Macroscopic Hexagonal Co3O4 Tubes Derived from Controllable Two-Dimensional Metal-Organic Layer Single Crystals: Formation Mechanism and Catalytic Activity

Article abstract of DOI:10.1021/acs.inorgchem.9b03396

Macroscopic Co₃O₄ hexagonal tubes were successfully synthesized using hollow two-dimensional (2D) MOL (metal-organic layer) single crystals as sacrificial templates. The hollow 2D MOL single crystals were prepared under hydrothermal conditions with acetonitrile (MeCN) as an interference agent. The formation of hollow-structured 2D MOL single crystals was tracked by time-dependent experiments, and two simultaneous paths - namely, the crystal-to-crystal transformation in solution and the dissolution + migration (toward the external surface) of inner crystallites - were identified as playing a key role in the formation of the unique hollow structure. The calculated change in Gibbs free energy (G = -1.18 eV) indicated that the crystal-to-crystal transformation was spontaneous at 393 K. Further addition of MeCN as an interference agent eventually leads to the formation of macroscopic hexagonal tubes. Among all of the as-synthesized Co₃O₄, Co-MeCN-O with a hexagonal tube morphology exhibited the best catalytic performance in toluene oxidation, it achieved a toluene conversion of 90% (T₉₀) at ～227 °C (a space velocity of 60 000 mL g^-1 h^-1) and the activity energy (E_a) is 69.5 kJ mol^-1. A series of characterizations were performed to investigate the structure-activity correlation. It was found that there are more structure defects, more adsorbed surface oxygen species, more surface Co³⁺ species, and higher reducibility at low temperatures on the Co-MeCN-O than on other Co₃O₄ samples; these factors are responsible for its excellent catalytic performance. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization showed that, when there is no oxygen in the atmosphere, the lattice oxygen may be involved in the activation of toluene, and the gas-phase oxygen replenished by the oxygen vacancies was essential for the total oxidation of toluene on the surface of the Co-MeCN-O catalysts, it also proves the importance of oxygen vacancies. Moreover, for the Co-MeCN-O catalysts, no obvious decrease in catalytic performance was observed after 120 h at 220 °C and it is still stable after cycling tests, which indicates that it exhibits excellent stability for toluene oxidation. This study sheds lights on the controllable synthesis of macroporous-microporous materials in single-crystalline form without an external template, and, thus, it may serve as a reference for future design and synthesis of hollow porous materials with outstanding catalytic performance.

Full text of DOI:10.1021/acs.inorgchem.9b03396

pubs.acs.org/IC
Article
Macroscopic Hexagonal Co3O4 Tubes Derived from Controllable
Two-Dimensional Metal-Organic Layer Single Crystals: Formation
Mechanism and Catalytic Activity
Mingyuan Zhang, Sibei Zou, Qian Zhang, Shengpeng Mo, Jinping Zhong, Dongdong Chen, Mingli Fu,
Peirong Chen, and Daiqi Ye*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Macroscopic Co3O4 hexagonal tubes were success-
fully synthesized using hollow two-dimensional (2D) MOL
(metal−organic layer) single crystals as sacrificial templates. The
hollow 2D MOL single crystals were prepared under hydrothermal
conditions with acetonitrile (MeCN) as an interference agent. The
formation of hollow-structured 2D MOL single crystals was
tracked by time-dependent experiments, and two simultaneous
pathsnamely, the crystal-to-crystal transformation in solution
and the dissolution + migration (toward the external surface) of
inner crystalliteswere identified as playing a key role in the
formation of the unique hollow structure. The calculated change in
Gibbs free energy (ΔG = −1.18 eV) indicated that the crystal-to-
crystal transformation was spontaneous at 393 K. Further addition of MeCN as an interference agent eventually leads to the
formation of macroscopic hexagonal tubes. Among all of the as-synthesized Co3O4, Co-MeCN-O with a hexagonal tube morphology
exhibited the best catalytic performance in toluene oxidation, it achieved a toluene conversion of 90% (T90) at ∼227 °C (a space
velocity of 60 000 mL g−1 h−1) and the activity energy (Ea) is 69.5 kJ mol−1. A series of characterizations were performed to
investigate the structure−activity correlation. It was found that there are more structure defects, more adsorbed surface oxygen
species, more surface Co3+ species, and higher reducibility at low temperatures on the Co-MeCN-O than on other Co3O4 samples;
these factors are responsible for its excellent catalytic performance. The in situ diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) characterization showed that, when there is no oxygen in the atmosphere, the lattice oxygen may be
involved in the activation of toluene, and the gas-phase oxygen replenished by the oxygen vacancies was essential for the total
oxidation of toluene on the surface of the Co-MeCN-O catalysts, it also proves the importance of oxygen vacancies. Moreover, for
the Co-MeCN-O catalysts, no obvious decrease in catalytic performance was observed after 120 h at 220 °C and it is still stable after
cycling tests, which indicates that it exhibits excellent stability for toluene oxidation. This study sheds lights on the controllable
synthesis of macroporous−microporous materials in single-crystalline form without an external template, and, thus, it may serve as a
reference for future design and synthesis of hollow porous materials with outstanding catalytic performance.
catalytic oxidation of VOCs, in terms of activity and
selectivity.16,17 Compared with metal oxides prepared via
traditional routes, the MOFs-derived metal oxide materials
generally have advantages such as larger surface area, higher
porosity, controllable morphology, and homogeneous heter-
oatom doping, which are considered to be crucial for
catalysis.18−23 Furthermore, the pyrolysis of MOFs could
easily lead to the formation of hollow-structured metal oxides,
1. INTRODUCTION
Co3O4 materials, because of their low-cost and interesting
physical and chemical properties, have been studied for many
applications, such as water oxidation, carbon monoxide (CO)
oxidation, and catalytic oxidation of volatile organic com-
pounds (VOCs).1−9 The morphology of Co3O4 plays a vital
role in catalysis10−13 and can vary with reagrd to different
preparation methods, such as hydrothermal methods,13 sol−gel
methods,5 template methods,14 coprecipitation methods,15 and
so on. Even though researchers are still exploring new ways to
synthesize Co3O4 with unique structures for better catalytic
performance.
Received: November 19, 2019
Recently, it was found that metal oxides prepared using
metal−organic framework (MOF) pyrolysis exhibit unique
physicochemical properties and are advantageous for the
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
min−1, and held for 5 h. The final products were called Co-MeCN-O,
Co-DMF-O, Co-EtOH-O, Co-H2O−O, Co-MeOH-O, Co-IPA-O,
and Co-EG-O, respectively.
2.2. Catalyst Characterization. Physicochemical properties of
these cobalt-based compounds and oxides were measured via various
techniques, including XRD, SEM, BET, H2-TPR, O2-TPD, XPS, and
in situ DRIFTS. The detailed characterization procedures are
described in the Supporting Information.
2.3. Catalytic Evaluations. The catalytic activity of toluene
oxidation over all the cobalt oxides was evaluated in a tubular fixed-
bed microreactor system (Φ = 10.0 mm). The feeding gas contains
1000 ppm toluene and 20%O2/N2, with a weight hourly space
velocity (WHSV) of 60 000 mL g−1 h−1 in the testing process. The
specific test conditions are described in the Supporting Information.
which favors rapid mass transport, because of their hierarchical
porous structures and high density of metal sites.24−28
The synthesis strategies of hollow-structured materials
derived from MOFs can be sorted into two categories, namely,
the pyrolysis of hollow-structured MOFs into hollow oxides,
and the directional conversion to hollow oxides from as-
synthesized MOFs with predesigned morphology and
composition.27 Typical synthetic routes include gradually
dissolution-regrowth, template-mediated assembly, interfacial
ion exchange, selective etching, self-catalytic pyrolysis, and
heterogeneous contraction.29−35 Recently, Shen et al.36
reported the ordered and highly oriented macropores within
MOFs single crystal, opening up a new area, namely, three-
dimensional (3D)-ordered materials containing both macro-
pores and micropores. The ordered macroporous−micro-
porous structure was synthesized by the template of
polystyrene nanosphere and a heterogeneous double-solvent-
induced nucleation method. This hierarchical framework
improved mass-diffusion properties and showed superior
catalytic activity, relative to the other ZIF-8 catalysts. The
synthesis of mesoporous or macroporous hierarchical crystal-
line porous materials remains an important goal;37,38 the
typical synthetic routes of the hollow structure have been
studied very clearly, but in single-crystalline form, they are still
difficult to design and synthesize.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Structural Characterization. Figure
1 illustrates the preparation of MOFs-derived Co3O4 schemati-
Herein, we report the a controllable synthesis of macro-
scopic hexagonal Co3O4 tubes derived from two-dimensional
(2D) MOLs single crystals and discuss the formation process
of the unique hollow structure. Different interference solvents
are used in the hydrothermal synthesis process, leading to
Co3O4 with different morphologies after templates pyrolysis.
When MeCN is introduced as an interference solvent, the
macroscopic hexagonal tubes form and the entire 3D hollow-
morphology MOFs remained monocrystalline. We performed
time-dependent experiments to track the formation mechanism
of the hollow morphology. The catalytic oxidation of toluene is
also studied and the hollow-morphology Co3O4 showed a
better performance than other Co3O4 derived from other 2D
MOLs.
Figure 1. Schematic illustrations of the synthesis process for the
MOFs-derived Co3O4: (a) molecular structure of 3,5-pyridinedicar-
boxylic and Co(NO3)2 acid, (b) the CoNO6 decahedron 2D MOLs
are yellow polyhedral and the topology is 6,3 and (c) stacking of the
2D MOLs by hydrogen-bonding interactions (dashed bonds). (d)
Typical SEM images of as-synthesized Co3O4.
cally, and all the Co3O4 samples were similarly prepared.
Specifically, the 2D MOL templates were synthesized through
a typically hydrothermal method, and its structure was
identified previously by X-ray single-crystal diffraction as
[Co(3,5-pydc)(H2O)2]n (Figures 1a and 1b).39,40 During
synthesis, sodium hydrate was used to assist ligand
dehydrogenation. Briefly, 3,5-pyridinedicarboxylic acid with
double molar Co(NO3)2·6H2O were mixed in solution,
immediately forming a pink powder of [Co(3,5-pydc)-
(H2O)2]n when no interference agent is present or the
interference agent is ethanol, methanol, N,N-dimethylforma-
mide, isopropanol, or ethylene glycol. However, when
acetonitrile was used as an interference agent, a crystalline
powder of [{Co(3,5-pydc)(H2O)4}·H2O]n was obtained (see
Figure S1 in the Supporting Information).41 After maintaining
at 120 °C for 72 h, the color of the crystalline powder changed
from pink to red. The corresponding crystal structures are
shown in Figures 1b and 1c. The thermogravimetric analyses
(TGA) result shows that the structure frame was collapsed at
350 °C in the nitrogen atmosphere and lost most components
after 500 °C (see Figures S2a and S2b in the Supporting
Information). Therefore, after two-stage calcination, the MOF
template was removed and Co3O4 was harvested (see Figure
1d).
2. EXPERIMENTAL SECTION
2.1. Catalyst Preparation. All chemicals were used without
further purification in this work, and all of them are analytical grade.
All of the MOF templates were prepared through hydrothermal
conditions but in different solvents. In a typical method, 2 mmol 3,5-
pyridinedicarboxylate (HPYDC) and 5 mmol NaOH were dissolved
in 60 mL of mixture of H2O and acetonitrile (MeCN)\N,N-
dimethylformamide (DMF)\ethanol (EtOH)\H2O\methanol (MeO-
H)\isopropanol (IPA)\ethylene glycol (EG) with a 1:1 ratio, 4 mmol
Co(NO3)2·6H2O was dissolved in 20 mL of the same mixture of
solvents. These two solvents were mixed and a lot of pink powder
came out. Then, stirring was performed for 5 min and the resulting
mixture was transferred to a 100 mL Teflon-lined stainless-steel
autoclave. We then held the temperature at 120 °C for 72 h in an
electric oven. After heat preservation, the autoclave was cooled to
room temperature in the natural environment. Afterward, the red
crystals were collected after washing with ethanol several times and
dried at 80 °C for 12 h. The products that came from different
solvents were named Co-MeCN, Co-DMF, Co-EtOH, Co-H2O, Co-
MeOH, Co-IPA, and Co-EG, respectively.
Further products were made from the above-mentioned crystals
through a two-stage calcination method. First, they are calcined in N2
at a heating ramp of 2 °C min−1 from 30 °C to 500 °C and held for 5
h. Then they were cooled naturally to room temperature, calcined in
dry air from room temperature to 350 °C at a heating ramp of 2 °C
B
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
The as-obtained products were investigated via powder X-
ray diffraction (PXRD) to confirm their structural properties.
Figure 2a shows that these MOFs templates retained as
Figure 2. PXRD patterns of (a) the MOFs templates and (b) the
Co3O4 products.
crystalline [Co(3,5-pydc)(H2O)2]n. Because we avoided grind-
ing all of the samples, the discrepancies of peak intensities is
derived from the preferential orientation, so that we can ensure
their structural integrity. Figure 2b shows that all the products
after two-stage calcinations display diffraction peaks at similar
positions and with similar relative intensities. The reflections at
∼19.0°, 31.4°, 36.7°, 38.5°, 44.9°, 59.3°, and 65.1° correspond
to the different planes of the spinel Co3O4 phase (space group:
Fdm (No. 227), JCPDS Card No. 42−1467), including (111),
(220), (311), (222), (400), (511), and (440).42,43 No peaks
were observed from other Co3O4 phases, proving that all of the
Co3O4 products have a spinel Co3O4 phase. The crystallite size
was calculated according to the (311) diffraction peak using
the Scherrer equation, and all the results are summarized in
Table 1.44 It shows that all of the products have a relatively
similar size, in the range of 10−16 nm.
Scanning electron microscopy (SEM) studies were per-
formed to investigate the different morphology of the products.
As shown in Figure 3, two-stage calcination did not change the
morphologies of the samples. In addition, it can be clearly
observed that the Co−H2O sample and Co−H2O−O sample
exhibited four-edge strip morphology (panels a1 and a2 in
Figure 3). When EtOH was added as an interference agent, the
four-edge strip morphology was retained, but the aspect ratio
was increased (panels b1 and b2 in Figure 3). The products
became hexagonal prisms (panels c1 and c2 in Figure 3) when
DMF was added, and macroscopic hexagonal tubes with a pore
size of ∼100 μm were obtained when MeCN was used as an
interference agent (see panels d1, d2, c1, and c2 in Figure 3).
During synthesis, we can observe that the building block
coordination polymers (CPs) in the form of crystalline
powders were constructed in the solvent immediately when
the ligand and the metal ions were mixed. As we know, the
Figure 3. SEM images of the as-synthesized MOFs templates and
Co3O4 samples: (a1) Co−H2O, (b1) Co-EtOH, (c1) Co-DMF, (d1)
Co-MeCN, (a2) Co−H2O−O, (b2) Co-EtOH-O, (c2) Co-DMF-O,
(d2) Co-MeCN-O, (e1) Co-MeCN, and (e2) Co-MeCN-O.
water has the strongest polarity in common solvents, when we
add some other polarity solvent as the interference agent, the
certain crystalline planes of the CPs could selectively absorb
them and adjust the morphology by anisotropic growth.41 It is
suggested that the stronger polarity of the interference agent
caused greater variation in morphology of the CPs. In addition,
Table 1. Textural Properties, Crystallite Size, and Conversion of Toluene on the Different Co3O4 Samples
pore volume
crystallite size
(nm)
T50%
(°C)
T90%
(°C)
Co3+/(Co3+ + Co2+)
(%)
Oads/(Oads + Olatt
(%)
)
sample
(cm3 g−1
)
surface area (m2 g−1
)
Co-MeCN-O
Co−H2O−O
Co-EtOH-O
Co-DMF-O
0.41
0.43
0.35
0.42
123.84
160.08
93.80
10.70
11.81
10.81
12.90
214
231
224
248
227
238
241
258
56.64
55.49
54.72
54.36
57.08
56.84
54.24
43.78
116.98
C
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 4. Time-dependent crystal growth of Co-MeCN. (a) SEM images and photographs of the powder collected after different growth times. (b)
PXRD patterns of the powder collected after different growth times. (c) Diagram of the hollow structure formation process.
we also tried MeOH, EG, and IPA as the interference solvents.
The MeOH had nearly the same polarity of EtOH and showed
almost the same result (see Figure S3a in the Supporting
Information). The weak polarity solvent IPA and nonpolar
solvent EG showed little influence of the morphology (see
Figures S3b and S3c in the Supporting Information).
bigger, and even cannot hold the hollow hexagonal prism
morphology. Taken together, the reactants transformed from 1
to 2 over time. In the beginning, the ligand and the metal ions
were present in high concentration in the reaction solution,
and assembled to powder crystals; when 2 started to crystallize,
the concentration was very low, so at that time, the ligand and
metal ions over 1 play the role of seed to help 2 crystallize on
1. 1 was probably covered by 2 at the beginning, but over time,
the inner crystallites of 1 dissolved and migrated out to grow
outside as 2. This process provided the original hollow inner
surface. The change in Gibbs free energy of the transition from
1 to 2 was calculated via density functional theory (DFT),
according to the methods described in the Supporting
Information. The DFT calculation result is ΔG = −1.18 eV,
which suggested that the reaction was spontaneous at 393 K;
this is consistent with the observed results. At the same time,
since the entire system tends toward the low-energy state, the
process of inner crystallites 2 dissolve and outside crystallites
grow, also following this trend.26 At that moment, certain
crystalline facets of 2 were selectively absorbing the
interference agent MeCN and adjusting the morphology to
macroscopic hexagonal tubes. To support this, we designed an
experiment, after the pink powder crystals came out, the solid
and the mother liquid were separated by filtering, then the
solid was washed by mixed solution (MeCN:H2O = 1:1)
several times to remove the ligand and metal ions, then
transferred 80 mL of the same mixed solution to a Teflon-lined
autoclave; the mother liquid was also put into the Teflon-lined
autoclave and both of them were heated at 120 °C for 72 h.
SEM analysis of the as-synthesized products shows that,
without the mother liquid, the powder crystals cannot form a
hollow hexagonal prism structure; in another autoclave, with
only mother liquid, only a few red crystals can be found and
also cannot form the hollow hexagonal prism structure (see
Figure S4 in the Supporting Information). Therefore, in this
reaction, the formation mechanism of the hollow structure was
divided into two simultaneous paths: one is the process of
crystal-to-crystal transformation in the solution, which
provided the original hollow inner surface, and the other one
3.2. Formation Mechanism of the Macroscopic
Hexagonal Tubes. When the MeCN became an interference
agent, the morphology became macroscopic hexagonal tubes
with infrequent micrometer-sized super hollow holes. The
single-crystal structure shows that the structure had no
changes, even becoming macroscopic hexagonal tubes (see
Table S1 in the Supporting Information). To the best of our
knowledge, this is the largest macroscopic hexagonal tube
morphology that has been reported.45,46 A time-dependent
reaction experiment then was conducted to study how this
special morphology formed. The reactants were prepared using
a one-pot method and divided into reactors, and solvothermal
treatment was used for different periods (1, 2, 4, 8, 12, 24, 72,
and 96 h) at 120 °C. Photographs of the solids after centrifugal
washing were taken (Figure 4a), as shown in Figure 4a; up to 2
h of reaction, all of the products are pink powder crystals, and
after 4 h of reaction, the red crystal is presented. The time-
dependent PXRD shows that the pink powder crystals
represent the crystal of [{Co(3,5-pydc)(H2O)4}·H2O]n (1),
and the pure red crystal is the crystal of [Co(3,5-pydc)-
(H2O)2]n (2) (Figure 4b). As time goes by, there are more and
more red crystals but fewer powder crystals. For longer periods
up to 96 h, all of the powder crystals disappear and only red
crystals are left. The time-dependent PXRD also followed this
process. Time-dependent scanningelectron microscopy (SEM)
analysis shows that, until 4 h, the morphology is mainly four
edge strips ∼1 μm in size (Figure 4a). According to the PXRD
analysis, it is the powder crystal of 1 (Figure 4b). After 8 h, the
morphology changed to hollow quadrilateral prisms. After 12
h, the hollow quadrilateral prisms became bigger. After 24 h,
the hollow quadrilateral prisms changed to hollow hexagonal
prisms. As time goes by, the hollow hexagonal prisms became
bigger and bigger, the hollow pores also became bigger and
D
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
is the process of inner crystallites dissolving and migrating out
to grow outside to reduce surface energy. Both paths are
indispensable. Their combined action with MeCN eventually
led to the formation of macroscopic hexagonal tubes. This
formation process provides a proposal on the controllable
synthesis of macroporous−microporous materials in single-
crystalline form without an external template, and thus may
serve as reference for the design and synthesis of hollow
porous materials.
3.3. Physicochemical Characterization and Catalytic
Performance. After two-stage calcination, the MOFs were
changed to Co3O4, with the respective morphology largely
retained. Multiple characterization techniques were used to
examine the physical and chemical properties of the as-
synthesized Co3O4, and the catalytic performance of Co3O4
was evaluated using toluene oxidation as a model reaction.
The N2 adsorption−desorption isotherms and pore size
distributions of the prepared Co3O4 samples are shown in
Figure S5 in the Supporting Information. All of the samples
display typical hysteresis loops and classical type-IV isotherms,
showing that all the Co3O4 samples exist mesoporous
structures.47 The BET area and pore volume are summarized
in Table 1. The pore volume of all the samples is ∼0.40 cm3
g−1, and the specific surface area is in the range of 93.80−
160.08 m2 g−1. All of the Co3O4 samples exhibits higher
specific surface area and pore volume than the traditional
Co3O4 catalysts; the higher specific area and pore volume
indicate more reaction area and is more conducive to the
adsorption of reactants.
X-ray photoelectron spectroscopy (XPS) was used to
analyze the oxidation states and the surface compositions of
the Co3O4 samples. As shown in Figure 5a, the Co 2p3/2
spectra at ∼780.0 eV could be further deconvoluted into two
component peaks and two shakeup satellite peaks: the shoulder
peak at ∼779.9 0.1 eV is associated with Co3+ and the peak
at ∼781.4 0.1 eV is associated with Co2+, while the peak at
784.8 0.1 eV corresponds to the shakeup satellite peaks of
Figure 5. XPS spectra of different Co3O4 catalysts: (a) Co 2P and (b)
O 1s.
Raman scattering was used to study the defects; the Raman
spectra of the Co3O4 catalysts are shown in Figure S8 in the
Supporting Information.53 Five different Raman bands can be
observed (A1g + Eg + 3F2g) at 194, 472, 514, 610, and 680
cm−1 in the 100−850 cm−1 range, corresponding to the
different modes of crystalline Co3O4.54 The weak Raman peaks
located at 194, 514, and 610 cm−1 can be associated with
tetrahedral sites (CoO4) characteristics. The band at 472 cm−1
is attributed to the Eg symmetry, and the strong band located
in 680 cm−1 is assigned to the symmetry of A1g which is
attributed to the octahedral sites (CoO6) of the Co3O4
phase.53 Furthermore, the A1g symmetry in the Co-EtOH-O,
Co-H2O-O, and Co-MeCN-O are located at 673 cm−1, 671,
and 663 cm−1, respectively, and are shifted to a lower
wavenumber position than that of Co-DMF-O. Usually, the
red-shift in the A1g symmetry is induced by a more defective
structure of the catalyst, which is beneficial to increase the
density of surface oxygen vacancies.51 According to the Raman
results, the abundance of surface oxygen vacancies follows the
order Co-MeCN-O > Co-H2O-O > Co-EtOH-O > Co-DMF-
O, which matches well with the XPS results.51,55
Temperature-programmed reduction in H2(H2-TPR) was
used to investigate the reducibility of the Co3O4 catalysts. As
shown in Figure S9a in the Supporting Information, there are
two peaks presented in the H2-TPR profiles, corresponding to
a reduction process with two steps. The first peak centered at
∼210 °C is attributed to the reduction from Co3+ to Co2+ and
is accompanied by a structural change from Co3O4 to CoO.
The second peak centered at ∼305 °C is associated with the
further reduction of CoO to simple elemental cobalt.51,54
Clearly, the reduction peak at ca. 210 °C of the Co-MeCN-O
Co3+ and the peak at 789.3
0.1 eV corresponds to the
shakeup satellite peaks of Co2+.4,48 The O 1s spectra at ∼529.9
eV can be deconvoluted into three peaks: at ∼529.8 eV,
corresponding to surface lattice oxygen (Olatt) species; at 531.2
eV, corresponding to surface-adsorbed oxygen (Oads) species;
and at 533.1 eV, corresponding to chemisorbed water
(Ow).12,49 Quantitative analyses via integrating the areas of
fitted peaks of the Co 2p3/2 and O 1s spectra obtained the
relative atomic ratios of Co3+/(Co3+ + Co2+) and Oads/(Oads
+
Olatt) on the surface (see Table 1). Both ratios follow the order
Co-MeCN-O > Co−H2O−O > Co-EtOH-O > Co-DMF-O.
According to previous studies,8,50,51 more surface Oads often
means more surface oxygen vacancies, in which the increased
Co3+ concentration can impact oxygen desorption, further
impacting the catalytic performance. To verify the presence of
oxygen vacancies, electron spin resonance (ESR) measure-
ments were performed. As shown in Figure S6 in the
Supporting Information, in comparison with other Co3O4,
the Co-MeCN-O showed a stronger signal at g = 2.004, which
can be attributed to the oxygen vacancy of nanomaterials.52 It
is expected that the Co-MeCN-O sample with the most surface
Co3+ and Oads will lead to more oxygen vacancies, which may
exhibit the best catalytic performance among all of the Co3O4
samples (see Figure S7 and Table S2 in the Supporting
Information).
E
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
sample showed higher intensity than the other three samples,
indicating a higher concentration of Co3+ ions, inconsistent
with the XPS results. The weak peak observed at 85−90 °C for
all the samples can be attributed to the surface-active oxygen
species on the Co3O4 catalysts, suggesting the adsorption of
molecular oxygen species by the oxygen vacancies.54,56 These
H2-TPR results showed good agreement with the Raman
results. All of the above results manifest that the more-
adsorbed oxygen species and richer Co3+ concentration of Co-
MeCN-O enhanced the reducibility, which corresponds to the
XPS results.
Temperature-programmed desorption of O2 (O2-TPD)
experiments were used to study the interaction between the
Co3O4 surface and adsorbed oxygen species. Generally, there
are four different types of oxygen species: oxygen vacancies in
adsorption molecular oxygen species (Ov), oxygen ions
adsorbed on the surface (Oads), lattice oxygen (Olatt,s) on the
surface, and lattice oxygen in bulk phase (Olatt,b). The
temperature for 90% toluene conversion) is 227 °C. In
contrast, the toluene conversions are significantly lower at 227
°C, namely, 64.37% for Co-EtOH-O, 35.70% for Co-H2O-O,
and only 6.18% for Co-DMF-O (see Figure S10 and Table S2
in the Supporting Information). The catalytic performance of
Co-MeCN-O is also superior, compared with previously
reported pure cobalt-based catalysts (see Table S3 in the
Supporting Information). Furthermore, the linear Arrhenius
plots of toluene conversion over different catalysts were
achieved at a low conversion (<20%). As shown in Figure 6b,
the apparent activation energy (Ea) values of the different
samples follow the order Co-DMF-O (128.9 kJ/mol) > Co-
EtOH-O (94.5 kJ/mol) > Co-H2O-O (84.6 kJ/mol) > Co-
MeCN-O (69.5 kJ/mol). Usually, the lower Ea value exhibits
superior catalytic activity, and the results confirm that the Co-
MeCN-O sample indicates the highest catalytic activity in
toluene combustion. In addition, to verify the stability of the
Co-MeCN-O, stability tests (at 220 °C for 120 h) and cycling
tests over Co-MeCN-O were performed; the toluene
conversion remained at 75% with little change (Figure 6c),
and the curves in cycling tests overlapped well after three
consecutive tests (Figure 6d). SEM and PXRD analyses were
used to observe the catalyst after three reaction cycles. As
shown in Figure S11a in the Supporting Information, the
morphology of the catalyst remained almost the same. The
PXRD of the used catalyst was well retained, and no additional
peaks from other species were observed (see Figure S11b in
the Supporting Information). The TGA curve of the fresh Co-
MeCN-O showed only 3.5% weight loss, which can attributed
to the desorption of water (see Figure S12a in the Supporting
Information), and the TGA curve of the cycling test sample
showed only 0.73% weight loss; from these observations, it can
be concluded that the Co-MeCN-O does not easily accumulate
carbon during reaction (see Figure S12b in the Supporting
Information).
In situ DRIFTS analysis was performed to track the
instantaneous composition on the surface of the Co-MeCN-
O catalyst, which can study the useful oxygen species and the
formed intermediates species in the toluene oxidation process.
As shown in Figures 7a and 7b, the Co-MeCN-O exposed in
500 ppm toluene/N2 or toluene/air at different temperatures.
As the temperature rises, various species occurred changes.
The band at ∼1450−1598 cm−1 can be ascribed to ν(CC)
vibrations of the aromatic rings skeleton, the band at 3070
cm−1 can be attributed to the methyl (−CH3) group, and the
band at 1025 cm−1 can be ascribed to vibration of the benzene
rings on C−H in-plane bending, indicating that the toluene is
adsorbed on the surface of the Co-MeCN-O.58 The bands at
1306 cm−1 are ascribed to vibrations of methylene (−CH2)
groups with C−H bending, indicating that the CH3 groups
from toluene was cracked into methylene and formed benzyl
(C6H5−CH2) on the surface of Co-MeCN-O.51 The bands at
∼1135, 1157, 1177, and 1180 cm−1 are attributed to the
stretching vibrations of C−O from typical alkoxide species
(C6H5−CH2−O). The band at 1215 and 1234 cm−1 are
ascribed to the stretching vibrations of C−O from the
phenolate (C6H5−OH) on the surface.59 The bands at
∼1652 cm−1 are attributed to the stretching vibrations of
ν(CO), indicating the generation of benzaldehyde species
(C6H5−CHO). The band at ∼1541 cm−1 is ascribed to the
formate species (−COO), suggesting the formation of
benzoate species (C6H5−COO), which is the key intermediate
during toluene oxidation.51 As shown in Figures 7a and 7b, on
desorption tendency follows the order Ov > Oads > Olatt,s
>
13,57
Olatt,b
.
As observed in Figure S9b in the Supporting
Information, the desorption peaks are roughly divided into
regions of <450 °C, 450−700 °C, and >700 °C, which
correspond to active oxygen on the surface (Ov and Oads),
lattice oxygen on the surface, and lattice oxygen in bulk phase.
All of the Co3O4 samples presented three peaks and the peak
intensity of Co-MeCN-O sample is greater than that observed
for other samples, indicating that the Co-MeCN-O sample has
more plentiful surface active oxygen species (Ov and Oads).
Usually, the more-intense desorption peaks and lower
beginning oxygen desorption temperatures lead to better
catalytic performance. Consequently, Co-MeCN-O will have
better catalytic activity; the Raman and H2-TPR analyses also
support this result.
The relationship between toluene conversion and reaction
temperature over different Co3O4 catalysts is shown in Figure
6a. The increase in toluene conversion can be clearly observed
as the temperature rises and reached 100% at 270 °C over all
of the catalysts. The Co-MeCN-O sample shows the best
catalytic activity, the T50 value (the temperature for 50%
toluene conversion) is 214 °C and the T90 value (the
Figure 6. (a) Catalytic performance of the Co3O4 catalysts; (b)
Arrhenius plots of the Co3O4 catalysts; (c) results from stability tests
of Co-MeCN-O at 220 °C; and (d) results from cycling tests over Co-
MeCN-O (toluene oxidation conditions: 1000 ppm toluene, 20 vol %
O2/N2, and WHSV = 60 000 mL g−1 h−1).
F
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
of oxygen species adsorbing on the surface by accelerating the
adsorption of molecular O2.63 Meanwhile, the adsorbed oxygen
species on the surface can be easy to desorb and is considered
to play the main role in the total oxidation of VOCs.64 Based
on the Raman results (Figure S8) and XPS results (Table 1), it
can be concluded that the structural defects and surface
adsorbed oxygen species are the reason for the excellent
catalytic activity of Co-MeCN-O in toluene oxidation. The
result of in situ DRIFTS can indicate that the oxygen vacancies
replenished the gas-phase oxygen and played a vital role during
the toluene oxidation; it also proves the importance of oxygen
vacancies. Furthermore, the low-temperature reducibility of
Co-MeCN-O may enhance the catalytic performance.65
During the reaction, toluene is oxidized along with a
transformation from Co3+ and Co2+.13 At the same time, the
increase in the content of high-valence metal ions promotes
the reactivity of oxygen species near the metal ions and the
chemical potential.13 Overall, the oxidation state of cobalt also
affects the catalytic performance.66 According to Raman, H2-
TPR, and XPS analyses, the excellent catalytic activity of Co-
MeCN-O is due to better reducibility and a higher abundance
of surface Co3+ among all the Co3O4 samples.
4. CONCLUSIONS
In summary, macroscopic hexagonal Co3O4 tubes were
successfully synthesized through the thermolysis of hollow
2D MOL single crystals. The hollow 2D MOL single crystals
were synthesized via the hydrothermal process, with
acetonitrile as the interfering agent. Moreover, based on the
time-dependent tracking experiments, a formation mechanism
including two complementary paths is first discovered to reveal
the formation of hollow single-crystalline 2D MOLs, namely,
the crystal-to-crystal transformation in the solution and the
dissolution + migration (toward the external surface) +
recrystallization of inner crystallites to reduce surface energy.
The calculation of the change in Gibbs energy (ΔG = −1.18
eV) also illustrates that the crystal-to-crystal transformation
was spontaneous at 393 K. Under the combined action with
MeCN, macroscopic hexagonal tubes can be finally obtained.
Among all of the as-synthesized Co3O4, the Co-MeCN-O
hexagonal tube morphology shows the best catalytic perform-
ance, with regard to toluene oxidation (T90 = 227 °C, Ea = 69.5
kJ mol−1). In addition, for the Co-MeCN-O, the catalytic
efficiency has no significant decrease after 120 h at 220 °C, and
it is stable after the cycling test. At the same time, Co-MeCN-
O also shows a greater number of structure defects, more
surface adsorbed oxygen species, lower temperature reduc-
ibility, and greater surface Co3+ concentration. All of these
features together lead to better catalytic activity. In addition,
the in situ DRIFTS shows that, during the toluene total
oxidation, the lattice oxygen on the surface of the catalyst may
be involved in the toluene initial activation, but the oxygen
vacancies that replenished the gas-phase oxygen played a vital
role in the toluene total oxidation; it also proves the
importance of oxygen vacancies. Our work provides a proposal
on a controllable synthesis of macroporous−microporous
materials in single-crystalline form without external template
and, therefore, may serve as a reference for the design and
synthesis of hollow porous materials in the future. It also
provides more selection possibility of catalyst in practical
applications.
Figure 7. (a, b) In situ DRIFTS results for Co-MeCN-O at different
temperatures: N2 + toluene (panel ((a)) and air + toluene (panel
((b)). (c) Schematic diagram of the important components in toluene
degradation.
the surface of Co-MeCN-O, the key intermediates are similar
in different atmospheres. During the toluene adsorption
process, the key intermediates can be clearly observed and
the bands of the key products increased with the rising
temperature until 200 °C. In the toluene/N2 conditions, after
the temperature is >200 °C, the intensity of key intermediate
bands and the toluene showed no obvious change (Figure 7a).
However, in contrast, when the gas-phase oxygen was added,
the intensity of the toluene and key intermediates decreases as
the temperature increases (Figure 7b). The above results show
that the lattice oxygen and the gas-phase oxygen in the Co-
MeCN-O can both involve the initial activation of toluene,
because, on the surface of the catalyst, the gas-phase oxygen
species could be reconstituted to the surface lattice oxygen;
only gaseous oxygen can be used for the deep oxidation of
toluene. On the surface of Co-MeCN-O catalysts, according to
the in situ DRIFTS result, the process of toluene catalytic
oxidation shows that the Mars−van Krevelen (MvK)
mechanism and the Langmuir−Hinshelwood (L-H) mecha-
nism operate at the same time.60 In addition, the reaction
mechanism should mainly follow the pathway shown in Figure
7c: toluene → benzyl radical → benzaldehyde → benzoate →
formate species → carbon dioxide and water.61,62
Significantly, the catalytic performance of Co3O4 is
associated with many aspects, such as reducibility, oxygen
vacancies, morphology, etc. Generally, the generation of
surface oxygen vacancies is the main reason for the defective
structure of spinel Co3O4, which often leads to the formation
G
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
ASSOCIATED CONTENT
ACKNOWLEDGMENTS
■
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03396.
We acknowledge the group of Prof. Hongxia Xi at South China
University of Technology (SCUT) for providing computa-
tional resources.
Supplemental figures and tables (PDF)
REFERENCES
■
(1) Li, Y.; Wang, H.; Li, Y.; Wang, Q.; Li, D.; Wang, R.; He, B.;
Gong, Y. 2D metal-organic-framework array-derived hierarchical
network architecture of cobalt oxide flakes with tunable oxygen
vacancies towards efficient oxygen evolution reaction. J. Catal. 2018,
364, 48−56.
(2) Zhang, M.; de Respinis, M.; Frei, H. Time-resolved observations
of water oxidation intermediates on a cobalt oxide nanoparticle
catalyst. Nat. Chem. 2014, 6, 362.
(3) Wang, H.-Y.; Hung, S.-F.; Chen, H.-Y.; Chan, T.-S.; Chen, H.
M.; Liu, B. In Operando Identification of Geometrical-Site-Depend-
ent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016,
138 (1), 36−39.
(4) Mo, S.; Li, S.; Li, J.; Deng, Y.; Peng, S.; Chen, J.; Chen, Y. Rich
surface Co(iii) ions-enhanced Co nanocatalyst benzene/toluene
oxidation performance derived from CoIICoIII layered double
hydroxide. Nanoscale 2016, 8 (34), 15763−15773.
AUTHOR INFORMATION
■
Corresponding Author
Daiqi Ye − School of Environment and Energy, South China
University of Technology, Guangzhou 510006, China; National
Engineering Laboratory for VOCs Pollution Control Technology
and Equipment, Guangzhou Higher Education Mega Centre,
Guangzhou 510006, People’s Republic of China; Guangdong
Provincial Key Laboratory of Atmospheric Environment and
Pollution Control (SCUT), Guangzhou Higher Education
Mega Centre, Guangzhou 510006, People’s Republic of China;
Email: cedqye@scut.edu.cn
Authors
Mingyuan Zhang − School of Environment and Energy, South
China University of Technology, Guangzhou 510006, China;
orcid.org/0000-0001-7447-8005
́
́
́
(5) Rokicinska, A.; Natkanski, P.; Dudek, B.; Drozdek, M.; Litynska-
́
́
Dobrzynska, L.; Kustrowski, P. Co3O4-pillared montmorillonite
catalysts synthesized by hydrogel-assisted route for total oxidation
of toluene. Appl. Catal., B 2016, 195, 59−68.
Sibei Zou − School of Environment and Energy, South China
University of Technology, Guangzhou 510006, China
Qian Zhang − School of Environment and Energy, South China
University of Technology, Guangzhou 510006, China
Shengpeng Mo − School of Environment and Energy, South
China University of Technology, Guangzhou 510006, China
Jinping Zhong − School of Environment and Energy, South
China University of Technology, Guangzhou 510006, China
Dongdong Chen − School of Environment and Energy, South
China University of Technology, Guangzhou 510006, China
Mingli Fu − School of Environment and Energy, South China
University of Technology, Guangzhou 510006, China; National
Engineering Laboratory for VOCs Pollution Control Technology
and Equipment, Guangzhou Higher Education Mega Centre,
Guangzhou 510006, People’s Republic of China; Guangdong
Provincial Key Laboratory of Atmospheric Environment and
Pollution Control (SCUT), Guangzhou Higher Education
Mega Centre, Guangzhou 510006, People’s Republic of China;
orcid.org/0000-0002-7678-1953
Peirong Chen − School of Environment and Energy, South
China University of Technology, Guangzhou 510006, China;
National Engineering Laboratory for VOCs Pollution Control
Technology and Equipment, Guangzhou Higher Education
Mega Centre, Guangzhou 510006, People’s Republic of China;
Guangdong Provincial Key Laboratory of Atmospheric
Environment and Pollution Control (SCUT), Guangzhou
Higher Education Mega Centre, Guangzhou 510006, People’s
Republic of China; orcid.org/0000-0003-1014-8044
(6) Tang, W.; Xiao, W.; Wang, S.; Ren, Z.; Ding, J.; Gao, P.-X.
Boosting catalytic propane oxidation over PGM-free Co3O4 nano-
crystal aggregates through chemical leaching: A comparative study
with Pt and Pd based catalysts. Appl. Catal., B 2018, 226, 585−595.
(7) Zhang, Q.; Mo, S.; Chen, B.; Zhang, W.; Huang, C.; Ye, D.
Hierarchical Co3O4 nanostructures in-situ grown on 3D nickel foam
towards toluene oxidation. Mol. Catal. 2018, 454, 12−20.
(8) Mo, S.; Li, S.; Ren, Q.; Zhang, M.; Sun, Y.; Wang, B.; Feng, Z.;
Zhang, Q.; Chen, Y.; Ye, D. Vertically-aligned Co3O4 arrays on Ni
foam as monolithic structured catalysts for CO oxidation: effects of
morphological transformation. Nanoscale 2018, 10 (16), 7746−7758.
(9) Zhong, J.; Zeng, Y.; Chen, D.; Mo, S.; Zhang, M.; Fu, M.; Wu, J.;
Su, Z.; Chen, P.; Ye, D. Toluene oxidation over Co3+-rich spinel
Co3O4: Evaluation of chemical and by-product species identified by in
situ DRIFTS combined with PTR-TOF-MS. J. Hazard. Mater. 2020,
386, 121957.
(10) Xie, X.; Li, Y.; Liu, Z.-Q.; Haruta, M.; Shen, W. Low-
temperature oxidation of CO catalysed by Co3O4 nanorods. Nature
2009, 458, 746.
(11) Ma, C. Y.; Mu, Z.; Li, J. J.; Jin, Y. G.; Cheng, J.; Lu, G. Q.; Hao,
Z. P.; Qiao, S. Z. Mesoporous Co3O4 and Au/Co3O4 Catalysts for
Low-Temperature Oxidation of Trace Ethylene. J. Am. Chem. Soc.
2010, 132 (8), 2608−2613.
(12) Wang, K.; Cao, Y.; Hu, J.; Li, Y.; Xie, J.; Jia, D. Solvent-Free
Chemical Approach to Synthesize Various Morphological Co3O4 for
CO Oxidation. ACS Appl. Mater. Interfaces 2017, 9 (19), 16128−
16137.
(13) Ren, Q.; Mo, S.; Peng, R.; Feng, Z.; Zhang, M.; Chen, L.; Fu,
M.; Wu, J.; Ye, D. Controllable synthesis of 3D hierarchical Co3O4
nanocatalysts with various morphologies for the catalytic oxidation of
toluene. J. Mater. Chem. A 2018, 6 (2), 498−509.
(14) Wang, Q.; Chen, J.; Zhang, H.; Wu, W.; Zhang, Z.; Dong, S.
Porous Co3O4 nanoplates with pH-switchable peroxidase- and
catalase-like activity. Nanoscale 2018, 10 (40), 19140−19146.
(15) Deng, S.; Liu, X.; Chen, N.; Deng, D.; Xiao, X.; Wang, Y. A
highly sensitive VOC gas sensor using p-type mesoporous Co3O4
nanosheets prepared by a facile chemical coprecipitation method.
Sens. Actuators, B 2016, 233, 615−623.
(16) Chen, X.; Chen, X.; Yu, E.; Cai, S.; Jia, H.; Chen, J.; Liang, P. In
situ pyrolysis of Ce-MOF to prepare CeO2 catalyst with obviously
improved catalytic performance for toluene combustion. Chem. Eng. J.
2018, 344, 469−479.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03396
Funding
National Natural Science Foundation of China (No.
51878292, 51108187, 51672273, B5151050) Science and
Technology Planning Project of Guangdong Province China
(No. 2015B0202236002) Guangdong Natural Science Foun-
dation (No. 2016A030311003).
Notes
The authors declare no competing financial interest.
H
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(17) Luo, Y.; Zheng, Y.; Zuo, J.; Feng, X.; Wang, X.; Zhang, T.;
Zhang, K.; Jiang, L. Insights into the high performance of Mn-Co
oxides derived from metal-organic frameworks for total toluene
oxidation. J. Hazard. Mater. 2018, 349, 119−127.
(36) Shen, K.; Zhang, L.; Chen, X.; Liu, L.; Zhang, D.; Han, Y.;
Chen, J.; Long, J.; Luque, R.; Li, Y.; Chen, B. Ordered macro-
microporous metal-organic framework single crystals. Science 2018,
359 (6372), 206−210.
(37) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G.
D. Generalized syntheses of large-pore mesoporous metal oxides with
semicrystalline frameworks. Nature 1998, 396 (6707), 152−155.
(38) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. An ordered
mesoporous organosilica hybrid material with a crystal-like wall
structure. Nature 2002, 416 (6878), 304−307.
(39) Plater, M. J.; Roberts, A. J.; Howie, R. A. Hydrothermal
Synthesis of Co(pdc)·2H2O (pdc = 3,5-Pyridinedicarboxylate). A
Two-dimensional Coordination Polymer. J. Chem. Res., Synop. 1998,
No. 5, 240−241.
(40) Whitfield, T.; Zheng, L.-M.; Wang, X.; Jacobson, A. J. Syntheses
and characterization of Co(pydc)(H2O)2 and Ni(pydc)(H2O) (pydc
= 3,5-pyridinedicarboxylate). Solid State Sci. 2001, 3 (8), 829−835.
(41) Liu, K.; Shen, Z. R.; Li, Y.; Han, S. D.; Hu, T. L.; Zhang, D. S.;
Bu, X. H.; Ruan, W. J. Solvent induced rapid modulation of micro/
nano structures of metal carboxylates coordination polymers:
mechanism and morphology dependent magnetism. Sci. Rep. 2015,
4, 6023.
(42) Lou, X. W.; Deng, D.; Lee, J. Y.; Feng, J.; Archer, L. A. Self-
Supported Formation of Needlelike Co3O4 Nanotubes and Their
Application as Lithium-Ion Battery Electrodes. Adv. Mater. 2008, 20
(2), 258−262.
(43) Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.;
Kisailus, D.; Zhao, H.; Tang, Z.; Wang, D. Accurate Control of
Multishelled Co3O4 Hollow Microspheres as High-Performance
Anode Materials in Lithium-Ion Batteries. Angew. Chem., Int. Ed.
2013, 52 (25), 6417−6420.
(44) Wei, T.-Y.; Chen, C.-H.; Chien, H.-C.; Lu, S.-Y.; Hu, C.-C. A
Cost-Effective Supercapacitor Material of Ultrahigh Specific Capaci-
tances: Spinel Nickel Cobaltite Aerogels from an Epoxide-Driven Sol-
Gel Process. Adv. Mater. 2010, 22 (3), 347−351.
(45) Yu, L.; Hu, H.; Wu, H. B.; Lou, X. W. Complex Hollow
Nanostructures: Synthesis and Energy-Related Applications. Adv.
Mater. 2017, 29 (15), 1604563.
(46) Yu, L.; Wu, H. B.; Lou, X. W. D. Self-Templated Formation of
Hollow Structures for Electrochemical Energy Applications. Acc.
Chem. Res. 2017, 50 (2), 293−301.
(47) Bai, B.; Li, J. Positive Effects of K+ Ions on Three-Dimensional
Mesoporous Ag/Co3O4 Catalyst for HCHO Oxidation. ACS Catal.
2014, 4 (8), 2753−2762.
(48) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L.
Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and
High Surface Area for the Oxygen Evolution Reaction. Angew. Chem.,
Int. Ed. 2016, 55 (17), 5277−5281.
(49) Seefeld, S.; Limpinsel, M.; Liu, Y.; Farhi, N.; Weber, A.; Zhang,
Y.; Berry, N.; Kwon, Y. J.; Perkins, C. L.; Hemminger, J. C.; Wu, R.;
Law, M. Iron Pyrite Thin Films Synthesized from an Fe(acac)3 Ink. J.
Am. Chem. Soc. 2013, 135 (11), 4412−4424.
(50) Bai, G.; Dai, H.; Deng, J.; Liu, Y.; Wang, F.; Zhao, Z.; Qiu, W.;
Au, C. T. Porous Co3O4 nanowires and nanorods: Highly active
catalysts for the combustion of toluene. Appl. Catal., A 2013, 450,
42−49.
(51) Zhao, S.; Hu, F.; Li, J. Hierarchical Core-Shell Al2O3@Pd-
CoAlO Microspheres for Low-Temperature Toluene Combustion.
ACS Catal. 2016, 6 (6), 3433−3441.
(52) Mo, S.; Zhang, Q.; Li, J.; Sun, Y.; Ren, Q.; Zou, S.; Zhang, Q.;
Lu, J.; Fu, M.; Mo, D.; Wu, J.; Huang, H.; Ye, D. Highly efficient
mesoporous MnO2 catalysts for the total toluene oxidation: Oxygen-
Vacancy defect engineering and involved intermediates using in situ
DRIFTS. Appl. Catal., B 2020, 264, 118464.
(53) Liu, Q.; Wang, L.-C.; Chen, M.; Cao, Y.; He, H.-Y.; Fan, K.-N.
Dry citrate-precursor synthesized nanocrystalline cobalt oxide as
highly active catalyst for total oxidation of propane. J. Catal. 2009, 263
(1), 104−113.
(18) Chen, Y.-Z.; Zhang, R.; Jiao, L.; Jiang, H.-L. Metal-organic
framework-derived porous materials for catalysis. Coord. Chem. Rev.
2018, 362, 1−23.
(19) Lee, K. J.; Lee, J. H.; Jeoung, S.; Moon, H. R. Transformation
of Metal-Organic Frameworks/Coordination Polymers into Func-
tional Nanostructured Materials: Experimental Approaches Based on
Mechanistic Insights. Acc. Chem. Res. 2017, 50 (11), 2684−2692.
(20) Zhang, H.; Nai, J.; Yu, L.; Lou, X. W. Metal-Organic-
Framework-Based Materials as Platforms for Renewable Energy and
Environmental Applications. Joule 2017, 1 (1), 77−107.
(21) Kalmutzki, M. J.; Hanikel, N.; Yaghi, O. M. Secondary building
units as the turning point in the development of the reticular
chemistry of MOFs. Sci. Adv. 2018, 4 (10), No. eaat9180.
(22) Robatjazi, H.; Weinberg, D.; Swearer, D. F.; Jacobson, C.;
Zhang, M.; Tian, S.; Zhou, L.; Nordlander, P.; Halas, N. J. Metal-
organic frameworks tailor the properties of aluminum nanocrystals.
Sci. Adv. 2019, 5 (2), No. eaav5340.
(23) Men, Y.; Liu, X.; Yang, F.; Ke, F.; Cheng, G.; Luo, W. Carbon
Encapsulated Hollow Co3O4 Composites Derived from Reduced
Graphene Oxide Wrapped Metal-Organic Frameworks with Enhanced
Lithium Storage and Water Oxidation Properties. Inorg. Chem. 2018,
57 (17), 10649−10655.
(24) Zhou, L.; Zhuang, Z.; Zhao, H.; Lin, M.; Zhao, D.; Mai, L.
Intricate Hollow Structures: Controlled Synthesis and Applications in
Energy Storage and Conversion. Adv. Mater. 2017, 29 (20), 1602914.
(25) Wang, Z.; Yu, R. Hollow Micro/Nanostructured Ceria-Based
Materials: Synthetic Strategies and Versatile Applications. Adv. Mater.
2019, 31 (38), 1800592.
(26) Zhang, Z.; Chen, Y.; Xu, X.; Zhang, J.; Xiang, G.; He, W.;
Wang, X. Well-Defined Metal-Organic Framework Hollow Nanoc-
ages. Angew. Chem., Int. Ed. 2014, 53 (2), 429−433.
(27) Cai, Z. X.; Wang, Z. L.; Kim, J.; Yamauchi, Y. Hollow
Functional Materials Derived from Metal-Organic Frameworks:
Synthetic Strategies, Conversion Mechanisms, and Electrochemical
Applications. Adv. Mater. 2019, 31 (11), 1804903.
(28) Zhang, J.; Li, F.; Chen, W.; Wang, C.; Cai, D. Facile synthesis
of hollow Co3O4-embedded carbon/reduced graphene oxides nano-
composites for use as efficient electrocatalysts in oxygen evolution
reaction. Electrochim. Acta 2019, 300, 123−130.
(29) Tian, W.; Hu, H.; Wang, Y.; Li, P.; Liu, J.; Liu, J.; Wang, X.; Xu,
X.; Li, Z.; Zhao, Q.; Ning, H.; Wu, W.; Wu, M. Metal-Organic
Frameworks Mediated Synthesis of One-Dimensional Molybdenum-
Based/Carbon Composites for Enhanced Lithium Storage. ACS Nano
2018, 12 (2), 1990−2000.
(30) Lee, I.; Choi, S.; Lee, H. J.; Oh, M. Hollow Metal-Organic
Framework Microparticles Assembled via a Self-Templated Formation
Mechanism. Cryst. Growth Des. 2015, 15 (11), 5169−5173.
(31) Liu, W.; Huang, J.; Yang, Q.; Wang, S.; Sun, X.; Zhang, W.; Liu,
J.; Huo, F. Multi-shelled Hollow Metal-Organic Frameworks. Angew.
Chem., Int. Ed. 2017, 56 (20), 5512−5516.
(32) Sen, U.; Erkartal, M.; Kung, C.-W.; Ramani, V.; Hupp, J. T.;
Farha, O. K. Proton Conducting Self-Assembled Metal-Organic
Framework/Polyelectrolyte Hollow Hybrid Nanostructures. ACS
Appl. Mater. Interfaces 2016, 8 (35), 23015−23021.
(33) Zhan, G.; Zeng, H. C. A Synthetic Protocol for Preparation of
Binary Multi-shelled Hollow Spheres and Their Enhanced Oxidation
Application. Chem. Mater. 2017, 29 (23), 10104−10112.
(34) Liu, S.; Wang, Z.; Zhou, S.; Yu, F.; Yu, M.; Chiang, C.-Y.; Zhou,
W.; Zhao, J.; Qiu, J. Metal-Organic-Framework-Derived Hybrid
Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen
Reduction and Evolution. Adv. Mater. 2017, 29 (31), 1700874.
(35) Zhang, H.; Zhou, W.; Dong, J.; Lu, X. F.; Lou, X. W.
Intramolecular electronic coupling in porous iron cobalt (oxy)-
phosphide nanoboxes enhances the electrocatalytic activity for oxygen
evolution. Energy Environ. Sci. 2019, 12 (11), 3348−3355.
I
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
́
́
́
(54) de Rivas, B.; Lopez-Fonseca, R.; Jimenez-Gonzalez, C.;
́
Gutierrez-Ortiz, J. I. Synthesis, characterisation and catalytic perform-
ance of nanocrystalline Co3O4 for gas-phase chlorinated VOC
abatement. J. Catal. 2011, 281 (1), 88−97.
(55) Zhao, S.; Li, K.; Jiang, S.; Li, J. Pd-Co based spinel oxides
derived from pd nanoparticles immobilized on layered double
hydroxides for toluene combustion. Appl. Catal., B 2016, 181, 236−
248.
(56) Yu, Y.; Takei, T.; Ohashi, H.; He, H.; Zhang, X.; Haruta, M.
Pretreatments of Co3O4 at moderate temperature for CO oxidation at
−80°C. J. Catal. 2009, 267 (2), 121−128.
(57) Bai, B.; Arandiyan, H.; Li, J. Comparison of the performance for
oxidation of formaldehyde on nano-Co3O4, 2D-Co3O4, and 3D-
Co3O4 catalysts. Appl. Catal., B 2013, 142−143, 677−683.
(58) Sun, H.; Liu, Z.; Chen, S.; Quan, X. The role of lattice oxygen
on the activity and selectivity of the OMS-2 catalyst for the total
oxidation of toluene. Chem. Eng. J. 2015, 270, 58−65.
(59) Chen, X.; Chen, X.; Yu, E.; Cai, S.; Jia, H.; Chen, J.; Liang, P. In
situ pyrolysis of Ce-MOF to prepare CeO2 catalyst with obviously
improved catalytic performance for toluene combustion. Chem. Eng. J.
2018, 344, 469−479.
(60) Wang, X.; Zhao, W.; Wu, X.; Zhang, T.; Liu, Y.; Zhang, K.;
Xiao, Y.; Jiang, L. Total oxidation of benzene over ACo2O4 (A = Cu,
Ni and Mn) catalysts: In situ DRIFTS account for understanding the
reaction mechanism. Appl. Surf. Sci. 2017, 426, 1198−1205.
(61) Dong, C.; Qu, Z.; Qin, Y.; Fu, Q.; Sun, H.; Duan, X. Revealing
the Highly Catalytic Performance of Spinel CoMn2O4 for Toluene
Oxidation: Involvement and Replenishment of Oxygen Species Using
In Situ Designed-TP Techniques. ACS Catal. 2019, 9 (8), 6698−
6710.
(62) Mo, S.; Zhang, Q.; Sun, Y.; Zhang, M.; Li, J.; Ren, Q.; Fu, M.;
Wu, J.; Chen, L.; Ye, D. Gaseous CO and toluene co-oxidation over
monolithic core-shell Co3O4-based hetero-structured catalysts. J.
Mater. Chem. A 2019, 7 (27), 16197−16210.
(63) Pan, X.; Yang, M.-Q.; Fu, X.; Zhang, N.; Xu, Y.-J. Defective
TiO2 with oxygen vacancies: synthesis, properties and photocatalytic
applications. Nanoscale 2013, 5 (9), 3601−3614.
(64) Bahlawane, N. Kinetics of methane combustion over CVD-
made cobalt oxide catalysts. Appl. Catal., B 2006, 67 (3), 168−176.
(65) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Venezia, A. M.;
Deganello, G. Co3O4/CeO2 composite oxides for methane emissions
abatement: Relationship between Co3O4-CeO2 interaction and
catalytic activity. Appl. Catal., B 2006, 66 (3), 217−227.
(66) Mo, S.; Li, S.; Li, W.; Li, J.; Chen, J.; Chen, Y. Excellent low
temperature performance for total benzene oxidation over meso-
porous CoMnAl composited oxides from hydrotalcites. J. Mater.
Chem. A 2016, 4 (21), 8113−8122.
J
https://dx.doi.org/10.1021/acs.inorgchem.9b03396
Inorg. Chem. XXXX, XXX, XXX−XXX