Study On Electrocatalytic Reduction Of Nitrate Nitrogen By Metal Oxide Catalystabstract

High nitrate concentrations in drinking water can lead to health problems,and managing the nitrogen cycle has been considered a major challenge by the National Academy of Engineering.Nitrate in water has been extensively studied in order to balance the nitrogen cycle Electrocatalytic nitrate reduction.Electrocatalytic reduction of nitrate converts NO₃^-to N₂ or NH₃ via electricity.Synthesis of NH₃ with nitrate has important economic and social significance,because it is an important chemical raw material and carbon-free fuel.The synthesis mechanism of NH₃ by NO₃^--N is complex,and the hydrogen evolution reaction reduces the yield and current efficiency of NH₃.Transition metal oxides（such as Cu O,TiO₂,etc.）have attracted much attention because of their ability to form oxygen vacancies（Ov）to improve NRR performance.However,how to enrich Ov in oxides（MOx）to provide enough H_（ads）to supply NRR process and the recovery of Ov has become an urgent problem.The+4 valence metal ions in MnO₂and TiO₂ can be reduced to+2 and+3 valence states,so MnO₂ and TiO₂ can easily form Ov.In this paper,MnO₂ and TiO₂were modified in two ways to obtain more Ov and promote the conversion of nitrate to ammonia.In the MnO₂ catalytic system,one superior dual-site NRR electrocatalyst that is composed of the Ov-enriched MnO₂ nanosheets（MnO₂-Ov）and Pd nanoparticles（deposited on MnO₂）is constructed over the three-dimensional porous nickel foam（Pd-MnO₂-Ov/Ni foam）.In a continuous-flow reaction cell,this electrode delivers a NO₃^--N conversion rate of 642 mg N m^-2 _electrode h^-1 and a NH₃ selectivity of 87.64%at-0.85 V vs.Ag/Ag Cl when feeding 22.5 mg L^-1 of NO₃^--N.Increasing the feeding NO₃^--N.concentration and flow rate can further lift the conversion rate to 1933 and1171 N m^-2 _electrode h^-1,respectively.Impacts of the NO₃^--N feeding concentration,flowing rate,solution p H,Pd loading mass,the coexisting anions and dissolved organic organisms on the NRR performances of Pd-MnO₂-Ov/Ni foam were also investigated.The combination of experimental characterizations and theoretical calculations reveal that the MnO₂-Ov adsorbs,immobilizes,and activates the NO₃^-and N-intermediates,while the Pd supplies the Ov sites with sufficient adsorbed hydrogen for both the NRR and Ov refreshment.In TiO₂ catalytic system,TiO₂ structure was modified by doping N element.N-TiO₂ catalysts with different amount of nitrogen doping were prepared by calcination of Ti N at different temperatures.Compared with pure phase TiO₂,doped TiO₂ has better performance of electrocatalytic reduction of nitrate,and with the reduction of nitrogen content,nitrate reduction efficiency and Faraday current efficiency both increase first and then decrease in a volcanic trend.N-TiO₂ fired at 650℃has the best effect.The most removal rate of NO₃^-can reach 90.28%,and the selectivity of NH₃ can reach86.32%.Combined with the characterization and experimental results,it is shown that the doping of N makes the N-TiO₂ catalyst have two crystal phases of rutile and anatase,and the content of oxygen vacancy decreases with the decrease of nitrogen content,but the gap width increases gradually,and the electron transport capacity on the catalyst changes Therefore,N doping can regulate TiO₂ crystal phase structure,oxygen vacancy content and electron transfer speed,thus significantly improving the electrocatalytic nitrate reduction performanceThis study provides two facile and effective strategies to improve the catalyst removal rate for nitrate removal.The results deepen the understanding of the NRR mechanism and provide a feasible method for improving the recovery of ammonia resources by electrocatalytic reduction of nitrate.The practical feasibility from NO₃^-to NH₄⁺is of great significance for the application of electrocatalytic reduction technology in practical environmental remediation.