Kinetics of Hydride Transfer from Catalytic Metal-Free Hydride Donors to CO2.

Selective reduction of CO2 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 CO2. The observed dependence of experimental and calculated HT barriers on the thermodynamic driving force was modeled by 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 CO2 would exhibit sluggish kinetics (<100 turnovers per second at -0.1 eV driving force), indicating that the conventional HT may not be an appropriate mechanism for solar conversion of CO2 to formate. We propose that the conventional HT mechanism should not be considered for CO2 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 CO2 reduction to formate.