Water-Resistant Pt Sites in Hydrophobic Mesopores Effective for Low-Temperature Ethylene Oxidation

The structure–activity relationship of silica-supported Pt catalysts in aerobic oxidation of 50 ppm ethylene was studied at 0 °C with a fixed-bed flow reactor and in situ characterization techniques using Fourier-transform infrared (FTIR) spectroscopy. The activity of all Pt catalysts examined here decreased by water molecules formed during stoichiometric oxidation of ethylene and became stable steadily. A mesoporous silica-supported Pt catalyst improved its steady-state activity after calcination of the support in air at 800 °C, whereas no such effect was observed for a nonporous silica support. CO-pulse titration, H2O adsorption measurements, 29Si MAS NMR, and in situ FTIR along with catalytic activity studies revealed that the activity of the mesoporous silica-supported Pt catalyst is higher than that of nonporous silica-supported ones, despite showing similar hydrophobicity and low Pt dispersion. In situ characterization using CO as a molecular probe indicates that a part of the Pt surface inside hydrophobic mesopores is not involved in the hydrogen-bonding network among physisorbed water molecules and surface SiOH groups even after full hydration of the catalyst surface, and bare Pt sites are expected to work more effectively for ethylene oxidation. Such a "hydrophobic Pt surface" can only be formed on a hydrophobic mesoporous silica support, which is probably because of Pt nanoparticles surrounded by a hydrophobic siloxane network entirely. A unique environment derived from the condensed siloxane network and restricted mesopores contributes largely to the high activity of Pt nanoparticles for low-temperature oxidation of trace ethylene.