Ce LIII-edge X-ray Absorption Research Articles

Redox behaviour and solid solubility of cerium ortho-niobates

Using Ce LIII edge X-ray absorption near edge spectroscopy (XANES), it is shown that acceptor dopants introduced to cerium members of the rare earth ortho-niobate series, Ce1-xSrxNbO4±δ and Ce1-xCaxNbO4±δ, are charge compensated by the formation of holes on the cerium sublattice. These spectroscopic studies are complemented by structural studies, using X-ray and neutron powder diffraction, determining that the solubility limit of the strontium and calcium dopants within the CeNbO4+δ structure is ~10% and ~30% respectively. Under oxidising conditions, the Ce3+/Ce4+ redox couple facilitates reversible redox processes, and it is observed that the Ce1-xSrxNbO4±δ and Ce1-xCaxNbO4±δ materials form commensurate and incommensurately modulated oxygen hyperstoichiometric phases as a function of temperature. Under reducing atmospheres, this redox activity is suppressed, and charge-compensating Ce4+ holes are annihilated.

Surface speciation and interactions between adsorbed chloride and water on cerium dioxide

Ceria particles with different specific surface areas (SSA) were contaminated with chloride and water, then heat treated at 500 and 900 °C to investigate sorption behaviour of these species on metal oxides. Results from x-ray photoelectron spectroscopy and infrared spectroscopy showed chloride and water adsorption onto particles increased with surface area and that these species were mostly removed on heat treatment (from 6.3 to 0.8 at% Cl- on high SSA and from 1.4 to 0.4 at% on low SSA particles). X-ray diffraction revealed that chloride was not incorporated into the bulk ceria structure, but crystal size increased upon contamination. Ce LIII-edge x-ray absorption spectroscopy confirmed that chloride was not present in the first co-ordination sphere around Ce(IV) ions, so was not bonded to Ce as chloride in the bulk structure. Sintering of contaminated high SSA particles occurred with heat treatment at 900 °C, and they resembled low SSA particles synthesised at this temperature. Physical chloride-particle interactions were investigated using electron microscopy and energy dispersive x-ray analysis, showing that chloride was homogeneously distributed on ceria and that reduction of porosity did not trap surface-sorbed chloride inside the particles as surface area was reduced during sintering. This has implications for stabilisation of chloride-contaminated PuO2 for long term storage.

Luminescence of Ce3+ in aluminophosphate glasses prepared in air

Control of activators in solid state matter is the most important focus for phosphor applications. However, melt-quenching from an inert atmosphere is restricted by the constraints of a purged electric furnace. Here, we have examined the optical and emission properties of Ce-doped 25Al2O3−75P2O5 glass, which is the stoichiometric chemical composition of the Al(PO3)3 crystal, prepared in an air atmosphere. The decay constant (τ1/e) and the internal quantum yield of the Ce-doped glasses depend on the Ce concentration. The Ce LIII-edge X-ray absorption near-edge structure analysis shows that the Ce3+/(Ce3+ + Ce4+) ratio increases with increasing Ce concentration, which indicates that the starting material, Ce(OCOCH3)3·H2O, affects the redox state during air melting. Although the glass was prepared in air, the Ce3+/(Ce3+ + Ce4+) ratios and the internal quantum yields of the glasses were found to be at least 65% and ~60%, respectively.

Photocatalysis of S-metolachlor in aqueous suspension of magnetic cerium-doped mTiO2 core-shell under simulated solar light.

Magnetic cerium-doped mesoporous titanium dioxide was synthesized by combining sol-gel method and calcination using tetrabutanate and ammonium cerium nitrate as precursors and Pluronic P123 as a template coating on iron oxide covered with carbon in ethanol. The magnetic Ce-doped catalyst showed only anatase structure with a slight increase in lattice parameters compared to the undoped catalyst. The Ce LIII-edge X-ray absorption near-edge spectroscopy (XANES) spectra showed Ce3+, and the cerium substitution doping into titanium dioxide was proposed. Degradation of S-metolachlor in aqueous magnetic photocatalyst suspension followed (pseudo) first-order kinetics in the presence of 0.5gL-1 of γ-Fe2O3@C@0.16mol% Ce-mTiO2 with a half-life of 55.18±1.63min. Fifteen degradation products were identified, and their transformation routes of the photocatalytic degradation were then proposed. Complementary toxicity assessment of the treated S-metolachlor solution was undertaken with Environment Canada's algal microplate assay measuring growth inhibition (72-h IC50) in the freshwater chlorophyte Pseudokirchneriella subcapitata. This test method revealed a significant decrease in toxicity (1.7-fold reduction after 180min of irradiation treatment), thereby confirming that the by-products formed following photocatalysis would be less harmful from an environmental point of view. Photocatalytic degradation of S-metolachlor thus appears to hold promise as a cost-effective treatment technology to diminish the presence of this herbicide in aquatic systems.

Cerium sequestration and accumulation in fractured crystalline bedrock: The role of Mn-Fe (hydr-)oxides and clay minerals

This study focuses on the mechanisms of Ce sequestration and accumulation in the fracture network of the upper kilometer of the granitoid bedrock of the Baltic Shield in southeast Sweden (Laxemar area, Sweden). The material includes 81 specimens of bulk secondary mineral precipitates (“fracture coatings”) collected on fracture walls identified in 17 drill cores, and 66 groundwater samples collected from 21 deep boreholes with equipment designed for retrieval of representative groundwater at controlled depths. The concentrations of Ce in the fracture coatings, although varying considerably (10–90th percentiles: 67–438mgkg−1), were frequently higher than those of the wall rock (10–90th percentiles: 70–118mgkg−1). Linear combination fitting analysis of Ce LIII-edge X-ray absorption near-edge structure (XANES) spectra, obtained for 19 fracture coatings with relatively high Ce concentrations (⩾145mgkg−1) and a wide range of Ce-anomaly values, revealed that Ce(IV) occurs frequently in the upper 10m of the fracture network (Ce(IV)/Cetotal=0.06–1.00 in 8 out of 11 specimens) and is mainly associated with Mn oxides (modeled as Ce oxidatively scavenged by birnessite). These features are in line with the strong oxidative and sorptive capacities of Mn oxide as demonstrated by previous studies, and abundant todorokite and birnessite-like Mn oxides identified in 3 out of 4 specimens analyzed by Mn K-edge X-ray absorption spectroscopy (XAS) in the upper parts of the fracture network (down to 5m). For a specimen with very high Ce concentration (1430mgkg−1) and NASC-normalized Ce anomaly (3.63), the analysis of Ce XANES and Mn XAS data revealed (i) a predominance of Ce oxide in addition to Ce scavenged by Mn oxide; and (ii) a large fraction of poorly-crystalline hexagonal birnessite and aqueous Mn2+, suggesting a recent or on-going oxidation of Mn2+ in this fracture. In addition, the Ce oxide precipitates on this fracture observed by in situ SEM-EDS contained considerable amounts of Mn. These spectroscopic and microscopic features led us to suggest that the remarkable accumulation of Ce(IV) in this fracture is a result of repeated formation and dissolution of Mn oxides, that is, formation of Mn oxide followed by oxidative scavenging of Ce as Ce oxide nanoparticles, which largely remained during the subsequent reductive dissolution of the Mn oxides. In addition, the XANES data indicate that goethite has the capability to oxidize Ce at near-neutral pH under our experimental conditions (goethite reacted with 0.001M Ce for 48h in a glove box with O2 <1ppm). This previously unrecognized Ce oxidation pathway also seems to contribute to a minor extent to the oxidative scavenging of Ce in the fracture network. Trivalent Ce in the fracture coatings, in particular below 2.5m, is mainly sorbed as inner-sphere complexes on clay minerals. Taking into account the facts that Ce in the present groundwater is scarce and modeled to be largely complexed with humic substance, it is argued that the inner-sphere complexes were mainly formed from past (Paleozoic) hydrothermal fluids.

The effect of oxygen fugacity, melt composition, temperature and pressure on the oxidation state of cerium in silicate melts

Ce LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from −10 to +11 in logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer), temperatures (1300–1500°C), and pressures (1atm and 1GPa). The oxidation state ratio of Ce, Ce4+/∑Ce (where ∑Ce=Ce3++Ce4+), was determined from the spectra allowing the relationships between Ce4+/∑Ce and fO2, temperature, pressure and melt composition to be determined. Ce4+/∑Ce varied systematically with fO2 from 0 to ~0.8 over the range of conditions studied. Ce4+ is stabilised relative to Ce3+ by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4+). Ce4+ in an Fe2+-bearing melt is not preserved on cooling to a glass due to the reaction Ce4++Fe2+=Ce3++Fe3+. Ce4+/∑Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.

Investigation of the hydrothermal crystallisation of the perovskite solid solution NaCe1−La Ti2O6 and its defect chemistry

Perovskites of nominal composition NaCe1−xLaxTi2O6 (0≤x≤1) crystallise directly under hydrothermal conditions at 240 °C. Raman spectroscopy shows distortion from the ideal cubic structure and Rietveld analysis of powder X-ray and neutron diffraction reveals that the materials represent a continuous series in rhombohedral space group R3c. Ce LIII-edge X-ray absorption near edge structure spectroscopy shows that while the majority of cerium is present as Ce3+ there is evidence for Ce4+. The paramagnetic Ce3+ affects the chemical shift and line width of 23Na MAS NMR spectra, which also show with no evidence for A-site ordering. 2H MAS NMR of samples prepared in D2O shows the inclusion of deuterium, which IR spectroscopy shows is most likely to be as D2O. The deuterium content is highest for the cerium-rich materials, consistent with oxidation of some cerium to Ce4+ to provide charge balance of A-site water.

Ceria-Doped Ni/SBA-16 Catalysts for Dry Reforming of Methane

Ni-ceria nanoparticles (Ni/Ce = 1/1) in the cage-like pores of SBA-16 were prepared and evaluated in methane dry reforming reactions. Coexistence of ceria in NiCe/SBA-16 resulted in forming uniformly sized Ni particles (av. 5.7 nm) within the mesopores of SBA-16, because of the confinement effect from the framework of SBA-16 and the strong interaction between Ni and ceria. Ceria addition facilitated the reduction of NiCe/SBA-16 compared with Ni/SBA-16, and Ce3+ was the dominant species in both fresh and used NiCe/SBA-16 catalysts, as determined by Ce LIII-edge X-ray absorption near-edge structure (XANES). The methane conversion was much more stable on NiCe/SBA-16 than on Ni/CeO2 and Ni/SBA-16 in the methane dry reforming at 973 K during a 100 h reaction period; the deactivation of the Ni catalyst and the collapse of the SBA-16 framework were preferably suppressed for NiCe/SBA-16 under the reaction conditions. The remarkable effect of ceria on the structural stability of both the active Ni particles and th...

Enhanced luminescence induced by change of cerium oxidation states in Li-codoped Lu2SiO5:Ce3+ phosphors

(Lu0.995−xCe0.005Lix)2SiO5 (x=0.0000–0.0100) phosphors were synthesized by sol–gel route, the luminescence intensity of the phosphor was improved by a factor of 2.3 at x=0.01 in comparison with the Li-free sample. This luminescence enhancement effect has been investigated by Ce LIII-edge X-ray absorption near edge structure spectroscopy. The underlying reason of enhanced luminescence was revealed. It is safely concluded that the enhanced luminescence by Li+ codoping in Lu2SiO5:Ce3+ phosphors is originated from the increase of the population of Ce3+. This result could also help us to understand the luminescence enhancement effect by Ca2+, Mg2+ or Na+ codoping in Lu2SiO5:Ce3+ phosphors.

Visible-Light-Absorbing Lindqvist-Type Polyoxometalates as Building Blocks for All-Inorganic Photosynthetic Assemblies

This study reports that polynuclear charge-transfer complexes consisting of Ce3+ and a Lindqvist-type polyoxometalate center, either W6O192− or Mo6O192−, function as an all-inorganic visible-light-absorbing chromophore. The polynuclear W6O19/Ce and Mo6O19/Ce complexes showed an intense absorption band from the UV region to 520 and 600 nm, respectively. The FTIR and Ce LIII-edge X-ray absorption measurements confirmed that the visible absorption is due to the oxo-bridged W6+/Ce3+ and Mo6+/Ce3+. As polyoxometalates have efficient redox and catalytic properties, their visible-light sensitization could provide a new building block for fabricating inorganic, molecular-based photosynthetic assemblies.

Magnetic properties and electronic structures of intermediate valence systemsCeRhSi2 andCe2Rh3Si5

The crystal structures and the physical (magnetic, electricaltransport and thermodynamic) properties of the ternary compoundsCeRhSi2 andCe2Rh3Si5 (orthorhombicCeNiSi2- andU2Co3Si5-type structures, respectively) were studied over wide ranges of temperature and magnetic fieldstrength. The results revealed that both materials are valence fluctuating systems, in line withprevious literature reports. Direct evidence for valence fluctuations was obtained by means of CeLIII-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy.The experimental data were confronted with the results of ab initio calculations of theelectronic band structures in both compounds.

Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates

Anchored visible light-absorbing metal oxide nanoclusters, consisting of trivalent cerium ions and Keggin-type polyoxotungstates (PW12O403− and CuIIPW11O397−), have been synthesized on the pore surface of mesoporous silica (MCM-41) as a new class of visible light sensitive photocatalysts. The diffuse reflectance UV−vis, FT-Raman, and Ce LIII-edge X-ray absorption measurements showed that grafting of trivalent cerium ions to the preimmobilized PW12O403− cluster on the silica surface provided a broad absorption tail extending from the UV region to 530 nm assigned to a metal-to-metal charge transfer (MMCT) transition of an oxo-bridged binuclear WVI−O−CeIII unit. Photoexcitation of WVI−O−CeIII → WV−O−CeIV MMCT in the anchored clusters initiated the photooxidation reactions of gaseous isopropanol to CO2, whereby WV and CeIV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW12O403− cluster, it was also confirmed that the present methods can be extended to metal-substituted polyoxotungstates. Grafting of trivalent cerium ions to the copper-monosubstituted cluster (CuIIPW11O397−) produced an oxo-bridged trinuclear CuII/WVI/CeIII unit and exhibited photoactivity 4 times higher than that for the binuclear WVI/CeIII cluster. CuII-substituted polyoxotungstates are known to initiate a multielectron dioxygen reduction reaction and have a unique redox property in that the two-electron reduction of CuII to Cu0 occurs in a concerted manner. Therefore, the enhanced photoactivity for the trinuclear unit demonstrates that the activation of a single Cu center from CuII to Cu0 is accomplished by sequential photoinduced MMCT through the confined WVI−O−CeIII linkages, which results in acceleration of the photocatalytic two-electron O2 reduction process.

Observation of the non-Fermi-liquid behavior in the evolution from heavy-fermion to valence-fluctuation behavior in Ce(Pt 1−xRh x)Si 2 series

We report the results of electrical resistivity, magnetic susceptibility, heat-capacity, and Ce-L III-edge X-ray absorption spectrum (XAS) measurements for a solid solution series Ce(Pt 1− x Rh x )Si 2 ( x = 0 , 0.2, 0.4, 0.6, 0.8, 1.0). The results reveal that CePtSi 2 is an antiferromagnetic heavy-fermion compound with T N ≈ 1.5 K . As Rh content increases, the Kondo effect gradually dominates over the magnetic ordering and drives the system towards an intermediate-valence (IV) regime with nearly quenched magnetism. Besides, we also observed non-Fermi-liquid behaviors in CePt 0.4Rh 0.6Si 2. These results indicate that at x ≈ 0.6 , the system is located in the vicinity of a quantum critical point of the evolution.

Spin frozen in the Kondo-lattice compound: CeCuSi 2

In this paper, we present the experimental results of X-ray powder diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements as well as Ce-L III-edge X-ray absorption spectrum of the Ce-based intermetallic compound CeCuSi 2. The results revealed that CeCuSi 2 is a Kondo-lattice compound with no superconducting or magnetic-phase transition above 0.4 K. In addition, we found spin-glass behavior in the DC susceptibility measurements. The AC susceptibility measurements and the magnetic entropy calculation also confirm the presence of the spin-glass phase. The possible formation mechanism for the spin-frozen state is also discussed in this paper.

Unusual increase of the Kondo effect by hydrogenation: case of the ternary silicide CeNiSi

The ternary silicide CeNiSi, which crystallises in the tetragonal LaPtSi-type structure (a=4.070(2)Å, c=14.030(5)Å), absorbs hydrogen at P(H2)=2MPa and T=393K to form the stable hydride CeNiSiH0.8(1). The hydrogenation induces (i) an anisotropic variation of the unit cell parameters (a=4.040(1)Å and c=14.253(4)Å) without significant increase of the unit cell volume (+0.1%) and (ii) a strong increase of the Kondo interaction in comparison with the initial ternary silicide. This last evolution is observed for the first time during insertion of H-atoms into CeNiX (X=np1 or np2 element) compounds. X-ray diffraction analysis, magnetic susceptibility and Ce LIII-edge X-ray absorption spectroscopy measurements performed on CeNiSi and CeNiSiH0.8(1) are reported and compared with those obtained on the isostructural RENiSiH1.0(1) hydrides with RE=La and Nd.

Correlation between oxygen content δ and metal–insulator transition in Sm1.85Ce0.15CuO4+δ antiferromagnetic superconductors

The role of oxygen content parameter δ on the metal–insulator transition and superconductivity in oxygen-reduced Sm1.85Ce0.15CuO4+δ (−0.06<δ<0.08) antiferromagnetic superconductors (TN(Sm)∼4.6K) are reported. A superconductor–insulator transition was observed near δ∼0.05, with Tc (onset) of 11K for δ∼0.045, 18K for δ∼0.04, 20K for δ∼0.03 and maximum Tc up to 24K for δ∼−0.02. The relative oxygen content δ was determined using the Sm2CuO4 as the standard and based on Ce4+ from Ce LIII-edge X-ray absorption spectra. For tetragonal c-axis aligned Sm1.85Ce0.15CuO3.98 powder (Tc=24K), a non-s-wave or dirty d-wave-like T2 power law was observed for T<0.5Tc from in-plane penetration depth Δλab(T)=λab(T)−λab(0) data.

On unstable magnetic ordering in Ce 3d compounds and their hydrides

57Fe Mössbauer, X-ray photoemission and Ce LIII edge X-ray absorption measurements on CeFe2, selected pseudobinaries Ce(Fe1 − yXy)2 and some of their hydrides were carried out to shed light on magnetic ordering in these materials. While the hydrides order ferromagnetically in the temperature range below Tc, a double transition is observed in the pseudobinaries. At Tc, ferromagnetic ordering sets in, followed by an antiferromagnetic phase below a lower-lying critical temperature Tc2, their magnetic ordering type having been found to be of some complexity. In the parent compounds, cerium is of a non-integral valency, i.e. it contains 4f0 admixture. On hydrogen absorption it changes to 3+, corresponding to 4f1.