Valence State and Catalytic Role of Cobalt Ions in Cobalt TiO2 Nanoparticle Photocatalysts for Acetaldehyde Degradation under Visible Light

Cobalt–carbon–sulfur-codoped anatase photocatalysts (size <9 nm) were synthesized by hydrolyzing titanium(IV) isopropoxide in the presence of cobalt salts and ammonium thiocyanate at room temperature. The annealed photocatalysts were characterized with nitrogen adsorption–desorption isotherms, energy-dispersive X-ray (EDX), X-ray diffraction (XRD), UV–visible diffuse reflectance, transmission electron microscopy (TEM), X-ray photon spectroscopy (XPS), and photoluminescence (PL) measurements. Characterization results showed that the doped cobalt, carbon, and sulfur of the TiO 2 photocatalysts have some beneficial effects on specific surface area, visible light absorption, hydroxyl radical generation, and electron–hole pair separation. For the acetaldehyde degradation reaction, the activity of the photocatalysts follows the order 2% Co(III)/(C, S)–TiO 2 ≈ 2% Co(II)/(C, S)–TiO 2 > 1% Co(II)/(C, S)–TiO 2 > 2% Co(II)–TiO 2 > 2% Co(III)–TiO 2 > (C, S)–TiO 2 under visible light and 2% Co(II)/(C, S)–TiO 2 > 1% Co(II)/(C, S)–TiO 2 > 2% Co(II)–TiO 2 ≈ (C, S)–TiO 2 > TiO 2 –P25 under UV light. It was concluded that the doped cobalt ions play a vital role and act as active sites in the photodegradation reaction. A reaction mechanism is proposed that may explain the synergistic effect of the codopants in visible-light-induced photodegradation of acetaldehyde. This new photocatalyst system, Co/(C, S)–TiO 2, can have other potential applications that only need visible light as energy input.