An exceptionally active and selective Pt-Au/TiO2 catalyst for hydrogenation of the nitro group in chloronitrobenzene

Article abstract of DOI:10.1039/c1gc16001j

Adding a very small amount of Pt entities (0.01-0.03 wt%) onto the Au surface of a Au/TiO₂ catalyst is found to be an efficient approach to improve the catalytic activity of Au for the hydrogenation of p-chloronitrobenzene (p-CNB), without loss of selectivity towards p-chloroaniline (p-CAN). The effect of catalyst amount, reaction temperature, H₂ pressure and reaction time on p-CNB hydrogenation was studied with 0.02wt%Pt-0.5wt%Au/TiO₂ (Pt_0.0002-Au_0.005/ TiO₂). The selectivity to p-CAN could be up to 100% at complete conversion of p-CNB with reaction temperatures at or below 333 K. The catalyst also exhibited perfect stability. The catalyst structure was characterized by TEM and XRD, and the mechanism of the high activity of the catalyst was discussed.

Full text of DOI:10.1039/c1gc16001j

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PAPER
An exceptionally active and selective Pt–Au/TiO catalyst for
2
hydrogenation of the nitro group in chloronitrobenzene†
a
a
a
a
b
Daiping He,* Xiangdong Jiao, Ping Jiang, Jian Wang and Bo-Qing Xu
Received 14th August 2011, Accepted 28th September 2011
DOI: 10.1039/c1gc16001j
Adding a very small amount of Pt entities (0.01–0.03 wt%) onto the Au surface of a Au/TiO
catalyst is found to be an efficient approach to improve the catalytic activity of Au for the
hydrogenation of p-chloronitrobenzene (p-CNB), without loss of selectivity towards
2
p-chloroaniline (p-CAN). The effect of catalyst amount, reaction temperature, H
reaction time on p-CNB hydrogenation was studied with 0.02wt%Pt–0.5wt%Au/TiO
). The selectivity to p-CAN could be up to 100% at complete conversion of
2
pressure and
2
(
Pt0.0002–Au0.005/TiO
2
p-CNB with reaction temperatures at or below 333 K. The catalyst also exhibited perfect stability.
The catalyst structure was characterized by TEM and XRD, and the mechanism of the high
activity of the catalyst was discussed.
1
. Introduction
Chloroanilines are an important class of industrial interme-
diates for a variety of specific and fine chemicals, including
1
pharmaceuticals, dyes, herbicides and pesticides. p-CAN, a
high production volume compound, can be synthesised via Fe
promoted reduction of p-CNB in acid media (Bechamp reaction)
or by selective hydrogenation using transition-metal catalysts.
Industrial implementation of the Bechamp reaction is no longer
viable due to the generation of significant amounts of toxic
metal waste which requires costly and laborious separation and
Scheme 1 The hydrogenation reaction pathways of p-CNB.
2
disposal. Liquid phase catalytic hydrogenation has emerged
sponding p-CAN is always accompanied by some dechlorination
reaction. Furthermore, hydrogen chloride (HCl) produced from
the dechlorination contributes to the corrosion of the reactor.
To achieve high selectivity towards p-CAN, great attempts
have been made to tailor the hydrogenation selectivity of
supported metal catalysts toward a specific hydrogenation of
the nitro group in p-CNB, e.g., by modifying the active metal
as a cleaner alternative to the Bechamp process with higher
3
associated product yields. However, the economic benefits are
dependent on catalyst performance, where unwanted C–Cl bond
hydrogenolysis is difficult to fully avoid. Reaction selectivity
is critical as p-CNB hydrogenation can generate a range of
intermediates and by-products as shown in Scheme 1, which
presents the reaction pathways proposed for batch liquid phase
5
6
with different promoters or a second metal component, by
4
hydrogenation.
7
assembling metal-colloids onto suitable oxide nanoparticles or
With a variety of catalysts (e.g., platinum, palladium,
rhodium, nickel), the hydrogenation of p-CNB to the corre-
8
using amorphous structures and by using delicate approaches
9
for catalyst preparation. However, the side reaction of hy-
drodechlorination still cannot be avoided completely.
Having started a couple of decades ago, catalysis by gold
continues to show a fascinating future in the field of catalysis.
Nanosized Au particles in interaction with a variety of support
materials showed special selectivity in catalyzing a number of
a
Key Lab of Advanced Materials, College of Chemistry, Chongqing
10
Normal University, Chongqing, 401331, China.
E-mail: hedaiping@126.com; Fax: (+86)23-6536-2777;
Tel: (+86)23-6536-2777
b
11
12
Innovative Catalysis Program, Key Lab of Organic Optoelectronics &
reactions including CO oxidation, alcohol oxidation, olefin
Molecular Engineering, Department of Chemistry, Tsinghua University,
Beijing, 100084, China. E-mail: bqxu@mail.tsinghua.edu.cn;
Fax: (+86)10-6279-2122; Tel: (+86)10-6277-2592
†
1
13
14
epoxidation, vinyl chloride synthesis, selective hydrogenation
of unsaturated hydrocarbons, aldehydes and ketones. For the
reduction of nitro compounds with H , Au/TiO , Au/Fe
15
16
O
3
Electronic supplementary information (ESI) available: See DOI:
0.1039/c1gc16001j
2
2
2
and Au/SiO
2
catalysts were recently found to be very specific in
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 111–116 | 111

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reducing the nitro group in various compounds containing other
was supported by the observed increase in the selectivity of the
desired p-CAN with increasing p-CNB conversion. Increasing
17
functional groups. Our previous work also found that selective
production of chloroanilines from chloronitrobenzenes without
any dechlorination was feasible by catalytic hydrogenation
the amount of Pt up to 0.03 wt% in the Pt–Au0.005/TiO catalyst
2
resulted in continued shortening of the reaction period for
achieving a complete conversion of p-CNB. For instance, a
reaction period of 40 min was found to be long enough to pro-
18
over Au/ZrO
2
.
However, the hydrogenation activity of Au
nanoparticles was often much lower than those of the platinum-
19
group metals, most probably due to an “intrinsic” nobleness of
Au for H activation, that the dissociative adsorption of H on
duce a p-CNB conversion of 100% over the Pt0.0003–Au0.005/TiO
catalyst. With the amount of Pt in the Pt–Au0.005/TiO increasing
2
2
2
2
Au is an activated process and is restricted to the edge and corner
sites or at the interface perimeter around the Au nanoparticles in
contact with oxide support. Such low hydrogenation activity
of the Au nanoparticles has been a major obstacle in view of
practical applications.
continuously, the reaction period for achieving a complete
conversion of p-CNB was further reduced, but the selectivity
of p-CAN began to decrease due to hydrodechlorination, and
so the selectivity of aniline (AN) increased. For example, the
selectivities for p-CAN and AN were 99.9% and 0.1% over
20
23
In the present study, we report that adding a very small
amount of Pt entities (0.01–0.03 wt%) onto the Au surface of a
Pt0.0004–Au0.005/TiO , respectively. Galvagno et al. reported that
2
the activity of bimetallic M–Pt is enhanced at low M/Pt ratios,
whereas at high M content the platinum is poisoned. With the
Au/TiO catalyst is an efficient approach to improve the catalytic
2
activity of Au for the hydrogenation of p-chloronitrobenzene,
without changing the selectivity propensity of the Au nanopar-
ticles for the reaction. Our results can provide the basis for the
development of a clean, high efficient route for the production
of p-CAN under mild conditions.
amount of Pt in the Pt–Au0.005/TiO
Au/Pt ratio reduced, so that the activity of Pt–Au0.005/TiO
probably enhanced to result in hydrodechlorination. Therefore,
2
increasing continuously, the
2
was
the amount of Pt in the Pt–Au0.005/TiO
wt%.
2
may be restricted to 0.03
The effect of the Pt0.0002–Au0.005/TiO
2
catalyst amount ex-
pressed as weight percent of catalyst based on p-CNB was
studied in the range of 0–9% (w/w) (Fig. 1). The conversion
of p-CNB varied linearly with the increase in catalyst amount
up to 6.4% and then levelled off. Therefore, it appears that up to a
catalyst amount of 6.4% (w/w) the reaction is mainly controlled
by either the rate of mass transfer from the bulk liquid to the
catalyst surface or the intrinsic kinetics of the reaction. Since,
for the size of the catalyst used and the conditions employed,
the diffusion of hydrogen from the bulk liquid to the solid is
estimated to be unimportant, the data appears to represent the
true kinetics of the process at a catalyst amount of 6.4% (w/w),
2
. Results and discussion
Table 1 shows the catalytic results of p-CNB hydrogenation over
a Pt–Au/TiO catalyst at 323 K, 1.0 MPa H . Pt/TiO and
Au/TiO catalysts were also included for comparison. Blank
tests showed no reaction of p-CNB in the absence of the
2
2
2
2
catalyst. The conversion of p-CNB over the 0.01 wt%Pt/TiO
2
(
(
Pt0.0001/TiO
Au0.005/TiO
2
) was not detected at all. The 0.5 wt%Au/TiO
2
2
) produced a low p-CNB conversion (only 0.9%)
were loaded with a very
in 1 h. When the same Au0.005/TiO
2
small amount of Pt (only 0.01 wt%), however, a dramatically
higher p-CNB conversion (99.2%) was achieved under the same
reaction conditions, and the undesired hydrodechlorination
reaction usually observed over other metal catalysts was not
detected, which indicated that the C–Cl bond in the p-CNB
molecule was kept intact during the reaction. In addition, the
selectivity for p-CAN increased from 94.7% to 99.9%, most
probably due to the intermediates (4, 4¢-dichloroazoxybenzene
and 4, 4¢-dichloroazobenzene) produced in the reaction process
being further hydrogenated to the desired product p-CAN, which
-
1
temperature of 323 K and 1.0 MPa H for 97.5 mg mL of
2
p-CNB.
21
22
Table 1 Selective hydrogenation of p-CNB over Pt/TiO
2
, Au/TiO
2
and
a
2
Pt–Au/TiO catalysts
Sel. (%)
React.
p-CNB
b
Catalyst
time (min) Conv. (%) p-CAN AN
Others
Fig. 1 Effect of Pt0.0002–Au0.005/TiO catalyst amount on hydrogenation
2
of p-CNB. 0.39 g p-CNB in 4.0 mL ethanol (solvent), 323 K, 1.0 MPa
Pt0.0001/TiO
Au0.005/TiO
Pt0.0001–Au0.005/TiO
Pt0.0002–Au0.005/TiO
Pt0.0003–Au0.005/TiO
Pt0.0004–Au0.005/TiO
Pt0.0005–Au0.005/TiO
2
60
60
60
45
40
38
37
n.d.
0.9
99.2
100
100
100
100
n.d.
94.7
99.9
100
100
99.9
99.5
n.d.
n.d.
n.d.
n.d.
n.d.
0.1
n.d.
5.3
0.1
n.d.
n.d.
n.d.
n.d.
H
2
, 30 min, stirring speed: 900 rpm.
2
2
2
2
2
2
We investigated the influence of reaction temperature on the
hydrogenation of p-CNB over the Pt0.0002–Au0.005/TiO
2
catalyst
under 1.0 MPa H . The results, summarized in Table 2, show
2
0.5
that the increase in reaction temperature resulted in continuous
increase in both p-CNB conversion and the selectivity to p-
CAN without the formation of any decholorination product.
The outstanding selectivity for p-CAN (100%) at complete
a
n.d.: not detected. Reaction conditions: 25.0 mg catalyst, 0.39 g p-CNB
in 4.0 mL ethanol (solvent), 323 K, 1.0 MPa H
,4¢-dichloroazoxybenzene and 4,4¢-dichloroazobenzene.
2
, stirring speed: 900 rpm.
b
4
1
12 | Green Chem., 2012, 14, 111–116
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Table 2 Effect of reaction temperature on selective hydrogenation of
Table 4 Effect of reaction time on selective hydrogenation of p-CAB
a
a
p-CNB over Pt0.0002–Au0.005/TiO
2
over Pt0.0002–Au0.005/TiO
2
Sel. (%)
Sel. (%)
React.
temp. (K)
p-CNB
Conv. (%)
React.
time (min)
p-CNB
Conv. (%)
p-CAN
AN
Others
p-CAN
AN
Others
b
b
3
3
3
3
3
03
13
23
33
43
10.1
55.2
100
100
100
93.9
97.3
100
100
99.7
n.d.
n.d.
n.d.
n.d.
0.2
6.1
2.7
15
30
45
60
51.8
83.4
100
100
100
100
100
97.1
98.7
100
100
100
100
99.9
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
2.9b
b
1.3
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
c
0.1
120
1
2
80
40
a
n.d.: not detected. Other reaction conditions: 25.0 mg catalyst, 0.39
c
0.1
g p-CNB in 4.0 mL ethanol (solvent), 1.0 MPa H , 1 h, stirring speed:
2
b
c
a
9
00 rpm. 4,4¢-dichloroazoxybenzene and 4,4¢-dichloroazobenzene. N-
n.d.: not detected. Other reaction conditions: 25.0 mg catalyst, 0.39 g
p-CNB in 4.0 mL ethanol (solvent), 323 K, 1.0 MPa H , stirring speed:
00 rpm. 4,4¢-dichloroazoxybenzene and 4,4¢-dichloroazobenzene. N-
ethyl-p-chloroaniline.
2
b
c
9
ethyl-p-chloroaniline.
conversion of p-CNB was obtained at 323–333 K. However,
a hydrodechlorination process was observed with further in-
creasing of the reaction temperature up to 343 K, and N-ethyl-
p-chloroaniline, a condensation product of p-CAN with the
solvent (ethanol), was also detected. Therefore, the optimum
reaction temperature may be restricted to 323–333 K.
selectivity is difficult to maintain once the substrate has been
exhausted completely over most metal-supported catalysts, since
in the absence of a substrate the catalytic hydrodechlorination
reaction will occur at a much faster rate. To our surprise, the high
selectivity could be maintained even when the reaction time was
extended by several hours after p-CNB was exhausted.
H
2
pressure has a significant effect on the substrate reactivity
and product selectivity (Table 3). The conversion of p-CNB and
selectivity to p-CAN were seen to increase with the H pressure.
The conversion of p-CNB and selectivity to p-CAN were 63.2%
and 97.9%, respectively, at 0.5 MPa H . The conversion increased
to 100% and the selectivity reached 100% at 1.0 MPa H . The
utilization rate in the reaction was ca. 89% by theoretical
calculations according to the following formula: % utilization
To explore the stability of the Pt0.0002–Au0.005/TiO for p-CNB
2
hydrogenation, the catalyst was separated from the reaction
system after the p-CNB hydrogenation reaction had completed,
it was washed with ethanol, dried under vacuum and then
reused for the same reaction. After reusing it five times, the
p-CNB conversion and p-CAN selectivity data obtained over
this recovered catalyst were found similar to those over the fresh
2
2
2
H
2
-
3
rate = m/M¥4/(25¥10 ¥10/22.4), where m is the mass of p-
CNB; M the mole mass of p-CNB. It is interesting to note that
the selectivity to p-CAN could be maintained even when the
one (Table 5). The results suggest that the Pt0.0002–Au0.005/TiO
catalyst is very stable and suitable for practical applications.
2
The X-ray diffraction (XRD) patterns of the TiO
2
,
reaction was conducted at 4 MPa H
that the undesired hydrodechlorination can be avoided over the
catalyst even when very high H pressure is
used for the reaction.
The effect of the reaction time for the hydrogenation of p-
was studied at 323 K, 1.0 MPa
. As shown in Table 4, both p-CNB conversion and the
selectivity for p-CAN increased along with the reaction time.
When the reaction was continued for 45 min, we observed a
complete conversion of p-CNB and 100% selectivity for p-CAN.
It is known for the catalytic reaction of interest, that a high
2
. These data demonstrate
Au0.005/TiO
2
and Pt0.0002–Au0.005/TiO
2
catalysts are given in Fig.
2. Peak assignments for TiO
2
(Fig. 2a, 2q = 25.2, 37.8, 48.1, 53.9
◦
Pt0.0002–Au0.005/TiO
2
2
and 55.1 ) are consistent with the (101), (004), (200), (105) and
(211) planes associated with tetragonal anatase (JCPDS-ICDD
21-1272). Apart from the diffraction peaks of the anatase TiO
there were no peaks in the XRD patterns of the Au0.005/TiO
(Fig. 2b) and the Pt0.0002–Au0.005/TiO
phases were detected. Moreover, neither the Au0.005/TiO
presented obvious change compared to
in the XRD spectra (Fig. 2). This result can presumably
be ascribed to the combinations of low contents of doping Au
2
,
CNB over Pt0.0002–Au0.005/TiO
2
2
H
2
2
(Fig. 2c), i.e., no new
nor
2
the Pt0.0002–Au0.005/TiO
2
the TiO
2
Table 5 Reusability of the Pt0.0002–Au0.005/TiO
2
catalyst for the selective
a
Table 3 Effect of H
2
pressure on selective hydrogenation of p-CAB
2
hydrogenation of p-CAB
a
over Pt0.0002–Au0.005/TiO
Sel. (%)
Sel. (%)
p-CNB
Conv. (%)
H
2
pressure
p-CNB
Cycle
p-CAN
AN
Others
n.d.
b
(
MPa)
Conv. (%)
p-CAN
AN
Others
1
2
3
4
5
100
100
n.d.
n.d.
n.d.
n.d.
n.d.
b
0
1
2
4
.5
.0
.0
.0
63.2
100
100
100
97.9
100
100
100
n.d.
n.d.
n.d.
n.d.
2.1
96.5
95.6
95.1
94.8
99.9
99.9
99.9
99.9
0.1b
n.d.
n.d.
n.d.
0.1b
0.1
b
0.1
a
a
n.d.: not detected. Other reaction conditions: 25.0 mg catalyst, 0.39 g
n.d.: not detected. Reaction conditions: 25.0 mg catalyst, 0.39 g p-CNB
p-CNB in 4.0 mL ethanol (solvent), 323 K, 1 h, stirring speed: 900 rpm.
,4¢-dichloroazoxybenzene and 4,4¢-dichloroazobenzene.
in 4.0 mL ethanol (solvent), 323 K, 1.0 MPa H , 45 min, stirring speed:
2
b
b
4
900 rpm. 4,4¢-dichloroazoxybenzene and 4,4¢-dichloroazobenzene.
Green Chem., 2012, 14, 111–116 | 113
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Gold particle size and distribution can have a direct impact
on the hydrogenation activity where the presence of nanoscale
Au particles (<10 nm) has been deemed essential for significant
25
activity, with a decrease in efficiency for larger Au particles.
This interpretation cannot be applied in the present work, for
these Au particles in the Pt0.0002–Au0.005/TiO did not change
2
obviously in size when Pt was added (Fig. 3a,b). The hydrogena-
tion activity of the Au nanoparticles was often much lower than
those of the platinum-group metals. Some observations show
that the low hydrogenation activity of the Au nanoparticles is
most probably due to an “intrinsic” nobleness of Au for H
activation, and that the dissociative adsorption of H on Au is
2
2
Fig. 2 XRD patterns of TiO
2 2
(a), Au0.005/TiO (b) and Pt0.0002–
an activated process and is restricted to the edge and corner
sites or to the interface perimeter around the Au nanoparticles
Au0.005/TiO
2
(c).
20
that are in contact with the oxide support. Therefore, the
activity enhancement of Au nanoparticles by the deposition of
Pt entities for the chemoselective hydrogenation of p-CNB could
and Pt below the detection limits of the XRD, small grain size,
and homogeneous distribution.
In order to obtain explicit details of Au and Pt particle
be explained by a synergic effect of Pt, Au and the TiO
2
support
size, Au0.005/TiO
2
and Pt0.0002–Au0.005/TiO were examined with
2
(
Fig. 4). It seems reasonable that the Pt sites were responsible
transmission electron microscopy (TEM) in the dark field mode.
As can be seen from the TEM images (Fig. 3a,b), small bright
for the dissociative activation of H molecules while Au sites,
2
especially the perimeter interfaces around the Au particles in
contact with the TiO support for the adsorption-activation of
spots can be discerned from the background over Au0.005/TiO
. A combined EDX analysis, using an
2
2
and Pt0.0002–Au0.005/TiO
2
the substrate molecules. The activated H atoms would then
migrate by spilling over toward nearby Au sites, especially
electron beam of 0.8 nm in diameter, showed that the signal of
Au could be detected only at the small bright spots (Fig. 3b1,b2).
No Pt signals were observed in the EDX spectrum (Fig. 3b2).
The crystal lattice spacing of a selected metal particle (Figure
S1, ESI†) is rather uniform and consistent with that of Au single
perimeter interfaces between TiO and Au nanoparticles for
2
a fast reaction with the adsorbed substrate. The dramatic
enhancement of the Au activity by the Pt entities would suggest
that the overall reaction rate was determined by the activation
24
crystallites, suggesting that the Pt and Au are probably not
alloyed. These are probably due to the low content of Pt in
of H
well maintained selectivity propensity of the Au nanoparticles
in the hydrogenation reactions, regardless of Pt in the Pt0.0002
catalyst, would indicate that such chemoselective
2
or by the supply of H atoms. On the other hand, the
the Pt0.0002–Au0.005/TiO
2
(only 0.02 wt%). Therefore, we conclude
that the small bright spots in Fig. 3b are the image of the Au or
Au and Pt species. The Au or Au and Pt particles in the Pt0.0002
catalyst have an average diameter of 5 nm, with a
–
Au0.005/TiO
2
–
hydrogenation selectivity was governed by the adsorption mode
of the substrate molecules on the Au sites, especially perimeter
Au0.005/TiO
2
size distribution from 3 to 6 nm (Fig. 3b), which is consistent
with the absence of characteristic diffraction peaks of Au and
interfaces between the TiO and Au nanoparticles.
2
Pt in the XRD pattern of Pt0.0002–Au0.005/TiO
2
, suggesting that
are in a homogenous dispersion
without any obvious aggregation.
the Au and Pt species on TiO
2
Fig. 4 Pt promotion of the hydrogenation catalysis of Au nanoparticles.
3
. Conclusions
In conclusion, adding a small amount of Pt entities (0.01–
.03 wt%) onto the Au surface of a Au/TiO catalyst is an
0
2
Fig. 3 TEM images of Au0.005/TiO
2
(a), Pt0.0002–Au0.005/TiO
2
(b) and
efficient approach to improve the catalytic activity of Au for
EDX spectra of the selected area 1 (b1) and 2 (b2).
the hydrogenation of p-chloronitrobenzene, while the C–Cl
1
14 | Green Chem., 2012, 14, 111–116
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bond in the p-CNB molecule remained intact. XRD, TEM
and catalytic results implied that this unusually high reaction
rate and selectivity might be due to the Pt sites acting to
A sample for TEM examination was made by placing a drop
of the catalyst powder dispersion in ethanol on a carbon-coated
copper grid, followed by drying at room temperature.
activate the H
2
molecules while the Au sites, especially perimeter
interfaces around the Au particles in contact with the TiO
2
4.3 Catalysis procedure
support serve to activate p-CNB by chemisorption. There was a
remarkable effect of the Pt loading and reaction temperature on
p-CAN selectivity. Excess amounts of Pt (>0.03 wt%) and high
reaction temperatures will cause the occurrence of the undesired
Catalytic tests were performed with magnetic stirring in a 25 mL
stainless steel autoclave containing 0.39 g p-CNB in 4.0 mL
ethanol (solvent). The catalyst (Pt–Au0.005/TiO powder, particle
2
size 0.1 mm) used in each run of the reaction was 25.0 mg.
The overall molar p-CNB/Au ratio in the reaction mixture was
catalytic hydrodechlorination reaction of p-CNB. As for Pt0.0002
–
Au0.005/TiO
2
, the selectivity for p-CAN could be up to 100% at
3
902. The reactor was flushed five times with 0.5 MPa H
2
before
complete conversion of p-CNB with reaction temperatures at or
below 333 K. In addition, the catalyst was quite stable and could
be used repetitively without significant deactivation. These facts
allow us to view it as a prospective heterogeneous catalyst for
selective production of p-CAN from p-CNB.
it was pressurized to the desired H pressure and placed into an
2
oil bath maintained at the reaction temperature. The reaction
was stopped at a selected time by cooling the reactor in an ice–
water bath. Analysis of the reaction products was done on an
HP6890A GC instrument with a HP-5 capillary column with a
FID detector, using benzene as an internal standard.
4
. Experimental
Acknowledgements
4
.1 Catalyst preparation
2
-1
This work was financially supported by the Chongqing Educa-
tion Commission (KJ110621), the Chongqing Normal Univer-
sity (10XLR021).
TiO with a BET surface area of 42 m g and an average grain
2
size of 35 nm was prepared by hydrothermal homogeneous
precipitation according to the following procedure: 6 g urea
was added into 50 mL of 16.5% (w/v) Ti(SO
4
2
) aqueous
solution (Beijing Chem. Co.). After stirring for 30 min at room
temperature, the mixture was transferred into an 80 mL teflon-
lined autoclave for hydrothermal reaction under 433 K for 20
h and cooled to room temperature naturally. The resultant
precipitate was filtered and washed with distilled water until it
was free from sulfate ions, then dried at 383 K overnight. Finally,
Notes and references
1
2
3
4
A. S. Travis, in The Chemistry of Anilines, ed. Z. Rappoport,
John Wiley & Sons, Ltd, Chichester, England, 2007, pp. 715–
782.
H. U. Blaser, U. Siegrist, H. Steiner and M. Studer, in Fine Chemicals
through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001, pp.
3
89–406.
the TiO
for 5 h. The Pt–Au/TiO
PtCl for the precursors of gold and platinum by deposition–
precipitation with urea. Typically, 2.0 g of TiO was suspended
2
was obtained by calcination in flowing air at 873 K
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catalyst was prepared using HAuCl
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,
H
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-
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-1
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(1.015 ¥ 10 mol L ),
-
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-1
-1
H
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PtCl
6
(4.1 ¥ 10 mol L ) and urea (0.2 mol L ). The mixture
7
9–86.
was heated to 353 K and maintained at this temperature for 4 h
under vigorous stirring and allowed to cool to room temperature
overnight. The precipitate was isolated and washed thoroughly
with distilled water by filtration until it was free from chloride
ions. Finally, the sample was dried at 383 K for 12 h and calcined
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1
-
1
at 473 K for 5 h in flowing air (60 mL min ). The amount of
gold and platinum in the catalyst was 0.49 wt% and 0.018 wt%,
respectively, according to ICP–AES analysis.
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2
9
B. J. Zuo, Y. Wang, Q. L. Wang, J. L. Zhang, N. Z. Wu, L. D. Peng,
4
.2 Catalyst characterization
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2
10 G. C. Bond, C. Louis and D. T. Thompson, Catalysis by Gold,
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catalyst were analyzed by means of inductively coupled plasma–
atomic emission spectroscopy (ICP–AES, Leeman Prodigy)
after the sample was dissolved in aqua regia. X-ray diffraction
1
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some sample on a glass slide. The transmission electron mi-
croscopy (TEM) measurement was performed on a JEOL JEM-
1
3 M. D. Hughes, Y. J. Xu, P. Jenkins, P. McMorn, P. Landon, D. I.
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000EX microscope operated at an accelerating voltage of 120
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unit was used to collect the EDX spectra for elemental analysis.
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