HYDROGENATION OF ETHYLENE BY ZINC OXIDE .I. ROLE OF SLOW HYDROGEN CHEMISORPTION

An experimental basis is provided for dividing hydrogen adsorption on zinc oxide into two types. Type I is rapid and reversible; type II is irreversible and occurs rapidly initially but slowly in the latter stages. Type I hydrogen is responsible for the observed OH and ZnH bands in the infrared and the principal source of hydrogen for the hydrogenation of ethylene. Type II does not participate in the hydrogenation of ethylene at room temperature but modifies the catalyst and enhances its activity. Neither the hydrogen adsorption nor ethylene adsorption is markedly affected by pretreating the catalyst with oxygen. The catalyst is poisoned for ethylene hydrogenation by exposure of the catalyst to oxygen provided chemisorbed hydrogen is present before the pretreatment. If care is taken to exclude chemisorbed hydrogen, the effect of oxygen pretreatment is relatively slight; hence, water (or its precursor) appears to be the poison. It is suggested that nonstoichiometry is not required for the development of active sites for hydrogenation. Instead, it is proposed that the strained sites, perhaps formed by dehydration, as suggested by Weller and Voltz, are the active sites.