  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.
