Adsorption and Oxidation of SO2 on the Surface of TiO2 Nanoparticles: The Role of Terminal hydroxyl, Oxygen Vacancy-Ti3+ States
Herein, the absorption and oxidation reaction of SO2 on TiO2 nanoparticles at 296 K under various environmental conditions (humidity, UV irradiation, and ozone copresence) were investigated by using flow chamber reaction system, synchrotron X-ray absorption near-edge structure (XANES) and high resolution synchrotron X-ray photoelectron spectroscopy (XPS) measurements. The results showed that TiO2NP catalysis SO2 oxidation to sulfate was at an extremely rapid rate. The proper relative humidity, UV irradiation and co-presence of ozone all markedly promoted SO2 oxidation on TiO2NPs. High resolution XPS unraveled that the terminal hydroxyl (OHt) and oxygen vacancy (VO)-Ti3+ states on TiO2NPs were the activate sites for SO2 adsorption and oxidation. The data of XPS measurements suggest that SO2 was adsorbed on OHt at Ti3+ in the vicinity of VOs to form HSO3-/SO32-. The resulted adsorbed-SO32- could bound to surface bridging O (Ob) atom and transformed into SO42-. H2O molecule could dissociate on VOs-Ti3+ into two bridging hydroxyl (OHb), subsequently forms new Ob, which provides activate O sites for the adsorbed-HSO3-/SO32- and oxidized them into SO42- on the surface of TiO2NPs. The co-presence of O3 could promote H2O dissociation into OHb, promoting the formation of Ob. The co-presence of O3 may also promote the adsorbed H2O dissociation into TiO2-O2- and hydroxyl radical (OH) on VOs, facilitating the oxidation of adsorbed HSO3-/SO32-. Under UV irradiation, new VOs were created via oxidation of lattice O by photo-generated hole, resulting in increased Ob and subsequently enhanced oxidation of adsorbed HSO3-/SO32- on TiO2NPs.