Hexamolybdenum clusters supported on graphene oxide: visible-light induced photocatalytic reduction of carbon dioxide into methanol

1. Hexamolybdenum Clusters supported on Graphene Oxide:
Visible-Light Induced Photocatalyst for Reduction of Carbon
Dioxide into Methanol
1

2. Introduction
 Burning of fossil fuel for meeting our energy demands has raised concentration of
CO2 upto 400 ppm.
 Dissolved CO2 makes acidification of sea water which reduces calcites needed by
marine animals to form shells.
 Under ground storage in oil well can burst suddenly like Lake Nyos in Cameroon
and asphyxiated 1700 people.
 In some algal strain like Schizochytrium sp. and Botryococcus braunii oil
content can go as high as 80%
 But conversion efficiency of photosynthesis is approximately 1% and cultivation of a
single biological species will undoubtedly raise environmental challenges.
Photocatalytic conversion of CO2 by using sunlight to valuable products
can provide energy as well level off rising CO2 concentration.
2

3.  Metal cluster is a multimettalic oxide or halides aggregates.
 Metal clusters due to presence of multi-metallic centers can undergo multi-electron
redox process which is an essential requirement for reduction of carbon dioxide.
The main drawback of metal clusters are their homogeneous nature and non-recyclability.
 Octahedral metal clusters of molybdenum (Mo6) are built up from [Mo6Xi
8Xa
6]2- building
blocks (X) halogen and/or chalcogen, i ) inner, a ) apical
 The halogen hydroxy or ligands situated on apical position are labile and can be
replaced with donor ligands like pyridine derivatives, etc.
 They strongly absorbs in visible region near 500 nm
[Mo6Xi
8Xa
6]2- cluster
Why clusters ?
3

4. A schematic illustration of (a) GO nanosheet decorated with various oxygen
functionalities (b) immobilization of Cs2Mo6Bri
8Bra
6 / (TBA)2Mo6Bri
8Bra
6 clusters
on the GO nano-sheets, and (c) molecular structure of Mo6 cluster representing
position of inner and apical ligands
Synthesis of graphene oxide supported Mo-cluster
=
50 mg clusters
water and ethanol(2:1)
50 mL GO (2.24 mg/mL)
4

5. (a) GO nanosheets (b) GO-Cs2Mo6Bri
8Bra
x and (c)
GO-(TBA)2Mo6Bri
8Bra
x composites.
FESEM micrographs and element
mapping
(a) GO-Cs2Mo6Bri
8Bra
x (b) GO-(TBA)2Mo6Bri
8Bra
x
HRTEM images
5

6. (a) GO, Cs2Mo6Bri
8Bra
x clusters and GO- Cs2Mo6Bri
8Bra
x composite; and (b) GO,
(TBA)2Mo6Bri
8Bra
6 clusters and GO-(TBA)2Mo6Bri
8Bra
6 composite.
FTIR spectra
6

7. High resolution C 1s XPS spectra
7

8. 745 740 735 730 725 720
6000
7000
8000
9000
GO-Cs2
Mo6
Br
i
8
Br
a
x
GO-(TBA)2
Mo6
Br
i
8
Br
a
x
O (KVV)
Cs 3d3/2
Cs 3d5/2
Intensity,cps
Binding Energy, eV
The Cs 3d XPS spectra Cs 3d5/2 and 3d 3/2 (a) Cs 3d
(b) Mo 3d
8

9. (c) Br 3d regions
(d) Br 3d region
Cluster
Br/Mo Found = 2.6
theoretical value= 2.33
After immobilization
Br/Mo Found =1.6
theoretical = (1.33).
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10. DT-TGA patterns
10

11. UV-visible spectra
Tauc plot for band gap calculation
11

12. Methanol Yield from different components
GC chromatogram of photoreaction product
GC calibration curve for the quantification
of methanol
12

13. Entry Catalyst React. Precursor Cat./mg Visible Light
Illumination
T/ h TON
1 Nil CO2 Nil Yes 24 -
2 GO- Cs2Mo6Bri
8Bra
x CO2 100 No 24 -
3
4
5
GO- Cs2Mo6Bri
8Bra
x
Cs2Mo6Bri
8Bra
6
(TBA)2Mo6Bri
8Bra
6
N2
CO2
CO2
100
Eq. to GO-CsMo6
Eq. to GO-
TBAMo6
Yes
Yes
Yes
24
24
24
-
3.30
2.75
6 GO CO2 100 Yes 24 0.04
7 GO- Cs2Mo6Bri
8Bra
x CO2 100 Yes 24 19.0
8 GO-
(TBA)2Mo6Bri
8Bra
x
CO2 100 Yes 24 10.3
Table: Photocatalytic reduction of CO2 into methanol under controlled experimental conditions
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14. Recycling experiments using (a) GO- (TBA)2Mo6Bri
8Bra
x and (b) GO- Cs2Mo6Bri
8Bra
x.
GO-(TBA)2Mo6Bri
8Bra
x - 7.00 %
GO-Cs2Mo6Bri
8Bra
x - 4.14 %
Fresh catalyst GO-(TBA)2Mo6Bri
8Bra
x - 7.16%
Fresh catalyst GO-Cs2Mo6Bri
8Bra
x - 4.98%
Mo content
14

15. Plausible mechanism of photoreduction of CO2 into methanol catalyzed
by GO-hexamolybdenum composite
15

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17. Thanks
A “Scientist” never get old,
B’COZ
everyday he learns something new !
Anonymous
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