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

Self-cleaning surfaces containing TiO₂ nanoparticles have been postulated to efficiently remove NOx from the atmosphere. However, UV irradiation of NO_x 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 NO_{3-, 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 NO3- 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.}