H2 Evolution Reaction Pathway of Cobalt Bis-bipyridyl Planar Catalyst Revealed by ab-initio Simulations

Ligand type and ligand coordination around Cobalt photo-catalysts influence the reaction mechanism of H2 production via water splitting. In this work, we investigated the H2 evolution reaction mechanisms of a Cobalt based bis-bipyridyl planar water reduction catalyst in an explicit solvent environment using Ab-initio Molecular Dynamics (AIMD), Metadynamics (MTD), Density Functional Theory (DFT) and Free Energy Perturbation Theory.We modeled each intermediate steps of the H2 evolution reaction pathways and results reveal that the Cobalt-based planar catalyst exhibits a preference for a pathway involving two successive reductions followed by two protonation steps (EECC). Following the first reduction, an energy barrier hinders conformational changes in water molecules within the first solvation shell, preventing Co-proton direct coordination required for the first protonation step of the sequential electron and proton transfer mechanism (ECEC). The second reduction step of the EECC mechanism allows for Co-proton direct interaction, utilized for the initial protonation step. Subsequent protonation in this reaction pathway results in H2 production and its release into the solvent.Free energy calculations show that the singlet spin state is more favorable than the triplet spin state following the first reduction. Moreover, reduction free energy of the second reduction is 0.19 eV less energetic than that observed in the first reduction step. Our study provides a fundamental conclusion that, although the sequential electron and proton transfer reaction mechanism (ECEC) is shown to be straightforward for octahedrally coordinated Co-based polypyridyl catalysts, it is not favorable for the Co-based bis-bipyridyl planar catalyst. Following the mechanism involving two successive reductions followed by two protonation steps (EECC), instead, the Co-based planar catalyst produces H2 and allows its diffusion into the solvent. This study also reveals that ligand type as well as ligand coordination around the Cobalt center affect the favorable reaction mechanism, improving our understanding of designing an efficient catalyst for H2 production through water splitting reactions.