ACIDO-BASIC PROPERTIES OF SUPPORTED LACRO3 CATALYSTS

Samples with different concentrations in Cr3+, La3+ supported on preco ated alumina with LaAlO3 phase, have been prepared by means of the imp regnation method. Structural analysis shows the presence of the perovs kite-type LaCrO3 phase for unsupported catalyst and LaCrO3 + LaAlO3 fo r supported ones. In this latter case, the LaCrO3 phase is observed wh en the concentration in Cr3+ is higher than 6,8 mu mol./m(2)(similar t o 4.10(18) Cr3+/m(2)). The surface/volume ratio of supported catalysts decreases gradually with the increase of Cr3+, La3+ loading and a dra stic loss of surface area is observed for unsupported LaCrO3. Surface analysis by mean; of XPS techniques for defining the surface concentra tion and oxidation states of cations, shows the presence of two Ols ph otolines that correspond to lattice (lower binding energy BE) and adso rbed (higher BE) oxygen. The energy of the former peak is almost the s ame as that measured for usual basic simple oxides. Chromium and lanth anum are in the trivalent state. Their concentrations at the surface i ncrease with the loading increase. Ammonia adsorption as well as isopr opyl alcohol conversion show that the number of NH3 molecules adsorbed and that of acetone formed per Cr3+ ion are higher for diluted sample s i.e. with a Cr3+ concentration lower than 29,1 mu mol.Cr3+/m(2) (sim ilar to 18.10(18) ions Cr3+/m(2)).