Study of the electrochemical oxidation of ammonia on platinum in alkaline solution: Effect of electrodeposition potential on the activity of platinum

The electrochemical activity for the oxidation of ammonia of Pt electrodes, prepared by pulse potentiostatic electrodeposition on glassy carbon, was investigated by cyclic voltammetry and chronoamperometry. The morphology of the platinum deposits was observed by scanning electron microscopy. It was demonstrated that the crystallographic structure of the Pt electrodes probed by electrochemistry, instead of their morphology, has an important impact on the activity of platinum for the oxidation of ammonia in alkaline media. A deposition sequence, where a very negative reducing potential in the hydrogen evolution region and an oxidative potential in the H-desorption region are applied, leads to the formation of preferentially oriented Pt (1 0 0) electrodes. The presence of large (1 0 0) terraces enhances the Pt electrode activity for the oxidation of ammonia but has no impact on its poisoning by reaction intermediates. The stability of these electrodes was investigated by applying a repeated oxidation/reduction cycling procedure. It was found that when the upper potential is less positive than 0.6 V (beginning of the formation of Pt oxides) the preferential orientation of platinum is not affected by the oxidation/reduction cycling whereas for more positive upper potential limit, long range order of Pt is perturbed. As a consequence, the electrochemical activity of the resulting electrodes for the oxidation of ammonia significantly decreased in the latter case.