SOLID-STATE EQUILIBRIA IN THE SYSTEM AL2O3-LA2O3-CR2O3 - REACTIVITY CATALYST SUPPORT LACRO3/AL2O3/

The zone of the ternary system Al2O3-La2O3-Cr2O3 delimited by LaCrO3, LaAlO3, Al2O3 and Cr2O3 has been examined. The existence of an extende d field of mixed crystals with rhombohedral symmetry LaAl1-xCrxO3 (0 l ess than or equal to x less than or equal to 0.85) has been verified. On the other hand for x > 085, the solids show XR diffraction spectrog rams typical of the orthorhombic phase LaCrO3. It has been found a sma ll isomorphous substitution of Al3+ by Cr3+ in the beta-alumina type p hase, leading to the formation of the LaAl11-x CrxO18 series (0 less t han or equal to x less than or equal to 0.75), moreover, the existence of Al2-xCrxO3 (0 less than or equal to x less than or equal to 2) mix ed crystals field has been confirmed. Finally it has been experimental ly verified that catalytic systems formed by mechanical mixtures of La CrO3 and Al2O3 prepared anyhow (LaCrO3 dispersed or supported on Al2O3 ) result thermodynamically unstable whatever the composition. (C) 1998 Elsevier Science Limited. All rights reserved.