【作者】 李磊；

【导师】 李白海；

【作者基本信息】 电子科技大学 ， 工程硕士（专业学位）， 2020， 硕士

【摘要】 氨资源作为工业和农业中的最重要原料之一,人类社会对其需求十分巨大。然而目前工业制氨的反应条件严苛,能源消耗巨大,造成的环境污染严重。电催化固氮作为近年来兴起的研究方向,具有常温常压和反应速率可控等明显优势,引起了广大科研人员的研究兴趣。电催化反应过程中,催化剂的选择是其中最重要的一环。近年来,单原子与纳米材料相结合,通过引入单原子调制配位环境的电子结构和几何结构,从而展现独特的催化活性、稳定性与选择性,已经在多种电催化领域取得重要进展。单原子催化剂在电催化氮还原领域的催化活性和反应机理研究目前还相对欠缺。本文旨在研究单原子修饰MoSSe和Mo₂TiC₂O₂二维材料在固氮领域的潜在应用,通过对一系列单原子筛选得到一些具有前景的固氮催化剂材料,并通过电子结构和电荷转移等信息研究其催化反应机理。本文具体研究内容包括如下:（1）通过对一系列单原子修饰的S或Se空位缺陷MoSSe纳米片的N₂和第一步加氢产物NNH的吸附能筛选,得到了具有最具前景的Mo单原子MoSSe催化剂。通过计算研究氮还原（NRR）过程全路径我们发现,Mo单原子S侧沉积表现出最高的NRR活性,仅具有0.49 eV的决速步能垒。对比纯MoSSe和Mo修饰MoSSe态密度发现,S空位中引入Mo单原子将在能带中引入杂质能级,有利于催化过程中的电子转移。通过计算对比析氢（HER）竞争反应能垒发现,HER可以被有效抑制,同时过渡态计算表明嵌入的Mo原子迁移至另一邻位S空位需要高达2.85 eV,其团聚被有效阻止。最后通过对比一系列单原子修饰MoSSe的NNH吸附能(E_*NNH)与其相应的限制电位（U_L）我们发现,这两者存在非常良好的线性关系,即U_L随E_*NNH的增强而降低。这样的线性关系为单原子修饰MoSSe在NRR方面应用提供了一定指引。（2）受实验在Mo₂TiC₂O₂纳米片上成功制备Pt单原子高效析氢催化剂的启发,我们利用DFT理论计算研究了一系列其他单原子修饰Mo₂TiC₂O₂在NRR方面的应用潜力。基于前人的工作结论,我们将第一和最后一步加氢反应能垒设为潜在的决速步,并筛除掉这两步反应能垒高于0.43的单原子催化剂。我们提出的这项标准成功筛选得出了Zr、Mo、Hf、Ta、W、Re和Os七种有前景的NRR催化剂。全路径计算结果表明Zr体系具有最高的NRR催化活性,其U_L仅为-0.15 V,这是目前报道过的最低值。Bader电荷分析表明Zr具有最高的正电中心,并且Zr的引入明显提升了催化剂在费米能级位置的占据态。HER能垒计算对比发现,Zr体系具有很高的NRR催化选择性,并且分子动力学和单原子体系形成能结果证明其具有高热力学稳定性和实验合成可行性。因此Zr单原子修饰的Mo₂TiC₂O₂催化剂具有非常高的在电催化NRR方面的应用潜力。（3）基于对单原子MXene的NRR研究,MXene表面O空位对电催化反应中间体的选择性加氢作用,我们尝试将这种机制引入到更需要选择性加氢过程的CO还原反应过程中。结果表明,这种存在空间限制作用的反应位点的确能够有效地促进CO选择性还原得到CH₄产物。其中W单原子体系展示出最高的CO还原活性,其PDS能垒仅为0.23 eV。最后,我们发现ΔG_ads（*COH）可以作为一个简单的描述符用于描述这种带有空间位阻体系的CORR活性。更多还原

【Abstract】 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 thatΔG_ads（*COH）can be used as a simple descriptor to describe the CORR activity of this system with steric hindrance.更多还原