Unraveling the low-temperature activity of Rh-CeO catalysts in CO oxidation: probing the local structure and Red-Ox transformation of Rh species

The local structure of the active sites is one of the key aspects of establishing the nature of the catalytic activity of the systems. In this work, a detailed structural investigation of the Rh-CeO 2 catalysts prepared by the co-precipitation method was carried out. The application of a variety of physicochemical methods such as XRD, Raman spectroscopy, XPS, TEM, TPR-H 2 , and XAS revealed the presence of highly dispersed Rh 3+ species in the catalysts: Rh 3+ single ions and RhO x clusters. The substitution of Ce 4+ ions by Rh 3+ species, which provided a strong distortion of the CeO 2 lattice, is shown. XAS data ensured the refinement of the Rh local structure. It was shown that single Rh 3+ sites located next to each other can merge the formation of RhO x clusters with Rh local environment close to the one in Rh 2 O 3 and CeRh 2 O 5 oxides. The distortion of the CeO 2 lattice around single and cluster rhodium species had a beneficial effect on the catalytic activity of the samples in low-temperature CO oxidation (LTO-CO). TEM, XAS, and in situ XRD data allowed establishing the structural transformations of the catalysts under Red-Ox treatments. The reduction treatment led to Rhn metallic cluster formation localized on defects of the reduced CeO 2− δ . The reduced sample demonstrated efficient CO conversion at 0 °C. However, this system was not stable: its contact with air led to ceria reoxidation and partial reoxidation of Rh to highly dispersed Rh 3+ species at room temperature, while heating in an oxidizing atmosphere resulted in the complete reoxidation of metallic rhodium species. The results of the work shed light on the structural aspects of the reversibility of the Rh-CeO 2 catalysts based on the highly dispersed Rh 3+ species under treatment in the reaction conditions. We established the structural features of Rh single atom and RhO x clusters in CeO 2 , their Red-Ox reversibility and low temperature CO oxidation activity.