Evidence for Decoupled Electron and Proton Transfer in the Electrochemical Oxidation of Ammonia on Pt(100)

The two traditional mechanisms of the electrochemical ammonia oxidation consider only concerted proton-electron transfer elementary steps and thus they predict that the rate-potential relationship is independent of the pH on the pH-corrected RHE potential scale. In this letter we show that this is not the case: the increase of the solution pH shifts the onset of the NH₃-to-N₂ oxidation on Pt(100) to lower potentials and also leads to higher surface concentration of formed NO_ad before the latter is oxidized to nitrite. Therefore, we present a new mechanism for the ammonia oxidation that incorporates a deprotonation step occurring prior to the electron transfer. The deprotonation step yields a negatively charged surface-adsorbed species that is discharged in a subsequent electron transfer step before the N-N bond formation. The negatively charged species is thus a precursor for the formation of N₂ and NO. The new mechanism should be a future guide for computational studies aiming at the identification of intermediates and corresponding activation barriers for the elementary steps. Ammonia oxidation is a new example of a bond-forming reaction on (100) terraces that involves decoupled proton-electron transfer.