A Study On The Performance Of Pd-based Catalysts In Electrochemical Synthesis Of Ammonia At Ambient Temperature And Pressure

The main way of nitrogen fixation in industry is the Haber–Bosch,but this process requires harsh reaction conditions,high cost and causes environmental pollution.Electrochemical synthesis of ammonia has attracted the attention of researchers because of its advantages including mild working conditions,environmental protection and simple process.At present,the biggest problem in electrochemical ammonia synthesis under ambient conditions is the lack of high-performance catalysts.In this paper,electrocatalytic ammonia synthesis on Pd-Mn composite catalysts and Pd_xAu_y/C alloy catalysts were studied under ambient conditions.The Pd-Mn composite catalysts can be prepared by dispersing Pd nanoparticles on the MnO₂through the polyol reduction method.The results show that Pd-Mn has synergistic catalytic effect in ammonia synthesis,and performance of Pd-Mn composite catalysts in electrocatalytic ammonia synthesis is better than the single metal component catalysts Pd/C and MnO₂.The possible mechanism is Mn adsorbs a large amount of N₂at first because of its strong adsorption capacity for N₂,and then Pd achieves hydrogenation for N₂through its unique hydrogenation ability.This mechanism effectively combining two different advantages of Pd and Mn in the ammonia synthesis process.γ-MnO₂is the best carrier for Pd of the three.Pd nanoparticles can be homogeneously dispersed on its surface at a p H of 7,and the particle size is about 4 nm.The ammonia synthesis rate and Faraday efficiency of Pd/γ-MnO₂catalyst can achieve 19.72μg·mg_Pd^-1h^-1and 8.4%at-0.05 V vs.RHE,which is higher than other Pd-Mn series catalysts under the same conditions.A series of Pd_xAu_y/C alloy catalysts with different Pd-Au ratios can be prepared by co-reduction method.The results show that Pd_xAu_y/C alloy catalysts have good performance of electrochemical ammonia synthesis.The higher proportion of Au,the higher overpotential required for the catalyst to reach the highest ammonia synthesis rate.Otherwise,the experimental results show that the addition of Au can improve the Faraday efficiency of Pd-based catalyst in ammonia synthesis.When the ratio of Pd and Au is 5:1 and 3:1,the ammonia synthesis rate is higher than Pd/C.The ammonia synthesis rate of Pd₅Au/C is 12.22μg·mg_pd^-1h^-1at-0.10 V vs.RHE,which is the highest among these series catalysts,and the Faraday efficiency can reach 3.7%.Pd-based catalysts can increase its long-term stability by introducing Au.The stability of Pd₁Au₁/C is the highest among them.During 12 hours continuous electrolysis,the electrochemical ammonia synthesis rate and Faraday efficiency of the Pd₁Au₁/C catalyst is only decayed by 15%.The stability of Pd₁Au₁/C catalyst is superior to other Pd_xAu_y/C alloy catalysts which have different Pd-Au ratios and Pd/C catalyst.