Highly efficient, selective, and durable photocatalytic system for CO2 reduction to formic acid

Article abstract of DOI:10.1039/c5sc02018b

We discovered an extremely suitable sacrificial electron donor, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole, for the selective photocatalytic reduction of CO₂ to formic acid using a Ru(ii)-Ru(ii) supramolecular photocatalyst. The efficiency, durability, and rate of photocatalysis are significantly increased (Pdbl;_HCOOH = 0.46, TON_HCOOH = 2766, TOF_HCOOH = 44.9 min^-1) in comparison with those using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole or 1-benzyl-1,4-dihydronicotinamide.

Full text of DOI:10.1039/c5sc02018b

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Highly efficient, selective, and durable
photocatalytic system for CO2 reduction to formic
Cite this: DOI: 10.1039/c5sc02018b
acid†
c
ab
Yusuke Tamaki,ab Kazuhide Koike and Osamu Ishitani
*
We discovered an extremely suitable sacrificial electron donor, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-
dihydro-1H-benzo[d]imidazole, for the selective photocatalytic reduction of CO2 to formic acid using
Received 5th June 2015
Accepted 28th August 2015
a
Ru(II)–Ru(II) supramolecular photocatalyst. The efficiency, durability, and rate of photocatalysis
are significantly increased (FHCOOH 0.46, TONHCOOH 2766, TOFHCOOH in
¼
¼
¼
44.9 minꢀ1
)
DOI: 10.1039/c5sc02018b
comparison with those using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole or 1-benzyl-1,4-
www.rsc.org/chemicalscience
dihydronicotinamide.
CO2 to formic acid, which consist of [Ru(dmb)m(BL)3ꢀm]
2+ (dmb
Introduction
¼ 4,40-dimethyl-2,20-bipyridine, BL ¼ 1,2-bis(40-methyl-[2,20-
bipyridin]-4-yl)ethane, m ¼ 0, 1, 2) as a photosensitizer unit and
[Ru(dmb)n(BL)2ꢀn(CO)2]2+ (n ¼ 0, 1) as a catalyst unit.6 In the
photocatalytic reactions, a model compound of coenzyme
NAD(P)H called 1-benzyl-1,4-dihydronicotinamide (BNAH,
Chart 1) was employed as a sacricial electron donor. In
particular, a trinuclear complex with two photosensitizer units
and one catalyst unit (Ru2–Ru(CO), Chart 1) exhibited the
A tremendous amount of carbon dioxide, the most oxidized
form of carbon, has been produced from the combustion of
carbon-containing materials such as fossil fuels, plastics, and
foods. Because carbon dioxide is a primary greenhouse gas that
contributes signicantly toward global warming, the produc-
tion of CO2 must be reduced. Therefore, a technology that
produces useful energy-rich chemicals from CO2 with solar light
as an energy source should be developed in the future.
highest photocatalytic activity.
A high turnover number
(TONHCOOH ¼ 671) and turnover frequency (TOFHCOOH
¼
Although hydrogen gas has garnered attention as a carbon-
free fuel, it is constrained by its physical and chemical proper-
ties such as difficulties in transportation because of its low
density and risk of explosion. Formic acid, one of the two-
electron reduced compounds of CO2, is a promising hydrogen
storage medium because it can easily be split by catalysts into
hydrogen and CO2 under mild conditions and it exists in the
liquid form with a higher energy density at ambient tempera-
ture.1,2 The hydrogenation reaction of CO2 has been actively
studied owing to its ability to produce formic acid.3–5 For
example, Himeda, Fujita, and co-workers recently reported iri-
dium(III) catalysts for efficient hydrogenation of CO2 under mild
conditions.5 Another method for the production of formic acid
without H2 is the photocatalytic reduction of CO2.
11.6 minꢀ1) for the formation of formic acid were observed.
However, the quantum yield for formic acid formation was
relatively low (FHCOOH ¼ 0.061), even compared with that for
supramolecular photocatalytic systems employing the same
photosensitizer units and cis,trans-[Re(BL)(CO)2(L)2]2+-type
complexes as catalyst units. These systems selectively produced
CO, with FCO values ranging from 0.10 to 0.15.7
We also reported that BNAH has several disadvantages as a
sacricial electron donor for the improvement of photocatalysis
in supramolecular systems.6–8 First, the efficiency of quenching
the excited state of the Ru(II) photosensitizer unit with BNAH is
relatively low. For example, even a 0.1 M solution of BNAH
quenched only 59% of the excited photosensitizer unit in
Ru2–Ru(CO). Second, a relatively fast back electron transfer
occurs from the reduced form of the photosensitizer unit to the
oxidized form of BNAH, i.e., BNAHc+. Third, accumulation of
the BNA dimers (BNA2), which were produced from the depro-
tonation of BNAHc+ and following coupling processes, prevents
further progression of the photocatalytic reaction. This is
because BNA2 is more effective than BNAH at quenching the
excited photosensitizer unit of the supramolecular photo-
catalysts. Owing to these limitations, we can conclude that the
reported values of the photocatalytic abilities of Ru2–Ru(CO) do
not reect their actual potential.
To the best of our knowledge, we were the rst to report
supramolecular photocatalysts for the selective reduction of
aDepartment of Chemistry, Graduate School of Science and Engineering, Tokyo
Institute of Technology, 2-12-1-NE-1 O-okayama, Meguro-ku, Tokyo, 152-8550,
Japan. E-mail: ishitani@chem.titech.ac.jp
bCREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi-city,
Saitama, 322-0012, Japan
cNational Institute of Advanced Industrial Science and Technology, Onogawa 16-1,
Tsukuba 305-8569, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5sc02018b
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Chart 1 Structures and abbreviations of Ru2–Ru(CO), model mononuclear complexes, BI(OH)H, BIH, and BNAH. The counter anions of the
ꢀ
complexes were PF6
.
We recently discovered that compared with BNAH, a benzi- was 0.15 mM, the absorbance of the solution was greater than
midazoline derivative (BIH, Chart 1) can function as a much 3 with a 1 cm path length. The solution was mixed vigorously
better sacricial electron donor for the excited state of during the irradiation process. Using 0.025 mM Ru2–Ru(CO),
[Ru(N^N)3]2+-type (N^N ¼ diimine ligand) complexes.8 The the number of absorbed photons could be calculated from the
efficiency, durability, and formation rate of CO generated from change in absorbance at 480 nm during irradiation.
the photocatalytic reduction of CO2 with the Ru(II)–Re(I)
LIC2 was used for determining the TONHCOOH: a wavelength
supramolecular photocatalyst (Ru–Re) were substantially of light >500 nm was used for excitation. The photocatalyst
improved with BIH (FCO ¼ 0.45, TONCO ¼ 3029, TOFCO
¼
concentration was 0.025 mM.
35.7 minꢀ1) compared to those with BNAH (FCO ¼ 0.15,
LIC3 was used for determining the TOFHCOOH: a wavelength
TONCO ¼ 207, TOFCO ¼ 4.7 minꢀ1). BIH could quantitatively of light >420 nm was used for excitation. The photocatalyst
quench the excited state of the Ru(II)–Re(I) photocatalyst. BIH is concentration was 0.025 mM.
more effective than BNAH in decreasing the efficiency of the
Fig. 1 shows the UV-vis absorption spectra of the reductants,
back electron transfer from its oxidized form to the reduced BIH and BI(OH)H, and their respective oxidized compounds,
form of the photocatalyst. BIH serves as a two-electron donor, BI+ and BI(Oꢀ)+ along with the photocatalyst, Ru2–Ru(CO).
whereas BNAH can only donate one electron to the photo- Because none of these compounds absorbed wavelength of light
catalyst because of the coupling reaction of BNAc. The two- longer than 440 nm, the excitation light was selectively absor-
electron oxidized product of BIH (BI+) did not prevent the bed by Ru2–Ru(CO) in the LIC1 and LIC2 conditions in a mixed
progress of the photocatalytic reaction.
solvent system of N,N-dimethylformamide (DMF) and trietha-
We applied similar strategies in this study and implemented nolamine (TEOA) (4 : 1 v/v), which was also used as the solvent
a new reductant to clarify the “real” photocatalytic abilities of in the photocatalytic reactions. Because the irradiated light was
Ru2–Ru(CO) for the reduction of CO2. We found that the pho- partially absorbed by BI(Oꢀ)+ in LIC3, the TOFHCOOH values
tocatalysis of Ru2–Ru(CO) is signicantly better than those were obtained in the rst stage of the photocatalytic reaction
employing BNAH or BIH if another benzimidazoline derivative when only a small amount of BI(Oꢀ)+ had accumulated in the
with an –OH group at the o-position of the phenyl group, i.e., reaction solution (10 min irradiation).
1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imid-
We rst attempted to use BIH as a reductant. A mixed
azole9–12 (BI(OH)H, Chart 1), is used. In particular, this method solution of DMF and TEOA (4 : 1 v/v) containing Ru2–Ru(CO)
gave the highest rate for the photocatalytic reduction of CO2 of (0.025 mM) and BIH (0.1 M) was irradiated (>500 nm, LIC2)
all previously reported visible-light-driven photocatalytic under a CO2 atmosphere. The resulting photocatalytic products
systems (TOFHCOOH ¼ 44.9 minꢀ1).
Results
We determined the quantum yield (F), turnover number (TON),
and turnover frequency (TOF) of the reduction product(s) to
evaluate the photocatalysts. The following reaction conditions
were selected. The details and justications for these conditions
are described in ESI.†6
Light irradiation condition 1 (LIC1) was used to determine
the FHCOOH: a 480 nm monochromatic light with 5.0 ꢁ 10ꢀ9
einstein sꢀ1 intensity was used for excitation. Quantum yields
Fig. 1 UV-vis absorption spectra of BIH (black solid line), BI+ (black
broken line), BI(OH)H (red solid line), BI(Oꢀ)+ (red broken line), and
were determined using two different concentrations of the
photocatalyst, i.e., 0.15 mM and 0.025 mM. When [Ru2–Ru(CO)] Ru2–Ru(CO) (blue line, 1/10 intensity) in DMF–TEOA (4 : 1 v/v).
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Table 1 Photocatalytic properties using Ru2–Ru(CO)a
TONb (selectivityc/%)
FHCOOH
d
e
f
TOFHCOOH
/minꢀ1
kq
g
Reductant
HCOOH
CO
H2
0.15 mM
0.025 mM
/108 Mꢀ1 sꢀ1
hq
BI(OH)H
BIH
BNAH
2766 (87)
641 (72)
562 (91)
215 (7)
237 (27)
29 (5)
212
13
29
0.46
0.18
0.06
0.42
0.17
0.04
44.9
10.2
7.8
9.9
11.0
0.20
0.99
0.99
0.59
a
A CO2 saturated DMF–TEOA (4 : 1 v/v) solution consisting of a sacricial electron donor (0.1 M) and Ru2–Ru(CO) was irradiated. The concentration
of Ru2–Ru(CO) was 0.15 mM or 0.025mM for the measurement of F (LIC1) and 0.025 mM for the measurement of TON and TOF (LIC2 and LIC3,
b
c
d
respectively). A 2 mL solution was irradiated in LIC2 for 20 h. Selectivity of the products. A 4 mL solution was irradiated using LIC1 (light
intensity: 5.0 ꢁ 10ꢀ9 einstein sꢀ1). A 4 mL solution was irradiated using LIC3. Quenching rate constant of emission from the photosensitizer
e
f
g
unit by a sacricial electron donor. Quenching fractions of emission, which is calculated as hq ¼ kqsem[reductant]/(1 + kqsem[reductant]), from
the photosensitizer unit using 0.1 M of a sacricial electron donor, where the emission lifetime of Ru2–Ru(CO) (sem) was 726 ns (ref. 6).
included both formic acid (TONHCOOH ¼ 641) and CO (TONCO
¼
Therefore, we substituted the reductant BIH with another
237). Note that the substitution of BIH with BNAH under this benzimidazoline derivative with an OH group, BI(OH)H,13
same reaction condition gives formic acid as a main product which can supply two electrons and two protons from one
and CO as an extremely minor product (TONHCOOH ¼ 562, molecule.9–11 This substitution of BIH with BI(OH)H using the
TONCO ¼ 29). Therefore, the yield for the selective formation of same photochemical reaction condition (LIC2) produced formic
formic acid decreased from 91% to 72% when BIH was used as acid with a much higher TON (TONHCOOH ¼ 2766) and selec-
the reductant (Table 1). Markedly, BIH could function as a tivity (87%) (eqn (1), Fig. 2a). When Ru2–Ru(CO) or irradiation
fascinating sacricial electron donor for CO2 reduction with was excluded from the experimental condition, no formic acid
Ru–Re to give CO as the main product (FCO ¼ 0.45, TONCO
¼
was produced. The amount of formic acid produced during the
3029, TOFCO ¼ 35.7 minꢀ1). These values are substantially photocatalytic reaction without TEOA (TONHCOOH ¼ 1096 aer 8
greater than those obtained with BNAH.8
h irradiation) was lower than that with TEOA (TONHCOOH
¼
The total reaction scheme for the photocatalytic reduction of 1504) (Fig. S2†). Using a mixed system of model complexes
CO2 to CO using Ru–Re and BIH was reported as follows.
(Chart 1), i.e., [Ru(dmb)3]2+ (Ru, dmb ¼ 4,40-dimethyl-2,20-
bipyridine) and cis-[Ru(dmb)2(CO)2]2+ (Ru(CO)), both the TON
(TONHCOOH ¼ 1969) and the selectivity (78%) of formic acid
formation were also lower than those using Ru2–Ru(CO)
(Fig. 2b). The quantum yields for formic acid formation
exhibited very high values; FHCOOH ¼ 0.46 with [Ru2–Ru(CO)] ¼
0.15 mM and FHCOOH ¼ 0.42 with [Ru2–Ru(CO)] ¼ 0.025 mM.
The reaction rate was also very high (TOFHCOOH ¼ 44.9 minꢀ1).
To the best of our knowledge, the turnover frequency value in
particular is the highest of those reported for visible-light-
driven photocatalysts for CO2 reduction. As shown in Fig. 2b
The formation of formic acid from CO2 might require two
protons along with the two required electrons. This may induce
a shortage of protons in the solution during the photocatalytic
reaction process.
CO2 + 2eꢀ + 2H+ / HCOOH
Fig. 2 (a) Photocatalytic formation of formic acid ( ), CO ( ), and H2 ( ) as a function of irradiation time. (b) Photocatalytic formation of
formic acid using BI(OH)H ( ), BIH ( ) or BNAH ( ) as a sacrificial electron donor: a CO2 saturated DMF–TEOA (4 : 1 v/v, 2 mL) solution containing
Ru2–Ru(CO) (0.025 mM) and a sacrificial electron donor (0.1 M) was irradiated using lex > 500 nm light (LIC2). Open circles ( ) show the results
obtained from the photocatalysis using BI(OH)H (0.1 M) and a mixed system of [Ru(dmb)3]2+ (0.05 mM) and cis-[Ru(dmb)2(CO)2]2+ (0.025 mM)
instead of Ru2–Ru(CO).
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and Table 1, the previously described photocatalysis of
Ru2–Ru(CO) signicantly increased when BI(OH)H was used
instead of BIH or BNAH.
Ru2ꢀRuðCOÞ ð0:025 mMÞ=hnð . 500 nmÞ; 25 h
CO2
HCOOH;CO; H
2
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
(1)
BIðOHÞH ð0:1 MÞ in DMFꢀTEOA ð4 : 1 v=vÞ
TON : 2766; 215; 212
In the photoinduced electron transfer process, the quench-
ing rate constant of emission from the photosensitizer unit of
Ru2–Ru(CO) by BI(OH)H (kq ¼ 9.9 ꢁ 108 Mꢀ1 sꢀ1) was signi-
cantly larger than that by BNAH (kq ¼ 2.0 ꢁ 107 Mꢀ1 sꢀ1). This
result is reasonable because BI(OH)H (E1/2(BI(OH)H/BI(OH)Hc+)
¼ ꢀ0.06 V)12 is a better reducing agent than BNAH (Eꢂ(BNAH/
BNAHc+) ¼ 0.20 V).14 Under photocatalytic reaction conditions
where the [reductant] ¼ 0.1 M, the reductive quenching by
BI(OH)H proceeded almost quantitatively (hq ¼ 99%) and was
1.7 times more efficient than that by BNAH (hq ¼ 59%). This
could be one of the reasons BI(OH)H provides a higher photo-
catalytic efficiency than BNAH.15
Fig. 3 13C{1H} NMR spectra of the reaction solution before (a) and
after (b) 13.5 h of irradiation: DMF-d7-TEOA (4 : 1 v/v) solution con-
taining BI(OH)H (0.1 M), Ru2–Ru(CO) (0.5 mM), and 13CO2 (569 mmHg)
was irradiated at >420 nm using a xenon lamp.
A labeling experiment using 13CO2 was conducted to deter-
mine the source of the carbon atoms in the produced formic
acid. Prior to irradiation, a strong signal attributed to 13CO2 was
observed at 125.6 ppm in a 13C{1H} NMR spectrum of a DMF-d7-
TEOA (4 : 1 v/v) solution consisting of Ru2–Ru(CO) (0.5 mM) and
BI(OH)H (0.1 M) under a 13CO2 (569 mmHg) atmosphere
(Fig. 3a). Irradiation at lex > 420 nm for 13.5 h caused both a
decrease in the 13CO2 signal and appearance of a strong signal at
168.2 ppm, which was attributed to an equilibrium mixture of
Fig. 4 1H NMR spectrum after 13.5 h irradiation for the same sample
shown in Fig. 3.
1
H13COOH and H13COOꢀ (Fig. 3b). In the H NMR spectrum of
the same solution, a doublet (1JCH ¼ 188 Hz) at 8.52 ppm was
attributed to the proton of an equilibrium mixture of H13COOH
and H13COOꢀ, which should be coupled with the 13C atom. The
peak area of a singlet at 8.52 ppm was approximately 1/100 of the
peak area of the doublet and was attributed to the equilibrium
mixture of H12COOH and H12COOꢀ (Fig. 4). This peak area ratio
was consistent with the contamination of 12CO2 in the used
13CO2 (99%). The same J value from the 13C–H coupling (188 Hz)
was also observed in the 13C NMR spectrum that was measured
1
without H decoupling (Fig. 5). These results clearly show that
formic acid was produced through the reduction of CO2.
The simultaneous consumption of BI(OH)H and the
production of its oxidized compound during the photocatalytic
reaction were quantitatively analyzed by high-performance
liquid chromatography (HPLC). The oxidized compound of
BI(OH)H, a two-electron oxidized and two-proton releasing
compound (BI(Oꢀ)+), was quantitatively produced (eqn (2)).
1
Fig. 5 13C NMR spectrum without H decoupling after 13.5 h irradia-
tion for the same sample shown in Fig. 3.
equivalent. For example, aer a 3 h irradiation period, 55 mmol
of BI(OH)H was consumed and 54 mmol of BI(Oꢀ)+ and 54 mmol
of the two-electron reduced compounds were produced.
Therefore, it is clear that BI(OH)H acts as both a two-electron
donor and a two-proton donor, and the electrons and material
balances in the photocatalytic reaction can be represented in
eqn (3). Moreover, it should be noted that although BIH func-
tions as a two-electron donor, only one proton was released in
the two-electron oxidation process (eqn (4)).8
(2)
Ru2ꢀRuðCOÞ=hv
HCOOH þ BIðOꢀÞþ (3)
Fig. 6 shows that the amounts of consumed BI(OH)H and the
generated BI(Oꢀ)+, and the sum of the generated two-electron
reduced compounds (formic acid + CO + H2) were almost
CO þ BIðOHÞH
ƒƒƒƒƒƒƒƒƒƒƒƒ!
2
DMFꢀTEOAð4 : 1 v=vÞ
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Fig. 8 Formation of Ruꢀ: (a) DMF–TEOA (5 : 1 v/v) solution containing
Ru (0.3 mM) and BI(OH)H (0.1 M) was irradiated at lex ¼ 532 nm with a
Nd:YAG pulse laser under degassed conditions at room temperature.
The red line shows the fitting results (ESI†).
Fig. 6 Photocatalytic production of the reduction products (HCOOH
+ CO + H2, ) and BI(Oꢀ)+ ( ), and the consumption of BI(OH)H ( ): a
CO2 saturated DMF–TEOA (4 : 1 v/v, 2 mL) solution containing BI(OH)
H (0.1 M) and Ru2–Ru(CO) (0.025 mM) was irradiated using lex
500 nm light (LIC2).
>
This time course of the Ruꢀ production was tted using
numerical analysis based on kinetics, as shown in Scheme 1,8
where I0 is the light intensity absorbed by Ru, 1(*Ru) and 3(*Ru)
are singlet and triplet metal-to-ligand charge-transfer (MLCT)
excited states of Ru, respectively, (Ruꢀ/BI(OH)Hc+) is an ion
pair produced via the reductive quenching of 3(*Ru) by
BI(OH)H, and B is a base, i.e., TEOA or BI(OH)H. The quenching
rate constant of 3(*Ru) was obtained with a Stern–Volmer
analysis of emission from 3(*Ru) (kq ¼ 1.17 ꢁ 109 Mꢀ1 sꢀ1). The
increase in Ruꢀ was tted using eqn (5), for which the deriva-
tion is shown in the ESI.†
(4)
To clarify the two-electron donation processes of BI(OH)H, a
laser ash photolysis method was employed. A DMF–TEOA
mixed solution containing BI(OH)H (0.1 M) and Ru (0.3 mM),
which was used as a model of the photosensitizer unit, was
irradiated at lex ¼ 532 nm using a Nd:YAG pulse laser. The time-
resolved UV-vis absorption spectrum is shown in Fig. 7. At 100 ns
aer the laser excitation, new absorption was observed at 500–
550 nm. This is attributed to the one-electron reduced species of
Ru, i.e., Ruꢀ,8 which should be produced via the reductive
quenching of excited Ru by BI(OH)H. Fig. 8 shows the time
course of [Ruꢀ] measured at labs ¼ 550 nm, which increased in
two steps. The initial rapid increase was completed within the
laser pulse, and the concentration of Ruꢀ increased up to
approximately 10 ms. These results clearly indicate that BI(OH)H
acts as a two-electron donor with one-photon excitation of Ru.
Since BI(OH)c, which can be produced via the deprotonation of
BI(OH)Hc+, has strong reducing power (Eopx ¼ ꢀ1.96 V vs.
d[Ruꢀ]/dt ¼ kesc[(Ruꢀ/BI(OH)Hc+)] + ket[Ru][BI(OH)c]
ꢀ krec2[Ruꢀ][BI(OH)Hc+]
(5)
Based on these analyses, the rate constants of the electron
transfer from BI(OH)c to the ground state of Ru (ket), of the back
electron transfer from Ruꢀ to BI(OH)Hc+ (krec2), and of the
deprotonation of BI(OH)Hc+ (kdp) were 9 ꢁ 109 Mꢀ1 sꢀ1, 5 ꢁ 109
Mꢀ1 sꢀ1, and 2 ꢁ 105 sꢀ1, respectively.
The formation quantum yields for the one-electron-reduced
species (OERS) (FOERS) of the model of the photosensitizer unit,
Ru, using BI(OH)H, BIH, and BNAH were also determined to
compare their abilities as sacricial electron donors (Table 2). A
DMF–TEOA (5 : 1 v/v) solution containing Ru (0.1 mM) and one
Fc+/Fc),12 it should be able to donate an electron to another Ru
red
1/2
molecule even in the ground state (E ¼ ꢀ1.81 V).
Fig. 7 Time-resolved UV-vis absorption spectrum at 100 ns after the
laser excitation: a DMF–TEOA (5 : 1 v/v) solution containing Ru
(0.3 mM) and BI(OH)H (0.1 M) was irradiated at lex ¼ 532 nm with a
Nd:YAG pulse laser under degassed conditions at room temperature.
Scheme 1 Kinetics of Ruꢀ production using BI(OH)H.
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Table 2 The accumulation quantum yields of OERS of Ru using
BI(OH)H, BIH, or BNAH as a reductanta
FOER
Light intensity
/ 10ꢀ9 einstein sꢀ1
BI(OH)H
BIH
BNAH
11
4.3
1.1
0.45
0.54
0.64
0.59
0.74
0.95
0.066
0.087
0.14
a
A 4 mL Ar-saturated DMF–TEOA (5 : 1 v/v) solution consisting of the
reductant (0.1 M) and Ru (0.1 mM) was irradiated at 480 nm.
Fig. 9 Stern–Volmer plots of emission quenching of Ru2–Ru(CO) by
BI(OH)H in DMF–TEOA (4 : 1 v/v); the excitation wavelength was 530 nm.
of the reductants (0.1 M) was irradiated with three different
light intensities. The production of Ruꢀ was monitored with a
UV-vis absorption spectrometer. A greater light intensity
induced a lower quantum yield in all cases (Table 2). Irradiation
with a greater light intensity should result in higher concen-
trations of both Ruꢀ and the one-electron oxidized species of
the reductant (QH+). This should, in turn, induce more rapid
back electron transfer from Ruꢀ to QH+. When BI(OH)H was
used as the reductant, the values of FOER were 5–7 times greater
than those by using BNAH. This ratio is consistent with the
(6)
(7)
quantum yields of production for formic acid, i.e., FHCOOH
-
(BI(OH)H)/FHCOOH(BNAH) ¼ 7.7 with a light intensity of 5.0 ꢁ
10ꢀ9 einstein sꢀ1 (LIC1). Therefore, the main reasons for the
signicant increase in the FHCOOH value when BI(OH)H is
substituted for BNAH are ascribed to an increase in the elec-
tron-donating ability and suppression of the back electron
transfer (Table 2 and Fig. 7).8
Discussion
(8)
From the steady-state measurements of BI(OH)H decay and
BI(Oꢀ)+ formation and the transient measurement of Ruꢀ by
laser ash photolysis, we can conclude that BI(OH)H acts as a
two-electron donor with one-photon excitation of Ru2–Ru(CO)
during photocatalysis. As the initial process, excited Ru2–Ru(CO)
was reductively quenched by BI(OH)H giving the OERS of the
photosensitizer unit and BI(OH)Hc+ at a rate constant of kq
¼
9.9 ꢁ 108 Mꢀ1 sꢀ1, which was obtained from linear Stern–Volmer
plots (Fig. 9, eqn (6)). The second step is most likely the depro-
tonation of BI(OH)Hc+ by a base (B), i.e., TEOA or BI(OH)H to
produce BI(OH)c (eqn (7)). The intramolecular electron transfer
should occur from the OERS of the photosensitizer unit to the
(9)
catalyst unit (eqn (8)). Because BI(OH)c is a strong reducing
red
1/2
agent, it can donate an additional electron to Ru2–Ru(CO) (E
¼
(10)
ꢀ1.87 V) and/or possibly to a reaction intermediate derived from
the OERS of Ru2–Ru(CO) (Ru2–Ru(Intermediate), eqn (10)) in the
ground state (eqn (9) and (12)). Excitation of the photosensitizer
unit of Ru2–Ru(Intermediate) might allow the acceptance of an
additional electron from BI(OH)H (eqn (11)). In the last step,
BI(OH)+ should release an additional proton to produce a zwit-
terion BI(Oꢀ)+ as a dead-end oxidized compound of BI(OH)H
(eqn (13)). Therefore, a single-photon excitation of Ru2–Ru(CO)
should allow BI(OH)H to donate two electrons and two protons
via a sequence of electron transfer, deprotonation, electron
transfer, and deprotonation processes.
(11)
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described above, the difference between BI(OH)H and BIH is
most likely attributed to the number of released protons per
donated electrons. A one-photon excitation of Ru2–Ru(CO)
allows BI(OH)H to donate two electrons and two protons
(eqn (2)) in a step-by-step process. Conversely, BIH can only
donate two electrons and one proton (eqn (4)).8 In the case of
BNAH, only one electron and one proton can be donated
(eqn (14)), where the ratio between the donated electron and
proton is similar to BI(OH)H.6,7 In other words, BIH can only
donate half the amount of protons for every electron; however,
both BI(OH)H and BNAH can donate an equal amount of
protons for every electron.
(12)
(13)
As described above, the photocatalytic CO2 reduction to
formic acid proceeded using BI(OH)H without TEOA; however
the TONHCOOH was lower than that using BI(OH)H with TEOA
(Fig. S2†). As previously reported, TEOA does not quench the
3MLCT excited state of the photosensitizer unit of Ru2–Ru(CO),
which means that it should function only as a base in the
photocatalytic reaction. In the absence of TEOA, a proton from
BI(OH)Hc+ should be captured by another molecule of BI(OH)H.
Since the rates of the deprotonation process of BI(OH)Hc+ and
the back electron transfer process from the OERS of
Ru2–Ru(CO) to BI(OH)Hc+ compete with each other (Scheme 1),
slower deprotonation should lower the electron-donating ability
of BI(OH)H.
Although BIH has a slightly stronger electron-donating
ability than BI(OH)H (Table 2), FHCOOH and the selectivity
toward the formation of formic acid using BIH were both
signicantly lower than those in the case of BI(OH)H (Table 1).
As previously reported, a system that used both Ru–Re as the
photocatalyst and BIH as the reductant showed an extremely
high efficiency, durability, and rate for the photocatalytic
reduction of CO2 (FCO ¼ 0.45, TONCO ¼ 3029).8 A system that
utilized Ru2–Ru(CO) as the photocatalyst and BIH as the elec-
tron donor showed an eight-fold increase in the production of
CO (TONCO ¼ 237), whereas the amount of formic acid
(TONHCOOH ¼ 641) was almost equivalent to the results
obtained with BNAH (TONCO ¼ 29, TONHCOOH ¼ 562). As
(14)
To determine the effects of the proton concentration in the
solution on the photocatalysis of Ru2–Ru(CO), a photocatalytic
reaction that uses BIH in the presence of a phenol (0.1 M) as a
proton source was investigated (Fig. 10). In the initial stage of
the photocatalytic reaction (1 h irradiation), the TONs for for-
mic acid and CO were 406 and 122, respectively. This is similar
to a system containing BI(OH)H (TONHCOOH ¼ 484, TONCO
¼
53), where the TONHCOOH is 2.3 times larger than a system with
BIH (TONHCOOH ¼ 180, TONCO ¼ 46). This result clearly indi-
cates that BIH decreases the formation of formic acid, which is
caused by a shortage of protons as BIH can only supply half the
amount of protons for every donated electron but conversion of
CO2 to HCOOH requires both two electrons and two protons.16,17
Candidates for the Ru2–Ru(Intermediate) might be formato
and carboxylato complexes as the catalyst unit, i.e., [Ru(BL)2-
(CO)(OC(O)H)]18 and [Ru(BL)2(CO)(COOH)]19 (eqn (10)). Sulli-
van, Meyer, and co-workers reported an electrocatalytic reduc-
tion of CO2 to formic acid using the hydrido complex cis-
[Ru(bpy)2(CO)H]+ (bpy ¼ 2,20-bipyridine).18 They suggested that
the one-electron reduction of cis-[Ru(bpy)2(CO)H]+ and the
subsequent insertion of CO2 gives cis-[Ru(bpy)2(CO)(OC(O)H)]0.
A subsequent additional electron reduction leads to the loss of
the formate anion and cis-[Ru(bpy)2(CO)H]+ is reformed. If a
similar mechanism occurs in the photocatalytic reaction of
Ru2–Ru(CO), the formation of the hydrido complex might
become more favorable if BI(OH)H is used instead of BIH. This
is because BI(OH)H can supply more protons during the pho-
tocatalytic reaction.
Experiments
Fig. 10 Photocatalytic formation of formic acid ( ) and CO ( ) using General procedures
BI(OH)H (0.1 M, blue), BIH and phenol (0.1 M each, red) or BIH (0.1 M,
black): a CO2 saturated DMF–TEOA (4 : 1 v/v, 2 mL) solution con-
taining Ru2–Ru(CO) (0.025 mM), a sacrificial electron donor (0.1 M)
The UV-vis absorption spectra were measured with a JASCO
V-565 spectrophotometer. 1H NMR spectra were obtained using
and a proton source (0.1 M) was irradiated by lex > 500 nm light (LIC2). a JEOL AL400 (400 MHz) system to identify the synthesized
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compounds in solutions of either CDCl3 or acetone-d6. The than 420 nm obtained using a 500 W xenon lamp with a cut-off
tetramethylsilane protons contained in CDCl3 and the residual lter. The residual carbon and proton of DMF-d7 was used as an
protons of acetone-d6 were used as an internal standard for internal standard for these measurements.
these measurements. The emission spectra were measured
using a JASCO FP-6500 spectrouorometer. The emission
quenching experiments were performed on Ar-saturated solu-
tions containing Ru2–Ru(CO) and ve different concentrations
of a sacricial electron donor. Quenching rate constants kq were
calculated from linear Stern–Volmer plots for the luminescence
of the 3MLCT excited state of the photosensitizer units together
with knowledge of their lifetimes.
Time-resolved UV-vis absorption spectroscopy
Second-harmonic-generation light at 532 nm produced with a
Spectra-Physics Quanta-Ray LAB-150-10 pulsed Nd:YAG laser
was used for excitation (10 ns FWHM). An Ushio 300 W Xe arc
lamp was operated in a pulse-enhanced mode (500 ms duration)
using an XC-300 power supply and a YXP-300 light pulsar (Eagle
Shoji) as a monitoring light source. The monitoring light beam
was passed through the quartz cuvette (10 ꢁ 10 ꢁ 40 mm) that
contained a sample and was directed into an R926 photo-
multiplier tube (Hamamatsu Photonics) on a Jobin-Yvon HR-320
monochromator. Time proles of the monitoring light intensity
were stored using a LeCroy WaveRunner 640zi oscilloscope
(4 GHz bandwidth). Transient spectra were obtained with an
Andor Technology iStar H320T-18F-03 (690 channels; minimum
gate width: 5 ns) ICCD detector head mounted on the HR-320
monochromator. A DMF–TEOA (5 : 1 v/v) solution that con-
tained Ru (0.3 mM), and BI(OH)H or BIH (0.1 M) was degassed
by freeze–pump–thaw method prior to the laser ash photolysis.
Photocatalytic reactions
The experimental details of photo-irradiation conditions of the
photocatalytic reactions are shown in the ESI.†6 All of the
experiments used 0.1 M concentrations of BI(OH)H, BIH, or
BNAH in a solvent mixture of DMF and TEOA (4 : 1 v/v). The
solutions were purged with CO2 for 20 min before irradiation.
The concentrations of CO and H2 were analyzed by a GC-TCD
(GL science GC323). Formic acid was analyzed using a capillary
electrophoresis system (Otsuka Electronics Co. CAPI-3300I). To
quantify the amount of formic acid, the photocatalytic reaction
solution was pretreated by diluting the solution 10 times with
H2O. As DMF is hydrolyzed to formic acid in the presence of a
base,20 a nonirradiated photocatalytic reaction solution of
saturated CO2, which suppresses the hydrolysis of DMF by
acting as an acid, was employed as a reference. The reference
solution was also measured before and aer the quantication
of formic acid, and its value was subtracted from the quantied
formic acid. It should be emphasized that substantially smaller
amounts of formic acid were produced by the hydrolysis of DMF
in comparison with the amounts of formic acid produced by the
photocatalytic reduction of CO2. This is probably due to the fact
that CO2 works as an acid. For example, in the photocatalytic
reaction that utilizes BI(OH)H under LIC2 (Fig. 2, Table 1),
138 mmol of formic acid was produced by the photocatalytic
reduction of CO2 aer a 20 h irradiation period. In contrast,
only 2.0 mmol of formic acid was produced in the reference
solution.
Quantum yield measurements of the one-electron reduction
of Ru
The quantum yield measurements were performed in a quartz
cubic cell (light pass length: 1 cm) consisting of Ru (0.1 mM) and a
sacricial electron donor (0.1 M) in a DMF–TEOA (5 : 1 v/v, 4 mL)
solution. Aer purging with Ar for 20 min, the solution was irra-
diated using a 500 W xenon lamp with a 480 nm (FWHM ¼ 10 nm)
band pass lter (Asahi Spectra Co.) and a 5 cm long CuSO4 solu-
tion (250 g Lꢀ1) lter. The UV-vis absorption spectral changes
during irradiation were measured with a Photal MCPD-2000
spectrophotometer. During irradiation, the temperatures of the
solutions were maintained at 25 ꢂC using an IWAKI constant
temperature system CTS-134A. The incident light intensity was
determined using a K3Fe(C2O4)3 actinometer and the number of
absorbed photons were calculated on the basis of the absorbance
changes at an irradiation wavelength of 480 nm. The amounts of
produced Ruꢀ were calculated using the molar absorptivity of Ruꢀ,
which was obtained by the electrochemical reduction of Ru in an
acetonitrile solution containing Et4NBF4 as an electrolyte.7
The HPLC analyses of BI(OH)H and BI(Oꢀ)+ were accom-
plished using a JASCO 880-PU pump with a Develosil ODS-UG-5
column (250 ꢁ 4.6 mm), a JASCO 880-51 degasser, and a JASCO
UV-2070 detector. The column temperature was maintained at
30 ꢂC with a JASCO 860-CO oven. The mobile phase was a 6 : 4
(v/v) mixture of acetonitrile and a NaOH–KH2PO4 buffer solu-
tion (pH 7) with a ow rate of 0.5 mL minꢀ1. The retention times
were 23.8 min (BI(OH)H) and 6.3 min (BI(Oꢀ)+).
Materials
DMF was dried over molecular sieves 4A and distilled under
reduced pressure (10–20 mm Hg). TEOA was also distilled under
reduced pressure (<1 mm Hg). Both solvents were kept under Ar
before use. All other reagents were of reagent-grade quality and
used without further purication. Ru2–Ru(CO),6 BI(OH)H,9,12
BI(Oꢀ)+,21 and BIH9,12 were prepared according to the reported
methods.
Labeling experiments using 13CO2
The 13CO2 experiment was performed in a DMF-d7-TEOA solu-
tion containing Ru2–Ru(CO) (0.5 mM) and BI(OH)H (0.1 M). The
tube was deaerated using the freeze–pump–thaw method before
13CO2 (569 mmHg) was introduced into the solution. 13C{1H},
Conclusion
13C, and 1H NMR spectra were measured with a JEOL AL300
1
(75 MHz for 13C NMR and 300 MHz for H NMR) before and We found that BI(OH)H is a suitable sacricial electron donor
aer 13.5 h of irradiation using light with a wavelength of more for the photochemical reduction of CO2 to formic acid by the
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utilization of Ru2–Ru(CO) as the photocatalyst. The efficiency, 10 E. Hasegawa, S. Takizawa, T. Seida, A. Yamaguchi,
durability, and rate of the photocatalytic reaction using
N. Yamaguchi, N. Chiba, T. Takahashi, H. Ikeda and
Ru2–Ru(CO) were all signicantly increased when BI(OH)H
K. Akiyama, Tetrahedron, 2006, 62, 6581–6588.
was employed as a sacricial reductant (FHCOOH ¼ 0.46, 11 E. Hasegawa, H. Hirose, K. Sasaki, S. Takizawa, T. Seida and
TONHCOOH ¼ 2766, TOFHCOOH ¼ 44.9 minꢀ1) relative to BIH or
N. Chiba, Heterocycles, 2009, 77, 1147–1161.
BNAH. The electron balance and the material balance of the 12 X.-Q. Zhu, M.-T. Zhang, A. Yu, C.-H. Wang and J.-P. Cheng, J.
photocatalysis was also claried, where one molecule of
Am. Chem. Soc., 2008, 130, 2501–2516.
BI(OH)H donates two electrons and two protons to one mole- 13 Deprotonation of the hydroxy group of BI(OH)H should not
cule of CO2 to produce one molecule each of BI(Oꢀ)+ and formic
acid.
proceed in the DMF–TEOA (4 : 1 v/v) solution because the p–
p* absorption band of BI(OH)H observed at lmax ¼ 275 nm
did not change at all between in the presence and absence of
TEOA (Fig. S1†).
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