Surface Properties of Reduced and Stoichiometric TiO₂ As Probed by SO₂ Adsorption

The adsorption and photochemical properties of reduced and stoichiometric anatase TiO₂ nanoparticles, prepared by annealing in vacuum and air, respectively, at different temperatures up to 500 °C and 2 days have been investigated. Combined X-ray diffraction and Raman spectroscopy results suggest that vacuum annealing leads to a defective, oxygen vacancy rich surface region with an accompanying decrease of the crystalline core. The surface chemical properties of the reduced and calcined TiO₂ nanoparticles were studied by means of SO₂ adsorption measured by in situ diffuse reflectance Fourier transform spectroscopy. On pristine TiO₂ nanoparticles SO₂ adsorption leads to a broad absorbance band centered at 1140 cm^–1. In contrast, SO₂ does not adsorb on stoichiometric TiO₂ obtained after long-term annealing in air at 500 °C. However, after the same heat treatment in vacuum, SO₂ is shown to bind strongly on well-defined adsorption sites associated with a narrow absorbance band at 1150 cm^–1. The increased adsorption on reduced TiO₂ is attributed to formation of subsurface oxygen vacancies and reactive Ti³⁺ species at the surface that promote SO₂ bonding. A surface-sulfite species (HSO₃^–) was identified as the major adsorbate on both the as-prepared and the vacuum-annealed sample, and a formation mechanism involving reaction with hydrogen from surface hydroxyl groups is proposed. During UV illumination, SO₂ is photoadsorbed on TiO₂ during SO₂ exposure in an inert He gas atmosphere. In contrast to dark SO₂ adsorption, this reaction does not involve surface defects, since the concentration of photoadsorbed SO₂ did not significantly change on the deeply reduced TiO₂ nanoparticles. On the basis of these findings, a new mechanism for the formation of surface-bound SO₃^2– during UV illumination on the stoichiometric surface is proposed, which should be generally applicable for other similar adsorbates and semiconducting oxides.