Mechanism for NO₂ Storage in Barium Oxide Supported on Magnesium Oxide Studied by in Situ Raman Spectroscopy

The mechanism for NO 2 storage in barium oxide supported on magnesium oxide (BaO/MgO) was investigated. In situ Raman spectroscopy was used to follow temporal changes in the composition of the catalyst that occurred during the reaction with NO 2 . Up to catalyst temperatures of 400 °C, the formation of an intermediate barium−nitro phase precedes the formation of nitrate ions, which upon further exposure lead to the formation of bulk nitrates. In contrast, preoxidation of the catalyst or exposure to oxygen leads to the formation of peroxides that enhance the rate for nitrate formation while the formation of nitro species is suppressed above 300 °C. The spectroscopic results are in agreement with a reaction mechanism that has recently been proposed by Broquist et al. ( J. Phys. Chem. B 2002, 106, 137) on the basis of DFT calculations.