Redox Properties Of Ceria Research Articles

Effect of temperature on polaronic transport in CeO2 thin-film.

The outstanding catalytic property of cerium oxide (CeO2) strongly depends on the polaron formation due to the oxygen vacancy (V̈O) defect and Ce4+ to Ce3+ transformation. Temperature plays an important role in the case of polaron generation in CeO2 and highly influences its electrical transport properties. Therefore, a much needed attention is required for detailed understanding of the effect of temperature on polaron formation and oxygen vacancy migration to get an idea about the improvement in the redox property of ceria. In this work, we have probed the generation of polarons in CeO2 thin-film deposited on a silicon (Si) substrate using the resonance photoemission spectroscopy (RPES) study. The RPES data show an increase in polaron density at the substrate-film interface of the thermally annealed film, indicating the formation of an interfacial Ce2O3 layer, which is, indeed, a phase change from the cubic to hexagonal structure. This leads to a modified electronic band structure, which has an impact on the capacitance-voltage (C-V) characteristics. This result nicely correlates the microscopic property of polarons and the macroscopic transport property of ceria.

Screened hybrid functionals applied to ceria: Effect of Fock exchange

We investigate how the redox properties of ceria are affected by the fraction of Fock exchange in screened HSE06-based hybrid density functionals, and we compare with $\mathrm{PBE}+U$ results, and with experiments when available. We find that using $\ensuremath{\sim}15%$ Fock exchange yields a good compromise with respect to structure, electronic structure, and calculated reduction energies, and represents a significant improvement over the $\mathrm{PBE}+U$ results. We also investigate the possibility to use a computationally cheaper HSE06//$\mathrm{PBE}+U$ protocol consisting of structure optimization with $\mathrm{PBE}+U$, a subsequent lattice parameter rescaling step, and, finally, a single-point full hybrid calculation. We find that such a composite computational protocol works very well and yields results in close agreement with those where HSE06 was used also for the structure optimization.

Simultaneous Ni Doping at Atom Scale in Ceria and Assembling into Well-Defined Lotuslike Structure for Enhanced Catalytic Performance.

Oxide materials with redox capability have attracted worldwide attentions in many applications. Introducing defects into crystal lattice is an effective method to modify and optimize redox capability of oxides as well as their catalytic performance. However, the relationship between intrinsic characteristics of defects and properties of oxides has been rarely reported. Herein, we report a facile strategy to introduce defects by doping a small amount of Ni atoms (∼1.8 at. %) into ceria lattice at atomic level through the effect of microstructure of crystal on the redox property of ceria. Amazingly, a small amount of single Ni atom-doped ceria has formed a homogeneous solid solution with uniform lotuslike morphology. It performs an outstanding catalytic performance of a reduced T50 of CO oxidation at 230 °C, which is 135 °C lower than that of pure CeO2 (365 °C). This is largely attributed to defects such as lattice distortion, crystal defects and elastic strain induced by Ni dopants. The DFT calculation has revealed that the electron density distribution of oxygen ions near Ni dopant, the reduced formation energy of oxygen vacancy originated from local chemical effect caused by local distortion after Ni doping. These differences have a great effect on increasing the concentration of oxygen vacancies and enhancing the migration of lattice oxygen from bulk to a surface which is closely related to optimized redox properties. As a result, oxygen storage capacity and the associated catalytic reactivity has been largely increased. We have clearly demonstrated the change of crystal lattice and the charge distribution effectively modify its chemical and physical properties at the atomic scale.

Insight into the Deactivation of Au/CeO2 Catalysts Studied by In Situ Spectroscopy during the CO-PROX Reaction

The activity and deactivation of Au/CeO2 catalysts in the PROX reaction were measured during long-term experiments (TOS = 160 h) by in situ DRS-UV–vis spectroscopy. During the reaction, the support showed a decrease in band gap energy. The activity and selectivity to CO2 were closely linked to the reduction of Aun+ species, the sintering of the reduced Au species, and to changes in the Ce3+/Ce4+ ratio. This ratio increased during long-term PROX experiments, showing a decrement of surface oxygen atoms from the CeO2 phase which participated in the reaction, after which the total oxygen fed to the reaction was wholly consumed. Also, almost all of the Au3+ species initially present in the catalysts were reduced to Au+ and Au0. Results suggest that the activity loss is connected to the fast reduction of Au species and is also linked to changes in the redox properties of ceria, breaking the cycle that provides oxygen to the reaction. Sintering of the Au nanoparticles occurs in a longer time scale. On the other ...

Quantitative depth profiling of Ce(3+) in Pt/CeO2 by in situ high-energy XPS in a hydrogen atmosphere.

The redox property of ceria is a key factor in the catalytic activity of ceria-based catalysts. The oxidation state of well-defined ceria nanocubes in gas environments was analysed in situ by a novel combination of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) and high-energy XPS at a synchrotron X-ray source. In situ high-energy XPS is a promising new tool to determine the electronic structure of matter under defined conditions. The aim was to quantitatively determine the degree of cerium reduction in a nano-structured ceria-supported platinum catalyst as a function of the gas environment. To obtain a non-destructive depth profile at near-ambient pressure, in situ high-energy XPS analysis was performed by varying the kinetic energy of photoelectrons from 1 to 5 keV, and, thus, the probing depth. In ceria nanocubes doped with platinum, oxygen vacancies formed only in the uppermost layers of ceria in an atmosphere of 1 mbar hydrogen and 403 K. For pristine ceria nanocubes, no change in the cerium oxidation state in various hydrogen or oxygen atmospheres was observed as a function of probing depth. In the absence of platinum, hydrogen does not dissociate and, thus, does not lead to reduction of ceria.

Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria

The influence of water on the redox properties of ceria is pivotal to its widespread exploitation spanning a variety of applications. Ab initio simulation techniques based on DFT-GGA+U are used to ...

Study by electrical conductivity measurements of semiconductive and redox properties of ceria and phosphated ceria catalysts

Pure ceria and two surface-phosphated ceria containing different amounts of phosphorus, 1.1P/CeO2 and 2.2P/CeO2, catalysts for isobutane oxydehydrogenation, were characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and was followed with time during sequential exposures to air, nitrogen, isobutane–air mixture (reaction mixture) and isobutane–nitrogen mixture in conditions similar to those of catalysis. All the materials appeared to be n-type semiconductors under air with quasi-free electrons as the main charge carriers and their electrical conductivity increased following the order: CeO2<1.1P/CeO2<2.2P/CeO2. While CeO2 and 1.1P/CeO2 remained n-type semiconductors when contacted with isobutane–nitrogen or isobutane–air mixture, 2.2P/CeO2 became a p-type semiconductor with positive holes as the main charge carriers. For 2.2P/CeO2 catalyst the oxydehydrogenation reaction involves surface lattice O− species, whereas for CeO2 surface lattice O2− anions are involved. For 1.1P/CeO2 catalyst the reaction mechanism involves both surface lattice O2− anions and surface lattice O− species. These results explained the difference in catalytic performances encountered on these solids. In all cases the overall reaction mechanism can be assimilated to a Mars and van Krevelen mechanism.

Study of nano-structured ceria for catalytic CO oxidation

Mesoporous, nano-structured ceria has been synthesized using a low-cost biopolymer chitosan as a template, and the same material has been studied for its CO oxidation activity. The ceria thus prepared shows low crystallite size (6.5 nm), high surface area (BET-SA 144.2 m2 g−1) and mesopores (32.4 A) without ordered structure. Different ceria samples have been synthesized by manipulating the chitosan : cerium ratio and synthesis temperature. These ceria samples have been characterized using XRD, BET-SA, SEM, FTIR, UV-DRS, PL and TEM techniques. The ceria samples show fluorite structure with cubic symmetry and increased lattice volume. The nano-structured ceria thus synthesized lowers the CO oxidation temperature by 30–60 °C as compared to the commercial ceria. The improved catalytic activity primarily appears to be the effect of improved surface area and pore characteristics of nano-structured ceria. Also the expansion in lattice cell volume leads to the improvement in redox properties of ceria, thereby facilitating the formation of surface oxygen vacancies. Such mesoporous ceria without ordered structure could be useful as an oxidation catalyst as well as a support for various catalysts. The platinum incorporated mesoporous ceria also shows excellent CO oxidation.

Effect of gallia doping on the acid–base and redox properties of ceria

This work presents the synthesis and characterization of cerium oxide doped with gallium, which were prepared by co-precipitation in basic aqueous solution followed by calcination at 773 K. XRD and Raman spectroscopy suggested that Ga(III) cations were incorporated into the ceria crystal structure. Moreover, surface Ga-H species were detected by FTIR with a 100 cm −1 downshift of the ν(Ga-H) peak, which indicated that Ga(III) were incorporated onto the surface lattice of the ceria. Results of CO 2 and CO chemisoption, as well as, TPR-H 2 and TPR-CO, showed that the acid–base (decreased basicity) and redox (enhanced reducibility) properties of the gallium-doped ceria materials were modified as compared with the pure cerium dioxide. The catalytic oxidation of CO was investigated over the oxides.

Ceria in catalysis: From automotive applications to the water–gas shift reaction

AbstractCeria is a crucial component of automotive catalysts, where its ability to be reduced and re‐oxidized provides oxygen storage capacity. Because of these redox properties, ceria can greatly enhance catalytic activities for a number of important reactions when it is used as a support for transition metals. For reactions that use steam as an oxidant (e.g., the water–gas‐shift reaction and steam reforming of hydrocarbons), rates for ceria‐supported metals can be several orders of magnitude higher than that for ceria or the transition metal alone. Because the redox properties of ceria are strongly dependent on treatment history and the presence of additives, there are significant opportunities for modifying catalysts based on ceria to further improve their performance. This article will review some of the contributions from my laboratory on understanding and using ceria in these applications. © 2010 American Institute of Chemical Engineers AIChE J, 2010

Propane reforming on Ni–Ru/GDC catalyst: H2 production for IT-SOFCs under SR and ATR conditions

Abstract Steam and auto-thermal reforming of propane over a Ni–Ru/GDC catalyst prepared by hydrothermal method were investigated under intermediate temperature solid oxide fuel cell (IT-SOFC) operating conditions. Such an approach is propaedeutical for the application of the catalyst in IT-SOFC. At reaction temperature higher than 700 °C, under both steam reforming (SR) and auto-thermal reforming (ATR), high propane conversion and syngas (H 2 + CO) productivity were obtained. In SR significant amount of filamentous carbon was formed mainly at 600 °C, with subsequent catalyst deactivation. In ATR, coke formation was completely depressed and the catalyst resulted to be very stable in all investigated reaction conditions. Notwithstanding the oxygen storage capability and redox properties of ceria in promoting both oxygen activation and carbon residues gasification, without oxygen in gas phase the concentration of oxygen vacancies itself were not able to inhibit carbon deposition. On the other hand, the presence of oxygen in the reaction stream slightly negatively reflects on syngas production, since it promotes CO 2 and H 2 O formation; however, the loss in selectivity is widely compensated by the absence of deactivation phenomena.

Insights into the redox properties of ceria-based oxides and their implications in catalysis

In the present work, the redox properties of ceria and related materials will be considered with a focus on these effects that are relevant in catalysis. Particularly, after briefly pointing out the relevant redox chemistry and characterization of ceria in classical TWC application, we will describe how the reduction/oxidation characteristics of ceria are of help in other applications, like the development of advanced ceria–zirconia based catalyst for auto exhaust treatment, and the elimination of soot from diesel engine exhaust. In the first example, different catalyst formulations based on ceria and zirconia are examined to see how catalytic activity is related with composition and a simple redox mechanism model for CO oxidation is also derived. A similar approach is used to characterize the role of ceria in the combustion of soot.

Structural and Redox Properties of Ceria in Alumina-Supported Ceria Catalyst Supports

Several alumina-supported ceria samples with varying ceria contents (1, 2, 10, and 39 wt %) have been studied by different techniques (XRD, Raman, TEM-EDS, UV−vis DRS, XANES, and EPR), aiming to establish a complete model on the structural and redox characteristics of ceria entities in these samples. Two general types of ceria entities are observed: aggregated crystalline ceria species (3D-Ce) and dispersed ceria species in the form of two-dimensional patches (2D-Ce). The relative amounts of each of these species do not show a linear relationship with the cerium loading: as the cerium content is increased, a trend toward formation of 3D-Ce, to the detriment of 2D-Ce, is observed. The EPR data evidence differences in chemical (redox) behavior between 2D-Ce, 3D-Ce, and unsupported ceria. Thus, the main differing characteristic of some 3D-Ce particles with respect to unsupported ceria, which holds true for all 3D-Ce formed with weight percent ≤ 10, is the absence of associated vacancies upon outgassing at ...

Magnetic study of the interaction of hydrogen with a Pt/CeO 2–Al 2O 3 catalyst: influence of the presence of chlorine

The influence of a chlorinated precursor on the redox properties of ceria was studied by comparing two Pt/CeO 2–Al 2O 3 catalysts prepared from hexachloroplatinic acid and platinum acetylacetonate. In absence of chlorine, a dispersion value higher than 100% has been measured by irreversible hydrogen chemisorption. To interpret this result, the interaction of H 2 with the support and the two catalysts were studied by magnetic measurements. The variations of the Ce 3+ content have given evidence that different redox processes may occur. The hydrogen spillover is clearly observed in presence of platinum with the phenomenon being more pronounced for the ex-acetylacetonate catalyst. The limited amount of hydrogen irreversibly adsorbed at 294 K on the support for this free chlorine reduced catalyst is sufficient to explain the excessive value obtained for the platinum dispersion. The results show a fixation of chlorine on the supported ceria phase which inhibits the mobility of hydroxyl species responsible for the hydrogen spillover. Moreover, independently to the presence of chlorine, a reduction at 573 K seems to stabilise a significant fraction of Ce 3+ ions against reoxidation under air at room temperature. This was not the case for bulk ceria and probably originates from ceria/alumina interactions occurring after reduction.

Influence of the redox properties of ceria on the preparation of three-way automotive platinum—rhodium/ alumina—ceria catalysts

The redox properties of ceria were investigated by temperature-programmed reduction performed under hydrogen and the catalysts were evaluated by determining the temperatures at which 50% conversion for propene and propane oxidation were reached. The results obtained show that ceria plays a decisive role within the preparation of bimetallic PtRh/Al 2O 3CeO 2 catalysts by successive impregnations (SI) when reduced capping cerium atoms (Ce 3+) are present on the parent Pt/Al 2O 3CeO 2 catalyst before introduction of the rhodium salt. Thus, rhodium deposition would occur by a redox reaction between Ce 3+ and Rh 3+. The preparation procedure of PtRh/Al 2O 3CeO 2 catalysts by the SI method increases the thermal stability of the metallic species under oxidizing conditions. This improvement would be the consequence of a higher resistance to the formation of PtRh alloys with surface enrichment in rhodium.