KINETICS AND MECHANISM FOR THE H D EXCHANGE BETWEEN ETHYLENE AND DEUTERIUM OVER PT(111)/

The H/D exchange between ethylene and hydrogen (or deuterium) on Pt(11 1) surfaces was monitored by laser-induced thermal desorption in combi nation with Fourier-transform mass spectrometry (LITD/FTMS) and by ref lection-absorption infrared spectroscopy (RAIRS). In the case of coads orbed C2D4 + H-2 the LITD experiments show that the H/D exchange react ion takes place at temperatures as low as 215 K, well below those need ed for the decomposition of ethylene to ethylidyne. It was also found that exchange on adsorbed C2D4 is significantly easier with H-2 than v iith C2H4, indicating that this H/D substitution requires surface hydr ogen but not the dissociation of ethylene, and suggesting that ethyl m oieties are the most probable intermediates for the reaction. It was a lso determined that the dosing order of the reactants influences the k inetics of the reaction, which means that specific surface ensembles a re required in the exchange; both the rate and the degree of H/D subst itution in ethylene are increased by dosing hydrogen (deuterium) befor ehand. In addition to the LITD experiments, the rate of formation of e thylidyne that takes place above 250 K was also monitored with RAIRS. It was shown that, when coadsorbed with deuterium, the extent of deute rium substitution in the resulting ethylidyne is largely determined by H/D exchange between the original ethylene molecules and surface hydr ogen, and that direct H/D exchange between ethylidyne and surface hydr ogen is measurable only at low ethylidyne coverages and higher tempera tures. Finally, it was established that the rate of ethylidyne formati on does not depend on the coverage of hydrogen on the surface, a resul t that points to an ethylidene intermediate in that reaction. The data are discussed on the basis of a mechanism in which the H/D exchange a nd ethylidyne formation are parallel processes which involve different surface species. (C) 1998 Academic Press.