INFRARED-SPECTROSCOPY STUDY OF THE SPECIES ARISING DURING NO2 ADSORPTION ON TIO2 (ANATASE)

The compounds formed during adsorption of NO2 on anatase are studied by IR spectroscopy. It is established that the first compounds that arise are Ti-O2NOH groups formed with the predominant participation of the H-bonded hydroxyl groups of the support. The increase in amount of added NO2 leads to its interaction with the isolated hydroxyls and formation of monodentate, bidentate, and bridge nitrates and water. Adsorption on coordinatively unsaturated acid-base pairs (Ti4+-O2-) is the last to proceed and is accompanied by disproportionation and formation of nitrates (on the titanium cations) and NO+ (on the oxygen anions). Thermal treatment in vacuum of samples saturated by NO2 leads to (i) desorption of the compounds in a reverse sequence with respect to their appearance and (ii) conversion of part of these compounds into nitro species. Saturation of the anatase surface with nitrogen dioxide leads to blocking of the Lewis acid sites and generation of Bronsted acidity at the expense of the basic surface hydroxyl groups on the support. Ammonia adsorption on such a surface is accompanied by NH4+ formation and bidentate/bridge-monodentate nitrate regrouping.