PHASE-COMPOSITION AND MECHANISM OF FORMATION OF BA-BETA-ALUMINA-TYPE SYSTEMS FOR CATALYTIC COMBUSTION PREPARED BY PRECIPITATION

A novel preparation method is proposed for Ba-beta-Al2O3-type systems to be used in high-temperature catalytic combustion, consisting of precipitation in aqueous medium. Differential thermal analysis-thermogravimetry, X-ray diffraction, Fourier transform-infrared, TEM and surface area data are presented for Ba-Al-0 samples with Al/Ba atomic ratios in the range 14-9 calcined at different temperatures up to 1670 K. The final materials consist of a barium-poor Ba-beta(I)-Al2O3 phase together with 1 % alpha-Al2O3 in the case of Al/Ba = 1 4 and of a barium-rich Ba-beta(II)-Al2O3 Phase together with 2% BaAl2O4 in the case of Al/Ba = 9. A monophasic sample with a Ba-beta-Al2O3 structure is obtained in the case of Al/Ba = 12; this phase is constituted by the simultaneous presence of both beta(I) and beta(II) structure types. Two routes operate above 1370 K in the formation of the Ba-beta-Al2O3 phases involving solid-state reactions between gamma-Al2O3 and BaAl2O4, and gamma-Al2O3 and dispersed barium compounds. Based on the analogies between the structures of gamma-Al2O3 and of the Ba-beta-Al2O3 phases it is suggested that the formation of the latter occurs via diffusion of barium ions within oxygen close-packed planes of the gamma-Al2O3-type spinel structure.