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

Abstract

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.