Improvement in Catalytic Performance of Carbon Nanotube-supported Metal Nanoparticles by Coverage with Silica Layers

Abstract

Carbon nanotube (CNT)-based composites have attracted a great deal of interest because of their potential application in catalysts. These composites are frequently prepared by the addition of metals or metal oxides to the outer surface of CNT. However, the surface modification of CNT is generally a challenging task, because their surface is chemically inert. In addition, metal or metal oxide particles on CNT are easily aggregated when these are used as catalysts. Recently, we developed CNT-supported metal nanoparticle catalysts covered with silica layers a few nanometers thick. The catalysts show high catalytic activity in spite of coverage of metal nanoparticles with silica layers. In addition, the silica layers which are wrapped around metal particles prevent the sintering of metal particles as well as the detachment of metal particles under severe reaction conditions. In addition, the CNT-supported metal particles covered with silica can be applied to catalysts for the oxygen reduction reaction at cathode in polymer electrolyte fuel cells (PEFCs). Pt catalysts supported on carbon black which are conventional catalysts in the state of the art PEFCs, are seriously deactivated under PEFC cathode conditions such as high potential, low pH and oxygen atmosphere due to the aggregation of Pt metal and the dissolution and re-deposition of Pt metal. We demonstrated that the silica-coated Pt/CNT is electrochemically active in spite of coverage of Pt metal with silica insulator. In addition, the coverage of Pt metal particles with silica layers inhibits the growth of Pt metal particles in size under cathode conditions in PEFCs. The silica-coated Pt/CNT thus shows high durability for the oxygen reduction reaction. The coverage with silica layers also prevents the dissolution of metals other than Pt under cathode conditions. Therefore, our silica-coating method is promising for the development of highly durable non-Pt metal catalysts for use in PEFC cathodes.