Modified graphene supported Ag-Cu NPs with enhanced bimetallic synergistic effect in oxidation and Chan-Lam coupling reactions

Article abstract of DOI:10.1039/d0ra01540g

Herein, well dispersed Ag-Cu NPs supported on modified graphene have been synthesized via a facile and rapid approach using sodium borohydride as a reducing agent under ambient conditions. Dicyandiamide is selected as an effective nitrogen source with TiO2 as an inorganic material to form two kinds of supports, labelled as TiO2-NGO and NTiO2-GO. Initially, the surface area analysis of these two support materials was carried out which indicated that N-doping of GO followed by anchoring with TiO2 has produced support material of larger surface area. Using both types of supports, ten nano-metal catalysts based on Ag and Cu were synthesized. Benefiting from the bimetallic synergistic effect and larger specific surface area of TiO2-NGO, Cu?Ag-TiO2-NGO is found to be a highly active and reusable catalyst out of other synthesized catalysts. It exhibits excellent catalytic activity for oxidation of alcohols and hydrocarbons as well as Chan-Lam coupling reactions. The nanocatalyst is intensively characterized by BET, SEM, HR-TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS. This journal is

Full text of DOI:10.1039/d0ra01540g

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Modified graphene supported Ag–Cu NPs with
enhanced bimetallic synergistic effect in oxidation
and Chan–Lam coupling reactions†
Cite this: RSC Adv., 2020, 10, 30048
*
Nitika Sharma, Anu Choudhary, Manpreet Kaur, Chandan Sharma, Satya Paul
and Monika Gupta
Herein, well dispersed Ag–Cu NPs supported on modified graphene have been synthesized via a facile and
rapid approach using sodium borohydride as a reducing agent under ambient conditions. Dicyandiamide is
selected as an effective nitrogen source with TiO2 as an inorganic material to form two kinds of supports,
labelled as TiO2–NGO and NTiO2–GO. Initially, the surface area analysis of these two support materials was
carried out which indicated that N-doping of GO followed by anchoring with TiO2 has produced support
material of larger surface area. Using both types of supports, ten nano-metal catalysts based on Ag and
Cu were synthesized. Benefiting from the bimetallic synergistic effect and larger specific surface area of
TiO2–NGO, Cu@Ag–TiO2–NGO is found to be a highly active and reusable catalyst out of other
synthesized catalysts. It exhibits excellent catalytic activity for oxidation of alcohols and hydrocarbons as
well as Chan–Lam coupling reactions. The nanocatalyst is intensively characterized by BET, SEM, HR-
TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS.
Received 18th February 2020
Accepted 6th August 2020
DOI: 10.1039/d0ra01540g
rsc.li/rsc-advances
nanoparticles as well as avoid the use of additional base in
stoichiometric amount. Recently, several approaches have been
Introduction
reported in literature which indicates that introduction of
nitrogen into graphene modulate its electronic as well as
chemical properties and hence make N-G more catalytically
active than graphene. For instance, Lin et al. have successfully
prepared nitrogen doped graphene (NG) from GO and urea at
800 ꢀC in an inert atmosphere, which has shown superior
catalytic activity towards ORR than Pt/C catalysts.3 Urea reacts
with oxygen-containing functional groups present in the GO,
where N content was increased upto 46%. Xiong et al. have
prepared N-doped graphene by thermal annealing of GO in
ammonia at different temperatures and used it as a conductive
support for Pt nanoparticles. The Pt/N-G catalysts showed
tremendous electrocatalytic activity towards methanol oxida-
tion as compared to undoped catalysts.4 This synergistic elec-
trochemical effect was probably due to the existence of
electronic interactions between Pt NPs and NG.
Further, to increase the surface area as well as thermal
stability, graphene has been immobilized onto various inor-
ganic materials such as silica, titania, zirconia, ceria etc. Zhang
et al. prepared a photocatalyst from polyelectrolyte/exfoliated
titania nanosheet/graphene oxide by occulation and calcina-
tion. The polyelectrolyte, PDDA (poly-diallyl-dimethyl-
ammonium chloride) acts as an effective binder to precipitate
GO and titania nanosheets, thereby enhancing the overall
performance of the catalyst signicantly.5 Liu et al. successfully
developed graphene-coated silica (GCS) which act as a highly
efficient sorbent. The prevention of dispersion in water by the
Graphene inherits remarkable electronic, thermal, optical and
mechanical properties, which make it attractive for use in the
eld of heterogeneous catalysis. However, due to aggregation1
between the layers of graphene, it has limited use in the eld of
catalysis. Graphene oxide (GO), generally obtained by the exfo-
liation of graphite2 has emerged as a new class of carbonaceous
water-compatible heterogeneous catalysts.3–6 Mostly, graphene
oxide (GO) and its derivatives have been used to synthesize
graphene-supported nano-metal catalysts because of the pres-
ence of plentiful oxygenated functional groups and topological
defects7 which serve as more favourable anchoring sites for the
precursors.8
Nowadays, heteroatom doping is extensively used to modify
the properties of GO and particularly N-doped graphene has
attracted sufficient scientic interest due to its alluring perfor-
mance in supercapacitors,9 lithium-ion batteries,10 catalyst
support11 and catalysis.12–14 Greater electronegativity of N as
compared to C along with the conjugation between nitrogen
lone pair of electrons and graphene p-system has found to tailor
the electronic properties of N-doped graphene. Also, N-modied
graphene has additional benets of introducing basic character
to the support material, which in turn stabilizes the metal
Department of Chemistry, University of Jammu, Jammu Tawi-180006, India. E-mail:
paul7@rediffmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01540g
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hydrophobic nature of graphene and adsorption capacity by other type, GO was modied with TiO2 rst to get GO–TiO2
agglomeration was overcome by coating it with silica.6 Among composite followed by doping with N using dicyandiamide to
various inorganic materials, nano-TiO2 (ref. 15–17) has received get nitrogen modied composite (NTiO2–GO). BET surface area
increased attention owing to its exceptional properties such as analysis indicated that TiO2–NGO composite has larger surface
non-toxicity, low cost and chemical stability as well as applica- area as compared to NTiO2–GO (Fig. 1). The existence of
tion in photocatalysis. Combining TiO2 with GO can facilitate synergistic effect between NGO and TiO2 would increase the
the electron transfer as GO binds with TiO2 by forming coor- surface area available for adsorption and improve the surface
dinate bonds between functional groups and Ti4+ centers and exposure of the catalytic active sites. Finally, Ag–Cu nano-
thus increases the thermal stability.
particles (NPs) with varied metal composition were immobi-
Although a variety of heterogeneous catalysts have been re- lized onto the support (TiO2–NGO and NTiO2–GO) so as to
ported in the literature for the oxidation of alcohols and obtain ten nanometal catalysts which were tested for the
hydrocarbons, and Chan–Lam coupling reactions, but the oxidation of alcohols and hydrocarbons, and Chan–Lam
development of a novel, versatile and recyclable catalytic system coupling reactions.
employing mild and inexpensive reaction conditions is still
Hence, the main aim of the present work was to get insight into
a challenging job. Hence, based on the principle of green the role of N-doping for the stabilization of metal nanoparticles
chemistry, we tried to develop a general and sustainable and to explore the mechanism behind the synergism between Ag–
protocol for the selective liquid-phase oxidation of alcohols and Cu NPs using different spectral studies. Out of the various cata-
hydrocarbons, and Chan–Lam coupling reactions. Further, the lysts synthesized, Cu@Ag–TiO2–NGO showed superior activity for
signicant enhancement in the activity and selectivity of the oxidations as well as for Chan–Lam coupling and was fully char-
synthesized catalyst was observed due to the presence of an acterized by several comprehensive techniques such as BET, SEM,
interesting and innovative support material which turned out to HR-TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS.
be highly useful for the stabilization of MNPs as well as hamper
the agglomeration and stacking of the GO sheets.
Experimental
Keeping in view the above facts in mind, we have prepared
Materials and characterization
two types of supports; one in which GO was rst modied with
nitrogen precursor, dicyandiamide (NGO) and then TiO2 was
incorporated onto its surface i.e. TiO2–NGO, whereas in the
The chemicals used were obtained from Aldrich Chemical
company and used further without purication. FTIR spectra of
Fig. 1 N2 adsorption–desorption isotherms of: (a) TiO2–NGO; (b) NTiO2–GO; (c) Cu@Ag–TiO2–NGO; (d) Cu@Ag–NTiO2–GO.
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the catalysts were obtained on FTIR spectrometer (Agilent formed was ltered, washed with distilled water, ethanol and
ꢀ
Technologies, L1600312 spectrum TWOLITA/ZnSe) while nally dried in an oven at 60 C overnight.
thermal stability of the prepared catalysts was measured by TGA
Synthesis of bimetallic Ag and Cu supported nano-metal catalysts.
analyzer (PerkinElmer, Diamond TG/DTA). XRD was obtained To a suspension of NTiO2–GO or TiO2–NGO (1 g) in ethanol : -
on a Bruker AXSD8 Advance X-ray diffractometer using Cu Ka water (1 : 1, 10 mL), AgNO3 (0.169 g, 0.1 mmol) was added and
radiations. SEM images were recorded using FEG SEM JSM- the resulting mixture was stirred for 20 min. To this suspension,
7600F Scanning Electron Microscope and HR-TEM images freshly prepared aqueous solution of NaBH4 (1.5 mmol, 5 mL)
using FEG, Tecnai G2, F30 Transmission Electron Microscope. was added over a period of 1 h and the stirring was continued for
Actual loading of metal was determined by ICP-AES analysis 3 h at room temperature. Now, Cu(NO3)2 (0.241 g, 0.1 mmol) was
using ARCOS, Simultaneous ICP spectrometer. EDX analysis added into the resultant mixture and further stirred for 20 min
was surveyed by using OXOFORD X-MAX Model JSM-7600F and followed by addition of freshly prepared aqueous solution of
XPS spectra of the catalysts were recorded on KRATOS ESCA NaBH4 (1.5 mmol, 5 mL) over a period of 1 h. Aer 12 h of
model AXIS 165 (Resolution) while CHN analysis was carried continuous stirring, the catalyst (Cu@Ag–NTiO2–GO or Cu@Ag–
out on Thermo Finnigan, FLASH EA 1112 series. BET (Bru- TiO2–NGO) was recovered by ltration and effectively washed
nauer–Emmett–Teller) specic surface area was determined with distilled water (3 ꢂ 10 mL), ethanol (3 ꢂ 10 mL) and dried in
from N2 adsorption–desorption isotherms using Belsorb Mini-X vacuum at room temperature overnight. Similarly, Ag@Cu–
analyzer. The mass spectra were analysed by ESI-esquire 3000 NTiO2–GO or Ag@Cu–TiO2–NGO were prepared following the
1
Bruker Daltonics spectrometer while H NMR (400 MHz) and same procedure as mention above, but here Cu(NO3)2 (0.241 g,
13C NMR (100 MHz) spectra of the synthesized products were 0.1 mmol) was added rst and then reduced, followed by the
recorded using chloroform-d on Bruker Advance III spectrom- addition of AgNO3 (0.169 g, 0.1 mmol) which was later reduced by
eter using TMS as an internal standard.
using NaBH4 solution. Cu–Ag alloy (Ag–Cu@TiO2–NGO or Ag–
Cu@NTiO2–GO) was prepared by stirring NTiO2–GO or TiO2–
NGO (1 g) in ethanol : water (1 : 1, 10 mL) for 20 min followed by
addition of Cu(NO3)2 (0.241 g, 0.1 mmol) and AgNO3 (0.169 g, 0.1
mmol) simultaneously. Then, freshly prepared aqueous solution
Catalyst preparation
Preparation of supports
Synthesis of NTiO2–GO. NTiO2–GO composite was prepared by of NaBH4 (3.0 mmol, 10 mL) was added dropwise over a period of
dispersing GO18 (1 g) into ethanol (40 mL) and ammonium 1 h and the stirring was continued for 12 h at room temperature.
hydroxide (5 mL) followed by ultrasonication for 1 h. This Aer the metal NPs were synthesized, they were separated by
solution was then transferred into a round bottom ask (100 ltration and washed with distilled water (3 ꢂ 10 mL) and
mL) and heated at 45 ꢀC under continuous stirring for 12 h ethanol (3 ꢂ 10 mL) respectively. The catalyst was dried in
followed by the addition of solution of tetrabutyl orthotitanate vacuum at room temperature overnight. Monometallic
(TBOT, 4 mL) in ethanol (30 mL) dropwise at a rate of 5 Cu@TiO2–NGO or Cu@NTiO2–GO and Ag@TiO2–NGO or
mL minꢁ1. The resultant TiO2–GO composite was ltered, Ag@NTiO2–GO were prepared by the similar methodology using
washed thoroughly with distilled water (5 mL), ethanol (5 mL), Cu(NO3)2 (0.241 g, 0.1 mmol) and AgNO3 (0.169 g, 0.1 mmol)
and dried in an oven at 60 ꢀC. Dicyandiamide (1 g) was respectively.
dispersed with TiO2–GO (2 g) in distilled water (100 mL) fol-
lowed by stirring at room temperature overnight. Water from
the well-dispersed suspension was removed, grey powdered
hybrid material was obtained, which was rst heated at a rate of
5 C min upto 200 C, maintaining this temperature for 1 h
and then, subsequently heating at 5 ꢀC minꢁ1 upto 350 ꢀC for 8
h. The as-synthesized dark grey powder was the desired NTiO2–
GO composite.
General procedure for the oxidation of alcohols and
hydrocarbons
For the oxidation, Cu@Ag–TiO2–NGO (0.1 g) was added to the
mixture of alcohol or hydrocarbon (1 mmol) and TBHP (1
mmol) in ethanol (8 mL) and the reaction mixture was stirred at
70 ꢀC for the desired time. Ethanol was then removed under
reduced pressure followed by addition of ethyl acetate (20 mL).
The catalyst was separated by ltration and washed with
deionized water (2 ꢂ 10 mL), ethyl acetate (3 ꢂ 5 mL) respec-
tively and dried under vacuum for further use. The organic layer
was washed with deionized water (3 ꢂ 20 mL) followed by
drying over anhydrous Na2SO4. Finally, the product was ob-
tained by column chromatography on silica gel (60–120 mesh)
using petroleum ether and ethyl acetate as eluting solvents.
ꢁ1
ꢀ
ꢀ
Synthesis of TiO2–NGO. Dicyandiamide (1 g) was mixed with
GO (2 g) in distilled water (100 mL) and was stirred well. Water
in the well-dispersed suspension was allowed to evaporate
which resulted in grey powdered hybrid materꢁia1l. This hybrid
ꢀ
ꢀ
material was rst heated at a rate of 5 C min upto 200 C,
maintaining this temperature for 1 h and then, subsequently
ꢁ1
ꢀ
ꢀ
heating at 5 C min upto 350 C for 8 h. The as-synthesized
dark grey powder was the desired NGO support. Now, TiO2–
NGO was prepared by ultrasonication of NGO (1 g) and
ammonium hydroxide (5 mL) in ethanol (40 mL) for 1 h. The
General procedure for Chan–Lam cross-coupling of aromatic
amines with arylboronic acids
resultant solution was then transferred into a round bottom
ꢀ
ask (100 mL) and heated at 45 C for 12 h with stirring, fol- The mixture of aryl amine (1 mmol), arylboronic acid (1 mmol)
lowed by addition of tetrabutyl orthotitanate (TBOT, 4 mL) in and Cu@Ag–TiO2–NGO (0.1 g) in water (5 mL) was stirred at
ethanol (30 mL) at the rate of 5 mL minꢁ1. TiO2–NGO composite 80 C for the appropriate time. Then, the catalyst was ltered
ꢀ
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Scheme 1 Schematic representation of the synthesis of Cu@Ag–TiO2–NGO.
off, washed with ethyl acetate (3 ꢂ 5 mL), deionized water (2 ꢂ monometallic Ag and Cu on modied GO were synthesized in
10 mL) and dried under vacuum. The ltrate was cooled to room order to study the synergism between two metals as well as
temperature, diluted with ethyl acetate (20 mL) and washed effect of N-doping prior to and aer modication of GO with
with brine solution followed by drying over anhydrous Na2SO4. TiO2. First of all, two different support materials, TiO2–NGO
Column chromatography on silica gel (60–120 mesh) using and NTiO2–GO were synthesized. In TiO2–NGO, GO was doped
ethyl acetate–petroleum ether as eluent was done to obtain the with nitrogen followed by anchoring of TiO2, whereas in NTiO2–
product.
The structures of the products were conrmed by 1H and 13
NMR and mass spectral data.
GO, rst anchoring of GO and TiO2 was carried out followed by
nitrogen doping using dicyandiamide. Initially, the surface area
analysis of these two support materials was carried out. The BET
surface area of TiO2–NGO was found to be 132 m2 gꢁ1 with pore
volume of 0.1289 cm3 gꢁ1, whereas NTiO2–GO has surface area
C
26.9 m2 gꢁ1 with pore volume 0.0486 cm3 gꢁ1
.
Results and discussion
Thus, it is concluded that N-doping of GO followed by
anchoring with TiO2 has produced support material of larger
surface area. This was further supported by the BET surface area
of synthesized Cu@Ag–TiO2–NGO as 98.0 m2 gꢁ1 with total pore
Herein, we report the synthesis of Ag–Cu bimetallic NPs deco-
rated on modied GO and its activity was evaluated for the
oxidation and C–N coupling reactions. Ten different catalysts
based on immobilization of bimetallic Ag–Cu and
Fig. 2 FEG-SEM images of Cu@Ag–TiO2–NGO.
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Fig. 3 HR-TEM images of: (a and b) Cu@Ag–TiO2–NGO; (c) histogram of metal NPs; (d) SAED pattern of Cu@Ag-TiO2-NGO.
volume as 0.131 cm3 gꢁ1, whereas Cu@Ag–NTiO2–GO as 21.3 support was occupied by the well dispersed Ag–Cu nano-
m2 gꢁ1 with total pore volume as 0.039 cm3 gꢁ1 (Fig. 1). It has particles. Initially, all catalysts were tested for oxidation (entry
been rationalized that the surface area of the catalyst would be 2a, Table 3) and Chan–Lam coupling (entry 5c, Table 6) and
smaller as compared to that of the support as some surface of found that Cu@Ag–TiO2–NGO was the best among all the
Fig. 4 EDX spectrum of Cu@Ag–TiO2–NGO.
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Fig. 5 CHN analysis of Cu@Ag–TiO2–NGO.
synthesized catalysts and so was fully characterized by BET, conrmed that the nanoparticles were evenly dispersed on the
SEM, HR-TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS. support surface. The histogram (Fig. 3c) shows an average size
An illustration of the preparation protocol for Cu@Ag–TiO2– of metal nanoparticles as 15–20 nm, while the corresponding
NGO is shown in Scheme 1. GO was rstly prepared by SAED pattern (Fig. 3d) shows concentric rings and dots which
Hummer's method followed by its thermal annealing with indicated that the nanoparticles are polycrystalline in nature.
dicyandiamide, which resulted in spontaneous conversion of
Aer carrying out the surface analysis, qualitative as well as
graphene oxide into nitrogen doped graphene oxide. Further, in quantitative analysis was done by means of Energy Dispersive X-
order to increase the thermal stability and surface area of ray (EDX), CHN and Inductively Coupled Plasma Atomic Emis-
modied graphene oxide, it was immobilized with TiO2. Finally, sion Spectroscopy (ICP-AES) analysis. 2.35 wt% Ag and 1.71 wt%
Ag(0) and Cu(0) nanoparticles were in situ synthesized and Cu were loaded onto TiO2–NGO as indicated by ICP-AES. EDX
immobilized onto the support and then fully characterized by analysis conrms that the catalyst is composed of C, O, Ti, Ag
several comprehensive techniques in order to determine their and Cu, thus indicating the successful graing of Ag–Cu
physicochemical compositions.
nanoparticles onto the modied graphene oxide (Fig. 4). The
The surface morphology of the catalyst was determined by existence of C, N and H in the Cu@Ag–TiO2–NGO was
Field Emission Gun Scanning Electron Microscopy (FEG-SEM) conrmed by CHN analysis, which clearly indicates that N
which clearly indicates that TiO2 embedded on GO sheets was doping was successfully done upto good extent (8.15%) by using
almost spherical in shape and distributed throughout its dicyandiamide as N precursor (Fig. 5).
surface (Fig. 2). This spherical shape avoids agglomeration,
which increases the surface area of the catalyst as well as its to examine the chemical composition of the materials in the
catalytic activity. catalyst. Here, the surface functionality of Cu@Ag–TiO2–NGO,
FTIR analysis is one of the most efficient and facile method
HR-TEM micrographs of Cu@Ag–TiO2–NGO (Fig. 3a and b) TiO2–NGO and NTiO2–GO was analyzed by FTIR study as shown
conrmed the layered structure of GO with several 2D stacked in Fig. 6. The typical peaks corresponding to functional groups
graphene sheets decorated by spherical TiO2 nanoparticles, in GO included O–H stretching (3400 cmꢁ1), C–H stretching
signifying a successful negative–positive electrostatic attraction
mechanism.19 The dark mesh on titania particles and GO sheets
Fig. 6 FTIR spectra of Cu@Ag–TiO2–NGO, NTiO2–GO and TiO2– Fig. 7 TGA graph of GO, TiO2–NGO, NTiO2–GO and Cu@Ag–TiO2–
NGO.
NGO.
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NGO. The incorporation of metallic Ag NPs is conrmed by the
diffraction peaks at 38.08, 44.24, 64.50 and 77.51ꢀ which were
assigned to the face-centered cubic (fcc) planes viz. (111), (200),
(220), and (311) of Ag nanoparticles.30 Due to amorphous nature
and good dispersion of copper nanoparticles, the distinct peak
with less intensity at 2q ¼ 43.4ꢀcorresponds to diffraction from
(111) plane of the fcc structure of metallic Cu31 was also
observed. In bimetallic Ag–Cu nanoparticles, all the above Ag
and Cu diffraction peaks with less intensity were observed,
thereby indicating that bimetallic nanoparticles consists of Ag
as well as Cu phases. Also, the XRD pattern of Cu@Ag–TiO2–
NGO has shown two characteristic peaks corresponding to (220)
and (222) planes appearing at 2q ¼ 42.10 and 60.0ꢀ, which
conrms the formation of Ag–Cu alloy.32 Also, no oxide peak at
61.7ꢀ due to CuO and 37.5ꢀ due to Cu2O phase was observed in
the bimetallic spectra signifying the oxide free Ag–Cu bimetallic
system.33 The characteristic diffraction peak at 2q ¼ 10.264ꢀ
correspond to the (002) plane of graphene oxide while the
presence of oxygen functionalities in graphene oxide34,35 was
conrmed by the appearance of peak at 12.5ꢀ. A broad peak at
2q ¼ 26.75ꢀ with a lower intensity corresponds to RGO,36 which
may be formed during the catalyst preparation while the intense
peak at 2q ¼ 25.889ꢀ corresponds to the (011) plane of TiO2.37
Hence, XRD pattern provides evidence for the co-existence of
NGO/TiO2 as well as copper and silver nanoparticles in the
nanocomposite.
XPS measurements were performed to examine the chemical
valence state of Ag–Cu nanoparticles. A survey scan (Fig. 9)
revealed the presence of elements C, N, Ag and Cu in the cata-
lysts respectively. The C 1s spectrum (Fig. 9A) could be decon-
voluted into ve peaks at 284.5 eV, 286.1 eV, 287.4 eV, 288.9 eV
and 289.7 eV, which were attributed to C–C, C–N, C–O, C]O
and O–C]O bonds respectively.38 The high resolution N 1s
spectrum (Fig. 9B) is very useful to explore the different types of
nitrogen functionalities in the catalyst. The binding energy
centered at 398.7 eV, 400.1 eV, 401.4 eV and 403.2 eV can be
assigned to the pyridinic N, pyrrolic N, graphitic N and
oxidized N, respectively.39,40 Fig. 9C(a) shows Ag 3d XPS spectra
of Ag@TiO2–NGO, where Ag 3d spectra was characterized by
doublet peak at 368.2 and 374.1 eV corresponding to Ag 3d5/2
and Ag 3d3/2 respectively, associated with pure Ag nano-
particles.41 Fig. 9C(b) and D(a) shows Ag 3d and Cu 2p core-level
spectrum of Cu@Ag–TiO2–NGO, in which Ag 3d binding energy
values does not depict much shi as compared to its mono-
metallic counterpart, while the Cu 2p reveals the presence of
several peaks with binding energies at 932.4 eV, 952.6 eV,
934.1 eV and 954.8 eV.40
Fig. 8 XRD spectra of Cu@Ag–TiO2–NGO.
(2930 cmꢁ1), C]O stretching (1635 cmꢁ1), C]C stretching
(1575 cmꢁ1), C–O stretching (1247 cmꢁ1), C–OH stretching
(1116 cmꢁ1) and C–O–H bending (1435 cmꢁ1), which conrmed
that the oxygen-containing functional groups were successfully
incorporated onto NTiO2–GO and TiO2–NGO surface.20 The
strong absorption at 536–684 cmꢁ1 is attributed to the stretch-
ing vibrations of Ti–O in TiO2.21 The peak corresponding to
C^N bond appeared at 2249 cmꢁ1, while the peak at 1383 cmꢁ1
corresponds to the stretching modes of C]N/C–N bonds.22,23
These results demonstrate the successful incorporation of N-
containing moieties over the surface of graphene oxide.
TGA analysis was employed to investigate the thermal
stability and degradation process of the catalyst under thermal
treatment (Fig. 7). It was remarkably noted that both types of
supports (TiO2–NGO and NTiO2–GO) illustrate better thermal
stability as compared to GO. However, addition of Ag–Cu
nanoparticles lowered the overall thermal stability of the
synthesized catalyst. This thermal degradation could be
accounted from the catalytic behavior of the nanoparticles
which increases the depolymerization as well as reduce the
activation energy during thermal process.24–26 The thermo-gram
suggests that the initial weight loss observed at the temperature
ꢀ
lower than 175 C which could be attributed to the removal of
The characteristic peak at 932.4 eV and 952.6 eV are attrib-
uted to Cu 2p3/2 and Cu 2p1/2 peaks of metallic copper respec-
tively, while the peaks at 934.1 eV and 954.8 eV correspond to Cu
2p3/2 and Cu 2p1/2 peaks of Cu2+ respectively. The presence of
Cu2+ was also conrmed by shake-up satellite peak at 941.8 eV,
which can be ascribed to the aerial oxidation of Cu atoms
present on the surface during sample preparation. Further, to
explore the mechanism behind the synergism between Ag–Cu
NPs, we have also recorded the XPS spectra of the reused cata-
lysts (aer ve catalytic runs) in each case and comparative
trapped water molecules between the GO layers. Further, weight
loss from 175–205 ꢀC could be assigned to the decomposition of
oxygen-containing functional groups from the surface of GO.27
The major weight loss above 470 ꢀC could be due to the
combustion of the carbon skeleton of GO.28,29 Hence, from the
TGA analysis, it can be inferred that the synthesized catalyst is
stable up to 175 ꢀC and further increase in temperature will
decompose the catalyst making it ineffective. Fig. 8 represents
the XRD of Cu@Ag–TiO2–NGO, Cu@TiO2–NGO and Ag@TiO2–
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Fig. 9 (A) XPS spectrum of C 1s; (B) XPS spectrum of N 1s; (C) XPS spectra depicting comparison of Ag 3d region, (a) monometallic Ag@TiO2–
NGO; (b) bimetallic Cu@Ag–TiO2–NGO; (c) reused Cu@Ag–TiO2–NGO in case of oxidation; (d) reused Cu@Ag–TiO2–NGO in case of C–N
coupling. (D) XPS spectra depicting comparison of Cu 2p region, (a) bimetallic Cu@Ag–TiO2–NGO; (b) reused Cu@Ag–TiO2–NGO in case of
oxidation; (c) reused Cu@Ag–TiO2–NGO in case of C–N coupling.
analysis has been carried out. Cu 2p and Ag 3d region of the possibly be due to the presence of TBHP used as an oxidant and
reused catalyst in case of C–N coupling [Fig. 9C(d) and D(c)] so the synergism existing between Ag and Cu nanoparticles is
between 4-methoxyaniline and phenyl boronic acid and oxida- not clearly visible in terms of binding energy values [Fig. 9C(c)
tion [Fig. 9C(c) and D(b)] of 4-chlorobenzyl alcohol were and D(b)]. All these points proved useful in formulating the
compared with the respective regions of the fresh catalyst mechanisms and also help in explaining the existence of
[Fig. 9C(b) and D(a)].
interactions existing between Ag and Cu NPs.
Considering the case of C–N coupling, Fig. 9C(b), C(d), D(a)
and D(c) shows the comparative analysis of high resolution Ag
3d and Cu 2p spectra of fresh and reused Cu@Ag–TiO2–NGO. A
signicant negative shi of the binding energy for Ag 3d (0.4 eV)
and Cu 2p (0.2 eV) of reused catalyst relative to that of the fresh
catalyst is identied, leading to lower electron density on Ag and
higher on Cu, which can be attributed to the electronic syner-
gism between Ag and Cu nanoparticles. Further, a decrease in
the intensity of Cu2+ along with an appreciable increase in the
intensity of Cu(0) was observed in the reused catalyst which
indicates that most of the Cu exist in the metallic form in the
reused catalyst. This can be ascribed to the dri of electron
density from silver to copper during the reaction, which is
actually responsible for the enhanced catalytic activity of the
catalyst. This point has been further explained in proposed
mechanism part. However, in case of reused catalyst obtained
aer oxidation, no noticeable change in magnitude as well as
intensity of peaks of Ag and Cu was observed. This could
Catalytic testing
Oxidation. Ten catalysts based on modied graphene were
prepared and the catalytic activity of each catalyst was tested for
the oxidation of uorene using TBHP (1 mmol) as an oxidant,
EtOH as solvent and 70 ꢀC as the optimal reaction temperature
to select the most active catalyst. Out of ten prepared catalysts,
Cu@Ag–TiO2–NGO was found to be the best as it gave high yield
in less time (Table 1). The effect of critical parameters such as
oxidant, solvent and temperature in the oxidation of primary,
secondary alcohols and hydrocarbons in the presence of
Cu@Ag–TiO2–NGO were studied (Table 2). Lower yields were
obtained in case of H2O and CH3CN (entries 1–3, 11 and 12,
Table 2) while the improved yields were obtained with
EtOH : H2O (entries 4 and 5, Table 2), but the promising results
ꢀ
were obtained in EtOH using TBHP at 70 C (entry 7, Table 2).
Also, among different oxidants i.e., air, O2, H2O2 and TBHP, the
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Table 1 Optimization of reaction conditions for the oxidation of fluorenea
Entry
Catalyst
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
Ag–Cu@TiO2–NGO
Ag@Cu–TiO2–NGO
Cu@Ag–TiO2–NGO
Cu@TiO2–NGO
2
2
2
2
2
2
2
2
2
2
72
80
82
68
74
65
73
80
60
65
Ag@TiO2–NGO
Ag–Cu@NTiO2–GO
Ag@Cu–NTiO2–GO
Cu@Ag–NTiO2–GO
Cu@NTiO2–GO
Ag@NTiO2–GO
a
Reaction conditions: uorene (1 mmol), TBHP (1 mmol), catalyst (0.1 g), ethanol (5 mL) at 70 ꢀC. b Column chromatography yield.
best results were obtained with TBHP in terms of time, yield and found to be oxidized smoothly with good yields (2j–2l, Table 3).
selectivity. Hence, to carry out the oxidation reaction, EtOH, Entries 2f and 2j were common products in case of secondary
TBHP and 70 ꢀC were selected as the optimized solvent, oxidant alcohols as well as hydrocarbons (Table 3). Further, in order to
and temperature respectively. Next, we investigated the scope of evaluate the efficiency of our protocol for the selective liquid-
the optimized protocol for primary alcohols, secondary alcohols phase oxidation of alcohols and hydrocarbons, the activity of
and hydrocarbons.
Cu@Ag–TiO2–NGO was compared with some of the reported
It was found that the reaction with alcohol bearing electron- heterogeneous catalytic systems and the results clearly indicate
donating group proceeds smoothly in less time as compared to the supremacy of the synthesized catalyst as compared to other
those containing electron withdrawing group (2b and 2d, Table catalysts (Table 4). Also, it was inferred that Ag/TiO2 and Cu/
3). Good yields without over-oxidized products i.e. carboxylic TiO2 (without graphene) exhibit low catalytic activity as
acids were obtained for the selective oxidation of a variety of compared to Cu@Ag–TiO2–NGO indicating that TiO2–NGO
primary alcohols (2a–2e, Table 3). The application of this plays a prominent role as a composite material in stabilizing the
protocol was investigated further for secondary alcohols, which metal NPs owing to its high surface area and thermal stability.
provided good results (2f–2j, Table 3). Hydrocarbons were also
Table 2 Effect of various reaction parameters for the oxidation of alcohols and hydrocarbons in the presence of Cu@Ag–TiO2–NGOa
Temperature
S. no
Solvent
Oxidant
(ꢀC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
H2O
H2O
H2O
Air
O2
TBHP
O2
TBHP
O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
60
60
60
60
60
80
70
80
60
50
60
80
2
2
2
2
2
2
2
2
2
2
2
2
No reaction
40
60
50
70
55
82
83
80
75
38
46
EtOH : H2O (3 : 1)
EtOH : H2O (3 : 1)
EtOH
EtOH
EtOH
EtOH
EtOH
CH3CN
CH3CN
9
10
11
12
a
Reaction conditions: uorene (1 mmol), TBHP (1 mmol), Cu@Ag–TiO2–NGO (0.1 g, Cu ¼ 2.69 mol%, Ag ¼ 2.17 mol%), ethanol (5 mL) at 70 ꢀC.
b
Column chromatography yield.
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Table 3 Cu@Ag–TiO2–NGO catalysed oxidation of alcohols and hydrocarbonsa,b
a
b
Reaction conditions: alcohol or hydrocarbon (1 mmol), TBHP (1 mmol), Cu@Ag–TiO2–NGO (0.1 g), ethanol (5 mL) at 70 ꢀC. Column
chromatography yield.
as the N in the catalyst imparts basic character, thereby
avoiding the use of additional base in the reaction. The test
reaction was optimized by using different solvents (H2O,
EtOH, CH3CN and DMF) and H2O was found to be the best,
whereas others were proved to be signicantly less effective
(Table 5). The test reaction was performed using 0.1 g of the
Cu@Ag–TiO2–NGO in the absence of base using H2O as
C–N bond formation
The best catalyst i.e. Cu@Ag–TiO2–NGO which we obtained
during the optimization in case of oxidations was chosen to
perform C–N cross-coupling reaction. In this work, we initially
selected the coupling reaction of phenyl boronic acid with
aniline in the absence of base as the model reaction to opti-
mize the reaction parameters and good results were obtained
Table 4 Comparison of the catalytic activity of the present catalyst with reported catalytic systems for the selective liquid-phase alcohol
oxidation
Catalyst
Reaction conditions
Time
Yield (%)
Reference
STA-12(Co)
4-Chlorobenzyl alcohol, TBHP, ethylacetate, 60 ꢀ
C
3h
5 min
5 h
58
90
78
75
57
62
50
81
99
99
85
42
43
44
44
Ag–ZnO nanocomposite
[Cu2(m-O2CC6H5)4(4-Etpy)2]
[Cu2(m-O2CC6H5)4(4-DMAP)2]
Cu/TiO2
Ag/TiO2
Au/TiO2
LaCrO3
Pd-G/SBA-16 G
Pt@CHs
4-Chlorobenzyl alcohol, TBHP, acetonitrile, 80 ꢀC
4-Chlorobenzyl alcohol, TBHP, methanol, 65 ꢀ
4-Chlorobenzyl alcohol, TBHP, methanol, 65 ꢀ
4-Chlorobenzyl alcohol, TBHP, EtOH, 70 ꢀC
4-Chlorobenzyl alcohol, TBHP, EtOH, 70 ꢀC
C
C
5 h
1.5 h
1.5 h
2 h
2.5 h
7 h
This work
This work
45
46
47
48
Benzyl alcohol, TBHP, solvent free, 94 ꢀ
C
4-Chlorobenzyl alcohol, TBHP, solvent-free, 90 ꢀ
Benzyl alcohol, air, toluene, 100 ꢀC, K2CO3
Benzyl alcohol, O2, toluene, 80 ꢀC, KOH
4–Chlorobenzyl alcohol, TBHP, EtOH, 70 ꢀC
C
3 h
1.5 h
Modied graphene based AgCu(0)
bimetallic catalyst (Cu@Ag–TiO2–NGO)
This work
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Table 5 Effect of various reaction parameters for C–N coupling in the
presence of Cu@Ag–TiO2–NGOa
with phenylboronic acid to give the corresponding N-arylated
products with improved yields (5d and 5e, Table 6). Further,
promising results were obtained with 4-uoroaniline, but no
product was obtained with 2-uoroaniline. In case of C–N
coupling between 4-bromoaniline and phenyl boronic acid (5e,
Table 6), small amount of C–C coupled product as the side
reaction was also observed. The desired N-arylation product
was obtained in good yield with 4-methoxyaniline as compared
to 3-methoxyaniline (5b and 5c, Table 6). However, no product
formation was observed with 2-methoxyaniline even aer
a long time. Hence, it was inferred that reaction of para- or
meta-substituted anilines gave good yield as compared to
ortho-substituted anilines, which were found to be less reactive
due to steric hindrance. Indoles and aliphatic amines undergo
coupling very slowly and the corresponding products could
not be isolated. C–N coupling was also explored using a variety
of p- or m-substituted arylboronic acids under the optimal
reaction conditions and moderate results were obtained (5f–
5i, Table 6).
Temperature
S. no.
Solvent
(ꢀC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
Water
Water
Water
Acetonitrile
Toluene
RT
80
100
RT
RT
70
RT
RT
60
8
3
25
75
76
35
35
55
35
35
53
64
3.75
6.5
6
5.5
6
5
5
5
Ethanol
Methanol
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
H2O/EtOH (1 : 1)
70
a
Reaction conditions: aniline (1 mmol), phenylboronic acid (1 mmol),
b
Cu@Ag–TiO2–NGO (0.1 g) in H2O (5 mL). Column chromatography
yield.
ꢀ
a solvent at 80 C. To explore the scope and generality of aryl
C–N bond formation, both electron-donating and electron-
withdrawing substituted aromatic amines were tested under
the optimized reaction conditions and it has been found that
substrates containing electron-withdrawing groups (F and Br)
at the para-position of the amines undergo coupling smoothly
Proposed mechanisms
C–N coupling. In order to get insight into the mechanistic
aspect behind the synergism between Ag and Cu nanoparticles,
XPS spectra of the reused catalysts (aer ve catalytic runs) was
recorded. A proposed mechanism for the Chan–Lam coupling
Table 6 Cu@Ag–TiO2–NGO catalyzed Chan–Lam cross-coupling reaction between arylamines and arylboronic acid in watera,b
a
Reaction conditions: aryl amine (1 mmol), arylboronic acid (1 mmol), Cu@Ag–TiO2–NGO (0.1 g) in H2O (5 mL) at 80 ꢀC. b Column chromatography
yield.
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increased electron density on Cu which enhances the oxidative
addition step. Thus, it can be concluded that it is the synergistic
effect between Ag and Cu metal nanoparticles which forms the
underlying principle behind the good catalytic activity of the
prepared catalyst.
Oxidations
To gain an insight into the reaction mechanism of the oxidation
of alcohols and hydrocarbons, and to discover if the radical
process is involved in the catalytic reaction, a radical-trapping
experiment using TEMPO, a radical scavenger was performed.
The reaction in case of 2a, Table 3 was performed rst for
40 min and the product was obtained with 50% yield. To this
reaction mixture, TEMPO (1 mmol) was added and the reaction
was further carried out for 90 min (completion time without
TEMPO). No signicant conversion was observed, which clearly
indicates that the oxidation reaction occurred via radical
intermediates. Moreover, the comparative XPS spectra of the
fresh and reused Cu@Ag–TiO2–NGO does not provided direct
evidence of the intense electronic interactions between Ag and
Cu nanoparticles but the enhanced catalytic activity of the
catalyst convinced us that some sort of electronic interactions
between Ag and Cu nanoparticles as observed in C–N coupling
also occurs during oxidation but due to the presence of TBHP
not much change was observed in binding energy values.
Hence, beneting from the enhanced bimetallic synergistic
effect, a plausible radical mechanism involving the synergistic
Fig. 10 Plausible mechanism for Cu@Ag–TiO2–NGO catalyzed C–N
coupling reaction: (I) oxidative addition; (II) transmetallation; (III)
reductive elimination.
between aromatic amines and phenyl boronic acid is shown in
Fig. 10. The reaction was proposed to proceed through a cata-
lytic cycle involving three main steps i.e. oxidative addition,
transmetallation and reductive elimination. The decrease in the
binding energy values of Ag and Cu as depicted by XPS spectrum
of the reused catalyst in case of C–N coupling [Fig. 9C(d) and
D(c)], indicates transfer of electrons from Ag to Cu leading to
Fig. 11 Probable mechanistic path for Cu@Ag–TiO2–NGO catalyzed oxidation of alcohols and hydrocarbons.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
We thank the Head, ACMS, IIT Kanpur for XPS study; Head,
SAIF, IIT Bombay for ICP-AES, HR-TEM and CHN studies;
AMRC, IIT Mandi for SEM, EDX, FT-IR and PXRD studies; and
Department of Chemistry, University of Jammu for BET, FTIR
and TGA. Financial assistance from the Department of Science
& Technology under PURSE programme; UGC New Delhi under
SAP; and to authors N. S., A. C. (RUSA-2.0), M. K. (SRF, CSIR) and
C. S. (SRF, UGC) is gratefully acknowledged.
Fig. 12 Recyclability of Cu@Ag–TiO2–NGO for oxidation (2a, Table 3)
and Chan–Lam coupling reaction (5c, Table 6) under optimized
reaction conditions.
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