Direct Molten Polymerization Synthesis of Highly Active Samarium Manganese Perovskites with Different Morphologies for VOC Removal

Article abstract of DOI:10.1021/acs.inorgchem.8b01125

A morphology-controlled molten polymerization route was developed to synthesize SmMnO₃ (SMO) perovskite catalysts with netlike (SMO-N), granular-like (SMO-G), and bulk (SMO-B) structures. The SMO perovskites were formed directly by a molten polymerization method, and their morphologies were controlled by using the derivative polymers as templates. Among all catalysts, the porous SMO-N exhibited the highest activity, over which the toluene, benzene, and o-xylene were completely oxidized to CO₂ at 240, 270, and 300 °C, respectively, which was comparable to that of typical noble-metal catalysts. The apparent activation energies of toluene over SMO-N (56.4 kJ·mol^-1) was much lower than that of SMO-G (70.8 kJ·mol^-1) and SMO-B (90.1 kJ·mol^-1). Based on the results of scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and H₂ temperature-programmed reduction characterization, we deduce that the excellent removal efficiency of volatile organic compounds (VOCs) over SMO-N catalyst was attributable to the special structure, high surface Mn⁴⁺/Mn³⁺ and O_latt/O_ads molar ratios, and strong reducibility. Due to the high activity, low cost, and simple preparation strategy, the SMO catalyst is a promising catalyst for VOC removal.

Full text of DOI:10.1021/acs.inorgchem.8b01125

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Direct Molten Polymerization Synthesis of Highly Active Samarium
Manganese Perovskites with Different Morphologies for VOC
Removal
Lizhong Liu, Hongbo Zhang, Jinping Jia, Tonghua Sun,* and Mengmeng Sun
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P.R.
China
S
* Supporting Information
ABSTRACT: A morphology-controlled molten polymerization route was
developed to synthesize SmMnO3 (SMO) perovskite catalysts with netlike
(SMO-N), granular-like (SMO-G), and bulk (SMO-B) structures. The SMO
perovskites were formed directly by a molten polymerization method, and
their morphologies were controlled by using the derivative polymers as
templates. Among all catalysts, the porous SMO-N exhibited the highest
activity, over which the toluene, benzene, and o-xylene were completely
oxidized to CO2 at 240, 270, and 300 °C, respectively, which was comparable
to that of typical noble-metal catalysts. The apparent activation energies of
toluene over SMO-N (56.4 kJ·mol−1) was much lower than that of SMO-G
(70.8 kJ·mol−1) and SMO-B (90.1 kJ·mol−1). Based on the results of scanning
electron microscopy, transmission electron microscopy, X-ray photoelectron
spectroscopy, and H2 temperature-programmed reduction characterization, we
deduce that the excellent removal efficiency of volatile organic compounds (VOCs) over SMO-N catalyst was attributable to the
special structure, high surface Mn4+/Mn3+ and Olatt/Oads molar ratios, and strong reducibility. Due to the high activity, low cost,
and simple preparation strategy, the SMO catalyst is a promising catalyst for VOC removal.
poly(methyl methacrylate) microspheres as templates and
INTRODUCTION
■
applied them to the study of catalytic oxidation of toluene or
Volatile organic compounds (VOCs), as the byproducts of the
chemical and manufacturing industries, are endangering the
atmosphere and human health. Many countries and organ-
izations have constituted stringent legislations and regulations
to curb the emission of VOCs.1−3 Among the present
technologies for removal of VOCs, catalytic oxidation is the
most promising approach as it could convert VOCs into CO2,
H2O, and other less hazardous compounds at low temper-
ature.4−7 Until now, researchers have developed a large
number of catalysts, such as noble-metal materials8,9 and
single10,11 or hybrid transition metal oxides12,13 for catalytic
oxidation of VOCs. Although noble-metal catalysts exhibit
great catalytic properties at low temperature, there are many
problems, including low thermal stability, high cost, easy
sintering, and susceptibility to poisoning, limiting their further
applications. By contrast, transition metal oxides have been
considered as the candidate catalysts for VOC oxidation due to
their low cost and great thermal stability.12−15 Therefore, it is
very important to develop advanced transition metal oxide
catalysts with excellent performance for catalytic oxidation of
VOCs.
23
́
́
methanol. Suarez-Vazquez et al. reported the synthesis of
SrTi1−xBxO3 (B = Cu and Mn) perovskites with dendritic
morphology via a hydrothermal route, presenting its catalytic
activity with complete conversion of toluene to CO2 at 350 °C.
However, those methods either are complex or require harsh
conditions. It is known that large quantities of catalysts are
required to cast honeycomb or catalytic filter modules for the
construction of industrial catalysts.24 This means that it is
necessary to develop simple fabrication processes for scale-up
production of catalysts with high catalytic activity.
The textural properties of catalysts, such as the crystal plane,
crystallinity, and morphology, is related to their catalytic
behavior.25 The morphology control has been a novel
approach for adjusting the catalytic performance of perovskites,
and the synthesis strategy is a key factor to control their
morphologies.26,27 To tackle the aforementioned challenges,
we herein present an industrially scalable method to prepare
SmMnO3 (SMO) perovskites with netlike, granular-like, and
bulk structures. During the process, the SmMnO3 perovskite
was synthesized by a molten polymerization strategy, and the
morphology control of SMO perovskites was realized by citric
acid polymerization, wherein the molten metal salts served as a
As a potential substitute of noble-metal catalyst and one of
the common transition metal oxides, the ABO3 perovskite-type
oxide has received extensive attention in the catalytic oxidation
of VOCs.16−20 Si et al.21 and Liu et al.22 prepared a three-
dimensional structure of 3D LaMnO3 perovskites by using
Received: April 24, 2018
© XXXX American Chemical Society
A
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solvent, and the polymerized derivatives were used as
templates to control their morphologies. The preparation is
not only simple and efficient in enhancing the nature of the
catalysts but also effective in advancing catalytic activity on
VOC (toluene, o-xylene, and benzene) oxidation. Utilizing
various catalyst characterizations, the relationship between
physicochemical performance and catalytic behavior as well as
mechanisms of VOC oxidation was also explored. More
importantly, the catalytic activity of SMO-N is also much
1 9
h i g h e r t h a n t h a t o f 3 D - L a M n O 3
a n d
28
La0.6Sr0.4Fe0.8Bi0.2O3−δ
.
EXPERIMENTAL SECTION
■
Figure 1. XRD patterns of SMO-N, SMO-G, and SMO-B.
Sample Preparation. The SmMnO3 perovskites with different
morphologies were synthesized via a one-step molten polymerization
strategy. The procedure of SMO-N was as follows: 1.23 g of
manganese acetate tetrahydrate, 2.22 g of samarium(III) nitrate
hexahydrate, and 3.15 g of citric acid (CA) were mixed. Next, the
mixture was heated to 750 °C at a heating rate of 2 °C·min−1 and then
held for 2 h in a electrical resistance furnace (YFX7/120-60) under air
atmosphere. The amount of CA was adjusted to 2.10 and 1.05 g to
obtain SMO-G and SMO-B, repectively, and no CA was added to
prepare the sample designated NCA.
structures. Compared with those of SMO, NCA had some
noticeable impurities (e.g., Sm2O3 and MnOx), which could be
confirmed by the XRD result of NCA (Figure S1). It is inferred
that when there is no CA, the distribution of the metal salts
was nonuniform, which restricted the precursors from
contacting each other adequately.
The morphologies of the samples could be clearly revealed
by scanning electron microscopy (SEM) and and transmission
electron microscopy (TEM) (Figure 2a−h). As shown in
Figure 2a,e, the SMO-B sample exhibited a compact surface,
suggesting that SMO-B had experienced serious agglomeration,
which could cause low surface area and small amounts of active
sites.21 In contrast, it could be evidently observed from the
SEM image in Figure 2b,c that the obtained SMO-N sample
exhibited a three-dimensional interconnected netlike structure,
and its arrangement became relatively orderly. Meanwhile, the
SMO-N sample manifested small sizes in comparison with
those of SMO-B, indicating that the agglomeration of the
sample was reduced to some extent. Moreover, it could also be
seen that a large number of nanopores and cavities with
different diameters were present on the SMO-N sample
(Figure 2f,g), which might be attributed to the release of gas
from the decomposition of carbon materials. The special
structure may be advantageous for the toluene molecule to
approach the catalyst interior and enhance the area of toluene
contact with the catalyst, thus favoring the occurrence of a
catalytic reaction. Moreover, the SMO-G sample, as shown in
the SEM and TEM images (Figure 2d,h), displayed disordered
granular-like nanoparticles, revealing that the precursors of the
catalyst were overdispersed. Those results showed that by
adjusting the polymerization of the CA monomer, the
morphology of the catalyst can be effectively modified.
A simultaneous differential scanning calorimetry/thermog-
ravimetric analysis (DSC-TGA) trace of the mixed system is
provided in Figure 3a,b. The curve in Figure 3a shows
significant weight loss at about 80 °C, which should result from
the loss of water in the system; the first endothermic peak at
ca. 70 °C in the DSC curve (Figure 3b) was caused by the
metal salts melting. The adjacent endothermic peak at 95 °C
may be attributed to the dissolution of citric acid in molten
salt. The weight loss peak at 118 °C was associated with the
dehydration due to the polymerization. A strong exothermic
peak at 342 °C in the DSC curve and a weight loss peak at 336
°C in the DTG curve indicated that the CA derivatives were
gradually self-combusted and decarbonized.
Catalytic Evaluation. The catalytic activities of the samples for
the catalytic oxidation of VOCs were evaluated in a continuous flow
fixed bed quartz microreactor (inner diameter = 6.0 mm). Then, 0.15
g of the sample was used to measure the activity (height of catalyst
bed = 160 mm). The VOC source was produced via an air-blowing
strategy, passing airflow through a bottle containing pure VOC chilled
in an ice−water isothermal bath. The feed gas contained 1000 ppm
VOC, air (21% O2 and 79% N2), and saturated vapor (RH = 100%).
The total flow rate was 80 mL·min−1, giving a weight hourly space
velocity (WHSV) of 32000 mL·g−1·h−1. In addition, when the effect
of WHSV on the catalytic activity was evaluated, the amount of
catalyst was adjusted to 0.2 g (WHSV = 24000 mL·g−1·h−1) or 0.1 g
(WHSV = 48000 mL·g−1·h−1). To investigate the influence of water
vapor, the moisture of the catalytic system was also regulated (RH =
0, 50, and 100%). The concentration of VOCs was analyzed by a gas
chromatograph (Shimadzu, GC-1020) equipped with a flame
ionization detector (FID). The CO2 was measured using a gas
chromatograph (HXSP, GC-950) equipped with a FID detector and a
methane conversion oven. Additionally, the VOC conversion, αvoc
,
and the CO2 yield, YCO , were calculated by the following equations:
2
[CxHy]in − [CxHy]out
αvoc
=
× 100%
× 100%
[CxHy]in
[CO2]out − [CO2]in
x[CxHy]in
YCO
=
2
where [CxHy]in, [CxHy]out, [CO2]in, and [CO2]out denote the inlet and
outlet concentrations of VOC and CO2, respectively.
RESULTS AND DISCUSSION
■
Figure 1 shows the X-ray diffraction (XRD) patterns of the as-
prepared materials under different preparation conditions. It
can be found that the XRD pattern of three samples depicted
diffraction peaks in the 2θ range of 15−70°, which well
corresponded to the XRD pattern of the standard SmMnO3
(JCPDS PDF# 25-0747), indicating that the SmMnO3
perovskite structure was formed after the calcination process.29
In addition, it should be noted that the diffraction peaks of the
catalysts became gradually broader in order SMO-B < SMO-N
< SMO-G, showing the decrease in the crystalline degree of
the SmMnO3 catalyst was due to the enhanced amount of CA,
which indicated that there may be differences in their
The synthetic mechanism of SMO with different morphol-
ogies is illustrated in Figure 4. As is known, the melting points
of manganese acetate and samarium trinitrate are much lower
B
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Figure 2. SEM images of SMO-B (a), SMO-N (b,c), and SMO-G (d); TEM images of SMO-B (e), SMO-N (f,g), and SMO-G (h) samples.
Figure 3. (a) TGA-DTG and (b) DSC traces of the mixed system.
N2 adsorption−desorption isotherms and pore-size distribu-
tions of SMO-N, SMO-G, and SMO-B perovskites are
displayed in Figure 5a,b, and the Brunauer−Emmett−Teller
Figure 5. (a) N2 adsorption−desorption isotherms and (b) pore-size
distributions of the three samples.
Figure 4. Schematic illustration of SMO synthesis.
than that of CA. Therefore, under heat, the manganese acetate
and samarium trinitrate were first melted into a liquid, and the
citric acid was dissolved quickly into the liquid to form a
homogeneous mixture. Afterward, citric acid underwent
polymerization to generate CA-derived polymers with complex
structures,30 wherein the samarium−manganese metal salts
doped in CA-derived polymers would be homogeneously
dispersed in the polymers (Figure S2). Namely, the as-
obtained polymer was used as a template for the growth of
perovskite precursors. Therefore, the change in the amount of
CA could lead to different morphologies of catalyst precursors.
As the temperature continued to increase, the derived
polymers were decomposed and gradually decarbonized.
Eventually, the different morphologies of SMO catalysts were
generated at high temperature.31
(BET) surface areas are summarized in Table 1. As shown in
Figure 5a, the isotherms of each sample showed a typical
IUPAC type II pattern with slit-type pores (H3-type) of the
BET hysteresis loop in the P/P0 range of 0.55−1.0, which are
characteristic of the combination of mesoporous and macro-
porous structures.32,33 Figure 5b shows the pore-size
distribution scattering from 4 to 110 nm of each sample,
which is consistent with the result of the isotherms. In
addition, the BET surface area of SMO-G, SMO-N, and SMO-
B was 27.7, 25.6, and 13.2 m2·g−1, respectively, which was
much higher than that of LaMnO3 (7.3 m2·g−1),32
La0.6Sr0.4Fe0.8Bi0.2O3−δ (4.5 m2·g−1),34 La0.6Sr0.4MnO3 (2.6
m2·g−1),35 and CeO2/LaCoO3 (17.6 m2·g−1).36 The relatively
high specific surface area could contribute to the improvement
of catalytic activity for VOC oxidation.37
C
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Table 1. Surface Element Compositions, BET Surface Areas, and H2 Consumption of Samples
XPS (molar ratio)
H2 consumption (mmol/g)
samples
Sm/Mn
O
latt/Oads
Mn4+/Mn3+
SBET (m2·g−1
)
T < 310 °C
310 < T < 578 °C
T > 578 °C
total
SMO-N
SMO-G
SMO-B
1.02
1.11
1.25
1.27
0.87
0.95
1.02
0.79
0.53
25.6
27.7
13.2
0.28
0.15
0.09
1.10
0.84
0.65
1.60
1.72
1.91
2.98
2.71
2.65
The information on surface element compositions, metal
oxidation states, and adsorbed species of SMO perovskites
with different morphologies can be obtained by an X-ray
photoelectron spectroscopy (XPS) technique. The Mn 2p3/2
and O 1s XPS spectra of the SMO-N, SMO-G, and SMO-B are
presented in Figure 6a,b, and the quantitative analysis results
Figure 7. H2-TPR profiles of SMO-N, SMO-G, and SMO-B.
the peak at 427 °C was assigned to the single-electron
reduction of Mn3+ located in a coordination-unsaturated
microenvironment, and the reduction peak about 726 °C
was from the reduction of Mn3+ to Mn2+.41−43 The
temperatures of all reduction peaks of SMO-N were much
lower in comparison to those of SMO-G and SMO-B,
suggesting that the three-dimensional network with porous
structure was beneficial for the reduction of the sample. In
order to facilitate the comparison of the hydrogen
consumption of samples, the reduction of each sample was
performed with three regions (e.g., less than 310, 310−578,
and greater than 578 °C, respectively). With regard to the
removal of the surface oxygen adspecies, the SMO-N displayed
the best consumption of H2 (0.28 mmol·g−1) below 310 °C by
comparison to that of SMO-G (0.15 mmol·g−1) and SMO-B
(0.09 mmol·g−1) (Table 1). In the range of 310−578 °C, all
three samples showed a greater amount of hydrogen
consumption, and the H2 consumption of SMO-N, SMO-G,
and SMO-B was 1.10, 0.84, and 0.65 mmol·g−1, respectively; in
the high-temperature region (above 578 °C), the amount of
hydrogen consumed was 1.60, 1.72, and 1.91 mmol·g−1 for
samples SMO-N, SMO-G, and SMO-B.
Catalytic Performance. Before testing the catalytic
activity, we performed the blank test under the condition of
no catalyst, and no conversion of VOCs was observed below
350 °C. The result showed that there was no occurrence of
homogeneous reactions under the adopted reaction conditions.
The catalytic activities of toluene, benzene, and o-xylene
oxidation over as-prepared SMO catalysts are displayed in
Figure 8a−d, where the measured conditions were as follows:
WHSV = 32000 mL·g−1·h−1, RH = 100%, VOC concentration
= 1000 ppm. Figure 8a shows the toluene conversion over the
three samples, and the SMO-N presented the best catalytic
activity. The T10%, T50%, and T90% (the reaction temperatures vs
toluene conversions of 10, 50, and 90%, respectively) were
utilized to evaluate the activities of the samples for catalytic
oxidation of toluene. The values of T10%, T50%, and T90% were
170, 190, and 215 °C for SMO-N, 175, 206, and 228 °C for
SMO-G, and 188, 223, and 258 °C for SMO-B, respectively.
The result indicated that SMO-N held a lower light-off
temperature.
Figure 6. (a) Mn 2p2/3 XPS spectra; (b) O 1s XPS spectra of SMO-
N, SMO-G, and SMO-B.
are summarized in Table 1. The asymmetrical XPS peaks can
be decomposed by curve fitting. As shown in Figure 6a, the
three components divided from the Mn 2p2/3 XPS spectrum at
642.9, 641.7, and 640.7 eV were attributed to the surface Mn2+,
Mn3+, and Mn4+, respectively.38 As known, the oxidation
reaction occurs on the surface of a catalyst, and the catalytic
activity could be associated with the extent of the Mn4+
⇔
Mn3+ redox process; the higher molar ratio of surface Mn4+/
Mn3+ was conducive to improving the redox cycles of
manganese ions, thus enhancing its catalytic capacity.20 For
the as-prepared catalyst, the molar ratios of surface Mn4+/Mn3+
were 1.05 (SMO-N), 0.79 (SMO-G), and 0.53 (SMO-B),
indicating that the porous SMO-N may hold higher catalytic
activity for VOC oxidation. Additionally, it can also be found
from O 1s XPS spectra (Figure 6b) of the samples that the
adsorbed oxygen species (O2 , O22−, or O−), surface lattice
−
oxygen (Olatt), and carbonate (CO32−) or hydroxide (OH−),
located at 528.9, 530.6, and 532.8 eV, respectively, were
copresent on the surface of the catalysts.39,40 The surface Olatt/
Oads increases in the order SMO-B < SMO-G < SMO-N,
indicating the increase in Olatt concentration, which could give
rise to improved catalytic activity of SMO-N. From Table 1,
furthermore, the surface Sm/Mn molar ratios for samples
SMO-N, SMO-G, and SMO-B were 1.03, 1.15, and 1.21,
respectively, indicating the surface of SMO-N catalyst held
more abundant Mn elements.
The reducibility is an important parameter that affects the
catalytic activity of the catalysts. The reducibility of SMO-N,
SMO-G, and SMO-B was acquired by a H2 temperature-
programmed reduction (TPR) experiment, as shown in Figure
7a,b, and their corresponding H2 consumption is listed in
Table 1. For the SMO-N, the reduction peak at 293 °C was
attributed to the surface oxygen adspecies of the catalyst, and
the peak at 349 °C was due to the reduction of Mn4+ to Mn3+;
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that toluene oxidation over SMO perovskites should adhere to
a Mars−van Krevelen mechanism,22,44 wherein toluene
molecules are attracted by high valent manganese ions and
react with the surface Olatt over catalysts to convert into H2O
and CO2, creating oxygen vacancies; afterward, gas-phase
oxygen molecules are activated by oxygen vacancies to
generate active oxygen, supplementing the consumption of
the Olatt due to toluene oxidation. Compared with SMO-G and
SMO-B, the SMO-N took on higher low-temperature
reducibility and higher molar ratios of Mn4+/Mn3+ and Olatt/
Oads, promoting the conversion of Mn4+ ⇔ Mn3+ and O2 →
Olatt, thereby improving the catalytic activity of the catalysts for
oxidation of toluene. The obvious porous and cavity structures
could also give the toluene molecules easy access into the
inside of SMO-N, further accelerating the catalytic reaction
process.
Furthermore, it can be found that SMO-N also has good
catalytic activities for benzene and o-xylene oxidation (Figure
8d). Over the SMO-N catalyst, toluene, benzene, and o-xylene
can be completely oxidized to CO2 at 240, 270, and 300 °C,
Figure 8. (a) Toluene conversion versus reaction temperature over
SMO-N, SMO-G, and SMO-B; (b) Arrhenius plots of the three
samples for toluene oxidation; (c) effect of WHSV on toluene
oxidation over SMO-N; (d) CO2 yield from toluene, benzene, and o-
xylene oxidation versus reaction temperature over SMO-N.
45
respectively, which was lower than that over Pd/ZrO2 (280
°C based on toluene oxidation), Au/CeTi46 (350 °C based on
47
toluene oxidation), Au/Al2O3 (290 °C based on benzene
48
oxidation), Ag/α-MnO2 (400 °C based on benzene
oxidation), or Pt/zeolite49 (350 °C based on o-xylene
oxidation). The difference in reactivity among VOCs over
SMO-N may be attributed to the degree of substrate
adsorption and structural effects.50 Moreover, by comparing
the toluene conversion and the yield of CO2 (Figure 8a,d), it
can also be observed that there is a difference in the numerical
values at the low-temperature reaction stage, indicating that
there may be some intermediates not completely mineralized
in the exhaust gas.51
Moreover, in the case of excess oxygen, the reaction of
toluene oxidation obeys a first-order kinetic mechanism with
respect to toluene concentration (c, mol·g−1): r = −kc = (−A
exp(−Ea/RT))c, where r, k, A, and Ea are the reaction rate
(mol·g−1·s−1), rate constant (s−1), pre-exponential factor, and
apparent activation energy (kJ·mol−1), respectively. The
Arrhenius plots for toluene conversion over the three samples
at the toluene conversion of <20% are illustrated in Figure 8b.
It is observed that the Arrhenius plots were in good linear
relationship of ln k versus 1000/T for all catalysts, and the Ea
values increased in the order SMO-N (56.4 kJ·mol−1) < SMO-
G (70.8 kJ·mol−1) < SMO-B (90.1 kJ·mol−1), suggesting that
toluene could be oxidized more readily over SMO-N.
Additionally, the Ea value (56.4 kJ·mol−1) for toluene oxidation
over SMO-N was also much lower than those of GC-LaMnO3
(84.0 kJ·mol−1),32 3DOM LaMnO3 (66.0 kJ·mol−1),19 3DOM
Eu0.6Sr0.4FeO3 (78.8 kJ·mol−1),34 La0.6Sr0.4Fe0.8Bi0.2O3−δ (127.7
kJ·mol−1),28 and MnOx/Al2O3 (133.0 kJ·mol−1).13 The results
further confirmed the excellent catalytic activity of SMO-N
catalyst at low temperature.
In addition, the catalytic stability of SMO-N was investigated
(Figure 9). The tests were performed under feed gas
The effect of different WHSV values on toluene conversion
were also examined over SMO-G. As shown in Figure 8c, the
catalytic activity of SMO-N gradually decreased with the
increase of WHSV values. The T10%, T50%, and T90%, values
were 160, 180, and 198 °C at WHSV = 24000 mL·g−1·h−1,
respectively, which were 10, 10, and 17 °C lower than those of
WHSV = 32000 mL·g−1·h−1. With the WHSV value increased
from 32000 to 48000 mL·g−1·h−1, the catalytic activity
decreased. The results showed that the WHSV value has a
great influence on the performance of catalytic oxidation of
toluene. In addition, Ea values of different WHSV values over
the SMO-N catalyst were also acquired and are shown in
Figure S3. We found that the Ea values at three different
WHSV values were extremely similar, showing that the space
velocity hardly affected the apparent activation energy,
indicating the Olatt plays the key role in toluene oxidation
over SMO catalysts.
Figure 9. Catalytic stability of SMO-N at WHSV = 32000 mL·g−1·
h−1.
conditions, including 1000 ppm toluene, air (21% O2 and
79% N2), and water vapor (RH = 0, 50, 100%). It could be
found that the change of humidity did not affect the
conversion of toluene but led to the decrease in the CO2
yield at 230 °C. It has been reported that the oxidation of
toluene follows a series of consecutive steps (toluene →
benzaldehyde → benzoic acid → aleic acid → H2O and
CO2).51 Namely, toluene was first oxidized to benzaldehyde
and benzoic acid, which consumed a small amount of O2 with
almost no release of CO2. Therefore, the introduction of
Accordingly, by comparing the catalytic activity and
characterization results of the three samples, we speculated
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adequate water did not affect the toluene conversion. However,
the oxygen concentration obtained by SMO-N decreased with
the introduction of water vapor, which was negative for O2
consumption steps, particularly for the oxidation containing
carboxylic group intermediates. Hence, the addition of water
had a distinct impact on CO2 yield in comparison to that with
toluene conversion. In addition, when the water was removed,
the SMO-N could achieve 99.8% CO2 yield at 230 °C; once
the RH is restored to 100%, the CO2 yield could also be
recovered, indicating that the SMO-N maintains high stability.
The catalytic stability of SMO-N at different temperatures was
also studied. As shown in Figure S5, at 190 and 170 °C, the
toluene conversion over SMO-N was stable at 49.6 and 10.9%,
respectively, which further demonstrated its high stability.
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CONCLUSION
■
The samarium manganese perovskites were synthesized by a
molten salt method, and the morphologies were successfully
controlled by using the derivative polymers as templates. The
catalytic activities of three samples were evaluated for toluene,
o-xylene, and benzene oxidation in wet air. The results showed
that the SMO-N perovskite possessed the most prominent
catalytic activity and stability, over which the toluene, benzene,
and o-xylene can be completely oxidized to CO2 at 240, 270,
and 300 °C, respectively. It is concluded that the special
structure, high oxygen adspecies concentration and surface
Mn4+/Mn3+ atomic ratios, and better low-temperature
reducibility played vital roles in deciding the catalytic
performance.
́
́
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01125.
XRD patterns of NCA sample and the materials at
different calcination temperatures and with different
morphologies, Arrhenius plots of SMO-N at different
WHSV values, catalytic stability study of SMO-N for
toluene oxidation at different temperatures, and
characterization of samples (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*Tel/fax: +86 21 54742817. E-mail: sunth@sjtu.edu.cn.
ORCID
Jinping Jia: 0000-0003-2409-338X
Tonghua Sun: 0000-0003-3803-0232
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the National Natural
Science Foundation of China (Grant Nos. 21507082 and
21377083), China Postdoctoral Science Foundation funded
project (Grant No. 2015M581625), and the special develop-
ment fund of Shanghai Zhangjiang National Innovation
Demonstration Zone (Grant No. 201505-Q P-B108-006).
(18) Ciambelli, P.; Cimino, S.; De Rossi, S.; Faticanti, M.; Lisi, L.;
Minelli, G.; Pettiti, I.; Porta, P.; Russo, G.; Turco, M. AMnO3 (A =
F
DOI: 10.1021/acs.inorgchem.8b01125
Inorg. Chem. XXXX, XXX, XXX−XXX

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