Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.