CATALYTIC FUEL COMBUSTION, REACTIONS AND CATALYSTS .14. THERMAL-STABILITY OF THE LA2O3-AL2O3 SYSTEM

The x-ray diffraction technique was used to study the thermal stabilit y of the La/Al2O3 system at temperatures in the range from 873 to 1573 K and at lanthanum concentrations from 0 to 13 wt %. It was shown tha t the stabilization of transient forms of alumina occurs in the presen ce of lanthanum ions. The stability of the La2O3-Al2O3 system is gover ned by the nature of the initial low-temperature aluminum oxide, the w ay lanthanum is introduced, and the La3+ concentration. The role playe d by the resultant alumina-based oxide compounds of lanthanum in the p hase transformation of Al2O3 is discussed. A hypothesis was put forwar d that the formation of the x-ray-amorphous solid solution of La3+ in gamma-Al2O3 at low temperatures (less than or equal to 873 K) became p ossible. The La3+ ions present in the gamma-Al2O3 Support restrict the diffusion of aluminum ions and prevent the appropriate rearrangement of the structure to produce the high-temperature alumina modifications . The above solid solution is a precursor of a La2O3 . 11Al(2)O(3) pha se. It is also shown that both LaAlO3 and La2O3 could not be considere d as the stabilizers for the transient alumina forms.