Reactions of acetylene during hydrogenation on Pd black catalyst

Abstract

Deuteration of trace amounts of acetylene in the presence and absence of ethylene has been studied on Pd black catalyst in a continuous flow reactor. At nonsteady state the Pd black catalysts showed very low selectivities, whereas higher selectivities were obtained at steady state. Pretreatment with air and H 2 yielded a catalyst with higher selectivity than the same catalyst treated only with H 2. The order in acetylene pressure passed from positive to negative with increasing p C 2 H 2 both in the presence and absence of ethylene. The deuterium distribution of the ethylene formed showed that hydrogen originating from acetylene also took part in the hydrogenation reaction; thus, part of the acetylene dissociatively adsorbed on the surface. Using [ 14C]C 2H 2 it was demonstrated that at low acetylene partial pressure the main route of acetylene hydrogenation was the formation of ethane, ethylene, and C 4 hydrocarbons. Using [ 14C]C 2H 4 it was also shown that at certain level of acetylene partial pressure the formation of ethane from ethylene was completely terminated. The observed kinetic data are discussed and it is suggested that different surface species such as dissociatively and associatively adsorbed acetylene as well as ethylidyne species are present on the palladium surface. Experimental conditions, such as catalyst pretreatment and partial pressure of the reactants, can influence the relative concentrations of these surface species and can also change the routes of surface reactions, leading to different reaction products from acetylene. The reaction mechanism proposed is discussed based on different organometallic and spectroscopic evidence published recently.