Computer Simulation of NO-NH₃ Reaction on Metal Oxide Catalysts

Abstract

Computer simulation method has been applied to the analysis of the kinetics of NO-NH₃ reaction on metal oxide catalysts. Rate constants of elementary steps in the NO-NH₃ reaction have been determined by the least squares method for V₂O₄ and Fe₂O₃ catalysts to elucidate the differences in their catalytic properties. The results showed that the adsorption of NH₃ is faster on V₂O₄ than on Fe₂O₃, whereas the adsorption of NO and the reaction of adsorbed NO with adsorbed hydrogen are faster on Fe₂O₃ than on V₂O₄. The effects of surface oxygen on the rates of NO-NH₃ and NO-H₂ reactions have been studied on the vanadium oxide catalyst. It has been found that the rate of NO-NH₃ reaction is markedly enhanced by surface oxygen (V=O species) whereas the rate of NO-H₂ reaction is significantly decreased. The promotive effect of the surface oxygen in the former reaction has been shown to arise from the follo-wing two actions of surface oxygen ; the removal of adsorbed hydrogen dissociated from NH₃ and the acceleration of the adsorption of NO. On Fe₂O₃, MnO₂, Cr₂O₃, ZnO and NiO catalysts . (group A catalysts), the activity for NO-NH₃ reaction has been found to be parallel to that for NO-H₂ reaction. The correlation has been explained in terms of the difference in the rates of the reactions of adsorbed NO with adsorbed hydrogen derived from NH₃ or H₂ ; the larger the rate constants of the steps, the faster the rates of both NO-H₂ and NO-NH₃ reac-tions. Further, guides to the improvement of V₂O₄ and Fe₂O₃ catalysts for NO-NH₃ reaction were given on the basis of the rate constants obtained.