Unraveling the Abnormal Positive Effect from Halogen Ion Adsorption on Pd-Catalyzed Electro-hydrogenation of Benzaldehyde

The adsorption of halogen ions on the catalyst surface is generally believed to poison the catalyst surface, thus reducing the efficiency of the electrocatalytic reaction. In this work, we demonstrate that the strong adsorption of halogen ions on the surface of Pd nanowires can effectively promote the electrocatalytic hydrogenation of benzaldehyde and enhance the faradaic efficiency (FE) of the reaction. The unexpected promotional behavior correlates directly with the strength of halogen ion adsorption on the Pd surface, as revealed by systematic electrochemical and in situ electrical transport spectroscopy (ETS) investigations. Strongly adsorbed iodide ions effectively suppress both H adsorption and H absorption ( in situ formation of PdH x ) on the surface of the Pd electrode, consequently inhibiting the competing hydrogen evolution reaction (HER). Furthermore, the reduced surface-active hydrogen coverage diminishes the bimolecular surface reaction (Langmuir–Hinshelwood) mechanism for benzaldehyde hydrogenation, instead redirecting the reaction pathway dominance toward the proton-coupled electron transfer (PCET) mechanism.