VALENCE STATES OF VANADIA-ON-TITANIA SILICA AND MOLYBDENA-ON-SILICA CATALYSTS AFTER REDUCTION AND OXIDATION

The behavior of vanadia-on-titania/silica and molybdena-on-silica catalysts during reduction by ammonia and oxidation by nitric oxide or oxygen at elevated temperatures was studied. The heat evolved during adsorption, reduction, and oxidation was measured by means of a differential scanning calorimeter. Ammonia reduces the catalysts to a certain extent under the formation of N2 and H2O. On the basis of the heat measured, the amount of reduced species was calculated. The reduced catalysts were oxidized by NO and O2, producing an amount of heat that corresponds to the respective reactions. The valence changes of V(V) to V(IV) and Mo(VI) to Mo(IV) were confirmed by means of X-ray photoelectron spectroscopy. The results of the differental scanning calorimetry measurements were used to calculate initial apparent rates of reduction and of oxidation. It was found that the rate of oxidation by O2 was about 5 times higher than that by NO. Moreover, the initial apparent rate of reduction by NH3 of the molybdena catalyst was significantly higher than that of the vanadia catalyst. These results were used to explain the differences in reactivity of the gas mixture of NO, NH3, and O2 over the catalysts; namely, NO does not participate in the reaction over the molybdena catalyst, while it does over the vanadia catalyst. © 1990 American Chemical Society.