Application And Research Of Palladium-Based Heterojunction In The Electrochemical Synthesis Of Ammonia

Electrochemical synthesis of ammonia can produce ammonia from nitrogen gas（N₂）in the electrolytes by the electrocatalysts at a specific potential under normal temperature and pressure.It is considered to be the most potential candidate as the traditional Haber-Bosch process for the synthesis of ammonia in the future due to its simple operation,environmental protection,clean and low energy consumption.However,electrochemical synthesis of ammonia encounters some tricky problems and thus limits in its applications.For example,the catalysts exhibit limited electrocatalytical active for the synthesis of ammonia.The competition of hydrogen evolution reaction（HER）leads to too low catalytic efficiency.The catalytic mechanism is not clear.In order to solve these problems,theoretical calculation was firstly utilized to demonstrate the possibility of Pd-based heterostructure in the electrochemical synthesis of ammonia in this thesis.Subsequently,Pd-based heterostructures were prepared by electrochemical technologies and applied into the electrochemical synthesis of ammonia.The main research contents are as follows:（1）Density functional theory（DFT）was used to calculate and design optimized Pd/Pd O heterojunction catalysts with different oxygen content.By the Gibbs adsorption energy of N₂/N and H,we found moderate oxygen content can inhibit HER competition that was beneficial for the progress of electrochemical synthesis of ammonia,and N₂was preferentially adsorbed at the junction of the two phases of the heterojunction by distal adsorption mode.In addition,by the density of states of the four catalyst models,it was obtained that with the increase of O content,the conductivity of the catalysts and adsorption capacity for N₂gradually reduced,which further proves that the moderate oxygen content in the catalyst can not only inhibition of HER but also enhance electrochemical synthesis of ammonia.Furthermore,N₂catalytic reduction mechanism of the catalysts with three oxygen contents was calculated.The Pd/Pd O heterojunction with moderate oxygen content has been demonstrated the optimal catalytic mechanism for electrochemical synthesis of ammonia through the distal association mechanism.（2）Three kinds of Pd/PdO heterojunction catalysts with different oxygen content were synthesized by electrodeposition.These catalysts have been applied in the electrochemical synthesis of ammonia.SEM,TEM,XPS have been utilized to characterize the structure,morphology,and composites of the heterojunction catalysts.Through the test of electrochemical performance,it is found that the Pd/Pd O heterojunction catalyst had good electrochemical synthesis of ammonia performance,the maximum ammonia yield was 11.0μg·h^-1mg^-1_cat,the Faraday efficiency（FE）was as high as 22.2%,and it had good stability and selectivity.Furthermore,in-situ Raman and in-situ Infrared spectra（IR）technology were used to explore the catalytic mechanism.It was found that Pd/Pd O heterojunction catalyst with moderate oxygen content via a distal association mechanism to implement the N₂reduction reaction,that was the optimal catalytic mechanism for electrochemical synthesis of ammonia,which is highly consistent with theoretical research.（3）In view of the excellent performances of Pd/PdO heterojunction catalyst for the synthesis of ammonia,the Pd O/WO_xheterojunction catalyst was synthesized by the co-deposition method.The electrodeposition conditions have been systematically investigated to obtain the optimal Pd O/WO_xheterojunction electrocatalysts.Under the optimal experimental conditions,the ammonia yield of the Pd O/WO_xheterojunction electrocatalysts was as high as 47.1μg·h^-1mg^-1_cat.The Faraday efficiency（FE）was 12.5%,and it represented a good stability.