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 N{sub 2} and H{sub 2}O. On the basis of the heat measured, the amount of reduced species was calculated. The reduced catalysts were oxidized by NO and O{sub 2}, 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 differential scanning calorimetry measurements were used to calculate initial apparent rates of reduction and of oxidation. It was found that the rate of oxidation by O{sub 2} was about 5 times higher than that by NO.