Decomposition of nitric oxide over barium oxide supported on magnesium oxide. 4. In situ Raman characterization of oxide phase transitions and peroxide species by O-18-labeling
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Time-resolved in situ Raman spectroscopy was used to follow the decomposition of supported crystalline BaO2 into defect-rich BaO. This decomposition occurred under O2 partial pressures which were small compared to those expected from a calculated phase diagram. However, the O2 partial pressures at which this BaO2 decomposition takes place are much greater than those which are reached during the catalytic decomposition of NO over this material. Additionally, supported crystalline BaO2 was never observed during the actual catalytic reaction. Therefore, it must be concluded that crystalline BaO2 does not play a role in the catalytic reaction. In addition to crystalline BaO2, with a major Raman band at 848 cm-1 and components at 826 and 814 cm-1, another peroxide species which had a band at 947 cm-1 was identified by 18O isotopic labeling. Other bands located in this region of the spectrum may be due to peroxide ions in defect-rich BaO/BaO2; however, 18O isotope labeling did not provide definitive evidence for their assignment. The peroxide species with a Raman band at 947 cm-1 was stable only in the presence of gas-phase O2 and responded very rapidly to changes in the O2 partial pressure. Therefore, it is likely that these peroxide ions are located at or close to the surface of defect-rich BaO crystals. It was previously shown that the species responsible for the Raman band at 947 cm-1 immediately reacted with NO to form an active Ba-nitro intermediate. This peroxide species on defect-rich BaO may therefore play an important role in the catalytic decomposition of NO.
