Visible light-assisted reduction of CO2 into formaldehyde by heteroleptic ruthenium metal complex-TiO2 hybrids in an aqueous medium

Article abstract of DOI:10.1039/c9gc03549d

The photocatalytic reduction of CO₂ with its simultaneous functionalization is a profound journey to achieve under an ambient condition. In the current research, precedence exists for the formation of HCHO, HCOOH, CO, CH₄, and CH₃OH after the reduction of CO₂ under suitable conditions. In this progression, HCHO is considered to be a reactive molecule, which occurs in the photocatalysis under suitable condition observed in the photocatalytic process. Herein, we report CO₂ reduction to formaldehyde via heterogeneous photocatalysis in an aqueous medium at pH 7. The as-synthesized hybrid photocatalyst is capable of being active under visible light (λ > 420 nm) by utilizing the heteroleptic ruthenium metal complex over TiO₂ nanoparticles via covalent interactions. The major reaction product was identified as formaldehyde, while trace amounts of CO and CH₄ were also detected in the presence of triethanolamine (TEOA) as a sacrificial donor. The maximum turnover number (720) for HCHO was obtained based on the metal complex used over the surface after 5 h visible light irradiation. Furthermore, formaldehyde (in situ) was utilized for the reaction with primary amines (aniline, 4-aminobenzoic acid) to form the corresponding imines under visible light. Directed by mechanistic studies, the results indicate for the first time that the C₁ reduced product of CO₂ in a heterogeneous medium can be utilized for synthesis of useful products.

Full text of DOI:10.1039/c9gc03549d

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DOI: 10.1039/C9GC03549D
Journal Name
ARTICLE
Visible light assisted reduction of CO into formaldehyde by
2
heteroleptic ruthenium metal complex-TiO hybrid in aqueous
medium
2
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
a
Alok Kumar, RajakumarAnathakrishnan*
www.rsc.org/
Photocatalytic reduction of CO
condition. In the current research, precedent exists for the formation of HCHO, HCOOH, CO, CH4, and CH
reduction of CO under provided suitable conditions. In this progression, HCHO is considered a very reactive molecule;
therefore, it remains to be observed in the photocatalytic process. Herein, we report CO reduction to formaldehyde via
heterogeneous photocatalysis in aqueous medium at pH 7. Synthesized hybrid photocatalyst capable of active under
visible light (λ > 420 nm) by utilizing the heteroleptic ruthenium metal complex over the TiO nanoparticles through
covalent interaction. The major reaction product was identified as formaldehyde, a trace amount of CO and CH were
2
and its simultaneous functionalization is a profound journey to achieve in an ambient
3
OH after
2
2
2
4
detected in the presence of triethanolamine (TEOA) as a sacrificial donor. The maximum turnover number (720) for HCHO
was obtained based on the metal complex used over the surface after 5 h visible-light irradiation. Furthermore,
formaldehyde (in situ) was utilized for reaction with primary amine (aniline, 4-aminobenzoic acid) into the corresponding
imine under the visible light. Directed by mechanistic studies, the results demonstrate for the first time that C
product of CO in a heterogeneous medium can be utilized for useful products.
1
reduced
2
under UV light.7, 8 In additions, the reduced bandgap
heterostructures were well explored by utilizing noble
metals and graphene to improve the properties under the
Introduction
The visible light assisted photocatalytic reduction of CO to
2
useful products has become considerable interest due to
the sustainable development of the environment and fossil
fuels scarcity. Imitating from nature (photosynthesis), there
were enormous efforts been continuously attempted to
utilize the sunlight in the presence of H
laboratory scale.1 Both H
consequently strenuous to oxidized and reduced at normal
conditions. The significant challenges for CO reduction are
product selectivity as well as conversion efficiency at the
provided system. This is due to the presence of highest
oxidations sate of carbon atoms (CO
electron and proton pathways to reduce the products such
3
as HCOOH, CO, CH OH, HCHO, etc. Therefore, increased
selectivity, convenient methods for catalysis, and high yield
of product are immensely desirable in today research. For
this purpose, various wide-bandgap semiconductors like
9
, 10
visible light.
However, the primary constraints are
related to non-aqueous solvents, mixed solvents, and
complication in product selectivity in a confined medium.
On the other hand, CO reduction in H O is dominated by
2 2
competitive hydrogen evaluation reaction (HER) on the
catalyst surface. Therefore, it has become a challenge to
control over the thermodynamics and kinetics of reaction in
a water medium; however, and the progression has not
2
O and CO
are stable molecules,
2
in the
-4
2
O and CO
2
2
1
1, 12
been limited in current research.
A molecular catalyst (homogeneous) such as ruthenium,
osmium, iridium, and cobalt complexes are known to give
2
) that follows a multi-
the selectivity as well as good quantum yield for CO
2
1
3, 14, 15
reduction in a reliable and suitable system.
Frederick
5
, 6
et al. had used an aqueous homogeneous medium to
produce methanol and formate from CO by Ru(II)
chromophore. Hamers and Einaga et al. had shown the
electrocatalytic reduction of aqueous CO to formaldehyde
2
1
6
17
2
2 2
TiO , ZnO, and others were introduced for CO conversion
1
8
over the boron-doped diamond (BDD) electrode, and
other groups had demonstrated direct formaldehyde
1
9
synthesis using polyhydride ruthenium complex. The
method described by Gratzel for dye-sensitized solar cells
a. Department of Chemistry, Environmental Materials & Analytical Chemistry
Laboratory, Indian Institute of Technology, Kharagpur 721302, India.
*
e-mail: raja.iitchem@yahoo.com
has paid attention to utilizing
semiconductor under visible light. Among the family of
a wide band gap
†
Footnotes relating to the title and/or authors should appear here.
2
0
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
2
semiconductor, TiO is well known non-toxic, bio-friendly,
readily available semiconductor which extensively being
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(c) The free NH rich surface could modulaViteew Artthicele OCnlOine
studied.7, 21-23 There were various anchor groups explored
over the TiO surface to increase the efficiency and stability
of photocatalytic activities in the past. Scheme 1a
represented the catalytic reactions of CO in aqueous
film resulting
2
DOI: 10.1039/C9GC03549D
2
adsorption over the heterogeneous surface which may lead
to increase the rate of CO reduction. Interestingly, the
hybrid catalyst has shown the excellent photocatalytic
activity towards the CO reduction yielding the selective
product (HCHO) after 5 h irradiation in aqueous medium
Scheme 1b). Furthermore, in situ formaldehyde was
2
4
2
2
medium using dye-sensitized (N719) TiO
mixture of products.23
2
2
(
allowed to react with amine (aniline, 4-aminobenzoic acid)
resulting into imine formation. The mechanistic pathways
of the nature of active species during the catalytic process
are discussed. We demonstrate the use of CO
2
reduced
product as C can further be used in heterogeneous
1
photocatalytic system under visible light by green
approach.
Schemes 1 (a) CO
product. (b) Represent the CO
RuL1) immobilized TiO hybrid (This work).
2
reductions over the TiO
2
film using N719 dye give mixture of
2
reduction by synthesized ruthenium complex
(
2
Results and discussion
Ligands L1, L2 (abbreviated L1OH, L2NO) and metal
complexes were synthesized by adapting from previous
Our group had reported the utilization of Ru(II)-complex as
homogeneous and heterogeneous conditions for the
photocatalytic organic transformation and environmental
2
9, 30
literature with slight modification (Scheme S1†).
L1 and
applications.2
5,
26
L2 gives significant yield 86% and 93% respectively. The
metal complexes were synthesized in ethanol medium.
Notably, here we aimed to introduced
benzimidazole containing ruthenium metal complexes with
two different functionality (-COOH, -NO for TiO
sensitizations (Scheme 2). A few numbers of literature
associated with both functional group (-COOH and -NO
have been introduced to understand the sensitization
[
Ru(bpy)
2 2
]Cl was used as a precursor for the synthesis of
2
)
2
2
+
2+
complex [RuL1(bpy)
2 2
] and [RuL2(bpy) ] abbreviated as a
RuL1OH and RuL2NO respectively. The details of synthetic
procedure have been discussed in supporting information.
All the ligands and metal complexes were well
2
)
2
7, 28
process of semiconductor.
The prominent advantage of
1
3
characterized by Proton NMR, CNMR, FTIR, Maldi-TOF MS
as described in supporting information (SI†, Fig. S1-S10).
The electronic photophysical properties of the metal
complexes were characterized by UV-Vis and fluorescence
using benzimidazole spacer in the complexes, which
enhances the better electron-donating capability hence,
electronic (ground state) properties of ruthenium metal
complex can be tuned. The strategy behind for choosing
this phenanthroline chelate site linked via benzimidazole
spacer with a different terminal functional group (-COOH, -
3
spectroscopy in CH CN solvents whereas stated otherwise.
Complex (RuL1OH) is very sensitive to the pH, soluble in
polar solvents and has an excellent hydrogen bonding site.
The details photophysical properties concerning the pH of
2
NO ) are as follows. (a) Metal complex exhibits enhanced
lifetime in aqueous and protic solvents, which has useful
advantages in photocatalysis to overcome the
recombination processes, and (b) Metal complex is
3
0
metal complex RuL1OH have been described elsewhere.
However, RuL2NO has preferably only one side (NH-proton)
of hydrogen bonding. Lifetime measurements were carried
out using time-correlated single photon counting (TCSPC) in
2
+
3
photostable over classical [Ru(bpy )] in aqueous media.
CH
the fluorescence lifetime 164 ns and 64 ns respectively
Table S1†). Both the metal complexes were immobilized on
synthesized TiO nanoparticle in suitable solvent (CH CN or
3
CN at pH 7. Complexes RuL1OH and RuL2NO revealed
(
2
3
DMF) at room temperature. Though both the solvents gave
same result, the DMF solvent had been used to estimate
the amount of incorporated metal complex into the TiO
surface to avoid any concentration error. Detailed synthesis
of TiO nanoparticles are mentioned in SI† (Scheme S2†).
2
2
The loaded amount of metal complex over the surface was
estimated by UV-Vis spectroscopy. The results gave 0.125
µM/g of RuL1OH and 0.093 µM/g of RuL2NO over the TiO2,
respectively. Hence, it is clear that metal complexes were
2
Scheme 2 TiO Hybrid photocatalyst of ruthenium (II) heteroleptic metal complex
anchored on surface. Two functional groups namely, a carboxylic acid group and
nitro group respectively. Excited state ruthenium (II) injects the electron into
2
successfully anchored on the TiO surface. Binding of metal
conduction band of TiO
reduction.
2 2
. The conduction band is actively participating for CO
complexes with TiO was studied by ATR-FTIR, diffuse
2
reflectance spectroscopy (DRS), cyclic voltammetry, and
Raman spectroscopy.
2
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decrease in energy of the ligand π* orbital due to electron-
Photophysical and Electrochemical Properties
View Article Online
33
10.1039/C9GC03549D
withdrawing properties of carboxylaDtOeI.: Both the hybrid
catalysts are capable of adsorbing visible light up to 600
nm. The corresponding band gap energy shifted from 3.2
eV to 2.5 eV as obtained from Kubelka–Munk (K-M method)
The UV-Vis spectra of the metal complexes RuL1OH and
RuL2NO shows the two transition band at 288 nm and 457
nm which correspond to π to π* (transition) and metal to
3
1
34
ligand charge transfer (MLCT) of ruthenium center (Fig. 1).
equation (Fig 1, inset). Furthermore, Similar effect
On the other hand, the MLCT band was significantly red-
shifted for RuL2NO which is presumably because of
observed in the fluorescence spectra of the complex
[RuL2NO] which is slightly red-shifted on contrary to
RuL1OH (Fig.S11†). Besides, the presences of benzimidazole
free proton in complexes and acid proton that could show
flexible properties with respect to the pH of medium under
investigation. Hence, we restricted our photocatalytic
studies at a neutral aqueous medium. Metal complex
Immobilized TiO2 nanoparticle can be preferably
photoexcited under the irradiation of a wavelength greater
than 400 nm. Both the complexes are presented MLCT
2
electron-withdrawing substituents (NO
S11†. Diffuse reflectance spectra of the hybrid catalyst
TiO /RuL1OH, TiO /RuL2NO) showed MLCT transition band
2
) as shown in Fig
(
2
2
and can significantly be perceived at 463 nm which is due to
ruthenium complexes (Fig. 1). In addition, similar effect was
attributed to solid-state of pure metal complex (RuL2NO),
which revealed bathochromic shift in important MLCT
transition as compared to RuL1OH complex. The more
electronegative ligand (electron-withdrawing at terminal
site) will lead to attract electron from the metal center.
transition band over TiO surface. However, despite the
several reports the stability and accountability of NO over
2
3
2
Therefore, electron density will be delocalized.
the TiO surface have not been fully understood in reaction
2
Consequently, delocalization prefers larger effective
conjugation, which results in bathochromic shift of MLCT
transition.
medium.27, 28 The various binding mechanism of –COOH
group over the TiO surface was proposed previously.35 The
2
room temperature photoluminescence spectrum of
nanocrystalline TiO
S13b . The TiO photoluminescence exhibits Fabry-Perot
fringes due to interference between backscattered and
2
and hybrid catalyst are shown in Fig.
†
2
forward-scattered PL light. The emission wavelength of TiO
is almost 535 nm which reflects partial oxygen vacancy.
2
6
3
Moreover, after immobilization an additional emission
intensity was observed to red region (630 nm) in hybrid due
to the intact ruthenium complexes. In addition, Ruthenium
complex is well known for its variable oxidation state
2
+
3+
(
Ru /Ru ). Hence, the electrochemical properties of the
complexes and hybrid catalysts were examined in purified
acetonitrile by cyclic voltammetry (CV). The typical CV scan
of the metal complexes revealed quasi reversible nature
with distinct anodic (oxidation) and cathodic (reduction)
peak (Fig.S14†). The results are depicted in Table 1. A
positive potential, vs. Ag/AgCl, RuL1OH shows couple
Fig. 1 (a) UV-Vis absorptions spectra of the metal complex (RuL1OH) measured in
3
+
2+
CH
TiO
3
CN (dotted line, ii). (i) Solid-state DRS of TiO
2
nanoparticles. (iv, iii) For hybrids
/RuL1OH (inset).
(Ru /Ru ) at 1.01 V with a peak separation of 70 mV.
2
/RuL1OH, TiO /RuL1NO, respectively. Tauc plot for TiO
2
2
However, the classical [Ru(bpy) ]2+ couple (Ru3+/Ru2+) was
3
3
7
2+
3
reported vs. Ag/AgCl as 1.25 V. On contrary to [Ru(bpy) ]
However, a small red-shift in the hybrid TiO
indicates the strong interaction through carboxyl group of
metal complex (Fig S12†). This effect is consistent with a
2
/RuL1OH
nearly 0.24 V less positive potential was observed for
RuL1OH, suggesting that L1 is moderately donor than 2,2’-
bipyridine (bpy) ligand.
Table 1 Electrochemical data and respective energy.
aE1/2 [V]
ox
aE1/2 [V]
red
bHOMO [eV]
-5.43
bLUMO [eV]
-3.16
cE0-0 [eV]
2.10
dEred [V]
0.84
0.68
eEox [V]
-1.09
-1.02
-0.89
Sample
RuL1OH
RuL2N
1.01
0.97
1.25
-1.26
-1.31
-1.34
-5.39
-5.67
-3.11
-3.08
1.99
Ru[(bpy)
3
]2+
2.14
g
0.80
aEpa = anodic peak potential, Epc = cathodic peak potential, and E1/2 = (Epc + Epa)/2 versus AgCl. HOMO and LUMO levels were determined using equations :
c
f
ox
red
c
d
*
E
E
HOMO (eV) = -e(E1/2 + 4.42), and ELUMO (eV) = -e(E1/2 + 4.42). E0-0 from the fluorescence data. Excited-state reduction potential estimated using Ered =
red
e
*
ox
g
1/2 + E0-0. Excited-state oxidation potential estimated using Eox = E1/2 - E0-0. Reference 37.
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Moreover, the photo- switching pattern of TiO and hybrid
2
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catalyst is analyzed. For hybrid cataly
on/Ioff) upon irradiation of light found (Fig. 3b). However,
TiO /RuL2NO was not shown much difference.
D
s
O
t
I
i
:
n
1
c
0
r
.1
e
0
a
3
s
9
e
/C
s
9
inGC
c
0
u
3
r
5
r
4
e
9
n
D
t
(I
2
Furthermore, electrochemical impedance spectroscopy
EIS) for the hybrid catalysts (TiO /RuL1OH, and
TiO /RuL2NO) were carried out to investigate the charge
(
2
2
transfer efficiency of the hybrid. The impedance was
measured with the bias of -0.7 V under dark and light. The
Nyquist plots are shown in Fig. S15†. The experimental data
Fig. 2 Cyclic voltammogram for hybrid catalyst recorded in MeCN 0.2 mM of
TBABF
6
are shown with symbols while solid line represents fit
obtained using Nova 2.1 software. Nyquist plot showed two
semicircles. However, such semicircle was not identified in
3
+
2+
2
Moreover, the hybrid TiO presents the (Ru /Ru ) couple
TiO
high impedance may be related to large resistance for TiO
However, in light the impedance has been significantly
reduced. The electron lifetime (τ ) was calculated to
investigate the backward electron transfer from TiO
2
/RuL1OH hybrid. In the dark both the hybrid presented
at 1.12 V with same peak separation. A negative potential
reduction peak of the complexes and hybrid catalyst shows
at -1.26 V and -0.715 V respectively (Fig. 2). The hybrid
catalyst was almost 0.11 V more positive value than
RuL1OH strongly suggesting that metal complex
2
.
n
2
3
8
conduction band to electrolyte. The values were 28.94
and 25.23 ms for RuL1OH and RuL2NO, respectively. The
immobilized over the TiO
2
surface. Furthermore, the
corresponding HOMO and LUMO energy level of complexes
and hybrid catalyst were calculated (Table 1). The LUMO
energy level (-3.16 V) of the complex is higher than the
n
higher τ manifests low recombination at the electrolyte
3
9
interface.
3
5
conduction band of TiO
possibility of photoexcited electron into the conduction
band of TiO is thermodynamically more reliable in both
2
(-4.2 eV vs NHE). Therefore, the
FTIR and Surface morphology studies
2
complexes even without any applied bias.
Furthermore, ATR-FTIR of the pure metal complexes shows
-1
the major IR characteristic peak at 1704 cm for C=O
To understand the photo efficacy of the prepared hybrid
catalysts, photo response was measured by shining light
from a xenon lamp (model no. 66902; Newport Corp. USA)
over the Indium Tin Oxide (ITO) electrode. The current was
measured between two contacts (sandwiched between ITO
electrode, 3 mm distance apart) using a Keighley sources
meter (Model 2400). Fig. 3a shows the dark-current-voltage
-1
-1
stretching mode whereas 1624 cm and 1380 cm , indicate
-
22
asymmetric and symmetric vibrations of -COO group. The
-1
peak at 1406, 1431, 1550 cm is signature of bipyridyl
ligand vibration. Hence, FTIR spectra indicated the
successful binding of complex over the TiO
bidentate covalent attachment mode (Fig. 4a). To further
confirm the complexes over the TiO surface the Raman
scattering spectra were recorded at excitations wavelength
2
surface with
2
(I-V) plot in both positive and negative bias which is almost
Ohmic in nature for all the hybrid catalyst. We have
identified that hybrid photocatalyst shows law dark current
5
1
14 nm. Spectra exhibits the Raman active mode for TiO
2
-1
-1
-1
-1
40 cm (E
g
), 397 cm (B1g), 515 cm (A1g), and 640 cm
(0.005 nA) at 0.5 V, and reveals a good photocurrent
(E
g
) respectively (Fig. 4b). All these lines are characteristic of
response after illuminations of Xenon light source. It can be
noticed that 8 times (0.04 nA) increase in the current was
found apparently at the same potential bias. However, the
half less than increase current (0.02 nA) was observed for
40
-1
anatase TiO
2
nanoparticles. The band at 640 cm is due to
symmetric Ti-O vibrations form A1g symmetric modes,
-1
-1
whereas as other two bands (397 cm , 515 cm )
41
6
corresponds to the degenerates modes of TiO octahedra.
TiO
and evidence for the presence of immobilized ruthenium
complexes over the TiO surface.
2
/RuL2NO hybrid catalyst. The increase in conductivity is
Moreover, at higher wavenumber in the ranges of 1000-
-1
2
200 cm the characteristic weak line of ruthenium metal
complexes appears in hybrid TiO (Fig. S16d ). The shoulder
peak appears in the range of 1250-1360 due to C-C ring and
2
†
2
-1
C-O stretching modes whereas 1380 cm reveals for
symmetric COO stretching. The fact that no sign of C=O
vibrations is visible. However, the presence of COO
-
42
-
strongly suggests the coordination of metal complex over
the TiO
2
nanoparticles via bidentate bridging. It is
worthwhile noticing that the relative Raman intensity got
4
3
increased after adsorption of complexes. However, there
is no change in energy was observed for RuL1OH (Fig
S16b
slightly shifted towards lower region whereas Raman
intensity remains almost constant (Fig S16c ).
†). In addition, the wavenumber of hybrid (RuL2NO)
Fig. 3 (a) Dark and photocurrent behaviour of hybrid photocatalysts
measured on ITO glass (I-V curves). (b) Transient photoresponse of hybrid
photocatalyst at on/off condition.
†
4
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suggested that the metal complex uniformly VbieiwndArstictleoOntlhinee
surface of anatase TiO NPs.
2
DOI: 10.1039/C9GC03549D
Fig. 4 (a) ATR-FTIR spectra of metal complexes RuL1OH, and hybrid
-
1
(
TiO
Raman spectra of TiO
TiO2/RuL2NO) with excitation wavelength at 514 nm.
2
/RuL1OH, TiO
2
RuL2NO) in the frequency range 400-1000 cm . (b)
2
and hybrid photocatalyst (TiO /RuL1OH,
2
The full widths at half-maximum (FMWH) were calculated
with respect to E (140) phonon mode (table S2 ). The
hybrid (TiO /RuL2NO) shows significant change in FWHM as
compare to pure TiO
g
†
2
2
. This change is due to phonon
confinement effect.44 The surface morphology of the hybrid
was obtained by using the field emission scanning electron
microscope (FESEM) and high-resolution transmission
electron microscope (HRTEM) as shown in Fig. S17
SEM images of the hybrid show spherical particles with
average diameter of 75 nm (Fig.S17c ). TEM analysis of the
†. The
†
hybrid provides the details views of surface morphology
which further confirm 75 nm spherical particles size. It is
important to note that there are no changes in morphology
of the particles perceived with the attached metal complex.
HRTEM lattice image gives the 0.35 nm d spacing which
corresponds to thermodynamically stable 101 facets of
4
5
anatase TiO
2
nanoparticles. In addition HRTEM images of
hybrid (TiO /RuL1OH) were
pure TiO and immobilised TiO
2
2
2
compared as shown in Fig.5a-d. The lattice fringes and
SEAD pattern confirm the particles are highly crystalline as
synthesized. The HRTEM image of pure TiO shows clear
2
fringes pattern and there is no trace of core (amorphous)
was found on contrary to complex (RuL1OH) immobilized
2 2
TiO (Fig. 5c). Lattice fringes of TiO /RuL1OH demonstrate a
thin core layer where the fringes are not crystalline this
indicates that presence of metal complex (RuL1OH) over
surface, and amorphous nature is due to ligand (Fig.5d). In
addition, the amorphous core is not visible for TiO
complex. RuL2NO contains NO functional group that lead
to weak interaction through TiO . Therefore, it can be
anticipated that metal complex has been successfully
incorporated on oxide surface. The active functional group
2
/RuL2NO
2
2
Fig. 5 HRTEM images of as prepare hybrid catalyst RuL1OH/TiO
of TiO (b) TiO /RuL1OH (c). Lattice fringes of TiO (d). Lattice fringes of
TiO /RuL1OH. HAADF-STEM images of hybrid photocatalyst and TiO (e, h).
EDS mapping of elements for Ti (f), O (g), N (I), and Ru (m).
(a). HRTEM
2
2
2
2
2
2
(
-COOH) capable of covalent interaction over the TiO
surface whereas NO prefer non-covalent interaction
electrostatic). Moreover, for further confirmation of the
2
2
(
Surface Composition, Chemical State, and Surface area
elemental compositions of the particles, we have
performed STEM-EDS elemental mapping. Fig. 5e-m shows
the TEM images of individual hybrid TiO
2
NPs and a
Understanding the chemical state of the element presents
in the sample, the X-Ray photoelectron spectroscopy (XPS)
has been carried out at room temperature. The typical XPS
corresponding STEM mapping of different elements that
are present in the sample. The mapping nicely executes
element present and their uniform distribution in the
spectra of the hybrid are shown in Fig. S34
†. The doublet
4
+
hybrid TiO
metal, carbon, and more importantly nitrogen coming from
the ligands (EDX, Fig. S36 ). In addition, the pure TiO
2
NPs. We can quickly identify the ruthenium
binding energy peaks (458.2 eV and 464.3 eV) for Ti (2p3/2)
4
+
46
2
and Ti (2p1/2) are agreement with the TiO nanoparticles.
†
2
Fig. 6 shows the deconvoluted XPS of hybrid for elements
STEM-EDS elemental mapping has been compared as
shown in Fig.5e-g. Hence, evidence from the result,
present. The binding energy peak at 530.5 eV and 531.6 eV
-
are attributed to O and Ti-OH from TiO
2
surface. However,
a peak at 531.6 eV has also some contributions for C=O
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Besides, some efforts have been made to introduce amine-
group of the metal complex. Furthermore, C1s show the
two characteristic peaks of binding energy at 284 eV and
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4
8
functionalized solid-materials to enha
D
n
O
c
I
e
: 1
C
0.
O
10
2
39
a
/
d
C
s
9
o
G
r
C
p
0
t
3
io
5
n
49D
.
2
85.8 eV respectively. The former is designated for
As conspicuous from FTIR and Raman spectra, the metal
complexes are covalently attached over the surface. Hence,
advantage of this metal complex has free NH group which may
pyridine, whereas later peak is contributed by C=O, C-C, and
C-O groups. Small peaks at approximately 280.3 eV
demonstrate the presence of Ru species (Ru3d3/2).
Moreover, it should be noted that there is a substantial
overlap between C1s and Ru3d3/2 binding energy; hence it
is not well resolved.
2
+
47
promote CO
motivated us, to perform CO
The results showed nearly 4.5 times higher (17.23 cm3/g)
uptake of CO than pure TiO2 (3.32 cm3/g). Interestingly, at
low CO pressure, the TiO /RuL1OH increased stiffly,
whereas TiO and TiO /RuL2N showed linear adsorption as
depicted in Fig. 7a. Furthermore, the crystallographic
structure information of TiO and hybrid was obtained by
XRD. All these characteristic peaks matched with the
standard anatase TiO Pattern (JCPDS File Card No. 21-
272). The crystallinity of the TiO remains intact after
2
adsorption on catalyst surface. These finding
2
adsorption experiment at 298.0 K.
2
2
2
2
2
2
2
1
2
immobilization result no change in 2θ values. However, the
intensity got increased in case of hybrid photocatalyst.
Photocatalytic activity
There are enormous efforts have been attempted by
utilising ruthenium metal complex sensitised nanoparticles,
pd-doped TiO
the reduced product of CO
environments (solvents, pH), experimental condition and
loading amount of noble metals (Table S3 ). The
synthesized hybrid catalyst was employed for CO reduction
2
, and ruthenium doped TiO
2
. We found that
2
specially depends on supplied
Fig. 6 (a) XPS spectra of hybrid photocatalyst RuL1OH/TiO2. Survey spectrum,
†
(
a) C1s and Ru3d, (b) Ti2p, (c) O1s (d) N1s, after immobilization of ruthenium
metal complexes.
2
(
λ > 420 nm, 250 W lamp) in aqueous medium at pH 7. For a
typical run, a powder of hybrid catalyst (RuL1OH, 1 g/L) was
dispersed thoroughly in aqueous medium (10 ml), and 0.8
mM of triethanoloamine was added. The solution was
Furthermore, in a heterogeneous system, all the reactions
will take place over the surface when we deal with the solid
powder catalyst. Therefore, this is a considerable interest to
explore the surface area with adsorption and desorption
properties of the catalyst under investigations. Based on
allowed for visible light irradiations (λ > 420 nm) under CO
2
atmosphere. The temperature was maintained at 25 ⁰C
through water circulation in glass jacket reactor. After 5 h
continuous irradiation the liquid sample was analysed by
UV-Vis spectroscopy. The spectra show the significant
transitions at almost 286 nm which is characteristic of
the N
hybrid (TiO
Emmett–Teller) BET surface area at 25 m /g (TiO
2
-adsorption isotherm the TiO
2
particles and their
2
/RuL1OH, TiO /RuL2NO) show (Brunauer–
2
2
2
), 63.8
2 2
m /g (TiO /RuL1OH), and 52 m /g (TiO /RuL2NO)
2
2
carbonyl group (C=O, n to π*) transition (Fig.S19b
†).
Therefore, liquid product was analyzed in HPLC by using the
different standards such as HCOOH, HCHO, and CH OH. The
3
major product was identified as formaldehyde.
respectively. The increased surface area of the material is
possibly due to attached ruthenium metal complex. The
amine functional group have widely been used for CO trap
2
in industries due to formation of carbamate later gives
carbonate upon hydrolysis.
Fig. 7 (a) CO
measured at 298 K. (b) Powder XRD pattern of TiO
photocatalyst (TiO2/RuL1OH).
2
adsorption isotherm of TiO
2
and hybrid photocatalyst
2
, simulated and hybrid
Fig. 8 Trace GC chromatogram matched with the slandered sample. CO peak
2
has not shown here due to over-saturation from the sample. Injected volume
1
0 ml.
6
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However, there was no additional peak observed for
formate and methanol. We further extended the analysis to
determine any gaseous products formed using gas analyzer
reduced C
known as Mannich reaction. Gener
amine with aldehyde is indeed common technique to
generate the imine. The formaldehyde and aniline
reaction in acidic medium has been reported in the
1
building block. Fundamentally this reaction is
View Article Online
D
a
O
ll
I
y
:
,
10
c
.1
o
0
n
39
d
/
e
C
n
9
s
G
a
C
t
0
io
3
n
549
o
D
f
5
2
(Thermo Fisher Scientific GC-Trace 1110) equipped with FID
detector.
5
3
literature and essential for industrial application.
Therefore, two primary amines (aniline and 4-aminobenzoic
acid) were used in our system for in situ photocatalytic
processes. The idea of taking 4-aminobenzoic acid is could
provide the conceptual mechanistic insight into catalytic
system.
4
The result shows CO and CH were produced (trace amount)
with a turnover number (TON) 17 µmol (136), and 8 µmol
64) respectively (Fig. 8). Hence, we continued
(
formaldehyde as a standard for quantitative analysis of
reduced products. Firstly, known concentrations of
formaldehyde were prepared in deionized water and
calibrated with HPLC. The linear regression line was
Table 2 Photocatalytic reaction for 8 h using hybrid catalyst under CO
and in situ reaction with amineb
atmospherea
2
4
9
2
obtained R (0.997 %) close to unity. Fig. 9a shows the HPLC
Product/µmol (TON)
chromatograms of reaction mixture which had been taken
at different interval of time after irradiations. The product
5
0
HCHO
CO
CH
4
Conv.
(
(
HCHO) was quantitated by following reported method.
The reaction product yield 90 µmol of a catalyst with TON
720) was estimated which is very significant in terms of
selectivity for CO reduction. In additions, the conventional
Entry
Photocatalyst
%)b
65
1
2
3
4
TiO
TiO
2
/RuL1OH
/RuL2NO
90(720)
0.8(12)
N.D
17 (136)
< 0.2
N.D.
8 (64)
N.D
N.D.
N.D
(
2
28
N.D.
N.D
2
TiO
TiO
2
/Ru(bpy)
/RuN179
3
method is used to detect the formaldehyde by using 2,4-
dinitrophenylhydrazine (DNPH) which gives 2,4-
R
2
80 (N.D)
N.D
dinitrophenylhydrazone
after
reaction
with
aConditions: hybrid photocatalyst 1 g/L in aqueous solution, 0.8 mmol
formaldehyde.51 Fig. 9b presents the HPLC of DNPH and
of TEOA, CO2 pressure, visible light irradiation (λ > 420 nm),
b
formaldehyde. These results further confirmed the CO
reduced product in our experimental condition.
2
Determined by GC using bromobenzene as an internal standard.
Aniline and 4-aminobenzoic acid were taken as amine sources. R =
Reference 23.
After 8 h irradiation (λ ˃ 420 nm) of reaction in H
2
O with
0.05 mmol) distilled aniline GC-MS spectra were analyzed
Fig. S20-S24 ). Interestingly, we identified 4-(4-
(
(
†
a
aminobenzyl) aniline with m/z (198) and methylene-aniline
Intermediate) as new species. Furthermore, respective
(
conversion (%) and selectivity were determined by using
bromobenzene as internal standard in GCMS. Similarly, the
possible identified intermediate for 4-aminobenzoic acid is
shown in Fig. S25-S27
the stepwise reaction of CO
†
. Our studies categorically provide
reduced product and insights
Fig. 9 (a) HPLC chromatogram kinetics of reaction mixture measures H
CH CN (60:40), and (b) 2,4 DNPH treated reaction mixture.
3
2
O:
2
direct observation of formaldehyde in a visible light assisted
heterogeneous catalytic system. Furthermore, for the
efficacy of electron transfer mechanism in catalytic process,
we were intrigued by the idea of paraquat (Methyl
Moreover, the same experiment was repeated with the
hybrid catalyst (RuL2NO, 1 g/L) at the aforementioned
conditions. Surprisingly, we found a few products with the
hybrid catalyst (RuL2NO, 1.0 g/L). We end up with the
2+
2+
viologen, MV ) one-electron reduction process. The MV
is employed as suitable electron acceptor as well as pH-
54
independent reversible redox potential (0.45 V vs NHE).
mixture of products (HCHO, CO, CH
4
) under the identical
+.
Furthermore, it can be readily identified (MV ) by their
unique characteristic blue color upon anaerobic irradiation
to light in the presence of a suitable catalyst or donor
condition which is difficult to quantify. The stability of the
metal complex over the surface is less due to electrostatic
interaction with NO
transition band can be perceived in liquid reaction mixture
after catalysis. The result suggesting that NO groups are
2
group. Consequently, an MLCT
-
(
EDTA, Cl , etc). However, upon aerobic exposure, the it
+.
forms MV which gives green fluorescence by formation of
2
1
’,2’-dihydro-1,1-dimethyl-2’-oxo-4,4-bipyridylium cation.
In general, the freshly prepared aqueous solution of MV
55,
not adequate for the sensitization process hence, the
efficiency of catalyst in visible light is less. In order to
understand the photo-assisted reduced product selectivity
as well as to identify the origin of formaldehyde, we
performed in situ reaction with amine. Sabo-Etienne et al.
56
2+
is not fluorescence in aqueous medium. Hence, we
preferred to investigate the electron transfer process by
2+
2+
2
using MV . The freshly prepared solution in D O of MV
was irradiated under the visible light (aerobic condition) in
the presence of catalyst without allowing any donor.
has first demonstrated the borane reduction CO
2
catalyzed
by polyhydride ruthenium complex to formaldehyde and
1
HNMR were recorded systematically concerning time. The
1
9
trapped in situ condition by aromatic amine. This method
2+
kinetic of MV after irradiation is shown in Fig.10. We
found that the chemical shift is deshielded at around 0.016
is fascinating to provide reliable information about the
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Ti3+ ions in the g < 2 were observed on anatase TiO
ppm in 180 min. The deshielding is attributed to increases
electron densities at carbon center. In addition, the UV-Vis
spectra show bathochromic shift after catalysis and emit at
2
after
View Article Online
5
8
excitation of solid in visible light. DO
corroborate the activity of catalyst in visible light. The
process of CO reduction mechanism in the aqueous
medium has been proposed previously.
based on data obtained by performing CO
T
I
h
: 1
e
0.1
r
0
e
3
s
9
u
/
l
C
t
9
s
G
s
C
t
0
r
3
o
5
ngly
4
9
D
around 528 nm (Fig. S18
green fluorescence in D O at λ ≤ 365 nm is shown in Fig. 10.
We further confirmed the products in LC-MS (Fig. S28-
S29 ). The results strongly support the efficiency of catalyst
performance in aqueous medium.
†
). The characteristic emission of
2
5
9, 60
2
Therefore,
reduction with
2
†
hybrid catalyst in heterogeneous medium, we introduced
the mechanism for catalytic process in our experimental
condition (Scheme 3). First, surfaced ruthenium complex
was excited in visible light and transfer an electron to the
2 2
conduction band of TiO . TiO nanoparticles are capable of
adsorbing CO on the surface and activated through
2
+
conduction band electrons. The sources of H occurred
from water in our system. Initially formic acid was formed.
However, the formic acid was converted to formaldehyde
after 5 h continuous CO
concentration of formic acid was decreased for time and
applied CO pressure as a result, not detectable by
2
pressure and irradiation. The
2
analytical technique. Besides, amine (aniline) was reacted
with formaldehyde to form N-hydroxymethyl aniline
+
activated by H which is coming from water splitting at
initial stage. Additionally, this attained equilibrium with N-
methylideneanilinium (1).
1
2+
2
Fig. 10 H NMR kinetic of MV in D O measures after visible light catalysis in
the absence of donor (TEOA).
3
+
Moreover, to further confirm the existence of Ti , low
temperature EPR spectra of TiO and hybrids (TiO /RuL1OH,
TiO /RuL2NO) were recorded at 100 K. The solid samples
were irradiated in in visible light (λ ˃ 420 nm) for 2 h. Fig. 11
2
2
2
3
+
significantly showed the EPR signal of Ti (paramagnetic),
while pure TiO is inactive. The observed g value (1.953,
.968) was calculated which corresponds to paramagnetic
2
1
3
+
Ti center.
Scheme 3 Proposed mechanism of reaction based on identified compounds
in GCMS.
Finally, it gives the N-(p-aminobenzyl) aniline (2) after
reaction with aniline. Interestingly, we have recognized a
defined m/z (107.02) for 4-aminobenzylium ion. This is the
crucial step for the formation of 4,4’-methylene
diphenyldiamine after rearrangement and industrially
2 2 2
Fig. 11 (a) EPR spectra of TiO , TiO /RuL1OH. (b) TiO /RuL2NO. All
measurements were carried out at 100K after visible light illumination (2 h).
6
1
Surprisingly, we have noticed one additional peak having g-
important reaction. Our results are evident to support for
2
value is close to 2.0 appears for hybrid TiO /RuL1OH. The
direct capturing of formaldehyde from CO reduction.
2
origin of this peak is due to ruthenium metal complex which
5
7
was generated during photo excitation in visible light. The
3
+
Ti has also been observed for hybrid (TiO
2
/RuL2NO).
Moreover, the control experiments were repeated by
adapting the same procedure (I) in the dark, (II) nitrogen
atmosphere, and (III) blank (without catalyst). None of
three conditions gained a single species of CO reduced
2
products in both UV-Vis and HPLC characterization. Again
However, there is no ruthenium signal was identified
perhaps due to low concentration. In addition, the intensity
was relatively lower in both the sample analysis.
Characteristic EPR signals of TiO
2
and hybrids attributed to
8
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the reaction was analyzed in acetonitrile provided the
identical conditions. Fig. 12 shows ATR-FTIR kinetics of
reaction mixture after irradiation. The donor is completely
consumed suggesting the potential activation of catalyst
View Article Online
4
Fig. 13. (a) The time courses of HCHO, CO, and CH rmation by visible-light
DOI :f o1 0.1039/C9GC03549D
irradiation (λ ˃ 420 nm) under CO
2
atmosphere. (b) Recyclability of hybrid
catalyst TiO /RuL1OH for HCHO formation in aqueous medium.
2
under visible light. The hybrid catalysts (TiO
2
/RuL1OH,
TiO /RuL2NO were isolated after catalysis and
2
the reaction by using different concentration (5-10 mmol)
of the ruthenium metal complex. We found that the
formaldehyde formation was not reliable after 10-15 h
irradiation. However, the usage of 10 mmol of RuL1OH, we
end up with 15 µM of formaldehyde (in 20 h). Furthermore,
we had tried to capture formaldehyde (in situ) using the
aniline and 4-aminobenzoic acid. The 4-aminobenzoic acid
solid products were isolated after the reaction. We
identified that a new C-H stretching frequency appeared
characterized by DRS. The DRS spectra show the significant
MLCT transition for ruthenium complex (RuL1OH). The
blank reaction (without CO2, catalyst) was performed to
further conformed the stability of metal complex in course
3
of catalytic process. Catalyst (1 g/L) in CH OH solvents was
irradiated (λ > 420 nm). Aliquot did not show signatures of
complexes apparently in FTIR spectra. However, solid
catalyst showed significant stretching frequency of metal
complex.
-1
around 2900-3000 cm . Moreover, the characteristic of N-
H for IR absorption frequency decreases relatively (Fig
S30†). The liquid aniline was difficult to isolate from a
homogeneous medium. However, we performed the
preparatory TLC in a mixture of DCM (2%) and hexane by
adding 0.01% of triethylamine. The LCMS spectra of the
samples were recorded and then m/z at 107 were
interpreted as shown in Fig. S31
using the 10 mmol of the ruthenium metal complex the
conversion of CO to formaldehyde is slow. Furthermore,
the in situ traps by using the amine are also low almost < 50
conversion. The RuL1NO had not yielded any products
†. The results suggest that
2
%
even after 24h irradiation. Moreover, the hybrid catalysts
were tested in the acidic medium. The reaction is
dominated by hydrogen evolution at pH ≤ 5.5. We have
further characterized the product in Trace GC-Trace 1110
equipped with FID, TCD2 and TCD3. We identified around
Fig. 12 FTIR spectra of blank reaction (without CO
TEAO).
2 3
) in CH CN with the donor
(
Furthermore, the recyclability experiments of catalytic
reaction were carried out upto eight cycles (Fig.13b). After
10 µmol H
However, we could not get any C
liquid reaction mixture (in HPLC). Further, the TiO
and TiO /RuL1NO could not give any product in the basic
2
is produced at per gram of catalyst (Fig. S32†)
reduced product in the
/RuL1OH
.
1
2
each cycle, the catalyst was washed with H O and then
2
reused. We notice that even after fifth subsequent catalytic
cycle, the yield of formaldehyde remains almost the same.
A slight decrease of product (10-15%) occurred after sixth
cycle, it could be due to very minor loss of catalyst (fine
particle) during the washing and recycling steps. The metal
2
alkaline medium.
Experiments
Materials and Methods
The entire chemicals were taken by the commercial sources
and used without further purification. 2,2-bipyridyl, 1, 10-
2 3 2
Phenanthroline (phen), aniline, D O, and RuCl .xH O had
purchased from Merck. Aniline was used after distillation.
Ammonium acetate, KBr, 4-formylbenzoic acid, and 4-nitro
benzaldehyde were taken from Spectrochem. GC grade
solvents were used for reactions and HPLC grade solvent
was used for HPLC analysis. UV-Vis, DRS, and fluorescence
spectra were recorded with Shimadzu (Model UV-2450)
complex can also be reduced over the TiO
repeated treatment (long run, say after eighth recycle) as
observed from DRS spectra (Fig S33 ). The recyclability test
2
surface after
†
confirmed that metal complex is robust, stable and capable
of used for several catalytic cycles without significant loss of
their catalytic activity. The post characterization of catalyst
was analysed after the dried. XPS survey revealed the
2
presence of ruthenium over the TiO surface (Fig.S35†).
Furthermore, the turnover number of each product vs
irradiation time was plotted (Fig.13a). We observed the
formaldehyde formation increases with respect to time. In
addition, further experiments were carried out in an
aqueous medium to understand the role of the ruthenium
metal complex alone. We have systematically investigated
spectrophotometer, a Varian Cary 500 UV−vis−NIR (BaSO
background) spectrophotometer, and a Hitachi (Model F-
4
1
13
7
000) spectrofluorimeter, respectively. H and C (400
MHz) were measured using Bruker Lambda spectrometer in
DMSO-d , D O, and CDCl . MALDI-TOF MS Studies were
6
2
3
performed by using BrukerUltrafleXtreme instrument in
which DHB (2,5-dihydroxy benzoic acid) used as matrix.
Solid and liquid sate cyclic voltammetry was studied using a
BASi Epsilon electrochemical work station in MeCN and
AgCl used as a reference electrode. The XPS spectrum of
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CO reduced product
the hybrid photocatalyst, acquired using a PHI 5000
VersaProbe II instrument. The emission profiles of metal
complexes were analyzed using the time-correlated single-
photon counting (TCSPC) picosecond spectrophotometer in
2
View Article Online
푆electivity =
2
DOI: 10.1039/C9GC03549D
Reduction products (mol)
CH
3
CN at pH 7. FT-IR spectra were recorded using Perkin-
and hybrid catalyst
The experiments were repeated twice to calculate the mol
and average TON has been reported.
Elmer Spectrum II. Powder XRD of TiO
2
were obtained using a Bruker Apex-2 X-ray diffractometer
with CuKα radiation (λ = 1.5418 Å) and scan rate 0.5 cm.
FESEM, HRTEM, and STEM, images of TiO and hybrid
2
Conclusions
catalyst were recorded using a Supra 40 scanning electron
microscope (Carl Zeiss Pvt. Ltd), and JEOL JEM2010
transmission electron microscope over carbon-copper grids.
Raman spectra of the synthesized sample were measured
using a Renishaw Raman microscope equipped with a diode
laser excitation source at 532 nm. BET surface area of
We have successfully developed an efficient and reliable
hybrid photocatalyst for CO
medium. Metal complexes are used as sensitizer over the
surface of TiO (average size: 70 nm). The NO group is
found to be insufficient to provide the electron over the
TiO (CB) due to weak interaction. It may bind to the
surface employing electrostatic interaction rather than –
COOH which is covalently linked. Formaldehyde is a C
reduced product of CO and challenging to identify in an
aqueous medium. We have described the stepwise
reduction of CO yielding formaldehyde including CO and
CH was minor products. The product is almost 68%
2
reduction in the aqueous
2
2
2
hybrid catalyst and N
obtained using a QuantachromeAutosorb-iQ instrument.
CO adsorptions studies were performed at 298 K, and all
2
sorption isotherms (77 K) were
1
2
2
the samples were degassed for overnight before analysis.
EPR experiments were carried out using Bruker ELEXSYS
2
5
80 X-band EPR spectrometer at 100K after visible-light
4
irradiation (λ > 420 nm).
selective. We have first time used the aniline to identify the
formaldehyde in situ reaction during the heterogeneous
photocatalytic process. The mechanistic investigations are
well supported through GC-MS, HPLC, Trace GC, and LC-MS.
In additions, by using Benzimidazole spacer provided the
long-live time of excited-state electron which brings faster
and selective reduced products. Moreover, it can also help
Photocatalytic reaction
A suspension of photocatalyst (1 g/L) in a reaction solution
(
10 ml H
2
O) was taken in a quartz tube (50 ml). The solution
purged continuously. 0.8
was evacuated first, and CO
2
mmol of Triethanolamine was added to the mixture.
Suspensions were irradiated using 250 W tungsten lamp (λ
to adsorb the CO
ring. The reusability of catalyst (TiO
2
on surface due to free NH group in the
/RuL1OH) revealed that
2
>
420 nm) combined with NaNO
2
aqueous solution filter.
it can be used for more than eight cycles effectively without
significant loss. This work contributes to the utilization of
CO into useful products under the visible light irradiation
2
The temperature of solution was maintained at 298 ± 3 K
through continuous flow of water chiller. Formaldehyde
(
HCHO) was in the liquid phase was analyzed by UHPLC
by a green approach, which will be extended for many
future applications.
equipped with diode array detection (Thermo Fisher Dionex
UltiMate 3000 SD). Standard formalin solution containing
3
7 wt% in H
2
O (10-15 % MeOH stabilizer) was used. The
were
Conflicts of interest
gaseous reaction products, such as CO and CH
4
analyzed using Thermo Fisher Scientific Trace-GC 1110
equipped with an FID detector (molecular sieve 5Å packed
column). For the direct observations of formaldehyde
initially 0.05mmol of aniline was introduced to the reaction
mixture. In this condition the reaction time was increased
The authors declare no conflict of interest.
Acknowledgements
3
h (Total 8 h) for reaction between formaldehyde and
aniline. The liquid product was directly analyzed in GC-MS
Thermo Scientific Trace 1300 gas chromatography and ISQ
Authors thank IITKGP for essential infrastructure and DST-
FIST funded facilities (XRD and NMR) at Department of
Chemistry. AK thank the Department of Science and
Technology (DST), New Delhi for INSPIRE Fellowship.
(
Single Quadrupole MS) using FID detector. The same steps
were repeated for 4-aminobenzoic acid reaction. We
estimated the photocatalytic activity of hybrid catalyst after
each photocatalytic reaction using turnover no (TON) and
selectivity (equations 1 and 2).
Notes and references
1
.
X. Chang, T. Wang and J. Gong, Energy Environ. Sci.,
016, 9, 2177-2196.
2
Product (mol)
TON =
1
2.
T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath,
T. S. Teets and D. G. Nocera, Chem. Rev., 2010, 110,
Used metal complexes (mol)
6
474-6502.
1
0 | J. Name., 2012, 00, 1-3
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3
4
5
6
7
8
9
.
.
.
.
.
.
.
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Please Gd roe en no tC ha ed mj u iss tt r my argins
Page 12 of 13
ARTICLE
Journal Name
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Z. He, L. Wen, D. Wang, Y. Xue, Q. Lu, C. Wu, J. Chen
View Article Online
DOI: 10.1039/C9GC03549D
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2 | J. Name., 2012, 00, 1-3
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Green Chemistry
View Article Online
DOI: 10.1039/C9GC03549D
Table of Contents
Outline: CO reduction was carried out in aqueous medium at neutral pH 7.0. We identified
2
the 68 % selective products as formaldehyde after 5 h under visible light (λ ˃ 420 nm)
irradiation. Furthermore, formaldehyde has been used to react with aniline in situ condition
under visible light. The source of hydrogen is coming from water, which provided the reliable
environment to react with amine. The mechanism of reaction is well investigated and
supported by GC-MS., HPLC, and Trace GC with multiple detectors.