N=N bond cleavage of azobenzene through Pt/TiO2 photocatalytic reduction

Article abstract of DOI:10.1039/b001062f

TiO₂ photocatalytic 2e^--reduction of azobenzene to hydrazobenzene is found to occur at λ(ex) > 300 nm while loading of nanometer-sized Pt particles on TiO₂ induces N=N bond cleavage via 4e^--reduction; only photoisomerization occurs in the absence of TiO₂.

Full text of DOI:10.1039/b001062f

NNN bond cleavage of azobenzene through Pt/TiO2 photocatalytic reduction
Hiroaki Tada,*a Masanobu Kubo,b Yoichi Inubushic and Seishiro Itob
a Environmental Science Research Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, 577-8502, Japan.
E-mail: h-tada@apsrv.apch.kindai.ac.jp
b Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae,
Higashi-Osaka, 577-8502, Japan
c
Fuji Pigment Co. Ltd., 2-23-2, Obana, Kawanishi, Hyogo 606-0015, Japan
Received (in Cambridge, UK) 7th February 2000, Accepted 27th April 2000
TiO2 photocatalytic 2e2-reduction of azobenzene to hydra-
zobenzene is found to occur at lex > 300 nm while loading
of nanometer-sized Pt particles on TiO2 induces NNN bond
cleavage via 4e2-reduction; only photoisomerization occurs
in the absence of TiO2.
The electronic absorption spectrum of AB showed two
absorption bands at 423 and 321 nm assignable to the n?p* and
p?p* transitions, respectively, for the trans isomer. The
absorptivity of the p?p* band for the trans isomer is 3.3 times
that for the cis isomer.11 The visible absorption at l > 400 nm
vanishes when the NNN bond of AB is broken. Accordingly, the
n?p* band is a good indication of the presence of the NNN
bond. Without either TiO2 (or Pt/TiO2) or irradiation, the
intensity of the n?p* band was almost invariant, while that of
the p?p* band significantly decreased. This fact suggests that
only trans–cis isomerization occurs under these conditions.12
Also, Pt loading on TiO2 increased the rate of isomerization in
the dark. This is probably due to the decrease in the energy
barrier for molecular rotation around the NNN bond with
adsorption of AB on Pt surfaces. No products other than AB
were detected from the irradiated solution by HPLC, which
supports the above conclusion.
Fig. 1 shows the variation of the concentrations of AB and
products in the presence of TiO2 as a function of irradiation time
(t). AB is slowly reduced to hydrazobenzene (HAB) with a
selectivity of 97% at 0 < t < 3 h. Since the turnover frequency
is calculated to be ca. 3 at t = 3 h, this reaction can be regarded
as photocatalytic. In the absorption spectra, the n?p* band
gradually weakened concurrently with a rapid decrease in the
p?p* band intensity. Providing direct evidence for cleavage of
the NNN bond.
From the viewpoint of “green chemistry”, it is important to
develop new processes for synthesizing useful compounds or
detoxifying harmful compounds utilizing solar energy. Hetero-
geneous photocatalytic oxidations derived from valence band
holes (h+vb) are attracting a great deal of attention for
application to environmental problems.1,2 Much less interest
has been shown in reductive photochemistry despite the fact
that conduction band electrons (e2cb) have a potential to induce
highly selective reduction because of their mild reducing
power.3–6 Most of the azo dyes used widely in textile industries
are carcinogenic and resistant to bacterial degradation, thus
requiring effective wastewater treatment. The groups of Kiwi7
and Oliveira-Campos8 have recently reported photocatalytic
oxidation of azo dyes using TiO2 and Fe2O3. On the other hand,
to our knowledge, the present work is the first study on
heterogeneous photocatalytic reduction of azo dyes. Particular
emphasis is placed on the loading effect of Pt nanoparticles on
a TiO2-photocatalyst.
Anatase TiO2 particles (average diameter = 180 nm, BET
surface area = 9.0 m2 g21) were supplied from Tayca Co. (JA-
1) and 0.1 wt% Pt was deposited on them by photodeposition
(Pt/TiO2).9 The particles (20 mg) were suspended in a 1.0 3
1024 M solution [50 mL, solvent H2O–EtOH (9/1 v/v)] of
azobenzene (AB, > 95%, Tokyo Kasei Co.) in a double-
jacketed cell. After the suspension had been purged with N2 for
15 min, irradiation (lex > 300 nm) was carried out with a 400
W high-pressure mercury arc (H-400P, Toshiba); the light
intensity integrated from 320 to 400 nm (I320–400) was measured
as 3.4 mW cm22. N2 bubbling and magnetic stirring of the
suspension were continued throughout the irradiation while the
reaction temperature was maintained at 31 ± 1 °C by circulating
thermostatted water around the cell through the outer jacket.
Product analysis was performed by both UV–VIS spectroscopy
and high performance liquid chromatography [HPLC measure-
ment conditions: column = Fluofix INW425 4.6 3 250 mm
(NEOS); mobile phase H2O–MeOH (1/1 v/v); flow rate = 3 mL
min21; l = 230 nm].
High-resolution transmission electron micrograph (HRTEM)
images of Pt/TiO2 demonstrated that Pt particles of diameter
2–5 nm are dispersed on the surface of TiO2. The degree of
adsorption of AB increased with loading of Pt (4.3 3 1027
mol g21 for TiO2 and 2.6 3 1026 mol g21 for Pt/TiO2 at an
equilibrium concentration of 4.0 3 1025 M), whereas the
degree of adsorption of EtOH was essentially invariant with Pt
loading. This finding indicates that AB and EtOH preferentially
adsorb on Pt and TiO2 surfaces of Pt/TiO2, respectively. The
interaction between AB and Pt would involve both s-bonding
[p orbital (AB) ? d orbital (Pt)] and p-backbonding [d orbitals
(Pt) ? p* orbital (AB)]. Aliphatic alcohols are known to adsorb
strongly on the surface of TiO2.10
Fig. 1 TiO2 photocatalytic reduction of AB at 31 ± 1 °C: initial pH = 6.4;
TiO2 20 mg/50 mL.
Fig. 2 shows time profiles of the concentrations of AB and
products with irradiation in the presence of Pt/TiO2. The rate of
reduction of AB to HAB markedly increases with loading of Pt
[conversion ca. 100%, selectivity (HAB) = 91% at t = 1 h].
Noticeably, further reduction of HAB to aniline (AN) occurs
[selectivity(AN) = 19.2% at t = 3 h]. In the absorption spectra,
the n?p* band of AB completely disappeared at t = 1 h and
new absorption bands appeared at 280 and 230 nm at 3 h that are
in accordance with the positions for the n?p* and p?p*
DOI: 10.1039/b001062f
Chem. Commun., 2000, 977–978
This journal is © The Royal Society of Chemistry 2000
977

induction period. Further the surface multi-electron transfer is
thought to be assisted by an electron-pool effect of Pt with a
large work function.
In conclusion, the 2e2-reduction of AB to HAB proceeds
selectively by using TiO2 as a photocatalyst, whereas only
photoisomerization occurs in the absence of TiO2. Loading of Pt
on TiO2 not only accelerates the reduction but also enables the
4e2-reduction of AB to AN. Our work thus represents a novel
method for treating wastewater containing azo dyes via
reductive cleavage of the NNN bonds.
We express sincere gratitude to Professor Masakuni Yoshi-
hara (Kinki University) for permission to use HPLC and to Dr
Tomoki Akita (Osaka National Research Institute) for the
HRTEM measurements.
Notes and references
1 Photocatalytic Purification and Treatment of Water and Air, ed. F. D.
Ollis and H. Al-Ekabi, Elsevier Science, Amsterdam, 1977.
2 A. Mills and S. Le Hunte, J. Photochem. Photobiol. A, 1997, 108, 1.
3 J. L. Ferry and W. H. Glaze, Langmuir, 1998, 14, 3551.
4 H. Tada, K. Teranishi, Y. Inubushi and S. Ito, Chem. Commun., 1998,
2345.
5 W. Choi and M. R. Hoffmann, J. Phys. Chem., 1996, 100, 2161.
6 P. Piccinini, C. Minero, M. Vincenti and E. Pelizzetti, J. Chem. Soc.,
Faraday Trans., 1997, 93, 1863.
7 J. Bandara, J. A. Mielczarski and J. Kiwi, Langmuir, 1999, 15, 7680.
8 M. S. Goncalves, A. M. F. Oliveira-Campos, E. M. M. S. Pinto, P. M.
S. Plasencia and M. J. R. P. Queiroz, Chemosphere, 1999, 39, 781.
9 TiO2 particles (10 g) were dispersed in a 5.52 3 1024 M aqueous
solution (245 mL) of H2PtCl6·6H2O, and then 5 mL of MeOH was
added as a reducing agent. After the suspension had been allowed to
stand overnight with stirring in the dark, it was irradiated with UV-light
(lex > 300 nm) for 1 h under an N2 atmosphere.
Fig. 2 Pt/TiO2 photocatalytic reduction of AB at 31 ± 1 °C: initial pH = 6.5;
Pt(0.10 wt%)/TiO2.
bands, respectively, of AN. These facts are consistent with the
results of Fig. 2. The pH of the Pt/TiO2 system decreased from
6.5 (t = 0) to 5.6 (t = 1 h), while the corresponding change was
very small for the unmodified TiO2 system (D pH = 0.2).
Product analysis by gas chromatography confirmed generation
of MeCHO and CO2 in each system. Evidently, EtOH acts as a
reductant in the present photocatalytic reaction.13 It has been
established that EtOH exerts a current doubling effect in TiO2
photocatalysis,14 which would also be responsible for the
reduction to proceed.
The results above clarified the two effects of Pt loading; one
is to increase the rate of 2e2-reduction (AB?HAB) and the
other is to enable 4e2-reduction (AB?AN). Pt loading
enhances both the adsorption of AB (adsorption effect)4 and the
10 R. I. Bickley, Heterogeneous Photocatalysis, ed. M. Schiavello, John
Wiley & Sons, Chichester, 1997, pp. 87–107.
11 J. Griffiths, Chem. Soc. Rev., 1972, 1, 48.
12 G. S. Hartley, Nature, 1937, 140, 281.
charge separation of e2cb···h+ pairs (charge separation
vb
13 H. Tada, K. Teranishi and S. Ito, Langmuir, 1999, 15, 7084.
14 S. R. Morrison and T. Freund, Electrochim. Acta, 1968, 13, 1968.
15 P. Pichat and J.-M. Herrmann, Photocatalysis Fundamentals and
Applications, ed. N. Serpone and E. Pelizzetti, John Wiley & Sons, New
York, 1989, pp. 217–250.
effect).15 In addition, selective adsorption of the reactant (AB)
on reduction sites (Pt) and the reductant (EtOH) on oxidation
sites (TiO2) is observed (reasonable delivery effect).16 The
remarkable increase in the reduction rate can be explained in
terms of these effects. Also, inspection of Fig. 1 and comparison
with Fig. 2 suggests that Pt loading leads to the absence of an
16 H. Tada, K. Teranishi, Y. Inubushi and S. Ito, Langmuir, 2000, 16,
3304.
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Chem. Commun., 2000, 977–978