CHEMISORPTION OF ETHYLENE, PROPYLENE AND ISOBUTYLENE ON ORDERED SN PT(111) SURFACE ALLOYS/

The adsorption and decomposition of a series of alkenes - ethylene, pr opylene and isobutylene - on Pt(111) and the (2 x 2) and (root 3 x roo t 3)R30 degrees Sn/Pt(111) surface alloys was investigated with temper ature programmed desorption (TPD), low energy electron diffraction (LE ED) and sticking coefficient measurements. In the case of ethylene, th is study extends a previous investigation [M.T. Paffett, S.C. Gebhard, R.G. Windham and B.E. Koel. Surf. Sci. 223 (1989) 449]. We report for the first time a (2 root 3 x 2 root 3)R30 degrees ordered structure f or ethylene chemisorbed on the (2 x 2)Sn/Pt(111) surface alloy. In gen eral for these small alkenes, sticking coefficient and TPD measurement s show little or no effect of alloyed Sn on either the initial stickin g coefficient or the saturation coverage of these molecules on the two Sn/Pt(111) ordered surface alloys when compared with the clean Pt(111 ) surface at 100 K. As was reported previously for ethylene, these Pt- Sn surface alloys have weaker propylene and isobutylene chemisorption bonding and strongly suppressed decomposition. Considering propylene a nd isobutylene as methyl-substituted ethylene reveals a small trend to ward decreasing adsorption energy and on all of these surfaces as the amount of methyl substitution increases. The propylene adsorption ener gy decreases from 17.4 to 14.8 and then to 11.7 kcal mol(-1) as the su bstrate is changed from Pt(111) to the (2 x 2) and the (root 3 x root 3)R30 degrees alloys, as estimated by TPD peak temperatures. The isobu tylene adsorption energy decreases from 17.1 to 14.7 and then to 10.8 kcal mol(-1) for the same series. This implicates that the ''Pt-only'' three-fold hollow sites are very important for strong alkene chemisor ption, since removal of these sites on the (root 3 x root 3)R30 degree s alloy causes a sharper decrease in the adsorption energy than expect ed based upon the changes observed for the (2 x 2) alloy. Even though propylene and isobutylene contain allylic C-H bonds that are much weak er than the vinylic C-H bonds in ethylene, only ca. 5-7% as much propy lene and isobutylene decomposes on the (2 x 2) alloy compared to the P t(111) surface and no decomposition occurs on the (root 3 x root 3)R30 degrees alloy. This result shows the importance of adjacent ''Pt-only '' three-fold hollow sites in alkene decomposition. (C) 1997 Elsevier Science B.V.