A TEMPERATURE-PROGRAMMED DESORPTION STUDY OF THE REACTION OF METHYLACETYLENE ON PT(111) AND SN PT(111) SURFACE ALLOYS/

he adsorption and reaction of methylacetylene (H3CC equivalent to CH) on Pt(111) and the p(2 x 2) and (root 3x root 3)R30 degrees-Sn/Pt(111) surface alloys were investigated with temperature programmed desorpti on, Auger electron spectroscopy and low energy electron diffraction. H ydrogenation of methylacetylene to form propylene is the most favored reaction pathway on all three surfaces accounting for ca 20% of the ad sorbed monolayer. Addition of Sn to the Pt(lll) surface to form these two ordered surface alloys suppresses the decomposition of methylacety lene to surface carbon. The alloy surfaces also greatly increase the a mount of reversibly adsorbed methylacetylene, from none on Pt(lll) to 60% of the adsorbed layer on the (root 3 x root 3)R30 degrees - Sn/Pt( 111) surface alloy. Methylacetylene reaction also leads to a small amo unt of desorption of benzene, along with butane, butene, isobutylene a nd ethylene. There is some difference in the yield of these other reac tion products depending the Sn concentration, with the (2 x 2)-Sn/Pt(1 11) surface alloy having the highest selectivity for these. Despite pr evious experiments showing cyclotrimerization of acetylene to form ben zene on the Pt-Sn surface alloys, the analogous reaction of methylacet ylene on the alloy surfaces was not observed, that is, cyclotrimerizat ion of methylacetylene to form trimethylbenzene. It is proposed that t his and the high yield of propylene is due to facile dehydrogenation o f methylacetylene because of the relatively weak H-CH2CCH bond compare d to acetylene. The desorption of several C-4 hydrocarbon products at low (<170 K) temperature indicates that some minor pathway involving C -C band breaking is possible on these surfaces. (C) 1998 Elsevier Scie nce B.V. All rights reserved.