Fast synthesis and formation mechanism of γ-MnO2 hollow nanospheres for aerobic oxidation of alcohols

Article abstract of DOI:10.1016/j.materresbull.2010.05.014

γ-MnO₂ hollow nanospheres of about 300-800 nm in size have been synthesized by a fast 1-h 2-step process in the presence of an excess amount of Mn²⁺ in aqueous solution without using any templates, hydrothermal processes and catalyti

Full text of DOI:10.1016/j.materresbull.2010.05.014

Materials Research Bulletin 45 (2010) 1218–1223
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Fast synthesis and formation mechanism of
g-MnO2 hollow nanospheres for
aerobic oxidation of alcohols
Xiaobo Fu a,b, Jiyun Feng a, , Huang Wang a,b, Ka Ming Ng a,b
*
a Nano and Advanced Materials Institute Limited, Clear Water Bay, Hong Kong
b Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
A R T I C L E I N F O
A B S T R A C T
Article history:
g
-MnO2 hollow nanospheres of about 300–800 nm in size have been synthesized by a fast 1-h 2-step
process in the presence of an excess amount of Mn2+ in aqueous solution without using any templates,
hydrothermal processes and catalytic routes. The evolution of morphologies evidenced that the fast
Received 18 November 2009
Received in revised form 24 February 2010
Accepted 18 May 2010
formation mechanism of the
g
-MnO2 hollow nanospheres in the presence of the excess amount of Mn2+
-MnO2 hollow nanospheres
Available online 25 May 2010
in solution followed the ‘‘Ostward ripening’’ process. The as-synthesized
g
showed high catalytic activity and selectivity in aerobic oxidation of various alcohols which was
attributed to their hollow nature and larger BET specific surface area.
ß 2010 Elsevier Ltd. All rights reserved.
Keywords:
A. Nanostructures
A. Oxides
B. Crystal growth
D. Catalytic properties
1. Introduction
Usually, the MnO2 crystallization proceeds through two steps: a
disordered [14] or layered [15,16] manganese oxide precursor is
The synthesis of metal oxides with controlled morphology has
shown profound significance in physical chemistry [1,2]. Recently,
manganese dioxide (MnO2) hollow structures have become attrac-
tive owing to their specific physical and chemical properties, such as
large surface area, low density and good permeability [3–8]. Three
methodshave been developed tosynthesize MnO2 hollowstructures
including template based methods, hydrothermal processes, and
catalytic routes. For template method [6,9,10], the removal of
templates by acid or base etching, or calcination, may damage the
desired hollow structures, and the subsequent purification proce-
dures could significantly increase the costs. For hydrothermal
processes [8,11–13], even though the hollow structures can form
with regular morphology, the operating conditions are severe and it
is difficult to be produced on a large scale. For the catalytic route, Xie
and co-workers [3] used a solution based catalytic method to
formed, followed by an aging process to form the final product with
ordered structure. The structure and the morphology of the final
product are dependent on the aging process. In addition, a high
aging temperature can accelerate the crystallization process of
MnO2 in solution [17]. Based on these findings, herein, we report a
fast 1-h 2-step process for synthesizing novel
g-MnO2 hollow
nanospheres in solution without using any templates, catalysts
and hydrothermal processes. This process consisted of the reaction
between MnSO4 and KMnO4 in aqueous solution in the presence of
an excess amount of Mn2+ at room temperature for 0.5 h to obtain a
solid precursor, followed by an aging of the solution at 60 8C for
another 0.5 h to obtain the
g-MnO2 hollow nanospheres. The as-
synthesized -MnO2 hollow nanospheres were characterized by
g
powder X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and BET analyses.
Their fast formation mechanism at 60 8C in solution was discussed,
and their catalytic activities were also evaluated in the aerobic
oxidation of various alcohols.
fabricate different
a-MnO2 hollow structures. Their catalytic route
castnew lighton the developmentof new superstructures. However,
the time for synthesizing the MnO2 hollow structures by those
methods is very long. Thus, in spite of many methods reported for
synthesizing MnO2 hollow structures, a fast and simple method
without using any templates, catalysts, and hydrothermal processes,
has not been fully developed. Developing such a method is expected
to have both academic and industrial significance.
2. Experiments
2.1. Synthesis
Our synthesis is based on the following reaction between
MnSO4 and KMnO4 in aqueous solution.
* Corresponding author. Fax: +852 23588113.
3Mn2þðaqÞ þ 2MnO4ꢀðaqÞ þ 2H2OðlÞ ! 5MnO2ðsÞ þ 4HþðaqÞ (1)
E-mail addresses: kejyfeng@ust.hk, kejyfeng@gmail.com (J. Feng).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.05.014

X. Fu et al. / Materials Research Bulletin 45 (2010) 1218–1223
1219
In the synthesis, 8.50 g analytical grade MnSO4ꢁH2O and
3.43 g analytical grade KMnO4 powders were mixed together
(MnSO4/KMnO4 mole ratio = 2.3), and ground to fine powder in
an automatic mortar at room temperature for 1 h. Then, the
mixed powder was transferred to 200 mL distilled water in a
beaker under continuous stirring. After that, 100 mL of 1 M
H2SO4 solution was immediately added to the beaker to
accelerate the reaction rate and remove potassium ions. The
solution was firstly kept at room temperature with vigorous
stirring for 0.5 h to obtain the precursor. Then, the solution was
transferred to a water bath at 60 8C with a condensator for
another 0.5 h with vigorous stirring, which was an aging process
to accelerate the crystallization of the precursor and form the
MnO2 hollow nanospheres. Finally, the precipitates were filtered
and rinsed with distilled water and ethanol to remove any
chemical species possibly remaining in the final product. The
resultant product was then dried in an oven at 70 8C overnight.
Some control experiments were also conducted which will be
discussed later.
2.2. Characterization
Fig. 1. XRD patterns for (a) the precursor obtained at solution reaction for 0.5 h at
room temperature, and (b) the as-synthesized product.
Phase identification and structure analysis of the
precursor and the as-synthesized product were performed on
a PAnalytical X’pert Pro X-ray Diffractometer equipped with Cu
˚
2.3. Evaluation of catalytic activity
K
a radiation (l = 1.54178 A), operating at 40 kV and 40 mA. The
morphologies of the two products were studied by a field
emission scanning electron microscope (FESEM, JEOL JSM-
6700F) with an accelerating voltage of 5 kV and a transmission
electron microscope (JEOL-2010F) with an accelerating voltage
of 200 kV, equipped with a Bruker Energy Dispersive Spectrom-
eter (EDS). The BET specific surface area was determined by
For a typical procedure of the alcohol oxidation, toluene (10 mL)
and the substrate (1 mmol) were first added to a 25 mL round-
bottomed flask containing 0.05 g catalyst. The mixture was then
stirred under reflux at 110 8C with air bubbling, which was
controlled by a mass flow controller with a flow rate of 10 mL/min.
After reaction for 4 h, the reaction mixture was cooled down to
room temperature. The filtrate was carefully analyzed using a gas
chromatography/mass spectrometry (GC/MS) fitted with a HP-
1MS column.
N2 adsorption at 77 K using
a Micromeritics ASAP 2000
system after the sample was degassed in vacuum at 130 8C
overnight.
Fig. 2. SEM images of the precursor obtained at solution reaction for 0.5 h at room temperature (a and b) and the as-synthesized product (c and d) (the inset is a higher
magnification of the solid nanosphere).

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X. Fu et al. / Materials Research Bulletin 45 (2010) 1218–1223
3. Results and discussion
50 nm emerged on the surface of the solid nanosphere. Fig. 2c
shows that the overall morphology of the as-synthesized product
was similar to that of the precursor. However, the length of the
nanorods increased from about 50 nm to about 100 nm, implying
that the length of the nanorods increased quickly at 60 8C within
0.5 h in solution in the presence of excess amount of MnSO4. In
addition, the SEM image of a broken nanosphere in Fig. 2d reveals
that the nanospheres were hollow. The morphology of the hollow
nanospheres is significantly different from that of the hollow
urchin structure consisting of long and uniform fibers pointing
outward reported by other researchers [3,8,10,13]. Elemental
analysis indicated that only trace amount (less than 0.1%) of K+ was
detected in both the precursor and the as-synthesized product.
The TEM images in Fig. 3a and b reveal that the precursor was
made of aggregated solid nansopheres. On the contrary, the dark-
bright contrast between the boundary and the center of the
nanospheres in Fig. 3c and d strongly confirms that the as-
synthesized product consisted of aggregated hollow nanospheres.
Fig. 1 shows the XRD patterns of the precursor and the as-
synthesized product, indicating that the precursor was poorly
ordered while the as-synthesized product was in
crystalline phase because all its major diffraction peaks can be
assigned to the orthorhombic phase of -MnO2 with lattice
g-MnO2
g
˚
˚
˚
constants a = 6.36 A, b = 10.15 A and c = 4.09 A (JCPDS 14-644). The
XRD results also reveal that -MnO2 crystalline phase formed
g
quickly from the poorly ordered precursor within 0.5 h aging at
60 8C in the presence of the excess amount of MnSO4 in solution. In
addition, the BET specific surface area of the as-synthesized
product was determined to be 158 m2 gꢀ1. The SEM image in
Fig. 2a shows that the precursor consisted of aggregated nano-
spheres with diameter about 300–800 nm. A broken nanosphere in
the image (as shown in the inset of Fig. 2a) reveals that these
nanospheres were solid. The SEM image in Fig. 2b depicts that
many nanorods with diameter about 10 nm and length about
Fig. 3. TEM images of the precursor obtained at solution reaction for 0.5 h at room temperature (a and b) and the as-synthesized product (c and d), SAED pattern (e) and
HRTEM image (f) of the s-synthesized product.

X. Fu et al. / Materials Research Bulletin 45 (2010) 1218–1223
1221
Fig. 4. A TEM image of the product obtained by 0.5 h room temperature reaction
followed by another 2.5 h aging at room temperature.
The characteristic distances of the
g-MnO2 hollow nanospheres,
even though without preferred orientation of the crystallites, could
be seen, which is shown in the SAED pattern in Fig. 3e. Fig. 3f is a
HRTEM image of a single nanorod located at a hollow structure,
confirming that the nanorod is crystalline; the lattice spacing of
0.40 nm is consistent with that of the
g-MnO2 (1 2 0) plane.
Therefore, both SEM and TEM images confirm that the novel
hollow nanospheres could be obtained via 0.5 h reaction in
solution at room temperature followed by another 0.5 h aging at
60 8C in solution in the presence of an excess amount of MnSO4. In
other words, a higher temperature in the aging process led to a fast
formation of the
g-MnO2 hollow nanospheres. In order to verify
this, a control experiment was conducted. After 0.5 h reaction at
room temperature, the reaction solution was still kept at room
temperature, even though the aging time was extended to 2.5 h,
only solid nanospheres were obtained, as shown in Fig. 4.
Many efforts have been made to synthesize MnO2 materials in
aqueous acidic solutions under reflux and with varying oxidizing
agents (KMnO4) concentrations. However, to the best of our
knowledge, no intensive study had been made on the varying the
Fig. 5. TEM images of the product obtained by using Mn(AC)2 (a) or ZnSO4 (b) to
replace the excess amount of MnSO4 in solution.
MnSO4 concentrations to form the
Our control experimental results indicate that the
g
-MnO2 hollow nanospheres.
-MnO2 hollow
hollow nanospheres. At the same time, the weight of the product
(about 5.73 g) is larger than the theoretical weight (about 4.72 g)
whichꢀcan be calculated using the reaction between Mn2+ and
oxidation of the excess amount of Mn2+ by the oxygen also took
place in solution as displayed in Eq. (2).
g
nanospheres could not be formed when the mole ratio of the
MnSO4 to KMnO4 was less than 2.3 (the stoichiometric ratio of the
MnSO4 to KMnO4 is 1.5), demonstrating that the excess amount of
the MnSO4 in solution played an important role in the formation of
the hollow nanospheres and the grinding process was not a key
MnO4
, implying that a secondary reaction involving the
step. However, Mn2+ or SO4
may be responsible for the
2ꢀ
Mn2þðaqÞ þ O2ðgÞ ! MnO2ðsÞ
(2)
formation of the -MnO2 hollow nanospheres, but which one
g
plays the real role had to be investigated. Thus, a control
experiment was conducted, in which MnSO4 and KMnO4 were
used as the starting materials to react firstly in stoichiometric
ratio, then Mn(AC)2 was added to the system to replace the
The MnO2 generated through this reaction would promote the
re-crystallization of the solid nanospheres.
The results above enable us to propose a formation mechanism
for the g-MnO2 hollow nanospheres in this fast 1-h 2-step process,
corresponding excess amount of MnSO4 in the synthesis of the
g
-
as shown in Fig. 6. The initial reaction at room temperature
between Mn2+ and MnO4 was too fast to enable orientation
ꢀ
MnO2 hollow nanospheres. We found that the -MnO2 hollow
g
nanospheres still formed, as shown in Fig. 5a. In another control
experiment, ZnSO4 was used to replace the corresponding excess
amount of MnSO4 to verify the function of the SO42ꢀ. However,
between the sheets of MnO6 octahedron, leading to the formation
of poorly ordered solid nanospheres. Then, fast growth of the
nuclei occurred. After that, owing to the complete consumption of
the KMnO4, the system was transformed into a thermodynamically
stable state. As the system was in an acidic condition and in the
absence of the complexant, it seems that Mn species were
only
Therefore, it is clearly revealed that the excess amount of the Mn2+
in solution was responsible for the formation of the -MnO2
g-MnO2 solid nanospheres formed, as shown in Fig. 5b.
g

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X. Fu et al. / Materials Research Bulletin 45 (2010) 1218–1223
Fig. 6. Schematic illustration of the fast formation mechanism of the
g
-MnO2 hollow nanospheres via an Ostwald ripening process.
favorable to dissolve [17]. Therefore, the early precipitate might
not reserve its shape and disordered structure.
small crystals because of the energy difference between them
[20].
Basically, the growth of MnO2 mainly follows three process-
es: dissolution-crystallization, oriented attachment and lateral
growth [10,17]. In our process, the nanorods were connected by
several nanoparticles, as can be clearly seen in Fig. 7. This may
be caused by a dissolution-crystallization first, followed by an
oriented attachment, which was also observed by Adachi et al.
[18,19] and Penn et al. [19]. As the aging process proceeded, the
crystallites in the inner core, having higher surface energy than
those on the outer surfaces, were easily dissolved in the
presence of the excess amount of the Mn2+ in solution. Then, the
outside nanorods served as sites for subsequent crystallization
process of the cores, and they grew at the expense of the small
crystallites in the cores. The direct evidence for the process is
that the size and length of the nanorods increased from about
5 nm and 50 nm to about 10 nm and 100 nm, as mentioned in
SEM and TEM characterizations. Finally, an interior cavity
gradually formed. However, it should be stressed that the
process could hardly happen or need a very long time at room
temperature. Because an increased temperature could effective-
ly accelerate the crystallization of MnO2 in solution as reported
by Portehault et al. [17], we accelerated the crystallization of the
MnO2 solid nanospheres by transferring the reaction system
from room temperature to 60 8C water bath for aging. Finally,
Conventional MnO2 have been used for allylic and benzylic
oxidations, and their catalytic activities depend on the prepara-
tions, compositions, crystal structures, and morphologies [21]. The
novel
aerobic oxidation of various alcohols, and the results are listed in
Table 1, indicating that the -MnO2 hollow nanospheres gave
g-MnO2 hollow nanospheres were used as a catalyst in the
g
excellent conversions and selectivity both for saturated and
unsaturated alcohols. Compared with the MnO2-OMS reported
[21], our
g-MnO2 hollow nanospheres showed higher catalytic
activities and selectivity under identical reaction conditions, which
Table 1
Aerobic oxidation of alcohols using
g-MnO2 hollow nanospheres as catalysts.
Substrates
Products
Time Conversion Selectivity TONa
(h)
4
(%)
(%)
100
>99
1.73
4
4
100
100
>99
>99
1.73
1.73
hollow
in solution. All the results demonstrated that the fast formation
mechanism of
g-MnO2 nanospheres quickly formed within 0.5 h aging
g
-MnO2 hollow nanospheres at 60 8C in the
presence of the excess amount of Mn2+ in solution followed an
Ostwald ripening process, in which the initial formation of tiny
crystalline nuclei in a supersaturated medium is followed by
crystal growth and the larger crystals grow at the expense of the
4
4
97
98
>99
>99
1.73
1.68
4
4
100
50
>99
>99
1.73
0.87
b
4
4
40
90
>99
>99
0.69
1.56
b
b
Reaction conditions: 1 mol cinnamyl alcohol, 10 mL toluene, 0.05 g catalyst,
T = 110 8C and pressure =0.1 MPa; flow rate of air: 10 mL/min.
a
Turn over number based on substrate per mole MnO2.
Fig. 7.
A high resolution SEM image of the as-synthesized g-MnO2 hollow
b
From Ref. [21].
nanospheres.

X. Fu et al. / Materials Research Bulletin 45 (2010) 1218–1223
1223
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g
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g-MnO2
hollow nanospheres by a 1-h 2-step process without using any
templates, catalysts, and hydrothermal processes, which is the
shortest reaction time reported in the literature to date. This
process consisted of the reaction between MnSO4 and KMnO4 in
aqueous solution in the presence of an excess amount of Mn2+ at
room temperature for 0.5 h, followed by an aging of the solution at
60 8C for another 0.5 h. The
g-MnO2 hollow nanospheres with a
diameter of about 300–800 nm consisted of nanorods with
diameter about 8–10 nm and length about 50–100 nm. The excess
amount of the Mn2+ in solution was found to be the key factor in
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the formation of the
formation mechanism of the
60 8C in the presence of the excess amount of Mn2+ in solution
followed the ‘‘Ostward ripening’’ process. The as-synthesized
g
-MnO2 hollow nanospheres. The fast
g
-MnO2 hollow nanospheres at
g
-
MnO2 hollow nanospheres showed high catalytic activity and
selectivity in aerobic oxidation of various alcohols because of their
larger BET specific surface area. This facile 1-h 2-step synthetic
process provides a new perspective of the fast synthesis of
hollow nanospheres.
g-MnO2
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