Colloids and Surfaces
A: Physicochemical and Engineering Aspects 161 (2000) 439 – 446
Surface to bulk characterization of phosphate modified
aluminas
Gamal A.H. Mekhemer, Ahmed K.H. Nohman *, Nasr E. Fouad,
Hussein A. Khalaf
Chemistry Department, Faculty of Science, Minia Uniersity, El -Minia 61519, Egypt
Received 16 November 1998; accepted 10 March 1999
Abstract
-Al
2
O
3
, dried alumina gel, as well as their phosphated forms using (NH
4
)
2
HPO
4
were prepared by wet
impregnation and calcined at 870 K. Resulted samples were subjected to investigate the consequent bulk [X-ray
powder diffractometry (XRD), diffuse reflectance spectroscopy (DRS) and infrared spectroscopy (IR)] and surface
(texture, N
2
-adsorption and surface acid properties, pyridine adsorption). Results indicated no detectable bulk phase
changes due to phosphation. However, the phosphated gel sample reveals the highest S
BET
. Surface stabilization of
phosphate species by -Al
2
O
3
or gel is indicated, leading to modifications on surface hydroxyl and hence surface
acidity. The phosphated gel sample exhibits the strongest acidity (both Bronsted and Lewis). © 2000 Elsevier Science
B.V. All rights reserved.
Keywords: Aluminas; Phosphated aluminas; Bulk and surface characterization; Acid behaviour
www.elsevier.nl/locate/colsurfa
1. Introduction
The transition aluminas (-, -, - and -Al
2
O
3
)
are widely used in industry as adsorbents, cata-
lysts or catalyst carriers, coatings, and soft abra-
sives, because of their fine particle size, high
surface area and the catalytic activities [1 – 4]. All
transition aluminas are reproducible and stable at
room temperature, but the sequence of their ther-
mal transformation is irreversible. In various cata-
lytic applications, the active surface layer of
alumina are subjected to severe chemical and/or
physical damages, among which trace chemical
contamination and thermal stress are most impor-
tant. As for the latter effect, it can produce loss of
surface area, pore blockage and undesirable crys-
tal phase transitions. For this reason, catalytic
alumina is often doped with small amounts of
foreign anions or cations, that are empirically
observed to improve the overall performance of
the substrate and are supposed to act as crystal
phase and/or surface area stabilizer.
Phosphates have been claimed to play the role
of support stabilizers [5], and to modify in a
potentially convenient way some of the acid – base
* Corresponding author. Fax: +20-86-342601.
E-mail address: rumenia@rusys.eg.net (A.K.H. Nohman)
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
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