Enhanced catalytic performances of in situ-assembled LaMnO3/Δ-MnO2 hetero-structures for toluene combustion

Article abstract of DOI:10.1016/j.cattod.2018.07.040

It is of fundamental significance to rationally design the potent toluene-oxidation nanocatalysts for alleviating the release of toluene, one hazardous volatile organic compound, from industrial processes, oil products and fitments materials. The manganese oxide has been developed as the extremely robust combustion materials for a wide spectra of application involving methane combustion, CO oxidation and so forth, but still has been haunted with the poor behavior for toluene combustion. Herein, we successfully demonstrate a one-step scenario wherein the well-structured LaMnO₃ perovskites are in situ assembled over δ-MnO₂ for the first time. Various characterizations unravel that the LaMnO₃ perovskites interact with δ-MnO₂ to result in good catalytic toluene combustion behavior at the Mn/La ratio of 15. Boosted active phase-carrier interplays, improved redox abilities, and tuned Mn⁴⁺/Mn³⁺ couples ultimately result in the decent toluene combustion property with the full toluene oxidation at 275 °C and T₉₀ at 258 °C. The nanostructured scalable MnO₂-based materials are promising catalyts that can be extended to degrade alkane, alcohols, aldehydes and benzene series.

Full text of DOI:10.1016/j.cattod.2018.07.040

Accepted Manuscript
Title: Enhanced catalytic performances of in situ-assembled
LaMnO /␦-MnO hetero-structures for toluene combustion
3
2
Authors: Jingsi Yang, Luming Li, Xiushan Yang, Shuang
Song, Jing Li, Fangli Jing, Wei Chu
PII:
S0920-5861(18)31095-2
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.07.040
CATTOD 11585
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
16-3-2018
16-5-2018
14-7-2018
Please cite this article as: Yang J, Li L, Yang X, Song S, Li J, Jing F,
Chu W, Enhanced catalytic performances of in situ-assembled LaMnO /␦-
3
MnO2 hetero-structures for toluene combustion, Catalysis Today (2018),
https://doi.org/10.1016/j.cattod.2018.07.040
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Enhanced Catalytic Performances of in Situ-Assembled LaMnO
Hetero-structures for Toluene Combustion
3
/δ-MnO
2
a, b
a, b
b
c
b
b
Jingsi Yang , Luming Li , Xiushan Yang , Shuang Song , Jing Li , Fangli Jing ,
a, b, c
Wei Chu*
a
b
c
Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu 610065, China
School of Chemical Engineering, Sichuan University, Chengdu 610065, China
The College of Architecture and Environment, Sichuan University, Chengdu 610065, China
Corresponding author：Wei Chu (Dr. Prof.)
Email addresses : chuwei1965@scu.edu.cn; chuwei1965scu@163.com
Graphical Abstract

Highlights



The well-structured LaMnO
3
perovskites were assembled in situ over δ-MnO .
2
The catalytic activity was enhanced significantly for toluene catalytic combustion.
Improved redox abilities and more active lattice oxygen were the main reason of good
performance.
Abstract:
It is of fundamental significance to rationally design the potent toluene-oxidation
nanocatalysts for alleviating the release of toluene, one hazardous volatile organic
compound, from industrial processes, oil products and fitments materials. The
manganese oxide has been developed as the extremely robust combustion materials
for a wide spectra of application involving methane combustion, CO oxidation and so
forth, but still has been haunted with the poor behavior for toluene combustion.
Herein, we successfully demonstrate a one-step scenario wherein the well-structured
LaMnO
characterizations unravel that the LaMnO
in good catalytic toluene combustion behavior at the Mn/La ratio of 15. Boosted
3
perovskites are in situ assembled over δ-MnO for the first time. Various
2
3
perovskites interact with δ-MnO to result
2
4
+
3+
active phase-carrier interplays, improved redox abilities, and tuned Mn /Mn
couples ultimately result in the decent toluene combustion property with the full
o
o
toluene oxidation at 275 C and T90 at 258 C. The nanostructured scalable MnO
based materials are promising catalyts that can be extended to degrade alkane,
alcohols, aldehydes and benzene series.
2
-
Keywords: Layered manganese oxide; Gunpowder-like combustion; Catalyst;
Perovskite oxide; Catalytic combustion of toluene

1
. Introduction
With increasing industrialization in the world, the decomposition of aromatic
hydrocarbon, halogenoalkane, and oxygenates has received more and more attentions
[
in environment-protecting fields 1-]. The varied concentrations and constituents of
volatile organic compounds (VOCs) lead to different degree of damage to human
body like respiratory tract and lung, even cause extremely serious lung cancer [4].
Among technologies extensively applied in constraint on exhausting VOCs
contaminant, catalytic combustion as one of extraordinarily potential measurements is
thoroughly studied in scientific researches [5, 6].
According to the latest researches results, the noble metals (mainly Pd, Pt and
Ag) catalysts have shown their excellent efficiency in catalytic oxidation of VOCs,
some stubborn obstructions like their high cost and weak thermal stability are still
required to surmount [7, ]. Conversely, the heterogeneous transition oxides catalysts
(
MnO
with good catalytic performance in degrading VOCs. Especially, MnO
comparable to Pt/Al in terms of the combustion of VOCs due to its multivalent
nature and nonstoichiometric composition [9-]. Marco Piumetti et al [14]. prepared
three types of mesoporous manganese oxide catalysts (Mn , Mn and Mn ) by
x
, CoO
x
, CuO, CeO
2
) became much more attractive since they also exhibited
x
was
2
O
3
2
O
3
3
O
4
x
O
y
means of solution combustion, and the results showed that manganese oxides with the
highest metal oxidation state and the highest amount of electrophilic oxygen on the
surface exhibited the highest oxidation activity. Zhang et al [15]. synthesized different
crystallographic polymorphs (α-, β-, γ- and δ-MnO
phase exhibits the best catalytic performance of completely oxidizing HCHO due to
its inner tunnel and lattice oxygen. δ-MnO as a promising catalyst performed better
than the rest when it was further applied for the destruction of gaseous Hexachloro
benzene, pentachloro benzene, and polychlorinated biphenyls [16]. Since δ-MnO
2
) and found that δ-MnO
2
crystal
2
2
exhibits so many superiorities in contrast with other polymorphs, it could potentially
be employed as a support in applications of supported catalysts. So far many attempts

have been focused on doping heterogeneous elements (such as Ti, Cu, Ce) on δ-MnO
2
to improve catalytic performance [17-].
Perovskite-type metal oxides with the advantages of adequate catalytic activity
and good thermal stability have been studied for several decades in the research field
of efficiently eliminating VOCs [20]. Previous works have systematically studied
partially replaced A- or B- sites by other elements in ABO
crucial distortion of crystalline lattice and transforms the primary inert structures
21, ]. The good catalytic performances were connected with essential
3
structure, which creates
[
physicochemical properties such as favourable oxidation-reduction capacity and
[
abundant oxygen vacancy 23-]. The three-dimensional ordered macroporous
perovskite materials (3DOM) LaMnO
3
or La
x
Sr1-xCoO were synthesized to improve
3
specific surface area, the resulting catalysts after loading noble metal or transition
[
]
metal oxides gave better catalytic performances 26, 27 . Another pathway for
fabricating high-performance perovskite catalysts could be obtained via acid
treatment of 3DOM-structured ABO to partly dissolve A-sites and regulate B-sites
3
cations on the surface, interface between metal oxide and perovskite was therefore
[
fabricated, which was further good to enhance the catalytic activity 28-2930]. The
obtained catalysts indicate good catalytic behaviors in CO oxidation and toluene
combustion because the attraction of La cations by O onion at the interface can
weaken and elongate Mn-O bonds [31]. Several researchers have prepared
perovskites anchored on metal oxides, such as La0.8Ce0.2MnO
3
/CeO
2
[32],
LaCoO [33]. It is beneficial to create LaMnO /MnO
3
/Ce1−xZr
x
O
2
3
2
interface to obtain
promising catalytic ability of toluene oxidation, while the conventional methods such
as sol-gel method and solid-state method have problems with both modulating the
formation of LaMnO
3
and immobilizing LaMnO on specific crystalline phase δ-
3
MnO
2
2
. Hence tackling the aforementioned challenges of constructing the LaMnO
interface is still essential.
3
/δ-
MnO
Herein, we assembled for the first time LaMnO
3
on δ-MnO
2
via facile
gunpowder-like combustion (GLC) method, and tested the catalytic behavior for the

total catalytic oxidation of toluene. In our previous work, self-propagating method has
been developed to synthesize various layered MO /δ-MnO (M=Ce, Cu, Zn, Ni)
hybrids for toluene combustion [34]. During GLC process which generates different
spark phenomenon in this paper, LaMnO /δ-MnO catalysts can be obtained from
x
2
3
2
fundamental raw materials rather than secondary manufacturing materials.
Meanwhile, samples prepared by sol-gel method and solid state method were futher
studied for comparison of the enhancement effect between LaMnO
3
and δ-MnO in
2
GLC. The relationship between catalytic activities and physicochemical properties
was discussed through a string of characterization techniques.
2
. Experimental
2
.1. Sample preparations
.1.1. Gunpowder-like combustion (GLC) method
2
A series of catalysts with different molar ratio of lanthanum and manganese was
synthesized via gunpowder-like combustion (GLC) with potassium permanganate and
lanthanum acetate as metal precursors. Firstly a certain amount of KMnO and
La(CH COO) were severally grounded for several minutes until the generation of
4
3
3
homogeneous precursors, followed by commixing two raw materials (Mn/La) at the
molar ratio of 15:1. Then the abrasive was ignited on a smooth glass accompanying
sporadic sparks. The cooled samples were washed and filtered with deionized water
for three times. Assuming complete reaction, KMnO as oxidizing agent reacted with
4
La(CH
3
COO)
3
. The total reaction mechanism could mainly be concluded by redox
o
reaction . Finally as-obtained catalyst was further desiccated at 100 C for 12 h in an
electric oven. The products (Mn/La=15:1) are named as GLC-15. Other specimens
was derived by similar procedures except the Mn/La molar ratio which was adjusted
to 3:1, 5:1, 8:1, 11:1 and 18:1, and the as-synthesized specimens were named as GLC-
3
, GLC-5, GLC-8, GLC-11 and GLC-18 respectively.

2
.1.2. Solid state method (SS )
In typical solid state preparation [35], a certain amount of KMnO
4
and
La(CH COO) was separately grounded for several minutes to obtain the
3
3
homogeneous substances, followed by commixing two raw materials (Mn/La) at the
molar ratio of 15:1. Then the mixture was transferred in a porcelain boat then heated
o
o
at a rate of 2 C/min up to 400 C and kept for 2 h under static air. At final stage,
samples were washed and filtered with deionized water for three times, and the SS15
o
sample was obtained after desiccating at 100 C overnight.
2
.1.3 Citric acid sol-gel method ( CA )
In the sol-gel process [36], 50 wt% manganese nitrate solution and lanthanum
acetate with a La/Mn molar ratio of 1:15 were dissolved in 30 mL deionized water.
Then, the citrate acid (with 20% excess of the total molars of the cations) were added
into the solution above. The obtained solution was magnetically stirred for 30 min and
o
then concentrated by evaporation at 80 C for 3 h until the generation of a spongy
o
solid. The solid product was further desiccated at 100 C overnight and finally
o
o
calcined at 500 C for 4 h at a heating rate of 2 C/min to obtain the CA15-500
sample. Another CA15-700 specimen was derived by similar procedures except the
o
o
step-wise thermal annealing at 500 C for 2h then heated up to 700 C and kept for 4h.
2
.2. Sample characterizations
X-ray diffraction was employed to identify the crystallinity and phase structure of all
samples with Cu Kα (λ=1.542 Å) as excitation source. The invariable parameters
including the scanning ranges from 5° to 80° and the scanning rate of 0.03 °/s were
used at the equipment (CX-2700). According to the Scherrer’s equation, the crystallite
size of catalysts was evaluated from the strongest reflection peaks corresponding to
birnessite at 2θ=12.3° and lanthanum manganate at 2θ=32.3°. Specific surface area,
pore size and pore volume were calculated via BJH method and identified by N
2
adsorption-desorption isotherms from an instrument Quantachrome Chemisorption.

o
Before analyzing the samples, each was treated at 200 C for 4 h under one over ten of
barometric pressure to desorb superficial foreign gas.
During the temperature-programmed reduction of hydrogen (H -TPR), 30 mg as-
2
synthesized catalysts were used on a chemisorption analyzer with TCD in a straight
o
quartz reactor, followed by pretreating at 300 C for 1 h with N
2
flowing. After cooled
down to room temperature, the reducing gas (5 % H
2
/N
2
) flowed into straight quartz
o
o
reactor at a rate of 30 mL/min. Temperature program was set from 50 C to 900 C at
o
a ramp rate of 10 C/min, and hydrogen consumption was instantaneously recorded by
TCD detector.
The microstructure of samples and regional Energy Dispersive Spectrometer
(
EDS) were observed on a scanning electron microscopy (SEM) from TESCAN
MAIA 3 LMHA apparatus at 15 kV. In addition, the elementary dispersibility of
lanthanum, manganese and potassium was detected by mapping analysis.
Transmission electron microscopy (TEM) and high-resolution TEM were carried out
to perform the microstructural presentation of all samples. Surface analysis was
performed to inquire the chemical state of catalysts by X-ray photoelectron
spectroscopy from Kratos XSAM800.
2
.3 Evaluation of toluene removal activity
In this paper, catalytic combustion of toluene was presented as the model reaction of
degrading VOCs. The catalytic activity test for toluene combustion was evaluated via
a quartz micro-reactor (i.d.=8 mm) under continuous flowing dry air. In the middle of
the quartz bed 100 mg sample (40-60 meshes) was loaded for each run and the quartz
micro-reactor was set in the center of the electric stove. As for toluene source, 1000
ppm toluene which could be adjusted via altering air flow meter was generated by
continuously bubbling toluene in the vessel immobilized in an ice water bath. The
overall flowing mixture (1000 ppm toluene + O
2
+ N
2
(balance)) passing through the
quartz bed was determined at 50 mL/min, giving a SV of 30,000 mLgcatal 1h-1. Prior to
-
catalytic activity test, the thermal pretreatment of samples were heated from room

o
o
temperature to 300 C with a linear rate of 5 C/min and kept for 1h to activate the
catalysts. The pretreatment could obviate the impact of impurity adsorption
simultaneously for toluene conversion. After that, the reactor was cooled down
o
naturally to 150 C and maintained at least for an hour to reach dynamic adsorption
equilibrium of toluene. The concentration of outlet toluene was analyzed by a online
gas chromatograph (SP-6890) equipped with a flame ionization detector (FID) and a
thermal conductivity detector (TCD). At each temperature test point, the temperature
program was maintained no less than half an hour to make the system come to
dynamic balance. No other chromatographic peaks were detected. The outlet gas was
import into TCD detector for examining CO product and FID detector for toluene.
2
The toluene conversion was calculated by the toluene number moles in and out via the
following formula:
N
tol, in  Ntol, out

% 
100%
Ntol, in
Where Ntol,in and Ntol,out represented the toluene number moles in and out respectively.
3
. Results and discussion
3
.1. Phase and structural properties of catalysts
XRD patterns are used to measure the crystal structure of all samples and the results
are presented in Fig. 1. The crystallite phase sizes of catalysts are calculated via
Scherrer’s equation and listed in Table 1. Each catalyst is inferred by PDF standard
cards to analyze the status of the crystalline phases.
In the XRD patterns, the diffraction peaks at 2θ=12.3° (0 0 1), 25.1° (0 0 2), 35.4°
(
2 0 0), 36.2° (1 1 0), 39.6° (1 1 1), 56.3° (1 1 3), 65.6° (0 2 0) belong to δ-MnO
2
(
PDF 80-1098), evidencing that GLC samples distinctly show lamellate birnessite
structure except GLC-3. The obtained lamellate birnessite phase is decomposed from
KMnO . Dissimilarly, the 2θ peaks of GLC-3 are mainly located at 32.3° (0 0 2),
5.5° (2 0 2), 47.3° (0 4 0), 57.7° (1 2 3), matching with orthorhombic system of
4
4

La0.88MnO2.82 (PDF 89-0685), it means that the high ratio of La/Mn can favorably
bring about the formation of La-Mn solid solution rather than birnessite, the
perovskite phase took the leading position in GLC-3 and inhibited the crystal growth
of the birnessite. Meanwhile 2θ at 15.7°, 27.2°, 28.0° are put down to La(OH)
3
(PDF
3
6-1481) due to the incomplete thermal decomposition of lanthanum acetate. Notably,
the concomitant diffraction peaks of δ-MnO and La0.88MnO2.82 can also be observed
2
for all GLC samples. In addition, the different dosage of lanthanum acetate can
markedly influence the appearance of phase in GLC samples. Further inspection into
the zoom-in diffraction peaks at 2θ=12.3° (001), 25.1° (002) (Fig. S1) reveals a
asymptotic left-shift from 12.6° for GLC-5 to 12.2° for GLC-18 with the lessening in
the La content. It can be possibly attributed to the gradual lattice dilation caused by
3
+
the partial substitution of the large-sized lanthanum cations (La =1.36 Å) in the
manganese octahedral lattice structure. The remarkable peak at 2θ=32.3° could
confirm that pittance of La atoms are crystallized with Mn atoms and generates into
the perovskite phase, explicitly substantiating that the present gunpowder-like
combustion scenario can potently enable forming the perovskite phase. It could be
seen that the asymptotically attenuated diffraction intensity at 2θ=32.3° occurs with
the decreasing concentration of lanthanum, indicating the diminution of grain size in
Table 1. Hence, XRD data clearly confirm that a composite hetero-structure of the
La-Mn solid solution assembled on the lamellate birnessite structure which can be
straightforwardly achieved as also verified by TEM and HRTEM results
hererinbelow. Noteworthy, the used catalysts can still maintain the almost identical
XRD patterns (Fig. S2) to the fresh counterparts with the stable crystallite sizes,
connoting the good structural tenability.

Fig. 1 (A) XRD patterns of GLC samples. (a) GLC-3; (b) GLC-5; (c) GLC-8; (d) GLC11; (e)
GLC-15; (f) GLC-18, respectively. (B) XRD patterns of as-prepared (g) CA15-700, (h)
CA15-500 and (i) SS15 samples.
XRD data of other samples fabricated from traditional methods are presented in
Fig. 1(B). The XRD pattern of SS15 well accords with the δ-MnO (PDF 80-1098)
2
with monoclinic symmetry. No other impurity phases indexed into perovskite or
lanthanum oxides can be found because higher temperature was required to form the
o
LaMnO
3
perovskite structure. However, higher temperature (>450 C) treatment
simultaneously resulted in the formation of other transition crystal phases like α-/γ-
MnO
2
[37]. As for catalysts derived from traditional sol-gel preparation, CA15-500
o
calcinated at 500 C displays the characteristic diffraction peaks of the tetragonal
single Mn
perovskite structure detected. It can be supposed that thermal treatment of 500 C
could lead to a multiple manganese oxidation state of Mn composed by Mn
and MnO [38-]. In contrast, the CA15-700 demonstrates the coexistence of cubic
3
O
4
phase (PDF 24-0734), and there is no typical diffraction peaks of
o
3
O
4
2
O
3
MnO (PDF 78-0424) and La0.88MnO2.82 (PDF 89-0685), as affirmed by the presence
of peaks at 2θ=34.9°, 40.5°, 58.7°, 73.8° and 2θ=32.3°, 45.5°, 47.3°, 57.7°
o
respectively. After calcination at 700 C, thermodynamically preferential perovskite
3
+
phase was formed via a strong combination of part of Mn with La
2
O
3
at elevated
temperature, which was in good agreement with representative hydrogen consumption
3
+
2+
peaks of Mn →Mn within the perovskite structure in H -TPR data. SS15 and
2

CA15-700 prepared via traditional methods presents different phase status from GLC
samples which produces a better hybrid composites that show an interaction effect
between LaMnO
3
and δ-MnO at their interface.
2
#
Crystallite sizes of δ-MnO
2
and La0.88MnO2.82 phases were calculated through Scherer formation
from half width of (2θ=12.2°) and (2θ=28.55°)) diffraction peak in the XRD patterns.
*
§
Surface areas calculated by the multi-BET method.
Yield = W Catalyst/W Precursor * 100%.
2
N adsorption-desorption analysis was carried out to measure specific surface area
(
SSA) of all catalysts, And the results were listed in Table 1, SSA of the GLC-
2
derived samples witnesses a rise from 10.0 to 32.1 m /g when enlarging the Mn/La
ratio. This is a result that the amount of gaseous products produced from lanthanum
acetate were coincident with the decreasing dosage of lanthanum acetate, then tender
sparks could refrain from the sintering of pores. The gradual enlargement of the pore
3
3
volume from 0.028 cm /g to 0.089 cm /g and average pore diameter from 8.7 nm to
4.9 nm direct affect the performance of GLC catalysts. On the other hand, excessive
loading of LaMnO might hinder the high specific surface areas of modified δ-MnO ,
1
3
2
which is accorded with TEM figures. On the main isotherms depict an type-IV shape
with the obvious H2 (or H3 or H1) hysteresis loop and the aggrandized gas uptake at
high pressures, suggesting a linear monolayer-multilayer adsorption nature and a
robust capillary condensation process emerging in a large number of mesopores (Fig.
S3). Coupled with SEM images below, textural properties closely depend on
morphological evolution with the varied La/Mn dosage. It can be logically deduced
that this observed hysteresis behavior with horizontal branches ensues from the
adsorption-desorption in narrow slit-like pores at low Mn/La ratio yet from the
superposition of the plate-like birnessite particles with slit-shaped pores under the
high Mn/La ratio. Thus, the straticulate morphology at a highest Mn/La ratio of 18
and 15 shows the small thinness, leading to the highest SSA in shape contrast with
other clip accumulation at low Mn/La dosages. As seen in Table 1, the SSA of SS15
2
2
is 23.5 m /g which is similar to that of GLC-18. In addition, GLC-15 (23.5 m /g)

2
possesses the considerably higher SSA than SS15 (16.1 m /g) although the both
samples had the same La/Mn ratio, such a result implied the GLC method can create
the bigger pores with a broader pore size distribution (Fig.S3). For GLC-15, gaseous
products were instantaneously generated, punching more pore structures than SS15
from the thermal decomposition, which is stimulative for catalytic reaction. Compared
with CA15-500 with CA15-700, the sintering at high temperature was responsible for
2
2
the decrease of SSA from 57.9 m /g to 40.9 m /g, accompanied by the formation of
perovskite phase. CA15-700 owes higher SSA than GLC-15, but the XRD data
display bigger grain size, which is a disadvantageous factor to catalytic oxidation of
toluene as proved later.
3
.2. Catalytic performances for toluene catalytic combustion
The catalytic performances of as-synthesized samples for toluene combustion
were tested at different temperatures with total flow rate of 50 mL/min corresponding
SV=30,000 mLgcatal 1h-1. Catalytic activity measurements was evaluated by the
reaction temperature T50 (representing the temperature at 50% toluene conversion)
and T90 (representing the temperature at 90% toluene conversion) respectively listed
in Table 2. Fig. 2a clearly demonstrates that the dosage of lanthanum acetate leads to
an important shift of the catalytic activity over GLC samples, which indicates the
smaller addition of lanthanum acetate are favorable for exhibiting good catalytic
properties. The addition of lanthanum acetate directly influence the grain size of perovskite in
GLC samples as presented in XRD. The total oxidation of toluene over GLC-15 was
-
o
achieved at 275 C, the best activity among all samples due to the small grain size, the
4
+
wider fissure, more active lattice oxygen and higher Mn content, as will be further
o
o
discussed below. Whether at 250 C (Fig. 2b) or 270 C(Fig. S4) the GLC-15
exhibits the superior catalytic ability in all GLC samples. T90 disclosed the order of
o
catalytic performance for the different samples to be GLC-15(258 C) ≈ GLC-18(260
o
o
o
o
o
C) > GLC-11(269 C) > GLC-8(290 C) > GLC-5(303 C) > GLC-3(over 330 C),
which is well consistent with the grain size and SSA data. Remarkably, in all samples
o
o
T
50 drops from 330 C on GLC-3 to 248 C on GLC-15, a substantial decline beyond

o
8
2 C in the catalytic evaluation of 1000 ppm toluene. Combining with XRD results,
perovskite phase took the leading position in the GLC-3, and it can be speculated that
the compounds of lanthanum cover the surface of GLC-3, and large-grained particles
2
with the lowest specific surface area of 10 m /g inhibits the catalytic activity of GLC-
3
.
Fig. 2 (a) Temperature dependence of toluene conversions over the GLC catalysts under the
-
1 -1
condition of toluene concentration =1000 ppm and SV= 30000 mLg-catal h . (b) Toluene
o
conversion at 250 C over GLC-x.
Compared with other samples from different method in Fig. 3, GLC-15 sample
showed the superior efficiency in removing toluene due to its special hetero-structure.
SS15 obtained from the calcination treatment displays the half toluene conversion at
o
o
2
62 C and a T90 of 295 C. The catalytic activity of CA15-700 was more inferior
o
o
o
(
T
50=309 C and T90=328 C) than that of the CA15-500 sample (T50=278 C and
o
T
90=316 C). The depressed activity in CA samples may be attributed to the high
2
o
temperature sintering which induces the decrease in SSA from 57.9 m /g at 500 C to
2
o
4+
3+
4
0.9 m /g at 700 C. In addition, as shown below the reduction of coupled Mn /Mn
was probably one of the main reasons for the activity fading. It was demonstrated that
LaMnO phase in GLC-derived catalysts exhibited a synergistic improvement effect
with δ-MnO for the catalytic activity than other catalysts (SS15, CA15-700). Dai. et
al. [42] has synthesized a series of MnO /3DOM LaMnO with the strong interaction
effect between LaMnO and MnO to obtain better catalytic activity. In our case,
3
2
x
3
3
x

GLC-15 with special LaMnO /birnessite shows the significantly promoted activity for
3
toluene better than CA15-700 and SS15, which was probably associated with the
lattice oxygen activity and low-temperature reducibility. Moreover the typical 0.5%
o
Pt/Al
2
O
3
catalyst attained the 100% toluene conversion at approximately 300 C under
[
]
the identical tested condition 35 . GLC-15 matched the 0.5% Pt/Al
2
3
O on deep
oxidation of toluene at high temperature, which suggested GLC-15 was an efficient
catalyst for the total removal of volatile organic compounds like toluene.
Fig. 3 Conversion of toluene to CO
the four samples.
2
and H O as a function of the reaction temperature over
2
3
.3. Morphology and reducibility of as-prepared samples

(a)
(b)
1
μm
5
00 nm
(c)
(d)
5
00 nm
1 μm
Fig. 4. SEM images of the catalysts: (a) GLC-3; (b) GLC-8; (c) GLC-15; (d) GLC-18.
Scanning electron microscopy was performed to examine the morphorstructure of
GLC samples, denoted in Fig. 4. It is clearly seen that the morphology is sensitive to
+
the La/Mn ratio because the presence of K exerts a pivotal impingement on the
+
crystal growth mechanism. When KMnO
4
starts to decompose, the formed K can
first be tethered to surfaces of the octahedral MnO
6
during the GLC synthesis, and
3
+
then replaced by La as transient nucleation sites for bonding with the high valent
3
+
Mn and initiating the sustained in situ assembly of the LaMn-based perovskite phase
over the MnO substrate, ultimately giving rise to a perovskite/MnO hybrid hetero-
2
2
structure with continuous, seamless and intimately contacted interfaces [43]. Within
the thus-obtained hetero-structure, more higher valence state Mn, beneficial for
toluene oxidation, are exposed, whilst La-O bonds around the interface periphery
would weaken and stretch the bond between manganese and oxygen, which facilitates
[
]
better catalytic activity for toluene combustion 31 . For this process, lanthanum
acetate at a dwindling dosage thus can induce a more peaceable redox reaction,
corresponding to the obtained thinner thickness and higher SSA in MnO
2
. In view of
the growth mode, the high La loading level finally gives a relatively regular cube with

a size of approximately 100 nm on GLC-3 but a laminar architecture 20 nm roughly
on GLC-18. In particular, the typical laminas of GLC-15 randomly stacked and
overlapped each other to contribute to more mesopores, as also verified by N -
2
porosimetry. More momentously, element mapping (Fig. S5) manifests relative
homogeneous distribution of La and Mn throughout the part surface of GLC-15
framework, while the slight La dots clearly suggests the good distribution of
prevoskite structure on the manganese oxide in short range. Meantime, EDS gives a
La atom percentage of 1.4%, much lower than that of Mn (74.5%), suggesting the
main surface elemental composition was birnessite and perovskite was distributed on
the surface. The catalytic performance and adsorbability are relevant to the surface
active sites thus the distribution of lanthanum, the crystal size and the superficial flaw
are sensitive factors.
Since GLC-15 performed the best catalytic activity for toluene combustion, the
TEM characterization was used to learn the postural existence of perovskite and
birnessite and to analyse the effect of different synthesis routes for La doped
manganese oxides. There has been a few reports about grain boundaries between two
different phases on the catalysts surface impacting on the enhancement of oxygen
mobility and activation of molecular oxygen [44-].

Fig. 5 TEM and HRTEM images of GLC samples and other samples from different methods:
a,b) GLC-3, (c,d) GLC-8, (e,f) GLC-15, (g,h) GLC-18, (i,j) SS15, (k,l) CA15-500, (m,n)
CA15-700.
(
In Fig. 5 (a)(c), the TEM images demonstrate GLC-3 and GLC-8 present large
particles which is similar microtopography to SEM results. The observed 0.27 nm and
.35 nm lattice fringe is ascribed to the (0 0 2) and (1 1 1) crystallographic planes of
La0.88MnO2.82 (PDF 89-0685). It can be clearly seen from GLC-15 and GLC-18 that
the layered sheet-like structures belong to δ-MnO was decorated by the black spots
i.e. perovskite phase. When doping high ratio La, the perovskite phase covers the
surface of δ-MnO substrate (Fig S8), and the mantle was changed to black spots after
reducing the dosage of lanthanum acetate. As a result, the catalytic activity of GLC-
5 and GLC-18 performed better than other samples. Moreover, There is an obvious
difference that GLC-15 and GLC-18 present lamellate morphology of δ-MnO but
SS15 only shows the pulverous aggregation. The lattice spacing of 0.29 nm detected
0
2
2
1
2

from SS15 and CA15-500 was assigned to the (1 0 1) of La
2
3
O (PDF 05-0602). And
TEM results confirmed the formation of perovskite structure in GLC-15 synthesized
through gunpowder-like combustion while SS15 and CA15-500 samples without high
temperature roasting present lanthanum oxide. In GLC preparation procedure,
momentary spark creates high temperature to attain the formation energy of
perovskite structure accompanying by generating birnessite-type MnO
perovskite/MnO hybrid hetero-structure contributes to the enhanced performance of
toluene combustion.
2
. The special
2
The H
-TPR experiment is the crucial characterization to analyze the redox ability
2
properties and oxygen maneuverability of the as-prepared catalysts. The excellent
low-temperature reducibility of GLC-15 sample may result in the superior catalytic
performance of toluene combustion. As presented in Fig. 6, it should be noted that the TPR
profiles of GLC samples apart from GLC-3 displayed four envelopes peaks named as
o
o
β
1
, β
2
, β
3
, β
4
. The first peak β ranging from 200 C to 250 C represents the removal
1
of oxygen species which are most weakly bound to the catalyst surfaces. According to
the reference [47,], the reductive process of Mn metal oxides could be divided into
o
three steps: the peak centered near 290 C ( β
2
) indicated the reduction of high
4
+
3+
oxidation states (Mn ) to lower valence (Mn ); the spiculate peak ( β
3
) in the middle
temperature window can be determined as partial reduction of Mn within the
octahedral MnO with different configurations [49]. The β peaks with the maximum
reduction temperatures are attributed to the successive reductions of Mn . In
addition, H -consumption from β to β could be vested in the whole procedure of
MnO → Mn → Mn → MnO, corresponding to the peak proportion ratio of
:1:2. GLC-15 and GLC-18 demonstrate the same first hydrogen consumption peak at
3
+
6
4
3
+
2
2
4
K
x
2
2
O
3
3
O
4
3
2
o
o
o
84 C while the first peak of GLC-3 was located at 323 C putting a gap of 39 C,
which means the lattice oxygen in GLC-15 and GLC-18 is more active than others.
4
+
Apparently, the total consequence demonstrates that a substantive portion of Mn in
o
δ-MnO
2
of GLC samples has been reduced to lower valence state of 2+ below 400 C.
La O is irreducible
during the reduction process but presence of
It should be noted that 2 3

perovskite structure impacts the reduction ability δ-MnO
With the reduced content of
of
.
2
lanthanum acetate, the interior structure of catalysts were transformed. From SEM
and XRD results, GLC-15 with low lanthanum loading level presents small particles
leading to high surface energy [50], thus GLC-15 can be reduced at lower
temperature. The TPR results show in GLC samples except GLC-3 the special
birnessite type is the dominating existence phase and the first reductive peak is in
accord with the order of catalytic activity. It has to be mentioned that the location of
the H consumption peaks from all samples also rely on the grain sizes, where small
2
quantities of the metal oxide species could be reduced first. The formation of bigger
perovskite particles may led to the reduced temperature shifting to higher temperature
location. The hetero-structure in GLC-15 created by high dispersed LaMnO could
3
vivify the surface lattice oxygen which benefits the catalytic activity of toluene
oxidation.
(
b)
(
a)
2
Fig. 6 H -TPR profiles of CA samples, SS15 samples and GLC samples with different La/Mn
molar ratios.
H
2
-TPR results of samples prepared from different methods also discussed. For
o
CA15-500, the reduction peak at 487 C can be assigned to the reduction of Mn
3
O
4
(
Mn
3
4
O → MnO). There is no peak observed in high-temperature region at CA15-

5
00, which performs typical reduction procedure of Mn
3
4
O in accordance with
[
]
o
references 14 . In contrast, the weak peak at 803 C implies the formation of
o
o
perovskite structure in CA15-700. but no obvious peak appear at 250 C -235 C
region which demonstrates GLC-15 has a better redox ability, resulting in distinctive
behaviors of catalytic activities. The SS15 sample also demonstrates the same
reduction peaks, a birnessite-type manganese oxide without characteristic reductive
peaks of perovskite oxide. The mainly reductive temperature of SS15 are close to the
o
temperature of GLC-15, while SS15 attains the toluene conversion of 90% at 295 C
inerter than GLC-15, verifying the facilitative function of hetero-interface in GLC-15.
Our previous work has show the increase of the surface oxygen mobility leads to the
phenomena that the reduction temperatures significantly shift to lower regions [51-].
It can be supposed that the oxygen mobility is increased in GLC-15 due to the special
hetero-structure of perovskite/birnessite.
3
.4. Surface Composition, Metal Oxidation State and Oxygen Species
Further information about surface elemental species was investigated by XPS
measurement. Fig. 7 displays the Mn2p3/2 and O1s XPS spectra of the six samples.
4
+
3+
The molar ratio of Mn to Mn was listed in Table 2 with data of Olatt/Oads
.
Apparently, it can be found that in O1s spectra the area could be separated to three
bands. The peaks at around 529.9-530.2 eV, 530.7-531.5 eV and 533.0-533.7 eV are
assignable to the surface lattice oxygen (Olatt), surface adsorbed oxygen species (such
as O , O and OH group) and adsorbed molarcular H
O [53], respectively. According to
2
−
−
2
Mars-van Krevelen mechanism, the adsorbed VOCs react with oxygen in the active
sites, resulting in the reduction of the metal oxide. Then the VOCs reactant was
oxidized by environmental oxygen along with the re-oxidation of the reduced metal
oxide. The lattice oxygen has a stimulative influence on the catalytic activity of
toluene combustion as Quan et al. has discussed [54].
Higher molar ratio of Olatt/Oads is beneficial for redox process in MVK mechanism
contributing to the higher performance of catalysts. At the decrescent La loading level

surface Olatt/Oads tends to nonlinearly increases, which is similar to Ce doped δ-MnO
2
[
55]. The GLC-15 possesses the highest ratio Olatt/Oads of 1.35, corresponding to the
enhanced redox reactivity certified by TPR characterization, which potentially
contributed to improved catalytic ability for toluene combustion. Reversely, GLC-3
exhibited the least ratio at only 0.26, due to abundant surface oxygen specie arising
from incomplete decomposition of lanthanum acetate generated hydroxy species. It
can be supposed that the increase of lanthanum leads to the decline ratio of Olatt/Oads
which is in agreement with the catalytic performance order: GLC-15 ≈ GLC-18 >
SS15 > GLC-8 > CA15-700 > GLC-3. The discrepancy of the Olatt/Oads molar ratio
,
caused by the reaction dosage of La(CH
metal oxide support interactions between LaMnO
samples obtained from different preparation exhibits low Olatt/Oads at 0.63 for CA15-
00 and 0.70 for SS15, which is fairly homologous with catalytic performance.
Luxuriant lattice oxygen on GLC-15 can be easily derivable to the perfect
performance of toluene oxidation on MnO catalysts.
3
COO)
3
suggests the existence of the strong
3
and δ-MnO support. Other
2
7
2
Fig. 7 (a) O 1s and (b) Mn 2p3/2 regions of XPS spectra of six samples: (a) CA15-700; (b)
SS15; (c) GLC-18; (d) GLC-15; (e) GLC-8; (f) GLC-3.
4
+
3+
According to the references [56-], an increase in the surface Mn /Mn ratio will
beneficially promote the catalytic performance of toluene combustion. Dai et al. 43]
[
reported that the MnO
x
-loaded 3DOM LaMnO
3
exhibited the highest surface
4
+
3+
Mn /Mn molar ratio which was responsible for the excellent catalytic performance

on toluene. In Fig. 7, the asymmetrical peak of Mn 2p3/2 XPS spectra was fitted to the
4
+
3+
Mn species located at 643.0-643.7 eV and Mn species located at 641.5-641.9 ev
4
+
3+
[
(
58-]. It can be observed that GLC-15 gave the highest molar ratio of Mn /Mn
4
+
0.56) indicative of the enrichment of Mn , corresponding to the best catalytic
4
+
3+
activity. Along with the increase of lanthanum, the Mn /Mn ratio was shifted from
4
+
3+
0
.56 to 0.32. Apparently, in Table 2, the discrepancy in the surface Mn /Mn molar
ratio evolved with the doping perovskite phase on the surface. On the other hand, the
diversity of metal oxide-support interactions also had an effect on the amount of Mn4+
3
+
species. It can be supposed that during the reaction La could anchor Mn to form
4
+
perovskite exposing more Mn on the surface hetero-structure. Considering other
4
+
3+
hybrid compounds, the surface element Mn /Mn ratios ofCA15-700 coupled with
LaMnO /MnO and of SS15 with the La /δ-MnO were also estimated by
characteristic peak areas of Mn 2p3/2, giving the sequence of GLC-15 (0.56) >
SS15(0.52) > CA15-700 (0.38). Further investigating the XPS results and H -TPR
3
x
2
O
3
2
2
4
+
results, higher content Mn and luxuriant lattice oxygen promoted by synergic effect
can contribute to the low-temperature reducibility and the good ability of deeply
oxidizing toluene.
3
.5. Catalytic stability
The catalytic stability is further evaluated on typical GLC-15 under GHSV=30,000
mLgcatal 1h-1 for four consecutive runs, as shown in Fig. 8. The results obtained
showed that the GLC-15 samples slightly decreased after third run, clearly recovering
the catalytic activity at the fourth run as same as the fresh catalysts. Specifically, the
phenomenon may be attributed to the catalyst activation process, denoting that GLC-
-
1
5 could present the stable catalytic ability.

(b)
(a)
Fig. 8 Toluene conversion vs reaction time over GLC-15 (a) during four consecutive cycles and
(b) under 25 h continuous test.
Total toluene conversion can be approximately attained for all four cycles at trial
o
temperature of 280 C (Fig. S7). Moreover, the durability of GLC-15 was also
o
evaluated at 280 C, manifesting the stable toluene conversion over 98% during 1500
min. More importantly, the GLC-15 catalyst can retain initial crystalline phase after
four time runs or long-term test as confirmed by the XRD spectra, implying the highly
efficient GLC-15 is a promising MnO -based catalyst for eliminating toluene.
2
4
. Conclusion
In summary, in situ assembling the well-structured and small-sized LaMnO
perovskites over δ-MnO has successfully synthesized via a facile one-step protocol,
and samples using the same recipe in GLC-15 has also been synthesized by different
methods. In this GLC-15 nanocatalyst, there were hetero-structure in δ-MnO
substrates loading some LaMnO perovskites. With the hetero-structure, the catalyst
appreciably enhances active phase-carrier interplays, improving the redox abilities,
3
2
2
3
4
+
3+
and modulating the Mn /Mn couples. Such optimized functions can finally impart
o
the good toluene combustion property with the full toluene oxidation at 275 C and

o
T
90 at 258 C. the catalyst has shown stable cyclicality under the certain condition, as
well as the structural tenability. The nanostructure multifarious MnO -based materials
2
are promising catalyst that can be extended to degrade alkane, alcohols, aldehydes and
benzene series.
Acknowledgements
This work was financially supported by the National Natural Science Foundation
of China (21476145). The authors greatly appreciate the technical support from
Institute of New Energy and Low Carbon Technology.
References
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Table 1. Detailed data of specific surface area (SBET), crystallite size and the product yield.
MnO
2
La0.88MnO2.82
Average pore
diameter
SSA* Pore volume
Yield§
(%)
sample
La:Mn crystalline crystalline size
2
3 −1
(
m /g)
(cm g )
size (nm)#
(nm)#
(
nm)
8.7
9.1
GLC-3
GLC-5
1:3
1:5
N.A.
45.1
44.3
39.1
17.8
16.3
16.6
N.A.
N.A.
65.0
53.9
14.3
13.0
11.6
10.6
N.A.
24.9
N.A.
13.2
10.5
16.6
0.028
0.024
0.043
0.060
0.075
0.089
0.036
0.150
0.167
45.0
47.0
54.1
55.2
54.2
48.6
50.0
N.A.
N.A.
GLC-8
1:8
10.3
9.8
GLC-11
GLC-15
GLC-18
SS15
1:11
1:15
1:18
1:15
1:15
1:15
24..5
23.5
32.1
16.1
40.9
57.9
13.0
11.1
9.0
CA15-700
CA15-500
14.6
11.6

Table 2. Physical and chemical combined results of H
2
-TPR, surface elemental compositions,
and toluene combustion activity.
o
4
+
3+ Catalytic activity ( C)
First peak of
Mn /Mn
Sample
Olatt/Oads
o
TPR ( C)
(2p3/2)
T
50
T
90
GLC-3
GLC-5
323
312
303
297
284
284
288
509
296
0.26
N.A.
0.62
N.A.
1.35
1.24
0.70
0.63
N.A.
0.32
> 330
277
258
254
248
252
262
309
279
/
N.A.
0.47
N.A.
0.56
0.53
0.51
0.38
N.A.
303
290
269
258
260
295
328
316
GLC-8
GLC-11
GLC-15
GLC-18
SS15
CA15-700
CA15-500