Geometric requirements for hydrocarbon catalytic sites on platinum surfaces.

Vibrational spectroscopic measurements and density functional calculations were used to identify a preferential catalytic mechanism for the transformation of acetylene, HC-CH, to vinylidene, C-CH2, on surfaces of Pt-Sn ordered alloys. In this mechanism, two adjacent Pt atoms adsorb an acetylene molecule and a third neighboring Pt atom is required for stabilizing the reacting H atom during the transformation. Therefore, unlike a direct H shift along the C-C bond in organometallic compounds with a single transition-metal atom, this mechanism has a geometric site requirement of three adjacent Pt atoms in the form of a three-fold site. The same geometric site requirement is identified for preferential C-H bond cleavage of acetylene with the formation of adsorbed C-CH and H species. In the absence of three-fold Pt sites, the reaction mechanism changes, and reactions of H transfer and C-H bond cleavage are suppressed.