Ab Initio Investigation of Covalently Immobilized Cobalt-Centered Metal-Organic Catalysts for CO₂Reduction: The Effect of the Substrate on the Reaction Energetics

Recently, experimental studies of covalently immobilized CO₂reduction organometallic catalysts have reported remarkable activity; however, the reason for this and the underlying mechanisms are not currently understood. To advance our understanding of such systems, we perform ab initio calculations investigating how covalent immobilization of such molecular catalysts and the substrate geometry affect the reaction pathways. We address this as realistically as possible by doing a survey of possible structures including common surface defects and nanotubes, as well as linkers. In particular, we study covalently immobilized cobalt-centered phthalocyanine (CoPc) and tetraphenylporphyrin (CoTPP), as used in CO₂electroreduction reactions (CO₂ERR), immobilized on pristine and defective graphene and single-walled carbon nanotubes (SWCNTs), with and without linkers. The bonding energies of the CoPc and CoTPP catalysts to the different substrates are found to be consistently stronger on the SWCNTs and on defect sites, suggesting that such structures will be the anchoring sites for these immobilized molecular catalysts. Covalently immobilized CoPc and CoTPP catalysts show improved CO₂ERR pathway performance compared to their homogeneous analogues. Favorable reaction pathways are found for upright bonded CoPc and CoTPP on a Stone-Wales defect and an octagon-pentagon line defect, respectively, in graphene. CoPc immobilized via a pyridine linker is found to have the most favorable reaction pathway due to strong exothermic behavior of CO₂adsorption and COOH formation while having a weak endothermic final step for CO desorption, consistent with its excellent experimental performance. We attribute this to unoccupied d_z²states just above the Fermi level. On comparing the calculated free energy reaction pathways with corresponding available experimental results of catalyst performance, we find consistent agreement. The present study provides a new detailed understanding into the covalent immobilization and function of these organometallic catalysts and the role that the charge, defects, and structure have on the free energy reaction pathways for CO₂ERR.