Hybrid Ni-Al layered double hydroxide/graphene composite supported gold nanoparticles for aerobic selective oxidation of benzyl alcohol

Article abstract of DOI:10.1039/c5ra05436b

In this work, a Ni-Al layered double hydroxide/graphene (NiAl-LDH/RGO) nanocomposite which was synthesized by introducing NiAl-LDH on the surface of graphene oxide (GO) and simultaneously reducing graphene oxide without any additional reducing agents was utilized as the support for Au nanoparticles. Raman spectroscopy and XPS analysis revealed that the NiAl-LDH/RGO composite had both defect sites and oxygenic functional groups in RGO to control the directional growth of Au nanoparticles and lead to a small particle size. Compared to an Au catalyst supported on single GO and RGO or NiAl-LDH, this composite-supported Au catalyst (Au/NiAl-LDH/RGO) exhibited superior catalytic activity and stability in the selective oxidation of benzyl alcohol using molecular oxygen under low pressure. Improved activity was mainly ascribed to the small Au particle size effect caused by RGO and the contribution of basic sites in NiAl-LDH. Moreover, the preferable catalytic stability of the Au/NiAl-LDH/RGO catalyst was attributed to the defect sites and oxygenic functional groups in RGO which anchored the Au NPs and prevented the agglomeration, meanwhile, the agglomeration of RGO was inhibited by the introduction of NiAl-LDH.

Full text of DOI:10.1039/c5ra05436b

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: D. Li, M. Miao, J.
Feng, Q. Jin, Y. He, Y. Liu, Y. Du and N. Zhang, RSC Adv., 2015, DOI: 10.1039/C5RA05436B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

RSC Advances
Page 1 ofJ9ournal Name
Dynamic Article Links ►
View Article Online
DOI: 10.1039/C5RA05436B
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Hybrid Ni-Al layered double hydroxide/graphene composite supported
gold nanoparticles for aerobic selective oxidation of benzyl alcohol
M. Y. Miao, J. T. Feng *, Q. Jin, Y. F. He, Y. N. Liu, Y. Y. Du, N. Zhang, D. Q. Li *
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
In this work, a Ni-Al layered double hydroxide/graphene (NiAl-LDH/RGO) nanocomposite which was
synthesized by introducing NiAl-LDH on the surface of graphene oxide (GO) and simultaneously
reducing graphene oxide without any additional reducing agents was utilized as the support for Au
nanoparticles. Raman spectroscopy and XPS analysis revealed that NiAl-LDH/RGO composite had both
10 defect sites and oxygenic functional groups in RGO to control the directional growth of Au nanoparticles
and lead to a small particle size. Compared to Au catalyst supported on single GO and RGO or NiAl-
LDH, this composite-supported Au catalyst (Au/NiAl-LDH/RGO) exhibited superior catalytic activity
and stability in the selective oxidation of benzyl alcohol using molecular oxygen under low pressure.
Improved activity was mainly ascribed to the small Au particle size effect caused by RGO and the
15 contribution of basic sites in NiAl-LDH. Moreover, preferable catalytic stability of Au/NiAl-LDH/RGO
catalyst was attributed to the defect sites and oxygenic functional groups in RGO which anchored the Au
NPs and prevented the agglomeration, meanwhile, the agglomeration of RGO was inhibited by the
introduction of NiAl-LDH.
Graphene oxide (GO) is the derivative of graphene,18 it not only
INTRODUCTION
retains the advantages of graphene but also have certain amounts
50 of oxygenic functional groups such as hydroxyl, epoxide and
carboxyl groups to promote the oxidation reaction.19,20 By the
process of chemical reduction, most of the oxygenic functional
groups in GO could be reduced and leave behind lots of
topological defects and carbon vacancies, while accomplishing
55 the transformation from GO to the reduced graphene oxide
(RGO). 21 Because of the functional groups as well as the carbon
vacancies and defects present in RGO, the sorption and
intercalation of ions and molecules are possible. 22 This feature,
together with the high specific surface area offers RGO a
60 promising candidate for catalyst supports in selective oxidation
reaction. However, a major practical shortcoming of GO or RGO
is the tendency to aggregate,23,24 resulting in the decrease of
catalytic activity and poor stability. To overcome such issues, it is
considerable to bring in other guest functional materials to
65 modify them.25
Layered double hydroxides (LDHs), with the general formula
of [M1−x2+Mx3+(OH)2]x+(An−)x/n·mH2O, have attracted a great deal
of interests in the application of heterogeneous catalysis because
of their wide variety of properties such as the different metal
70 cations on hydrotalcite layer with tunable compositions, alkali
property and thermal stability.26-29 Taking advantages of their
unique tunable properties, LDHs are suitable for fabricating
graphene-based composites.30,31 Li et al. reported the synthesis
and catalytic performance of ZnCr-LDH/graphene composite as
75 the catalyst for visible-light-responsive photocatalytic toward
rhodamine degradation.32 They found that the growth of LDH on
20 Selective oxidation of benzyl alcohol is an important reaction for
the production of benzaldehydes, which are widely used both as
high value components of dyestuffs, pharmaceuticals, perfumes,
agrochemicals and as fine chemical intermediates.1-4 The
traditional process to produce benzaldehyde has many problems
25 such as the difficulty of product separation and the discharge of
caustic liquid or gas. Direct oxidation of benzyl alcohol with
molecular oxygen (O2) as the terminal oxidant has attracted
considerable attention from the viewpoint of green sustainable
chemistry.5 To date, developing catalysts for solvent-free
30 oxidation of benzyl alcohol with high activity and selectivity is
still a fundamental challenge.6 Compared with other noble metals
(Pd, Pt, etc.), supported Au nanoparticles (NPs) with higher
selectivity towards target product and stability, have been used as
a promising catalyst in selective oxidation of benzyl alcohol.7-9
35 However, a common feature of Au NPs catalysts is the need of
strongly alkaline medium (typically NaOH or K2CO3) which was
believed to facilitate the β-hydride elimination step, making
purification
a
challenge and limiting their industrial
application.10-12 An efficient way to circumvent these difficulties
40 is to develop new types of Au NPs supports that can be finely
tuned with controllable surface properties to maximize the
catalytic activity.13
Graphene, a unique crystal of monolayer carbon atoms,
densely packed into
a two-dimensional (2D) honeycomb
45 lattice,14-17 has received tremendous attentions as a promising
catalyst support due to its large specific surface area, outstanding
corrosion resistance and excellent thermal conductivity.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

RSC Advances
Page 2 of 9
the graphene sheets was very effective in preventing graphene
method. A certain amount of GO was dispersed into 50 mL of a
sheets restack to form layered graphite structure. Furthermore, the
alkali medium used for the growth of LDH could also
mixture of NaOH (14.0 mmol) and Na2CO3 (4.2 mmol) solution
View Article Online
upon 30 min of ultrasonication. Next, both the mixture and
DOI: 10.1039/C5RA05436B
simultaneously reduce GO to reduced graphene oxide (RGO), 45 another salt solution containing Ni(NO3)2·6H2O (4.8 mmol) and
5
which has less oxygenic functional groups to improve the
hydrophilicity of GO.33 They also reported a graphene-supported
Ni nanocatalyst prepared via self-reduction of hybrid Ni-Al
layered double hydroxide/graphene and it exhibited superior
Al(NO3)3·9H2O (1.6 mmol) were added drop-wise together under
vigorous stirring at room temperature to obtain a suspension.
Subsequently, NaOH solution (0.2 M) was titrated with the
solution for adjusting the pH value of the solution to 10.5. The
catalytic performance in the liquid phase selective hydrogenation 50 suspension was then crystallized for the LDH phase for 10 h at
o
10 of cinnamaldehyde to hydrocinnamaldehyde.34 To the best of our
knowledge, utilizing LDH/graphene composites as the catalyst
support for selective oxidation reaction has not been reported yet.
Herein, in this work, we reported the research on the specific
150 C, followed by filtering and washing with deionized water
o
until pH 7. The solid was dried at 60 C for 12 h in a vacuum
oven to obtain NiAl-LDH/RGO composite. For comparison, both
the pure NiAl-LDH and RGO were prepared. The NiAl-LDH was
combination of NiAl-LDH and RGO as the support of Au catalyst 55 fabricated in the absence of GO under the above identical
15 for selective oxidation of benzyl alcohol. Initially, we synthesized
GO by a modified Hummers method, whereafter, NiAl-LDH was
prepared by coprecipitation method on the surface of GO, which
involved the nucleation and growth of NiAl-LDH and the
reduction of graphene oxide without the addition of any reducing
20 agents. Au nanoparticles were then immobilized on the support of
NiAl-LDH/RGO by using sol-immobilization method (Fig. 1). As
comparison, Au nanoparticles were also loaded on pure GO,
RGO and NiAl-LDH using the same method. XRD, FT-IR, TEM,
Raman spectra, BET, ICP and XPS were employed to investigate
25 the composition and structure of the catalysts and revealed the
promotion effect of NiAl-LDH/RGO support on the catalytic
performance for Au catalyst.
experimental conditions, and the RGO was prepared by reducing
GO in NaOH (0.8 M) solution.
Au/NiAl-LDH/RGO catalyst prepared by sol-immobilization
method
60 Supported Au catalyst was prepared using a sol-immobilization
method. An aqueous solution of HAuCl4·3H2O of the desired
concentration, Poly(vinyl alcohol) (PVA) (1 wt% aqueous
solution) and an aqueous solution of NaBH4 (0.1 M) was
prepared. The catalyst comprising Au nanoparticles with 1 wt%
65 total metal loading on NiAl-LDH/RGO support was prepared as
follows: The required amount of a PVA solution was added
(PVA/Au (wt/wt)=1.2) to an aqueous HAuCl4 solution; a freshly
prepared solution of NaBH4 (NaBH4/Au (mol/mol)=5) was then
added to form a dark-brown sol. After 2 h of sol generation, the
70 colloid was immobilized by adding NiAl-LDH/RGO under
vigorous stirring conditions at room temperature. After 2 h the
slurry was filtered, the catalyst washed thoroughly with distilled
o
water and dried at 60 C overnight. This sample is denoted as
Au/NiAl-LDH/RGO. For comparative study, Au catalysts
75 supported on pure GO and NiAl-LDH were also prepared as
stated above keeping the total metal loading of 1% wt and
denoted as Au/GO, Au/RGO and Au/NiAl-LDH, respectively.
Characterizations
Powder XRD patterns were recorded on a Shimadzu XRD-600
80 X-ray powder diffractometer (Cu Kα radiation, λ = 0.15406 nm)
between 3° and 70° 2θ, with a scan speed of 10° min-1. Fourier
transform infrared (FT-IR) spectra were recorded on a Bruker
Vector 22 spectrometer using the KBr pellet technique (1 mg of
sample in 100 mg of KBr). Transmission electron microscopy
85 (TEM) was carried out on a Hitachi H-800 transmission electron
microscope with an accelerating voltage of 100 kV. The Raman
Fig.1 Schematic illustration of preparing the Au/NiAl-LDH/RGO catalyst.
30 Experimental Section
Materials
spectra were obtained on
a Jobin Yvon Horiba HR800
spectrometer using a 532 nm line of Ar+ ion laser as the
excitation source at room temperature. BET surface area of
90 catalyst supports was analyzed by low temperature N2
adsorption/desorption method on a Micromeritics Surface Area &
Porosity Gemini VII 2390 (before the BET measurement, the
samples were degassed at 100 °C for 15 h). Elemental analysis
was performed using a Shimadzu ICPS-75000 inductively
95 coupled plasma emission spectrometer (ICP-ES) and an
Elementar Vario EL elemental analyzer. X-ray photoelectron
spectroscopy (XPS) was recorded on a VG ESCALAB 2201 XL
spectrometer with a monochromatic Mg Kα X-ray radiation
Purified natural graphite powder was purchased from Beijing
OuHe Technologies co., ltd. Other reagents were A.R. grade and
used without further purification. The water used in all the
35 experiments was deionized and had an electrical conductivity less
than 10–6 S·cm-1.
Synthesis of NiAl-LDH/RGO composites
The starting material, graphite oxide (GO), was synthesized by a
modified Hummers method. GO was exfoliated using an
40 ultrasonication method in ultrasonic bath. NiAl-LDH/RGO
composite was then fabricated directly by the coprecipitation
2
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 9
RSC Advances
(1253.6 eV photons). Binding energies were calibrated based on
the graphite C 1s peak at 284.5 eV. The morphology and size
reduction of GO to RGO. However, the pattern of composite
shows similar diffraction peaks with NiAl-LDH, suggesting the
distribution of the samples were examined using JEOL J-2100 45 formation of NiAl-LDH, and the depDoOsiIt:io10n.10o3Vf9ie/wCN5AiRrAtiAcl0l-eL5O4Dn3lH6inBe
high resolution transmission electron microscopy (HRTEM).
nanoplatelets on the surface of the resulting RGO weakened the
surface energy of graphene sheets and inhibited the restacking of
the exfoliated RGO.37 Moreover, comparing NiAl-LDH/RGO
composite to pure NiAl-LDH, there is an obvious decrease in the
50 intensity of the diffraction peaks, due to the disorder in the
stacked structure.38
5
Benzyl alcohol oxidation
Benzyl alcohol oxidation was carried out in a stainless steal
autoclave with a nominal volume of 100 ml. In a typical reaction,
400 mg catalyst and 40 mL benzyl alcohol were added into the
reactor, and then purged with O2 five times before closing and the
10 pressure was maintained at 2 bar (relative pressure). The reactor
with the reaction mixture was kept in a heating block, heating to
140 oC under vigorous stirring using a magnetic bar inside the
reactor (stirring rate 1000 rpm). After 10 h reaction time, the
reaction mixture was cooled down to room temperature and the
15 catalyst was separated from the reaction mixture by
centrifugation, thoroughly washed with acetone for recycle.
During the reaction, an aliquot of the clear supernatant (0.5 mL)
was periodically taken from the reaction mixture, filtered and
diluted with mesitylene (0.5 mL, internal standard for GC
20 analysis), and then analyzed by an Agilent 7890B gas
chromatograph equipped with a flame ionisation detector and a
30 m CP-Wax 52 CB column. The products were identified by
comparison with known standards. For the quantification of the
amounts of reactants consumed and products generated, an
25 internal standard method was used. The carbon mass balance was
proved for the experiments with a maximal difference of ±5%.
Fig. 3 FT-IR spectra of GO (a), RGO (b), NiAl-LDH (c) and NiAl-
LDH/RGO (d).
55
The FT-IR spectra of GO, RGO, NiAl-LDH and NiAl-
LDH/RGO samples are shown in Fig. 3. In the IR spectrum of
GO (Fig. 3a), the absorption peak around 1729 cm-1 is assigned to
C=O stretching vibration of −COOH groups, and the absorption
around 1622 cm-1 is associated with the stretching vibration of
Results and discussion
Characterization of NiAl-LDH/RGO composite
60 carbon backbone (C=C/C−C).35 The three stretching vibrations of
C−OH, C−O−C and C−O are located around 1393, 1221, 1056
cm-1 respectively. In addition, the broad absorption bands
between 3670 and 3260 cm-1 belonged to hydroxyl stretching
band (ν O−H) caused by −COOH groups and water molecules.39
65 For RGO obtained from the reduction of GO in NaOH solution
(Fig. 3b), the most obvious broad absorption is at 1572 cm-1,
assigned to the skeletal aromatic vibration, other absorption peaks
cannot be clearly identified. The FT-IR spectra of NiAl-LDH
shown the peak of asymmetric stretching vibrations of carbonate
70 anions at 1375cm-1, 40 and the bands appearing at 600 cm-1 and
550 cm-1 are attributed to the characteristic bending vibrations of
metal−oxygen (M−O) bond and metal−hydroxyl (M−OH) bond
in the lattice of LDH.41 However, as for NiAl-LDH/RGO, the
absorption bands related to C=O and C−O stretching vibrations
75 cannot be clearly observed comparing with pure GO. Meanwhile,
the absorption peaks for the stretching vibrations of the carbon
30 Fig. 2 XRD patterns of GO (a), NiAl-LDH (b) and NiAl-LDH/RGO (c).
Fig. 2 shows the powder XRD patterns of GO, NiAl-LDH and
NiAl-LDH/RGO. The XRD pattern of the precursor GO shows a
strong characteristic (001) diffraction at 2θ=11.5° (Fig. 2a),
corresponding to an interlayer distance of 0.78 nm, which can be
35 attributed to the formation of oxygenic functional groups on the
graphite sheets.35 The pattern of NiAl-LDH (Fig. 2b) exhibits the
two-dimensional hydrotalcite-like characteristic reflections of
(003), (006), and (110) peaks, the d spacing value of the (003)
diffraction peak of LDH phase is about 0.76 nm, demonstrating
40 the intercalation of CO32- anions into the interlayer of LDH.36 As
expected, no characteristic diffraction of GO was observed in the
composite sample (Fig. 2c), which could be attributed to the
2−
backbone (1572 cm-1) and CO3 anions (1375 cm-1) were
observed. The results of FT-IR spectra further confirmed the
reduction of GO and the presence of LDH phases in the NiAl-
80 LDH/RGO composite.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

RSC Advances
Page 4 of 9
oxygenic functional groups in carbon materials could also result
40 in over-oxidation of alcohols. 44 In contrast to pure GO, the peak
areas of C−OH, C−O−C and −COOHDOgrI:o1u0p.1s03V9fieo/wCr 5ARrNtiAci0leA5O4l-n3l6inBe
LDH/RGO composite decreased significantly and the peak area
ratio of the oxygen-containing bonds to the total carbon bonds
reduced to 36.72%, which further proved the effective and
45 sufficient reduction of GO to RGO during the in situ growth
process of the NiAl-LDH. In this process, NaOH severing as the
precipitating agent for NiAl-LDH formation could also provide
alkaline ambient for the deoxygenation of oxygenic functional
groups in GO.32
Fig. 4 TEM images of GO (a), NiAl-LDH (b), NiAl-LDH/RGO (c) and
the size distribution of NiAl-LDH nanoplatelets.
The morphology and structure of GO, NiAl-LDH and NiAl-
LDH/RGO were investigated by means of TEM measurements,
as shown in Fig. 4 As for GO (Fig. 4a), a transparent ultrathin
sheet structure with a few thin ripples within the flat surface can
be observed, which shows that GO sheets were efficiently
exfoliated.31 The TEM image of NiAl-LDH/RGO (Fig. 4c) shows
5
10 a marked contrast with pure GO, the thin graphene sheets were
decorated densely by nanosized NiAl-LDH particles and no
obvious aggregate is observed, which is in accord with the XRD
patterns. The NiAl-LDH nanoplatelets grew parallel or
perpendicular to the surface of RGO. It is worth mentioning that
15 the average crystallite size of the NiAl-LDH in NiAl-LDH/RGO
composites is about 61.8 nm, significantly lower than that of pure
NiAl-LDH (Fig. 4b) obtained under the similar conditions (175.1
nm). Herein, we supposed that the oxygenic functional groups on
the surface of GO could promote the sorption and intercalation of
20 metal ions and act as anchoring sites for metal nanoparticles,
which controlled the directional growth of NiAl-LDH
nanoplatelets and led to a smaller particle size, consistent with
other literature reports. 42, 43
50
Fig. 6 Raman spectra of GO (a) and NiAl-LDH/RGO (b).
In the Raman spectra of GO and NiAl-LDH/RGO composites,
the two intense peaks at around 1358 and 1590 cm-1 can be
respectively assigned to the D band and G band of the carbon
55 materials, which corresponding to the vibrations of carbon atoms
with dangling bonds in plane terminations of disordered graphite
and the first order scattering of sp2 bonds of carbon atoms.45 By
quantitatively measuring the intensity ratio of the D and G bands
(ID/IG), the ordered and disordered crystal structures of carbon
60 can be evaluated. For the D/G band intensity ratio of NiAl-
LDH/RGO composite (ID/IG = 1.13), there is an increased ID/IG
intensity ratio in comparison with that of GO (ID/IG = 0.90),
indicating that the composite decorated with NiAl-LDH
nanoplatelets could lead to a more disordered structure. This
65 result could be ascribed to the decrease in the size of the in-plane
sp2 domains as well as the increase of some un-repaired
topological defects and carbon vacancies caused by the removal
of oxygen functional groups from GO during the growth of NiAl-
LDH nanoplatelets.46 Similar to the oxygenic functional groups,
70 the added defect sites on the surface of RGO in NiAl-LDH/RGO
composite could also adsorb the metal ions and be utilized as
anchoring centers for the nucleation and dispersion of the loading
metal nanoparticles and lead to a smaller particle size.47In
summary, even with less oxygenic functional groups, the NiAl-
75 LDH/RGO composite could still impact the particle size
distribution and surface morphology of the loaded Au NPs, as it
has plenty of both defect sites and oxygenic functional groups.
25
Fig. 5 C1s XPS of GO (a) and NiAl-LDH/RGO (b).
The chemical structure of GO and NiAl-LDH/RGO was further
explored by the XPS technique. As shown in Fig. 5, the C1s
spectra of both GO and NiAl-LDH/RGO can be deconvoluted
into four separate peaks corresponding to different carbon bonds
30 including C＝C and/or C−C groups (284.5 eV), C−OH groups
(286.0 eV), C−O−C groups (286.6 eV) and −COOH groups
(288.5 eV).23 In the spectrum of GO (Fig. 5a), the peak area ratio
of the oxygen-containing bonds to the total carbon bonds was
calculated to be 60.07%, accounting for a large degree of
35 oxidation. The plenty of oxygenic functional groups on the
surface of GO could control the directional growth of NiAl-LDH
nanoplatelets or Au NPs and therefore lead to a small particle
size. However, some literatures reported that the superabundant
Support effect and catalytic performance of the NiAl-
LDH/RGO supported Au catalyst
80 In order to take into consideration the texture properties of
Au/GO, Au/RGO, Au/NiAl-LDH and Au/NiAl-LDH/RGO, the
Brunauer-Emmett-Teller surface areas (SBET) of the different
4
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 5 of 9
RSC Advances
supports were presented in Table 1, as well as the Au loading of
the catalysts determined by ICP analysis. The SBET of GO, RGO
and NiAl-LDH were 136.93 m2/g, 141.89 m2/g and 61.18 m2/g,
respectively, and increased to 172.47 m2/g after hybrid. In
general, high surface area could lead to small particle size, high
dispersion of active components, and therefore superior catalytic
performance.22 As for the experimental Au loadings employed in
the final catalysts, the Au content in Au/GO is much lower than it
in Au/NiAl-LDH with the same preparation process. Excluding
View Article Online
DOI: 10.1039/C5RA05436B
5
10 the experimental errors, this phenomenon might be attributed to
high hydrophilicity of GO leading to the loss in the preparation of
catalyst. However, the Au loadings in Au/RGO and Au/NiAl-
LDH/RGO is up to 0.65 wt. % and 0.66 wt. %, indicating that the
RGO and NiAl-LDH/RGO composite have less oxygenic
15 functional groups and therefore reduce the hydrophilicity of GO.
50
Fig. 8 Au 4f XPS of Au/silicon wafers (a), Au/GO (b), Au/NiAl-LDH (c)
and Au/NiAl-LDH/RGO (d).
The catalytic oxidation of benzyl alcohol under solvent-free
o
condition was investigated at 140 C with 2 bar oxygen using a
55 stainless steal autoclave and the results were shown in Fig. 9 and
Table 1. The benzyl alcohol conversion of 1% Au/RGO is nearly
same with 1% Au/GO catalysts and both in a relative low level.
However, the benzaldehyde selectivity of Au/RGO catalysts was
significantly higher than the catalysts supported with pure GO,
60 which confirmed that superabundant oxygenic functional groups
in GO facilitated the over-oxidation of benzyl alcohol and
decrease the benzaldehyde selectivity. The benzyl alcohol
conversion of 1%Au/NiAl-LDH/RGO catalyst reached ca. 62%
after 10 h, higher than Au/GO, Au/RGO and Au/NiAl-LDH
65 catalysts, which illustrated a very pronounced promotion of NiAl-
LDH/RGO support to the oxidation of benzyl alcohol. The
catalytic performance of the physical mixture of Au/GO and
Au/NiAl-LDH catalysts as shown in Table.1 was between the two
single support catalysts and much lower than Au/NiAl-
70 LDH/RGO catalyst, which indicated that there was synergistic
effect between RGO and NiAl-LDH in the composite support.
The activity of Au/NiAl-LDH/RGO catalyst was 8.7 times higher
than Au/GO catalyst, attributing to the introduction of NiAl-LDH
with an abundance of basic sites and transition metal cations. 1.2
75 times higher than Au/NiAl-LDH catalyst can be ascribed to
smaller Au particle size benefiting from the oxygenic functional
groups and defect sites in RGO.
Fig. 7 HRTEM images and the size distribution of 1% Au/GO (a), 1%
Au/NiAl-LDH (b), 1% Au/NiAl-LDH/RGO (c).
Fig. 7 shows the HRTEM images of the Au catalysts supported
20 on GO, NiAl-LDH and NiAl-LDH/RGO and the corresponding
particle size distributions by measuring more than 200 particles
from different regions. In the catalyst samples supported on GO
and NiAl-LDH/RGO (Figure 7a and c), the average size of Au
particles were 2.92 nm and 2.63 nm, respectively, which were
25 much smaller than that supported on NiAl-LDH (4.18 nm).
Smaller particle size of Au NPs on GO was attributed to the
oxygenic functional groups on the surface of support, which
provided anchoring centers for Au NPs. As for Au/NiAl-
LDH/RGO, although there was a decrease in the oxygenic
30 functional groups on the surface of RGO, the added defect sites in
NiAl-LDH/RGO composite could also impact the particle size
distribution of the loading Au NPs and the large specific surface
area was also a key factor. Moreover, the Au NPs supported on
pure GO tended to aggregate in a small region, which will also
35 cause an adverse impact on the catalytic activity.
Fig. 8 displays the Au 4f XPS spectra of Au/silicon wafers,
Au/GO, Au/NiAl-LDH and Au/NiAl-LDH/RGO. The peaks with
binding energies at 83.0-84.0 eV and 86.8-87.8 eV can be
assigned to the electron transitions of Au 4f7/2 and Au 4f5/2
,
40 respectively.50 Compared with the Au NPs supported on chemical
inert carrier (silicon wafers), the Au 4f7/2 peak of Au NPs in
Au/NiAl-LDH and Au/NiAl-LDH/RGO samples were shifted to
lower binding energies by about 0.6 eV and 0.3 eV respectively,
indicating that there was a stronger interaction between the Au
45 particles and the NiAl-LDH or NiAl-LDH/RGO support.
However, no such shift was observed for the Au/GO catalyst. In
consequence, the introduction of the NiAl-LDH could strengthen
the metal-support interaction and enhance the alcohol oxidation
activity of catalyst.
Fig. 9 Catalytic performance of 1% Au catalysts supported on GO, NiAl-
80
LDH and NiAl-LDH/RGO.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

RSC Advances
Page 6 of 9
Table 1. Catalytic performance of Au catalysts with different supports.
Specific
surface area
(m2/g)
Au
loading
(wt. %)
Mean Au
size (nm)
Conversion
Selectivity
(%)
TON[a]
2604.7
2724.6
12024.9
17893.4
9686.8
View Article Online
DOI: 10.1039/C5RA05436B
Catalysts
Au/GO
(%)
136.93
141.89
61.18
172.47
—
0.52
0.65
0.82
0.66
0.75
2.92
2.89
4.18
2.63
—
7.1
60.6
82.1
63.3
65.2
60.2
Au/RGO
6.3
Au/NiAl-LDH
Au/NiAl-LDH/RGO
Au/GO + Au/NiAl-LDH[b]
51.8
62.0
38.2
[a] TON values were calculated with respect to the total loading of Au after 10 h reaction.
[b] Au/GO + Au/NiAl-LDH is the physical mixture of Au/GO and Au/NiAl-LDH
5
Even the Au loadings of four catalysts are different, the trend of
selectivity was significantly higher than the catalysts supported
TON in Table.1 was in accordance with the activity, and the 45 with pure GO.
Au/NiAl-LDH/RGO catalyst had the highest TON. In addition,
when comparing the selectivity towards benzaldehyde at iso-
conversion of benzyl alcohol, the selectivity of Au/NiAl-
10 LDH/RGO catalyst is nearly equal with Au/NiAl-LDH catalyst
but considerably higher than Au/GO catalyst, which may be
attributed to the surface properties of LDH and less oxygenic
functional groups of RGO, preventing the over-oxidation of
benzyl alcohol.
15
Besides the target product of benzaldehyde, there were still a
number of by-products formed by oxidation of benzyl alcohol
e.g. toluene, benzoic acid, benzyl benzoate. Fig. 10 shows the
selectivity towards by-products as a function of benzyl alcohol
conversion for the Au/GO, Au/NiAl-LDH and Au/NiAl-
20 LDH/RGO catalysts. All three catalytic selectivity to toluene
maintained in a relative low level (﹤10%). In the case of Au/GO
catalyst, the major by-product in benzyl alcohol oxidation was
benzoic acid, and a few benzyl benzoate was also produced.
However, the reaction catalyzed by Au/NiAl-LDH and Au/NiAl-
25 LDH/RGO barely produced benzoic acid but the selectivity
towards benzyl benzoate increased to ca. 35%. It is possible that
benzaldehyde reacted with benzyl alcohol to form a hemiacetyl
and it was oxidized to benzyl benzoate, which means there are
two different reaction routes catalyzed by Au/GO and Au/NiAl-
30 LDH catalyst.51 Compared with Au/GO catalyst, the Au/NiAl-
LDH exhibited preferable selectivity to the target product, and the
Au/NiAl-LDH/RGO catalyst inherited this advantage.
The effect of the GO/NiAl-LDH ratio on activity and
selectivity was investigated and the results were shown in Fig. 11.
35 The GO/NiAl-LDH ratio was achieved by turning the mass ratio
of GO and Ni(NO3)2·6H2O precursor. An increase in catalytic
activity was observed with the content of NiAl-LDH increasing.
This increase of activity reached
a maximum with a
GO/Ni(NO3)2·6H2O mass ratio of 1:2.8. A further increase in the
40 NiAl-LDH content resulted in a decrease in the catalytic activity.
It is important to note that in the presence of a minor amount of
NiAl-LDH (1:1.4 GO/ Ni(NO3)2·6H2O mass ratio), there was no
obvious increase in the catalytic activity, but the benzaldehyde
Fig. 10 Selectivity to toluene, benzoic acid and benzyl benzoate as a
function of benzyl alcohol conversion over 1% Au catalysts supported by
GO, NiAl-LDH and NiAl-LDH/RGO.
6
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 7 of 9
RSC Advances
This phenomenon is similar with the fact that the selectivity of
Au/RGO is better than Au/GO, mainly attributed to the reduction
of GO in accompany with the growth of NiAl-LDH. In summary,
the optimum selective oxidation of benzyl alcohol to
benzaldehyde occured for the GO/ Ni(NO3)2·6H2O 1:2.8 mass
ratio support with the yield of 40%.
View Article Online
DOI: 10.1039/C5RA05436B
5
Fig. 12 Reusability of 1% Au catalysts supported by GO, NiAl-LDH and
45
NiAl-LDH/RGO.
Fig. 11 The effect of GO/NiAl-LDH ratio on the benzyl alcohol
conversion, selectivity to benzaldehyde and yield (reaction time: 10 h).
10
The reusability of the catalyst is also one of the key factors.
After reaction, the catalysts were separated from the reaction
mixture by centrifugation, thoroughly washed with acetone and
then reused in the next reaction under the same conditions. The
reusability of the catalysts supported on GO, NiAl-LDH and
Fig. 13 HRTEM images and the size distribution of the used Au/GO (a),
Au/NiAl-LDH (b) and Au/NiAl-LDH/RGO (c).
15 NiAl-LDH/RGO were investigated for three successive reactions
and the results were shown in Fig. 12. After third recycling, the
conversion of Au/GO and Au/NiAl-LDH catalysts dropped by
49.8% and 18.3%, respectively, while for Au/NiAl-LDH/RGO
catalyst, the conversion only dropped by 10.0% and the
20 selectivity of benzaldehyde still reached ca. 68%. HRTEM
images of the used Au catalysts are shown in Fig. 13. For
Au/NiAl-LDH catalyst, the average size of Au NPs after three
times used was up to 8.93 nm, and the obvious agglomeration of
Au NPs was observed. Therefore, the decrease in catalytic
25 activity for Au/NiAl-LDH catalyst was mainly due to the
agglomeration of Au NPs. A obvious deactivation was also
observed in the catalyst supported on pure GO, however, the Au
NPs were still uniformly scattered on the surface of both GO and
NiAl-LDH/RGO support without obvious agglomeration, and the
30 average size of Au NPs in the used Au/GO and Au/ NiAl-
LDH/RGO catalyst were 5.15 nm and 4.93 nm. As the particle
sizes of Au NPs supported on GO and NiAl-LDH/RGO were
similar, the main reason for the deactivation could possibly be the
agglomeration of GO, which partly covered the active sites. In
35 contrast, the Au/NiAl-LDH/RGO catalyst was found to be fully
reusable which attributed to the defect sites and oxygenic
functional groups in RGO could anchor the Au NPs and prevent
the agglomeration, meanwhile, the agglomeration of RGO was
inhibited by the growth of NiAl-LDH. Therefore, the Au/NiAl-
40 LDH/RGO catalyst possessed not only higher activity and
benzaldehyde selectivity but also good stability during the
solvent-free oxidation of benzyl alcohol.
Conclusions
50 In the present work, a novel Au nanocatalyst supported on NiAl-
LDH/RGO nanocomposite has been presented. The composite
support was prepared via a coprecipitation process of NiAl-LDH
on the surface of GO accompanied by the reduction of GO to
RGO in alkaline solution. The formation of NiAl-LDH/RGO
55 composite was confirmed by XRD and FT-IR. TEM observation
of NiAl-LDH/RGO confirmed that the graphene sheets were
decorated densely by nanosized NiAl-LDH particles and no
obvious aggregate was observed. Raman spectroscopy and XPS
analysis revealed that there were both defect sites and oxygenic
60 functional groups in NiAl-LDH/RGO composite, which could be
utilized as anchoring centers for the nucleation and dispersion of
the Au nanoparticles and led to a smaller particle size. The
average size of Au nanoparticles supported on GO, NiAl-LDH
and NiAl-LDH/RGO were 2.92 nm, 4.18 nm and 2.63 nm,
65 respectively. The synthesized Au/NiAl-LDH/RGO catalyst
showed preferable catalytic performance including activity,
selectivity and stability in the selective oxidation of benzyl
alcohol. Improved activity was mainly ascribed to the small size
of Au nanoparticles which controlled by RGO. Preferable
70 selectivity to benzaldehyde was owing to the surface properties of
NiAl-LDH as well as appropriate oxygenic functional groups in
RGO which prevented the over-oxidation of benzyl alcohol.
Moreover, superior catalytic stability of Au/NiAl-LDH/RGO
catalyst was attributed to the defect sites and oxygenic functional
75 groups in RGO which anchored the Au NPs and prevented the
agglomeration, meanwhile, the agglomeration of RGO was
inhibited by the introduction of NiAl-LDH. Thus, it can be seen
that NiAl-LDH/RGO composite maintained the advantages of
both RGO and NiAl-LDH, which enable it as a promising
80 catalyst support in the selective oxidation of benzyl alcohol.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

RSC Advances
Page 8 of 9
22 G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth and
Acknowledgements
R. Mülhaupt, J. Am. Chem. Soc., 2009, 131, 8262-8270.
60 23 P. Sharma, G. Darabdhara, T. M. ReDdOdI:y1,0.A10.3V9iBe/wCo5AraRrthiAc,0le5OP4n.3l6inBe
Bezboruah, P. Gogoi, N. Hussain, P. Sengupta, M. R. Das,
Catalysis Communications, 2013, 40, 139-144.
24 B. J. Hong, O. C. Compton, Z. An, I. Eryazici and S. T.
Nguyen, ACS nano, 2011, 6, 63-73.
This work was supported by National Natural Science
Foundation, the 973 Project (2011CBA00506), Beijing Natural
Science Foundation (2132032), the Fundamental Research Funds
for the Central Universities (YS1406) and the Beijing
Engineering Center for Hierarchical Catalysts.
5
65 25 P. Sharma, G. Darabdhara, T. M. Reddy, A. Borah, P.
Bezboruah, P. Gogoi, N. Hussain, P. Sengupta and M. R.
Das, Catalysis Communications, 2013, 40, 139-144.
26 K. M. Parida, L. Mohapatra, Chem. Eng. J, 2012, 179, 131-
139.
70 27 G. L. Fan, F. Li, D. G. Evans and X. Duan, Chemical Society
Reviews, 2014, 43, 7040-7066.
28 M. F. Shao, F. Y. Ning, J. W. Zhao, M. Wei, D. G. Evans and
X. Duan, Adv. Funct. Mater., 2013, 23, 3513-3518.
29 B. Ballarin, A. Mignani, E. Scavetta, M. Giorgetti, D. Tonelli,
Notes and references
* State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, 100029, Box 98, 15 Bei San Huan
10 East Road, Beijing, 10002 , China
Fax: +86 (10) 6442 5385; Tel: +86 (10) 6445 1007;
E-mail address:
lidq@mail.buct.edu.cn (D. Q. Li)
fengjt@mail.buct.edu.cn (J. T. Feng)
15
20
25
30
35
40
1
Z. Guo, B. Liu, Q. H. Zhang, W. P. Deng, Y. Wang and Y. H.
Yang, Chem. Soc. Rev., 2014, 43, 3480-3524.
N. Dimitratos, J. A. Lopez-Sanchez, D. Morgan, A. F. Carley,
R. Tiruvalam, C. J. Kiely, D. Bethella and G. J. Hutchings,
Phys. Chem. Chem. Phys., 2009, 11, 5142-5153.
W. Feng, G. J. Wu, L. D. Li and N. J. Guan, Green Chem.,
2011, 13, 3265-3272.
A. Thomas, Q. He and J. K. Edwards, Faraday Discuss.,
2011, 152, 381-392.
Y. L. Hong, X. L. Jing, J. L. Huang, D. H. Sun, T. Odoom-
Wubah, F. Yang, M. M. Du, and Q. B. Li, ACS Sustainable
Chem. Eng., 2014, 2, 1752-1759.
2
75
80
85
90
E. Boanini, C. Mousty and V. Prevot, Langmuir, 2012, 28,
15065-15074.
30 K. M. Parida, M. Sahoo, S. Singha, J. Catal., 2010, 276, 161-
169.
31 Z. Gao, J. Wang, Z. S. Li, W. L. Yang, B. Wang, M. J. Hou,
Y. He, Q. Liu, T. Mann, P. P. Yang, M. L. Zhang and L. H.
Liu, Chem. Mater., 2011, 23, 3509-3516.
32 M. Lan, G. L. Fan, L. Yang and F. Li, Ind. Eng. Chem. Res.,
2014, 53, 12943-12952.
33 X. B. Fan, W. C. Peng, Y. Li, X. Y. Li, S. L. Wang, G. L.
Zhang and F. B. Zhang, Adv. Mater., 2008, 20, 4490-4493.
34 R. F. Xie, G. L. Fan, Q. Ma, L. Yang and F. Li, J. Mater.
Chem. A, 2012, 00, 1-3.
35 L. L. Geng, S. J. Wua, Y. C. Zou, M. J. Jia, W. X. Zhang, W.
F. Yan, G. Liu, Journal of Colloid and Interface Science,
2014, 42, 71-77.
3
4
5
6
7
8
V. R. Choudhary, R. Jha and P. Jana, Green Chem., 2007, 9,
267-272.
N. Dimitratos, J. A. Lopez-Sanchez and G. J. Hutchings,
Chem. Sci., 2012, 3, 20-44.
G. L. Hallett-Tapley, M. J. Silvero, C. J. Bueno-Alejo, M.
González-Béjar, C. D. McTiernan, M. Grenier, J. C. Netto-
Ferreira and J. C. Scaiano, J. Phys. Chem. C, 2013, 117,
12279-12288.
36 H. Wang, X. Xiang and F. Li, J. Mater. Chem., 2010, 20,
3944-3952.
37 L. Li, R. Z. Ma, Y. Ebina, K. Fukuda, K. Takada and T.
Sasaki, J. Am. Chem. Soc., 2007, 129, 8000-8007.
95 38 J. Xu, S. Gai, F. He, N. Niu, P. Gao, Y. J. Chen and P. P.
Yang, J. Mater. Chem. A, 2014, 2, 1022-1031.
39 C. L. Bao, L. Song, W. Y. Xing, B. H. Yuan, C. A. Wilkie, J.
L. Huang, Y. Q. Guo and Y. Hu, J. Mater. Chem., 2012, 22,
6088-6096.
9
L. C. Wang, Y. M. Liu, M. Chen, Y. Cao, H. Y. He and K. N.
Fan, J. Phys. Chem. C, 2008, 112, 6981-6987.
10 T. A. G. Silva, R. Landersb and L. M. Rossi, Catal. Sci.
Technol., 2013, 3, 2993-2999.
11 A. Villa, C. E. Chan-Thaw, G. M. Veith, K. L. More, D. Ferri
and L. Prati, ChemCatChem, 2011, 3, 1612-1618.
12 S. E. Davis, M. S. Ide and R. J. Davis, Green Chem., 2013,
15, 17-45.
100 40 S. L. Ma, C. H. Fan, L. Du, G. L. Huang, X. J. Yang, W. P.
Tang, Y. Makita and K. Ooi, Chem. Mater., 2009, 21, 3602-
3610.
13 F. Z. Zhang, X. F. Zhao, C. H. Feng, B. Li, T. Chen, W. Lu,
X. D. Lei and S. L. Xu, ACS Catal., 2011, 1, 232-237.
45 14 A. Mondal and N. R. Jana, ACS Catal., 2014, 4, 593-599.
15 M. I. Katsnelson, Materials today, 2007, 10, 20-27.
16 L. Liao, H. L. Peng and Z. F. Liu, J. Am. Chem. Soc., 2014,
136, 12194-12200.
41 Y. J. Feng, Y. Jiang, Q. Huang, S. T. Chen, F.B. Zhang, P. G.
Tang and D. Q. Li, Ind. Eng. Chem. Res., 2014, 53, 2287-
105
2292.
42 X. Q. Xie, J. L. Long, J. Xu, L. M. Chen, Y. Wang, Z. Z.
Zhang and X. X. Wang, RSC Advances, 2012, 2, 12438-
12446.
17 M. Sun, H. J. Liu, Y. Liu, J. H. Qu and J. H. Li, Nanoscale,
50
2013, 00, 1-3.
18 N. N. Zhang, H. X. Qiu, Y. Liu, W. Wang, Y. Li, X. D. Wang
and J. P. Gao, J. Mater. Chem., 2011, 21, 11080-11083.
19 B Garg, Y. C. Ling, Green Materials, 2013, 1, 47-61.
20 D. R. Dreyer, H. P. Jia and C. W. Bielawski, Angew. Chem.
Int. Ed., 2010, 49, 6813-6816.
43 F. F. Ren, C. Q. Wang, C. Y. Zhai, F. X. Jiang, R. R. Yue, Y.
K. Du, P. Yang and J. K. Xu, J. Mater. Chem. A, 2013, 1,
7255-7261.
44 E. G. Rodrigues, J. J. Delgado, X. Chen, M. F. R. Pereira and
J. J. M. Órfão, Ind. Eng. Chem. Res., 2012, 51, 15884-15894.
110
55
21 E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura, and I.
Honma, Nano Lett., 2009, 9, 2255-2255.
8
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 9 of 9
RSC Advances
45 M. X. Li, J. E. Zhu, L. L. Zhang, X. Chen, H. M. Zhang, F. Z.
Zhang, S. L. Xu and D. G. Evans, Nanoscale, 2011, 3, 4240-
4246.
View Article Online
DOI: 10.1039/C5RA05436B
46 X. Wang, S. Zhou, W. Y. Xing, B. Yu, X. M. Feng, L. Song
and Y. Hu, J. Mater. Chem. A, 2013, 1, 4383-4390.
47 W. H. Fang, J. S. Chen, Q. H. Zhang, W. P. Deng and Y.
Wang, Chem. Eur. J., 2011, 17, 1247-1256.
5
48 J. Wang, X. J. Lang, B. Zhaorigetu, M. L. Jia, J. Wang, X. F.
Guo and J. C. Zhao, ChemCatChem, 2014, 6, 1737-1747.
10 49 J. T. Feng, C. Ma, P. J. Miedziak, J. K. Edwards, G. L. Brett,
D. Q. Li, Y. Y. Du, D. J. Morganb and G. J. Hutchings,
Dalton Trans., 2013, 42, 14498-14508.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9