Photocatalytic reduction of nitro compounds using TiO2 photocatalyst by UV and Vis dye-sensitized systems

Article abstract of DOI:10.1002/cjoc.201190094

Nitro-aromatic compounds can be photocatalytically reduced into the corresponding amine-aromatic compounds using TiO₂ as a photocatalyst in the UV/TiO₂/holes scavenger and Vis/TiO₂/dye-sensitized systems. In the UV/TiO₂/holes scavenger system, reaction substrate alcohols such as methanol could be used as the holes scavengers, and in the Vis/TiO₂/dye-sensitized system, substrate alcohols could be oxidized to the corresponding aldehydes with high selectivity. When methanol was used as the holes scavengers and the illumination time was 6 h, 87.2% of p-nitrotoluene could be photocatalytically reduced into p-toluidine. In the Vis/TiO ₂/dye-sensitized system, the effect of aromatic alcohols for the photocatalytic reduction of nitrobenzene was better than that of other alcohols. At the same time, aromatic alcohols can be easily oxidized, and the production efficiencies of the corresponding aldehydes were higher than those of other alcohols. The possible reaction mechanisms were also proposed.

Full text of DOI:10.1002/cjoc.201190094

FULL PAPER
Photocatalytic Reduction of Nitro Compounds Using TiO2
Photocatalyst by UV and Vis Dye-sensitized Systems
Chen, Shifu*(陈士夫)
Zhang, Huaye(张华烨)
Yu, Xiaoling(余晓玲)
Liu, Wei(刘伟)
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, China
Nitro-aromatic compounds can be photocatalytically reduced into the corresponding amine-aromatic com-
pounds using TiO as a photocatalyst in the UV/TiO /holes scavenger and Vis/TiO /dye-sensitized systems. In the
2
2
2
UV/TiO /holes scavenger system, reaction substrate alcohols such as methanol could be used as the holes scaven-
2
gers, and in the Vis/TiO /dye-sensitized system, substrate alcohols could be oxidized to the corresponding alde-
2
hydes with high selectivity. When methanol was used as the holes scavengers and the illumination time was 6 h,
8
7.2% of p-nitrotoluene could be photocatalytically reduced into p-toluidine. In the Vis/TiO /dye-sensitized system,
2
the effect of aromatic alcohols for the photocatalytic reduction of nitrobenzene was better than that of other alcohols.
At the same time, aromatic alcohols can be easily oxidized, and the production efficiencies of the corresponding al-
dehydes were higher than those of other alcohols. The possible reaction mechanisms were also proposed.
Keywords photochemistry, synthetic, method, nitro-aromatic compounds, UV/TiO /holes scavenger, Vis/TiO /
2
2
dye-sensitized systems
4
Introduction
Recently, Zhao and co-workers reported that the
selective oxidation of alcohols was achieved under visi-
ble-light irradiation in a system containing dye-sensi-
Semiconductor photocatalysis has attracted much
research attention in the past decades owing to their ap-
plication in environmental purification, water splitting,
and conversion of solar energy into electrical energy,
tized TiO
,2,6,6-tetramethylpiperidinyloxyl, TEMPO). In the
dye-sensitized system, the dye, rather than the TiO
2
and nitroxyl radicals (such as
2
1
-6
2
etc. It is known that when the surface of semiconduc-
tor absorbs photons and its energy is equal to or higher
than that of the band gap, the low-energy electron of
valence band is excited to conduction band, and elec-
tron-hole pairs will be produced on the surface of semi-
conductor. Photo-excited electron has a higher reducing
power, and photo-excited hole has a higher oxidizing
power. If these photo-excited electron-hole pairs do not
recombine, they would have the reactions of reduction
catalyst, absorbs visible light and is excited. The process
involves two important steps of electron transfer: (1) the
electron injection from the excited dye molecule to the
2
conduction band of TiO generating dye radical; and (2)
the dye-radical promoted oxidation of TEMPO to
＋
TEMPO , which can selectively oxidize the alcohols to
－
－
aldehydes. Dioxygen reacts with e to form O
cb
2
(
which reacts further to give H
cited state of the dye molecules to inject electrons into
the conduction band of TiO incessantly. In the system,
2 2
O ) and enables the ex-
7
-9
and oxidation, respectively. The photoinduced reduc-
tion of nitrobenzene should attract considerable interest
in view of pursuing environmentally ‘benign’ or ‘green’
synthesis, since it consumes low-energy photons and
occurs under atmospheric pressure and at room tem-
perature. Therefore, the study on photocatalytic reduc-
tion by semiconductor has great significance. Ferry et
2
only when electrons are captured by dissolved oxygen
in the solution, can the reaction of selective oxidation of
alcohols proceed. The above result really enlightened us.
If dissolved oxygen is absent and nitro compounds are
added into the reaction system, the selective oxidation
of alcohols and photocatalytic reduction of nitro com-
pounds will be achieved under visible-light irradiation
1
0
11
al. and Flores et al. have reported the results of
photocatalytic reduction of aromatic nitro-compounds in
the presence of sacrificial electron donors, respectively.
We have reported the results of photocatalytic reduction
of nitrobenzene to aniline using titanium dioxide pow-
der as photocatalyst, under the protection of nitrogen
and in the presence of holes scavenger, such as metha-
2
in a system containing dye-sensitized TiO . At the same
－
time, dye radicals were stable as O , and H
2
O
2
were
2
not formed in the system.
In this paper, we first study the photocatalytic reduc-
tion of aromatic nitro-compounds to amine-aromatic
compounds using TiO
2
as a photocatalyst under
1
2
nol, ethanol.
*
E-mail: chshifu@chnu.edu.cn; Tel.: 0086-0561-3806611; Fax: 0086-0561-3803141
Received May 3, 2010; revised September 26, 2010; accepted November 22, 2010.
Project supported by the National Natural Science Foundation of China (Nos. 20673042, 20973071) and the Key Project of Science and Technology
Research of Ministry of Education of China (208062).
Chin. J. Chem. 2011, 29, 399— 404
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
399

Chen et al.
FULL PAPER
UV-light irradiation. Then, the feasibility of selective
oxidation of alcohols and photocatalytic reduction of
aromatic nitro-compounds under visible-light irradiation
in a system containing dye-sensitized TiO was investi-
2
gated. And the reaction mechanisms were also studied.
and filtered through a 0.2 µm millipore filter to remove
the particles. The filtrate was then analyzed. For the
purpose of determining the reproducibility of the results,
at least duplicated actions were carried out under each
condition to seek average results.
Analysis
Experimental
The main products for photocatalytic reduction of
aromatic nitro-compounds are aromatic amino-com-
pounds. The products for selective oxidation of alcohols
are aldehydes. The production efficiency of aromatic
amino-compounds or aldehydes and the residual effi-
ciency of aromatic nitro-compound or alcohol for each
sample were calculated from the following formulas:
Materials
Nitrobenzene, aniline, p-nitrotoluene, m-nitrotoluene,
o-nitrotoluene, p-toluidine, m-toluidine, o-toluidine and
the anthraquinone dye Alizarin Red (AR) were pur-
chased from Shanghai Chemical Reagent Ltd. TiO
2
powder used in the experiments was prepared by ther-
mal decomposition and calcination of a colloidal solu-
tion made by hydrolysis of titanium tetraisopropoxide
η
η
1
2
＝P
＝R
t
/C
0
0
×100%
×100%
(1)
(2)
[Ti(iso-OC
3
H
7
)
4
] in our laboratory. From the analysis, it
sample contains mainly anatase
is known that the TiO
2
t
/C
with the mean grain size of 30 nm. Aromatic alcohols,
aliphatic alcohols such as benzenemethanol, p-methyl
benzyl alcohol, propenol, heptanol, butanol, methanol
where η
is the production efficiency of aromatic
1
amino-compound or aldehyde; η
ciency of aromatic nitro-compound or alcohol; P
amount of aromatic amino-compound or aldehyde in
solution after illumination for t; R is the amount of
aromatic nitro-compound or alcohol in solution after
illumination time t; C is the total amount of aromatic
2
is the residual effi-
(MeOH), ethanol (EtOH) and other chemicals used in
t
is the
the experiments were purchased from other Chinese
chemical reagent company without further purification.
They are of analytically pure grade. Deionized and dou-
bly distilled water was used throughout this study.
t
0
nitro-compound or alcohol in solution before illumina-
tion.
Photoreaction apparatus and procedure
The experiments were carried out in a photoreaction
apparatus. The photoreaction apparatus consists of two
Alcohols and aldehydes in the solutions were ana-
lyzed with a SCHIMADU GCMS-QP2010 Plus. The
structures of the products were confirmed by GC-MS
comparison with standard samples. Aromatic ni-
tro-compounds concentrations were also determined by
gas-chromatography with hydrogen flame detector
(SCHIMADU GCMS-QP2010). (GC conditions: chro-
2
1
2,13
parts.
The first part is an annular quartz tube. A 375
W medium pressure mercury lamp (Institute of Electric
Light Source, Beijing) with a maximum emission at
about 365 nm and a 500 W Xenon lamp (Institute of
Electric Light Source, Beijing) with a maximum emis-
sion at about 470 nm, were used as UV and visible light
sources, respectively. The wavelength of the visible
light is controlled through a cutoff filter (λ＞420 nm,
Instrument Company of Nantong, China). The lamp is
laid in an empty chamber of the annular tube, and run-
ning water passes through an inner thimble of the annu-
lar tube. Owing to continuous cooling, the temperature
of the reaction solution is maintained at approximately
matographic column: AC20; carrier gas: N ; flow rate:
30 mL/min; sample injection volume:
4
µL).
Amino-compound concentrations in the solutions were
determined spectrophotometrically using naphthalene
ethylenediamine azo as a developer.
Results and discussion
2
UV/TiO /holes scavenger system
3
0 ℃. The second part is a sealed beaker of 10 cm di-
－
1
ameter. At the beginning of the experiment, the reaction
solution (UV/TiO
The fixed amount of TiO was 4.0 g•L , methanol
2
2
/holes scavenger system and Vis/
was chosen as holes scavenger. Initial pH of reaction
TiO
2
/dye-sensitized system, volumes are 300 and 100
solution is 4.0 and initial concentration of aromatic ni-
－
4
－
1
mL, respectively) containing reactants and photocatalyst
was put in the sealed beakers, and a magnetic stirring
device was used to stir the reaction solution. The dis-
tance between the light source and the surface of the
reaction solution is 11 cm. Nitrogen was passed through
the solution for 0.5 h before illumination to remove the
dissolved oxygen in the reaction solution. In order to
disperse the photocatalyst powder, the suspensions were
ultrasonically vibrated for 20 min prior to irradiation.
After illumination, the samples were taken from the re-
action suspension, centrifuged at 7000 r/min for 20 min
tro-compounds was 7.3×10 mol•L . The relation-
ships among the residual efficiency of aromatic ni-
tro-compounds, the production efficiency of aromatic
amino-compounds and the illumination time were in-
vestigated. The results are shown in Figures 1 and 2.
From Figures 1 and 2, it can be seen that the produc-
tion efficiency of aromatic amino-compounds increases
and the residual efficiency of p, m, o-nitrotoluene de-
creases with the increase of the illumination time. When
the illumination time increases from 1 to 6 h, the pro-
duction efficiency of p-toluidine increases from 27.2%
4
00
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 399— 404

2
Photocatalytic Reduction of Nitro Compounds Using TiO Photocatalyst
1
1,12
to 87.2%, and the residual efficiency of p-nitrotoluene
decreases from 46.1% to 1.39%. For m-toluidine, the
production efficiency increases from 17.5% to 72.7%,
and the residual efficiency of m-nitrotoluene decreases
from 60.5% to 10.5%. For o-toluidine, the production
efficiency increases from 14.5% to 67.7%, and the re-
sidual efficiency of o-nitrotoluene decreases from
muir-Hinshelwood kinetics model.
The kinetic
curves obtained in the solvent are characteristic for the
formal first-order kinetics, and are fitted to the exponen-
tial function:
C＝A＋B exp(－kt)
(3)
where t is illumination time; C is the concentration of p,
m, o-toluidine or p, m, o-nitrotoluene; k is the rate con-
stant of the decrease of aromatic nitro-compounds or the
production of amine-aromatic compounds; A and B are
69.5% to 15.5%. It is clear that the production effi-
ciency of aromatic amino-compound and the residual
efficiency of aromatic nitro-compounds are greatly re-
lated to the structure of nitro-aromatic compounds. The
degree for photocatalytic reduction of aromatic nitro-
compounds is as follows: (p-nitrotoluene)＞(m-nitro-
toluene)＞(o-nitrotoluene).
t
constants. C is concentration at the time of t.
According to the relationship between the concentra-
tion of aromatic nitro-compounds or aromatic amine-
compounds and the illumination time, the kinetics equa-
tions for photocatalytic reduction of p, m, o-nitrotoluene
－
4
in methanol are C
t
＝7.3×10 /exp(0.7121t), C
t
＝7.3×
－
4
－
4
1
0
/exp(0.3850t) and C
t
＝7.3×10 /exp(0.3211t),
respectively.
The kinetics equations for the formation of p, m,
－
4
o-toluidine in methanol are C
t
＝ － 8.01 × 10
－
4
[
exp(－0.2934t)－1], C
t
＝－8.01×10 [exp(－0.1998t)
－
4
－
1] and C
t
＝－8.01×10 [exp(－0.1735t)－1], re-
spectively.
It is believed that the photocatalytic reduction reac-
tion of aromatic nitro-compounds occurs on the surface
14,15
of TiO
are created on the surface of TiO
such as methanol adsorbed on the surface of TiO
2
.
Under UV illumination, electron-hole pairs
. Holes scavengers
pre-
2
2
Figure 1 Relationship between the residual efficiency of aro-
matic nitro-compounds and the illumination time.
vent the recombination of electron-hole pairs by trap-
ping holes, and the photoexcited electrons are strong
enough to reduce aromatic nitro-compounds adsorbed
on the surface of TiO
2
to aromatic amino-com-
1
1,12,14
pounds.
In the process, the alcohols as holes
scavengers were oxidized to the corresponding alde-
hydes followed by the mineralization of aldehydes into
1
0,11,16,17
CO
It was proposed that the photocatalytic reduction of
nitro-aromatic compounds was a 6-electron reac-
2 2
and H O.
1
2,14,15
tion.
nitrobenzene is as follows: RNO
RNH
The process of photocatalytic reduction of
2
→RNO→RNHOH→
2
.
Figure 2 Relationship between the production efficiency of
aromatic amino-compounds and the illumination time.
Theoretically, if aromatic nitro-compounds are pho-
tocatalytically reduced completely without by-products,
In the process, the main intermediates of the
photoreduction of nitrobenzene (i.e., nitrosobenzene and
N-hydroxylaniline) have been detected in our laboratory.
It can be seen that the reduction efficiency is influenced
by a series of processes and factors including the elec-
1
mol of aromatic nitro-compounds can produce 1 mol
aromatic amino-compounds. From Figure 1 and Figure
, it is clear that, when the illumination time is 8 h,
2
aromatic nitro-compounds such as p-nitrotoluene can
not be photocatalytically reduced to amino-compound
completely, and it produced 10% or more by-products.
It has been reported that the photocatalytic reduction
kinetics for all substrates are found to obey Lang-
2
tron transfer from the conduction band of TiO to a nitro
compound, the back-electron transfer from a nitro radi-
cal anion to the valence band, and proton availabil-
1
2,15
ity.
Chin. J. Chem. 2011, 29, 399— 404
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
401

Chen et al.
FULL PAPER
Based on the above results, if nitrobenzene is chosen
as the deputy, the possible schematic diagram of photo-
catalytic reduction of aromatic nitro-compounds in the
AR/TiO
Vis, (5) AR/TiO
2
＋O
2
＋Vis, (3) AR/TiO
2
＋N
2
, (4) TiO
2
＋N
2
＋
2
＋N
2
＋Vis. The results showed that
there were no obvious reactions for the photocatalytic
reduction of aromatic nitro-compounds and selective
oxidation of alcohols under conditions 1, 3 and 4. The
selective oxidation of alcohols may be achieved under
condition 2, but the photocatalytic reduction of aromatic
2
UV/TiO /holes scavenger system is proposed in Figure
3
.
2
nitro-compounds did not take place. Only when TiO ,
dye, nitrogen and visible-light irradiation are present,
can the photocatalytic reduction of aromatic nitro-
compounds and selective oxidation of alcohols occur.
In order to evaluate the photocatalytic reduction of
aromatic nitro-compounds and selective oxidation of
alcohols, latent solvent carbon tetrachloride and chloro-
form were chosen as reaction solvents, and nitrobenzene
was chosen as a deputy of aromatic nitro-compounds.
Reaction condition is as follows: the volume of the al-
cohol is 5.0 mL; the amount of TiO
centration of nitrobenzene is 0.0406 mol•L , and the
2
is 0.4 g; the con-
－
1
－
4
－
1
concentration of AR used is 5.6×10 mol•L . The
results are shown in Table 1.
Figure 3 Schematic diagram of photocatalytic reduction of
nitrobenzene in the UV/TiO /holes scavenger system.
From Table 1, it can be seen that, for the photocata-
lytic reduction of nitrobenzene, aromatic alcohols and
aliphatic alcohols of short chain are better substrates.
When benzenemethanol and p-methyl benzyl alcohol
were used as substrates, the production efficiency of
aniline is the highest. When aliphatic alcohols were
used as substrates, it is plausible that the aliphatic alco-
hols of short chain are better than those of long chain.
However, the production efficiency of aniline is not
correlative with the chain length of aliphatic alcohols. It
was reported that when reaction substrate has lower
viscosity, higher polarity, lower polarisability and
higher rate of polarity/polarisability, it is favorable for
the photocatalytic reduction of aromatic nitro-com-
2
2
Vis/TiO /dye-sensitized system
In this paper, we used the commercial anthraquinone
dye Alizarin Red (AR) as the sensitizer. It is known that
AR has a high extinction coefficient (εmax＝28000) at
wavelengths of 400—500 nm in the visible region and a
redox potential, which is suitable for effectively driving
the corresponding events of electron transfer from the
2
excited state of AR to the conduction band of TiO . The
0
redox potential E
(－1.57 V vs. standard hy-
AR*/AR+i
drogen electrode, SHE) is lower than Ecb of TiO
2
4
,18,19
(
－0.50 V vs. SHE).
The photocatalytic reactions were carried out at five
20
pounds. From Table 1, it is clear that, the easier alco-
hol is oxidized, the more likely nitro-compounds are
different experimental conditions: (1) AR/TiO
2
＋O
2
, (2)
a
Table 1 Results of the photocatalytic reduction of nitrobenzene and selective oxidation of alcohols
－
1
Production efficiency of aldehyde
Aniline/(mol•L )
Entry
Substrate
Product
t/h
18
18
A
B
A
B
－
－
4
4
－
－
4
4
1
2
85.2
85.1
2.52× 10
2.76× 10
2.36× 10
80.1
80.6
2.26× 10
－
－
4
5
－
－
4
5
3
4
—
OH
＝
O
18
20
55.7
52.1
58.0
50.5
1.41× 10
3.83× 10
1.88× 10
3.85× 10
－
－
5
4
－
－
5
4
5
6
19
30
47.9
70.4
47.4
71.0
4.02× 10
2.89× 10
4.10× 10
3.05× 10
－
－
5
5
－
－
5
4
7
8
30
31
47.4
44.6
46.3
44.7
6.92× 10
9.86× 10
8.92× 10
1.03× 10
a
A: Carbon tetrachloride as solvent, B: Chloroform as solvent.
4
02
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 399— 404

2
Photocatalytic Reduction of Nitro Compounds Using TiO Photocatalyst
reduced. This may be attributed to the fact, the easier
alcohol is oxidized, the more favorable it is for the elec-
tron transfer between dye and photocatalyst, which re-
sults in the increase of the production efficiency of ani-
line.
duced to dye. The above process achieves the cycle of
dye and the dye radical. The photoexcited electrons are
strong enough to reduce aromatic nitro-compounds ad-
sorbed on the surface of TiO to aromatic amino-com-
2
pounds. At the same time, it enables the excited state of
the dye molecules to inject electrons into the conduction
From Table 1, it can be seen that for the selective
oxidation of alcohols, the aromatic alcohols substrates
such as benzenemethanol and p-methyl benzyl alcohol
are easily oxidized, and the production efficiencies of
the corresponding aldehydes are higher than those of
other alcohols. However, aliphatic alcohols exhibited
relatively low activity, and with the increase in chain
length, the activity decreases gradually. Aromatic alco-
hol is oxidized more easily than aliphatic alcohol. The
reason is that although both of them contain OH group
in their structure, the hydrocarbyls connected to OH
group are different. The phenyl in the hydrocarbyl of
aromatic alcohol is a powerful electrophilic group, and
is able to induce α-H on carbon atoms connected to OH
group to activate, and causes aromatic alcohol to be
oxidized more easily. However, the hydrocarbyl in ali-
phatic alcohol does not have the ability to activate α-H.
The aliphatic alcohol with short chain is easier to be
oxidized than that with long chain. The reason is that,
the longer the carbon chain is, the stronger electron do-
nating group it will be, which causes α-H to be less ac-
tivated.
2
band of TiO incessantly.
Based on the above results, a possible mechanism is
proposed in Figure 4.
Figure 4 Possible mechanisms of photocatalytic reduction of
nitrobenzene in the Vis/TiO /dye-sensitized system.
2
2
1-23
It was reported
2
that in the dye/TiO -photosensi-
tized reaction, the dye radicals were transient, active
species decomposed to small molecules and even fur-
ther to CO by complicated dioxygen-involved reactions.
It is interesting that propenol can be oxidized to the
corresponding unsaturated aldehyde. The results corre-
2
In the dye/TiO
2
-photosensitized system, TEMPO is
4
spond with the previous report. At the same time, the
necessary to maintain the stability of dye radicals and
4
results also showed that the selectivity of the oxidation
of alcohols was high, and all of them exceeded 96%.
From Table 1, it can also be seen that the latent solvents
have no obvious effect on the photocatalytic reduction
of nitrobenzene and selective oxidation of alcohols in
high selectivity. However, in the Vis/TiO /dye-sensi-
2
tized system, because there was no dissolved oxygen in
the system and aromatic nitro-compounds acted as
electrons scavenger, the dye radicals were stable in the
reaction process. The conclusion was verified in our
laboratory because in the reaction process, no fragments
of dye degradation were found in the chromatographic
analysis.
2
the Vis/TiO /dye-sensitized system.
The photocatalytic reduction of other aromatic ni-
tro-compounds, such as p-nitrotoluene, m-nirotoluene
and o-nirotoluene were also investigated. The result
showed that when methanol was used as a reaction sub-
strate and the illumination time was 20 h, the production
concentrations of corresponding amino-compounds for
p-nitrotoluene, m-nitrotoluene and o-nirotoluene were
Conclusions
In UV/TiO /holes scavenger system, photocatalytic
2
reduction of nitro-aromatic compounds to amine-aro-
matic compounds can be processed in the presence of
sacrificial electron donor such as methanol. When illu-
minated for 6 h, 7.3×10 mol•L of p-nitrotoluene
can be photocatalytically reduced completely, and more
than 87.2% of p-nitrotoluene can form p-toluidine in
－
4
－
4
－
4
－
1
2
.38×10 , 1.92×10 and 1.87×10 mol•L , re-
spectively. From the above results, compared with the
selective oxidation of alcohols, it is clear that, the effi-
ciency of the photocatalytic reduction of aromatic ni-
tro-compounds is lower. This may be attributed to low
efficiency of the electron injection from the excited dye
－
4
－
1
methanol. In Vis/TiO /dye-sensitized system, the sub-
2
molecule to the conduction band of TiO
tron transfer from the conduction band of TiO
compound and proton availability.
2
and the elec-
to a nitro
strate alcohols could be oxidized to the corresponding
aldehydes with high selectivity, and nitro-aromatic
compounds could be photocatalytically reduced into the
corresponding amine-aromatic compounds. Although
the efficiency of photoreduction is low in the system, it
achieves double purpose for the selective oxidation of
alcohols and the photocatalytic reduction of nitro-aro-
matic compounds.
2
It is known that, in the dye-sensitized system, the
dye, rather than the TiO photocatalyst, absorbs visible
2
4
light and is excited. The photoexcited electron injection
from the excited dye molecule to the conduction band of
TiO
2
generates dye radical. The dye radical can selec-
tively oxidize the alcohols to aldehydes, and it was re-
Chin. J. Chem. 2011, 29, 399— 404
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403

Chen et al.
FULL PAPER
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