Reaction selectivity as a test for catalysis on exposed metal

Abstract

The metal sensitive selectivities of hydrogenation reactions are employed to differentiate between metal catalysis and spillover catalysis. Two catalyst metal specific reactions, dihydrogen addition to 2-hexyne and 1,6-dimethylcyclohexene, are studied on silicon supported palladium and platinum films covered with silica overlayers. At temperatures near 200°C selectivities observed are characteristic of the metal in the underlayer. Since such films have been shown not to contain transition metal in the silica layer, the results suggest that cracks in the silica are present during reaction. In contrast, the selectivity observed at temperatures below 100°C is not that of the metal underlayer and is therefore consistent with a spillover phenomenon. The method represents a general test which may also prove useful to rule out transition metal catalysis in studies invoking a hydrogen spillover mechanism.