On the Mechanism for the Hydrogenation of Olefins on Transition-Metal Surfaces: The Chemistry of Ethylene on Pt(111)

Abstract

The chemistry of ethylene on Pt(111) single-crystal surfaces was chosen here to represent olefin hydrogenation reactions on transition-metal catalysts. In vaccum the hydrogenation of ethylene was proven to proceed via a stepwise incorporation of hydrogen atoms on the clean surface, but under high pressures the catalyst was shown to become covered with carbonaceous deposits soon after exposure to the reactant gases. The species that compose the strongly bonded hydrocarbon fragments were identified as ethylidyne, a C2 moiety where one carbon atom sits on a 3-fold hollow site on the surface and is single-bonded to a methyl group directly above it. In order to better understand the role of the ethylidyne layer in the hydrogenation reaction, the mechanism of the conversion of ethylene to ethylidyne was studied in some detail. Even though no simple scheme has been found to explain this surface process so far, our studies have led to the rejection of previously suggested two-step pathways involving either vinyl ...