Preparation, characterization, and catalytic performance of PdPt/3DOM LaMnAl11O19 for the combustion of methane

Article abstract of DOI:10.1016/j.apcata.2018.05.022

Three-dimensionally ordered macroporus LaMnAl₁₁O₁₉ (3DOM LMAO) and its supported Pd, Pd–Pt, and Pt nanoparticles were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods, respectively. The 1100 °C-calcined 3DOM LMAO support possessed a hexaaluminate phase, and its supported noble metal samples displayed a high surface area of 26–29 m²/g. A Pd–Pt alloy was generated in the PdPt/3DOM LMAO samples, and the particle sizes of the noble metal nanoparticles (NPs) were 3–5 nm. The 0.97 wt% Pd/3DOM LMAO (0.97 P d/3DOM LMAO) sample possessed the highest surface adsorbed oxygen concentration and the best low-temperature reducibility, showing the highest catalytic activity (T_10% = 259 °C, T_50% = 308 °C, and T_90% = 343 °C at SV = 20,000 mL/(g h)) for methane combustion. The 1.14 wt% Pd_2.8Pt/3DOM LMAO (1.14Pd_2.8Pt/3DOM LMAO) sample performed the best (T_10% = 284 °C, T_50% = 372 °C, and T_90% = 456 °C at SV = 20,000 mL/(g h)) among the PdPt/3DOM LMAO samples. Doping of Pt to the Pd-based catalyst could improve the H₂O-, CO₂-, and SO₂-resistant ability without significant influence on thermal stability, although 1.14Pd_2.8Pt/3DOM LMAO was less active than 0.97 P d/3DOM LMAO for methane combustion. The ex situ X-ray photoelectron spectroscopy was used to explore the formation of active PdO species during the oxidation processes of Pd in 1.14Pd_2.8Pt/3DOM LMAO and 0.97Pd/3DOM LaMnAl₁₁O₁₉ at different temperatures.

Full text of DOI:10.1016/j.apcata.2018.05.022

Accepted Manuscript
Title: Preparation, characterization, and catalytic performance
of PdPt/3DOM LaMnAl O for the combustion of methane
1
1
19
Authors: Peng Xu, Xing Zhang, Xingtian Zhao, Jun Yang,
Zhiquan Hou, Lu Bai, Huaiqiu Chang, Yuxi Liu, Jiguang
Deng, Guangsheng Guo, Hongxing Dai, Chak-Tong Au
PII:
S0926-860X(18)30244-8
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.05.022
APCATA 16671
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
6-3-2018
30-4-2018
21-5-2018
Please cite this article as: Xu P, Zhang X, Zhao X, Yang J, Hou Z, Bai L, Chang H,
Liu Y, Deng J, Guo G, Dai H, Au C-Tong, Preparation, characterization, and catalytic
performance of PdPt/3DOM LaMnAl O for the combustion of methane, Applied
1
1
19
Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.05.022
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Preparation, characterization, and catalytic performance of PdPt/3DOM
LaMnAl11O19 for the combustion of methane
a,b
a
a
a
a
b
b
Peng Xu , Xing Zhang , Xingtian Zhao , Jun Yang , Zhiquan Hou , Lu Bai , Huaiqiu Chang ,
a
a
a,
a,
c,
Yuxi Liu , Jiguang Deng , Guangsheng Guo *, Hongxing Dai *, Chak-Tong Au *
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
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National
Center for Nanoscience and Technology, Beijing 100190, China
c
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong
*
To whom correspondence should be addressed:
Profs. Guangsheng Guo and Hongxing Dai
Tel. No.: +8610-6739-6118; fax: +8610-6739-1983
E-mail addresses: guogs@bjut.edu.cn (G.S. Guo); hxdai@bjut.edu.cn (H.X. Dai)
Prof. Chak-Tong Au
Tel. No.: +852-3411-7810
E-mail address: pctau@hkbu.edu.hk
1

Graphical Abstract:
PdPt/3DOM LaMnAl O
1
1
19
CH4
CO2
H O
2
O2
Research Highlights
►
3DOM LaMnAl11
PdPt/3DOM LMAO is prepared using the polyvinyl alcohol-protected reduction method.
0.97Pd/3DOM LMAO outperforms 1.14Pd2.8Pt/3DOM LMAO for methane combustion.
1.14Pd2.8Pt/3DOM LMAO shows stronger H O-, CO -, and SO -resistant ability.
The ex situ XPS technique is used to explore the formation of active PdO species.
O19 (3DOM LMAO) is fabricated using the PMMA-templating method.
►
►
►
2
2
2
►
2

Abstract:
Three-dimensionally ordered macroporus LaMnAl11O19 (3DOM LMAO) and its supported
Pd, PdPt, and Pt nanoparticles were prepared using the polymethyl methacrylate-templating
o
and polyvinyl alcohol-protected reduction methods, respectively. The 1100 C-calcined
3
DOM LMAO support possessed a hexaaluminate phase, and its supported noble metal
2
samples displayed a high surface area of 2629 m /g. A PdPt alloy was generated in the
PdPt/3DOM LMAO samples, and the particle sizes of the noble metal nanoparticles (NPs)
were 35 nm. The 0.97 wt% Pd/3DOM LMAO (0.97Pd/3DOM LMAO) sample possessed the
highest surface adsorbed oxygen concentration and the best low-temperature reducibility,
o
o
o
showing the highest catalytic activity (T10% = 259 C, T50% = 308 C, and T90% = 343 C at SV
=
20,000 mL/(g h)) for methane combustion. The 1.14 wt% Pd2.8Pt/3DOM LMAO
o
o
(
1.14Pd2.8Pt/3DOM LMAO) sample performed the best (T10% = 284 C, T50% = 372 C, and
o
T
90% = 456 C at SV = 20,000 mL/(g h)) among the PdPt/3DOM LMAO samples. Doping of
Pt to the Pd-based catalyst could improve the H
2
O-, CO
2
-, and SO -resistant ability without
2
significant influence on thermal stability, although 1.14Pd2.8Pt/3DOM LMAO was less active
than 0.97Pd/3DOM LMAO for methane combustion. The ex situ X-ray photoelectron
spectroscopy was used to explore the formation of active PdO species during the oxidation
processes of Pd in 1.14Pd2.8Pt/3DOM LMAO and 0.97Pd/3DOM LaMnAl11
temperatures.
O19 at different
Keywords: Three-dimensionally ordered macropore; Bimetallic nanoparticle; Supported
noble metal catalyst; Methane combustion; Ex situ XPS.
3

1. Introduction
As the main component of natural gas, methane is an important clean energy which
contains low levels of nitrogen and sulfur contaminants [13]. Methane attracts much interest
because of its abundant reserves. The highly stable property of methane, however, makes it
difficult to be converted directly. On one hand, methane combustion needs high temperatures
o
up to 1400 C to realize the complete combustion, in which the nitrogen in air and the trace
amounts of nitrogen and sulfur in natural gas can be oxidized into noxious NO
On the other hand, the uncompleted combustion causes the release of unburned methane,
which can induce a green house effect 20 times higher than CO [5]. Catalytic combustion is a
promising technology to lower the temperatures of methane combustion, which can reduce
NO formation. One of the most important issues is to develop a high-efficiency and stable
catalyst that can guarantee the complete combustion of methane at low temperatures.
Hexaaluminates (AAl12 19, in which A is an alkali, alkaline earth or rare earth element) are
x
and SO
x
[4].
2
x
O
regarded as an excellent catalyst candidate for the combustion of methane [6,7]. Such
o
materials are usually obtained after calcination at above 1200 C and thus possess good
thermal stability. When the Al ions are partially substituted by transition metal (e.g., Mn, Fe,
Co or Cr) ions, the resulting hexaaluminates are active for methane combustion. Among the
substituted materials, LaMnAl11
O19 shows the best catalytic activity for methane combustion.
Catalytic performance of LaMnAl11
O19 can be further enhanced by tuning its morphology and
porous structure or loading a certain amount of noble metal(s). Due to high surface areas that
facilitate good dispersion of active components (e.g., nanosized transition metal oxides and
noble metal nanoparticles (NPs)), three-dimensionally ordered macroporous (3DOM)
4

supports are applied in a number of catalytic systems previously reported by our group
810], and most of their supported catalysts showed enhanced performance for the
[
combustion of organics.
Among the supported noble metal catalysts, the supported Pd catalysts exhibit the best
activity in the oxidation of methane [1113], but supported Pd-only catalysts could not
tolerate the exposure of trace amounts of SO
2
or CO for a long time. To overcome this
2
problem, our strategy is to dope a certain amount of the second noble metal into the
palladium, hence forming a bimetallic catalyst. Platinum has been shown to be active in
methane combustion at low temperatures and the doping of Pt could prevent the growth of Pd
and PdO particles [15,16]. It has been reported that supported bimetallic catalysts could
improve the stability and poison-resistant ability, as compared with supported Pd-only
catalysts [17]. For example, the temperature at a methane conversion of 10% was as low as
o
2
28 C over PdPt bimetallic NPs supported on TiO
2
in the cyclo-olefins reaction [18];
o
PdPt/ceria-stabilized ZrO
2
was the most active in the range of 620800 C [19]. It has been
also observed that addition of a certain amount of Pt could increase the activity of Pd with
improved ability resistant to steam and sulfur dioxide because Pt did not adsorb SO [1921].
2
Therefore, the sulfur dioxide-resistant ability of Pd-based catalysts can be enhanced by
doping Pt.
The re-organization behaviors of supported bimetallic catalysts have been studied using the
extended X-ray absorption fine structure [22], X-ray diffraction, in situ infrared spectroscopy
[23], and other techniques [24]. Deep understanding on the re-organization phenomenon of
bimetallic catalytic systems, however, is not enough. The peak position shifts in temperature
5

programmed desorption (O -TPD) profiles are considered as a proof of interactions between
2
two metals. Catalytic activity and stability of bimetallic catalyst systems can be tested
directly, but how Pt restrains the formation process of PdO at low temperatures is still
interesting. Furthermore, the distribution ratio of PdO to Pd species on the surface of catalyst
with an increased temperature is also unknown. In order to solve these problems, we build up
an ex situ X-ray photoelectron spectroscopic (XPS) system, which can process the catalyst in
different gases and temperatures, and finally gain the most fresh surface XPS information
without exposure to the air.
In this work, we prepared PdPt NPs supported on 3DOM LaMnAl11O19, characterized
their physicochemical properties, evaluated their catalytic activities for methane combustion,
and further explored the poison-resistant ability of typical catalysts. We also used the ex situ
XPS technique to explore the formation of active PdO species during the oxidation processes
of Pd in PdPt/3DOM LaMnAl11O19 and in Pd/3DOM LaMnAl11O19 at different temperatures.
2. Experimental
2
.1. Catalyst preparation
The 3DOM LaMnAl11
O
19 support with an average pore diameter of ca. 116 nm was
prepared according to the procedures described elsewhere [8,9]. The PdPt NPs were
synthesized using the polyvinyl alcohol (PVA, MWaver. = 10,000 g/mol)-protected reduction
method with NaBH
catalyst was 1.00 wt%. The Pd/Pt molar ratios in the catalysts were 1.0 : 0, 3.0 : 1.0, 1.0 : 1.0,
.0 : 3.0, and 0 : 1.0, respectively. The typical fabrication procedures are as follows: (i) A
4
as the reducing agent. The total loading of the noble metal(s) in each
1
desired amount of PVA (noble metal/PVA mass ratio = 1.0 : 1.5) was added into the PdCl
2
6

aqueous solution in an ice bath under fiercely stirring for 1 h; (ii) the desired amount of
PtCl aqueous solution (0.010 mol/L) was added into the above mixed aqueous solution
under continuously stirring for 1 h; and (iii) a desired amount of NaBH aqueous solution
2.00 g/L) was rapidly injected to form a brown suspension, thus obtaining the uniform Pd,
H
2
6
4
(
PdPt or Pt NPs.
The 3DOM LaMnAl11
O
19-supported PdPt catalysts were prepared using the gas-bubble-
19 was added into the
Pd, PdPt or Pt NPs suspension with a theoretical noble metal loading of 1.00 wt%. The
assistant adsorption method. The desired amount of 3DOM LaMnAl11
O
above noble metal sol was subjected to ultrasonic (60 kHz) treatment for 2 min. After N
2
bubbling (100 mL/min) for 4 h, the Pd, PdPt or Pt NPs could be completely adsorbed on the
surface of 3DOM LaMnAl11
O19. All of the solids were filtered, washed with 2.00 L of
o
o
deionized water, dried at 80 C for 12 h, and calcined in air at a ramp of 2 C/min from room
o
temperature (RT) to 550 C and kept at this temperature for 2 h, thus obtaining the
PdPt/3DOM LaMnAl11
using the inductively coupled plasmaatomic emission spectroscopy (ICPAES), and the
results reveal that the actual catalyst compositions were 0.97 wt% Pd/3DOM LaMnAl11
.14 wt% Pd2.8Pt /3DOM LaMnAl11 19, 1.12 wt% Pd Pt /3DOMLaMnAl11 19, 1.06 wt%
Pd Pt2.6/3DOM LaMnAl11 19, and 0.94 wt% Pt/3DOM LaMnAl11 19, which were denoted as
O19 samples. The exact contents of noble metal(s) were investigated
19
O ,
1
1
O
1
1
O
1
O
O
0
1
.97Pd/3DOM LMAO, 1.14Pd2.8Pt/3DOM LMAO, 1.12PdPt/3DOM LMAO,
.06PdPt2.6/3DOM LMAO, and 0.94Pt/3DOM LMAO, respectively. For the purpose of
comparison, we also prepared the 0.91 wt% Pd/Al
2
O
3
(denoted as 0.91Pd/Al
2
3
O ) sample
using the same method.
7

2
.2. Catalyst characterization
Noble metal loadings in the catalysts were investigated using the ICPAES technique on
the thermo elemental IRIS intrepid ER/S apparatus. Crystal phases of the samples were
obtained on a Bruker/AXS D8 Advance diffractometer with Cu K radiation. Surface areas
and pore parameters of the samples were measured via nitrogen adsorption at the liquid N
2
temperature on a Micromeretics ASAP 2020 apparatus. Scanning electron microscopic (SEM)
and transmission electron microscopic (TEM) images as well as selected-area electron
diffraction (SAED) patterns were recorded on the Hitachi SU8220 and FEI F20 instruments,
respectively. The surface elemental compositions and chemical states of the samples were
gained using the XPS technique on a Thermo Fisher Scientific ESCALAB 250 Xi
spectrometer, with the monochromatic Al K X-ray source (hv = 1486.6 eV). All of the
binding energies (BEs) of the elements in the samples were calibrated by using the C 1s
binding energy (284.6 eV) of adventitious carbon. Another separated vacuum chamber was
built up on the instrument with a temperature-variable sample holder and double gas path
system, which could realize the ex situ characterization of the catalysts. The samples were
handled in an O2 flow of 17 mL/min for the heating and cooling process.
Hydrogen temperature-programmed reduction (H -TPR) experiments were carried out on a
2
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
o
microreactor (i.d. = 4 mm) and pretreated in an O
2
flow of 30 mL/min at 300 C for 1 h. After
being cooled to RT at the same atmosphere, the pretreated sample was exposed to a flow (50
o
mL/min) of 5% H
2
95% Ar (v/v) mixture and heated at a ramp of 10 C/min from RT to 900
8

oC. The alteration in H
concentration of the effluent was monitored online by the chemical
2
adsorption analyzer. The reduction peak was calibrated against that of the complete reduction
of a known standard powdered CuO (Aldrich, 99.995%).
In situ diffuse reflectance Fourier transform infrared (DRIFT) spectra of the samples in the
−1
1
900–2300 cm range were recorded using a Nicolet 6700 spectrometer (32 scans, MCT
−1
detector) with a resolution of 4 cm at 25 °C. The operation conditions were as follows: (i)
o
the sample was purged with a N
2
flow (30 mL/min) at 250 C for 1 h to remove the surface
adsorbed species and cooled to RT in the same atmosphere, (ii) CO was then adsorbed on the
sample in a (10 vol% CO + 90 vol% N ) flow (30 mL/min) at RT for 1 h to reach saturation
adsorption, (iii) the sample was finally purged with a N flow (30 mL/min) for 1 h to remove
2
2
the CO in the gas phase; and (iv) the DRIFT spectrum of the sample was recorded in a N
flow (30 mL/min).
2
2
.3. Catalytic activity measurement
Catalytic activities of the samples for methane combustion were measured in a continuous
flow fixed-bed quartz tubular microreactor (i.d. = 6.0 mm). 0.050 g of the sample (40–60
mesh) was diluted by 0.25 g of quartz sand (40–60 mesh) to minimize the influence of hot
o
spots. All of the samples were pretreated in an oxygen flow of 30 mL/min at 300 C for 1 h.
The feed was composed of 1.5 vol% CH
4
+ 98.5 vol% air, the total flow was 16.7 mL/min,
and the space velocity was 20,000 mL/(g h). The total feed flow was increased with a rise in
space velocity. In the case of water vapor addition, 5.0 vol% H
2
O was introduced by passing
and SO addition, 5.0
from a SO cylinder
o
the feed stream through a water saturator at 34 C. In the case of CO
2
2
vol% of CO
2
from a high-purity (99.9 %) CO
2
cylinder or 100-ppm SO
2
2
9

(
balanced with N ) was introduced via a mass flow controller. The reaction product gases
2
were analyzed by an online gas chromatograph (GC-2010, 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 1000 column (3 m in length)
for permanent gas detection. The balance 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
9
0 %, respectively. CH
4
conversion was defined as (cinlet – coutlet)/cinlet × 100 %, where the cinlet
and coutlet are the inlet and outlet CH
4
concentrations in the feed stream, respectively.
3
. Results and discussion
3
.1. Crystal phase composition, surface morphology, pore structure, and surface area
XRD can provide useful information of the crystal phase compositions of a sample. Fig. 1
o
o
shows the XRD patterns of all of the samples. The diffraction signals at 2 = 18.6 , 31.8 ,
o
o
o
o
3
3.8 , 35.8 , 59.7 , and 66.5 were assigned to the hexaaluminate phase [25,26], as well
indexed in Fig. 1f, which was in good accordance with the diffraction signals of LaMnAl11
JCPDS PDF# 36-1317). The detection of sharp XRD peaks indicates a good crystallinity of
O
19
(
o
o
each sample. XRD peaks of the PdO or Pd phase should appear at 2 = 33.8 and 40.1 [1].
No obvious diffraction signals attributed to the Pd and/or Pt phase(s), however, were
observed. This result might be caused due to the low loadings of noble metal NPs and their
good dispersion on the surface of the support.
10

(
f)
e)
d)
(
(
(c)
(b)
(a)
10
20
30
40
50
60
70
80
2
Theta (Deg.)
Fig. 1. XRD patterns of (a) 3DOM LMAO, (b) 0.97Pd/3DOM LMAO, (c) 1.14Pd2.8Pt/3DOM LMAO, (d)
1
.12PdPt/3DOM LMAO, (e) 1.06PdPt2.6/3DOM LMAO, and (f) 0.94Pt/3DOM LMAO.
Fig. 2 shows the SEM images of the as-prepared samples. The 3DOM LaMnAl11
O
19
support displayed a high-quality 3DOM structure (Fig. 2a). After loading the noble metal(s),
the macroporous morphology (Fig. 2bf) was still retained, and the average pore sizes were in
the range of 97113 nm (Table 1). The pore sizes of the supported noble metal samples were
obviously smaller than the average diameter of the PMMA microsphere template, which was
o
a result due to the skeleton shrinking of 3DOM LaMnAl11
O
19 after calcination at 1100 C.
Fig. 2. SEM images of (a) 3DOM LMAO, (b) 0.97Pd/3DOM LMAO, (c) 1.14Pd2.8Pt/3DOM LMAO, (d)
11

1
.12PdPt/3DOM LMAO, (e) 1.06PdPt2.6/3DOM LMAO, and (f) 0.94Pt/3DOM LMAO.
Good distributions of the noble metal NPs on the surface of the 3DOM LMAO support and
high-quality 3DOM structures can be clearly seen from the TEM images (Fig. 3) of the
samples. It is also observed from the SAED pattern (inset of Fig. 3a) that there were multiple
electron diffraction rings of the support, indicating that the 3DOM LMAO support was
polycrystalline. The noble metal NPs were uniformly dispersed on the surface of 3DOM
LMAO (Fig. 3bf). After analyzing more than 70 noble metal NPs in each sample, one can
find that the average particle sizes of the noble metal(s) were in the range of 3.94.4 nm (Fig.
3
hl). Typical TEM mapping image of the Pd and Pt in 1.12PdPt/3DOM LMAO is shown in
Fig. 3g. The element distribution and signal intensity of the Pd and Pt were quite similar,
which means that a PdPt alloy was formed in the highly dispersed PdPt NPs of the
1
.12PdPt/3DOM LMAO sample. In the preparation and calcination processes of the
bimetallic noble metal NPs-loaded catalysts, H PtCl and PdCl were thoroughly mixed. After
2
6
2
reduction, the PdPt atom clusters were assembled and grew up to form an alloy NPs of
PdPt. In one of our previous studies [8], we used the same method to prepare the
AuPd/3DOM Mn
2
3
O samples, and confirmed the formation of AuPd alloy in the samples.
Furthermore, as shown in Fig. 3g, the Pd and Pt in 1.12PdPt/3DOM LMAO were uniformly
distributed in the noble metal NPs. In other words, an alloy of PdPt might be formed in the
sample.
12

Fig. 3. TEM images as well as the SAED pattern (inset of (a)), (hl) particle-size distributions, and average
noble metal particle sizes (d) of (a) 3DOM LMAO, (b, h) 0.97Pd/3DOM LMAO, (c, i) 1.14Pd2.8Pt/3DOM
LMAO, (d, j) 1.12PdPt/3DOM LMAO, (e, k) 1.06PdPt2.6/3DOM LMAO, (f, l) 0.94Pt/3DOM LMAO, and
(g) TEM mapping image of Pd and Pt in 1.12PdPt/3DOM LMAO.
(
A)
(B)
2
1
1
00
50
00
1
.0
0.8
.6
(e)
(e)
(d)
(
d)
c)
0
(c)
(
0.4
0.2
0.0
(b)
5
0
(b)
a)
(a)
(
0.0
0.2
0.4
0.6
0.8
1.0
0
50
100
150
200
Relative pressure p/p0
Pore diameter (nm)
Fig. 4. (A) Nitrogen adsorptiondesorption isotherms and (B) pore-size distributions of (a) 0.97Pd/3DOM
LMAO, (b) 1.14Pd2.8Pt/3DOM LMAO, (c) 1.12PdPt/3DOM LMAO, (d) 1.06PdPt2.6/3DOM LMAO, and
13

(e) 0.94Pt/3DOM LMAO.
Table 1. Surface areas, pore volumes, average pore sizes, noble metal particle sizes, nominal and actual
noble metal loadings, and actual Pd/Pt molar ratios of the 3DOM LMAO-supported Pd, PdPt or Pt samples.
Average
Noble metal
particle sizea
(nm)
Nominal noble
metal loading
(wt%)
Actual noble
Actual Pd/Pt
Surface area Pore volume
Sample
pore sizea
metal contentb molar ratiob
(wt%)
2
3
(
m /g)
(cm /g)
(nm)
0
1
1
.97Pd/3DOM LMAO
28.8
0.06
0.06
0.06
97
4.2
4.3
3.9
1.00
1.00
1.00
0.97
1.14
1.12
_
.14Pd2.8Pt/3DOM LMAO 28.9
107
98
2.80
1.00
.12PdPt/3DOM LMAO
26.3
1
0
.06PdPt2.6/3DOM LMAO 29.0
0.06
0.06
101
113
4.4
4.1
1.00
1.00
1.06
0.94
0.38
_
.94Pt/3DOM LMAO
27.6
a
Data were estimated according to the TEM images of the samples;
b
Data were obtained by the ICPAES technique.
Fig. 4 shows the N adsorptiondesorption isotherms and pore-size distributions of the
2
porous samples, and the surface areas and textural properties of the samples are summarized
in Table 1. Each of the samples displayed a typical II isotherm with a H3 hysteresis loop in
the relative pressure (p/p ) range of 0.8–1.0 (Fig. 4A), indicating the existence of a
0
macroporous structure which was confirmed by the pore-size distributions (Fig. 4B) of the
2
samples. As shown in Table 1, surface areas of the samples were similar (26.329.0 m /g),
3
and all of the porous samples possessed a pore volume of 0.06 cm /g. Hence, the surface area
14

was not the main factor to influence the catalytic activity of the samples.
3
.2. Surface composition, metal oxidation state, and oxygen species
XPS is a surface-sensitive technique that can provide the information of surface element
compositions, metal oxidation states, and oxygen species [27]. Fig. 5 shows the Mn 2p, O 1s
and Pd 3d XPS spectra of the as-obtained samples. Since Mn possessed different chemical
2
+
3+
states (i.e., Mn and Mn ) on the surface of LaMnAl11
O19 [28], the Mn 2p XPS signals could
be decomposed into three components at BE = 640.4, 642.0, and 643.7 eV (Fig. 5A), which
2
+
3+
2+
were assignable to the surface Mn and Mn species and the satellite of Mn species
[29,30], respectively. Table 2 summarizes the quantitative surface element compositions of the
2
+
3+
samples. It is seen that the Mn /Mn molar ratio declined from 0.81 on 0.97Pd/3DOM
LMAO to 0.63 on 1.06PdPt2.6/3DOM LMAO. The results indicate that (i) the electronic
3+
interaction between Mn and PdPt alloy NPs became weaker with a drop in Pd content of
2
+
the sample, and (ii) the rise in Mn species concentration resulted in an increase in surface
oxygen vacancy concentration. Usually, O molecules are apt to be adsorbed at the oxygen
2
vacancies of a (mixed) metal oxide. The increased concentration of oxygen vacancies was
favorable for the activation and adsorption of oxygen molecules to be active oxygen species,
which would be beneficial for the redox-involving reactions [31].
The O 1s XPS spectrum of each sample could be decomposed into three components at BE
=
530.4, 531.6, and 533.3 eV (Fig. 5B), attributable to the surface lattice oxygen (Olatt),
, O
2
or O ), and adsorbed hydroxyl or carbonate species

adsorbed oxygen (Oads, e.g., O
2
2
[
14], respectively. The Oads species are important in the low-temperature oxidation of organics
32], and the increased Oads species concentration could enhance the oxidation activities of
[
15

catalysts [33,34]. As shown in Table 2, the Oads/Olatt molar ratio decreased from 0.70 on
.97Pd/3DOM LMAO to 0.35 on 1.06PdPt2.6/3DOM LMAO. This result indicates that
0
addition of Pt could not help to activate oxygen molecules.
We recorded the Pd 4d spectra of the samples, as shown in Fig. S1. The binding energy of
Pd 4d was near 0 eV, and the signals were also very weak. Therefore, it is difficult to obtain
the chemical states of Pd in the samples using the Pd 4d spectra. As shown in Fig. 5C, the
asymmetric Pd 3d XPS spectrum of each sample could be decomposed into four components:
0
The ones at BE = 335.4 and 340.5 eV were assignable to the surface Pd species, whereas the
2
+
ones at 337.5 and 342.6 eV were ascribable to the surface Pd species [35,36]. Apparently, Pd
species mainly existed in the form of PdO (more than 81.64 %) on 0.97Pd/3DOM LMAO
2
+
0
(
Fig. 5C(a)). With the addition of Pt to Pd, the Pd /Pd ratio decreased to 0.37 on
2
+
0
1
.06PdPt2.6/3DOM LMAO. It is known that the higher surface Pd /Pd molar ratio means a
stronger ability to activate oxygen molecules, and hence giving rise to a better catalytic
performance.
Since the XPS signals of Pt 4f (BE = 7078 eV) were overlapped with those of Al 2p, it is
hard to quantify the surface Pt species using the Pt 4f spectra. Hence, we recorded the Pt 4d
spectra of the samples, as shown in Fig. 5D. The asymmetrical Pt 4d signal of each sample
could be decomposed into two components: The one at BE = 314.8 eV was assigned to the
0
2+
surface Pt species, and the one at BE = 317.5 eV was attributed to the surface Pt species
2
+
0
[
37]. With the rise in Pt content of the bimetallic sample, the Pt /Pt molar ratio decreased
from 0.84 (on 1.14Pd2.8Pt/3DOM LMAO) to 0.55 (on 1.06PdPt2.6/3DOM LMAO).
16

3+
(A) Mn2p
Mn
2+
Mn
(
a)
b)
c)
d)
e)
(
(
(
(
660
655
650
645
640
635
Binding energy (eV)
O latt
O ads
(
B) O1s
OOH or carbonate
(a)
(b)
(c)
(d)
(e)
5
36
534
532
530
528
526
Binding energy (eV)
2
+
(C) Pd3d
Pd
0
Pd
(a)
(b)
(c)
(d)
344
342
340
338
336
334
332
Binding energy (eV)
0
(D) Pt 4d
2+
Pt
Pt
(b)
(c)
(
d)
e)
(
326
324
322
320
318
316
314
312
310
Binding energy (eV)
17

Fig. 5. (A) Mn 2p, (B) O 1s, (C) Pd 3d, and (D) Pt 4d XPS spectra of (a) 0.97Pd/3DOM LMAO, (b)
1
0
.14Pd2.8Pt/3DOM LMAO, (c) 1.12PdPt/3DOM LMAO, (d) 1.06PdPt2.6/3DOM LMAO, and (e)
.94Pt/3DOM LMAO.
2
Table 2. Surface element compositions and H consumption of the samples.
2
+
3+
2+
0
2+
0
a
Mn /Mn
Oads/Olatt
Pd /Pd
Pt /Pt
H
2
consumption (mmol/gcat
)
Sample
o
molar ratio
0.81
molar ratio
0.70
molar ratio
81.64
molar ratio
< 400 C  400
Total
1.70
0
1
1
.97Pd/3DOM LMAO
.14Pd2.8Pt/3DOM LMAO
.12PdPt/3DOM LMAO
–
1.16
1.02
0.86
0.67
0.66
0.54
0.52
0.54
0.30
0.32
0.77
0.68
0.63
0.65
0.52
0.42
0.35
0.37
4.14
0.41
0.37
–
0.84
0.74
0.55
0.60
1.54
1.40
0.97
0.98
1
0
.06PdPt2.6/3DOM LMAO
.94Pt/3DOM LMAO
a
2
Data were estimated by quantitatively analyzing the H -TPR profiles.
3
.3. Reducibility
(
A) 173
(
a)
b)
1
86
(
2
02
(
c)
d)
e)
2
15
(
2
61
(
2
00
400
600
800
o
Temperature ( C)
18

1
6
(B)
(a)
1
4
(b)
1
2
1
0
8
6
(
c)
(
d)
4
2
(e)
2
.20
2.25
2.30
2.35
-
1
1
000/T (K )
Fig. 6. (A) H
2
-TPR profiles and (B) initial H
2
consumption rate as a function of inverse temperature of (a)
0
.97Pd/3DOM LMAO, (b) 1.14Pd2.8Pt/3DOM LMAO, (c) 1.12PdPt/3DOM LMAO, (d) 1.06PdPt2.6/3DOM
LMAO, and (e) 0.94Pt/3DOM LMAO.
Fig. 6A illustrates the H -TPR profiles of the noble metal-loaded catalysts. The
2
monometallic Pd-loaded 0.97Pd/3DOM LMAO sample had a continuous H consumption
2
o
from 10 to 400 C (Fig. 6A(a)). The reduction of the PdO species usually happened in this
temperature range [37]. During the reduction process, the highly dispersed PdO species were
first converted to metallic palladium and palladium hydride. Palladium hydride then
o
decomposed bellow 80 C, which could be identified from the TPR profiles. For the bimetallic
PdPt-loaded samples, however, the decomposition peak of such a hydride declined to very
low level or even could not been seen. The absence of hydrogen desorption in H -TPR
2
profiles indicates the formation of PdPt alloy, which might not be able to form palladium
hydride.
PdO and PtO
2
that had a strong interaction with the support are usually reduced in the
o
temperature range of 230400 C [38,39]. The three-dimensionally ordered macroporous
structure was beneficial for the high dispersion of the noble metal NPs, thus the reduction
19

o
temperature of the noble metal NPs decreased to 173 C for 0.97Pd/3DOM LMAO and 261
oC for 0.94Pt/3DOM LMAO. The hydrogen consumption decreased with the drop in Pd/Pt
o
ratio from 2.8 : 1.0 to 1.0 : 2.6. The hydrogen consumption above 400 C was due to the
3
+
2+
reduction of Mn to Mn [28]. Compared with the reduction process [14], the hydrogen
consumption of the noble metal NPs-loaded 3DOM LMAO was higher than that of 3DOM
LMAO. This result could be ascribed to the interaction between the support and the noble
metal alloy NPs.
o
The H
2
consumption of the samples below 400 C increased in the order of 0.94Pt/3DOM
LMAO (0.66 mmol/gcat)  1.06PdPt2.6/3DOM LMAO (0.67 mmol/gcat) < 1.12PdPt/3DOM
LMAO (0.86 mmol/gcat) < 1.14Pd2.8Pt/3DOM LMAO (1.02 mmol/gcat) < 0.97Pd/3DOM
LMAO (1.16 mmol/gcat). Usually, low-temperature reducibility intimately relates to the initial
H
2
consumption rate (the initial 25 % of the first reduction peak of each catalyst [40]). The
calculated initial H consumption rate versus inverse temperature of the samples is depicted in
Fig. 6B. The initial H consumption rate (i.e., low-temperature reducibility) increased in the
sequence of 0.94Pt/3DOM LMAO < 1.06PdPt2.6/3DOM LMAO < 1.12PdPt/3DOM LMAO <
.97Pd/3DOM LMAO  1.14Pd2.8Pt/3DOM LMAO. It is in basic accordance with the
sequences in Oads concentration (Table 2) and catalytic performance (below).
2
2
0
3
.4. In situ DRIFT spectra of CO adsorption
In situ DRIFT spectra of CO adsorption on the samples can provide valuable information of

1
the interactions between Pd and Pt. The absorption band at 1942 cm (Fig. 7a) was assigned
to the bridge-bonded CO species on the metallic Pd, which was greatly weakened on the

1
1
.14Pd2.8Pt/3DOM LMAO sample (Fig. 7b). The band centered at 2064 cm (Fig. 7a)
20


1
disappeared on the 1.14Pd2.8Pt/3DOM LMAO sample. The band at 2090 cm on

1
0
7
.97Pd/3DOM LMAO (Fig. 7a) [41,42] or at 2088 cm [43] on 0.94Pt/3DOM LMAO (Fig.
c), which could be attributed to the linearly adsorbed CO species, shifted to the one at 2085

1
cm on 1.14Pd2.8Pt/3DOM LMAO (Fig. 7b). This result might be due to formation of the
PdPt alloy, which changed the metallic states of Pd and Pt.
-1
-1
2
090 cm
-1
1942 cm
2
064 cm
(
a)
b)
-
1
1
2
2
085 cm
088 cm
(
-
(
c)
(
e)
2250
2200
2150
2100
2050
2000
1950
-
1
Wavenumber (cm )
Fig. 7. In
spectra of CO adsorption on (a) 0.97Pd/3DOM LMAO, (b) 1.14Pd2.8Pt/3DOM LMAO, and (c)
.94Pt/3DOM LMAO.
situ DRIFT
0
3
.5. Catalytic performance
Lowering the light-off and combustion temperatures are the final aim for methane catalytic
combustion. A lower light-off temperature is beneficial to reduce the energy consumption and
environmental contamination. Fig. 8 shows the activities and specific reaction rate of the as-
prepared catalysts for methane combustion at SV = 20,000 mL/(g h). The methane conversion
increased monotonously with the rise in temperature over each catalyst. The T10%, T50%, and
o
T
90% over 0.97Pd/3DOM LMAO were 259, 308, and 343 C (Fig. 8A(a)), whereas those over
21

o
1
.06PdPt2.6/3DOM LMAO were 385, 487, and 549 C (Fig. 8A(d)). The rise in Pt content of
the noble metal alloy-loaded sample depressed the catalytic activity (Fig. 8A(bd)), both T10%
o
and T90% declined by ca. 100 C from over 1.14Pd2.8Pt/3DOM LMAO to over
1
4
.06PdPt2.6/3DOM LMAO. The T10%, T50%, and T90% over 0.94Pt/3DOM LMAO were 338,
o
37, and 515 C (Fig. 8A(e)), which were higher than those over 1.06PdPt2.6/3DOM LMAO.
As it was shown in Fig. 8B, the specific reaction rate of the prepared catalysts at any
temperature was in accordance with the activities order. The results demonstrate that the noble
metal alloy-loaded sample with a higher Pd content gave rise to a better catalytic performance
for methane combustion.
100
o
(A)
o
469 C
o
o
3
43 C
456 C
515 C
o
5
49 C
80
60
40
20
0
d
e
o
o
3
08 C
372 C
c
o
4
87 C
o
o
4
37 C
4
01 C
b
a
o
o
284 C
o
2
59 C
338 C
o
3
85 C
200
300
400
500
o
Reaction temperature ( C)
(B)
1
1
6
2
8
4
0
d
e
c
b
a
3
Reaction temperature ( C)
00
400
500
o
Fig. 8. (A) Methane conversion and (B) specific reaction rate as a function of reaction temperature of (a)
22

0
.97Pd/3DOM LMAO, (b) 1.14Pd2.8Pt/3DOM LMAO, (c) 1.12PdPt/3DOM LMAO, (d) 1.06PdPt2.6/3DOM
LMAO, and (e) 0.94Pt/3DOM LMAO at SV = 20,000 mL/(g h). The reactant composition: 1.5 vol% CH
8.5 vol% air.
4
+
9
1
00
o
4
56 C
o
5
32 C
80
60
40
20
0
c
o
3
72 C
o
4
59 C
b
a
o
2
84 C
o
3
81 C
2
00
250
300
350
400
450
500
550
o
Reaction temperature ( C)
Fig. 9. Methane conversion as a function of temperature over 1.14Pd2.8Pt/3DOM LMAO at SV = (a)
2
9
0,000 mL/(g h), (b) 40,000 mL/(g h), and (c) 80,000 mL/(g h). The reactant composition: 1.5 vol% CH
8.5 vol% air.
4
+
In order to compare the effect of SV on the activity of the Pt-doped Pd catalyst, we
measured the methane conversions over 1.14Pd2.8Pt/3DOM LMAO at different temperatures
and SVs, and the results are shown in Fig. 9. It can be observed that methane conversion
decreased with the rise in SV from 20,000 to 80,000 mL/(g h), and the T10% increased by 97
o
o
C from 284 to 381 C. Compared with the T10% and T90% over 0.97Pd/3DOM LMAO [14],
o
those over 1.14Pd2.8Pt/3DOM LMAO decreased by ca. 45 and 65 C, respectively. The results
indicate that the doping of Pt to Pd made the catalyst much more suffered from the SV effect.
In order to better compare the catalytic performance, we prepared the 0.91Pd/Al
2
O sample
3
23

and measured its catalytic activity for methane combustion under the same conditions, as
shown in Fig. S2. Apparently, our 1.14Pd2.8Pt/3DOM LMAO sample performed much better
than the 0.91Pd/Al
2
3
O sample, demonstrating that the bimetallic PdPt sample was superior to
the monometallic Pd sample in catalyzing the combustion of methane. Cargnello et al. [1]
prepared the Pd@CeO
2
/Al
2
O
3
catalyst with a core-shell supramolecular structure, observed
o
that methane was completely converted at about 400 C even at a SV of 200,000 mL/(g h).
Our 1.14Pd2.8Pt/3DOM LMAO sample was inferior to the Pd@CeO
catalytic activity for methane combustion.
2
/Al
2
3
O sample in
On-stream methane oxidation experiments were carried out to explore the stability of the
typical catalysts. Except that 0.97Pd/3DOM LMAO showed the best catalytic performance
o
and over which about 87 % methane was converted at 350 C, the other experiments were all
o
carried out at 450 C and SV = 40,000 mL/(g h), as shown in Fig. 10A. The 3DOM LMAO
support showed a good thermal stability, which could be attributed to the stable crystal
o
structure after calcination at 1100 C. When the noble metals were loaded on the surface of the
3
DOM LMAO support, all of the catalysts possessed a good thermal stability after 30 h of on-
stream methane combustion, during which methane conversion almost kept unchanged with a
slight fluctuation. The results demonstrate that the 3DOM LMAO-supported noble metal NPs
were catalytically stable under the adopted conditions.
We examined the stability and hysteresis effects of the 0.91Pd/Al
2
O
3
and
1
.14Pd2.8Pt/3DOM LMAO samples at SV = 20,000 mL/(g h) when the temperature increased
o
o
from 200 to 850 C and decreased from 850 to 200 C, and the results are shown in Figs. S3
and 10B. It is known that the catalytic activity is associated with the chemical state of noble
24

metals, and the oxidized noble metals show better activity than the metallic noble metals for
methane combustion. Comparing with the activity when the temperature increased, there was
a noticeable decrease in activity when the temperature decreased over the 0.91Pd/Al
O
2 3
sample (Fig. S3). Over the 1.14Pd2.8Pt/3DOM LMAO sample, however, there was no
significant difference in activity when the temperature increased and decreased in the
o
temperature ranges of 200450 and 650850 C (Fig. 10B), except that a big drop in activity
o
occurred at around 550 C (a result possibly due to the change in chemical state of the PdPt
NPs in the sample). The activity recovery during the temperature drop process was related to
the re-oxidation of Pd [19]. Therefore, the above results demonstrate that the introduction of
o
Pt to Pd could stabilize the catalytic activity below 450 C.
(A)
100
a
80
60
40
20
c
b
e
d
0
5
10
15
20
25
30
On-stream reaction time (h)
1
00 (B)
80
60
40
20
0
1
00
200
300
400
500
600
700
800
900
o
Reaction temperature ( C)
25

Fig. 10. (A) Methane conversion versus on-stream methane combustion time over (a) 0.97Pd/3DOM
o
LMAO at 350 C and SV = 40,000 mL/(g h), (b) 1.14Pd2.8Pt/3DOM LMAO, (c) 1.12PdPt/3DOM LMAO,
o
(
d) 1.06PdPt2.6/3DOM LMAO, and (e) 0.94Pt/3DOM LMAO at 450 C and SV = 40,000 mL/(g h); and (B)
methane converison versus reaction temperature over 1.14Pd2.8Pt/3DOM LMAO at SV = 20,000 mL/(g h)
o
o
when the temperature increased from 200 to 850 C and decreased from 850 to 200 C. The reactant
composition: 1.5 vol% CH + 98.5 vol% air.
4
3
.6. Effects of water vapor, carbon dioxide, and sulfur dioxide on catalytic activity
100
(A)
H O on
2
a
80
60
40
20
0
H O off
H O on
2
2
b
e
H O on
2
H O off
2
H O off
2
5
10
15
20
25
30
On-stream reaction time (h)
100
(
B) CO on
O treatment
2
2
a
80
60
40
20
0
CO off
2
CO on
O treatment
2
2
b
e
CO off
CO on
2
O treatment
2
2
CO off
2
1
0
20
30
40
On-stream reaction time (h)
26

100
(C)
80
60
40
20
0
a
SO on
2
SO off
2
c
SO on
2
SO off
2
b
SO on
2
SO off
2
e
SO on
SO off
2
2
SO off
2
d
SO on
2
5
10
15
20
On-stream reaction time (h)
Fig. 11. Effects of (A) 5.0 vol% H
2
2 2
O, (B) 5.0 vol% CO , and (C) 100 ppm SO on methane conversion over
o
(
a) 0.97Pd/3DOM LMAO at 350 C and SV = 40,000 mL/(g h), (b) 1.14Pd2.8Pt/3DOM LMAO, (c)
o
1
4
.12PdPt/3DOM LMAO, (d) 1.06PdPt2.6/3DOM LMAO, and (e) 0.94Pt/3DOM LMAO at 450 C and SV =
4 2 4
0,000 mL/(g h). The reactant composition: 1.5 vol% CH + 93.5 vol% air + 5.0 vol% H O, 1.5 vol% CH
+
2 4 2
93.5 vol% air + 5.0 vol% CO or 1.5 vol% CH + 98.5 vol% air + 100 ppm SO .
Methane combustion can release a huge amount of heat, moisture, and carbon dioxide as
well as a trace amount of sulfur dioxide. All of these products might be influence the activity
of the catalyst. Fig. 11 shows the on-stream methane combustion activities of the samples in
the presence of 5.0 % H
2
O, 5.0 vol% CO
2
or 100 ppm SO at SV = 40,000 mL/(g h).
2
The effect of water vapor on catalytic activity has previously been studied [44]. The
introduction of water to the reactant feed could cause a adverse effect because of the
competitive adsorption of water and reactant molecules. When 5.0 vol% water vapor was
added to the catalytic system, methane conversion dropped from 86.1 to 74.0 % over
0
.97Pd/3DOM LMAO and from 36.6 to 25.3 % over 0.94Pt/3DOM LMAO after 25 h of
reaction. Over 1.14Pd2.8Pt/3DOM LMAO, however, methane conversion decreased slightly
from 55.2 to 50.9 %) after 25 h of reaction. It means that the doping of Pt was beneficial for
(
27

the improvement in water-resistant ability. When the water was cut off, however, methane
conversions were recovered over all of the catalysts. The partial deactivation of the Pd-based
catalyst was mainly due to formation of hydroxyls on the surface of the catalyst, causing the
covering of the active sites. Although the water influence on catalytic activity was a reversible
process, the doping of Pt with a small amount could help the Pd-based catalyst to keep the
original catalytic activity.
Since a lot of carbon dioxide is produced during the methane combustion process, the
active sites would be occupied by the formed carbonate species, thus decreasing the activity
of the catalyst. As shown in Fig. 11B, methane conversion decreased from 86.6 to 82.5 %
after the introduction of 5.0 vol% CO
oxidized in an O
could tolerate the influence of CO
Therefore, a small amount of Pt doping to Pd (1.14Pd2.8Pt/3DOM LMAO) could improve the
CO -resistant ability.
Usually, catalytic activity decreases when the Pd-based catalysts are poisoned by a trace
amount of SO during natural gas combustion. After introducing 100 ppm SO to the catalytic
system, methane conversion over 0.97Pd/3DOM LMAO quickly declined from ca. 81 to
5 %, however, the 0.94Pt/3DOM LMAO sample exhibited a good SO -resistant ability
without distinctive alternation in methane conversion. With the rise in Pt content of the loaded
Pd-Pt alloy, the influence due to the SO poisoning on methane combustion activity was
2
, but it was recovered when the CO -treated sample was
2
o
2
flow (30 mL/min) at 450 C for 2 h. The 0.94Pt/3DOM LMAO sample
2
without obvious decline in catalytic activity (Fig. 11B(b)).
2
2
2
5
2
2
weakened. Methane conversion decreased by ca. 20, 16, and 5 % over 1.14Pd2.8Pt/3DOM
LMAO, 1.12PdPt/3DOM LMAO, and 1.06PdPt2.6/3DOM LMAO, respectively.
28

3
.7. Effect of Pt addition to the formation of PdO
From the above catalytic performance of the samples with different Pd/Pt molar ratios, we
know that addition of a small amount of Pt to Pd could improve the sulfur dioxide-, carbon
dioxide-, and moisture-resistant ability, although Pt doping could not decrease methane
combustion temperatures. According to the literature [45,46], PdO was the most important
active species during the methane combustion processes. It seems that formation of PdO was
suppressed since the catalytic activity was not improved after addition of Pt to Pd. We
speculate how the Pt component affected formation of the PdO species. We herein employed
the ex situ XPS technique to monitor the formation process of PdO in the 1.14Pd2.8Pt/3DOM
LMAO sample, and at the same time the 0.97Pd/3DOM LMAO sample was also taken into
consideration for comparison.
The oxidation experiments were carried out in an oxygen atmosphere at a pressure of 1000
mbar for 1 h, and the results are shown in Fig. 12. The samples were freshly prepared without
calcination. It can be realized form Fig. 12A and C that the oxidation process of Pd NPs
related to the reaction temperature. Even though the experiment was undertaken under oxygen
condition, formation of PdO was still preferred at higher temperatures. PdO species with a
content of more than 98 % in the total Pd amount were generated on the surface of the
o
0
.97Pd/3DOM LMAO sample at 300 C, which was in good accordance with its high
catalytic activity at low temperatures.
As for the 1.14Pd2.8Pt/3DOM LMAO sample, the formed PdO species were less than 4 %
o
in the total Pd content at 350 C, which was obviously lower than that (98 %) in the
o
0
.97Pd/3DOM LMAO sample. Even when the reaction temperature increased to 450 C, only
29