(728b) Revised Mechanism of La Stabilization for La-Doped Alumina Catalyst Supports

Mesoporous alumina in the gamma phase is widely used as a catalyst support material due to its high surface area and favorable catalytic activity. Many of its applications employ high enough temperatures for both sintering and phase transformation (to the alpha phase) to occur, causing a substantial decrease in both surface area and catalytic activity. The addition of a few weight percent lanthanum (1-5%) to the alumina stabilizes the gamma phase, increasing the temperature at which these undesirable effects occur by at least 100°C. Previous studies have concluded that the La stabilizes the alumina by forming a lanthanum aluminate layer at the surface which inhibits sintering and decreases the rate of alpha-phase nucleation, thereby postponing phase transformation. We have performed EXAFS studies of La-doped alumina supports which contradict this model, showing that the La forms neither LaAlO₃ nor La₂O₃ at the surface and is indeed not present in sufficient quantities (with 3 wt% doping) to form such layers. In addition to these EXAFS studies, we have studied both the surface area (through BET measurements) and the phase progression (through PDF and Rietveld analyses of x-ray diffraction data) of both pure alumina and La-doped alumina supports as a function of temperature. Through these studies and symmetry considerations, we have formulated a revised mechanism of phase stabilization for La-doped alumina supports which we will discuss here.