The influence of temperature and structure of palladium surfaces on the adsorption of ethylene and hydrogenation of adsorption complexes by molecular and atomic hydrogen

Abstract

A measurement of the work-function change, combined with the volumetric method and gas product analysis were used for investigation of the influence of temperature and palladium surface structure on the adsorption of ethylene and hydrogenation of its adsorption complexes by molecular and atomic hydrogen. It was verified that on palladium the highest activity for the C-H bond splitting of hydrocarbons is found on the adsorption sites corresponding to low-coordination surface atoms. The activation energy of the C-H bond dissociation is very low (ED ##i 2 kJ/mol); as a result, in the early stages of surface coverage, hydrogen appears on the surface - together with the formation of surface adsorption complexes. The presence of hydrogen is the main reason for the non-linearity of the work-function changes, observed for ethylene adsorption in the low-coverage region. Stable dehydrogenated surface particles are formed also by self-hydrogenation of ethylene in the higher-coverage region. With increasing temperature, the extent of dehydrogenation of the adsorbed complexes also increases - while the influence of molecular hydrogen on the work-function of the surface with pre-adsorbed ethylene becomes less significant. The interaction of atomic hydrogen with ethylene adsorption-complexes caused in all cases irreversible changes of the surface work-function.