Research On Electronic Regulation And Performance Of Nitrogen Reduction Electrocatalysts

Ammonia is vital to many industrial production processes and the entire human life.It is one of the key factors that determine the development of human society.In2016,globally the amount of produced NH₃ approximately reached 170 million tons,and it is expected to increase at an annual rate of 2.3%.Today,the most widely used synthetic ammonia technology for artificial nitrogen fixation is the Haber-Bosch method,and its output has exceeded 90%of the total synthetic ammonia output.However,this reaction is a highly energy-consuming and environmentally unfriendly reaction process.The annual energy consumption accounts for more than 2%of the total global energy consumption,and the annual carbon dioxide emissions account for1.6%of the total global emissions.Therefore,the search for a green,environmentally friendly,low-consumption and high-efficiency approach for N₂ reduction reaction to produce ammonia has attracted more attention of researchers.Among them,under mild conditions,the electrocatalytic ammonia synthesis method using water and N₂ as raw materials is considered to be one of the possible alternatives to the Haber-Bosch method.However,due to the difficulties of the inertness of nitrogen,the low solubility of nitrogen in aqueous solution,the difficult kinetics owing to the multi-step electron transfer proton coupling process and the strong competitive reaction-hydrogen evolution reaction,there is an urgent need to design efficient and stable catalysts to promote the process of electrocatalytic nitrogen reduction to synthesize ammonia.At present,the development of electrocatalytic nitrogen reduction reaction under mild conditions is still in its infancy,and the electrocatalytic performance indicators of the reported catalysts are not satisfactory.This thesis mainly focuses on the metal-based materials,regulates the electronic structure of the material by multiple methods,combines various characterization methods and theoretical calculation simulations to explore the relationship between its electronic structure and the performance of electrocatalytic nitrogen reduction reaction.The research contents mainly include the following aspects:1.Inspired by the natural biological nitrogenase,Mo is one of the most interesting candidate elements for N₂ reduction reaction catalysts.Designing materials with unique electronic structures and active centers that can activate N(?)N triple bond like the Fe Mo cofactor in nitrogenase,will provide a feasible way to achieve efficient electrocatalytic nitrogen reduction.Introducing oxygen vacancies is an effective strategy to achieve this.Oxygen vacancies are rich in local electrons,which can enhance the adsorption and activation of noble gases.At the same time,through structural optimization such as amorphization strategy,additional defect sites can be introduced to improve the catalytic performance.Therefore,we design the catalyst that combine the amorphous structure with molybdenum oxide.The amorphous Mo O_x material prepared by a simple hydrothermal method exhibits excellent catalytic performance for electrocatalytic nitrogen reduction,with a synthetic ammonia yield rate of 12.70?g h^-1 mg^-1_cat.and a Faradaic efficiency of 7.06%at-0.4V.Amorphous Mo O_x materials have a large number of coordinatively unsaturated sites,surface defects,and unique electronic structures,which are beneficial to the adsorption and activation of nitrogen,thereby enhancing the performances for electrocatalytic nitrogen reduction reaction.2.Through theoretical calculations,it is found that reducing the size of Rh nanoparticles can make it exhibit a high spin polarization state.And the high spin polarization state has a promoting effect on the adsorption and activation of nitrogen in the nitrogen reduction reaction.As a proof-of-concept experiment,Rh/CNT is synthesized using a simple method with Rh nanoparticles supported on carbon nanotubes and is applied as catalyst in electrocatalytic nitrogen reduction reaction under normal temperature and pressure.At a relatively low potential,Rh/CNT exhibits excellent catalytic performances for nitrogen reduction reaction,with a high ammonia synthesis rate of 26.91?g h^-1 mg^-1_cat.(-0.1 V vs.RHE),and a Faradaic efficiency of23.48%(0 V vs.RHE).In addition,the detailed reaction mechanism in electrocatalytic nitrogen reduction reaction is calculated by density functional theory.The high spin and charge density caused by the size effect and the substrate effect are conducive to the adsorption and activation of N₂,reduce the Gibbs free energy of the potential determine step,and enhance the electrocatalytic activity of Rh/CNT for nitrogen reduction reaction.This work further reveals the effect of the unique electronic structure of the catalyst on the catalytic performance of electrocatalytic nitrogen reduction reaction.3.To further study the structure-activity relationship between the electronic structure of catalysts and the performance of electrocatalytic nitrogen reduction,we propose a material design strategy to optimize the active site and electronic structure of perovskite catalysts by doping.By changing the doping amount of Co in La Ni O₃,a series of Co-LNO materials is obtained.And it is found that with the amount of Co doping increasing,the electrocatalytic performance of Co-LNO for nitrogen reduction reaction is positively correlated with its effective magnetic moment.Among them,Co-LNO-3 shows the best catalytic performances with ammonia yield rate of 14.57?g h^-1mg^-1_cat.and Faradaic efficiency of 26.44%at-0.1 V vs.RHE.Various means of representation and the results of density functional theory indicate that the doping of Co provides better active site,and it works with the oxygen vacancies generated in the synthesis process to adjust the electron spin structure of the catalyst and reduce Gibbs free energy of the potential determine step(*NN?*NNH).Furthermore,the calculation results of the partial electron density state show that there is a strong coupling between the d orbital of the active site of Co on OV-Co-LNO and the molecular orbital of N₂,which fundamentally determines its excellent adsorption and activation of N₂.