IR spectroscopy study of NO adsorption and NO + O2 co-adsorption on Al2O3

Abstract

Adsorption of NO on Al₂O₃ leads to the formation of small amounts of surface nitro–nitrito (1461, 1318, 1228, 1220 and 1074 cm⁻¹) and nitrato (1652 cm⁻¹) complexes. Introduction of oxygen into the system causes oxidation of the NO₂⁻ compounds to nitrates (1628, 1591, 1565, 1298, 1259 and 1042 cm⁻¹) whose concentration increases with time and with the oxygen amount. The nitrates obtained in this way are stable up to 723 K. They increase the Lewis acidity of Al³⁺ sites in the vicinity, as a result of which mononitrosyls of the type Al³⁺(NO₃⁻)-NO (1938 cm⁻¹) are formed. In addition, weakly bound N₂O₃ (1895, 1565, 1298 cm⁻¹) and N₂O₄ (1752 cm⁻¹) are obtained. Analysis of the spectra in the 2900–2400 cm⁻¹ region shows the presence of combination bands at 2883, 2624 and 2546–2526 cm⁻¹, which can be useful for the spectral identification of the surface anionic nitrogen-oxo species.