Search for reactive intermediates in catalytic oxidation with hydrogen peroxide over amorphous niobium(V) and tantalum(V) oxides

Abstract

The reactive oxygen intermediates generated on surface of amorphous Nb2O5 and Ta2O5 upon interaction with aqueous H2O2 were identified. The role of dehydroxylation of the surface, pH of H2O2 solution, and the presence or absence of oxygen in gas phase were studied in depth by UV–Vis, FTIR, Raman, and EPR techniques. The study revealed high ability of the amorphous Nb2O5 to form peroxo, superoxo, and radical hydroxyl species upon contact with hydrogen peroxide. This process depends on the level of surface hydroxylation which is lower for amorphous Ta2O5 than amorphous Nb2O5. The relationship between the formation of superoxo and peroxo species and pH of the H2O2/H2O solution was proved. Superoxo species interact with the excess of H2O2 towards hydroxyl radicals, which are the most active species in catalytic oxidation. The role of pH was evident in the oxidation of glycerol. NaOH and link of superoxo species to niobium enhance the reaction rate by increase of hydroxyl radicals number. The hydroxyl radicals are also the active species in the formation of hydroxylated cyclohexene and finally cyclohexenediol in cyclohexene oxidation.