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

Abstract

The adsorption and photochemical properties of reduced and stoichiometric anatase TiO 2 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 TiO 2 nanoparticles were studied by means of SO 2 adsorption measured by in situ diffuse reflectance Fourier transform spectroscopy. On pristine TiO 2 nanoparticles SO 2 adsorption leads to a broad absorbance band centered at 1140 cm −1 . In contrast, SO 2 does not adsorb on stoichiometric TiO 2 obtained after long-term annealing in air at 500 °C. However, after the same heat treatment in vacuum, SO 2 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 TiO 2 is attributed to formation of subsurface oxygen vacancies and reactive Ti 3+ species at the surface that promote SO 2 bonding. A surface-sulfite species (HSO 3 − ) 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, SO 2 is photoadsorbed on TiO 2 during SO 2 exposure in an inert He gas atmosphere. In contrast to dark SO 2 adsorption, this reaction does not involve surface defects, since the concentration of photoadsorbed SO 2 did not significantly change on the deeply reduced TiO 2 nanoparticles. On the basis of these findings, a new mechanism for the formation of surface-bound SO 3 2− during UV illumination on the stoichiometric surface is proposed, which should be generally applicable for other similar adsorbates and semiconducting oxides.

Photoinduced Adsorption and Oxidation of SO2 on Anatase TiO2(101)

“…The two negative peaks measured at 3725 and 3680 cm –1 are associated with the loss of surface hydroxyl groups during adsorption, and the peaks situated at 1377, 1305, and 1200–1150 cm –1 are related to the photo-oxidized species. 29 Surface hydroxyl groups are always expected to some extent on TiO 2 , especially in experiments conducted at atmospheric pressures, since TiO 2 is hygroscopic and water can adsorb dissociatively at surface defect sites and facets such as {001}. The negative bands at 3725 and 3680 cm –1 indicate to what extent hydroxyls have been replaced or otherwise overshadowed by the addition of new adsorbates.…”

“… 24 , 29 The intensity of this band has been shown to increase on reduced surfaces, where there are oxygen vacancies present in subsurface sites, and to disappear on stoichiometric and crystalline surfaces. 29 In contrast, our calculations show that surface-sulfite and surface-sulfate species are destabilized by subsurface oxygen vacancies on defect-free surfaces. The 1150 cm –1 signal in Figure 1 , representing TiO 2 with small concentration of defects, can thus be attributed mainly to the ν 2 (1) mode due to surface-sulfate.…”

“… 40 − 43 Previous studies of reduced anatase TiO 2 nanoparticles showed that subsurface oxygen vacancies enable SO 2 adsorption in the absence of UV illumination but that the same defects do not promote photoinduced adsorption, as on the defect-free surface. 29 …”

“… 29 In previous studies we have shown that SO 2 does not adsorb in the absence of UV illumination on crystalline and stoichiometric TiO 2 at room temperature, employing identical particles as those used in this work. 29 , 30 These conclusions are qualitatively supported by the work of Baltrusaitis et al, 24 showing much larger adsorption capacity on small 4 nm TiO 2 nanoparticles compared to 32 nm nanocrystals. Therefore, SO 2 adsorption on anatase TiO 2 provides a good example of how excited-state chemistries facilitate molecule adsorption.…”

Oxygen Vacant Semiconductor Photocatalysts

“…[167] (Figure 11 Besides the aqueous organic contaminates, photocatalysts with OVs have been employed to remove gaseous pollutants, for example, NO, Hg 0 , and SO 2 . [164,165,[168][169][170][171][172] Fe doped TiO 2 nanoparticles exhibited better catalytic properties of NO oxidation than pristine sample under visible light irradiation. [171] The incorporation of Fe ions facilitated the formation of OVs on the surface of TiO 2 , providing more adsorption and activation sites for NO oxidation reaction.…”