Design of a Photoactive Bimetallic Cu-Au@g-C3N4 Catalyst for Visible Light Driven Hydroxylation of the Benzene Reaction through C–H Activation

Article abstract of DOI:10.1002/ejic.201800622

In the present study, a new photoactive bimetallic Cu-Au@g-C₃N₄ catalytic system was designed by impregnating Cu and Au NPs over the graphitic carbon nitrite (g-C₃N₄) surface. Characterization of the catalyst us

Full text of DOI:10.1002/ejic.201800622

A Journal of
Accepted Article
Title: Design of a novel photoactive bimetallic Cu-Au@g-C3N4 catalyst
for visible light driven hydroxylation of benzene reaction via C-H
activation
Authors: Bishal Bhuyan, Meghali Devi, Debashree Borah, Siddhartha
Sankar Dhar, and Rajashree Newar
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800622
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European Journal of Inorganic Chemistry
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FULL PAPER
Design of a novel photoactive bimetallic Cu-Au@g-C
3
N catalyst
4
for visible light driven hydroxylation of benzene reaction via C-H
activation
Bishal Bhuyan, Meghali Devi, Debashree Bora, Siddhartha Sankar Dhar,* and Rajashree Newar
Abstract: In the present study, a new photoactive bimetallic Cu-
Au@g-C catalytic system was designed by impregnating Cu and
Au NPs over the graphitic carbon nitrite (g-C surface.
Characterization of catalyst using multiple techniques illustrated that
very small sized Cu and Au NPs were mono-dispersed on g-C
products. Even the involvement of less active transition metals
such as Fe, V, Cu, etc. have been investigated as catalysts in
conjunction with hydrogen peroxide.[10,11] As for example, Peng,[12]
3 4
N
N )
3 4
and his co-workers recently reported
a catalytic system
N
3 4
comprising [copper bis (dodecanesulfonate)] for the direct
hydroxylation of benzene (HOB) reaction and found that the
catalysts manifested the desired products with very low yield and
selectivity. Moreover, this protocol required high temperature (523
K) for the reaction to occur.[13] In this context, it is pertinent that
when these less active metals were immobilized over porous
materials such as silica[14] and also polymeric support,[15] it
significantly brought down the reaction temperature. They
manifested good catalytic performance, however, required
extended reaction time. As a result, it is reasonably important to
develop newer catalysts which can produce phenol from benzene
under milder conditions with high yields and selectivity.
surface, which were obtained by the in-situ reduction of the metal salts
using sodium borohydride. The as-synthesized bimetallic Cu-Au@g-
3 4
C N catalyst efficiently catalyzed the hydroxylation of benzene (HOB)
reaction via C-H activation under visible light irradiation at room
temperature. The dominant role of synergistic interaction between Cu
and Au NPs were further elucidated by high resolution transmission
electron microscopy (HR-TEM) studies and X-ray photoelectron
Spectroscopy (XPS) analysis. This study may provide a new avenue
for the preparation of photoactive bimetallic catalysts and offer an
insight to the synergistic interaction of bimetallic catalytic HOB
reactions.
Bimetallic catalysts combined with highly active noble metals and
highly selective common metals, such as Au and Cu, may be
considered as a rational choice of catalysts for this reaction
process, which could simultaneously reduce cost and produce
good yield and selectivity in the HOB reaction.[16] It is worth
mentioning that synergism between the two metals may improve
the catalytic performance of the bimetallic catalysts.[17] In addition,
for finding alternatives to traditional metal-based catalysts, it is
imperative to heterogenize them on surfaces for efficient recovery
and recycling. This concept of metal NPs loaded high surface
area supports can further improve the overall dispersion of the
Introduction
Phenol is considered to be an important organic intermediate
used in the synthesis of polymer resins, pharmaceutical products
along with its widespread application in preservatives and
fungicides. [1,2] The annual production of phenol is estimated to be
nearly about 7 million metric tonnes,[2] which accounts to its wide
applicability in numerous fields. Hock and Lang, in the year 1944,
first reported the synthesis of phenol via the cumene process.[3]
Nevertheless, the requirement of high pressure and temperature
along with specific involvement of Lewis acid catalysts added to
some of its negative aspects.[3,4] To simplify the synthetic pathway,
much emphasis was laid on newer catalysts, based on use of
noble metals like Pd and Pt along with their heterogeneous
entities in presence of hydrogen peroxide.[5] These alternative
pathways succeeded in eliminating the use of elevated pressure
and temperature conditions. However, the over-oxidation of the
desired product added to its main disadvantages, thereby limiting
metal NPs and thereby improve the catalytic performance and
reducing the costs of the bimetallic catalysts.[18] However, the
existing synthetic methods deployed for such fabrication of
catalysts over porous materials are indeed time consuming and
involve tedious work up procedures and harmful solvents. Usually,
these methods implicate laborious synthetic routes and costly
technique such as vapour deposition for loading metal NPs using
under harsh reaction conditions.[19] A host of literature also
H
2
reveals such preparation which involves multi-steps in fabrication
of supported bimetallic catalysts.[16,19]
its selectivity. Recently, metal free catalysts e.g. grapheme,[7]
C3N4[ and MWCNT[9] have also been exploited as catalysts for
the generation of phenol. Interestingly, these catalysts afford
superior selectivity of the reaction, but produce poor yield of
[6]
3 4
Graphitic carbon nitrides (g-C N ) are a class of carbon based
8]
material which have been demonstrated as a highly efficient and
[20]
versatile support material in recent times.
With a wealth of
attractive properties, such as chemical and thermal stability, low
density, super hardness, water resistivity and biocompatibility,
carbon nitrites have unambiguously become one of the most
Bishal Bhuyan, Meghali Devi, Debashree Bora, Dr. Siddhartha
Sankar Dhar, Rajashree Newar
Department of Chemistry
National Institute of Technology, Silchar
Silchar, 788010, Assam (India)
promising materials for surface modifications, light emitting
devices, photocatalysis and catalysis.[21-23] In addition, the ease of
preparation of these materials accounts to its advantageous
preference over other contemporary supports. Usually the
E-mail: ssd_iitg@hotmail.com
3 4
synthesis of g-C N is conducted in air atmosphere, without the
use of any inert gases. During the synthesis process, the
precursor material is thermo-chemically decomposed to porous
materials with high surface area. Until now, several reports on use
Supporting information for this article is given via a link at the end of
the document
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European Journal of Inorganic Chemistry
10.1002/ejic.201800622
FULL PAPER
of g-C
3
N
4
in important organic reactions including Friedal-crafts
At the foremost, FT-IR spectroscopy was performed to determine
[
24-
acylation, oxygen reduction reactions, etc. have been reported.
3 4
the chemical bonding and composition in g-C N and Cu-Au@g-
2
6]
It is important to note that g-C
3
N
4
is an inherently photoactive
C
3
N
4
, as shown in Fig. 1. The spectrum for pure g-C
3
N
4
(Fig. 1
[
26]
-
material with a suitable band gap of 2.73 eV, and thus promises
to be extensively used in a wide array of applications including
visible light mediated conversions. Pertinent to mention here that
metals viz. Au and Fe catalysts supported on g-C
been studied, which provides a cue for further research in this
area.[27]
(a)) shows a broad absorption band in the region 3100-3300 cm
1
, which suggests the stretching modes of N-H bonds of primary
[
29]
(-NH
2
) and secondary (N=H) amines in g-C
3
N
4
.
Additionally,
-1
N
4
have already
sharp bands centered in the range of 1250-1700 cm can be
assigned to stretching vibration modes of C-N heterocycles.[
Additionally, the peak at 809 cm corresponds to the presence of
characteristic ring breath modes of the s-triazine ring units.[ In
3
30]
-1
31]
As witnessed in recent years, the rational use of visible light in
important organic reactions has been the hotspot in the domain
case of Cu-Au@g-C
3
N
4
(Fig. 1(b)), the spectrum display almost
, likely because of the strong
. However, the strong absorption peaks of
were slightly shifted to lower wavenumbers in Cu-Au@g-
. These shifts suggest the successful immobilization of Cu
and Au NPs over g-C . Moreover, the peak intensities of the
Cu-Au@g-C catalyst were comparatively lower, validating the
of organic transformations for its outstanding results.[8] Apparently, the same pattern as that of g-C
3 4
N
the involvement of visible light in any catalytic system enhances
the performance of catalysts immensely.[8,28] As already
mentioned, the objective of the present study involves the
conversion of benzene into phenol in high yields and selectivity
via C−H activation. As a result, taking cue from all the
observations, we pursued visible light energy for the activation of
C−H bond in the current study. Herein, we developed a
photoactive catalyst comprising bimetallic Cu-Au NPs
IR response of g-C
3 4
N
g-C
3
N
4
3 4
C N
3
N
4
3
N
4
above conjecture.
3 4
immobilized on graphitic carbon nitrite (g-C N ) for the direct HOB
reaction. In the current work, g-C is used as a benign and
3
N
4
economically feasible photoactive support for the efficient room
temperature conversion of benzene to phenol using visible light.
Results and Discussion
3 4
In the present study, the preparation of g-C N was achieved via
thermal decomposition of melamine at 550° C. In the process,
melamine was initially decomposed to melem, which on further
heating transformed into melon. Under the specified reaction
conditions, melon gradually converted itself to g-C
Sequentially, Au and Cu metal salts were in-situ reduced over the
g-C nanosheets in presence of NaBH to afford Cu-Au@g-
. The systematic formation of Cu-Au@g-C3N4 catalyst is
3 4
N nanosheets.
3
N
4
4
3 4
C N
schematically represented in Scheme 1.
Figure 1. FT-IR analysis of (a) g-C3N4, (b) Cu-Au@g-C3N4
XRD analysis was performed to characterize the crystal structure
of the as-synthesized g-C
witnessed from Fig. 2(a), the XRD plot of g-C
diffraction peak at 27.5° which corresponds to the amorphous
nature of g-C . For Cu-Au@g-C , interestingly, the XRD
3
N
4
and Cu-Au@g-C
3
N
4
. As can be
3
4
N shows a broad
3
N
4
3 4
N
pattern changes. Four additional characteristic peaks for Au at 2θ
values of 38.19°, 44.29°, 64.8° and 77.72° (JCPDS file no. #89-
3
697) were observed (Fig. 2(b)). However, there were no
characteristic peaks for Cu witnessed, confirming that copper NPs
are amorphous in Cu-Au@g-C catalyst. The surface
morphology of bimetallic Cu-Au@ g-C was initially
characterized using SEM analysis. As shown in Fig. 3(a), the
surface is basically the g-C nanosheets that results from the
3
N
4
3
N
4
3
N
4
thermal treatment of melamine under moderate reaction
conditions. The SEM images with higher magnification displayed
that irregular metallic NPs are dispersed on the surface of g-C
3 4
N
3 4
Scheme 1. Schematic representation of the formation of CuAu@g-C N .
nanosheets (Fig. 3(b-d)). As witnessed, the sizes of the metallic
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European Journal of Inorganic Chemistry
10.1002/ejic.201800622
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NPs formed are relatively small. Additionally, the electron
diffraction studies (EDAS) pattern of the as-synthesized materials
further confirmed that the NPs formed are Cu and Au NPs and
In order to further assess the morphological characteristics of the
formed g-C
Fig. 4 displayed the TEM micrographs of pure g-C
nanosheets like structure are observed (Fig. 4(a)). The
amorphous nature of g-C nanosheets were further suggested
3
N
4
and Cu-Au@g-C
3
N
4
, TEM analysis was performed.
3
N
4
where
3 4
are homo-decorated on g-C N . The EDAS spectrum clearly
showed the presence of elements C, N, Cu and Au. It is pertinent
that the uniformity of the NPs may be associated to the simple in-
situ reduction process of the metallic ions (Cu2+ and Au3+)
3 4
N
from the electron diffraction (ED) patterns, as shown in Fig. 4(b).
As evidenced, there was no concentric ring patterns observed in
the ED patterns. The TEM micrographs were also recorded to
followed by their simultaneous deposition on the g-C
3 4
N
nanosheets.
3 4
discern the surface morphology of Cu-Au@g-C N , as shown in
Fig. 5. The images clearly revealed that large number of very
small nanoparticles of both Cu and Au are formed and dispersed
rather uniformly on the surface of g-C
large magnification showed that the formed nanoparticles of Cu
and Au are mono-dispersed on the surface of g-C nanosheets
Fig. 5(b,c)). The particle sizes of the metallic NPs were calculated
3 4
N . The TEM images with
3
N
4
(
from the TEM images and were found in the range 5-10 nm, with
an average particle size of 7 nm. The ED patterns of the as-
synthesized material revealed that the metallic NPs were poly-
crystalline (Fig. 5(d)).
3 4 3 4
Figure 2. XRD analysis of (a) g-C N , (b) Cu-Au@g-C N
3 4
Figure 4. TEM micrographs of (a) g-C N , (b) SAED patterns
Figure 3. SEM micrographs of (a,) Cu-Au@g-C
3 4
N , (b-d) high magnification
SEM images of Cu-Au@g-C , (e) EDAS pattern of Cu-Au@g-C N
3
N
4
3 4
3 4
Figure 5. TEM micrographs of (a,b) Cu-Au@g-C N , (c) high magnification TEM
image, (d) SAED patterns..
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European Journal of Inorganic Chemistry
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XPS analysis was also performed to elucidate the chemical state
In order to analyze the changes in the optical properties of Cu-
and surface chemical composition of pure g-C
. The XPS survey was initially examined for pure g-C
Accordingly, as shown in Fig. 6(a), one C 1s peak is observed at
3
N
4
and Cu-Au@g-
Au@g-C
, the samples were characterized by UV diffuse reflectance
spectroscopy and the results are displayed in Fig. 7. As can be
seen, the pure g-C has strong absorption in UV region with an
absorption edge at 460 nm (Fig. 7(a)). However, after the
formation of the bimetallic Cu-Au@g-C material, it showed
considerable absorption in the visible region. This observation
suggests that Cu-Au@g-C should possess higher
3 4
N after the loading of Cu and Au metallic NPs over g-
C
3
N
4
3
N
4
.
3 4
C N
2
84.5 eV, which represents the adventitious hydrocarbon from
3 4
N
XPS instrument.[32] Another sharp C 1s peak observed at 287.7
2
eV suggests the existence of the defect containing sp -bonded
carbon (C=N) for pure g-C
3 4
N
[33]
3
N
4
.
3 4
For Cu-Au@g-C N , the C 1s
XPS spectrum shows two peaks centered at 284.5 eV and 288.1
eV, respectively (Fig. 6(b)). Here, the peak located at 288.1 eV
3 4
N
photocatalytic activity in the visible region. In addition, the band
gap of the samples were also calculated, as can be seen in the
inset picture of Fig. 7. An empirical formulae was accordingly used
to calculate the band gap energy values (Eg) of the prepared
samples [37] as follows-
shows a slightly positive shift compared to that of g-C
Presumably, this change in the peak position can be ascribed to
the immobilization of Cu and Au NPs on g-C . The same pattern
was also observed for N 1s spectrum of g-C and Cu-Au@g-
(Fig. 6(c,d)). The characteristic peak of N for g-C (398.4
eV) was seen to be slightly shifted to 399.0 eV after the deposition
of Cu and Au NPs on g-C . Similar shifts in peak positions have
been observed by addition of metal oxide NPs such as NiMoO
over pure g-C
.[34] In addition, the high resolution XPS (HR-
3 4
N .
3
N
4
3
N
4
[훼ℎ휗 = 퐴(ℎ휗 − 퐸ꢀ)](1/2)
C
3
N
4
3 4
N
Where, 훼, ℎ휗, 퐴, 푎푛푑 퐸ꢀ denotes the absorption-coefficient,
photon energy, proportionality constant and band gap values,
respectively. The Eg values of the as-synthesized materials were
3
N
4
4
(1/2)
[3]
3
N
4
calculated by plotting the linear portion of [훼ℎ휗]
and it was found to be 2.73 eV, and 2.27 eV for pure g-C
Cu-Au@g-C , respectively.
Thus, from the above calculations, it is evident that the band gap
values of g-C is significantly reduced by the incorporation of
푉푠 ℎ휗(푒푉)
XPS) spectrum of Cu 2p can be deconvulated into two
asymmetric peaks located at 930.2 eV (Cu 2p3/2) and 949.9 eV
3 4
N
and
3
N
4
(
Cu 2p1/2) (Fig. 7 (e)). There are no peaks corresponding to Cu (II)
in the spectrum, which suggests that all Cu present are in the form
of elemental copper.[35] Moreover, the peak separation between
the two levels is 19.7 eV, which suggests the existence of Cu (0)
3 4
N
metallic Cu and Au NPs. This ultimately results in an increase in
the lifetime of the photogenerated charge carriers under visible
light irradiation, leading to an enhanced photoactivity.
3 4
metal in Cu-Au@g-C N . Likewise, the HRXPS spectrum of Au 4f
showed two peaks centered at 83.5 eV and 87.1 eV, which are
characteristic of the Au 4f7/2 and Au 4f5/2 core level binding
energies (Fig. 6(f)). These values are consistent with the values
reported[36] for metallic gold which further confirms the presence
0
of metallic gold (Au ). These results demonstrate that Cu-Au@g-
C
3
N
4
has been successfully formed and the metallic NPs formed
in-situ are in their elemental states.
Figure 7. UV-vis DRS spectra of (a) g-C
3
N
4
, (b) Cu-Au@g-C
3
N
4
The N
2
adsorption-desorption isotherms of g-C
3
N
4
and Cu-Au@
g-C N samples are presented in Fig. 8. These isotherms can be
3
4
classified as type II (according to IUPAC classification) with
narrow hysteresis loops, which is typical of mesoporous
materials.[ The specific surface areas of g-C
38]
N
4
and Cu-Au@g-
3
2
2
C
3
N
4
samples were estimated to be 23.9 m /g and 47.2 m /g,
respectively. It was clearly observed that of the two samples, the
Cu-Au@g-C N sample, which was prepared by immobilization of
3 4 3 4
Figure 6. XPS survey of (a,c) g-C N , (b,d,e,f) Cu-Au@g-C N
3
4
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European Journal of Inorganic Chemistry
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Au and Cu metal NPs over g-C
surface area, indicating that the Cu-Au@g-C
active sites where the catalytic reaction could occur.
3
N
4
, exhibited the largest specific
H activation hydroxylation of benzene at room temperature (Table
1, entries 9-13). To our delight, these bimetallic combinations
3
N
4
sample had more
served our esteemed purpose and amongst all, Cu-Au@g-C
gave excellent results in less than 30 min (Table 1, entry 13).
Evidently, the bimetallic Cu-Au@g-C catalyst outperformed
the monometallic Au@g-C and Cu@g-C . The effectiveness
3 4
N
3
N
4
3
N
4
3 4
N
of the bimetallic catalyst was nearly about three folds to the
corresponding Au and Cu monometallic catalysts. Considering
the fact that bimetallic Cu-Au@g-C
quantity of Au@g-C and Cu@g-C
for the same reaction under identical conditions (Table 1, entry
9). Interestingly, even after doubling the quantity did not led to
any significant improvement. These observations reasoned the
existence of a dominant synergistic effect in Cu-Au@g-C . The
3
N
4
contains more metals, the
3
N
4
3 4
N were doubled and tested
1
3
N
4
control reaction (dark conditions) was also executed with
bimetallic Cu-Au NPs, delivering very low performance, even after
extended reaction time (Table 1, entry 20). It further ascertained
3 4
the vital role of g-C N as a promoter under photochemical
conditions. After screening the active catalyst for C-H activation
hydroxylation of benzene, we next performed the optimization of
the catalyst amount and solvent. The optimized results are
summarized in Table. 1, entries 14-18. Initially, 50 mg of
2 3 4 3 4
Figure 8. N adsorption-desorption isotherm of (a) g-C N , (b) Cu-Au@g-C N
3 4
CuAu@g-C N was used to assess the reaction, which was
further reduced to 30 mg. It was clearly observed that there were
minimal changes in the reaction outcome, delivering the product
in 30 min (Table 1, entries 14,15). Still, on further reducing the
catalyst quantity to 15 mg led to an increase in reaction time and
over exposure to irradiation (Table 1, entry 16). It was therefore
noted that 30 mg of the catalyst was ideal for the efficient C-H
activation hydroxylation reaction. Sequentially, a vast array of
solvents were also screened. Acetonitrile was observed to be a
better solvent compared to water and methanol. Both water and
After the successful synthesis and detailed characterization of the
as-synthesized materials, their catalytic potential was
investigated in the hydroxylation of benzene to phenol. We
performed the hydroxylation of benzene to phenol under visible
3 4
light using a series of metals decorated on g-C N . It was believed
that g-C would provide itself as a promising candidate for this
3
N
4
catalytic assessment, owing to the presence of nitrogenous
chromophore.[39]
electron rich photoactive
This built-in
methanol lessened the effective performance of CuAu@g-C
3 4
N
chromophore of g-C N would unambiguously absorb visible light
3
4
catalyst (Table 1, entries 17,18). Taking into consideration of all
irradiation, thereby, accelerating C-H activation and thus helping
in crossing the activation energy for achieving phenol via benzene
the observations, it was concluded that 30 mg of CuAu@g-C
3
N
O
4
conversion.[
39]
was ideal for performing the HOB reaction in presence of H
and acetonitrile as the solvent.
2
2
Owing to the inherent photoactive characteristics
of g-C
immobilizing it with other metals. Accordingly, we prepared a
series of NPs decorated g-C materials including Fe @g-
, Cu@g-C , Pd@g-C , Ag@g-C and Au@g-C for
evaluating hydroxylation of benzene (HOB) with acetonitrile using
3 4
N , we contemplated to explore its effectiveness via
Table 1. Screening of catalysts for hydroxylation of benzene[a] to phenol
3
N
4
3 4
O
C
3
N
4
3
N
4
3
N
4
3
N
4
3 4
N
Entry
Catalyst
3 4
Fe @g-C N
Time
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
14 h
30 min
30 min
30 min
2 h
30 min
30 min
30 min
14 h
Conversion[b]
15 %
43 %
39%
1
2
3
4
5
6
7
8
9
O
3 4
H
Fe
2
O
2
as an oxidant under visible light irradiation. Eventually,
@g-C gave very low conversion rates, even after 14 h
Cu@g-C
Pd@g-C
Ag@g-C
3
3
3
N
4
3
O
4
3
N
4
N
N
4
4
25 %
34 %
56 %
60 %
65 %
70 %
72 %
73 %
69 %
99 %
99%
of continuous stirring (Table 1, entry 1). Shifting the metals from
Au@g-C
3
N
4
Fe
Table 1, entry 3) showed no significant improvement. Ag@g-C
and Au@g-C
3
1
3
O
4
to Cu (Cu@g-C
3
N
4
, Table 1, entry 2) and Pd (Pd@g-C
3
N
4
,
FePd@g-C
FeAg@g-C
3
N
4
3
N
4
N
3 4
3
N
4
gave moderate conversion rate with 25% and
4% respectively, however, took long reaction time of 14 h (Table
, entries 4,5). Evidently, these catalysts failed to efficiently
FeAu@g-C
FeCu@g-C
3
N
4
3
N
4
10
11
12
CuPd@g-C
AgPd@g-C
3
3
N
4
N
4
perform the visible light mediated HOB reaction. Next, we thrived
for bimetallic combinations at g-C and the results are
AuPd@g-C
CuAu@g-C
CuAu@g-C
CuAu@g-C
CuAu@g-C
CuAu@g-C
CuAu@g-C
3
N
4
3
N
4
13
3
3
3
3
3
3
N
4
4
4
4
4
4
[
[
[
c]
d]
e]
1
1
1
1
4
5
6
7
N
N
N
N
N
summarized in Table 1, entries 6-8. Apparently, it was observed
that there was slight increase in the conversion rates, yet, these
catalysts were non-active to oxidize benzene to phenol. As a
result, we opted for some other rational bimetallic combinations
99%
99%
[f]
79 %
83 %
51 %
25 %
-
[
[
[
[
g]
h]
i]
18
19
20
21
Cu@g-C
CuAu@g-C
No catalyst
3 4 3 4
N + Au@g-C N
viz.
Fe-Cu@g-C
3
N
4
,
3 4 3 4
Cu-Pd@g-C N , Ag-Pd@g-C N , Cu-
3 4
N
Au@g-C
3
N
4
and Au-Pd@g-C N
3 4
for the visible light mediated C-
j]
14 h
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European Journal of Inorganic Chemistry
10.1002/ejic.201800622
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molecule and the slow production of hydroxyl radicals result in an
uncatalyzed HOB reaction (Table 1, entry 21). As a result,
decreasing the stability of benzene and facilitating the faster
production of hydroxyl radicals are efficient approaches to
increase the reaction rate of this reaction. In this context, it has
been already reported that Cu NPs can activate the C-H bond of
benzene molecule.[42] and Au NPs can accelerate the production
of hydroxyl radicals. Moreover, from the UV-DRS studies, it is
Reaction conditions: [a]Benzene (1 mmol), CH
W domestic cool LED bulb, 50 mg catalyst; [b]Isolated yield of phenol; [c]50 mg of
3
2 2
CN (5.0 mL), 30% H O (1.1 mmol), 20
3 4 3 4 3 4
CuAu@g-C N N N
; [d]30 mg of CuAu@g-C ; [e]15 mg of CuAu@g-C ; [f]Water as a
solvent; [g]Ethanol as a solvent; physical mixture; Reaction was performed in the
[h]
[i]
dark; [j]No catalyst
The photoluminescence analysis was performed to elucidate the
recombination tendency of charge carriers in our synthesized
samples, as evidenced from Fig. 9. Basically, a PL spectrum
shows the transfer, migration, and recombination of
photogenerated electron hole pairs in the photocatalytic
3 4
evident that Cu-Au@g-C N photocatalyst have a narrow
bandgap of 2.27 eV, owing to which it can efficiently absorb visible
light. The PL studies on the other way suggested that the
photocatalyst can lengthen the separation time of the charge
carriers to a great extent, suggesting an enhanced photocatalytic
performance under visible light irradiation. Eventually, in the HOB
processes.[
40]
Hence, the PL studies could quantify the charge
. As evidenced from the PL
shows an emission band centered at
45 nm, which can be ascribed to the recombination process of
transfer dynamics in CuAu@g-C
3 4
N
reaction catalysed by Cu@g-C
activated, the rate of the generation of hydroxyl radical was not
significantly increased. In case of Au@g-C , the production of
3 4
N , although the benzene ring was
spectrum, the pure g-C
3
N
4
4
[
41]
3 4
N
self-trapped excitations in g-C
evident that the peak position of Cu-Au@ g-C
of pure g-C . However, the intensity of the emission spectra of
Cu-Au@ g-C is significantly decreased, which suggests that
separation of the charge carriers in Cu-Au@ g-C photocatalyst
is greatly increased compared to pure g-C . These findings
3
N
4
.
From the spectrum, it is
hydroxyl radical was promoted, however the aromatic ring of
benzene remained stable, resulting to inefficient performance. In
contrast, the HOB reaction catalyzed in presence of CuAu@g-
C N boosted the conversion rate due to the presence of
3 4
synergistic effect.
3
N
4
is similar to that
3
N
4
3
N
4
3
N
4
3
N
4
further ascertained the efficiency under visible light mediated
catalytic performance of the catalysts.
3 4
Scheme 2. Mechanistic pathway for efficient HOB reaction by CuAu@g-C N
under visible light irradiation.
3 4 3 4
Figure 9. PL spectra of (a) g-C N , (b) Cu-Au@g-C N .
To find additional evidence of the synergistic effect present in
CuAu@g-C , HR-TEM analysis was further evaluated (Fig. 10).
3
N
4
The main objective of the analysis was to discern the relative
position of the two metal NPs in the catalyst. As observed from
the HR-TEM micrographs in Fig. 10, the Cu and Au NPs are in
The plausible mechanism in the present study involves the
degradation of hydrogen peroxide into hydroxyl radicals over the
photo active bimetallic surface of the nanoparticles. Sequentially,
contact with each other in CuAu@g-C
of some interaction between these two NPs on the surface of g-
. Owing to their possible interaction, it is expected that a
synergism exists between Cu and Au NPs in the catalytic reaction,
thus enhancing the catalytic performance of Cu-Au@g-C
catalyst. Furthermore, XPS analysis was used to determine the
redox properties of CuAu@g-C catalyst. As shown in Fig. 11,
3 4
N , suggesting possibility
the molecular benzene embraces the g-C
covalent interactions and thereby gets activated by Cu-Au@g-
under visible light resulting in the facilitation of C-H bond.
3 4
N surface via non-
3 4
C N
3 4
C N
Finally, the generated hydroxyl radical react with benzene leading
to the formation of phenol (Scheme 2).[16] It is pertinent to mention
that in general, the stability of the aromatic ring of benzene
3
N
4
3
N
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201800622
FULL PAPER
significant shifts in binding energy were observed in both the Cu
was evident that there was a slight increase in particle size with a
range of 10-15 nm. All these observations unambiguously
suggest the true heterogeneous nature of the catalyst, which is a
prerequisite for practical applications.
2
p and Au 4f spectra of CuAu@g-C
Cu@g-C and Au@g-C . These findings suggest that there
was electron transfer between Cu and Au in bimetallic CuAu@g-
catalyst, leading to a significant synergism.
3 4
N , compared with that of
3
N
4
3 4
N
3 4
C N
Figure 10. TEM micrographs of (a) CuAu@g-C
3 4
N , (b) HR-TEM image, (d) HR-
TEM image showing the relative position of Cu and Au NPs in CuAu@g-C
N .
3 4
3 4
Figure 12. Recyclability of Cu-Au@g-C N in HOB reaction
Figure 11. XPS spectra of (a,b) Cu 2p and (c,d) Au 4f for Cu@g-C
, and CuAu@g-C
3 4
N , Au@g-
C
3
N
4
3 4
N .
After completion of the first cycle, the catalyst was recovered
using centrifuge, washed with acetone and dried under vacuum.
A new reaction was then performed with fresh benzene, under
3 4
similar conditions. The bimetallic CuAu@g-C N could be
Figure 13. (a) TEM image of spent Cu-Au@g-C
c) particle size distribution histogram.
3 4
N catalyst, (b) EDAX pattern,
recycled at least 5 times without losing its activity, as evidenced
from Fig. 12. The leaching test was performed for Cu and Au by
ICP-AES analysis using the filtrate and it was found that no Cu or
Au elements were present in the filtrate. We also observed that
amount of Cu and Au present in the spent catalyst is almost
similar to that of the fresh catalyst, as evidenced from the ICP-
AES analysis results (see ESM). Additionally, the TEM image of
the spent catalyst showed an almost similar particle distribution to
that of the fresh catalyst (Fig. 13(a)). EDAS pattern were also
recorded for the spent catalyst and it showed the presence of all
requisite elements, confirming that no leaching occurred, as
shown in Fig. 13(b). From the plot of particle size distribution, it
(
Conclusions
In summary, a novel synthetic route towards preparation of
photoactive bimetallic CuAu@g-C catalyst is demonstrated in
3
N
4
the present study. The photoactive catalyst was characterized by
multiple techniques including FT-IR, XRD, SEM, TEM, EDAS, UV-
DRS, BET and PL analysis to authenticate its successful
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European Journal of Inorganic Chemistry
10.1002/ejic.201800622
FULL PAPER
formation. The morphological studies revealed that large number
of very small nanoparticles of Cu and Au NPs (5-10 nm) were
centrifuged, washed with acetone and dried under vacuum to afford the
desired Cu-Au@g-C
Cu@g-C and Au@g-C
AuCl were synthesized using the same procedure.
3 4
To authenticate the successful formation, the obtained Cu-Au@g-C N
N
3 4
product. For comparison, individual samples of
decorated rather uniformly over g-C
photoactive bimetallic CuAu@g-C
3
N
4
surface. Apart from it, the
catalyst was applied
N
3 4
N
3 4
using the same amounts of Cu(NO and
3 2
)
3
N
4
3
towards the controlled synthesis of phenol from benzene. It was
clearly observed from the studies that our synthesized material
led to complete conversion of benzene to phenol (conversion
exceeding 99%) without forming any side products. Additionally,
the photoactive nature of the synthesized catalyst was further
illustrated using UV-DRS analysis. It is believed that this novel
and robust protocol for direct HOB reaction unambiguously would
open up new avenues in the domain of heterogeneous catalytic
systems.
catalyst was characterized using several techniques including XRD, FT-IR,
SEM, TEM, EDX, XPS, UV-DRS, BET and PL analysis.
General procedure for the hydroxylation of benzene
Benzene (1 mmol) and catalyst (30 mg) in acetonitrile (5 mL) were charged
into 25 mL side-armed round bottomed flask equipped with a magnetic
2 2
stirring bar. Sequentially, 30% H O (1 mmol) was added to the reaction
mixture and exposed to visible light irradiation using a 20 watt domestic
cool LED bulb. Eventually, the progress of the reaction was monitored by
TLC. After the completion of reaction, the catalyst was separated from the
reaction mixture followed by centrifugation. The obtained product was
extracted with ethyl acetate and further characterized by H and 13C NMR.
Experimental Section
1
Materials and Methods
1
Detailed presentation of physical data: m.p. 42° C; H NMR (300 MHz,
CDCl
3
, TMS) 7.24 δ (t, 2H, J= 2Hz), 6.94 δ (t, 1H, J=0.8Hz), 6.82 δ (d, 2H,
3 6 6
Melamine (C H N ), Gold Chloride (AuCl
3
), Copper Nitrate (Cu(NO
3
)
2
),
13
J= 2Hz), 5.54 δ (S, 1H); C NMR= δ (155.23, 129.89, 129.61, 120.84,
Sodium borohydride (NaBH
4
), Benzene (C H
6 6
) were purchased from
115.52, 115.41)
Merck India Ltd. and were used as received without any further purification.
Double distilled water was used throughout the experiments. FTIR spectra
were obtained using a KBr matrix via a Bruker 3000 Hyperion Microscope
with the Vertex 80 FT-IR system. XRD measurements of the samples were
carried out on a Bruker AXS D8-Advance powder X-ray diffractometer with
Cu-Kα radiation (λ=1.5418Å) with a scan speed of 2°/min. For assessing
their morphology, scanning electron microscopy (SEM JSM-7600F)
equipped with EDS was used. Transmission electron microscopy images
were obtained on a JEOL, JEM 2100 instrument. The UV–DRS analysis
was recorded by a Scinco 4100 apparatus. The X-ray photoelectron
spectroscopy (XPS) spectra were obtained, using a PHI 5000 Versa Prob
II XPS with AES Module. PL measurements were carried out on a Hitachi
Model No. F-4600 Fluorescence Spectrophotometer. 1H and 13C NMR
spectra of the compound were obtained using a Bruker AV III at 300 Hz.
Acknowledgements
The authors thank SAIF, IIT Bombay, STIC Cochin, ACMS IIT
Kanpur, and SAIF, Gauhati University for sample analysis. BB
acknowledges Director, NIT Silchar for providing institutional
fellowship.
Conflict of interest
The authors declare no conflict of interest.
3 4
Synthesis of graphitic carbon nitrite (g-C N )
Keywords: Carbon nitrites • photoactive bimetallic catalysts •
mono-dispersed NPs • catalytic HOB reaction • synergistic
interaction
3 4
The photoactive g-C N was prepared using a facile approach with
melamine as the precursor. Melamine (5 g) was taken and sequentially
calcined at 550° C in a muffle furnace for 3 h. The collected pale yellow
3 4
coloured powder was named g-C N and stored until further use.
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European Journal of Inorganic Chemistry
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Entry for the Table of Contents
FULL PAPER
Visible light mediated hydroxylation
of benzene via C-H activation with
Bishal Bhuyan, Meghali Devi,
Debashree Bora, Siddhartha Sankar
Dhar,* and Rajashree Newar
novel bimetallic Cu-Au@g-C
3 4
N
photocatalyst
Page No. – Page No.
Design of
bimetallic Cu-Au@g-C
a
novel photoactive
catalyst for
3
N
4
visible light driven hydroxylation of
benzene reaction via C-H activation
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