Solid–gas interaction of nitrogen oxide adsorbed on MnOx–CeO2: a DRIFTS study

Abstract

The formation of fluorite-type solid solutions with compositions (n)MnO_x–(1 − n)CeO₂ is effective in increasing adsorptive NO uptake. Solid–gas interactions for NO adsorbed onto preoxidized samples was studied by the use of diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) to elucidate the role of each oxide component. On exposure to a flowing gas containing 0.04% NO, several different types of nitrate and/or nitrite were yielded depending on temperature (25 and 150 °C), O₂ concentration (0, 2, and 10%) in the gas feed, and composition of the binary oxide (n = 0, 0.25, 0.5, and 1.0). On CeO₂ (n = 0), chelating nitrite (NO⁻ ₂) prevailed, while only small amounts of bidentate and unidentate nitrates (NO⁻ ₃) were formed at 150 °C. By contrast, adsorption onto MnO_x (n = 1.0) at 150 °C produced bidentate nitrate even in the absence of O₂, suggesting that the lattice oxygen takes part in the NO oxidation process. On MnO_x–CeO₂ (n = 0.25 and 0.5), chelating nitrite bound to Ce ions was increasingly converted to nitrate at higher O₂ concentration and at elevated temperature. A surface reaction model on the basis of the DRIFTS results reveals that the redox of Mn ions with simultaneous oxygen equilibration with the gas phase should play a key role in facilitating the oxidative adsorption of NO.