Aluminium(III) adsorption: A soft and simple method to prevent TiO 2 deactivation during salicylic acid photodegradation

Article abstract of DOI:10.1039/b416598e

Deposition of poisoning species on TiO₂ during salicylic acid photodegradation can be halted when Al(III) has been previously adsorbed on the catalyst surface; this widens the application of photocatalysis to more concentrated solutions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b416598e

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Aluminium(III) adsorption: a soft and simple method to prevent TiO2
deactivation during salicylic acid photodegradation{
Maria Isabel Franch, Jose´ Peral, Xavier Dome`nech and Jose´ A. Ayllo´n*
Received (in Cambridge, UK) 29th October 2004, Accepted 25th January 2005
First published as an Advance Article on the web 11th February 2005
DOI: 10.1039/b416598e
and photoxidation of SA lead to significant TiO2 deactivation. The
poisoning substance has been tentatively proposed to be some Cr–
Ln complex where L is a SA mineralization intermediate.12 SA
photodegradation on biased TiO2 electrodes has also been
observed to lead to surface deactivation that, in this case, has
been ascribed to the formation of (non-characterized) insulating
polymers.13
Deposition of poisoning species on TiO2 during salicylic acid
photodegradation can be halted when Al(III) has been
previously adsorbed on the catalyst surface; this widens the
application of photocatalysis to more concentrated solutions.
Semiconductor heterogeneous photocatalysis has attracted con-
siderable attention owing to its high capability to eliminate
?
The effect of SA concentration on photodegradation rate under
our experimental conditions was studied.{ The high radiation flux
used (8.31 6 1026 einstein L21 s21, uranil actinometer) is expected
to favour efficiency decay when strongly adsorbed organics are
present.14 Increasing SA concentration has a negative effect on
the observed degradation rate (Table 1). A possible explanation is
surface displacement of species like water or hydroxide by
salicylate complexes, which would cause lower yields of
hydroxyl radical formation.14,15 Besides, the amount of surface
hydroxides is directly related to the ability of the TiO2 surface to
adsorb oxygen molecules.16 When electron scavengers are scarce,
the adsorbate promoted electron–hole recombination becomes
favoured.14,17
biorecalcitrant contaminants by means of OH radicals generated
in mild conditions.1 To promote applications on a large scale, the
efficiency of this technique must be improved.2 In this sense,
catalyst deactivation is one of the major drawbacks of hetero-
geneous photocatalysis. The prevention of catalyst deactivation
would lead to lower economic costs and higher organic matter
mineralization efficiency with no need for catalyst regeneration
steps. We have previously reported that Al(III) adsorption
enhances the photocatalytic activity of TiO2 for maleic acid
mineralization.3 Here, it is shown that for salicylic acid (SA), a
model aromatic compound, this simple modification of TiO2 not
only noticeably enhances the rate of SA photodegradation but also
prevents the formation of poisoning species.
SA photodegradation is greatly enhanced by adsorbed Al(III)
(Fig. 1). Catalyst modification is simply done by mixing the TiO2
powder with an Al(NO3)3 solution.§ The Al(III) concentration used
is enough to saturate the TiO2 surface, which corresponds to a
surface concentration about 0.22 Al(III) atom per nm2. The lack of
SA degradation due to the presence of Al(III) in the aqueous phase
was proved by carrying out experiments in the absence of TiO2.
This means that a surface phenomenon must promote the
observed improvement in catalyst photoactivity.
TiO2 deactivation is often observed in gas–solid photocatalytic
processes, due to the accumulation of strongly adsorbed degrada-
tion by-products.4 In exchange, TiO2 poisoning in water suspen-
sions is rarely reported. Both the competitive adsorption of the
solvent and its ability to solve mineralization intermediates help to
keep the surface of the catalyst clean.5 The low organic
concentration usually employed allows the catalyst to be reused.6
Furthermore, the photocatalytic efficiency decay could be too slow
to be noticed on a single catalyst use and, in general, this is the
experimental procedure used. On the other hand, the studies that
report TiO2 deactivation during the photocatalytic degradation of
organic compounds in the aqueous phase7 pay little attention to
the nature of the poisoning species. In other studies, ageing or
deactivation in aqueous suspensions has been related to the
accumulation of inorganic byproducts,8 but the formation of
organic poisoning adsorbates should not be excluded.
The concentration of intermediates in solution is also affected by
the presence of Al(III).{ The direct oxidation pathway of SA
through catechol formation is favoured over the degradation route
through dihydroxybenzoic intermediates. The effect of Al(III) is
visible to the naked eye. While bare TiO2 turns a yellowish-brown
colour, the Al(III) modified catalyst remains nearly uncoloured.
Hence, the most remarkable effect due to the presence of Al(III) is
related to the intermediates accumulated on the catalyst surface.
Adsorbed intermediates were extracted by alkaline aqueous
SA photodegradation and adsorption has been subject of
numerous studies.9–11 Only in a few of these reports has catalyst
deactivation been observed. Simultaneous photoredution of Cr(VI)
1
solutions and analyzed by H and 13C-NMR and ESI-MS.{ In
the case of bare TiO2, the extracted substance corresponds to
678 a.m.u. oligomer (3 repeated units of 226 a.m.u.). The NMR
spectra indicate that the oligomer contains monomeric units whose
{ Electronic supplementary information (ESI) available: experimental
procedure for the extraction of the poisoning substances and additional
figures. Fig. S1: SA and intermediates evolution during the photodegrada-
tion processes. Fig. S2: detail of 1H-NMR spectrum of deactivating
species. Fig. S3: detail of 13C-NMR and DEPT-135 spectra of deactivating
species. Fig. S4: ESI-MS spectrum of deactivating species. Fig. S5: ATR-
FTIR spectrum of SA adsorbed on bare and Al(III) modified TiO2. See
http://www.rsc.org/suppdata/cc/b4/b416598e/
Table 1 Kinetic constant and initial rate for different [SA]0
[SA]0/mMa
0.05
1.7
0.10
1.5
0.25
1.2
0.50
1.0
1.00
0.17
r0/106 M min21
a
[SA]0 were calculated without considering adsorption.
*joseantonio.ayllon@uab.es
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1851–1853 | 1851

View Article Online
While almost complete deactivation is observed on bare TiO2
after the first run, the Al(III) modified catalyst performs higher
degradation yields than bare TiO2 and maintains the photo-
catalytic activity. The same SA addition was repeated twice. At the
end of the third and fourth one hour irradiation steps (not shown)
SA elimination (7% and 5% respectively) is only observed when
Al(III) is present. The decreasing percentage of eliminated SA can
be related to the catalyst dilution and the increase of intermediates
that might compete for the catalyst surface.
Under the experimental conditions of the assay described above,
the SA concentration was lowered 20% due to adsorption on bare
TiO2 whereas it was reduced only 12% in the presence of Al(III).
This suggests that Al(III) is blocking some of the SA adsorption
sites. Degradation rate enhancement is not, therefore, due to
increased SA concentration on or near the catalyst surface, as it
has been observed in lantanide-doped titania materials.21
Furthermore, the ATR-FTIR spectra (measured ex situ; Tensor
Bruker, equipped with MKII Golden Gate) of SA adsorbed onto
TiO2 does not show significant changes due to the presence of
Al(III).{ As a tentative explanation for the reported results, it seems
that adsorbed Al(III) avoids surface blocking by strong adsorbates,
favouring oxygen scavenging of photogenerated e2cb. This
decreases electron–hole recombination and probably prevents the
evolution of organic species through reductive pathways. Also, the
results above show that the presence of Al(III) leads to lower
amounts of adsorbed SA in the dark. In this sense, the positive
effect of Al(III) could involve, to some extent, a lower adsorption
of the photogenerated intermediates.
Fig. 1 Effect of adsorbed Al(III) on TiO2 (1.5 g L21) on the evolution of
remaining SA with irradiation time. pH0 5 3.0; T 5 25.0 uC. [SA]0 is the
value without considering adsorption. Square symbols, 0.25 mM; round
symbols, 1.00 mM. Black symbols, bare TiO2 catalyst; empty symbols,
Al(III) modified TiO2 catalyst.
structure is related to 1,3-dioxanol rings. This substance is
chemically far from SA. Dioxolane-type intermediates have been
found after photodegradation of salicylate modified TiO2
nanoparticles.18 When Al(III) is adsorbed over the titania surface,
no substances different from those observed in the aqueous phase
are detected by the alkaline extraction treatment. Hence, the
oligomer species must be connected with the poisoning phenom-
enon. TiO2 deactivation has been associated to the deposition of
polymers.19 However, polymeric nature is not a reason to be a
poisoning species. To block the TiO2 photocatalytic activity, the
adsorbate must be chemically inert or able to act as a
recombination centre.20 To get a deeper insight into the ability
of Al(III) to prevent catalyst deactivation, an experiment where the
same amount of SA was periodically added to the reactor was
performed (Fig. 2).
The addition of a soluble Al(III) salt is a straightforward way to
improve the photocatalytic activity of TiO2. The easy and mild
procedure reported here is an interesting alternative to the
literature methods (doped materials preparation or composite
materials preparation) which require thermal treatments.
The authors wish to thank to CICYT (project PPQ2002-04060-
C02-01) and the European Commission (CADOX project, EVK1-
CT-2002-00122) for financial support.
Maria Isabel Franch, Jose´ Peral, Xavier Dome`nech and Jose´ A. Ayllo´n*
Departamento de Qu´ımica, Universidad Auto´noma de Barcelona, 08290,
Cerdanyola del Valle`s, Spain. E-mail: joseantonio.ayllon@uab.es;
Fax: +34 93581 2920; Tel: +34 93581 2176.
Notes and references
{ TiO2 Degussa P-25 (0.600 g) was added to a SA aqueous solution at
different concentrations (0.400 L). The pH was adjusted to 3.0, with diluted
HClO4 or NaOH, and the suspension was sonicated for 10 minutes, and
then magnetically stirred in the dark for 30 minutes. After this, irradiation
with a 125 W medium pressure Hg-lamp was carried out under air
bubbling. A water-cooled Pyrex glass jacket filtered UV (l , 290 nm) and
IR radiations. Under these experimental conditions, neither air stripping
nor direct photolysis contribute significantly to SA elimination. The
photocatalytic process was stopped by filtration. The concentration of SA
and intermediates was measured by HPLC with a UV–Visible diode array
absorbance detector; stationary phase: Hypersyl ODS column 5 mm 25 6
0.46 cm; mobile phase: acetonitrile–water (40 : 60) mixture at pH 3
(phosphate buffer) at isocratic flow rate (0.75 mL min21).
§ In the experiments to study the Al(III) effect, the TiO2 (0.600 g) was
suspended into 2.0 6 1024 M Al(NO3)3 aqueous solution (0.20 L). After
adjusting the pH value to 3.0, the suspension was sonicated for 10 minutes
and stirred for 30 minutes in the dark. Next, 0.200 L of SA solution at
different initial concentrations (twice the concentration used in the
corresponding bare TiO2 assay) were added, and the pH readjusted to
3.0 if necessary.
Fig. 2 Effect of adsorbed Al(III) on TiO2 (1.5 g L21) on the evolution of
remaining SA with irradiation time at pH0 5 3.0; T 5 25.0 uC. The initial
[SA] was 5.0 1024 M. The solution was irradiated for one hour. Four
samples of 5 mL each were removed from the reactor at fixed times. After
one hour, the UV lamp was switched off and 20 mL of 2.0 1023 M SA was
added. Next, the suspension was allowed to equilibrate in the dark for
30 minutes before irradiating for one more hour.
1852 | Chem. Commun., 2005, 1851–1853
This journal is ß The Royal Society of Chemistry 2005

View Article Online
1 J-M. Herrmann, C. Guillard, J. Disdier, C. Lehaut, S. Malato and
J. Blanco, Appl. Catal., B, 2002, 35, 281; M. R. Hoffmann, S. T. Martin,
W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69; A. Mills and
S. Le Haunte, J. Photochem. Photobiol., A, 1997, 108, 1.
2 A. Mills and S-K. Lee, J. Photochem. Photobiol., A, 2002, 152, 233.
3 M. I. Franch, J. Peral, X. Dome`nech, R. F. Howe and J. A. Ayllo´n,
Appl. Catal., B, 2005, 55, 105.
4 O. Carp, C. L. Huisman and A. Reller, Prog. Solid State Chem., 2004,
32, 33.
5 G. Marc`ı, M. Addamo, V. Augugliaro, S. Coluccia, E. Garc´ıa-Lo´pez,
V. Loddo, G. Martra, L. Palmisano and M. Schiavello, J. Photochem.
Photobiol., A, 2003, 160, 105.
P. Mandelbaum, M. Matsuyoshi, S. Schiller, S. A. Bilmes and
M. A. Blesa, Langmuir, 1998, 14, 868.
12 G. Colon, M. C. Hidalgo and J. A. Navio, J. Photochem. Photobiol., A,
2001, 138, 79.
13 M. E. Calvo, R. J. Candal and S. A. Bilmes, Catal. Today, 2002, 76,
133–139.
14 J. Cunningham, G. Al-Sayyed, P. Sedlak and J. Caffrey, Catal. Today,
1999, 53, 145.
15 R. Morand, K. Noworyta and J. Augustynski, Chem. Phys. Lett., 2002,
364, 244.
16 K. Chhor, J. F. Bocquet and C. Colbeau-Justin, Mater. Chem. Phys.,
2004, 86, 123.
6 X. Z. Li, H. Liu, L. F. Cheng and H. J. Tong, Environ. Sci. Technol.,
2003, 37, 3989.
7 K. V. S. Rao, M. Subrahmanyam and P. Boule, Appl. Catal., B, 2004,
49, 2398; H. Kominami, H. Kumamoto, Y. Kera and B. Ohtani,
J. Photochem. Photobiol., A, 2003, 160, 99; J.-M. Wu and T.-W. Zhang,
J. Photochem. Photobiol., A, 2004, 162, 171.
8 S. P. Cho, S. C. Hong and S.-I. Hong, Chem. Eng. J., 2004, 98, 245;
P. Fernandez-Iban˜ez, J. Blanco, S. Malato and F. J. de las Nieves,
Water Res., 2003, 37, 3180.
17 M. R. Prairie, L. R. Evans, B. M. Stange and S. L. Martinez, Environ.
Sci. Technol., 1993, 27, 1776; S. T. Martin, J. M. Kesselman, D. S. Park,
N. S. Lewis and M. R. Hoffmann, Environ. Sci. Technol., 1996, 30,
2535.
18 O. V. Makarova, T. Rajh, M. C. Thurnauer, A. Martin, P. A. Kemme
and D. Cropek, Environ. Sci. Technol., 2000, 34, 4797.
19 H. Einaga, S. Futamura and T. Ibusuki, Phys. Chem. Chem. Phys.,
1999, 1, 4903; O. d’Hennezel, P. Pichat and D. F. Ollis, J. Photochem.
Photobiol., A, 1998, 118, 197.
9 S. Tunesi and M. Anderson, J. Phys. Chem., 1991, 95, 3399.
10 A. Mills, C. E. Holland, R. H. Davies and D. Worsley, J. Photochem.
Photobiol., A, 1994, 83, 257.
11 A. D. Weisz, L. Garcia Rodenas, P. J. Morando, A. E. Regazzoni and
M. A. Blesa, Catal. Today, 2002, 76, 103; A. E. Regazzoni,
20 T. Minabe, D. A. Tryk, P. Sawunyama, Y. Kikuchi, K. Hashimoto and
A. Fujishima, J. Photochem. Photobiol., A, 2000, 137, 3; Y. Cho and
W. Choi, J. Photochem. Photobiol., A, 2002, 148, 129.
21 K. T. Ranjit, I. Willner, S. H. Bossmann and A. M. Braun, J. Catal.,
2001, 204, 305.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1851–1853 | 1853