Selective oxidation of crotyl alcohol by Au: X Pd bimetallic pseudo-single-atom catalysts

Article abstract of DOI:10.1039/d0cy01387k

AuPd bimetallic single-atom catalysts are being extensively studied as selective catalysts for hydrogenation and oxidation reactions due to their high selectivity. Previous work in our group has shown that alloy and core-shell AuPd nanoparticle catalysts can selectively oxidize crotyl alcohol to crotonaldehyde at room temperature in base-free conditions. In this work, we discuss the synthesis, extensive characterization, and activity for crotyl alcohol oxidation across a series of AuxPd catalysts (x = 4, 3, 2, and 1) made by both co-reduction and sequential reduction strategies, in order to examine whether single-atom systems can lead to improved activity and/or selectivity for this reaction. X-ray absorption spectroscopy data shows that both co-and sequentially-reduced Au4Pd catalysts have very small Pd-Pd coordination numbers, with values of 1.2 ± 0.3 and 1.6 ± 0.3, respectively, which indicates that they are closest to single-atom systems. The co-Au4Pd catalyst, with the lowest Pd-Pd CN, also exhibits the highest selectivity for the selective oxidation of crotyl alcohol to crotonaldehyde. We were further able to enhance the selectivity of the AuPd nanoparticle catalysts by incorporating vinyl acetate as a hydride scavenger. We show in this paper that dispersing Pd in a Au matrix can lead to very selective catalysts while also lowering the amount of Pd needed in the system.

Full text of DOI:10.1039/d0cy01387k

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Selective oxidation of crotyl alcohol by AuxPd
bimetallic pseudo-single-atom catalysts†
Cite this: DOI: 10.1039/d0cy01387k
*
Brandon A. Chivers and Robert W. J. Scott
AuPd bimetallic single-atom catalysts are being extensively studied as selective catalysts for hydrogenation
and oxidation reactions due to their high selectivity. Previous work in our group has shown that alloy and
core–shell AuPd nanoparticle catalysts can selectively oxidize crotyl alcohol to crotonaldehyde at room
temperature in base-free conditions. In this work, we discuss the synthesis, extensive characterization, and
activity for crotyl alcohol oxidation across a series of AuxPd catalysts (x = 4, 3, 2, and 1) made by both co-
reduction and sequential reduction strategies, in order to examine whether single-atom systems can lead
to improved activity and/or selectivity for this reaction. X-ray absorption spectroscopy data shows that both
co- and sequentially-reduced Au4Pd catalysts have very small Pd–Pd coordination numbers, with values of
1.2 0.3 and 1.6 0.3, respectively, which indicates that they are closest to single-atom systems. The co-
Au4Pd catalyst, with the lowest Pd–Pd CN, also exhibits the highest selectivity for the selective oxidation of
crotyl alcohol to crotonaldehyde. We were further able to enhance the selectivity of the AuPd nanoparticle
catalysts by incorporating vinyl acetate as a hydride scavenger. We show in this paper that dispersing Pd in
a Au matrix can lead to very selective catalysts while also lowering the amount of Pd needed in the system.
Received 9th July 2020,
Accepted 18th September 2020
DOI: 10.1039/d0cy01387k
rsc.li/catalysis
on a support surface. Conventional techniques include TEM
techniques such as high-angle annular dark-field (HAADF)
Introduction
Metallic nanomaterials such as metal nanoparticles (NPs) and
nanoclusters (NCs) are well-known materials in the catalysis
field for catalyzing numerous reactions such as oxidation,1–6
hydrogenation,7–12 and C–C coupling13–15 reactions. Recently,
there has been a great deal of research on single-atom
catalysts (SACs), in which catalytic metal atoms are singly
isolated within a matrix of another metal or on a support
material.9,12,16–29 Common metals that are used to synthesize
SACs include Pt,30–33 Au,34 Ir,35 Rh,36 Pd,8,25,37–39 Fe,40,41 and
Co,42 with Pd and Pt being used most frequently. The intrigue
behind SACs is due to their ability to catalyze numerous
reactions with high selectivity while using a much lower metal
loading relative to their NP and NC counterparts.8,16–18,21,43–46
In the case of SACs, each metal atom is theoretically
contributing to the reaction. In contrast, sub-surface metal
atoms in NPs and NCs can be unused throughout a catalytic
cycle.16,17 Not only has there been catalytic improvements
shown with SACs, but also economic benefits as the cost of
making such catalysts can be much lower because less metal
is needed. Thorough characterization processes are required
to determine whether a SAC has atomic dispersion of metal
imaging or high-resolution transmission electron microscopy
(HR-TEM).27,37,38,47
Perhaps the most powerful
characterization tool is the use of synchrotron radiation,
where techniques such as X-ray absorption spectroscopy (XAS)
can be used to determine local atomic information on a metal
center in a catalyst.48–50 Through fitting of the EXAFS region,
valuable information such as coordination numbers and
bond distances can be determined for a metal center of
interest, which allows one to piece together the atomic
structure of a catalyst.16,24,33,38,49,51–58
The selective oxidation of alcohols has been studied
extensively by our group and others using monometallic Au and
Pd catalysts, as well as bimetallic AuPd catalysts with varying
metal loadings and catalyst sizes.4,23,24,44,45,51,52,59–71 Pd is a
powerful metal catalyst for oxidation and hydrogenation
reactions, while Au contributes to bolster the performance of Pd
through synergistic electronic effects.4,8–10,12,44,51–53,72–78 We
have previously shown that bimetallic PVP-stabilized AuPd NP
catalysts are more active and selective for alcohol oxidation
reactions than their monometallic Pd and Au NP counterparts.
Additionally, Balcha et al.24 showed that the selective oxidation
of allylic alcohols such as crotyl alcohol could be carried out at
room temperature (25 °C) and base-free conditions, and that
the most effective catalysts were sequentially reduced systems
that had approximate core–shell structures and Au : Pd ratios of
1 :3. Similarly, Abis et al.60 recently synthesized an AuPd/TiO2
Department of Chemistry, University of Saskatchewan, 110 Science Place,
Saskatoon, Saskatchewan, S7N 5C9, Canada. E-mail: robert.scott@usask.ca
† Electronic supplementary information (ESI) available: Fitted EXAFS spectra in
R-space. See DOI: 10.1039/d0cy01387k
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catalyst for the plasmonic oxidation of glycerol at 90 °C under
base-free conditions, with a maximum activity was seen at an
Au: Pd ratio of 2 : 1. Thus, while it has been shown that AuPd
systems are excellent selective oxidation catalysts under various
reaction conditions, we wish to see if selectivity can be
enhanced by completely dispersing the Pd in the Au matrix.
SACs are becoming more prominent in the catalysis
field, with numerous reports of SACs being used for
Co-reduction synthesis of AuxPd catalysts
Co-reduced AuxPd (x = 4, 3, 2, and 1) nanoparticles (NPs)
were synthesized by modifying procedures from Tsunoyama
et al.,87 and Balcha et al.24 1.0 g PVP (MW ∼ 58 000) was
dissolved in 5.8 mL H2O in a 100 mL round-bottom flask at
room temperature (22 °C). The solution was then placed in
an ice-bath and cooled to 0 °C. To this cooled solution, 6.2
mL K2PdCl4 (9.0 × 10−5 mol Pd) and 1, 2, 3, or 4 equiv. of
HAuCl4·3H2O (9.0 × 10–5 mol Au to 3.6 × 10–4 mol Au) was
added depending on the desired ratio of Pd/Au. In a separate
100 mL round-bottom flask, 1.0 g PVP (MW ∼ 58 000) was
dissolved in 6.0 mL H2O and cooled to 0 °C in an ice bath.
After 20 minutes of stirring, 2.0 mL of ice-cold NaBH4 (5
equiv. to the amount of Au) was added, followed by stirring
for another 10 minutes. The stirring speed was increased to
1600 rpm, and the reducing agent solution was added to
solution containing the metal salt as quickly as possible. The
final solution changed from a yellow–brown solution to a
dark brown–black solution immediately after the solutions
were mixed. This dark solution was then stirred for 30
minutes in an ice bath, followed by stirring at 30 minutes at
room temperature (22 °C). Once the stirring was completed,
the catalyst solution was placed in cellulose dialysis
membranes with a molecular weight cutoff of 12 000 g mol−1
and dialyzed for 24 hours in 500 mL of deionized water
without stirring. A final [Pd] of 3.0 mM was achieved, with
the final [Au] = 3.0 mM, 6.0 mM, 9.0 mM, and 12 mM
depending on the Pd/Au ratio.
oxidation,
hydrogenation,
and
C–C
coupling
reactions.5,8,10,29,32,38,40,41,44,79–84 AuPd SAC systems have been
studied for selective hydrogenation catalysts, due to the high
activity of Pd for such reactions.8,28,45,59,85,86 Liu et al.8 designed
an AuPd SAC for the selective hydrogenation of 1-hexyne to
1-hexene and showed an improvement in the selectivity of an
AuPd SAC compared to
a Pd NP catalyst under room
temperature (25 °C) conditions. While there are countless
examples of AuPd nanoparticle systems catalyzing selective
oxidation reactions, very few examples exist employing
bimetallic AuPd SACs.44,45 Recently, Zhu et al.45 successfully
synthesized a collection of dilute AuPd nanoalloys with varying
Au : Pd atomic ratios which were thoroughly characterized via
TEM imaging and XAS to evaluate the atomic structure of the
nanoalloys. The catalytic activity was evaluated for the oxidation
of benzyl alcohol at 90 °C in the absence of a base, and it was
determined that the Au : Pd catalyst with a 1 : 1 ratio showed the
highest activity and selectivity for the oxidation reaction.
Herein, we report the synthesis of co-reduced and
sequentially-reduced AuxPd catalysts with varying Au : Pd
ratios (x = 4, 3, 2, and 1), and characterize them by TEM and
XAS. For the Au4Pd catalysts, EXAFS modelling shows that Pd
is in a pseudo-single-atom coordination environment in the
Au NP matrix. The AuxPd catalysts are then evaluated for the
selective oxidation of crotyl alcohol; results from these
selective oxidation reactions show that the pseudo-single-
atom catalyst (co-Au4Pd) exhibits the highest activity and
selectivity towards crotonaldehyde. We also show that one
can further enhance the selectivity through the addition of a
sacrificial alkene, vinyl acetate, to the system.
Sequential reduction synthesis of core–shell AuxPd catalysts
Sequentially-reduced core–shell AuxPd (x = 4, 3, 2, and 1) NPs
were synthesized with slight modifications of literature
protocols.52 First, an Au NP seed solution was synthesized
([Au] = 18.2 mM), and then K2PdCl4 was reduced using
ascorbic acid in the Au NP solution to give sequentially-
reduced AuxPd.
Au NPs were synthesized as follows: 2.0 g PVP (MW ∼
58 000) was dissolved in a mixture of water/methanol (15 mL/
16 mL) in a 100 mL round bottom flask. The flask was placed
in an ice bath and cooled to 0 °C before the addition of 473
mg HAuCl4·3H2O (6.0 × 10–4 mol Au). In another 100 mL
round bottom flask, 1.0 g PVP (MW ∼ 58 000) was dissolved
in a water/methanol mixture (3 mL/3 mL), placed in an ice
bath and cooled to 0 °C. After 20 minutes, 2.0 mL of ice-cold
NaBH4 (7 equiv. to Au) was added and stirred for a further 10
minutes. The stirring speed was increased to 1600 rpm, and
the solution containing the reducing agent was added to the
flask containing the Au salt as quickly as possible. The
solution changed from a yellow solution to a dark red
solution upon NaBH4 addition. This solution was left to stir
in an ice bath for 30 minutes, following by stirring at room
temperature (22 °C) for 30 minutes. The dark red solution of
Au NPs was then dialyzed as detailed earlier. After dialysis,
the Au NP solution was concentrated by removing methanol
to give an aqueous solution with a final [Au] of 18.2 mM. The
Experimental
Materials
Potassium tetrachloropalladate(II) (K2PdCl4, 98%, Sigma-
Aldrich), gold(III) chloride trihydrate (HAuCl4·3H2O, ≥99.9%
trace metal basis, Sigma-Aldrich), polyvinylpyrrolidone (PVP,
M.W. 58 000, Alfa Aesar), sodium borohydride (NaBH4, 98%,
Sigma-Aldrich), L-ascorbic acid (C6H8O6, reagent grade,
crystalline, Sigma-Aldrich), aluminum oxide (Al2O3, pore size
58 Å, ∼150 mesh, Sigma-Aldrich), crotyl alcohol (CH3-
CHCHCH2OH, mixture of cis and trans, Sigma-Aldrich),
vinyl acetate (VA, C4H6O2, stabilized for synthesis, Sigma-
Aldrich), acetone (Certified ACS, ≥99.5%, Fisher), methanol
(CH3OH, Certified ACS, ≥99.8%, Fisher), and ethyl acetate
(CH3COOC2H5, ACS reagent, ≥99.5%, Sigma-Aldrich) were
used as received. Milli-Q (Millipore, Bedford, MA) deionized
water (18 MΩ cm) was used throughout.
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methanol was removed by stirring the post-dialysis solution
at 1200 rpm and heating it in a water bath to 40 °C under
high vacuum conditions.
+800 eV at each edge. Pd and Au foils were used for
references at the Pd K-edge and Au LIII-edge, respectively. All
measurements were conducted in both transmission and
fluorescence modes at room temperature (22 °C), with AuxPd
samples loaded onto alumina at 2.0% by weight Pd (Au
weight percentage varied with Pd/Au ratio) using an acetone
precipitation method.90 These solid powders were placed into
a Teflon sample holder and sealed with Kapton tape. XANES
and EXAFS data analysis was performed using the software
package IFEFFIT.91 fcc bulk lattice parameters (i.e., first shell
coordination numbers of 12) were used to fit the Pd and Au
reference foil data and determine the amplitude reduction
Sequentially-reduced AuxPd NPs were synthesized by
mixing 1.0 g PVP (MW ∼ 58 000) with 9.0 × 10–5 mol Pd
(K2PdCl4) and 18.2 mM Au NPs so that the Au ratio to Pd
was 1 : 1, 2 : 1, 3 : 1, or 4 : 1 (9.0 × 10–5 mol Au to 3.6 × 10–4
mol Au). This mixture was stirred at 800 rpm and cooled
in an ice bath to 0 °C for 5 minutes before increasing the
stir rate to 1600 rpm and adding 10 equiv. of ascorbic
acid (equiv. with respect to the moles of Pd, 159 mg, 9.0 ×
10–4 mol ascorbic acid) to reduce the Pd. The solution
colour slowly changed from dark red to black after stirring
the solution in ice at 1400 rpm for 1 hour. After all
reactants were added, the final volume of the solution was
adjusted to be 30.0 mL. The solution was dialyzed
overnight using similar dialysis procedures as noted above.
The final solution had a [Pd] = 3.0 mM, with the [Au] =
3.0 mM, 6.0 mM, 9.0 mM, and 12 mM depending on the
Pd/Au ratio.
2
factor (So2) for both foils. For the Au foil, So was determined
to be 0.90 and 0.88 on BioXAS and HXMA beamlines,
2
respectively. For the Pd foil, So was determined to be 0.82
2
on both BioXAS and HXMA. These So values were fixed and
used to determine the EXAFS parameters for all samples. The
soft X-ray microcharacterization beamline (SXRMB) 06B1-1
(energy range, 1.7–10 keV; resolution, 1.0 × 104 ΔE/E) at the
CLS was used for collecting XANES spectra at the Pd LIII-
edge. SXRMB samples were prepared in two ways. Solid
samples were loaded onto double-sided tape, and
fluorescence data was collected under a helium atmosphere.
Liquid samples were analyzed by loading solution into SPEX
CertiPrep Disposable XRF X-Cell sample cups and covering
the cup with a 4 μm ultralene window film. Data was
collected in fluorescence mode.
Characterization
UV-vis spectra were obtained using a Varian Cary 50 Bio UV-
visible spectrophotometer with a scan range of 200–1000 nm
and an optical path length of 1.0 cm. Transmission electron
micrographs (TEM) were taken with an HT7700 TEM
operating at 100 kV. TEM grids were prepared by placing a
few drops of catalyst in Milli-Q deionized water onto a lacey
carbon TEM grid (Electron Microscopy Sciences). Particle
sizes were measured using the software ImageJ by measuring
at least 70 particles for each sample.88 All X-ray photoelectron
spectroscopy (XPS) measurements were collected using a
Kratos (Manchester, UK) AXIS Supra system at the
Saskatchewan Structural Sciences Centre (SSSC). This system
is equipped with a 500 mm Rowland circle monochromated
Al Kα (1486.6 eV) source and combined hemispherical
analyzer (HSA) and spherical mirror analyzer (SMA). A spot
size of hybrid slot (300 × 700 microns) was used. All survey
scan spectra were collected in the −5–1200 eV binding energy
range in 1 eV steps with a pass energy of 160 eV. High-
resolution scans of 2 regions were also conducted using 0.05
eV steps with a pass energy of 20 eV. An accelerating voltage
of 15 keV and an emission current of 10 mA was used for the
analysis. The XPS spectra were analyzed using the CasaXPS
software program.89 High-resolution scans were obtained for
Pd 3d and Au 4f peaks, which were then calibrated to a C 1s
peak at 284.8 eV.
Catalytic oxidation of crotyl alcohol
The oxidation of crotyl alcohol was performed following
previous literature protocols as detailed in Maclennan et al.51
Co- and sequentially-reduced AuxPd samples, with [Pd] = 3.0
mM and varying [Au] depending on the Pd/Au ratio, were
studied in this catalytic oxidation reaction. To a 50 mL
3-neck round-bottom flask, 5.0 mL of the catalyst solution
was added. A condenser was then attached to the middle
neck of the flask and the other two necks covered with septa.
A needle was connected to one end of the flask and the tip
placed into the catalyst solution in order to sparge the
solution with O2(g) during the reaction. The system was
purged with O2(g) for 20 minutes before adding 250 equiv. of
crotyl alcohol (0.32 mL, for a substrate : Pd ratio of 250 : 1)
via a syringe needle. The reaction was then stirred at 1400
rpm for 3 hours at 22 °C. After the 3 hour reaction, 2.0 mL
of the reaction mixture was collected in
a vial and
transferred to 2.0 mL of ethyl acetate. This was done by
mixing the reaction mixture with ethyl acetate (1.0 mL per
extraction) by shaking it vigorously in a vial, and then the
contents were transferred to two 2.0 mL centrifugation tubes
and centrifuged at 6000 rpm for 10 seconds. The top layer of
the neat extract was collected for analysis, while the bottom
water/catalyst layer was discarded. Conversion, selectivity,
and turnover numbers for the reaction were obtained from
gas chromatography (GC) using an FID detector (Agilent
Technologies 7890A) and an HP-Innowax capillary column.
The hard X-ray microanalysis beamline (HXMA) 061D-1
(energy range, 5–30 keV; resolution, 1 × 10–4 ΔE/E) and
biological X-ray absorption spectroscopy beamline (BioXAS)
07ID-2M (energy range, 5–32 keV; resolution, 5 × 10–5 ΔE/E)
at the Canadian Light Source (CLS) were used for collecting
X-ray absorption spectra (XANES and EXAFS) at the Pd
K-edge and Au LIII-edge. The energy scan range for
measurements on both beamlines was between −200 eV to
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The ethyl acetate extract was used for GC–FID analysis. Two
reactions were run for each AuxPd sample with three
injections per reaction.
The oxidation of crotyl alcohol was also performed in the
absence of O2(g), using vinyl acetate as a sacrificial alkane.92
For this reaction, 5.0 mL of the AuxPd catalyst solution was
delivered into a 100 mL round-bottom flask. 0.35 mL of vinyl
acetate was then added, and the mixture stirred at 1400 rpm.
After 2 minutes of stirring, 0.32 mL of crotyl alcohol was
added (1 : 1 vinyl acetate : crotyl alcohol ratio), and the
reaction was continued for 3 hours before GC–FID analysis.
Product extraction was performed in the same way previously
mentioned. GC–FID calibration curves were run for this
reaction using the starting substrate crotyl alcohol, and the
three main products crotonaldehyde, 1-butanol, and 3-buten-
1-ol. Six different concentrations were run for each reactant,
with three injections performed at each concentration. Linear
calibration curves were obtained for all species with R2 values
of ∼0.99 for all systems.
Results and discussion
Synthesis and characterization of co- and sequentially-
reduced AuxPd catalysts
The concentration of Pd was kept constant for the crotyl
alcohol oxidation reactions at 3.0 mM. This value was chosen
with the idea of evaluating these catalysts with XAS
measurements. In order to obtain high-quality XAS data that
exhibits a deep k-space, a higher concentration of metals
needed to be used. Therefore, the concentration of Pd was
3.0 mM, while the [Au] increased from 3.0 mM to 12 mM,
Fig. 1 TEM images of a) co-Au4Pd, b) co-Au3Pd, c) co-Au2Pd, d) co-
AuPd, e) seq-Au4Pd, f) seq-Au3Pd, g) seq-Au2Pd, and h) seq-AuPd
catalysts.
depending on the Au : Pd ratio. Since
a high metal
concentration was used, the size of these catalysts was
monitored by TEM to ensure that particle agglomeration and
growth did not occur, while retaining relatively monodisperse
particle sizes. Both co-reduction and sequential reduction
strategies were used to obtain AuxPd catalysts. The co-
reduced (co-) AuxPd catalysts were made by reducing an Au
and Pd precursor in the presence of a stabilizer, while
sequentially-reduced (seq-) AuxPd catalysts were made by first
reducing an Au precursor to give Au NPs, then reducing a Pd
precursor into the Au NP matrix. Fig. 1 shows TEM images
for each ratio of co- and seq-AuxPd catalysts. A complete table
of average particle sizes and standard deviations are listed in
Table 1.88
From Table 1, the average particle size for the Au NP seed
solution is 3.6 1.1 nm. As Pd is added into the system, the
seq-AuxPd particle sizes increase as the ratio of Au : Pd is
decreased towards unity (i.e., 1 : 1). Expected particle sizes
can be calculated for the seq-AuxPd catalysts using eqn (1):93
1
3
ꢀ
ꢁ
VPd½Pdꢀ
VAu½Auꢀ
D ¼ Dcore 1 þ
(1)
where Dcore is the particle size of the Au NPs, VPd is the molar
volume of Pd, [Pd] is the concentration of Pd, VAu is the
Table 1 Average TEM particle sizes
For the co-AuxPd catalysts, particle sizes vary between 3.0
and 3.6 nm. For the seq-AuxPd catalysts, particle sizes vary
between 3.6 and 4.9 nm. The particle sizes of each catalyst
for both reduction methods are all within similar ranges,
which would suggest that particle growth or agglomeration is
not occurring, meaning these catalysts are stable under the
reduction conditions during their synthesis. Smaller NP
catalysts ensure that the surface of the catalyst can be better
studied using XAS, as a larger fraction of atoms is on the
surface of the NPs.
Catalyst
Particle size (nm)
co-Au4Pd
co-Au3Pd
co-Au2Pd
co-AuPd
seq-Au4Pd
seq-Au3Pd
seq-Au2Pd
seq-AuPd
Au NPs
3.1 0.9
3.0 1.1
3.6 1.0
3.5 1.3
3.6 0.9
3.7 0.8
4.0 1.0
4.9 1.3
3.6 1.1
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molar volume of Au, and [Au] is the concentration of Au. For
the seq-AuxPd catalysts, particle sizes of 3.8, 3.9, 4.1, and 4.4
nm were predicted for Au : Pd ratios of 4 : 1, 3 : 1, 2 : 1, and 1 :
1, respectively. These values correspond well with the
measured particle sizes in Table 1.
To investigate whether secondary nucleation of Pd is
occurring in the sequentially reduced system, a histogram of
particle sizes is shown in Fig. 2. Histograms were generated by
plotting the raw count of binned particle sizes for each
sequentially-reduced catalyst and Au NPs and then fitting
normal distribution curves to the raw data. The highest count
of particle sizes occurs between 3–5 nm for each system,
including the Au NPs. As Pd is introduced into the system (i.e.,
to form seq-AuxPd), a new distribution of smaller particle sizes
is not observed. This would indicate that a secondary nucleation
event of new Pd NPs is not occurring, suggesting the system is
bimetallic, though the catalysts are characterized further below
to show they are indeed bimetallic.
Fig. 3 contains UV-vis spectra comparing Au NP seeds with
the bimetallic Au4Pd NP system after Pd is deposited on the
surface, as well as the co-reduced Au4Pd NP system. A typical
absorption peak is seen ∼550 nm for Au NPs which is caused
by surface plasmon resonance.94 In the case of co-Au4Pd, no
SPR peak is seen in the system. This would point towards
Fig. 3 Comparison of UV-vis spectra for co-Au4Pd, seq-Au4Pd, and
Au NPs.
respectively.95 Fig. 4b shows Au 4f7/2 and 4f5/2 peaks for co-
Au4Pd, with binding energies of 82.8 and 86.4 eV,
respectively. These values also correlate well with metallic
Au(0) in the literature at 83.2 and 86.9 eV, respectively.96
Fig. 4c shows seq-Au4Pd fitted peaks for Pd 3d5/2 and 3d3/2
with binding energies of 334.6 and 339.9 eV, respectively.
Again, these values correlate well to Pd(0) from the
literature.95 Lastly, Fig. 4d shows seq-Au4Pd fitted peaks for
Au 4f7/2 and 4f5/2 with binding energies of 83.0 and 86.6 eV,
respectively. These values correlate well with the previously
mentioned literature.96 From this analysis, it can be
concluded that both Au and Pd are in their metallic forms on
the surface of the catalyst, meaning a full reduction has
occurred. The surface analysis also confirms that both
metallic Au and Pd are present in the system, though more
in-depth results from X-ray absorption spectroscopy (XAS)
will be discussed below to provide further conclusions on the
make-up of these catalysts at each Au : Pd ratio.
Au–Pd interactions becoming more prominent, and
a
bimetallic alloy structure for the NPs.24 On the other hand,
for the seq-Au4Pd catalyst, a slight peak can be seen at ∼520
nm after the Pd reduction, which is dampened and shifted to
lower energy compared to the pure Au NP plasmon peak.
This indicates that Au–Au interactions are still prevalent in
the system, though a dampening of the peak is consistent
with the addition of Pd on the surface of Au.24,68,93
XPS studies were performed to characterize the surface
composition of both co- and sequentially-reduced Au4Pd
catalysts. Fig. 4 shows XPS spectra for both co- and seq-Au4Pd
samples. In Fig. 4a, Pd 3d5/2 and 3d3/2 peaks were fit for co-
Au4Pd, giving binding energies of 334.6 and 339.9 eV,
respectively. These values correlate reasonably well with
literature to metallic Pd(0) at 335.1 eV and 340.4,
Fig. 5 contains XANES data at the Pd LIII-edge, showing a
reference Pd foil compared to each ratio of co-AuxPd
Fig. 4 Fitted XPS spectra for (a) co-red Au4Pd Pd 3d peak, (b) co-red
Au4Pd Au 4f peak, (c) seq-Au4Pd Pd 3d peak, and (d) seq-Au4Pd Au 4f
peak.
Fig. 2 Distribution of particle sizes measured for Au NPs and all seq-
AuxPd catalysts.
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Fig. 5 Pd LIII-edge XANES data of co-AuxPd samples.
catalysts. The white-line feature at 3173 eV is caused by the
excitation of electrons from the 2p3/2 to 4d band in Pd.61,97
The increase in white line intensity relative to Pd foil would
suggest for each co-AuxPd catalyst that electrons are being
withdrawn from the 4d band of Pd by Au in the
catalyst.51,98,99 Also, a shift in the energy of the white line is
observed for co-AuxPd catalysts, further concluding that
electrons are being withdrawn from the 4d band of Pd by Au.
This allows one to conclude that our catalyst is a bimetallic
AuPd system as these apparent shifts in white line intensity
and energy correlate well with the literature from our group
and others.51,61,98,99 We are uncertain as to why the co-Au2Pd
system showed the most electron-deficient Pd.
The bimetallic nature of both co- and sequentially-reduced
AuxPd catalysts was evaluated via XAS. Fig. 6 contains XAS
data collected for co-AuxPd catalysts at each ratio of Au : Pd
(i.e., 4 : 1, 3 : 1, 2 : 1, and 1 : 1). Fig. 6a shows XANES spectra
from the Pd K-edge for each co-AuxPd catalyst. The Pd K-edge
measures the transition from a 1s orbital to 5p orbital in Pd
at the edge, with a white line observed near 24 350 eV. Fig. 6b
shows XANES spectra from the Au LIII-edge for each co-AuxPd
catalyst. A shift to lower white line energy for the bimetallic
samples relative to the Au foil would indicate an increase in
electron density of the 5d band of Au.24,98,100 This result,
combined with results discussed from Fig. 5, shows that Au
is withdrawing electrons from Pd, increasing the electron
density of the Au 5d band and depleting electrons from the
4d band of Pd. Fig. 6c and d contain k-space data collected
on the Pd K-edge and Au LIII-edge, respectively, for all co-Aux-
Pd catalysts. A comparison between the bimetallic samples
and monometallic reference foils shows an obvious shift in
periodicity at both metal edges beyond a wavenumber of 4
Å−1. This indicates there is a strong Au–Pd interaction in the
co-AuxPd catalysts, providing further proof that they are
bimetallic.56 The R-space data at the Pd K-edge and Au LIII-
edge for co-AuxPd catalysts are shown in Fig. 6e and f. More
intense peaks correlate to larger M–M coordination numbers
for the metal center being analyzed. In Fig. 6e, the sharp
peak seen slightly below 2.5 Å for the Pd reference foil is due
Fig. 6 X-ray absorption spectroscopy data for co-AuxPd catalysts. (a)
Pd K-edge XANES, (b) Au LIII-edge XANES, (c) Pd K-edge k-space, (d)
Au LIII-edge k-space, (e) Pd K-edge R-space, and (f) Au LIII-edge
R-space.
to first shell Pd–Pd interactions, while significantly reduced
Pd–M peak intensities are seen for the co-AuxPd catalysts.
Fitted EXAFS spectra which are phase-corrected for both the
Pd–Pd peak and Au–Au peak are included in the ESI.† Fig. 6e
suggests that a minute Pd–M interaction is observed for the
co-reduced systems, indicating that Pd is likely located on
the surface of Au. To quantitatively show this, a further
discussion on the fitting of this data will be included later in
this section. Fig. 6f shows the R-space data at the Au LIII-edge
for all co-reduced AuxPd catalysts as well as Au foil. A strong
first shell Au–Au interaction is seen slightly above 2.5 Å,
while a strong Au–Pd interaction is observed slightly below
2.25
Å for each co-AuxPd sample. This strong Au–Pd
interaction in the co-AuxPd catalysts suggests that these
systems are alloys. There is a qualitative shift in the co-
reduced spectra versus the Au foil, further providing proof of
strong Au–Pd interactions.
Fig. 7 shows the XANES region at the Pd LIII-edge for
sequentially-reduced catalysts, where an increase in white line
intensity at 3173 eV, as well as a shift in white line energy is
observed. As discussed for the co-reduced system, an increase
in white line intensity and shift to higher energy indicates
electron withdrawal from the 4d band of Pd, though the shift
is less dramatic for the seq-AuxPd system. Overall the higher
white line intensity of the seq-AuxPd samples, as compared to
the co-AuxPd samples, suggests that Pd has reduced 4d
occupancy in the seq-AuxPd system. This may be due to more
of the Pd being on the surface in this system.
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indicating that an increase in electron density in the 5d band
of Au is not as prominent for these samples. This is likely
because most Au is not in direct contact with Pd in these
samples (as is shown below). Fig. 8c and d show the k-space
data collected on both metal edges for the seq-AuxPd
catalysts. No significant shifts in the periodicity of the wave
are observed at either metal edge, though the amplitude of
the wave at the Au LIII-edge correlates well with the Au
reference foil, indicating that Au is in
a near-bulk
coordination environment. Again, this will be discussed in
more detail later, though a suggestion can be made that the
seq-AuxPd catalysts take on a core–shell morphology.24 Fig. 8e
contains R-space data for each seq-AuxPd catalyst at the Pd
K-edge as well as a Pd reference foil. Like the co-reduced
catalysts, an intense feature just below 2.5 Å is observed for
the reference foil, though weak Pd–M interactions are
Fig. 7 Pd LIII-edge XANES data for seq-AuxPd samples.
observed for the seq-AuxPd catalyst. There is
a slight
Fig. 8 contains XANES and EXAFS spectra for seq-AuxPd
catalysts at each ratio of Au : Pd (i.e., 4 : 1, 3 : 1, 2 : 1, and 1 : 1).
Fig. 8a shows XANES spectra from the Pd K-edge for all seq-
AuxPd catalysts. As previously mentioned, this is the
transition from a 1s to 5p orbital for Pd, with a white line
observed near 24 350 eV. The spectra closely resemble that of
Pd foil at the edge suggesting that Pd is reduced in all the
samples, though the dampened EXAFS region suggests much
lower coordination numbers for Pd in the bimetallic NPs.
Fig. 8b shows the Au LIII-edge XANES data for each seq-AuxPd
sample. No obvious shift in the white line is observed,
qualitative difference in peak shapes between the co- and seq-
AuxPd catalysts, which will be discussed in more detail below.
Fig. 8f shows the R-space data collected on the Au LIII-edge
for each seq-AuxPd catalyst and an Au reference foil. A strong
Au–Au interaction is observed for the Au reference foil as well
as the seq-AuxPd catalysts just above 2.5 Å, indicating a near-
bulk Au environment in the seq-AuxPd catalysts.
Fig. 9 provides an in-depth comparison of the Pd K-edge
and Au LIII-edge between both co- and seq-AuxPd catalysts at
4 : 1 and 1 : 1 Au : Pd ratios. In Fig. 9a, R-space data at the Pd
K-edge is shown for co- and seq-AuxPd samples. As mentioned
previously, the reduction in amplitude for the Pd–Pd just
below 2.5 Å indicates that most of the Pd is on the surface of
both catalysts. The low intensity of the Pd–Pd peak may
indicate low coordination numbers (CNs) for the Pd–Pd bond
in both catalysts, though the data must be fit to quantify this
Pd–Pd interaction in terms of CNs. Small features can be seen
slightly shifted to a higher radial distance for both catalysts,
indicating Au–Pd bonding. A slightly further shift is seen for
co-Au4Pd and co-AuPd relative to the seq-AuxPd catalysts,
indicating that Pd may be in more of an isolated environment
compared to the seq-AuxPd catalysts and Pd–Pd CNs are lower
for each ratio of Au : Pd, though further quantification was
done through fitting the EXAFS data. Fig. 9b contains the Au
LIII-edge R-space data for co- and sequentially-reduced AuxPd
catalysts at the same ratios. Qualitatively, there are nuanced
differences in line shapes between the co- and seq-AuxPd
Fig. 8 X-ray absorption spectroscopy data for seq-AuxPd catalysts. (a)
Pd K-edge XANES, (b) Au LIII-edge XANES, (c) Pd K-edge k-space, (d)
Au LIII-edge k-space, (e) Pd K-edge R-space, and (f) Au LIII-edge
R-space.
Fig. 9 R-Space data comparing co-AuxPd and seq-AuxPd catalysts at
(a) Pd K-edge and (b) Au LIII-edge.
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catalysts. The co-AuxPd catalysts show a stronger Au–Pd
interaction just below 2.25 Å than the seq-AuxPd catalysts,
indicating more Au–Pd mixing in the co-reduced catalysts. A
more significant difference in the R-space data between co-
and seq-AuxPd is seen as the seq-AuxPd takes on a very similar
shape to the Au foil. This would qualitatively suggest that Au is
in a near-bulk environment in the seq-AuxPd catalyst with very
strong Au–Au interactions, whereas a strong Au–Au interaction
is not as prevalent in co-AuxPd catalysts. Further quantitative
conclusions can be made in regard to Pd–M and Au–M
interactions by modelling CNs for Pd–Pd, Pd–Au, Au–Au, and
Au–Pd interactions through EXAFS data fitting using the
IFEFFIT software package.
Simultaneous EXAFS fitting of both the Pd K-edge and Au
LIII edge was done using AuPd alloy fcc models, and fitting
results are shown in Tables 2 and 3. In Tables 2 and 3, the
element that comes first symbolizes the absorbing atom, while
the second element symbolizes the neighbouring atom that
scatters photoelectrons released by the absorbing atom (e.g. a
PdAu interaction is a Pd absorbing atom with neighbouring
first shell Au atoms). Both first shell Pd–Pd and Pd–Au
interactions were quantified for all the co-reduced systems.
For the Au4Pd co-reduced NPs, the small CN of 1.2 0.3 for
the Pd–Pd interaction, along with a significant CN of 8.7 0.8
for the Pd–Au interaction indicates that the Pd is nearly in a
1.2 0.3 was also quantified. As the ratio of Au : Pd decreases
from 4 : 1 to 1 : 1, the CN of Au–Au decreases while the CN of
Au–Pd increases. At an Au : Pd ratio of 1 : 1, the CN for Au–Au
is 5.8 0.9, and for Au–Pd is 3.7 0.6. To conclude, the nearest
catalyst to a SAC is the co-Au4Pd system, with all other ratios
showing a trend towards AuPd catalysts with more contiguous
Pd domains (i.e., higher coordination between Pd atoms). The
total Pd–M and Au–M CNs can be calculated by adding
together Pd–Au with Pd–Pd, and Au–Pd with Au–Au,
respectively, to confirm which metal is preferentially on the
surface of the catalysts. As the ratio of Au : Pd decreases from
4 : 1 to 1 : 1, the total Pd–M CN decreases, while Au–M
increases, indicating that Pd is initially predominantly on the
surface of the 4 : 1 catalyst and not as predominantly on the
surface as the ratio approaches unity. The bond distances
listed are in good agreement with the literature44,51 on
bimetallic AuPd systems, with Pd–Pd distances at 2.79 0.01 Å
and Au–Au distances of 2.83 0.01 Å. These values are slightly
lower than the reference foils, which is typical for NP
systems.102,103
Compared to the co-reduced samples, slight increases in
Pd–Pd CNs become apparent for the sequentially reduced
samples. For the seq-Au4Pd system, a CN of 1.6 0.3 for Pd–
Pd and 6.1 0.5 for Pd–Au is observed. As the ratio of Au : Pd
decreases, the CN of Pd–Pd increases while Pd–Au decreases,
with the seq-AuPd system showing 6.0 0.4 for Pd–Pd, and
3.2 0.6 for Pd–Au. This is a similar trend observed in the
co-AuxPd catalysts. For seq-Au4Pd, a large CN of 9.5 0.4 for
Au–Au is observed. The seq-AuPd catalyst has an Au–Au CN
of 11.2 1.1, resembling an Au fcc lattice (CN of 12). This is
a sign that all Au atoms are in the interior of the NPs as Au :
Pd ratios approach unity. The insignificant Au–Pd interaction
for each sequentially-reduced catalyst along with large Pd–Pd
single-atom environment (i.e.
a
pseudo-single-atom-
catalyst).44,101 The co-reduced systems showed a slightly
increased Pd–Pd CN of 2.5 0.6, 2.5 0.4, and 2.4 1.2, for 3 :
1, 2 : 1, and 1 : 1 Au : Pd systems, respectively. This small
increase in Pd–Pd CN signals that Pd is associated with
neighbouring Pd atoms more often within the core or on the
surface of Au. Notably, the Pd–Au CN begins to decrease with
the Au : Pd ratio, going from 7.2 0.9 down to 6.0 2.4 for the
3 : 1 and 1 : 1 samples, respectively. Au–Au and Au–Pd first shell
interactions were also quantified for all the co-reduced
catalysts. A large CN of 7.1 0.7 for the first shell Au–Au in the
co-Au4Pd catalyst was seen, indicating a large number of Au–
Au neighbours. A low CN for Au–Pd in the co-Au4Pd catalyst of
and Au–Au interactions would indicate
a
core–shell
morphology rather than an alloy.24,51,102 Bond distances of
∼2.70, ∼2.73, and ∼2.83 Å for Pd–Pd, Pd–Au, and Au–Au
bonds are in good agreement with previous work.51 The
shorter bond distances seen for Pd–Pd and Pd–Au bonds
Table 2 EXAFS fitting parameters for co-AuxPd catalysts
Catalyst
Bond
CN
R (Å)
σ2 (Å2)
E0 (eV)
R-Factor
co-Au4Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
8.7 0.8
1.2 0.3
7.1 0.7
1.2 0.3
7.2 0.9
2.5 0.6
6.3 0.8
1.6 0.3
7.3 0.9
2.5 0.4
6.5 1.2
2.7 0.5
6.0 2.4
2.4 1.2
5.8 0.9
3.7 0.6
2.80 0.01
2.79 0.02
2.83 0.01
2.80 0.01
2.80 0.01
2.79 0.01
2.82 0.01
2.80 0.01
2.80 0.01
2.79 0.01
2.82 0.01
2.80 0.01
2.80 0.01
2.77 0.03
2.82 0.01
2.80 0.01
0.008 0.001
0.006 0.002
0.009 0.001
0.006 0.002
0.007 0.001
0.007 0.002
0.008 0.001
0.004 0.001
0.008 0.002
0.005 0.001
0.009 0.002
0.007 0.001
0.008 0.006
0.005 0.004
0.008 0.001
0.007 0.001
−4.1 0.6
−4.1 0.6
4.7 0.4
4.7 0.4
−1.7 0.4
−1.7 0.4
4.2 0.5
4.2 0.5
−5.4 0.4
−5.4 0.4
5.2 0.6
5.2 0.6
−6.2 0.8
−6.2 0.8
6.1 0.6
6.1 0.6
0.008
co-Au3Pd
co-Au2Pd
co-AuPd
0.016
0.007
0.023
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Table 3 EXAFS fitting parameters for seq-AuxPd catalysts
Catalyst
Bond
CN
R (Å)
σ
2 (Å2)
E0 (eV)
R-Factor
seq-Au4Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
Pd–Au
Pd–Pd
Au–Au
Au–Pd
6.1 0.5
1.6 0.3
9.5 0.4
NDa
5.2 0.5
1.9 0.3
9.5 0.4
0.5 0.1
5.7 0.7
3.3 0.5
12.2 0.7
ND
2.73 0.01
2.68 0.02
2.84 0.01
ND
2.74 0.01
2.69 0.02
2.83 0.01
2.74 0.01
2.71 0.01
2.68 0.02
2.84 0.01
ND
0.008 0.001
0.007 0.002
0.010 0.001
ND
0.006 0.001
0.005 0.002
0.009 0.001
0.004 0.001
0.011 0.002
0.008 0.001
0.011 0.001
ND
−4.4 0.7
−4.4 0.7
3.5 0.6
ND
−10.5 0.8
−10.5 0.8
3.0 0.6
3.0 0.6
−2.9 1.0
−2.9 1.0
4.9 0.7
ND
0.017
seq-Au3Pd
seq-Au2Pd
seq-AuPd
0.020
0.017
0.015
3.2 0.6
6.0 0.4
11.2 1.1
ND
2.73 0.02
2.73 0.01
2.84 0.01
ND
0.008 0.001
0.009 0.001
0.009 0.001
ND
−7.6 0.6
−7.6 0.6
4.6 1.2
ND
a
ND = not detected. This indicates that the errors in the EXAFS parameter were larger than the value obtained.
(compared to the co-reduced system) are also consistent with
phase-separation of the Pd and Au phases, and the lower Pd–
Au bond length may be due to lattice mismatches at the Pd–
Au interface. Similar to what was seen in the co-reduced
system, the seq-Au4Pd system is the closest to a SAC.
Selective oxidation of crotyl alcohol using co- and seq-
reduced AuxPd catalysts
The co-reduced and sequentially reduced AuxPd catalysts
were monitored for the base-free, room-temperature selective
Table 4 Crotyl alcohol oxidation resultsa
Catalyst
Conversion
Compound
Selectivity
TON
TOF (h−1
)
co-Au4Pd
39
24
21
10
21
15
12
9
9
2
3
1
1
1
1
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
77
17
6
2
1
3
98 22
33
20
18
8
7
1
3
co-Au3Pd
65
21
15
67
17
17
36
22
42
62
20
18
48
25
27
35
28
37
27
29
45
0.0
1
1
1
3
1
1
1
1
1
1
1
1
4
2
2
3
1
2
2
1
1
59
53
24
51
37
29
23
0.8
17
4
8
2
1
3
2
1
co-Au2Pd
co-AuPd
1
seq-Au4Pd
seq-Au3Pd
seq-Au2Pd
seq-AuPd
17
12
10
8
1
1
1
1
1
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
Au NP controlb
Pd NP controlc
0.3
7
0.3
6
100.0
0.0
70
22
3-Buten-1-ol
8
a
All reactions were run for 3 hours, stirring at 1400 rpm at room temperature (22 °C). The Pd: substrate ratio used for all reactions was 1 : 250
b
c
(5 mL of sample, 0.32 mL of crotyl alcohol). [Pd] = 3 mM for all AuxPd catalysts. [Au] = 18.2 mM. [Pd] = 3.0 mM. The average TOF was
calculated by dividing the TON by 3 hours.
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oxidation of crotyl alcohol to evaluate the selectivity of each
catalyst in producing crotonaldehyde. Table 4 shows the
results from this reaction with co- and seq-reduced AuxPd
catalysts, as well as monometallic Au and Pd NPs. Both the
monometallic NP catalysts showed <10% conversion to the
three products of the crotyl alcohol oxidation,
crotonaldehyde, 1-butanol, and 3-buten-1-ol. Each bimetallic
catalyst was more active (i.e., higher conversion) and more
selective to crotonaldehyde than either monometallic catalyst.
The pseudo-single-atom co-Au4Pd catalyst showed the
greatest selectivity towards crotonaldehyde formation, at
77%. The largest conversion of 39 9% was also observed for
this catalyst. Notably, the selectivity towards crotonaldehyde
decreased from the co-Au4Pd catalyst to the co-AuPd system,
which had a selectivity of 36%. This result would suggest that
not only is the near SAC system more selective, the selectivity
is also related to the minimal Pd–Pd interactions (Table 2)
that exist for this catalyst. An increase in side-product
formation from the 3 : 1 catalyst down to 1 : 1 would indicate
increased amounts of Pd–H species present in the catalyst,
which is directly related to the increase in the CNs of Pd–
Pd.51,52 As Pd forms continuous domains on the surface of
Au, Pd–H species can be directly adjacent to other Pd
catalytic sites, which adsorb the alkene, increasing the
likelihood of side-products to form. Comparing the co-
believe this may be due to a secondary, much slower, redox
mechanism in which Pd is oxidized on the Pd nanoparticle
surface, followed by stoichiometric reactions of Pd(II) salts
with crotyl alcohol to give Pd(0) and crotonaldehyde.104 We
have previously shown via in situ XAS that this mechanism is
not available in bimetallic PdAu systems, as the Pd is
tremendously stable to over-oxidation in the bimetallic
particles.51 Turnover numbers (TONs) were generated based
on the Pd : substrate ratio in our catalysts, which was 1 : 250,
and average turnover frequencies (TOFs) were calculated by
dividing the TON by 3 hours. The TON and TOF values can
be compared to previous work by Maclennan et al.51 using a
seq-AuPd3 catalyst, which used a higher ratio of Pd to Au
than our work and different concentrations of each metal
([Pd] = 9.36 mM, [Au] = 3.13 mM). The same Pd : substrate
ratio of 1 : 250 was used in this work, as well as similar
reaction conditions at ambient temperature (25 °C). In that
work, a conversion of 72.4% and selectivity of 62.8% was
obtained, with a TON of 180. Our best catalyst, co-Au4Pd,
showed lower conversion (39
9%) and better selectivity
(77%). The lower conversion is likely due to the lower Pd
concentration used. In this work, we were able to
significantly reduce the amount of Pd in the catalyst, while
improving the selectivity towards crotonaldehyde. Crotyl
alcohol oxidations have also been studied by our group and
others, although typically using much harsher reactions
(higher T, presence of bases) and at lower metal
concentrations, which make comparisons difficult.23,24,61
A comparison study was performed to show that vinyl
acetate (VA) can also be used as a sacrificial alkane to “clean”
the surface of the catalyst, similar to how O2(g) removes Pd–H
species.92 This study was done to prove that Pd–H species
lead to the production of undesired side-products, and to
mitigate side-product formation to drive selectivity towards
crotonaldehyde. The result of substituting O2(g) with VA is
noted in Table 5. The co-Au4Pd and co-AuPd catalysts were
compared, as well as the seq-Au4Pd and seq-AuPd catalysts.
reduced catalyst to the sequentially reduced system,
a
decrease in aldehyde selectivity is observed for the
sequentially reduced system at each Au : Pd ratio relative to
the co-reduced system. The selectivity to the aldehyde
decreases from 62% for seq-Au4Pd down to 27% for seq-
AuPd. Control reactions were performed to show that Au NPs
are nearly inert for the oxidation reaction, while Pd NPs show
high selectivity, but low conversion. These controls are
consistent with previous work.61 It is interesting to note that
the Pd only system, while showing poor activity, does have
moderate selectivity towards crotonaldehyde (while addition
of Au seems to lower selectivity at lower Au : Pd ratios). We
Table 5 Crotyl alcohol oxidation results using vinyl acetatea
Catalyst
Conversion
Compound
Selectivity
88
6.6 0.3
5.9 0.1
TON
113
TOF (h−1
)
co-Au4Pd
45
17
30
13
3
1
8
1
1
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
Crotonaldehyde
1-Butanol
1
2
38
14
25
11
2
1
7
1
1
co-AuPd
69
13
18
78
11
11
50
20
30
13
80
7
4
2
2
1
1
1
1
1
1
42 21
seq-Au4Pd
seq-AuPd
Vinyl acetate
75
32
6
1
1
3-Buten-1-ol
Crotonaldehyde
1-Butanol
3-Buten-1-ol
a
All reactions were run for 3 hours, stirring at 1400 rpm at room temperature (22 °C). The Pd: substrate ratio used for all reactions was 1 : 250
(5 mL of sample, 0.32 mL crotyl alcohol), while the substrate : VA ratio was kept at 1 : 1 (0.35 mL VA).
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For all four catalysts, increased conversions and selectivities Acknowledgements
are observed, with the selectivity for the co-Au4Pd system
We would like to thank the National Sciences and
Engineering Research Council (NSERC) and the University of
Saskatchewan for funding. We would also like to thank Drs.
Yongfeng Hu, Qunfeng Xiao, Ning Chen, Weifeng Chen, and
Roman Chernikov for their help in collecting XAS data on the
SXRMB, HXMA, and BioXAS beamlines. X-ray absorption
spectroscopy experiments at the Canadian Light Source, a
national research facility of the University of Saskatchewan,
which is supported by the Canada Foundation for Innovation
(CFI), the Natural Sciences and Engineering Research Council
(NSERC), the National Research Council (NRC), the Canadian
Institutes of Health Research (CIHR), the Government of
Saskatchewan, and the University of Saskatchewan.
increasing from 77 2% using O2(g) to 88 1% using VA.
Similarly, for seq-Au4Pd, the selectivity increased from 62
1% to 78 1% using O2(g) and VA, respectively. The reduction
of side-product formation would indicate the VA cleans the
surface of the catalyst by removing Pd–H species more
efficiently than O2(g); this may be due to the fact that VA is in
the same phase as the substrate, while O2 has to diffuse into
the liquid phase which sets up possible mass transfer
limitations. The significance of using VA is that it is
inexpensive, small amounts of it are needed for the oxidation
reaction (1 : 1 VA : substrate), and a pressurized system of
oxygen gas is not needed. The use of VA also makes it easier
to replicate the oxidation reaction, as everything can be
thrown into “one-pot” and left to react for a desired amount
of time. Additionally, while the use of VA as the oxidant may
be impractical due to subsequent issues in separating the
products of the reaction, the results show that one of the root
causes of poorer selectivity for this reaction when using
oxygen as the oxidant is likely caused by mass-transfer
challenges of oxygen into the liquid phase.
Notes and references
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catalysts with
x = 4, 3, 2, and 1 were synthesized,
characterized, and evaluated as selective catalysts in the
oxidation of crotyl alcohol to crotonaldehyde. TEM, XPS, and
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synthesized, with sequentially-reduced AuxPd nanoparticles
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metallic Au and Pd were present in each catalyst system, and
Pd LIII edge XAS data showed that Pd was 4d electron
deficient in the AuPd systems. Modelling of the EXAFS data
confirmed that co-Au4Pd and seq-Au4Pd nanoparticles
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catalysts showed more contiguous domains of Pd. Results
from the oxidation of crotyl alcohol showed that the co-Au4Pd
system was not only more active but also most selective in
forming crotonaldehyde. As the ratio of Au : Pd decreased in
both co- and seq-AuxPd catalysts, activity and selectivity
decreased. These catalysts were compared to monometallic
Au and Pd NPs and were all more active and selective than
the monometallic catalysts. Finally, we showed that vinyl
acetate could be used as a more effective oxidant in this
system which promoted higher selectivity towards
crotonaldehyde formation.
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