Synthesis and characterization of some nanostructured composite oxides for low temperature catalytic combustion of dilute propane

Article abstract of DOI:10.1039/c7ra03916f

Powders of five nanosized perovskites, SrMnO₃, SrCoO₃, GdAlO₃, FeMnO₃, La_0.6Pb_0.2Mg_0.2MnO₃, and four ferrospinels, MgFe₂O₄, Ni_0.5Co_0.5Fe₂O₄, Ni_0.5Co_0.5Fe_1.9Sc_0.1O₄, Ni_0.5Co_0.5Fe_1.8Sc_0.2O₄, were prepared by a sol-gel self-combustion technique. The powders were catalytically tested in the flameless combustion of dilute propane at atmospheric pressure. All samples had nanocrystalline structures and the crystallite sizes were in the range of 26-89 nm. Catalytic testing showed a significant dependence of the catalytic performance on the catalyst composition. As experiments revealed, the La_0.6Pb_0.2Mg_0.2MnO₃ perovskite catalyst, with a specific surface area of 8.6 m² g^-1, exhibits the best performance in propane conversion, of about 92% at 350 °C. The lowest catalytic efficiency was obtained for FeMnO₃ and SrCoO₃ catalysts. Different reactivities of active oxygen species involved in the catalytic oxidation reaction determine registered differences in catalytic activity of oxide compounds. The results suggest that the La_0.6Pb_0.2Mg_0.2MnO₃ perovskite can be a suitable catalyst for catalytic combustion of dilute propane at low temperatures (below 400 °C).

Full text of DOI:10.1039/c7ra03916f

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Synthesis and characterization of some
nanostructured composite oxides for low
temperature catalytic combustion of dilute
propane
Cite this: RSC Adv., 2017, 7, 27863
a
and L. Leontieab
*
C. Doroftei
Powders of five nanosized perovskites, SrMnO3, SrCoO3, GdAlO3, FeMnO3, La0.6Pb0.2Mg0.2MnO3, and four
ferrospinels, MgFe2O4, Ni0.5Co0.5Fe2O4, Ni0.5Co0.5Fe1.9Sc0.1O4, Ni0.5Co0.5Fe1.8Sc0.2O4, were prepared by
a sol–gel self-combustion technique. The powders were catalytically tested in the flameless combustion
of dilute propane at atmospheric pressure. All samples had nanocrystalline structures and the crystallite
sizes were in the range of 26–89 nm. Catalytic testing showed a significant dependence of the catalytic
performance on the catalyst composition. As experiments revealed, the La0.6Pb0.2Mg0.2MnO3 perovskite
catalyst, with a specific surface area of 8.6 m2 ꢀ1, exhibits the best performance in propane conversion,
g
of about 92% at 350 ꢁC. The lowest catalytic efficiency was obtained for FeMnO3 and SrCoO3 catalysts.
Different reactivities of active oxygen species involved in the catalytic oxidation reaction determine
registered differences in catalytic activity of oxide compounds. The results suggest that the
La0.6Pb0.2Mg0.2MnO3 perovskite can be a suitable catalyst for catalytic combustion of dilute propane at
low temperatures (below 400 ꢁC).
Received 6th April 2017
Accepted 22nd May 2017
DOI: 10.1039/c7ra03916f
rsc.li/rsc-advances
compounds, ABO3 (A is usually a rare earth and B is a transi-
tion metal),12–16 and ferrospinel-type compounds, AFe2O4 (A is
a divalent metal).17–20
1. Introduction
The progressive increase of volatile organic compound (VOC)
emissions requires the development of new technologies for
their removal from the atmosphere.1–3 Compared to conven-
tional thermal incineration, catalytic combustion is considered
a far more efficient method for removing low concentrations of
VOCs from gaseous streams.2,4 Catalytic combustion is also
more economical because it can operate at low temperatures
(<500 ꢁC) with diluted effluent streams (<1% VOC). Catalytic
combustion performance is strongly dependent on the type of
catalyst used. Two categories of catalysts have been typically
employed for VOC oxidation reactions: noble metals and tran-
sition metal oxides.5–11 Noble metal catalysts display, in general,
high hydrocarbon combustion activity, but they are expensive
and more prone to deactivation by poisoning. The transition
metal oxide catalysts are, generally, less active than noble
metals, but more resistant towards poisoning and bear a lower
cost.7,11
The preparation of catalysts with nanosized particles is the
key to an efficient catalytic performance. In nanostructured
materials, the interface between nanoparticles and surrounding
medium plays a more important role than in the bulk materials.
Moreover, the strong curvature of nanoparticles due to their
small radius leads to an increased number of the structural
defects at the nanoparticle surface, enhancing the surface
reactivity. Various synthesis methods have been tested to obtain
catalyst materials with superior microstructure, e.g. sol–gel,21
coprecipitation,22 or combustion reaction.23 Oliva et al.24 found
that the preparation procedure can exert a remarkable inuence
on the physico-chemical characteristics and catalytic properties
of the obtained materials.
In the present work a nonconventional method, sol–gel self-
combustion,25 followed by a thermal treatment, was used to
prepare low cost catalysts – perovskite and ferrospinel nano-
powders. In this processing technology, the heat generated by
a rapid exothermic combustion reaction was used for the
synthesis reaction of oxide ceramics. The method requires basic
equipment and assures a high purity and homogeneous mixing
on the atomic scale. The ceramic materials prepared by sol–gel
self-combustion have superior properties with respect to those
made by conventional methods and are cheaper to produce. We
analyzed comparatively the catalytic performances of the
Recently, considerable attention has been directed towards
catalysts based on mixed metal oxides such as perovskite-type
aAlexandru Ioan Cuza University, Integrated Center for Studies in Environmental
Science for North-East Region (CERNESIM), 11 Carol
I Blvd, 7000506 Iasi,
Romania. E-mail: docorneliug@gmail.com; docorneliu@yahoo.com; Tel: +40 730
719855
bAlexandru Ioan Cuza University, Faculty of Physics, 11 Carol I Blvd, 7000506 Iasi,
Romania
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obtained nanopowders utilized in the catalytic ameless crystallite size was estimated based on XRD peak broadening,
combustion of the dilute propane. The inuence of various using the Scherrer equation:28
parameters such as chemical composition of the catalysts,
0:9l
DXRD
¼
;
(1)
crystallite size, surface specic area, reaction temperature and
conversion degree of the propane has been investigated. The
structural and morphological characterization of the materials
was performed using X-ray diffraction (XRD), Scanning Electron
Microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area
analysis, Energy Dispersive X-ray spectroscopy (EDX) and X-ray
photoelectron spectroscopy (XPS).
b cos q
where l is wavelength of Cu-Ka1 radiation, b is the full width at
half maximum of the peak, and q denotes its Bragg diffraction
angle. A scanning electron microscope (JEOL-200 CX) was used
to visualize the surface morphology of samples. A Quantach-
rome automated gas adsorption system (Quantachrome
Instruments) was used to record N2 adsorption isotherms at 77
K. The powders were degassed at 300 ꢁC for 1 h before the
measurements. The BET specic surface area (SBET) was deter-
mined from nitrogen sorption data using the BET equation.29
The elemental composition of the particle surface was exam-
ined with an Energy Dispersive X-ray Spectrometer (EDX) using
a voltage of 20 kV. The X-ray (Al-Ka radiation) photoelectron
spectroscopy (XPS) was used to distinguish the oxidation state
of the cations present on the surfaces.
2. Experimental
2.1. Catalysts preparation and characterization
Five nanograined perovskite powders of nominal compositions:
SrMnO3, SrCoO3, GdAlO3, FeMnO3, La0.6Pb0.2Mg0.2MnO3 and
four nanograined ferrospinel powders of nominal composi-
tions: MgFe2O4, Ni0.5Co0.5Fe2O4, Ni0.5Co0.5Fe1.9Sc0.1O4, Ni0.5
-
Co0.5Fe1.8Sc0.2O4 were prepared by sol–gel self-combustion
procedure followed by heat treatment at 900 ꢁC or 1000 ꢁC
(depending on the nature of crystalline phase of materials). We
used metal nitrates, ammonium hydroxide and polyvinyl
alcohol (PVA) as starting materials. A solution containing metal
nitrates was mixed with an aqueous solution (10% concentra-
tion) of polyvinyl alcohol. A small amount of NH4OH solution
(10% concentration) was dropped to adjust the pH value to
about 8. This produced a sol oꢁf metal hydroxides and ammo-
nium nitrate. By drying at 100 C for 12 h, the sol was turned
into a dried gel. The dried gel was ignited in a corner and
a combustion reaction spontaneously propagated through the
2.2. Catalytic activity measurements
Catalyst activity tests were performed at atmospheric pressure,
in a laboratory scale with a ow type set-up (ow rate of 100 cm3
minꢀ1, propane concentration in air of 1–2& and the gas hourly
space velocity, GHSV, of 5100 hꢀ1), previously described in ref.
30. The catalyst powder (0.3–0.5 g) was sandwiched between two
layers of quartz wool in a quartz tubular microreactor (f ¼ 7
mm) placed in an electric furnace. The increase of the
temperature was made in steps of 50 C, from 50 to 550 C. As
a result of catalytic combustion, at every predetermined
temperature, the gas concentration at the reactor exit was
smaller than the inlet gas concentration. The catalytic activity of
the nine oxide materials under study was evaluated in terms of
propane conversion (to carbon dioxide) rate over these
materials:26,30,31
ꢁ
ꢁ
ꢁ
whole gel. The obtained powders were calcinated at 500 C to
eliminate the residual organic compounds. Finally, the calcined
powders were annealed in air at 900 ꢁC (ferrite powders) or
ꢁ
1000 C (perovskite powders). Some details on the preparation
procedure can be found in earlier publications.26,27 A total of 9
different samples were synthesized by sol–gel self-combustion,
as mentioned in Table 1.
cin ꢀ cout
C ¼
ꢂ 100 ð%Þ;
(2)
cin
The crystal structure and phase composition of the samples
were analyzed by XRD. X-ray diffraction measurements of the
powders were performed at room temperature using a PAN-
ALYTICAL X' PERT PRO MPD powder diffractometer with Cu-
Ka1 radiation (l ¼ 0.15405 nm). The XRD patterns were regis-
tered between 20 and 80ꢁ (2q) at a rate of 2ꢁ minꢀ1. The average
where cin and cout represent the inlet and outlet propane
concentration, respectively, measured with a photoionization
detector (PID-TECH) for VOCs, at several temperatures. Data
were collected when the ameless catalytic combustion reached
a steady state, aer about 30 minutes at each temperature.
These experiments were repeated decreasing the temperature
and similar results were obtained, indicating the stability of
ferrites and perovskites over time, and the absence of the
deactivation.
Table 1 Ferrites and perovskites prepared by sol–gel self-combustion
procedure
Sample symbol
Chemical composition
Heat treatment
3. Results and discussion
3.1. Materials characterization
F1
F2
F3
F4
P1
P2
P3
P4
P5
MgFe2O4
900 ꢁC, 20 min
900 ꢁC, 4 h
900 ꢁC, 4 h
900 ꢁC, 4 h
1000 ꢁC, 7 h
1000 ꢁC, 7 h
1000 ꢁC, 7 h
1000 ꢁC, 7 h
1000 ꢁC, 2 h
Ni0.5Co0.5Fe2O4
Ni0.5Co0.5Fe1.9Sc0.1O4
Ni0.5Co0.5Fe1.8Sc0.2O4
SrMnO3
SrCoO3
FeMnO3
The XRD patterns at room temperature of the nine powdered
samples are shown in Fig. 1. Well dened sharp peaks can be
clearly seen in the diffraction patterns and show that as-
synthesized materials have a good crystalline quality. Crystal-
line phases identied by comparing to a known standard,
average crystallite size DXRD, derived from XRD data, specic
GdAlO3
La0.6Pb0.2Mg0.2MnO3
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(c) The average crystallite size decreased by scandium
incorporation in Ni–Co ferrite (F3, F4 samples). This may be due
to a structural disorder induced by the larger Sc-ions, which can
lead to some delay in the growth of the ferrite crystallites;
(d) The SBET values are smaller for perovskites and ferrites
containing only two cations and these results appear to be
caused by an effective increase in the degree of crystallinity as
a result of the heat treatment. These values are comparable to
those reported by other authors,32 for similar chemical
compositions.
The microstructure plays a major role in the performance of
a ceramic catalyst and for this reason it was examined by SEM.
The surface morphology of the nine materials is shown in SEM
micrographies given in Fig. 2. As can be seen, the grains have
a rounded shape and the pores are not intragranular. These
images also show that samples exhibit similar morphologies,
being composed of aggregates of nanograins with irregular
shapes and sizes. The images also reveal the presence of large
interaggregate pores.
The surface elemental composition of the materials was
inspected by EDX analysis. In Fig. 3 the EDX spectra for four
catalyst materials are given and their elemental compositions
are summarized in Table 3. Each provided composition repre-
sents the mean value of three determinations at different points
of each sample. One can remark that the chemical composition
Fig. 1 Powder X-ray diffraction patterns of the investigated samples. of the samples is close to the nominal one, i.e. the M/Mt (see
(*) – Main phases in studied ferrites; (ꢃ) – main phases in studied
Table 3) ratios are close to the theoretical values given in
parenthesis. This is proof of the homogeneous distribution of
the elements in the solids prepared by sol–gel self-combustion
technique.
perovskites.
surface area SBET and pore volume obtained from nitrogen
adsorption/desorption isotherms at 77 K, are summarized in
Table 2.
On the basis of data presented in Fig. 1 and Table 2, the
following remarks can be made:
(a) All samples were indexed as perovskite- or ferrospinel-
type structures, without presence of any foreign phase;
(b) Irrespective of phase composition, all materials show
nanoscale crystallinity. The average crystallite size, DXRD, was
found to be in the range of 26–89 nm. The smallest crystallites
were identied in La–Pb–Mg manganite perovskite powder;
The XPS technique was used to distinguish the oxidation
state of the cations present on the surfaces. Among the studied
ferrites are presented the XPS spectra of the Ni0.5Co0.5Fe1.8
-
Sc0.2O4 ferrite and among the studied perovskites are presented
the XPS spectra of the La0.6Pb0.2Mg0.2MnO3 perovskite. Fig. 4(a)
and (b) show the Fe 2p and Sc 2p, respectively, XPS spectra of
Ni0.5Co0.5Fe1.8Sc0.2O4 ferrite. However, the analyses of Fe 2p
spectra are relatively complex for ferrites. The divalent Fe 2p3/2
peak at 709.5 eV and the trivalent Fe 2p3/2 peak at 711.2 eV are
respectively associated to satellite peaks at 715.5 eV and
719.0 eV.19,33–35 In the case of the sample with Sc0.2 the observed
Fe 2p3/2 peaks with binding energies (BE) between 711.2 eV and
Table 2 Structure characteristics of investigated samples
Pore volume
Sample symbol
Crystalline phases
DXRD (nm)
SBET (m2 gꢀ1
)
(cm3 gꢀ1
)
F1
F2
F3
F4
P1
P2
P3
P4
P5
Cubic spinel
Cubic spinel
Cubic spinel
Cubic spinel
Hexagonal perovskite
Hexagonal perovskite
Cubic perovskite
Orthorhombic perovskite
Cubic perovskite
41.8
41.7
35.2
34.8
88.9
59.9
59.2
39.6
25.8
4.0
26.5
31.7
32.3
2.2
1.9
3.2
9.8
8.6
0.0060
0.0053
0.0072
0.0089
0.0010
0.0030
0.0044
0.0018
0.0021
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Fig. 2 SEM micrographs of the investigated samples.
713.0 eV, and the associated satellite at around 716.3 eV and energies of the elements present in the La0.67Pb0.33MnO3 re-
720.0 eV (Fig. 4a), conrm the predominance of Fe3+ and a small ported by Kowalik et al.39 are the following: 641.1 eV for Mn3+
fraction of Fe2+ present on the sample surface.19,35,36 The 2p3/2, 643.2 eV for Mn4+ 2p3/2, 645.7 eV for Pb 4p3/2 and 2.02 for
observed BE shis can be attributed to the different surround- the Mn3+/Mn4+ ratio. The binding energies of the perovskite
ings of the Fe3+ ions in the A and B sites and of the Sc3+ ions catalyst used by us (La0.6Pb0.2Mg0.2MnO3) are characterised by
(Fig. 4b) in the B (octahedral) site within the ferrite structure.
values very close to those reported above: 641.5 eV for Mn3+
Fig. 5(a) and (b) show the Mn 2p and La 3d, respectively, XPS 2p3/2, 643.1 eV for Mn4+ 2p3/2, 644.9 eV for Pb 4p3/2, 653.1 eV for
spectra of La0.6Pb0.2Mg0.2MnO3 perovskite. The La 3d level is Mn3+ 2p1/2, 654.5 eV for Mn4+ 2p1/2, 834.2 eV for La 3d5/2
characterised by a double peak for each components (La 3d3/2 (838.4 eV for satellite), 850.9 eV for La 3d3/2 (855.2 eV for
and La 3d5/2). The Mn 2p peak was decomposed in 2 compo- satellite) and 1.40 for the Mn3+/Mn4+ ratio.
nents. The full width at half maximum (FWHM) of which were
imposed to be equal: a component at 653.6 attributed to Mn
37
2p1/2 and a component at 641.9 attributed to Mn 2p3/2
.
The
3.2. Catalytic activity
binding energies of the elements present in the parent perov-
skite (LaMnO3) reported by Arendt et al.38 are the following:
641.9 eV for Mn 2p3/2, 834.0 eV for La 3d5/2, 851.0 eV for La 3d3/2
529.1 eV for O(I) 1s and 530.6 eV for O(II) 1s. The binding
The catalytic combustion tests of propane over the nine catalyst
samples were performed as a function of temperature. Propane
oxidation reaction on the catalyst surface takes place according
to a radical mechanism consisting of several stages. Global
,
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Fig. 3 EDX spectra for MgFe2O4 (F1), Ni0.5Co0.5Fe1.8Sc0.2O4 (F4), SrMnO3 (P1) and La0.6Pb0.2Mg0.2MnO3 (P5).
oxidation reaction of propane, summing all stages of this inuence of the chemical composition of the catalyst on the
mechanism, will be as follows:
propane conversion at different temperatures. It should be
noted that no catalytic activity was observed at temperatures
below 100 ꢁC. Moreover, the catalytic activity of SrCoO3, GdAlO3
and FeMnO3 in propane combustion reaction started at
temperatures over 200 ꢁC and temperatures above that level had
rather limited inuence on their activity.
C3H8 + 5O2 ¼ 3CO2 + 4H2O.
(3)
In one of these stages catalyst contribution occurs, which
will speed up the process of combustion through its interven-
tion from time to time, until all residues of hydrocarbon species
are consumed, nally yielding CO2 and H2O.
Several repetitions of tests led to similar results, thus indi-
cating reproducibility of actual experimental data. Some of
these results were published in previous articles.30,40
Results showing the temperature dependence of propane
conversion in its combustion reaction over all catalysts are
presented in Fig. 6. The catalysts exhibited substantial differ-
ences in catalytic activity, in the temperature range of 200–
400 ꢁC. The conversion data in Fig. 6 suggest a signicant
Among the nine catalysts, the La0.6Pb0.2Mg0.2MnO3 perov-
skite showed the highestꢁpropane combustion activity at low
temperatures (below 350 C). This catalꢁyst was able to convert
ꢁ
50% of propane at temperature of 224 C and 90% at 320 C.
The high improved catalytic activity may be attributed to the
very small crystallites (26 nm), as well as to the oxygen vacancies
generated by the presence of both manganese ions with variable
valence,41,42 fact conrmed by us by analyzing XPS spectra in
Fig. 5a. The partial substitution of La3+ ions by lower valence
ions (as Pb2+–Mg2+) in LaMnO3 perovskite leads to the oxidation
Table 3 EDX analysis of some metal oxide catalystsa,b
MgFe2O4
GdAlO3
SrMnO3
SrCoO3
Ni0.5Co0.5Fe1.8Sc0.2O4
La0.6Pb0.2Mg0.2MnO3
O (at%) 54.44
O (at%) 64.18
Gd (at%) 17.82
Al (at%) 18.00
Gd/Mt 0.5 (05)
Al/Mt 0.5 (0.5)
O (at%) 62.11
O (at%) 60.80
Sr (at%) 20.84
Co (at%) 18.36
Sr/Mt 0.53 (0.5)
Co/Mt 0.47 (0.5)
O (at%) 49.39
Ni (at%) 8.09
Co (at%) 9.82
Sc (at%) 3.34
Fe (at%) 29.36
Ni/Mt 0.16 (0.17)
Co/Mt 0.19 (0.17)
Sc/Mt 0.07 (0.07)
Fe/Mt 0.58 (0.60)
O (at%) 63.85
Mg (at%) 15.08
Fe (at%) 30.48
Mg/Mt 0.33 (0.33)
Fe/Mt 0.67 (0.66)
Sr (at%) 21.50
Mn (at%) 16.39
Sr/Mt 0.56 (0.5)
Mn/Mt 0.44 (0.5)
La (at%) 11.61
Mn (at%) 18.04
Mg (at%) 4.02
Pb (at%) 2.47
La/Mt 0.3 2 (0.3)
Mn/Mt 0.50 (0.5)
Mg/Mt 0.11(0.1)
Pb/Mt 0.07 (0.1)
a
b
Mt is total amount of metallic composition on the surface. In parenthesis are given theoretical values.
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Fig. 4 XPS spectra of the Fe 2p (a) and Sc 2p (b) of the Ni0.5Co0.5Sc0.2Fe1.8O4 powder.
of Mn3+ to Mn4+ and the formation of oxygen vacancies (active a high conversion degree of propane, of 88–89%, was obtained
sites for oxygen adsorption).41,42 The oxygen vacancies are over the two catalysts.
essential for the adsorption/dissociation of oxygen molecules
Even at low conversions, CO was not observed among the
during the catalytic oxidation reaction and strongly affect the nal products and this indicates that propane was completely
catalytic activity of the catalyst. More oxygen vacancies involve converted to CO2 and H2O.
a larger density of adsorbed oxygen species (Oꢀ, O2ꢀ, O2ꢀ),
weakly anchored on the catalyst surface, which favors the VOC sion was registered for SrCoO3 (32% conversion rate at 500 C)
oxidation. The larger the number of oxygen ions adsorbed, the and FeMnO3 (13% conversion rate at 500 ꢁC), although our
The less efficient catalytic performance in propane conver-
ꢁ
faster the gas oxidation reaction would be.
previous work40 showed that these perovskites exhibit high
From Fig. 6 one can also remark that SrMnO3 perovskite and catalytic activity towards acetone conversion. The reasons for
Ni0.5Co0.5Fe1.8Sc0.2O4 ferrite show similar catalytic activities in such a selective catalytic activity of SrCoO3 and FeMnO3 are not
the temperature range 150–350 ꢁC, even if these catalysts yet clear. A dependence of activity on the surface area was not
display different compositions and crystallographic structures. found. Other factors, such as structural defects or oxygen
Both catalysts can convert 50% of propane in CO2 and H2O at mobility, are likely to control the catalytic activity of oxide
the same low temperature, of about 240 ꢁC. Moreover, at 350 ꢁC catalysts.
Fig. 5 XPS spectra of the Mn 2p and Pb 4p (a) and La 2p (b) of the La0.6Pb0.2Mg0.2MnO3 powder.
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La0.6Pb0.2Mg0.2MnO3 catalyst in combustion reaction of
propane is comparable to that of the Pt/Al2O3 noble metal
catalyst, which could achieve 50% conversion of propane at
325 C and 90% at 400 C.43
ꢁ
ꢁ
It can be assumed that the catalytic combustion of propane
vapor takes place in the presence of excess oxygen. The apparent
activation energies (Ea) for the catalytic reactions were calcu-
lated by means of the Arrhenius-type plot of the natural loga-
rithm of the reaction rate (k) at very low conversion, below 20%,
versus inverse temperature (1/T).44–47 The Arrhenius equation
can be given in the form:44,48
k ¼ A exp(ꢀEa/RT)
(4)
where k, A, R and Ea are the reaction rate, preexponential factor,
gas constant and apparent activation energy, respectively.
Table 4 also includes the values of the kinetic parameters
(apparent activation energy, Ea, and reaction rate, k) for propane
oxidation over the nine catalysts. Reaction rates are normalized
to the BET surface area, in order to compare the specic cata-
lytic activities of the catalysts. One can observe that the
normalized reaction rate changes from 3.8 ꢂ 10ꢀ2 mmol (sꢀ1
mꢀ2) to 11 ꢂ 10ꢀ2 mmol (sꢀ1 mꢀ2), in dependence of catalyst
composition. The highest value of the reaction rate [11 ꢂ 10ꢀ2
mmol (sꢀ1 mꢀ2)] was obtained for propane combustion over
La0.6Pb0.2Mg0.2MnO3 catalyst, which proved the best propane
Fig. 6 Conversion rate versus temperature in the catalytic combus-
tion of propane over the nine catalyst samples: MgFe2O4 (F1), Ni0.5
-
Co0.5Fe2O4 (F2), Ni0.5Co0.5Fe1.9Sc0.1O4 (F3), Ni0.5Co0.5Fe1.8Sc0.2O4
(F4), SrMnO3 (P1), SrCoO3 (P2), FeMnO3 (P3), GdAlO3 (P4) and La0.6
-
Pb0.2Mg0.2MnO3 (P5).
It is interesting to note, from the results presented in Fig. 6,
that the La0.6Pb0.2Mg0.2MnO3 catalyst, which shows the highest
propane combustion activity at low temperatures, is a pure
cubic perovskite as is the FeMnO3 catalyst, which exhibits very
low propane combustion activity. This suggests that the high
catalytic activity of the rst may be related to the presence of the
structural defects (oxygen vacancies) created by the decreasing
crystallite size (Table 2).
The main indicator of catalytic activity of a given catalyst is
typically temperature T50 – the temperature required for 50%
conversion of a gas. At T50 temperature the catalytic activity is
sufficiently high and the interactions between catalyst surface
and reactants are intense. The lower this parameter is, the
higher the activity of the catalyst is. In Table 4 the values of T50
and T90 (the temperature required for 90% propane conversion)
for all catalysts are listed. The T50 and T90 temperatures for the
most active catalyst, La0.6Pb0.2Mg0.2MnO3, are much lower than
those for the other catalysts. As present studies revealed, the
degree of propane conversion for SrCoO3 and FeMnO3 perov-
skite catalysts is smaller than 50%. The catalytic performance of
ꢁ
conversion rate, of 92%, at 350 C (see Fig. 6).
In Table 4 one can observe the wide variation in determined
activation energies from 28 kJ molꢀ1 to 85 kJ molꢀ1. The
smallest value (28 kJ molꢀ1) of the activation energy was regis-
tered for La0.6Pb0.2Mg0.2MnO3 catalyst, for which the reaction
rate was the highest. The differences observed between the
activation energies of the nine catalysts allow one to speculate
that the nature of the catalytic sites differs from one catalyst to
another. We mention that the values of the kinetic parameters
provided here are comparable to those presented by other
authors for other oxides and VOCs.49,50
The present study suggests that La0.6Pb0.2Mg0.2MnO3
perovskite catalyst can be a promising candidate to substitute
highly expensive noble metal catalysts used in propane
combustion. Further investigations of other ferrite and perov-
skite compositions are required, in order to nd new catalyst
compositions with better performances.
Table 4 T50, T90, propane conversion at 400 ꢁC and kinetic parameters
Symbol
T50 (ꢁC)
T90 (ꢁC)
Conv400 (%)
Reaction ratea (mmol sꢀ1 m2)
Activation energyb (kJ molꢀ1
)
F1
F2
F3
F4
P1
P2
P3
P4
P5
311
300
267
240
240
—
—
438
224
—
76
86
88
90
90
26
8
8.14 ꢂ 10ꢀ2
4.57 ꢂ 10ꢀ2
4.66 ꢂ 10ꢀ2
5.53 ꢂ 10ꢀ2
9.8 ꢂ 10ꢀ2
3.8 ꢂ 10ꢀ2
6.1 ꢂ 10ꢀ2
7.8 ꢂ 10ꢀ2
11.0 ꢂ 10ꢀ2
75
82
65
50
31
48
80
71
28
418
432
380
400
—
—
—
320
35
92
a
b
Reaction rate at low conversion per unit surface area of catalyst. Apparent activation energy for low conversions.
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22 A. B. Gadkari, T. J. Shinde and P. N. Vasambekar, Mater.
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4. Conclusions
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Composite oxides catalysts (ferrites: MgFe2O4, Ni0.5Co0.5Fe2O4,
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to propane combustion at low temperatures: a 92% conversion
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