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

A deep understanding of the ammonia oxidation reaction(AOR) overplatinum surfaces may facilitate the use of ammonia as a carbon-freesource for energy storage and conversion. Herein, using an unprecedentedexperimental 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 inAOR 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 AORonset potential was 0.40 V, and the surface intermediates were NH x,ads and N2H y,ads, which were the main precursors of N-2, considereda major product. N2H4, NO, and NH2OH were considered minor products in this potential region. In thehigh-potential region, from 0.82 V, adsorbed O2- was the main oxygenated surface species, owing to the strong interactionsbetween OH- and oxidized Pt. We found that NO andN(2)O play a key role as reaction intermediates. Anotherremarkable result is the detection of HN3 as a gaseousproduct. NO2, N2H4, and NH2OH were considered the minor products. The nitrite and nitrate detectedby IC were solution-phase products of the AOR at high potentials.The real-time identification of seven gaseous products, viz., N-2, NO, N2H4, NH2OH, HN3, N2O, and NO2, and two solution-phaseproducts, NO2 (-) and NO3 (-), enabled us to propose AOR mechanistic pathways, openingmore possibilities for the electrochemical generation of high-value-addednitrogenated products depending on Pt surface conditions.

Automated summary

This study investigates the ammonia oxidation reaction (AOR) over platinum surfaces using an unprecedented experimental approach. By combining online electrochemical mass spectrometry (OLEMS) and ion chromatography (IC), the researchers were able to detect multiple AOR products and demonstrate the variation in AOR selectivity depending on surface conditions. They identified the major and minor products, proposed AOR mechanistic pathways, and opened up possibilities for the electrochemical generation of nitrogenated products.