A polymeric-semiconductor-metal-complex hybrid photocatalyst for visible-light CO2 reduction

Article abstract of DOI:10.1039/c3cc45532g

A polymeric carbon nitride semiconductor is demonstrated to photocatalyse CO₂ reduction to formic acid under visible light (λ > 400 nm) with a high turnover number (>200 for 20 hours) and selectivity (>80%), when coupled with a molecular ruthenium complex as a catalyst. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc45532g

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DOI: 10.1039/C3CC45532G
Graphical and textual abstract
D
CO2
h+ e−
HCOOH
D+
Visible light
(λ > 400 nm)
Carbon nitride
(C3N4)
C3N4, coupled with a ruthenium complex, photocatalyses CO2 reduction into HCOOH
under visible light (λ > 400 nm).

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DOI: 10.1039/C3CC45532G
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
A polymeric-semiconductor/metal-complex hybrid photocatalyst for
visible-light CO2 reduction†
Kazuhiko Maeda,*a,b Keita Sekizawa,a Osamu Ishitania
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A
B
mpg-C3N4
A polymer semiconductor of carbon nitride is demonstrated
to photocatalyse CO2 reduction into formic acid under visible
light (λ > 400 nm) with a high turnover number (>200 for 20
hours) and selectivity (>80%), when coupled with a molecular
1.0
0.8
0.6
0.4
0.2
0.0
Ru/mpg-C3N4
Ru in methanol
50
55
60
10 ruthenium complex as a catalyst.
Perfect structure of C3N4
N
Photocatalytic reduction of CO2 is becoming an important
research subject in modern chemistry in order to address the
depletion of carbon resources and the suppression of global
warming as well as to accomplish CO2-reduction half cycle in
15 artificial photosynthesis.1,2 Certain metal complexes based on
rhenium or ruthenium are known to catalyse CO2 reduction into
CO or HCOOH photocatalytically or electrochemically with high
selectivity and quantum yields.3–6 From the viewpoint of large-
scale application and efficient solar energy utilization, however,
20 semiconductor-based heterogeneous photocatalysts would be
more advantageous over molecular-based homogeneous catalysts,
considering the superior oxidation ability to utilize a mild
reductant (ideally, water) and potential recyclability.7 Several
kinds of heterogeneous photocatalysts based on inorganic
25 compounds for CO2 reduction have been developed,8–11 and some
have achieved the reaction using water as an electron donor.10,11
However, the development of photochemical CO2 reduction
catalysts that are sufficiently active, stable, inexpensive, and
capable of harvesting visible photons still remains a challenge in
30 chemistry.
300
400
500
600
700
N
N
N
≡
Wavelength / nm
N
N
N
C
N
N
N
Conduction band
(C2s,2p orbitals)
CO2/HCOOH
(–0.59 V)
Tri-s-triazine (melem) unit
–1
0
CO2/CO
(–0.51 V)
Band gap
(~2.8 eV)
+1
+2
Valence band
(N2p orbitals)
Structure of Ru
65
Fig. 1 (A) Structures of C3N4 and the ruthenium complex (Ru)
employed in this study. (B) UV-visible absorption spectrum of Ru in
methanol (11.8 µM), and diffuse reflectance spectra of mpg-C3N4 and
Ru/mpg-C3N4. (C) Schematic band structure diagram of mpg-C3N4, along
with some redox potentials.
70
Here we develop a hybrid photocatalyst consisting of C3N4 and
a small fraction of a ruthenium complex, cis,tans-[Ru{4,4’-
(CH2PO3H2)2-2,2’-bipyridine}(CO)2Cl2] (abbreviated as Ru),
which respectively work as light-harvesting unit and a catalytic
centre for CO2 reduction. This simple binary composite is capable
75 of converting CO2 into HCOOH with a selectivity of >80% and a
turnover number (TON) of >200, both of which are the highest
among heterogeneous photocatalytic CO2 reduction systems
reported to work under visible light.19,20 The structure of C3N4
and the ruthenium complex employed in this study is illustrated
80 in Fig. 1A.
Carbon nitride has several proposed allotropes with diverse
properties, but the graphitic phase is regarded as the most stable
under ambient conditions. A polymeric carbon nitride, melon, has
been reported by Berzelius and Liebig in 1834.21 Here graphitic
85 carbon nitride having a mesoporous structure (referred to as mpg-
C3N4 hereafter) was employed as a light-harvesting unit. The
preparation detail was given in ESI. The successful preparation of
mpg-C3N4 having a 12 nm pore was confirmed by nitrogen
adsorption/desorption experiment at 77 K (Fig. S1). The specific
90 surface area of mpg-C3N4 used in this study was 180 m2 g–1.
As indicated by the yellow color, mpg-C3N4 exhibits a steep
absorption edge at 450 nm with a tailing that extends to longer
We have explored new functions of earth-abundant
carbon/nitrogen-based polymer semiconductors especially for
artificial photosynthesis.12–15 Recently, we reported that carbon
nitride (C3N4) functions as a stable photocatalyst for water
35 reduction and oxidation under visible light (λ > 420 nm).12,13
Another important and encouraging application of C3N4 in
artificial photosynthesis is photocatalytic CO2 reduction. To
accomplish the real artificial photosynthesis applicable in large-
scale, a CO2-reduction photosystem that consists of an earth-
40 abundant material is undoubtedly desirable. While C3N4 has been
reported to activate CO2 at 423 K under a pressurized condition
in the dark,16 our initial attempt showed that C3N4 alone did not
give any reduction product of CO2 under visible light at ambient
condition. Very recently, it was claimed that C3N4 was capable of
45 reducing CO2 into CO, CH3OH or C2H5OH.17,18 However, it
should be noted that these reports lack data for isotope labelling
experiments to clarify the origin of CO2 reduction products, and
the availability of C3N4 for CO2 reduction still remains unknown.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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wavelengths, as shown in Fig. 1B. On the other hand, the
conduction and valence band, which are respectively consumed
ruthenium complex (Ru) has a negligible absorption band in
visible light region. Therefore, it is in principle possible to
activate almost only C3N4 under irradiation with wavelength
longer than 400 nm.
Electrochemical impedance spectroscopy analysis, which was
conducted according to our previous report,15 indicated n-type
semiconducting character of mpg-C3N4 due to the positive slope
in the Mott–Schottky plot (Fig. S2). The estimated flat-band
by CO2 reduction with the aid of the Ru catalyst and the
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oxidation of TEOA.
DOI: 10.1039/C3CC45532G
As shown in Fig. S4, the amount of produced HCOOH
50 increased with irradiation time, and the rate of HCOOH
production was found to depend on the loading amount of Ru on
mpg-C3N4. The selectivity of HCOOH formation exceeded 80%
(Table 1, entries 7–9). When 7.8 µmol g–1 of Ru was employed
as a catalyst, the turnover number (TON) with respect to the
5
10 potential was –1.29 V versus Ag/AgCl reference electrode at pH 55 loaded Ru after 20 h of reaction exceeded 200, providing
6.6 (i.e., –1.09 V versus normal hydrogen electrode at pH 6.6),
which is thermodynamically negative enough to allow for
photocatalytic CO2 reduction (CO2/HCOOH, –0.59 V; CO2/CO,
–0.51 V at pH 6.6). An illustration of band-gap structure of mpg-
15 C3N4 is shown in Fig. 1C.
By suspending mpg-C3N4 powder in methanol dissolved with
Ru, which was in prior synthesized according to the literature,22
under continuous stirring overnight at room temperature, Ru
adsorbed almost quantitatively on mpg-C3N4 (up to 39.2 µmol g–
20 1). The coverage of Ru (39.2 µmol g–1) on mpg-C3N4 corresponds
approximately to 8%. The adsorbed Ru was also confirmed by
FT-IR spectroscopy. As shown in Fig. S3, two peaks assigned to
the vibration mode of two carbonyl groups in Ru were clearly
observed in the spectrum of Ru/mpg-C3N4. The peak positions of
25 the carbonyl groups in Ru/mpg-C3N4 are almost the same as
those in Ru, indicating no significant electronic interaction
between mpg-C3N4 and the loaded Ru.
Visible-light CO2 reduction was then attempted using mpg-
C3N4 catalysts, and the results are summarized in Table 1. mpg-
30 C3N4 without any modification gave no reduction product of CO2
(entry 1), generating just small amount of H2. However,
modification of mpg-C3N4 with Ru as a catalytic active site for
CO2 reduction resulted in clearly observable HCOOH formation
as the major product, with CO and H2 as minor products (entry 2).
35 A composite of Ru and an insulator (Al2O3) produced only trace
amount of H2, with no CO2-reduction products (entry 3),
indicating Ru loaded on the surface of Al2O3 alone does not have
CO2 reduction capability under visible light.23 Control
experiments also showed that neither HCOOH and CO was
40 produced when Ru/mpg-C3N4 or CO2 was absent (entries 4 and 5,
respectively). In addition, the amount of CO2-reduction products
was very small in the absence of an electron donor, here
triethanolamine (TEOA) (entry 6). These results strongly
suggests that mpg-C3N4 undergoes photoexcitation upon visible
45 light (λ > 400 nm), generating electrons and holes in the
confirmation of catalytic HCOOH formation. It should be noted
that the TON achieved in this study was much higher than that
recorded using an expensive tantalum-based semiconductor
material in a similar reaction condition: TONs = 89 for 20 h (ref.
60 19), 118 for 60 h (ref. 20). It clearly demonstrates the great
potential of carbon nitride polymers for photocatalytic CO2
reduction with visible light. One plausible explanation of the
catalyst deactivation is that the loaded Ru desorbed after
photocatalytic reaction, as confirmed by UV-visible spectroscopy.
65
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
70
350 400 450 500 550 600
Wavelength / nm
75
Fig. 2 An action spectrum of HCOOH production by Ru/mpg-C3N4
photocatalyst (blue plot), along with diffuse reflectance spectrum of
Ru/mpg-C3N4 (red curve). Reaction conditions: photocatalyst, Ru (39.2
µmol g–1)-loaded mpg-C3N4 10 mg; solution, a mixture of acetonitrile and
80 triethanolamine (4:1 v/v) 10 mL; reaction vessel, Pyrex top-irradiation
type cell with septum (26 mL capacity); light source, 300 W xenon lamp
with a band pass filter of 400 nm; light intensity, 30 mW.
Fig. 2 shows an action spectrum of photocatalytic CO2
reduction into HCOOH using Ru/mpg-C3N4. The apparent
85 quantum yield (AQY) for the HCOOH production decreased with
increasing wavelength of the incident light, primarily because of
a decrease in the absorbance, and finally reached zero for 550 nm
irradiation. This result clearly indicates that the reaction proceeds
through visible-light absorption by mpg-C3N4. The maximum
90 AQY obtained in this study was ~1.5% at 400 nm (Table S1). It
should be noted that an abundant C/N-based polymer
Table 1. Results of visible-light CO2 reduction (λ > 400 nm)a
Entry
Photocatalyst
Amount of Ru
Solution
Atmosphere
Amount of products (5 h) / nmol
loaded / µmol g–1
H2
CO
HCOOH
1
2
3
4
5
6
7
8
9
mpg-C3N4
Ru/mpg-C3N4
Ru/Al2O3
0
Acetonitrile/Triethanolamine
Acetonitrile/Triethanolamine
Acetonitrile/Triethanolamine
Acetonitrile/Triethanolamine
Acetonitrile/Triethanolamine
Acetonitrile
Acetonitrile/Triethanolamine
Acetonitrile/Triethanolamine
Acetonitrile/Triethanolamine
CO2
CO2
CO2
CO2
Argon
CO2
CO2
CO2
CO2
354
1267
7
0
580
0
0
0
160
1433
2499
2997
0
5455
0
0
0
110
15022
17248
19345
7.8
7.8
0
7.8
7.8
23.5
39.2
78.4
None
0
Ru/mpg-C3N4
Ru/mpg-C3N4
Ru/mpg-C3N4
Ru/mpg-C3N4
Ru/mpg-C3N4
3772
0
1332
1482
1273
a Reaction conditions: photocatalyst, Ru-loaded mpg-C3N4 8.0 mg; solution, a mixture of acetonitrile and triethanolamine (4:1 v/v) 4 mL; reaction vessel,
Pyrex test tube with a septum (11 mL capacity); light source, 450 W xenon lamp with a NaNO2 solution filter.
2 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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Page 4 of 4
semiconductor achieved photocatalytic performance comparable
improvement of activity and refinement of the structure of each
to an expensive metal-based semiconductor photocatalyst.19
To investigate the origin of HCOOH and CO generated during
the reaction, the reduction of 13C-labeled CO2 (610 Torr) was
conducted using Ru/mpg-C3N4 under visible light. As shown in
Fig. S5, the 13C–NMR spectrum of the supernatant solution after
60 component are necessary toward large-scale practical application,
we have thus addressed an important firDstOsI:t1ep0.1t0oV3wi9ea/wrCdA3rCatCirc4tlei5fOi5cn3il2ianGle
photosynthesis through CO2 reduction using a heterogeneous
photocatalyst.
5
K. M. acknowledges a start-up funding of Faculty of Science,
5 h of photoreaction indicated a clear peak assigned to H13COOH 65 Tokyo Institute of Technology, and Grant-in-Aid for Scientific
at δ = 160 ppm. In contrast, performing the same reaction but
Research on Innovative Areas (Project No. 25107512).
Acknowledgements are also extended to a PRESTO/JST program
“Chemical Conversion of Light Energy” and Grant-in-Aid for
Young Scientists (A) (Project No. 25709078).
with unlabelled CO2 did not result in the appearance of the
10 H13COOH peak. In the 1H–NMR spectrum of the reactant
solution irradiated under 13CO2 atmosphere, a doublet (J13
=
CH
188 Hz) was observed between 8.23 and 8.60 ppm, attributed to
protons bound to the 13C atom in H13COOH. Only a singlet was,
on the other hand, observed at 8.42 ppm in the solution reacted
15 under unlabelled CO2 atmosphere. These results clearly indicate
that the produced HCOOH during the photoreaction using
Ru/mpg-C3N4 almost entirely resulted from CO2 reduction.
70 Notes and references
a Department of Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo
152-8550, Japan. Fax: +81-3-5734-2284; Tel: +81-3-5734-2239; E-
mail: maedak@chem.titech.ac.jp
75 b Precursory Research for Embryonic Science and Technology (PRESTO),
Japan Science and Technology Agency (JST) 4-1-8 Honcho Kawaguchi,
Saitama 332-0012, Japan.
The origin of CO generated during the photoreaction as a
minor product was also examined by means of gas
20 chromatography and the combined mass-spectroscopy (GC-MS).
The result indicates that approximately 87% of the produced CO
originated from photocatalytic reduction of CO2, while the
leftovers of 13% was produced from Ru (Fig. S6). FT-IR analysis
showed that the original biscarbonyl structure of Ru disappeared
25 after reaction, giving a different spectral profile (Fig. S3). It is
known that such a structural change in a metal complex catalyst
sometimes occurs during CO2 photoreduction in a homogeneous
system.5 It has been also reported that even without irradiation,
the biscarbonyl structure of a ruthenium complex undergoes a
30 change upon reaction with triethanolamine.6 Considering the fact
that the photocatalytic activity of Ru/mpg-C3N4 for HCOOH
production remains almost unchanged even after 5 h of reaction
(Fig. S4 and Table S1), the Ru complex after structural change
may be related to the active species of CO2 reduction catalysis.
† Electronic supplementary information (ESI) available: experimental
details, additional reaction and characterization data.
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35
On the basis of these results, it is reasonable to conclude that
CO2 reduction into HCOOH occurs photocatalytically on mpg-
C3N4 modified with a Ru complex as a catalyst in the presence of
TEOA as an electron donor, ruling out the degradation effect of
mpg-C3N4. Compared to CO2 reduction photosystems reported so
12 X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.
100
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40 far, our photocatalytic system has several feasible aspects as
follows. First, C3N4 has the ability to oxidize water into
molecular oxygen with visible light, as we have explored so
far,12,13 meaning that the present CO2-reduction photosystem
could in principle be coupled with a water oxidation system to
45 create a real artificial photosynthetic assembly that works under
visible light. Most importantly, in contrast to inorganic
compounds, one can control both bulk and surface properties of
C3N4 based on synthetic protocol by organic chemistry,15 thereby
modulating the band-gap structure and introducing a desired
50 organic moiety that anchors a metal-complex catalyst. Besides,
C3N4 is structurally flexible, exhibiting various shapes with the
aid of a hard template such as silicas during the synthesis.14 It
would also become possible to replace an expensive Ru complex
catalyst with a base-metal complex, with fine-tuning of the
55 molecular structure for optimal operation. Bourrez et al. recently
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conduct reaction using just Ru in a homogeneous system.
24 M. Bourrez, F. Molton, S. Chardon-Noblat and A. Deronzier, Angew.
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reported that
a
manganese-based complex exhibited
electrocatalytic activity for CO2 reduction into CO, although
TON was not very high (TON = 13 for 4 h).24 Although further
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3