Preparation Of Sulfur Modified Three-dimensional Porous Carbon Material And Study On Electrocatalytic Conversion Performance Of Nitrogen

Ammonia plays an important role in the economy and development of human society.Ammonia is not only the main upstream product of nitrogen-containing chemical reagents,but also an important raw material for the production of fertilizer.At present,the ammonia production mainly depends on energy-intensive Haber-Bosch industrial ammonia technology.But with the development of the society,the earth resources,limited fossil fuel reserves,intensifying environmental pollution problem increasingly prominent,the scientists had to find the environment friendly and energy sustainable ways of new type of synthetic ammonia.In order to alleviate the traditional ammonia process because of harsh production conditions brought about by energy consumption,serious pollution and other problems.In recent years,with the deepening of research by scientists,electrochemical ammonia synthesis method has been considered as one of the most promising new ammonia synthesis technologies for realizing catalytic conversion of N₂ due to its mature mechanism,environmental friendliness and renewable energy sources.The catalyst was the key to reduce the reaction energy barrier and activate the reactants.Due to the strong bond energy(940.95k J·mol^-1)of N₂ molecules,it was difficult to be activated to participate in the synthesis reaction.In the process of ammonia synthesis,high efficiency and high selectivity catalyst was needed to reduce the activation energy of N?N reaction.Therefore,the design of catalysts with excellent synthesis performance was the key to achieve efficient synthesis of ammonia under mild conditions.Among them,precious metal catalysts are the earliest studied catalysts for electrocatalytic ammonia synthesis.Although most precious metal catalysts show excellent performance for ammonia synthesis,problems such as high cost,scarce resources and low utilization rate of active metals restrict the large-scale preparation and industrial application of precious metal catalysts.The development of transition metal catalysts has made up the defect of precious metal in cost to some extent,but ion pollution and easy agglomeration of catalytic centers have become the bottleneck of the development of transition metal catalysts.In recent years,nonmetallic catalysts have emerged in the field of energy storage and electrocatalysis due to their advantages of low cost,abundant reserves,structural diversity and tunability.The purpose of this thesis was to design and prepare a high-efficiency carbon-based nonmetallic catalyst for the study of electrocatalytic performance of nitrogen reduction under mild conditions.In this study,a series of three-dimensional porous carbon catalysts modified with sulfur were prepared for the electrocatalytic synthesis of ammonia by using the method of low temperature sulfur-carrying with the stable three-dimensional porous carbon as the carrier,and all of them showed excellent electrochemical catalytic stability and high selectivity of N₂catalytic conversion.The research work was mainly divided into the following four parts:1.Using self-assembled three-dimensional graphene(3DG)as the support,sulfur modification in three-dimensional graphene support to obtain a composite catalyst(S/3DG),for the study of electrocatalytic nitrogen reduction performance under mild conditions.The effects of nitrogen penetration time,ventilation rate and membrane service time on the reaction results were systematically investigated.The optimal test conditions of catalytic reaction were determined as nitrogen penetration time of 30 min,ventilation rate of 20m L/min,and ultrasonic cleaning of membrane was needed to ensure the accuracy of the experimental results.The unique cross-linked three-dimensional structure of 3DG provided a conductive network for electron transport,which greatly improves the reaction electron transport efficiency.Moreover,the increase of defect density of 3DG due to partial sulfur doping leads to the increase of catalytic active sites,which contributed to the improvement of catalytic performance.Under the optimum test voltage of-0.6V,the ammonia yield of S/3DG was 38.81?g _NH3mg^-1_cat.h^-1,and Faraday efficiency was 7.72%.The stable catalytic performance of the catalyst was proved by six cycle tests,and the excellent selectivity of the catalyst to N₂reduction was proved by the detection of no by-product N₂H₄.2.Using ordered mesoporous carbon material CMK-3 as sulfur carrier,catalyst S/CMK-3 was prepared,and the performance of ammonia synthesis was tested.On the one hand,due to the limiting effect of the CMK-3 channel,a large number of sulfur nanoparticles grow in the CMK-3 channel,which improves the hydrophobicity of the catalyst to inhibit HER reaction.On the other hand,the doping of some sulfur atoms in the carbon material causes the change of the electronic structure of the neighboring carbon atoms.According to Raman data,the value of I_D/I_G increases from 0.90 to 0.99,indicating that the disorder of the catalyst increases.In addition,the ordered through-hole of CMK-3 also provided a channel for electron transport,which improves the Faraday efficiency of ammonia synthesis.The results show that the ammonia yield of catalyst S/CMK-3 was 31.19?g _NH3mg^-1_cat.h^-1 and Faraday efficiency was 13.36%at-0.3V.3.Using the biomass porous carbon(DSPC)obtained from durian shell pyrolysis as the carrier,the preparation conditions for obtaining the biomass multi-porous carbon carrier suitable for electrocatalytic reaction were determined by investigating the roasting temperature,C/base ratio and activation time.When the carbon/base ratio was 1:4,the calcination temperature was900°C and activated for 3 h.Three catalysts,N-DSPC,S/DSPC and S/N-DSPC,were prepared by nitrogen doping,sulfur modification and sulfur-nitrogen co-doping respectively,and their performance of electrochemical synthesis of ammonia was tested.The results showed that the synergistic effect of diheteroatom doping could significantly improve the catalytic performance of the catalyst.Under the same test conditions,compared with N-DSPC and S/DSPC,the S/N-DSPC catalyst obtained the highest ammonia yield of 32.05?g _NH3mg^-1_cat.h^-1.4.Nitrogen-doped multistage porous carbon(N-MPC)derived from zeolite-like metallic organic framework material(MAF)constructed with nitrogen ligands was used as a carrier to carry sulfur at low temperature,and the catalyst(S/N-MPC)was successfully obtained,and the electrocatalytic nitrogen fixation was studied.First of all,the unique multi-stage pore structure of N-MPC provides effective space for sulfur modification,nitrogen adsorption,and electrolyte infiltration.Second,in the S/N-MPC catalyst,the synergistic effect of the diatomic doping of S and N increased the catalytic activity,and the best ammonia yield and Faraday efficiency were 45.51?g _NH3mg^-1_cat.h^-1 and 25.16%at-0.3 V and-0.2 V.The DFT results confirmed that the reaction path of the catalyst S/N-MPC is an alternate hydrogenation path in the association mechanism.The activation energy required in the rate-determining step of the ammonia synthesis process was reduced from 2.32 eV to 2.23 eV by the modification of sulfur.