Electrochemical Methods for Assessing Kinetic Factors in the Reduction of CO2 to Formate: Implications for Improving Electrocatalyst Design

摘要

Thermochemical insights are often employed in the rationalization of reactivity and in the design of electrocatalysts for CO2 reduction reactions targeting C-H bond-containing products. This work identifies experimental methods for assessing kinetic aspects of reactivity. These methods are illustrated using [Fe4N(CO)(12)](-), which produces formate from CO2 at -1.2 V versus SCE in either a MeCN/H2O solvent (95:5) or pH 6.5 buffered water. Elementary rates for each reaction step are identified along with the rate determining step (RDS) as C-H bond formation. Transition state kinetics were determined from an Eyring analysis for the rate-determining C-H bond formation step using temperature-dependent electrochemical measurements. A lower measured Delta G double dagger (298 K, 12.3 +/- 0.1 kcal mol(-1)) in a pH 6.5 aqueous solution, compared with a Delta G double dagger(298 K) of 15.0 +/- 0.1 kcal molt in a MeCN/H2O solvent (95:5), correlates with faster observed reaction rates and provides a kinetic rationalization for the solvent-dependent chemistry. Taken together, the experimentally determined kinetic insights highlight that enhancement of the local concentration of CO2 at catalyst-hydride sites should be a primary focus of ongoing catalyst design. This will both enhance reaction rates and increase selectivity for C-H bond formation over competing H-H bond formation, because that step is fast in H-2 evolution reactions.