Highly Active Mn3-xFexO4 Spinel with Defects for Toluene Mineralization: Insights into Regulation of the Oxygen Vacancy and Active Metals

Article abstract of DOI:10.1021/acs.inorgchem.9b02105

A series of highly defected Mn_3-xFe_xO₄ spinels with different amounts of oxygen vacancies and active metals were successfully synthesized by regulating the insertion of Fe ions into the crystal structure of Mn₃O₄ via self-polymerizable monomer adjustment of the molten Mn-Fe salt dispersion. The characterization of X-ray diffraction, Raman, scanning electron microscopy, X-ray photoelectron spectroscopy, and N₂ adsorption-desorption showed that the doping of Fe increased the lattice defects, oxygen vacancy concentration, specific surface area, mesoporosity, and catalytic properties compared to Cu ions doping. Temperature-programmed reduction with hydrogen and oxygen pulse chemisorption tests determined that the doping level of Fe ions had an important influence on the oxygen vacancy content and the dispersion of active metals on the catalysts' surfaces. For the best Mn-dispersed and most active Mn_2.4Fe_0.6O₄ catalyst, a long-term toluene oxidation measurement running for 120 h of uninterrupted reaction, at the low temperature of 240 °C, high humidity (relative humidity = 100%), and high weight hourly space velocity of 60000 mL·g^-1·h^-1, was also carried out, which indicated that the catalyst possessed high stability and endurability. Moreover, the continuous oxidation route and internal principle for toluene oxidation were also revealed by the in situ diffuse-reflectance infrared Fourier transform spectroscopy and gas chromatography-mass spectrometry techniques and deep dynamics study.

Full text of DOI:10.1021/acs.inorgchem.9b02105

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Highly Active Mn Fe O Spinel with Defects for Toluene
3
−x
x 4
Mineralization: Insights into Regulation of the Oxygen Vacancy and
Active Metals
Lizhong Liu,* Jiangtian Sun, Jiandong Ding, Yan Zhang, Tonghua Sun,* and Jinping Jia§
,
†
‡
†
†
,§
†School of Chemistry and Chemical Engineering, Nantong University, 9 Seyuan Road, Nantong 226019, Jiangsu Province,P. R.
China
‡
Department of Chemistry, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Road, Suzhou Dushu Lake Science and Education
Innovation District, Suzhou Industrial Park, Suzhou 215123, P. R. China
§
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P. R.
China
*
S Supporting Information
ABSTRACT: A series of highly defected Mn3−xFe O spinels with different
x
4
amounts of oxygen vacancies and active metals were successfully synthesized
by regulating the insertion of Fe ions into the crystal structure of Mn O via
3
4
self-polymerizable monomer adjustment of the molten Mn−Fe salt dispersion.
The characterization of X-ray diffraction, Raman, scanning electron
microscopy, X-ray photoelectron spectroscopy, and N2 adsorption−
desorption showed that the doping of Fe increased the lattice defects, oxygen
vacancy concentration, specific surface area, mesoporosity, and catalytic
properties compared to Cu ions doping. Temperature-programmed reduction
with hydrogen and oxygen pulse chemisorption tests determined that the
doping level of Fe ions had an important influence on the oxygen vacancy
content and the dispersion of active metals on the catalysts’ surfaces. For the
best Mn-dispersed and most active Mn Fe O catalyst, a long-term toluene
2.4 0.6
4
oxidation measurement running for 120 h of uninterrupted reaction, at the
low temperature of 240 °C, high humidity (relative humidity = 100%), and high weight hourly space velocity of 60000 mL·g ·
−
1
−
1
h , was also carried out, which indicated that the catalyst possessed high stability and endurability. Moreover, the continuous
oxidation route and internal principle for toluene oxidation were also revealed by the in situ diffuse-reflectance infrared Fourier
transform spectroscopy and gas chromatography−mass spectrometry techniques and deep dynamics study.
1
. INTRODUCTION
many researchers are currently paying close attention to studies
18,20−22
on transition-metal oxides.
Spinel catalysts with AB O structure, as one of the potential
Volatile organic compounds (VOCs), as precursors of
photochemical smog and ozone, are attracting wide attention
because of their toxic, mutagenic, carcinogenic, and teratoge-
2
4
substitutes for noble metals, has received extensive attention in
the removal of VOCs because of its polyvalence, affordability,
1
−3
netic nature.
89 harmful air pollutants indicated in CAAA90, is a typical
gaseous pollutant that is relatively difficult to remove at low
Among various VOCs, toluene, as one of the
23−29
and low toxicity.
It has been reported that heteroatom-
1
doped spinels can create plenty of oxygen defects, which could
improve performance because they can be used as reaction
centers for the migration and complementation of oxygen
4
−6
temperature because of its chemical stability.
various control technologies, such as photodegradation,
biodegradation, physical separation, plasma oxidation,
and catalytic oxidation, have been developed for toluene
removal, and catalytic oxidation is recognized as the best
economical and efficient technology because it can convert
toluene to carbon dioxide (CO ) and water (H O) at relatively
low temperature.
oxidation is catalysts, which commonly include noble metals
At present,
7
3
0
species. Hammiche-Bellal et al. prepared a CoFe O
2
4
8
9
10
composite spinel above 500 °C by a coprecipitation method,
11
31
achieving good removal of ethanol. Behar et al. studied the
Cu Mn O composite spinel for toluene oxidation, display-
1
.5
1.5
4
32
ing high removal efficiency (RE). Castano et al. investigated
̃
the doped spinel MnMgAlO catalyst for toluene removal,
2
2
x
1
2,13
As is known, the key of catalytic
obtaining increased catalytic activity. However, it should be
inappropriate to attribute the high activity of spinels to
1
4−16
and transition-metal oxides.
Noble metals possess high
catalytic activity, but their expense, sintering, coke, etc., limit
Received: July 14, 2019
11,17−19
their widespread use in VOC oxidation.
Therefore,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
1
heteroatom doping while ignoring how doped heteroatoms
affect its catalytic activity. Besides, the conventional syntheses
of spinel catalysts must be a multistep independent
manipulation, which is also distinctly relatively time-consum-
ing and requires harsh conditions.
We herein first reported a facile and scalable method to
synthesize a highly active heteroatom-doped spinel catalyst
energy (E
, kJ·mol ) for toluene mineralization were calculated byeqs
a
1
−4
[
C H ] − [C H ]
7
8 in
7
8 out
RE =
× 100%
× 100%
[
C H ]
(1)
(2)
(3)
7
8 in
[CO2]out − [CO2]in
ME =
7
[C H ]
7
8 in
(
Mn3−xFe O ) at relatively low temperature. Regulation of the
x 4
[
C H ] Q ME
oxygen vacancy induction and dispersion of active metals were
adjusted by controlling the ratio of precursors in molten metal
salts. Combined with various characterizations, the influence of
the doping ion type and degree on the microstructure and
performance of the catalysts was investigatived in depth, and
the important intrinsic link between the structure and catalytic
behavior was revealed. Ultimately, the effects of the Fe doping
amount on the catalytic performance and the long-term
catalytic oxidation evaluation of toluene on the preferred active
catalyst under simulated realistic exhaust conditions were also
explored.
7
8 in
r =
mcat
d(ln k)
Ea = −R
1
T
d(
)
(4)
where [C H ] , [C H ] , [CO ] , and [CO ] named the inlet and
7
8
in
7
8
out
2
in
2 out
−
3
outlet concentrations of toluene and CO (mol·mL ); r, Q, k, R, and
2
−
1
−1
−1
T are the reaction rate (mol·g ·s ), volumetric flow (mL·s ), rate
−
1 −1
constant (mL·g ·s ), molar gas constant, and reaction temperature
K), respectively.
(
3
. RESULTS AND DISCUSSION
2
. EXPERIMENTAL SECTION
Figure 2a shows the X-ray diffraction (XRD) results for both
the doped and undoped spinels. The diffraction peaks of all
2
.1. Preparation of Catalysts. A series of Mn3−xFe O (x = 0.5,
x 4
0
.6, 0.7, and 1.0) catalysts were synthesized by a one-step heat
treatment and are shown in Figure 1. The typical preparation
Figure 1. Representation of the Mn3−xFe O spinels.
x
3
Figure 2. (a) XRD patterns, (b) Raman spectra, (c) N adsorption−
procedure of Mn Fe O is as follows: 2.00 g of Fe(NO ) ·9H O,
2
2
.4 0.6
4
3
3
2
desorption isotherms, and (d) pore-size distributions of Mn O ,
4
.90 g of Mn(CH COO) ·4H O, and 7.88 g of citric acid were
3
4
3
2
2
Mn3−xCu O , and Mn Fe O (x = 0.6).
mingled; afterward, the above mingled system was heated up to 380
x
4
3−x
x
4
−
1
°
C at 2 °C·min and kept there for 2 h in a muffle furnace, which was
placed in a fume hood with exhaust gas absorption. For comparison,
catalysts according to that of the spinel Mn O (JCPDS 18-
3
4
Fe(NO ) ·9H O was replaced with 1.21 g of Cu(NO ) ·3H O to
3
3
2
3
3
2
0
803) could be identifiable, indicating that the Mn3−xCu O
obtain Mn3−xCu O , and the amounts of Mn(CH COO) ·4H O and
x 4
x
4
3
2
2
and Mn3−xFe O samples showed crystalline structures similar
citric acid were projected to be 4.90 and 6.30 g to obtain Mn O , and
x
4
3
4
the sample obtained after removal of Mn(Ac) ·4H O was γ-Fe O .
to that of Mn
3
O
4
. After the adjunction of Fe and Cu to the
2
2
2
3
The vendor details are provided in section S1.
.2. Catalytic Evaluation. The catalytic activities of the catalysts
synthesis system, the crystal growth of the original Mn O
3 4
2
obviously became disturbed and the diffraction peaks of new
for toluene oxidation were tested in a continuous-flow fixed-bed
quartz microreactor (i.d. = 6.0 mm). A 100 mg sample was employed
to assess the performance. The air stream, containing 1000 ppm of
toluene and saturated steam produced by airblowing, passed through
samples were broader compared to Mn O , which is attributed
3
4
to the structural damage due to anisotropic expansion and
33
contraction of the unit cell parameters, suggesting that Fe
−
1
and Cu ions can get into the framework of Mn O and take
3 4
the sample layer at 100 mL·min− to obtain a weight hourly space
1
−1
over the sites of Mn during the crystal growth process, thus
leading to crystalline defects being introduced to the catalyst,
which helped to generate more oxygen vacancies. Besides, it
should be noted that the sample doped with Cu also displayed
velocity (WHSV) of 60000 mL·g ·h . The moisture in the catalytic
system was adjusted [relative humidity (RH) = 0, 55, and 100%] to
measure the influence of steam on the performance. The RE and
mineralization efficiency (ME) of toluene and apparent activation
B
DOI: 10.1021/acs.inorgchem.9b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Element Compositions, Surface Areas, and Peak Areas of Raman Spectra at 584−598 cm−1 of Catalysts
feed (molar ratio)
Mn/Fe (Cu)
XPS (molar ratio)
Oads/Olatt
(Mn3+ + Mn )/Mn
2+
4+
SBET (m ·g−1)
2
peak area
a
sample (x = 0.6)
Mn/Fe (Cu)
Mn O4
Mn3−xFe O4
Mn3−xCu O4
0
4.00
4.00
0
4.16
5.12
0.47
0.89
0.53
1.88
2.33
2.04
23.4
84.8
28.9
690.2
1630.6
840.1
3
x
x
aComprehensive area of the peak at 584−598 cm−1 over Raman spectra.
some traces of Cu Mn O (JCPDS 35-1030), while the
N adsorption−desorption isotherms and pore-size distribu-
3−x x 4 3−x x 4 3 4
in Figure 2c,d. As shown in Figure 2c, the isotherms of the
1.4
1.6
4
2
sample doped with Fe displayed no clear evidence for the
formation of other types of metal oxides. This behavior may be
tions of Mn Fe O , Mn Cu O , and Mn O are displayed
3+
interpreted by the ionic radius. The radius of Fe (0.55 Å) is
catalysts showed a typical IUPAC type IV pattern, which is
3+
34,45,46
lower than that of Mn (0.58 Å), which allowed the
representative of a mesoporous material.
Figure 2d
incorporation of Fe3 in the Mn2 sites of Mn O .
+
3+
31
shows that the pore-size distributions were from 2 to 50 nm,
confirming the presence of mesopores. Moreover, the
Brunauer−Emmett−Teller (BET) surface areas of
3
4
+
Nevertheless, the ionic radius of Cu (0.73 Å) was higher
than that (0.67 Å) of Mn , impeding the incorporation of
Cu in the Mn sites of Mn O , keeping Cu from entering
into the Mn O crystals but causing it to bind to Mn to form
2+
34
2+
2+
2+
Mn Fe O , Mn Cu O , and Mn O were 84.8, 28.9, and
3
4
3−x
x
4
3−x
x
4
3
4
3+
2
−1
23.4 m ·g (Table 1), respectively. It is obvious that the
addition of Fe led to a larger surface area, which should be
attributed to the formation of more mesoporous structures on
the catalyst surface, increasing the capacity of the catalytic
3
4
another crystal structure (Cu Mn O ). Additionally, it can
1.4
1.6
4
be found from Figure S4 that the X-ray photoelectron
spectroscopy (XPS) characteristic peak of Cu appeared at
3+
35
47
9
43.7 eV attributed to Cu , demonstrating the existence of
oxidation of toluene. Meanwhile, generated Cu Mn O
1
.4
1.6
4
trivalent Cu. Indeed, many researchers have reported the
may have filled most of the mesopores, which weakened the
number of mesopores in the catalysts and resulted in the
insertion of Cu not significantly advancing the surface areas of
the samples.
3
+
existence of Cu species in some solid oxides such as
3
6
37
38
La2−xSr CuO , La CuO , Bi−Sr−Ca−Cu−O oxides,
x
4
2
4
3
9
40
3+
YBa Cu O6+y, and Cu/Al O , wherein Cu was found.
2
3
2
3
3
9
Su et al. proposed the following transport behaviors of Cu
ions under suitable conditions (eqs 5 and 6).
The scanning electron microscopy (SEM) images of three
samples are displayed in Figure 3. From Figure 3a,b, it can be
obviously seen that there is a large amount of irregular porosity
in the wrinkled Mn O catalyst. After Fe ion incorporation
+
+
3+
2
Cu2 ⇔ Cu + Cu
(5)
3
4
(
Figure 3c,d), a large number of relatively ordered mesopores
with a pore size of 19 ± 5 nm appeared in the catalyst
Mn3−xFe O ), which greatly improved its specific surface area.
1
2
+
=
3+
Cu + O ⇔ O + Cu
2
(6)
(
x
4
The degree of Cu2 disproportionation was temperature-
+
The porous structure and high surface area are important to
advancing the activity of the catalyst. This was conducive to
dependent, and Cu3 increases with increasing O content.
+
2
4
5,48
Similarly, the above transport behaviors should also exist in our
the proximity of toluene molecules to catalysts.
the addition of Cu ions completely changed the original
morphology of Mn , and the SEM images of Mn3−xCu O
x 4
However,
preparation system of Cu−Mn oxide, causing the presence of
3
+
Cu , which is inserted into the Mn O lattice because of the
3
O
4
3
4
lower ionic radius of Cu3 compared to that of Mn .
+
3+
showed spheroidal nanoparticles, which may result in an
insignificant rise in the surface area of the catalyst (Figure 3e,f).
Analysis of the SEM images was consistent with that of the N
Obviously, the Fe ions are more easily inserted into Mn O
3
4
than Cu ions, thus creating more surface defects for the
catalyst.
2
adsorption−desorption isotherms of the catalysts.
The vibrational behavior of the lattices and structural
divergence of three samples were analyzed by Raman
spectroscopy, and all peaks of the samples were deconvoluted
The Mn 2p
and O 1s XPS spectra of Mn O ,
3
/2 3 4
Mn3−xFe O , and Mn Cu O are presented in Figure 4a,b.
The molar ratio of the surface Mn/Fe for Mn3−xFe O was
4.16, which was very close to that of the feed (Mn/Fe = 4/1),
indicating that Fe ions were uniformly doped into Mn O .
However, the molar ratio of the surface Mn/Cu (5.12) for
Mn3−xCu O was much higher than the feed molar ratio of
x
4
3−x
x
4
x
4
41
by the Gaussian method (Figure 2b). For pure Mn O , four
3
4
broad peaks are clearly presented: the T symmetry modes at
2
g
3
4
−
1
−1
3
00 and 598 cm , the E symmetry mode at 356 cm , and
the A1g symmetry “breathing” mode at 645 cm . With the
g
−1
42
x
4
addtion of Fe and Cu, the peaks for the T (1), E , and A
Mn/Cu (4/1), showing that some Cu was not incorporated
into Mn O . As shown in Figure 4a, the three components
2g
g
1g
symmetry modes all showed negative shifts to some extent,
hinting that the existence of crystal defects is due to the
insertion of Fe and Cu into the Mn O lattice through the
3
4
divided from the XPS spectrum of Mn 2p at 643.1, 641.9,
3
/2
4
+
3+
2+
and 640.7 eV correspond to the surface Mn , Mn , and Mn ,
respectively. The surface molar ratios of (Mn + Mn ) to
3
4
1
8,43,44
48
2+
3+
partial substitution of Mn ions.
The oxygen vacancy
4
+
concentration can be indirectly tested in the region of 584−
Mn for Mn3−xFe O (2.33) and Mn Cu O (2.04) were
x 4 3−x x 4
−1
27,41
5
98 cm .
The corresponding peak areas are shown in
higher than that of Mn O (1.88), suggesting that the doped
3 4
2
+
Table 1, and the oxygen vacancy concentrations were in the
order of Mn3−xFe O > Mn Cu O > Mn O . The increased
Fe and Cu ions can result in an increase of the surface Mn
3
+
and Mn . The generation of oxygen vacancies (denoted as Δ)
x
4
3−x
x
4
3
4
49
defective sites facilitated the catalytic oxidation of toluene
because they serve as reaction centers wherein the reactive
species can be migrated and supplemented over oxygen
vacancies. The results are consistent with those of XRD.
can be on the basis of eqs 7 and 8. Namely, the presence of
more low-valent Mn facilitates the appearance of more
crystalline defects and oxygen vacancies in the catalysts, thus
accelerating the process of converting O to activated oxygen
2
C
DOI: 10.1021/acs.inorgchem.9b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. SEM images of (a and b) Mn O , (c and d) Mn Cu O , and (e,f) Mn Fe O (x = 0.6).
3
4
3−x
x
4
3−x
x
4
2
4,50
Besides, the surface molar ratio of (Mn2+
species.
+
been removed well in the preparation process. The O /O
ads latt
3
+
4+
Mn )/Mn in Mn3−xFe O was also much higher than that of
Mn3−xCu O . This may be ascribed to the fact that Fe ions
have a greater chance of being inserted into the unit cell of
Mn O compared to Cu because of the relatively difficult
molar ratio can also indirectly show the concentration of the
oxygen vacancies because Oads originated from the dissociation
x
4
3
+
x
4
49
of adsorbed O on oxygen vacancies. As shown in Table 1,
2
3+
3
4
the increase of the O /O ratio followed the order of Mn O
ads
latt
3
4
2
+
3+
conversion of Cu to Cu .
(0.47) < Mn3−xCu O (0.53) < Mn3−xFe O , which also
x 4 x 4
represented the abundance of surface oxygen vacancies.
The type and mobility of oxygen species were ascertained
through temperature-programmed desorption with oxygen
Mn4 − O2− − Mn4+ → Mn3+ − Δ − Mn3+ + 1 O2↑
+
2
(
7)
(
O -TPD; Figure 4c). Considering that oxygen desorption
2
Mn3+ − O2− − Mn3+ → Mn2+ − Δ − Mn2+ + 1 O2↑
commonly occurs below 400 °C and that above 400 °C is
assigned to the release of bulk oxygen, the O -TPD peaks of
19
2
2
(
8)
three samples below 400 °C were analyzed by curve-fitting
Figure 4b shows the O 1s XPS spectra of all samples. The
surface-adsorbed oxygen species (O , O , or O ) at 529.8
eV, lattice oxygen (O ) at 531.4 eV, and carbonate (CO
or hydroxide (OH ) at 533.3 eV, respectively, appeared on the
catalysts’ surfaces.
showed that CO3 or OH on the surface of the catalyst has
(Figure 4d). The three peaks for Mn O correspond to the
3 4
−
2−
−
physisorbed molecular oxygen (126 °C), the chemisorbed
oxygen atoms on the surface (203 °C), and the chemisorbed
2
2
2
3
−
)
latt
−
11
oxygen atomss in the lattice layer of the surface (281 °C).
1
1,45
The virtually indistinguishable peak
After doping Fe and Cu ions, the peak positions of all oxygen
2
−
−
desorption for Mn3−xCu O and Mn Fe O shifted toward
x 4 3−x x 4
D
DOI: 10.1021/acs.inorgchem.9b02105
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Inorganic Chemistry
Article
discovered that the pure Mn O has a relatively low number of
3
4
oxygen vacancies and low surface areas (Table 1), and this was
detrimental to the Oads generation and contact between the
catalyst and VOC to some extent. However, when Fe and Cu
were added into the system, the acquired Mn3−xFe O and
x
4
Cu Mn O held a higher proportion of O and a larger
x
3−x
4
ads
specific surface area, thus bringing better activity in comparison
with that of Mn O . In addition, the complete incorporation of
3
4
Fe into Mn O crystals can cause more crystal defects, thereby
3
4
greatly increasing the oxygen vacancy concentration and
further enhancing the active species of catalysts. This promotes
a higher catalytic activity of Mn3−xFe O compared to
x
4
Mn3−xCu O .
x
4
In order to explore the intrinsic characteristics of catalysts,
we herein introduced the apparent activation energy, which
19
can exclude the effect of specific surface area. The E values
a
of toluene mineralization (Figure 5b) increased in the order of
−
1
−1
Mn3−xFe O (70 kJ·mol ) < Mn Cu O (127 kJ·mol ) <
x
4
3−x
x
4
−
1
Mn O (135 kJ·mol ), suggesting that Mn Fe O has the
3
4
3−x
x
4
highest surface oxygen species, which were also easier to
release, allowing toluene to be more susceptible to deep
oxidation over Mn3−xFe O -0.6. Moreover, the E value over
Figure 4. (a) Mn 2p3/2 XPS spectra. (b) O 1s XPS spectra. O -TPD
2
x
4
a
profiles of Mn O , Mn Fe O , and Mn Cu O (x = 0.6): (c) 80−
−1
3
4
3−x
x
4
3−x
x
4
Mn3−xFe O -0.6 (70 kJ·mol ) was also much lower than those
x
4
9
00 °C; (d) 80−400 °C.
−
18
1
18
of Mn−Fe oxide (127.2 kJ·mol ; deep oxidation), MnO2
−
1
−1
(
153.0 kJ·mol ; deep oxidation), CeO (90.9 kJ·mol ; deep
2
17 17
the low-temperature range, illustrating that the active oxygen
could be release more readily. Moreover, Mn3 Fe O
−1
oxidation), Mn−Ce oxide (88.1 kJ·mol ; deep oxidation),
−x
x
4
−1
53
Cu Mn O (111.2 kJ·mol ; total oxidation), MnO/CeO
1
.5
1.5
4
2
possessed an oxygen desorption capacity superior to that of
Mn3 Cu O , manifesting more surface defects in the
−
1
54
(
>127.6 kJ·mol ; total oxidation), MnO /Al O (133.0 kJ·
x 2 3
54
−x
x
4
−1
−1
mol ; total oxidation), and Ni Zn Fe O (94 kJ·mol ;
0
.5
0.5
2
4
Mn3 Fe O catalyst.
−x
x
4
55
total oxidation). Generally, the activity of catalysts was
decided by their structures, metal states, and redox properties,
which can be increased by an optimized preparation
Before the performance of the samples was assessed, a blank
measurement was executed with no catalyst, and the removal
of toluene was not found below 350 °C, indicating that, under
the reaction conditions adopted, no homogeneous reaction
18,56
method.
Therefore, we reckoned that the distinction
4
5
between the E value over Mn Fe O and the literature
a
3−x
x
4
took place. Figure 5a shows the evaluation of toluene
values should be ascribed to a high oxygen vacancy
concentration in Mn3−xFe O . According to the above test
x
4
results, the influence of the internal and external mass diffusion
on the catalytic reaction rate of toluene oxidation can be
excluded (Figure S6).
The reducibility of samples with different contents of Fe
evaluated by H -TPR is displayed in Figure 6a,b and listed in
2
Figure 5. (a) ME of toluene. (b) Dynamic study of the behavior of
catalysts in deep oxidization of toluene over Mn O , Mn Cu O ,
3
4
3−x
x
4
and Mn3−xFe O (x = 0.6).
x
4
oxidation. Over Mn O , Mn Fe O , and Mn Cu O , the
3
4
3−x
x
4
3−x
x
4
ME of toluene was enhanced with a rise in the temperature,
and Mn3−xFe O presented the most activity among of all of
Figure 6. (a) H -TPR profiles and (b) initial H consumption rates at
x
4
2
2
the samples. T and T (temperatures vs ME of 50 and 90%)
low temperature of Mn3−xFe
x
O
4
with various amounts of Fe.
5
0
90
were used to appraise the catalytic performance of three
catalysts. For Mn3−xFe O , the values of T and T90 for
Table 2. The peaks of Mn O at 323 and 469 °C were due to
3 4
x
4
50
toluene mineralization were 199 and 224 °C, which were much
the conversion of MnO to Mn O and Mn O to MnO,
2 3 4 3 4
27
lower in comparison with those of Mn3−xCu O (232 and 256
respectively. After Fe was incorporated into Mn O , the peak
3 4
x
4
°
C) and Mn O (255 and 315 °C), showing that Mn Fe O
belonging to the conversion of Mn O to MnO shifted
3
4
3−x
x
4
3 4
possesses a lower light-off temperature. Generally, catalytic
oxidation of the VOC at low reaction temperature is affected
obviously, and it appeared at 460 °C for Mn Fe O , 447 °C
2.5 0.5 4
for Mn Fe O , 457 °C for Mn Fe O , and 480 °C for
2
.4 0.6
4
2.3 0.7
4
11,19,45,51,52
by the Oads concentration and specific surface area.
Combined with the above various characterizations, it can be
Mn FeO , respectively. Also, the reduction temperature tends
2 4
to decrease first and then increase gradually with an increase of
E
DOI: 10.1021/acs.inorgchem.9b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. H Consumption, O Uptake, Metal Dispersion, and TOF of Mn Fe O
4
2
2
2.4 0.6
a
sample
Mn O4
Mn2.5Fe0.5O4
Mn2.4Fe0.6O4
Mn2.3Fe0.7O4
ICP-Mn (wt %) total H uptake (μmol·gcat−1) O uptake at 350 °C (μmol·gcat−1) Mn dispersion (%) TOF at 160 °C (10−6 s−1
)
2 2
72.3
59.8
57.2
54.5
47.8
0
3785
4581
5067
5359
5991
11648
17299
18266
23296
19025
13558
0
33
42
56
48
39
0
0.05
3.53
6.52
5.81
1.23
0
3
Mn FeO4
2
γ-Fe O3
2
a
Mn dispersion: fraction of Mn atoms at the catalyst surface.
the iron content. The incorporation of a small number of Fe
ions led to a loose lattice of Mn O , which was favorable for
3
4
3
+
the contact of Mn with H . Furthermore, it can be also seen
2
that the peaks at 416, 540, and 619 °C for Mn FeO were
2
4
assigned to the conversion of Fe O to Fe O , Fe O to FeO,
2
3
3
4
3
4
1
8,27
and FeO to Fe.
Namely, Fe O was first reduced to Fe O
2 3 3 4
before Mn O was reduced to MnO. Therefore, the addition of
3
4
excessive Fe into Mn O resulted in more Fe O oxide films on
3
4
3
4
the catalysts’ surfaces during the reduction process, which
3+
delayed the normal reduction of Mn . Besides, the total H
2
consumption of Mn3 Fe O was improved in comparison with
−x
x
4
3
+
0
Figure 7. (a) Temperature-dependent changes of the TOF values. (b)
Samples with different amounts of Fe versus TOF values at 160 °C
Mn O because Fe was finally reduced to Fe . The total H
3
4
2
consumption increased in the order of Mn O < Mn Fe O
3+
2+
4+
3
4
2.5 0.5
4
and molar ratios of (Mn + Mn )/Mn and O /O .
ads
latt
<
Mn Fe O < Mn Fe O < Mn FeO < γ-Fe O (Table
2.4 0.6 4 0.3 0.7 4 2 4 2 3
2
). Moreover, the reducibility of catalysts was evaluated by an
tendencies, demonstrating that the suitable amount of Fe
incorporated could also increase the concentration of low-
valence Mn and Oads on the surface of catalysts, further
suggesting that more oxygen vacancies were present in
Mn Fe O .
initial H uptake rate at low temperature (Figure 6b). The
reducibility at low temperature was decreased in the proper
order of Mn Fe O > Mn Fe O > Mn Fe O >
Mn FeO > Mn O , suggesting that Mn Fe O has more
oxygen vacancies and surface active oxygen atoms as well as
better catalytic activity.
In addition, it should be noted that only active metals were
considered to play major roles in VOC oxidation rather than
all metal ions of catalysts. Therefore, the dispersion of surface
Mn of Mn3 Fe O is necessary for evaluation. According to
the H -TPR results and considering the fact that these samples
2
2
.4 0.6
4
2.3 0.7
4
2.5 0.5
4
2
4
3
4
2.4 0.6
4
2
.4 0.6
4
Moreover, the intermediates in toluene oxidation over
Mn Fe O were also detected by in situ diffuse-reflectance
2
.4 0.6
4
infrared Fourier transform spectroscopy (DRIFTS) and gas
chromatography−mass spectrometry (GC−MS). It can be
observed from in situ Fourier transform infrared spectroscopy
(Figure 8a) that a series of surface structural changes for
−x
x
4
2
were synthesized at a relatively low temperature and their
performance was tested below 350 °C (Figure S7), we herein
selected 350 °C as the oxygen pulse chemisorption temper-
5
7
ature. The O uptake of catalysts at 350 °C is listed in Table
2
2
. The dispersion of surface Mn of Mn Fe O (56%) was
2.4 0.6 4
higher than those of Mn O (33%), Mn Fe O (48%),
3
4
2.3 0.7
4
Mn Fe O (42%), and Mn FeO (39%), suggesting that
2
.5 0.5
4
2
4
utilization of Mn was the highest in Mn Fe O and a
2.4 0.6
4
maximum number of oxygen vacancies were present in the
catalyst.
The catalytic performance of various amounts of Fe doped
in Mn O was also tested for low-temperature oxidation of
Figure 8. (a) Change of DRIFTS of Mn3−xFe O for toluene
oxidation, (b) GC−MS results of the exhaust gas.
x
4
3
4
toluene. Because toluene oxidation occurred at the active sites
of catalysts, the catalytic activities of samples were estimated by
using turnover frequency (TOF). As depicted in Figure 7a, the
TOF values enhanced in the sequence of Mn O < Mn FeO <
Mn Fe O have emerged. With the temperature-dependent
2
.4 0.6
4
−
1
3
4
2
4
changes, a band appears at 3070 cm , corresponding to C−H
of the vinyl stretching vibration. The band at 2345 cm was
assigned to the CO vibration, and the peaks at 1644 cm
58
−1
Mn Fe O < Mn Fe O < Mn Fe O , which is
2
.5
0.5
4
2.3
0.7
4
2.4
0.6
4
−
1
consistent with the dispersion trend of Mn. The results
showed a better catalytic performance of Mn Fe O ,
2
2
.4 0.6
4
were the vibration mode of −OH and the H O deformation
2
27,58
indicating that the suitable incorporation of Fe can play a
positive role in the catalytic activity. Moreover, according to
the TOF values of Mn Fe O at 160 °C, the changes in the
vibration, respectively.
The strong bands at 1596, 1554,
−
1
and 1411 cm were due to the C−C and CO stretching
1
8,59
vibrations as well as carbonate species,
respectively.
3
−x
x
4
3
+
2+
4+
−1
ratios of (Mn + Mn )/Mn and O /O were also
Moreover, the vibration modes at 1308 and 1180 cm were
according to the ν (COO) stretching vibration. These results
ads
latt
27
evaluated by XPS. As shown in Figure 7b, the curves of the
s
TOF values, the ratios of (Mn3 + Mn )/Mn , and O /O
+
2+
4+
showed that toluene over Mn Fe O was oxidized to
ads
latt
2.4 0.6
4
with different contents of Fe displayed volcano-shaped
−CHO, −COOH, or −COOC− organics. According to the
F
DOI: 10.1021/acs.inorgchem.9b02105
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
detection of exhaust gas by GC−MS (Figure 8b), it can be
found that acetic acid, benzaldehyde, 2-(5H)-furanone, citric
acid, and benzoic acid coexisted in the exhaust gas after low-
temperature catalytic oxidation of toluene. Combined with
DRIFTS and GC−MS characterizations, the possible degra-
dation path of toluene undergoes the production of
benzaldehyde, benzoic acid, 2-(5H)-furanone, citric acid, and
acetic acid, further oxidation to organic salts, thus conversion
to inorganic salts; ultimately, the inorganic salts decomposed
to CO . Besides, the activated O molecules can supplement
4. CONCLUSION
A series of Mn3−xFe O spinels with different amounts of
x
4
oxygen vacancies were synthesized by regulating the insertion
of Fe ions. The influence of the doping ion type and ion
doping degree on the oxygen vacancy concentration and
catalytic performance of catalysts was studied. The improve-
ment of physical−chemical properties such as the structures,
lattice defects, and morphologies leads to the better low-
temperature activity for toluene oxidation over Mn3−xFe O
x
4
2
2
catalyst. For the most active Mn Fe O catalyst, its increased
2.4 0.6 4
the spent active oxygen over oxygen vacancies.
performance can be due to the higher concentration of
The catalytic stability of Mn Fe O was also carried out
3+
2.4 0.6
4
structural defect and oxygen vacancy and larger (Mn
+
under long time-on-stream conditions (120 h of uninterrupted
reaction). As shown in Figure 9, the experiment can be divided
2+
4+
Mn )/Mn and O /O ratios. The test under simulated
ads
latt
realistic exhaust gas also showed that Mn Fe O is a superior
2
.4 0.6
4
catalyst (T50 = 199 °C and T90 = 224 °C for toluene
−
6
−1
mineralization; TOF160 °C = 6.52 × 10 s ) with stability
(
long-term toluene oxidation test running for 120 h) and
satisfied endurability to high humidity (RH = 55% and 100%).
With systemic dynamic research on toluene oxidation over
Mn Fe O , the principles for toluene activation and CO
2
2
.4 0.6
4
production were proven to be dissimilar. We can rationally
anticipate that the Mn Fe O spinel is an effective and
2
.4 0.6
4
promising catalyst for the removal of VOCs.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02105.
■
*
S
Figure 9. Catalytic stability of Mn3−xFe O for the RE and ME of
x
4
−
1
toluene at different humidity, 240 °C, and WHSV of 60000 mL·g ·
Characterization, XRD patterns of γ-Fe O and
2
3
−
1
h .
Mn3−xFe O before and after catalytic reaction, EDS
x 4
mapping spectrum of Mn3−xCu O and Mn Fe O , Cu
x
4
3−x
x
4
2
p XPS spectra of Mn3−xCu O , RE of toluene over
x 4
into three stages. Obviously, the arbitrarily adjustment of the
Mn3−xFe O , Mn Cu O , and Mn O , effect of the
WHSV on toluene mineralization, and RE of toluene
over samples with different contents of Fe (PDF)
x
4
3−x
x
4
3
4
H O contents did not impact the RE of toluene at 240 °C for
2
9
0 h of uninterrupted reaction, except the ME of toluene.
Because we know that, under heating conditions, the adsorbed
oxygen was excited and overflowed from the oxygen vacancies
to demineralize toluene molecules, the absorbed oxygen was
supplemented by the capture and dissociation of gaseous
oxygen over oxygen vacancies. Therefore, the constant flow of
oxygen is necessary for the oxidation reaction to continue.
During the oxidation reaction, toluene was first oxidized to
benzaldehyde and benzoic acid, which consumed just a little
AUTHOR INFORMATION
Corresponding Authors
E-mail: Lzliu@ntu.edu.cn (L.L.).
E-mail: sunth@sjtu.edu.cn (T.S.).
ORCID
Lizhong Liu: 0000-0002-6171-8513
Tonghua Sun: 0000-0003-3803-0232
Jinping Jia: 0000-0003-2409-338X
■
*
*
11,45
oxygen and hardly released CO2.
Nevertheless, the amount
of oxygen trapped by the Mn Fe O catalyst was greatly
2.4 0.6
4
reduced by the addition of H O, which was made against
2
Notes
consuming oxygen steps especially to the oxidation of
The authors declare no competing financial interest.
intermediates including −COOH. Consequently, the added
H O could play a distinct effect on the toluene ME compared
2
ACKNOWLEDGMENTS
The authors are grateful for financial support from the
National Natural Science Foundation of China (Grant
1876107).
■
to the RE of toluene. Following the first stage, the oxidation
experiment of toluene with a decrease in the reaction
temperature from 240 to 110 °C was performed (RH =
2
1
00%). The test results were similar to those of Figures 5a and
S5, implying that the Mn Fe O catalyst has good stability.
2.4 0.6
4
REFERENCES
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DOI: 10.1021/acs.inorgchem.9b02105
Inorg. Chem. XXXX, XXX, XXX−XXX