Characterization of boria-alumina mixed oxides prepared by a sol-gel method. Part 1: NMR characterization of the xerogels

While boria-alumina mixed oxides have been widely used for various catalytic reactions, their structure has never been satisfactory elucidated and is still a matter of debates. The present paper deals with the elucidation of the structure of boria-alumina prepared by a sol-gel method with B / Al atomic ratio varying from 0.013 to 1.643. The powders were prepared by hydrolysis of aluminum tri-sec-butoxide in presence of (NH4)2B4O7.4H2O. Then, the solid-state magic angle spinning NMR spectroscopy was used to characterize the obtained solids in the dried state (xerogels), the calcined state being the object of a next paper. Both 27Al and 11B MAS-NMR spectra were recorded with subsequent simulation of these last ones. This allowed to build a consistent structural model of these xerogels, taking into account the evolutions of calculated parameters such as the quadrupolar interaction, the real chemical shift, the relative quantity of respectively BO3, BO4, tetrahedral, pentahedral and octahedral aluminum species as a function of the B / Al atomic ratio. As a result, it was found that for B / Al 0.06, BO3 chains attached to these tetrahedral aluminum species are formed and start to grow through the matrix. Further, for B / Al > 0.15, when the boron loading increases, the BO3 chains progressively emerge outside of the matrix, crossing it over. Furthermore, for B / Al > 0.26, some pentahedral aluminum species are formed supposedly due to the considerable steric strains afforded by some particular aluminum atoms. In brief, the presented model elucidates the structure of the dried alumina-based xerogels and is the key starting point to explain the structure of the oxides obtained after calcination and presented in the following paper

Characterization of boria-alumina mixed oxides prepared by a sol-gel method. Part 2: Characterization of the calcined xerogels

Sol-gel boria-alumina mixed oxides with a wide range of B / Al atomic compositions have been characterized as dried xerogels in the preceding paper in which a structural model with four domains of composition was proposed. The present paper presents the results obtained on the same series of solids after calcination at 500°C. It was found that the structure of the dried xerogels has a strong influence on the structure of the final oxides. In particular, in the dried xerogel state from B / Al ~ 0.15, BO3 chains crossed over the solid matrix and this ratio was also a crucial limit for the calcined solids structure. Indeed, during calcination some BO3 chains were volatilized, giving a measured B / Al ratio lower than the theoretical one; the oxides were dislocated and their SSA brutally increasing from ~ 500 m2.g-1 up to ~ 650 m2.g-1. Cleavage resulted in the exposition of new external surfaces that exhibited BO3OH species incrusted in the host alumina matrix, conferring then an epitactic character to the solids. Further, the 11B MAS-NMR spectra of the solids exhibited a feature constituted of BO3 and BO4 (hydrated surface BO3 species) components that were resolved by a software simulation. While for B / Al 0.15. This increase originated in a strain on the new BO3 species trapped along the cleaved surfaces. Further, XRD and XPS results showed that while for B / Al 0.15 a mixed phase was progressively formed. In good agreement, the 27Al MAS-NMR spectra of the solids with high B / Al ratio were similar to that expected for model boria-alumina mixed phases. In addition, the BO4 / BO3 ratios calculated by simulation of the 11B MAS-NMR spectra simulation were consistent with the XPS results. We calculated the proportion of B species (in wt.%) present on the surface of the solids (i.e. BO4 species), which can be potentially involved in catalytic reactions. It was remarkable that the tendency observed for the BO4 wt.% as a function of the B / Al ratio was consistent with a boria-alumina phase diagram previously proposed by Gielisse et al.

Characterization of Sol-Gel derived scintillating LuBO3 films doped with rare earth ions.

Rare earth doped LuBO3 thin films have been prepared by combining sol-gel process and coatings techniques such as spin coating and spray pyrolysis. Annealing treatment results in the crystallization of the film as vaterite phase and incorporation of the doping ions in solid solution. XPS and RBS spectrocopies showed that the composition of the films is close to the nominal one. Adventitious carbon has been observed and attributed to incomplete pyrolysis of metal-organic precursors. XPS concentrations profiles show a good homogeneity for the films. RBS demonstrated some inter-diffusion between amorphous carbon substrate and borate films resulting in a gradient of carbon at the interface between the substrate and the film itself. Finally scintillation spectra have been recorded and demonstrate the potentiality of these films to be used as X-ray intensifying screens

Surface characterization of sol-gel derived scintillating rare-earth doped Lu2SiO5 thin films

International audienceRare earth doped Lu2SiO5 thin films have been prepared by combining sol-gel process and spin coating. Annealing treatment results in the crystallization of the film and efficient incorporation of rare earth doping ions. XPS and RBS spectrocopies showed that the composition of the films is close to the nominal one. Adventitious carbon has been observed and attributed to incomplete pyrolysis of metal-organic precursors. XPS concentrations profiles show a good homogeneity for the films. RBS demonstrated some inter-diffusion between amorphous carbon substrate and silicate films resulting in a gradient of carbon at the interface between the substrate and the film itself

Study of the atomic distribution in NiO—MoO

The application of ISS in order to investigate the surface composition of a NiO—MoO3/γAl2O3 system is discussed. It is found that the experimental conditions and sample conditioning (in particular, energy of the probe ion, time of 4He+ sputtering, time of degazing) influences the intensity ratio of the elements leading to difficulties in the description of the surface in terms of atomic distribution. Atom shielding effects which origin comes from different atomic radii of the species involved in this study and some contamination are supposed to be the origin of the difficulties in interpretation. Considerations about the shielding effect lead us to the conclusion that there is no bilayer Mo—Ni—Al on our sample

Évolution des silicates dans les milieux interstellaires, circumstellaires et cométaires (le rôle de l'irradiation et de la température)

Suite au développement des techniques d'observation et d'analyse, notre connaissance de la poussière silicatée dans les environnements astrophysiques a beaucoup évolué. Le cycle de vie de la poussière débute aœc sa formation autour des étoiles en fin de vie. Par la pression des vents stellaires, elle est ensuite injectée dans le milieu interstellaire (MIS) dans lequel elle transite. Enfin, elle peut être incorporée dans les disques d'accrétion autour des étoiles jeunes. Lors de ces différentes étapes, elle est soumise à diverses sollicitations qui induisent des modifications structurales ou chimiques. L'un des objectifs de cette thèse est d'abord de déterminer les modifications chimiques et morphologiques de la poussière silicatée quand elle est soumise à une irradiation ionique dans les ondes de choc de supernovae. Pour cela, nous avons effectué des expériences d'irradiation ionique (H+ et He+) à basse énergie in situ dans un spectroscope de photoélectrons (XPS). Les suivis chimiques et morphologiques ont été réalisés par XPS et par microscopie à force atomique. Le second objectif de cette thèse est d'indiquer comment la poussière amorphe évolue lors de son incorporation dans les régions internes du dIsque d'accrétion autour des étoiles jeunes. Nous avons ainsi mis en place des traitements thermiques de silicates amorphes in situ dans un microscope électronique en transmission (MET) et en four sous atmosphère contrôlée. Les évolutions microstructurales et chimiques ont été observées à l'aide d'un MET associé à un spectroscope à dispersion d'énergie. Nous avons montré qu'une irradiation ionique induit des modifications chimiques et morphologiques. Dans le milieu mterstellaire, les ondes de choc de supernovae sont donc des évènements très propices à modifier les matériaux initialement issus des étoiles en fin de vie. Les produits de recristallisation obtenus suite aux recuits sont très dépendants de la pression partielle d'oxygène imposée. Les microstructures obtenues sont souvent comparables avec celles observées dans les objets les plus primitifs de notre système solaire (poussière interplanétaire ou cométaire) La recristallisation du précurseur interstellaIre dans la partie interne du disque d'accrétion est donc un moyen pour former des phases qui sont ensuite incorporées dans les astéroïdes ou les comètes.LILLE1-BU (590092102) / SudocSudocFranceF

Phases et sites actifs, intermédiaires réactionnels et espèces adsorbées en catalyse sur particules métalliques (analyses spectroscopiques in situ)

Une combinaison de techniques d'analyse in situ (XPS, IR et Raman) a permis de caractériser la nature de la phase active et des espèces adsorbées sur des catalyseurs à base de Pd au cours de la décomposition de NO et de sa réduction par CO. L'étude XPS montre des variations de l'état chimique du Pd, variant du Pd métallique à plusieurs formes de Pd oxydé en fonction des atmosphères et températures réactionnelles. Cette évolution de la phase active est confirmée par spectroscopie Raman de résonance qui permet la caractérisation directe de l'oxyde PdO. Différentes espèces adsorbées sur Pd0 (NOads,COads,Nads, nitro-nitrito, nitrito et nitro) et sur alumine (nitrite, nitrate et carbonate) ont été caractérisées. L'ensemble des données spectroscopiques est discuté au regard des performances catalytiques afin d'aboutir à une meilleure compréhension des mécanismes réactionnels en catalyse de post-combustion automobile. Enfin, l'oxydation de CH4 a été étudiée par spectroscopie Raman operando.LILLE1-BU (590092102) / SudocSudocFranceF