Understanding Hydrogen Atom and Hydride Transfer Processes during Electrochemical Alcohol and Aldehyde Oxidation

Selective oxidation of alcohols and aldehydes to their corresponding carboxylic acids is of broad importance for applications in organic synthesis. These reactions can be performed at the anode of electrochemical and photoelectrochemical cells that produce fuels at the cathode (e.g., water reduction to form H2), allowing for the production of valuable products at both electrodes. NiOOH is among the most promising electrocatalysts for selectively converting alcohols and aldehydes to carboxylic acids. Recent work has revealed that electrochemical alcohol and aldehyde oxidation can occur through two different pathways, one via hydrogen atom transfer and the other via hydride transfer; however, details of these mechanisms are yet to be elucidated. In this work, we examined the effect of pH and the concentration of aliphatic and aromatic alcohols and aldehydes to determine the key factors and steps that affect the kinetics of the two oxidation pathways. Through these experiments, we obtained a comprehensive mechanistic understanding of the two pathways. These results were complemented by kinetic isotope effect experiments to further probe which steps control the rate of each pathway. We then used the understanding these experiments enabled, coupled with computational results, to propose that the ability of alcohols and aldehydes to outcompete OH– for adsorption on Ni4+ sites is a key predictor of which oxidation pathway will be favored.