Photocatalytic reduction of carbon dioxide to methanol using a ruthenium trinuclear polyazine complex immobilized on graphene oxide under visible light irradiation

Article abstract of DOI:10.1039/c4ta01494d

A ruthenium trinuclear polyazine complex was synthesized and subsequently immobilized through complexation to a graphene oxide support containing phenanthroline ligands (GO-phen). The developed photocatalyst was used for the photocatalytic reduction of CO₂ to methanol, using a 20 watt white cold LED flood light, in a dimethyl formamide-water mixture containing triethylamine as a reductive quencher. After 48 h illumination, the yield of methanol was found to be 3977.57 ± 5.60 μmol g_cat ^-1. The developed photocatalyst exhibited a higher photocatalytic activity than graphene oxide, which provided a yield of 2201.40 ± 8.76 μmol g_cat^-1. After the reaction, the catalyst was easily recovered and reused for four subsequent runs without a significant loss of catalytic activity and no leaching of the metal/ligand was detected during the reaction.

Full text of DOI:10.1039/c4ta01494d

Journal of
Materials Chemistry A
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PAPER
Photocatalytic reduction of carbon dioxide to
methanol using a ruthenium trinuclear polyazine
complex immobilized on graphene oxide under
visible light irradiation†
Cite this: J. Mater. Chem. A, 2014, 2,
11246
*
Pawan Kumar, Bir Sain and Suman L. Jain
A ruthenium trinuclear polyazine complex was synthesized and subsequently immobilized through
complexation to a graphene oxide support containing phenanthroline ligands (GO-phen). The developed
photocatalyst was used for the photocatalytic reduction of CO2 to methanol, using a 20 watt white cold
LED flood light, in a dimethyl formamide–water mixture containing triethylamine as a reductive
ꢁ1
quencher. After 48 h illumination, the yield of methanol was found to be 3977.57 ꢀ 5.60 mmol gcat
.
Received 28th March 2014
Accepted 17th April 2014
The developed photocatalyst exhibited a higher photocatalytic activity than graphene oxide, which
provided a yield of 2201.40 ꢀ 8.76 mmol gcatꢁ1. After the reaction, the catalyst was easily recovered and
reused for four subsequent runs without a significant loss of catalytic activity and no leaching of the
metal/ligand was detected during the reaction.
DOI: 10.1039/c4ta01494d
www.rsc.org/MaterialsA
Carbon dioxide (CO2) is an extremely stable molecule, which is including its excellent thermal conductivity, high mechanical
generally produced by the combustion of fossil fuels and strength and carrier mobility, and fascinating chemical prop-
respiration. The recycling of CO2 to form useful products by erties.6a–c Owing to the presence of ample oxygen-containing
activation/reduction is a challenging task, requiring appro- functionalities and its high surface area, it has emerged as a
priate catalysts and energy input. In this context, visible light- promising material for the heterogeneization of homogeneous
driven photoreduction of CO2 to high energy products, such as metal complexes.7a–c Metal-free graphene-based materials, due
methanol, may provide a solution for both the shortage of fossil to their surface charge separation abilities, can serve as catalysts
fuels and global warming.1 Many semiconductors, such as TiO2, in energy production applications.8 In this context, Hsu et al.
ZnO, and CdS, have been known to act as photocatalysts for CO2 reported the use of graphene oxide as a photocatalyst for CO2
reduction, albeit with the major drawbacks of lower quantum reduction, however the yield was 0.172 mmol gcat hꢁ1.9 The
ꢁ1
yields and poor selectivities.2 Molecular catalysis using transi- yield can further be increased by the anchoring of visible light-
tion metal complexes, such as rhenium(I) bipyridine, ruth- absorbing molecules to the graphene oxide support.10 In the
enium(II) polypyridine carbonyl, cobalt(II) trisbipyridine, and present paper, we have used ruthenium polyazine complexes as
cobalt(III) macrocycles, as catalysts with a photosensitizer can anchoring units as they strongly absorb visible light and, by
reduce CO2 with a relatively high quantum yield and high changing the ligand and the number of complex units, the
selectivity of products.3 However, the homogeneous nature of absorption range can be tuned.11 We have synthesized ruthe-
these catalysts makes the process unviable from economical as nium trinuclear polyazine complexes and then anchored these
well as environmental viewpoints. This problem can be solved to the graphene oxide support through complex formation with
by the anchoring of these complexes to a photoactive support, phenanthroline ligands. The synthesized photocatalyst was
which not only enables the facile recovery of the catalyst, but used for the photoreduction of CO2 to methanol under visible
may also allow the photoactive support and complexes to work light irradiation, and provided a methanol yield of 3977.57 ꢀ
synergistically for better electron transfer to CO2.4
5.60 mmol gcatꢁ1 aer 48 h in the presence of triethylamine as a
Since the discovery of graphene in 2007,5 it has been devel- sacricial donor.
oped rapidly because of its excellent physical properties,
Synthesis and characterization of the
catalyst
Chemical Sciences Division, CSIR-Indian Institute of Petroleum Mohkampur,
Dehradun-248005, India. E-mail: suman@iip.res.in; Fax: +91-135-2660202; Tel:
+91-135-2525788
Graphene oxide was prepared by the oxidation of graphite
under harsh conditions, by following a modied Hummers
method.12 The high specic surface area and easy access to
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta01494d
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oxygen-containing functionalities on the surface of the gra-
phene oxide sheets enables the facile immobilization of the
metal complexes on the graphene oxide support. Prior to
immobilization, the GO was treated with chloroacetic acid to
convert the OH and epoxy groups to –COOH groups, which were
subsequently treated with thionyl chloride and then used for
the graing of the 1,10-phenanthroline-5-amine ligands to the
GO surface. Ruthenium trinuclear complex 1 was synthesized by
following a literature procedure, as shown in Scheme 1. Finally,
the trinuclear complex 1 was immobilized via complexation
with the phenanthroline ligands graed on the surface of gra-
phene oxide. The schematic representation of the synthesis of
the ruthenium(II) trinuclear complex immobilized on graphene
oxide (Ru–phen-GO) 2 is shown in Scheme 2.
The N2 adsorption–desorption isotherms of 2 are depicted in
Fig. S1.† The supported material retains the characteristics of a
type IV isotherm and mesoporosity.13 The BET surface area
(SBET), total pore volume (VP) and mean pore diameter (rp) for
GO were found to be 85.54 m2 gꢁ1, 0.1123 cm3 gꢁ1 and 5.5566
nm, respectively (Fig. S2a†). The SBET of 2 was 27.961 m2 gꢁ1
,
which was assumed to be due to the successful attachment of
the complex to the surface of GO. The total pore volume and
mean pore diameter of 2 were 0.1089 cm3 gꢁ1 and 15.577 nm,
respectively (Fig. S2b†). The changes in the surface properties of
GO conrmed the successful attachment of the ruthenium
complexes to the GO surface.
XRD patterns were used to study changes in structure
(Fig. 1). The XRD spectrum of GO, as shown in Fig. 1a, gives a
characteristic diffraction peak at 10.80 (001), related to the GO
interlayer spacing of approximately 0.74 nm.14 Aer immobili-
zation of the ruthenium complex 1, this peak disappeared and
another broad diffraction peak for graphite (002) at 2q value
ꢂ26ꢃ appeared, indicating that exfoliation of the layered Ru–
phen-GO was achieved.15 The XRD pattern clearly indicates that
the material was amorphous in nature.
The surface morphologies of GO and Ru–phen-GO were
investigated via scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). The SEM image of GO
(Fig. 2a) exhibited typical two-dimensional, multilayered
nanosheets with macroscopic wrinkling, as documented in the
literature. The SEM image of Ru–phen-GO 2 (Fig. 2b) exhibited
an agglomerated, layered structure incorporating macromolec-
ular complex moieties between the sheets. The EDX pattern
Scheme 2 Synthesis of Ru–phen-GO 2.
clearly indicates the presence of the ruthenium complex in the
synthesized material 2 (Fig. 2c).
The TEM images of 2 show twisted and crumpled nanosheets
(Fig. 3). It can be seen that the crumpled nanosheets are in an
agglomerated phase. Furthermore, the appearance of dark
spots indicates that the ruthenium complexes were successfully
graed to the GO nanosheets (Fig. 3b). In addition, the ruthe-
nium complex moieties were intercalated between GO's sheets
and the morphology of the sheets remained intact during the
immobilization of the complex. The selected area electron
diffraction (SAED) pattern of the catalyst shows that the mate-
rial was amorphous in nature (Fig. 3c).
Fig. 4 shows the FT-IR spectra of the ruthenium trinuclear
complex 1, GO and GO-supported complex 2. The pure support
GO (Fig. 4b) exhibited bands at 3409, 1720, 1621, 1220, and 1058
cmꢁ1 due to the stretching modes of the O–H, C]O, C]C, C–O
Scheme 1 Synthesis of ruthenium trinuclear complex 1.
Fig. 1 XRD patterns: (a) GO and (b) Ru–phen-GO 2.
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The UV-Vis absorption spectra of the ruthenium trinuclear
complex 1, GO and GO-immobilized complex 2 are shown in
Fig. 5. The absorption spectrum of complex 1 (Fig. 5a) shows a
sharp peak at 260 nm due to a p / p* transition and a shoulder
at 425 nm due to metal-to-ligand charge-transfer (MLCT).18 The
absorption spectrum of GO (Fig. 5b) indicates the characteristic
absorption bands near to 230 nm due to the p / p* transitions
of aromatic ring electrons and a small hump near to 300 nm due
to the n / p* transitions of carbonyl group electrons.19 Aer
the immobilization of complex 1, the absorption peak of GO
exhibited a red shi at 300 nm (Fig. 5c). This was due to the
coordination of the phenanthroline ligand to ruthenium
complex 1. Because of the large extent of the conjugated system
in phenanthroline, the MLCT transition appeared at a low
energy in the GO-supported complex.20 Moreover, a red shi in
the absorption spectrum of the GO-immobilized ruthenium
complex conrmed the successful attachment of the ruthenium
complexes to the GO support.
X-ray photoelectron spectroscopy (XPS) was used to further
conrm the attachment of the Ru-complex to the phen-GO
support. Fig. 6a depicts the survey spectrum of 2, where the key
elements such as Ru, N, O and C can be clearly identied.
Furthermore, high-resolution XPS spectra at the C (1s) and Ru
(3d) regions (Fig. 6b) revealed three prominent peaks at 284.76
eV, 281.94 eV and 285.85 eV, which are assigned to C (1s), Ru
(3d5/2) and Ru (3d3/2) respectively, evidencing the successful
immobilization of the ruthenium complexes on the GO
support.21
Fig. 2 SEM images of (a) GO and (b) Ru–phen-GO 2 and the (c) EDX
pattern of 2.
The thermal stability of the developed photocatalyst 2 was
determined by TGA (Fig. 7). The thermogram of GO (Fig. 7a)
ꢃ
shows apprꢃox. 50% weight loss up to 400 C. The rst weight
Fig. 3 TEM images of (a) GO and (b) Ru–phen-GO 2, and the (c) SAED
pattern of 2.
loss at 100 C was due to the loss of water, and another major
weight loss from 150 ꢃC to 250 ꢃC was due to the loss of oxygen-
containing functionalities of GO.22 The thermogram of 2
(Fig. 7b) shows a three-step weight loss pattern at temperatures
ranging from 50 C to 800 C. The rst weight loss at temper-
atures ranging from 50 ꢃC to 250 ꢃC was attributed to the
removal of physically adsorbed water. The second region
and C–O–C bands, respectively.16 Compared with the pure
support GO, the strong band at 1680 cmꢁ1, assigned to amide
groups (C]O–NH),17 and a few peaks of ruthenium complex 1
at 1180 cmꢁ1 and 1090 cmꢁ1 clearly supported the successful
attachment of complex 1 to GO.
ꢃ
ꢃ
ꢃ
between 250 and 450 C was due to the combustion of amine
and unreacted oxygen-carrying functionalities, i.e. those not
Fig. 5 UV-Vis absorption spectra of (a) Ru-complex 1, (b) GO and (c)
Fig. 4 FTIR spectra of (a) Ru-complex 1, (b) GO and (c) Ru–phen-GO 2. GO-attached complex 2.
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Fig. 6 XPS spectra: (a) a survey scan of Ru–phen-GO 2 and (b) C (1s)
and Ru (3d).
used for attachment of the ruthenium complex. The third
region was mainly related to the decomposition of the ligands of
the ruthenium complexes in the temperature range from 450 to
Fig. 8 Tauc plot for calculating the band gap of GO.
ꢃ
800 C.
the major reduction product. The analysis of the gaseous phase
did not show the presence of CO, CH4 or any other possible
product from the reduction of CO2. Samples taken at different
time intervals were analyzed by injecting them into the gas
chromatograph with the help of an auto-sampler, followed by
the quantitative measurement of their methanol yield. The
methanol (MeOH) formation rate, RMeOH (mmol gcatꢁ1), was
calculated as a function of the reaction time and plotted in
Fig. 9. The results clearly indicated that the GO-immobilized
Ru-complex exhibited a higher catalytic activity, compared to
GO. Aer illumination for 48 h in visible light, using GO and
GO-immobilized Ru-complex in the presence of triethylamine
as sacricial donor, the yields of methanol were found to be
2201.40 ꢀ 8.76 and 3977.57 ꢀ 5.60 mmol gcatꢁ1, respectively. The
higher yield of methanol using the GO-immobilized Ru-
complex is attributed to the better dispersion of the ruthenium
complexes attached to the sheets and the visible light-mediated
electron transition in the complex, with the fast transfer of
electrons to the conduction band of graphene oxide.
The immobilization of ruthenium complex 1 on graphene
oxide enhanced its photocatalytic activity signicantly and
provided absorption in the visible light region. This was further
conrmed by determining the optical band gap of the synthe-
sized graphene oxide with the help of a Tauc plot with linear
extrapolation (Fig. 8). Owing to the amorphous nature and non-
uniform oxidation levels of GO sheets, a sharp adsorption edge
was not obtained in the Tauc plot. We observed an approximate
band gap range between 2.9–3.7 eV, which is in good agreement
with the existing literature reports.9 The obtained band gap
value of GO is quite large, and therefore the transition of the
electrons from the valence band to the conduction band in
visible light cannot be possible. However, the immobilization of
the ruthenium complex moieties on the GO support provided
an efficient visible light photocatalyst for CO2 activation.23
The photocatalytic reduction of CO2
First, the catalytic activity of GO and Ru–phen-GO was tested for
Blank experiments, in the absence of a photocatalyst under
the photoreduction of CO2 in a water–DMF mixture, using tri- visible light irradiation, as well as in the dark using the GO-
ethylamine as a sacricial donor under visible light irradiation.
Aer the photoreduction, a 1 ml liquid sample was withdrawn
and analyzed in a GC-FID equipped with a 30 m long Stabilwax®
w/Integra-Guard® column. The methanol yield was used to
evaluate the performance of the catalysts, as it was obtained as
immobilized ruthenium complex under identical experimental
conditions, showed that there was no organic product found for
longer periods of exposure.
Fig. 9 Conversion of CO2 to methanol over time using (a) photo-
catalyst 2 and (b) GO.
Fig. 7 TGA plots of (a) GO and (b) Ru–phen-GO 2.
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Isotopic labeling experiments with
13CO2
In order to conrm the methanol formation through CO2
reduction, instead of the photo-dissociation of the solvent or
graphene oxide support, isotopic labeling experiments were
performed with 13CO2 . The photocatalytic experiment was
carried out by using 13CO2 in place of 12CO2 under identical
experimental conditions. The reaction product was analyzed by
GC-MS. We obtained the distinct peak associated with 13CH3OH
(m/z 33) instead of 12CH3OH (m/z 32) (Fig. S9†). These data
conrmed that methanol was produced directly from the pho-
tocatalytic reduction of CO2 instead of any photo-dissociation of
the carbon-containing catalyst or solvent. Furthermore, a blank
experiment without CO2 under otherwise identical experi-
mental conditions did not produce methanol.
Aer the photocatalytic reaction, the catalyst was recovered
by centrifugation and recycled for four subsequent runs under
the described experimental conditions (Fig. 10). As shown in
Fig. 10, the methanol yield was similar in all of the cases. These
results conrmed the stability as well as the efficient recycling
of the synthesized ruthenium complex immobilized to gra-
phene oxide for the photoreduction of CO2. Furthermore, ICP-
AES analysis of the recovered catalyst showed that there was no
signicant leaching during the reaction, and the concentration
of the ruthenium in the recovered catalyst was found to be 4.12
wt%. This is in good agreement with the concentration in the
fresh catalyst (4.14%).
Scheme 3 Possible mechanism of the reaction.
inject electrons into the conduction band (CB) of GO, which are
subsequently used for the reduction of adsorbed CO2 to meth-
anol on the surface of GO. Triethylamine acts as a sacricial
agent and provides electrons to the oxidized ruthenium for the
continuation of the process.
Ru-bpm-phen + hn / (Ru+-bpmꢁ)*phen (S1)
(Ru+bpmꢁ)*phen + TEA / [(Rubpm)phenꢁ]* + TEA+
[(Rubpm)phenꢁ]* / Ru-bpm-phen + eꢁ (CB of GO)
eꢁCB (GO) + CO2 + H+ / CH3OH
Moreover, the possible photocatalytic mechanism of the Ru–
phen-GO has been demonstrated, for the better understanding
of the CO2 photoreduction process (Scheme 3). As shown in the
Tauc plot, the wide band gap of GO restricts its excitation in
visible light. However, the synthesized ruthenium complex can
absorb strongly in the visible light region and be excited from
the S0 to S1 state. In the synthesized complex, metal units are
attached by bridging bipyrimidine (bpm) ligands, therefore the
electron density of the conjugated p* orbitals of the ligands
decreases and electrons ow through the bridging ligands to
another metal centre (the antenna effect).24 Finally phen ligands
Conclusions
We have synthesized a novel heterogeneous graphene-immo-
bilized ruthenium trinuclear catalyst for the photocatalytic
reduction of carbon dioxide to methanol under visible light
irradiation. Owing to its wide band gap, graphene oxide alone
does not undergo excitation in visible light. The ruthenium
complex, because of its better visible light absorbance, facili-
tates the transportation of electrons from the ruthenium
macromolecule to GO's conduction band. Aer 48 h of irra-
diation, the yield of methanol using the ruthenium complex
immobilized to graphene oxide was found to be 3977.57 ꢀ
5.60 mmol gcatꢁ1. This value is much higher than that of GO
alone (2201.40 ꢀ 8.76 mmol gcatꢁ1). To the best of our
knowledge, this is the rst report describing the use of a
graphene oxide-immobilized metal complex as an efficient
photoredox catalyst for the photoreduction of CO2. We believe
that the results reported in the present work will open up new
possibilities for developing efficient photocatalysts for the
production of high value products from CO2 in a sustainable
way.
We kindly acknowledge the Director, CSIR-IIP for his kind
permission to publish these results. PK is thankful to CSIR,
New Delhi for his research fellowship under the Emeritus
Scientist Scheme. The analytical division of the institute is
kindly acknowledged for providing support in analysis of
samples. DST, New Delhi is kindly acknowledged for nancial
assistance.
Fig. 10 Recycling of the Ru–phen-GO catalyst for the photoreduction
of CO2 to methanol.
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Synthesis of graphene oxide
Experimental
Graphene oxide (GO) was synthesized from graphite akes
using a modied Hummers method.12 In a typical experiment,
concentrated H2SO4 (68 ml) was added to a ask containing
graphite akes (2 g) and sodium nitrate (0.75 g) while stirring at
0 ꢃC (ice bath). Aer that, KMnO4 (9.0 g) was added slowly to this
mixture and stirred for 5 days. 100ꢃml diluted H2SO4 (5 wt%)
was added to this and heated at 90 C for 2 hours with contin-
uous stirring. The subsequent addition of 30 wt% H2O2 solution
(approximately 5.4 ml) followed by stirring for 2 h at room
temperature afforded graphene oxide, which was collected by
centrifugation. The as synthesized material was subsequently
washed with H2SO4 (3 wt%), H2O2 (0.5 wt%), HCl (3 wt%) and
nally with distilled water.
Materials
The 2,20-bipyridine (bpy, 99%), graphite akes, 2,20-bipyr-
imidine (bpm, 95%), ruthenium chloride trihydrate and 1,10-
phenanthroline-5-amine (phen-NH2, 97%) purchased from
Aldrich were of analytical grade and used without further
purication. Ammonium hexauoro phosphate (99.9%) and
chloroacetic acid (99%) were of analytical grade and procured
from Alfa Aesar. All other chemicals were of A. R. grade and used
without further purication.
Techniques used
Fourier transform infrared spectra were recorded on a Perkin-
Elmer spectrum RX-1 IR spectrophotometer using a potassium
bromide window. Scanning electron microscopy (SEM)
imaging of the synthesized GO for determining its rough
structure was executed using a eld emission scanning elec-
tron microscope (Jeol Model JSM-6340F). The ne structures of
the materials were determined with high resolution trans-
mission electron microscopy (HR-TEM), using an FEI-Tecnai
G2 Twin transition electron microscope operating at an
acceleration voltage of 200 kV. For HR-TEM, a very dilute
aqueous dispersion of the catalyst was deposited on a carbon-
coated copper grid. The absorption spectra of the unattached
Ru-complex 1 in acetonitrile and the solid GO-attached Ru
catalyst were collected on a Perkin Elmer lambda-19 UV-Vis-
NIR spectrophotometer using a 10 mm quartz cell, and BaSO4
as a reference. X-ray powder diffraction patterns were recorded
on a Bruker D8 Advance diffractometer at 40 kV and 40 mA
with Cu Ka radiation (l ¼ 0.15418 nm) for conrming the
synthesis of GO and its functionalization, and the sample for
XRD was prepared on a glass slide by adding the well-dispersed
catalyst in a slot and drying properly. Surface properties like
the Brunauer–Emmett–Teller (BET) surface areas, Barrett–
Joiner–Halenda (BJH) porosities and pore volumes of GO and
the GO-attached Ru-complex were examined by N2 adsorption–
desorption measurements at 77 K by using the VP; Micro-
meritics ASAP 2010. Thermogravimetric analyses (TGAs) of GO
and the GO-attached Ru-complex, for evaluating the thermal
stability of the catalyst, were carried out using a thermal
analyzer TA-SDT Q-600. Analyꢃsis was carried out in the
temperature range of 40 to 800 C under nitrogen ow with a
Synthesis of carboxylated graphene oxide25
Graphene oxide (0.4 g) was dispersed in distilled water (200 ml)
with the help of ultrasonication. The obtained suspension was
treated with NaOH (2.4 g) and chloroacetic acid (2.0 g) and then
sonicated for 3 h. This step converted the hydroxyl (–OH) and
epoxy groups of GO to carboxylic acid groups (–COOH). Diluted
HCl was added to the suspension containing GO–COOH to
achieve a neutral pH. Finally the obtained GO–COOH was
collected by centrifugation and washed with distilled water, and
then dried.
Synthesis of phenanthroline (phen) ligand functionalized
graphene oxide10a,23
Carboxylated graphene oxide (GO–COOH) was treated with
thionyl chloride (25 ml) under reux for 12 h to convert the
carboxylic acid groups of GO into carbonyl chloride groups.
Aer the completion of the reaction, excess thionyl chloride was
removed under distillation at reduced pressure. The synthe-
sized GO–COCl was washed with THF (3 times) and dried in a
vacuum. The obtained GO–COCl was taken in DMF, treated with
1,10-phenanthroline-5-amine (phen-NH2, 0.5 g) and triethyl
amine (0.5 ml) and then reuxed under a nitrogen atmosphere
for 18 h. Finally, the phenanthroline functionalized graphene
oxide support (Phen-GO) was separated by centrifugation and
washed thoroughly with ethanol, and dried under vacuum.
Synthesis of [Ru(bpy)2bpm]$2PF6 complex26
heating rate 10 ꢃC minꢁ1. 1H NMR and 13C NMR spectra of the Ru(bpy)2Cl2$2H2O27 (1.0 g 2.0 mmol) and 2-20-bipyrimidine
ruthenium complex were obtained at 500 MHz using a Bruker (0.98 g, 6.2 mmol) were taken in a 100 ml round-bottomed ask
Avance-II 500 MHz instrument. To determine the ruthenium and then a deoxygenated ethanol–water mixture was added (50
content of the catalyst, inductively coupled plasma atomic ml, 2 : 1 v/v), followed by reuxing for 3 h with stirring under a
emission spectrometry (ICP-AES) analysis was carried out using nitrogen atmosphere. Aer being cooled at room temperature,
an ICP-AE spectrometer (DRE, PS-3000UV, Leeman Labs Inc, the mixture was treated with a saturated solution of ammonium
USA). The metal from 0.05 g catalyst was leached out using hexauorophosphate and then placed in a refrigerator at 0 ꢃC to
conc. HNO3, and the nal volume was made up to 10 ml by precipitate the complex. The precipitated complex was sepa-
adding distilled water. ESI-MS spectra of the synthesized rated via membrane ltration and then dissolved in acetone,
ruthenium trinuclear complex 1 were recorded on a Thermo and re-precipitated with diethyl ether. The complex was further
Exactive Orbitrap system in HESI mode. CHN analysis was puried by column chromatography (alumina) using toluene–
done to conrm that metal had not leached and that the acetonitrile (3/2 v/v) as eluent (UV-Vis absorbance – 430, 286,
complex was intact during its synthesis.
249 nm).
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as a result of the title reaction, 1.0 ml of the nal reaction
solution was used for the GC measurements. The concentration
of methanol was calculated by integrating the peak area for the
characteristic methanol band in the chromatogram (Fig. S8†).
Blank control experiments were conducted to ensure that
methanol production was due to the photoreduction of CO2,
and to eliminate the surrounding interference. One blank test
was carried out by illuminating the solution in the absence of
the photocatalyst, and another was performed in the dark using
a photocatalyst under identical experimental conditions. An
additional blank test was performed by illuminating the reac-
tion mixture in the presence of the photocatalyst by using N2
rather than CO2. No product was detected in the above three
blank tests even aer longer periods of exposure.
Synthesis of ruthenium trinuclear complex
[{(bpy)2Ru(bpm)]2RuCl2$4PF6}] 1 28
In a typical experiment, [Ru(bpy)2(bpm)](PF6)2 (0.865 mmol),
RuCl3$3H2O (0.4605 mmol) and LiCl (4.715 mmol) were taken
in a 100 ml round-bottomed ask and then ethanol was added
(100 ml). The resulting mixture was reuxed with stirring for 24
h under a nitrogen atmosphere. The reaction mixture was
cooled to room temperature and then a saturated solution of
NH4PF6 was added to precipitate complex 1. For purication,
the obtained complex was dissolved in the minimum amount of
acetone and re-precipitated with diethyl ether. Further puri-
cation was done by column chromatography using alumina as
the stationary phase and toluene–acetonitrile (v/v, 2 : 1) as a
mobile phase. UV-Vis absorbance was found at l max 425 nm
and 260 nm. ESI-MS: 1827 [M+ ꢁ 2Clꢁ + 2H+], 1745 [M+ ꢁ PF6 ꢁ
6H+], 1583 [M+ ꢁ 2PF6 ꢁ Fꢁ], 1423 [M+ ꢁ 3PF6 ꢁ Clꢁ ꢁ 3H+] and
1314[M+ ꢁ 4PF6 ꢁ 2H+]. 1H and 13C NMR and ESI-MS spectra of
the synthesized complex are shown in the ESI (Fig. S3–S6†).
Notes and references
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Immobilization of complex 1 to the phen-GO support29
Phen-GO and the [(bpy)2Ru(bpm)RuCl2(bpm)Ru(bpy)2](PF6)4
complex were added to a round-bottomed ask containing
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Photocatalytic CO2 reduction experiment
The photocatalytic reduction was carried out under visible light
by using a 20 watt white cold LED ood light (model no. -HP-FL-
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(dia. 5 cm) was kept about 3 cm away from the light source and
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with DMF (30 ml), triethylamine (10 ml) and deionized water (10
ml) and then the solution was degassed by purging with
nitrogen for 30 min under vigorous stirring. Then the photo-
catalyst (0.1 g) was added and the resulting suspension was
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light source and the samples were collected at 2 h intervals. The
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removed with the help of a syringe lter (2 nm PTFE, 13 mm
diameter). The quantitative determination of methanol over
time was performed using a gas chromatography-ame ioniza-
tion detector (GC-FID) equipped with a 30 m long Stabilwax® w/
Integra-Guard® column, at a ow rate of 0.5 ml minꢁ1, an
ꢃ
injector temperature of 250 C and an FID temperature of 275
ꢃC. A calibration curve was used for quantication and for the
conrmation of the linear response of the GC-FID system 10 (a) R. Yamuna, S. Ramakrishnan, K. Dhara, R. Devi,
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N. K. Kothurkar, E. Kirubha and P. K. Palanisamy, J.
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