Study On The Construction Of Catalytic Sites And Structure-activity Relationship Towards Electrochemical Nitrogen Reduction Under Mild Conditions

Ammonia is an important chemical raw material,and human demand for ammonia is increasing year by year.At the same time,the hydrogen storage capacity of ammonia is as high as 17.6%,and it is easy to liquefy.It can be stored and transported by tankers and existing pipe networks.Its decomposition products are H2 and N2,and do not contain C or N oxides.It is environmentally friendly and is a hydrogen storage material with great development potential.Facing increasingly serious environmental pollution and energy shortages,ammonia synthesis has become a hot spot and focus of recent scientific research.The high stability of nitrogen molecules makes the hydrogenation reaction particularly difficult.At present,industrial ammonia synthesis uses a high temperature and high pressure synthesis method,but a large amount of carbon dioxide and other greenhouse gases are generated during the synthesis process,and the process flow of this method is complicated.The equilibrium conversion rate of hydrogen is low.Therefore,researchers are constantly looking for new and efficient ammonia synthesis methods,such as photocatalysis,arc discharge,and plasma synthesis.However,electrocatalysis with controllable yield at room temperature and pressure has attracted the attention of a large number of researchers due to its simple equipment and mild conditions.The research on electrocatalytic nitrogen synthesis of ammonia under mild conditions has brought new opportunities for "green nitrogen fixation" and the storage of renewable electric energy.However,the low efficiency of the catalyst for the synthesis of ammonia,poor product selectivity,and fuzzy reaction mechanism have severely restricted the development of this direction.Therefore,this thesis will focus on the design of a new type of "high-efficiency" electrocatalyst as the main research content,develop a universal synthesis method for a new type of catalyst with high active site exposure,adjust its composition,structure and composition,and prepare a highly active catalyst;optimize the reaction system,establish scientific and unified testing standards.The research content mainly includes the following aspects:1.In order to reduce the amount of precious metals and adjust the electronic structure of precious metals,we used sodium borohydride and tannic acid as reducing agents and dispersants,and highly conductive RGO as a carrier,designed and synthesized PdCu nanocluster catalysts,and successfully proved that the catalyst can electrocatalytic nitrogen reduction under normal temperature and pressure.As the active site for nitrogen reduction,Pd is a very good hydrogen storage material,so the surface will be covered with a large amount of hydrogen,thereby weakening its own nitrogen reduction performance.The introduction of non-precious metal copper,on the one hand,reduces the amount of precious metal Pd,on the other hand,it can accelerate the desorption of hydrogen on the surface of the catalyst.Compared with the single-component catalyst,the output of ammonia has been significantly improved.By adjusting the ratio of adding Pd and Cu,the adsorption and activation of nitrogen by the catalyst can reach the best state.Amorphous catalysis will expose more unsaturated coordination active sites and enhance electrocatalytic activity.Finally,the catalyst achieved an ammonia yield rate of 2.8 ?g mgcat·-1 h-1 at-0.2 V.2.Based on the fact that amorphous materials can expose a large number of dangling bonds and unsaturated sites,we introduced metastable CeOx to successfully prepare amorphous Au nanoclusters,anchor the composite to RGO,and improve the conductivity of the material.While reducing the amount of precious metals,it can effectively improve the catalytic activity of precious metals.Experiments have proved that the catalyst can realize electrocatalytic nitrogen reduction,and its performance is improved compared to pure Au catalyst.Compared with the crystalline Au supported on RGO,the performance of the amorphous catalyst is significantly improved.When the precious metal Au loading is 1.31 wt%,the highest ammonia yield can reach 8.3 ?g mgcat.-1 h-1,and the faraday efficiency is 10.10%.3.Using TiO₂ as the substrate,different reduction methods were used to control the particle size of the supported Au to study the size effect of the electrocatalytic nitrogen reduction of gold nanoparticles on the TiO₂ substrate.Finally,it is concluded that the sub-nanocluster Au(?0.5 nm)supported on titanium dioxide obtained at room temperature and pressure with tannic acid as the reducing agent has better catalytic activity.Through testing and characterization,we found that the Au of the sub-nanocluster forms an Au-O-Ti bond with the titanium dioxide substrate,and the formation of this bond facilitates the adsorption and activation of nitrogen molecules,thereby enhancing the activity of the catalyst for nitrogen reduction to synthesize ammonia.The catalyst synthesis method is simple,and only a small amount of noble metal catalysts are needed to realize the efficient nitrogen reduction synthesis ammonia reaction,which is more conducive to the wide application in the future.Finally,the catalyst can achieve an ammonia production rate of 21.4 ?g mgcat.-1 h-1 and a faraday efficiency of 8.11%at potential of-0.2 V.4.The doping of B will generate Lewis acid centers to control the electronic structure of the material,and theoretical calculations prove that the doping can effectively promote the formation of*N2H.We synthesized B-RGO material with low-cost boric acid and graphene oxide,and studied the effect of doping amount on nitrogen reduction activity.Based on its high conductivity,high electrochemical active area and effective Lewis acid catalytic sites,the non-metallic catalyst achieves a faraday efficiency of 6.7%at-0.2 V and ammonia production rate of 2.51 ?g mgcat.-1 h-1 at-0.3 V.The activity is 11 times and 4 times than that of undoped materials,respectively.