Heterogeneous Catalytic Hydrogenation: Is Double Bond/Surface Coordination Necessary?

The hydrogenation of unsaturated molecules is a key reaction in heterogeneous catalysis, with very broad applications in chemistry. The accepted scheme today is that the double bond is first chemisorbed on the catalyst’s surface, such as platinum, and that hydrogen is transferred on the adsorbed molecule by a concerted mechanism through a triangular three-member ring transition state. In this Letter, we show that an alternative mechanism is possible, where the double bond is not coordinated with the surface but approaches above the H atom leading to a six-member ring transition state. Such a mechanism is demonstrated from first-principle calculations for the case of butadiene hydrogenation on platinum and on a Pt surface modified by alloying with Sn. The hydrogenation elementary step on an uncoordinated C═C bond of butadiene shows a low activation barrier (∼20 kJ·mol −1 ). The cases where this pathway is globally favorable, with a small energy cost for C═C bond decoordination, are finally discussed.