Deciphering Complex Interfacial Dynamics via Operando Microbalance during Potential-Modulated Alkyne Hydrogenation

Applied potentials can alter heterogeneous catalysis, but the roles of ions, solvent, and reactants at the polarized catalyst-electrolyte interface remain unclear. Here, we track potential-driven interfacial mass redistribution during aqueous Pd-catalyzed alkynol semi-hydrogenation using an operando electrochemical quartz crystal microbalance (EQCM). By combining gravimetric measurements with a volumetric occupancy model, we quantify the competitive redistribution of ions, reactants, and hydrogen at the solid-liquid interface. The results show that hydrated Na+ promotes cathodic interfacial mass accumulation, forming a cation-bridged microenvironment that enriches alkynol and limits H2 availability. Positive polarization disrupts this microenvironment, promotes product removal, and restores surface hydrogen. Based on these results, we developed an oscillating potential program (100 s at –0.9 V, 20 s at +0.5 V) to control reactant enrichment, product removal, and hydrogen availability, increasing alkenol selectivity from ∼50% to 97% while enhancing activity by 173%.