In situ Raman spectroscopy of peroxide ions on barium/magnesium oxide catalysts
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In situ Raman spectroscopy has been used to show that peroxide ions are present on Ba/MgO catalysts at temperatures up to 800°C. The surface O22- ions are believed to be responsible for the activation of CH4 during the oxidative coupling reaction which occurs at these elevated temperatures. At 100°C the peroxide ion spectrum is characterized by a major line at 842 cm-1, with minor peaks 829 and 821 cm-1. As the temperature was increased, all of the peaks shifted to lower wavenumbers and broadened, until at 800°C only a broad asymmetric band remained. Introduction of 18O2 also caused the peaks to shift to lower wavenumbers, consistent with the assignment of the spectra to peroxide ions. In the presence of CO2 and H2O and O22- ions reacted to form carbonate ions at T ≥ 500°C, which is in agreement with the poisoning effect of CO2 in the catalytic experiments. At Ba loadings ≤2 mol % a two-dimensional layer of BaO2 is present on the surface of MgO, but at larger loadings crystallites of BaO2 contribute significantly to the Raman spectra.
author list (cited authors)
Lunsford, J. H., Yang, X., Haller, K., Laane, J., Mestl, G., & Knoezinger, H.