The role of adsorbates in the electrochemical oxidation of ammonia on noble and transition metal electrodes

The activity for ammonia oxidation and the intermediates formed during the reaction have been studied on platinum, palladium, rhodium, ruthenium, iridium, copper, silver and gold electrodes. The activity in the selective oxidation to N2 is related directly to the nature of the species at the surface: the electrode is active if NHads (or NH2,ads) is present and deactivates when Nads is present. The potential at which NHads or Nads is formed is metal dependent. The observed trend in the strength of adsorption of Nads is Ru>Rh>Pd>Ir>Pt≫Au, Ag, Cu. This trend corresponds well with the trend observed in the calculated heat of adsorption of atomic nitrogen, with iridium being an exception. Platinum is the best catalyst for this reaction because Nads is formed at high potential, compared to rhodium and palladium, but seems to stabilize NHads rather well. Gold, silver and copper do not form NHads or Nads, and show only an activity when the surface becomes oxidized. The metal electrodissolution is enhanced by ammonia under these conditions. Most metals produce oxygen-containing products, like NO and N2O, at potentials where the surface becomes oxidized.