Controlling Acetylene Adsorption and Reactions on Pt-Sn Catalytic Surfaces

Acetylene reactivity as a function of Sn concentration on Pt catalytic surfaces was studied by comparing adsorption and reactions of regular and deuterated acetylene at 90-1000 K on three surfaces, Pt(111), Pt3Sn/Pt(111), and Pt2Sn/Pt(111), using high-resolution electron energy loss spectroscopy, temperature-programmed desorption, and density functional theory calculations. The strongly adsorbed di-sigma/pi-bonded acetylene species, which dominate on pure Pt, were not detected on the Pt-Sn surfaces. The presence of Sn is also shown to suppress acetylene decomposition and, as a result, to maintain adsorbed acetylene in the molecular form as weakly adsorbed pi- and di-sigma-bonded species. The destabilization of adsorbed acetylene makes associative reactions with the formation of dimers (C-4 hydrocarbons) and trimers (benzene) progressively more energetically favorable with increasing Sn concentration. Acetylene adsorption modes and reactions on Pt catalytic surfaces can, therefore, be controlled with Sn alloying. The concentration of Sn needs to be an optimal level for catalytic activity since all hydrocarbon species bind preferentially only to Pt sites.