Formation mechanism of O2- radical anions in the adsorption of NO + O2 and NO2 + O2 mixtures on ZrO2 according to EPR and TPD data

The effects of various factors on the formation of 02 radical anions in the adsorption of an NO + O-2 or NO2 + O-2 Mixture on ZrO2 were studied. It was found that the thermal stability of the O-2(-) species depends on the composition of the adsorbed gas. It was suggested that nitrogen oxide complexes on ZrO2 centers are responsible for the formation of O-2(-). These centers are formed upon the treatment of the oxide in a vacuum; however, they are different from both coordinatively unsaturated Zr4+ cations (NO adsorption centers at 77 K) and Zr4+-O--O--Zr4+ centers, at which O-2(-) are formed because of the adsorption of H-2 + O-2. Based on the experimental data, the mechanism of O-2(-) formation in the adsorption of an NO + O-2 mixture is discussed.