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Graphical abstract
A hierarchically porous zeolite Beta as a carrier was designed via a combined approach of
surfactant-assisted hydrothermal treatment and the following alkali etching. Then, the Cu and Mn
species with high dispersity were co-loaded into the carrier. The obtained catalyst exhibits highly
efficient and stable catalytic activity, as well as an excellent resistance to water vapor in soot catalytic
oxidation
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Journal Name
RSC
Publishing
ARTICLE
This journal is © The Royal Society of Chemistry 2013 J. N am e., 2 01 3, 00, 1- 3 | 1
Cite this: DOI: 10.1039/x0xx00000 x
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cu/Mn co-Loaded Hierarchically Porous Zeolite Beta：
：：
：A Highly
Efficient Synergetic Catalyst for Soot Oxidation
Xiaoxia Zhou, Hangrong Chen*, Guobin Zhang, Jin Wang, Zhiguo Xie, Zile Hua, Lingxia
Zhang and Jianlin Shi*
A hierarchically porous zeolite Beta has been designed via a facile combined surfactant-assisted
hydrothermal treatment and subsequent alkali etching approach. High contents of the Cu and Mn species
were then highly dispersed into the as-synthesized hierarchically porous zeolite Beta. The prepared
composite catalyst exhibits highly active, stable and recyclable catalytic performances for soot oxidation,
and the extraordinarily low T
50
(260 ℃) and T
90
(300 ℃) values can be achieved in the presence of NO.
A synergetic catalytic effect between valence-varied active species Cu
n+
and Mn
n+
has been proposed
which is featured with the promoted generations of highly active oxygen species O
-
/O
2-
and active
intermediate NO
2
. Besides, the obtained catalyst shows an excellent water-resistance performance in the
soot oxidation thanks for the hydrophobic crystalline zeolite framework and the interaction between soot
and water vapor.
Introduction
Currently, more and more diesel engines are being used on vehicles
due to their high thermal and economical efficiency. However, the
particulate matter (PM, mainly containing soot) and NO
x
emitted
from diesel engines have been arousing increasingly serious
environmental problems.
1-4
Therefore, the efficient treatment of soot
particles has become one of the most important issues in
environment protection. Generally, a diesel particulate filter (DPF) is
used for the emission reduction of soot particles,
5
and the collected
soot particles in DPF can be oxidized by oxygen at a relatively high
temperature (>700 ℃). However, the diesel exhaust temperature is
usually in between 150 to 400 ℃, which is too low to ignite soot
combustion by oxygen, thus leading to the inactivation of the DPF
owing to the accumulation of soot particles.
6
Therefore, one of the
most hopeful and ideal strategies is how to design and preparation of
a highly efficient catalyst to promote the soot oxidation at much
lower temperatures.
Several catalysts, such as ceria-based compounds,
6-7
and
perovskite-like oxides
8-9
have been reported but showed low
catalytic activities for the oxidation of soot nanoparticles. Very
recently, Corro et al investigated the catalytic behaviors of Ag, Cu
and Au loaded fumed SiO
2
for the diesel soot oxidation,
10-11
and it
was found that the Ag/SiO
2
catalysts exhibited strong catalytic
performance for diesel soot oxidation while the catalytic activity of
the Cu/SiO
2
nearly completely disappeared during the subsequent
reaction cycles.
10
In addition, Liu and Zhao et al
12-13
reported
Pt
2.0
/CeO
x
-loaded three-dimensionally ordered macroporous (3DOM)
structure as a novel catalyst, which showed high catalytic
performances at rather low T
50
(330 ℃) and T
90
(365 ℃) values for
the soot oxidation. But they mainly aimed at the catalytic activity,
lacking of the researches on water-resistance and cycling
performance of the catalyst.
12
Generally, zeolite Beta with unique
three-dimensional network of large pores (12MR) exhibits much
higher surface area than medium pores of zeolite ZSM-5 (10MR),
which is much more helpful to the uniform dispersion of active
species. The existence of large amount of macropores (> 50 nm) in
zeolite often induces the structural collapse during heat-treatment.
Therefore, we believe that the hierarchically porous zeolite Beta
containing an inter-penetrating micro-mesoporous system, which has
higher surface area and thermal/hydrothermal stability, is more
practical and favorable for soot oxidation. In addition, the further
investigation of real-world catalysis under the simulated exhaust gas
involving a large quantity of moisture is highly desired.
Unfortunately, most of the reported noble metal-based catalysts with
the excellent catalytic activity showed poor water-resistance and
cycle property in the soot oxidation. Therefore, it is still
enormous challenges to develop the non noble metal-based catalyst
in the soot oxidation.
Herein, a novel hierarchically porous zeolite Beta as the support
has been successfully prepared via a surfactant-assisted
hydrothermal approach, followed by a facile alkali etching, which
has firstly applied to the oxidation of soot nanoparticles.
It is well
known that zeolite Beta is a kind of crystalline aluminosilicate with
better hydrophobicity than most of metal oxides due to the presence
of extensive [SiO
4
] tetrahedrons in the framework, which is
beneficial for the water-resistance. Besides, zeolite Beta can produce
abundant acid sites and oxygen vacancies due to the introduction of
heteroatom Al,
14-15
which is beneficial to the adsorption of O
2
.
In
addition, it was demonstrated that the presence of strong acid, i.e.
H
2
SO
4
, could promote the NO
2
formation and soot oxidation.
16
Therefore, we believe that the rich acid sites, especially the strong B
acid in zeolite framework,
can also promote the NO
2
formation and
soot oxidation. Additionally, the distinctively hierarchical porous
structure is expected to greatly facilitate the mass transport of guest
molecules
17-21
(e.g., soot particles), and ensure the full contact
between soot particles and active species in the pore channels, and
consequently promote the soot oxidation. Therefore, the hierarchical
porous zeolite Beta could be an excellent carrier capable of
significantly promoting the soot oxidation. The active Cu and Mn
species were afterwards co-dispersed in/on the carrier by a co-
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impregnation method. The obtained bi-functional catalyst Cu/Mn co-
loaded hierarchically porous Beta (CuMn-HBeta) shows remarkable
catalytic activity, excellent water-resistance and good recyclability in
the soot oxidation process. Finally, the possible synergetic catalytic
mechanism of soot oxidation on this prepared catalyst CuMn-HBeta
is also proposed and detailedly discussed.
Experimental Section
Chemicals and reagents
Tetraethylammonium hydroxide (TEAOH, 25 wt%) and
cetyltrimethylammonium chloride (CTAB, 99%) were obtained from
Shanghai J&K, China. Silicic acid hydrate(H
2
SiO
3
, AR, Sinopharm
Chemical Reagent Co., Ltd)
The synthesis of samples
Preparation of the mesoporous Beta (MBeta)
First, 0.15 g KCl was added into 6 mL distilled water and 14.4 g
TEAOH solution. Then, 50 mmol H
2
SiO
3
were dissolved into the
above solution and stirred at 313 K. Second, the solution containing
0.033g NaOH and 2 mmol NaAlO
2
were slowly added into the
resultant solution and further stirred at 313 K for 8 h. Finally, 1 g
CTAB was added into the obtained solution and further stirred at
353 K for 8 h. Next, the obtained mixed solution was hydrothermal
treated 24 h at 423 K, after that, the products were washed with
distilled water and dried at 373 K for 10 h. The final product MBeta
was obtained after calcination at 823 K for 8 h to remove any
organics.
Herein, the introduction of alkali metal ions K
+
can improve the
crystallization kinetics of zeolite, which can increase the
crystallization rate and shorten the hydrothermal crystallization time
of zeolite. The choice of H
2
SiO
3
as silicon source can shorten
induction period of nucleation and increase the crystallization rate.
Preparation of the hierarchically porous Beta (HBeta)
The obtained MBeta (0.5 g) was added to 50 mL distilled water
containing 0.25 mmol NaOH and stirred at 80 ℃ for 8 h, and the
HBeta with wide pore size distribution can be obtained. Before all
catalysis tests, the catalyst HBeta was ion-exchanged three times
with NH
4
+
by using a 10% NH
4
NO
3
solution and then calcined in air
at 550 °C for 4 h to transfer it into the H form zeolite Beta.
Preparation of the Cu-HBeta, Mn-HBeta and the CuMn-HBeta
10 mmol copper(II) nitrate (Cu(NO
3
)
2
·3H
2
O) were dissolved in
20 mL distilled water and 1 mL ethanol mixed solution to form a
homogeneous solution, and then 0.5 g HBeta sample was added into
the solution and stirred at 353 K for 8 h. After that, the sample was
washed with deionized water and then dried at 373 K overnight.
Finally, the sample Cu-HBeta was synthesized after calcination at
823 K for 6 h.
1 mmol potassium permanganate (KMnO
4
) was dissolved in 20
mL distilled water and 1 mL ethanol mixed solution to form a
homogeneous solution, and then and 0.5 g HBeta sample was added
into the solution and stirred at 353 K for 8 h. After that, the sample
was washed with deionized water and then dried at 373 K overnight.
Finally, the sample Mn-HBeta was synthesized after calcination at
823 K for 6 h.
1 mmol KMnO
4
and 10 mmol (Cu(NO
3
)
2
·3H
2
O) were dissolved
in 20 mL distilled water and 1 mL ethanol mixed solution to form a
homogeneous solution, and then 0.5 g HBeta sample was added into
the solution and stirred at 353 K for 8 h. After that, the sample was
washed with deionized water and then dried at 373 K overnight.
Finally, the sample CuMn-HBeta was synthesized after calcination
at 823 K for 6 h. The contents of Cu and Mn-loading were controlled
by adjusting concentrationof Cu(NO
3
)
2
and KMnO
4
in the solution.
Preparation of the Al-MCM-41
The mesoporous material Al-MCM-41 was obtained by a
conventional hydrothermal system. First, the aluminum isopropoxide
(0.2 mmol) was dissolved in 120 mL deionized water and 1.2 mmol
NH
3
solution, and then the cetyltrimethylammonium chloride (0.85
mmol) was added to the above solution to form a homogeneous
solution. Second, the tetraethoxysilane (5 mmol) was slowly added
to the mixture solution under strongly stirring. Afterward, the
obtained solution was hydrothermal treated at 393 K and then was
dried at 333 K. The final product Al-MCM-41was obtained after
calcination at 823 K for 8 h. Finally, the larger mesoporous was
created by the similar alkali (NaOH) etching process.
Preparation of the CuMn-Al-MCM-41and CuMn-MBeta
The sample CuMn-Al-MCM-41 and CuMn-MBeta were
prepared by a simple co-impregnating method, similar to above
synthesis process.
Characteristics
Powder X-ray diffraction (XRD) patterns of the prepared
samples were tested on a Rigaku D/Max 2200PC diffractometer with
Cu Kα radiation (40 kV and 40 mA). The nitrogen sorption and
desorption curves were tested using Micromeritics Tristar 3000 at 77
K, and the specific surface area and the pore size distribution were
calculated using the Brunauer-Emmett-Teller (BET) and Barrett-
Joyner-Halenda (BJH) methods, respectively. Transmission Electron
Microscopic (TEM) imaging was performed on a JEOL-2010F
electron microscope operated at 200 kV. Field Emission Scanning
Electron Microscope (FE-SEM) and element mapping images were
obtained on Hitachi S-4800. The temperature programmed
desorption of ammonia (NH
3
-TPD) were performed on a on
Micromeritics Chemisorb 2750 instrument. 50 mg of sample was
heated at 823 K in N
2
for 1 h and after cooling to 373 K, then
ammonia adsorption was carried out for 30 min. The NH
3
-TPD of
the samples was carried out by temperature programed from 373 to
823 K at a heating rate of 10 K min
−
1
under N
2
flow (25 mL/min).
The desorption amount of NH
3
was measured using a thermal
conductivity detector (TCD). The temperature programed reduction
with hydrogen (H
2
-TPR) experiments were also performed on a on
Micromeritics Chemisorb 2750 instrument under the mixture of 5%
H
2
in N
2
flow (25 mL/min) on 50 mg catalyst with a heating rate of
10 K/min, respectively. The uptake amount of H
2
was measured
using a thermal conductivity detector (TCD). The contents of Cu and
Mn were measured by using Inductively Coupled Plasma Atomic
Emission Spectroscopy (ICP-AES) analyzer on a Vista AX. X-ray
photoelectron spectroscopy (XPS) signals were recorded on an
ESCAlab250 instrument. The fitting of Mn element was carried out
by using Gaussian fitting parameters.
The FT-IR spectra for pyridine
adsorption were recorded on a Bruker spectrometer equipped with a
MCT detector. All samples were treated at 423 K for 60 min before
pyridine adsorption. Next, pyridine was adsorbed for 10 min and
then evacuated for 30 min at room temperature. Finically, all the
spectra could be collected and subtracted with the background
reference.
Catalytic activity measurement
The oxidation of NO
The NO oxidation was performed in a quartz tubular fixed-bed
flow reactor. The feed gas composition contains 500 ppm NO, 10%
O
2
and balance N
2
, W/F = 0.03 (g·s)/mL. The total flow rate of the
feed gas was 200 mL/min and the space velocity is 120000 h
-1
. The
NO and NO
2
concentrations in the inlet and outlet gas were online
monitored by the NO
x
analyzer (Thermo Fisher 42i-LS).
The oxidation of soot particles
Carbon black from Degussa (particle size was 10-50 nm) was
used as the model soot particles. The model soot particles (10 mg)
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and catalyst (100 mg) were mixed at weight ratios of 1:10 with a
spatula to simulate the loose contact mode. Then, the mixture was
placed in a quartz tube after mixing with 200 mg silica pellets. A gas
mixture of 500 ppm NO、10% O
2
and balance N
2
was feed with a
flow rate of 200 mL/min and the space velocity is 120000 h
-1
, W/F =
0.03 (g·s)/mL. The analysis of the products was performed using an
online GC analyzer equipped with FID detectors. The catalytic
activity of soot oxidation was evaluated by the values of T
10
, T
50
and
T
90
, which were defined as the temperatures at 10%, 50% and 90%
of soot oxidation, respectively.
Results and Discussion
The synthesis of Cu/Mn co-loaded hierarchically porous
zeolite Beta
The schematic drawing of the synthesis of CuMn-HBeta is
shown in Scheme 1. The MBeta containing an extensive wormlike
small mesopore network (around 4 nm in diameter) was firstly
obtained by a common surfactant CTAB assisted hydrothermal route,
followed by selective alkali (NaOH) etching to obtain the HBeta,
which created abundant large mesopores (dozens of nanometers in
diameter), besides the micropores and small mesopores, as shown in
Step 1. Next, the CuMn-HBeta was successfully synthesized by a co-
impregnation method. Herein, Cu/Mn species could be uniformly co-
dispersed in/onto HBeta carrier owing to the large surface area and
3D hierarchically porous structure, as shown in Step 2.
Scheme 1. Schematic drawing of the formation of the sample CuMn-HBeta.
Step 1: creation of large mesopores by NaOH solution etching; Step 2: co-
dispersion of Cu and Mn species an/in hierarchically porous Beta matrix.
Structural Characteristics
FE-SEM images of different samples shown in Figure S1a-b
reveal that zeolite Beta is composed of small nanocrystals, and the
MBeta becomes rough and porous, which can be ascribed to the
generation of the porous structure after adding the CTAB molecules.
The HBeta (Figure S1c) presents a looser and more porous structure
than MBeta (Figure S1b), and there are two types of mesopores
presented in the sample HBeta: small mesopores and large
mesopores, as shown in Figure S1d, which can be ascribed to the
stacking between zeolite nanoparticles and the alkali etching process,
respectively, thus the sample HBeta shows a hierarchically porous
structure with wide pore size distribution. After loaded with Cu and
Mn species, the obtained CuMn-HBeta still keeps the zeolite Beta
morphology and porous structure (Figure 1a-b). In addition, The
XRD pattern of the sample CuMn-HBeta (Figure S2) clearly exhibits
the diffraction peak of typical zeolite Beta structure, and the
Cu
1.5
Mn
1.5
O
4
spinel phase, as a highly active species, can be detected
in CuMn-HBeta sample, which is favorable to enhance the catalytic
activity.
22
During the inorganic salts co-impregnation process, there
is no denying that some cations (i.e. H
+
, Al
3+
) of zeolite framework
can be replaced by Cu and Mn ions after co-loading Cu and Mn
species due to the strong ion exchange ability of zeolite, which
makes zeolite framework structure inevitably be destroyed in a
certain extent, therefore, the intensity of XRD peak of CuMn-HBeta
decreased with the loading of Cu and Mn species (Figure S2).The
typical TEM image in Figure 1c indicates that the CuMn-HBeta has
an apparent mesoporous structure and the Cu/Mn active components
have been highly co-dispersed in the HBeta. The high-magnification
TEM image has been inserted in Figure 1c, which further indicates
that the metal oxides with the size smaller than 20 nm have been
highly dispersed in the carrier HBeta. The EDS spectrum of selected
area of the CuMn-HBeta in Figure 1d confirms the co-existence of
Si, Al, O, Cu and Mn elements. In addition, the element mappings of
CuMn-HBeta in Figure 1e-h further confirm high dispersity of both
Cu (9.0 wt%) and Mn species (9.0 wt%).
The N
2
adsorption/desorption isotherms and the pore size
distribution curves of the samples MBeta, HBeta and CuMn-HBeta
are shown in Figure S3, and the corresponding results of pore
structure parameters are summarized in Table S1. All the samples
exhibit typical type IV isotherms, confirming the presence of
mesopore structure. For the reference sample MBeta with well-
defined mesopores of 3.8 nm in diameter, the BET surface area and
total pore volume were calculated to be 583 m
2
g
−1
and 0.41cm
3
g
−1
respectively. Thereinto, the mesopore surface area and mesopore
volume were 208 m
2
g
−1
and 0.30 cm
3
g
−1
, respectively. The carrier
HBeta shows a remarkably increased N
2
adsorption volume at
elevated relative pressure in the N
2
adsorption/desorption isotherm,
confirming the presence of large mesopores (7-60 nm). Compared
with MBeta, HBeta shows a little decreased BET total surface area
(402 m
2
g
−1
) but the increased mesopore surface area (281 m
2
g
−1
).
Moreover, both the total pore volumes (0.65 cm
3
g
−1
) and mesopore
pore volume (0.58 cm
3
g
−1
) of HBeta are higher than that of the
MBeta, indicative of the successful creation of 3D hierarchical pore
structure. After loading with Cu and Mn species, both BET surface
area and total pore volume of CuMn-HBeta show considerable
reduction, due to some of active Cu and Mn species dispersion into
HBeta mesopore channels. Nevertheless, the prepared CuMn-HBeta
maintains the original pore size distributions within 7-50 nm, large
BET total surface area of 289 m
2
g
−1
and mesopore surface area of
196 m
2
g
−1
, as well as high total pore volume (0.42 cm
3
g
−1
) and
mesopore pore volume (0.36 cm
3
g
−1
). In addition, the Si/Al ratio of
MBeta is 22, and the value reduces to 10 for the HBeta, which was
attributed to the loss of Si in zeolite framework during the alkali
treatment. Interestingly, the Si/Al value of CuMn-HBeta increased to
12, resulted from the exchange of some Al
3+
cations in zeolite
framework by Cu/Mn ions after co- loading Cu/Mn species.
Figure 1. a-b) Typical FE-SEM, c) low-magnification and high-
magnification TEM images (inset) and d) the corresponding EDS figure of
the CuMn-HBeta, e-h) the element mappings of Si, Cu O and Mn of the
CuMn-HBeta.
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Figure 2. a) Cu 2p photoeletron spectrum, b) Cu LMM Auger spectrum, c)
Mn 2p and d) O 1s photoeletron spectra of CuMn-HBeta. Peak fitting is
carried out on the 2p 3/2 peak of Cu element.
Table 1. The XPS surface composition analysis of Cu and O of the catalyst
CuMn-HBeta: binding energy (eV) and percentage of total area.
Element Cu O
Cu
+
Cu
2+
O
I
O
II
O
III
Binding Energy
/ eV
931.3
934.1
530.1
531.6
532.9
Percentage of
total area./ %
17.2
82.8
32.1
38.4
29.5
XPS analysis results of the prepared CuMn-HBeta (Figure 2a-c)
indicate that both Cu and Mn species present variable valences. For
the sample CuMn-HBeta, the Cu2p3/2 photoemission peak presents
two components: the high intensity peak located at 934.1 eV is
assigned to Cu
2+
ion and the low intensity one at 931.3 eV
corresponds to Cu
+
ion (Figure 2a), which accounts for 82.8% and
17.2% of the total Cu content, respectively (Table 1). In addition, it
is found that the Cu2p3/2 photoelectron spectrum for the CuMn-
HBeta contains high intensity satellite peaks (denote with *) at ca. 9
eV above the main peak, further indicating the presence of Cu
2+
(Figure 2a). The binding energies of the CuLMM Auger peaks of the
sample CuMn-HBeta at 569.2 and 570.3 eV, as shown in Figure 2b,
can be well-ascribed to the Cu
2+
and Cu
+
,
respectively,
23a
suggesting
the absence of Cu in the CuMn-HBeta. The Mn2p3/2 spectrum level
at 642.2 eV can be attributed to an oxidation state between Mn
3+
and
Mn
4+
, as shown in Figure 2c.
The O 1s XPS peak of the sample CuMn-HBeta can be
deconvoluted into three components at about 530.1, 531.6 and 532.9
eV after Gaussian fitting, as shown in Figure 2d, which can be
ascribed to lattice oxygen O
Ⅰ
, hydroxyl oxygen O
Ⅱ
and adsorbed
oxygen O
III
, respectively
23b-c
. The lattice oxygen O
I
can be derived
from O
2-
ions in the CuO
x
and MnO
x
lattices of CuMn-HBeta.
Besides, as high as 29.5% (Table 1) of surface adsorbed oxygen (O
Ⅲ
)
can be found in this sample (Figure 2d), indicating that the sample
CuMn-HBeta is more active for the adsorption of oxygen species
probably owing to the existence of oxygen vacancies in this sample.
The formation of the oxygen vacancy
Herein, the generation of oxygen vacancy origins from the
following respects: 1) the escape of crystal oxygen in the heat-
treatment process, as shown in reaction 1; 2) the doping of the
heteroatom Al in [SiO
4
], as shown in reaction 2; 3) the interaction
between low-valence state Cu
+
and high-valence state MnO
2
, as
shown in reaction 3; 4) the interaction between low-valence state
Mn
3+
and high-valence state CuO, as shown in reaction 4. Especially,
a large number of oxygen vacancies can generate through the
interactions between valence-varied Cu and Mn (reaction 3 and 4).
Ocrystal 1/2 O2 + VO
.
.
(1)
Al
3+
+
[SiO
4
]
Al
S
i
+ V
O
.
.
.
(2)
Cu
+
+
MnO
2
Cu
2+
+ MnO
2
-
x
+ V
O
.
.
(3)
Mn
3+
+
CuO
Mn
4+
+ CuO
1
-
x
+ V
O
.
.
(4)
Effects of active species and the carrier on soot oxidation
From H
2
-TPR profiles (Figure 3), it can be found that the
reference single loaded sample Mn-HBeta shows two clear reduction
peaks at around 350 and 420 ℃, which are ascribed to the reduction
of Mn
4+
→Mn
3+
and Mn
3+
→Mn
2+
, respectively.
24-25
In addition, the
reduction peak of another reference Cu-HBeta sample related to
Cu
2+
→Cu
+
can be observed at around 215 ℃.
26a
More interesting,
the H
2
-TPR profile of co-loaded sample CuMn-HBeta shows a
distinctively different redox behavior from either Cu-HBeta or Mn-
HBeta. The reduction peaks significantly shift to lower temperature
region, indicating that the redox behavior of catalyst can be greatly
promoted after co-loading the Cu and Mn species. This is probably
attributed to the strong interaction between valence-varied active
species MnO
x
and CuO
x
in which the mobility of oxygen species can
be greatly promoted, similar to the previous report.
26b
Therefore, in
the soot oxidation, the co-loaded sample CuMn-HBeta shows an
excellent catalytic performance with much lower T
50
(260 ℃) and
T
90
(300 ℃) values than those of the references Cu-HBeta and Mn-
HBeta, as shown in Table 2 (Entry 4, 5 and 7), which is mainly
ascribed to the strong redox behaviors of valence-varied Cu
n+
and
Mn
n+
.
Besides the active components, the catalyst carrier with strong
acid and large surface area also plays an important role in the
catalytic reaction. The large surface area of this prepared
hierarchically porous HBeta is beneficial for both high dispersion of
active species and the enlarged contact area between active species
and soot nanoparticles. In addition, the acidity of catalyst carrier is
another important factor in the soot oxidation, i.e. the soot oxidation
Figure 3. The H
2
-TPR profiles over the sample CuMn-HBeta and the
references Cu-HBeta and Mn-HBeta.
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Figure 4. FTIR pyridine adsorption of the samples HBeta and CuMn-HBeta.
Table 2. The catalytic performances of soot oxidation in the presence of NO
over different catalysts or without catalyst (loose contact condition).
Entry
Sample Cu
[d]
(wt%)
Mn
[d]
(wt%)
T
10
(℃)
T
50
(℃)
T
90
(℃)
S
CO2
(%)
1 Pure soot
[a]
- - 480 570 655 60.0
2 Pure soot
[b]
- - 220 370 495 86.0
3 HBeta
[a]
- - 455 545 600 97.0
4 Cu-HBeta
[a]
9.0 - 290 350 500 >99.9
5 Mn-HBeta
[a]
- 9.0 290 485 500 >99.9
6 CuMn-MBeta
[a]
9.0 9.0 280 380 430 >99.9
7 CuMn-HBeta
[a]
9.0 9.0 180 260 300 >99.9
8 CuMn-Al-
MCM-41
[a]
9.0 9.0 240 330 400 >99.9
9 CuMn-
HBeta(aged)
[a,c]
9.0 9.0 200 255 305 >99.9
[a]
Reaction condition: 10% O
2
and 500 ppm NO in N
2
, total flow is 200 mL
min
-1
; the space velocity is 120000 h
-1
; W/F = 0.03 (g·s)/mL; the mass ratio
(catalyst/soot) is 10:1.
[b]
Reaction condition: 10% O
2
and 500 ppm NO
2
in N
2
,
total flow is 200 mL min
-1
; the space velocity is 120000 h
-1
; W/F = 0.03
(g·s)/mL; the mass ratio (catalyst/soot) is 10:1.
[c]
Aged condition: 6% H
2
O in
N
2
, 800 ℃ for 36 h.
[d]
The contents of Cu and Mn were measured by ICP-
AES analyzer.
can be promote in the presence of acidic support HBeta, as shown in
Table 2 (Entry 3). In order to investigate the effect of different
catalyst carriers, a reference CuMn-Al-MCM-41 with same loading
amount of active components and similar mesopore size distribution
(7-50 nm) has been prepared for comparison. The NH
3
-TPD profiles
shown in Figure S4, indicate that all samples MBeta, HBeta, CuMn-
Al-MCM-41 and CuMn-HBeta show a strong desorption peak at
lower temperature range (< 200 °C), which can be assigned to
ammonia desorption from weak acid site. For the reference MBeta
and HBeta, a small peak at about 440 °C occurs, which is assigned to
ammonia desorption from strong acidic sites related to framework Al
atoms.
27a
More interesting, the prepared sample CuMn-HBeta
exhibits a weak desorption peak at a higher temperature (540 °C),
which can be attributed to the strong acid sites related to framework
Al atoms and the highly dispersed metal oxides
27a
. Furthermore, the
FT-IR analysis of adsorbed pyridine as probe molecules has been
conducted for the samples HBeta and CuMn-HBeta (Figure 4). The
band at 1547 cm
−
1
indicates the presence of Brønsted acidic centre,
which is related to the acidic proton of Si-O(H)-Al group, and the
band at 1445 cm
−
1
can be attributed to Lewis acidic centre (Al
3+
).
27b
In addition, the band at 1483 (CuMn-HBeta) and 1491 cm
−
1
(HBeta)
can be related to the interaction between pyridine and both Brønsted
and Lewis acidic centres. The relative concentrations of Brønsted
acid, Lewis acidic and the B/L ratios of samples HBeta and CuMn-
HBeta can be calculated from the area of PyH+ and PyL peaks (1545
and 1450 cm
-1
), as shown in Figure 4. The CuMn-HBeta shows a
little lower B/L ratio (0.07) compared to HBeta (0.10), indicating the
inevitable loss of proton acid in zeolite after co-loading the Cu and
Mn species. The results of soot catalytic oxidation are shown in
Table 2 (Entry 7 and 8). It is clear that CuMn-HBeta shows much
higher catalytic activity with lower temperatures of T
50
(260 °C) and
T
90
(300 °C) than those of the reference CuMn-Al-MCM-41 with
higher T
50
(330 °C) and T
90
(400 °C) temperatures, confirming the
skeleton acidity presented in the HBeta carrier, especially the distinct
strong B acid, can greatly promote soot oxidation.
Effect of the water vapor on the soot oxidation
In order to investigate the effect of water vapor on the soot
oxidation in the catalytic reaction system, two similar soot oxidation
processes over the catalyst CuMn-HBeta were conducted with and
without H
2
O vapor, as shown in Figure 5a. It is clear that the water
vapor in the reaction gas is favorable for the soot oxidation at higher
temperatures (> 350 ℃), which is consistent with the previous report
of Chun et al., since the activation energy of carbon with water could
be greatly reduced from 60±2 kJ/mol without water, to 40±2 kJ/mol
in the presence of water,
28
indicating that the water vapor could
involve the soot oxidation reaction and lowered the activation energy
of soot oxidation. Furthermore, the catalytic activity of the soot
oxidation on the catalyst CuMn-HBeta with different amount of
water vapor was tested in the presence of NO, as shown in Figure 5b.
Though the soot oxidation efficiency slightly reduces at lower
temperatures (< 350 ℃) with the increased contents of water vapor,
the catalyst CuMn-HBeta keeps very stable and high catalytic
activity for soot oxidation under varied water contents from 1.0 to 11
vol.%, especially at higher temperatures (> 350 ℃). The above
results indicates that a certain amount of water vapor can be firstly
adsorbed onto the catalyst and partly cover some active sites, thus
leading the slightly reduced catalytic activity at lower temperatures
(< 350 ℃), next, the adsorbed water vapor on the catalyst can
involve in the soot oxidation and the covered active sites can re-
expose at higher temperatures (> 350 ℃), which endows the
prepared catalyst with good water-resistance. In addition, the strong
hydrophobicity of crystalline zeolite Beta framework also prevents
water molecules from accessing most active sites on the catalyst
CuMn-HBeta to some extent, thus further improving the water-
resistance of the catalyst.
Figure 5. a) The soot oxidation over the catalyst CuMn-HBeta in the
different reaction conditions (Reaction condition: 10% O
2
; 500 ppm NO in
N
2
; 6% H
2
O; total flow is 200 mL min
-1
; the space velocity is 120000 h
-1
;
W/F = 0.03 (g·s)/mL; the mass ratio (catalyst/soot) is 10:1); b) the influence
of H
2
O vapor contents on the soot oxidation over the sample CuMn-HBeta in
the soot catalytic oxidation (Reaction condition: 10% O
2
; 500 ppm NO in N
2
;
total flow is 200 mL min
-1
; the space velocity is 120000 h
-1
, W/F = 0.03
(g·s)/mL; the mass ratio (catalyst/soot) is 10:1).
The thermal stability and the reusability of catalyst
CuMn-HBeta
The cycle performance and hydrothermal stability of the catalyst
CuMn-HBeta have been further investigated. It can be found that the
prepared sample CuMn-HBeta maintains high catalytic activity even
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after 5 cycle tests (Figure S5), which can be ascribed to the highly
dispersed active species Cu
n+
and Mn
n+
on the carrier, as well as the
stable crystalline structure of zeolite carrier at elevated temperatures.
It is worth noting that the slightly decreased catalytic activity in the
cycle tests is attributed to the inevitable loss of the catalyst during
recovering after each cycle test. The anti-aging performance
experiments of CuMn-HBeta were conducted in the condition of 6%
H
2
O in N
2
at 800 ℃ for 36 h. It is interesting that the catalyst still
keeps an excellent catalytic activity in the soot oxidation after aging
(Table 2, Entry 9), which can be attributed to the support HBeta with
high thermal and hydrothermal stability. Besides, the similar results
of N
2
adsorption/desorption (Figure S6 and Table S1), NH
3
-TPD
(Figure S4), XPS (Figure S7 and Table S2) analyses and the same
Si/Al ratio of the catalyst CuMn-HBeta before and after aging (Table
S1) also confirm that the CuMn-HBeta has the excellent texture and
chemical structure stability.
The possible catalytic mechanism for soot oxidation
The soot oxidation is a typical heterogeneous catalytic reaction,
and the crucial issue affecting the catalytic performance is the
amount of active sites and the contact area between the catalyst and
soot particles. Therefore, the design of larger mesopore for the
catalyst is an effective approach for improving the soot oxidation
performance. Our previous work demonstrated that the introduction
of hierarchically porous structure in CuO
x
/CeO
2
co-loaded ZrO
2
-
TiO
2
nanocomposite can greatly promote the soot oxidation in the
presence of NO/O
2
.
29
Therefore, it is expected that the sample
CuMn-HBeta can show a much superior catalytic activity than the
CuMn-MBeta without hierarchical pore structure, as shown in Table
2 (Entry 6 and 7), since it provides the quick diffusion and transfer
channels for soot nanoparticles in large and three-dimensional pore
structure.
.
.
O
2
+ V
O
O
ads
(5)
.
.
N
O + VO NOads
(6)
Based on above results and discussions, a possible mechanism
for soot oxidation on CuMn-HBeta can be proposed and the
pathways are summarized in Scheme 2. First of all, the oxygen can
be adsorbed on the surface of the catalyst CuMn-HBeta due to the
presence of a large amount of oxygen vacancies (Figure 2d), leading
to the generation of lots of surface adsorbed oxygen (O
ads
) (Figure
2d), as shown in reaction 5. Besides, the NO molecules could also be
adsorbed (NO
ads
) on the surface of the catalyst CuMn-HBeta with
the action of oxygen vacancy, as shown in reaction 6. Secondly, the
highly active oxygen species (O
-
/O
2-
) can be created by the
synergetic effect from the interaction between valence-varied Cu
n+
and Mn
n+
(Step 1). In detailed, the synergetic effect between valence-
varied Cu
n+
and Mn
n+
may be explained as follows: the highly active
Cu
+
can donate an electron to Mn
4+
and generates Mn
3+
and Cu
2+
by
the interaction between valence-varied Cu
n+
and Mn
n+
. Next, the low
valence Mn
3+
can be oxdized to Mn
4+
by the adsorbed O
ads
and
donates electrons to O
ads,
thus the O
ads
is activated and generates the
rich active oxygen species (O
-
/O
2-
). Meanwhile, the high valence
Cu
2+
can be reduced back to low valence Cu
+
by adsorbing the NO
ads
and captureing electrons from it, thus NO
ads
can be activated and
generates NO
+
species. Consequently, the large number of NO
2
can
be formed by the interreaction between activated NO
+
and O
-
/O
2-
species. The similar NO
ads
and O
ads
activation processes by Mn
4+
and
Cu
+
, respectively, can also happen in cycle during the catalytic
process, also leading to the formation of intermediate NO
2
. Herein,
the synergetic effect between Cu
n+
and Mn
n+
could be featured with
one component activation by the other, i.e., the activation of Cu
n+
by
Mn
n+
, or vice versa, which is consistent with our previous researches.
Scheme 2. The possible catalytic pathways for the soot oxidation in the
presence of NO and O
2
on the sample CuMn-HBeta. Step 1: the generation of
highly active oxygen species O
-
/O
2-
and NO
2
; Step 3: the soot nanaparticles
in the presence of highly active oxygen O
-
/O
2-
and highly active intermediate
NO
2
.
30-31
Next, the produced highly active species NO
2
and O
-
/O
2-
will
spill over onto the surface of soot particles and then involve in the
soot oxidation (Step 2).
During the soot oxidation, soot can be expressed as C
bulk
C, (the
C
bulk
and C represent the internal and external carbon, respectively),
since the oxidation proceeds from the outside to inside. The soot
surface was firstly oxidized to form the functionalized groups
C
bulk
C(O) under the attack by NO
2
and O
-
/O
2-
, afterwards, the
C
bulk
C(O) will decompose into C
bulk
and C(O), and the oxidized
carbon groups C(O) continue to be oxidized into CO/CO
2
.
The
strong B acid on the HBeta carrier is believed to benefit the
decomposition of C(O) from the functionalized groups of C
bulk
C(O)
and thus increase the C-oxidation rate, according to kröcher’s
report.
16
As a result, the C
bulk
can be completely oxidized into
CO/CO
2
by NO
2
and O
-
/O
2-
(Step 2). The selectivity of produce CO
2
is nearly 100% in the presence of catalysts due to high active species.
In order to confirm that the generated NO
2
could be acted as a highly
active intermediate for the direct oxidization of soot particles into
CO/CO
2
,
32
the similar soot oxidation in the absence of NO
2
without
using catalyst was conducted for comparison, as shown in Table 2
(Entry 1). It is clear that it needs much high temperatures (T
50
=
570 ℃ and T
90
= 655 ℃) to oxidize the pure soot nanoparticles with
only NO, however, the temperatures (T
50
and T
90
) of soot oxidation
can be greatly reduced in the presence of NO
2
, as shown in Table 2
(Entry 2), which definitely confirm the stronger oxidative activity of
NO
2
. More interesting, our prepared CuMn-HBeta shows much high
catalytic activity with lower temperatures of T
10
(180℃) and T
50
(260℃) for soot oxidation, as shown in Table 2 (Entry 7). Therefore,
it is believed that excellent catalytic activity of the CuMn-HBeta at
low temperatures (e.g., < 220℃) can be mainly ascribed to the large
amount of active oxygen species from the synergetic effect between
valence-varied Cu
n+
and Mn
n+
. While, the generation of the
intermediate NO
2
derived from this high active catalyst will greatly
facilitate the soot oxidation at the temperatures of above 220 ℃. In
addition, the CuMn-HBeta showed a much higher catalytic activity
for the oxidation of NO than both references Cu-HBeta and Mn-
HBeta (Figure S7), further confirming the synergetic effect between
Cu
n+
and Mn
n+
could facilitate the oxidation of NO into NO
2.
Herein,
the conversion profiles of NO catalytic oxidation on all the samples
Cu-HBeta, Mn-HBeta and CuMn-HBeta show no overlap with the
thermodynamic equilibrium line at high temperature, as shown in
Figure S8, indicating that the adsorbed NO on the samples can be
easily desorbed from the catalysts at elevated temperature, leading to
the decrease of the conversion of NO at high temperature.
In brief, the following contributions are responsible for the
reasons of high catalytic activity of the sample CuMn-HBeta: 1) the
acidic HBeta carrier can promote soot oxidation (Table 2, Entry 1
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and 3); 2) the hierarchical pore structure can improve the diffusion
and transport of soot particles and increase the contact areas between
soot particles and active species, accelerating the soot oxidation
(Table 2, Entry 6 and 7); 3) the presence of large amount of adsorbed
oxygen (Figure 2d, XPS of O 1s); 4) the strong synergetic effect
between valence-varied Cu
n+
and Mn
n+
is greatly beneficial to the
generation of oxygen vacancies (Reaction 3 and 4), highly active
oxygen species and NO
2
(Scheme 2).
Conclusions
In summary, a novel CuMn-HBeta catalyst with hierarchical
mesopore structure has been synthesized using zeolite Beta as
catalyst carrier, followed by an alkali etching, and a facile co-
impregnation process of co-loading with high content of active Cu
and Mn species. The obtained CuMn-HBeta catalyst exhibits an
excellent and highly stable catalytic activity with low light-off
temperatures of T
50
(260 ℃) and T
90
(300 ℃), and high CO
2
selectivity (> 99.9 %), for soot oxidation in the presence of NO. The
superior catalytic performance can be attributed to the creation of 3D
hierarchically porous structure, the highly dispersed active species
and the synergetic catalytic effect between valence-changeable Cu
n+
and Mn
n+
, which promote the generations of active oxygen species
O
-
/O
2-
and active intermediate NO
2
, together accelerating the soot
oxidation. More importantly, the catalyst also shows good water-
resistance
resulting from the hydrophobic and stable crystalline
zeolite framework, as well as the interaction between soot and water
vapor, which largely prevent the deactivation of the catalyst.
Especially, the prepared CuMn-HBeta also shows excellent
recyclability and anti-aging performance in the soot oxidation. Such
composite catalyst as a high-active solid acid catalyst, as we
believed, can meet much critical requirements for the on-line soot
removal from diesel engine exhausts in moving vehicles, and can
also be applied in the many other redox catalytic reactions.
Acknowledgement
This research was sponsored by National Key Basic Research
Program of China, (2013CB933200), National 863 plans projects
(2012AA062703), China National Funds for Distinguished Young
Scientists (51225202), Key Program for Science and Technology
Commission of Shanghai Municipality (11JC1413400).
Notes and references
*Prof. H. Chen and Prof. J. Shi Corresponding-Author.
Dr. X. Zhou, G. Zhang, J. Wang, Z. Xie, Z, Hua, L. Zhang Wu.
State Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of
Sciences, 1295 Ding-Xi Road, Shanghai 200050, PR China
E-mail: hrchen@mail.sic.ac.cn (Hangrong Chen); jlshi@mail.sic.ac.cn
(J.L. Shi).
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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