| Nitrate,as the highest oxidation form of nitrogen,is stable in groundwater and surface water,and has become a common chemical form of nitrogen pollution in the world.Ammonia is one of the most common basic industrial chemicals.It not only plays an important role in agriculture,textile industry,plastics and pharmaceutical industry,but also shows its prospect as the next generation of energy carrier due to its high energy density(4.3 kWh/kg)and clean emission.From the perspective of environmental protection and energy carrier,it has great attraction to convert nitrate into ammonia in water.Electrochemical reduction of nitrate to ammonia as become a hot topic in recent years.In this paper,three-dimensional honeycomb porous carbon was prepared firstly,and then Cu2O nanoparticles were loaded into the honeycomb porous carbon by chemical reduction co deposition method.The morphology and electrochemical properties of Cu2O/Sn-MPC were further optimized by changing the type of doped metal and the content of Sn,Cu2O nanoparticles were uniformly loaded in three-dimensional porous carbon,and the peak current was-7.70 mA/cm2 at-0.8 V vs.SCE.Combined with physical characterization and electrochemical test,Cu2O/Sn-MPC,Cu2OMPC,Sn-MPC and MPC were compared,the results show that the introduction of Sn element promotes the dispersion of Cu2O and improves the catalytic activity and utilization.In contrast,Cu2O/Sn-MPC exhibited a corrected nitrate reduction potential and a higher peak current.In order to further study the behavior and mechanism of reduction NO3-on the electrode,based on the electrochemical test results of Cu2O/Sn-MPC at different nitrate concentration and scanning rate,the reduction of NO3-to NO2-is mainly controlled by adsorption,the reduction to NH4+ is controlled by diffusion.According to the electrochemical performance at different temperatures,the activation energy of NO3-reduction was calculated,and Cu2O/Sn-MPC showed the lowest activation energy,so the efficient electrocatalytic reduction of nitrate was realized.Based on the best Cu2O/Sn-MPC catalyst,the mechanism of Cu2O active site reducing NO3-to NH4+ is studied by DFT calculation.First of all,the adsorption structure and free energy of the reactants,intermediates and products on the Cu2O surface along the path of NO3-reduction to NH4+has been calculated by DFT,The electrochemical reduction of NO3-to NH4+ is an exothermic reaction,which is very advantageous from the thermodynamic point of view.Secondly,the partial density of States and differential charge density further explain that the interaction between Cu2O and NO3-depends on van der Waals force,the high hybrid strength promoted the efficient reduction of NO3-.The above experimental and theoretical studies have proved that Cu2O/Sn-MPC catalyst has high NO3-reduction activity and high NH4+ selectivity.In order to better apply Cu2O/Sn-MPC electrode to the electrochemical reduction of nitrate in groundwater,the influencing factors of NO3--N electrolysis system were optimized.The supporting electrolyte,current density,electrolysis time and initial ammonia concentration were explored.When the supporting electrolyte is 0.2 M Na2SO4,NO3--N concentration is 10 mmol/L,the current is-20 mA,the electrolysis time is 10 h,The conversion of nitrate to ammonia was 77.22%,the selectivity of ammonia is 64.97%.In a word,through the optimization of catalyst and mechanism,combined with the optimization of electrolysis conditions,The Cu2O/Sn-MPC electrode developed in this paper shows a good application prospect in electrolytic reduction of NO3--N to NH4+-N. |
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