Characterisation of an RuO2·xH2O colloid and evaluation of its ability to mediate the oxidation of water

Abstract

A colloid of RuO₂·xH₂O supported by polybrene was prepared, characterised and assessed as an O₂ catalyst. Thermal analysis (t.g.a. and d.t.g.a.) of RuO₂·xH₂O precipitated from the colloid indicated the presence of both weakly and tightly bound water. Dynamic light scattering indicated that the coagulated colloidal particles were large (d= 825 nm) and positively charged. Transmission electron microscopy demonstrated that the colloidal particles were themselves aggregates of crystallites too small ( < 10 nm) to be clearly resolved. The colloid proved unstable towards coagulation under conditions of high electrolyte concentration ( 10^–3 mol dm^–3) even when the electrolyte was H₂SO₄. In the presence of Ce^IV ions the colloid did show some activity as an O₂ catalyst (O₂ yield = 73%) but also underwent some anodic corrosion to RuO₄(27%). At low concentrations of Ce^IV ions (4.5 × 10⁵ mol dm^–3) and colloid ([RuO₂]= 0.02–0.00125 g dm^–3) the colloid appeared to mediate the oxidation of polybrene over that of water by the Ce^IV ions. Kinetic studies performed under these conditions and in the presence of a high constant background concentration of polybrene (0.015 g dm^–3) showed the kinetics to be biphasic with an initial fast step (associated with charging of the catalyst) followed by a second step which was proportional to the concentrations of both Ce^IV ions and colloid. Under conditions where no extra polybrene was added to dilutions of the colloid some O₂ evolution was observed (ca. 20%) and the kinetics of Ce^IV disappearance once again appeared biphasic, although more complicated and difficult to interpret.