Effects of divalent metal ions of hydrotalcites on catalytic behavior of supported gold nanocatalysts for chemoselective hydrogenation of 3-nitrostyrene

Article abstract of DOI:10.1016/j.jcat.2018.05.007

The effect of the divalent metal ions on the hydrotalcite (HT) (MAl-HT; M = Mg, Zn, Ni)-supported thiolated Au₂₅ nanoclusters (NCs) as the precatalysts for the chemoselective hydrogenation of 3-nitrostyrene to 3-vinylaniline was investigated. The highest chemoselectivity was obtained over the Au₂₅/ZnAl-HT-300 (calcined at 300 °C) catalyst, with a maintained selectivity of desired product above 98%. The Au₂₅/NiAl-HT-300 catalyst exhibited the highest activity, although the particle size of gold (3.2 nm) was greater than those of the Au₂₅/MgAl-HT-300 (2.2 nm) and Au₂₅/ZnAl-HT-300 (1.7 nm) catalysts. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results of CO adsorption revealed that Ni interacting intimately with gold could be reduced easily, which affected the catalytic behavior of the Au₂₅/NiAl-HT-300 catalyst. Furthermore, the results of the in situ DRIFTS of the adsorption of nitrostyrene at 10 bar of hydrogen suggested that, besides the condensation route, it also followed the direct route to produce aniline on Au₂₅/NiAl-HT-300, which was different from the other two catalysts. This work provides new insight into the support effect over the gold catalysts for selective hydrogenation reactions.

Full text of DOI:10.1016/j.jcat.2018.05.007

Journal of Catalysis 364 (2018) 174–182
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Effects of divalent metal ions of hydrotalcites on catalytic behavior
of supported gold nanocatalysts for chemoselective hydrogenation
of 3-nitrostyrene
a
a
a
a,b
Yuan Tan a,b, Xiao Yan Liu a, , Lin Li , Leilei Kang , Aiqin Wang , Tao Zhang
⇑
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China
b University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of the divalent metal ions on the hydrotalcite (HT) (MAl-HT; M = Mg, Zn, Ni)-supported thio-
lated Au25 nanoclusters (NCs) as the precatalysts for the chemoselective hydrogenation of 3-nitrostyrene
to 3-vinylaniline was investigated. The highest chemoselectivity was obtained over the Au25/ZnAl-HT-
300 (calcined at 300 °C) catalyst, with a maintained selectivity of desired product above 98%. The
Au25/NiAl-HT-300 catalyst exhibited the highest activity, although the particle size of gold (3.2 nm)
was greater than those of the Au25/MgAl-HT-300 (2.2 nm) and Au25/ZnAl-HT-300 (1.7 nm) catalysts.
The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results of CO adsorption
revealed that Ni interacting intimately with gold could be reduced easily, which affected the catalytic
behavior of the Au25/NiAl-HT-300 catalyst. Furthermore, the results of the in situ DRIFTS of the adsorp-
tion of nitrostyrene at 10 bar of hydrogen suggested that, besides the condensation route, it also followed
the direct route to produce aniline on Au25/NiAl-HT-300, which was different from the other two cata-
lysts. This work provides new insight into the support effect over the gold catalysts for selective hydro-
genation reactions.
Received 11 February 2018
Revised 7 May 2018
Accepted 7 May 2018
Keywords:
Au25
Gold clusters
Hydrotalcite
Nitrostyrene
Selective hydrogenation
Ó 2018 Published by Elsevier Inc.
1. Introduction
chemoselective for the reduction of substituted nitroaromatic
compounds. Compared with the MgO and SiO2 supports, the
Functionalized aromatic amines are generally produced by
selective hydrogenation of the corresponding nitro compounds.
They are important industrial intermediates for the production of
a range of pharmaceuticals, polymers, agrochemicals, herbicides,
dyestuffs, and fine chemicals [1–7]. It was reported that the sup-
ported gold catalysts exhibited high chemoselectivity for the
hydrogenation of the functionalized nitroaromatics [8–11]. It was
proved that the hydrogenation reaction happened at the interfaces
between gold and the support [12,13]. Thus, the catalytic perfor-
mance of the gold catalysts for this reaction could be greatly
affected by the nature of the support [9,12–14].
Al2O3-supported gold nanocatalyst with small gold particle size
(ꢀ2.7 nm) was reported to exhibit the best catalytic activity for
the selective hydrogenation of 4-nitrostyrene [12]. The coordina-
tively unsaturated Au atoms on gold nanoparticles (NPs) and the
base–acid sites on Al2O3 were claimed to be responsible for the
activation of H2 [12]. Although the support itself was not active
for the hydrogenation reaction, it played an important role in
affecting the reactivity and selectivity of the gold catalysts. There-
fore, it is necessary to investigate the support effect for better
understanding of the catalytic mechanism and the rational design
of efficient gold catalysts.
Boronat et al. [9] reported that, over the Au/TiO2 catalyst, the
nitro group instead of the olefinic group could be preferentially
adsorbed at the interfaces between gold and the support, which
enabled the catalyst to exhibit high chemoselectivity for the hydro-
genation of the nitro group. However, such superior adsorption
was not observed over the Au/SiO2 system, which made it not
Controlling the size of the gold NPs is very important for study-
ing the support effect [15,16]. Recently, thiolated Au25 NCs
appeared as representative precursors for synthesizing supported
gold catalysts, because they could be synthesized easily, and the
size fit in the interest of the gold catalysis for many redox reactions
[17–27]. Previously, we developed a gold catalyst with high
chemoselectivity to 3-vinylaniline in the hydrogenation of 3-
nitrostyrene using ZnAl-HT supported thiolated Au25 NCs as a pre-
catalyst [28]. We found that, unlike other gold catalysts [9,12]. The
⇑
Corresponding author.
E-mail address: xyliu2003@dicp.ac.cn (X.Y. Liu).
https://doi.org/10.1016/j.jcat.2018.05.007
0021-9517/Ó 2018 Published by Elsevier Inc.

Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
175
nitro group and the vinyl group were not competitively adsorbed
onto the surface of the Au25/ZnAl-HT catalyst, while only the for-
mer could be adsorbed and hydrogenated. However, the intrinsic
origin of the high chemoselectivity for this catalyst is not yet clear.
In order to gain deep insight into this question, in this work, we
tried to study the support effect on the catalytic performance of
gold for this reaction. Unlike the previous study on the support
effect for the chemoselective hydrogenation of functionalized
nitroaromatics over gold catalysts, we used three different hydro-
talcites (MAl-HT, M = Mg, Zn, Ni) with similar structures as support
precursors. We selected atomically precise thiolated Au25 NCs as
the precursor of gold to exclude the influence of the preparation
method on the formation of gold particles. The performance of
the three catalysts for the chemoselective hydrogenation of 3-
nitrostyrene was obviously different.
get a good signal/noise ratio, the loading of Au was increased to
10 wt% for characterization by energy-dispersive X-ray mapping
(EDS-mapping) and XAS.
For comparison, the Au/MAl-HT-300 catalysts were prepared by
the deposition–precipitation (DP) method. In a typical synthesis,
an aqueous solution of HAuCl4 (20 mM, 5 mL) was added to the
suspension solution under vigorous stirring, in which 2.00 g of
the hydrotalcite powder (MAl-HT) was included. Then 1 M of
NaOH solution was used to adjust the pH value to 10. The reaction
was allowed to proceed at room temperature for 12 h. Then a solid
was obtained after filtering, washing, and drying. Before the cat-
alytic test, the solids were calcined at 300 °C for 2 h, with a heating
rate of 5 °C/min.
2.2. Catalytic test
A series of characterizations were employed to explore the key
factors influencing the catalytic performances. High-angle annular
dark-field-scanning transmission electron microscopy (HAADF-
STEM) was used to figure out the size distributions of the Au par-
ticles. CO2 temperature-programmed desorption (CO2 TPD) and
in situ DRIFTS of CO adsorption were utilized to test the surface
properties of the catalysts. The reaction routes were investigated
by in situ DRIFTS of 3-styrene adsorption at 10 bar of H2. The rea-
sons that led to the different catalytic performances of the three
catalysts were discussed based on the above results. This work will
be beneficial for understanding the origin of chemoselectivity and
activity over supported gold catalysts for this kind of reactions.
Catalytic testing of the chemoselective hydrogenation of 3-
nitrostyrene was carried out in a stainless steel autoclave equipped
with a pressure gauge under magnetic stirring. Before reaction, a
mixture of 3-nitrostyrene (0.2 M) and toluene and o-xylene (0.1
M) totaling 2 mL was put into the vessel. Then certain amounts
of catalysts were introduced into the autoclave. After sealing, the
autoclave was flushed with hydrogen six times and then pressur-
ized at 10 bar. To initiate the reaction, the reactor was heated to
90 °C in a water bath without stirring until the temperature
reached the specified value. After reaction, the product was con-
densed and analyzed by gas chromatography/mass spectrometry.
The turnover frequency (TOF) was measured when the conversion
of the substrate was below 20% and calculated in consideration of
the total loading of gold [10,28–30] and also its dispersion [31].
2. Experiment
2.1. Preparation of the catalysts
2.3. Characterization
The Au25 NCs were prepared by a NaOH-mediated NaBH4 reduc-
tion method according to the previous work [21,28]. Typically, 5.0
mL of the aqueous solution of HAuCl4 (110 mM) and 150 mL of cys-
teine solution (5.5 mM) were successively added to 200 mL of
ultrapure water under stirring. Then 30 mL of 1 M NaOH solution
was introduced into the above mixture. After 15 min of stirring,
excessive sodium borohydride was added to the above solution,
followed by vigorous stirring for 3 h. Finally, the products were col-
lected and washed with ethanol–water (V/V = 3:1). The Au25 NCs
were then obtained by lyophilization. The UV–vis spectrum of
the Au25 NCs is shown in Fig. S1 in the Supplementary Information.
The MAl-HTs were prepared as follows. Solution A was obtained
by adding M(NO3)2Á6H2O (0.21 mol) and Al(NO3)3Á9H2O (0.07 mol)
to 200 mL of ultrapure water. Solution B was prepared by adding
NaOH (0.438 mol) and Na2CO3 (0.113 mol) to 200 mL of ultrapure
water. Then solution A was slowly pumped into solution B (3 mL/
min) under constant stirring in the water bath at 75 °C. The gel was
aged at 75 °C for 24 h and the solid obtained was filtered and
washed with water and ethanol until the pH value of the filtrate
became neutral. The precipitates were dried in an oven at 80 °C
overnight to obtain the MAl-HTs.
The supported Au25 NCs catalysts were prepared as follows.
Au25 NCs (30 mg) were dispersed into 10 mL of ultrapure water.
Then 2.00 g of the MAl-HT supports was added into the above sus-
pension under vigorous stirring. After 1 h, the product was washed
with ultrapure water and collected by centrifugation (8000 rpm, 6
min). The residue was then freeze- dried for 10 h. The obtained
samples were defined as Au25/MAl-HT. Before the catalytic test,
the precursors were calcined at 300 °C for 2 h, with a heating rate
of 5 °C/min, and were denoted as Au25/MAl-HT-300. The loadings
of Au in the Au25/MAl-HT-300 catalysts were determined by induc-
tively coupled plasma spectrometry (ICP-AES). They were 1.02,
0.96, and 1.06% when the M was Mg, Zn, and Ni, respectively. To
The actual Au loadings were measured with an ICP-AES on an
IRIS Intrepid II XSP instrument (Thermo Electron Corporation).
The UV–visible spectra were recorded on a Cintra (GBC) apparatus
with water as a reference at room temperature. The Au25 NCs were
dissolved in the water for measurement. A continuous mode was
used in the wavelength range from 190 to 900 nm at a scanning
speed of 100 nm minÀ1. The X-ray powder diffraction (XRD) pat-
terns were determined on a PW3040/60 X’Pert PRO (PANalytical)
diffractometer equipped with
a CuKa radiation source (k =
0.15432 nm) operating at 40 kV and 40 mA. The HAADF-STEM,
the high-resolution transmission electron microscopy (HRTEM),
and the EDS mapping images were recorded on a JEOL JEM-
2100F microscope equipped with STEM dark-field (DF) and Oxford
detectors at 200 kV. CO2 TPD was conducted on a Micromeritics
AutoChem II 2920 automated catalyst characterization system.
The CO2 molecules were detected by an OmniStar mass spectrom-
eter (MS) equipped with the software quadstar 32-bit.
The in situ CO-DRIFTS spectra were acquired with a BRUKER
Equinox 55 spectrometer equipped with a MCT detector in the
range 400–4000 cmÀ1. An attenuated total reflection infrared
(ATR-IR) spectroscope was equipped with a DLaTGS detector and
the spectrum was acquired with a Bruker Vertex 70 V spectrome-
ter. The experiment was operated at room temperature and atmo-
spheric pressure. Before the test, the catalyst was dispersed into
10% of the ethanol/water and the mixture treated with ultrasound
for 30 min. Then the suspension was added dropwise onto the sur-
face of the diamond crystal on the instrument at a temperature of
90 °C. After drying, the background spectrum was recorded and the
substrate solution was added. To get the signal of the adsorbed
substrates, the spectrum was recorded when the liquid was
evaporated.
The in situ DRIFTS of nitrostyrene were recorded with a BRUKER
Equinox 55 spectrometer equipped with a MCT detector in the

176
Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
range 800–2000 cmÀ1 with a resolution of 4 cmÀ1. Before the test,
the catalyst was packed into a sample cell with a ZnSe window. The
background spectrum was recorded at 25 °C under atmospheric
pressure in helium. Then, with the protection of pure helium, 5
to 1000 min, the selectivity of 3-vinylaniline was maintained, with
a trace of azo compound as the by-product. 3-Ethylaniline (D) was
not detected, indicating that overhydrogenation of the substrate
did not happen in this case, which agreed well with our previous
work [28]. Over the Au25/NiAl-HT-300 catalyst (Fig. 1c), the nitro
group was hydrogenated preferentially at the first stage. When
the reaction proceeded for about 75 min, 3-ethylaniline appeared,
indicating the hydrogenation of the C@C bond. After about 140
min, the desired product was almost completely overhydrogenated
to 3-ethylaniline. The result suggested that the nitro group and the
vinyl group might be competitively adsorbed on the Au25/NiAl-HT-
300 catalyst, which behaved like the traditional gold systems (Au/
TiO2 [9], Au/Al2O3 [12], etc.). Over the three catalysts, 3-
ethylnitrobenzene (C) was not detected in the products, indicating
that the vinyl group could not be preferentially hydrogenated. To
sum up, the TOF values of the three catalysts were in the following
ll of 3-nitrostyrene was introduced into the sample for adsorption.
Subsequently, 10 bar of hydrogen was introduced into the cell and
the reaction temperature was increased to a fixed temperature
with a heating rate of 15 °C/min. After obtaining the steady state,
the spectra of 3-nitrostyrene hydrogenation process were collected
at 50, 60, 70, 80, and 90 °C, respectively.
The X-ray absorption near-edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS) spectra at the
Au LIII edge were recorded at the BL14W1 Shanghai Synchrotron
Radiation Facility (SSRF) [32], Shanghai Institute of Applied Physics
(SINAP), China. A double Si (1 1 1) crystal monochromator was
used for the energy selection. The energy was calibrated by Au foil.
The spectra were all collected at room temperature in the trans-
mission mode. The data were analyzed by the Athena software
package, with Artemis for fitting. The ranges used for data fitting
sequence:
Au25/MgAl-HT-300 < Au25/ZnAl-HT-300 < Au25/NiAl-
HT-300, as listed in Table 1.
To confirm the different selectivity over the three catalysts, con-
trol experiments were conducted under the same reaction condi-
tions to test the reactivity of the nitro group and the vinyl group
using the styrene and the artificial mixture of nitrobenzene and
styrene as substrates (Table 2). Only 2.0–5.5% of the styrene could
be converted to ethylbenzene over the Au25/MgAl-HT-300 catalyst,
in k-space and R-space were
D DR 1.2–3.3 Å,
k 3–12.5 ÅÀ1 and
respectively.
3. Results and discussion
which implied its low selectivity for the vinyl group. Over the Au25
/
ZnAl-HT-300 catalyst, the styrene could hardly be converted, no
matter if there was nitrobenzene in the substrate or not. However,
the nitrobenzene and styrene could be completely converted over
the Au25/NiAl-HT-300 catalyst. This indicated that the Au25/NiAl-
HT-300 catalyst was highly active for the hydrogenation of both
the nitro group and the vinyl group. The results agreed well with
the kinetic data in Fig. 1.
3.1. Catalytic performances
The distributions of the products with reaction time over the
catalysts for the hydrogenation of 3-nitrostyrene (A) are shown
in Fig. 1. One can see that the desired product, 3-vinylaniline (B),
over the different catalysts increased linearly in the initial stage.
Among them, the Au25/MgAl-HT-300 catalyst showed the lowest
activity (Fig. 1a). Even when the reaction proceeded for 400 min,
the 3-nitrostyrene still had not been converted completely, with
a conversion of 88.2% and a selectivity of 96.3%. The Au25/ZnAl-
HT-300 catalyst showed relatively moderate activity but the high-
est selectivity (Fig. 1b). After about 240 min, the 3-nitrostyrene
was converted completely, and the selectivity of the 3-
vinylaniline reached 99.3%. When the reaction time was prolonged
3.2. Effect of particle sizes of Au
Well-defined layered structure characteristics of the supports
and HT-supported Au25 NCs were confirmed by XRD measurement,
as shown in Fig. 2. Blank MgAl-HT, ZnAl-HT, and NiAl-HT (a, b, c)
displayed patterns similar to those of the uncalcined samples
Fig. 1. The distributions of the products with reaction time for the hydrogenation of 3-nitrostyrene over the catalysts in the presence of H2: (a) Au25/MgAl-HT-300; (b) Au25
/
ZnAl-HT-300; (c) Au25/NiAl-HT-300. Reaction conditions: 3-nitrostyrene: 0.4 mmol; catalyst: 50 mg; Au: ꢀ0.63 mol%; H2 pressure: 10 bar; reaction temperature: 90 °C.

Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
177
Table 1
Catalytic results of the hydrogenation of 3-nitrostyrene over the supported gold catalysts.
a
b
Entry
Catalyst
Size (nm)
Time (h)
Conv. (%)
Sel. (%)
TOF (hÀ1
)
TOF (hÀ1
)
1
2
3
4
5
6
Au25/MgAl-HT-300
Au25/ZnAl-HT-300
Au25/NiAl-HT-300
Au/MgAl-HT-300
Au/ZnAl-HT-300
Au/NiAl-HT-300
2.2 0.8
1.7 0.4
3.2 1.0
5.7 12.9
2.6 0.8
11.9 5.2
6.5
4
1.25
16.5
16.5
16.5
88.2
96.9
95.0
3.0
93.8
69.4
96.3
99.3
96.9
>99
98.1
9.5
36.9
71.1
133.4
0.3
15.6
9.0
67.1
101.6
351.1
1.4
33.2
74.8
Note: Reaction conditions: 3-nitrostyrene: 0.4 mmol; catalyst: 50 mg; Au: ꢀ0.63 mol%; H2 pressure: 10 bar; reaction temperature: 90 °C; solvent: 2 mL toluene.
a
The TOF was calculated based on the total loading of gold [10,28–30].
The TOF was calculated based on the dispersion of gold [31].
b
Table 2
Control experiments over the three catalysts.
Entry
Catalysts
Conversion (%)
Styrenea
Styreneb
Nitrobenzeneb
1
2
3
Au25/MgAl-HT-300
Au25/ZnAl-HT-300
Au25/NiAl-HT-300
5.5
1.0
100
2.0
0.9
100
100
100
100
Note: Reaction conditions: temperature 90 °C; H2 pressure 10 bar; solvent 2 mL toluene; reaction time 4 h.
a
Substrate: 0.4 mmol styrene.
Substrate: 0.4 mmol styrene and 0.4 mmol nitrobenzene.
b
(a1, b1, c1): the obvious characteristic reflections of the layered
structure with a series of narrow, symmetric, and strong peaks at
low 2h angles (<25°) [33]. After calcination at 300 °C, the three cat-
alysts (a2, b2, c2) exhibited the patterns of the mixed oxide, which
resembled those of MgO, ZnO, and NiO. Nevertheless, none of the
diffraction peaks associated with Au were observed. This meant
the gold NCs might be highly dispersed on the supports or the
Au face-centered cubic structures could be covered by the diffrac-
tion peaks of the support.
To get the size distributions of the gold particles, we conducted
HADDF-STEM characterization. From the images shown in Fig. 3,
we can see that the average particle sizes of Au25/MgAl-HT-300,
Au25/ZnAl-HT-300, and Au25/NiAl-HT-300 catalysts were 2.2 0.
8 nm (Fig. 3a1), 1.7 0.4 nm (Fig. 3b1), and 3.2 1.0 nm
(Fig. 3c1), respectively. Among them, the Au25/ZnAl-HT-300 cata-
lyst showed the smallest average particle size and the narrowest
size distribution. According to our previous work [28], the epitaxial
interaction between the gold and the ZnAl-HT support and the for-
mation of an AuASAZn bond might stabilize the gold particles. In
this work, by the HRTEM characterization, the lattice matching
between gold and the support were also found to exist in the
Au25/MgAl-HT-300 (Fig. 3a2) and Au25/NiAl-HT-300 (Fig. 3c2) cat-
alysts, which accorded with the previous work [33]. However, the
size distributions of gold nanoparticles over these catalysts were
different, which might be due to the different capability of the sup-
ports to stabilize gold NPs.
To illustrate the effect of particle sizes of Au on the selective
hydrogenation of 3-nitrostyrene, we synthesized the correspond-
ing gold catalysts over the three HT supports by deposition–precipi
tation (DP), and denoted them as Au/MAl-HT-300. We calculated
the average particle sizes of gold over these catalysts according
to the HAADF-STEM images (Fig. S2). The average particle sizes
of gold over Au/MgAl-HT-300, Au/NiAl-HT-300, and Au/ZnAl-HT-
300 catalysts were 5.7 12.9, 11.9 5.2, and 2.6 0.8 nm, respec-
tively. They were larger than those of the corresponding Au25
/
MAl-HT-300 catalysts. The size of the gold NPs depended on the
nature of the support and the synthesis method. The residual sulfur
of the thiolated Au25 NCs was claimed to be critical for stabilizing
the gold particles [28]. In the present work, according to the EDS
mapping (Fig. S3) and EXAFS (Table S1 and Figs. S4 and S5) results,
we demonstrated that residual sulfur still remained after calcina-
tion at 300 °C over the Au25/MAl-HT-300 samples, which might
form the AuASAM bonds and help to prevent the agglomeration
of gold particles.
Thus, the catalytic performances were influenced accordingly.
As shown in Table 1, the TOF values of the Au/MAl-HT-300 cata-
lysts were much lower than those of the corresponding Au25
/
MAl-HT-300 catalysts, which are visualized in Fig. 4a. The reason
that the Au/MAl-HT-300 catalysts were inferior to the Au25/MAl-
HT-300 catalysts was the poor dispersion of gold on the samples
obtained by the DP method. According to the plot of the TOF values
with the average particle sizes of gold (Fig. 4b), the size of gold can
influence the catalytic activity. The smaller particles rendered
Fig. 2. XRD patterns of (a) MgAl-HT; (a1) Au25/MgAl-HT; (a2) Au25/MgAl-HT-300;
(b) ZnAl-HT; (b1) Au25/ZnAl-HT; (b2) Au25/ZnAl-HT-300; (c) NiAl-HT; (c1) Au25/NiAl-
HT; (c2) Au25/NiAl-HT-300.

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Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
Fig. 3. HAADF-STEM and HRTEM images of the Au25/MgAl-HT-300 (a1, a2), Au25/ZnAl-HT-300 (b1, b2), and Au25/NiAl-HT-300 (c1, c2) catalysts.
Fig. 4. The TOF values for hydrogenation of 3-nitrostyrene over different gold catalysts with respect to different supports (a) and the diameter of the gold particles (b). A and
star mean catalysts prepared by impregnation (Au25 as the precursor of the gold); B and ball mean catalysts prepared by DP.
higher activity than the bigger ones, in which the TOF values were
improved exponentially with the decrease of the particle sizes of
gold. Notably, the NiAl-HT- supported gold catalysts were beyond
the law, because the Ni species probably promoted the dissociation
of hydrogen [34–36]. The TOF values calculated by considering the
dispersion of gold [30] were also plotted with the mean particle
sizes of gold (Fig. S6), which displayed a tendency similar to that
in Fig. 4.
conducted to figure out the basicity of the supports and the corre-
sponding Au catalysts. As shown in Fig. 5, the MgAl-HT-300 sup-
port and the Au25/MgAl-HT-300 catalyst displayed the strongest
basicity: the amounts of basic sites calculated according to the des-
orbed CO2 were 58.6 and 33.0 lmol/g, respectively. They were fol-
lowed by the NiAl-HT-300 support and the Au25/NiAl-HT-300
catalyst, where the amounts of basic sites decreased to 12.9 and
11.5
l
mol/g, respectively. The ZnAl-HT-300 (5.8
lmol/g) support
and the Au25/ZnAl-HT-300 (4.7
lmol/g) showed the weakest basic-
ity. According to the literature, the nitro group could be adsorbed
easily on the basic surface [37,38]. The Au25/MgAl-HT-300 catalyst
showed the lowest activity, although it exhibited the strongest
basicity. Too strong basicity of the support (MgO, MgAl-HT, etc.)
3.3. Effect of the properties of the surfaces
Generally, the basicity of the catalysts could affect the reactivity
for the hydrogenation of nitroaromatics [12,37]. Here, CO2 TPD was

Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
179
the Au–Ni/SiO2 bimetallic system [41]. The catalytic behavior of
the Au25/NiAl-HT-300 catalyst was similar to that of the Au–Ni
alloy catalysts, on which the vinyl group could be hydrogenated
after the hydrogenation of the nitro group [43].
For most hydrogenation reactions over the supported gold cat-
alysts, the activation and dissociation of hydrogen was the rate-
determining step [44–46]. In this work, to clarify the active site
for hydrogen dissociation, blank experiments were conducted to
evaluate the performance of the three supports. The results indi-
cated that the supports themselves were not active for the hydro-
genation of 3-nitrostyrene under the same reaction conditions as
the catalysts, although they could adsorb the nitro groups of the
substrates as well as the corresponding gold catalysts (Fig. S9).
We can reasonably speculate that the gold on the surface of the
catalysts played a key role in the dissociation of hydrogen. The
activation of hydrogen on the gold surfaces was investigated by
Corma’s group by density functional theory (DFT) calculations
[13]. They discovered that the gold atoms active for H2 dissociation
were neutral or had a net charge close to zero. Here, we found
there was metallic gold on the surfaces of the three catalysts,
which acted as the active site for the activation of hydrogen. The
high selectivity for the hydrogenation of the ANO2 over the three
catalysts at the beginning stage was attributed to the metallic gold.
However, the metallic nickel on the surface of the Au25/NiAl-HT-
300 catalyst could help to activate hydrogen and lead to the hydro-
genation of the vinyl bond after the nitro group was hydrogenated.
Fig. 5. CO2 TPD spectra of the Au25 NCs catalysts and the corresponding supports:
(a) MgAl-HT-300; (b) Au25/MgAl-HT-300; (c) ZnAl-HT-300; (d) Au25/ZnAl-HT-300;
(e) NiAl-HT-300; (f) Au25/NiAl-HT-300.
was detrimental to the hydrogenation of the nitro group, which has
been verified by Shimizu et al. [12]. But the intrinsic reason was
not clear. In our work, the Au25/NiAl-HT-300 catalyst with medium
basicity showed the highest activity, which might be attributed to
the easy activation of hydrogen over the Ni-based supports [34,35].
The in situ DRIFTS of CO adsorption was conducted to charac-
terize the properties of the surfaces of the three catalysts. As
shown in Fig. 6, the obvious band at around 2098 cmÀ1 could be
ascribed to the CO molecules adsorbed on metallic gold [39,40].
This was in good agreement with the results of XANES and EXAFS
(Figs. S7, S8). However, one more band at 2048 cmÀ1 appeared over
the Au25/NiAl-HT-300 catalyst, which could be ascribed to CO
adsorption on metallic nickel [41,42]. The reduction of the nickel
in the support might be promoted by the gold, as manifested in
3.4. Reaction route
The generally accepted reaction mechanism for the hydrogena-
tion of aromatic nitro compounds was divided into the direct route
and the condensation route [47]. Aniline derivatives were reported
to form on the Pt/CaCO3 or Pt/C–H3PO4 catalysts through both
routes, whereas on the Au/TiO2 catalyst, the reduction of aromatic
nitro compounds did not follow the condensation route but only
the direct route [48]. In this work, the azo compounds were
detected during the catalytic processes of the three catalysts
Fig. 6. In situ CO-DRIFTS spectra of adsorbed CO over Au NCs catalysts: (a) Au25/MgAl-HT-300; (b) Au25/ZnAl-HT-300; (c). Au25/NiAl-HT-300.

180
Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
Fig. 7. In situ DRIFTS spectra of adsorbed nitrostyrene over the catalysts: (a) Au25/MgAl-HT-300; (b) Au25/ZnAl-HT-300; (c) Au25/NiAl-HT-300 at 25 °C (1), 50 °C (2), 60 °C (3),
70 °C (4), 80 °C (5), and 90 °C (6) before (1) and after (2–6) introduction of 10 bar of H2.
implied that the reaction path on the Au25/NiAl-HT-300 catalyst
also followed the direct route as well as the condensation route.
Based on the above results and our kinetic data over the three
catalysts, we propose a reaction pathway for the hydrogenation
of 3-nitrostyrene over the three catalysts (Scheme 1). There was
a condensation route over the three catalysts: The 3-nitrostyrene
was first reduced to the nitroso compound and then to the hydrox-
ylamine compound, followed by one molecule of the nitroso com-
pound and a molecule of the hydroxylamine producing one
molecule of the azo compound, which was consecutively reduced
to the hydrazo and aniline compounds. As indicated by the
in situ DRIFTS results in Fig. 7, there was also a direct route over
the Au25/NiAl-HT-300 catalyst, which permitted direct transforma-
tion from nitrostyrene to aniline compounds. Furthermore, overhy-
drogenation to aminoethylbenzene could also happen over the
Au25/NiAl-HT-300 catalyst. The difference in reaction pathways
of Au25/NiAl-HT-300 from those of the others might be caused by
the appearance of the metallic nickel, which could enhance the
reactivity and influence the selectivity.
Scheme 1. Proposed reaction mechanism for the hydrogenation of 3-nitrostyrene
over the three catalysts.
4. Conclusions
(Fig. S9), suggesting that they were following the condensation
route to produce 3-vinylaniline. To further investigate the reaction
path of the hydrogenation processes over the three catalysts, we
employed in situ DRIFTS under 10 bar of hydrogen to detect the
intermediate products. The adsorption spectra are displayed in
Fig. 7. Before the hydrogen was introduced, the bands at 1540
The effect of the divalent metal ions in the hydrotalcite on the
catalytic performance of the Au25/MAl-HT-300 catalyst for the
hydrogenation of 3-nitrostyrene was investigated. The three cata-
lysts exhibited different catalytic performance. The highest TOF
was obtained over the Au25/NiAl-HT-300 catalyst, but severe over-
hydrogenation to 3-ethylaniline was observed with prolonged
reaction time. Inferior activity was obtained over the Au25/ZnAl-
HT-300 and Au25/MgAl-HT-300 catalysts. The selectivity for the
desired product was highest over the Au25/ZnAl-HT-300 catalyst,
on which the selectivity of 3-vinylstyrene could be maintained at
above 98% even after the nitro group of the substrate was con-
verted completely. When CO was used as the probe molecule,
the in situ DRIFTS results showed that the gold in a metallic state
over the three catalysts led to superior hydrogenation of the nitro
group. Meanwhile, the new absorption band at 2048 cmÀ1 due to
the metallic Ni over the Au25/NiAl-HT-300 catalyst indicated that
Ni intimately interacted with Au could be reduced at room temper-
ature, which contributed to the high reactivity and overhydrogena-
tion. Based on the in situ DRIFTS results of 3-nitrostyene
adsorption at 10 bar of hydrogen, the three catalysts all followed
the condensation route, while the direct route only appeared on
the Au25/NiAl-HT-300 catalyst. Accordingly, the effect of the diva-
lent metal ions on the reaction pathways of the Au25/MAl-HT-300
and 1358 cmÀ1 due to the
m
as(NO2) and
ms(NO2) of the adsorbed
nitro species [12,49] and the band at 1633 cmÀ1 due to the
m
(C@C) [9] were observed over the three catalysts. When the pres-
sure of the hydrogen was enhanced to 10 bar, the IR band at
1558 cmÀ1 appeared on all of the catalysts with increasing temper-
ature, which could be assigned to the vibrations of the N@N bond
[50]. This agreed well with the results of chromatographic analysis
(Fig. S10). That meant that the reduction of nitrostyrene on the
Au25/MAl-HT-300 catalysts followed the condensation route. The
bands at 1620, 1604, and 1493 cmÀ1 were ascribed to aniline
[12,51], which was produced during the hydrogenation of the sub-
strate. The intensity of these bands appeared much higher over the
Au25/NiAl-HT-300 than on the other two catalysts. This suggested
that the Au25/NiAl-HT-300 catalyst exhibited the highest catalytic
activity, in accordance with the reactivity evaluation results. The
new bands at 1583 cmÀ1 (ANO) [52,53] and 1462 cmÀ1 (ANOH)
[49] were observed on the Au25/NiAl-HT-300 catalyst but not on
the Au25/MgAl-HT-300 and Au25/ZnAl-HT-300 samples. This

Y. Tan et al. / Journal of Catalysis 364 (2018) 174–182
181
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Supporting information
UV–visible spectra of the thiolated Au25 NCs, XANES and EXAFS
results of the three catalysts, HAADF-STEM images of the gold cat-
alysts prepared by the DP method, the ATR-FTIR spectra of the 3-
nitrostyrene on the three catalysts and supports, and some other
results are available free of charge at http://pubs.acs.org.
Acknowledgments
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The authors are grateful to the National Natural Science Foun-
dation of China (21522608, 21476227, 21776271, and
21573232), the National Key Projects for Fundamental Research
and Development of China (2016YFA0202801), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB17020100). The Department of Science and Technology of
Liaoning province under Contract 2015020086-101 is also
acknowledged. We also thank the BL14W at the Shanghai Syn-
chrotron Radiation Facility (SSRF) for the XAFS experiments. The
authors declare no competing financial interest.
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