Cobalt-mediated activation of peroxymonosulfate (PMS) has been widely investigated for the oxidation of organic pollutants. Herein, we employ cobalt-doped Black TiO₂ nanotubes (Co-Black TNT) for the efficient, stable, and reusable activator of PMS for the degradation of organic pollutants. Cobalt-doped Black TNTs induce the activation of PMS by itself and stabilized oxygen vacancies that enhance the bonding with PMS and provide catalytic active sties for PMS activation. A relatively high electronic conductivity associated with the coexistence of Ti⁴⁺ and Ti³⁺ in Co-Black TNT enables an efficient electron transfer between PMS and the catalyst. As a result, Co-Black TNT is an effective catalyst for PMS activation leading to the degradation of selected organic pollutants when compared to other TNTs (TNT, Co-TNT, and black TNT) and other Co-based materials (Co₃O₄, Co-TiO₂, CoFe₂O₄, and Co₃O₄/rGO). The observed organic compound degradation kinetics are retarded in the presence of methanol and natural organic matter as sulfate radical scavengers. These results demonstrate that sulfate radical is the primary oxidant generated via PMS activation on Co-Black TNT. The strong interaction between Co and TiO₂ through Co-O-Ti bonds and rapid redox cycle of Co²⁺/Co³⁺ in Co-Black TNT prevents cobalt leaching and enhances catalyst stability over a wide pH range and repetitive uses of the catalyst. Electrode-supported Co-Black TNT facilitates the recovery of the catalyst from the treated water.

A variety of combinations of CdS, TiO₂, and Pt in preparing the hybrid catalysts were studied for hydrogen production under visible light (λ > 420 nm) irradiation. The preparation method sensitively influenced the activity of the ternary hybrid catalysts. The formation of the potential gradient at the interface between CdS and TiO₂ is necessary in achieving the efficient charge separation and transfer and how the platinum as a cocatalyst is loaded onto the CdS/TiO₂ hybrid catalysts determines the overall hydrogen production efficiency. The common method of photoplatinization of CdS/TiO₂ hybrid [Pt-(CdS/TiO₂)] was much less efficient than the present method in which Pt was photodeposited on bare TiO₂, which was followed by the deposition of CdS [CdS/(Pt-TiO₂)]. The CdS/(Pt-TiO₂) has the hydrogen production rate ranging (6–9) × 10⁻³ mol h⁻¹ g⁻¹, which is higher by a factor of 3–30 than that of Pt-(CdS/TiO₂). The photocatalytic activity of the ternary hybrid catalysts was extremely sensitive to where the platinum is loaded. The photoactivity of the hybrid catalyst was also assessed in terms of the photocurrent collected by the methyl viologen electron shuttle in the catalyst suspension. CdS/(Pt-TiO₂) generated higher photocurrents than Pt-(CdS/TiO₂) by a factor of 2–7. The extreme sensitivity of the preparation method to the hydrogen production activity should be taken into account when hybrid photocatalysts are designed and prepared.