Structural and Catalytic Features of Metal Nanoparticles Encapsulated in a Hollow Carbon Sphere

Abstract

A ligand-free metal nanoparticle encapsulated in a porous hollow carbon cage was prepared by surface coverage of the metal nanoparticle core with a silica layer and a carbon-filled mesoporous silica overlayer followed by chemical etching of siliceous components. A detailed investigation by using microscopic, spectroscopic, and gas-adsorption techniques clarified structural characteristics of the unique core-shell composite. The core–shell composite based on Pt metal nanoparticle core was found to work as a highly active catalyst for liquid-phase hydrogenation of nitrobenzene and olefins, as well as reductive alkylation of aniline with acetone. For the Rh metal-based composite, we demonstrated the synergetic catalytic function for aromatic and heterocyclic compounds in water, which cannot be achieved by conventional Rh nanoparticles stabilized with organic agents. Moreover, the core-hollow shell composite based on Pd metal nanoparticle core was proved to have high catalytic activity and regeneration ability for aerobic oxidation reaction of alcohols. These specific catalytic functions are derived from the structural features, i.e., the physically unstable “naked” metal nanoparticles are protected by the porous carbon shell which also provides nanosized reaction space at the surface of the metal nanoparticle core.