Sol–Gel Synthesis and Distinctive Structural Features of Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> Solid Solutions

We have synthesized Ce_xZr_1–xO₂ solid solutions via the thermal decomposition of xerogels of different compositions prepared by drying appropriate hydrosols. The synthesized materials have been characterized by thermal analysis, X-ray diffraction, and Raman spectroscopy. The results demonstrate that the solid solutions are formed at relatively low temperatures (450–600°C). The Ce_xZr_1–xO₂ samples with x = 0.5–0.9 consist of a cubic solid solution. At a lower CeO₂ content (x = 0.2), the material consists of a mixture of cubic and tetragonal phases.     

Nanostructured Ce1−xZrxO2 solid solutions produced by mechanochemical processing

The nanostructured Ce_1−xZr_xO₂ solid solutions (x ≤ 0.2) have been successfully synthesized from CeCl₃–ZrCl₄–NaOH mixtures by mechanochemical processing as a gradual transformation, involving in-situ CeO₂ and amorphous ZrO₂ formation as intermediates. Solid solutions type-Ce_1−xZr_xO₂ along with NaCl as diluent were obtained at different milling times, with a final composition of Ce_0.8Zr_0.2O₂ after 5 h and 15 h under high and low energetic milling conditions, respectively. The NaCl formed during the mechanochemical reaction, which is eliminated by washing after calcination of the as-milled sample, allows to obtain a Ce_0.8Zr_0.2O₂ solid solution with high surface area and nanometric grains. The nanostructured Ce_0.8Zr_0.2O₂ solid solution shows good thermal stability after prolonged heating at 600 °C. However, the addition of an extra amount of diluent during mechanochemical reaction evidences a detrimental effect, avoiding the formation of solid solution or deteriorating the textural/microstructuctural characteristics of the obtained Ce_1−xZr_xO₂ solid solution. Removal of NaCl previous to calcination improves notably the textural/microstructural characteristics of the Ce_0.8Zr_0.2O₂ solid solution.     

Microwave-induced combustion synthesis and electrical conductivity of Ce1−xGdxO2−1/2x ceramics

Ce_1−xGd_xO_2−1/2x nanopowder were successfully synthesized by microwave-induced combustion process. For the preparation, cerium nitrate, gadolinium nitrate hexahydrate, and urea were used for the microwave-induced combustion process. The process took only 30 min to obtain Ce_1−xGd_xO_2−1/2x powders. The exo-endo temperature, phase identification, and morphology of resultant powders were investigated by TG/DTA, XRD, and SEM. The as-received Ce_1−xGd_xO_2−1/2x powders showed that the average particle size ranged from 18 to 50 nm, crystallite dimension varied from 11 to 20 nm, and the specific surface area was distribution from 16 to 46 m²/g. As for Ce_1−xGd_xO_2−1/2x ceramics sintered at 1450 °C for 3 h, the bulk density of Ce_1−xGd_xO_2−1/2x ceramics were over 91% of the theoretical density, the maximum electrical conductivity, σ_700 °C = 0.017 S/cm with minimum activation energy, E_a = 0.869 eV was found at Ce_0.80Gd_0.20O_1.90 ceramic.     

Synthesis of monophasic Ce<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript> solid solution by microwave-induced combustion method

Nanocrystalline monophasic Ce_0.5Zr_0.5O₂ solid solution (1:1 molar ratio) has been synthesized by microwave-induced combustion method in a modified domestic microwave oven (2.45 GHz, 700 W) in approximately 40 min from cerium nitrate and zirconium nitrate precursors using urea as ignition fuel. For the purpose of better comparison, a Ce _x Zr_{1 − x} O₂ solid solution (1:1 molar ratio) was also synthesized by a conventional co-precipitation method from nitrate precursors and subjected to different calcination temperatures. The synthesized powders of both methods were characterized by means of X-ray powder diffraction, thermogravimetry/differential thermal analysis, scanning electron microscopy, and BET surface area techniques. Oxygen storage capacity (OSC) measurements were performed to understand the usefulness of these materials for various applications. The characterization results reveal that the sample obtained by microwave-induced combustion-synthesis route exhibits homogeneous monophasic Ce_0.5Zr_0.5O₂ solid solution whereas the co-precipitated sample displays compositional heterogeneity. The OSC measurements reveal that the materials synthesized by both methods exhibit comparable oxygen vacancy content (δ).     

The synthesis of mesoporous Ce1?xZrxO2 by modified evaporation-induced self-assembly method

Mesoporous Ce_1−x Zr _x O₂ with high surface area was synthesized using a modified evaporation-induced self-assembly method that combined citric acid as complexing agent and cetyltrimethyl ammonium bromide as surfactant. The samples with different Ce/Zr molar ratio were characterized by thermogravimetry and differential thermal analysis, X-ray diffraction, transmission electron microscopy, selected area electron diffraction, Brunauer–Emmett–Teller (BET), and Barrett–Joyner–Halenda methods. It was found that when the Zr molar fraction was larger than 0.3, a mixture of cubic phase and tetragonal phase was formed. Ce_0.7Zr_0.3O₂ solid solution had the largest BET surface area (217 m² g⁻¹) and mesoporous structure. The catalytic performances of mesoporous Ce_1−x Zr _x O₂ for CO oxidation were examined. Mesoporous Ce_0.7Zr_0.3O₂ solid solution demonstrated the best catalytic activity due to the high surface area and an enhanced redox property caused by appropriate Zr⁴⁺ incorporation.     

Thermal stability and photoluminescence of Zr1−xCexO2 (0 ≤ x ≤ 1) nanoparticles synthesized in a non-aqueous process

Complete range Zr_1−xCe_xO₂ (0 ≤ x ≤ 1) nanoparticles are synthesized by the thermal decomposition of metal-organic precursors (zirconium acetylacetonate and cerium acetylacetonate) in oleylamine. XRD and HRTEM indicate that all of the as-prepared nanoparticles are single-crystal, and the crystallinity becomes better with increasing Ce content. The Zr_1−xCe_xO₂ nanoparticles with Ce content larger than 1/8 crystallize in cubic fluorite phase. XRD measurements on the as-prepared and calcinated ZrO₂ samples reveal that the tetragonal ZrO₂ nanoparticles are stable below 600 °C, and the lattice parameters of ZrO₂ nanoparticles decrease with decreasing particle size. The thermal stability of the cubic phase increases with increasing Ce content. The UV–vis absorption spectra reveal that the band gap energy increases with increasing cerium content. Room-temperature photoluminescence (PL) spectra of pure ZrO₂ nanoparticles show strong emission peaks centered at about 441 nm at room temperature, which is attributed to the ionized oxygen vacancies in the nanoparticles. On the other hand, room-temperature PL spectra of the as-prepared CeO₂ nanoparticles shows two peaks at 417 and 436 nm, which might arise from the transition from the cerium 4f and to the oxygen 2p band (valence band) in CeO₂ and the presence of oxygen vacancies, respectively.     

Surface and structural characterisation of coprecipitated CexZr1?xO2 (0?≤?x?≤?1) mixed oxides

Ce _x Zr_1−x O₂-mixed oxides with three different Ce/Zr ratios (Ce_0.8Zr_0.2O₂, Ce_0.5Zr_0.5O₂ and Ce_0.2Zr_0.8O₂) along with pure cerium and zirconium oxides were prepared by coprecipitation of the metal hydroxides in alkali media and subsequent calcinations at 500 °C, using two different cerium precursors (Ce(NO₃)₃·6H₂O or (NH₄)₂Ce(NO₃)₆). These samples were characterised by N₂ adsorption at −196 °C, XRD, Raman spectroscopy, XPS and H₂-temperature programmed reduction. Besides, the two mixed oxides with higher cerium content were calcined at higher temperature (1000 °C) with the additional purpose of studying their thermal stability and phase homogeneity. XRD and Raman spectroscopy confirm a significant improvement in the insertion of zirconium cations into the ceria lattice when the samples Ce_0.8Zr_0.2O₂ and Ce_0.5Zr_0.5O₂ are synthesised with (NH₄)₂Ce(NO₃)₆ instead of Ce(NO₃)₃·6H₂O. This is attributed to a more homogeneous coprecipitation of cerium and zirconium hydroxides, leading to mixed oxides with better bulk oxygen mobility and smaller lattice parameter. Moreover, the mixed oxides prepared with the (NH₄)₂Ce(NO₃)₆ precursor and calcined at 1000 °C exhibit a single phase whereas phase segregation occurs in the counterpart mixed oxides prepared with the Ce(NO₃)₃·6H₂O precursor. XPS analysis reveal correlations among O/(Ce + Zr) surface atomic ratio and total cerium content for both cerium precursors. Among the samples calcined at 1000 °C, Ce_0.8Zr_0.2O₂ synthesised with Ce(NO₃)₃·6H₂O is the only one that preserves the low-temperature surface reduction peak, and also shows a BET surface area slightly higher than those of the rest of samples calcined at high temperature.     

Iron-stabilized nanocrystalline ZrO2 solid solutions: Synthesis by combustion and thermal stability

The synthesis of Fe³⁺-stabilized zirconia by the nitrate/urea combustion route was investigated. Using several characterization techniques, including X-ray diffraction, field-emission-gun scanning electron microscopy and notably Mössbauer spectroscopy, it was possible to determine the appropriate amount of urea that allows to obtain a totally stabilized Zr_0.9Fe_0.1O_1.95 solid solution. The nanocrystalline zirconia solid solution is mostly tetragonal, but the presence of the cubic phase could not be ruled out. An in-depth study of the thermal stability in air showed that the Fe³⁺ solubility in the stabilized solid solution starts to decrease at about 875 °C which results in the formation of hematite (possibly containing some Zr⁴⁺) at the surface of the zirconia grains and further provokes the progressive transformation into the monoclinic zirconia phase.     

Synthesis, structure and oxygen-storage capacity of Pr1−xZrxO2−δ and Pr1−x−yPdyZrxO2−δ

Nanocrystalline Pr_1−xZr_xO_2−δ (0 ≤ x ≤ 1) and Pr_1−x−yPd_yZr_xO_2−δ (x = 0.50, y = 0.02) solid solutions have been synthesized by a single step solution combustion method. The whole range of solid solution compositions crystallize in cubic fluorite structure. The lattice parameter ‘a’ linearly varied up to x = 1.0. Oxygen-storage capacity (OSC) and redox properties of Pr_1−xZr_xO_2−δ (0.0 ≤ x ≤ 0.8) solid solutions have been investigated by temperature-programmed reduction (TPR) and are compared with those of Ce_1−xZr_xO₂. Pr_1−xZr_xO_2−δ exhibited H₂ uptake and CO oxidation at a lower temperature than Ce_1−xZr_xO₂. Small amount of Pd ion (y = 0.02) substitution was found to bring down the temperature of oxygen release-storage significantly.     

Effect of Ca doping on the performance of CeO<Subscript>2</Subscript>–NiO catalysts for CH<Subscript>4</Subscript> catalytic combustion

Methane catalytic combustion was carried out over the Ce_0.9–xNi_0.1Ca_xO_δ (0 < x ≤ 0.3) catalysts prepared by a citric acid complexation–combustion method. When x ≤ 0.1, the presence of Ca can enhance the surface area and reduce the crystalline size, and improve the reduction of the dispersed NiO species in catalyst, resulting in an improvement of the catalytic activity of Ce_0.9–xNi_0.1Ca_xO_δ. The XRD and Raman results show that Ce_0.9–xNi_0.1Ca_xO_δ (x ≤ 0.1) solid solution can form by Ni and Ca incorporation in the CeO₂ lattices. TEM and etching results reveal that part of Ni disperses well on the surface of Ca-doped sample. FT-IR testing shows that with an increase in Ca amount (x > 0.1), more carbonate species (mainly carbonate calcium) can form on the catalyst surface, which would severely debase the catalytic activity of Ce_0.9–xNi_0.1Ca_xO_δ.     

Formation and thermal properties of fluorite-pyrochlore composite structure in La2(ZrxCe1 − x)2O7 oxide system

Rare-earth oxides of La₂(Zr_xCe_1 ? x)₂O₇ for thermal barrier coatings (TBCs) are fabricated via a solid-state reaction at 1600 °C. As the phase formation, microstructure, and thermal properties of these oxides are examined, a fluorite–pyrochlore composite structure is found in the La₂(Zr_xCe_1 ? x)₂O₇ system. This composite structure is composed of coarse Ce-rich fluorite and fine Zr-rich pyrochlore grains. From XRD and microstructural analysis, the lattice parameter and volume fraction of each phase are evaluated in order to obtain the intrinsic thermal conductivity value of composite-structured oxide with porosity calibration. The thermal conductivity of the composite structure is similar to that of pyrochlore La₂Zr₂O₇, which is attributed to phonon scattering by phase boundaries.     

Ferroelectric, dielectric and piezoelectric properties of Pb1?xCex(Zr0.60Ti0.40)O3, 0?≤?x?≤?0.08

The ferroelectric, dielectric and piezoelectric properties of compositions Pb_1−x Ce _x (Zr_0.60Ti_0.40)O₃, (x = 0.0, 0.01, 0.02, 0.04, 0.06 and 0.08) are studied. The above compositions are prepared from their constituent oxides, calcined at 900 °C for 4 h and various phases present are characterized by X-ray diffraction (XRD) technique. The above powders are uniaxially pressed into circular compacts, sintered at 1,250 °C for 2 h, electroded, poled at 2 kV/mm D.C. voltage and their electrical properties are measured. The XRD analysis shows the presence of rhombohedral phase up to 2 mol% ceria while tetragonal phase found at higher concentrations. It is observed that the ferroelectric, dielectric and piezoelectric properties increase with the addition of ceria with a maximum at 2 mol% and then decreases. The higher piezo properties associated with low ceria concentration are attributed to rhombohedral phase.     

Synthesis and properties of Ce1−xGdxO2−x/2 solid solution prepared by flame spray pyrolysis

Flame spray pyrolysis, which produces ultrafine particles, was applied to the synthesis of Ce_1−xGd_xO_2−x/2 solid solutions by substituting Gd from a mole fraction of 0–0.40. The solubility limit of Gd in the Ce_1−xGd_xO_2−x/2 solid solution produced by flame spray pyrolysis was between 0.25 and 0.30, which is consistent with the reported value. The as-prepared Ce_1−xGd_xO_2−x/2 particles had a square morphology and a nanometer range in the equivalent diameter. The small particle size made it possible to reduce the sintering temperature of the Ce_1−xGd_xO_2−x/2 solid solution from 1650 °C to 1400 °C for the ceria-based solid electrolytes produced by the solid state preparation. The maximum ionic conductivity was achieved when the mole fraction of Gd was 0.25. The mole fraction for the highest ionic conductivity was the same as the particles produced by hydrothermal synthesis. However, the ionic conductivity of the Ce_1−xGd_xO_2−x/2 prepared by the flame spray pyrolysis (1.01 × 10⁻² S/cm at 600 °C) was higher than that prepared by the hydrothermal synthesis (7.53 × 10⁻³ S/cm at 600 °C).     

RxCe0.8-xZr0.2O2催化剂的制备及催化碳烟燃烧性能研究

采用溶胶凝胶法制备一系列R_xCe_(0.8-x)Zr_(0.2)O_2(R=Mg,Ca,Sr和Ba,x=0,0.1,0.2和0.3)催化剂,并用X射线衍射(XRD)、比表面积(BET)、扫描电镜(SEM)、氢气程序升温还原(H_2-TPR)和程序升温氧化(TPO)等技术对催化剂进行表征,同时考察该系列催化剂催化碳烟燃烧活性。研究结果表明,Zr~(4+)均能进入CeO2晶格中形成具有立方萤石结构的固溶体。在不同的接触条件下,样品Ba_(0.1)Ce_(0.7)Zr_(0.2)O_2催化碳烟燃烧活性均最高。在一系列样品Ba_(0.1)Ce_(0.7)Zr_(0.2)O_2中,催化剂与碳烟紧密接触条件下其催化碳烟燃烧时更能够反映出催化剂内在活性的大小,而松散接触时则更易受到接触条件的影响。     

Synthesis and microstructure characterization of tetragonal Zr1–xTixO2 (x = 0–1) solid solutions

Oxide powders of Zr_1–xTi_xO₂ (x = 0–1) solid solutions with micron-sized particles were synthesized via a solution combustion method. The synthesis process and Zr/Ti molar ratio were optimized to produce powders with the tetragonal crystal structure. X-ray diffraction, Raman spectroscopy and transmission electron spectroscopy results confirm that a full crystallization microstructure with the single tetragonal phase is obtained after calcination at 600 °C while maintaining the crystallite size <30 nm. Zr/Ti oxide mixtures with Zr ≥ 67 mol% exhibit a tetragonal crystal structure and the embedding Ti in ZrO₂ improves the structure stability. The nitrogen sorption results indicate that the powders possess mesoporous morphology with medium specific surface areas (∼10–50 m²/g). Chemical stability tests show that these powders are relatively stable with negligible removal of titanium and zirconium after elution by 0.5 mol/L HCl. Density functional theory was used to calculate the most stable structure with low energy for the selected composition.     

Large‐Pore Mesoporous CeO2–ZrO2 Solid Solutions with In‐Pore Confined Pt Nanoparticles for Enhanced CO Oxidation

Active and stable catalysts are highly desired for converting harmful substances (e.g., CO, NO_x) in exhaust gases of vehicles into safe gases at low exhaust temperatures. Here, a solvent evaporation–induced co‐assembly process is employed to design ordered mesoporous Ce_xZr_1?xO₂ (0 ≤ x ≤ 1) solid solutions by using high‐molecular‐weight poly(ethylene oxide)‐block‐polystyrene as the template. The obtained mesoporous Ce_xZr_1?xO₂ possesses high surface area (60–100 m² g^?1) and large pore size (12–15 nm), enabling its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without blocking pore channels. The obtained mesoporous Pt/Ce_0.8Zr_0.2O₂ catalyst exhibits superior CO oxidation activity with a very low T₁₀₀ value of 130 °C (temperature of 100% CO conversion) and excellent stability due to the rich lattice oxygen vacancies in the Ce_0.8Zr_0.2O₂ framework. The simulated catalytic evaluations of CO oxidation combined with various characterizations reveal that the intrinsic high surface oxygen mobility and well‐interconnected pore structure of the mesoporous Pt/Ce_0.8Zr_0.2O₂ catalyst are responsible for the remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/Ce_xZr_1?xO₂‐s with small pore size (3.8 nm), ordered mesoporous Pt/Ce_xZr_1?xO₂ not only facilitates the mass diffusion of reactants and products, but also provides abundant anchoring sites for Pt nanoparticles and numerous exposed catalytically active interfaces for efficient heterogeneous catalysis.     

Influence of Ce0.68Zr0.32O2 solid solution on depositing γ-alumina washcoat on FeCrAl foils

The influence of Ce_0.68Zr_0.32O₂ solid solution on properties of γ-alumina based washcoat on FeCrAl foils was investigated. FeCrAl foils were pretreated at 950°C in air for 10 h before coating washcoat. Different amounts of Ce_0.68Zr_0.32O₂ solid solution were added into ã-alumina-based slurries. The properties of washcoats were measured by ultrasonic vibration and thermal shock test, SEM, BET and XRD. The results show that the addition of Ce_0.68Zr_0.32O₂ solid solution into slurries can improve γ-Al₂O₃-based washcoat adhesion on FeCrAl foils. The more the Ce_0.68Zr_0.32O₂ solid solution added into slurries, the higher was the specific surface area of aged samples. XRD characterization proved that ceria-zirconia solid solution can inhibit the transformation of γ-A1₂O₃ crystal into others at 1050°C for 20 h.     

Enhanced reduction properties of mesostructured Ce0.5Zr0.5O2 solid solutions

Mesostructured Ce_0.5Zr_0.5O₂ solid solutions (M-CZ) were hydrothermally synthesized using Gemini surfactant as the template. X-ray diffraction (XRD), small-angle X-ray diffraction (SAXRD), N₂ adsorption–desorption isotherms and high-resolution transmission electronic microscopy (HRTEM) were adopted to characterize the samples. The product had a surface area of 123.5 m² g⁻¹ with maximum oxygen storage capacity (OSC) of 0.58 mol O₂/mol Ce. Oxygen anions in the M-CZ can be repeatedly released and resumed during the redox recycles. Reduction of Ce⁴⁺ to Ce³⁺ or lower valence and Zr⁴⁺ to Zr³⁺ were fulfilled with an obvious color change during the temperature programmed reduction (TPR) process, while the crystal structure of the product remained unchanged even after severe reduction. The mesostructure of the product can improve the reductive ability of Ce⁴⁺ and Zr⁴⁺ cations, which was beneficial to the enhancement of OSC.     

Facile synthesis of cubic fluorite nano-Ce1−xZrxO2 via hydrothermal crystallization method

Nano-Ce_1?xZr_xO₂ (x = 0.15, 0.25, 0.5) were synthesized via co-precipitation using NH₄OH as precipitant and hydrothermal crystallization. The XRD results confirmed that the cubic fluorite nano-Ce_1?xZr_xO₂ can form in NH₄OH solution (pH > 10) at 150 °C for 12 h, and well crystallized 20–50 nm nano-Ce_1?xZr_xO₂ were obtained at 200 °C for 22 h. The crystal growth of Ce_1?xZr_xO₂ was suppressed under higher OH^? concentration and crystallite size decreased with increasing concentration of NH₄OH. Ce3d XP spectra showed that the main valence state of the cerium on Ce_1?xZr_xO₂ surface is +4, and substituting Ce⁴⁺ with Zr⁴⁺ has no obvious influence on Ce³⁺/Ce⁴⁺ ratio.     

Effects of Zr/Ce molar ratio and water content on thermal stability and structure of ZrO2–CeO2 mixed oxides prepared via sol–gel process

ZrO₂–CeO₂ mixed oxides were synthesized via sol–gel process. Thermal stability, structure and morphology of samples were investigated by powder X-ray diffraction, FT-Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. In this approach, the solvent composition and Zr/Ce molar ratio have great influences on the structure and morphology of final products. With decreasing water content in the mixed solvent, specific surface area of powders increased and the single tetragonal phase was obtained. Only when the volume ratio of water and ethanol and the Zr/Ce molar ratio were 1:1, tetragonal t″-Zr_0.5Ce_0.5O₂ could be stabilized in powders at temperature as high as 1000 °C. Meanwhile, tetragonal (t′) and (t″) phases coexisted in Zr_0.5Ce_0.5O₂ solid solution without peak splitting after calcination at 1100 °C, further transforming into cubic and tetragonal (t′) phases at 1200 °C. The effective activation energy for Zr_0.5Ce_0.5O₂ nanocrystallite growth during annealing is about 5.24 ± 0.15 kJ/mol.