Electrochemically Promoted Oxidation Mechanisms in Plasmonic Catalysis of Decarboxylation

Surface plasmon (SP)-driven catalytic decarboxylation reactions have garnered considerable attention in recent years. However, the mechanism of SP-induced decarboxylation within an electrochemical environment remains systematically unexplored. Herein, we investigate the electrochemical behavior of 4-mercaptobenzoic acid (4-MBA) within a Au nanoparticles/4-MBA molecular self-assembly layer/Au(111) configuration under varying pH conditions by using surface-enhanced Raman spectroscopy. It is found that 4-MBA efficiently converts to thiophenol (TP) in neutral/alkaline solutions. The TP conversion rate increased with applied potential from 10% to 40% at pH = 7, and from 20 to 50% at pH = 9. However, no TP formed at pH 1. In situ shell-isolated nanoparticle-enhanced Raman spectroscopy confirms that the decarboxylation is induced by SP-generated charge carriers. These reveal that increasing potential in neutral/alkaline solutions elevates OH^- concentration within the electrical double layer, and significantly lowers the energy barrier for hot holes (h⁺) to combine with OH^-. This promotes the formation of ^•OH radicals, triggering decarboxylation. Conversely, at pH = 1, extremely low OH^- concentration results in a prohibitively high barrier for H₂O oxidation to ^•OH by h⁺, inhibiting the reaction. This work provides molecular-level insights into the electrochemically promoted oxidation mechanisms governing plasmonic catalytic decarboxylation.