Mechanism of the Electrochemical Oxidation of Ammonia over Platinum: An in Situ attenuated Total Reflection Infrared Study

The recent development of anion exchange membranes (AEMs) has increased the potential and the importance of ammonia as a fuel for anion exchange membrane fuel cells (AEMFCs). Although the reaction mechanism for the electrochemical ammonia oxidation over Pt electrode has been extensively studied, there is no report on the detection of the prospected intermediates and/or the poisoning species on Pt electrode. In this study, the electro-oxidation of ammonia over Pt electrode in alkaline aqueous solutions was studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. In the presence of NH 3 , the band ascribable to the HNH bending mode of adsorbed NH 3 was confirmed at 1662−1674 cm −1 in the potential range of 0.1−1.1 V. The intensity of this band decreased continuously with a rise in potential, indicating the oxidative consumption of adsorbed ammonia. In response to this behavior, the band at 1269 cm −1 appeared alternatively above 0.2 V, and its intensity reached the local maximal value at ca. 0.4 V. Note that this potential of ca. 0.4 V agreed well with the onset potential of ammonia oxidation, ca. 0.45 V, in the linear sweep voltammogram. This 1269 cm −1 band was assigned to the NH 2 wagging mode of N 2 H 4 , which was one of the active intermediates, N 2 H x + y ,ad ( x = 1 or 2, y = 1 or 2), according to the mechanism proposed by Gerischer and Mauerer. To the best of our knowledge, this is the first report for the detection of N 2 H 4 as a reaction intermediate over Pt electrode. Furthermore, the formation of bridged NO was also observed above the onset potential of ammonia oxidation, ca. 0.5 V. Such adsorbed NO species probably inhibit the electrochemical reaction due to the occupation of reaction sites at higher potential. Figure 1