Ceria Samples Research Articles

Low-Temperature Reducibility of MxCe1–xO2 (M = Zr, Hf) under Hydrogen Atmosphere

Redox cycles utilize the reversible oxygen release/uptake of cerium oxide in a variety of renewable energy applications such as fuel cells, water gas shift reactions, and solar thermochemical fuel production. For all applications the degree of reduction/oxidation determines the overall performance. In this study we report on the redox behavior of MxCe1–xO2−δ (M = Zr, Hf; x = 0, 0.15, 0.2) solid solutions monitored by high-temperature in situ X-ray diffraction. During reduction in H2 at 600 °C and the successive formation of Ce3+ and oxygen vacancies, the lattice of ceria expands up to 0.3%. The lattice expansion of hafnium-doped ceria samples is 4 times larger than in zirconium-doped or undoped ceria, indicating drastically higher extents of oxygen vacancy formation. The same trends are validated using temperature-programmed reduction measurements. Complete reoxidation of the MxCe1–xO2−δ solid solutions in air at 600 °C is reflecting the reversibility of the redox process. Scanning electron microscopy and...

The effect of dopants on the redox performance, microstructure and phase formation of ceria

A solid state reaction is employed to investigate the influence of ZrO2, HfO2, Pr6O11, TiO2 and Li2O doping on CeO2 for a possible use in solar thermochemical redox cycles. Ceramics with a macroscopic interconnected porosity, necessary for high mass transport during redox cycles, are produced by the addition of spherical carbon particles prior to sintering. Partial closure of porosity is detectable when CeO2 is doped with Pr, Zr or Hf, while Li co-doping retains interconnected porosity more effectively than other doped or pristine ceria samples. In dense ceramics, microstructures reveal a reduction of the average grain size of pristine CeO2 with increasing Zr and Hf dopant concentration. These trends are validated using Pechini synthesized materials of the same composition. The reduction in grain size is even more pronounced for Pr doped CeO2 and Li doped Hf0.1Ce0.9O2, while TiO2 doping induces softening of samples under operating conditions (>1500 °C) limiting its use for high temperature applications. The redox performance of MxCe1−xO2−δ (M = Zr, Hf; 0 ≤ x ≤ 0.2) can be increased significantly with increasing Zr and Hf dopant concentration. At x = 0.2 (Zr, Hf) the fuel production rates are doubled as compared to pristine CeO2. The redox performance of Hf doped CeO2 remains stable upon co-doping with Li+.

Effect of Microgravity on Synthesis of Nano Ceria

Cerium oxide (CeO2) was prepared using a controlled-precipitation method under microgravity at the International Space Station (ISS). For comparison, ceria was also synthesized under normal-gravity conditions (referred as control). The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size analysis results indicated that the ceria particles grown in space had lower surface area and pore volume compared to the control samples. Furthermore, the space samples had a broader pore size distribution ranging from 30–600 Å, whereas the control samples consisted of pore sizes from 30–50 Å range. Structural information of the ceria particles were obtained using TEM and XRD. Based on the TEM images, it was confirmed that the space samples were predominantly nano-rods, on the other hand, only nano-polyhedra particles were seen in the control ceria samples. The average particle size was larger for ceria samples synthesized in space. XRD results showed higher crystallinity as well as larger mean crystal size for the space samples. The effect of sodium hydroxide concentration on synthesis of ceria was also examined using 1 M and 3 M solutions. It was found that the control samples, prepared in 1 M and 3 M sodium hydroxide solutions, did not show a significant difference between the two. However, when the ceria samples were prepared in a more basic medium (3 M) under microgravity, a decrease in the particle size of the nano-rods and appearances of nano-polyhedra and spheres were observed.

Investigation of long term reactive stability of ceria for use in solar thermochemical cycles

The use of an intermediate reactive material composed of cerium (IV) oxide (ceria) is explored for solar fuel production through a CO2-splitting thermochemical redox cycle. To this end, powder and porous ceria samples are tested with TGA (thermogravimetric analysis) to ascertain their maximum fuel production potential from the CeO2 → CeO2−δ cycle. A maximum value of the non-stoichiometric reduction factor δ of ceria powder was 0.0383 at 1450 °C. The reactive stability of a synthesized porous ceria sample is then observed with carbon dioxide splitting at 1100 °C and thermal reduction at 1450 °C. Approximately 86.4% of initial fuel production is retained after 2000 cycles, and the mean value of δ is found to be 0.0197. SEM (scanning electron microscopy) imaging suggests that the porous ceria structure is retained over 2000 cycles despite apparent loss of some surface area. EDS (energy dispersive x-ray spectroscopy) line scans show that oxidation of porous ceria becomes increasingly homogenous throughout the bulk material over an increasing number of cycles. Significant retention of reactivity and porous structure demonstrates the potential of porous ceria for use in a commercial thermochemical reactor.

Enhanced hydrothermal stability and oxygen storage capacity of La3+ doped CeO2-γ-Al2O3 intergrowth mixed oxides

La3+-doped CeO2-γ-Al2O3 intergrowth mixed oxide powders were synthesized using an improved amorphous citric precursor (IACP) method and then steam-aged with 90% steam and 10% air for 12h at 788°C. The synthesized materials were characterized through X-ray diffraction, energy-dispersive X-ray spectrometry, H2 temperature programmed reduction, Brunauer–Emmett–Teller, and oxygen storage/release capacity (OSC) measurements. La3+ was successfully doped into the CeO2-γ-Al2O3 lattice to form an intergrowth system. Strong intergrowth interactions were observed between CeO2, La2O3, and γ-Al2O3 crystallites. The (Ce0.92La0.08)0.4Al0.6O2−z(CLA) sample showed a maximum OSC of 979μmolg−1; however, the pure ceria sample showed a low OSC of 315μmolg−1. The CLA samples maintained a relatively high OSC of 880μmolg−1 even after hydrothermal treatment. These results may be attributed to the incorporation of La3+ ions into the ceria lattice to form a CeO2–La2O3 solid solution as well as to the intergrowth interactions between the CeO2–La2O3 solid solution and γ-Al2O3. Such interactions affected structural homogeneity and oxygen vacancy formation and inhibit the CLA particles from sintering during steam-aging.

Elimination of Surface Adsorbates on Gadolinium Doped Ceria for Electrochemical Strain Microscopy

Use of solid oxide fuel cells for the conversion of electrochemical energy has a number of advantages over current technologies. These systems offer high efficiencies for energy conversion, a choice of inexpensive hydrocarbon fuels, and environmentally friendly operation. The mechanisms which govern the functionality of these materials may be elucidated using scanning probe microscopy based techniques. However, the formation of surface adsorbates complicates their use. One such technique, electrochemical strain microscopy, quantifies the mechanical response of a surface resulting from the application of an AC bias through a cantilever. We have used this methodology in conjunction with a number of strategies to understand the behavior of gadolinium doped ceria, a widely used electrolyte material, while circumventing the issues that arise at the material surface. Sections of the ceria sample were ablated using a focused ion beam to expose a surface previously shielded from the ambient environment, then subsequently studied with electrochemical strain microscopy. We find that the ablation process produced a surface morphology exhibiting an enhanced electromechanical response similar to polycrystalline ceria at the grain boundaries that has previously been attributed to the space-charge effect. Thermogravimetric analysis, between 20°C and 400°C, revealed a plateau in weight around 200°C and 350°C, suggesting these are points that surface adsorbates may be removed from the electrolyte. We are currently studying the ablated regions at 250°C in a controlled environment to analyze the strain response under these conditions.

Ce K edge XAS of ceria-based redox materials under realistic conditions for the two-step solar thermochemical dissociation of water and/or CO2.

X-ray absorption spectroscopy was used to characterise ceria-based materials under realistic conditions present in a reactor for solar thermochemical two-step water and carbon dioxide splitting. A setup suitable for in situ measurements in transmission mode at the cerium K edge from room temperature up to 1773 K is presented. Time-resolved X-ray absorption near-edge structure (XANES) data, collected for a 10 mol% hafnium-doped ceria sample (Ce0.9Hf0.1O2-δ) during reduction at 1773 K in a flow of inert gas and during re-oxidation by CO2 at 1073 K, enables the quantitative determination of the non-stoichiometry δ of the fluorite-type structure. XANES analysis suggests the formation of the hexagonal Ce2O3 phase upon reduction in 2% hydrogen/helium at 1773 K. We discuss the experimental limitations and possibilities of high-temperature in situ XAS at edges of lower energy as well as the importance of the technique for understanding and improving the properties of ceria-based oxygen storage materials for thermochemical solar energy conversion.

Effect of grain size and microstructure on radiation stability of CeO2: an extensive study.

To investigate the variation in the radiation stability of ceria with microstructure under the electronic excitation regime, ceria samples sintered under different conditions were irradiated with high energy 100 MeV Ag ions. The ceria nanopowders were synthesized and sintered at 800 °C (S800), 1000 °C (S1000) and 1300 °C (S1300), respectively. The samples with widely varying grain size, densities and microstructure were obtained. The pristine and irradiated samples were studied by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). None of the samples amorphized up to the highest fluence of 1 × 10(14) ions per cm(2) employed in this study. XRD and Raman studies showed that the sample with lowest grain size suffered maximum damage while the sample with largest grain size was most stable and showed little change in crystallinity. Raman spectroscopy indicated the enhanced formation of Ce(3+) and related defects in the sample with larger grain size after irradiation. The most intriguing result was the absence of Ce(3+)-related defects in the sample with lowest grain size which actually showed maximum damage upon irradiation. The XPS studies on S800 and S1300 provided concrete evidence for the presence of Ce(3+) and oxygen ion vacancies in S1300. The grain boundaries and grain size dependent stability have been discussed.

The role of metal–support interaction on catalytic methane activation

In this work, we study CH4 chemisorption over Pt-containing catalysts at low temperature (200–450°C) in a fixed bed reactor. We report the systematic investigation of the effect of temperature, support (ceria vs. silica), and Pt content, on the amount of chemisorbed CH4 and the structure and reactivity of adsorbed CHx species, as formed during 1min exposure of the catalyst to the CH4 flow. The total CH4 chemisorbed and the state of carbonaceous film were determined by Temperature Programmed Desorption (TPD) and Temperature Programmed Surface Reaction (TPSR), as performed immediately upon catalyst exposure to the CH4 flow. The hydrogenation of carbon ad-species produced only methane. It was found that the strong metal–support interaction, present in ceria samples, plays an important role in CH4 conversion, and lends further evidence to the hypothesis that the metal–support interface causes a large spillover of adsorbed C and H species from Pt onto ceria – thus clearing the Pt active sites for further CH4 dehydrogenation. Not only did the ceria based samples activate up to 5 times more CH4 than the silica based samples, but the nature of the adsorbed CHx species and how they bond to the Pt surface varied on different supports. Thus, both the catalyst structure and operating conditions were found to play key roles in controlling the amounts of chemisorbed CH4 and the structure and the reactivity of the adsorbed carbonaceous film, so they should be carefully considered when rationally designing catalysts for methane activation and conversion.

Synthesis of Co/Ni Unitary‐ or Binary‐Doped CeO2 Mesoporous Nanospheres and Their Catalytic Performance for CO Oxidation

AbstractIn this paper, pure ceria, ceria doped with transition metal ions (Co2+, Ni2+), and Co/Ni binary‐doped ceria mesoporous notched hollow nanospheres were prepared from a one‐step solvothermal synthesis. By introducing metal ions, the composition can be freely manipulated. The morphologies and crystalline structures of the products were characterized in detail by XRD, TEM, SEM, and HRTEM. The surface compositions of the as‐prepared ceria samples were detected by Raman spectroscopy, energy‐dispersive X‐ray spectrometry (EDS), and X‐ray photoelectron spectroscopy (XPS). The surface areas and pore‐size distributions of the as‐obtained doped ceria mesoporous nanospheres were investigated by N2 adsorption–desorption measurements. Temperature‐programmed reduction measurements under H2 (H2‐TPR) showed the better reduction behavior of the doped ceria samples. Preliminary CO catalytic oxidation experiments indicated that the doped ceria samples showed strikingly higher catalytic activity, owing to the intrinsic surface defects of the samples. In addition, the as‐obtained ceria nanospheres can be used as excellent supports for gold nanoparticles to remove CO by catalytic oxidation; therefore, they demonstrate a promising potential in environmental remediation. This one‐step synthesis is a versatile approach and it could be extended to other binary or ternary metal oxide systems.

Comparison of the catalytic benzene oxidation activity of mesoporous ceria prepared via hard-template and soft-template

Mesoporous ceria samples were prepared both by nanocasting using SBA-15 mesoporous silica as hard-template and by evaporation-induced self-assembly using F127 triblock copolymer as soft-template. The samples were characterized by TEM, XRD, TPR, XPS and nitrogen adsorption. Ceria prepared with hard-template appeared as arrays of connected rods with high specific surface area and highly defective internal structure that facilitates bulk reduction by hydrogen at temperature as low as 400°C. The soft-template made sample exhibited morphology of three dimensionally linked filaments with low extent of internal defects. Hydrogen reduction of the soft-template made ceria was confined to the surface region for reduction temperature as high as 700°C. The hard-template made ceria was much more active than the soft-template made ceria for benzene oxidation at low temperature, giving a T50 at 257°C. The activity of the soft-template made ceria was greatly enhanced by etching with 2M NaOH at 80°C overnight. The high benzene oxidation activity of both the hard-template made and the sodium hydroxide treated soft-template made ceria were attributed to the generation of surface defects by the action of sodium hydroxide. The defect sites adsorb active oxygen that effect benzene oxidation below 300°C.

Effect of dopants [Gd3+, Sm3+, Ca2+, Sr2+ and Ba2+] on the performance characteristics of ceria based electrolytes for application in solid oxide fuel cells

Doped CeO2 based materials are now-a-days proposed as alternate electrolyte materials for solid oxide fuel cells (SOFCs) working at low temperature (~723 – 873 K). In this research work, nanoparticles of CeO2 doped with Gd3+, Sm3+, Ca2+, Sr2+ and Ba2+were prepared by a simple homogeneous chemical precipitation method. The prepared materials (after heat treatment at 1023 K for 2 hours) were systematically characterized by XRD, EDAX analysis, FTIR , particle size analysis and SEM. Lattice parameters were calculated from the XRD data. The XRD results indicate that all the doped ceria samples studied are single phase with a cubic fluorite structure. The average particle size of the doped ceria powder was about 48 – 115 nm and the particles have shown narrow particle size distribution patterns. AC impedance spectroscopy studies performed on the sintered specimens have shown better oxide ion conductivity values and hence these materials may be suitable for application as electrolyte materials in solid oxide fuel cells working at low temperature (~723 – 873 K). ________________________________________GRAPHICAL ABSTRACT

Production of cerium-141 using ceria and nanoceria powder: a potential radioisotope for simultaneous therapeutic and diagnostic applications

Cerium-141 [T 1/2 = 32.5 days, β 1 = 580.4 keV (30 %), β 2 = 435.0 keV (70 %), γ = 145.4 keV (48 %)], has been introduced as a radionuclide for therapy while can be used in diagnosis as well. In this study, nuclear model calculation on 141Ce production was investigated via the 140Ce(d,p)141Ce, 142Ce(d,dn)141Ce, 141Pr(n,p)141Ce, and 140Ce(n,γ)141Ce nuclear reactions. 140Ce was irradiated by thermal neutron at the Tehran Research Reactor according to the 140Ce(n,γ)141Ce reaction. In addition, the obtained activity of the produced 141Ce was compared with the theoretical calculations. The results showed that the end of bombardment activities of 141Ce is 631.64 MBq theoretically and 611.60 MBq experimentally for ceria powder. The activities of similar samples of ceria (CeO2) powder in two forms of nano-particles (nanoceria) and bulk were compared after bombardment by thermal neutrons. The results showed that activities of nanoceria were less than the bulk form of ceria.

Experimental evidences of the relationship between reducibility and micro- and nanostructure in commercial high surface area ceria

Two commercial high BET surface areas CeO2 are characterized, as received and after calcination at different temperatures, to better understand the relationship between reducibility and micro- and nanostructure. Combination of TPR data, Rietveld analysis of XRD diagrams and HREM/HAADF-STEM suggest that the nano-particles are responsible for Ce4+ reduction in the moderate temperature range. Results obtained in this work show that previous models based on kinetic and theoretical analysis can be fully supported by morphological characterization. This study unveils the key parameters that must be known in order to select appropriate commercial ceria samples.

Preparation and characterization of Ce1–xDyx–ySryO2–δ system

The effect of dysprosium and strontium on the total ionic conductivity of ceria in the system Ce1–xDyx–ySryO2–δ was studied. In this system, few compositions were prepared with x=0.15, y=0.015, 0.03 and 0.045 by modified sol–gel process using maltose and pectin as organic precursors. Rietveld refinement of XRD patterns confirms the cubic structure with space group. SEM images show relatively uniform grains with clean and distinct grain boundaries. Four probe AC impedance measurements were carried out to evaluate the total ionic conductivity in the temperature range of 150–500 °C and frequency range of 40 Hz–1 MHz. The composition Ce0.85Dy0.12Sr0.03O2–δ shows higher electrical conductivity than single-doped ceria samples.

Reduction Characteristics of Ceria under Ethanol Steam Reforming Conditions: Effect of the Particle Size

Reducibility of ceria under steam reforming conditions and the effect of particle size on its reducibility was examined using two ceria samples with distinctly different mean particle sizes (3.5 nm versus 120 nm), but with similar polyhedral morphologies. The degree of reduction from Ce4+ to Ce3+ was characterized by temperature programmed reduction (TPR) and in situ X-ray absorption near edge structure spectroscopy (XANES) where the nanopolyhedra were observed to reduce much more readily compared to the larger particle-size sample. There was also significant reduction of the nanopolyhedra under ethanol steam reforming conditions. Ceria nanopolyhedra exhibited significantly more Ce3+ sites which contributed to a lower occurrence of surface acidic sites. The acidic/basic sites were probed by probe molecules such as pyridine and CO2 through in situ diffuse reflectance infrared spectroscopy (DRIFTS). The particle size also showed major differences in the steam reforming activity of ceria, with nanopolyhedra with a 3.5-nm mean particle size exhibiting significantly higher carbon cleavage and ethanol dehydration activity than its counterpart of 120 nm mean particle size.

Copper doped ceria nanospheres: surface defects promoted catalytic activity and a versatile approach

In this paper, Cu2+ doped ceria nanospheres were produced from a one-step hydrothermal synthesis. The products were characterized in detail at the structural and electronic level by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). XRD results revealed that the doped ceria was free of other crystallographic phases, such as CeOHCO3, CuO or Cu2O. By introducing various reactant molar ratios of Cu:Ce, the composition can be freely manipulated. The H2-TPR measurement shows the improved reduction behaviour of the doped ceria samples prepared by us. A preliminary CO catalytic oxidation experiment indicated that the Cu2+ doped ceria samples showed strikingly higher catalytic activity, due to the intrinsic surface defects of the samples. In addition ceria doped with other transition metal ions (M2+), such Co2+, Ni2+ and Mn2+, has also been successfully prepared. Considering this one-step synthesis as a versatile approach, it can be further extended to other binary metal oxide systems.

Heavily impregnated ceria nanoparticles with europium oxide: spectroscopic evidences for homogenous solid solutions and intrinsic structure of Eu3+-oxygen environments

We report on the homogeneity, structural and luminescence properties of ceria nanoparticles doped with Eu3+. Eu3+ in concentration of 1, 5 and 10 wt% was incorporated via wetness impregnation into preformed ceria nanoparticles followed by calcination in air at 1000 °C. A remarkable homogeneity of Eu3+-ceria solid solutions is measured for ceria grown by citrate and micro-emulsion methods using Raman, Diffuse Reflectance in UV–Vis, photoluminescence spectroscopies and X-ray diffraction, even for the Eu3+ concentration of 10 wt%. The emission properties of all Eu3+-doped ceria samples are well-characterized by a two main centre model assigned to perturbed and isolated Eu3+ centres. These centres correspond to Eu3+ located in the nearest (local symmetry lower than cubic and Eu3+-oxygen coordination lower than eight) and next-nearest-neighbour positions (cubic local symmetry and eightfold Eu3+-oxygen coordination) to oxygen vacancy, respectively. With increase of Eu3+ concentration, both the oxygen vacancy concentration and the relative contribution of the perturbed Eu3+ centre to the total emission increase. It is established that the characteristic emission and excitation spectra of the two main Eu3+ centres as well as the overall multisite distribution of Eu3+ within ceria lattice are intrinsic properties of Eu3+-doped ceria since these do not depend on synthesis route, nanoparticle size and Eu3+ concentration.

Some Structural Aspects of Ionic Conductivity in Co-Doped Ceria-Based Electrolytes

Abstract The sinters of co-doped ceria solid solutions with the formula of Ce0.85Sm0.15-x RxO1.9, where R = Y, Gd, Pr, Tb, Ox-0.15, were obtained from powders synthesised by Pechini method. The linear variation of cell parameter a vs. chemical composition was observed for Ce0.85Sm0:15-xRxO1.9, where R = Y, Gd, Tb, 0 <x<0.15 samples. However, the introduction of Pr3+ into Ce0.85Sm0.15-x PrxO1.9 caused a small deviation from linearity due to possible changes in the valence from Pr3+ to Pr4+. The determined values of oxide transference number tion for Ce0.85Sm0.15-xRxO1.9, R = Y, Gd in the temperature range 400-750°C and partial oxygen pressure from 10-6 to 1 atm were close to 1, which indicated that materials investigated exhibited practically pure ionic oxide conductivity. On the other hand, the introduction of Tb3+ or Pr3+ higher than x>0.05 into solid solution Ce0.85Sm0.15-xRxO1.9, R = Tb, Pr caused a decrease in the ionic transference number tion below 1 due to an increase in partial electronic conduction. This fact limiting investigated co-doped terbia and samaria or samaria and praseodymia ceria-based solid solutions for the further application as oxide electrolytes in solid oxide fuel cells. The analysis of bulk and grain boundary values indicated that partial substitution of Sm3+ by Y3+ or Gd3+ caused slight improvements in the ionic conductivity of Ce0.85Sm0.15-xRxO1.9. The highest ionic conductivity was found for solid solution with chemical composition Ce0.85Sm0.1Y0.05O1.9. The selected co-doped ceria samples were tested as solid electrolytes in solid oxide fuel cells operating in the intermediate temperature range 500-750°C.

Improving the catalytic activity of CeO2/H2O2 system by sulfation pretreatment of CeO2

Organic pollutants can be effectively degraded by CeO2/H2O2 system, the catalytic activity of which is restricted by over-complexation of H2O2 with CeO2. In this study, nano ceria was pretreated simply in different ways. Degradation of AO7 was employed to evaluate the catalytic activity of the pretreated ceria samples in the presence of H2O2 in pre-adsorbed mode (AO7 pre-adsorbed on CeO2 before the addition of H2O2) and pre-mixed mode (CeO2 pre-mixed with H2O2 before the addition of AO7). Two sulfated ceria samples (H2SO4–CeO2 and Na2SO4–CeO2) exhibit enhanced catalytic performance in both modes compared with un-pretreated bare CeO2. XRD, BET, FTIR and potentiometric titration methods were used to characterize the pretreated ceria samples. The sulfated samples display stronger surface acidity due to the sulfate ions anchored on their surface. Raman and XPS measurements were carried out to analyze the samples after reacting with H2O2. A mechanism to explain the improved catalytic activity of sulfated CeO2 is proposed that the active surface peroxide species (Ce(III)O2H−), which are excessive due to the over-complexation of H2O2 with surface Ce3+ sites, can be decomposed into high-active hydroxyl radicals under surface acidic environment.