H₂ Evolution Reaction Pathway of a Cobalt Bis-bipyridyl Planar Catalyst Revealed by Ab Initio Simulations

Ligand type and ligand coordination around cobalt photocatalysts influence the reaction mechanism of H2 production via water splitting. We investigated the H2 production mechanisms of a cobalt bis-bipyridyl planar water reduction catalyst in an aqueous solution using ab initio molecular dynamics, metadynamics, density functional theory, and free energy perturbation theory. Modeling each intermediate step of the reaction mechanism reveals that the cobalt-based planar catalyst exhibits a preference for a pathway involving two successive reductions, followed by two protonation steps. The reason for this preference is the presence of an energy barrier for the conformational change of water molecules in the first solvation shell following the first reduction. This barrier prevents direct coordination of cobalt with protons, which is necessary for the initial protonation step in the sequential electron- and proton-transfer mechanisms. In the mechanism involving two successive reductions followed by two protonations, however, the second reduction step facilitates direct interaction between cobalt and protons, enabling the initial protonation. Following the second protonation of this mechanism, H2 is produced and released into the solvent. Reduction free energy calculations reveal that, following the first reduction, the singlet spin state of the system is more favorable than the triplet spin state. The reduction free energy of the second electron transfer is 0.19 eV less energetic than that of the first reduction reaction.