Highly efficient Pd catalysts loaded on La1?xSrxMnO3 perovskite nanotube support for low-temperature toluene oxidation

Article abstract of DOI:10.1016/j.jallcom.2021.159575

Perovskite oxides are among the most promising thermal catalysts for catalytic oxidation of volatile organic compounds due to their specific structures, low cost, and good activities in catalytic oxidation. In this work, we developed a nanocasting array method for preparing nanotube perovskite oxides (N-LMO) with high surface areas. N-L_1?xS_xMO (x = 0.2, 0.5, 0.8) samples were prepared by partly replacing lanthanum ions with strontium. Meanwhile, this process will increase the Mn⁴⁺/Mn³⁺ ratio. The precious metal palladium was loaded on the N-L_0.8S_0.2MO surface and used for low-temperature (T₉₀ = 166 °C) degradation under the feed gas (50 ppm toluene) stream containing 5 vol% of water. This was achieved due to the interaction between Pd and N-L_0.8S_0.2MO. Cycling experiments verify that the samples of 2 wt% Pd@N-L_0.8S_0.2MO has good stability. The excellent catalytic performance makes our strategy an effective method for developing high-efficiency catalysts for low-temperature catalytic oxidation.

Full text of DOI:10.1016/j.jallcom.2021.159575

Journal of Alloys and Compounds 871 (2021) 159575
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Highly efficient Pd catalysts loaded on La Sr MnO perovskite nanotube
1
−x
x
3
support for low-temperature toluene oxidation
Xunxun Li, Dongyun Chen, Najun Li, Qingfeng Xu, Hua Li, Jinghui He, Jianmei Lu⁎
Collaborative Innovation Center of Suzhou Nano Science and Technology, College of Chemistry Chemical Engineering and Materials Science Soochow University,
199 Ren’ai Road, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Perovskite oxides are among the most promising thermal catalysts for catalytic oxidation of volatile organic
compounds due to their specific structures, low cost, and good activities in catalytic oxidation. In this work,
we developed a nanocasting array method for preparing nanotube perovskite oxides (N-LMO) with high
Received 5 January 2021
Received in revised form 3 March 2021
Accepted 16 March 2021
Available online 18 March 2021
surface areas. N-L1−x
strontium. Meanwhile, this process will increase the Mn /Mn ratio. The precious metal palladium was
loaded on the N-L0.8 0.2MO surface and used for low-temperature (T90 = 166 °C) degradation under the feed
gas (50 ppm toluene) stream containing 5 vol% of water. This was achieved due to the interaction between
Pd and N-L0.8 0.2MO. Cycling experiments verify that the samples of 2 wt% Pd@N-L0.8 0.2MO has good
x
S MO (x = 0.2, 0.5, 0.8) samples were prepared by partly replacing lanthanum ions with
4+
3+
S
Keywords:
Catalytic oxidation
S
S
Pd/L1-x
x
S MO
Toluene
Introducing strontium
Nanocasting
stability. The excellent catalytic performance makes our strategy an effective method for developing high-
efficiency catalysts for low-temperature catalytic oxidation.
© 2021 Published by Elsevier B.V.
1. Introduction
performance in toluene oxidation (T90 = 224 °C). The main function
of the metal ion at the A position in a lanthanum-based perovskite is
Volatile organic compound (VOC) emissions from industrial
processes cause a range of environmental pollution problems such
as formation of O , secondary organic aerosols, and photochemical
smog [1]. Various techniques, e.g., catalytic photolysis, plasma dis-
charge, adsorption, and catalytic oxidation, have been developed for
decreasing the amounts of toluene and other harmful organic pol-
lutants in the atmosphere [2,3]. Catalytic oxidation can convert VOCs
to stabilize the perovskite structure, whereas the transition-metal
ion at the B position plays a major catalytic role. Replacing the A site
metal with low-valent metal ions affects the valence state of the
transition metal at the B site and generates oxygen vacancies [11,12].
This is therefore an efficient method for improving the catalytic
ability of a perovskite oxide. Some studies have shown that the
substitution of strontium for lanthanum considerably improves the
catalytic performance [13–15]. It has been reported that LSMO has a
positive effect on the catalytic activity in methane oxidation [16].
The catalytic performance mainly depends on the Mn4+/Mn ratio
and the specific surface area.
3
2 2
to CO and H O, without secondary pollution, and has been ex-
tensively studied [4–6]. Our aim therefore was to develop an effi-
cient, environmentally friendly, and energy-saving catalyst for the
catalytic oxidation of toluene.
3+
In recent years, lanthanum-based perovskites (LaBO
3
) have at-
The high temperatures used for perovskite oxide synthesis decrease
the specific surface area, and this affects the catalytic performance. This
problem can be solved by using nanocasting in the synthetic process.
Use of a material with a large specific surface area and pore size as a
hard template, e.g. poly(methyl methacrylate) (PMMA) [17], KIT-6 [18],
or SBA-15 [19,20] enables synthesis of perovskites with fixed
morphologies and high specific surface areas by casting and removing
the template. Li et al. studied the catalytic oxidation of methane over
tracted much attention in the field of catalytic oxidation due to their
low cost, structural stability, good thermal stability, and good redox
capabilities [7–9]. The physical and chemical properties of per-
ovskites are strongly affected by their preparation method. Zhang
et al. compared the performances of LaMnO
by citrate sol–gel, glycine combustion, and coprecipitation methods
in the catalytic oxidation of toluene [10]. They found that LaMnO
3
perovskites prepared
3
synthesized by a citrate sol–gel method gave the best catalytic
3
three-dimensionally ordered La0.6Sr0.4MnO prepared by using PMMA
microspheres as a hard template [21]. The specific surface area of the
2
−1
catalyst was 32–40 m g , and it showed good catalytic activity. Freddy
et al. synthesized a LaNiO perovskite with a high specific surface area
150 m g ) by nanocasting with SBA-15 as a hard template [20]. Use of
⁎
3
Corresponding author.
E-mail addresses: dychen@suda.edu.cn (D. Chen), lujm@suda.edu.cn (J. Lu).
2
−1
(
https://doi.org/10.1016/j.jallcom.2021.159575
0925-8388/© 2021 Published by Elsevier B.V.

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
SBA-15, which has a high specific surface area and regular channel
structure, as a hard template effectively disperses perovskite particles
and decreases the particle size.
Precious-metal catalysts give excellent performances but their high
cost and tendency to agglomerate limit their use. Recently, supported
precious-metal catalysts have received widespread attention because
of their excellent catalytic performances in VOC degradation. Loading
precious metals on the surface of a support can significantly increase
the catalytic activity and this has become an effective method for
improving catalytic performances [22,23]. A Pd-based catalyst was
(La(NO
3
)
3
.6H
2
O, 99.99% metals basis), Pluronic P123 (PEG-PPG-PEG),
citric acid and strontium nitrate (Sr(NO
Sigma-Alorich. Manganese (Ⅱ) nitrate 50% aqueous solution (Mn
(NO ), Ethanol absolute, TEOS hydrochloric acid, and sodium tet-
rachloropalladate(II) (Na PdCl ) was brought from Tokyo Chemical
3 2
) ) were purchased from
3 2
)
2
4
Industry Co. Ltd. Toluene was purchased from Messer Air Liquide,
Ltd. (China).
2
.2. Synthesis of ordered mesoporous SBA-15
dispersed on UiO-66 [24,25], SBA-15 [26], and ZrO
precious-metal accumulation. Ren et al. prepared Pd–Ce/γ-Al
alysts and investigated Pd/Ce bimetal interactions in toluene catalytic
oxidation [22]. However, there have been few reports of palladium
loading on mesoporous perovskite oxides. The interactions between
precious metals and oxide supports need further study.
2
[27] to prevent
cat-
The hard template (ordered mesoporous SBA-15) were firstly
synthesized. 4 g of P123 was dissolved in 65 mL distilled water and
0 mL hydrochloric acid (37 wt%) under magnetic stirring at room
2 3
O
1
temperature. Then P123 was used as a template structure directing
agent (SDA). When the solution becomes clear, 4.16 g of TEOS was
added drop by drop under constant stirring. The mixed solution was
then continuously stirred for 24 h in a water bath at 38 °C. Then, they
were transferred to Teflon-lined stainless steel autoclave and main-
tained at 110 °C for 24 h. After cooling to room temperature, the pro-
duct was filtered and dried at 60 °C overnight. Finally, the white
powder was calcined at 550 °C for 6 h with a ramping rate of 5 °C/min
in the muffle furnace to remove the SDA.
We investigated the effects of Sr doping and the interactions
between Pd and lanthanum-based perovskites on the catalytic oxi-
x
dation of toluene by preparing a series of Pd/L1−xS MO (x = 0.2, 0.5,
0
.8) catalysts by nanocasting with SBA-15 as a hard template. A
schematic diagram of the preparation is shown in Scheme 1. The
structures and physicochemical properties of the synthesized cata-
lysts were investigated by using various techniques such as X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM), high-resolution (HR)-TEM, X-ray pho-
2
.3. Synthesis of mesoporous LaMnO
3
and La0.8Sr0.2MnO
3
2 2
toelectron spectroscopy (XPS), H -TPR, O -TPD, Fourier-transform
infrared (FT-IR) spectroscopy, and Brunauer–Emmett–Teller (BET)
analysis. The nanotube perovskite oxide (N-LMO) sample N-
4
mmol of La (NO .6H O, 4 mmol of Mn(NO )
3
)
3
2
3 2
were dissolved
in 5 mL of distilled water and 15 mL of ethanol absolute to obtain a
homogeneous solution. Then 4 mmol of citric acid acting as a com-
plexation were slowly added into the above solution under magnetic
stirring at room temperature for 1 h. 1 g of SBA-15 was dispersed in
the total solution under constantly stirring and maintained for 12 h.
In order to obtain the powder, this solution was dried in an oven for
3
La0.8Sr0.2MnO gave an excellent catalytic performance due to its
4
+
3+
high Mn /Mn ratio and abundant lattice oxygens. Precious-metal
loading gave 2 wt% Pd@N-La0.8Sr0.2MnO , which had excellent cat-
3
alytic ability, mainly because of interactions between palladium and
the perovskite oxide.
6
h at 80 °C. After being fully grinded, the precursor was calcined in
the muffle furnace at 500 °C for 5 h, then they were heated up to
700 °C for 8 h. Throughout the heating process, the heating rate was
2. Experimental section
5
°C/min. Finally, the SBA-15 template was removed by being dis-
2.1. Chemicals and materials
persed in 2 M NaOH at 70 °C for 6 h. The final product was denoted
as N-LMO. Simultaneously, we adjusted the ratio of La/Sr to prepare
All chemicals are analytically pure and can be used directly
x 3 x
La1−xSr MnO (x = 0.2, 0.5, 0.8) and named as N-L1−xS MO, which
without further processing. Lanthanum nitrate hexahydrate
was described in the support information.
Scheme 1. Schematic diagram of synthesis of Pd/LSMO nanocomposites.
2

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
Fig. 1. XRD patterns of prepared catalysts: (a) XRD patterns of a N-LMO, b N-L0.8
S
0.2MO, c L0.5
S
0.5MO, and d L0.2
S
0.8MO and (b) XRD patterns of samples with various palladium
loadings.
For comparison, we synthesized bulk LMO using a traditional
citric acid sol-gel method. The detailed synthesis steps were dis-
played in the support information.
recorded as C. The conversion rate of toluene can be calculated as
follows:
C0
C0
C
=
× 100%
The feed stream introduces 5 vol%H
(1)
2
.4. Synthesis loading precious of mesoporous 1 wt% Pd@
La0.8Sr0.2MnO , 2 wt% Pd@La0.8Sr0.2MnO , 4 wt% Pd@La0.8Sr0.2MnO
wt% Pd@La0.8Sr0.2MnO
2
O through a water saturator
3
3
3
,
to study the effect of water vapor on the catalytic activity of the
supported precious metal catalyst, and calculate it by the following
6
3
formula: ψ = φP
v
/P, where ψ is the water vapor content and φ is the
In a typical preparation process, sodium tetrachloropalladate(II)
relative humidity, P
v
is the saturated vapor pressure of water at a
is deposited on the surface of the catalyst by immersion method, and
then they was reduced in a hydrogen atmosphere. The preparation of
the catalyst loaded with 1% palladium would be introduced in detail.
certain temperature, and P is the standard atmospheric pressure.
Turnover frequencies (TOF) was calculated based on the fol-
lowing equation:
First, we put 120 mg of catalyst N-La0.8Sr0.2MnO
After being uniformly dispersed by ultrasound，0.83 mL sodium
tetrachloropalladate(II) (Na PdCl ) (1 g/250 mL) was added to the
solution at 60 °C for 8 h. Then, the black powder was obtained.
Finally, they were calcined in 10% H /N atmosphere at 250 °C for
h. The sample obtained is named 1 wt% Pd@La0.8Sr0.2MnO . We
3
in 15 mL of ethanol.
MPd
1
TOFpd(S ) = Xtoluene Ftoluene
2
4
mCatXpdDPd
(2)
Where Xtoluene denotes the toluene conversion at certain at certain
temperature; Ftoluene is the toluene flow rate in unit of mol s−1; mCat
is the catalyst amount. TOFPd is the turnover frequency based on Pd
dispersion; MPd denotes the molar weight of Pd (106.4 g mol ); XPd
is the weight fraction of Pd in the sample as determined by ICP-OES.
2
2
2
3
adjusted the volume of sodium chloroplatinate to achieve the pre-
paration of different loading ratios.
−1
2.5. Characterization
3
. Results and discussion
All of the catalysts were characterized by a series of techniques.
More specifically, X-ray diffraction (X′ Pert-Pro MPD) was used to
analyze structure of the catalysts. The scanning electron microscopy
3
.1. Crystal-phase composition
XRD was used to determine the structures of the perovskite oxide
(
SEM), the transmission electron microscopic (TEM) and high-
catalysts. Fig. 1a shows the diffraction patterns of N-La1−xSr
ovskite oxide catalysts with x = 0, 0.2, 0.5, and 0.8. The XRD pattern of
LaMnO is shown in the Supporting Information (Fig. S1). All the dif-
fraction peaks in the catalyst XRD patterns exactly correspond to those
of the standard LaMnO sample (PDF#75-0440) [28]. This indicates that
the N-La1−xSr MnO samples retained the perovskite structure during
the Sr doping process. However, the diffraction peaks of SrCO (JCPDS
5-0418) are present in the N-La0.2Sr0.8MnO pattern, which indicates
x 3
MnO per-
resolution transmission electron microscopic (HRTEM) were applied
to observe the surface morphology of the perovskite catalyst. The
data of specific surface area and pore size distribution were obtained
3
from N
XPS, ESCALAB 250Xi) was used to analyze valence state of the
surface elements of the catalyst. The actual content of palladium is
determined by ICP-AES (Varian 710-ES). H -TPR was performed to
observe the reducibility of catalysts. O -TPD was used to analyze
2
adsorption–desorption. X-ray photoelectron spectroscopy
3
(
x
3
3
2
0
3
2
that the introduction of excessive strontium into the lanthanum man-
ganate will result in the formation of strontium carbonate [29]. The
peak corresponding to the C═O group vibration is present in the FT-IR
spectrum of N-La0.2Sr0.8MnO (Fig. S2). This is attributed to SrCO for-
3 3
mation due to the high proportion of Sr. The infrared spectrum can also
prove that most of the SBA-15 template was removed by etching with
oxygen species. Fourier transform Infrared (FT-IR) was utilized for
analyzing chemical bonds. The toluene concentration was monitored
online by a gas chromatograph (GC-2010 Plus) equipped with a mass
spectrometer (MS-QP2020).
2
.6. Measurement of catalytic activity
sodium hydroxide. With increasing Sr content, the N-La1−xSr
perovskite pattern showed a diffraction peak corresponding to the
LaMnO orthorhombic structure, with a slight shift toward a smaller 2θ
value. This indicates expansion of the N-La1−xSr MnO cell volume [30].
The N-La1−xSr MnO peak intensities are lower than those of bulk
LaMnO , which suggests that the nanoparticles obtained by the nano-
casting method are smaller than those in bulk LaMnO [19]. The in-
x 3
MnO
The catalytic performance of the catalyst was measured by the
3
catalytic oxidation of toluene. 50 mg of catalyst was packed into a
U-shaped quartz reaction tube, and the degradation ability of the
catalyst was tested through a fixed bed reactor. The initial gas is
x
3
x
3
3
5
(
0 ppm of toluene in the air at
WHSV = 36,000 mL/(h g)), and its concentration is recorded as C
addition, the toluene concentration through the reactor was
a
rate of 30 mL/min
3
0
. In
tensity of the diffraction peak was further weakened with increasing Sr
content. This indicates that the introduction of Sr limits nanoparticle
3

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
Table 1
BET surface areas, pore volumes, La/Sr ratios, Mn4+/Mn3+ and Olatt/Oads
.
2
3
La/Sra
Mn4+/Mn3+b
O
latt/Oadsb
Sample
BET surface area (m /g)
Pore volume (cm /g)
BJH pore size (nm)
LMO
N-LMO
N-L0.8
N-L0.5
N-L0.2
21
142
244
239
205.81
0.17
0.46
0.64
0.62
0.52
33.66
10.07
6.58
6.25
5.87
–
–
4.70
1.09
0.28
0.51
0.44
0.92
0.89
0.84
0.77
0.90
1.09
0.65
0.31
S
S
S
0.2MO
0.5MO
0.8MO
a
Determined by the ICP-AES technique.
Determined by the XPS.
b
growth [29]. The powder XRD patterns of N-La0.8Sr0.2MnO
3
with various
between the reactant molecules and the catalyst, thereby improving the
catalytic activity of the catalytic oxidation of toluene.
amounts of supported Pd are shown in Fig. 1b. The Fig. 1b clearly shows
that loading Pd on the perovskite oxide did not lead to the appearance
of new diffraction peaks. This indicates that the formed Pd nano-
particles have low content, uniform loading and small size. The BET data
x 3
The structures and element distributions of Pd@N-La1−xSr MnO
were further explored by using high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM), HR-TEM, and energy-
dispersive X-ray (EDX) mapping. As shown in Fig. 3d, clear lattice fringes
3
for mesoporous N-La0.8Sr0.2MnO with various Pd loadings are sum-
marized in Table 1 and S1.
of Pd@N-La1−xSr
and 0.230 nm interplanar crystal spacings correspond to the (110) and
111) crystal faces of La0.8Sr0.2MnO and Pd. No SrCO lattice fringes
were observed. Palladium nanoparticles loaded with different propor-
tions have different sizes. As shown in Fig. S7, Pd nanoparticles are
dispersed on the surface of ordered mesoporous perovskite. It could be
found that the mean size of Pd nanoparticles are gradually increasing
and the data displayed in Fig. S8. The size of Pd nanoparticles values are
x 3
MnO can be observed in the HR-TEM image; the 0.274
(
3
3
3.2. Structure and morphology determination
The morphologies and structures of SBA-15 and the N-La1−xSr
x
MnO
3
catalysts were investigated by SEM and TEM (Fig. 2a-f). Fig. 2a and d
shows the SBA-15 mesoporous nanotube array morphology. The worm-
like morphology and view parallel to the axis of the cylindrical channel
can be clearly seen. Images of the N-LMO and N-L0.8
obtained by nanocasting are shown in Fig. 2b, c, e, and f. It can be seen
from Fig. 2 that the morphologies of N-LMO and N-L0.8 0.2MO corre-
spond exactly to the morphology of SBA-15, and they all have an or-
dered nanotube structure. This indicates that lanthanum manganate
perfectly replicates the ordered mesoporous structure of SBA-15. The
morphology of unmodified LMO is shown in Fig. S3. We can clearly see
that the ordered nanotube perovskite prepared by the nano-casting
method can greatly reduce the accumulation of perovskite. The
perovskite oxide of the mesoporous nanotube array can provide a good
support for dispersing precious metals. This promotes the contact
3.79, 3.68, 4.04 and 5.62 nm, respectively. The HAADF-STEM image in
S0.2MO catalysts
Fig. 3b confirms that the morphology consists of ordered mesoporous
nanotube arrays. In addition, STEM EDX mapping confirmed that the
elements La, Mn, O, Sr and Pd were present and uniformly distributed.
From the above results, we can draw the conclusion that the ordered
S
x 3
mesoporous Pd@N-La1−xSr MnO was successfully synthesized.
3.3. Surface compositions, metal oxidation states, and oxygen species
XPS is used to identify the elements and chemical valences on
solid-sample surfaces. The XPS spectra of LMO, N-LMO, and N-
Fig. 2. SEM and TEM images of (a, d) SBA-15, (b, e) LMO and (c, f) L0.8S0.2MO.
4

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
Fig. 3. (a) SEM, (b) HAADF-STEM, (c) TEM, (d) HR-TEM, and (e) STEM- HAADF images of 2 wt% Pd@N-La0.8Sr0.2MnO
3
; STEM EDX mapping images of (f) La, (g) Mn, (h) O, (i) Sr and
(
k) Pd.
La1−xSr
x
MnO
3
(x = 0.2, 0.5, 0.8) are shown in Fig. S4. The spectra of
increased the oxygen species content. The Olatt/Oads ratio of N-
La0.8Sr0.2MnO (1.09) was greater than that of the N-LMO perovskite
(0.90). This indicates that the oxygen mobility of N-La0.8Sr0.2MnO is
higher. Lattice oxygen migration directly generates oxygen vacancies.
This is conducive to adsorption of gaseous oxygen, which supplements
the strontium-containing catalysts (Fig. S4a) clearly show the pre-
sence of strontium, and the strontium peak intensity increases with
increasing amount of doped strontium. The survey XPS spectra of
the catalysts with supported Pd species are shown in Fig. 4a-d.
Fig. 4b shows the XPS spectra of the catalysts with various Pd
loadings (1, 2, 4 and 6 wt%). The Pd 3d spectra (Fig. 4b), contain four
3
3
the lattice oxygen. The Olatt/Oads ratios of N-La0.5Sr0.5MnO
La0.2Sr0.8MnO are 0.65 and 0.31, respectively,which are significantly
lower N-La0.8Sr0.2MnO . This indicates that high doping with Sr is not
3
and N-
3
sub-peaks from the perovskite-supported Pd. The peaks at 335.4 and
3
0
3
3
40.8 eV correspond to catalyst surface Pd species, and the peaks at
conducive to the formation of perovskite crystals. Lattice defects in the
perovskite oxide increase the Sr content, which corresponds to weak-
ening of the XRD diffraction peak.
It can be seen from Fig. 4c that the Olatt/Oads ratio is increasing. In
order to investigate the surface adsorption and lattice oxygen spe-
37.0 and 342.2 eV correspond to catalyst surface Pd2+ species [31]. It
clearly shows that palladium mainly exists in the form of zero-valent
palladium. The Mn /Mn ratio decreased with increasing palla-
dium loading. This is attributed to interactions between Pd and
4
+
3+
N-La0.8Sr0.2MnO
tion [16]:
3
, and it can be represented by the following equa-
cies, O
orbed O
oxygen species on the surface of catalysts (Oads) and desorbed within
50–800 °C were denoted as lattice oxygen species (Olatt). We can
2
-TPD was performed over the three catalysts (Fig S10). Des-
2
species below 450 °C were denoted as surface adsorption
Pd0 + Mn4+ Pd + + Mn3+
(3)
4
observe from the Fig. S10 that oxygen species mainly exist in the
catalyst in the form of lattice oxygen.
The Mn 2p XPS spectra are shown in Fig. 4d and S4c. The Mn 2p
peaks at 641.3 and 642.8 eV are attributed to Mn and Mn , re-
spectively [32]. The surface Mn4+/Mn3+ ratio clearly increased after
3+
4+
These results show that the manganese valence, type of oxygen
species, and interactions between the metal and the support are
closely related to the catalytic performance. Scheme 2 shows the
changes in the perovskite structure that are caused by strontium
doping and loading with precious-metal species.
Sr incorporation. This indicates that replacement of lanthanum with
low-valent strontium leads to an increase in the manganese valence
to maintain electrical neutrality. The Mn4+ ion is strongly oxidizing
and is conducive to redox cycles; it plays a vital role in improving the
catalytic performance in toluene degradation [33]. N-La0.8Sr0.2MnO
3
has the highest Mn4 /Mn ratio; this is one of the reasons why it
+
3+
3.4. Surface areas and pore structures
gives the best performance. Palladium loading on N-La0.8Sr0.2MnO
3
3
+
increased the Mn content. This indicates that the interactions be-
tween the metal and the support are beneficial to the catalytic
reaction.
The N
tributions for LMO, N-LMO, and N-La1−xSr
shown in Fig. 5. All the samples give a type IV isotherm with a type
hysteresis loop located in the relative pressure range 0.6–1.0 [35].
2
adsorption–desorption isotherms and the pore size dis-
x
MnO (x = 0.2, 0.5, 0.8) are
3
The O 1s spectra of the samples, which are shown in Fig. 4c and S4d,
were used to examine the oxygen storage capacity and oxygen mobility
of the perovskite. The O 1s peaks at 529.4, 531.1 and 532.5 eV are at-
tributed to lattice oxygen species (Olatt), surface adsorbed oxygen spe-
cies (Oads), and molecular water or hydroxyl species adsorbed on the
surface [34]. It should be noted that alkali etching and Sr doping
H
3
This suggests that the catalysts prepared by nanocasting and alkali
etching were mesoporous. Morphological modification increased the
2
−1
2 −1
surface area seven-fold, from 21.03 m g (LMO) to 142.95 m g
(N-LMO). Introduction of strontium enlarged the hysteresis loop,
which indicates significant increases in the mesoporosity and
5

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
3 3 3 3
Fig. 4. XPS spectra of 1 wt% Pd@N-La0.8Sr0.2MnO , 2 wt% Pd@N-La0.8Sr0.2MnO , 4 wt% Pd@N-La0.8Sr0.2MnO , 6 wt% Pd@N-La0.8Sr0.2MnO : (a) full, (b) Pd 3d, (c) O 1s, and (d) Mn 2p
spectra. ((1, 2, 4, 6) wt% Pd in the figure represents (1, 2, 4, 6) wt% Pd@N-La0.8Sr0.2MnO ).
3
specific surface area. This shows that the introduction of Sr limits the
growth of perovskite grains. This result is consistent with the
XRD data.
formation of by-products. Fig. 6a shows the toluene conversions over
the perovskite samples without a loaded precious metal. The cata-
lytic activity of the LMO sample was lower than those of the N-LMO
samples. This indicates that the surface area is one of the factors that
affect the catalytic performance. The replacement of lanthanum by
strontium significantly improves the catalytic performance. This is
attributed to an increase in the manganese valence. And according to
x 3
The Barrett–Joyner–Halenda (BJH) pore size of N-La1−xSr MnO
3
−1
was about 810 nm, and the pore volume was 0.45–0.65 cm g . The
BET surface areas, pore volumes, and BJH pore sizes of the samples
are summarized in Table 1. The porous structure and high surface
area facilitate improvement of the catalytic performance in toluene
degradation by promoting contact between the reactant molecules
the results of XPS, the samples of N-L0.8
S
0.2MO has the highest ratio
of Mn4+/Mn , which displayed in the Table 1. It has been reported in
3+
4+
and the catalyst. The N-L0.8
S
0.2MO sample has the largest surface
the literature that a high content of Mn promotes catalytic oxi-
dation [33,36,37]. Fig. 6b shows that loading precious metals on the
perovskite further improves the catalytic activity. This is mainly at-
tributed to interactions between precious metals and perovskites.
The decrease in the Mn4+ content and increase in the Pd content
observed from the XPS results proves that such interactions occur.
The differences between the catalytic performances of samples with
and without a loaded precious metal can be further clarified by com-
paring the values of T50, T90, and T100 (representing the reaction tem-
peratures for 50%, 90% and 100% toluene conversion); these are listed in
area and pore volume. This provides good conditions for the contact
between the toluene gas and the catalyst, and facilitates the deso-
rption of the product.
2+
3.5. Catalytic performance
The catalytic performances of all the synthesized catalysts in
toluene catalytic oxidation were investigated. The catalysts enabled
complete conversion of toluene to H O and CO without the
2
2
Scheme 2. Perovskite oxide structural changes.
6

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
Fig. 5. Nitrogen adsorption–desorption isotherms and corresponding pore diameter distribution curves for LMO, N-LMO, N-L0.8S0.2MO, N-L0.5S0.5MO and N-L0.2S0.8MO.
Table S2. Meanwhile, compared with other catalysts, we consider 2 wt%
Pd@N-L0.8 0.2MO be a promising catalyst for the degradation of toluene
Table S3). The T90 values for LMO, N-LMO, N-L0.8 0.2MO, and 2 wt% Pd@
0.2MO are 220, 209, 177 and 166 °C, respectively (Fig. 6d). In addi-
tion, the catalyst of 2 wt% Pd@L0.8 0.2MO still maintains a conversion
rate of over 90% after three cycles at 170 °C，which are showed in Fig.
S6. We tested the stability of the catalyst to oxidation toluene. We found
that the catalyst has good stability (Fig. 6c). The toluene conversion rate
remains constant in the long-term catalytic oxidation of toluene. A good
performance depends on three factors: a high surface area, which im-
proves contact between the reactants and the catalyst; the introduction
of strontium, which increases the manganese valence; and interactions
between palladium and the perovskite. It has been reported in the lit-
erature that interactions between the metal and the support improve
loaded onto the N-L0.8
lower temperatures. The result suggests the presence of a strong in-
teraction between Pd nanoparticles and N-L0.8 0.2MO support in 2 wt%
Pd@N-L0.8 0.2MO.
A proposed mechanism for the enhanced catalytic activities
achieved with N-L0.8 0.2MO and 2 wt% Pd@L0.8 0.2MO is shown in
Scheme 3. In the absence of a precious metal, toluene molecules are
first adsorbed on the perovskite surface and converted to CO and H
under the effect of surface lattice oxygens and manganese. In this
process, the surface lattice oxygens are consumed and oxygen va-
cancies are generated. Internal lattice oxygens migrate to the surface
and gaseous oxygen can easily supplement the lattice oxygens at low
temperatures [40]. After Pd loading, the catalytic reaction is mainly
promoted by the interactions between Pd and the manganese at the B
sites in the perovskite oxide. It is also adsorbed by toluene and is
converted to a pollution-free product through catalytic oxidation. The
interactions between precious metals and perovskites significantly
improve the catalytic performance compared with those of non-pre-
cious-metal supported catalysts. The interactions between precious
metals and perovskites are reflected by XPS data.
S0.2MO surface, the reduction peaks shifted to
S
(
L
S
S
0.8
S
S
S
S
S
2
2
O
the catalytic activity and selectivity [27,38,39]. The H
was performed to determine the reducibility of the catalysts. The H
TPR profiles of the two samples are exhibited in Fig. S9, The reduction
reaction occurred over the N-L0.8 0.2MO with following features: the
reduction peak at 326 °C is attributed to the reduction of Mn to Mn
2
-TPR experiment
2
-
S
4+
3+
,
as well as the removal of overstoichiometric oxygen. When Pd was
Fig. 6. (a) Toluene conversions as function of reaction temperature over LMO, N-LMO, N-L0.8
catalysts with various Pd loadings. Reaction conditions: 50 ppm toluene, weight hourly space velocity = 36,000 mL h
S
0.2MO, N-L0.5
S
0.5MO, and N-L0.2
S
0.8MO (b) Toluene conversions over Pd@LSMO
−1
−1
g , under the feed gas (50 ppm toluene) stream containing
5
vol% of water. (c) Stability test of catalyst 2 wt% Pd@L0.8
S
0.2MO. (d) The temperature difference of each catalyst when the toluene conversion rate reaches 90%.
7

X. Li, D. Chen, N. Li et al.
Journal of Alloys and Compounds 871 (2021) 159575
Scheme 3. Proposed mechanism for catalytic oxidation of toluene on LSMO and Pd@LSMO.
4. Conclusions
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