Photocatalytic production of hydrogen peroxide by two-electron reduction of dioxygen with carbon-neutral oxalate using a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalyst

Article abstract of DOI:10.1039/c2cc34170k

Efficient photocatalytic production of hydrogen peroxide (H ₂O₂) from O₂ and oxalate has been made possible by using a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalyst in an oxygen-saturated mixed solution of a buffer and acetonitrile with a high quantum yield of 14% (maximum 50% for the two-electron process) at λ = 334 nm and a high H₂O₂ yield of 93% at λ > 340 nm.

Full text of DOI:10.1039/c2cc34170k

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COMMUNICATION
Photocatalytic production of hydrogen peroxide by two-electron
reduction of dioxygen with carbon-neutral oxalate using
a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalystw
a
a
a
ab
Yusuke Yamada, Akifumi Nomura, Takamitsu Miyahigashi and Shunichi Fukuzumi*
Received 11th June 2012, Accepted 26th June 2012
DOI: 10.1039/c2cc34170k
Efficient photocatalytic production of hydrogen peroxide (H O )
2
prevention of back electron transfer. Photocatalytic H O produc-
2 2
2
from O
2
and oxalate has been made possible by using a 2-phenyl-4-
2
tion via the O reduction using various electron donors such as
(
1-naphthyl)quinolinium ion as a robust photocatalyst in an oxygen-
alcohols and acetic acid has been examined by various systems
6
7
saturated mixed solution of a buffer and acetonitrile with a high
quantum yield of 14% (maximum 50% for the two-electron process)
using semiconductors or organic photosensitizers. It is desired to
use oxalate as an electron donor for the photocatalytic production
at k = 334 nm and a high H
2
O
2
yield of 93% at k 4 340 nm.
2 2
of H O , because oxalate stored in the edible plants is easily
extracted with boiling water but disposed as a waste material.
However, such carbon-neutral oxalate has yet to be utilized as an
Hydrogen peroxide (H ) has been recognized not only as a
2
O
2
8
efficient electron donor in previous reports, because oxalate
clean oxidant but also as a selective oxidant for various
1
oxidation processes. H O is also a potential candidate for
includes the C–C bond which is kinetically stable under ambient
9
conditions.
2
2
a clean liquid fuel in the next generation, because H O
2
2
Herein, we report photocatalytic H O production by O
2
generates electric power by using an H O fuel cell and emits
2
2
2
2
2
only water and oxygen after the reaction. Currently H O
reduction with oxalate as an electron donor using a 2-phenyl-
+
-(1-naphthyl)quinolinium ion (QuPh –NA, Scheme 1a), which
2
2
4
supplied to industry is mainly produced by the anthraquinone
process, which requires costly hydrogen and large energy
forms the long-lived electron-transfer state upon the photoexcita-
1
0
tion with strong oxidation ability, as a photocatalyst. In the
photocatalytic system, the quantum yield of H O production was
consumption for extraction of H
2
O
2
from an organic reaction
medium. A much simpler and less energy consuming process
should be developed for the wide applications of H
can be simply prepared by reduction of atmospheric O
2
3
2
2
determined by a conventional method using an actinometer.
Nanosecond laser flash photolysis and kinetic measurements were
2
2
O .
H
2
O
2
ꢀ
ꢀ
+
performed to reveal that the quinolinyl radical (QuPh –NA
+
)
by an organic molecule, which possesses high reducing ability.
Oxalate is a compound capable of acting as a strong reductant
produced by the photoirradiation of QuPh –NA reduces O to
2
0
4a
produce H O as indicated in Scheme 1b.
2 2
(
E = À0.475 V vs. NHE) and can be easily found in soil and
4b
The photocatalytic H O production has been performed by
2 2
leaves of various vegetables in a high content of B1 wt%.
photoirradiation (l 4 340 nm) of an oxygen-saturated
1.3 mM) mixed solution (2.0 mL) of a phosphate buffer
Thus, a natural system utilizes oxalate as an electron donor for
producing H in the enzymatic active centre of oxalate
oxidase, which catalyzes the oxidation of oxalate, reduction of
(
(
(
2 2
O
300 mM, pH 6.0; pH was adjusted by NaOH) and acetonitrile
+
MeCN) [1 : 1 (v/v)] containing QuPh –NA (0.22 mM) and
5
O
2
to H
2
O
2
and formation of two moles of CO
COOH) + O - 2CO + H
In this process, CO is solely produced as a by-product. A
2
[eqn (1)].
oxalate (1.5 mM). Fig. 1 shows the time courses of H
2 2
O and
(
2
2
2
2 2
O (1)
CO2 production in the photocatalytic reaction. From the
slopes of these lines in Fig. 1, the rate of CO
À1
2
evolution was
2
determined to be 0.45 mmol h , which is nearly double of
property of oxalate, which can be converted into CO
2
after
À1
H O production rate of 0.23 mmol h , in agreement with the
2
2
oxidation, benefits efficient photocatalytic reactions because of
stoichiometry in eqn (1).
a
Department of Material and Life Science, Division of Advanced
Science and Biotechnology, Graduate School of Engineering,
Osaka University, ALCA, Japan Science and Technology Agency
(JST), Suita, Osaka 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Department of Bioinspired Science, Ewha Womans University,
Seoul 120-750, Korea
b
w Electronic supplementary information (ESI) available: Experimental
procedures, time courses of H production (Fig. S1), CV of oxalic
acid (Fig. S2), quantum yield determination (Fig. S3) and transient
2
O
2
+
Scheme 1 (a) Structure of QuPh –NA and (b) the overall photocatalytic
cycles for H production.
absorption spectra in the presence of O
c2cc34170k
2
(Fig. S4). See DOI: 10.1039/
2 2
O
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8329–8331 8329

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Fig. 1 Time courses of H
production of H and CO
l 4 340 nm) of an oxygen-saturated mixed solution (2.0 mL) of a
phosphate buffer (pH 6.0) and MeCN [1: 1 (v/v)] containing
+
QuPh –NA (0.22 mM) and oxalate (1.5 mM).
2
O
2
and CO
2
production. The photocatalytic
Fig. 2 (a) Recyclability tests of H
2
O
2
production under photoirradiation
O
2 2
2
was performed by photoirradiation
(l 4 340 nm) of an oxygen-saturated mixed solution (2.0 mL) of a buffer
(
+
solution (pH 7.0) and MeCN [1 : 1 (v/v)] containing QuPh –NA (0.22 mM)
and (COOH) (3.0 mM). Relative activity of each cycle was determined by
2
referring to the catalytic activity of the 1st cycle. (b) UV-vis absorption
spectra of the solutions before starting the photocatalytic reaction (solid line)
and after the 5th cycle (dashed line).
The photocatalytic H O production was executed using
2
2
oxalate with different concentrations ranging from 1.5 mM to
+
production rates referring to the first cycle. More than 90% activity
6
.0 mM and QuPh –NA (0.22 mM) in a mixed solution of a
phosphate buffer (200 mM, pH 7.0) and MeCN [1 : 1 (v/v),
.0 mL] as shown in Fig. S1 in ESI.w The concentration of H in
the reaction solution increased linearly for initial 200 min and then
the H production rate decreased at prolonged irradiation time
due to the consumption of oxalate. The initial (90 min) H
2 2
of the first cycle was maintained for each cycle with the H O yield
of each cycle higher than 75%. Oxalate acts as a two-electron
+
reductant, thus, the total turnover number of QuPh –NA for five
+
cycles reached more than 100. The robustness of QuPh –NA was
2
2 2
O
2 2
O
also assured by insignificant change in the UV-vis spectra. Fig. 2b
compares the UV-vis absorption of the solutions before starting the
photocatalytic reaction (solid line) and after the 5th cycle (dashed
2 2
O
À1
production rates increased from 0.53 mmol h (1.5 mM oxalate) to
.6 mmol h (6.0 mM oxalate) in proportion to the concentration
À1
1
+
line). Thus, the organic photocatalyst of QuPh –NA is stable
of oxalate. The H O yields calculated based on oxalate reached
2
2
during the recycling tests.
+
Laser flash photolysis measurements of QuPh –NA were con-
93% in the solution containing 1.5 mM oxalate. The maximum
concentration of oxalate in the mixed solution is 6.0 mM, however,
a further increase in oxalate concentration is possible in a mixed
solution of pure water and MeCN [1 : 1 (v/v)]. Upon increasing the
ꢀ
ꢀ
+
ducted to detect the electron-transfer state (QuPh –NA ) in the
absence and presence of oxalate as shown in Fig. 3. Laser excitation
at 355 nm of a mixed solution of an aqueous buffer (pH 7.0) and
concentration of oxalate to 12 mM, the initial (30 min) reaction rate
À1
+
MeCN [1 : 1 (v/v)] containing QuPh –NA (0.056 mM) results in
reached 6.3 mmol h . On the other hand, the H
2
O
2
yield was less
ꢀ
ꢀ
+
formation of the electron-transfer state (QuPh –NA ) with a
than 30%, because of an increase in pH during the reaction. A
further increase in the concentration of oxalate to 200 mM did not
lead to a dramatic increase in the H O production rate.
quantum yield of 0.34, which is well matched with the previously
10,11
reported value.
ꢀ
ꢀ
+
Formation of QuPh –NA was confirmed by
2
2
ꢀ
the transient absorption at lmax = 420 nm (QuPh moiety) and
The efficiency of the photocatalytic reaction can be evaluated
by the quantum yield (F) of the products, where the F value
was defined as the mole number of H O produced divided by
ꢀ
+
6
90 nm (NA
solution in the absence of oxalate as shown in Fig. 3a.
Additionally, a broad band appearing at around 1000 nm evidences
moiety) at 2.4 ms after photoirradiation of the
10,11
2
2
that of photons absorbed by the photocatalyst. In previous
reports, the F value of H production has been determined to
be 4.2% by photoirradiation (340 nm) of an oxygen-saturated
ꢀ
ꢀ
+
formation of the p-dimer radical cation, [(QuPh –NA )-
2 2
O
+
QuPh –NA)], in the solution.
10,11
(
In the presence of oxalate
8a
(6.0 mM), the decay behaviour of these characteristic bands in
Fig. 3b differs from the case in the absence of oxalate in Fig. 3a.
buffer (pH 7.5–8.0) containing ZnO colloid and oxalate. The
F value of the H production in a mixed solution of water
2 2
O
+
and MeCN [1: 1 (v/v)] containing QuPh –NA (0.057 mM) and
oxalate (200 mM) was determined using a ferrioxalate actino-
meter to be 14% (Fig. S3, ESIw), which is more than three times
of the F value obtained with ZnO. Such high quantum yield is
originated from the high oxidizing and reducing ability of
ꢀ
ꢀ
+
photogenarated QuPh –NA , Ered = 1.87 V and Eox
=
À0.90 V vs. SCE in MeCN, which are sufficient for oxidation
1
2
.
0
of oxalate and reduction of O
The robustness of the photocatalytic system for H
2 2
O produc-
tion was also examined by photoirradiation (l 4 340 nm) of an
oxygen-saturated mixed solution (2.0 mL) of a buffer (pH 7.0) and
+
MeCN containing QuPh –NA (0.22 mM) and oxalate (3.0 mM).
+
Fig. 3 Transient absorption spectra of QuPh –NA (0.056 mM) in a
mixed solution of a deaerated phosphate buffer (pH 7.0) and MeCN
The calculated amount of oxalic acid was added to a reaction
solution after each reaction for four times. The relative activity of
each reaction cycle is indicated in Fig. 2a in terms of H O
[
1 : 1 (v/v)] at 298 K taken at 2.4 ms (red) and 24 ms (blue) after
nanosecond laser excitation at 355 nm (a) in the absence and (b) in the
presence of oxalic acid (6.0 mM).
2
2
8
330 Chem. Commun., 2012, 48, 8329–8331
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9
À1 À1
s , which is close to
The absorption band at l = 420 nm remained at 24 ms after
photoexcitation, on the other hand, the absorption bands at l =
linear plot in Fig. 4d to be 4.6 Â 10 M
1
the diffusion limited value. The second-order rate constant
2
ꢀ
+
ꢀ
+
6
90 nm and at around 1000 nm assigned to the NA and p-dimer
of electron transfer from oxalate to the NA
moiety was
À1 À1
s , which is only 1/300 of kred(O2). Thus, in the
10,11
radical cation, respectively,
7
disappeared in this period. This
1.6 Â 10 M
ꢀ
+
ꢀ
observation suggests that electron transfer from oxalate to the
+
reaction solution, the reduction of O by QuPh –NA
2
ꢀ
NA
moiety of the p-dimer radical cation occurs as predicted
is faster than the oxidation of oxalate even though the
concentration of O (1.3 mM) is smaller than the concen-
tration of oxalate (0–6 mM). These values support the reaction
ꢀ
+
by the higher one-electron reduction potential of the NA moiety
2
10
(Ered = 1.87 V vs. SCE) than the onset oxidation potential of
oxalate (0.80 V vs. SCE in a deaerated mixed solution of a buffer
and MeCN [1 : 1 (v/v)] as shown in Fig. S2 in ESIw).
2
mechanism shown in Scheme 1b, in which the O reduction by
ꢀ
QuPh occurs first and then the remaining electron transfer
À
ꢀ
+
ꢀ
+
to NA occurs to oxidize oxalate.
with oxalate using
The electron transfer from oxalate to the NA
moiety of
from (COO )
2
the p-dimer radical cation was monitored by the decay curves
of absorption at 690 nm at various concentrations of oxalate
as shown in Fig. 4a. The rate obeyed pseudo-first-order
2 2
This is the first report to produce H O
an electron donor–acceptor linked dyad as a robust photo-
catalyst. Oxalate acts as an efficient electron donor in the
photocatalytic two-electron reduction of O to produce H O
2
kinetics and the pseudo-first-order rate constant (kobs
)
2
2
increased linearly with increasing concentration of oxalate.
with a high F value (14%). Laser flash photolysis measurements
manifested that electron transfer from the photogenerated
The second-order rate constant (k ) of electron transfer from
ox
ꢀ
+
oxalate to the NA moiety of the p-dimer radical cation
was determined from the slope of a linear plot in Fig. 4b to be
ꢀ
ꢀ
+
+
+
p-dimer radical cation [(QuPh –NA )(QuPh –NA)] to O
2
+
ꢀ
occurs first to afford QuPh –NA , followed by electron
7
À1 À1
ꢀ
+
1
.6 Â 10 M
s
.
transfer from oxalate to NA
.
ꢀ
The electron transfer from QuPh to oxygen was monitored
This work was supported by Grants-in-Aid (No. 20108010
to S.F. and 24350069 to Y.Y.) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan and NRF/
MEST of Korea through the WCU (R31-2008-000-10010-0)
and GRL (2010-00353) Programs.
by the decay curves of absorption at 420 nm shown in Fig. 4c
in the presence of various concentrations of oxygen. The rate
obeyed pseudo-first-order kinetics and the pseudo-first-order
rate constant (kobs) increased in proportion to concentration
of oxygen. The second-order rate constant (kred(O2)) of electron
ꢀ
Notes and references
transfer from QuPh to O was determined from the slope of a
2
1
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¨
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Fig. 4 (a) Decay time profile of absorption at 690 nm due to
+
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ꢀ
ꢀ
QuPh –NA
at various concentrations of oxalate (0.38 mM, red;
1
.50 mM, blue; 2.3 mM, green; 3.0 mM, black) in the presence of
+
+
QuPh –NA (0.056 mM). QuPh –NA
ꢀ
ꢀ
was produced by the laser
excitation (l = 355 nm) of a deaerated mixed solution of a phosphate
buffer (pH 7.0) and MeCN [1 : 1 (v/v)]. (b) Plot of the pseudo-first-
order rate constant (kobs) for electron transfer from oxalate to
9
S. Yamazaki, Y. Yamada, N. Fujiwara, T. Ioroi, Z. Shiroma,
H. Senoh and K. Yasuda, J. Electroanal. Chem., 2007, 602, 96.
ꢀ
ꢀ
+
QuPh –NA
vs. [oxalate]. (c) Decay time profile of absorption at
10 H. Kotani, K. Ohkubo and S. Fukuzumi, Faraday Discuss., 2012,
155, 89.
ꢀ
ꢀ
+
20 nm due to QuPh –NA with various concentrations of O
4
2
(red,
1
1 Y. Yamada, T. Miyahigashi, H. Kotani, K. Ohkubo and
S. Fukuzumi, J. Am. Chem. Soc., 2011, 133, 16136.
0
.0081 mM; blue, 0.016 mM; green, 0.024 mM; pink, 0.033 mM;
+
purple, 0.049 and gray, 0.065 mM) in the presence of QuPh –NA
1
2 (a) S. Fukuzumi, H. Miyao, K. Ohkubo and T. Suenobu, J. Phys.
Chem. A, 2005, 109, 3285; (b) S. Fukuzumi and S. Kuroda, Res.
Chem. Intermed., 1999, 25, 789.
(
0.056 mM). (d) Plot of the pseudo-first-order rate constant (kobs) for
+
ꢀ
ꢀ
2 2
electron transfer from QuPh –NA to O vs. [O ].
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8329–8331 8331