Ce L3-edge Research Articles

High quantum yield luminescence and scintillation properties of high-Ce-doped MgF2–Al2O3–B2O3 glasses and their glass structure

Ce-doped inorganic single crystals are currently used in scintillators in various instruments because of their outstanding luminescence properties. However, these crystals are expensive to manufacture and are inefficient in terms of their energy consumption. In this study, xCeF3-doped 40MgF2–20Al2O3–40B2O3 glasses (x = 0−30 mol%) were prepared by a melt-quenching method in N2 atmosphere, and their photoluminescence, scintillation, and dosimetry properties were investigated. The X-ray absorption fine edge spectroscopy of the Ce L3-edge indicated that the doped Ce exists in the form of Ce3+ ions, and that Ce4+ does not form because of the low basicity of the glass. The glasses underwent photoluminescence at 380 nm due to the 5d-4f transition of Ce3+, with a very high quantum yield of up to ∼100 % and low concentration quenching. The peak shape of the X-ray-induced luminescence was similar to that of the PL with a decay time of 110−70 ns. The light yields for gamma rays (137Cs source) were obtained from the pulse-height spectra, and the highest light yield among the present samples was 1071 photons/MeV with the energy resolution of 17 %. The thermoluminescence glow curves had peaks at 365 and 460 K and a good linear relationship existed in the range of 0.5–1000 mGy, with the dose response being comparable to that of a commercial dosimeter. The origin of the high quantum yield was probed by analyzing the glass structure using molecular dynamics simulation based on a graph neural network. A strong tendency to selectively bind cations and anion pairs was found: Mg preferably binds to F, B preferably binds to O, and Al binds to O and F, which can be explained by soft and hard acid and base theories. Because of this selectivity the glass forms fluoride- and oxide-rich domains, which seem to be responsible for the more effective dispersal of the luminescent center. The results of this study are expected to contribute to the development of glasses with enhanced photoluminescence and scintillation luminescence properties.

Electronic-Structure Interpretation: How Much Do We Understand Ce L3 XANES?

Historically, cerium has been attractive for pharmaceutical and industrial applications. The cerium atom has the unique ability to cycle between two chemical states (Ce(III) and Ce(IV)) and drastically adjust its electronic configuration: [Xe] 4f15d16s2 in response to a chemical reaction. Understanding how electrons drive chemical reactions is an important topic. The most direct way of probing the chemical and electronic structure of materials is by X-ray absorption spectroscopy (XAS) or X-ray absorption near-edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode. Such measurements at the Ce L3 edge have the advantage of a high penetration depth, enabling in-situ reaction studies in a time-resolved manner and investigation of material production or material performance under specific conditions. But how much do we understand Ce L3 XANES? This article provides an overview of the information that can be extracted from experimental Ce L3 XAS/XANES/HERFD data. A collection of XANES data recorded on various cerium systems in HERFD mode is presented here together with detailed discussions on data analysis and the current status of spectral interpretation, including electronic structure calculations.

Probing the Long- and Short-Range Structural Chemistry in the C-Type Bixbyite Oxides Th0.40Nd0.48Ce0.12O1.76, Th0.47Nd0.43Ce0.10O1.785, and Th0.45Nd0.37Ce0.18O1.815 via Synchrotron X-ray Diffraction and Absorption Spectroscopy.

The long- and short-range structural chemistry of the C-type bixbyite compounds Th0.40Nd0.48Ce0.12O1.76, Th0.47Nd0.43Ce0.10O1.785, and Th0.45Nd0.37Ce0.18O1.815 is systematically examined using synchrotron X-ray powder diffraction (S-PXRD), high-energy resolution fluorescence detection X-ray absorption near edge (HERFD-XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS) measurements supported by electronic structure calculations. S-PXRD measurements revealed that the title compounds all form classical C-type bixbyite structures in space group Ia3̅ that have disordered cationic crystallographic sites with further observation of characteristic superlattice reflections corresponding to oxygen vacancies. Despite the occurrence of oxygen vacancies, HERFD-XANES measurements on the Ce L3-edge revealed that Ce incorporates as Ce4+ into the structures but involves local distortion that resembles cluster behavior and loss of nearest-neighbors. In comparison, HERFD-XANES measurements on the Nd L3-edge supported by electronic structure calculations reveal that Nd3+ adopts a local coordination environment similar to the long-range C-type structure while providing charge balancing for the formation of oxygen defects. Th L3-edge EXAFS analysis reveals shorter average Th-O distances in the title compounds in comparison to pristine ThO2 in addition to shorter Th-O and Th-Ce distances compared to Th-Th or Ce-Ce in the corresponding F-type binary oxides (ThO2 and CeO2). These distances are further found to decrease with the increased Nd content of the structures despite simultaneous observation of the overall lattice structure progressively expanding. Linear combination calculations of the M-O bond lengths are used to help explain these observations, where the role of oxygen defects, via Nd3+ incorporation, induces local bond contraction and enhanced Th cation valence, leading to the observed increased lattice expansion with progressive Nd3+ incorporation. Overall, the investigation points to the significance of dissimilar cations exhibiting variable short-range chemical behavior and how it can affect the long-range structural chemistry of complex oxides.

Investigation of cerium as a surrogate for tetravalent actinides in monazite-type compounds

The incorporation of tetravalent cerium into the monazite structure via Ca(II)-coupled substitution was investigated using a solid state and a co-precipitation route for the synthesis of the La1-2xCaxCexPO4 solid solution. Based on powder XRD measurements and elemental mappings an optimised synthesis procedure was developed that averts the formation of secondary phases and allows the stabilisation of tetravalent cerium with a ratio of up to 0.21 Ce(IV)/Ce(III). In-situ HERFD-XANES measurements at the Ce L3 edge at up to 800°C were performed to study the cerium oxidation state during the phase transformation from rhabdophane to monazite, revealing an unexpected non-linear behaviour as well as a charge-directing effect of the lanthanum cation.

Investigating cerium redox changes between aluminosilicate glass and melt: A multispectroscopic approach.

Redox control of glasses is paramount both to their fusion process and to obtaining the desired properties of high technological glasses. However, the link between melting parameters, such as temperature, furnace atmosphere, or quenching rate, and the redox state of the final products is poorly understood. In this work, in situ x-ray absorption near-edge structure (XANES) data at Ce L3-edge data were acquired at high temperatures on cerium-containing sodium aluminosilicate glasses, allowing the determination of thermodynamic constants necessary to predict the cerium redox state over a wide temperature range (900-1500 °C). The results obtained were compared to the Raman spectra of samples quenched at different temperatures. Our findings demonstrate that the quench performed was fast enough to block the cerium oxidation state, meaning the redox measured at room temperature is representative of a high temperature state. This was further verified by room temperature Raman spectroscopy, where a relationship was found between the spectra and melting conditions. Wet chemical analysis, XANES at Ce L3-edge, Raman spectroscopy, and optical absorption spectroscopy were successfully used to determine the redox state of cerium in aluminosilicates.

Operando Tracking the Interactions between CoOx and CeO2 during Oxygen Evolution Reaction

AbstractCeO2 greatly enhances the electrocatalytic oxygen evolution reaction (OER) activity of CoOx, though the enhancement mechanism beyond this synergy is yet to be understood. Here, operando hard X‐ray absorption spectroscopy (hXAS) is applied to monitor the Co K edge and Ce L3 edge in CoOx/CeO2 to shed light on the evolution of the Co and Ce oxidation states during OER. In addition, ex situ soft XAS (sXAS) characterizations provide information on the irreversible surface‐specific transformations of the Co L3 edge as well as of the O K edge. Combining the operando and ex situ spectroscopic characterizations with comprehensive electrochemical analyses, it is confirmed that CeO2 is not the active center for the OER. However, coupling CeO2 with CoOx introduces significant modifications in the Co and O species at the CoOx surface and alters the flat band potential (Efb), leading to more favorable Co oxidation state transformations during OER and possibly modifying the preferential reaction pathway. This work establishes the connections between electronic structures, Co oxidation state and the OER reaction mechanism for CoOx/CeO2 composites electrodes.

Mechanism of NH3-SCR over P/CeO2 Catalysts Investigated by Operando Spectroscopies.

This study reports a comprehensive investigation into the active sites and reaction mechanism for the selective catalytic reduction of NO by NH3 (NH3-SCR) over phosphate-loaded ceria (P/CeO2). Catalyst characterization and density functional theory calculations reveal that H3PO4 and H2P2O6 species are the dominant phosphate species on the P/CeO2 catalysts under the experimental conditions. The reduction/oxidation half-cycles (RHC/OHC) were investigated using in situ X-ray absorption near-edge structure for Ce L3-edge, ultraviolet-visible, and infrared (IR) spectroscopies together with online analysis of outlet products (operando spectroscopy). The Ce4+(OH-) species, possibly adjacent to the phosphate species, are reduced by NO + NH3 to produce N2, H2O, and Ce3+ species (RHC). The Ce3+ species is reoxidized by aqueous O2 (OHC). The results from IR spectroscopy suggest that the RHC initiates with the reaction between NO and Ce4+(OH-) to yield Ce3+ and gaseous HONO, which then react with NH3 to produce N2 and H2O via NH4NO2 intermediates.

Using Ex Situ and In Situ HERFD-XANES to Reveal the Superior Oxidation and Reduction Cycling of Ceria Nanocubes Dispersed in Silica Aerogel.

The oxygen storage capacity of ceria-based catalytic materials is influenced by their size, morphology, and surface structure, which can be tuned using surfactant-mediated synthesis. In particular, the cuboidal morphology exposes the most reactive surfaces; however, when the capping agent is removed, the nanocubes can agglomerate and limit the available reactive surface. Here, we study ceria nanocubes, lanthanum-doped ceria nanocubes, and ceria nanocubes embedded inside a highly porous silica aerogel by high-energy resolution fluorescence detection-X-ray absorption near edge spectroscopy at the Ce L3 edge. In situ measurements showed an increased reversibility of redox cycles in ceria nanocubes when embedded in the aerogel, demonstrating enhanced reactivity due to the retention of reactive surfaces. These aerogel nanocomposites show greater improvement in the redox capacity and increased thermal stability of this catalytic material compared to the surfactant-capped nanocubes. Ex situ measurements were also performed to study the effect of lanthanum doping on the cerium oxidation state in the nanocubes, indicating a higher proportion of Ce4+ compared to that of the undoped ceria nanocubes.

Synthesis, Crystal Structure, Local Structure, and Magnetic Properties of Polycrystalline and Single-Crystalline Ce2Pt6Al15

Asymmetry, such as non-centrosymmetry in the crystal or chiral structure and local symmetry breaking, plays an important role in the discovery of new phenomena. The honeycomb structure is an example of an asymmetric structure. Ce2Pt6Al15 is a candidate for a frustrated system with honeycomb Ce-layers, which have been reported to show near the quantum critical point. However, the ground state of Ce2Pt6Al15 depends on the sample, and analysis of the crystal structure is difficult due to the presence of stacking disorder. We synthesized polycrystalline Ce2Pt6Al15 using arc melting method (AM-Ce2Pt6Al15) and single-crystalline Ce2Pt6Al15 using flux method (F-Ce2Pt6Al15). The prepared samples were characterized by electron probe micro-analysis (EPMA), single and powder X-ray diffraction methods, measured magnetic properties and X-ray absorption spectroscopy (XAS). The composition ratio of AM-Ce2Pt6Al15 was stoichiometric, although it contained a small amount (i.e., a few percent) of the impurity Ce2Pt9Al16. Meanwhile, the composition ratio of F-Ce2Pt6Al15 deviated from stoichiometry. The X-ray absorption fine structure (XAFS) spectrum of AM-Ce2Pt6Al15 at the Ce L3-edge was similar to that of CeF3, which possesses the Ce3+ configuration, indicating that the valence of Ce in Ce2Pt6Al15 is trivalent; this result is consistent with that for the magnetic susceptibility. To determine the precise structure, we analyzed the extended X-ray absorption fine structure (EXAFS) spectra of Ce L3- and Pt L3-edges for Ce2Pt6Al15, and found that the EXAFS spectra of Ce2Pt6Al15 can be explained not as a hexagonal Sc0.6Fe2Si4.9-type structure but, instead, as an orthorhombic structure with honeycomb structure.

On the effect of metal loading on the reducibility and redox chemistry of ceria supported Pd catalysts.

The effect of Pd loading on the redox characteristics of a ceria support was examined using in situ Pd K-edge XAS, Ce L3-edge XAS and in situ X-ray diffraction techniques. Analysis of the data obtained from these techniques indicates that the onset temperature for the partial reduction of Ce(IV) to Ce(III), by exposure to H2, varies inversely with the loading of Pd. Whilst the onset and completion temperatures of the reduction of Ce(IV) to Ce(III) are different, both samples yield the same maximal fraction of Ce(III) formation independent of Pd loading. Furthermore, the partial reduction of Ce is found to be concurrent with the reduction of PdO and demonstrated that the presence of metallic Pd is necessary for the reduction of the CeO2 support. Upon passivation by room temperature oxidation, a full oxidation of the reduced ceria support was observed. However, only a mild surface oxidation of Pd was identified. The mild passivation of the Pd is found to lead to a highly reactive sample upon a second reduction by H2. The onset of the reduction of Pd and Ce has been demonstrated to be independent of the Pd loading after a mild passivation with both samples exhibiting near room temperature reduction in the presence of H2.

Probing Multiconfigurational States by Spectroscopy: The Cerium XAS L3 -edge Puzzle.

The Ce L3 edge XAS spectra of CeO2 and cerocene [Ce(C8 H8 )2 ] were calculated with relativistic ab-initio multireference wavefunction approaches capable of reproducing the observed spectra accurately. The study aims to resolve the decades-long puzzle regarding the relationship between the number and relative intensities of the XAS peaks and the 4f electron occupation in the ground state (GS) versus the core-excited states (ESs). CeO2 and cerocene exemplify the different roles of covalent bonding and wavefunction configurational composition in the observed intensity patterns. Good agreement is found between the calculated GS 4f-shell occupations and the value derived from XAS measurements using peak areas (nf ). The identity of the two-peaked Ce L3 edge is fully rationalized from the perspective of the relaxed wavefunctions for the GS and core ESs. The states underlying the different peaks differ from each other in a surprisingly simple way that can be associated with 4f1 vs. 4f0 sub-configurations. Furthermore, part of one of the cerocene spectral peaks is associated with 4f2 sub-configurations. The pattern therefore reveals excited states that can be interpreted in terms of Ce IV and III oxidation numbers, as long assumed, with Ce II states additionally appearing in the cerocene spectrum. While this work demonstrates the rough accuracy of the conventional approach to determining nf from Ce L3 -edge XAS, limitations are highlighted in terms of the ultimate accuracy of this approach and the potential of observing new types of excited states. The need to determine the sources of nf by calculations, is stressed.

Thermally stable surfactant-free ceria nanocubes in silica aerogel

Surfactant-mediated chemical routes allow one to synthesize highly engineered shape- and size-controlled nanocrystals. However, the occurrence of capping agents on the surface of the nanocrystals is undesirable for selected applications. Here, a novel approach to the production of shape-controlled nanocrystals which exhibit high thermal stability is demonstrated. Ceria nanocubes obtained by surfactant-mediated synthesis are embedded inside a highly porous silica aerogel and thermally treated to remove the capping agent. Powder X-ray Diffraction and Scanning Transmission Electron Microscopy show the homogeneous dispersion of the nanocubes within the aerogel matrix. Remarkably, both the size and the shape of the ceria nanocubes are retained not only throughout the aerogel syntheses but also upon thermal treatments up to 900 °C, while avoiding their agglomeration. The reactivity of ceria is measured by in situ High-Energy Resolution Fluorescence Detected - X-ray Absorption Near Edge Spectroscopy at the Ce L3 edge, and shows the reversibility of redox cycles of ceria nanocubes when they are embedded in the aerogel. This demonstrates that the enhanced reactivity due to their prominent {100} crystal facets is preserved. In contrast, unsupported ceria nanocubes begin to agglomerate as soon as the capping agent decomposes, leading to a degradation of their reactivity already at 275 °C.

Structure of Reduced Cerium Oxide Ultrathin Films on Pt(111): Local Atomic Environment and Long‐Range Order

AbstractTo optimize the catalytic functionality of cerium oxide it is important to understand the structural modifications associated with reduction and the role of the proximity of metals, which are often coupled with the oxide in the applications. For this purpose, the evolution of the short‐ and long‐range structure of cerium oxide ultrathin epitaxial films and nanostructures supported on Pt(111) is investigated using X‐ray absorption spectroscopy at the Ce L3 edge and surface X‐ray diffraction, during reduction by thermal treatments in vacuum. In epitaxial nanoislands reduction is associated with a contraction of the Ce–O distance and with the appearance of CePt bonds. The formation of a phase with a (2 × 2) periodicity after a thermal treatment at 1023 K is ascribed to the formation of a Pt5Ce alloy. Films of 3 nm thickness do not show, on average, significant structural modifications with the same thermal treatment, consistent with the hypothesis that the reduction involves only the topmost surface layers and it does not influence significantly the bulk structure of the material. This study demonstrates a strong interaction between cerium oxide and platinum, which has implications for the reactivity and stability of catalysts based on metals combined with reducible oxides.

Local structure displacements in La[formula omitted]Ce[formula omitted]OBiSSe as a function of Ce substitution

We have investigated the local structure of layered La1−xCexOBiSSe system by Bi L3-edge extended X-ray absorption fine structure (EXAFS) measurements for different Ce substitutions. Ce L3-edge X-ray absorption spectroscopy (XAS) has been used to evaluate the Ce valence responsible for the self-doping in this system. We have found that the local distortion, determined by the separation between two Bi-Ch distances within the BiCh2-layer (Ch=S,Se), is quickly suppressed by Ce substitution while the axial Bi-S2 bond elongates. Ce L3-edge XAS reveals a coexistence of Ce3+ and Ce4+ in which the Ce4+ weight decreases, an indication of a partial breaking of RE-S-Bi (RE=La/Ce) charge transfer channel with Ce substitution. The results suggest that interaction between REO spacer layer and BiCh2 layer, dictated by the out-of-plane Bi-S2 distance, has a significant role in triggering superconductivity in the title system, with the in-plane distortion controlling the charge mobility within the BiCh2-layer.

The effect of Sm addition on structure, redox properties and catalytic activities for water gas shift reaction of ceria-based support

Oxygen vacancy in ceria-based support is a dominate factor for catalyzing the water gas shift reaction by faster a redox cycle between Ce4+ and Ce3+ which is one of the elementary step in WGS reaction mechanism. Therefore, in this work, CeO2-based support was incorporation by Sm ion in order to produce the oxygen vacancy defect site. Ce1-xSmxO2 (x = 0.010, 0.050, and 0.10 at%) mixed oxide supports were synthesized by urea gelation to investigate the role of Sm addition into ceria-based support on surface, structural and redox properties which was altered to their catalytic activities. XRD and Raman spectroscopy results indicated the incorporation of Sm3+ into CeO2 lattice which gave rise to unit cell enlargement and led to structural distortions inside CeO2 which produced strain and unbalanced charge and then finally facilitated oxygen vacancy formation. XANES and EXAFS results also evidenced the replacement of Sm3+ into Ce4+ site in CeO2 lattice. H2-Temperature programmed reduction and temperature-resolved X-ray absorption spectroscopy (under reduction condition) were employed to investigate the reducibility of Ce1-xSmxO2 supports. XANES spectra of Ce L3 edges displayed lower surface reduction temperature (Ce4+ to Ce3+) which corresponded to H2-TPR results. The results of catalytic activity tests for water gas shift reaction showed that the activity of 1 wt%Pd/Ce1-xSmxO2 was higher than that of 1 wt%Pd/CeO2. The role of Sm on enhancing WGS reaction rate were to increase CO molecules adsorption by promoting the dispersion of active metal site on catalyst surface and to facilitate the redox cycle between Ce4+ and Ce3+ of ceria support by oxygen vacancies formation. Therefore, combined these two effects the WGS activities were enhanced. Moreover, by using the results from our previous works together with the results from this work, we can evidence that speeding up the different elementary steps in the reaction mechanism, the WGS rate can also be enhanced.

Investigation of Factors That Affect the Oxidation State of Ce in the Garnet-Type Structure.

Oxide materials that adopt the garnet-type structure (X3A2B3O12) have received attention for a wide variety of applications, one of which is as potential wasteforms for the sequestration of radioactive actinide elements. The actinides are able to be accommodated in the eight-coordinate X site of the garnet structure. This study focuses on the investigation of Ce substitution into the X site as a surrogate for Pu because of their similar chemical properties. This is accomplished through analysis of the Y3- zCe zAlFe4O12 (0.05 ⩽ z ⩽ 0.20) materials. The effects of the Ce concentration, oxidation state of the Ce in the starting materials, annealing environment, and cooling rate on the local structure and Ce and Fe oxidation states were investigated through analysis of the powder X-ray diffraction patterns and Ce L3-edge and Fe K-edge X-ray absorption spectroscopy (XANES) spectra. Analysis of Ce L3-edge XANES spectra indicated that Ce was present as both 3+ and 4+ oxidation states, the ratios of which depended on the synthetic conditions. The largest concentration of Ce4+ was observed when the materials were postannealed at 800 °C following synthesis of the materials at 1400 °C. Variations in the Ce oxidation state are the result of the temperature-dependent Ce3+/Ce4+ redox couple, with Ce4+ being favored at lower temperatures. Analysis of the Fe K-edge spectra indicated that Fe was only present in the 3+ oxidation state and the Fe coordination number increased with increasing concentration of Ce4+, which is necessary to charge balance the system. The materials in this study can be described as an oxygen-intercalated garnet-type structure with the formula Y3- zCe zAlFe4O12+δ.

Band gap engineering, electronic state and local atomic structure of Ni doped CeO2 nanoparticles

In cerium dioxide (CeO2) as semiconductor compound, the tuning of band gap energy is a pivotal feature for visible light applications. In this report, nanoparticles (NPs) of Ce1 − xNixO2 (0.0 ≤ x ≤ 0.10) were synthesized through co-precipitation route. The structural, electronic state and optical band gap were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, Raman scattering and UV–Vis spectroscopy. XRD results revealed the single-phase nanocrystalline behavior of CeO2 with cubic fluorite structure. The electronic configuration of the samples, probed via NEXAFS spectra at Ce M5, 4 edges, demonstrated that the cerium exists in Ce4+ and Ce3+ mixed valence states. The formation of Ce3+ ions in the vicinity of oxygen vacancies (Ov) were assessed also via Raman scattering and XANES spectra at Ce L3 edge. The estimated concentrations of Ce3+ were increased from 6 to 13% as the Ni content increase from 0 to 10% respectively. Here, the inclusion of Ni2+ ions into CeO2 network induced additional Ov and simultaneously reduced the optical band gap from 3.9 eV for pure CeO2 to 2.6 eV for 10% Ni doped CeO2 NPs. Therefore, the oxygen loss population seem to be responsible for the band gap reduction. The role of Ov for creating profound donor band near the conduction band and narrowing the band gap energy were discussed.

Towards the surface hydroxyl species in CeO2 nanoparticles.

Understanding the complex chemistry of functional nanomaterials is of fundamental importance. Controlled synthesis and characterization at the atomic level is essential to gain deeper insight into the unique chemical reactivity exhibited by many nanomaterials. Cerium oxide nanoparticles have many industrial and commercial applications, resulting from very strong catalytic, pro- and anti-oxidant activity. However, the identity of the active species and the chemical mechanisms imparted by nanoceria remain elusive, impeding the further development of new applications. Here, we explore the behavior of cerium oxide nanoparticles of different sizes at different temperatures and trace the electronic structure changes by state-of-the-art soft and hard X-ray experiments combined with computational methods. We confirm the absence of the Ce(iii) oxidation state at the surface of CeO2 nanoparticles, even for particles as small as 2 nm. Synchrotron X-ray absorption experiments at Ce L3 and M5 edges, combined with X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS) and theoretical calculations demonstrate that in addition to the nanoceria charge stability, the formation of hydroxyl groups at the surface profoundly affects the chemical performance of these nanomaterials.

Oxidation State and Dielectric Properties of Ceria-Based Catalysts by Complementary Microwave Cavity Perturbation and X-Ray Absorption Spectroscopy Measurements

The three-way catalytic converter (TWC) has become an integral part of the modern exhaust gas aftertreatment for gasoline engines. Some years ago, radio frequency technology has been successfully tested for monitoring and controlling ceria based TWC oxygen storage by contactless measuring the dielectric catalyst properties inside a cavity resonator. Applying the cavity perturbation method, we present results on CeO2, the oxygen storage component of three-way catalysts, and on differently prepared Pt–ceria model catalysts and combine it with in situ X-ray absorption spectroscopic (XAS) measurements conducted at the Ce L3-edge. The dielectric properties of the oxidized and reduced Pt–ceria and pure ceria powders were determined at 1.2 GHz between 250 and 550 °C. As the experiments show, the reduction of the material changes both, the polarization and the occurring dielectric losses. The complementary in situ XAS measurements, which allow monitoring variations in the redox state of ceria, gave a precise evaluation of the ceria reduction and oxidation. In addition, a substantial improvement of the low-temperature reducibility of ceria in the presence of Pt was demonstrated by both measurement techniques, in agreement with earlier studies. Furthermore, time resolved XAS data obtained during reducing/oxidizing cycles at 250 °C and 350 °C unraveled the faster and more pronounced reduction of ceria at higher temperature. Both methods proved to be valuable methods to provide insight into the oxygen storage and release process of ceria based materials, and capable to discriminate the impact of the noble metal or the ceria surface area in great detail, even under dynamic conditions, which is highly relevant for studying the function of state-of-the art three-way catalysis and giving input for OSC modeling studies.

Surface exsolution and local atomic structure of amorphous CeTiOx

We made precipitated nano-ceria (∼5 nm) on the surface of the catalyst by heat treatment of Ce-supersaturated amorphous CeTiOx to improve the oxygen storage properties of CeO2. The catalysts were prepared by sol-gel methods and TiO2 nanoparticles were preferentially generated as a core material to form selective Ce-supersaturated structure on the catalyst surface. Reaction temperature and amount of doping element are optimized to induce selective crystallization of CeO2. CeCe (2nd shell) bond around 0.38 nm of Ce L3-edge extended X-ray absorption fine structure is reduced and nanostructure of precipitated ceria on the surface is observed by HREM. The catalyst is present as amorphous with precipitated nano-CeO2 on the surface. The de-NOx efficiency of the catalyst, which has precipitated CeO2, improves by ∼50% owing to the simultaneous reactions of the nano CeO2 and the amorphous CeTiOx.