Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd alloy nanoparticles: Highly active catalysts for methane combustion

Article abstract of DOI:10.1016/j.cattod.2016.05.035

Three-dimensionally ordered macroporous CoCr₂O₄ (3DOM CoCr₂O₄) and its supported Au–Pd alloy (xAuPd_y/3DOM CoCr₂O₄; x = 0.98 and 1.93 wt%; Pd/Au molar ratio (y) = 1.93–1.96) nanocatalysts were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods, respectively. Physicochemical properties of the samples were characterized by means of a number of techniques, and their catalytic activities were evaluated for methane combustion. The 3DOM CoCr₂O₄ and xAuPd_y/3DOM CoCr₂O₄ samples possessed a high-quality 3DOM structure and a surface area of 33–36 m²/g. The Au–Pd alloy nanoparticles (NPs) with an average size of 3.3 nm were uniformly dispersed on the surface of the samples. The 1.93AuPd_1.95/3DOM CoCr₂O₄ sample showed the best catalytic performance: the T_10%, T_50%, and T_90% (temperatures required for achieving methane conversion of 10, 50, and 90%, respectively) were 305, 353, and 394 °C at a space velocity (SV) of 20,000 mL/(g h). The effects of SV, water vapor, and sulfur dioxide on the catalytic activity of the 1.93AuPd_1.95/3DOM CoCr₂O₄ sample were also examined. It is concluded that the excellent catalytic performance of 1.93AuPd_1.95/3DOM CoCr₂O₄ was associated with the higher surface area and adsorbed oxygen species concentration, better low-temperature reducibility, and strong interaction between Au–Pd alloy NPs and 3DOM CoCr₂O₄.

Full text of DOI:10.1016/j.cattod.2016.05.035

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Contents lists available at ScienceDirect
Catalysis Today
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Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd
alloy nanoparticles: Highly active catalysts for methane combustion
Zhiwei Wang, Jiguang Deng∗, Yuxi Liu, Huanggen Yang, Shaohua Xie, Zhixing Wu,
Hongxing Dai∗
Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, Key Laboratory of Advanced
Functional Materials, Education Ministry of China, 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
a r t i c l e i n f o
a b s t r a c t
Article history:
Three-dimensionally ordered macroporous CoCr2O4 (3DOM CoCr2O4) and its supported Au–Pd alloy
(xAuPdy/3DOM CoCr2O4; x = 0.98 and 1.93 wt%; Pd/Au molar ratio (y) = 1.93–1.96) nanocatalysts were
prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction
methods, respectively. Physicochemical properties of the samples were characterized by means of a
number of techniques, and their catalytic activities were evaluated for methane combustion. The 3DOM
CoCr2O4 and xAuPdy/3DOM CoCr2O4 samples possessed a high-quality 3DOM structure and a surface area
of 33–36 m2/g. The Au–Pd alloy nanoparticles (NPs) with an average size of 3.3 nm were uniformly dis-
persed on the surface of the samples. The 1.93AuPd1.95/3DOM CoCr2O4 sample showed the best catalytic
performance: the T10%, T50%, and T90% (temperatures required for achieving methane conversion of 10, 50,
and 90%, respectively) were 305, 353, and 394 ◦C at a space velocity (SV) of 20,000 mL/(g h). The effects
of SV, water vapor, and sulfur dioxide on the catalytic activity of the 1.93AuPd1.95/3DOM CoCr2O4 sam-
ple were also examined. It is concluded that the excellent catalytic performance of 1.93AuPd1.95/3DOM
CoCr2O4 was associated with the higher surface area and adsorbed oxygen species concentration, better
low-temperature reducibility, and strong interaction between Au–Pd alloy NPs and 3DOM CoCr2O4.
© 2016 Elsevier B.V. All rights reserved.
Received 5 January 2016
Received in revised form 11 April 2016
Accepted 11 May 2016
Available online xxx
Keywords:
Three-dimensionally ordered macropore
Au–Pd alloy nanoparticle
Cobalt chromate
Supported noble metal catalyst
Methane combustion
1. Introduction
bustion was due to the formation of PdOx at the top layers above
250 ◦C. Although some noble metals are expensive, one can insert a
As the main component of natural gas, methane is widely used
in the field of chemical industry, transportation, and private fami-
lies. In the traditional applications, however, methane combustion
usually undertakes above 1200 ◦C, and this combustion process
inevitably gives rise to harmful gases, such as NOx and CO, which
can cause serious pollution to the atmosphere environment. Cat-
alytic combustion of methane is an effective pathway to decrease
the emissions of NOx and CO, in which the key issue is the avail-
ability of high-performance catalysts.
Supported noble metal Pd or Pt shows excellent catalytic per-
formance at low temperatures for methane combustion [1]. For
instance, Li et al. [2] reported that Co3O4 nanosheet-supported
5 wt% Pd exhibited a high activity for methane combustion due to
the strong interaction between Pd2+ and Co3O4. Xu et al. [3] pointed
out that the good performance of Pd/ꢀ-Al2O3 for methane com-
less cheap metal to form a bimetal alloy catalyst. Au is an ideal can-
didate. The addition of Au to Pd could greatly enhance the catalytic
activity and stability. In recent years, some researchers observed
that Au–Pd alloys were highly active in the combustion of toluene,
alcohols, and methane [4–7]. For example, Goodman et al. [6] inves-
tigated the promotional effect of Au in the Au–Pd alloy catalyst,
and found that the Au could isolate the Pd site. After studying the
Au–Pd/TiO2 catalysts for the solvent-free oxidation of primary alco-
hols to aldehydes, Hutchings et al. [7] pointed out that the Au could
act as an electronic promoter for Pd.
Transition-metal (e.g., Mn, Co, Cr and Fe) oxides [8–10] are gen-
erally considered as efficient catalyst support because of lower
cost and relative abundant resources. Among the transition-metal
oxide catalysts, Cr-based spinel-type oxides (e.g., CoCr2O4) exhibit
fair catalytic activities in dichloromethane oxidation [11], NOx
removal [12], and methane combustion [13–15]. Li et al. [14]
observed that cobalt-chromium oxide with the cobalt/chromium
atomic ratio of 1: 2 exhibited good performance for methane com-
bustion. However, their cobalt–chromium oxide catalysts were
∗
Corresponding authors.
E-mail addresses: jgdeng@bjut.edu.cn (J. Deng), hxdai@bjut.edu.cn (H. Dai).
http://dx.doi.org/10.1016/j.cattod.2016.05.035
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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ticles: Highly active catalysts for methane combustion, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.035

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irregular in morphology and no presence of pores. In the past
years, three-dimensionally ordered macroporous (3DOM) materi-
als have been intensively studied for the applications in various
fields (such as semiconductoring, photonics, electrochromics, and
catalysis [16–18]), because they have several advantages of higher
surface areas, easy mass transfer, and suitability for dispersion of
active components. To the best of our knowledge, there have so far
been no reports on the preparation and catalytic activities of 3DOM
CoCr2O4-supported Au–Pd alloy nanoparticles (NPs) for methane
combustion in the literature.
were 0.98 and 1.93 wt% in xAuPdy/3DOM CoCr2O4, respectively,
and the corresponding real Pd/Au molar ratios were 1.93 and 1.95.
For comparison, the Bulk CoCr2O4 and xAuPdy/Bulk CoCr2O4
samples were also prepared using the above same methods without
adding the PMMA template. The results of the ICP–AES stud-
ies reveal that the real Au–Pd loading (x) was 1.98 wt% in the
xAuPdy/Bulk CoCr2O4 sample, with the real Pd/Au molar ratio being
1.96.
All chemicals (A.R. in purity) were purchased from Beijing
Chemical Reagent Company and used without further purification.
Previously, our group prepared the 3DOM or three-
dimensionally ordered mesoporous cobalt oxide-supported
gold-palladium alloy nanocatalysts, and investigated their cat-
alytic performance for the oxidation of carbon dioxide, toluene,
and methane [19–21]. It is found that the optimal gold/palladium
atomic ratio was ca. 1: 2. In the present study, we adopted the
polymethyl methacrylate (PMMA)-templating and polyvinyl alco-
hol (PVA)-protected reduction strategies to generate the 3DOM
CoCr2O4 and xAuPdy/3DOM CoCr2O4 nanocatalysts, characterized
their physicochemical properties, and evaluated their catalytic
activities for methane combustion.
2.2. Catalyst characterization
Elemental analyses of the noble metal loadings were 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 diffractometer
with Cu K␣ radiation and nickel filter (ꢁ = 0.15406 nm). Scan-
ning electron microscopic (SEM) images of the samples were
recorded on a Gemini Zeiss Supra 55 apparatus (operating at 10 kV).
Transmission electron microscopic (TEM) images and selected-
area electron diffraction (SAED) patterns of the samples were
obtained using the JEOL-2010 equipment (operating at 200 kV).
BET (Brunauer–Emmett–Teller) surface areas and pore-size distri-
butions 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 measurement.
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 1 s bind-
ing energies (BEs) of surface species using Mg K␣ (hv = 1253.6 eV)
as the excitation source.
2. Experimental
2.1. Catalyst preparation
The well-arrayed PMMA microspheres with an average diam-
eter of ca. 300 nm were synthesized according to procedures
described elsewhere [22].
The 3DOM CoCr2O4 was prepared via the PMMA-templating
Hydrogen temperature-programmed reduction (H2-TPR)
experiments were carried out on a chemical adsorption analyzer
(Autochem II 2920, Micromeritics). Before TPR measurement,
ca. 50 mg of the sample (40–60 mesh) was loaded to a quartz
fixed-bed U-shaped microreactor (i.d. = 4 mm) and pretreated in
an O2 flow of 20 mL/min at 300 ◦C for 1 h. After being cooled at
the same atmosphere to RT, the sample was purged with a helium
flow of 30 mL/min for 15 min. Finally, the pretreated sample was
exposed to a flow (50 mL/min) of 5% H2–95% Ar (v/v) mixture and
heated at a ramp of 10 ◦C/min from RT to 900 ◦C. The alteration
in H2 concentration of the effluent was monitored online by the
chemical adsorption analyzer. The reduction peak was calibrated
against that of the complete reduction of a known standard
powdered CuO (Aldrich, 99.995%).
route. In
a
typical procedure, 6.66 mmol of Co(NO3)2·6H2O
and 13.3 mmol of Cr(NO3)3·9H2O were dissolved in 10 mL
of poly(ethylene glycol) (PEG, MW = 400 g/mol) and methanol
(MeOH) solution (PEG/MeOH volumetric ratio = 1: 9) at room tem-
perature (RT) under stirring for 30 min to obtain a transparent
solution. Then, 2.0 g of the PMMA hard template was soaked in the
above precursor solution for about 30 min. After that, the mixture
was filtered and dried at RT for 48 h. The as-obtained powders were
calcined in a N2 flow of 100 mL/min at a ramp of 1 ◦C/min from RT
to 300 ◦C and maintained at this temperature for 2 h, then cooled to
RT, and calcined in an air flow of 100 mL/min at a ramp of 1 ◦C/min
from RT to 300 ◦C and maintained at this temperature for 2 h, and
finally increased to 500 ◦C and maintained at this temperature for
4 h, thus generating the 3DOM CoCr2O4 sample.
The
3DOM
CoCr2O4-supported
gold-palladium
alloy
2.3. Catalytic evaluation
(xAuPdy/3DOM CoCr2O4) samples were prepared using the
PVA-protected reduction method. The typical preparation pro-
cedure was as follows: 1.8 and 3.6 mL (for x = 0.98 and 1.93 wt%,
respectively) of PVA aqueous solution (2.0 g/L) was added to 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, 2.1 and 4.1 mL
(for x = 0.98 and 1.93 wt%, respectively) of NaBH4 aqueous solution
(2.0 g/L) was quickly injected into the mixed noble metal aqueous
solution, generating a dark-brown sol. Subsequently, 0.3 g of the
3DOM CoCr2O4 support was then added to the dark-brown sol and
vigorously stirred for 6 h. Afterwards, the mixture solution was
filtered and washed with deionized water until there was no Cl−
ions in the supernatant were detected using an AgNO3 aqueous
solution (0.1 mol/L). The obtained wet solid was dried at 80 ◦C
for 10 h, and calcined in a muffle furnace at a ramp of 1 ◦C/min
from RT to 500 ◦C and maintained at this temperature for 4 h,
thus obtaining the xAuPdy/3DOM CoCr2O4 samples. The results of
inductively coupled plasma atomic emission spectroscopic (ICP-
AES) investigations reveal that the real loadings of noble metals
Catalytic activities of the samples were evaluated in a contin-
uous 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
sands (40–60 mesh). Prior to the measurement, the sample was
pretreated in an O2 flow of 20 mL/min at 300 ◦C for 1 h. After
being cooled to a given temperature, the reactant gas containing
methane was passed through the catalyst bed. The reactant mix-
ture gas was 2.5 vol% CH4 + 20 vol% O2 + 77.5 vol% N2 (balance),
and the total flow was 16.7 mL/min, giving a space velocity (SV)
of ca. 20,000 mL/(g h). In the case of water vapor introduction, 5.0
vol% H2O was introduced by passing the feed stream through a
water saturator at a certain temperature. In the case of SO2 intro-
duction, 100-ppm SO2 from a SO2 cylinder (balanced with N2) was
introduced to the reaction system through a mass flow controller.
Reactants and products were analyzed online by gas chromatogra-
phy (GC-14C, Shimadzu) equipped with a flame ionization detector
(FID) and a thermal conductivity detector (TCD), using a stabilwax
column (30 m in length) for methane separation and a Carboxen
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Pd or Au–Pd alloy phases were detected, indicating that the noble
metal NPs were highly dispersed on the surface of the samples. No
detection of noble metal phases might be also due to their lower
loadings.
Fig.
2 shows the SEM images of the porous samples. It
(e)
(d)
can be clearly seen that the 3DOM CoCr2O4 and xAuPdy/3DOM
CoCr2O4 samples displayed a high-quality 3DOM architecture
with a macropore diameter of 120–180 nm. The loading of Au–Pd
NPs did not destroy the macropore structure of CoCr2O4. Fig. 3
shows the TEM images and SAED patterns of the xAuPdy/3DOM
CoCr2O4 and 1.98AuPd1.96/Bulk CoCr2O4 samples. Apparently, the
xAuPdy/3DOM CoCr2O4 samples possessed a high-quality 3DOM
structure that was composed of interconnected macropores with
nanocrystalline skeletons, in agreement with their SEM observa-
tions; however, the 1.98AuPd1.96/Bulk CoCr2O4 sample exhibited a
sub-microsized morphology. The Au–Pd alloy NPs were uniformly
deposited on the surface of the samples. The sizes of Au–Pd alloy
NPs were in the range of 2–6 nm. After statistical analysis on the
sizes of 100 Au–Pd alloy NPs in the TEM images of the samples, the
particle size distributions of noble metal NPs are shown in Fig. 4. The
average sizes of noble metal NPs in 0.98AuPd1.93/3DOM CoCr2O4,
1.93AuPd1.95/3DOM CoCr2O4, and 1.98AuPd1.96/Bulk CoCr2O4 were
3.3, 3.4, and 3.5 nm, respectively. The recording of multiple bright
electron diffraction rings in the SAED patterns (Fig. 3) of the
0.98AuPd1.93/3DOM CoCr2O4, 1.93AuPd1.95/3DOM CoCr2O4, and
1.98AuPd1.96/Bulk CoCr2O4 samples reveals that these materials
were polycrystalline.
Fig. 5 shows the N2 adsorption-desorption isotherms and pore-
size distributions of the samples, and their textural properties are
summarized in Table 1. It can be observed from Fig. 5A that all of
the samples displayed a type II isotherm with a type H3 hysteresis
loop in the relative pressure (p/p0) range of 0.8–1.0, indicating that
these samples possessed a macroporous structure. The pore-size
distribution curves showed that each of the samples possessed a
pore-size distribution in the ranges of 2–8 and 50–180 nm, respec-
tively. The BET surface areas of the porous samples (3DOM CoCr2O4,
0.98AuPd1.93/3DOM CoCr2O4, and 1.93AuPd1.95/3DOM CoCr2O4)
were 33.2–35.6 m2/g, significantly higher than those (7.2–8.8 m2/g)
of the bulk counterparts (Bulk CoCr2O4 and 1.98AuPd1.96/Bulk
CoCr2O4).
(c)
(b)
(a)
10
20
30
40
50
60
70
80
2 Theta (Deg.)
Fig. 1. XRD patterns of (a) 3DOM CoCr2O4, (b) 0.98AuPd1.93/3DOM CoCr2O4, (c)
1.93AuPd1.95/3DOM CoCr2O4, (d) 1.98AuPd1.96/Bulk Co3O4, and (e) Bulk CoCr2O4.
1000 column (3 m in length) for permanent gas detection. The bal-
ance of carbon throughout the catalytic system was estimated to
be 99.5 1.5%. Catalytic activities of the samples were evaluated
using the temperatures (T10%, T50%, and T90%) required for achieving
methane conversions of 10, 50, and 90%, respectively. CH4 conver-
sion was defined as (cinlet–coutlet)/cinlet × 100%, where the cinlet and
coutlet are CH4 concentrations of the inlet and outlet feed stream,
respectively.
3. Results and discussion
3.1. Catalyst composition, crystal structure, morphology, particle
size distribution, and surface area
The results of ICP-AES investigations reveal that the chem-
ical compositions of xAuPdy/3DOM CoCr2O4 and xAuPdy/Bulk
CoCr2O4 samples were 0.98 wt% AuPd1.93/3DOM CoCr2O4, 1.93 wt%
AuPd1.95/3DOM CoCr2O4, and 1.98 wt% AuPd1.96/Bulk CoCr2O4
(denoted as 0.98AuPd1.93/3DOM CoCr2O4, 1.93AuPd1.95/3DOM
CoCr2O4, and 1.98AuPd1.96/Bulk CoCr2O4), respectively.
Fig. 1 shows the XRD patterns of the as-prepared samples. Com-
pared to the XRD pattern of the standard CoCr2O4 sample (JCPDS
PDF # 22–1084), one can realize that the CoCr2O4 in Bulk CoCr2O4,
1.98AuPd1.96/Bulk CoCr2O4 or xAuPdy/3DOM CoCr2O4 was cubic in
crystal structure, and all of the Bragg diffraction peaks could be
well indexed, as indicated in Fig. 1e. The loading of Au–Pd alloys
did not lead to any changes in crystal structure of CoCr2O4. No Au,
3.2. Surface composition, metal oxidation state, and oxygen
species
XPS is an effective technique to investigate the surface element
compositions, metal oxidation states, and adsorbed species of a cat-
alyst. Fig. 6 shows the Co 2p3/2, Cr 2p3/2, O 1s, Au 4f, and Pd 3d
XPS spectra of the samples. The asymmetrical Co 2p3/2 XPS spec-
trum of each sample could be decomposed to two components at
BE = 780.7 and 783.1 eV (Fig. 6A), assignable to the surface Co3+ and
Co2+ species [23], respectively. The presence of surface Co2+ in the
samples was also confirmed by the recording of a satellite signal at
Table 1
BET surface areas, pore diameters, pore volumes, average Au–Pd particle sizes, real Au and Pd contents, and real Pd/Au molar ratios of the as-prepared samples.
Sample
Surface area
(m2/g)
Pore diametera
(nm)
Pore volume
(cm3/g)
Average Au–Pd
particle sizeb
(nm)
Real Au contentc
(wt%)
Real Pd contentc
(wt%)
Real Pd/Au molar
ratioc (mol/mol)
3DOM CoCr2O4
0.98AuPd1.93/3DOM CoCr2O4 35.6
1.93AuPd1.95/3DOM CoCr2O4 34.9
1.98AuPd1.96/Bulk CoCr2O4
Bulk CoCr2O4
33.2
120–180
120–180
120–180
–
0.029
0.031
0.033
−
−
−
−
−
3.2
3.3
3.2
−
0.48
0.94
0.96
−
0.50
0.99
1.02
−
1.93
1.95
1.96
−
8.8
7.2
–
−
a
Data were estimated according to the estimation of macropores in the SEM images of the samples.
Data were estimated according to the estimation of noble metal particles in the TEM images of the samples.
Data were determined by the ICP–AES technique.
b
c
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Fig. 2. SEM images of (a, b) 3DOM CoCr2O4, (c, d) 0.98AuPd1.85/3DOM CoCr2O4, and (e, f) 1.93AuPd1.95/3DOM CoCr2O4.
Fig. 3. TEM images and SAED patterns (insets) of (a, b) 0.98AuPd1.85/3DOM CoCr2O4, (c, d) 1.93AuPd1.95/3DOM CoCr2O4, and (e, f) 1.98AuPd1.96/Bulk CoCr2O4.
BE = 787.2 eV. The surface Co3+/Co2+ molar ratios of the samples are
summarized in Table 2. The surface Co3+/Co2+ molar ratio signifi-
cantly decreased from 2.95 to 1.92–2.24 after the loading of Au–Pd
alloy NPs. In other words, the 3DOM CoCr2O4 in the supported
Au–Pd alloy samples would possess higher oxygen vacancy concen-
trations in the surface vicinity of 3DOM CoCr2O4 [24]. The Cr 2p3/2
XPS spectrum of each sample could be decomposed into three com-
ponents at BE = 575.4, 576.5, and 578.2 eV (Fig. 6B), attributable to
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Fig. 4. Au Pd particle size distributions of (A) 0.98AuPd1.93/3DOM CoCr2O4, (B) 1.93AuPd1.95/3DOM CoCr2O4, and (C) 1.98AuPd1.96/Bulk CoCr2O4.
500
400
300
200
100
0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(A)
(B)
(c)
(c)
(b)
(a)
(b)
(a)
0.0
0.2
0.4
0.6
0.8
1.0
0
50
100
150
200
Relative pressure
/
p p 0
Pore diameter (nm)
Fig. 5. (A) N2 adsorption desorption isotherms and (B) pore-size distributions of (a) 3DOM CoCr2O4, (b) 0.98AuPd1.93/3DOM CoCr2O4 and (c) 1.93AuPd1.95/3DOM CoCr2O4.
Table 2
Surface element compositions and H2 consumption of the samples.
Sample
Co3+/Co2+ molar
ratio (mol/mol)
Cr6+/Cr3+ molar
ratio (mol/mol)
O
ads/Olatt molar
Auı+/Au0 molar
ratio (mol/mol)
Pd2+/Pd0 molar
ratio (mol/mol)
H2 consumption
below 400 ◦Ca
(mmol/g)
ratio (mol/mol)
3DOM CoCr2O4
2.95
2.20
1.92
2.24
−
0.54
0.38
0.39
0.39
−
0.53
0.55
0.62
0.60
−
−
−
0.25
0.40
0.66
0.17
0.34
0.98AuPd1.93/3DOM CoCr2O4
1.93AuPd1.95/3DOM CoCr2O4
1.98AuPd1.96/Bulk CoCr2O4
Bulk CoCr2O4
0.09
0.17
0.12
−
0.11
0.23
0.20
−
a
Data were estimated by quantitatively analyzing the reduction peaks in the H2-TPR profiles.
the surface Cr(OH)3 or Cr2O3 [25], Cr3+ (occupied octahedral sites)
[11,14,26], and Cr6+ [14,27,28] species, respectively. The Cr6+/Cr3+
molar ratio decreased from 0.54 to 0.38–0.39 (Table 2) when the
Au–Pd alloy NPs were loaded on the surface of 3DOM CoCr2O4 or
Bulk CoCr2O4.
The components at BE = 83.5 and 87.2 eV were assigned to the sur-
face metallic gold (Au0) species, whereas the ones at BE = 84.5 and
88.3 eV were attributed to the surface oxidized gold (Auı+) species
[31,32]. As shown in Fig. 6E, the Pd 3d spectrum of each sample
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) species, while the
ones at BE = 337.9 and 343.2 eV were due to the surface oxidized
palladium (Pd2+) species [33]. Apparently, the Au␦+/Au0 or Pd2+/Pd0
molar ratio increased with the loading of Au–Pd alloy NPs, a result
possibly due to the strong interaction between noble metal NPs and
As shown in Fig. 6C, the asymmetric O 1 s XPS signal could
be deconvoluted to three components at BE = 530.1, 531.6, and
532.9 eV, ascribable to the surface lattice oxygen (Olatt), adsorbed
oxygen (Oads, e.g., O2−, O2
or O−), and carbonate or adsorbed
2−
water species [29,30], respectively. It has been known that the
surface adsorbed oxygen species play an important role in deep
oxidation of organics [29]. With the loading of Au–Pd alloy NPs, the
surface Oads/Olatt molar ratio increased and the 1.93AuPd1.95/3DOM
CoCr2O4 sample possessed the highest Oads/Olatt molar ratio (0.62).
The Au 4f XPS spectrum of each sample could be decomposed
into four components at BE = 83.5, 84.5, 87.2, and 88.3 eV (Fig. 6D).
cobalt–chromium oxide (Au0 or Pd0 + Co3+ → Au␦+ or Pd2+ + Co2+
;
Au0 or Pd0 + Cr6+ → Au␦+ or Pd2+ + Cr3+) [31,34], which coincides
with the drop in surface Co3+/Co2+ or Cr6+/Cr3+ molar ratio of the
samples.
Please cite this article in press as: Z. Wang, et al., Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd alloy nanopar-
ticles: Highly active catalysts for methane combustion, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.035

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(C) O 1s
530.1 eV
531.6 eV
780.7 eV
(A) Co 2p3/2
(B) Cr 2p3/2
576.5 eV
575.4 eV
783.1 eV
532.9 eV
578.2 eV
787.2 eV
(d)
(d)
(d)
(c)
(c)
(b)
(a)
(c)
(b)
(b)
(a)
(a)
775
779
783
787
791
527
529
531
533
535
572
574
576
578
580
582
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
(D) Au 4f
83.5 eV
335.5 eV
337.9 eV
(E) Pd 3d
340.9 eV
87.2 eV
88.3 eV
84.5 eV
(d)
343.2 eV
(d)
(c)
(b)
(c)
(b)
80
82
84
86
88
90
332
336
340
344
Binding energy (eV)
Binding energy (eV)
Fig. 6. (A) Co 2p3/2, (B) Cr 2p3/2, (C) O 1s, (D) Au 4f, and (E) Pd 3d XPS spectra of (a) 3DOM CoCr2O4, (b) 0.98AuPd1.93/3DOM CoCr2O4, (c) 1.93AuPd1.95/3DOM CoCr2O4, and
(d) 1.98AuPd1.96/Bulk CoCr2O4.
3.3. Reducibility
reducibility of a sample [38], and the results are shown in
Fig. 7B. Obviously, the low-temperature reducibility decreased in
the order of 1.93AuPd1.95/3DOM CoCr2O4 > 0.98AuPd1.93/3DOM
CoCr2O4 > 3DOM CoCr2O4 > 1.98AuPd1.96/Bulk CoCr2O4 > Bulk
CoCr2O4, which was basically in agreement with the sequence in
catalytic activity (shown below).
H2-TPR experiments were performed to investigate the
reducibility of the catalysts, and the results are illustrated in Fig. 7A.
For the 3DOM CoCr2O4 sample, there were three reduction peaks
at 282, 467, and 723 ◦C. According to the literature, the reduction
peak centered at 723 ◦C was assigned to the reduction of Co2+ to
the metallic cobalt [35]. The reduction peaks at 282 and 467 ◦C
were attributed to the reduction of Cr6+ to Cr3+ at the surface
and bulk of CoCr2O4 [36,37], respectively. Comparing to the Bulk
CoCr2O4, the 3DOM CoCr2O4 sample was more reducible at lower
temperatures. After loading the Au–Pd NPs on the 3DOM CoCr2O4
surface, the reduction peaks of the xAuPdy/3DOM CoCr2O4 and
1.98AuPd1.96/Bulk CoCr2O4 samples shifted to lower temperatures.
The first reduction peak was assignable to the reduction of cationic
Au or Pd and surface adsorbed oxygen species.
By quantitatively analyzing the reduction peaks in the H2-TPR
profiles, one can obtain the H2 consumption of the samples, as
summarized in Table 2. Apparently, the H2 consumptions of the
xAuPdy/3DOM CoCr2O4 and 1.98AuPd1.96/Bulk CoCr2O4 samples
were higher than those of the 3DOM CoCr2O4 and Bulk CoCr2O4
supports. Generally speaking, the initial (where less than 25%
oxygen in the sample is consumed for the first reduction peak) H2
consumption rate can be used to evaluate the low-temperature
3.4. Catalytic performance
Catalytic activities of the as-prepared samples were evalu-
ated for methane combustion, and the results are shown in
Fig. 8A. It is observed that the 3DOM CoCr2O4 and xAuPdy/3DOM
CoCr2O4 samples performed much better than the Bulk CoCr2O4
and 1.98AuPd1.96/Bulk CoCr2O4 samples, respectively. Compared to
the bulk sample, the 3DOM-structured catalysts displayed porous
structures and much higher surface areas. Generally speaking, com-
bustion of methane takes place on the surface of a catalyst. A
porous structure is beneficial for the dispersion of active compo-
nents as well as the adsorption and diffusion of reactants, whereas
a higher surface area can favor the accessibility of reactants to
the surface active sites [18]. Therefore, it is understandable that
the 3DOM-structured catalysts outperformed the bulk counterpart.
Furthermore, the loading of Au–Pd alloy NPs were beneficial for
the enhancement in catalytic performance of the sample. To better
Please cite this article in press as: Z. Wang, et al., Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd alloy nanopar-
ticles: Highly active catalysts for methane combustion, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.035

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7
8
285 o
C
(A)
(e)
(B)
654 o
C
6
4
2
0
(c)
130 o
C
692 o
C
(a)
521 o
C
C
72 o
C
(e)
856 o
C
112 o
C
637 o
C
(d)
252 o
259 o
C
465 o
(c)
C
615 o
C
119 o
C
485 o
C
(b)
(d)
(b)
282 o
C
723 o
C
467 o
C
(a)
1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3
30
230
430
630
830
Temperature (oC)
1000/ (K-1)
T
Fig. 7. (A) H2-TPR profiles and (B) initial H2 consumption rate as a function of inverse temperature of (a) 3DOM CoCr2O4, (b) 0.98AuPd1.93/3DOM CoCr2O4, (c)
1.93AuPd1.95/3DOM CoCr2O4, (d) 1.98AuPd1.96/Bulk CoCr2O4, and (e) Bulk CoCr2O4.
Table 3
Catalytic activities, TOF values, and specific reaction rates of the samples at SV = 20,000 mL/(g h).
Sample
Methane combustion activity
Methane combustion at 320 ◦
C
T10% (◦C)
T50% (◦C)
T90% (◦C)
TOFNoblemetal
TOFCoCr2O4
Specific reaction rate
(␮mol/(gcat s))
Specific reaction rate
(␮mol/(gNoblemetal s))
(×10−3 s−1
)
(×10−3 s−1
)
3DOM CoCr2O4
320
310
305
325
335
370
362
353
385
400
440
410
394
467
490
–
0.168
0.213
0.271
0.123
0.0936
0.744
0.931
1.15
0.541
0.413
–
0.98AuPd1.93/3DOM CoCr2O4
1.93AuPd1.95/3DOM CoCr2O4
1.98AuPd1.96/Bulk CoCr2O4
Bulk CoCr2O4
13.0
8.23
3.75
–
95.0
60.0
27.4
–
compare the activities of all of the samples, we use the reaction
temperatures (T10%, T50%, and T90%) at methane conversion = 10, 50,
and 90%, respectively, as summarized in Table 3. Among these sam-
ples, the 1.93AuPd1.95/3DOM CoCr2O4 sample performed the best:
T10% = 305 ◦C, T50% = 353 ◦C, and T90% = 394 ◦C at SV = 20,000 mL/(g h).
Obviously, the T90% over 1.93AuPd1.95/3DOM CoCr2O4 was lower
by 16, 46, 73, and 96 ◦C than those over 0.98AuPd1.93/3DOM
CoCr2O4, 3DOM CoCr2O4, 1.98AuPd1.96/Bulk CoCr2O4, and Bulk
CoCr2O4, respectively. Compared to 0.98AuPd1.93/3DOM CoCr2O4
or 1.98AuPd1.96/Bulk CoCr2O4, 1.93AuPd1.95/3DOM CoCr2O4 pos-
sessed higher surface Oads/Olatt, Auı+/Au0, and Pd2+/Pd0 molar
ratios (Table 2). It is well known that a larger amount of surface
Fig. 8. (A) Methane conversion and (B) specific reaction rate as a function of reaction temperature over the as-prepared samples at SV = 20,000 mL/(g h).
Please cite this article in press as: Z. Wang, et al., Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd alloy nanopar-
ticles: Highly active catalysts for methane combustion, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.035

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adsorbed oxygen species benefits the adsorption and activation of
reactants [29], and higher surface Au␦+/Au0 and Pd2+/Pd0 molar
ratios could improve the low-temperature reducibility (Fig. 7).
Therefore, we believe that the better catalytic performance of
1.93AuPd1.95/3DOM CoCr2O4 might be associated with its surface
element compositions and low-temperature reducibility.
As we know, there are several kinds of active sites (e.g., noble
metal, transition-metal mixed oxide, and interface between noble
metal NPs and transition-metal mixed oxide) in the reducible
mixed metal oxide-supported noble metal catalysts. Thus, it is
hard to calculate the turnover frequencies (TOFs). The TOFM
(TOFM = zC0/nM, where z is the conversion at a certain tempera-
ture, C0 (mol/s) is the initial methane concentration per second,
and nM (mol) is the molar amount of metal or mixed oxide
(M = Au–Pd or CoCr2O4)) and specific reaction rates normalized
per gram of catalyst or noble metals at a typical temperature
(e.g., 320 ◦C) were calculated according to the activity data and
amounts of Au–Pd and CoCr2O4 in the samples, as summarized in
introduced to the feedstock, methane conversion decreased from
ca. 76–69%; when water vapor was cut off, methane conversion
restored to the original value. The deactivation of this sample in the
presence of water vapor might be due to the competitive adsorp-
tion of water and methane molecules on the surface of the sample.
Therefore, introduction of moisture exerted a negative effect on
methane combustion, and the deactivation due to water vapor
addition was reversible. Similar phenomena have been reported
by other researchers [4,19–21].
Fig. 12. shows the effect of SO2 on methane combustion over
1.93AuPd1.95/3DOM CoCr2O4 at 394 ◦C and SV = 20,000 mL/(g h) in
the presence of 100 ppm SO2. It can be seen from Fig. 12. that
methane conversion slowly decreased from 90 to 80% after 20 h
of on-stream reaction. After the used sample was activated in O2 at
500 ◦C for 1 h, methane conversion slightly increased to ca. 85%. This
result demonstrates that deactivation of the 1.93AuPd1.95/3DOM
CoCr2O4 sample induced by SO2 addition was irreversible. Wu
et al. [20] reported that the SO2 introduced to the supported pal-
Table 3. It is seen that the highest TOFNoblemetal (13.0 × 10−3 s−1
)
was achieved over the 0.98AuPd1.93/3DOM CoCr2O4 sample (pos-
sible due to the lower loading of noble metals), whereas the
100
highest TOFCoCr2O4 (0.271 × 10−3 s−1
) was obtained over the
1.93AuPd1.95/3DOM CoCr2O4 sample. According to the XPS and
H2-TPR results (Figs. 6 and 7 and Table 2), we deduce that there
was a synergistic effect between Au–Pd NPs and 3DOM CoCr2O4.
As shown in Table 3, the TOFs normalized by the amount of
noble metal were larger than the TOFs normalized by the amount
of CoCr2O4, indicating that noble metal NPs were more active
than CoCr2O4. On the other hand, the TOFs normalized by the
amount of noble metal were quite different for the Au–Pd sup-
ported catalysts, suggesting that CH4 oxidation might not occur
predominantly on the noble metal sites. Hence, we believe that the
synergistic effect between Au–Pd NPs and 3DOM CoCr2O4 played
an important role in the oxidation of CH4. Fig. 8B shows the spe-
cific reaction rates normalized per gram of catalyst. The changing
trends in methane reaction rate versus temperature were simi-
lar to those in methane conversion versus temperature, and the
1.93AuPd1.95/3DOM CoCr2O4 sample exhibited the highest specific
reaction rate (1.15 ␮mol/(gcat s)) at 320 ◦C, although the high-
est specific reaction rate (95.0 ␮mol/(gNobl metal s)) at 320 ◦C was
achieved over the 0.98AuPd1.93/3DOM CoCr2O4 sample.
80
60
40
0
10
20
30
40
On-stream reaction time (h)
Fig. 9. Methane conversion as a function of on-stream reaction time over the
1.93AuPd1.95/3DOM CoCr2O4 sample at 394 ◦C and SV = 20,000 mL/(g h).
In order to examine the catalytic stability, we carried
out the 40-h on-stream methane oxidation over the best-
performing 1.93AuPd1.95/3DOM CoCr2O4 sample at 394 ◦C and
SV = 20,000 mL/(g h), and the result is shown in Fig. 9. No sig-
nificant loss in catalytic activity was observed. In other words,
the 1.93AuPd1.95/3DOM CoCr2O4 sample was catalytically durable
under the adopted conditions.
In the past years, methane combustion over various supported
noble metal catalysts has been reported. It is worth point-
ing out that xAuPdy/3DOM CoCr2O4 were superior in catalytic
methane combustion to 1 wt% Pd/ZrO2 [39], 4.5 wt% Ru/ZnAl2O4
[40], 3 wt% Ce30Cr-500/Al2O3 [41], Pd–PdOx–Pt/Al2O3 [42], and
La0.6Sr0.4MnO3 [43].
3.5. Effects of SV, water vapor, and sulfur dioxide
The effect of SV on methane combustion over the
1.93AuPd1.95/3DOM CoCr2O4 sample is shown in Fig. 10. It is
observed that methane conversion decreased with the rise in SV, a
result due to the shortening of contact time.
On-stream methane combustion over the 1.93AuPd1.95/3DOM
CoCr2O4 sample in the presence and absence of 5.0 vol% water
vapor at 378 ◦C and SV = 20,000 mL/(g h) was conducted to exam-
ine the effect of moisture on catalytic activity, and the result is
shown in Fig. 11. It is found that when 5.0 vol% water vapor was
Fig. 10. Effect of SV on methane combustion over the 1.93AuPd1.95/3DOM CoCr2O4
sample.
Please cite this article in press as: Z. Wang, et al., Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd alloy nanopar-
ticles: Highly active catalysts for methane combustion, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.035

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9
100
80
low-temperature reducibility and the highest adsorbed oxygen
species concentration, rendering it to show the best catalytic
performance (T50% = 353 ◦C and T90% = 394 ◦C) for methane combus-
tion at SV = 20,000 mL/(g h). The addition of moisture and sulfur
dioxide to the reaction system gave rise to deactivation of the
1.93AuPd1.95/3DOM CoCr2O4 sample, the deactivation induced by
water vapor addition was reversible but SO2 introduction caused
irreversible deactivation. It is concluded that the excellent catalytic
performance of 1.93AuPd1.95/3DOM CoCr2O4 was associated with
its high surface area and Oads concentration, good low-temperature
reducibility, and strong interaction between Au–Pd alloy NPs and
3DOM CoCr2O4.
H2O-free
60
H2O-free
5.0 vol% H2O
H2O on
H2O off
40
20
0
Acknowledgments
This work was supported by the NSF of China (21377008
and 21477005), National High Technology Research and Develop-
ment Program of China (2015AA034603), Beijing Nova Program
(Z141109001814106), NSF of Beijing Municipal Commission
of Education (KM201410005008), Foundation on the Creative
Research Team Construction Promotion Project of Beijing Munici-
pal Institutions, and Scientific Research Base Construction–Science
and Technology Creation Platform National Materials Research
Base Construction.
0
5
10
15
20
On-stream reaction time (h)
Fig. 11. Effect of water vapor on methane conversion over the 1.93AuPd1.95/3DOM
CoCr2O4 sample at 378 ◦C and SV = 20,000 mL/(g h).
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or SO4
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Please cite this article in press as: Z. Wang, et al., Three-dimensionally ordered macroporous CoCr2O4-supported Au–Pd alloy nanopar-
ticles: Highly active catalysts for methane combustion, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.05.035