Decomposition of nitric oxide over barium oxide supported on magnesium oxide .2. In situ Raman characterization of phases present during the catalytic reaction
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High temperature in situ Raman spectroscopy was used to determine the complex phase behavior during the decomposition of Ba(NO3)2 supported on MgO. The starting material, crystalline Ba(NO3)2, is stable up to 500C in a 1% NO in He gas mixture. Above this temperature or at lower NO partial pressures, the crystalline Ba(NO3)2 is transformed into an amorphous intermediate phase II' which contains nitrate and nitrite ions. This phase is gradually converted into phase II" containing nitrate ions, Ba-nitrito complexes, and the first traces of a Ba-nitro species. The next step in the decomposition is the formation of phase III, which consists mainly of Ba-nitro complexes and nitrate ions. Phase III is stable in decreasing NO pressures until its decomposition into defect-rich BaO, phase IV. A hysteresis was observed for the stability range of phase III as a function of decreasing or increasing NO partial pressures. The transformations between phase III and IV are characterized by isosbestic points in the Raman spectra. Thus, decomposition/reformation of phase III occurs directly without any detectable intermediates. The effect of temperature on the decomposition of phase III into the defect-rich BaO, as determined by Raman spectroscopy, is in good agreement with the calculated coexistence of BaO, BaO2, and Ba(NO3)2 phases. Moreover, there is a good correlation between the presence of phase III and the maximum catalytic activity for the decomposition of NO. These in situ Raman results strongly suggest that the observed activity of highly loaded Ba/MgO catalysts arises from the formation of phase III. The sharp falloff in the N2 formation rate at a particular temperature is related to the transition between phase III and phase IV. Transient in situ Raman experiments indicate that Ba-nitro complexes in phase III are taking part in the catalytic cycle as reaction intermediates.
