Preparation And Performance Investigation Of Highly-efficient Catalysts Towards Nitrogen/Nitrate Electroreduction To Ammonia

Ammonia(NH₃)as an important chemical,is widely applied in human society.NH₃ and its derivatives,including urea,constitute the important components of fertilizers.In addition,as the energy storage intermediate and carbon-free energy carrier,NH₃ has received much attention in hydrogen economy in recent years.Since the 20th century,the industrial NH₃production has relied on the traditional Haber-Bosch process.However,the harsh synthesis conditions with high temperature and pressure(400-500?,150-300 bar)and massive CO₂emission for H₂ preparation,have continued to aggravate the global energy consumption and greenhouse effect.Therefore,it is necessary and urgent to develop a low-energy,green and sustainable alternative method for NH₃ synthesis.Electrochemical NH₃ synthesis,using water as proton source and renewable energy as driving force,is expected to eliminate the disadvantages of fossil fuels as H and energy source,meanwhile reduce the carbon usage and emission in chemical industry,which is considered as a mild and sustainable NH₃synthesis process.N₂ as the main component in the air,is regarded as an ideal N source for synthesizing NH₃.Accordingly,the electrocatalytic N₂reduction reaction,if processing at ambient conditions,is a promising NH₃ production method.However,restricted by the high dissociation energy of N?N bond(941 kJ mol^-1),the limited N₂ solubility in H₂O,and the competition of hydrogen evolution reaction,the NH₃ yield and selectivity are still at low levels.Nitrate(NO₃^-)where the dissociation energy of the N=O bond(204 kJ mol^-1)is much lower than that of the N?N bond,is regarded as an alternative N source for electrochemical NH₃ synthesis under mild conditions.Besides,in view of the fact that NO₃^-is a common pollutant in surface and groundwater,NO₃^--NH₃ conversion can not only provide a green and sustainable NH₃ synthesis technology,but also alleviate the worldwide energy and pollution problems.Nevertheless,the efficient and selective conversion of NO₃^-to NH₃ remains a big challenge.Whether N₂ or NO₃^-,the NH₃ synthesis efficiency mainly depends on the design and regulation of catalysts.Hence,the development of high-activity,high-selectivity and high-stability catalysts is the key to realizing efficient electrochemical NH₃ synthesis.In this thesis,the main contents include the following aspects:1.Oxygen vacancies,due to the lower formation energy,are considered as the most common anion vacancies in transition metal oxides.Oxygen vacancies as the active center can enhance the adsorption and activation of inert gas,meanwhile reduce the energy barrier of the reaction.We adopt the hydrothermal method to synthesize the non-precious metal BiVO₄ catalyst,and introduce different concentrations of oxygen vacancies into BiVO₄ by adjusting the initial pH of the hydrothermal reaction.The as-prepared oxygen vacancies-rich BiVO₄ catalyst exhibits the excellent N₂ electroreduction performance at room temperature and pressure,including 8.60?g h^-1 mg^-1_cat.NH₃ yield rate and 10.04%Faradaic efficiency(FE)at-0.5 V vs.RHE,as well as the excellent stability during electrolysis.According to the density functional theory calculations,due to the introduction of oxygen vacancies,V atoms in BiVO₄ with lower coordination and higher spin-polarization with a localized magnetic moment become more favorable active sites.Accordingly,the adsorption and activation of N₂ have been greatly improved,which accounts for the excellent NH₃ synthesis performance of BiVO₄.2.In view of that Fe and Mo elements as components of nitrogenase have brilliant N₂adsorption capacity,meanwhile,Au atom with weak H adsorption have the potential to inhibit hydrogen evolution.Hence,we introduce Au nanoparticles(NPs)into iron molybdate(Fe₂(MoO₄)₃)to synthesize Au/Fe₂(MoO₄)₃ catalyst for the first time,and apply it as the N₂electroreduction working electrode.Combined with experimental and theoretical investigation,we find that the interaction at the interface between Au and Fe₂(MoO₄)₃ can regulate the electronic structure of Fe₂(MoO₄)₃,so that Mo atoms adjacent to Au NPs serve as active sites for not only reducing the reaction energy of the rate-determining step from0.98 to 0.6 e V,but also suppressing the hydrogen evolution reaction.Therefore,the as-prepared Au/Fe₂(MoO₄)₃ achieves 7.61?g h^-1 mg^-1_cat.NH₃ production rate and 18.79%FE at-0.4 V vs.RHE under ambient conditions.In addition,all NH₃ production in this work are monitored by nuclear magnetic resonance as the exclusive quantitative detection method(¹⁴N₂ or ¹⁵N₂).3.The number of active sites in the catalyst is an important factor limiting the NO₃^-reduction performance.When the material size is decreased to the nanometer scale,its surface area will significantly increase,and consequently expose more active sites.Meanwhile,cobalt oxide(Co₃O₄)is regarded as an ideal electrochemical cathode material due to the good catalytic activity and electrochemical durability.Therefore,we prepare Co₃O₄ NPs by precipitation method and apply them as electrocatalysts for NO₃^--NH₃conversion under environmental conditions.The small particle size,weak crystallinity and abundant exposed defect sites of Co₃O₄ NPs increase the catalytic active centers for the NO₃^-adsorption and the subsequent hydrogenation.The as-prepared Co₃O₄ NPs achieve the effective NH₃ current density as high as 383.4 m A cm^-2 at-0.8 V vs.RHE,corresponding to30.401 mg h^-1 cm^-2 NH₃ yield rate and 98.60%FE,which is much higher than that of commercial Co₃O₄.In addition,Co₃O₄ NPs show excellent stability during electrolysis.