Photocatalytic reduction of CO2to CO utilizing a stable and efficient hetero-homogeneous hybrid system

Article abstract of DOI:10.1039/c4ra08848d

The combination of the conventional inorganic semiconductor CdS and a homogeneous catalyst could reduce CO₂to CO economically under visible light with high selectivity, stability, and efficiency.

Full text of DOI:10.1039/c4ra08848d

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Photocatalytic reduction of CO2 to CO utilizing a
stable and efficient hetero–homogeneous hybrid
system†
Cite this: RSC Adv., 2014, 4, 44991
Received 18th August 2014
Accepted 8th September 2014
*
Zhigang Chai, Qi Li and Dongsheng Xu
DOI: 10.1039/c4ra08848d
www.rsc.org/advances
The combination of the conventional inorganic semiconductor CdS activated, the homogeneous process oen has high quantum
and a homogeneous catalyst could reduce CO2 to CO economically efficiencies and selectivities.2,10,11 However, despite the difficult
under visible light with high selectivity, stability, and efficiency.
separation of the catalysts, the instability of the photosensi-
tizers under long time irradiation12 strongly limits the applica-
tion of this method. In the heterogeneous process,
semiconductors generally act as the photocatalysts and CO2
molecules are reduced by the photo-generated electrons on the
surface of the semiconductor.4,13–16 The semiconductors have
better stability than the complex compounds in homogeneous
systems and they are easily to be separated from the solvents.
Nevertheless, due to the weak adsorption of CO2 on the surface
of semiconductor, the CO2 molecules could not be well acti-
vated and consequently, the quantum yields in heterogeneous
system are generally low.4,17,18
Since the homogeneous and heterogeneous processes have
high quantum efficiencies and stabilities, respectively, it is
reasonable to think that a hybrid of heterogeneous and homo-
geneous methods may be an ideal pathway with both good
stability and high efficiency. Semiconductor linked with ruthe-
nium complex has been reported to be a good photocatalytic
system to selectively reduce CO2 into HCOOH.19 In this system,
the semiconductor acts as the photosensitizer to absorb light
and the metal complex is used for activating and reducing CO2.
Because the conduction band minimum (CBM) of the semi-
conductor is more negative than the CO2 reduction potential in
complex, the photo-generated electrons in the semiconductor
can easily transfer to the metal complex and then reduce the
CO2 molecules. More recently, a cobalt-containing zeolitic imi-
dazolate framework (Co-ZIF-9) cooperating with a ruthenium-
based photosensitizer was used for CO2 conversion.20 But, the
ruthenium-based photosensitizers would lose their activities
slowly under sustained irradiation. More importantly, trietha-
nolamine (TEOA) was widely used as electron donor in most of
previous reports2,19–22 due to its ability of fast consuming of
photo-generated holes. However, much attention was paid on
the reduction reaction of CO2, and the oxidation products of
TEOA was not investigated which is indeed crucial for cost and
energy concerns. If TEOA was excessively oxidized or even
Although carbon dioxide (CO2) is widely regarded as a green-
house gas, which results in global warming, it is also one of the
most abundant carbon sources for fuels and organic materials.1
The xation and conversion of CO2 into chemical fuels or
industrial chemicals, which can be used within the current
energy and technology infrastructure, is a promising solution
from the viewpoint of realizing a sustainable society. However,
the conversion process is acceptable only if a renewable and
environmentally friendly energy source could be used for this
purpose. Photocatalytic reduction of CO2 under solar irradia-
tion (i.e. articial photosynthesis) is considered to be the most
ideal way for the conversion of CO2. Depending on the number
of electrons transferred, the reduction of CO2 can lead to the
formation of different products, such as, carbon monoxide,2
formic acid,3 formaldehyde,4 methanol,4 methane.5–7 Among of
them, carbon monoxide (CO), as one of the most important
chemical raw materials, has achieved great attentions due to its
various industrial applications; for instance, Fischer–Tropsch
synthesis.8
Typically, the photocatalytic reduction of CO2 can be divided
into two types: one is homogeneous process and the other is
heterogeneous process. In the homogeneous photocatalytic
process, metal complex compounds are usually used for
absorbing light and reducing CO2. The light absorption and
catalytic reduction process could be accomplished in a single
molecule or two molecules.9 Because CO2 could be coordinated
to the metal center of the complex molecules and thus be
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for
Structural Chemistry of Unstable and Stable Species, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, China. E-mail: dsxu@
pku.edu.cn; Fax: +86 10 62760360; Tel: +86 10 62760360
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08848d
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 44991–44995 | 44991

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mineralized into CO2, the whole reaction was not economical. is estimated to be 2.42 eV, which is consistent with the band gap
Furthermore, no light-energy would be stored in the whole (2.42 eV) of bulk CdS. The tail absorption in its red region may
reaction. Therefore, a mild oxidation of TEOA to a more valu- originate from the scattering of CdS particles. High resolution
able chemical seems an optimal choice. In order to achieve the transmission electron microscopy (HRTEM) image (Fig. S1†)
purpose, both appropriate valence band position and reaction shows that the size of CdS nanocrystals ranges from 5 to 8 nm,
condition are needed.
which is consistent with the XRD result.
Herein, we report an economical, stable and efficient hybrid
In the hybrid system, CdS nanoparticles and Co-bipy
catalytic system for photoreduction of CO2, which is obtained by complex were all added into the acetonitrile solution. The
simply mixing CdS nanoparticles (heterogeneous photosensi- photocatalytic experiments of CO2 reduction were carried out by
tizer) and CoCl2/2,20-bipyridine (Co-bipy, homogeneous cata- using visible light irradiation (l > 420 nm) with triethanolamine
lyst, n(bipy)/n(Co2+) ¼ 1) in an acetonitrile solution. It was found (TEOA) as electron donor under a pressure of 4 atm. When the
that TEOA was mildly oxidized rather than photo-degraded, and photocatalytic process started, large amounts of CO (23 mmol
it is estimated that about 43 kJ molꢀ1 of solar energy are stored per 3 h) was generated with a small amount of H2 (3 mmol per 3
in the photocatalytic reaction. Under visible light irradiation, h). Studies on the CO/H2 evolution as a function of irradiation
the CO2 molecules could be selectively reduced into CO with an time shows that the relationship between the amount of CO/H2
apparent quantum yield as high as 1.0% at l ¼ 470 nm. We produced and the irradiation time is linear (Fig. 2b). The rate of
believe the use of organic solvent (acetonitrile) and the fast CO and H2 evolution per gram of CdS was calculated to be 844
interfacial electron transfer increase the stability of the whole and 122 mmol hꢀ1, respectively. Although photocatalytic reac-
system. Moreover, the ratio of CO and the byproduct H2 could tion rates cannot be accurately compared, the above rates was
easily be tuned from 0.7 to 7.0 in this system, which is very found to be about 7-fold higher than that in the chemical
helpful for their applications as syngas.
bonded system.19 The total amount of CO was 62 mmol aer 8 h,
The cadmium sulde nanoparticles was prepared from thus the corresponding turnover number (TON) was calculated
sodium sulde and cadmium sulfate in an aqueous ammonia to be 4.1 with respect to the amount of Co2+. The measured
solution according to the previously reported method.23 The apparent quantum yield of CO formation at 1 atmosphere was
structure of the synthesized CdS nanocrystals is characterized 1.0% under monochromatic irradiation at l ¼ 470 nm. Since no
by X-ray powder diffraction (XRD). The pattern is illustrated in activity decreasing was observed within 8 h irradiation, it is
Fig. 1a and it could be indexed as pure zinc blende (cubic) phase indicated that the hybrid system is stable (Fig. 2b). Above results
of CdS, (JSPDS no. 10-0454) with prominent peaks corre- reveal that the hybrid system is an efficient and stable system
sponding to the reections at (111), (220), and (311) planes, for photocatalytic CO2 reduction. For comparison, we have done
respectively. The broadened peaks indicates the small crystallite three types of control experiments. When the reaction was
size of the as-prepared CdS. When the CdS powder is dispersed performed in dark, no CO or H2 was detected, which clearly
in methanol, a yellow dispersion is obtained (inset of Fig. 1b). indicated that the production of CO and H2 depended on the
Fig. 1b shows the UV-vis absorption spectrum of the CdS light. When CdS alone was used, large amounts of H2 (9.5 mmol
dispersed in methanol. The most direct way of extracting the per 3 h) was produced and only trace amounts of CO was
optical band gap is to simply determine the wavelength at which detected. This result suggests that the CdS particles could easily
the extrapolations of the base line and the absorption edge generate H2 from the oxidative dehydrogenation of TEOA.
cross. Using this method, the band gap absorbing edge is about However, due to the poor adsorption of CO2 on their surfaces
512 nm. Correspondingly, the band gap of the as-obtained CdS and strongly negative single-electron reduction potential of
Fig. 1 Characterization of the CdS nanocrystals: (a) XRD pattern. (b) UV-vis absorbing spectrum measured under transmission mode. Inset
shows the image of the CdS suspension.
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Fig. 4 Influence of bipy/Co2+ ratio on CO/H2 generation.
abundance) was assigned to be molecular ion peak of 13CO. The
above results approved that the produced CO in our hybrid
organic system only comes from the reduction of CO2.
The effect of bipy and Co2+ on photocatalytic reduction of
CO2 was investigated. Fig. 4 shows the ratio of bipy to Co2+
affects both CO and H2 generation. When bipy was absence,
large amounts of H2 was generated while CO evolution was
inhibited. Aer addition of equivalent amounts of bipy, the
amount of CO evolution was dramatically increased, while the
amount of H2 evolution was decreased. This can be attributed to
2+
Fig. 2 (a) Control experiments after 3 h irradiation. (b) Gas produced
as a function of irradiation time for CdS/Co-bipy.
the fact that formation of Co(bipy)x can decrease the reduc-
tion potential of Co2+/Co+. Therefore, the electron-transfer from
CdS to Co center was accelerated, which was favorable for CO2
reduction. When the ratio of bipy to Co2+ was increased from 1
to 3, CO generation was decreased slightly which may be due to
lack of vacant coordination site for CO2 binding at the Co
center. The highest CO selectivity was obtained when n(bipy)/
n(Co2+) ¼ 1.
CO2,24 CdS can hardly reduce CO2 to CO. When Co-bipy alone
was used, no CO or H2 was produced, indicating that the Co-
bipy complex itself has no photocatalytic activity.
To verify the source of CO, 13C labeled experiment was per-
formed by using 13CO2 as reactant. The evolution of CO was
analyzed by gas chromatography mass spectrometry (GC-MS).
Fig. 3 is the mass spectra of CO peak obtained under 12CO2 and
13CO2, respectively. Under 12CO2 (99% abundance), m/z ¼ 28
(99% abundance) was assigned to be the molecular ion peak of
The inuence of CO2 pressure in our photocatalytic system
was also examined. Fig. 5 shows that the production of CO
increased with increasing pressure of CO2 while the production
of H2 decreased. It was reasonably to think that since the
reduction reactions of H+ and CO2 are competitive in the same
system, higher CO2 concentration in solution is more favorable
12CO. Under 13CO2 (98% abundance), m/z
¼
29 (98%
Fig. 3 Mass spectrum of CO peak obtained under 12CO2 and 13CO2,
respectively.
Fig. 5 Amount of CO and H2 as a function of CO2 pressure.
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Scheme 1 Left: photos of the solution before and after irradiation. Right: possible mechanism of photocatalytic CO2 reduction in the hybrid
system. S: solvent or bipy.
for CO2 reduction. As we know, CO/H2 mixture can be used to
The high efficiency and stability of the hybrid system may be
synthesize fuels and other valuable chemicals. Herein, the ratio due to the following aspects. Firstly, the position of CBM of CdS
of CO/H2 mixture can be easily modulated from 0.7 to 7 by (ꢀ0.9 V vs. NHE, pH ¼ 7)28 is more negative than the potential of
controlling pressure of CO2, which makes them have potential CO2/CO (ꢀ0.53 V vs. NHE, pH ¼ 7), therefore photocatalytic
applications in chemical industry.
reduction of CO2 into CO by CdS is thermodynamically feasible.
A possible mechanism (Scheme 1) of photocatalytic CO2 Besides, due to the activation of CO2 through coordination to
reduction in the hybrid system was proposed based on the Co center, the rate of CO2 reduction was dramatically increased
previous assumption.25 First of all, CdS particles absorb light (Fig. 2a). Secondly, CdS suffers from stability problem when O2
and generate active electron–hole pair. For the reduction part, or H2O exists.29 And, in aqueous, CdS could generate H2 at a
the electron reduces CoII to CoI which is the active intermediate. high rate when using sacricial agent.30,31 In present system,
We observed the evidence of the generation of CoI (an air- because organic solvent was used and O2 was absent, the
sensitive dark blue solution, UV-vis sepectra was shown in stability problem of CdS due to O2 and H2O was avoided.
Fig. S4†) which could also be obtained by chemical reduction of Besides, the photo-generated electrons of CdS can be quickly
[Co(bipy)3]2+ with Na–Hg.26 Then, CO2 coordinates to CoI consumed by Co-bipy, meanwhile the holes can be quickly
through electrophilic attack.25 The subsequent decomposition consumed by TEOA, thus, the fast interfacial electron transfer
of CoIII–COOH complex forms CoIII–CO complex. CO is gener- (IFET) also inhabits the photo-corrosion of CdS. The high
ated through a ligand-dissociation reaction. Eventually, CoIII stability of CdS in our system was conrmed by the cyclic
was reduce to the primary CoII by either electron in CdS or CoI. experiment (Fig. S5†), and no activity decrease for CO genera-
For the oxidation part, TEOA was oxidized by the hole in CdS. tion was found aer eight cycles. The above aspects result in the
The primary oxidation product of TEOA was generally consid- high efficiency and stability of the hybrid system.
ered to be TEOA+.27 Through analyzing the solution aer irra-
diation by GC-MS (see Fig. S3†), the nal oxidation product of
TEOA was identied as N,N-bis(2-hydroxyethyl)glycine (Scheme
S1B†) which is a more valuable chemical than TEOA. This
Conclusions
In summary, a hybrid photocatalytic system has been fabricated
to selectively reduce CO2 to CO under visible light irradiation.
Our results suggest that the mixture of metal complex and CdS
nanoparticles could offer an economical, efficient and stable
system. The measured apparent quantum yield of CO formation
at 1 atmosphere was as high as 1.0% under monochromatic
irradiation at l ¼ 470 nm. We believe that the photocatalytic
CO2 reduction based on CdS in organic phase using the hybrid
system could be further extended to couple with a specic
oxidation reaction of organic molecules. Therefore, it is helpful
for developing a new atomically economical and efficient arti-
cial photosynthesis system.
means the oxidation reaction is economically acceptable. Due to
the appropriate valence band position of CdS and the absence
of water and oxygen, TEOA was mildly oxidized rather than
photo-degraded. It is reasonable to speculate that the nal
oxidation product may originate from further the oxidation of 2-
(bis(2-hydroxyethyl)amino)acetaldehyde (A) when it was
exposed in air, because the possible mechanism of generation
of A from TEOA was proposed in previous report.27 If this is the
case, it is estimated that about 43 kJ molꢀ1 of solar energy are
stored in reaction (1) through molecular mechanics calculation
(Table S1†).
(1)
Acknowledgements
The possible mechanism of the oxidation of TEOA is showed
in Scheme S1.† H2 originates from reduction of protons by two
ways: one is from CdS; one is from the Co-bipy.
This work is supported by NSFC (Grant nos 50821061,
51121091, 21133001), MSTC (Grant nos 2014CB239303,
2013CB932601, 2011CB808702).
44994 | RSC Adv., 2014, 4, 44991–44995
This journal is © The Royal Society of Chemistry 2014