ON THE MECHANISM FOR THE HYDROGENATION OF OLEFINS ON TRANSITION-METALSURFACES - THE CHEMISTRY OF ETHYLENE ON PT(111)

The chemistry of ethylene on Pt(111) single-crystal surfaces was chose n here to represent olefin hydrogenation reactions on transition-metal catalysts. In vaccum the hydrogenation of ethylene was proven to proc eed via a stepwise incorporation of hydrogen atoms on the clean surfac e, but under high pressures the catalyst was shown to become covered w ith carbonaceous deposits soon after exposure to the reactant gases. T he species that compose the strongly bonded hydrocarbon fragments were identified as ethylidyne, a C-2 moiety where one carbon atom sits on a 3-fold hollow site on the surface and is single-bonded to a methyl g roup directly above it. In order to better understand the role of the ethylidyne layer in the hydrogenation reaction, the mechanism of the c onversion of ethylene to ethylidyne was studied in some detail. Even t hough 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 or ethylidene intermediates. Vinyl moieties were shown to undergo a series of reactions and to form a family of intermediates, including ethylene, before ultimately tran sforming into ethylidyne. The involvement of ethylidene in any simple two-step mechanism was shown to also be inconsistent with results from kinetic studies using trideuteroethylene. Finally, the participation of ethyl groups was discarded on the grounds that they decompose readi ly via a beta-hydride elimination step into ethylene. The participatio n of any of those intermediates in the mechanism for ethylene conversi on could nevertheless be possible if they were to reach a fast pre-equ ilibrium with the chemisorbed ethylene and to then decompose slowly to ethylidyne. Unfortunately, further testing of this hypothesis is hind ered by the added complications due to the non-first-order kinetics of the ethylidyne formation and the competition of that reaction with ot her H/D exchange and hydrogenation steps. It is also not clear yet wha t role these ethylidyne moieties may play in the mechanism of the high -pressure catalytic reactions, but several options are discussed here, including the possibility of them acting either as hydrogen transfer agents or simply as site blockers on the surface.