Photocatalysis of a dinuclear Ru(II)-Re(I) Complex for CO2reduction on a solid surface

Article abstract of DOI:10.1021/jacs.0c09170

The development of CO2-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO2-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru(II)-complex photosensitizer and a Re(I)-complex catalyst fixed on the surface of insulating Al2O3 particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO2 atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru(II) and Re(I) complexes attached to an Al2O3 surface, the photocatalytic activity was much lower. The activity of the Al2O3-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO2 reduction. Coadsorption of a Ru(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the Al2O3 particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO2 reduction, which are composed of metal-complex photocatalysts and solid materials.

Full text of DOI:10.1021/jacs.0c09170

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Photocatalysis of a Dinuclear Ru(II)−Re(I) Complex for CO2
Reduction on a Solid Surface
Daiki Saito, Yasuomi Yamazaki, Yusuke Tamaki, and Osamu Ishitani*
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ABSTRACT: The development of CO2-reduction photocatalysts is one
of the main targets in the field of artificial photosynthesis. Recently,
numerous hybrid systems in which supramolecular photocatalysts
comprised of a photosensitizer and catalytic-metal-complex units are
immobilized on inorganic solid materials, such as semiconductors or
mesoporous organosilica, have been reported as CO2-reduction
photocatalysts for various functions, including water oxidation and
light harvesting. In the present study, we investigated the photocatalytic
properties of supramolecular photocatalysts comprised of a Ru(II)-
complex photosensitizer and a Re(I)-complex catalyst fixed on the
surface of insulating Al2O3 particles: the distance among the supra-
molecular photocatalyst molecules should be fixed. Visible-light
irradiation of the photocatalyst in the presence of an electron donor
under a CO2 atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear
Ru(II) and Re(I) complexes attached to an Al2O3 surface, the photocatalytic activity was much lower. The activity of the Al2O3-
supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of
photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of
the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by
the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited
photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface.
The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular
photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO2 reduction. Coadsorption of a Ru(II)
mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst
on the surface of the Al2O3 particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of
CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic
systems for CO2 reduction, which are composed of metal-complex photocatalysts and solid materials.
affords useful chemicals, e.g., CO and HCOOH, as two-
electron-reduced compounds (eqs 1 and 2).3
INTRODUCTION
■
Global warming and the depletion of fossil resources are
serious threats to the future of human beings. Artificial
photosynthesis in which CO2 is converted to useful high-
energy chemicals using solar light as the energy source is one
of the most promising technologies for addressing both of
these problems. However, various functionalities must be
added to artificial photosynthesis systems if they are to be
practically useful. In such systems, CO2 must be photochemi-
cally reduced because it is a highly oxidized stable carbon
compound. Photochemical electron transfer should be used for
this purpose where, in principle, one-photon excitation induces
only one-electron transfer. However, one-electron reduction of
CO2 requires very high energy (E0 = −1.9 V vs NHE), and the
Since some metal-complex catalysts can accept two electrons
and introduce them to activated CO2 to give CO and/or
HCOOH,4−9 various photocatalytic systems for CO2 reduction
have been constructed using such catalysts together with
redox-photosensitizer metal complexes that initiate photo-
chemical one-electron transfer from a reductant to the
catalyst.10−14 Recently, so-called supramolecular photocatalysts
in which photosensitizer and catalysts units are connected have
Received: August 26, 2020
product CO2 is too active to be stored.1,2 Conversely,
−•
multielectron reduction (typically two-electron reduction) of
CO2 drastically lowers the reduction potential of CO2 and
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been widely researched owing to their superior photocatalytic
abilities compared to those of the corresponding mixed
systems in solution.15−21 It has been reported that rapid
intramolecular electron transfer from the reduced photo-
sensitizer unit to the catalyst unit is a crucial process in the
reduction of CO2 in supramolecular photocatalytic sys-
tems.22,23
sensitizer unit to the catalyst unit.22,23,42,43 However, reduction
of CO2 to CO or HCOOH requires two electrons, as
described above, but the photosensitizer unit can supply only
one electron to the catalyst unit. Can a photosensitizer
photochemically supply another electron to a catalyst unit that
has already accepted one electron and reacted with CO2? Does
disproportionation of two reduced supramolecular photo-
catalyst molecules give “two-electron reduced” species upon
their collision in homogeneous solution? Although the answers
to these questions are important for developing more useful
hybrid solid systems with metal-complex photocatalysts, the
reported hybrid systems comprised of metal complexes and
photofunctional solid materials promote their photocatalytic
reactions via many and complicated processes, and this
complexity makes it difficult to answer these questions.
Immobilization of the metal-complex photocatalysts onto
nonphotoactive solid surfaces could provide a selective control
method by introducing distance between the photosensitizer
and the catalyst in mixed systems and also between the
supramolecular photocatalyst molecules as well. In other
words, changing the adsorption density of metal complexes
with strong anchoring groups on insulators can systematically
change surface distances. This could provide important
information for answering the questions above.
E0/V vs NHE
CO2(g) + 2e− + 2H+ → CO + H2O −0.52 V
(1)
CO2(g) + 2e− + 2H+ → HCOOH −0.61 V
(2)
Although there are highly efficient photocatalytic systems for
CO2 reduction consisting of metal complexes alone,10−14 these
photocatalysts require a sacrificial electron donor to promote
CO2 reduction owing to their weak oxidation powers in the
excited state. Furthermore, their light-absorption abilities are
insufficient for the efficient use of solar light. Thus, in order to
realize artificial photosynthesis for use in practical technolo-
gies, the addition of water oxidation and light-harvesting
functions to such photocatalytic systems is necessary.
A useful strategy for the addition of such functions to metal-
complex photocatalysts is the hybridization with functional
solid materials.24−32 Recently, supramolecular photocatalysts
have been composited with various solid materials to fabricate
hybrid photocatalysts.33−37 The combination of supramolecu-
lar photocatalysts with semiconductors improves the photo-
chemical oxidation power of such photocatalytic systems.38 As
a typical example, a photoelectrochemical cell consisting of a
Ru(II)−Re(I) supramolecular photocatalyst fixed on a p-type
semiconductor such as NiO and CuGaO2 as a photocathode
and an n-type semiconductor photoanode such as CoOx/
TaON can photocatalytically reduce CO2 to CO using water as
a reductant and visible light as energy.39,40 Such high reduction
and oxidizing powers are produced by stepwise excitation of
the n-type semiconductor electrode and the Ru(II)-complex
photosensitizer unit of the supramolecular photocatalyst. This
is the so-called Z-scheme mechanism. A hybrid system
comprised of a Ru(II)−Re(I) supramolecular photocatalyst
fixed to the surface of periodic mesoporous organosilica
(PMO), where the walls contained acridone molecules as light
absorber units, has also been reported as a photocatalytic
system with light-harvesting functionality.41 In this system,
photon energy absorbed by approximately 40 acridone units in
the wall is efficiently transferred to one Ru(II) photosensitizer
unit and initiates efficient photocatalytic CO2 reduction.
In these hybrid systems containing solid materials, the
supramolecular photocatalysts work much better than a mixed
system of the corresponding mononuclear complexes.
Although we may assume that one of the reasons for this is
the rapid intramolecular electron transfer between the
photosensitizer and catalyst units in the supramolecular
photocatalyst, no systematic investigation into the superiority
of supramolecular photocatalysts over the corresponding
mixed systems on solid materials has been reported. There
are several important mechanistic questions regarding CO2
reduction using supramolecular photocatalysts, both on the
surfaces of solid materials and in homogeneous solutions. One
such question is how the catalyst unit can obtain the second
electron. It has already been reported that the first reduction
process proceeds via reductive quenching of the excited
photosensitizer unit by a reductant followed by rapid
intramolecular electron transfer from the reduced photo-
Herein, a supramolecular photocatalyst consisting of a
Ru(II) photosensitizer with methyl phosphonic acid groups
as anchors and a Re(I) catalyst (RuRe in Chart 1) was
Chart 1. Structures and Abbreviations of the Metal
Complexes Used in This Study
adsorbed on Al2O3 insulator nanoparticles, and the effects of
the adsorption density of RuRe on its photocatalytic ability for
CO2 reduction were investigated in detail. Photocatalytic CO2
reduction was also conducted by using mononuclear Ru and
Re complexes (Chart 1) immobilized on the Al2O3 together,
and the results are compared with those using RuRe.
RESULTS AND DISCUSSION
■
The complexes RuRe, Ru, and Re, which each have a diimine
ligand with two methylphosphonic acid anchor groups (Chart
1), were synthesized according to previous reports.41,44,45 In
this study, SiO2 (AEROSIL OX 50) and Al2O3 (AEROXIDE
Alu C) were used as solid insulators to adsorb the metal
complexes on their surfaces because photochemical electron
transfer between the complex and the solid material can be
avoided. Since these solid particles, of which the average
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AA (μmol g−1) × R2 × 3.14 × N
particle sizes are 200−300 nm, are nonporous, the average
distances between the metal complexes absorbed on the
surface can be estimated from the surface area of the particles
and the number of adsorbed metal complex molecules. Al2O3
or SiO2 particles were added into an MeCN solution
containing the metal complex or complexes, and the
suspension was stirred under Ar atmosphere at room
temperature for 7 days. The particles were collected by
filtration and washed with MeCN several times until the filtrate
did not contain the metal complex(es). The amount of the
metal complex adsorbed was determined by measuring the
amounts of the metal complex dissolved in the solution before
and after the addition of the particles. We assessed the
desorption of the complexes from the solid materials by the
following method: The solid material adsorbing RuRe as
described above (RuRe/Al2O3 and RuRe/SiO2, the adsorption
density: 5 μmol g−1) was dispersed into a mixed solution of
N,N-dimethylacetoamide (DMA)−triethanolamine (TEOA)
(4:1, v/v) in the dark for 60 h, and then the solid was filtered.
The UV−vis absorption spectrum of the filtrate was measured
to determine the amount of RuRe desorbed from the solid
material. In the case of RuRe/Al2O3, only 1% of the adsorbed
RuRe was desorbed, while 12% was desorbed in the case of
RuRe/SiO2. Therefore, we chose Al2O3 particles as the solid
material for all subsequent experiments. The amounts of the
metal complex(es) adsorbed (AA) were obtained by using eq
3.
A
SC (%) =
× 100
specific surface area (m2 g−1
)
(4)
Figure 2. Molecular lengths of RuRe optimized by MM2 calculation.
The specific surface area was measured by nitrogen
adsorption/desorption isotherm analysis and the Brunauer−
Emmett−Teller equation (Figure S1). For example, the SC of
the sample with AA = 130 μmol g−1 was calculated as 93 9%.
This suggests that the Al2O3 surface of this sample is very
densely covered, even with a single layer of RuRe. The AA
(SC) value was reproductively controlled between 5 μmol g−1
(5%) and 110 μmol g−1 (80%) by changing the amount of
Al2O3 added into the RuRe solution.
Figure 3a shows the diffuse reflectance FT-IR spectrum of
RuRe/Al2O3 (AA = 5 μmol g−1). CO stretching bands of the
Re unit are observed at ν(CO) = 2022, 1910, and 1900 cm−1,
which is in agreement with the FT-IR absorption spectrum of
RuRe measured in MeCN solution. Figure 3b shows the
diffuse reflectance UV−vis spectrum of RuRe/Al2O3 (AA = 5
μmol g−1) and the UV−vis absorption spectrum of RuRe in
MeCN. Absorption bands attributed to singlet metal-to-ligand
charge transfer (1MLCT) absorption were observed at around
460 nm in both spectra.
The coadsorption of both Ru and Re onto Al2O3 was
performed according to the following stepwise procedure
because of the low solubility of Re in MeCN: First, the Al2O3
particles were added to a MeOH solution containing Re, and
the solution was stirred at room temperature for 7 days. The
dispersion was filtered, and the obtained solid was washed
using MeOH. The solid was added into an MeCN solution
containing Ru, and the solution was stirred again for 7 days.
The same subsequent filtration and washing procedures were
used to obtain (Ru + Re)/Al2O3, and the desorption of the
complexes was measured by the same procedure as that used
for RuRe/Al2O3. The desorption was found to be very low
(<4%).
Abefore − Aafter
50 [μM] × 50 [mL]
X [mg]
AA (μmol g−1) =
×
Abefore
(3)
where Abefore and Aafter are the absorbances of RuRe, of which
the initial concentration was 50 μM in 50 mL of MeCN, in the
filtrates before and after the adsorption procedure (the
washing solvent was added into the filtrate, and the amount
of the solution was adjusted to be the same, i.e., 50 mL) and X
is the amount of Al2O3 particles added. Figure 1 shows the
Photocatalytic CO2 reduction was carried out as follows:
The RuRe/Al2O3 particles were dispersed in DMA−TEOA
(4:1, v/v) containing 1-benzyl-1,4-dihydronicotinamide
(BNAH) as a one-electron donor at a concentration of 0.1
M. These suspensions were saturated with CO2 and irradiated
at λex > 480 nm using a high-pressure Hg lamp equipped with a
K2CrO4 (30% w/w, d = 1 cm) aqueous solution filter. In each
sample, the amount of the photosensitizer unit in the
photocatalytic reaction was adjusted to 100 nmol. For example,
the amounts of RuRe/Al2O3 particles added were 20 and 2 mg
for AA = 5 and 50 μmol g−1, respectively. To minimalize the
differences between the dispersed reaction solutions due to the
effect of light scattering, untreated Al2O3 particles were
additionally introduced into the reaction solution to make
the total amount of RuRe/Al2O3 and Al2O3 20 mg. During
irradiation, the reaction solution was stirred using a magnetic
stirrer.
Figure 1. Adsorption amounts of RuRe on Al2O3.
adsorption curve for RuRe on Al2O3 surfaces. Almost all of the
dissolved RuRe in the solution is adsorbed onto Al2O3 when
AA is below 110 μmol g−1, and the maximum is AA = 130
μmol g−1.
Surface coverages of RuRe on Al2O3 (SC) were estimated
using eq 4 where R is the short molecular radius of RuRe (R =
0.6 nm) calculated using the MM2 molecular mechanics
program (Figure 2).
C
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Figure 3. (a) FT-IR spectra of RuRe adsorbed on Al2O3 and RuRe dissolved in MeCN. (b) UV−vis diffuse reflectance and absorption spectra of
RuRe/Al2O3 and RuRe.
Figure 4 shows the time profiles of product generation in
photochemical CO2 reduction using RuRe/Al2O3 (a and b)
and (Ru + Re)/Al2O3 (c and d), for which AA is (5 and 5
μmol g−1) and (50 and 50 μmol g−1). In both cases using
RuRe/Al2O3, CO was photocatalytically produced as a major
product with high selectivity (ΓCO = 80% and 92%) and
relatively high turnover numbers (TONCO = 60 and 86,
respectively). It is important to note that the photocatalytic
reduction of CO2 proceeded even in the system using RuRe/
Al2O3 with very low AA (Figure 4a), where the distance
between the neighboring RuRe molecules on the Al2O3 is
much larger than the molecular size of RuRe, as described
later, and BNAH can provide only one electron upon
excitation of the photosensitizer unit of RuRe because the
one-electron oxidized and deprotonated compound, i.e., BNA•,
quickly dimerizes to 4,4′-BNA2 or 4,6′-BNA2 (eq 5), neither of
systems in solution, and this process is a through-bond
electron transfer.22,23
In the cases of the mixed mononuclear system, i.e., (Ru +
Re)/Al2O3, only intermolecular electron transfer from the
reduced Ru to Re can trigger the photocatalytic reaction,
which is therefore slower or not possible, especially in the low-
AA case. Photocatalytic CO formation did not proceed in the
cases using only Ru or Re adsorbed on Al2O3 (Ru/Al2O3, Re/
Al2O3, Figure S2).
which can reduce RuRe in the ground state. Therefore, we can
conclude that the photocatalytic CO formation proceeds via
step-by-step double excitations of the photosensitizer unit of
RuRe in the ground state, which are followed by reductive
quenching processes of its excited state by BNAH each time,
for an isolated RuRe molecule on Al2O3.
Figure 4. Time profiles of CO (red), HCOOH (blue), and H2
(green) production during photocatalytic reactions over (a) RuRe/
Al2O3 (AA = 5 μmol g−1), (b) RuRe/Al2O3 (AA = 50 μmol g−1), (c)
(Ru + Re)/Al2O3 (AA = 5 + 5 μmol g−1), and (d) (Ru + Re)/Al2O3
(AA = 50 + 50 μmol g−1).
Although CO was also photocatalytically produced over (Ru
+ Re)/Al2O3 with both higher AAs (Figure 4d: AA = 50 μmol
g−1 and 50 μmol g−1), the TONCO and ΓCO values were much
lower (TONCO = 25, ΓCO = 85%) than those for the
supramolecular systems (TONCO = 86, ΓCO = 92%). For the
lower-adsorption-density case (Figure 4c: AA = 5 μmol g−1),
the effectiveness of the supramolecular photocatalyst was more
apparent: TONCO = 60 and ΓCO = 80% in the system using
RuRe/Al2O3 while TONCO = 5 and ΓCO = 32% in the system
using (Ru + Re)/Al2O3. These results clearly indicate that the
covalent bonding between the Ru photosensitizer and Re
catalyst units in the supramolecular photocatalysts plays a
crucial role in the photocatalytic CO2 reduction on the solid
surface. It has been reported that rapid electron transfer
proceeds from the one-electron-reduced Ru photosensitizer
unit, which is produced by photochemical reductive quench-
ing, to the Re catalyst unit in the supramolecular photocatalyst
Figure 5 shows the time profile for the TON values of CO
formation in the photocatalytic reaction using RuRe/Al2O3
with various AAvalues (5−110 μmol g−1). In all cases, CO was
produced with high selectively (ΓCO > 80%) with 60 h of light
irradiation, and small amounts of HCOOH and H2 were
detected as minor products. Both the turnover number and
frequency of CO formation (TONCO and TOFCO) showed
different trends depending on the AA. We can assess the
durabilities of the RuRe/Al2O3 systems using the TONCO
values obtained after 120 h irradiation. Conversely, the TOFCO
value in the initial stage of the reaction can be used as an
indicator of the rate of CO formation, which here is defined as
the value of the TONCO value at 1 h of irradiation. The data
are summarized in Table 1. The TOFCO was lower in the case
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single exponential function (Figure S3), the obtained emission
lifetime (τem) was dependent on AA; i.e., a higher AA led to a
shorter τem. For example, τ = 762 ns at AA = 5 μmol g−1 and τ
= 685 ns at AA = 110 μmol g−1. However, the emission
lifetime did not change upon decreasing the intensity of the
excitation light using a 10% neutral density filter. Therefore,
triplet−triplet annihilation should not proceed. The rate
constants of nonradiative and radiative deactivation (kr and
knr) were calculated using the values of the emission quantum
yield and the emission lifetime: knr increased with increasing
adsorption density while kr did not change. These results
strongly suggest that self-quenching occurred on the Al2O3
surface. This is likely to be one of the reasons for the lower ηq
value in the cases with higher AA.
Figure 6 shows the Stern−Volmer plots for the quenching of
emissions from RuRe/Al2O3 by BNAH, all of which exhibited
Figure 5. Time courses of CO formation during the photocatalytic
reactions using RuRe/Al2O3 with different AA values during
irradiation at λex > 480 nm in DMA/TEOA (4:1, v/v) containing
BNAH (0.1 M) under CO2 atmosphere (a) up to 5 h irradiation and
(b) up to 120 h irradiation.
with the higher AA (for example, 14 h−1 at 5 μmol g−1 and 4
h−1 at 110 μmol g−1). Interestingly, TONCO was higher for
higher AA up to AA = 80 μmol g−1 (56 at 5 μmol g−1 and 134
at 80 μmol g−1). Although in the case for AA = 110 μmol g−1,
TONCO was slightly lower than that for AA = 80 μmol g−1, and
it was still higher than that for the other cases (TONCO = 118).
Data for emission from the Ru(II) photosensitizer unit on
the Al2O3 and its quenching by BNAH are summarized in
Table 1. The quenching fraction (ηq) indicates how much the
photosensitizer excited state was reductively quenched by
BNAH (0.1 M) and is calculated using eq 6 with the Stern−
Volmer constant (KSV).
Figure 6. Stern−Volmer plots of the quenching of emission from
RuRe on Al2O3 surfaces at different adsorption densities. Excitation
wavelength = 510 nm. The solvent was DMA/TEOA (4:1, v/v).
good linearity, thus giving the reductive quenching rate
constant of the excited Ru(II) photosensitizer unit on the
Al2O3 (kq in Table 1). The kq value also decreased when the
AA increased. This suggests that a higher adsorption density of
RuRe on Al2O3 caused a higher steric interaction among the
neighboring RuRe molecules on Al2O3, hindering the approach
of BNAH to the excited Ru photosensitizer unit owing to the
bulkiness of the Re unit. The slower reductive quenching
should also lower the TOFCO value. It is noteworthy that
BNAH acts as a one-electron donor with the one-photon
0.1 × KSV
ηq =
1 + 0.1 × KSV
(6)
Although all emission decays from the excited RuRe/Al2O3 in
suspended solutions could be reasonably fitted by using a
a
Table 1. Photophysical, Quenching, and Photocatalytic Behaviors of RuRe/Al2O3
b
c
d
e
f
g
h
AA, μmol g−1
τ, ns
Φem
kr, 105 s−1
knr, 105 s−1
K
SV, M−1
ηq
kq, 106 M−1 s−1
TOFCO, h−1
TONCO
Γ
CO, %
r, nm
5
762
711
695
687
685
550
0.12
0.11
0.10
0.10
0.10
0.08
1.5
1.5
1.5
1.5
1.5
1.5
11.6
12.6
12.9
13.0
13.1
16.7
2.3
2.1
1.9
1.7
1.3
2.0
0.19
0.17
0.16
0.15
0.12
0.17
3.0
2.9
2.8
2.5
1.9
3.6
14
12
11
6
4
48
56
78
115
134
118
68
85
93
98
98
91
6.3
2.6
2.0
1.6
1.3
1.4
30
50
80
110
i
j
5 and 95
707
a
b
c
d
A DMA/TEOA mixed solution (4:1, v/v) under CO2. λex = 510 nm. Rate constants for radiative deactivation calculated as kr = Φem/τem. Rate
e
f
constants for nonradiative deactivation calculated as knr = kr/Φem − kr. Quenching fraction in the presence of BNAH (0.1 M). TOFCO calculated
g
as [produced CO during initial 1 h (mol)]/[used RuRe (mol) × irradiation time (h)]. Selectivity of CO formation calculated as (produced CO)/
h
(total of CO, HCOOH, and H2) at 120 h. Average distance between metal complexes calculated as [(specific surface area (m2 g−1))/(adsorption
i
j
density (mol g−1) × NA × 3.14)]1/2 × 2. (RuRe + Ru)/Al2O3. Based on RuRe used. The data after irradiation for 60 h.
E
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excitation of the Ru photosensitizer unit because of the rapid
dimerization of BNA• (eq 5). However, since CO formation
from CO2 requires two-electron reduction, the reductive
quenching of the excited Ru photosensitizer unit by BNAH
should proceed twice. Therefore, these higher AA values may
have multiple effects on lowering TOFCO
.
For long irradiation times, the durability of RuRe/Al2O3
increases upon increasing AA (Figure 5). For example, at AA =
5 μmol g−1, photocatalytic CO formation was terminated after
15 h irradiation with TONCO ∼ 60. Conversely, the sample
with AA = 110 μmol g−1 continuously produced CO for over
120 h, and TONCO was 118 after 120 h irradiation. It is
noteworthy that the initial speed of the CO formation over
RuRe/Al2O3 with AA = 110 μmol g−1 was much slower than
that with AA = 5 μmol g−1. Therefore, the formation amounts
were reversed after ∼30 h irradiation. The rapid termination of
the photocatalytic CO formation was observed when AA ≤ 30
μmol g−1, while the photocatalysis continued for longer when
AA ≥ 50 μmol g−1, especially at 80−110 μmol g−1. Table 1
summarizes TONCO after 120 h irradiation using RuRe/Al2O3
with various AA values.
Figure 8. Relationship between Φem and the average inter-RuRe
distance for RuRe/Al2O3 after 30 h of photocatalytic reaction in
DMA/TEOA (4:1, v/v) at 480 nm excitation under a CO2
atmosphere.
Al2O3 samples with various r values. Φem drastically decreased
with the r value when r < 2.6 nm, and the Φem values for r =
2.6 and 6.3 nm were similar and lower than the others. It is
noteworthy that the emission efficiencies of the sample with
the larger r value before the photocatalysis tended to be higher,
and all of the values were higher than those after the
photocatalytic reaction (Table 1). It has been reported that
photochemical ligand substitution of the Ru photosensitizer
unit occurs when one-electron-reduced species (OERS) of the
Ru photosensitizer are excited in solution.46 The products
were [Ru(diimine)2(solvent)2]2+-type complexes, which do not
emit at room temperature in solution.
The suspensions after photocatalytic reactions for 0, 30, and
60 h were filtered, and the diffuse reflectance UV−vis spectra
of the obtained solid samples were measured (Figure 9). In the
case of AA = 5 μmol g−1, the broad absorption at the 460 nm,
which is attributed to the 1MLCT band of the Ru
photosensitizer unit, decreases, and a new absorption at longer
wavelength appears. This also strongly indicates that ligand
substitution of the Ru photosensitizer unit occurs during the
photocatalytic reaction, giving the [Ru(diimine)2(solvent)2]2+-
type complex(es). In the case of AA = 110 μmol g−1, such
spectral changes were slower, as shown in Figure 9b.
Figure 7. Average metal-to-metal distances for RuRe. (A) Average
distance between RuRe at 5 μmol g−1. (B) Average distance between
RuRe at 110 μmol g−1.
The results presented above and those in previous reports
dealing with homogeneous systems18,46 strongly suggest that
the decomposition, i.e., the ligand-substitution reaction of the
Ru photosensitizer unit of RuRe, occurred during the
photocatalytic reaction, especially for the RuRe/Al2O3 samples
with r ≥ 2.6 nm. This reaction induced loss of the
photocatalytic activity of RuRe/Al2O3. In other words, higher
r values led to more rapid decomposition of the Ru
photosensitizer unit of RuRe on Al2O3, and this is most likely
one of the main reasons that the durability of the photocatalyst
is lower for RuRe/Al2O3 samples with higher r values,
especially those with r ≥ 2.6 nm. Therefore, the durability
and photocatalytic activity of RuRe (except for the initial
reductive quenching process of the excited RuRe by BNAH)
could be increased by closely arranging the RuRe molecules on
the Al2O3 surface. Since RuRe cannot move on the Al2O3
surface, it does not readily contact with other neighboring
RuRe moieties at low adsorption density with r > 2.6 nm,
which is similar to the molecule length along the bond between
the Ru and Re units of RuRe (Figure 2). One possible
mechanism for the increase in the photocatalyst durability
upon introducing shorter distances between the RuRe
molecules on Al2O3 is the donation of one electron from an
OERS of RuRe to another OERS (disproportionation) and/or
The average distance between the centers of neighboring
RuRe molecules on Al2O3 (r) is calculated using eq 7 (Table
1).
specific surface area (m2g−1
)
r =
× 2
AA (mol g−1) × N × 3.14
(7)
A
For example, r is 1.3 and 6.3 nm when AA is 110 and 5 μmol
g−1, respectively (Figure 7). Thus, rapid termination of
photocatalytic CO formation was observed when r ≥ 2.6
nm, while the photocatalysis continued for an extended
duration when r ≤ 2.0 nm, especially 1.3−1.6 nm. The
molecular dimensions of RuRe (Figure 2) are 1.2 nm
(diameter of the Ru photosensitizer unit) and 2.4 nm (length
along the bond between the Ru and Re units). These results
indicate that the durability of the photocatalyst decreased
when the average distance between the RuRe molecules on
Al2O3 was longer than the length of RuRe (Figure 7).
The dispersion after the photocatalytic reaction for 30 h was
filtered, and the obtained solid was dispersed again into a
DMA/TEOA (4:1, v/v) solution in the absence of BNAH in
order to measure the emission quantum yields from the Ru
photosensitizer unit (Φem). Figure 8 shows Φem for the RuRe/
F
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Figure 9. Diffuse reflectance UV−vis spectra of RuRe/Al2O3 after photocatalytic reaction: (a) 5 μmol g−1; (b) 110 μmol g−1. The spectra are
normalized at 400 nm.
the intermediate derived from the OERS of RuRe, since CO
formation from CO2 requires two-electron reduction. To
obtain two electrons via photochemical reduction of the
complex(es) by BNAH, RuRe needs to be excited twice and
quenched by BNAH each time. However, the side reaction
and/or decomposition of the one-electron-reduced complexes
may proceed before the second photon absorption during the
photocatalytic reactions, even when using a high-power light
source such as the 500 W high-pressure mercury lamp
employed in the present study. If the neighboring OERS of
RuRe can donate an electron to another OERS (or the
intermediate), their reaction speed should increase in order to
decrease the residence time of the OERS species in the
reaction solution.
Figure 10. Time profile of TONCO during photocatalytic reaction
over (RuRe + Ru)/Al2O3 in DMA/TEOA (4:1, v/v) containing 0.1
M BNAH under a CO2 atmosphere. Irradiation at λex > 480 nm using
a Hg lamp. In each sample, the adsorption density of RuRe was
In order to clarify whether a second electron reduction
donation process from an adjacent OERS species of RuRe
accelerates the photocatalytic CO2 reduction, a model
adjusted to AA = 5 μmol g−1. The adsorption density of Ru was
maintained at AA = 95 μmol g−1 (red), 85 μmol g−1 (purple), 50
μmol g−1 (green), 20 μmol g−1 (yellow), and 0 μmol g−1 (blue). The
amount of RuRe was fixed at 10 nmol, which is much smaller
compared to the case shown in Figure 4a (100 nmol). The black plots
with dotted line show CO formation using (Ru+Re)/Al2O3 (AA = 95
and 5 μmol g−1). The amounts of Re in all the samples were 10 nmol.
mononuclear analogue of the Ru photosensitizer unit (Ru)
was coadsorbed on RuRe/Al2O3. RuRe/Al2O3 (AA = 5 μmol
g−1) synthesized as described above was dispersed into MeCN
solutions containing various concentrations of Ru, and the
suspensions were stirred for 2 days. The codoped samples,
labeled (RuRe + Ru)/Al2O3, were collected by filtration and
washed in MeCN several times. The photocatalytic abilities of
the dispersions of (RuRe + Ru)/Al2O3 (2 mg which contains
10 nmol of RuRe) in DMA/TEOA (4:1, v/v, 4 mL) solutions
containing BNAH (0.1 M) were investigated (λex > 480 nm
under a CO2 atmosphere), and Figure 10 shows the results.
The photophysical and quenching behaviors of (RuRe + Ru)/
Al2O3 (AA = 5 μmol g−1 and 95 μmol g−1, respectively) are
also summarized in Table 1. Since the formation amounts of
CO in the initial stage of the photocatalytic reaction using 2
mg of RuRe/Al2O3, which contained 10 mg of RuRe in the
absence of Ru, were lower than the detection limit, in Figure
10, the data in the case using 100 nmol of RuRe, of which the
solution contained 10 times more RuRe/Al2O3 (5 μmol g−1)
particles (Figure 4a), were divided by 10 and are shown as 0
μmol g−1 (blue plots) in Figure 10. The coadsorption of Ru
clearly induced higher photocatalytic activity compared to that
using RuRe/Al2O3 without Ru (CO formation: 0.60 μmol for
60 h irradiation). Upon increasing the amount of Ru on
RuRe/Al2O3 from AA = 20 to 95 μmol g−1, an increase in CO
formation from 1.7 to 7.1 μmol at 60 h of light irradiation was
observed. It should be noted that the system using Ru/Al2O3
(AA = 100 μmol g−1) in the absence of RuRe produced only a
small amount of CO (0.13 μmol, Figure S2), and the durability
drastically increased when the total adsorption amounts of
RuRe and Ru were AA ≥ 55 μmol g−1, where the average
distance between the complexes is r ≤ 2 nm. These results
clearly indicate that the OERS species of Ru, which are
produced upon photochemical reduction by BNAH, can
donate one electron not only to RuRe but also to the OERS
of RuRe and/or the intermediate derived from the OERS of
RuRe. Consequently, consumption of the OERS of RuRe
should be accelerated, and the durability of RuRe should be
increased. Therefore, a similar process should also work in the
case of photocatalytic reactions using RuRe/Al2O3; i.e., the
OERS of RuRe donates one electron to another OERS
(disproportionation) and/or the intermediate derived from the
OERS of RuRe to allow two-electron reduction of CO2 to CO.
Interestingly, the photocatalytic ability of the sample with both
the model mononuclear Ru and Re complexes with high
adsorption density, i.e., (Ru + Re)/Al2O3 (both AAs = 50
μmol g−1), was much lower than that of RuRe /Al2O3 (AA =
50 μmol g−1) (Figures 4b and 4d), and, even at a low
adsorption density such as AA = 5 μmol g−1 (r = 6.3 nm),
RuRe works as a photocatalyst for CO2 reduction. These
results strongly suggest that CO formation via double
excitation of the same Ru photosensitizer unit in one molecule
of RuRe also proceeds as described previously. The
bimolecular mechanism described in this section can accelerate
G
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photocatalytic CO2 reduction using RuRe. The black plots
with a dotted line in Figure 10 show CO formation in the
system using (Ru + Re)/Al2O3 (AA = 95 and 5 μmol g−1)
(TONCO was 64 after irradiation for 60 h), which was much
lower compared to that in the case using (RuRe + Ru)/Al2O3
with the same amount of Ru adsorbed sample (red plots in
Figure 10, Table 1). This result clearly indicates that
connection between the photosensitizer and the catalyst
using the bridging ligand is important for improving photo-
catalysis even on the solid surface probably due to the rapid
electron transfer from the OERS of the photosensitizer unit to
the catalyst unit via though-bond electron transfer (within
several nanoseconds).22
photosensitizer unit, lowering the quenching fraction (ηq) and
suppressing the production yield of the OERS of RuRe.
However, when r is equal to or shorter than the distance over
which neighboring RuRe molecules can contact with each
other, another mechanism of the photocatalytic reduction of
CO2 comes into play. This mechanism proceeds via the
disproportionation of two OERSs of RuRe and/or electron
donation from the OERS of RuRe to the intermediate.
Based on these investigations, we could successfully
construct an efficient and durable photocatalytic system for
CO2 reduction using a minimal amount of Re metal, i.e.,
coadsorption of a small amount of RuRe with an excess
amount of the photosensitizer mononuclear complex (Ru) on
Al2O3.
It is noteworthy that the amount of CO formation in the
case using (RuRe + Ru)/Al2O3, which contained only 10 nmol
of RuRe, could produce over 7 μmol of CO for 60 h irradiation
with the assistance of the coadsorbed Ru (190 nmol). It is a
larger amount than that produced by using a 10 times higher
amount of RuRe in the case of RuRe/Al2O3 with AA = 5 μmol
g−1 (5.6 μmol of CO formation). It should be also noted that
the TOFCO of (RuRe + Ru)/Al2O3 (AA = 5 and 95 μmol g−1)
(TOFCO = 48) was much higher than those in the cases using
RuRe/Al2O3 (Table 1). It is also noteworthy that the kq value
of (RuRe + Ru)/Al2O3 (AA = 5 and 95 μmol g−1) was larger
even compared to that of RuRe/Al2O3 (AA = 5 μmol g−1)
even though the redox potential of the excited Ru should be
very similar to that of the excited Ru unit of RuRe (Table 1).
This clearly indicates that the steric hindrance made by the Re
unit on the surface of Al2O3 makes reductive quenching of the
excited Ru unit less efficient in RuRe; in other words, the
coadsorption of Ru can make both durability and efficiency of
RuRe as the photocatalyst much better. Coadsorption of redox
photosensitizers can reinforce the photocatalysis of the
supramolecular photocatalyst.
These investigations provide new architectures for the
construction of efficient and durable hybrid photocatalytic
systems composed of metal-complex photocatalysts and solid
materials for CO2 reduction; e.g., the chemical bonding
between a redox photosensitizer and a catalyst, i.e., the
supramolecular photocatalyst structure, has big advantages
compared to the mixed system of the corresponding
mononuclear complexes on a solid surface, and second
electron donation to the reduced catalyst unit from outside
can drastically increase the durability of the supramolecular
photocatalyst.
EXPERIMENTAL SECTION
General Procedures. H NMR spectra were measured using a
■
1
JEOL ECA-400II. For the measurement of 31P NMR, a JEOL LA400
was used. A Shimadzu LCMS-2010A was used for electrospray
ionization mass spectrometry (ESI-MS). A JASCO V-670 was used
for UV−vis absorption spectra and diffuse reflection UV−vis spectra.
A quartz cell (optical path length of 1 cm) and a reflection
measurement unit (JASCO, sample part Φ = 5 mm) equipped with an
integrating sphere were used, respectively. Emission quantum yields
were determined using a Hamamatsu Photonics absolute PL quantum
yield measurement system C9920-01 (excitation light source: 150 W
Xe lamp; detector: PMA-12 multichannel CCD). Emission lifetimes
were obtained using a Horiba FluoroHub time-correlated singl-
photon-counting system. The excitation light source was an LED
pulse lamp (NanoLED, 510 nm), and a TBX-04. BELLSORP-miniII
was used for nitrogen adsorption/desorption measurement (measure-
ment temperature: 77 K). Brunauer−Emmett−Teller (BET) specific
surface areas were calculated from values in the region where the
relative pressure was 0.08−0.2 in the adsorption isotherm after
heating at 333 K for 2 h.
CONCLUSIONS
■
A dinuclear complex consisting of Ru(II) photosensitizer and
Re(I) catalyst units, RuRe, fixed on an Al2O3 surface can
photocatalyze CO2 reduction with high selectivity for CO
formation. The photocatalytic ability of this system is much
higher than that using a mixture of the corresponding
mononuclear complexes fixed on the same Al2O3 surface, i.e.,
(Re + Ru)/Al2O3. This clearly demonstrates the superiority of
the supramolecular photocatalyst, in which the photosensitizer
and catalyst complexes are connected with an ethylene chain,
compared to the corresponding mixed system consisting of
mononuclear photosensitizer and catalyst complexes fixed on a
solid surface. The rapid intramolecular electron transfer from
the photochemically reduced photosensitizer unit to the
catalyst unit plays an important role in the superiority of the
supramolecular photocatalyst, especially on the surface of the
solid material. The photocatalytic CO formation proceeds via
step-by-step double excitations of the same Ru photosensitizer
unit in one molecule of RuRe followed by the reductive
quenching of the excited state by BNAH each time, at least in
the case of a RuRe molecule isolated on Al2O3.
All chemicals and solvents were commercial reagent quality and
used without purification unless otherwise stated. DMA was dried
over 4 Å molecular sieves and distilled under reduced pressure. TEOA
was distilled under reduced pressure. BNAH was synthesized
according to the literature procedure.47
Synthesis of RuRe, Ru, and Re Adsorbed on Al2O3. The
RuRe, Ru, and Re were synthesized according to the previously
reported method.41,44,45 The desired amount of Al2O3 was dispersed
in acetonitrile (20 mL), and another acetonitrile solution containing
the metal complex (50 μM) was added into it. This suspension was
stirred for 1 week under an Ar atmosphere. The suspension was
filtered, and the obtained solid was washed with acetonitrile several
times.
Photocatalytic Reactions. Photocatalytic reactions were per-
formed using 11 mL test tubes. DMA/TEOA (4:1, v/v) solutions (4
mL) containing BNAH (0.1 M) were bubbled with CO2 for 20−30
min in a test tube and then dispersed using a magnetic stirrer. A high-
pressure Hg lamp (300 W, Eikosya Co.) equipped with a uranyl glass
and a K2CrO4 solution filter (λ < 480 nm) were used as a light source.
During irradiation, the reaction solution was stirred and kept at
approximately 25 °C using a constant-temperature water tank. The
The speed and durability of the photocatalysis were strongly
and oppositely dependent on the distance (r) between the
neighboring RuRe molecules; i.e., shorter r induced slower CO
formation in the initial stage of the photocatalytic reaction but
higher durability at longer irradiation time. Steric hindrance
between neighboring RuRe molecules owing to the short r
suppresses the access of the reductant (BNAH) to the excited
H
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produced CO and H2 were analyzed by a GC-TCD (GL science
GC323). Formic acid was analyzed using a capillary electrophoresis
system (Agilent 7100) with a buffer solution (pH = 6.0) consisting of
quinolinic acid, hexadecyltrimethylammonium hydroxide, and 2-
amino-2-hydroxymethyl-1,3-propanediol as the electrolyte.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c09170.
■
sı
Adsorption/desorption isotherm for Al2O3; time courses
of CO, HCOOH, and H2 using Re/Al2O3 and Ru/
Al2O3; emission decay curves for RuRe/Al2O3 (PDF)
AUTHOR INFORMATION
Corresponding Author
Osamu Ishitani − Department of Chemistry, Tokyo Institute of
Technology, Meguro-ku, Tokyo 152-8550, Japan; orcid.org/
0000-0001-9557-7854; Email: ishitani@chem.titech.ac.jp
■
Authors
Daiki Saito − Department of Chemistry, Tokyo Institute of
Technology, Meguro-ku, Tokyo 152-8550, Japan
Yasuomi Yamazaki − Department of Chemistry, Tokyo Institute
of Technology, Meguro-ku, Tokyo 152-8550, Japan;
orcid.org/0000-0002-8640-7367
Yusuke Tamaki − Department of Chemistry, Tokyo Institute of
Technology, Meguro-ku, Tokyo 152-8550, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09170
(18) Umemoto, A.; Yamazaki, Y.; Saito, D.; Tamaki, Y.; Ishitani, O.
Synthesis of a Novel Re(I)-Ru(II)-Re(I) Trinuclear Complex as an
Effective Photocatalyst for CO2 Reduction. Bull. Chem. Soc. Jpn. 2020,
93, 127−137.
(19) Cheung, P. L.; Kapper, S. C.; Zeng, T.; Thompson, M. E.;
Kubiak, C. P. Improving Photocatalysis for the Reduction of CO2
through Non-covalent Supramolecular Assembly. J. Am. Chem. Soc.
2019, 141, 14961−14965.
(20) Tamaki, Y.; Ishitani, O. Supramolecular Photocatalysts for the
Reduction of CO2. ACS Catal. 2017, 7, 3394−3409.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Number
JP20H00396 and JP17H06440 in Scientific Research on
Innovative Areas “Innovations for Light-Energy Conversion
(I4LEC)”.
(21) Cancelliere, A. M.; Puntoriero, F.; Serroni, S.; Campagna, S.;
Tamaki, Y.; Saito, D.; Ishitani, O. Efficient trinuclear Ru(ii)-Re(i)
supramolecular photocatalysts for CO2 reduction based on a new tris-
chelating bridging ligand built around a central aromatic ring. Chem.
Sci. 2020, 11, 1556−1563.
(22) Yamazaki, Y.; Ohkubo, K.; Saito, D.; Yatsu, T.; Tamaki, Y.;
Tanaka, S.; Koike, K.; Onda, K.; Ishitani, O. Kinetics and Mechanism
of Intramolecular Electron Transfer in Ru(II)-Re(I) Supramolecular
CO2-Reduction Photocatalysts: Effects of Bridging Ligands. Inorg.
Chem. 2019, 58, 11480−11492.
(23) Koike, K.; Grills, D. C.; Tamaki, Y.; Fujita, E.; Okubo, K.;
Yamazaki, Y.; Saigo, M.; Mukuta, T.; Onda, K.; Ishitani, O.
Investigation of excited state, reductive quenching, and intramolecular
electron transfer of Ru(II)-Re(I) supramolecular photocatalysts for
CO2 reduction using time-resolved IR measurements. Chem. Sci.
2018, 9, 2961−2974.
(24) Sato, S.; Morikawa, T.; Saeki, S.; Kajino, T.; Motohiro, T.
Visible-Light-Induced Selective CO2 Reduction Utilizing a Ruthe-
nium Complex Electrocatalyst Linked to a p-Type Nitrogen-Doped
Ta2O5 Semiconductor. Angew. Chem., Int. Ed. 2010, 49, 5101−5105.
(25) Suzuki, T. M.; Yoshino, S.; Takayama, T.; Iwase, A.; Kudo, A.;
Morikawa, T. Z-Schematic and visible-light-driven CO2 reduction
using H2O as an electron donor by a particulate mixture of a Ru-
complex/(CuGa)1‑xZn2xS2 hybrid catalyst, BiVO4 and an electron
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