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Langmuir 1996, 12, 1926-1927
Reversibly Adsorbed π-Bonded Ethene on Pt(111) Surfaces by Infrared Reflection Absorption Spectroscopy
Scheme 1. Adsorption Structures of Ethene
Jun Kubota, Shigeru Ichihara, Junko N. Kondo, Kazunari Domen,* and Chiaki Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226 Japan Received October 11, 1995. In Final Form: January 2, 1996
Adsorption of ethene on metal surfaces has been extensively investigated by various methods.1,2 Especially, single-crystal surfaces of Pt and Rh are known to be highly active for ethene decomposition and hydrogenation, and there are many reports on ethene adsorption and reactions on the surfaces.3-7 Under ultrahigh-vacuum conditions (10-8 Pa), ethene is known to adsorb on the Pt(111) surface via di-σ-coordination below 250 K and to transform to ethylidyne (CH3Ct) at around 270 K.3-7 The ethylidyne further decomposes above 450 K accompanied by desorption of hydrogen. A (2×2) ordered structure is formed at saturation coverage of ethylidyne on the Pt(111) surface (coverage θCCH3 ) 0.25).5 It was also reported that ethene adsorbs with π-coordination on clean Pt(111) at temperature as low as 37 K and that the species is regarded as the physisorbed ethene.8 The π-bonded ethene, on the other hand, has been observed on corrugated surfaces such as Pt(110)9,10 and Pt(210)11 at 93-100 K, and on oxygen or alkali metal modified Pt(111) surfaces.4,12-14 Schematic drawings of π- and di-σ-bonded ethene and ethylidyne are illustrated in Scheme 1. High-pressure reactors combined with ultrahighvacuum (UHV) chambers have been used to carry out kinetic studies and postreaction analysis, and the mechanism of ethene hydrogenation over metal surfaces has been revealed.15,16 Hydrogenation of ethylidyne was found to be slower than that of ethene molecules, and the ethylidyne species is considered not to be the reaction intermediate of catalytic hydrogenation of ethene. Spectroscopic characterization of the surfaces under in situ conditions of ethene hydrogenation is noted recently. The hydrogenation of ethylidyne on Pt(111) at high hydrogen pressures (>104 Pa) was investigated by IRAS.17 In this paper, ethene adsorption under relatively high pressures (ca. 10-3 Pa) studied as one of the elementary (1) Bertolini, J. C.; Massardier, J. The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis vol. 3; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984, p 107. (2) Sheppard, N. Annu. Rev. Phys. Chem. 1988, 39, 589. (3) Somorjai, G. A.; Van Hove, M. A.; Bent, B. E. J. Phys. Chem. 1988, 92, 973. (4) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 117, 685. (5) Kesmodel, L. L.; Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979, 70, 2180. (6) Zaera, F. J. Phys. Chem. 1990, 94, 5090 and references therein. (7) Cremer, P.; Stanners, C.; Niemantsverdriet, J. W.; Shen, Y. R.; Somorjai, G. Surf. Sci., in press. (8) Hugenschmidt, M. B.; Dolle, P.; Jupille, J.; Cassuto, A. J. Vac. Sci. Thechnol. 1989, A7, 3312. (9) Yagasaki, E.; Mazel, R. I. Surf. Sci. 1989, 222, 430. (10) Yagasaki, E.; Backman, A. L.; Chen, B.; Masel. R. I. J. Vac. Sci. Technol. 1990, A8, 2616. (11) Backman, A. L.; Masel. R. I. J. Phys. Chem. 1990, 94, 5300. (12) Cassuto, A.; Mane, M.; Hugenschmidt, M.; Dolle, P.; Jupille, J. Surf. Sci. 1990, 237, 63. (13) Windham, R. G.; Bartram, M. E.; Koel, B. E. J. Phys. Chem. 1988, 92, 2862. (14) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf. Sci. 1988, 193, 387. (15) Zaera, F.; Somorjai, G. A. J. Am. Chem. Soc. 1984, 106, 2288. (16) Rodriguez, J. A.; Goodman, D. W. Surf. Sci. Rep. 1991, 14, 1 and references therein. (17) McDougall, G.; Yates, H. Spec. Publ. R. Soc. Chem. 1992, 114, 109.
steps of ethene hydrogenation by IRAS is described. It has been demonstrated that reversibly adsorbed π-bonded ethene exists on Pt(111) at 112 K. The interaction of π-bonded ethene with di-σ-bonded ethene and ethylidyne is also described. The experimental setup was similar to that described previously.18 Briefly, a JEOL JIR-100 FTIR spectrometer was used at 4 cm-1 resolution with an InSb (>1850 cm-1)/ HgCdTe (
