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 NH_x,ads and N₂H_y,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 O^2– 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.