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 × 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 ν 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-NO x species during N 2 purges, NO/N 2, O 2 or H 2 O/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.