Determination of cerium(III) and cerium(IV) in nanodisperse ceria by chemical methods

Percentages of different valence cerium species have been determined in powdery samples, redispersible compositions, and aqueous sols of nanodisperse ceria prepared from cerium(IV) and cerium(III) salts by various methods with or without organic stabilizers. Cerium(III) is shown to be virtually absent in nearly all of the CeO₂ samples studied. Organic stabilizers are shown to be capable of reducing cerium(IV) in aqueous CeO₂ sols.     

Metal–metal charge transfer and interfacial charge transfer mechanism for the visible light photocatalytic activity of cerium and nitrogen co-doped TiO2

Nanosized cerium and nitrogen co-doped TiO₂ (Ce–TiO_2?xN_x) was synthesized by sol gel method and characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), FESEM, Fourier transform infrared, N₂ adsorption and desorption methods, photoluminescence and ultraviolet–visible (UV–vis) DRS techniques. PXRD analysis shows the dopant decreases the crystallite sizes and slows the crystallization of the titania matrix. XPS confirm the existence of cerium ion in +3 or +4 state, and nitrogen in ?3 state in Ce–TiO_2?xN_x. The modified surface of TiO₂ provides highly active sites for the dyes at the periphery of the Ce–O–Ti interface and also inhibits Ce particles from sintering. UV–visible DRS studies show that the metal–metal charge transfer (MMCT) of Ti/Ce assembly (Ti⁴⁺/Ce³⁺ → Ti³⁺/Ce⁴⁺) is responsible for the visible light photocatalytic activity. Photoluminescence was used to determine the effect of cerium ion on the electron–hole pair separation between the two interfaces Ce–TiO_2?xN_x and Ce₂O₃. This separation increases with the increase of cerium and nitrogen ion concentrations of doped samples. The degradation kinetics of methylene blue and methyl violet dyes in the presence of sol gel TiO₂, Ce–TiO_2?xN_x and commercial Degussa P25 was determined. The higher visible light activity of Ce–TiO_2?xN_x was due to the participation of MMCT and interfacial charge transfer mechanism.     

Thermodynamics of the formation of cerium(IV) malonate complex and the kinetics of its redox decomposition

Thermodynamic and kinetic characteristics of cerium(IV) malonate complex formed in the first stage of cerium(IV) oxidation by malonic acid H₂Mal are studied using a spectrophotometer, a photometer, and a pH-meter at a ionic strength of I = 2 in the pH region of 0.3–1.6 in a sulfuric acid medium at a temperature of 296.8 K. Its composition is found to be CeOHMal⁺. The form of organic ligand is Mal^2?; the thermodynamic parameters of its formation and kinetic parameters of its intramolecular redox decomposition are determined. The most likely scheme of the initial stages of redox proceeding in the Ce⁴⁺–SO ₄ ^2- –H₂Mal system is discussed, and a quantitative model of it is proposed.     

The use of cerium(IV) phosphate for the gravimetric determination and separation of cerium

A method for the gravimetric determination of cerium as Ce₃(PO₄)₄ is described. Cerium can be separated from many metals in this form, as well as from permanganate and dichromate; the cerium separated can then be titrated with iron(II) solution. The method was verified for the determination of cerium in a rare earth concentrate.     

Quantitative Separation of Pd(II) by TLC of Metal Ions on Cerium (IV) Antimonate

Abstract

A quantitative TLC separation of metal ions using cerium (IV) antimonate as the solid phase are described. Using an acetonitrile- HNO₃ solvent Pb²⁺ can be separated quantitatively (2–10 μg) from several other ions. A quantitative spectrophotometric assay using p-nitroso-dimethylaniline is proposed.     

Determination of traces of lanthanides in cerium oxide by liquid chromatography with on-column preconcentration

Summary Traces of lanthanides can be effectively separated from other lanthanides in major concentrations and other elements by liquid chromatography on a column of Separon SGX C18 silica gel modified by sorption of ammonium dodecylsulphate or ammonium octanesulphonate and determined spectrophotometrically with Arsenazo III. Isocratic or stepwise acidity gradient elution with 90 mmol/l of ammonium 2-hydroxy-2-methylpropionate (HIBA) or 60 mmol/l of ammonium citrate was used. Interferences by the elements up to 10³–10⁴ concentration ratio can be eliminated. Lanthanides at mg/g level can be determined in cerium oxide with a relative standard deviation s_r<2.5% (n=5).     

Ruthenium(III)‐mediated oxidation of D‐mannitol by cerium(IV) in aqueous sulfuric acid medium: A kinetic and mechanistic approach

The oxidation of D ‐mannitol by cerium(IV) has been studied spectrophotometrically in aqueous sulfuric acid medium at 25°C at constant ionic strength of 1.60 mol dm^?3. A microamount of ruthenium(III) (10^?6 mol dm^?3) is sufficient to enhance the slow reaction between D ‐mannitol and cerium(IV). The oxidation products were identified by spot test, IR and GC‐MS spectra. The stoichiometry is 1:4, i.e., [D ‐mannitol]: [Ce(IV)] = 1:4. The reaction is first order in both cerium(IV) and ruthenium(III) concentrations. The order with respect to D ‐mannitol concentration varies from first order to zero order as the D ‐mannitol concentration increases. Increase in the sulfuric acid concentration decreases the reaction rate. The added sulfate and bisulfate decreases the rate of reaction. The active species of oxidant and catalyst are Ce(SO₄)₂ and [Ru(H₂O)₆]³⁺, respectively. A possible mechanism is proposed. The activation parameters are determined with respect to a slow step and reaction constants involved have been determined. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 440–452, 2010     

Trivalent‐Intermediate Valent Cerium Ordering in Ce2RuZn4

The new intermetallic cerium compound Ce₂RuZn₄ was synthesized from the elements in a sealed tantalum tube in a water‐cooled sample chamber of an induction furnace. Ce₂RuZn₄ crystallizes with a new structure type: P4/nmm, Z = 2, a = 719.6(1), c = 520.2(1) pm, wR2 = 0.0816, 273 F² values and 15 variables. The structure contains two crystallographically independent cerium atoms: Ce1 with CN 16 (12 Zn + 4 Ce) and Ce2 with CN 14 (2 Ru + 8 Zn + 4 Ce). Based on the interatomic distances the two sites can be assigned to trivalent Ce1 and intermediate valent Ce2. The trivalent‐intermediate valent cerium ordering is underlined by magnetic susceptibility measurements. Ce₂RuZn₄ shows modified Curie‐Weiss behaviour in the temperature range 10–290 K with an experimental magnetic moment of 2.57(1) μ_B per formula unit. Thus only half of the cerium atoms are trivalent in Ce₂RuZn₄. A remarkable feature of the Ce₂RuZn₄ structure are short Ce2–Ru distances of 260 pm. The crystal chemistry of Ce₂RuZn₄ is discussed.     

Manganese(II) catalysed oxidation of glycerol by cerium(IV) in aqueous sulphuric acid medium: a kinetic and mechanistic study

The manganese(II) catalysed oxidation of glycerol by cerium(IV) in aqueous sulphuric acid has been studied spectrophotometrically at 25 °C and I = 1.60 mol dm⁻³. Stoichiometry analysis shows that one mole of glycerol reacts with two moles of cerium(IV) to give cerium(III) and glycolic aldehyde. The reaction is first order in both cerium(IV) and manganese(II), and the order with respect to glycerol concentration varies from first to zero order as the glycerol concentration increases. Increase in sulphuric acid concentration, added sulphate and bisulphate all decrease the rate. Added cerium(III) retards the rate of reaction, whereas glycolic aldehyde had no effect. The active species of oxidant and catalyst are Ce(SO₄)₂ and [Mn(H₂O)₄]²⁺. A mechanism is proposed, and the reaction constants and activation parameters have been determined.     

Separation of rare earth elements in the tributyl phosphate–Ln(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>–Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript> system in the counter current process

The 40-step extraction process to separate rare earth elements (RЕEs) according to the praseodymium–cerium line with the use of mixer–settler extractors in a 100% TBP–Ln(NO₃)₃–Ca(NO₃)₂ system is implemented. A lanthanum–cerium concentrate containing less than 0.03 wt % of the remaining REEs is obtained. The flow diagram of the separation process of a rare earth (RE) concentrate isolated from phosphogypsum is considered.     

Nature of influence exerted by Na<Subscript>2</Subscript>SiF<Subscript>6</Subscript> on REE recovery from orthophosphoric acid solution in the course of CaSO<Subscript>4</Subscript>·0.5H<Subscript>2</Subscript>O crystallization

Chemical and X-ray phase analyses were used to study the influence exerted by Na₂SiF₆ on the isomorphic inclusion of cerium into the structure of CaSO₄?0.5H₂O precipitates formed from solutions of phosphoric acid hemihydrate (38 wt % P₂O₅). The poorly soluble suspensions of CaSO₄?0.5H₂O precipitates can serve as adsorbents for cerium compounds, with CaSO₄?0.5H₂O–NaCe(SO₄)₂?H₂O and CaSO₄?0.5H₂O–CePO₄?0.5H₂O solid solutions formed. The introduction of Na₂SiF₆ makes the sorption properties CaSO4?0.5H2O several times better because the Na₂SiF₆ phase is a source of sodium cations and creates the necessary Na: Ce ratio of 1: 1 for extracting cerium from the liquid phase into a precipitate in the form of a CaSO₄?0.5H₂O–[NaCe(SO₄)₂?H₂O + CePO₄?0.5H₂O] solid solution. Under the industrial conditions in which extraction phosphoric acid is manufactured, a similar isomorphous capture of rare-earth elements of the cerium group (La–Sm) may occur in joint precipitation of CaSO₄?0.5H₂O and Na₂SiF₆.     

Separation and determination of rhenium with 5,6-diphenyl-2,3-dihydro(asym)triazine-3-thione

5,6-DiphenyI-2,3-dihydro(asym)triazine-3-thione can be used as a gravimetric reagent for the determination of rhenium (4–50 mg) in the presence of 0.7–1.7 M hydrochloric acid and tin(II) chloride. The composition of the dried complex, which can be weighed directly, is [ReO(C₁₅H₁₀N₃S)₂]₂. The method is simple and rhenium can be determined in the presence of a number of foreign ions.     

Thermal decomposition of cerium(III) perchlorate

Thermal decomposition of Ce(ClO₄)₃ ? 9H₂O and Ce(ClO₄)₃ to give cerium(IV) dioxide in the temperature range 240–460°C was studied by DSC–TGA, X-ray powder diffraction, IR and mass spectroscopy. The thermolysis of these salts was shown to proceed through the stage of formation of intermediate product supposedly cerium oxoperchlorate. The thermal decomposition of cerium(III) perchlorate hydrate at 460°C leads to formation of nanocrystalline cerium dioxide with particle size of 13 nm.     

Trivalent‐Intermediate Valent Cerium Ordering in CeRuSn – A Static Intermediate Valent Cerium Compound with a Superstructure of the CeCoAl Type

New equiatomic stannide CeRuSn was synthesized from the elements by arc‐melting. CeRuSn was investigated by X‐ray powder and single crystal diffraction: C2/m, a = 1156.1(4), b = 475.9(2) and c = 1023.3(4) pm, β = 102.89(3)°, wR2 = 0.0466, 1229 F² values and 38 variables. CeRuSn adopts a superstructure of the monoclinic CeCoAl type through a doubling of the subcell c axis. In the superstructure two crystallographically independent cerium sites occur. Based on the interatomic distances the two sites can be assigned to trivalent Ce2 and intermediate valent Ce1. This trivalent‐intermediate valent cerium ordering is underlined by magnetic susceptibility measurements χ_m(T): below 150 K χ_m, measured with decreasing temperature, follows a Curie‐Weiss law χ_m = C_m/(T–θ_p) giving C_m = 0.38 emuK/mol as Curie constant per CeRuSn mol; a value showing that only half of the cerium atoms are trivalent in CeRuSn (C_m = 0.807 emuK/mol for one free Ce³⁺ ion). A remarkable feature of the CeRuSn structure are the short Ce1–Ru1 (233 pm) and Ce1–Ru2 (246 pm) distances. The crystal chemistry of CeRuSn is discussed on the basis of a group‐subgroup scheme.     

Thermodynamic and Kinetic Parameters of Cerium(IV) Complexes with Some Dicarboxylic Acids

The thermodynamic and kinetic parameters of the cerium(IV) complexes formed in the initial stage of oxidation of dicarboxylic acids (H₂L), like pentanedioic, butanedioic, propanedioic, and ethanedioic acids, by cerium(IV) sulfate were studied by the spectrophotometric and pH-potentiometric methods with the aid of integral kinetic methods at an ionic strength I = 2 mol/L within the pH range of–0.3–1.6 in a sulfuric acid medium and at temperature of 293.15 K. The composition of these complexes, the form of organic ligand existence therein, the thermodynamic parameters of their formation, and the kinetic parameters of their intramolecular redox decomposition were determined. Linear correlations between the found thermodynamic and kinetic parameters of the examined complexes [CeOHL]⁺ were obtained. The rate equation of the redox process occurring in the systems Ce⁴⁺–H₂L was established and the corresponding reaction model was considered.     

Valence fluctuations,crystal structure stability and the effects of alloying

The systematics of the effects of alloying on the valence of cerium are discussed. In addition to factors such as the size and valence of the substitute atom and resulting “lattice pressure” and motion of the f level, the possible role of crystal structure stability is examined. In particular, a detailed analysis of both on cerium site and off cerium site alloying of CePd₃ suggests that crystal structure stability considerations are of over-riding importance for this system. For the Cu₃Au structure, the existence of a maximum number N_max of electrons per formula unit, which is determined from studies of Zr(Rh,Pd)₃, allows one correctly to predict the concentrations at which trivalence and saturated valence occur in six related pseudobinary alloy systems. However, these predictions are based on a model which requires:

• 1.(a) that in the mixed valence regime the valence of cerium adjusts so that the total number of electrons per cell is constant at N_max, and
• 2.(b) that cerium be tetravalent in CeRh₃.

The latter requirement is in strong disagreement with studies using microscopic probes such as L_III, absorption, X-ray photoelectron spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS). Nonetheless, the predictive ability of the model is striking and cannot be dismissed as merely fortuitous.     

Einkristalle des Cer(III)‐Borosilicats Ce3[BSiO6][SiO4]

Single Crystals of the Cerium(III) Borosilicate Ce₃[BSiO₆][SiO₄] Colorless, lath‐shaped single crystals of Ce₃[BSiO₆]‐ [SiO₄] (orthorhombic, Pbca; a = 990.07(6), b = 720.36(4), c = 2329.2(2) pm, Z = 8) were obtained in attempts to synthesize fluoride borates with trivalent cerium in evacuated silica tubes by reaction of educt mixtures of elemental cerium, cerium dioxide, cerium trifluoride, and boron sesquioxide (Ce, CeO₂, CeF₃, B₂O₃; molar ratio 3 : 1 : 3 : 3) in fluxing CsCl (700 °C, 7 d) with the glass wall. The crystal structure contains eight‐ (Ce1) and ninefold coordinated Ce³⁺ cations (Ce2 and Ce3) surrounded by oxygen atoms. Charge balance is achieved by both discrete borosilicate ([BSiO₆]^5– ≡ [O₂BOSiO₃]^5–) and ortho‐silicate anions ([SiO₄]^4–). The former consists of a [BO₃] triangle linked to a [SiO₄] tetrahedron by a single vertex. The anions form layers in [001] direction alternatingly built up from [BSiO₆]^5– and [SiO₄]^4– groups while Ce³⁺ cations are located in between.     

X-ray diffraction and thermodynamic characteristics for tellurite of the composition Li<Subscript>2</Subscript>CeTeO<Subscript>5</Subscript>

Tellurite of the composition Li₂CeTeO₅ is synthesized by solid-phase method from cerium(IV) and tellurium(IV) oxides and lithium carbonate. The type of syngony, the unit cell parameters, and the compound’s X-ray and pycnometry densities are determined via X-ray diffraction analysis. The isobaric heat capacity of lithium–cerium tellurite is studied by means of dynamic calorimetry in the temperature range of 298.15–673 K; the results serve as the basis for deriving C _p ^° ~ f(T) dependency equations and determining the compound’s thermodynamic functions. λ-shaped anomalous effects, due probably to Type II phase transitions, are found on the C _p ^° ~ f(T) dependence.     

A Combined Experimental and Theoretical Study of the Versatile Reactivity of an Oxocerium(IV) Complex: Concerted Versus Reductive Addition

A combined experimental and theoretical investigation on the cerium(IV) oxo complex [(L_OEt)₂Ce(=O)(H₂O)] ⋅ MeC(O)NH₂ ( 1 ; L_OEt⁻=[Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]⁻) demonstrates that the intermediate spin-state nature of the ground state of the cerium complex is responsible for the versatility of its reactivity towards small molecules such as CO, CO₂, SO₂, and NO. CASSCF calculations together with magnetic susceptibility measurements indicate that the ground state of the cerium complex is of multiconfigurational character and comprised of 74 % of Ce^IV and 26 % of Ce^III. The latter is found to be responsible for its reductive addition behavior towards CO, SO₂, and NO. This is the first report to date on the influence of the multiconfigurational ground state on the reactivity of a metal–oxo complex.     

Cerium-based catalysts for N-oxidation of pyridine-based derivatives: homogeneous and heterogeneous systems

Herein, the catalytic properties of the cerium (IV) salt, cerium (IV)-sandwiched polyoxometalate (POM) and cerium (IV)-sandwiched polyoxometalate intercalated in layered double hydroxides (LDHs) in the H₂O₂-based green oxidation reactions have been evaluated. These cerium (IV)-based systems were applied as homogeneous and heterogeneous catalysts for the oxidation of pyridines. Despite the fact that the cerium (IV)-sandwiched polyoxometalate as a homogeneous reaction system gives good results, there are some disadvantages in recovery and reusability process. To overcome these problems, new nano catalyst was synthesized by intercalation of the Cerium (IV)-sandwiched polyoxometalate into tris(hydroxymethyl) aminomethane-modified layered double hydroxides (Tris-LDH-CO₃). The as-prepared nanocomposite was characterized and used as an effective heterogeneous catalyst for the oxidation of pyridines under mild conditions in the presence of H₂O₂ as an oxidant. The new heterogeneous nanocomposite can be recovered and reused easily from the reaction media at least ten times without significant decrease in catalytic activity.

Open image in new window