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

Whether SO42- 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 V-RHE, 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 SO42- in the bulk solution. This is supported by the pHand composition-dependent change of the electrochemical potential of related processes and the fact that only a single band for Scontaining 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).

Automated summary

This study systematically investigates the effect of pH on sulfate adsorption on Pd(111) electrode using cyclic voltammetry. It is found that the onset potential for sulfate adsorption is positively correlated to the solution pH, and in alkaline solutions, sulfate adsorption overlaps with the OHad region. By analyzing the thermodynamic equilibrium potential and the Faradic charge due to adsorption, it is proposed that SO4* is the dominating adsorbate on Pd(111), regardless of whether the main precursor is HSO4- or SO42- in the solution. This has important implications for the understanding of interfacial electrochemical processes, particularly pH-modulated electrochemical reactions.