Description
The main objective of this work is to study heterogeneous catalysts in situ and operando for ammonia oxidation under conditions approaching industrial settings, with the aim of linking morphology and surface structure to chemical selectivity. Currently, the macroscopic structural changes associated with this industrial process are partially known, but their role in selectivity remains unclear. The proposed approach is based on three different platinum model catalysts: large single crystals, submicron single crystals, and assemblies of iso-oriented submicron single crystals.
Three techniques were primarily used: Bragg coherent diffraction imaging (BCDI) and surface X-ray diffraction (SXRD) at the SixS beamline of SOLEIL synchrotron, and X-ray photoelectron spectroscopy (XPS) at the B07-C beamline of DIAMOND synchrotron. The catalytic activity was monitored in parallel with each technique for each sample, temperature and gas compositions by tracking the partial pressures of the three oxidation products: nitrogen (N₂), nitric oxide (NO), and nitrous oxide (N₂O).
BCDI enables the retrieval of detailed information on single particles, such as shape, facets, surface and interface tension, compression, and the nature of some defects. The average particle behavior was analyzed using SXRD on epitaxial nanoparticles. Additionally, SXRD was employed to study the behavior of individual facets namely, {111} and {100} on large single crystals. XPS was performed on single crystals to determine the major surface species associated with each condition. The parameter space explored involved a few selected temperatures ranging from 300°C to 600°C for a chosen set of ammonia-to-oxygen ratios in the mbar regime.
The study revealed the existence of two distinct families of particles, potentially exhibiting different behaviors during the reaction, as well as the alternative responses of individual facets to the gas environment.
