Synthesis Of Iron-based Catalysts And Their Electrocatalystic Performance For Ammonia Synthesis By Nitrate Reduction

The excessive discharge of industrial wastewater and the extensive use of agricultural nitrogen fertilizers make the concentration of nitrates in water bodies increasingly high,which seriously threatens the sustainable development of ecosystems and the safety of human life with the rapid development of industry and agriculture.Various treatment technologies have been proposed for the removal of nitrate in water bodies so as to deal with the harm caused by nitrate pollution.Electrocatalytic reduction as a green and economical technology for treating nitrate and synthesizing ammonia barge it to the forefront.This technolory seemed to have considerable because of simple and efficiency,low cost and controllable treatment products.Design of catalyst is one of the important links in the electrocatalytic nitrate reduction process.Fe-based catalysts which low cost and high activity have been widely applied in recent years.However,Fe-based catalysts have more problems such as aggregation and leaching during the reaction process because of ferromagnetic properties,thus limiting their large-scale use in practical applications.Therefore,it is important to develop novel preparation methods and design and construct high-performance iron-based catalysts to achieve high activity and selectivity for ammonia synthesis by electrocatalytic reduction of nitrates.Based on this,the experiment is mainly divided into the following three parts:1.A series of iron-based catalysts with different morphologies,crystalline phases and surface states,Fe₃（CO）₁₂,Fe₃（CO）₁₂-80,Fe⁰/Fe₃O₄-165,Fe₃O₄-250,and systematically evaluated the performance of these catalysts for electrocatalytic nitrate reduction to ammonia.The optimal reaction conditions were obtained by optimizing the applied cathodic reaction potential,the initial nitrate concentration,the initial p H of the electrolyte,and the type of electrolytic cell.Under the optimal reaction conditions,the best electrocatalytic performance of the Fe₃O₄ nanomicrosphere catalyst Fe⁰/Fe₃O₄-165,obtained at a pyrolysis temperature of 165°C and with a surface enriched with zero-valent iron（Fe⁰）,was achieved with NH₄⁺yields,selectivity and FE up to 5059μg h^-1 mg^-1_cat,respectively,after 2 h of electrocatalytic nitrate reduction reaction in a neutral electrolyte.50.12%and 55%,which were significantly better than other Fe-based catalysts.Combined with the characterization results,it is clear that the excellent catalytic performance of Fe⁰/Fe₃O₄-165 is attributed to its small grain size and abundant Fe⁰ species on the surface,which can enhance the performance of electrocatalytic nitrate reduction for ammonia synthesis through the synergistic interaction with Fe₃O₄.Meanwhile,the nitrate removal rate could reach 92%after 10 h of electrocatalytic reduction reaction in 1000 ppm NO₃^-+0.5 M Na₂SO₄ electrolyte with Fe⁰/Fe₃O₄-165 as the catalyst,indicating its potential for practical application.2.In order to solve the problems that iron-based catalysts are prone to agglomeration and leaching during the catalytic reaction and poor electrical conductivity,and to further improve the activity and stability of iron-based catalysts for electrocatalytic nitrate reduction of ammonia,we have constructed iron-doped macroporous carbon composite catalysts Fe/C by using potassium citrate as the carbon source,obtaining macroporous carbon carriers through high-temperature desorption,and further loading iron carbonyl molecular cluster compounds after low-temperature pyrolysis.A series of Fe/C catalysts with different Fe content,dispersion,presence and surface states were prepared by modulating the ratio of Fe carbonyl molecular cluster compounds and macroporous carbon carriers,and the performance of these catalysts for electrocatalytic reduction of ammonia by nitrate was systematically evaluated.Under the optimal reaction conditions,the Fe/C-12catalysts prepared with a 12:1 mass ratio of macroporous carbon and iron carbonyl molecular cluster compounds exhibited the best electrocatalytic performance,with NH₄⁺yields,selectivity and FE up to 7166μg h^-1 mg^-1_cat,84.62%and 75.53%,which was significantly better than other Fe-based composite catalysts and Fe⁰/Fe₃O₄catalysts.Meanwhile,the nitrate removal rate could reach 92.1%after 10 h constant pressure electroreduction treatment of 1000 ppm NO₃^-+0.5 M Na₂SO⁴ electrolyte.Combined with the characterization results,it is clear that the excellent catalytic performance of Fe/C-12 originates from the Fe active species with good surface dispersion,the abundant Fe²⁺on the Fe surface,the excellent charge transfer energy and the synergistic effect between the carrier and active components.3.Due to the complex environment of the nitrate-containing water bodies actually treated,the catalysts used are required to have a high degree of environmental adaptability.In order to improve the environmental adaptability of the Fe-based catalysts,we prepared a composite catalyst Fe/C₃N₄ with highly stable C₃N₄as the carrier and Fe as the active center by introducing heteroatom N into the carrier based on the catalysts prepared in the previous chapter.The preparation conditions and catalytic reaction conditions of the catalysts were optimized using a similar method as in the previous chapter.The results showed that the optimal Fe/C₃N₄-5catalyst with uniform pore size distribution,large specific surface area,rich nitrogen content and low pyrrole nitrogen content could achieve efficient reduction of high nitrate concentrations,while having a wide p H adaptability.The NH₄⁺yield was maintained at about 26%in the p H=1-11 range,and the selectivity could reach more than 60%,showing high and excellent environmental adaptability.Meanwhile,the nitrate removal rate could reach 86.5%after 10 h constant pressure electroreduction treatment of 1000 ppm NO₃^-+0.5 M Na₂SO₄ electrolyte。...