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