Electrochemical behaviour of olefins: oxidation at ruthenium-titanium dioxide and iridium-titanium dioxide coated electrodes

The electrocatalytic behaviour of a series of olefins was studied on thermally prepared Ti/MO2 and Ti/M0.3Ti0.7O2 electrodes (M = Ru, Ir) in 1.0 M HClO4 in mixed solvent (AN/H2O, 40/60 v/v). The voltammetric investigation was limited to the potential region preceding the OER on these electrodes materials (E < 1.2 V vs SSCE). Aliphatic olefins (isophorone and cyclohexene) are inactive while the aromatic olefins show a single (safrole) or two (isosafrole) oxidation peaks. The overall catalytic activity of these electrode materials is about the same for both substrates. However, when morphological effects (differences in electrode surface area) are taken into account, normalizing the geometric current density (or faradaic charge) per surface site activity, a slightly better efficiency of the active surface sites is observed for Ru-based electrodes when compared to the equivalent Ir-based materials. Partial substitution of the noble metal catalysts by TiO2 results in a synergetic effect depressing the efficiency of the active surface sites of the TiO2-stabilized electrocatalysts. The decrease with potential cycling of the substrate oxidation current is attributed to dimeric/polymeric film formation blocking the electrode surface. Reflectance and FTIR spectroscopy as well as ohmic resistance data support film formation.