Research On The Electrocatalytic Nitrogen Fixation Properties Of Transition-Metal Decorated Two-Dimensional Materials

As one of the most important raw materials in industry and agriculture,ammonia resources have huge demand from human society.However,the current industrial ammonia production has severe reaction conditions,huge energy consumption,and serious environmental pollution.Electrocatalytic nitrogen fixation,as a promising research field that has emerged in recent years,has obvious advantages such as moderate reaction temperature,controllable pressure and reaction rate,which has attracted lots of research interest of researchers.In the electrocatalytic reaction,the selection of catalyst is the most important part.In recent years,the combination of single atoms and nanomaterials,through introducing single atoms to modulate the electronic structure and geometry of the coordination environment,exhibiting unique catalytic activity,stability and selectivity,has made significant progress in a variety of electrocatalytic fields.However,studies on the catalytic activity and reaction mechanism of single-atom catalysts?SACs?in the field of electrocatalytic nitrogen reduction are still relatively lacking.The purpose of this paper is to investigate the potential applications of single-atom-decorated MoSSe and Mo₂TiC₂O₂ 2D materials in the field of nitrogen fixation.By screening a series of different single atoms,some promising nitrogen-fixing catalyst materials are obtained,where their catalytic mechanism will be further studied by calculating the electronic structure and charge transfer.The specific research contents of this article include the following:?1?The most promising Mo single-atom MoSSe catalyst was obtained by screening the adsorption energy of N₂ and the first hydrogenated NNH intermediate of a series of single-atom-modified S or Se vacancy-defective MoSSe nanosheets.By studying the full path of the nitrogen reduction reaction?NRR?process,we find that the S-side deposition of Mo single atom exhibits the highest NRR activity,with only a maximum barrier of0.49 eV.Comparing the density of states between pure MoSSe and Mo-decorated MoSSe,it is found that the introduction of Mo single atom in the S vacancy will produce impurity energy levels in the band gap,which is beneficial to the electron transfer.By calculating the energy barrier of hydrogen evolution reaction?HER?,we found that HER can be effectively inhibited,while the transition state calculations show that the embedded Mo atom needs to reach 2.85 eV to migrate to another adjacent S vacancy,which will effectively prevent its agglomeration.Finally,by comparing the NNH adsorption energy(E_*NNH)of a series of single-atom-decorated MoSSe with its corresponding limiting potential?U_L?,we find that there is a very good linear relationship,that is,U_L decreases with the increase of E_*NNH.Such a linear relationship provides some guidance for the application of the single-atom decorated MoSSe in NRR.?2?Inspired by the experimental successful synthesis of single Pt anchored Mo₂TiC₂O₂ nanosheets for high-efficiency HER,we investigated the potential NRR application of a series of other single-atom-decorated Mo₂TiC₂O₂.Based on the conclusions of previous work,we set the reaction barrier of the first and the last hydrogenation processes as possible potential determination steps?PDS?,and screened out SACs with reaction barriers higher than 0.43 eV in these two steps.Our proposed criteria successfully screened seven promising NRR catalysts including Zr,Mo,Hf,Ta,W,Re and Os systems.The calculation results of full NRR reaction paths show that the Zr-SAC possess the highest NRR catalytic activity,and its U_L is only-0.15 V,which is the lowest value ever reported.Bader charge analysis shows that Zr has the highest positively charged center,and the introduction of Zr significantly increases the occupied state of the catalyst at the Fermi level.The HER barrier calculations exhibited that the Zr-SAC has a higher catalytic selectivity towards NRR,and the results of molecular dynamics and formation energy of Zr-SAC prove that it has high thermodynamic stability and experimental feasibility.Therefore,the Zr single-atom decorated Mo₂TiC₂O₂ catalyst has very high application potential in electrocatalytic NRR.?3?Based on the NRR study of single-atom MXene,the selective hydrogenation of the electrocatalytic reaction intermediates will be leaded by the surface O vacancy of MXene.So,we tried to introduce this mechanism into the CO reduction reaction process.The results show that this kind of reaction site with steric hindrance can indeed effectively promote the selective reduction of CO to obtain CH₄ products.Among them,the W single-atom system shows the highest CO reduction activity,and its PDS energy barrier is only0.23 eV.Finally,we found that?G_ads?*COH?can be used as a simple descriptor to describe the CORR activity of this system with steric hindrance.