Selective Photocatalytic Reduction of 4-Nitrophenol to 4-Aminophenol over Ru-Doped TiO₂: Effect of Ru Loading and Reaction Parameters

Nitrophenolic compounds are among the most widespread persistent organic pollutants found in industrial wastewater, with 4-nitrophenol (4-NP) being particularly concerning due to its high toxicity and chemical stability. The selective conversion of 4-NP into 4-aminophenol (4-AP), a valuable intermediate in the pharmaceutical and agrochemical industries, represents an attractive and sustainable alternative to conventional degradation processes. In this work, Ru-doped TiO₂ photocatalysts were synthesized via a modified sol–gel method, and their performance in the selective photocatalytic reduction of 4-NP under UV irradiation using 365 nm LEDs was investigated. Different Ru loadings (0.05–0.2 mol%) were evaluated to determine the optimal catalyst composition. In addition, the effects of catalyst dosage, pollutant concentration, and Na₂SO₃ scavenger concentration were systematically studied to optimize the photocatalytic process. The synthesized materials were characterized by XRD, Raman spectroscopy, FT-IR, UV-Vis diffuse reflectance spectroscopy, BET analysis, SEM, and EDX mapping. Among the investigated samples, the Ru–TiO₂ photocatalyst containing 0.1 mol% Ru exhibited the highest photocatalytic activity. Under optimized operating conditions (catalyst concentration of 1.5 g/L and 4-NP concentration of 10 mg/L), nearly complete conversion of 4-NP was achieved within 180 min of UV irradiation, with a corresponding 4-AP yield of approximately 95%. Moreover, the catalyst showed excellent stability over repeated photocatalytic cycles without significant loss of activity. The obtained results demonstrate the crucial role of ruthenium in promoting selective photocatalytic reduction pathways under UV irradiation and highlight the potential of Ru–TiO₂ photocatalysts for the sustainable conversion of nitroaromatic compounds and wastewater treatment applications.