Visible light assisted photocatalytic reduction of CO2 using a graphene oxide supported heteroleptic ruthenium complex

Article abstract of DOI:10.1039/c4gc01400f

A new heteroleptic ruthenium complex containing 2-thiophenyl benzimidazole ligands was synthesized using a microwave technique and was immobilized to graphene oxide via covalent attachment. The synthesized catalyst was used for the photoreduction of carbon dioxide under visible light irradiation without using a sacrificial agent, which gave 2050 μmol g^-1 cat methanol after 24 h of irradiation.

Full text of DOI:10.1039/c4gc01400f

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Visible light assisted photocatalytic reduction of CO2 using graphene
oxide supported heteroleptic ruthenium complex
Pawan Kumar,a Amit Bansiwal,b Nitin Labhsetwarb and Suman L. Jaina*
5
aChemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun-248005 (India); *Email: Suman@iip.res.in
bCSIR-National Environmental Engineering Research Institute (CSIR-NEERI), Nehru Marg, Nagpur-440020, India
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
further be increased by anchoring of visible light absorbing
molecule to the support of graphene oxide. Recently we have
50 reported graphene oxide immobilized cobalt phthalocyanine for
the photoreduction of CO2 under visible light irradiation. The
yield of methanol after 48 hours of reaction was found to be
3781.8881ꢁmol.gꢀ1ꢀcat.22a Subsequently we reported graphene
oxide immobilized ruthenium trinuclear complexes for the
55 photoreduction of CO2 and the yield of methanol after 48 h
illumination was found to be 3977.57±5.60 ꢁmol.gꢀ1 cat.22b The
methanol yield using these immobilized catalysts was found to be
much hogher as compared to the graphene oxide alone, however
the need of triethylamine as sacrificial donor makes the
60 developed methods of less practical utility. In continuation to our
ongoing research, herein we present a novel approach for
synthesizing heteroleptic ruthenium complex and its
immobilization to graphene oxide by covalent bonding.
Activation of the synthesized graphene oxide immobilized
65 ruthenium catalyst under visible light in the absence of sacrificial
donor led to the reduction of CO2 into methanol and afforded
methanol yield 2050 ꢁmol.gꢀ1 cat after 24 h of irradiation.
A
new heteroleptic ruthenium complex containing 2-
10 thiophenyl benzimidazole ligands was synthesized using
microwave technique and immobilized to graphene oxide via
covalent attachment. The synthesized catalyst was used for
the photoreduction of carbon dioxide under visible light
irradiation without using sacrificial agent, which gave 2050
15 µmol.g-1 cat methanol after 24 h of irradiation
Finding cleaner energy sources to satisfy the world’s growing
demand is one of society’s foremost challenges for the next halfꢀ
century. Utilization of solar energy through the generation of
hydrogen from water splitting1 and/or through the photocatalytic
20 reduction of carbon dioxide2 to fuels (CO, CH3OH, and CH4) has
been widely accepted to be a realistic solution to fulfill the energy
demands. The subject has been recently reviewed in a number of
critical reviews3 as well as a significant number of patents4ꢀ9 have
been granted in last few decades showing the conisderable
25 progress towards practical utility of photogeneration of hydrogen
from water or of fuels, such as carbon monoxide, methanol and/or
methane from the photoinduced reduction of carbon dioxide.
Despite of various known heterogeneous photocatalysts such as
metal oxides, metal doped oxides, mixed metal oxides and
30 composites,10ꢀ16 transition metal complexes such as rhenium(I)
bipyridine, ruthenium(II) complexes, cobalt (II) trisbipyridine,
cobalt (III) macrocycles, metalloporphyrins and nickel tetraꢀaza
macrocycles (cyclams) have also been reported in the literature to
carry out the photocatalytic reduction of CO2 to valuable C1
35 building blocks.17 However, homogeneous nature of these
catalysts makes the process unviable from economic as well as
environmental viewpoints.18 This problem can be solved by
anchoring of such homogeneous complexes to photoactive
support, which not only provide facile recovery of the catalyst,
40 but also photoactive support and complexes may work
synergistically to provide better electron transfer to CO2.19
Synthesis and characterization of the catalyst
At first 2ꢀthiophenyl benzimidazole ligand was synthesized from
70 the reaction of oꢀphenylene diamine and thiophene 2ꢀ
carboxaldehyde in the presence of cadmium chloride.23 As
prepared sulphur containing ligand was used for the synthesis of
corresponding ruthenium complex24 by reacting it with
Ru(bpy)2Cl2 under microwave irradiation as shown in Scheme 1.
75 The synthesized homogeneous heteroleptic ruthenium complex
was subsequently immobilized to graphene oxide support via
covalent attachment. For this purpose, graphene oxide was
obtained from oxidation of graphite with KMnO4 and H2SO4
according to Hummer’s method.25 Occurrence of plenty oxygen
80 containing functionalities in the form of free –OH, ꢀCOOH and
epoxide groups and high specific surface area made graphene
oxide an ideal material for grafting of homogeneous complexes.
In most of the literature reports, the oxygen containing functional
groups presented at edges of GO were used for immobilization
85 and therefore low loading of catalyst was obtained.26 Here, we
have also targeted the epoxide groups located on the basal plane
Chemically derived graphene oxide (GO) owing to the presence
of ample oxygen functionalities and higher surface area, it has
come out to be an outstanding support matrix for immobilizing
45 homogeneous metal complexes.20 By using graphene oxide as
photocatalyst, Hsu et. al reported 0.172 ꢁmol.gꢀ1 cat hꢀ1 methanol
as photoreduction product of carbon dioxide.21 The yield can
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of sheets for the attachment of metal complex. Thus, we have 35 like CO, CH4 etc. The absence of peak in GCꢀFID and GCꢀMS
treated the GO with chloroacetic acid to convert –OH and
epoxide groups in to–COOH groups. Thus obtained COOH
functionalized graphene oxide was readily converted to –COCl
functionalized GO by treating it with thionyl chloride. The
for any other possible liquid product, and in GCꢀRGA (TCDꢀ
FID) for any possible gaseous product inferred the high
selectivity (Catalytic Selectivity = 1) of the catalyst 2 for
methanol formation. As methanol was the major photoreduction
5
obtained chemically functionalized COClꢀGO support was treated 40 product, its formation rate, RMeOH (µmol/g.cat) as a function of
with ruthenium complex 1 to give graphene oxide immobilized
ruthenium catalyst 2 as shown in Scheme 2.
reaction time was calculated and plotted in Fig. 1. It can be
clearly seen that after 24 hours of illumination, the methanol
yield for catalyst 2 was found to be 2050 ꢁmol.gꢀ1 cat in the
absence of sacrificial agent. However, the use of graphene oxide
45 as photocatalyst provided only 482 ꢁmol.gꢀ1cat yield of methanol
under otherwise identical experimental conditions. The quantum
yield (ϕ) for methanol formation was estimated to be 0.180 for GOꢀ
Ru catalyst 2 and 0.044 for GO. Further, to evaluate the effect of
ruthenium complex units attached to GO, we conducted CO2
50 reduction experiments by using GOꢀCOOH, 5% RuCl3/GO and
equimolar homogeneous complex 1 as presented in GOꢀRu 2. The
methanol yield using these catalysts was found to be 320, 739 and
1048 µmol/g.cat respectively and the corresponding quantum yield
(ϕ) was obtained 0.029, 0.067 and 0.09 respectively. These results
55 suggested that the incorporation of ruthenium complex 1 units to GO
sheet enhanced the photocatalytic activity of GO significantly, which
is most likely due to the better inflow of electrons to the conduction
band of GO.
10
Scheme 1: Synthesis of 2ꢀthiophenylbenzimidazole ligand and
heteroleptic Ruthenium (II) complex 1
Three blank experiments i.e. i) visible light irradiation without
60 catalyst 2; ii) under dark with catalyst 2; and iii) purging with N2
instead of CO2 were also performed. There was practically no
photoreduction product found in all the above mentioned blank
experiments even after the long period of illumination.
Scheme 2: Synthesis of GOꢀRu catalyst 2
15 The loading of ruthenium complex on GO support was found to
be 0.51 mmol/g as determined by ICPꢀAES analysis. The
complete characterization of the catalyst including surface
properties, XRD, FTIR, SEM, TEM, UVꢀVis and TGA are given
in the supporting information of the manuscript (See ESI file).
65
Fig. 1: CO2 to methanol formation rate a) blank reaction b) using GOꢀ
COOH c) GO d) 5% RuCl3 /GO e) Ru complex equimolar amount to GOꢀ
Ru catalyst 2 and c) GOꢀRu catalyst 2
20 The photocatalytic activation of CO2
After the photocatalytic reaction, catalyst was easily recovered by
centrifugation and reused for recycling experiments. It can be
The photocatalytic activity of synthesized GO and GOꢀRu
catalyst 2 was tested for the photoreduction of CO2 in water and
DMF system saturated with CO2 without using sacrificial agent
under visible light irradiation. After various period of irradiation,
25 1ꢁl sample was withdrawn and analyzed in a GC/FID equipped
with a 30 m long Stabilwax® w/IntegraꢀGuard® column. For
maintaining the accuracy of measurements, the sample was
injected with the help of autosampler. The obtained peak area
was correlated with standard calibration curve for quantitative
30 determination of methanol. Methanol yield was used to evaluate
the performance of the catalysts as it was obtained as the major
reduction product. To determine the gaseous products, 20 ꢁL
sample was injected in GCꢀRGA (TCDꢀFID). The gas phase
analysis did not show the presence of any possible byꢀproduct
70 seen from Fig. 2 that the recovered catalyst exhibited almost
similar activity for the photoreduction of CO2 as the fresh one.
There was no leaching of metal/ ligand observed, which was
further ascertained by ICPꢀAES analysis of recovered catalyst.
The recovered catalyst after three runs gave ruthenium content of
75 5.07 % which was nearly similar to the fresh one (5.15%)
considering the experimental error.
2
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optical band gap, as determined by Tauc’s plot.29 This value was
35 well below for better visible light mediated transitions.
Fig. 2: Reuse experiments for catalyst 2
Isotopic tracer experiments
Fig 4: Cyclic voltametry curve of homogeneous Ru complex 1
For establishing the origin of methanol from CO2, instead of
organic matter presented in the reaction media or catalyst, we
performed isotopic tracer experiment by using 13CO2 in place of
12CO2 while all other conditions were kept identical. After
illumination of the photoreaction mixture, the product was
analyzed by GCꢀMass, giving 13CH3OH at m/z 33 instead of
Based on the band gap values a plausible mechanism for the
photoreduction of CO2 to methanol is shown in Scheme 3.
40 Oxygen containing functionalities in GO converts aromatic sp2
carbon into sp3 carbons so 2D network of sp2 and sp3 domain is
created on GO’s surface. Aromatic sp2 hybridized carbon
containing domain behave like conduction band from which
electrons can freely move with minimum resistance. While sp3
45 carbon containing domain have tightly held electrons and behave
as valance band. Thus, a number of semiconductor zones evolve
on the surface of graphene oxide.30 As shown in Tauc plot of GO,
the large band gap prevents the transition of electrons from
valance band to conduction band in visible light. Because of
50 small difference of HOMOꢀLUMO, the synthesized ruthenium
complex can absorb strongly in visible region. So it can easily get
excited after absorbing visible light and transfer electrons to
conduction band of GO. Finally these electrons used for the
reduction of CO2 adsorbed on the surface of GO to methanol.22,31
55 Ru(HOMOꢀLUMO) → Ru*(HOMO+ + LUMOeꢀ) MLCT
Ru*(HOMO+ + LUMOeꢀ)→ Ru*(HOMO+ + LUMO) + eꢀ (CB of
GO)
5
10 12CH3OH m/z 32 (Fig. S19).
To explain the better photoactivity of catalyst, optical band gap
values were calculated. Tauc plot of ruthenium complex 1 Fig. 3a
gave the optical band gap value 1.90 eV to 2.29 eV, which was a
strong indication of visible light active nature of complex 1.27 For
15 GO Fig. 3b, instead of a sharp optical band gap, a range of band
gap (2.9ꢀ3.7 eV) was obtained that was due to the uneven
oxidation of sheets. This obtained value was in well conformity
with the reported literature value.21,28 While in case of GOꢀRu
catalyst 2, the value of band gap was found to be 1.15 eV, and 2.9
20 eV respectively. The value of band gap at 1.15 eV suggested that
the attachment of ruthenium complex 1 to GO support enhanced
the photoactivity of material significantly. Another band gap
value at 2.9 eV, corresponding to the conjugated aromatic ring
system was observed most likely due to the participation of
25 oxygen containing groups in bond formation with Ru complex 1
and therefore, numerous aromatic domains were evolved on the
sheets of the catalyst 2.
Ru*(HOMO+ + LUMO) + eꢀ(derived from water splitting) →
Ru(HOMOꢀLUMO)
60 6eꢀ CB (GO) + CO2 + 6H+(derived from water splitting) →
CH3OH + H2O
Scheme 3: Plausible mechanism of photoreaction
65 Conclusions
We have successfully developed and demonstrated a novel
heteroleptic ruthenium(II) complex immobilized to graphene
oxide as an efficient heterogeneous photocatalyst for the
photocatalytic reduction of CO2 to methanol without using
70 sacrificial agent and under visible light irradiation. After the
photoreduction, the catalyst was easily recovered by
centrifugation and reused for subsequent runs. The recovered
Fig. 3: Tauc plot for calculating band gap of a) Ruthenium complex 1 b)
30
GO c) GOꢀRu catalyst 2
Further experimental data of cyclic voltametry gave the
difference of HOMOꢀLUMO (half wave potential, E1/2). The
value of 1.915 eV (Fig. 4, Table S1) was in well conformity of
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aChemical Sciences Division, CSIR-Indian Institute of Petroleum,
Dehradun-248005, India
Tel. 91-135-2525788; Fax: 91-135-2660202; Email: suman@iip.res.in
20 bEnvironmental Materials Division, CSIR-National Environmental
Engineering Research Institute (CSIR-NEERI), Nagpur-India
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DOI: 10.1039/C4GC01400F
Visible light assisted photocatalytic reduction of CO2
using graphene oxide supported heteroleptic ruthenium
complex
Pawan Kumar, Amit Bansiwal, Nitin Labhsetwar and Suman L. Jain
5
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Journal Name, [year], [vol], 00–00 | 5