Abstract:
In the hydrogenation of acetylene over Pd + SiO2, Rh + SiO2, Ir + SiO2 and Pd + Al2O3, the activities of the catalysts progressively decrease to a steady state constant activity. This deactivation is irreversible at 298 K and can only be effected by acetylene + hydrogen reaction mixtures; acetylene alone produces no significant deactivation. The selectivity and overall reaction orders are independent of the catalytic activity. Adsorptions of [14C]C2H2 and [14C]C2H4 show that the deactivation is associated with the progressive build up of permanently retained acetylenic species on the catalyst surface. Three types of adsorbed acetylene are recognised. The results are interpreted in terms of the hydrogenation reaction occurring by a hydrogen-transfer mechanism between a dissociatively adsorbed C2Hx species and associatively adsorbed acetylene, which forms an overlayer on the dissociatively adsorbed acetylene. The permanent retention of acetylenic species may be due to surface polymer formation.
