Synthesis of Imines by Selective Oxidation of Amines on alloy nanoparticles of palladium and gold under visible light irradiation at ambient temperatures

Article abstract of DOI:10.1007/s10562-019-03079-w

Abstract: Imines are important intermediates in organic synthesis. To avoid derivatives, the research on direct oxidized amines as an alternative method is imminent. The gold (Au)–palladium (Pd) alloy nanoparticles (NPs) catalyst shows a favourable activi

Full text of DOI:10.1007/s10562-019-03079-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-03079-w
Synthesis of Imines by Selective Oxidation of Amines on alloy
nanoparticles of palladium and gold under visible light irradiation
at ambient temperatures
Eerdemutu1 · Lijun Ding1 · Chun Li1 · Shuang Zhao1 · Xianliang Sheng1
Received: 21 October 2019 / Accepted: 13 December 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Imines are important intermediates in organic synthesis. To avoid derivatives, the research on direct oxidized amines as an
alternative method is imminent. The gold (Au)–palladium (Pd) alloy nanoparticles (NPs) catalyst shows a favourable activ-
ity and high selectivity on benzylamine oxidation reaction under light irradiation. The yield of the reaction on the alloy NPs
with a Au:Pd ratio of 1:1.86 in the dark was 36% at 45 °C, while the yield increased to 95% under light illumination. The
study demonstrates diferent properties from alloy NP to pure Au NP and Pd NP catalysts which may be caused by increased
surface charge heterogeneity in the alloy system. The conversion rate of the reaction can also be enhanced by light irradiation
and heat. The light energy which utilized by Localized Surface Plasmon Resonance efect of gold in alloy system can highly
enhanced the efciency of reaction. These fndings provide useful guidelines for designing efcient alloy catalysts with a
plasmonic metal and a catalytically active transition metal for various organic syntheses driven by sunlight.
Graphic Abstract
Keywords Gold–palladium alloy · Benzylamine oxidation · Imines · Photocatalysis
1 Introduction
Imines are important synthetic intermediates in the syn-
thesis of fne chemicals, pharmaceuticals, and agricultural
chemicals [1-3]. Generally, synthesis of imine derivatives
involves the condensation reaction of amine with carbonyl
* Xianliang Sheng
etu2017@imau.edu.cn
1
College of Science, Inner Mongolia Agricultural University,
Hohhot 010018, People’s Republic of China
Vol.:(0123456789)
1 3

Eerdemutu et al.
compound. To avoid the problem caused by the excessively
active activity of ketones or aldehydes, oxidation of amines,
as an alternative strategy to the direct oxidation of amines,
has been recently studied [4, 5]. A number of ruthenium
complexes have been studied for the amine oxidation, how-
ever they only can oxidize limited numbers of benzylamine
at high temperature (usually above 100 °C). Research on
noble metals such as gold powder or Au, Pd and Pt support
on Al2O3 or TiO2 gives another pathway to produce imines
but the reaction condition is limited in O2 atmosphere under
5 atm and temperature as high as 100 °C [6, 7]. Gold–pal-
ladium nanoparticles supported on porous steel fbers can
drive the reaction under heating, while high pressure (5 bar
O2) and high temperature (100 °C) is still needed [8]. Pho-
tocatalytic processes driven by carbon nitride (graphitic-
C3N4) was used to synthesize imine from amine under 5 bar
O2 pressure at 60–80 °C with visible light irradiation [9].
This process can utilize the light source as a clean and green
energy. This reaction can be performed under moderate con-
ditions using high energy irradiation source such as UV irra-
diation or microwave [10, 11]. However these energy source
results in relatively lower product selectivity.
28]. The catalytic efciency of the alloy NP is superior to
those of pure AuNPs or PdNPs because of the diference of
electro negativity between Au (2.54) and Pd (2.2). In addi-
tion, charge heterogeneity of alloy NP surface is expected,
which can enhance the interaction in alloy NPs and the elec-
trophilic or nucleophilic of reactant molecules [29]. When
the alloy NPs under light irradiation, the high energy elec-
trons will be generated on Au due to the LSPR efect and
the electron will fow to the Pd sites on the alloy NP surface
which is caused by the electronegativity diference [30].
This process will form a relatively high energy state at the
Pd sites of the alloy NP surface where it is more activated
towards reactant molecules. These properties of Au–Pd alloy
NPs catalyst provides a possibility of driving reactions under
light irradiation with a moderate temperature.
In the present study, the reaction of oxidize benzylamine
to imine is studied to confrm the activity of Au–Pd alloy
NP catalyst under the light irradiation. In addition, the prop-
erties of the alloy catalyst with diferent Au–Pd ratio are
investigated. The overall performance of these alloy catalysts
shows a promising activity towards benzylamine oxidation
comparing with the pure Au NPs and Pd NPs. Moreover, the
performance of the alloy NPs catalysts can be signifcantly
enhanced by visible light irradiation and optimized ratio of
alloy metal.
Palladium is widely used as unique and highly efcient
catalysts for many important organic reactions [12-14]. In
order to improve the reaction efciency, high temperature
and high pressure are in used, which are energy intensive.
Despite their high yields, it is very difcult to recover the
Pd after the reaction due to the homogeneity of the catalyst,
for example, the separation of Suzuki reaction products from
ionic liquids and homogeneous transition metal catalyst via
solvent extraction [15-17].
2 Experimental
2.1 Catalyst Preparation
It is well known that gold nanoparticles (AuNPs) exhibit
a strong visible light absorption due to the localized sur-
face plasmon resonance (LSPR) efect [18, 19], and they
also can absorb ultraviolet (UV) light inducing inter-band
electron transitions (from 5d to 6sp) [20, 21]. The LSPR
is a collective oscillation of the free (conduction) electrons
in metal NPs when they resonate with the electromagnetic
feld of the incident light with certain natural frequency [22].
The conduction electrons of Au NPs gain energy from the
irradiation, hence electrons with high energy at the surface
is able to activate molecules adsorbed on the NPs for chemi-
cal reactions [23]. The advantage is that the light-excited
electrons at the surface of the NPs can interact with reac-
tant molecules efciently and avoid energy loss caused by
electron migration. Hence, one can utilise this mechanism
to drive the reaction with light instead of heat at moderate
temperature [24-26].
Catalysts with 3 wt% metal loaded on ZrO2 which include
pure gold NPs, pure palladium NPs and four catalysts of
Au and Pd alloy NPs with diferent Au:Pd ratios were pre-
pared 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 a solu-
tion from 15.2 mL of 0.01 M HAuCl4 aqueous solution and
28.3 mL of 0.01 M PdCl2 aqueous solution under magneti-
cally stirring. 20 mL of 0.53 M lysine was then added into
this suspension under vigorous stirring. The stirring was
prolonged for 30 min, and then 10 mL of 0.35 M NaBH4
solution was added dropwise to the suspension in 20 min
period, followed by 10 mL of 0.3 M hydrochloric acid. The
mixture was aged for 24 h and then the solid was separated
by centrifuge, washed with water and ethanol, and dried at
60 °C in air. Catalysts with diferent Au:Pd ratios were pre-
pared in a similar method but using diferent quantities of
HAuCl4 aqueous solution or PdCl2 aqueous solution.
Au–Pd alloy NPs has been used as a new type of visible
light photocatalysts to combine the excellent catalytic activ-
ity of PdNPs with the light absorbance of AuNPs [27]. This
alloy NPs has been reported and used for thermal reactions
like oxidize alcohols and synthesize hydrogen peroxide [8,
1 3

Synthesis of Imines by Selective Oxidation of Amines on alloy nanoparticles of palladium and…
2.2 Catalyst Characterization
A transmission electron microscopy (TEM) studies of the
catalysts were carried out on a Philips CM200 TEM with an
accelerating voltage of 200 kV. The contents of Au and Pd
of the prepared catalysts were determined by energy disper-
sion X-ray spectrum (EDX) technique using the attachment
to a FEI Quanta 200 Environmental SEM. UV–Vis spectra
of the catalysts were examined by a Varian Cary 5000 spec-
trometer. The infrared emission spectroscopy (IES) meas-
urements were carried out on a Digilab FTS-60A spectrom-
eter equipped with a TGS detector, which was modifed by
replacing the IR source with an emission cell.
2.3 Activity Test
Fig. 1 TEM image of Au–Pd@ZrO2 catalyst and particle size distri-
The suspension of catalyst, solvent and the reactant in a
Pyrex glass fask 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 inten-
sity 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 by using an
air-conditioner attached to the chamber. The reaction system
under light illumination was maintained at the same tem-
perature as the corresponding reaction system in the dark
to ensure that the comparison is meaningful. The reaction
conditions of each reaction are given briefy as footnotes
of Table 1. At given irradiation time intervals, 2 mL ali-
quots were collected, centrifuged, and then fltered through
bution of Au–Pd@ZrO2 used in this study. The arrows indicate Au–
Pd NPs
a Millipore flter (pore size 0.45 μm) to remove the catalyst
particulates. The fltrates were analysed by Gas Chromatog-
raphy (6890 Agilent Technologies) with a HP-5 column to
measure the concentration change of reactants and products.
The product analysis used an Agilent Technologies 6890 gas
chromatograph equipped with Agilent Technologies 5973 N
mass selective detector and an Agilent HP-5MS column.
Table 1 Catalytic oxidation of benzylamine into imines with the Au–Pd@ZrO2 photocatalysts of various Au:Pd molar ratios under visible light
irradiation (number in red colour) and in the dark (number in black colour)
NH
hv O2
N
2
Au-PdAlloy NPs
R
R
R
TOF/h−1
Light
R
Au:Pda
Yield/%
Light
Selec./%
Light
TON
Light
Q.Y./%
Dark
Dark
Dark
Dark
H
H
H
H
H
H
H
1:1.86
1:1.00
1:5.58
1:0.62
1:0
95
84
80
74
26
30
28
36
39
24
35
12
21
16
96
99
95
99
97
89
94
98
98
96
98
98
83
90
87
85
64
59
28
21
28
33
39
19
38
16
15
16
0.91
0.88
0.67
0.62
0.29
0.22
0.29
0.34
0.41
0.20
0.39
0.16
0.16
0.17
1.7
1.3
1.6
1.2
0.4
0.7
0.3
0:1
1:1 mixb
aMolar ratio
b25mg Au@ZrO2 and 25 mg Pd@ZrO2. Reaction conditions: 1 mmol of reactant, 50 mg (containing 3% of metals) of catalyst in 10 ml CH3CN
solvent at 45 °C and 1 atm of O2 for 96 h. TON and TOF were calculated based the total amount of metal(s)
1 3

Eerdemutu et al.
(AuNPs, PdNPs and Au–Pd alloy NPs) on the ZrO2 sup-
ports display high levels of absorption in the UV and visible
range, indicating that NPs are able to absorb solar energy
efciently. The absorption peak at 520 nm in the spectrum
of the pure AuNP sample is due to the LSPR efect of the
AuNPs [15, 16]. When the AuNP is loaded on support, this
LSPR peak may shift and broaden due to the interaction
with the support [27]. The gold LSPR peak disappears in
the spectra of Au–Pd alloy NP sample but after investiga-
tion by achieve the reactions with diferent wavelength, we
found that the LSPR efect can still enhance the catalysts
performance under light irradiation.
3 Result and Discussion
TEM analysis of the alloy NPs (Fig. 1) shows the catalysts
morphology and that the mean diameters of the Au–Pd
alloy NPs are about 4.5 nm. More than 60% particles have a
particle size of 4–5 nm. There is no particle obvious larger
than 10 nm or less than 1 nm. The previous researches have
shown that the binary Au–Pd alloy NPs synthesized by the
same method have a structure of spherical central distri-
bution [31]. The catalyst with this size distribution being
applied here is due to the size and dispersion of the Au NPs
could signifcantly infuences their catalytic performance
[32]. The larger particle size gives stronger light absorp-
tion due to the LSPR efect which is important for the pho-
tocatalytic performance. The smaller particle size trend to
have a larger specifc surface area which has more change
to interact with reactant [33]. In a previous study we found
an optimized particle size of 5 nm in mean size, which can
engender strong LSPR absorption while have a relatively
large specifc surface area to accommodate reactant mol-
ecules for reaction.
The oxidative coupling of benzylamines over the cata-
lysts with diferent gold and palladium molar ratio was
examined and the results are listed in Table 1. Pure AuNPs
exhibit low activity (with a yield of 12%) for the oxidation
of benzylamine into imine in the dark. The activity of the
pure PdNPs for this reaction is better than those observed
with AuNPs, but still relatively low (yield 21%). The alloy
NPs exhibited improved activity under the same condition,
the yield is 36% when the catalyst with the optimal Au:Pd
ratio (1:1.86) was used. The light irradiation signifcantly
enhanced catalytic activity of all catalysts while the depend-
ence of the yield on the Au:Pd ratio has a similar trend to
that of the reaction in the dark. The irradiation has little
impact on the selectivity of product. Similar result on the
optimal Au:Pd molar ratio for the Au–Pd alloy NP catalysts
has been reported by other groups testing their reactions at
relatively high temperatures (≥190 °C) and high pressure
[36, 37].
The observed UV–Vis light absorption of the sample
(Fig. 2) is evidence showing the alloy formation in the
Au–Pd NPs. The spectrum of the Au–Pd alloy NPs on ZrO2
support is clearly diferent from the spectra of the pure metal
AuNPs, PdNPs and the mixture of AuNPs with PdNPs. This
diference is believed that caused by the association with the
formation of Au–Pd alloy NPs [6, 11, 34]. The band-gap of
the ZrO2 carrier is about 5 eV [35], which exhibits no visible
light absorption at wavelengths above 400 nm.
Benzylamines with electron-donating substituted groups
(CH3 and OCH3) and electron-withdrawing substituted
group (Cl) could produce corresponding imines with high
yields. Use the optimal Au:Pd catalyst can achieve more than
90% yield on all three substituent group. The conversion
rate of electron-donating groups without light irradiation
is higher than the electron-withdrawing group but similar
when light is introduced. The benzylamine with CH3 and
OCH3 groups normally are easier to get high yield than Cl
group which proved by the result of dark reaction under
both pure and alloy metal catalyst. However, the conversion
rate of Cl group have more diference between light and
dark reaction, especially the conversion increased from 2
to 85% for Au:Pd molar ratio at 1:0.62. This increase trend
is diferent from the OCH3 and H group and maybe cause
by the diferent properties of electron-donating groups and
electron-withdrawing group. Table 2 demonstrate that, when
using alloy catalysts, light irradiation has more infuence on
electron-withdrawing group than electron-donating groups
and gold content may have strongly impact on the conversion
of electron-withdrawing.
Therefore, the ZrO2 support itself does not contribute
to the photocatalytic activity. The metal loaded catalysts
Fig. 2 Difuse refectance UV–visible (DR-UV–Vis) spectra of the
Au–Pd@ZrO2 catalyst and their comparison with pure Au@ZrO2,
Pd@ZrO2 and mechanical mixture of both pure metal AuNPs PdNPs
To better understand the enhancement of the catalytic
performance of the Au–Pd alloy NPs under light irradiation,
1 3

Synthesis of Imines by Selective Oxidation of Amines on alloy nanoparticles of palladium and…
Table 2 Catalytic oxidation
of benzylamine substituent
group into corresponding
imines with the Au–Pd@ZrO2
photocatalysts of various Au:Pd
molar ratios under visible light
irradiation (number in red
colour) and in the dark (number
in black colour)
R
Au:Pda
Yield/%
Light
Selec./%
Light
TON
Light
TOF/h−1
Light
Q.Y./%
Dark
Dark
Dark
Dark
H
1:1.86
1:1.86
1:1.86
1:5.58
1:0.62
1:0
94b
88b
98c
66c
85c
14c
42c
93b
81b
61b
24b
39b
32b
24
16c
4c
96
98
93
94
95
98
99
99
99
99
100
99
99
99
43
40
45
26
46
9
15
43
32
33
16
14
15
11
7
2
1
0
1
12
8
9
0.90
0.84
1.12
0.66
1.14
0.23
0.37
0.89
0.68
0.68
0.33
0.29
0.31
0.23
0.18
0.04
0.03
0.00
0.02
0.25
0.18
0.19
0.19
0.15
1.8
1.8
2.4
1.8
2.4
0.4
1.1
1.9
1.7
1.3
0.3
0.6
CH3
Cl
H
H
H
H
OCH3
H
H
H
2c
Tracec 99
0:1
2c
95
98
98
99
99
98
1:1.86
1:5.58
1:0.62
1:0
26b
21b
17b
14b
20b
99
100
100
9
7
H
0:1
aMolar ratio
bReaction time 48 h
cReaction time 40 h. Reaction conditions: 1 mmol of reactant, 100 mg (containing 3% of metals) of catalyst
in 10 ml CH3CN solvent at 45 °C and 1 atm of O2. TON and TOF were calculated based the total amount
of metal(s)
Fig. 3 The dependence of the catalytic activity of Au–Pd alloy NPs
for the benzylamine reaction on the intensity of diferent light irra-
diation. Both light driven reaction and the reaction in the dark were
conducted at 60 °C 1 °C
Fig. 4 The dependence of the catalytic activity of Au–Pd alloy NPs
for the benzylamine reaction on the intensity of diferent light irradia-
tion and temperature
the dependence of the catalysed reaction on the light’s wave-
length, intensity and temperature were investigated and pre-
sented in Fig. 3–6. Figure 3 shows the dependence of the
conversion of benzylamine reaction catalysed by the Au–Pd
alloy NPs (1:1.86) on the intensity of light irradiation. The
conversion almost increases linearly with the increasing
of light intensity. The contribution of the light irradiation
was obtained by subtracting the conversion efciency of
the dark reaction from the conversion efciency under light
at the same temperature. The photocatalytic reaction was
carefully maintained at same temperature as the reaction
in the dark at 60 1 °C. The relative contributions of light
irradiation to the conversion efciencies are shown in Fig. 3.
It is obvious the contribution rises linearly as the light inten-
sity increased. For example, under irradiation with a light
intensity of 0.2 W/cm2, 38% of the conversion is due to the
light irradiation while the reaction in the dark contributed
62% of the overall conversion at 60 1 °C. When the light
intensity increased to 1.2 W/cm2, 74% of the conversion rate
is due to light irradiation. This trend clearly demonstrates
that the light irradiation enhances the catalytic activity of
Au–Pd alloy NPs.
1 3

Eerdemutu et al.
The dependence of the conversion on the light irradia-
tion intensity at diferent temperature between 30 °C and
60 °C for the oxidative coupling catalysed by the Au–Pd
alloy NPs (1:1.86) is shown in Fig. 4. At all the tempera-
tures, light intensity leads to high conversion rate. Moreo-
ver, at higher temperature, the reaction proceeds faster. The
dependence of the reaction efciency on the light intensity
is almost linear at all the temperatures, but the slopes are
increases as the temperature was raised. The slope at 60 °C
is the largest (0.52) followed by those at 50 °C (0.35), 40 °C
(0.32) and 30 °C (0.22). Considering the reaction under the
same light intensity are two diferent temperatures. The same
amount of light energy input activates more benzylamine
molecules for reaction at the higher temperature than at the
lower one. According to the Bose–Einstein distribution, the
more adsorbed reactant molecules exist in excited states
at higher temperature, the more light excited NP conduc-
tion electrons are existed at higher energy levels. It means
that the reactant molecules (organic molecules or oxygen
molecules) require less energy to overcome the activation
barrier. The excited NP electrons deliver this reduced quan-
tum of energy required for the reaction to proceed. There
are more excited NP conduction electrons with sufcient
energy levels to drive the reaction at higher temperatures
[38]. Hence, the same light energy input (the same light
intensity) can activate the more reactant molecules adsorbed
on the NP surface for the reaction. Therefore, the reaction
under 60 °C light irradiation contributes the highest conver-
sion diferences between light and dark reactions, which can
be deduced from the explanation of Fig. 3.
and helped to tune the wavelength range that irradiated the
reaction systems. The dependence of the catalytic perfor-
mance on wavelength range for benzylamine is depicted in
Fig. 5. Cutting of light with wavelengths > 420 nm using
the flter, the conversion is about 5% less than that under the
irradiation between 400 and 800 nm. It means that the light
with wavelengths in the range between 420 and 800 nm,
could bring about nearly 90% of the conversion efciency
contributed by the light irradiation. Similarly, by applying
a flter that with a cut-of light with wavelength of 490 nm,
550 nm and 600 nm, the conversion drops to 70%, 48% and
38% respectively. The conversion without irradiation is 24%
which means under an irradiation that between 600–800 nm
only 14% (=38-24%) conversion contribution from photo-
reaction. For the wavelength 420–800 nm, 490–800 nm
and 550–800 nm this number is 66%, 46% and 24% respec-
tively. Set the total efect from light irradiation at wavelength
420–800 nm as 100%, the efect from light irradiation of
420–490 nm, 490–550 nm, 550–600 nm and 600–800 nm
can be calculated as 31%, 33%, 15% and 21%. From the
calculation, the irradiation in the 490–550 nm range has the
largest contribution to the conversion rate. The photons of
the light in 420–490 nm range have higher energy but the
irradiation has lower impact on the reaction than the irradia-
tion of 490–550 nm range. The rest of two ranges have even
lower ability to drive the reaction. The absorption of light
in the 490–550 nm range brings the best catalytic efciency,
and AuNPs has strong LSPR absorption (peaked at 520 nm).
It appears that the light absorption due to the LSPR efect
of gold nanostructures in the Au–Pd alloy NPs is the most
efcient driving for reaction.
To determine the wavelength of the light efect on the con-
version efciency, various optical low pass flters have been
introduced to flter light below specifc cut-of wavelengths
Fig. 5 The dependence of the catalytic activity of Au–Pd alloy NPs
for the benzylamine reaction on the wavelength of the light irradia-
tion. Both light driven reaction and the reaction in the dark were con-
ducted at 45 °C 1 °C
Fig. 6 The dependence of Au–Pd@ZrO2 performance on the Au:Pd
molar ratio of the alloy NPs in light enhanced reaction and in the dark
reaction for the benzylamine reactions in the present study
1 3

Synthesis of Imines by Selective Oxidation of Amines on alloy nanoparticles of palladium and…
To have an in-depth understanding of the alloy catalyst,
we plot the catalytic performance under light irradiation
and in the dark against the Au:Pd molar ratio of the alloy
NPs in Fig. 6 based on the data of Table 1. The irradiated
reaction achieved the highest yield of target products at the
molar ratio of 1:1.86. Alloy NPs with other Au:Pd ratios
(1:5.58, 1:1 and 1:0.62) are proved to be less active. Fur-
thermore, the dependence on the Au:Pd ratio of the alloy
NPs for the reaction activity in the dark is similar to under
irradiation, but exhibits much lower conversion efciency.
Figure 6 shows that light illumination and alloy composition
both have impact on catalytic performance of the alloy NPs,
where irradiation with light has more pronounced efect than
the Au:Pd ratio.
palladium until establish a new equilibrium (chemical poten-
tials are equal around the whole alloy NPs) and enhances
surface charge heterogeneity of the NPs.
The Au:Pd ratio can strongly infuence the charge distri-
bution and the charge heterogeneity of the cluster. When the
charge heterogeneity is increased, the adsorption reactant
molecules on the NPs is enhanced [8, 28, 40]. This could
result in higher probability that the reactant molecules are
adsorbed on the alloy NPs more than on AuNPs. When the
system is under irradiation, the conduction electrons of NPs
absorb light energy and are excited to high energy levels.
These excited electrons of the NPs have better ability to
migrate to the adsorbed benzyl amine molecules inducing
their reactions. The stronger the adsorption of benzyl amine
molecules on the NPs, the easier the electron transfer is.
The IES analysis with diferent temperature were con-
ducted to give an evidence of this simulation and shown in
Fig. 7. To verify the absorbed benzylamine on catalysts the
N–H bond and C=C stretch were used. On Au@ZrO2 cata-
lyst, the absorbed benzylamine can be detected at 200 °C
and 400-450°C. However, at temperature 425 °C and
450 °C the absorbed benzylamine began to desorb and the
peak strength is lower than that at 400 °C. When the tem-
perature goes to 475 °C, no obvious peak is shown in the
spectrum which means nearly no benzylamine absorbed
on the catalyst. The AuPd@ZrO2 catalyst shows much
stronger adsorption strength during the IES analysis. The
characteristic peak can be observed even at 600 °C. These
analyses gives us evidence that the AuPd alloy has larger
surface charge heterogeneity than pure gold which result
in a stronger adsorption strength and adsorption capacity.
The apparent activation energy was calculated from the
reaction results under light irradiation and in the dark by
applying the Arrhenius equation and using the reaction data
taken between 30–60 °C. The diference between the activa-
tion energies of the irradiated process and the process in the
dark (ΔEa) indicates the light irradiation contribution [39].
For benzylamine reaction, the activation energies of the light
enhanced process is 118.4 kJ·mol−1 and the process in the
dark is 131.5 kJ·mol−1. When the visible light illumination
exists, the activation energy reduces to 13.1 kJ·mol−1 which
is a deduction by 10%. The reason for the increased catalytic
activity of the alloy NPs may be that the surface charge het-
erogeneity of the alloy NPs is larger than that of the pure
metal NPs [8, 39]. This increased surface charge hetero-
geneity may cause by work function diferences between
pure gold (ΦAu~5.3 eV) and palladium (ΦPd~5.6 eV). In
the Au–Pd alloy system, electrons will fow from gold to
Fig. 7 IES spectra of ben-
zylamine absorbed catalysts at
diferent temperature. a Au@
ZrO2 catalyst; b AuPd@ZrO2
alloy catalyst
1 3

Eerdemutu et al.
could be extended to a variety of synthetic possess driven
by sunlight.
Acknowledgements This work is fnancially support by Scientifc
Research Program of Higher Education Institutions of Inner Mongolia
Autonomous Region (NJZY18054).
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Fig. 8 Proposed mechanism of Au–Pd alloy NPs@ZrO2catalyzed
oxidation of benzylamine
Generally, the synthesis of imine derivatives involves
condensation of an amine and a carbonyl compound. Pos-
sible mechanisms of the imine formation on noble metal
catalysts under heating were proposed [6, 7, 35, 41, 42].
However, the exact pathway of the reaction still cannot
be fully comprehended. The analysis of gas chromatog-
raphy–mass spectrometry can deduce a possible reaction
mechanism. The oxidation of benzylamine into diben-
zylimine reaction proceeds via two steps as shown in
Fig. 8. The reactant benzylamine initially absorbed on the
catalyst surface and then oxidized to benzaldehyde which
has a high reactivity with benzylamine. The intermediate
products benzamide and benzaldehyde were detected as
long as other by products. The produced benzaldehyde
would react with benzylamine quickly and form the fnal
product of imine. During the oxidation process, some ben-
zylamine were not deaminized and formed benzamide.
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In summary, photocatalytic oxidation of amines on Au–Pd
alloy NPs was achieved highly selective. The charge het-
erogeneity on the surface of alloy NP is greater than that
of AuNPs or PdNPs which leads to a stronger interaction
between the alloy NPs and reactant molecules. The Au:Pd
molar ratio of catalysts has important infuence on charge
heterogeneity and catalytic performance of the alloy NPs.
The alloy NPs absorbs visible light strongly contributed by
the LSPR efect of gold, which drive the reaction more ef-
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energy and facilitate reaction of the adsorbed benzyl amine
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