Photochemical reduction of carbon dioxide coupled with water oxidation using various soft-oxometalate (SOM) based catalytic systems

Article abstract of DOI:10.1039/c6ta02825j

Simultaneous CO₂ reduction and water oxidation as a coupled process is an important challenge in the quest of clean energy production. Herein, we report a metal oxide based heterogeneous catalytic system, which not only couples CO₂ reduction with water oxidation, but also provides very high turnover number for CO₂ reduction with scalability. Such a catalytic system can simultaneously oxidize water and release the generated electrons for reduction of CO₂ with a maximum turnover number and turnover frequency as high as 1.4 × 10⁶ and 610 s^-1, respectively, following effective catalyst concentration; whereas, turnover number and turnover frequency is 1366 and 1380 h^-1 per mole of catalyst, respectively. The starting materials for this catalytic process are CO₂ and water while the end products are oxygen and formic acid and in few cases, formaldehyde. The prospect of using the formic acid generated during our process in fuel cells to generate green energy is also worth mentioning.

Full text of DOI:10.1039/c6ta02825j

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PAPER
Photochemical reduction of carbon dioxide
coupled with water oxidation using various soft-
Cite this: J. Mater. Chem. A, 2016, 4,
oxometalate (SOM) based catalytic systems†
8875
Santu Das, Subharanjan Biswas, Tuniki Balaraju, Soumitra Barman,
*
Ramudu Pochamoni and Soumyajit Roy
Simultaneous CO2 reduction and water oxidation as a coupled process is an important challenge in the
quest of clean energy production. Herein, we report a metal oxide based heterogeneous catalytic
system, which not only couples CO2 reduction with water oxidation, but also provides very high turnover
number for CO2 reduction with scalability. Such a catalytic system can simultaneously oxidize water and
release the generated electrons for reduction of CO2 with a maximum turnover number and turnover
frequency as high as 1.4 ꢀ 106 and 610 sꢁ1, respectively, following effective catalyst concentration;
whereas, turnover number and turnover frequency is 1366 and 1380 hꢁ1 per mole of catalyst,
respectively. The starting materials for this catalytic process are CO2 and water while the end products
are oxygen and formic acid and in few cases, formaldehyde. The prospect of using the formic acid
generated during our process in fuel cells to generate green energy is also worth mentioning.
Received 6th April 2016
Accepted 5th May 2016
DOI: 10.1039/c6ta02825j
www.rsc.org/MaterialsA
photoelectrochemical CO2 reduction using Re-porphyrins, Co,
Ni cyclams and polyporphyrins, respectively. For CO2 reduction,
Introduction
four prime approaches can be identied as follows. (1) CO2
reduction based on macrocycles such as cyclams with Ni2+ and
Co2+.24,25 (2) Polypyrridyl based materials with Re+, Ru+ and
Mn.26–29 (3) Pd2+ based pincer type complexes reported by
Dubois30 and the related complexes of Ir pioneered by Peruz-
zini.31 (4) Cantat and co-workers have pioneered metal free CO2
reduction using nitrogen containing bases such as amidines
and guanidines with hydrosilanes as the reducing agents.32
Molybdenum alone has been shown to be capable of CO2
reduction.33 Likewise, water oxidation by metal complexes has
also attracted enormous attention since the seminal concept of
articial photosynthesis proposed by Nocera.2 The use of optical
semi-conductors coupled with a metal complex for CO2 reduc-
tion has been very recently proposed.33 The possibility of
achieving scalable coupled CO2 reduction with water oxidation
has also been proposed.2,4 However, to date and to the best of
our knowledge, no scalable approach for the realization of this
effort has been reported. Some related noteworthy efforts
warrant mention. For instance, Carpenter and co-workers have
explored the possibility of CO2 reduction without the presence
of any external reducing agent.9 With a prototype example of an
amine, they have shown this reduction in an indirect fashion.12
More recently, Yang et al.34 have shown the possibility of
coupling of CO2 reduction and water oxidation with nano-
composites of tantalum-based pyrochlore nanoparticles and
indium hydroxide where pyrochlore nanoparticles catalyze the
CO2 reduction reaction or water reduction reaction and indium
hydroxide catalyzes water oxidation. Though a very elegant
Nature performs CO2 reduction and couples it with water
oxidation regularly and with very high efficacy in photosyn-
thesis. CO2 emissions are ever increasing. Water and light are
amply abundant. Fossil fuel reserves are depleting and up-
stream C1 feed-stocks demands are steeply increasing. Hence,
the immediate question is can CO2 reduction and water oxida-
tion be coupled in a synthetic scalable catalytic system? Herein,
we report such a catalytic system based on three different metal
oxide based catalysts that can simultaneously oxidize water, and
release the generated electrons for the reduction of CO2 with
a high TON (turn over number). The starting materials of this
catalytic process are CO2 and water while the end products are
oxygen and formic acid and in a few cases, formaldehyde. Now
we describe briey the background of our present catalytic
process and the development of its design.
Our design of the present catalyst is centred on two
phenomena that are coupled together, viz., photo-chemical CO2
reduction1–13 and water oxidation.14–22 Photochemical CO2
reduction has attracted enormous interest in recent times.
During the 1980s Lehn23 and Sauvage24 pioneered the eld of
Eco-Friendly Applied Materials Laboratory (EFAML), Materials Science Centre,
Department of Chemical Sciences, Indian Institute of Science Education & Research,
Mohanpur Campus, Kolkata, 741246 West Bengal, India. E-mail: s.roy@iiserkol.ac.in
† Electronic
supplementary
information
(ESI)
available:
Additional
characterization and stability of the catalysts during the reactions and product
quantication table. CCDC 1057222. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c6ta02825j
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effort, this method also suffers from a negligible turnover
number. In another effort, Kudo et al.35 reported a method
coupling Ag co-catalyst loaded perovskites with the formula of
ALa4TiO4O15 (A ¼ Ca, Sr and Ba). The catalyst, although couples
CO2 reduction with water oxidation with simultaneous forma-
tion of HCOOH and CO, provides only a negligible yield
(maximum CO yield is 22 micromoles per hour with 0.3 g
catalyst loading). The single catalysis electrocatalytic CO2
reduction coupled with water oxidation reported by Meyer
group36,37 also deserves a mention. They have shown that a Ru(II)
polypyridyl carbene complex can catalyze the electrochemical
reduction of CO2 in water with added bicarbonate to give
synthesis gas (H2 and CO) mixtures at the cathode and water
oxidation to O2 at the anode. However, the problems of low
turnover numbers and scalability also occur. The issue of scal- Fig. 1 The structures of the three catalysts, {Mn6P3W24} is shown in
polyhedral representation while {Mo154} and {Mo132} are shown as
ability has been addressed in the recent work of the Yaghi and
a space-filling model. Color code: Mo, red; O, blue; Mn, yellow; W,
Chang group.1 They have shown that cobalt–porphyrin based
grey; P, pink.
covalent organic frameworks can be optimized to synthesize the
catalytic material, which shows very high efficiency towards the
reduction of CO2 to CO. The faradaic efficiency of this reaction
{Mn6P3W24}y, where y ¼ 931
{Mo132}n@RGO, where n ¼ 1064
(2)
(3)
was 90% with turnover numbers up to 290 000, with an initial
turnover frequency of 9400 hꢁ1
.
Here we take the next step. To perform photochemical CO2
reduction coupled with the water oxidation reaction, we chose
a set of various metal oxide clusters. It is already known from
earlier studies that metal oxide clusters can act as active and
efficient photocatalysts37,38 as well electrocatalysts39,40 in water
oxidation and water reduction41,42 reactions. These metal oxide
clusters are also cheap and green. To enhance the effective
surface area of the catalysts, instead of using the metal oxides in
the solid phase, we have prepared self-assembled vesicles with
where RGO stands for reduced graphene oxide.
Before preparing the effective catalysts we rst briey intro-
duce the metal oxide based molecular precursors of the cata-
lysts. We chose three metal oxide based clusters viz. {Mo154},
{Mn6P3W24} and {Mo132}.
Molecular structure of Mo154
the metal oxides. Such vesicle-like super-structure formation of {Mo154
}
is
a
giant wheel-shaped mixed-valence poly-
the metal oxide clusters is well known and studied by our group oxomolybdate cluster with the formula {Mo154O462H14(H2-
14ꢁ
and we have proposed them to be called so-oxometalates O)70
}
comprising 140 {MoO6} octahedra and 14 pentagonal
(SOMs).43–50 Very recently we have exploited various properties bipyramids of the type {MoO7}.58 It can be described as a tetra-
of SOMs and thereby, their usability in a large range of appli- decamer comprising 14 {Mo8} units with a central {MoO7}
cations have been explored. SOMs can undergo topological group. This {MoO7} is symmetrically connected to ve {MoO6}
transformation,51 can be used in patterning52,53 and as active octahedra by edge sharing. This builds a {(Mo)Mo5} pentagon.
particles.46 The catalytic activity of SOMs has also been explored Four of the {MoO6} octahedra are linked to further {MoO6}
in various types of reactions viz., in photo-polymerization54–56 octahedra via corner sharing to form the {Mo8} unit. The two
and photochemical water oxidation.57 In this article we have MoO6 octahedra, which are not directly connected to the central
shown that SOM type materials based on oxo-molybdate and MoO7 bipyramid are fused to neighbouring {Mo8} units through
oxo-tungstate clusters can efficiently catalyse the reduction of corners. Neighbouring {Mo8} groups are additionally fused
CO2 to HCOOH/HCHO by transferring the electrons and together by the {Mo2} units, thereby completing the inner-ring
protons generated from oxidation of water. The catalysts re- parts of the upper and lower half of the ring structure. The
ported here present a new opportunity as they provide a method complete construction of the ring is achieved by fusing the
that is scalable and fast with a very high turnover number.
second half through the 14 {Mo1} groups at the equator aer
rotating around 360/14ꢂ relative to the rst. Thus, the ‘giant-
wheel’ can be formed in terms of the three different building
blocks as [{Mo2}14{Mo8}14{Mo1}14]14ꢁ
.
Results and discussion
About the catalysts
We have synthesized crystals of these giant-wheel like
molybdenum blue compound, {Na15[{Mo126}VI{Mo28}VO462H14
(H2O)70 0.5
]
[{Mo124}VI{Mo28}VO457H14(H2O)
]
68 0.5$ca.
400H2O}
We applied the design principles in heterogeneous catalysis
mode to render the process scalable and rapid. Following the
above design principle three catalysts were synthesized (Fig. 1):
following a literature procedure.59 {Mo154} has a space group P1.
It is characterized by FT-IR (Fig. 2a), resonance Raman spec-
troscopy (Fig. 2b) and SEM (Fig. 3a). The IR spectrum
ꢀ
{Mo154}x, where x ¼ 1165
(1) shows characteristic peaks at 1629 (dH O), 1409, 1308, 1015, 955
2
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Fig. 3 (Left column) SEM images of 1, 2 and 3 (a, b & c, respectively).
Vesicle like morphology is seen in the case of 1 and 2. (Right column)
TEM image of 3 is shown in d–f. Both SEM & TEM images of 3 show
embedded Mo132 spheres on the surface of RGO sheet. Curves of RGO
sheets are shown by black arrows while white arrows indicate Mo132
spheres.
Fig. 2 Characterization of all the catalysts by FT-IR and Raman
spectroscopy, from the top to bottom row: catalyst 1, 2 and 3. The
presence of characteristics bands in the FT-IR (a, c, e) and Raman (b, d,
f) spectra are observed for all the catalysts. In the Raman spectrum of
catalyst 3 (Mo132@RGO), characteristic D and G bands are shifted
somewhat, which might be due to the interaction of Mo132 with the
RGO surface.
Molecular structure of {Mn6P3W24}
(nMo]O), 898, 801, 625 and 565 cmꢁ1
.
Using an excitation of
59
This polyoxometalate has the formula, {Mn6P3W24O94(H2-
O)2}17ꢁ (Fig. 1). It crystalizes in a monoclinic unit cell, which
consists of two Keggin type {Mn3PW9O34} units linked by one
{PW6O24} unit. The synthesized POM has C2v symmetry with
a banana shape. The molecular structure may also be called
a double sandwiched structure that implies the presence of two
distinct {Mn3O13} triads formed by octahedral units of {MnO6}.
In this structure, all six of the Mn centers are not in similar
environment as one of the terminal Mn atoms from each Keggin
type cage are coordinated with a water molecule. The Mn–Mn
distance in the crystals are not all identical with a minimum
the 1064 nm band, a resonance-Raman spectrum is obtained
with ve bands characteristic for the molybdenum blue species at
802 (nasMo–O), 531, 458, 325 (dO–Mo–O) and 215 (nasMo–O) cmꢁ1
.
60
The electronic absorption spectrum is dominated by two bands
characteristic for molybdenum blue species at 745 and 1076 nm
(Fig. 4a). The rst maxima is assigned to intervalence (MoV/MoVI)
charge transfer transitions (IVCT) and the second maxima
corresponds to the abundance of 28 MoV centers.61
˚
˚
distance of 3.30 A and maximum distance of 3.35 A. From these
Mn–Mn distances we can conclude that there is no Mn–Mn
magnetic coupling possible in the molecule. The oxidation state
of manganese is +II, which is conrmed from bond valance sum
method.63 The characteristic FT-IR peaks (cmꢁ1) are: 1617
On active catalyst (1)
Synthesis and characterization of {Mo154 1165 superstruc-
}
tures. The active catalyst is a SOM i.e., dispersion of {Mo154} in
water. {Mo154} undergoes self-assembly in a dispersion to form
a vesicle-like SOM superstructure (1). Each vesicle-like SOM
contains 1165 {Mo154}.62 The structure is designated to have the
(dH O), 1065 (nP–O), 932 (nW]O), 730 (nW–O–W) and 480 (nMn–O
)
2
cmꢁ1 (Fig. 2c). The Raman peaks (cmꢁ1) at: 488, 602, 797 and
933 are attributed to (nas,Mn–O), (nW]O) and (nP–O), respectively
(Fig. 2d). From EAS spectroscopy, the absorption maximum is
found to be at 250 nm (Fig. 4b). The structure can be obtained in
the CCDC database by quoting the CCDC no. 1057222.
A detailed account of the synthesis of {Mn6P3W24} is given in
the experimental section.
following formula: {Mo154}1165. Here 1165 rings are icosahe-
drally placed around 12 vertices of the quasi-icosahedron. The
structure is a hollow shell of {Mo154} rings, whose cavity and
exterior are lled with water molecules. The SEM image indi-
cates the spherical nature of the vesicles with dimension near
about 50 nm. The dynamic light scattering study of 1 deter-
mines the hydrodynamic radius of the SOM 1 as 51 nm (Fig. 4d).
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Fig. 4 UV-Vis spectroscopy of 1, 2 and 3 (upper panel) shows characteristic bands for {Mo154} (745 and 1076 nm), {Mn6P3W24} (250 nm) and
{Mo132} (450 nm). The magnified characteristics bands are shown in the insets. The lower panel represents the hydrodynamic radius obtained
from DLS spectroscopy of 1, 2 and 3 with an average of 51, 130 and 300 nm, respectively.
Mo5} building blocks are disposed at twelve vertices of an ico-
sahedron. Such disposition creates space for 30 linkers on
the icosahedron's surface, which simultaneously link to the
prevalent pentagonal building blocks. Omitting the oxygen
atoms for the sake of simplicity, the linkers are {Mo2(CH3COO)}
in the case of {Mo132}. Hence, the overall cluster formulation
can de described as [pentagon]12[linker]30 or {(Mo)Mo5}12-
On active catalyst (2)
Synthesis and characterization of {Mn6P3W24}931 based
SOMs. Taking the solid crystals, we prepared the SOMs of
{Mn6P3W24} in water. A dispersion of SOMs is prepared by just
shaking crystals of {Mn6P3W24} in water. We obtain a dispersion
full of blackberry type self-assembled SOMs43,44 as observed
from the SEM images (Fig. 3b). Each vesicle contains 931
{Mn6P3W24} units. From the EDS experiment we have shown the
ratio of P : Mn : W in crystal is 1 : 2.22 : 8.3, which is very close
to the ratio found in the crystal structure of compound
{Mn6P3W24}. The hydrodynamic radius of 2 is measured from
the DLS experiment, which shows an average hydrodynamic
radius of 130 nm (Fig. 4e). The SEM image also indicates the
radius of the vesicles is around 150 nm (Fig. 3b). We used this
SOM dispersion of 2 as an active catalyst in our experiments.
{Mo2(acetate)}30 or {Mo132(acetate)30} or {Mo132} or {Mo132O372
-
42ꢁ
(CH3COO)30(H2O)72
}
.
The IR spectrum (Fig. 2e) shows main peaks at 1635 (dH O),
2
ꢁ1
59
+
1400 (dCH , nsCOO, dasNH ), 1020, 748 and 565 cm
.
Raman
3
4
bands (Fig. 2f) are observed at 950 (nMo]O), 875, 375 and 314
cmꢁ1 and correspond to the irreducible representations, Hg and
A
1g (bands with the highest intensity). The electronic absorption
spectrum shows an intense band at 450 nm owing to Mo ) O
charge transfer (Fig. 4c).
Mo132 was synthesized following a literature procedure.59
Molecular structure of Mo132
42ꢁ
On active catalyst (3)
{Mo132} of the formula {Mo132O372(CH3COO)30(H2O)72
}
is
a type of molybdenum brown “Keplerate” anion with a diameter
Synthesis and characterization of {Mo132}1064@RGO. Since it
of 2.9 nm. This spherical cluster contains 132 molybdenum is known that Mo132 self-assembles in an aqueous solution to
centers of which 72 molybdenum centers are in the +VI oxida- form a SOM superstructure in dispersion.64 We then made
tion state and the remaining 60 molybdenum centers are in the a new composite material of Mo132 with reduced graphene oxide
+V oxidation state. The dark brown color of Mo132 arises due to (RGO). We synthesized RGO following a literature procedure.65
intervalence charge transfer between the MoV and MoVI centers. The Mo132@RGO composite SOM (3) was prepared by sonica-
Such a Keplerate cluster assembled with acetate as the ligand tion of a solution of Mo132 with RGO and is characterized by
has an icosahedral point group symmetry. 12 pentagonal {(Mo) HATR-IR (Fig. 2e), Raman (Fig. 2f) and UV-Vis spectroscopy
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Fig. 5 (a) MALDI-MS of the product after CO2 reduction. The formation of formic acid is observed as a peak is seen at 46.31. (b) The reaction of
aniline coupling to form amide. (c) MALDI-MS of amide. (d) NMR of amide formed.
(Fig. 4c). The morphology of the composites in the corre- water oxidation is achieved by incorporation of three modular
sponding dispersion was obtained from SEM and TEM (Fig. 3c functions via these metal-oxide based frameworks. (1) Simul-
and d–f, respectively). From the microscopy images it is taneous photo-activation of thousands of metal centres (elec-
observed that Mo132 single clusters are embedded on the tron and hole generation) to oxidize water, (2) water oxidation
surface of RGO. This depicts the nature of 3. In the Raman (O2, proton and electron generation) and (3) concomitant
spectrum we nd the characteristics peaks of RGO are slightly transfer of the generated protons and electrons to reduce CO2 to
shied in the composite material, which may be due to the HCOOH/HCHO by the catalyst framework.
interaction of the attached Mo132 with the RGO surface. All the
Now we describe the catalytic activity of 1, 2, and 3 in the
characteristic peaks of Mo132 are retained in the Raman spec- matter of the photochemical CO2 reduction reaction.
trum, which indicates the integrity of Mo132 is retained in the
composite. The DLS experiment further reveals that the average
hydrodynamic radius of the composite material is 300 nm
(Fig. 4f). The zeta potential of the composite material is
ꢁ40 mV, which indicates that the dispersion is negatively
charged stabilized due to anionic {Mo132} in the reaction
mixture (Fig. S10†). 1064 Mo132 cluster molecules are said to be
Photocatalytic reduction of CO2
In our present case we used 1, 2 and 3 as photocatalysts for
photochemical CO2 reduction. Among the three catalysts, 1 and
2 are one component catalysts while 3 is a two component
catalyst. RGO is used for the facile transport of electrons. The
principle product of CO2 reduction is formic acid; however, we
have also obtained formaldehyde in signicant amounts with
catalysts 1 and 3. In terms of formic acid production, with
respect to every unit of super-structure, 3 gives a maximum yield
involved to form
a single SOM super-structure in the
Mo132@RGO composite, which is the active catalyst. It is to be
noted that the photochemical reduction of CO2 coupled with
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of formic acid of 205 mmol with a turnover number of ca. 1.4 ꢀ 1 and 3; 2 selectively reduced CO2 to formic acid. No other
106 in water (while per mole turnover number is 1366). On the gaseous reduced product was formed during the course of CO2
other hand, 1 and 2 gives a TON of 0.9 ꢀ 106 and 0.25 ꢀ 106 reduction as conrmed by gas chromatography-mass spectros-
(whereas, per mole values are 778 & 270), respectively, at 0.15 copy (GC-MS). Formation of formic acid was further conrmed
mmol loading (for details on the turnover number calculation from the cyclic voltammogram (CV) where we observed a peak
see later). Here water gets oxidized to produce oxygen, protons around ꢁ0.60 V vs. Ag/AgCl electrode, which corresponds to the
and electrons; these protons and electrons are responsible for reduction potential of CO2/HCOOH (Fig. 6a). Simultaneously
the reduction of CO2.
water oxidation during the course of CO2 reduction was char-
The formation of formic acid was rst characterized by acterized using an YSI Clark type electrode. Further conrma-
matrix-assisted laser desorption ionization-mass spectroscopy tion of water oxidation was obtained from the CV where we
(MALDI-MS) using a a-cyano-4-hydroxycinnamic acid matrix observed a sharp increase in current at 1.2 V vs. Ag/AgCl elec-
(Fig. 5a) and quantied by high performance liquid chroma- trode (Fig. 6a), typical of water oxidation. All the reactions were
tography (HPLC) (against an external standard 0.1 M formic carried out at least ve times & the average data for the ve sets
acid). Furthermore, the produced formic acid was characterized were taken and analysed accordingly.
by coupling it with aniline in the presence of HATU (1-[bis-
Next we performed time-dependent experiments to investi-
(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium gate the kinetics of the reaction. It was observed that for 1 and 3
3-oxid hexauorophosphate) as a coupling agent to form the the reaction is almost completed within 40 minutes (Fig. 6b and
amide (Fig. 5b), which was detected by MALDI-MS using an i), while 2 takes 75 minutes to complete the reaction (Fig. 6b).
1
HCCA matrix (Fig. 5c) and H NMR spectroscopy (Fig. 5d). The These reactions are relatively rapid. This is probably due to the
generation of formaldehyde in the reaction mixture was quan- presence of a large number of active catalytic centers in our
tied using HPLC (against an external standard of 0.1 M catalysts.
formaldehyde). Formaldehyde was obtained only in the case of
Fig. 6 (a) Cyclic voltammetry of the reaction product after CO2 reduction coupled with water oxidation, (b) time-dependent formic acid
formation by photochemical carbon dioxide reduction using 1, 2, 3 as the catalyst at pH 5, (c) pH-dependent formic acid formation, (d) time-
dependent evolution of oxygen at pH 5, (e) pH-dependent O2 formation, (f, g & h) variation of catalyst 1 – {Mo154 1165
}
, 2 – {Mn6P3W24}931, 3 –
{Mo132 1064 on formic acid formation, respectively, (i) time-dependent generation of formaldehyde, catalyst 2 shows no formaldehyde gener-
}
ation, (j) pH variation with catalyst 1 and 3 to get maximum formaldehyde at pH 5 for both cases, and (k) & (l) variation of catalyst loading on
formaldehyde formation for 1 and 3, respectively.
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Table 1 Calculation of the number of metal oxide cluster molecules
per vesicle
oxometalate as phase instabilities set in. Thus, aer reaching
a certain value, the active surface area remains constant, which
is reected in the nature of product concentration prole with
catalyst loading (Fig. 6f–h, k and l).
Number of metal oxide
clusters present per vesicle of SOM
˚
s (A)
Catalyst
R (nm)
Mo132@RGO
Mn6P3W24
302
130
29.92
13.78
1064
931
Calculation of the active catalyst concentration and thus, the
TON of the reaction
In the case of 1, 2, and 3, 116.7 mmol, 40.6 mmol and 205 mmol
formic acid was obtained, respectively with a loading of
0.15 mmol catalyst in each case. The turnover numbers of the
reaction per mole of the catalysts are 778, 270 and 1366 for 1, 2,
and 3, respectively. We have already mentioned that these
oxometalates undergo self-assembly in a dispersion to form
a vesicle-like SOM super-structure.43,44,62,67 Therefore, the active
catalyst in reaction medium is different to that found for the
solid catalyst we have taken in the reaction. Also, due to the self-
assembly active surface area of all the catalysts increasing when
compared to the surface area of solid catalysts. We have calcu-
lated the active concentration of the catalysts, i.e., the number
of catalyst molecules forming the SOM vesicle in the dispersion
(Table 1).
Then, we investigated the effect of the pH of the reaction
medium on the photochemical CO2 reduction reaction as well
as the photochemical water oxidation reaction. CO2 reduction
increases with decreasing pH (Fig. 6c and j) while water oxida-
tion reaction is hindered (Fig. 6e). This can be explained by
considering three different reaction equilibria (eqn (4)–(6)).
2H2O / 4H+ + O2 + 4e
CO2 + 2H+ + 2e / HCOOH
CO2 + 4H+ + 4e / 2HCHO
(4)
(5)
(6)
Considering the effective concentration of the catalysts in
the reaction, we calculated the effective turnover number of the
catalysts. The results are summarized and tabulated in Table 2.
In the case of Mo154, the number of metal oxide cluster units
present in a single SOM vesicle is 1165.62 Thus, we have calcu-
lated the number of metal oxide clusters present per SOM
vesicle using eqn (7).
As we know that carbon dioxide reduction is a proton
coupled reaction and therefore, formation of the reduced
product is always favored with increasing proton concentration
in the reaction medium i.e. decreasing the pH of the reaction
medium. On the other hand, the water oxidation reaction
releases protons into the reaction medium and therefore,
removal of protons from the reaction medium enhances the
rate of water oxidation. Thus, with an increasing pH of the
reaction medium, the rate of water oxidation increases.
No. of metal oxide cluster unit ¼ (4pR2)/(72.006s2) ꢀ 60 (7)
Catalyst concentration dependent studies on the reactions
follow the normal trend of heterogeneous catalyst kinetics.66
Initially the concentration of reduced product increases with
the concentration of catalyst up to a certain limit of catalyst
loading. Then, the rate of the increment of product formation
decreases with catalyst loading and nally, the reaction
becomes independent of catalyst concentration. This is due to
the fact that such reactions are totally dependent on the active
surface area of the catalyst, which does not change aer
a certain loading of the catalyst. We already mentioned that
vesicle-like super-structures of oxometalates in the dispersion
are actual active catalysts, therefore the active surface area is
generally the surface area of a vesicle, which increases with
increasing radius (i.e. hydrodynamic radius) of the vesicle. It is
observed that aer certain loading of catalyst, the hydrody-
namic radius does not increase with the concentration of
R is the hydrodynamic radius of the SOM vesicle formed in the
dispersion. R is obtained using dynamic light scattering (DLS)
analysis. s is the diameter of the isolated single cluster, taking
the van der Waals radii of the constituent atoms.
We can now calculate the actual turnover number of this
reaction, which is obtained dividing the concentration of
product by the concentration of active catalyst vesicles in the
dispersion (Table 2). We have performed each experiment ve
times, so an average yield out of those sets of experiments has
been considered to calculate the turnover numbers. In this way
the turnover numbers were found to be 0.9 ꢀ 106, 0.25 ꢀ 106
and 1.4 ꢀ 106 for 1, 2, and 3, respectively.
The water oxidation kinetics for all three catalysts are iden-
tical to the carbon dioxide reduction kinetics (Fig. 6d). This
indicates that the water oxidation and carbon dioxide reduction
reactions take place simultaneously. This also indicates an
Table 2 Calculation of the turnover number per mole of catalyst and the ‘effective turnover number’
Amount of
solid catalyst molecule formed
Catalyst (p) (mmol)
catalyst vesicle (m) dispersion (q ¼ p/m) (nmol) (r) (mmol)
Number of catalyst Concentration of
Formic acid
TON per
TON with respect to
catalyst vesicle in
formed in reaction mole catalyst concentration of catalyst TOF
(s ¼ r/p)
SOM vesicle (t ¼ r/q)
(sꢁ1
)
1
2
3
0.15
0.15
0.15
1165
931
1064
0.129
0.161
0.140
116.7
40.6
205
778
270
1366
0.9 ꢀ 106
0.25 ꢀ 106
1.4 ꢀ 106
377
56
610
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interesting fact that carbon dioxide reduction in this reaction is the CV, we see no peak at ꢁ0.6 V in the absence of water, which
coupled with the water oxidation reaction. We will now check indicates the absence of any reduced products of CO2 like
whether these two processes are indeed coupled.
HCOOH in the reaction medium. Upon gradually increasing the
amount of water, there is an increase in the peak at ꢁ0.6 V along
with a simultaneous increase in the current at 1.2 V, which
indicates water oxidation and the production of O2. A similar
trend is seen in the amount of HCOOH and O2 formed. For
instance, the production of HCOOH increases upon increasing
the amount of water. The above results conrm that photo-
chemical CO2 reduction depends on photochemical water
oxidation and that these processes are coupled.
On the coupling of the redox processes: the dependency of
CO2 reduction on water oxidation
To check whether CO2 reduction depends on the water oxida-
tion reaction, we have performed the photochemical CO2
reduction reaction in dry dimethyl formamide (DMF), keeping
the other conditions unchanged. In this aprotic solvent, we
observed that there is no reduction of CO2 (Fig. 7a and c)
indicating that photochemical CO2 reduction may depend on
the photochemical water oxidation reaction. To further prove
On the reaction pathway
the dependency, we then performed the reaction using different Now, we can delineate the likely reaction pathway. To do so we
ratios of water and DMF. Upon gradually increasing the amount rst identied the essential components of the reaction: 1.
of water in the reaction mixture, we noticed a gradual increase Light: without light the reaction does not take place and 2.
in the amount of formic acid and evolved oxygen in the reaction Water: we have already seen that without water there is no CO2
system (Fig. 7b). We have also shown the increase in the reduction. All these demonstrate that the catalytic reaction has
generation of HCOOH and O2 by cyclic voltammetry (Fig. 7a). In three crucial components: light, water and CO2.
Fig. 7 (a) The CV of the solution after the reaction with a gradual increase in the water amount, (b) the increase in the amount of formic acid and
oxygen with an increase in the water amount in reaction system, (c) the amount of formic acid formation in 10 mL of water and 10 mL of DMF,
showing no formic acid formation in DMF, and (d) the reusability of all three catalysts shown up to 5 cycles of the reaction in terms of the yield of
formic acid.
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We now sketch a pathway for the reaction. We speculate that S13–S15†). For instance, we have reused the catalysts for ve
the initial clusters go to their excited state upon irradiation with cycles and have seen a maximum 25% decrease in the yield for
UV-light. Clusters in the excited state generate excitons that in the production of HCOOH, in the case of catalyst 3. In the case
turn create holes (h+) and electrons (eꢁ) in pairs. Now, the of 1, the decrease is 20% while 2 exhibits the maximum reus-
generated holes (h+s) oxidize H2O to produce oxygen and release ability with a decrease in yield of formic acid of only 12% aer
electrons and protons in the reaction.68 Proton coupled reduction ve cycles (Fig. 7d). Thus, all the catalysts can be reused further
of CO2 to form HCOOH/HCHO takes place by subsequent transfer to catalyse the same reaction.
of the electrons and protons released during water oxidation.
Thus, in this photochemical CO2 reduction coupled with water
oxidation water acts as the only electron source in the reduction
reaction. Aer each cycle the catalyst goes back to its initial state
and gets re-excited by light again and the catalytic cycle continues.
Now, to show the actual catalyst in the reaction medium, we
performed the photochemical CO2 reduction, keeping the other
conditions the same using precursor constituents of 1, 2 and 3.
No reaction was observed with those precursor constituents
(Table S1 in the ESI†). It implies that the vesicle-like SOM
superstructures composed of giant metal oxide clusters are only
responsible for the photochemical CO2 reduction reaction in
water. All these results are tabulated in Table S2.†
On the photochemical activity of the catalysts
To show that the catalysts are photochemically active under the
reaction condition, we have performed dye degradation studies
using 1, 2 and 3. We have used methyl orange for 1, methylene
blue for 2, while perylene tetracarboxylate was used in the case
of 3, to avoid the absorption maxima overlapping with the
catalysts. Methyl orange shows an absorption maxima at
455 nm, methylene blue shows at 560 nm and perylene tetra-
carboxylate has three bands at 385, 410 and 415 nm. The UV
spectra of the dye with the catalysts show that all of the catalysts
can degrade their corresponding dyes upon irradiation as
evident from the decreasing intensity of the UV-Vis maxima of
the dyes with time (Fig. 9).
The stability of catalysts 1, 2 and 3
Here, it is worth mentioning that the SOM-type catalysts
The catalysts are stable up to many cycles of the reaction. This is provide better catalytic activity than the conventional catalysts
shown by Raman and infrared spectroscopy (shown in Fig. 8 & owing to the simultaneous excitation of millions of photoactive
Fig. 8 The stability of all three catalysts after the reaction, as shown by Raman spectroscopy.
Fig. 9 Dye degradation studies of catalyst 1 using methyl orange, 2 using methyl blue and 3 using perylene tetracarboxylate. A gradual decrease
in the intensity of the dyes with the irradiated catalysts proves the photoactivity of the catalysts.
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clusters in the catalyst vesicles under photochemical conditions dried in air. Yield: 3.3 g (52% based on molybdate). Raman
due to the high number density of the molecules in the vesicles, spectrocopy conrmed the structure of the prepared Mo132
.
which is usually not the case in conventional semiconductors
used as photocatalysts.
Synthesis of RGO. Firstly, graphene oxide (GO) was prepared
via the modied Hummers' method.70 Typically, 50 g of H2SO4
(98%) and 2 g of graphite were placed in a reactor cooled to 0 ꢂC
using an ice-water bath. Aer stirring the suspension for
30 min, 0.3 g of KMnO4 was added in small portions keeping the
Experimental section
Materials and reagents
ꢂ
temperature in the reactor less than 10 C using an ice-water
bath. Aer stirring the mixture for 30 minutes, 6 g of KMnO4
All the materials were purchased from commercially available was further added to the suspension gradually. Then, the
sources and used without further purication. All glassware was reactor was heated for 30 minutes to keep the temperature
rst boiled in an acid bath then washed with water and nally constant at approximately 35 ꢂC. As the reaction progressed, the
cleaned with acetone. They were dried in hot air oven overnight. suspension became pasty and brown in color. At the end of this
We used doubly distilled deionized water in all the experiments. 30 min period, 90 mL of water was slowly stirred into the paste
Synthesis of Mo154
(Na15[MoVI126MoV28O462H14(H2O)70 0.5
to prevent violent effervescence, causing an increase in
]
[MoVI124MoV28O457H14 temperature from (90 to 95) ꢂC. The diluted suspension, now
(H2O)68]0.5$hydrate). To a solution of Na2MoO4$2H2O (3.0 g, 12.4 brown in color, was maintained at this temperature for 15 min.
mmol) in 10 mL of water, freshly powdered Na2S2O4 (0.2 g, 1.15 The suspension was then further treated with a mixture of 7 mL
mmol) was added (light yellow coloration). Immediately aer- of hydrogen peroxide (30%) and 53 mL of water to reduce the
wards, under continuous stirring, 30 mL of hydrochloric acid (1 residual permanganate and MnO2 to soluble MnSO4. The
M) was rushed into the solution (color changes to deep blue). suspension was ltered and washed with distilled water three
The solution was stirred in an open 100 mL Erlenmeyer ask for times and the solids were collected by ltration and dried at 45
further 10 min and then stored undisturbed in a closed ask at ꢂC. Graphene was prepared via a hydrothermal process using
20 ꢂC for 3 days. The precipitated blue crystals were removed by a basic solution. Generally, 2 g of GO was mixed in 80 mL of
ltration, washed quickly with a small amount of cold water and ultrapure water and the mixture was stirred for 4 h. The
dried at room temperature over CaCl2. Yield: 0.7 g (13.18% suspension was then ultrasonically treated for 1 h. A concen-
based on Mo).
trated ammonia solution (37 wt%) was used to adjust the
Synthesis and characterization of Na17[Mn6P3W24O94(H2O)2]. suspension to pH 10. The suspension was kept stirred overnight
Na2WO4$2H2O (11.873 g, 0.036 mol), Na2HPO4$7H2O (1.073 g, with a cover. Then, the mixture was transferred into a Teon-
0.004 mol) and Mn(OAc)2$4H2O (1.96 g, 0.008 mol) were mixed in lined autoclave (120 mL) and treated at 453 K for 18 h. The
30 mL of water. The color of the solution turned light yellow and precipitate was washed three times with ultrapure water/
then, the reaction mixture was heated to 100 ꢂC for 3 h. Aer ethanol. Aer ltration, the chemically reduced graphene oxide
heating, the color of the reaction mixture turned dark yellow. The was dried at 353 K.
reaction mixture was cooled to room temperature and then, the
Preparation protocol and stability of the Mo132@RGO
pH was adjusted to 6. The reaction mixture was then saturated composite. We have added a xed ratio of RGO and Mo132 and
with excess NaCl and nally ltered off. A clear yellow solution kept it for 1 week in the dark aer sonication for 4 h. We have
was obtained, which was stored in an open Erlenmeyer-ask for seen that only a RGO/Mo132 ratio of 1.2 : 1 provides a stable
crystallization. Aer 1 week pale yellow crystals were obtained. composite for which a dispersion is stable, even for months. So,
The crystals were ltered, washed with water and dried in vacuum this ratio was taken for characterization and the photochemical
overnight. Yield: 27% (based on Mn). Thermogravimetric esti- experiments.
mation shows 43 water molecules of hydration and agrees well
with that of the unit cell volume (Fig. S2†). Characteristic FT-IR
Characterization techniques
peaks (cmꢁ1): 1617, 1416, 1065, 932, 730, 480. EAS spectroscopy:
lmax ¼ 250 nm. The cif structure of the cluster is available on
Fourier transform infra-red spectroscopy (FT-IR). FTIR
spectroscopy of catalysts 1 and 2 was performed using KBr
discs. Initially a pellet was prepared from a mixture of KBr and
internet vide CCDC number 1057222.
Synthesis of Mo132@RGO
Synthesis of Mo132. The cluster was prepared according to 3 mg of 1 or 2. The FTIR spectrum was recorded using a Perkin
a literature procedure.69 N2H4$H2SO4 (0.8 g, 6.1 mmol) was Elmer Spectrum RX1 spectrophotometer with FTIR facility in
added to a solution of (NH4)6Mo7O24$4H2O (5.6 g, 4.5 mmol) the range 2000–400 cmꢁ1 for 1 & 2 and 3000–500 for 3. The
and CH3COONH4 (12.5 g, 162.2 mmol) in H2O (250 mL). The horizontal attenuated total internal reectance (HATR) spec-
solution was then stirred for 10 min (color change to blue green) trum of catalyst 3 and also for 1 and 2 were also acquired for the
and 50% CH3COOH (83.0 mL) was subsequently added. The liquid samples. In that case we placed 1 mL of the sample on
reaction solution, nowꢂgreen, was stored in an open 500 mL a zinc selenite palate and the IR spectrum was recorded within
Erlenmeyer ask at 20 C without further stirring (fume hood; a range of 2000–600 cmꢁ1. The spectrum obtained in this
slow color change to dark brown). Aer 4 days, the precipitated process is quite different from that obtained using the FT-IR
red-brown crystals of were ltered off over a glass fritt (D2), method. We observe broad peaks rather than sharp peaks,
washed with 90% ethanol, ethanol and diethyl ether, and nally which are obtained in the FT-IR spectrum. This is due to the
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ꢁ3
presence of water in the sample and the low concentration of peak was 7.83 e A and the deepest hole ꢁ10.784 e Aꢁ3. The
˚
˚
the sample.
unique crystal data number is CCDC 1057222.
Electronic absorption spectroscopy (EAS). Very dilute solu-
Electron paramagnetic resonance spectroscopy (EPR). A very
tions of 1–3 were taken in a quartz cuvette and the electronic well ground sample was poured into a thin EPR tube and the
absorption spectrum were recorded on a U-4100 spectropho- EPR spectrum was measured at room temperature on a Bruker A
tometer (Liquid) within the desired range.
300 electron paramagnetic resonance instrument in the range
Cyclic voltammetry (CV). A PAR model 273 potentiostat was of 1350 Gauss to 5350 Gauss.
used for the CV experiments. A platinum wire auxiliary elec-
Thermogravimetric analysis (TGA). TGA was performed
trode, a glassy carbon working electrode with surface area of using a Shimadzu DTG-60 thermal analyzer system at a heating
0.002826 cm2 and an aqueous Ag/Ag+ reference electrode, which rate of 10 ꢂC minꢁ1 to 500 ꢂC in a dried air atmosphere and the
was lled with saturated KCl solution, were used in a three air ow rate was 30 mL minꢁ1. The sample was loaded on an
electrode conguration. The scan rate was 0.5 V sꢁ1. The CV alumina pan.
spectrum was recorded in the range of ꢁ1.2 V to +1.3 V. The
1H NMR spectroscopy. 1H NMR spectroscopy was performed
blank refers to the amount of oxygen present in distilled water using a Bruker Avance 500 MHz spectrometer.
in our mentioned reaction conditions. The pH of the medium
General reaction procedure for the photo-catalytic CO2
was 7. 0.1 M KCl solution was used as the supporting electrolyte reduction and water oxidation reactions. The photo-catalytic
in all the experiments. All measurements were performed at carbon dioxide reduction reactions were performed as follows:
298 K under an inert atmosphere.
a desired amount of catalyst (1–3) was taken in 10 mL of
Raman spectroscopy. A LABRAM HR800 Raman spectrom- degassed double distilled water. The reaction mixtures were
eter was employed using the 633 nm line of a He–Ne ion laser then sealed and CO2 gas was purged for 2 h. The reaction
(l ¼ 633 nm) as the excitation source to analyze the sample.
mixtures were kept in a photo-reactor under UV-light (energy
Resonance Raman spectroscopy. A Bruker RFS27 spectrom- density 19 mW cmꢁ2 lamp, l ¼ 373 nm) for different time
eter was employed using the 1064 nm line as an excitation intervals. Then, 20 mL of the reaction mixture was taken out
source.
from the aliquot and diluted to 10 mL with doubly distilled
Matrix-assisted laser desorption ionization-mass spectrom- water. The aliquots were used directly for HPLC to quantify the
etry (MALDI-MS). Matrix-assisted laser desorption ionization liquid products with an external standard of 0.1 M formic acid
time-of-ight (MALDI-TOF) mass spectrometry was carried out and formaldehyde solution. Furthermore, a Clark type electrode
on a Bruker ultraeXtreme™ instrument equipped with a smart was used to measure the evolved oxygen due to water oxidation.
beam-II laser in the reector mode and 22 kV acceleration All of the results of these experiments are recorded in Fig. 6. For
voltage. 2,5-Dihydroxybenzoic acid (DHB, Bruker) was used as MALDI-MS experiment we co-crystalized the product with an
the matrix.
HCCA matrix prior to recording the MS data. From all the mass
Transmission electron microscopy (TEM). TEM (trans- measurement techniques we obtained the molecular ion peak
mission electron microscopy) images are taken with a JEOL JEM of formic acid.
2010 electron microscope.
Next we performed CV experiments with the reaction mass
Dynamic light scattering (DLS). DLS experiments were per- using KCl as an electrolyte within a potential range of +1.5 V to
formed using a Malvern Zetasizer instrument equipped with ꢁ1.2 V with respect to the Ag/AgCl reference electrode in
a 633 nm laser.
a standard 3-electrode system. We obtained a peak around
Scanning electron microscopy-energy-dispersive X-ray spec- ꢁ0.6 V, which is indicative of the formation of formic acid
troscopy (SEM-EDS). The images were recorded with a SUPRA 55 from carbon dioxide. To further prove the formation of formic
VP-41-32 scanning electron microscope and analyzed using the acid in the reaction medium we performed a coupling reaction
Smart-SEM version 5.05 Zeiss soware. The SEM sample was with the resulting reaction solution. An excess solution of
prepared by drop casting a very dilute dispersion onto a silicon aniline (100 mL) in 2 mL of acetonitrile was added to the
wafer and drying in a dust free area.
reaction mixture and 20 mg HATU (1-[bis(dimethylamino)
Single crystal X-ray diffraction (SCXRD). A pale yellow crystal methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexa-
of 2 was selected on a SuperNova, Dual, Cu at zero, Eos uorophosphate) was added as a coupling agent. The reaction
diffractometer. The crystal was kept at 100.00(12) K during data mixture was stirred for 1 h at room temperature resulting in
collection. Using Olex2, the structure was solved with the the formation of the amide in the reaction medium. The
superip structure solution program using charge ipping and organic components were extracted with EtOAc (3 ꢀ 10 mL)
rened with the ShelXL renement package using least squares and then, the EtOAc was removed in vacuo. Then, the solid
minimization. Single intensity data was collected with graphite- mass was further dissolved in acetonitrile to perform GC-MS
monochromatic Mo-Ka radiation (m ¼ 20.898). Among the and MALDI-MS following the above mentioned procedure.
23 969 unique reections (Rint ¼ 0.0505, 2qmax ¼ 50.06) 18 947 Aer purication we further characterized the amide using 1H
were considered to be observed with (I > 2s(I)). The nal cycle of NMR spectroscopy.
renement, including the atomic coordinates, anisotropic
High performance liquid chromatography (HPLC). All the
thermal parameters, (W, P, and Ni atoms), and isotropic reaction samples were monitored by an HITACHI-HPLC
thermal parameters (Na and O atoms), converged at R ¼ 0.0869 system equipped with binary 2130 pumps, a manual sampler
ꢂ
and Rw ¼ 0.221 (I > 2s(I)) In the nal difference map the highest and 2490 refractive index detector, maintained at 50 C. The
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products were separated on a sugar ion-exclusion column
(250 ꢀ 4.8 mm) maintained at 60 ꢂC using water as the mobile
phase at a 0.8 mL minꢁ1 ow rate. The HPLC system was
controlled and processed by Inkarp soware. A standard for-
mic acid and formaldehyde solution was prepared and cali-
brated. Each product sample was diluted with a known
volume of milli-Q water before analysis to prevent overloading
the column. All experiments were carried out ve times and
the average values were reported within a standard deviation
of <5.0%.
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We have shown a scalable pathway for photochemical carbon
dioxide reduction coupled with water oxidation using three
different SOM type heterogeneous mixed valent metal oxide
based clusters with the highest turnover numbers recorded till
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achieving an effective turnover number as high as 1.4 ꢀ 106.
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carbon dioxide reduction model system where water oxidation
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sunlight does not escape our attention. The end product formic
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Acknowledgements
SR gratefully acknowledges the grants obtained from IISER-
Kolkata, India, DST-fast track and BRNS-DAE. SD and TB
acknowledge IISER-K; SB and SB (Barman) acknowledge UGC
and RP acknowledges DST for fellowship. This paper is dedi-
¨
cated to Prof. Achim Muller.
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