Sunlight-driven hydrogen peroxide production from water and molecular oxygen by metal-free photocatalysts

Article abstract of DOI:10.1002/anie.201407938

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H₂O₂) is a very important subject. Early reported processes, however, require hydrogen (H₂) and palladium-based catalysts. Herein we propose a photocatalytic process for H₂O₂ synthesis driven by metalfree catalysts with earth-abundant water and molecular oxygen (O₂) as resources under sunlight irradiation (δ>400 nm). We use graphitic carbon nitride (g-C₃N₄) containing electrondeficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence-band potential of the catalysts, while maintaining sufficient conduction-band potential for O₂ reduction. Visible light irradiation of the catalysts in pure water with O₂ successfully produces H₂O₂ by oxidation of water by the photoformed valence-band holes and selective two-electron reduction of O₂ by the conduction band electrons.

Full text of DOI:10.1002/anie.201407938

Angewandte
Chemie
DOI: 10.1002/anie.201407938
Photocatalysis
Sunlight-Driven Hydrogen Peroxide Production from Water and
Molecular Oxygen by Metal-Free Photocatalysts**
Yasuhiro Shiraishi,* Shunsuke Kanazawa, Yusuke Kofuji, Hirokatsu Sakamoto,
Satoshi Ichikawa, Shunsuke Tanaka, and Takayuki Hirai
Abstract: Design of green, safe, and sustainable process for the
gases. A new method capable of producing H O without H is
2 2 2
synthesis of hydrogen peroxide (H O ) is a very important
therefore desired.
Photocatalytic H O production on semiconductor mate-
rials, such as titanium dioxide (TiO ), has also been studied.
2
2
2
subject. Early reported processes, however, require hydrogen
H ) and palladium-based catalysts. Herein we propose
2
2
[6]
(
2
a photocatalytic process for H O synthesis driven by metal-
The reactions are usually carried out by UV irradiation (l <
2
2
free catalysts with earth-abundant water and molecular oxygen
O ) as resources under sunlight irradiation (l > 400 nm). We
400 nm) of an O -saturated water with TiO in the presence of
2
2
(
alcohols as the electron and proton donor. Photoexcitation of
2
ꢀ
+
use graphitic carbon nitride (g-C N ) containing electron-
TiO produces the electron (e ) and positive hole (h ) pairs.
3
4
2
+
deficient aromatic diimide units as catalysts. Incorporating the
diimide units positively shifts the valence-band potential of the
catalysts, while maintaining sufficient conduction-band poten-
The h oxidize alcohol and produce aldehyde and protons,
while the e promote two-electron reduction of O2 and
ꢀ
produce H O [Eq. (1) and (2)].
2
2
tial for O reduction. Visible light irradiation of the catalysts in
2
R-CH OH þ 2 hþ ! R-CHO þ 2 Hþ
ð1Þ
ð2Þ
pure water with O successfully produces H O by oxidation of
2
2
2
2
water by the photoformed valence-band holes and selective
O þ 2 Hþ þ 2 eꢀ ! H O
2
2
2
two-electron reduction of O by the conduction band electrons.
2
These reactions proceed without H at room temperature,
2
H
ydrogen peroxide (H O ) is a clean oxidant that emits
2 2
therefore are a safe H O synthesis. The selectivity for H O
2
2
2
2
only water as a byproduct and is widely used for pulp
formation is, however, very low (< 6%). This is because one-
[
1]
bleaching, disinfection, and organic synthesis. H O has also
received much attention as an energy carrier for fuel cells,
offering an alternative to H because it is soluble in water and
can be used in an one-compartment cell for electricity
generation. H O is currently manufactured in industry by
the anthraquinone method based on oxidation of anthrahy-
droquinone by O2; however, it requires a regeneration of
anthrahydroquinone by hydrogenation of the formed anthra-
quinone with H on palladium-based catalysts. Alternative to
2
2
electron reduction of O occurs mainly and produces a super-
2
[
7]
oxide (COOH) radical [Eq. (3)], thus suppressing two-
2
electron reduction of O [Eq. (2)].
2
[
2]
2
2
O
2
þ Hþ þ eꢀ ! OOH
C
ð3Þ
[3]
Earlier, we found that graphitic carbon nitride (g-C N ),
3 4
a metal-free polymeric semiconductor with a graphitic stack-
[8]
ing structure of melem sheets, promotes selective two-
electron reduction of O and produces H with greater than
90% selectivity under sunlight irradiation (l > 400 nm). The
selective two-electron reduction of O is due to the efficient
2
the high-energy-consuming two-step process, an one-step
H O synthesis with H and O has been studied extensively
2
O
2 2
[9]
2
2
2
2
[
4]
[5]
with Pd or Au–Pd bimetallic catalysts. This direct synthesis
quantitatively produces H O , but requires extreme care
because of the potentially explosive nature of H /O mixed
2
formation of 1,4-endoperoxide species on the melem unit
2
2
ꢀ
(Scheme 1). The photoformed e are localized at the C1 and
2
2
ꢀ
N4 positions of melem (Scheme 1a). The e reduces O and
2
creates a superoxo radical (Scheme 1b). This is rapidly
ꢀ
reduced by another e at the para position and produces
[
*] Dr. Y. Shiraishi, S. Kanazawa, Y. Kofuji, H. Sakamoto, Prof. T. Hirai
Research Center for Solar Energy Chemistry, and Division of
Chemical Engineering, Graduate School of Engineering Science
Osaka University, Toyonaka 560-8531 (Japan)
1,4-endoperoxide species (Scheme 1c), which is readily trans-
formed to H O . The rapid endoperoxide formation (Sche-
2
2
me 1b!c) suppresses the COOH radical formation [Sche-
E-mail: shiraish@cheng.es.osaka-u.ac.jp
me 1b!d; Eq. (3)], thus promoting selective two-electron
Dr. S. Ichikawa
Institute for NanoScience Design
Osaka University (Japan)
reduction of O [Eq. (2)].
2
A green and sustainable process for photocatalytic H O
synthesis requires the use of earth-abundant water in place of
alcohol. If this is achieved, oxidation of water by the
photoformed h produces O and protons [Eq. (4)], while
2
2
Dr. S. Tanaka
Department of Chemical, Energy and Environmental Engineering
Kansai University (Japan)
+
2
ꢀ
the e promote two-electron reduction of O [Eq. (2)]. H O
2
2
2
[
**] This work was supported by the Grant-in-Aid for Scientific Research
can therefore be synthesized from water and O2 under
sunlight irradiation with 100% atom efficiency [Eq. (5)].
(
No. 26289296) from the Ministry of Education, Culture, Sports,
Science and Technology (Japan) (MEXT).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407938.
2
H O þ 4 hþ ! O þ 4 Hþ
ð4Þ
2
2
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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.
Angewandte
Communications
[14]
materials.
As summarized in Table 1, the gray-brown
powders of g-C N /PDI obtained have surface areas similar
3
4
x
2
ꢀ1
to g-C N (5–8 m g ). As shown in Figure S1 in the Support-
3
4
[
a]
Table 1: Results for photocatalytic H O production
2
2
Entry
Catalyst
Surface
area
H O2
[mmol]
2
[
c]
2
ꢀ1 [b]
[m g
]
[
d]
1
2
3
4
5
6
7
TiO2
57
-
-
7.8
190
5.6
7.1
-
-
<0.2
<0.2
<0.2
<0.2
<0.2
22.4
50.6
50.2
50.3
[
e]
Au/TiO2
F-TiO2
g-C N
mpg-C N
g-C N /PDI
g-C N /PDI
st reuse
nd reuse
3
4
3
4
3
4
33
3
4
51
[f]
1
2
[f]
8
9
g-C N /PDI
6.7
-
-
39.4
<0.2
<0.2
3
4
59
[
g]
N,N’-dipropylPDI
g-C N +N,N’-dipropylPDI
[
h]
10
3
4
Scheme 1. Selective H O production on g-C N under visible light
2
2
3
4
irradiation. For more details on (a)–(d) see text.
[a] Reaction conditions: water (30 mL), catalyst (50 mg), O (1 atm),
2
l>420 nm (intensity at 420–500 nm: 26.9 Wm ), time (48 h). [b] BET
ꢀ2
surface area. [c] Determined by redox titration with KMnO (detection
4
limit: 0.2 mmol). [d] JRC-TIO-4 TiO supplied from the Catalyst Society of
2
2
H O þ O ! 2 H O
ð5Þ
Japan (equivalent to Degussa P25; anatase/rutile=ca. 83/17). [e] The
amount of Au loaded is 1 wt%, and the average diameter of Au particles
is 3.4ꢁ0.9 nm. [f] Catalyst was reused after simple washing with water
2
2
2
2
The g-C N catalyst is, however, unsuccessful for H O
2
followed by drying in vacuo. [g] 50 mg. [h] g-C
dipropylPDI (25 mg).
N (25 mg) and N,N’-
3 4
3
4
2
[
9]
production from water and O2, because the top of the
valence band (VB) lies at approximately 1.4 V (versus the
[10]
normal hydrogen electrode (NHE), pH 7),
which is insufficient for water oxidation (ca.
[
11]
0
.8 V)
because of the small thermodynamic
driving force. Improving g-C N to promote water
3
4
oxidation while maintaining high selectivity for
two-electron reduction of O is necessary.
2
Herein we report that a simple modification
of g-C N facilitates H O production from water
3
4
2
2
and O . Aromatic diimides are an important class
2
of n-type organic semiconductors for application
to organic field-effect transistors (OFETs) owing
[
12]
to their high electron mobility and stability.
Incorporating the diimides into the semiconduc-
tors positively shifts the oxidation and reduction
[
13]
potentials due to their high electron affinity.
We incorporated pyromellitic diimide (PDI),
a simple aromatic diimide, into the g-C N net-
3
4
work by
a
facile thermal condensation
(
Scheme 2). The obtained g-C N /PDI catalysts,
3
4
when activated by visible light, successfully
oxidize water owing to the positively shifted VB
levels, while maintaining high selectivity for two-
electron reduction of O . This thus facilitates
2
H O synthesis from water and O under sunlight
2
2
2
irradiation.
The g-C N /PDI catalysts containing differ-
3
4
x
ent amounts of PDI units (x [mol%] = 33, 51, and
9) were prepared by calcinating a mixture of
melem and pyromellitic dianhydride at 598 K
5
Scheme 2. a) Synthesis of g-C N /PDI , b) their proposed structure, and c) the struc-
3
4
x
ture of N,N’-dipropylPDI. C gray, H white, N blue, and O red spheres; the second
Scheme 2a), according to procedure for related layer is shown in green for clarity.
(
2
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Chemie
ing Information, transmission electron microscopy (TEM)
images of the catalysts show sheet-like structures, as is the
the results obtained by 48 h reaction. Bare TiO (entry 1)
2
scarcely produces H O (less than detection limit, 0.2 mmol).
2
2
[
15]
[6f]
[6d]
case for g-C N . X-ray diffraction (XRD) of g-C N (Fig-
TiO loaded with Au or modified with fluoride, which
3
4
3
4
2
ure S2) shows a peak at 2q = 27.4 (d = 0.325 nm) assigned to
have been proposed for H O2 production, are inactive
2
[16]
(
002) packing of the melem sheets. Increasing the amounts
(entries 2 and 3). g-C N (entry 4) does not produce H O .
3
4
2
2
of PDI units decreases its intensity, along with a formation of
new two peaks at 2q = 19.0 (d = 0.467 nm) and 29.6 (d =
Mesoporous g-C N (mpg-C N ) with a large surface area
3 4 3 4
2
ꢀ1
[21]
(190 m g ) prepared by a silica-templated polymerization
is also inactive (entry 5). In contrast, g-C N /PDI produce
0
.302 nm). These are assigned to p,p-stacking of PDI
3
4
x
[17]
units and donor–acceptor interaction between melem and
very large amounts of H O (entries 6–8); the amount of H O
2 2 2 2
formed on g-C N /PDI (50.6 mmol) is more than 250-fold of
3 4 51
[18]
PDI units, respectively. These data suggest that, as shown in
Scheme 2b, the PDI units are randomly incorporated within
the melem sheet and the sheets are layered multiply. As
shown in Figure S3, X-ray photoelectron spectroscopy (XPS)
that obtained with g-C N (< 0.2 mmol). As shown by entries 9
3
4
and 10, N,N’-dipropylPDI (Scheme 2c), a reference com-
pound for PDI, and a mixture of g-C N and N,N’-dipro-
3
4
2
of g-C N (N1s level) shows three peaks assigned to sp -
pylPDI scarcely produces H O . These data suggest that
2 2
3
4
hybridized N atoms of melem at 398.5 eV (red, Scheme 2a),
trigonal N atoms of the melem center at 399.4 eV (blue,
Scheme 2a), and terminal amine N atoms at 401.0 eV (green,
incorporating PDI units within the g-C N network facilitates
3 4
H O production from water and O .
2
2
2
Table S1 shows the results of photoreaction on g-C N /
3
4
[
19]
Scheme 2a), respectively. Incorporating PDI units creates
a new peak at 400.1 eV, assigned to imide N atoms of the PDI
PDIx with 2-PrOH as a sacrificial electron donor. The
selectivity for the amount of H O2 formed relative to the
2
[20]
units (purple, Scheme 2). Integrating these signals there-
fore allows rough determination of the mole fraction of PDI
units (x) within g-C N /PDI .
amounts of photooxidation products (acetone and CO ) is
2
about 90%, which is similar to that obtained with g-C N . This
3
4
suggests that g-C N /PDI selectively promote two-electron
3
4
x
3
4
x
[
9]
Diffuse-reflectance UV/Vis spectra of g-C N /PDI (Fig-
reduction of O as does g-C N . Figure S6 shows the change
2 3 4
3
4
x
ure S4) show absorption in the visible region, as is the case for
g-C N ; their band-gap energies are 2.4–2.8 eV. Electrochem-
in the amounts of H O formed during reaction in a water/O
2 2 2
system with g-C N /PDI . The rate of H O evolution is
3
4
3
4
x
2
2
ical Mott–Schottky plots of g-C N /PDI (Figure S5) show
almost constant even after prolonged irradiation, indicating
that the catalysts produce H O without loss of activity. The g-
3
4
x
typical n-type character. The obtained flat band potentials
and band-gap energies provide the band structures of g-C N /
2
2
C N /PDI catalyst, when reused for further reaction
3
4
3
4
51
PDI (Figure 1). Both conduction band (CB) and VB levels
(entry 7, Table 1), shows almost the same activity as the
fresh sample. In addition, the recovered catalyst shows similar
X-ray diffraction (XRD) pattern to that of the fresh one
x
(
Figure S2). These data suggest that the catalyst is stable and
reusable for further reaction.
Figure S7 shows the action spectrum for H O forma-
[22]
2
2
tion on g-C N /PDI . The apparent quantum yields (FAQY)
3
4
51
agree with the absorption spectrum of the catalyst, indicating
that its band-gap excitation promotes water oxidation and O2
reduction. Note that FAQY at 420 nm is 2.6%, which is similar
to that for overall water splitting on a highly active solid-
[23]
solution catalyst (ca. 2.5% at 420 nm). Ab initio calcula-
tions based on the density functional theory (DFT) were
performed within the Gaussian03 program to clarify the
effect of PDI unit on the electronic structure of g-C N with
3
4
simple melem and melem-PDI models (Figure 2). Their main
electronic transitions (S !S ) are HOMO!LUMO and
0
1
HOMO!LUMO + 2, respectively. Incorporating PDI units
Figure 1. Electronic band structures of g-C N and g-C N /PDI . Photo-
graphs show the corresponding powders of the samples.
3
4
3
4
x
decreases both S and S levels; this agrees with the observed
0
1
VB and CB levels (Figure 1). The electrons on both HOMO
and LUMO + 2 of the melem-PDI model are located mainly
at the melem unit with partial distribution to the PDI units.
become more positive by the incorporation of PDI units
[
13]
[13]
owing to their high electron affinity, with the CB levels still
This is reflected by the high electron affinity of PDI unit.
[11]
more negative than the reduction potential of O (0.08 V).
The electrons on HOMO are located at the N2 and N6
positions of the melem unit, and those on LUMO + 2 are at
the C1 and N4 units. These data suggest that these respective
atoms on the melem units behave as the oxidation (N2 and
N6) and reduction (C1 and N4) sites, as is the case for g-
2
These data suggest that g-C N /PDI may possess enhanced
3
4
x
capability for water oxidation with sufficient potentials for O2
reduction.
Photocatalytic reactions were performed by photoirradia-
tion of pure water (30 mL) containing respective catalysts
[
9]
C N4 (Scheme 1).
3
(
50 mg) by a Xe lamp (l > 420 nm) with magnetic stirring
The mechanism for H O formation on the photoexcited
2
2
[
24]
under O atmosphere (1 atm) at 298 K. Table 1 summarizes
g-C N /PDI can be explained as Scheme 3. Photoexcitation
2
3
4
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3
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[25]
to in-plane bending mode of imide moiety. Both
ꢀ
1
the 709 and 982 cm
bands are assigned to
ꢀ1
breathing mode of melem, and the 753 cm band
is assigned to out-of-plane bending mode of
melem. Figure 3a shows the Raman shift of the
melem-PDI model obtained by DFT calculation.
The four bands obtained (619, 665, 766, and
[26]
ꢀ
1
1
004 cm ) are similar to the observed ones (Fig-
ure 3A), indicating that the calculation precisely
represents the electronic structure of g-C N /PDI.
3
4
Figure 3B shows the Raman spectrum of g-C N /
3
4
PDI51 recovered after photoreaction in a 2-PrOH/
ꢀ1
O system. A new broad band appears at 892 cm .
2
Figure 3b depicts the calculated Raman shift of
1
,4-endoperoxide species adsorbed on the melem-
PDI system. The three bands obtained (837, 890,
ꢀ1
and 906 cm ) are assigned to the C1–O symmetric,
O–O stretching, and C1–O asymmetric vibrations
Figure 2. Energy diagrams and interfacial plots of main orbitals for melem (left) and of the endoperoxide species, respectively. They
melem-PDI (right) models calculated at the DFT level (B3LYP/6-31g(d)).
agree reasonably with the observed broad band at
ꢀ
1
8
92 cm (Figure 3B). Figure 3C shows the Raman
spectrum of g-C N recovered after photoreaction.
3
4
ꢀ
+
ꢀ1
by two photons creates two sets of e /h pairs (Scheme 3a).
A similar band appears at 891 cm , assigned to the 1,4-
endoperoxide species. These data indicate that two-electron
reduction of O2 is indeed promoted on the g-C N /PDI
ꢀ
The formed e are localized at the C1 and N4 positions of the
melem unit, whereas h are at the N2 and N6 positions. The h
oxidizes water (Scheme 3b), and the e on either C1 or N4
+
+
3
4
ꢀ
surface (Scheme 3).
position reduces O , producing a superoxo radical (Sche-
me 3c). This radial is rapidly reduced by the e at the para
The activity of H O2 formation increases with the
incorporation of PDI units (Table 1). The reaction consists
2
2
ꢀ
position, producing 1,4-endoperoxide species (Scheme 3d).
Protonation of the species produces H O .
of water oxidation and O reduction. To clarify the activity of
2
these half reactions, water oxidation with AgNO3 as an
electron acceptor and O2 reduction with 2-PrOH as an
electron donor were carried out. As shown in Figure S8 and
S9, the activity of both reactions increases with the amount of
PDI units. The rate of water oxidation with AgNO on g-C N /
2
2
The 1,4-endoperoxide formation on g-C N /PDI
3
4
(
Scheme 3d) is confirmed by Raman spectroscopy. As
shown in Figure 3A, fresh g-C N /PDI exhibits four bands
3
4
51
ꢀ1
ꢀ1
at 645, 709, 753, and 982 cm . The 645 cm band is assigned
3
3
4
ꢀ1
PDI51 is 0.8 mmolh . This is much smaller than
that of O reduction with 2-PrOH (42 mmolh )
ꢀ
1
2
but similar to that of H O formation in a water/
2
ꢀ
2
1
O system (1.1 mmolh , Figure S6). This result
2
suggests that water oxidation is the rate-deter-
mining step for H O formation. The positive
2
2
shift of the VB level by incorporating PDI units
Figure 1) therefore facilitates water oxidation
and promotes H O formation.
(
2
2
Of the catalysts, g-C N /PDI shows the
3
4
51
highest activity (Table 1). The catalyst contain-
ing a larger amount of PDI units (g-C N /PDI )
3
4
59
shows decreased activity despite its more pos-
itive VB level. The half reactions show similar
tendency of the activity (Figure S8 and S9). As
shown in Figure 1, incorporating PDI units also
positively shifts the CB level. One of the
possible reasons for the low activity is therefore
the insufficient reduction capability. In addi-
tion, as shown in Scheme 3, the melem unit acts
as the active sites for both water oxidation and
O reduction. Incorporating larger amounts of
2
PDI units decreases the number of melem units
exposed on the catalyst. This may be another
possible reason for the low activity. The g-C N /
Scheme 3. Proposed mechanism for H O formation on the photoactivated g-C N /PDI
from water and O2. For more details on (a)–(d) see text.
2
2
3
4
[
24]
3
4
4
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Angewandte
Chemie
Figure 4. a) Spectral irradiance for sunlight (green, Nov 23, 2013 at
:00–17:00, north latitude 34.78, east longitude 135.58; light intensity
8
ꢀ2
at 300–500 nm, 2.8 mWcm ) and solar simulator (orange) without
and (blue) with l>420 nm filter (light intensity at 300–500 nm and
ꢀ2
4
20–500 nm are 2.9 and 1.3 mWcm , respectively). b) Time-depen-
dent change in H O evolution during photoreaction with g-C N /PDI
51
2
2
3
4
in a water/O system under irradiation by solar simulator (orange)
2
without and (blue) with l>420 nm filter.
These data suggest that g-C N /PDI is successfully activated
3
4
by visible region light within sunlight and promotes efficient
H O production.
2
2
In summary, we found that sunlight activation of g-C N /
3
4
PDI produces H O from water and O . This promotes water
2
2
2
Figure 3. Raman spectra for A) fresh g-C N /PDI , and B) g-C N /
3
4
51
3
4
oxidation and two-electron reduction of O , facilitating
PDI51 recovered after photoreaction, and C) g-C N recovered after
2
3
4
photoreaction (12 h) in a 2-PrOH solution with O . Calculated shift for
efficient production of H O . At present, the catalytic activity
2
2
2
a) the melem-PDI model and b) the model with 1,4-endoperoxide
species (B3LYP/6-31g(d)).
is insufficient; the amount of H O formed by 48 h reaction is
2 2
approximately 30 mmol (1 mm). Activity improvement is
necessary for practical application. Nevertheless, the present
photoprocess has significant advantages: 1) metal-free cata-
lyst, 2) cost-free light (sunlight), 3) earth-abundant resources
PDI51 catalyst has appropriate VB and CB levels and
relatively large number of active sites (melem units), thus
facilitating efficient H O production.
(water and O ), and 4) mild reaction conditions (atmospheric
2
pressure and ambient temperature). The basic concept
presented herein based on the band engineering of g-C N
4
2
2
Efficient H O formation on g-C N /PDI requires visible
2
2
3
4
3
light irradiation (l > 400 nm); UV irradiation decreases the
efficiency. Photoreaction with a solar simulator as a light
source confirms this. As shown in Figure 4a, spectral irradi-
ance of the simulator (Figure 4a, orange) ranges from UV to
visible region and is similar to that of sunlight (Figure 4a,
with aromatic diimide may contribute to the design of safe,
green, and sustainable H O synthesis by sunlight.
2
2
Received: August 4, 2014
Published online: && &&, &&&&
green). Figure 4b shows the time-dependent change in H O2
2
Keywords: graphitic carbon nitride · hydrogen peroxide ·
oxygen · photocatalysis · water
evolution with g-C N /PDI in a water/O2 system under
3
4
51
.
irradiation by a solar simulator. Irradiation of entire wave-
length light (Figure 4b, orange) successfully produces H O at
2
2
the early stage, but the formation rate decreases with time.
This is due to the decomposition of the formed H O by
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2
2
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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ꢀ
+
e /h pairs and their consumption is tentatively proposed in
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ꢀ
+
stage, the position of another two e /h pairs on the melem unit
4893 – 4908.
and the mechanism for O formation are unclear.
2
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[28] This is confirmed by photoirradiation of a water (30 mL) with
H O (100 mmol) under Ar. The rate of H O decomposition
2
2
2
2
ꢀ1
under > 300 nm irradiation is 4.5 mmolh , which is similar to
ꢀ1
that obtained with 50 mgg-C N /PDI (4.8 mmolh ). In con-
3
4
51
trast, the decomposition rates under > 420 nm irradiation with-
ꢀ
1
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out and with catalyst are 0.5 and 0.7 mmolh , respectively. These
data indicate that photocatalytic decomposition of H O scarcely
2
2
occurs and direct decomposition by absorbing UV light is the
crucial factor for H O decomposition.
2
2
6
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These are not the final page numbers!

Angewandte
Chemie
Communications
Photocatalysis
Y. Shiraishi,* S. Kanazawa, Y. Kofuji,
H. Sakamoto, S. Ichikawa, S. Tanaka,
T. Hirai
&&&& — &&&&
Peroxide made by sunbathing: Sunlight
irradiation (l>400 nm) of graphitic
carbon nitride (g-C N ) containing elec-
and O . This metal-free H O synthesis is
2 2 2
driven by oxidation of water by the
photoformed valence-band holes and
Sunlight-Driven Hydrogen Peroxide
Production from Water and Molecular
Oxygen by Metal-Free Photocatalysts
3
4
tron-deficient aromatic diimide units
successfully produces H O from water
selective two-electron reduction of O by
2
the conduction-band electrons.
2
2
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7
These are not the final page numbers!