A temperature programmed desorption study of the reaction of methylacetylene on Pt(111) and Sn/Pt(111) surface alloys

The adsorption and reaction of methylacetylene (H₃CC≡CH) on Pt(111) and the p(2 x 2) and (√3 × √3)R30^○-Sn/Pt(111) surface alloys were investigated with temperature programmed desorption, Auger electron spectroscopy and low energy electron diffraction. Hydrogenation of methylacetylene to form propylene is the most favored reaction pathway on all three surfaces accounting for ca 20% of the adsorbed monolayer. Addition of Sn to the Pt(111) surface to form these two ordered surface alloys suppresses the decomposition of methylacetylene to surface carbon. The alloy surfaces also greatly increase the amount of reversibly adsorbed methylacetylene, from none on Pt(111) to 60% of the adsorbed layer on the (√3 × √3)R30^○-Sn/Pt(111) surface alloy. Methylacetylene reaction also leads to a small amount of desorption of benzene, along with butane, butene, isobutylene and ethylene. There is some difference in the yield of these other reaction products depending the Sn concentration, with the (2 x 2)-Sn/Pt(111) surface alloy having the highest selectivity for these. Despite previous experiments showing cyclotrimerization of acetylene to form benzene on the Pt-Sn surface alloys, the analogous reaction of methylacetylene on the alloy surfaces was not observed, that is, cyclotrimerization of methylacetylene to form trimethylbenzene. It is proposed that this and the high yield of propylene is due to facile dehydrogenation of methylacetylene because of the relatively weak H-CH₂CCH bond compared to acetylene. The desorption of several C₄ hydrocarbon products at low (<170 K) temperature indicates that some minor pathway involving C-C bond breaking is possible on these surfaces.