Inhibition Effect of Surface Oxygenated Species on Ammonia Oxidation Reaction

The ammonia oxidation reaction (AOR) cannot proceed on Pt surface at potentials corresponding to the formation of surface oxygenated species (Oads and OHads), it is thus suspicious that, in addition to the Nads, the surface oxygenated species may also play an inhibitive role. Yet this is difficult to be proven in aqueous media where the AOR is always accompanied with the water oxidation reaction. In the present work, we carry out differential electrochemical mass spectroscopy (DEMS) studies of the AOR in nonaqueous media where no surface oxygenated species is involved, and the results turn out be remarkably different from those obtained in KOH solution. It is evident that, without the blocking of oxygenated species, the Pt surface can remain continuously active for AOR, and N2 is the dominant product. More strikingly, Pd becomes highly active for the AOR in nonaqueous media, a result enormously different from that in KOH solution where Pd exhibits a very low catalytic activity because of severer surface passivation by oxygenated species. This work provides compelling evidence for the inhibition effect of surface oxygenated species on the AOR, and illuminates our understanding of relevant reaction mechanisms.