Cerium Valence Research Articles

Effect of glass content on the phase assemblage and processing requirements of zirconolite glass-ceramics for actinide immobilisation

Zirconolite is a candidate wasteform for actinides (U/Pu/Th), and glass incorporation to form glass-ceramics provides flexibility to accommodate heterogeneous wastes while simplifying processing requirements; however, the glass content required to optimise these benefits remains unestablished. This systematic study investigated the effect of glass content on zirconolite crystallisation, phase assemblage, phase composition, cerium valence states and partitioning. Cerium-bearing zirconolite glass-ceramics were fabricated by targeting zirconolite (Ca0.8Ce0.2ZrTi1.6Al0.4O7; Ce as actinide surrogate) with varying amounts (0–100 vol%) of glass addition (NaAl1.5B0.5Ca0.7Ti0.2Si2O8.6). Differential scanning calorimetry data revealed that glass addition lowered the zirconolite crystallisation temperature (1283°C to 1200°C). X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy analyses showed that glass addition lowered the temperature (1320°C to 1270°C) required to fabricate phase-pure zirconolite glass-ceramics and minimise CeO2/ZrO2 phases, with > 90 % of cerium preferentially partitioned into zirconolite. X-ray absorption near edge spectroscopy data showed predominantly Ce4+ within zirconolite as targeted.

The reactivity of CeO2 towards MoO3 in air atmosphere – reinvestigation

AbstractIt is known that both the type and possibilities of application of cerium molybdates(VI) largely depend on the methods of their synthesis. Despite this, information on the type of molybdates(VI) formed as a result of a waste-free and environmentally friendly reaction occurring in the solid state between CeO2 and MoO3 in the air atmosphere, are divergent. The conducted research indicates that CeO2 and MoO3 react in air and in the temperature range of 500–650 °C to form two compounds, i.e. Ce5Mo8O32 and/or Ce2Mo4O15. Only the Ce5Mo8O32 compound, for the first time, was obtained as a pure phase. The synthesis of Ce5Mo8O32 takes place through an intermediate stage. In this stage, with the evolution of oxygen, the compound Ce2Mo4O15 is formed, which then reacts with excess CeO2 to Ce5Mo8O32. The obtained compound was characterized by XRD, DTA–TG, FTIR and UV–Vis/DRS methods. Ce5Mo8O32 has a green-olive colour and a density of 4.82 ± 0.05 g cm−3. It was found that this compound melts incongruently at the temperature of 960 ± 5 °C with the separation of solid CeO2. The value of the energy gap Eg ~ 2.59 eV allows the Ce5Mo8O32 compound to be classified as a semiconductor. The previously unknown properties of the compound with mixed cerium valence (Ce 4 3+ Ce4+Mo 8 6+ O32), characterized in this work, will constitute the basis for its application research.

Selective inhibition of partial EMT-induced tumour cell growth by cerium valence states of extracellular ceria nanoparticles for anticancer treatment

Targeting specific tumour cells and their microenvironments is essential for enhancing the efficacy of chemotherapy and reducing its side effects. A partial epithelial-to-mesenchymal transition state (pEMT, with a hybrid epithelial/mesenchymal phenotype) in tumour cells is an attractive targeting for anticancer treatment because it potentially provides maximal stemness and metastasis relevant to malignant cancer stem cell-like features. However, treatment strategies to target pEMT in tumour cells remain a challenge. This study demonstrates that extracellular cerium oxide nanoparticles (CNPs) selectively inhibit the growth of pEMT-induced tumour cells, without affecting full epithelial tumour cells. Herein, highly concentrated Ce3+ and Ce4+ ions are formed on CNP-layered poly-L-lactic acid surfaces. Cell cultures of pEMT-induced and uninduced lung cancer cell lines on the CNP-layered substrates allow the effect of extracellular CNPs on tumour cell growth to be investigated. The extracellular CNPs with dominant Ce3+ and Ce4+ ions were able to trap pEMT-induced tumour cells in a growth-arrested quiescent/dormant or cytostatic state without generating redox-related reactive oxygen species (ROS), i.e. non-redox mechanisms. The dominant Ce3+ state provided highly efficient growth inhibition of the pEMT-induced tumour cells. In contrast, the dominant Ce4+ state showed highly selective and appropriate growth regulation of normal and tumour cells, including a mesenchymal phenotype. Furthermore, Ce4+-CNPs readily adsorbed serum-derived fibronectin and laminin. Cerium valence-specific proteins adsorbed on CNPs may influence receptor-mediated cell-CNP interactions, leading to tumour cell growth inhibition. These findings provide new perspectives for pEMT-targeting anticancer treatments based on the unique biointerface of extracellular CNPs with different Ce valence states.

Modification of Ce3+ luminescence in Li+ and Mg2+ codoped GAGG:Ce scintillators: A comparative study using synchrotron radiation

The luminescence characteristics of Gd3Al2Ga3O12:Ce3+ garnet scintillator crystals codoped with Li+ and Mg2+ ions were investigated. The excitation and emission spectra were obtained within the energy range of 3.6-21.6 eV, at both liquid helium and room temperature conditions to examine the effects of Li+ and Mg2+ codoping on cerium valence states, gadolinium-cerium energy transfer, and defect formation. Codoping with Mg2+ was found to efficiently convert Ce3+ to Ce4+, evidenced by significant changes in the shapes of the excitation spectra profile for Ce3+ luminescence in the exciton range, while Li+ had little impact. The presence of the high concentration of Ce4+ ions hampered the Gd3+→Ce4+ energy transfer. Redshifts in Ce3+ emission peaks indicated codopant-induced perturbations to the crystal field environment. The significant changes in the location of the Ce3+ excitation bands of Ce3+ luminescence in the exciton range further suggested alterations in the elemental distributions by the creation of complex defect clusters, particularly with Mg2+ codoping. The results demonstrate that Li+ and Mg2+ uniquely impact cerium valence, energy transfer processes, and structural properties in GAGG:Ce crystals.

The Role of Cerium Valence in the Conversion Temperature of H2Ti3O7 Nanoribbons to TiO2-B and Anatase Nanoribbons, and Further to Rutile.

CeO2-TiO2 is an important mixed oxide due to its catalytic properties, particularly in heterogeneous photocatalysis. This study presents a straightforward method to obtain 1D TiO2 nanostructures decorated with CeO2 nanoparticles at the surface. As the precursor, we used H2Ti3O7 nanoribbons prepared from sodium titanate nanoribbons by ion exchange. Two cerium sources with an oxidation state of +3 and +4 were used to obtain mixed oxides. HAADF-STEM mapping of the Ce4+-modified nanoribbons revealed a thin continuous layer at the surface of the H2Ti3O7 nanoribbons, while Ce3+ cerium ions intercalated partially between the titanate layers. The phase composition and morphology changes were monitored during calcination between 620 °C and 960 °C. Thermal treatment led to the formation of CeO2 nanoparticles on the surface of the TiO2 nanoribbons, whose size increased with the calcination temperature. The use of Ce4+ raised the temperature required for converting H2Ti3O7 to TiO2-B by approximately 200 °C, and the temperature for the formation of anatase. For the Ce3+ batch, the presence of cerium inhibited the conversion to rutile. Analysis of cerium oxidation states revealed the existence of both +4 and +3 in all calcined samples, regardless of the initial cerium oxidation state.

Pressure-induced valence changes of cerium in some cerium-based compounds

In the present work, we investigate how pressure affects the valence change of cerium in cerium-based compounds CeX (X= O, S, Te, P, As, Sb, and Bi). The valence of cerium at a certain high pressure is estimated by comparing the experimentally observed unit cell volume and the calculated unit cell volume of stable trivalent cerium ion in the compound. The change in unit cell volume with pressure is estimated by employing the Birch equation of state. It is found that the valence of Ce in the CeX is significantly influenced by both the applied pressure as well as the chemical environment around the Ce ion. J. Bangladesh Acad. Sci. 46(2); 147-154: December 2022

High-sensitive XANES analysis at Ce L2-edge for Ce in bauxites using transition-edge sensors: Implications for Ti-rich geological samples

Accurate determination of cerium (Ce) valence state is important for interpreting the Ce anomaly in geological archives for (paleo)redox reconstruction. However, the routine application of Ce L3-edge X-ray absorption near-edge structure (XANES) spectroscopy for detecting trace Ce in geological samples can often be restricted by coexisting titanium (Ti) due to the proximity of their fluorescence emission lines. Therefore, the signal-to-noise ratio of Ce L3-edge XANES spectra may not be sufficiently high for high-quality spectroscopic analysis. This study introduces a semi-quantitative approach appropriate for Ti-rich, Ce-dilute geological materials by synchrotron-based X-ray measurement at the Ce L2-edge. First, the results confirm that Ce L2-edge XANES spectra are able to avoid overlapping Ti Kβ emissions and provide more reliable information on the Ce valence state in Ti-rich materials relative to L3-edge XANES. Moreover, the application of transition-edge sensor (TES) could reach the higher sensitivity with better energy resolution than conventional silicon drift detector (SDD) to detect fluorescence X-ray (Ce Lβ1). The investigation on bauxites developed from the Columbia River Basalts shows that combining Ce L2-edge XANES and TES allows for resolving weak Ce fluorescence lines at the L2-edge from Ti-rich, Ce-dilute samples (Ti/Ce mass ratio up to ∼6000, tens of ppm Ce). The outcome emphasizes the practical possibility of investigating Ce redox state in Ti-rich geological samples.

Antiferromagnetism and mixed valency in the new Kondo lattice compound Ce3Rh4Sn7

Single crystals of a new ternary compound Ce3Rh4Sn7 were synthesized by flux method and its crystal structure and magnetic properties were studied by single crystal X–ray diffraction and magnetic susceptibility, specific heat and resistivity measurements. Ce3Rh4Sn7 crystallizes in a new monoclinic crystal structure, space group P21∕m (No. 11) with Z = 2 and lattice parameters a = 9.8708(5) Å, b = 4.5779(3) Å and c = 14.2381(7) Å, β = 109.535(5)∘, and V = 606.35(6) Å3. The structure exhibits three inequivalent Ce-sites. A striking structural motif is the tetragonal pyramid of Rh-ions surrounding the Ce2 yielding an extreme short Ce2–Rh1 spacing of d = 2.7486(10) Å. This strong hint to mixed valent cerium behavior is supported by the entropy analysis. We argue that the Ce2–ion is in a nonmagnetic Ce3+δ state while the other 2 Ce–sites are in a trivalent state. At low temperatures these latter become partially Kondo screened (TK ≈ 4 K) and order antiferromagnetic below TN = 2.31 K. The ordering is accompanied by a superzone gap-opening in direction of the magnetic propagation vector Q = (0, 0, l).

Photo-oxidation and kinetics of gaseous elemental mercury using CeO2/TiO2{001} and {101} photocatalysts in the atmosphere of multiple air pollutants

In this study, the photo-oxidation and kinetics of elemental mercury (Hg0) with self-prepared TiO2 and CeO2/TiO2 at 100–200 ℃ were explored. The surface properties of the photocatalysts were characterized before conducting the Hg0 photo-oxidation experiments. Both field emission scanning electron microscopy and high-resolution transmission electron microscopy showed that the grain size of TiO2 {101} was smaller than TiO2 {001}. XPS spectral showed that cerium (Ce) valence shifting between CeO2 and Ce2O3 generated strong oxidant O2-. Chemisorbed oxygen (Oα) was found to be more active than lattice oxygen (Oβ). Doping CeO2 to anatase-TiO2 effectively enhanced the photo-oxidation of Hg0. Higher Hg0 photo-oxidation efficiency and better thermal stability were observed in CeO2/TiO2{001} than CeO2/TiO2{101}. The photo-oxidation efficiency of Hg0 decreased with reaction temperature. In N2+Hg0+NO, low NO concentration (< 100 ppm) suppressed and high NO concentration (>300 ppm) enhanced the oxidation of Hg0. In N2+Hg0+SO2, SO2 competed with Hg0 for active sites on the surface of photocatalysts and suppressed the photo-oxidation of Hg0 since sulfate and sulfite as reaction products were observed on the surface of photocatalysts. Both facets could not prevent from being poisoned, however, CeO2/TiO2{001} could decelerate sulfur poisoning. On simulation by LH kinetic model, the reaction rate constant increased and the equilibrium constant decreased with reaction temperature, indicating that the photo-oxidation of Hg0 was dominated by physical adsorption.

A Novelty Analytical Approach to Determine Oxidation States in Complex Refractory Oxides Containing Iron, Uranium, Cerium and other Mixed Valence Cations

Titrimetry technique has been used for the first time to determine the oxidation state in complex refractory oxides containing iron, uranium, cerium, and other mixed valence cations. In particular, sodium molybdate (3Na2O-4MoO3), used as an oxidative solvent for metal oxides. Cerium (IV) sulfate solution which prepared with sulfuric acid was used as a titrant. The excess Ce (IV) is back titrated with Fe (II) standard solution with indicator ferroin. The mechanism and the results have been interpreted.

Efficient removal arsenate from water by biochar-loaded Ce3+-enriched ultra-fine ceria nanoparticles through adsorption-precipitation

Biochar-loaded Ce3+-enriched ultra-fine ceria nanoparticles (Ce-BC) were used as a novel nanostructured adsorbent for the removal of arsenate (As(V)) from aqueous solutions. The effect of cerium valence on As(V) adsorption and the mechanism of As(V) adsorption onto Ce-BC were investigated using batch experiments and a series of spectroscopy detection technologies. The adsorption isotherm data fitted with the Langmuir model, with maximum As(V) sorption capacity of 219.8 mg g−1 at pH 5.0 and 25 °C. The adsorption kinetics fitted well with the pseudo-second-order model. Ce3+ on the surface of Ce-BC plays an important role in the adsorption of As(V). The decrease in Ce3+ concentration from 60.1% to 48.9% on the Ce-BC surface, significantly decreased the adsorption of As(V) on Ce-BC. Furthermore, a strong affinity between As(V) and Ce3+-enriched Ce-BC was revealed, resulting in irreversible adsorption. Most importantly, the adsorbed As(V) could further react with Ce3+ of the ultra-fine cerium oxide nanoparticles in Ce-BC to form rod-like CeAsO4 precipitates. Through the novel adsorption-precipitation process, Ce-BC can be used to remove trace As(V).

The preparation temperature influences the physicochemical nature and activity of nanoceria.

Cerium oxide nanoparticles, so-called nanoceria, are engineered nanomaterials prepared by many methods that result in products with varying physicochemical properties and applications. Those used industrially are often calcined, an example is NM-212. Other nanoceria have beneficial pharmaceutical properties and are often prepared by solvothermal synthesis. Solvothermally synthesized nanoceria dissolve in acidic environments, accelerated by carboxylic acids. NM-212 dissolution has been reported to be minimal. To gain insight into the role of high-temperature exposure on nanoceria dissolution, product susceptibility to carboxylic acid-accelerated dissolution, and its effect on biological and catalytic properties of nanoceria, the dissolution of NM-212, a solvothermally synthesized nanoceria material, and a calcined form of the solvothermally synthesized nanoceria material (ca. 40, 4, and 40 nm diameter, respectively) was investigated. Two dissolution methods were employed. Dissolution of NM-212 and the calcined nanoceria was much slower than that of the non-calcined form. The decreased solubility was attributed to an increased amount of surface Ce4+ species induced by the high temperature. Carboxylic acids doubled the very low dissolution rate of NM-212. Nanoceria dissolution releases Ce3+ ions, which, with phosphate, form insoluble cerium phosphate in vivo. The addition of immobilized phosphates did not accelerate nanoceria dissolution, suggesting that the Ce3+ ion release during nanoceria dissolution was phosphate-independent. Smaller particles resulting from partial nanoceria dissolution led to less cellular protein carbonyl formation, attributed to an increased amount of surface Ce3+ species. Surface reactivity was greater for the solvothermally synthesized nanoceria, which had more Ce3+ species at the surface. The results show that temperature treatment of nanoceria can produce significant differences in solubility and surface cerium valence, which affect the biological and catalytic properties of nanoceria.

Effect of air annealing on the optical properties and luminescence performance of Ce:YAG ceramics for light-emitting diodes at different Ce concentrations

(Y1-x%Cex%)3Al5O12 (x = 0.2,0.4,0.6,0.8,1.0) transparent ceramics were fabricated by vacuum sintering technology, followed by air annealing at different temperatures. Transmittance of ceramics, valence of cerium, and luminescent properties with varying annealing temperatures are studied in detail. The negative effect of Ce3+ oxidation induced by annealing gets increasingly evident when Ce concentration increases. Collaborating Ce:YAG ceramics with InGaN blue chips, light-emitting diodes (LEDs) with superior performance were constructed. The relationships between Ce concentration, annealing temperature, and luminous flux of LEDs are elucidated, showing that the optimized annealing temperature of Ce:YAG ceramics decreases from 1200 °C to 900 °C as Ce concentration increases from 0.2 at% to 1.0 at%. The luminous fluxes of optimized LEDs increase by ∼10 % compared with that of unannealed LEDs.

Preparation of cerium molybdates and their antiviral activity against bacteriophage Φ6 and SARS-CoV-2

Two cerium molybdates (Ce2Mo3O12 and γ-Ce2Mo3O13) were prepared using either polymerizable complex method or hydrothermal process. The obtained powders were almost single-phase with different cerium valence. Both samples were found to have antiviral activity against bacteriophage Φ6. Especially, γ-Ce2Mo3O13 exhibited high antiviral activity against both bacteriophage Φ6 and SARS-CoV-2 coronavirus, which causes COVID-19. A synergetic effect of Ce and molybdate ion was inferred along with the specific surface area as key factors for antiviral activity.

Improved performance of Ti-based conversion coating in the presence of Ce/Co ions: Surface characterization, electrochemical and adhesion study

Abstract Novel chromium-free conversion coatings based on titanium-cerium (Ti/Ce-CC) and titanium-cobalt (Ti/Co-CC) were prepared to enhance the anti-corrosion resistance and adhesion property of the organic coating applied on Al-2024 substrate. The surface characteristics of the modified titanium conversion coating (Ti-CC) were evaluated by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Tungsten microelectrode and contact angle measuring device. The electrochemical behavior of conversion coatings was evaluated by electrochemical impedance spectroscopy (EIS) measurement. Anti-corrosion and adhesion properties of polyurethane coating were evaluated by EIS, salt spray test and pull-off measurement. FE-SEM and AFM results show that by the addition of cerium and cobalt ions, the morphology of Ti-CC was completely changed. Oxide and hydroxide form of the different valence of cerium and cobalt ions were existed on the surface based on XPS spectra. The maximum surface energy is obtained for Ti/Ce-CC. Electrochemical data revealed that the Ti/Ce-CC enhance the anti-corrosion resistance to a significant degree (271.11 × 103 Ω.cm2) compare to the bare sample (3.70 × 103 Ω.cm2). The polyurethane coating applied on Ti/Ce-CC showed minimum delamination, highest anti-corrosion resistance and better adhesion strength in comparison with the Ti/Co-CC, Ti-CC and bare coated samples.

Atomistic origin of high-concentration Ce3+ in {100}-faceted Cr-substituted CeO2 nanocrystals

Improving the potential of promising CeO2-based nanocatalysts in practical applications requires an atomic-scale analysis of the effects of active dopants on the distribution of Ce valence states and the formation of oxygen vacancies (VOs). In this study, a Cr dopant is introduced into the cubic {100}-faceted CeO2 nanocrystals (NCs) with an average size of 7.8 nm via supercritical water. The Cr dopants substitute Ce sites in the amount of approximately 3 mol%. Based on the aberration-corrected STEM-EELS, the effects of Cr dopant on the distribution of cerium valence states are investigated layer by layer across the ultrafine Cr-substituted CeO2 NC perpendicular to the {100} exposed facet. It is found that an increased amount of Ce3+ cations is present in Cr-substituted CeO2 NCs, particularly in the internal atomic layers, compared to the pristine CeO2 NCs. The atomic-scale analysis of the local structure combined with theoretical calculations demonstrates that Cr dopant reduces the formation energy of VOs and increases the mobility of oxygen atoms for the nano-sized CeO2. These effects, in principle, result in an improved oxygen storage capacity and provide a fundamental understanding of role of the dopant in the formation and distribution of VOs in the doped CeO2 NCs.

Ce0.5La0.5Ni9Ge4 compound: Why is cerium valence not 3+?

Ce0.5La0.5Ni9Ge4 compound shows both Fermi and non-Fermi Liquid behaviour. Cerium is considered a magnetic atom and literature usually shows its electronic valence as Ce3+ with j=5/2 total angular momentum. In this paper we use two-channel Anderson Model and also density functional theory (DFT) to address this information. The Numerical Renormalization Group (NRG) is used to diagonalize this model and to obtain electronic specific heat versus temperature curves, from which entropy and degrees of freedom of the Anderson Model are taken. Both NRG and DFT indicate that Cerium atom must have Ce1+ electronic valence and j=9/2 total angular momentum.

Synergetic Li–Ca Co‐Doping for Ce Valence Conversion in Yttrium Aluminum Garnet

Herein, optical‐quality single crystals of yttrium aluminum garnet (YAG):Ce,Ca,Li are grown by the vertical Bridgman method. Substitution and charge compensation mechanisms of Li+ are considered based on optical absorption in as‐grown and γ‐ray‐irradiated single crystals and their luminescence decay and rise time under X‐ray excitation. The measurements are conducted on single crystals with impurity concentrations varying over a wide range. A suitable combination of Ca and Li concentration is able to control the cerium valence, which plays a major role in the time response under ionizing radiation.

Effect of the Ce3+ concentration on laser-sintered YAG ceramics for white LEDs applications

This paper describes the laser sintering and luminescence properties of Y3Al5O12 (YAG) phosphors doped with different concentrations of Ce3+ and synthesized using the polymeric precursor method. The ceramics were sintered by a new laser sintering technique in which a CO2 laser is employed as the heating source. The resultant ceramics exhibited a homogeneous microstructure with narrow grain size distribution and high relative density. The introduction of Ce3+ ions led to luminescence quenching at a concentration above 0.5 mol% and a redshift of the emission spectrum band with increasing cerium concentration. The excitation spectrum showed two characteristic bands centered at 340 nm and 460 nm and a relative change in their intensity by change the Ce3+ concentration. The presence of a single valence (Ce3+) of cerium was determined by X-ray absorption near edge structure measurements.

Dual-valence Ce doped UV-shielding glasses with high transparency and stability

On account the urgent demand for UV radiation protection in diverse applications, intensive efforts have been devoted to UV-shielding materials, ranging from wide-bandgap semiconductors to organo-inorganic composite films. However, they constantly suffer from insufficient stability, complex preparation and low transparency in visible region. Here, we report a new UV-shielding inorganic robust glasses through manipulating valence of cerium. Via conventional melt-quenching method, dual valence Ce-doped aluminoborate glasses were successfully prepared in air atmosphere. By taking advantage of the co-absorption of Ce3+ and Ce4+, glass samples present prominent UV-absorbing capability in UVA, UVB and UVC region, and they are highly transparent in the visible-wavelength region. More importantly, compared with current UV resistant organic-inorganic films, glass samples still maintain excellent UV protection ability and high transparency after surviving 120 h in harsh environment, including high temperature, acid and alkaline environment. These results indicate that dual valence Ce-doped glasses, with high anti-UV ability, good visible transparency and excellent stability, could be excellent UV-shielding materials for diverse environment, such as greenhouse, vehicle windows and museums.