Theoretical study on the catalytic activity of palladium for the hydrogenation of acetylene

Abstract

The catalytic activity of palladium for the hydrogenation reaction of acetylene is studied by the ab initio quantum chemical method with the use of the cluster model. Based on our previous theoretical result that the hydrogen molecule is easily adsorbed dissociatively on the Pd2 fragment, we investigate the reactivity of this absorbed hydrogen for acetylene in two modes of the reaction, namely the Eley-Rideal (ER) mode and the Langmuir-Hinshelwood (LH) mode. Full catalytic reaction cycles are studied for both modes. From the study of the ER mode, it is shown that these hydrogens are almost as reactive as free atomic hydrogens in spite of the existence of the Pd-H bonds binding these hydrogens on the surface. The barrier of the reaction is calculated to be 32 kcal/mol. The electronic origin of the catalytic activity of palladium is shown to be the electron correlation and the participation of the dangling bond of the palladium surface. The nature of the reaction is clarified by analyzing the force and electron density along the reaction process. From the study of the LH mode, which is more realistic as a model of the catalytic reaction than the ER mode, it is shown that the two-step mechanism involving vinyl radical as a surface reaction intermediate is more preferable than the one-step simultaneous mechanism. The barriers of these catalytic cycles of the reactions are calculated to be 7 and 28 kcal/mol, respectively. The mechanism of the catalytic selectivity that acetylene is selectively converted to ethylene (not to ethane) in the ethylene-acetylene mixture is clarified.