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

Time-resolved in situ Raman spectroscopy was used to follow the decomp osition of supported crystalline BaO2 into defect-rich BaO. This decom position occurred under O-2 partial pressures which were small compare d to those expected from a calculated phase diagram. However, the O-2 partial pressures at which this BaO2 decomposition takes place are muc h greater than these which are reached during the catalytic decomposit ion of NO over this material, Additionally, supported crystalline BaO2 was never observed during the actual catalytic reaction. Therefore, i t must be concluded that crystalline BaO2 does not play a role in the catalytic reaction. In addition to crystalline BaO2, with a major Rama n band at 848 cm(-1) and components at 826 and 814 cm(-1). another per oxide species which had a band at 947 cm(-1) was identified by O-18 is otopic labeling. Other bands located in this region of the spectrum ma y be due to peroxide ions in defect-rich BaO/BaO2; however, O-18 isoto pe labeling did not provide definitive evidence for their assignment. The peroxide species with a Raman band at 947 cm(-1) was stable only i n 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 peroxi de ions are located at or close to the surface of defect-rich BaO crys tals. It was previously shown that the species responsible for the Ram an band at 947 cm(-1) immediately reacted with NO to form an active Ba -nitro intermediate. This peroxide species on defect-rich BaO may ther efore play an important role in the catalytic decomposition of NO.