Au-Pd nanoalloys supported on Mg-Al mixed metal oxides as a multifunctional catalyst for solvent-free oxidation of benzyl alcohol

Article abstract of DOI:10.1039/c3dt51855h

Au-Pd nanoalloys supported on Mg-Al mixed metal oxides prepared using sol-immobilisation are found to be highly efficient and reusable catalysts for the solvent-free oxidation of benzyl alcohol using molecular oxygen under low pressure. When using this support alloying Pd with Au resulted in an increase in both activity and selectivity to benzaldehyde and moreover an improved resistance to catalyst deactivation compared with the monometallic Pd and Au catalysts. The turnover number for the Au/Pd 1:1 molar ratio catalyst achieved 13000 after 240 min and the selectivity to benzaldehyde was maintained at 93%; this high catalytic activity can be retained in full after three successive uses. The ensemble and electronic effect of Au-Pd nanoalloys were studied by IR spectroscopy using CO chemisorption, XPS and HRTEM. Moreover, the bifunctional nature of the acid-base MgAl-MMO support was found to be important as the acid sites are considered to be responsible for the improvement of catalytic activity; while, the basic sites gave rise to high selectivity. A possible mechanism with Au-Pd nanoparticles as the active sites has been proposed, illustrating that the oxidation of benzyl alcohol can proceed through the cooperation between the Au-Pd nanoalloys and the base/acid sites on the surface of the support.

Full text of DOI:10.1039/c3dt51855h

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absence of acidity can switch-off the disproportionation reaction
of benzyl alcohol and thus exhibit much higher selectivity to the
metal loading of 1% wt.
Monometallic catalysts containing only gold or palladium were
aldehyde; however, this desirable selectivity was accompanied by 60 prepared using a similar methodology, and these are denoted 1%
32
low benzyl alcohol conversion. Hence, developing new types of
metal catalyst that can exhibit controllable surface acidic or basic
properties of supports could be of value. Layered double
hydroxide (LDH), known as a family of synthetic anionic clays,
Au/MgAl-MMO, 1% Pd/MgAl-MMO, 0.65% Au/MgAl-MMO
and 0.35% Pd/MgAl-MMO respectively.
Catalyst characterization: Powder XRD patterns were
recorded on a Shimadzu XRD-600 X-ray powder diffractometer
5
0
5
0
5
are a class of two-dimensional brucite Mg(OH) -like layered 65 (Cu K radiation, λ = 0.15406 nm) between 3º and 70º 2θ, with a
2
a
–
1
inorganic materials. They can be expressed by the general
formula [M 1-xM (OH) ] (A )x/n·mH O. As a consequence
x 2 2
of the tailored structure-design and controlled accessibility to
catalytic sites, LDH provide potential opportunities for designing
scan speed of 10 º min . Elemental analysis was performed using
a Shimadzu ICPS-75000 inductively coupled plasma emission
spectrometer (ICP-ES) and an Elementar Vario EL elemental
analyzer. The morphology, size distribution, lattice fringe and
2
+
3+
x+
n-
33
1
1
2
2
and innovating novel catalyst supports, catalyst precursors as well 70 crystal boundaries of the samples were examined using JEOL J-
3
4-36
as actual catalysts.
In particular, calcination of LDHs at
2100 high-resolution transmission electron microscopy
(HRTEM). UV-visible analysis of the colloidal sols was used to
monitor the intensity and position of the plasmon resonance band
intermediate temperatures (450-600 °C) often gives poorly
crystallized mixed metal oxides (MMO), which possess higher
surface area and tunable surface acidic/basic properties.
37
of gold using
a
Shimadzu UV-2501PC UV-visible
adsorption–
Knowing that MgAl-MMO has both strong acid and base sites on 75 spectrophotometer. The low temperature
N
2
their surfaces, herein, we report the research on the specific
combination of bimetallic Au-Pd nanoparticles with the acid-base
bifunctional material of MgAl-MMO as a novel heterogeneous
catalyst for solvent-free oxidation of benzyl alcohol using
desorption experiments were carried out on a Micromeritics
Surface Area & Porosity Gemini VII 2390. XPS measurements
were performed on a Kratos Axis Ultra DLD photoelectron
spectrometer using monochromatic Al Kα radiation. The mixed
molecular oxygen. HRTEM, XPS, In situ FT-IR of CO and 80 metal oxides were calibrated to the Mg 2p photoelectron peak at
NH /CO -TPD were employed to investigate the structure and 49.6 eV as measured from pure MgO and MgAl samples and
3
2
2 4
O
3
8
composition of the catalyst and reveal the synergistic effect
between Au and Pd as well as the unique cooperative interaction
between the bimetallic Au-Pd and surface acidic/basic sites on the
MMO support.
consistent with that reported in the literature.
CO and NH -TPD of the catalysts were conducted on a
Micromeritics ChemiSorb 2720. About 100 mg of samples were
2
3
o
85 loaded in a quartz reactor and heated in He for 60 min at 120 C,
followed by treating with the probe molecules (25 mL/min) for
60 min at room temperature. Weakly adsorbed probe molecules
were removed by flowing He for 120 min, and then desorption
Experimental
3
3
4
4
5
5
0
5
0
5
0
5
Support preparation: MgAl-MMO support was derived from
an MgAl-LDH precursor. MgAl-LDH precursor was prepared by
o
process was carried out from room temperature to 800 C in He
o
9
0
flow with the rate of 10 C/min.
means of a hydrothermal synthesis method. Mg(NO
3
)
2
·6H
O and urea with an Mg /Al /urea molar ratio of
:1:7 were dissolved in 80 mL deionized water to make a mixed
2
O,
In situ Fourier-transformed infrared absorption spectroscopy of
CO experiments were carried out in a custom built quartz cell
equipped with KBr windows allowing sample activation and
successive measurements in the range of 292–823 K, at pressures
2
+
3+
3 3 2
Al(NO ) ·9H
2
solution. The resulting solution was transferred to an autoclave
and aged at 150 °C for 6 h. The precipitate was then centrifuged
and thoroughly washed with deionized water until the filtrate
reached pH 7. After drying and calcination at 450 °C for 4 h,
MgAl-LDH precursor transformed into MgAl-MMO support.
Catalyst preparation: Supported Au, Pd and Au-Pd catalysts
were prepared using a sol-immobilization method. An aqueous
solution of PdCl and HAuCl ·3H O (Johnson Matthey) of the
2 4 2
desired concentration was prepared. For the bimetallic catalyst
with the Au:Pd molar ratio of 1:1, the detailed preparation
-4
9
5
as low as 10 . The catalysts were pressed into self-supporting
pellets and activated in the same cell used for the measurement.
The thermal treatments were performed either in dynamic
vacuum or under static conditions (no flux), according to
procedures discussed below. In situ FTIR spectra on the
-1
1
1
1
1
00 investigated catalysts were recorded at a resolution of 0.964 cm
on a Nicolet 380 instrument. After nitrogen pre-treatment at 100
o
o
C for 45 minutes and hydrogenation treatment at 100 C for 60
minutes, the catalyst was scanned to get a background record
procedure employed is described below. To an aqueous PdCl
HAuCl solution of the desired concentration, the required
amount of a PVA solution (1 wt %) was added (PVA/(Au+Pd)
wt/wt)=1.2); a freshly prepared solution of NaBH (0.1 M,
NaBH /(Au+Pd) (mol/mol)=5) was then added to form a dark-
2
and
-3
below a pressure of 10 Pa. And then the catalyst was exposed to
4
05 a CO flow for 60 minutes. Sample scanning for adsorbed CO on
-
catalysts was conducted after the pressure was reduced below 10
Pa again.
(
4
3
4
Benzyl alcohol oxidation: Benzyl alcohol oxidation was
carried out in a 50 mL glass stirred reactor. In a typical reaction,
10 10 mg catalyst and 1 mL benzyl alcohol were charged into the
brown sol. After 30 min of sol generation, the colloid was
immobilized by adding support under vigorous stirring conditions.
The amount of support material required was calculated so as to
have a total final metal loading of 1 % wt. After 1 h the slurry
was filtered, the catalyst washed thoroughly with distilled water
and dried at 120 °C overnight. This sample is denoted as 1% Au-
Pd/MgAl-MMO. Other bimetallic catalysts with different molar
ratio of Au/Pd were prepared as stated above keeping the total
2
reactor which was then purged with O three times before closing
and the pressure was maintained at 1 bar (relative pressure). The
reactor with the reaction mixture was kept in a heating block,
which was preheated to the reaction temperature. The reaction
15 mixture was stirred at 1000 rpm using a magnetic bar inside the

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DOI: 10.1039/C3DT51855H
reactor. After a specific time, the stirring was stopped and the
reactor was rapidly cooled in an ice bath. After cooling for 10
min, the reactor was opened slowly and the contents were
centrifuged.
5
0
5
An aliquot of the clear supernatant reaction mixture (0.5 mL)
was diluted with mesitylene (0.5 mL, external standard for GC
analysis) Samples were removed periodically and analysed by
GC. For the analysis of the products a GC–MS (Waters, GCT
Premier) and GC (a Varian star 3400 cx with a 30 m CP-Wax 52
CB column) were employed. The products were identified by
comparison with known standards. For the quantification of the
amounts of reactants consumed and products generated, an
external standard method was used. The selectivity was
calculated as mol of product formed per mol of reacted benzyl
alcohol and the carbon balance was within ±5%.
1
1
45 Figure 2 Selectivity to benzaldehyde as a function of benzyl
alcohol conversion
Result and discussion
As we have previously reported, benzaldehyde, the target
product, can be formed by both selective oxidation and
Synergistic effect of Au and Pd
3
2
The catalytic oxidation of benzyl alcohol under solvent-free 50 disproportionation of benzyl alcohol. In the present work, we
o
conditions has been investigated at 140 C with 1 bar oxygen
using a glass reactor and the results are shown in Figure 1. The
monometallic Pd/MgAl-MMO catalysts exhibited very high
initial activity, but the selectivity to benzaldehyde decreased
quantify these two reactions separately and try to understand the
effect of the nature of the catalyst on these two reactions. The
separation and quantification of these two reactions has been
performed using the following underlying premise. Benzaldehyde
2
2
3
3
0
5
0
5
significantly with the time on line. In the case of 1% Pd/MgAl- 55 is formed from the reactions of both oxidation and
MMO, the selectivity to benzaldehyde decreased to <80% after
240 min. As for the Au/MgAl-MMO catalysts, the benzyl alcohol
conversions were much lower and the Au loading had little
influence on the catalytic activity, indicating the pure Au catalyst
disproportionation, therefore it cannot be used to quantify either
reaction. However, toluene is formed exclusively from
disproportionation; therefore the amount of toluene can be used
as a measure of the extent of the disproportionation reaction,
is less active for this reaction. In contrast, the bimetallic 60 since in this reaction equimolar amounts of toluene and
AuPd/MgAl-MMO catalyst was both active and selective for this
reaction. The conversion of benzyl alcohol reached ca. 93% after
benzaldehyde are produced. The turnover numbers for the
oxidation reaction (TON ), disproportionation reaction (TON )
O
D
2
9
40 min and the selectivity towards benzaldehyde maintained ca.
3%. Furthermore, when expressed in terms of the conversion (F-
and the overall reaction (TONTot) were calculated using the
following equations and the results are plotted in Figure 3.
ig. 2) it is possible to compare the selectivities towards
benzaldehyde at iso-conversion (since in oxidation reactions
selectivity can be a function of conversion). It is apparent that the
bimetallic Au-Pd/MgAl-MMO catalyst gave much higher
benzaldehyde selectivity than both pure Pd and Au catalyst at iso-
conversion which illustrates a very pronounced synergistic effect
between Au and Pd.
65
2*moltol
TOND
TONO
TONTot



molmetal
molPhCHO  moltol
molmetal
molPhCH2OH converted
molmetal
4
0
7
0
Figure 3 Reaction time profiles of the turnover numbers over
Figure 1 Catalytic performance of MgAl-MMO supported 1%
MgAl-MMO supported 1% Au-Pd, 1% Pd, and 1% Au
Au-Pd, 1% and 0.65% Au, 1% and 0.35% Pd and MgO supported
1
% Au-Pd
It is apparent that the pure Au catalyst was almost inactive for
the disproportionation reaction and the TONTot was very close to

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DOI: 10.1039/C3DT51855H
the TON
o
, which indicates that the oxidation reaction was the
analysis for the deactivated catalyst, indicating that Au leaching is
not the sole reason for the activity loss. The deactivation could
possibly be due to the aggregation and poisoning of Au
nanoparticles. A slight deactivation was also observed in the pure
only active reaction over the pure Au catalyst. However, a small
increase of TON
while the TON
D
was observed over the Pd catalyst after 30 min,
decreased slightly. This illustrates that the
O
5
0
5
0
5
0
oxidation reaction was suppressed after 30 min and a very slow 50 Pd catalyst; however, Pd leaching was observed to be negligible.
disproportionation reaction was observed exclusively with the
pure Pd catalyst, however, the reaction was largely inhibited after
This indicates that agglomeration and poisoning could also be the
main reasons for the deactivation of Pd catalyst. In contrast, the
bimetallic Au-Pd catalyst was found to be fully reusable and no
metal leaching was observed. In summary, the bimetallic Au-Pd
3
0 min. After alloying Au with Pd, the TON remained at a low
D
level, while the TONTot increased to 13000 after 240 min which
1
1
2
2
3
was evidently higher than that of the pure Pd (8610) and Au 55 catalyst possessed not only higher activity and benzaldehyde
catalyst (7445). These results further confirm the positive
synergistic effect of alloying gold and palladium for the selective
oxidation of benzyl alcohol.
selectivity but also preferable stability during the solvent-free
oxidation of benzyl alcohol.
Besides toluene derived from disproportionation reaction, there
are still a number of by-products that can be formed by over-
oxidation of benzyl alcohol eg. benzoic acid, benzyl benzoate,
3
9
benzene, dibenzylether and dibenzyl acetal. Figure 4 shows the
selectivity towards major by-products formed by over-oxidation
of benzyl alcohol as a function of benzyl alcohol conversion for
the Au, Pd and bimetallic Au-Pd catalysts. In the case of Pd
catalyst, the selectivity to benzene, benzoic acid and benzyl
benzoate increased significantly with the benzyl alcohol
conversion increasing. For the pure Au catalyst, although the
selectivity to benzene and benzoic acid maintained in a low level
(<1%), the selectivity towards benzyl benzoate dramatically
increased to 13% when the benzyl alcohol conversion reached
Figure 5 Reusability of MgAl-MMO supported 1% Au-Pd, 1%
Pd, and 1% Au
4
0%. It is possible that benzaldehyde reacts with benzyl alcohol
6
6
7
7
0
5
0
5
1
8
to form a hemiacetyl and this is oxidised to benzyl benzoate.
However, the selectivities to all the three by-products over Au-Pd
catalyst were much lower, which demonstrates that alloying Pd
with Au could effectively suppress the over-oxidation of benzyl
alcohol.
The effect of the Au/Pd molar ratio on activity and selectivity
were then investigated and the results are shown in Figure 6. An
increase in catalytic activity was observed on increasing the Pd
content. This increase of activity reached a maximum with an
Au/Pd molar ratio of 1:2. A further increase in the Pd content
resulted in a decrease in the catalytic activity. It is important to
note that even in the presence of a minor amount of gold or
palladium (i.e., in the case of 1:5 and 5:1 Au/Pd molar ratios) an
increase in the catalytic activity was observed with respect to the
monometallic Au and Pd catalysts. However, by raising the molar
fraction of Pd above 0.5, the selectivity to benzaldehyde dropped
to 86% for the Au/Pd 1:2 catalyst. Therefore, the optimum
selective oxidation of benzyl alcohol to benzaldehyde occurs for
the Au/Pd 1:1 molar ratio catalyst with the yield of 87%.
Figure 4 Selectivity to benzene, benzoic acid and benzyl
benzoate as a function of benzyl alcohol conversion over MgAl-
MMO supported 1% Au-Pd, 1% Pd, and 1% Au
3
4
4
5
0
5
After reaction with benzyl alcohol for 120 min, the catalysts
were separated from the reaction mixture by centrifugation,
thoroughly washed with acetone, and then reused in the next run
under the same conditions. The reusability of the catalysts was
investigated for three successive reactions and the results are
shown in Figure 5. There was a considerable decrease in catalytic
activity over the monometallic Au catalyst during these recycling
tests. However, only 6.5% Au leaching is confirmed by ICP
Figure 6 The effect of Au/Pd molar ratio on the benzyl alcohol
conversion (squares), selectivity to benzaldehyde (circles) and
yield (Reaction time: 240 min)

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DOI: 10.1039/C3DT51855H
Characterizations of MgAl-MMO supported Au, Pd and Au-
Pd catalysts
To investigate the structure of the prepared Au, Pd and Au-Pd
nanoparticles, we have initially characterized the colloidal sols
with UV-vis spectroscopy and XRD. Figure 7 shows UV-vis
spectra of Au, Pd, and Au-Pd bimetallic nanoparticles. The
appearance of the plasmon resonance with very weak intensity
was observed at about 518 nm which is characteristic for gold
5
0
5
4
0
nanoparticles with particle sizes below 10 nm. However, as
expected there is no surface plasmon band in the Pd sol. The
disappearance of the gold surface plasmon band in the spectrum
of the bimetallic Au-Pd sol can be attributed to changes in the
band structure of the Au nanoparticles due to alloying with Pd.
These results suggest the formation of bimetallic nanoalloys,
1
4
1
1
which is in agreement with our previously reported data.
Figure 7 UV-vis spectra of Au, Pd, and bimetallic Au-Pd
nanoparticles
2
0
The XRD patterns of Au, Pd, and bimetallic Au-Pd
nanoparticles, shown in Figure 8, exhibit two diffraction peaks in
the range of 30 ° < 2θ < 70 ° which can be indexed to the (111)
4
2
and (200) of the fcc structure of monometallic Au and Pd. An
obvious shift from pure Au to Pd was observed in the diffraction
peaks of the bimetallic Au-Pd particles, indicating the formation
of Au-Pd alloys. The d111 and d200 of Pd (0.225 nm and 0.195 nm),
Au (0.235 nm and 0.205 nm) and Au-Pd (0.230 nm and 0.200 nm)
were calculated according to Bragg formula.
2
5
Figure 9 HRTEM images of 1% Au/MgAl-MMO (a), 1%
Pd/MgAl-MMO (b), 1% Au-Pd/MgAl-MMO (c) and 1% Au-
Pd/MgO (d)
3
4
4
5
5
0
5
0
HRTEM images of the MgAl-MMO supported Au, Pd and
bimetallic Au–Pd (Fig. 9 a~c) show that the nanoparticles in all
the catalysts are evenly distributed on the surface of MgAl-MMO.
The corresponding particle size distributions by measuring more
than 200 particles from different regions are also shown. The Au
particles cover a wider size distribution and the average size (3.0
nm) is slightly higher than that of Pd (2.8 nm) and Au-Pd (2.7
nm), but these are very similar and there is no visible difference
in the morphology of metal nanoparticles. Most of the
nanoparticles were cub-octahedra composed of (111) and (100)
facets, with the remainder being multiply twinned structures, such
as decahedra and icosahedra composed of only (111) facets. The
lattice structure of the bimetallic Au-Pd can be observed with
3
0
Figure 8 XRD patterns of Au, Pd and bimetallic AuPd
nanoparticles
0
.230 nm and 0.200 nm of the lattice spacing measured for the
adjacent (111) and (100) facet, which are in between the values of
Pd (0.225 nm and 0.195 nm) and Au (0.235 nm and 2.04 nm) and

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DOI: 10.1039/C3DT51855H
consistent with the XRD results and the values reported in the
2
1,40,43
references.
This change in the lattice constant may lead to
both geometric and electronic effects, which in turn might
contribute to the interesting chemical properties and enhanced
catalytic behaviour we observe. Moreover, no obvious phase
contrasts were observed suggesting the absence of core-shell
structures, nor were structural incoherencies apparent (assignable
to separate monometallic particles) in the bimetallic Au-Pd
catalyst, suggesting the absence of segregated monometallic
nanoparticles.
5
0
5
0
5
0
5
0
1
1
2
2
3
3
4
IR spectroscopy of CO chemisorption is an excellent method to
probe the geometric and electronic structures of bimetallic
4
4,45
surfaces.
Fig. 10 compares IR spectra of CO adsorbed on
MgAl-MMO supported Pd, Au and AuPd. It is known that CO
adsorbs on Pd in two modes, namely, linear and multi-
coordinated; while, CO adsorption onto Au surfaces is weak, and
no adsorption band of CO can be generally observed at room
temperature. In comparison with the monometallic Pd catalyst,
the overall spectral intensity of the bimetallic Au-Pd catalyst
decreases significantly. This is because the number of Pd active
sites for CO adsorption was reduced due to the alloying of Pd
with Au. Moreover, a striking feature in Fig. 10 is that the
spectral intensity of a sharp band observed at wavenumbers <
Figure 11 Au 4f XPS patterns of the pure Au/MgAl-MMO
catalyst and bimetallic Au-Pd/MgAl-MMO catalyst
5
5
6
6
7
7
0
5
0
5
0
5
To explore the possibility of electronic interactions between
Au and Pd, XPS measurements were performed and the results
are shown in Figure 11. Analysis of the Pd is not possible with
these low loaded samples as the Pd peaks are obscured by the Mg
Auger bands. Therefore, we present only results for the Au 4f
signal, although this itself is complicated slightly due to the
overlap of the Au 4f5/2 peak with the Mg 2s photoelectron line.
To account for this, we apply a fixed spin orbit splitting of 3.67
eV for the Au 4f doublet, with an area ratio of 4:3 and assume the
FWHM of both peaks are equal.
-1
46,47
2
000 cm , assigned to the multi-coordinated CO species,
decreases markedly with the addition of Au. The area ratio of
linear-bound to multi-bound bands, A /A , increased from 0.1 to
.16, indicating that the addition of Au effectively blocked the
l
m
0
sites consisting of two or three adjacent Pd atoms. This trend is in
agreement with the literature report that the gold component can
separate the contiguous Pd sites in the Au−Pd alloy catalysts until
For the Au/MgAl-MMO catalyst, two Au signals are obtained
from the results of a perfect fit, at binding energies of 83.1 and
1
8,47,48
the surface Pd atoms are isolated completely by Au.
8
5.0 eV. The binding energy (BE) of the latter species is higher
According to the presented catalytic data, the single Pd sites,
isolated by Au atoms, are suggested to facilitate the coupling of
surface species to products while inhibiting the formation of
undesirable by-products. This ensemble effect of Au-Pd alloy has
also been considered as the origin of unusual reactivity in the
reaction of acetylene coupling, selective oxidation of glycerol,
52
than may be attributed to final state effects alone and therefore
this shift is due to a dominance of initial-state effects for a
cationic Au species and which is likely to be stabilised from
+
53
0
bonding to f-centres. The lower BE state is attributed to Au ,
the low BE a consequence of charge transfer from support to
54,55
nanoparticle.
For the bimetallic Au-Pd catalyst, again two
4
2, 49-51
2 2 2
N O decomposition as well as the direct synthesis of H O .
gold species are determined from the fitting analysis. However,
Moreover, it can be seen that the bridged CO on Pd atoms is
slightly blue-shifted with the addition of Au, implying there is
charge transfer from Pd to Au due to the large electronegativity of
+
whilst the Au species remains at ca. 85 eV, a distinct shift (0.5
eV) to lower BE for the lower BE Au species was observed. Such
shifts suggests that there is charge transfer from Pd to Au as a
result of alloy formation and consistent with the results from
HRTEM and CO-IR. These synergistic electronic effects in
which Au atoms draw electron density away from Pd atoms
enhanced the interaction of Pd atoms with the substrate, and
4
3
Au.
5
6
thereby increased the catalytic activity.
Support effect of the Au-Pd catalysts
8
0
An understanding of the role of the support may also lead to
significant improvements in supported metal catalysts for alcohol
oxidation. For this purpose, Au-Pd nanoparticles were
immobilized on the commercial MgO as a comparable sample for
the Au-Pd/MgAl-MMO catalyst. As shown in Figure 1 and 2, in
comparison of MgAl-MMO supported Au-Pd catalyst, the Au-
Pd/MgO catalyst exhibited similar selectivity to benzaldehyde,
but with much lower benzyl alcohol conversion. After 240 min
reaction, the conversion of benzyl alcohol reached only ca. 46%.
This confirmed the support is of importance in respect to the
8
5
Figure 10 Transmission FTIR spectra of CO adsorption on
monometallic Pd/MgAl-MMO, Au/MgAl-MMO and bimetallic
Au–Pd/MgAl-MMO catalyst
4
5

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DOI: 10.1039/C3DT51855H
origin of the catalyst activity in benzyl alcohol oxidation as we
HRTEM images and the corresponding size distribution of Au–
Pd nanoparticles supported on MgO are shown in Fig. 9 (d). In
contrast to the Au-Pd/MgAl-MMO catalyst, the Au-Pd/MgO
1
3,57
have noted previously.
However, both of the pristine MgAl-
MMO and MgO support are almost inactive in our case. Hence,
the benefit of utilizing the MgAl layed mixed oxide as a support 50 catalyst exhibited a boarder size distribution and slightly larger
5
0
5
0
5
can be attributed to a cooperative interaction between AuPd
nanoparticles and the support surface. Figure 12 shows the XRD
patterns of the supports and the corresponding catalysts. As
expected, the reflections characteristic of a poorly crystalline
mean size of Au-Pd particles (3.1 nm). Most of smaller particles
in the Au-Pd/MgO catalyst are cub-octahedral, which are
consistent with those in the Au-Pd/MgAl-MMO catalyst, while
the larger particles have an irregular shape. From the high
MgO-like phase was observed in the prepared MgAl-MMO 55 magnified image, we can observe that these irregular particles are
1
1
2
2
support which is similar to that reported in the literature for
generally composed of two or three Au-Pd alloy cubooctahedra.
The difference in the size distribution of Au-Pd supported on
MgAl-MMO and MgO can be ascribed to the distinction of their
surface area. Typically, the fraction of coordinatively unsaturated
5
8
MgAl-MMO.
However, after immobilisation of metallic
nanoparticles, the original reflections of the MgAl-MMO have
been lost and replaced by the characteristic peaks of an MgAl-
LDHs phase. This is due to treatment with the solvent during the 60 atoms which play dominant roles in the oxidation of benzyl
1
9, 30
immobilization step LDH layers spontaneously reconstruct their
alcohol increases with decreasing metal nanoparticle size.
5
9
original form. During the process of sol immobilisation, the
MgAl-MMO support was dispersed in the aqueous solution for 1
h, reconstruction of the LDH structure was therefore observed as
However, this subtle difference in the size of Au-Pd particles is
not considered to be the main reason for the disparity in the
catalytic activity observed for Au-Pd/MgAl-MMO and Au-
shown by the reappearance of the characteristic (003), (006), (009) 65 Pd/MgO. To investigate the difference between the catalysts, the
6
0
and (110) reflections. In the case of Au-Pd/MgO catalyst, both
the characteristic reflections of MgO and Mg(OH) phase were
observed, indicating a large amount of MgO converted to
acidity and basicity of the support were determined and the
results are shown in Figure 13. The CO -TPD profile of the
2
2
MgAl-MMO support can be deconvoluted in three main
desorption peaks, reaching maximum desorption rate at ~233,
6
1
Mg(OH)
2
during the immobilisation of Au-Pd. No XRD
reflections for Au-Pd species were found in the XRD patterns 70 318 °C and 495 °C which can be attributed to the weak, moderate
which is due to both small particle sizes and low metal loadings.
5
9
and strong basic sites, respectively. The weak and moderate
basic sites are predominant, representing most contribution of the
o
total evolved CO
2
. The peak below 120 C was assigned to the
2
desorption of the physically adsorbed CO . In the case of MgO
7
8
8
9
9
5
0
5
0
5
support, the profile also contains three main overlapping peaks;
while significant shift to higher temperatures were observed,
illustrating higher basic strengths. The contribution of the strong
basic sits for MgAl-MMO support is much lower than that for
MgO support, indicating the incorporation of alumina could
suppress the formation of strong basic sites. Considering the
relative contributions of the NH desorption peaks (Fig. 13 b), it is
3
apparent that the MgAl-MMO support possesses at least two
acidic sites; however, the MgO support, as expected, does not
possess any acidic sites. The temperature and intensity of the NH
desorption peak indicates that the MgAl-MMO support possesses
relative strong acidity. In conclusion, the NH -TPD and CO -TPD
3
Figure 12 XRD patterns of the pristine MgAl-MMO and MgO
support and the supported Au, Pd and Au-Pd catalysts
3
2
studies reveal that the MgO support has only basic sites, as is to
be expected; however, the MgAl-MMO support possesses both
acidity and basicity. With the present information on surface
acidity, it is not possible to assess the type of acid sites and basic
sites (Bronsted or Lewis), as the TPD cannot distinguish between
the two. However, these data can aid in determining the origins of
the catalytic performance.
Based on the significant support effects in the Au-Pd catalyzed
oxidation of benzyl alcohol we have observed, we suggest that
acid–base properties of the support play pivotal roles in
determining both activity and selectivity. The Au-Pd/MgO
catalyst with stronger basicity but no acidity affords very high
selectivity to benzaldehyde but quite low activity, whereas the
3
3
4
4
0
5
0
5
To gain more insight into differences between the catalysts,
nitrogen adsorption-desorption measurements were performed to
shed light on the pore structure of the support. Due to the phase
transformation of both MgAl-MMO and MgO during the sol-
immobilization process, each support was treated with deionized
water for the subsequent characterisation. The specific surface
2
area of the water treated MgAl-MMO support (96 m /g) is much
2
higher than that of MgO support (28 m /g) and also MgAl-LDH
2
precursor (26 m /g). This may have resulted from the formation
of a large number of shrunk MgAl-LDH crystallites during the
calcination process and the aggregation of the pores in the
5
8
microcrystallites. After complete regeneration of the LDH
structure during the sol-immobilization process, a large number
of pores were preserved. As a consequence of the difference in
specific surface area of MgAl-MMO and MgO, AuPd
nanoparticles are expected to have narrower size distribution and
therefore smaller average size on the support of MgAl-MMO.
1
00 Au-Pd/MgAl-MMO catalyst with both acidity and basicity
exhibited not only high activity but also excellent selectivity.
Therefore, we consider that the acid sites of the support was
response for the improvement of benzyl alcohol conversion;
while, the basic sites gave rise to high selectivity to benzaldehyde.

View Article Online
DOI: 10.1039/C3DT51855H
4
5
5
6
6
5
0
5
0
5
which passes through an abstraction of the proton by the hydroxyl
group on the support, to give an alkoxide intermediate at the
interface and form a water molecule. We speculate that this step
may determine the selectivity, because both Au-Pd/MgAl-MMO
and Au-Pd/MgO catalyst with basic sites showed very high
selectivity to benzaldehyde. However, the strength of basic sites
seems to have little impact on the selectivity. In the second step,
the intermediate undergoes coordination to form a metal-H bond
with the coordinately unsaturated active centre to afford unsteady
metal-alcoholate-LDH species, which undergoes a β-hydride
elimination to give metal-hydride species at the catalyst interface
together with the corresponding carbonyl compound. This is
expected to be the rate-determining step, and the coordinatively
unsaturated metal atoms are more active for the cleavage of this
C-H bond. Hence, the catalyst with smaller Au-Pd particles
exhibited higher activity. The third step will be the rapid
oxidation of the metal hydride by oxygen to form water promoted
by the acid sites of the support while recovering the initial
metallic site, thereby completing the catalytic cycle. The
proposed mechanism provides fundamental reasons for the higher
activity of Au-Pd nanoparticles on acid–base bifunctional support
2 3
Figure 13 CO and NH -TPD of deionized water treated MgAl-
MMO support and MgO support
5
Mechanism of operation of the bimetallic Au-Pd/MgAl-MMO
catalyst
It is generally accepted that the alcohol oxidation over both Pd
and Au nanoparticles involves β-H elimination of a dissociated
alcohol on the metal surface, which is considered to be the rate-
determining step, followed by reaction of oxygen with M–H
(MgAl-MMO) than on basic MgO.
1
1
2
2
3
3
4
0
5
0
5
0
5
0
6
2-64
species.
The process of β-H elimination is also considered to
be the rate-determining step over the bimetallic Au-Pd
nanoparticles in our case. The highly improved TON and
selectivity to benzaldehyde of the bimetallic Au-Pd catalyst can
be attributed to the synergistic effect of Au and Pd. The electronic
effects in which Au atoms draw electron density away from Pd
atoms enhance the interaction of Pd atoms with the substrate, and
thereby increased the catalytic activity; while the geometrical
effect in which Au atoms isolated Pd sites inhibited the pathway
to the undesired by-products.
In addition to the synergistic effect of Au and Pd, we have
demonstrated that the nature of the support also play pivotal roles
in the oxidation of benzyl alcohol. On the one hand, the MgAl-
LDH support exhibit both basicity and acidity. A base which acts
to cleave the O-H bond of the alcohol to form an alkoxide
intermediate is often added as a promoter into the catalytic
Scheme 1 The possible reaction pathway of the solvent free
oxidation for benzyl alcohol over Au-Pd/MgAl-MMO catalyst
7
0
6
5
system for aerobic oxidation of alcohols. Therefore, the basic
sites on the MgAl-LDH support in our case play a key role in the
activation of the O-H bond of the alcohol, to facilitate formation
of an adsorbed alkoxide intermediate. While, the acid sites (i.e.,
Conclusions
We have synthesized a series of MgAl-MMO supported Au, Pd
and Au-Pd catalysts by a sol-immobilization method and
investigated their structural and electronic properties as well as
their catalytic behaviour in the solvent-free oxidation of benzyl
alcohol with molecular oxygen. Alloying Pd with Au results in an
increase of both activity and selectivity to benzaldehyde and,
moreover, an improved resistance to catalyst deactivation
compared with the monometallic Pd and Au catalysts. The TON
over the Au/Pd 1:1 molar ratio catalyst achieved 13000 after 240
min and the selectivity to benzaldehyde was maintained at 93%
throughout and the catalysts are fully reusable. XRD, UV-vis and
HRTEM confirmed the formation of Au-Pd nanoalloys with the
average size of 2.7 nm instead of core-shell bimetallic particles
and segregated monometallic particles. The ensemble and
electronic effect between Pd and Au in the Au-Pd/MgAl-MMO
δ+
AlO–H ) might participate in the reaction of the metal hydride
with oxygen to form water and recover the initial metallic
7
8
8
5
0
5
2
5,30
site.
Simultaneously, AuPd nanoparticles showed a relative
narrower size distribution and thus smaller average size on the
support of MgAl-MMO. The decrease of particle size would
increase the amount of coordinatively unsaturated atoms which
are also beneficial for the improvement of catalytic activity.
Based on our experimental results and the previous
3
0,59
findings,
we propose a possible mechanism with Au-Pd
nanoparticles as active sites (Scheme1), illustrating that the
oxidation of benzyl alcohol proceeds through the cooperation
between the bimetallic Au-Pd nanoparticles and the base-acid
sites on the surface of the MgAl-LDH support. In the first step,
δ-
benzyl alcohol attacks a basic Mg–OH site on the MgAl-LDH,

View Article Online
DOI: 10.1039/C3DT51855H
Hutchings, Science, 2006, 311, 362.
9 L. Kesavan, R. Tiruvalam, M. H. Ab Rahim, M. I. bin Saiman, D. I.
Enache, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor,
catalyst were revealed by IR spectroscopy of CO chemisorption
and XPS analysis. The electronic effects in which Au atoms draw
electron density away from Pd atoms enhanced the interaction of
Pd atoms with the substrate, and thereby increased the catalytic
activity. While, the geometrical effect in which Au atoms isolated
Pd sites inhibited the pathway to the undesired by-products.
In order to investigate the support effect, Au-Pd/MgO catalyst
was prepared for comparison. The promotional effect of the
MgAl-MMO support on the catalytic activity for benzyl alcohol
oxidation suggested that the acid sites of the support was
response for the improvement of catalytic activity; while, the
basic sites gave rise to high selectivity. A possible mechanism
with Au-Pd nanoparticles as active sites was proposed,
illustrating that the oxidation of benzyl alcohol would proceed
1
6
7
7
5
0
5
D. W. Knight, C. J. Kiely and G. J. Hutchings, Science, 2011, 311, 195.
2
0 J. Pritchard, L. Kesavan, M. Piccinini, Q. He, R. Tiruvalam, N.
5
0
5
Dimitratos, J. A. Lopez-Sanchez, A. F. Carley, J. K. Edwards, C. J. Kiely
and G. J. Hutchings, Langmuir, 2010, 26, 16568.
21 N. Dimitratos, A. Villa, D. Wang, F. Porta, D. S. Su and L. Prati, J.
Catal., 2006, 244, 113.
2
2 D. Wang, A. Villa, P. Spontoni, D. S. Su and L. Prati, Chem. Eur. J.
2010, 16, 10007.
1
23 Z. Yin, M. F. Chi, Q. J. Zhu, D. Ma, J. M. Sun and X. H. Bao, J. Mater.
Chem, A, 2013, 1, 9157.
2
4 A. Villa, D. Wang, N. Dimitratos, D. S. Su, V. Trevisan and L. Prati,
Catal. Today, 2010, 150, 8.
5 S. J. Mejía-Rosales, C. Fernández-Navarro, E. Pérez-Tijerina, D. A.
Blom, L. F. Allard and M. José-Yacamán, J. Phys. Chem. C 2007, 111,
2
1
through the cooperation between the Au-Pd nanoalloys and the 80 1256.
2
2
2
4
6 K. Shimizu, K. Sugino, K. Sawabe and A. Satsuma, Chem. Eur. J.,
009, 15, 2341.
7 J. Chen, Q. Zhang, Y. Wang and H. Wan, Adv. Synth. Catal. 2008, 350,
53.
base/acid sites on the surface of the support.
Acknowledgments
8
5
28 A. Villa, C. E. Chan-Thaw, G. M. Veith, K. L. More and D. Ferri, L.
Prati, Chem. Cat. Chem., 2011, 3, 1612.
The work was financially supported by the 973 Project of China
(2011CBA00506), the Support Plan Project of China
2
9 H. Sun, F. Z. Su, J. Ni, Y. Cao, H. Y. He and K. N. Fan, Angew. Chem.
Int. Ed., 2009, 48, 4390.
0 W. Fang, J. Chen, Q. Zhang, W. Deng and Y. Wang, Chem. Eur. J.,
2132032) and China Scholarship Council. We also thank Cardiff 90 2011, 17, 1247.
2
0
(2012BAE06B08), the Natural Science Foundation of Beijing
3
(
University for financial support.
31 D. I. Enache, D. W. Knight and G. J. Hutchings, Catal. Lett., 2005, 103,
4
3
3.
2 M. Sankar, E. Nowicka, R. Tiruvalam, Q. He, S. H. Taylor, C. J. Kiely,
D. Bethell, D. W. Knight and G. J. Hutchings, Chem. Eur. J., 2011, 17,
6524.
2
5
References
9
5
3
3
2
3
3 D. G. Evans, R.C.T. Slade, Struct. Bond., 2006, 119, 1.
4 J. T. Feng, Y. J. Lin, D. G. Evans, X. Duan, D. Q. Li, J. Catal., 2009,
66, 351.
1
2
3
4
S. E. Davis, M. S. Ide and R. J. Davis, Green Chem, 2013, 15, 17.
R. Ferrando, J. Jellinek and R. L. Johnston, Chem. Rev., 2008, 108, 847.
D. S Wang and Y. D. Li, Adv. Mater., 2011, 23, 1044.
5 L. He, Y. Q. Huang, A. Q. Wang, Y Liu, X. Y. Liu, X. W. Chen, J.
M. Sankar, N. Dimitratos, P. J. Miedziak,P. P. Wells, C. J. Kiely and G. J.
1
1
1
1
1
1
1
00 José Delgado, X. D. Wang and T. Zhang, J. Catal., 2013, 298, 1.
6 B. M. Choudary, S. Madhi, N. S. Chowdari, M. L. Kantam and B.
Sreedhar, J. Am. Chem. Soc., 2002, 124, 14127.
7 X. D. Lei, F. Z. Zhang, L. Yang, X. X. Guo, Y. Y. Tian, S. S. Fu, F.
3
3
4
4
5
5
6
0
5
0
5
0
5
0
Hutchings, Chem. Soc. Rev., 2012, 41, 8099.
M. Contea, A. F. Carleya, G. Attarda, A. A. Herzing, C. J. Kiely and G. J.
Hutchings, J. Cata., 2008, 257, 190.
F. Ye, H. Liu, W. W. Hu, J. Y. Zhong, Y. Y. Chen, H. B. Cao and J. Yang,
3
5
3
6
Li, D. G. Evans and X. Duan, AIChE J., 2007, 53, 932.
05 38 B. R. Strohmeier, Surf. Sci. Spectra, 1994, 2, 121.
3
F. Carley and G. J. Hutchings, Catal. Today, 2007, 122, 407.
4
4
Dalton Trans., 2012, 41, 2898.
7 G. L. Brett, Q. He, C. Hammond, P. J. Miedziak, N. Dimitratos, M.
Sankar, A. A. Herzing, M. Conte, J. A. Lopez-Sanchez, C. J. Kiely, D. W.
Knight, S. H. Taylor, and G. J. Hutchings, Angew. Chem., 2011, 123,
1
8
9 D. I. Enache, D. Barker, J. K. Edwards, S. H. Taylor, D. W. Knight, A.
0 R. J. Nikhil, G. Latha, and M. Catherine J., Langmuir, 2001, 17, 6783.
1 J. A. Lopez-Sanchez, N. Dimitratos, P. Miedziak, E. Ntainjua, J. K.
0318.
G. ten Brink, I. W. C. E. Arends and R. A. Sheldon, Science, 2000, 287,
10 Edwards, D. Morgan, A. F. Carley, R. Tiruvalam, C. J. Kiely and G. J.
Hutchings, Phys.Chem.Chem.Phys., 2008, 10, 1921.
1636.
9
2
1
A. Abad, P. Concepcin, A. Corma and H. García, Angew. Chem. Int. Ed.,
005, 44, 4066.
0 N. Dimitratos, J. A. Lopez-Sanchez, D. Morgan, A. F. Carley, R.
4
2 Y. W. Lee, N. H. Kim, K. Y. Lee, K. Kwon, M. Kim and S. W. Han, J.
Phys. Chem. C, 2008, 112, 6717.
3 X. Wei, X. F. Yang, A. Q. Wang, L. Li, X. Y. Liu, T. Zhang, C.Y. Mou
15 and J. Li, J. Phys. Chem. C, 2012, 116, 6222.
4
Tiruvalam, C. J. Kiely, D. Bethell and G. J. Hutchings, Phys. Chem. Chem.
Phys., 2009, 11, 5142.
4
5
4
4
4 F. Gao, Y. L.Wang and D. W. Goodman, J. Am. Chem. Soc., 2009, 131,
734.
5 N. Toshima and T. Yonezawa, New J. Chem., 1998, 22, 1179.
6 J. Rebelli, M. Detwiler, S. Ma, C. T. Williams and J. R. Monnier, J.
11 P. J. Miedziak, Q. He, J. K. Edwards, S. H. Taylor, D. W. Knight, B.
Tarbit, C. J. Kiely and G. J. Hutchings, Catal. Today, 2011, 163, 47.
5
1
S. E. Davis, M. S. Ide and R. J. Davis, Green. Chem., 2013, 15, 17.
2 M. Pagliaro, S. Campestrini and R. Ciriminna, Chem. Soc. Rev., 2005,
20 Catal., 2010, 270, 224.
7 H. L. Abbott, A. Aumer, Y. Lei, C. Asokan, R. J. Meyer, M. Sterrer, S.
Shaikhutdinov and H.-J. Freund, J. Phys. Chem. C, 2010, 114, 17099.
34, 837.
3 M. Sankar, E. Nowicka, R. Tiruvalam, Q. He, S. H. Taylor, C. J. Kiely,
D. Bethell, D. W. Knight and G. J. Hutchings, Chem. Eur. J., 2011, 17,
4
1
4
3
8 M. S.vChen, D. Kumar, C. W. Yi and D. W. Goodman, Science, 2005,
10, 291.
6
1
524.
4 A. Abad, P. Concepcin, A. Corma,and H. García, Angew. Chem. Int.
25 49 A. F. Lee, C. J. Baddeley, C. Hardacre, R. M. Ormerod, R. M. Lambert,
G. Schmid and H. J. West, Phys. Chem., 1995, 99, 6096.
5
2
5
Ed.,2005, 44, 4066.
5 F. Z. Su, Y. M. Liu, L. C. Wang, Y. Cao, H. Y. He and K. N. Fan,
Angew. Chem. Int. Ed., 2008, 120, 340.
1
0 D. Wang, A. Villa, F. Porta, L. Prati and D. S. Su, J. Phys. Chem. C,
008, 112, 8617.
1 H. C. Ham, G. S. Hwang, J. Han, S. W. Nam and T. H. Lim, J. Phys.
1
1
6 C. Della Pina, E. Falletta, M. Rossi,J. Catal.., 2008, 260, 384.
7 A. S. K. Hashmi and G. J. Hutchings, Angew. Chem. Int. Ed., 2006, 45,
30 Chem. C, 2009, 113, 12943.
2 M. Baron, O. Bondarchuk, D. Stacchiola, S. K. Shaikhutdinov and H.-
J. Freund, J. Phys. Chem. C, 2009, 113, 6042.
7896.
8 D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F.
Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W. Knight and G. J.
5
1

View Article Online
DOI: 10.1039/C3DT51855H
5
3 R. P. Socha, E. Zackiewicz, N. Spiridis and J. Korecki, Surf. Interface
Anal., 2010, 42 , 536.
4 M. Murdoch, G. I. N. Waterhouse, M. A. Nadeem, J. B. Metson, M. A.
5
Keane, R. F. Howe, J. Llorca and H. Idriss, Nature Chem., 2011, 3, 489.
55 S. Arrii, F. Morfin, A. J. Renouprez and J. L. Rousset, J. Am. Chem.
Soc., 2004, 126, 1199.
5
0
5
0
5
5
5
6 W. Hou, N. A. Dehm and R. W.J. Scott, J. Catal., 2008, 253, 22.
7 M. Sankar, E. Nowicka, P. J. Miedziak, G. L. Brett, R. L. Jenkins, N.
Dimitratos, S. H. Taylor, D. W. Knight, D. Bethell and G. J. Hutchings,
Faraday Discuss., 2010, 145 , 341.
1
1
2
2
5
8 X. D. Lei, W. Lu, Q. Peng, H. Y. Li, T. Chen, S. L. Xu and F. Z. Zhang,
Appl. Catal. A: Gen., 2011, 399, 87.
5
6
9 T. Chen, F. Z. Zhang and Y. Zhu, Catal. Lett., 2013, 143, 206.
0 J. T. Feng, X. Y. Ma, Y. F. He, D. G. Evans and D. Q. Li, Appl. Catal.
A: Gen., 2012, 10-20, 413.
1 R. Giorgi, C. Bozzi, L. Dei, C. Gabbiani, B. W. Ninham and P.
Baglioni, Langmuir, 2005, 21, 8495.
2 M. Conte, H. Miyamura, S. Kobayashi and V. Chechik, J. Am. Chem.
6
6
Soc., 2009, 131, 7189.
63 Hi. Tsunoyama, H. Sakurai, Y. Negishi and T. Tsukuda, J. Am. Chem.
Soc., 2005, 127, 9374.
6
6
1
4 A. Abad, A. Corma and H. García, Chem. Eur. J., 2008, 14, 212.
5 T. Ishida, M. Nagaoka and T. Akita, M. Haruta, Chem. Eur. J., 2008,
4, 8456.