María José Capitán

Molecular dynamics studies of the structure of γ-alumina

Abstract Molecular dynamics (MD) studies of γ-Al 2 O 3 using a pairwise additive interaction potential of Paulings type are reported. MD simulations on a medium-sized cluster of γ-Al 2 O 3 (Al 576 O 864 ) show a picture of its structure which is in agreement with neutron diffraction results and lead to an interpretation of its main properties. MD simulations at several temperatures and velocity autocorrelation functions are also reported.

Surface models for γ-Al2O3 from molecular dynamics simulations

Molecular dynamics simulations for γ-Al2O3 and La3+-doped γ-Al2O3 crystals have been performed. From bulk simulations a description of the crystal-terminating layers in both doped and pure alumina solids is reported. The molecular dynamics simulations allow us to describe systematically for the first time the surface of the γ-Al2O3 crystal, taking into account the actual stoichiometry of the solid. This description results in a model in which the number of different surface sites is increased with respect to the previously reported models. The number of different surface sites may account for the IR spectra of hydroxy groups adsorbed on γ-Al2O3.

X-Ray Diffraction Study of Oxide Scales Formed at High Temperatures on AISI 304 Stainless Steel After Cerium Deposition

High-resolution X-ray diffraction has been usedto characterize the structure of oxide scales grownduring oxidation at 1173 K of cerium-modified stainlesssteels. The oxide scales consist of a mixture of Fe2O3 andCr2O3, as well as spinels. Severaloxidation treatments prior to cerium deposition havebeen applied. New phases, which are cerium-related,appear, depending on the preoxidation treatment. Bycomparing these results with previous ones onlanthanum-modified AISI 304 stainless steel, somepossible explanations for the reactive-element effectare proposed.

Phase Behavior of Aqueous Dispersions of Mixtures of N-Palmitoyl Ceramide and Cholesterol: A Lipid System with Ceramide#Cholesterol Crystalline Lamellar Phases

Ceramides are particularly abundant in the stratum corneum lipid matrix, where they determine its unusual mesostructure, are involved in the lateral segregation of lipid domains in biological cell membranes, and are also known to act as signaling agents in cells. The importance attributed to ceramides in several biological processes has heightened in recent years, demanding a better understanding of their interaction with other membrane components, namely, cholesterol. Structural data concerning pure ceramides in water are relatively scarce, and this is even more the case for mixtures of ceramides with other lipids commonly associated with them in biological systems. We have derived the thermotropic binary phase diagram of mixtures of N-palmitoyl- D-erythro-sphingosine, C16:0-ceramide, and cholesterol in excess water, using differential scanning calorimetry and small- and wide-angle X-ray diffraction. These mixtures are self-organized in lamellar mesostructures that, between other particularities, show two ceramide to cholesterol crystalline phases with molar proportions that approach 2:3 and 1:3. The 2:3 phase crystallizes in a tetragonal arrangement with a lamellar repeat distance of 3.50 nm, which indicates an unusual lipid stacking, probably unilamellar. The uncommon mesostructures formed by ceramides with cholesterol should be considered in the rationalization of their singular structural role in biological systems.

EXAFS and DRIFTS study of lanthanide doped rhodium catalysts

The activity for the production of methane in the hydrogenation of carbon oxides has been measured over rare earth oxide (Ln = La, Ce, Sm, Yb, Lu) promoted Rh/Al2O3 catalysts in a fixed bed flow reactor at atmospheric pressure. The catalyst structures have been determined by EXAFS experiments at the K edge of the rhodium metal. From the EXAFS data it is concluded that the average coordination number of rhodium depends on the rare earth cation used as promoter. The Rh average coordination number follows the trend obtained from CO chemisorption data. The correlation of the activity results with the structural data demonstrates that the activity towards methane production decreases on increasing the metal dispersion, this being the main effect the promoter causes on the rhodium catalyst.

Onset of perovskite formation in the catalytic system La2O3/γ-Al2O3

Molecular dynamics simulations of model systems for the surface interaction of lanthanum oxide supported on γ-alumina have been carried out at 1500 K. The onset of formation of perovskite-like phases has been analysed in samples containing four different concentrations of lanthanum oxide. A mechanism of the formation of perovskite-like polyhedra is proposed. This mechanism essentially involves a displacement of an oxide ion associated to an octahedral aluminum by a lanthanum ion and appears to be independent of La2O3 loadings.

Anomalous scattering study of oxide scales formed at 1173 K on surface modified stainless steel

The effect of lanthanum deposits on AISI 304 grade stainless steels has been studied by anomalous scattering, X-ray diffraction, Rutherford backscattering spectrometry (RBS) and scanning electron microscopy coupled with a energy dispersive spectrometer (SEM-EDX). The lanthanum deposits were obtained by the pyrosol method which implies the interaction at 473 K of an aerosol of lanthanum nitrate with the steel surface. The deposits enhance the corrosion resistance of the stainless steels specimens under synthetic air at 1173 K by a factor of three with respect of the non-treated steels. The main crystalline component of the protective oxide scale is demonstrated to be Fe2O3 . This result is explained on the basis of the oxide crystallinity and the chemical reactions at the alloy surface during the deposition procedure.

Micropore formation mechanisms in γAl2O3

Abstract A set of twenty molecular dynamics simulations of the surface of γ-Al 2 O 3 were performed at different densities and temperatures in order to investigate the formation mechanisms and structure of micropore systems. It is found that a severe reconstruction takes place at high densities which involves ion migration and a partial amorphization process of the surface. At low densities a different process takes place which does not involve ionic migration but only a local atomic rearrangement. The statistics of size distribution and volume per unit mass calculated from the molecular dynamics simulations are within the ranges experimentally obtained and theoretically predicted.

Kinetic salt effects in the bromide oxidation by bromate

The kinetic salt effect on the oxidation of bromide ion by bromate have been studied. It is concluded that the salt effects are the result of ion-solvent interactions and seem to be connected not only to the strength of these interactions, as measured by the lowering of solvent activity, but also by the structural characteristics of ion-solvent interactions. This conclusion is based on the use of Pitzers treatment to estimate the activity coefficients.

EXAFS data analysis for lanthanide sesquioxides

The EXAFS spectra of lanthanide sesquioxides (Lu2O3, Sm2O3, La2O3) at the lanthanide LIII-edge are analysed. The complex coordination polyhedron around the lanthanide cation is modelled with the minimum number of coordination shells that still allows the acquisition of coordination numbers and shell distances in agreement with the radial distribution functions from crystallographic data. Theoretical phase shifts and backscattering amplitudes with an amplitude reduction factor, S20= 0.73, are reliable for reproducing the experimental EXAFS data. A model with one Lu–O and two Lu–Lu shells simulates the Lu coordination polyhedron in C-Lu2O3, while for La2O3 the model includes two oxygen shells for simulating the nearest neighbours and one longer distance that averages that of the La–La pairs. The coordination around Sm in Sm2O3 is the most complex and two-shell models are needed to simulate the Sm–O and Sm–Sm absorber–backscatterer pairs. The models obtained are applied in the EXAFS analysis of dispersed Ln2O3/Al2O3 samples, where X-ray diffraction fails to detect the structure adopted by the lanthanide phase. The results show that in an Sm2O3/Al2O3 sample calcined at 800 °C, very small Sm2O3 particles are formed. In an La2O3/Al2O3 sample with low loading the analysis procedure allows the detection of the aluminium atoms that are present with the oxygens around the lanthanum ions, thus suggesting the incipient formation of a bidimensional LaAlO3 phase.