High temperature thermal stabilization of alumina modified by lanthanum species

The effects of precursor pretreatment and addition methods of lanthanum species on stabilization of alumina (surface area loss, phase transformations and high temperature interaction with lanthanum species) have been investigated by BET specific surface area measurements (BET). X-ray powder diffraction (XRD), N-2 adsorption-desorption isotherms. thermal analysis and X-ray photoelectron spectroscopy (XPS) in the range of 600-1150 degreesC. Although powder La2O3, which is mechanically mixed with gamma -Al2O3 or pseudo boehmite. can effectively retard the alpha phase transformation by solid phase interaction with Al2O3, it does not show a positive effect on retarding the loss of surface area. Compared with the direct impregnation of gamma -Al2O3, the gelation of pseudo boehmite by acidification accelerates phase transformations and weakens the stabilizing influence of lanthanum species. At 600 degreesC and for atomic ratio of La/Al up to 0.1 or at 1150 degreesC and La/Al less than or equal to0.02, the lanthanum species is highly dispersed in alumina. With the increase of calcination temperature or lanthanum content, lanthanum species is present as dispersed La2O3, LaAlO3 and crystalline La2O3. At T less than or equal to 1000 degreesC the surface area loss of alumina is mainly attributed to the sintering of particles. The follow-up loss at T>1000 degreesC results from both sintering and phase transformations. The highly dispersed lanthanum species retard both sintering and phase transformations, and their associated surface area loss. However, the formation of LaAlO3 mainly retards the surface area loss resulting from the alpha phase transformation. Having considered the purely mechanical mixing effect of additive on the surface area loss of alumina, an influence criterion of lanthanum species on retarding the surface area loss whether resulting from sintering or from or phase transformation at high temperature greater than or equal to 1000 degreesC has been proposed in this paper. (C) 2001 Elsevier Science B.V. All rights reserved.