Materials Sciences and Applications, 2013, 4, 447-452
http://dx.doi.org/10.4236/msa.2013.48054 Published Online August 2013 (http://www.scirp.org/journal/msa)
447
Nanocrystalline Mixed Oxides Containing Magnesium
Prepared by a Combined Sol-Gel and Self-Combustion
Method for Catalyst Applications
Nicolae Rezlescu1, Elena Rezlescu1, Liliana Sachelarie2, Paul Dorin Popa1, Corneliu Doroftei1,
Maria Ignat3
1National Institute of Research and Development for Technical Physics, Iasi, Romania; 2Apolonia University, Iasi, Romania; 3Al. I.
Cuza University, Iasi, Romania.
Received June 13th, 2013; revised July 12th, 2013; accepted July 20th, 2013
Copyright © 2013 Nicolae Rezlescu et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
MgFe2O4 spinel ferrite and La0.6Pb0.2Mg0.2Mn O3 perovskite nanopowders were synthesized by a combined sol-gel and
self-combustion method and heat treatment. The morphological and structural characterization of the obtained powders
has been performed with various techniques: X-ray diffraction (XRD), SEM observations, EDAX spectroscopy and
BET analysis. The samples have been catalytically tested in flameless combustion reaction of acetone, benzene, pro-
pane and Pb free gasoline at atmospheric pressure. The results revealed a higher catalytic activity of La0.6Pb0.2Mg0.2
MnO3 perovskite than that of MgFe2O4 ferrite. This higher catalytic activity can be ascribed to smaller crystallite size
(27 nm), larger surface area (8.5 m2/g) and the presence of manganese cations with variable valence (Mn3+ - Mn4+). The
current results suggest that La0.6Pb0.2Mg0.2MnO3 perovskite is preferable to the Mg ferrite and that it can be a promising
catalyst for acetone and propane combustion at low temperatures.
Keywords: Oxides; Sol-Gel-Self-Combustion; Microstructure; SEM; XRD; Catalytic Properties
1. Introduction
The aim of the present work is to comparatively estimate
the physical and catalytic properties of two magnesium
containing oxide compounds prepared by sol-gel-self-
combustion: MgFe2O4 spinel ferrite and La0.6Pb0.2Mg0.2
MnO3 perovskite. The role of the Mg ion in a ceramic is
to prevent the growth of the grains by reducing the grain
boundary mobility.
In the last years, the use of perovskite or spinel type
oxide compounds as catalyst has been widely investi-
gated in order to find a catalyst with high thermal stabil-
ity [1] and low temperature activity [2]. Transition metal
perovskites as LaMnO3 are known to be very good oxi-
dation catalyst. The partial substitution of La3+ ions by
lower valence ions (such as Pb2+, Mg2+, Ca2+) produces
the partial oxidation of Mn3+ to Mn4+ ions and the in-
crease in oxygen vacancies which enhance the catalytic
activity of the perovskite. The stability of Mn4+ ions
seems to be the most important factor in the catalytic
activity of perovskite manganites [3]. Saracco et al. [4]
reported the positive effect of the Mg substitution in the
basic LaMnO3 perovskite on the catalytic activity of the
resulting perovskite.
Oliva et al. [5] found that the preparation procedure
can have a remarkable effect on the physico-chemical
characteristics and the catalytic properties of perovskites.
In the present work we applied a nonconventional pro-
cedure which is a combined sol-gel and self-combustion
method [6] followed by thermal treatment. In this proce-
dure the heat generated by an exothermic combustion
reaction was used for synthesis reaction of the oxide ce-
ramics. The intimate mixing of constituent ions so that
nucleation and crystallization can occur at relatively low
temperature is the main feature of this method. Sol-gel-
self-combustion method offers a number of advantages
including homogeneous mixing (on the atomic scale),
low energy cost, easy manufacturing and the control of
the grain size by subsequent heat treatments.
The phase composition, microstructural features,
compositional homogeneity, specific surface area and
pore size of the obtained powders were studied by X-ray
powder diffraction analysis (XRD), scanning electron

Nanocrystalline Mixed Oxides Containing Magnesium Prepared by a Combined Sol-Gel
and Self-Combustion Method for Catalyst Applications
448
microscopy (SEM) coupled with energy dispersive X-ray
analysis (EDAX) and nitrogen adsorption/desorption at ~77
K. The obtained nanopowders (La0.6Pb0.2Mg0.2MnO 3 and
MgFe2O4) have been tested as catalysts for the catalytic
flameless combustion of acetone, benzene, propane and
Pb free gasoline. The effect of the phase composition and
structure on the performance of the two catalyst samples
has been investigated.
2. Experimental
2.1. Sample Preparation
MgFe2O4 spinel ferrite and La0.6Pb0.2Mg0.2MnO3 perov-
skite have been prepared by sol-gel-self-combustion me-
thod [6], using metal nitrates, ammonium hydroxide and
polyvinyl alcohol as starting materials. This method in-
cluded the following steps: 1) dissolution of metal ni-
trates in deionized water; 2) polyvinyl alcohol (10%
concentration) addition to nitrate solution to make a col-
loidal solution; 3) NH4OH (10% concentration) addition
to increase pH to about 8; 4) stirring at 80˚C to turn the
sol of metal hydroxides into gel; 5) drying the gel at
100˚C; 6) self-combustion of the dried gel; 7) calcina-
tions at 500˚C for 30 min of the burnt powder to elimi-
nate any residual ceramic compound; 8) heat treatment of
the powders. MgFe2O4 powder was treated at 900˚C for
10 min and La0.6Pb0.2Mg0.2MnO3 was treated at 1000˚C
for 320 min. The higher temperature for longer time in-
terval for perovskite was preferred for two major reasons.
First, the heat released by the combustion reaction is not
sufficient to raise the system temperature to a level that
allows the growth of the perovskite crystallites. Second,
La0.6Pb0.2Mg0.2MnO3 is an oxide compound and it is pos-
sible that the migration of ions required for the formation
of the perovskite structure demands some residence time
at high temperature. In Figure 1 is given the flow dia-
gram for preparing process by sol-gel self-combustion
and heat treatment. By this procedure oxide compounds
can be prepared at much lower temperature than by the
conventional solid state reaction method.
2.2. Characterization Techniques
Crystal structure and phase composition of the samples
were analyzed by XRD. X-ray diffraction measurements
of the powders were performed at room temperature us-
ing PANANALYTICAL X’ PERT PRO MPD powder
diffractometer and CuKα radiation (λ = 1,542,512 Ǻ).
The spectra were scanned between 20 and 80˚ (2θ) at a
rate of 2˚/min. The average crystallite size was evaluated
based on XRD peak broadening using the Scherer equa-
tion D = 0.9 λ/βcosθ, were λ is radiation wavelength
(0.15405 nm) of CuKα, β is the half width of the peak
and θ is the Bragg diffraction peak angle. A scanning
Solution of
nitrates
Polyvinyl
alcohol solution
N
H
4
OH
solution
Colloidal solution
Hydroxide sol
Heat treated powder
Viscous gel
Dried gel
N
anocrystaline
p
owde
r
Calcined powder
Control of pH
Stirri ng
Drying in air
Selfcombustion
Calcination at 500 oC
Heat treatment
Figure 1. Flow diagram for preparing the oxide nanopow-
ders by combined sol-gel and self-combustion route.
electron microscope (JEOL-200 CX) was used to observe
the surface morphology. Textural characteristics were
investigated by means of specific surface area deter-
mined by BET (Brunauer-Emmett-Teller) [7] method
from the nitrogen sorption isotherms at 77 K. Adsorp-
tion/desorption isotherms were determined with NOV-
A-2200 apparatus. The pore size distribution (PSD) curves
were obtained from sorption isotherms using BJH (Bar-
ret-Joyner-Halenda) method [7]. The chemical composi-
tion of the surface particles was examined with Energy
Dispersive X-ray Spectrometer (EDS).
3. Results and Discussion
The XRD patterns at room temperature of the heat
treated samples are shown in Figure 2. Results revealed
that all samples were monophase without any other sec-
ond phase. Cubic spinel structure (Fd3m space group)
was identified for MgFe2O4 sample and cubic perovskite
structure (Fm3m space group) for La0.6Pb0.2Mg0.2MnO3.
The broadening of the peaks implies the generation of
crystallites in the nanosize range. The lattice parameters
and average crystallite size derived from XRD data are
given in Table 1. The values of the lattice parameters
almost coincide with those presented in the literature in
analogous compounds [8]. It is evident the nanosized
crystallinity and that the crystallite size and density be-
have inversely to each other with respect to Mg concen-
tration. Smaller crystal size (27.28 nm) and higher den-
sity (8.33 g/cm3) were found in La0.6Pb0.2Mg0.2MnO3
perovskite which contains 20% molar Mg only.
SEM micrographs showing surface morphology of the

Nanocrystalline Mixed Oxides Containing Magnesium Prepared by a Combined Sol-Gel
and Self-Combustion Method for Catalyst Applications
449
Figure 2. XRD patterns for La0.6Pb0.2Mg0.2MnO3 and MgFe2O4 nanopowders.
Table 1. Structure characteristics.
Sample composition Lattice parameter
(nm)
Crystal
size
(nm)
dx
(g/cm3)
MgFe2O4 0.8366 41.78 4.55
La0.6Pb0.2Mg0.2MnO3 0.7740 27.28 8.33
two powders are shown in Figure 3. One can note sig-
nificant differences in the microstructure of the two sam-
ples. Small agglomerates of fine grains with irregular
shape can be observed in the perovskite powder, whereas
MgFe2O4 spinel powder is characterized by the presence
of dense and larger agglomerates.
The EDAX patterns confirm the homogeneous mixing
of atoms in both samples and the purity of the chemical
compositions. Figure 4 shows EDAX spectrum for
MgFe2O4 and Table 2 shows the elemental composition.
The Mg/Fe ratio is found close to the theoretical value
(0.5) and this is proof of homogeneous distribution of the
elements in the solid.
Nitrogen adsorption/desorption isotherms at ~77 K
were used to obtain information about the specific sur-
face area SBET, the pore volume and the pore size in the
ceramic particles. The isotherms for the both samples are
presented in Figure 5. As can be seen, the desorption
branch does not follow the adsorption branch, but forms
a hysteresis loop of type H3 according to the Interna-
tional Union of Pure and Applied Chemistry (IUPAC)
classification [9]. H3 type hysteresis is typically for ma-
terials with an interparticle mesoporosity (pore size 2 -
50 nm) [7]. However, a clear decision with regard to the
type of hysteresis loop is not always possible. The factors
which determine the shape of the hysteresis loop are still
not fully known for disordered pore system [10].
Pore size distribution curves (PSD by BJH method) for
both samples obtained from N2 (~77 K) sorption iso-
therms are given inset of Figure 5. There are two distinct
ranges for pore size distribution for MgFe2O4. These are
between 2.7 and 3 nm and between 5.5 and 6.3 nm. The
pore size distribution of La0.6Pb0.2Mg0.2MnO3 was found
to be different from that of MgFe2O4. Four ranges with
different pore sizes were evidenced: two narrow ranges,
from 3 to 3.5 nm and from 4.8 to 5.2 nm, and two wider
pore ranges, from 6.8 to 8.5 nm and from 13 to 15 nm.
But all the pore sizes are within the mesoporous region.
The BET surface area, pore volume and average pore
diameter are given in Table 3. BET specific surface area
(8.5 m2/g) of La0.6Pb0.2Mg0.2MnO3 is higher than that of
MgFe2O4 (4.0 m2/g). Larger pore volume and smaller
particle size determine the increased SBET value of the
perovskite. The characteristics of MgFe2O4 are compara-
ble to those of other spinel ferrites reported in [11].
The catalytic testing of the two catalyst samples in the
flameless combustion of four VOCs (acetone, propane,
benzene and Pb free gasoline) was carried out at atmos-
pheric pressure in a flow-type set-up previously de-
scribed by us [12]. The catalyst powder (0.3 - 0.5 g) was
sandwiched between two layers of quartz wool in a
quartz tubular micro-reactor (Φ = 7 mm) placed in an
electrical furnace. The increase of the temperature was
made stepwise. At every predetermined temperature, as a
result of catalytic combustion, the gas concentration at
the exist of reactor will be smaller than the inlet gas con-
centration. The degree of conversion of gases over cata-
lysts at a certain temperature was calculated as:
in out
in
100%
cc
Conv c

,
where cin and cout are the inlet and outlet gas concentra-
tion, respectively, measured by a photo-ionization detec-
tor (PID-TECH) for VOCs. Data were collected when the
flameless catalytic combustion had reached a steady state,
after about 20 minutes at each temperature. These ex-

Nanocrystalline Mixed Oxides Containing Magnesium Prepared by a Combined Sol-Gel
and Self-Combustion Method for Catalyst Applications
450
1μm
MgFe2O4
1μm
La0.6Pb0.2Mg0.2MnO3
Figure 3. SEM micrographs for La0.6Pb0.2Mg0.2MnO3 and MgFe2O4 nanopowders.
Table 2. EDAX analysis of MgFe2O4 powder.
O (at%) 50.34
Mg (at%) 17.60
Fe (at%) 31.99
Mg/Fe 0.55
Figure 4. EDAX spectrum for MgFe2O4 ferrite heat treated
at 900˚C for 10 min.
periments were repeated decreasing the temperature and
similar results were obtained.
The catalytic reactions were investigated in the 20˚C -
550˚C range. Figure 6 plots the gas conversion over the
two catalysts as a function of reaction temperature. One
can note the followings:
1) The catalytic activity of the two nanomaterials in
the gas combustion is strongly influenced by the reaction
temperature. Increasing the reaction temperature pro-
motes gas conversion over the two catalysts.
2) The reactions involving spinel type oxide occur at
higher temperatures than in the presence of the perov-
skite catalyst.
3) The activities of the two catalysts differ signifi-
Table 3. Surface characteristics of MgFe2O4 and La0.6Pb0.2
Mg0.2MnO3 powders.
Characteristics MgFe2O4 La0.6Pb0.2Mg0.2MnO3
BET surface area (m2/g) 4.0 8.5
BJH pore volume (cc/g) 0.006 0.021
BJH average pore size (nm) 4.25 7.50
Particle size DBET* (nm) 33.7 89.2
*DBET was calculated using SBET [7].
cantly (Figure 6). From all experiments, La0.6Pb0.2Mg0.2
MnO3 perovskite resulted to be more active than Mg
Fe2O4 ferrite. The conversion degree of the gases over
perovskite catalyst may even exceed 90% at 500˚C,
whereas over spinel catalyst the conversion is below 80%.
The worst result was obtained for Pb free gasoline com-
bustion over spinel catalyst: 34% conversion at 550˚C.
The temperature of the 50% conversion of a gas, T50,
was used to estimate the catalytic activity of the two
catalysts: MgFe2O4 spinel and La0.6Pb0.2Mg0.2MnO3 pe-
rovskite. At T50 temperature the catalytic activity for the
total oxidation of gases is sufficiently high. The lower
T50 is, the higher the activity. The T50 values for each
catalyst and each VOC (except for Pb free gasoline) are
plotted in bar diagram presented in Figure 7. The T50
values for benzene and acetone conversion over La0.6
Pb0.2Mg0.2MnO3 are in agreement with those reported by
Spinicci et al. [13]. For gasoline conversion over MgFe2O4
catalyst, T50 exceeds 550˚C.
The catalyst stability of the two materials was studied
and no deactivation was observed in any of the samples
after 24 hours.
The better catalyst performance of La0.6Pb0.2Mg0.2
MnO3 perovskite in comparison with MgFe2O4 spinel
can be attributed to smaller crystallite size (27 nm), lar-
ger surface area (8.5 m2/g) and the presence of manga-

Nanocrystalline Mixed Oxides Containing Magnesium Prepared by a Combined Sol-Gel
and Self-Combustion Method for Catalyst Applications
451
Relative pressu r e, P/P0
Adsorbed volume, cc/g STP
0
5
10
15
20
25
30
0 0.2 0.4 0.6 0.8 1
0 51015 20
Pore diameter
,
nm
dVd
3.3
5.0
7.8
13.9
La0.6Pb0.2Mg0.2MnO3
0
1
2
3
4
5
00.2 0.40.6 0.8 1
Relative pressu re , P/P0
Adsorbed volume, cc/g STP
MgFe 2O4
2 4 6 8 10
Pore diameter, nm
dVd
5.7
2.8
Figure 5. N2 adsorption/desorption isotherms at 77 K of the two powders: La0.6Pb0.2Mg0.2MnO3 perovskite and MgFe2O4
spinel ferrite ( : the adsorption branch and ▪ □: the desorption branch). Inset: the pore size distribution graphs.
(a) (b)
Figure 6. Conversion of acetone, propane, benzene and Pb free gasoline vs. temperature over La0.6Pb0.2Mg0.2MnO3 perovskite
and MgFe2O4 spinel catalysts.
nese cations with variable valence. Redox titration me-
thod showed that La0.6Pb0.2Mg0.2MnO3 perovskite con-
tains a considerable concentration of Mn4+ ions (35%) in
addition to Mn3+ ions, which form to compensate the
charge change caused by Mg2+ and Pb2+ substitutions for
La3+. The presence of Mn3+ - Mn4+ ions implies oxygen
vacancies which favor the increase in the oxygen ion
species (O−, O2
−, O2−) adsorbed on perovskite surface
[14]. The larger the number of oxygen ions adsorbed, the
faster the oxidation of gases would be. The gas oxidation
activity over catalyst is related to the ability of surface
adsorbed oxygen to activate gas and to the ability of gas
phase oxygen to fill surface oxygen vacancies. We have
not excluded the involvement of the lattice oxygen al-
though its mobility is smaller than that of surface ad-
sorbed oxygen. The formation of the defective structures
in the perovskite structure by partial substitution of La by
ions of lower valence facilitates the mobility of the lattice
oxygen (via oxygen vacancy mechanism [15]) and the
material will be more active for oxidation reactions [16].
4. Conclusion
In this work the combined sol-gel and self-combustion
method has been employed to prepare MgFe2O4 spinel
and La0.6Pb0.2Mg0.2MnO3 perovskite for catalyst applica-
tions. It is an inexpensive method and the obtained prod-

Nanocrystalline Mixed Oxides Containing Magnesium Prepared by a Combined Sol-Gel
and Self-Combustion Method for Catalyst Applications
452
Figure 7. Bar diagram for the temperature of 50% conver-
sion, T50, of acetone, propane and benzene on MgFe2O4
spinel and La0.6Pb0.2Mg0.2MnO3 manganite catalysts.
ucts were pure and presented nanosized crystallinity.
Both samples have been tested in the catalytic combus-
tion of acetone, propane, benzene and Pb free gasoline.
La0.6Pb0.2Mg0.2MnO 3 catalyst is more active at low tem-
peratures compared to MgFe2O4 catalyst. Higher cata-
lytic activity of the perovskite (over 90% gas conversion)
is related to smaller crystallite size (27 nm), higher spe-
cific area (8.5 m2/g) and the presence of manganese
cations with variable valence (Mn3+ - Mn4+). La0.6Pb0.2
Mg0.2MnO3 perovskite can be a promising catalyst for
catalytic combustion of acetone and propane at low tem-
peratures (bellow 300˚C).
5. Acknowledgements
This work was supported by a grant of the Romanian
National Authority for Scientific Research, CNST-UE-
FISCDI, project number PN-II-ID-PCE-2011-3-0453.
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