Decomposition of Nitric Oxide over Barium Oxide Supported on Magnesium Oxide. 4. In Situ Raman Characterization of Oxide Phase Transitions and Peroxide Species by ¹⁸O-Labeling

Time-resolved in situ Raman spectroscopy was used to follow the decomposition of supported crystalline BaO 2 into defect-rich BaO. This decomposition occurred under O 2 partial pressures which were small compared to those expected from a calculated phase diagram. However, the O 2 partial pressures at which this BaO 2 decomposition takes place are much greater than those which are reached during the catalytic decomposition of NO over this material. Additionally, supported crystalline BaO 2 was never observed during the actual catalytic reaction. Therefore, it must be concluded that crystalline BaO 2 does not play a role in the catalytic reaction. In addition to crystalline BaO 2, 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 18 O isotopic labeling. Other bands located in this region of the spectrum may be due to peroxide ions in defect-rich BaO/BaO 2; however, 18 O 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 O 2 and responded very rapidly to changes in the O 2 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.