Two decades of ceria nanoparticle research: structure, properties and emerging applications

Cerium oxide nanoparticles (CeNPs) are versatile materials with unique and unusual properties that vary depending on their surface chemistry, size, shape, coating, oxidation states, crystallinity, dopant, and structural and surface defects. This review encompasses advances made over the past twenty years in the development of CeNPs and ceria-based nanostructures, the structural determinants affecting their activity, and translation of these distinct features into applications. The two oxidation states of nanosized CeNPs (Ce3+/Ce4+) coexisting at the nanoscale level facilitate the formation of oxygen vacancies and defect states, which confer extremely high reactivity and oxygen buffering capacity and the ability to act as catalysts for oxidation and reduction reactions. However, the method of synthesis, surface functionalization, surface coating and defects are important factors in determining their properties. This review highlights key properties of CeNPs, their synthesis, interactions, and reaction pathways and provides examples of emerging applications. Due to their unique properties, CeNPs have become quintessential candidates for catalysis, chemical mechanical planarization (CMP), sensing, biomedical applications, and environmental remediation, with tremendous potential to create novel products and translational innovations in a wide range of industries. This review highlights the timely relevance and the transformative potential of these materials in addressing societal challenges and driving technological advancements across these fields.

Ni-Ru supported on CeO2 obtained by mechanochemical milling for catalytic hydrogen production from ammonia

Developing active and stable catalysts for carbon-free hydrogen production is crucial to mitigate the effects of climate change. Ammonia is a promising carbon-free hydrogen source, as it has a high hydrogen content and is liquid at low pressure, which allows its easy storage and transportation. We have recently developed a nickel-based catalyst with a small content of ruthenium supported on cerium oxide, which exhibits high activity and stability in ammonia decomposition. Here, we investigate mechanochemical milling for its synthesis, a faster and less energy-consuming technique than conventional ones. Results indicate that mechanochemical synthesis increases catalytic activity compared to the conventional incipient wetness impregnation method. The interaction between the metal precursors and the support is key in fine-tuning catalytic activity, which increases linearly with oxygen vacancies in the support. Moreover, the mechanochemical method modifies the oxidation state of Ni and Ru species, with a variation depending on the precursors.

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.

Specific reactivity of 4d and 5d transition metals supported over CeO2 for ammonia oxidation

Pt/CeO2 catalysts were most active in selective catalytic oxidation of ammonia, where Pt triggered the activation of surface lattice oxygen, and the dehydrogenation of ammonia assisted by surface lattice oxygen was the rate-determining step.

Direct Observation of Rhodium Ex-Solution from a Ceria Nanodomain and Its Use for Hydrogen Production via Propane Steam Reforming

The ex-solution phenomenon has received attention as a promising technique to prepare highly durable heterogeneous catalysts. Perovskite materials have been mainly used as host oxides for ex-solution, but their small surface areas have limited their practical use. Here, Rh was ex-solved by reducing Rh-doped ceria solid solution, and nanosized Rh catalysts with a high surface area of 70.7 m²/g were prepared. The Rh nanoparticles ex-solved from the ceria nanodomains were directly monitored by in situ transmission electron microscopy. The Rh nanoparticles whose sizes are 2-3 nm were not coarsened during the propane steam reforming process carried out at 700 °C for 65 h, leading to high resistance against sintering and coke formation. On the contrary, the Rh catalyst simply deposited on CeO₂ was significantly sintered after the reaction, and the size of Rh nanoparticles increased to 25 nm, resulting in severe coke formation. Our work shows that ex-solution from a ceria-based nanodomain can be a good way to prepare metal nanoparticle catalysts with a large surface area and excellent durability for gas-phase reactions at high temperatures.

Theoretical Studies on the Stability and Reactivity of the Metal-Doped CeO2(100) Surface: Toward H2 Dissociation and Oxygen Vacancy Formation

Surface doping is a common method to improve the performance of nanostructured materials. Different dopants will affect the structure and catalytic reactivity of the support. For a comprehensive understanding of the doping effects of metals doped into CeO₂, we conducted density functional theory (DFT) studies on the stabilities and geometry structures of transition-metal atoms (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au) doped into CeO₂(111), (110), and (100) surfaces. Moreover, the reactivity for H₂ dissociation and oxygen vacancy formation are systematically investigated on M-doped CeO₂(100) surfaces. The greater the binding energies of doped M atoms on the CeO₂ surface, the more difficult the formation of oxygen vacancies. The doped Co and Ir atoms do not directly participate in H₂ activation but serve as a promoter to make the H-H bond to break easily. The Cu, Ru, Pd, Ag, Pt, and Au atoms could act as the catalytically active center for H₂ dissociation and greatly reduce the activation energy barrier. Besides, it is easier to generate H₂O (W^M) and a surface oxygen vacancy from the intermediate H2^M/H4^M than from H3^M/H5^M, which is related to the acid-base interaction between H_Ce/M* and H_O* in H2^M/H4^M. This work could provide theoretical insights into the atomic structure characteristics of the transition-metal-doped CeO₂(100) surface and give ideas for the design of hydrogenation catalysts.

Atomic structures and local electronic properties of K- and Rh-modified ceria/Pt(111) inverse model catalysts

Ceria has been widely applied as a support in heterogeneous catalysis due to its unique capability to store and release oxygen. As a typical inverse model catalyst, a ceria/Pt(111) system has attracted much attention due to its strong metal-oxide interaction. The structural and electronic properties of the ceria/Pt(111) system can be effectively modified by the introduction of alien K and Rh atoms. Here, the K- and Rh-modified ceria/Pt(111) inverse model catalysts have been investigated with high resolution scanning tunneling microscopy and apparent local work function measurement. The experimental results indicate that the K atoms prefer to occupy the top sites of the stoichiometric ceria, while the Rh atoms are prone to stay at the electron-rich ceria island edges. The K and Rh atoms act as an electron donor and acceptor on ceria/Pt(111), respectively. Such a study on the modification of the ceria-based catalysts should help understand strong metal-oxide interaction in heterogeneous catalysis at the atomic level.

Pt/CeO2 and Pt/CeSnOx Catalysts for Low-Temperature CO Oxidation Prepared by Plasma-Arc Technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeO_x and Pt/CeSnO_x for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnO_x and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt²⁺ and Pt⁴⁺ ions in the catalyst Pt/CeO_x, whereas only the state Pt²⁺ of platinum could be detected in the Sn-modified catalyst Pt/CeSnO_x. Insertion of Sn ions into the Pt/CeO_x lattice destabilizes/reduces Pt⁴⁺ cations in the Pt/CeSnO_x catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce³⁺ ions. Our DFT calculations corroborate destabilization of Pt⁴⁺ ions by incorporation of cationic Sn in Pt/CeO_x. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions.

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).