Synthesis Of Copper-Based Catalysts And Its Performance For Electrochemical Nitrate Reduction

In recent years,the nitrate（NO₃^-）content in surface and ground water is increasing due to the acceleration of social industrialization,the excessive use of chemical fertilizers,the unreasonable disposal of animal excrement,and other reasons.It will destroy the nitrogen cycle in the biosphere resulting in eutrophication of the water and other environmental problems if the concentration of nitrate is too high.Furthermore,the NO₃^-could be converted into NO₂^-in the human body,which may cause“Blue Baby Syndrome”and improve the incidence of cancer.It is a serious threat to human health.In order to solve the problem of nitrate pollution,researchers have developed plenty of treatment methods including biological denitrification,physical methods（ion exchange,electrodialysis,reverse osmosis）,and reduction by active metal,etc.However,these methods are difficult to achieve large-scale industrial application because of various problems such as secondary treatment of generated sludge,complicated process,strict control of reaction conditions,and expensive equipment but low efficiency,etc.The method of electrochemical reduction can be employed for the degradation of nitrate and generation of useful value-added product named ammonia.The reaction process utilizes water as the proton source and it is driven by green energy.Besides,it can be easily operated and controlled.Nowadays,it has attracted extensive attention from scientific researchers.To design catalysts with low cost,high activity,and long-term stability is the key aspect in electrochemical research.Copper-based materials are inexpensive and exhibit high activity for the degradation of NO₃^-but have poor stability and will generate a large amount of by-products（esp NO₂^-）.To address these issues,this paper has optimized copper-based catalysts and studied the performance characteristics of electrochemical nitrate reduction under different systems.The main contents are as follows:（1）A series of Metal-Organic Frameworks（MOFs）derivatives are synthesized by a simple ion exchange method combined with a subsequent annealing step.The Cu-Ru@C-0.5 catalyst exhibits optimal nitrate reduction performance.In the electrolyte of0.1 M Na₂SO₄ containing 50μg m L^-1 NO₃^-,the removal rate of NO₃^-is 100%,together with Faradaic efficiency of 90.4%,ammonia selectivity of 84.7%,NO₂^-selectivity of6.6%,and the ammonia yield of 1265.4μg h^-1 mg^-1 at-0.6 V（vs.RHE）.It not only has excellent cycling stability but also performs well in electrolytes with different ion environments.This is mainly because the highly graphitized carbon substrate possesses excellent conductivity and it prevents the aggregation of particles.In addition,the ultrafine Cu particles doped with Ru facilitate the contact between the active sites and the electrolyte.What’s more,the strong electronic interaction between Cu and Ru is beneficial to the adsorption of NO₃^-and the generation of active hydrogen.（2）By using electrospinning technology and subsequent heat treatment process,the copper and cobalt bimetallic nanoparticles are loaded on nitrogen-doped carbon nanofibers to prepare highly efficient catalysts with various metal ratios.Among them,Cu₃Co₁/NCNFs exhibits the highest nitrate degradation activity to synthesis ammonia.In the electrolyte of 1 M KOH containing 0.05 M NO₃^-,the ammonia yield can reach14283μg h^-1 mg^-1,together with Faradaic efficiency of 95.2%,ammonia selectivity of87.4%,NO₂^-selectivity of 4.5%,and the high concentration of nitrate could be completely degraded within 5 h at-0.5 V（vs.RHE）.Its excellent catalytic performance is mainly attributed to the following aspects including the conductivity of the nitrogen-doped carbon nanofibers,the excellent contact between the electrolyte and catalyst provided by one-dimensional structure of cross-linking,and the improvement of ammonia selectivity by the highest proportion of graphitic nitrogen.Moreover,the Co-N_xbonds will strengthen the electron transfer and adjust the adsorption of intermediate.