ion M1 and the target atom M2•

5

Chapter 2

Et

Fig. 2.1 Schematic representation of the scattering process.

(2.1)

where

(2.2)

During LEIS experiments Ei, 9 and M1 are fixed. The final energy of the ions increases

with the mass of the target atom, thus the LEIS-(energy-)spectrum is in fact an equivalent

of a mass spectrum of the surface atoms. It follows that the mass resolution is at a

maximum if the difference in mass between incident ion and target atom is small and the

scattering angle 9 is close to 180°.

LEIS can be made sensitive to the outermost atomie layer only when noble gas ions are used as projectiles. The high electron affinity in combination with the large differentlal scattering cross sections due to the low energies in LEIS ensures that a large proportion of

the ions is neutralized during the interaction with the target. The peaks in the LEIS

spectrum can be regarded as surface peaks as the contribution of deeper layers to the

peaks is negligible. However, a background in the spectrum, from neutralized particles

being reionized prior to leaving the surface after scattering events in the bulk, is often

present. A typical spectrum, 9 at 142°, is shown in figure 2.2.

6

Theoretica[ aspects of LEIS

-. c ::1

.Q

750

:a 5oo -~ ën c .!! c

Si . A

surface peak. " ---" 11 11 I I

I I

I

I

I I

(j) 250 üj

! double inelastic background I collisions

\ ~~-,-< \I ...1 _,.,., \ \ .-~'"' \ , ...

- \ I ..,_

1250 1750 2250 Flnal energy (eV)

Fig. 22 A typical LEIS spectrum, 3 keV 4He+ from a silicon surface.

2.3 Quantification

The intensity Si of ions scattered at element i can be described by:

da1 • S1=1-P1 N1TcR dC (2.3)

where I =incident ion flux

do/d!l = differentlal cross sectlon for scattering from element i

Pt = ion fractlon after scattering from element i at fixed scattering angle Ni = numerical density of element i per surface area

T = transmission of the analyser ( ,.."E1)

c = instromental factor depending on the analyser and detectlon efficiency

R = constant to take into account the effect of the roughness of the surface

The differentlal cross sectlon and the ion fractlon will be explained in more detail in the

next paragraphs. The influence of surface roughness and matrix effects on the

quantlficatlon in LEIS will be discussed in chapter 4.

7

Chapter 2

2.3 .1 The differential cross section

The differential cross section is defined as the fraction of the incident intensity scattered

into a unit solid angle at a given direction. When the interaction between the ion and the

target atom is known, the cross section can be calculated. The interaction can be described

by an interatomic, repulsive potenriaL In the energy range used in LEIS, 0.1 - 10 keV, this

interaction is determined by the nuclear charges and the screening by the electron clouds. The screened coulomb potenrial used in LEIS has the general form [1]:

zze2 V(r) 1 2 ~(!'.)

4n:Ecf a

where cp(r/a) is the screening function and a the screening length. The Molière approximation of the Thomas-Fermi screening function is used and can be written as:

r r r r cl»(-)=0.3Sexp( -o.3-) +0.55exp( -1.2-) +O.lOexp( -6-) a a a a

with

0.88534izo

(2.4)

(2.5)

a=----2 (2.6)

99% of the total primary ions) provides the surface

sensitivity of LEIS. From the toplayer, generally, a few percent of the particles is

backscattered as an ion, as is the case of noble gas ions. The ion fraction is strongly

dependent on the primary energy of the ions, the scattering angle and the ion-atom

combination. The exact behaviour of the ions in an ion-atom combination is still not well

understood. Therefore direct quantification of the signals of LEIS is still impossible, unless

calibration against the pure elements is employed.

8

Theoretica[ aspects of LEIS

HELIUM

ts ls ts

(a) (b) (c)

AUG ER V ALENCE-LEVEL CORE-LEVEL NEUTRALIZA TION RESONANCE RESONANCE

NEUTRALIZA TION NEUTRALIZA TION

He ... (1s) --He0(1s2) He.(1s) --He*(1s2s) He•(1s) -He0 (1s 2)

Fig. 23 Mechanisms of neutralization of noble gas ions at solid surfaces.

The basic mechanisms for the neutralization, of ions in LEIS, were frrst described by

Hagstrum [2] and are shown in tigure 2.3. Auger neutralization is thought to be the

dominant neutralization process in most cases.

Hagstrum developed a physical model for the Auger neutralization process for metal

surfaces, based on the idea that the transition rate for neutralization is exponentially

dependent on the distance between the ion and the surface. This idea leads to equation 2.7,

where V; and v1 are the veloeities of the projectile befm-e and after collision respectively,

and vc is the characteristic velocity or neutralization constant, specific for each element.

(2.7)

The expression can be interpreted as an expression for the survival probability of the

projectile ion, scattered at a eertaio angle 8, which depends on the time the ion spends within a eertaio distance of an atom. The neutralization constant is in the order of

magnitude of Hf rn/s for He+ scattering. Despite a large number of supporting measurements [3,4], the validity of this model is still debatable [5,6]. Recently there bas

been increasing interest in valenee level resonance neutralization, which is affected by

changes in workfunctions of the target material as shown by Souda et al. [7]. For He+

scattering on a number of target atoms also core level resonance neutralization is

observed, but they are well understood and are only important in special cases (In, Pb, Ge,

Bi, Sb)[3].

9

Chapter 2

2.4 References

[1] H. Niehus, W. Heiland and E.Taglauer, Surface Science Reports 17 (1993) 221

[2] H.D. Hagstrum, Phys. Rev. 96 (1954) 336 [3] H.H. Brongersmaand T.M. Buck, Nucl. Instr. Meth. 132 (1976) 559

[4] M. Beekschutte and E. Tagtauer, Nucl. Instr. Meth. B 78 (1993) 29 [5] L.K. Verhey, B. Poetserna and A.L. Boers, Nucl. Instr. Meth. 132 (1976) 565 [6] D.I. O'Connor, Y.G. Shen, I.M. Wilson and R.I. MacDonald, Surf. Sci. 197 (1988)

277 [7] R. Souda, T. Aizawa, K. Miura, C. Oshima, S. Otani and Y. lshizawa, Nucl. Instr.

Meth. B 33 (1988) 374

10

Experimental

Chapter 3

Experimental

3.1 Introduetion

It is obvious that in surface science any additional contamination of the surface should be

prevented. Therefore effective preparation of a clean sample surface is essential, while

contamination must be avoided during measurements. Operation in high and ultra high

vacuum (UHV) is generally necessary. The presence of a native oxide and hydroxide

means that the final surface preparation must be carried out in the UHV chamber. In case

of metal surfaces, this can be done by the ion beam used for the analysis. Exposing the surface to high doses of heavy inert gas ions (Ar+ or the like) bas a peeling effect on the surface, thus residual oxygen, carbon and hydroxide groups, present on the surface of the

target, are removed by sputtering. Though the sputtering damages the surface, the structure

can be restored for most metals and alloys by annealing after the sputtering process.

However, when dealing with micro crystallites and oxides, as most catalysts are, this is

impossible as the sputtering process destroys the fragile structure. A pretreatment of

hearing the oxide in a mild oxidizing atmosphere is often sufficient to remove the

contamination. To keep sputtering effects as low as possible, while measuring, a light inert gas ion (He+)

is used and the iondoseis kept toa minimum in order to retard the unavoidable damage. The experimental considerations used for the low-energy ion scattering measurements are

described in detail in this chapter. The main features of other analysis techniques used in

this work are also described.

3.2 The NODUS

In this study the LEIS measurements were performed on the NODUS. The original set-up was frrstly described by Brongersma et al. [1]. It was developed as an essenrial NOn

Destructive Ultra Sensitive technique for surface analysis. In figure 3.1 the configuration

used nowadays is drawn. A mono-energetic ion beam is produced in the ion source. The

ions are mass selected, deflected and focused perpendicular onto the target, which is

placed on a carousel. Twelve sample bolders can be stored on the carousel. This way the

user is able to analyse different samples onder identical scattering conditions. The ions

scatteredover 142° are collected and energy analysed in a cylindrical mirror analyser (CMA) described in more detail below.

11

Chapter 3

Fig. 3.1

12

Schematic representation of the LEIS apparatus NODUS. 1. ion source, 2. mass filter, 3. dejlection plates, 4. einze/lens, 5. aperture, 6. CMA, 7. carousel, 8. turbo molecular pump, 9. ion getter pump, 10,11. valve, 12. gas

inlet needie valve.

Experimenta/

3.2.1 The UHV system

To achieve ultra high vacuum (UHV) and to rnaintaio it, the system must be pumped

continuously. The NODUS is built with a differential pumping system which contains a

small turbo molecular pump near the ion source, a turbo molecular pump just before the

aperture and an ion getter and a titanium sublimation pump, cooled with liquid nitrogen, in

the main chamber, see tigure 3.1. The background pressure is kept at about 1 *1o-9mbar and increases to 1 *10"8mbar when the ion beam is operated. The pressure raise is due to

the inert noble gas when operaring the ion source, and is too low to influence the

measurements.

3.2 .2 The ion souree

The ion source, a Leybold IQE 12/38, is shown schematically in tigure 3.2. The unit consists in principle of four parts; a ring-shaped filament cathode, an anode cage, an electron repelient and an ion extractor. Electroos emitted by the filament catbode are

accelerated towards the anode cage. The electroos may be captured at the anode, but it is

more likely that they pass through the cage. When the electroos have passed the cage, the

electroos are accelerated back to the cage by the repulsive electron shield. The cage region

may be passed several times before being captured at the anode cage. When gas is

introduced in the cage region, the gas atoms will be ionized by collisions with the

accelerated electrons. The positive ions are caught in the cage region. The positive ions are pulled out of the cage with an extraction electrode. The extracted ions are then accelerated to the desired energy, mass selected by a mass filter (an adjustable electtic and

magnetic field, perpendicular toeach other and to the beam). The beam is directed onto

the target via an aperture, a set of deflection plates and a series of electrastatic lenses. Just

in front of the sample an einzel lens can be used to focus the beam from about 5 mm to 1 mm diameter. A typical beam current is 60 nA.

3.2 .3 The cylindrical mirror analyser

To be able to work with low ion doses, a kind of cylindrical mirror analyser (CMA) is

used to collect the scattered ions effectively. With a conventional hemispherical analyser

only a smallsolid angle can be selected. A CMA, with cylindrical symmetry, selects a

complete cone. A disadvantage of a CMA is that it cannot be mounted easily, so the solid

angle is fixed. In the NODUS only ions scatteredover an angle of 142° are selected. In

tigure 3.3 the CMA in the NODUS is schematically shown. The scattered ions pass the

frrst slit and are deflected by a homogeneaus electrastatic field.

13

Chapter 3

4

1

Fig. 32

Fig. 33

)

I 2 3

Detail of the ion souree of the NO DUS, Leybold IQE 12138. /.electron

repel/ent, 2. anode (cage), 3. cathode (ring), 4. ion extractor.

:

Experimental

3.2 .4 Veteetion of the ions

To detect the ions, eight channeltrons are placed in a ring-shaped series behind the second

slit of the CMA. A channeltron is a snail-shell shaped glass tube. The inside is covered

with an electron multiplying materiaL The impact of an ion leads to a cascade of

secondary electrons, into the channeltron. This results in a pulse which can be detected. A

problem that frequently arises with the use of channeltrons is the variabie detection

efficiency. Different impact energies give different pulses, and even the place where a

channeltron is hit results in varying pulses. Because a discriminator level for detection is

necessary to suppress noise, and tuning the channeltrons to saturation was impossible, the

detection efficiency behaviour is not completely clear. Therefore, to get round the

problems of the different detection efficiency of the channeltrons, the results in chapter 4

are not presented in a way in accordance with Hagstrums model, as described in more

detail in [2], but in comparison toa calibration sample.

3.2.5 Charging of samples

When the ion beam hits the surface of a sample, most ions are neutralized. This implies a

charge transport to the target surface. The surface will charge when it is insulating. The

charging must be compensated because it will lead to a shift in the energy spectrum. The

shift is due to a deceleration of the ions before and an acceleration after the collision with

the target, thus the scattering conditions are changed. The charging can be compensated by

flooding the surface with low-energetic thermal electrons. The electrons are emitted from a

ring-shaped filament, and directed to target via deflection plates, see figure 3.3. Because

the sample is flooded from all sides, even rough and porous surfaces are fully charge

compensated.

3.3 Other characterization techniques

3.3 .1 BET surface area

The specific surface area of the catalytically active powders, in chapter 5 and 6, was determined by nitrogen physisorption. It is based on multilayer adsorption described by the

adsorption-isotherm of Brunauer, Emmet and Teller, the BET -equation:

V Cp, (3.1) VIII (1-p,)-(l-p, +Cp,)

where V = adsorbed volume

15

Chapter 3

V m = 'monolayer capacity', the volume necessary to fill a monolayer

Pr = p/po = relative vapour pressure

C = a constant Po = saturation pressure at the adsorption temperature

C and V m can be calculated from a series of measurements where the relative vapour

pressure is changed and the adsorbed volume is measured. When liquid nitrogen is used,

the specific surface area can be determined, where an adsorbed N2 molecule is 16.2 Á2

•

3.3 .2 Catalytic Activity

Activity measurements of catalysts, as in chapter 5 and 6, can be carried out in a

microreactor system. An example is given in figure 3.4.

Fig 3.4

-~-----~::.::-------

E

Gas Analysis

------· B

------ c

------ D

~out

The kinetics set-up, a microreactor system. The gas mixture f/ows through

the temperature controlled reactor, where A: thermocouples, B: heat jacket,

C: catalyst bed, D: sample loop, E: gas chromatograph including integrator

and recorder for data processing. C3 is propane.

Reactant gases and a carrier gas, usually an inert gas as He, flow through a bed of catalyst

in a pressure and temperature controlled atmosphere. The catalyst bed is placed a quartz tubular plug flow reactor where the temperature is checked by one or more thermocouples.

16

Experimental

Via a sample loop the reactor products are analysed by a gas chromatograph, so activity

and selectivity of the catalyst can be determined.

3.3 .3 X -ray diffraction

X-ray diffraction (XRD) is basedon the diffraction of X-rays by crystallographic planes.

Interference of diffracted waves from successive planes indoces maxima when the path

difference is an integral number n of wavelengtbs A., according toBragg's law:

nl=2dsin8 (3.2)

where d is the spacing of parallel atomie planes and 8 the diffraction angle. All crystalline

matenals are charactenzed by a specific set of values corresponding to various planes. The

bulk crystal structure can be determined by XRD.

3.4 Experimental procedure

Most studied matenals are rough insuiaring powders. The powders are pressed to wafers in

a tantalum or a lead disk. The sample bolders are placed in a transfer system and carried

to the preparation room, where the samplescan be pretreated. Usually the pretreatment

requires hearing the sampletoabout 2000 C in 20 mbar oxygen for 15 minutes. In this

way the largest part of contamination (typically carbon) is removed. Also most hydroxyl

groups present on the surface are removed by this pretteatment After the pretteatment the sample is moved to the carousel of the NODUS. The sample can now be analysed with the ion beam. The energy analysis is computer controlled. To avoid unnecessary

destruction of the surface only an interval of interest is scanned. In most cases the frrst

spectrum differs from the following spectra because during the first minutes hydroxyl

groups still present on the surface are removed. During the measurement of a spectrum at

a target current of 60 nA, the target surface is exposed to anion dose of about 5*1014

particles of He+ per cm2• Assuming a sputter yield for Heions on surfaces of 10%, during

20 successive spectra 1 monolayer is removed, since the surface density is of the order of

1015 atoms/cm2• To improve resolution, up to 10 successive spectra are added, after the confrrmation that the successive spectra are of identical shape.

3.5 References

[1] H.H. Brongersma, N. Hazewindus, J.M. van Nieuwland, A.M.M. Otten and A.J.

Smets, Rev. Sci. Instr. 49 (1978) 707

17

Chapter 3

[2] S.N. Mikhailov, R.J.M. Elfrink, J.-P. Jacobs, L.C.A. van den Oetelaar, P.J. Scanion and H.H. Brongersma, submitted to Nucl. Instr. Meth. B

18

Quantification in LEIS

Chapter 4

Quantification of the composition of alloy and oxide surfaces using Low-Energy Ion Scattering·

*The contentsof this chapter is submitted to the Journal ofVacuum Technology A by

J.-P. Jacobs, S. Reijne, R.JM. Elfrink, SN. Mikhailov, M. Wuttig and H.H. Brongersma,

and presentedat the 40th symposium of the American Vacuum Society, November 1993,

Florida, U.SA.

4.1 Introduetion

In various fields of science and technology, such as heterogenous catalysis, IC-technology,

corrosion protection, and ceramics, there is a wide interest in the surface structure and

chemica! composition of pure metals, metal oxides, and alloys.

Low-energy ion scattering (LEIS) is a surface sensitive technique that selectively probes

the outermost atomie layer, see chapter 2 and 3 and [1,2]. A beam of mono-energetic inert

gas ions (He+, Ne+, Ar+, 0.5-10 ke V) is directed onto the target. Since the energy loss of

the scattered ion depends on the mass of the target atom, the atomie composition of the

target can be derived from the energy distributton of the scattered ions, see chapter 2. The

high neutralization probability and large scattering cross section of the inert gas ions

ensures the monolayer sensitivity.

The intensity (Si) of the ions scattered from element i on the surface can be described by

equation (2.3). This can also be written as:

(4.1)

where R is again the influence of roughness, Tti is the calibrated sensitivity for element i, and ei is the surface coverage [1,3]. Most of the factors described, can be calculated or

detennined experimentally. The ion fraction, which is strongly ion and target element

dependent, and the influence of the roughness are, however, two factors which hamper the

direct quantification. Generally, quantification in LEIS is basedon calibration against pure

elements. For this purpose eq. (4.1) can be used. This metbod is only valid when the

influence of the roughness is known and neutralization behaviour is identical to that of the

calibration target. So, when the neutralization on element i is not influenced by the

chemica! environment of the atom of element i (no matrix effects) direct calibration is

possible.

Opinions in literature about possible matrix effects vary. On the one hand, recent results

on different carbon species show that for graphitic carbon the sensitivity for 2 ke V

19

Chapter 4

incident 4He+ and a scattering angle of 136° is about 240 times lower than for carbidic

carbon [4]. On the other hand, strong matrix effects reported for oxides [5] were later

shown to be due to special atomie properties of the elements involved and independent of

the matrix [6]. Studies on different clean metals showed no work function-related

neutralization behaviour[7], also the adsorption of CO had no effect [8]. However, a large

decreasein the ion yields was reported when different alkali metals were deposited [8-10].

In alloys Novaeek and Varga [11] and Ackennans et al. [3] reported the absence of matrix

effects in NiPt and CuPd alloys. Currently the absence or presence of matrix effects in

low-energy ion scattering cannot be predicted.

Except for elements where a direct resonant charge exchange process dominates the

neutralization, e.g. Pb or Bi, the ion fraction can, in a first approximation, be written as:

(4.2)

where V; and v1 are the veloeities of the ion bef01-e and after collision respectively, and vc is the characteristic velocity for a given ion-target combination. Since the ion fraction

changes with the energy of the ions, a change in the magnitude of the neutralization due to

the matrix can be observed by comparing the signals of the different systems as a function

of primary energy. If no matrix effects are present the experimentally detennined

concentrations of the compound will not depend on the primary energy , which greatly

simplifies the quantification.

The analysis of oxides is not straighûorward. In heterogeneous catalysis, for example, highly porous supports are used to improve catalytic performance and thennal stability.

These powder supports (e.g. y-Al20 3, a-Al20 3, SiO~ are non-conductive and can have

specific surface areas up toa few hundred m2/g. It was reported by Margraf et al. [11] that

for different aluminum oxides the signa! will decrease by more than a factor of 5 due to

surface roughness. Calculations of Nelson [21] on the influence of surface roughness on

the intensity of LEIS-signals predict a decrease by a factor of about 1. 7 when introducing

roughness.

To obtain a better insight into the factors which affect the quantification, more

investigations are needed. The aim of this work is to detennine the presence or absence of

matrix effects in LEIS for aluminum. Therefore, Al has been studied in various chemica!

environments (metal, alloy, oxide). Al20 3 is of great importance in heterogenous catalysis

as a support material. Related samples containing Ni are also studied. Furthennore the

influence of roughness on the LEIS signa! is studied by comparing optically flat surfaces,

with very rough powders used in catalysis.

20

Quantification in LEIS

4.2 Experimental

The LEIS experiments were performed using the low-energy ion scattering set-ups,

NODUS and MINI-MOBIS. Their basic design is the same and has been described in

more detail in chapter 3 and [13]. The primary ions are generaled in a Leybold ion souree

and are directed perpendicular onto the target. The ions scatteredover 142° (NODUS) and

136° (MINI-MOBIS) are analysed by a kind of cylindrical mirror analyser (CMA). In the

NODUS apparatus charging of insuiaring samples can be effectively compensated by

flooding the surface from all sides with low-energy electrons. The MINI-MOBIS is

equipped with a sputter ion souree (Leybold type IQE 12/38) at grazing incidence (15°).

Oxides are measured in the NODUS, while alloys are studied using the MINI-MOBIS. In both machines there is a facility to heat the samples up to 1000 K. The nominal base pressure in the MINI-MOBIS is in the low 10"10 mbar range, while in the NODUS a base

pressure in the low 10"9 mbar range can be maintained. The base pressure is higher

because in the NODUS highly porous powder catalysts are measured. Durlog the

operation the pressure will increase in both set-ups to the about 1 *10"8 mbar. This increase

is due to the inert gas from the ion source, which will not affect the measurements.

The following samples were used in the analysis: Al (polycrystalline), Ni{100}, NiAl{ 110} [14], Ni8J>lw (polycrystalline), AggoA120 (polycrystalline)[15], y-Al20 3 (AKZO,

-powder), a-Al20 3 (Fluka A.G., powder), a-Al20 3 sapphire{ 11 02} (Crystal Systems), NiO

(Johnson Matthey GmbH, powder), 14 wt%-Ni/y-Al20 3 (catalysts prepared by atomie layer epitaxy (ALE) [16], powder).

The metals and alloys were cleaned by standard procedures of sputter-anneal cycles until

judged clean by LEIS measurements, i.e. typically less than the detection limit of a few %

of a monolayer of oxygen and carbon. The sapphire single crystal was heated in 1 bar oxygen at 1400 K for 24 h to obtain a well-structured surface. However, the surface was then contaminated due to alkali diffusion. Therefore, these oxide surfaces were Ar-sputter

cleaned prior to analysis. All powders were pressed into wafers.

It has been reported in literature [14] that a very thin alumina film (=5 Á) can be grown

on a NiA1{110} alloy aftera saturation with oxygen (> 1200 L). This system is used in

catalysis as a model system for the alumina support. In this study it is a suitable model

system for a smooth aluminum oxide surface in comparison to the smooth sapphire and

rough oxide powders.

4.3 Results and discussion

The measurements on the Ni-Pt and the Ag-Al alloy were performed by R.J.M. Elfrink

[25]. He also reproduced the results on NiAl { 110}. Figure 4.1 shows typical LEIS-spectra

21

Chapter 4

spectra of the different systems. The spectra shown were all measured using a 3ke V 4He

ion beam. Spectra presented in one figure can be compared on an absolute scale since the

experimental conditions were kept constant.

......... tn ~ () ........ >a ~ ·-tn c Cl) ~ c ·-1 en -w ..J

3000

2000

1000

Fig. 4.1

Ag

l N

... P I ~

••

-- NiAI(llO}

Ni8J>~0 AgsoA12o

Al

11 11 11

•• 11 11

•• •• •• •' I •• 1 1 11 11 11 •• I I 11 I : 11

.,J 11 I

- ... ~ I

1000

500

10001500200025003000 Final energy (eV)

A 1--· y-All~3 -- sapphire ' - NîJvAIQ 11 IJ•~ 3 1 I - - oxidized Ni I I

I \ I I

I \

1250 1750 2250 Final energy (eV)

Typical LEIS spectra using 3 keV 4He+. a. alloys, b. oxides.

The LEIS-signal intensities are calibrated against the signal produced by the pure metal sample. In this way, only ions which pass through the analysing and detection system with the same energy are compared. The results are, therefore, presenled in such a way that the

signals do not have to be corrected for experimental factors such as the transmission of the

CMA or the efficiency of the channeltrons used in the detection, as mentioned in chapter

3. The results for the calibrated Al signals can be found in figure 4.3. The calibrated Ni

signals are shown in tigure 4.4.

4.3.1 Alloys

The results for the alloys are presented in a different way using calibration against the

pure metals, as is shown in figure 4.2. The signals in this tigure are corrected for the

atomie densirles in the different samples, where for the polycrystalline materials closed-

packed surfaces are assumed. If (for flat surfaces) no matrix effects are present, the ion

fraction p+ does not depend on the chemica! environment, and thus the signal is only

22

Quantification in LEIS

dependent on the coverage and a calibrated sensirivity factor (see equarion 4.1). This

sensirivity factor is the calibrarion against the pure element (corrected for the atomie

density). From this factor, the absolute coverage can be calculated. The coverages of the

different components must add up to 100%.

.......... '#. ..._.. 100 CD C') 80 as a.. CD > 60 0 ()

CD 40 () as ..... a.. :l 20 en

0

Fig. 42

total 0 tot~ 100 • ~ • 0

80 •A~

60

40

0 4He 20 e 3He

0 1000 2000 3000 1000 2000 3000

Results of the eaUbration as a function of primary energy. a. Ni8rf't20, b. AgsoAl2o·

For NisJ>tw and AggoA120, it is shown in fig. 4.2 a,b that the calculated composirions do

not depend on the ion energy. The results for the NisoPtw alloy confrrm the results

obtained by Novaeek and Varga [10] that no matrix effect is present. At every primary

energy the results are reproduced quantitarively and the coverages add up to 100%. This is

also true for AggoA120 and also holds for the experiments with 3He. The neutralizarion

behavior is expected to be the same for the two He isotopes, the only difference being the

velocity of the incident and scattered ions [17]. In the NisoPtw alloy an enrichment of Pt is

found which is probably due to preferenrial sputtering. The surface concentrarions of

AggoA120 reflect the bulkvalues in accordance with Dirks and Brongersma [15].

So far the alloys seem to behave straightforwardly: there are no matrix effects. The results

on the NiAl alloy (see fig. 4.3 for the Al signal and fig. 4.4 for the Ni signal) show that

the calibrated Ni signal is not dependent on the primary energy while the calibrated Al

signal decreases with increasing primary energy.

A possible effect would be shielding of Al by a preferenrial contaminarion during the

measurement. But this effect would cause the Al signal to increase with increasing energy,

23

Chapter 4

...-.. ffl. "'""" ~ 0 a. -;(

en

60

40

-:c 20 en

0

Quantification in LEIS

Al-pure to NiAl-alloy. In references [19,20] it is shown that, for the surface, the total

density of states (DOS) of NiAl { 110} exhibits a ftlled d-band. The calculations imply that,

effectively, electrons are transferred from nickel to aluminum. In the alloy the Ni d-band

is tilled However there is a larger transfer of the sp-electrons to the aluminum. The

stronger localization in comparison to the pure aluminum may be the cause for the decrease of the neutralization, which would result in a larger ion fraction. More thorough

and detailed theoretica! and experimental investigations, currently underway [24], are

needed to give a more conclusive answer on this subject.

4.3.2 Oxides

Some typical LEIS-spectra of the measured oxides are shown in tigure 4.1 b. From the

spectrum of the oxidized NiAl, one can distinguish the Al and 0 peaks originating from

the scattering from the uppermost atomie layer. The background which extends to about

2250 eV is related to ions which have penetraled the bulk and after back-scattering have

been reionized prior to leaving the surface. Because the background starts just below the

energy position of the Ni single scattering peak, it can be concluded that Ni atoms can be

found just below the surface layer. This is in agreement with the models of the oxidized

NiAl reported in literature [14].

For the oxides, tigure 4.3 shows that there is no change in the LEIS-signal intensity ratios

as a function of the incident energy. From this it could be concluded that the

neutralization behaviour for the metal (Al polycrystalline), the Al in the alloy AggoA120,

and in the oxides (powders, and smooth surfaces) is the same. This implies that the work

function and energy distribution of the valenee electrons do not affect the neutralization in

this case: direct quantitication is possible. However, only about 20% of the Al on the

smooth oxides in comparison to the Al polycrystalline calibration samples detected LEIS.

The aluminum density in the sapphire, oxidized NiAl { 110} and the Al polycrystalline

sample differs by a factor between 1.5 and 3.8 depending on the model of the surface. The

decrease of a factor of 5 in the Al LEIS-signal cannot be explained on the basis of the bulk structure decrease in the Al density. Van Leerdam et al. [ 6] showed that for silica the

decrease of the LEIS intensity was in agreement with the difference in surface density.

Our results on silica confmned this. For the alumina two possible explanations can be

considered: physical shadowing and blocking of the aluminum by an oxygen overlayer or

by a change in neutralization of the ions (matrix effects). The last explanation seems not

to be valid since the signal of the Al as a function of the primary energy is not affected by the change from metal to oxide, as is shown in tigure 4.2. This leaves the effective

shielding of the Al atoms by oxygen; thus, the Al concentration on the uppermost surface

layer does not reflect the bulk structure. However, why there should be preferenrial

shielding of the aluminum atoms on a sputtered surface is not clear. Nevertheless, since

aluminum oxides are known not to reduce under ion bombardment, coupled with the fact

25

Chapter 4

that the oxygen atoms are mobile onder such conditions, the surface composition found is

plausible. This means that quantitative comparison of the different alominurn oxides is

possible.

4.3.3 Surface roughness

The results on the study of the roughness can be extracted from fig. 4.3 for the Al and fig. 4.4 for the Ni. We will concentrale on the alominurn oxides. The measured AVO ratios are summarized in table 4.1. From this it can be concluded that all the spottered alumina have essentially the same ratio, which is to be expected as the bulk stoichiometry is the same.

Since it is not expected that a short sputter cycle will influence the macroscopie

roughness, one can use the Al signals of the spottered surfaces to quantify the influence of

the surface roughness, see table 4.1.

ratio SA/So SA/So Roughness

material clean spottered S rouf)l/S smooth Al Al

-a-Al20 3 sapphire { 11 02} 2.5 1.0 a-Al20 3 2.3 2.5 0.6 y-Al20 3 1.7 2.5 0.6 Al20:/NiAl { 110} 2.8

Table 4.1 Aluminum/Oxygen LEIS signa! ratios and the influence of roughness (3 keV 4He+).

The influence of roughness on the LEIS signals has been described by Nelson (figure 4.5, geometry A [21]) and has been adapted to oor geometry of normal incidence and

backscattering (135°). The physical screening by a two dimensional surface of tilted cubes

is calculated negleering any change of neutralization. A normal distribution for the

possible orientations of the cubes is used following Nelson. The geometry (B) is shown in

fig. 4.5. The shielding on a tilted surface covered by spheres (C) is also calculated. Finally geometry BandCare combined in D, tilted cubes with a surface of small spheres. When

the spheres are considered to be small compared to the cubes, only the geometry

determines the shielding, not the dimensions. The geometry of the surface roughness is

considered to be the same in all azimuthal directions. The calculations show a decrease by

a factor of approximately two at a rms slope of 4

Quantijication in LEIS

.c ... 0 0 E U)

1.0

0.9

0.8

' 0.7 .c C) ~

~ -0.6

0.5

Fig. 4.5

,-----... ...... ... ...... ... ...... ... D' ...

0 10

' ... ' '

20

' ' ' \

30

\

(rms slope in degrees)

B

D

40

Reduction in the intensity due to shadowing as a function of the RMS slope

of the surface for different geometries. A: tilted cubes (45° incidence, 9fr

scattering [10]). B: tilted cubes (normal incidence, 135° scattering). C:

tilted surface of spheres ( normal inc, 135° scattering). D: tilted cubes

covered with spheres (normal inc, 135° scattering).

The measurements on the very rough powdered wafers and the smooth single crystals (fig.

4.3) are in agreement with the calculations, using a simple physical shielding model.

Obviously this does not imply that our simple model is a good representation of the rough

surface. However the combination of the physical shielding and the increasing density

when the surface is tilted lead to a decrease of not more than a factor 2. This is found for

27

Chapter 4

a number of different systems including the alumina and silica, see appendix B for the

latter. From these results, it follows that the roughness influences the LEIS-intensities

although the effect is not dramatic. There is no significant influence of the surface

roughness between the a-Al20 3 (5.5 m2/g) and y-Al20 3 (269 m

2/g), thus the large

difference in specific surface area is not reflected in the LEIS-intensities. Furthermore, from the fact that the lines of the smooth and rough surfaces in the figures 4.3 and 4.4 are

parallel, it can be concluded that no significant change in the neutralization behaviour is

found that can be attributed to the surface roughness.

4.3 .4 Surface composition of powders

The surface compositions of the different aluminum oxides exhibit different AVO ratios, as is shown in table 4.1. It is therefore possible to distinguish the y-Al20 3 from a-Al20 3• Van

Leerdam [22] points out that the difference in the AVO ratio between the two aluminas

can be described in terms of the exposure of specific crystallographic planes of the alumina crystallites. The surface of the y-Al20 3, a defect spinel, could be assigned toa

D{ 110} spinel surface plane. This plane contains only octahedrally coordinated aluminum

cations. The results are in agreement with recent studies on other catalytically active

spinels [23].

The fact that LEIS can provide quantitative information offers new possibilities to check

the validity of the surface stoichiometry of a model system. The oxidized NiAl{ 110} for example shows an alumina overlayer which contains more Al than is found in y-Al20 3, and even more than for a-Al20 3• An explanation would be that the thin oxide layer is still

y- Al20 3 but a different plane is exposed in the powder from that in the single crystal

surface, due to pinning of the crystal structure by the underlying NiAl metal. Efforts to

use differentfacesof a-Al20 3 to study the preferenrial exposure of one of the low-index

planes in the powder a-alumina were not successful. No alkali-free surface could be

obtained after annealing at the required annealing temperature of 1400 Kin oxygen.

4.4 Conclusions

No matrix effects were found on NisoP~o and AggoA120 alloys. On NiAl{llO} a significant

deviation for the calibrated Al signal was interpreted as a change in the neutralization due

to the matrix.

Even for the oxides no change in the neutralization behaviour was found. Quantification is possible. The surface composition of the aluminum oxides determined by LEIS show less

aluminum than expected from bulk densities.

28

Quantification in LEIS

When comparing a metal (Al or Ni) to an alloy (AggoA120 or Pt80~H20) and an oxide (Al20 3 or NiO) no matrix effects could be determined. Quantification of the LEIS results when

performing high resolution depth profiling is straight forward in many cases. One should,

however, never discount the effect fully as is shown in the case of the NiAl.

The results on the influence of the roughness show that when the scattering conditions are

kept constant, different powders can be compared on an absolute scale without correcting

for the exact surface roughness. This consequently facilitates the absolute comparison of

LEIS results from different powders, e.g. when studying catalysts of different pretteatment

or loading. One should, however, prepare the wafers reproducibly. lf the powder, for

example, is not pressed the LEIS intensity will decrease considerably since the

macroscopie density of the surface is very low.

LEIS can provide quantitative comparison between oxides. This offers new possibilities to

check the validity of the surface composition of a model system in comparison to the

powder systems used in catalysis.

4.5 References

[1] H.H. Brongersmaand G.C. van Leerdam, in "Fundamental Aspectsof

Heterogeneons Catalysis studied by Partiele Beams", eds. H.H. Brongersmaand

R.A. van Santen, NATO ASI Series B 265, Plenum Press, New York 1991, 283.

[2] H. Niehus, W. Heiland, E. Taglauer, Surface Science Reports 17 (1993) 213

[3] P.A.J. Ackermans, G.C.R. Krotzen and H.H. Brongersma, Nucl. Instr. Meth. B 45

(1990) 384

[4] L.C.A van den Oetelaar, S.N. Mikhailov and H.H. Brongersma, Nucl. Instr. Meth

B, in press.

[5] R.C. McCune, J. Vac. Sci. Technol. 18 (1981) 700.

[6] G.C. van Leerdam and H.H. Brongersma, Surf. Sci. 254 (1991) 153.

[7] D.J. O'Connor, Y.G. Shen, J.M. Wilson and R.J. MacDonald, Surf. Sci. 197 (1988)

277

[8] M. Beckschulte and E. Taglauer, Nucl. Instr. Meth. B 78 (1993) 29.

[9] R. Souda, T. Aizawa, K. Miura, C. Oshima, S. Otani, Y. Ishizawa, Nucl. Instr. Meth. B 33 (1988) 374.

[10] M.J. Ashwin, D.P. Woodruff, Surf. Sci. 244 (1991) 247.

[11] P. Novaeek and P. Varga, Surf. Sci. 248 (1991) 183.

[12] R. Margraf, H. Knözinger and E. Taglauer, Surf. Sci. 211!212 (1989) 1083.

[13] H.H. Brongersma, N. Hazewindus, J.M. van Nieuwland, A.M.M. Otten and A.J.

Smets, Rev. Sci. Instrum. 49 (1978) 707.

[14] M. Wuttig, W. Hoffmann, R. Jaeger, H. Kuhlenbeck and H.Freund, Mat. Res. Soc.

29

Chapter 4

Symp. Proc. 221 (1991) 143

R.M. Jaeger, H. Kuhlenbeck, H.-J. Freund, M. Wuttig, W. Hoffmann, R. Franehy

and H. lbaeh, Surf.Sei. 259 (1991) 235

[15] A.G. Dirks and H.H. Brongersma, J. Electrochem. Soc. 127 (1980) 2043

[16] J.-P. Jaeobs, J.G.H. Reintjes, H.H. Brongersma, L.P. Lindfors and 0. Jylhä, Catal.

Lett., aeeepted for pubHeation

[17] P.A.J. Aekermans, M.A.P. Creuwels, H.H. Brongersmaand P.J. Seanlon, Surf.Sei.

227 (1990) 361

[18] D.R. Muilins and S.H. Overbury, Surf. Sei. 199 (1988) 141

[19] S.-C. Lui, M.H. Kang, E.J. Mele, E.W. Plummerand D.M. Zehner, Phys. Rev. B

39 (1989) 13 153

[20] S.-C. Lui, J.W. Davenport, E.W. Plummer, D.M. Zehner, G.W. Fernando, Phys.

Rev B 42 (1990) 1582

[21] G.C. Nelson, J. Appl. Phys. 47 (1976) 1253.

[22] G.C. van Leerdam, PhD. Thesis, Eindhoven University of Teehnology, The

Netherlands, 1991

[23] J.-P. Jaeobs, A. Maltha, J.G.H. Reintjes, J. Drimal, V. Ponee and H.H. Brongersma,

J. Catal., aeeepted for pubHeation

[24] R.J.A. van den Oetelaar, Master Thesis, Eindhoven University of Teehnology, The

Netherlands, to be publisbed

[25] R.J.M. Elfrink, Master Thesis, Eindhoven University of Technology, The

Netherlands, february 1994

30

The surface offerrites

Chapter 5

The surface structure of catalytically active spinels, Ferrites·

*This study is performed by J.-P. Jacobs, S. Reijne, MR. Anantharamans, R.HR. Smitl, K. Seshan6 and HR. Brongersma, and is presented at the SON Catalysis meeting, January

3 and 4 (1994), Lunteren, the Netherlands. (' Faculty ofChemistry, Twente University, s

on leave from the Cochin University of Science & Technology, India)

5.1 Introduetion

Perrites are well known as magnetic materials, which are used in electronics, as recording

media or transformer materials. Recently there has also been an increasing interest in the

use of small ferrite particles in catalysis e.g. as a catalyst in the production of alkenes. Alkenes can be produced from alkanes by oxidative dehydrogenation. These reacrions

usually require high temperatures, where the dehydrogenated products react rapidly with

oxygen. However, high selectivity can be obtained in the production of butene and

butadiene from n-butane using ZnFe20 4 [1], and from butene to butadiene using MgF~04 [2]. Perritescan have large surface areas and have a spinel structure related to y-Al20 3, a

very common support material in catalysis.

The structure and properties of spinels are of wide interest for various applications. On

some alloys, as NiCr, a spinel structured overlayer is formed which prevents corrosion.

Also spinels can be found in catalysis as support material as well as active oxides. The general formula fora spinel compound is AB20 4, or A8B160 32 per unit cell. There are three

possible cation distributions over the 8 tetrabedral and 16 octabedral interstices. These are;

the normal distribution, where the tetrabedral positions are occupied by the A cations, and

the octabedral by the B cations, a random distribution and the inverse distribution, with 8

B earlons in tetrabedral and 8 A and 8 B earlons in octahedral positions. All ferrites

except Zn and Cd ferrite are known to be inverse [3]. What valency and coordination is

responsible for the catalytic activity and selectivity? In earlierstudies it has been

concluded that the ions in the tetrabedrally coordinated sites are either inactive or make

only a minor contribution to the activity of spinel structured catalysts [4-7].

The inactivity of the tetrabedral positioned cations can originate from stronger metal-

oxygen honds due to lower valency or coordination number. Also possible is that the

reactants can not access these positions. This can be explained if the surface structure of

spinels is considered. The unit cell of a spinel consists of 32 cubic-close-packed oxygen

anions. In case of a normal 2-3 spinel such as ZnF~04, 8 of the 64 tetrabedral interslices are occupied by divalent metal cations, and 16 of the 32 octahedral sites are filled with

trivalent metal cations. In case of an inverse spinel, like MgP~04, 8 Pe3+ ions are in

31

Chapter 5

Fig. 5.1

32

(111) A (111) 8

(110) c (11 0) D

(1 00) E (1 00) F

The low index planes of a normal spinel structure, notation as ref. [13]. The open spheres represent the oxygen anions, the solid spheres the

octahedrally coordinated cations and the cross-hatched spheres the

tetrahedrally coordinated cations.

The surface offerrites

tetrabedral interstices, while the divalent Mg ions accupy actabedral positions. Considering

only low index planes, as is accepted in literature [7-9], 6 different planes can be distinguished. Following the notation of Knözinger and Ratnasamy [9], these are A(lll),

B(lll), C(llO), 0(110), E(lOO) and F(lOO) as shown in figure 5.1. All planes except

B(lll) and D(llO) have both tetra and actabedral sites on the surface. In the latter two,

only actabedral positions are exposed.

Results using LEIS in combination with catalytic activity measurements showed for a

series of Zn1_xMnxA120 4 spinels, ZnMn20 4, ZnCo20 4 and CoA120 4, that only actabedral

sites are present on the surface [4]. Is this also observed for the ferrites? lf so, does the

change of the surface affect the catalytic activity? A number of ferrites were prepared and

stuclied with LEIS. Activity measurements were perfonned on propane conversion at the

Twente University, though a high selectivity topropene is not expected.

5.2 Ex perimental

5.2 .1 Catalyst preparation

The preparation of the different oxides required strict controL A smal! impurity can lead to

large differences on the surfaces of solids. Therefore all chemieals were taken fresh and of

high purity (p.a.) from Merck. The iron oxides Fe30 4, y-F~03 and a-Fe20 3 were synthesized at low temperatures from iron oxalate precursors. The various products were

obtained by decomposition from these iron oxalate precursors. Fe~04.2H20 was prepared as a fine, yellow, crystalline powder, by precipitation. FeC12.4H20 was dissolved in

distilled water and treated with oxalic acid, also in aqueous medium. The solution was

maintained at 500C until the precipitation was complete. The precipitate was then washed

and driedat lOOOC. The decomposition schemes were basedon the reacrions as from

Ananthraman et al. [ 17].

Fe30 4 (magnetite)

1. Fe~04.2H20 --> FeO + CO + C02 + 2H20 2a. 4Fe0 --> Fe30 4 + Fe

3Fe + 4ll:z0 --> Fe30 4 + 4H2 2b. 3Fe0 + H20 --> Fe30 4 + H2

These reacrions take place in an inert atmosphere of oxygen free nitrogen. The

nitrogen was bubbled through water at 32°C and flowed continuously over the

oxalate. The oxalate was isothennally decomposed at 5000C for 4 h resulting in a

fine black powder.

y-Fe20 3 (maghemite)

was directly produced from F~04• The magnetite was cooled in the same atmosphere to 250°C. Moist air was flowed through the system for 2 h, leading to

33

0

Chapter 5

gamma ferric oxide.

a-F~03 (haematite) Fe~04.2H20 --> Fe~04 + 2H20 2Fe~04 + 0 2 --> Fe20 3 +CO+ C02 The decomposition of the oxalate dihydrate took place in air at 6()(fC for 3 h.

ZnF~04 Zinc ferrite was produced with the low temperature preparalive technique described

by Sato et al. [18]. A 0.1 M aqueous solution of zinc nitrate and a 0.2 M aqueous

solution of iron (lil) nitrate were prepared separately. 100 ml of each were mixed

and heated to 44°C. While stirring a solution of NH3 was added until the pH

reached a value of 10. The (co-)precipitates were wasbed and driedat 1000C

ovemight. Foliowed by calcination at 5000C fine crystallites of ZnF~04 were formed.

MgFe20 4 The production of MgF~04 at low temperatures caused too many problems. Traces of a-F~03 were found using coprecipitation as described by Yang et al. [2].

Both magnesium and zinc ferrite were therefore also produced at elevated temperatures

employing a ceramic technique. The appropriate amounts of ZnO (MgO) and a-Fe20 3 were mixed thoroughly. The mixture was prefired at 5000C for 6 hand mixed again. The final sintering took place at 1 000°C for 24 h.

5.2 .2 Characterization of the catalysts

Nitrogen physisorption data (BET-surface areas) of the resultant matenals were obtained

on a Micromeretics ASAP 2400 adsorption system, see section 3.3.1 for more details.

XRD pattems todetermine the powders were obtained in a Philips diffractometer, using

Cu Ka radiation, operaring at 40 keV and 40 mA, see section 3.3.3.

Catalytic activity measurements were carried out in a conventional continuous

microreactor system, see section 3.3.2. Por more details about the set-up used see [19].

The microreactor operated at approximately atmospheric pressure using propane (99.5%

purity), oxygen as oxidant and helium as inert carrier gas. The sample was heated in a

series of steps of 25° from 3000C to 500°C while a sequence of measurements was carried

out during each temperature step. Each step was maintained for 2 h. The testing procedure

was as follows:

Catalyst sample

Reactor

Bed dimensions

Gas flow

34

ca. 300 mg, in the form of grains of 0.3-0.6 mm diameter, diluted

with ca. 600 mg quartz grains.

Quartz tubular plug flow reactor, intemal diameter 4 mm, heated

length 40 cm.

4 mm diameter, length 52 mm/g diluted catalyst.

150 mVmin, consisring of 22.5 mVmin (15 vol%) propane, 7.5

The surface offerrites

Pressure drop

Residence time Analysis

mVmin (5 vol%) oxygen and 120 mVmin (80 vol%) helium.

Less than 20 mm Hg. 0.173 s.

Hewlett Packani 5880 A gas chromatograph, equipped with a TCD

detector and fitted with a MS 5A column for separation of CO and

0 2 and a Hayesep Q column for separation of the other products.

Propane conversion: Amount of carbon in all products divided by the amount of carbon

in propane in an analysis taken at room temperature.

Rate of prop. cons: Product of interpolated propane conversion and molar flow rate of

propane, divided by the specific surface area of the catalyst.

The surface composition of the catalysts was determined using low-energy ion scattering

(LEIS). With LEIS, the outermost atomie layer of a solid only is characterized, see chapter

2 and [20,21]. The experiments were performed with the LEIS instrument NODUS, of

which the basis design is described in chapter 3.2 and [22]. In this apparatus it is possible to compensate for surface charging, which is the main problem in most surface techniques, by flooding thermal electroos over the surface from all sides. The base pressure in the system is about 1 *10-9 mbar, though the pressure increases to 1 *10-8 mbar during the measurement, principally due to the inert gas of the ion beam. This increase, however does

not affect the measurements.

The powders used for the LEIS experiments were pressed into pellets. A 3 ke V 4He+ ion

beam was used for the LEIS measurements. Recently it was found that the influence of

surface roughness, see chapter 4 or [23], is not as large as reported previously [24]. The

scattering conditions were kept constant during the experiments, therefore the LEIS signals

of different powder catalysts, despite differences in surface areas, can be compared. The

surface of the ferrites reduces quickly under ion bombardment, nevertheless, a few

successive spectra remaio identical and can, therefore, be added.

5.3 Results

X-Ray diffraction pattems indicated only monophasic and pure oxides except for the

magnesium ferrite produced by co-precipitation. This showed traces of a-phase iron oxide

and was therefore excluded from the activity measurements. A typical XRD pattem is shown in figure 5.2. Some typical LEIS spectra from the spinels are shown in figure 5.3.

As can beseen the surface peak of oxygen is similar in the spectra. The MgFez04 sample

clearly shows a significant amount of Mg. Calibration against the pure, polycrystalline

metals showed that Mg and Fe are present on the surface in an equal amount.

35

Chapter 5

t -• ~ . • -~ .. -• c • .. c -

Fig. 52

-tn ... (,) ->a ... ·-tn c Cl) ... c ·-I en -w ..J

Fig. 53

36

100 111

10

440 111 111

411

111

0

10 80 70 80 10 40 30 20 10 0

28 [0 )-

A typical XRD pattern ofmonophasic Fe30 4•

0 Mg

1

Fe

1 .. ,, r' I I

I I I I

2000 I I

' ' ,, ~ ' 1\ ' ' ' ,, \"' '• I \ '" '" •'' ... ~ ,,, ' "''• ''. I \ I ' 11 , 1500 .... ~- ,, l ' "I t ~/ ... ..,I""'"'' J ,.., \.,' \ Z n

1000 I\.. tftiV' ~ Fe30 4, 60nA, 27sec

111 ./"

----- MgFe20 4, ceramic -- ZnFe20 4, ceramic

500 \

0 1000 1500 2000 2500 Typical LEIS spectra of the ferrites. Measured for 27 s!channel at 60 nA.

The surface offerrites

The iron peak from the ZnF~04 and the Fe30 4 sample have the same peak areas, where the iron peak of the magnesium ferrite prepared by the ceramic technique is about half the

size. The mass difference of Fe and Zn is small. The LEIS peaks overlap and, therefore,

cause some difficulties in the precise determination of peak areas. However, using peak

deconvolution [ 4], just a small amount of Zn is detected on the surf ace. This supports the idea that only octahedral sites are exposed, because the Zn ions occupy the tetrabedral

interslices of the ferrite and no Zn is seen on the surface. The LEIS results are listed in table 5.1.

Catalyst

F~04 ZnFe20 4 MgFe20 4 a-F~03 y-F~03 ZnFe20 4 MgF~04

Table 5.1

LEIS peak area (1{}3 #) BET area propane conv. Metbod 0 Mg Fe Zn m2/g oo-s mol m-2s-1)

prec1p. ox. 5.2 24.4 8.35 5.0

ceramic 4.8 26.5

Chapter 5

In the table, also, the specific surface areas and the rate of propane consumption at 3500C

are listed. The both by the ceramic technique produced ferrites have specific surface areas

of less than 1 m2/g. The BET surface areas of these compounds are less reliable. The error

in the other BET results is about 10%. As the ferrites are not particularly selective in the

dehydrogenation of propane, only a-Fe20 3 and F~04 produced a reasonable amount of propene and the remaining ferrites merely produced C02, the propane conversion bas been

taken as activity criterion.

Consictering the catalysts with a specific surface area larger than 1m2/g, the a-phased iron

oxide shows the highest consumption of propane. The activity was so high that the

temperature could not be optimally controlled, suggesting means that the real temperature

is slightly higher than 350°C. This can affect the activity, though taking this into account

its' conversion is at leastabout a factor of 2 higher than the propane conversion of y-

F~03. This is in agreement with the LEIS results. The propane consumption of the

coprecipitated ZnF~04 is comparable to that of the y-F~03. However, this is different for the F~04 sample. The activity is only half the activity of y-F~03, despite the similar surfaces. The correlation of the activity and LEIS will be discussed in detail in the next

section.

5.4 Discussion and conclusions

This investigation was performed to obtain more information about the surface of spinel

structured ferrites, either normal or inverse, in combination with activity measurements, to

support the present ideas about spinel surfaces. The value of this combination bas already

been shown by Jacobs et al. [4].

The LEIS results on the zinc, magnesium and pure ferrite confmn the ideas of Shelef and

Yao [4,10], Beaufils and Barbaux [11,12] and Jacobs et al. [4] about spinel surfaces.

Ziólkowski and Barbaux [7] predicted from semi empirica! calculations that for Co30 4 spinels the A(l11) and D(110) planes are preferred on the surface whereas Shelef and Yao

et al. [5, 10] using low-energy ion scattering (LEIS) concluded that, despite the low

resolution of their experiments, tetrabedral sites are not on the surface. This was also

found by Beaufils and Barbaux [11,12], who concluded using surface neutron differential

diffraction that the surface of the normal spinels MgA120 4 and Co30 4 only consistedof

(110) and (111) planes, for the Co containing spinel being limited to B(111) and 0(110).

Furthermore they suggested that 80% of the exposed facesof y-Al20 3 are (110). This plane

was also suggested by Van Leerdam [13]. Also various XPS-studies on spinels were

reported [14,15,16]. Allen et al. [16] concluded that the bulk composition of a number of

mixed transition metal oxides is reflected on the spinel surface. It must be noted that the

information depth of XPS is several atomie layers, in contrast to LEIS, where the

38

The surface offerrites

infonnation depth is limited to only the frrst atomie layer.

Our results support the ideas described above. Like in other spinels, ferrites expose only

cationsin octahedrally coordinated sites. Therefore, in ZnF~04, a normal spinel, a low Zn concentration is detected with LEIS. This cannot be ascribed to a low sensitivity of LEIS

for Zn, since zincis clearly observed in the pure metal and ZnO [25]. In MgFe20 4 both Mg and Fe are found to be present in equal amounts on the surface, since they are equally

distributed over the octahedral sites due to the inversion [3]. F~04 exposes the same amount of iron as ZnF~04 or y-Fe20 3 on the surface, as is shown by the LEIS results. But due tothefact that Fe30 4 is an inverse spinel, both Fe2+ and Fe3+ are expected.

The activity measurements correlate nicely with these findings. It is found that the Fe3+ on

the surface determines the activity in the test reaction. The activity of y-F~03 and ZnF~04 is similar, while Fe30 4 is only half the active. This can be ascribed to the inversion as discussed above. MgF~04 exhibits a lower activity than the ZnFe20 4 or y-F~03, but the expected decrease of a factor of 2 is not found. This is probably due to the

large error in the surface area measurements. This provides the opportunity to the tailoring

of a specific catalyst by the choice of catalytically active cations which have strong

octahedral site preferenee energies.

The results also show that both the y-F~03 and ZnF~04 have a similar surface. y-F~03 can be transfonned to a-F~03 at less than 500°C, while, on the contrast, ZnFe20 4 has a high thennal stability. This phenomenon is also observed for respectively y-Al20 3 and

ZnA~04• One can imagine that in this way the commonly used y-Al20 3 can be exchanged by ZnA120 4 (or y-Fe20 3 by ZnF~04) to obtain a stabie catalyst for dehydrogenation reactions, which take place at elevated temperatures. That this is indeed possible will be

shown in the next chapter.

5.5 References

[1]. H. Annendáriz, G.A. Aguilar-Rfos, P. Salas, M.A. Valenzuela, I. Schifter, H.

Arriola and N. Nava, Appl. Catal. A 92 (1992) 29

[2]. B.L. Yang, D.S. Cheng and S.B. Lee, Appl. Catal. 70 (1991) 161

[3]. F.C. Romeijn, Philips Res. Rep. 8 (1953) 304

[4]. J.-P. Jacobs, A. Maltha, J.G.H. Reintjes, J. Drimal, V. Ponec and H.H. Brongersma,

J. Catal., accepted for pubHeation

[5]. H.C. Yao and M. Shelef, J. Phys. Chem. 78 (1974) 2490

[6]. B.L. Yang, S.F. Chan, W.S. Chang and Y.Z. Chen, J. Catal. 130 (1991) 52

[7]. J. Ziólkowski and Y. Barbaux, J. Mol. Catal. 67 (1991) 199

[8]. B.C. Lippens and J.J. Steggerda in "Physical and Chemica! Aspects of Adsorbents

39

Chapter 5

and Catalysts", Ed. B.G. Linsen, Academie Press, New York 1970, 171

[9]. H. Knözinger and P. Ratnasamy, Catal. Rev.-Sci. Eng. 17 (1979) 31

[10]. M. Shelef, M.A.Z. Wheeler and H.C. Yao, Surf. Sci. 47 (1975) 697

[11]. J.P. Beaufils and Y. Barbaux, J. Chim. Phys. 78 (1981) 387 (French)

[12]. J.P. Beaufils and Y. Barbaux, J. Appl. Cryst. 15 (1982) 301

[13]. G.C. van Leerdam, PhD. Thesis, Eindhoven University of Technology, The

Netherlands 1991 [14]. V.A.M. Brabers, F.M. van Setten and P.S.A. Knapen, J. Solid State Chem. 49

(1983) 93

[15]. S. Vepfek, D.L. Cocke, S. Kehl and H.R. Oswald, J. Catal. 100 (1986) 250

[16]. G.C. Allen, S.J. Harris, J.A. Jutson and J.M. Dyke, Appl. Surf. Sci. 37 (1989) 111

[17]. M.R. Anantharaman, S.S. Shewale, V. Rao, K. Seshan and H.V. Keer, Indian J.

Chem. A 21 (1982) 714

[18]. T. Sato, K. Hanada, M. Seki and T. Iijima, Appl. Phys. A 50 (1990) 13

[19]. R.H.H. Smits, K. Seshan and J.R.H. Ross, Catal. Today 16 (1993) 513

[20]. H.H. Brongersmaand G.C. van Leerdam in "Fundamental Aspectsof

Heterogeneons Catalysis Studied by Partiele Beams", Eds. H.H. Brongersmaand

R.A. van Santen, NATO-ASI Series B 265, Plenum, New York 1991, 283

[21]. H. Niehus, W. Heiland and E. Taglauer, Surf. Sci. Rep. 17 (1993) 213

[22]. H.H. Brongersma, N. Hazewindus, J.M. van Nieuwland, A.M.M. Otten and A.J.

Smets, Rev. Sci. Instr. 49 (1978) 707 [23]. J.-P. Jacobs, S. Reijne, R.J.M. Elfrink, S.N. Mikhailov, M. Wuttig and H.H.

Brongersma, J. Vac. Techn. A, submitted [24]. R. Margraf, H. Knözinger and E. Taglauer, Surf. Sci. 211/212 (1989) 1083

[25]. H.H. Brongersma and J.-P. Jacobs, Appl. Surf. Sci., in press

40

The surface of zinc aluminates

Chapter 6

The influence of the preparation metbod on the surface structure of ZnA120 4*

*The contentsof this chapter wil/ be submitted to the Journalof Molecular Catalysis, by

MA. Valenzuela1.2, J.-P. Jacobl, S Reijnt!, B. Zapata1, P .Bosclf and HR. Brongersmd (1/nstituto Mexicano del Petróleo, IBP Avenida de los 100 metros #500, México 07300

D.F., Mexico 2Universidad Autónoma Metropolitana-Iztapalapa, Department of Chemistry, P.O. Box 55-

534, México 09340 D.F., Mexico 3Faculty of Physics and Schuit Institute of Catalysis, Eindhoven University ofTechnology,

P.O. Box 513, 5600MB Eindhoven, The Netherlands)

6.1 Introduetion

Catalysts used in heterogeneons catalysis can be divided into two groups. Most reacrions

on metals run in an adsorbed overlayer, in which the catalyst itself is at most indirectly

chemically active. Other catalysts, in most cases active oxides, participate in a more active

role. In selective oxidation reacrions lattice oxygen appears in the products, leaving an oxygen vacancy which can be replenisbed by gaseous molecular oxygen. This is known as

the Mars-van Krevelen mechanism [1]. The activity and selectivity depends on the

properties of the catalyst: morphology of the material, defects, strength and coordination of

cation-anion bonds, for instance. Not only oxidation reactions, but also selective rednetion (e.g. nitro-benzene to nitroso-benzene [2]), or dehydrogenation of alkanes [3,4] are among

the scope of applications. In all cases the outermost atomie layer of the catalyst is the

limiting, but also activaring factor.

Generally, a catalysts' support must have a large surface area of some 10 or more square

meters per gram. Such a requirement is fulfilled by highly porous powders. This study

involves zinc aluminates, which are used as a catalyst support to form thermally-stable

dehydrogenation catalysts. Zinc aluminate (ZnA120 4) bas a spinel structure. The structure and properties of spinels are of wide interest for various applications. Also in catalysis

spinels are found in a number of systems, e.g. y-Al20 3, a very common support material.

Supported oxide catalysts [5,6] and catalytically active oxides mayalso be spinels [2,4]. In search for improved properties of dehydrogenation catalysts, interest has been focused on

spinel-type structures like magnesium and zinc aluminates. This because their main

characteristics: hydrophobicity, high mechanica! resistance and low surface acidity [3],

make them well suited to perfonn in the conditions imposed by dehydrogenation reacrions of light alkanes.

41

Chapter 6

Aluminates are known to be normal spinels [7,8]. What valency and coordination is

responsible for the catalytic activity and selectivity? From earlier studies it bas been concluded that the ions in the tetrabedrally coordinated sites are either inactive or

contribute little to the activity of spinel structured catalysts [9-12].

The inactivity of the tetrabedral positioned cations could originate from stronger metal-

oxygen honds due to lower valency or coordination number. Also it is imaginable that the

reactants can not access these positions. This can be explained considering the surface

structure of spinels. The unitcellof a spinel contains 32 cubic-close-packed oxygen anions. In the case of a normal 2-3 spinel such as ZnA120 4, 8 of the 64 tetrabedral

interslices are occupied by divalent metal cations and 16 of the 32 octahedral sites are

filled with trivalent metal cations. Considering only low index planes, as is common in

literature [ 12-14], 6 different planes can be distinguished. Following the notation of

Knözinger and Ratnasamy [14] all planes except B(111) and 0(110) have both tetra and

octahedral sites on the surface. The latter two only expose octahedral positions.

Ziólkowski and Barbaux [12] predicted from calculations that for powders of CÜJ04 spinels the A(111) and 0(110) planes are preferred on the surface. This was also found by

Beauflls and Barbaux [15,16] who concluded, using surface neutron differential diffraction,

that the surface of the normal spinels MgA120 4 and Co30 4 only consisled of (110) and

(111) planes; the Co containing spinel being limited to B(111) and 0(110). Forthermore

they suggested that 80% of the exposed faces of y-Al20 3 are (110). This plane was also suggested by Van Leerdam [17]. Allen et al. [18] concluded from XPS measurements that

the bulk composition of a number of mixed transition metal oxides is reflected on the

spinel surface. It must be noted that the information depth of XPS is several atomie 1ayers, in contrast to LEIS, where the information depth is limited to only the fust atomie layer.

Recent results using LEIS in combination with catalytic activity measurements showed

that, for several Co, Mn and Al containing spinels, only octahedral sites are present on the

surface [9], this was also found in ferrites, see chapter 5. In the present study the effect of

the preparation metbod and addition of some impurities to zinc aluminates is examined.

Calcium bas been reported in previous work [ 19] to increase the specific surface area. Tin is used in Pt-Sn/Al20 3 catalysts, which bas a wide industrial application in oil-refining processes [20]. Pt/ZnA120 4 and Pt-Sn!ZnA120 4 seem to bemost promising catalysts with a

higher thermal stability [21,22]. The prepared samples were used to support platinum and were tested in an isobutane dehydrogenation reaction.

The present investigation focuses on the influence of the varloos preparation methods and

on the effect of calcium and tin on the ZnA120 4 spinel surface. The results will be used to

interpret activity measurements. The value of this kind of correlation bas already been

shown by Jacobs et al. [9] and Darligues et al. [23] among others.

42

The surface of zinc aluminates

6.2 Experimental

6.2.1 Catalyst preparation

Three different preparation methods are compared. One pure zinc aluminate, a 3 wt% tin (Sn) doped and a 3 wt% calcium (Ca) doped Zn~04 have been prepared by coprecipitation from nitrates. From an initia! pH of 2, the pH was brought to 7 .5, through

add