Cobalt phthalocyanine immobilized on graphene oxide: An efficient visible-active catalyst for the photoreduction of carbon dioxide

Article abstract of DOI:10.1002/chem.201304189

New graphene oxide (GO)-tethered-Co^II phthalocyanine complex [CoPc-GO] was synthesized by a stepwise procedure and demonstrated to be an efficient, cost-effective and recyclable photocatalyst for the reduction of carbon dioxide to produce methanol as the main product. The developed GO-immobilized CoPc was characterized by X-ray diffraction (XRD), FTIR, XPS, Raman, diffusion reflection UV/Vis spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR, XPS, Raman, UV/Vis and ICP-AES along with elemental analysis data showed that Co^II-Pc complex was successfully grafted on GO. The prepared catalyst was used for the photocatalytic reduction of carbon dioxide by using water as a solvent and triethylamine as the sacrificial donor. Methanol was obtained as the major reaction product along with the formation of minor amount of CO (0.82%). It was found that GO-grafted CoPc exhibited higher photocatalytic activity than homogeneous CoPc, as well as GO, and showed good recoverability without significant leaching during the reaction. Quantitative determination of methanol was done by GC flame-ionization detector (FID), and verification of product was done by NMR spectroscopy. The yield of methanol after 48 h of reaction by using GO-CoPc catalyst in the presence of sacrificial donor triethylamine was found to be 3781.8881 μmolg^-1cat., and the conversion rate was found to be 78.7893 μmolg^-1cat.h^-1. After the photoreduction experiment, the catalyst was easily recovered by filtration and reused for the subsequent recycling experiment without significant change in the catalytic efficiency. Very photoactive! Cobalt phthalocyanine grafted to the chemically functionalized graphene oxide was found to be an efficient heterogeneous visible-light-induced photoredox catalyst for the photoreduction of carbon dioxide to methanol in a very good yield. The developed photocatalyst exhibited superior activity compared with the existing photocatalytic systems and gave methanol as the major reaction product (see scheme).

Full text of DOI:10.1002/chem.201304189

DOI: 10.1002/chem.201304189
Full Paper
&
Heterogeneous Catalysis
Cobalt Phthalocyanine Immobilized on Graphene Oxide: An
Efficient Visible-Active Catalyst for the Photoreduction of Carbon
Dioxide
Pawan Kumar,[a] Arvind Kumar,[b] Bojja Sreedhar,[c] Bir Sain,[a] Siddharth S. Ray,[b] and
Suman L. Jain*[a]
Abstract: New graphene oxide (GO)-tethered–CoII phthalo-
cyanine complex [CoPc–GO] was synthesized by a stepwise
procedure and demonstrated to be an efficient, cost-effec-
tive and recyclable photocatalyst for the reduction of carbon
dioxide to produce methanol as the main product. The de-
veloped GO-immobilized CoPc was characterized by X-ray
diffraction (XRD), FTIR, XPS, Raman, diffusion reflection UV/
Vis spectroscopy, inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES), thermogravimetric analysis
(TGA), Brunauer–Emmett–Teller (BET), scanning electron mi-
croscopy (SEM), and transmission electron microscopy (TEM).
FTIR, XPS, Raman, UV/Vis and ICP-AES along with elemental
analysis data showed that CoII–Pc complex was successfully
grafted on GO. The prepared catalyst was used for the pho-
tocatalytic reduction of carbon dioxide by using water as
a solvent and triethylamine as the sacrificial donor. Methanol
was obtained as the major reaction product along with the
formation of minor amount of CO (0.82%). It was found that
GO-grafted CoPc exhibited higher photocatalytic activity
than homogeneous CoPc, as well as GO, and showed good
recoverability without significant leaching during the reac-
tion. Quantitative determination of methanol was done by
GC flame-ionization detector (FID), and verification of prod-
uct was done by NMR spectroscopy. The yield of methanol
after 48 h of reaction by using GO–CoPc catalyst in the pres-
ence of sacrificial donor triethylamine was found to be
3781.8881 mmolgꢀ1 cat., and the conversion rate was found
to be 78.7893 mmolgꢀ1cat.hꢀ1. After the photoreduction ex-
periment, the catalyst was easily recovered by filtration and
reused for the subsequent recycling experiment without sig-
nificant change in the catalytic efficiency.
Utilization of carbon dioxide as a sustainable C1 building block
has emerged as an important area of research in last two dec-
ades mainly due to the concurrently problems of global warm-
ing, as well as depletion of fossil fuels.[1] The effective use of
clean and abundant solar energy might provide a viable ap-
proach to solve the problems of energy and environmental
crisis by a photocatalytic reduction of carbon dioxide to valu-
able chemicals. Although a number of improved catalytic sys-
tems, such as nanosized titanium oxide, titanium oxide doped
with transition metals, noble metals, and mixed with another
metal oxide have been emerged as potent catalysts for the
photoreduction of CO2, but their quantum yields and selectivi-
ties of products are low.[2–3] In the recent years, photocatalytic
systems, including transition-metal complexes, such as ruthe-
nium(II) polypyridine carbonyl complex, cobalt(II) trisbipyridine,
cobalt(III) macrocycles, rhenium and iridium complex with
a photosensitizer have emerged to be efficient catalytic sys-
tems to reduce CO2 with relatively high quantum yield and
high selectivity of products.[4] However, the difficult recovery
and non-recycling ability of a homogeneous complex make
these systems less attractive. To overcome these problems, im-
mobilization of homogeneous metal complexes to solid sup-
port is an obvious way to combine the advantages of homoge-
neous catalysts, for example, high reactivity, selectivity, and
heterogeneous catalysts, such as facile recovery and recycling
of the catalyst.[5] In this context, a number of homogeneous
photoredox complexes have been supported to various sup-
ports, such as TiO2, organic polymers, ion-exchange resins.
However, in most of the cases, noncovalent attachment of the
metal complex to support materials renders to the leaching of
the active species during the reaction.[6]
[a] P. Kumar, Dr. B. Sain, Dr. S. L. Jain
Chemical Sciences Division, CSIR-Indian Institute of Petroleum
Mohkampur, Dehradun 248005 (India)
Fax: (+91)135-2660202
E-mail: suman@iip.res.in
[b] A. Kumar, Dr. S. S. Ray
Analytical Sciences Division, CSIR-Indian Institute of Petroleum
Mohkampur, Dehradun 248005 (India)
[c] Dr. B. Sreedhar
Inorganic and Physical Chemistry Division
CSIR-Indian Institute of Chemical Technology
Hyderabad, 500607, Andhra Pradesh (India)
Metallophthalocyanines and their derivatives have been
widely used as photosensitizers for various applications in a va-
riety of fields due to their unique properties, such as excellent
semiconductivity, photoconductivity, thermal, and chemical
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304189.
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stability. Most of the studies conducted to date on metal
phthalocyanines for photocatalytic reduction of CO2 were per-
formed either in homogeneous solution or by supporting
them to other semiconductor materials, such as TiO2.[7] In this
context, Zhou et al.[8] reported the use of in situ synthesized
TiO2/CoPc nanocomposite for the photocatalytic reduction of
carbon dioxide to formic acid, formaldehyde, and methanol.
Graphene oxide (GO), an oxidized form of grapheme, has
emerged to be a promising material, particularly as a precursor
to prepare reduced GO (rGO), chemically functionalized gra-
phene, and grapheme-based composite materials.[9,10] Very re-
cently, Hsu et al.[11] have reported GO as a promising photocat-
alyst for the conversion of CO2 to methanol with the conver-
sion rate of 0.172 mmolgꢀ1 cat.ꢀ1 hꢀ1 under visible light.
vide great potential as a support material for various cata-
lysts.[13] Prior to the immobilization, the GO was treated with
chloroacetic acid to convert the epoxy groups into carboxy
(COOH) groups, which were subsequently used for the grafting
of CoPc to the GO surface. The schematic representation of
the synthesis of CoPc immobilized on GO (GO–CoPc) is shown
in Scheme 1.
The morphology and structure of as-prepared GO and GO–
CoPc catalyst was investigated by scanning electron microsco-
py (SEM) and transmission electron microscopy (TEM). As pre-
sented in Figure 1b, the SEM image of GO–CoPc shows twisted
Herein, we describe the successful synthesis of covalently
anchored cobalt phthalocyanine to the GO, its characterization,
and use for the photocatalytic reduction of the carbon dioxide
to methanol with improved product yield and selectivity. The
oxygen functionalities on GO are targeted for stable and effi-
cient anchoring of CoPc catalyst on nanosheets of GO through
covalent attachment. The prepared catalyst exhibited higher
photocatalytic activity under visible-light irradiation and gave
methanol with improved yield and selectivity. The quantitative
and qualitative determination of the methanol formation was
Figure 1. SEM image of a) GO and b) GO-CoPc.
1
performed by GC and H NMR spectroscopy.
and crumpled nanosheets in an agglomerated phase with lot
of wrinkled features of GO–CoPc catalyst. The TEM images
shown in Figure 2a and b also reveal the nanoscopic features
with layered structure of GO–CoPc catalyst (Figure S1 in the
Supporting Information). The TEM image of carboxylic group
(COOH) containing GO before immobilization of CoPc is shown
in Figure 2c. The dark spots in the TEM image (Figure 2c) are
probably due to the COOH functionalities located on the sur-
Synthesis and characterization of catalyst
GO was obtained from the oxidation of graphite with KMnO4
and H2SO4 following the literature procedure.[12] High specific
surface area of GO along with easily accessible ample oxygen
functionalities decorated on both side of GO nanosheets pro-
Scheme 1. Synthesis of GO-grafted CoPc.
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broad diffraction of graphite (002) at 2q value of approximately
268 appeared, indicating that exfoliation of the layered CoPc–
GO was obtained.[15]
The porosity characteristics of the samples were investigated
by measuring the N2 adsorption/desorption isotherms at 77 K
(Figure 4). Total pore volume and surface area of synthesized
Figure 2. TEM image of a) GO; b) GO-CoPc; c) GO-COOH; and d) SAED pat-
tern of GO-CoPc.
face of GO. Selected area electron diffraction pattern of GO–
CoPc shows that material was amorphous in nature (Fig-
ure 2d).
XRD patterns were used to study the changes in structure
(Figure 3). The XRD spectrum of GO shown in Figure 3a gives
characteristic diffraction peak at 10.88 (001), corresponding to
a GO interlayer spacing of approximately 0.74 nm.[14] After im-
mobilization of CoPc, this peak disappeared, and another
Figure 4. Adsorption/desorption curve of a) GO and b) GO-CoPc.
GO was found to be 0.1212 cm3 gꢀ1 and 87.247 m2 gꢀ1, respec-
tively. Total pore diameter of GO was found to be 5.5566 nm
that was in agreement to mesoporous nature.[16] The adsorp-
tion/desorption isotherm was of type IV (Figure 4). The negligi-
ble N2 adsorption in GO and GO–CoPc suggested the filled in-
terlayer galleries and the restacked graphitic structure in GO.
Obtained value of SBET 12.34 m2 gꢀ1 for GO–CoPc suggested
successful attachment of CoPc at the surface of GO. Total pore
volume and mean pore diameter of GO–CoPc was measured
Figure 3. XRD spectra of a) GO and b) GO-CoPc.
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to be 0.09 cm3 gꢀ1 and 30.158 nm, respectively (Figure S2 in
the Supporting Information).
The UV/Vis spectra of GO and GO–CoPc are shown in
Figure 5. The UV/Vis spectra of CoPc reveals two main bands,
Figure 6. IR spectrum of a) CoPc; b) GO; and c) GO-CoPc.
Further XPS and Raman analyses provided essential and
useful information for the covalent attachment of the CoPc to
GO. The N1s XPS spectrum of CoPc clearly indicated that the
peaks of nitrogen functionalities appeared at 398.95 (CꢀN
bonds), 400.43 (NH2), and 401.58 eV (C=N bonds; Figure 7a). In
contrast, CoPc-immobilized GO showed the shifting of bands
and a considerable intensity enhancement of the peak at
400.69 eV due to the electron-withdrawing effect of GO (Fig-
ure 7b).
Furthermore, in the Raman spectrum of GO, there are two
prominent bands at about 1363 (D band) and 1608 (G band)
cmꢀ1 (532 nm excitation; Figure 8a). In contrast, both the D
and G bands of GO–CoPc were found to be slightly shifted to
the lower-wave numbers, and a new band at 1512 cmꢀ1 was
assigned to pyrrole C=C stretching mode of CoPc (Figure 8b).
Figure 9 shows the TGA curves of the as-prepared GO and
GO–CoPc catalysts. The GO showed initial weight loss near
1088C, evidently owing to evaporation of water molecules,
which are held in the material (Figure 9a). The second signifi-
cant weight loss was observed in the range of 180–2408C, and
is due to thermal decomposition of oxygen-carrying functional-
ities. The thermogram of the GO–CoPc catalyst illustrates an
exothermic major weight loss over a wide range of tempera-
ture (300–5008C) on account of the slow decomposition of the
phthalocyanine moieties: this indicates that the catalyst is suffi-
ciently thermally stable.
Figure 5. UV/Vis spectra of a) CoPc; b) GO; and c) GO-CoPc.
that is, Soret band at l=300 nm and a Q band at 660 nm (Fig-
ure 5a).[16] The UV/Vis spectra of GO shows a characteristics
peak at nearly 230 nm, which suggests that the synthesized
GO possess an ordered graphitic structure due to the p–p*
transition of C=C(Figure 5b). Solid UV/Vis spectra of GO-at-
tached CoPc indicates the significant shifting of the absorption
band of GO to a longer wavelength at 300 nm, which clearly
indicates that CoPc was successfully attached to GO (Fig-
ure 5c).
Figure 6 shows the FTIR spectra of as-prepared GO and GO–
CoPc catalysts. The FTIR spectrum of GO shows vibrational
bands at approximately 3410 cmꢀ1 due to hydroxyl stretching
vibrations in COOH and/or intercalated water (Figure 6b). The
The photocatalytic reduction of CO2
strong band at 1720 cmꢀ1 (n˜C O) belongs to carboxylic acid
=
groups, 1630 cmꢀ1—to aromatic C=C, and the bands at
1227 cmꢀ1 and 1058 cmꢀ1 are attributed to the CꢀOH and
alkoxy CꢀO stretching vibrations, respectively. After the immo-
bilization reaction, several new peaks appeared in the FTIR
spectrum of GO–CoPc. The characteristic peak of C=O at
1720 cmꢀ1 disappeared, and a new peak corresponding to
amide (C(O)NH) stretching vibration at n˜ =1654 cmꢀ1 revealed
the formation of amide bond between CoPc and GO.
First, the reactivity of the GO, physical 1:1 mixture of GO+
CoPc and GO–CoPc, was tested for the photocatalytic reduc-
tion of the CO2 by using triethylamine as sacrificial donor and
water as a solvent (Figure 10). After the photoreduction, 1 mL
liquid sample was withdrawn and analyzed in a GC-FID appara-
tus equipped with a 30 m long Stabilwax w/Integra-Guard
column. Methanol yield was used to evaluate the performance
of the catalysts, because it was obtained as the major reduc-
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Figure 9. TGA pattern of a) GO and b) GO-CoPc.
Figure 7. N1s XPS spectra of a) CoPc and b) GO-CoPc.
Figure 10. Reaction illumination conditions.
shown in Figure 11. The results clearly indicate that the GO–
CoPc exhibited higher catalytic activity compared to the GO
and the physical 1:1 mixture of GO–CoPc. After illumination for
48 h in visible light by using GO–CoPc as catalyst in the pres-
ence of triethylamine as sacrificial donor, the yield of methanol
was found to be 3781.8881 mmolgꢀ1cat. or production rate
78.7893 mmolgꢀ1cat.hꢀ1. The analysis of gaseous phase by GC
showed 99.17% CO2 and 0.82% CO after 48 h of the reaction.
In the GC analysis of gaseous phase, we did not observe the
presence of methane and other possible products from reduc-
tion of carbon dioxide. Formic acid, formaldehyde, and formate
were not detected in liquid-phase analysis. Blank experiments,
in the absence of GO–CoPc, as well as in the dark reaction,
showed that there was no organic product found for long peri-
ods of CO2 photoreduction.
Figure 8. Raman spectra of a) GO and b) GO-CoPc.
tion product. The methanol-formation rate, RMeOH (mmolgꢀ1 cat.)
Further, we tested the recycling of the GO–CoPc catalyst for
the photoreduction of the CO2. After completion of the reac-
as a function of reaction time was calculated and plotted as
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GO. This significantly promoted the reaction and greatly en-
hanced the photoefficiency of CO2 with H2O to produce
CH3OH.[24] Main advantage of attachment of CoPc to central
plane of GO was better charge injection to conduction band of
GO. Further, the use of sacrificial donor (triethylamine, TEA)
that provides electrons to phthalocyanine also increased yield
of methanol.[25]
*
CoPc þ visible light ¼ CoPc ðexcited singlet stateÞ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
þ
ꢀ
*
CoPc ¼ CoPc þ e GO ðelectron transfer in CBÞ
CoPcþ þ TEA ¼ CoPc þ TEAþ
eꢀGO ðelectron in CBÞ þ CO2 þ Hþ ¼ CH3OH
Figure 11. Methanol-conversion rate for a) GO; b) GO:Co-Pc (1:1); and c) GO-
CoPc.
In conclusion, we have demonstrated that GO-immobilized
cobalt phthalocyanine is an effective, recyclable, and cost-ef-
fective photocatalyst for the photoreduction of CO2 to metha-
nol. Our studies showed that the presence of CoPc to the GO
surface play an important role, the methanol conversion rate
can be achieved up to 3781.8881 mmolgꢀ1 cat. under visible-
light irradiation for 48 h. These conversion rates are much
higher than that of GO. The chemical attachment of CoPc on
GO surface provides the higher yield of the desired methanol
compared to the physical mixing of the GO and CoPc in 1:1
ratio. CoPc can be promoted by visible light to produce elec-
tron–hole pairs. Attachment of CoPc to GO prohibits the re-
combination of hole and electron. In addition, CoPc rapidly
transfers excited electrons to the conduction bands of the GO,
which provides the efficient photoreduction of the CO2 with
water to produce methanol.
tion, the catalyst was separated by centrifugation and reused
for the subsequent run. ICP analysis of recovered catalyst after
one cycle showed cobalt content 1.05 wt%, indicating that
small leaching had occurred during the reaction.
Moreover, the possible photocatalytic mechanism of the
GO–CoPc has been demonstrated for the better understanding
of the CO2 photoreduction process (Scheme 2). It is reported in
Experimental Section
Materials
Graphite flakes, cobalt(II) phthalocyanine (97%), and triethylamine
(99%) were purchased from Sigma–Aldrich. Potassium permanga-
nate 99.0%, sodium nitrate 99.0%, concentrated sulfuric acid, hy-
drogen peroxide (30%), hydrochloric acid, chloroacetic acid, NaOH,
thionyl chloride, chlorosulfonic acid (99%), acetonitrile, and HPLC-
grade water were purchased from Merck (India). All chemicals and
solvents were of analytical grade and used as received. Cobalt–
phthalocyaninetetrasulfonamide complex was synthesized by fol-
lowing the literature procedure.[26]
Scheme 2. Possible mechanistic pathway of CO2 reduction.
the literature that presence of various oxy functionalites in GO
creates a 2D network of infintite sp2 and sp3 domains.[18,11] Due
to presence of these domains on GO sheet it behaves as it has
enormous semiconductor devices on its surface.[19] These sp2
carbon atoms in sheets domain have properties similar to gra-
phene, and electron can flow in this region with minimum re-
sistance: it works like a conduction band. Because the nature
of sp3 carbon atoms is such that the associated electrons are
not mobile, these domains serve as a valance band. GO works
like a semiconductor with valance and conduction bands.[20–22]
Because CoPc absorbs strongly in visible region, Pc got excited
after absorbing photons and formed S0 to S1 state.[23] CoPc rap-
idly transferred excited electrons to conduction band (CB) of
GO, and then CO2 adsorbed on GO surface can accept elec-
trons that are to be reduced. We assumed that the covalent at-
tachment of CoPc to the functional groups of GO provides the
reduced band gap between valence and conduction bands of
Techniques used
UV/Vis absorption spectra for GO and GO–CoPc dispersions were
recorded with a Perkin–Elmer lambda-19 UV/Vis-NIR spectropho-
tometer by using a 10 mm quartz cell. Solid-state spectra were re-
corded by using BaSO4 as a reference. FTIR analysis was conducted
by Perkin–Elmer Spectrum RX-1 IR spectrophotometer. HR-TEM of
the GO and GO–CoPc was executed by using FEI-TecnaiG2 TwinTEM
operating at an acceleration voltage of 200 kV. SEM analysis was
performed by Jeol Model JSM-6340F. For FE-SEM analysis, aqueous
dispersions of GO and GO–CoPc were deposited on the glass
slides, whereas very dilute aqueous suspensions were deposited
on carbon-coated copper grids for HR-TEM analysis. X-ray powder
diffraction (XRD) analyses were carried out by using a Bruker D8
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Synthesis of carboxylated GO[27]
Advance diffractometer at 40 kV and 40 mA with CuKa radiation
(l=0.15418 nm). Sample for XRD was prepared by the deposition
of well-dispersed graphene–CoPc on glass slide followed by
drying; the analysis was performed by using cobalt as the target
material. Thermogravimetric analyses (TGA) of these samples were
carried out by using a thermal analyzer TA-SDT Q-600. All samples
were analyzed in the temperature range of 40 to 9008C at a heat-
ing rate of 108Cminꢀ1 under the nitrogen flow. The porous proper-
ties of GO and GO–CoPc catalysts were examined by N2 adsorp-
tion/desorption isotherms at 77 K and the related data (surface
area, SBET; pore volume, PV; Micromeritics ASAP 2010) were calcu-
lated. XPS measurements were obtained on a KRATOS-AXIS 165 in-
strument equipped with dual aluminum–magnesium anodes by
using MgKa radiation (hn=1253.6 eV) operated at 5 kV and 15 mA
with pass energy 80 eV and an increment of 0.1 eV. To overcome
the charging problem, a charge neutralizer of 2 eV was applied
and the binding energy of C1s core level (BEffi84.6 eV) of adventi-
tious hydrocarbon was used as a standard. The XPS spectra were
fitted by using a nonlinear square method with the convolution of
Lorentzian and Gaussian functions, after a polynomial background
was subtracted from the raw spectra. The conversions and selectiv-
ity of the products were determined by high-resolution GC-FID
(Varian CP-3800). 1H NMR spectra of the products were recorded
on 500 MHz by using Bruker Avance-II 500 MHz instrument. ICP-
AES analysis was carried out by inductively coupled plasma atomic
emission spectrometer (ICP-AES, DRE, PS-3000UV, Leeman Labs
Inc., USA). Samples for ICP-AES were prepared by leaching out
0.01 g of sample with HNO3 (conc.), and then heated for 30 min
and volume to 10 mL.
In a typical experiment, GO (200 mg) was dispersed in deionized
water (100 mL) by using ultrasonicator to give a concentration of
2 mgmLꢀ1. The obtained suspension was treated with NaOH
(1.2 g) and chloroacetic acid (1.0 g) for 3 h over ultrasonicator bath
to convert the OH groups into COOH groups. The resulting GOꢀ
COOH product was neutralized by using a dilute HCl solution and
purified by repeated washing with deionized water and dried
under vacuum.
Immobilization of CoPc–tetrasulfonamide to carboxylated
GO[28,29]
Carboxylated GO (GOꢀCOOH; 200 mg) was added in DMF (1 mL)
and treated with excess thionyl chloride at 658C for 12 h to obtain
its acyl chloride derivative. Unreacted thionyl chloride was re-
moved by vacuum distillation. The product was washed with THF
(three times) and vacuum dried. The obtained GO-Cl (100 mg) was
heated at reflux with cobalt phthalocyanine tetrasulfonamide [Co-
Pc-(SO2NH2)4 complex (100 mg) in DMF under nitrogen atmos-
phere. The GO-immobilized CoPc complex was separated from the
mixture by filtration, washed with ethanol (3–4 times) by using
centrifugation and membrane filtration. The GO immobilized CoPc
complex was dried in an oven. The analytical values for the GO-
supported CoPc were found to be C 59.67, H 3.073, N 5.468, and S
4.75 %. The cobalt content in synthesized GO–CoPc was estimated
by ICP-AES analysis, and the value was found to be 1.13 wt% Co.
Based on the value of cobalt, the loading of the CoPc in GO–CoPc
After the photoreduction, the mixture was analyzed by GC. The
gaseous phase was analyzed by GC-thermal conductivity detector
(TCD) and GC-FID (Agilent 7890 A GC system) by using a column
(RGA, refinery gas analyzer) at the flow rate (H2 35, air 350, make-
up flow 27 mLminꢀ1, for TCD reference flow-45 mLminꢀ1, helium
flow 2 mLminꢀ1), injector temperature 2208C, TCD detector tem-
perature and FID detector temperature(2208C). Liquid samples
were analyzed with GC-FID model Varian CP3800 (column specifica-
tion: Stabilwax w/Integra-Guard, length 30 m, 0.25 ID) at the flow
rate 0.5 mLminꢀ1, injector temp 2508C, FID detector temperature
2758C. Autosampler (model no. Varian CP 8410) was used for in-
jecting the same amount of sample (1 mL). Calibration curve was
plotted with the help of auto sampler by injecting one microliter
sample (Figure S3 in the Supporting Information).
was found to be 2.3 mmolgꢀ1
.
Photocatalytic CO2-reduction experiment
Photocatalytic experiment was performed in borosil cylindrical
vessel (100 mL) of 5 cm diameter (Figure 1). Photoirradiation was
carried out under visible light by using 20 W white cold LED flood
light (model no. HP-FL-20W-F-Hope LED Opto-Electric Co., Ltd.). In-
tensity of the light at vessel was measured by intensity meter and
was found to be 75 Wmꢀ2. The vessel was charged initially with
water (40 mL) and triethylamine (10 mL), and then the solution
was degassed by continuous purging of nitrogen for 15 min. CO2
was bubbled through the solution for at least 30 min to saturate
the solution, then catalyst (100 mg) was added to the above-de-
scribed solution. The vessel was tightly closed during the reaction
and stirred continuously by a magnetic stirring bar to prevent sedi-
mentation of the catalyst. Samples were collected after every 2 h
by using a long needle, and the catalyst was removed with syringe
filter (2 nm PTFE, 13 mm diameter). Quantitative determination
was done by using GC-FID (flow rate 0.5 mLminꢀ1, injector temper-
ature 2508C, FID detector temperature 2758C). A calibration curve
was prepared for quantification and for confirmation of linear re-
sponse of GC-FID system.
Synthesis of graphene oxide (GO)
Graphene oxide (GO) was synthesized from graphite flakes by
using modified Hummers methods.[11] In a typical synthesis, con-
centrated H2SO4 (34 mL) was added into a flask containing graphite
flakes (1 gm) and sodium nitrate (0.75 gm) under stirring at 08C
(ice bath). Approximately 4.5 gm of KMnO4 was added gently over
20 min to the mixture and kept under stirring at RT for five days.
After that, diluted H2SO4 (50 mL, 5 wt%) was added into the mix-
ture and heated to 908C under continuous stirring for 2 h. Then,
H2O2 (30 wt%; 2.7 mL) was added into the solution and stirred for
another 2 h under ambient conditions. After completion of the re-
action, the mixture was centrifuged (6000 rpm, 15 min) and
washed with H2SO4 (3 wt%), H2O2 (0.5 wt%), and HCl (3 wt%), and
then repeatedly washed with distilled water, until the pH of the fil-
trate became neutral. Finally, the prepared GO was dispersed in
distilled water and the homogeneous GO dispersion was then cen-
trifuged and filtered.
Blank reactions were conducted to ensure that methanol produc-
tion was due to the photoreduction of CO2 and to eliminate sur-
rounding interference. One blank was Vis-illuminated without the
catalyst, and another one was kept in the dark with the catalyst
and CO2 under the same experimental conditions. An additional
blank test was Vis-illuminated with the catalyst filling N2 rather
than CO2. No product was detected in the above-described three
blank tests.
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Published online on && &&, 0000
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FULL PAPER
Very photoactive! Cobalt phthalocya-
nine grafted to the chemically function-
alized graphene oxide was found to be
an efficient heterogeneous visible-light-
induced photoredox catalyst for the
photoreduction of carbon dioxide to
methanol in a very good yield. The de-
veloped photocatalyst exhibited superi-
or activity compared with the existing
photocatalytic systems and gave metha-
nol as the major reaction product (see
scheme).
&
Heterogeneous Catalysis
P. Kumar, A. Kumar, B. Sreedhar, B. Sain,
S. S. Ray, S. L. Jain*
&& – &&
Cobalt Phthalocyanine Immobilized on
Graphene Oxide: An Efficient Visible-
Active Catalyst for the Photoreduction
of Carbon Dioxide
Chem. Eur. J. 2014, 20, 1 – 9
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9
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ