Mixture of fuels approach for the synthesis of SrFeO3-δ nanocatalyst and its impact on the catalytic reduction of nitrobenzene

Article abstract of DOI:10.1021/ic502121k

A modified solution combustion approach was applied in the synthesis of nanosize SrFeO_3-δ; (SFO) using single as well as mixture of citric acid, oxalic acid, and glycine as fuels with corresponding metal nitrates as precursors. The synthesized and calcined powders were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis and derivative thermogravimetric analysis (TG-DTG), scanning electron microscopy, transmission electron microscopy, N₂ physisorption methods, and acidic strength by n-butyl amine titration methods. The FT-IR spectra show the lower-frequency band at 599 cm^-1 corresponds to metal-oxygen bond (possible Fe-O stretching frequencies) vibrations for the perovskite-structure compound. TG-DTG confirms the formation temperature of SFO ranging between 850-900 °C. XRD results reveal that the use of mixture of fuels in the preparation has effect on the crystallite size of the resultant compound. The average particle size of the samples prepared from single fuels as determined from XRD was ～50-35 nm, whereas for samples obtained from mixture of fuels, particles with a size of 30-25 nm were obtained. Specifically, the combination of mixture of fuels for the synthesis of SFO catalysts prevents agglomeration of the particles, which in turn leads to decrease in crystallite size and increase in the surface area of the catalysts. It was also observed that the present approach also impacted the catalytic activity of the SFO in the catalytic reduction of nitrobenzene to azoxybenzene.

Full text of DOI:10.1021/ic502121k

Article
pubs.acs.org/IC
Mixture of Fuels Approach for the Synthesis of SrFeO3−δ
Nanocatalyst and Its Impact on the Catalytic Reduction of
Nitrobenzene
†
‡
†
,†
Akula Naveenkumar, Praveena Kuruva, Chikkadasappa Shivakumara, and Chilukoti Srilakshmi*
†
‡
Solid State and Structural Chemistry Unit (SSCU), and Materials Research Centre (MRC), Indian Institute of Science (IISc),
Bangalore 560012, Karnataka, India
*
S Supporting Information
ABSTRACT: A modified solution combustion approach was
applied in the synthesis of nanosize SrFeO3−δ (SFO) using
single as well as mixture of citric acid, oxalic acid, and glycine
as fuels with corresponding metal nitrates as precursors. The
synthesized and calcined powders were characterized by
Fourier transform infrared spectroscopy (FT-IR), X-ray
diffraction (XRD), thermogravimetric analysis and derivative
thermogravimetric analysis (TG-DTG), scanning electron
microscopy, transmission electron microscopy, N physisorp-
2
tion methods, and acidic strength by n-butyl amine titration
methods. The FT-IR spectra show the lower-frequency band at
99 cm− corresponds to metal−oxygen bond (possible Fe−O
1
5
stretching frequencies) vibrations for the perovskite-structure
compound. TG-DTG confirms the formation temperature of SFO ranging between 850−900 °C. XRD results reveal that the use
of mixture of fuels in the preparation has effect on the crystallite size of the resultant compound. The average particle size of the
samples prepared from single fuels as determined from XRD was ∼50−35 nm, whereas for samples obtained from mixture of
fuels, particles with a size of 30−25 nm were obtained. Specifically, the combination of mixture of fuels for the synthesis of SFO
catalysts prevents agglomeration of the particles, which in turn leads to decrease in crystallite size and increase in the surface area
of the catalysts. It was also observed that the present approach also impacted the catalytic activity of the SFO in the catalytic
reduction of nitrobenzene to azoxybenzene.
9
1
. INTRODUCTION
of 1000−1200 °C. This conventional solid-state reaction
process suffers from many disadvantages such as high reaction
temperature, large particle size formation, loss of chemical
homogeneity, and low sinterability. Different synthetic
approaches have been optimized to prepare ferrites, such as
Among ferrites, strontium ferrite SrFeO3−δ (0 < δ < 0.5)
perovskite has attracted increasing attention due its strong
oxygen-deficient composition for various applications. This
wide range of nonstoichiometry, implying different oxidation
states for iron cations, and the presence of oxygen vacancies
make this system very attractive for catalysis, for instance, for
10
11
12
solvothermal, hydrothermal synthesis, forced hydrolysis,
1
3
water-in-oil (w/o) inverse microemulsion, citric acid
1
4
15
1
2
precursor method, microwave-assisted coprecipitation, and
16
methane combustion and oxidative dehydration of ethane.
SrFeO3 has also been used as an electrode material in solid
liquid-phase deposition, by exploiting the decomposition of
−δ
17
suitable single-source precursors and by coprecipitation
oxide fuel cell and has been considered as a possible candidate
for high-temperature solid-state electrochemical devices in
particular as dense oxygen separation membranes for partial
7
18
methods. However, to the best our knowledge, there are no
reports on the synthesis of SrFeO3−δ using solution-combustion
synthesis process. A wide variety of inorganic materials has
3
−6
oxidation of methane to syngas.
Teraoka and co-workers
19
been prepared using solution-combustion process. It is fast
and simple compared to other methods of synthesis and yields
high-purity, homogeneous, crystalline oxides in a short
preparation time and with less net energy input. In this
approach, the physical properties such as surface area, particle
size distribution, and agglomeration of the particles in the final
product depend on the flame temperature, which in turn
have shown that substituted SrCo Fe1−xO3−δ has exceptionally
x
high oxygen-transport rates, and thus the interest in these
materials is increasing. The nonstoichiometric chemistry of this
system, which can also be traced back to the different synthesis
routes used for its preparation, has been thoroughly
investigated, and four distinct compounds with nominal
compositions of Sr Fe O
structurally described.
(n = 2, 4, 8, and ∞) have been
n
n
3n−1
8
Traditionally, SrFeO3−δ has been prepared by conventional
solid-state reaction of Fe O and SrO in the temperature range
Received: August 31, 2014
2
3
©
XXXX American Chemical Society
A
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depends on the nature of fuel and fuel to oxidant ratio. In
solution combustion synthesis, the type and amount of the fuel
plays an important role in determining the morphology,
composition, and particulate properties of the final product.
Mixture of fuels approach facilitates the reduction in particle
Table 1. List of SrFeO3−δ Samples Prepared and Their Fuel
Compositions
sample name
fuel composition
0.4 M citric acid (CA)
SFO-1
SFO-2
SFO-3
SFO-4
SFO-5
SFO-6
SFO-7
20
0.4 M oxalic acid (OA)
size compared to the product formed using single fuel.
0.4 M glycine (Gly)
Overall mixture of fuels approach gives a good control over
flame temperature. Specifically, there are no reports available on
using mixture of fuels for the synthesis of SrFeO3−δ by solution
combustion method.
0.2 M oxalic acid + 0.2 M citric acid
0.2 M citric acid + 0.2 M glycine
0.2 M oxalic acid + 0.2 M glycine
0.134 M oxalic acid + 0.134 M citric acid + 0.134 M glycine
The synthesized SrFeO3−δ catalysts have been evaluated for
the catalytic reduction of nitrobenzene to azoxybenzene. Azoxy
compounds have received much attention because of their
size was obtained by line-broadening method using Scherrer
and Warren formula:
21
physiological activities and utilization in liquid crystal systems.
Azoxybenzenes (AZYs) have been utilized as dyes, analytical
reagents, reducing agents, stabilizers, and polymerization
inhibitors. The most common approach for the synthesis of
AZY is the reduction of aromatic nitro compounds by a variety
of homogeneous and heterogeneous catalysts. These methods
0.9λ
D = ( 2
2 1/2
B − b ) cos θ
where D is the crystallite size, λ the wavelength of the radiation,
θ the Bragg’s angle, and B and b are the full width at half
maxima (fwhm) observed for the sample and standard,
respectively.
22
include reduction of nitrobenzene with glucose/NaOH,
2
3
24
25
26
sodium arsenate, Mg reagent, KBH4, PH , and yellow
3
2
7
28
phosphorus. Thallium-mediated reduction of nitrobenzene
is reported as a well-known method for the synthesis of AZY.
Zinc in the presence of acetic acid and acetic anhydride, Se,
and Te reagents were used as suitable reagents for
nitrobenzene conversion. Recently, we have reported Fe-
FT-IR spectra were measured on a PerkinElmer FT-IR-300
spectrometer using KBr pellet method. The Brunauer−
Emmett−Teller (BET) surface area of the samples was
29
30
3
1
measured using a Model-Quantachrome Autosorb iQ
2
automated gas sorption analyzer. Samples were initially
pretreated in N at 300 °C for 4 h, and the BET surface area
doped BaTiO as a highly efficient catalyst for the catalytic
2
3
32
and pore volume of the catalysts were obtained by adsorbing
reduction of nitrobenzene to synthesize AZY. In the present
study, we synthesized for the first time SrFeO3−δ nanocatalysts
by solution combustion process using combination of mixture
of fuels and applied these catalysts for the catalytic reduction of
nitrobenzene to AZY. The above catalysts were characterized
using Fourier transform infrared (FT-IR) spectroscopy, X-ray
diffraction (XRD), thermogravimetric analysis and derivative
thermogravimetric analysis (TG-DTG), scanning electron
microscopy (SEM), transmission electron microscopy
N at liquid nitrogen temperature, −196 °C.
The TG-DTG analysis was carried out on a Mettler-Toledo
apparatus. In a typical experiment, sample weight of 10−20 mg
was analyzed in the N flux in the temperature ranging from 25
2
2
to 1000 °C and at a heating rate of 10 °C/min.
The acidic strength of the solid samples was measured by the
potentiometric titration method. A known mass of solid
suspended in acetonitrile was stirred for 3 h, and the suspension
was titrated with a solution of 0.05 N n-butylamine in
acetonitrile, at a flow rate of 0.02 mL/min; the variation in
the electrode potential was measured with a potentiometric
titrator using a standard calomel electrode. The acidity of the
catalyst measured by this technique enables determination of
the total number of acidic sites and their strength. To interpret
the results, it is suggested that the initial electrode potential (Ei)
be taken as maximum acidic strength of the surface sites and
the range; where the plateau is reached (meq/g) can be
considered as the total number of acid sites. The acidic strength
of the surface sites can be assigned according to the following
(TEM), and N2 physisorption methods, and their acidic
strength was assessed by potentiometric titration method.
2. EXPERIMENTAL SECTION
Synthesis of SrFeO3−δ by Solution Combustion Method. The
starting materials used in this study are Sr(NO ) ·6H O (Sigma-
3
2
2
Aldrich), Fe(NO ) ·9H O (Merck), oxalic acid (Merck), citric acid
3
3
2
(
Merck), and glycine (Merck). Stoichiometric amounts of oxidants
such as strontium nitrate hexahydrate (0.2M), iron nitrate non-
ahydrate (0.2M), and the fuel, for example, citric acid (0.4M),
respectively, were dissolved in minimum quantity of water, and the
formed solution was kept on a hot plate preheated at 180 °C; the
solution boils, foams, catches fire, and burns with an incandescent
flame to yield SrFeO3−δ (SFO) precursor with the evolution of gases
33
ranges very strong site E > 100 mV; strong site 0 < E < 100
mV; weak site −100 < E < 0 mV; very weak site E < −100 mV.
Field emission scanning electron microscope (FESEM),
model INSPECTTM S50, was used for morpholological
studies. TEM model JEM-2010, JEOL, Tokyo, Japan, was
used to study the morphology of the sample.
i
i
i
i
13
as reported in the literature. Finally, the prepared precursor was
ground to a fine powder and calcined at 900 °C for 12 h in a static air
furnace. The fuel and the oxidant (metal nitrates) molar ratio in the
reaction mixture was maintained at unity. A stoichiometric
composition is defined as the composition where ratio of oxidizing
to reducing valency is equal to unity and ensures that the evolution of
heat is maximized. List of the samples prepared by the above-described
method and their fuel compositions are given in Table 1.
4
. CATALYTIC ACTIVITY
Catalytic reduction with propan-2-ol as hydrogen donor was
carried out under continuous stirring in a two-necked 100 mL
round-bottom flask fitted with a reflux condenser with H gas
2
bubbling through the reaction mixture. Analysis was done by
withdrawing definite aliquots of the sample at regular intervals.
In a typical experiment, 150 mg of the catalyst was dispersed in
a solution containing 20 mmol of nitrobenzene, 20 mmol of
KOH pellets, and 20 mL of propan-2-ol. The mixture was
3
. CATALYST CHARACTERIZATION
The crystallinity and phase identification of the powders was
determined by using Philips X-ray diffractometer (X’pert MPD
3040) with Cu Kα as radiation source. The average crystallite
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stirred and heated under reflux for 2−6 h in an oil bath in the
040−0905. Traces of Sr Fe O at corresponding 2θ = 32.14°
10−x
4
3
presence of H gas. The products were analyzed on the basis of
was found as minor phase in all the samples except in the
sample prepared with oxalic acid (SFO-2). It indicates that the
oxalic acid is the suitable fuel for obtaining the pure phase of
SrFeO3−δ. The average crystallite size calculated from XRD line
broadening is shown in Table 3. It is observed from the table
that the crystallite size of the samples decreased with mixture of
2
their retention times using a gas chromatograph-mass
spectrometer.
5. RESULTS AND DISCUSSION
5.1. X-ray Diffraction. Figure 1 shows the indexed powder
20
XRD patterns of SrFeO3−δ calcined powders at 900 °C for 12 h.
fuels compared to the single fuels. It was reported that
mixture of fuels prevents the agglomeration of particles, and
combustion reaction will take place at lower temperatures than
the single fuels. This leads to decrease in average crystallite size
of the samples.
5
.2. FT-IR Spectroscopy of SFO Samples at Various
Temperatures. FT-IR spectra of as-synthesized samples are
shown in Figure 2(I). The IR bands in the region of 1350−
460 cm− correspond to the symmetric NO and asymmetric
1
1
2
stretching vibration of nitrate group. The less-intense bands
−1
that appeared in the region of 810−820 cm are due to N−O
−
1
stretching, and bands at 700−740 cm are due to NO
2
bending vibrational mode of strontium nitrate phase present
in the SFO samples. Infrared vibrations corresponding to traces
of Fe O at 564 cm and SrCO at 862 cm were also
−1
−1
3
4
3
observed in the spectra.
FT-IR spectra of SFO samples calcined at 900 °C measured
−1
in the range of 400−2000 cm are shown in Figure 2(II).The
spectra of all the SFO samples showed a broad absorption band
−1
in the region of 590−599 cm correspond to Fe−O stretching
−1
vibration. The broad band contains shoulder at 545 cm . This
indicates the formation of SrFeO3−δ, and it was reported that
the appearance of the two bands was due to defective SrFeO3−δ
compound formation. This defect structure arises due to
2+
34
production of oxygen vacancies by Sr .
Figure 1. XRD patterns of the SFO samples calcined at 900 °C. (a)
SFO-1 (b) SFO-2 (c) SFO-3 (d) SFO-4 (e) SFO-5 (f) SFO-6 (g)
SFO-7; *Sr Fe O10−x.
During the thermal treatment of the strontium ferrite
precursor at temperatures up to 450 °C the major crystalline
phase present is strontium nitrate. This can be designated as
stage 1. Heating the precursor at 600 and 700 °C produces
strontium carbonate as well as iron oxide (Fe O ). This can be
4
3
̅
SrFeO2.97 phase with cubic symmetry and Pm3m space group is
observed in all the samples and confirmed the same by
comparing the d values with the standard JCPDS Pattern No.
3
4
termed as stage 2. The final stage, stage 3, gives the formation
Figure 2. FT-IR spectra of SFO (I) as-synthesized samples and (II) calcined at 900 °C. (a) SFO-1 (b) SFO-2 (c) SFO-3 (d) SFO-4 (e) SFO-5 (f)
SFO-6 (g) SFO-7.
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Table 2. FTIR Phase Identification of the SFO Samples Calcined at Various Temperatures
a
IR band position (cm‑1)
sample
temperature (°C)
phase identification
SFO-1 to 7
ambient
Sr(NO3)2
1629 (w), 1382 (vs), 1442 (m), 816 (w,sh), 738 (w,sh)
Fe O (traces)
561(m)
3
4
SrCO (traces)
862 (vw)
3
SFO-1, 3, 5, 6, 7
200
Sr(NO3)2
1634 (w), 1385 (vs), 1444 (m)
Fe O (traces)
560−585 (m)
3
4
SrCO (traces)
858 (m,sh), 706, and 698 (vw)
1627 (m), 1438 (w), 1384 (vs), 815 (w,sh), 732 (w,sh)
564
3
SFO-2, 4
200
300
Sr(NO3)2
Fe O (traces)
3
4
SFO-1, 4, 5, 6
Sr(NO3)2
1633 (m), 1447 (w), 1384 (vs)
530−565 (w)
Fe O (traces)
3
4
SrCO (traces)
856, 695−707 (vw)
3
SFO-2, 3, 7
SFO-1, 5
300
450
Sr(NO3)2
SrCO3
1632 (m), 1445 (w), 1384 (s,sh), 815 (w,sh), 737 (w,sh), 531(w, b)
1635 (w), 1447 (vs), 1385 (w,sh), 858 (m,sh), 699 (w)
Fe O4
3
560 (m)
SFO-2 to 4 and 7
Sr(NO3)2
1636 (m), 1454 (m), 1384 (v,s), 698, 736 (vw)
Fe O4
3
561 (m)
traces of SrCO3
Sr(NO3)2
857 (w, sh)
SFO-6
1631, 1384, 584
SFO-1 to 7
600
600
SrCO3
1631(w), 1455 (vs), 1385 (vw), 858 (m,sh), 699, 705 (vw)
Fe O4
3
570 (m)
SFO-6
Sr(NO3)2
1633 (w), 1455 (m), 1384 (vs)
859 (w,sh)
traces of SrCO3
Fe O4
3
555 (m)
SFO-1 to 7
SFO-1 to 7
700
900
SrCO3
1456 (s), 857 (m,sh), 698−705 (vw)
570−580 (m)
590−599 (b, m), 545 (sh)
Fe O4
3
SrFeO3−δ
a
Phases identified according to reference FT-IR spectra of the corresponding compounds (Figure S1 given in Supporting Information).
of the required ferrite on heating the sample to temperatures of
00 °C. Table 2 summarizes the FTIR spectra of the products
°C showed the presence of iron oxide (Fe O ) and the
3 4
9
formation of strontium carbonate (SrCO ). At 600 °C
3
strontium nitrate has disappeared totally to form strontium
carbonate product, which is either produced by reaction of
strontium oxide formed from decomposition of strontium
Table 3. Physical Properties of the Various SFO Materials
BET
pore
acidic
strength
(meq/g)
XRD
crystallite
size (nm)
SEM
particle size
(nm)
nitrate or direct reaction of strontium nitrate with CO evolved
surface area volume
2
2
catalyst
(m /g)
(mL/g)
during combustion of fuels or by interaction with atmospheric
SFO-1
SFO-2
SFO-3
SFO-4
SFO-5
SFO-6
SFO-7
27
34
58
47
30
52
60
0.0048
0.0064
0.0105
0.0087
0.0049
0.0112
0.013
170
169
230
122
181
110
195
49.4
46.9
35.4
29.0
30.4
27.7
26.2
240
CO to yield the carbonate as given in (eq 1).
2
270
Stage 3: FTIR and XRD at higher temperatures, 900 °C
(Figures 1 and 2(II)). The samples obtained at 900 °C showed
the presence of SrFeO3−δ as major diffraction pattern and
03−210
03−250
02−302
06−280
03−290
Sr Fe O as a trace compound in most of the samples. It is
3
4
10−x
evident that at these higher temperatures the strontium
carbonate has decomposed according to equation given
below. The SrO has subsequently reacted with the iron oxide
Fe O to produce the required ferrite. All the corresponding
spectra of the SFO samples calcined from 200 to 700 °C are
given in the Supporting Information.
observed at various temperatures and their IR band positions.
Explanations of the reaction mechanisms involved in each of
the three stages from the thermal decomposition/oxidation are
given below.
3
4
SrCO (s) → SrO(s) + CO (g)
Stage 1: FTIR analysis of samples heat treated from ambient
to 450 °C (Figure 2(I) and Figure S2 given in Supporting
Information). The SFO precursor powders at room temper-
ature showed strontium nitrate as the main crystalline phase
and traces of Fe O and SrCO . In samples produced at 200,
3
2
5
.3. Scanning Electron Microscopy. Figure 3 shows the
SEM images of all the SFO samples. Because of their magnetic
properties, highly dense microstructure exhibiting considerable
agglomeration is apparent in all the samples. In general,
spherical particles with homogeneous size distribution are
observed in all the samples. The grain size of the samples
calculated from SEM is given in Table 3 and is in the range of
210−302 nm. Hexagonal platelike structures were observed in
the SFO-7 sample, which was synthesized using mixture of all
three fuels. Hexagonal platelike formation was further
confirmed with TEM analysis of SFO-7 sample (Figure 3h).
3
4
3
3
00, and 450 °C the presence of the strontium nitrate is still
evident except in the samples of SFO-1 and SFO-5made at 450
C where strontium carbonate (SrCO ) and iron oxide (Fe O )
°
3
3
4
present as major crystalline phases, which indicate the
beginning of decomposition of the strontium nitrate.
Stage 2: FT-IR analysis of the samples heat treated at 600
and 700 °C. (Figure S2, Supporting Information) The FT-IR
analysis of samples produced by heat treatment at 600 and 700
D
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Figure 3. SEM images of the various SFO materials. (a) SFO-1 (b) SFO-2 (c) SFO-3 (d) SFO-4 (e) SFO-5 (f) SFO-6 (g) SFO-7 (h) TEM image of
one platelet of SFO-7.
5
.4. Thermogravimetric and Derivative Thermogravi-
decrease in acidic strength of the catalysts does not have any
impact on the conversion of nitrobenzene.
metric Analysis. TG-DTG curves of the SFO precursor
samples with a heating rate of 10 °C/min in nitrogen
atmosphere are shown in Figure 4a−e. The TG curves of the
SFO precursor samples exhibit four distinct weight loss steps.
The first weight loss observed from ambient to 205 °C is
attributed to the loss of absorbed water. The second weight loss
from 280 to 370 °C observed mainly in Figure 4a,e and the
corresponding endothermic peak in DTG at 360 °C could be
due to decomposition of organics, and the third weight loss
observed between 430 and 580 °C showed a sharp decrease
due to decomposition of strontium nitrate into strontium
carbonate. This was confirmed with the FT-IR analysis of the
samples heat treated from 450 to 900 °C (see Table 2).
Selectivity toward AZY was found to increase in the catalysts
obtained from mixture of fuels. The increase in selectivity is
mainly due to increase in surface area and pore volume of the
catalysts as given in Table 3. Selectivity obtained over SFO
catalysts prepared from single fuels was in the range of 72−74%
whereas the selectivity obtained over SFO catalysts prepared
from mixture of fuels was between 75 and 82%. The raise in
selectivity could also be due to decrease in acidic strength of the
catalysts. It clearly indicates that the formation of AZY requires
redox nature of the catalysts, which can be readily tuned via
mixing of fuels during catalyst preparation. Mixing of fuel
approach also prevents the agglomeration of the particles,
which was evidenced by increase in surface area of the catalyst
as well as pore volume (Table 3). The increase in selectivity is
also in good agreement with the decrease in crystallite size of
the catalysts obtained from XRD.
Formation of SrCO from strontium nitrate proceeds either by
interaction of strontium nitrate with CO2 evolved during
organics decomposition or by interaction with atmospheric
3
CO as follows (eq 1)
2
It can be inferred from the above results that the slight
decrease in selectivity of the catalysts prepared with single fuels
is due to formation of aniline as side product due to high acidic
strength of the catalysts (Figure 6, SFO-1 to SFO-3) compared
with SFO catalysts (Figure 6, SFO-4 to 7) obtained from
mixture of fuels. It seems that the decrease in acidic strength
might favor the coupling reaction between nitrosobenzene and
hydroxylamine intermediates, leading to the formation of AZY.
Mechanism of the reaction has been discussed in our previous
paper (ref 32).
Sr(NO ) + CO → SrCO + 2NO + 1/2O
(1)
(2)
(3)
3
2
2
3
2
2
SrCO → SrO + CO
3
2
Fe O + 3SrO + O → 3SrFeO
3−δ
3
4
2
The final weight loss, observed above 850 °C, was due to
decomposition of strontium carbonate into SrO (eq 2), and
subsequent reaction with Fe O in the presence of O leads to
3
4
2
the formation of SrFeO3−δ compound (eq 3). The changes
observed in the TG-DTG of the samples are in good agreement
with the FT-IR results of the samples heat treated from 200 to
6
. CONCLUSIONS
9
00 °C.
5.5. Catalytic Reduction of Nitrobenzene. The catalytic
Nanocrystalline SrFeO3−δwas synthesized using single and
mixture of various fuels by solution combustion method. The
synthesis of nanocrystalline SrFeO3−δ using the mixture of citric
acid, glycine, and oxalic as fuels influenced the morphology,
microstructure, and crystallite size of the samples. Various
intermediate phases formed during heat treatment of the SFO
precursors at various temperatures to transform to SrFeO3−δ
were identified using FT-IR spectroscopy. TG-DTG and FT-IR
analyses confirm that strontium carbonate completely decom-
posed to SrO at temperatures above 825 °C, which
reduction of nitrobenzene to AZY is illustrated in Scheme 1.
The activity profiles of nitrobenzene obtained over various
SrFeO3−δ catalysts are shown in Figure 5. It was observed that
conversion of nitrobenzene was low with SFO catalysts
prepared by using single fuels and in the range of 67−80%
compared to SFO catalysts obtained from mixture of fuels
where conversion obtained was in the range of 80−91%. It can
be observed from Figure 6 that the increase in conversion is
independent of the acidic strength of the catalysts as the
E
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Figure 4. TG and DTG curves of SFO precursor samples. (a) SFO-1 (b) SFO-2 (c) SFO-3 (d)SFO-4 (e) SFO-5 (f) SFO-6 (g) SFO-7.
Scheme 1. Catalytic Reduction of Nitrobenzene to AZY
with oxalic acid as fuel. High surface area and pore volume of
SrFeO3−δ catalysts obtained from mixture of fuels could be due
to prevention of agglomeration of particles and leads to a
combustion reaction that will take place at lower temperatures
than that of the single fuels. A crystallite size of 26 nm was
achieved with the sample prepared from combination of three
fuels. Thus, it is concluded that by using mixture of fuels one
can not only reduce the exothermicity of the combustion
reaction but also reduce the particle size to a greater extent.
subsequently reacts with Fe O to form strontium ferrite. XRD
3
4
studies confirm that pure phase of SrFeO3−δ can be obtained
F
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ACKNOWLEDGMENTS
■
C.S.L. wishes to thank to department of Science and
Technology for financial support through INSPIRE Faculty
award.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
E-mail: ch.srilakshmi@sscu.iisc.ernet.in.
■
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Notes
The authors declare no competing financial interest.
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