Chemistry of NOx on TiO2 surfaces studied by ambient pressure XPS: Products, effect of UV irradiation, water, and coadsorbed K +

Abstract

Self-cleaning surfaces containing TiO2 nanoparticles have been postulated to efficiently remove NOx from the atmosphere. However, UV irradiation of NOx adsorbed on TiO2 also was shown to form harmful gas-phase byproducts such as HONO and N2O that may limit their depolluting potential. Ambient pressure XPS was used to study surface and gas-phase species formed during adsorption of NO2 on TiO2 and subsequent UV irradiation at λ = 365 nm. It is shown here that NO3 -, adsorbed on TiO2 as a byproduct of NO2 disproportionation, was quantitatively converted to surface NO2 and other reduced nitrogenated species under UV irradiation in the absence of moisture. When water vapor was present, a faster NO 3 - conversion occurred, leading to a net loss of surface-bound nitrogenated species. Strongly adsorbed NO3 - in the vicinity of coadsorbed K+ cations was stable under UV light, leading to an efficient capture of nitrogenated compounds. © 2013 American Chemical Society.