Performance and kinetic study on Pd/OMS-2 catalyst for CO catalytic oxidation: effect of preparation method

Article abstract of DOI:10.1007/s11164-016-2743-0

Manganese oxide octahedral molecular sieves (OMS-2) synthesized from hydrothermal (H-OMS-2), reflux (R-OMS-2), co-precipitation (C-OMS-2), and solid phase (S-OMS-2) methods were impregnated with palladium and used for CO catalytic oxidation. Preparation methods presented an obvious effect on the morphology and catalytic activity of Pd/OMS-2 catalysts for CO oxidation. The hydrothermal synthesized OMS-2 (Pd/H-OMS-2) exhibited more ordered nanorod structure and higher crystallinity than Pd/R-OMS-2, Pd/C-OMS-2, and Pd/S-OMS-2. Further surface analysis indicated that different preparation methods of synthesizing OMS-2 and the impregnation process followed have a significant effect on the chemical states of Mn and O over the final Pd/OMS-2 products. The kinetics studies showed the trend of apparent activation energy (E_a) over different catalysts: Pd/H-OMS-2 (18.19?kJ/mol)?

Full text of DOI:10.1007/s11164-016-2743-0

Res Chem Intermed
DOI 10.1007/s11164-016-2743-0
Performance and kinetic study on Pd/OMS-2 catalyst
for CO catalytic oxidation: effect of preparation
method
1
1
1
•
Qiulin Zhang
1
•
Qixian Liu
1
•
Ping Ning
1
•
Xin Liu Lisi Xu
1
Yankang Duan
•
•
Zhongxian Song
1
Tengfei Zhang
•
Received: 17 June 2016 / Accepted: 12 September 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Manganese oxide octahedral molecular sieves (OMS-2) synthesized from
hydrothermal (H-OMS-2), reflux (R-OMS-2), co-precipitation (C-OMS-2), and
solid phase (S-OMS-2) methods were impregnated with palladium and used for CO
catalytic oxidation. Preparation methods presented an obvious effect on the mor-
phology and catalytic activity of Pd/OMS-2 catalysts for CO oxidation. The
hydrothermal synthesized OMS-2 (Pd/H-OMS-2) exhibited more ordered nanorod
structure and higher crystallinity than Pd/R-OMS-2, Pd/C-OMS-2, and Pd/S-OMS-
2
. Further surface analysis indicated that different preparation methods of synthe-
sizing OMS-2 and the impregnation process followed have a significant effect on
the chemical states of Mn and O over the final Pd/OMS-2 products. The kinetics
studies showed the trend of apparent activation energy (E ) over different catalysts:
a
Pd/H-OMS-2
(
(18.19 kJ/mol) \ Pd/R-OMS-2
22.57 kJ/mol) \ Pd/S-OMS-2 (29.44 kJ/mol). Over 99 % of the CO conversion
(21.56 kJ/mol) \ Pd/C-OMS-2
was obtained at 35 °C by the optimal Pd/H-OMS-2 catalyst.
Keywords Pd/OMS-2 Á CO oxidation Á Ordered nanorod Á Catalytic activity Á
Activation energy (E )
a
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2743-0)
contains supplementary material, which is available to authorized users.
&
Ping Ning
ningping_58@126.com
1
Faculty of Environmental Science and Engineering, Kunming University of Science and
Technology, Kunming 650500, People’s Republic of China
123

Q. Zhang et al.
Introduction
Carbon monoxide emitted from automobile exhausts, industrial sources, and other
human sources has become a major air pollutant. It is of great practical and
academic value to explore catalytic oxidation of CO. To date, a large number of
transitional metal oxide catalysts and noble metal-containing catalysts [1] have been
explored for CO oxidation below 100 °C. Among these catalysts, supported Au [2],
Ag [3], and platinum group metals (PGMs) [1], as well as spinal Co O [4],
3
4
nonstoichiometric MnO [5], and CuO-based catalysts [6] have been have been
x
widely studied and proven to be highly active for low-temperature CO oxidation.
Pd-containing catalysts have been employed by the three-way catalyst (TWC), and
catalytic oxidation of CO and hydrocarbons (HC) from automobile and exhausts [7]
due to their superior low-temperature catalytic activity for CO oxidation [8].
MnOx components with good oxygen-storing capacity (OSC) and superior
hypoxia tolerance have been widely employed for the Hopcalite catalysts [9] and
low-temperature catalytic oxidation of CO [10]. It has been reported that a
2
-
mechanism on the MnO surface exists as follows: CO (ads) ? O (lattice) ? 2
2
4
?/3?
3?/2?
Mn
? CO (ads) ? 2 Mn
[11]. Previous reports [11, 12] have indicated
that low-temperature activity of CO catalytic oxidation over different MnO was in
2
x
the order of MnO [ Mn O [ Mn O . Comparing with Mn O and Mn O , MnO
2
2
3
3
4
2
3
3
4
2
exhibits higher thermal and chemical stability. In addition, different morphologies
and structures of MnO also have obvious effect on the catalytic activity [13, 14]. It
2
is reported [15] that manganese oxide octahedral molecular sieve has one-
dimensional channel structure, which may simultaneously facilitate internal
diffusion of CO, enhancing structural stability and oxygen storage and release
capacity during catalytic oxidation processes. OMS-2 supported noble metal
catalysts are proved to be active for CO catalytic oxidation [16, 17]. To date,
however, lowering the content of noble metals and enhancing low-temperature
catalytic activity remains hard work, since cost and light-off temperature are top
concerns for catalytic combustion of CO-containing industrial exhausts [18].
Generally, it is critical to optimize morphology and porous structure and
chemical sate of surface species to reduce palladium content, cost and enhancing
low-temperature catalytic activity of CO oxidation [19, 20]. Nowadays, mesoporous
metal oxides have been regarded as a new generation of catalysts or catalysts
support, which are known for their thermal stability, narrow pore size distribution,
superior redox properties and potential promotion effect on dispersion of surface
active species [21, 22]. Octahedral molecular sieves (OMS-2) with high crystallinity
and ordered mesoporous structure is easy to be synthesized via eco-friendly ways
without any soft or hard templates [23, 24].
In this work, hydrothermal, reflux, co-precipitation and solid phase method have
been adopted to synthesize manganese oxide octahedral molecular sieve. As
prepared manganese oxide octahedral molecular sieves have been impregnated with
Pd for low-temperature CO catalytic oxidation. Further explorations of structural
and surface analysis as well as kinetic study have been employed to characterize the
catalysts.
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
Experimental
Catalyst preparation
All the chemicals are of analytical grade without the requirement for further
purification.
Hydrothermal method
Samples of 4.73 g of Mn(CH COO) Á4H O, 4.4 g of KMnO , 1 mL of concentrated
3
2
2
4
nitric acid were sequentially dispersed into 64 mL of distilled water. The mixed
solution was transferred into a Teflon-lined stainless steel autoclave and heated for
2
4 h at 100 °C. The resulting product was filtered, washed with deionized water and
then dried at 100 °C overnight. The resulting solid was calcined at 400 °C in air for
h to obtain the support and signed with H-OMS-2.
4
Reflux method
Samples of 5.688 g of KMnO4 and 6.127 g of manganese acetatetetrahydrate
(
Mn(CH COO) Á4H O) were dissolved in 50 mL distilled water respectively at room
3
2
2
temperature. The two types of solution were then mixed and heated at 120 °C with
stirring. The mixed solution was refluxed at 120 °C for 24 h. The resultant material
was filtered, washed with distilled water, and then dried at 100 °C overnight. The final
product was calcined at 400 °C for 4 h in air and denoted as R-OMS-2.
Co-precipitation method
A sample of 32.34 g aqueous solution of Mn(NO ) (50 wt% of Mn(NO ) ) was
3
2
3 2
dissolved in 150 mL of distilled water. Ammonia was then added to adjust to a pH
of 9–10 and agitated for 1 h at room temperature and then heated at 80 °C for 5 h.
The product was filtered, washed with distilled water and dried at 100 °C overnight.
The product was calcined at 400 °C for 4 h in air and denoted as C-OMS-2.
Solid-state method
Samples of 2.37 g of KMnO and 5.39 g of Mn(CH COO) Á4H O were ground and
4
3
2
2
mixed. The resultant uniform mixture was placed into a round-bottom flask
(
250 mL). It was then heated to 80 °C for 4 h. The product was filtered, washed
with distilled water, and dried at 100 °C overnight. Finally, it was calcined at
00 °C for 4 h in air. The obtained sample was denoted as S-OMS-2.
4
Impregnation of palladium
Pd/OMS-2 catalysts were prepared by impregnation method. OMS-2 was incipi-
ently impregnated into the aqueous solution of Pd(NO ) with nominal Pd content of
3
2
123

Q. Zhang et al.
1
% (wt%) for 3 h at 70 °C, followed by drying at 105 °C overnight and calcining
at 400 °C for 3 h.
Characterizations of the catalysts
X-ray powder diffraction (XRD) analysis was performed on a BrukerD8 Advance
diffractometer system, with operating voltage of 40 kV and current of 40 mA, using
˚
Cu-Ka radiation (k = 1.5406 A) and a graphite monochromator. The samples were
investigated in the 2h range of 10°–70° at a scanning speed of 0.03°/S. X-ray
photoelectron spectroscopy (XPS) tests were carried out on an ULVAC PHI 5000
Probe-II spectrometer using Al K radiation under UHV. The C1s peak (284.8 eV)
a
was used for the calibration of binding energy values. Surface atomic concentrations
of Mn, Pd, and O, as well as peak areas were determined by the XPS Peak Fit
(
(
Version 4.0, AISN Software Inc.) software. Transmission electron microscopy
TEM) tests were measured on a FEI Tecnai G220 transmission electron
microscope, using an accelerating voltage of 200 kV. N adsorption/desorption
2
tests were carried out on a Micromeritics TriStar II 3020 instrument at 77 K using
liquid N as adsorbent. Specific surface areas (SSAs) were determined by the
2
Brunauer–Emmett–Teller (BET) equation. The pore volume was determined from
the from the adsorption isotherms. Pore size distributions were determined by the
non-local density functional theory (NL-DFT). The H -temperature programmed
2
reduction (H -TPR) experiments were carried out on a homemade quartz-tube fixed-
2
bed reactor (i.d. 3 nm). A 10 mg sample and 40 mg quartz sand were mixed and
filled for each measurement. The catalyst was pre-treated in flow N (30 mL/min) at
2
4
00 °C for 40 min and then cooled to room temperature. After that, all H -TPR runs
2
were performed from 0 to 900 °C under 5 % H in Ar with a heating rate of 8 °C/
min. A TCD was used to detect the consumption of H .
2
2
Catalytic tests of CO oxidation
The catalyst was pretreated in air flow at 225 °C for 40 min, to remove surface
impurities. CO oxidation reaction was carried out in quartz-tube reactor (i.d. 6 mm)
under atmospheric pressure. 200 mg of (60–80 mesh) catalyst was filled for each
reaction. An K-type thermocouple was enclosed to monitor the reaction temper-
ature. The reaction gases were as follow: 1 % of CO (vol%), 10.5 % of O (vol%)
2
balanced with N . The total flow rate was 50 mL/min and the gas hourly space
2
velocity was 15,000 mL/(g h). The gas components outlet were detected by a gas
chromatograph with a TDX-01 column, a methanator and a flame ionization
detector (FID) under steady-state conditions. The CO conversion was calculated as
follow:
½
COŠ À ½COŠ
in
out
CO conversion ð%Þ ¼
 100 %
½COŠ
in
where [CO] represents the CO concentration in the feed gas, [CO] represent the
in
out
CO concentrations in the effluent gases.
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
Fig. 1 Conversion of low-temperature CO catalytic oxidation over Pd/OMS-2 catalysts (1.0 vol% CO,
10.5 vol% O2, N
-1
-1
h )
2
balanced, with the total flow rate of 50 mL/min and GHSV of 15,000 mL g
Results and discussion
Catalytic activity of CO
Catalytic activity on the Pd/OMS-2 catalysts are given in Fig. 1. It can be seen from
Fig. 1 that the catalytic activity of CO oxidation on Pd/OMS-2 catalysts decreased
Fig. 2 XRD patterns of various Pd/OMS-2 catalysts
123

Q. Zhang et al.
Fig. 3 TEM images of catalysts: A and b Pd/H-OMS-2, B and b Pd/R-OMS-2, C and c Pd/S-OMS-2,c
D and d Pd/C-OMS-2
following the trend of Pd/H-OMS-2 [ Pd/R-OMS-2 [ Pd/C-OMS-2 [ Pd/S-OMS-
2
. All of the Pd/OMS-2 catalysts obtained over 99 % of CO catalytic conversion
below 85 °C, while pure OMS-2 attained over 99 % of CO conversion at 170 °C.
The Pd/H-OMS-2 catalyst exhibited the best catalytic conversion of CO. Over 99 %
of CO catalytic conversion could be obtained at 35 °C. In contrast, the catalytic
activity of Pd/C-OMS-2, Pd/R-OMS-2, and Pd/S-OMS-2 catalysts are lower than
that of Pd/H-OMS-2. Therefore, it can be concluded from Fig. 1, that the
hydrothermal method is preferred to prepare Pd/OMS-2 catalyst in terms of catalytic
conversion of CO in the present research.
XRD analysis
XRD patterns of Pd/OMS-2 catalysts are listed in Fig. 2. Pd/OMS-2 catalysts show
no obvious diffraction peaks of palladium species in the forms of amorphous phase
or highly dispersed species beyond the detection limits of XRD technique; EDAX
(
Fig. S5) indicates that palladium exists. Pd/H-OMS-2, Pd/R-OMS-2, Pd/S-OMS-2
show obvious characteristic diffraction peaks at 2h = 12.7°, 18.0°,25.8°, 28.7°,
3
MnO (110), (200), (220), (310), (211), (301), (411), (600), (521), and (002),
7.5°, 42.0°, 50.0°, 56.5°, 60.5°, and 65°, corresponding to the crystal planes of
2
indicating the presence of octahedral molecular sieve (OMS-2) structure
16, 19, 23]. Pd-C/OMS-2 catalyst exhibits only three obvious characteristic
[
diffraction peaks of MnO at 2h = 37.5°, 42.08, and 56.58. Pd/H-OMS-2 shows
2
stronger diffraction peaks of MnO than other catalysts, indicating that the highest
crystallinity of Pd/H-OMS-2. Combining with catalytic activity test (Fig. 1), high
crystallinity of MnO is preferred to increase the low-temperature catalytic activity
2
2
over OMS-2 supported palladium catalysts. While further studies are expected to
explore the correlations between catalytic activity, crystallinity, and other physio-
chemical properties.
TEM analysis
Transmission electron microscope (TEM) images and FE-SEM images of various
Pd/OMS-2 catalysts are, respectively, presented in Fig. 3 and Fig. S1-S4 in the
Electronic Supplementary Information (ESI). Pd/H-OMS-2 and Pd/R-OMS-2
exhibit highly dispersed regular single-crystal nanorod structure with single
diameter of 35–55 nm and length of over 200–400 nm, while Pd/S-OMS-2 and
Pd/C-OMS-2 show morphology of obvious piled pores, amorphous phase, and
mixed crystal of sponge-like accumulated nanoparticles with single particle size of
1
0–20 nm and total particle size of 150–200 nm. The octahedral structure of MnO2
on Pd/H-OMS-2, Pd/R-OMS-2, and Pd/S-OMS-2 can be observed from the small
illustrations. TEM images of Pd/OMS-2 show the lattice fringes are 0.71, 0.698,
0.689, 0.49, and 0.31 nm corresponding to the (110), (200), and (301) plane [25],
consistent with the XRD analysis (Fig. 2). Pd/S-OMS-2 possesses two different
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
123

Q. Zhang et al.
planes. In terms of catalytic activity (Fig. 1), the morphology and catalytic activity
are dramatically affected by the preparation methods, and the OMS-2 (Pd/H-OMS-
2
) prepared by hydrothermal method exhibited ordered Mn nanorod structure,
resulting in the improvement of catalytic activity for CO oxidation. While piled
pores, amorphous phase, and mixed crystal of sponge-like accumulated nanopar-
ticles cannot facilitate catalytic activity in the present research, the high crystallinity
of nanorod and octahedral structure is generally regarded as a potential factor of
decreasing specific surface area (SSA). Thus, further research of SSA and porous
properties, as well as other surface analysis of the catalysts are expected.
N adsorption desorption tests
2
The N adsorption/desorption isotherms of various Pd/OMS-2 samples are shown in
2
Fig. 4. N adsorption/desorption isotherms over all samples listed showed a typical
2
type of IUPAC IV isotherm with H2 hysteresis loops indicating the existence of
mesoporous structures. Compared with H-OMS-2, the N adsorption/desorption
2
hysteresis loop of Pd/H-OMS-2 become wider, indicating the increase of pore volume
over Pd/H-OMS-2, after impregnation with palladium. Specific surface area (SSA),
pore volume, and pore size of the various catalysts are shown in Table 1. The average
pore size and pore volume of the Pd/H-OMS-2 are increased after impregnation with
palladium. SSA of the various catalysts increased in the order of Pd/H-OMS-2 \ Pd/
R-OMS-2 \ Pd/C-OMS-2 \ Pd/S-OMS-2, while low-temperature catalytic activity
(
Fig. 1) of all the catalysts decreased with the increased SSA. Pore size distributions
2
Fig. 4 N adsorption–desorption isotherms of the various catalysts at 77 K
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
Table 1 BET specific surface area (SSA), pore volume and pore size of the various catalysts
b
Sample
Surface
Average pore
b
diameter (nm)
Pore volume
3
(cm /g)
a
2
area (m /g)
H-OMS-2
53
53
10.9
15.3
16.6
10.2
8.1
0.13
0.22
0.30
0.26
0.25
Pd/H-OMS-2
Pd/R-OMS-2
Pd/C-OMS-2
Pd/S-OMS-2
67
88
110
a
Calculated by BET method
b
Calculated by BJH method
Fig. 5 Pore size distributions of the different Pd/OMS-2 catalysts
over all catalysts are shown in Fig. 5, all peaks of pore size distribution at 5–45 nm,
corresponding to major characters of mesoporous, which were consistent with the
characters of N -adsorption and desorption isotherms. Pore size distribution of
2
H-OMS-2 could be narrowed after impregnated with palladium. Pd/H-OMS-2 and Pd/
R-OMS-2 exhibit single peaks at 24 ± 1 nm and major pore size distributions at
2
0–35 nm, respectively. Pd/C-OMS-2 exhibits double peaks at 8 and 24 nm, the
average pore size of is 10 nm, while Pd/S-OMS-2 exhibits single peak of pore size
distribution at 9 nm with average pore size of 8 nm. In conclusion, the low-
temperature CO oxidation activity increases with the increase of average pore size. In
other words, in terms of internal diffusion during heterogeneous catalysis, a relative
large diameter of mesoporous size and narrow pore size distribution is preferred to
enhance the low-temperature catalytic conversion of CO [26].
123

Q. Zhang et al.
XPS analysis
X-ray photoelectron spectroscopy (XPS) analysis was used to analyze the chemical
state and surface atomic concentration of Pd, Mn, and O. Pd 3d peaks located at
3
43.0 and 337.5 ± 1 eV (Fig. 6a) indicate the presence of Pd (II) and the absence of
Pd (IV) species [27, 28]. Fig. 6b shows the XPS spectra of Mn 2p over various
samples. It can be observed from the Fig. 6b that Mn 2p3/2 and Mn 2p1/2 of Pd/C-
OMS-2 display binding energy at 640.6, 641.8, and 643.1 eV, which are due to the
2?
3?
4?
valence state of the surface Mn , Mn , and Mn ions, but Mn 2p3/2 and Mn 2p1/2
of Pd/H-OMS-2, Pd/R-OMS-2, and Pd/S-OMS-2 display in binding energy at 641.1,
6
42.3, and 643.6 eV, this suggests that binding energy of Pd/H-OMS-2, Pd/R-OMS-2,
and Pd/S-OMS-2 migrate to the higher numerical value [29]. A quantitative analysis
of peaks areas over the Mn 2p spectra have been carried out and the surface
3
/2
2?
3?
4?
element molar ratios of Mn /Mn /Mn are summarized in Table 2. The XPS
spectra of O1s are shown in Fig. 6c. The main asymmetric peaks of O1s at a binding
energy of 529–530 eV can be attributed to surface lattice oxygen (O ) and binding
a
energy of O1s at 531–532 eV can be ascribed to the surface adsorbed oxygen (Ob)
[30, 31]. Pd/H-OMS-2, Pd/C-OMS-2, and Pd/S-OMS-2 exhibit decreased binding
energy of lattice oxygen. In general, the chemical states and molar ratios of surface
oxygen and lattice oxygen have a great effect on the catalytic activity at low
temperature in the process of the CO catalytic oxidation [12–16]. It can be inferred
From Table 2, that the surface Oads/(Olatt ? Oads) molar ratios of Pd/H-OMS-2, Pd/R-
OMS-2, Pd/C-OMS-2, Pd/S-OMS-2 were 15, 9, 25, and 12 %, respectively, which
have no incremental relative changes on the catalytic activity. It is interesting that Pd/
H-OMS-2 and Pd/R-OMS-2 possess higher catalytic activity and binding energy of O
1
s and Mn 2p than Pd/C-OMS-2 and Pd/S-OMS-2. In addition, impregnation of
palladium into H-OMS-2 lead to the binding energy of O 1s shifted to a higher level.
To our best knowledge, a relatively higher charged surface species might be preferred
to enhance catalytic activity over palladium-based catalysts [32, 33].
H -TPR analysis
2
The hydrogen temperature-programmed reduction (H -TPR) experiments were
2
performed to investigate redox properties of the catalysts. The H -TPR profiles of
2
different catalysts are shown in Fig. 7. The H -TPR of each catalyst at 0–75°C are
2
shown in Fig. 7a. All catalysts exist peaks can be attributed to the reduction of the
surface scattered PdO [34]. Pd/H-OMS-2 exhibits three continuous light peaks of H2
consumption between 0 and 70 °C, indicating existence of multi-Pd (II) species
[
35, 36]. The first slight peak of H consumption and the second peaks over Pd/H-
2
OMS-2 were attributed to the reduction of highly dispersed surface PdO and MnOx
interacted PdO, the third peak was attributed to the reduction of surface Pd–O–Mn
species or hydrogen overflow [37, 38]. Pd/R-OMS-2 showed two H -consumption
2
peaks between 20 and 57 °C, which was attributed to the reduction of surface PdO and
the reduction of surface Pd(II) (or Pd
consistent with our XPS analysis (Fig. 6). Pd/H-OMS-2, Pd/R-OMS-2, Pd/C-OMS-2,
) from Pd–O–Mn structure, respectively,
SMSI
and Pd/S-OMS-2 exhibit peaks and shoulders at 104–180 °C, which was attributed to
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
Fig. 6 XPS spectra of the
catalysts: a Pd 3d peaks,
b Mn 2p peaks, and
c O 1s speaks
123

Q. Zhang et al.
Table 2 Surface abundance (molar ratios. %) of various species over all samples
Sample
Surface atomic concentration (%)
a
2?
Mn \Mn \Mn /Mntol
3?
4?
b
O
Mn
Pd
O
ads/Otol
H-OMS-2
66.0
68.3
66.0
65.7
66.3
33.4
31.1
33.5
33.7
33.1
–
8
10
8
45
46
43
42
45
45
46
48
48
45
Pd/H-OMS-2
Pd/R-OMS-2
Pd/C-OMS-2
Pd/S-OMS-2
0.6
0.5
0.6
0.6
15
9
9
25
12
10
10
Surface Pd, Mn, and O species over Pd/OMS-2 could be quantitatively analyzed as XPS peaks of Pd 3d,
Mn 2p, and O 1s respectively
a
Oads
Oads=Otol ¼
 100 %
O
adsþOlatt
Mn2þnMn nMn4þ
3þ
3þ
b
Mn2þnMn3þnMn4þ=Mntol ¼
2þ
4þ Â 100 %
Mn þMn þMn
the further reduction of subsurface PdSMSI or reduction of surface MnO promoted by
2
the hydrogen overflow of surface exposed palladium [35, 36]. It should be noted that
H-OMS-2 shows two shoulders and a peak at 330, 345, and 375 °C, respectively,
which was ascribed to the gradual reduction of MnO ? Mn O ? Mn O ? MnO
2
3
4
2 3
process [11, 13, 14]. Thus, it can be concluded the strong interaction of Pd–O–Mn
structure may inhibit the reduction surface Pd(II) species, while the reduction of high-
n?
valence Mn can be facilitated by the presence of Pd–O–Mn structure. In conclusion,
SSA and the start temperature of H consumption uptakes over various catalysts
2
increased in the orders of Pd/H-OMS-2 \ Pd/R-OMS-2 \ Pd/C-OMS-2 \ Pd/S-
OMS-2, while crystallinity and low-temperature catalytic activity decreased in the
orders of Pd/H-OMS-2 [ Pd/R-OMS-2 [ Pd/C-OMS-2 [ Pd/S-OMS-2.
The kinetic study of Pd/OMS-2 catalysts
Turn-over frequency (TOF) over various catalysts are listed in Table 3. It can be
inferred from Table 3 that catalytic conversion of CO oxidation increased with the
increase of TOF over different catalysts, except Pd/C-OMS-2. Generally, a higher
TOF is preferred to enhance catalytic activity. However, in order to find more
adequate proof of influencing kinetic factors, further research of reaction rates and
activation energies have been carried out.
Kinetic studies are employed to evaluate the catalytic reaction apparent
activation energy. In order to determine the order of reaction with respect to CO,
the concentration of CO was varied from 5000 to 10,000 ppm. The test temperature
of Pd/H-OMS-2 and Pd/R-OMS-2 at 50 °C,the Pd/C-OMS-2 and Pd/S-OMS-2 at
8
5 °C, the flow rate was 168 mL/min and only 10 mg of catalyst were used to
maintain\20 % CO of conversion. The rates of CO oxidation were calculated using
the following equation [38]:
Rðmol/gsÞ ¼ ðUCO Â V Â XCOÞ=m
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
2 2
Fig. 7 H -TPR profiles of the various catalysts (under 5 % H in Ar with a heating rate of 8 °C/min)
where UCO represents the volume fraction of CO, X is the CO conversion, V
represents the reaction mixture flow rate (mL/min), m represents the catalyst weight
(
g). The calculated rates were presented in Fig. 8, it can be seen that the rates of CO
oxidation over all catalysts increase linearly with the increase of CO concentration.
The reaction order with respect to CO was calculated to be nearly 1 [39]. Overall,
the kinetic results indicated that the rate of CO conversion had a positive depen-
dence on CO concentration. The activation energy (E ) was calculated using the
a
Arrhenius equation [40]:
123

Q. Zhang et al.
Table 3 Turn-over frequency (TOF) of various catalysts
Temperature (°C)
Sample/TOF (s
25
30
35
40
45
50
55
60
a
-1
)
Pd/H-OMS-2
Pd/R-OMS-2
Pd/C-OMS-2
Pd/S-OMS-2
a
3.69
1.05
0.13
0.28
3.82
1.54
0.25
0.42
4.07
1.98
0.36
0.67
4.19
2.01
0.53
0.78
2.11
0.92
1.06
2.15
1.05
1.28
1.34
1.54
1.64
1.76
-1
co/Vco Qco represents the gas flow rate under the actual condition (mL s ), X represents
TOF = Qco
X
the CO conversion, Vco represents the adsorption quantity of catalysts which calculated by CO-TPD area
Fig. 8 Dependence of CO conversion rate on CO inlet concentration on Pd/OMS-2 catalysts (Pd/H-
OMS-2 and Pd/R-OMS-2 at 50 °C,Pd/C-OMS-2 and Pd/S-OMS-2 at 85 °C)
ꢀ
ꢁ
0
kà ¼ À V =W lnð1 À xÞ
kà ¼ A expðÀEa=RTÞ
where V represents the gas flow rate under the actual condition (ml min ),
0
-1
W represents the catalyst weight (g), x represents the CO conversion, E represents
a
apparent activation energy, A represents pre-exponential factor. The Arrhenius plots
of CO reaction rates over Pd/OMS-2 catalysts were shown in Fig. 9. The apparent
activation energy of the reaction are 29.44, 22.57, 21.56, and 18.1 kJ/mol for Pd/S-
OMS-2, Pd/C-OMS-2, Pd/R-OMS-2, and Pd/H-OMS-2 respectively. It can be
1
23

Performance and kinetic study on Pd/OMS-2 catalyst for…
Fig. 9 Arrhenius plot of CO catalytic oxidation on Pd/OMS-2 catalysts
indicated that the preparation methods of OMS-2 had a significant effect on the
activation energy. The Pd/H-OMS-2 shows the lowest apparent activation energy,
which indicated that surface adsorbed oxygen species are easily activated at low
temperature.
Conclusions
Manganese oxide octahedral molecular sieve were synthesized by different
methods. Over 99 % of CO conversion can be obtained by the optimal Pd/H-
OMS-2 catalyst at 35 °C. High-crystalline nanorod OMS-2 with large pore
diameters is preferred to enhance CO catalytic oxidation over OMS-2 supported
Pd catalysts. Hydrothermally synthesized Pd/H-OMS-2 exhibited the largest pore
diameters, highest catalytic activity and crystallinity. SSA, activation energy (Ea)
and the start temperature of H2 consumption uptakes over various catalysts
increased in the orders of Pd/H-OMS-2 \ Pd/R-OMS-2 \ Pd/C-OMS-2 \ Pd/S-
OMS-2, while catalytic activity decreased with the increase of SSA, activation
energy (E ), and the start temperature of H consumption uptake.
a
2
Acknowledgments This work is supported by the National Natural Science Foundation of China (No.
1307047).
2
123

Q. Zhang et al.
References
1
2
3
4
5
6
7
8
. E.D. Park, D. Lee, H.C. Lee, Catal. Today 139, 280–290 (2009)
. Z. Ma, S. Dai, Nano. Res. 4, 3–32 (2011)
. X.D. Zhang, Z.P. Qu, F.L. Yu, Y. Wang, Chin. J. Catal. l34, 1277–1290 (2013)
. X. Xie, Y. Li, Z.Q. Liu, M. Haruta, W. Shen, Nature 458, 746–749 (2009)
. V. Iablokov, K. Frey, O. Geszti, N. Kruse, Catal. Lett. 134, 210–216 (2010)
. M.F. Luo, Y.P. Song, J.Q. Lu, X.Y. Wang, Z.Y. Wang, J. Phys. Chem. 111, 12686–12692 (2007)
. T. Kobayashi, T. Yamada, K. Kayano, Appl. Catal. B Environ. 30, 287–292 (2001)
. G.N. Li, L. Li, Y. Yuan, J.J. Shi, Y.Y. Yuan, W.R. Zhao, L.S. Jian, Appl. Catal. B 158–159, 341–347
(
2014)
. T. Biemelt, K. Wegner, J. Teichert, M.R. Lohe, J. Martin, J. Grothe, S. Kaskel, Appl. Catal. B 184,
08–215 (2016)
9
2
1
1
1
1
1
1
1
0. X.L. Guo, J. Li, R.X. Zhou, Fuel 163, 56–64 (2016)
1. K. Ramesh, L.W. Chen, F.X. Chen, Y. Liu, Z. Wang, Y.F. Han, Catal. Today 13, 477–482 (2008)
2. J.S. Park, D.S. Doh, K.Y. Lee, Top. Catal. 10, 127–131 (2000)
3. R. Xu, X. Wang, D.S. Wang, K.B. Zhou, Y.D. Li, J. Catal. 237, 426–430 (2006)
4. S.H. Liang, F. Teng, G. Bulgan, R.L. Zong, Y.F. Zhu, J. Phys. Chem. C 112, 5307–5315 (2008)
5. H.C. Genuino, M.S. Seraji, Y.T. Meng, D. Valencia, S.L. Suib, Appl. Catal. B 63, 361–369 (2015)
6. N.N. Opembe, C. Guild, C. King’ondu, N.C. Nelson, I.I. Slowing, S.L. Suib, Ind. Eng. Chem. Res.
5
3, 19044–19051 (2014)
7. O. Sanz, J.J. Delgado, P. Navarro, G. Arzamendi, L.M. Gandia, M. Montes, Appl. Catal. B 110,
23–237 (2011)
1
2
1
1
2
2
8. X. Liu, P. Ning, L.S. Xu, Q.X. Liu, Z.X. Song, Q.L. Zhang, RSC Adv. 6, 41181–41188 (2016)
9. Z.Q. Zou, M. Meng, Y.Q. Zha, J. Phys. Chem. C 114, 468–477 (2010)
0. S. Hinokuma, H. Fujii, M. Okamoto, K. Ikeue, M. Machida, J. Mater. Chem. 22, 6183–6190 (2010)
1. Z.J. Yang, J.J. Wei, H.X. Yang, L. Liu, H. Liang, Y.Z. Yang, Eur. J. Inorg. Chem. 21, 3354–3359
(2010)
2
2. A. Taguchi, F. Sch u¨ th, Microporous Mesoporous Mater. 77, 1–45 (2005)
23. X.S. Liu, J.Q. Lu, X.X. Wang, M.F. Luo, Chin. J. Catal. 31, 181–185 (2010)
24. R.N. DeGuzman, Y.F. Shen, E.J. Neth, S.L. Suib, C.L. O’Young, S. Levine, J.M. Newsam, Chem.
Mater. 6, 815–821 (1994)
25. X. Gao, H.A. Lu, C. Zhen, X.X. Jia, J. Mineral. Petrol. 3, 52–57 (2005)
26. Y. Ren, Z. Ma, R.E. Morris, Z. Liu, F. Jiao, S. Dai, P.G. Bruce, Nat. Commun. 4, 2015 (2013)
27. X.P. Cao, L.L. Cao, W.Q. Yao, X.Y. Ye, Surf. Interface Anal. 24, 662–666 (1996)
28. P. Kast, M. Friedrich, D. Teschner, F. Girgsdies, T. Lunkenbein, R.N. Alnoncourt, M. Behrens, R.
Schlogl, Appl. Catal. A 502, 8–17 (2015)
29. D. Fang, J.L. Xie, H. Hu, H. Yang, F. He, Z.B. Fu, Chem. Eng. J. 271, 23–30 (2015)
30. A. Bensalem, F. Bozon-Verduraz, M. Delamar, G. Bugli, Appl. Catal. A 121, 81–93 (1995)
31. S. Hamoudi, F. Larachi, A. Adnot, A. Sayari, J. Catal. 185, 333–344 (1999)
32. M.S. Kim, S.H. Chung, C.J. Yoo, M.S. Lee, I.H. Cho, D.W. Lee, K.Y. Lee, Appl. Catal. B 142–143,
354–361 (2013)
33. X. Yuan, G. Sun, H. Asakura, T. Tanaka, X. Chen, Y. Yuan, G. Laurenczy, Y. Kou, P.J. Dyson, Y.
Ning, Chem. Eur. J. 19, 1227–1234 (2013)
3
3
3
3
4. S.Y. Lin, L.Y. Yang, X. Yang, R.X. Zhou, Chem. Eng. J. 247, 42–49 (2014)
5. R.X. Zhou, B. Zhao, B.H. Yue, Appl. Surf. Sci. 254, 4701–4707 (2008)
6. B.H. Yue, R.X. Zhou, Y.J. Wang, X.M. Zheng, Appl. Surf. Sci. 252, 5820–5828 (2006)
´
7. M.C. Alvarez-Galv a´ n, B. Pawelec, V.A. de la Pe n˜ a O’Shea, J.L.G. Fierro, P.L. Arias, Appl. Catal. B
51, 83–91 (2004)
3
3
4
8. R.V. Gulyaev, E.M. Slavinskaya, S.A. Novopashin, D.V. Smovzh, A.V. Zaikovskii, D.Y. Osadchii,
O.A. Bulavchenko, S.V. Korenev, A.I. Boronin, Appl. Catal. B 47, 132–143 (2014)
9. J.L. Ayastuy, A. Gurbani, M.P. Gonz a´ lez-Marcos, M.A. Gutierrez-Ortiz, Ind. Eng. Chem. Res. 48,
5633–5641 (2009)
0. Z.M. Wu, L. Zhang, Q.Q. Guan, M.L. Fu, D.Q. Ye, T. Wu, Mater. Res. Bull. 70, 567–572 (2015)
1
23