H2 Evolution Reaction Pathway of Cobalt Bis-bipyridyl Planar Catalyst Revealed by ab-initio Simulations
Creators: name: Guclu, Ipek
name: Kiser, Ayas
name: Gurdal, Yeliz
Publisher: name: Zenodo
Year: 2024
Subjects: Ab-initio Molecular Dynamics
Metadynamics
Density Functional Theory
Reduction free energy
Hydrogen evolution reaction

description: 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.
description: An open source CP2K/QUICKSTEP package was used for the Ab-initio molecular Dynamics (AIMD) and Metadynamics (MTD) simulations. Electronic structure calculations were performed at the Kohn-Sham Density Functional Theory (DFT) level using Gaussian and plane wave (GPW) formalism. Valence electrons were explicitly described, and the interactions between the valence electrons and the atomic cores were described using norm-conserving Goedecker-Teter-Hutter (GTH) pseudo potentials. We have performed AIMD simulations in order to model initial state of the catalyst/water system and the intermediate steps of the sequential electron and proton transfer reaction mechanism (ECEC) as well as pathway involving two successive reductions followed by two protonations (EECC). Perdew-Burke Ernzerhof's (PBE) Generalized Gradient Approximation (GGA) has been used as the exchange-correlation functional in all AIMD simulations. Van der Waals (vdW) interactions have been computed self-consistently using the Vydrov-Voorhis (rVV-10) van der Waals density functional. Following, AIMD simulations in canonical (NVT) ensemble have been carried out in order to model intermediate states of the ECEC and EECC reaction mechanisms. The  first protonation intermediate step of the ECEC reaction mechanism has been modeled using MTD simulations.

The electronic structure alterations of each intermediate steps and the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals of the selected AIMD snapshots were calculated using hybrid density functional, PBE0/rVV10. PBE0 Hartree-Fock exchange fraction parameter was setto 0.25. The auxiliary density matrix method (ADMM) that ensures a signicant speeding while employing hybrid density functionals have been used for all hybrid DFT calculations.

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