Preparation Of Non-noble Metal Catalysts And Their Properties For Electrocatalytic Ammonia Synthesis

Ammonia（NH₃）as a common chemical is widely used in industry,agriculture,medicine and other industries,and plays an irreplaceable role in the development of the global economy.In addition,as a transportable hydrogen carrier with high liquefaction temperature and low pressure,it is a potential energy carrier.The traditional Haber-Bosch method needs to be synthesized under high pressure and high temperature,and it also produces a large amount of carbon dioxide,which is extremely unfriendly to the environment.In this paper,the development of a new non-polluting and high-efficiency ammonia production process has become the consensus of global researchers.Electrocatalysis has become an important research direction due to the advantages of mild conditions,energy saving and environmental protection.Common electrocatalytic ammonia synthesis mainly includes two pathways:electrocatalytic nitrogen reduction to ammonia（NRR）and electrocatalytic nitrate reduction to ammonia（NitRR）.The NRR can convert N₂ and H₂O into NH₃ at normal temperature and pressure,which has the advantages of simple operation and environmental friendliness.The required electrical energy can be obtained through the conversion of renewable energy such as wind energy and solar energy.The NitRR uses NO₃^-as the nitrogen source,and can controllably synthesize ammonia under the action of the catalyst.The reaction can use nitrate in wastewater as a raw material,which can not only obtain NH₃ at normal temperature and pressure,but also is expected to solve the problem of nitrate pollution.However,the reaction mechanisms of NRR and NitRR are complex,and there are problems such as low ammonia yield and poor selectivity.Research on low-cost,high-conversion catalysts is the key to breaking through the difficulties.Among them,precious metal catalysts have outstanding performance,but their shortcomings of limited reserves and high price greatly limit their further development,while non-precious metals have become an important research direction due to their low price and abundant reserves.In summary,this work aims to design a non-precious metal catalyst with excellent performance and economical practicality for electrocatalytic ammonia synthesis（NRR/NitRR）reaction,which mainly includes the following two contents:（1）Using a simple wet-chemical method,small（5.9 nm）and well-distributed tin（Sn）nanoparticles supported on carbon black(Sn _SC/C)were prepared for NRR.Due to the addition of an appropriate amount of sodium citrate,C₆H₅O₇^3-covered the small Sn nanoparticles,which could slow down the growth rate of metal particles and prevent the small nanoparticles from agglomerating into large nanoparticles,so the Sn nanoparticles were small in size.The ultra-small metal size has a larger specific surface area and more active sites,as well as better electrical conductivity,which can efficiently catalyze the occurrence of NRR.At-0.4V（vs RHE）,the highest NH₃ yield was 17.28μg h^-1 mg^-1,corresponding to a Faradaic efficiency（FE）was 14.87%.At-0.2 V（vs RHE）,the FE was 22.76%,corresponding to an NH₃ yield was 13.87μg h^-1 mg^-1.At the same time,the stability of Sn _SC/C catalyst was also shown by cycling experiments and transmission electron microscopy.（2）Using a simple chemical method,Cu Mo/C catalysts with small particles（2.3 nm）and uniform distribution were prepared for NitRR.Since the interaction between metals promotes electron transfer,improves the utilization of active materials and reduces the reaction barrier,the NitRR can be efficiently catalyzed.Different proportions of metals have a great influence on the electronic structure changes.Among Cu Mo/C catalysts with different metal ratios,Cu₄Mo₆/C is the most prominent.At-1.1V（vs RHE）,the ammonia yield was 3283.36μg h^-1mg^-1,corresponding to FE was 41.20%.At-1.0 V（vs RHE）,the NH₃ yield was 2525.70μg h^-1 mg^-1,corresponding to FE was 56.45%.At-1.0 V（vs RHE）,there was no obvious change in NH₃ yield and FE after 10 consecutive NitRR,showing a prominent stability.