Photocatalytic Degradation of Salicylic Acid in Water Under Different Irradiation Conditions in the Presence of Doped (N, Fe) Titanium Dioxide

Developing titanium dioxide (TiO2) samples able to absorb and effectively utilize both ultraviolet radiation and visible light will lead to considerable reduction in costs of photocatalytic purification of natural and waste waters. Using the sol–gel method, we synthesized nanoscale titanium dioxide samples doped with nitrogen or co-doped with nitrogen and iron ions, with the samples having a narrower band gap of 2.59–2.99 eV. The synthesized N-TiO2 and (Fe,N)-TiO2 samples and the standard titanium dioxide Degussa P-25 are compared in terms of their activities in photocatalytic oxidation of aqueous salicylic acid solutions (0.2 mmol/L, pH 4) by dissolved oxygen or hydrogen peroxide upon irradiation with light from a SVD-120 lamp (λ > 200 nm) or a metal-halide GE CMH70 lamp (λ > 360 nm). The relative photocatalytic activity of doped TiO2 samples in oxidation of aqueous salicylic acid solutions depends on their iron content (0–1% Fe), the type of oxidant used, and the irradiation conditions. In photocatalytic oxidation with oxygen, samples N-TiO2 (λ > 200 nm) and (Fe,N)-TiO2 (1.0% Fe) (λ > 360 nm) exhibit the highest degree of mineralization of salicylic acid among the doped TiO2 samples, while (Fe,N)-TiO2 (0.5% Fe) is the most effective in degradation of salicylic acid by hydrogen peroxide. Using hydrogen peroxide raises the degree of photocatalytic mineralization of salicylic acid in the presence of any of the synthesized samples under both types of irradiation conditions, compared to its oxidation by dissolved oxygen. In addition, the use of H2O2 levels off the differences in the activity between synthesized TiO2 samples with different doping level of iron that are observed in the O2/TiO2/UV photocatalytic system upon irradiation with UV–visible light (λ > 200 nm), while the extent of salicylic acid degradation in the presence of most active synthesized catalysts is considerably closer to that achieved with the standard photocatalyst.