Enhancing Aqueous Chlorate Reduction Using Vanadium Redox Cycles and pH Control

Chlorate (ClO3-) is a toxic oxyanion pollutant from industrial wastes, agricultural applications, drinking water disinfection, and wastewater treatment. Catalytic reduction of ClO3- using palladium (Pd) nanoparticle catalysts exhibited sluggish kinetics. This work demonstrates an 18-fold activity enhancement by integrating earth-abundant vanadium (V) into the common Pd/C catalyst. X-ray photoelectron spectroscopy and electrochemical studies indicated that V-V and V-IV precursors are reduced to V-III in the aqueous phase (rather than immobilized on the carbon support) by Pd-activated H-2. The V-III/IV redox cycle is the predominant mechanism for the ClO3- reduction. Further reduction of chlorine intermediates to Cl- could proceed via V-III/IV and V-IV/V redox cycles or direct reduction by Pd/C. To capture the potentially toxic V metal from the treated solution, we adjusted the pH from 3 to 8 after the reaction, which completely immobilized V-III onto Pd/C for catalyst recycling. The enhanced performance of reductive catalysis using a Group 5 metal adds to the diversity of transition metals (e.g., Cr, Mo, Re, Fe, and Ru in Groups 6-8) for water pollutant treatment via various unique mechanisms.