Adsorption and oxidation of SO2 on the surface of TiO2 nanoparticles: the role of terminal hydroxyl and oxygen vacancy–Ti3+ states

Abstract

Herein, the absorption and oxidation reactions of SO₂ on TiO₂ nanoparticles (TiO₂ NPs) at 296 K under various environmental conditions (humidity, UV irradiation, and ozone copresence) were investigated by using a 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 oxidation of SO₂ to sulfate via TiO₂ NP catalysis happened at a very rapid rate. The appropriate relative humidity, UV irradiation and co-presence of ozone all markedly promoted SO₂ oxidation on TiO₂ NPs. High resolution XPS unraveled that the terminal hydroxyl (OH_t) and oxygen vacancy (V_O)–Ti³⁺ states on TiO₂ NPs were the active sites for SO₂ adsorption and oxidation. The data of XPS measurements suggest that SO₂ was adsorbed on a OH_t next to a Ti³⁺ V_O and reacted to form HSO₃⁻. HSO₃⁻ can then transform into SO₃²⁻via transfer of a proton. The resulting adsorbed SO₃²⁻ could bind to a surface bridging O (O_b) atom and transform into SO₄²⁻. A H₂O molecule could dissociate on V_O–Ti³⁺ into two bridging hydroxyl (OH_b) groups, subsequently forming new O_b, which provides an active O site for the adsorbed HSO₃⁻/SO₃²⁻ and oxidizes them into HSO₄⁻/SO₄²⁻ on the surface of the TiO₂ NPs. The copresence of O₃ could promote H₂O dissociation into OH_b, promoting the formation of O_b. The copresence of O₃ may also promote the dissociation of adsorbed H₂O into TiO₂–O₂⁻ and hydroxyl radicals (˙OH) on V_Os, facilitating the oxidation of adsorbed HSO₃⁻/SO₃²⁻. Under UV irradiation, new V_Os were created via oxidation of lattice O by photo-generated holes, resulting in increased O_b and subsequently enhanced oxidation of adsorbed HSO₃⁻/SO₃²⁻ on TiO₂ NPs.