Construction of the multi-layer TiO2-NTs/Sb-SnO2/PbO2 electrode for the highly efficient and selective oxidation of ammonia in aqueous solution: Characterization, performance and mechanism

Electrochemical oxidation is a promising technology to convert ammonia nitrogen to environmental-friendly gaseous nitrogen, but the high cost of the precious metal as an electrode limits its application. Herein, the multi-layer TiO2-NTs/Sb-SnO2/PbO2 electrode, a cheap and efficient dimensionally stable anode (DSA), was constructed for selective electrocatalytic oxidation of ammonia. The surface morphology, crystalline structure, and composition of the TiO2-NTs/Sb-SnO2/PbO2 electrode were investigated by the field emission scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The synergistic effect of the multi-layer structures of the electrode contributed to a stable structure and efficient electrocatalytic activity. Accelerated lifetime measurements and electrode recycling tests proved that the electrode possessed outstanding stability and long life with continuously used for 935 h. Compared with the commercial Ti/RuO2-IrO2, the TiO2-NTs/Sb-SnO2/PbO2 electrode displayed a more excellent performance of chlorine evolution and higher removal efficiency of ammonia. Under the optimal conditions, 30 mg/L of ammonia nitrogen could be completely removed with 92.3% selective conversion to gaseous nitrogen after 60 min. Moreover, the difference between oxygen evolution potential and chlorine evolution potential exceeded 0.45 V on the TiO2-NTs/Sb-SnO2/PbO2 electrode, which enhanced the anodic chlorine evolution reaction. Hypochlorite radical (ClO•) was supposed as the dominant oxidant to transform ammonia in water, which could promote ammonia removal and the selectivity of gaseous nitrogen with minimizing by-products formation such as nitrite and nitrate, etc. Therefore, this study suggests that the constructed multi-layer TiO2-NTs/Sb-SnO2/PbO2 electrode might be a promising anode for the electrochemical oxidation of ammonia nitrogen to N2 in an aqueous solution.