DFT Study on the Electrocatalytic Reduction of CO2 to CO by a Molecular Chromium Complex

A variety of molecular transition metal-based electrocatalysts for the reduction of carbon dioxide (CO2) have been developed to explore the viability of utilization strategies for addressing its rising atmospheric concentrations and the corresponding effects of global warming. Concomitantly, this approach could also meet steadily increasing global energy demands for value-added carbon-based chemical feedstocks as nonrenewable petrochemical resources are consumed. Reports on the molecular electrocatalytic reduction of CO2 mediated by chromium (Cr) complexes are scarce relative to other earth-abundant transition metals. Recently, our group reported a Cr complex that can efficiently catalyze the reduction of CO2 to carbon monoxide (CO) at low overpotentials. Here, we present new mechanistic insight through a computational (density functional theory) study, exploring the origin of kinetic selectivity, relative energetic positioning of the intermediates, speciation with respect to solvent coordination and spin state, as well as the role of the redox-active bipyridine moiety. Importantly, these studies suggest that under certain reducing conditions, the formation of bicarbonate could become a competitive reaction pathway, informing new areas of interest for future experimental studies.

为应对大气中二氧化碳（carbon dioxide, CO₂）浓度持续升高及其引发的全球变暖效应，研究人员已开发出多种过渡金属基分子电催化剂（electrocatalysts）用于二氧化碳还原反应，以期探索可行的资源化利用策略。与此同时，随着不可再生石化资源的逐步耗竭，该技术路线亦可满足全球对高附加值碳基化工原料不断增长的能源需求。相较于其他地壳储量丰富的过渡金属，以铬（chromium, Cr）配合物介导的分子电催化CO₂还原相关研究报道仍较为匮乏。近期，本课题组报道了一种铬配合物，可在低过电位下高效催化CO₂还原为一氧化碳（carbon monoxide, CO）。本文通过密度泛函理论（density functional theory）计算研究，揭示了全新的反应机理认知：涵盖动力学选择性的起源、中间体的相对能量分布、与溶剂配位及自旋态相关的物种演化，以及氧化还原活性联吡啶（bipyridine）配体的作用机制。尤为重要的是，本研究表明，在特定还原条件下，碳酸氢根的生成可能成为竞争性反应路径，为未来实验研究指明了新的方向。