Reserch On Interface Structure Of Catalyst And Its Catalytic Characteristics For Electrochemical Synthesis Of Ammonia

Ammonia is one of the most annually produced inorganic compounds in the world.It is also an intermediate for the production of chemical fertilizers and important chemical raw materials.It plays an important role in the development of the national economy.Ammonia is mainly synthesized from hydrogen and atmospheric nitrogen by the Haber-Bosch process.The Haber-Bosch process requires usually high-temperature and high-pressure operating conditions,inevitably leading to high energy consumption with low conversion rate.and contributing to the greenhouse gas emissions.It is urgent to develop green ammonia synthesis technology under mild conditions.Electrochemical ammonia synthesis can break the thermodynamic energy barrier of nitrogen activation under driving by electrical energy,and can realize the synthesis of ammonia from water and nitrogen at normal temperature and pressure,thus attracting widespread attention.In this paper,activated carbon?AC?and carbon spheres?CS?are used as carriers,respectively.Ni O/AC and Ni O/CS electrocatalysts are prepared by the equal volume impregnation method,and characterized by XRD,SEM,CV and EIS.Electrochemical synthesis of ammonia is carried out using KOH-Na OH molten salt as the electrolyte,and water and nitrogen as raw materials.Microscopic analysis shows that the active component of the catalyst is Ni O,which uniformly supports on the surface of the AC in form of Ni O nanorods with the length of about 400 nm and diameter of about 20 nm.Too high or too low nickel-carbon ratio in the catalyst lead to decrease in the performance of catalyst.Increase in the calcination temperature also cause the decrease in the catalyst performance.Followed by continuing to increase the temperature,the ammonia production rate and FE reduce.As the reaction temperature of ammonia synthesis increases,both the ammonia production rate and Faraday efficiency?FE?increase.Increase of the electrolysis voltage should promote the rate of ammonia production,but the FE decreases.At 1.75 V,the ammonia production rate can reach a maximum of 7.96×10^-9mol·s^–1·cm^–2,and the maximum FE can reach 14.23%at 1.15 V.Compared with stainless steel mesh,the use of tin-lead alloy cathode can improve ammonia production.The ammonia synthesis process is divided into two steps:electrolysis and ammonia production.As a result,the ammonia production rate and FE are significantly improved.The introduction of a small amount of Fe to Ni O/AC catalyst can improve the selectivity of the catalyst.The reaction path for electrochemical synthesis of ammonia from H₂O and N₂at atmospheric pressure:Ni O is electro-reduced to Ni at the cathode,and the generated Ni reacts with H₂O and N₂under the action of Ni O/AC to generate ammonia.The catalyst was prepared by using glucose as carbon source and Ni?NO₃?_2·6H₂O as nickel source through hydrothermal,impregnation and calcination steps.The results show that the prepared catalyst has a spherical structure with a diameter about5.4?m.Ni O nanorods grown in situ on the CS surface are with a length about 400nm and a diameter about 100 nm.Cyclic voltammetry studies show that Ni O in the catalyst can be reduced to Ni under the electric field,and CS can inhibit the hydrogen evolution reaction.In the preparation process of the catalyst,either too high or too low nickel-carbon ratio reduce the performance of the catalyst.The appropriate ratio of nickel to carbon is 3:10.Adding the additive of cetyltrimethylammonium bromide should reduce the catalytic ammonia production over Ni O/CS.The increase in electrolysis voltage significantly increase the ammonia production rate.Ammonia production rate reaches a maximum value of 2.72×10^-8mol·s^–1·cm^–2at 1.55 V,but the FE is significantly reduced.FE reaches a maximum value of 69.56%at 1.15 V.Compared with the Ni O/AC catalyst,the ammonia production rate is increased by an order of magnitude,and the FE is increased by about 4.9 times.