The Intricate Sabatier Optimality of Anion Electroreduction and Its Consequences for Nitrate Reduction

The Sabatier principle is arguably among the most extended concepts in catalysis. It molded the idea of optimality as a balance between two undesired extremes of strong and weak binding. In electrocatalysis, Sabatier-type volcano plots have had tremendous success for reactions with coupled proton-electron transfers involving neutral reactants. However, we show here that the concept ought to be revised when charged reactants are involved. Specifically, we analyze anion electroreduction mediated by specific anion adsorption. We observe drastic changes in free-energy diagrams and volcano plots compared to the reduction of neutral species: Specific adsorption is destabilized by the potential, the subsequent hydrogenation is potential-independent, and the next step is stabilized by the potential. Consequently, volcano plots become mobile, such that strong and weak binding turn into potential-dependent notions. We illustrate these intriguing phenomena for nitrate electroreduction on transition metals, a promising yet currently suboptimal electrocatalytic process for wastewater treatment.