Tracking Facet-Dependent Surface Transformations of Ceria Nanoparticles from UHV to Ambient Pressure via Infrared Spectroscopy

Understanding how surface structures and coordination environments of metal oxide powders evolve from ultrahigh vacuum (UHV) to ambient pressure is a central challenge in heterogeneous catalysis. Surface-ligand infrared spectroscopy (SLIR) provides in situ, pressure-independent access to these transformations. We report a systematic IR spectroscopic study of ceria nanoparticles with distinct morphologies─octahedra, rods, cubes, and sponges─using CO as a probe molecule. By combining UHV-FTIRS and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), we track morphology-dependent vibrational signatures across wide pressure and temperature ranges. Facet-specific CO bands are identified using polarization-resolved infrared reflection absorption spectroscopy (IRRAS) reference data from low-index ceria single crystals, enabling assignments to oxidized and reduced surface sites. Despite the complexity of the spectra, a coherent interpretation is achieved, offering insights into the atomic-level behavior of pure ceria catalysts. Remarkably, only the nanooctahedra exhibit the expected CO adsorption response at atmospheric pressure, while all other shapes show reversible dynamic changes.