Gold nanoparticle-decorated graphene oxide: Synthesis and application in oxidation reactions under benign conditions

Article abstract of DOI:10.1016/j.molcata.2016.07.047

Gold nanoparticles (～13?nm, 4?wt% Au) dispersed on reduced graphene oxide (Au/rGO) surface were prepared via the reduction of gold (III) chloride using sodium citrate on reduced graphene oxide support. The chemical, morphological, and size analyses of Au/rGO by XRD, XPS, TEM, HR-TEM/EDS, AAS, and Raman spectroscopy revealed that the hybrid material comprised of a well-defined rGO support decorated with gold nanoparticles. The ensuing catalyst was evaluated for the oxidation of ethyl benzenes (yields up to 95%) and oxidative esterification of aldehydes (73–97%) using oxygen as an environmentally friendly oxidant in good to excellent yields; reusable and recyclable catalyst displayed high activity without the use of toxic organic solvents.

Full text of DOI:10.1016/j.molcata.2016.07.047

Accepted Manuscript
Title: Gold nanoparticle-decorated graphene oxide: Synthesis
and Application in oxidation reactions under benign
conditions
Author: Radka Pocklanova Anuj K. Rathi Manoj B. Gawande
Kasibhatta Kumara Ramanatha Datta Vaclav Ranc Klara Cepe
Martin Petr Rajender S. Varma Libor Kvitek Radek Zboril
PII:
S1381-1169(16)30297-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.07.047
MOLCAA 9981
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
24-5-2016
25-7-2016
26-7-2016
Please cite this article as: Radka Pocklanova, Anuj K.Rathi, Manoj B.Gawande,
Kasibhatta Kumara Ramanatha Datta, Vaclav Ranc, Klara Cepe, Martin Petr, Rajender
S.Varma, Libor Kvitek, Radek Zboril, Gold nanoparticle-decorated graphene oxide:
Synthesis and Application in oxidation reactions under benign conditions, Journal of
Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.07.047
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Gold nanoparticle-decorated graphene oxide: Synthesis and
Application in oxidation reactions under benign conditions
Radka Pocklanova, Anuj K. Rathi, Manoj B. Gawande,* Kasibhatta Kumara Ramanatha Datta, Vaclav Ranc,
Klara Cepe, Martin Petr, Rajender S. Varma, Libor Kvitek and Radek Zboril*
Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical
Chemistry, Palacky University, Šlechtitelů 27, 783 71, Olomouc, Czech Republic.

GRAPHICAL ABSTRACT
Gold nanoparticles (~13 nm, 4% Au) dispersed on reduced graphene oxide (Au/rGO) were prepared via
reduction of gold (III) complex and used for the oxidation reactions under benign conditions.
Highlights
Gold nanoparticles (10-13 nm, 4% Au) dispersed over the reduced graphene oxide support (Au/rGO).
High efficiency for the oxidation of ethyl benzene and the oxidative esterification of aldehydes.
Good to excellent yields using oxygen as an environmentally-friendly oxidant.
Recyclable up to five cycles without decreasing its catalytic activity.
ABSTRACT
Gold nanoparticles (~13 nm, 4wt% Au) dispersed on reduced graphene oxide (Au/rGO) surface were prepared via the reduction of
gold (III) chloride using sodium citrate on reduced graphene oxide support. The chemical, morphological, and size analyses of
Au/rGO by XRD, XPS, TEM, HR-TEM/EDS, AAS, and Raman spectroscopy revealed that the hybrid material comprised of a well-
defined rGO support decorated with gold nanoparticles. The ensuing catalyst was evaluated for the oxidation of ethyl benzenes
(
yields up to 95%) and oxidative esterification of aldehydes (73-97%) using oxygen as an environmentally friendly oxidant in good
to excellent yields; reusable and recyclable catalyst displayed high activity without the use of toxic organic solvents.
Keywords: Reduced graphene oxide/gold nanocomposite • Recyclable catalysts • Oxidation reactions • Sustainable protocol •
Heterogeneous catalysis

1. Introduction
Graphene oxide (GO) is hydrophilic 2D carbon material containing important oxygen bearing functional groups (e.g. hydroxyl,
carboxyl, carbonyls or epoxides) and is generally prepared via chemical functionalization of graphene [1-2]. The presence of
functional oxygen groups on the GO surface provides reactive sites for the immobilization of metals via carbon surface
chemistry and facilitates the dispersion in water [3-5].
This abundance of functionalities on GO surface has been used for the post-synthetic modifications via the dispersion of
metal nanoparticles [6-8]. The ensuing composites have potential applications in chemical sensing, hydrogen energy storage,
biomedicine, environmental remediation, catalysis to name a few [9-18]. Importantly, metal NPs on graphene support increase
the catalytic activity of nanoparticles due to their spatial distribution which decreases their aggregation and facilitates to
increase more surface active sites [19-20].
The surface chemistry of carbon materials and their modification with metal nanoparticles is the key factor for catalytic
performance based on the amount of O- containing functional groups, which have the strong catalytic effect and generate
possible adsorbing sites for metal nanoparticles [21-22].
The syntheses of metal NPs/graphene nanocomposites have been investigated using various protocols including chemical
reduction [23], microwave irradiation [24-25], sonolysis [26-27], thermal exfoliation [28], or impregnation using hydrogen gas
reduction hydrothermal method [29]. Some of them are plagued by complicated protocols, which include the usage of
2
elevated temperatures, organic solvents, and H atmosphere [29-30].
In recent years, Au NPs themselves and their supported composite forms have proven their prowess as active catalysts for a
variety of heterogeneous organic transformations [31-35]. The design and progress of new catalytic protocols for the selective
oxidation of hydrocarbons into the analogous oxygen containing desired products namely aldehydes, ketones, alcohols,
epoxides, and carboxylic acids is vital and imperative reactions in the pharmaceutical industry. Lately, synthesis of
acetophenone from ethylbenzene via the selective oxidation process has become a topic of specific interest because
acetophenone is essentially used as raw material for the production of pharmaceuticals, perfumes, alcohol, esters, and
aldehydes. In general, such carbonyl compounds have been conventionally obtained via Friedel-Crafts acylation of benzene
with an acyl halide or acid anhydride in the attendance of stoichiometric amounts of homogeneous Lewis acids (e.g., FeCl
3
,
TiCl , AlCl , BF , SnCl ) or strong protic acids (e.g. HF, H SO ); this reaction pathway generates a large volume of toxic and
4
3
3
4
2
4
corrosive waste [36]. Though homogeneous catalysts display good performance and selectivity, but the practical problems
encountered in their use entails difficulty in product separation, and aggregation of active catalytic sites, which have curtailed
their industrial applications. Heterogeneous catalysts have an advantage as they can be separated at the end of the reaction
by simple filtration; efforts are afoot to make use of some heterogeneous catalysts for the oxidation of hydrocarbons [37-40].
Carboxylic acid derivatives are important building blocks and are widely used in pharmaceuticals, polymers, agrochemicals
and natural products synthesis [41-43]. Among the available synthetic methods, an effective route has been developed via
oxidative esterification of the readily available aldehydes or alcohols [44-45]. However, some of these procedures require the
use of stoichiometric amounts of heavy-metal oxidants [46]. Consequently, it is important to find sustainable benign reaction
conditions for the oxidative esterification of aldehydes.
There are some reports on Au nanoparticles supported graphene/reduced graphene oxide via synthetic procedures namely surfactant-
free synthesis of Au-graphene hybrids for high-performance oxygen reduction reaction[47], in-situ synthesis of reduced graphene oxide
and gold [48], one-step synthesis of hybrid gold nanoparticle–graphene oxide nanosheets through electrostatic self-assembly [49], one-
pot preparation of reduced graphene oxide (RGO) or Au (Ag) nanoparticle-RGO hybrids using chitosan as a reducing and stabilizing
agent [50] and Au nanoparticles decorated on the surface of the rGO via spontaneous chemical reduction of HAuCl
In continuation of our research work on the design and development of nanocatalysts and their ‘greener’ applications, [1,52-
5] herein, we report the synthesis of Au/rGO nanocomposites for oxidation of various substituted
4
by RGO [51].
5

ethyl benzenes and aerobic oxidation of aldehydes to the corresponding desired products. Synthesis of Au/rGO composite by
using sodium citrate which serves both, as reducing and stabilizing agent, produced nm-sized Au particles that are well-
dispersed on rGO support (Scheme 1).
2. Experimental Section
2.1 Catalysts Preparation
2.1.1 Synthesis of graphene oxide (GO)
Graphite powder was prepared using Hummer´s method [56]. Typically, graphite (1 g) and sodium nitrite (0.5 g) were stirred in
concentrated H SO (23 mL) at 0 °C and then KMnO (3.0 g) was added to this solution within 15 minutes, while maintaining the
2
4
4
temperature below 20 °C. The reaction mixture was allowed to reach 35 °C and stirred for additional 30 minutes. Distilled water (46 mL)
was then added slowly (exothermic process) and the resulting mixture was stirred at 98 °C for 15 minutes, then cooled to room
2 2
temperature. 30% H O (1mL) was then added to it followed by 140 mL distilled water. The resulting material was intensively washed
several times with distilled water followed by centrifugation at 6000 rpm to remove any residual impurities and the obtained material
was dried in vacuum oven to obtained graphene oxide (GO) powder.
2
.1.2 Synthesis of reduced graphene oxide/gold nanocomposite (Au/rGO).
4 2
Graphene oxide (1.5 mg/mL) was dispersed in 10 mL of distilled water followed by the addition of 50 mL (1 mol/L) of HAuCl ·3H O
solution. This suspension was sonicated for 15 minutes and aged for 30 min to allow the interaction of gold ions with the graphene
oxide surface. The solution was heated until 80 °C and then an aqueous solution of sodium citrate (0.3 mol/L, 1 mL) was added
dropwise into it. The reaction was maintained at this temperature for 1 h. The resulting nanocomposite was then washed with distilled
water and ethanol to eliminate the excess of free gold nanoparticles and dried under vacuum oven.
2.1.3 Preparation of gold nanoparticles.
A 5 mL solution of Au NPs was prepared by the reduction of HAuCl
presence of polyvinylpyrrolidone (PVP, 1 wt%, 400 μL). The NaBH solution was added to the solution containing PVP and HAuCl
the reaction mixture was then magnetically stirred for two minutes to obtain final solution of Au NPs.
4
(0.1 M, 1.4 mL) using NaBH
4
solution (0.1 M, 3.2 mL) in the
4
4
and
2.1.4 General procedure for the oxidation of ethyl benzenes.
A mixture containing substrate (0.5 mmol), t-butyl hydroperoxide (TBHP) (1 mmol) and Au/rGO (15 mg) were mixed in acetonitrile (5
mL). The resulting mixture was stirred at 70°C for 24 h. Progress of the reaction was monitored by TLC (silica gel; n-hexane/ethyl
acetate). After desired time, the crude material was extracted with ethyl acetate and concentrated under reduced pressure. The
obtained material was analyzed by gas chromatography using dodecane as internal standard.
2.1.5 General procedure for oxidative esterification of aldehydes.
To a stirred mixture of aldehyde (0.5 mmol), potassium carbonate (8 mol%) in methanol (2 mL), Au/rGO (0.3 mol%) was added. The
resulting mixture was heated at 65 °C for appropriate reaction time (Table 4) under O atmosphere and progress of the reaction was
2
monitored by TLC (silica gel; n-hexane/ethyl acetate). After the desired time, the reaction mass was analyzed by gas chromatography
using dodecane as internal standard.

2
.2 Characterization techniques
The chemical nature of this hybrid material was studied using X-ray diffraction (XRD), and the morphology of the synthesized
Au/rGO nanocomposites was probed using microscopic techniques, including transmission electron microscope (TEM), high
resolution transmission electron microscopy (HRTEM) and high angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) analysis. The structural information X-ray photoelectron spectroscopy (XPS), Raman
spectroscopy and Gold quantity are measured by atomic absorption spectroscopy (AAS). (See SI for details of various
techniques).
3. Results and Discussion
The XRD pattern for GO has a dominant peak at 2θ =13° corresponding to an interlayer spacing of 0.79 nm (Figure 1), which
decreased rapidly in Au/rGO diffractogram thus indicating the substantial reduction of GO to rGO (See Figure S1) [57]. This
2
reduction process represents reestablishment of conjugated graphene network sp as shown in the case of epoxide ring-
opening reaction [58]. Au NPs dispersed over rGO display new diffraction peaks at 2θ = 44.6°, 52.1°, 76.7°, 93.3°, 98.5°,
which correspond to the crystal planes of (111), (200), (220), (311) and (222) of Au fcc structure. These results imply that gold
nanoparticles are anchored through functional groups (Figure 1).
TEM and HRTEM/HAADF inspections of Au/rGO show that Au nanoparticles were homogeneously deposited on the surface
of the rGO sheets (Figure 2). They are distributed without any agglomeration onto the surface of rGO sheet with the size of
gold NPs being 12.5 ± 2.6 nm (see histogram Figure S2); TEM images confirm that there are no Au NPs outside of rGO
sheets. The thin layer and structure of the graphene sheet with some corrugation are suggesting a flexible structure of the
rGO support. Additionally, the high resolution image of Au/rGO composite shows the oriented lattice of Au NPs (Figure 2a).
The d spacing value of 0.23 nm corresponds with Au (111) fcc structure. The electron diffraction pattern of Au NPs on rGO
support is depicted in Figure S3.
A closer inspection of the synthesized catalyst, analysed by HAADF STEM (Figure 2, b-h), clearly indicates the presence of
Au, C and O. The homogeneous distribution of oxygen containing functional groups help anchor Au NPs over reduced
graphene oxide sheets. The wt% of gold metal in Au/rGO nanocomposites was found to be 4.0% by atomic absorption
spectroscopy (AAS).
Furthermore, the as-prepared nanomaterial was characterized using Raman spectroscopy. The recorded Raman spectra of
GO, raw graphite (labelled as GR), and Au/rGO are depicted in Figure 3 which contain characteristic signals for carbon
materials GO and Au/rGO comprising G-band, localized at 1572 cm-1 and D band, present at 1334 cm ; a moderate red shift,
of 5 cm-1 was observed as the maximum of G band shifted from 1588 cm-1 (observed at GO) to 1593 cm-1 (observed at
Au/rGO). This shift signify an interaction between Au nanoparticles and GO substrate as described by Subrahmanyam and
co-workers [59].
-1
The XPS spectrum of Au/rGO nanocomposites showed Au 4f line with the position of the Au 4f7/2 at 84.12 eV and Au 4f5/2 at
87.75 eV, perfectly matching with metallic Au (Figure 4a) [60]. The XPS spectrum of the C1s depicting the fraction of carbon
participating in C-O single bond, carboxyl-, carbonyl- and C-C bonds is indicative of the fact that graphene oxide is in reduced
2
state (Figure 4b). The binding energy of C-C bonding is situated at 284.78 eV, which signalized the peak for sp -hybridized
carbon. The shifted peaks at 286.17 eV, 289.04 eV, 290.30 eV were assigned for the C-O, C=O, O-C=O functional group,
respectively [61]. Based on the intensity; carbon bonding is expectedly the most abundant than oxygenated functional groups.
It has been reported and the results here concur that GO is suitable for the synthesis of graphene-metal nanoparticles due to

its oxygen containing groups [62]. The XPS results in the Figure S4 and Figure S5, show GO and rGO C1s line respectively,
where we clearly see that the oxygen functionalities in rGO decreased as compared to GO.
Catalytic activity of Au/rGO nanocomposites
The catalytic activity of Au/rGO catalyst was assessed for the oxidation of ethyl benzenes and its derivatives. Initially, to
optimize the reaction condition ethyl benzene was chosen as a model substrate and a series of experiments with varying
o
amounts of Au/rGO (0.10-0.35 mol%) in the presence of TBHP at 65 C were examined (Table 1). It became clear that
reaction did not proceed without the catalysts or with just rGO alone (without Au NPs) (Table 1, entries 1 and 2). Further, the
results revealed that 0.35 mol% of Au/rGO catalyst was the optimum amount for quantitative conversion of the ethyl benzene
(Table 1, entry 6) while decrease in catalyst quantity to 0.10 mol%, and 0.25 mol%, provided less products formation 35%
and 75%, respectively (Table 1, entries 4 and 5). Notably, a reaction carried out under optimized conditions with pure Au
nanoparticles afforded comparative lower yield (Table 1, entry 3). These results highlight the significance of reduced
graphene oxide as a support for anchoring of gold nanoparticles [63]. Additionally, the oxidation of ethyl benzene was studied
at time interval 6, 12, 18 and 24 h; results are summarised in Table S1 (Figure S6a).
With this optimized reaction in hand, further substrate scope was explored for the substituted ethyl benzenes such as 4-
chloroethyl benzene, 4-bromoethyl benzene, 4-nitroethylbenzene, 3-aminoethyl benzene and 4-methylethyl benzene;
corresponding desired products were obtained in good to excellent yields and selectivity for respective ketones (Table 2). It
was noticed that electron withdrawing groups such as 4-chloroethyl benzene, 4-bromoethyl benzene, and 4-nitroethylbenzene
(Table 2, entries 3-5) gave comparatively less yields (55-85%), while ethyl benzene and 3-aminoethyl benzene (Table 2,
entries 1 and 5) showed excellent yields of the corresponding ketones (> 94%) including propyl benzene which is also
showed quantitative yield (Table 2, entry 7).
Furthermore, the catalytic activity of Au/rGO nanocatalyst was explored for the aerobic oxidative esterification of aldehydes using eco-
friendly oxygen as an oxidant (Table 3). Control experiments indicated that the reaction of 4 chlorobenzaldehyde in methanol did not
proceed without the catalyst or the base, but combination of rGO/Au catalyst and the base delivered the product (Table 3, entry 4). To
optimize the conditions, the influence of the various bases such as K
2
CO
3
, Na
2
CO
3
, K
3
PO
4
and Cs
2
CO
3
were analysed (Table 3, entries
CO within 3
7-10). The time factor at regular interval also investigated; the highest conversion of product was obtained using 8 mol% K
2
3
h (Table 3, entry 7 and Figure S6b). Notably, the catalytic efficiency of Au NPs without rGO support gave poor yield (28%) (Table 3,
entry 6). After optimization of the reaction conditions, the catalytic scope of the reaction was further scrutinized using various
substituted aldehydes bearing both electron withdrawing and electron donating groups and the products were obtained in good to
excellent yields (73-97%) within 3-8h (Table 4, entries 1-6).
The reusability of the heterogeneous catalyst is an important factor from commercial and economic points of view. The stability of the
Au/rGO catalyst and its activity was scrutinized by recycling experiment for the oxidation of ethyl benzene and the oxidative
esterification reaction under optimised conditions. In each cycle, the Au/rGO catalyst was recovered by centrifugation, washed with

o
water, followed by ethanol. The recovered catalyst was dried in vacuum oven at 60 C and employed for the next reaction. For both
reactions, the recovered catalyst was successfully recycled five times without affecting its activity (See SI, Table S2). A detailed
experimental investigation revealed that the reaction proceeded via heterogeneous catalysis as we did observe trifling soluble Au
species after five reaction cycles (e.g. 0.13% and 0.17% of Au in ethyl benzene oxidation and oxidative esterification reactions,
respectively). It should be mentioned that the catalytic activity of Au/rGO nanocomposites for these oxidation reactions are comparable
or better in terms of yield when assessed against the reported protocols;[40, 64-65] earlier described procedures are often time
consuming and some of them lack the critical reusability option [64, 66-67].
Conclusions
In summary, we have successfully demonstrated that gold nanoparticles, with a mean diameter of ~13 nm, can be homogeneously
deposited on the surface of reduced graphene oxides using a simple protocol. The ensuing Au/reduced graphene oxide catalyst could
be effectively deployed for the oxidation of ethyl benzenes (55-95%) and for the aerobic oxidation of aldehydes to esters (73-97%)
using eco-friendly oxygen as an oxidant. Importantly, the catalytic activity of gold supported on rGO substrate is significantly higher
when compared to Au NPs used without the graphene oxide support. The excellent reusability outcomes reaffirm a great potential for
Au/rGO catalyst in alkane oxidations and aerobic oxidative esterification reactions. This environmentally-friendly protocol facilitates easy
recyclability of the catalyst, up to five times without a loss of efficiency, and uses a benign reaction medium which may bode well for the
future exploration of similar strategies.
Acknowledgements
The authors thank Ms. J. Stranska for TEM analysis and acknowledge support from the Ministry of Education, Youth and Sports of the
Czech Republic (LO1305), by Technology Agency of the Czech Republic "Competence Centres" (project TE01020218), by the
Operational Program Education for Competitiveness
-
European Social Fund (project CZ.1.07/2.3.00/30.0041 and
CZ.1.07/2.4.00/31.0189) of the Ministry of Education, Youth and Sports of the Czech Republic. The work is also funded by the Palacky
University Institutional support, IGA grant (Project No. IGA-PrF-2015-017) and internal grant of Palacky University, IGA-PrF-2015-022.
References
[
[
[
1] V. Georgakilas, M. Otyepka, A.B. Bourlinos, V. Chandra, N. Kim, K.C. Kemp, P. Hobza, R. Zboril, K.S. Kim, Chem. Rev. 112 (2012) 6156-6214.
2] O.C. Compton, S.T. Nguyen, Small 6 (2010) 711-723.
3] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc.
Rev. 39 (2010) 228-240.
[4] C. Hontoria-Lucas, A.J. López-Peinado, J.D. López-González, M.L. Rojas-Cervantes, R.M. Martín-Aranda, Carbon 33 (1995) 1585-1592.
[5] V. Georgakilas, J. N. Tiwari, K. C. Kemp, J. A. Perman, A. B. Bourlinos, K. S. Kim and R. Zboril, Chem. Rev. (116) 2016, 5464–5519.
[6] H.C. Schniepp, J.-L. Li, M.J. McAllister, H. Sai, M. Herrera-Alonso, D.H. Adamson, R. K. Prud’homme, R. Car, D. A. Saville, I. A. Aksay, J. Phys.
Chem. B. 110 (2006) 8535-8539.
[
7] H. Yang, J. Jiang, W. Zhou, L. Lai, L. Xi, Y. M. Lam, Z. Shen, B. Khezri and T. Yu, Nanoscale Res. Lett. 6 (2011)1-8.
8] M. J. McAllister, J.-L. Li, D.H. Adamson, H.C. Schniepp, A.A. Abdala, J. Liu, M. H-Alonso, D. L. Milius, R. Car, R. K. Prud’homme, I. A. Ahsay,
Chem. Mater. 19 (2007), 4396-4404.
[
[
[
[
[
[
[
[
[
9] R. Ma, T. Sasaki, Acc. Chem. Res. (48) 2015, 136-143.
10] V. Chabot, D. Higgins, A. Yu, X. Xiao, Z. Chen, J. Zhang. Energ. Environ. Sci. 7 (2014) 1564-1596.
11] D. Chen, H. Feng, J. Li, Chem. Rev. 112 (2012) 6027-6053.
12] C. Chung, Y.-K. Kim, D. Shin, S.-R. Ryoo, B.H. Hong, D.-H, Acc. Chem. Res. 46 (2013) 2211-2224.
13] K.P. Loh, Q. Bao, G. Eda, M. Chhowalla, Nat. Chem. 2 (2010) 1015-1024.
14] Y. Zhu, D.K. James, J.M. Tour, Adv. Mater. 24 (2012) 4924-4955.
15] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R. S. Ruoff, Adv. Mater. 22 (2010) 3906-3924.
16] S. Navalon, A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Chem. Rev. 114 (2014) 6179-6212.

[
[
[
[
[
[
[
[
17] X.-K. Kong, C.-L. Chen, Q.-W. Chen, Chem. Soc. Rev., 2014, 43, 2841-2857.
18] J. Zhao, M. Sun, Z. Liu, B. Quan, C. Gu and J. Li, Sci. Rep. 5 (2015)16019.
19] Q. Zhuo, Y. Ma, J. Gao, P. Zhang, Y. Xia, Y. Tian, X. Sun, J. Zhong, X. Sun, Inorg. Chem. 52 (2013) 3141-3147.
20] G. Goncalves, P.A.A.P. Marques, C.M. Granadeiro, H.I.S. Nogueira, M.K. Singh, J. Grácio, Chem. Mater. 21(2009) 4796-4802.
21] A. Mondal, N.R. Jana, ACS Catal. 4 (2014) 593-599.
22] H. Sun, S. Liu, G. Zhou, H.M. Ang, M.O. Tadé, S. Wang, ACS Appl. Mater. Inter. 4 (2012) 5466-5471.
23] Y. Si, E.T. Samulski, Chem. Mater. 20 (2008) 6792-6797.
24] H.M.A. Hassan, V. Abdelsayed, A.E.R.S. Khder, K.M. AbouZeid, J. Terner, M.S. El-Shall, S. Al-Resayes, A. A. El-azhary, J. Mater. Chem 19
(2009) 3832-3837.
[
[
[
[
[
[
25] D. Marquardt, C. Vollmer, R. Thomann, P. Steurer, R. Mülhaupt, E. Redel, C. Janiak, Carbon 49 (2011)1326-1332.
26] K. Vinodgopal, B. Neppolian, I.V. Lightcap, F. Grieser, M. Ashokkumar, P.V. Kamat, J. Phys. Chem. Lett. 1 (2010) 1987-1993.
27] B. Neppolian, A. Bruno, C.L. Bianchi, M. Ashokkumar, Ultrason. Sonochem. 19 (2012) 9-15.
28] H.-B. Zhang, J.-W. Wang, Q. Yan, W.-G. Zheng, C. Chen, Z.-Z. Yu, J. Mater. Chem. 21 (2011) 5392-5397.
29] S.M. Choi, M.H. Seo, H.J. Kim, W.B. Kim, Synthetic Met. 161 (2011) 2405-2411.
30] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Valamakanni, R. S. Ruoff, Nano Lett. 9 (2009) 1593-1597.
[31] A.S.K. Hashmi, G.J. Hutchings, Angew. Chem. Int. Ed. 45 (2006) 7896-7936.
[32] C. Marsden, E. Taarning, D. Hansen, L. Johansen, S. K. Klitgaard, K. Egeblad and C. H. Christensen, Green Chem. 10 (2008) 168-170.
[
[
[
[
[
[
[
[
[
[
[
[
[
[
33] S.K. Movahed, M. Fakharian, M. Dabiri, A. Bazgir, RSC Adv. 4 (2014) 5243-5247.
34] P.T. Yin, S. Shah, M. Chhowalla, K.-B. Lee, Chem. Rev. 115 (2015) 2483-2531.
35] H. Zhang, D. Hines, D.L. Akins, Dalton Trans. 43 (2014) 2670-2675.
36] G. A. Olah, Friedel–Crafts Chemistry, Wiley-Interscience, New York, 1963.
37] F. Rajabi, R. Luque, J.H. Clark, B. Karimi, D.J. Macquarrie, Catal. Commun. 12 (2011) 510-513.
38] C. He, J. Li, J. Cheng, L. Li, P. Li, Z. Hao, Z. P. Xu, Ind. Eng. Chem. Res. 48 (2009) 6930-6936.
39] R.K. Jha, S. Shylesh, S.S. Bhoware, A.P. Singh, Micropor. Mesopor. Mat. 95 (2006) 154-163.
40] M. Arshadi, M. Ghiaci, A. Rahmanian, H. Ghaziaskar, A. Gil, Appl. Catal. B. 119 (2012) 81-90.
41] D. Kaminskyy, A. Kryshchyshyn, I. Nektegayev, O. Vasylenko, P. Grellier, R. Lesyk, Eur. J. Med. Chem. 75 (2014) 57-66.
42] C. Ballatore, D. M. Huryn and A. B. Smith, ChemMedChem 8 (2013) 385-395.
43] H.-J. Song, Y.-X, Liu, Y.-X. Liu, Y.-Q. Huang, Y.-Q. Li, Q.-M. Wang, Bioorg. Med. Chem. Lett. 24 (2014) 5228-5233.
44] J.N. Rosa, R.S. Reddy, N.R. Candeias, P.M.S.D. Cal, P.M.P. Gois, Org. Lett. 12 (2010) 2686-2689.
45] R. Lerebours, C. Wolf, J. Am. Chem. Soc. 128 (2006) 13052-13053.
46] B. Neises, W. Steglich, Angew. Chem. Int. Ed. 17 (1978), 522-524.
[47] H. Yin, H. Tang, D. Wang, Y. Gao, Z. Tang, ACS Nano 2012, 6, 8288–
8297.
[
48] X. Dong, W. Huang, P. Chen, Nanoscale Res Lett 2011, 6:60.
49] Y. Choi, H. S. Bae, E. Seo, S.Jang, K. H. Park, B.-Su Kim, J. Mater.
Chem., 2011, 21, 15431–15436.
[
[50] Y. Guo, X. Sun, Y. Liu, W. Wang, H. Qiu, J. Gao, Carbon 2012, 50,
2513–2523.
[51] S. K. Movahed, M. Fakharian, M. Dabiri, A. Bazgir, RSC Adv., 2014, 4, 5243–5247.
[
[
[
[
52] M.B. Gawande, S.N. Shelke, R. Zboril, R.S. Varma, Acc. Chem. Res. 47 (2014) 1338-1348.
53] M.B. Gawande, P.S. Branco, R.S. Varma, Chem. Soc. Rev. 42 (2013) 3371-3393.
54] M.B. Gawande, A. Rathi, I.D. Nogueira, R.S. Varma, P.P. Branco, Green Chem. 15 (2013) 1895–1899
55] M.B. Gawande, P.S. Branco, I.D. Nogueira, C.A.A. Ghumman, N. Bundaleski, A. Santos, O. M. N. D. Teodoro, R Luque, Green Chem. 15 (2013)
682-689.
[56] D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A.
Slesarev, L. B. Alemany, W. Lu, J. M. Tour ACS Nano, 2010, 4 (8),
4806–4814.
[
[
[
[
[
[
57] Y. Guo, X. Sun, Y. Liu, W. Wang, H. Qiu, J. Gao, Carbon 50( 2012) 2513-2523.
58] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon 45 (2007) 1558-1565.
59] K.S. Subrahmanyam, A.K. Manna, S.K. Pati, C.N.R. Rao, Chem. Phys. Lett. 497 (2010)70-75.
60] J. Han, Y. Liu, R. Guo, J. Am. Chem. Soc., 131 (2009) 2060-2061.
61] J. Shen, T. Li, Y. Long, M. Shi, N. Li, M. Ye, Carbon 50 (2012) 2134-2140.
62] W. Wang, D. He, J. Duan, S. Wang, Hongshang Peng, Hongpeng Wu, Ming Fu, Yongsheng Wang, Xiqing Zhang, Chem. Phys. Lett. 582 (2013)
119–122.
[63] M. Yang, M. Zhou, A. Zhang, C. Zhang, J. Phys. Chem. C.116 (2012) 22336-22340.

[
[
[
[
64] J.-B. Feng, J.-L. Gong, Q. Li, X.-F. Wu, Tetrahedron Lett. 55 (2014) 1657-1659.
65] M. Ghiaci, Z. Sadeghi, M.E. Sedaghat, H. Karimi-Maleh, J. Safaei-Ghomi, A. Gil, Appl. Catal. A. 381 (2010) 121-131.
66] Y. Li, L. Wang, R. Yan, J. Han, S. Zhang, Catal. Sci. Technol. 5 (2015) 3682-3692.
67] B. Gutmann, P. Elsner, D. Roberge, C.O. Kappe, ACS Catal. 3 (2013) 2669-2676.

Figure 1. XRD patterns (Co-Kα) of GO, Au (reference), rGO and Au/rGO .

Figure 2. (a) TEM image of Au/rGO with selected detail of single Au nanoparticles (inset ; scale 10 nm), (b) HAADF image of Au/rGO
TEM image of gold nanoparticles onto graphene sheet, (c-h) elemental mapping of C, Au, O, Au/C, Au/O and Au/C/O, respectively.

Figure 3. Raman spectrum of Au/rGO composite along with GO and graphite (GR)) spectra for comparison.

Figure 4. XPS spectra of Au/rGO nanocomposite: (a) Au 4f line (b) C1s line. The position of the lines for metallic gold and different C-
bonded species are denoted by arrows.

Scheme 1. Synthesis of Au/rGO nanocomposites.

Table 1. Optimization of oxidation of ethyl benzene.

Entry
Catalyst
Conversion (%)
1
2
rGO
--
0
0
3
4
5
6
Au NPs (0.35 mol%)
Au/rGO (0.1 mol%)
Au/rGO (0.25 mol%)
Au/rGO (0.35 mol%)
70
35
75
100
Reaction condition: Ethyl benzene (0.5 mmol), TBHP (1 mmol), Au/rGO (0.1-0.35 mol%), acetonitrile (5 mL), internal standard: dodecane, 24
h.

Table 2. Oxidation of various substituted benzenes catalyzed by Au/rGO.a
b
-1
Substrate
Reactant
Yield (%)
TON
TOF (h )
Entry
Conversion
Ketone Alcohol
(
%)
1
00
95
5
83
4
3
1
2
.
.
8
2
18
67
100
100
3
.
85
75
15
13
70
60
3
9
6
8
2.5
4
5
.
.
9
55
94
27
4
45
82
2
4
9
9
6
.
1
00
94
6
82
4
7
.
a
. b
Reaction conditions: Substrate (0.5 mmol), TBHP (1 mmol), Au/rGO (30 mg, 0.35 mol%), acetonitrile (5 mL), 24 h, 70°C Yield calculated
on the basis of GC analysis by using dodecane as internal standard.

Table 3. Optimization of oxidative esterification of 4-chlorobenzaldehyde catalyzed by Au/rGO.a
Time
b
Entry
Catalyst
Base
Yield (%)
(h)
1
2
3
4
5
6
7
8
9
---
---
12
12
12
12
12
3
NR
NR
NR
60
---
4 mol% K
---
2
CO
3
Au/rGO
Au/rGO
---
4 mol% K
8 mol% K
8 mol% K
8 mol% K
2
2
2
2
CO
CO
CO
CO
3
3
3
NR
28
Au NPs
Au/rGO
Au/rGO
3
3
97
8 mol% Na
8 mol% Cs
8 mol% K PO
4
2
CO
3
8
65
Au/rGO
Au/rGO
2
CO
3
8
8
40
10
3
3
5
a
b
Reaction conditions: 4-Chlorobenzaldehyde (0.5 mmol), Au/rGO (0.3 mol%), MeOH (2 mL), O
basis of GC analysis by using dodecane as internal standard, NR- No reaction.
2
, 65 °C. Yield calculated on the

Table 4. Aerobic oxidative esterification of different aldehydes catalyzed by Au/rGO nanocomposite.a
Entry
Substrate
Product
Time
h)
Yieldb
%
TON
TOF (h-1)
(
1
.
.
3
97
80
27
91
75
78
25
26
2
3
3
95
85
3
.
.
4
5
5
70
14
82
68
13
5.
6.
8
73
60
8
aReaction conditions: Aldehyde (0.5 mmol), K
bYield calculated on the basis of GC analysis by using dodecane as internal standard.
CO
(8 mol%), Au/rGO (15 mg, 0.3 mol%), MeOH (2 mL), O
, 65 °C;
2
3
2