Selective reduction of CO<sub>2</sub> to formate
represents an ongoing challenge in photoelectrocatalysis. To provide
mechanistic insights, we investigate the kinetics of hydride transfer (HT) from
a series of metal-free hydride donors to CO<sub>2</sub>. The observed
dependence of experimental and calculated HT barriers on the thermodynamic
driving force was modeled using the Marcus hydride transfer formalism to obtain
the insights into the effect of reorganization energies on the reaction
kinetics. Our results indicate that, even if the most ideal hydride donor were
discovered, the HT to CO<sub>2</sub> would exhibit sluggish kinetics (less than
100 turnovers at 0.1 eV driving force), indicating that the conventional HT may
not be an appropriate mechanism for Solar conversion of CO<sub>2</sub> to
formate. We propose that the conventional HT mechanism should not be considered
for CO<sub>2</sub> reduction catalysis and argue that the orthogonal HT
mechanism, previously proposed to address thermodynamic limitations of this
reaction, may also lead to lower kinetic barriers for CO<sub>2</sub> reduction
to formate.
Kinetics of Hydride Transfer from Metal-Free Hydride Donors to CO2
