Unveiling the particle size effect and surface reactivity of Pt/C nanoparticles for ammonia electrooxidation using in situ infrared spectroscopy

The ammonia electrooxidation reaction (AmER) has attracted considerable attention due to its potential for hydrogen storage and transportation, as well as its possible application in direct ammonia fuel cells. In the present work, we studied ammonia electrooxidation on carbon-supported Pt/C nanoparticles (NPs) of four average sizes of 1.3, 2.2, 2.8, and 4.2 nm. Carbon-supported Pt NPs with a 20 wt% metal loading were synthesized using the polyol method, and the control of the synthesis solution pH allowed the formation of Pt NPs of different average sizes, which was confirmed by TEM. The onset potential was more negative for the smallest nanoparticles (1.3 nm) compared to those for the larger ones. Pt/C with a mean particle size of 2.2 nm showed better stability while exhibiting comparable activity to the 1.3 nm particles. As revealed by in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), the oxidation products included N–H species, azide ions, and nitrate and nitrite compounds. The N–H stretching peak was observed at about 2800 cm⁻¹ on the Pt surface and in the bulk of the electrolyte. However, the intensity of peaks corresponding to the reaction products was different on the surface of Pt and in the bulk of the electrolyte. NO₂⁻ was mostly observed in the bulk of the electrolyte. In contrast, NO₃⁻ was present on the Pt surface. PM-IRRAS demonstrated that the particle size affected the catalytic activity of Pt/C NPs but not their selectivity. In addition, the PM-IRRAS technique allowed, for the first time, distinguishing both symmetric and asymmetric N–O bonds that were not observed previously using IR spectroscopy during ammonia electrooxidation.