Highly Active Gold and Gold–Palladium Catalysts Prepared by Colloidal Methods in the Absence of Polymer Stabilizers

Article abstract of DOI:10.1002/cctc.201700483

Supported gold and gold–palladium nanoparticles were found to be effective catalysts for the selective oxidation of glycerol and benzyl alcohol. The properties and stabilities of catalysts are often sensitive to factors such as the dimensions, shape, and

Full text of DOI:10.1002/cctc.201700483

Accepted Article
Title: Highly Active Gold and Gold-Palladium Catalysts Prepared by
Colloidal Methods in the Absence of Polymer Stabilizers
Authors: Laura Abis, Simon Freakley, Georgios Dodekatos, David
Morgan, Meenakshisundaram Sankar, Nikolaos Dimitratos,
Qian He, Christopher Kiely, and Graham John Hutchings
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ChemCatChem
10.1002/cctc.201700483
COMMUNICATION
Highly Active Gold and Gold-Palladium Catalysts Prepared by
Colloidal Methods in the Absence of Polymer Stabilizers
[
a]
[a]
[b]
[a]
Laura Abis , Simon J. Freakley , Georgios Dodekatos , David J. Morgan , Meenakshisundaram
Sankar , Nikolaos Dimitratos , Qian He , Christopher J. Kiely[a,c], Graham J. Hutchings[a]*
[
a]
[a]
[a]
polymers.17 The stabilization effect is provided by anionic (BR
species (where R = H or OH) that originate from the hydrolysis
of the reducing agent (NaBH ) present in the solution, in addition
)-
Abstract: Supported gold and gold-palladium nanoparticles are
effective catalysts for the selective oxidation of glycerol and benzyl
alcohol. Catalytic properties and stability are often sensitive to
factors such as the dimensions, shape and composition of the metal
nanoparticles. Although colloidal methods provide an easy and quick
way to synthesize supported metal catalysts, they typically involve
using polymers such as polyvinyl alcohol (PVA) or
polyvinylpyrrolidone (PVP) as steric stabilizers, which can
sometimes be detrimental in subsequent catalytic reactions. Here we
report the synthesis of supported gold and gold-palladium
nanoparticles without the addition of stabilizing polymers. The
catalysts prepared with and without the addition of polymers
performed very similarly in the selective oxidation of glycerol and
benzyl alcohol suggesting that polymers are not essential to make
active catalysts for these reactions. Thus, this new stabilizer free
method provides a facile and highly effective way of circumventing
the inherent problems of polymer stabilizers when preparing gold
and gold palladium catalysts.
4
4
to chloride anions from the metal precursors. These anions
when adsorbed onto the Au surface provide a degree of
18
electrostatic stabilization to the nanoparticles. The resultant
colloidal gold sol was reported to be an active homogeneous
catalyst for the reduction of 4-nitrophenol to 4-nitroaniline.
However, the main issue of synthesising and utilising metal
colloids without stabilizers is their long-term stability and
resistance to sintering. Herein we describe the synthesis of TiO
2
supported Au and Au-Pd nanoparticles by a variant of the sol-
immobilisation method previously reported in the literature, in
which polymers were not added to act as steric stabilizers. The
catalysts were investigated for the selective oxidation of glycerol
and benzyl alcohol, as they have been previously shown to be
active for these reactions.1
9-21
The catalytic activities,
selectivities and physical characteristics of the stabilizer free
SF) materials are compared to those of catalysts prepared by
(
conventional methods using PVA and PVP.
Colloidal solutions were prepared by reducing aqueous solutions
of Au or Au-Pd precursors with NaBH either in the presence of
4
PVA, PVP or under stabilizer free (SF) conditions. After 30 min
of colloid generation the nanoparticles were immobilised by the
2 2 4
addition of TiO and acidification with H SO before being filtered,
Supported gold and gold-palladium nanoparticles are
exceptionally effective catalysts for selective oxidation.1-5
Structural properties of these catalysts (i.e., particle size,
composition, oxidation state) have a significant influence on
catalytic activity, selectivity and stability.6-10 Therefore, an
important objective is to control these parameters in order to
synthesise effective catalysts. The generation of colloidal metal
washed and dried. Glycerol oxidation was studied in the
presence of base for the monometallic stabilizer free (SF) 1% wt
Au/TiO
2
material and the counterpart catalysts that use either
10-
sols typically involves polymer additives as particle stabilizers.
PVA or PVP as a polymer stabilizer.
1
3
It is well recognised that ligand stabilization of the colloidal
metal sol is a crucial step in order to produce small metal
nanoparticles with a narrow particle size distribution.11 In many
examples, the presence of polymer ligands such as
polyvinylpyrrolydone (PVP) or polyvinylalcohol (PVA) are
necessary to prevent agglomeration of particles, and hence an
overall increase in mean particle size, during the preparation
1
00
100
90
80
70
60
50
40
30
20
10
0
90
8
0
1
4
70
60
procedure. However, the presence of these polymer ligands
can lead to several disadvantages in catalytic applications, since
they can partially or completely block the access of reactant
molecules to the metal surface, thereby decreasing the catalyst
5
0
15
40
30
activity. Refluxing the catalysts in water to remove water
soluble polymers has been shown to be an effective way to
increase catalytic activity for gas phase reactions, while
calcination treatments to combust the protective polymer ligands
can result in an increase in the mean particle size and carbon
2
0
0
0
1
1
0, 16
deposition.
It has recently been reported by Deradet et al.
that it is possible to prepare dilute stable colloidal solutions of Au
with a mean particle diameter of 3.2 nm without the addition of
30
60
90
120
150
180
210
240
Time / min

PVP▲PVA SF
[
a]
L. Abis, Dr. S. J. Freakley, Dr. D. J. Morgan, Dr. M. Sankar, Dr. N.
Dimitratos, Dr. Q. He, Prof. C. J. Kiely, Prof. Gr. J. Hutchings
Cardiff Catalysis Institute, Cardiff University, Main Building, Park
Place, Cardiff, CF103AT, UK
E-mail: hutch@cf.ac.uk
G. Dodekatos
Figure 1. Comparison of the glycerol oxidation activity for sol-immobilised
monometallic 1% Au / TiO catalysts prepared either stabilizer free (SF), or in
the presence of PVA or PVP ligands. Key: Bold lines - glycerol (GLY)
2
conversion; dotted lines - tartronate (TA) selectivity; dashed lines - glycerate
(GA) selectivity. Reaction conditions: 110 mg catalyst, 60 °C, 3 bar O2, 1200
[
[
b]
c]
rpm, GLY/Au=500:1(mol), NaOH/GLY=2:1 (mol), total volume 10 mL.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1 D-
45470 Mülheim an der Ruhr, Germany
Department of Materials Science and Engineering, Lehigh
University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA
Figure 1 shows that complete glycerol conversion was achieved
after 4 h for all the three variants (PVA, PVP and SF) of
monometallic Au/TiO catalysts with similar conversion versus
2
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201700483
COMMUNICATION
time-on-line profiles. The main products obtained were glycerate
preparation of Au-Pd catalysts without the addition of stabilizing
polymers ligands results in materials with similar catalytic
performance to those prepared in the presence of PVA or PVP
for the selective oxidation of glycerol. To further confirm the
similarity of the PVA and SF variants of the bimetallic materials,
the catalytic activity for benzyl alcohol oxidation was also
investigated. Figure 3 demonstrates that the PVA and SF
variants of the Au-Pd catalysts had very similar reaction profiles
in terms of benzyl alcohol conversion and selectivity towards
benzaldehyde and toluene. Once again, there was no
substantial difference in the catalytic activity and selectivity
performance for the two catalysts implying that the materials are
similar irrespective of the presence or absence of the polymer
stabilizer during the initial colloid preparation.
(
GA) and tartronate (TA) with a combined selectivity typically
above 70%. The remaining 30% comprised minor products such
as oxalate, lactate, glycolate, formate and acetate. Overall, the
product distribution was broadly similar for all three catalysts.
During the course of the reaction, the selectivity towards GA and
TA remained constant for all three catalysts tested, indicating
that they were relatively stable to further oxidation under the
reaction conditions employed in the presence of glycerol. This
also suggests that GA and TA are both produced as primary
oxidation products and that TA is not predominantly generated
via a consecutive oxidation pathway. This hypothesis was
confirmed by performing experiments under identical conditions,
2
but with GA as the starting material, using the Au/TiO catalysts
prepared with PVA and SF (Figure S1). These experiments
show that GA oxidation was three-to-four times slower than
glycerol oxidation, indicating that the relative stability of GA and
TA in the glycerol oxidation reaction is due either to lower
relative reaction rates or competitive adsorption of glycerol and
GA. A comparison of the activity for glycerol oxidation of the
monometallic catalysts synthesized with and without polymeric
stabilizing ligands showed no significant difference in conversion,
implying that polymer stabilizers are not necessary to produce
highly active catalysts. Comparing the selectivity towards the
major products, a 10-15% difference in the selectivity towards
TA and GA was observed between the SF and the PVA/PVP
stabilized catalysts. The selectivity at iso-conversion throughout
the reaction towards TA followed the order SF > PVA > PVP,
while the opposite trend was observed for selectivity towards GA.
This suggests that the nature and presence of the polymer can
alter reaction selectivity through either particle size effects or by
partial ligand coverage of the active surface of the nanoparticles.
100
90
80
70
60
50
40
30
20
100
90
80
70
60
50
40
30
20
10
0
10
0
30
60
90
120
150
180
210
240
Time / min

PVP▲PVA SF
Figure 2. Comparison of the glycerol oxidation activity for sol-immobilised
bimetallic 1% Au-Pd/TiO catalysts samples synthesized either with
2
PVA/PVP stabilizer addition or under stabilizer-free conditions. Key: Bold
lines - glycerol (GLY) conversion; dotted lines - tartronate (TA) selectivity;
dashed lines - glycerate (GA) selectivity. Reaction conditions: 110 mg
catalyst, 60 °C, 3 bar O2, 1200 rpm, GLY/Au=500:1(mol), NaOH/GLY=2:1
Figure 2 presents catalytic results from the comparable set of
2
bimetallic 1% wt Au-Pd/TiO sol-immobilised materials for
(mol), total volume 10 mL.
glycerol oxidation. Again, all three catalyst variants (PVA, PVP
and SF) showed similar reaction profiles in terms of glycerol
conversion indicating that the presence of a polymer during the
preparation of the catalyst is not required to achieve high
catalytic activity, with full conversion being reached after 2 h of
reaction. The trends in selectivity were not as pronounced as
those observed for the corresponding monometallic Au catalysts
at short reaction times. At reaction times exceeding 120 min, GA
selectivity decreased with a concurrent increase in TA selectivity.
It is also worth noting that in this case, further oxidation of GA
could occur when glycerol was no longer available in the
reaction mixture, in contrast to the situation found for
monometallic Au catalysts. Figure S1 also contains results of GA
1
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100
90
80
70
60
50
40
30
20
10
0
9
0
0
8
70
60
5
4
3
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1
0
0
0
0
0
0
2
oxidation with Au-Pd/TiO samples prepared with PVA and SF.
These bimetallic materials showed higher reaction rates towards
GA oxidation than the corresponding monometallic counterparts.
In common with the monometallic Au catalysts, no further
oxidation of GA was observed during glycerol oxidation while
non reacted glycerol is still present in the reaction mixture;
however, in the case of both Au-Pd materials, further oxidation
did occur when 100% glycerol conversion was reached with the
30
60
90
120
150
180
Time / min

PVA  SF
Figure 3 – Comparison of PVA and stabilizer free (SF) 1% Au-Pd / TiO
2
catalysts for the solvent-free oxidation of benzyl alcohol. Key: Bold lines -
benzyl alcohol (BnOH) conversion; dotted lines - benzaldehyde (BnCHO)
selectivity; dashed lines - =toluene (Tol) selectivity. Reaction conditions: 20
Au-Pd (SF)/TiO
than the Au (PVA)/TiO
2
catalyst having a higher GA oxidation activity
variant. These results suggest that the
mg catalyst, BnOH/metals= 28000 (mol), 120 °C, 1 bar O2, 1000 rpm. To
2
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ChemCatChem
10.1002/cctc.201700483
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elucidate any structural differences of the SF materials with
their PVA or PVP variants, electron microscopy studies were
performed on the unused materials. The particle size
size despite lower amounts of surface Au which could
suggest that the smaller particles formed when using PVA are
not as effective in this reaction despite the increase in surface
Au or Pd. The higher overall selectivity towards TA displayed
by the SF-type materials also does not correlate with the
measured mean particle size, suggesting that the stabilizing
ligands in these catalysts do play some role in determining
the selectivity to specific products.
distributions, after immobilisation on the TiO
2
support, as
determined from bright field TEM micrographs of the
monometallic Au catalysts prepared by the SF, PVA and PVP
methods are shown in Figures 4 and S2. Comparisons of the
particle size distributions in Figure 4 show that materials
prepared with polymer addition tend to have a narrower
particle size distribution and lower mean particle size (PVA:
2.7 ± 0.6 nm; PVP: 3.5 ± 1 nm) than the stabilizer free sample
(a)
(b)
(
5.4 ± 1.6 nm). Analysis of the particle size distributions of the
bimetallic Au-Pd catalysts prepared by the SF, PVA and PVP
methods (also shown in Figure 4) indicate a similar trend in
particle size distributions and mean particle size (PVA: 3.2 ± 1
nm; PVP: 3.0 ± 1.2 nm; SF: 3.9 ± 1.3 nm). The structures of
supported nanoparticles in the 1% Au/TiO
2
(Figure 5 (a, b))
and 1% Au-Pd/TiO (Figure 5 (c, d)) catalysts prepared by SF
2
methods were also studied using aberration corrected
scanning transmission electron microscopy (AC-STEM). The
results did not show any significant variations in metal particle
morphology or metal support interaction compared to
catalysts prepared with PVA and PVP polymers as reported
(
c)
(d)
in our previous studies.22 For the bimetallic 1% Au-Pd/TiO
2
SF catalyst, the nanoparticles were found to be alloyed as
expected, as confirmed by X-ray energy dispersive
spectroscopy (X-EDS) (Figure 5(d)).
1
% Au / TiO2
1% AuPd / TiO
2
Figure 5 – Representative high angle annular dark field (HAADF) STEM
images of (a, b) 1% Au/TiO and (c, d) 1% Au-Pd/TiO materials prepared
via the stabilizer-free route. The inset in (d) shows the corresponding X-ray
energy dispersive spectrum (XEDS) obtained from the particle imaged,
indicating that it is a bimetallic alloy. The Si K signal is an artefact arising
from internal fluorescence of the detector.
2
2
PVP
PVP
2
0
0
20
10
3.5±1.0 nm
3.0±1.2 nm
1
0
0
0
PVA
PVA
3
2
1
30
3.2±1.0 nm
2
For the bimetallic 1% Au-Pd/TiO catalysts, the results
2.7±0.6 nm
0
0
0
20
10
0
obtained for glycerol and benzyl alcohol oxidation reactions
demonstrate that stabilizing ligands are not necessary in the
sol-immobilisation synthesis method to produce materials that
achieve high catalytic activity. For glycerol oxidation,
selectivity profiles for the sol-immobilised Au-Pd catalysts
operating at similar conversion levels, while not as
pronounced as the monometallic Au catalysts, do still show
that the SF variant has higher selectivity towards TA, in
addition to an enhanced oxidation of the GA to TA after the
glycerol has been consumed.
SF
SF
2
0
0
0
20
10
0
5
.4±1.6 nm
3.9±1.3 nm
1
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
Particle Size / nm
Particle Size / nm
Figure 4 – Particle size distributions derived from bright field TEM images
of more than 100 monometallic Au or bimetallic Au-Pd particles prepared
either stabilizer free, or with PVA and PVP ligands present.
X-ray photoelectron spectroscopy (XPS) (Table 1) was
carried out on all samples and confirmed the similarity of the
sol-immobilised materials produced with and without the
addition of polymer stabilizers. The binding energy of the Au
Although having a slightly larger mean size and a broader
particle size distribution, the catalysts prepared using the SF
route perform in a very similar manner to their counterparts
prepared with PVA or PVP. This suggests that while the
catalysts prepared with stabilisers have a higher number of
surface Au or Pd atoms the glycerol oxidation reaction rate is
not proportional to this alone. In fact the activity of these
catalysts is determined by the balance between optimum
particle size and the availability of the nanoparticles to carry
out the reaction. Previous studies23 have demonstrated that
catalysts with particle size around 5 nm were in fact more
active than analogous catalysts with smaller mean particle
(
84.2-83.9 eV) indicates the presence of metallic Au in all of
the monometallic samples as expected.24 Analysis of the
bimetallic catalysts indicated a similar Au : Pd surface ratio in
all samples with the Pd being present in the metallic state as
indicated by its binding energy (335.0 - 335.5 eV). To
evaluate the effect of the polymer on the stability of the
catalysts during reaction, both the monometallic and the
bimetallic catalysts were re-used for glycerol oxidation and
the catalytic results are shown in Table 1. Both monometallic
and bimetallic catalysts prepared with PVP were shown to be
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Table 1. Summary of physical characteristics determined by XPS and TEM analysis of monometallic Au and bimetallic Au-Pd sol-immobilised catalysts
supported on TiO prepared either under stabilizer free (SF) conditions, or in the presence of PVA or PVP ligands. Also presented are the corresponding
catalyst re-usability measurements determined from repeated glycerol oxidation testing. The standard error in the glycerol oxidation conversion results is ± 3%.
2
Catalyst
Stabilizer
Identity
Mean Particle
Size (nm)
Au 4f binding
energy.
Pd 3d
binding
energy
Au:Pd
atomic
ratio
Glycerol
Glycerol
Conversion at 60
Conversion at 60
st
nd
(eV)
min 1 use
min 2 use
(eV)
(%)
(%)
1
/
% Au
TiO
SF
5.4
2.7
3.5
3.9
3.0
3.2
84.2
84.2
84.2
84.0
84.0
83.9
-
-
-
-
-
58
52
42
94
95
91
48
45
40
87
89
94
2
PVA
PVP
SF
-
1
% Au-Pd
/
335.0
335.5
335.4
1.5
1.4
1.4
TiO
2
PVA
PVP
Reaction conditions for 1st test: 110 mg catalyst, 60 °C, 3 bar O
conditions for re-usability test: 60 mg catalyst, 60 °C, 3 bar O , 1200 rpm, GLY/Au=500:1(mol), NaOH/GLY=2:1 (mol), total volume 10 mL.
2
, 1200 rpm, GLY/Au=500:1(mol), NaOH/GLY=2:1 (mol), total volume 10 mL. Reaction
2
stable to re-use within experimental error. In contrast, catalysts
prepared with PVA, which is known to be prone to removal from
the metal surface in aqueous solutions at elevated
temperatures , showed a decrease in conversion of around 6%
for both the mono- and bi-metallic catalyst systems. Catalysts
prepared via SF methods also showed a deactivation on re-use
of around 10 % conversion in the monometallic case and 7% in
the bimetallic case. While the degree of deactivation of the SF
materials is greater than that observed for the catalysts prepared
with stabilizers, their absence do not lead to excessive leaching
Acknowledgements
We thank Dr Peter Miedziak for carrying out BF-TEM analysis of
the samples and MaxNet Energy for financial support. CJK
gratefully acknowledges funding from the National Science
Foundation Major Research Instrumentation program (GR#
MRI/DMR-1040229).
1
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2
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removes complications in terms of subsequent dissolution of the
protective polymer into the reaction media and the blocking of
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The results also suggest that the presence of the stabilizing
ligands can also affect the selectivity to certain products in
selective alcohol oxidation reactions while not markedly effecting
the overall conversion levels.
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Catalysis : Au Naturale – We show
that the presence of polymer
stabilisers is not necessary to
produce catalysts that are highly
active for the selective oxidation of
glycerol and benzyl alcohol. Gold
and gold palladium nanoparticles
prepared in the absence of PVA and
PVP stabilisers show identical
activity to those prepared with
stabilisers but crucially the polymer
Laura Abis, Simon J. Freakley,
Georgios Dodekatos, David J. Morgan,
Meenakshisundaram Sankar, Nikolaos
Dimitratos, Qian He, Christopher J.
Kiely, Graham J. Hutchings*
Page No. – Page No.
Highly Active Gold and Gold-
Palladium Catalysts Prepared by
Colloidal Methods in the Absence of
Polymer Stabilizers
Stabiliser Free Catalysts
additives
alter
the
reaction
selectivity.
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