Thermal chemistry of C3 allyl groups on Pt(111)

The thermal chemistry of allyl iodide and of allyl bromide adsorbed on Pt(111) was investigated by temperature-programmed desorption (TPD) and reflection-absorption infrared (RAIRS) spectroscopies. On the clean Pt(111) surface, both allyls were found to bond to the surface via the halogen atom in a gauche conformation and with the C-C-C plane close to parallel to the surface. In the presence of surface hydrogen, allyl iodide was found to tilt backward such that the C-C-C plane adopts a more vertical orientation on the surface; in this more upright geometry, both cis and gauche isomers were identified. With both molecules the facile thermally activated cleavage of the carbon-halogen bond results in the formation of C-3 allylic species on the surface. A eta (1) allylic moiety was identified by RAIRS in the high-exposure regime of allyl iodide after annealing to 200 K, and the formation of a second eta (3) allylic intermediate was inferred from TPD experiments with coadsorbed hydrogen and deuterium. At low surface concentrations, allyl moieties mainly dehydrogenate, ultimately to H-2(g) and C-ads At coverages approaching monolayer, on the other hand, some of the allyls hydrogenate to propene around 310 K via a process which relies on the decomposition of a few of the same surface allyl moieties as the source of hydrogen. Hydrogenation to propene can be enhanced and induced at much lower temperatures by predosing hydrogen on the surface. This propene is seen to desorb in two temperature desorption peaks. The first, at 185 K, is likely to come from direct hydrogenation of the eta (1) allyl intermediate, and does not involve any appreciable H-D exchange. The higher temperature desorption regime is observed around 250 K, and yields not only propene but also significant amounts of propane. In this case, a eta (3) allylic species is believed to hydrogenate to di-sigma bonded propane, and to subsequently undergo multiple propene-propyl-propene surface interconversions to produce extensive H-D exchanged products.