Photocatalytically green synthesis of H2O2 using 2-ethyl-9,10-anthraquinone as an electron condenser

Article abstract of DOI:10.1039/c4ra09702e

A high level (～9 mM) of hydrogen peroxide is photocatalytically synthesized by using O₂ as an oxidant and 2-ethyl-9,10-anthraquinone (EAQ) as an electron condenser. The photo-catalytic efficiency of the EAQ-assisted H₂O₂ production is ca. 10-fold higher than that of H₂ generation with Pt/P25. This journal is

Full text of DOI:10.1039/c4ra09702e

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Photocatalytically green synthesis of H O using 2-
2
2
ethyl-9,10-anthraquinone as an electron
Cite this: RSC Adv., 2014, 4, 52199
condenser†
Received 3rd September 2014
Accepted 10th October 2014
Dandan Zhang, Gangqiang Xu, Tao Chen and Feng Chen*
DOI: 10.1039/c4ra09702e
www.rsc.org/advances
3
+
À
4+
À
A high level (ꢀ9 mM) of hydrogen peroxide is photocatalytically
Ti (e ) + O / Ti –O2c
(1)
(2)
(3)
2
2
synthesized by using O as an oxidant and 2-ethyl-9,10-anthraquinone
3
Ti (e )–O
+
À
À
+
4+
À
2
c
+ H / Ti – O
2
H
(
EAQ) as an electron condenser. The photo-catalytic efficiency of the
EAQ-assisted H
O
2 2
production is ca. 10-fold higher than that of H
2
4
Ti – O
+
À
+
4+
2
H + H / Ti + H
2
O
2
generation with Pt/P25.
3
+
À
The photoelectron on the TiO surface, Ti (e ), reduces the
2
5a
adsorbed O2 and produces a superoxo radical. Then the
Hydrogen peroxide (H
2
O
2
) is a clean oxidant that emits only
superoxo radical is transformed to a hydroperoxo specie via
further reduction, and protonated to produce H O . Unfor-
2 2
water as a byproduct and is widely used in industries for organic
7,8
1
synthesis, pulp bleaching, waste-water treatment, and as a
2
tunately, the hydroperoxo species are easily decomposed by
disinfectant. Traditionally, H
2
O
2
is produced with an anthra-
with O
multistep route via an anthraquinone/
anthrahydroquinone cycle. Recently, from the viewpoint of
green and sustainable chemistry, direct synthesis of H O from
À
8
further reduction with e to produce hydroxyl radical, which
quinone oxidation (AO) process which combines H
through
2
2
has been frequently veried with Electron Paramagnetic Reso-
a
8,9
nance (EPR) technique.
2
2
3+
Ti (e )– O
À
À
+
4+
À
2
H + H / Ti –cOH + HO
(4)
3
H
2
and O
potentially explosive nature of H
serious risk to the operation process; further, highly pure H
is indispensable which requires signicant energy costs in both
the AO process and the H /O direct synthesis process. To
2
has attracted a lot of attention. However, the
/O gas mixture brings a
gas
2
2
Also, the back reaction of the hydroperoxo specie with photo-
2
6
holes is highly possible.
4+
Ti – O
À
+
4+
4+
À
+
+ H
2
2
H + h / Ti –O
H 4 Ti –O
c
(5)
2
2
2
circumvent the above disadvantages, photocatalytic attempts
have begun to emerge nowadays. By using methanol/ethanol as
4
10
So far, H O -assisted photocatalysis is inevitable, which
2
2
a sacricial electron donor, direct synthesis of H O is reported limits the H O concentration (from tens of mM to several mM)
2
2
2 2
5
to be available.
However, direct photocatalytic generation of H
in most reported systems for directly photocatalytic H O2
from production. Hereinaer, we introduce an experimental attempt
2
2
O
2
methanol is not an easy task due to the undesirable rapid to photocatalytically produce H O of high levels by innovatively
2
2
5c
reverse reactions as well as the over-reduction of surface employing 2-ethyl-9,10-anthraquinone (EAQ) as an electron
hydroperoxo species. As we know, the photocatalytic produc- condenser. The photogenerated electrons are trapped by EAQ
6
tion of H
below.
2
c,7,8
O
2
is a multistep process, which is briey shown during the photocatalytic process by forming 2-ethyl-9,10-
5
anthrahydroquinone (H EAQ), which subsequently induces
2
reduction of O
2 2 2
molecules into H O by simple air bubbling in
dark (eqn (6), Fig. S1†).
Key Laboratory for Advanced Materials, Institute of Fine Chemicals, East China
University of Science and Technology, 130 Meilong Road, Shanghai, 200237, PR
China. E-mail: fengchen@ecust.edu.cn
H
2
EAQ + O
2
/ EAQ + H
2
O
2
(6)
†
Electronic supplementary information (ESI) available: Synthesis and
characterizations of the Pt/P25 catalyst, photocatalytic synthesis,
determination of H , hydrogen evolution, by-products analysis with HPLC
and LC-MS, mechanism of H generation, reduction kinetics of EAQ and the
Pt nanoclusters deposited P25 TiO
of 0.4 wt%) was employed as the photocatalyst, which was
prepared with a typical photocatalytic reduction method and
2
(Pt/P25, with a Pt percent
2 2
H O
2 2
O
11
2 2
O
corresponding reaction rate constants. See DOI: 10.1039/c4ra09702e
had a light gray color. Pt/P25 contains just similar TiO lattice
2
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phase composition with that of P25 (rutile/anatase: 20/80,
Fig. S2†), of which Pt nanoclusters of ca. 2 nm are well depos-
ited on the TiO grains (20–30 nm, Fig. S3†). Alternatively, some
2
12a
12b
other TiO composites, such as Ag/TiO
and MoS /TiO ,
2 2
2
2
can also be used as the photocatalyst in this work.
The photocatalytic reaction was carried out by using meth-
anol (100 mL) as the solvent and the hole scavenger, EAQ as the
electron condenser, and N
2
bubbling to keep the system
anaerobic. The dosage of Pt/P25 was 0.2 g. Pt/P25 has signicant
photoabsorption at wavelength less than 406 nm (Fig. S4†),
while EAQ extends its photoabsorption to 471 nm (Fig. S4†).
Hence, the reaction solution was irradiated under either direct
irradiation (300 W Xe lamp, PLS-SXE300) or visible irradiation
(
with an optical lter, l > 420 nm). Fig. 1A shows the photo-
catalytic generation of H and the corresponding control
reactions. H is kept absent throughout the reaction under
2 2
O
2 2
O
visible irradiation with photocatalyst or under direct light irra-
diation without the Pt/P25 photocatalyst, but generated
smoothly under direct irradiation with photocatalyst. As EAQ
absorbs both UV and visible light while TiO2 can only be
aroused with UV light, it suggests that the H O is generated via
2 2
a photocatalysis-related process. Briey, photogenerated elec-
trons in the Pt/P25 photocatalyst reduce the EAQ molecules to
give H
2
EAQ, while the holes at the valence band of Pt/P25 are
EAQ solution is
captured by hole scavenger, methanol. When H
2
air bubbled, the reaction of oxygen with H EAQ leads to the
2
formation of H O . Notably, the H O production rate is ca. 10-
2
2
2 2
fold than that of H with Pt/P25 according to their initial slopes
2
as shown in Fig. 1A.
Fig. 1B shows the generating proles of H
EAQ dosages in this photocatalytic system. The H
2
O
2
with various
2 2
O is initially
Fig. 1 (A) Photocatalytic generation of H
tion with photocatalyst, (b) visible irradiation with photocatalyst, (c)
direct irradiation without photocatalyst, and (d) photocatalytic
2
O
2
under (a) direct irradia-
produced efficiently in all cases; however, at the EAQ dosage of
À1
1
.0 g L , the H
2
O
2
production gets slower and reaches its generation of H2 under direct irradiation. (B) H O generation with
2
2
À1
À1
À1
À1
maximum at a reaction time of ca. 1.5 h. The measured and the EAQ dosages of (a) 1.0 g L , (b) 2.0 g L , (c) 3.0 g L , (d) 4.0 g L , (e)
tted maximal produced H O concentrations are 2.6 mM and
2 2
À1
6
.0 g L with the Pt/P25 photocatalyst.

2
.5 mM, respectively. Further prolonging the irradiation time
leads to a slow decrease in H concentration. The exhaustion
of EAQ would be the main constraint that limits the maximal
level in this reaction. Meanwhile, two possible reasons can
also be responsible for the adverse H concentration
2 2
O
concentration of 9.1 mM is produced with EAQ dosage of 4.0 g
À1
L
at a reaction time of 8.2 h, which should be ascribed to that
2 2
H O
more concentrated EAQ benets the photoelectron scavenge
and thus favors the formation of H EAQ. However, as the light
2 2
O
decrease, which we observed in Fig. 1B: one is the back-reaction
2
absorption region of TiO
increasing the EAQ dosage seems adverse to the initial gener-
ation rate of H . EAQ of high concentrations would signi-
2
mostly overlaps with that of EAQ,
of H EAQ with the photogenerated holes, which slowly becomes
2
serious at higher H EAQ concentration. The other is the
2
2 2
O
chemical deterioration of EAQ; as we know, the chemical
structure change of EAQ, especially on its anthracene ring,
cantly compete with Pt/P25 on photon absorption and thus
hinder some the photoexcitation of Pt/P25 photocatalyst.
Nevertheless, a high initial EAQ dosage would enhance the
would alter the redox potential of EAQ/H
production from O via H EAQ.
Although a H concentration of 2.5 mM is quite high
among the literature reported H levels with the directly
2
EAQ and disable the
H
2
O
2
2
2
reachable H EAQ level under irradiation; i.e., the generated
2
O
2
2
amounts of H O with higher EAQ dosages would slowly catch
2
O
2
2 2
4
up with and even surpass those with lower EAQ dosages as
shown in Fig. 1B. However, much longer irradiation times are
photocatalytic synthesis, optimizing the reaction parameters to
get a better production is preferred. Increasing the EAQ dosage
À1
required for reaching the maximal H EAQ levels with the higher
to 2.0 g L changes much on the H O generation. As shown in
2
2
2
EAQ dosages.
Fig. 1B, the H
2 2
O concentration further increases and reaches
The side reactions during the EAQ-assisted photocatalytic
H O production are undesired and adverse in this work, just as
2 2
those in the industrial AO process. Therefore, the intermediates
and the products of the reaction solution were monitored with
its maximum (5.3 mM) at an irradiation time of 3.9 h, which is
À1
ca. 2.1-fold than that with EAQ dosage of 1.0 g L . As shown in
Table 1 (also Table S1†), the maximal H concentrations
become higher at higher EAQ dosages; a maximal H
2 2
O
2 2
O
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Table 1 Maximal concentrations and corresponding yields of H O with various EAQ dosages
EAQ Concentration
Maximal H
(mM)
2
O
2
concentration
À1
a
a
a
(g L
)
(mM)
Optimal reaction time (h)
Maximal yield (%)
1
2
3
4
6
.0
.0
.0
.0
.0
4.23
8.46
12.70
16.93
25.39
1.4
3.9
6.5
8.2
2.5
5.3
8.3
9.1
59.0
62.4
65.7
54.0b
34.4
b
b
9.0
8.7
a
b
Fitting data in Fig. 1B. Fitting data at 9 h.
HPLC (Fig. S5†) and LC-MS (Fig. S6 and S12†) techniques.
Table 2 lists all the substances that were detected out in the
solution. Correspondingly, a schematically photo catalytic
2
reaction of EAQ is proposed in Fig. 2. H EAQ is obtained from
EAQ via two successive single-electron reduction processes.
However, an inverse oxidation is also possible to the semi-
hydroanthraquinone intermediate, which leads to the forma-
tion of oxo-H EAQ1 and oxo-H EAQ2 intermediates. Due to the
2
2
strong reducibility of H EAQ, oxo-H EAQ1 and oxo-H EAQ2,
2
2
2
they would quickly react with oxygen molecules to produce
and transform themselves back to EAQ, oxo-EAQ1 and
oxo-EAQ2, respectively. A prolonged photocatalytic process
induces a deep hydrogenation of H EAQ, which results the
formation of 2-ethyl-9-anthranol, 2-ethyl-10-anthranol, 2-ethyl-
0-H-9-anthranone (EAN1) and 2-ethyl-9-H-10-anthranone
EAN2). These four deep hydrogenation products are not
2 2
H O
2
Fig. 2 Proposed photocatalytic reduction reaction pathways for EAQ.
1
(
Table 2 Chemical substances detected out with LC-MS after photocatalysis (air bubbled)
Time (min)
Formula
Substance
Structure
Meas. m/z
Pred. Mw
252.122
1
1.24
1.52
C
16
16
H
12
12
O
O
3
3
Oxo-EAQ1
253.120
1
C
H
Oxo-EAQ2
253.120
252.122
1
1
1
1
4.98
5.27
7.32
7.79
C
16
C
16
C
16
C
16
H
14
H
14
H
14
H
14
O
O
O
O
2-Ethyl-9-anthranol
2-Ethyl-10-anthranol
EAN1
223.110
223.109
223.108
223.108
222.111
222.111
222.111
222.111
EAN2
1
2
8.83
0.41
C
16
16
H
12
12
O
O
2
2
EAQ
—
—
C
H
EAQ isomer
—
237.089
236.091
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2
2
the last substance (isomer of EAQ) identied from the reaction
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2 2
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2
2
À1
with 4.0 g L EAQ for 8.2 h irradiation, which is advantageous
comparing with the directly photocatalytic route for H O
2
2
production (from tens of mM to several mM). The enhanced
formation is due to the electron condensing with EAQ and
the “off-site” H generation strategy, which circumvents the
adverse over-reduction or back-reaction of hydroperoxo species
the precursor of H ) during the photocatalytic process. It
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H O
2 2
O
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2 2
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Acknowledgements
This work was supported by the National Nature Science
Foundations of China (21177039) and the Innovation Program
of Shanghai Municipal Education Commission (13ZZ042).
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52202 | RSC Adv., 2014, 4, 52199–52202
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