Meng Zheng, Luo Jianmin, Li Weiyang, Mirica Katherine A
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States.
Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755, United States.
J Am Chem Soc. 2020 Dec 30;142(52):21656-21669. doi: 10.1021/jacs.0c07041. Epub 2020 Dec 11.
The use of reticular materials in the electrochemical reduction of carbon dioxide to value-added products has the potential to enable tunable control of the catalytic performance through the modulation of chemical and structural features of framework materials with atomic precision. However, the tunable functional performance of such systems is still largely hampered by their poor electrical conductivities. This work demonstrates the use of four systematic structural analogs of conductive two-dimensional (2D) metal-organic frameworks (MOFs) made of metallophthalocyanine (MPc) ligands linked by Cu nodes with electrical conductivities of 2.73 × 10 to 1.04 × 10 S cm for the electrochemical reduction of CO to CO. The catalytic performance of the MOFs, including the activity and selectivity, is found to be hierarchically governed by two important structural factors: the metal within the MPc (M = Co vs Ni) catalytic subunit and the identity of the heteroatomic cross-linkers between these subunits (X = O vs NH). The activity and selectivity are dominated by the choice of metal within MPcs and are further modulated by the heteroatomic linkages. Among these MOFs, exhibited the highest selectivity toward CO product (Faradaic efficiency FE = 85%) with high current densities up to -17.3 mA cm as a composite with carbon black at 1:1 mass ratio) at a low overpotential of -0.63 V. Without using any conductive additives, the use of directly as an electrode material was able to achieve a current density of -9.5 mA cm with a FE of 79%. Mechanistic studies by comparison tests with metal-free phthalocyanine MOF analogs supported the dominant catalytic role of the central metal of the phthalocyanine over Cu nodes. Density-functional theory calculations further suggested that, compared with the NiPc-based and NH-linked analogs, CoPc-based and O-linked MOFs have lower activation energies in the formation of carboxyl intermediate, in line with their higher activities and selectivity. The results of this study indicate that the use of 2D MPc-based conductive framework materials holds great promise for achieving efficient CO reduction through strategic ligand engineering with multiple levels of tunability.
在将二氧化碳电化学还原为增值产品的过程中，使用网状材料有潜力通过以原子精度调节骨架材料的化学和结构特征来实现对催化性能的可调控制。然而，这类体系的可调功能性能仍在很大程度上受到其低电导率的阻碍。这项工作展示了使用由铜节点连接的金属酞菁（MPc）配体构成的四种具有系统结构类似物的导电二维（2D）金属有机框架（MOF），其电导率为2.73×10至1.04×10 S cm，用于将CO电化学还原为CO。发现MOF的催化性能，包括活性和选择性，由两个重要的结构因素分级控制：MPc（M = Co对Ni）催化亚基内的金属以及这些亚基之间杂原子交联剂的身份（X = O对NH）。活性和选择性由MPc内金属的选择主导，并通过杂原子键进一步调节。在这些MOF中，作为与炭黑质量比为1:1的复合材料，在-0.63 V的低过电位下，对CO产物表现出最高的选择性（法拉第效率FE = 85%），电流密度高达-17.3 mA cm。在不使用任何导电添加剂的情况下，直接将用作电极材料能够实现-9.5 mA cm的电流密度，FE为79%。通过与无金属酞菁MOF类似物的对比测试进行的机理研究支持了酞菁中心金属相对于铜节点的主导催化作用。密度泛函理论计算进一步表明，与基于NiPc和NH连接的类似物相比，基于CoPc和O连接的MOF在羧基中间体形成中具有更低的活化能，这与其更高的活性和选择性一致。这项研究的结果表明，使用基于二维MPc的导电框架材料通过具有多级可调性的策略性配体工程在实现高效CO还原方面具有巨大潜力。
