Propriétés acido-basiques des catalyseurs LaCrO3 supportés

Acido-basic properties of supported LaCrO₃ catalysts

¹ Laboratoire d'Études de Solides Catalyseurs, Département de Chimie, Faculté des Sciences, 1060 Tunis, Tunisie
² Laboratoire d'Études de la Réactivité Catalytique des Surfaces et Interfaces, URA 1498 du CNRS, Université Louis Pasteur, Institut le Bel, 67070 Strasbourg, France

Résumé

Des catalyseurs contenant différentes concentrations en Cr³⁺, La³⁺ supportés sur une alumine préalablement enrobée avec une phase LaAlO₃, ont été préparés par imprégnation. La structure cristalline, la surface BET, la nature des ions superficiels, la quantité de NH₃ adsorbée ainsi que la conversion de l'isopropanol ont été étudiées en fonction de la concentration en Cr³⁺. Il est montré que la capacité adsorptive et l'activité déshydrogénante, ramenées par ion Cr³⁺, sont plus importantes lorsque la concentration est inférieure à 29,1 μmpl.Cr³⁺/m² (∼18,10¹⁸ ions Cr³⁺/m²).

Abstract

Samples with different concentrations in Cr³⁺, La³⁺ supported on precoated alumina with LaAlO₃ phase, have been prepared by means of the impregnation method. Structural analysis shows the presence of the perovskite-type LaCrO₃ phase for unsupported catalyst and LaCrO₃ + LaAlO₃ for supported ones. In this latter case, the LaCrO₃ phase is observed when the concentration in Cr³⁺ is higher than 6.8 mol./m² (∼4.10¹⁸ Cr³⁺/m²). The surface/volume ratio of supported catalysts decreases gradually with the increase of Cr³⁺, La³⁺ loading and a drastic loss of surface area is observed for unsupported LaCrO₃. Surface analysis by means of XPS techniques for defining the surface concentration and oxidation states of cations, shows the presence of two O1s photolines that correspond to lattice (lower dinding energy BE) and adsorbed (higher BE) oxygen. The energy of the former peak is almost the same as that measured for usual basic simple oxides. Chromium and lanthanum are in the trivalent state. Their concentrations at the surface increase with the loading increase. Ammonia adsorption as well as isopropyl alcohol conversion show that the number of NH₃ molecules adsorbed and that of acetone formed per Cr³⁺ ion are higher for diluted samples i.e. with a Cr³⁺ concentration lower than 29.1 mol.Cr³⁺/m² (∼18.10¹⁸Cr³⁺/m²).