Construction Of Tin-based Catalysts And Their Application In Electrocatalytic Synthesis Of Ammonia

Ammonia,plays a crucial role in the development of modern agriculture and industry.Compared with the traditional Haber-Bosch ammonia synthesis method in industry,electrocatalytic nitrogen reduction reaction（NRR）synthesis of ammonia is considered to be a promising and environmentally friendly strategy for ammonia synthesis.The construction of suitable electrocatalysts is the key to realize efficient electrocatalytic NRR.Among them,tin,is a non-precious metal and non-transition metal element.Tin-based materials have been widely used in catalysis,energy storage,photovoltaics and other fields.Among tin-based materials,Snsites have weak activity for hydrogen evolution reaction（HER）and strong adsorption capacity for nitrogen,which are considered as potential ideal electrocatalytic nitrogen reduction catalysts.However,pure tin-based materials inhibit the progress of electrocatalytic reactions due to their poor electrical conductivity.Therefore,it is necessary to improve the interfacial electron transfer of tin-based materials through doping or recombination strategies to facilitate the subsequent electrochemical processes.For the electrocatalytic NRR,from the perspective of bionics,nitrogenase is the main reaction medium for biological nitrogen fixation,among which iron,molybdenum and sulfur are the important elements composing nitrogenase.Introducing elements such as iron,molybdenum,and sulfur into tin-based catalysts can not only improve the electron transport ability of tin-based materials,but also build more active sites for nitrogen reduction.The specific research contents of this thesis are as follows:1.Fe-SnO₂ nanosheets were constructed by the method of hydrothermal combined with high temperature annealing.The composition,morphology and element valence state of the samples were characterized by XRD,SEM,TEM and XPS.The results of transient photovoltage test prove that Fe doping strategy can effectively improve the electron transport ability of SnO₂.The test results of electrocatalytic NRR show that Fe-SnO₂ can achieve the best yield at-0.8 V vs.RHE(NH₃ yield:28.45μg h^-1 mg^-1_cat,FE:6.54%),at the same potential,it is 2.10 times the NH₃ yield of pure SnO₂(NH₃ yield:13.52μg h^-1 mg^-1_cat).And in the cycle stability and long-term stability test,it can still maintain a good nitrogen reduction ability.The Fe-SnO₂ was modeled by VASP and the reaction pathway of NRR was studied.The calculation results of density functional theory（DFT）showed that the Fe-doping strategy could enhance the adsorption of nitrogen at the adjacent sites,and the reaction proceeded according to the alternate hydrogenation pathway.2.Firstly,MoO₃ nanowires were prepared by a hydrothermal method,and then Sn²⁺was attached to the MoO₃ surface by a one-step solvothermal method,and at the same time,the hard template was etched by DMF.After high temperature annealing,Mo-SnO₂ nanoparticles supported on carbon shells were prepared.The composition,morphology and element valence state of the samples were tested by XRD,SEM,TEM,XPS and other characterization methods.By adjusting the ratio of Mo to Sn,a series of Mo-SnO₂/C with different ratios were prepared.In the experiment of electrocatalytic nitrogen reduction,it was found that when Mo:Sn=1:2,Mo-SnO₂/C can be achieved the best NH₃ yield(24.03μg h^-1 mg^-1_cat)at-0.8V vs.RHE,which was 1.81 times that of pure SnO₂ at the same potential.The results of transient photovoltage test prove that Mo-SnO₂ and carbon shell can delay the release rate of interfacial electrons,improve the electron utilization rate in electrochemical tests,and realize the continuous and stable electrocatalytic NRR.3.The Sn-Tu precursor,thiourea and glucose were mechanically mixed by ball milling,and the ball-milled samples were annealed in an argon atmosphere to obtain SnS/NSC mesoporous nanosheets.The successful preparation of SnS/NSC mesoporous nanosheets was confirmed by means of XRD,SEM,BET and Raman.By adjusting the annealing temperature,it was found that the SnS/NSC prepared at 800℃could achieve the best nitrogen reduction ability at-0.40 V vs.RHE(NH₃ yield:29.06μg h^-1 mg^-1_cat),which was 3.90 times that of SnS(NH₃ yield:7.45μg h^-1mg^-1_cat),which is 3.32 times that of NSC nanosheets(NH₃ yield:8.74μg h^-1 mg^-1_cat).It shows that the composite of SnS and NSC can effectively improve the NRR ability of tin-based materials.The test results of transient photovoltage revealed the electron transport mechanism of SnS/NSC.With the increase of annealing temperature,the time taken for electrons to return to the ground state gradually decreases,and when the temperature reaches 800℃,SnS/NSC exhibits excellent electron transport ability.At the same time,with the support of Snsites and S sites,SnS/NSC-800℃achieved nitrogen reduction tests at small overpotential.