Controllable Preparation Of Copper Based Nanomaterials And Application In Electrochemical Nitrate Reduction Reaction

The Haber-Bosch process used in the synthetic ammonia industry uses cheap iron catalyst to catalyze the conversion of N₂and H₂into ammonia,and the ammonia produced is further used in the production of nitrogen fertilizer.However,in the next century,human activities released incalculable nitrogen into the environment,resulting in global nitrogen cycle imbalance.Among them,the release of nitrate is the most harmful.In addition,the hydrogen used in the synthetic ammonia industry is obtained by water gas conversion or methane reforming,which consumes a lot of energy.The nitrate in water is converted into an ideal substitute for Haber-Bosch ammonia production in ammonia industry by electrochemical method.This process uses protons in water to replace hydrogen,which greatly improves the production cost and safety,and conforms to the development concept of green chemistry.In the agricultural field,urea is a very important nitrogen fertilizer and is the downstream product of synthetic ammonia industry.In the process of electrolytic nitrate reduction,further coupling carbon dioxide to obtain high value-added urea can not only further realize the green preparation of urea,but also expand the application of nitrate reduction.However,in the reduction of electrolytic nitrate,there are some key problems such as low activity of ammonia generation and low Faraday efficiency.In this thesis,copper-based catalysts are selected to design catalysts from the aspects of surface and interface structure and electronic structure control of catalysts,which are used to catalyze nitrate to ammonia and C-N coupling to urea.The main contents are summarized as follows:1.A two-step method was used to prepare a copper monoatomic catalyst loaded on a double-mesoporous nitrogen-doped carbon for the electro-catalytic reduction of nitrate（NO₃RR）to ammonia.This project aims to improve the NH₃production activity of NO₃^-by designing and constructing monatomic active sites to maximize the number of active sites and regulating the intrinsic activity of active sites.By coating Cu precursor in the ZIF-8 channel,it can be converted into Cu-N₄monoatomic active site at high temperature.Through theoretical calculation,it is found that the Cu-N₄active site formed reduces the critical step energy barrier from NO₃^-to NH₃relative to the Cu cluster.In addition,theoretical calculations also found that the Cu-N₄site promoted the conversion of the intermediate products*N₂O₃into*NO and*HNO₂instead of continuing hydrogenation to obtain N₂,significantly improving the selectivity of NO₃^-to NH₃.At the same time,in the process of high temperature calcination,ZIF-8 was transformed into a dual-mesoporous carbon carrier with different inner and outer pore sizes under the catalysis of Cu atoms.The mass transfer kinetics of NO₃^-and products can be improved by larger internal pore size.Based on the improvement of intrinsic active sites and mass transfer kinetics,the formation rate of ammonia reached 13.8 mol _NH3g_cat.^-1h^-1at-1.0 V（relative to reversible hydrogen electrode）on the carbon loaded Cu monoatomic catalyst with double mesoporous nitrogen,Faraday efficiency reaches 95.5%.Cu-N-C catalyst can simulate the scenario of industrial ammonia synthesis under the condition of high current density(about 200 m A cm^-2)and 9 times of electrolyte volume amplification,and can maintain the stability test of NO₃RR for 120 hours.After continuous electrolysis in the flow cell,3.6 L,0.1 M ammonia solution was finally obtained.This study provides a new idea for the design of high activity,high selectivity and practical electrocatalyst for nitrate reduction to ammonia.2.This thesis reports a layered two-dimensional nitrate reduction reaction（NO₃RR）.The results show that the insertion of potassium ion（K⁺）in situ electrochemistry enlarges the interlayer spacing,thus triggering the NO₃RR between layers.Obtainedα-Ni_0.902Cu_0.098（OH）₂ultra-thin nanosheet has high NO₃RR performance,and the ammonia yield is 13.4 mol _NH3g_cat.^-1h^-1at-0.6 V（RHE）.The Faraday efficiency of NO₃^-to NH₃is 98.9%,which is much higher than that of copper-based electrocatalysts.In addition,the catalyst has good cycle stability,can maintain 20 cycles continuously,and has no obvious activity and ammonia Faraday efficiency decay.At the same time,the results of in-situ electrochemical Raman spectroscopy revealed the reaction pathway of NO₃^-to NH₃.3.We report charge polarized Cu^δ+-Pd^δ–dual sites in Cu single atom alloy to stabilize the key C-and N-intermediates to promote C-N coupling.By precisely tuning atomic dispersion of Cu sites and optimizing the carrier,Cu₁Pd-Fe Ni（OH）₂composite catalyst achieves recorded urea production rate of 436.9 mmol g_cat.^-1h^-1and Faradaic efficiency of 66.4%,as well as ultra-long cycling stability（1000 h）.In situ spectroscopic results decode the formation of C-N bond by coupling of*CO and*NH₂.Theoretical calculation reveals that Cu doping in Pd lattice promotes the deep reduction of NO₃^-to*NH₂,and Pd-Cu dual sites lower the energy barrier of the pivotal C-N coupling.This work provides a new perspective for the design of efficient,selective and robust electrochemical carbon-nitrogen coupling catalysts from the perspective of monoatomic alloys.