Ru-modified TiO₂ photocatalysts with different Ru loadings (0.05, 0.1, and 0.2 mol%) were synthesized and investigated for the selective photocatalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) under UV irradiation. The materials were characterized by X-ray diffraction (XRD), Raman and Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), N₂ physisorption, UV–Vis diffuse reflectance spectroscopy, photoluminescence (PL), and photoelectrochemical measurements. Ru incorporation preserved the anatase structure, while significantly affecting the textural, surface, optical, and charge-carrier properties of TiO₂. The effect of Ru loading was particularly evident in the charge-carrier dynamics and photocatalytic behaviour. The 0.05 mol% Ru–TiO₂ sample exhibited the highest photocurrent response and the fastest 4-NP conversion kinetics, whereas the 0.1 mol% Ru–TiO₂ photocatalyst showed the highest selectivity toward 4-AP. The latter also exhibited the highest specific surface area (160 m2 g−1), the smallest crystallite size, enhanced surface hydroxylation, and the lowest band-gap energy. These results indicate that maximizing charge transport alone is insufficient to achieve the highest selectivity and that an appropriate balance between charge-carrier dynamics and surface properties is required. Under optimized operating conditions (0.1 mol% Ru–TiO₂, catalyst loading of 1.5 g L−1, initial 4-NP concentration of 0.072 mM, and Na₂SO₃ concentration of 3.97 mM), complete 4-NP conversion and a 4-AP yield of approximately 95% were achieved after 180 min of UV irradiation. The optimized photocatalyst maintained high conversion and 4-AP yield over three consecutive recycling cycles. Post-reaction XRF analysis showed no appreciable change in the Ru content, while XRD confirmed preservation of the crystalline structure, supporting the compositional, structural, and operational stability of the photocatalyst under the investigated conditions.Overall, the results demonstrate that Ru loading plays a key role in controlling the interplay between charge-carrier dynamics and surface properties of TiO₂, thereby determining the activity and selectivity of the photocatalytic reduction of 4-NP to 4-AP.
