Electrochemical amination of acetone using Ag as cathode: mechanism and role of Pb impurities for hydrogen transfer

Abstract

Electrochemical reductive amination of acetone using Ag electrodes is strongly enhanced by 1 ppm Pb in the solution but the underlying mechanisms and the nature of the Pb species remain poorly understood. To understand how trace elements alter reaction pathways and enhance catalytic performance, the catalytic behavior of Pb traces in electrochemical hydrogenation was theoretically investigated. Using computational modeling, we examined the formation of Pb hydrides (PbH₂ and PbH₄) on the electrified silver surface at the highly reducing potential used experimentally and their reactivity either at the polarized surface or in the solution in proximity of the electrode, including for hydrogen transfer towards the imine intermediate in the electrocatalytic amination of acetone with methylamine. Our results highlight a key role of Pb hydride species (PbH₂ at the electrode surface or PbH₄ in the near-electrode solution) for the imine hydrogenation step. In the absence of Pb, hydrogenations directly on the Ag electrode by proton-coupled electron transfer or by surface hydrides both suffer from high energy barriers, altogether explaining the experimental requirement of Pb traces for activity. One possible scenario is a dynamic cycle where PbH₄, a highly unstable compound, is formed by the reducing potential at the Ag electrode and acts as a short-lived hydrogen shuttle in the near-electrode solution, either homogeneously hydrogenating the imine or producing hydrogen evolution, by decomposition or interaction with water. Another scenario underlines PbH₂ as the main actor, formed on the electrode, and hydrogenating the imine on the Ag surface heterogeneously. This study proposes an original hypothesis for the role of Pb as a hydrogen transfer agent, offering a mechanistic understanding of its catalytic behavior. Our findings not only explain the enhanced efficiency and selectivity observed in Pb-catalyzed electrochemical hydrogenation but also open avenues for the rational design of advanced catalysts based on trace elements to optimize sustainable electrochemical processes.