Oxidation of carbon monoxide, hydrogen peroxide and water at a boron doped diamond electrode: the competition for hydroxyl radicals.

Boron doped diamond (BDD) electrodes have an extremely high over-voltage for oxygen evolution from water, which favours its use in oxidation processes of other compounds at high potentials. We used a rotating ring disc (RRDE) assembly and differential electrochemical mass spectrometry (DEMS) in order to monitor the consumption or the production of species in the course of the electrode processes. By intercepting the intermediate of the electrochemical water oxidation with chemical reactions we demonstrate clearly, albeit indirectly, that in the water oxidation process at BDD above 2.5 V the first step is the formation of (OH)-O-center dot radicals. The electro-oxidation of CO to CO2 at BDD electrodes proceeds only via a first attack by (OH)-O-center dot radicals followed by a further electron transfer to the electrode. At potentials below the onset of oxygen evolution from water, H2O2 is oxidised by a direct electron transfer to the BDD electrode, while at higher potentials, two different reactions paths compete for the (OH)-O-center dot radicals formed in the first electron transfer from water: one, where these (OH)-O-center dot radicals react with each other followed by further electron transfers leading to O-2 on the one hand and one, where (OH)-O-center dot radicals react with other species like H2O2 or CO with subsequent electron transfers on the other hand.