ESR studies of O2- adsorbed on silica gel: Photoformation and rotational dynamics

Abstract

An ESR investigation is reported of oxygen radicals formed by UV radiation of silica gel surfaces in the presence of O2. The species O 2- and O3- are identified. The O2- radical shows a temperature dependent signal shape between -196 and 25 C which is interpreted in terms of Brownian rotational surface diffusion about an axis perpendicular to the O-O bond and to the surface. Correlation times for surface diffusion are estimated from comparison of observed and computer simulated spectra, and an activation energy for surface diffusion of 2.4+-0.8 kJ mol-1 calculated. Comparison with previously reported spectra for W/SiO2 and Ti/SiO2 surfaces indicates that the mobile O2- species formed in all three systems must be located on the SiO2 support, and that the mobility is due to the absence of strong interaction with transition metal sites.