Preparation Of Doped ZrO₂ And The Catalytic Performance Of Electrochemical Synthesis Of Ammonia

At present,the method of industrial Haber-Bosch is still the principal means of large-scale production of ammonia（NH₃）after a century,but it shows the disadvantages of high energy consumption and pollution.Electrochemical catalytic nitrogen reduction reaction（ENRR）is a promising green alternative to the Haber-Bosch method for NH₃ production.In the ENRR system,the design of catalyst has a crucial influence on the yield and efficiency of NH₃.Noble metal catalysts have good electrical conductivity,nitrogen（N₂）adsorption,and binding ability.Therefore,they show good potential for development,but their scarcity leads to limited large-scale applications.Hence,non-precious metal-based catalysts have emerged as a promising ENRR catalyst.Here,ZrO₂ was used to form different solid solution oxide by doping with metal cations,and 0.1 mol L^-1 Na₂SO₄ was used as the electrolyte and their performance of electrochemical NH₃ synthesis was investigated.The main research results are as follows:（1）Cerium-zirconium solid solution(Zr_xCe_1-xO₂)was synthesized by ammonia coprecipitation with Ce Cl₃·7H₂O and ZrCl₄.Whereafter,the Zr_xCe_1-xO₂ were annealed.The results showed that Zr_0.9Ce_0.1O₂ possessed the best ENRR catalytic performance with the highest NH₃ synthesis rate of 9.34×10^-11mol cm^-2 s^-1 at-1.35 V（vs.Ag/Ag Cl）,and the highest Faraday efficiency（FE）of 62.68%at-1.3 V（vs.Ag/Ag Cl）.The Zr_0.9Ce_0.1O₂ had the most oxygen vacancies,which can provide more active sites to interact with N₂ for its cleavage,smaller charge transfer resistance(R_ct),and relatively larger bilayer capacitance(C_d1).As a result,the Zr_0.9Ce_0.1O₂ can effectively suppress the hydrogen evolution reaction（HER）,which improved the FE and gave it excellent electrocatalytic performance.（2）Hf-doped zirconium-hafnium solid solution(Zr_xHf_1-xO₂)was prepared by sol-gel method with ZrCl₄,HfCl₄,and citric acid as raw materials.The test results showed that Zr_0.6Hf_0.4O₂possessed the best ENRR catalytic performance with the highest NH₃ synthesis rate of 9.04×10^-11 mol cm^-2 s^-1 at-1.3 V（vs.Ag/Ag Cl）when the FE was 16.02%.Structural characterization proved that the Zr_0.6Hf_0.4O₂ solid solution had the best crystallinity with both monoclinic and cubic crystalline phases and good dispersion.The coexisting crystalline phase and excellent dispersion can provide more active sites to participate in the N₂reduction reaction,thus endowing Zr_0.6Hf_0.4O₂ with the best catalytic performance.（3）A series of yttria oxide stabilized zirconia（YSZ）catalysts were prepared using a two-step hydrothermal method with YN₃O₉·6H₂O,ZrCl₄,and urea.The samples were calcined at 600℃for 2 h.The rate of NH₃ synthesis at-1.0 V（vs.Ag/Ag Cl）for the sample with Y doping of 0.1（10YSZ-600）was1.58×10^-10 mol cm^-2 s^-1 with an FE of 57.21%.The structural characterization showed that the structure of YSZ was transformed into a tetragonal phase with the increase of Y doping and the calcination temperature of the sample,which increased the C_d1 and electrochemical specific surface area and promotes the ENRR.