Role of hydrogen in olefin isomerization and hydrogenation: a molecular beam study on Pd model supported catalysts.

The role of surface, and subsurface hydrogen species in olefin cis-trans isomerization and hydrogenation over a model Pd/Fe(3)O(4)/Pt(111) catalyst was investigated by pulsed molecular beam experiments and infrared reflection-absorption spectroscopy. We show that non-equivalent hydrogen species are involved in the two reaction pathways: whereas cis-trans isomerization proceeds with the surface hydrogen species, the presence of hydrogen absorbed in the subsurface region of Pd particles is required for the hydrogenation pathway. The activity and selectivity toward both reaction channels was found to significantly change on Pd particles when they are modified with strongly dehydrogenated carbonaceous deposits. Sustained hydrogenation activity was observed only on C-precovered particles, whereas sustained cis-trans isomerization proceeds on both C-free and C-containing catalyst. We discuss the possible microscopic origins of this effect.