EPR study of ceria-silica and ceria-alumina catalysts: Localization of superoxide radical anions

Oxygen adsorption experiments were performed on evacuated and prereduced CeO2/SiO2 and CeO2/Al2O3 catalysts with and without platinum. Considerable amounts of the superoxide radical ions were stabilized on all the samples. Signal parameters suggest Ce4+-O2− positioning for all detectable superoxide species. Physisorbed oxygen broadens O2− signal beyond detection for all the alumina-based samples, while the same procedure for all the silica-based samples did not change signal shape of O2− species. Detectable O2− species are localized in the bulk of ceria and the nature of support (silica or alumina) determines the number of oxygen vacancies and the rate of electron transfer. XRD data suggest that for alumina-based samples small and/or thin islands of ceria dominate, while comparatively large ceria particles are stabilized on the surface of silica-based samples with the same ceria content. Average size of ceria crystallites is still not determining factor and cannot account for the observed differences. Higher concentrations of paramagnetic species may be stabilized on alumina-based samples and thus, sensor-like behavior towards gaseous oxygen at room temperature was detected for them-oxygen admission reversibly changes superoxide lineshape. For silica samples, only minor changes of O2− lineshapes were typical upon the change of the partial pressure of oxygen at ambient and low temperatures. Addition of platinum has little effect on parameters of the O2− signal, except an enhancement of the superoxide decay in the reducing media. Possible site for O2− stabilization inside the lattice of CeO2 was proposed and relevance of the observed effects to the redox catalysis discussed.