Hydrogen and anion adsorption at platinum single crystal electrodes in phosphate solutions over a wide range of pH

The adsorption of hydrogen and anions at platinum single crystal, mainly Pt(111), electrodes in phosphate buffer solution was studied by electrochemical and in-situ FTIR measurements over a wide range of pH and potential. Phosphoric acid and its anions behaved essentially similarly to sulphuric acid, being different from perchloric acid. The weakly adsorbed hydrogen was not affected by the kind of anions and pH. Strongly adsorbed hydrogen on Pt(111) split into two parts at pH = ca. 5. These two parts united again at pH > 12, giving the same peak (P_0,1) as observed in 0.1 M NaOH. A potential shift in the positive direction past the double layer region caused a change of the adsorbed PO₄^3- to the protonated adsorbate, HPO₄^2- at high pH and of the adsorbed HPO₄^2- to H₂ PO₄^- at neutral pH, respectively. Comparison of the electrochemical results with the in-situ FTIR results suggested the importance of adsorbed OH in the change of the adsorbed phosphate species. The effect of the surface structure was not significant for phosphate adsorption.