Polarization-Induced Local pH Swing Promotes Pd-Catalyzed CO2 Hydrogenation

Abstract

Electrochemical polarization can dramatically promote the rate of concurrent nonfaradaic catalytic reactions, but the mechanistic basis for these promotion effects at solid-liquid interfaces remains poorly understood. Herein, we establish a mechanistic framework for nonfaradaic promotion in aqueous media that operates via a local pH swing induced by a concurrent faradaic reaction. As a model system, we examined the kinetics of nonfaradaic Pd-catalyzed CO2 hydrogenation to formate and find that the reaction can be promoted by a combination of high alkalinity and high CO2 concentration. In bulk electrolyte, alkalinity and CO2 concentration are inversely correlated to each other as set by the CO2/bicarbonate equilibrium. We show that this impasse can be overcome by using electrical polarization to generate a nonequilibrium local environment that has both high alkalinity and high CO2 concentration. We find that this local pH swing promotes the rate of nonfaradaic CO2 hydrogenation to formate by nearly 3 orders of magnitude at modest potential bias. The work establishes a rigorous mechanistic model of nonfaradaic promotion in aqueous media and provides a basis for enhancing hydrogenation catalysis under mild conditions via electrical polarization.