DECOMPOSITION OF NITRIC-OXIDE OVER BARIUM OXIDE-SUPPORTED ON MAGNESIUM-OXIDE .3. IN-SITU RAMAN CHARACTERIZATION OF THE ROLE OF OXYGEN

Time-resolved high-temperature in situ Raman spectroscopy was successf ully used to study details of the complex reaction network between gas phase oxygen and the species present in highly loaded Ba/MgO catalyst s. The decomposition of the catalyst precursor Ba(NO3)(2) to an amorph ous phase containing nitrate and nitrite ions (phase II') occurs at hi gher temperatures in the presence of O-2 than in He. Thus, the presenc e of O-2 stabilizes the precursor Ba(NO3)(2). In the presence of O-2, phase III, containing the Ba-nitro complex, is not observed, and the d ecomposition of the amorphous phase II' directly reacts to BaO2. Defec t-rich BaO rather than BaO2 is formed when the decomposition is in pur e He. Gas-phase oxygen, therefore, exerts a strong effect on the stabi lity of the phases present on the catalyst. In addition, it was shown that oxygen, added to the gas feed during catalytic NO decomposition, reduces the number of active Ba-nitro complex species via oxidation to nitrates. This reaction is identified with the inhibiting effect of o xygen on the catalytic activity. Crystalline BaO2 reacts with NO to fo rm nitrates, nitrites, and Ba-nitro complexes, depending on the O-2 an d NO partial pressures and reaction temperatures. At lower temperature s, phase II' is formed, while at elevated temperatures, the reaction r esults in phase III containing Ba-nitro complexes. The formation of th ese phases from BaO2 and their interconversions confirms the previousl y established phase diagram and suggests the role of BaO2 in the activ ation of NO. Crystalline BaO2, however, was never detected under catal ytic conditions. It has been shown that, in the absence of crystalline BaO2, NO activation occurs on O-2(2-) ions that are present in or on defect-rich BaO. Peroxide species, therefore, may play an important ro le in the catalytic cycle. A catalytic cycle is proposed in which the intermediate Ba-nitro species are formed from the reaction of NO with O-2(2-) ions in or on defect-rich BaO. In the rate-determining step, t he intermediate Ba-nitro species react with a second gas phase, or wea kly adsorbed, NO molecule to produce N-2 and O-2. Oxygen reforms the a ctivating peroxide species.