Design Of Zirconium-based And Iron-based Electrocatalysts And Their Nitrogen Fixation Performance

NH₃ plays an indispensable role in agriculture and in the chemical industry.However,its large-scale production still relies deeply on the century-old Haber–Bosch process under high temperature and pressure along with CO₂ emission and fossil fuel consumption.The electrocatalytic N₂ reduction reactions（NRR）for NH₃ production is favorable approaches to avoid these issues because it is carbon-neutral and energy-saving.The biggest problem faced in electrocatalytic nitrogen reduction ammonia synthesis process is the lower yield and selectivity of ammonia,and how to achieve high yield and Faraday efficiency is the focus of future research.There are three main types of NRR catalysts reported so far:transition metals-based catalysts,noble metal catalysts and non-metallic catalysts.Among them,the yield of transition metals-based catalysts is generally low,while the catalytic performance of precious metal catalysts is good,but the high cost and scarcity limit their large-scale applications.Transition metals-based materials are receiving increasing attention due to their special d-orbital structures,easy synthesis,controlled stability and lower cost.But d-orbital electrons can also favor the formation of metal-H bonds,leading to low selectivity in electrocatalytic nitrogen reduction ammonia synthesis reactions.In addition to this,there is the issue of N₂ activation.To address the above problems,this thesis focuses on the design of zirconium-based and iron-based catalysts in transition metals and their performance in electrocatalytic nitrogen reduction for ammonia synthesis.The catalysts were rationally designed by modulating defects and surface modification to improve their electrocatalytic nitrogen reduction performance.And the theoretical calculation results are combined to reveal the NRR reaction mechanism.The main research work of this thesis is as follows:（1）Zr reserves are abundant and inexpensive,and the resources are widely distributed,which has the prospect of large-scale industrial applications.According to previous reports,it is known that Zr metal surface has strong ability to adsorb N atoms under certain electrochemical windows.This facilitates the electrocatalytic nitrogen reduction reaction.Therefore,we focused our research on Zr-based catalysts.We applied commercial ZrO₂ nanoparticles for the first time in the field of electrocatalytic nitrogen reduction for ammonia synthesis.In 0.1 M HCl,the NH₃ yield rate reached24.74μg h^-1 mg^-1_cat.when the potential value reached-0.45 V,the FE was:5.0%.（2）ZrS₂ has attracted much attention and the corresponding nanowires,nanotubes and nanobelts have been synthesized in the past decade.Because of its high electrical conductivity,it allows for more efficient electron transfer and facilitates applications in electrocatalysis.To enhance the adsorption of N₂,on the basis of ZrO₂,ZrS₂ nanofibers with sulfur vacancy defects were prepared using electrostatic spinning technique and capped tube vulcanization method.By electrochemical tests,the FE reached a maximum value of 10.33%at a potential value of-0.30 V in 0.1 M HCl,and the NH₃production rate reached 30.72μg h^-1 mg^-1_cat.at a potential value of-0.35 V.（3）Fe,which is abundant and cheaply available,is also used in industrial ammonia synthesis catalysts.Phosphide is a good electron conductor material,but it is also a typical catalyst for hydrogen evolution.In order to attenuate the hydrogen evolution reaction,we used hydrophobic alkylthiols covered with Fe₃P to give it excellent performance in electrocatalytic nitrogen reduction for ammonia synthesis.After a series of electrochemical tests and calculations,the ammonia production rate reached 1.80×10^-10 mol s^-1 cm^-2 at a potential value of-0.30 V in 0.1 M Na₂SO₄,with a FE of 11.22%.Compared with Fe₃P unmodified by alkylthiols,the NH₃ production rate(2.16×10^-11 mol s^-1 cm^-2)was increased by 8.33 times and the FE（0.9%）was increased by 12.6 times.Theoretical calculations showed that the modification of alkylthiols not only improved the hydrophobicity of Fe₃P,but the formation of Fe-S bonds also changed its chemical activity.