Enhancing the visible-light photoactivity of silica-supported TiO₂ for the photocatalytic treatment of pharmaceuticals in water

Catalyst samples based on SiO₂-supported TiO₂ were prepared with the incorporation of Ag (metal), S (nonmetal), and ZnO@S (semiconductor and nonmetal). The materials were evaluated regarding their morphological, optical, and crystalline properties as well as their photoactivity under visible and ultraviolet light toward the degradation rate of a model emerging pollutant, acetaminophen (ACT). All modified materials exhibited improved performance over the undoped catalyst. The Ag-doped catalyst achieved the largest degradation under visible radiation (about 30% in 120 min), whereas under ultraviolet irradiation, the ZnO@S-doped sample exhibited the best performance (about 62% in 120 min). A Doehlert design was carried out to evaluate the influence of pH and temperature on the photoactivity of Ag-TiO₂/SiO₂. In addition, the role of each reactive species in the photodegradation reaction was investigated by radical scavenger experiments, and the superoxide radical anion O₂^•- was shown to be the predominant reactive species. The stability of the Ag-TiO₂/SiO₂ material under ultraviolet and visible light was confirmed after five successive operation cycles, showing a reasonable (about 50%) loss of activity under visible irradiation and a slight improvement (about 13%) under UV light, as a result of the photo-reduction of Ag⁺. Lastly, the effect of the initial pollutant concentration showed that ACT degradation using Ag-TiO₂/SiO₂ follows the Langmuir-Hinshelwood kinetics, with intrinsic reaction rate k = 2.71 × 10^-4 mmol L^-1 min^-1 under visible-light radiation.