Oxidation State Of Cerium Research Articles

Evaluation and X-Ray Induced Modification of the Cerium Oxidation State in Cerium Calixarene Complexes

The cerium oxidation state in novel calixarene-supported cerium(IV) β-diketonate complexes [p-tBu-calix[4](OMe)2(O)2]Ce(acac)2 (1) and [p-tBu-calix[4](OMe)2(O)2]Ce(hfac)2 (2), which are a new class of potential precursors for homogeneous oxidative transformations, has been determined using X-ray photoelectron spectroscopy (XPS). Cerium oxidation states between 3.6 and 3.65 were detected, distinctly different from their nominal value of +4. An X-ray induced photoreduction of these compounds was detected. Because of the observed stability of the X-ray modified oxidation state under ambient conditions this effect might be used for a long-standing fine tuning of the Ce oxidation state in cerium calixarenes.

Formation of the cerium orthovanadateCeVO4:DFT+Ustudy

We report density functional theory calculations of the structural, electronic, and thermodynamic properties of cerium orthovanadate CeVO4 employing the local density approximation LDA , generalized gradient approximation GGA-PBE , LDA+U, and GGA-PBE+U functionals. The LDA+U, GGA-PBE+U, LDA, and GGA-PBE equilibrium volumes deviate by −2.4%, +3.6%, −7.4%, and −0.8%, respectively, from experimental results. DFT+U DFT predicts an antiferromagnetic ferromagnetic insulating metallic ground state, which is in agreement with experimental observations. DFT+U yields Ce and V ions in the III+ and V+ oxidation state, respectively. CeVO4 can be obtained by the reaction between Ce2O3 and V2O5 1 2Ce2O3 s + 1 2V2O5 s →CeVO4 s under an inert atmosphere, which is described as exoenergetic H0 =1.6−1.8 eV by all functionals. The reaction 1 2Ce2O3 s + 1 2V2O5 s →CeO2 s +VO2 s is exoenergetic with H0 =0.75, 0.25, 1.70, and 1.24 eV for LDA+U, GGA-PBE+U, LDA, and GGA-PBE, respectively. Hence, VV+ is more easily reduced to VIV+ than CeIV+ to CeIII+, but the difference is small as obtained with DFT+U, PBE+U, in particular. The variation of this reaction energy is due to the different performance of the various approaches for the description of the change in oxidation state of cerium, IV+ to III+ J. L. F. Da Silva et al., Phys. Rev. B 75, 045121 2007 . The small difference between the VV and CeIV reducibilities may have consequences for the use of CeO2 as support of V2O5 catalysts in selective oxidation.

Low-Temperature Redox Properties of Nanocrystalline Cerium (IV) Oxides Revealed by in Situ XANES

Time-resolved, in situ XANES studies of some cerium titanate pyrochlores and a fluorite-type bismuth-cerium oxide under reducing and oxidizing gas atmospheres reveal that reduction of cerium occurs at temperature below 500 degrees C in each of the materials. Cerium oxidation state during these treatments is quantified by deconvolution of the Ce L-III-edge XANES signal using fitted Lorentzians and comparison to data from reference materials of known average oxidation state. In the case of the cerium titanate pyrochlore, complementary XANES experiments at the Ti K-edge show that average titanium oxidation state is largely unaffected on reduction of the sample, although small changes in titanium local environment are apparent that are most likely due to thermal expansion effects as well as an expansion of the structure due to the reduction of cerium. Reoxidation of the pyrochlore is extremely facile, occurring even under hydrogen, which may be attributed to an increase in crystallite size and loss of oxide-rich surfaces, which is backed up by TEM observations.

Corrosion protection of aluminum borate whisker reinforced AA6061 composite by cerium oxide-based conversion coating

Cerium oxide-based conversion coatings on Al18B4O33w/6061Al composite surface were obtained by immersing the composite into a cerium chloride (CeCl3) solution containing various concentrations of hydrogen peroxide (H2O2). Coating microstructures were observed by scanning electron microscopy (SEM). Coating composition and the cerium oxidation state were characterized using energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS) respectively. Electrochemical methods were used to study the coatings corrosion resistance. Polarization experiments in 3.5% NaCl solution showed that the sensitivity to pitting corrosion for the conversion coated composites was much lower than that of the untreated samples. The results indicated that the uniformity of the coating had the biggest effect on corrosion behavior of the composite. The coating obtained from a Ce-solution containing 100 ml/L H2O2 (30 wt.%) produced better corrosion resistance due to the surface being covered by uniform conversion coating.

Quantitative analyses of oxygen release/storage and CO2 adsorption on ceria and Pt–Rh/ceria

The CO/O2 and CO2 pulse experiments were carried out to acquire useful information about oxygen release/storage and CO2 adsorption on ceria and Pt–Rh/ceria. In the CO pulse experiments at 500 °C, ca. 60% of CO uptake was released as CO2 while the rest of CO uptake was retained as carbon residuals on the surfaces of both samples. The carbon residuals could be removed when O2 was provided. In the CO2 pulse experiments, the adsorption of CO2 was found to relate to the temperatures and the oxidation states of surface cerium. The reduced Ce3+ sites (O vacancies) were responsible for the adsorption of CO2 at the temperature of 500 °C. In addition, the molar ratios of CO2 adsorption to O vacancies (38–39%) were in agreement with the ratios of carbon residuals to CO uptake ( ca. 40%) measured in the CO pulse experiments. Quantitative analyses of oxygen release/storage and CO2 adsorption implied that in the process of oxygen release, carbon residuals were possibly in the form of a carbonate-like species due to the adsorption of CO2 onto the reduced Ce3+ sites.

Redox behaviour of cerium oxychloride in molten MgCl 2–NaCl–KCl eutectic

The electrochemical behaviour of cerium oxychloride in MgCl 2–NaCl–KCl ternary eutectic was investigated by cyclic voltammetry at 823 K. The cyclic voltammogram of UO 2Cl 2–CeOCl in MgCl 2–NaCl–KCl eutectic shows two peaks during the cathodic sweep as well as anodic sweep. The reduction of UO 2 2+ is by a single step two-electron transfer and that of CeO + is by a single step one-electron transfer. The reduction of CeO + was found to be quasi-reversible. The reduction potentials of UO 2 2+/UO 2 and CeO +/CeO versus Ag(I)/Ag reference electrode at 823 K are 0.103 and −0.299 V, respectively. The diffusion coefficient of CeO + at 823 K is in the range of (1.7–1.9) × 10 −5 cm 2 s −1. The cyclic voltammogram for 0.015 mol% CeOCl shows an additional peak during the anodic sweep at −0.056 V, which is being attributed to monolayer dissolution of CeO at the glassy carbon working electrode. Electrochemical impedance data of 0.015 mol% CeOCl in MgCl 2–NaCl–KCl eutectic at the open circuit potential was fitted to a Randles cell from which the heterogeneous rate constant was estimated. X-ray photoelectron spectroscopy was used to confirm that the oxidation state of cerium in the eutectic is +3.

Phase stability in ceria-zirconia binary oxide nanoparticles: The effect of the Ce3+ concentration and the redox environment

The stability of the cubic phase (c′) of Ce1−xZrxO2−y nanoparticles was studied by x-ray absorption near edge spectroscopy (XANES), time-resolved high temperature x-ray diffraction (XRD), and room temperature XRD. A clear relationship between the phase stability of the c′ phase and the Ce3+ concentration was found in zirconia-doped ceria, prepared in an oxidizing environment. The percentage of the 3+ oxidation state of cerium was measured from the relative Ce3+ peak intensity at the CeLIII edge in XANES. A concentration of 11% of the larger Ce3+ ions, among all cerium ions, helps to release the local stress induced by the smaller Zr4+ ions and stabilizes the c′ phase even under high zirconia concentrations of 40%–60%. A smaller particle size enhances this effect. XANES results at the ZrLIII edge supported the cubic phase stabilization. When the homogenization anneal was performed under a reducing environment instead of in air, the solubility limit of the cubic phase Ce1−xZrxO2−y was extended to above 90% zirconia, while the Ce3+ concentration increased, reaching 94% of all [Ce] in Ce0.1Zr0.9O2−y, which confirms the crucial role of Ce3+ ions in stabilizing the cubic phase as well as the importance of the gaseous environment during the homogenization anneal.

Cerium treatments for temporary protection of electroplated steel

The present work aims at evaluating the corrosion performance of a cerium immersion treatment on zinc electroplated. For comparison, zinc coatings pretreated with a commercial chromate treatment were also examined. The corrosion behaviour of pretreated zinc surfaces under immersion in sodium chloride was investigated by potentiodynamic polarization to evaluate the effect of cerium on the oxygen reduction reaction. The influence of immersion time, temperature of cerium solutions, stirring influence, cerium concentrations, cerium salt, pH of the solution and oxidation state of cerium are also investigated. Scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) were used to characterize the sample surfaces after treatments. In order to compare the corrosion performance of cerium pretreated samples, an accelerated test in a humid chamber was carried out. The results indicate that the cerium film formed on the zinc surfaces markedly suppressed the cathodic process. The accelerated corrosion test shows that white rust formation is delayed although the protection given by this treatment is less efficient than with commercial chromate treatments.

XAFS and spectroscopic characterization for the structure and electronic structure of Ce-DMDPhPDA complexes in methanol

The structural parameters and the electronic structure of the complex between cerium(III) and N,N′-dimethyl-N,N′-diphenylpyridine-2,6-carboxyamide (DMDPhPDA) were studied by XAFS, and both luminescence and excitation spectroscopes. We investigated the fundamental aspects of this material as the ligand is promising for the separation of trivalent actinides and lanthanides. The Ce-K XAFS spectra were measured at the SPring-8 synchrotron facility in Japan. It was determined that the bond distances between the carbonyl oxygen and cerium, and between the pyridyl nitrogen and cerium were about 2.49 and 2.67 Å, respectively. The angle between the Ce-N bond and the pz axis of the lone pair of the pyridyl nitrogen is 180° for the solution state, suggesting covalent interaction between cerium and the nitrogen of the DMDPhPDA ligand. Emission from this system is severely quenched, and red-shifts, and supports the presence of a strong inner sphere complexation between cerium and the nitrogen atom of the pyridyl group. From the XANES spectrum, the oxidation state of cerium was determined to be trivalent.

X-ray photoelectron spectroscopy of CeO 2–Na 2O–SiO 2 glasses

A series of (CeO 2) x –(Na 2O) 0.3–(SiO 2) (0.7− x) glasses, where 0.025 ≤ x ≤ 0.075, have been synthesized and investigated by mean of X-ray photoelectron spectroscopy (XPS). The Ce 3d spin-orbit doublet was curve fitted in order to quantify the proportions of each cerium oxidation state in these glasses. It was found that Ce ions are predominantly in the Ce(III) state in glasses with compositions x ≤ 0.075, while mixed Ce valences were found in the glass with composition x = 0.10. The O 1s spectra have also been curve fitted with two components, one from bridging oxygen (BO) and the other from non-bridging oxygen atoms (NBO). The measured number of NBO, based on the fact that only oxygen atoms in the site Si–O–Na + contribute to the NBO peak, was found to be constant at ∼35% for all samples, in good agreement with the value calculated from the glass composition and inductively coupled plasma (ICP) suggesting that Ce ions enter the network as a glass intermediate. The thermal measurements done on these glasses agree well with the XPS findings.

Cerium oxidation state in ceria nanoparticles studied with X-ray photoelectron spectroscopy and absorption near edge spectroscopy

Abstract X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy experiments are used to investigate the oxidation state of cerium ions in ceria nanoparticles. A comparison of results shows that XPS yields a higher concentration of Ce3+ ions, even after analysis with a core–shell model. Three factors are proposed for the discrepancy between results: surface reduction of ceria in the XPS vacuum chamber enhanced by X-ray radiation, fast reduction dynamics associated with ceria nanoparticles, and a diffuse depth profile of the Ce3+ concentration inside ceria particles. Our results suggest that the high-vacuum XPS studies of ceria have overestimated the Ce3+ concentration in ceria nanoparticles under ambient condition. More importantly, we have demonstrated the importance of using complimentary surface analysis techniques to investigate the valence state of ceria nanoparticles.

Correlation of the oxidation state of cerium in sol–gel glasses as a function of thermal treatment via optical spectroscopy and XANES studies

Sol–gel glass matrices containing lanthanides have numerous technological applications and their formation involves several chemical facets. In the case of cerium, its ability to exist in two different oxidation states or in mixed valence state provides additional complexities for the sol–gel process. The oxidation state of cerium present during different facets of preparation of sol–gel glasses, and also as a function of the starting oxidation state of cerium added, were studied both by optical spectroscopy and X-ray absorption near-edge structures (XANES). The findings acquired by each approach were compared. The primary focus was on the redox chemistries associated with sample preparation, gelation, and thermal treatment. When Ce 3+ is introduced into the starting sols, the trivalent state normally prevails in the wet and room temperature-dried gels. Heating in air at >100 °C can generate a light yellow coloration with partial oxidation to the tetravalent state. Above 200 °C and up to ∼1000 °C, cerium is oxidized to its tetravalent state. In contrast, when tetravalent cerium is introduced into the sol, both the wet and room temperature-dried gels lose the yellow–brown color of the initial ceric ammonium nitrate solution. When the sol–gel is heated to 110 °C it turns yellowish as the cerium tends to be re-oxidized. The yellow color is believed to represent the effect of oxidation and oligomerization of the cerium–silanol units in the matrix. The luminescence properties are also affected by these changes, the details of which are reported herein.

XPS Investigation of ceramic matrixes for disposal of long-living radioactive waste products

The synthesis of ceramic matrixes for the long-term storage of highly active radionuclide wastes and determination of physical and chemical forms of radionuclides in them is one of the important problems in radioecology. It enables to create purpose fully materials for the long-term storage of radionuclides. In the present work the samples of ceramics [CaCe0.9Ti2O6.8(I) and CaCeTi2O7(II}] formed under various conditions were investigated with the X-ray photo electron spectroscopy. It is necessary for synthesis of ceramic matrixes, for the disposal of the plutonium and others tetravalent actinides. A technique was developed for the determination of cerium oxidation state (Ce3+ and Ce4+) on the basis of the X-ray photo electron spectroscopy spectral structure characteristics. It was established that the sample (I) formed at 300 MPa and T = 1400 ?C in the air atmosphere contained on the surface two types of cerium ions in the ratio ? 63 atomic % of Ce3+ and 37 atomic % of Ce4+, and the sample (II) formed at 300 MPa and T= 1300 ?C in the oxygen atmosphere contained on its surface two types of cerium ions also, but in the ratio ? 36 atomic % of Ce3+ and 64 atomic % of Ce4+. It was established that on the surface of the studied ceramics carbonates of calcium and/or cerium could be formed under influence of the environment that leads to the destruction of ceramics.

1D‐Polyoxometalate‐Based Composite Compounds − Design, Synthesis, Crystal Structures, and Properties of [{Ln(NMP)6}(PMo12O40)]n (Ln = La, Ce, Pr; NMP = N‐methyl‐2‐pyrrolidone)

AbstractThe structures of three 1:1 composite compounds prepared with the polyoxometalate (POM) [PMo12O40]3− and the cations [Ln(NMP)6]3+ [Ln = La (1), Ce (3), Pr (2); NMP = N‐methyl‐2‐pyrrolidone] exhibit two types of zig‐zag chains with alternating cations and anions through Mo−Ot−Ln−Ot−Mo links in the crystal. The compounds were characterized by IR, UV, and ESR spectroscopy, single‐crystal X‐ray structural analysis, and by a study of their thermal properties. In all the compounds, the La3+, Pr3+, and Ce3+ centers are eight‐coordinate with the oxygen atoms in bicapped trigonal‐prismatic geometries. The variation of the average Ln−O separations along the La, Ce, and Pr series is consistent with the effects of the lanthanide contraction. The results of the single‐crystal X‐ray diffraction analyses and the IR spectroscopic studies are in agreement and both show the metal cation units are coordinatively bonded to the Keggin clusters. The UV spectra of the title compounds suggest that their structures are entirely disrupted in dilute solution. Low‐temperature ESR spectra indicate that thermal electron delocalization occurs among the Mo atoms in the three compounds. The results of cyclic voltammetry (CV) show that compounds 1, 2, and 3 all undergo five two‐electron reversible reductions and that the [PMo12O40]3− anions are the active centers for electrochemical redox reactions in solution, while the corresponding cations have only a small effect on the electrochemical properties. Studies of magnetic properties show that 1 is diamagnetic, while 2and 3 exhibit antiferromagnetic Pr−Pr or Ce−Ce exchange interactions. In addition to the results of CV, the average bond lengths and the magnetic properties of compound 3 indicate that the oxidation state of cerium is III in the compound of formula [{Ce(NMP)6}(PMo12O40)]n. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Assembly of 3‐D Network Structures Containing Oxydiacetate and CeIII or CeIV

AbstractReaction of CeCl3·7H2O with Na2(oda) (oda = O(CH2CO2)22— oxydiacetate) in a 2:3 ratio gives the neutral cerium(III) complex [Ce2(oda)3(H2O)3]·9H2O (1). Treatment of a 1:3 mixture of CeCl3·7H2O and H2oda in water with 4 molar equivalents of NaOH also gives 1 but, with a larger excess of NaOH, the tri‐sodium salt Na3[Ce(oda)3]·9H2O (2) is isolated. Formation of a tri‐ammonium analogue of 2 can be achieved by neutralisation of an aqueous solution of CeCl3·7H2O and H2(oda) in a 1:3 ratio by NH4OH, giving (NH4)3[Ce(oda)3]·7H2O (3). Use of the cerium(IV) reagent (NH4)2[Ce(NO3)6] with Na2(oda) results in reduction to cerium(III) under ambient conditions and isolation of 1. However, in the absence of light this reaction yields crystals of the novel cerium(IV) heterobimetallic [Ce(oda)3Na4(NO3)2] (4). Each of these complexes exhibit a 3‐D network structure having a common nine‐coordinate [Ce(oda)3]n— (n = 2 or 3) subunit, irrespective of the oxidation state of cerium. In 1, six [Ce(oda)3]3— anions are connected, through bridging bidentate carboxylates, to a second Ce3+ site further coordinated by three water molecules. In contrast, the ammonium salt 2, displays isolated [Ce(oda)3]3— anions, devoid of further carboxylate bonding, but enmeshed within a network of hydrogen‐bonded NH4+ cations and water molecules. The remarkable structure of 4 consists of infinite 2‐D sheets of [Na2(NO3)]+ pillared by [Ce(oda)3]2— units, the connectivity arising by multidentate nitrate and carboxylate bridging.

Bond Valence Sums in Coordination Chemistry. The Calculation of the Oxidation State of Cerium in Complexes Containing Cerium Bonded Only to Oxygen

A total of 119 CeO(n)fragments with n = 3-12 were analyzed by using the bond valence sum, or BVS, method to yield new R(0) values for Ce(III)-O of 2.121(13) A and for Ce(IV)-O of 2.068(12) A. These R(0) values can be used to calculate the oxidation state of Ce in complexes where Ce is bonded only to O ligands. Furthermore, the average R(0) value of 2.094 A gives a good indication of whether the oxidation state of the Ce ion is +3 or +4 from the observed distances without any assumptions. The fact that complexes with coordination numbers of 10-12 are in agreement is significant since this study is the first example which indicates that high coordination numbers also follow BVS rules. The Ce-O distances used in deriving the R(0) values for the +3 and +4 complexes are tabulated as a function of coordination number and have a wide range of values, but the average Ce-O distance reflects the requirement that the BVS must equal the oxidation state. Several examples are given where the oxidation state of the Ce ion is apparently incorrectly assigned, as well as cases where problems with the X-ray structure determinations are indicated by a disagreement between the postulated and calculated oxidation state.

Determination of the oxidation state of cerium in rocks by Ce L III-edge X-ray absorption near-edge structure spectroscopy

Details of the determination of the Ce(IV)/Ce(III) ratio by Ce L III-edge X-ray absorption near-edge structure spectroscopy are given. Cerium(III) shows a single peak in the XANES region while a distinct doublet was observed for Ce(IV) species. Each spectrum can be fitted by a combination of Arctangent and Lorentzian functions. A linear combination of XANES spectra for Ce(III) and Ce(IV) species can simulate XANES spectra of Ce in natural rocks. The peak area ratio of the Lorentzian functions for the Ce(IV) and Ce(III) species is correlated with the mole ratio of Ce(IV) to Ce(III). This relation was used to determine Ce(IV)/Ce(III) ratios in natural rocks and Ce sorbed onto α-Fe 2O 3 and δ-MnO 2 prepared in the laboratory. The determined Ce(IV)/Ce(III) ratios were compared with the degree of Ce anomaly found in rare earth element (REE) patterns, their relative abundance of REE being normalized by appropriate reference material. The observed Ce(IV)/Ce(III) ratios determined by XANES were consistent with the degree of Ce anomalies in REE partitioning patterns between water and α-Fe 2O 3 or δ-MnO 2, suggesting the validity of the present method. The established method was applied to the determination of the Ce(IV)/Ce(III) ratio in the apatite fossil, conodont. The degree of Ce anomaly of conodont has been regarded as providing a record of palaeoredox variation in ancient oceans. It is expected that the Ce anomaly coupled with the Ce(IV)/Ce(III) ratio will provide useful geochemical information such as elucidation of palaeoredox by its application to conodont.

Characterization of cerium-based conversion coatings for corrosion protection of aluminum alloys

Cerium-based conversion coatings were formed by a spontaneous reaction between a water-based solution containing CeCl 3 and aluminum alloy 7075-T6 substrates. Coating performance was evaluated in neutral salt fog according to ASTM B117. Coating microstructure and thickness were observed by scanning electron microscopy (SEM). Coating composition and the cerium oxidation state were characterized using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) respectively. The morphology and salt fog performance of cerium conversion coatings were affected by pre-treatment of the panel prior to coating. The best pre-treatment consisted of desmutting, degreasing, and acid activation. After immersion in the coating solution for 30 s, Ce-rich deposits formed on the 7075 surface. After 5 min, coatings consisted of Ce-rich particles in a Ce-containing matrix. Immersion times of 5 min or longer produced coatings that could pass published military requirements for conversion coating performance in neutral salt fog. XPS analysis showed that the coatings contained Ce 4+.

Ensuring complete absence of Ce(IV) and measurement of the stability constant of the fluoride complex of Ce(III)

Literature survey revealed a wide variation in the measured stability constant values of the aqueous fluoride complexes of trivalent cerium. This could be due to inadequate care for full conversion and maintenance of the oxidation state of cerium to trivalent state. In the present work quinhydrone has been used to ensure complete absence of Ce(IV) and the stability constant of CeF 2+ in 1 M NaClO 4 has been measured potentiometrically using a fluoride ion selective electrode. Log β 1 obtained in this work was 2.936±0.024 and fitted well with the general trend of stability constants of the lanthanide (rare earths) fluorides in aqueous solution.

Optical properties of cerium and europium doped fluoroaluminate glasses

Fluoroaluminate glasses with the composition 36.5 AlF 3:13.5 YF 3:20 CaF 2:10 MgF 2:10 SrF 2:10 BaF 2 were doped with cerium and europium under various melting conditions. Depending on the melting conditions, different oxidation states of cerium and europium were obtained in the glasses. Using UV-visible absorption and fluorescence spectroscopy, these glasses were characterized. Cerium doped fluoroaluminate glasses had broad UV emission but exhibited apparent concentration quenching after concentrations of less than 1 mol% CeF 4. Europium doped glasses exhibited a broad UV/blue emission due to Eu 2+ and while concentration quenching was not observed with increased levels of EuF 3, the ratio of Eu 2+/Eu 3+ decreased with increasing total europium content. The apparent obeyance of the tarnishing model indicates that the Eu 2+/Eu 3+ ratio might be increased by increased exposure to H 2.