Photocatalytic and electrocatalytic reduction of CO2 to methanol by the homogeneous pyridine-based systems

Article abstract of DOI:10.1016/j.apcata.2016.04.003

The visible light driven pyridine catalytic reduction of CO₂ was performed in aid of ruthenium phenanthroline complex photosensitizer. The light absorbance and utilization of the catalytic system was identified using UV-vis absorption and emission spectroscopic techniques. Under the dark conditions, electrochemical properties of the catalytic system were evaluated by compared the redox properties under N₂ and CO₂ pressure, respectively. Then, the photocatalytic CO₂ reduction with this catalytic system was studied under the visible light (λ > 420 nm) irradiation. The gaseous and liquid products of the CO₂ reduction were analyzed using gas chromatography and liquid chromatography. Methanol was the main product and there were no other products determined in the present system. The photocatalytic system showed a high product selectivity and efficient productivity with 60 μmol/L of methanol as solo product under the optimized conditions.

Full text of DOI:10.1016/j.apcata.2016.04.003

Applied Catalysis A: General 520 (2016) 1–6
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Photocatalytic and electrocatalytic reduction of CO2 to methanol by
the homogeneous pyridine-based systems
Wei Wanga, Junxiao Zhangb, Hui Wanga,∗, Lianjia Chena, Zhaoyong Bianb,∗
a College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, PR China
b College of Water Sciences, Beijing Normal University, Beijing 100875, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The visible light driven pyridine catalytic reduction of CO2 was performed in aid of ruthenium phenan-
throline complex photosensitizer. The light absorbance and utilization of the catalytic system was
identified using UV–vis absorption and emission spectroscopic techniques. Under the dark conditions,
electrochemical properties of the catalytic system were evaluated by compared the redox properties
under N2 and CO2 pressure, respectively. Then, the photocatalytic CO2 reduction with this catalytic sys-
tem was studied under the visible light (␭ > 420 nm) irradiation. The gaseous and liquid products of the
CO2 reduction were analyzed using gas chromatography and liquid chromatography. Methanol was the
main product and there were no other products determined in the present system. The photocatalytic
system showed a high product selectivity and efficient productivity with 60 ␮mol/L of methanol as solo
product under the optimized conditions.
Received 7 January 2016
Received in revised form 23 March 2016
Accepted 5 April 2016
Available online 6 April 2016
Keywords:
Photocatalysis
CO2 reduction
Methanol
Metal complex
Homogeneous catalysis
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
methanol, and methane [15,22–24]. Nevertheless, examples of
selectivelight-drivenCO2 conversionto reducedcarbonproductsin
The conversion of CO2 to useful fuels powered by renewable
energy has dramatically an effect in the energy and environmen-
tal fields. The attempts to fulfill the conversion are hot research
spots and attract considerable attention [1–10]. The electrochem-
ical reduction of CO2 is one such process, which has been studied
and produces alkanes, alcohols, and other desirable products under
suitable reaction conditions [11]. However, this attractive tech-
nology for the production of carbon based chemicals suffered
by the low selectivity, poorly defined reaction mechanisms, and
large overpotentials which induced huge energy consumption [12].
Recently, the photocatalytic reduction of CO2 is an attractive route
that can take advantage of the renewable and abundant energy of
the sun for long-term CO2 utilization [13–21].
There are several ways to reduce CO2 with the assistance of
renewable solar energy, and these methods can be divided into
two major categories: homogeneous photoreduction by a molecu-
lar catalyst and hetergeneous photoreduction by a semiconductor
catalyst [14]. Semiconductors such as TiO2 and SiC had been widely
studied as heterogeneous catalysts for the photochemical conver-
sion of CO2 to a variety of carbon products such as carbon monoxide,
heterogeneous systems were limited to the wide band gap, UV light
responsible materials that do not exhibit high selectivity toward
a single carbon product [25], aside from select transition-metal
doped silicates [26–29]. Homogeneous molecular systems offer an
alternative strategy for solar CO2 fixation that allows for modu-
lar tuning of their performances via synthetic chemistry [16,30].
In aqueous systems, the pyridinium ion (pyrH+) has been reported
to act as a homogeneous catalyst for the electrochemical reduc-
tion of CO2 with 30% Faradaic yields for methanol formation on
hydrogenated palladium electrodes [31].
Photocatalytic reduction of CO2 has become an interesting
research topic because of the potential utilization of renewable
solar energy [32–34]. Photochemical reductions of CO2 with selec-
tive product formation using rhenium polypyridine catalysts had
been extensively studied, however, rhenium complexes mainly
absorb the UV light and could not utilize the full solar spectrum
[35,36]. And the visible light photocatalytic CO2 reduction system
suffered from the noble-metal catalysts that achieve low turnover
numbers and/or selectivity [16]. Related systems with [Ru(bpy)3]2+
as a photosensitizer and transition metal complexes have reported
with rare yield of CO but concomitant H2 production with a high
yield [37–41]. Highly efficient and selective photocatalysts that
could function under sunlight are important for solar hydrocar-
bon production [34,42]. It is urged to achieve sustainable, solar
∗
Corresponding author.
E-mail addresses: wanghui@bjfu.edu.cn (H. Wang), bian@bnu.edu.cn (Z. Bian).
http://dx.doi.org/10.1016/j.apcata.2016.04.003
0926-860X/© 2016 Elsevier B.V. All rights reserved.

2
W. Wang et al. / Applied Catalysis A: General 520 (2016) 1–6
CO2 conversion to a predominant product with high selectivity and
activity.
In this article, a visible light driven CO2 reduction with pyridine
as catalyst was reported. To conform the efficiency of this system,
electrochemical technologies such as cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) was used to investigate the
redox properties of the pyridine catalyst and ruthenium complex
photosensitizer. Absorption spectra of the system was studied to
evaluate the light utilization. In a homemade photocatalytic reac-
tor, the photocatalytic reduction of CO2 was performed under the
visible light irradiation. The solo determined product of the reduc-
tion was methanol.
2. Experimental
2.1. Synthesis of Ru complex
[Ru(phen)3](PF6)2 was prepared with slight modifications
to a literature method [43]. RuCl3 (0.42 g, 2 mmol) and 1,10-
phenanthroline (1.09 g, 6 mmol) were added in an ethanol solution
and heated to reflux with stirring under an atmosphere of N2 for
8 h. Then the solution was cooled down to room temperature. Sub-
sequently NH4PF6 (3.26 g, 20 mmol) saturated solution was added
to the solution to form a red precipitate. The solid was filtered and
dried under vacuum overnight.
Fig. 1. Scheme of the photocatalytic reaction device.
long pass filter (␭ > 420 nm) that was immersed in water. Light was
focused onto the stirred sample and irradiated for 1–6 h. After irra-
diation, aliquots were analyzed by CP-3800 gas chromatography
(Varian, USA) coupled with a flame ionization detector (GC-FID).
The reaction apparatus is shown in Fig. 1.
2.2. Analytical method
3. Results and discussion
1H NMR spectra was recorded at 25 ◦C at Bruker AV500 500 MHz
Nuclear Magnetic Resonance spectrometer. The chemical shift was
adjusted to zero with tetramethylsilane as a reference substance.
UV–vis absorption spectra was acquired on UV-2600 spectrom-
eter (Shimadzu, Japan) using screw cap quartz cuvettes of 1 cm
pathlength. All absorption spectra were recorded at room temper-
ature and all samples were prepared in acetonitrile: water = 1:1.
Concentration of the solution was 0.02 mmol/L and scan wave-
length range was 200–800 nm.
3.1. Synthesis and characterization of photosensitizer
NMR spectrum of [Ru(phen)3](PF6)2 complex is shown in Fig. 2.
1H NMR (500 MHz, CDC13): ı 8.76 (dd, 6H), 8.22 (dd, 6H), 7.92 (dd,
6H) which were in accord with refer data [44].
Absorption spectra and emission spectra of ruthenium complex
photosensitizer are shown in Fig. 3.
As shown in the UV–vis absorption spectrum of ruthenium com-
plex (Fig. 3(a)), the dominant feature in the visible region was
a broad metal to ligand charge transfer (MLCT) absorption band
centered at 451 nm that presume the complex had a certain of
absorption and response in the visible region. In the UV region,
the intense bands were assigned to the ligand-based ␲-␲* transi-
tion (1IL) of phenanthroline ligand [45]. As shown in Fig. 3(b), the
excited ruthenium complex photosensitizer gave a strong emission
centered at 600 nm. CV and DPV of ruthenium complex are shown
in Fig. 4.
As shown in Fig. 4(a), the first oxidation potential was occurred
at 1.20 V corresponding to the peak of DPV at 1.20 V, which repre-
sented the oxidation potential of [RuII(phen)]2+ losing one electron
shown in Eq. (1). Subsequent, reduction potential occurred at
−1.38 V, −1.64 V, −1.85 V corresponding to the peaks of DPV at
−1.38 V, −1.64 V, −1.85 V, which can be speculated by the reduction
process of Eqs. (2)–(4) [43].
The photoluminescence (PL) spectra was measured by the F-
7000 (Hitachi, Japan) using screw cap quartz cuvettes of 1 cm
pathlength. All absorption spectra were recorded at room temper-
ature and all samples were prepared in acetonitrile: water = 1:1.
Concentration of the solution was 0.02 mmol/L and scan wave-
length range was 500–800 nm.
Electrochemical measurements of the complex are performed
on a CHI660D electrochemical workstation (Chenhua, China) using
a standard three-electrode cell with a platinum wire as work-
ing electrode, a standard Ag/AgCl in saturated KCl as reference
electrode, and a platinum wire as counter electrode. Tetrabutylam-
monium hexafluorophosphate (0.1 mol/L) and potassium chloride
(0.5 mol/L) were used as electrolyte reagents. CV measurement
of pyridine (5.24 mmol/L) with different scanning speeds, pH and
different electrolyte concentration were performed. Prior to mea-
surement, the solutions were sparged with N2 and CO2 gas for
40 min, respectively.
[RuII(phen)3]2+−e− → [RuIII(phen)3]3+E1/2 = 1.20 V(1)
2.3. Visible-light photoredox method for CO2 reduction
[RuII(phen)3]2+ + e− → [RuII(phen)2(phen−)]1+E1/2 = −1.38 V(2)
[RuII(phen)2(phen−)]1+ + e− → [RuII(phen)(phen−)2]•E1/2 = −1.64 V(3)
[RuII(phen)(phen−)2]• + e− → [RuII(phen−)3]−E1/2 = −1.85 V(4)
It indicated that [RuII(phen−)3]− was of vital importance for
photoinduced electron transfer, which certified why the complex
was good at photocatalysis.
[Ru(phen)3](PF6)2 (0.020 mmol/L), pyridine (50 mmol/L), KCl
(0.1 mmol/L) and ascorbic acid (0.2 mmol/L) were mixed in 25 mL of
acetonitrile solution and added in a 50 mL glass tube reactor. The pH
of solution was adjusted to 4, 5, and 6 respectively with hydrochlo-
ric acid and sodium hydroxide. The samples were charged with a
stir bar and the system was sealed with a rubber septum which
was tightened with a copper wire. The sample solution and reactor
headspace was saturated with CO2. The samples were irradiated by
a 500 W Xe arc lamp. The wavelength of light was controlled by a

W. Wang et al. / Applied Catalysis A: General 520 (2016) 1–6
3
Fig. 2. NMR spectrum of [Ru(phen)3](PF6)2 complexes.
2.0
1.6
1.2
0.8
0.4
0.0
20
(a)
(a)
CH CN:H O
3
2
10
0
-10
-20
300
400
500
600
700
1
0
-1
-2
Wavelength(nm)
Potential(V)
35
30
25
20
15
10
5
1000
800
600
400
200
0
(b)
(b)
CH CN:H O
3
2
0
1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
Potential(V)
500
600
700
800
Wavelength(nm)
Fig. 4. CV (a) and DPV (b) of complexes with tetrabutylammonium hexafluorophos-
phate (0.1 mol/L) and [Ru(phen)3](PF6)2 (5 mmol/L) in acetonitrile solution; scan
rate, 100 mV/s; All potentials were in volts vs. the Fc+/Fc couple.
Fig. 3. UV-vis absorption spectrum (a) and emission spectrum of complexes (b) in
CH3CN:H2O (1:1) solutions.
In order to investigate the effect of the sacrificial agent on the
photocatalytic reduction of CO2, ascorbic acid was chosen respec-
tively as a quencher to test fluorescence quenching experiment of
ruthenium complex. The effect of different concentration of ascor-
bic acid in the quenching experiment is shown in Fig. 5.
As shown in Fig. 5(a), the emission intensity of the ruthenium
complex photosensitizer decreased with the increase of the con-
centration of ascorbic acid as the quenching agent. Fig. 5(b) shows
that relationship between emission peak intensity/initial inten-
sity and concentration of ascorbic acid was linear with R2, 0.9914
which proved that ascorbic acid was the appropriate sacrificial
agent in experiment of photocatalytic reduction of CO2. The results
indicated that ascorbic acid could efficiently quench the excited
ruthenium complex photosensitizer to form reduced species which
could transfer electrons to the CO2 reduction reaction.
As shown in Fig. 6, the reduction peak under CO2 pressure
was significantly greater than that under N2, which proved that
it occurred redox reaction between pyridine catalyst and CO2.
In these processes, pyridine was protonated to form pyridinium
PyH+. PyH+ was reduced to form PyH+ after accept an electron
under the reduction potential. Then PyH+ could react with CO2 to
form PyCOOH+ in the homogeneous phase which becomes further
reduced into CH3OH through a series of subsequent reduction steps
[42].
To investigate the rate determined step in these processes, CV
spectra were repeated with different scanning speeds which are
shown in Fig. 7.
As shown in Fig. 7(a), the reduction current became bigger with
the increase of the scan rate. In order to study whether the electrode
was affected by adsorption and diffusion controlled, peak current
was plotted with scan speed and the square root of the scanning
rate, respectively. Fig. 7(b) and (c) show that relationship between
peak current and scan rate was linear with R2, 0.9316 which were
3.2. Evaluation of pyridine for electrocatalytic CO2 reduction
CV of pyridine is performed under N2 and CO2 pressure in Fig. 6.

4
W. Wang et al. / Applied Catalysis A: General 520 (2016) 1–6
1500
1200
900
600
300
0
-0.20
CO
(a)
0.00 μmol/L
2
(a)
1000 mV/s
5 mV/s
0.66 μmol/L
1.12 μmol/L
5.09 μmol/L
7.64 μmol/L
11.45 μmol/L
34.36 μmol/L
-0.15
-0.10
-0.05
0.00
0.05
0.10
500
550
600
650
700
750
800
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
Wavelength(nm)
Potential(V)
2.5
2.0
1.5
1.0
0.5
0.0
0.18
0.15
0.12
0.09
0.06
0.03
(b)
(b)
2
R =0.9316
2
R
=0.9914
0
4
8
12
16
20
0
200
400
600
800
1000
Concentrations of ascorbic acid(μmol/L)
-1
)
Scan rate(mV s
Fig. 5. Different concentrations of ascorbic acid quenching (a) and linear depen-
dence of emission intensity versus different concentration of ascorbic acid in
degassed acetonitrile: water (1:1) excited at 551 nm (top) and 600 nm (bottom)
at room temperature; 0.02 mmol/L Ru complex (b).
0.25
0.20
0.15
0.10
0.05
(c)
-0.06
N
2
2
R =0.9962
CO
-0.04
-0.02
0.00
0.02
0.04
2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
1/2
-1 1/2
Scan rate (mV s
)
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
Fig. 7. CV of different scan rate (a) and linear dependence of peak current versus
the scan rate (b) linear dependence of peak current versus the square root of the
scan rate from 5 to 1000 mV/s (c).
Potential(V)
Fig. 6. The reduction pattern of pyridine under N2 and CO2 pressure with concentra-
tion of 10 mmol/L pyridine, 0.5 mol/L potassium chloride solution; scan rate, 5 mV/s;
pH 5.
at approximately −0.6 V that increases with decreasing pH (hence
increasing [PyH+]). The large observable reductive feature at
approximately −0.6 V was assigned to PyH+ reduction to pyridine.
The catalytic potentials decreased when the solution pH changed
from 2 to 4 and kept stable during the pH of 4–5.3. After that,
the catalytic potentials increased with the increase of solution pH.
Therefore, the solution pH was set as 5 to keep the catalytic poten-
tial low [47].
In the process of photocatalysis, the electrolyte concentration
plays an important role. Fig. 9 shows the effect of the concentration
of pyridine on the reduction current. The difference of reduction
peaks current between CO2 and N2 mapping with the concentration
of pyridine are shown in Fig. 9(b).
same as the square root of the scanning rate following R2, 0.9962. All
the result indicated that the electrode was affected by adsorption
and diffusion controlled [43,46–49]. This means that the reaction
rate is not controlled by the number of active sites on the elec-
trode surface, but rather the spread of CO2 in the solution. At the
reduction potential of pyridine, the reaction of CO2 and pyridine
was diffusion-limited controlled. Upon addition of CO2, the dif-
fusion limited current increased remarkably, while the potential
shifted catholically, and the reversibility in the return oxidation
wave was lost due to the chemical reaction between CO2 and the
electrocatalyst [43].
As shown in Fig. 9(a), the intensity of reduction peak increased
significantly with the increase of pyridine during the low pyridine
concentration from 2.24 mmol/L to 5.24 mmol/L. In order to make
it clear that effect of the concentration of pyridine, the difference of
reduction peak currents under CO2 and N2 pressure mapping with
To study the effect of pH on the reduction current, the results
are shown in Fig. 8.
Fig. 8 shows the effect of solution pH on the catalytic cur-
rent density under CO2 pressure. An irreversible reduction peak

W. Wang et al. / Applied Catalysis A: General 520 (2016) 1–6
5
-0.20
-0.15
-0.10
-0.05
0.00
pH=5
60
50
40
30
20
10
0
(a)
pH=4
pH=2
pH=4
pH=5
pH=5.3
pH=6
pH=6
no ascorbic acid
no pyridine
0.05
0.10
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0
1
2
3
4
5
6
Potential(V)
Time(h)
Fig. 10. Methanol concentration versus pH and reaction time.
-0.20
-0.15
-0.10
-0.05
0.00
-0.70
-0.65
-0.60
-0.55
-0.50
the concentration of pyridine was studied and shown in Fig. 9(b).
With the increase of pyridine concentration, the catalytic current
difference increased till to a platform. It can be speculated that
the redox reaction CO2 may be limited by the high concentration
of pyridine [50]. The photogenerated electron from the photosen-
sitizer by the visible light irradiation should be transferred to a
reaction center before the recombination of the photoinduced elec-
tron and holes. When the active sites on the surface of the catalyst
matched the photoinduced electrons, the CO2 reduction will per-
form smoothly. If the catalyst concentration was much high than
photoinduced electrons could be generated by the photosensitizer,
it would have a negative impact on the rate of electron transfer
and catalytic effect. When pyridine concentration was higher than
6.35 mmol/L, there was another redox reaction which was not the
conversion of CO2 to methanol that need to be further studied. The
experiment results showed that the best appropriate concentration
of pyridine was 2–6 mmol/L which could be prepared with different
conditions.
(b)
Peak current
Peak potential
2
3
4
pH
5
6
Fig. 8. CV under CO2 with different pH (a) and linear of pH with peak current and
potential (b).
(a)
3.3. Experiment and reaction mechanism of photocatalytic
system
The pH value of the solution was the most important factor for
the catalytic current. Hence, in the photocatlytic reaction of the CO2
conversion, the solution pH was chosen as an effect parameter for
the product yield. The yield of methanol with different pH and time
are shown in Fig. 10.
As shown in Fig. 10, with the increase of reaction time, the yield
of methanol increased. And the system in pH 5 showed highest
catalytic efficiency than that in pH 4 and 6. When the pH was 5 and
the reaction time was 6 h, the yield of methanol reached 60 ␮mol/L.
However, the yield of methanol was negligible when pH was 6,
because H+ was the vital to the process of photocatalycal reaction
and a certain amount of H+ can enhance the activity of catalyst [34].
The product methanol could not be determined when the ascorbic
acid or pyridine were absent in the system.
(b)
Pyridine and Pyridinium (pyrH+) were the species of interest
in this work due to its proposed role in the CO2 catalytic reduc-
tion. It could be inferred that at pH = 5, pyrH+ participated in one of
the several irreversible reactions: pyridinyl radical formation, dihy-
dropyridine formation, and reduction of the weak acid protons to
dihydrogen.
Although the CO2 photoreduction mechanism is complex, it is
well known that pyridine (Py) catalyzed the CO2 reduction through
the PyCOOH• formation. There were consequential steps in this
reaction. Protonated pyridine (PyH+) could be reduced to pyridyl
radical (PyH•) after getting an electron from one electron reduced
photosensitizer. Then, the PyH• species reacted with CO2 to gener-
ate a PyCOOH• through a proton transfer process. Finally, methanol
Fig. 9. CV and current difference with pyridine concentration diagram (a) and rela-
tion between peak currents deviation through N2 and CO2 and concentration of
pyridine (b).

6
W. Wang et al. / Applied Catalysis A: General 520 (2016) 1–6
was formed from PyCOOH• through a series of proton-coupled elec-
tron transfer [42].
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The photocatalytic reduction of CO2 to methanol was per-
formed in the present pyridine catalysis system under visible
light irradiation. [Ru(phen)3](PF6)2 was prepared as a photosen-
sitizer and showed excellent visible light utilization activity and
electron transfer properties. And in electrochemical experiments,
pyridine showed clear catalytic reaction under CO2 pressure com-
pared that under N2 pressure. The solution pH had important effect
on the catalytic properties. By controlling variable methods for
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catalytic reduction of CO2 was achieved: exposure to the light of
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[Ru(phen)3](PF6)2 as the chromophore; 5.24 mmol/L pyridine as
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Acknowledgements
This work was supported by the Beijing Higher Education Young
Elite Teacher Project (No. YETP0773), the National Natural Science
Foundation of China (No.51278053 and 21373032), and grant-in-
aid from Kochi University of Technology.
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