Paper SS+AS+EN-WeA3
Removal of Surface Carbon from Pt(111) by Hydrogenation via an Ethylidyne Intermediate

Wednesday, October 21, 2015, 3:00 pm, Room 113

Transition metals that are used to catalyze reactions of hydrocarbons are often deactivated by the deposition of unreactive carbon on the catalyst surface. The structure and properties of the deposited carbon are often poorly defined. We have investigated the reactivity of carbon deposited onto a Pt(111) surface through exposure to acetylene at 750 K. At this temperature the acetylene is completely dehydrogenated leaving only carbon the surface. Earlier work had shown that the carbon deposited in this way largely consists of C₂ molecules. We have used reflection absorption infrared spectroscopy (RAIRS) to characterize the reactivity of the deposited carbon under ambient pressures of H₂(g) up to 10 torr. The results show that C₂ can be hydrogenated to ethylidyne (CCH₃) and that the ethylidyne is slowly hydrogenated to ethane, which desorbs thus removing carbon from the surface. The maximum coverage of the C₂ molecules can be deduced from comparison with the peak areas measured with RAIRS for ethylidyne formed from ethylene exposure, which is known to give an ethylidyne coverage of 0.25 monolayer. Auger electron spectroscopy confirms that surface carbon is removed by hydrogenation under these conditions. In separate experiments based on comparisons of s- and p-polarized RAIR spectra in which both surface and gas phase species can be simultaneously monitored, we have shown that surface ethylidyne is a spectator species as gas phase acetylene is converted first to gas phase ethylene and then to gas phase ethane. Although ethylidyne is a spectator species in the hydrogenation of ethylene and acetylene to ethane over Pt(111), in the case of C₂ hydrogenation, ethylidyne plays the role of a reaction intermediate.