Computational Modeling of Electrocatalysts for CO2 Reduction: Probing the Role of Primary, Secondary, and Outer Coordination Spheres.

Abstract

ConspectusIn the search for efficient and selective electrocatalysts capable of converting greenhouse gases to value-added products, enzymes found in naturally existing bacteria provide the basis for most approaches toward electrocatalyst design. Ni,Fe-carbon monoxide dehydrogenase (Ni,Fe-CODH) is one such enzyme, with a nickel-iron-sulfur cluster named the C-cluster, where CO₂ binds and is converted to CO at high rates near the thermodynamic potential. In this Account, we divide the enzyme's catalytic contributions into three categories based on location and function. We also discuss how computational techniques provide crucial insight into implementing these findings in homogeneous CO₂ reduction electrocatalysis design principles. The CO₂ binding sites (e.g., Ni and "unique" Fe ion) along with the ligands that support it (e.g., iron-sulfur cluster) form the primary coordination sphere. This is replicated in molecular electrocatalysts via the metal center and ligand framework where the substrate binds. This coordination sphere has a direct impact on the electronic configuration of the catalyst. By computationally modeling a series of Ni and Co complexes with bipyridyl-N-heterocyclic carbene ligand frameworks of varying degrees of planarity, we were able to closely examine how the primary coordination sphere controls the product distribution between CO and H₂ for these catalysts. The secondary coordination sphere (SCS) of Ni,Fe-CODH contains residues proximal to the active site pocket that provide hydrogen-bonding stabilizations necessary for the reaction to proceed. Enhancing the SCS when synthesizing new catalysts involves substituting functional groups onto the ligand for direct interaction with the substrate. To analyze the endless possible substitutions, computational techniques are ideal for deciphering the intricacies of substituent effects, as we demonstrated with an array of imidazolium-functionalized Mn and Re bipyridyl tricarbonyl complexes. By examining how the electrostatic interactions between the ligand, substrate, and proton source lowered activation energy barriers, we determined how best to pinpoint the SCS additions. The outer coordination sphere comprises the remaining parts of Ni,Fe-CODH, such as the elaborate protein matrix, solvent interactions, and remote metalloclusters. The challenge in elucidating and replicating the role of the vast protein matrix has understandably led to a localized focus on the primary and secondary coordination spheres. However, certain portions of Ni,Fe-CODH's expansive protein scaffold are suggested to be catalytically relevant despite considerable distance from the active site. Closer studies of these relatively overlooked areas of nature's exceptionally proficient catalysts may be crucial to continually improve upon electrocatalysis protocols. Mechanistic analysis of cobalt phthalocyanines (CoPc) immobilized onto carbon nanotubes (CoPc/CNT) reveals how the active site microenvironment and outer coordination sphere effects unlock the CoPc molecule's previously inaccessible intrinsic catalytic ability to convert CO₂ to MeOH. Our research suggests that incorporating the three coordination spheres in a holistic approach may be vital for advancing electrocatalysis toward viability in mitigating climate disruption. Computational methods allow us to closely examine transition states and determine how to minimize key activation energy barriers.