Structure-performance relationships of MnO2 nanocatalyst for the low-temperature SCR removal of NOX under ammonia

Article abstract of DOI:10.1039/c6ra03108k

To investigate the corresponding relationship between catalytic efficiency and structure, MnO₂ nanomaterials (nanospheres, nanosheets, nanorods) have been prepared successfully, and were thoroughly characterized by SEM and TEM. Furthermore, the selective catalytic reduction (SCR) performance of NO_X under ammonia was used as an indicative reaction. Among the MnO₂ nanomaterials with different morphologies, it was found that their SCR activities showed an interesting variation tendency: nanospheres > nanosheets > nanorods of MnO₂. The NO conversion ratio of the MnO₂ nanospheres could reach 100% from 200 to 350 °C. Moreover, in order to study the probable mechanism for the best removal efficiency of the nanospheres, XRD, H₂-TPR, NH₃-TPD, BET, XPS and in situ DRIFTS were performed in detail. It is found that surface chemisorbed oxygen, specific surface area, reducibility and acid sites have great influence on the NO removal efficiency in the SCR reaction. In addition, how several process parameters affect the NO_X removal efficiency was carried out, such as time, H₂O and SO₂.

Full text of DOI:10.1039/c6ra03108k

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PAPER
Structure–performance relationships of MnO2
nanocatalyst for the low-temperature SCR removal
of NOX under ammonia†
Cite this: RSC Adv., 2016, 6, 54926
a
a
a
b
Yi Li, Yanping Li, Yuan Wan, Sihui Zhan, Qingxin Guanb and Yang Tianc
*
*
To investigate the corresponding relationship between catalytic efficiency and structure, MnO2
nanomaterials (nanospheres, nanosheets, nanorods) have been prepared successfully, and were
thoroughly characterized by SEM and TEM. Furthermore, the selective catalytic reduction (SCR)
performance of NOX under ammonia was used as an indicative reaction. Among the MnO2
nanomaterials with different morphologies, it was found that their SCR activities showed an interesting
variation tendency: nanospheres > nanosheets > nanorods of MnO2. The NO conversion ratio of the
MnO2 nanospheres could reach 100% from 200 to 350 ꢀC. Moreover, in order to study the probable
mechanism for the best removal efficiency of the nanospheres, XRD, H2-TPR, NH3-TPD, BET, XPS and in
situ DRIFTS were performed in detail. It is found that surface chemisorbed oxygen, specific surface area,
reducibility and acid sites have great influence on the NO removal efficiency in the SCR reaction. In
addition, how several process parameters affect the NOX removal efficiency was carried out, such as
time, H2O and SO2.
Received 2nd February 2016
Accepted 23rd May 2016
DOI: 10.1039/c6ra03108k
www.rsc.org/advances
doped vanadium content. Chen11 found that novel Fe–Mn
mixed-oxide catalysts showed the highest removal performance
1. Introduction
and 100% selectivity of N2. Among the many transition metal
oxides, manganese oxides have attracted much attention due to
their high catalytic activity, good redox properties, relatively low
toxicities, good sintering resistance and low cost. As we all
know, MnO2 has a redox cycle process between Mn2+ and Mn4+,
providing oxygen mobility in the oxide lattice. However, Mn4+
ions are expected to exhibit higher activity compared to Mn3+
and Mn2+ in catalytic reactions.12,13 Manganese oxide-based
catalysts (MnOX) exhibited a relatively high activity for the
SCR reaction, such as MnOX/CeO2,14,15 MnOX/CuO,16,17 MnOX/
ZrO2–CeO2,18 MnOX/TiO2,19,20 MnO2/Fe2O3,11,21 MnO2/NiCo2O4,22
and so on.
As we all know, SCR degradation of NOX with NH3 can be
affected by many factors, for instance, the oxidation state of the
transition metal element, the shape, the crystallinity and the
specic surface.23 However, few studies have focused on the
corresponding relation between catalytic efficiency and
morphology or dimensions. For example, Bai24 et al. studied
MnO2 catalyst with different dimensions. They found that 3D-
MnO2 obtained the best catalytic properties, which is due to
abundant surface adsorbed oxygen species, better reducibility
and more Mn4+ ions. Zhao23 found that Co3O4 nanorods showed
much higher efficiency in SCR degradation of NOX with NH3
than Co3O4 nanoparticles, which is due to more Co3+ adsorp-
tion on the Co3O4 nanorods. Gao et al.18 studied the
morphology-dependent properties of MnOX/ZrO2–CeO2 nano-
structures for NOX reduction with NH3. They found that the
Emission of nitrogen oxides (NOX) from the industrial
combustion of coal and fossil fuels has been one of the major
causes of environmental pollution. NOX contributes to the
formation of photo-chemical smog and acid rain, and ozone
layer depletion, and have drawn wide concern due to their high
toxicity to human health.1–6 Many approaches have been
developed to remove NOX, among which SCR degradation of
NOX with NH3 has been proven to be an effective technology.
As typical commercial catalysts, vanadium-based catalysts
exhibit higher catalytic activity and selectivity in the SCR
degradation of NOX with NH3. However, some inherent draw-
backs still exist, such as the poor low temperature SCR activity,
the toxicity of VOX, and the narrow operating temperature
window.7–9 Therefore, it is necessary to develop several new
types of catalysts that contain various metal oxides (Mn, Fe, Ni,
Cu, Ce oxides) on different commercial supports. For example,
Liu10 found that the addition of Ce not only enhanced the
activity and alkali resistance of V2O5/TiO2 but also reduced its
aDepartment of Chemistry, Tianjin University, Tianjin 300072, P. R. China. E-mail:
liyi@tju.edu.cn
bCollege of Environmental Science and Engineering, Nankai University, Tianjin
300071, P. R. China. E-mail: sihuizhan@nankai.edu.cn; Fax: +86-22-23502756; Tel:
+86-22-23502756
cDepartment of Chemistry, Capital Normal University, Beijing 100048, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra03108k
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MnOX/ZrO2–CeO2 nanorods achieved signicantly higher NOX measurements of the MnO2 materials were carried out on an X-
removal efficiency than the nanocubes and nanopolyhedra, ray diffractometer (Rigaku D/Max 2200 PC) over a 2q range
which is attributed to more Mn4+ species, adsorbed surface between 10ꢀ and 80ꢀ with the electric current being xed at 40
oxygen and oxygen vacancies.
mA and the voltage being operated at 40 kV, with a graphite
In this paper, pure MnO2 nanocrystals with different monochrometer and Cu Ka radiation (l ¼ 0.15418 nm) at room
morphologies, including bulk particles, nanorods, nanosheets temperature. Temperature-programmed reduction by hydrogen
and nanospheres were synthesized successfully and their cata- (H2-TPR) was conducted on a chemisorption analyzer (Micro-
lytic activities in the NH3-SCR reaction were investigated, indi- meritics Autochem 2920 II instrument) containing a thermal
cating that the MnO2 nanospheres exhibited the highest NO conductivity detector (TCD). The experimental pretreatment
conversion ratio in the NH3-SCR reaction. Furthermore, its was as follows. MnO2 material was outgassed at 100 ꢀC for 1 h in
reaction mechanism was proposed using the analysis of SEM, owing Ar with about 30–40 mg different catalysts. These
ꢀ
TEM, BET, XRD, NH3-TPD, H2-TPR, XPS and in situ DRIFTS.
samples were heated up to 700 C under a 10% H2/Ar gas ow
(50 mL minꢁ1) at a rate of 10 C min aer cooling to room
ꢁ1
ꢀ
temperature. Temperature-programmed desorption by
2. Experimental
2.1. Preparation of materials
ammonia (NH3-TPD) was performed on a chemisorption
analyzer (Quantachrome Autosorb iQ-C-TCD-MS). Approxi-
mately 100 mg ꢀMnO2 samples were saturated with highly puri-
All of the chemicals were analytical grade without further
purication. Manganese(II) chloride tetrahydrate (MnCl2$4H2O)
and urea were purchased from Sigma-Aldrich. Manganese
sulfate (MnSO4) and ammonium persulfate (NH4)2S2O8 were
purchased from Aladdin Industrial Inc. PEG 400, potassium
permanganate (KMnO4) and sodium dodecyl benzene sulfonate
(SDBS) were purchased from Jiangtian Chemical Technology
Co. Ltd. (Tianjin, China).
The preparation methods of the three precursors were as
follows: the b-MnO2 nanorods25 were synthesized by a hydro-
thermal method. In a typical process, 0.2 g KMnO4 and 4 mL
PEG (400) were added into a 100 mL Teon-sealed autoclave.
Then 80 mL deionized water was added into the autoclave and
kept under rapid magnetic agitation at room temperaꢀture for 10
min. The Teon-sealed autoclave was heated to 160 C for 5 h.
The synthesis process of g-MnO2 nanosheets26 was similar to
the above steps. 1.3 g SDBS and 0.4 g MnCl2$4H2O were added
into 45 mL deionized water, then 0.2 g urea was dropped into
this homogeneous solution under stirring. Aer continuous
stirring for 30 min, the mixture was transferred into a Teon-
ꢀ
ed NH3 at 50 C for 1 h and then ushed at 50 C for 1 h to
remove physically adsorbed NH3 aer degassing at 300 ꢀC for 2
h in owing N2. In the end, these samples were heated from 200
ꢀC to 700 ꢀC at a heating rate of 10 ꢀC minꢁ1. The X-ray
photoelectron spectroscopy (XPS) of the three different MnO2
samples was carried out on a spectrometer (Thermal ESCALAB
250, UK) with Al Ka radiation, and the binding energies were
calibrated by using C 1s (C 1s ¼ 284.6 eV). The specic surface
areas and the pore size distributions of the three different
shapes of MnO2 nanocrystals were observed respectively by the
Brunauer–Emmett–Teller (BET) method, a cylindrical pore
model (BJH method) and the adsorption branches. The in situ
diffuse reectance infrared Fourier transform spectroscopy (in
situ DRIFTS) was carried out on a spectrometer (Thermo Nicolet
iS50ꢀFTIR). Prior to each test, the MnO2 catalyst was disposed at
ꢀ
350 C in owing argon for 60 min and then cooled to 150 C.
The sample spectra were recorded using the KBr pellet method
by collecting 32 scans at a resolution of 4 cmꢁ1 in the range of
400–4000 cmꢁ1
.
ꢀ
sealed autoclave and maintained at 110 C for 12 h.
In a similar process, 1.5 g MnSO4 and 23 g (NH4)2S2O8 were
2.3. Catalytic performance tests
prepared to synthesize g-MnO2 nanospheres,27 which were
The catalytic activity tests for the three different MnO2 samples
were conducted in a xed-bed quartz reactor (inner diameter of
10 mm) with 500 mgꢀof samples measuring 40–60 mesh and
ranging from 50–400 C under atmospheric pressure. The SCR
reaction activity measurement by ammonia was as follows: 500
mg catalyst was put in the reactor. The total ow rate of the
mixed gases (500 ppm NH3, 500 ppm NO, 5% H2O (when used),
3% O2, all balanced by nitrogen) was 200 mL minꢁ1, amounting
to a gas hourly space velocity (GHSV) of 2.8 ꢂ 104 hꢁ1. The
concentrations of O2, NO, NOX and NO2 were continually
monitored by an FTIR spectrometer (GASMET FTIR DX4000,
Finland). All the concentrations of the outlet gases were recor-
ꢀ
dissolved in 50 mL deionized water and maintained at 110 C
for 10 h in a Teon-sealed autoclave.
Then the obtained three precursors were ltered and washed
with deionized water and absolute ethanol several times,
respectively. Aer drying at 60 ꢀC in an oven, the resulting
powders were nally calcined at 400 ꢀC for 5 h in air.
As a comparison, the MnO2 bulk nanoparticles were
prepared directly by calcining Mn(NO3)2$4H2O (97%) at 400 ꢀC
for 5 h.
2.2. Characterization
MnO2 nanomaterials with different morphologies were inves- ded when the SCR process reached a steady-state aer half an
tigated by eld-emission scanning electron microscopy hour at each temperature. To further count the activity of the
(FESEM) (JSM-6700F), and the size and morphology of the three different shaped MnO2 samples, the NO conversion was
samples were observed by transmission electron microscopy obtained from the following equation, where NOout is the
(TEM) using a JEOL Model JEM-1200EX instrument with an concentration of the outlet NO and NOin is the concentration of
accelerating voltage of 200 kV. The X-ray diffraction (XRD) the inlet NO.
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ꢀ
ꢁ
½NOꢃ ꢁ ½NOꢃ
in
out
NO conversionð%Þ ¼
ꢂ 100
½NOꢃ
in
NO oxidation to NO2 and N2 selectivity were obtained by
using the following equations, where NOX is the sum of the NO
and NO2 concentrations.
½NO2ꢃ ꢁ ½NO2ꢃ
out
in
NO conversion to NO2 ð%Þ ¼
ꢂ 100
½NOꢃ
in
N2 selectivity ð%Þ ¼
ꢀ
ꢁ
2½N2Oꢃ
out
1 ꢁ
ꢂ 100
½NOxꢃ þ ½NH3ꢃ ꢁ ½NOxꢃ ꢁ ½NH3ꢃ
in
in
out
out
In order to parallelly compare the activities of the three
different MnO2 samples, we performed turnover frequency
(TOF) experiments. The total ow rate of the mixed gases (500
ppm NH3, 500 ppm NO, 3% O2, all balanced by nitrogen) was
3000 mL minꢁ1, which is equivalent to a gas hourly space
velocity (GHSV) of 4.2 ꢂ 105 hꢁ1. By assuming that the reaction
components were free of diffusion limitations, TOF values can
be calculated according to the following equation:
Fig. 1 SEM and HRTEM images of MnO2 nanorods (a, d), MnO2
Â
Ã
FNOXNO
nanosheets (b, e) and MnO2 nanospheres (c, f).
TOF ¼
hꢁ1
22:4 nMn4þ
Fig. 1e. The interplanar distance of the MnO2 nanosheets is 2.25
nm, which corresponds to the (022) crystal plane. In Fig. 1f, the
MnO2 nanospheres are composed of numerous randomly
oriented nanorods, and the interplanar distance is 3.398 nm,
corresponding to the (101) crystal plane.
FNOXNO
Rate ¼
22:4 W
where FNO is the concentration of inlet NO (L minꢁ1), XNO is the
4+
NO conversion, W is the catalyst weight (g), and nMn is the
mole number of Mn4+ ions calculated by H2-TPR.
Fig. 2 shows the XRD patterns of the MnO2 materials with
different morphologies. The diffraction peaks of MnO2 bulk
3. Results and discussion
3.1. Structural properties
The SEM images of the obtained MnO2 nanostructures are
shown in Fig. 1. A large number of MnO2 nanorods are pre-
sented in Fig. 1a, where the nanorods are shown to have
a smooth surface and typical rod-like shape. These high-purity
MnO2 nanorods are about 100 nm in diameter and their
lengths are in the range of 2–10 mm. Meanwhile, from the SEM
image in Fig. 1b, the product is presented as a uniform hexag-
onal nanosheet with a side length of 1.2 mm and a thickness of
approximately 200 nm. The surfaces of these uniform hexagonal
nanosheets are smooth. Fig. 1c shows urchin-like nanospheres
which are composed of numerous randomly oriented nanorods,
and the diameters are about 4 mm. These nanorods are 400–800
nm in length and 100–150 nm in width.
The detailed morphology structures of the MnO2 nanorods,
nanosheets and nanospheres are further analyzed by HRTEM in
Fig. 1. As shown in Fig. 1d, it can be seen that the MnO2
nanorods are well crystallized and the interplanar distances are
3.098 nm and 2.35 nm, corresponding to the (110) and (101)
Fig.
2 XRD patterns of the MnO2 nanomaterials with different
morphologies, such as nanorods, nanosheets, nanospheres and bulk
crystal planes, respectively. A pretty morphology is displayed in particles.
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nanoparticles can be well identied as b-MnO2 (JCPDS 24-
0735).28 The MnO2 nanorods can be determined as tetragonal-
phase b-MnO2 (JCPDS 24-0735), which are produced by the
pyrolysis of MnOOH nanorods, as illustrated in Fig. 2. The
˚
˚
lattice constants are calculated, and a is 4.38 A and c is 2.88 A,
˚
which are very consistent with the reported values (a ¼ 4.40 A
29
˚
and c ¼ 2.87 A). The intensity ratios of (110) and the other
reections are different, a result of the preferential growth of
the materials. The mechanism has been proved using many
one-dimensional nanocrystals.30 The MnO2 hexagonal nano-
sheets could be identied as g-MnO2 (JCPDS 30-0820), which is
from the calcination of MnCO3 nanosheets and can be easily
˚
˚
indexed to the lattice constants (a is 2.80 A, b is 2.80 A, c is 4.45
˚
A). The XRD patterns of MnO2 nanospheres show that the
calcined products were g-MnO2 (JCPDS 30-0820). From the full
width at half maximum and intensity of the peaks, the diffrac-
tion peaks of the MnO2 nanospheres are higher than that of the
MnO2 nanosheets, suggesting that the MnO2 nanospheres ob-
tained high crystallinity.
The N2 adsorption–desorption isotherms and the pore size
distribution, and the specic surface area and pore volume of
the MnO2 bulk nanoparticles, MnO2 nanorods, MnO2 nano-
sheets and MnO2 nanospheres are shown in Fig. 3 and Table 1,
respectively. In Fig. 3a, it can be seen that the MnO2 nano-
spheres and nanosheets all have a typical type IV isotherm with
type-H3 hysteresis loops according to the denition of IUPAC.
The hysteresis loops indicate the presence of mesopores.31,32
However, both the MnO2 bulk nanoparticles and MnO2 nano-
rods did not show the typical IV-type isotherm, indicating that
the MnO2 bulk nanoparticles and MnO2 nanorods have no
mesoporous channels. It can be further observed over the N2
adsorption–desorption isotherms that the turning point of the
nitrogen-adsorbed volume for MnO2 nanorods and nano-
spheres is 0.9, and the N2 adsorbed volume further increases
sharply under higher relative pressures, which is ascribed to the
large mesopores (>10 nm) in the samples.33 As illustrated in
Fig. 3b, the MnO2 nanospheres show a wide peak at 40–60 nm,
which might correspond to the staggered arrangement of
numerous randomly oriented MnO2 nanorods. The conse-
quence is consistent with the results of SEM and TEM analysis.
The MnO2 nanosheets exhibit a sharp peak at around 8 nm,
which might result from the packing of the nanosheets. The
pore distribution of MnO2 bulk nanoparticles and MnO2
nanorods shows that a few large mesopores exist. As shown in
Table 1, the specic areas of the MnO2 bulk nanoparticles and
MnO2 nanorods are lower than the MnO2 nanosheets and
nanospheres, which is in good conformity with the results of
NO removal efficiency in the SCR reaction. Furthermore, it can
be seen that the specic surface area of the MnO2 nanosheets is
almost equal to the nanospheres at about 16 m2 gꢁ1. However,
the pore volume of MnO2 nanospheres (0.14 cm3 gꢁ1) is
Fig. 3 N2 adsorption–desorption isotherms (a) and pore diameter
distribution (b) of the MnO2 nanomaterials with different
morphologies.
nanospheres.34 It is reported that the larger specic surface area
and porous structures can offer more active sites and increase
the adsorption of reactants in the SCR reaction.35,36 The specic
surface area and pore volume of the MnO2 nanospheres are
higher than the MnO2 nanosheets and nanorods, which is
benecial for the exposure of more active sites and adsorption
of the reactants. From what has been discussed above, we may
draw the conclusion that the MnO2 nanospheres obtained the
Table 1 Specific area, pore volume and pore diameter distribution of
the MnO2 samples
Specic area Pore volume Pore diameter
signicantly higher than that of the nanosheets (0.07 cm3 gꢁ1), Materials
(m2 gꢁ1
)
(cm3 gꢁ1
)
(nm)
which is due to them being composed of numerous randomly
MnO2 bulk
oriented nanorods of urchin-like MnO2 nanospheres.24 The
nanoparticles
5.6
0.01
3.83
similar specic surface area and large differences in the total
pore volume of the MnO2 nanosheets and nanospheres are due
to the pore diameters of the nanosheets being less than for the
MnO2 nanorods
MnO2 nanosheets
MnO2 nanospheres
9.6
16.0
15.7
0.02
0.07
0.14
24.78
12.58
68.74
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highest catalytic activity in the SCR reaction and it might be due
to the large specic surface area and pore volume.37 This is
consistent with the above discussion.
The chemical states of the elements in the near-surface
region were investigated by XPS. The survey spectra of three
different samples (Fig. 4a) show the existence of Mn and O, with
C from the reference. Moreover, the atom molar rate of Mn and
O is about 1 : 2 in the survey spectra, demonstrating the
formation of MnO2. As illustrated in Fig. 4b, two XPS binding
energies of Mn 2p1/2 and Mn 2p3/2 at 654.1 eV and 642.4 eV were
observed for MnOX. It can be seen that the binding energy
separation is 11.7 eV between the Mn 2p1/2 and Mn 2p3/2
states.38 As shown in Fig. 4b, Mn 2p1/2 can be divided into two
characteristic peaks which can be ascribed to Mn4+ and Mn3+
ions displayed with binding energies of 654.8 eV and 654.1 eV,
respectively. Moreover, Mn 2p3/2 can be divided into two char-
acteristic peaks which can be ascribed to Mn4+ and Mn3+ ions
displayed with binding energies of 642.9 eV and 641.8 eV,
respectively. The relative atomic percentages of Mn3+ and Mn4+
over the surface of the MnO2 catalysts are shown in Table 2,
respectively. We nd that the MnO2 nanospheres display the
highest Mn4+/Mnn+ molar ratios. As we all know, higher oxida-
tion state manganese species are more active for the SCR reac-
tion over manganese-based catalysts. It is well known that Mn4+
ions play a major role in low-temperature SCR reactions, which
could promote the oxidation of nitric oxide to nitrogen
dioxide.24,39 Therefore a high Mn4+/Mnn+ ratio will be conducive
to the low-temperature SCR reaction. The Mn4+ content in the
MnO2 nanospheres (55.6%) was much higher than that in the
MnO2 nanosheets (49.1%) and nanorods (37.2%), which is
consistent with the results of SCR activity.
The O 1s XPS spectra of the three samples in Fig. 4c were
deconvoluted using a curve-tting procedure, and contain two
kinds of distinguished surface oxygen species. The lower
binding energy peak at 529.0–530.0 eV might be assigned to the
surface lattice oxygen (Ob), such as defective oxides or surface
oxygen ions bonded to manganese in a coordinatively saturated
environment, and the higher binding energy peak at 531.0–
532.0 eV could belong to the surface chemisorbed oxygen (Oa).39
Generally, the surface chemisorbed oxygen (Oa) is more reactive
than the surface lattice oxygen (Ob), which is due to the higher
mobility of Oa. Hence a high Oa/(Oa + Ob) ratio will be conducive
to the SCR reaction.40 The Oa ratio between the MnO2 nano-
spheres (43.72%) and nanosheets (47.9%) has no signicant
difference. The Oa rate of the MnO2 nanospheres and nano-
sheets were much higher than that of the MnO2 nanorods
(36.73%), which corresponds to the lowest NO conversion effi-
ciency of the MnO2 nanorods in the SCR reaction.
To investigate the reducibility of the MnO2 samples with
different morphologies in the SCR reaction, including bulk
nanoparticles, nanorods, nanosheets and nanospheres, the H2-
TPR spectra of the MnO2 catalysts are shown in Fig. 5a. It can be
seen that the three reduction peaks of the MnO2 bulk nano-
particles ranging from 350 to 600 ꢀC are attributed to the
Fig. 4 XPS spectra of the MnO2 nanorods, MnO2 nanosheets and
MnO2 nanospheres.
reduction of MnO2 to Mn2O3, Mn2O3 to Mn3O4 and Mn3O4 to overlapping strong reduction peaks. The rst reduction peak
MnO, respectively.34,41 Moreover, the TPR spectra of the MnO2 may be the conversion of MnO2 to Mn2O3, and the second
nanorods, nanosheets and nanospheres all show two reduction peak may be attributed to the reduction of Mn2O3 to
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Table 2 XPS results of the MnO2 samples
catalysts, which might imply that the MnO2 nanospheres have
a wider operating temperature window. All the features corre-
spond to the SCR activity results. The reduction peaks 1 and 2 of
the MnO2 nanorods correspond to H2 consumptions of 8.35 and
3.58 mmol gꢁ1, respectively. The reduction peaks 1 and 2 of the
MnO2 nanosheets correspond to H2 consumptions of 6.01 and
4.55 mmol gꢁ1, respectively. Meanwhile, we nd that the
reduction peaks 1 and 2 of MnO2 nanospheres correspond to H2
consumptions of 7.22 and 4.01 mmol gꢁ1 and are located at
277 ꢀC and 422 ꢀC, respectively. From the above we can come to
the conclusion that the total hydrogen consumption of the
samples approaches the theoretical hydrogen consumption
(11.49 mmol gꢁ1). The MnO2 samples show the rst reduction
step of MnO2 to Mn2O3 (peak 1), which corresponds to the
reduction of Mn4+ to Mn3+.42 Therefore, we can calculate the
Mn4+ total active sites of the MnO2 nanospheres, nanosheets
and nanorods to be 0.835, 0.601 and 0.702 mmol, respectively.
The H2 consumption of the reduction peaks of the MnO2
nanosheets are less than that of the MnO2 nanospheres, and the
MnO2 nanospheres obtained a higher NO conversion efficiency
than the MnO2 nanosheets (Table 3).
Surface Atomic Concentration (%)
2p
MnO2 materials Mn4+/Mnn+ Mn3+/Mnn+ Oa (531.2) Ob (529.5)
Nanorods
Nanosheets
Nanospheres
37.2
49.2
55.6
62.8
50.8
44.4
36.73
47.90
43.72
63.27
52.10
56.28
Temperature-programmed desorption by ammonia (NH3-
TPD) experiments were performed in order to investigate the
surface acidity of the catalyst. As we all know, ammonia storage
capacity plays an important role in the SCR reaction system.
Thus, it is necessary to research the adsorption–desorption
behavior of the catalyst by NH3.43 Based on analysis in previous
+
reports, the thermal stability of the NH4 associated with the
Brønsted acid sites was lower than the NH3 associated with the
Lewis acid sites. The low-temperature desorption peak below
+
ꢀ
350 C corresponds to the NH4 desorbed from Brønsted acid
ꢀ
sites and the high-temperature desorption peak above 400 C
corresponds to NH3 desorbed from Lewis acid sites.44 As shown
in Fig. 5b, two obvious desorption peaks were observed for the
MnO2 nanospheres. The lower temperature desorption peak of
+
the MnO2 nanospheres at 310 ꢀC is associated with NH4
adsorbed on the Brønsted acid sites and the higher temperature
ꢀ
desorption peak at 550 C is associated with NH3 adsorbed on
the Lewis acid sites. Compared with the MnO2 nanospheres, no
Brønsted acid sites were seen for the MnO2 bulk nanoparticles,
MnO2 nanorods and MnO2 nanosheets. Moreover, the intensity
of the higher temperature desorption peak of the MnO2 nano-
spheres is also higher than the three other samples. As we all
know, Lewis acid sites exert a major inuence in SCR systems,
which may be a very signicant reason for the SCR reaction
Fig. 5 H2-TPR (a) and NH3-TPD (b) patterns of the MnO2 bulk nano-
particles, MnO2 nanorods, MnO2 nanosheets and MnO2 nanospheres.
Table 3 H2-TPR results of the MnO2 samples
MnO. These features correspond to the reduction of MnO2 with
low crystallinity.41 However, some differences in the four spectra
can be seen. The rst reduction peak of MnO2 nanospheres
appeared at lower temperature than the two other MnO2 cata-
lysts, which might imply that the MnO2 nanospheres have
better low temperature reducibility than the two other MnO2
catalysts. Furthermore, the reduction peak region of the MnO2
nanospheres is also wider than those of the two other MnO2
H2
Temperature
(ꢀC)
consumption
(mmol gꢁ1
)
Mn4+ total
Samples
Peak 1 Peak 2 Peak 1 Peak 2 active sites (mmol)
Nanorods
Nanosheets
299
309
409
412
422
8.05
6.01
7.22
3.58
4.55
4.01
0.805
0.601
0.722
Nanospheres 277
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activity of the MnO2 nanospheres being higher than those of the the plot. It can be seen that the activation energy of the MnO2
MnO2 bulk nanoparticles, MnO2 nanorods and MnO2 nano- nanospheres (20.5 kJ molꢁ1) is lower than the MnO2 nanosheets
sheets. All the features correspond to the SCR activity results.
(30.7 kJ molꢁ1) and nanorods (44.5 kJ molꢁ1), indicating that
As we all know, Mn4+ ions are most likely the active sites of surface adsorbed oxygen species are easily activated and favour
NO oxidation to NO2 because the oxidation process oen occurs enhanced SCR performance, which also corresponds to the SCR
from redox cycling of high and low valence state cations.45,46 results.
Therefore, the TOF was dened as the number of NOX converted
per Mn4+ ion. The catalytic activity can be seen from TOF
analysis (Fig. 6a), and different TOF values of the three samples
3.2. NH3-SCR activity
at six different temperature points was obtained, indicating that Stability tests and H2O resistance studies of the three different
the TOF values increase with increasing temperature. Moreover, catalysts in the SCR system were performed, and the results are
the TOF value of the MnO2 nanospheres is 0.07 hꢁ1 at 100 C, shown in Fig. 7. Fig. 7a showed the SCR by ammonia perfor-
ꢀ
and 0.15 hꢁ1 at 140 ꢀC. Furthermore, it can be seen that the TOF mance of the MnO2 with different structures and the traditional
values varied with the different morphology. The MnO2 nano- V2O5–WO3/TiO2 catalyst. It is seen that the NO conversion of all
spheres obtained the highest TOF values from 100 to 200 ꢀC of the catalysts increases at rst and nally decreases as the
over the MnO2 nanosheets and MnO2 nanorods. Therefore, it temperature is elevated. However, some inherent differences in
can be concluded that abundant Mn4+ ions in the MnO2 the catalyst performance are discovered. It can be seen that the
nanospheres promoted their catalytic performance in the SCR MnO2 nanospheres showed about 100% NOX conversion at 20ꢀ0–
reaction.45,46 The Arrhenius plots of NO oxidation of the three 300 C and obtained over 90% NOX conversion at 150–350 C
ꢀ
different samples are shown in Fig. 6b. Furthermore, the reac- with a wide operating temperature window. The MnO2 nano-
tion activation energy can be calculated based on the slope of sheets obtained an almost 100% conversion efficiency only at
200 ꢀC, with a narrow operating temperature window. The
MnO2 nanorods were also observed to show very low activity and
a best NO conversion of only 80% at 300 ꢀC in the investigated
temperature window in the SCR by NH3. In contrast, the MnO2
bulk nanoparticles obtained the lowest activity in the investi-
gated temperature window, and the traditional V2O5–WO3/TiO2
catalyst displays poor low temperature SCR activity. Comparing
all of the above catalysts, MnO2 nanosphere catalysts display
a wider operating temperature window and the highest
conversion efficiency over the other catalysts. As shown in
Fig. 7b, the NO conversion of the three different catalysts
showed little change during a 72 h continuous running dura-
tion, indicating that all these catalysts have excellent stability.
Compared to the samples before the SCR reaction, there is
almost no change of the morphology, surface area and crystal-
linity of the reacted samples, which was proved by TEM, SEM,
XRD and BET (Fig. S1–S3†). Therefore, aer the reaction the
three MnO2 materials still maintain the original structure,
which proves that the three MnO2 morphologies are still stable
aer the SCR reaction.
Fig. 7c indicates the effects of H2O on NH3-SCR activity. Aer
adding water, it can be seen that the NO conversion efficiency of
the MnO2 nanospheres decreased to 93% and then remained at
97% for the next 18 h. The NO conversion efficiency returned to
100% quickly aer the removal of H2O. Moreover, the NO
conversion efficiency of the MnO2 nanorods and nanosheets
decreased quickly aer the introduction of H2O. However the
NO conversion efficiency returned quickly aer cutting off the
H2O. All these features indicate the recoverable activity of the
samples in the NH3-SCR system. Hence, it can be trusted that
the inhibition effect of H2O was possibly attributed to the
competitive adsorption of H2O and NH3 onto the catalyst
surface.47 Meanwhile, Fig. 7d shows the effects of SO2 on the
NH3-SCR activity. The NO conversion of the MnO2 nanospheres
decreased to 87% with the introduction of SO2 and was main-
Fig. 6 TOF values (a) and Arrhenius plots of the NO oxidation rates (b)
of the MnO2 nanorods, MnO2 nanosheets and MnO2 nanospheres.
tained at 87%. Aer removing the SO2 gas, the NO conversion
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returned to 95%, suggesting that the MnO2 nanospheres have
a good SO2 resistance in the NH3-SCR reaction. The NO
conversion of the MnO2 nanorods and nanosheets decreased
rapidly aer the introduction of SO2. On removing the SO2 gas,
the NO conversion of the MnO2 nanosheet does not fully
recover, suggesting that manganese sulfate formed on the
catalyst surface.48 Compared with the MnO2 nanorods and
nanosheets, the MnO2 nanospheres obtained the highest SO2
resistance in the NH3-SCR reaction. Meanwhile, the co-effect of
SO2 and H2O was also studied. When SO2 and H2O were intro-
duced into the feed gases at the same time, the NOX conversion
ratios of the three MnO2 samples all decreased, but the NOX
conversion of the MnO2 nanospheres was still 70%. The above
results suggest that the MnO2 nanospheres had good SO2/H2O
durability. Additionally, N2 selectivity tests are shown in
Fig. S5.† As can be seen, the N2 selectivity decreases and N2O
concentration increases for all the catalyst with the elevating
temperature. Moreover, the MnO2 samples can obtain higher N2
selectivity at low temperature (<200 ꢀC). However, the N2
selectivity became lower in the high temperature SCR reaction.
It is now widely accepted that the enhancement of NO to NO2
over catalysts in the SCR reaction could signicantly promote
the catalytic activity, and can be attributed to the faster reaction:
NO + NO2 + 2NH3 / 3H2O + 2N2.49,50 Therefore, the experiments
of NO to NO2 in the absence of NH3 on the catalyst are studied.
As shown in Fig. 8, we can see that the oxidation of NO to NO2
increases at rst and nally decreases with the elevating
temperature. Moreover, the oxidation of NO to NO2 reaches the
ꢀ
maximum conversion at 350 C, and then decreases above 350
ꢀC, which is attributed to the thermal equilibrium composi-
tions. Meanwhile, the MnO2 nanospheres obtained a higher
activity of oxidation of NO to NO2 than the MnO2 nanosheets
and nanorods. The formation of NH4NO2 from NO2 and NH3 in
the SCR reaction has been reported.50,51 Therefore, a high NO2
concentration, or a high activity of oxidation of NO to NO2,
enhance the SCR activity.
Fig. 7 NO conversion (a), stability test (b), H2O resistance (c), effect of
SO2 (d) and co-effect of SO2 and H2O (e) on NO conversion for the
MnO2 nanorods, nanosheets and nanospheres at the optimum
temperature. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2,
5% H2O (when used), 5% SO2 (when used) and N2 balance gas.
Fig. 8 Oxidation of NO to NO2 over the MnO2 samples at different
temperatures.
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3.3. In situ DRIFTS
The acid site distribution and the gas molecule adsorption
behavior on the surface were studied by in situ DRIFTS experi-
ments. At rst the catalysts were purged with N2 for 1 h and
cooled down to 50 ꢀC. NH3 was then introduced into the IR cell
and spectra were recorded as a function of temperature. The in
situ FTIR spectra of adsorbed NH3 on the three different shaped
MnO2 nanomaterials at different temperatures are shown in
Fig. 9. With the increase of temperature, the intensities of all
the bands decreased. Several bands at 1387, 1300, 1143, 1106
and 993 cmꢁ1 were observed on the MnO2 nanorods catalyst in
Fig. 9a. The bands at 1300, 1143 and 1106 cmꢁ1 could be
ascribed to coordinated NH3 Lewis acid sites.52–54 The weak
band at 1387 cmꢁ1 may be due to the oxidization species of
adsorbed ammonia species.35,55 The band at 933 cmꢁ1 was
assigned to gas or weakly adsorbed NH3.35 As shown in Fig. 9b,
the NH3 adsorption peaks of the MnO2 nanosheets are at 1450,
1141, 1110 and 1082 cmꢁ1, of which the weak peak at 1450 cmꢁ1
+
can be assigned to the NH4 bound to the Brønsted acid sites,
and the adsorption peaks at 1141, 1110 and 1082 cmꢁ1 can be
ascribed to the NH3 bound to the Lewis acid sites.56,57 Several
bands at 1642, 1395, 1137 and 1113 cmꢁ1 can be found for the
MnO2 nanospheres in Fig. 9c. The adsorption peaks at 1642 and
+
1395 cmꢁ1 were ascribed to the NH4 bound to the Brønsted
acid sites. Two bands at 1113 and 1137 cmꢁ1 were also found,
which could be ascribed to coordinated NH3 Lewis acid sites. It
is evident that the band intensity of Lewis acid sites over the
MnO2 nanospheres is higher than that over the MnO2 nanorods
and nanosheets. All the characteristics corresponded to the
above analysis in NH3-TPD.
The in situ DRIFTS of NO + O2 adsorption over the three
different shaped MnO2 catalysts at different temperatures are
shown in Fig. 10. First of all the catalysts were purged with N2 for
1 h and cooled down to 50 ꢀC. NO + O2 was then introduced into
the IR cell, and spectra were recorded as a function of tempera-
ture. As shown in Fig. 10a, the band at 1379 cmꢁ1 could be
ascribed to the adsorbed NO3ꢁ, which is due to the dispropor-
tionation of chemisorbed NO2 on the sample surface.58 The
bands at 1559, 1312 and 1209 cmꢁ1 be assigned to bridged
nitrate, bidentate nitrates and bridging nitrites, respectively.57 As
shown in Fig. 10b, three adsorption bands were observed over the
MnO2 nanosheets catalyst. Besides the bands assigned to mon-
2ꢁ
odentate nitrate (1499 cmꢁ1), trans-N2O2 (1461 cmꢁ1) and M–
NO2 nitro compounds (1375 cmꢁ1) were also observed.53–55,59–61
Several bands at 1568, 1541, 1499, 1385, and 1215 cmꢁ1 were
observed on the MnO2 nanospheres catalyst shown in Fig. 10c.
The bands at 1541 cmꢁ1 and 1499 cmꢁ1 could be attributed to the
monodentate nitrate species and the bands at 1215 cmꢁ1 could
be assigned to the disproportionation of NO bridging
nitrates.54,55,62 The band at 1568 cmꢁ1 is in accordance with the
literature and can be assigned to the bidentate nitrate.52,63,64 The
band at 1385 cmꢁ1 is close to the value in the previous study and
could be attributed to monodentate nitrite.57 Remarkably, the
bands of the MnO2 nanospheres are much more than those of
the MnO2 nanorods and nanosheets. The above in situ DRIFTS
analyses show that the larger pore volume and hierarchical
Fig. 9 In situ DRIFTS of (a) NH3 adsorption on the MnO2 nanorods; (b)
NH3 adsorption on the MnO2 nanosheets; (c) NH3 adsorption on the
MnO2 nanospheres.
porous structure have enhanced the chemisorption and activa-
tion of reactant molecules on the MnO2 nanospheres, resulting
in the best NO conversion efficiency in NH3-SCR.
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g-MnO2 nanosheets obtained signicantly higher NO conver-
sions than the b-MnO2 nanorods and b-MnO2 bulk nano-
particles. As described in the above sections, the XPS results
show that g-MnO2 has more surface chemisorbed oxygen than
b-MnO2. According to the literature,24,38 the surface chem-
isorbed oxygen on g-MnO2 participates easier in the reaction
than b-MnO2, because the surface chemisorbed oxygen on
g-MnO2 is more active. All the above analysis implies that
surface chemisorbed oxygen is the main factor in promoting the
activation of NO and NH3. Based on the above analysis and
discussion of the XPS measurements, the MnO2 nanorods have
a relatively high Mn4+ ion content (61.59%) but the lowest
surface chemisorbed oxygen (36.73%) on the surface, and the
MnO2 nanosheets have abundant surface chemisorbed oxygen
(47.9%) but the lowest Mn4+ ion content (35.18%) on the
surface. Generally, the MnO2 nanospheres have the highest
Mn4+ ion content (78.35%) and higher surface chemisorbed
oxygen (43.72%). As we all know, Mn4+ ions and surface
chemisorbed oxygen play important roles in the low-
temperature SCR reaction. Therefore a high Mn4+/Mnn+ ratio
and Oa/(Oa + Ob) ratio will be conducive to the low-temperature
SCR reaction. As shown in Fig. 11, the rst oxidation process of
the MnO2 nanocatalysts is NO to NO2, which is consistent with
the reduction of Mn4+ to Mn2+. Furthermore, the TPD and in situ
DRIFTS results show that the MnO2 nanospheres have more
acid sites on the outside surface than the MnO2 nanosheets and
MnO2 nanorods. Meanwhile, the MnO2 nanospheres show
a higher catalytic effect than the MnO2 nanosheets. The MnO2
nanospheres have more exposed Mn4+ and acid sites due to
their three dimensional structure. However, the MnO2 nano-
sheets with a two dimensional structure are easy to pile up into
pieces, which may result in fewer Mn4+ and acid sites on the
outside surface. All the features correspond to the above results
of in situ DRIFTS, TPD and XPS. Moreover, the MnO2 nano-
spheres obtained higher TOF values from 100 to 200 ꢀC than
both the MnO2 nanosheets and MnO2 nanorods. Therefore, it
can be concluded that the abundant Mn4+ ions in the MnO2
nanospheres promote the catalytic performance in the SCR
reaction.45,46 Meanwhile, it can be found that activation energy
of the MnO2 nanospheres (20.5 kJ molꢁ1) is lower than the
MnO2 nanosheets (30.7 kJ molꢁ1) and nanorods (44.5 kJ molꢁ1),
indicating that surface adsorbed oxygen species are easily acti-
vated and in favour of SCR performance. This result is
Fig. 10 In situ DRIFTS of NO + O2 adsorption on the MnO2 nanorods
(a), NO + O2 adsorption on the MnO2 nanosheets (b); NO + O2
adsorption on the MnO2 nanospheres (c).
3.4. Reaction mechanism analysis
Herein, g-MnO2 nanospheres, g-MnO2 nanosheets, and b-MnO2
nanorods were found to display different catalytic effects.
Furthermore, it is found that the g-MnO2 nanospheres and
Fig. 11 Reaction mechanism of NH3-SCR on MnO2 with different
morphologies.
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Acknowledgements
This work was supported by the National Natural Science 23 B. Meng, Z. B. Zhao, X. Z. Wang, J. J. Liang and J. S. Qiu, Appl.
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University, Ministry of Education) (201401), and by the Natural 25 Z. C. Bai, B. Sun, N. Fan, Z. C. Ju, M. H. Li, L. Q Xu and
Science Foundation of Tianjin (Grant No. 15JCYBJC48400 and
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