Enhanced Catalytic Oxidation of Chlorobenzene over MnO2Grafted in Situ by Rare Earth Oxide: Surface Doping Induces Lattice Oxygen Activation

Article abstract of DOI:10.1021/acs.inorgchem.0c02197

A series of highly active MnO2@REOx (RE = Gd, Sm, Ce, and La) catalysts were successfully synthesized via in situ growth on the surface of MnO2, wherein the rare earth oxides were planted on the defect sites left by hydrogen peroxide etching of the surface of MnO2. Their physicochemical performance was investigated by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction with hydrogen (H2-TPR), and temperature-programmed desorption of oxygen (O2-TPD). The catalytic properties are compared through the catalytic oxidation of chlorobenzene. Among all catalysts, MnO2@GdOx showed better mobility of surface lattice oxygen and higher molar ratios of Mn4+/Mn3+ (1.10) and Oads/Olatt (0.45), which promoted the superior low-temperature catalytic activity for chlorobenzene oxidation. The long-term chlorobenzene oxidation test (18 h) at different temperatures and the experiments with a mixture of VOCs showed that the as-prepared catalyst not only possessed a high stability but also was suitable for the efficient and simultaneous removal of multicomponent VOCs (toluene, benzene, acetone, and chlorobenzene). This work also provided an idea for the further development of high-efficiency catalysts for the purification of VOCs from complex atmosphere environment.

Full text of DOI:10.1021/acs.inorgchem.0c02197

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Article
Enhanced Catalytic Oxidation of Chlorobenzene over MnO2 Grafted
In Situ by Rare Earth Oxide: Surface Doping Induces Lattice Oxygen
Activation
Lizhong Liu,* Ruoyu Liu, Tong Xu, Qiang Zhang, Yubin Tan, Qiaoling Zhang, Jiandong Ding,
and Yanfeng Tang*
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ABSTRACT: A series of highly active MnO2@REOx (RE = Gd, Sm, Ce, and La) catalysts were
successfully synthesized via in situ growth on the surface of MnO2, wherein the rare earth oxides
were planted on the defect sites left by hydrogen peroxide etching of the surface of MnO2. Their
physicochemical performance was investigated by scanning electron microscopy with energy-
dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), N2 adsorption−desorption, X-ray
photoelectron spectroscopy (XPS), temperature-programmed reduction with hydrogen (H2-
TPR), and temperature-programmed desorption of oxygen (O2-TPD). The catalytic properties
are compared through the catalytic oxidation of chlorobenzene. Among all catalysts, MnO2@
GdOx showed better mobility of surface lattice oxygen and higher molar ratios of Mn4+/Mn3+
(1.10) and Oads/Olatt (0.45), which promoted the superior low-temperature catalytic activity for
chlorobenzene oxidation. The long-term chlorobenzene oxidation test (18 h) at different
temperatures and the experiments with a mixture of VOCs showed that the as-prepared catalyst
not only possessed a high stability but also was suitable for the efficient and simultaneous
removal of multicomponent VOCs (toluene, benzene, acetone, and chlorobenzene). This work
also provided an idea for the further development of high-efficiency catalysts for the purification of VOCs from complex atmosphere
environment.
As a kind of transition metal oxide, manganese dioxide has
been widely studied due to its environmentally friendliness,
low cost, and good catalytic activity for VOCs oxidation.15−17
It has been reported that the catalytic activity and stability of
the heteroatom-modified manganese dioxide can better meet
the requirements of different harsh conditions.18−21 Chen et al.
synthesized manganese dioxide doped with iron and cerium,
which were effective to remove the aromatic VOCs.22,23 He et
al. prepared copper-doped manganese oxide catalysts and their
results showed that the doping of copper into manganese
oxides enriched the oxygen vacancies and increased the
catalytic activity.24 Cui et al. studied copper-doped manganese
dioxide catalyst and found that the incorporation can tune
oxygen vacancy concentration of MnO2.25 We also reported a
modified manganese oxide catalyst with iron, and it exhibited
excellent catalytic oxidation activity for toluene. However, it
was also found that the catalytic activity for chlorobenzene was
1. INTRODUCTION
As a global air pollutant, volatile organic compounds (VOCs)
are one of the main drivers of pollution such as urban haze,
photochemical smog, and atmospheric toxicity.1 Human-
generated emission of VOCs often originates from the tail
gas of waste incineration, electric furnace steel-making, iron ore
sintering, metal smelting, and coal-fired and coking industries,
which generally contain a large number of chlorinated aromatic
hydrocarbons, such as chlorobenzene with teratogenic,
carcinogenic, and mutagenic hazards to the human body.2−4
Therefore, VOC emission is restricted by more and more strict
regulations, which promotes paying more attention to the
development of efficient control technologies.5 Catalytic
oxidation technology has the advantages of low energy
consumption, high purification, and less secondary pollution,
so it is considered to be one of the most promising
technologies for the industrialized treatment of VOCs.6−8
The focus of catalytic oxidation technology is catalysts.9,10
Although noble metals have high catalytic activity, their high
cost, easy sintering at high temperature, and easy poisoning
restrict their further application.11,12 Compared with noble
metals, transition metal oxides have attracted much attention
due to their low cost, good antitoxic properties, and wide
sources.8,13,14
Received: July 23, 2020
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not enough.7 Less attention was paid to the fact that the
catalytic oxidation of VOCs depends on the active species on
the surface, and the active species located in the bulk cannot
participate in the reaction. Besides, the difficulty of
chlorobenzene oxidation is that chlorine ions easily invade
the oxygen vacancies on the catalyst’s surface, inhibiting the
regeneration of adsorbed oxygen. Generally, the chloride ions
can be extruded via lattice oxygen, but the required
temperature is too high.26 Hence, the question of how to
modify the surface of MnO2 with heteroatoms to enhance the
low-temperature mobility of lattice oxygen is the key to
improving the catalytic activity for the oxidation of chlorine-
containing VOCs.
[CxHy]in × αVOC
rVOC
=
mcat
(1)
[CxHy]in × YCO
2
rCO
=
2
mcat
(2)
(3)
d(ln rVOC
)
Ea(VOC) = −R ×
Ea(CO2) = −R ×
1
d
(T )
d(ln rCO
)
2
1
d
(T )
(4)
where [CxHy]in, rVOC, rCO , kVOC, kCO , Ea(VOC), and Ea(CO2) are the
2
2
Rare earth elements have high oxygen affinity, which can
provide catalysts with excellent oxygen transfer capabilities.27
In this work, rare earth oxides (GdOx, SmOx, CeOx, and LaOx)
were directly grown in situ on the surface of manganese
dioxide, wherein the hydrolysis of rare earth ions and the
etching of manganese dioxide surface by hydrogen peroxide
were carried out simultaneously. Combined with various
characterization techniques (SEM-EDS, XPS, H2-TPR O2-
TPD, etc.), the reason for the difference in the ability of
different rare earth elements to activate lattice oxygen on the
surface was revealed. Ultimately, the long-term evaluation of
chlorobenzene oxidation as well as the catalytic oxidation of
toluene, benzene, acetone, and mixed multicomponent VOCs
over the preferred active catalyst were also explored.
concentration of VOC in inlet (mol/m3), reaction rates (mol/(g·s))
of VOC conversion and CO2 yield, and apparent activation energies
of VOC conversion and CO2 yield.
3. RESULTS AND DISCUSSION
3.1. Characterization. All samples were characterized by
SEM-EDS. It can be observed from SEM image in Figure S1
that the morphology of pure MnO2 mainly presented as
nanosheets, wherein some also contained a few nanorods. After
the in situ modification via the rare earth elements, the
morphology of each catalyst has not changed significantly
(Figure 1a−d). Including Mn (Figure 1e−h) and O (Figure
2. EXPERIMENTAL SECTION
2.1. Preparation of Catalysts. A series of MnO2@REOx (RE =
La, Ce, Sm, and Gd) catalysts were synthesized by in situ growth of
rare earth oxides on the surface of MnO2 as shown in Scheme 1. A
Scheme 1. Schematic Illustration of MnO2@REOx Synthesis
typical preparation procedure of MnO2@GdOx was as follows: First,
0.5 mmol (0.2234 g) of gadolinium nitrate hexahydrate was dissolved
in 50 mL of 95% alcohol solution; afterward, 1.0 g of manganese
dioxide was dispersed in the above solution by ultrasound. Then, 0.85
g (750 μL) of a 30% hydrogen peroxide solution was added under
continuous stirring. After 30 min, the mixed solution was filtered, and
the obtained precipitate was heated to 450 °C for 3 h at a heating rate
of 5 °C/min in a muffle furnace. The obtained sample was recorded as
MnO2@GdOx.
For comparison, gadolinium nitrate hexahydrate was replaced with
0.5 mmol (0.2143 g) of lanthanum nitrate hexahydrate, 0.5 mmol
(0.2220 g) of samarium nitrate hexahydrate, and 0.5 mmol (0.2149 g)
of ceric nitrate hexahydrate to obtain MnO2@LaOx, MnO2@SmOx,
and MnO2@CeOx, respectively. In addition, MnO2 was prepared by a
hydrothermal reaction according to previous researches (for details,
see the Supporting Information, section S2).15
Figure 1. SEM images of MnO2@GdOx (a), MnO2@SmOx (b),
MnO2@CeOx (c), and MnO2@LaOx (d). EDS elemental mapping
images of Mn from MnO2@GdOx (e), MnO2@SmOx (f), MnO2@
CeOx (g), and MnO2@LaOx (h). EDS elemental mapping images of
O from MnO2@GdOx (i), MnO2@SmOx (j), MnO2@CeOx (k), and
MnO2@LaOx (l). EDS elemental mapping images of Gd from
MnO2@GdOx (m), Sm from MnO2@SmOx (n), Ce from MnO2@
CeOx (o), and La from MnO2@LaOx (p).
1i−l) elements, the EDS mapping images of MnO2@GdOx,
MnO2@SmOx, MnO2@CeOx, and MnO2@LaOx showed the
uniform distribution of Gd, Sm, Ce, and La elements (Figure
1m−p), suggesting that Gd, Sm, Ce, and La elements have
been successfully inserted on the surface of MnO2. In addition,
according to ICP analysis of samples, it can be confirmed that
the rare earth elements have been successfully combined with
MnO2 (Table 1). In fact, rare earth ions (RE3+) are easily
hydrolyzed in solution, thus forming the H+ ions and
2.2. Catalytic Evaluation. The performance of catalysts for VOC
oxidation was assessed in a successive-flow fixed-bed quartz
microreactor (I.D. = 6.0 mm) with continuous heating. First, 0.2 g
of sample powder was employed to test the activity. The stream with
air, the saturated steam (RH = 100%), and 500 ppm of chlorobenzene
were passed through the layer with a total flow rate of 100 mL/min.
VOC conversion (αVOC), CO2 yield (YCO ), and apparent activation
2
energy (Ea, kJ/mol) were calculated by eqs 1−4:32
B
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Table 1. Surface Element Compositions and Hydrogen Uptake of MnO2, MnO2@CeOx, MnO2@LaOx, MnO2@GdOx, and
MnO2@SmOx
peak position and molar ratio of elements by XPS
RE/Mn (mol %) by
ICP
H2 uptake of Mn4+→Mn3+
catalysts
Mn4+
Mn3+
Olatt
Oads
Mn4+/Mn3+ ΔEs(eV) Oads/Olatt
(mmol/g)
MnO2@GdOx
MnO2@SmOx
MnO2@CeOx
MnO2@LaOx
MnO2
4.85
4.82
4.88
4.80
0
642.84
642.89
642.90
642.91
643.18
641.62
641.64
641.67
641.69
641.88
529.27
529.28
529.31
529.33
529.64
531.18
531.20
531.24
531.25
531.32
1.10
0.98
0.81
0.79
0.64
4.62
4.64
4.69
4.71
4.73
0.45
0.42
0.36
0.34
0.29
50693
49367
49050
48718
47241
RE(OH)3 precipitates (reaction I). The H+ ion generated by
this can participate in the reaction between manganese dioxide
and hydrogen peroxide (reaction II), causing the tetravalent
manganese on the surface of the catalyst to be reduced into
divalent manganese and leave the defect sites.5 Eventually, the
combined reaction is completed, shown as reaction III.
Besides, the resulting rare earth hydroxide colloids can be
adsorbed into the defect sites by electrostatic action. After
further treatment, the composites with rare earth were formed.
N2 adsorption−desorption isotherms and pore-size distribu-
tion of all samples are shown in Figure 3a,b. The N2
RE3+ + 3H2O ↔ RE(OH)3 + 3H+
(I)
MnO2 + 2H+ + H2O2 → Mn2+ + O2↑ + 2H2O
(II)
3MnO2 + 2RE3+ + 3H2O2
→ 3Mn2+ + 3O2↑ + 2RE(OH)3↓
Figure 3. (a) N2 adsorption−desorption isotherms and (b) pore size
distributions of MnO2, MnO2@CeOx, MnO2@LaOx, MnO2@GdOx,
and MnO2@SmOx.
(III)
Figure 2 displays the XRD patterns of MnO2, MnO2@GdOx,
MnO2@SmOx, MnO2@CeOx, and MnO2@LaOx. The XRD
adsorption−desorption isotherms (Figure 3a) of MnO2,
MnO2@CeOx, MnO2@LaOx, MnO2@GdOx, and MnO2@
SmOx showed the typical IUPAC type IV pattern with a H3
type hysteresis loop, implying the existence of the slit
nanopores formed by stacking layered nanoparticles.28 More-
over, the pore size-distribution of all samples was between 2−
10 nm (Figure 3b). Compared with MnO2, the pore size
distributions of MnO2@CeOx, MnO2@LaOx, MnO2@GdOx,
and MnO2@SmOx were slightly shifted to larger pore sizes. As
calculated by the BET method, the specific surface areas of
each sample are 27.84 m2/g for MnO2, 19.13 m2/g for
MnO2@GdOx, 20.82 m2/g for MnO2@SmOx, 21.44 m2/g for
MnO2@CeOx, and 18.08 m2/g for MnO2@LaOx, respectively.
The slightly increase in pore size distribution and decrease in
specific surface area may be due to the rare earth oxides (LaOx,
CeOx, SmOx, and GdOx) loading on the surface of manganese
dioxide.
In order to further explore the effect of different rare earth
oxides on the surface characteristics of manganese dioxide, the
oxidation states of Mn and active oxygen species on the surface
of MnO2, MnO2@CeOx, MnO2@LaOx, MnO2@GdOx, and
MnO2@SmOx were obtained by XPS. The O 1s, Mn 2p3/2 and
Mn 3s XPS spectra of all samples are displayed in Figure 4a−c,
and the quantitative analysis is shown in Table 1. As presented
in Figure 4a, three components divided from Mn 2p3/2 XPS
spectra of MnO2 related to Mn4+ (643.18 eV), Mn3+ (641.88
eV), and Mn2+ (640.80 eV), respectively.7,29 After the rare
earth elements were grown on the surface of MnO2, the peak
positions of Mn4+, Mn3+, and Mn2+ shifted toward a lower
binding energy compared to that of MnO2. It is suggested that
the addition of rare earth oxides increased the electron cloud
density of Mn4+, Mn3+, and Mn2+ on the surface of MnO2.5
Meanwhile, we also found an increase of the Mn4+/Mn3+ molar
Figure 2. XRD patterns of MnO2, MnO2@GdOx, MnO2@SmOx,
MnO2@CeOx, and MnO2@LaOx. Powder Diffraction File (PDF) no.
24-0735, Joint Committee on Powder Diffraction Standards
(JCPDS).
patterns of all samples displayed the same diffraction
characteristic peaks at 2θ = 28.68, 37.33, 41.01, 42.82, 56.65,
59.37, 67.24, and 68.55°, which corresponded to the (110),
(101), (200), (111), (211), (220), (310), and (221) planes of
MnO2 (PDF no. 24−0735, JCPDS), indicating all catalysts
showed the crystalline structures similar to that of MnO2.
Meanwhile, no other obvious diffraction peaks corresponding
to GdOx, SmOx, CeOx, and LaOx were found.
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Figure 4. (a) Mn 2p3/2, (b) Mn 3s, and (c) O 1s XPS spectra of MnO2, MnO2@CeOx, MnO2@LaOx, MnO2@GdOx, and MnO2@SmOx.
ratios in the order of 0.64 (MnO2) < 0.79 (MnO2@LaOx) <
0.81 (MnO2@CeOx) < 0.98 (MnO2@SmOx) < (1.10)
MnO2@GdOx. That is, MnO2@GdOx achieved the highest
molar ratios of Mn4+/Mn3+ and had a better transformation of
Mn4+ ↔ Mn3+, which is instrumental in the migration of active
oxygen species and the catalytic oxidation of VOC.8 Rare earth
elements have a strong oxygen affinity.27,29 During the
formation of MnO2@REOx, this can cause the electrons in
Mn2+ (0.067 nm) are smaller than those of La3+ (0.103 nm),
Ce3+ (0.102 nm), and Sm3+ (0.096 nm) and Mn4+, Mn3+, and
Mn2+. During the formation of the composites, the manganese
ions removed by the hydrogen peroxide can enter the lattice
gap of the rare earth oxides. The smaller radius gap between
manganese and rare earth ions was more likely to cause the
distortion of crystal structure of rare earth oxides, thereby
increasing the surface oxygen vacancies and enhancing the
surface adsorbed oxygen content. In addition, the analysis of
the catalyst surface by XPS can also reveal the presence of Gd,
Sm, Ce, and La elements (Figure S2).
Mn−O to shift, thereby promoting the conversion of Mn3+
→
Mn4+. Meanwhile, the larger the electronegativity of rare earth
oxide, the stronger the attraction of the rare earth element to
oxygen; the increase of electronegativity follows the sequence
of LaOx < CeOx< SmOx < GdOx.30 Therefore, among the four
rare earth modified catalysts, MnO2@GdOx should theoret-
ically have the highest proportion of tetravalent manganese.
In addition, it can be observed from Figure 4b that the Mn
3s XPS spectra exhibits two separate peaks. The average
oxidation state (AOS) of Mn on the surface of MnO2 can be
assessed by the magnitude of Mn 3s doublet splitting
(ΔEs):16,31
O2-TPD technology can directly reflect the type and
mobility of oxygen species on the catalyst’s surface. As
revealed in Figure 5a, after the manganese dioxide was
AOS = 8.956 − 1.126ΔEs
(5)
As illustrated in Table 1, the AOS increases in the sequence of
MnO2@GdOx (4.336) > MnO2@SmOx (4.316) > MnO2@
CeOx (4.266) > MnO2@LaOx (4.246) > MnO2 (4.226). The
higher AOS also hints the higher fraction of Mn4+, which is
consistent with the characterization result of Mn 2p3/2 XPS
spectra.
Figure 5. O2-TPD profiles of MnO2, MnO2@GdOx, MnO2@SmOx,
MnO2@CeOx, and MnO2@LaOx: (a) 50−820 °C; (b) 50−500 °C.
The peaks from the O 1s XPS spectra of MnO2 at 532.5,
531.32, and 529.24 eV belong to adsorbed oxygen species
modified with Gd, Sm, Ce, and La, its characteristic peak of
oxygen in bulk at 536 °C shifted up to 30 °C for MnO2@
GdOx, 29 °C for MnO2@SmOx, 26 °C for MnO2@CeOx, and
24 °C for MnO2@LaOx, respectively, which indicated that the
formed rare earth oxides films on MnO2 surface hindered the
overflow of bulk oxygen. Considering that during the catalytic
reaction the migration temperature of active oxygen was
usually below 500 °C, the O2-TPD profiles of all catalysts
below 500 °C were further analyzed (Figure 5b). These peaks
of MnO2 belong to physisorbed molecular oxygen (141 °C),
chemisorbed oxygen atoms on the surface (199 °C), and
oxygen in surface lattice (395 °C), respectively.5,7 After doping
of Gd, Sm, Ce, and La ions, the peaks corresponding to the
physisorbed molecular oxygen and oxygen in surface lattice
moved toward a lower temperature, while the peak
corresponding to the chemisorbed oxygen atoms transferred
to a higher temperature. The loading of rare earth oxides on
the surface of MnO2 caused the reduction in the specific
surface area, weakening the catalyst’s capability of adsorbing
O2, dropping the desorption temperature of O2 on the catalyst
surface. Besides, the increase in desorption temperature of the
(Oads), lattice oxygen (Olatt), and CO3 or OH−, respectively
2−
(Figure 4c).27,32 It can be found from Table 1 that the doping
of rare earth oxides on the surface of MnO2 also caused a
significant decrease in the binding energies of lattice oxygen
and adsorbed oxygen, raising the electron cloud densities of
Olatt and Oads, which was conducive to the movement of lattice
oxygen to the oxygen vacancy, thus removing the chlorine
atoms from the oxygen vacancies. At the same time, it also
helped the adsorbed oxygen species to attack the carbon
cations (C+) left via the dechlorination of chlorobenzene.
Furthermore, the Oads/Olatt molar ratios of five catalysts
increased in the order of MnO2 (0.29) < MnO2@LaOx (0.34)
< MnO2@CeOx (0.36) < MnO2@SmOx (0.42) < MnO2@
GdOx (0.45). In situ modification of Gd on the surface of
manganese dioxide can increase the adsorbed oxygen content
to a greater extent, suggesting that MnO2@GdOx has a better
catalytic activity for chlorobenzene oxidation. This may be
related to the smallest radius of Gd3+ ion among the four rare
earth ions. The differences between the radii of Gd3+ (0.094
nm) and those of Mn4+ (0.053 nm), Mn3+ (0.058 nm), and
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catalysts with rare earth elements can be attributed to the
strong attraction of rare earth oxides. More importantly, the
doping of rare earth elements induced the desorption of
surface lattice oxygen at a lower temperature, which facilitated
the rapid migration of surface lattice oxygen into oxygen
vacancies for the removal of chlorine atom from the oxygen
vacancies, thereby restoring the relationship of conversion−
regeneration between oxygen vacancies and adsorption oxygen,
further maintaining the high degradation of chlorobenzene.
Figure 6 shows the H2-TPR profiles of MnO2, MnO2@
GdOx, MnO2@SmOx, MnO2@CeOx, and MnO2@LaOx. For
Figure 7. (a) Chlorobenzene conversion vs reaction temperature over
different catalysts. (b) CO2 yield from chlorobenzene oxidation vs
reaction temperature over different catalysts. Dynamic study into the
behavior of different catalysts in oxidization of chlorobenzene (c) and
the yield of CO2 (d).
Figure 6. H2-TPR profiles of MnO2, MnO2@GdOx, MnO2@SmOx,
MnO2@GdCeOx, and MnO2@LaOx.
chlorine atoms cannot be removed quickly, then the oxygen
vacancies would lose their capability to generate adsorbed
oxygen.34,36 The above characterizations via XPS O2-TPD and
H2-TPR showed that Gd modification brought better surface
lattice mobility and helped to squeeze chlorine atoms out from
the oxygen vacancies, thereby restoring the ability of the
oxygen vacancies to produce adsorbed oxygen, promoting the
catalytic oxidation of chlorobenzene. In addition, it is worth
noting that when the reaction temperature is lower than 240
°C, the conversion of chlorobenzene over CeOx is higher than
that over MnO2, which was attribute to the better oxygen
storage capacity of cerium oxide than manganese dioxide.37
The mineralization of chlorobenzene over different catalysts
is shown in Figure 7b. With the increase in the reaction
temperature, the CO2 yields of all samples increased
simultaneously. The CO2 yields of SmOx, LaOx, and GdOx
were almost zero, indicating that the active oxygen on the
surface of SmOx, LaOx, and GdOx was difficult to activate in
large quantities. The descending order of CO2 yields for all
MnO2, the characteristic peaks at 331 and 420 °C correspond
to the reduction of Mn4+ → Mn3+ and Mn3+ → Mn2+,
respectively.5,33 However, when MnO2 was modified with
GdOx, SmOx, CeOx, or LaOx, both of the above characteristic
peaks appeared to move significantly toward a higher
temperature, showing that the formation of rare earth oxides
films on the MnO2 surface delayed the reduction of manganese
ions.34 In addition, the H2 consumption of Mn4+ → Mn3+ for
the five catalysts is also calculated. As shown in Table 1, the
order of the increase in H2 consumption is as follows: MnO2
(47241 mmol/g) < MnO2@LaOx (48718 mmol/g) < MnO2@
CeOx (49050 mmol/g) < MnO2@SmOx (49367 mmol/g) <
MnO2@GdOx (50693 mmol/g). This result was consistent
with that of XPS. It also suggested that rare earth oxides with
strong electronegativity had a better ability to induce the
formation of high valence manganese on the catalyst’ surface.
3.2. Catalytic Activity Test. Before performance evalua-
tion, the influence of interal and exteral diffusion and mass
transfer have been excluded. Figure 7a emerges the activity
comparison of chlorobenzene oxidation over different catalysts.
As the catalytic reaction temperature continued to increase, the
catalytic activity of all samples increased. Among them, the
catalytic activities of SmOx, GdOx, and LaOx were poor, and
increasing the reaction temperature did not significantly
improve their catalytic activity. Over the entire reaction
temperature range, the catalytic activities of catalysts modified
by rare earth elements for the conversion of chlorobenzene
were higher than the activity of MnO2 and the decreasing
activity of modified catalysts in the catalytic oxidation of
chlorobenzene was in the order of MnO2@GdOx > MnO2@
SmOx > MnO2@CeOx > MnO2@LaOx. Gd-modified man-
ganese dioxide has a higher ability to catalyze the oxidation of
chlorobenzene. The degradation of chlorobenzene first occurs
with the cleavage of the C−Cl bond.35 When chlorobenzene
molecule is close to the catalyst’s surface, the chlorine atom is
easily captured by oxygen vacancies, thus remaining in the
oxygen vacancies after the breaking of C−Cl bonds. If the
samples is as follows: MnO2@GdOx > MnO2@SmOx
>
MnO2@CeOx > MnO2@LaOx > CeOx > LaOx ≈ SmOx ≈
GdOx. This sequence reflected the migration and conversion
capacity of active species. High transfer efficiency between
lattice oxygen, oxygen vacancy, and adsorbed oxygen is
conducive to generating more active oxygen to attack
chlorobenzene molecules, achieving high-efficiency mineraliza-
tion. Moreover, the T20%, T50%, and T90% values (temperature
vs chlorobenzene oxidation of 20%, 50%, and 90%) were also
applied to evaluate their performance. As shown in Table 2, the
temperature values of CO2 yield for all samples are obviously
greater than that of chlorobenzene conversion, implying the
presence of byproducts (e.g., alcohols and phenol species) in
the low-temperature catalytic oxidation of chlorobenzene.34,36
The apparent activation energy (Ea) is an inherent
characteristic of a catalyst which has nothing to do with the
specific surface area, concentration, and temperature.38 As
displayed in Figure 7c,d and Table 2, the Ea(VOC) and
Ea(CO2) of chlorobenzene oxidation increased in the order of
MnO2@GdOx (43.24 and 51.31 kJ/mol) < MnO2@SmOx
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Table 2. Comparison of Catalytic Activities over Different Catalysts
conversion
CO2 yield
apparent activation energy
samples
T20%/°C
T50%/°C
T90%/°C
T20%/°C
T50%/°C
T90%/°C
Ea(VOC) (kJ/mol)
Ea(CO2) (kJ/mol)
MnO2@GdOx
MnO2@SmOx
MnO2@CeOx
MnO2@LaOx
MnO2
216
237
250
259
267
281
261
276
290
303
310
337
308
319
344
350
n/a
n/a
240
259
266
275
287
337
282
296
309
318
327
n/a
320
345
350
n/a
n/a
n/a
43.24
58.12
70.11
75.33
93.02
87.09
51.31
71.58
80.34
82.25
114.67
119.17
CeOx
(58.12 and 71.58 kJ/mol) < MnO2@CeOx (70.11 and 80.34
kJ/mol) < MnO2@LaOx (75.33 and 82.25 kJ/mol) < MnO2
(93.02 and 114.67 kJ/mol). This suggested that MnO2@GdOx
had the best mobility of lattice oxygen and the highest ratio of
adsorbed oxygen, which facilitated the considerable degrada-
tion of chlorobenzene over MnO2@GdOx. Meanwhile, by
comparing the values of Ea(VOC) and Ea(CO2), it can be
found that the Ea(CO2) value of each catalyst is significantly
higher than its Ea(VOC) value, which is a result of the
degradation of chlorobenzene being a multistep process. The
conversion of chlorobenzene only requires the first step of C−
Cl bond cleavage, but the continued oxidation to carbon
dioxide requires more oxidative decomposition processes, for
example, chlorobenzene → phenol → hydroquinone → ... →
small molecule organic matter → CO2,34 which is obviously
bound to consume more energy. Furthermore, by testing the
catalytic activity of catalysts with different gadolinium content
in the catalytic oxidation of chlorobenzene, it can be found that
the catalytic activity of the obtained catalyst was the best when
the input amount of gadolinium was 0.5 mmol (Figure S3). In
addition, the activity comparison of MnO2@GdOx with other
catalysts is shown in Table S1.
conversion of chlorobenzene was also restored, but the yield of
CO2 decreased slightly. This shows that the part of active sites
on the surface of MnO2@GdOx were occupied by chlorine
atoms after the cooling reaction. Although the temperature was
subsequently increased, the current temperature of 325 °C did
not push all the chlorine atoms out of the oxygen vacancies,
which requires a higher reaction temperature. Moreover, from
SEM-EDS elemental mapping images of MnO2@GdOx after
chlorobenzene oxidation (Figure S4), the presence of chlorine
element can be observed, suggesting that chlorine was not
easily removed from the surface. Besides, the MnO2@GdOx
also displayed a good recyclability (Figure S5).
In addition, the catalytic oxidation of toluene, benzene, and
acetone were also investigated. As exhibited in Figure 9a−c,
The long-term stability of the best active catalyst (MnO2@
GdOx) was also studied. As shown in Figure 8, the stability test
Figure 9. Catalytic oxidation of (a) 1000 ppm toluene, (b) 1000 ppm
benzene, (c) 1000 ppm acetone, and (d) a mixture of VOCs: acetone
(440 ppm), toluene (159 ppm), benzene (147 ppm), and
chlorobenzene (58 ppm) over MnO2@GdOx.
Figure 8. Long-term stability of MnO2@GdOx for catalytic oxidation
of chlorobenzene at 300 and 325 °C.
MnO2@GdOx also shows high catalytic activity for each single-
component VOC. The lowest temperatures of completely
mineralized VOCs over MnO2@GdOx are 225 °C for toluene
oxidation, 250 °C for benzene oxidation, and 225 °C for
acetone oxidation. The difference between VOC conversion
and CO2 yield at various temperatures was relatively small,
implying that MnO2@GdOx catalyst had a good ability of
catalytic oxidation of VOC in deep. Besides, considering that
real exhaust, such as the exhaust from spraying and
pharmaceuticals, usually contains a mixture of acetone,
toluene, benzene, and chlorobenzene, the catalytic activity of
MnO2@GdOx on the mixture VOCs has also been studied. As
demonstrated in Figure 9d, the mixed components can be
completely converted at 350 °C. When the reaction temper-
can be divided into three intervals. In the first interval of 6 h,
the catalytic reaction temperature was maintained at 325 °C,
and the catalytic activity of MnO2@GdOx was maintained at
over 99% of chlorobenzene conversion and over 94% of CO2
yield. As the reaction temperature dropped to 300 °C, the
conversion and mineralization of chlorobenzene obtained by
MnO2@GdOx dropped to 84% and 66%, respectively. At the
same time, the gap between the conversion of chlorobenzene
and the yield of CO2 in this interval increased significantly,
indicating that the reduction of the reaction temperature led to
a substantial decrease in the active oxygen content, which is
insufficient to fully mineralize the chlorobenzene. In addition,
when the reaction temperature was raised to 325 °C, the
F
https://dx.doi.org/10.1021/acs.inorgchem.0c02197
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
ature was higher than 225 °C, the order of VOC conversion
was as follows: acetone > toluene > benzene > chlorobenzene;
when the reaction temperature was lower than 200 °C, the
order of VOC removal was acetone > chlorobenzene > toluene
> benzene. This change in the sequence may be related to the
catalytic reaction mechanism at different reaction temper-
atures. At low temperature, the degradation of VOCs mainly
relied on the mutual conversion between adsorbed oxygen and
oxygen vacancies on the catalyst surface. The dissociation
energy of C−Cl bond (96 kcal/mol on the phenyl group) is
lower than that of C−H bond (110 kcal/mol on the phenyl
group).34 Therefore, the C−Cl is easier to break than C−H.
When the reaction was located in a high temperature zone, the
lattice oxygen on the surface of the catalyst would be greatly
excited, which has an additive effect on the organic carbon
chain, thereby increasing the conversion of toluene and
benzene.
China; orcid.org/0000-0002-6171-8513; Email: lzliu@
ntu.edu.cn
Yanfeng Tang − Nantong Key Lab of Intelligent and New
Energy Materials, School of Chemistry and Chemical
Engineering, Nantong University, Jiangsu Province, Nantong
226019, PR China; Email: tangyf@ntu.edu.cn
Authors
Ruoyu Liu − Nantong Key Lab of Intelligent and New Energy
Materials, School of Chemistry and Chemical Engineering,
Nantong University, Jiangsu Province, Nantong 226019, PR
China
Tong Xu − Nantong Key Lab of Intelligent and New Energy
Materials, School of Chemistry and Chemical Engineering,
Nantong University, Jiangsu Province, Nantong 226019, PR
China
Qiang Zhang − Nantong Key Lab of Intelligent and New Energy
Materials, School of Chemistry and Chemical Engineering,
Nantong University, Jiangsu Province, Nantong 226019, PR
China
4. CONCLUSION
MnO2@GdOx, MnO2@SmOx, MnO2@CeOx, and MnO2@
LaOx catalysts were successfully prepared by in situ etching
growth on a manganese dioxide surface. Combined with
characterization via XPS, H2-TPR, O2-TPD, and the
chlorobenzene oxidation, the effect of the modification of
different rare earth elements on the surface structure and
catalytic activity of catalysts were revealed. The improvement
in physicochemical performance can be due to the higher
concentration of surface structure defects and electronegativity.
Lattice oxygen migration ability and the molar ratios of Oads/
Olatt and Mn4+/Mn3+ change from high to low in the order of
MnO2@GdOx > MnO2@SmOx > MnO2@CeOx > MnO2@
LaOx> MnO2, consistent with the order of their catalytic
oxidation of chlorobenzene. The systemic dynamic research
indicated that the Ea(VOC) and Ea(CO2) values of
chlorobenzene oxidation over MnO2@GdOx (43.24 and
51.31 kJ/mol) were lower than those of MnO2@SmOx
(58.12 and 71.58 kJ/mol), MnO2@CeOx (70.11 and 80.34
kJ/mol), MnO2@LaOx (75.33 and 82.25 kJ/mol), CeOx
(87.09 and 119.17 kJ/mol), and MnO2 (93.02 and 114.67
kJ/mol). Long-term stability tests and catalytic oxidation
experiments of a mixture of VOCs showed that MnO2@GdOx
is an excellent catalyst for VOC removal.
Yubin Tan − Nantong Key Lab of Intelligent and New Energy
Materials, School of Chemistry and Chemical Engineering,
Nantong University, Jiangsu Province, Nantong 226019, PR
China
Qiaoling Zhang − Nantong Key Lab of Intelligent and New
Energy Materials, School of Chemistry and Chemical
Engineering, Nantong University, Jiangsu Province, Nantong
226019, PR China
Jiandong Ding − Nantong Key Lab of Intelligent and New
Energy Materials, School of Chemistry and Chemical
Engineering, Nantong University, Jiangsu Province, Nantong
226019, PR China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02197
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support from the National
Natural Science Foundation of China (Grant No. 21776140)
and the Large Instruments Open Foundation of Nantong
University (Grant No. KFJN2027, KFJN2025, KFJN2018).
ASSOCIATED CONTENT
■
sı
* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Authors
Lizhong Liu − Nantong Key Lab of Intelligent and New Energy
Materials, School of Chemistry and Chemical Engineering,
Nantong University, Jiangsu Province, Nantong 226019, PR
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