N-Doped TiO2-Nb2O5 Sol–Gel Catalysts: Synthesis, Characterization, Adsorption Capacity, Photocatalytic and Antioxidant Activity

TiO2-based semiconductors are formidable photocatalysts for redox reaction applications. Although N-doped TiO2-Nb2O5 catalysts have already been explored in the literature, studies on their antioxidant activity are scarce, and systematic investigations on the effects of synthesis parameters over a wide range of %Nb and NH4OH concentrations are limited. In addition, the relationship between optimal pH and %Nb has not yet been adequately explored. In the present work, the sol–gel synthesis of N-doped TiO2-Nb2O5 catalysts was optimized using a design of experiments approach focused on photocatalysis, adsorption, and antioxidant applications. The samples were characterized by TGA, SEM/EDS, XRD, PZC tests, photoacoustic spectroscopy, and N2-adsorption/desorption experiments. The salicylic acid (SA) degradation tests and DPPH radical scavenging assays demonstrated the superior photocatalytic activity (up to 72.9% SA degradation in 30 min, pH 5) and antioxidant capacity (IC50 = 88.9 μg mL−1) of pure TiO2 compared to the N-doped TiO2-Nb2O5 catalysts. The photocatalytic activity, however, proved to be intensely dependent on the pH and %Nb interaction, and at pH 3, the 25Nb-1N-400 catalyst promoted more significant SA degradation (59.9%) compared to pure TiO2 (42.8%). In the methylene blue (MB) adsorption tests, the catalysts N-doped TiO2-Nb2O5 showed removals at least seven times greater than TiO2 catalysts without Nb.