Sulfated TiO2: Preparation, Properties, and Applications in Photocatalysis

Sulfated titanium dioxide represents a class of material that combines the intrinsic semiconductor photocatalytic activity of TiO2 with the superacid properties induced by surface sulfate coordination. This chapter provides a comprehensive review of the synthesis, surface structure, and photocatalytic applications of sulfated titania, with particular emphasis on the mechanistic connections between surface chemistry and photocatalytic performance. The superacidity of sulfated TiO2, with a Hammett acidity of H0 ≈ −14 exceeding that of pure sulfuric acid, originates from a bond order redistribution within the surface-coordinated sulfate group. Inductive electron withdrawal by S6+ through the bridging S–O–Ti linkages depletes electron density at surface Ti4+ centers, generating strong Lewis acid sites, while the resulting polarization of the terminal S=O bonds activates adjacent surface hydroxyls as Brønsted acid sites. The balance between these two site types is governed by surface sulfate coverage and can be systematically tuned through the choice of preparation method and calcination temperature, as demonstrated by operando FTIR studies using pyridine as a molecular probe. Three principal routes to sulfated TiO2 are reviewed: wet impregnation with sulfuric acid or sulfate salts, photofixation of SO2, and solid-state thermolysis of titanyl sulfate (TiOSO4). The latter approach yields highly homogeneous sulfate-terminated anatase nanoparticles with tunable acidity in a single calcination step. Surface sulfate also stabilizes the photocatalytically preferred anatase polymorph against transformation to rutile, raising the transition temperature by up to 200°C. The coordination modes of surface sulfate, namely monodentate, bidentate chelating, and bidentate bridging, are characterized by their distinct infrared spectroscopic signatures, and the structural heterogeneity of the sulfated surface, including the possible formation of polysulfate species at high loadings, is discussed in the context of its influence on acid site distribution. The enhancement of photocatalytic activity by surface sulfation is attributed to four complementary mechanisms: prolonged electron-hole pair lifetime through transient electron trapping by electrophilic sulfate groups, increased reactant adsorption at the enriched acid site surface, modulation of intermediate binding that suppresses deactivating side reactions, and thermal stabilization of the photoactive anatase phase. These effects are illustrated through studies of both gas-phase photocatalytic oxidation of volatile organic compounds, including acetaldehyde, toluene, and organophosphates, and aqueous-phase degradation of organic pollutants. Taken together, the evidence establishes sulfated TiO2 as a multifunctional photocatalyst in which superacid surface chemistry and semiconductor photophysics act synergistically, offering practical advantages over conventional TiO2 in applications requiring sustained activity and resistance to intermediate poisoning.