Chemistry of Acetylene on Platinum (111) and (100) Surfaces (Chemisorption/Ultra-High Vacuum/Metal Surface Chemistry/Hydrocarbon Reactions) E

Proc. Nati Acad. Sci. USA Vol. 78, No. 11, pp. 6571-6575, November 1981 Chemistry

Chemistry of acetylene on platinum (111) and (100) surfaces (chemisorption/ultra-high vacuum/metal surface chemistry/hydrocarbon reactions) E. L. MUETTERTIESt, M.-C. TSAit, AND S. R. KELEMENt tMaterials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720; and Department of Chemistry, University of California, Berkeley, California 94720; and tCorporate Research Science Laboratories, Exxon Research and Engineering Company, P.O. Box 45, Linden, New Jersey 07036 Contributed by Earl L. Muetterties, July 24, 1981

ABSTRACT An ultra-high vacuum experimental study of face generated showed the 5 x 20 low energy electron diffiac- acetylene chemisorption on Pt(111) and Pt(100) and ofthe reaction tion pattern. The 5 X 20 surface converted to the 1 X 1 structure of hydrogen with the acetylene adsorbate has established distin- with exposure to acetylene (5). The 1 x 1 surface is stabilized guishing features ofcarbon-hydrogen bond breaking and making by the presence of acetylene and is present during the hydro- processes as a function ofpressure, temperature, and surfacecrys- genation and dehydrogenation processes discussed in this tallography. The rates for both processes are substantially higher article. on the Pt(100) surface. Net acetylene-hydrogen processes, in the temperature range of20°C to -13rC, are distinctly different on the two surfaces: on Pt(100) the net reaction is hydrogen exchange RESULTS ('H. H exchange) and on Pt(III) the only detectable reaction is hydrogenation. Stereochemical differences in the acetylene ad- Acetylene is the simplest unsaturated hydrocarbon. Its geo- sorbate structure are considered to be a contributing factor to the metric structure, electronic structure, and organic chemistry differences in acetylene chemistry on these two surfaces. are quite precisely defined (6-8). The molecular details ofacetylene interactions with inorganic and organometallic complexes Essential to an in-depth understanding of hydrocarbon chem- also are relatively well defined (9, 10). Catalytic acetylene istry mediated at metal surfaces is the delineation ofthe surface chemistry based on solid state or solution state catalysts is well electronic, topological, and compositional features that signif- explored in terms ofreaction products, although the molecular icantly affect carbon-hydrogen bond making and breaking pro- features of the catalytic reaction mechanism are not fully de- cesses. Because the activation energy for carbon-hydrogen fined (8, 9, 11). The basic features of acetylene chemisorption bond breaking is typically much lower than for carbon-carbon on metal surfaces is a much investigated area, especially the bond breaking, it is, in principle, experimentally easier to study chemisorption of acetylene on crystallographically defined this type ofreaction. Using primarily the techniques ofthermal metal surfaces under ultra high vacuum conditions. However, desorption spectroscopy, displacement reactions, and isotopic full understanding at the molecular level has not been reached labeling under ultra-high vacuum conditions, Friend and Muet- for the acetylene surface case. A careful analysis ofthe acetylene terties (1) have successfully defined molecular features ofben- surface science literature (12-28) [an excellent general discus- zene and toluene chemistry on a range ofnickel surface planes. sion is that of Demuth (12)] reveals certain complications com- We describe here an analogous study of acetylene chemisorp- mon to surface chemistry, especially hydrocarbon surface chem- tion and acetylene reactions on the flat platinum (111) and (100) istry. The complications are fully recognized by the surface planes. This study establishes the temperatures required for scientists but are often ignored in the discussion ofspecific sci- carbon-hydrogen bond making and for carbon-hydrogen bond entific investigations. Critical limitations or complications are: breaking for the acetylene molecule on these platinum surfaces (i) As the complexity ofthe adsorbate species increases from and also sets certain limits for the compositional and stereo- an atom to a diatomic to a polyatomic molecule, the probability chemical features of the chemisorbed species derived from that compositionally or stereochemically differentiable adsor- acetylene. bate states will be formed under a specific set of experimental conditions generally increases. (ii) The sensitivities ofthe various common electron spectro- EXPERIMENTAL scopic or diffraction techniques to a given adsorbate species Reagents. Acetylene, purchased from Matheson, was passed obviously are not necessarily directly related. Thus, in the case through a -780C cold trap before admission to the manifold of chemisorption of a molecule that gives rise to two or more leading to the ultra high vacuum chamber. Perdeuteroacet- differentiable adsorbate species, the possibility arises that dif- ylene, 99 atom % C22H2, was purchased from Merck; mass spec- ferent techniques will primarily sense different adsorbate spe- trometric analysis indicated the C22H2 concentration was cies. Low energy electron diffraction, for example, senses only -95%. Hydrogen (99.95%) and deuterium (99.7%) were pur- adsorbates that have long-range order. chased from Matheson (East Rutherford, NJ) and Liquid Car- (iii) The lifetime of a chemisorption state or set of states is bonic (Chicago), respectively. finite. This state may alter during the time period ofa spectro- Procedure. The basic ultra-high vacuum system, the ex- scopic or diffraction study and, ofcourse, the electron or photon perimental protocols, and the source and the cleaning proce- beam may effect surface state transformations. dure for the platinum (111) crystals have been described in full (iv) The character of the chemisorption state can be a very in earlier papers (2-4). The cleaning procedure for Pt(100) is sensitive function of temperature, pressure (background pres- that described by Fischer et al (5), and the clean platinum sur- sure), surface contaminants, and surface coverages. Molecules present in the ultra-high vacuum system can and do react with The publication costs ofthis article were defrayed in part by page charge the metal surface or with the chemisorbed species. In fact, as payment. This article must therefore be hereby marked "advertise- is now recognized (23), hydrogen reacts with acetylene chem- ment" in accordance with 18 U. S. C. §1734 solely to indicate this fact. isorbed on Pt(111). 6571 Downloaded by guest on September 29, 2021 6572 Chemistry: Muetterties et al Proc. Natt Acad. Sci. USA 78 (1981) (v) In specific spectroscopic or diffraction studies, there nat- A Pt(lll)-'H-C22H2 surface was formed at temperatures in urally are attempts to enhance spectral or diffraction features. the range of - 100C to +500C by first chemisorbing hydrogen In these attempts, time scale or seemingly minor temperature, and then perdeuteroacetylene. Thermal desorption immedi- pressure, or coverage variations may not be explicitly re- ately after the adsorption processes yielded an H2 maximum at ported-yet minor excursions in such experimental variables 120TC, characteristic of a Pt(lll)-'H surface; no 2H2or 'H2H may wreak substantial changes in surface adsorbate species. All was detected in this low-temperature region. Characteristic of of these are real problems for the seemingly simple system of Pt(111)-C22H2, there were two 2H2 desorption maxima at acetylene chemisorption on metal surfaces and are particularly ==2450C and --3800C. A small 1H2H desorption maximum at evident for the chemisorption of acetylene on Pt(111). 2450C and a very small 1H2H maximum at 3800C were also ob- The states of acetylene chemisorption on Pt(111) have been served. The appearance of 'H2H is largely ascribed to the over- examined by thermal desorption spectroscopy (13), ultraviolet lap of the broad 1200C and 240'C desorption peaks for photoemission spectroscopy (12-14, 16), low energy electron Pt(lll)-'H and the low-temperature acetylene decomposition diffraction (17-22), and high-resolution electron energy loss process, respectively, and to a small amount (==5%) of C21H2H spectroscopy (23-26). Most scientific attention has been given impurity in the C22H2 sample. Hence, under these conditions, recently to the data and conclusions derived from the last two there was no detectable reaction between C22H2 and the hy- techniques. Although originally the data from these two tech- drogen atoms on the surface. niques were referenced to a "stable" Pt(*11)-C2H2 state, the In contrast to the above experiment at 200C and low hydro- data are relevant only to a species or set of species generated gen surface activities, Pt(111)-'H-C22H2 surfaces heated to by a higher temperature reaction between hydrogen and the temperatures above 500C for periods of about 1-5 min yielded original chemisorbed acetylene species (see below). This ad- a significantly different desorption pattern, as shown in Fig. 1 sorbate has been described as CH3C (21, 22), CH2CH (13), and for a 135°C reaction. As in the lower temperature experiments, CH3CH (23-26). Careful photoemission studies done as a func- only 'H2 showed a desorption maximum in the temperature tion oftemperature for C2H2 chemisorbed on Pt(111) at or near 200C are reported to be consistent with oa-bonded HCCH or CCH2 species (12-14). Analogous photoemission studies for Pt(100)-C2H2 at 20'C suggest that the major adsorbate is a com- plexed HCCH species (5, 15). Our studies of acetylene chemisorption on Pt(111) and on Pt(100) definitively establish the temperatures at which C-H bond breaking and bond making processes occur at moderate rates. Our detection ofcarbon-hydrogen bond breaking at low temperatures, 20-135°C, was based on isotopic labeling or ex- 0 c change ofhydrogen and deuterium and the established fast site 0) 4.- exchange of 1H and 2H atoms on a metal surface. This probe c sensed only those surface carbon-hydrogen bond breaking processes in which a C-H bond was converted to a Ptdi i-H 0 bond and in which the Pt nfci-H species had afinite lifetime. 2 A carbon-hydrogen bond breaking process for an adsorbed 0 HCCH species that proceeded simply by a 1,2 shift ofa hydrogen atom to give CCH2 would not be detected by our experimental procedure. Consistent with earlier reports, we found that the thermal desorption spectrum for Pt( 11)-C21H2, formed at 0-20°C, consisted of two broad hydrogen (H2) desorption peaks with max- t f imum desorption rates occurring at =240°C and =380°C with 2450C 3800C relative intensities of1:1. For Pt(111)-C22H2, the two 2H2 thermal desorption maxima were at 245°C and 380°C. At high initial Temperature (OC) acetylene coverages (>0.5 to 0.6 of a monolayer), the thermal desorption spectrum for Pt(111)-C21H2 also showed a small FIG. 1. A Pt(111)-1H-C22H2 surface was formed at -200 and (<25%) reversible desorption peak for acetylene at 160°C. At then was heated to 13500 for 1 min. Aftercooling to -35TC, the thermal showed all desorption (decomposition) experiment was performed. Presented comparable high coverages, Pt(111)-C21H2-C22H2 above are the thermal desorption spectra in which the desorption of three possible acetylene molecules desorbed at =1600C. At- 1H2 (upper curve), 1H2H (middle curve), and 2H2 (lower curve) were tempts to displace acetylene from Pt(111)-C2H2 at 200C with monitored by mass spectrometry. Plotted are the mass 2, 3, and 4 ion strong field ligands such as trimethylphosphine and methyl is- intensities as a function of temperature (the heating rate was 250 sec ocyanide were unsuccessful-in sharp contrast to the facile dis- and the rate was linear above -60OC). The low-temperature maximum placement of benzene from Pt(111)-C6H6 by trimethylphos- for 1H2 is in the region characteristic of Pt(lll)-1H. The absence of thermal desorption of acetylene from 'H2H or 2H2 desorption in this low-temperature range established phine. However, unequivocally that C-2H bonds were not broken at measurable rates Pt(111)-C2H2-P(CH3)3 did yield far more acetylene than did an to form Pts,fc6-H bonds up to and at 13500 within the time scale of analogous acetylene coverage on Pt(l1) alone. No benzene was this experiment. The high-temperature desorption maxima at -24000 detected in thermal desorption experiments involving and -38000 characteristic of C-H bond breaking for the adsorbate Pt(lll)-C2H2a species are detectable for all three species, 1H2, lIzH1, and 2H2, show- Thermal desorption of hydrogen from Pt(111) is a sensitive ing that C-'H bonds were formed in the surface reaction of 'H and 022H2 at temperatures of 13500 (or below). The much larger intensity function ofcoverage and heating rate but the typical range for of the 24000 desorption peaks relative to the 380°C peak for 'H2 and the maximum after adsorption near 20(C is -50-150'C; the 1H2H does not necessarilyr imply that specific (in a structural or ster- temperature maximum for deuterium is slightly higher than for eochemical context) H(2H) bond types are broken at 24000; the hydrogen at comparable coverages. intensity differences may reflect largely a kinetic isotopic effect. Downloaded by guest on September 29, 2021 Chemis": Muetterties et al. Proc. Natd Acad. Sci. USA 78 (1981) 6573 region characteristic ofPt( 11)-'H(2H). All changes were in the =240C and =380TC peaks. Substantial 'H2 and 'H2H maxima were observed near 240TC with small maxima for these two molecules at -380TC. The integrated 1H2, 1H2H, and 2H2 intensities for the 240TC and 380TC maxima were in the relative ratio of I24 > I3wc with an apparent maximum ratio ofabout 2:1. The results establish that no Ptsu-2H bonds are broken up to 135TC and that the net reaction between C22H2 and 1H adsorbed on this Pt(111) surface is hydrogenation. The higher temperature. (50-135TC) reactions between adsorbed C22H2 and H on Pt(111) indicated that reactions should occur at a measurable rate at 25TC. To test this implication, a (0y Ad070 Pt(O11)-C22H? surface, generated and maintained at 20TC, was 0 0.40 treated to an H2 atmosphere of 1i-0 torr (1 torr = 133 Pa) for 20 min. A subsequent thermal desorption experiment established that carbon-hydrogen bonds had been formed under I. these conditions and that no carbon-deuterium bonds had been cleaved: there was only an 'H2 desorptionpeak in the low-tem- 100 200 300 400 500 perature region characteristic of Pt(111)- H, a very small 1H2 and larger 1H2H and 2H2 peaks at 240°C, and essentially only Temperature (0C) 'H2H (small) and 2H2 peaks at 3800C. FIG. 2. Hydrogen desorption spectra obtained by Fischer et al. (5) In the thermal pretreatment ofPt( 1l)-'H-C22H2, there was for the thermal desorption (decomposition) of Pt(100)-C2H2 as a func- a direct correlation between the increase in the 'H2 and 1H2H tion of fractional coverage, 6, of the platinum surface. The essential desorption peaks at r240°C and 380°C, and the development desorption line shapes and the temperatures for the desorption max- of a 2 x 2 low energy electron diffraction pattern for ima were fully reproduced in this present study; the temperature maximum for 2H2 desorption from Pt(100)-C22H2 were shifted to slightly Pt(11I)-'H-C22H2, which pattern is the diagnostic feature for higher temperatures, -5C. The weak, lowest temperature maximum the acetylene adsorbate species identified (21, 22) as CH3C in observedforthis Pt(100)-C2lH2 system is closetoamaximum observed the low energy electron diffraction studies of Somorjai and co- for Pt(100)-H, and, therefore, may represent desorption of 1H2 from workers (21, 22). Ptf.,c6-H species formed by partial dehydrogenation of the acetylene Acetylene chemisorbed on Pt(100) in an irreversible fashion. adsorbate at temperatures between about 50TC and 160TC. Rapid heating of Pt(100)-C21H2 (or C22H2) yielded no detectable acetylene or any other hydrocarbon such as benzene. Hy- only trace amounts of 1H2H and no 2H2 detected at the two drogen, formed by carbon-hydrogen bond scission ofacetylene major desorption temperatures characteristic of the carbon- adsorbed at coverages of -0.5, evolved with two major de- hydrogen (deuterium) bond breaking processes. sorption peaks at -260°C and =320°C. There were, however, additional small maxima, and the spectral profile was dependent DISCUSSION upon coverage, as shown in Fig. 2. No attempt is made here to mechanistically interpret the fine structure ofthe desorption Isotopic labeling ofthe acetylene-hydrogen reaction on the two spectra: because the platinum 5 X 20 - 1 x 1 surface recon- atomically flat platinum (111) and (100) planes has allowed iden- struction is a function of adsorbate coverage and potentially a tification of the temperatures at which carbon-hydrogen bond function of temperature, there is a reasonable probability that making and bond breaking processes proceed at moderate rates there are more than one structurally or stereochemically dif- under low hydrogen presures of 10- to 10-1" torr. Quantita- ferentiable adsorbates derived from Pt(100) and acetylene. tively and qualitatively, the acetylene-hydrogen reaction is dif- The chemistry ofacetylene on Pt(100) was substantially different on the Pt(111) and Pt(100) surfaces. The most distin- ferent from that on Pt(111) in that carbon-hydrogen bond mak- guishing feature for Pt(111) is the absence of measurable ing and also bond breaking was far more facile on Pt(100). Ex- carbon-hydrogen bond breaking (to form Pt,f,6-H species) up posure of Pt(100)-C22H2, formed at 20°C, to a hydrogen to temperatures of at least 120TC, whereas the process is de- pressure of 10- to 10-` torr for a period of 5-15 min yielded tectable at 20TC for Pt(100). For Pt(100), the net reaction be- adsorbate species that contained a substantial amount of car- tween adsorbed acetylene and hydrogen atoms at 20-135TC is bon-hydrogen bonds as evidenced by large 'H2 and 'H2H de- hydrogen exchange (1H-2H exchange), whereas on Pt(111) the sorption maxima at 260°C and 320°C in the subsequent thermal only reaction is hydrogenation. desorption (decomposition) experiment. A slight modification In the initial interaction of acetylene with the flat Pt( 1) of this experiment established that carbon-deuterium bond surface plane, the carbon-carbon bond probably is in a plane breaking occurred at 35°C. A Pt(100)-C22H2 state formed at nearly parallel to the metal surface plane in order to allow max- =35°C was treated with 1H2 at 10-6 torr for 10 min. Then the imal overlap ofthe Grand ir* acetylene orbitals with appropriate thermal desorption (decomposition) experiment was per- metal surface orbitals. The situation for the Pt(100) flat plane formed. The major desorption peaks in the region characteristic may be analogous, but the possibility of significant tipping of of carbon-hydrogen bond breaking were largely those of 'H2 the C-C bond vector with respect to the surface plane is higher and 1H2H with only a small amount of2H2. Hydrogen-deuterium (see Fig. 3). At moderate temperatures, there should be some exchange at the adsorbate carbon sites was nearly complete reorganization [often referred to as rehybridization of the under these conditions. All integration data indicated that the HCCH framework (5, 12, 13, 15)], and if molecular metal co- net reaction on this surface was 'H-2H exchange. ordination chemistry is an appropriate guide for this acetylene At higher temperatures of 70-130TC, exposure of Pt(100)- chemistry, the two carbon-hydrogen vectors should be tipped, C22H2 to a hydrogen pressure of 10-7 torr led to a rapid hydro- within a plane normal to the surface and away from the surface gen-deuterium exchange. After exposure of several minutes, plane. This type ofinteraction is consistent with ultraviolet pho- the thermal desorption spectra consisted largely of 'H2 with toemission data (12, 13) for Pt(lll)-C2H2. Furthermore, the Downloaded by guest on September 29, 2021 6574 Chemistry: Muetterties et al. Proc. Nad Acad. Sci. USA 78 (1981)

FIG. 3. Scale model of a three-atom and four-atom section of Pt(011) and Pt(100) with the two carbon atoms (smaller, dark spheres) of a HCCH adsorbate that illustrates the topographically defined potential for tipping of the C-C bond vector of the acetylene adsorbate, with the constraint that both carbon atoms maintain abonding interaction with surface platinum atoms, with respect tothe surface plane forthe two platinum surfaces. This potential for tipping could be realized in the adsorbate ground state structure or in thermally excited states. Clearly, the potential is greater for the (100) surface. With C-C bond distances in the range of 1.30-1.40 A, the angles generated by the C-HC vector and the surface plane are -13° and -45° for the (111) and (100) surfaces, respectively.

relatively high activation energy required for carbon-hydrogen thereby enhance the probability of facile carbon-hydrogen bond scission (to form Pts,-,,e~H species) on Pt(1l1)-C2H2 bond breaking and ultimately an exchange process. Recently, strongly implicates a stereochemistry in which the C-H hy- Casalone et al. (33) have proposed on the basis ofa low-energy drogen atoms are well removed from the surface metal atoms electron diffraction study ofNi(100)-C2H2 that the C-C bond (29-31). Carbon-hydrogen bond scission for what was initially vector is tipped by some 500 from the surface plane-a value Pt(1ll)-C21H2 was measurably fast only at temperatures near close to the geometric estimate noted in Fig. 3. 240NC (this statement relates to the time scale of seconds and It is important to note that in all results and discussions re- pressures of 10-10 torr ofthe thermal desorption experiment). lating to the particular process ofcarbon-hydrogen bond mak- Rapid carbon-hydrogen bond breaking (to form Pt .u-H spe- ing that the rates are a very sensitive function ofhydrogen pres- cies) was not observed even at 1350C within a time interval of sure, which establishes the thermodynamic activity of the minutes for the Pt(lll)-C22H2 state, formed initially at 20TC. surface hydrogen atoms. The low rates reported herein are sim- Thus, following the above reasoning, we suggest that the ad- ply a reflection of the very low pressure conditions. Hydrogen sorbate species formed from the hydrogenation ofPt(l11)C21H2 and acetylene are converted to ethylene and then to ethane on does not have any ofthe hydrogen atoms near the surface metal a platinum surface at ambient pressures and temperatures, al- atoms. Of the three models proposed for this species-CH3C, though even under these conditons the rates are not excep- (21, 22, 28), CH2CH (13, 32), and CH3CH (23-26)-the first, tionally high. We saw no evidence that the adsorbate was re- an ethylidyne species proposed by Kesmodel et al. (21, 22) to moved from the Pt(100) surface, presumably as ethane, by have the C-C bond vector normal to the surface plane, is fully passing hydrogen over the Pt(100)-C2H2 surface at 20TC and consistent with our chemical and isotopic labeling experiments 10-6 torr pressure within a 25-min period as judged by Auger simply because this model would have all hydrogen atoms far analysis and by a final thermal desorption experiment. removed from the surface, plane. This consideration does not In the thermal decomposition ofacetylene in Pt(111)-C21H2 exclude the other two proposed models, because either ofthese formed at 20TC, the relative intensities of the two 1H2 desorp- could have a stereochemistry (29) such that all hydrogen atoms tion peaks are approximately 1:1, suggestive ofan intermediate would be relatively distant from surface metal atoms. C2H state in the thermal decomposition ofPt(111)-C21H2. This In contrast to the case with Pt(11), we found carbon-hydrogen may be the case, but we do not believe that these data alone bond breaking in the acetylene adsorbate on Pt(100) to be meas- allow such a conclusion because of the very broad character of urable at temperatures of200C. In light ofthis facile bond break- the two desorption peaks (there may in fact be more than two ing process and of the ultraviolet photoemission data (5, 15), desorption peaks) and because ofthe possibility that more than which are consistent with a slightly reorganized or "rehybrid- one differentiable adsorbate species is present at these higher ized" (5, 15) acetylene framework for Pt(100)-C2H2, we suggest temperatures. The relative intensities ofthe two hydrogen de- that either the C-C bond vector is tipped with respect to the sorption peaks- for the higher temperature species generated metal surface plane or the HCCH adsorbate species is not co- from Pt(111)-C21H2 and hydrogen are approximately 2:1, planar through some torsional deformation, a deformation dis- suggestive ofaC2H3 adsorbate species; however, forthe reasons cussed by Felter and Weinberg (27). Either possibility could stated above, we do not believe that the data from the desorp- bring one or both hydrogen atoms sufficiently close to the sur- tion (decomposition) experiments alone allow such a conclusion face as to generate a C-H-Pt multicenter bond (29-31) and to be made. Downloaded by guest on September 29, 2021 Chemistry: Muetterties et al. Proc. Nad Acad. Sci. USA 78 (1981) 6575

We note a curious coincidence in the temperature four the 5. Fischer, T. E., Kelemen, S. R. & Bonzel, H. P. (1977) Surf. Sci. maximum rate of hydrogen evolution in the highest tempera- 64, 157-175. process The temperature 6. Callomon, J. H. & Stoicheff, B. P. (1957) Can. J. Phys. 35, ture decomposition for Pt(O11)-C2H2. 373-382. is =38O0C. It is coincident with the highest temperature de- 7. Palke, W. E. & Lipscomb, W. N. (1966)1. Am. Chem. Soc. 88, composition peak for Pt(111)-C2H4, Pt(111)-C6H6, and 2384-2393. Pt( 1l)-cyclohexene. This may be no more than a coincidence 8. Rutledge, T. F. (1968) Acetylenic Compounds (Reinhold, New or it may reflect a common feature in the ultimate decompo- York). sition mechanism on this platinum surfiace ofhydrocarbon ad- 9. Otsuka, S. & Nakamura, A. (1976) Adv. Organomet. Chin. 14, or at least certain interrelated hydrocarbons (ethylene 245-283. sorbates 10. Muetterties, E. L., Rhodin, T. N., Band, E., Brucker, C. F. & can be dehydrogenatedto acetylene, acetylene can be catalyt- Pretzer, W. R. (1979) Chem. Rev. 79, 91-136. ically trimerized to benzene, and cyclohexene, can be dehy- 11. Parshall, G. W. (1980) Homogenous Catalysis (Wiley, New York). drogenated to benzene on platinum). For Pt(100), the acet- 12. Demuth, J. E. (1979) Surf. Sci. 84, 315-328. ylene desorption spectrum is a function ofadsorbate coverage 13. Demuth, J. E. (1979) Surf. Sci. 80, 367-387. in that there is no high-temperature hydrogen peak at low cov- 14. Demuth,' J. E. (1977) Chem. Phys. Lett. 45, 12-17. erages Benzene shows a different coverage depen- 15. Fischer, T. E. & Kelemen, S. R. (1978) Surf Sd. 74, 47-53. (Fig. 2). 16. Lo, W. J., Chung, Y. W., Kesmodel, L L., Stair, P. C. & So- dence; nevertheless, benzene and acetylene have similar high- moijai, G. A. (1977) Solid State Coinmun. 22, 335-337. temperature H2 peaks at high adsorbate coverages on Pt(100). 17. Morgan, A. E. & Somorjai, G. A. (1969) J. Chem. Phys. 51, In summary, we note that the substantive differences in the 3309-3320. acetylene surface chemistry between Pt(111) and Pt(100) appear 18. Kesmodel, L. L., Stair, P. C., Baetzold, R. C. & Somoijai, G. A. to be intimately associated with the topographical differences (1976) Phys. Rev. Left. 36, 1316-1319. differences may be ob- 19. Stair, P. C. & Somorjai, G. A. (1977) J. Chem. Phys. 66, between these two surfaces. Similar 2036-2044. served for acetylene chemistry on other second and third row 20. Kesmodel, L. L., Baetzold, R. C. & Somorjai, G. A. (1977) Surf. group VIII metal surfaces and the differences can be directly Sci. 66, 299-320. related to the topographical features of the metal substrate. 21. Kesmodel, L. L., Dubois, L. H. & Somorjai, G. A. (1978) Chem. Salmer6n and Somorjai (34) have studied 'H-2H exchange Phys. Lett. 56, 267-271. reactions, mediated on a Pt(111) surface, between 2H2 and a 22. Kesmodel, L. L., Dubois, L. H. & Somoijai, G. A. (1979) J. variety of hydrocarbons, including acetylene. Their experi- Chemi. Phys. 70, 2180-2188. 23. Ibach, H., Hopster, H. & Sexton, B. (1977) AppI Phys. 14, mental results for acetylene on Pt(111) are similar to those de- 21-24. scribed in this article. 24. Ibach, H., Hopster, H. & Sexton, B. (1977) Appl. Surf. Scd. 1, 1-24. Suggestions and critical comments by Prof. R. M. Gavin and Mr. Kirk 25. Ibach, H. & Lehwald, S. (1978)J. Vac. Sci. Technol. 15, 407-415. Shanahan are gratefully acknowledged. This work was supported by the 26. Baro, A. Me & Ibach, H. (1981)J. Chem. Phys. 74, 4194-4199. Director, Office ofEnergy Research, Office ofBasic Energy Sciences; 27. Felter, T. E. & Weinberg, W. H. (1981) Surf. Sci. 103, 265-287. Chemical Sciences Division of the U.S. Department of Energy under 28. Howard, M. H., Kettle, S. F. A., Oxton, I. A., Powell, D. B., Contract W-7405-ENG48. E.L.M. is indebted to the Miller Institute Sheppard, N. & Skinner, P. (1981)J. Chem. Soc. Faraday Trans. for Basic Research in Science for a grant in the form of a Miller 77, 397-404. Professorship. 29. Gavin, R. M., Reutt, J. & Muetterties, E. L. (1981) Proc. NatL Acad. Sci. USA 78, 3981-3985. 1. Friend, C. M. & Muetterties, E. L. (1981) J. Am. Chem. Soc. 30. Muetterties, E. L. (1981) Am. Chem. Soc. Symp. Ser. 155, 103, 773-779. 273-297. 2. Friend, C. M., Gavin, R. M., Muetterties, E. L. & Tsai, M.-C. 31. Beno, M. A., Williams, J. M., Tachikawa, M. & Muetterties, E. (1980)1. Am. Chem. Soc. 102, 1717-1719. L. (1981)J. Am. Chean. Soc. 103, 1485-1492. 3. Muetterties, E.L. & Tsai, M.-C. (1980) BulL Soc. Chim. Beig. 89, 32. Demuth, J. E. (1980) Surf. Sci. 93, L82-L88. 813-817. 33. Casalone, G., Cattania, M. G. & Simonetta, M. (1981) Surf. Sci. 4. Friend, C. M., Stein, J. & Muetterties, E. L. (1981) J. Am. 103, L121-L125. Chem. Soc. 103, 767-772. 34. Salmer6n, M. & Somoijai, G. A. J. Phys. Chean., in press. Downloaded by guest on September 29, 2021