cs501963v_si_001.pdf (329.95 kB)
A Mn Bipyrimidine Catalyst Predicted To Reduce CO2 at Lower Overpotential
journal contribution
posted on 2015-04-03, 00:00 authored by Yan Choi Lam, Robert J. Nielsen, Harry B. Gray, William
A. GoddardExperimentally, [(L)Mn(CO)3]− (where
L = bis-alkyl-substituted bipyridine) has been observed to catalyze
the electrochemical reduction of CO2 to CO in the presence
of trifluoroethanol (TFEH). Here we report the atomistic level mechanism
of complete catalytic cycles for this reaction, on the basis of DFT
calculations (B3LYP-D3 with continuum solvation) of the free energies
of reaction and activation, as well as reduction potentials for all
catalytically relevant elementary steps. The highly exergonic homoconjugation
and carbonation of TFE– play critical roles in reaction
thermodynamics and kinetics, the overall half-reaction being 3CO2 + 2TFEH + 2e– → CO + H2O + 2[F3CCH2OCO2]− (calculated standard reduction potential: −1.49 V vs SCE).
In the catalytic cycle for CO formation, CO2 coordinates
to [(L)Mn(CO)3]− (1a, L
= bpy), and the adduct is then protonated to form [(L)Mn(CO)3(CO2H)] (3a). 3a subsequently
reacts to form [(L)Mn(CO)4]0 (5a) via one of two pathways: (a) TFEH-mediated dehydroxylation to [(L)Mn(CO)4]+ (4a), followed by one-electron
reduction to 5a, or (b) under more reducing potentials,
one-electron reduction to [(L)Mn(CO)3(CO2H)]− (3′a), followed by
dehydroxylation to 5a. Pathway b has a lower activation
energy by 2.2 kcal mol–1. Consequently, the maximum
catalytic turnover frequency (TOFmax) is achieved at ∼−1.75
V vs SCE (∼0.25 V overpotential). For the analogous bipyrimidine
compound (not yet studied experimentally), reduction of 3b to 3′b occurs at a potential 0.5
V more positive than that of 3a, and the overpotential
required to achieve TOFmax is predicted to be lower by
∼0.25 V. This improvement is, however, achieved at the price
of a lower TOFmax, and we predict that 1b has
superior TOF at potentials above ∼−1.6 V vs SCE. In
addition, the various factors contributing to product selectivity
(CO over H2) are discussed.