Renaud Revel

Nanocrystalline brookite with enhanced stability and photocatalytic activity: influence of lanthanum(III) doping.

Metastable TiO(2) polymorphs are more promising materials than rutile for specific applications such as photocatalysis or catalysis support. This was clearly demonstrated for the anatase phase but still under consideration for brookite, which is difficult to obtain as pure phase. Moreover, the surface doping of anatase with lanthanum ions is known to both increase the thermal stability of the metastable phase and improve its photocatalytic activity. In this study, TiO(2) nanoparticles of almost only the brookite structure were prepared by a simple sol-gel procedure in aqueous solution. The nanoparticles were then doped with lanthanum(III) ions. The thermal stability of the nanoparticles was analyzed by X-ray diffraction and kinetic models were successfully applied to quantify phases evolutions. The presence of surface-sorbed lanthanum(III) ions increased the phase stability of at least 200 °C and this temperature shift was attributed to the selective phase stabilization of metastable TiO(2) polymorphs. Moreover, the combination of the surface doping ions and the thermal treatment induces the vanishing of the secondary anatase phase, and the photocatalytic tests on the doped brookite nanoparticles demonstrated that the doping increased photocatalytic activity and that the extent depended on the duration of the sintering treatment.

Use of polyols as particle size and shape controllers: application to boehmite synthesis from sol–gel routes

Polyols were successfully used as size and shape controllers of oxide nanoparticles synthesized by soft chemistry in aqueous solution. The efficiency of acyclic polyols as a complexing agent depends obviously on the number of OH groups bonded to the carbon chain (and thus on the carbon chain length), but also on their stereochemistry. This innovating way to control morphology has been experienced for the synthesis of boehmite nanoparticles, whose morphology variations related to xylitol adsorption (C5 alditol) have been previously reported. The use of polyols during synthesis causes a modification of the usual morphologies observed, specifically resulting in an increase of (101) faces area. It is evidenced here that the variations of the nanoparticle aspect ratio are related to polyol complexing ability, and more specifically to molecule topology and configuration. Indeed, the morphology variations increase as a function of polyol carbon chain length and number of hydroxyl groups, and is much pronounced for stereoisomers exhibiting hydroxyl groups all oriented on the same side of the molecule (threo-threo sequences). Thanks to these various polyols used, we showed how the progressive levels of complexing ability allow us to tune boehmite particle size and shape. Material characterizations were performed using relevant methods such as X-ray diffraction powder pattern simulation and zetametry in addition to transmission electron microscopy. Since γ-alumina is obtained from boehmite by a topotactic transformation, we expect that this method will provide a promising way to control surface properties of γ-alumina, an important industrial catalyst support.

Thermal Stability of Brookite - TiO2 Nanoparticles with Controlled Size and Shape: in-situ studies by XRD

TiO2 is a material of great interest for many technological applications, such as photocatalyst or catalyst support. As the material properties depends on the polymorph used: rutile, anatase or brookite, it is of the foremost importance to know the thermal stability of each polymorph. The anatase to rutile phase transition has been largely studied. On the contrary, very few work refer to the brookite to rutile transition. In this study, a set of nanoparticles (~ 10 nm) of pure brookite with an isotropic shape was synthesized and sintered under various conditions. In situ X-ray diffraction (XRD) measurements were carried out from 600 to 750°C, without detection of the anatase polymorph. The influence of the annealing gas atmosphere on the brookite to rutile transformation, and the influence of adsorbed cations on the particles and the type of aggregation on the transformation kinetics are presented and discussed.

Synthesis and Surface Reactivity of Nanocomposite Support Al2O3/α-Al2O3

A simple method leading to the preparation of homogeneous macroporous high surface area alumina composites is presented in this work. A simple incipient wetness impregnation of aluminum nitrate on α-Al 2 O 3 leads to high surface area macroporous composites. The impregnated phase is however not homogeneously dispersed on the support porosity, e.g. an important fraction of the α-Al 2 O 3 crystals remains free from the impregnated phase. A high dispersion of the impregnated phase is obtained after a supplementary step of basic impregnation. After this step, the impregnated phase completely covers the surface of the support, leading to highly homogeneous composites. Moreover, a precise control of the pH was found to be the key factor to control the morphology of the impregnated phase, and then the physical properties of the composites.

Characterization of the acid-base properties of transition aluminas by model reaction and correlation with IR study

The transformation of a cyclopentanol/cyclohexanone mixture allows us to estimate simultaneously the acidity and the basicity of catalysts. Two reactions take place: the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. This reaction was carried out at two temperatures over four aluminas. Theta alumina seems to be the most basic of the aluminas tested. Correlation between model reaction and IR study were also discussed.