A facile template-free preparation of porous manganese oxides by thermal decomposition method

Article abstract of DOI:10.1134/S0020168510010127

Porous Manganese oxides with large surface area were prepared by a facile template-free method. Highly crystallized MnCO₃ precursors with micron size in high dispersivity were prepared in water first. Thermal treatment was carried out with the obtained composite at different temperatures to produce porous manganese oxides. Thermogravimetry and differential thermal analysis (TG-DTA), X-ray powder diffraction (XRD), Scanning electron microscopy (SEM) and N₂ adsorption-desorption measurements were applied to characterize the porous manganese oxides. Amorphous MnO₂ obtained at 400°C shows a mesoporous structure and has a large specific surface area of 135.1 m²/g. Well crystallized α-Mn₂O₃ crystal with surface area of 18.51 m²/g macroporous framework was obtained at 600°C.

Full text of DOI:10.1134/S0020168510010127

ISSN 0020ꢀ1685, Inorganic Materials, 2010, Vol. 46, No. 1, pp. 51–54. © Pleiades Publishing, Ltd., 2010.
A Facile TemplateꢀFree Preparation of Porous Manganese Oxides
by Thermal Decomposition Method1
Xianzhong Zhanga, Linhai Yuea, Dalai Jinb, and Yifan Zhengc
aChemistry Department, Zhejiang University, Hangzhou 310027, People’s, Republic of China
bCenter of Materials Engineering, Zhejiang SciꢀTech University, Xiasha University Town,
Hangzhou 310018, People’s Republic of China
cCollege of Chemical Engineering and Materials Science, Zhejiang University of Technology,
Hangzhou 310014, People’s Republic of China
eꢀmail: zjchem_yue@126.com
Received January 12, 2009
Abstract—Porous Manganese oxides with large surface area were prepared by a facile templateꢀfree method.
Highly crystallized MnCO3 precursors with micron size in high dispersivity were prepared in water first. Therꢀ
mal treatment was carried out with the obtained composite at different temperatures to produce porous manꢀ
ganese oxides. Thermogravimetry and differential thermal analysis (TGꢀDTA), Xꢀray powder diffraction
(XRD), Scanning electron microscopy (SEM) and N2 adsorptionꢀdesorption measurements were applied to
characterize the porous manganese oxides. Amorphous MnO2 obtained at 400°C shows a mesoporous strucꢀ
ture and has a large specific surface area of 135.1 m2/g. Well crystallized
of 18.51 m2/g macroporous framework was obtained at 600°C.
αꢀMn2O3 crystal with surface area
DOI: 10.1134/S0020168510010127
1
INTRODUCTION
template agents were required and the purity of the
products was uncertain as removing the precursors.
Furthermore, the use of soft template caused low crysꢀ
Porous oxides of transition metals hold many
advantages over aluminosilicate materials in areas of
electromagnetics, photoelectronics and catalysis
because transition metal atoms can exist in various
oxidation states [1]. In particular, manganese oxides
have attracted considerable attentions due to their
wide applications as catalysts [2], adsorbents [3], magꢀ
netic materials [4], sensors [5] and supercapacitor
electrode materials [6]. Since manganese oxides have
various advantages such as low cost, ecoꢀfriendly
property and fascinating application potentials, many
efforts have been devoted to the synthesis of porous
manganese oxides around the world.
tallinity. Its low heatꢀresistivity above 400 C tended to
°
cause pore structure collapse at high temperature. The
strategy without the assistance of templates seems
more desirable in the preparation of porous mangaꢀ
nese oxides.
In this paper, a facile but novel templateꢀfree
approach was reported for preparing porous mangaꢀ
nese oxides. Micron sized MnCO3 crystal was syntheꢀ
sized by a simple precipitation reaction. Porous manꢀ
ganese oxides were then successfully prepared by
direct thermal decomposition of precursor in static air
without any additives. Amorphous MnO2 with mesoꢀ
porous structure and crystallized pure ꢀMn2O3 with
α
Most of the methods applied in synthesizing the
porous manganese oxides were reported to use of temꢀ
plate technique [7] in the past decade. Xu and his coꢀ
workers synthesized amorphous MnO2 via reduction
route using supramolecular as template and the prodꢀ
uct exhibited excellent electrochemical is property [8].
The amorphous mesoporous MnO2 was reported to be
prepared by sonochemical method using surface modꢀ
ified SBAꢀ15 as hard template. It showed a specific
surface area of 91 m2/g [9]. Macroporous Mn2O3 was
synthesized by Andreas Stein Group using polystyrene
spheres as template, and the product showed a BET
surface area of 20 m2/g [10]. Although templates facilꢀ
itated the formation of the pore structure, expensive
macroporous structure were obtained. Similar method
has been applied in the formation of microporous
Mn2O3 by thermal decomposition of monoclinic manꢀ
ganese oxalate in our previous work [11].
EXPERIMENTAL
Synthesis. All the chemical reagents are of analytiꢀ
cal grade and purchased from Shanghai Chemical
Reagent Company which were used without further
purification. 0.2 mol/L Na2CO3 solution was first preꢀ
pared, then 0.2 mol/L MnCl2 solution was dripped
down in to the Na2CO3 solution while strong magnetic
stirring was applied at 30
°
C. Keep stirring for 30 minꢀ
1
The article is published in the original.
utes after the reaction. The precipitation was aged
51

52
XIANZHONG ZHANG et al.
I
I
3
2
1
10
20
30
40
50
60
70
, deg
10
20
30
40
50
60
70
, deg
2θ
2θ
Fig. 3. XRD patterns of the calcined samples: CT400 (
CT600 ( ), CT800 ( ).
1),
Fig. 1. XRD pattern of the precursor MnCO
.
2
3
3
statically for 4 h and then filtered, washed with disꢀ recorded on a Hitachi Sꢀ4700 model scanning elecꢀ
tilled water for several times and dried in an oven at tron microscopy. TGꢀDTA curves were determined by
90
°
C
for 12 h.
Thermal treatment was carried out with the
obtained precursor in a muffle furnace at temperatures
of 400, 600, 800 for 2 h, the heating rate during calꢀ
cinations was controlled to be 10 C/min. The final
a WCTꢀ1 apparatus at a heating rate of 10 C/min in
°
atmosphere. N2 adsorptionꢀdesorption isotherms were
measured in a NOVA 1000e instrument at 77.36 K.
The resulting isotherms were analyzed by the BET
method.
°
C
°
products were labeled as CT400, CT600, CT800.
RESULTS AND DISCUSSION
Characterization. Xꢀray diffraction (XRD) of the
powder sample was performed by a Philips X Pert
Figure 1 shows the XRD pattern of the precursor.
The sharp diffraction peaks of the pattern can be
indexed to the 012, 104, 110, 113, 202, 024, 116, 122,
214 and 301 reflections of the rhodochrosite structure
correspondingly according to the standard card
JCPDS (44ꢀ1427). No characteristic peaks of other
impurities were detected, indicating the high purity of
the precursor. The crystallite size of the precursor estiꢀ
mated by the Scherrer formula is about 22 nm.
MPD diffractometer using Cu
K
radiation (
λ
=
α
0.15405 nm). Scan rate of
were applied to record the pattern in the
75 . The operation voltage and current were 40 kV and
45 mA respectively. SEM images of the samples were
4
°
/min and step size of 0.04
°
2
θ
range 10 –
°
°
m, %
100
4
The thermal behavior of precursor MnCO3 crystal
was illustrated in Fig. 2. The TG curve shows a large
90
weight loss range from the beginning to 520
°
C. The
2
initial weight loss around 100 is attributed to the loss
°
C
DTA
TG
80
70
60
50
40
of water which is corresponding to the first endotherꢀ
mic peak on the DTA curve. It can be seen from the
0
TG curve that an indistinct inflexion point near 360 C
°
can be discerned by the slope change of the TG curve
as shown in Fig. 2. A phase transformation is considꢀ
ered to be possible around this inflexion point. The
–2
–4
weight loss at 140–360 C is 24.77%.
°
Inflexion point
200
Figure 3 shows the XRD patterns of the asꢀpreꢀ
pared samples calcined at different temperatures. As
400
Temperature,
600
0
for the sample CT400 which was calcined at 400
the diffused broad peaks at = 37.12 , 42.24 , and
56.18 can be identified to the amorphous MnO2
according to JCPDS (30ꢀ0820). While for CT600 and
°
C,
°
C
2
θ
°
°
°
Fig. 2. TGꢀDTA curves for the precursor MnCO
.
3
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2010

A FACILE TEMPLATEꢀFREE PREPARATION OF POROUS MANGANESE OXIDES
53
500 nm
500 nm
1 μm
(b)
(а)
500 nm
500 nm
(c)
(d)
Fig. 4. SEM images of the precursor (a), sample CT400 (b), sample CT600 (c), sample CT800 (d).
CT800, the sharp diffraction peaks can be identified to coarse. After heating at 400°C, no obvious pore strucꢀ
be bixbyite (24ꢀ0508) as shown in Fig. 3 (scan 2and 3).
ture can be observed with the sample CT400 as shown
in Fig. 4b. The surface morphology changed little
compared with the precursor. It was interesting to
observe macroporous structure (Fig. 4c) with sample
CT600. The pore size and shape were irregular. With
According to Biernacki’s report [12], MnO was
first formed accompanied with the emission of CO2
during the first stage of the thermal decomposition of
MnCO3. Then MnO was prone to be oxidized by O2 in
the air simultaneously. The total reaction equation is
shown as following:
the heating temperature increasing up to 800 C, the
°
pore structure collapsed to form flat lamella as shown
in Fig. 4d.
2MnCO3 + O2
2MnO2 + 2CO2.
(1)
The theoretical value of the solid weight loss during
this process is 24.37%, which is very close to our
detected result above. With the increase of temperaꢀ
ture, it was considered that the irregular order of the
amorphous phase recombined to form crystallized
phase for receiving enough energy. The reaction is
shown as following:
Figure 5 shows representative nitrogen adsorpꢀ
tion/desorption isotherms of sample CT400 and
CT600. The isotherm of CT400 is type IV according to
the classification of IUPAC. The hysteresis loop
obtained from sample CT400 mainly present in the
range of 0.4–0.9 P/P0. Such adsorption behavior indiꢀ
1
2
2MnO2
Mn2O3 +
ꢀ
O2.
(2) cated the presence of the mesoporous structure. The
specific surface area of this sample is 135.1 m2/g as calꢀ
This reaction is considered to happen above 500
according to the TG curve, accompanied with a small
exothermic peak close to 520 on the DTA curve as
shown in Fig. 2.
°
C
culated by the Brunauer–Emmett–Teller (BET) forꢀ
mula. The sample CT600 exhibits an infrequent type
III isotherm which indicates the presence of
macroporous structure. The long hysteresis loop at
P/P0 = 0.16–0.98 indicates the broad pore size distriꢀ
bution. The specific surface area of sample CT600 is
only 18.51 m2/g, which may be caused by the vast
°
C
The morphology of the precursor as well as the calꢀ
cined samples was detected by SEM measurement.
The MnCO3 crystal was spherical or nutꢀlike which
was aggregated by subꢀcrystalline of 20–30 nm as
shown in Fig. 4a. The surface of the composite is transformation from mesoporous to macroporous.
INORGANIC MATERIALS Vol. 46
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54
XIANZHONG ZHANG et al.
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180
160
CT400
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CONCLUSIONS
Porous manganese oxides are facilely prepared by a
templateꢀfree thermal decomposition method.
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simple precipitation reaction. Porous manganese
oxides were then obtained by the controlled thermal
decomposition of the precursor MnCO3. Amorphous
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°
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remained the surface morphology of the precursor and
had a specific surface area of 135.1 m2/g. Highly crysꢀ
tallized macroporous Mn2O3 was prepared at 600 C
°
which had a specific surface area of 18.51 m2 g–1. This
approach commendably simplified the preparation of
mesoporous MnO2 and macroporous Mn2O3 with brief
process and low cost. Such method is supposed to be
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INORGANIC MATERIALS Vol. 46
No. 1
2010