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 successfully used to study details of the complex reaction network between gas phase oxygen and the species present in highly loaded Ba/MgO catalysts. The decomposition of the catalyst precursor Ba(NO 3 ) 2 to an amorphous phase containing nitrate and nitrite ions (phase II‘) occurs at higher temperatures in the presence of O 2 than in He. Thus, the presence of O 2 stabilizes the precursor Ba(NO 3 ) 2 . In the presence of O 2, phase III, containing the Ba−nitro complex, is not observed, and the decomposition of the amorphous phase II‘ directly reacts to BaO 2 . Defect-rich BaO rather than BaO 2 is formed when the decomposition is in pure He. Gas-phase oxygen, therefore, exerts a strong effect on the stability 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 oxygen on the catalytic activity. Crystalline BaO 2 reacts with NO to form nitrates, nitrites, and Ba−nitro complexes, depending on the O 2 and NO partial pressures and reaction temperatures. At lower temperatures, phase II‘ is formed, while at elevated temperatures, the reaction results in phase III containing Ba−nitro complexes. The formation of these phases from BaO 2 and their interconversions confirms the previously established phase diagram and suggests the role of BaO 2 in the activation of NO. Crystalline BaO 2, however, was never detected under catalytic conditions. It has been shown that, in the absence of crystalline BaO 2, NO activation occurs on O 2 2- ions that are present in or on defect-rich BaO. Peroxide species, therefore, may play an important role 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, the intermediate Ba−nitro species react with a second gas phase, or weakly adsorbed, NO molecule to produce N 2 and O 2 . Oxygen reforms the activating peroxide species.