Pure and PO4-doped aluminas (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 (γ-Al2O3) to high-temperature spinel aluminas (δ, θ-Al2O4), and delays somewhat the phase transition from spinel alumina to the corundum phase (α-Al2O3). Phosphates have a positive effect on surface area and porosity only for the corundum phase obtained at T≥1450 K, in which the particles morphology is modified with respect to pure α-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) AlIV ions) present on the regular crystal planes. Meanwhile, the presence of phosphates produces an appreciable increase of the number of strong cus AlIV Lewis acid sites present in crystallographically and/or coordinatively defective configurations. When the bulk transition to α-Al2O3 has occurred in systems treated at T≥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
Surface characterization of modified aluminas. III. Surface features of PO4-doped Al2O3
MORTERRA, Claudio;MAGNACCA, Giuliana;
1995-01-01
Abstract
Pure and PO4-doped aluminas (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 (γ-Al2O3) to high-temperature spinel aluminas (δ, θ-Al2O4), and delays somewhat the phase transition from spinel alumina to the corundum phase (α-Al2O3). Phosphates have a positive effect on surface area and porosity only for the corundum phase obtained at T≥1450 K, in which the particles morphology is modified with respect to pure α-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) AlIV ions) present on the regular crystal planes. Meanwhile, the presence of phosphates produces an appreciable increase of the number of strong cus AlIV Lewis acid sites present in crystallographically and/or coordinatively defective configurations. When the bulk transition to α-Al2O3 has occurred in systems treated at T≥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
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