Leandro M. Acuña

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.

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.

Physicochemical properties of nanostructured Pd/lanthanide-doped ceria spheres with high catalytic activity for CH4 combustion

In this work, nanostructured Ce0.9Gd0.1O2−δ (GDC) and Ce0.9Pr0.1O2−δ (PrDC) spheres previously obtained by microwave assisted hydrothermal homogeneous co-precipitation were impregnated with 1 wt% Pd by the incipient wetness impregnation by an aqueous Pd2+ solution. Their properties were characterized by synchrotron radiation X-ray diffraction (SR-XRD), X-ray absorption near-edge spectroscopy (XANES) and scanning and high resolution electron microscopy with X-ray spectroscopy. Spherical particles with average diameters around 200 nm were found to consist of crystallites of average size, 10 nm, with small particles of PdO finely dispersed over the sphere surface. In situ XRD and XANES experiments were carried out under reducing and oxidizing conditions in order to investigate the redox behaviour of these materials and to evaluate the effect of Pd loading on the oxidation state of Ce. All of the lanthanide-doped ceria (LnDC) supports were found to have a cubic crystal structure (Fm3m space group). An increase in lattice parameters was observed under reducing conditions which was attributed to the reduction of Ce4+ ions to the larger Ce3+ ions, and to the associated increase in oxygen vacancy (VO) concentration. Addition of Pd to the LnDC spheres increased their Ce3+ content. Finally, catalytic tests for CH4 combustion were performed on the LnDC and Pd/LnDC nanocatalysts. The best performance was observed in samples with 1 wt% Pd loading, which exhibited T50 values (temperature at which 50% of CH4 conversion was reached) close to 310 °C. These values are 220 °C and 260 °C lower than those obtained for nanostructured PrDC and GDC spheres alone, respectively.

Study Of Phase Transition In Nanostructured ZrO2‐CeO2 Solid Solutions By Synchrotron Radiation

In this work we studied the tetragonal to cubic (t‐c) phase transition as function of temperature in compositionally homogeneous ZrO2‐CeO2 powders by synchrotron radiation X‐ray diffraction (SR‐XRD). Transitions are correlated with changes in the local order of the second coordination shell of Zr atom, studied by EXAFS.