Titanium Containing γ -MnO2 (TM) Hollow Spheres: One-step synthesis and catalytic activities in li/air batteries and oxidative chemical reactions

Article abstract of DOI:10.1002/adfm.201001080

Titanium containing γ -MnO₂ octahedral molecular sieves having hollow sphere structures are successfully prepared for the fi rst time using a one-step synthesis method. Titanium cations are used as structure-directing agents in the synthesis pr

Full text of DOI:10.1002/adfm.201001080

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Titanium Containing γ-MnO2 (TM) Hollow Spheres:
One-Step Synthesis and Catalytic Activities in Li/Air
Batteries and Oxidative Chemical Reactions
By Lei Jin, Linping Xu, Christine Morein, Chun-hu Chen, Monique Lai,
Saminda Dharmarathna, Arthur Dobley, and Steven L. Suib*
nanostructures have been success-
fully prepared.[3–6] However, it is still
a significant challenge to develop low
Titanium containing γ-MnO2 octahedral molecular sieves having hollow
sphere structures are successfully prepared for the first time using a one-step
synthesis method. Titanium cations are used as structure-directing agents in
the synthesis process. The assembly of the hollow spheres is carried out at
the beginning of the process. Various techniques including XRD, N2 adsorp-
tion, SEM, EDX, RAMAN, TEM, XPS, and TGA are employed for the materials
characterization. Ti is incorporated into the MnO2 framework in isolated sites,
and TiO2 phases (anatase and rutile) are not observed. When introduced
in medium-sized lithium-air batteries, the materials give very high specific
capacity (up to 2.3 A h g−1 ). These materials are also catalytically tested in the
oxidation of toluene with molecular oxygen at atmospheric pressure, showing
significant oxidative catalytic activities in this difficult chemical reaction.
temperature,
facile,
solution-based,
shape-controlled, and template free self-
assembly routes for the formation of
hierarchical structures.
Nanostructured hollow spheres and
their related structures have either pos-
sess, or are proposed to have, diverse and
fascinating applications, such as cata-
lysts,[7] drug delivery,[8] high efficiency
photodecomposition,[9] and low density
batteries or devices.[10] Their syntheses
often rely on complicated templating
approaches, in which hard (e.g., inor-
ganic, metal, and polymer particles) or
soft sacrificial templates (e.g., supramolecular assemblies of
surfactant and polymer) were used to create a hollow struc-
ture.[11–14] However, template contamination mostly decreases
the activity of synthesized materials and the removal of resi-
dues is difficult.
Titanium containing materials have attracted attention in
catalysis, solar cells, gas sensors, and semiconductors in recent
decades.[15–19] For example, there is a great interest in titanium
containing zeolite catalysts for selective oxidation recently. The
titanium silicalites with MFI (TS-1) or MEL (TS-2) zeolite struc-
tures are remarkable redox catalysts in numerous friendly envi-
ronmental applications.[20] Titanium-containing zeolites have
become some of the most commercially significant catalysts
for green peroxide oxidation of organic molecules.[21] However,
titanium-containing γ-MnO2 octahedral molecular sieves (TM)
with significantly enhanced catalytic activity has rarely been
reported.
Manganese oxide structures can form mixed-valent octahedral
molecular sieves (OMS). These OMS materials have applications
in energy storage, acid catalysts, and ion-exchange processes.[22–27]
An electrochemically and catalytically active manganese oxide
OMS is γ-MnO2. This form of MnO2 octahedral molecular sieve
(also called chemical manganese dioxide, CMD), a disordered 1 ×
1 and 1 × 2 tunnel structure corner shared with MnO6 octahedral
chains,[28] is one of these most useful manganese oxides and has
been widely applied in the battery industry because of its high
activity, low cost of raw materials, and low toxicity. Conventionally,
γ-MnO2 was synthesized by electrolysis (EMD), redox reactions,
1. Introduction
The preparation of nanomaterials of defined structures and
composition, with tailored properties, has attracted immense
scientific and technological interest due to the fact that mor-
phologies and structures play a very important role in deter-
mining the catalytic, magnetic, and electric properties of
materials.[1,2] Control of structures of nanomaterials has been
one of the most challenging issues faced by materials sci-
entists. Many efforts have been focused on self-assembly of
nano-particles into two –dimensional (2D) and three-dimen-
sional (3D) well-ordered hollow spheres, nanorods/nanowires
and several hierarchical structures. For example, hierarchical
ZnO nanostructures, penniform BaWO4 nanostructures,
a trigonal Se nanowire network, and dandelion-like CuO
∗
[ ] L. Jin, Dr. L. Xu, C.-h. Chen, M. Lai, S. Dharmarathna,
Prof. S. L. Suib
Department of Chemistry
University of Connecticut
Institute of Materials Science
University of Connecticut
U-3060, 55 North Eagleville Rd., Storrs, Connecticut 06269 (USA)
E-mail: Steven.Suib@uconn.edu
C. Morein, Dr. A. Dobley
Yardney Technical Products Inc.
82 Mechanic Street, Pawcatuck, Connecticut 06379 (USA)
DOI: 10.1002/adfm.201001080
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or disproportionation. However, to the best of our knowledge,
the γ-MnO2 prepared using the above methods having irregular
morphologies, low surface area (<80 m2 g−1 ), and easy one-step
synthesis of γ-MnO2 hollow spheres with high surface area
(>87 m2 g−1 ) has never been reported.
Li/air batteries have recently attracted interest due to the
largest theoretical specific energy (11 972 W h kg−1 ) among all
practical electrochemical couples.[29–32] In a Li/air battery, the
cathode material, oxygen is absorbed directly from the environ-
ment during the discharge process. The electrochemical reac-
tions involved are described below:
4Li(s) + O2(g) → 2Li2O(s)V = 2.91V
(1)
(2)
Figure 1. A representative XRD pattern recorded for γ–MnO2 synthesized
in this work. a) pure γ–MnO2; b) 2 at% Ti/Mn; c) 4 at% Ti/Mn; d) 8 at%
Ti/Mn.
2Li(s) + O2(g) → Li2O2(s)V = 3.10V
The air cathodes are composed of a mixture of carbon powder,
O2 reduction catalysts, and a binder, and the nature of the cat-
alyst plays a key role in delivering high specific capacities.[29]
γ–MnO2 based materials have been widely used in traditional
batteries. However, the use of γ–MnO2 materials as efficient
O2 reduction catalysts in the Li/air battery is novel.
Development of efficient heterogeneous catalysts for a solvent
free toluene oxidation system has also received considerable
attention due to their convenient use and facile recycling.[33–37]
For example, Wang et al. reported liquid phase toluene oxidation
with molecular oxygen over copper-based metal oxides. In this
process, however, in order to observe good conversion, a closed
capacity autoclave, oxygen pressure, and higher temperature
(T > 160 °C) were used. These common reaction conditions are
too dangerous due to the explosion hazards of toluene-oxygen
vapors at high reaction temperatures.[28,38] From our previous
work, we investigated the solvent free atmospheric oxidation of
toluene in the presence of molecular oxygen using synthesized
γ-MnO2 OMS as the efficient heterogeneous catalyst.[28] More
efficient oxidation catalysts are still needed.
match the reference data of a pure γ-MnO2 phase.[39] The broad-
ening of the (201) peak is known to occur for certain types of
random intergrowths of ramsdellite (1 × 2) tunnel structure
and pyrolusite (1 × 2 tunnel structure) of manganese dioxide
based on the “De Wolff model” published in 1959.[40] Materials
with different loadings of titanium ions were also investigated
by XRD. Without Ti loading, a pure γ-MnO2 phase (1 × 1 and
1 × 2 intergrowth tunnel structure manganese oxide) was
obtained. After Ti-loading, as shown by Figure 1b for a typical
sample with Ti/Mn = 2:100 atomic ratio (TM-3), all the peaks
can be assigned to the γ-MnO2 structure, but with broader peaks.
The greater amount Ti loading led to broader patterns. These
patterns are evidence of smaller particle size and higher surface
area than found for the non-Ti loaded γ-MnO2 materials. After
8at%Tiloading,theγ-MnO2 structurewasmaintained(Figure1d)
with no other phases, including Ti or TiO2 phases present.
However, under similar synthesis conditions, an anatase TiO2
sample was obtained when no MnSO4 and KMnO4 were used.
In this work, a simple one-step precipitation synthesis of
titanium containing γ-MnO2 OMS hollow spheres with enhanced
catalytic activity is first reported without adding surfactants or tem-
plates. Herein, Ti ion was used as the morphology directing agent
in the precipitation process. With the addition of titanium ions,
the obtained γ-MnO2 octahedral molecular sieves have hollow
spherical morphologies and high surface areas (>87 m2 g−1 ).
A high charge storage capacity (2.2 A h g−1 carbon) was
obtained in the lithium air battery tests by using the reported
titanium containing γ-MnO2 hollow spheres as the O2 reduction
catalyst. In addition, synthesized γ-MnO2 OMS have promising
catalytic activities in the atmospheric oxidation of toluene in
the presence of molecular oxygen.[28] In this work, the obtained
γ-MnO2 has significantly enhanced catalytic activity in this dif-
ficult liquid phase toluene oxidation process.
2.2. Surface Area (SA), Porosity, and Energy Dispersive X-ray
Spectroscopy (EDX)
N2 physisorption for selected titanium containing γ-MnO2
material (TM-1, Ti/Mn = 8:100, 8 at%) shows a typical isotherm
with hysteresis for that of porous materials, shown in Figure 2.
The insert in Figure 2 shows a broad mesopore distribution cal-
culated by the BJH method. The 8% Ti containing γ-MnO2 has
pores with average diameters of 37 Å. The BET surface area of
the material is higher than those of materials without Ti incor-
poration, as summarized in Table 1. EDXS data reveal the exist-
ence of manganese and titanium. No potassium was obtained.
Fixed ratios of Ti/Mn around 0.02, 0.04, and 0.08 were observed
as designed. The EDXS results are also summarized in Table 1.
2. Results
2.3. Scanning Electron Microscope (SEM) and Transmission
Electron Microscopy (TEM)
2.1. X-ray Diffraction (XRD)
Figure 3 shows typical morphologies of pure γ-MnO2 and
the selected Ti-containing γ-MnO2 (TM-1). A nano-fiber like
The phase purity of the products was investigated by conven-
tional XRD experiments, as shown in Figure 1. All of the peaks
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8 at% Ti). In addition, compared to the pure
γ-MnO2, after Ti incorporation, the materials
did not show any extra weight loss peaks.
2.5. Raman Spectroscopy
Raman spectroscopy was used to study
the presence of Ti in the framework of the
γ-MnO2 octahedral molecular sieves. Various
authors have used Raman techniques to
study transition metal atoms in the frame-
Figure 2. Pore size distribution of selected Ti containing γ–MnO2 (TM-1).
work of molecular sieves.[44–47] Raman spectra
of titanium containing γ-MnO2 are shown in
Figure 5. The peaks of MnO2 located around
600–650 cm−1 are overlapped with a strong band of TiO2 in this
region. Raman patterns of pure anatase TiO2 clearly showed
peaks at 394.2, 513.4 and 637.6 cm−1 . Physically mixed 8 mol%
TiO2 with γ-MnO2 (pure γ-MnO2 synthesized using the proce-
dure mentioned above) also showed peaks at 394.2 cm−1 and
513.4 cm−1 , indicating an anatase tetragonal structure assigned
to the 2B1g mode (394.2 cm−1 and 513.4 cm−1 ) for the space
group D4h/19. However, the synthesized Ti containing manga-
nese oxides showed no peaks in this area. With increasing con-
tents (higher Ti/Mn molar ratio) in the samples, the peaks of
MnO2 located around 600 – 650 cm−1 were shifted and broaden.
No additional peaks were present for any TiO2 crystalline phase
or Ti=O bond vibrations (from TiO2+ species).
morphology for pure γ-MnO2 (Figure 3a) with an average diam-
eter of about 50 nm was observed. However, after Ti incor-
poration, the morphology changed to hollow nanospheres
composed of nano-flakes with an average flake thickness
of about 15 nm. Both SEM and TEM images show a uni-
form morphology for the Ti-containing γ-MnO2 and the pure
γ-MnO2.
The hollow nature of the synthesized material (TM-1) was
confirmed by TEM imaging (Figure 3e and 3f). The diam-
eters of the hollow shells range from 200 to 500 nm. The
high magnification image reveals a shell thickness of 50 nm
(Figure 3f). Increasing the magnification in Figure 3f clearly
reveals the lattice fringes of a flake showing the lattice spacing
of the (110) planes, d ∼ 3.2 Å. No extra phases were observed for
the hollow spheres even down to the 5 nm scale. The features
observed in the TEM images also show broken lattice fringes,
which correspond to structural defects and distortion of γ -MnO2
as reported in the literature.[40–43] These defects and distortions
are important for the catalytic activity of the γ-MnO2.[41]
2.6. X-ray Photoelectron Spectroscopy (XPS)
XPS was used to analyze the binding energy (BE) values
and the atomic surface concentration of the corresponding
elements present in the as-obtained Ti containing γ–MnO2
molecular sieve (8 at% Ti/Mn). The XPS survey spectrum
reveals that the 8% Ti containing γ–MnO2 sample consists of
the elements Ti, Mn, and O, no other impurities such as potas-
sium ions or elemental sulfur are detected. The Mn 2p core
level spectrum (Figure 6a inset) illustrates that the observed
values of the binding energies for Mn 2p 3/2 and Mn 2p 1/2
(641.6 and 653.1 eV, respectively) are in good agreement with
the literature values for MnO2. The Ti 2p core level spectrum
(Figure 6a inset) showed binding energies of 458.8 eV and
464.3 eV for Ti 2p 3/2 and Ti 2p 1/2 respectively. To define
the difference in BE values from Ti-O-Ti binding present in
TiO2 and Ti-O-Mn in the γ–MnO2 framework is difficult. The
2.4. Thermogravimetric Analysis (TGA)
The thermal stability of the synthesized materials was analyzed
by TGA measurements in N2 atmosphere. As illustrated in
Figure 4, all of the materials only show major weight losses for
temperatures higher than 450 °C. The weight loss is related to
release of oxygen in the structure causing a structural collapse
similar to other OMS materials. With increasing of Ti incorpo-
rating levels from 0% to 8%, temperatures for structural col-
lapse decrease from around 520 °C (pure γ-MnO2) to 450 °C
(TM-3, 2 at% Ti), 460 °C (TM-2, 4 at% Ti), and 470 °C (TM-1,
T a b le 1 . BET surface area of synthesized materials and EDXS results on titanium analysis.
Sample ID
BET surface area [m2 g−1
]
EDXS analysis
Theoretical [at %]
Detecting Elements
Experimental [at %]
18
59
77
87
Ti
Ti
Ti
Ti
0
2
4
8
0
Pure γ–MnO2
2% Ti/Mn
4% Ti/Mn
8% Ti/Mn
1.9
3.5
7.5
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depend strongly on the surface concentra-
tion of the atoms present, thus making the
interpretation of the data with respect to Ti
more complicated.
2.7. Catalytic Results
Electrocatalytic activities of the selected Ti
containing γ-MnO2 (TM-1) were tested in a
lithium-air battery. Table 2 lists the specific
capacities in three battery cells. An average
specific capacity of 2.2 A h g−1 of carbon
delivered by using the prepared material as
the O2 reduction catalyst was obtained. The
high specific capacity indicates that the mate-
rial we report here is an efficient O2 reduction
catalyst in the application of Li/air battery.
On the other hand, in our previous work, we
have reported that γ-MnO2 was very active
in atmospheric toluene oxidation. By using
the synthesized γ-MnO2, 17.6% conversion
was achieved. In this work, we compared
the Ti incorporated γ-MnO2 hollow sphere
materials with the unincorporated material
we reported before. As shown in Table 3,
the catalytic activity increased from 17%
to 31% with increasing Ti content in the
materials, indicating that the as-synthesized
hollow spheres are more efficient catalysts
than a previously reported γ-MnO2 for tol-
uene oxidation at atmospheric pressure and
in an easily operated reaction system.
Figure 3. SEM images of γ–MnO2 spheres. A) morphology of the pure γ-MnO2; B) overall
morphology of TM-1 (8% Ti); C) detailed views on average sized spheres; D) a detailed view
on an individual sphere; and E) TEM over all image of the synthesized hollow spheres, F) high
magnification of (E).
interpretation of XPS spectra of Ti species present in γ–MnO2
3. Discussion
framework is complex. However, Figure 6b shows the core
level spectrum of O 1s, and two binding energies of 530.2 eV
and 532.2 eV were observed. The BE of 530.2 eV can be
assigned to the Mn-O binding in MnO2, and the BE of 532.2 eV
might belong to the Ti-O-Mn species, suggesting that the Ti
species are incorporated into the γ–MnO2 framework. Because
XPS is a surface technique, the photoelectron peaks would
3.1. Titanium Incorporation
There are three possible forms for Ti to be present in the prod-
ucts: Free TiO2+ cations in the tunnels, framework incorporation
in the OMS materials, or TiO2. The tunnels of γ-MnO2 with a
size of 2.3Å × 2.3Å and 2.3Å × 4.6Å might not accommodate
titanium species since the total lengths of Ti-O bonds are
around 3.7 Å. On the other hand, γ-MnO2 does not support
ion-exchange activity.[48] After an overnight ion-exchange with
300 mL of 0.5 M sodium sulfate solution, γ-MnO2 shows similar
Ti/Mn molar ratios of about 0.08:1 from EDX results. There-
fore, the only possibility for Ti with such a high percentage in
the total manganese seems to be incorporation of Ti4+ cations
into the framework of γ-MnO2 because the cations in the tun-
nels can be ion-exchanged with the cations in the solution and
Ti/Mn ratio should be decreased after ion-exchange. Similar
sizes, charges, and coordination numbers of manganese and Ti
cations might contribute to the incorporation of Ti into MnO6
octahedral units. Crystal radii of octahedral Ti4+, Mn3+ and Mn4+
in crystals are 0.75 Å, 0.72 Å, and 0.67 Å respectively. Ti4+ may
substitute for Mn4+ without causing much structural disorder
or serious charge imbalances as shown in XRD patterns.
Figure 4. TGA plot of the prepared Ti containing γ–MnO2 materials.
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Figure 5. Normalized Raman spectra of the Ti containing γ–MnO2 hollow
spheres.
Figure 6. a) XPS survey scan and high-resolution scan (inset) of Ti 2p
and Mn 2p; b) XPS high-resolution scan of O 1s. (The C1s peak was used
as an internal reference to correct for charge shifts)
TGA measurements of the Ti containing γ-MnO2 samples
showed that with different amounts of Ti content, no extra
weight loss peaks were observed. The major weight loss peaks
of all the Ti containing γ-MnO2 materials (TM-1, TM-2, and
TM-3) shifted toward lower temperatures than that of the pure
γ-MnO2 material. Among the Ti containing materials, however,
the major weight loss peaks slightly increased with a decrease in
Ti loading levels. This indicates that the thermal stability of the
Ti doped γ-MnO2 materials is slightly enhanced on increasing
the Ti-loading. Physical modifications unlikely cause all these
effects. Therefore, there must be some chemical modifications
of the γ-MnO2 structure, such as the introduction of Ti in the
crystal structure of γ-MnO2, in order to make this happen.
Results of Raman spectra also suggested framework tita-
nium incorporation in γ-MnO2. As shown in Figure 5, no
extra stretching vibrational bands which can be indexed to
Ti = O bonds (from TiO2+ species) or TiO2 crystal structures
were observed. Peaks of MnO2 located around 600 – 650 cm−1
were shifted and broadened with increasing Ti content (larger
Ti/Mn molar ratio). The shifted peak position might due to the
asymmetric stretching vibration of the Ti-O-Mn bonds. These
observations, along with peak broadening, the smaller crystal-
line size, and the BE of O 1s from XPS results suggest that the
titanium might be incorporated into the framework of γ-MnO2
octahedral molecular sieves.
3.2. Effects of the Reaction Parameters on the Transformation
and Structure of the Obtained Titanium Containing γ-MnO2
Materials
Various amounts of titanium incorporation, distinct metal
cation incorporation, various starting Mn2+/Mn7+ ratios, and
different oxidant precursors have played important roles in
determining the morphology and crystal structure of the final
T a b le 2 . Electrocatalytic capacities of the TM-1 material in the Li-air battery test.
Specific Capacity [mA h g−1 of Carbon]
Cell ID
Initial Potential [V]
Discharge Capacity [mA h]
1
2
3
3.30
3.30
3.30
57.3
62.5
55.4
2361
2317
1867
2182
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T a b le 3 . Results of toluene atmospheric oxidation by the selected titanium containing γ–MnO2 (TM-1) materials.
Sample ID
Sel Bzdh [%] [a]
Sel Bzal [%] [b]
Sel Bzad [%] [c]
Conv [%]
TON [d]
19
29
23
30
28
20
19
20
17
51
55
49
50
16
17
19
31
undoped – γ-MnO2
2% Ti – γ-MnO2 (TM-3)
4% Ti – γ-MnO2 (TM-2)
8% Ti – γ-MnO2 (TM-1)
20
22
36
[a] Benzaldehyde. [b] Benzyl alcohol. [c] Benzoic Acid. [d] Turn over number. Reaction conditions: 70 mg of catalysts was mixed ultrasonically in 10 mL toluene and
0.1 mol% AIBN. Reflux at 110 °C for 20 h while bubbling O2 with constant flow rate of 9 mL min−1 . Turn Over Number = mol of converted toluene per mol of catalysts used =
(Conv. × 10 mL × 0.87 g.mL−1 /92.4 g. mol −1 )/(0.07 g/87 g.mol−1 ).
products. When the amounts of titanium incorporation were
varied from 2% to 8% of the total manganese with other param-
eters unchanged, the structure of the materials did not signif-
icantly change based on the XRD pattern shown in Figure 1.
The 8% Ti/Mn material has broader XRD peaks than the 2%
Ti/Mn material, which can be due to higher surface areas and
smaller particle sizes. Figure 7 shows the SEM images of mate-
rials with different ratios of Ti/Mn. Without titanium incorpo-
ration, the material is made up of homogenous nanofibers that
are around 1 μm in length and 50 nm in diameter. However,
with 2% titanium incorporation, the morphology changed to
become spherical like materials. With increasing amounts of
titanium incorporation, the spherical like materials became
more homogeneous, which suggests that the titanium ion is a
hollow sphere-like morphology-directing agent for γ -MnO2.
In order to confirm that titanium ions behave like mor-
phology directing agents, distinct metal ions which have
similar ionic radii were also investigated while keeping all
the other parameters the same. In the process, we chose
V(IV, 0.72 Å), Cr(III, 0.76 Å), Fe(III, 0.79 Å), and Co(III,
0.75 Å) independently whose crystal radii is close to that of Ti(IV,
0.75 Å). Figure 8 shows the morphology of the materials. As
can be seen, none of these materials showed a hollow sphere-
like morphology, and only titanium ion acts as a hollow sphere-
like morphology directing agent in the synthesis of γ-MnO2.
The titanium containing γ-MnO2 (TM) was successfully pre-
pared by a simple in-situ precipitation method. In acidic media,
a redox reaction between MnO4− and Mn2+ is expected to spon-
taneously produce MnO2 based on Equations (3) and (4).
MnO−4 (1) + 4H+ (1) + 3e → MnO2(s) + 2H2O(1)E◦ = 1.68V
(3)
MnO2(s) + 4H+ (1) + 2e → Mn2+ (1) + 2H2O(1)E◦ = 1.22V
(4)
In this process, KMnO4 solution was designed to be added
dropwise into a hot Mn2+ solution (85 °C) to provide excess
Mn2+ during the reaction. A spontaneous redox reaction occurs
between the reactants dissolved in water. Since γ-MnO2 has
point defects of Mn4+ cations replaced by Mn2+ cations,[40]
Mn4+/Mn2+ replacement occurred immediately with energy
−
provided by heating during the addition of MnO4 , γ-MnO2 was
formed.[28]
Herein, we discuss the amount of MnO4− addition on γ-MnO2
structure formation. During the process, we added 150 mL
of 0.7 M KMnO4 solution instead of 100 mL into the same
amount of Mn2+ hot solution with the same feeding rate
(0.67 mL min−1 ) and kept all the other conditions the same.
Samples were collected at different times: 1 min, 5 min,
10 min, 15 min, and 20 min. XRD results (Figure 9) show that
Figure 8. SEM images of γ–MnO2 with different transit metal ion incorpa-
ration. a) 8 at% Fe (III); b) 8 at% V (IV); c) 8 at% Cr (III); d) 8 at%
Co (III).
Figure 7. SEM images of γ–MnO2 spheres with various amounts of
titanium.
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Figure 9. XRD patterns in terms of different reaction time.
Figure 11. XRD patterns in terms of different amount of oxidants added.
(∗) γ–MnO2
the structure did not change in the first 15 min (100 mL KMnO4
solution was added) when using the exact same procedure as
before. After 15 min’s addition, more KMnO4 was added into
the slurry. Since most of the free Mn2+ has been precipitated
to form γ-MnO2, Mn4+/Mn2+ replacement can not take place
because of the lack of excess Mn2+, and the cryptomelane struc-
ture started to form, as shown in Figure 10.
In an effort to further understand the formation of the
γ-MnO2 crystal structure, we also investigated the effect
of oxidants on the γ-MnO2 structure formation. Figure 11
shows the XRD results from different oxidants with different
reduction potentials. K2S2O8 has a higher reduction potential
(Equation 5) than that of KMnO4, and the XRD result showed
that the γ-MnO2 structure was maintained, whereas the
structure of manganese oxide changed when using the same
concentration of KClO3 as the oxidant. Results obtained
from using different oxidants indicate that utilization of
a strong oxidant is important in formation of the γ-MnO2
structure.
has been followed by examining the products harvested at dif-
ferent intervals of reaction times. Figure 12 shows representa-
tive SEM images of the products obtained after various reaction
times, which have been demonstrated to be pure γ-MnO2
crystals by related XRD measurements. From the SEM image
of the sample produced at 1 min, we can clearly observe the
existence of many small balls, and individual spheres with a
diameter of less than 300 nm. After longer refluxing time, the
morphologies are more clear, which may be due to increasing
crystallinity as shown in XRD (Figure 9).
A self-transformation process by the localized Ostwald rip-
ening could explain the formation mechanism of the titanium
containing γ-MnO2 (TM-1) hollow spheres. Similar mecha-
nisms have been used to prepare different hollow spheres
recently.[49–55] The schematic diagram in Figure 13 shows the
possible formation mechanism of the titanium containing
−
γ-MnO2 (TM-1) hollow sphere. In the beginning (Step 1), MnO4
reacts with Mn2+ to form nanospheres directed by titanium
ions and the nanospheres showed solid interiors as examined
by TEM. With addition of more KMnO4 solution, crystallites
located on the outermost surface would serve as nucleation
seeds for the γ-MnO2 crystallization process. The outer crys-
tallites started to grow larger after attracting the smaller crys-
tallites underneath, and the continued surface growth on the
outer most surface resulted in a core-shell morphology (Step 2).
As a result of his process, the core size was then reduced gradu-
ally while the vacant volume inside the sphere was enlarged
(Step 3), which can be demonstrated by the observation of
the morphology through SEM and TEM at a reaction time of
30 min. The vacant space between core and the shell was gen-
erated through Ostwald ripening with the mass being trans-
ported. The inner core can be easily merged into particles on
the outer surface with enough ripening time, and a hollow
structure formed (Step 4).
S2O28− (aq) + 2H+ (aq)2e → 2HSO−4 (aq)E ◦ = 2.1V
(5)
ClO−3 (aq) + 3H+ (aq) + 2e
(6)
→ HClO2(aq) + H2O(1)Eo = 1.24
3.3. Hollow Sphere Structure Evolution
To shed light on the formation mechanism of these novel tita-
nium containing γ-MnO2 hollow spheres, their growth process
3.4. Electrocatalytic and Oxidative Catalytic Activities
One of the greatest successes for the Ti containing γ-MnO2
(TM) is the electrocatalytic activity in the Li/air battery test.
γ-MnO2 has been proven to be one of the most useful manga-
nese oxides and has been widely applied in the battery industry
due to its high activity.[28] However, among all the manganese
oxide based materials, γ-MnO2 does not have high activity in the
Figure 10. XRD patterns of materials prepared with different oxidants.
(∗) Cryptomelane MnO2
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γ –MnOOH manganite material earlier
reported as catalysts in lithium air battery
tests. This is the highest specific capacity
reported in Li/air battery tests using γ-MnO2
based materials as O2 reduction catalysts.
Another great success for the titanium
containing γ-MnO2 materials is the enhanced
oxidative catalytic activities. As reported ear-
lier,[28] solvent free atmospheric toluene oxi-
dation has attracted special attention as a
promising environmentally friendly reaction.
However, this reaction remains a challenge
due to the difficulty of oxidizing sp3 hybrid-
ized carbon in inactive hydrocarbons. Pure
γ-MnO2 prepared by the co-precipitation
method was found to be the highest efficient
catalyst (17% conversion in 20 h) in earlier
work. Here, as can be seen in Table 3, the
new Ti containing γ-MnO2 reported here can
catalyze 31% toluene conversion under the
same conditions. This much higher oxida-
tive catalytic activity than the pure γ-MnO2,
indicates the strong oxidative activity of this
material. In order to further confirm the high
oxidative catalytic efficiency of this material,
we also studied the selected Ti containing
γ-MnO2 (TM-1, 8 mol% Ti/Mn) as a het-
Figure 12. FESEM images for samples prepared at 1 min, 2 min, 5 min, 10 min, 15 min,
30 min, 1h, 5h, and 72h.
electrocatalytic reduction of O2 in metal-air batteries.[56] Earlier
work suggests that γ–MnOOH manganite materials have very
high specific capacities (up to 2.2 A h g−1 of carbon).[57] Here we
tested the selected titanium containing γ-MnO2 (TM-1) in the
same medium-sized lithium-air battery apparatus as reported
before. Results are listed in Table 2 and show that capacities
up to 2.4 A h g−1 of carbon were obtained, indicating similar
high electrocatalytic activity of O2 reduction as that of using
erogeneous catalyst for the anisyl alcohol oxidation reaction. A
promising high conversion (88%) with high selectivity (100%)
on oxidized product (anisic aldehyde) were achieved in 4 h.
These are improved catalytic properties as compared to other
reported work using manganese oxide based heterogeneous cat-
alysts under the same conditions.[58] Thus, these Ti containing
γ-MnO2 new materials also open up new possibilities for oxida-
tive catalytic reactions.
Figure 13. Schematic illustration of the formation process of hollow spheres and the corresponding SEM and TEM images.
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120 to 700 °C with a heating rate of 10 °C min−1 . The Raman spectra
were taken at room temperature in the spectral range 100–2000 cm−1
using a Renishaw 2000 Raman microscope system, which includes an
optical microscope and a CCD camera for multichannel detection. XPS
was done on a Leybold-Heraeus model 3000 spectrometer upgraded with
a SPECS EA10MCD energy analyzer. The titanium containing γ–MnO2
powder samples were pressed into indium foil to minimize charging.
The average C 1s binding energy found on the indium was used as an
internal reference to correct for charge shifts.
The selected Ti containing γ-MnO2 catalyst (TM-1) was incorporated
into hand coated air electrodes for use in Li-air cells following previous
procedures.[57] The air cathodes were combined with a Setella separator,
and a lithium metal anode, and then placed into a reusable cell case. The
electrolyte used was 1 M LiPF6 1:1:1 EC: DEC: DMC (where EC is ethylene
carbonate, DEC is diethyl carbonate, and DMC is dimethyl carbonate).
All of the cells built were placed into gas tight bags which were flushed
and filled with oxygen gas. In addition all of the cells were placed into
a temperature controlled incubator set at 25 °C. The cells were then
discharged at a current density of 0.15 mA cm−2 using a Maccor test unit.
All the catalytic reactions were carried out in a three-necked round-
bottom flask equipped with a condenser; a 9 mL min−1 of O2 fl ow into
the solution at atmospheric pressure, and the flask was placed in an oil
bath at a temperature of 110 °C. In a typical reaction, toluene (10 mL,
0.094 mol) with 0.1 mol% of AIBN (2,2’-azobis(isobutyronitrile), a
very commonly used radical initiator for the polymerization of vinyl
monomers[11]) and 70 mg of catalyst (pretreated at 120 °C for 6 h) were
introduced into the reactor. After refluxing for 20 h, the products were
dissolved in acetonitrile and analyzed with an HP 5890 Series II gas
chromatograph coupled with an HP 5971 mass detector. The column
used was a nonpolar cross-linked siloxane (HP-1) with dimensions of
12.5 m × 0.2 m × 0.33 μm .
4. Conclusions
In conclusion, the Ti-containing γ-MnO2 (TM) hollow spheres
have been successfully synthesized for the first time by a simple
co-precipitation method. Titanium cations were used as struc-
ture directing agents in the synthesis process. Characterization
results suggest that the Ti species were incorporated into the
framework of the γ-MnO2 hollow spheres and no separate ana-
tase or rutile phase were observed. This special structural mate-
rial prepared by the synthesis procedure disclosed in this paper
shows high specific capacity (2.2 A h g−1 of carbon) in the Li/air
battery tests and strong oxidative catalytic activity in the atmos-
pheric toluene oxidation reaction (conversion of 31% obtained
in 20 h reaction, highest conversion reported) in comparison
with earlier work. Moreover, a self-transformation process by
localized Ostwald ripening can explain the formation mecha-
nism of the titanium containing γ-MnO2 hollow spheres. This
was further characterized by SEM and TEM analyses. This
work provides a novel and easy pathway for the synthesis of
hollow spheres with strong electrocatalytic and oxidative cata-
lytic activities in Li-air batteries and chemical oxidation reac-
tions respectively, offering a new material platform for batteries
and chemical reactions.
5. Experimental Section
Synthesis of the Titanium Containing γ-MnO2 Octahedral Molecular
Sieves: The γ-MnO2 was prepared by an in situ precipitation method. In
a typical synthesis, MnSO4 (3.55 g) was dissolved in 100 mL distilled
water, TiOSO4.xH2SO4 (Alfa-Aesar, 78 wt.% TiOSO4) was then added
into the solution with various Ti/Mn atomic ratios of 2:100 (TM-3),
4:100 (TM-2), and 8:100 (TM-1) at room temperature, along with 6 mL
of concentrated nitric acid. The obtained suspension was stirred at room
temperature for 1 h until all the TiOSO4 was dissolved and the solution
was clear (Solution A). Solution B was prepared by dissolving 1.098 g
of potassium permanganate in 100 mL distilled water (0.07 M). The
solution B was then added dropwise into the hot aqueous solution A
(90 °C, refluxing) in 15 min (flow rate of 6.7 mL min−1 ) under vigorous
stirring. After refluxing the resulting solution for 24 h, the precipitate was
filtered and washed with distilled water to remove by-products, followed
by drying overnight at 100 °C.
Acknowledgements
We acknowledge support of the NSF GOALI CBET 0827800, and the
Chemical Sciences, Geosciences, and Biosciences Division, Office of
Basic Energy Sciences, Office of Science, US Department of Energy. We
also thank Yardney Techincal Products, Inc. for lithium air battery tests,
Dr. Heng Zhang for the help of XPS analysis, Dr. Francis Galasso, and
Dr. Raymond Joesten for many helpful discussions.
Received: May 27, 2010
Revised: June 28, 2010
Published online: August 16, 2010
Characterization: The structure of the prepared materials was first
analyzed with powder X-ray diffraction (XRD) using a Scintag XDS 2000
diffractometer with a Cu- Kα X-ray source (= 1.5418 nm) and a beam
voltage of 45 KV and 40 mA beam current. Morphology of the material
was analyzed with a Zeiss DSM 982 Gemini field emission scanning
electron microscope (FESEM). Powder specimens were dispersed in
ethanol, dropped onto a gold coated silicon wafer, and then the dried
wafer was mounted onto a stainless steel sample holder. High resolution
transmission electron microscopy (HRSEM) studies were carried out
using a JEOL 2010 UHR FasTEM operating at an accelerating voltage of
200 KV and equipped with energy dispersive X-ray spectroscopy (EDS).
The samples were prepared by dispersing the material in 2-propanol.
Then a drop of the dispersion was placed onto a carbon-coated copper
grid and allowed to dry. The chemical composition of the samples
was determined by EDS. The surface areas and porsize distributions
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