EPR monitoring of redox processes in transition metal oxide catalysts.

Transition metal oxides (TMO) are widely used as catalysts of a number of catalytic reactions, mainly of redox nature. This is due to the unique ability of transition metal ions to change easily their oxidation state, coordination, and/or arrangement of the coordination polyhedra. At present, the only abundant, direct information available about paramagnetic centers formed during interaction of the reagents with the TMO surface stems from EPR studies. This technique is very sensitive and allows detection of very small concentration of paramagnetic sites. Moreover, analysis of the EPR spectrum offers a direct approach to the determination of symmetry and electronic structure of the centers concerned. Such information is of particular value for understanding the operating reduction/reoxidation mechanisms, with immediate implications for solid-state structural studies, catalysis and so forth. The present work shows two aspects of the use of EPR for monitoring of redox processes with the participation of TMO: a) investigation of the reaction kinetics and b) description of the individual active sites involved in the catalytically important redox processes. The presented examples include: a) kinetic descriptions (including mathematical models) of reduction and oxidation processes in the vanadia-molybdena catalysts occurring upon interaction with propene and oxygen, b) identification of paramagnetic centers formed in the reduced molybdena, c) analysis of the butene interaction with a nickel catalyst, and d) description of the radical oxygen species on the surface of manganese-containing catalysts.