Direct decomposition of nitric oxide over perovskite-type catalysts: Part II. Effect of oxygen in the feed on the activity of three selected compositions Original Research Article

Direct decomposition of nitric oxide over three selected perovskites, both in the absence and the presence of oxygen added to the reaction gas mixture, was investigated at a steady state under a wide range of experimental conditions in a plug-flow reactor with 1 g catalyst. Temperatures between 723 and 923 K were covered. The catalysts (La0.87Sr0.13Mn0.2Ni0.8O3−δ, La0.66Sr0.34Ni0.3Co0.7O3−δ, and La0.8Sr0.2Cu0.15Fe0.85O3−δ) were prepared with specific surface area of 12.7, 9.0 and 10.1 m2/g, respectively. When no oxygen was added to the feed, these perovskites showed an improved activity in comparison with other literature known materials, except Cu-ZSM-5 zeolites. Oxygen has an inhibiting effect, which depends strongly on temperature and varies with flow-rate (contact time) and composition. It is relatively weak at lower temperatures and higher contact times and increases between about 773 and 873 K. Between 873 and 923 K, where the oxygen inhibition decreases with temperature, the rate of decomposition can be best expressed by a simple model: r=kNOPNO/(1+KPO2). For the low temperature range, the NO decomposition is best described by a model, in which oxygen is an inhibitor but also reactant. Overall data analysis permits to assume that oxygen takes part in the formation of an intermediate giving dinitrogen and that inhibition by oxygen is caused by its parallel participation in the formation of surface nitrates.