Rodolfo O. Fuentes

Synchrotron X‐ray diffraction study of the tetragonal–cubic phase boundary of nanocrystalline ZrO2–CeO2 synthesized by a gel‐combustion process

The crystal structures of a number of nanocrystalline ZrO2–CeO2 solid solutions, synthesized by a pH-controlled nitrate-glycine gel-combustion process, were studied. By using a synchrotron X-ray diffractometer, small peaks of the tetragonal phase, which correspond to forbidden reflections in the case of a perfect cubic fluorite structure, were clearly detected. By monitoring the most intense of these reflections, 112, as a function of the CeO2 content, the tetragonal–cubic phase boundary was found to be at 85 (5) mol% CeO2. For a CeO2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t′ form) was detected, whereas, between 68 and 85 mol% CeO2, the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t′′ form) was verified. Finally, solid solutions with higher CeO2 contents exhibit the cubic fluorite-type phase.

Nanoparticulate ceria–zirconia anode materials for intermediate temperature solid oxide fuel cells using hydrocarbon fuels

Solid Oxide Fuel Cells (SOFCs) represent an attractive technology for the conversion of chemical to electrical energy because of their high efficiencies and low environmental impact, and because of the useful, high grade heat also generated. Direct utilisation of hydrocarbons in SOFCs would contribute to the more sparing utilisation of remaining fossil fuel reserves. In the longer term, this technology could be extended to work with more sustainable biofuel and waste-derived feedstocks. In this work, nanoparticulate Ceria–Zirconia mixed oxides, Ce1−xZrxO2 (x = 0.1, 0.25, 0.5, 0.75 and 0.9), were studied with a view to their application as anode materials in intermediate temperature (IT) SOFCs using hydrocarbon fuels. Impedance spectra were recorded in symmetrical cells under reducing conditions using gadolinium-doped ceria (GDC) as the electrolyte material. The spectra were analysed in terms of a double fractal finite length Gerischer impedance model. The model parameters were found to have monotonic dependences on temperature and more complex relationships with respect to Zr content. Diffusion-related processes and the electrochemical reaction were fastest for intermediate Zr contents while the chemical exchange reaction rate increased with decreasing Zr content. As a result, anode catalysts with 10 and 25 mol% Zr showed the lowest polarisation resistances of only 0.17 and 4.52 Ω cm2 at 700 °C in humidified 5% H2 and humidified 5% CH4, respectively. These values represented an approximately two-fold improvement on the pure ceria electrode. This performance compares very favourably with the currently most promising candidate anode materials applied in SOFCs for use with hydrocarbon fuels.

Local structure of the metal-oxygen bond in compositionally homogeneous, nanocrystalline zirconia-ceria solid solutions synthesized by a gel-combustion process

Compositionally homogeneous ZrO2–CeO2 nanopowders have been characterized by Raman and extended x-ray absorption fine structure (EXAFS) spectroscopies. These techniques revealed a tetragonal-to-cubic phase transition as a function of CeO2 content, as observed in a previous synchrotron x-ray diffraction study. The tetragonal–cubic phase boundary was found to be at (85 ± 5) mol% CeO2. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the Zr–O first neighbour coordination sphere, while the Ce–O coordination sphere preserves its cubic symmetry over the whole composition range.

Local atomic structure in tetragonal pure ZrO2 nanopowders

The local atomic structures around the Zr atom of pure (undoped) ZrO2 nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2 nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to the z direction; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

Preparation and characterisation of nanostructured gadolinia-doped ceria tubes

In this work, nanostructured gadolinia-doped ceria tubes (GdxCe1−xO2−x/2 with x = 0.1 and 0.2) were synthesised following a very simple, high yield procedure and their properties were characterised by XRD and by electron microscopy (SEM and HRTEM). Tubes of both oxide compositions were comprised of nanocrystals that exhibited the cubic phase (Fmm space group). In SEM, the tubes were found to have lengths of around 2 µm, diameters of around 700 nm and wall thicknesses of about 10 nm. The SEM and TEM results showed that individual tubes were composed of a thin sheet of nanoparticles curved round to form the tubular structure. The size of these primary nanoparticles was calculated from the peak-broadening seen in the XRD results. Average crystallite sizes of 7.8 and 9.4 nm were found for Gd0.1Ce0.9O1.95 and Gd0.2Ce0.8O1.9, respectively. Electron microscopy observations confirmed the size range of these nanoparticles by direct observation. The nanostructured Gd0.1Ce0.9O1.95 tubes exhibited a higher value of specific surface area, at 97 m2 g−1, than the other composition (61 m2 g−1).

Nanostructured terbium-doped ceria spheres: effect of dopants on their physical and chemical properties under reducing and oxidizing conditions

In this work, nanostructured Ce1−xTbxO2−δ (x = 0.1 and 0.2) spheres were synthesized by microwave assisted hydrothermal homogeneous co-precipitation and their properties were characterized by synchrotron radiation X-ray diffraction (SR-XRD), X-ray absorption spectroscopy (XAS) and scanning and high resolution electron microscopy (SEM and HRTEM) with energy dispersive X-ray spectroscopy (EDS). Spherical particles with average diameters around 200 nm were obtained in excellent yields. In order to compare the effect of the morphology on the physico-chemical properties, terbium-doped ceria nanopowders were also synthesized by a cation complexation method. In situ SR-XRD, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) experiments were carried out under reducing and oxidizing conditions in order to investigate the redox behaviour of these materials and to evaluate the oxidation state ratios in the Ce3+/Ce4+ and Tb3+/Tb4+ couples. All of the Ce1−xTbxO2−δ samples were found to have a cubic crystal structure (Fmm space group). The spheres were composed of nanoparticles with an average crystallite size of about 10 nm. In situ XRD experiments showed an increase in lattice parameters on reduction which was attributed to the reduction of Ce4+ and Tb4+ cations to Ce3+ and Tb3+, which have larger radii, and to the associated increase in the VO concentration. The effect of the synthesis method on structural properties was evident in that the percentage of Tb present as Tb3+ in the nanostructured spheres was larger than that in the nanopowders of the same elemental composition.

Catalytic Behavior of PdO ∕ NiO ∕ SDC Composites for Partial Oxidation of Methane: Application as Anodes of Single-Chamber IT-SOFCs

PdO/NiO/SDC composites (SDC: Sm 2 O 3 -doped CeO 2 ) have been proposed to be excellent anodes for single-chamber, intermediate-temperature solid oxide fuel cells (SC-IT-SOFCs) operated under mixtures of CH 4 and air, reaching high power densities at relatively low working temperatures. In this work we evaluated the catalytic activity of these composites in the partial oxidation of CH 4 , finding that their high performance is due to the excellent catalytic properties of these materials. The effect of the addition of PdO was investigated by analyzing NiO/SDC and PdO/NiO/SDC composites under different reaction temperatures and CH 4 :O 2 ratios. Finally, PdO/NiO/SDC composites were evaluated under operation in SC-IT-SOFCs similar to those reported in the literature using SDC electrolytes and Sm 0.5 Sr 0.5 CoO 3 cathodes. Our results confirmed the feasibility of SC-IT-SOFCs.

Structural, physical and chemical properties of nanostructured nickel-substituted ceria oxides under reducing and oxidizing conditions

This work reports the synthesis of nanostructured Ce1−xNixO2−δ (x = 0.05, 0.1, 0.15 and 0.2) oxides prepared by a cation complexation route and with the main objective of studying their redox properties using a combination of electron microscopy, synchrotron radiation X-ray diffraction (SR-XRD) and X-ray absorption near-edge spectroscopy (XANES). The Ce1−xNixO2−δ series of nanopowders maintain the cubic crystal structure (Fm3m space group) of pure ceria, with an average crystallite size of 5–7 nm indicated by XRD patterns and confirmed by transmission electron microscopy. In situ SR-XRD and XANES carried out under reducing (5% H2/He; 5% CO/He) and oxidizing (21% O2/N2) atmospheres at temperatures up to 500 °C show a Ni solubility limit close to 15 at% in air at room temperature, decreasing to about 10 at% after exposure to 5% H2/He atmosphere at 500 °C. At room temperature in air, the effect of Ni on the lattice parameter of Ce1−xNixO2−δ is negligible, whereas a marked expansion of the lattice is observed at 500 °C in reducing conditions. This is shown by XANES to be correlated with the reduction of up to 25% of Ce4+ cations to the much larger Ce3+, possibly accompanied by the formation of oxygen vacancies. The redox ability of the Ce4+/Ce3+ couple in nanocrystalline Ni-substituted ceria is greatly enhanced in comparison to pure ceria or achieved by using other dopants (e.g. Gd, Tb or Pr), where it is limited to less than 5% of Ce cations.

High-temperature X-ray powder diffraction study of the tetragonal-cubic phase transition in nanocrystalline, compositionally homogeneous ZrO2-CeO2 solid solutions

, CO and hydrocarbons from automotiveexhausts , as anodes in fuel cell technologies, and others. Inparticular, the metastable forms of the tetragonal phase havebeen investigated widely since they are the most suitable forapplications Trovarelli, 2002; Di Monte and Kaspar, 2005 .The crystal structure of compositionally homogeneousZrO

Performance of Single-Chamber Intermediate-Temperature SOFCs Operated in Methane/Air Mixtures using PdO/NiO/CeO2-Sm2O3 Anodes

In recent years, new concepts have been explored in order to find anodes adequate for operation with hydrocarbon fuels with high catalytic activity at intermediate temperatures, since the usual internal reforming of methane requires very high temperatures. In this sense, single-chamber SOFCs have attracted great attention because they take advantage of the ambient oxygen to produce H2 and CO by the partial oxidation of the hydrocarbon. In this work, we studied the performance of single-chamber intermediatetemperature SOFCs operated with methane as fuel using PdO/NiO/CeO2-Sm2O3 anodes. CeO2-Sm2O3 and Sm0.5Sr0.5CoO3 were used as electrolyte and cathode, respectively. All these materials were prepared by low-cost wet chemical methods. Our results showed that the incorporation of PdO in the anode allows to reach high peak power densities, of about 100 mW/cm 2 at 600°C.