N-Doped TiO[sub.2]-Nb[sub.2]O[sub.5] Sol–Gel Catalysts: Synthesis, Characterization, Adsorption Capacity, Photocatalytic and Antioxidant Activity

TiO[sub.2] -based semiconductors are formidable photocatalysts for redox reaction applications. Although N-doped TiO[sub.2] -Nb[sub.2] O[sub.5] 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 NH[sub.4] OH 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 TiO[sub.2] -Nb[sub.2] O[sub.5] 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 N[sub.2] -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 (IC[sub.50] = 88.9 μg mL[sup.−1] ) of pure TiO[sub.2] compared to the N-doped TiO[sub.2] -Nb[sub.2] O[sub.5] 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 TiO[sub.2] (42.8%). In the methylene blue (MB) adsorption tests, the catalysts N-doped TiO[sub.2] -Nb[sub.2] O[sub.5] showed removals at least seven times greater than TiO[sub.2] catalysts without Nb.