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 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

Training the trainers workshop: an outcome evaluation

Bibliography: p. 200-206

Mise en évidence par rpe et rmn. large bande de l’influence d’un niveau de surface oh/ohsur les propriétés catalytiques de ZnO

Les interactions entre l’eau résiduelle et l'oxygène sur l'oxyde de zinc ont été étudiées à l’aide des techniques de RPE et de RMN large bande appliquées respectivement à l’analyse des variations du signal des électrons de conduction et du spectre du proton. Les résultats obtenus s’interprètent quantitativement en supposant l’existence d’un niveau de surface OH/OH- qui affecte les réactions de désorption et d’adsorption de l’oxygène. La réaction de coadsorption du monoxyde de carbone et de l’oxygène peut également être interprétée compte tenu de la présence de ce niveau de surface

Fast magic angle spinning solid-state 51V NMR characterization of vanadium oxide catalysts supported on TiO2

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