ETHYLENE DIMERIZATION OVER A MODEL SN PT(111) CATALYST/

The dimerization reaction of ethylene was studied over Pt(111) and (sq uare-root3 x square-root3)R30-degrees-Sn/Pt(111) model catalysts at mo derate pressures (20-100 Torr). The catalyst surfaces were prepared an d characterized in a UHV surface analysis system and moderate pressure catalytic reactions were conducted with an attached batch reactor. Th e overall catalytic activity of the (square-root3 x square-root3)R30-d egrees-Sn/Pt(111) surface alloy for C4 products was slightly higher th an that at Pt(111). In addition to the dimerization reaction, hydrogen olysis of ethylene to propane and methane was also observed, with the (square-root3 x square-root3)R30-degrees-Sn/Pt(111) surface alloy less active than Pt(111). Among the C4 products, butenes and n-butane were the major components. Carbon buildup was observed to be significant a bove 500 K with the (square-root3 x square-root3)R30-degrees-Sn/Pt(111 ) surface alloy much more resistant than Pt(111). The dimerization of ethylene was not eliminated by the presence of surface carbonaceous de posits and even at significant surface coverages of carbon the model c atalysts exhibited significant activitis. The results are discussed in terms of the surface chemistry of ethylene and the previously reporte d catalytic reactions of acetylene trimerization and n-butane hydrogen olysis at these surfaces.