Micro-and Nano-scale Construction Of Cu-based Composites And Their Electrocatalytic Mechanism For Nitrate Removal

Nitrate(NO3-)pollution has become a major environmental issue of concern in China and the world,and the accumulation of NO3-in the aqueous environment poses a great risk to human health.It is important to develop a green electrochemical denitrification technology to realize the selective conversion of NO3-to N2.Currently,the development of electrochemical denitrification technology is limited by the activity and stability of electrocatalysts.And the sluggish reaction kinetics,low N2 selectivity and competing side reactions become the main bottlenecks of electrochemical nitrate reduction reaction(NO3-RR).Therefore,there is an urgent need to design and develop electrocatalysts with both high efficiency,stability,and high selectivity as well as electrochemical reactors to promote the development and practical application of electrochemical denitrification technology.In this study,three Cu-based nanoelectrodes with high electrocatalytic activity and selectivity,namely Cu2O/Cu,Pd-Cu/Cu2O,and reconstructed Cu(OH)2 catalysts,were prepared and used for the electrocatalytic reduction of NO3-in water.The catalytic reaction efficacy and related influencing factors were also studied.The effects of microscopic morphology,interfacial structure,and electron transport properties on NO3-RR were analyzed to study the correlation between the structure and properties of Cu-based catalysts and catalytic reduction efficiency.Combining multiple in situ/ex situ characterization techniques and theoretical studies,the surface reconstruction of Cu-based catalysts during the electrochemical nitrate reduction process and nitrate reduction mechanism were all revealed.The main research results are as follows.(1)An efficient,stable,and three-dimensional Cu2O/Cu catalyst was synthesized using an in situ electrochemical reconstruction strategy.The Cu2O/Cu catalyst was able to rapidly reduce NO3-in water with the conversion of 99.8%and the reaction rate of 4.82×10-2 min-1 at-1.4 V vs Ag/AgCl.Under the anodic electrochlorination,the active chlorine species could effectively convert the intermediate product ammonia(NH4+)to N2,achieving 96.8%of N2 selectivity,indicating that the NO3-pollutant could be completely removed from water by the two-stage route of "N03-→NH4+→N2".During the electroreduction process,Cu2O undergoes a self-reduction reaction to form Cu2O/Cu with an electron transfer process at the interface.Most importantly,Cu2O sites in Cu2O/Cu catalysts exhibit the same atomic hydrogen(*H)supply properties as noble metal catalysts,which facilitates NO3-RR via the atomic*H-mediated reduction pathway.Theoretical studies suggested that rapid*NO3 adsorption,sufficient*H supply,and rapid reduction of*NO2 to*NO,and highly selective conversion of*NO to*NOH are key to promoting the rapid reduction of NO3-on the Cu2O/Cu catalyst surface.In addition,the Cu2O/Cu catalyst demonstrated good catalytic/chemical stability in continuous cycle tests.(2)Based on the above research basis,Pd/Cu synergistic catalytic system(Pd-Cu/Cu2O)was constructed using Cu/Cu2O nanorods as precursors for achieving efficient electrocatalytic reduction of NO3-.In the chloride ion-containing system,the NO3-conversion,reaction rate constant,and N2 selectivity were 100%,3.49×10-2 min-1,and 100%,respectively,at-1.3 V vs Ag/AgCl.It was shown that the multiphase heterogeneous interface between Cu-Pd and Cu2O triggered an inhomogeneous space charge distribution and enhanced interfacial polarization.The Pd species made the 3d orbitals of Cu more polarizable,forming electron-deficient Pd sites and electron-rich Cu sites,thus promoting the adsorption of NO3-at the Pd sites and further reduction at the adjacent Cu sites.In situ characterization and theoretical studies showed that Pd-Cu/Cu2O inhibited the generation of*NOH and promoted the formation of the key intermediate*N,and the sufficient atomic*H accelerated the hydrogenation process of*N on the catalyst surface.Therefore,compared with Cu/Cu2O catalysts,Pd-Cu/Cu2O catalysts possessed unparalleled electrocatalytic activity and adsorption properties,making them one of the most promising catalysts for electrocatalytic NO3-RR.In addition,the Pd-Cu/Cu2O catalyst had strong NO3-enrichment by reaction kinetics study and theory simulation analysis,which could achieve efficient nitrate electroreduction even in low concentration of NO3-containing wastewater.(3)Rational design of electrochemical reactors is also the key to the development of electrochemical denitrification technology.In this study,a"zero-gap" electrocatalytic system was constructed for electrocatalytic NO3-RR using a three-dimensional self-supported Cu(OH)2 nanoelectrode as the cathode.The "zero-gap" electrochemical reactor is characterized by the extremely small inter-electrode spacing(d=0.1 mm),which effectively reduces the Ohmic drop between electrodes,thus improving the electrocatalytic nitrate reduction efficiency and reaction rate,and exhibits high stability and low energy consumption.At an initial concentration of 100 mg-N·L-1 and a current density of 10 mA·cm-2,the "zero-gap" electrocatalytic system achieved the highest N2 selectivity(80.4%)and the lowest energy consumption(0.12 kWh·g-1),exceeding the performance of most transition metal-based electrocatalysts in the chloride-free system.A linear fit revealed a positive correlation between current efficiency and N2 selectivity(R2=0.940),and shortening the electrode spacing could significantly improve the current efficiency.In the "zero-gap"electrocatalytic system,the N2 selectivity of the electrocatalytic system was improved by increasing the coverage of key intermediates*N on the catalyst surface and promoting N-N bonding due to the significant increase in current efficiency.The design of electrochemical reactors to improve nitrate reduction efficiency and N2 selectivity has not been reported in previous studies,and this strategy provides a new avenue for building high-performance electrocatalytic systems.The above research results provide a theoretical basis and a technical support for the treatment of excess nitrate in water using Cu-based nanoelectrodes and "zero-gap" electrochemical reactors,which have good application prospects.