调节金属有机框架的配体场实现在工业电流密度下将CO_(2)高选择性还原为碳氢化合物

Tailoring ligand fields of metal‐azolate frameworks for highly selective electroreduction of CO_(2) to hydrocarbons at industrial current density

电化学CO_(2)还原反应(eCO_(2)RR)利用可再生能源将CO_(2)转化为增值化学品,因而得到广泛关注.然而,目前已报道的电催化剂倾向于将CO_(2)转化为HCOOH和CO,而不是增值化学品.金属有机框架(MOF)由于具有较大的比表面积、可调的结构和丰富的活性位点,已被广泛应用于多相催化、气体储存和气体分离等领域.MOF是研究结构-性能关系的理想平台.金属多氮唑框架(MAF)是一类具有较好化学稳定性的MOF,但在电催化方面的研究很少.本文合成了三种具有相同四铜(Ⅱ)簇单元的金属多氮唑框架(Cu-BTP,Cu-BTTri和Cu-BTT),分别由吡唑、三氮唑和四氮唑衍生配体构成.配体的pKa值依次为H_(3)BTP(pKa=19.8)>H_(3)BTTri(pKa=13.9)>H3BTT(pKa=4.9),表明与BTTri3-和BTT3-相比,BTP3-具有最高的配体场强或配位能力.在相同条件下,对三种MAFs材料进行了电催化CO_(2)还原性能测试.其中,基于吡唑配体的Cu-BTP在流动电解池中电流密度可达1.25 A cm-2,碳氢产物的选择性达到82%(CH_(4)为60%,C_(2)H_(4)为22%).在1.25 A cm-2下连续运行60 h后,催化剂性能未见明显变化.三种MAFs材料对碳氢产物选择性的顺序为Cu-BTP(81%)>Cu-BTTri(60%)>Cu-BTT(49%),与配体的pKa顺序一致,说明配体场的场强对eCO_(2)RR的烃类产物选择性有重要影响.13CO_(2)同位素电化学实验证实了气液产物中的碳来自于CO_(2).X射线衍射图谱、扫描电镜、透射电镜、X射线吸收光谱和X射线光电子能谱结果表明,Cu-BTP具有较好的稳定性.密度泛函理论(DFT)计算揭示了三种MAFs的eCO_(2)RR反应机制.Cu-BTP(Cu:+0.95,N:-0.75)中铜离子和配位氮原子的电荷均低于Cu-BTTri(Cu:+0.99,N:-0.28)和Cu-BTT(Cu:+1.02,N:-0.16),该结果归因于配位N原子周围的电子密度与pKa数值呈正相关.最强的路易斯碱度和配体场场强导致Cu-BTP中金属簇的Cu-N4位点上电子密度最大,因此金属中心的路易斯碱度增强最多.相应地,Cu(Ⅱ)在Cu-BTP中的晶体场稳定化能值(218.48 kJ mol^(-1))大于Cu-BTTri(215.59 kJ mol^(-1))和Cu-BTT(208.23 kJ mol^(-1)),Cu-BTP的价带最大值(VBM)和导带最小值(CBM)的能级也最高.根据Koopmans定理,VBM越高说明催化剂的供电子能力越强,越高的VBM能级越靠近*CO的π*未占据轨道,这不仅有利于Lewis酸CO_(2)的吸附和活化,也有利于*CO和*CHO中间体通过π反键相互作用的吸附和活化.因此,*CO和*CHO在Cu-BTP,Cu-BTTri和Cu-BTT上的吸附能依次降低.总体而言,三种MAF金属中心的路易斯碱度依次为Cu-BTP>Cu-BTTri>Cu-BTT,与它们的eCO_(2)RR催化性能一致.综上,本文通过调节MAF的有机配体的场强度来调控eCO_(2)RR中金属活性位点与关键中间体(*CO,*CHO和*COCHO)的相互作用,进而生成碳氢化合物.其中,具有最强路易斯碱配体的MAF材料在安培级电流密度下对CO_(2)深度还原为碳氢化合物具有较高的选择性,为理性开发设计MOF及相应高性能电催化剂提供了新思路.

The catalytic activity of a metal complex is often contingent upon factors such as its valence state,coordination configuration of the metal ion,and coordination ability of the ligand.Hence,revealing the influence of the ligand field of the active metal center on the selectivity of the product for electrochemical CO_(2) reduction is crucial.Herein,three isostructural metal-azolate frameworks(MAFs)(Cu‐BTP,Cu‐BTTri,and Cu‐BTT)with the same cyclic tetracopper(II)cluster units,namely[Cu3(BTP)2](Cu‐BTP,H_(3)BTP=1,3,5-tris(1H-pyrazol-4-yl)benzene),[Cu_(3)(BTTri)_(2)](Cu‐BTTri,H_(3)BTTri=1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene),and[Cu_(3)(BTT)_(2)](Cu‐BTT,H3BTT=1,3,5-tris(2H-tetrazol-5-yl)benzene)were synthesized using pyrazolate-,triazolate-,and tetrazolate-based ligands,respectively.The synthesized MAFs were subjected to analysis to evaluate their electrochemical capabilities for reducing CO_(2) under identical reaction conditions.Among them,the pyrazolate MAF,Cu‐BTP,delivers a current density of 1.25 A cm−2 in a flow cell device with the highest Faradiac efficiency for hydrocarbons(CH_(4),60%;C_(2)H_(4),22%).Furthermore,the system shows no obvious degradation over 60 h of continuous operation.The order of selectivity of the three MAFs for hydrocarbon production is consistent with the corresponding pKa values of the azolate ligands.Theoretical calculations show that a stronger Lewis basicity of the organic ligand,resulting in a stronger ligand field strength,is conducive to strengthening the binding of metal centers with key intermediates,such as*CO and*CHO.This ultimately leads to the deep reduction of CO_(2) to hydrocarbons.