Metal–Support Interaction: Group VIII Metals and Reducible Oxides

Abstract

This chapter discusses the interaction of small metal particles with reducible oxide supports, and focuses on Group VIII (or Group 8-10) metals supported on TiO2. The strong suppression in catalytic activity and chemisorption capacity observed on TiO2-supported catalysts after reduction at high temperatures can be, at least partially, explained by a geometric blocking of sites by TiO2 species. The dominant mechanism of formation of TiO2 species on metal particles is migration during the high-temperature reduction, but some deposition of metal precursors during preparation may also occur. This picture finds an analog in the structure of the bimetallic clusters observed in Group VIII-Group Ib catalysts, in the sense that the structure sensitive reactions, e.g., alkane hydrogenolysis, are most affected whereas only modest effects are observed for structureinsensitive reactions, e.g., hydrogenation-dehydrogenation. The migration of reduced species from the support is accompanied by the formation of metal-Ti bonds, which provide the thermodynamic driving force for the migration. The metal-support bonding is not identical to that in the intermetallic compounds because of the associated oxygen that imparts cationic character to the Ti and Group VIII metal partner; in other words, it may be analogous to the Group VIII-Ti bonding that persists on intermetallic compounds surfaces after oxygen chemisorption.