Mechanism and kinetics of the selective hydrogenation of ethyne and ethene

A review of the kinetics and mechanism for the selective hydrogenation of ethyne and ethene on palladium catalysts is presented. The progress made in the last fifteen years is mainly discussed. It has become clear that the classical view, where the selectivity of the reaction was believed to be due to the thermodynamic factor is an over-simplification. Currently, it is generally assumed that at least two different sites are active during the selective hydrogenation, one of these might possibly involve the support. Ethene hydrogenation also occurs in the presence of high ethyne concentrations, which cannot be explained by the classical theory. Besides the two main hydrogenation reactions and the oligomerisation, there exists a direct route from ethyne to ethane, which, however, is only of minor importance. Possibly due to the rather complex nature of the system, there have been relatively few kinetic studies presenting practical rate expressions.