Mixed-valence Cerium Research Articles

Synthesis of Nanoceria with Varied Ratios of Ce3+/Ce4+ Utilizing Soluble Borate Glass.

Mixed-valence cerium oxide nanoparticles (nanoceria) have been investigated with pronounced interest due to a wide range of biomedical and industrial applications that arises from its remarkable redox catalytic properties. However, there is no understanding of how to control the formation of these two types of nanoceria to obtain Ce3+/Ce4+ ratios required in various applications. In this work, using a soluble borate glass, nanoceria with specific ratios of Ce3+/Ce4+ are created and extracted via controlled glass-melting parameters. Glass embedded with nanoceria as well as nanoceria extracted from the glass were studied via XANES and fitted with the Multivariate Curve Resolution (MCR) technique to calculate the ratio of Ce3+/Ce4+. Results show that mixed-valence nanoceria with specific ratios are hermetically sealed within the glass for long durations. When the glass dissolves, the mixed-valence nanoceria are released, and the extracted nanoceria have unchanged Ce3+/Ce4+ ratios. Furthermore, TEM investigation on released nanoceria show that the nanoceria consist of several different structures. Although nanocrystal structures of Ce7O12, Ce11O20, and Ce2O3 contribute to the reduced state, a new quasi-stable phase of CeO1.66 has been observed as well.

Oxygen Vacancy-Rich Mixed-Valence Cerium MOF: An Efficient Separator Coating to High-Performance Lithium-Sulfur Batteries.

Mixed-valence metal-organic frameworks (MOFs) have exhibited unique potential in fields such as catalysis and gas separation. However, it is still an open challenge to prepare mixed-valence MOFs with isolated Ce(IV, III) arrays due to the easy formation of CeIII under the synthetic conditions for MOFs. Meanwhile, the performance of Li-S batteries is greatly limited by the fatal shuttle effect and the slow transmission rate of Li+ caused by the inherent characteristics of sulfur species. Here, we report a mixed-valence cerium MOF, named CSUST-1 (CSUST stands for Changsha University of Science and Technology), with isolated Ce(IV, III) arrays and abundant oxygen vacancies (OVs), synthesized as guided by the facile and elaborate kinetic stability study of UiO-66(Ce), to work as an efficient separator coating for circumventing both issues at the same time. Benefiting from the synergistic function of the Ce(IV, III) arrays (redox couples), the abundant OVs, and the open Ce sites within CSUST-1, the CSUST-1/CNT composite, as a separator coating material in the Li-S battery, can remarkably accelerate the redox kinetics of the polysulfides and the Li+ transportation. Consequently, the Li-S cell with the CSUST-1/CNT-coated separator exhibited a high initial specific capacity of 1468 mA h/g at 0.1 C and maintained long-term stability for a capacity of 538 mA h/g after 1200 cycles at 2 C with a decay rate of only 0.037% per cycle. Even at a high sulfur loading of 8 mg/cm2, the cell with the CSUST/CNT-coated separator still demonstrated excellent performance with an initial areal capacity of 8.7 mA h/cm2 at 0.1 C and retained the areal capacity of 6.1 mA h/cm2 after 60 cycles.

Coordination-Driven Self-Assembly of a 2D Graphite-Like Framework Constructed from High-Nuclear Ce10 Cluster Encapsulated Polyoxotungstates.

It is challenging to explore and prepare high-nuclear lanthanide (Ln) cluster-encapsulated polyoxometalates (POMs). Herein, we fabricate an unprecedented Ce10-cluster-embedded polyoxotungstate (POT) (TMA)14H2[Ce(III)(H2O)6]{[Ce(IV)7Ce(III)3O6(OH)6(CO3)(H2O)11][(P2W16O59)]3}·41H2O (1) (TMA = tetramethyleneamine) by coordination-driven self-assembly strategy, which contains the largest Ce cluster [Ce(IV)7Ce(III)3O6(OH)6(CO3)(H2O)11] (Ce10) in all the Ln-containing POM chemistry to date. Self-assembly of the in situ dilacunary [P2W16O59](12-) fragments and mixed Ce(3+) and Ce(4+) ions by means of coordination-driven force results in a novel 2D graphite-like framework constructed from mixed-valent cerium(III/IV) cluster {Ce10} encapsulated poly(POT) units and Ce(3+) ions. The most remarkable feature is that the skeleton of the centrosymmetric Ce10-cluster-embedded POT trimer contains three dilacunary [P2W16O59](12-) fragments trapping a novel {Ce10} cluster via 18 terminal-oxo and three μ4-oxo atoms.

Facile green synthesis of palladium quantum dots@carbon on mixed valence cerium oxide/graphene hybrid nanostructured bifunctional catalyst for electrocatalysis of alcohol and water

Electrocatalytic oxidation of ethylene glycol at Pd quantum dots@C–CeOx/RGO electrode.

Antiferromagnetic ordering in a mixed-valent cerium compound CeRuSn

We report the results of extensive neutron powder diffraction experiments on a ternary compound CeRuSn in a temperature range from 2 to 520 K. At room temperature CeRuSn orders in a monoclinic structure that is related to the CeCoAl type by the doubling of the $c$-axis parameter caused by the splitting of the original single sites for each element into two sites. Upon cooling a very broad hysteretic structural transition has been observed in CeRuSn leading to the presence of higher-ordered variants having multiple $c$-axis parameters. Depending on the temperatures, one or the other variant seems to be more stable, but the sample as a whole becomes multiphase. We find also evidence for further symmetry lowering at low temperatures. CeRuSn is found to exhibit an antiferromagnetic order below ${T}_{N}=2.8(1)$ K. The order is described by two incommensurate propagation vectors ${q}_{1}=(0,0,\ensuremath{\approx}0.30)$ and ${q}_{2}=(0,0,\ensuremath{\approx}0.43)$ with respect to the doubled crystal structure variant. Although the details of the magnetic structure remain elusive, the best agreement has been found for a complicated distorted cycloidal magnetic structure with Ce moments of $\ensuremath{\approx}$0.7 ${\ensuremath{\mu}}_{B}$ being confined to the $ac$ plane.

Solid-state 119Sn NMR and M¨ossbauer Spectroscopic Studies of the Intermediate-valent Stannide CeRuSn

The ternary stannide CeRuSn is a static mixed-valent cerium compound with an ordering of trivalent and intermediate-valent cerium on two distinct crystallographic sites. 119Sn Mössbauer spectra showed two electronically almost identical tin atoms at 323 K, while at 298 K and below (77 and 4:2 K) two tin sites can clearly be distinguished. Solid-state 119Sn NMR experiments were performed to probe the local hyperfine fields at the two different Sn sites. Powder 119Sn NMR spectra are nicely fitted with two Sn sites with nearly the same magnetic anisotropy, but with different absolute shift values. Both Sn sites are strongly affected by crossover-like transitions between 100 and 280 K. This local-site study confirms the superstructure modulations found in previous investigations. Towards lower temperatures the powder NMR spectra are broadened giving strong precursor evidence for the antiferromagnetically ordered ground state.

Cerium oxide nanoclusters: commensurate with concepts of polyoxometalate chemistry?

The mixed-valent cerium(III/IV) oxide clusters {Ce(10)} and {Ce(22)}, derived from condensation reactions of cerium carboxylate coordination polymers, exhibit molecular growth tendencies similar to those of 'classical' group V and VI polyoxometalates, but with increasing nuclearity approach the structure of the parental oxide, CeO(2).

Pseudotetragonal Structure of Li2+xCex3+Ce12−x4+F50: The First Mixed Valence Cerium Fluoride

The crystal structure of the new Li(5.5)Ce(12)F(50) compound has been fully characterized by single-crystal and synchrotron powder X-ray diffraction. An accurate pseudotetragonal structure was described in the monoclinic P2(1) space group with 68 independent crystallographic sites. The Li(5.5)Ce(12)F(50) composition belongs to the Li(2+x)Ce(x)(3+)Ce(12-x)(4+)F(50) solid solution. Its structure consists of an opened fluorine framework where a channel network allows the intercalation of relatively mobile lithium cations, inducing the formation of the mixed-valence cerium (the intercalation of Li(+) leads to the reduction of a part of Ce(4+) to Ce(3+)). One part of the lithium ions, necessary for the electroneutrality of the tetravalent equivalent cerium fluoride (Li(2)Ce(12)F(50) composition), is in a locked fluorine polyhedron. Only the supplementary x amount of lithium is able to be exchanged in Li(2+x)Ce(x)(3+)Ce(12-x)(4+)F(50). The structure of Li(2+x)Ce(x)(3+)Ce(12-x)(4+)F(50) is a rearrangement, due to lithium intercalation, of the base CeF(4) structure. Bond valence calculation on Ce sites, Ce coordination polyhedra volumes, and a calculated Ce cationic radius give the indication of a partial long-range ordering of trivalent and tetravalent cerium cations in specific slabs of the structure. (7)Li NMR spectroscopy and XPS analyses have confirmed all of the structure details.

New Open‐Framework Mixed‐Valence Chromium(III) Cerium(III)/(IV) Sulfate: CrCeIII7CeIV6 (HSO4)6 (SO4)21×75H2O.

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Hydrogen absorption in CeNiAl: Formation of crystalline and amorphous hydride phases

Hydriding studies on mixed valent intermetallic compound, CeNiAl, have been carried out. Absorption as well as desorption routes have been employed to find the composition range of various hydride phases. Stable hydride composition, CeNiAlH x, with a maximum value of x = 3.2, is obtained. No single phase crystalline hydride could be stabilized for x < 1.9. Further, it is found that any attempt to stabilize composition(s) with x < 1.9, by desorbing hydrogen from higher hydrides, results in the formation of an amorphous hydride phase. Hydrogen induced amorphization is being reported for the first time in Fe 2P structure. X-ray diffraction studies of the hydride with x = 3.2, as well as neutron diffraction studies on a deuteride of the same composition show that the hydriding (deuteriding) results in transformation from the Fe 2P type hexagonal (space group P 6 ¯ 2 m ) to AlB 2 type hexagonal (space group P6/ mmm) structure. Structural parameters and D atom occupancies are derived from the neutron diffraction measurements on CeNiAlD 3.2. Magnetization measurements show that the structural transformation is also accompanied by a transformation from the mixed valent cerium in the parent compound to trivalent Ce in the hydride phases.

Resonant inelastic X-ray scattering from highly correlated Ce multilayers

The 2p 6→2p 5(4f5d) +1→2p 63d 9(4f5d) +1 resonant inelastic X-ray scattering (RIXS) process is studied in [(5 Å) La/(10 Å) Ce/(5 Å) La/(30 Å) Fe]× n multilayers. RIXS is a bulk sensitive probe which, as shown here, provides information on the correlation energies in mixed-valent cerium systems.

The Fermi Surface of Itinerant 4f Electrons in CeNi

Based on the itinerant-electron model for the 4f electrons, the energy band structure and the Fermi surface is calculated for the mixed-valence cerium compound CeNi and its reference material LaNi by a self-consistent relativistic APW method with the exchange and correlation potential in the local-density approximation. Both CeNi and LaNi are compensated metals, and their hole and electron Fermi surfaces are multiply-connected sheets with open orbits, the direction of which is consistent with the high-field magnetoresistance. Though they are quite different from each other, the Fermi surfaces for CeNi and LaNi can explain reasonably well both the magnitude and the angle dependence of the de Haas-van Alphen effect frequency branches. These theoretical results suggest that the 4f electrons may be itinerant in the ground state in CeNi, as they are in CeSn 3 .

Resistivity anomalies of dilute Zr1−xRExAl2 alloys

Abstract In ZrAl2 the resistivity increment of stable lanthanum and praseodymium impurities is larger than that of mixed valent cerium impurities. All increments increase with increasing temperature. These results are discussed in terms of charge density contrast scattering and of a reduction in the local phonon frequencies on the rare earth impurities.

High pressure resistivity results on mixed valent systems in which magnetic order is important

We discuss high pressure resistivity results on mixed valent cerium and europium systems in which magnetic order plays an important role. In the CeMn 2Si 2-based systems the magnetic order is due to the manganese sublattice, and the cerium mixed valent/heavy fermion effects persist at least in the antiferromagnetically ordered phase of these systems. In the EuO system, the europium ferromagnetic order is dramatically enhanced precursive to a high pressure hybridization-gap-mixed-valent phase.

CeRhIn: A new mixed-valent cerium compound.

A new ternary equiatomic compound, CeRhIn, has been synthesized which is found to crystallize in the hexagonal ${\mathrm{Fe}}_{2}$P structure. The unit-cell volume of this compound deviates from that expected on the basis of lanthanide contraction, indicating the nontrivalent state of the Ce ions in this compound. The magnetic susceptibility of CeRhIn shows a broad maximum at about 150 K. Such a maximum is a characteristic feature of the mixed-valent cerium compounds. The resistivity of CeRhIn due to magnetic scattering also shows a broad maximum at about 140 K. Thus from lattice volume, susceptibility, and resistivity measurements, it is concluded that cerium ions are in a mixed-valent state in CeRhIn.

Correlation effects in the electronic structure and photoemission spectra of mixed-valence cerium compounds

The electronic ground states and core-hole and valence-hole excited states of so-called tetravalent and mixed-valence Ce compounds are studied taking into account strong correlations among $4f$ electrons. By an analysis of the core-level photoemission satellites, Ce${\mathrm{O}}_{2}$, which has been thought to be tetravalent, is identified as a mixed-valence system with $\ensuremath{\sim}0.6 4f$ electron, where the magnetic moment of the $4{f}^{1}$ configuration is quenched via singlet coupling between the $4f$ electron and the O $2p$ hole. $4f$-derived satellite features in the resonant photoemission spectrum of Ce${\mathrm{O}}_{2}$ are calculated, which qualitatively explain resonant photoemission spectra of tetravalent Ce intermetallic compounds. The core-level and $4f$-derived photoemission spectra of mixed-valence CeN are also studied by considering only the $4f$-electron---N $2p$-hole coupling as in Ce${\mathrm{O}}_{2}$ and neglecting the $4f\ensuremath{-}5d$ mixing. It is also suggested that for the $\ensuremath{\alpha}$ phase of Ce metal the absence of a localized magnetic moment, volume collapse, and a $4{f}^{0}$ signal in the core-level photoemission spectra are explained by this type of configuration-mixing description.

A XANES study of mixed-valence transition-metal oxides and rare-earth alloys

Energies of the 1s → 3d. 1s → 4s and 1s → 4p transitions of Mn, Fe and Co in a series of oxides have been obtained from X-ray absorption near-edge structure (XANES). Bivalent metal oxides do not show separate features in XANES due to the two oxidation states of the metal, but the spectroscopic transition energies. $E_\gamma$: as well as the 3d-4s and 3d-4p energy differences. $\delta$E, increase smoothly with the oxidation number of the metal or its effective atomic charge, q. The variation of transition energies as well as $\delta$E values with q is best described by the parabolic relation. $E_\gamma (or \deltaE) = aq + bq^2$. Mixed-valence cerium and ytterbium alloys distinctly show transitions due to the valence states in the XANES.

3d and 4d core level X-ray photoelectron spectroscopy of mixed-valence CePd3

3d and 4d core-level XPS spectra for CePd3, a mixed-valence system, have been measured. Each spectrum exhibits two sets of structures, each corresponding to one of the valence states of cerium. Thus the usefulness of XPS, which has so far not been used extensively to investigate the mixed-valence cerium systems, is pointed out.