Oxidation State Of Ce Research Articles

Reactivity of a Chloride Decorated, Mixed Valent CeIII/IV38-Oxo Cluster.

A cerium-oxo nanocluster capped by chloride ligands, [CeIV38-nCeIIInO56-(n+1)(OH)n+1Cl51(H2O)11]10- (n = 1-24), has been isolated from acidic chloride solutions by using potassium counterions. The crystal structure was elucidated using single crystal X-ray diffraction. At the center of the cluster is a {Ce14} core that exhibits the same fluorite-type structure as bulk CeO2, with eight-coordinate Ce sites bridged by tetrahedral oxo anions. The {Ce14} is further surrounded by a peripheral shell of six tetranuclear {Ce4} subunits that are located on each of the faces of the core to yield the {Ce38} cluster. The surface of the cluster is capped by 51 bridging/terminal chloride ligands and 11 water molecules; the anionic cluster is charge balanced by potassium counterions that exist in the outer coordination sphere. While assignment of the Ce oxidation state by bond valence summation was ambiguous, Ce L3-edge X-ray absorption, X-ray photoelectron, and UV-vis-NIR absorption results were consistent with a CeIII/CeIV cluster. Systematic changes in the XANES and UV-vis-NIR absorption spectra over time pointed to reactivity of the cluster upon exposure to air. These changes were examined using single crystal X-ray diffraction, and a clear single-crystal-to-single-crystal transformation was captured; an overall loss of surface-bound chlorides and water molecules as well as new μ2-OH sites was observed on the cluster surface. This work provides a rare snapshot of metal oxide cluster reactivity. The results may hold implications for understanding the physical and chemical properties of ceria nanoparticles and provide insight into the behavior of other metal-oxo clusters of significant technological and environmental interest.

Luminescence and physical properties of Ce3+-doped potassium gadolinium phosphate glasses for radiation detector application

This work aimed to study the effect of Gd2O3 and CeF3 modified on potassium gadolinium phosphate glasses scintillators in 17K2O: xGd2O3: (82.5-x)P2O5: 0.5CeF3 (where x = 9, 11, 13, 15 and 17 mol%) and 17K2O: 17Gd2O3: (66-x)P2O5: xCeF3 (where x = 0.1, 0.3, 1, 1.5, 2, 3 and 5 mol%) systems produced by the melt quenching technique. The physical, optical, and scintillation properties of the specimens were measured and discussed. The density and molar volume of the glasses trend to increases with increasing Gd2O3 concentration, while an invariable for all CeF3 concentration. The photoluminescence and x-ray luminescence spectrums show the highest value at 9 and 17 mol% of Gd2O3 concentration, respectively, which shows the decay time between 15.02 and 22.84 ns. XANES results shows the main oxidation state of Ce is 3+ ion for all glasses. However, the photoluminescence and x-ray luminescence spectrums show the maximum value at 5 mol% of CeF3 concentration, which shows the fast decay time between 16.26 and 36.02 ns. These results of the x-ray induced luminescence and the fast decay time in nanoseconds level are possible applied as well as the scintillation materials for radiation detector application.

Heterometallic Clusters with Multiple Rare Earth Metal-Transition Metal Bonding.

Although a series of complexes with rare earth (RE) metal-metal bonds have been reported, complexes which have multiple RE-Rh bonds are unknown. Here we present the identification of the first example of a molecule containing multiple RE-Rh bonds. The complex with multiple Ce-Rh bonds was synthesized by the reduction of a d-f heterometallic molecular cluster Ce{N[(CH2CH2NPiPr2)RhCl(COD)]3} with excess potassium-graphite. The oxidation state of Ce in 3a appears to be a mixture of Ce(III) and Ce(IV), which was confirmed by X-ray photoelectron spectroscopy, magnetism, and theoretical investigations (DFT and CASSCF). For comparison, the analogous species with multiple La(III)-Rh and Nd(III)-Rh bonds were also constructed. This study provides a possible route for the construction of complexes with multiple RE metal-metal bonds and an investigation of their potential properties and applications.

Synthesis and characterisation of HIP Ca0.80Ce0.20ZrTi1.60Cr0.40O7 zirconolite and observations of the ceramic\u2013canister interface

A sample of zirconolite with nominal composition Ca0.80Ce0.20ZrTi1.60Cr0.40O7 was processed via Hot Isostatic Pressing (HIP), with a dwell temperature and pressure of 1320 °C/100 MPa maintained for 4 h. The produced wasteform was characterised by powder XRD, SEM–EDS, Ce L3 and Cr K-edge XANES. A significant portion of the Ce inventory did not fully partition within the zirconolite phase, instead remaining as CeO2 within the microstructure. Inspection of the stainless steel–ceramic interface detailed the presence of an interaction region dominated by a Cr-rich oxide layer. No significant Cr or Fe migration was observed, although a greater concentration of perovskite was observed at the periphery, relative to the bulk ceramic matrix. The X-ray absorption features of Cr remained analogous with Cr3+ accommodation within TiO6 octahedra in the zirconolite matrix. The absorption edge of Ce was comprised of contributions from zirconolite-2M and unincorporated CeO2, with an average oxidation state of Ce3.9+. As zirconolite-2M accounted for > 92 wt% of the overall phase assemblage, it is clear that the dominant oxidation state of Ce in this phase was Ce4+.Graphic abstract

Titania nanotube array supported nanoceria with redox cycling stability ameliorates oxidative stress-inhibited osteogenesis

The core catalytic property of cerium oxide nanoparticles (CeNPs) has been proposed to be due to the redox cycling capability of Ce4+/Ce3+ oxidation state. Thus far there are few reports on controlling the cycling stability of Ce4+/Ce3+ couple in biological fluids which is the most important parameter in the application of CeNPs as a regenerative nano-antioxidant. In this study, vertically aligned titania nanotube array supported CeNPs (TiNTA-CeNPs), regardless of the pre-dominant Ce oxidation state, retained the cycling capability of Ce4+/Ce3+ in H2O2-containing phosphate-buffered saline (PBS) compared with Ti supported CeNPs. This was because preferable phosphate adsorption on surface Ti3+ in TiNTA-CeNPs preserved Ce3+ active sties. Accordingly, Ce3+-rich TiNTA-CeNP1 and Ce4+-rich TiNTA-CeNP2 in PBS exhibited more sustained superoxide dismutase and catalase mimetic response, respectively. A correlation between electronic band structures of Ce-Ti mixed oxides and redox potential of reactive oxygen species (ROS) aided in interpretation of enhanced redox cycling capability and enzyme-like activities. Less Fenton-active TiNTA-CeNP2 exhibited a greater ability to protect pre-osteoblasts against ROS-induced oxidative stress in vitro. To demonstrate in vivo osteoprotective effect of TiNTA-CeNP2, a rat model of oxidative stress-related osteoporosis was established. The results indicated that TiNTA-CeNP2 held great potential for ameliorating osteogenesis in oxidative stress-related bone diseases.

RETRACTED ARTICLE: Structural, dielectric and electrical properties of cerium-modified strontium manganite ceramics

In this communication, structural, dielectric, spectroscopic and electrical characteristics of cerium-modified strontium manganite perovskite of a composition SrMn0.9Ce0.1O3, (SMCO) prepared by a high-temperature solid-state reaction technique have been reported. The preliminary structural analysis of SrMnO3 exhibits hexagonal (P63/mmc) crystal structure, whereas SMCO, synthesized under identical conditions, shows a tetragonal (I4/mmm) structure. The average crystallite size and lattice strain of SMCO using X-ray data were found to be 74 nm and 0.107%, respectively. The surface morphology, study by the scanning electron microscopy (SEM), shows distinct grains of average size of 19.2 μm. The X-ray photoelectron spectroscopy (XPS) study confirms the oxidation state of Mn and Ce as Mn4+ and Ce4+ and composition of SMCO compound. The grains and the grain boundaries play an important role to explain the conduction mechanism. The bulk resistance (Rb) decreases from 1.020 × 105 Ω at 25 °C to 1.096 × 103 Ω at 500 °C. This behaviour of decrease in resistance with the increase in temperature shows semiconductor (negative temperature coefficient resistance) nature of the material at high temperatures. The variation of the activation energies with temperature suggests that the ac conductivity is thermally activated. The immobile charge carriers at low temperatures and defects and oxygen vacancies at high temperatures are responsible for the thermally activated conduction mechanism. Detailed studies of electrical parameters as a function of frequency at different temperatures using dielectric and impedance spectroscopy of SMCO have provided conduction mechanism and structural properties relationship.

Intrinsic catalase-mimicking MOFzyme for sensitive detection of hydrogen peroxide and ferric ions

Lanthanide metal-organic frameworks (Ln-MOFs) exhibit outstanding properties, including high chemical and thermal stability, as well as their promising enzyme-mimic activity (MOFzyme). A new cerium MOFs (Ce-MOF) was synthesized via a solvothermal method using organic linker 4,4′,4″-nitrilotribenzoic acid (H3NTB). The crystallinity of the prepared material was confirmed using X-ray diffraction (XRD). The coordination between Ce and H3NTB was approved using Fourier transforms infrared (FT-IR). The chemical and oxidation state of Ce-MOF was characterized using X-ray photoelectron spectroscopy (XPS). The analysis of XPS data suggested the presence of two oxidation states of cerium, i.e., Ce(III) and Ce(IV). According to the imaging using a scanning electron microscope (SEM) and transmission electron microscope (TEM), Ce-MOF exhibits hexagonal rods’ morphology. Ce-MOF offered thermal stability up to 350 °C. It showed a fluorescence emission signal at wavelength 550 nm with a Stock’s shift of 200 nm. Ce-MOF’s application as a probe for sensing hydrogen peroxide (H2O2) and ferric ions (Fe3+) was reported. Ce-MOF exhibited peroxidase MOFzyme activity. Thus, it offered a turn-on emission upon interaction with different concentrations of H2O2. It detects Fe3+ ions with a limit of detection (LOD) and a linear range of 0.016 µM and 0.016–0.133 µM, respectively. The study also investigated the mechanisms of sensing for H2O2 and Fe3+ ions. The changes in the emission signals were due to the intrinsic catalase activity of Ce-MOF. Ce-MOF offered MOFzyme properties providing a promising feature for biocatalysis and biosensing applications.

Structural, Morphological, and Electrochemical Performance of CeO2/NiO Nanocomposite for Supercapacitor Applications

The composite of ceria has been widely studied as an electrode material for supercapacitors applications due to its high energy density. Herein, we synthesize CeO2/NiO nanocomposite via a hydrothermal route and explore its different aspects using various characterization techniques. The crystal structure is investigated using X-ray diffraction, Fourier transform infrared, and Raman spectroscopy. The formation of nanoflakes which combine to form flower-like morphology is observed from scanning electron microscope images. Selected area scans confirm the presence of all elements in accordance with their stoichiometric amount and thus authenticate the elemental purity. Polycrystalline nature with crystallite size 8–10 nm having truncated octahedron shape is confirmed from tunneling electron microscope images. Using X-ray photoelectron spectroscopy the different oxidation states of Ce and Ni are observed which play the role of active sites in the electrochemical performance of this nanocomposite material. Cyclic Voltammetry(CV) measurements at different scan rates and Galvanic Charge Discharge (GCD) measurements at different current densities are performed to probe the electrochemical response which revealed the potential of CeO2/NiO nanocomposite as an electrode material for energy storage devices.

Cerium(iv) complexes with guanidinate ligands: intense colors and anomalous electronic structures.

A series of cerium(iv) mixed-ligand guanidinate–amide complexes, {[(Me3Si)2NC(NiPr)2]xCeIV[N(SiMe3)2]3−x}+ (x = 0–3), was prepared by chemical oxidation of the corresponding cerium(iii) complexes, where x = 1 and 2 represent novel complexes. The Ce(iv) complexes exhibited a range of intense colors, including red, black, cyan, and green. Notably, increasing the number of the guanidinate ligands from zero to three resulted in significant redshift of the absorption bands from 503 nm (2.48 eV) to 785 nm (1.58 eV) in THF. X-ray absorption near edge structure (XANES) spectra indicated increasing f occupancy (nf) with more guanidinate ligands, and revealed the multiconfigurational ground states for all Ce(iv) complexes. Cyclic voltammetry experiments demonstrated less stabilization of the Ce(iv) oxidation state with more guanidinate ligands. Moreover, the Ce(iv) tris(guanidinate) complex exhibited temperature independent paramagnetism (TIP) arising from the small energy gap between the ground- and excited states with considerable magnetic moments. Computational analysis suggested that the origin of the low energy absorption bands was a charge transfer between guanidinate π orbitals that were close in energy to the unoccupied Ce 4f orbitals. However, the incorporation of sterically hindered guanidinate ligands inhibited optimal overlaps between Ce 5d and ligand N 2p orbitals. As a result, there was an overall decrease of ligand-to-metal donation and a less stabilized Ce(iv) oxidation state, while at the same time, more of the donated electron density ended up in the 4f shell. The results indicate that incorporating guanidinate ligands into Ce(iv) complexes gives rise to intense charge transfer bands and noteworthy electronic structures, providing insights into the stabilization of tetravalent lanthanide oxidation states.

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]- layers.

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite “charge-neutral perovskite layers” by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [CeIVTa2O7] was synthesized through a soft chemical oxidative reaction based on anionic [CeIIITa2O7]− layers. The Ce oxidation state for the charge-neutral [CeIVTa2O7] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [CeIIITa2O7]− layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [CeIVTa2O7] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [CeIVTa2O7] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [CeIVTa2O7] layers to form anionic [CeIIITa2O7]− layers. Furthermore, the anionic [CeIIITa2O7]− layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the “charge-neutral” perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.

A Unified Approach to Dot-, Rod-, and Sheet-MOFs and Electrochemistry of Exemplars Containing Y, Ce, Gd, and Benzene-Tetracarboxylate

Metal-Organic Frameworks made from discrete multi-metal-ion units in the shape of clusters and rods (termed dot-MOFs and rod-MOFs) are well-known. Here, we introduce MOFs with multi-metallic units in the form of sheets, sheet-MOFs. We show exemplars of all three type of units based on structures containing Y3+, Ce3+, or Gd3+ linked by benzene-1,2,4,5-tetracarboxylate to give crystals of a dot-MOF in H2NMe2[Y(btec)(H2O)] CTH-14, a sheet-MOF in [Ce3 (btec)(Hbtec)(NO3)2]·½dmf CTH-15, and a rod-MOF in [Gd2(Hbtec)2](4,4’-azopyridine) CTH-16. Cyclic voltammetry shows that sheet-MOF CTH-15 stabilize the Ce(IV) oxidation state and suggests catalytic ability for electrochemical nitrate reduction. Given the fact that sheet-MOFs represent an intellectual advance in the evolution of the MOF field, a unified approach is proposed for the topological classification of dot-, rod- and sheet-MOFs. It is suggested that the stability of MOFs follow in the trend dot

The Effect of Temperature on the Stability and Cerium Oxidation State of CeTi2O6 in Inert and Oxidizing Atmospheres.

Aliquots of well-characterized Ce-brannerite were annealed at different temperatures under N2 and synthetic air atmospheres. The autoreduction of cerium at temperature was observed using thermogravimetry to monitor the mass lost as O2 was evolved. It has been shown that the brannerite structure is stable with a small fraction of Ce3+, charge-balanced by O vacancies. The range of stability was determined to be Ce4+0.975Ti2O5.95, the fully oxidized end-member, to Ce3.87+0.975Ti2O5.886, as reduced by annealing under N2 at 1075 °C. Higher temperatures under N2 led to further reduction of Ce and collapse of the brannerite structure. Ce-brannerite remained stable on heating to 1300 °C in synthetic air, with multiple steps of oxidation and reduction corresponding to changes in the average Ce oxidation state. We propose that the autoreduction of Ce at temperature is an important factor in the overall thermodynamic stability of Ce-brannerite at temperature and has a large impact on the energetics of formation of Ce-brannerite.

Redox Active Hexanuclear Mixed Valence Dicationic Ce(III)/Ce(IV) Coordination Clusters

A series of mixed valence hexanuclear dicationic coordination clusters containing two µ4‐O2– bridges with general formula M2+(X–)2 [M = CeIV2CeIII4(µ4‐O)2(L–R)4(val)6(H2O)2; 12+, 32+: R = H, 22+: Me; 12+, 22+: X = CeIII2(val)3(NO3)4, 32+: X = NO3] were synthesized, isolated and characterized. The reaction of cerium(III)nitrate hexahydrate with Schiff base ligand (H2L–R) and o‐vanillin (val‐H) under basic condition led to the formation of M2+(X–)2 and the reaction progress was monitored by mass spectrometric studies. The molecular structures of all complexes were unambiguously characterized by single‐crystal X‐ray diffraction. The magnetic susceptibility measurements of M2+(X–)2 showed that the Ce(III) ions are weakly antiferromagnetically coupled. Thermal stability and molar conductivity of M2+(X–)2 were also studied. Complex 32+(X–)2 was further studied by X‐ray photoelectron spectroscopy to confirm the oxidation states of Ce(III/IV) ions. The M2+ cation was shown to catalyze the TEMPO‐free oxidation of functionalized benzyl alcohols quantitatively to the corresponding benzaldehyde derivatives at 100 °C in presence of aerial O2 using DMF as the solvent.

Effect of glass composition on the crystallization of CePO4–borosilicate glass composite materials

Glass–ceramic composite materials are composed of ceramic crystallites within a glass matrix. These materials have been suggested as nuclear waste forms; however, the formation of targeted crystal phases remains a challenge to utilizing these materials as waste forms. The crystallization of composite materials consisting of CePO4dispersed in borosilicate glass at low temperatures was investigated in this study. The basicity of the borosilicate glass matrix was decreased to favour Ce3+and the formation of CePO4at low annealing temperatures using a one-step coprecipitation method. The basicity of the glass was lowered by removing the network modifier oxides from the glass composition. The crystalline phases, Ce oxidation states, and elemental distribution in CePO4–borosilicate glass composite materials were studied by powder X-ray diffraction, Ce L1- and L3-edge X-ray absorption near-edge spectroscopy, and scanning electron microscopy/energy dispersive X-ray spectroscopy. Lowering the glass basicity was successful in favouring the formation of CePO4and had the added benefit of reducing the crystallization of unwanted crystal phases.

New photoactive mesoporous Ce-modified TiO2 for simultaneous wastewater treatment and electric power generation

In the present paper is presented an efficient strategy for synthesis of mesoporous TiO2 modified with different Ce concentrations through a sol–gel process in the presence of triblock Pluronic P123 as structure directing agent, integrated with evaporation-induced self- assembly (EISA) approach. The nanocomposite consisted mostly of small crystallite of anatase. The presence of a new crystal phase corresponding to cerium titanate was evidenced in Ce-modified powder samples. All materials were characterized by SEM and TEM microscopies, UV–vis, XPS and N2 adsorption-desorption isotherms in order to examine the textural and structural characteristics and the chemical nature of their surface. Furthermore, the photoelectrochemical characterization evidenced the effect of the TiO2 mesoporous structure, the amount of cerium and the oxidation state of Ti and Ce in the new photocatalysts on the enhanced photovoltaic performance. The photocatalytic activity of the as-synthesized materials was evaluated for phenol photodegradation in aqueous media and the reactive species involved in photocatalytic process were established by using some radical scavengers in the photodegradation experiments. Based on the obtained results, these new Ce-modified mesoporous TiO2 photocatalysts could be considered for degradation of organic compounds from wastewaters by advanced oxidation processes or for photoanodes construction for efficient photoelectrochemical fuel cell (PFC) systems.

Mineralogical and geochemical characteristics of the iron–duricrust deposit in Adi-Daero area, northwestern Tigray, Ethiopia: implication for the origin and controlling factors

ABSTRACT Mineralogical and geochemical variations of elements from the Adi-Daero iron–duricrust were studied using petrographic, X–ray diffraction and geochemical analyses. Petrographic studies of the samples confirmed the presence of nodular and colloform textural features. The XRD analysis showed the presence of haematite, goethite, quartz, kaolinite and ilmenite are the mineral constituents. Major–element geochemistry was determined by ICP-AES, and trace and REEs were analysed using ICP-MS. Of the major oxides, Fe2O3 (6.8–72.93 wt-%) and SiO2 (11.07–77.75 wt-%) showed significant variation with generally increasing and decreasing trends from the iron-rich duricrust to the bedrock, respectively. In the profile studied, the alkali and alkaline earth elements (Ca, Mg, Na and K) were highly depleted, suggesting that iron–duricrust near–complete leaching. Elevated values of Sr and Ba in the lower part of the profile are probably due to their abundance in the protolith material (sandstone). A considerable enrichment of V and Cr, and Zr and REEs in the iron-rich duricrust and clay-rich horizon compared to the bedrock confirms their substitution for the Feoxyhydroxides, occurring in heavy mineral phases and adsorbed onto clay minerals, respectively. The La/Y <1 indicates an acidic environment in the upper part of the profile (AD-1 to AD-6; 0.64–0.99) and La/Y >1 a basic environment in the basal part of the profile (AD-7 to AD-12; 1.13–1.58). Similarly, the (La/Yb)N ratio of samples AD-1 to AD-6 (6.81–8.05) is significantly lower than that of AD-7 to AD–12 (10.09–24.54), reflecting an acidic environment. Positive Ce anomalies in the samples of AD-1 to AD-2 (1.17–1.2) are linked to the existence of cerianite due to change in oxidation state of Ce while in AD-8 and AD-9 (0.92–0.93), it is related to adsorption of Ce ions onto clay particles protolith. Therefore, the absolute enrichment of iron in the Adi-Daero iron–duricrust deposit was most likely an in situ process that involved vertical transfer from the iron-poor sandstone source rock.

The Role of Ceria in a Hybrid Catalyst toward Alkaline Water Oxidation.

Ceria-based catalysts for the oxygen evolution reaction (OER) have received keen interest in recent years owing to their potential excellent cost performance. However, despite a flurry of research activities, the mechanism on how ceria activates those hybrid catalysts is still puzzling. Herein, by controllably modifying the oxidation state of Ce in ceria, it was revealed that creating Ce3+ species, which are redox-coupled to Ce4+ under OER conditions, could enhance the conductivity and optimize the OH* binding, leading to greatly improved OER activity of the catalysts. More importantly, the ceria-based hybrid catalysts also exhibited excellent long-term stability even when operating at a high current density of 50 mA cm-2 in the strong alkaline electrolyte of 6 m KOH for more than 50 h. This work unveils the underlying role of ceria in improving the activity/stability of ceria-based catalysts and opens the way to design and fabricate ceria-based electrocatalysts for water splitting.

Quantification of anthropogenic and geogenic Ce in sewage sludge based on Ce oxidation state and rare earth element patterns

Emissions of Ce from anthropogenic activities (anthropogenic Ce) into urban wastewater systems and the environment result from its widespread industrial use (abrasives, catalysts, nanotechnology). Because Ce in sewage sludge can also be of geogenic origin, the quantification of anthropogenic Ce in sewage sludge remains elusive. In this study, we evaluated the suitability of Ce oxidation state and rare earth element (REE) patterns for the quantification of anthropogenic Ce fractions in sewage sludge. A diverse set of soil samples served to gain baseline information on geogenic Ce. Geogenic Ce in the soils was characterized by high Ce(III) fractions (≥70%) and their REE patterns were comparable to the REE patterns of the upper continental crust. The sewage sludges contained on average ∼80% Ce(IV) (range 18–108%), pointing to the importance of anthropogenic inputs of Ce(IV). The quantification of the anthropogenic Ce fraction based on Ce oxidation state, however, was associated with considerable uncertainty because geogenic and anthropogenic Ce cannot exclusively be assigned to Ce(III) and Ce(IV), respectively. The REE patterns of most sewage sludges indicated a clear enrichment of Ce compared to heavier REE. Based on the assumption that the industrially used Ce is free of (most) other REE, we estimated the fraction of anthropogenic Ce in the sludges based on individual Ce/REE ratios. For the individual sludges the anthropogenic contributions were very variable (10–100%) but consistent fractions were obtained for individual sludges when calculated based on Ce/Dy (dysprosium), Ce/Er (erbium) and Ce/Eu (europium) ratios. Electron microscopy analysis of sludges dominated by anthropogenic Ce revealed that the Ce was mostly contained in nanoscale particles devoid of elements characteristic of Ce-bearing minerals. Thus, anthropogenic Ce contents derived from REE patterns may be used to validate current mass flow models for engineered CeO2 nanoparticles.

Partitioning of Ce, as a simulant for Pu, in a multiphase ceramic nuclear waste form

AbstractA pyrochlore‐rich multiphase ceramic waste form was prepared by the alkoxide‐route and consolidated by sintering in air. Cerium (Ce) was used as a surrogate for plutonium (Pu), which was found in both the major pyrochlore and minor perovskite phases, but not the minor rutile phase. A synchrotron‐based fluorescence X‐ray absorption near‐edge spectroscopy (XANES) imaging technique was employed to determine the Ce oxidation state in each of the two Ce‐containing phases. It was shown that the major pyrochlore phase contains only tetravalent Ce, whereas the minor perovskite phase incorporates exclusively trivalent Ce. The effect of sintering atmosphere was also investigated, which revealed a very different phase assemblage when sintered in argon (Ar). A major perovskite phase was produced under Ar with a minor zirconolite‐4M phase, both containing trivalent Ce. The composition of the minor rutile phase remained the same whether it was sintered in air or Ar. The results demonstrate the importance of controlling the oxidation state of multivalent waste ions in the design of ceramic waste forms.

Electronic conductivity in gadolinium doped ceria under direct current as a trigger for flash sintering

In this paper, we discuss the progression of flash sintering in 10mol% gadolinium-doped ceria specimens. Flash transition is correlated with the generation of n-type electronic conductivity in air under direct-current bias. Its origin is attributed to partial reduction of the material which propagates from the cathodic-to-anodic region. The phenomenon was observed, in-situ, by monitoring the development of electrochemical blackening during the incubation period of the flash-experiment. Anomalous features, including shift in the valence band edge, shrinkage of the band-gap and change in the oxidation state of Ce in flashed samples was confirmed by X-ray photoelectron spectroscopy and diffuse reflectance measurements.