O− radical anions on oxide catalysts: Formation, properties, and reactions

Abstract

A systematic in situ EPR study of processes yielding O− radical anions on the surface of oxide dielectrics (MgO, CaO), semiconductors (ZnO, TiO2), supported systems (V/SiO2), and zeolite FeZSM-5 is reported. Methodological approaches to the study of O-radical anions are considered for the cases in which these species are directly undetectable by EPR. Particular attention is focused on the development of methods of investigation of so-called α-oxygen on the FeZSM-5 surface, which is an O− radical anion stabilized on the paramagnetic ion Fe3+. The reactions involving α-oxygen and the analogous reactions known for O-radical anions stabilized on the oxide surface are demonstrated to occur in similar ways. The photostimulated formation of spatially separated electron and hole centers on the surface of oxide systems is most likely due not to charge separation, but to the spatial separation of the radicals resulting from the homolytic photodissociation of chemisorbed water. A scheme is suggested for this process on the partially hydroxylated MgO surface.