The electrical double layer on oxides: Specific adsorption of chloride and methylviologen on ruthenium dioxide

The double-layer properties of colloidal RuO₂, prepared by thermal decomposition of RuCl₃ at 420°C, have been studied by potentiometric acid-base titrations in combination with electrophoretic mobility measurements. The point of zero charge (pzc) in KNO₃ solutions was found to be pH 5.75 ± 0.05, and the isoelectric point (iep) is positioned at pH 5.8. From the total capacitance of the double layer at the pzc an electrochemical surface area of 21.5 m²/g has been found, which is equal to the BET surface area. The capacitance of the inner part of the double layer (C_i) is 300 μF/cm², which is high compared to C_i on AgI and Hg, but of the same order as that commonly found for oxides. This subject is briefly discussed. The surface charge (σ₀) as a function of pH could be fitted satisfactorily with a simple double-layer model. In the presence of KCl the pzc and the iep are shifted to higher and lower pH, respectively, indicating specific adsorption of Cl⁻ ions. The ionic composition of the double layer as a function of σ₀ and the specific adsorption of Cl⁻ at the pzc have been calculated by a straightforward thermodynamic analysis combined with diffuse double-layer theory. Methylviologen (MV²⁺) also adsorbs specifically and at negative surface charges superequivalent adsorption can take place. In the presence of an excess of KNO₃, specific adsorption of MV²⁺ is no longer noticeable. Some consequences for the catalytic reduction of water by RuO₂ in the presence of MV²⁺ are considered.