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

摘要

Time-resolved in situ Raman spectroscopy was used to follow the decomposition of supported crystal BaO↓(2) into defect-rich BaO. This decomposition occurred under O↓(2) partial pressures which were small comp to those expected from a calculated phase diagram. However, the O↓(2) partial pressures at which this B 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 ac catalytic reaction. Therefore, it must be concluded that crystalline BaO↓(2) does not play a role in the catalytic reaction. !n 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 label 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 rap to changes in the O↓(2) partial pressure. Therefore, it is likely that these peroxide ions are located at or c to the surface of defect-rich BaO crystals. It was previously shown that the species responsible for the Ran band at 947 cm↑(-1) immediately reacted with NO to form an active Ba-nitro intermediate. This per0x species on defect-rich BaO may therefore play an important role in the catalytic decomposition of NO.