Functionalization of Carbon Nanotube-Supported Precious Metal Catalysts by Coverage with Metal Oxide Layers

Precious metals such as Pd, Pt, and Rh have been utilized as the active components of many catalysts. Owing to the high cost of these metals, their loadings in catalysts must be reduced. However, reducing loadings without compromising catalytic performance is difficult. Precious metal catalysts are inevitably deactivated by severe reaction conditions (e.g., high temperature and pressure), which tend to increase of the size of the precious metal particles. Here, we review our recent work on the functionalization of supported precious metal catalysts by coverage with layers of metal oxides such as silica. Coverage with silica layers a few nanometers thick prevents aggregation of the metal particles as well as their detachment from the supports under severe reaction conditions. For example, coverage of supported Pt and Pd electrocatalysts in polymer electrolyte fuel cells with silica layers effectively improves catalyst durability by inhibiting not only particle aggregation but also the diffusion of dissolved metal species out of the catalysts. Coverage of supported precious metal photocatalysts with titania layers also enhances catalytic activity.