A novel Ru/TiO2 hybrid nanocomposite catalyzed photoreduction of CO2 to methanol under visible light

Nanoscale
PAPER
Cite this: Nanoscale, 2015, 7, 15258
Received 5th June 2015,
Accepted 10th August 2015
DOI: 10.1039/c5nr03712c
www.rsc.org/nanoscale
A novel Ru/TiO2 hybrid nanocomposite catalyzed
photoreduction of CO2 to methanol under
visible light†
Pawan Kumar,a
Chetan Joshi,a
Nitin Labhsetwar,b
Rabah Boukherroubc
and
Suman L. Jain*a
A novel in situ synthesized Ru(bpy)3/TiO2 hybrid nanocomposite is developed for the photoreduction of
CO2 into methanol under visible light irradiation. The prepared composite was characterized by means of
SEM, TEM, XRD, DT–TGA, XPS, UV–Vis and FT-IR techniques. The photocatalytic activity of the syn-
thesized hybrid catalyst was tested for the photoreduction of CO2 under visible light using triethylamine
as a sacrificial donor. The methanol yield for the Ru(bpy)3/TiO2 hybrid nanocomposite was found to be
1876 μmol g−1
cat (ϕMeOH 0.024 mol Einstein−1
) that was much higher in comparison with the in situ syn-
thesized TiO2, 828 μmol g−1
) and the homogeneous Ru(bpy)3Cl2
complex, 385 μmol g−1
).
Introduction
The development of visible light driven photocatalysts that can
use clean, abundant and safer solar energy for the reduction of
CO2 to high value-added chemicals is an area of tremendous
importance.1
Transformation of CO2 to fuels via photocatalytic
means may provide a long term solution for the shortage of
fossil fuels and the global warming.2
The first report on the
photoreduction of CO2 to organic compounds under UV light
using TiO2 as a photocatalyst dates back to 1979.3
However,
the low quantum efficiency due to a relatively large band gap
of TiO2 semiconductor focuses the interest towards enhancing
the light response of TiO2 in the visible region.4
In this regard,
many methods to widen the absorption wavelength of TiO2,
i.e. band gap engineering, doping with metals, nano-sized
TiO2 particles, TiO2-based binary catalysts, etc. have been
developed.5
Recently, transition metal-based complexes due to
their wide absorption range in the visible region and higher
efficiencies have been considered to be superior catalysts for
photoreduction of CO2 to high value chemicals.6
A number of
metal complexes, such as ruthenium(II) polypyridine carbonyl
complex,7
cobalt(II) trisbipyridine,8
cobalt(III) macrocycles9
and
rhenium(I) bipyridine (bpy) complexes10
have been used as
effective catalysts for CO2 reduction, providing a relatively high
quantum yield and high selectivity of the products. Among the
known transition metal complexes, tris-(2,2′-bipyridine) ruthe-
nium(II) chloride (Fig. 1), owing to its excellent photophysical
and photochemical properties has extensively been used as a
photosensitizer for various applications including CO2 photo-
reduction.11
However, the difficult recovery and non-recycling
ability of this and other homogeneous complexes make their
utility limited. One of the logical solutions to overcome these
problems is to anchor these complexes to nanocrystalline semi-
conductor supports which not only provide facile recovery of the
catalyst, but also photoactive support and the complex may
work synergistically to provide better electron transfer to CO2.12
In continuation of our ongoing research on photocatalytic
reduction of CO2,13
we herein report an efficient Ru(bpy)3/TiO2
Fig. 1 Structure of the Ru(bpy)3Cl2 complex.
†Electronic supplementary information (ESI) available: GC chromatograms of
reaction products and calibration curve for methanol analysis. See DOI: 10.1039/
c5nr03712c
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
Environmental Materials Division, CSIR-National Environmental Engineering
Research Institute (CSIR-NEERI), Nagpur, India
c
Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN) UMR
CNRS 8520, Université Lille1, Avenue Poincaré-BP60069, 59652 Villeneuve d’Ascq
Cédex, France
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hybrid nanocomposite prepared by the in situ grafting of
Ru(bpy)3Cl2 complex to nanocrystalline TiO2 for the photo-
reduction of CO2 to methanol under visible light.
Results and discussion
Synthesis and characterization of the catalyst
The required Ru(bpy)3/TiO2 hybrid was synthesized by grafting
of the (Ru(bpy)3Cl2) complex to the in situ synthesized TiO2
through a precipitation method14
as shown in Scheme 1. In a
typical synthesis, titanium tetrachloride (TiCl4) was added to an
ethanolic solution containing triethanolamine; the resulting
precipitate was dissolved in water and added with the
Ru(bpy)3Cl2
15
complex. The obtained solution was re-precipi-
tated in ammoniacal solution under sonication to give the
desired hybrid Ru(bpy)3/TiO2 nanocomposite. For comparison
in situ TiO2 was synthesized by using the same procedure
without adding the ruthenium complex. Furthermore, to
compare the activity and stability of the synthesized hybrid i.e.
Ru(bpy)3/TiO2, we also synthesized a ruthenium complex
grafted to the commercially available P25 TiO2 support via an
adsorption method using similar equivalents of Ru(bpy)3Cl2 to
that in the hybrid Ru(bpy)3/TiO2.
The surface morphology of the in situ synthesized TiO2 and
Ru(bpy)3/TiO2 was determined by scanning electron
microscopy (SEM) (Fig. 2). The SEM image of the in situ syn-
thesized TiO2 showed small sized particles (25–35 nm) of irre-
gular shape which remained almost unchanged in the hybrid
Ru(bpy)3/TiO2. The EDX pattern of in situ TiO2 indicated the
presence of Ti and O elements (Fig. 2c), whereas the additional
peak of Ru and Cl in the EDX of Ru(bpy)3/TiO2 confirmed the
presence of these elements in the synthesized hybrid (Fig. 2c
and d). FE-SEM elemental mapping showed the uniform and
homogeneous distribution of ruthenium in the composite
(Fig. 2e and f). For determining the fine structure of the cata-
lyst, HR-TEM analysis was performed (Fig. 3). The HR-TEM
image of in situ TiO2 at the 10 nm scale showed rough imper-
fect structures with fringes having 0.35 nm inter planar dis-
tance due to the (101) crystalline planes of TiO2 (Fig. 3a). This
morphology remained almost unchanged in the hybrid
Ru(bpy)3/TiO2 composite (Fig. 3b) with (101) diffraction
planes, suggesting that the presence of the ruthenium
complex did not influence the crystallization phase of TiO2
(Fig. 3b). SAED patterns of in situ TiO2 and the hybrid Ru-
(bpy)3/TiO2 exhibited more number of rings due to the diffrac-
tion of different planes of TiO2, confirming the polycrystalline
nature of the material (Fig. 3c and d). Furthermore, STEM
elemental mapping of the Ru(bpy)3/TiO2 composite showed
that the ruthenium complex was equally distributed through-
out the composite (Fig. S1†). The EDX pattern confirmed the
presence of ruthenium in the composite (Fig. S1†).
Vibrational spectra of the homogeneous Ru(bpy)3Cl2
(Fig. 4a) complex revealed characteristic peaks at 1037, 1376,
1633, 2258, 2294 and 2412 cm−1
due to the ring vibrations of
bipyridine.16
The FTIR spectrum of in situ TiO2 exhibited peaks
at 1044 cm−1
due to Ti–O, 1401 and 1627 cm−1
due to interca-
lated water molecules (Fig. 4b).17
The broad peak at 3410 cm−1
was also observed due to –OH functionalities and adsorbed water
molecules. For the in situ synthesized hybrid Ru(bpy)3/TiO2, the
appearance of the corresponding peaks of both components i.e.Scheme 1 Synthetic outline of the in situ Ru(bpy)3/TiO2 photocatalyst.
Fig. 2 SEM images of: (a) in situ TiO2; (b) Ru(bpy)3/TiO2; EDX pattern of
(c) in situ TiO2; (d) Ru(bpy)3/TiO2; elemental mapping of (e) in situ TiO2;
(f) Ru(bpy)3/TiO2.
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TiO2 and Ru(bpy)3Cl2 (Fig. 4c) confirmed the incorporation of
the ruthenium complex into the hybrid catalyst.
The XRD diﬀraction pattern of pure Ru(bpy)3Cl2 gave dif-
fraction peaks at the 2θ value of 17.6°, 26.0°, 28.1°, 29.1°,
31.0°, 39.4°, 44.4°, 46.4°, 54.5°, 62.5° and 71.6° due to various
planes of the crystalline complex (Fig. 5a). X-ray analysis of
in situ TiO2 gave characteristic diﬀraction peaks of both
anatase and the small amount of the rutile phase of TiO2. The
peaks at the 2θ value of 27.7° (101), 36.2° (004), 44.1° (200),
54.3° (105), 56.9° (211), 62.9° (118) and 69.9° (116) were
assigned to the anatase tetragonal TiO2 as confirmed by JCPDS
card no. 21-1272, while the peaks at 44.2° (210), 46.7° (101),
64.2° (002) and 70.0° (220) were attributed to the rutile form of
TiO2 as confirmed by JCPDS card no. 88-1175 (Fig. 5a).18
In
the XRD pattern of the hybrid Ru(bpy)3/TiO2, both anatase and
rutile TiO2 peaks were observed, suggesting that grafting of the
ruthenium complex did not change the phase pattern of TiO2
in the hybrid. A less intense peak at 26.1° observed in the
in situ synthesized Ru(bpy)3/TiO2 hybrid was assumed to be
due to the presence of the ruthenium complex (Fig. 5b).
The surface properties of the in situ synthesized TiO2 and
hybrid Ru(bpy)3/TiO2 catalysts were investigated with the help
of a nitrogen adsorption–desorption isotherm. As shown in
Fig. 6, the loop of the isotherm of the synthesized materials
was of Type-(IV) with the mean pore diameter between 2 and
50 nm, indicating the mesoporous nature of the materials.19
The BET surface area (SBET), total pore volume (VP) and mean
pore diameter (rp) (Fig. 6a) for in situ TiO2 were determined to
be 35.24 m2
g−1
, 0.05 cm3
g−1
and 7.58 nm, respectively while
for the in situ synthesized hybrid Ru(bpy)3/TiO2, these values
were found to be 26.96 m2
g−1
, 0.06 cm3
g−1
and 9.37 nm,
respectively (Fig. 6b).
Fig. 4 FTIR spectra of (a) Ru(bpy)3Cl2; (b) in situ TiO2; (c) Ru(bpy)3/TiO2.
Fig. 5 XRD patterns of: (a) Ru(bpy)3Cl2; (b) in situ TiO2; (c) in situ
Ru(bpy)3/TiO2.
Fig. 6 Adsorption desorption patterns of: (a) in situ TiO2; (b) Ru(bpy)3/
TiO2.
Fig. 3 HR-TEM images of: (a) in situ TiO2; (b) Ru(bpy)3/TiO2; (c) SAED
patterns of (c) in situ TiO2; and (d) in situ Ru(bpy)3/TiO2.
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The electronic absorption spectrum of Ru(bpy)3Cl2 in aceto-
nitrile shows the characteristic inter ligand π → π* transition at
290 nm and another sharp hump at 455 nm due to the metal-
to-ligand dπ → π* charge transition (Fig. 7a).20
UV-Vis spectrum
of in situ TiO2 gave a sharp peak at 300 nm due to the tran-
sitions of electrons from the valence band to the conductance
band (Fig. 7c).21
While for the in situ synthesized hybrid
Ru(bpy)3/TiO2, a sharp peak at 305 nm due to TiO2 and a short
hump at 460 nm due to the presence of Ru(bpy)3Cl2 are
observed. These values clearly indicated that the synthesized
hybrid catalyst is visible light active (Fig. 7b). The low intensity
of the hump at 460 nm is probably due to the lower loading of
the ruthenium complex in the synthesized hybrid.
The optical band gap of the synthesized materials was
determined with the help of Tauc’s plot to confirm their
visible light activity (Fig. 8). The ruthenium complex shows
two band gaps, the first one at 4.05 eV due to inter-ligand π →
π* transition corresponding to 305.6 nm and the second one
at 2.55 eV due to the metal to ligand charge transfer (MLCT)
corresponding to 485.4 nm. The band gap (485.4 nm) associ-
ated with MLCT transition proves the visible light absorption
of the Ru(bpy)3Cl2 complex (Fig. 8a). Whereas, for TiO2, the
obtained band gap value of 3.15 eV is associated with
393.0 nm (Fig. 8b). For the hybrid Ru(bpy)3/TiO2 catalyst the
band gap value was found to be 2.65 eV (467.1 nm) due to
MLCT transition of the Ru(bpy)3Cl2 complex (Fig. 8c). This
reduced value of the band gap confirmed the visible light
activity of the synthesized hybrid.
The surface chemical composition of the synthesized TiO2
and hybrid Ru(bpy)3/TiO2 was investigated with XPS (Fig. 9). In
the case of TiO2 and Ru(bpy)3/TiO2 two characteristic peak
components at 464.23 and 458.39 eV assigned respectively to
Ti 2p1/2 and Ti 2p3/2 confirmed the anatase Ti(IV) form of
titania (Fig. 9a).22
The high resolution O 1s XPS spectrum of
in situ TiO2 showed two peaks at 532.3 and 529.83 eV due to
–OH and Ti–O bonds, respectively. However for the hybrid Ru-
(bpy)3/TiO2 the peak for the O 1s region at 531.00 eV due to
–OH was found to be decreased in intensity. This is most likely
due to the grafting of complex moieties between the crystal
lattice followed by subsequent removal of water molecules
(Fig. 9b). However, the peaks due to ruthenium could not be
detected probably due to the lower loading of the complex or
overlapping with the C 1s peak. The N 1s XPS spectrum dis-
played two peak components at 401.17 and 399.20 eV due to
CvN and C–N of bipyridine which confirmed the presence of
the ruthenium complex in the synthesized hybrid Ru(bpy)3/
TiO2 (Fig. 9c).
The thermal degradation pattern of the synthesized
materials was determined by thermogravimetric analysis
(TGA). As shown in Fig. 10a, the thermogram of in situ TiO2
exhibited a major weight loss in the range of 225–350 °C
which is most likely due to the loss of oxygen during crystalli-
zation at higher temperatures. The thermogram of the in situ
Fig. 9 Wide scan XPS spectra of in situ TiO2 and Ru(bpy)3/TiO2: (a) Ti
2p; (b) O 1s and (c) N 1s region.
Fig. 7 UV/Vis absorption spectra of (a) Ru(bpy)3Cl2; (b) Ru(bpy)3/TiO2;
(c) in situ TiO2.
Fig. 8 Tauc plot for calculation of the band gap of: (a) Ru(bpy)3Cl2; (b)
in situ TiO2; (c) Ru(bpy)3/TiO2.
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synthesized hybrid Ru(bpy)3/TiO2 showed two weight losses
(Fig. 10b). The first one at around 95 °C due to the loss of
adsorbed water on the surface of the catalyst.23
The second
weight loss at 350 °C was probably due to the partial degra-
dation of the complex moieties and carbonaceous matter
along with the loss of oxygen during crystallization.
The photo-catalytic reduction of CO2
The photocatalytic reduction experiments were carried out by
using a 20 watt white cold LED light in a water/DMF mixture
using triethylamine as a sacrificial donor. After 24 h of visible
light illumination in the presence of a Ru(bpy)3/TiO2 photo-
catalyst, methanol was obtained as the major liquid product of
CO2 reduction as determined by GC-FID (Fig. S2†). Gaseous
products were analyzed by GC-TCD and GC-FID equipped with
RGA column; only oxygen and unreacted CO2 could be
detected in the gaseous phase. This analysis suggested that
the synthesized hybrid catalyst was highly selective for the pro-
duction of methanol from photo-reduction of CO2. The quanti-
tative determination of the produced methanol was performed
with GC-FID by plotting a calibration curve (Fig. S4†). The
obtained area in the peak of the GC chromatogram was corre-
lated with the amount of methanol (in μmol g−1
cat) with the
help of the calibration curve. Since methanol was obtained as
the major reaction product, a graph between methanol for-
mation vs. time was plotted (Fig. 11). It is clearly seen in
Fig. 11c that the hybrid Ru(bpy)3/TiO2 catalyst after 24 h of
irradiation time gave methanol yield of 1876 μmol g−1
cat,
which is much higher in comparison with the methanol yield
of 828 μmol g−1
cat obtained with in situ TiO2 (Fig. 11b).
The formation of methanol under visible light using in situ
TiO2 can be explained by the presence of defects (energy
levels) within the band gap of TiO2.24
The value of the
quantum yield (ϕMeOH) for the photogenerated methanol
using the in situ hybrid Ru(bpy)3/TiO2 was found to be
0.024 mol Einstein−1
, while for in situ TiO2 this value was
found to be very low i.e. 0.010 mol Einstein−1
. To establish the
higher stability and better performance of the in situ Ru(bpy)3/
TiO2 hybrid catalyst, we also synthesized P25 TiO2 grafted
ruthenium complex via adsorption of the Ru(bpy)3Cl2 complex
on P25 TiO2 and checked its photocatalytic activity for CO2
reduction under identical conditions. The yield of methanol
after 24 h irradiation was found to be 1102 μmol g−1
cat with a
quantum yield of 0.014 mol Einstein−1
. Furthermore, we tested
the photocatalytic activity of homogeneous Ru(bpy)3Cl2 (using
the equimolar amount as presented in the hybrid Ru(bpy)3/
TiO2) for photoreduction of CO2 under identical conditions.
The yield of methanol after 24 h irradiation using the homo-
geneous ruthenium complex was found to be 385 μmol g−1
cat
with a quantum yield of 0.005 mol Einstein−1
. Three blank
reactions were carried out to confirm the origin of methanol
as a result of photoreduction of CO2 and not from any other
organic compound. In all blank experiments, no methanol was
formed which clearly indicated that methanol was formed due
to the photo-reduction of CO2 (Fig. 11a). Moreover, an equal
yield of methanol was obtained by using aprotic solvent aceto-
nitrile in place of DMF confirming that the reaction was not
sensitive to the solvent and the protons required for CO2
reduction were not derived from the solvent molecules
(Fig. S3†). To confirm that the protons required for the
reduction of CO2 were originated from water, we also quanti-
fied the evolved oxygen by GC-TCD. The amount of oxygen
obtained, 2364 µmol g−1
cat, was 1.26 times more than the
obtained methanol yield. This value was found to be in well
accordance with the reaction stoichiometry as shown in the
proposed mechanism.
In order to check the eﬀect of the light wavelength on the
methanol yield, we calculated the methanol quantum yield (ϕ)
at diﬀerent wavelengths of light by using the homogeneous
Ru(bpy)3Cl2 complex, in situ TiO2 and in situ Ru(bpy)3/TiO2
hybrid catalyst. For the Ru(bpy)3Cl2 complex the maximum
apparent quantum yield (AQY) was determined to be
Fig. 11 Methanol formation from CO2 photoreduction: (a) for blank
reaction; (b) using homogeneous Ru(bpy)3Cl2 complex; (c) in situ TiO2;
(d) Ru-complex adsorbed on P25 TiO2; (e) in situ Ru(bpy)3/TiO2.Fig. 10 DT-TGA diagrams of: (a) TiO2; (b) Ru(bpy)3/TiO2.
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at 450 nm, while for in situ TiO2 this
value was found to be 0.01 mol Einstein−1
at 400 nm. For the
hybrid Ru(bpy)3/TiO2 catalyst the AQY was found to be
at 450 nm (Fig. 12). As shown in Fig. 12,
the absorption pattern of the synthesized hybrid Ru(bpy)3/TiO2
matches well with the absorption profile of the ruthenium
complex (Ru(bpy)3Cl2), which suggested that the photoreduc-
tion of CO2 proceeded through excitation of the sensitizer
ruthenium complex followed by injection of the electrons to
the conduction band of TiO2.
Furthermore, to evaluate the effect of ruthenium loading,
we have synthesized a number of hybrid Ru(bpy)3/TiO2 cata-
lysts by varying the amount of Ru(bpy)3Cl2 as 2.7, 5.4 and
10.8 mg. Among all the synthesized catalysts, the hybrid syn-
thesized using 5.4 mg of Ru(bpy)3Cl2 was found to be the best
one and afforded the maximum methanol yield. However
further increase of the Ru(bpy)3Cl2 amount to 10.8 mg did not
enhance the activity of the composite to any significant extent
(Fig. S12†). This may be due to generation of more electron
and hole pairs than the adsorbed CO2 molecules available on
the surface of the catalyst.
Furthermore, the origin of methanol from CO2 was con-
firmed by performing the isotopic labeling experiments using
13
CO2 in place of 12
CO2. After 24 h of visible light irradiation
the photoreduction product was analyzed by GC-MS. The mass
spectrum of the product showed a peak at 33 m/z, which con-
firmed that methanol was generated from the photoreduction
of CO2 and not from the degradation of other organic com-
ponents (Fig. S6 and S7†).
Furthermore, the recycling of the recovered Ru(bpy)3/TiO2
hybrid catalyst was tested for five subsequent runs and the
yield of methanol remained almost same as that obtained with
the fresh catalyst (Fig. 13). The recycling experiments con-
firmed that the catalyst was robust enough for recycling and
can be used for several runs without significant loss of its cata-
lytic activity. Furthermore, the ruthenium content of the cata-
lyst after five recycling experiments was determined by
ICP-AES analysis and it was found to be 1.34 wt%; this value
was comparable to the Ru content of the freshly synthesized
catalyst (1.38 wt%). On the other hand, the ruthenium
complex adsorbed on P25 TiO2 showed significant leaching
with a ruthenium content of 0.24 wt% even after the first recy-
cling experiment. The results clearly indicated the superiority
of the in situ synthesized hybrid catalyst in terms of recyclabil-
ity and stability.
After performing the five recycling experiments, the
Ru(bpy)3/TiO2 photocatalyst was characterized with FTIR, XRD,
FE-SEM, FE-SEM EDX, elemental mapping, UV-Vis spectro-
photometry and ICP-AES (Fig. S8–S11†). FTIR, XRD and SEM
showed no significant change in the structural composition of
the catalyst. The FE-SEM EDX pattern displayed nearly the
same intense peaks as that of the freshly synthesized catalyst.
Elemental mapping showed well distribution of ruthenium as
in the freshly synthesized catalyst. The UV-Vis spectra of the
in situ Ru(bpy)3/TiO2 catalyst after five recycling experiments
exhibited a similar absorption pattern. These analysis results
along with the ICP-AES analysis suggested that the synthesized
catalyst was quite stable and remained almost unchanged after
being used for several runs.
For defining better catalytic activity of the in situ Ru(bpy)3/
TiO2 catalyst, we have proposed a possible mechanistic
pathway for the photoreduction of CO2 under visible light
irradiation (Fig. 14). Owing to the wide band gap of TiO2
(3.2 eV), it cannot give visible light mediated transitions.25
Incorporation of the photo-sensitizer Ru(bpy)3Cl2, which
strongly absorbs in the visible region increases the absorption
pattern of TiO2 towards visible light. Thus, after absorbing the
visible light, photo-sensitizer ruthenium units in the hybrid
Ru(bpy)3/TiO2 become excited and transfer electrons in the
conduction band of TiO2.26,27
In this way, a continuous injec-
tion of electrons in the conduction band of TiO2 facilitates the
reduction of CO2 adsorbed on the surface of the nanocompo-
site. Triethylamine acts as a sacrificial donor and provides
necessary electrons for the reaction and gets transformed into
Fig. 13 Catalyst recycling data for four cycles.Fig. 12 Quantum yield of methanol at different wavelengths by using
(a) Ru(bpy)3Cl2; (b) in situ TiO2; (c) in situ Ru(bpy)3/TiO2; (d) UV–Vis spec-
trum of Ru(bpy)3Cl2.
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its degradation products.28
Water splitting provides protons
for the reaction (Fig. 14).
½RuðbpyÞ3Cl2Š þ hν ! ½RuðbpyÞ3Cl2Š*ðS1Þ
½RuðbpyÞ3Cl2Š*ðS1Þ ! ½RuðbpyÞ3Cl2Š*ðS3Þ
½RuðbpyÞ3Cl2Š*ðS3Þ þ TiO2 !
½RuðbpyÞ3Cl2Šþ
þ 6eÀ
ðin conduction band of TiO2Þ
6eÀ
ðin conduction band of TiO2Þ þ CO2 þ 6Hþ
! CH3OH þ H2O
½RuðbpyÞ3Cl2Šþ
þ TEA ! ½RuðbpyÞ3Cl2Š þ TEA°þ
TEA°þ
ðdegradationÞ ! Imine ! degradation products
3H2O ! 6Hþ
þ 3=2O2 þ 6eÀ
Conclusions
We have developed a novel hybrid Ru(bpy)3/TiO2 catalyst syn-
thesized via an in situ grafting method, providing uniformly
distributed TiO2 nanoparticles grafted with ruthenium
complex moieties. The synthesized hybrid catalyst was used
for the photoreduction of CO2 under visible light using tri-
ethylamine as a sacrificial donor. The grafted sensitizer
Ru(bpy)3 units absorb visible light and transfer electrons to
the conduction band of TiO2, which are subsequently used for
the CO2 reduction to methanol. The synthesized hybrid cata-
lyst was found to be highly selective and provided methanol as
the major reaction product without any evidence for the for-
mation of any other liquid as well as gaseous product. The
developed hybrid catalyst gave 1876.6 μmol g−1
cat (ϕMeOH
) methanol after 24 h of visible light illu-
mination that was much higher than 828 μmol g−1
cat (ϕMeOH
) obtained for in situ TiO2. The present
methodology provides a very facile approach for the synthesis
of heterogeneous photocatalytic materials, which selectively
produced methanol that may serve as an alternative fuel in the
near future.
Experimental
Material
2,2′-Bipyridine (99%), ruthenium chloride trihydrate, titanium
tetrachloride were purchased from Aldrich. They were of
analytical grade and used without further purification. Trietha-
nolamine (99%) was of analytical grade and procured from
Alfa Aesar. All other chemicals were of A.R. grade and used
without further purification.
Characterization
The rough surface morphology of the synthesized catalysts was
determined using FE-SEM (Jeol Model JSM-6340F), while the
fine structure was examined with High Resolution Trans-
mission Electron Microscopy (HRTEM) using JEOL 2100 LaB6
working at 200 kV equipped with a STEM module and an
Oxford SDD 80 mm2
EDX detector. The images were taken
with a GATAN Orius 200D for medium resolution and a GATAN
UltraScan 1000 for high resolution. Samples for HR-TEM were
prepared by making a dilute aqueous dispersion of the
samples and depositing them on a carbon coated copper TEM
grid. Fourier transform infrared (FTIR) spectra were recorded
on a Perkin–Elmer spectrum RX-1 IR spectrophotometer using
a potassium bromide window. UV-Visible absorption spectra
of Ru(bpy)3Cl2 in acetonitrile, in situ TiO2 and in situ Ru(bpy)3/
TiO2 were collected on a Perkin Elmer lambda-19 UV-VIS-NIR
spectrophotometer using a 10 mm quartz cell, using BaSO4 as
the reference. The crystalline phase of the samples was deter-
mined using the X-ray powder diﬀraction pattern with a
Bruker D8 Advance diﬀractometer at 40 kV and 40 mA with
Cu Kα radiation (λ = 0.15418 nm). The samples for XRD were
prepared on a glass slide by adding a well dispersed catalyst in
slot and drying properly. The surface properties like Brunauer–
Emmett–Teller (BET) surface area, Barrett–Joyner–Halenda
(BJH) porosity, pore volume of in situ TiO2 and in situ
Ru(bpy)3/TiO2 were examined by N2 adsorption–desorption iso-
therms at 77 K by using VP. Micromeritics ASAP2010. Thermal
stability of samples was evaluated by thermogravimetric ana-
lyses (TGA) using a thermal analyzer TA-SDT Q-600. Analysis
was carried out in the temperature range of 40 to 800 °C under
nitrogen flow with a heating rate of 10 °C min−1
. The ruthe-
nium content of the catalyst was measured on an Inductively
Coupled Plasma Atomic Emission Spectrometer (ICP-AES,
DRE, PS-3000UV, Leeman Labs Inc, USA). For ICP-AES, 0.05 g
of the catalyst was leached out using Conc. HNO3 and the final
volume was made up to 10 ml by adding distilled water. LED
light was used for irradiation of the reaction mixture (Model:
HP-FL-20W-F, Hope LED Opto-Electric Co., Ltd). It has the fol-
lowing specification – chip: KWE, Bridgelux, Epistar COB LED,
Fig. 14 Plausible mechanism of CO2 reduction of the in situ Ru(bpy)3/
TiO2 nanocomposite.
Paper Nanoscale
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20 W, lum output: 80–90 lm w−1
, beam angle: 120 degree and
wavelength λ > 400 nm (maximum at 515 nm). Fig. S5† shows
the spectral power distribution (spectrum profile) for diﬀerent
wavelengths.
Synthesis of the in situ Ru(bpy)3/TiO2 hybrid14
Ru(bpy)3Cl2 was synthesized by following the literature
method.15
For the in situ Ru(bpy)3/TiO2 synthesis, in a mixed
solution of triethanolamine (9.5 mL) and 20 mL of ethanol,
4.2 mL of titanium tetrachloride (TiCl4) was added dropwise.
The obtained solid complex of titanium was dissolved in
100 mL distilled water. This mixture was stirred for 30 min for
making a transparent solution. To this solution, 5.4 mg of Ru-
(bpy)3Cl2 was added, and then ammonia was added dropwise
into the orange transparent solution with stirring until the pH
value of the solution reached 10. This content was sonicated
for 2 h by maintaining the temperature of the mixture below
35 °C. The resulting mixture was centrifuged for collecting the
precipitated in situ Ru(bpy)3/TiO2 and washed twice with deio-
nized water and once with ethanol and further dried at 50 °C
under vacuum overnight. The ruthenium content (determined
by ICP-AES) was 1.38 wt% or 0.18 mmol Ru(bpy)3Cl2 g−1
cat.
For comparison studies, in situ TiO2 was synthesized using a
similar method except without adding Ru(bpy)3Cl2. Further-
more, we synthesized the Ru(bpy)3Cl2 complex adsorbed on
P25 TiO2 by dissolving Ru(bpy)3Cl2(0.09 mmol) in acetonitrile
(10 mL) and then added P25 TiO2 (0.5 g). The mixture was
concentrated under reduced pressure and then dried. The
ruthenium content in the synthesized Ru(bpy)3/P25 TiO2 was
determined to be 1.34 wt% by ICP-AES analysis.
Photocatalytic CO2 reduction experiment
The photocatalytic CO2 reduction experiment was carried out
in a borosil vessel having 5 cm diameter charged with a water/
DMF/mixture (3 : 1 : 1) purged with nitrogen gas for 30 min to
remove the dissolved gases. After that, CO2 was purged
through the solution for additional 30 min to saturate the solu-
tion. Then a 100 mg catalyst was added to this mixture and the
vessel was sealed with a septum and illuminated with a
20 watt LED (Model no. HP-FL-20W-F-Hope LED Opto-Electric
Co., Ltd) with stirring. The vessel was placed 3 cm away from
the light source and the intensity at the surface of the vessel
was found to be 75 W cm−2
. The whole system was insulated
from the external interference by placing it in a dark box. For
checking the reaction progress, after every two hours interval
samples were collected with the help of a needle and analyzed
with GC-FID (Varian CP3800, column specification – Varian
capillary column, CP Sil 24CB LOW BLEED/MS 30 m 0.25 mm,
0.25 μm # CP 5817) at the flow rate 0.5 mL min−1
, injector
temperature 250 °C, and FID detector temperature 275 °C. For
GC analysis, samples were injected with the help of an auto-
sampler. For quantification of produced methanol and ensur-
ing its linear sensitivity toward various concentrations, a
calibration curve was plotted. To confirm that the produced
methanol was the CO2 photoreaction product, we have per-
formed several blank reactions: (1) in the presence of the cata-
lyst and CO2 without illumination, (2) in the absence of the
catalyst but with visible light illumination of the vessel in the
presence of CO2 and (3) visible light illumination in the pres-
ence of the catalyst but using N2 instead of CO2. In all the
above blank reactions, no liquid product was detected. Fur-
thermore, we have carried out the photoreaction by using an
aprotic organic solvent: acetonitrile : water : triethylamine
under identical conditions. By using this solvent system the
yield of methanol was found to be similar to DMF. Gaseous
products were analyzed with GC-FID and GC-TCD equipped
with RGA (refinery gas analyzer) column. Recycling experi-
ments were carried out to check the robustness of the catalyst
for the long time reusability. After completion of the reaction,
the catalyst was separated by centrifugation, washed with
ethanol, dried at 50 °C and reused for the subsequent
reaction.
Acknowledgements
Authors are thankful to Director IIP, for granting permission
to publish these results. PK is thankful to CSIR, New Delhi for
providing research fellowship. CJ kindly acknowledges CSIR,
New Delhi for providing technical HR under XII five year pro-
jects. Analytical department is kindly acknowledged for the
analysis of samples.
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A novel Ru/TiO2 hybrid nanocomposite catalyzed photoreduction of CO2 to methanol under visible light

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