催化学报 ›› 2023, Vol. 53: 102-108.DOI: 10.1016/S1872-2067(23)64519-X

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调节金属有机框架的配体场实现在工业电流密度下将CO2高选择性还原为碳氢化合物

薛欢a, 黄嘉润a, 王志烁a, 赵振华a, 石文b, 廖培钦a,*(), 陈小明a   

  1. a中山大学化学学院, 生物无机与合成化学教育部重点实验室, 广东广州510275
    b中山大学化学学院, 广东广州510275
  • 收稿日期:2023-07-05 接受日期:2023-09-20 出版日期:2023-10-18 发布日期:2023-10-25
  • 通讯作者: *电子邮箱: liaopq3@mail.sysu.edu.cn (廖培钦).
  • 基金资助:
    国家重点研究发展计划(2021YFA1500401);国家自然科学基金(21890380);国家自然科学基金(21821003);国家自然科学基金(22371304);广东省科技创新战略专项市县科技创新支撑项目(STKJ2023078);广东省科技重点项目(2020B010188002);广东省珠江人才计划地方创新研究团队项目(2017BT01C161)

Tailoring ligand fields of metal-azolate frameworks for highly selective electroreduction of CO2 to hydrocarbons at industrial current density

Huan Xuea, Jia-Run Huanga, Zhi-Shuo Wanga, Zhen-Hua Zhaoa, Wen Shib, Pei-Qin Liaoa,*(), Xiao-Ming Chena   

  1. aMOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat−Sen University, Guangzhou 510275, Guangdong, China
    bSchool of Chemistry, Sun Yat-Sen University, Guangzhou 510275, Guangdong, China
  • Received:2023-07-05 Accepted:2023-09-20 Online:2023-10-18 Published:2023-10-25
  • Contact: *E-mail: liaopq3@mail.sysu.edu.cn (P.-Q. Liao).
  • Supported by:
    The National Key Research and Development Program of China(2021YFA1500401);The National Nature Science Foundation of China(21890380);The National Nature Science Foundation of China(21821003);The National Nature Science Foundation of China(22371304);Science and Technology Innovation Special Support Project of Guangdong Province City County, China(STKJ2023078);Science and Technology Key Project of Guangdong Province, China(2020B010188002);Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program(2017BT01C161)

摘要:

电化学CO2还原反应(eCO2RR)利用可再生能源将CO2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CO2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-BTTriCu-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H3BTP (pKa = 19.8) > H3BTTri (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CO2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% (CH4为60%, C2H4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eCO2RR的烃类产物选择性有重要影响. 13CO2同位素电化学实验证实了气液产物中的碳来自于CO2. 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eCO2RR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CO2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-BTTriCu-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eCO2RR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eCO2RR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CO2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.

关键词: 金属有机框架, 金属氮酸盐框架, 碳氢化合物, 电催化,

Abstract:

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 CO2 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, H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene), [Cu3(BTTri)2] (Cu-BTTri, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), and [Cu3(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 CO2 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 (CH4, 60%; C2H4, 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 CO2 to hydrocarbons.

Key words: Metal-organic framework, Metal-azolate framework, Hydrocarbons, Electrocatalysis, Copper