Adsorption and reactivity of hydrous iron oxide nanoparticles on boron-doped diamond

Hydrous iron oxide nanoparticles of 8-16 nm diameter are adsorbed from an aqueous sol onto the surface of polished borondoped diamond electrodes. Boron-doped diamond acts as an inert substrate with low background currents and a wide potential window, allowing electrochemical and electrocatalytic properties of an electrically non-conducting material such as hydrous iron oxide to be studied. The electrochemical reduction of hydrous ferric oxide in aqueous phosphate buffer at pH 5 at a potential of ca. -0.2 V vs. SCE is accompanied by stripping of the deposit, which allows the determination of the approximate coverage of the oxide on the boron-doped diamond electrode, typically 1.4 x 10^-9 mol cm^-2. Next, hydrous ferric oxide is shown to act as an efficient electrocatalyst for the oxidation of hydroxide to dioxygen. The process is proposed to occur with apparently one electron per hydroxide via a C_het,revE_revC_het,irrev-type process, initiated by deprotonation of an active iron oxide surface site. It is also demonstrated that the reaction of hydrogen peroxide on hydrous ferric oxide surfaces (a heterogeneous Fenton-type process) is similarly triggered by the deprotonation of an active surface site.