Ammonia selective catalytic reduction of NO over Fe/Cu-SSZ-13

Article abstract of DOI:10.1039/c4ra09290b

A series of CHA-type Fe_x/Cu-SSZ-13 catalysts were prepared by liquid ion exchange method based on the one-pot synthesized Cu-SSZ-13 catalyst. The catalysts were applied to the selective catalytic reduction of NO with NH₃ and were characterized by means of XRD, N₂ adsorption-desorption, UV-Vis-DRS, H₂-TPR, XPS and in situ DRIFTS. Fe_x/Cu-SSZ-13 catalysts achieved high NH₃-SCR activity, excellent N₂ selectivity from 150 °C to 550°C, high hydrothermal stability and were highly resistant to high space velocity. The results may be attributed to the synergistic effects between copper and iron species. The results of H₂-TPR, UV-Vis DRS and XPS indicated that more active sites and stronger redox capacity contributed to the excellent NH₃-SCR performance upon the addition of Fe species into Cu-SSZ-13. In situ DRIFTS results showed that NO⁺ and NO^3- generated from NO + O₂ adsorption were the key intermediates and were further reduced to N₂ by NH₃. This journal is

Full text of DOI:10.1039/c4ra09290b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Ammonia selective catalytic reduction of NO over
Fe/Cu-SSZ-13
Cite this: RSC Adv., 2014, 4, 52130
a
ab
ac
Ranran Zhang, Yonghong Li†* and Tieli Zhen
A series of CHA-type Fe
one-pot synthesized Cu-SSZ-13 catalyst. The catalysts were applied to the selective catalytic reduction of
NO with NH and were characterized by means of XRD, N adsorption–desorption, UV-Vis-DRS, H -TPR,
XPS and in situ DRIFTS. Fe /Cu-SSZ-13 catalysts achieved high NH -SCR activity, excellent N selectivity
x
/Cu-SSZ-13 catalysts were prepared by liquid ion exchange method based on the
3
2
2
x
3
2
ꢀ
ꢀ
from 150 C to 550 C, high hydrothermal stability and were highly resistant to high space velocity. The
results may be attributed to the synergistic effects between copper and iron species. The results of
Received 26th August 2014
Accepted 10th October 2014
H
2
-TPR, UV-Vis DRS and XPS indicated that more active sites and stronger redox capacity contributed to
the excellent NH -SCR performance upon the addition of Fe species into Cu-SSZ-13. In situ DRIFTS
results showed that NO and NO generated from NO + O
were further reduced to N by NH
3
DOI: 10.1039/c4ra09290b
+
3ꢁ
2
adsorption were the key intermediates and
www.rsc.org/advances
2
3
.
capacity. Additionally, the synthesis of SSZ-13 zeolite typically is
to use amines or quaternary ammoniums as an organic
Introduction
Nitrogen oxide from diesel exhausts is one of the important structure-directing agent. For example, the SSZs were synthe-
pollutants leading to photochemical smog, acid rain and ozone sized by using N,N,N-trimethyl-1-adamantammonium
Nowadays, under oxygen-rich conditions, the hydroxide (TMAdaOH) as and
1–3
10
depletion.
a
template by Zones
removal of NO remains a challenge in the eld of environmental coworkers. TMAdaOH is the most common template but the
catalysis since conventional three-way catalysts are of low effi- cost is higher. Therefore, it is urgently desirable to synthesize
ciency. The selective catalytic reduction with NH is considered the Cu-SSZ-13 zeolite with a controllable metal loading using
3
to be the most effective technology for the removal of NO from low-cost templates.
diesel engine exhausts. The rst commercialized catalyst V
2
O
5
–
At present, many metal complexes were widely employed to
4
11,12
WO
3
/TiO
2
has been widely employed in NH
3
-SCR. However, it direct the framework of zeolites. Corma
has directly
exhibited poor thermal stability, low space velocity and narrow synthesized Cu-SAPO-34 using Cu-triethylenetetramine as a
13
temperature window in diesel vehicle applications. Recently, template. Corma's group has elaborately reported the one-pot
iron and copper based zeolites were widely employed in NH3- synthesized Cu-SSZ-13 zeolite for NH -SCR using a rational
3
SCR due to their wider temperature window and higher thermal mixture of templates, which exhibited controllable copper
stability than vanadium catalysts. In particular, Cu/zeolite loading and higher catalytic activity. The CHA-type Cu-SSZ-13
catalysts showed remarkable low-temperature activity, high N
2
exhibited superior catalytic properties for NH
-SCR. Recently, a high hydrothermal stability, superior N selectivity and excel-
copper-exchanged SSZ-13 zeolite with CHA crystal structure lent resistance to high space velocity. In the practical applica-
was reported, which became a promising catalyst for NH
-SCR tion conditions, space velocity of diesel exhaust varies from
3
-SCR, such as
5
selectivity and hydrothermal stability for NH
3
2
6,7
3
ꢁ
1
ꢁ1
8
compared with Cu-beta and Cu-ZSM-5 zeolites, because of its 10 000 h to 100 000 h . Therefore, the one-pot synthesized
higher SCR activity, N selectivity and high hydrothermal Cu-SSZ-13 catalyst is a prime candidate for the NO
stability. But it was difficult to control Cu loading in H-SSZ-13 from diesel engine exhaust.
x
elimination
2
9
zeolite due to the small porosity and limited ionic exchange
Additionally, Fe/zeolite catalysts, such as Fe-ZSM-5, have also
been widely used for NH -SCR, which displayed high NO
3
ꢀ
conversion at high temperature (>350 C). However, Fe-ZSM-5
a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
suffered from NH3 inhibition effect and propene
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China
b
14–16
poisoning.
Most recently, a series of heterobimetallic
National Engineering Research Center for Distillation Technology, Tianjin 300072,
zeolites have been synthesized by introducing a secondary
P. R. China. E-mail: yhli@tju.edu.cn; Fax: +86 22 27404705; Tel: +86 22 27404701
ext. 8858
metal cation M using ion exchange method. Cu–Fe/ZSM-5
c
17
Jinan Engineering Vocational College, Jinan, Shandong 250000, P. R. China
catalysts prepared by Sultana using subsequent ion-exchange
†
Postal address: School of Chemical Engineering and Technology, Tianjin
18
and by Zhang using incipient-wetness-impregnation method
University, 92 Weijin Road, Tianjin 300072, P. R. China.
52130 | RSC Adv., 2014, 4, 52130–52139
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
showed higher NO conversion compared with Fe/ZSM-5 and Cu/ areas were calculated according to standard BET method in
19
ZSM-5. Yang has given details about the synthesis of CuFe-SSZ- 0.05–0.35 partial pressure range and the pore volumes were
3
+
1
3 by incorporating Fe into copper-exchanged Cu-SSZ-13, and depended on the V–t plot method according to the desorption
the catalyst exhibited complementary advantages and syner- isotherm.
2 2
gistic effects. But the Fe /Cu-SSZ-13 catalysts based on the one- Temperature-programmed reduction by H (H -TPR) tests
x
pot synthesized Cu-SSZ-13 zeolite have not been reported so far. were performed using a XianQuan TP-5079 apparatus equipped
In this paper, we attempt to develop a more effective catalyst for with a thermal conductivity detector to monitor the hydrogen
NH -SCR and modify Cu-SSZ-13 with iron. The main task consumption signal. During H -TPR experiments, each time
3 2
focused on the following aspects: (1) the inuence of Fe incor- 30 mg powder samples (40–60 mesh) were loaded in the quartz
ꢀ
poration on the structure of Cu-SSZ-13 and chemical state of Cu tube reactor and heated from room temperature to 900 C at a
ꢀ
ꢁ1
and Fe species. (2) The inuence of Fe incorporation on the heating rate of 10 C min in the atmosphere of 8% H /N (v/v)
2
2
ꢁ1
redox capacity and the probable active sites. (3) The possible with a total gas ow 30 mL min . H O and CO were removed
2
2
ꢀ
reaction mechanism.
by a cooling trap (60 C) and a lter packed with CaO + NaOH
before the outlet gases entering the TCD.
UV-visible diffuse reectance spectra (UV-vis DRS) were
collected in a Hitachi UV3010 (Japan) spectrometer equipped
with a 60 mm diameter integrating sphere. Prior to a
measurement each time, BaSO4 was used as the reference
material to record baseline spectrum.
Experimental
Preparation of Fe /Cu-SSZ-13 catalysts
x
Cu-SSZ-13 zeolite was prepared via the typical one-pot hydro-
thermal method with the raw material molar ratio of
X-ray Photoelectron Spectra (XPS) were collected on a Perkin-
Elmer PHI-1600 ESCA spectrometer using Mg Ka (hn ¼
5.0Na
2
O : 1.0Al
2
O
3
: 10.0SiO
2
: 200H
2
O : 2.0Cu–TEPA.
The
20
difference from Xie's research was the enhanced alkaline
medium, including the amount of NaOH and Cu–TEPA. All
chemicals were purchased from commercial chemical suppliers
and used directly without further purication. In a typical
treatment, the as-made Cu-SSZ-13 precursor was exchanged
ꢁ
19
1
253.6 eV, 1 eV ¼ 1.603 ꢂ 10
J) as X-ray source. The pressure
ꢁ
7
of the analysis chamber maintained 2 ꢂ 10 Pa. The binding
energies were calibrated by C 1s peak at B.E. ¼ 284.6 eV as an
internal standard.
ꢀ
In situ DRIFTS spectra were recorded by a Fourier transform
infrared spectrometer (Nicolet 6700) equipped with a high
environment cell tted with ZnSe window, a Smart collector and
with NH NO solution at 80 C water bath for 10 h twice, fol-
4
3
lowed by ltration, washing with deionized water and drying at
ꢀ
ꢀ
1
10 C overnight. The product obtained was calcined at 550 C
for 8 h. The nal product Cu-SSZ-13 was obtained. A series of
Fe /Cu-SSZ-13 catalysts (x represented the molar ratio of Fe to
a MCT detector cooled by liquid N
Harrick reaction chamber and heated to 500 C under N
atmosphere with a total ow of 100 mL min for 30 min to
2
. The sample was loaded in a
ꢀ
2
x
ꢁ
1
Cu in catalysts) with variable concentrations of ferric nitrate
and xed copper content were prepared by ion exchange in
aqueous solution. In a typical synthesis process, a series of
ferric nitrate solution with different concentrations were rstly
purge. Then the sample was cooled to experimental tempera-
2
ture and the IR background spectrum was taken at N ow by
ꢁ
1
accumulating 64 scans with a resolution of 4 cm . The outlet
stream was analyzed by online mass spectrometer (OmniStar
GSD-301).
prepared. Then a certain amount of Cu-SSZ-13 was slowly added
ꢀ
into Fe(NO
3
)
3
$9H
2
O solution and was exchanged at 80 C in
water bath heating for 8 h under vigorously stirring. This was
ꢀ
followed by drying at 110 C for 12 h and calcining in air at
Catalytic performance tests
ꢀ
600 C for 5 h.
3
The NH -SCR activity evaluation experiments were conducted
in a xed bed quartz micro reactor (inner diameter 5 mm),
which loaded required amounts of catalyst (20–40 mesh size
Catalysts characterization
The crystal structures of catalysts were investigated by means of powder) at ambient condition. A K-type thermocouple (o.d.1
X-ray powder diffraction (XRD) which were carried out on a mm) was directly inserted into the middle of catalyst bed from
D8-Focus X-ray diffractometer (Bruker Company) using Cu Ka the bottom of the reactor and connected to a temperature
(
l ¼ 0.15418 nm) as radiation source. The operating voltage and programmed control instrument to measure the reaction
current were 40 kV and 40 mA respectively, in the 2q range from temperature. The feed gases were composed of 500 ppm NO,
5
identied by matching measured diffraction lines with refer- SO
ence patterns in JCPDS standard card.
ꢀ
ꢀ
ꢀ
ꢁ1
to 40 at a scanning speed of 5 min . The main phase was 500 ppm NH
3
, 5 vol% O
(when used), 5 vol% CO
a total ow rate of 300 mL min , corresponding to a GHSV
2
, 5 vol% H
2
O (when used), 100 ppm
2
2
(when used) and balance N
2
, with
ꢁ
1
ꢁ
1
ꢁ1
x
The concentration of Cu and Fe in Fe /Cu-SSZ-13 catalysts from 100 000 h to 600 000 h based on various volumes of
was determined by ICP-AES measurement. The BET (Brunauer– the catalyst. The water vapor was pumped by an injection
Emmett–Teller) surface areas and pore volumes of these cata- pump (LSP01-1A, Longer Pump Inc.) and an evaporator. Mass
lysts were determined from nitrogen adsorption–desorption ow controllers were calibrated by a soap-bubble meter. The
isotherm at 77 K on a Micromeritics Tristar-3000 analyzer. Prior concentration of NO and NOx were recorded by an online
ꢀ
to the measurements, catalysts were degassed at 90 C for 1 h chemiluminescent analyzer (Model KM9106, Kane Inc.), when
ꢀ
and at 300 C for another 3 h under N
2
atmosphere. All surface the NO conversion was constant and a steady state was
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 52130–52139 | 52131

View Article Online
RSC Advances
Paper
2
achieved aer 1 h. The N selectivity experiment was per- Physicochemical properties of the catalysts
formed through collecting NO, NO and N O concentrations
with a Thermo Nicolet IS10 Fourier-transform infrared (FTIR)
2
2
Fig. 2 displayed N adsorption–desorption isotherms of Cu-SSZ-
2
1
3 and Fe
adsorption and desorption process of all catalysts were revers-
ible, as well as N adsorption increased rapidly at low relative
pressure and reached a stable value at a certain relative
adsorption saturation pressure (P/P ). Clearly, all isotherms
x 2
/Cu-SSZ-13. It can be seen from Fig. 2 that N
spectrometer. NO conversion and N2 selectivity of NH -SCR
3
were respectively dened as:
2
NO conversion (%) ¼ (1 ꢁ [NO
x
]
out/[NO
x
]in) ꢂ 100; [NO
x
]
0
¼
[NO] + [NO
2
]
belonged to type-I according to the IUPAC classication, a
characteristic of microporous materials. Table 1 showed the
N
2
selectivity (%) ¼ ([NO]in + [NH
3
]
in ꢁ [NO
2 out
]
physicochemical properties of Cu-SSZ-13 and Fe /Cu-SSZ-13
x
ꢁ
2 ꢂ [N
2
O]out)/([NO]in + [NH
3
]
in) ꢂ 100
8
catalysts. Kwak reported that the order of pore sizes for Cu-
SSZ-13, Cu-ZSM-5 and Cu-beta was Cu-SSZ-13 < Cu-ZSM-5 <
Cu-beta and thought that the smaller pores seem to be more
favorable to N
2
selectivity. In this paper, the catalysts possessed
Results and discussion
XRD patterns results
larger specic surface areas and smaller pore sizes, which
would be in favor of NH oxidation and N selectivity. The N2
3 2
21
adsorption–desorption isotherm shape remained unchanged
aer Fe modication. However, the BET specic surface area
and pore volume decreased aer Fe incorporation into Cu-SSZ-
13. The results indicated that Fe species covered the external
Fig. 1a showed the XRD patterns of Cu-SSZ-13 and Fe /Cu-SSZ-
x
1
2
3 catalysts. All the catalysts showed characteristic peaks in
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
q ¼ 9.438 , 13.941 , 15.959 , 17.821 , 20.540 , 25.060 , with a
ꢀ
ꢀ
ꢀ
typical CHA structure diffraction peaks (2q ¼ 9.5 , 14.0 , 16.1 ,
ꢀ ꢀ ꢀ
1
7.8 , 20.7 , 25.0 ), and exhibited high crystallinity. The XRD
results indicated that the incorporation of Fe species did not
cause the destruction of the original zeolite structure. It was
reported that CuO or a-Fe O would contribute to NH oxidation
2
3
3
at high temperature, which adversely affected the reduction of
NO with NH . However, the phases of CuO (PDF# 48-1548) and
a-Fe
and Fe
3
2
O
3
species (PDF# 33-0664) were not observed in Cu-SSZ-13
/Cu-SSZ-13, indicating that Cu and Fe species as amor-
x
phous oxides were well dispersed on the surface of catalysts,
which would be favorable to improve NH -SCR activity. The XRD
3
proles were performed to probe possible structural changes
aer Fe1.32/Cu-SSZ-13 undergoing hydrothermal aging under
ꢀ
1
0% H O, 5% O and N at 600, 750 and 800 C, respectively.
2
2
2
The results were shown in Fig. 1b. Little or no changes were
ꢀ
ꢀ
observed for aging Fe1.32/Cu-SSZ-13 at 600 C and 750 C, which
indicated that these zeolite structures remained largely intact
during the hydrothermal aging. However, as the aging temper-
ꢀ
ature was above 800 C, Fe1.32/Cu-SSZ-13 gave rise to the
Fig. 2
2 x
N adsorption–desorption isotherms of Cu-SSZ-13 and Fe /
complete collapse of the zeolite structure.
Cu-SSZ-13 catalysts.
Fig. 1 XRD patterns of the one-pot synthesized Cu-SSZ-13 and iron-exchanged Fe
Fe1.32/Cu-SSZ-13 at 600, 750 and 800 C (b).
x
/Cu-SSZ-13 (a), the XRD patterns of hydrothermal aging
ꢀ
52132 | RSC Adv., 2014, 4, 52130–52139
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
a
Table 1 Physicochemical properties of the Cu-SSZ-13 and Fe
x
/Cu-SSZ-13 catalysts
Surface area
Pore volume
Pore size
(nm)
Si/Al ratio
ICP
Cu content
(wt%)
Fe content
(wt%)
2
ꢁ1
3
ꢁ1
Sample
(m g
)
(cm g
)
Cu-SSZ-13
431.39
415.56
364.79
348.36
21.08
432.33
397.09
375.04
0.197
0.153
0.146
0.142
0.002
0.160
0.146
0.136
1.91
2.32
2.24
2.09
8.02
2.15
2.36
2.35
3.56
3.59
7.41
5.47
0.00
4.92
Fe1.03/Cu-SSZ-13
Fe1.03/Cu-SSZ-13-600
Fe1.03/Cu-SSZ-13-750
Fe1.03/Cu-SSZ-13-800
Fe1.32/Cu-SSZ-13
Fe5.03/Cu-SSZ-13
Fe7.91/Cu-SSZ-13
3.55
3.55
3.54
4.67
1.38
1.21
5.37
6.07
8.36
a
Cu and Fe content represent the relative percentage of Cu and Fe in the total content of zeolite.
+
ꢀ
2+
surface of Cu-SSZ-13 and blocked the microporous channels. Cu , the peak at 367 C to the reduction of stable Cu located in
+
ꢀ
+
Especially, Fe1.32/Cu-SSZ-13 showed larger BET surface area and six-membered rings to Cu and at 884 C to the reduction of Cu
pore volume. It indicated that the strong interaction between iron to Cu . Therefore, the reduction peaks at 222 C and 256 C were
0
ꢀ
ꢀ
2
+
+
and copper species suppressed Cu species aggregation, which attributed to the reduction of Cu to Cu . Aer Fe species
would block micropores, and Fe species nely dispersed in the incorporation into Cu-SSZ-13, the H -TPR reduction tempera-
catalyst. Additionally, the BET surface area and pore volume ture shied to higher temperature and the reduction peaks
decreased with aging temperature increasing over Fe1.32/Cu-SSZ- appeared at around 400 C, 430 C, 466 C, 520 C and 650 C.
3, which indicated that hydrothermal treatment had a serious The reason might be that the strong interaction between copper
impact on the pore structure. Especially, the pore structure was or iron metallic component and support in Fe /Cu-SSZ-13 made
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1
x
17
almost completely destroyed for Fe1.32/Cu-SSZ-13-800.
them difficult to be reduced. Sultana assigned the peaks at
around 360 C, 450 C and 677 C to the reduction of Fe to Fe
or to Fe
ꢀ
ꢀ
ꢀ
3+
2+
(
3ꢁd)+
(3ꢁd)+
2+
0
, Fe
to FeO and Fe to Fe , respectively.
Results of H -TPR
Therefore, the iron reduction peaks located at high temperature
became clear but copper peaks became weak aer Fe loading.
Furthermore, the peaks became broader aer Fe loading, indi-
cating that Fe species existed in several forms. The results
indicated that the high-temperature conversion of NO could be
improved by enhancing redox ability of Fe species. Thus the
operating temperature window was further widened.
2
In order to gain understanding of the redox properties of the
catalysts, the H -TPR of Cu-SSZ-13 and Fe /Cu-SSZ-13 were
2
x
measured. The results of H -TPR were shown in Fig. 3. The three
main hydrogen reduction peaks with different peak areas were
observed over Cu-SSZ-13 at 222 C, 256 C and 825 C, respec-
tively. The peaks demonstrated that isolated Cu was reduced
2
ꢀ
ꢀ
ꢀ
2
+
2
+
+
+
0
22
by the two steps (Cu / Cu , Cu / Cu ). Gao reported that
the H
2
-TPR characteristic peaks of Cu-SSZ-13 were ascribed to
UV-vis DRS spectra
ꢀ
the following situations: the peak at 257 C was attributed to the
reduction of unstable Cu inside the cage of CHA structure to
Fig. 4 showed the UV-Vis spectra of as-synthesized Cu-SSZ-13
and Fe /Cu-SSZ-13. Cu-SSZ-13 displayed one main peak at 225
x
2
+
Fig. 3 TPR profiles of Cu-SSZ-13 (a), Fe1.03/Cu-SSZ-13 (b), Fe1.32/Cu- Fig. 4 UV-Vis DRS spectra of Cu-SSZ-13 (a), Fe1.03/Cu-SSZ-13 (b),
SSZ-13 (c), Fe5.03/Cu-SSZ-13 (d) and Fe7.91/Cu-SSZ-13 (e).
Fe1.32/Cu-SSZ-13 (c) and Fe5.03/Cu-SSZ-13 (d).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 52130–52139 | 52133

View Article Online
RSC Advances
Paper
2
+
nm, which was attributed to a highly dispersed Cu species.
Adsorption peaks at 210 nm over Cu-SSZ-13 and Fe /Cu-SSZ-13
x
were ascribed to the charge-transfer band from lattice oxygen
2
+
to isolated Cu ion, the peak at ꢃ400 nm to O
/ Cu
bridge
2+
23
2
charge-transfer in [Cu O] . UV-Vis spectra could identify
different Cu and Fe species within a certain wavelength range.
Generally, the bands below 300 nm for Fe-zeolites were attrib-
3
+
3+
uted to isolated Fe , including isolated tetrahedral Fe and
3
+
octahedral Fe at 210 nm and 270 nm, the bands between 300
and 400 nm to oligomeric clusters, the bands above 400 nm to
large Fe O particles and above 450 nm to d-d transition of
2
3
24,25
a-Fe O .
The results revealed that the bimetallic Fe–Cu
2
3
catalysts contained several active sites. Cooper species were in
2
+
forms of isolated Cu and iron species were in three types of
3
+
x y 2 3
isolated Fe , Fe O oligomer and a-Fe O . Obviously, the bands
were broadened and shied to higher wavelengths with Fe
loading increasing. It suggested that the peaks exhibited a red
shi due to more d-orbitals of electronic transition. According
to the results of UV-Vis DRS and the following NH -SCR activity,
3
we deduced that the deNO performances might be improved by
x
3
+
oxo-Fe
a-Fe
and oligomeric Fe
x
O
y
clusters, while aggregated
18,26
2
O
3
was unfavorable to NH
3
-SCR.
The results of XPS
In order to understand the surface elementary composition of
the catalysts and the interaction between Cu and Fe species, Cu-
SSZ-13 and Fe1.32/Cu-SSZ-13 samples were investigated by XPS
technique. The XPS results of Cu 2p for Cu-SSZ-13 and Fe1.32
/
Cu-SSZ-13 catalysts, including the deconvolution results with
GL function, were present in Fig. 5a and b. All samples exhibited
Cu 2p3/2 and Cu 2p1/2 peaks at approximate B.E. of 933.0 eV and
Fig. 5 XPS results of Cu 2p of Cu-SSZ-13 (a) and Fe1.32/Cu-SSZ-13
952.5 eV, respectively. The Cu 2p3/2 transition was deconvoluted catalysts (b); XPS results of Fe 2p of Fe1.32/Cu-SSZ-13 (c).
into three peaks at 936.7, 934.3 and 933.7 eV. In previous paper,
it was reported that Cu 2p3/2 transition below 933.0 eV was
0
assigned to metallic copper (Cu ) and Cu
2
O, while above 933 eV copper to iron. The Fe Oy active sites enhanced the redox
x
2+
27
to different Cu . Correspondingly, the peak at 933.7 eV was capacity due to more oxygen vacancies, which were favorable to
assigned to the agglomerated CuO particles,
28,29
at 934.3 eV to NH -SCR. The shake-up satellite located at 718.0 eV, with
3
2
+
isolated Cu coordinated to tetrahedral oxygen atoms and the energies 7 eV higher than Fe 2p3/2, was associated with the Fe
2
+
band located at 936.7 eV to Cu in octahedral coordination, as 2p3/2 peak.
30
shown in Fig. 5a. The characteristic satellites appeared at B.E.
of 943.0 eV in all catalysts, with energies 10 eV higher than the
Cu 2p3/2 signal. These shake-up peaks could be assigned to the
In situ DRIFTS studies
charge transfer between the 3d orbital of transition metal and In order to further understand the interaction of NO + O
p orbital of surrounding oxygen ligands. The weak shake-up NH over Fe1.32/Cu-SSZ-13 catalyst, the catalyst was exposed to
satellites were also related to Cu . The results showed that NO + O and NH . Firstly, the sample was pretreated in N (the
2
and
2
3
2
+
2
3
2
ꢁ
1
ꢀ
B.E. of Cu 2p3/2 (933.8 eV) in Fe1.32/Cu-SSZ-13 presented a blue total ow of 100 mL min ) at 500 C for 30 min and then cooled
ꢀ
shi compared with that of Cu-SSZ-13 (933.5 eV), indicating the to 175 C. Aer lasting for 10 min, the sample was adsorbed
weak interaction between iron and copper species.
Fig. 5c showed the XPS spectra of Fe 2p for Fe1.32/Cu-SSZ-13 Before 500 ppm NH
catalyst. The characteristic peaks of Fe 2p3/2 and Fe 2p1/2 were SSZ-13, N was introduced for another 30 min to remove phys-
located at 711.6 and 724.4 eV, respectively. The Fe 2p3/2 peak ical adsorbed NO + O
was deconvoluted into two main contributions. The bands The DRIFTS spectra were shown in Fig. 6. Upon introducing
located at 710.9 and 713.4 eV were assigned to FeO and Fe O , NO + O , the bands at 2170, 1694, 1612, 1574, 1450, 1373 and
with 500 ppm NO + 5% O
2
for 30 min at reaction temperature.
3
was passed over pre-adsorbed Fe1.32/Cu-
2
2
.
2
3
2
ꢁ1
31
2+
ꢁ1
respectively. The B.E. of Fe is lower due to the interaction 1236 cm appeared in IR spectra. The band at 2170 cm was
between iron and the surrounding atom, such as Cu, leading to assigned to NO , at 1694 cm to dimer (NO)
an increase in the oxygen vacancies and charge transfer from NO, at 1612 cm to gas-phase NO
+
19
ꢁ1
2
from adsorbed
from adsorption of pre-
ꢁ
1
2
52134 | RSC Adv., 2014, 4, 52130–52139
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
ꢀ
Fig. 6 DRIFTS spectra of adsorbed 500 ppm NO + 5% O
2
+ 500 ppm NH
3
over Fe1.32/Cu-SSZ-13 at 175 C.
ꢁ
1
22ꢁ 32
ꢁ1
mixed NO + O
1
nitrate species.
2
and at 1373 cm to cis-N
2
O
.
The bands at 3735 cm were assigned to O–H stretching vibration of struc-
ꢁ
1
ꢁ1
500 cm to 1650 cm were assigned to different nitro and ture hydroxyl group and the surface silanol hydroxyl group,
3
3,34
39
However, it was difficult to precisely identify respectively. The results showed that ammonia adsorption
these bands because of their similar vibrations in different consumed both structure hydroxyl and silanol hydroxyl. At the
3
5,36
nitrate species. Ma and co-workers
1
assigned the IR bands at end of ammonia treatment, some NO
x
signals completely dis-
ꢁ1
ꢁ1
601 (1600 cm ), 1546, 1362 and 1235 cm to monodentate appeared. The signals of N
nitrates over Cu-SSZ-13, at 1574 cm to bidentate nitrates, at online mass spectrometry due to NO reduction.
x
500 cm to nitrites. Iwasaki and co-workers thought that the
2
and H
2
O were also observed from
ꢁ1
8
ꢁ
1
37
+
3ꢁ
1
In our study, the intermediates of NO and NO played an
ꢁ
1
+
3ꢁ
bands at 1656 and 1590 cm were attributed to the nitro and important role in NH
3
-SCR. The reduction of NO and NO by
3
8
41,42
41
nitrate groups respectively. Shan thought the bands at 1601 NH
3
has been reported in previous literatures.
reported the reduction of HNO by NO and the result showed
3
Yeom
ꢁ1
ꢁ1
and 1232 cm were attributed to bridging nitrate, at 1578 cm
to bidentate nitrate and at 1549 cm to monodentate nitrate. that NO and HNO established the following equilibrium: NO +
ꢁ1
3
ꢁ
1
Therefore, the bands at 1612, 1574, 1450 and 1236 cm were HNO
assigned to nitro and nitrate species adsorbed on transition fast reaction with NO
metal. kinds of reactions ((i) HNO
Aer NH passed over the sample, there was time delay of according to Yeom's report. So we speculated that the possible
more than 4 min before the bands at 1373, 1694 and 2170 cm
disappeared, suggesting that the zeolite adsorbed NH until
saturation on the active sites and the cis-N O , dimer (NO)2
3
# NO
2
+ HONO. The peak of HONO was invisible due to
or NH in Fig. 6. N was yielded by two
3
+ NH ; (ii) HNO + NO + NH )
2
3
2
3
3
3
3
ꢁ1
reaction process was shown in Scheme 1.
3
2
ꢁ
2
2
+
32
and NO exhibited activity. Aer NH adsorption, the peaks Catalytic performance tests
3
representing two distinct ammonia species appeared in IR
3
The inuence of ion exchange level on NH -SCR. The activ-
ꢁ
1
spectra. The bands at 3335, 3184, 1617, 1277 and 1165 cm
were assigned to Lewis acid site-adsorbed ammonia, while the
ities of a series of Fe /Cu-SSZ-13 and Cu-SSZ-13 catalysts were
x
investigated under the “standard SCR” condition. Fig. 7a
ꢁ
1
bands at 3274 and 1450 cm were Brønsted acid site-adsorbed
showed the NO conversion results over Cu-SSZ-13 and Fe
x
/Cu-
39
ammonia species. In the stretching vibrations region of N–H
ꢀ
ꢀ
SSZ-13 catalysts in the temperature range of 150 C to 550 C
ꢁ
1
bonds, the band at 3184 cm was assigned to coordinated
ꢁ1
under a high hourly space velocity of 150 000 h . The NO
ꢁ
1
ammonia and the bands at 3335 and 3274 cm were assigned
to ammonia ions. Furthermore, in the bending vibrations
ꢀ
conversion increased rapidly below 200 C and slowly declined
ꢀ
above 400 C with the temperature increasing and reached
ꢁ
1
+
region, the peak at 1450 cm was assigned to NH
4
on Brønsted
ꢀ
ꢀ
ꢀ
above 85% in the temperature range of 175 C to 450 C over Cu-
ꢁ
1
acid site and at 1165, 1277, 1617 cm to the NH
3
coordinated to
ꢀ
SSZ-13. Especially in the region of 175 C to 350 C, the NO
conversion remained above 90%. Compared with Cu-SSZ-13,
33,40
the Lewis acidic sites.
The results indicated that Brønsted
acid (1450 cm ) and Lewis acid (1617 cm ) sites were involved
in SCR at low temperature. The negative peaks at 3604 and
ꢁ
1
ꢁ1
Fe
x
/Cu-SSZ-13 with a low iron exchange level exhibited higher
NO conversion within a broader temperature window of 175 C
ꢀ
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 52130–52139 | 52135

View Article Online
RSC Advances
Paper
ꢀ
to 550 C. Particularly, NO conversion was improved remarkably
ꢀ
(
above 85%) above 400 C. Thus, Fe /Cu-SSZ-13 catalysts have
x
enlarged the operating temperature window. Clearly, high
temperature activity was further improved due to the synergistic
effects between Cu and Fe species. However, Fe /Cu-SSZ-13
x
zeolites with higher iron exchange level resulted in a signi-
cant decrease of NO conversion in the whole temperature
window. In order to investigate the effect of ion exchange level
on NO conversion, the average conversion of NO in the
ꢀ
ꢀ
temperature range of 200 C to 400 C was calculated and was
shown in Fig. 7b as a function of Fe/Cu (molar ratio). The results
of Fig. 7b suggested that the suitable iron loading could
promote NO conversion, while the excessive iron loading could
block the “channel” of zeolites. This could also be explained by
2
N adsorption–desorption results. Therefore, the choice of the
ion-exchange concentration was also very important for
improving the NO conversion.
Scheme 1 Reaction process of NH
Fe1.32/Cu-SSZ-13 at 175 C.
3
and pre-adsorbed NO + O
2
over
ꢀ
ꢁ
1
Fig. 7 NO conversion over Cu-SSZ-13 and Fe
x
/Cu-SSZ-13 catalysts with variable molar ratio of Fe to Cu under GHSV of 150 000 h in the
temperature range of 150 C to 550 C as a function of reaction temperature (a), the average NO conversion over Fe /Cu-SSZ-13 as the molar
ratio of Fe to Cu (b) and N selectivity as a function of reaction temperature (c).
ꢀ
ꢀ
x
2
52136 | RSC Adv., 2014, 4, 52130–52139
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Most notably, the results in Fig. 7c showed that the NH
3
oxidation produced NO at high temperature and the
concentrations of N O and NO were less than 10 ppm and N
availability of NH was limited, leading to the decrease of NO
3
2
2
2
selectivity reached almost 100% in the whole operating conversion. However, very high NO conversion (>80%) was
ꢀ ꢀ
temperature range over Fe1.32/Cu-SSZ-13.
Effect of space velocity on NH -SCR activity. In the practical results suggested that Cu-SSZ-13 catalyst showed superior
application conditions, space velocity of diesel exhaust varies resistance to the high space velocity.
due to the different driving conditions. Therefore, the effect of In order to further understand the resistance to gas hourly
space velocity on NH -SCR activity was evaluated and the results space velocity property of Fe /Cu-SSZ-13 zeolites, Fe1.32/Cu-SSZ-
were shown in Fig. 8. 13 was investigated under GHSV of 100 000 h to 200 000 h
obtained from 200 C to 450 C under all GHSV conditions. The
3
3
x
ꢁ
1
ꢁ1
.
Fig. 8a showed the effect of space velocity on NO conversion As shown in Fig. 8b, Fe /Cu-SSZ-13 catalysts achieved the
x
ꢀ
over Cu-SSZ-13 catalyst. Obviously, NO conversion decreased similar NO conversion above 200 C under the different space
ꢁ
1
ꢁ1
with the GHSV increasing from 100 000 h to 600 000 h in velocity, indicating that they were effectively resistant to high
ꢀ
ꢀ
the temperature range of 150 C to 550 C. The inuence was gas hourly space velocity.
not clearly observed above 400 C, but the decrease of catalytic
activity below 300 C was remarkable. On the one hand, the the results of anti-poisoning performance over Cu-SSZ-13 and
ꢀ
2 2 2 x
H O, SO and CO tolerance of Fe /Cu-SSZ-13. Fig. 9 showed
ꢀ
short duration of exposure between NO and NH
3
resulted in the Fe1.32/Cu-SSZ-13 catalysts. Fig. 9a showed the SO
2
tolerance and
decrease of NO conversion. On the other hand, non-selective
H
2
O tolerance results of Cu-SSZ-13 and Fe1.32/Cu-SSZ-13 at
3
Fig. 8 NH -SCR activity under different GHSV over Cu-SSZ-13 catalyst (a) and Fe1.32/Cu-SSZ-13 catalyst (b).
ꢀ
ꢁ1
Fig. 9 NO conversion over Cu-SSZ-13 and Fe1.32/Cu-SSZ-13 catalysts at 300 C under GHSV of 150 000 h in the co-presence of H
2 2
O + SO
(
a); H O + CO (b).
2
2
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 52130–52139 | 52137

View Article Online
RSC Advances
Paper
ꢀ
ꢁ1
3
00 C under a space velocity of 150 000 h . When SO
2
was The SCR activity result was in accordance with XRD and BET
added into the reaction system, NO conversion decreased results. Compared with Cu-SSZ-13-600, Fe1.32/Cu-SSZ-13-600
slightly because of SO2 competitive adsorption with reaction showed higher NO conversion, indicating that Fe1.32/Cu-SSZ-
gases, but NO conversion restored aer SO was removed. The 13 was more robust and resistant to relatively harsh hydro-
2
presence of H
to the A decreased and E
leading to the decrease of the reaction rate. Since the co-
existence of SO and water vapor in the combustion exhaust
oen leads to the deactivation of the SCR catalysts, therefore, it
is necessary to investigate the effects of SO and H O on NO In conclusion, a series of CHA-type Fe /Cu-SSZ-13 catalysts were
2
O slightly decreased NO conversion. This was due thermal treatment than Cu-SSZ-13. This result was related to the
a
increased in Arrhenius equation, higher Si/Al ratio in Fe1.32/Cu-SSZ-13 than Cu-SSZ-13.
43
2
Conclusions
2
2
x
conversion. However, when SO and H O were simultaneously prepared by liquid ion exchange method on the basis of Cu-SSZ-
2
2
added into feed gases, NO conversion rapidly decreased from 13. The characteristics of the catalysts were investigated by
5% to 25% and hardly restored to the original level over Cu- means of XRD, N adsorption–desorption, H -TPR, UV-Vis DRS
9
2
2
SSZ-13. The result could be explained that ammonium sulfate and XPS. The catalytic performances and the reaction mecha-
and copper sulfate formed on the surface of the catalyst and nism were studied in NH
blocked the active sites, resulting in the catalyst permanent respectively.
3
-SCR system and in situ DRIFTS,
44
poisoning. Compared with Cu-SSZ-13, Fe1.32/Cu-SSZ-13 cata-
Zeolite structure remained unchanged aer Fe incorporation
lyst displayed better resistance to water and sulfur dioxide. into Cu-SSZ-13 as shown in XRD patterns. H -TPR results indi-
2
When 100 ppm SO and 5% H O were present in the feed gases, cated that Fe /Cu-SSZ-13 showed good redox ability within a
2
2
x
NO conversion decreased from 98% to 86% and then restored wider temperature range than Cu-SSZ-13, due to the high-
aer they were removed. Fig. 9b showed that pure CO and temperature redox performance and rich forms of iron
the co-presence of CO and H O in diesel exhaust had no effect species. The XPS and UV-Vis DRS results suggested that there
on NO conversion. The conclusion was in accordance with were more active sites, such as Fe and Fe
previous ref. 45. and there was strong interaction between Fe and Cu species.
Inuence of hydrothermal aging of the catalysts on activity. The results were favorable to NH -SCR activity. In situ DRIFTS
In order to further investigate hydrothermal stability, the Cu- results suggested that both Brønsted and Lewis acid sites were
2
2
2
3
+
x y
O aer Fe loading
3
2
ꢁ
+
SSZ-13 and Fe1.32/Cu-SSZ-13 catalysts were treated in 10% involved in NH -SCR. At low temperature, cis-N O , NO and
3
2 2
ꢀ
3ꢁ
H O, 5% O and N at 600, 750 and 800 C for 8 h to probe the NO were main active groups and reacted with adsorbed NH3.
2
2
2
inuence of hydrothermal aging on the catalytic activity. Fig. 10
The catalytic performances of Fe /Cu-SSZ-13 were improved
x
showed the SCR activities of fresh and hydrothermal aging aer iron incorporation, due to the synergistic effects between
samples. NO conversion of hydrothermally treated samples iron and copper species. Particularly, when the molar ratio of Fe
decreased on the whole temperature range, especially, at high to Cu approached 1.0, the bimetallic zeolite showed the
temperature. Furthermore, the deNO
3 decreased with increasing aging temperature. Fe1.32/Cu- metallic zeolite. A wider operating temperature window and
SSZ-13-600 and Fe1.32/Cu-SSZ-13-750 achieved more than 85% high N selectivity (>98%) were obtained over Fe1.32/Cu-SSZ-13.
x
activity of Fe1.32/Cu-SSZ- comprehensive advantage compared with the parent mono-
1
2
ꢀ
ꢀ
NO conversion from 200 C to 350 C. However, when the aging NO conversion was more than 85% in the temperature range of
ꢀ
ꢀ
ꢀ
temperature was 800 C, NO conversion decreased signicantly. 175 C to 550 C. Although water tolerance and sulfur tolerance
increased over Fe /Cu-SSZ-13 compared with Cu-SSZ-13, NO
conversion rapidly decreased from 98% to 86% and hardly
restored under the co-existence of H O and SO . Therefore, the
O and SO tolerance
x
2
2
development of catalysts with high H
needs to be further investigated.
2
2
Acknowledgements
We acknowledge gratefully the nancial subsidy of the Program
of Universities' Innovative Research Terms (no. IRT0936).
Notes and references
1
2
H. Bosch and F. Janssen, Catal. Today, 1988, 2, 369–521.
V. I. Parvulescu, P. Grange and B. Delmon, Catal. Today,
1
998, 46, 233–316.
P. Granger and V. I. Parvulescu, Chem. Rev., 2011, 111, 3155–
207.
M. Kobayashi, R. Kuma and A. Morita, Catal. Lett., 2006, 112,
37–44.
3
4
3
Fig. 10 NO conversion of Cu-SSZ-13 and Fe1.32/Cu-SSZ-13 after
hydrothermal aging.
52138 | RSC Adv., 2014, 4, 52130–52139
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
5
6
7
M. Colombo, I. Nova and E. Tronconi, Catal. Today, 2010, 24 X. Shi, F. Liu, L. Xie, W. Shan and H. He, Environ. Sci.
51, 223–230. Technol., 2013, 47, 3293–3298.
D. W. Fickel, E. D'Addio, J. A. Lauterbach and R. F. Lobo, 25 X. Shi, F. Liu, W. Shan and H. He, Chin. J. Catal., 2012, 33,
Appl. Catal., B, 2011, 102, 441–448. 454–464.
I. Bull, R. S. Boorse, W. M. Jaglowski, G. S. Koermer, A. Moini, 26 M. S. Kumara, M. Schwidder, W. Gr u¨ nert, U. Bentrup and
1
J. A. Patchett, W. M. Xue, P. Burk, J. C. Dettling and
M. T. Caudle, US. Pat., 0 226 545, 2008.
J. H. Kwak, R. G. Tonkyn, D. H. Kim, J. Szanyi and
C. H. F. Peden, J. Catal., 2010, 275, 187–190.
A. Br u¨ ckner, J. Catal., 2006, 239, 173–186.
27 T. Ghodselahi, M. A. Vesaghi, A. Shaekhani, A. Baghizadeet
and M. Lameiial, Appl. Surf. Sci., 2008, 255, 2730–2734.
28 M. H. Kim, I. S. Nam and Y. G. Kim, J. Catal., 1998, 179, 350–
360.
8
9
S. J. Schmieg, S. H. Oh and C. H. Kim, Catal. Today, 2012,
1
84, 252–261.
0 S. I. Zones, J. Chem. Soc., Faraday Trans., 1991, 87, 3709–
716.
29 C. D. Wagner, Handbook of X-ray photoelectron spectroscopy,
ed. G. E. Muilenberg, Perkin-Elmer, USA, 1st edn, 1979, ch.
3, pp. 74–80.
1
1
1
1
1
1
3
1 R. M. Franco, M. Moliner, C. Franch, A. Kustov and 30 D. Chadwick and T. Hashemi, Corros. Sci., 1978, 18, 39–51.
A. Corma, Appl. Catal., B, 2012, 127, 273–280.
2 R. M. Franco, M. Moliner, P. Concepcion, J. R. Thogersen
and A. Corma, J. Catal., 2014, 314, 73–82.
31 S. J. Roosendaal, B. Van Asselen, J. W. Elsenaar,
A. M. Vredenberg and F. H. P. M. Habraken, Surf. Sci.,
1999, 442, 329–337.
3 R. M. Franco, M. Moliner, J. R. Thogersen and A. Corma, 32 Y. Peng, C. Wang and J. Li, Appl. Catal., B, 2014, 144, 538–
ChemCatChem, 2013, 5, 3316–3323. 546.
4 J. Y. Luo, X. Hou, P. Wijayakoon, S. J. Schmieg, W. Li and 33 D. Wang, L. Zhang, K. Kamasamudram and W. S. Epling,
W. S. Epling, Appl. Catal., B, 2011, 102, 110–119. ACS Catal., 2013, 3, 871–881.
5 M. Devadas, O. Kr ¨o cher, M. Elsener, A. Wokaun, G. Mitrikas, 34 K. I. Hadjiivanov, Catal. Rev., 2000, 42, 71–144.
N. S ¨o ger, M. Pfeifer, Y. Demel and L. Mussmann, Catal. 35 L. Ma, J. Li, Y. Cheng, G. Cavataio, R. W. McCabe, L. Fu and
Today, 2007, 119, 137–144.
6 L. Ma, J. Li, Y. Cheng, C. K. Lambert and Li. Fu, Environ. Sci. 36 S. Yang, C. Wang, J. Li, N. Yan, L. Ma and H. Chang, Appl.
Technol., 2012, 46, 1747–1754. Catal., B, 2011, 110, 71–80.
7 A. Sultana, M. Sasaki, K. Suzuki and H. Hamada, Catal. 37 M. Iwasaki, K. Yamazaki, K. Banno and H. Shinjoh, J. Catal.,
Commun., 2013, 41, 21–25. 2008, 260, 205–216.
8 T. Zhang, J. Liu, D. Wang, Z. Zhao, Y. Wei, K. Cheng, G. Jiang 38 W. Shan, F. Liu, H. He, X. Shi and C. Zhang, Chem. Commun.,
and A. Duan, Appl. Catal., B, 2014, 148, 520–531. 2011, 47, 8046–8048.
9 X. Yang, Z. Wu, M. Moses-Debusk, D. R. Mullins, 39 H. Zhu, J. H. Kwak, C. H. F. Peden and J. Szanyi, Catal. Today,
J. Li, Appl. Catal., B, 2014, 156–157, 428–437.
1
1
1
1
S. M. Mahurin, R. A. Geiger, M. Kidder and C. K. Narula, J.
Phys. Chem. C, 2012, 116, 23322–23331.
0 L. Xie, F. Liu, L. Ren, X. Shi, F. Xiao and H. He, Environ. Sci.
Technol., 2014, 48, 566–572.
2013, 205, 16–23.
40 H. Sj ¨o vall, E. Fridell, R. J. Blint and L. Olsson, Top. Catal.,
2007, 42, 113–117.
41 Y. H. Yeom, J. Henao, M. J. Li, W. M. H. Sachtlera and
E. Weitz, J. Catal., 2005, 231, 181–193.
2
2
1 J. H. Park, H. J. Park, J. H. Baik, I. S. Nama, C. H. Shinb,
J. H. Leec, B. K. Choc and S. H. Ohc, J. Catal., 2006, 240, 42 M. Aschi and F. Grandinetti, Chem. Phys. Lett., 1997, 267, 98–
7–57. 104.
2 F. Gao, E. D. Walter, E. M. Karp, J. Luo, R. G. Tonkyn, 43 C. Ciardelli, I. Nova, E. Tronconi, B. Konrad, D. Chatterjee,
4
2
2
J. H. Kwak, J. Szanyi and C. H. F. Peden, J. Catal., 2013,
00, 20–29.
K. Ecke and M. Weibel, Chem. Eng. Sci., 2004, 59, 5301–5309.
44 X. Zhang, B. Shen and K. Wang, Ind. Eng. Chem. Res., 2013,
19, 1272–1279.
3
3 S. T. Korhonen, D. W. Fickel, R. F. Lobo, B. M. Weckhuysen
and A. M. Beale, Chem. Commun., 2011, 47, 800–802.
45 T. Yu, J. Wang, M. Shen and W. Li, Catal. Sci. Technol., 2013,
3
, 3234–3241.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 52130–52139 | 52139