Visible light enhanced oxidant free dehydrogenation of aromatic alcohols using Au-Pd alloy nanoparticle catalysts

Article abstract of DOI:10.1039/c3gc41866a

We find that visible light irradiation of gold-palladium alloy nanoparticles supported on photocatalytically inert ZrO₂ significantly enhances their catalytic activity for oxidant-free dehydrogenation of aromatic alcohols to the corresponding a

Full text of DOI:10.1039/c3gc41866a

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Visible light enhanced oxidant free
dehydrogenation of aromatic alcohols using
Cite this: Green Chem., 2014, 16, 331
Au–Pd alloy nanoparticle catalysts†
Sarina Sarina,a Sagala Bai,b Yiming Huang,a Chao Chen,a Jianfeng Jia,c Esa Jaatinen,a
Godwin A. Ayoko,a Zhaorigetu Bao*b and Huaiyong Zhu*a
We find that visible light irradiation of gold–palladium alloy nanoparticles supported on photocatalytically
inert ZrO2 significantly enhances their catalytic activity for oxidant-free dehydrogenation of aromatic
alcohols to the corresponding aldehydes at ambient temperatures. Dehydrogenation is also the dominant
process in the selective oxidation of the alcohols to the corresponding aldehydes with molecular oxygen.
The alloy nanoparticles strongly absorb light and exhibit superior catalytic and photocatalytic activity
when compared to either pure palladium or gold nanoparticles. Analysis with a free electron gas model
for the bulk alloy structure reveals that the alloying increases the surface charge heterogeneity on the
alloy particle surface, which enhances the interaction between the alcohol molecules and the metal NPs.
The increased surface charge heterogeneity of the alloy particles is confirmed with density function
theory applied to small alloy clusters. Optimal catalytic activity was observed with a Au : Pd molar ratio of
1 : 186, which is in good agreement with the theoretical analysis. The rate-determining step of the de-
hydrogenation is hydrogen abstraction. The conduction electrons of the nanoparticles are photo-excited
by the incident light giving them the necessary energy to be injected into the adsorbed alcohol mole-
cules, promoting the hydrogen abstraction. The strong chemical adsorption of alcohol molecules facili-
tates this electron transfer. The results show that the alloy nanoparticles efficiently couple thermal and
photonic energy sources to drive the dehydrogenation. These findings provide useful insight into the
design of catalysts that utilize light for various organic syntheses at ambient temperatures.
Received 6th September 2013,
Accepted 16th October 2013
DOI: 10.1039/c3gc41866a
www.rsc.org/greenchem
development of new solar cells and the production of hydro-
gen and oxygen from water.2 To date, only limited progress has
Introduction
Photocatalytic processes are generally able to drive reactions at been reported in applying TiO2 and Nb2O5 semiconductor
ambient temperature and pressure. As a result, product selec- photocatalysts to synthesize organic chemicals.3–5 This is
tivity is improved and unstable intermediates of thermal reac- largely because of the limited light absorption and low
tions may be produced as products of photocatalytic reactions. efficiency of these photocatalysts. Due to their band structure,
However, the bulk of all reported photocatalytic studies focus the well-known TiO2 photocatalysts only show significant light
on semiconductor photocatalysts such as TiO2, ZnO and CdS absorption of ultraviolet (UV) light.1,2,6,7 Consequently, the
and their application in decomposing organic pollutants,1 the efficiency of the semiconductor photocatalysts, expressed as
photon yield, is typically low.7 To overcome this, various
methods have been developed to produce efficient visible light
photocatalysts from semiconducting materials.8 In recent
years, photocatalysis with nanoparticles (NPs) of plasmonic
metals, such as gold (Au), silver (Ag) and copper (Cu), has
attracted significant interest because of their exceptional
absorption of visible light and thus their potential as visible
light photocatalysts.9–17 Plasmonic metal nanoparticles exhibit
strong visible-light absorption due to the localised surface
plasmon resonance (LSPR).18–20 In addition they also strongly
absorb ultraviolet (UV) light due to inter-band electron tran-
sitions (e.g. for Au, between the 5d and 6sp bands).21–23 The
aSchool of Chemistry, Physics and Mechanical Engineering, Queensland University of
Technology, Brisbane, QLD4001, Australia. E-mail: hy.zhu@qut.edu.au;
Fax: +61 7 3138 1804; Tel: +61 7 3138 1581
bSchool of Chemistry, Inner Mongolia Normal University, Hohhot, China.
E-mail: zrgt@imnu.edu.cn
cSchool of Chemical and Material Science, Shanxi Normal University, Linfen 041004,
China
†Electronic supplementary information (ESI) available: Experimental section,
estimation of Au–Pd alloy NPs’ ionic property by free gas model, proposed mech-
anism of oxidative coupling of benzylamine (A) and aromatic alcohol oxidation
(B) over Au–Pd alloy NPs under visible light irradiation, calculation of apparent
activation energy for the reactions. See DOI: 10.1039/c3gc41866a
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NP conduction electrons gain the irradiation energy, resulting related to corrosion and plating out on the reactor wall, hand-
in high energy electrons at the surface. These energetic elec- ling, recovery, and reuse of the catalyst represent serious limit-
trons, which remain in an excited ‘hot’ state for up to ations of these processes.38 Solid catalysts active in the liquid
0.5–1 ps,24 can promote chemical reactions of molecules on phase under mild conditions have a much broader application
the surface of the NPs.25,26
range. Relatively few heterogeneous Pd (supported Pd nanoparti-
The underpinning photocatalytic mechanisms of the Au cles and PdII) are available. For example, Pd on hydrotalcite,39,40
and Ag NPs supported by insulating solids with very wide carbon,39,41 Al2O3,39 SiO2,39 pumice,42 SiO2–Al2O3 mixed oxide,43
band gaps are quite distinct from those of semiconducting TiO244 and polymer supported Pd.45 Those catalysts, both in the
photocatalysts.9–12,25,26 These highly efficient NP photocata- metallic NPs or immobilized ionic state, can catalyse the benzyl
lysts have several important advantages over their extensively alcohol oxidation at elevated temperatures and/or high pressure.
studied semiconductor counterparts. (1) The NP conduction Recently it was reported that activation of molecular oxygen is
electrons gain light energy, resulting in high energy electrons the key step in selective oxidation of aromatic alcohols using
at the NP surface which is advantageous for activating mole- TiO2 as photocatalysts under UV irradiation.
cules adsorbed on the NPs for chemical reactions. (2) Since
The use of Au and Pd alloy NPs as visible light photocata-
both light harvesting and the catalysing reaction take place on lysts for the selective oxidation of aromatic alcohols will have
the NPs, the photon efficiency is not significantly affected by an underlying mechanism different from those aforemen-
photo-excited charge migration. (3) The density of the conduc- tioned processes. It is known that during selective oxidation,
tion electrons at the NP surface is much higher than that at the the aromatic alcohols may undergo abstraction of a hydrogen
surface of any semiconductor, so once photo-energized more atom bonded to the α-carbon atom (the carbon atom of the
electrons are available to drive reactions. (4) The metal NPs have methylene group bonded to the hydroxyl group), which is
a strong affinity for some reactants, such as CO and organic denoted as α-H, followed by abstraction of the hydrogen atom
compounds, making the NPs superior photocatalysts for organic from the hydroxyl group.46,47 If the first hydrogen abstraction
synthesis reactions compared to semiconductor photocatalysts.
is the rate-determining step and can take place via a photo-
However, the total number of chemical reactions that can catalytic process, high reaction temperature and high oxygen
be catalysed by the three plasmonic metals is relatively few pressure are not necessary for selective oxidation. In the
when compared to those thermally catalysed by non-plasmonic present study, we find that light irradiation can drive the α-H
transition metals. To develop catalysts for light driven syn- abstraction of aromatic alcohols with Au–Pd alloy NPs and
thesis of a broad range of organic chemicals, we presented a thus the transformation from alcohol into the corresponding
unique but viable approach: alloying gold and a transition aldehyde can be achieved in an oxidant free environment at
metal such as Pd, which is thermally catalytically active for ambient temperatures. The α-H abstraction is the rate-deter-
many reactions.13 Thus, the light energy absorbed by the gold mining step of the selective oxidation. Theoretical calculations
can enhance the intrinsic catalytic activity of Pd at moderate by two independent methods show that the alloying of gold
temperatures. A typical example is the selective (or partial) oxi- and palladium enhances the interaction between the alcohol
dation of aromatic alcohols to the corresponding aldehydes, molecules and the alloy NPs. The strong interaction facilitates
which can be catalysed by Pd catalysts but not Au catalysts. the transfer of light excited electrons of the alloy NPs to the
Here we show that Au–Pd alloy NPs exhibit superior catalytic alcohol molecules adsorbed on the NPs, and such an electron
activity when exposed to visible light irradiation at ambient transfer enables the hydrogen abstraction under moderate con-
temperatures than that displayed by NPs made from either ditions. Understanding this mechanism is useful for develop-
pure component metals.
ing photocatalytic processes for other important syntheses.
Selective oxidation of alcohols is widely considered one of
the most fundamental transformations in both laboratory and
industrial synthetic chemistry because the product carbonyl
compounds can serve as important and versatile intermediates
for fine chemical synthesis.27–30 Permanganate and dichro-
Results and discussion
Performance of the photocatalysts
mate have been traditionally employed31,32 but they are expens- In the present study, Au and Pd alloy NPs with varying relative
ive and/or toxic. For environmental and economic reasons, ratios were prepared on a ZrO2 support (abbreviated as Au–
metal-catalysed reactions using molecular oxygen as an Pd@ZrO2, and details are given in ESI†). The metal nanoparti-
oxidant are particularly attractive. Many of the reactions are cles on the support were well dispersed avoiding aggregation
conducted under heating and/or high pressure to achieve a of the particle. ZrO2 has a wide band gap (5 eV), and exhibits
better reaction efficiency. Since the first successful example of no visible light absorption. Hence, the support does not con-
palladium-catalysed aerobic oxidation of alcohols in 1977 by tribute to photocatalytic activity. Fig. 1 shows the photo-
Blackburn and Schwartz,33 subsequent efforts have extended catalytic performance of the Au–Pd@ZrO2 photocatalysts with
the substrate scope and efficiency of palladium catalysts. various Au : Pd mass ratios in catalysing the dehydrogenation
However, the reported PdII salt based homogeneous of aromatic alcohols. All experiments were conducted
catalysts34–37 require high catalyst concentration and an excess under an Ar gas atmosphere after a strict freeze–pump–thaw
of ligands or bases. On an industrial scale the problems degassing process to remove O2. The conversion rates achieved
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Fig. 1 Dehydrogenation of aromatic alcohols with the Au–Pd@ZrO2 catalysts of various Au : Pd mass ratios under visible light irradiation (blue bar)
and in the dark (black bar). All of the data represent the averages of triplicate runs with a mean variation of less than 3%. The quantum yield (Q.Y., %)
and its calculation method are given in ESI.† A(1) Benzyl alcohol dehydrogenation using the catalyst of 3% Au–Pd@ZrO2; the reaction proceeded for
5 h. A(2) 4-Methyloxy benzyl alcohol dehydrogenation, reaction for 2 h. A(3) 1-Phenylethanol dehydrogenation, reaction for 22 h. Reaction con-
ditions: 2 mmol of the reactant, 50 mg of catalyst in trifluorotoluene solvent at 45 °C and under 1 atm of Ar after O2 removal using freeze–pump–
thaw degassing. B(1) Benzyl alcohol dehydrogenation, 2% Au–Pd@ZrO2, reaction for 16 h. B(2) 2-Phenylethanol dehydrogenation, 2% Au–Pd@ZrO2,
reaction for 16 h. B(3) 3-Phenylpropanol dehydrogenation, 2% Au–Pd@ZrO2, reaction for 16 h. Reaction conditions: 1 mmol of the reactant, 50 mg
of catalyst in trifluorotoluene solvent at 45 °C and under 1 atm of Ar after O2 removal using freeze–pump–thaw degassing.
under light irradiation (in blue) are compared with those alcohols into the corresponding aldehydes with Au–Pd@ZrO2
obtained in the dark (in black).
The results in Fig. demonstrate that visible light nificantly enhances the dehydrogenation.
irradiation increased the product yield of the dehydrogenation Compared to the Au–Pd@ZrO2 catalyst containing 3% of
photocatalysts is dehydrogenation, and light irradiation sig-
1
product of aromatic alcohols – corresponding aldehydes. For alloy NPs, the Au–Pd@ZrO2 catalyst with lower metal loading
example, the conversion rate of benzyl alcohol with the Au– (2%), the reaction time required to achieve the same conver-
Pd@ZrO2 catalyst with an Au : Pd mass ratio of 1 : 1 is 100%. In sion rate with identical reaction conditions was longer, as
contrast the rate is 43% when the reaction was conducted in shown in Fig. 1B. The impact on the photocatalytic activity of
the dark. Similar trends are observed in other reactants, for varying mass ratios of the two metals with 2% metal loading
example, the conversion rate of 4-methoxybenzyl alcohol is has the same trend as that observed with the catalysts with 3%
99% under light irradiation but 54% in the dark at the same metal loading.
reaction temperature. Blank experiments without metal NPs
As can be seen in Fig. 1, the alloy NP catalysts exhibited sig-
were also conducted with just the photocatalytically inactive nificantly higher activity than pure Au NPs (Au : Pd ratio of
ZrO2 supports dispersed in a toluene solution of benzyl 1 : 0) or pure Pd NPs (Au : Pd ratio of 0 : 1) for the dehydrogena-
alcohol. Under otherwise identical conditions, no alcohol con- tion of alcohols under visible light irradiation (>420 nm). This
version was observed under visible light irradiation or in the fact implies that Au–Pd@ZrO2 catalysts are not collections of
dark. We found that at identical reaction temperatures and Au NPs and Pd NPs; instead, Au and Pd exist as binary alloy
identical reaction periods, the conversion rates of the oxygen- particles in these samples.
free dehydrogenation of aromatic alcohols (under an argon
Transmission electron microscopy (TEM) analysis of the
atmosphere) are comparable to those of the selective oxidation NPs (Fig. 2) shows that the mean diameters of the Au–Pd alloy
conducted under an oxygen atmosphere. This demonstrates NPs are less than 10 nm. Fig. 2B is a line profile of the energy
that the rate-determining step of the transformation from dispersion X-ray (EDX) spectrum of a typical Au–Pd alloy NP in
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photocatalytic activity. The absorption peak at 520 nm in the
spectrum of the pure Au (3 wt%) sample is due to the LSPR
absorption of the Au NPs.21,25,26,48–50,52–55 The presence of the
support and its interaction with the Au NP can shift the reso-
nance to longer wavelengths and broaden the LSPR absorption
peak. The LSPR absorption band of Pd NPs is in the UV range at
a wavelength of 330 nm.18 However, this absorption is not
observed in the extinction spectrum of the pure Pd NPs on ZrO2.
Interestingly, we observed a clear light absorption near the
Pd absorption band (330 nm) in the spectrum of Au–Pd alloy
particles on ZrO2 (trace b, Fig. 4B), which is believed to be
associated with Au–Pd alloy NPs.18 In the spectrum of the alloy
sample, the characteristic LSPR absorption peak of Au NPs at
520 nm is much weaker when compared to the spectrum of
the pure Au sample, but larger than the absorption observed
for the pure Pd sample.
Fig. 2 (A) TEM image of 1.5% Au–1.5% Pd@ZrO2 catalyst. The arrows
indicate Au–Pd NPs. (B) High resolution TEM images of an alloy particle
in the catalyst. (C) EDX spectrum line profile analysis of a typical Au–Pd
NP indicated along the blue arrow in Fig. 2A, providing information of
the elemental composition and distribution of the NP.
The influence of light irradiation
The wavelength and intensity are important irradiation energy
parameters and, hence, they are the critical factors influencing
the performance of photocatalysts in reactions. The depen-
dence of the photocatalytic dehydrogenation of benzyl alcohol
Fig. 2A, which shows the elemental distribution along the
radial direction of the metal NP. The line profile indicates that
the NP consists of both Au and Pd distributed spherically
around a common centre. This means that the two metals
exist as binary alloy NPs in this sample. No diffraction peaks
corresponding to either metallic Au or metallic Pd were
observed from X-ray diffraction (XRD) patterns of the Au, Pd
and Au–Pd samples (not shown) because of the low metal
content of the samples.
The formation of Au–Pd alloy NPs is also supported by the
significant change of light absorption properties of the sample
as shown in Fig. 3.48–50 Therefore, the catalytic properties of
this sample resulted from Au–Pd alloy NPs rather than a collec-
tion of discrete Au NPs and Pd NPs.
ZrO2 has a band gap of about 5 eV,51 and exhibits weak
visible light absorption with no charge carriers generated under
irradiation of light with wavelengths above 400 nm (Fig. 3).
Therefore, the ZrO2 support by itself does not contribute to
Fig. 4 (A) The dependence of the catalytic activity of the Au–Pd@ZrO2
catalyst for the benzyl alcohol dehydrogenation on the wavelength of
light irradiation. Both the light-driven reaction and the reaction in the
dark were conducted at 45 °C 1 °C. (B) Action spectrum based on the
data in (A), and the light absorption spectra of Au–Pd@ZrO2 and
Au@ZrO2 are given for comparison.
Fig. 3 Diffuse reflectance UV-visible (DR-UV-vis) spectra of the photo-
catalysts and ZrO2 support.
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on the wavelength and intensity was investigated. The results
are illustrated in Fig. 4.
A different wavelength range is found to produce the most
significant enhancement in performance of the pure Au NP
Optical filter glasses were applied to block irradiation below photocatalyst from that observed for Au–Pd@ZrO2. In our
specific cut-off wavelengths. Therefore, we were able to tune recent study on selective reactions catalysed by the Au NPs
the wavelength of the light to clarify the influence of the wave- under visible light, acetophenone hydrogenation to 1-phenyl
length range on the catalytic activity of the Au–Pd@ZrO2 cata- ethyl alcohol and styrene oxide reduction to styrene,14,15 light
lyst for benzyl alcohol oxidation. When light with wavelengths irradiation in the wavelength range between 490 nm and
in the full 400 nm to 800 nm range irradiated the reaction 550 nm made the most important contribution to driving the
system a 100% reaction conversion was observed. Applying a reaction. The contributions from light in this wavelength
filter that blocked wavelengths below 490 nm resulted in a range for the two reactions are 41% and 65%, respectively. The
decrease in the conversion of the reaction to 83%. Increasing contribution of light in the wavelength range of 550–600 nm is
the cut-off wavelength to 550 nm and then 600 nm resulted in much less (24% and 0% for the two reactions, respectively),
the conversion decreasing to 65% and 46%, respectively. Given while for the Au–Pd@ZrO2 catalyst in the present study, the con-
that the thermal conversion rate at this temperature is 44%, tribution of light within the wavelength range of 550–600 nm is
the contribution of light irradiation in the wavelength range of even more significant than that of light in the range of
400–800 nm to the overall catalytic activity is 56%.
490–550 nm. In other words, the effective wavelength range of
From the results acquired from the irradiation of tuned Au–Pd alloy NPs is broader (490–600 nm) than that of Au NPs.
wavelength we can estimate the contribution of the light in a This phenomenon can be attributed to the formation of the
narrow wavelength range. For example, the yield of the reaction alloy NPs, and it also reveals that Au–Pd alloy NPs have
is 83% when the light with wavelengths below 490 nm was cut the potential to utilize light energy from a wider portion of the
off. Since the reaction temperature was held constant, the con- visible light spectrum for enhancing reactions than pure Au NPs.
tribution of the thermal reaction remains constant regardless
The impact of the light intensity on the catalyst perform-
of the filter used. Therefore, the observed decrease in the yield ance was investigated while keeping other experimental con-
of 17% (= 100%–83%) can be attributed to light irradiation by ditions unchanged. Fig. 5 shows the rate of benzyl alcohol
wavelengths in the 400 nm–490 nm range. The enhancement dehydrogenation over the Au–Pd@ZrO2 catalyst with a Au : Pd
in the yield due to the light irradiation by wavelengths in the molar ratio of 1 : 1 as a function of light intensity at 45 °C
490 nm–800 nm range is 39% (= 83%–44%), which accounts
for 47% of the overall yield achieved (83%). When the system
was irradiated with light with wavelengths in the 600 nm–
800 nm range, the enhancement in the yield decreased to 4%.
As shown by the results in Fig. 4A, light irradiation in the wave-
length range of 490 nm–600 nm results in the largest enhance-
ment in yield, accounting for 66% of the total light irradiation
enhancement (conversion rate difference between photo-
catalytic reaction and that observed in the dark), 32% results
from the light irradiation with wavelengths between 490 nm
and 550 nm, 34% from light between 550 nm and 600 nm,
while the light with wavelengths between 400–490 nm and
600–800 nm account for the remaining 34%. The total light
energy absorbed by the NPs was estimated from the overlap of
the absorption spectrum of the Au–Pd@ZrO2 catalyst, and the
spectral distribution of the light irradiated. It was found that
36.2% of the total light energy absorption by the NPs was in
the 490 nm–600 nm wavelength range. Plotting the enhance-
ment caused by light irradiation from the different wavelength
ranges (the vertical axis on the right hand side) against the
light wavelength reveals the action spectrum (Fig. 4B). It shows
which light wavelengths are most effectively used in specific
chemical reactions. Given that the LSPR peak of Au NPs is in
the wavelength range between 500 nm and 600 nm, these
results suggest that for Au–Pd@ZrO2 photocatalysts, it is the
gold that harvests visible light and that the gold nanostructure’s
LSPR plays an important role in enhancing the reaction yield in
alloy NP catalysed reactions. Furthermore, from the spectrum, it
Fig. 5 Light intensity dependent photocatalytic activity of Au–Pd alloy
is seen that the rate-determining step of the alcohol dehydro-
genation should occur with Au–Pd alloy NPs.
NPs for the transformation from benzyl alcohol into benzaldehyde at
45 °C (A) and 60 °C (B).
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1 °C and 60 °C 1 °C, respectively. When the light intensity
increased (the reaction temperature of the reaction mixture
was controlled at 45 °C; the only parameter changed is light
intensity), the conversion of benzyl alcohol oxidation increased
linearly up to a light intensity of 0.8 W cm−2. Further increase
in light intensity results in much greater rate increases, and
the relation between light intensity and reaction rate becomes
nonlinear. This is a feature of the chemical processes driven
by the light excited electrons of metals.16 It is also possible
that when the light intensity is very high, multi-photon absorp-
tion occurs, increasing the number of excited metal electrons
with sufficient energy to drive the reactions.
The light induced enhancement on the conversion was
calculated by subtracting the observed conversion of a reaction
performed in the dark from the conversion observed under
light irradiation performed at the same temperature. This
allows the photo-induced and thermal contributions to the
conversions to be determined and expressed as a percentage
for each process, as shown in Fig. 6. It shows clearly that
higher light intensities result in a larger light enhanced contri-
bution to the total conversion rate.
Fig. 7 Dependence of the photocatalytic rate of benzyl alcohol oxi-
dation on reaction temperature at a constant light intensity.
example), increasing operating temperature results in a signifi-
cant increase of the photocatalytic reaction rate. There are two
critical aspects of the temperature effect on the photocatalytic
rate. First, the relative population of the adsorbed reactant
molecule with excited states increases according to the Bose–
Einstein distribution at higher temperatures, which means
that the reactant molecule (benzyl alcohol or oxygen molecule)
will require less energy to overcome the activation barrier, and
this “less energy” could be provided by light irradiation.16,56
Second, at higher temperature more electrons of the alloy NPs
are in higher energy levels which can be excited by light
irradiation yielding more electrons with sufficient energy to
induce reactions in the alcohol molecules adsorbed on the
alloy NPs.
The influence of reaction temperature
Increasing operating temperature was observed to increase the
photocatalytic rate of benzyl alcohol oxidation. Fig. 7 shows
that at a constant light intensity (we applied 0.4 W cm−2 as an
The influence of the Au : Pd component ratio
As shown by the results in Fig. 1, the Au : Pd mass ratio of the
alloy particles is a key influencing factor on the catalytic per-
formance of the oxidant free dehydrogenation of the aromatic
alcohols both under light illumination and in the dark. All
reactions achieved the highest yield of target products when
the Au : Pd mass ratio of the alloy NPs was 1 : 1 (molar ratio of
1 : 1.86). Alloy NPs with other Au : Pd mass ratios proved to be
much less active. When the reactions were performed in the
dark, the same dependence on the Au : Pd mass ratio was
observed but with much lower conversion efficiencies. Cata-
lytic processes driven by heating Au–Pd alloy catalysts have
been reported in the literature. It was found that Au–Pd alloy
NPs could catalyse the hydrogenation of cinnamaldehyde to
cinnamyl alcohol.57 Toshima et al. reported that Au–Pd alloy
NPs were more active in catalyzing the hydrogenation of
1,3-cyclooctadiene than either gold or palladium alone.58,59
A possible explanation for the superior catalytic activities of
Au–Pd alloy NPs to NPs consisting of either a pure component
is that Au can isolate active Pd sites within bimetallic
systems.60 In the present study, to confirm the significant role
that the Pd sites at the alloy interface play in the catalytic
Fig. 6 Light intensity dependent activity of Au–Pd@ZrO2 photocatalysts
on oxidant free dehydrogenation of benzyl alcohol.
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processes, a sample of NPs with a Pd core and an Au shell was oxidation and thus enhance the catalytic activity. Furthermore,
prepared by reducing HAuCl4 on the Pd-ZrO2 support with H2. the Fermi level in alloy NPs, Φalloy, is higher than that in pure
This sample has almost no Pd sites at the NP surface and Pd NPs, ΦPd, so that the transfer of electrons at the Fermi level
exhibited low activity for selective oxidation of benzyl alcohol of the alloy NPs to the benzyl alcohol molecule adsorbed on
(10% conversion compared to 100% achieved by the Au–Pd the NPs is easier compared with that from the Fermi level of
alloy NPs with a similar Au : Pd ratio). However, the existence pure Pd NPs to the adsorbed molecule. The electron transfer
of Pd sites at the alloy surface does not explain why the causes the transformation of benzyl alcohol molecules as dis-
optimal catalytic activity was observed when the Au : Pd molar cussed later. The light absorption of gold results in energetic
ratio is 1 : 1.86. The underlying cause of this dependence on conduction electrons, which are in an even higher energy level
alloy composition is believed to be related to the higher Φalloy* and have a better ability to migrate to the Pd sites on
surface charge heterogeneity of the alloy NPs, compared with the surface. This further increases the possibility of electron
those of either pure component NPs.13,61 Pure palladium has a transfer from the alloy NPs to the reactant molecules.
slightly larger work function (ΦPd ∼ 5.6 eV) than pure gold
Since the increased surface charge heterogeneity of the
(ΦAu ∼ 5.3 eV), so once the two metals are in contact, electrons alloy NPs is due to the electron redistribution between Au and
will redistribute between gold and palladium until equilibrium Pd, we estimated the increase (details are provided in ESI†) by
is reached and the chemical potentials are equal everywhere in using a free electron gas model.51 The analysis reveals that the
the NPs (see Fig. 8). This electron redistribution between Au number of electrons transferred between the two metals, (ΔN),
and Pd enhances surface charge heterogeneity of the NPs (the is maximum when the molar ratio of the two metals in the
surface charge of an NP of pure metal is not homogeneous). alloy NPs is 1 : 1.86. A plot of the electron transfer (ΔN; refer to
The greater surface charge heterogeneity results in an the axis on the left hand side) predicted by the model as a
enhanced interaction between the alcohol and the NP. The function of the gold mole fraction (%) in the Au–Pd alloy NPs
enhanced interaction may lower the activation energy of the is shown in Fig. 8C. The catalytic conversion rates (refer to the
Fig. 8 Schematic profiles showing the impact of alloying and visible light irradiation of an Au–Pd alloy NP. (A) The surface electronic properties of
the alloy NPs are different from those of pure gold NPs as there are Pd islets on the alloy NP surface. The Pd sites are electron-rich because Pd has a
slightly larger work function than gold and electrons will flow from gold to palladium until equilibrium is reached (the chemical potentials of the
electrons are equal in the two metals, being Φalloy). (B) The light absorption by gold results in energetic conduction electrons, which migrate to the
Pd sites on the surface. The Fermi level of the alloy NPs under light irradiation is higher (Φalloy*) than that without irradiation (Φalloy), which increases
the charge transfer possibility from the NP to the reactant molecule. Thus the surface Pd sites with energetic electrons could exhibit significantly
enhanced catalytic activity even at ambient temperatures. Since in such an alloy NP structure the energy of incident light is very efficiently utilised to
enhance the intrinsic catalytic activity of palladium, efficient photocatalysts may be developed from it for the synthesis of organic chemicals. (C)
Electron transfer from gold to palladium in the alloy NPs, expressed as ΔN, varies with the composition of the alloy NPs (the curve). ΔN reaches a
maximum when the atomic ratio of Au and Pd in alloy NPs is 1 : 1.86 (mass ratio 1 : 1). The information on gold content of the alloy NPs (horizontal
axis) and photocatalytic conversion (vertical axis on the right hand side) of the reactions in the present study is also given in the figure (the symbols).
The photocatalytic efficiency of the alloy NP photocatalysts is strongly correlated to the electron transfer ΔN.
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axis on the right hand side) of the photocatalysts for oxidant apparent activation energy.14,15 For example, as illustrated in
free dehydrogenation of benzyl alcohol are also given (the Fig. 9C, the apparent activation energy for the dehydrogena-
symbols; refer to the axis on the right hand side). The results tion in the dark is ∼74.5 kJ mol−1 and it is ∼58.7 kJ mol−1 for
in Fig. 8C demonstrate a strong correlation between ΔN and the reaction under visible light illumination. The apparent
the catalytic conversion efficiency of reactants over the alloy activation energy in the dark is in agreement with those
NP catalysts. Note that this electron transfer ΔN analysis uses reported by Bavykin et al.64 (79 kJ mol−1 for the ruthenium-
parameters of bulk particles and is independent of the size of catalyzed oxidation of benzyl alcohol), and by Ilyas et al.65
alloy particles.
(77.8 kJ mol−1 for a Pt/ZrO2 catalyst system). Visible light
We also carried out simulations using the density function irradiation reduces the activation energy of the partial oxi-
theory (DFT) for electron states with and without light dation by 15.8 kJ mol−1, which represents 21% of the acti-
irradiation; the irradiation wavelength range between 532 and vation energy.
535 nm is chosen, which is around the LSPR absorption of Au.
It was reported that the first step of alcohol oxidation with
Calculation capacity limitations of our DFT simulation necessi- a Pd catalyst was alcohol dehydrogenation, which resulted in
tated the examination of a Pd32, Au32, and Au12Pd20 cluster. adsorbed hydrogen on the Pd surface and the formation of
The Au : Pd ratio of the Au12Pd20 cluster is 1.67 close to the aldehyde.66 The rate-determining step of the alcohol oxidation
ratio of 1 : 1.86 for the optimal Au–Pd@ZrO2 photocatalyst. on gold catalysts is believed to be the hydrogen abstraction
The detailed calculation method and the calculated Mulliken from alcohol and the formation of Au–H species; the role of
charge distributions are given in ESI.† The DFT simulation oxygen in this reaction is to remove hydrogen from the gold
results confirm that charge heterogeneity exists even in the surface, leading to the catalytic cycle.67,68 In order to clarify
monometallic Pd clusters and monometallic Au clusters the reaction mechanism we investigated the hydrogen abstrac-
(Fig. 9), and the alloy structure of Au and Pd increases the tion from alcohol to the surface of Au–Pd NPs. 2,2′,6,6′-Tetra-
charge heterogeneity of the NP surface. This result is consist- methylpiperidine N-oxyl (TEMPO) is
a good hydrogen
ent with that of the free electron gas model analysis and pre- abstractor, and can abstract hydrogen from the surface of
vious reports.62,63 Furthermore, the comparison of simulation metals to form hydroxylamine but not from alcohol mole-
results without irradiation (blue lines) with the results under cules.69 If the addition of TEMPO can result in alcohol dehy-
irradiation (red lines) suggests that light irradiation also pro- drogenation in the absence of oxygen, then it proves that
motes the charge heterogeneity in Au–Pd alloy NPs.
hydrogen transfer from the alcohol to Au–Pd NPs occurs in the
The apparent activation energies of the oxidant free dehy- reaction because the TEMPO can only abstract the hydrogen
drogenation of benzyl alcohol under light irradiation and in on the NP surface. Here TEMPO plays a role similar to oxygen:
the dark were derived from the kinetic data of the reaction at removing hydrogen from the NP surface to complete the cata-
different temperatures (details are provided in ESI†) using the lytic cycle so that the catalytic conversion of alcohol to the
Arrhenius equation. The difference between the activation corresponding aldehyde could proceed.
energies under light irradiation and in the dark (ΔEa) indicates
Benzyl alcohol dehydrogenation with Au–Pd@ZrO2 (Au : Pd
the contribution of the light irradiation to reducing the ratio of 1 : 1.86) catalyst at 45 °C under light irradiation was
Fig. 9 The optimized geometry and the natural charge distributions of the Au32 cluster (A) and Au12Pd20 clusters (B) in the ground state and the
considered excited state. (C) Apparent activation energy reduction of benzyl alcohol dehydrogenation caused by the light irradiation on the Au–Pd
alloy NP photocatalyst.
338 | Green Chem., 2014, 16, 331–341
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conducted under a N2 atmosphere with 300 mg of TEMPO from the hydroxyl group of the transient anion species pro-
added to the reaction system. After visible light irradiation, ceeds readily producing aldehyde as the final product while
21% of the benzyl alcohol was converted to benzaldehyde. No the negative charge of the transient anions returns to the alloy
conversion was observed in the dark at 45 °C. When the reac- NPs. The charge injection to the reactant on the plasmonic
tion was conducted at 80 °C in the dark, 30% of the benzyl metal NPs and the charge return to the NPs after reaction are
alcohol was converted by the TEMPO. Evidently in the absence known processes.25,26 Oxygen or TEMPO takes away the hydro-
of oxygen, TEMPO could abstract hydrogen atoms and drive gen on the NP surface yielding water or hydroxylamine,
alcohol oxidation either under visible light irradiation or in respectively, and thus, the NPs’ ability for dehydrogenation of
the dark at a higher temperature. Blank experiments of alcohol is restored.
TEMPO addition without metal NPs (ZrO2 support only) were
The conduction electrons of the alloy NPs can gain the
also conducted at both 45 °C and 80 °C. No conversion was energy of the incident visible light via the LSPR effect and
observed even under visible light, indicating that TEMPO inter-band transition. These energetic electrons are available
could not abstract hydrogen atoms directly from the alcohol to Pd sites at the NP surface because of electron–electron col-
but could capture them from the surface of Au–Pd NPs.
lisions and electron redistribution between Au and Pd. The Pd
It is known that light excited electrons of plasmonic metal sites have good affinity to the aromatic alcohol molecules and
NP can populate unoccupied orbitals of the molecules the interaction between the alcohol molecules and the NP
adsorbed on the NPs yielding a transient anion species.16,25,70 surface is enhanced by the surface charge heterogeneity of the
Results of a DFT simulation (the detailed calculation method alloy NPs. The strong interaction facilitates the transfer of the
is given in ESI†) show that in a benzyl alcohol molecule, the light excited electrons of the NPs to the adsorbed alcohol
distances between the α-C and the two H atoms are 1.098 and molecules. As a result, the catalytic activity of alloy NPs is sig-
1.100 Å, respectively, while in the transient benzyl alcohol nificantly enhanced at ambient temperatures, which allows the
anion species one C–H distance remains at 1.100 Å while the alloy NPs to efficiently catalyse the selective oxidation of aro-
other elongates to 1.104 Å. The energy required to break one of matic alcohols to the corresponding aldehydes and ketones.
the C–H bonds at the α-C in the molecule is 371 kJ mol−1, but This explains our observations that the conversion of a reac-
only 242 kJ mol−1 is required to break the longer C–H bond of tion under light irradiation is always higher than that of the
the α-C in the transient anion species. Hence the light corresponding reaction in the dark, and that the selective oxi-
irradiation can facilitate the hydrogen abstraction from the α-C dation of benzyl alcohol could proceed at moderate tempera-
through the excitation of NP electrons to the benzyl alcohol tures (e.g. <45 °C) under irradiation but not in the dark.
molecules adsorbed on them. Therefore, the irradiation
reduces the apparent activation energy of the reaction.
On the basis of these facts, a tentative mechanism for selec-
tive benzyl alcohol oxidation is proposed as depicted in
Conclusions
Scheme 1. The results of the influence of light intensity and In summary, it was found for the first time that visible light
the action spectrum indicate that the rate determining step of could significantly enhance the performance of Au–Pd alloy
the partial oxidation takes place on the surface of the sup- NPs supported on ZrO2 for the dehydrogenation of aromatic
ported alloy NPs. The first step should be the abstraction of alcohols to yield the corresponding aldehydes in the absence
α-H atoms from the alcohol molecules. Once this abstraction of oxygen at ambient temperature. The rate determining step
is completed the subsequent abstraction of the hydrogen atom of the dehydrogenation is the abstraction of α-H atoms from
the alcohol molecules. The dehydrogenation is also the rate
determining step of selective oxidation of the alcohols to the
corresponding aldehydes with molecular oxygen. The results
of both the free electron gas model analysis and DFT simu-
lation indicate that the alloy structure of Au and Pd increases
the charge heterogeneity of the NP surface, which enhanced
interaction between the alloy NPs and the alcohol molecules
adsorbed on the NPs. The combination of light absorption of
alloy NPs, the enhanced interaction and the intrinsic catalytic
activity of the transition metal leads to a unique structure
where the absorption of visible and UV radiation can yield
energetic electrons available at catalytically active transition
metal sites on the NP surface promoting the reaction of the
molecules adsorbed on the NPs. The optimal activity for the
alloy NPs was observed with an Au : Pd molar ratio of 1 : 1.86,
being in good agreement with the simulation results. The Au–
Pd@ZrO2 is an example of the alloy NPs formed by gold and a
catalytically active transition metal, which can be used as a
Scheme 1 Proposed mechanism of aromatic alcohol oxidation over
Au–Pd alloy NPs under visible light irradiation.
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