Photocatalytic CO2 reduction in N,N-dimethylacetamide/water as an alternative solvent system

Article abstract of DOI:10.1021/ic500050q

N,N-Dimethylacetamide (DMA) was used for the first time as the reaction solvent in the photocatalytic reduction of CO₂. DMA is highly stable against hydrolysis and does not produce formate even if it is hydrolyzed. We report the catalytic activities of [Ru(bpy)₂(CO)₂](PF ₆)₂ (bpy = 2,2′-bipyridine) in the presence of [Ru(bpy)₃](PF₆)₂ as a photosensitizer and 1-benzyl-1,4-dihydronicotinamide (BNAH) as an electron donor in DMA/water. In the photochemical CO₂ reduction, carbon monoxide (CO) and formate are catalytically produced, while dihydrogen (H₂) from the reduction of water is scarcely evolved. We verified that BNAH is oxidized to afford BNA dimers during the photocatalyses in DMA/water. The plots of the production for the CO₂ reduction versus the water content in DMA/water show that the 10 vol % water content gives the highest amount of the reduction products, whose reaction quantum yields (φ′) are determined to be 11.6% and 3.2% for CO and formate, respectively. The results are compared with those in the N,N-dimethylformamide (DMF)/water system, which has been typically used as the solvent system for the CO₂ reduction.

Full text of DOI:10.1021/ic500050q

Article
pubs.acs.org/IC
Photocatalytic CO2 Reduction in N,N‑Dimethylacetamide/Water as an
Alternative Solvent System
Yusuke Kuramochi,† Masaya Kamiya,† and Hitoshi Ishida*,†,‡
†Department of Chemistry, Graduate School of Science, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa
252-0373, Japan
‡Precursory Research for Embryonic Science (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: N,N-Dimethylacetamide (DMA) was used for
the first time as the reaction solvent in the photocatalytic
reduction of CO2. DMA is highly stable against hydrolysis and
does not produce formate even if it is hydrolyzed. We report
the catalytic activities of [Ru(bpy)2(CO)2](PF6)2 (bpy = 2,2′-
bipyridine) in the presence of [Ru(bpy)3](PF6)2 as a
photosensitizer and 1-benzyl-1,4-dihydronicotinamide
(BNAH) as an electron donor in DMA/water. In the
photochemical CO2 reduction, carbon monoxide (CO) and
formate are catalytically produced, while dihydrogen (H2)
from the reduction of water is scarcely evolved. We verified that BNAH is oxidized to afford BNA dimers during the
photocatalyses in DMA/water. The plots of the production for the CO2 reduction versus the water content in DMA/water show
that the 10 vol % water content gives the highest amount of the reduction products, whose reaction quantum yields (Φ′) are
determined to be 11.6% and 3.2% for CO and formate, respectively. The results are compared with those in the N,N-
dimethylformamide (DMF)/water system, which has been typically used as the solvent system for the CO2 reduction.
CO2 reduction, a careful calibration is required to quantify
formate produced in CO2 reduction.
INTRODUCTION
■
Metal complexes can be designed on the molecular levels,
enabling us to provide highly active catalysts. The photo-
catalytic CO2 reduction using metal complexes has attracted
much attention toward construction of artificial photosynthetic
systems and solar fuels.1 In most cases of catalytic CO2
reduction by metal complexes, the reduction products are
carbon monoxide (CO) and/or formate (HCOO−). While the
artificial photosynthetic systems have recently been developed
by combining semiconductors with ruthenium bipyridyl
carbonyl complexes such as [Ru(bpy)2(CO)2]2+, trans-(Cl)-
Ru(bpy)(CO)2Cl2, and the derivatives as the CO2 reduction
catalysts,2 the evaluations of the catalytic activities of the metal
complexes toward CO2 reduction are still carried out in organic
homogeneous solution.3
N,N-Dimethylformamide (DMF) has been well-used as the
reaction solvent for the CO2 reduction due to the high
solubilities of CO2 and the metal complexes.3−5 However, Vos
et al. have recently pointed out a problem; namely, that DMF
readily hydrolyzes to give formate in the presence of water and
amine, making it difficult to quantify formate for the CO2
reduction.6 They have also indicated that hydrolysis of DMF
would occur not during CO2 reduction but in the course of an
analytical process such as ion exchange chromatographic
analysis, which usually requires the addition of water to
quantify formate. As long as DMF is used as the solvent for
In this Report, we propose N,N-dimethylacetamide (DMA)
as an alternative solvent for the photocatalytic CO2 reduction.
To evaluate the effectiveness of this solvent, we selected the
photocatalytic system consisting of [Ru(bpy)2(CO)2](PF6)2
(bpy = 2,2′-bipyridine) as the catalyst, [Ru(bpy)3](PF6)2 as
the photosensitizer, and 1-benzyl-1,4-dihydronicotinamide
(BNAH) as the electron donor (Chart 1). The photocatalytic
system, which was originally reported by Ishida and Tanaka et
al., has yielded both CO and formate as the CO2 reduction
products in DMF/water in the absence of amines such as
triethanolamine or triethylamine.5 The selectivity for CO/
formate in the CO2 reduction has been elucidated by the
equilibrium reactions among the carbonyl complex ([Ru-
(bpy)2(CO)2]2+), the carboxylic acid complex ([Ru-
(bpy)2(CO)(C(O)OH)]+), and the η1-CO2 adduct ([Ru-
(bpy)2(CO)(CO2)]).7 In the catalysis, water acts as the proton
source to accept the oxide anion from the coordinated CO2 to
produce CO and OH−. This is in contrast to the catalytic
systems consisting of the rhenium complexes, which selectively
produce CO with CO32− by the oxide-transfer reaction between
two CO2 molecules.3d,e,8 The catalytic CO2 reduction systems
using DMF/water as the solvent should be reevaluated
Received: October 4, 2013
Published: March 14, 2014
© 2014 American Chemical Society
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Chart 1. Structures of the Ruthenium(II) Complexes, the Electron Donor, and the Solvents
solution containing [Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M),
[Ru(bpy)3](PF6)2 (5.0 × 10−4 M), and BNAH (0.10 M) in an
NMR tube (i.d. = 4 mm) was bubbled with 13CO2 gas, which was
generated by addition of 1.0 M sulfuric acid to Ba13CO3 powder. After
photoirradiation using a 400 W high-pressure mercury lamp (λ > 400
nm; Riko Kagaku, L-39 cutoff filter) in a merry-go-round irradiation
apparatus (Riko Kagaku, RH400−10W) for 3 h, the 13C NMR spectra
were measured on a Bruker AVANCE II 600 spectrometer.
particularly for formate production. We then examine the
photocatalyses in DMA/water, and the results are compared to
the ones in DMF/water. We also investigate the oxidation
products of BNAH and the effect of the water contents in the
DMA/water on the photocatalytic CO2 reduction.
EXPERIMENTAL SECTION
■
Quantum Yield Determination. The quantum yields of the
photochemical CO2 reduction were determined using K3[Fe(C2O4)3]
as the chemical actinometer.12 The definition of the quantum yields
for the photochemical CO2 reduction is discussed in this Report. The
experimental procedure using the chemical actinometer is the
following: the DMA/water (9:1 v/v, 4 mL) solution containing
[Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M), [Ru(bpy)3](PF6)2 (5.0 ×
10−4 M), and BNAH (0.10 M) in a square quartz cuvette (1.0 cm) was
bubbled with CO2 gas for 30 min and was irradiated with a 500 W
superhigh-pressure mercury lamp (Ushio, USH-500D) through a
Toshiba Y-43 glass filter (λ > 400 nm) for 5, 10, and 15 min. The rate
of the photochemical CO2 reduction was determined by amounts of
CO and formate. The photochemical generation of Fe(II) ion from
K3[Fe(C2O4)3] was quantified by reacting with 1,10-phenanthroline,
followed by measuring the absorption spectra on a Shimadzu
MultiSpec-1500 spectrometer. The quantum yields were also
determined with a Shimadzu absolute photoreaction quantum yields
measurement system QYM-01 equipped with a 460 nm band-pass
filter and a 300 W Xe lamp and were confirmed to be the same results
as the values obtained using the aforementioned chemical actinometer.
HPLC Analyses for Oxidation Products of BNAH. To pursue
the oxidation products of the electron donor, BNAH, the reaction
solutions were analyzed with high performance liquid chromatography
(HPLC). Analyses were performed on a Shimadzu LC-10Ai system
equipped with a Tosoh TSKgel ODS-80Ts column (4.6 mm i.d. × 15
cm). A CH3OH/water (55:45 v/v) solution was used as an eluent with
flow rate of 1.0 mL/min (column temperature: 313 K), and a
Shimadzu SPD-M20A prominence diode array was used as the
detector. The retention times under the analytical conditions were
determined using the authentic samples: BNAH 9.6 min, 4,6′-BNA2
7.0 and 9.6 min, 4,4′-BNA2 15.7 min, and BNA+ 3.4 min.
Materials. The ruthenium complexes [Ru(bpy)2(CO)2](PF6)2 and
[Ru(bpy)3](PF6)2 were prepared according to the literature.9 BNAH
and the oxidized dimer products (4,4′-BNA2 and two diastereoisomers
of 4,6′-BNA2) were prepared according to the literature10 and stored
in a refrigerator. DMA (Wako, dehydrate), DMF (Watanabe
Chemical, special grade), DMA-d9 (Cambridge Isotope Lab.), DMF-
d7 (TCI), and Ba13CO3 (Cambridge Isotope Lab.) were used as
supplied. No residual triethylamine was observed in the solvents on
the NMR analyses. High-purity water (resistivity: 18.2 MΩ cm) was
obtained by an ultrapure water system (RFU424TA, Advantec).
Photocatalytic CO2 Reduction. Ar-saturated DMA/water or
DMF/water solutions (5 mL) of [Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4
M), [Ru(bpy)3](PF6)2 (5.0 × 10−4 M), and BNAH (0.10 M) were
placed in quartz tubes (23 mL volume, i.d. = 14 mm). Each solution
was bubbled through a septum cap with CO2 gas for 20 min. Ten
tubes were set in a merry-go-round irradiation apparatus (Riko
Kagaku, RH400−10W) and then were irradiated using a 400 W high-
pressure mercury lamp at λ > 400 nm (Riko Kagaku, L-39 cutoff filter).
The reaction temperature was maintained at 298 3 K by using a
water bath. After they were irradiated for a definite time, the gaseous
phase was analyzed for CO and H2 content on a gas chromatography
(GC) system based on a Shimadzu GC-2014. The product gases (0.10
mL) were injected with a gastight syringe into the GC equipped with
successive Porapak-N, Molecular Sieve 13X, and Shimalite-Q columns
(stainless steel columns). N2 (>99.9995%) was used as the carrier gas.
CO was methanized through a Shimadzu MTN-1 methanizer,
followed by detection with flame ionization detector (FID). H2 was
detected with thermal conductivity detector (TCD). Formate was
extracted as formic acid with ethyl acetate prior to the GC analyses as
follows.11 A portion of the resulting solution (2 mL) was placed in a
sample tube. After adding water to adjust the water content to 50 vol
%, 1.0 M sulfuric acid was added (10 vol % of the resulting DMA/
water solution). The same amount of ethyl acetate as that of the
acidified resulting solution was added, and the mixed solution was
shaken for 10 s and stood for 2 min to extract formic acid. The upper
phase (2 μL) was injected into a Shimadzu GC-2014 equipped with
DB-WAX columns (i.d. 0.53 mm, 15 m × 2). Formic acid was detected
with FID after methanization by a Shimadzu MTN-1 methanizer. The
reliability of quantification of formate on the GC was confirmed by the
fact that the results matched with the ones using an Otsuka Electrons
CAPI-3300I capillary electrophoresis analyzer within experimental
errors. Time dependence of the reaction was pursued by analyzing
each reaction tube with different irradiation times.
Quenching Experiments. Emission from the excited states of
[Ru(bpy)3](PF6)2 in the Ar-saturated DMA/water solutions were
recorded on a Hitachi F-4500 spectrometer (λex = 453 nm) in the
absence and presence of the quencher BNAH or BNA2. The Stern−
Volmer relationship (eq 1) was obtained by the plots of the relative
emission intensity (I0/I) versus the concentration of the quencher (Q
= BNAH or BNA2):
I0/I = 1 + KSV[Q] = 1 + kqτ[Q]
(1)
where I0 and I represent the intensity at 620 nm in the absence and the
presence of the quencher, respectively, and Ksv, kq, and τ are the
Stern−Volmer constant, the quenching rate constant, and the emission
lifetime, respectively. The emission lifetimes were measured at 298 K
13C NMR Study for Photocatalytic CO2 Reduction. The DMA-
d9/water (9:1 v/v, 0.83 mL) or DMF-d7/water (9:1 v/v, 0.83 mL)
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with a FluoroCube fluorescence lifetime spectrometer (Horiba Jobin
Yvon) using a 455 nm laser diode (NanoLED).
Electrochemical Measurements. Cyclic voltammograms (CV)
and differential pulse voltammograms (DPV) were obtained by a Bio-
Logic VSP potentiostat using an EC-Lab software. Measurements were
conducted in DMA/water containing tetrabutylammonium perchlo-
rate (nBu4NClO4, 0.10 M) as a supporting electrolyte under Ar using a
BAS microcell. As the electrodes, a BAS glassy-carbon working
electrode, a BAS Pt counter electrode, and a BAS RE-7 reference
electrode (Ag/AgNO3 0.01 M in acetonitrile) were used.
RESULTS AND DISCUSSION
■
Photocatalytic CO2 Reduction in DMF/Water. Photo-
catalytic CO2 reduction has been carried out by photo-
irradiating (λ > 400 nm) the CO2-saturated DMF/water (9:1,
v/v) solution containing [Ru(bpy)2(CO)2]2+ (1.0 × 10−4 M),
[Ru(bpy)3]2+ (5.0 × 10−4 M), and BNAH (0.10 M), which act
as the catalyst, the photosensitizer, and the sacrificial electron
donor, respectively. In general, the amines (e.g., triethanol-
amine and triethylamine) or ascorbate has been utilized as the
electron donor for photochemical H2 evolution. One reason
why we have chosen BNAH is that the amines are known not
to work as the electron donor in aqueous solution, in which
amines are protonated and lose the electron-donating
abilities.5a On the other hand, we have carried out the
photocatalytic CO2 reductions using sodium ascorbate (0.10
M) instead of BNAH as the electron donor in DMF/water
(1:1, v/v) containing [Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M)
and [Ru(bpy)3](PF6)2 (5.0 × 10−4 M); however, no reduction
product of CO2 was obtained. This is another reason why we
have selected BNAH. In this Report, we have irradiated with
visible light (>400 nm) for all the photocatalytic reactions
because neither [Ru(bpy)2(CO)2]2+ nor BNAH has any
appreciable absorption above 400 nm. In this condition, the
photosensitizer [Ru(bpy)3]2+ was selectively excited by the
photoirradiation.
Figure 1. Photoirradiation time dependence of products in the (a)
CO2 and (b) Ar-saturated DMF/water (9:1, v/v) solution containing
[Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M), [Ru(bpy)3](PF6)2 (5.0 ×
10−4 M), and BNAH (0.10 M): CO ( ), HCOOH ( ), H2 (Δ), and
■
○
CO+HCOOH (+).
The time courses of the photochemical reduction in a CO2-
saturated and Ar-saturated DMF/water (9:1, v/v) solution are
shown in Figure 1a,b, respectively. The gaseous products (CO
and H2) are analyzed with GC, and formate is also quantified
with GC by acidifying it to formic acid. Thus, we denote
“HCOOH” in the figures as analyzed by GC, although the word
“formate” is used in the text. The 13C NMR experiment in
DMF-d7/water (9:1 v/v) using 13CO2 indicates that CO and
formate originate from CO2 (see Supporting Information,
Figure S1); however, 25 μmol of formate is detected in the
photoreaction under Ar atmosphere (Figure 1b). It suggests
that the hydrolysis of DMF partly occurs. Time-independent
detection of formate strongly suggests that the decomposition
of DMF occurs not during the photoirradiation but during the
GC analyses. Actually, the capillary electrophoresis analyses
scarcely detected formate in the Ar-saturated photoreaction
solution. Figure 1a shows production of approximately 110
μmol (TON ≈ 220) of formate and 55 μmol (TON 110) of
CO at 2 h, but we estimate the formate production by CO2
reduction at 85 μmol (TON 170) by taking account of the
production (25 μmol) from hydrolysis of DMF. Figure 1b
shows that catalytic evolution of dihydrogen (H2; 43 μmol,
TON 85) occurs by the reduction of water in Ar-saturated
solution. On the other hand, H2 scarcely evolves in CO2-
saturated solution (Figure 1a), indicating that the catalyst
selectively reduces CO2.
simple solution to avoid the formate production from DMF
hydrolysis. To search for the alternative solvent, we initially
examined the photocatalytic CO2 reduction in some common
solvents such as acetone, ethanol, and acetonitrile instead of
DMF (Supporting Information, Figure S2). However, the
catalytic activities are dramatically depressed in such solvent
systems. In the solvent systems we have examined, DMA gives
a good result. DMA has higher resistance to hydrolysis than
DMF, and DMA does not afford formate but acetate even if it is
hydrolyzed.13 Figure 2a,b shows the time courses of the
photoreaction in CO2 and Ar-saturated DMA/water (9:1, v/v),
respectively. Figure 2b shows no formate yield in the absence of
CO2, indicating that the use of DMA enables us to quantify
formate with GC, which requires heat. Figure 2a shows that 56
μmol (TON 112) of formate and 82 μmol (TON 164) of CO
yield at 2 h.
We carried out the photoreduction in the 13CO2-saturated
DMA-d9/water (9:1, v/v) solution and confirmed that CO and
formate come from CO2 (Figure 3). As also observed in DMF-
d7/water, no other 13C peaks including H13CO3 or 13CO3
−
2−
originating from the 13CO2 reduction are found. This result
supports the idea that the catalytic reaction by [Ru(bpy)2-
(CO)2]2+ proceeds not via the oxide-transfer between two CO2
molecules but by the proton-coupled electron transfer.3d,e,7 The
NMR spectra of the resulting DMA/water solution did not
show any signals of acetic acid, indicating that the
decomposition of DMA did not occur during the photo-
Photocatalytic CO2 Reduction in DMA/Water. It has
been considered that changing the solvent system would be a
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and quantification of the BNA dimers will be described in the
next section.
The initial reaction rate for the CO2 reduction, namely, the
production rates for the sum of CO and formate in DMA/
water, is comparable to that in DMF/water (see Figures 1a and
2a). However, the rate in the DMA/water system seems to
become slow after 1 h of photoirradiation. This is because the
result in DMF/water contains the formate production by the
hydrolysis of DMF. If the amounts of formate produced under
Ar are subtracted from the total ones, the results become quite
comparable to the results in DMA/water.
Although the catalytic activities in DMF/water after the
correction are similar to those in DMA/water, the selectivities
for formate/CO are different. In DMA/water, CO rather than
formate evolves (see Figure 2a). On the other hand, the
amounts of formate in DMF/water before the correction look
larger than those of CO (Figure 1a), but after the correction
the amounts of formate become quite similar to those of CO.
The selectivity for formate/CO has been explained by the
equilibrium reactions among the carbonyl, the carboxylic acid,
and η1-CO2 complexes: the use of strong acids with low pKa
gives CO, while weak acids with high pKa provide formate.7
The results might be elucidated by assuming that H2O acts as
the stronger acid in DMA than it does in DMF. Otherwise the
η1-CO2 complex ([Ru(bpy)2(CO)(CO2)]), which would be a
precursor for formate production, might be stabilized in DMF/
water rather than DMA/water. The latter assumption would be
led from the report that the η1-CO2 complex is stabilized by the
hydrogen bonding with some solvent molecules.15 The reason
remains unclear, and the research on the CO/formate
selectivity is now in progress.
Determination of Oxidation Products of BNAH. It has
been known that BNAH can act as not only a one-electron but
also a two-electron donor (Supporting Information, Scheme
S1). One-electron oxidation of BNAH followed by deprotona-
tion gives a more powerful reductant, BNA•, than BNAH. In
the case of the two-electron oxidation process the BNA• is
further oxidized to give BNA+. The one-electron oxidation
process gives BNA2 via the coupling of the BNA•. Thus, we
have identified and quantified the oxidation product of BNAH
by using the reversed-phase HPLC on the resulting solution of
the photochemical CO2 reduction. As the oxidation products of
BNAH, the stereoisomers of 4,6′-BNA2 and 4,4′-BNA2 are
observed, while no BNA+ is detected (Supporting Information,
Figure S3). This result is consistent with that observed in the
13C NMR spectra.14 In the DMA/water solvent system, water
molecules are considered to accept the proton from the
oxidized BNAH. The total amount of BNA2 increases,
accompanied with decreasing amounts of BNAH, and reaches
0.043 M after photoirradiation for 4 h (Figure 4). The trace of
BNA2 is matched to the total amount of CO and formate,
which are quantified within experimental errors by GC. This
indicates that the stoichiometry of the reaction is expressed as
eq 2.
Figure 2. Photoirradiation time dependence of products in the (a)
CO2 and (b) Ar-saturated DMA/water (9:1, v/v) solution containing
[Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M), [Ru(bpy)3](PF6)2 (5.0 ×
10−4 M), and BNAH (0.10 M): CO ( ), HCOOH ( ), H2 (Δ), and
■
○
CO+HCOOH (+).
Figure 3. The 13C NMR spectra of DMA-d9/water (9:1, v/v) solution
containing [Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M), [Ru(bpy)3]-
(PF6)2 (5.0 × 10−4 M), and BNAH (0.10 M) (a) before and (b) after
bubbling with 13CO2 gas followed by photoirradiation (λ > 400 nm)
●
for 3 h. The peaks marked with (×), ( ), and (*) are assigned to
DMFA, BNAH, and BNA dimers,14 respectively.
chemical reaction. The peaks for BNAH in the 13C NMR
spectrum decrease after the photoirradiation. With decreasing
peaks for BNAH, the new peaks appear, which are not assigned
to BNA+ but to BNA dimers. This suggests that BNA dimers
form as the oxidation products of BNAH. The identification
The decrease of BNAH in Figure 4 does not obey the first-
order reaction kinetics, and it gradually becomes slow with
passing time. The similar slowdown in the reaction rate has
been observed after 2 h in the photocatalytic CO2 reduction in
DMA/water (9:1, v/v), as shown in Figure 2a. The phenomena
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consisting mainly of 405, 436, and 546 nm wavelengths. For the
photochemical CO2 reduction, the quantum yield Φ can be
defined as the molar fraction of the product of CO2 to the
incident photon: the Φ values for CO and formate production
are 5.8% and 1.6%, respectively. The total quantum yield
corresponding to the photochemical CO2 reduction is 7.4%.
We also utilized a Shimadzu absolute photoreaction quantum
yields measurement system QYM-01 to determine the reaction
quantum yields. The system monitors the light intensities
before and after absorption of the reaction solution and
estimates the absorbed light intensity. When irradiated at 460
nm through a monochromator from a 300 W Xe lamp, the
reaction quantum yields are determined to be ΦCO = 5.5% and
ΦHCOOH = 1.8%. Even though the procedure is quite different
from the relative method using the chemical actinometer, the
quantum yields obtained are almost the same.
The almost-quantitative formation of BNA2 as the oxidation
product of BNAH indicates that one photon induces one
electron transfer in the photochemical reaction. Since the CO2
reduction to CO or formate requires two-electron transfer, the
photochemical quantum yield Φ′ can be defined by the molar
fraction of electron used for the CO2 reduction to the incident
photon, that is, by twice the value of Φ. According to the
definition, the quantum yields of the photocatalytic CO2
reduction in DMA/water (9:1, v/v) are determined to be
Φ′CO = 11.6% and Φ′HCOOH = 3.2%, and the total yield
Φ′′(CO+HCOOH) is 14.8%. On the other hand, the Φ′(CO+HCOOH)
in DMF/water (9:1, v/v) is estimated to be 16.5% (literature
17.5%).5a The similar values in DMF/water and DMA/water
indicate that DMA can be a good alternative solvent for DMF
in this photochemical reaction.
Effects of Water Content on Catalytic Activity. We
further examined the effects of water content in the CO2-
saturated DMA/water solutions on the products of the
photochemical reactions (Figure 5a). As observed in the
DMF/water solution (Supporting Information, Figure S7a), the
total amount of CO and formate is largest at 10 vol % water
content and decreases to approximately one-fourth of that
when going from 10 to 50 vol % water content. While the
largest production of CO is observed at 10 vol % water content,
the amounts of formate are similar among the 0, 10, and 20 vol
% water content solutions. The smaller production at 0 than at
10 vol % water content indicates that water contributes to the
proton source or/and transportation of the protons for the CO2
reduction.5a The experiments under the Ar-saturated solutions
do not afford formate (Figure 5b), in contrast to the reactions
in the DMF/water solutions (Supporting Information, Figure
S7b).
Increasing the water content, the photocatalytic abilities of
CO2 reduction dramatically decrease. This tendency is also
observed in the quenching rate constants of the excited states of
[Ru(bpy)3]2+ quenched by BNAH or BNA2. The quenching
rate constants with BNAH and BNA2 in the other DMA/water
solvent systems are determined from the data shown in
Supporting Information, Figures S4 and S5. The Stern−Volmer
constants and the quenching rate constants are summarized in
Table 1. The quenching fractions (ηq) of the emission from the
excited [Ru(bpy)3]2+ complex with BNAH are also calculated.
In all the solvent systems, the excited states of [Ru(bpy)3]2+ are
efficiently quenched; however, the efficiencies decrease by
increasing the water contents. In the photocatalytic CO2
reduction system, the reduced species of [Ru(bpy)3]2+ is
considered to act as an electron relay to the catalyst
Figure 4. Concentration change of the oxidation and reduction
products by the photoirradiation (λ > 400 nm) of the reaction solution
containing [Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M), [Ru(bpy)3]-
(PF6)2 (5.0 × 10−4 M), and BNAH (0.10 M) in the CO2-saturated
●
DMA/water (9:1, v/v) solution: BNAH ( ), two stereoisomers of
○
4,6′-BNA2 and 4,4′-BNA2 (Δ), and CO+HCOOH ( ). The
simulation curve of [BNAH] was calculated by using the quenching
rate constants of BNAH (kq = 2.6 × 108 M−1 s−1) and BNA2 (kq = 1.1
× 109 M−1 s−1) and the emission lifetime of the excited [Ru(bpy)3]2+
(0.84 μs). The simulation curve of [BNA2] was estimated by the
equation [BNA2] = 1/2 × (0.10 − [BNAH]).3c
could be explained by the quenching of the excited state of
[Ru(bpy)3]2+ with BNA2. It has been reported that BNA2 is a
good electron-transfer quencher for the excited [Ru(bpy)3]2+;
however, the BNA2 and [Ru(bpy)3]+ resulting from the
+
quenching efficiently cause the back electron transfer to
regenerate BNA2 and [Ru(bpy)3]2+.3g To determine the
quenching rate constants, we carried out quenching experi-
ments by BNAH and BNA2 in various DMA/water solvent
systems (Supporting Information, Figure S4 and S5). In the
DMA/water (9:1, v/v) solution, where the emission lifetime of
the excited [Ru(bpy)3]2+ is 0.84 μs, the quenching rate constant
of BNA2 (kq′ = 1.1 × 109 M−1 s−1) is 4 times larger than that of
BNAH (kq = 2.6 × 108 M−1 s−1). In Figure 4, the concentration
changes of BNAH and BNA2 are simulated based on the
quenching rate constants by BNAH and BNA2. The simulation
curves closely match with the experimental plots as well as the
slowdown of the reaction rate after 2 h.
We also carried out an experiment in which BNAH is further
added to the resulting solution after 3 h of photoirradiation
(Supporting Information, Figure S6). The reaction has once
slowed down; however, it has recovered after addition of
further BNAH. This result strongly suggests that the inactivity
of the reaction as observed in Figure 2a does not come from
light degradation of [Ru(bpy)2(CO)2]2+ or [Ru(bpy)3]2+ but is
due to both the decrease of BNAH and the quenching of the
excited [Ru(bpy)3]2+ ion by the BNA dimer.
Reaction Quantum Yields. We determined the quantum
yields for the photocatalytic CO2 reduction in the DMA/water
(9:1, v/v) solution containing [Ru(bpy)2(CO)2](PF6)2 (1.0 ×
10−4 M), [Ru(bpy)3](PF6)2 (5.0 × 10−4 M), and BNAH (0.10
M) by using the ferrioxalate actinometer. We used a superhigh-
pressure Hg lamp equipped with a cutoff filter (>400 nm). As
[Ru(bpy)3]2+ absorbed the light as the photosensitizer, this
combination of lamp and filter results in irradiation light
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Inorganic Chemistry
Article
while the reduction potentials of [Ru(bpy)3]2+ become more
negative with increasing water content. These results lead to
the decrease of the Gibbs free energy changes (−ΔGET) in the
electron transfer with increasing water content. It indicates that
the electron transfer from BNAH to the excited states of
ruthenium tris(bipyridine) complex becomes slower with
increasing water content in the solution.
CONCLUSION
■
We demonstrated that DMA can be a good alternative solvent
for DMF in the photocatalytic CO2 reduction, where
[Ru(bpy)2(CO)2]2+, [Ru(bpy)3]2+, and BNAH act as the
catalyst, the photosensitizer, and the sacrificial electron donor.
The photoreduction of CO2 proceeded efficiently in the DMA/
water (9:1, v/v) solution system, of which the catalytic activities
are comparable to those in DMF/water. Furthermore, DMA
does not cause the contamination of formate by the hydrolysis,
while DMF is partly hydrolyzed to afford formate not during
the photoreaction but during the GC analyses. The solvent
system using DMA instead of DMF enables us to quantify
formate even under harsher analytical conditions such as GC
analyses. Even if the amounts of formate yielded by hydrolysis
of DMF under Ar are subtracted from the amounts of formate
produced in the CO2 reduction in DMF/water (9:1, v/v), the
amounts are larger than those in the CO2 reduction in DMA/
water (9:1, v/v). The reasons for the difference in the CO/
formate selectivity between DMF and DMA still remains
unclear, and research is now in progress.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectra (13C NMR) in DMF-d7/water, photocatalytic CO2
reduction in several common solvents, scheme for oxidation
process of BNAH, HPLC analyses for the oxidation products of
BNAH, Stern−Volmer plots for the quenching by BNAH and
4,6′-BNA2, time-courses of photocatalytic CO2 reduction with
addition of BNAH, effect of water contents in DMF/water on
photocatalytic CO2 reduction, and the redox potentials of
BNAH and [Ru(bpy)3]2+ in DMA/water. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 5. Effects of water content in the (a) CO2 and (b) Ar-saturated
DMA/H2O solution on the products in the photochemical reaction (λ
> 400 nm, 1 h). [Ru(bpy)2(CO)2](PF6)2 (1.0 × 10−4 M),
[Ru(bpy)3](PF6)2 (5.0 × 10−4 M), and BNAH (0.10 M): CO ( ),
○
■
HCOOH ( ), H2 (Δ), and CO+HCOOH (+).
[Ru(bpy)2(CO)2]2+. Thus, the decrease of the quenching
efficiencies would be a reason why the photocatalytic activities
depress at the higher water contents in the DMA/water
solution systems.
To know the effects of the water content, we measured the
oxidation potentials of BNAH and the reduction potentials of
[Ru(bpy)3]2+ with various water contents in DMA/water. As
shown in Supporting Information, Table S1, the oxidation
potentials of BNAH are independent of the water content,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ishida@sci.kitasato-u.ac.jp. Phone: (+81)42-778-9925.
Fax: (+81)42-778-9925.
Notes
The authors declare no competing financial interest.
Table 1. Solvent Effects on Quenching Rate Constants of Excited [Ru(bpy)3]2+ by BNAH and 4,6′-BNA2 in DMA/Water at 298
K
BNAH
kq, M−1 s−1
4,6′-BNA2
a
b
c
d
b
c
water content, vol %
τ, μs
Ksv, M−1
ηq, %
Ksv′, M−1
kq′, M−1 s−1
0
0.88
0.84
0.84
0.82
0.80
394
217
105
80
4.5 × 108
2.6 × 108
1.3 × 108
9.8 × 107
5.5 × 107
98
96
91
89
81
1330
915
540
389
242
1.5 × 109
1.1 × 109
6.4 × 108
4.7 × 108
3.0 × 108
10
20
30
40
44
a
b
c
d
Emission lifetime of [Ru(bpy)3]2+. Stern−Volmer constants. Quenching rate constants. Quenching fraction of emission from [Ru(bpy)3]2+ in
the presence of 0.1 M BNAH, calculated as 0.1 KSV /(1 + 0.1KSV).
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Article
(b) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518−536.
ACKNOWLEDGMENTS
■
We thank Dr. Yusuke Tamaki, Dr. Keita Sekizawa, and Prof.
Osamu Ishitani (Tokyo Institute of Technology) for help with
capillary electrophoresis analyses, Dr. Yasushi Nakata (Horiba,
Ltd.) for emission lifetime experiments, and Mr. Kyohei
Fukaya, Mr. Akito Enomoto, and Mr. Makoto Yoshida for
experimental supports. This work was supported by the
PRESTO Program of JST.
(13) Kankaanpera, A.; Scharlin, P.; Kuusisto, I.; Kallio, R.; Bernoulli,
̈
E. J. Chem. Soc., Perkin Trans. 2 1999, 169−174.
(14) Moracci, F. M.; Liberatore, F.; Carelli, V.; Arnone, A.; Carelli, I.;
Cardinali, M. E. J. Org. Chem. 1978, 43, 3420−3422.
(15) (a) Tanaka, H.; Tzeng, B.-C.; Nagao, H.; Peng, S.-M.; Tanaka,
K. Inorg. Chem. 1993, 32, 1508−1512. (b) Tanaka, H.; Nagao, H.;
Peng, S.-M.; Tanaka, K. Organometallics 1992, 11, 1450−1451.
REFERENCES
■
(1) (a) Takeda, H.; Ishitani, O. Coord. Chem. Rev. 2010, 254, 346−
354. (b) Tanaka, K. Chem. Rec. 2009, 9, 169−186. (c) Morris, A. J.;
Meyer, G. J.; Fujita, E. Acc. Chem. Res. 2009, 42, 1983−1994.
(d) Marks, T. J.; et al. Chem. Rev. 2001, 101, 953−996. (e) Fujita, E.
Coord. Chem. Rev. 1999, 185−186, 373−384.
(2) (a) Suzuki, T. M.; Tanaka, H.; Morikawa, T.; Iwaki, M.; Sato, S.;
Saeki, S.; Inoue, M.; Kajino, T.; Motohiro, T. Chem. Commun. 2011,
47, 8673−8675. (b) Sato, S.; Arai, T.; Morikawa, T.; Uemura, K.;
Suzuki, T. M.; Tanaka, H.; Kajino, T. J. Am. Chem. Soc. 2011, 133,
15240−15243. (c) Sato, S.; Morikawa, T.; Saeki, S.; Kajino, T.;
Motohiro, T. Angew. Chem., Int. Ed. 2010, 49, 5101−5105. (d) Arai,
T.; Sato, S.; Uemura, K.; Morikawa, T.; Kajino, T.; Motohiro, T. Chem.
Commun. 2010, 46, 6944−6946.
(3) (a) Morimoto, T.; Nakajima, T.; Sawa, S.; Nakanishi, R.; Imori,
D.; Ishitani, O. J. Am. Chem. Soc. 2013, 135, 16825−16828.
(b) Morimoto, T.; Nishiura, C.; Tanaka, M.; Rohacova, J.;
Nakagawa, Y.; Funada, Y.; Koike, K.; Yamamoto, Y.; Shishido, S.;
Kojima, T.; Saeki, T.; Ozeki, T.; Ishitani, O. J. Am. Chem. Soc. 2013,
́
135, 13266−13269. (c) Voyame, P.; Toghill, K. E.; Mendez, M. A.;
Girault, H. H. Inorg. Chem. 2013, 52, 10949−10957. (d) Tamaki, Y.;
Koike, K.; Morimoto, T.; Yamazaki, Y.; Ishitani, O. Inorg. Chem. 2013,
52, 11902−11909. (e) Tamaki, Y.; Koike, K.; Morimoto, T.; Ishitani,
O. J. Catal. 2013, 304, 22−28. (f) Costentin, C.; Drouet, S.; Robert,
́
M.; Saveant, J.-M. Science 2012, 338, 90−94. (g) Tamaki, Y.;
Morimoto, T.; Koike, K.; Ishitani, O. Proc. Natl. Acad. Sci. U.S.A.
2012, 109, 15673−15678.
(4) (a) Lehn, J. M.; Ziessel, R. J. Organomet. Chem. 1990, 382, 157−
173. (b) Hawecker, J.; Lehn, J. M.; Ziessel, R. J. Chem. Soc., Chem.
Commun. 1985, 56−58.
(5) (a) Ishida, H.; Terada, T.; Tanaka, K.; Tanaka, T. Inorg. Chem.
1990, 29, 905−911. (b) Ishida, H.; Tanaka, K.; Tanaka, T. Chem. Lett.
1988, 339−342. (c) Ishida, H.; Tanaka, K.; Tanaka, T. Chem. Lett.
1987, 1035−1038.
(6) Paul, A.; Connolly, D.; Schulz, M.; Pryce, M. T.; Vos, J. G. Inorg.
Chem. 2012, 51, 1977−1979.
(7) (a) Ishida, H.; Fujiki, K.; Ohba, T.; Ohkubo, K.; Tanaka, K.;
Terada, T.; Tanaka, T. J. Chem. Soc., Dalton Trans. 1990, 2155−2160.
(b) Ishida, H.; Tanaka, H.; Tanaka, K.; Tanaka, T. J. Chem. Soc., Chem.
Commun. 1987, 131−132. (c) Ishida, H.; Tanaka, K.; Tanaka, T.
Organometallics 1987, 6, 181−186. (d) Ishida, H.; Tanaka, K.; Tanaka,
T. Chem. Lett. 1985, 405−406.
(8) (a) Agarwal, J.; Fujita, E.; Schaefer, H. F., III; Muckerman, J. T. J.
Am. Chem. Soc. 2012, 134, 5180−5186. (b) Agarwal, J.; Sanders, B. C.;
Fujita, E.; Schaefer, H. F., III; Harropcand, T. C.; Muckerman, J. T.
Chem. Commun. 2012, 48, 6797−6799.
(9) (a) Kelly, J. M.; O’Connell, C. M.; Vos, J. G. J. Chem. Soc., Dalton
Trans. 1986, 253−258. (b) Suzuki, K.; Kobayashi, A.; Kaneko, S.;
Takehira, K.; Yoshihara, T.; Ishida, H.; Shiina, Y.; Oishi, S.; Tobita, S.
Phys. Chem. Chem. Phys. 2009, 11, 9850−9860.
(10) (a) Mauzerall, D.; Westheimer, F. H. J. Am. Chem. Soc. 1955, 77,
2261−2264. (b) Ohnishi, Y.; Kitami, M. Bull. Chem. Soc. Jpn. 1979, 52,
2674−2677.
(11) Cicchillo, R. M.; Zhang, H.; Blodgett, J. A. V.; Whitteck, J. T.;
Li, G.; Nair, S. K.; van der Donk, W. A.; Metcalf, W. W. Nature 2009,
459, 871−874.
(12) (a) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook
of Photochemistry; 3rd ed.; CRC Press: Boca Raton, FL, 2006.
3332
dx.doi.org/10.1021/ic500050q | Inorg. Chem. 2014, 53, 3326−3332