Synthesis and characterization of Mn(II) and Mn(IV) oxide
nanoparticles inside mesoporous carbon CMK-3
Holger Huwe and Michael Fröba*
Institute of Inorganic and Analytical Chemistry, Justus Liebig University Giessen,
Heinrich-Buff-Ring 58, 35392 Giessen, Germany
Via an intra-pore synthesis manganese(IV) oxide nanoparticles were synthesized inside the pore
system of mesoporous carbon CMK-3. The formation of manganese(IV)oxide almost exclusively
within the pore system and preservation of the host structure was proved by powder X-ray
diffractometry, nitrogen physisorption and transmission electron microscopy. The manganese(IV)
oxide was reduced to manganese(II) oxide inside the mesopores. The local structure of both
materials was determined by X-ray absorption spectroscopy. The mesoporous carbons and the hostguest compounds are of great interest for several uses, as catalyst supports and electronic
applications [1, 2].
The synthesis of the host-guest compound is similar to that reported in [1]. Three cycles of wet
impregnation with manganese(IV) nitrate solution in ethanol, drying and calcination were applied
to form the Mn(IV) oxide almost exclusively inside the pore system. The formation of Mn(IV)
oxide nanoparticles in the pore system and its preservation was confirmed with XRD and
physisorption measurements (not depicted).
To gain knowledge about the oxidation state of the manganese, XAS measurements at the Mn Kedge were carried out. All spectra were recorded at liquid nitrogen temperature with a Si(111)
monochromator. Due to the low amount of manganese inside the samples the spectra were recorded
up to 5 times in fluorescence mode and averaged for data analysis to reveal a better signal to noise
ratio.
For data analysis the programs WinXAS [3] and Feff [4] were used. The shells were extracted by a
Fourier filter using a Bessel window in the range between 1.8 and 4 Å. The number of parameters
in the fits were all smaller than the possible number of independent parameters.
The recorded absorption spectra are divided into two regions: The first is assigned to the X-ray
absorption near-edge structure (XANES) and the second to the extended X-ray absorption fine
structure (EXAFS).
The results of the XANES investigations show a reduction of manganese(IV) to manganese(II) as it
can be seen from Figure 1. The principle component analysis of the XANES spectra is listed in
table 1. From these data we can conclude that the original material at the beginning of the reduction
was pristine manganese(IV) oxide and the after the complete reduction process pristine
manganese(II) oxide was obtained.
Table 1: Results of the first Linear Combination XANES fit.
sample /
reference component
MnO2 @CMK-3
MnO
MnO2
Mn2O3
Mn
principle component
of the first fit (p.c.) [%]
-44
99
-0.4
-54
E [eV] sample / p.c. [%] E [eV]
-0.03
-0.022
3.1
0.5
527
MnO@CMK-3
98
-87
-3.9
-22
0.0007
14
0.03
-3.6
Figure 1a: Normalized Mn-K-XANES
spectra of the host/guest compounds and
their respective bulk materials.
Figure 1b: Fourier transforms (not phaseshift corrected) of the respective Mn K-edge
XAFS oscillations of the host/guest
compound and their belonging bulk
materials
The data resulting from the EXAFS investigation are listed in table 2 and 3. The resulting fits
confirm our assumption that the nanoparticles that were synthesized in the pores of CMK-3 show
the crystal structure of manganese(IV) oxide. The following reduction procedure lead to
manganese(II) oxide as it can be concluded from the results listed in table 3. A complete reduction
to manganese(0) could not be achieved under these reduction conditions.
Table 2: Refined structure parameters extracted from the Mn-K EXAFS of the host/guest
compound and bulk material as well as corresponding results extracted from single crystal data.
shell
Mn-O
Mn-O
Mn-Mn
Mn-O
Mn-Mn
Mn-O
CN
R [Å]
MnO2 @CMK-3
1.01
1.89
3.01
1.92
1.22
2.94
1.39
3.40
2.98
3.35
-
CN
R [Å]
bulk MnO2
2.00
1.90
3.99
1.90
1.91
2.85
3.78
3.30
7.71
3.46
3.55
3.46
CN
R [Å]
single crystal data
2
1.88
4
1.89
2
2.87
4
3.34
8
3.42
4
3.43
Table 3: Refined structure parameters extracted from the Mn-K EXAFS of the reduced host/guest
compound and reduced bulk material and the corresponding results extracted from single crystal
data.
shell
Mn-O
Mn-Mn
Mn-O
Mn-Mn
CN
R [Å]
MnO@CMK-3
3.45
2.20
7.32
3.09
4.26
3.72
-
CN
R [Å]
bulk MnO
6.00
2.23
11.82
3.16
7.42
3.91
3.21
4.46
CN
R [Å]
single crystal data
6
2.22
12
3.14
8
3.85
6
4.44
Acknowledgements
The authors thank Hasylab@Desy for financial support and providing beamtime and Dr. Edmund
Welter and Ulf Brüggmann for technical and research support at the beamline A1.
References
[1] H. Huwe, M. Fröba, Microporous Mesoporous Mater., 60, 151, (2003)
[2] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B, 103, 7743, (1999)
[3] T. Ressler, J. Synchrotron Rad., 5, 118, (1998)
[4] A.L. Ankudinov, B. Ravel, J.J. Rehr, and S.D. Conradson, Phys. Rev. B, 58, 7565, (1998)
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