Acid treated Sr-substituted LaCoO3 perovskite for toluene oxidation

Article abstract of DOI:10.1016/j.catcom.2021.106314

For A-site substituted AA'BO₃ perovskite, the doped A' element is easy to migrate to the surface and form segregation of A'O_x, which could partially block active B sites and thus reduce catalytic activity. In the present work a La_0.8Sr_0.2CoO_3-δ perovskite-type solid was first prepared and then treated with acetic acid to eliminate SrO segregation on its surface. The obtained catalyst (LCSO-a) possessed a significantly enhanced catalytic activity for toluene oxidation in the 220–260 °C range. This promotion effect might be attributed to the increase in the concentration of active surface oxygen species and the reduction in their binding strength. In addition, the LCSO-a catalyst showed very good stability and water resistance.

Full text of DOI:10.1016/j.catcom.2021.106314

Catalysis Communications 155 (2021) 106314
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Catalysis Communications
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Short communication
Acid treated Sr-substituted LaCoO3 perovskite for toluene oxidation
Yonghui Wei*, Lei Ni, Minxia Li, Jili Zhao
College of Chemistry and Biology, Beihua University, Jilin 132013, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
For A-site substituted AA’BO3 perovskite, the doped A’ element is easy to migrate to the surface and form
segregation of A’Ox, which could partially block active B sites and thus reduce catalytic activity. In the present
work a La0.8Sr0.2CoO3-δ perovskite-type solid was first prepared and then treated with acetic acid to eliminate
SrO segregation on its surface. The obtained catalyst (LCSO-a) possessed a significantly enhanced catalytic ac-
tivity for toluene oxidation in the 220–260 ◦C range. This promotion effect might be attributed to the increase in
the concentration of active surface oxygen species and the reduction in their binding strength. In addition, the
LCSO-a catalyst showed very good stability and water resistance.
Toluene oxidation
Perovskite
Sr segregation
Acid treatment
1. Introduction
et al. [22] prepared LaMnO3 perovskites and Sr2+- and Cu2+-doped
perovskites with enhanced catalytic activity for phenol oxidation. On
As a derivative of benzene and one of typical volatile organic com-
pounds (VOCs), toluene is a commonly used solvent in the chemical
industry and a significant contributor to the formation of photochemical
smog [1–3]. Among the current technologies for VOCs elimination, such
as toluene, catalytic oxidation has been considered as one of the most
effective due to its low operation temperature, high purification effi-
ciency, recoverable heat, and no secondary pollution [4–8]. Traditional
catalysts of noble metals usually exhibit higher catalytic activity
compared with those of metal oxides. However, their high cost and
significant poisoning tendency limit their further considerations for
VOCs catalytic applications [9–11]. Therefore, the development of new
and efficient catalysts is a research hotspot in VOCs emission control
technology.
the other hand, it has been recently proposed that doped A’ cation may
partially migrate from the bulk to the surface of perovskite (surface
segregation) [23,24]. Shao-Horn and co-workers [23] found that Sr
segregation emerged and the SrO/La0.8Sr0.2CoO3-δ structure was formed
when Sr was doped into the LaCoO3. B site (transition metal element) is
considered as the active site of ABO3 in VOCs oxidation [25–27], and Sr
segregation on the surface of La0.8Sr0.2CoO3-δ would partially cover
active B sites, and therefore reducing catalytic activity.
The A-site doping in perovskites brings two aspects of influencing the
VOCs oxidation. In one hand, the valence of B site is tuned, or the
mobility of active surface oxygen is enhanced, which both benefit the
improvement of catalytic activity. On the other hand, the doped A’ site is
easy to migrate to the surface and form A’Ox segregation, which could
partially block active B sites and thus deteriorate the catalytic activity.
The synthesis temperature is one of the key factors that influence the
formation of A’Ox segregation [24]. Usually, the negative effect is
weaker than the positive effect, and the catalytic performance is
enhanced after A-site doping. It appears to the best of our knowledge
that not so much attention was given to the negative effect in the
literature.
Perovskites, with the general formula ABO3, are considered prom-
ising candidates for replacing noble metals in several heterogeneous
catalytic applications due to their flexible composition, good redox
properties, superior thermal stability, and relatively low price [12–15].
However, the catalytic performance of ABO3 is usually not satisfactory
in the case of VOCs oxidation reactions. Many methods were used to
improve their activities in recent years [16–20]. The most common
approach is to substitute A site with a lower valence A’ cation to tune the
valence of B site or enhance the mobility of active surface oxygen spe-
cies. Weng et al. [21] synthesized Ca2+ and Mg2+ modified LaCoO3
perovskites and found that the introduction of Ca2+ and Mg2+ were
beneficial for the toluene oxidation process, which reduced significantly
the apparent activation energy (Ea) of LaCoO3 to only 34 kJ/mol. Chang
Here, LaCoO3 (LCO) perovskite was doped with Sr and La0.8Sr0.2
-
CoO3-δ (LSCO) was formed at a high-temperature (900 ◦C) treatment to
enlarge the negative effect. The La0.8Sr0.2CoO3-δ showed lower catalytic
activity than LaCoO3 in toluene oxidation. Subsequently, a selective
method for the removal of surface SrO was adopted (Scheme 1). The
obtained catalyst (LSCO-a) showed a remarkably improved catalytic
* Corresponding author.
E-mail address: bhhxwyh@163.com (Y. Wei).
https://doi.org/10.1016/j.catcom.2021.106314
Received 16 November 2020; Received in revised form 31 March 2021; Accepted 4 April 2021
Available online 7 April 2021
1566-7367/© 2021 The Author(s).
Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Y. Wei et al.
Catalysis Communications 155 (2021) 106314
activity. H2-TPR and XPS were carried out to gather information about
possible reasons for the different catalytic performance of the three
perovskite-type materials investigated for toluene oxidation. It is the
first time the influence of A’Ox segregation and the effective solution to
such a negative effect in VOCs oxidation is investigated to the best of our
knowledge. The results of this work provide an effective way for the A-
site doped perovskites materials to show their intrinsic catalytic activ-
ities in VOCs oxidation.
hydrogen temperature-programmed reduction (H2-TPR) techniques,
and N2 physical adsorption measurements (77 K) for textural analysis.
Details of the characterization procedures are provided in the Electronic
Supporting Information (ESI).
2.4. Catalytic performance evaluation
The catalytic activities (in terms of toluene conversion) of toluene
oxidation were evaluated in a continuous flow fixed-bed quartz micro-
reactor (i.d. = 6.0 mm). The gaseous toluene was generated by passing
2. Experimental
N
2 gas flow through a bottle containing pure toluene chilled in an ice-
2.1. Synthesis of monodisperse PMMA microspheres
water isothermal bath. The feed gas contained 2000 ppm toluene,
20% O2, 3 or 5% H2O (when needed) and N2 (balance gas) at a total flow
rate of 100 mL/min. The catalyst sample used was 0.05 g (40–60 mesh),
giving a GHSV of ~120,000 mL/(g h). The concentration of toluene at
the outlet of microreactor was monitored online by a gas chromatograph
(Agilent 7890A) equipped with a flame ionization detector (FID) and a
thermal conductivity detector (TCD). Further details are described in the
ESI. When we evaluated the effect of GHSV on the catalytic activity for
the LSCO-a sample, the amount of catalyst used was 0.025 g (GHSV =
240,000 mL/(g h) or 0.1 g (GHSV = 60,000 mL/(g h). To test the cat-
alytic stability of the LSCO-a sample, an uninterrupted reaction exper-
iment was performed which involved three cycles (heating up and
cooling down) of catalytic tests at the temperature range of 210–280 ◦C.
The apparent activation energy of the toluene oxidation reaction (Ea, kJ/
mol) was evaluated after conducting kinetic experiments in the tem-
perature range of 210–250 ◦C, where toluene conversions were kept
below 20%. Internal and external mass transport effects were checked
according to the experimental criteria reported [28].
Monodisperse polymethyl methacrylate (PMMA) microspheres were
used as the hard template. At first, 3.0 mmol K2S2O8 was added into a
three-necked flask with 1500 mL deionized water at 70 ◦C under
vigorous stirring until a transparent solution was obtained. The resulting
solution was then degassed by continuously flowing N2 gas. Next, 115
mL methyl methacrylate was poured into the above solution while the
temperature stayed at 70 ◦C. The mixtures were subsequently stirred at
70 ◦C for 1 h until a white suspension was obtained. After cooling to
room T, the suspension was transferred to appropriate bottles for
centrifugation (75 min), and the resulting solid obtained was washed
with abundant deionized water. Then the precipitate was dried by a
water bath at 80 ◦C for 5 h and then at room temperature for 48 h before
ground well in fine powder (sample code PMMA).
2.2. Catalysts preparation
The three-dimensionally ordered macroporous (3DOM) LaCoO3 and
La0.8Sr0.2CoO3-δ samples were prepared by a template method. A certain
amount of La(NO3)3⋅6H2O, Co(NO3)2⋅6H2O, Sr(NO3)2 (only used for
La0.8Sr0.2CoO3-δ sample) metal precursors and deionized water were
mixed under stirring for 1 h. Then ~2 g PMMA template was impreg-
nated with the above solution (thoroughly wetted). Next, the excessive
liquid was filtered and the solid was dried at room temperature for 24 h.
The resulting solid was then calcined as follows: the first calcination step
involved a heating rate of 1 ◦C/min up to 300 ◦C in N2 gas flow, and the
temperature of the solid was kept at 300 ◦C for 3 h. After cooling to room
temperature, the calcination atmosphere was switched to air for the
second calcination step that involved a heating rate of 1 ◦C/min up to
300 ◦C, and a stay at this temperature for 2 h. Subsequently, the solid
was heated up in air to 900 ◦C and held for 4 h. The heating rate was kept
at 1 ◦C/min. The obtained two perovskite materials, undoped and Sr-
doped LaCoO3 were denoted as LCO and LSCO, respectively.
3. Results and discussion
3.1. Catalysts crystal structure
Powder X-ray diffraction (XRD) was used to investigate the crystal
structure of the three perovskite-type materials as shown in Fig. S1 (ESI).
It is found that the XRD peaks of these samples in the 10–80◦ 2θ range
could be well correspond to the standard ABO3 perovskites (refer to PDF
#48–0123 and #46–0704). All these three samples were single-phase
and of rhombohedral structure. Generally, the crystal structure (ortho-
rhombic, rhombohedral or monoclinic) of perovskite is strongly
dependent on the synthesis method. By adopting various preparation
approaches with or without a template, rhombohedral and hexagonal
perovskites could be generated. The structure of LSCO was the same as
that of LCO, implying that Sr-doping did not change the crystal structure
of the perovskite material. After HAC treatment, the intensity of XRD
peaks for the LSCO-a catalyst sample was slightly weaker than that of
LSCO, suggesting that the degree of crystallinity of the former perovskite
decreased after HAC treatment.
LSCO-a sample was prepared by a selective removal of Sr method, in
which the LSCO powder was immersed in diluted acetic acid (HAC)
solution and kept for 10 h. The solid was then filtered, washed and dried
at 60 ◦C overnight. The final product was denoted as LSCO-a.
2.3. Catalysts characterization
3.2. Morphology and texture of perovskites
All the samples were characterized by means of powder X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and
Fig. S2 illustrates the morphological differences of these three
perovskite-type materials. It can be observed (Fig. S2(a)) that the PMMA
Scheme 1. Synthesis route of the LSCO-a catalyst sample.
2

Y. Wei et al.
Catalysis Communications 155 (2021) 106314
spheres show a regular arrangement, whereas the LCO and LSCO solids
possess high-quality chain-like ordered macroporous morphologies
(Fig. S2(b) and S2(c), respectively). The average pore size was ~50 nm,
and the average wall thickness ~22 nm. Fig. S2(d) reveal the change of
the 3DOM structure after HAC treatment. The original smooth surface of
the macropores became rumpled and the well-ordered structure became
a little bit disordered. The surface area results (Table 1) from the N2
adsorption–desorption isotherms and the pore-size distribution of the
three samples (Fig. S3, ESI) were consistent with the information SEM
images provided. The surface area was increased after Sr doping, ca.
from 8.3 m2/g (LCO) to 12.8 m2/g (LSCO), while a further increase to
17.4 m2/g (LSCO-a) after HAC treatment was noted. It is suggested that
the shrinkage in volume of the treated materials could be responsible for
the morphological and surface area changes. Normally, a porous struc-
ture of high surface area is beneficial for the improvement of its catalytic
performance per gram basis due to the increased density of surface
catalytic sites and for faster diffusion rates (increased pore size). Thus,
the LSCO-a sample with the largest specific surface area (BET) and mean
pore size facilitated toluene intra-particle diffusion rates and increased
activity for toluene oxidation.
provided, the surface Co2+/Co3+ atom ratio was practically the same for
the three solid surfaces but not in the case of Oads/Olatt atom ratio. The
latter varies in the 0.79–1.19 range (Table 1). One possible explanation
for this behavior could be the charge imbalance created by the divalent
Sr substitution for the trivalent La. Because the highest stable oxidation
state for cobalt is +3, this charge imbalance can only be neutralized with
O vacancies, which could be associated with the weakly bonded Oads
.
After doping Sr into the LCO, the Oads in the bulk LSCO should had been
increased with the increase of O vacancies formation. But the surface of
LSCO was covered by a large amount of SrO, which restricted the for-
mation of surface Oads. Therefore, the Oads/Olatt ratio was decreased
after LCO doping with Sr. After HAC treatment, surface Sr was removed
to a large extent and the resulting LSCO-a solid caused an increased
value for the Oads/Olatt surface ratio. The increased specific surface area
might have also played a role in the increase of Oads/Olatt ratio after HAC
treatment. The rise in surface active oxygen species concentration,
especially for Oads, might be linked to the enhanced catalytic perfor-
mance of LSCO-a sample for the total oxidation of toluene.
H2-TPR experiments were conducted to investigate the reducibility
of the samples, and their profiles are illustrated in Fig. 2. The hydrogen
consumption over cobalt perovskites generally involves two tempera-
ture regions, i.e., at 300–500 ◦C corresponding to the reduction of Co3+
to Co2+, and at 500–700 ◦C for the reduction of Co2+ to metallic cobalt
(Co0). The Co3+ reduction peak played an important role in the low-
temperature reducibility of the catalysts. Compared with the LCO sam-
ple, the Co2+ reduction peak for the LSCO sample shifted from 600 to
570 ◦C as opposed to the Co3+ reduction peak for the LCO sample (ca.
403 and 441 ◦C) which appeared earlier than that of LSCO (ca. 422 ◦C).
The latter result suggests that Sr doping did not improve the low tem-
perature reducibility of LCO sample. For the LSCO-a sample, the Co3+
reduction peak shifted to a clearly lower temperature, ca. 338 ◦C. The
improved low-temperature reducibility of the LSCO-a sample could be
linked to the improvement of catalytic performance for toluene oxida-
tion [29,30].
3.3. Catalyst surface composition and metal oxidation states
XPS experiments were carried out to provide information about
surface elemental composition, including types of surface oxygen and
cobalt species for the examined catalytic samples. Fig. 1 presents the Co
2p3/2 and O 1 s XPS spectra recorded over the samples, and related
estimated values of surface composition parameters are listed in Table 1.
It is found that the surface Sr/La atom ratio for LSCO is 0.73, which is
higher than the theoretical value 0.25. This indicates the enrichment of
the surface of LSCO with Sr element. After HAC treatment, the surface
Sr/La atom ratio for LSCO-a (0.26) shows an obvious reduction, which
demonstrates the removal of Sr from the surface.
The XPS calculated value of La/Co ratio is also listed in Table 1. It is
shown that La/Co atom ratio for LCO is 1.89, which is higher than the
theoretical value (1.00). This result suggests that the native surface of
LCO perovskite is preferentially occupied by La cations, which is in
accord with earlier reports [25–27]. When LCO was doped with Sr the
surface La/Co atom ratio for LSCO decreased to 1.37. This reduction
could be attributed to the partially substituted La for Sr. The proportion
for Sr substitution was 0.2, and the corresponding La/Co ratio should be
1.512. However, the surface La/Co atom ratio for LSCO is 1.37, which is
lower than the theoretical value (1.512). This result indicates that bulk
Sr element migrated on the sample surface, leading to the reduction of
surface La composition. After HAC treatment, the surface La/Co atom
ratio for LSCO-a becomes 1.13, which is the minimum value among the
three samples. It is suggested that partial amount of surface La might had
been removed by acid treatment after Sr substitution.
3.4. Catalytic performance
The evaluation of the catalytic activity of the three perovskite ma-
terials (LCO, LSCO and LSCO-a) in terms of toluene conversion for the
oxidation of toluene in the 210–280 ◦C range is shown in Fig. 3(a). A
blank activity test in the absence of catalyst showed that below 300 ◦C
the conversion of toluene under the examined feed gas composition and
GHSV was practically zero. The toluene conversion is increased with
increasing reaction temperature for all three catalysts. The LSCO cata-
lyst shows lower conversion values than the LCO sample in the whole
temperature range. However, after the applied HAC treatment, the
LSCO-a shows the best performance among the three samples, especially
in the 220–260 ◦C range. In particular, at 250 ◦C the conversion of
toluene is 80% for the LSCO-a compared to only 20% for the LSCO
catalyst. Toluene was completely oxidized to CO2 and H2O over the
three cobalt-based perovskite materials, and no partial oxidation prod-
ucts were detected in agreement to the very good carbon material bal-
ance (ca. 99.5%). T50 and T90 (the reaction temperature corresponding
to toluene conversion of 50% and 90%, respectively) were used to
As shown in Fig. 1, surface Co3+ and Co2+ species, as well as surface
lattice oxygen (Olatt), adsorbed oxygen (Oads, e.g., O2ꢀ , O22ꢀ , or Oꢀ ), and
surface carbonates (CO23ꢀ ) are present on the surfaces of these three
catalytic samples. The surface Co2+/Co3+ and Oads/Olatt atom ratios can
markedly influence the VOCs catalytic oxidation performance of cobalt
oxide. According to the quantitative analysis of the XPS spectra
Table 1
Element composition, BET surface area, toluene oxidation activity, and apparent.
XPS
SBET (m2/g)
toluene oxidation activity and apparent activation energy
Sr/La
La/Co
Co2+/Co3+
molar ratio
Oads/Olatt
T50
T90
Ea
molar ratio
molar ratio
molar ratio
(◦C)
(◦C)
(kJ/mol)
LCO
–
0.73
0.26
1.89
1.37
1.13
0.29
0.32
0.31
0.90
0.79
1.19
8.3
256
264
239
272
295
257
189
203
151
LSCO
LSCO-a
12.8
17.4
Activation energy (Eα) for the LCO, LSCO and LSCO-a catalytic materials.
3

Y. Wei et al.
Catalysis Communications 155 (2021) 106314
Fig. 1. (a) Co 2p3/2 and (b) O 1 s XPS spectra of the LCO, LSCO and LSCO-a samples.
exposed surface Co cations concentration increased significantly, giving
rise to the improved performance illustrated in Fig. 3(a). By comparing
the catalytic activity and characterization results presented in this work,
it can be found that there was an obvious correlation of surface oxygen
species concentration, and low-temperature reducibility with toluene
oxidation activity. After normalizing the activity results shown in Fig. 3
(a) per unit surface area, the same activity trend among the three cat-
alytic materials remained. The apparent activation energy (Ea) was
estimated, and results are presented in Fig. 3(b) using the Arrhenius
relationship (R2 values better than 0.991). The LSCO-a catalyst with the
lowest Ea value exhibits the highest catalytic activity for toluene com-
bustion at low temperatures. The differences in Ea values are most likely
generated by the intrinsic activity of surface oxygen species. It should be
noted that the LSCO-a sample possessed the highest Oads/Olatt ratio.
It is well known that the presence of H2O in flue gas affects toluene
oxidation activity on the perovskites catalysts. The effects of H2O (3%
and 5%) on the activity of the LSCO-a at 240 and 260 ◦C were studied
and results are shown in Fig. S4. It can be observed that there was a
slight decrease in toluene conversion upon introduction of water in the
feed gas stream over the LSCO-a catalyst, which was stabilized after ~2
h in the the stream with water. At 240 ◦C, the final decrease of toluene
conversion after adding 3% and 5% H2O was 2.4% and 3.2%, respec-
tively. At 260 ◦C, the corresponding values were 4.6% and 7.2%,
respectively. It should also be noted that toluene conversion was almost
restored after excluding H2O from the reactant feed gas stream. These
results suggest that the LSCO-a sample possessed good tolerance to H2O,
which is promising for practical applications.
Fig. 2. H2-TPR profiles of the LCO, LSCO and LSCO-a samples.
compare the catalytic activities of the three samples. For the LCO, LSCO
and LSCO-a samples, the values of T50 obtained were 256, 264, and
239 ◦C, respectively, and of T90 were 272, 295 and 257 ◦C, respectively
(Table 1). Therefore, it can be deduced that the activities of the three
samples enhanced in the order of LSCO < LCO < LSCO-a. The oxidation
of toluene at low temperatures usually occurs on the surface transition
metal cation sites (such as Co cations). The surface of LSCO sample was
preferentially occupied by Sr cations, as proved by the XPS results, and
active Co sites were partially covered causing reduction in activity. After
the HAC treatment, surface Sr was removed to a large extent, and
To test the catalytic stability of the LSCO-a sample, an uninterrupted
reaction experiment was performed which involved three consecutive
runs of catalytic tests (heating up and cooling down), and results are
Fig. 3. (a) Activity profiles and (b) Arrhenius plots of the LCO, LSCO and LSCO-a samples. Reaction conditions: 2000 ppm toluene, 20% O2, and N2 (balance gas),
GHSV = 120,000 mL/(g h).
4

Y. Wei et al.
Catalysis Communications 155 (2021) 106314
shown in Fig. S5 (ESI). The running time of the whole stability test was
~45 h, and the observed result is that the activity of LSCO-a remained
practically unchanged within 45 h of reaction. The influence of GHSV on
the toluene conversion was also investigated and results are presented in
Fig. S6 (ESI). In the case of LSCO-a sample, the T50 and T90 values were
239 and 257 ◦C at GHSV = 120,000 mL/(g h), respectively. These T50
and T90 values are 12 and 16 ◦C lower, respectively, than those achieved
at the highest GHSV of 240,000 mL/(g h). When the GHSV was
decreased from 120,000 to 60,000 mL/(g h), the toluene conversions
increased. This behavior is characteristic of a relatively fast kinetic rate
of toluene oxidation compared to the rates of mass transport.
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Appendix A. Supplementary data
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