Cerium and tin oxides anchored onto reduced graphene oxide for selective catalytic reduction of NO with NH3 at low temperatures

Article abstract of DOI:10.1039/c8ra05151h

A series of cerium and tin oxides anchored on reduced graphene oxide (CeO₂-SnO_x/rGO) catalysts are synthesized using a hydrothermal method and their catalytic activities are investigated by selective catalytic reduction (SCR) of NO with NH₃ in the temperature range of 120-280 °C. The results indicate that the CeO₂-SnO_x/rGO catalyst shows high SCR activity and high selectivity to N₂ in the temperature range of 120-280 °C. The catalyst with a mass ratio of (Ce + Sn)/GO = 3.9 exhibits NO conversion of about 86% at 160 °C, above 97% NO conversion at temperatures of 200-280 °C and higher than 95% N₂ selectivity at 120-280 °C. In addition, the catalyst presents a certain SO₂ resistance. It is found that the highly dispersed CeO₂ nanoparticles are deposited on the surface of rGO nanosheets, because of the incorporation of Sn⁴⁺ into the lattice of CeO₂. The mesoporous structures of the CeO₂-SnO_x/rGO catalyst provides a large specific surface area and more active sites for facilitating the adsorption of reactant species, leading to high SCR activity. More importantly, the synergistic interaction between cerium and tin oxides is responsible for the excellent SCR activity, which results in a higher ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺), higher concentrations of surface chemisorbed oxygen and oxygen vacancies, more strong acid sites and stronger acid strength on the surface of the CeSn(3.9)/rGO catalyst.

Full text of DOI:10.1039/c8ra05151h

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Cerium and tin oxides anchored onto reduced
graphene oxide for selective catalytic reduction of
NO with NH3 at low temperatures
ab
Cite this: RSC Adv., 2018, 8, 36383
Yanli Wang,a Ying Kang,a Meng Ge,a Xiu Zhanga and Liang Zhan
*
A series of cerium and tin oxides anchored on reduced graphene oxide (CeO2–SnOx/rGO) catalysts are
synthesized using a hydrothermal method and their catalytic activities are investigated by selective
catalytic reduction (SCR) of NO with NH3 in the temperature range of 120–280 ꢀC. The results
indicate that the CeO2–SnOx/rGO catalyst shows high SCR activity and high selectivity to N2 in the
temperature range of 120–280 ꢀC. The catalyst with a mass ratio of (Ce + Sn)/GO ¼ 3.9 exhibits NO
conversion of about 86% at 160 ꢀC, above 97% NO conversion at temperatures of 200–280 ꢀC and
higher than 95% N2 selectivity at 120–280 ꢀC. In addition, the catalyst presents a certain SO2
resistance. It is found that the highly dispersed CeO2 nanoparticles are deposited on the surface of
rGO nanosheets, because of the incorporation of Sn4+ into the lattice of CeO2. The mesoporous
structures of the CeO2–SnOx/rGO catalyst provides a large specific surface area and more active sites
for facilitating the adsorption of reactant species, leading to high SCR activity. More importantly, the
synergistic interaction between cerium and tin oxides is responsible for the excellent SCR activity,
which results in a higher ratio of Ce3+/(Ce3+ + Ce4+), higher concentrations of surface chemisorbed
oxygen and oxygen vacancies, more strong acid sites and stronger acid strength on the surface of the
CeSn(3.9)/rGO catalyst.
Received 15th June 2018
Accepted 18th October 2018
DOI: 10.1039/c8ra05151h
rsc.li/rsc-advances
carbon materials (activated carbon,8 activated carbon bers,9
carbon nanotubes10–12 and honeycomb activated carbon13,14).
1. Introduction
Among the novel SCR catalyst candidates, MnOx–CeO2
catalysts have attracted much attention due to their high NOx
removal activities and resistance to SO2 poisoning in the low
Nitrogen oxides (NOx) emitted from ue gases are considered to
be one of the major air pollutants, and must be removed before
emission. Selective catalytic reduction (SCR) of NO with NH3
has become the mainstream strategy of industrial application
and research due to its high NOx removal efficiency and low
energy consumption.1,2 Up to now, commercial SCR catalysts
(such as V2O5/TiO2, V2O5–WO3/TiO2) must be operated in a high
temperature range of 100–200 C.15,16 The promoting effect of
ꢀ
CeO2 is attributed to its unique redox property and excellent
oxygen storage capacity,17,18 which associates with the formation
of oxygen vacancies. As literatures reported,7,18 the replacement
of partial cerium atoms in the CeO2 lattice by the other transi-
tion metal ions caused the distortion of CeO2 lattice and
generated defects, resulting in improved thermal stability and
increased SCR activity. However, it should be pointed out that
MnOx–CeO2 based catalysts exhibit low SCR activities especially
in the presence of SO2. To improve the resistance to SO2
poisoning in NO removal, the MnOx–CeO2 catalysts modied
with SnO2 have been developed. Chang et al.19,20 reported SnO2-
modied MnOx–CeO2 catalysts enhanced the SCR activity,
ꢀ
temperature range of 350–450 C to avoid catalyst deactivation
3,4
by SO2, while the stack gas temperatures in many industrial
ꢀ
boilers are only about 120–250 C. To satisfy the high reaction
temperature (350–450 ꢀC) for V2O5/TiO2 based catalysts, existing
boiler systems must be coupled with a heating system or the
ue gases are preheated, resulting in high energy consumption.
Therefore, various novel low temperature SCR catalysts are
designed and developed for practical NOx removal, such as
V2O5, CeO2, MnOx, MnOx–CeO2 supported on TiO2 (ref. 5–7) or
ꢀ
broadened the operating temperature window (80–300 C) and
exhibited good SO2 resistance. Our recent studies21 revealed
that spherical activated carbons supported SnOx–CeO2–MnOx
catalyst (SnCeMn/SACs) with a molar ratio of Sn/Mn ¼ 0.25
yielded higher than 95% NO conversion in the temperature
range of 140–280 ꢀC, and showed high SO2 resistance. At 240 ꢀC,
NO conversion over SnCeMn/SACs gradually decreased from
96% in the absence of SO2 to about 77% in 480 min upon the
aState Key Laboratory of Chemical Engineering, Key Laboratory for Specially
Functional Polymers and Related Technology of Ministry of Education, Shanghai
Key Laboratory of Multiphase Materials Chemical Engineering, East China
University of Science and Technology, Shanghai 200237, China. E-mail: zhanliang@
ecust.edu.cn; Fax: +86 21 64252914
bCAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, China
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introduction of SO2 in the feed gas. Recently, it has also been
demonstrated CeO2–SnOx catalysts showed high SCR activities,
which was attributed to the strong synergistic effect between Ce
and Sn species, leading to increased surface acidity of Lewis
acid sites.22 Based on the high activities of CeO2–SnOx based
catalysts, it is necessary the development of CeO2–SnOx in
nanoscale to further improve their SCR activities. As a novel
carbon nanomaterial, reduced graphene oxide (rGO) has been
attracted attention, because of its unique two-dimensional
microstructure and the existence of a certain amount of
oxygen-containing functional groups on it.23,24 Therefore, the
combination of rGO nanosheets and CeO2–SnOx are expected to
achieve nanostructured CeO2–SnOx that provides larger reac-
tion interfaces, leading to the enhancement of the catalytic
activity.
2.2 Activity test
The activity test was carried out in a xed-bed reactor with
a diameter of 12 mm. At steady state, the feed gases are con-
sisted with 500 ppm NO, 500 ppm NH3, 5 vol% O2, 200 ppm SO2
(when used), and balance N2. The total ow rate was controlled
at 500 mL minꢁ1, where the gas hourly space velocity (GHSV) is
62 500 hꢁ1, and the reaction temperature was regulated from
120 to 280 ꢀC. The concentrations of NO and O2 in the inlet and
outlet gases were continually monitored on-line by a ue gas
analyzer (MRU VARIO PLUS, Germany). The effluent gas
concentrations of NO and N2O were monitored by Nicolet (FTIR,
´ ´
PROTEGE460, America). The SCR behaviors of the catalysts are
expressed by NO conversion and N2 selectivity, which were
calculated as literature reported.27
In this work, a series of CeO2–SnOx anchoredon reduced
grapheme oxide (CeO2–SnOx/rGO) catalysts are synthesized
using hydrothermal method and their activities for SCR of NO
are evaluated. The effects of SnOx on the activities, structures
and surface properties of CeO2/rGO catalyst are also investi-
gated. The catalysts are characterized to understand the phys-
ical and chemical properties of CeO2–SnOx/rGO as well as the
relationship between structure and catalytic activity. The ob-
tained CeO2–SnOx/rGO catalysts exhibit high SCR activities in
the low temperature range of 120–280 ꢀC, resulted from the
highly dispersed CeO2 nanoparticles, mesoporous structures
with high surface area and the synergistic interaction between
cerium and tin oxides.
2.3 Characterization
Nitrogen adsorption/desorption isotherms of the samples were
performed by Micromeritics ASAP 2020 M analyzer, and the
pore size distributions were obtained by the Barrett–Joyner–
Halenda (BJH) model. Raman and Fourier transform infrared
(FT-IR) spectra were recorded on a Spex 1403Raman spectrom-
eter and a Nicolet 5700 FTIR spectrometer, respectively. Powder
X-ray diffraction (XRD) patterns of the samples were recorded by
a Riguku D/Max 2550 diffractometer using Cu Ka radiation. The
lattice parameters and crystallite sizes of all samples were
calculated according to Bragg's law and the Debye–Scherrer
equation, respectively. The microstructures of the samples were
investigated using
a JEOLJSM-6360LV scanning electron
microscope (SEM) and a JEOL JEM-2010 transmission electron
microscopy (TEM). The surface compositions of the catalysts
were analyzed by X-ray photoelectron spectroscopy (XPS) with
a PHI 5000 VersaProbe system. NH3-temperature programmed
desorption (NH3-TPD) experiments were carried out on an
adsorption apparatus (TP-5080, Xianquan Co.) with a thermal
conductivity detector (TCD). 0.1 g sample was loaded into the
2. Experimental
2.1 Catalyst preparation
All the chemical reagents purchased from Aldrich were analyt-
ical grade. The graphene oxide (GO) was prepared using the
modied Hummers.25 The CeO2–SnOx/rGO catalysts were
synthesized by the hydrothermal method using GO, Ce(NO3)3-
$6H2O and SnCl4$5H2O as the precursors. In a typical proce-
dure, 100 mg GO was initially dispersed in 50 mL deionized
water by ultrasonic treatment for 3 h. Then appropriate
amounts of Ce(NO3)3$6H2O and SnCl4$5H2O were dropped
slowly into the GO slurry and the mixture was stirred for 1 h.
Subsequently, ammonia solution was added into the above
solution under vigorously stirring until the pH value reached.11
Aer stirring for 30 min, the obtained suspension was trans-
ferred into a 100 mL Teon-lined stainless autoclave and heated
at 180 ꢀC for 12 h. Finally, the product was washed with
ꢀ
reactor and pretreated at 300 C for 30 min in He stream, and
then cooled to 100 ꢀC in He. Then the He ow was switched to
a stream containing 5 vol% NH3/He to adsorb NH3 for 60 min at
100 ꢀC. And subsequently the sample was purged with He at the
same temperature. Finally, the sample was heated in He from
ꢁ1
ꢀ
ꢀ
100 C to 800 ꢀC at a heating rate of 10 C min
.
3. Results and discussion
3.1 SCR activities
deionized water and then freeze-dried for 24 h, followed by Fig. 1 presents the SCR activities versus reaction temperature
calcination at 300 ꢀC for 2 h in N2 atmosphere. The molar ratio over the CeO2/rGO, SnOx/rGO and CeO2–SnOx/rGO catalysts.
of Sn/Ce in all CeO2–SnOx/rGO catalysts is controlled at 0.5, The CeO2/rGO catalyst exhibits a relatively high SCR activity,
which was reported to be the optimum ratio of Ce–Sn/TiO2 and NO conversion increases from 34% at 120 ꢀC to 68% at
catalyst for NO removal.26 Besides, the mass ratios of (Ce + Sn)/ 180 ꢀC, and is more than 83% from 200 to 280 ꢀC. Although NO
GO is 1.3, 2.6 and 3.9, respectively. The resultant catalysts are conversion of SnOx/rGO catalystis less than 15% in the
ꢀ
denoted as CeSn(m)/rGO, where m presents the mass ratio of (Ce temperature range of 120–280 C, interestingly, the CeSn(3.9)/
+ Sn)/GO. For example, CeSn(1.3)/rGO refers to the mass ratio of rGO catalyst shows a higher SCR activity than both of CeO2/
(Ce + Sn)/GO is 1.3 in the CeO2–SnOx/rGO catalyst. For rGO and SnOx/rGO catalysts. NO conversion of the CeSn(3.9)/
ꢀ
comparison, CeO2/rGO and SnOx/rGO were also synthesized rGO catalyst can achieve about 86% at 160 C, and above 97%
using the similar procedure.
at temperatures of 200–280 ꢀC, respectively. These results
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obviously and greatly enhanced with increasing mass ratio of
(Ce + Sn)/GO at different reaction temperatures. NO conversion
ꢀ
of CeSn(2.6)/rGO catalyst can increase to about 91% at 200 C,
and about 98% at temperatures of 240–280 ꢀC. NO conversion of
CeSn(3.9)/rGO catalyst can reach about 64% at 120 ꢀC, about
ꢀ
ꢀ
86% at 160 C and about 97% at 200 C. And it shows nearly
99% NO conversion in the temperature range of 240–280 ꢀC.
This SCR activity result is better than the data reported in the
literatures for CeO2–SnOx (ref. 22), CeO2/TixSn1ꢁxO2 (ref. 28)
and H-CeSnTiOx (ref. 29) catalysts in the temperature range of
ꢀ
120–280 C. The increase in SCR activity with increasing mass
ratio of (Ce + Sn)/GO may be attributed to the increase of active
sites on the surface of catalyst. Fig. 2b shows the N2 selectivity at
different temperatures over CeSn(1.3)/rGO, CeSn(2.6)/rGO and
CeSn(3.9)/rGO catalysts. The N2 selectivity of all the catalysts
decreases with increasing temperature, however, higher than
95% N2 selectivity is still obtained at 280 ꢀC. This result is
similar to that reported on other CeO2–SnOx based cata-
lysts.22,28,29 Therefore, the CeSn(3.9)/rGO catalyst is very active
and highly selective for the SCR of NO.
Fig. 1 NO conversions over various catalysts at different reaction
temperatures.
indicate that the interaction between CeO2 and SnOx is bene-
cial to promote the SCR activity of CeO2/rGO catalyst.
Since Fig. 1 indicates that the addition of SnOx has a positive
effect on the SCR activity of CeO2/rGO catalyst, the SCR activities
of CeO2–SnOx/rGO catalysts with different mass ratios of (Ce +
Sn)/GO were further investigated. As shown in Fig. 2a, all the
CeO2–SnOx/rGO catalysts exhibit high SCR activities and NO
conversions of all the catalysts increase with increasing reaction
temperature. NO conversion of CeSn(1.3)/rGO catalyst is less
3.2 Morphological and structural characterization of the
catalysts
3.2.1 Morphology. The morphology and microstructure of
the CeSn(3.9)/rGO catalyst are characterized using TEM. As
shown in Fig. 3a, CeO2 nanoparticles with the average size of
about 6 nm are dispersed homogeneously on the rGO nano-
sheets, and the HRTEM image reveals that the visible lattice
fringes with a lattice spacing of 0.31 and 0.26 nm, corre-
sponding to the (111) and (220) crystal planes of CeO2 (Fig. 3b).
Compared with CeO2/rGO, the size of metal oxides nano-
particles in the CeSn(3.9)/rGO signicantly becomes smaller
(Fig. 3c). The coexistence of cerium and tin oxides decreases the
nanoparticle size, indicating that there exists strong interaction
between CeO2 and SnOx.30 Interestingly, although the (111)
crystal plane of CeO2 can be also clearly observed (Fig. 3d) on
the CeSn(3.9)/rGO, the (110) crystal plane of SnO2 with a lattice
ꢀ
than 30% in the low temperature range of 120–160 C, and it
reaches about 80% at 280 ꢀC. Furthermore, NO conversion is
Fig. 2 NO conversions (a) and N2 selectivity (b) over CeO2–SnOx/rGO Fig. 3 TEM and HRTEM images of CeO2/rGO (a and b) and the
catalysts with different mass ratios of (Ce + Sn)/GO.
CeSn(3.9)/rGO catalyst (c and d).
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spacing of 0.33 nm cannot be observed.31 Therefore, the highly prevent the agglomeration of CeO2 and SnOx nanoparticles,
dispersed nanoparticles aer the addition of SnOx are expected agreeing with the TEM result. It is worth noting that the average
to be favourable for the enhancement of the SCR activity.
pore size of CeO2–SnOx/rGO catalysts is much smaller than that
3.2.2 Physical and chemical properties. The nitrogen of the CeO2/rGO catalyst (Fig. 4b), suggesting the structural
adsorption/desorption isotherms of CeO2/rGO and CeO2–SnOx/ change of the catalysts to some extent. Therefore, it can be
rGO catalysts exhibit typical type-IV curves with a hysteresis concluded that the mesoporous structures with larger specic
loop (Fig. 4a), suggesting that there exists mesoporous struc- surface area of CeO2–SnOx/rGO catalysts can provide more
tures produced during the hydrothermal process. CeO2/rGO has surface active sites for facilitating the adsorption of reactant
a specic surface area of 133.4 m2 gꢁ1 and an average pore size species, leading to the increase in SCR activity.
of about 8.43 nm (Table 1). The addition of SnOx to CeO2/rGO
To determine the total contents of CeO2 and SnOx in the
causes an increase in specic surface area, and the specic CeO2–SnOx/rGO catalysts, TG were carried out from 25 to 800 ꢀC
surface areas of CeSn(1.3)/rGO, CeSn(2.6)/rGO and CeSn(3.9)/ in air atmosphere (Fig. 4c). From TG curves, a small weight loss
rGO are 249.1, 220.8 and 197.7 m2 gꢁ1, respectively. The below 150 ꢀC is attributed to the removal of physically adsorbed
specic surface areas of these CeO2–SnOx/rGO catalysts are water in the samples. And the obvious weight loss between 280
much larger than the data reported for CeO2–SnOx (63 m2 gꢁ1),22 and 500 ꢀC is ascribed to the combustion of grapheme into CO2.
CeO2/TixSn1ꢁxO2 (75.6 m2 gꢁ1
)
and H-CeSnTiOx (133.6 m2 The residual weight aer TG tests is determined to be 67.0%,
28
gꢁ1),29 which may be explained because rGO nanosheets can 78.9% and 84.4% for CeSn(1.3)/rGO, CeSn(2.6)/rGO and
Fig. 4 (a) N2 adsorption–desorption isotherms, (b) corresponding pore size distributions, (c) TG curves of the various catalysts, (d) Raman spectra
of rGO and the CeSn(3.9)/rGO catalyst.
Table 1 Physical properties of various catalystsa
b
SBET (m2 gꢁ1
)
Vtotal (cm3 gꢁ1
)
Average pore size (nm)
Lattice parameter (A)
Crystallize sizec (nm)
˚
Sample
CeO2/rGO
133.4
249.1
220.8
197.7
0.28
0.19
0.21
0.20
8.43
3.29
3.76
4.04
5.419
5.361
5.369
5.364
13.3
3.7
4.4
5.5
CeSn(1.3)/rGO
Ce(Sn(2.6)/rGO
CeSn(3.9)/rGO
a
SBET: BET specic surface area; Vtotal: total pore volume. b Calculated from the diffraction peak of the (111) plane. c Calculated from the XRD peak
of the (111) plane.
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CeSn(3.9)/rGO, respectively, corresponding to the total contents catalyst. This result further suggests Sn4+ can be incorporated
of CeO2 and SnOx in the samples. into the lattice of CeO2 to form the CeO2–SnOx solid solution.
To analyze the chemical structure of CeSn(3.9)/rGO catalyst, Such catalyst solid solution phenomenonon was also reported
the Raman spectroscopy is explored as shown in Fig. 4d. Two by Chen et al. on CuO/CexSn1ꢁxO2 catalysts (1 ꢁ x # 0.2), which
characteristic peaks at about 1350 (D band) and 1596 cmꢁ1 (G found the formation of homogeneous solid solutions between
band) can be clearly observed in both rGO and the CeSn(3.9)/ CeO2 and SnO2.41 Similar explanations were also elucidated by
rGO catalyst, corresponding to the sp3 and sp2 carbon atoms, Machida et al. on MnOx–CeO2 binary oxides, where the
respectively.32 This phenomenon conrms the existence of rGO replacement of Ce4+ by Mn3+ in the uorite structure results in
in the obtained sample. The ratio of relative intensity ID/IG the formation of a solid solution between Mn2O3 and CeO2.16,42
reects the degree of graphitization, defects and the domain Additionally, CeO2 diffraction peaks of CeO2–SnOx/rGO cata-
size of graphitization. The ID/IG value of CeSn(3.9)/rGO catalyst lysts are broader than those of individual CeO2/rGO and the
(0.99) is higher than that of rGO (0.92), indicating much more crystallize size of CeO2–SnOx/rGO catalysts is smaller than that
defects in the CeSn(3.9)/rGO catalyst. Another peak at 460 cmꢁ1 of CeO2/rGO (Table 1), further conrming the formation of
is also observed in the Raman spectrum of CeSn(3.9)/rGO CeO2–SnOx solid solution resulted from the interaction between
catalyst, corresponding to the F2g vibration mode of CeO2 in cerium and tin species, which leads to higher dispersion of the
the uorite structure,20,33 which indicates the presence of CeO2 metal oxides on the surface of CeO2–SnOx/rGO. Therefore, the
nanoparticles in the CeSn(3.9)/rGO catalyst. Unfortunately, the improved dispersion of cerium and tin species on the catalyst
characteristic peaks at about 610 cmꢁ1 corresponding to SnOx surface plays a positive effect on SCR activity, which is consis-
cannot be observed,34 suggesting that no pure SnOx phase is tent with SCR activity results.
formed. It is worth noting that two broad peaks at 263 and
To illustrate the different types of chemical functional
599 cmꢁ1 can be also observed in the CeSn(3.9)/rGO catalyst groups in the obtained samples from GO to the CeSn(3.9)/rGO
(the inset in Fig. 4d), which are related to the existence of oxygen catalyst, FT-IR spectra were carried out. As shown in Fig. 5b,
vacancies.35 The generation of oxygen vacancies could increases the representative absorption peaks ofGO can be observed,
the oxygen storage capacity and transfer ability between Ce3+ including the stretching vibration peak of O–H group at
and Ce4+, which contributes to enhancing the SCR activity.36
approximately 3420 cmꢁ1, the C]O (COOH) stretching vibra-
The XRD patterns of the CeO2–SnOx/rGO catalysts with tion peak at 1720 cmꢁ1, the aromatic C]C stretching vibration
different mass ratios of (Ce + Sn)/GO were characterized, along peak at approximately 1624 cmꢁ1, the C–OH deformation
with that of CeO2/rGO for comparison (Fig. 5a). CeO2/rGO vibration peak at 1407 cmꢁ1, and the alkoxy C–O stretching
catalyst shows characteristic diffraction peaks located at 28.5ꢀ, vibration peak at 1080 cmꢁ1 43,44 For the CeSn(3.9)/rGO catalyst,
.
33.1ꢀ, 47.5ꢀ and 56.4ꢀ, which can be assigned to (111), (200), the spectrum peaks of C]O, C–OH and C–O are signicantly
(220) and (311) planes of cubic CeO2 crystalline (JCPDS card no. weakened. This result further conrms GO is reduced to rGO
34-0394), respectively.37,38 For CeO2–SnOx/rGO catalysts, only during the hydrothermal treatment, agreeing well with the
diffraction peaks of cubic CeO2 are observed and the typical Raman result.
characteristic peaks of SnO2 located at 26.7ꢀ, 33.9ꢀ and 51.6ꢀ
To further determine the chemical composition of the CeO2–
can not be observed,39,40 in accordance with TEM and Raman SnOx/rGO catalyst, the XPS analysis was performed. Fig. 6
results, indicating the replacement of Ce4+ by Sn4+ due to the presents the XPS survey spectra of CeSn(1.3)/rGO and CeSn(3.9)/
smaller ionic radius of Sn4+ than Ce4+(the former is 0.069 nm, rGO, revealing the presence characteristic peaks of cerium, tin,
the latter is 0.087 nm). Meanwhile, no a broad peak around 26ꢀ carbon and oxygen. And the detail surface atomic concentra-
related to the restacked rGO is detected, suggesting the as- tions of Ce, Sn, C, O and the relative atomic ratios of Ce3+/(Ce3+
prepared grapheme oxide is well exfoliated. It is worth point- + Ce4+) are summarized in Table 2. Fig. 7a shows the XPS spectra
ing out that the diffraction peaks of CeO2–SnOx/rGO catalysts of C 1s. Three peak at 284.8, 286.6 and 289.0 eV are observed,
slightly shi to higher angle compared to those of CeO2/rGO corresponding to the C–C, the C–O, and O–C]O groups,
Fig. 5 (a) XRD patterns of the various catalysts. (b) FT-IR spectra of GO and the CeSn(3.9)/rGO catalyst.
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The O 1s XPS spectra are tted into two peaks and the results
are shown in Fig. 7b. Two peaks at 529.5–530.2 eV and 531.2–
532.0 eV correspond to the peaks of lattice oxygen (Oa) and
chemisorbed oxygen (Ob), respectively.37 As shown in Fig. 7b
and Table 2, the Oa concentration of CeSn(3.9)/rGO markly
increases compared with that of CeSn(1.3)/rGO, due to
increased lattice oxygen in SnOx and CeO2 with the increasing of
(Ce + Sn)/GO mass ratio. It is worth noting that the Ob
concentration on CeSn(3.9)/rGO (13.57%) is higher than that on
CeSn(1.3)/rGO (9.92%), indicating increased chemisorbed
oxygen concentration on the surface of CeSn(3.9)/rGO. It has
been generally accepted that the chemisorbed oxygen species
(Ob) are more active than the lattice oxygen species (Oa) due to
their higher oxygen mobility.47 Furthermore, the higher
concentration of Ob is benecial for the oxidation of NO to NO2,
which accelerates the “fast SCR” process (4NH3 + 2NO + 2NO2
/ 4N2 + 6H2O) and hence promotes the catalytic activity.48
The Ce 3d XPS spectra are shown in Fig. 7c. The u000, u00, u, v000,
v00, and v peaks are assigned to Ce4+, while the peaks labeled u0
and v0 are attributed to Ce3+.37,49 For the two catalysts, the
intensities of the u0 and v0 peaks for Ce3+are much weaker than
those of the u000, u00, u, v000, v00, and v peaks for Ce4+, suggesting
that Ce4+ and Ce3 coexist and the main valence state of cerium is
Ce4+. It is noted that the intensities of Ce4+ and Ce3+ peaks in
CeSn(3.9)/rGO are much stronger than those in CeSn(1.3)/rGO,
indicating increased concentrations of Ce4+ and Ce3+. The ratios
of Ce3+/(Ce3+ + Ce4+) can be determined by the area ratio of Ce3+
Fig. 6 XPS survey spectra of the different catalysts.
Table 2 XPS results of the different catalysts
Atomic
ratio (%)
Surface atomic
concentration (%)
O
Ce3+
(Ce3+ + Ce4+
)
/
Sample
Ce
Sn
C
Oa
Ob
CeSn(1.3)/rGO 2.80 4.09 64.87 18.33
9.92 18.7
CeSn(3.9)/rGO 5.59 7.59 40.12 33.13 13.57 19.8
respectively.45 The low intensities of C–O, and O–C]O func-
tional groups in the obtained samples indicate the reduction of
GO to rGO during the hydrothermal process,46 which is
consistent with the Raman and FT-IR results.
species. As illustrated by Table 2, the ratio of Ce3+/(Ce3+ + Ce4+
)
in CeSn(3.9)/rGO (19.8%) is slightly higher than that in
CeSn(1.3)/rGO (18.7%). It has been reported that the existence
Fig. 7 XPS spectra of C 1s (a), O 1s (b), Ce 3d (c) and Sn 3d (d) for the different catalysts.
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of Ce3+ can create charge imbalance to form oxygen vacancies CeSn(3.9)/rGO, two obvious desorption peaks above 350 C are
and unsaturated chemical bonds,26,50 leading to the increase in observed. It is widely accepted that the position of NH3
chemisorbed oxygen on the catalyst surface, in accordance with desorption peak is connected with the acid strength and the
ꢀ
the above results of O 1 s spectra. Thus, the higher ratio of Ce3+
/
area of desorption peak is proportional to the acid amount.
(Ce3+ + Ce4+) in CeSn(3.9)/rGO implies the generation of more Compared with CeO2/rGO, the desorption peaks of CeSn(1.3)/
oxygen vacancies. The increase of oxygen vacancies on the rGO and CeSn(3.9)/rGO markly shi to higher temperature,
catalyst surface could result in more gaseous oxygen being suggesting that the strength of strong acid sites become
supplied on the catalyst.51 Moreover, higher concentration of stronger aer the addition of SnOx. Meanwhile, the desorption
oxygen vacancies is advantageous to the improvement of oxygen peak areas of CeSn(1.3)/rGO and CeSn(3.9)/rGO are larger than
mobility.52 Both of them could facilitate the activation and that of CeO2/rGO, indicating that there are more strong acid
transportation of the active oxygen species in the SCR reactions, sites on them. It could be attributed to the larger specic
which result in the excellent SCR activity of CeSn(3.9)/rGO surface area of CeSn(1.3)/rGO and CeSn(3.9)/rGO (Table 1). The
catalyst, as evidenced by the results shown in Fig. 2a.
amount of strong acid sites on CeSn(3.9)/rGO is lower than that
Fig. 7d presents the Sn 3d XPS spectra of SnOx/rGO, of CeSn(1.3)/rGO, probably due to the decrease in specic
CeSn(1.3)/rGO and CeSn(3.9)/rGO. The binding energies of Sn surface area (Table 1). Additionally, the desorption peak
3d3/2 and Sn 3d5/2 in SnOx/rGO are located at 495.9 and 487.5 eV, temperature of CeSn(3.9)/rGO is higher than that of CeSn(1.3)/
respectively, indicating the existence of Sn4+ 21,40 Compared with rGO, suggesting that the strength of strong acid sites on
.
SnOx/rGO, the peaks of Sn 3d3/2 and Sn 3d5/2 in CeSn(1.3)/rGO CeSn(3.9)/rGO is stronger. The stronger acid sites could be
and CeSn(3.9)/rGO catalysts slightly shi to lower binding related to the stronger interaction between CeO2 and SnOx on
energies. The shi in binding energy indicates that there are CeSn(3.9)/rGO. Therefore, CeSn(3.9)/rGO has more amount and
excess electrons around Sn atoms on CeSn(1.3)/rGO and stronger strength of strong acid sites, which facilitates the
CeSn(3.9)/rGO, and the existence of synergistic interaction adsorption and activation of NH3, resulting in the higher SCR
between cerium and tin oxides is further demonstrated by the activity of CeSn(3.9)/rGO.
following redox equilibrium: Sn4+ + 2Ce3+ 4 Sn2+ + 2Ce4+.53 The
above XPS results demonstrate that relatively high concentra-
3.4 Effect of SO2 on SCR activity
tions of chemisorbed oxygen species, oxygen vacancies and
Ce3+/(Ce3+ + Ce4+) are benecial for the SCR activity of
CeSn(3.9)/rGO catalyst.
The actual ue gas still contains some residual SO2 even aer
desulfurization, and thus it is necessary to investigate the SO2
resistancꢀe of CeSn(3.9)/rGO catalyst in the temperature range of
120–280 C. Fig. 9 presents the effect of SO2 on SCR activity of
CeSn(3.9)/rGO catalyst at 240 ꢀC. Aer the introduction of
200 ppm SO2 into the feed stream, NO conversion gradually
decreases from about 95% to about 63% in 180 min. Aer SO2 is
shut off, NO conversion cannot be recovered to the original
value, but NO conversion still maintains at about 65%. Such
results suggest that CeSn(3.9)/rGO catalyst has a certain SO2
resistance. Since CeSn(3.9)/rGO catalyst not only exhibits high
SCR activity in the temperature range of 120–280 ꢀC but also
presents good SO2 resistance, CeSn(3.9)/rGO catalyst seems to
have promising prospect for NO removal at low temperature.
3.3 NH3-TPD analysis
It is generally recognized that the adsorption and activation
(partial oxidation: NH3(ads) / NH2(ads)) of NH3 is a key step in
the NH3-SCR reaction.54,55 Therefore, the adsorption and acti-
vation of NH3 is promoted by increasing the surface acidity of
the catalyst, which is favorable for improving SCR activity. NH3-
TPD experiments were carried out to measure the acidity of the
obtained catalysts. Fig. 8 shows the NH3-TPD curves for CeO2/
rGO, CeSn(1.3)/rGO and CeSn(3.9)/rGO. For CeO2/rGO, one
desorption peak appeared at 150–320 ꢀC is assigned to the weak
acid sites, while another broad desorption peak at 350–620 ꢀC is
assigned to the strong acid sites. For CeSn(1.3)/rGO and
Fig. 9 Effect of SO2 on SCR activity of CeSn(3.9)/rGO catalyst at
Fig. 8 NH3-TPD profiles of the various catalysts.
240 ꢀC.
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Paper
12 S. X. Cai, H. Hu, H. R. Li, L. Y. Shi and D. S. Zhang, Nanoscale,
2016, 8, 3588–3598.
4. Conclusions
13 Y. L. Wang, Z. Y. Liu, L. Zhan, Z. G. Huang, Q. Y. Liu and
J. R. Ma, Chem. Eng. Sci., 2004, 59, 5283–5290.
14 Y. L. Wang, X. X. Li, L. Zhan, C. Li, W. M. Qiao and L. C. Ling,
Ind. Eng. Chem. Res., 2015, 54, 2274–2278.
15 Z. M. Liu, Y. Yi, S. X. Zhang, T. L. Zhu, J. Z. ZhuandJ and
G. Wang, Catal. Today, 2013, 216, 76–81.
The CeO2–SnOx anchored on reduced graphene oxide (CeO2–
SnOx/rGO) catalysts were successfully synthesized using
hydrothermal method. The CeO2–SnOx/rGO catalysts show high
SCR activities in the temperature range of 120–280 ꢀC. About
ꢀ
86% NO conversion at 160 C and more than 97% NO conver-
ꢀ
sion in the temperature range of 200–280 C are achieved over
16 G. Qi, R. T. Yang and R. Chang, Appl. Catal., B, 2004, 51, 93–
106.
the CeSn(3.9)/rGO catalyst. All the catalysts yield higher than
95% N2 selectivity at 120–280 ꢀC. Additionally, CeSn(3.9)/rGO
catalyst presents a certain SO2 resistance. The excellent SCR
activity can be attributed to the highly dispersed CeO2 nano-
particles on the rGO nanosheets, and its mesoporous structures
with large specic surface area. More importantly, there are
higher ratio of Ce3+/(Ce3+ + Ce4+), higher concentrations of
surface chemisorbed oxygen and oxygen vacancies, more
amount of strong acid sites and stronger acid strength on the
surface of CeSn(3.9)/rGO catalyst, because there exists the
synergistic interaction between cerium and tin oxides. These
factors contribute to higher SCR activity of CeSn(3.9)/rGO
catalyst. Further experiments are underway to elucidate the
mechanisms of SO2 deactivation and SCR reaction over CeO2–
SnOx/rGO catalyst.
17 B. M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant
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18 A. Trovarelli, F. Zamar, J. Llorca, C. deLeitenburg, G. Dolcetti
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21 Y. L. Wang, Z. G. He, L. Zhan and M. Ge, New Carbon Mater.,
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22 Z. M. Liu, X. Feng, Z. Z. Zhou, Y. J. Feng and J. H. Li, Appl.
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Conflicts of interest
24 S. F. Pei and H. M. Cheng, Carbon, 2012, 50, 3210–3228.
25 W. S. Hummers and R. E. Offeman, Preparation of graphitic
oxide, J. Am. Chem. Soc., 1958, 80, 1339.
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27 P. Li, Q. Y. Liu and Z. Y. Liu, Ind. Eng. Chem. Res., 2011, 50,
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There are no conicts to declare.
Acknowledgements
This work was nancially supported by National Science
Foundation of China (No. 51472086, 51002051, 20806024) and
CAS Key Laboratory of Carbon Materials (No. KLCMKFJJ1703).
28 L. Zhang, L. L. Li, Y. Cao, Y. Xiong, S. G. Wu, J. F. Sun,
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