| With the development of industry and agriculture,nitrate pollution caused by excessive fertilization,improper treatment of industrial wastewater and domestic sewage poses a threat to the natural environment and human health.Electrocatalytic technology shows the advantages of using clean electrons as reducing agent and less secondary pollution,and is often used to remove nitrate.Ammonia,as an important raw material and energy carrier for the production of fertilizers,currently mainly relies on the Haber-Bosch（H-B）method,which operates under harsh conditions and is an energy-and pollution-intensive process.Therefore,it is of great significance to reduce nitrate pollutants to high added value ammonia under mild conditions by electrocatalytic method.However,complex eight-electron reactions need higher requirements on cathode materials,and noble metal catalysts usually have excellent catalytic performance,but they are expensive.In view of the problems of ammonia selectivity and low ammonia synthesis efficiency of transition metal-based catalysts,the transition metal cobalt with high ammonia selectivity was selected as the research object,and the performance of ammonia synthesis and the structure-activity relationship between catalyst and catalytic activity were investigated by ZIF-67 derived Co-based catalytic materials modified by interface engineering and size-controlled engineering.The main findings are as follows:（1）Co nanoparticles rich nitrogen-doped carbon materials（Co/NC-x）were prepared by direct pyrolysis ZIF-67,and applied to the electrocatalytic nitrate reduction to ammonia synthesis.It was found that Co/NC-800 exhibited a higher current density and a lower starting potential than other counterparts,illustrating that Co/NC-800 had higher nitrate reduction activity.The yield of NH3 can reach up to 1352.5μg h-1 mgcat-1 at-1.7 V vs.Ag/Ag Cl,and the NH3 Faraday efficiency and selectivity can reach 81.2%and 98.2%at-1.2 V vs.Ag/Ag Cl.Combined with electrochemical experiments and DFT calculation results,it is confirmed that Co nanoparticles are the main catalytic active sites and the interface effect between Co nanoparticles and N-doped carbon（NC）enhances electron transfer.The introduction of Co0 significantly reduced the adsorption energy of NO2-over Co/NC-800,promoted the desorption of NO2-,facilitated the conversion to NH3,and improved the yield and selectivity of NH3.（2）In order to solve the problem of reduction of active sites caused by spontaneous agglomeration of Co atoms during high-temperature pyrolysis of ZIF-67 and to further improve ammonia yield and Faraday efficiency,Co/NC was regulated on the basis of maintaining the original dodecahedron by adding organic acids to the precursors.The characterization showed that the modified EDTA-Co/NC,CA-CO/NC and VC-Co/NC still maintained the original dodecahedral structure,and the size effect was induced by different organic acid coordination.In which the average particle size of Co nanoparticles of the modified EDTA-Co/NC is about 17.9 nm,the reduction of size can provide more active sites,and the proportion of Co0 and pyrrole nitrogen is higher than counterparts.Moreover,Co0 was proved to be the main active site of NO3RR ammonia synthesis reaction,so EDTA-Co/NC exhibits high NH3 yield of 6458.69μg h-1 mgcat-1at-1.6 V vs.Ag/Ag Cl,and the NH3 Faraday efficiency of 87.81%.Additionally,the crude ammonium chloride was successful separated and purified from electrolyte,indicating that electrocatalytic reduction of nitrate ammonia synthesis has certain feasibility and application prospects. |
