Challenges in Electrocatalysis of Ammonia Oxidation on Platinum Surfaces: Discovering Reaction Pathways

A deep understanding of the ammonia oxidation reaction (AOR) over platinum surfaces may facilitate the use of ammonia as a carbon-free source for energy storage and conversion. Herein, using an unprecedented experimental approach of combining online electrochemical mass spectrometry (OLEMS) and ion chromatography (IC) with high-area Pt/C surfaces, many AOR products were simultaneously detected and the variation in AOR selectivity depending on the surface conditions was demonstrated. In the low-potential region of 0.40–0.82 V, the adsorbed OH– was the dominant oxygenated surface species. The AOR onset potential was 0.40 V, and the surface intermediates were NHx,ads and N2Hy,ads, which were the main precursors of N2, considered a major product. N2H4, NO, and NH2OH were considered minor products in this potential region. In the high-potential region, from 0.82 V, adsorbed O2– was the main oxygenated surface species, owing to the strong interactions between OH– and oxidized Pt. We found that NO and N2O play a key role as reaction intermediates. Another remarkable result is the detection of HN3 as a gaseous product. NO2, N2H4, and NH2OH were considered the minor products. The nitrite and nitrate detected by IC were solution-phase products of the AOR at high potentials. The real-time identification of seven gaseous products, viz., N2, NO, N2H4, NH2OH, HN3, N2O, and NO2, and two solution-phase products, NO2– and NO3–, enabled us to propose AOR mechanistic pathways, opening more possibilities for the electrochemical generation of high-value-added nitrogenated products depending on Pt surface conditions.