Chemisorption of ethylene, propylene and isobutylene on ordered Sn/Pt(111) surface alloys

The adsorption and decomposition of a series of alkenes - ethylene, propylene and isobutylene - on Pt(111) and the (2 x 2) and (root 3 x root 3)R30 degrees Sn/Pt(111) surface alloys was investigated with temperature programmed desorption (TPD), low energy electron diffraction (LEED) and sticking coefficient measurements. In the case of ethylene, this 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 for ethylene chemisorbed on the (2 x 2)Sn/Pt(111) surface alloy. In general for these small alkenes, sticking coefficient and TPD measurements show little or no effect of alloyed Sn on either the initial sticking 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 and isobutylene as methyl-substituted ethylene reveals a small trend toward decreasing adsorption energy and on all of these surfaces as the amount of methyl substitution increases. The propylene adsorption energy decreases from 17.4 to 14.8 and then to 11.7 kcal mol(-1) as the substrate 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 isobutylene 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 chemisorption, since removal of these sites on the (root 3 x root 3)R30 degrees alloy causes a sharper decrease in the adsorption energy than expected based upon the changes observed for the (2 x 2) alloy. Even though propylene and isobutylene contain allylic C-H bonds that are much weaker than the vinylic C-H bonds in ethylene, only ca. 5-7% as much propylene and isobutylene decomposes on the (2 x 2) alloy compared to the Pt(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.