Activity and Selectivity in Nitroarene Hydrogenation over Au Nanoparticles on the Edge/Corner of Anatase

Article abstract of DOI:10.1021/acscatal.6b00530

Highly selective hydrogenation of molecules containing more than one reducible group is always challenging. Here we report an efficient strategy for rational preparation of highly selective gold-based catalysts in the hydrogenation of substituted nitroarenes by positioning gold nanoparticles on the edge/corner sites of anatase. Mechanistic studies reveal that the catalyst with gold nanoparticles on the edge/corner sites of anatase could form unique sites for selective adsorption and activation of nitro groups, thus leading to high activity and selectivity. This strategy for preparation of supported gold catalysts opens a new door for the design of highly efficient heterogeneous catalysts in the future.

Full text of DOI:10.1021/acscatal.6b00530

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Article
Activity and Selectivity in Nitroarene Hydrogenation
over Au Nanoparticles on the Edge/Corner of Anatase
Liang Wang, Jian Zhang, Hong Wang, Yi Shao, Xiaohui
Liu, Yan-Qin Wang, James P. Lewis, and Feng-Shou Xiao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00530 • Publication Date (Web): 19 May 2016
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Activity and Selectivity in Nitroarene Hydrogenation over Au Nano-
particles on the Edge/Corner of Anatase
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Liang Wang,*a Jian Zhang,a Hong Wang,*b Yi Shao,c Xiaohui Liu,c Yan-Qin Wang,c James P.
Lewis,b and Feng-Shou Xiao*a,d
a Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou
310028, China
b Department of Physics and Astronomy, West Virginia University, Morgantown WV 26506-6315, USA
c Institute of Catalysis, East China University of Science and Technology, Shanghai 200237, China
d
Key Lab of Biomass Chemical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027,
China.
ABSTRACT: Highly selective hydrogenation of molecules containing more than one reducible groups is always chal-
lenging. Here we report an efficient strategy for rational preparation of highly selective gold-based catalysts in hydro-
genation of substituted nitroarenes by positioning gold nanoparticles on the edge/corner sites of anatase. Mechanistic
studies reveal that the catalyst with gold nanoparticles on the edge/corner sites of anatase could form unique sites for
selective adsorption and activation of nitro groups, thus leading to high activity and selectivity. This strategy for prepa-
ration of supported gold catalysts opens a new door for design of highly efficient heterogeneous catalysts in the future.
KEYWORDS: gold nanoparticle, nitrobenzene hydrogenation, high activity, anatase, adsorption, selectivity
field, but the poor activities of Au/TiO2 catalysts still
hinder the practical applications.2,31,35,36 An attractive
strategy for enhancing catalytic activities of the Au/TiO2
catalysts is to rationally design their structures, and suc-
cessful examples are preparation of highly active cata-
lysts for the water-gas shift and various oxidations,39-40,41-
44 but it is still not successful for enhancement of catalyt-
ic activity in the nitroarene hydrogenation by adjusting
the structure of the Au/TiO2 catalysts.
To understand the dependence of the Au/TiO2 struc-
ture on the activity in the nitroarene hydrogenation, a
theoretical simulation for the adsorption of nitrobenzene
molecule on the Au/TiO2 with different structures has
been studied. Very interestingly, it is observed that the
adsorption of the nitrobenzene on the edge/corner sites
of TiO2 support is stronger than that on the (101) facet of
TiO2 (See the details in Figure S1). Based on the well-
known knowledge that the adsorption is a very im-
portant step in hydrogenation of nitroarenes,38 it is sug-
gested that the Au nanoparticles loaded on the
edge/corner sites of TiO2 support (Au/TiO2-EC) could
improve the catalytic activities. As we expected, when the
Au/TiO2-EC is used to catalyze hydrogenation of 3-
nitrostyrene, the turnover frequency of 279 h-1 is ob-
tained. This value is much higher than those (<173 h-1) of
the conventional Au nanocatalysts.35 More importantly,
the chemoselectivity to 3-aminostyrene could reach as
high as 99.0%. Our study reveals that the specific loading
methodology might open a new door for design and
preparation of highly efficient catalysts with simultane-
ous activities and selectivities in the hydrogenations.
1. INTRODUCTION
The metal-catalyzed hydrogenations with molecular
hydrogen have been regarded as a great discovery in the
production of fine chemicals and pharmaceuticals.1-9 In
these reactions, when more than one reducible groups
are present on substrate molecules, selective hydrogena-
tion of the target group is challenging because the side-
reactions easily occur on the non-target groups.10-20 One
efficient approach for solution of this problem is to poi-
son/block some of the atoms on the catalyst surface by
additives such as quinoline, carbon monoxide, thiol lig-
ands, and metal-based modifiers.21-25 Because numerous
studies have demonstrated the differences in catalytic
activity or selectivity when reactions occurs on different
sites of the metal nanocrystals,26-28 poisoning/blocking
the sites for undesired reactions could achieve encourag-
ing progress for improving catalytic selectivities.29,30
A
prominent example is the industrial Lindlar catalyst.
However, in most of these cases, catalytic activities are
remarkably reduced due to poisoning the catalysts.21,22 In
addition, leaching of these additives in the catalytic sys-
tems is difficult for the catalyst recycling. Therefore, it is
strongly desirable to simultaneously enhance the catalyt-
ic activity and selectivity in the hydrogenation.
Hydrogenation of substituted nitroarenes is chosen
as a model in this work because these hydrogenated
products are industrially important intermediates for the
formation of fine chemicals and high-performance rub-
bers and polymers.2,31-38 Recently, it is reported that TiO2
supported Au nanocatalysts (Au/TiO2) are successful for
selective hydrogenation of substituted nitroarenes to
anilines by selectively adsorption of the nitro groups.2,38
This discovery leads to a series of investigations in this
2. EXPERIMENTAL SECTION
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2.1 Materials. All reagents were of analytical grade drogen was introduced and kept at a desired pressure
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and used as purchased without further purification.
2.2 Synthesis. The anatase support Au nanocata-
lyst with Au nanoparticles mostly loaded on the
edge/corner sites of anatase (Au/TiO2-EC) was synthe-
sized according to our previous work.44 As a typical run
for synthesizing Au/TiO2-EC, 250 mg of polyvinylpyrrol-
idone (PVP, K13-K18, average molecular weight at
~10,000) was added into HAuCl4 solution (0.55 mmol of
Au and 300 ml of water) and stirred at 0 °C for 1 hour.
Then NaBH4 solution (400 mg of NaBH4 in 20 ml of wa-
ter) was rapidly added under vigorous stirring at 0 °C for
another 1 hour. Followed by adding 10 g of anatase, the
mixture was further stirred at 40 °C under vacuum to
evaporate the water. Finally, the solid powder was
washed with a large amount of water for many times and
calcined at 400 °C for 4 hours in oxygen with pro-
grammed increase of temperature (from room tempera-
ture to 300°C at 1°C/min, after 2 h, increase to 400 °C at
1°C/min) to remove PVP. After washing with NaBH4 so-
lution for 8 h at room temperature, the Au/TiO2-EC
sample with Au content at 0.91 wt% was obtained.
The conventional Au nanocatalyst with Au nanopar-
ticles mostly loaded on the anatase facet sites (Au/TiO2-
Con) was synthesized by deposition-precipitation meth-
od. As a typical run, 10 g of anatase was added to 500 ml
of HAuCl4 (0.55 mmol of Au) and urea (50 mmol) solu-
tion. After stirring at 90 °C for 4 hours in a closed reactor
kept away from the light, the liquid mixture was cooled
in an ice bath at 0 °C, followed by adding NaBH4 solution
(400 mg of NaBH4 in 20 ml of water). Finally, the solid
sample was filtrated and washed with a large amount of
water, dried at 100 °C for 12 hours, and calcined at
400 °C for 4 hours. After washing with NaBH4 solution
for 8 h at room temperature, the Au/TiO2-Con sample
with Au content at 0.87 wt% was obtained.
2.3 Characterization. XPS spectra were per-
formed on a Thermo ESCALAB 250with Al Ka radiation
at h = 90° for the X-ray sources; the binding energies
were calibrated using the C1s peak at 284.9 eV. IR spec-
trawere recorded using a Nicolet NEXUS 670 FT-IR
spectrometer equipped with a MCT detector and ZeSe
windows and a high temperature reaction chamber. The
nitrobenzene steam was introduced into the system by
Ar flow.The content of Au was determined from induc-
tively coupled plasma (ICP) with a Perkin-Elmer Plasma
40 emission spectrometer. The position of Au nanoparti-
cles was demonstrated by high-resolution transmission
electron microscopy (HR-TEM) over a JEOL JEM-
3000F microscope equipped with a field-emission gun.
By analyzing more than 100 Au nanoparticles randomly
selected in the HR-TEM images for each sample, we con-
firm that 86% of the Au nanoparticles of Au/TiO2-Con
sample are located on the (101) surface of anatase, while
more than 75% of the Au nanoparticles of Au/TiO2-EC
are on the edge/corner sites of anatase support. These
detailed information in catalyst structures of Au/TiO2-
EC and Au/TiO2-Con was included in our previous
work.44
(measured at the reaction temperature) and the reaction
system was heated to a given temperature (measured
with a thermometer in an oil bath). After the reaction,
the product was taken out of the reaction system and
analyzed by gas chromatography (GC-14C, Shimadzu,
using a flame ionization detector) with a flexible quartz
capillary column coated with OV-17 or FFAP. The recy-
clability of the catalyst was tested by separating it from
the reaction system by centrifugation, washing with a
large quantity of methanol, and drying at 80 °C for 6
hours. In the recyclable test of Au/TiO2-EC, the Au con-
centration in the liquor is less than 5 ppb by ICP analysis
after separating the used solid catalyst, indicating that no
Au leaching occurred during the reaction.
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3. RESULT and DISCUSSION
3.1 Kinetic study. The Au nanoparticles supported
on the edge/corner sites of anatase were synthesized
according to the literature (Au/TiO2-EC).44 For compari-
son, the control catalyst of conventional Au/TiO2 with Au
nanoparticles mostly attached on the facet of TiO2 sup-
port was also synthesized, denoted as Au/TiO2-Con. The
Au loading were established to be 0.91 and 0.87 wt% by
inductively coupled plasma (ICP) analysis, respectively.
(A)
(B)
Au/TiO2-EC
Au/TiO2-Con
1000
800
600
400
200
0
Au/TiO2-EC
Au/TiO2-Con
-11.0
-11.5
-12.0
-12.5
-13.0
60 70 80 90 100 110 120
T (oC)
2.5
2.6
1/T (10-3 K-1
2.7
2.8
)
(C) 8
(D)
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Au/TiO2-EC
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Au/TiO2-EC
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0.8
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0
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4
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Concentration of Nitrobenzene (10-1 mol L-1
)
PH2 (bar)
Figure 1. Kinetic studies in nitrobenzene hydrogenation
over Au catalysts. (A) TOFs of the Au/TiO2-EC and
Au/TiO2-Con in hydrogenation of nitrobenzene. (B) Exper-
imental points correlating the inverse of the absolute tem-
perature (1/T) with the initial reaction rates (ln r0) over the
Au/TiO2-EC and Au/TiO2-Con catalysts. (C) Dependences of
reaction rate on nitrobenzene concentration over the
Au/TiO2-EC and Au/TiO2-Con catalysts. (D) Dependences
of reaction rate on hydrogen pressure over the Au/TiO2-EC
and Au/TiO2-Con catalysts.
We start from a kinetic study on the hydrogenation
of nitrobenzene over the Au-based catalysts. The anatase
support without Au species is inactive for the reaction.
By studying the turnover frequencies (TOFs) according
to the nitrobenzene conversion at the start of each reac-
tion, it is found that the Au/TiO2-EC always exhibited
higher TOF values than Au/TiO2-Con at various reaction
2.4 Catalytic tests. The hydrogenation reactions
were performed in a high-pressure autoclave with a
magnetic stirrer (1200 rpm). Typically, the substrate,
catalyst, and solvent were mixed in the reactor and
stirred for 15 min at room temperature. Then, pure hy-
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temperatures (Figure 1A), implying the high activity.
TPD process. Desorption conditions: (a) 50 C, 10 min, (b)
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Based on the initial reaction rate (ro) values at different
reaction temperature (Figure S2), the apparent activa-
tion energies (Ea) of Au/TiO2-EC and Au/TiO2-Con are
calculated to be ~26 and ~40 KJ mol-1, respectively (Fig-
ure 1B). The different Ea values indicate that the conver-
sion of nitrobenzene over Au/TiO2-EC is much easier
than that over Au/TiO2-Con.
50 C, 30 min, (c) 100 C, 10 min, (d) 100 C, 20 min, (e)
100 C, 30 min, (f) 150 C, 10 min, (g) 150 C, 20 min, (h)
150 C, 30 min, (i) 200 C, 10 min, (j) 200 C, 20 min, (k)
200 C, 30 min, (l) 300 C, 10 min, (m) 300 C, 20 min, and
(n) 300 C, 30 min; (C) Changes in IR band intensities dur-
ing the TPD process.
Furthermore, the dependences of ro values on nitro-
benzene concentration and gaseous hydrogen pressure
were performed to understand the kinetic reaction or-
ders. Very clearly, the Au/TiO2-EC always exhibits higher
ro values than Au/TiO2-Con under either different nitro-
benzene concentration or different hydrogen pressure
(Figures 1C and 1D). Importantly, at low nitrobenzene
concentration (<0.05 mol L-1), the rates over Au/TiO2-
Con increase more significantly than those of Au/TiO2-
EC, yielding the kinetic orders for nitrobenzene concen-
tration at 0.27 and 1.11 for Au/TiO2-EC and Au/TiO2-Con,
respectively. Under constant concentration of nitroben-
zene, the reaction rates over Au/TiO2-Con is constant
with increasing the hydrogen pressure (Figure 1D), indi-
cating the zero kinetic order. In this case, the activation
of nitrobenzene acted as a rate control step over
Au/TiO2-Con in the hydrogenation of nitrobenzene. In
comparison, the rates over Au/TiO2-EC increase with the
hydrogen pressure, giving the reaction kinetic order at
0.85. This phenomenon indicates that the Au/TiO2-EC
breaks through the limitation of nitrobenzene activation,
exhibiting hydrogen activation as a control step.
By the kinetic studies, it is identified unequivocally
that the Au/TiO2-EC is more catalytically efficient than
the Au/TiO2-Con in the hydrogenation of nitrobenzene
(Figure S3). Considering that the weak transfer limita-
tion of the reaction substrates on these nano-sized ana-
tase supported Au catalysts, the distinguishing apparent
Ea values and reaction kinetic orders suggest that the
Au/TiO2-EC catalyst should have superior ability than
Au/TiO2-Con in activating nitro groups.
3.2 Nitro group-adsorption IR. It has been re-
ported that IR spectroscopy is powerful for understand-
ing the adsorption of nitro molecules on solid sup-
port.38,45 Therefore, in order to understand the interac-
tion between nitrobenzene and catalysts, we conducted
the FT-IR spectra of nitrobenzene adsorbed on Au/TiO2-
EC, Au/TiO2-Con, and anatase. The Au/TiO2-EC gives
strong absorbance bands at 1527, 1491, and 1349 cm-1
assigned to the nitro groups (Figure 2A). The bands at
1527 and 1349 cm-1 have already been definitely under-
stood as the asymmetric stretching(νasym) vibration fre-
quency of nitro group adsorbed on Ti-OH group and
symmetric stretching(νsym) vibration frequency of nitro
group adsorbed on the electron-rich surface Ti atoms,
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e
(A) X 8
X 8
d
c
b
a
1700
1600
Wavenumber (cm-1)
1500
1400
1300
X 8
e
d
(B)
X 8
1349
(A)
(B)
1527
1491
c
1491
1527
1349
1589
1589
n
b
Au/TiO2-EC
Au/TiO2-Con
m
l
a
k
j
Anatase
1700
1600
1500
1400
1300
Wavenumber (cm-1)
1800 1700 1600 1500 1400 1300
Wavenumber (cm-1
i
)
Figure 3. FT-IR spectra of the nitrobenzene adsorbed on
the (A) Au/TiO2-Con and (B) anatase. Desorption tempera-
ture at (a) 50 C, (b) 100 C, (c) 150 C, (d) 200 C, and (e)
250 C.
h
g
(C)
f
νasym(1491)
e
d
(A)
x
8
e
d
νsym(1349)
c
x
8
b
νasym(1527)
a
c
a
m n
c
g
k
l
b
d e f
Desorption Conditions
h i
j
1700
1600
1500
1400
1300
Wavenumber (cm-1)
b
Figure 2. (A) IR spectra of nitrobenzene adsorbed on the
Au/TiO2-EC, Au/TiO2-Con, and anatase; (B) IR spectra of
the adsorbed nitrobenzene on the Au/TiO2-EC during the
a
1700
1600
1500
1400
1300
Wavenumber (cm-1)
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ventional νasym band of nitro group (1527 cm-1), indicating
that the N=O bonds are significantly weaken by
(B)
1
2
3
4
interacting with the Au/TiO2-EC catalyst. Therefore, the
1491-cm-1 band should be favorable for the hydrogena-
tion of nitro groups.
b
Figure 2B shows the FT-IR spectra of the adsorbed
nitrobenzene on the Au/TiO2-EC during the temperature
programmed desorption (TPD) process in flowing Ar.
Correspondingly, the spectral changes of the νsym(1349
cm-1), νasym(1491 cm-1), and νasym (1527 cm-1) bands in in-
tensity are given in Figure 2C. Clearly, the νsym (1349 cm-1)
and νasym (1527 cm-1) bands exhibit significant decrease in
intensity with increasing the desorption temperature. At
150 C, the extremely weak νsym(1349 cm-1) and νasym(1527
cm-1) bands confirm that most of the nitro groups are
desorbed from the corresponding sites (Figure 2B-h). At
the same time, the νasym(1491 cm-1) band becomes
stronger by increasing the temperature from 50 to 150 C
(Figures 2B and 2C), which is different from the general
phenomenon observed on the bands of 1349 and 1527
cm-1. This phenomenon might be attributed to the trans-
fer of nitrobenzene molecules, which were desorbed
from the electron Ti atoms and/or Ti-OH groups by in-
creasing temperature, and re-adsorbed asymmetrically
on the unique strong adsorption sites, leading to the en-
hancement in the νasym(1491 cm-1) band intensity. Nota-
bly, further increasing the temperature (>200 C) could
also lead to a decreased intensity of the νasym(1491 cm-1)
band, but obvious peak could still be detected even at a
high temperature of 300 C (Figure 2B-n). These obser-
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1400
1300
Wavenumber (cm-1)
Figure 4. (A) IR spectra of the nitrobenzene adsorbed on
Au/TiO2-EC under flowing H2/Ar at 90 C for (a) 0 min, (b)
2 min, (c) 3 min, (d) 9 min, and (e) 10 min; (B) IR spectra of
the nitrobenzene adsorbed on Au/TiO2-EC under flowing Ar
at 90 C for (a) 0 min and (b) 30 min.
respectively.36,45 Notably, the νsym band at 1349 cm-1 is
undetectable on the Au/TiO2-Con and anatase (Figure
2A), due to the lack of electron-rich Ti atoms, as con-
firmed by the XPS spectra (Figure S4).
Additionally, on the IR spectrum of nitrobenzene on
the Au/TiO2-EC, the band at 1491 cm-1 (Figure 2A), which
is rarely reported yet, is reasonably assigned to another
asymmetrical adsorption type of nitro group on the
Au/TiO2-EC. It is worth noting that the band of nitro
group at 1491 cm-1 has significant red shift from the con-
(A)
a
c
b
Au/TiO2-Con
Au/TiO2-EC
(C)
(B)
2109
C
2084
Au
N
O
Ti
O
2200
2100
2000
1900
1800
Wavenumber (cm-1)
Figure 5. (A) The Fukui Function distribution of contour (a) Au13, (b) Au13/TiO2 model with no oxygen vacancy
(Relation to Au/TiO2-con), and (c) Au13/TiO2 with oxygen vacancy (Relation to Au/TiO2-EC, the contour iso value is
0( )
0.025). The yellow density indicates that the Fukui Function at that site is positive 푓 푟 > 0, while the blue color
0( )
density indicates that the Fukui Function at that site is negative 푓 푟 < 0; (B) CO-adsorption IR spectra of the
Au/TiO2-EC and Au/TiO2-Con; (C) The zoomed-in image of the most energetic favourable structure of nitrobenzene
attached on Au13/TiO2 model with one oxygen vacancy.
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vations indicate unique stability of the 1491 cm-1 species
interaction. In these optimized models, we have imple-
1
2
3
4
5
6
7
8
due to the strong adsorption of nitro groups. In contrast,
the IR bands of nitrobenzene adsorbed on Au/TiO2-Con
(Figure 3A) and TiO2 (Figure 3B) are very weak after de-
sorption at 150 C, due to the lack of strong adsorption
sites for nitro groups.
mented Fukui function as the principle factor to illus-
trate the potential possibility of the models for adsorb-
ing/interacting with nitro group, because the Fukui
Function was defined as the first derivative of the elec-
tron density at a position r.46,47 As shown in Figure 5A-a,
the Fukui Function of Au13 itself is symmetric. However,
once attached on the anatase nanosupport, the Fukui
Function of Au13 presented distinguished changes in
terms of the distribution, which proves the anatase sup-
port introduced significant impact on Au nanoparticle
(Figure 5A-b). Interestingly, the model with oxygen va-
cancy exhibits a more electrophilic region around the
TiO2-Au13 interface (Figure 5A-c). To experimentally con-
firm this on the synthesized catalysts, we perform CO-
adsorption IR spectra on Au/TiO2-EC and Au/TiO2-Con
samples, because CO is known as an electron-rich mole-
cule and sensitive to determine the electronic structure
of the solid surfaces. As shown in Figure 5B, the liner CO
vibrational frequency on the Au/TiO2-EC was 2109 cm-1,
25 cm-1 of shift higher than that of the Au/TiO2-Con
(2084 cm-1). This blue shift indicates the stronger bind-
ing of CO on the Au/TiO2-EC than the Au/TiO2-Con, be-
cause the CO benefits more to adsorb on the electrophilic
sites.48,49
Importantly, after introduction of H2 to the
Au/TiO2-EC adsorbed nitrobenzene, the intensity of the
νasym(1491 cm-1) band decreased more immediately and
significantly than those of νasym (1527 cm-1) and νsym (1349
cm-1) in the IR spectra (Figure 4A), which is different
from the phenomenon during the desorption process in
following Ar under the same temperature (Figure 4B).
These results might suggest that the nitro groups on
νasym(1491 cm-1) band are favorable to be reduced, be-
cause these nitro groups are highly activated by the
strong adsorption, which could explain the origin of
higher activity of the Au/TiO2-EC than the Au/TiO2-Con.
3.3 DFT simulations. To further understand the
adsorption behavior of nitrobenzene on the catalysts, we
performed density functional theory (DFT) simulations
to understand the catalytic mechanism in principle. The
computational models with Au13 clusters supported on
anatase were built and optimized (Figure S5). According
to our previous study in Au-anatase nanostructure, the
model with Au13 on the edge/corner sites of anatase,
which simulates Au/TiO2-EC catalyst, is favorable to
produce oxygen vacancies on the Au-anatase interface
due to the strong interaction between Au and Ti.44 In
contrast, the model of Au13 on the {101} facet of anatase,
which simulates Au/TiO2-Con catalyst, has weak Au-Ti
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Furthermore, we introduce a nitrobenzene molecule
to the model of Au13 on the edge/corner sites of anatase.
By attaching nitrobenzene on different positions of the
model, we found that the most energetic favorable ad-
sorption for nitrobenzene can occur near the oxygen va-
cancy on the Au-TiO2 interface (Figure S6B). The nitro
Table 1. Catalytic data in the hydrogenation of different molecules over various catalysts.a
Conv.
(%)
Sel.c
Substrate
Catalyst
Conditionsb
(%)
3-nitrostyrene
Au/TiO2-EC
Au/TiO2-Con
Au/TiO2-Con
Au/TiO2
99.0
55.5
71.1
99.0
90 C/10 bar/4.5 h
90 C/10 bar/4.5 h
90 C/10 bar/7 h
120 C/9 bar/6 h
93.3
93.1
95.9d
0d
98.5
99.5
98.3
60.0
99.3
67.3
Au-Pd/TiO2
Au/TiO2-EC
Au/TiO2-Con
Au/TiO2-EC
Au/TiO2-Con
120 C/9 bar/0.03 h
100 C/10 bar/2 h
100 C/10 bar/2 h
100 C/10 bar/2 h
100 C/10 bar/2 h
2-chloro-4-nitrophenol
2-nitroacetophenone
99.5
91.0
99.0
97.0
a Reaction conditions: 0.8 mmol of nitrobenzene, 40 mg of catalyst, 3 ml of toluene, the molecular formula of substrates, and
products are presented in Scheme S1; b Reaction temperature/H2 pressure/reaction time; c Selectivity to the corresponding
substituted aniline product; d The data are from Ref. 2.
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group of nitrobenzene interacts with one Au atom via corresponding amines at 99.0-99.5%, which are much
1
forming a Au-O interaction (Figure S7). Meanwhile, the
Ti atom nearby also interacts with this oxygen atom,
which leads to a noticeable lengthening of the N-O bond
(Figure 5C). It is important to remark that the oxygen
vacancy locates right next to this Au atom. There is also a
distortion of the benzene ring due to the interaction be-
tween some of the carbon atoms and the Au atoms upper
body. It turns out this attaching manner leaves the other
oxygen atom of nitro group easily being attacked by the
coming hydrogen species, which initiates the hydrogena-
tion reaction to occur at the attaching site (See details in
Figures S6 and S7). Meanwhile, this adsorption behavior
is unavailable when the oxygen vacancy is artificially
eliminated on the catalyst model. Such changes in simu-
lation indicate the important role of catalyst structure.
Combining with the results in nitrobenzene adsorption-
IR spectra, it could be proposed that the strong adsorp-
tion and activation of nitro group on the Au/TiO2-EC
lead to the high catalytic activity in hydrogenation of
nitrobenzene.
3.4 Hydrogenation selectivity. The hydrogena-
tion of 3-nitrostyrene was performed to study the cata-
lytic selectivity, because the vinyl groups are very active
in the hydrogenation. Although Au nano catalysts have
been reported to exhibit more than 90% selectivity in the
hydrogenation of 3-nitrostyrene to 3-aminostyrene,2 it is
surprising to observe that the selectivity could be further
improved over the Au/TiO2-EC, giving the 3-
aminostyrene selectivity at 99.0%, as shown in Table 1.
Under the same reaction conditions, the Au/TiO2-Con
gives really lower conversion and selectivity at 55.5 and
93.3%, respectively. This phenomenon is reasonably at-
tributed to the selectively and strongly adsorbing the
nitro group of 3-nitrostyrene molecule on Au/TiO2-EC
(Figures S8-S11). In addition, the chemoselective reduc-
tion of nitro group in the presence of carbonyl, phenol,
and chlorine groups has been studied through the hy-
higher than those (91.0-97.0%, Table 1) over the
Au/TiO2-Con catalyst, indicating the excellent selectivity
of Au/TiO2-EC catalyst. Based on these results, one can
assume that the Au/TiO2-EC is more specific than
Au/TiO2-Con to catalyze the reduction of nitro group due
to the strong adsorption of nitro group on Au/TiO2-EC,
as confirmed by the IR study. To check this hypothesis,
hydrogenation of various groups were carried out sepa-
rately by employing as substrates. As summarized in Ta-
ble 2, both Au/TiO2-EC and Au/TiO2-Con are active for
the hydrogenation of all substrates. When nitrobenzene,
styrene, acetophenone and chlorobenzene were physical-
ly mixed as substrate, the Au/TiO2-EC is only active for
hydrogenation of the nitrobenzene, where the hydro-
genation of other groups (e.g. -Cl, -C=O, -C=C) are pre-
vented in the complete hydrogenation reactions. In com-
parison, the Au/TiO2-Con is still active for hydrogena-
tion of the nitrobenzene, styrene, and acetophenone (Ta-
ble 2). These results indicate the obvious advantage of
the Au/TiO2-EC in selective hydrogenations, compared
with the Au/TiO2-Con.
3.5 Recyclability test. In the hydrogenation of 3-
nitrostyrene, the Au/TiO2-EC catalyst could be easily
recycled by filtration after each reaction run with negli-
gible Au leaching as confirmed by ICP analysis. Conse-
quently, it exhibits stable catalytic performances during
continuous reaction cycles. For example, after 9 recycles,
it still gives the conversion of 3-nitrostyrene higher than
97.0% and selectivity of 3-aminostyrene higher than
99.0% (Figure 6A). Very importantly, it is worth noting
that the reaction rate of 3-nitrostyrene at the beginning
of each run is very similar, indicating the good stability
of the Au/TiO2-EC catalyst (Figure 6B).
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3.6 Large-scale reaction. When the amount of 3-
nitrostyrene in the hydrogenation is a 4.8-g scale, the
Au/TiO2-EC still gives the conversion at 96.7% and 3-
aminostyrene selectivity at 99.0% (Scheme S2). In this
case, the turnover frequency (TOF) of the Au/TiO2-EC
could reach to 279 h-1 calculated from the total amount of
Au in the reaction system. This TOF value (279 h-1),
drogenation
of
2-chloro-4-nitrophenol
and
2-
nitroacetophenone (Scheme S1). In these cases, the
Au/TiO2-EC always exhibits excellent selectivity to the
Table 2. TOFs of the Au/TiO2-EC and Au/TiO2-Con in the hydrogenation of various substrates.a
TOF (h-1)b
Catalyst
Nitrobenzene
Styrene
150
Acetophenone
Chlorobenzene
Au/TiO2-EC
Au/TiO2-Con
Au/TiO2-ECc
545
375
469
154
177
--d
<10
<10
--d
168
--d
--d
Au/TiO2-Conc
298
34
21
a Reaction conditions: 0.3 mmol of substrate, 10 bar of H2, 5 mg of catalyst, and 3 ml of toluene; b Calculated from all the Au atoms in
the reaction system; c A mixture of nitrobenzene (0.3 mmol), styrene (0.3 mmol), acetophenone (0.3 mmol), and chlorobenzene (0.3
mmol) were employed as substrates; d Undetectable.
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ACS Catalysis
which even obtained under relatively mild reaction con- this work can be potentially used to develop novel effi-
ditions, is still much higher than that (<173 h-1) of the
conventional Au nanocatalysts.35 Furthermore, systemat-
ic investigations were carried out by employing various
substrates (Table S1 and S2), showing that the Au/TiO2-
EC catalyst is generally active for chemoselective hydro-
genation of different substituted nitroarenes (Table S1).
The excellent catalytic activity and selectivity combined
with the generalized applicability make the Au/TiO2-EC
catalyst potentially useful for wide applications for
chemoselective hydrogenation of substituted nitroarenes.
cient catalysts for chemoselective hydrogenations in the
future.
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ASSOCIATED CONTENT
Supporting Information. The XPS, IR, DFT and detailed
catalytic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
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Corresponding Author
* E-mail: liangwang@zju.edu.cn (L.W.);
* E-mail: hong.wang@mail.wvu.edu (H.W.)
* E-mail: fsxiao@zju.edu.cn (F.S.X.)
Conversion of 3-nitrostyrene
Selectivity of 3-aminostyrene
(A)
100
80
60
40
20
0
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work is supported by National Science Foundation
(DMREF CHE-1434378), National Natural Science Founda-
tion of China (21273197, U1462202 and 21403192).
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(B)
30 min
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Run number
Rum Number
Figure 6. Recycling tests of selective hydrogenation of 3-
nitrostyrene over the Au/TiO2-EC catalyst. The reaction
conditions are the same to those described in Table 1. (A)
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4. CONCLUSIONS
In summary, we have reported Au nanoparticles
supported on the edge/corner sites of anatase are highly
active and selective for the hydrogenation of substituted
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