Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction under photoirradiation

Article abstract of DOI:10.1016/j.jphotochem.2015.06.014

In this study, the photochemical reaction of a metal-complex-based catalyst was analyzed by electrochemical measurements. A well-known catalyst for the CO₂ reduction reaction, meso-tetraphenylporphyrin iron(III) chloride (Fe(tpp)Cl), was selected as the target analyte. Although the analysis of the electrochemical response of Fe(tpp)Cl under photoirradiation with conventional cyclic voltammetry (CV) was not allowed, the adaptation of thin layer cyclic voltammetry (TLCV) enabled us to detect the photochemical reaction of Fe(tpp)Cl. The influence of photoirradiation on the electrochemical property of Fe(tpp)Cl was investigated both under Ar and CO₂ atmospheres. Although the thin layer cyclic voltammograms of Fe(tpp)Cl upon photoirradiation under an Ar atmosphere were almost the same as those measured in the dark, the measurements under a CO₂ atmosphere clearly indicated the change of the electrochemical response upon photoirradiation. The detailed analysis of this phenomenon revealed that the photoinduced decarbonylation reaction regenerates the original [Fe^II(tpp)] complex under photoirradiation.

Full text of DOI:10.1016/j.jphotochem.2015.06.014

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Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction
under photoirradiation
Arisa Fukatsua,b, Mio Kondoa,b,c,d, Yuki Okabea,b, Shigeyuki Masaokaa,b,c,
*
a
Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science, 5-1, Higashiyama, Myodaiji, Okazaki, Aichi 444-8787,
Japan
b
SOKENDAI (The Graduate University for Advanced Studies), Shonan Village, Hayama, Kanagawa 240-0193, Japan
Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, 38, Nishigo-naka, Myodaiji, Okazaki, Aichi 444-8585, Japan
ACT-C, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
c
d
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study, the photochemical reaction of
a metal-complex-based catalyst was analyzed by
Received 20 March 2015
Received in revised form 12 June 2015
Accepted 16 June 2015
Available online xxx
electrochemical measurements.
A well-known catalyst for the CO2 reduction reaction, meso-
tetraphenylporphyrin iron(III) chloride (Fe(tpp)Cl), was selected as the target analyte. Although the
analysis of the electrochemical response of Fe(tpp)Cl under photoirradiation with conventional cyclic
voltammetry (CV) was not allowed, the adaptation of thin layer cyclic voltammetry (TLCV) enabled us to
detect the photochemical reaction of Fe(tpp)Cl. The influence of photoirradiation on the electrochemical
property of Fe(tpp)Cl was investigated both under Ar and CO2 atmospheres. Although the thin layer cyclic
voltammograms of Fe(tpp)Cl upon photoirradiation under an Ar atmosphere were almost the same as
those measured in the dark, the measurements under a CO2 atmosphere clearly indicated the change of
the electrochemical response upon photoirradiation. The detailed analysis of this phenomenon revealed
that the photoinduced decarbonylation reaction regenerates the original [FeII(tpp)] complex under
photoirradiation.
Keywords:
Electrochemistry
CO2 reduction
Iron porphyrin
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
energy conversion reactions. However, in general, the electrochemi-
cal and photochemical properties of molecular catalysts are
Artificial photosynthesis, which is a representative example of
solar-energy conversion systems, has attracted much attention due
toitspotentialapplicationinrenewableenergy[1].To constructsuch
artificial photosynthetic systems, the development of efficient
catalysts for reactions involving multi-electron transfer, such as
CO2 reduction and water oxidation, is essential. There are two
primary approaches to developing such catalysts: homogeneous [2]
and heterogeneous systems [3]. Homogeneous catalysts such as
metal complexes are advantageous for the analysis of their reaction
mechanisms, including the respective photochemical and electro-
chemical processes, to allow for further modification of the catalyst
design at the molecular level. To understand the detailed reaction
mechanisms,theelectrochemicalresponseofthecatalystsshouldbe
investigated under photoirradiation because the electrochemical
process proceeds subsequent to the photochemical process in solar-
separately evaluated using different experimental setups. To the
best of our knowledge, there have been no reports on the
electrochemical analysis of molecular catalytic systems in homoge-
neous solutions under photoirradiation.
Recently, we established a method to perform electrochemical
analysis of photoirradiated solutions [4]. In our previous report, we
measured cyclic voltammograms of ferrocene (Fe(C5H5)2) under
photoirradiation in a conventional electrochemical setup and
obtained sigmoidal-shaped voltammograms that were unsuitable
for the detailed analysis of the electrochemical processes. This
result suggested that the photoirradiation may have generated
convection due to local increases in temperature, and the mass
transfer subsequently induced by this convection affected the
electrochemical responses. Based on this result, we aimed to
decrease the unfavorable electrochemical response under photo-
irradiation and found three techniques (measurements with a
rotating disk electrode (RDE), fast scanning, and thin layer cyclic
voltammetry (TLCV)) that could suppress the aforementioned
unfavorable current change. We also succeeded in detecting the
electrochemical response of photochemically generated molecules
using TLCV techniques.
* Corresponding author at: Department of Life and Coordination-Complex
Molecular Science, Institute for Molecular Science, 5-1, Higashiyama, Myodaiji,
Okazaki, Aichi 444-8787, Japan.
E-mail address: masaoka@ims.ac.jp (S. Masaoka).
http://dx.doi.org/10.1016/j.jphotochem.2015.06.014
1010-6030/ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Fukatsu, et al., Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction under photoirradiation,
J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.014

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As a further extension of our study, in this work, we applied our
photo-electrochemical method for the analysis of catalytic
systems. As a catalytic system, the CO2 reduction reaction is an
attractive target because molecules that can catalyze CO2
reduction are longstanding targets for direct electrolytic fuel
synthesis. Moreover, the capture and the efficient use of CO2 is an
important issue to resolve not only the shortage of fossil fuels but
also global warming caused by increasing atmospheric CO2
concentrations. In this contribution, the influence of photo-
irradiation on the electrochemical CO2 reduction reaction cata-
lyzed by meso-tetraphenylporphyrin iron(III) chloride (Fe(tpp)Cl)
[5,6] was analyzed using the electrochemical technique.
[6]. Under photoirradiation using a Xe lamp with a CM-1 cold
mirror (400–800 nm), the shape of the voltammogram dramati-
cally changed, and the current that was attributed to the
convection induced by the photoirradiation was observed. These
results indicated that the analysis of the photoelectrochemical
response of Fe(tpp)Cl was not allowed with the conventional
technique. By contrast, the measurements using the TLCV
technique with a layer thickness (d,Fig. 1c) of 0.25 mm gave
different results. As shown in Fig. 1b, under dark conditions, three
reversible redox waves were observed at approximately the same
potentials as those obtained in the conventional measurement.
Under photoirradiation, the same three reversible reduction waves
were observed, maintaining the shape of the voltammogram.
Therefore, the convection induced by photoirradiation was
suggested to be suppressed in the TLCV measurements, and it
was confirmed that the electrochemical measurement of Fe(tpp)Cl
under photoirradiation could be performed using the TLCV
technique.
2. Results and discussion
A
cyclic voltammogram of Fe(tpp)Cl measured by the
conventional technique is shown in Fig. 1a. The voltammograms
were measured in 0.1 M tetra(n-butyl) ammonium perchlorate
(TBAP)/N,N-dimethylformamide (DMF). Under an Ar atmosphere
in dark conditions, the voltammogram displayed three reversible
reduction waves at E1/2 (half-wave potentials) = ꢀ0.6, ꢀ1.5 and
ꢀ2.1 V vs. ferrocenium/ferrocene (Fc+/Fc), which were assigned as
Fe(III)/Fe(II), Fe(II)/Fe(I) and Fe(I)/Fe(0) redox couples, respectively
TLCV measurements of Fe(tpp)Cl were also performed under
a CO2 atmosphere to analyze the catalytic reduction of CO2. In
the voltammogram of Fe(tpp)Cl under dark conditions (Fig. 2),
the first and second reduction peaks were observed at the exact
same potentials as those observed under Ar conditions.
Fig.1. (a) Cyclic voltammograms (d = 5 mm) with Fe(III)/Fe(II), Fe(II)/Fe(I) and Fe(I)/Fe(0) redox waves. (b) Thin layer cyclic voltammograms (d = 0.25 mm) with Fe(III)/Fe(II), Fe
(II)/Fe(I) and Fe(I)/Fe(0) redox waves. (c) Schematic illustration of the thickness of the solution layer. CV measurements were performed with Fe(tpp)Cl (1 mM) in a 0.1 M TBAP
/ DMF solution with photoirradiation (red, 400 nm <
l
< 800 nm) and without photoirradiation (blue) under an Ar atmosphere (WE: GC; CE: Pt wire; RE: Ag+/Ag; scan rate:
20 mV sꢀ1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Thin layer cyclic voltammograms (d = 0.25 mm) of Fe(tpp)Cl (1 mM) in a 0.1 M TBAP / DMF solution with photoirradiation (red, 400 nm < l< 800 nm) and without
photoirradiation (blue) under a CO2 atmosphere (WE: GC; CE: Pt wire; RE: Ag+/Ag; scan rate: 20 mV sꢀ1). (a) Voltammograms with a catalytic current. (b) Enlarged
voltammograms focusing on Fe(III)/Fe(II) and Fe(II)/Fe(I) redox waves. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
Please cite this article in press as: A. Fukatsu, et al., Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction under photoirradiation,
J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.014

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3
Fig. 3. Thin layer chronoamperograms (E = ꢀ2.3 V vs. Fc+/Fc, d = 0.25 mm) of Fe(tpp)Cl (1 mM) in
a 0.1 M TBAP / DMF solution with photoirradiation (red,
400 nm < l
< 800 nm) and without photoirradiation (blue) under a CO2 atmosphere (WE: GC; CE: Pt wire; RE: Ag+/Ag). (a) Time vs. Current, (b) Time vs. Coulombs.
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Additionally, large irreversible cathodic currents were observed at
the potential corresponding to the Fe(I)/Fe(0) redox couple (ꢀ2.1 V
vs. Fc+/Fc), indicating the electrocatalytic CO2 reduction to CO. In
the subsequent anodic scan in the positive direction, the
irreversible wave was observed at approximately Epa = ꢀ1.7 V vs.
Fc+/Fc, and the original re-oxidation waves observed at
Epa = ꢀ0.6 and ꢀ1.5 V under an Ar atmosphere were not detected
(Fig. 2, blue line). By contrast, under photoirradiation, the two
reversible redox waves at E1/2 = ꢀ0.6 and ꢀ1.5 V and the catalytic
currents at approximately ꢀ2.1 V were observed (Fig. 2, red line).
Fig. 4. Thin layer cyclic voltammograms (d = 0.25 mm) of Fe(tpp)Cl (1 mM) in a 0.1 M TBAP / DMF solution for which the negative edges of the potential sweep were ꢀ2.5 V
(blue), ꢀ2.2 V (green) and ꢀ2.1 V (red) (WE: GC; CE: Pt wire; RE: Ag+/Ag; scan rate: 20 mV sꢀ1). (a) Voltammograms showing the negative edges of potential sweeping without
photoirradiation. (b) Enlarged voltammograms focusing on Fe(III)/Fe(II) and Fe(II)/Fe(I) redox waves without photoirradiation. (c) Voltammograms showing the negative
edges of potential sweeping with photoirradiation. (d) Enlarged voltammograms focusing on Fe(III)/Fe(II) and Fe(II)/Fe(I) redox waves with photoirradiation. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: A. Fukatsu, et al., Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction under photoirradiation,
J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.014

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Fig. 5. Thin layer cyclic voltammograms (d = 0.25 mm) of Fe(tpp)Cl (1 mM) in a 0.1 M TBAP / DMF solution with photoirradiation in various light intensities (red,150 W; green,
300 W) and without photoirradiation (blue) under a CO2 atmosphere (WE: GC; CE: Pt wire; RE: Ag+/Ag, scan rate: 20 mV sꢀ1). (a) Voltammograms showing a catalytic current.
(b) Enlarged voltammograms focusing on Fe(III)/Fe(II) and Fe(II)/Fe(I) redox waves. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
It should be noted that the intensity of the irreversible wave at
Epa = ꢀ1.7 V, which was observed in the measurements under dark
conditions, dramatically decreased in the voltammogram under
photoirradiation. These results clearly indicated that photo-
irradiation largely affected the redox properties of Fe(tpp)Cl
under a CO2 atmosphere.
To further understand this phenomenon, several electrochemi-
cal measurements were performed in thin-layer conditions under
a CO2 atmosphere. First, chronoamperometry was performed both
under photoirradiation and in the dark. The solution of Fe(tpp)Cl
(1 mM) in DMF containing TBAP (0.1 M) was electrolyzed at the GC
electrode with a cell operating potential of ꢀ2.3 V vs. Fc+/Fc while
keeping the layer thickness as 0.25 mm. As shown in Fig. 3,
constant currents flowed during the 300 s of electrolysis, and the
intensities of the currents were almost the same irrespective of the
photoirradiation. From these results, it was assumed that the
catalytic current of CO2 reduction catalyzed by Fe(tpp)Cl was not
affected by photoirradiation.
Second, the origin of the irreversible peak at approximately
ꢀ1.7 V under the dark conditions was investigated by changing the
negative edges of the potential sweep. Fig. 4 shows the voltammo-
grams of Fe(tpp)Cl for which the negative edges of the potential
sweep were ꢀ2.5 (blue line), ꢀ2.2 (green line) and ꢀ2.1 V (red line).
Although the shapes of the voltammograms were almost the same
when the measurements were performed under photoirradiation,
a clear difference was observed in the measurement under the dark
condition. For the negative edge located at ꢀ2.1 V, which was more
positive than the CO2 reduction potential catalyzed by Fe(tpp)Cl,
the second redox wave was reversibly observed. By shifting the
negative edges to the negative region, the irreversible peak at
ꢀ1.7 V gradually appeared, and the intensity of the peak at
Epa = ꢀ1.5 V decreased. These observations suggested that the flow
of the large catalytic current induced the generation of the
irreversible peak at ꢀ1.7 V.
peak around ꢀ1.7 V was presumed to be effectively suppressed by
photoirradiation.
The aforementioned observations enabled us to describe the
effect of photoirradiation on the redox properties of Fe(tpp)Cl in
combination with the proposed catalytic cycle. The proposed
reaction mechanism of electrocatalytic CO2 reduction by Fe(tpp)Cl
was reported by Costentin et al. [6] and is summarized in
Scheme 1. Initially, Fe(tpp)Cl is reduced from Fe(III) to Fe(0) by the
sequential one-electron reduction processes; these processes
were observed in the measurement under an Ar atmosphere
(vide supra). The generated Fe(0) species react with CO2, and the
CO2-bound complexes forms. Subsequent adduction of acid (in
this case, the hydrated water of the catalyst) and the CꢀꢀO bond
cleavage reaction in coordination with proton transfer result in
the formation of the CO-bound complex. Further reaction with the
Fe(0) species leads to the generation of CO. It was also reported
that CO generated by the catalytic reaction could bind to catalysts
with the Fe(I) state as the side reaction of the CO2 reduction
reaction by Fe(tpp)Cl (I in Scheme 1), and the electrochemical
response corresponding to this side reaction was observed at
approximately ꢀ1.7 V. Similarly, the irreversible peak, which was
attributed to the side reaction, was detected at ꢀ1.7 V in our
experiments using the TLCV technique in the dark condition.
Furthermore, we found that the intensity of the peak decreased
upon photoirradiation and, in some conditions, the electrochemi-
cal response of the side reaction could be suppressed.
A
reasonable explanation of this phenomenon is given by consider-
ing the photoinduced decarbonylation from carbonyliron tetra-
phenylporphyrin [7]. The oxidative Fe–CO reassociation
([FeI(tpp)]ꢀ + CO ! [FeII(tpp)CO] + eꢀ, I in Scheme 1) occurs at
E ꢁ ꢀ1.7 V regardless of photoirradiation. Under photoirradiated
condition, the photoinduced decarbonylation reaction of [FeII(tpp)
CO] proceeds to afford [FeII(tpp)] ([FeII(tpp)CO] + h
n
! [FeII(tpp)] +
CO) [7]. The photochemically generated [FeII(tpp)] is successively
Finally, the dependence of the generation of the irreversible
peak on the light intensity was investigated. Fig. 5 shows the cyclic
voltammogram of Fe(tpp)Cl under photoirradiation with different
intensities of light (150 and 300 W). When the light intensity was
increased, the peak height of the irreversible wave at ꢀ1.7 V
decreased, and the original re-oxidation wave at Epa = ꢀ1.5 V
increased. From these results, the generation of the irreversible
reduced ([FeII(tpp)]
+
eꢀ
!
[FeI(tpp)]ꢀ, E1/2 ꢁ ꢀ1.5 V, II in
Scheme 1). As a result, the current attributed to the oxidative
Fe–CO reassociation (I in Scheme 1) is offset by the current
attributed to the reduction of the photochemically generated
[FeII(tpp)] (II in Scheme 1). In total, the electrochemical response
of the side reaction in the catalytic system could be hindered by
photoirradiation.
Please cite this article in press as: A. Fukatsu, et al., Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction under photoirradiation,
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5
Scheme 1. Proposed mechanism of the catalytic reaction [6]. The coordination of solvent molecules and counter anions is in equilibrium [8] and omitted for clarity.
3. Conclusions
photoinduced CO dissociation from [FeII(tpp)CO] based on the
analysis of these electrochemical responses in combination with
the proposed reaction mechanism. These results show that our
method presented in this contribution has the possibility of
analyzing various catalytic reactions. Further analysis of this
reaction and other catalytic reactions by thin layer techniques is
currently being investigated.
In conclusion, we first demonstrated the electrochemical
analysis of a catalytic system under photoirradiation. A well-
known catalyst for CO2 reduction, Fe(tpp)Cl, was selected as a
target analyte, and the electrochemical response of the catalyst
was successfully investigated using the TLCV technique. Under a
CO2 atmosphere, the catalytic current originating from CO2
reduction was observed both under the dark and light conditions.
Under the dark conditions, an irreversible re-oxidation wave was
observed at ꢀ1.7 V vs. Fc+/Fc in the voltammogram. Under
photoirradiation, on the other hand, the intensity of the irrevers-
ible peak dramatically decreased; however, the intensities of the
reversible peaks originating from the redox of the catalyst
increased. Further electrochemical measurements with various
conditions revealed that the generation of the irreversible peak
depended on the negative edges of the potential sweep and the
intensity of the light used in photoirradiation. Moreover, it was
found that the original [FeII(tpp)] complex is regenerated by the
4. Experimental
4.1. Materials
Pyrrole was purchased from Sigma–Aldrich Co., LLC. Benzalde-
hyde, propanoic acid, hydrochloric acid and ferrocene (Fc) were
purchased from Wako Pure Chemical Industries, Ltd. FeCl2 4H2O
ꢂ
and N,N-dimethylformamide (DMF) were purchased from Kanto
Chemical Co., Inc. Tetra(n-butyl) ammonium perchlorate (TBAP)
was purchased from Tokyo Chemical Industry Co., Ltd. All of the
solvents and reagents were of the highest quality available and
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Lab software. The measurements were performed using the system
shown in the previous report [4] (Fig. 6) to avoid photoirradiation
of the reference electrode and the counter electrode. The photo-
irradiation was performed using an ILC Technology CERMAX LX-
300 Xe lamp (operated at 150 W unless otherwise mentioned)
equipped with a CM-1 cold mirror (400 nm < l< 800 nm). In all
cases, a platinum wire counter electrode (diameter 0.5 mm, from
BAS) and a glassy carbon (GC) disk working electrode (diameter
3 mm, from BAS) were used. The working electrode was treated
between scans by means of polishing with 0.05-mm alumina paste
(from BAS) and washing with purified H2O. For all measurements,
an Ag+/Ag reference electrode was used. The supporting electrolyte
was 0.1 M TBAP. The potentials reported within these measure-
ments were referenced to the Fc+/Fc couple at 0 V.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “AnApple” (No. 25107526), a Grant-
in-Aid for Young Scientists (A) (No. 25708011) (to S.M.), a Grant-in-
Aid for Challenging Exploratory Research (No. 26620160) (to S.M.)
and a Grant-in-Aid for Young Scientists (B) (No. 24750140) (to M.
K.) from the Japan Society for the Promotion of Science. The
authors thank T. Yano for manufacturing the photoelectrochemical
cell and T. Omaru for making the glass cell.
Fig. 6. Schematic illustration of the custom-made photoelectrochemical cell.
were used as received, except for TBAP and DMF for the
electrochemical measurements. TBAP was recrystallized from
absolute ethanol and dried in vacuo. DMF was degassed and
purified under N2 atmosphere using a GlassContour solvent system
(Nikko Hansen Co., Ltd.).
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under argon unless otherwise mentioned. A three-electrode
configuration was employed in conjunction with a Biologic SP-
50 potentiostat that interfaced with a computer with Biologic EC-
Please cite this article in press as: A. Fukatsu, et al., Electrochemical analysis of iron-porphyrin-catalyzed CO2 reduction under photoirradiation,
J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.014