Controllable construction of Ce-Mn-Ox with tunable oxygen vacancies and active species for toluene catalytic combustion

Article abstract of DOI:10.1002/aoc.5958

A series of Ce-Mn-O_x catalysts synthesized under different hydrothermal conditions were evaluated by catalytic removal of toluene. The results of characterization showed that the contents of oxygen vacancies and active species in catalysts were crucial for the catalytic oxidation process. The concentration of Ce³⁺, Mn³⁺, and adsorbed oxygen associated with structural defects in Ce-Mn-O_x catalysts could be controlled by hydrothermal conditions, which were considered to promote redox capacity and improve catalytic oxidation performance. In addition, suitable synthetic conditions could increase the S_BET and V_p of catalysts. Among the prepared catalysts, CM-100 showed the best catalytic performance due to the generation of more defective oxygen and active species (Ce³⁺, Mn³⁺, and surface-adsorbed oxygen). In addition, the CM-100 catalyst showed satisfactory water resistance and stability.

Full text of DOI:10.1002/aoc.5958

Received: 9 May 2020
DOI: 10.1002/aoc.5958
Revised: 8 July 2020
Accepted: 17 July 2020
F U L L P A P E R
Controllable construction of Ce-Mn-Ox with tunable oxygen
vacancies and active species for toluene catalytic
combustion
Zhenzhen Huang1
Xuejun Zhang2
Zhaodong Wang4
| Jinggang Zhao2 | Zhongxian Song3,4
| Yanli Mao4 | Heng Zhao2 | Min Zhao2 | Shuixia Liu5
| Wei Liu2
|
|
1School of Water Conservancy and
Engineering, Zhengzhou University,
Zhengzhou, 450001, People's Republic of
China
2Shenyang University of Chemical
Technology, Shenyang, 110142, People's
Republic of China
3College of Chemistry, Zhengzhou
University, Zhengzhou, 450001, People's
Republic of China
4Faculty of Environmental and Municipal
Engineering, Henan Key Laboratory of
Water Pollution Control and Rehabilitation
Technology, Henan University of Urban
Construction, Pingdingshan, 467036,
People's Republic of China
A series of Ce-Mn-Ox catalysts synthesized under different hydrothermal
conditions were evaluated by catalytic removal of toluene. The results of
characterization showed that the contents of oxygen vacancies and active
species in catalysts were crucial for the catalytic oxidation process. The concen-
tration of Ce3+, Mn3+, and adsorbed oxygen associated with structural defects
in Ce-Mn-Ox catalysts could be controlled by hydrothermal conditions, which
were considered to promote redox capacity and improve catalytic oxidation
performance. In addition, suitable synthetic conditions could increase the SBET
and Vp of catalysts. Among the prepared catalysts, CM-100 showed the best
catalytic performance due to the generation of more defective oxygen and
active species (Ce3+, Mn3+, and surface-adsorbed oxygen). In addition, the
CM-100 catalyst showed satisfactory water resistance and stability.
5Henan Key Laboratory of Polyamide
Intermediates, Pingdingshan, 467000,
People's Republic of China
K E Y W O R D S
catalytic oxidation, hydrothermal conditions, redox capacity, structure defects, surface
adsorption oxygen
Correspondence
Zhongxian Song, Zhaodong Wang,
Faculty of Environmental and Municipal
Engineering, Henan Key Laboratory of
Water Pollution Control and
Rehabilitation Technology, Henan
University of Urban Construction.
Pingdingshan, 467036, People's Republic
of China.
Email: songzhongxian@126.com;
wangzdhncj@126.com
Xuejun Zhang, Shenyang University of
Chemical Technology, Shenyang 110142,
People's Republic of China.
Email: xjzhang_syict@163.com
Zhenzhen Huang and Jinggang Zhao contribute equally to this article.
Appl Organomet Chem. 2020;e5958.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5958

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HUANG ET AL.
Funding information
Doctoral Research Start-up Project of
Henan University of Urban Construction,
Grant/Award Number: 990/Q2017011;
Educational Department of Liaoning
Province, Grant/Award Number:
LZ2019002; Henan Key Scientific
Research Projects, Grant/Award Number:
20A610003; Liaoning Province Doctoral
Startup Fund, Grant/Award Number:
20170520402; National Natural Science
Foundation of China, Grant/Award
Number: 21872096; School-Enterprise
Cooperation Fund, Grant/Award Number:
NT-2019-004; Young Teacher Foundation
of Henan University of Urban
Construction, Grant/Award Number:
YCJQNGGJS201903
1 | INTRODUCTION
CeO2-CoOx,[27] and CeO2-MnOx.
Of these catalysts,
[28]
CeO2-MnOx has been extensively studied due to the spe-
cial and ideal interactions between Ce and Mn species.
Venkataswamy et al.[29] revealed that the presence of
more surface hydroxyl-like groups and oxygen defects in
Ce0.7Mn0.3O2-δ catalyst were beneficial to catalytic activ-
ity. Liao et al.[30] proved that Mn0.85Ce0.15 catalyst pos-
sesses more oxygen vacancies due to the generation of
more Ce-Mn-O solid solution, which is crucial in improv-
ing the catalytic activity of the catalyst. Delimaris
et al.[31] claimed that the MnOx-CeO2 with an appropriate
Mn/(Mn + Ce) atomic ratio (0.25) could reduce more
Ce4+ ions and increase the specific surface area, which
could enhance the catalytic performance of toluene in
practice. Du et al.[32] reported that Mn0.6Ce0.4O2 synthe-
sized by a hydrothermal approach contributed to the
presence of Mn–Ce solid solution, which not only
improved the surface area, but also enhanced the oxygen
mobility and reducibility of the catalysts. Although
Ce-Mn-Ox catalysts have been studied widely in the cata-
lytic oxidation field of VOCs, the effect of hydrothermal
conditions on oxygen vacancies and active species in cat-
alysts has rarely been mentioned. This paper explores the
influence of hydrothermal conditions on the structure
defects, active species, and catalytic oxidation perfor-
mance of Ce-Mn-Ox catalysts.
As typical emissions of industrial processes, transport
and indoor decorations, volatile organic compounds
(VOCs) are considered to contribute to the generation of
haze, photochemical smog, and various potential toxic-
ities that are hazardous to the human body and the
atmospheric environment.[1–4] Efficient technology is
therefore urgently needed to eliminate the emission of
high concentrations of VOCs. Several techniques are
available for the abatement of VOCs, such as thermal
incineration, adsorption, catalytic combustion, and
photocatalytic processes.[5–7] Of these, catalytic combus-
tion has attracted much attention due to its low operation
temperature, high efficiency, economical feasibility, and
lack of secondary pollution.[8,9]
For catalytic oxidation, it was necessary to develop a
catalyst with excellent performance in VOC catalytic
combustion. Noble metal catalysts show remarkable cata-
lytic oxidation activity of VOCs compared to non-noble
metal catalysts. However, the practical application of
noble metal catalysts is limited by expensive prices and
the deactivation from sintering or poisoning.[10–12]
Various non-noble metal catalysts have therefore been
developed, including transition metal catalysts,[13,14] rare
earth metal catalysts,[15,16] and supported catalysts.[17,18]
As inexpensive and abundant materials, ceria materials
have been explored extensively in the field of VOC
catalytic elimination, which could be attributed to ade-
quate oxygen storage capacity (OSC) and excellent redox
performance resulting from the easy and reversible redox
In the light of the above aspects, a series of Ce-Mn-Ox
catalytic materials were synthesized by a hydrothermal
method using different hydrothermal treatment tempera-
tures. The catalytic activities for toluene over the
samples were evaluated with 500 ppm of toluene as the
removal target. Furthermore, the physical properties of
all samples were measured via X-ray diffraction (XRD),
Raman spectra, and N2 adsorption–desorption technol-
ogy. The surface ion distributions of all catalysts were
evaluated by X-ray photoelectron spectroscopy (XPS).
cycle of Ce3+$ Ce4+ [15,19,20]
.
In addition, due to further
improvement of physicochemical properties, transition
metal modified ceria-based catalysts have been studied,
such as CeO2-CuOx,[21] CeO2-TiOx,[22] CeO2-FeOx,[23]
CeO2-
CrOx,[24]
CeO2-NiOx,[25]
CeO2-ZrOx,[26]

HUANG ET AL.
3 of 12
The mobility of the oxygen species was tested by
oxygen temperature-programmed desorption technology
(O2-TPD). The reducibility of catalysts was measured by
the hydrogen temperature-programmed reduction tech-
nique (H2-TPR). Water resistance and durability tests
were performed over specific catalysts.
evaluating. The feed gas containing 500 ppm toluene
with gas hour space velocity (GHSV) of 60,000 hr–1
was composed of toluene, oxygen and nitrogen (20
vol% O2/N2). Before heating, the simulated feed gases
were introduced into the gas path system to stabilize the
equipment for 1 hr. A gas chromatograph (FULI 9790II)
matched with a flame ionization detector was used to
analyze the concentrations of toluene and reaction prod-
ucts in the inlet and outlet gas streams. The water resis-
tance procedures were as follows: the balance gas (N2)
was directly introduced into the reaction system to test
the dry catalytic performance of the catalyst, then the
balance gas was first introduced into the water, and
then introduced into the mixed gas system to detect the
difference in catalytic performance. This operation was
repeated multiple times to test the water resistance.
2 | EXPERIMENTAL
2.1 | Chemicals and materials
The chemicals involved in this experiment were
A.R. grade and were used directly without further
purification. The chemicals used included toluene,
manganese(II) nitrate (Mn(NO3)2), cerium(III) nitrate
hexahydrate (Ce (NO3)3ꢀ6H2O), and NH3ꢀH2O solution.
The toluene conversion (Ct) and the CO2 yield ðCCO
were calculated using the following formulas:
Toluene conversion (Ct)
Þ
2
2.2 | Catalyst synthesis
ðtolueneÞin −ðtolueneÞout
Ct =
× 100%
The Ce-Mn-Ox samples (25 wt% MnO2/CeO2) were syn-
thesized by a hydrothermal method at different treatment
temperatures, i.e. 60, 80, 100, 120, and 140ꢁC. In a typical
run, a suitable stoichiometric amount of manganese(II)
nitrate was first completely dissolved in deionized water
(50 ml). The corresponding amount of cerium(III) nitrate
hexahydrate was then completely dissolved in the solu-
tion. The pH value of the mixed solution was increased
to 10 using drops of concentrated NH3ꢀH2O solution
while stirring. The resulting sediment was stirred mag-
netically for 4 hr at room temperature. Subsequently, the
turbid liquid was hydrothermally treated for 12 hr in a
stainless-steel autoclave at a corresponding temperature.
Next, the slurry was washed to pH neutral with deionized
water and the precipitate was obtained by filtration. The
collected sample was dried in an oven at 105ꢁC for 12 hr.
A muffle furnace was used to calcine the samples at
550ꢁC for 5 hr. The resulting samples were labeled
CM-60, CM-80, CM-100, CM-120, and CM-140. The pre-
pared catalysts were pressed into sheets by a tablet press
and sifted into particles (40–60 mesh) for later use.
ðtolueneÞin
CO2 yield ðCCO
Þ
2
ðCCO
Þ
2
out
CCO
=
× 100%
2
7 × ðtolueneÞin
where (toluene)in and (toluene)out denote the toluene
concentrations of the inlet and outlet of the catalyst eval-
uation system, respectively, and ðCCO
Þ
is the CO2 con-
out
centration of the evaluation system outlet.
2
The normalized reaction rate (r, molꢀm−2ꢀs−1) of
toluene over the catalysts was calculated as:
F
P
r =
×
× Ct × ðtolueneÞin
mcat × SBET RT
where F is the gas flow rate, mcat is the catalyst mass,
SBET is the specific surface area of the catalyst, P is the
reaction system pressure, R is the proportional constant,
and T is the reaction temperature.
2.3 | Catalytic activity evaluation
2.4 | Catalyst characterization
The catalytic performance experiments for toluene
oxidation were implemented in a temperature range of
210–270ꢁC at atmospheric pressure in a conventional cat-
alyst evaluation system equipped with fixed-bed quartz
tube (internal diameter = 8.0 mm), purchased from
Kunming City, Yunnan province, China. Catalyst parti-
cles (0.1 ml) were packed in a microreactor before
Nitrogen adsorption and desorption experiments of the
as-synthesized materials were carried out at 77 K using
an SSA-6000 instrument (Technology Company of
Builder Electronic, Peking, China). The SBET of every
sample was determined using the linear portion of the
Brunauer–Emmett–Teller (BET) model. The desorption
branches of isotherms were utilized to measure the pore

4 of 12
HUANG ET AL.
volume (Vp) and pore size distribution using the
Barrett–Joyner–Halenda formula. All samples were pre-
liminarily outgassed at 250ꢁC for 3 hr under vacuum.
The powder XRD was carried out on an Advance Bruker
diffractometer (D8) made in Germany using Cu Kα radia-
tion. The diffraction angle (2θ) range was 10–80ꢁ and the
scanning step size was 0.02ꢁ with a scanning speed of
1.5 s. Raman spectra were acquired using a Raman
microscope (Renishaw-2000, England). The excitation
source was a 532-nm wavelength Ar ion laser with
2 cm−1 plus resolution. The XPS spectra were recorded
on an ESCALAB 250Xi system made in the United States
with Al Kα as the X-ray exciting source. The C 1 s peak at
284.6 eV was used as the reference for the binding ener-
gies (BE). The H2-TPR tests for the prepared catalysts
were carried out on a chemisorption analyzer (PCA-1200;
Builder Electronic Technology Company, Beijing, China)
equipped quartz U-shaped tubular reactor. A sample of
20 mg was loaded into the tubular reactor in a flat posi-
tion and pretreated in N2 at 400ꢁC for 40 min before mea-
surement. The reactor with the catalyst was then cooled
to 100ꢁC. The reduction process was performed from
100 to 900ꢁC with a heating rate of 10ꢁC/min under a
mixed gas atmosphere of 5 vol% H2/Ar (30 ml/min). A
thermal conductivity detector (TCD) was used to monitor
the H2 consumption. The O2-TPD tests were measured
via the same analyzer as used for the H2-TPR tests. First,
200 mg of the catalyst was put into a tubular reactor in a
flat position and preheated in an He flow of 30 ml/min at
200ꢁC for 1 hr. The catalyst was cooled to 50ꢁC and
exposed to a mixed gas atmosphere (20 vol% O2/He) for
60 min (30 ml/min). After the purging process, TPD
analysis was performed by temperature programming
from 50 to 650ꢁC at a rate of 10ꢁC/min in an He
atmosphere. Simultaneously, the TCD recorded the
desorbed oxygen signal.
3 | RESULTS AND DISCUSSION
3.1 | Measurements of toluene catalytic
activity
The catalytic performances of the Ce-Mn-Ox samples
were evaluated for toluene oxidation and the results are
shown in Figure 1a,b. Toluene conversion and CO2 yield
increased with increasing temperature for all catalysts
but to different extents. As shown in Figure 1a, toluene
was completely converted below 260ꢁC over all catalysts.
The reaction process included the complete decomposi-
tion of intermediate products under high temperature
conditions. Here, T90 was used as an index to evaluate
whether the catalytic performance of the catalyst met the
research purpose. The T90 values were used to compare
the catalytic oxidation performances of the Ce-Mn-Ox
catalysts (Table 1) and the order of the catalytic
activity was found to be CM-100 (240ꢁC) > CM-120
(246ꢁC) > CM-80 (249ꢁC) > CM-60 (249.5ꢁC) > CM-140
(250ꢁC). The CM-100 catalyst shows relatively high
activity and its toluene conversion reached 90% at 240ꢁC,
followed by the CM-120 catalyst and three other catalysts.
The CO2 selectivity of Ce-Mn-Ox materials (Figure 1b)
shows that the CO2 yield order is not exactly consistent
with the order of T90 values. The CM-80 catalyst showed
the worst CO2 yield, but the CM-100 and CM-120
catalysts still presented excellent CO2 selectivity. This
indicated the slight differences of deep oxidation perfor-
mance to toluene via Ce-Mn-Ox catalytic materials syn-
thesized under different hydrothermal conditions.
TABLE 1 The results of catalytic activities, N2 adsorption–
desorption, and Raman studies for Ce-Mn-Ox catalysts
Catalyst
CM-60
T90 (ꢁC)
249.5
249
SBET (m2/g)
Vp (cm3/g)
0.264
Cov
a
93
18.1%
18.9%
22.4%
19.6%
17.6%
CM-80
109
104
98
0.286
CM-100
CM-120
CM-140
240
0.303
3.2 | N2 adsorption–desorption studies
246
0.277
250
78
0.251
The pore structural properties of all the Ce-Mn-Ox mate-
rials were measured by N2 adsorption–desorption tests,
aCalculated by Raman peak areas.
FIGURE 1 The catalytic oxidation
performance of (a) toluene and (b) CO2 yield
over Ce-Mn-Ox catalysts

HUANG ET AL.
5 of 12
and the relevant graphs and data are shown in Figure 2
and Table 1. From Figure 2a, it can be seen that all the
Ce-Mn-Ox catalysts display type IV isotherms and hyster-
esis loops (H3) in a high relative pressure (P/P0) range
(0.7–1.0), which indicates the generation of mesoporous
structures in all catalyst materials.[10,32,33] Remarkably,
although the isotherms of all catalysts were similar in
shape, the precise locations of the hysteresis loops of dif-
ferent catalysts appeared at different P/P0 ranges, which
implies that the pore structure of the catalysts is affected
by the hydrothermal conditions.[20,34] This can be con-
firmed by the different shapes of the pore size distribu-
tion curves (Figure 2b). The pore size distribution curves
of all the catalysts have two obvious peaks at ca. 3 nm
and ca. 12 nm, much larger than the kinetic diameter of
the toluene molecule (ca. 0.6 nm), which was considered
to facilitate the diffusion of toluene molecules through
the pores.[35] The SBET and Vp of all the prepared samples
are also listed in Table 1. The sequences for SBET and Vp
are CM-80 (109 m2/g) > CM-100 (104 m2/g) > CM-120
(98 m2/g) > CM-60 (93 m2/g) > CM-140 (78 m2/g) and
CM-100 (0.303 cm3/g) > CM-80 (0.286 cm3/g) > CM-120
(0.277 cm3/g) > CM-60 (0.264 cm3/g) > CM-140
(0.251 cm3/g), respectively. The literature reports that
higher SBET and Vp values are beneficial to overcome
mass transfer resistance and improve the adsorption
performance of VOCs, resulting in the superior catalytic
performance.[10,32] The results show that the CM-100,
CM-120, and CM-80 catalysts had higher SBET and Vp
values than CM-60 and CM-140, contributing to better
catalytic activity. It is worth noting that the CM-100
catalyst did not show the maximum SBET, which suggests
that the physical structure of the catalyst is not the only
factor affecting the catalytic performance of the catalyst.
3.3 | XRD studies
The effect of hydrothermal temperatures on the phase
composition of Ce-Mn-Ox catalysts was measured by
XRD and the powder diffraction patterns are depicted in
Figure 3. From Figure 3a, all the diffraction patterns of
the prepared samples can be attributed to the characteris-
tics of cubic fluorite ceria (JCPDS No. 89–8,436). The
values of 2θ for the peaks are ca. 28.9ꢁ, 33.3ꢁ, 48.1ꢁ, 56.9ꢁ,
60.1ꢁ, 70.3ꢁ, 77.2ꢁ, and 79.7ꢁ, which were attributed to
the (111), (200), (220), (311), (222), (400), (331), and (420)
characteristic lattice planes of ceria, respectively.[32,36–38]
Furthermore, the absence of diffraction peaks for manga-
nese oxide indicates that the Mn ions might be incorpo-
rated in the CeO2 lattice structure to form solid solution.
Another possibility is that the highly dispersed oxide par-
ticles sizes were too small to be detected by XRD.[39–42] In
addition, the partial magnification of cubic fluorite CeO2
(111 characteristic peaks) shown in Figure 3b displays
the peak deviation phenomenon, which provides evi-
dence for manganese cations being incorporated in cubic
fluorite CeO2. Considering the smaller radii of Mn3+
(0.65 Å) and Mn4+ (0.53 Å) than that of Ce4+ (1.11 Å),
this led to the formation of Ce-Mn-O solid solution.[43–45]
The presence of the solid solution was considered to
promote the emergence of structural defects (such as
FIGURE 2 (a) N2 adsorption–desorption
isotherms and (b) pore-size distribution curves
for Ce-Mn-Ox catalysts
FIGURE 3 (a) XRD patterns and
(b) partially enlarged XRD detail of Ce-Mn-Ox
catalysts

6 of 12
HUANG ET AL.
oxygen vacancies), which could increase the oxygen
storage/release capacity of catalysts, resulting in better
catalytic oxidation activity. The effect of different
hydrothermal conditions on the structure of Ce-Mn-Ox
catalysts therefore might lead to different element
valence states and redox properties, which was discussed
in detail below.
oxygen species and weaken the Ce–O bond, resulting in
the promotion of oxygen mobility.[49]
To explore the effect of oxygen vacancies on catalytic
ability, the broad peaks were divided and fitted into two
groups (Figure 4b), representing the Mn3O4 phase and
oxygen vacancies. The relative oxygen vacancy concen-
trations (Cov) were estimated by the ratio of A600/A462
temporarily. A600 and A462 represent the peak areas of
bands at about 600 and 462 cm−1, and the relevant data
are summarized in Table 1. The relevant values followed
the order CM-100 (22.4%) > CM-120 (19.6%) > CM-80
(18.9%) > CM-60 (18.1%) > CM-140 (17.6%), which is
consistent with catalytic activity. Obviously, different
hydrothermal conditions lead to different amplitudes of
defects in the crystal structure of the catalysts, which fur-
ther affects the distribution of active species, leading to
different catalytic oxidation capacities. The structural
defect and active species contents in the catalysts can
therefore be controlled by adjusting the hydrothermal
temperature.
3.4 | Raman analyses
Raman spectroscopy was used to determine the
metal–oxygen vibration properties of the Ce-Mn-Ox
lattice and the results are shown in Figure 4. All the
catalysts exhibited the sharp characteristic F2g band
(ca. 462 cm−1) and this can be attributed to the Ce4+–O
symmetrical stretching vibration in the cubic fluorite-
structure CeO2 phase.[21,23,30] It should be noted that the
frequencies of the CM-60, CM-80, CM-100, and CM-120
samples were lower than that of CM-140. Furthermore,
the intensity and width of the F2g peak for all the cata-
lysts were observed at different frequencies, and the
CM-100 catalyst had a broader and weaker character
than the other samples. The deformation and/or offset
phenomena of the F2g band were attributed to the suc-
cessful incorporation of manganese ions into the CeO2
lattice structure and distortion of the Ce–O bonding sym-
metry, resulting in the generation of Mn-Ce-O solid solu-
tions and oxygen vacancies.[29,42,46] Thus, CM-100 might
generate more structure defects due to its larger deforma-
tion. Another broad, weak peak (550–700 cm−1) was
observed for all the Ce-Mn-Ox materials and can be
attributed to the overlap phenomenon between the char-
3.5 | XPS analyses
The atomic states on the surface of Ce-Mn-Ox materials
synthesized under different hydrothermal conditions
were studied by XPS and the spectra of Ce 3d, Mn 2p,
and O 1 s are shown in Figure 5. According to the Ce 3d
spectra (Figure 5a), the spectra for all catalysts were fitted
into eight contributions and the features assigned to the
[50–53]
spin-orbit components of Ce 3d3/2 and Ce 3d5/2.
As
can be seen from Figure 5a, the characteristic peaks of
the Ce4+ 3d5/2 species are labelled V, V00 and V000, and the
Ce4+ 3d3/2 species are labelled U, U00 and U000.[54,55] The
couple peaks V0 and U0 are assigned to Ce3+ 3d5/2 and
Ce3+ 3d3/2, respectively.[20,56] This phenomenon indicates
that the Ce3+ and Ce4+ species coexist on the catalyst sur-
face. It should be emphasized that although the catalysts
show similar XPS spectra, the exact BE values of the dif-
ferent catalysts are not completely consistent, which
implies that the chemical environment of ceria is affected
by different hydrothermal temperatures, resulting in
acteristic peaks at ca. 640 and ca. 600 cm−1 [39]
It has
.
been reported that the peaks at 640 cm−1 could be attrib-
uted to the characteristic stretching mode of the manga-
nese oxidation bond from the Mn3O4 phase in Ce-Mn-Ox
catalysts.[30,47,48] Moreover, the characteristic peaks at
about 600 cm−1 could be assigned to the existence of oxy-
gen vacancies.[30,47,48] It is widely believed that the gener-
ation of oxygen vacancies is a key factor in promoting the
catalytic performance of VOCs, which could activate the
FIGURE 4 (a) Raman spectra and
(b) fitting spectra of Ce-Mn-Ox materials

HUANG ET AL.
7 of 12
FIGURE 5 XPS spectra for (a) Ce
3d, (b) Mn 2p, and (c) O 1 s, and (d) the
relation between normalized reaction
rate and XPS results
different metal valence states and redox abilities in the
catalysts.[47,57] It ahs been reported that the hydrothermal
temperature may affect the crystal growth process of
synthetic materials, which contributes to the heterogene-
ity in atomic arrangement structure and ion distribu-
tion.[58,59] According to the analysis of peak areas, the
relative densities of Ce3+ (Ce3+/(Ce3++Ce4+)) in all the
catalytic materials were calculated and the relevant data
are presented in Table 2. Obviously, when the hydrother-
mal temperature of Ce-Mn-Ox increased from 60 to
100ꢁC, the Ce3+ content also increased and reached
24.0% at CM-100 (Table 2). As the temperature rose
further, the Ce3+ content began to decline and the ten-
dency followed the sequence of 24.0% CM-100 > 21.2%
CM-120 > 19.1% CM-80 > 18.6% CM-60 > 18.1% CM-
140, which shows that hydrothermal conditions have
different amplitude effects on the chemical perfor-
mance of catalytic materials and the CM-100 catalyst
has the highest relative density of Ce3+. The literature
shows that the generation of Ce3+ implies the creation
of oxygen defects based on the electroneutrality princi-
ple, which is thought to enhance the adsorption capac-
ity of reactive oxygen species on the surface of the
catalytic material. Generation of Ce3+ associated with
the presence of oxygen defects is therefore crucial to
the oxygen motility participating in oxidation mecha-
nism, and thus gave excellent catalytic oxidation of tol-
uene over CM-100 catalyst.[15,32,60,61]
The spectra of Mn 2p for all the prepared catalysts are
presented in Figure 5b. Based on previous literature
reports, the Mn 2p3/2 spectra could be split into two com-
ponents. The BE around 641.3 and 642.6 eV could be
attributed to the presence of Mn3+ and Mn4+ species,
respectively, indicating the coexistence of two kinds of
Mn ions on the surface of the catalysts.[30,39,62] It is worth
noting that although all catalysts showed similar Mn
2p3/2 spectra, the exact location of the peaks is not
entirely consistent. In particular, the BE values of peaks
TABLE 2 The results of XPS and H2-TPR experiments for Ce-Mn-Ox catalysts
P
b
Catalysts
CM-60
Ce3+/(Ce3++Ce4+
18.6%
)
Mn3+/(Mn3++Mn4+
)
Oα/(Oα + Oβ)
30.9%
ra
HCL
1.59
1.87
1.99
1.69
1.41
HCc
48.1%
48.3%
56.6%
52.6%
47.8%
0.620
0.623
1.068
0.873
0.614
2.32
2.63
2.72
2.40
2.14
CM-80
19.1%
31.1%
CM-100
CM-120
CM-140
24.0%
33.1%
21.2%
32.7%
18.1%
30.8%
aMeasured at 240ꢁC, (10−3 μmolꢀm−2ꢀs−1).
bH2 consumption calculated from low temperature (120–540ꢁC) H2-TPR peak areas (mmol/g).
cH2 consumption calculated from total H2-TPR peak areas (mmol/g).

8 of 12
HUANG ET AL.
for CM-60, CM-80, CM-100, and CM-120 were lower than
that of CM-140. This implies the existence of various
interactions between cerium and manganese oxides,
which may be considered evidence of defects in the cata-
lytic materials and contributes to the generation of more
low-valence Mn species (Mn3+) over the CM-60, CM-80,
CM-100, and CM-120 catalysts.[47,63,64] In addition, it has
been reported that the valence state of manganese is con-
nected to the hydrothermal treatment temperature,
which contributes to the difference in the amplitudes of
Mn–O bonding capacity and reciprocity between Ce and
Mn species in the crystallization process.[65,66] To discuss
the specific influence of hydrothermal conditions on
manganese valencies in the prepared catalytic materials,
the corresponding peak areas were integrated to calculate
the relative Mn3+ concentrations (Mn3+/(Mn3++Mn4+))
and the pertinent data are displayed in Table 2. With
increasing hydrothermal treatment temperatures, the
relative concentration of Mn3+ first increased and then
decreased, and the highest Mn3+ content for CM-100
was 56.6%. The order of Mn3+ species concentrations
in the catalysts were 56.6% CM-100 > 52.6% CM-
120 > 48.3% CM-80 > 48.1% CM-60 > 47.8% CM-140.
Apparently, the hydrothermal temperature has a large
influence on the manganese valence state distribution,
which is due to the different cerium manganese inter-
actions caused by different conditions. It ahs been
reported that the lower oxidation state of manganese
possesses the desired adsorption capacity of surface
oxygen species and contributes to the generation of
crystal defects, which is beneficial to improve the cata-
lytic properties of catalysts.[63,67,68] Thus, CM-100
exhibited superior catalytic removal performance for
toluene compared to the other catalysts.
data are shown in Table 2. With increasing hydrother-
mal temperature from 60 to 100ꢁC, the Oα concentra-
tions in Ce-Mn-Ox materials go up to higher values. As
the temperature rises further (>100ꢁC), Oα content
begins to decline, which shows the similarly changing
tendencies as Ce3+ and Mn3+ contents in catalysts. The
relative concentration of Oα follows the sequence: 33.1%
CM-100 > 32.7% CM-120 > 31.1% CM-80 > 30.9%
CM-60 > 30.8% CM-140, which indicates that hydro-
thermal temperatures have a great influence on the dis-
tribution of oxygen species in catalysts, and the CM-100
sample showed the most adsorption oxygen. It has been
reported that surface-adsorbed oxygen as the active sub-
stance plays a critical role in oxidation reactions due to
its excellent mobility compared to lattice oxygen, which
facilitates the oxidation reaction of toluene at low
temperatures.[39,57,71,72]
In addition, the normalized reaction rate (r, molꢀm−2ꢀs−1)
was calculated at 240ꢁC to investigate the effect of Ce3+,
Mn3+, and Oα contents on the catalytic properties of Ce-Mn-
Ox catalytic materials. The pertinent data are listed in
Table 2. Figure 5d shows the relationship between the nor-
malized reaction rate and the relative concentrations of
Ce3+, Mn3+, and Oα. The r value increased with the increase
of active species, which confirms the acceleration in the
toluene catalytic process resulting from Ce3+, Mn3+. and Oα
species. This indicates that suitable synthesis conditions
could produce more Mn3+, Ce3+, and Oα species resulting
from the satisfactory interactions between Ce and Mn
species. In other words, more active sites and species could
be generated on the Ce-Mn-Ox catalyst surface, which was
may promote the reaction in unit area.[15,73] Therefore, the
CM-100 catalyst shows the best oxidation ability in toluene
catalytic removal.
The O 1 s core level spectra of Ce-Mn-Ox catalytic
materials prepared under different hydrothermal condi-
tions are illustrated in Figure 5c. It can be seen that all
the spectra are composed of two oxygen chemical states
and the lower BE signal at ca. 529.2 eV (called Oβ) is
characteristic of lattice oxygen, such as O2−, while the
second band at ca. 531.2 eV (called Oα) is associated
with surface-adsorbed oxygen species such as O22−, O−,
and O2−. The latter are associated with structural
defects.[25,28,69,70] As can be seen from Figure 5c, there
are differences in BE values between the XPS spectra of
Ce-Mn-Ox catalysts and this can be attributed to the
discrepant synergetic interaction between Mn–O and
Ce–O bonds, resulting from the electron-transfer pro-
cess and/or change in structural arrangement, consis-
tent with the XPS results of Ce 3d and Mn 2p.[32,47]
Additionally, the ratio of Oα/(Oα + Oβ) can be used to
quantitatively investigate the effect of different hydro-
thermal conditions on oxygen species and the pertinent
3.6 | H2-TPR studies
H2-TPR studies were performed to demonstate the reduc-
ibility of the prepared catalytic materials, which were
synthesized under different hydrothermal conditions.
The hydrogen consumption profiles are displayed in
Figure 6 and the quantitative analysis is shown in
Table 2. It can be seen from Figure 6a that all the cata-
lysts have a high temperature reduction peak at about
640–900ꢁC and a wide overlapping peak at low tempera-
ture (ca. 120–540ꢁC), which is in accordance with
previous reports.[31,74] The high-temperature hydrogen
consumption peak centered at ca. 760ꢁC can be assigned
to the reduction process of bulk CeO2, while the low-
temperature reduction process is the two-step reduction
of manganese oxide accompanied by partial reduction of
surface CeO2.[31,74] The low-temperature reduction

HUANG ET AL.
9 of 12
FIGURE 6 (a) H2-TPR profiles and
(b) initial H2 consumption rates for Ce-Mn-Ox
catalysts
processes were all fitted into two parts. As shown in
Figure 6a, the peak centered at ca. 220ꢁC can be assigned
to the reduction process of Mn4+ to Mn3+ and the second
peak centered at ca. 350ꢁC can be attributed to the reduc-
tion process of Mn3+ to Mn2+ together with reduction of
partial ceria.[42,46,62,74] It is important to note that the
locations of the low-temperature reduction peaks for
Ce-Mn-Ox catalysts synthesized at different hydrothermal
temperatures were inconsistent. In particular, the
low-temperature reduction of CM-100 sample occurred at
the lowest temperature (dotted line), which indicates that
hydrothermal treatment at 100ꢁC for Ce-Mn-Ox samples
has better reducibility than that of the other catalysts.
This phenomenon might be due to the generation of
more defective or active oxygen species from the Ce-Mn-
O solid solutions. It was considered to possess superior
oxygen mobility resulting from the lower bond energy of
Mn-O-Ce than that of Ce-O bond. Thus, the hydrother-
mal process at 100ꢁC might be the best synthesis condi-
tion for Ce-Mn-Ox catalysts.[39,63] In addition, according
to the quantitative calculation of related reduction peak
areas, the hydrogen consumption (HC) of the prepared
Ce-Mn-Ox catalytic materials was quantitatively esti-
reverse order to T90, which suggests that rinitial at low
temperature plays a crucial role in the catalytic process
and CM-100 should have the best reduction performance.
However, although the CM-100 and CM-120 catalysts
exhibited prominent rinitial, the overall sequence and cata-
lytic activity were not absolutely consistent. This also
shows that the catalytic performance of the catalyst is
affected by a variety of factors.
3.7 | O2-TPD studies
O2-TPD was used to study the types and mobility of
oxygen species in Ce-Mn-Ox catalytic materials. As
depicted in Figure 7, two temperature intervals could be
identified in all profiles of Ce-Mn-Ox samples except for
the CM-80 catalyst. The desorption phenomenon of lat-
tice oxygen (Olat) from the bulk of composite materials
occurred at high temperature (>500ꢁC). The desorption
phenomenon of surface adsorbed oxygen appeared at low
temperature (<500ꢁC).[10,33,63,76] The absence of a desorp-
tion peak for Olat in the CM-80 catalyst was probably due
to its weak desorption signal overlapped with the stron-
ger desorption signal of Osur, which could be attributed
to fewer lattice defects in the catalyst. Furthermore, the
mated and the related data are listed in Table 2. The total
P
H2 consumption ( HC) decreased in the order of CM-
100 (2.72 mmol/g) > CM-80 (2.63 mmol/g) > CM-120
(2.40 mmol/g) > CM-60 (2.32 mmol/g) > CM-140
(2.14 mmol/g). Furthermore, the low-temperature
(ca. 120–540ꢁC) H2 consumption (HCL) showed the
same trend and followed the order CM-100
(1.99 mmol/g) > CM-80 (1.87 mmol/g) > CM-120
(1.69 mmol/g) > CM-60 (1.59 mmol/g) > CM-140
(1.41 mmol/g). The CM-100 catalyst showed the largest
HC, which was attributed to the reduction of more
surface-active species resulting from the presence of more
structural defects. It is worthwhile mentioning that the
order of T90 and HC were not exactly consistent, which
indicates that HC is only one aspect of evaluating
catalytic activity.[75] Therefore, the redox properties of
Ce-Mn-Ox catalysts were explored using the initial hydro-
gen consumption rate (rinitial). As can be seen from
Figure 6b, the order of rinitial has approximately the
FIGURE 7 O2-TPD profiles of Ce-Mn-Ox catalysts

10 of 12
HUANG ET AL.
FIGURE 8 (a) Water resistance
and (b) durability with water resistance
of CM-100 catalyst
shape of the desorption peaks in all the catalysts were
asymmetric, which could be attributed to the step-by-step
desorption resulting from the uneven particle size distri-
bution and/or different atomic structures. This suggests
that although the catalysts have the same elemental com-
position, their oxygen species might exhibit different
properties due to the different synthesis conditions.[77]
Another interesting finding is that the CM-60, CM-80,
and CM-140 catalysts show higher initial peak tempera-
tures than CM-100 and CM-120. In addition, the intensi-
ties of the low-temperature desorption peaks for CM-80,
CM-100, and CM-120 samples were stronger compared to
CM-60 and CM-140. The literature reports that low-
temperature (<500ꢁC) oxygen species are weakly
adsorbed on the surface of catalysts and the contents
could be enriched by the weaker interaction of M–O
bonds (M = Mn3+ or Ce3+) resulting from structure
defects (such as oxygen vacancy) in the catalyst frame-
work.[68,70,78] Thus, the lower initial peak temperature
and stronger peak intensity imply that more oxygen spe-
cies with high mobility exist in CM-100 and CM-120,
which in tun implies the presence of more structure
defects. Hence, the surface adsorption oxygen plays a sig-
nificant role in the catalytic oxidation process of toluene
due to its active chemical properties.[25,30,70,79] The CM-
100 sample should therefore exhibit higher catalytic
activity in toluene combustion.
can be seen from Figure 8a, when water vapor (5 vol.%)
was introduced in the reaction system, the conversion
dropped from ca. 82.5% to ca. 79.5%. The conversion ret-
urned to its original level quickly after removing the
water vapor. The decrease in activity was because of com-
petitive adsorption between toluene molecules and water
vapor on the active sites of the CM-100 sample. The
recovery of activity might be due to the rapid removal of
residual water vapor from the reactor at high tempera-
tures. After three cycles of the same operation, although
the conversion showed a slight decline, the catalytic per-
formance remained at a high level, which indicates that
better water resistance could be observed on CM-100 cat-
alyst for toluene abatement and the decline in activity
was reversible. Considering the durability of the catalyst
during the reaction, the stability of the CM-100 catalyst
was tested at 238ꢁC, with a water resistance test at the
end. As shown in Figure 8b, after 500 min of continuous
use of the catalyst, the toluene conversion showed a
slight decline and still maintained better water resistance,
which implied that the CM-100 sample has an ideal sta-
bility for toluene oxidation. It is worth mentioning that
after removing the water vapor, the toluene conversion
exceeded the original conversion. This phenomenon
might be due to the transient residue of hydroxyl species
from water on the catalyst surface, which could facilitate
a catalytic reaction within a short time as the reactive
oxygen species.[28,80]
3.8 | Water resistance and durability
studies
4 | CONCLUSIONS
According to the above analysis, CM-100 displayed the
best catalytic performance compared to the other cata-
lysts in a dry mixed atmosphere. However, the actual
exhaust emissions were sometimes wet, and always
included some water vapor. Therefore, water resistance is
an importamt index to assess the catalytic oxidation per-
formance of the catalysts. The cyclic water resistance
experiment over CM-100 catalyst for the toluene removal
process was tested at 237ꢁC with GHSV of 60,000 h−1. As
In summary, the catalytic activity of toluene was evalu-
ated over Ce-Mn-Ox catalysts synthesized under different
conditions. The characterization results showed that
excellent texture properties could be detected in CM-80,
CM-100, and CM-120 catalysts, such as appropriate SBET
and Vp values. In addition, the structural defects (such as
oxygen vacancy) and active species (such as Ce3+, Mn3+
,
and surface adsorbed oxygen) in the catalysts could be
controlled by adjusting the hydrothermal conditions. The

HUANG ET AL.
11 of 12
relative concentrations of Cov, Ce3+, Mn3+, and Oα, and
the normalized reaction rate were surprisingly consistent
with catalytic activity and the T90 values followed the
order CM-100 (240ꢁC) < CM-120 (246ꢁC) < CM-80
(249ꢁC) < CM-60 (249.5ꢁC) < CM-140 (250ꢁC). The phe-
nomena supported the accelerated function to catalytic
oxidation performance resuting from the tunable oxygen
vacancies and active species in the Ce-Mn-Ox catalysts.
Among the prepared catalyats, CM-100 showed the best
redox performance due to its higher concentration of
structure defects and active species. Furthermore, CM-
100 exhibited satisfactory water resistance and stability,
which shows its great potential for practical application.
[8] X. J. Zhang, J. G. Zhao, Z. X. Song, H. Zhao, W. Liu, Z. A. Ma,
M. Zhao, B. L. Zhao, React. Kinet. Mech. Cat. 2019, 128, 271.
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W. Rojek, T. Machej, R. P. Socha, L. Litynska-Dobrzynska,
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ACKNOWLEDGMENTS
[14] G. Q. Li, C. H. Zhang, Z. Wang, H. Huang, H. Peng, X. B. Li,
Appl. Catal. A: Gen. 2018, 550, 67.
[15] L. Wang, Y. F. Wang, Y. Zhang, Y. B. Yu, H. He, X. B. Qin,
B. Y. Wang, Catal. Sci. Technol. 2016, 6, 4840.
[16] B. D. Rivas, N. Guillén-Hurtado, R. López-Fonseca,
F. Coloma-Pascual, A. García-García, J. I. Gutiérrez-Ortiz,
A. Bueno-López, Appl. Catal. B: Environ. 2012, 121, 162.
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[19] D. H. Shang, Q. Zhong, W. Cai, Appl. Surf. Sci. 2015, 325, 211.
[20] F. Y. Hu, J. J. Chen, Y. Peng, H. Song, K. Z. Li, J. H. Li, Chem.
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This work was supported financially by the National
Natural Science Foundation of China (No. 21872096),
the Educational Department of Liaoning Province
(NO. LZ2019002), Henan Key Scientific Research Projects
(No. 20A610003), the Doctoral Research Start-up Project
of Henan University of Urban Construction (No. 990/
Q2017011), the Young Teacher Foundation of Henan
University of Urban Construction (No. YCJQNGGJS20
1903), the Liaoning Province Doctoral Startup Fund
(No. 20170520402), and the School-Enterprise Coopera-
tion Fund (No. NT-2019-004).
[21] Y. J. Luo, K. C. Wang, Y. X. Xu, X. Y. Wang, Q. R. Qian,
Q. H. Chen, New J. Chem. 2015, 39, 1001.
AUTHOR CONTRIBUTIONS
Zhenzhen Huang: Data curation; formal analysis.
Jinggang Zhao: Data curation; formal analysis.
Zhongxian Song: Conceptualization; methodology. Wei
Liu: Supervision. Yanli Mao: Supervision. Heng Zhao:
Software. min zhao: Software. Shuixia Liu: Validation.
Zhaodong Wang: Project administration.
[22] Z. N. Shi, P. Yang, F. Tao, R. X. Zhou, Chem. Eng. J. 2016,
295, 99.
[23] W. Wang, Q. Zhu, Q. G. Dai, X. Y. Wang, Chem. Eng. J. 2017,
307, 1037.
[24] P. Yang, Z. N. Shi, S. S. Yang, R. X. Zhou, Chem. Eng. Sci.
2015, 126, 361.
[25] Z. Hu, S. Qiu, Y. You, Y. Guo, Y. L. Guo, L. Wang,
W. C. Zhan, G. Z. Lu, Appl. Catal. B: Environ. 2018, 225, 110.
[26] H. F. Lu, Y. Zhou, W. F. Han, H. F. Huang, Y. F. Chen, Appl.
Catal. A: Gen. 2013, 464, 101.
[27] H. Wu, G. Pantaleo, G. D. Carlo, S. Guo, G. Marcì,
P. Concepción, A. M. Venezia, L. F. Liotta, Catal. Sci. Technol.
2015, 5, 1888.
[28] L. L. Zhao, Z. P. Zhang, Y. S. Li, X. S. Leng, T. R. Zhang,
F. L. Yuan, X. Y. Niu, Y. J. Zhu, Appl. Catal. B: Environ. 2019,
245, 502.
[29] P. Venkataswamy, D. Jampaiah, K. N. Rao, B. M. Reddy, Appl.
Catal. A: Gen. 2014, 488, 1.
[30] Y. N. Liao, M. L. Fu, L. M. Chen, J. L. Wu, B. C. Huang,
D. Q. Ye, Catal. Today 2013, 216, 220.
ORCID
Zhenzhen Huang https://orcid.org/0000-0003-0829-
4472
Zhongxian Song https://orcid.org/0000-0002-4733-3676
Wei Liu https://orcid.org/0000-0002-6083-9778
Xuejun Zhang https://orcid.org/0000-0002-2279-3297
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