Reactivity of a hydroxylated alumina surface in the presence of NO diluted in N2:: A PM-IRRAS in situ investigation

The interaction of NO (900 ppm NO/N-2, 2 x 10(5) Pa, RT) with a hydroxylated alumina surface was investigated by in situ polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS). The resulting spectra reveal two distinct periods, namely 0-1 day and 1-13 days of exposure. During the first period, NO exposure primarily leads to the formation of nitrites (1523, 1470, 1330, 1235, and 1130 cm(-1)), HONO (1235 cm(-1)), and perhaps also some nitrates (1650-1620 cm(-1)) and hyponitrites (1340 cm(-1)). After one day of exposure, the transformation of nitrites into nitrates is demonstrated by the decrease in intensity of the band at 1235 cm(-1), attributed to the v(1) stretching vibration of bridging bidentate nitrites and also by the presence of an isosbestic point at 1260 cm(-1). Various nitrate species including monodentate, bidentate, bridged, and water-solvated nitrates (1600-1280 cm(-1)) are then formed on the surface. The stability of the formed ad-NOx species during N-2 purges, NO/N-2, O-2 or H2O/O-2 exposures was also investigated in detail in this study. These experiments revealed an equilibrium at the surface between water-solvated and oxide-coordinated nitrates.