Photoreduction of CO2 to methanol with hexanuclear molybdenum [Mo6Br14]2- cluster units under visible light irradiation

Article abstract of DOI:10.1039/c3ra47255h

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 acetonitrile/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 = tetrabutylammonium). After 24 h illumination, the yield of methanol was 6679.45 and 5550.53 μmol g ^-1 cat. using Cs₂[Mo₆Br₁₄] and (TBA)₂[Mo₆Br₁₄] cluster compounds as photocatalysts, respectively.

Full text of DOI:10.1039/c3ra47255h

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Photoreduction of CO to methanol with hexanuclear molybdenum
2
2
-
[
Mo Br ] cluster units under visible light irradiation.
6
14
a
a
b
b
c*
Pawan Kumar , Subodh Kumar , Stéphane Cordier , Serge Paofai , Rabah Boukherroub and Suman L.
Jain
a*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
Octahedral molybdenum clusters were found to be efficient visible
light induced homogeneous photocatalysts for the reduction of
processes for energy storage, as singlet oxygen photocatalysts
and as bioꢁimaging agents. However, to the best of our
1
1
1
0
carbon dioxide (CO ) to methanol. Photoreduction was carried out 55 knowledge, these cluster complexes have not been investigated
2
so far for the photocatalytic reduction of CO2.
by using 20 watt white cold LED flood light in dimethyl
In the present paper, we wish to report on the use of octahedral
molybdenum clusterꢁbased compounds Cs [Mo Br ] (i.e.
formamide/water or acetonitrile/water solutions containing
triethylamine as a reductive quencher. Among the two cluster-based
compounds, Cs [Mo Br ] exhibited higher photocatalytic efficiency
2
6
8
i
a
i
a
Cs [Mo Br Br ]) and (TBA) Mo Br (i.e. (TBA) [Mo Br Br ],
2
6
8
6
2
6
8
2
6
8
6
2
6
14
6
6
7
7
0
5
0
5
TBA = 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
sacrificial donor.
1
5
and afforded higher yield of methanol than (TBA) Mo Br (TBA =
2 6 14
tetrabutylammonium) After 24 h illumination , the yield of
.
-1
methanol was 6679.45 and 5550.53 µmol.g cat using Cs [Mo Br ]
2
6
14
and (TBA) [Mo Br ] cluster compounds as photocatalysts,
2
6
14
The synthesis and characterizations of Cs [Mo Br ] and
2
6
8
respectively.
1
2
(TBA) [Mo Br] have been fully described in the literature.
The structures are based on [Mo X L ] cluster units depicted
in Fig. 1 as anionic building blocks and Cs and (TBA) as
2
6
14
i
a
2ꢁ
6
8
6
2
2
3
3
4
4
5
0
5
0
5
0
5
0
The continously increasing concentration of carbon dioxide
+
+
(
CO ) in the atmosphere has become one of the most serious
2
1
counter cations. .
problems with regard to the greenhouse effect. Photocatalytic
reduction of CO with water for the production of hydrocarbons
2
could be one of the viable solutions to address the issues related
2
to the climate change as well as energy shortage. Over the past
few decades, extensive efforts have been devoted to develop
visible light active photocatalytic materials as visible light is
3
ubiquitous, renewable and clean source of energy. Although
semiconductors such as TiO , ZnS, and CdS have widely been
2
used as photocatalysts for CO reduction, their quantum yields
2
4
and selectivities of products are low.
Homogeneous
photocatalysts, including transition metal complexes such as
5
ruthenium(II) polypyridine carbonyl complex,
cobalt(II)
6
7
80
trisbipyridine, and cobalt(III) macrocycles have been widely
investigated for this reaction. Enzyme catalysts have also been
used for such photoreduction processes. Transition metal based
i
a
2ꢁ
8
Fig. 1. Schematic representation of the [Mo
6
Br
8
L
6
]
cluster
unit.
molecular complexes are advantageous due to their high quantum
efficiencies 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
overcome by using high nuclearity transition metal cluster
i
aꢁa
a
8
5
Initially, Mo Br (Mo Br Br
Br ) was prepared by the
4/2 2
6
12
6
8
reaction of Br with Mo at 750 °C. Then an excision reaction was
2
performed between a stoichiometric amount of MoBr and CsBr
2
3
2ꢁ
at 850 °C for two days. Afterwards, Cs
dry acetone, filtrated and recrystallized in pure form.
[Mo Br]14 was obtained by precipitation after dissolution
of Cs [Mo Br ] in a mixture of ethanol and water followed by
2 6
Mo Br14 was dissolved in
complexes having the basic formula, [M (ꢀ ꢁX) L ] , which is
6
8 6
i
a
2ꢁ
also written as [Mo X L ] (i=inner ; a=apical; X=halogen,
6
8
6
9
90 (TBA)
2
6
L=halogen or functional organic ligand)
.
In such faceꢁcapped
metal atom clusters, the metallic electrons are delocalized on all
the metal centers leading to particular intrinsic properties
2
6
14
addition of a twoꢁfold excess of (TBA)Br. Absorption spectra of
the synthesized Moꢁclusters are displayed in Fig. 2. They exhibit
a strong absorption band near 300 nm originating from π–π*
absorption, and the two bands at 360ꢁ390 and 400–450 nm
corresponding to MLCT singlet and triplet absorptions,
(
magnetic, optical, redox processes). These nanosized molecular
units exhibit a large absorption window from UV to visible as
well as a large emission window from red to infrared due to the
9
5
1
0
delocalization of valence electrons on all metal centers.
1
3
respectively. Furthermore, the Tauc plot was used to determine
Molybdenum halide based cluster complexes have been known
since a long time and have been used for photochemical
the bandgap as of the photocatalysts (Fig. 3). The value
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DOI: 10.1039/C3RA47255H
associated with the point of intersection of the line tangent to the 35 product was further confirmed by HRGCꢁMS and HPLC analysis
plotted curve (inflection point with the horizontal axis (hν axis)
gives the band gap Eg. Eg values of 2.42 and 2.41 eV are
(Figs. S2 & S3).
Methanol was identiffied as the major reaction product using the
developed catalystic system. The stoichiometry of the reaction is
given in equation 1.
deduced from the Tauc plots for Cs Mo Br
and
(TBA) [Mo Br] , respectively, indicating that these clusters can
2
6
14
5
2
6
14
absorb visible light.
40
+
ꢁ
CO + 6H + 6e → CH OH + H O……………………..…(Eq. 1)
2
3
2
Quantitative measurement of methanol was carried out by using
GCꢁFID. A calibration curve was plotted by injecting an exact
amount of methanol (1 ꢀL) using auto samples (Fig. S4). To
evaluate the concentration of methanol produced as a result of
the cluster catalyzed reaction, 1.0 ꢀL of the final 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 photcatalyst and another under dark condition without
4
5
5
5
0
5
Fig. 2: UVꢁVis spectra of Mo clusters.
irradiation using Cs [Mo Br ]
cluster as catalyst under
2
6
14
described experimental conditions, showed that there was no
organic product formation for long periods of exposure (48 h).
E
g
1
0
g
Fig. 3: Tauc plot for band gap (E ) determination of Moꢁclusters.
Fig. 4. Methanol yield for CO
2
2 6
photoreduction using a) Cs [Mo Br14] and b)
(
TBA)
2
[Mo Br14]).
6
The as obtained catalysts were tested for the photoreduction of
CO . The photocatalytic reduction of CO using as synthesized
2
2
To establish the formation of methanol from the photocatalytic
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 comparable
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, confirming that the
methanol is being formed from the photoreduction of CO2.
Moꢁclusters as catalyst was carried out at room temperature
under visible light irradiation by using 20 watt white cold LED
flood light in DMF, triethylamine as a sacrificial donor and water
as a source of protons. After the photoreduction, 1 ꢂL liquid
sample was withdrawn and analyzed in a GC/FID equipped with
a 30 m long Stabilwax® w/IntegraꢁGuard® 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 (ꢀmol/g cat.) in the photocatalytic
reduction of CO by using Cs [Mo Br ] and (TBA) [Mo Br ]
6
6
7
7
0
5
0
5
1
2
2
3
5
0
5
0
2
2
6
14
2
6
14
are plotted in Fig. 4. The results suggest that molybdenum
Importantly, the yield of methanol observed in the present
cluster Cs [Mo Br ] exhibits higher photocatalytic activity as
2
6
14
reduction of CO
metal based complexes such as cobalt phthalocyanine (1032
mol/g cat), ruthenium phosphine (2210 ꢀmol/g cat)ꢁcatalyzed
2
is higher than the values previously reported for
compared to the (TBA) [Mo Br ].
A
fter 24 h visible light
2
6
14
irradiation, the yield of methanol using Cs [Mo Br ] and
2
6
14
ꢀ
(
TBA) [Mo Br ] as catalysts under identical experimental
2 6 14
14
photoreductions of CO to methanol.
2
ꢁ
1
conditions was 6679.45 and 5550.53 ꢂmol.g cat, respectively.
The turnover numbers for MeOH formation were 13.1 and 12.1
using Cs [Mo Br ] and (TBA) [Mo Br ], respectively. The
The exact mechanism of the reaction is not clear at this stage.
However, based on the literature reports it is assumed that the
Moꢁcluster molecule might be photoactivated to the triplet metalꢁ
2
6
14
2
6
14
quantum yields of methanol formation were determined to be
.061 and 0.051 for Cs [Mo Br ] and (TBA) [Mo Br ],
3
toꢁligand charge transfer ( MLCT) excited state, which is
0
2
6
14
2
6
14
quenched by tertiary amine (triethylamine), as the first step of
respectively. The formation of methanol as the major reaction
2
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the photocatalytic reaction, to generate the oneꢁelectron reduced 45 as measured by intensity meter. The vessel was initially charged with DMF
1
5,16
(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,
50 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 filter (2nm PTFE, 13 mm diameter).
Quantitative determination was achieved by using Gas Chromatography FID
using a flow rate of 0.5 mL/min, injector temperature: 250 °C, and FID
(
OER) species of the molybdenum complexes.
In the next
step, the intermediate OER species react with CO to give radical
2
ꢁ
.
anion of carbon dioxide (CO2 ), which subsequently reacts with
a proton to produce methanol as depicted in Scheme 1.
5
5
6
6
5
0
5
detector temperature: 275 °C. A calibration curve was used for quantification
and for confirmation of linear response of GCꢁFID system.
Blank reactions were conducted to ensure that methanol production was due
2
to the photoreduction of CO , and to eliminate 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
photocatalyst under the identical experimental conditions. An additional
blank test was performed by illuminating the reaction mixture in the presence
2 2
of photocatalyst by filling N rather than CO . No product was detected in the
above three blank tests.
Notes and References
1
). M. M. Halmann and M. Steinberg, Greenhouse Gas Carbon Dioxide
Mitigation Science and Technology, Lewis Publishers, 1999.
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Grimes, ACS Nano, 2010, 4, 1259–1278; c) M. Cokoja, C. Bruckmeier,
B. Rieger, W. A. Herrmann and F. E. Kuhn, Angew. Chem. Int. Ed.
(
TBA) [Mo Br ]
cluster
compounds exhibit
excellent
75
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14
2
011, 50, 8510–8537.
photocatalytic performance for the photoreduction of CO under
2
). a) H. Fujiwara, H. Hosokawa, K. Murakoshi, Y. Wada,S. Yanagida, T.
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visible light irradiation to give methanol selectively. Among the
two clusterꢁbased compounds, Cs [Mo Br ] exhibited higher
2
6
14
photocatalytic efficiency and afforded higher yield of methanol
than (TBA) Mo Br . After 24 h visible light irradiation, the
8
8
9
9
0
5
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2
6
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ꢁ
1
yields of methanol were 6679.45 and 5550.53 ꢂmol.g cat using
Cs [Mo Br ] and (TBA) [Mo Br ] as photocatalysts,
2
6
14
2
6
14
6
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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
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Nord Pas de Calais region.
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a
3
3
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5
Chemical Sciences Division,CSIR-Indian Institute of Petroleum, Dehradun-
48005, India; Tel.: +91-135-2525788; Fax. +91-135-2660202; Email:
suman@iip.res.in
Université de Rennes 1, Institut Sciences Chimiques de Rennes, UR1-CNRS
226, Equipe Chimie du Solide et Matériaux, Campus de Beaulieu, CS
74205, 35042 Rennes Cedex, France
Institut de Recherche Interdisciplinaire (IRI, USR CNRS 3078), Université
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Experimental procedure for photocatalytic reduction of CO2
2
4
0
Photocatalytic reduction of CO
cm in diameter. Photoirradiation was carried out under visible light by using
0 watt white cold LED flood light (Model No.ꢁHPꢁFLꢁ20WꢁFꢁHope LED
2
was performed in a 60 mL borosil tube of 4 110 14. a) S. Liu, Z. Zhao and Z. Wang, Photochem. Photobiol. Sci., 2007, 6,
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OptoꢁElectric 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/m
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2-
Visible light induced hexanuclear molybdenum [Mo Br ]
6
14
cluster units for the photocatalytic reduction of CO to methanol
2
a
a
b
b
Pawan Kumar , Subodh Kumar , Stéphane Cordier , Serge Paofai Rabah
c
a*
Boukherroub * and Suman L. Jain
5
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5