This document summarizes the use of octahedral molybdenum cluster compounds Cs2[Mo6Br14] and (TBA)2[Mo6Br14] as photocatalysts for the reduction of carbon dioxide to methanol under visible light irradiation. Cs2[Mo6Br14] exhibited higher photocatalytic activity than (TBA)2[Mo6Br14], producing 6679.45 and 5550.53 mmol g−1 of methanol, respectively, after 24 hours of illumination. The clusters have suitable band gaps and light absorption properties for driving the photocatalytic reaction. This study demonstrates the potential of molybdenum cluster complexes for catalyzing important reactions using solar

1. Photoreduction of CO2 to methanol with
hexanuclear molybdenum [Mo6Br14]2À
cluster
units under visible light irradiation†
Pawan Kumar,a
Subodh Kumar,a
St´ephane Cordier,b
Serge Paofai,b
Rabah Boukherroub*c
and Suman L. Jain*a
Octahedral molybdenum clusters were found to be efficient visible
light homogeneous photocatalysts for the reduction of carbon dioxide
(CO2) to methanol. Photoreduction was carried out by using 20 watt
white cold LED flood light in dimethyl formamide/water or acetoni-
trile/water solutions containing triethylamine as a reductive quencher.
Among the two cluster-based compounds, Cs2[Mo6Br14] exhibited
higher photocatalytic efficiency and afforded higher yield of methanol
than (TBA)2[Mo6Br14] (TBA ¼ tetrabutylammonium). After 24 h illumi-
nation, the yield of methanol was 6679.45 and 5550.53 mmol gÀ1
cat.
using Cs2[Mo6Br14] and (TBA)2[Mo6Br14] cluster compounds as
photocatalysts, respectively.
The continuously increasing concentration of carbon dioxide
(CO2) in the atmosphere has become one of the most serious
problems with regard to the greenhouse effect.1
Photocatalytic
reduction of CO2 with water for the production of hydrocarbons
could be one of the viable solutions to address the issues related
to climate change as well as energy shortage.2
Over the past few
decades, extensive efforts have been devoted to develop visible
light active photocatalytic materials as visible light is ubiqui-
tous, renewable and clean source of energy.3
Although semi-
conductors such as TiO2, ZnS, and CdS have widely been used as
photocatalysts for CO2 reduction, their quantum yields and
selectivities of products are low.4
Homogeneous photocatalysts,
including transition metal complexes such as ruthenium(II)
polypyridine carbonyl complex,5
cobalt(II) trisbipyridine,6
and
cobalt(III) macrocycles7
have been widely investigated for this
reaction. Enzyme catalysts have also been used for such
photoreduction processes.8
Transition metal based molecular
complexes are advantageous due to their high quantum effi-
ciencies and high selectivity of products. However, in some
instances, a weak absorption in the visible region makes the
utility of these complexes limited. This limitation can be over-
come by using high nuclearity transition metal cluster
complexes having the basic formula, [M6(m3
-X)8L6]2À
, which is
also written as [Mo6Xi
8La
6]2À
(i ¼ inner; a ¼ apical; X ¼ halogen,
L ¼ halogen or functional organic ligand).9
In such face-capped
metal atom clusters, the metallic electrons are delocalized on all
the metal centers leading to particular intrinsic properties
(magnetic, optical, redox processes). These nanosized molec-
ular units exhibit a large absorption window from UV to visible
as well as a large emission window from red to infrared due to
the delocalization of valence electrons on all metal centers.10
Molybdenum halide based cluster complexes have been known
since a long time and have been used for photochemical
processes for energy storage, as singlet oxygen photocatalysts
and as bio-imaging agents.11
However, to the best of our
knowledge, these cluster complexes have not been investigated
so far for the photocatalytic reduction of CO2.
In the present paper, we wish to report on the use of octa-
hedral molybdenum cluster-based compounds Cs2[Mo6Br14]
(i.e. Cs2[Mo6Bri
8Bra
6]) and (TBA)2[Mo6Br14] (i.e. (TBA)2[Mo6-
Bri
8Bra
6], TBA ¼ tetrabutyl ammonium) as efficient visible light
active photocatalysts for the photoreduction of CO2 to methanol
by using water as the reaction medium and triethylamine as a
sacricial donor.
The synthesis and characterizations of Cs2[Mo6Br14] and
(TBA)2[Mo6Br]14 have been fully described in the literature.12
The structures are based on [Mo6Xi
8La
6]2À
cluster units depicted
in Fig. 1 as anionic building blocks and Cs+
and (TBA)+
as
counter cations.
Initially, Mo6Br12 (Mo6Bri
8Bra–a
4/2Bra
2) was prepared by the
reaction of Br2 with Mo at 750
C. Then an excision reaction was
performed between a stoichiometric amount of MoBr2 and CsBr
at 850
C for two days. Aerwards, Cs2Mo6Br14 was dissolved in
dry acetone, ltrated and recrystallized in pure form.
a
Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun-248005,
India. E-mail: suman@iip.res.in; Fax: +91-135-2660202; Tel: +91-135-2525788
b
Universit´e de Rennes 1, Institut Sciences Chimiques de Rennes, UR1-CNRS 6226,
Equipe Chimie du Solide et Mat´eriaux, Campus de Beaulieu, CS 74205, 35042
Rennes Cedex, France
c
Institut de Recherche Interdisciplinaire (IRI, USR CNRS 3078), Universit´e Lille 1, Parc
de la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve d'Ascq, France
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47255h
Cite this: RSC Adv., 2014, 4, 10420
Received 3rd December 2013
Accepted 3rd February 2014
DOI: 10.1039/c3ra47255h
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2. (TBA)2[Mo6Br]14 was obtained by precipitation aer dissolution
of Cs2[Mo6Br14] in a mixture of ethanol and water followed by
addition of a two-fold excess of (TBA)Br. Absorption spectra of
the synthesized Mo-clusters are displayed in Fig. 2. They exhibit
a strong absorption band near 300 nm originating from p–p*
absorption, and the two bands at 360–390 and 400–450 nm
corresponding to MLCT singlet and triplet absorptions,
respectively.13
Furthermore, the Tauc plot was used to deter-
mine the bandgap as of the photocatalysts (Fig. 3). The value
associated with the point of intersection of the line tangent to
the plotted curve inection point with the horizontal axis (hn
axis) gives the band gap Eg. Eg values of 2.42 and 2.41 eV are
deduced from the Tauc plots for Cs2Mo6Br14 and (TBA)2-
[Mo6Br]14, respectively, indicating that these clusters can absorb
visible light.
The as obtained catalysts were tested for the photoreduction
of CO2. The photocatalytic reduction of CO2 using as synthe-
sized Mo-clusters as catalyst was carried out at room tempera-
ture under visible light irradiation by using 20 watt white cold
LED ood light in DMF, triethylamine as a sacricial donor and
water as a source of protons. Aer the photoreduction, 1 mL
liquid sample was withdrawn and analyzed in a GC/FID
equipped with a 30 m long Stabilwax® w/Integra-Guard®
column. Methanol yield was used to evaluate the performance
of the catalysts as it was obtained as the major reduction
product. Methanol (MeOH) formation rate, RMeOH (mmol gÀ1
cat.) in the photocatalytic reduction of CO2 by using
Cs2[Mo6Br14] and (TBA)2[Mo6Br14] are plotted in Fig. 4. The
results suggest that molybdenum cluster Cs2[Mo6Br14] exhibits
higher photocatalytic activity as compared to the (TBA)2-
[Mo6Br14]. Aer 24 h visible light irradiation, the yield of
methanol using Cs2[Mo6Br14] and (TBA)2[Mo6Br14] as catalysts
under identical experimental conditions was 6679.45 and
5550.53 mmol gÀ1
cat., respectively. The turnover numbers for
MeOH formation were 13.1 and 12.1 using Cs2[Mo6Br14] and
(TBA)2[Mo6Br14], respectively. The quantum yields of methanol
formation were determined to be 0.061 and 0.051 for
Cs2[Mo6Br14] and (TBA)2[Mo6Br14], respectively. The formation
of methanol as the major reaction product was further
conrmed by HRGC-MS and HPLC analysis (Fig. S2 and S3†).
Methanol was identied as the major reaction product using
the developed catalytic system. The stoichiometry of the reac-
tion is given in eqn (1).
CO2 + 6H+
+ 6eÀ
/ CH3OH + H2O (1)
Quantitative measurement of methanol was carried out by
using GC-FID. A calibration curve was plotted by injecting an
exact amount of methanol (1 mL) using auto samples (Fig. S4†).
To evaluate the concentration of methanol produced as a result
of the cluster catalyzed reaction, 1.0 mL of the nal reaction
solution was used for GC measurements. The concentration of
methanol was calculated by integrating the peak area for the
characteristic methanol band in the chromatogram (Fig. S1†).
Blank experiments, one under visible light irradiation
without using photocatalyst and another under dark condition
Fig. 1 Schematic representation of the [Mo6Bri
8La
6]2À
cluster unit.
Fig. 2 UV-Vis spectra of Mo clusters.
Fig. 3 Tauc plot for band gap (Eg) determination of Mo-clusters.
Fig. 4 Methanol yield for CO2 photoreduction using (a) Cs2[Mo6Br14]
and (b) (TBA)2[Mo6Br14].
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3. without irradiation using Cs2[Mo6Br14] cluster as catalyst under
described experimental conditions, showed that there was no
organic product formation for long periods of exposure (48 h).
To establish the formation of methanol from the photo-
catalytic reduction of CO2 and not deriving from DMF, we have
performed the photoreduction of CO2 in another polar aprotic
solvent i.e. acetonitrile under identical experimental conditions.
The yield of methanol in acetonitrile was found to be compa-
rable to that obtained in DMF (Fig. S3†). Furthermore, we carried
out the photoreaction by using acetonitrile and catalyst in the
absence of carbon dioxide under identical reaction conditions.
No methanol formation was observed, conrming that the
methanol is being formed from the photoreduction of CO2.
Importantly, the yield of methanol observed in the present
reduction of CO2 is higher than the values previously reported
for metal based complexes such as cobalt phthalocyanine
(1032 mmol gÀ1
cat.), ruthenium phosphine (2210 mmol gÀ1
cat.)-catalyzed photoreductions of CO2 to methanol.14
The exact mechanism of the reaction is not clear at this
stage. However, based on the literature reports it is assumed
that the Mo-cluster molecule might be photoactivated to the
triplet metal-to-ligand charge transfer (3
MLCT) excited state,
which is quenched by tertiary amine (triethylamine), as the rst
step of the photocatalytic reaction, to generate the one-electron
reduced (OER) species of the molybdenum complexes.15,16
In the
next step, the intermediate OER species react with CO2 to give
radical anion of carbon dioxide (CO2
Àc
), which subsequently
reacts with protons to produce methanol as depicted in
Scheme 1.
In summary, we have demonstrated that Cs2[Mo6Br14] and
(TBA)2[Mo6Br14] cluster compounds exhibit excellent photo-
catalytic performance for the photoreduction of CO2 under
visible light irradiation to give methanol selectively. Among the
two cluster-based compounds, Cs2[Mo6Br14] exhibited higher
photocatalytic efficiency and afforded higher yield of methanol
than (TBA)2[Mo6Br14]. Aer 24 h visible light irradiation, the
yields of methanol were 6679.45 and 5550.53 mmol gÀ1
cat.
using Cs2[Mo6Br14] and (TBA)2[Mo6Br14] as photocatalysts,
respectively. The results provided in this work hold promise in
view of various photocatalytic applications of the Mo6 clusters
under solar light irradiation.
We are thankful to the Director, CSIR-IIP for his kind
permission to publish these results. PK acknowledges the CSIR,
New Delhi, for the Research Fellowship. RB acknowledges
nancial support from the CNRS, the Universit´e Lille1 and the
Nord Pas de Calais region.
Experimental procedure for
photocatalytic reduction of CO2
Photocatalytic reduction of CO2 was performed in a 60 mL
borosil tube of 4 cm in diameter. Photoirradiation was carried
out under visible light by using 20 watt white cold LED ood
light (Model no.-HP-FL-20W-F-Hope LED Opto-Electric Co. Ltd).
The reaction vessel was kept about 2 cm far away from the light
source and the intensity of the light at the vessel was 85 W mÀ2
as measured by intensity meter. The vessel was initially charged
with DMF (30 mL), triethylamine (10 mL) and deionized water
(10 mL) and then the solution was degassed by purging nitrogen
for 30 min under vigorous stirring. Then the photocatalyst
(0.1 g) was added to the solution and the resulting mixture was
saturated with carbon dioxide. The vessel was sealed, irradiated
with light source and the samples were collected in every 2 h
intervals. The samples were collected by using a long needle and
the catalyst was removed with the help of a syringe lter (2 nm
PTFE, 13 mm diameter). Quantitative determination was ach-
ieved by using Gas Chromatography FID using a ow rate of
0.5 mL minÀ1
, injector temperature: 250
C, and FID detector
temperature: 275
C. A calibration curve was used for quanti-
cation and for conrmation of linear response of GC-FID
system.
Blank reactions were conducted to ensure that methanol
production was due to the photoreduction of CO2, and to elim-
inate surrounding interference. One blank was carried out by
illuminating the solution in the absence of photocatalyst, and
another was performed in the dark in the presence of photo-
catalyst under the identical experimental conditions. An addi-
tional blank test was performed by illuminating the reaction
mixture in the presence of photocatalyst by lling N2 rather than
CO2. No product was detected in the above three blank tests.
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