Colloids and Surfaces
A: Physicochemical and Engineering Aspects 203 (2002) 87 – 95
Surface and catalytic properties of Co
3
O
4
/Al
2
O
3
as
influenced by ZnO
M. Mokhtar
a
, H.G. El-Shobaky
b,
*, A.S. Ahmed
b
a
Department of Physical Chemistry, National Research Centre, Dokki, Cairo, Egypt
b
Department of Chemistry, Faculty of Science, Cairo Uniersity, Cairo, Egypt
Received 15 February 2001; accepted 8 October 2001
Abstract
The effects of ZnO-treatment (2.1–7.7 wt.%) of Co
3
O
4
/Al
2
O
3
system (32 wt.% Co
3
O
4
) on solid – solid interaction
between the catalyst and its support, surface and catalytic properties were investigated. The techniques employed were
XRD, nitrogen adsorption at -196 °C and catalysis of CO by O
2
at 60–100 °C using a static method. The results
obtained revealed that ZnO-treatment of the investigated system conducted at 400 °C increased the degree of
dispersion of Co
3
O
4
phase via decreasing its particle size. Opposite effect was observed upon treatment with 4 wt.%
ZnO followed by heating at 600 °C. The doping process conducted at 400 and 600 °C resulted in no significant
changes in the BET-surface areas of Co
3
O
4
/Al
2
O
3
system, while the doping process conducted at 500 °C resulted in
a significant progressive increase in the BET-surface areas of the treated adsorbents. The doping process followed by
calcinations at 400 and 500 °C resulted in widening of the pores present. The catalytic activity was found to increase
by increasing the amount of dopant present for the solids precalcined at 400 and 500 °C reaching to a maximum limit
in presence of 4 wt.% ZnO. The increase was, however, more pronounced for the solids precalcined at 400 °C. The
maximum increase in the catalytic activity, expressed as reaction rate constant (k ), for the solids precalcined at
400 °C was 171, 288 and 350% for the reaction conducted at 60, 80 and 100 °C, respectively. The doping process
conducted at 600 °C resulted in a progressive decrease in the catalytic activity of the treated solids. The observed
increase in the catalytic activity due to ZnO-doping at 400 and 500 °C was discussed in terms of an effective increase
in the degree of dispersion of Co
3
O
4
crystallites. The decrease in the activity due to doping at 600 °C has been
tentatively discussed in terms of a possible coverage of the surfaces of Co
3
O
4
with ZnO film and also to a possible
enhancement of diffusion of cobalt species in Al
2
O
3
lattice sites. ZnO-doping of Co
3
O
4
/Al
2
O
3
did not modify the
mechanism of the catalysed reactions but changed the concentration of catalytically active constituent taking part in
the catalysed reaction without changing their energetic nature. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: CO-oxidation by O
2
; Co
3
O
4
/Al
2
O
3
; ZnO-doping
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1. Introduction
Cobalt has been one of the most widely used
metals for Fischer – Tropsch catalysts since the * Corresponding author.
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