Surface studies of nano-dimensional particle size iron-manganese spinel mixed oxides

Article abstract of DOI:10.1139/v98-186

Monophase binary iron-manganese spinel oxides of high dispersity are prepared by thermal decomposition of carbonate precursors in the limits 0.33 ≤ Fe/Mn ≤ 2, at temperatures 570-770 K. Interparticle meso- and macropores contribute to the surface morpholo

Full text of DOI:10.1139/v98-186

1363
Surface studies of nano-dimensional particle
size iron–manganese spinel mixed oxides
Ellie L. Uzunova, Dimitar G. Klissurski, and Stella D. Nemska
Abstract: Monophase binary iron–manganese spinel oxides of high dispersity are prepared by thermal decomposition
of carbonate precursors in the limits 0.33 ≤ Fe/Mn ≤ 2, at temperatures 570–770 K. Interparticle meso- and macropores
contribute to the surface morphology of the spinel oxides, composed from spherical-shaped particles with diameters in
the range 7–12 nm. The cation distribution of manganese ferrite corresponds to Mn0.85Fe0.15[Mn0.14Fe1.86]O4+δ. Surface
analysis by ESCA reveals high concentration of nonstoichiometric oxygen and presence of Mn(III) and Mn(IV) is
indicated in the X-ray photoelectron spectra. Reduction with hydrogen proceeds in two steps and is shifted to higher
temperatures with increasing the iron content.
Key words: ferrite, spinel, Fe, Mn.
Résumé : Les oxydes binaires monophases de Fe et Mn de type spinel avec une dispersion très haute sont preparés par
une décomposition thermique des carbonates comme précurseurs dans les limites des rapports 0.33 ≤ Fe/Mn ≤ 2 à
température 570–770 K. La morphologie des oxydes est formeé‚ par des particules sphériques au diamètre 7–12 nm et
des meso- et macro-pores interparticulaires. La distribution des cations correspond à la formule
Mn0.85Fe0.15[Mn0.14Fe1.86]O4+δ. Une haute concentration d’oxygène non-stoichiométrique à la surface a été etablie par
ESCA. Les spectres XPS indiquent aussi une présence de Mn(III) et Mn(IV). La réduction par l’hydrogène est
effectuée en deux étapes et exige une plus haute température avec l’augmentation du contenu du fer.
Mots clés : ferrite, spinel, Fe, Mn.
Uzunova et al. 1366
decomposition should be performed with care to avoid re-
duction (6). Transition-metal binary oxides and particularly
Spinel oxides, containing manganese and iron, are inter-
those, containing manganese, exist in a broad range of struc-
esting as magnetic materials and are also acknowledged for
ture types, depending on the composition and the oxidation
their catalytic properties. Binary spinel oxides, containing
state of the cations. Their reactivity in oxidation reactions is
transition metal cations in higher oxidation state, are very
governed by the particular forms of adsorbed oxygen and the
active combustion catalysts for CO and hydrocarbons (1–3).
surface oxygen concentration. Cation-deficient oxides of
Catalysts in the Fischer–Tropsch synthesis are developed on
high-dispersity and with Lewis-base properties are generally
the basis of different iron–manganese oxide compositions
obtained by the thermal decomposition of basic carbonates
(4). For all these applications, low particle dimensions of the
(8, 9).
oxide phase is essential, and it is usually achieved by the
The present study reports on the structure and morphology
“soft chemistry” synthesis methods in the low-temperature
of high-dispersity iron–manganese binary spinel oxides. The
range of thermal decomposition and the choice of an appro-
relation between surface composition, metal cation oxidation
priate precursor compound, containing all metal cations in a
states, and oxidation reactivity is discussed.
single crystalline phase (5–11). The precursor route may di-
rect the synthesis towards particular chemical architecture
(11). Nitrate salt decomposition favours higher oxidation
states of the metal cations in the final product, due to the ox-
idative media, created during the thermolysis (5). Oxalates
form mixed crystals with a number of transition metal cat-
ions and are often employed as precursors, but their thermal
The iron–manganese spinel oxides were synthesised by
thermal decomposition of calcite-like carbonate precursors
at temperatures in the range 570–870 K. The carbonates
were coprecipitated from a solution of Fe(II) and Mn(II)
chlorides, taken in a desired molar ratio and total Me(II)
concentration 0.6 mol/dm3 with a sodium carbonate solution
at pH 9. The iron and manganese content was determined by
X-ray fluorescence analysis. The XRD analysis was per-
formed on a diffractometer with CuKα monochromatized ra-
diation. Moessbauer spectra were recorded in constant
acceleration mode at room temperature. The source used
was a 57Co/(Cr matrix), and the isomer shifts were referred
to α-Fe. Full-sorption nitrogen isotherms at 77 K were re-
Received January 22, 1998.
E.L. Uzunova,1 D.G. Klissurski,1 and S.D. Nemska.
Institute of General and Inorganic Chemistry, Bulgarian
Academy of Sciences, Sofia 1113, Bulgaria.
1Author to whom correspondence may be addressed.
Telephone: +(359 2) 9793592. Fax: +(359 2) 705024.
E-mail: BANCHEM@BGEARN.BITNET;
inorg_ic@bgcict.acad.bg
Can. J. Chem. 76: 1363–1366 (1998)
© 1998 NRC Canada

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Can. J. Chem. Vol. 76, 1998
Table 1. Unit cell parameters and chemical composition of iron–
Fig. 1. Moessbauer spectrum of manganese ferrite, synthesized at
manganese spinel oxides.
870 K.
Unit cell parameters
o
Sample code
Chemical formulae
(A)
MF31
MF21
MF11
MF12
Mn2.25Fe0.75O4
Mn2.05Fe0.95O4
Mn1.51Fe1.49O4
Mn0.99Fe2.01O4
a = 5.76
a = 5.75
a = 8.475
a = 8.440
c = 9.35
c = 9.32
Table 2. Moessbauer parameters of manganese ferrite.
Hyperfine
magnetic field,
Isomeric shift (mm/s)
Heff (kOe)
δ(A)
0.32
δ(B)
0.43
A
523
B
486
Table 2. The isomer shifts of the components correspond to
iron(III) cations in cubic spinels, with zero quadrupole split-
tings. The cation distribution, deduced from the ratio of the
areas of the two components and based on chemical analysis
data can be written as Mn0.85Fe0.15[Mn0.14Fe1.86]O4+δ. This
formula composition is consistent with the low affinity of
Mn(II) and the preference of Mn(III) for octahedral coordi-
nation, evidenced by the value of the crystal field stabiliza-
tion energy (14). The effective hyperfine magnetic field at
Fe57 in both tetrahedral and octahedral sites is stronger,
compared to the one reported by Sawatsky et al. (13) for
high-temperature syntheses. We have observed a similar ef-
fect with cobalt ferrite, where the samples, synthesised at
low temperature (670 K) possess higher internal magnetic
field (15). This result indicates, that the presence of cation
vacancies (8, 9) is rather a surface phenomenon and does not
interfere with the A–A, B–B and A–B iron cation interac-
tions.
corded on a Omnisorp 100 equipment in a continuous flow
regime. Particle dimensions were determined by transmis-
sion electron microscopy. X-ray photoelectron spectra (XPS)
were registered on a ESCALAB MkII spectrometer with
AlKα radiation (1486.6 eV). Temperature-programmed re-
duction with hydrogen (TPR) was performed in a flow sys-
tem with 5% H2 in helium at flow rate 40 mL/min and
heating rate 20 K/min.
Structure and cation distribution
The samples with Fe/Mn ratios in the range 0.33 ≤ Fe/Mn
≤ 2, decomposed at 670 K are with spinel structure, the first
two being tetragonal (Table 1). The slight decrease of unit-
cell parameters for samples with higher Fe/Mn ratio is con-
sistent with the hypothesis of Mn(III)–Fe(III) substitution on
octahedral sites. Comparison of the effective high-spin octa-
hedral ionic radii (Mn3+ = 0.645 Å, Fe3+ = 0.645 Å) (12)
does not support directly this statement; however, in iron-
rich samples the cooperative effect of Jahn–Teller Mn3+–
[O2–]6 local distortions is diminished, leading to a regular
cubic spinel structure and smaller unit-cell size (8, 9). At
temperatures above 770 K, the spinel oxides undergo phase
segregation to the individual oxides: manganese oxides
(MnO and Mn2O3) from the manganese rich spinels and α-
Fe2O3 from the spinels with Fe/Mn > 2. Manganese ferrite is
of relatively higher thermal stability and remains as a single
phase spinel up to 870 K. A stable manganese ferrite phase
is obtained also at temperatures above 1300 K, while in the
temperature range 870–1200 K a mixture of spinel and α-
phase exist.
Morphology and pore distribution study
The full-sorption isotherms of all samples suggest pres-
ence of meso- and macropores (16). While the major contri-
bution in pore volume for samples MF31 and MF21 comes
from pores of radii 100–300 Å, the iron-rich samples contain
predominantly smaller pores of radii 20–40 Å (Table 3). For
all samples, the pore distribution is rather broad, and there is
a significant part of macropores of radii above 250 Å. The
particles are of spherical shape, and their effective diameters
are within 70–120 Å, measured by TEM. Surface area in-
creases with the Fe/Mn ratio due to both the smaller particle
size of the ferrite samples and to the more developed mes-
opore structure.
The Moessbauer spectra of samples, synthesised at 670 K
contain a central doublet, resulting from small particle
superparamagnetism. This effect may be produced also by
the presence of tetrahedral site cation vacancies (8).The dou-
blet remains even at sintering temperatures of 870 K, except
in the case of manganese ferrite, where a typical sextet is
registered (Fig. 1). The spectrum was computer fitted, as-
suming two components that are due to iron cations on tetra-
hedral (A) and octahedral (B) positions in the spinel
structure (13). The Moessbauer parameters are presented in
Surface analysis
The electron binding energies are summarized in Table 4
and compared to reference measurements, reported for simi-
lar oxides. The peak position of Mn 2p3/2 is shifted towards
higher binding energy, and the peak maxima correspond to a
value within the range for MnO2 and Mn2O3, rather than
MnO (17). The Fe 2p3/2 and O1s peak maxima are in agree-
ment with the values, reported earlier for ferrites (15, 17).
Considerable broadening is observed for Mn 2p3/2 and Fe
2p3/2, but all peaks are symmetric in shape, Fig. 2.
© 1998 NRC Canada

Uzunova et al.
1365
Table 3. Pore size distribution and specific surface areas of Fe–Mn spinel oxides.
Pore volume (cc/g) at radii (Å)
BET surface
area (m2/g)
Sample
10–20
20–30
30–40
40–50
50–100
100–200
200–300
>300
Total
<0.01
0.01
0.01
0.03
0.01
<0.01
0.05
<0.01
<0.01
0.13
<0.01
<0.01
0.09
<0.01
0.03
0.06
0.05
0.01
0.13
0.08
0.06
0.02
0.08
0.06
0.04
0.03
0.06
0.08
0.04
MF31
MF21
MF11
MF12
44
67
124
135
0.10
0.33
0.56
0.43
0.09
0.08
0.04
Table 4. Electron binding energies for iron–manganese spinel oxides (eV).
Sample
Mn 2p3/2
FWHMb
Fe 2p3/2
FWHMb
O1s
FWHMb
MF31
MF21
MF11
MF12
MnO
Mn3O4
Mn2O3
MnO2
642.0
642.8
642.5
3.8
3.9
3.8
3.8
710.5
711.3
711.4
711.0
3.9
3.8
3.7
3.9
530.2
530.6
530.4
530.3
2.2
1.9
2.0
1.8
642.7
640.9a
641.7a
641.8a
642.4a
(643.1)c
aReference 17.
bFull width at half maximum.
cThis study.
Fig. 2. The Mn 2p3/2, Fe 2p3/2, and O1s spectra of iron–manganese spinel oxides, synthesized at 670 K.
The oxygen-to-metal cation ratio on the surface slightly
increases with the iron bulk concentration, Fig. 3. The non-
stoichiometric surface oxygen exceeds the quantity expected
for spinel oxides M3O4, where both divalent and trivalent
cations take place in the structure; sesquioxides M2O3,
where all cations are in the trivalent state; and even the oxy-
gen content for M(IV)O2. The very high oxygen-to-metal
cation ratio on the surface, together with the shift to higher
binding energies in the Mn 2p3/2 spectra can be interpreted
by suggesting presence of manganese in a higher oxidation
state: Mn(III) and Mn(IV). The surface cation distribution in
terms of iron/manganese ratio corresponds to the bulk stoi-
chiometry in the range 0.33 ≤ Fe/Mn ≤ 1, Fig. 4. Iron-dom-
inated compositions MF12 and MF13 possess manganese-
enriched surface, suggesting that the occurrence of surface
cation vacancies is mainly on behalf of the Fe(III) cations.
Temperature-programmed reduction
Two peaks are registered in the TPR spectra of iron–man-
ganese spinel oxides, Fig. 5. The first one does not change
significantly with sample composition and corresponds to
reduction of nonstoichiometric surface oxygen. The second
peak covers a broad temperature range and shifts to higher
temperature for the iron-rich samples, Fe/Mn ≥ 1. The triva-
lent cations of manganese and iron are less stable and are
easily reduced to the divalent oxidation state, followed by
the reduction steps Fe(II) → Fe and Mn(II) → Mn. These
processes give rise to a single broad peak in the TPR pat-
© 1998 NRC Canada

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Can. J. Chem. Vol. 76, 1998
Fig. 3. Oxygen to metal cation surface ratio as a function of
bulk iron/manganese ratio.
Fig. 5. Temperature programmed reduction of iron–manganese
spinel oxides, synthesized at 670 K; (a) MF31; (b) MF21;
(c) MF11; (d) MF12.
Fig. 4. Surface iron/manganese ratio as a function of the bulk
one.
The authors thank for financial support the Bulgarian Na-
tional Science Fund at the Ministry Of Science and Educa-
tion under contract X-628.
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The high-dispersity spinel iron–manganese oxides, syn-
thesized from carbonate precursors, contain a large excess of
surface oxygen, related to surface cation deficiency and pre-
sence of manganese cations in a higher oxidation state:
Mn(III) and Mn(IV). Reduction with hydrogen proceeds in
two steps and very high temperature is needed for the com-
plete reduction of iron-rich compositions.
© 1998 NRC Canada