EFFECT OF COMPOSITION ON THE POINT OF ZERO CHARGE OF RUO2+TIO2 MIXED OXIDES

RuO2 + TiO2 mixed oxides have been prepared by thermal decomposition of RuCl3 + TiCl4 at 450-degrees-C. The structural features have been investigated by X-ray diffraction, while the Braunauer-Emmett-Teller method has been used to determine the surface area of the powders. The point of zero charge (pzc) has been measured by potentiometric titration of oxide suspensions in KNO3 solution. The charge-pH curves have been transformed into capacitance values which have been used at the pzc to estimate in-situ the real surface area and the inner layer capacitance. In the absence of RuO2, TiO2 is mainly in the anatase form. The rutile form of TiO2 has a pzc close to that of RuO2 which results in a pzc varying very little between 40 and 100% RuO2. The same is the case with the real surface area. This is in accord with the performances of electrodes showing RuO2-like properties down to 30-40% RuO2. The behaviour of RuO2 + TiO2 oxides is structurally, electronically and chemically dominated by that of the RuO2 component.