Methane-Driven Photocatalytic Nitrate Reduction to Ammonia over Cu-Modified TiO2

Abstract Photocatalytic reduction of NO3– in water to NH3 offers an attractive route for converting wastewater into a nitrogen resource; however, previous studies rely on liquid organic sacrificial agents that are not environmentally benign. Herein, we demonstrate the use of CH4 as an environmentally benign gaseous sacrificial agent for photocatalytic NO3– reduction to NH3 over Cu-modified TiO2. Under optimized conditions, the system achieved an NH3 production rate of 9.5 μmol g–1 h–1, with 97% NO3– conversion and 78% NH3 selectivity, while CO2 was the solo carbon-containing product formed from CH4. Mechanistic studies revealed that CH4 reacted with hydroxyl radicals generated via water oxidation over the photocatalyst, thereby suppressing charge recombination and enhancing electron utilization for NO3– reduction to NH3 at the Cu sites. In addition, CH4 suppressed overoxidation of the Cu sites, helping to maintain low-valent Cu species that are active for NO3– reduction. The reaction system exhibited stable performance over four cycles and was further applicable to the reduction of other oxyanion pollutants, such as BrO3–. This approach avoids secondary contamination caused by sacrificial agents and provides a simple and practical strategy for coupling methane utilization with photocatalytic reduction processes.