Decomposition of nitric oxide over barium oxide supported on magnesium oxide .1. Catalytic results and in situ Raman spectroscopic evidence for a barium-nitro intermediate
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Barium oxide supported on magnesium oxide (Ba/MgO) exhibits unusual behavior as a catalyst for NO decomposition at Ba loadings of 11 mol % or geater. The catalytic activity is characterized by a sharp decrease in activity when the reaction temperature exceeds a certain value that depends on the partial pressures of NO and O2. The fall-off temperatures are between 630 and 770C for concentrations between 1% and 4% NO in helium. At 700C with 4% NO, the N2 formation rate was 2.5 mol g-1 s-1 for a 14 mol % Ba/MgO catalyst. The discontinuity in activity is accompanied by an abrupt change in activation energy for the reaction, which suggests a change in mechanism. In situ Raman spectroscopy was used to follow temporal changes in the composition of the catalyst that occurred after step changes in temperature of NO concentration. A comparison of spectroscopic and catalytic results indicates that a barium-nitro phase is an intermediate in the catalytic cycle below the fall-off temperature. Nitro species at the surface apparently react with NO to form the decomposition products, N2 and O2. This is believed to be the rate-limiting step in the catalytic cycle. Peroxide ions may be involved in the formation of the nitro intermediate.