Effect of pH on Sulfate Adsorption on the Pd(111) Electrode

Whether SO4 2– or HSO4 – is the dominating adsorbate on the Pd(111) electrode, and what is­(are) the driving force responsible for its­(their) formation are fundamental issues in interfacial electrochemistry that remain unsolved. In this work, the effect of pH on (bi)­sulfate adsorption on Pd(111) in electrolytic solutions of pH values from 0 to 13 is systematically studied by cyclic voltammetry. The onset potential for (bi)­sulfate adsorption is ca. 0.23 V vs the reversible hydrogen electrode (RHE) in solution at pH = 1. When the solution pH increases from 0 to 3, the peak potential of sulfate adsorption shifts positively with a slope of ca. 20–40 mV/pH. In alkaline solutions at pH > 10, sulfate adsorption takes place at E > 0.55 VRHE, falling into the potential region where OHad occurs. After carefully analyzing the related onset/peak potential of (bi)­sulfate desorption/adsorption to the thermodynamic equilibrium potential of possible electrode reactions and the corresponding Faradic charge due to adsorption, we propose that the adsorbate on Pd(111) is most probably SO4*, no matter whether the main precursor for its adsorption is HSO4 – or SO4 2– in the bulk solution. This is supported by the pH- and composition-dependent change of the electrochemical potential of related processes and the fact that only a single band for S-containing adsorbate is detected by infrared spectroscopy. The fact that the onset potential for sulfate adsorption is negative compared to the potential of zero charge of Pd(111) suggests that pH-modulated electrochemical potential of the reactants, rather than the pH-dependent surface charging behavior, is the dominant factor of the observed pH effect on sulfate adsorption on Pd(111).