Publication date: 15th December 2025
The main objective of this work is to study heterogeneous catalysts in situ and operando for ammonia oxidation under conditions that approach industrial settings, with the aim of linking morphology and surface structure to chemical selectivity. Although the macroscopic structural changes that accompany this industrial process are partially known, their influence on selectivity remains unclear. The proposed approach relies on three different platinum model catalysts: large single crystals, sub‑micron single crystals, and assemblies of iso‑oriented sub‑micron single crystals.
Three techniques were primarily employed: Bragg coherent diffraction imaging (BCDI) and surface X-ray diffraction (SXRD) at the SIXS beamline of the SOLEIL synchrotron, and X‑ray photoelectron spectroscopy (XPS) at the B07‑C beamline of the DIAMOND synchrotron. For each sample, temperature, and gas composition, the catalytic activity was monitored in parallel with each technique by tracking the partial pressures of the three oxidation products: nitrogen (N₂), nitric oxide (NO), and nitrous oxide (N₂O)(1).
Bragg coherent diffraction imaging (BCDI) provides detailed information on individual particles shape, exposed facets, surface and interface tension, elastic strain, and the nature of certain defects. The average particle behavior was examined with surface X‑ray diffraction (SXRD) on epitaxial nanoparticles, while SXRD was also used to probe the response of specific crystal facets, namely {111} and {100}, on large single crystals. X‑ray photoelectron spectroscopy (XPS) was carried out on single crystals to identify the dominant surface species present under each set of conditions(2).
The parameter space explored comprised a limited number of temperatures (300-600 °C) and a selected range of ammonia‑to‑oxygen ratios in the millibar pressure regime.
The study revealed two distinct families of particles, each likely to exhibit different behavior during the reaction, as well as the differing responses of individual facets to the gas environment.
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