SURFACE CHARACTERIZATION OF MODIFIED ALUMINAS .3. SURFACE-FEATURES OF PO4-DOPED AL2O3

Pure and PO4-doped aluminas (approximate to 3% P2O5), pretreated at three different temperatures (773, 1273, and 1473 K), have been compared by XRD, TEM, and FTIR spectroscopy. The addition of phosphates does not modify the phase transition of low-temperature spinel aluminas (gamma-Al2O3) to high-temperature spinel aluminas (delta, theta-Al2O3), and delays somewhat the phase transition from spinel alumina to the corundum phase (alpha-Al2O3). Phosphates have a positive effect on surface area and porosity only for the corundum phase obtained at T greater than or equal to 1450 K, in which the particles morphology is modified with respect to pure (alpha-Al2O3). The effect of PO4-doping is appreciable on surface basicity and acidity. The weak basicity of alumina is gradually eliminated, with increasing firing temperature. The presence of phosphates increases the strong surface acidity of aluminas: phosphates tend to collect preferably on the flat patches of regular crystal planes, and so doing decrease the amount and increase the strength of the Lewis acid sites (coordinatively unsaturated (cus) Al-IV ions) present on the regular crystal planes. Meanwhile, the presence of phosphates produces an appreciable increase of the number of strong cus Al-IV Lewis acid sites present in crystallographically and/or coordinatively defective configurations. When the bulk transition to alpha-Al2O3 has occurred in systems treated at T greater than or equal to 1400 K, the samples retain surface properties reminiscent of those of the transition alumina phases. The diverse opinion of investigators that phosphates act toward alumina as phase stabilizing needs some corrections and additions: the positive role of phosphates on the alumina support implies the stabilization of higher amounts of the strong Lewis acid sites that are most likely to be important in catalytic applications. (C) 1995 Academic Press, Inc.