C. Morterra et al., SURFACE CHARACTERIZATION OF MODIFIED ALUMINAS .3. SURFACE-FEATURES OFPO4-DOPED AL2O3, Journal of catalysis, 152(2), 1995, pp. 384-395
Pure and PO4-doped aluminas (approximate to 3% P2O5), pretreated at th
ree different temperatures (773, 1273, and 1473 K), have been compared
by XRD, TEM, and FTIR spectroscopy. The addition of phosphates does n
ot modify the phase transition of low-temperature spinel aluminas (gam
ma-Al2O3) to high-temperature spinel aluminas (delta, theta-Al2O3), an
d delays somewhat the phase transition from spinel alumina to the coru
ndum phase (alpha-Al2O3). Phosphates have a positive effect on surface
area and porosity only for the corundum phase obtained at T greater t
han or equal to 1450 K, in which the particles morphology is modified
with respect to pure (alpha-Al2O3). The effect of PO4-doping is apprec
iable on surface basicity and acidity. The weak basicity of alumina is
gradually eliminated, with increasing firing temperature. The presenc
e of phosphates increases the strong surface acidity of aluminas: phos
phates tend to collect preferably on the flat patches of regular cryst
al 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 phos
phates produces an appreciable increase of the number of strong cus Al
-IV Lewis acid sites present in crystallographically and/or coordinati
vely 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 tra
nsition alumina phases. The diverse opinion of investigators that phos
phates 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 s
ites that are most likely to be important in catalytic applications. (
C) 1995 Academic Press, Inc.