Adsorbate-Induced Restructuring of Surfaces - American Chemical

Chapter 15

Adsorbate-Induced Restructuring of Surfaces

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 13, 2016 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch015

Surface Thermodynamic Puzzle G. A. Somorjai Department of Chemistry, University of California, Berkeley, CA 94720 and Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720

Most solid surfaces restructure when clean. Absorbates that chemically bind cause additional marked restructuring of metal substrates that is well demonstrated by recent low energy electron diffraction (LEED)-surface crystallography studies. The rearrangement of atoms in the solid substrate may be local or long range and can occur on time scales of adsorption (less than 10 seconds). Atomically rough surfaces rearrange more readily and at lower temperatures and are also more chemically active in breaking bonds. Surface thermodynamic data are missing and very much needed to elucidate the nature and reasons for surface restructuring. Adsorbate induced restructuring can explain the need for thermal activation of chemical bond breaking, the dominant role of rough surfaces in dissociative chemisorption and catalytic activity and the structure insensitivity of a class of catalyzed reactions. -3

+3

In the beginning of the century, one of the major thermodynamic puzzles, the discrepancy i n the equilibrium constant of ammonia as determined by Haber and Nernst has induced the rapid development of physical chemistry(l). The data points reported by these two giants of chemistry are shown i n Figure 1. The equilibrium constant as determined i n their laboratories was very d i f f e r e n t and the search for the reasons of the discrepency lead to the development of low temperature heat capacity measurements, commerical high pressure reactor technology and to the accelerated development of c a t a l y t i c research and catalyst based chemical technologies. The use of catalysts and the establishment of high pressure reactor technology was needed to s h i f t the equilibrium between nitrogen, hydrogen and ammonia.

0097-6156/90AM28-0218$06.25/0 © 1990 American Chemical Society

In Bonding Energetics in Organometallic Compounds; Marks, Tobin J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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Figure 1 . The equilibrium constant f o r the synthesis of ammonia from nitrogen and hydrogen as determined by Nernst and Haber as a function of temperature.



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BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS

Modern surface chemistry and heterogeneous c a t a l y s i s have many puzzles that await s o l u t i o n . Even the use of s i n g l e c r y s t a l surfaces shown i n Figure 2 and c h a r a c t e r i z a t i o n techniques that provide information about the atomic s t r u c t u r e , composition and oxidation state of surface atoms on the molecular l e v e l have not provided solutions to these puzzles as yet. One of the most i n t e r e s t i n g puzzles i s the need for 1) thermal a c t i v a t i o n to break large binding energy chemical bonds. Discovery of t h i s phenomenon was i n the 1930's when dinitrogen was adsorbed on i r o n and i t s low heat of adsorption was measured. As the temperature was increased above around 130K a large exotherm was observed i n d i c a t i n g d i s s o c i a t i v e adsorption of dinitrogen(2). That i s , at t h i s temperature molecular nitrogen dissociated to make iron-nitrogen bonds as the atomic nitrogen chemisorbed. S i m i l a r observation for the thermal a c t i v a t i o n of chemisorbed hydrocarbons i s shown i n Figure 3. Ethylene, propylene and higher o l e f i n s adsorbed on platinum decompose upon h e a t i n g Q ) . Hydrogen desorbs, leaving a p a r t i a l l y dehydrogenated organometallic fragment on the metal surface. The decomposition occurs sequentially at well-defined temperatures. Why strong 100-200 k c a l chemical bonds break by changing the temperature a few degrees for a given chemisorbed molecule on a given surface i s one of the puzzles of modern surface chemistry. While the molecular structure before and a f t e r the thermal decomposition i s w e l l characterized (as shown for ethylene i n Figure 4), the reasons for t h i s t r a n s i t i o n and the mechanism for i t s occurrence are not understood. 2) Rough Surfaces Do Chemistry. Another puzzle i s the way rough surfaces do chemistry, both bond breaking and c a t a l y s i s . The rougher, more open the surface i s the more l i k e l y that i t breaks chemical bonds and at a lower temperature. Figure 5 shows three c r y s t a l faces of platinum, the f l a t , stepped and the kinked surface. Figure 6 shows the stepwise decomposition of ethylene on the f l a t (111) surface of n i c k e l as compared to the stepped surface of n i c k e l ( 4 ) . The same decomposition pathway i s found on both of these surfaces. However, on the stepped surface decomposition occurs at a much lower temperature, below 150K, as compared to the n i c k e l (111) face which shows chemical bond breaking only at about 230K. The a c t i v i t y of rough surfaces can perhaps be best demonstrated by studying the behavior of hydrogen on f l a t , stepped and kinked surfaces of platinum. When hydrogen i s adsorbed on these surfaces and then a temperature programmed thermal desorption experiment i s c a r r i e d out, i t i s found that hydrogen desorbs at the maximum rate at the lowest temperature from the f l a t surface while from stepped and kink surfaces desorption occurs at higher temperatures (Figure 7). The higher temperature desorption peaks are associated with the hydrogen desorption from steps and kinks r e s p e c t i v e l y ( l ) . S i m i l a r l y , the c a t a l y t i c H2/D2 exchange also occurs by at least an order of magnitude higher reaction p r o b a b i l i t y at steps as compared to the f l a t surface(£). This can be studied by a molecular beam surface s c a t t e r i n g process where a mixed H2+D2 beam i s scattered from a stepped platinum surface or a f l a t platinum surface and the r e s u l t i n g HD that forms upon s i n g l e s c a t t e r i n g i s measured. The presence of HD c l e a r l y indicates d i s s o c i a t i v e adsorption and reaction of hydrogen on the metal surface. As Figure



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2

Figure 2. Small area (1 cm ) single crystal surface that is used in surface science studies as well as a model heterogeneous catalyst. (Reproduced with permission from Lawrence Berkeley Laboratory.)



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_l

200

I

I

I

400

I 600

I

l_ 800

Τ (Κ)

Figure 3. Thermal desorption spectra for small alkenes adsorbed on the platinum (111) surface showing sequential hydrogen evolution and decomposition. (Reproduced from ref. 3. Copyright 1982 American Chemical Society.)




15. SOMORJAI

Figure 4. The surface structures of ethylene at 77K, 310K and 450K as determined by high r e s o l u t i o n electron energy loss spectroscopy and low energy electron d i f f r a c t i o n .


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[211]

PERIODICITY

Pt-(557)

PERIODICITY'

Pt-(679) Figure 5. Theflatstepped and kinked surfaces of platinum. (Photo credit: Lawrence Berkeley Laboratories. Reproduced with permission from ref. 5. Copyright 1980 Elsevier Science Publishers.)


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SOMORJAI

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C H (g) 2

Ni(111)

C H 2

4

230 Κ

4

C H


2

2

400 Κ

+ 2H

C H or C H + H (g) 2

2

C H (g) 2

- 250 Κ

Adsorbate-Induced Restructuring of Surfaces - American Chemical

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