Chemistry of NO2 on oxide surfaces:: Formation of NO3 on TiO2(110) and NO2⇆O vacancy interactions

Synchrotron-based high-resolution photoemission, X-ray absorption near-edge spectroscopy, and first-principles density functional (DF) slab calculations were used to study the interaction of NO2 with a TiO2(110) single crystal and powders, of titania. The main product of the adsorption of NO2 on TiO2(110) is surface nitrate with a small amount of chemisorbed NO2. A similar result is obtained after the reaction of NO2 with polycrystalline powders of TiO2 or other oxide powders. This trend, however, does not imply that the metal centers of the oxides are unreactive toward NO2. An unexpected mechanism is seen for the formation of NO3. Photoemission data and DF calculations indicate that the surface nitrate forms through the disproportionation of NO2 on Ti sites (2NO(2,ads) --> NO3,ads + NOgas) rather than direct adsorption of NO2 on O centers of titania. Complex interactions take place between NO2 and O vacancies of TiO2(110). Electronic states associated with O vacancies play a predominant role in the bonding and surface chemistry of NO2. The adsorbed NO2, on its part, affects the thermochemical stability of O vacancies, facilitating their migration from the bulk to the surface of titania. The behavior of the NO2/titania system illustrates the importance of surface and subsurface defects when using an oxide for trapping or destroying NOx species in the prevention of environmental pollution (DeNOx operations).