A highly active and anti-coking Pd-Pt/SiO2 catalyst for catalytic combustion of toluene at low temperature

Article abstract of DOI:10.1016/j.apcata.2016.10.016

Catalytic combustion has been identified as one of the most efficient and economically feasible routes for removing volatile organic compounds. In this work, supported noble-metal (Pd and Pt) catalysts for catalytic combustion of toluene were prepared by an improved incipient wetness impregnation method. Controlled amounts of oleic acid (OA) were introduced into the aqueous solution of metal salts in this new preparation method, which improved Pd and Pt dispersion remarkably on SiO₂, as confirmed by XRD and TEM. XPS results showed that introduction of OA in the catalyst preparation resulted in increased proportion of metallic palladium. All of the activities in catalysts with OA were much higher than those of the catalysts without OA in the same catalyst system. Among the catalysts prepared, the 0.25%Pd-0.25%Pt/SiO₂–OA catalyst exhibited the best catalytic activity for toluene oxidation, and the T₉₈ of toluene was only 160 °C under the conditions of toluene concentration at 1000 ppm and SV at 60,000 mL/(g h). Furthermore, the 0.25%Pd-0.25%Pt/SiO₂–OA catalyst maintained high catalytic activity in the active stability test at 160 °C, and the result of TG indicated that no coke was formed on the spent catalyst.

Full text of DOI:10.1016/j.apcata.2016.10.016

Applied Catalysis A: General 529 (2017) 60–67
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A highly active and anti-coking Pd-Pt/SiO catalyst for catalytic
2
combustion of toluene at low temperature
∗
Hui Wang, Wei Yang, Penghui Tian, Jie Zhou, Rui Tang, Shengji Wu
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Xiasha University Park, Hangzhou, Zhejiang 310018, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2016
Received in revised form
Catalytic combustion has been identified as one of the most efficient and economically feasible routes
for removing volatile organic compounds. In this work, supported noble-metal (Pd and Pt) catalysts for
catalytic combustion of toluene were prepared by an improved incipient wetness impregnation method.
Controlled amounts of oleic acid (OA) were introduced into the aqueous solution of metal salts in this
new preparation method, which improved Pd and Pt dispersion remarkably on SiO2, as confirmed by XRD
and TEM. XPS results showed that introduction of OA in the catalyst preparation resulted in increased
proportion of metallic palladium. All of the activities in catalysts with OA were much higher than those
of the catalysts without OA in the same catalyst system. Among the catalysts prepared, the 0.25%Pd-
2
3 September 2016
Accepted 17 October 2016
Available online 18 October 2016
Keywords:
Catalytic combustion
Platinum
Palladium
Volatile organic compounds (VOCs)
Toluene
0
.25%Pt/SiO2–OA catalyst exhibited the best catalytic activity for toluene oxidation, and the T98 of toluene
◦
was only 160 C under the conditions of toluene concentration at 1000 ppm and SV at 60,000 mL/(g h). Fur-
thermore, the 0.25%Pd-0.25%Pt/SiO2–OA catalyst maintained high catalytic activity in the active stability
◦
test at 160 C, and the result of TG indicated that no coke was formed on the spent catalyst.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
for low-concentration VOCs at low temperature [20–23]. Because
of their large surface areas, acidic properties, and high thermal and
hydrothermal stabilities, solid acid catalysts such as zeolites, ␥-
As one of the major components of air pollution, volatile organic
compounds (VOCs) are emitted mainly from transportation and
industrial processes [1,2]. VOCs are harmful to human health and
the environment because of their toxic, malodorous, carcinogenic
and mutagenic nature [3]. In addition, they are recognized as a
component and promoter of photochemical smog, which is found
in many large cities all over the world [4]. Therefore, methods
of eliminating VOCs have attracted considerable attention in the
light of stringent worldwide environmental legislation. Up until
now, a majority of the current industrial VOC abatement has been
carried out by catalytic combustion technology, which has been
identified as one of the most efficient and economically feasible
routes [5–7]. Consequently, the research and development of cata-
lysts for catalytic combustion has become a hotspot research area,
internationally [8,9].
Al O , ZrO and SiO2 have been extensively considered as catalyst
2 3 2
carriers for supported noble-metal catalysts [24–28]. For exam-
ple, S. Siffert and co-workers prepared Pd/FAU and Pd/BEA, which
were found to be powerful catalysts for the total oxidation of VOCs
[29]. Montes et al. carefully investigated the activity and stability
of Pt/Al O3 catalysts in the oxidation of toluene [30]. However, the
2
main drawback in using the solid acid catalysts as the catalyst car-
riers for catalytic combustion is their high tendency toward carbon
deposition, resulting in deactivation of the catalysts [31]. First, the
acidic sites always participate in coke production as a side reaction.
Second, the micropores in solid acid catalysts, especially in conven-
tional zeolites, significantly influence the mass transfer in catalytic
combustion. Currently, solid acid catalysts containing hierarchical
pores have been synthesized to improve the mass transfer [32,33].
S. Siffert et al. synthesized hierarchical bimodal macro-mesoporous
ZrO as the carriers for a Pd/ZrO catalyst that exhibited high activ-
The catalysts used for catalytic combustion include mainly per-
ovskites [10,11], transition metal oxides [12–14] and supported
noble-metal catalysts [15–19]. Among these, supported noble-
metal catalysts are preferred for catalytic combustion, especially
2
2
ity for VOCs oxidation [34]. Mesoporous Beta zeolite-supported
Pt catalyst was successfully prepared by Xiao et al., and showed
superior performance in the catalytic combustion of toluene [35].
Despite the good mass-transfer properties of mesoporous solid
acid catalysts, however, carbon deposition still occurred, only at a
∗ Corresponding author.
E-mail address: wushengji26@hdu.edu.cn (S. Wu).
http://dx.doi.org/10.1016/j.apcata.2016.10.016
0
926-860X/© 2016 Elsevier B.V. All rights reserved.

H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
61
◦
◦
slower rate. Therefore, it is still a challenge to achieve a highly active
and stable catalyst for catalytic combustion at low temperature.
Recently, Mo et al. reported a highly active and anti-coking
Ni-La O /SiO (silica gel) catalyst for syngas production from dry
H O at 110 C, the dried sample was calcined at 550 C for 4 h. The
2
catalysts prepared with OA and without OA were designated as
0.25%Pd-0.25%Pt/SiO –OA and 0.25%Pd-0.25%Pt/SiO , respectively.
The other Bimetallic Pd-Pt catalysts with various carriers were
prepared through a similar process. Furthermore, the supported
noble-metal catalysts were reduced by hydrogen for two hours
before catalytic activity testing.
2
2
2
3
2
CO for reforming methane [36]. The preparation process was sim-
2
ple and similar to the traditional incipient wetness impregnation
method. Small amounts of oleic acid (OA) were introduced into the
aqueous solution of metal salts in this new preparation method,
which improved nickel dispersion remarkably. The mechanism of
OA also has been investigated in detail by Mo et al.; it can be
described as follows [37]. Metal ions and carboxyl of OA can form
coordination compounds, which would self-assemble on the metal
ion species surface as a shell. The shell could prevent the core (metal
ion species) from agglomeration during thermal pretreatment. The
new method provided an inspiration, and we would like to design
a supported metal catalyst on silica in order to achieve a highly
active and anti-coking catalyst for catalytic combustion of VOCs at
low temperature. Fumed silica was used as the catalyst support;
it is an inexpensive raw material for making zeolites, and can be
commercialized for bulk production.
2.3. Catalyst characterization
X-ray diffraction (XRD) patterns were obtained with a RIGAKU
Ultimate IV diffractometer using Cu K␣ radiation. Thermal
gravimetric analysis (TGA) experiments were performed on a
Mettler-toledo thermogravimetric analyzer (TGA/DSC LF1600,
Switzerland) at the range of room temperature to 700 C at a heat-
ing rate of 20 C/min at atmospheric pressure under air atmosphere.
The flow rate of the air was 40 mL/min. Transmission electron
microscopy (TEM) experiments were performed on a JEM-2100
electron microscope (JEOL, Japan) with an acceleration voltage of
◦
◦
◦
200 kV. Nitrogen sorption isotherms at −196 C were measured
using an Autosorb-iQ system (Quantachrome Instruments). The
surface area was calculated by using the Brunauer–Emmett–Teller
2
. Experimental
(BET) method. The pore size distribution was calculated by using
2
.1. Materials
the Barrett-Joyner-Halenda (BJH) method. X-ray photoelectron
spectra (XPS) of the samples were recorded using a Thermo Sci-
entific ESCALAB 250Xi with Al K X-ray radiation for the X-ray
source. The binding energies (BEs) were calibrated against C1s
(284.8 eV) peaks. The samples were all reduced by H2 before
catalyst characterization of XRD, TEM, BET and XPS. Catalysts
were stored in sealed bags and not re-activated prior to XPS
measurements. Temperature-programmed desorptions of toluene
(toluene-TPD) were performed on a gas chromatograph (Kexiao,
GC1690) equipped with a flame ionization detector (FID). In a typi-
The fumed silica (7 nm particle size, specific surface area:
2
3
80 m /g) was provided by Evonik Degussa Corp. H-ZSM-5 (15 ␮m
2
particle size, Si/Al = 25, specific surface area: 340 m /g) was pro-
vided by Nankai Catalyst Corp. Silica gel (50 ␮m particle size,
2
specific surface area: 360 ± 30 m /g) was provided by Qingdao
Kaibang Material Corp. Metal nitrates of AR purity were purchased
from Aladdin. H PdCl and H PtCl6 were supplied by Hangzhou
2
6
2
Kaiming Catalyst Corp.
cal run, 100 mg of sample were pre-treated in a H flow (30 mL/min)
2
◦
◦
2.2. Catalyst preparation
at 300 C for 2 h and then cooled down to 20 C prior to the adsorp-
tion of toluene for 2 h. After saturation with toluene, the sample
was flushed with pure N2 (30 mL/min) for 2 h at 20 C. The pro-
◦
2.2.1. Monometallic catalyst
The catalysts were synthesized with an improved incipient
files of toluene-TPD were recorded online at a heating rate of
◦
◦
wetness impregnation method as described in references 36
and 37. Unlike in the traditional incipient wetness impregna-
tion method, oleic acid (OA) was introduced into the aqueous
solution of metal salts. The preparation process was as follows.
10 C/min until 800 C. The chemisorption of CO was performed on
a chemisorption analyzer (micromeritics Auto Chem II). In a typical
run, 100 mg of sample were pre-treated in a H2 flow (30 mL/min) at
◦
◦
400 C for 1 h and then cooled down to 50 C prior to the adsorption
of CO pulses.
First, 11.8 mg of H PdCl6 was dissolved in 4 mL of deionized
2
water, and then 14.5 ␮L of oleic acid (a molar ratio of OA/Pd = 1)
was added into the solution. After stirring for a few minutes at
room temperature, 1 g of fumed silica was introduced into the
above solution. Subsequently, the samples were impregnated at
room temperature for 10 h. Then the impregnated catalyst was
2.4. Catalytic activity tests
The catalytic combustion of toluene experiment was carried
out in a fixed-bed quartz reactor (i.d. 6 mm and length 300 mm)
at atmospheric pressure. The catalytic combustion of toluene over
noble metal catalyst was performed at the temperatures between
aged at room temperature for 10 h. After drying out the H O
2
◦
◦
at 110 C, the sample was calcined at 550 C for 4 h. The cat-
alysts prepared with OA and without OA were designated as
◦
140 and 210 C. The experiment over transition metal catalyst was
◦
0
.5%Pd/SiO –OA and 0.5%Pd/SiO , respectively. The 0.5%Pt/SiO ,
performed at the temperatures between 300 and 400 C. The typical
2
2
2
0
.5%Pt/SiO –OA, 5.0%CuO/SiO , 5.0%CuO/SiO –OA, 2.5%Fe O /SiO
experiment used 100 mg of catalyst (40–60 mesh) with a total flow
rate of the feed stream at 100 mL/min, giving a space velocity (SV) of
60,000 mL/(g h). The feed stream containing 1000 ppm toluene was
generated by bubbling air through a gas washing bottle containing
pure toluene chilled in an ice-water isothermal bath, and then fur-
ther diluted with another air stream. Fig.S1 shows the experimental
setup for the combustion of toluene. Prior to the catalytic reaction,
the supported noble-metal catalyst was reduced in an H2 atmo-
2
2
2
2
3
2
and 2.5%Fe O /SiO –OA catalysts were prepared with this new
2
3
2
method, using an appropriate amount of aqueous solution of
metal salts. Furthermore, the supported noble-metal catalysts were
reduced by hydrogen for two hours before catalyst characterization
and catalytic activity testing.
2.2.2. Bimetallic Pd-Pt catalyst
◦
First, 5.9 mg of H PdCl6 and 8.0 mg of H PtCl6 were dissolved
sphere at 300 or 400 C for 2 h. (The supported transition metal
2
2
◦
in 4 mL of deionized water, and then 14.5 ␮L of oleic acid (a molar
ratio of OA/Pd + Pt = 1) was added into the solution. After stirring for
few minutes at room temperature, 1 g of fumed silica was intro-
duced into the above solution. Subsequently, the samples were
impregnated at room temperature for 10 h. After drying out the
oxide catalyst was pretreated in an N2 flow at 400 C for 2 h.) The
concentration of the toluene at the outlet of the reactor was moni-
tored with a gas chromatograph (Kexiao, GC1690) equipped with a
flame ionization detector (FID). The concentration of the oxidative
products (CO2 and CO) was monitored with a gas chromatograph

6
2
H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
Table 1
XPS results for the various catalysts.
0
2+a
Catalyst
Pd species
Peak area
Pd 3d5/2
Pd /Pd
Pd 3d3/2
0
.5%Pd/SiO2
Pd0
Pd
312.8
130.3
130.7
80.1
2.4
2.8
2
+
+
0.5%Pd/SiO2–OA
Pd0
351.3
125.9
252.4
140.2
2
Pd
a
Peak area ratio of Pd 3d5/2.
particles were heterogeneously dispersed on the 0.5%Pd/SiO2 cat-
alyst (Fig. 2A–b). Furthermore, the mean particle size and standard
deviation of the Pd particles were calculated for the TEM images in
Figs. 2 and S1. These statistics reveal that the mean particle size of
the Pd particles was 15.5 nm with a standard deviation of 4.2 nm on
the 0.5%Pd/SiO2 catalyst (Fig. 2B–b). By contrast, the mean particle
size of the Pd particles was 11.3 nm, and the standard deviation only
1
.1 nm, on the 0.5%Pd/SiO –OA catalyst (Fig. 2B–a). The TEM results
Fig. 1. XRD patterns of 0.5%Pd/SiO2, 0.5%Pd/SiO2–OA, 0.5%Pt/SiO2, and
.5%Pt/SiO2–OA samples.
2
0
hence previously indicated that the addition of OA could effectively
promote the dispersion of Pd in accordance with the results of XRD.
XPS measurements were subsequently performed to determine
the oxidation state of Pd in the catalysts following reduction by H2
and storage in air at room temperature. Fig. S2 represents the Pd
(
Kexiao, GC1690) equipped with a thermal conductivity detector
(
TCD) using a Carboxen packed column (TDX01, 2 m × 3 mm). The
removal of toluene was calculated on the basis of toluene consump-
tion. The selectivity to CO2 was calculated by toluene conversion
3
d XPS spectra of 0.5%Pd/SiO2 and 0.5%Pd/SiO –OA. The Pd 3d
2
5/2
,
2
+
peak lying at around 340.6 eV is related to oxidized palladium Pd
and outlet concentration of CO . The removal efficiency of toluene
2
while the Pd 3d5/2 peak located at around 335.6 eV can be attributed
was calculated using the following equation:
to metallic palladium Pd0 [38,39]. Pd0 species are regarded as cat-
[
toluene]in − [toluene]out
alytically active centers for the conversion of toluene into CO and
H O [38–40]. After deconvolution, the ratios of Pd /Pd in the both
samples could be estimated, as shown in Table 1. The Pd /Pd
ratio of the 0.5%Pd/SiO –OA catalyst was higher than that of the
Toluene conversion (%) =
× 100%
2
[
toluene]in
0
2+
2
0
2+
2
3
. Results and discussion
0
.5%Pd/SiO2 catalyst. Thus, XPS results show that introduction of
OA in the catalyst preparation results in increased proportion of
3
.1. Catalyst characterization
0
Pd , resistant to re-oxidation during air contact. According to the
0
literature, highly dispersed Pd clusters are capable of re-oxidation
Fig. 1 shows the X-ray diffraction patterns of 0.5%Pd/SiO2,
.5%Pd/SiO –OA, 0.5%Pt/SiO2 and 0.5%Pt/SiO –OA samples.
0
during air (or oxygen) contact [38,41]. The high proportion of Pd in
0
2
2
the 0.5%Pd/SiO –OA catalyst is consistent with the highly dispersed
2
Notably, the XRD pattern of the 0.5%Pt/SiO2 catalyst exhibited
several well-resolved characteristic peaks (2 Theta = 39.8, 46.2 and
6
that of the 0.5%Pt/SiO –OA catalyst. Similar results were detected
for 0.5%Pd/SiO2 and 0.5%Pd/SiO –OA as well. The Pd characteristic
peak of 0.5%Pd/SiO2 was stronger than that of 0.5%Pd/SiO –OA.
The metallic particle size is calculated with Scherrer formula on the
basis of XRD results. The mean particle size of the Pt particles was
0
Pd clusters observed by TEM.
◦
Fig. 3 shows the toluene-TPD profiles of 0.5%Pd/SiO2 and
7.5 ) associated with Pt particles; this was much stronger than
0
.5%Pd/SiO –OA. Obviously, 0.5%Pd/SiO –OA showed much higher
2
2
2
◦
desorption temperatures (112 and 492 C) than 0.5%Pd/SiO (95
and 338 C), suggesting that 0.5%Pd/SiO –OA adsorbs toluene more
2
2
◦
2
2
strongly than 0.5%Pd/SiO . The first step of catalytic reaction is
2
adsorption. Thus, it can be speculated that the 0.5%Pd/SiO –OA
2
catalyst is beneficial to activate toluene molecular at lower tem-
perature during the catalytic combustion of toluene.
2
0.0 nm on the 0.5%Pt/SiO catalyst, whereas the mean particle size
2
of the Pt particles was only 7.6 nm on the 0.5%Pt/SiO –OA catalyst.
2
The Pd particle sizes of the 0.5%Pd/SiO2 and 0.5%Pd/SiO –OA
2
catalyst were 13.9 nm and 12.0 nm, respectively. The results of
CO chemisorption led to the similar conclusion in accordance
with the results of XRD. The Pd particle size of the 0.5%Pd/SiO2
3.2. Catalytic performance
3.2.1. Catalytic combustion of toluene over various monometallic
catalysts
and 0.5%Pd/SiO –OA measured by CO chemisorption was 20.6 nm
2
and 18.0 nm. The results suggested that Pt and Pd particles on
the catalysts with OA were highly dispersed. Fig. S2 shows the
Fig. 4 shows the dependencies of the catalytic activities on reac-
tion temperature in the combustion of toluene over 0.5%Pd/SiO2,
X-ray diffraction patterns of 5.0%CuO/SiO2 and 5.0%CuO/SiO –OA.
0.5%Pd/SiO –OA, 0.5%Pt/SiO2, 0.5%Pt/SiO –OA, 5.0%CuO/SiO2,
2
2
2
The characteristic peaks of CuO were observed clearly for the
5.0%CuO/SiO –OA, 2.5%Fe O /SiO and 2.5%Fe O /SiO –OA cata-
2 2 3 2 2 3 2
5
.0%CuO/SiO2 catalyst. By contrast, the characteristic peaks of
lysts. To compare the catalytic activities, the T98 of toluene over
the catalysts was estimated from Fig. 4. Notably, the activities
of the catalysts with OA were all much higher than those of the
CuO were much weaker for 5.0%CuO/SiO –OA, indicating that CuO
particles on the catalysts were also highly dispersed through the
addition of OA. These results are in agreement with references
2
catalysts without OA in the same catalyst system. For example, the
◦
[
36,37].
T98 of toluene was only 180 C over 0.5%Pt/SiO –OA, which was
2
◦
◦
Fig. 2A displays the TEM images of the 0.5%Pd/SiO –OA and
about 20 C lower than that (200 C) over 0.5%Pt/SiO , as shown
in Table 2. The T98 of toluene was 190 C over 0.5%Pd/SiO –OA,
which was about 15 C lower than that (205 C) over 0.5%Pd/SiO .
2
2
◦
0.5%Pd/SiO2 samples, exhibiting direct evidence for the presence
2
◦
◦
of Pd nanoparticles. Clearly, Pd particles were homogeneously dis-
2
persed on the 0.5%Pd/SiO –OA catalyst (Fig. 2A–a). However, Pd
Meanwhile, the selectivity to CO was calculated. All the supported
2
2

H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
63
Fig. 2. (A) TEM images and (B) Pd particle size distributions of (a) 0.5%Pd/SiO2–OA, and (b) 0.5%Pd/SiO2 samples.
noble-metal catalysts showed nearly 100% selectivities to CO2
at the temperature of their own T98, and no CO was detected
by on-line TCD during the reaction. The carbon balance reached
9
9.3 ± 4.2%. Moreover, when transition metal oxide CuO or Fe O
2
3
was loaded as the active component instead of using a noble metal,
the addition of OA could still remarkably enhance the activities of
the catalysts for toluene conversion. For example, the conversion
◦
of toluene over 5.0%CuO/SiO –OA was 94.7% at 385 C, but the
2
conversion over 5.0%CuO/SiO was only 17.9% at the same reaction
2
temperature. The conversions of toluene over 2.5%Fe O /SiO –OA
2
3
2
◦
and 2.5%Fe O /SiO were 94.3% and 14.4% at 385 C, respectively.
2
3
2
Consequently, the above results indicated that the improved
incipient wetness impregnation through the addition of OA could
effectively promote the activities of the catalysts, due to the
high dispersion of active component confirmed by XRD and TEM.
Notably, the activities of the 0.5%Pd/SiO –OA catalyst was higher
2
than that of the 0.5%Pd/SiO2 catalyst, which are positively related
0
to their proportion of Pd species observed from XPS results
(Table 1). Besides, the addition of OA could also result in a stronger
adsorption of toluene in the catalysts to promote the activities of
the catalysts (Fig. 3).
Fig. 3. Toluene-TPD profiles of 0.5%Pd/SiO2 and 0.5%Pd/SiO2–OA.
3
.2.2. Catalytic combustion of toluene over the bimetallic
alysts. Thus, it can be speculated that the bimetallic Pd-Pt–OA
catalyst is beneficial to activate toluene molecular at lower tem-
perature during the catalytic combustion of toluene. Fig. 6 displays
the dependencies of the catalytic activities on reaction tempera-
ture in the combustion of toluene over the bimetallic Pd-Pt–OA
catalyst and the monometallic Pd–OA and Pt–OA catalyst. In
the case of the bimetallic catalyst with the same total metal
Pd-Pt–OA catalysts
In order to further improve the activity of the supported noble-
metal catalyst, the bimetallic Pd-Pt–OA catalyst was prepared, since
there is a synergic interaction between the particles of Pd and Pt
as reported in the literature [42–44]. Fig. 5 shows the toluene-
TPD profiles of 0.5%Pd/SiO –OA, 0.5%Pt/SiO –OA and 0.25%Pd-
2
2
0
.25%Pt/SiO –OA catalysts. The 0.25%Pd-0.25%Pt/SiO –OA catalyst
loading, the 0.25%Pd-0.25%Pt/SiO –OA catalyst exhibited much
2
2
2
possessed the highest desorption temperatures among these cat-
better catalytic activity for toluene oxidation than did either the

6
4
H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
Fig. 4. Catalytic combustion of toluene over (A) 0.5%Pd/SiO2, 0.5%Pd/SiO2–OA, (B) 0.5%Pt/SiO2, 0.5%Pt/SiO2–OA, (C) 5.0%CuO/SiO2, 5.0%CuO/SiO2–OA, (D) 2.5%Fe2O3/SiO2 and
.5%Fe2O3/SiO2–OA catalysts under the conditions of toluene concentration at 1000 ppm and SV at 60,000 mL/(g h) (n = 3).
2
Table 2
Catalytic data in the combustion of toluene over various catalysts.
Catalyst
CTola (ppm)
SVb [mL/(g h)]
Activity
Ref.
◦
T98 ( C)
TOF[molTol/(molPdPt s)]
◦
◦
C
160
C
190
−
−
3
2
0
0
0
0
0
0
0
0
0
0
0
1
1
.5%Pd/SiO2
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
60000
60000
60000
60000
60000
60000
70000
70000
60000
60000
60000
60000
40000
205
190
180
160
200
250
198
197
200
220
170
195
180
2.34 × 10
1.45 × 10
This work
This work
This work
This work
This work
[34]
[39]
[40]
[29]
[29]
.5%Pd/SiO2–OA
.5%Pt/SiO2–OA
.25%Pd-0.25%Pt/SiO2–OA
.05%Pd-0.05%Pt/SiO2–OA
.5%Pd/mesoporous ZrO2
.5%Pd/Al-SBA-15
.5% Pd/ZK-50
.5%Pd/HBEA
.5%Pd/HFAU
.5%Pt/KZSM-5
.0%Pt/HBEA
−
−
3
2
6.92 × 10
1.88 × 10
[45]
[35]
[46]
1.63 × 10−
3
.0%Pt/CeO2-AC
monometallic 0.5%Pd/SiO –OA or 0.5%Pt/SiO –OA catalyst. It can
with the catalytic performance mentioned above, i.e., the bimetal-
lic Pd-Pt–OA catalyst is more active than the monometallic Pd–OA
and Pt–OA catalyst. These catalytic data are also much more attrac-
tive than those of reported in literature [29,34–40,45,46], as briefly
summarized in Table 2, which indicates that The bimetallic Pd-
2
2
◦
be seen that the T98 of toluene could be as low as 160 C over
.25%Pd-0.25%Pt/SiO –OA, indicating that the bimetallic active
0
2
component led to a considerable increase in the catalytic oxidation
of toluene. Furthermore, the 0.25%Pd-0.25%Pt/SiO –OA catalyst
2
◦
showed nearly 100% selectivity to CO2 at 160 C, and no CO was
Pt/SiO –OA catalyst is really an active catalyst for the catalytic
2
detected by on-line TCD during the reaction. To compare the
toluene catalytic activity over various catalysts accurately, the
turnover frequencies (TOF) of toluene combustion were deter-
mined from the toluene conversion curves under differential
conditions (Table 2). It can be seen that the TOF value agrees well
combustion of toluene.
The influence of the Pd/Pt molar ratio on the catalytic activity
of the bimetallic Pd-Pt–OA catalyst was also tested, as is shown in
Fig. S3. When the Pd/Pt molar ratio was controlled to between 0.5
and 1, the bimetallic Pd-Pt–OA catalyst with a total Pd-Pt loading

H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
65
Fig. 5. Toluene-TPD profiles of 0.5%Pd/SiO2–OA, 0.5%Pt/SiO2–OA and 0.25%Pd-
.25%Pt/SiO2–OA.
Fig. 7. Dependencies of catalytic activities on reduction temperature by H2 in the
0
catalytic combustion of toluene over 0.25%Pd-0.25%Pt/SiO –OA catalyst (n = 3).
2
Fig. 6. Catalytic combustion of toluene over the 0.5%Pd/SiO2–OA, 0.5%Pd/SiO2–OA
and 0.25%Pd-0.25%Pt/SiO2–OA catalysts (n = 3).
Fig. 8. Effect of metal loading on the catalytic activities in the combustion of toluene
over the bimetallic Pd-Pt/SiO2 catalysts (n = 3).
of 0.5% showed the optimal catalytic activity for the combustion of
toluene. Significantly, the optimal hydrogen reduction temperature
was distinct for the preparation of each of the various supported
noble-metal catalysts. According to the results from both the lit-
erature [35] and this work (Fig. S4), the reduction temperatures of
effect on the quantity of active component. Nevertheless, the 0.05%
Pd-0.05%Pt/SiO –OA catalyst still exhibited high catalytic activity
2
for toluene conversion, despite exceedingly low noble-metal load-
ing. Thus, this highly active catalyst with low noble-metal loading
has broad prospective industrial application.
◦
the monometallic Pd and Pt catalysts herein were 400 and 300 C,
respectively. The catalytic activities of the bimetallic Pd-Pt catalysts
were also investigated by varying the reduction temperature from
3
.2.4. Catalytic activities of the Pd-Pt–OA catalysts with various
carriers
Since a catalyst carrier is one of the key components of a sup-
◦
2
60 to 400 C, and the results are shown in Fig. 7. Apparently, the
optimal reduction temperature for the 0.25%Pd-0.25%Pt/SiO –OA
2
◦
catalyst was 300 C.
ported catalyst, the influence of a catalyst carrier on catalytic
activity was investigated. Fig. 9 displays the toluene conversion
as a function of temperature over the bimetallic Pd-Pt–OA cat-
alysts, with various catalyst carriers. In order to compare the
catalytic activities, the T98 and T50 of toluene were estimated from
3.2.3. Catalytic activities of the Pd-Pt–OA catalysts with low
noble-metal loading
From an economic point of view, a highly active catalyst with
low platinum and palladium component is desirable. Fig. 8 displays
the effect of metal loading on the catalytic activities in the com-
◦
Fig. 9. T98 (T50) values were 160 (146), 170 (153), and 175 (167) C
over 0.25%Pd-0.25%Pt/SiO –OA (fumed silica), 0.25%Pd-0.25%Pt/H-
2
bustion of toluene over the bimetallic Pd-Pt/SiO –OA catalysts. To
ZSM-5–OA and 0.25%Pd-0.25%Pt/SiO –OA (silica gel), respectively.
2
2
compare the catalytic activities, the T98 of toluene was estimated
Obviously, the bimetallic Pd-Pt–OA catalyst showed the best per-
formance for toluene conversion when fumed silica was used as
the catalyst support. In fact, fumed silica is one of cheapest raw
materials for making zeolites, including ZSM-5, and can be com-
mercialized for bulk production; hence this low-cost material is
from Fig. 8. The T98 values over 0.25%Pd-0.25%Pt/SiO –OA, 0.1%Pd-
2
0
2
.1%Pt/SiO –OA and 0.05%Pd-0.05%Pt/SiO –OA were 160, 175 and
00 C, respectively. Obviously, the lower the metal loading, the
2
2
◦
lower the catalytic activity, because the metal loading had a direct

6
6
H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
Fig. 11. DTG-TGA curves of the spent 0.25%Pd-0.25%Pt/SiO2–OA (a, c) and 0.25%Pd-
.25%Pt/H-ZSM-5–OA (b, d) catalysts after the active stability reaction for 16 h.
0
Fig. 9. Effect of the catalyst carriers on the catalytic activities in the combustion of
toluene over the bimetallic 0.25%Pd-0.25%Pt catalysts (n = 3).
suggested that coke was formed on the spent 0.25%Pd-0.25%Pt/H-
ZSM-5–OA catalyst, resulting in deactivation of the catalyst.
TGA is a common method employed to examine the combustion
behavior of solid materials. Therefore, TGA was applied to evaluate
the combustion behavior of the coke formation on catalysts. Fig. 11
shows the DTG-TGA curves of the spent 0.25%Pd-0.25%Pt/SiO –OA
2
and 0.25%Pd-0.25%Pt/H-ZSM-5–OA catalyst after an active stability
reaction for 16 h. The DTG curve of 0.25%Pd-0.25%Pt/H-ZSM-5–OA
showed two peaks. The negative peak, centered at the low tem-
◦
perature, 63 C, was attributed to water desorption, while the
◦
other peak, at high temperature, 441 C, could be ascribed to coke
formation during the reaction. Interestingly, the spent 0.25%Pd-
0
.25%Pt/SiO –OA catalyst did not show any peak in the DTG curve
2
at high temperature, but had one peak at low temperature. There-
fore, this phenomenon indicated that no coke was formed on
the spent 0.25%Pd-0.25%Pt/SiO –OA catalyst. Meanwhile, the TGA
2
curves show quantitative analyses of weight losses. The weight loss
◦
at below 170 C is attributed to water desorption, while the weight
◦
loss at 180–540 C should be assigned to coke formation. Based on
the weight loss observed in the TGA curve, the weight loss for coke
◦
Fig. 10. The active stability test in the combustion of toluene over the 0.25%Pd-
formation at 180–540 C of 0.25%Pd-0.25%Pt/SiO2–OA (0%) was
0
.25%Pt/SiO2–OA and 0.25%Pd-0.25%Pt/H-ZSM-5–OA catalysts under the conditions
much less than that of 0.25%Pd-0.25%Pt/H-ZSM-5–OA (2.9%), indi-
cating that 0.25%Pd-0.25%Pt/H-ZSM-5–OA had much stronger coke
◦
of reaction temperature at 160 and 170 C, respectively.
formation than 0.25%Pd-0.25%Pt/SiO –OA. This phenomenon could
2
be attributed to the following reason. 0.25%Pd-0.25%Pt/SiO –OA
showed BET surface area and average pore-size at 360.9 m /g and
2
expected to further raise the potential of the Pd-Pt/SiO –OA cata-
lyst in industrial application.
2
2
4
0
5.2 nm, as summarized in Table S1 and Fig. S5. Though 0.25%Pd-
.25%Pt/H-ZSM-5–OA showed the similar BET surface area, its
3
.2.5. Catalyst life and coke formation
Fig. 10 shows the dependencies of the catalytic activities on
average pore-size was just 0.5 nm. 0.25%Pd-0.25%Pt/SiO2–OA with
mesoporosity was favorable for mass transfer in the catalytic com-
bustion of toluene. Consequently, the 0.25%Pd-0.25%Pt/SiO2–OA
catalyst had high catalytic activity and stability in the reaction of
catalytic combustion.
reaction time in the combustion of toluene over the 0.25%Pd-
.25%Pt/SiO –OA and 0.25%Pd-0.25%Pt/H-ZSM-5–OA catalysts. The
0
2
reaction temperatures of 0.25%Pd-0.25%Pt/SiO –OA and 0.25%Pd-
2
0
1
.25%Pt/H-ZSM-5–OA for the active stability test were 160 and
70 C, respectively. When the reaction time was less than 12 h,
◦
4. Conclusions
both the catalysts could obtain high toluene conversion (over
6%) at their own reaction temperatures. When the reaction time
9
Pd-Pt/SiO –OA catalysts were successfully prepared by
2
increased from 12 to 16 h, the 0.25%Pd-0.25%Pt/SiO –OA catalyst
improved incipient wetness impregnation with fumed silica.
The addition of OA could effectively promote the activities of
the catalysts, due to the high dispersion of Pd and Pt particles,
2
still exhibited nearly complete conversion of toluene. In con-
trast, the toluene conversion gradually decreased and remained at
about 86% over the 0.25%Pd-0.25%Pt/H-ZSM-5–OA catalyst over the
confirmed by characterization data. The bimetallic Pd-Pt/SiO –OA
2
same time period. It was found that the 0.25%Pd-0.25%Pt/SiO –OA
catalyst exhibited much better catalytic activity for toluene oxi-
dation than the monometallic Pd/SiO –OA or Pt/SiO –OA catalyst.
2
catalyst exhibited high catalytic activity and stability. Moreover,
the color of the 0.25%Pd-0.25%Pt/H-ZSM-5–OA catalyst turned to
tawny after the active stability reaction, whereas the color of
2
2
Most importantly, the Pd-Pt/SiO –OA catalyst exhibited high cat-
2
alytic activity and stability in the reaction of catalytic combustion.
Based on the weight loss observed in the TGA curve, no coke was
the 0.25%Pd-0.25%Pt/SiO –OA stayed the same. This phenomenon
2

H. Wang et al. / Applied Catalysis A: General 529 (2017) 60–67
67
formed on the spent Pd-Pt/SiO –OA catalyst. Furthermore, the
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Acknowledgements
Financial support from the National Natural Science Foundation
of China (Grant 21406044), the Zhejiang Province Nature Science
Foundation of China (Grant LY12E06001), and the Science and
Technology Planning Project of Zhejiang Province, China (Grant
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