1998
Langmuir 1991, 7, 1998-1999
Reversibility of C1 Hydrogenation-Dehydrogenation Reactions on Platinum Surfaces under Vacuum Francisco Zaera Department of Chemistry, University of California, Riverside, California 92521 Received June 20,1991. In Final Form: July 29, 1991
In this communication we present spectroscopic evidence for the formation of methyl and methylene groups as productst of methyl iodide decomposition on a Pt(ll1) surface. We also show thermal programmed desorption spectra obtained from surfaces where deuterium was coadsorbed with methyl iodide in order to study the kinetics of hydrogenation-dehydrogenation steps involving those surface species. D2 coadsorption allowed for the detection of methane molecules with different degrees of deuterium substitution and provided information on the extent to which reversible H-D exchange takes place. We found that even though the hydrogenation of methyl groups to methane occurs with very high selectivity if hydrogen (or deuterium) is coadsorbed on the surface, the final hydrogenation step is preceded by multiple exchange reactions. The product distribution seen for methane desorption after coadsorbing deuterium and normal methyl iodide also suggests that methylidyne (CH) moieties are formed on this platinum surface. from the cracking patterns of each isotropicallysubstituted In this communication we present both kinetic and methane molecule as discussed e1se~here.l~ Even though spectroscopicevidence for the formation of adsorbed methyl (CH3)and methylene (CH2)moieties on Pt(ll1)surfaces most of the desorbing methane consists of either CHI or CH3D originating from direct hydrogenation (or deuteriand additional kinetic data which suggest that methylidyne (CH) groups form on the same substrate as well. ation) of surface methyl groups, detectable amounts of Methyl and methylene fragments have already been CHzD2, CHD3, and CD4 are also produced. These molecules must be the result of the hydrogenation (deuteriisolated on many clean transition-metal surfaces,'-" but ation) of intermediates that have gone through surface even though the interconversion between those species has been proposed to occur in many catalytic p r o c e ~ s e s , ~ ~ - ~H-D ~ exchange steps, and we propose that such exchange occurs via an initial dehydrogenation of methyl groups to such reactions have not yet been studied in sufficient methylene moieties. detail.15 We have previously shown that methyl groups can be prepared on Pt(ll1) surfaces by thermal decomOur TPD spectra also indicate that multiple H-D position of methyl iodide: a method we use here to study exchange is favored in these experiments. Indeed, while their surface chemistry. We report thermal programmed the CD4 yield amounts to about 8%,CHzDz corresponds desorption data which indicate that hydrogenation-deto only about 3% (Figure 1, right panel). This product hydrogenation reactions on those adsorbed hydrocarbon distribution can only be explained by a kinetic argument fragments are reversible even under vacuum conditions. that includes an additional surface reaction, which we suggest to be the formation of methylidyne fragments (CX, Thermal programmed desorption (TPD) and reflectionadsorption infrared spectroscopy (RAIRS) experiments where X stands for either H or D). Whereas the methyl were performed in separate ultrahigh-vacuum (UHV) groups responsible for the production of monodeuterichambers, each equipped with the required instrumenated methane are stable surface species, methylene must tation for surface cleaning and characterization.1G18 Kibe much more reactive and undergo reversible interconnetic evidence for the formation of three different surface version to methylidyne before it finally hydrogenates to methane.20 Additional evidence for this mechanism is intermediates on Pt(ll1) surfaces was obtained by using TPD. Figure 1shows an example of the results obtained contained in the peak shapes for each desorbing species from normal methyl iodide coadsorbed with deuterium which will be discussed in a full report to be published latter. The product distribution seen here could alteron Pt(ll1); methane was always the main desorption product in these experiments. The data shown here were natively be explained by proposing a concerted (SN2) step for the H-D exchange in the methylene groups, but even obtained by simultaneously monitoring all peaks in the though such possibility cannot be completely ruled out 16-20 amu region and by deconvolving the contributions based solely on our experimental results, we believe it to be unlikely because the geometry of the transition state (1)Zhou, X.-L.; Yoon, C.; White, J. M. Surf. Sci. 1988,206, 379. (2)Zhou, X.-L.; Solymosi, F.; Blase, P. M.; Cannon, K. C.; White, J. complex in this case would need to be quite restrictive, M. Surf. Sci. 1989, 219, 294. and that would require a significant drop in entropy that (3)Zhou, X.-L.; White, J. M. Surf. Sci. 1988, 194, 438. would in turn manifest as a very low preexponential factor (4)Lee, M. B.; Yang, Q. Y.; Ceyer, S. T. J.Chem. Phys. 1987,87,2724. (5)Solymosi, F.;Kiss, J.; Revesz, K. J. Phys. Chem. 1990, 94, 2224. for the reaction rate. (6)Zaera, F.;Hoffmann, Helmuth J. Phys. Chem., in press. Spectroscopic evidence for the formation of both meth(7)Liu, Z.-M.; Costello, S. A.; Roop, B.; Coon, S. R.; Akhter, S.; White, yl and methylene moieties comes from RAIRS data as J. M. J. Phys. Chem. 1989,93, 7681. (8)Steinbach, F.;Kiss, J.; Krall, R. Surf. Sci. 1985, 157, 401. those shown in Figure 2.6 These were obtained by (9)McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1984, 148, 292. normalizing the 1000 scan spectra from adsorbed methyl (10)George, P. M.; Avery, N. R.; Weinberg, W. H.; Tebee, F. N. J.Am. iodide by similar spectra for the clean surface, each taken Chem. SOC.1983,105, 1393. (11)Henderson, M. A.;Radloff, P. L.; White, J. M.; Mims, C. A. J. with 4-cm-I resolution. Methyl formation is evidenced by Phys. Chem. 1988,92, 4111. the disappearance of the main peak at 2112 cm-l due to (12)Biloen, P.; Sachtler, W. M. H. Aduan. C a t d 1981, 30, 165. the symmetric C-D stretch of molecularly adsorbed meth(13)Brady, R. C.; Pettit, R. J. Am. Chem. SOC.1981, 103, 1287. (14)Rofer-DePoorter, C. K. Chem. Rev. 1981,81, 447. yl iodide above 240 K, which occurs while a new sharp (15)Radloff, P. L.; Mitchell, G. E.; Greenlief, C. M.; White, J. M.; peak grows at 2092 cm-I for the symmetric C-D stretch Mims, C. A. Surf. Sci. 1987, 183, 377. (16)Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci., in press. (17)Zaera, F. Surf. Sci. 1989,219, 453. (la)Zaera, F. J. Vac. Sci. Technol., A 1989, 7,640.
0743-7463/91/2407-1998$02.50/0
(19)Zaera, F.J . Phys. Chem. 1990, 94, 8350. (20)Kemball, C. Catal. Rev. 1971, 5, 33.
0 - 1991 American Chemical Societv
Langmuir, Vol. 7,No. 10, 1991 1999
Letters
r
Methane TPD 2L
Product distribution
D2+ 2L C%l /Pt(lll), 1OOK
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I
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. CD4
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Temperature / K
Figure 1. Methane thermal desorption spectra after sequential adsorption of 2 L Dz + 2 L CHsI on Pt(ll1) at 100 K. The left frame displaysTPD spectra for methane with all possible deuterium isotopic substitutionswhile the right frame shows the resulting product distribution. The yields reported here were reproducible to better than 0.005.
I CD3l I P t ( l l 1 ) RAIRS I
I
~0.0001 T / K
-
"y 2092 cni'
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Figure2. Reflection-absorption infrared spectrafrom saturation coverages of deuteriated methyl iodide adsorbed on Pt(ll1) as a function of annealing temperature. Only the region for the C-D stretchingmodes, between 1900and 2300 cm-', is displayed here. Our peak assignment is also shown.
of chemisorbed methyl moieties.6 Another small peak due to the methyl symmetric deformation (umbrella) mode is also seen at 1244 cm-l in RAIRS experiments using normal methyl iodide. Additionally, the spectra in Figure 2 display a broad feature around 2040 cm-' corresponcing to the symmetric C-D stretch in methylene (CD2) groups that grows upon heating briefly to 280 K at the expense of the signal from methyl moieties. I t should be noticed that each peak seen in these spectra must be assigned to a different species because the surface selection rule that applies in RAIRS precludes the possibility of having both symmetric and asymmetric C-D stretching modes being active at the same time in most adsorption geometries.21 Our assignments are consistent with the general observation in organometallic compounds that bridging me(21) Bradshaw, A. M. Appl. Surf. Sci. 1982, 11/12, 712.
thylene vibrational modes appear at lower frequencies than those from methyl groups.22 In summary, we have provided spectroscopic evidence for the formation of both methyl and methylene groups on Pt(111)surfaces and have shown thermal programmed desorption data which indicate that those intermediates can be hydrogenated to methane even under our low pressure conditions. More interestingly, the number of isotopic substitutions in methane desorbing from surfaces with coadsorbed D2 and CH3I displays a "U" shaped distribution that argues for the existence of an additional unstable moiety, most likely methylidyne, and for the rapid interconversion between such intermediate and methylene groups before full hydrogenation to methane takes place. Similar kinetic observations have been reported previously in studies on the H-D exchange of saturated hydrocarbons under catalytic condition~,2~*~"~5 but this is to the best of our knowledge the first time where such reversible hydrogenation-dehydrogenation reactions have been seen under vacuum. We discuss the implications of our results for catalytic reaction mechanisms in a separate report,26and a full account of our experiments will include a detailed analysis of the corresponding kinetic parameter~.~~ Acknowledgment. Financial support for this research was provided by a grant from the National Science Foundation (CHE-9012560). (22) Maslowsky, E., Jr. Vibrational Spectra of Organometallic Compounds; Wiley: New York, 1977. (23) Taylor, T. I. In Catalysis, Volume V; Emmett, P. H., Ed.;Reinhold New York, 1957. (24) Anderson, J. R. Rev. Acre Appl. Chem. 1957, 7, 165. (25) Zaera, F.; Somorjai, G. A. J . Phys. Chem. 1985,89,3211. (26) Zaera, F. Catal. Lett., in press. (27) Zaera, F. Submitted for publication in Surf.Sci.
