Simultaneous Photocatalytic Abatement of NO and SO2: Influence of the TiO2 Nature and Mechanistic Insights

Background: TiO2 is currently being incorporated into several construction materials, such as cement and asphalt because this photocatalyst can act as a passive system to reduce the concentration of typical urban pollutants like NOx and SO2 under solar illumination. Objective: In order to get further insights on the possible influence of the interaction between common pollutants, the present work investigates the mechanism of NOx photo-oxidation in the presence of SO2 traces over TiO2 samples of different textural and morphological characteristics. Methods: The performance for the photo-oxidation of NOx and SO2 in a dry air stream over TiO2 samples, both commercial and lab-prepared by hydrothermal and thermal methods, was evaluated by means of FTIR analyses of the gas phase. These materials were characterized by XRD, N2 adsorption isotherms, and DR UV-vis spectroscopy. Mechanistic studies were performed by in situ DRIFT under UV irradiation. Results: Photocatalytic tests showed a very efficient removal of the two selected pollutants using most of the TiO2 samples. In the case of SO2, elimination of these molecules is due not only to photocatalytic oxidation but also to a significant extent, to adsorption. Although in shorter periods, no byproducts are generated, following irradiation for several hours, the production of NO2 progressively increases and reaches 100 % selectivity over some photocatalyst. In situ DRIFTS analyses show the evolution of the surface composition and reveal the formation of the different types of surface nitrates with different symmetry. Under these operating conditions, a minor amount of sulfates are also formed. Conclusions: The presence of a low concentration of SO2 does not appear to be detrimental for NO removal. NO2 formation is delayed on the TiO2 samples with high specific surface area, which also tend to be more active. The spectroscopic results confirm the involvement of surface hydroxyls in the formation of adsorbed nitrate species.