Design And Preparation Of VS₂ Catalysts For Ammonia Production By Electrocatalytic Nitrogen Reduvtion

Ammonia has a wide range of applications in the chemical industry,pharmaceutical intermediates and fertilizer synthesis.Aslo,ammonia is considered as a promising carbon-free energy storage carrier due to its high energy density.The Haber-Bosch process is usually used for industrial ammonia production.However,this process has been largely limited by the high energy consumption and large greenhouse gas emissions.Alternatively,electrocatalytic N2 reduction reaction(NRR)can directly synthesize ammonia from water and air under mild conditions,which is recognized as a competitive route to replace the Haber-Bosch process for ammonia synthesis.However,the NRR has a low ammonia yield and Faraday efficiency due to high N2 activation energy barrier and competitive hydrogen evolution reaction(HER),To this end,reasonably design and develop efficient electrocatalysts is crucial for improving NRR efficiency.In this thesis,VS2 was prepared and used as NRR electrocatalyst.The active sites of VS2 for electrocatalytic NRR was identified and the strategy of inhibiting hydrogen evolution reaction on VS2 was explored.(1)Inspired by the biological nitrogen fixation of nitrogenase(cofactors such as FeMo,FeFe and FeV protein)under environmental conditions,flower-like VS2(FL-VS2)was synthesized by simple hydrothermal method and used in electrocatalytic ammonia production.FL-VS2 showed a high activity for NRR with an ammonia yield of 34.62 μg h-1 mgcat-1,a Faraday efficiency(FE)of 2.09%,which was much better than that of spherical VS2(SL-VS2).A poisoning experiment shows that the V site of FL-VS2 was the active site for NRR catalysis.First-principles calculations reveal that the S-edge site and V-edge site of FL-VS2 are easy to chemiadsorption protons H and N2,respectively.Theoretical studies also show that the transformation from N2 to NH3 is prefer to follow the Hybrid pathway.(2)In order to restrain the competitive hydrogen evolution reaction at the VS2,the edge S was partially removed to enhance the NRR performance.After calcined in argon at 350℃ for 1 hour,the obtained VS2-350 showed high NRR activity with an ammonia yield of 20.29 μg h-1 mgcat-1,a Faraday efficiency(FE)of 3.86%,which was much better than that of VS2(VNH3:15.92 μg h-1 mgcat-1,FE:1.69%).We found that the removal of edge S is an effective strategy of suppressing HER at VS2.Mechanism studies show that the transformation of N2 to NH3 follows the Hybrid 2 pathway,and the removal of edge S is beneficial to the chemisorption and activation of N2.In summary,FL-VS2 and VS2-350 were designed and prepared as efficient NRR electrocatalysts.Combined with experimental results and theoretical calculations,we verified the V is the active site of VS2 for NRR.An efficient strategy of partially removing edge S was developed to further improve the NRR performance by inhibiting hydrogen evolution reaction.These in-depth studies and corresponding strategies provide new avenue for developing high performance electrocatalysts with enhanced surface adsorption and activation ability of N2,as well as high ability of inhibiting HER.