Chemistry of NO_x 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 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.

TiO2 alterations with natural aging: Unveiling the role of nitric acid on NIR reflectance

“…In particular, nitric acid and sulfuric acid may be generated by the degradation of main atmospheric pollutants NOx and SO2, which produce NO3 -and SO4 2-, respectively. Equations 1 and 2 describe the case of nitrates formation, as more likely to be observed in current environments where large quantities of NOx are released [14,30,31,36].…”

“…The solar spectral reflectance was characterized for unexposed samples, and each six months; here we report the results after two years. Accelerated exposure: Accelerated exposure consisted in immersing membrane samples in different aqueous solutions for 24 h, focusing on two acid solutions -HNO3 and H2SO4 -to reproduce the contact of the materials with sulfuric or nitric acids generated by pollution or by TiO2 itself as byproduct of its photoactivity [14,30,31]. The list of solutions employed is given in Table 2.…”

“…These may be either organic -industrial wastewaters, volatile organic compounds (VOCs) emitted by exhausts, combustion gases, etc. [9][10][11] -or inorganic -nitrogen oxides [12][13][14]. Moreover, changes in wettability upon UV irradiation lead to a superhydrophilic state which, coupled with photocatalysis, results in the so called self-cleaning effect [15].…”

The Environmental Photochemistry of Oxide Surfaces and the Nature of Frozen Salt Solutions: A New in Situ XPS Approach

“…This experimental configuration allows the in situ investigation of photoactive materials under atmospherically relevant conditions and light while, compared to other setups [26], reducing the extent of gas phase photochemistry induced by UV irradiation.…”

“…The latter can be affected also by photochemical processes, such as the photochemistry of nitrogen oxides on mineral oxide particles contained in atmospheric mineral dust plumes uplifted from arid areas, which represents another important playground for atmospheric chemistry and climate science. So far, only few experiments have attempted to probe surface species on, e.g., ice [24] or mineral oxides [25,26] under atmospheric conditions. Indeed, the high water vapor pressure at environmentally relevant temperature (0.01-1 mbar) makes it difficult to use surface sensitive probes, thus pointing out the importance and potential of near ambient pressure XPS for environmental research.…”