Catalytic performance of cobalt oxide-supported gold-palladium nanocatalysts for the removal of toluene and o-xylene

Article abstract of DOI:10.1016/S1872-2067(16)62569-X

Using the molten salt and polyvinyl alcohol-protected reduction method, we fabricated Co₃O₄ octahedron-supported Au-Pd (x(AuPd_y)/Co₃O₄; x = (0.18, 0.47, and 0.96) wt%; y (Pd/Au molar ratio) = 1.85?1.97) nanocatalysts. The molten salt-derived Co₃O₄ sample possessed well-defined octahedral morphology, with an edge length of 300 nm. The Au-Pd nanoparticles, with sizes of 2.7?3.2 nm, were uniformly dispersed on the surface of Co₃O₄. The 0.96 (AuPd_1.92)/Co₃O₄ sample showed the highest catalytic activity for toluene and o-xylene oxidation, and the temperature required for achieving 90% conversion of toluene and o-xylene was 180 and 187 °C, respectively, at a space velocity of 40000 mL/(g–h). The high catalytic performance of Co₃O₄ octahedron-supported Au-Pd nanocatalysts was associated with the interaction between Au-Pd nanoparticles and Co₃O₄ and high concentration of adsorbed oxygen species.

Full text of DOI:10.1016/S1872-2067(16)62569-X

Chinese Journal of Catalysis 38 (2017) 207–216
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue on the International Symposium on Environmental Catalysis (ISEC 2016))
Catalytic performance of cobalt oxide‐supported gold‐palladium
nanocatalysts for the removal of toluene and o‐xylene
Zhiwei Wanga, Yuxi Liua, Tao Yangb, Jiguang Denga,*, Shaohua Xiea, Hongxing Dai a,#
a Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, and Laboratory of Catalysis
Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of
Technology, Beijing 100124, China
b Mechanical Engineering Department, University of Aveiro, 3810‐193 Aveiro, Portugal
A R T I C L E I N F O
A B S T R A C T
Article history:
Using the molten salt and polyvinyl alcohol‐protected reduction method, we fabricated Co3O4 octa‐
hedron‐supported Au‐Pd (x(AuPdy)/Co3O4; x = (0.18, 0.47, and 0.96) wt%; y (Pd/Au molar ratio) =
1.851.97) nanocatalysts. The molten salt‐derived Co3O4 sample possessed well‐defined octahedral
morphology, with an edge length of 300 nm. The Au‐Pd nanoparticles, with sizes of 2.73.2 nm,
were uniformly dispersed on the surface of Co3O4. The 0.96(AuPd1.92)/Co3O4 sample showed the
highest catalytic activity for toluene and o‐xylene oxidation, and the temperature required for
achieving 90% conversion of toluene and o‐xylene was 180 and 187 °C, respectively, at a space
velocity of 40000 mL/(g·h). The high catalytic performance of Co3O4 octahedron‐supported Au‐Pd
nanocatalysts was associated with the interaction between Au‐Pd nanoparticles and Co3O4 and high
concentration of adsorbed oxygen species.
Received 17 September 2016
Accepted 27 October 2016
Published 5 February 2017
Keywords:
Molten salt
Cobalt oxide
Au‐Pd nanoparticle
Volatile organic compound
Catalytic oxidation
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
performance for the removal of VOCs at low temperatures
[6–14]. However, the presence of moisture can greatly inhibit
It is well known that volatile organic compounds (VOCs, e.g.,
toluene and o‐xylene) are not only detrimental to the environ‐
ment but also harmful to human health [1–3]. Catalytic oxida‐
tion is considered to be one of the most effective methods for
the removal of VOCs because of its high removal efficiency,
absence of secondary pollution, and low fossil cost [4,5]. The
key issue for this technology is the development of efficient
catalysts.
the catalytic activity of supported single noble metal nanocata‐
lysts [13,14]. In previous studies, researchers have reported
that supported noble metal alloy nanocatalysts exhibit unique
catalytic performance, compared with the corresponding sup‐
ported single noble metal nanocatalysts [15–19]. For example,
our group found that three‐dimensionally ordered
macroporous Mn2O3‐supported Au‐Pd nanocatalysts exhibited
much better catalytic performance for toluene oxidation than
supported Au or Pd nanocatalysts [16].
Recently, supported noble metal (e.g., Au, Pd, Pt) nanocata‐
lysts have gained much attention because of their high catalytic
Among the transition metal (e.g., Mn, Fe, Zn, Ni and Co) ox‐
* Corresponding author. Tel: +86‐10‐67396118; Fax: +86‐10‐67391983; E‐mail: jgdeng@bjut.edu.cn
# Corresponding author. Tel: +86‐10‐67396118; Fax: +86‐10‐67391983; E‐mail: hxdai@bjut.edu.cn
This work was supported by the National Natural Science Foundation of China (21377008, 21477005, U1507108), National High Technology Re‐
search and Development Program of China (2015AA034603), Beijing Nova Program (Z141109001814106), and Natural Science Foundation of Bei‐
jing Municipal Commission of Education (KM201410005008).
DOI: 10.1016/S1872‐2067(16)62569‐X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 2, February 2017

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Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
ides [20–24], cobalt oxides have been widely used as supports
and catalysts for the removal of VOCs [15,25–28]. Recently,
much attention has been paid to the support morphology be‐
cause of its possible effects on the catalytic performance. Cobalt
oxides with various morphologies (e.g., sphere, cube, belt,
nanowire, and rod shapes) have been generated via the solu‐
tion method [29–32]. Recently, our group found that the mol‐
ten salt method is convenient for the preparation of transition
oxides with defined morphologies (e.g., Mn2O3 nanowires) [33].
As an extension of this work, we fabricated Co3O4 octahedrons
via the molten salt method and used them to support Au‐Pd
nanocatalysts fabricated via the polyvinyl alcohol (PVA)‐pro‐
tected reduction method; we then investigated their catalytic
performance for the removal of toluene and o‐xylene.
2.2. Catalyst characterization
Elemental analysis of the noble metal loadings was per‐
formed using the ICP‐AES technique on a Thermo Electron IRIS
Intrepid ER/S spectrometer. X‐ray diffraction (XRD) patterns of
the samples were recorded on a Bruker D8 Advance diffrac‐
tometer with Cu Kα radiation and a nickel filter ( = 0.15406
nm). Scanning electron microscopy (SEM) images of the sam‐
ples were recorded on a Gemini Zeiss Supra 55 apparatus (op‐
erating at 10 kV). Transmission electron microscopy (TEM)
images and selected‐area electron diffraction (SAED) patterns
of the samples were obtained using a JEOL‐2010 (operating at
200 kV). High‐angle annular dark field (HAADF) images and
element mapping were acquired on an FEI G2 80–200/
Chemi‐STEM Cs‐corrected transmission electron microscope
with a probe corrector. BET (Brunauer‐Emmett‐Teller) surface
areas of the samples were measured via N2 adsorption at 196
°C on a Micromeritics ASAP 2020 analyzer with the samples
being degassed at 250 °C for 2.5 h under vacuum before meas‐
urement. X‐ray photoelectron spectroscopy (XPS, VG CLAM 4
MCD analyzer) was used to determine the Co 2p, O 1s, Au 4f, Pd
3d, and C 1s binding energies (BEs) of surface species using Mg
K (hv = 1253.6 eV) as the excitation source. To remove the
adsorbed water and carbonate species on the surface, the sam‐
ples were pre‐treated in O2 (flow rate = 20 mL/min) at 300 °C
for 1 h and then cooled to RT, followed by transferring the
pre‐treated samples into the spectrometer in a transparent
Glove Bag (Instruments for Research and Industry, USA) filled
with helium. The pre‐treated samples were outgassed in the
preparation chamber (10–5 Torr) for 0.5 h and then introduced
into the analysis chamber (3  10–9 Torr) for XPS spectrum
recording. The C 1s signal at 284.6 eV was taken as a reference
for BE calibration.
Hydrogen temperature‐programmed reduction (H2‐TPR)
experiments were carried out on a chemical adsorption ana‐
lyzer (Autochem II 2920, Micromeritics). Before TPR meas‐
urement, ca. 50 mg of the sample (40–60 mesh) was loaded to a
quartz fixed‐bed U‐shaped microreactor (i.d. = 4 mm) and pre‐
treated in an O2 flow of 30 mL/min at 250 °C for 1 h. After being
cooled under the same atmosphere to 30 °C, the sample was
purged with a helium flow of 30 mL/min for 15 min. Finally,
the pretreated sample was exposed to a flow (30 mL/min) of
5% H2‐95% Ar (v/v) mixture and heated at a ramp rate of 10
°C/min from 30 °C to 900 °C. The alteration in H2 concentration
of the effluent was monitored online by the chemical adsorp‐
tion analyzer. The reduction peak was calibrated against that of
the complete reduction of a known standard powdered CuO
(Aldrich, 99.995%).
2. Experimental
2.1. Catalyst preparation
Co3O4 octahedrons were fabricated via the molten salt
method. Typically, 29.4 g of NaNO3, 0.6 g of NaF, and 10 g of
CoCl2 were well ground and then calcined in a muffle furnace at
350 °C for 2 h. The as‐obtained solid was dissolved in deionized

water and washed several times to remove Na+, NO3 and F
ions. The final Co3O4 octahedron sample was obtained after
being dried in an oven at 80 °C.
The
Co3O4
octahedron‐supported
gold‐palladium
(x(AuPdy)/Co3O4) samples were prepared using the PVA‐pro‐
tected reduction method. The typical preparation procedures
were as follows. A 0.6, 1.5, or 3 mL of PVA aqueous solution
(2.0 g/L) was added to a PdCl2 (1.5 mmol/L) and HAuCl4 (1.5
mmol/L) mixed aqueous solution in an ice bath under vigorous
stirring for 30 min. Then, 0.7, 1.7, or 3.4 mL of NaBH4 aqueous
solution (2.0 g/L) was quickly injected into the mixed noble
metal aqueous solution, generating a dark‐brown sol. Subse‐
quently, 0.5 g of the Co3O4 support was added to the
dark‐brown sol and vigorously stirred for 8 h. The mixture
solution was filtered and washed with deionized water, until no
Cl ions in the supernatant were detected using AgNO3 (0.1
mol/L) aqueous solution. The obtained wet solid was dried at
80 °C for 10 h and calcined in a muffle furnace at a ramp rate of
1 °C/min from room temperature (RT) to 300 °C and main‐
tained at this temperature for 4 h, thus obtaining the
x(AuPdy)/Co3O4 samples. The results of inductively coupled
plasma atomic emission spectroscopic (ICP‐AES) investigations
reveal that the real loadings of noble metals were (0.18, 0.47,
and 0.96) wt% in x(AuPdy)/Co3O4, respectively, and the corre‐
sponding real Pd/Au molar ratios were 1.85, 1.93, and 1.92.
For comparison, the x(AuPdy)/bulk Co3O4 samples were also
prepared using thermal decomposition of cobalt nitrate at 600
°C for 4 h and the PVA‐protected reduction method. The results
of the ICP‐AES studies reveal that the real Au‐Pd loading (x)
was 0.95 wt% in the x(AuPdy) /bulk Co3O4 sample, with the real
Pd/Au molar ratio being 1.97.
Oxygen temperature‐programmed desorption (O2‐TPD) was
carried out on the same apparatus used in the H2‐TPR experi‐
ments. Prior to O2‐TPD experiment, 30 mg of the sample was
preheated in an O2 flow of 30 mL/min at 300 °C for 1 h. After
cooling to RT, the sample was kept in the same flow for another
1 h. Before the sample was heated from RT to 750 °C at a ramp
of 10 °C/min, a helium flow of 40 mL/min was employed to
remove the O2 for 1 h. The oxygen concentration in the effluent
was continuously monitored by a thermal conductivity detec‐
All chemicals (A.R. in purity) were purchased from Beijing
Chemical Reagent Company and used without further purifica‐
tion.

Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
209
tor. The oxygen desorption peak was calibrated according to
the results of the oxygen pulse experiment.
2.3. Catalytic performance evaluation
Catalytic activities of the samples were evaluated in a con‐
tinuous flow fixed‐bed quartz tubular microreactor (i.d. = 6.0
mm). 50 mg of the sample (40–60 mesh) was diluted with 0.25
g of quartz sand (40–60 mesh). Prior to the measurement, the
sample was pretreated in an O2 flow of 20 mL/min at 250 °C for
1 h. After being cooled to a given temperature, the reactant gas
containing toluene or o‐xylene was passed through the catalyst
bed. The reactant mixture gas was 0.1% toluene or o‐xylene +
O2 + N2 (balance), and the total flow was 33.6 mL/min, giving a
toluene or o‐xylene/O2 molar ratio of 1/400 and a space veloc‐
ity (SV) of ca. 40000 mL/(g·h). The 0.1% toluene or o‐xylene
was generated by passing a N2 flow through a bottle containing
pure toluene or o‐xylene in a low‐temperature thermostatic
bath. In the case of water vapor introduction, 3.0 vol% H2O was
introduced by passing the feed stream through a water satura‐
tor at 24 °C. Reactants and products were analyzed online by
gas chromatography (GC‐14C, Shimadzu) equipped with a
flame ionization detector (FID) and a thermal conductivity de‐
tector (TCD), using a stabilwax column (30 m in length) for
toluene or o‐xylene separation and a Carboxen 1000 column (3
m in length) for permanent gas detection. Conversion was de‐
fined as (cinlet – coutlet)/cinlet × 100%, where cinlet and coutlet are the
inlet and outlet toluene or o‐xylene concentrations in the feed
stream, respectively. We used a mass spectrometer (Hiden
HPR20) to detect the products and found that CO2 and H2O
were the only products in the combustion of toluene and
o‐xylene. The balance of carbon throughout the investigation
was estimated to be (99.5  1.5)%.
(5)
(4)
(3)
(2)
(1)
10
20
30
40
50
60
70
80
2 /(o)
Fig. 1. XRD patterns of (1) Co3O4, (2) 0.18(AuPd1.85)/Co3O4, (3)
0.47(AuPd1.93)/Co3O4, (4) 0.96(AuPd1.92)/Co3O4, and (5) 0.95(AuPd1.97)/
bulk Co3O4.
3.1. Characterization results
Fig. 1 shows the XRD patterns of the as‐prepared samples.
Compared with the XRD pattern of the standard Co3O4 sample
(JCPDS PDF #42‐1467), it is clear that cobalt oxide in
0.95(AuPd1.97)/bulk Co3O4 and x(AuPdy)/Co3O4 was cubic in
crystal structure, and all of the diffraction peaks could be well
indexed (Fig. 1(5)). The loading of Au‐Pd nanoparticles did not
cause any changes in the crystal structure of Co3O4. In addition,
diffraction peaks ascribed to the Au, Pd, or Au‐Pd phases were
not detected, indicating that the noble metal nanoparticles
might be well dispersed on the surface of Co3O4.
As can be seen from Fig. 2, the Co3O4 support derived via the
molten salt method possessed octahedral morphology, with an
edge length of 300 nm, while the Co3O4 support derived via the
thermal decomposition of nitrates had an irregular morpholo‐
3. Results and discussion
(e)
(d)
(a)
(f)
(b)
(g)
(c)
5 nm
50 nm
50 nm
1 µm
200 nm
(h)
(j)
(i)
Co3O4 (1 1 1)
0.461 nm
Co3O4 (1 1 1)
0.461 nm
Co3O4 (1 1 1)
0.461 nm
100 nm
20 nm
5 nm
5 nm
5 nm
Fig. 2. SEM images of (a,b) Co3O4 and TEM images and SAED patterns (insets) of (c,d) 0.18(AuPd1.85)/Co3O4, (e,f) 0.47(AuPd1.93)/Co3O4, (g,h)
0.96(AuPd1.92)/Co3O4, and (i,j) 0.95(AuPd1.97)/bulk Co3O4.

210
Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
100
100
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
(d)
(c)
80
60
40
20
0
80
60
40
20
0
1.0-2.0 2.0-3.0 3.0-4.0 4.0-5.0
AuPd particle size (nm)
2.0-3.0 3.0-4.0 4.0-5.0
AuPd particle size (nm)
2.0-3.0 3.0-4.0 4.0-5.0
AuPd particle size (nm)
1.0-2.0 2.0-3.0 3.0-4.0
AuPd particle size (nm)
Fig. 3. Au‐Pd particle size distributions of (a) 0.18(AuPd1.85)/Co3O4, (b) 0.47(AuPd1.93)/Co3O4, (c) 0.96(AuPd1.92)/Co3O4, and (d) 0.95(AuPd1.97)/bulk
Co3O4.
gy. There were uniformly dispersed Au‐Pd nanoparticles on the
surface of octahedral Co3O4. The average sizes of Au‐Pd nano‐
particles were in the range of 2.7−3.2 nm (Fig. 3 and Table 1).
The lattice spacings (d value) of Co3O4 in the x(AuPdy)/Co3O4
and 0.95(AuPd1.97)/bulk Co3O4 samples was estimated to be ca.
0.461 nm (Fig. 2(f, h, j)), rather close to that of the (111) plane
of standard Co3O4 (JCPDS PDF #42‐1467). The linearly aligned
bright electron diffraction spots in the SAED patterns (insets of
Fig. 2(d, f, h, and j)) suggest that the Co3O4 supports were single
crystalline.
The STEM and HAADF element mapping images of the
0.96(AuPd1.92)/Co3O4 sample (Fig. 4) confirmed that Au and Pd
species were uniformly dispersed on the surface of octahedral
Co3O4. In other words, an Au‐Pd alloy may have been formed.
Shown in Fig. 5 are the Co 2p3/2, O 1s, Au 4f, and Pd 3d XPS
spectra of the samples. The asymmetrical Co 2p3/2 XPS spec‐
trum (Fig. 5(a)) of each sample could be decomposed into two
components at BE = 780.1 and 781.7 eV, attributed to the sur‐
face Co3+ and Co2+ species [14,34], respectively. The weak satel‐
lite signal at BE = 785.7 eV also confirmed the presence of sur‐
face Co2+ in the samples [35]. Because of the interaction be‐
tween noble metal NPs and Co3O4 (Au0 + Co3+  Au+ + Co2+ or
Pd0 + Co3+  Pd2+ + Co2+), the surface Co3+/Co2+ molar ratios of
the samples decreased from 1.41 to 0.90 with an increase in the
loading of Au‐Pd nanoparticles from (0 to 0.96) wt% (Table 2).
The asymmetrical O 1s XPS spectrum (Fig. 5(b)) of each sample
could be decomposed to three components at BE = 529.9,
531.5, and 533.3 eV, assignable to the surface lattice oxygen
(Olatt), adsorbed oxygen (Oads, e.g., O2 , O22–, or O–), and car‐
–
bonate or adsorbed water species [36,37], respectively. It is
well known that the surface adsorbed oxygen species play an
important role in deep oxidation of organics [38]. An increase
in the surface Oads species could enhance the catalytic activity of
a catalyst. As shown in Table 2, the surface Oads/Olatt molar ratio
increased from 0.49 to 2.36 with an increase in the loading of
Au‐Pd nanoparticles from (0 to 0.96) wt%. The Au 4f XPS spec‐
trum of each sample could be decomposed into four compo‐
nents at BE = 83.5, 84.5, 87.2, and 88.3 eV (Fig. 5(c)). The ones
at BE = 83.5 and 87.2 eV were attributed to the surface metallic
gold (Au0) species, whereas the ones at BE = 84.5 and 88.3 eV
were attributed to the surface oxidized gold (Auδ+) species
[39,40]. As shown in Fig. 5(d), the Pd 3d spectrum of each sam‐
ple could be deconvoluted into four components at BE = 335.5,
337.9, 340.9, and 343.2 eV. The components at BE = 335.5 and
340.9 eV were due to the surface metallic palladium (Pd0) spe‐
cies, while the ones at BE = 337.9 and 343.2 eV were due to the
surface oxidized palladium (Pd2+) species [41–43]. Apparently,
the surface Auδ+/Au0 or Pd2+/Pd0 molar ratio also increased
with a rise in the loading of Au‐Pd nanoparticles because of the
interaction between Au‐Pd nanoparticles and Co3O4 [39,44].
The H2‐TPR profiles of the present catalysts are shown in
Fig. 6. For the Co3O4 octahedron sample, there were two
asymmetrical reduction peaks, at 386 and 497 °C, because of
(a)
(b)
(c)
Table 1
BET surface areas, average Au‐Pd particle sizes, real Au and Pd contents,
and real Pd/Au molar ratios of the as‐prepared samples.
O
Real contentb
(wt%)
Average
Au‐Pd
particle
size a (nm)
Real
50 nm
25 nm
50 nm
Surface
area
Pd/Au
molar
ratio b
Sample
(m2/g)
Au
—
Pd
—
(d)
(e)
(f)
Co3O4
1.2
1.2
1.3
1.3
8.6
—
—
0.18(AuPd1.85)/Co3O4
0.47(AuPd1.93)/Co3O4
0.96(AuPd1.92)/Co3O4
0.95(AuPd1.97)/bulk Co3O4
2.8
2.9
2.7
3.2
0.09 0.09
0.23 0.24
0.47 0.49
0.46 0.49
1.85
1.93
1.92
1.97
Co
Au
Pd
25 nm
25 nm
25 nm
a Estimated according to the estimation of noble metal particles in the
Fig. 4. HAADF‐STEM and elemental scanning images of
0.96(AuPd1.92)/Co3O4.
TEM images of the samples.
b Determined by the ICP‐AES technique.

Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
211
(a) Co 2p3/2
(b) O 1s
780.1
529.9
781.7
531.5
533.3
785.7
(5)
(5)
(4)
(3)
(2)
(4)
(3)
(2)
(1)
(1)
775
778
781
784
787
790
527
529
531
Binding energy (eV)
533
535
Binding energy (eV)
(c) Au 4f
(d) Pd 3d
83.5
335.5
87.2
340.9
84.5
88.3
337.9
343.2
(5)
(4)
(5)
(4)
(3)
(2)
(3)
(2)
80
82
84
86
88
90
332
334
336
338
340
342
344
Binding energy (eV)
Binding energy (eV)
Fig. 5. (a) Co 2p3/2, (b) O 1s, (c) Au 4f, and (d) Pd 3d XPS spectra of (1) Co3O4, (2) 0.18(AuPd1.85)/Co3O4, (3) 0.47(AuPd1.93)/Co3O4, (4)
0.96(AuPd1.92)/Co3O4, and (5) 0.95(AuPd1.97)/bulk Co3O4.
the reduction of Co3O4 to CoO and CoO to metallic cobalt
[45,46], respectively. When the Au‐Pd nanoparticles were
loaded on the surface of octahedral or bulk Co3O4, the two re‐
duction peaks were shifted to lower temperatures because of
the interaction between Au‐Pd nanoparticles and the support.
This interaction enhanced the low reducibility of the supported
catalysts, which could greatly improve the catalytic activity of
these samples. The H2 consumption of the present catalysts
was in the range of 13.03–14.27 mmol/g.
Shown in Fig. 7 are the O2‐TPD profiles of the as‐prepared
samples. The weak signal at low temperature (< 600 °C) is due
to the desorption of surface chemisorbed oxygen (O2 , O22− or
O−) species [26,47]. The peaks above 600 °C are attributable to
the desorption of lattice oxygen (O2−) species. To some extent,
the loading of AuPd nanoparticles enhanced the adsorbed oxy‐
gen species amount of the sample (Table 2), indicating that the
loading of AuPd nanoparticles improved the oxygen activation
ability of x(AuPdy)/Co3O4 [48,49].
−
Table 2
Surface element compositions, O2 desorption, and H2 consumption of the samples.
H2 consumption a (mmol/g)
O2 desorption b (mmol/g)
Co3+/Co2+ Oads/Olatt
molar ratio molar ratio molar ratio molar ratio
Auδ+/Au0
Pd2+/Pd0
Sample
I
—
II
III
IV
Total
200600 °C
0.089
Co3O4
1.41
1.28
1.07
0.90
1.09
0.49
1.20
1.38
2.36
0.62
—
—
—
2.70 10.59 13.29
0.18(AuPd1.85)/Co3O4
0.47(AuPd1.93)/Co3O4
0.96(AuPd1.92)/Co3O4
0.95(AuPd1.97)/Bulk Co3O4
0.28
0.44
0.49
0.32
0.36
0.53
0.56
0.39
1.90 2.87 5.40
3.38 3.78 3.47
3.78 3.91 3.74
1.64 2.83 6.09
2.86 13.03
2.65 13.28
1.61 13.04
3.71 14.27
0.072
0.095
0.120
0.046
a Estimated by quantitatively analyzing the reduction peaks in the H2‐TPR profiles.
b Estimated by quantitatively analyzing the desorption peaks in the O2‐TPD profiles.

212
Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
119

300

600
305

450
240
(5)
(5)
(4)
(3)
(2)
(1)
Ⅳ
Ⅱ
Ⅰ
Ⅲ
125
301
261
Ⅳ
Ⅲ
Ⅰ Ⅱ
(4)
(3)
133
302
276
Ⅲ
Ⅰ Ⅱ
Ⅳ
307
262
146
Ⅰ
Ⅳ
Ⅲ
Ⅱ
(2)
(1)
497
386
Ⅳ
Ⅲ
30 130 230 330 430 530 630 730 830
Temperature (oC)
50
150
250
350
450
550
650
750
Temperature (oC)
Fig. 6. H2‐TPR profiles of (1) Co3O4, (2) 0.18(AuPd1.85)/Co3O4, (3)
0.47(AuPd1.93)/Co3O4, (4) 0.96(AuPd1.92)/Co3O4, and (5) 0.95(AuPd1.97)/
bulk Co3O4.
Fig. 7. O2‐TPD profiles of (1) Co3O4, (2) 0.18(AuPd1.85)/Co3O4, (3)
0.47(AuPd1.93)/Co3O4, (4) 0.96(AuPd1.92)Co3O4, and (5) 0.95(AuPd1.97)/
bulk Co3O4.
o‐xylene. Furthermore, with an increase in the loading of Au‐Pd
nanoparticles, the present x(AuPdy)/Co3O4 catalyst exhibited
higher catalytic activity, and the 0.96(AuPd1.92)/Co3O4 sample
performed best. Under the mentioned reaction conditions, the
temperatures required for 10%, 50%, and 90% conversion of
toluene or o‐xylene (denoted T10%, T50%, and T90%) over the
present catalysts are summarized in Table 3. With an increase
in the loading of Au‐Pd nanoparticles from 0 to 0.96 wt%, T50%
and T90% over the x(AuPdy)/Co3O4 nanocatalysts decreased
from 262 to 171 °C and from 268 to 180 °C, respectively, for
toluene oxidation; and from 285 to 181 °C and from 293 to 187
°C, respectively, for o‐xylene oxidation. The 0.96(AuPd1.92)/
Co3O4 sample exhibited higher catalytic activity than the
0.95(AuPd1.97)/bulk Co3O4 sample. T50% and T90% over the mol‐
ten salt‐derived Co3O4 supported Au‐Pd nanocatalysts de‐
creased by 2030 °C.
3.2. Catalyst activity
The complete oxidation of toluene and o‐xylene was used to
evaluate the catalytic activities of the as‐prepared samples. In
the blank experiments (only quartz sand was loaded in the
micro‐reactor) under the conditions of toluene or o‐xylene
concentration = 0.1%, toluene or o‐xylene/O2 molar ratio =
1/400, and SV = 40000 mL/(g·h), no significant toluene and
o‐xylene conversions were observed at 400 °C for toluene or
o‐xylene combustion. These results indicate that no significant
homogeneous reactions took place under the adopted reaction
conditions. The catalytic activities of the present catalysts are
shown in Fig. 8. Apparently, the conversions of toluene and
o‐xylene increased with a rise in the reaction temperature. The
loading of Au‐Pd nanoparticles on the surface of octahedral
Co3O4 benefited the catalytic combustion of toluene and
For the transition metal oxide‐supported noble metal cata‐
100
(b)
100
(a)
80
80
(4)
(3)
(5) (2)
(1)
(4)
(3)
(2)
(1)
60
40
20
0
(5)
60
40
20
0
100
150
200
Temperature (oC)
250
300
100
150
200
Temperature (oC)
250
300
Fig. 8. (a) Toluene conversion and (b) o‐xylene conversion as a function of reaction temperature over (1) Co3O4, (2) 0.18(AuPd1.85)/Co3O4, (3)
0.47(AuPd1.93)/Co3O4, (4) 0.96(AuPd1.92)/Co3O4, and (5) 0.95(AuPd1.97)/bulk Co3O4 under the conditions of toluene or o‐xylene concentration = 0.1%,
toluene or o‐xylene/O2 molar ratio = 1/400, and SV = 40000 mL/(g·h).

Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
213
100
Table 3
Catalytic activities of the catalysts at SV = 40000 mL/(g·h).
80
60
40
20
0
Toluene oxidation
(°C)
o‐Xylene oxidation
(°C)
Sample
T10% T50% T90%
238 262 268
175 200 206
165 183 192
153 171 180
T10% T50% T90%
276 285 293
190 210 223
173 193 203
165 181 187
190 200 215
Co3O4
0.18(AuPd1.85)/Co3O4
0.47(AuPd1.93)/Co3O4
0.96(AuPd1.92)/Co3O4
0.95(AuPd1.97)/bulk Co3O4 170 195 200
0
5
10
15
20
25
30
On-stream reaction time (h)
lysts, it is difficult to define the active sites and to calculate the
number of active sites. In the present study, all of the species,
such as Au, Pd, and AuPd nanoparticles, and the interface be‐
tween AuPd nanoparticles and Co3O4, could act as active sites
for VOC oxidation. That is to say, the accurate calculation of
TOFs might be impossible. However, the redox ability and oxy‐
gen vacancy concentration of the catalyst certainly played an
important role in the oxidation removal of VOCs. As can be seen
from the results of H2‐TPR, O2‐TPD, and XPS characterizations,
the loading of AuPd nanoparticles on the surface of Co3O4 en‐
hanced the low‐temperature reducibility and surface oxygen
species. Therefore, it is understandable that 0.96(AuPd1.92)/
Co3O4 performed better than Co3O4, 0.18(AuPd1.85)/Co3O4, and
0.47(AuPd1.93)/Co3O4. Because of the high price of Au and Pd,
we did not investigate the catalytic activities of supported cat‐
alysts with larger loadings (> 1 wt%) of AuPd nanoparticles.
Because the adopted reaction conditions have a great influ‐
ence on the catalytic performance, comparison is difficult
among various catalysts in terms of activity, selectivity, and
stability. Table 4 summarizes the catalytic activities of
0.96(AuPd1.92)/Co3O4 and catalysts reported in the literature
for toluene or o‐xylene oxidation under similar reaction condi‐
tions. We can see that 0.96(AuPd1.92)/Co3O4 performed better
than 0.90(Au1Pd2)/Ce0.6Zr0.3Y0.1O2 (T90% = 218 °C at SV = 20000
mL/(g·h)) [17], 1.5Pd‐Au/Ti‐Zr (T90% = 230 °C at SV = 200 h–1)
[50], 1.5Pd‐Au/TiO2 (T90% = 220 °C at SV = 200 h–1) [51],
1.0Pd/meso‐Co3O4 (T90% = 230 °C at SV = 200 h–1) [52],
1.0Pd/meso‐Co3O4 (T90% = 249 °C at SV = 200 h–1) [48], and
7.4Au/Co3O4 (T90% = 218 °C at SV = 20000 mL/(g·h)) [29], and
exhibited similar or inferior catalytic activity to
1.0Au‐Pd/3DOM Co3O4 (T90% = 179 °C at SV = 20000 mL/(g·h))
[15], 0.97(Au1Pd2)/meso‐Cr2O3 (T90% = 176 °C at SV = 20000
Fig. 9. Toluene conversion as a function of on‐stream reaction time
over the 0.96(AuPd1.92)/Co3O4 sample at 180 °C and SV = 40000
mL/(g·h).
mL/(g·h)) [53] and 6.5Au/meso‐Co3O4 (T90% = 138 °C at SV =
20000 mL/(g·h)) [12].
To probe the catalytic stability of 0.96(AuPd1.92)/Co3O4, we
conducted a 30‐h on‐stream reaction experiment at 180 °C, and
the result is shown in Fig. 9. Apparently, there were no signifi‐
cant losses in catalytic activity within 30 h of on‐stream reac‐
tion time, indicating that the 0.96(AuPd1.92)/Co3O4 sample was
catalytically stable. As expected, toluene or o‐xylene conversion
increased with a decrease in SV (Fig. 10(a) and (b)) from 60000
to 20000 mL/(g·h), indicating that the extension in contact
time was favorable for the conversion of toluene or o‐xylene.
To examine the effect of moisture on the catalytic activity of
0.96(AuPd1.92)/Co3O4, we introduced 3 vol% water vapor, and
the result is shown in Fig. 11. It can be seen that because of the
competitive adsorption between water vapor and toluene mol‐
ecules on the surface of the catalyst, toluene conversion de‐
creased from 80% to 74% with the addition of 3 vol% water
vapor. However, when the water vapor was cut off, toluene
conversion returned to its original values. That is to say, the
deactivation was reversible.
4. Conclusions
Octahedral Co3O4 and its supported Au‐Pd nanocatalysts
were fabricated via the molten salt and PVA‐protected reduc‐
tion methods, respectively. The molten salt‐derived Co3O4 oc‐
tahedron was cubic, with a single crystalline structure. The
Table 4
Comparison of catalytic activities over various catalysts for the oxidation of VOCs as reported in the references.
Catalyst
0.96(AuPd1.92)/Co3O4
1.5Pd‐Au/Ti‐Zr
1.0Au‐Pd/3DOM Co3O4
0.90(Au1Pd2)/Ce0.6Zr0.3Y0.1O2
1.5Pd‐Au/TiO2
1.0Pd/mes‐Co3O4
0.97(Au1Pd2)/mes‐Cr2O3
1.0Pd/mes‐Co3O4
7.4Au/Co3O4
Reactant
Toluene
Toluene
Toluene
Toluene
Toluene
o‐xylene
Toluene
o‐xylene
Toluene
Toluene
VOCs concentration (ppm) Space velocity (mL/(g·h))
T90% (°C)
180
230
Ref.
This work
[50]
1000
1000
1000
1000
1000
150
40000
200 h–1
40000
179
[15]
20000
218
220
204
176
249
285
138
[17]
[51]
[52]
[53]
[48]
[35]
[12]
200 h–1
200 h–1
20000
60000
20000
20000
1000
150
1000
1000
6.5Au/mes‐Co3O4

214
Zhiwei Wang et al. / Chinese Journal of Catalysis 38 (2017) 207–216
100
80
60
40
20
0
100
(b)
(a)
80
(1)
(2)
(3)
(2)
(3)
(1)
60
40
20
0
120
140
160
180
200
120
140
160
Temperature (oC)
180
200
Temperature (oC)
Fig. 10. Effect of space velocity on (a) toluene and (b) o‐xylene oxidation over the 0.96(AuPd1.92)/Co3O4 sample. (1) 20000 mL/(g·h); (2) 40000
mL/(g·h); (3) 60000 mL/(g·h).
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3.0 vol% water
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Graphical Abstract
Chin. J. Catal., 2017, 38: 207–216 doi: 10.1016/S1872‐2067(16)62569‐X
Catalytic performance of cobalt oxide‐supported gold‐palladium
nanocatalysts for the removal of toluene and o‐xylene
o-xylene
o-xylene
Au-Pd/Co3O4
CO2
Zhiwei Wang, Yuxi Liu, Tao Yang, Jiguang Deng*, Shaohua Xie, Hongxing Dai*
Beijing University of Technology, China;
University of Aveiro, Portugal
toluene
Co3O4 octahedron‐supported Au‐Pd nanocatalyst exhibited high catalytic activ‐
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