Wu Yueshen, Liang Yongye, Wang Hailiang
Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States.
Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States.
Acc Chem Res. 2021 Aug 4. doi: 10.1021/acs.accounts.1c00200.
ConspectusMolecular catalysts, often deployed in homogeneous conditions, are favorable systems for structure-reactivity correlation studies of electrochemical reactions because of their well-defined active site structures and ease of mechanistic investigation. In pursuit of selective and active electrocatalysts for the CO reduction reactions which are promising for converting carbon emissions to useful fuels and chemical products, it is desirable to support molecular catalysts on substrates because heterogeneous catalysts can afford the high current density and operational convenience that practical electrolyzers require. Herein, we share our understanding in the development of heterogenized metal phthalocyanine catalysts for the electrochemical reduction of CO. From the optimization of preparation methods and material structures for the electrocatalytic activity toward CO reduction to CO, we find that molecular-level dispersion of the active material and high electrical conductivity of the support are among the most important factors controlling the activity. The molecular nature of the active site enables mechanism-based optimization. We demonstrate how electron-withdrawing and -donating ligand substituents can be utilized to modify the redox property of the molecule and improve its catalytic activity and stability. Adjusting these factors further allows us to achieve electrochemical reduction of CO to methanol with appreciable activity, which has not been attainable by conventional molecular catalysts. The six-electron reduction process goes through CO as the key intermediate. Rapid and continuous electron delivery to the active site favors further reduction of CO to methanol. We also point out that, in homogeneous electrocatalysis where the catalyst molecules are dissolved in the electrolyte solution, even if the molecular structure remains intact, the actual catalysis may be dominated by molecules permanently adsorbed on the electrode surface and is thus heterogeneous in nature. This account uses our research on CO electroreduction reactions catalyzed by metal phthalocyanine molecules to illustrate our understanding about heterogeneous molecular electrocatalysis, which is also applicable to other electrochemical systems.
综述
分子催化剂通常在均相条件下使用，由于其活性位点结构明确且易于进行机理研究，是电化学反应结构-反应性关联研究的理想体系。为了寻找用于将碳排放转化为有用燃料和化学产品的一氧化碳还原反应的选择性和活性电催化剂，将分子催化剂负载在基底上是很有必要的，因为非均相催化剂能够提供实际电解槽所需的高电流密度和操作便利性。在此，我们分享我们对用于一氧化碳电化学还原的异质化金属酞菁催化剂开发的理解。从优化制备方法和材料结构以提高对一氧化碳还原为一氧化碳的电催化活性，我们发现活性材料的分子水平分散和载体的高电导率是控制活性的最重要因素之一。活性位点的分子性质使得基于机理的优化成为可能。我们展示了如何利用吸电子和给电子配体取代基来改变分子的氧化还原性质并提高其催化活性和稳定性。进一步调整这些因素使我们能够实现将一氧化碳电化学还原为甲醇，并具有可观的活性，这是传统分子催化剂无法实现的。六电子还原过程以一氧化碳作为关键中间体。快速且持续地向活性位点传递电子有利于将一氧化碳进一步还原为甲醇。我们还指出，在均相电催化中，催化剂分子溶解在电解质溶液中，即使分子结构保持完整，实际催化作用可能由永久吸附在电极表面的分子主导，因此本质上是非均相的。本综述利用我们对金属酞菁分子催化的一氧化碳电还原反应的研究来说明我们对非均相分子电催化的理解，这也适用于其他电化学体系。
