Enhancing catalytic performance of palladium in gold and palladium alloy nanoparticles for organic synthesis reactions through visible light irradiation at ambient temperatures

Article abstract of DOI:10.1021/ja400527a

The intrinsic catalytic activity of palladium (Pd) is significantly enhanced in gold (Au)-Pd alloy nanoparticles (NPs) under visible light irradiation at ambient temperatures. The alloy NPs strongly absorb light and efficiently enhance the conversion of several reactions, including Suzuki-Miyaura cross coupling, oxidative addition of benzylamine, selective oxidation of aromatic alcohols to corresponding aldehydes and ketones, and phenol oxidation. The Au/Pd molar ratio of the alloy NPs has an important impact on performance of the catalysts since it determines both the electronic heterogeneity and the distribution of Pd sites at the NP surface, with these two factors playing key roles in the catalytic activity. Irradiating with light produces an even more profound enhancement in the catalytic performance of the NPs. For example, the best conversion rate achieved thermally at 30 C for Suzuki-Miyaura cross coupling was 37% at a Au/Pd ratio of 1:1.86, while under light illumination the yield increased to 96% under the same conditions. The catalytic activity of the alloy NPs depends on the intensity and wavelength of incident light. Light absorption due to the Localized Surface Plasmon Resonance of gold nanocrystals plays an important role in enhancing catalyst performance. We believe that the conduction electrons of the NPs gain the light absorbed energy producing energetic electrons at the surface Pd sites, which enhances the sites' intrinsic catalytic ability. These findings provide useful guidelines for designing efficient catalysts composed of alloys of a plasmonic metal and a catalytically active transition metal for various organic syntheses driven by sunlight.

Full text of DOI:10.1021/ja400527a

Article
pubs.acs.org/JACS
Enhancing Catalytic Performance of Palladium in Gold and Palladium
Alloy Nanoparticles for Organic Synthesis Reactions through Visible
Light Irradiation at Ambient Temperatures
Sarina Sarina,† Huaiyong Zhu,*,† Esa Jaatinen,† Qi Xiao,† Hongwei Liu,† Jianfeng Jia,‡ Chao Chen,†
and Jian Zhao†
†School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane QLD4001, Australia
‡School of Chemical and Material Science, Shanxi Normal University, Linfen 041004, China
S
* Supporting Information
ABSTRACT: The intrinsic catalytic activity of palladium (Pd)
is significantly enhanced in gold (Au)-Pd alloy nanoparticles
(NPs) under visible light irradiation at ambient temperatures.
The alloy NPs strongly absorb light and efficiently enhance the
conversion of several reactions, including Suzuki-Miyaura cross
coupling, oxidative addition of benzylamine, selective oxidation
of aromatic alcohols to corresponding aldehydes and ketones,
and phenol oxidation. The Au/Pd molar ratio of the alloy NPs
has an important impact on performance of the catalysts since
it determines both the electronic heterogeneity and the
distribution of Pd sites at the NP surface, with these two
factors playing key roles in the catalytic activity. Irradiating with light produces an even more profound enhancement in the
catalytic performance of the NPs. For example, the best conversion rate achieved thermally at 30 °C for Suzuki-Miyaura cross
coupling was 37% at a Au/Pd ratio of 1:1.86, while under light illumination the yield increased to 96% under the same
conditions. The catalytic activity of the alloy NPs depends on the intensity and wavelength of incident light. Light absorption due
to the Localized Surface Plasmon Resonance of gold nanocrystals plays an important role in enhancing catalyst performance. We
believe that the conduction electrons of the NPs gain the light absorbed energy producing energetic electrons at the surface Pd
sites, which enhances the sites’ intrinsic catalytic ability. These findings provide useful guidelines for designing efficient catalysts
composed of alloys of a plasmonic metal and a catalytically active transition metal for various organic syntheses driven by
sunlight.
this process the NPs’ conduction electrons gain the irradiation
energy resulting in high energy electrons at the surface, which is
desirable for activating molecules on the NPs for chemical
reactions. Photocatalysts consisting of gold and silver NPs, that
function via this mechanism have been developed.4−9 The
obvious advantage of such a process is that the energy of
incident light is very efficiently coupled into the reactant
molecules, and not dispersed into the other components of the
reaction system, such as the container, solvent and the material
that supports the NPs.11−13 New catalyst structures could be
designed that exploit the vast potential of this process where we
use light, instead of heat, as the energy source to drive reactions
with unprecedented efficiency.
1. INTRODUCTION
Palladium (Pd) is well-known to be catalytically active for many
reactions of important organic syntheses.1−3 Many of these
reactions are conducted under heating to improve the yield and
achieve better reaction efficiency. As thermal enhancement of
catalytic performance is energy intensive, it would be a
significant breakthrough if we could enhance the catalytic
activity of palladium at ambient temperatures using sunlight,
the reliable, abundant, and “green” energy source.
While many of the organic synthesis reactions catalyzed by
homogeneous Pd catalysts may achieve high yields, it is difficult
to recover the catalysts postreaction. We aim to develop
efficient, heterogeneous catalysts that function under mild
ambient conditions and that can be recovered readily for reuse.
It is well-known that gold nanoparticles (AuNPs) exhibit
strong visible-light absorption due to the localized surface
plasmon resonance (LSPR),4−7 and that they also absorb
ultraviolet (UV) light due to interband electron transitions
(from 5d to 6sp).8−10 The LSPR effect is the collective
oscillation of conduction electrons in the NPs, which resonate
with the electromagnetic field of the incident light. Through
With this goal in mind, we have incorporated Pd into AuNPs
and utilize the engineered properties of the composite alloy
NPs to catalyze various reactions with visible light. The catalytic
properties of the alloy NP are significantly different from those
of pure AuNP or PdNP because of the effect that alloying has
Received: January 16, 2013
Published: March 25, 2013
© 2013 American Chemical Society
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with scanning beam diameter down to 1.0 nm. X-ray diffraction
(XRD) patterns of the sample powders were collected using a Philips
PANalytical X’pert Pro diffractometer. CuKα radiation (λ = 1.5418 Å)
and a fixed power source (40 kV and 40 mA) were used. DR-UV−vis
spectra of the sample powders were examined by a Varian Cary 5000
spectrometer.
on the NP’s surface electronic properties. Given that the
electronegativity of Pd (2.20) is lower than that of Au (2.54)
there will be a charge heterogeneity at the alloy NP surface,
with both electron-rich sites and slightly positively charged sites
present.14−17 The electronic heterogeneity means that inter-
actions between the alloy NPs and both electrophilic or
nucleophilic reactant molecules can be enhanced.18,19 In
addition, upon formation of the alloy, electrons will flow
across the Au and Pd interface until the electron chemical
potential equalizes throughout the alloy NP. Then when the
NP is irradiated with light, additional electrons energized
through light absorption are also transported since they can
remain in an excited “hot” state for up to 0.5−1 ps.8,20 This
results in energetic electrons collecting at the Pd sites on the
NP surface. Thus, it is reasonable to expect that under visible
and UV irradiation the Pd sites with the energetic electrons at
the alloy NPs surface will exhibit superior catalytic activity to
that of pure palladium NPs alone. This significant enhancement
of the intrinsic catalytic activity of palladium makes it possible
to apply the alloy NP catalysts to drive various reactions at
ambient temperatures with light.
Activity Test. The suspension of catalyst powder, solvent and the
reactant was placed in a chamber in which a 500 W Halogen lamp
(from Nelson, wavelength in the range 400−750 nm) was used as a
light source and the light intensity was usually 0.30 W/cm2 (except for
the experiments investigating the impact of the intensity). The
temperature of the reaction system was carefully controlled with an air-
conditioner attached to the chamber. The reaction system under light
illumination was maintained at the same temperature as the
corresponding reaction system in the dark to ensure that the
comparison is meaningful. The details of the reaction systems are
given briefly as footnotes of Table 1 for each reaction. At given
irradiation time intervals, 2 mL aliquots were collected, centrifuged,
and then filtered through a Millipore filter (pore size 0.45 μm) to
remove the catalyst particulates. The filtrates were analyzed by gas
chromatography (HP6890 Agilent Technologies) with a HP-5 column
to measure the concentration change of reactants and products.
3. RESULTS AND DISCUSSION
In the present study, several organic reactions were studied
for confirming the general applicability of Au−Pd alloy NP
catalysts for various reactions under visible light irradiation.
Suzuki-Miyaura cross-coupling is a widely used reaction for the
formation of C−C bonds,2,21,22 and oxidative addition of
benzylamine is an effective route to produce imine (C−N
coupling).23−25 Imine derivatives are important building blocks
for the synthesis of fine chemicals and pharmaceuticals,26,27 and
the strategy for the direct oxidation of amines has attracted
much interest.25,28−30 Selective (partial) oxidation of aromatic
alcohols to the corresponding aldehydes or ketones with O2 is
also a class of fundamental reactions for organic synthesis.31
The product carbonyl compounds can serve as important and
versatile intermediates for fine chemicals synthesis.32−35 In
catalyzing these reactions, we found that the alloy NPs
outperformed pure NPs of Au and Pd and that, in addition,
when irradiated with visible light the performance of the alloy
NPs was significantly enhanced.
Transmission electron microscopy (TEM) analysis of the alloy
NPs (Figure 1A−C) shows that the mean diameters of the Au−
Pd alloy NPs are less than 8 nm (Figure 1B). Figure 1D is a line
profile analysis of the energy dispersion X-ray (EDX) spectrum
for a typical Au−Pd alloy NP in Figure 1A, showing that the
NP consists of both Au and Pd distributed spherically around a
common center, which means that the two metals exist as
binary alloy NPs in this sample.
2. METHODS
Catalyst Preparation. Catalysts with 3 wt % of pure gold NPs on
ZrO2 (labeled 3%Au), 3 wt % of pure Pd NPs on ZrO2 (3%Pd) and
three catalysts of Au and Pd alloy NPs supported by ZrO2 (abbreviated
Au−Pd@ZrO2), with different Au/Pd ratios were prepared by
impregnation-reduction method. For example, 1.5 wt %Au−1.5 wt %
Pd/ZrO2 was prepared by the following procedure: 2.0 g ZrO2 powder
was dispersed into 15.2 mL of 0.01 M HAuCl4 aqueous solution and
28.3 mL of 0.01 M PdCl2 aqueous solution were added while
magnetically stirring. A total of 20 mL of 0.53 M lysine was then added
into the mixture with vigorous stirring for 30 min. To this suspension,
10 mL of 0.35 M NaBH4 solution was added dropwise in 20 min,
followed by an addition of 10 mL of 0.3 M hydrochloric acid. The
mixture was aged for 24 h and then the solid was separated, washed
with water and ethanol, and dried at 60 °C. The dried solid was used
directly as catalyst. Catalysts with other Au/Pd ratios were prepared in
a similar method but using different quantities of HAuCl4 aqueous
solution or PdCl2 aqueous solution.
Catalyst Characterization. A TEM study and line profile analysis
by the energy dispersion X-ray spectrum technique carried out on a
Philips CM200 TEM with an accelerating voltage of 200 kV were used
to characterize the catalysts. The Au and Pd content of the prepared
catalysts were determined by EDX technology using the attachment to
a FEI Quanta 200 Environmental SEM. The element line scanning was
conducted on a Bruker EDX scanner attached to JEOL-2200FS TEM
Figure 1. (A) TEM image of Au−Pd@ZrO2 catalyst. The arrows
indicate Au−Pd NPs. (B) Particle size distribution of Au−Pd@ZrO2
used in this study. (C) High resolution TEM images of an alloy
particle in catalyst. (D) The line profile analysis of EDX spectra for a
typical Au−Pd NP indicated by the red square and the inset in (A),
providing the information of the elemental composition and
distribution of the NP.
Evidence of the formation of Au−Pd alloy NPs is seen in the
observed light absorption of the sample as shown in Figure 2.
Here, the spectrum of the Au−Pd alloy NPs sample is clearly
different from the spectra of the pure metal NPs. This is
believed to be associated with the formation of Au−Pd alloy
NPs.36−38 ZrO2 has a band gap of about 5 eV39 and exhibits
weak visible light absorption at wavelengths above 400 nm, and
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of 1:1.86. The results of reactions in the dark at same
temperature and performance of Au@ZrO2 and Pd@ZrO2
catalysts are also provided for comparison. The overall metal
content in the three catalysts was the same being 3 wt % of the
catalyst. As can be seen in Table 1, the irradiation increased the
product yield of the Suzuki coupling at 30 °C to 96% (from
37% in the dark), and the yield of oxidation of benzylamine at
45 °C to 95% (from 36% in the dark at 45 °C), and the yield of
selective oxidation of benzyl alcohol to 99% (from 44% in dark
at 45 °C). Control experiments using ZrO2 (without Au−Pd
alloy NPs) as the catalyst were performed. No conversion was
observed for the reaction when the system was illuminated with
incident light nor when the reaction was allowed to proceed in
the dark. This confirms that the catalytic activity is due to the
Au−Pd alloy NPs and that the catalytic enhancement observed
when the system was irradiated with light is also caused by the
NPs. These results indicate that the Au−Pd alloy NPs function
as photocatalysts and that under visible light irradiation the
catalytic performance of palladium for these organic synthesis
reactions at ambient temperatures is significantly enhanced.
The data obtained from the three reactions will be discussed in
more detail later.
The results in Table 1 show an interesting trend. For
reactions conducted in the dark where the PdNPs exhibit
substantial activity and the AuNPs showed some, albeit lower,
activity, such as the first four reactions in Table 1, the Au−Pd
alloy NPs exhibited superior photocatalytic activity to both the
AuNPs and the PdNPs. However, for reactions that pure
AuNPs exhibit significant activity but PdNPs do not, such as
the last two reactions in Table 1, the photocatalytic
performance of the Au−Pd alloy NPs was inferior to that
observed for the pure AuNPs. Thus, for this second class of
Figure 2. Diffuse reflectance UV−visible (DR-UV−vis) spectra of the
Au−Pd@ZrO2 catalyst and their comparison with pure Au@ZrO2 and
Pd@ZrO2. The inset is a photograph of the catalysts.
therefore, the ZrO2 support itself does not contribute to
photocatalytic activity. In contrast, all the Au, Pd, and Au−Pd
alloy catalysts on the ZrO2 supports, display high levels of
absorption in the UV and visible range, indicating that solar
energy is strongly coupled to the metal NPs. The absorption
peak at 520 nm in the spectrum of the pure AuNP sample
corresponds to the LSPR of the AuNPs.4−7 The presence of the
support and its interaction with the AuNPs can shift and
broaden this peak. The AuNPs also absorb UV irradiation
through interband (5d → 6sp) transitions.8−10 The LSPR for
the pure PdNPs is deep within the UV region so its light
absorption at solar wavelengths occurs through both LSPR and
interband electron transition contributions.
Table 1 shows the results of light-enhanced reactions using
the Au−Pd@ZrO2 catalyst with an optimal Au/Pd molar ratio
Table 1. Performance of the Catalysts of Au−Pd Alloy NPs, Pure AuNPs, and Pure PdNPs Supported on ZrO2 for Different
a
Types of Reactions
a
The data of the reaction in the dark at the same temperature is provided for comparison. The detailed data, including turnover number, turnover
frequency, and quantum yield, are given in Tables 2−4.
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Table 2. Suzuki-Miyaura Cross-Coupling Reactions Catalyzed with the Au−Pd@ZrO2 Catalysts of Various Au/Pd Molar Ratios
a
under Visible Light Irradiation (Numbers in Red Color) and in the Dark (Numbers in Black Color)
a
[a] Molar ratio; [b] Reaction time 6 h; [c] Reaction time 5 h; [d] Reaction time 2 h; [e] Reaction time 4 h; [f] Reaction time 22 h. Reaction
conditions: 1 mmol of aryl iodide, 1.5 mmol of aryl boronic acid, 50 mg (containing 3% of metals) of catalyst and 3 mmol of base K2CO3 in DMF/
H2O = 3:1 (solvent) at 30 °C and 1 atm of argon. TON and TOF were calculated based the total amount of metal(s). The calculation method of
quantum yield (Q.Y.) is given in Supporting Information (SI).
Table 3. Catalytic Oxidation of Benzylamine into Imines with the Au−Pd@ZrO2 Photocatalysts of Various Au/Pd Molar Ratios
a
under Visible Light Irradiation (Number in Red Color) and in the Dark (Number in Black Color)
a
[a] Molar ratio; [b] Reaction time 96 h; [c] Reaction time 40 h; [d] Reaction time 48 h. Reaction conditions: 1 mmol of reactant, 50 mg
(containing 3% of metals) of catalyst in CH3CN solvent at 45 °C and 1 atm of O2. TON and TOF were calculated based the total amount of
metal(s). The calculation method of Q.Y. is given in SI.
reactions, where the catalytic activity is Au dominated, the Au−
Pd alloy NPs did not perform better than the mono metal NPs.
These results imply that, in reactions where the catalytic activity
is Pd dominated, the intrinsic catalytic activity of palladium is
significantly enhanced in the alloy NPs by light irradiation, even
when at ambient temperatures. This provides a general guiding
principle for determining under what conditions that light-
enhanced performance of alloy NP catalysts can be expected as
well as providing design criteria for selecting transition metals
to alloy with gold to catalyze specific reactions with light driven
processes. These concepts will be demonstrated in the
following discussion.
PdNPs are not effective catalysts in the hydroamination of
alkyne or the reduction of nitrobenzene to azobenzene (entries
5 and 6 in Table 1). Because the catalytically active sites for
these two reactions are not the Pd sites at the surface, it is not
expected that the photocatalytic performance of the alloy NP
would be any better than that of AuNPs. The significant role
that the Pd sites at the external surface of the alloy NPs play in
some of the catalytic processes, such as the selective oxidation
of benzyl alcohol, was demonstrated by preparing a sample of
NPs with a Pd core and an Au shell by reducing HAuCl4 on a
Pd-ZrO2 support with H2. This sample has almost no Pd sites
at the NP surface and exhibited low activity for selective
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Table 4. Catalytic Oxidation of Aromatic Alcohols with the Au−Pd@ZrO2 Catalysts of Various Au/Pd Molar Ratios under
a
Visible Light Irradiation (Number in Red Color) and in the Dark (Number in Black Color)
a
[a] Molar ratio; [b] Reaction time 5 h; [c] Reaction time 4 h; [d] Reaction time 2 h; [e] Reaction time 20 h; [f] Reaction time 22 h. Reaction
conditions: 2 mmol of the reactant, 50 mg (containing 3% of metals) of catalyst in trifluorotoluene solvent at 45 °C and 1 atm of O2. TON and TOF
were calculated based the total amount of metal(s).
oxidation of benzyl alcohol (10% conversion compared to
100% achieved by the Au−Pd alloy NP).
Generally, the synthesis of imine derivatives involves
condensation of an amine and a carbonyl compound. To
solve the problem caused by the excessively active nature of
ketones or aldehydes, an alternative strategy to the direct
oxidation of amines has attracted much interest.25,28−30,41
Ruthenium based catalysts have been developed for this
purpose,42 but they oxidize only a limited range of amines
into imines at relatively high temperature (mostly >110 °C). A.
We also found that the performance of the Au−Pd@ZrO2
catalysts strongly depends on the Au/Pd molar ratio for the
first three reactions in Table 1 both in the presence and absence
of light. The detailed data of the three reactions, Suzuki-
Miyaura cross-coupling, oxidative addition of benzylamine, and
selective oxidation of aromatic alcohols, are provided in Tables
2−4.
́
Grirrane et al. reported 5% Pd/TiO2 catalyst can oxidize
benzylamine at 100 °C temperature and 5 atm O2 pressure,
with a product yield that may reach 75% after 30 h (5% Au/
TiO2 and Pt/TiO2 show catalytic activity under the same
reaction conditions and reaction time with product yields of
98% and 22%).28 Photocatalytic processes with carbon nitride
(graphitic, C3N4) for oxidative coupling of benzylamine were
revealed by F. Su et al., with the reactions taking place under 5
atm O2 pressure at 60−80 °C with visible light irradiation.
These high reaction temperatures and pressure indicate that
this reaction requires critical conditions when using supported
noble metal nanoparticle as the catalyst. The most moderate
reaction conditions reported for this reaction so far is the UV
light photocatalytic oxidation of amines with TiO2.43 However,
the UV light irradiation may result in a relatively lower product
selectivity.
The first successful example of palladium-catalyzed aerobic
oxidation of alcohols was reported by Blackburn and Schwartz
in 1977,44 however, only a few heterogeneous catalysts of
supported PdNPs and PdII species are available to date. For
example, Pd on hydrotalcite,45,46 carbon,45,47 Al2O3,45 SiO2,45
pumice,48 SiO2−Al2O3 mixed oxide,49 TiO2,50 and polymer-
There are many reports on the Suzuki-Miyaira coupling
reactions via homogeneous catalytic processes catalyzed by
palladium complexes with various ligands. With these
homogeneous catalytic processes it is difficult to recycle the
catalyst, which is an important consideration from an industrial
application perspective and in minimizing the impact on the
environment.40 The Suzuki-Miyaira coupling can also be
catalyzed by heterogeneous catalysts of the supported PdNPs,
but all of them are at elevated temperatures (≥60 °C). For
example, PdNPs on mesoporous silica MCM41 achieved a yield
of 93% at 78 °C after 5 h. When the NPs were supported on
carbon nanotubes, the yield was 87−94% at 70−100 °C. A
table summarizing the various reaction conditions previously
considered and details of the catalyst used are provided in SI
(Table S1). This table shows that the Au−Pd alloy NP catalysts
under investigation here can drive the same reactions at much
lower temperatures (only 30 °C) under visible light irradiation
while still achieving excellent yields. This means that by alloying
with Au, we introduce light, which enhances the performance of
Pd catalyst for synthesis reactions.
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supported Pd.51 As shown in Table S2 (in SI), those catalysts,
both in the metallic NPs or immobilized ionic state, can
catalyze the benzyl alcohol oxidation at elevated temperatures
and high pressure. The process with PdNPs on hydrotalcite
achieved a yield of 99% in an hour at 90 °C. Au−Pd alloy NPs
supported on TiO2 were also used as the catalyst of this
reaction, which exhibited improved catalytic activity (a yield
≤70% in 8 h), compared to PdNPs, but at a higher temperature
(100 °C) and pressure (2 atm O2). The lower yield could be
attributed to the lower stability of the product, benzaldehyde, at
higher temperatures and pressures. The selective oxidation with
O2 is an exothermic reaction. Hence, the photocatalytic process
at low reaction temperatures proposed in the present study is
thermodynamically preferred. Utilizing light energy to promote
the efficiency of this process requires low energy input.
The results in Tables 1−4 show that, without light
irradiation, pure AuNPs exhibit no catalytic activity for
Suzuki-Miyaura cross-coupling, a low activity (with a yield of
12%) for the oxidation of benzylamine into imine, and very low
activity (yield of 3%) for the oxidation of benzyl alcohol. The
activity of the pure PdNPs for these three reactions are better
than those observed with AuNPs, but still relatively low (yields
are between 11 and 21%). The alloy NPs exhibited improved
activity for the reactions in the dark. For example, for the three
reactions, the yields achieved with the alloy NPs with the
optimal Au/Pd ratio of 1:1.86 are between 36 and 44%. The
underlying cause of this dependence on alloy composition has
not been fully understood, yet. We note that the optimal Au:Pd
molar ratio for the Au−Pd alloy NP catalysts used for reactions
at high temperature (≥190 °C) and high pressure, reported by
other groups,52,53 is similar to that in the present study.
Visible light irradiation of the alloy NP catalysts significantly
increased the yields of the reactions (up to 95 and 100%). It
enhances the conversion rate of the reactions while it has little
impact on the product selectivity (see Tables 2−4).
Figure 4. Dependence of the catalytic activity of Au−Pd alloy NPs for
the Suzuki-Miyaura coupling reaction on the wavelength of the light
irradiation. Both light driven reaction and the reaction in the dark were
conducted at 30 1 °C.
To ensure that thermal effects could be discounted, the
temperature of the reaction mixture was carefully controlled
and maintained at 30 °C
1 °C. The contribution of the
conversion efficiency that can be attributed to light irradiation
was calculated by subtracting the conversion efficiency of the
reaction in the dark from that observed when the system was
irradiated, with both reactions occurring at same temperature.
Here the conversion efficiency observed in the absence of light
is the thermal contribution. The relative contributions of light
and thermal processes to the conversion efficiencies are shown
in Figure 3. It can be seen, the higher the light intensity, the
greater the contribution due to light irradiation to the overall
conversion rate. For example, when the light intensity is 0.2 W/
cm2, only 35% of the conversion results from light irradiation
with 65% attributed to the thermal effects at 30 °C 1 °C.
When the light intensity is 0.4 W/cm2, 60% of the conversion is
due to light irradiation. For the selective oxidation of benzyl
alcohol, the contribution attributed to light irradiation also
increased from 24% at 0.1 W/cm2 to 36, 49, and 56% at 0.2,
0.3, and 0.4 W/cm2 (Figure S2 in SI). This trend clearly
demonstrates the enhancement that light irradiation produces
in the catalytic activity of Au−Pd alloy NPs.
To better understand the enhancement of the catalytic
performance that occurs through light irradiation of the Au−Pd
alloy NPs, the dependence of the catalyzed reaction on the
light’s wavelength and intensity were investigated. The specific
example of Suzuki-Miyaura coupling is used with the results
shown in Figures 3 and 4.
As shown in Figure 3, increasing the light intensity resulted
in an almost linear increase in the conversion of the Suzuki-
Miyaura coupling reaction catalyzed by the Au−Pd alloy NPs.
To determine the effect on the conversion efficiency that the
wavelength of the light used to irradiate the systems has, we
used various optical low pass filters to block light below specific
cutoff wavelengths. Thus, we could tune the wavelength range
that irradiated the reaction systems. The dependence of the
catalytic performance on wavelength range for the Suzuki
coupling reaction is illustrated in Figure 4 and the results for
benzyl alcohol oxidation are provided in Supporting
Information as Figure S3.
Without any filters, light with wavelengths between 400−800
nm illuminated the system and a conversion efficiency of 95%
was observed. Applying a filter that transmitted wavelengths in
between 490−800 nm saw the conversion efficiency decrease to
80%. When the light was filtered so that only wavelengths
between 550−800 nm were transmitted the conversion
dropped to 62%, while filtering the incident light to transmit
wavelengths between 600−800 nm saw the conversion decrease
further to 41%. Given that the reaction conversion in the dark
at this temperature was 37%, irradiating the system with light
that has wavelengths longer than 600 nm produces little change
(4% = 41−37%) to the reaction. In contrast the contribution
from the light with wavelengths in the range of 400−600 nm is
55% (=96−41%), accounting for 57% of the total conversion
Figure 3. The dependence of the catalytic activity of Au−Pd alloy NPs
for the Suzuki-Miyaura coupling reaction on the intensity of the light
irradiation. Both light driven reaction and the reaction in the dark were
conducted at 30 1 °C.
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Figure 5. Dependence of Au−Pd@ZrO2 performance on the Au/Pd molar ratio of the alloy NPs in light-enhanced reaction (A) and in the dark
reaction (B) for the three reactions in the present study. (C) Apparent activation energies of the reactions are calculated for the visible-light-
enhanced reaction and the reaction in the dark, contribution of light irradiation is calculated from difference of conversion of two processes (with
and without light) and presented by percentage.
rate (55/96 × 100%) and over 91% of the contribution from
visible light irradiation [54/(96−37) × 100%]. Similarly, we
can calculate the contribution to the conversion efficiency for
any of the specific wavelength ranges used. As illustrated in
Figure 4, light in the wavelength ranges 490−550 nm and 550−
600 nm account for 30 and 35%, respectively, of the light-
induced conversion (conversion difference of visible-light-
enhanced reaction and the reaction in the dark), while light
in the wavelength ranges of 400−490 nm and 600−800 nm
contribute the remaining 35%.
Similarly, in the selective oxidation of benzyl alcohol,
irradiation with light with wavelengths between 490−550
(32%) and 550−600 nm (34%) has the largest effect on
conversion efficiency, while light at other wavelengths ranges
produces the remaining 34% of light-induced conversion
(Figure S2 in SI). A common feature for these light-enhanced
reactions is that the largest contribution to the catalytic
performance results from radiation with wavelengths between
490 and 600 nm (>64%). The energy absorbed by the NPs
from light in this wavelength range was estimated from the
overlap of the absorption spectrum of the Au−Pd alloy NPs
with the spectral irradiance of incandescent light used (Figure
S4 in SI), to be 36.2% of the total light energy absorbed by the
NPs. Given that the LSPR peak of AuNPs is in this wavelength
range, these results suggest that the gold in the alloy NPs
functions as an antenna for visible light absorption.
It was also found that the wavelength range producing the
largest performance enhancement with the Au−Pd alloy NPs
(490−600 nm) is broader than that displayed by AuNPs. In our
recent study on selective reactions catalyzed by AuNPs
illuminated with visible light (acetophenone hydrogenation to
1-phenyl ethyl alcohol and styrene oxide reduction to
styrene),54,55 light in the wavelength range 490 to 550 nm
was found to provide the main contribution that drove these
reactions, as shown in Figure S4 in SI. The light-induced
contributions attributed to this wavelength range for the two
reactions were 41 and 65%, respectively. The contribution of
light in the wavelength range of 550−600 nm was much less
(24 and 0%). While for the Au−Pd alloy NPs described in this
present study, the contribution of light with wavelengths in the
range 550−600 nm is much more significant. This observation
confirms that Au−Pd alloy NPs have the potential to utilize
visible light energy from broader portion of the spectrum than
AuNPs in light-enhanced reactions.
the observations of the same reactions performed in the dark
are shown in Figure 5B for comparison. All three reactions
achieved the highest yield of target products when the molar
ratio was 1:1.86. Alloy NPs with other Au/Pd ratios (1:5.58, 1:1
and 1:0.62) proved less active. Interestingly, the dependence on
the Au/Pd ratio of the alloy NPs for the reactions in the dark is
similar to that observed for the light-enhanced reactions but
with much lower conversion efficiencies. The figure shows
while light illumination and alloy composition both impact on
catalytic performance of the alloy NPs, that irradiation with
light has a more pronounced effect than the Au/Pd ratio.
A comparison between the contributions to catalytic
performance of light irradiation and the alloy component
ratio of Au and Pd was performed. We define the contribution
of ratio (CoR) as the difference between the conversion rates of
thermal reactions undertaken without light when catalyzed by
alloy NPs with different Au/Pd ratios; and the contribution of
light irradiation (CoL) as the increase in the conversion rate
from that observed in the dark when the incident light is
introduced. For the Suzuki-Miyaura coupling (Table 2), the
CoR calculated between any two Au/Pd ratios is in the range of
4 to ∼31%, while the CoL is in the range of 22 to ∼59%, which
indicates that the light irradiation enhances the catalytic activity
for this reaction more significantly than the component ratio.
For the other two reactions (Tables 3 and 4), this trend is even
more significant: CoRs of the benzylamine oxidation and
benzyl alcohol oxidation are in the ranges 1 to ∼15% and 1 to
∼17%, respectively, which are much lower than the CoLs of 39
to ∼59% and 41 to ∼61%. These results demonstrate that a
greater improvement in the catalytic performance of Au−Pd
alloy NPs is obtained with light irradiation than by varying the
alloy component ratio.
One possible explanation for these observations is that the
charge heterogeneity at the alloy NPs’ surface results in the
improved catalytic activity of the alloy structure,15−17 as the
dependence of the performance on the ratio was observed in
reactions in the dark with a similar trend to that observed with
the light driven reactions (Figure 5A,B). At the Au/Pd ratio of
1:1.86, the surface charge heterogeneity is optimal for the
catalysis both in the presence and absence of light. To further
confirm the charge heterogeneity at the surface of the alloy
NPs, simulations using the density function theory (DFT) were
carried out for electron states with and without light irradiation;
the wavelength of the light is chosen around the SPR
absorption of Au in Figure 2 (530 and 535 nm). Calculation
capacity limitations of our DFT simulation necessitated the
examination of a small gold cluster Au32 and an alloy cluster
The dependence of the catalytic performance of three light-
enhanced reactions on Au/Pd molar ratio of the alloy NPs is
illustrated in Figure 5A based on the data of Tables 2−4, while
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Au12Pd20 as a first step. The charge heterogeneity, indicated by
the natural charge distributions, of the two clusters in the
ground state (corresponding to the state in the dark) and an
excited state (corresponding to that under light irradiation)
were simulated. The detailed calculation method and the
calculated natural charge distributions are given in SI (TextS2).
The results of the calculation confirm that a charge
heterogeneity exists even in the pure gold Au32 clusters and
that alloying increases the charge heterogeneity of the Au12Pd20
cluster surface (Figure S5 in SI). This charge heterogeneity is to
be expected and in agreement with previous reports.14,56 When
the alloy cluster is irradiated with light, the conduction
electrons absorb energy, raising the cluster into one of the
excited states. Interestingly for both clusters, the calculated
natural charge distribution for the excited state is similar to that
calculated for the ground state. Because the charge distribution
of the cluster and, therefore, the charge heterogeneity, is
strongly influenced by the Au/Pd ratio, changing this ratio will
alter the charge distributions of ground and excited states in a
more or less similar manner. By extension, varying the Au/Pd
ratio will produce the same trend in the catalytic performance
of the Au−Pd@ZrO2 alloy NPs, regardless of whether the
reaction system is irradiated with light or in the dark. The
increased charge heterogeneity means that interactions between
the alloy NPs and reactant molecules is enhanced.12,13 There is
a much higher probability that the reactant molecules are
adsorbed on the alloy NPs than on AuNPs. When the alloy NPs
are irradiated with light, the conduction electrons are elevated
into excited states through absorption of light energy increasing
the NPs capability of inducing reactions involving the adsorbed
reactant molecules, over that seen when the same reaction is
performed in the dark.
Utilizing this knowledge and information from literature, we
propose reaction pathways for three reaction processes that use
the Au−Pd@ZrO2 photocatalyst. The first key step of Suzuki-
Miyaura coupling is the activation of iodobenzene on the
electron-rich Pd sites at the surface of Au−Pd alloy NPs (as
illustrated in S1A in SI), which is made possible because of the
intrinsic catalytic activity of palladium to Suzuki-Miyaura
coupling.40,57 Following on from this, the aromatic borate
species react with the activated aryl species from the aryl iodide
on the electron-rich surface Pd sites (so-called transmetalation).
The final step is reductive elimination of the cross-coupling
product R1Ph-PhR2 molecule, which returns the initial alloy
surface, completing the photocatalysis cycle. Light irradiation
significantly enhances the surface charge heterogeneity, which
subsequently increases the interaction (absorption of reactant
molecule on NP surface) between reactant molecules and Au−
Pd alloy NPs. This then results in a higher catalytic activity
when the system is irradiated with light than when it is in the
dark.
We calculated the apparent activation energy of the reaction
under light irradiation and in the dark by applying the
Arrhenius equation and using the reaction data taken at several
temperatures, dependent on the reaction, in the range of 30 to
70 °C. For each reaction, the difference between the activation
energies of the light-enhanced process and the process in the
dark (ΔEa) indicates the contribution due to light irradiation in
reducing the apparent activation energy.54 For example, as
illustrated in Figure 5C, the apparent activation energy for the
Suzuki-Miyaura coupling in the dark is ∼49.2 kJ/mol, while it is
∼33.7 kJ/mol for the reaction under visible light illumination.
Visible light irradiation reduces the activation energy of Suzuki-
Miyaura coupling by 15.5 kJ/mol, which represents 31% of the
activation energy. Similarly, the activation energies of selective
oxidation of benzyl alcohol and benzylamine oxidative coupling
are reduced by 15.8 and 13.1 kJ/mol, respectively, which means
that visible light irradiation reduces the apparent activation
energy of these two reactions by 21 and 10%, respectively.
The apparent quantum yield of the reactions with the Au−
Pd@ZrO2 catalysts when illuminated with visible light were
estimated by subtracting the conversion rate of a reaction
performed in the dark from that observed when the system was
irradiated with light and performed at the same temperature.
The yields are 2.7, 3.8, and 8.1%, respectively, for three
reactions, which is much higher than that of the processes with
the well-known TiO2-based photocatalyst under UV light
(1.5%).12,60 It should be noted that the yields reported here are
approximations, as the absorption spectra shown include both
absorption and scatter. As a consequence, the actual light
absorption is lower than that derived from the absorption
spectra, which means that the actual yields will be even higher
than those we have estimated. This demonstrates that in these
alloy NP structures the energy of incident light is very
efficiently utilized to enhance the intrinsic catalytic activity of
palladium.
Finally, the general concept that organic syntheses can be
driven efficiently by sunlight at ambient temperatures with alloy
NP catalysts composed of a plasmonic metal and a catalytically
active transition metal is supported by the results presented
here with Pd and Au.
4. CONCLUSIONS
The findings in the present study demonstrate that the intrinsic
catalytic activity of Pd can be significantly enhanced at ambient
temperatures by light irradiation of alloy Au−Pd NPs. In
essence, this is because the charge heterogeneity of the alloy
NP surface is greater than that of AuNP surface, leading to a
stronger interaction between the alloy NPs and reactant
molecules. The conduction electrons of the NPs gain the
absorbed light energy, generating energetic conduction
electrons at the surface Pd sites to which the reactant molecules
have an affinity. The energetic electrons at the sites enhance
their intrinsic catalytic ability to catalyze reactions. The Pd sites
and the charge heterogeneity at the NP surface play key roles in
the catalytic reactions. Because the distribution of Pd sites and
charge heterogeneity at the NPs depends on the Au/Pd molar
ratio, the ratio has an important influence on the catalytic
performance of the alloy NPs. Because little input energy is
consumed by other components of the reaction system, such as
the solvent, support of the NPs, the atmosphere or container,
this catalyst structure is highly efficient for driving various
chemical reactions with sunlight. The knowledge acquired in
this study may inspire further studies in new efficient catalysts
For the process of oxidative coupling of benzylamine into
imines (shown in Scheme S1B), the most probable mechanism
is that benzylamine is first oxidized into benzaldehyde by
abstraction of α-H from the −CH2− group, followed by the
nucleophilic attack of these nascent aldehydes by the unreacted
amines to yield the corresponding imines.23,24 The abstraction
of α-H from the −CH2− group and formation of alloy-H
species also take place in the first step of the pathway for the
selective oxidation of aromatic alcohols to the corresponding
aldehydes or ketones,58,59 as shown in Scheme S1C. The
dehydrogenated species then undergo a C−H bond cleavage,
yielding aldehyde or ketone as the product.
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ASSOCIATED CONTENT
* Supporting Information
Detailed data, schemes, and methods. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: hy.zhu@qut.edu.au.
■
Notes
(38) Chen, Y. H.; Tseng, Y. H.; Yeh, C. S. J. Mater Chem. 2002, 12,
1419.
The authors declare no competing financial interest.
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(40) Molnar, A. Chem. Rev. 2011, 111, 2251.
ACKNOWLEDGMENTS
The authors also gratefully acknowledge financial support from
the Australia Research Council (ARC DP110104990).
■
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