Highly Efficient and Selective Photocatalytic CO2 Reduction to CO in Water by a Cobalt Porphyrin Molecular Catalyst

Article abstract of DOI:10.1021/acscatal.8b04975

The performance of a water-soluble cobalt porphyrin ([{meso-tetra(4-sulfonatophenyl)porphyrinato}cobalt(III)], CoTPPS) as a catalyst for the photoreduction of CO₂ in fully aqueous media has been investigated under visible light irradiation using [Ru(bpy)₃]²⁺ as a photosensitizer and ascorbate as a sacrificial electron donor. CO is selectively produced (>82%) with high efficiency (926 TON_CO; TON_CO = turnover number for CO). Upon optimization, selectivities of at least 91% are achieved. Efficiencies up to 4000 TON_CO and 2400 h^-1 TOF_CO (TOF_CO = turnover frequency for CO) are reached at low catalyst loadings, albeit with loss in selectivity. This work successfully demonstrates the ability of CoTPPS to perform highly efficient photoreduction of CO₂ in water while retaining its high selectivity for CO formation.

Full text of DOI:10.1021/acscatal.8b04975

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Article
2
Highly Efficient and Selective Photocatalytic CO Reduction
to CO in Water by a Cobalt Porphyrin Molecular Catalyst
Arnau Call, Mihaela Cibian, Keiya Yamamoto, Takashi Nakazono, Kosei Yamauchi, and Ken Sakai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04975 • Publication Date (Web): 17 Apr 2019
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§
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Arnau Call,*† Mihaela Cibian,*†‡ Keiya Yamamoto,†‡ Takashi Nakazono,†‡ Kosei Yamauchi,†‡
#
Ken Sakai*†‡
‡Department of Chemistry, Faculty of Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan.
†International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Ni-
shi-ku, Fukuoka 819-0395, Japan.
#Center of Molecular Systems (CMS), Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan.
Supporting Information Placeholder
ABSTRACT: The performance of a water-soluble cobalt porphyrin (CoTPPS) as a catalyst for the photoreduction of CO2 in fully
aqueous media has been investigated under visible light irradiation using [Ru(bpy)3]2+ as a photosensitizer and ascorbate as a
sacrificial electron donor. CO is selectively produced (> 82%) with high efficiency (926 TONCO). Upon optimization, selectivities
of at least 91% are achieved. Efficiencies up to 4000 TONCO and 2400 h-1 TOFCO are reached at low catalyst loadings, albeit with
loss in selectivity. This work successfully demonstrates the ability of CoTPPS to perform highly efficient photoreduction of CO2
in water while retaining its high selectivity for CO formation.
KEYWORDS: CO2 reduction, cobalt porphyrin, photocatalysis, selectivity, molecular catalysis
sion when photo-driven by [Ru(bpy)3]2+ in the presence of
ascorbate (AscHNa) as
a
sacrificial electron donor
(SED).9a,9b,10 For [Ni(cyclam)]2+ derivatives, it was shown that
the TONCO value can be maximized to 38 but with loss in
selectivity (SelCO2 < 10%). On the contrary, SelCO2 can be
maximized up to 94% but by sacrificing the efficiency
(TONCO = 2.2).10c,11 The overall catalytic performances of
[Ni(cyclam)]2+ derivatives were significantly improved by
using ZnSe quantum dots as visible light photosensitizer (PS)
even though the selectivity still remained relatively low
(TONCO = 283; SelCO2 = 34%).9e Similarly, the photocatalysis
of CdS quantum dots anchored with nickel terpyridine cata-
lysts under visible light resulted in high selectivity but low
TONCO values (TONCO = 20; SelCO2 = 90%).9c Furthermore,
Robert and co-workers reported that the trimethylamine-
functionalized iron tetraphenylporphyrin (FeTMA) can
achieve high selectivity for CO formation (SelCO2 = 95%)
when used in a photocatalytic system driven by an organic
dye, in a H2O:CH3CN (9:1 v/v) solvent mixture.9d For this
system the TONCO value was reported to be 120 after 4 days
of visible light irradiation. More recently, Weiss and co-
workers assembled the positively charged FeTMA catalyst
with negatively charged colloidal CuInS2/ZnS quantum dots
to reduce CO2 to CO with high selectivity (SelCO2 = 99%) in
water (TONCO = 450 after 30 h of irradiation).12 Importantly,
the highly selective systems show rather low activity, and the
overall efficiency is often raised only by sacrificing the selec-
tivity, although one excellent example of a cobalt catalyst
achieving both high efficiency and high selectivity
(TONCO=1380, SelCO2 =95%) was very recently reported while
this study was under the review process.13
Photocatalytic conversion of CO2 into chemical feed-
stocks is considered as a promising but challenging strategy
to achieve an energy sustainable society.1 One of the key
goals is to develop systems capable of selectively and effi-
ciently photocatalyzing CO2 reduction under visible light
irradiation.2 From a practical viewpoint, the CO2 reduction
must be coupled with water oxidation in order to gain the
electrons and protons required to drive the reduction pro-
cesses.3 Moreover, our intention has been to pursue a possi-
bility of driving all reactions in fully aqueous media in hope
to achieve the more practically useful photosynthetic systems.
Nevertheless, the presence of water often promotes the pho-
tocatalytic proton reduction, thermodynamically and kinet-
ically more favorable, resulting in decreased efficiency and
selectivity for CO2 reduction.4
Up to now, a large number of homogeneous5 and hetero-
geneous2 photocatalytic systems for CO2 reduction have been
developed. Most of them exhibit reasonably high catalytic
performances when the photochemical reactions are con-
ducted in either non-aqueous solvents6 or water-organic
solvent mixtures.7 However, photocatalytic systems showing
both high activity and high selectivity in fully aqueous media
are extremely rare and are predominantly noble metal-
based.8 Selected examples of first-row transition metal cata-
lysts for photocatalytic CO2-to-CO conversion in water, of
major interest in this study, are presented in Figure 1 (see
also Table S1), highlighting the best performances in terms of
selectivity and/or efficiency.9 Co and Ni tetra-azamacrocycles
were reported to promote photocatalytic CO2-to-CO conver-
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82% (Table S2, entry 1), which is considerably high among
those previously reported for the photocatalytic CO2-to-CO
HIGH
SELECTIVITY
1
conversion in fully aqueous media (Figure 1 and Table S1).
2
3
4
5
6
7
8
9
PS: [Ru(bpy)3]2+
TONCO = 2
SelCO2 = 87%
Control experiments were performed and are summa-
rized in Table S2 (see also Figures S1 and S2). In the absence
of CoTPPS in CO2-saturated solution (Table S2, entry 2), only
a trace amount of CO forms (4.2 µmol), but a remarkably
large amount of H2 (36.8 µmol) evolves. Importantly, only a
trace amount of formate is generated in both the presence
and absence of CoTPPS (6.6 µmol and 5.0 µmol, respectively).
In the absence of CO2 (under N2-saturated condition), small
amounts of CO and H2 form only when CoTPPS is present
(Figure 2B and Table S2, entries 3 and 4). Neither CO nor H2
evolves in the absence of CoTPPS under N2. As mentioned
above, H2 production significantly increases in the absence of
CoTPPS under CO2 atmosphere, suggesting that the degrada-
tion of [Ru(bpy)3]2+ is greatly enhanced by the presence of
CO2 to yield the chemical species catalytically active for H2
evolution (Figure 2B). This behavior is consistent with the
reports by Grant et al. and others, in which the [Ru(bpy)2]n+
carbonyl-type photodegradation products, such as
[Ru(bpy)2(CO)H]+ and [Ru(bpy)2(CO)2]2+, were proposed to
be the active catalysts for H2 and formate production.8a,10b,16
Furthermore, no reduced products form in the absence of
either the photosensitizer or a suitable sacrificial electron
donor (SED) (Table S2, entries 5 and 6). It was also con-
firmed that the pH remained unchanged after the photolysis
experiments and that the substantial diminishment is seen in
SelCO2 (45%) at pH 5.3 (CO2-purged phosphate buffer). All
these results indicate that the cobalt porphyrin is necessary
to catalyze the CO2 reduction into CO. An isotopic labelling
experiment conducted under a 13CO2 atmosphere led to the
formation of 13CO as a major product, thus confirming that
CO is originated from CO2 reduction (Figure S1).
PS: Purpurin
TONCO = 120
SelCO2 = 95%
CoTPPS
PS: CdS
PS: [Ru(bpy)3]2+
TONCO = 926
SelCO2 = 82%
TONCO = 20
SelCO2 = 90%
PS: ZnSe
TONCO = 283
SelCO2 = 34%
HIGH
EFFICIENCY
PS: RuP/ZrO2
TONCO = 5
SelCO2 = 19%
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Efficiency
Selectivity
Ref [9a] Ref [9b] Ref [9c] Ref [9d] Ref [9e] This work
Figure 1. Selected photocatalytic systems for CO2-to-CO
reduction in aqueous media (PS: photosensitizer).
Herein we report on the performance of CoTPPS, [{meso-
tetra(4-sulfonatophenyl)porphyrinato}cobalt(III)], as a cata-
lyst for CO2-to-CO photoconversion when employed in the
[Ru(bpy)3]2+/sodium ascorbate (AscHNa) system. We have
found that CoTPPS is an outstanding catalyst not only due to
its unprecedentedly high activity (TONCO = 4000, TOFCO
=
2400 h-1, SelCO2 = 41 %) but also due to its ability to yield a
high TONCO value (TONCO = 926) while also retaining a high
selectivity for CO2-to-CO conversion (SelCO2 = 82%) under
optimal conditions (Figure 1). Moreover, selectivities of at
least 91% are achieved with high efficiency (TONCO = 400).
We note that the previous studies of cobalt porphyrins in
photocatalytic CO2 reduction were only carried out under
non-aqueous conditions.14,15
Photocatalytic CO2 reduction assays were performed us-
ing CO2-saturated aqueous bicarbonate buffer (pH 6.7) in the
presence of AscHNa (0.1 M) under visible light irradiation
(Figures 2A and 2B). Photoirradiation of the solution con-
taining CoTPPS (10 µM) and [Ru(bpy)3]2+ (500 µM) affords
CO as a major product (92.6 µmol, 2.26 mL, TONCO = 926,
TOFCO = 456 h-1) (Figure 2A). Dihydrogen (19.7 µmol of H2)
also evolves, together with a trace amount of formate (6.6
µmol). The selectivity for CO2 reduction vs. water reduction
(SelCO2 = mol (CO)/(mol (H2) + mol (CO)) is estimated to be
Catalytic assays employing CoCl2 and/or the metal-free
TPPS ligand in equimolar loadings yield a negligible amount
of CO, while the amount of H2 generated is higher than that
formed using CoTPPS (Table S2, entries 7-9). Mercury poi-
soning experiments and assays performed with cobalt nano-
particles (Co NP, size 28 nm) demonstrate the homogeneous
nature of the catalysis by CoTPPS (Table S2, entries 10 and 11,
Figures S3 and S4). The effect of varying either the AscHNa
concentration or the light intensity was also examined (Fig-
ures S6-S10).
C
A
B
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0
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0
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0
H2 without CoTPPS
H2 without CoTPPS
H2 with CoTPPS
H2 with CoTPPS
90
CO without CoTPPS
CO without CoTPPS
CO with CoTPPS
CO with CoTPPS
60
30
0
Bicarbonate
CO2 pH 6.7
Phosphate
CO2 pH 6.9
Phosphate
N2 pH 6.9
0
1
2
3
4
Bicarbonate
N2 pH 9.1
Time (h)
Figure 2. A) Photocatalytic activity of CoTPPS. Amount (circles) and rate (triangles) profiles for CO (red) and H2 (blue) produc-
tion. B and C) Amounts of products at 4 h irradiation in the presence and the absence of CoTPPS measured for N2- or CO2-
saturated solutions in a bicarbonate (B) (Table S2, entries 1-4) and phosphate (C) buffer.17 Photolysis carried out in a 10 mL CO2-
saturated solution containing CoTPPS (10 µM), [Ru(bpy)3]Cl2 (500 µM), 0.1 M buffer, and 0.1 M AscHNa, at 25 ºC, for 4 h using Xe
lamp irradiation (λ > 400 nm, 179 mW·cm-2). The experiments were performed at least in duplicate (error 1-15%).
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0
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0
200 400 600 800 1000
[Ru(bpy)32+] (mM)
[CoTPPS] (mM)
[CoTPPS] (mM)
Figure 3. Kinetic studies by varying A) [Ru(bpy)32+] using 10 µM CoTPPS and B-D) [CoTPPS] using 500 µM [Ru(bpy)3]2+. E
and F) Selectivities for the experiments shown in A and B, respectively. Photolysis carried out in a 10 mL CO2-saturated solution
containing CoTPPS, [Ru(bpy)3]Cl2, 0.1 M NaHCO3, and 0.1 M AscHNa, at 25 ºC, for 4 h using Xe lamp irradiation (λ > 400 nm, 179
mW·cm-2).
The quantum yield (ΦCO), estimated on the basis of the
amount of photons absorbed (see Experimental Section), was
determined as 0.81% using a 428-nm light, in agreement with
the multistep nature of the photocatalytic CO2 reduction.
The corresponding value for the photocatalysis using an iron
porphyrin catalyst and a tris(phenylpyridinato)iridium(III)
PPS], reaching its maximum at around 30 µM. However, the
rate gradually decreases at higher catalyst loadings. This
behavior is attributable to the increased light absorption by
CoIITPPS (ε412 nm = 1.6 x 105 M-1cm-1), interfering with the light
absorption at the MLCT band of [Ru(bpy)3]2+ (ε450 nm = 1.4 x
104 M-1cm-1) (Figure S18).20 The TONCO exponentially increas-
es with decreasing [CoTPPS]. Under the optimal conditions
which maximize the TONCO, values of TONCO = ca. 4000 and
TOFCO = 2400 h-1 are attained at [CoTPPS] = 0.5 µM, albeit
photosensitizer in acetonitrile was reported as ΦCO
=
0.0013%.18
Employing a phosphate buffer (pH =6.9), the superior se-
lectivity of CoTPPS for CO2 reduction vs. water reduction at
the same pH can be verified by observing the products in the
presence and absence of CO2 (Figure 2C and Figure S11). The
H2 evolution under N2 is largely enhanced by the presence of
CoTPPS, confirming that it is indeed an active H2 evolution
catalyst.19 Remarkably, though, in the presence of CO2, CO is
formed preferentially over H2 (SelCO2 = 82%).
with a lower apparent21 selectivity for CO formation (SelCO2
=
41%) (Figure 3D). To the best of our knowledge, these values
are among the highest values yet reported for the first-row
transition metal molecular catalysts for the photocatalytic
reduction of CO2 to CO under visible light in fully aqueous
media (Table S1). In sharp contrast, H2 generation is marked-
ly prevented by raising the catalyst concentration over the
entire range studied, strongly supporting the conclusion that
H2 evolution is predominantly catalyzed by the degradation
products of [Ru(bpy)3]2+ (e.g., cis-[Ru(bpy)2(CO)H]+), as
discussed above. Perhaps, the efficient catalytic cycle of CO2
reduction by CoTPPS plays a role as an inhibitor for either
the generation of the [Ru(bpy)2(CO)H]+ species or the cata-
lytic cycle of H2 generation itself. This inhibition effect be-
comes more obvious at higher [CoTPPS]/[Ru(bpy)32+] ratios,
resulting in the maximum SelCO2 value of 91% when the
[Ru(bpy)3]2+ concentration is minimized (50 µM) (Figures 3E
and 3F). Under these conditions, the photocatalytic activity
remains sufficiently high (TONCO = 400, TOFCO = 31 h-1).
Although the CO formation ceases after 4 h (Figure 2A), a
similar CO production profile and high selectivity (SelCO2
=
91%) can be restored by adding the initially added amount of
[Ru(bpy)3]Cl2 (50 µM) (Figure S12). In contrast, the re-
addition of CoTPPS and/or AscHNa did not restore the initial
activity. The degradation of [Ru(bpy)3]2+ is thus concluded to
be the major cause for the cease of photocatalysis, as also
demonstrated by photostability experiments (Figures S13 and
S14). Upon increasing the [Ru(bpy)3]2+ concentration, both
the rate and the total amount of CO formation saturate at
about 400 µM (Figures 3A, S15, and S16). Therefore, in the
lower [Ru(bpy)3]2+ concentration domain, the photosensiti-
zation limits the overall rate of photocatalysis.
Interestingly, the present photocatalytic system preserves
its activity even in the presence of a large amount of O2 in
the headspace (p(O2)= 0.3, [O2]= 12.3 mM, even though inhi-
bition in CO formation occurs due to the concomitant pro-
motion of O2 reduction (Figures S19-S21).22
The net performance is also dependent on the catalyst
concentration, where two different regimes are identified
(Figures 3, B-E, and S17). In agreement with the molecular
nature of the catalysis, at low catalyst concentrations, the CO
formation rate shows an enhancement with increasing [CoT-
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0
-5
20
15
10
5
On the basis of our experimental and theoretical results,
A
B
under Ar
under CO2
together with those described in the literature,23,24 a possible
catalytic cycle is proposed in Scheme 1. The starting diaqua-
cobalt(III) species [CoIIITPPS(H2O)2]+ (pKa1=7.02)25 is readily
reduced to the CoII species [CoIITPPS(H2O)]26 in the presence
of a large excess of AscH- (Scheme 1 and Figures S22 and
S23).27 The photocatalytic cycle is initiated via light absorp-
tion by [Ru(bpy)3]2+, reductive quenching of [Ru(bpy)3]2+* by
AscH- (kq = 1.9 x 107 M-1·s-1) (Figures S24 and S25), and elec-
tron transfer from [RuII(bpy)2(bpy•-)]+ (Ep = -1.49 V vs. SCE) to
[CoIITPPS(H2O)] (E1/2 = -0.92 V) to afford [CoITPPS]- (Figure
4A-C).
1
2
3
4
5
6
7
8
[Ru(bpy)3]Cl2
Ep = -1.49 V
-10
-15
-20
-25
-30
-35
-40
0
(CoIITPPS/[CoITPPS]-)
E1/2 = -0.92 V
-5
-10
-1.2
-1.0
E (V) vs. SCE
-0.8
-0.6
-1.6
-1.2
-0.8
-0.4
E (V) vs. SCE
C
0
-10
-20
-30
-40
-50
-60
9
10 µA
CV
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The electrochemistry results (Figure 4) reveal that this
species is further reduced to give a porphyrin-reduced inter-
mediate [CoI(TPPS•-)]2- at -1.26 V vs. SCE (Figure 4C; both CV
and SWV), which appears to be coupled with the rise of
catalytic current. We note that upon introduction of CO2
there is no meaningful change at the first reduction wave
(E1/2 = -0.92 V vs. SCE) (Figure 4A). Under the catalytic CO2
reduction conditions, the reduction potential of [Ru(bpy)3]2+
(-1.49 V vs. SCE, Figure 4B) is negative enough to reduce
[CoITPPS]- and generate the reactive [CoI(TPPS•-)]2- species.
Importantly, the above-mentioned second reduction peak
potential is consistent with that computed by DFT in this
work (-1.32 V vs. SCE; see Figure 5). Moreover, these values
are also in line with the porphyrin-based reduction poten-
tials reported for the free TPPS ligand as well as for some
M(TPPS) derivatives (M = Zn, Pd) in aqueous media (-1.16 V
to -1.23 V vs. SCE).28 The CO2-binding with [CoI(TPPS•-)]2- has
exothermic path (ΔG = -4.2 kcal·mol-1) with a lower activa-
tion barrier (ΔG╪ = 7.6 kcal·mol-1) to form CO2-bound spe-
cies in comparison with the CO2-binding path with
[CoITPPS]- (see Figure 5). However, we do not completely
rule out the latter path, since it can also result in the same
porphyrin-reduced species [CoIII(TPPS•-)(CO22-)]2- and is only
slightly uphill in energy (ΔG = 1.2 kcal·mol-1, ΔG╪ = 9.9 kcal·
mol-1) (Table S23). We thus assume that the CO2-binding to
[CoI(TPPS•-)]2- is the major path under these experimental
conditions. Although some reports describe the CO2-bound
species as [CoI(P•-)(CO2)]2- (P = porphyrin),24b based on our
SWV
CoIITPPS/[CoITPPS]-
E1/2 = -0.92 V
[CoITPPS]-/[CoI(TPPS•-)]2-
Ep = -1.26 V
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
E (V) vs. SCE
Figure 4. A) Cyclic voltammograms (CV) of CoTPPS (1 mM)
in a 0.1 M phosphate buffer under CO2 (red) and under Ar
(blue) at pH 6.7. B) Square-wave voltammogram (SWV) of
[Ru(bpy)3]Cl2 (1 mM) in a 0.1 M NaHCO3 buffer under CO2 at
pH 6.7. C) CV (red) SWV (green) of CoTPPS (1 mM) in a 0.1
M NaHCO3 buffer under CO2 at pH 6.7. Scan rate 0.1 V·s-1, T
= 21 ºC.
•
2-
DFT results we adopt the formulation of [CoIII(TPPS -)(CO2
)]2- (see SI for details), in accord with the results of XANES
measurements carried out for the CO2-bound cobalt catalysts
with tetra-azamacrocyclic ligands.29 The spin density plots as
well as the spin density values shown in Tables S4 and S7
provide strong support for the anion radical form of the
porphyrin (e.g., TPPS•-), indicating that the major radical
character is delocalized over the porphyrin ring system.
•
2-
Notably, under CO2, the formation of [CoIII(TPPS -)(CO2
)]2- is significantly preferred over the formation of hydride
species (e.g., [CoIII(TPPS•-)(H)]-) leading to H2 evolution, as
demonstrated by the high SelCO2 values (Figures 2 and 3).
Our DFT results also indicate that further reduction of the
porphyrin-reduced intermediate [CoIII(TPPS•-)(CO22-)]2- into
[CoIII(TPPS2-)(CO22-)]3- (Ecalc
=
-1.71
V
vs. SCE) by
Scheme 1. Proposed reaction scheme for the CO2-to-CO
photoconversion in the [Ru(bpy)3]2+/ascorbate/CoTPPS sys-
tem in water. The peripheral charges of the sulfonated
groups are neglected for simplicity and clarity.
[RuII(bpy)2(bpy•-)]+ is rather uphill by 5.1 kcal·mol-1, not likely
to compete well with the alternative protonation path shown
in Scheme 1 and Figure 5.
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Figure 5. Free energy diagram computed by DFT for the catalytic cycle by CoIIP(H2O) (P = non-substituted porphyrin), promot-
ed by the reductive equivalent of the [Ru(bpy)3]2+/[Ru(bpy)3]+ couple at pH = 6.7 (see Figure S27 for details)
The next step in the photocatalytic cycle is the protona-
tion of [CoIII(TPPS•-)(CO22-)]2- into [CoIII(TPPS•-)(CO2H-)]-,
which is downhill by -3.1 kcal·mol-1 (Figure 5). The subse-
quent C-O bond cleavage coupled with protonation and
water elimination is more highly downhill (by -17.2 kcal·mol-
1), affording [CoIITPPS(CO)], for which the low-spin CoII d7
configuration is confirmed by DFT (Tables S6 and S9). Final-
ly, simple dissociation of CO from [CoIITPPS(CO)] (G = -0.6
kcal·mol-1) may occur to give [CoIITPPS], but the aqua-
ligated species [CoIITPPS(H2O)] is computed to be the major
species in solution (Figure 5).
the sample solutions were maintained at 25 ºC during the
spectrophotometric measurements. ESI-TOF mass spectra
were recorded on a JEOL JMS-T100LC mass spectrometer in
positive or negative ion mode. 1H NMR spectra were acquired
on a JEOL JNM-ESA 600 MHz spectrometer. Luminescence
spectra were recorded on a Shimadzu RF5300PC spectro-
fluorophotometer. Cyclic voltammetric experiments were
recorded on a BAS ALS Model 602DKM electrochemical
analyzer using a three electrode system consisting of a glassy
carbon working electrode, a platinum wire counter electrode,
and a saturated calomel reference electrode (SCE) for the
measurements in aqueous solutions. All reported potentials
are given relative to SCE. The quantification of gases was
done using a Shimadzu GC-14A gas chromatograph equipped
with a molecular sieve 13X-S Å column of 2 m x 3 mm i.d., at
30 ºC. The injection of the sample gas (200 µL) was per-
formed manually using a gas-tight syringe and the output
signal from the thermal conductivity detector of the gas
chromatograph was analyzed using a Shimadzu C-R8A inte-
grator. The CO and H2 peaks were determined using a cali-
bration curve which had been previously obtained employing
standard CO and H2 gases. Photoirradiation experiments
were carried out with an ILC Technology CERMAX LX-300
Xe lamp (179 mW·cm-2, 220 A) equipped with a CM-1 cold
mirror which reflects lights in the range of 400 << 800 nm.
The photolysis vial (21 mL) was immersed in a 25 ºC water
bath to remove IR radiation and to eliminate the tempera-
ture effects.
In conclusion, while CoTPPS efficiently catalyzes the re-
duction of water to H2 under N2, it selectively and efficiently
catalyzes CO2-to-CO conversion under CO2 even in fully
aqueous media. Among the first-row transition metal mo-
lecular catalysts promoting CO2-to-CO photoconversion in
water, CoTPPS exhibits high activity while retaining its high
selectivity for CO2 reduction vs. water reduction. Further
studies focused on the detection and characterization of the
key intermediates are still needed. We also note the im-
portance of developing new photosensitizers robust enough
to avoid in situ generation of any active water reduction
catalysts. The extended studies are in progress in our group.
All solvents and reagents were
of the highest quality available and were used as received.
TPPS (meso-tetrakis(4-sulfophenyl)porphyrin) was pur-
chased from Tokyo Chemical Industry Co., Ltd. and it was
used as received. All other reagents were purchased from
Kanto Chemicals Co., Inc. and used without further purifica-
tion. Purification of water (0.055 µS) was performed with a
RFD250RB water distillation apparatus.
In a typical photocatalytic run, a 10 mL
aqueous solution containing CoTPPS (10 µM), [Ru(bpy)3]Cl2
(500 µM), sodium ascorbate (0.1 M), and NaHCO3 (0.1 M),
was purged with CO2 (purity ≥ 99.995 %) for 15 min prior to
irradiation. The temperature was held constant at 25 ºC
throughout the experiment. The gases generated from the
reaction during the photolysis were quantified by GC-TCD
measurements of 200 µL aliquots from the reaction vial
headspace. The experiments were performed at least in du-
UV-Visible absorption spectra were
recorded on a Shimadzu UV2450SIM spectrophotometer. All
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Page 6 of 9
plicate (error 1-15%). It was found during this study that the
purity of the compounds is very important for the reproduci-
bility of the photocatalytic experiments.
After adjusting the pH at 7.5 by adding 1 M NaOH, the solu-
tion was refluxed for 24 h and allowed to cool to room tem-
perature. The solution was then filtered for the removal of
insoluble materials. The filtrate was then reduced and passed
through a column containing a Dowex 50W-X8 cation-
exchange resin in the H+ form (50-100-mesh) in order to
remove residual cobalt ions present in the reaction mixture.
The CoTPPS was precipitated as a purple solid from a wa-
ter/acetone solvent mixture. The precipitate was collected by
filtration, washed with acetone and diethyl ether, and dried
in vacuo. Yield 65%. MS (ESI – MeOH, negative mode) (m/z):
328.98 [C44H24N4S4O12Co]3- (100%) (calcd: 328.99). Elemental
analysis of CoTPPS calcd: (%) for C44H27N4S4O12Co·12H2O
(1207.1): C 43.78, H 4.26, N 4.64; found: C 44.07, H 4.26, N
4.63.
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A 2.5 mL aqueous solution
containing CoTPPS (10 µM), [Ru(bpy)3]Cl2 (500 µM), sodium
ascorbate (0.1 M), and phosphate buffer (0.1 M), was purged
with Ar for 15 min, followed by 13CO2 bubbling (pHfinal= 6.7).
The 13CO generated during irradiation was detected by GC-
MS (HP 6890 Series GC SYSTEM, 5973 Mass Selective Detec-
tor). The 13CO2 gas was produced by adding water to a mix-
ture of ascorbic acid with NaH13CO3 (99 atom % 13C, Cam-
bridge Isotope Laboratories, Inc.).
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The amount of formate in the
liquid phase was determined by the following method. After
4 hours of irradiation, the reaction solvent was removed by
freeze-drying. The resulting residue was dissolved in 0.8 mL
D2O and 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium
salt (TSP-d4, 2 µmol) was added. After filtering the solution
Density functional theory (DFT) calcu-
lations were performed using the Gaussian 09 package of
programs31 to better understand the electronic structures of
some possible intermediates and the energy diagram for the
catalytic cycle of CO2 reduction by the cobalt porphyrin. In
our calculations, the simplest non-substituted cobalt porphy-
rin system (P) (e.g., without the four sulfonatophenyl moie-
ties in CoTPPS) has been adopted. Therefore, for the com-
puted model systems, descriptions such as [CoIIP] and [CoIP]-
will be used to describe the species of interest. The structures
were fully optimized at the B3LYP level of calculations with
the effect of solvation in water taken into consideration by
using the polarizable continuum model (PCM) method.32
Calculations were typically performed using the 6-31G* basis
set applied to all atoms. For some of the species, calculations
were also performed using the SDD basis set adopted for the
Co ion with the 6-31G* basis set applied to the remaining
HCNO atoms. Plots showing the spin density distribution
were generated using GaussView 5.0.9.33 Detailed description
is given in SI.
1
through Celite, the H NMR spectrum was measured and the
amount of formate was determined using the relative inte-
grated intensity of its signal with respect to that of TSP-d4
added as an internal standard.
Cata-
lytic reactions varying the amount of O2 in the reaction ves-
sel were performed in a 10 mL aqueous solution containing
CoTPPS (70 µM), [Ru(bpy)3]Cl2 (500 µM), sodium ascorbate
(0.1 M), NaHCO3 (0.1 M), purged with CO2 (purity ≥
99.995 %) for 15 min. After the CO2 purging, the correspond-
ing amount of O2 was introduced into the reaction vessel
with a syringe through the septum. Before irradiation, the
solution was vigorously shaken during 1 min and left under
stirring for 30 min in order to dissolve the O2 into the solu-
tion. The gases generated from the reaction during the pho-
tolysis were quantified by the analysis of an aliquot of gas
(200 µL) from the headspace of the reaction vial by GC-TCD
measurements. The experiments were performed at least in
duplicate (error 1-15%).
Determination of the Quantum Yield.18 Considering
the fact that the reduction of CO2 to CO is a two-electron
process, the overall quantum yield of the process (ΦCO) was
calculated using the following equation:
*E-mail: arnau_call@chem.kyushu-univ.jp
*E-mail: cibian_mihaela@chem.kyushu-univ.jp
*E-mail: ksakai@chem.kyushu-univ.jp
푛푢푚푏푒푟 표푓 퐶푂 푚표푙푒푐푢푙푒푠 푥 2
훷퐶푂
=
푥 100
푛푢푚푏푒푟 표푓 푝ℎ표푡표푛푠 푎푏푠표푟푏푒푑
§ AC and MC contributed equally to this work.
The number of CO molecules can be determined from
the moles of CO in the sample headspace (obtained by GC
measurements as described above) and Avogadro’s number
(6.022×1023). The number of photons absorbed (1.81×1018
photons·s-1) was estimated by measuring the incident light
power inside the reaction vessel (140 mW·cm-2) (an Optical
Power Meter TQ8210 with Optical Sensor Q82017A from
Advantest was used), taking the photon wavelength equal to
428 nm (a set of interference filters L39 + SV490 (380 < λ <
490 nm) was used) and considering an illuminated area of 6
cm2. Under our conditions the complete absorption of pho-
tons is satisfied due to the high absorbance of the solution at
the wavelength adopted. The obtained quantum yield is
0.81% at 1 hour of irradiation.
The authors declare no competing financial interests.
The Supporting Information is available free of charge on the
ACS Publications website.
Additional data (control and optimization experiments,
addition and photostability experiments, electrochemistry
data, quenching experiments, and DFT results) (PDF).
CoTPPS [{meso-tetra(4-
sulfonatophenyl)porphyrinato}cobalt(III)]·12H2O was prepa-
red as previously described.30. CoCl2·6H2O (0.143 g, 6 mmol),
and TPPS (0.090 g, 1 mmol) were dissolved in water (25 mL).
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ACS Catalysis
Enabling Durable, Selective CO2 Reduction under Visible Light in
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Superstructures of Quantum Dots and Iron Porphyrins for Reduction
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(17) The pH was measured after bubbling with either N2 or CO2. The
phosphate buffer at pH 6.9 under CO2 (1 atm) was prepared by
bubbling CO2 through an aqueous solution containing Na3PO4 (0.1
M) and AscHNa (0.1 M), which initially displayed pH 11.8 under air.
The bicarbonate buffer at pH 6.7 was similarly prepared by bubbling
CO2 through an aqueous solution containing NaHCO3 (0.1 M) and
AscHNa (0.1 M), which initially displayed pH 9.1. The product yields
for the CO2 reduction under the bicarbonate (pH = 6.7) (Figure 2B,
left) and phosphate (pH = 6.9) (Figure 2C, left) conditions were
similar in both the presence and absence of CoTPPS. Neither CO nor
H2 evolves under N2, in the absence of CoTPPS, under both
bicarbonate (pH = 6.7) and phosphate (pH = 6.9) conditions.
(18) Bonin, J.; Robert, M.; Routier, M. Selective and Efficient
Photocatalytic CO2 Reduction to CO Using Visible Light and an
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This work was supported by JSPS KAKENHI Grant Number
JP18H01996 (Grant-in-Aid for Scientific Research (B)) and by
JSPS KAKENHI Grant Number JP18H05171 in a Grant-in-Aid
for Scientific Research on Innovative Areas “Innovations for
Light-Energy Conversion (I4LEC)”. This work was also sup-
ported by the International Institute for Carbon Neutral
Energy Research (WPI-I2CNER), sponsored by the World
Premier International Research Center Initiative (WPI),
MEXT, Japan. MC thanks le Fonds du Québec pour la recher-
ché sur la nature et les technologies (FRQNT) for a PD scho-
larship. The authors are also grateful to Kyushu University
Department of Chemistry and I2CNER services and person-
nel.
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Carbon Dioxide in Water. 2. Mechanism from First Principles. J.
Phys. Chem. A 2010, 114, 10174-10184; (c) Shen, J.; Kolb, M. J.; Göttle,
A. J.; Koper, M. T. M. DFT Study on the Mechanism of the
Electrochemical Reduction of CO2 Catalyzed by Cobalt Porphyrins. J.
Phys. Chem. C 2016, 120, 15714-15721; (d) Göttle, A. J.; Koper, M. T. M.
Proton-Coupled Electron Transfer in the Electrocatalysis of CO2
Reduction: Prediction of Sequential vs. Concerted Pathways Using
DFT. Chem. Sci. 2017, 8, 458-465.
(31) M. J. Frisch, et al., Gaussian 09 (Revision C.01), Gaussian Inc.,
Wallingford, CT, 2009.
(32) (a) Barone, V.; Cossi, M.; Tomasi, J. Geometry Optimization of
Molecular Structures in Solution by the Polarizable Continuum
Model. J. Comput. Chem. 1998, 19, 404-417; (b) Cossi, M.; Scalmani,
G.; Rega, N.; Barone, V. New Developments in the Polarizable
Continuum Model for Quantum Mechanical and Classical
Calculations on Molecules in Solution. J. Chem. Phys. 2002, 117, 43-
54; (c) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical
Continuum Solvation Models. Chem. Rev. 2005, 105, 2999-3094.
(33) R. Dennington and T. Keith and J. Millam, GaussView (Version
5), Semichem Inc., Shawnee Mission, KS, 2009.
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(25) Ashley, K. R.; Leipoldt, J. G. Kinetic and Equilibrium Study of
the
Reaction
of
(meso-Tetrakis(p-
sulfonatophenyl)porphyrinato)diaquocobaltate(III) with Pyridine in
Aqueous Solution. Inorg. Chem. 1981, 20, 2326-2333.
(26) Based on our DFT results, three forms [CoII(P)], [CoII(P)(H2O)],
and [CoII(P)(H2O)2] coexist in aqueous media for this oxidation state
with the relative abundances 11, 72, and 17%, respectively (see Table
S23).
(27) The starting CoIIITPPS species (Soret = 426 nm) is readily
reduced to CoIITPPS (Soret = 412 nm) in the presence of the large
excess of AscH- (Eox (AscHNa) = +0.54 V vs. SCE); (CoTPPSIII/II – Ered
:
0.10 V (irr); -0.43 V (irr); Eox: 0.40 V (irr) vs. SCE (Li, X.; Zhang, L.;
Mu, J.; Qiu, J. Nanoscale Res. Lett. 2008, 3, 169-178).
(28) Kalyanasundaram, K.; Neumann-Spallart, M. Photophysical and
Redox Properties of Water-Soluble Porphyrins in Aqueous Media. J.
Phys. Chem. 1982, 86, 5163-5169.
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ACS Catalysis
Water-soluble cobalt porphyrins are highly efficient catalysts for the CO2-to-CO photoreduction in fully aqueous
media while retaining a high selectivity in CO formation vs. H2 formation.
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
> 400 nm
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Photocatalytic reduction of CO2 to CO in water
Homogeneous system
Highly efficient AND selective
CO
H2
[Ru(bpy)3]2+
e-
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phosphate
phosphate
CoTPPS
CO2 pH 6.9
N2 pH 6.9
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