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 N(2) is related directly to the nature of the species at the surface: the electrode is active if NH(ads) (or NH(2,ads)) is present and deactivates when N(ads) is present. The potential at which NH(ads) or N(ads) is formed is metal dependent. The observed trend in the strength of adsorption of N(ads) is Ru > Ph > Pd > Ir > Pt much greater than 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 N(ads) is formed at high potential, compared to rhodium and palladium, but seems to stabilize NH(ads) rather well. Gold, silver and copper do not form NH(ads) or N(ads), 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 N(2)O, at potentials where the surface becomes oxidized. (C) 2001 Elsevier Science B.V. All rights reserved.