Origin of the High Activity of Porous Carbon-Coated Platinum Nanoparticles for Aerobic Oxidation of Alcohols

Abstract

In an attempt to clarify the origin of the high activity and durability for aerobic oxidation of alcohols over a platinum (Pt)−carbon composite, i.e., Pt nanoparticles embedded in microporous carbon (nPt@hC), the catalytic reaction mechanism and microstructure of Pt nanoparticles were investigated in detail. By means of kinetic analyses, catalytic oxidation on nPt@hC was found to proceed through the formation of Pt-alcoholates, the β-hydride elimination to form Pt-hydride species (Pt−H), and oxidation of Pt−H with molecular oxygen. It was also revealed that the β-hydride elimination step was a rate-determining step in this reaction. These findings and results of structural studies indicate that the achievement of high catalytic activity on nPt@hC is due to stabilization of its transition state of a positively charged carbocationic component by the electron-rich carbon matrix surrounding Pt nanoparticles, leading to lowering activation energy. Moreover, detailed investigation of the surface characteristics of Pt nanoparticles in nPt@hC after catalytic reactions by using various analytical methods revealed that the durability of nPt@hC for aerobic oxidation of alcohols is due to the suppression of aggregation of Pt nanoparticles and prevention of chemical poisoning of Pt surfaces.