Characterization and Catalyst Development - American Chemical

Chapter 20

Characterization of Supported Metal Catalysts by X-ray Photoelectron Spectroscopy The Problem of the Binding Energy Reference

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2016 | http://pubs.acs.org Publication Date: October 3, 1989 | doi: 10.1021/bk-1989-0411.ch020

Robert F. Hicks Department of Chemical Engineering, University of California, Los Angeles, C A 90024-1592

The energy levels for x-ray photoemission from metal particles on insulat ing supports are described. Metal particles on insulators are not grounded to the spectrometer. The vacuum levels of the metal particle and the insu lator align at some unknown potential relative to the vacuum level of the spectrometer. Referencing the binding energy to an internal standard, as is usually done for catalysts, introduces two unknowns in the binding energy equation: the work function of the metal and the work function of the internal standard. If the support is a semiconductor or semimetal, or the metal particles are small ( is the spectrometer work function, and $ is the metal particle work function. s

M

Bradley et al.; Characterization and Catalyst Development ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

216

CHARACTERIZATION AND CATALYST DEVELOPMENT

Spectrometer

hv


-kin

-F

/,

'W///A

• M 3d 5/2 Sample Holder

Metal Sample

Figure 1. A schematic of the energy levels for photoemission from a metal sample. Spectrometer

-kin

F -F

b

r-

/

Egap //////

/

I2p M3d5/2 Sample Holder

Insulated Metal Particle

V

Insulator

Figure 2. A schematic of the energy levels for photoemission from a large metal particle on an insulator.


20.

HICKS

217

Characterization of Supported Metal Catalysts

If one is confident that the catalyst support is an insulator, then equation (2) can be used to estimate the binding energy of the metal. However, some independent measure ment is needed to obtain values of § and V . A value for the metal work function cannot be assumed a priori. The work function can vary ± 0 . 5 eV depending on the structure of the surface and the coverage of adsorbed molecules (32). As stated earlier, V depends on a number of variables which are difficult to measure. M

p

p

It has been suggested that these problems can be overcome by using an internal standard, such as the C Is peak of the carbon impurity, or the A u 4f 7/2 peak of vapor deposited gold (3133). If equation (2) holds for the internal standard as well, the unk nown V can be eliminated by subtraction:


p

E

E

U

E

E

B* ~ B = kto - kki + ^Au ~

0)

However, equation (3) still contains two unknowns: the work function of the metal and the work function of the gold deposit. A multilaboratory study has shown that neither carbon nor gold provide a suitable internal standard for catalyst supports (34). Alternatively, the binding energy scale has been referenced to a core level of the support metal cation, for example, the Si 2p peak of silica. This is no improvement. The work function of a high surface area, amorphous catalyst support has never been measured. Photoemission from Small Metal Clusters on Insulators So far the discussion has focused on large metal particles on insulators. These particles exhibit "bulk-like" electronic properties: their valence band is similar to the valence band of a 1 c m metal foil. Ultraviolet photoemission studies have shown that the "bulk-like" band structure is attained for particles larger than 25-50 angstroms in diameter (35). X-ray photoemission from metal clusters smaller than 25 angstroms in diameter is even more problematic than for large particles. When clusters are deposited on insulating substrates, positive shifts in the metal binding energies are observed with decreasing cluster size. These shifts result from a number of competing phenomena (35-41): the charge on the cluster during the lifetime of the core hole, the reduced screening of the core hole, the decomposition of the valence band, and the rehybridization of the valence orbitals. The first two are final state effects, while the latter two are initial state effects. The final state cluster charge can dominate the binding energy shift, and extends over a range of sizes where the cluster exhibits metallic behavior, i.e., 10 to 25 angstroms (40,41). The insulat ing support will reduce the magnitude of the cluster charge by forming an opposite image charge. The image charge will depend on the cluster-substrate geometry and the polarizability of the substrate (40). 2

Recent experiments by Citrin and coworkers (41) have clarified the role of the sup port in photoemission from small metal clusters. They condensed several monolayers of krypton onto either platinum or sodium metal substrates. By varying the thickness of the krypton from one to ten monolayers, the surface could be converted from metal to semimetal to insulator. The krypton peak position provides a direct measure of the sample vacuum level (32). The krypton layers are thin, less than 10 monolayers, so that the vacuum level is determined by the metal substrate. Onto the krypton layers, sodium clus ters were deposited at varying coverages. Shifts in the Kr 4s and Na 2p binding energies were recorded relative to the Fermi level of the grounded substrate. The results obtained by Citrin and coworkers are shown in Figure 3. For sodium clusters on a metal support (wigl M L Kr/Pt, filled circles), the Kr 4s binding energy decreases with cluster coverage. This shows that the Fermi levels of the sodium and plati num equilibrate. As the sodium is added, the work function decreases from the value for platinum to the value for a sodium film. Conversely, the N a 2p peak position does not shift with cluster coverage. The rapid electron transfer between the sodium and platinum prevents any accumulation of charge on the cluster in the photoemission final state (41).



218


COVERAGE OF Na (atoms/cm2)

Figure 3. The dependence of the K r 4s and Na 2p binding energies on the coverage of sodium, the thickness of krypton, and the substrate. (Reproduced with permission from Ref. 41. Copyright 1987 The American Physical Society.)



20.

HICKS


219

For sodium deposited on insulating supports (8 M L Kr/Pt, open circles; 6 M L Kr/Na, open squares), the K r 4s peak position does not change with sodium cluster cover age. Here, the krypton keeps the clusters from transferring charge with the underlying metal. Proof that die metal substrate determines the vacuum level is demonstrated by the shift in the K r 4s peak when platinum (open circles) is substituted for sodium (open squares). The 2 eV shift is approximately equal to the work function difference between platinum and sodium metal. Since the sodium clusters on the insulating krypton are aligned with the vacuum level, the Na 2p lines also maintain a constant 2 eV separation for the two different substrates used. In this particular experiment, the vacuum level is known and we can "correct" the position of the Na 2p line. The increasing Na 2p binding energy with decreasing cluster size can be attributed to the positive charge left on the clus ter in the core ionized final state. Semimetal or semiconductor supports were also simulated by depositing 3 mono layers of krypton on platinum (half filled circles in Figure 3). In this case the results are intermediate between metal and insulator. The K r 4s peak shifts with cluster coverage, indicating a decrease in the substrate work function. This occurs because the sodium clus ters are able to transfer charge with the platinum and are grounded to it as in the case of a metal support. At the same time, the Na 2p peak shifts to higher binding energy with decreasing cluster size. This indicates that the clusters are charged in the photoemission final state as in the case of an insulating support. A n explanation of why the clusters can be grounded to the semimetal substrate, but still accumulate a final state cluster charge is given in reference 41. The significance of these results for catalysts comprised of small metal clusters is twofold: (1) The clusters will exhibit large positive binding energy shifts that are simply an artifact of the photoemission process, i.e., the charge resulting from the ionized core. This final state charge is sensitive to the support composition through its image charge. (2) On semiconductor or semimetal supports, the cluster Fermi level may drift towards that of the support and spectrometer, creating a partially charged condition for the initial state of the system. Implications for Metal-Support Interactions The results of XPS studies of metal-support interactions (9.10.18.20-25) can be re examined in light of the analysis presented above. A l l of the binding energy measurements made in these studies were referenced to an internal standard. For large metal particles, the core level shifts may be explained by any combination of the following: a.

the binding energy of the metal is affected by a metal-support interaction.

b.

the work function of the metal is affected by a metal-support interaction, or a change in the structure of the metal surface.

c.

the work function of the reference material changes.

d.

the conductivity of the support changes.

For small metal clusters, the core level shifts may be explained by all of the above plus a final state effect, i.e., the charge on the ionized cluster. In an X P S experiment, it is impos sible to distinguish between these different phenomena. Therefore, binding energy shifts, or a lack thereof, cannot be taken as evidence for or against an electronic interaction with the metal.


220


In spite of this drawback, there is still much to be gained from X P S characteriza tion of supported metal catalysts. Among these are the interconversion of metal salts into oxide and metal during catalyst pretreatment, the identification of poisons, and the distribu tion of metal within a zeolite or a porous pellet. Ackno wled gments


The author wishes to thank T. H . Fleisch for his strong support of this work, J. Z . Shyu, J. Schreiner, and Amoco Corporation for many revealing X P S experiments, and P. H . Citrin for helpful discussions and for the use of his data. Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

27. 28. 29.

Delgass, N . W.; Hughes, T . R.; Fadley, C . S. Catal. Rev. 1970, 4, 179. Hercules, D . M . Anal. Chem. 1970, 42, 20A. Brinen, J. S. Accts. Chem. Res. 1976, 9, 86. Schwab, G . M. Trans. Faraday Soc. 1946, 42, 689. Schwab, G . M. Discuss. Faraday Soc. 1950, 8, 166. Boudart, M . ; Djega-Mariadassou, G . Kinetics of Heterogeneous Catalytic Reactions; Princeton University Press: Prince ton, N J , 1984. Tauster, S. J.; Fung, S. C.; Baker, R. T . K.; Horsley, J. A . Science 1981, 211. 1121. Horsley, J. A . J. Amer. Chem. Soc. 1979, 101, 2870. Fleisch, T. H . ; Hicks, R. F.; Bell, A . T. J. Catal. 1984, 87, 398. Fleisch, T. H . ; Bell, A . T.; Regalbuto, J. R.; Thomson, R. T.; Lane, G . S.; Wolf, E . E . ; Hicks, R. F. Stud. Surf. Sci. Catal. 1988, 38, 791. Santos, J.; Phillips, J.; Dumesic, J. A . J. Catal. 1983, 81, 147. Dumesic, J. A . ; Stevenson, S. A . ; Sherwood, R. D.; Baker, R. T . K . J. Catal. 1986, 99, 79. Belton, D. N . ; Sun, Y . M . ; White, J. M. J. Phys. Chem. 1984, 88, 5172. Resasco, D. E.; Haller, G . L. J. Catal. 1983, 82, 279. Hicks, R. F.; Yen, Q. J.; Bell, A . T . J. Catal. 1984, 89, 498. Rieck, J. S.; Bell, A . T . J. Catal. 1986, 99, 262. Rieck, J. S.; Bell, A . T . J. Catal. 1986, 99, 278. Regalbuto, J. R.; Fleisch, T . H . ; Wolf, E . E. J. Catal 1987, 107, 114. Regalbuto, J. R.; Allen, C . W.; Wolf, E . E. J. Catal. 1987, 108, 304. Vedrine, J. C.; Dufaux, M . ; Naccache, C.; Imelik, B . J. Chem. Soc. Faraday Trans. I 1978, 74, 440. Foger, K.; Anderson, J. R. J. Catal. 1978, 54, 318. Kao, C . C.; Tsai, S. C.; Bahl, M . K.; Chung, Y . W.; L o , W. J. Surf. Sci. 1980, 95, 1. Sexton, B. A . ; Hughes, A . E . ; Foger, K . J. Catal. 1982, 77, 85. Chien, S. H . ; Shelimov, B. N . ; Resasco, D . E . ; Lee, E . H . ; Haller, G . L. J. Catal. 1982, 77, 301. Huizinga, T.; Van T . Blik, H . F. J.; Vis, J. C.; Prins, R. Surf. Sci. 1983, 135, 580. Seigbahm, K . ; Nordling, C.; Fahlman, A . ; Nordberg, R.; Hamrin, K . ; Hedman, J.; Johansson, G . ; Bergmark, T.; Karlsson, S. E . ; Lindgren, I.; Lindberg, B. Nova Acta Regiae Societatis Scientiarum Upsaliensis Ser. IV 1967, V o l . 20. p. 1. Fadley, C . S. In Electron Spectroscopy: Theory. Techniques, and Applications: Brundle, C . R.; Baker, A . D . , Eds.; Academic Press: New York, 1978. Delgass, W. N . ; Haller, G . L.; Kellerman, R.; Lunsford, J. H . Spectroscopy in Heterogeneous Catalysis; Academic Press: New York, 1979. Feldman, L. C.; Mayer, J. W. Fundamentals of Surface and Thin Film Analysis: Elsevier Science Publishing: New York, 1986.


20. 30. 31. 32. 33. 34.


35. 36. 37. 38. 39. 40. 41.

HICKS


221

Lewis, R. T.; Kelley, M. A . J. Electron Spectrosc. Rel. Phenom. 1980, 30, 105. Edgell, M. J.; Baer, D R.; Castle, J. E. Appl. Surf. Sci. 1986, 26, 129. Wandelt, K . Stud. Surf. Sci. Catal. 1987, 32, 280. Evans, S. In Handbook of X - r a y and Ultraviolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden: Philadelphia, P A , 1977. Madey, T . E.; Wagner, C . D.; Joshi, A . J. Electron Spectrosc. Rel. Phenom. 1977, 10, 359. Takasu, Y . ; Unwin, R.; Tesche, B.; Bradshaw, A . M.; Grunze, M. Surf. Sci. 1978, 77, 219. Mason, M . G . Phys. Rev. B 1983, 27, 748. Citrin, P. H . ; Wertheim, G . K . Phys. Rev. B 1983, 27, 3176. Cheung, T . T. P. Surf. Sci. 1984, 140, 151. Parmigiani, F.; Kay, E . ; Bagus, P. S.; Nelin, C . J. J. Electron. Spectrosc. Rel. Phenom. 1985, 36, 257. Wertheim, G . K . ; DiCenzo, S. B.; Buchanan, D . N . E. Phys. Rev. B 1986, 33, 5384. Qiu, S. L.; Pan, X . ; Strongin, M . ; Citrin, P. H . Phys. Rev. B 1987, 36, 1292.

R E C E I V E D April 27, 1989


Characterization and Catalyst Development - American Chemical

Recommend Documents