Effect of pH on (bi)phosphate adsorption at Pd(111) electrode

Phosphoric acid/phosphate solutions are commonly used as buffer solutions in the fundamental studies in electrochemistry. Information on the nature of adsorbed (bi)phosphate as well as the domain structures is of great importance in unveiling the impact on electrochemical processes occurring at electrode electrolyte interface. In this work, the pH effect on the adsorption of phosphate related species on Pd(111) electrode has been investigated by cyclic voltammetry over a broad pH range from 1 to 14. After carefully analyzing the related onset desorption potential of (bi)phosphate adsorbate to the thermodynamic equilibrium potential of the corresponding electrode reactions, three different phosphate related adsorbates have been identified, which are highly pH-dependent. Our analysis reveals that the dominant phosphate anions in bulk solution undergo deprotonation upon adsorption. At pH<1.5, the main adsorbate on Pd(111) is H2PO4λ1−; in solutions with 1.5 < pH < 7, the main adsorbate changes into HPO4λ2−. At higher pHs, PO4λ3− on electrode surface dominates. The exact charging value of λi, which represents the true valence of the (bi)phosphate adsorbate, could be determined using theoretical calculations. In addition, our results can not rule out the possibility of co-adsorbed H3PO4 molecules on Pd(111) at pH < 1.5, which needs to be proved by infrared spectroscopy in the future.