Thermal conversion of chemisorbed acetylene to vinylidene and hydrogenation to ethylidyne on Rh{111}: A laser induced desorption study

The kinetics of the conversion of acetylene (HCCH) to vinylidene (CCH2) at the surface of Rh{111} has been studied using laser-induced thermal desorption (LITD). Acetylene is desorbed intact into the gas phase when the surface-is rapidly (similar to 10(10) K s(-1)) heated by a laser pulse, which provides a quantitative means of following the slow thermal conversion to vinylidene at the surface. This conversion is found to occur over the temperature range from 140 to 220 K. Temperature-jump isothermal studies of the conversion process were performed, and the results provide the basis for a one-step mechanism with a single activated complex involving the surface-assisted migration of one H atom across the C-C bond, with product inhibition of the reaction due to an activation energy barrier that increases strongly with the amount of vinylidene formed. With no surface vinylidene present the activation energy for the surface conversion is 39 kJ mol(-1), independent of acetylene coverage. If H-2 or D-2 are postdosed to the surface, ethylidyne (CCH3) is efficiently formed by direct H (D) atom addition to CCH2, and the product CCH3 is found not to inhibit the HCCH to CCH2 conversion.