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, HCCH, to vinylidene, CCH2, 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 CC 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 CH bond cleavage of acetylene with the formation of adsorbed CCH and H species. In the absence of three-fold Pt sites, the reaction mechanism changes, and reactions of H transfer and CH bond cleavage are suppressed.