A spectroscopic characterization of the structure of supported metal catalysts

A spectroscopic characterization of the structure of supported

metal catalysts

Citation for published version (APA):

Martens, J. H. A. (1988). A spectroscopic characterization of the structure of supported metal catalysts. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR282367

DOI:

10.6100/IR282367

Document status and date: Published: 01/01/1988

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Structure of Supported Metal Catalysts

A Spectroscopic Characterization of the

Structure of Supported Metal Catalysts

F:en Spectroscopische Karakterisering van de

Struktuur

van

Gedragen MetaalkaJalysaJoren

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van

de rector magnificus, prof. dr. F .N. Hooge, voor een commissie aangewezen door het college van

dekanen in het openbaar te verdedigen op dinsdag 22 maart 1988 te 14.00 uur

door

J. H. A. Martens

Dit proefschrift is goedgekeurd door de pronwtoren:

prof. dr. R. Prins

en

prof. dr. ir. D. C. Koningsberger

The research reported in this thesis has been carried out at the

Laboratory of Inorganic Chemistry and Catalysis at the Eindhoven

University of Technology and has been supported by the

Nether-lands Foundation for Chemical Research (SON) with financial aid

from the Netherlands Organization for the Advancement of Pure

aan mijn vader

en aan mijn moeder

aan Angeliene

Contents

Chapter

1

Introduction

1

1.1

Catalysis

1

1.2

Heterogeneous Catalysis

2

1.3

Characterization of Supported Catalysts

7

1.4

Scope of this Thesis

8

1.5

References

8

Chapter

2

Experimental Techniques

11

2.1

Catalyst Preparation

11

2.1.1

Introduction

11

2.

1.2

Pore Volume Impregnation

13

2.1.3

Ion Exchange

14

2.1.4

The Urea Method

15

2.2

Temperature Programmed Reduction

15

2.3

Hydrogen Chemisorption

17

2.4

Electron Spin Resonance Spectroscopy

18

2

.

5

Nuclear Magnetic Resonance Spectroscopy

20

2.6

Mossbauer Spectroscopy

21

2

.

7

Laser Raman Spectroscopy

23

2.8

ASED-MO Computations

25

2.9

EXAFS

28

2.9.1

Basic Principles

28

2.9.2

Fourier Transformations

34

2.9.3

Reference Compound and

37

Calculating Spectra

2.9.4

Data Analysis

38

2.

9.5

Experimental Method

43

2.10

References

44

Chapter

3

The Preparation of y-Al₂0₃ supported Monometallic

47

Rh and Pt and Bimetallic Rh-Pt Catalysts

3.1

Introduction

47

3.2

Experimental

48

3.2.1

NMR and Laser Raman Experiments

48

3.2.2

Adsorption Experiments

49

3.3 Results and Discussion

51

3.3.1 NMR and Laser Raman Experiments

51

3.3.2 Adsorption Experiments

56

3.3.3 TPR of Rh, Pt and Rh-Pt/ Al

₂

0

₃

67

3.4 Conclusions

71

3.5 References

72

Chapter 4Ferric Iron in Reduced Si0

₂

Supported

73

Fe-Ru and Fe-Pt Catalysts

Evidence from Mt>ssbauer Spectroscopy and

Electron Spin Resonance

4.1 Introduction

73

4

.

2 Experimental

75

4.3 Results and Discussion

76

4.4 Conclusions

81

4

.

5 References

81

Chapter 5Controlled Oxygen Chemisorption on an

83

Alumina Supported Rhodium Catalyst

The Formation of a Metal-Metal Oxide Interface

Determined by EXAFS

5.1 Introduction

83

5

.

2 Experimental

84

5

.

3 Results

86

5

.

4 Discussion

90

5.4.1 Rh/Al

₂

0

₃

after Reduction and Evacuation

90

5.4.2 A Model for the Oxidation of Metal Particles 95

5.4.3 Rh/ Al

₂

0

₃

after Oxygen Admission at 100 K 101

5.4.4 Rh/ A1

₂

0

₃

after Warming up to 300 K

102

5

.

4.5 General Remarks

104

5.5 Conclusions

108

5.6 References

109

Chapter 6 The Structure of

the

Metal-Support Interface

111

in Rh/ A1

₂

0

₃

Determined with the AS ED-MO Method

6

.

1 Introduction

111

6.2 Theoretical Method

112

6

.

4

Discussion

117

6.5

Conclusions

120

6

.

6

References

121

Chapter

7

The Structure of Rh/Ti0

₂

in the Normal and the

123

SMSI State as Determined by EXAFS and HRTEM

7.1

Introduction

123

7

.

2

Experimental

127

7

.

2.1

Catalyst Preparation

127

7

.

2.2

EXAFS Measurements

129

7

.

2

.

3

HRTEM Experiments

130

7.3

Results

132

7.3.1

Analysis of the EXAFS Spectra

132

7

.

3.2

Characterization with H RTEM

145

7.4

Discussion

147

7

.

4

.

1

Rh/Ti0

₂

after Reduction at

473

K

147

7.4

.

2

Rh/Ti0

₂

after Reduction at

723

K

151

7

.

4.3

Evacuation at

623

K

159

7

.

4

.

3

.

1

Rh/ Al

₂

0

₃

159

7.4.3

.

2

Rh/Ti0

₂

160

7.4.4

Oxygen Admiss

i

on at

100

K

160

7.4

.

4

.

1

Rh/ Al

₂

0

₃

160

7.4.4.2

Rh/Ti0

₂

161

7

.

4

.

5

Oxygen Admission at

300

K

162

7.4

.

5.1

Rh/A1

₂

0

₃

162

7.4.5.2

Rh/Ti0

₂

164

7.4

.

6

Different Rh

0

-Ti

n

+

Contributions

165

7.4.7

Comparison with Literature Data

167

7.5

Final Conclusions

170

7.6

Referen

c

es

173

Chapter

8

Strong Metal

-

Support Interactions

177

in Rh/Ti0

₂

Prepared with Ion Exchange

8.1

Introduction

177

8.2

Experimental

179

8

.

2

.

1

Catalyst Preparation

179

8

.

2

.

2

EXAFS M

e

asurem

e

nts

179

Chapter 9

8

.

4

Discussion

184

8

.

4.1

Rh/Ti0

₂

after Reduction at 494 K

184

8

.

4.2

Rh/Ti0

₂

after Reduction at 623 K

187

8.4

.

3

Rh/Ti0

₂

after Reduction at 773 K

190

8

.

4.4

General Remarks

191

8.5

Final Conclusions

194

8

.

6

References

195

EXAFS Evidence for Direct Rh

0

-Tan+ Bonding

197

and Coverage of the Metal Part

i

cles

in a Rh/Ta

₂

0

₅

Catalyst in the SMSI State

9.1

Introduction

197

9

.

2

Experimental

199

9.2

.

1

Catalyst Preparation

199

9

.

2.2

EXAFS Measurements

200

9.3

Results

201

9

.

3.1

Reference Compounds

201

9

.

3.2

Analysis of the EXAFS Spectra

203

9

.

4

Discuss

i

on

207

9.4.1

Rh/Ta

₂

0

₅

after Reduction at 523 K

207

9

.

4.2

Rh/Ta

₂

0

₅

after Reduction at 858 K

210

9

.

4.3

Rh/Ta

₂

0

₅

after Admission of 0

₂

215

in the S MS I State

9.5 Final Conclusions

9.6

References

216

217

Chapter 10

Concluding Remarks

219

Summary

225

Samenvatting

231

Dankwoord

237

Curriculum Vitae

240

Chapter 1

Introduction

1.1 Catalysis

Chemistry is an area of vital importance in todays society, and within chemistry. catalysis is a mainstay. not only in many indus-trial applications. but also in numerous processes in the chemistry of life. Catalysis performs a key role in processes such as the conversion of crude oil into a wide scale of useful products, in the preparation of many nutrients, in the conversion of coal via syn-thesis gas into products like alcohols, gasoline, and in the removal of noxious components from exhaust gases.

Catalysis is the science of accelerating chemical reactions that under normal conditions proceed only slowly or not at all. The rate of a chemical reaction can be controlled by a few parameters only: temperature, pressure and composition. lr:i addition, the choice of a suitable catalyst may change the reaction pathway. As a conse -quence, the overall reaction rate may be increased and/or new path -ways, and therefore new products, may become feasible. A chemi-cal reaction is the result of a collision of two or more molecules or atoms. The function of a catalyst is merely to capture the partici-pants of the reaction, to bring them in close contact and thus to guide them through some reaction pathway. The combination of catalyst and reactant( s) dictates the pathway. Two characteristics are important in describing a catalyst. First of all its activity. that is the rate at which the products are generated. The higher the activity, the better the catalyst. Secondly, there is the selectivity. In most cases, a catalyst produces a (wide) range of products, some

of which are useful and others are not. By selectivity in general we mean the fraction of (useful) products, usually expressed as a

percentage. A high selectivity indicates that the catalyst produces mainly the desired products.

Catalysts are known in may varieties, but in principle they can be classified into two categories : homogeneous and heterogeneous catalysts. Homogeneous catalysts can be mixed perfectly with the reactants,

i.e.,

up to molecular scale. Homogeneous catalysis mainly occurs in the liquid phase. The reactants and the catalyst are liquids or dissolved in the liquid phase. All the enzymes at work in our body are homogeneous catalysts. In heterogeneous catalysis. the catalyst. the reactants and the products are in separate phases and therefore the mixing is far from perfect. The catalyst is usually a solid and the reactants are liquids or gases. The automotive catalyst is an example of a heterogeneous catalyst. The reactants, hydrogen carbon monoxide and nitric oxides, and the products, water, carbon dioxide and nitrogen, are gaseous while the active catalyst is a combination of several precious metals supported on some (inert) support.

1.2 Heterogeneous Catalysis

In this thesis we will describe heterogeneous catalysts contain-ing precious metals. We have already met one application of such catalysts : the use of noble metals in automotive catalysis. Another important use of metals is in the Fischer Tropsch synthesis. In this process, carbon monoxide and hydrogen are combined to long-chain hydrocarbons and oxygen-containing organic molecules like alcohols ( 1-3). The way Fischer Tropsch catalysts and automotive catalysts work is very similar. The metals are capable of adsorbing the reac-tants and of splitting them into smaller fragments or atoms. On the surface of a suitable metal, sometimes with the help of addi-tives. these fragmented molecules rearrange, in the case of Fischer Tropsch synthesis to useful products and in the case of automotive

catalysis to harmless products. These products desorb and leave the surface of the catalyst as gases. Here we have encountered on of the most important aspects of heterogeneous catalysis : only the surface of the metals is exposed to the (gaseous or liquid) reactants and products. Therefore, only the metal atoms in the surface are active in the process. The atoms directly beneath the surface atoms may still play a (minor) role, while all the other atoms in the bulk are 'useless'. To give an impression of this 'waste' : in a metal sphere or ball of size 1 mm, only one in about 1.3 million atoms is a surface atom; in a metal particle of one micron, one in . approxi-mately 1.3 thousand atoms is in the surface of the particle and therefore potentially useful in a heterogeneous reaction. For inex-pensive and commonly available metals, this is hardly an objection.

For precious noble metals which are not only very expensive, but also sometimes very rare, this is worth consideration. The general aim therefore is to reduce the size of the metal particles and conse-quently to use the metal as efficiently as possible. This can be achieved by 'dispersing' the metal on an inert support material. A commonly used support is aluminum oxide, Al₂0₃, known as alumina. It has a typical surface area of several hundreds of square meters per gram. The surface area in a few grams of alumina (a tea spoon full) would typically cover the area of a football field.

Several techniques are available to implant large amounts of 'highly dispersed' metal particles on this surface. The size of these parti-cles is measured in nanometers (1 nm= 10-9 m) or in Angstroms

(1 A= 10-10 m). In most cases, the fraction of metal· atoms in the surface is close to unity and in the majority of cases above one half. Typically, the activity in Fischer Tropsch synthesis of one gram of alumina loaded with one percent rhodium exceeds the activity of one gram pure rhodium powder by one or two orders of magnitude, while the price of the alumina supported catalyst is lower by about two orders of magnitude.

Very small metal particles may not be metallic : their chemical properties,

i.e.

,

the properties of the surf ace atoms, may deviate from the properties of the surface atoms in larger metal particles

(4-6). Another important characteristic of small metal particles is, that they are more easily influenced than larger metal particles. A striking example is a phenomenon discovered in 1978 (7-9). For metal particles supported on oxides of transition metals. such as titania (Ti0_2), vanadia (V ₂0₃₎ and tantalum oxide (T a₂0_5), two different 'states' are accessible. There is a 'normal' state, in which the properties of the metal particles are comparable to the proper-ties of the same metal particles supported on inert oxides like alumina (A1₂0₃₎ and silica (Si0₂₎. The surface area of such parti-cles can be estimated by measuring the amount of gas that adsorbs on the surface of the particles. The other state is known as the Strong Metal-Support Interaction (SMSI) state. The properties differ markedly from the properties of the 'normal' metal particles. Most pronounced is the decrease of the amount of gas that the metal particles can adsorb. In either state, however, the basic structure of the particles is the same and the metal particles can be brought from the normal into the SMSI state and

vice versa.

After reduction at low temperatures the particles are in the 'normal' state and a subsequent reduction at high temperatures induces the SMSI state. After oxidation at mild temperatures (up to

500

K) and a subsequent reduction at low temperature. the 'normal' state is restored. Catalysts that can be brought into the SMSI state have one characteristic in common : in the temperature regime up to

800 K the support can be reduced to a suboxide and this suboxide can be re-oxidized to the original oxide. Thus, SMSI state and pres-ence of suboxides go together. The reduction of the support is in general catalyzed by the metal particles and is limited to the direct environment of the metal particles. These reduced suboxides have in general different properties than the original oxide. They may have semi-conducting or even meta II ic properties (7-9), or they may have an enhanced mobility ( 13-20). These special properties are thought to be responsible for the SMSI state. Several models have been proposed to explain the SMSI state. In the first model, already adopted by Tauster and his co-workers (7-9), the origin for this SMSI state is thought to be an enhanced interaction between metal particles and supporting oxide, due to the electronic properties of

the suboxide. In one of the first papers on SMSI (JO) even a direct bonding between metal atoms and Tin+ ions in the support was suggested. The formation of alloys is another model that may explain the incapability of adsorbing gases in the SMSI state ( 1-3.11 >. In this model it is assumed that during the reduction of the support (for example, Ti0₂₎ metallic titanium is formed which may diffuse into the metal particles and form alloy particles. It is known that the gas adsorption capacity of these alloys is very low ( 12). The third and last model that is important in explaining the SMSI state is the coverage model. It is assumed that reduced support species have an increased mobility and may cover the metal parti-cles, thus blocking the surface of the metal particles and decreasing their adsorption capacity. In many publications, evidence for cover-age has already been reported ( 13-20). In chapters 7, 8 and 9 of this thesis, we will discuss the SMSI phenomenon and its for origin rhodium catalysts supported on Ti0₂ and Ta₂0₅.

In the above discussion, it was assumed that pure metals were used to guide reacting molecules along their reaction pathway. This pathway can be modified by introducing additives on, in and/or beneath the surface of the metal particles. Like the support materi-als mentioned above, these additives influence the properties of (the surface atoms of) the metal particles.

We

can discern two classes of additives : promoters and (other) metals. The difference between the two classes can be found in their activities towards the desired process in the absence of their host metal. A promoter is, on its own, incapable of catalyzing the desired reaction. Combined with some active host metal, however, the activity of the host may be increased significantly. Promoters are found among alkali (Li, Na, K) ( 21-26), rare earth and some transition metal or metal oxides (V₂0_3, Mo0_3, Th0₂₎ (27,28). The other class of additives is in fact the class of active metals itself. The intention here is to 'combine' the properties of two (or more) active metals. There are cases known where the combination of two metals is 'better' than the 'sum' of the two monometallic cases, better in terms of activity and/or selectivity. For example, for the Fischer-Tropsch synthesis,

an increase in methanol and ethanol selectivities for Fe-Rh/Si0₂ catalysts ( 29) and an increase in ethane and propene selectivities for Co-Rh catalysts (JO) has been reported. The activity of bime-tallics has been the subject of many studies. The major problem is to prepare metal particles that contain both metals and to verify that indeed alloy formation has taken place. In most studies, TP R is used to investigate .the formation of alloy particles. In preparing bimetallic catalysts, two metal precursors are used. These precur

-sors may have a different reducibility. Thus, in Temperature Pro-grammed Reduction (T PR), they will be reduced separately. How-ever, when they are co-impregnated, already during the impregna-tion and subsequent drying step, particles containing both precur-sors may have been formed and in general, these particles will be reduced at the the reduction temperature of the component that is reduced most easily. This component, once reduced, can adsorb and dissociate hydrogen and thus can catalyze the reduction of the second component. In that case, TP R may indeed point to the for-mation of bimetallic particles. Using TPR, it was found that in Cu-Ni/Si0₂ alloy particles were formed (3/ ,32). Evidence has also been found for an intimate contact between the two constituent metals in Pt-Re/ Al₂0₃ catalyst <33,34). Oxidation at high tempera-tures caused segregation of platinum oxide and rhenium oxide. In (35-37), the formation of bimetallic Co-Rh catalysts supported on A1₂0_3, Ti0₂ and Si0₂ has been described. It was found that rho-dium aided the reduction af cobalt and that bimetallic particles were formed. During oxidation, segregation occurred, but this did not lead to the formation of monometallic particles during a subsequent reduction. For Co-Rh/Ti0₂ (36), it was found that CoRh₂0₄ was formed and that this mixed oxide was covered with Co₃0_4. Such a segregation may also occur in metallic particles : the component with the lowest sublimation enthalpy will be present preferably in the surface of the metal particles. For Co-Rh catalysts, it has been reported that the outer shell of the alloy particles was enriched in cobalt ( 37 ). Segregation may be enhanced by the gas atmosphere; this is known as gas induced surface enrichment. Clearly, bimetallic catalysis is a delicate subject; the structure and composition of the

alloy particles can be affected in numerous ways. In chapters 3 and 4, some aspects of bimetallic Rh-Pt/ Al 203, Fe-Ru/Si02 and

Fe-Pt/Si02 catalysts will be discussed.

1.3 Characterization of Supported Catalysts

In order to control and to steer the properties of supported metal particles, it is of prime importance to know and to under-stand their structure. Once their structure has been related to the catalytic action of the catalyst, one may be able to develop 'better' catalyst systems in a scientific manner. In determining the struc-ture of a heterogeneous catalyst, the support is the major obstacle. The active metal is present on the 'internal' surface, 'inside' the porous support material. The number of metal particles visible with an ordinary light microscopy is only a small fraction of the total amount of metal particles present in the specimen. This is a major disadvantage of characterization techniques that use radiation that cannot penetrate the support. In this thesis, we will describe the use of radiation of high enough energy to penetrate the support and thus to reach the metal particles. EXAFS uses high energy X-ray radiation, ESR and NMR use radio- and microwaves and IVlossbauer uses gamma radiation. However, the amount of support exceeds the amount of metal by one or two orders of magnitude. As a result, the signal-to-noise ratio and the separation of signals may become a draw back. Another way 'around the support' is to use gases which penetrate in the pores, reach the metal particles and are adsorbed on the surface of the particles (hydrogen chemisorp-tion) or which in some way react with the metal particles (tempera-ture programmed reactions such as reduction and oxidation). In chapter 2 these techniques will be discussed in more detail.

1.4 Scope of this Thesis

In this thesis we will focus on determining the structure of supported noble metal catalysts. Several interesting catalyst sys-tems have been studied. Chapter 2 describes the techniques used to prepare and study the catalysts. In chapter 3, preparational aspects of alumina-supported monometallic rhodium and bimetallic rhodium-platina catalysts will be discussed. Chapter 4 deals with the intriguing presence of ferric iron (Fe3

+)

in Si0₂ supported bime-tallic Fe-Ru and Fe-Pt catalysts, the existence of which has not been realized for a long time. In chapter 5, the structure of an alumina-supported rhodium catalyst during an oxidation process is described. Chapter 6 deals with a computational approach of sup-ported rhodium catalysts. In chapters 7, 8 and 9 EXAF S studies of catalysts that suffer from metal-support interactions will be highlighted : Rh/Ti0₂ and Rh/Ta₂0₅. In chapter 10, the results are discussed in a wider context and where possible interrelated.

1.5 References

1. Fischer, F.; Tropsch, H. Brennstof Chemie 1923, 4, 276

2. Fischer, F.; Tropsch, H. Brennstof Chemie 1924, 5, 201 3. Fischer, F.; Tropsch, H. BrennstofChemie 1924, 6, 217

4. Yao, H. C.; Yu Yao, Y F : Otto, K. J. Catal. 1978, 45, 120 5. Graydon, W F.; Langan, M. D. J. Catal. 1981, 69, 180

6. Hugues, F.; Besson, B.; Basset, J. M. J. Chem. Soc., Chem. Comm.

1980, 719

7. Tauster, S. J.; Fung, S. C.; Garten, R.L. J. Am. Chem. Soc. 1978,

100, 170.

9. Tauster, S J.; Fung, S C : Baker. R T. K : Horsley. J. A. Science (Washington, DC) 1981, 211. 1121.

10. Horsley. J. A. J. Am. Chem. Soc. 1979. 101. 2870

11. Beard, B C; Ross, P N. J. Phys. Chem. 1984. 90, 6811.

12. Brewer, L. "Phase Stability in Metals and Alloys": Rudman. P.; Stringer, J.; Jaffee, R, ed.; MacGraw-Hill: New York. 1967; pp 39-61. 13. Meriaudeau, P.; Dutel, J. F : Dufaux, M.; Naccache C "Studies of

Surface Science and Catalysis" 1982, 11.

14. Belton, D. N.; Sun, Y -M : White, J. M. ]. Phys. Chem. 1984, 88,

1690.

15. Belton, D. N.; Sun, Y -M.; White, J. M. J. Phys. Chem. 1984, 88,

5172.

16. Simoens, A J.; Baker, R. T. K.; Dwyer. D. J.; Lund. C R. F : Madon.

R. J. J. Catal 1984, 86. 359

17. Chung, Y M.; Xiong. G.; Kao, CC. J. Catal. 1984, 85, 237.

18. Sadeghi, H. R.; Henrich, V. E. J. Catal. 1984, 87, 279.

19. Sun, Y -M.; Belton. D. N.; White, J. M. J. Phys. Chem. 1986, 90,

5178.

20. Ko, G. S.; Gorte, R. J. J. Catal 1984, 90, 59.

21. Dry, M. E. "Catalysis"; Anderson, J. R.; Boudart, M., Eds.; Springer Verlag, 1981, Vol. I, p. 159

22. Anderson, R. B. "Catalysis"; Emmet, P. H , Ed.; Reinhold, New York, 1956, Vol. IV, p. 123.

23. Kikuzono, Y.; Kagami, S.; Naito, S.; Onishi, T., Tamaru, K. Far. Disc. Chem. Soc. 1981, 72, 135.

24. Vedage, G. A.; Himelfarb, P B.; Simmons, G. W.; Klier, K. Solid State Chem. 1985 (ACS Symposium series 279, Graselli, R. K.; Braz -dil. J. C.; Eds.)

25. Mori, T.; Masuda, H.; Imai, H.; Miyamoto, A.; Niizuma, H.; Hattori.

T.; Murakami, Y J. Molec. Catal. 1984, 25, 263.

26. Mori, T.; Miyamoto, A.; Takahashi, N.; Niizuma, H.; Hattori, T.; Murakami, Y. J. Catal. 1986, 102, 199.

27. Mori, T.; Miyamoto, A.; Takahashi, N.; Fukagaya, M.; Hattori, T.; Murakami, Y. J. Phys. Chem. 1986, 90, 5197.

28. Ichikawa, M.; Shikakura, K.; Kawai, M. "Heterogeneous Catalysis Related to Energy Problems", Proc. Symp. Dalian, China. 1982, A

-08-1.

29. Bhasin, M. M.; Bartley, W. J.; Ellgen, D. C.; Wilson, T. P J. Catal.

1978, 54, 120.

30. Villiger, P.; Barrault, J.; Barbier, J.; Leclerq, G.; Maurel, R. Bull. Soc.

Chim. Fr. 1979, 1-413.

31. Robertson, S. D.; McNicol, B. D.; de Baas, J. H.; Kloet, S C.; Jen

-kins, J. W J. Catal. 1975, 37, 424.

32. Jenkins, J. W; McNicol, B. D.; Robertson, S D. Chem. Tech 1975,

7, 316.

33. Wagstaff, N.; Prins, R J. Catal. 1979, 59, 435.

34. Wagstaff, N.; Prins, R J. Catal. 1979, 59, 445. 35. van ·t Blik, H.F. J.; Prins, R. J. Catal. 1986, 97, 188

36. Martens, J. H. A.; van ·t Blik, H. F. J.; Prins, R. J. Catal. 1986, 97, 200

37. van 't Blik, H. F. J.; Koningsberger, D. C.; Prins, R. J. Catal. 1986,

Chapter 2

Experimental Techniques

2.1 Catalyst Preparation

2.1.1

Introduction

For the properties such as activity, selectivity and stability of the eventual catalyst, the method of preparation is of crucial impor-tance. Prime consideration- always is to keep the size of the metal particles within acceptable ranges. The smaller the metal particles, the more active sites per gram of metal. Several techniques are known to bring forth small metal particles. The choice of the preparation method depends on the metal and the support to be used. We will describe three preparation methods which were used to prepare the catalysts that will be discussed in this thesis. The active material is in our case always a metal. It is impossible to directly dispers a metal homogeneously on a porous support. In most cases and in all cases discussed in this thesis, a precursor, usually a metal salt, is dissolved in a solute. This. solution can penetrate into the pores of the support and enable the metal precur-sor to be deposited on the internal surface area of the support. This is the part of the preparation in which the three methods differ. In paragraphs 2.1.2, 2.1.3 and 2.1.4 we will discuss this deposition of metal precursor for the three methods separately.

After fixing the metal precursor onto the support, the solvent is removed by filtering and drying and the precursor remains in the pores of the support. Sometimes not only the metal salt but in addition some residue originating from the solvent stays behind as well. To discard of this, a calcination step may be introduced,

i.e.,

the precursor catalyst is heated in air to a few hundred degree cen-tigrade. After calcination or drying, the metal precursor is brought into the active. metallic state by a reduction in hydrogen. After reduction, the catalyst is highly active and exposing the catalyst without further precaution to air would result in a process known as 'run away oxidation'. Already during the start of the oxidation pro-cess enough heat would be evolved to allow the oxidation propro-cess to continue in an uncontrollable way. The temperature of the parti-cles would reach a level at which the mobility of the partiparti-cles is high enough to start a sintering process, in which several smaller metal particles join to form larger metal particles. This process is of course to be avoided. Therefore, direct after reduction, the active catalyst is flushed with nitrogen in order to remove all hydrogen. Thereafter, oxygen is added carefully to the nitrogen feed. The oxygen content is increased slowly up to a level of 20%. This pro-cess is known as passivation : a careful and controlled oxidation of the metal particles. For noble metal particles, passivation is in gen-eral limited to the surface of the metal particles. The active metal is then covered by an oxide layer, is made 'passive' and can be stored in air to await further study. A reduction in hydrogen at low tem-peratures is in general enough to restore the catalyst to its active state.

In the discussion above, the first and most important part of the preparation, the introduction of the metal precursor onto the internal surface area of the support, has been skipped. We will now continue to describe this essential part for three methods. The methods are the pore volume impregnation method, the ion exchange method and the urea method.

2.1.2

Pore Volume Impregnation

The pore volume impregnation method is the method which is most frequently used in catalyst preparation because of the elegant simplicity of the method. The quantity of metal salt needed to prepare the catalyst is dissolved in the exact amount of solute needed to fill the pores of the support. We illustrate this by the fol-lowing example, in which we prepare 5 g of an alumina supported catalyst loaded with 2 wt% rhodium. The alumina used has an internal surf ace area of

180

m2 g-1 and a pore volume of

0.6

ml g-1.

The precursor is RhCl₃·3H₂0 with a molecular weight of 263.3 g. 5 g of the eventual catalyst will contain

4.9

g A1₂0₃ and

0.1

g Rh, which is equivalent to

0.2559

g RhCl₃·3H₂0 and has to be dissolved in

4.9*0.6

=

2.94

ml water. This solution is added slowly to the alumina, which is stirred vigorously in order to distribute the dis-solved precursor evenly over the support. Because of capillary forces, the solution is soaked up quickly in the pores of the support. It is important that the pores are just filled. When too little solvent is used, the precursor is spread only on part of the surface area of the support, and this may result in larger metal particles. When too much solvent is used, part of the precursor will end up on the rela-tively small external surface area of the support, which may result in a few but very large metal particles outside the pores. Because the pores have to be filled precisely, th is method is also known as the incipient wetness technique : when the pores are.just filled, on the verge of 'flowing over', the support starts to feel wet. After impregnating or incipiently wetting the support, the catalyst precur-sor is, as described above, dried carefully, calcined if necessary,. reduced in hydrogen and finally passivated.

2.1.3

Jon Exchange

A technique more sophisticated than the pore volume impreg

-nation method is the ion exchange technique. In this method,

cations are fixed to the internal surface area of the support

.

An

example is the ammonia ion NH

₄

+ which absorbs readily on

sup-ports like titania, Ti0

₂•

An ammonium solution (NH

₄

0H) is added

to the support and allowed to adsorb

.

After the adsorption process,

the support is filtered off

.

This support, saturated with ammonia,

is added to a solution of a metal salt,

e.

g,

an aqueous solution of

Rh ( N 0

₃

h

It is essential that the metal ions in the solution are

present as cations

.

In the case of Rh(N0

₃

b,

rhodium is present as

IRh(OH)n(H

₂

0)

₆

.

nJ(

3

·

n)+ complexes.

These (positively charged)

complexes exchange readily with the absorbed N H

₄

+ ions on the

support

.

In this way, an equal spread of the metal precursor over

the support can be achieved. If RhCl

₃

were used as metal precursor,

rhodium would be present as (H

₃

0+

+)

[RhCl

₃

(0H)n(H

₂

0)

3

_

nJn-complexes and these negatively charged nJn-complexes will not

exchange with the N H

₄

+ ions on the support

.

In this thesis, we will encounter an example in which no

specific counter ion has been adsorbed on the support to exchange

with a metal complex. The 4 wt% Rh/Ti0

₂

catalyst described in

chapter 7 has been prepared by exchanging Rh(N0

₃

h

with the

pro-tons present in the surface hydroxyl groups in Ti0

_2;

the Rh/Ti0

₂

catalyst described in chapter 8 has been prepared by ion exchange

.

.

using ammonia

.

After the metal precursor has been exchanged, the support is

filtered off and dried. In case ammonia has been used, a calcination

step is usually applied to remove the ammonia left behind on the

support. The dried or calcined catalyst is then reduced in hydrogen

and finally passivated.

2.1.4

The Urea Method

The urea method is founded on a controlled raise of the pH of a solution of the metal precursor and urea (CO(NH₂

)i)

in which the support is suspended ( 1,2). The solution is stirred vigorously. An adequate temperature is selected (

±

365 K) at which the urea decomposes slowly according to the following reaction :

As a result, the pH value of the solution slowly increases as the urea decomposes. At a certain pH value a metal hydroxide starts to form and precipitate on the support. Because the urea decompo-sition rate and thus the liberation of OW groups can be controlled by regulating the temperature, the precipitation of the metal precur-sor can be controlled elegantly. As a result, the method provides a homogeneous spread of metal hydroxide over the support, which is a good starting point for obtaining highly dispersed metal particles. After drying, a calcination step is essential in order to remove the urea left behind in the sample. Finally, calcination is followed by reduction and passivation.

2.2

Temperature Programmed Reduction

Temperature programmed reactions are used frequently to study the chemical behavior of supported metal catalysts. Most important among them is Temperature Programmed Reduction (TP R). Other examples are Temperature Programmed Desorption (TPD), Oxidation (TPO) and Sulfiding (TPS). The principles are the same for all temperature programmed reaction techniques. As the temperature of the catalyst is increased, some reaction of the

active phase with the gas atmosphere is studied. In TPR. the redu-cibility of a sample,

e

.

g,

an oxidized metal catalyst or a catalyst precursor, takes place. The catalyst is flushed with a mixture of 4% H2 in N2 and the temperature of the sample is increased at a

constant heating rate of 5 K min-1. The following reaction schemes illustrate the reduction processes that may proceed in case of a pre-cursor catalyst which has been prepared using RhCl_3, or in case of an oxidized rhodium catalyst :

2 RhCl3

+

3/2

H₂

+::±

2 Rh

+

3 HCI Rh203

+

3 _H2

+::±

2 Rh

+

3 _H20

By monitoring the consumption of hydrogen, one can get informa-tion on the reducinforma-tion process. The hydrogen uptake as a function of temperature is usually denoted as the 'TPR profile' of the sam-ple. The temperatures at which the reduction proceeds reveal what substance is being reduced and can be used as a 'fingerprint'. The amount of hydrogen consumed provides the reaction stoichiometry and/or the degree of reduction at a certain temperature during the process.

Just as the other temperature programmed techniques, TP R is used most often as a fingerprint technique. For detailed descrip

-tions, we refer to the review by Hurst (3) and a few of the earliest papers on TPR (4-11 ). The apparatus we used has been described extensively by Boer et al. ( 12).

2.3

Hydrogen Chemisorption

In supported metal catalysis, the amount of metal atoms exposed to the gas atmosphere is one of the most important characteristics of the catalyst. The dispersion of a catalyst is used to quantify this and is defined as the fraction of metal atoms in contact with the gas atmosphere. Adsorption and desorption tech-niques can be used to estimate the dispersion. These techtech-niques use a selected gas that adsorbs only on the metal particles and not on the support. Dependent on the specific technique used, the amount of gas that adsorbs or desorbs is measured. The hydrogen chemisorption technique used to measure the dispersion of the catalysts discussed in this thesis is extensively described in ( 13,14).

Briefly, the experimental procedure comprised the following steps. A catalyst sample, dried, calcined, passivated or oxidized, is reduced

in situ in 100% H₂ at the desired temperature. After the reduction

procedure. the sample is evacuated at some elevated temperature, in general 473 K. The chemisorption cell contains two sections, of which the exact volume is known. The first compartment is used as a reference chamber, the second contains the (now reduced and evacuated) catalyst sample. A stop cock separates the two sec-tions. After reduction and evacuation, a known amount of hydro-gen is admitted into the reference chamber. After the valve between the reference and catalyst section is openeq. hydrogen starts to adsorb on the catalyst. In most cases, adsorption is an activated and therefore slow process. To circumvent this, the catalyst is temporarily heated, usually to 473 K, to speed up the

adsorption process. After cooling down and equilibrating, the amount of gaseous hydrogen can be computed by measuring the pressure and the amount of adsorbed hydrogen can be calculated. The experiment is then continued in the desorption mode. The stop cock between the two chambers is closed and the hydrogen pres -sure in the reference chamber is lowered. After opening the stop cock, hydrogen desorbs from the catalyst because of the lower pres

equilibrium pressure. Thus. the ratio of the amount of adsorbed hydrogen and the amount of metal present. the H/M value. is moni-tored as a function of pressure. The linear part of this desorption isotherm is extrapolated to zero pressure in order to nullify small errors in the volume of the catalyst section and to eliminate adsorp-tion of hydrogen on the support.

As indicated in ( /4), this H/M value is not a direct measure for the dispersion. but can be used to compare catalysts and is therefore very useful as a fingerprint. However. since in the same paper the hydrogen chemisorption method has been calibrated with an independent technique. EXAFS. we can estimate very accurately. dispersion and particle sizes from the H/M values.

2.4 Electron Spin Resonance Spectroscopy

In case a catalyst contains paramagnetic centers. Electron Spin Resonance is a useful technique to study these centers. For an iso-lated electron. a paramagnetic center. two states are accessible, one with spin +1_/z

_(O'),

_{the other with spin -}1_/z

_(y)

_.

_{In the absence of a}

magnetic field these two states are degenerated :

E(

O')

=

E(

y).

A magnetic field removes this degeneracy. the two states differ in energy by

!iE =

£(0') -

E(y)

-

g

e

l3H

12.4.1]

in which

13

is the Bohr rnagneton and H is the magnitude of the magnetic field. The electron g-factor,

ge,

for an isolated electron in a spin-only case. is equal to 2.0023. Transitions between the two levels O' and

13

can be generated by a suitable electromagnetic

radia-tion. In practice. the frequency of the electromagnetic radiation is kept constant while the magnetic field is varied linearly with time.

When the photon energy h ii equals the difference in energy of the two states, transitions between ex and {3 may occur. This 1s accompanied by absorption of the electromagnetic radiation.

In practice, electrons are never isolated and we have to expand the theory to perturbed unpaired electrons. In case of a free elec-tron, the electron spin is solely responsible for the electrons mag-netic moment and the Hamiltonian H can be written as

H

gf3(H·S)

[2.4.2]

In relevant cases, however, the electron 'moves' in an orbit 'around' a nucleus and this gives rise to an orbital angular momentum cou-pling

(f3 (H ·L ))

and a spin-orbit coupling (.\

(L ·S)).

Conse-quently, the Hamiltonian can be written as

H =

g{3(H·S)

+

f3(H·L)

+

.\(L·S)

[2.4.3]

The complication induced by the latter two phenomena can be cir-cumvented be defining an effective spin Hamiltonian

Herr

which operates only on fictive spin states :

Herr

=

f3 (H

·g

err"S:)

[2.4.4]

The effects therefore manifest themselves in the value of the effective g-tensor

g

err·

Measuring the effective g-values therefore provides information on the magnitude of the orbital angular momentum and the spin-orbit coupling. Two important properties of the g-tensor need to be mentioned. The electron spin is coupled to its orbital momentum and therefore to the lattice in which the atom or ion is situated. Hence, relaxation phenomena are related to the effective g-value. Enhanced relaxations may be accompanied by large deviations of g

err

from 2.0023. The second property to

mention 1s the symmetry of the g-factor. A crystal field with

spherical symmetry gives rise to an isotropic

g

-

value

.

For an axial

field along the

z-axis.

two

g

-values,

gxx

=

gYY

and

g

₁₁•

may be

observed.

In Chapter 4, an ESR study of Fe

3+

ions

will be described.

Fe

3+

has a 3d

5

configuration and in a high spin case in an

octahedral crystal field. Fe

3+

has five unpaired electrons

.

Whenever

the site symmetry deviates slightly from perfect octahedral. which

is usually the case, this gives rise to a very characteristic ESR signal

centered at

g

=

4.2 (

15- 17).

2.5 Nuclear Magnetic Resonance Spectroscopy

For Nuclear Magnetic Resonance, the basic principles are the

same as those for ESR. While in ESR transitions between electron

spin states are observed, in NM R transitions between nuclear spin

states are studied. For nuclei with

I= +1

_{/z in a magnetic field,}

again two states are accessible

:

one with the nucleus' magnetic

moment parallel and the other anti-parallel to the external magnetic

field. When irradiated with

a

suitable electromagnetic radiation,

transitions between those states can be generated and an

absorp-tion of the electromagnetic radiaabsorp-tion can be observed. As

.

for ESR,

the frequency of the electromagnetic radiation is kept constant and

the absorbance is monitored as a function of the external magnetic

field. The magnetic field which the nucleus experiences is in general

not equal to the applied magnetic field : the electrons surrounding

the nucleus modify the external magnetic field. Thus, the shift in

NMR spectra gives information on the chemical environment of the

nucleus under study and is therefore

called

the chemical shift. For

example, the chemical

shift

for Pt in H

₂

PtCl

₆

is different from the

chemical shift of Pt in Na

₂

Pt(OH)

₆ ( 18,19)

and

can

therefore,

although both salts are present in the same sample, be used to

study these salts separately. In chapter 3 we will encounter an example in which 195Pt NM R is used to estimate the amount of Pt present in H₂PtCl₆ crystallites in an impregnated and dried catalyst sample.

2.6 Mossbauer Spectroscopy

In 1957, Rudolf

L.

Mossbauer demonstrated that nuclei can resonantly absorb gamma rays which originate from similar nuclei decaying from excited states ( 20-22). The basic principles are explained in Figure 2.1a. In this example, 57Co is used as a source and decays according to the scheme in Figure 2.1. The 14.4 keV emission can be used to generate transitions in 57Fe nuclei in the sample between I

=

1_{h and I}

=

%.

_{In practice, the energy of the}

emitted gamma quanta differs slightly from the energy needed to excite the 57Fe nucleus under study. To circumvent this, the source is give a velocity and because of the Doppler effect, the emitted energy is modulated slightly. The absorption therefore, is measured as a function of the Doppler velocity of the source and peak posi-tions and shifts are reported in mm s-1. This is ii lust rated In Figure 2.1b. There are three hyperfine interactions which are respon-sible for the fact that the 57Fe nuclei in the sample absorbs quanta of a different energy than those emitted by the excited 57 Fe nuclei in the source. The first is the isomer shift (J.S.) which is a meas-ure for the electron density around the nuclei under study. The iso-mer shift is caused by the Coulomb interaction between the posi-tively charged nucleus and the negaposi-tively charged s-electrons, whose wave functions overlap with the nucleus. The isomer shift gives information on the oxidation state of the iron. The second hyperfine interaction is the quadrupole splitting. In its ground state, the 57Fe nucleus has a spherical charge distribution and therefore no quadrupole moment. In the excited state, however, the nucleus has an ellipsoidally shaped charge distribution and therefore has a

Figure 2.1 Mossbauer Spectroscopy (a) Basic Principles

(b) Experimental set up

(c) Three basic Mossbauer spectra : (1) no hyperfine interactions (the spectrum of stainless steel), (2) the influence of quadru-pole splitting (the spectrum of sodium nirtoprusside) and (3) magnetic hyperfine splitting (the spectrum af a-Fe).

57Co

~lectron

b

CJ

\apture

..

source sample detector

9%

91%

137 keV

123 keV

- - -'.--- I= 3/ 2 --~-I=1/2 (14.4

k e V

-____L _ _ _ _ , __

Doppler velocity

I I I I I I I I I I I I I I I I I I I I I -10 -5 0 5 10

c

~

(1)

w

(21 (3)

positive electric quadrupole moment. In case the nucleus experi-ences an electric field gradient, two orientations are allowed for the quadrupole moment and a splitting of the excited level is observed (see Figure 2.1c). The splitting between the two excited levels t::i..£₀ is proportional to the electric field gradient at the nucleus; The third interaction is a magnetic hyperfine splitting. The magnetic moment of the excited nucleus will react on any external magnetic field, including the magnetic field induced by the surrounding electrons.

Two orientations are accessible for the ground state and four for the excited level. Thus, a splitting of the ground level and the excited level is observed (see Figure 2.1c). Therefore, for a nucleus with a magnetic moment, eight transitions are possible, two of which are forbidden, leaving six transitions.

Thus, isomer shift, quadrupole splitting and magnetic hyperfine splitting are used to identify the environment of 57Fe nuclei in a sample. Mossbauer spectroscopy can of course be used for other elements as well. Examples are Ir and Ru. In chapter 4 we will discuss an example in which the state of iron in Si0₂ supported bimetallic Fe-Ru and Fe-Pt catalysts is studied using Mossbauer spectroscopy.

2. 7 Laser Raman Spectroscopy

In Raman spectroscopy, the em1ss1on spectra of excited molecules are studied. A laser is used as a light source. The electric field

E

of light will give rise to a redistribution of the charge in a polarizable molecule and thus induce a dipole, the dipole moment

71

being equal to

in which O' is the polarizability of the molecule. For light with fre

-quency

v_0•

the dipole moment is equal to

µ -

0'£

₀

sin(21Tv

0

t)

[2.7.2)

When during the absorption process the excited molecule decays to

a state different from the ground state, a state with an internal

vibration with frequency

vi, the polarizabil ity O' will oscillate

accordingly :

[2

.

7.3)

and therefore

µ

[O'o

+

13

sin(217' v

i

_{t ) )Eosin(217' v0 t )}

[2.7.4)

-

O'oEosin(21Tvo

t)

+

1_hf3E

0 ,cos(21T(vo-vi)t) -

cos(21T(v 0+vi)t)]

The excited molecule will therefore emit radiation with frequencies

equal to

_{v0 (Rayleigh scattering), v0 - vi and v0}

+

vi, the Stokes

and Anti-Stokes lines respectively. The frequency shifts in Raman

therefore correspond directly to the frequency of the induced

vibra-tion. As in infrared (IR) spectroscopy, these frequencies are very

specific for the molecule under study. In chapter 3 we will discuss

the Raman spectra of the PtCl

₆2-

unit. The basic unit of this

molecule is an octahedron and three of its vibrations are Raman

active. Figure 2.2 shows the PtCl

₆2-

unit. For centro-symmetric

systems, the vibrations that are inactive in Raman spectroscopy,

are active in IR and

vice versa

.

However

,

IR turned out to be

inca-pable of_ detecting any vibrations in the PtCl

₆ 2-

unit of impregnated

and dried Pt/ A1

₂

0

₃

catalysts. Details of the Raman spectroscopy

experiments wil I be given in chapter 3.

Figure 2.2 The structure of H2PtCl6

Cl

Cl

I

/

Cl

I

/

Cl

Cl-Pt-Cl

···

··

Cl--Pt-Cl

/I

/I

Cl

.Cl

Cl

Cl

I

/

er

I

/

er

c1

Cl-Pt-Cl

···C

l--Pt

-

-Cl

/

I

.

/

I

Cl

Cl

Cl

Cl

I

/

Cl

I

/

Cl

-

P t -

·

·

C 1--Pt

--

-C 1

/

I

,,

I

Cl

Cl

Cl

,,

I/Cl

I/Cl

Cl

C 1

-

---Pt---C 1

·

···

C 1

-

Pt-C l

c1

/

I

c1

/

I

Cl

Cl

2.8 ASED-MO Computations

ASED is an acronym for atom superposition electron delocali-zation. The ASED-Molecular Orbital theory (23) has been applied in numerous and diverse studies to predict structures, reaction mechanisms and vibrational and electronic properties. The theory uses for input data the ionization potentials and valence state Slater orbital exponents for the constituent atoms (24-28). These parame-ters are sometimes altered, particularly in treatments of ionic hetero nuclear molecules, to ensure reasonably accurate calculations. The electronic charge density function of a molecule or solid can be par-titioned into components in any number of arbitrary ways. The ASED-MO theory is based on a partitioning of the density function

into free atomic components and the rest. The atomic components are spherically symmetric in a field-free space and are centered on

the nuclei. They follow the nuclei 'perfectly'. The remainder

changes its shape depending on the geometry. With respect to the nuclei. it is 'non-perfectly following'. Figure 2.3a shows an example of perfectly following atomic densities Pa and Pb for atoms a and b

in a diatomic molecule and a nonperfectly following charge density

Pnpf . The interaction energy is made up of a repulsive part which

can be computed from the perfectly following charge densities and an attractive part, which can be calculated from the nonperfectly following charge density. In Figure 2.3b both repulsive and attrac-tive energy terms and their sum, the interaction energy are shown

schematically. The first assumption in the ASED method is, that

the atoms are first instantaneously superimposed. Based on this, the repulsive energy term follows from

[2.8.1]

in which Z is the nuclear charge,

p

the atomic charge density

func-tion,

R

the coordinate of the nuclei and

r

the coordinate of the

elec-trons. ER is repulsive because nuclear repulsion energy is greater

than the attractive energy between nucleus b and Pa. The

non-perfectly following energy term is attractive because of the concen-tration of charge in the internuclear region due to bonding. The attractive energy term is given by

z

Rfb

r (

.

R.) d J i

- - b oo

J

Pnpf 1 • b

dR;

I

R;- r

I

drdR;

[2.8.2]

It is impossible to evaluate equation [2.8.2]. However, the

non-perfectly following or attractive energy term is due to electron delo-calization and is roughly equal to the difference in atomic and

Figure 2.3 ASED-MO terms for a diatomic molecule.

(a) The perfectly and non perfectly following charge densities for a diatomic molecule AB

(b) The attractive energy term. the repulsion term and the in-teraction energy as a function of interatomic distance.

a

_b

/ /

R

a-b

Etot

E

_npf

ER

molecular orbital energies. Thus, E att is successfully approximated

by

!).£mo

'°

n Eab

£., I I

[2.8.3)

which is a summation over the molecular orbitals i; n; is the orbital

occupation number (0, 1 or 2), Eia and Eib are the atomic orbital

energies (in practice, VSIP) and Eiab the molecular orbital energies.

The molecular orbital energies are the solutions of the (extended Hi.ickel) Hamiltonian with

HC1a

_ll

_-

_-(\ISJP)r

_[2

_.

₈

_.

₄₎

Ha

.

a

_-

₀

_[2

_.

_8.5)

l j

H/2j

1125(Hcza

+

Hbb) 5 ab

c-0.13R

• ll J J l j

[2

.

8.6)

in which S;'1/' is the overlap integral of a and b. R the internuclear distance. Finally. the total energy is the sum of

ER

and E

att

:

Etot -

L,

ER(a .b)

+

Eau

a >b

[2

.

8.7]

For a detailed discussion, we refer to (

23

,

29 ).

In chapter 6 we will describe the results of this kind of calculation for 10 atom rho-dium metal clusters supported on y-Al₂0_3. From these calcula-tions, we will be able to derive information about the binding of rhodium metal particles to the alumina support, the binding energy and the structure of the metal-support interface.

2.9 Extended

X-ray

Absorption Fine Structure

2.9.1

Basic Principles

EXAFS is an acronym for Extended X-ray Absorption Fine Structure. It refers to the 'wiggles', the fine structure that can be observed in the X-ray absorption spectra of condensed phases at the high energy side of absorption edges. Let us focus on a 1s elec-tron which is subjected to monochromatic X-ray radiation. When the photon energy tiw is lower than the binding Eb energy of the 1s electron, absorption will not take place. When the photon energy equals the binding energy. the 1s electron may be excited and this