Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt complex

Article abstract of DOI:10.1016/j.jphotochem.2017.09.019

Conversion of CO₂ into value-added chemicals by solar energy is one of the most promising pathways to partially address both energy crisis and global warming effect. The properties of reaction medium play a key role in determining the efficiency of CO₂ photoreduction and modulating the distribution of reaction products. Herein, we report the detailed investigation of solvent effects on the performance of photocatalytic CO₂ reduction by means of photo-/electro-chemical and spectroscopy techniques. The relationship between photochemical CO₂ performance and solvent properties (e.g., viscosity, solubility, and electrochemical behaviors) are systematically explored. The active intermediates for CO₂ photoreduction catalysis are examined by UV–vis absorption spectra. Moreover, a possible reaction pathway involving photoexcited charge transfers and intermediate species transformations in various solvents is proposed.

Full text of DOI:10.1016/j.jphotochem.2017.09.019

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JPC 10864 No. of Pages 6
Journal of Photochemistry and Photobiology A: Chemistry xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt
complex
Jinliang Lina,b, , Biao Qina,b, Guanglian Zhaoa,b
*
a
Department of Chemical and Engineering, Zunyi Normal College, 563000, Zunyi, China
Characteristics Key Laboratory of Qianbei, Zunyi Normal College, 563000, Zunyi, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Conversion of CO2 into value-added chemicals by solar energy is one of the most promising pathways to
partially address both energy crisis and global warming effect. The properties of reaction medium play a
key role in determining the efficiency of CO2 photoreduction and modulating the distribution of reaction
products. Herein, we report the detailed investigation of solvent effects on the performance of
photocatalytic CO2 reduction by means of photo-/electro-chemical and spectroscopy techniques. The
relationship between photochemical CO2 performance and solvent properties (e.g., viscosity, solubility,
and electrochemical behaviors) are systematically explored. The active intermediates for CO2
photoreduction catalysis are examined by UV–vis absorption spectra. Moreover, a possible reaction
pathway involving photoexcited charge transfers and intermediate species transformations in various
solvents is proposed.
Received 17 April 2017
Received in revised form 2 September 2017
Accepted 9 September 2017
Available online xxx
PACS:
82.30.Vy
82.33.-z
Keywords:
Photocatalysis
CO2 reduction
Solvent effect
Photochemistry
Electron transfer
© 2017 Published by Elsevier B.V.
1. Introduction
activated by transition-metal catalysts under light irradiation,
which coupled to appropriate reductants would efficiently
Photocatalytic reduction of CO2 by solar resource, a significant
half reaction of artificial photosynthesis, has been considered as
one promising solution to produce renewable energy and alleviate
global warmming [1,2]. Since the pioneer work of photocatalytic
reduction of CO2 in aqueous solution over semiconductor photo-
catalysts by Inoue et al. a variety of photochemical CO2 conversion
systems have been established and investigated [3]. Among the
developed CO2 reduction systems, gaseous systems for photo-
catalytic reduction of CO2 with vaporized H2O over solid catalysts
have been actively studied, which exhibit inspiring achievements
[4]. Materials such as TiO2, SrTiO3, CdS, zeolites, g-C3N4 polymers,
metal-organicn frameworks (MOF), and covalent organic frame-
works (COF) are demonstrated as efficient catalysts for CO2
photoreduction in liquid reaction mediums [1,5,6]. In the CO2
reduction systems involving metal based catalysts, the adsorption
promote the conversion of CO2. As a demonstration, Lehn’s group
developed a photochemical CO2 reduction system containing a
catalyst combination of [Ru(bpy)]3Cl2 (bpy = 2,20-bipyridine) and
CoCl2 to act as a photosensitizer and an electron mediator,
respectively, along with TEOA as the electron donor [8].
Cobalt complexes have been widely employed as an efficient
homogeneous cocatalysts to mediate CO2 photoreduction through
generating a Co(I) species transition state. The formation of Co-
ꢀ
CO2 adduct with bent geometry can substantially minimize the
activation energy of CO2 molecules for subsequent conversion
reactions. Xu et al. have developed an efficient hybrid CO2
photoreduction system by mixing CdS nanoparticles and CoCl2/
bpy in an acetonitrile solution, which affords a high apparent
quantum yield of 1.0% for CO formation under monochromatic
irradiation (l= 470 nm) [9]. Our previous work revealed that
ꢀ
strength of CO2 to the metal species on catalyst surface greatly
determines the activity and selectivity of the reaction [5,7]. Of note,
the thermodynamically stable linear CO2 molecules can also be
similar reaction mechanism can further be extended to both
heterogeneous and homogenous photocatalytic CO2 reduction
systems in acetonitrile solution [10]. Besides, a high-effciency
photocatalytic CO2 reduction system consisting of semiconductor
powders and a Ru-based complex is also achieved by Bhatt and
coworkers [11]. So far, the reported heterogeneous and homoge-
neous photocatalytic CO2 reduction systems are mainly operated in
liquid phase (e.g., suspension or solution). Results of these works
* Corresponding author at: Department of Chemical and Engineering, Zunyi
Normal College, Address: Ping’an Street (No.6), Honghuagang District, Zunyi City,
Guizhou Province, China.
E-mail address: jinliang_lin@163.com (J. Lin).
http://dx.doi.org/10.1016/j.jphotochem.2017.09.019
1010-6030/© 2017 Published by Elsevier B.V.
Please cite this article in press as: J. Lin, et al., Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt complex, J. Photochem.
Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.019

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JPC 10864 No. of Pages 6
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point out that the interaction between CO2 molecules and metal
species on catalysts surface is significantly influenced by reaction
medium, especially the reaction solvents.
transition was conducted in a sealed container adaptable for record
equipment, and exposed light irradiation for 10 min before test.
The influence of solvent on chemical equilibrium has long been
investigated, and can be mainly measurened by dielectric
constants [12]. It is well known that the first step in CO2
conversion involves the formation of a “CO2ꢀ” intermediate, which
requires a high reaction potential (e.g., ꢀ2.2 V vs. SCE, in DMF
solution) [13–15]. Previous studies demonstrate that the proper-
ties of used solvent can obviously decrease the energy barrier
during CO2 activation. For instance, using 1-ethyl-3-methylimi-
dazolium tetrafluoroborate ([EMIM]BF4) as solvent, the generation
2.3. Electrochemical measurements
Electrochemical measurements were conducted with a CHI
660E workstation in a conventional three electrode cell, using a Pt
plate as the counter electrode and saturated calomel electrode
(SCE) as the reference electrode. The photoelectrode was
immersed in a 0.1 M KCl aqueous solution. The photoelectrodes
were prepared by a typical coating method: the aqueous slurries of
CdS were quantificationally coated on ITO glass substrates[18]. The
films on ITO glass substrate were dried in air and annealed at 100 ꢂC
for 1 h as the final photoelectrodes. The electrolyte solution (ca.
60 m1), saturated with N2 or CO2 respectively and stirred by a
magnetic stirrer, contained the same ingredients as photocatalytic
reduction system in each solvent. The cell was then sealed and, at
the same time, the three electrodes were immersed into the
electrolyte because they were fixed on the cover of the cell. The
electrolysis cell was placed into a constant temperature water bath
of 20 ꢂC. Time dependent photocurrent curves were measured at
ꢀ1.2 V with amperometric i-t curve method; and the electro-
ꢀ
of CO2 anion only necessitates a much lower overpotential of
0.17 V due to the complex effect of EMIM [15]. Besides, it is also
reported that the selectivity of CO2 reduction is mainly depended
on the adsorption of CO2ꢀ on catalyst surface, which is also largely
affected the nature of reaction solvent. In low polar solvents, the
formed CO2ꢀ may be adsorbed strongly to the active surface sites of
photocatalyst by carbon atom of the anion. In this case, carbon
monoxide and formate are usually the main products. Alternative-
ly, the CO2ꢀ anion is prone to be stabilized by the solvent molecules
with high dielectric constant, resulting in weakened interaction
with photocatalyst, and thus generating formate as the product
[16]. Additionally, the solvent-dependent, charge-transfer equilib-
rium between active radicals is explored as well in organic
synthesis [12]. On the other hand, the CO2ꢀ anion would form a M-
catalytic behavior were test from
voltammetry model.
0
to ꢀ1.8 V with cyclic
2.4. Photocatalysis test
ꢀ
CO2 (M = metal) intermediate as the first reaction step in metal-
containing CO2 reduction systems, and the properties of reaction
solvent are considered to be of cꢀrucial importance to the
generation and stabilization of M-CO2 adducts [17].
All experiments were performed in a Schlenk flask (80 ml)
under an atmosphere pressure of CO2 (1 atm) [19]. In the Schlenk
flask, CdS (50 mg), bpy(15 mg), CoCl2 6H2O (10 mmol) were added
ꢁ
Based on the above considerations, we are interested in
conducting an intensive study on the influences of solvent
molecules to CO2 activation and conversion through the formation
in the mixture of solvent (5 ml) and triethanolamine (TEOA, 1 ml).
TEOA was used as the sacrificial agent. The system was subjected to
vacuum degassing and backfilling with pure CO2 gas. This process
was repeated three times and after the last cycle the flask was back
filled with CO2. Then the system was irradiated with two counter
non-focus 50 W white LED light source under vigorous stirring at
20 ꢂC controlled by a water-cooling system. The produced gases
(CO, H2) were detected by a gas chromatography (Agilent 7890,
Agilent Technologies) equipped with a packed column (TDX–
1 mesh 42/10). Ar was used as the carrier gas.
ꢀ
of M-CO2 adduct, which is of particular importance but rarely
reported. Herein, we demonstrate the detailed investigation of
solvent effects on the performance of photocatalytic CO2 reduction
by means of photo-/electro-chemical and spectroscopy techniques.
The relationship between photochemical CO2 performance and
solvent properties (e.g., viscosity, solubility, and electrochemical
behaviors) are systematically investigated. The active intermedi-
ates for CO2 photoreduction catalysis are examined by UV–vis
absorption spectra. Moreover,
a
possible reaction pathway
3. Results and disscussion
involving photoexcited charge transfers and intermediate species
transformations in various solvents is proposed.
Results of photocatalytic CO2 reduction performance in
different reaction solvents are listed in Table 1. With acetonitrile
(AN) as the reaction solvent under visible light irradiation for 2 h
(Entry 1, Table 1), the CO2 reduction system exhibits a high
2. Experimental section
2.1. Chemicals
photocatalytic activity, generating 42.6
7.4 mol of H2 evolution. When DMF is used as the reaction
solvent, the formation of CO is diminished to 29.4 mol (Entry 2,
Table 1), but with comparable CO selectivity relative to that in AN.
The generation and selectivity of CO in DMSO solvent is much
mmol of CO production and
m
All the chemicals were purchased from China Sinopharm
Chemical reagent Co. Ltd and without further purification.
Cadmium sulfide (CdS) is of high purity (99.999%), and other
chemicals including N, N-dimethylformamide (DMF), dimethyl
sulfoxide (DMSO), tetrahydrofufan (THF), acetonitrile (AN), trie-
m
thanolamine(TEOA), cobalt chloride hexahydrate (CoCl2
2,2-bipyridine (bpy) are of reagent grade. The used water used is
ꢁ
6H2O) and
Table 1
Photocatalytic CO2 reduction performance with various solvents as the reaction
medium.a
ultrapure with resistivity of ca.18 M
Vcm.
Entry
Solvent
Viscosity
(cp)
Solubility
(mol/L)
CO
mol]
H2
mol]
Sel.b
[%]
2.2. Characterization
[
m
[m
1
2
3
4
5
AN
0.37
0.92
2.24
0.55
1
0.28
0.20
0.14
0.21
0.03
42.6
29.4
8.9
24.7
0.23
7.4
4.8
8.0
2.6
1.9
85.2
86.0
52.7
90.5
10.8
X-ray diffraction (XRD) patterns were collected using a Bruker
D8 Advance X-ray diffractometer (Cu K 1 irradiation, = 1.5406 Å).
Absorption spectra were obtained on an UV–vis spectrophotome-
ter (Varian Cary 50 Conc). The metal-solvent and metal-ligand-
solvent were directly mixed before measurement. Detection of CoI
DMF
DMSO
THF
l
a
H2O
a
All the reactions were perfomed under 1 atm and at 20 ꢂC.
Sel. = mol CO/mol (H2 + CO) ꢃ 100.
b
Please cite this article in press as: J. Lin, et al., Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt complex, J. Photochem.
Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.019

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3
inferior among the examined aprotic solvents (Entry 3, Table 1),
mainly because of the low CO2 solubility and the high viscosity
hindering the reaction kinetics. Although the CO2 solubility in THF
is similar to that in DMF, the archived CO evolution is only
moderate (Entry 4, Table 1), but the selectivity of CO (90.5%) is the
highest. This high CO selectivity should be intimately related with
the high coordination ability of THF to Co-bpy complex [20]. Once
using H2O as a classic protic solvent, only a tiny amount of CO and
H2 is produced (Entry 5, Table 1), indicating that aprotic solvents
are more favorable for CO2 reduction in the developed reaction
system. Besides, it is worth mentioning that trace amount of formic
acid was detected when AN or H2O was employed as the reaction
solvent. The distribution of reaction products is consistent with the
results of reported works [21–24]. All these results suggest that the
CO2 solubility and the metal-coordinating capabilities of reaction
solvents impressively affect the performance of photocatalytic CO2
reduction. From Table 1, it is concluded that the order of CO
production is in good accordance with CO2 solubility in solvents,
and no obvious correlation with solvent viscosity is observed.
The above conclusions are also supported by the time courses of
products in different solvents. As shown in Fig. 1, the reaction rate
of CO2 reduction catalysis is largely determined by the properties
of used solvents. When the reaction is performed with AN as the
solvent, the catalytic system manifests the highest production of
CO and H2. After reaction for about 2 h, the generation of CO is
almost unchanged, due mainly to the consumption of electron
donor TEOA in the system. Using H2O as the reaction solvent, the
production of CO is negligible, but the evolution rate of H2 is nearly
maintains at constant. This observation is reasonable as the
presence of plentiful protons facilitates H2 generation. These
findings further demonstrate that the solubility of CO2 in the
solvent greatly influences the performance of CO2 photoreduction.
The electrochemical behaviors of CO2 reduction are then
inspected by cyclic voltammetry (CV). In these tests, the processes
of CO2 reduction can be monitored indirectly, which involves the
initial reduction of CoII complexes and their subsequent reaction
with CO2 to form Co-CO2 intermediates. The thus formed CoI
complexes are important redox active catalysts [8,9]. In the tests,
the reduction potential of CO2 is approaching to the thermody-
namic value. The obtained CV curves are shown in Fig. 2. In the
absence of CO2, the obtained redox potential (E00 = ꢀ0.7 V) in AN,
DMF, and DMSO (Fig. 2a–c) is attributed to the conversion of CoII
complex to CoI counterpart [25]. In this stage, the low valent Co
intermediates are stabilized by pyridine ligand through weak
coordination [26], promoting the activation of CO2 molecules via
nucleophilic attack. The activity of CO2 conversion reaction is
determined not only by the energy barrier of forming the
corresponding intermediates, but also by the solubility of CO2 in
the reaction solvent. No obvious signal is observed in THF solution
(Fig. 2d), possibly because of the generation of a light blue
precipitate as shown in Fig. S2. In H2O (Fig. 2e), one couple of redox
peaks are detected (E00 = ꢀ0.4 V, and E00 = ꢀ1.2 V), which is
different from that observed in organic solvents (Fig. 2a–c). This
finding may be caused by the protonation of CoI to produce CoI-H
complex, subsequently decreasing the reduction potential [27].
When CO2 is introduced (Fig. 2a–c), the corresponding peaks
observed in N2 shift toward anodic position with the potential of
about 500 mV, and the intensities of peak are enhanced consider-
ably. The markedly increased current densities indicate that the
reaction systems experience electrocatalytic CO2 reduction reac-
tions with high efficiency. With CO2 being introduced into THF, the
shape of the CV is almost unchanged compared with that in N2
saturated solution, consistent with the results in UV–vis measure-
ments. This phenomenon is mainly resulted from the coordination
of cobalt species with oxygen atom of ꢀthe solvent molecule, and
thus hindering the formation of M-CO2 and the charge transfers
afterwards. Besides, when the CO2 reduction reaction is conducted
in H2O, an obvious reduction peak (E00 = ꢀ0.4 V) is detected as well.
This result further suggests the proton-coupled CO2 reduction
mechanism.
During the processes of photocatalytic reactions, electron-hole
paires and some labile intermediates are formed upon light
irradiation [28]. The typical role of electron migration in organic
electrolytes could be reflected by transient photocurrent response
under light illumination. The photocurrent responses of the CO2
reduction systems with different solvents as the electrolytes are
displayed in Fig. S3. The photocurrent intensity indicates the
transfer kinetics of charge carriers. The trend of photocurrent
density is in good agreement with the order of CO2 photoreduction
activity in organic solvents as shown in Fig. 1. The observed
photocurrent generation in H2O in mainly attributed to H2
evolution reaction. These results reveal that the transportation
of photo-induced charges of CO2 conversion photocatalysis can be
accelerated by selecting appropriate solvent as the reaction
medium.
Absorption spectroscopy is employed to characterize the
interaction of solute-solvent in CO2 reduction system. The UV–
vis absorption spectra of solute molecules are influenced by
solvent molecules around and other environments. In order to
minimize the interferences caused by the surrounding factors, the
UV–vis absorption spectra are collected in solution by directly
dissolving Co2+ (CoCl2) in various solvents. As shown in Fig. 3, the
UV–vis absorption spectra obtained in all the organic solution
generally exhibit two absorption bands (line a–d). The absorption
Fig. 1. Production of CO and H2 in different solvents: (a) AN, (b) DMF, (c) THF, (d) DMSO, and (e) H2O.
Please cite this article in press as: J. Lin, et al., Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt complex, J. Photochem.
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Fig. 3. UV–vis absorption spectra of Co2+-solvent interaction recorded in different
solvents: (a) AN, (b) DMF, (c) DMSO, (d) THF, and (e) H2O.
tetrahedral complexes [29]. These results indicate the strong
interaction between the ligand and metal cation in the CO2 systems
when organic solvents are used as the reaction medium. However,
only a weak absorption peak (ca. 480 nm) is observed when the Co2
+ is dissolved in H2O [30]. These changes are mainly induced by the
alternation of the chemical nature of solvent molecules, such as the
charge transfers between solvent and solute, solvent dependent
aggregation and complexation.
Fig. 4 displays the UV–vis absorption spectra of the reaction
mixture with the presence of bpy. As can be seen, after the
introduction of bpy, the absorption position around 300 nm
experiences an obvious red-shift, highlighting the enhanced MLCT
process. While the absorption intensity at 550–750 nm is
weakened, which indicats that the cobalt center is favored to
coordinate with bpy, rather than the solvent molecules. It should
be noted that the tertiary amine TEOA may also contribute to the
solvent effect in photocatalytic CO2 reduction systems by serving
as a ligand to coordinate with cobalt center. Reults of UV–vis
absorption characterizations demonstrate that the ligand (e.g.,
bpy) in CO2 reduction system can alter the charge distribution of
Fig. 2. Cyclic voltammograms of CdS/ITO electrodes in various electrolyte: (a) AN,
(b) DMF, (c) DMSO, (d) THF, and (e) H2O.
bands located at around 300 nm are attributed to the metal-to-
ligand charge transition (MLCT) band. The absorption bands
positioned at 550–750 nm is due to the d–d transition in CoL4
Fig. 4. UV–vis absorption spectra of Co(bpy)nCl2 in different solution: (a) AN, (b)
DMF, (c) DMSO, (d) H2O, and (e) THF.
Please cite this article in press as: J. Lin, et al., Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt complex, J. Photochem.
Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.019

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5
Scheme 1. Diagram of solvent effect on photocatalytic CO2 reduction system.
Fig. 5. UV–vis absorption spectra of the Co(bpy)n+ intermediates formed in the CO2
reduction system with acetonitrile as the reaction solvent (a) before and (b) after
light irradiation. Inserts are the corresponding photographs [9].
transfer between the metal ion and coordinated CO2 is inevitably
influenced by the nature of reaction solvent. As demonstrated from
Scheme 1, a general explanation of solvent effects on photo-
catalytic CO2 reduction is that the photoexicetd electrons are apt to
be localized at oxygen atoms or carbon atoms of the metal-CO2
carbonate, which is determined by the interaction between solvent
and CO2 molecules.
In conclusion, we have demonstrated a detailed study of solvent
effects on photocatalytic CO2 reduction catalysis mediated by
cobalt complex. The properties of solvent play a crucial role in
determining the activity and selectivity of CO2 photoreduction.
Moreover, electrochemical and photochemical measurements are
proved to be of useful tools to provide new insights into the solvent
effect in photocatalytic CO2 reduction.
metal center, and eventually affect CO2 adsorption and the catalytic
performance of CO2 reduction system.
To investigate the experiences of CoII during CO2 photoreduc-
tion processes, after photocatalysis the reaction solution is
measured by UV–vis absorption spectroscopy. As shown in
Fig. 5, using AN as the solvent, the color of reaction mixture is
changed from orange to dark blue color upon light irradiation for
about 10 min. This observation is accompanied by the formation of
a new absorption band (ca. 500–700 nm) in the UV–vis absorption
spectrum, together with the blue shift of the initial absorption
bands at ca. 300 nm. The broad absorption band is attributed to the
intramolecular charge-transfer from metal center to pyridinium
moiety. Similar results are also detected when using other solvents
as the reaction medium (Fig. S4), mainly because of the generation
of a reducing CoI-complex during photocatalytic CO2 reduction
reaction. The findings of UV–vis absorption measurements further
indicate that the photochemical CO2 conversion reaction is
triggered by light irradiation. Interestingly, the color change of
reaction mixture is also found when N2 is used as the gas feedstock
instead of CO2 (Fig. S5), suggesting that the CoI-complex can form
as long as the process is operated in anaerobic condition.
Based on the results of CO2 reduction evaluation and photo-/
Acknowledgements
This research was sponsored by the Construction Projects of
Undergraduate Course Teaching, Department of Guizhou Provin-
cial Education (No. [2015]337), Guizhou Provincial Project on
Science and Technology, Department of Science and Technology of
Guizhou Province (No. [2017]1198), and Scientific Research
Foundation for Talented Scholars, Zunyi Normal College (BS
[2014]08).
Appendix A. Supplementary data
electro-chemcial characterizations,
a possible mechanism is
proposed as illustrated in Scheme 1. Under light irradiation,
semiconductor photocatalyst is excited and liberates electrons
from valence band to conduction band [31]. The photogenerated
electrons are then departed from the surface of photocatalyst and
grasped easily by Co (II) ion, thus generating the active
intermediate of CoI complex [8]. Meanwhile, H2 evolution reaction
through combination of CoI intermediate with protons to form
CoIII-hydride is probably similar to the CO2 conversion [27]. The
cobalt ion allows a strong interaction with CO2 to form a metal
carbonate. After the production of metal carbonate, the negative
charge of the oxygen atom of CO2 increased as a result of back-
donation from an occupied metal orbital to an unoccupied CO2 p*
orbital. Meanwhile the protonation is promoted and thus
providing oxygen atom from activated CO2 molecules by a proton
to yield the reduction product CO [32]. During the processes, the
properties of reaction solvent critically affect the generation and
stability of active species through solvation. In view of the
interaction between cobalt ion and solvent, the process of charge
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
jphotochem.2017.09.019.
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Please cite this article in press as: J. Lin, et al., Effect of solvents on photocatalytic reduction of CO2 mediated by cobalt complex, J. Photochem.
Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.019