Cobalt-Porphyrin Catalyzed Electrochemical Reduction of Carbon Dioxide in Water II: Mechanism from First Principles

TL;DR

This study uses first principles computational methods to elucidate the detailed mechanism of CO2 electrochemical reduction to CO catalyzed by cobalt porphyrin in water, highlighting water's stabilizing role and reaction energetics.

Contribution

It provides a comprehensive mechanistic analysis combining DFT and molecular dynamics, revealing water's critical role and the energetics of key reaction steps in cobalt porphyrin catalysis.

Findings

Water stabilizes key intermediates

Reaction is thermodynamically favorable with small barriers

Protonation and bond cleavage steps are elucidated

Abstract

We apply first principles computational techniques to analyze the two-electron, multi-step, electrochemical reduction of CO2 to CO in water using cobalt porphyrin as a catalyst. Density Functional Theory calculations with hybrid functionals and dielectric continuum solvation are used to determine the steps at which electrons are added. This information is corroborated with ab initio molecular dynamics simulations in an explicit aqueous environment which reveal the critical role of water in stabilizing a key intermediate formed by CO2 bound to cobalt. Using potential of mean force calculations, the intermediate is found to spontaneously accept a proton to form a carboxylate acid group at pH<9.0, and the subsequent cleavage of a C-OH bond to form CO is exothermic and associated with a small free energy barrier. These predictions suggest that the proposed reaction mechanism is viable if electron transfer to the catalyst is sufficiently fast. The variation in cobalt ion charge and spin states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the need for computing electrochemical potentials are emphasized.