Synthesis of Highly Active Sulfate-Promoted Rutile Titania Nanoparticles with a Response to Visible Light

Abstract

Highly active sulfate-promoted rutile titania (SO42-/TiO2) with smaller band gap was prepared by an in situ sulfation method, that is, under moderate conditions, sulfate-promoted rutile titania was directly obtained via precipitating Ti(SO4)2 in NaOH solution followed by peptizing in HNO3 without the phase transformation from anatase to rutile. Thus, the negative impacts of phase transformation from anatase to rutile on the structure, surface, and photoactivity properties of the catalysts due to higher calcination temperature can be avoided. The catalysts were characterized by means of thermal analysis, Brunauer−Emmett−Teller analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV−visible spectroscopy, FT-IR pyridine adsorption, and temperature-programmed desorption (TPD). The results show sulfate species are sensitive to the variation of calcination temperature. In the process of peptizing, sulfate species are homogeneously dispersed throughout the bulk of catalysts, allowing sulfate species to penetrate into the network of TiO2 effectively. After being calcined at 300 °C, sulfate species occupy oxygen sites to form Ti−S bonds, as evidenced by XPS results. As calcination temperature is further increased to 600 °C or above, the active sulfate species on the catalyst surface are destroyed, and the sulfate species in the network of TiO2 are expelled out onto the surface to form inactive sulfate species. Thus, Ti3+ defects will be produced on the catalyst surface. Accompanying this process, surface area is decreased promptly, and crystalline size is greatly increased via two fast growth phases due to the decomposition of sulfate species with different binding forces. Most importantly, the band gap of SO42-/TiO2 is remarkably shifted to the visible light region due to the formation of Ti−S bonds, and with increasing calcination temperature the visible light absorption capability is reduced due to breakage of Ti−S bonds. The excellent photoactivity of 300 °C calcined SO42-/TiO2 can be explained by its small crystalline size, high surface area, loose and porous microstructure, and the generation of Brønsted acidity on its surface.