Stability and Temperature-Induced Agglomeration of Rh Nanoparticles Supported by CeO₂

The effects of reduction by H₂ and by heat treatment in vacuum and in O₂ flow on Rh particle size changes of Rh/CeO₂ samples were studied by X-ray photoelectron spectroscopy (XPS), high-resolution electron microscopy (HRTEM), and CO adsorption followed by diffuse reflectance infrared spectroscopy (DRIFTS). Low-temperature (373–423 K) reduction of Rh without agglomeration is demonstrated. An average particle size of 2.3 ± 1.1 nm was measured by HRTEM regardless of the metal loading (1–5%). On Rh/CeO₂, a significant particle size increase of the Rh particles was detected on heating (773 K). In this work, we suggest that the temperature-induced surface decrease resulting from the sintering of Rh is favored only for well-dispersed particles. XP spectra revealed that the mobile oxygens of CeO₂ fundamentally determine the oxidation state of the supported metals. At elevated temperature, the oxidation of the reduced support surface as well as the metal component takes place because of the segregation of ceria oxygens. When the aggregated particles were reoxidized, the redispersion of Rh was observed probably because of the formation of Rh–O–Ce bonds