Unraveling the low-temperature activity of Rh-CeO2 catalysts in CO oxidation: probing the local structure and Red-Ox transformation of Rh3+ 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-CeO₂ 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-H₂, and XAS revealed the presence of highly dispersed Rh³⁺ species in the catalysts: Rh³⁺ single ions and RhO_x clusters. The substitution of Ce⁴⁺ ions by Rh³⁺ species, which provided a strong distortion of the CeO₂ lattice, is shown. XAS data ensured the refinement of the Rh local structure. It was shown that single Rh³⁺ sites located next to each other can merge the formation of RhO_x clusters with Rh local environment close to the one in Rh₂O₃ and CeRh₂O₅ oxides. The distortion of the CeO₂ 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 Rh³⁺ 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-CeO₂ catalysts based on the highly dispersed Rh³⁺ species under treatment in the reaction conditions.

Synthesis of Ce1−xPdxO2−δ Solid Solution in Molten Nitrate

CeO2-based solid solutions in which Pd partially substitutes for Ce attract considerable attention, owing to their high catalytic performances. In this study, the solid solution (Ce1−xPdxO2−δ) with a high Pd content (x ~ 0.2) was synthesized through co-precipitation under oxidative conditions using molten nitrate, and its structure and thermal decomposition were examined. The characteristics of the solid solution, such as the change in a lattice constant, inhibition of sintering, and ionic states, were examined using X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM−EDS), transmission electron microscopy (TEM)−EDS, and X-ray photoelectron spectroscopy (XPS). The synthesis method proposed in this study appears suitable for the easy preparation of CeO2 solid solutions with a high Pd content.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties.

Reversible low-temperature redox activity and selective oxidation catalysis derived from the concerted activation of multiple metal species on Cr and Rh-incorporated ceria catalysts

The ceria-based catalyst incorporated with Cr and a trace amount of Rh (Cr_0.19Rh_0.06CeO_z) was prepared and the reversible redox performances and oxidation catalysis of CO and alcohol derivatives with O₂ at low temperatures (<373 K) were investigated. In situ X-ray absorption fine structure (XAFS), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)-EDS/EELS and temperature-programmed reduction/oxidation (TPR/TPO) revealed the structures and redox mechanisms of three metals in Cr_0.19Rh_0.06CeO_z: dispersed Rh^3+δ species (<1 nm) and Cr^6-γO_3-x nanoparticles (∼1 nm) supported on CeO₂ in Cr_0.19Rh_0.06CeO_z were transformed to Rh nanoclusters, Cr(OH)₃ species and CeO_2-x with two Ce³⁺-oxide layers at the surface in a concerted activation manner of the three metal species with H₂.

Thermal stability and propane combustion activity of RhxCe1−xO2−y nanoparticles deposited on functionalized alumina

A new, effective method for stabilization of Rh_xCe_1−xO_2−y nanoparticles synthesized by the reverse microemulsion method by their deposition on functionalized alumina has been successfully applied for systems in a wide range of Rh contents (x ≤ 0.15).

The structure and catalytic properties of Rh-doped CeO2 catalysts

The PDF analysis with TEM, XPS and Raman spectroscopy indicates the formation of homogenous Rh_xCe_1−xO_2−δ nanocrystalline solid solutions.

Real-space observation of strong metal-support interaction: state-of-the-art and what's the next

The real-space resolving of the encapsulated overlayer in the well-known model and industry catalysts, ascribed to the advent of dedicated transmission electron microscopy, enables us to probe novel nano/micro architecture chemistry for better application, revisiting our understanding of this key issue in heterogeneous catalysis. In this review, we summarize the latest progress of real-space observation of SMSI in several well-known systems mainly covered from the metal catalysts (mostly Pt) supported by the TiO2 , CeO2 and Fe3 O4 . As a comparison with the model catalyst Pt/Fe3 O4 , the industrial catalyst Cu/ZnO is also listed, followed with the suggested ongoing directions in the field.