Study On Preparation And Performance Of Cathode Catalyst In Electrochemical Synthesis Of Ammonia

Ammonia synthesizes about 200 million tons per year.It is a key precursor for the production of chemical fertilizers.It is a convenient hydrogen carrier and emerging clean energy.It is widely used in industries such as industry,agriculture and medicine.It is one of the most important chemicals in the human and planetary ecosystem.One.Before the invention of synthetic ammonia,the ammonia on earth mainly came from the nitrogen-fixing reaction of nitrogen-fixing microorganisms.In 1909,the German chemist Haber produced ammonia from the air for the first time.In 1918,German chemist Bosch improved the high-pressure synthetic ammonia method pioneered by Haber,making it possible for synthetic ammonia to be industrially produced.Now this method of industrial synthesis of ammonia is called the Haber-Bosch method.However,the industrial synthesis of ammonia needs to be carried out under high temperature and high pressure.The high temperature and high pressure makes the reaction extremely energy-intensive,the reversible reaction conversion rate is low,and a large amount of greenhouse gases are also produced in the process.The electrochemical method can use H₂O as the hydrogen source and N₂as the nitrogen source to carry out ammonia synthesis at room temperature and normal pressure,which makes the future green synthetic ammonia industry an alternative route and has received widespread attention.Using N₂as the nitrogen source to obtain NH₃through the reduction and hydrogenation process requires destabilizing N≡N.In order to reduce the energy consumed in the process,a catalyst is required to have a strong adsorption and activation effect on N₂.At the same time,a competitive hydrogen evolution reaction will occur during the electrochemical synthesis of ammonia,which makes the Faraday efficiency of ammonia synthesis in this process low.Therefore,the reduction of N₂to synthesize ammonia by electrochemical methods has higher requirements on the catalyst.At present,the research on the electrochemical synthesis of ammonia is also focused on the study of catalysts.In this paper,inspired by the elements of Fe and Mo contained in biological nitrogenase,a series of transition metal sulfides have been prepared as catalysts.First,Fe₃S₄was synthesized by solvothermal method,and CoS₂and NiS₂were subsequently prepared.The NRR performance of these three catalysts in alkaline electrolyte was tested.The results showed that among the three transition metal catalysts,Fe₃S₄has the best catalytic performance for ammonia production.The rate reached 10.94 mg·h^-1·mg^-1_cat,and the efficiency was 4.44%.The catalytic performance of these three catalysts was tested under acidic conditions,and it was found that the ammonia yields of the three catalysts were improved compared with alkaline conditions.Fe₃S₄has the best catalytic performance,the synthetic ammonia yield reaches 14.51 mg·h^-1·mg^-1cat,and the efficiency is 15.12%.Subsequent use of simple hydrothermal method,using ammonium molybdate as the molybdenum source,and thiourea as the sulfur source,to synthesize the curd-shaped nano-molybdenum disulfide.The performance of electrochemical synthesis of ammonia was tested.The ammonia yield was 3.07 mg·h^-1·mg^-1_catand the Faraday efficiency was 3.34%,both of which were not high.In order to improve its catalytic performance,zinc oxide is introduced to increase its conductivity.Preliminary experiments used chemical methods to synthesize nano-sized zinc oxide.Then,the prepared molybdenum disulfide was added in the process of preparing zinc oxide,and the zinc oxide was allowed to grow on the molybdenum disulfide to form a composite material.The catalytic performance of the composite material was tested.The optimal ammonia yield and Faraday efficiency were 14.48 mg·h^-1·mg^-1_catand11.3%.Compared with pure MoS₂,the ammonia yield and Faraday efficiency of MoS₂-Zn O composites are greatly improved.At the same time,the effects of different temperatures（700°C,800°C,900°C,1000°C）on the catalytic performance of the catalyst were also explored,and it was found that the catalytic performance after 900°C firing treatment was the best,and the yield and Faraday efficiency were respectively 10.51 mg·h^-1·mg^-1_catand 15.1%.