Hydrogenation of 3-methyl-crotonaldehyde on the Pt(553) stepped surface: Influence of the structure and of preadsorbed tin

The hydrogenation of 3-methyl-crotonaldehyde was studied, in the gas phase, on a well-defined Pt(553) or Pt(S)-[S(111)x(111)] stepped surface and the results are compared with those of similar experiments on Pt(111). The selectivity is governed mainly by structural factors. Low-coordination step atoms create favorable hydrogenation sites for the production of saturated aldehyde, whereas (111) flat terraces lead to the formation of unsaturated alcohol. The influence of a preadsorbed submonolayer of tin was investigated on Pt(11) and Pt(553). The effect of this metallic additive depends on its local concentration and arrangement. Tin drastically changes the selectivity of Pt(553). At low coverage, tin, located in the vicinity of the steps, changes the selectivity to the benefit of the saturated alcohol. Al higher coverage (theta > 0.3), tin grows in islands on the terraces and the expected improvement in selectivity toward unsaturated alcohol was observed, suggesting a combination of electronic and geometric effects. (C) 1996 Academic Press, Inc.