Structure, Morphology and Reducibility of Epitaxial Cerium Oxide Ultrathin Films and Nanostructures

High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

The inverse catalyst 'cerium oxide (ceria) on copper' has attracted much interest in recent time because of its promising catalytic activity in the water-gas-shift reaction and the hydrogenation of CO₂. For such reactions it is important to study the redox behaviour of this system, in particular with respect to the reduction by H₂. Here, we investigate the high-temperature O₂oxidation and H₂reduction of ceria nanoparticles (NPs) and a Cu(111) support by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). After oxidation at 550 °C, the ceria NPs and the Cu(111) support are fully oxidized, with the copper oxide exhibiting a new oxide structure as verified by LEED and STM. We show that a high H₂dosage in the kilo Langmuir range is needed to entirely reduce the copper support at 550 °C. A work function (WF) difference of △ϕ_rCeria/Cu-Cu≈ -0.6 eV between the ceria NPs and the metallic Cu(111) support is measured, with the Cu(111) surface showing no signatures of separated and confined surface regions composed by an alloy of Cu and Ce. After oxidation, the WF difference is close to zero (△ϕ_Ceria/Cu-Cu≈ -0.1…0 eV), which probably is due to a WF change of both, ceria and copper.

Engineering the Multi-Enzymatic Activity of Cerium Oxide Nanoparticle Coatings for the Antioxidant Protection of Implants

Imbalance of oxidants is a universal contributor to the failure of implanted devices and tissues. A sustained oxidative environment leads to cytotoxicity, prolonged inflammation, and ultimately host rejection of implanted devices/grafts. The incorporation of antioxidant materials can inhibit this redox/inflammatory cycle and enhance implant efficacy. Cerium oxide nanoparticles (CONP) is a highly promising agent that exhibits potent, ubiquitous, and self-renewable antioxidant properties. Integrating CONP as surface coatings provides ease in translating antioxidant properties to various implants/grafts. Herein, we describe the formation of CONP coatings, generated via the sequential deposition of CONP and alginate, and the impact of coating properties, pH, and polymer molecular weight, on their resulting redox profile. Investigation of CONP deposition, layer formation, and coating uniformity/thickness on their resulting oxidant scavenging activity identified key parameters for customizing global antioxidant properties. Results found lower molecular weight alginates and physiological pH shift CONP activity to a higher H2O2 to O2 --scavenging capability. The antioxidant properties measured for these various coatings translated to distinct antioxidant protection to the underlying encapsulated cells. Information gained from this work can be leveraged to tailor coatings towards specific oxidant-scavenging applications and prolong the function of medical devices and cellular implants.

Synthesis and characterization of nanoceria for electrochemical sensing applications

Nanoceria (cerium oxide nanoparticles: CeO2-NPs) has received significant attention due to its biocompatibility, good conductivity, and the ability to transfer oxygen. Nanoceria has been widely used to develop electrochemical sensors and biosensors as it could increase response time, sensitivity, and stability of the sensor. In this review, we discussed synthesis methods, and the recent applications employing CeO2-NPs for electrochemical detection of various analytes reported in the most recent four years.

Cerium Oxide Nanoparticles: Advances in Biodistribution, Toxicity, and Preclinical Exploration

Antioxidant nanoparticles have recently gained tremendous attention for their enormous potential in biomedicine. However, discrepant reports of either medical benefits or toxicity, and lack of reproducibility of many studies, generate uncertainties delaying their effective implementation. Herein, the case of cerium oxide is considered, a well-known catalyst in the petrochemistry industry and one of the first antioxidant nanoparticles proposed for medicine. Like other nanoparticles, it is now described as a promising therapeutic alternative, now as threatening to health. Sources of these discrepancies and how this analysis helps to overcome contradictions found for other nanoparticles are summarized and discussed. For the context of this analysis, what has been reported in the liver is reviewed, where many diseases are related to oxidative stress. Since well-dispersed nanoparticles passively accumulate in liver, it represents a major testing field for the study of new nanomedicines and their clinical translation. Even more, many contradictory works have reported in liver either cerium-oxide-associated toxicity or protection against oxidative stress and inflammation. Based on this, finally, the intention is to propose solutions to design improved nanoparticles that will work more precisely in medicine and safely in society.

Dynamics of the Interaction Between Ceria and Platinum During Redox Processes

The work is focused on understanding the dynamics of the processes which occur at the interface between ceria and platinum during redox processes, by investigating an inverse catalytic model system made of ceria epitaxial islands and ultrathin films supported on Pt(111). The evolution of the morphology, structure and electronic properties is analyzed in real-time during reduction and oxidation, using low-energy electron microscopy and spatially resolved low-energy electron diffraction. The reduction is induced using different methods, namely thermal treatments in ultra-high vacuum and in H₂ as well as deposition of Ce on the oxide surface, while re-oxidation is obtained by exposure to oxygen at elevated temperature. The use of two different epitaxial systems, continuous films and nanostructures, allows determining the influence of platinum proximity on the stabilization of the specific phases observed. The factors that limit the reversibility of the observed modifications with the different oxidation treatments are also discussed. The obtained results highlight important aspects of the cerium oxide/Pt interaction that are relevant for a complete understanding of the behavior of Pt/CeO₂ catalysts.

Growth and structure of ultrathin praseodymium oxide layers on ruthenium(0001)

The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 °C, the oxide is shown to form hexagonally close-packed (A-type) Pr₂O₃(0001) islands that are up to 3 nm high. Depending on the local substrate step density, the islands either adopt a triangular shape on sufficiently large terraces or acquire a trapezoidal shape with the long base aligned along the substrate steps.

The cubic-to-hexagonal phase transition of cerium oxide particles: dynamics and structure

Cerium oxide is often applied in today's catalysts due to its remarkable oxygen storage capacity. The changes in stoichiometry during reaction are linked to structural modifications, which in turn affect its catalytic activity. We present a real-time in situ study of the structural transformations of cerium oxide particles on ruthenium(0001) at high temperatures of 700 °C in ultra-high vacuum. Our results demonstrate that the reduction from CeO₂ to cubic Ce₂O₃ proceeds via ordered intermediary phases. The final reduction step from cubic to hexagonal Ce₂O₃ is accompanied by a lattice expansion, the formation of two new surface terminations, a partial dissolution of the cerium oxide particles, and a massive mass transport of cerium from the particles to the substrate. The conclusions allow for new insights into the structure, stability, and dynamics of cerium oxide nanoparticles in strongly reducing environments.

Efficient Ceria-Platinum Inverse Catalyst for Partial Oxidation of Methanol

Ceria-platinum-based bilayered thin films deposited by magnetron sputtering were developed and tested in regard to their catalytic activity for methanol oxidation by employing a temperature-programmed reaction (TPR) technique. The composition and structure of the samples were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Both conventional (oxide-supported metal nanoparticles [NPs]) and inverse configurations (metal with oxide overlayer) were analyzed to uncover the structural dependence of activity and selectivity of these catalysts with respect to different pathways of methanol oxidation. We clearly demonstrate that the amount of cerium oxide (ceria) loading has a profound influence on methanol oxidation reaction characteristics. Adding a noncontinuous adlayer of ceria greatly enhances the catalytic performance of platinum (Pt) in favor of partial oxidation of methanol (POM), gaining an order of magnitude in the absolute yield of hydrogen. Moreover, the undesired by-production of carbon monoxide (CO) is strongly suppressed, making the ceria-platinum inverse catalyst a great candidate for clean hydrogen production. It is suggested that the methanol oxidation process is facilitated by the synergistic effect between both components of the inverse catalyst (involving oxygen from ceria and providing a reaction site on the adjacent Pt surface) as well as by the fact that the ability of ceria to exchange oxygen (i.e., to alter the oxidation state of Ce between 3+ and 4+) during the reaction is inversely proportional to its thickness. The increased redox capability of the discontinuous ceria adlayer shifts the preferred reaction pathway from dehydrogenation of hydroxymethyl intermediate to CO in favor of its oxidation to formate.