Sol-gel synthesis and characterization of mesoporous manganese oxide

Article abstract of DOI:10.1016/j.materresbull.2003.07.005

Mesoporous manganese oxide (MPMO) from reduction of KMnO₄ with maleic acid, was obtained and characterized in detail. The characterization of the material was confirmed by high-resolution transmission electron microscopy (HRTEM), X-ray powder diffractometry (XRD) and N₂ sorptometry. The results showed that MPMO is a pseudo-crystalline material with complex network pore structure, of which BET specific surface area is 297m²/g and pore size distribution is approximately in the range of 0.7-6.0nm. The MPMO material turns to cryptomelane when the calcinating temperature rises to 400°C. The optimum sol-gel reaction conditions are KMnO₄/C ₄H₄O₄ molar ratio=3, pH=7 and gelation time > 6h.

Full text of DOI:10.1016/j.materresbull.2003.07.005

Materials Research Bulletin 38 (2003) 1695–1703
Sol–gel synthesis and characterization of
mesoporous manganese oxide
Xinlin Honga, Gaoyong Zhanga,b,*, Yinyan Zhua, Hengquan Yanga
aSchool of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China
bChina Research Institute of Daily Chemical Industry, Wenyuan Street No. 34, Taiyuan 030001, PR China
Received 6 December 2002; received in revised form 2 July 2003; accepted 23 July 2003
Abstract
Mesoporous manganese oxide (MPMO) from reduction of KMnO4 with maleic acid, was obtained and
characterized in detail. The characterization of the material was confirmed by high-resolution transmission
electron microscopy (HRTEM), X-ray powder diffractometry (XRD) and N2 sorptometry. The results showed that
MPMO is a pseudo-crystalline material with complex network pore structure, of which BET specific surface area
is 297 m2/g and pore size distribution is approximately in the range of 0.7–6.0 nm. The MPMO material turns to
cryptomelane when the calcinating temperature rises to 400 8C. The optimum sol–gel reaction conditions are
KMnO4/C4H4O4 molar ratio ¼ 3, pH ¼ 7 and gelation time > 6 h.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; B. Sol–gel synthesis
1. Introduction
Porous manganese oxides are of considerable interest since these materials can be used in adsorption,
catalysis, batteries, and other applications [1]. So far, micro- and mesoporous manganese oxides, as
well as layered clay-like manganese oxide materials were prepared via a variety of routes. Among these
materials, mesoporous manganese oxide has been the focus of intense interest for its nanoscale pore
structure, high surface area and unique surface properties, which can reduce the solid-state diffusion
distances and maximize the fraction of the interface to improve the electrochemical and catalytical
behaviors of manganese oxides, especially when used as cathode material for lithium ion batteries [2].
Mesoporous manganese oxide with mixed-valent semiconducting property has been prepared
from Mn(OH)2 with cetyl trimethyl bromide(CTAB) surfactant as micellar template [3]. Novel
*
Corresponding author. Tel.: þ86-351-4044836; fax: þ86-351-4040802.
E-mail address: hongxinlin@vip.sina.com (G. Zhang).
0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2003.07.005

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tetraalkylammonium manganese oxide colloids with a plate-like 4–12 nm in diameter have been
synthesized from reduction of tetraalkylammonium permangate [4]. These colloids were prepared in a
delaminated form and can undergo self-assembly to produce layered structure, showing a unique
semiconducting property. Recently, Liu et al. succeeded in synthesizing a SiO2-pillared birnessite using
octylamine to preliminarily expand the layer structure and use tetraethoxylsilane (TEOS) as the pillaring
source [5]. Their results showed evidence of the formation of a pillared mesoporous structure. Hong et al.
prepared mesoporous manganese oxide using non-ionic surfactant (polyoxyethylene fatty alcohol, AEO9)
template in the sol–gel process of synthesizing cryptomelane [6]. BET surface area of the material is as
high as 387 m2/g, and its mesopore structure shows good thermal stability up to 500 8C in air.
Mesoporous manganese oxides have been obtained using a wide range of low temperature, liquid-
phase methods, and these materials are excellent precursors for other functional materials. Sol–gel
routes have been widely used in the synthesis of microporous (cryptomelane) and layered (birnessite)
manganese oxides from reduction of KMnO4 with organic reducing agents [7–9]. Little attention has
been focused on the relationship of these sol–gel processes, resulting mesoporous aerogels and the
colloidal suspensions derived from the synthesis.
Recently, synthesis and characterization of mesoporous manganese oxides via surfactant templating
method were reported [6]. Meanwhile, the rheology of the suspensions obtained during preparation was
also studied, and a unique oscillation behavior (apparent viscosity, storage modulus and loss modulus
all fluctuate with gelation time) exists in these suspensions [10]. In the present work, we obtained
mesoporous manganese oxides (MPMO) from reduction of KMnO4 with maleic acid, and characterized
this material in detail. The results show that MPMO is a pseudo-crystalline material with complex
network pore structure, of which BET specific surface area is 297 m2/g and pore size distribution is
approximately in the range of 0.7–6.0 nm. The MPMO material turns to cryptomelane when the
calcinating temperature rises to 400 8C. Furthermore, systematic study on this sol–gel process indicated
that the pore structures of manganese oxides synthesized are affected directly by several factors (molar
ratio (KMnO4/C4H4O4), pH, gelating time).
2. Experimental
2.1. Sample preparation
MPMO was typically synthesized by mild reduction of aqueous solution of KMnO4 with maleic acid
at a molar ratio of KMnO4/C4H4O4 ¼ 3 at room temperature. The resulting dark brown gel was kept at
room temperature in air for 24 h and then filtered and washed with double deionized water for six
times. After that, the gel was dried at 60 8C in air for about 20 h.
MPMO material was heated at 200, 300, 400 8C in air for 3 h, respectively. And the resultant
corresponding samples were marked as MPMO-200, MPMO-300 and MPMO-400.
2.2. Characterization techniques
High-resolution transmission electron microscopy (HRTEM) was carried out using a JEM-2010
JEOL (Japan) ultra high-resolution microscope operated at 200 kV and 100 mA. Test samples were
directly crushed and sprinkled onto carbon-coated grids (JEOL, Japan).

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X-ray powder diffractometry (XRD) was performed at room temperature, using a Rigaku
˚
diffractometer equipped with a source of Ni-filtered Cu Ka radiation (l ¼ 1:5406 A), and an X-ray
generator operated at 40 kVand 100 mA. The diffractograms were recorded in the range of 10–808 with
divergence and scaller slits of 18, at a step size of 0.028 and a step time of 0.12 s.
The specific surface area and pore size distribution were determined by BET [11], t-plot [12], and
BJH [13] analysis of N2 adsorption–desorption isotherms measured on the samples, using automatic
ASAP 2010 Micromeritics sorptometer (USA) equipped with an outgassing platform and on-line
data acquisition and handling system. The nitrogen gas was a 99.999% pure product of PRAXAIR
(Beijing, PR China), and the sample outgassing was carried out at 100 8C and 10À5 Torr for 5 h. The
reproducibility of the measurements was better than 98%.
3. Results and discussion
3.1. Material characteristics
3.1.1. Bulk properties
TEM micrographs obtained from MPMO (Fig. 1a–c) indicate that the material is a complex network
pore structure. The selected area electron diffraction (SAED) (Fig. 1a) pattern accounts for a relatively
pseudo-crystalline state (i.e. the microstructure of material is a mixture of orderly and disorderly
regions). The HRTEM micrograph of MPMO (Fig. 1d) reveals that the test material consists of some
layered domains.
The MPMO powders show nearly amorphous X-ray powder diffraction pattern (Fig. 1a), but some
˚
peaks are detected, such as 6.91, 3.65 A. These parameters are similar to those of synthetic birnessite
(OL-1) [9], indicating that there is some layered structure existing in MPMO. This is in accordance
with the HRTEM observation above. Thermal treatment of MPMO at 400 8C leads to the formation of
cryptomelane structure, whose XRD pattern is similar to that of [7]. After calcinating at 600 8C, the
˚
powders show sharp XRD pattern of cryptomelane with some peaks for bixbyite (Mn2O3, 3.85, 2.72 A)
emerged (Fig. 2c).
MPMO is a mixture of layered and disordered domain, and the transition of layered structure to
cryptomelane have not been reported in other manganese oxide materials sol–gel synthesized via
reduction of KMnO4 with maleic acid [7]. The similar conversion is just mentioned in the work of
spontaneous formation of manganese oxide helices [14], which is prepared from reduction of
tetramethyl ammonium permanganate with 2-butanol/H2O.
3.1.2. Pore structure
N2 adsorption–desorption isotherm (Fig. 3a) of MPMO belongs to BDDT type IV, indicating that the
test material expose mesoporous structure. The hysteresis loop in the isotherm is similar to type-H2,
which is associated with a more complex network pore structure [15]. This result is supported by
directly HRTEM observation of MPMO (Fig. 1). The BET surface area is 297 m2/g (Table 1), and pore
size distribution derived from the desorption branch is between 1.7 and 6.0 nm (Fig. 3a). The t-plot
curve for sample (Fig. 3b) shows a downward deviation to the straight line from around 0.7 nm, due to
micropore filling of nitrogen. This indicates that micropores with width around 0.7 nm exist in MPMO,
i.e. the layered structures observed from HRTEM and XRD. Meanwhile, t-plot analysis gives a

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Fig. 1. HRTEM photographs of MPMO materials.
Table 1
Pore properties of samples treated at different temperature
Sample
SBET (m2/g)
CBET
Smp (m2/g)
Sext (m2/g)
Vde (cm3/g)
Dde (nm)
MPMO
297
313
235
43
161
196
148
291
33
57
21
9
264
256
213
34
0.277
0.279
0.252
0.125
3.59
3.62
MPMO-200
MPMO-300
MPMO-400
3.72
10.38
Smp: surface area of micropore; Sext: surface area of external surface; Vde: pore volume calculated from desorption branch; Dde:
average pore diameter calculated from desorption branch and referred to mean hydraulic diameter Dp ¼ 4Vp=Sp.

X. Hong et al. / Materials Research Bulletin 38 (2003) 1695–1703
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Fig. 2. XRD patterns of samples. (a) MPMO, (b) MPMO-400, (c) MPMO-600.
micropore surface area (Smp) of 33 m2/g and external surface area (Sext) of 264 m2/g. This means that
the layered plates stack irregularly to form abundant mesopores.
Fig. 4 shows the N2 adsorption results of samples treated at different temperature. It proves that the
pore structure of MPMO material is stable to calcination up to 300 8C, because of the similar isotherms
and hysteresis loops of MPMO, MPMO-200 and MPMO-300 (Fig. 4a). With temperature increasing,
the two peaks of MPMO’s pore size distribution curve (Fig. 4b) exhibit different changes, one (around
3.2 nm) increases, and the other (around 5.0 nm) decreases. The enhanced peak is due to exposure of
new pores, which were occupied by water and oxalic acid molecules before thermal treatment. At the
same time, the peak (around 5.0 nm) decreased, because larger pores of MPMO readily collapse during
calcination.
The pore properties of samples treated at different temperature are shown in Table 1. It indicates that
this kind of irregularly stacked mesopore can withstand thermal treatment up to 300 8C. The CBET
constants are below 200 at lower 400 8C. Because CBET value is a measure of the strength of interaction
between the nitrogen molecule and pore surface, the low CBET value indicates the weak interaction
between the nitrogen molecule and pore surface due to the presence of relatively wide mesopore
structures. At 400 8C, CBET constant is 291, this is attribute to the formation of cryptomelane, in which
˚
a typical 4.6 A tunnel structure is in existence.
3.2. Sol–gel process
MPMO of high surface area is good precursor for preparing different functional materials, which can
be used in electrochemistry, catalysis, sensors, etc. The sol–gel process for MPMO is very sensitive to
variations in reaction conditions, and the resulting material is quite different in structure and properties.
Here, BET surface area was chosen as the main parameter to scale the relationship between the pore
structure and several reaction factors of the sol–gel process.

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Fig. 3. N2 adsorption–desorption results of MPMO material. (a) N2 adsorption–desorption isotherm and pore size distribution
curve, (b) t-plot curve.
3.2.1. KMnO4/C4H4O4 molar ratio
Results of sol–gel reactions with different molar ratio of KMnO4 and maleic acid are shown in Fig. 5.
Surface area of resulting sample reaches the highest at about 3. This can be easily understood to take
this sol–gel mechanism [16,17] into account:
3HOOCCH¼CHCOOH þ 10MnO4À þ 10H3Oþ ! 10MnO2 þ 18H2O þ 6CO2 þ 3C2O4H2
Following this mechanism, the optimum stoichiometry is KMnO4/C4H4O4 molar ratio ¼ 10=3 ꢀ 3:33.
Around this point, the sol–gel process is more complete and the attained network structure is more
thoroughly organized. Because there are enough cross-linking molecules (C2O4H2) to interconnect the
manganese oxide centers, this is important for building the network mesopore. As a consequence, less
maleic acid (high KMnO4/C4H4O4 molar ratio) leads to a quicker decrease in surface area.
3.2.2. pH effect
The initial pH value (adjusted with H2SO4 and KOH solution) of the solution affects directly the pore
structure of manganese oxide obtained (Fig. 6). For the construction of mesoporous manganese oxide, it
is better without any acid or base added into the starting solution. An increase in basicity accelerates
gelation, but does not improve the SBET of the products. This is due to the reaction of cross-linking

X. Hong et al. / Materials Research Bulletin 38 (2003) 1695–1703
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Fig. 4. N2 adsorption–desorption results of samples treated at different temperature. (a) N2 adsorption–desorption isotherms,
(b) pore size distribution curves.
molecules (oxalic acid) with base, which breaks up the network structure. When acidity of the initial
solution increases, the nature of the reaction changes from sol–gel to precipitation type. Because the
electrical double layer of the colloidal particle surface is depressed strongly with the addition of acid,
this reduces the electrostatic repulsive force produced by negative charges of colloidal surfaces, and is
harmful to the formation of manganese oxide gel structure.
3.2.3. Gelation time
Gelation is important for the sol–gel process to attain MPMO material of high surface area. Fig. 7
describes the time dependence of resulting pore structure. The result shows that the surface area of
sample is nearly constant, when gelating after 6 h. After 1 h of stirring rapidly, the colloidal dispersion
of manganese oxide begins to flocculate to form a continuous gel structure instead of individual flocs.
400
300
200
100
0
1.5
2
2.5
3
3.5
4
4.5
KMnO4/C 4H4O4 (molar ratio)
Fig. 5. KMnO4/C4H4O4 molar ratio effect on surface area of manganese oxides.

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300
250
200
150
100
50
0
0
2
4
6
8
10
12
14
16
pH value
Fig. 6. pH effect on surface area of manganese oxides.
400
300
200
100
0
0
5
10
15
20
25
Gelation time (hour)
Fig. 7. The relationship of gelation time and the surface area of manganese oxides.
The gel structures build up slowly with time, as the particles orient themselves towards positions of
minimum free energy under the influence of Brownian motion. The surface area of materials stays
about the same after 6 h and suggests that the gel microstructure is complete.
From the discussion above, optimum reaction conditions to synthesize MPMO material of high
surface area can be concluded, i.e. KMnO4/C4H4O4 molar ratio ¼ 3, pH ¼ 7 and gelation time > 6 h.
4. Conclusions
A simple sol–gel route has been developed for preparing MPMO with high surface area (297 m2/g),
of which pore distribution is locating at 0.7–6.0 nm. HRTEM, XRD and N2 sorptometry were used to
characterize this material, and the results showed that MPMO is a pseudo-crystalline material with
complex network pore structure. The material can withstand thermal treatment up to 300 8C and turns
to cryptomelane when the calcinating temperature rises to 400 8C. The effects of molar ratio (KMnO4/
C4H4O4), pH value and gelating time on the pore structure of MPMO were also discussed in detail, and
the optimum sol–gel reaction conditions is that KMnO4/C4H4O4 molar ratio ¼ 3, pH ¼ 7 and gelation
time > 6 h.
Acknowledgements
This research was funded by Fundamental Research Item (No. 2001ccc01300) of Technology
Ministry of China. The authors would like to thank Surfactant Key Lab of China Research Institute of
Daily Chemical Industry for its support.

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