MECHANISM OF THE CONVERSION OF ETHENE TO ETHYLIDYNE ON RHODIUM(111) - EVIDENCE FOR A VINYLIC INTERMEDIATE

Temperature-programmed static secondary ion mass spectrometry (TPSSIMS) and temperature-programmed desorption (TPD) have been used to study the mechanism of ethene to ethylidyne conversion on Rh(111). Ethene is adsorbed molecularly on Rh(111) at 100 K. During temperature-programmed reaction of small coverages of ethene, hydrogen accumulates on the surface at 140 K, followed by the formation of ethylidyne at 190 K. Scission of a C-H bond being the first step in the decomposition of ethene points to a vinyl (either eta(1)-CHCH2 or mu(3)-eta(2)-CHCH2) intermediate in the reaction to ethylidyne. The temperature at which the vinylic intermediate forms increases with increasing coverage until it coincides with the ethylidyne formation temperature, which remains at 190 K, irrespective of coverage. This behaviour is explained by an ensemble size requirement for C-H scission:the number of vacant ensembles decreases with increasing coverage, thus obstructing C-H cleavage, but increases again during the formation of ethylidyne. Similarities and differences in ethylidyne formation on Rh(111) and Pt(111) are discussed.