DECOMPOSITION OF NITRIC-OXIDE OVER BARIUM OXIDE-SUPPORTED ON MAGNESIUM-OXIDE .1. CATALYTIC RESULTS AND IN-SITU RAMAN-SPECTROSCOPIC EVIDENCEFOR A BARIUM-NITRO INTERMEDIATE

Barium oxide supported on magnesium oxide (Ba/MgO) exhibits unusual be havior as a catalyst for NO decomposition at Ba loadings of 11 mol % o r greater. The catalytic activity is characterized by a sharp decrease in activity when the reaction temperature exceeds a certain value tha t depends on the partial pressures of NO and O-2, The fall-off tempera tures are between 630 and 700 degrees C for concentrations between 1% and 4% NO in helium. At 700 degrees C with 4% NO, the N-2 formation ra te was 2.5 mu mol g(-1) s(-1) for a 14 mol % Ba/MgO catalyst. The disc ontinuity in activity is accompanied by an abrupt change in activation energy for the reaction, which suggests a change in mechanism. In sit u Raman spectroscopy was used to follow temporal changes in the compos ition of the catalyst that occurred after step changes in temperature or NO concentration. A comparison of spectroscopic and catalytic resul ts indicates that a barium-nitro phase is an intermediate in the catal ytic cycle below the fall-off temperature. Nitro species at the surfac e apparently react with NO to form the decomposition products, N-2 and O-2. This is believed to be the rate-limiting step in the catalytic c ycle. Peroxide ions may be involved in the formation of the nitro inte rmediate.