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J. Rathousky´ et al. / Catalysis Today 161 (2011) 202–208
togeneration of OH radicals responsible for the primary attack on
the organic matter as well as the break-up of the compactness and
lowering the adhesion of the carbonized layer, which can be due to
both liquid water and its vapour at higher temperatures.
4. Conclusion
The carried out photocatalytic experiments showed that meso-
porous TiO2 films with large surface area and pores of ca 10 nm
in size are efficient photocatalyst in the decomposition of thin
layers of oleic acid deposited by dip- or spray-coating on their
surface and are therefore possible candidates for self-cleaning coat-
ings. While nonanal and 9-oxononanoic acid were identified as
primary intermediates of the photocatalytic degradation of oleic
acid, azelaic acid and nonanoic acid were the secondary interme-
diates. These compounds have been shown to correspond with the
main products determined by quantum mechanical calculations of
photocatalytic degradation of model cis-3-hexenoic acid. In these
calculations the oxidative degradation was induced by an attack
of hydroxyl radical on the double bond. It has been found that the
eliminated hydroxyl radical could act catalytically and thus cause
accelerated degradation of unsaturated compound including fatty
acids.
Fig. 9. Time evolution of contact angle for water and mass of oleic acid deposited
on one-layer TiO2 film during the irradiation with UV light. Oleic acid was deposited
by spraying its 5% heptanes solution. Dotted and solid lines show two independent
experiments for different amount of deposited oleic acid.
including fatty acids. Further results of the theoretical study will
be described in detail in a separate article.
3.3. Photocatalytic performance followed by the contact angle
measurement
Acknowledgement
Photocatalytic experiments were aimed on the assessment of
the effect of the thickness of the mesoporous film of TiO2 and the
mass and way of the deposition of oleic acid on the photocatalytic
performance. Two different methods used for the spreading of oleic
acid provide deposits with completely different character, namely
its smooth and uniform layer by dip-coating or more or less inter-
connected droplets by spraying. These two variants could simulate
different situations which can occur in real applications.
Firstly, the photocatalytic performance of one- and two-layer
TiO2 films was compared, the oleic acid being deposited by dip-
coating from its5%heptanesolution. BothTiO2 films exhibitedafast
decrease in the contact angle for water corresponding to the high
rate of the photocatalytic transformation of oleic acid. At shorter
irradiation times, the degradation proceeded faster on two-layer
TiO2 film. In the case of higher concentrations of oleic acid (10%)
in its heptane solution used for dip-coating, the decrease in the
contact angle for water was slower. However, even for the larger
layer TiO2 film was only slightly worse.
The photocatalytic degradation of oleic acid deposited by spray-
ing on one-layer TiO2 film was followed also by a gravimetric
method (Fig. 9). During 30 h of irradiation, the mass of the organic
layer was gradually decreasing. A major decrease in mass occurred
within the first 20 h that correlates well with the contact angle
measurements.
To assess the limits of the photocatalysis on mesoporous layers
of TiO2, also the degradation of partly carbonized layers of oleic
acid was attempted, which is of course much more difficult. The
complete removal was possible only for thinner carbonized layers.
Thicker layers could not be removed even after prolonged UV irradi-
ation, probably due to their very limited UV transparency. However,
we found that the efficiency of removal was enhanced by higher
humidity and temperature during the photocatalytic process, espe-
cially in the saturated water vapour at 60 ◦C. The water vapour
seems to have twofold beneficial effects, namely an increased pho-
The authors are thankful to the Grant Agency of the Czech
Republic (grants no. 104/08/0435-1 and 203/08/H032) and to the
Ministry of Education, Youth and Sport of the Czech Republic
(project 1M0577).
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