Design And Preparation Of Double-active-site Nitrogen Reduction Catalyst And Its Performance In Ammonia Synthesis

As an essential chemical product,ammonia has irreplaceable important applications in the fields of industry,agriculture and security.Nowadays,the Haber-Bosch process is still the main way to synthesize ammonia,but it requires a number of energy for production and leads to severe carbon emission.In that case,alternative processes with mild production condition and less pollution are urgently needed.Inspired by the biological nitrogen fixation process carried out by natural microorganisms,Electrocatalytic Nitrogen Reduction（NRR）was proposed,which is driven by electricity and happens on the surface of catalysts with N₂ and H₂O as raw materials to produce ammonia under normal temperature and pressure.However,on the one hand,N₂ is a kind of highly inert component,which is difficult to be activated,and on the other hand,the main side reaction,Hydrogen Evolution Reaction（HER）,also occurs.Therefore,the NRR at ambient condition also faces the problem of low rate of ammonia production and Faraday efficiency.In response to the above problems,this thesis started with the hydrogen evolution reaction,and demonstrated the important role of the catalyst’s H⁺activation ability in the process of catalyzing the reduction of nitrogen to ammonia through the design of dual-active center NRR catalysts,and explored the improvement of the efficiency of nitrogen reduction to ammonia synthesis based on the above knowledge.The research results obtained in this paper are as follows:（1）The important effect of catalysts on the H⁺activation in the process of NRR is discussed.The catalysts can’t be able to exhibit electrocatalytic nitrogen reduction activity unless with strong H⁺activation ability and such property will affect the optimized potential of NRR.For verification,we firstly prepared a variety of HER catalysts by depositing Ni（OH）₂,Ni,Ni₂P and Ni（PO₃）₂on carbon cloths,and then obtained dual-active-site NRR catalysts by doping Mo element as the activation center of N₂.The results indicated that the catalysts with weak H⁺activation ability hardly exhibited NRR activity while Mo-Ni₂P with good HER activation has the best performances.Its highest ammonia production rate reaches to 8.8μg mg^-1h^-1,and the corresponding potential is shifting to a positive potential by 0.1 V than that of Mo-Ni（PO₃）₂with weaker H⁺activation ability which proves the enhancement of H⁺activation is beneficial to reduce the overpotential of NRR.Such results also confirm the two sides of HER towards NRR which is both competitive and collaborative.（2）Based on the above research,considering that HER acts as a dominated role and causes the low production rate of ammonia and low Faradic efficiency,we further achieved synchronized improvement of Faraday efficiency and ammonia production rate by increasing the density of NRR activation sites and adjusting the concentration of nitrogen activation element,Mo.The results show that after increasing the Mo:Ni element ratio to 1:1,the ammonia production rate and current efficiency of the catalyst have been significantly improved.At the same time,especially the highest Faraday current efficiency has been improved significantly,reaching 44.2%at-0.1 V potential.The research work proved the scientificity and feasibility of balancing the competitive reaction of hydrogen evolution by increasing the catalytic active center of nitrogen reduction,which has a good reference significance for the design of high-efficiency nitrogen reduction catalysts.