Introduction
TiO
2
is a photocatalyst used in wide range of environ-
mental processes because of its unique properties: it is
a semiconductor with band gap about 3.0 eV
possessing strong oxidative ability and non-toxicity
[1–3]. However, there are some disadvantages of its
use as a photocatalyst, namely the light adsorption in
UV light spectrum region only and the high
electron (e
–
) – hole (h
+
) recombination rate. These
disadvantages have been faced e.g. by doping TiO
2
with transition metal ions or by special heat treat-
ments [4, 5]. Some of the metal ions, like Ag
+
and Fe
3+
,
when used as additives, have provoked an additional
antibacterial activity of the titania photocatalyst [6–9].
It is well known that high specific surface area is a
desirable property of catalytic materials. The addition
of polyethylenglycol (PEG) to TiO
2
based gels was
reported to increase their surface area and to improve
their photocatalytic activity [10–15]. In this work, Fe
3+
and PEG with average molecular mass 600 (PEG
600
)
were used as dopants for TiO
2
based materials to
improve their properties, namely to extend the light
absorption into the visible light region, to reduce the
recombination of the photogenerated e
–
and h
+
and to
increase the surface area.
The purpose of this study is to characterize the
thermal behavior of Fe
2
O
3
/TiO
2
gel samples and the
effect of PEG
600
admixture on their microstructure
development. Methods of TG, DTA and EGA were
used to elucidate processes that take place during
heating of the Fe
2
O
3
/TiO
2
meso-porous materials. The
microstructure development was characterized by
emanation thermal analysis (ETA) [16, 17] under in situ
heating in air. The ETA was already used in our pre-
vious studies to monitor the microstructure development
of titania samples during heating of their precursors. A
good agreement was found between the ETA results,
surface area measurements and microstructure
characteristics obtained by electron microscopy [18].
Experimental
Preparation of the samples
Sol–gel technique was used to prepare Fe
3+
containing mesoporous titania gels with and without
polyethylene glycol (PEG
600
) addition. The amount
of 76.34 mL tetrabutylorthotitanate Ti(OBu)
4
(Fluka)
was dissolved in 245.84 mL ethanol (C
2
H
5
OH)
absolute (Riedel–de Haen) under vigorous stirring at
room temperature for 1 h. The used molar ratio was
Ti(OBu)
4
:C
2
H
5
OH= 0.093:2.465. The iron(III) nitrate
nonahydrate (Merck) was used as the precursor for
Fe
3+
and for water in the titania based gel preparation.
A solution of 13.58 mL Fe(NO
3
)
3
⋅9H
2
O in 100 mL
ethanol was added to the Ti containing solution and
stirred for 1 h. In order to provide acidic conditions the
amount of 5 mL 65% HNO
3
(Ferak) was added. The
PEG
600
containing sol (0.035 M) was prepared by
1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary
© 2005 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands
Journal of Thermal Analysis and Calorimetry, Vol. 80 (2005) 503–509
THERMAL BEHAVIOR OF Fe
2
O
3
/TiO
2
MESOPOROUS GELS
V. Balek
1*
, N. Todorova
2
, C. Trapalis
2
, V. Štengl
3
, E.Ve¹erníková
3
, J. Šubrt
3
, Z. Malek
1
and
G. Kordas
2
1
Nuclear Research Institute Re, plc, 250 68 Re, Czech Republic
2
Institute of Materials Science, NCSR DEMOKRITOS, 15310 Athens, Greece
3
Institute of Inorganic Chemistry, ASCR, 250 68 Re, Czech Republic
Titania-based photocatalytic materials were prepared by sol–gel method using Fe
3+
and polyethyleneglycol (PEG
600
) as additives.
Thermogravimetry (TG), differential thermal analysis (DTA) and evolved gas analysis (EGA) with MS detection were used to
elucidate processes that take place during heating of Fe
3+
containing titania gels. The microstructure development of the Fe
2
O
3
/TiO
2
gel samples with and without PEG
600
admixtures was characterized by emanation thermal analysis (ETA) under in situ heating in
air. A mathematical model was used for the evaluation of ETA results. Surface area and porosity measurements of the samples dried
at 120°C and the samples preheated for 1 h to 300 and 500°C were compared. From the XRD measurements it was confirmed that
the crystallization of anatase took place after thermal heating up to 600°C.
Keywords: annealing, DTA, EGA, emanation thermal analysis, Fe
2
O
3
/TiO
2
, porosity, sol-gel, surface area, thermogravimetry
* Author for correspondence: bal@ujv.cz