J. Phys. Chem. 1983,
87, 2469-2472
2489
Acetylene, Ethylene, and Arene Chemistry of Palladium Surfaces T. M. Gentle and E. L. Muetterties* Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, California 94720 (Received: April 8, 1983)
The chemisorption behavior of acetylene, ethylene, and benzene on the low Miller index planes of palladium is described. This behavior is very sensitive to the surface topography and surface composition, particularly to sulfur and phosphorus adatoms. Most complex was the acetylene chemistry where decomposition to carbon, reversible desorption, reversible C-H bond scission, trimerization to benzene, and hydrogenation to ethylene were five readily characterizable, competing reactions. In this context of acetylene chemistry, the palladium surfaces behave quite differently from those of nickel and platinum under ultrahigh vacuum conditions. Also, unlike nickel and platinum, palladium under ultrahigh vacuum conditions effected hydrogenation reactions such as the conversion of acetylene to ethylene.
tation of the surface and to probe for ordered chemisorption states. Gas composition was monitored with a quadrupole mass spectrometer (Uthe Technology International, Model 100C) in conjunction with a programmer which externally drove the mass spectrometer and stored the output. The palladium crystals, derived from a section of a single crystal (99.999%), were cut for proper orientation with a spark source and were polished. The crystals after mounting on the manipulator were cleaned by oxygen
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Introduction Acetylene, ethylene, and benzene comprise a basic and very important set of unsaturated organic molecules that are of general scientific and technological significance in catalytic chemistry. Understandably, these three organic molecules have been studied rather intensively by ultrahigh vacuum techniques on two technically important metals, nickel and platinum.1 We describe here in preliminary form the coordination chemistry of these three molecules, and derivatives thereof, on low Miller index planes of palladium. The chemistry is complex, sensitive to surface topography and to surface impurity atoms such as sulfur and phosphorus, and diverges from that of the congener metals, nickel and platinum. Most significantly, palladium ultrahigh vacuum chemistry models conventional palladium-catalyzed chemistry in hydrogenation reactions. To place this palladium hydrocarbon surface chemistry in perspective, we first discuss benzene and its derivatives followed by ethylene, and finally acetylene, and its derivatives, as the apparently most complex system. 23'4
treatment (10"5 torr at 400-500 °C) to remove carbon and by argon ion sputtering (600 eV at 400 °C) and annealing at 800 °C to remove oxygen. The crystal was heated on the back side with an electron gun to effect rapid and essentially linear heating rates for the thermal desorption experiments. All hydrocarbons employed in this study were of high purity (reagent grade); only acetylene had to be distilled prior to use to remove acetone. The aromatic hydrocarbons were predried over calcium hydride to remove traces of water. Procedures for the thermal desorption experiments (heating rates of 25 °C s'1) and the chemical displacement reactions have been described4 earlier. As a reference to hydrogen desorption experiments, we note that the range for H2 desorption maxima (coverage dependent) is similar for the three palladium low Miller index planes, 50-100 °C.
Experimental Procedure An ultrahigh vacuum chamber (Varían) was used for all experiments. The base pressure was 10'10 to 10'11 torr. Auger electron spectroscopy (retarding field analyzer) was used to monitor surface composition as was low-energy electron diffraction for sulfur coverages;5 *the latter technique was also employed to verify crystallographic orien-
Arene Chemistry Benzene chemisorption on the three palladium low Miller index planes was fully associative (molecular) at 20 °C as demonstrated through quantitative displacements of the benzene by trimethylphosphine. Rapid heating of Pd(XXX)-C6H6 surfaces led to competing reactions of thermal desorption and decomposition as observed for nickel and platinum surfaces. No reversible C-H bond
(1) General and specific references to acetylene and ethylene surface chemistry on Ni and Pt are listed below. Both molecules ultimately form a relatively stable ethylidyne (CCH3) species on Pt(lll). (2) (a) Fischer, T. E.; Kelemen, S. R.; Bonzel, . P. Surf. Sci., 1977, 64,157-75. (b) Demuth, J. E. Ibid.. 1979, 84, 315-28. (c) Ibid. 1979,80, 367-87. (d) Chem. Phys. Lett. 1977,45,12-7. (e) Fischer, T. E.; Kelemen, S. R. Surf. Sci. 1978, 74, 47-53. (f) Lo, W. J.; Chung, Y. W.; Kesmodel, L. L; Stair, P. C.; Somorjai, G. A.; Solid State Commun. 1977,22, 335-7. (g) Morgan, A. E.; Somorjai, G. A. J. Chem. Phys. 1969,51, 3309-20. (h) Kesmodel, L. L; Stair, P. C.; Baetzold, R. C.; Somorjai, G. A. Phys. Rev. Lett. 1976, 36, 1316-9. (i) Stair, P. C.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 2036-44. (j) Kesmodel, L. L.; Baetzold, R. C.; Somorjai, G. A. Surf. Sci. 1977,66,299-320. (k) Kesmodel, L. L; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1978,56, 267-71. (1) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180-8. (m) Ibach, H.; Hopster, H.; Sexton, B. Appl. Phys. 1977,14, 21-4. (n) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf. Sci. 1977,1,1-24. (o) Ibach, H.; Lehwald, S. J. Vac. Sci. Technol. 1978, 15, 407-15. (p) Baro, A. M.; Ibach, H. J. Chem. Phys. 1981, 74, 4194-9. (q) Felter, T. E.; Weinberg, W. H. Surf. Sci. 1981,103, 265-87. (r) Howard, . H.; Kettle, S. F. A.; Oxton, J. A.; Powell, D. B.; Sheppard, N.; Skinner, P. J. Chem. Soc., Faraday Trans. 2 1981, 77, 397-404. (s) Muetterties, E. L.; Tsai, M.-C.; Kelemen, S. R. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 6571. (3) See ref 4 and references therein for benzene and toluene studies on nickel and platinum surfaces. (4) (a) Friend, C. M.; Muetterties, E. L. J. Am. Chem. Soc. 1981,103, 773-9. (b) Tsai, M.-C.; Muetterties, E. L. Ibid. 1982,104, 2534-9. (c) J. Phys. Chem. 1982, 86, 5067. (5) Peralta, L.; Berthier, Y.; Huber, M. Surf. Sci. 1981, 104, 435.
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breaking was evident on Pd(100) or Pd(110) before or during thermal desorption in that initial chemisorption of C6H6 + C6D6 mixtures did not yield C6HXD6_X molecules in the thermal desorption experiments. However, H-D exchange between chemisorbed C6H6 and C6D6 was detected on Pd(lll): all possible C6HXD6_X molecules were observed in the thermal desorption experiment at the 250 °C maximum, but not in the low temperature maximum at +40 °C. The extent of exchange, however, was low,
