Preparation And Modification Of Bismuth-based Catalysts For Photoelectrocatalytic Ammonia Synthesis

Ammonia is considered not only a promising clean energy source,but also an indispensable commodity as an agricultural fertilizer and industrial chemical.In chemical industry,ammonia is mainly synthesized by the Haber-Bosch method,but the raw materials required for ammonia production in this way are obtained by gasification of coal and steam reforming of natural gas,which not only consumes a lot of electricity but also emits carbon dioxide.With the introduction of"carbon peaking"and"carbon neutral"policies in China,it is important to find efficient,stable and sustainable ammonia synthesis methods.In recent years,photoelectrocatalytic ammonia synthesis has been considered as a feasible way to achieve nitrogen reduction at ambient temperature and pressure,but the Faraday efficiency and ammonia yield of photoelectrocatalytic ammonia synthesis are still low due to practical barriers such as nitrogen inertia and competitive hydrogen precipitation reactions.Therefore,the search for superior catalysts is also a top priority at present.Bismuth-based catalysts are characterized by high chemical stability,low toxicity and low cost,but the catalyst itself has low catalytic performance for ammonia synthesis.The catalytic performance of the catalysts for nitrogen reduction reaction can be improved by the strategies such as reasonable construction of defective sites（e.g.doping,vacancies）,addition of co-catalysts or construction of heterojunctions that can effectively promote the migration and separation of carriers.To address the above strategies,this thesis carried out a study of related ammonia catalysts based on Bismuth-based catalysts:（1）The Co Fe-LDH/BiVO₄ composite photoanode material is prepared by electrochemical deposition,high-temperature calcination and hydrothermal method,and Ru/Cu₂O is used as the electrocathode to replace the costly Pt electrode,which in turn forms a three-electrode system with the Ag/Ag Cl chloride reference electrode.Co Fe-LDH has a narrow band gap and metallic properties that promote charge separation at the LDH/BiVO₄ phase interface,and the generated photoelectrons are transferred to the cathode through an external circuit to participate in the nitrogen reduction reaction.The photocurrent of the Co Fe-LDH/BiVO₄ photoanode reaches3.55 m A/cm² at a potential of 1.23 V（vs.RHE）under AM 1.5 G（100 m W/cm²）,and the starting potential is significantly reduced.The overall performance of the constructed photoanode-electrocathode system was improved when the photoanode was illuminated,and a better ammonia yield of 230μmol h^-1 g_cat^-1 as well as an optimum FE of 14.9%were obtained at+0.9V（vs.RHE）bias voltage.（2）Although the photoanode can provide more photoelectrons to promote the reaction of the system,it is also indirectly involved in the nitrogen reduction reaction.P-type BiVO₄ was prepared as a photocathode by hydrothermal method and loaded with Ni Fe OOH co-catalyst by impregnation method.The effects of different impregnation times of the co-catalyst on the photovoltaic performance and nitrogen fixation efficiency of P-type BiVO₄ were investigated in a three-electrode system.XPS demonstrated that the Ni Fe OOH co-catalyst had a large number of oxygen vacancies that could be used as catalytic active sites.Ni Fe OOH/p-BiVO₄exhibited more negative current density in light as well as nitrogen atmosphere,indicating that the catalyst was selective to nitrogen.The composite electrode material achieved an ammonia yield of 83.6×10^-3μmol h^-1 cm^-2(64.3 mmol h^-1 g_cat^-1)at a bias voltage of-0.15 V（vs.RHE）with a Faraday efficiency of 32.3%.（3）Bi OBr_xI_1-x solid solution was prepared by hydrothermal method using carbon paper as the carrier,and then combined with g-C₃N₄ with introduced nitrogen vacancies to form a composite photocathode material.XPS proved that the self-doping of bismuth halide oxide would not change its structure,while EPR proved that the nitrogen vacancy was successfully introduced into g-C₃N₄.BET shows that the composite has a larger specific surface area,which is conducive to nitrogen adsorption.Through DFT calculation,it is further proved that nitrogen vacancy is the active site for nitrogen adsorption and activation,and the remote pathway is easier to carry out the reaction.The NV-g-C₃N₄/Bi OBr_0.75I_0.25 exhibited excellent performance in the three-electrode system and at a bias voltage of-0.4 V（vs.RHE）:a rather high Faraday efficiency（16.5%）and nitrogen fixation efficiency(2.41 mmol h^-1 g_cat^-1).