Catalytic oxidation of ammonia I. Reaction kinetics and mechanism

The kinetics of ammonia oxidation over oxides of manganese, cobalt, copper, iron and vanadium have been studied. The proposed reaction mechanism involves oxygen adsorption (oxidation of the catalyst surface) and reduction of the surface with ammonia to form the reaction products. The latter step consists of several stages involving the intermediate formation of nitroxyl and imide species. The interaction of imide with nitroxyl leads to nitrogen while the reaction between two nitroxyls results in nitrous oxide. For this model rate equations have been deduced which describe the overall process and the parallel reactions of the formation of N2 and N2O. These equations are shown to be in accordance with the experimental data obtained. It has been found that the selectivity in mild oxidation (N2 formation) decreases and the selectivity in deep oxidation (N2O formation) grows with an increase in the surface coverage with oxygen θ. The values of θ increase with the ratio of partial pressures of O2 to NH3 (PO2PNH3) in the reaction mixture. The governing role of θ has been supported by experiments in which catalysts were reduced with ammonia in the absence of O2 in the gas phase. The reaction mechanism involving the formation of N2, N2O and NO is also considered. The corresponding rate equations have been derived, and the expected dependence of specificity on the reaction mixture composition and temperature has been examined.