Hydrogenation of acetylene over supported metal catalysts. Part 2.—[14C]tracer study of deactivation phenomena

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 C₂H_x 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.