Alcohol oxidations in aqueous solutions using Au, Pd, and bimetallic AuPd nanoparticle catalysts

Article abstract of DOI:10.1016/j.jcat.2007.10.025

Alcohol oxidations under mild conditions using polyvinylpyrrolidone (PVP)-stabilized Au, Pd and bimetallic AuPd nanoparticle catalysts in aqueous solutions have been investigated. The catalytic activities of the nanoparticles towards the oxidation of benzyl alcohol, 1-butanol, 2-butanol, 2-buten-1-ol and 1,4-butanediol indicate that bimetallic 1:3 Au:Pd nanoparticles have higher catalytic activities than Au, Pd and other bimetallic AuPd nanoparticles, and that selectivities towards specific products can often be tuned using bimetallic particles. In addition, advantages and disadvantages for the use of such nanoparticle catalysts as mild, environmentally-friendly oxidation catalysts are examined.

Full text of DOI:10.1016/j.jcat.2007.10.025

Journal of Catalysis 253 (2008) 22–27
www.elsevier.com/locate/jcat
Alcohol oxidations in aqueous solutions using Au, Pd, and bimetallic AuPd
nanoparticle catalysts
1
1
∗
Wenbo Hou , Nicole A. Dehm , Robert W.J. Scott
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada
Received 9 August 2007; revised 26 October 2007; accepted 31 October 2007
Abstract
Alcohol oxidations under mild conditions using polyvinylpyrrolidone (PVP)-stabilized Au, Pd and bimetallic AuPd nanoparticle catalysts in
aqueous solutions have been investigated. The catalytic activities of the nanoparticles towards the oxidation of benzyl alcohol, 1-butanol, 2-butanol,
2-buten-1-ol and 1,4-butanediol indicate that bimetallic 1:3 Au:Pd nanoparticles have higher catalytic activities than Au, Pd and other bimetallic
AuPd nanoparticles, and that selectivities towards specific products can often be tuned using bimetallic particles. In addition, advantages and
disadvantages for the use of such nanoparticle catalysts as mild, environmentally-friendly oxidation catalysts are examined.
©
2007 Elsevier Inc. All rights reserved.
Keywords: Catalysis; Alcohol oxidation; Nanoparticles; Bimetallic; Aqueous; Polymer
1
. Introduction
shown that microgel-stabilized Pd nanoclusters are effective for
the selective oxidation of secondary alcohols, while Abad et al.
[6,7] have noted that ceria-supported Au nanoparticle catalysts
are most suitable for the aerobic oxidation of allylic alcohols
and can effectively prevent the isomerization and hydrogena-
tion of C=C double bonds. Therefore there is still significant
demand for catalyst systems which can activate a wide range
of substrates and can be used under mild, aqueous conditions
[15,16,18].
Herein, we show that polyvinylpyrrolidone (PVP)-stabilized
bimetallic AuPd nanoparticle catalysts in aqueous solutions can
be used as effective catalysts for a wide range of alcohol ox-
idation reactions under mild conditions. This work builds on
the earlier observations by Tsunoyama and coworkers [18] that
PVP-stabilized Au nanoparticles could be successfully used for
the aerobic oxidation of benzylic alcohols in water. The Au
nanoparticles had high catalytic activities, and the PVP stabi-
lizer effectively prevented the agglomeration of nanoparticles
during the course of the catalytic oxidations in aqueous solu-
tion. Several groups have also recently examined supported-
bimetallic PdAu nanoparticles as catalysts with increased ac-
tivity and product selectivity of alcohol oxidation reactions.
Specifically, Enache et al. [10] and Dimitratos et al. [19] have
both observed that supported-AuPd bimetallic catalysts have
Oxidation of alcohols to their respective aldehyde or ketone
is a useful and fundamental organic reaction [1,2]. Tradition-
ally, this oxidation is performed using stoichiometric amounts
of oxidants, such as permanganate [3], chromate [2,4], or bro-
mate [5]. These methods produce a large amount of waste,
and are unacceptable in view of green chemical practices. Re-
cently, transition metal nanoparticle-catalyzed aerobic alcohol
oxidations have been investigated, and many have shown high
catalytic activities and good selectivities [6–17]. However, de-
spite this progress, there are still persistent problems in this
field. First, many oxidation reactions are conducted in organic
solvents or solvent-less conditions [6,7,10,12,13], and the re-
sulting mixtures of the organic substrates, products, solvents
and molecular oxygen can be quite dangerous. Also, some oxi-
dation reactions have to be performed under severe conditions,
such as high temperature and high oxygen pressure [6,7,14]. Fi-
nally, a number of catalysts have only been shown to be active
for specific types of alcohols. For example, Biffis et al. [8] have
*
Corresponding author. Fax: +1 306 966 4730.
E-mail address: robert.scott@usask.ca (R.W.J. Scott).
Both authors contributed equally to this work.
1
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.10.025

W. Hou et al. / Journal of Catalysis 253 (2008) 22–27
23
improved activity and selectivity of desired products for aer-
obic oxidations of alcohols, presumably due to synergetic elec-
tronic interactions between Au and Pd atoms in the individ-
ual nanoparticles [20]. Thus we wished to investigate whether
polymer-stabilized, solution-phase bimetallic AuPd nanopar-
ticles would also show significantly enhanced activities and
desirable product selectivities for aerobic alcohol oxidations.
Herein we report the synthesis of PVP-stabilized Au, Pd and
AuPd bimetallic nanoparticle catalysts, and show that the re-
sulting nanoparticles can catalyze the oxidation of benzyl al-
cohol, 1-butanol, 2-butanol, 2-buten-1-ol and 1,4-butanediol in
aqueous solutions under an oxygen atmosphere.
with other alcohols. For these studies, both the nanoparticle cat-
alyst and substrate amounts were increased by a factor of five,
which necessitated a doubling of the PVP stabilizer in order to
keep the particles stable during the oxidation of benzyl alcohol
and 1-butanol.
NMR, GCMS and GC samples were prepared as follows:
1 ml of the solution was taken out from the oxidation reaction
system and placed in a vial and 500 µl of 1.0 M hydrochlo-
−
4
ric acid (5 × 10 mol) was added. Then 1 ml of CDCl3 was
added, and the vial was shaken to transfer the substrates and
products into the organic phase. The CDCl3 was then extracted
1
and characterized by H NMR and/or GCMS, GC. Turnover
frequencies (TOFs), conversions and selectivities were deter-
mined from NMR. Turnover frequencies (TOFs) were deter-
mined from the slope of linear plots of turnover (mol product
(aldehyde + acid + benzoate)/mol Pd + Au) vs time for initial
studies and calculated using the molar ratio of the converted
substrate over the catalyst divided by the reaction time for sub-
sequent studies. Turnover numbers generated from NMR re-
sults were verified by GC or GCMS for several substrates; in
all cases, both methods gave comparable results. Control ex-
periments in which no nanoparticles were present showed no
background alcohol oxidation was occurring over 24 h.
2
. Experimental and methods
2
.1. Materials
Poly(N-vinyl-2-pyrrolidone) (M.W. 40,000), hydrogen tetra-
chloroaurate hydrate (99.9%), potassium tetrachloropalladate
(
1
9
99.99%), benzyl alcohol (99+%, ACS), 2-butanol (99.9%),
,4-butanediol (99.9%) and 2-buten-1-ol (predominantly trans,
7%) were purchased from Alfa and were used without further
purification. Sodium borohydride powder (98%) was obtained
from Aldrich and was used as obtained. K2CO3 (anhydrous)
was obtained from EMD Chemicals Inc. and was used without
further purification. n-Butanol (ACS) was obtained from EM
Science and was used without further purification. Deuterated
solvents were purchased from Cambridge Isotope Laboratories.
2.4. Characterization
UV–vis spectra were obtained using a Varian Cary 50 Bio
UV–Visible spectrophotometer with a scan range of 300–
1
1
8 Mꢀ cm Milli-Q water (Millipore, Bedford, MA) was used
900 nm with an optical path length of 1.0 cm. H NMR
throughout.
spectra were obtained using a Bruker 500 MHz Avance NMR
spectrometer; chemical shifts were referenced to the residual
protons of the deuterated solvent. Transmission electron micro-
graphs (TEM) were obtained with a Philips 410 microscope
operating at 100 keV. To prepare samples for TEM, a drop
of the solution containing the nanoparticles was placed on a
carbon-coated Cu grid which had been pretreated by plasma
discharge. Gas chromatography data was obtained using a HP
4890D gas chromatograph and a HP 3393A Integrator using
pure acetophenone as an internal standard. The identity of prod-
ucts was confirmed by GC-MS (GC EI + Magnet VG 70SE) in
subsequent studies.
2
.2. Preparation of PVP stabilized 1:3 Au:Pd nanoparticles
The following procedure was used to prepare PVP stabi-
lized 1:3 Au:Pd nanoparticles [18]. First, 0.188 ml of a 10 mM
−
6
potassium tetrachloropalladate solution (1.88 × 10 mol) and
0
.062 ml of a 10 mM hydrogen tetrachloroaurate trihydrate
−
6
solution (0.62 × 10 mol) was added to 2 ml of deionized
water with stirring. Next, 0.5 ml of a 1.39 mM PVP solution
−
7
(
3
6.95×10 mol) was added. This mixture was then stirred for
◦
0 min at 0 C. Finally, 0.25 ml of a fresh 0.10 M sodium boro-
−
5
hydride solution (2.5 × 10 mol) was added. After stirring for
an additional 30 min the PVP stabilized 1:3 Au:Pd NPs were di-
luted to 5.0 ml. Au, Pd and other bimetallic Au:Pd nanoparticles
were prepared as stated above keeping the total molar amount
of metal salt constant.
3. Results and discussion
Poly(N-vinyl-2-pyrrolidone) (PVP) stabilized Pd, Au, and
bimetallic AuPd particles were synthesized via NaBH4 reduc-
tion of aqueous solutions of the metal salts in the presence
of the polymer. TEM images of the resulting Pd, Au, and
AuPd nanoparticles stabilized with PVP are shown in Fig. 1.
The average size of the PVP stabilized Au and Pd nanoparti-
cles that were synthesized were measured through TEM was
found to be 3.3 ± 0.8 nm and 3.9 ± 2.8 nm, respectively, while
the bimetallic AuPd nanoparticles had average particle sizes
of 2.7 ± 0.7 nm (Au:Pd = 1:3), 2.7 ± 1.2 nm (Au:Pd = 1:1)
and 4.0 ± 1.7 nm (Au:Pd = 3:1). The PVP stabilized metal
nanoparticles are known to be weakly stabilized through mul-
tiple coordination of the amido sites of the PVP [18,21]. Fig. 2
2
.3. Oxidation reactions
The following procedure was used to oxidize benzyl alco-
hol [18]. First, 5.0 ml of the previously prepared nanoparticles,
−
4
5
1.9 mg of potassium carbonate (3.75 × 10 mol) and 2.5 ml
of deionized water were mixed together with vigorous stirring
at 1080 rpm with a magnetic stir bar. Next, 129 µl of benzyl
−3
alcohol (1.25 × 10 mol) was added (for a substrate:catalyst
ratio of 500:1). For alternative substrates the catalytic reactions
were performed as above but substituting the benzyl alcohol

24
W. Hou et al. / Journal of Catalysis 253 (2008) 22–27
Fig. 1. TEM images of PVP-stabilized (a) Au nanoparticles, (b) Pd nanoparticles, (c) 1:3 Au:Pd nanoparticles, (d) 1:1 Au:Pd nanoparticles, (e) 3:1 Au:Pd nanopar-
ticles.
stabilizers over all Au:Pd bimetallic ratios. Of note is the ab-
sence of any plasmon bands in the bimetallic nanoparticles,
which suggests that all particles are bimetallic with no sepa-
rate formation of pure Au nanoparticles [24].
The catalytic activity of the nanoparticles towards the oxi-
dation of benzyl alcohol was measured over seven hours (see
Section 2), and the formation of benzaldehyde and benzyl ben-
zoate was observed. GC and GCMS results confirmed the pres-
ence of benzyl benzoate and absence of benzylic acid as a
product. Quantitative yields and turnover frequencies of the
two products were determined based on the integration of the
respective peaks of the pure alcohol, aldehyde and benzoate.
For the PVP stabilized bimetallic nanoparticles (Table 1, en-
tries 1–5) the 1:3 Au:Pd nanoparticles showed the highest
turnover frequency for the oxidation of benzyl alcohol in air
at room temperature. Such enhancements of catalytic activity
have previously been seen for many AuPd systems [19,24],
and are thought to be due to synergistic electronic effects in
Fig. 2. UV–vis spectra of PVP-stabilized Au, Pd, and Au:Pd nanoparticles.
which Au atoms draw electron density away from Pd atoms,
thereby enhancing the interaction of Pd atoms with the sub-
shows representative UV–vis spectra of the AuPd bimetallic se-
strate [20]. The selectivity of each oxidation reaction was also
ries after reduction with sodium borohydride. UV–vis spectra
determined, and the results can be seen in Table 1 entries 1–5
of the AuPd nanoparticles show an exponentially increasing
absorbance toward higher energy; this is a consequence of inter-
band transitions of the newly formed bimetallic AuPd nanopar-
ticles [22,23]. In contrast, the PVP-stabilized Au nanoparticles
show a broad plasmon band around 515 nm after reduction,
which is in general agreement with TEM results (3.3± 0.8 nm).
These results show that as-synthesized nanoparticles with sim-
ilar particle sizes (2.5–4.0 nm) can be stabilized with the PVP
for each different type of nanoparticle. When pure Au nanopar-
ticles were used for the oxidation of benzyl alcohol, nearly
equal amounts of benzaldehyde and benzyl benzoate were pro-
duced, in agreement with previous work by Tsunoyama et
al. [18]. However, when Pd and AuPd bimetallic nanoparti-
cles were used for the oxidation, benzaldehyde was produced
preferentially with nearly 100% selectivity. Enache and co-
workers [10] previously investigated the oxidation of benzyl

W. Hou et al. / Journal of Catalysis 253 (2008) 22–27
25
Table 1
Turnover frequencies and selectivities for benzyl alcohol oxidation with PVP
stabilized Au, Pd and Au:Pd bimetallic nanoparticles
Entry
Type of
NP
Conditions
Turnover
frequency
Selectivity
Benzal-
dehyde
Benzyl
benzoate
−
1 a
(h
)
1
2
3
4
5
6
7
8
9
Au
298 K, air
298 K, air
298 K, air
298 K, air
298 K, air
8.6
8.6
7.1
11.5
6.2
3.9
26.2
3.7
52.8
86.1
96.9
97.9
100
100
98.1
100
47.2
13.9
3.1
2.1
0
0
1.9
0
3:1 Au:Pd
1:1 Au:Pd
1:3 Au:Pd
Pd
Au
1:3 Au:Pd
Pd
Au
1:3 Au:Pd
Pd
298 K, O
298 K, O
298 K, O
358 K, O
358 K, O
358 K, O
2
2
2
2
2
2
14.3
57.3
32.0
97.3
100
100
2.7
0
0
1
1
0
1
−
6
Note. Conditions: moles (Pd + Au) = 2.5 × 10
a
mol, Pd + Au:substrate =
5
00:1.
The turnover frequency is the mean value over 6 h measured by plots of
Fig. 3. Arrhenius plot of PVP-stabilized 1:3 Au:Pd nanoparticles at 358 K in
mol product (aldehyde + acid + benzoate)/mol Pd + Au) vs time. The values
do not include alcohol consumed via esterification with benzoic acid to form
the benzoate.
1
atm O .
2
O2, 358 K) were kinetically-limiting at the catalyst concentra-
tion chosen, or if there was still a mass-transport limitation
with respect to O2 under these conditions. Lowering the stir-
ring speed (from 1080 to 720 rpm) led to falling TOFs over
time, suggesting that mass-transport limitations become quite
strong at lower stirring rates. Attempts to lower the catalyst con-
centration to ensure the reaction was under kinetically-limiting
conditions invariably led to de-activation of the PdAu nanopar-
ticle catalysts within an hour. Indeed, others have previously
shown for Pd and Pt nanoparticle catalysts that kinetically-
limiting conditions can lead to over-oxidation and poisoning of
the particle surface, and that alcohol oxidations can be consid-
erably faster under mass-transport-limiting conditions, in which
just enough oxygen is present to clean the surface but not to
oxidize the metal surface [25,26]. Thus it is likely that mild
mass transport limitations exist for the reaction under these
conditions. Such mass-transport-limited conditions appear un-
avoidable as nanoparticle stability is severely problematic under
kinetically-limiting conditions. Finally, the measured effective
activation energy for the 1:3 Au:Pd nanoparticles was found
to be ca. 14 kJ/mol (Fig. 3), which is much lower than that
seen previously for pure PVP-stabilized Au and Pd nanoparti-
cles (20 and 33 kJ/mol, respectively), [18] which lends further
support to the likelihood that mass-transport limitations exist
under these conditions.
alcohol at 373 K with O2 as oxidant in the absence of sol-
vent using TiO2-supported PdAu catalysts, and also found that
the bimetallic catalysts were very active for this reaction with
high selectivity to benzaldehyde (ꢀ96%) at high conversion
rates.
The conditions were varied in order to maximize the cat-
alytic activity of the nanoparticles. Interestingly, it was found
that the TOF for the oxidation of benzyl alcohol by the
−
1
−1
1
:3 Au:Pd nanoparticles increased to 26.2 h from 11.5 h
upon replacing the atmosphere with pure O2 at 298 K, while
slight decreases were seen for both the pure Au and Pd nanopar-
ticles (Table 1, entries 6–8). While we are still uncertain as to
the origin of the decreasing catalytic activities for the pure Au
and Pd nanoparticles, the results for the 1:3 Au:Pd nanoparti-
cle catalysts suggest that under these experimental conditions
the solubility of oxygen in water was a rate-limiting step in
the catalytic oxidation reaction. Increasing the reaction tem-
perature to 358 K led to even further increases in the catalytic
activity for all three catalysts (Table 1, entries 9–11), with the
−
1
1
:3 Au:Pd nanoparticles showing a TOF of 57.3 h with 100%
selectivity towards benzaldehyde under these conditions, which
is four times greater than the activity of the Au nanoparticles
under these conditions and nearly two times greater than the
corresponding Pd nanoparticles. Interestingly, for reactions un-
der pure O2, the selectivity of PVP-stabilized Au nanoparticles
shifted nearly completely towards the formation of benzalde-
hyde, though we are still investigating the source of this selec-
tivity change. Finally, the catalysts retained their activity over
Intrigued by these initial results, we wished to examine the
generality of the alcohol oxidation reaction using the 1:3 Au:Pd
catalysts at scaled-up conditions. The catalytic activity of the
1:3 Au:Pd nanoparticles toward other alcohol substrates (1-bu-
tanol, 2-butanol, 2-buten-1-ol and 1,4-butanediol) were subse-
2
4 h reaction cycles under these conditions, and no precipita-
tion of the PVP-stabilized particles was seen over the course of
the reaction. Indeed, after undergoing an oxidation reaction for
quently examined under 1 atm O at 335 K; the results are
summarized in Table 2 and Scheme 1. For these studies, both
2
2
4 h under O2 at 358 K; the particle size as determined by TEM
the amount of nanoparticle catalyst and the substrate were in-
creased by a factor of five from earlier studies; this necessitated
a further doubling of the amount of PVP stabilizer in order to
keep the particles stable during the oxidation of benzyl alcohol
increased only very slightly from 2.7 ± 0.7 nm to 2.8 ± 0.6 nm.
The kinetics of the above reaction were carefully examined
to attempt to determine whether the final conditions used (1 atm

26
W. Hou et al. / Journal of Catalysis 253 (2008) 22–27
Entry
1
Reaction
2
3
4
5
Scheme 1. Alcohol oxidation reactions over 1:3 Au:Pd bimetallic nanoparticles.
Table 2
No selectivity towards the aldehyde was seen at any point
for the oxidation of 1-butanol, as butyric acid was always the
predominant product (>75% butyric acid, negligible amounts
of butyl butyrate were detected by GC). We speculate that the
catalytic oxidation of benzyl alcohol and 1-butanol reactions
slow down due to the buildup of benzoic acid and butyric acid
on the surface of nanoparticles, lowering the availability of
catalyst sites for alcohol oxidation. Alternatively, acid buildup
tends to lower the pH of the solution (typically around 11.5–
11.6 to start), and we have found that alcohol oxidations over
PVP-stabilized nanoparticles are much less effective at pHs be-
low 11. We note that no nanoparticle precipitation was seen for
either of these systems during the 24 h reaction. Strong deacti-
vation of AuPd and AuPt catalysts owing to the carboxylic acid
production has also been observed by others [19]. Support for
this conjecture is seen in the data for the oxidation of 2-butanol
and 1,4-butanediol using the 1:3 Au:Pd nanoparticle catalysts;
100% selectivity towards 2-butanone and γ -butyrolactone, re-
spectively, were seen at substantially higher TOFs than any of
the other substrates, with only very gradual changes in the TOF
over time.
The oxidation of 2-buten-1-ol using 1:3 Au:Pd nanoparti-
cles shows some interesting features. Three separate reactions
were seen: the oxidation reaction of 2-buten-ol to crotonalde-
hyde, the hydrogenation reaction of 2-buten-1-ol to 1-butanol
and the isomerization of 2-buten-1-ol into 3-buten-1-ol. After
one hour, the product selectivities were 34% crotonaldehyde,
26% 1-butanol, and 40% 3-buten-1-ol. After the first hour, no
further 3-buten-ol and very little 1-butanol were firmed, as the
reaction became much more selective towards the oxidation of
2-buten-ol to crotonaldehyde. Similar C=C double bond iso-
merization and hydrogenation products were reported by Abad
et al. [6,7], who suggested that Pd–H species were formed dur-
Turnover frequencies and selectivities for diverse substrate oxidations with PVP
stabilized 1:3 Au:Pd bimetallic nanoparticles
Entry Substrate
Conversion
%)
Turnover
frequency
Aldehyde or ketone
selectivity (%)
(
−
1 a
(h
)
1
h
8 h
1 h
90.5
26.4
100
8 h
39.7
23.1
100
77.6
100
b
1
2
3
4
5
Benzyl alcohol 14.8
33.6
74
74
125
67
1-Butanol
2-Butanol
2-Buten-1-ol
14.8
25.0
13.3
24.6
65.0
38.5
84.7
34.0
c
d
1,4-Butanediol 46.7
234
100
−
5
Note. Conditions: [Pd + Au] = 1.25 × 10 mol, Pd + Au:substrate = 500:1.
a
Turnover frequency was calculated over the first hour.
After 24 h.
After 5 h.
γ -Butyrolactone selectivity.
b
c
d
and 1-butanol. Substantial oxidations of all the alcohols stud-
ied were seen under these conditions. The rate of oxidation of
benzyl alcohol and 1-butanol were quite similar after one hour,
with conversions of 14.8%, but the oxidations of both substrates
slowed considerably over time, likely due to mass-transport
limitations at these higher catalyst concentrations. In addition,
we note that while after 1 h, the major product for the benzyl al-
cohol oxidation was benzaldehyde (90.5%), a great deal of the
aldehyde was subsequently oxidized to benzoic acid over 24 h
(
final selectivity 39.7% for the aldehyde). Note that benzyl ben-
zoate was a minor product under these new conditions; unlike
the earlier room temperature reactions with PVP-stabilized Au
nanoparticles in which benzyl benzoate was seen. Control ex-
periments with benzaldehyde in the absence of nanoparticles
indicate that disproportionation of the aldehyde over the course
of 24 h occurs, and may be a source of the lowered selectivity
of the benzyl alcohol oxidation.

W. Hou et al. / Journal of Catalysis 253 (2008) 22–27
27
ing the aerobic alcohol oxidation which promote C=C dou-
ble bond isomerization and hydrogenation. We found that no
isomerization and hydrogenation products were formed using
PVP stabilized Au nanoparticle catalyst at the same conditions.
However, it should be noted the pure Au nanoparticle catalyst
has a low catalytically-activity for this substrate, with a conver-
sion of 2-buten-1-ol to crotonaldehyde of 1.7% was seen after
one hour, followed by apparent deactivation of the catalyst.
The actual mechanism of alcohol oxidation of the AuPd cat-
alysts may involve several pathways, given that both pure Au
and Pd nanoparticles show activity (albeit lower for the pure
metals) for this reaction [6,18]. It has been postulated that alco-
hol oxidation over Pd nanoparticles involves β-H elimination
of a dissociated alcohol on the Pd surface, followed by re-
action of oxygen with Pd–H species, while alcohol oxidation
over Au surfaces involves primarily superoxo species via oxy-
gen activation over the Au surface [18]. The appearance of the
hydrogenation and isomerization products seen for the AuPd
catalysts for the oxidation of 2-buten-1-ol above (which are not
seen for pure Au catalysts), along with the maximum activity of
bimetallic catalysts with high Pd contents, both suggest that sur-
face Pd atoms are predominately the catalytically-active species
in these particles. This would be consistent with a synergetic
electronic effect, which has been used to explain enhancements
in other AuPd catalysts. However, it should be noted no fur-
ther buildup of the hydrogenation and isomerization products
of 2-buten-1-ol are seen after 1 h, suggesting that other cat-
alytic pathways (such as activation over Au atoms) may be
more predominant over longer time scales. In-situ production
cidate the mechanism(s) of alcohol oxidation reactions over
bimetallic catalysts.
Acknowledgments
We thank the NSERC and the University of Saskatchewan
for financial support of this project and Ken Thoms and the
Saskatchewan Structural Science Centre for help with GCMS
measurements.
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(
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(
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4
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In summary, PVP-stabilized 1:3 Au:Pd nanoparticles have
(
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catalyze the aerobic oxidations of aliphatic, allylic, phenylic
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In most cases, selective oxidations to aldehydes (or ketones)
were seen; however, alkanoic acids were the major products for
primary aliphatic alcohols and γ -butyrolactone was the only
product observed for the oxidation of 1,4-butanediol. Future
work will be focused on how to prevent catalyst poisoning in
this system, understanding structure–property relationships in
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(
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3
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