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Issue
J. Chim. Phys.
Volume 95, Number 9, October 1998
Page(s) 2015 - 2033
DOI http://dx.doi.org/10.1051/jcp:1998351
DOI: 10.1051/jcp:1998351


J. Chim. Phys. Vol. 95, N°9  p. 2015-2033

Propriétés acido-basiques des catalyseurs LaCrO3 supportés

H. Triki1, H. Batis1 and A. Katrib2

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

Abstract
Samples with different concentrations in Cr 3+, La 3+ supported on precoated alumina with LaAlO 3 phase, have been prepared by means of the impregnation method. Structural analysis shows the presence of the perovskite-type LaCrO 3 phase for unsupported catalyst and LaCrO 3 + LaAlO 3 for supported ones. In this latter case, the LaCrO 3 phase is observed when the concentration in Cr 3+ is higher than 6.8 mol./m 2 ( $\sim 4.10^{18}$ Cr 3+/m 2). The surface/volume ratio of supported catalysts decreases gradually with the increase of Cr 3+, La 3+ loading and a drastic loss of surface area is observed for unsupported LaCrO 3. 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 NH 3 molecules adsorbed and that of acetone formed per Cr 3+ ion are higher for diluted samples i.e. with a Cr 3+ concentration lower than 29.1 mol.Cr 3+/m 2 ( $\sim 18.10^{18}$Cr 3+/m 2).

Résumé
Des catalyseurs contenant différentes concentrations en Cr 3+, La 3+ supportés sur une alumine préalablement enrobée avec une phase LaAlO 3, ont été préparés par imprégnation. La structure cristalline, la surface BET, la nature des ions superficiels, la quantité de NH 3 adsorbée ainsi que la conversion de l'isopropanol ont été étudiées en fonction de la concentration en Cr 3+. Il est montré que la capacité adsorptive et l'activité déshydrogénante, ramenées par ion Cr 3+, sont plus importantes lorsque la concentration est inférieure à 29,1  $\mu$mpl.Cr 3+/m 2 ( $\sim 18,10^{18}$ ions Cr 3+/m 2).


Key words: pérovskite type LaCrO3 -- acido-basicité -- adsorption de NH3 -- conversion de l'isopropanol
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