Co2+ substituted La2CuO4/LaCoO3 perovskite nanocomposites: Synthesis, properties and heterogeneous catalytic performance

Article abstract of DOI:10.1039/c8nj04133d

Cobalt substituted La₂CuO₄/LaCoO₃ perovskite nanocomposites were prepared using a microwave combustion method. X-ray diffraction analysis showed that Co²⁺ substitution in La₂CuO₄ induced the formation of a secondary LaCoO₃ phase with rhombohedral structure along with the existing orthorhombic (La₂CuO₄) structure. The orthorhombic/rhombohedral structured nanocomposites possessed an average crystallite size in the range of 36-46 nm and 21-49 nm, respectively. The Rietveld analysis confirmed the formation of these two phases. The appearance of FT-IR bands around 682 and 512 cm^-1 was correlated to the orthorhombic stretching modes while the bands around 580 and 445 cm^-1 were associated with the rhombohedral stretching modes thereby confirming the two-phase perovskite system. An optical study revealed two linear regions depicting two band gaps related to the La₂CuO₄ and LaCoO₃ phases. The band gap was found to increase with an increase in the Co²⁺ doping fraction. Morphological observations using scanning electron microscopy showed intragranular pores and fused grains with distinct grain boundaries. The change in the magnetic properties was associated most probably with the exchange of the A and B sites within the La₂CuO₄ host lattice because of Co²⁺ doping, as well as the important phase composition of La₂CuO₄/LaCoO₃ obtained from the ferromagnetic-to-paramagnetic transition. The BET surface area of the pure and doped nanocomposites was found to vary considerably within the range of 12.2-43.7 m² g^-1. The as-fabricated La₂CuO₄/LaCoO₃ perovskite nanocomposites were evaluated for the conversion of glycerol to formic acid in a liquid phase batch reactor under atmospheric conditions. This La³⁺ based nanocomposite behaves as an efficient bifunctional catalyst with high conversion efficiency and selectivity around 96.5% and 95.1%, respectively.

Full text of DOI:10.1039/c8nj04133d

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2
+
Co substituted La CuO /LaCoO perovskite
2
4
3
nanocomposites: synthesis, properties and
heterogeneous catalytic performance
Cite this: New J. Chem., 2018,
42, 18128
a
a
b
c
M. Sukumar, L. John Kennedy, * J. Judith Vijaya,
B. Al-Najar and
c
M. Bououdina
Cobalt substituted La
method. X-ray diffraction analysis showed that Co substitution in La
secondary LaCoO phase with rhombohedral structure along with the existing orthorhombic (La
2 4 3
CuO /LaCoO perovskite nanocomposites were prepared using a microwave combustion
2+
2
CuO
4
induced the formation of a
4
CuO )
3
2
structure. The orthorhombic/rhombohedral structured nanocomposites possessed an average crystallite
size in the range of 36–46 nm and 21–49 nm, respectively. The Rietveld analysis confirmed the for-
ꢀ1
mation of these two phases. The appearance of FT-IR bands around 682 and 512 cm was correlated
ꢀ1
to the orthorhombic stretching modes while the bands around 580 and 445 cm were associated with
the rhombohedral stretching modes thereby confirming the two-phase perovskite system. An optical
2 4 3
study revealed two linear regions depicting two band gaps related to the La CuO and LaCoO phases.
2+
The band gap was found to increase with an increase in the Co doping fraction. Morphological
observations using scanning electron microscopy showed intragranular pores and fused grains with
distinct grain boundaries. The change in the magnetic properties was associated most probably with the
2+
exchange of the A and B sites within the La host lattice because of Co doping, as well as the
2
CuO
4
important phase composition of La CuO /LaCoO obtained from the ferromagnetic-to-paramagnetic
2
4
3
Received 14th August 2018,
Accepted 8th October 2018
transition. The BET surface area of the pure and doped nanocomposites was found to vary considerably
2
ꢀ1
within the range of 12.2–43.7 m
g
2 4 3
. The as-fabricated La CuO /LaCoO perovskite nanocomposites
were evaluated for the conversion of glycerol to formic acid in a liquid phase batch reactor under
DOI: 10.1039/c8nj04133d
3+
atmospheric conditions. This La based nanocomposite behaves as an efficient bifunctional catalyst
with high conversion efficiency and selectivity around 96.5% and 95.1%, respectively.
rsc.li/njc
with smaller size. A trivalent rare earth cation occupies the
1
. Introduction
A-site of A
Magnetic nanocomposites, either spinels or perovskites doped to electroneutrality of the oxides (A
with transition and rare earth metal ions, have been investi- La CuO is a perovskite type mixed oxide, having alternating
gated for various applications due to their unique structural, layers of a (LaO) perovskite structure and rock-salt CuO units
A BO compounds gen- (La2 Cu O ). The A-site (La ) cation has nine-fold coordi-
2 4
BO , while a divalent cation occupies the B-site, due
3
+
2+ 2ꢀ
7
2
B O
4
). In this class,
2
4
2
2
1
–5
3+
2+ 2ꢀ
8
3+
electrical and magnetic properties.
erally have an orthorhombic K NiF type structure, with the nation and the B-site (Cu ) cation has octahedral coordination,
2
4
4
2
+
2
4
6
9
8
Cmca space group and 4 formula units per unit cell (Z = 4).
respectively. La CuO is a p-type anti-ferromagnetic semiconductor
2 4
K
2
NiF -type (A BO = AO + ABO ) oxides belong to the perovs- that can naturally undergo a structural phase transition from
4
2
4
3
kite family, where A is a large metal cation such as rare earth orthorhombic to tetragonal either by doping impurities, by applying
and alkaline earth metal ions, while B is a transition metal ion pressure or by changing temperature. There are three major struc-
2 4
tural phases observed in the La CuO metal oxide: (i) low tem-
a
perature tetragonal (LTT), (ii) low temperature orthorhombic
Materials and Physics Division, School of Advanced Sciences, Vellore Institute of
Technology (VIT), Chennai Campus, Chennai 600 127, Tamil Nadu, India.
E-mail: jklsac14@yahoo.co.in, ljkennedy14@gmail.com; Fax: 044-39932555;
Tel: +91-44-39931326
(LTO), and (iii) high temperature tetragonal (HTT) with I /mmm,
4
10
Bmab, and P4 /ncm symmetry, respectively.
2
Another type of perovskite oxide with the formula of ABO3,
b
c
Catalysis and Nanomaterials Research Laboratory, Department of Chemistry,
Loyola College (Autonomous), Chennai 600 034, Tamil Nadu, India
Department of Physics, College of Science, University of Bahrain, P.O. Box 32038,
Bahrain
where the larger cation at the A-site has 12 coordination and the
1
1,12
smaller cation at the B-site has 6 coordination, respectively.
This
1
3
type of perovskite has a typical band gap ranging from 3 to 4 eV.
1
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LaCoO
NJC
3
belonging to the ABO
3
perovskite family is a mixed electro-
The role of microwaves in the formation of two perovskite-
nic and oxygen-ion conductor with remarkable electronic structure, like systems at a relatively lower temperature and a shorter time
thermal stability, ionic conductivity, electron mobility and redox was investigated. The synthesized La CuO /LaCoO perovskite
2
4
3
1
4
behavior. This type of perovskite structure is perfectly cubic, but nanocomposites were characterized by several techniques
the difference in the ionic radius of La (1.36 Å) and Co (0.9 Å) including XRD, FTIR, DRS, HR-SEM, EDX, VSM, TG/DTA and
distorts the cubic structure leading to the appearance of rhombo- BET techniques in order to determine the structural, optical,
3+
2+
15
hedral symmetry. LaCoO
changing properties with respect to temperature. At room tempera- evaluated for glycerol oxidation to explore their potential future
ture (293 K), LaCoO crystallizes in a rhombohedral structure with catalytic applications.
the R3%c space group and a unit cell consisting of 10 atoms.
LaCoO undergoes a gradual transition from a diamagnetic to a
3
also exhibits unique electronic phase and magnetic properties. The nanocomposites were then
3
1
6
3
2
. Experimental
paramagnetic semiconductor at room temperature while
semiconductor-to-metal transition is also possible at 500–600 K.
a
1
7
2+
2.1. Synthesis of Co doped La CuO /LaCoO perovskite
2
4
3
In recent years, composite materials have gained particular nanocomposites
interest because they offer numerous advantages, as they
La(NO
3
)
3
ꢁ6H
2
O, Co(NO
3
)
2
ꢁ4H
2
O and Cu(NO
3
)
2
ꢁ3H
2
O were used
significantly possess enhanced characteristics compared to
their individual constituents or conventional materials. In this
context, a nanocomposite can be defined as a combination of two
or more constituent materials or phases, showing high thermal
stability, high strength, chemical resistance, improved catalytic
as precursors and NH CONH as a fuel for the synthesis of the
composites. The precursors for La2 Co CuO and urea consti-
2
2
ꢀx
x
4
tuents in the desired proportions were dissolved in de-ionized
water and stirred for 45 min at room temperature to obtain
a homogeneous solution. The fuel to oxidizer ratio (F/O) was
equal to 1 as per the propellant chemistry concept. The required
amount of oxidizer-to-fuel molar ratio for the composition was
calculated based on the total oxidizing and reducing valences in
the oxidizer (lanthanum nitrate, cobalt nitrate, copper nitrate) and
dividing by the sum of the total oxidizing and reducing valences in
1
8–21
properties and so on.
the combination of perovskite based nanocomposites is scarce in
the literature. La CuO and LaCoO based-perovskite nanocompo-
sites can be synthesized by different physical and chemical routes
Despite many available nanocomposites,
2
4
3
6,13,22,23
8,24
25,26
such as, solid state reaction,
sol–gel,
combustion,
27,28
13
hydrothermal
perovskite oxides are widely used for catalytic activities
and high energy ball milling. These types of
39
the fuel (urea) as expressed in the following equation.
9,12,25,29,30
P
2
2
31
and in gas sensors and superconductor devices. The catalytic
all oxidizing and reducing elements in oxidizer
Oxidizer=Fuel ¼
P
3+
applications of lanthanum (La ) based materials have higher
carbonaceous oxidation activity while cobalt (Co ) containing
ð ꢀ 1Þ oxidizing and reducing elements in fuel
2+
(1)
28
perovskites exhibit excellent electrochemical properties. The role
of perovskites as oxidation catalysts may be associated with the
The homogeneous solution was exposed to microwave energy in a
microwave oven at 2.45 GHz, 900 W power for 15 min. Once the
temperature reached a threshold level, the precursor solution
underwent dehydration and combustion, resulting is a black fluffy
powder (LCM). This black powder contained metal hydroxides and
other impurity phases (see Fig. 3), and it was subsequently sub-
jected to calcination at 500 1C for 2 h, resulting in the formation of
2ꢀ
high mobility of O ions within the crystal lattice and their
migration to the surface, and the catalyst can provide additional
32
active sites for the oxidation of carbonaceous substances.
Velasquez et al. and Villoria et al. have prepared La CuO and
2
4
LaCoO perovskites individually by employing a self-combustion
3
method using glycine as a fuel followed by calcination at 700 1C for
25,26
6
h and 750 1C for 4 h, respectively.
These perovskites have
2 4 3
the crystalline La CuO and LaCoO phases of the perovskite
system. Since the final obtained products consisted of a combi-
nation of two perovskite phases, the samples were identified as
been used for the catalytic oxidation of glycerol and oxidative
reforming of diesel. However, reports on the preparation of
La CuO /LaCoO nanocomposites. Fig. 1 shows the schematic
La
are scarce. The present study focuses on the synthesis of Co
doped La CuO /LaCoO perovskite oxides employing the micro-
2
CuO
4
and LaCoO
3
using a microwave combustion method
2
4
3
2+
representation for the preparation of La CuO /LaCoO perovskite
2
4
3
nanocomposites. The compositions were prepared with the addi-
2
4
3
2+
2+
wave combustion technique. The effect of Co doping on La CuO
x 4
tion of cobalt (Co ) of different molar ratios in a La2ꢀxCo CuO
2
4
system with x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 and the samples are
under microwave influence for the appearance of a secondary
labeled LCC0, LCC1, LCC2, LCC3, LCC4 and LCC5, respectively.
3
perovskite phase (LaCoO ) is examined. In comparison to the
conventional heating methods, the microwave combustion
method is considered as a rapid process for the synthesis of
2
.2. Characterization
33–37
metal oxide materials.
The choice of oxidizer (precursor) and Thermal behaviour of the as-prepared powders was studied by
fuel (urea) in a suitable stoichiometric ratio will allow better thermogravimetric analysis using an Exstar TG/DTA 7200
ꢀ1
control over the combustion process. This results in the formation instrument from SII Nanotech at a heating rate of 5 1C min
of nanostructures with the desired phase formation and composi- under a nitrogen atmosphere. The phase composition has been
tion, or sometimes with the formation of an unexpected additional analysed by X-ray diffraction using a high resolution Rigaku
38
phase. In such case, the additional phase will disappear when Ultima IV diffractometer with CuKa radiation (l = 1.5406 Å).
subjected to calcination at moderate temperature. The refinements of the XRD patterns were performed using the
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2 4 3
Fig. 1 Schematic representation of the preparation of La CuO /LaCoO perovskite nanocomposites.
PDXL program based on the Rietveld method. A Thermo 30 m ꢂ 250 mm ꢂ 0.25 mm column. Helium gas was used as the
ꢀ1
scientific Nicolet 10 OMNI FTIR spectrophotometer was used carrier gas at a flow rate of 1.4 mL min , with a split ratio of 130/1
for the determination of surface functional groups. Diffuse and an injector temperature of 25 1C, and the oven temperature
reflectance UV-visible spectra were recorded using a Thermo profile was 80 1C for 5.0 min and increased to 100 1C with a
ꢀ
1
scientific Evolution 300 UV-visible spectrophotometer to ramping rate of 3 1C min , then maintained at the final tem-
estimate the energy bandgap. Morphological observations were perature for 1 min.
performed using a HITACHI S4800 high resolution field
scanning electron microscope (HR-SEM) equipped with a
HORIBA EMAX energy dispersive X-ray (EDX) detector for 3. Results and discussion
elemental composition analysis. Magnetic measurements were
carried out at room temperature using a PMC MicroMag 3900
model vibrating sample magnetometer (VSM) equipped with a
3
.1. TG/DTA analysis of the LCM sample
Thermogravimetric analysis of the black fluffy powder (LCM)
obtained as such from the microwave oven has been performed
to study the thermal behavior. The TG/DTA curve of the LCM
sample is given in Fig. 2. A slight weight loss (3%) between
1
T magnet and the magnetization curves were measured from
ꢀ
10 000 Oe to 10 000 Oe. N adsorption–desorption isotherms
of the porous double-perovskite nanocomposites were measured
using an automatic adsorption instrument (Micromeritics – ASAP
2
40 and 105 1C can be attributed to dehydration of water molecules
on the surface of the particles as confirmed by the appearance of
an endothermic peak in the DTA thermogram. The major weight
loss (23.2%) occurring between 105 and 460 1C is due to the
elimination of the hydroxyl group and other volatile components
present in the sample. However, beyond 460 1C, there is only a
gradual weight loss occurring in the sample (6.8%), which may be
caused by the elimination of tightly bound volatiles or structural
rearrangement taking place. Hence, calcination has been per-
2
020 unit). Prior to the measurement, the samples were degassed
at 150 1C for 12 h. The nitrogen adsorption–desorption isotherms
were recorded at a liquid nitrogen temperature of 77 K.
2.3. Catalytic activity tests
Selective liquid phase oxidation of glycerol was carried out in a
batch reactor at atmospheric pressure. The reaction parameters
are as follows: 50 mg of La CuO /LaCoO as the catalyst, 50 mmol
2
4
3
40
formed in the temperature range of 500 to 700 1C. The results
reveal that a single phase (orthorhombic) crystal structure is
formed with the studied temperature range. Moreover, the sample
calcined at 700 1C does not have much difference in the thermal
behavior compared to that of the sample calcined at 500 1C, and
hence in the present study, the calcination temperature has been
fixed at 500 1C.
of glycerol (C
3 8 3
H O ) as the reactant, 50 mmol of hydrogen peroxide
2 2
(H O ) as the oxidant and dimethyl sulfoxide (DMSO) as the
solvent. The above constituents were placed in a round bottom
two-necked flask (250 ml) equipped with a water condenser and
the temperature was maintained at 70 1C for 6 h and then cooled
to room temperature. The obtained product was collected and
analyzed using MAYURA 1100 gas chromatography (GC) equipped
with an Abel Bonded (AB-1, Capillary) column (30 m ꢂ 0.25 mm ꢂ
3.2. X-ray diffraction analysis
0
.25 mm) with an FID detector. The products were also analyzed by
GC-MS using an Agilent Technologies Model 7890A gas chromato- Fig. 3 shows the XRD patterns of the as-prepared La
graph, coupled to a mass detector model 5975C, with an HP-5MS, perovskite nanocomposites. The undoped La CuO
2
CuO
(LCC0) reveals
4 3
/LaCoO
2
4
1
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Fig. 2 TG-DTA spectrum of the LCM sample.
a single perovskite phase with an orthorhombic structure (space
group Bmab, JCPDS No: 88-0940). This indicates that the adopted
microwave combustion, followed by calcination at 500 1C for 2 h is
2 4
sufficient to prepare a single La CuO perovskite phase. The
characteristic peaks observed at 2y values around 24.31, 27.11,
3
5
1.11, 33.11, 33.41, 41.21, 41.71, 43.51, 47.81, 54.01, 54.41, 55.81,
8.01, 64.91, 65.11, 69.61, 70.01, 75.31, 76.51, 78.51 are indexed as the
(
(
111), (004), (113), (020), (200), (006), (115), (204), (220), (206),
117),(224), (133), (226), (135), (040), (400), (331), (228) and (333)
crystallographic planes, respectively. However a different trend was
2 4 3
Fig. 3 X-ray diffraction patterns of the La CuO /LaCoO perovskite
nanocomposites.
2+
2+
observed upon doping with Co . It was found that upon Co
doping, a new secondary LaCoO perovskite phase appears. The
substitution of La by Co ions results in the formation of two
phase-systems with orthorhombic (La CuO ) and rhombohedral
3
3+
2+
2
4
(
LaCoO ) structures respectively, indicating the formation of a
3
composite. Other than the appearance of the additional phase,
no complete phase transition has been observed. Moreover,
2 3 2 4
impurity phases like CoO, La O and CuO are absent. The La CuO
and LaCoO compounds have been indexed according to JCPDS
3
card No. 82-0169 and 48-0123 with the Cmca and R3%c space group,
respectively. The intensity of the peaks corresponding to the
2
+
rhombohedral phase gradually increases with increasing Co
content and dominated the LCC5 sample due to the stable LaCoO
3
2+
secondary phase (Fig. 4). As the Co doping concentration is
further increased, this effect was found to be more pronounced,
and the formation of a stable two-phase perovskite system is
confirmed.
The lattice parameters of the orthorhombic/rhombohedral
perovskite nanocomposites have been calculated from the X-ray
diffraction data using eqn (2) and (3)
2 4 3
Fig. 4 Phase composition (%) of the La CuO /LaCoO perovskite
nanocomposites.
1
h2 k2 l2
¼
þ
þ
(2)
(3)
parameters a, b and c as well as the unit cell volume (V) of both
the orthorhombic and rhombohedral perovskite phases are
illustrated in Table 1.
d2 a2 b2 c2
ꢀ
ꢁ
ꢀ
ꢁ
2
2
2
2
2
1
d2
h þk þl sin a þ 2ðhk þ kl þ hlÞ cos a ꢀ cos a
¼
The variation in the lattice parameters (see Table 1) is
2
2
3
a ð1 ꢀ 3 cos a þ 2 cos aÞ
2
+
2 4
attributed to the dissolution of Co within the La CuO host
3
+
where d is the inter-atomic spacing, a, b and c are the lattice lattice by occupying La sites since both ions have different
2
+
3+
constants and a is the diffraction angle. The calculated lattice ionic radii (0.9 Å and 1.36 Å for Co and La , respectively), in
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Table 1 Rietveld refined XRD parameters of the La
2
CuO
4
/LaCoO
3
perovskite nanocomposites
Sample Phase
Crystal
structure
Space Crystallite a
Lattice
Cell
3
code
composition
%
group size D (nm) Microstrain parameter (Å) volume (Å ) Fit parameters (%)
LCC0
La
2
CuO
4
100 Orthorhombic Bmab 39.6
89.5 Orthorhombic Cmab 46
0.045
a = 5.3589,
b = 5.3959,
c = 13.1525
a = 5.3676,
b = 13.1340,
c = 5.3907
a = 5.4680,
c = 13.2620
a = 5.3663,
b = 13.1320,
c = 5.3891
a = 5.4650,
c = 13.2470
a = 5.3702,
b = 13.1300,
c = 5.3860
a = 5.4670,
c = 13.2580
a = 5.3705,
b = 13.1360,
c = 5.3880
a = 5.4660,
c = 13.2460
a = 5.3985,
b = 13.1550,
c = 5.3739
a = 5.4310,
c = 13.4260
380.31
380.03
R
wp = 13.86
R
p
= 10.36
R
R
e
e
= 11.50
= 12.76
2
S = 1.20
w = 1.45
p
R = 9.93
2
LCC1
LCC2
LCC3
LCC4
LCC5
a
La
2
CuO
4
0.097
Rwp = 13.46
S = 1.00
w = 1.11
LaCoO
3
10.5 Rhombohedral R3%c
25
0.210
0.073
343.39
379.77
2
La CuO
4
4
4
4
77
Orthorhombic Cmab 44.6
Rwp = 14.44
R
p
= 10.82
R
R
R
R
e
= 13.09
2
S = 1.10
w = 1.21
LaCoO
3
23
Rhombohedral R3%c
21
0.260
0.138
342.63
379.77
2
La CuO
62.3 Orthorhombic Cmab 45
Rwp = 15.94
R
p
= 11.83
e
e
e
= 12.90
= 13.74
= 13.80
2
S = 1.19
w = 1.43
LaCoO
3
37.7 Rhombohedral R3%c
49
0.520
0.070
343.16
380.10
2
La CuO
50
Orthorhombic Cmab 36
Rwp = 14.84
p
R = 11.15
2
S = 1.07
w = 1.16
LaCoO
3
50
31
Rhombohedral R3%c
36
0.590
0.740
342.73
381.63
2
La CuO
Orthorhombic Cmab 43
Rwp = 17.17
p
R = 13.61
2
S = 1.24
w = 1.54
LaCoO
3
69
Rhombohedral R3%c
45
0.660
342.95
Average crystallite size D (nm) by Debye–Scherrer’s formula.
addition to the effect of cation concentration in a solid solution
by the miscibility limit of the corresponding phase formation.
The goodness of fit parameter (S) was calculated from
S = Rwp/R , where Rwp is the weighted profile and R is the
4
1
e
e
The average crystallite size (D) of the orthorhombic/rhom- expected weighted profile reliability factor, respectively. The
bohedral perovskite nanocomposites has been calculated using value of ‘S’ around unity indicates the excellent goodness of fit
3
8
the Debye–Scherrer formula:
and confirms that the refined parameters are determined more
precisely.
4
2
0
:89l
D ¼
(4)
3.3. FTIR analysis
b cos y
Fig. 6 shows the FTIR spectra of the La CuO /LaCoO perovs-
2
4
3
where, l is the X-ray wavelength, y is the Bragg diffraction angle kite nanocomposites. The broad band centered at about
ꢀ1
and b is the full width at half maximum (FWHM). Table 1 3747 cm can be associated with the O–H longitudinal stretching
indicates an average crystallite size for the undoped La
2
CuO
4
of vibration of the adsorbed water molecules, while the adsorption
2
+
ꢀ1
about 39.6 nm. The Co doped La
LaCoO
of 36–46 nm and 21–49 nm, respectively. The obtained changes nitrogen group due to the combustion process. In addition, the
2
CuO
4
(orthorhombic) and bands at 1732 and 1530 cm are ascribed to the H–O–H vibration.
ꢀ
1
3
(rhombohedral) possessed a crystallite size in the range The weak band at 1380 cm may be assigned to the residual
ꢀ
1
in the values of the crystallite size in both La CuO and LaCoO
two high intensity bands at 682 and 512 cm are correlated to the
phases can be associated with experimental factors such as the La–O and Cu–O stretching modes of the orthorhombic La CuO
2
4
3
2
4
2
+
43
concentration of Co dopant and intrinsic parameters such as phase. The LCC5 sample with a maximum amount of LaCoO3
the nature of the dopant and the type of crystal structure, for and a minimum amount of the La CuO phase (Fig. 4), shows the
instance orthorhombic and rhombohedral, due to the effect of stretching vibrations at 580 and 445 cm , which are characteristic
2
4
ꢀ
1
30
atomic arrangements on grain growth (preferred orientation bands of the LaCoO
during grain growth occurs along the facile direction that is the band intensity at 682 and 512 cm is due to the decrease
energetically favorable). in the amount of the orthorhombic phase alongside the increase
3
perovskite phase. The steady decrease of
ꢀ1
2+
Fig. 5 shows the Rietveld refinements performed on the in the amount of the rhombohedral phase with increasing Co
La CuO /LaCoO perovskite nanocomposites using PDXL soft- doping fraction.
2
4
3
ware. The refinements were carried out to confirm two phase
formation. The observed X-ray diffraction patterns of the
3.4. Optical properties
2
+
2 4 3 2 4
La CuO /LaCoO composites match well with the calculated The optical properties of the Co doped La CuO /LaCoO
3
2
ones (Fig. 5), as the values of w given in Table 1 are closer to 1. perovskite nanocomposites were studied using UV-Vis diffuse
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2 4 3
Fig. 5 Rietveld refined XRD patterns for the La CuO /LaCoO perovskite nanocomposites.
reflectance spectroscopy. The Kubelka–Munk (K–M) function This may be due to quantum confinement phenomena occur-
2
+
[
F(R)] is generally applied to convert the diffuse reflectance into ring at the nano-regime. With Co doping, two linear regions
the equivalent absorption coefficient (a) as given in eqn (5) and are observed corresponding to the La
2
CuO
4
and LaCoO
3
3
3
8
mostly used for analyzing the powder samples:
perovskite phases. The optical band gap of the pure LaCoO
2
4
2+
perovskite is 3.09 eV. With increasing Co content, the band
gap related to the first region corresponding to the ortho-
rhombic phase increases from 2.32 eV to 2.56 eV and that of
2
ð1 ꢀ RÞ
a ¼ FðRÞ ¼
(5)
2
R
where, F(R) is the K–M function, a is the absorption coefficient the rhombohedral phase increases from 2.94 eV to 3.09 eV
and R is the reflectance. Thus the Tauc relation becomes:
2 4
(Table 2). The increase in the bulk band gap for the La CuO /
2+
LaCoO
This is due to an increase in bond-mismatch, which is the
where n = 1/2 and 2 for direct and indirect transitions, respectively, variation of the lattice parameters a, b and c. The substitution of
3
nanocomposite may be due to the Co substitution effect.
n
F(R)hn = A(hn ꢀ E
g
)
(6)
2
3+
2+
thereby giving direct and indirect band gaps. The (F(R)hn) versus La (large radii) by Co (small radii) within the La CuO host
2
4
hn plots are shown in Fig. 7. The extrapolation of the linear region lattice will enhance the bond mismatch by contracting the (LaO)
2
2
to (F(R)hn) = 0 gives the band gap value. The optical band gap layers. This will lead to the expansion of CuO
value of the undoped La CuO perovskite is 1.88 eV, which is resulting in an increase in the band gap of the La
higher than the reported values of 1.30 eV and 1.24 eV.
2
and CoO
6
sheets,
and
2
4
2
CuO
4
8,44
8
3
LaCoO perovskite phases, respectively.
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ꢀ
1
is 2.547/3.528 memu g
(obtained after the removal of the
paramagnetic component by using the MicroMag software).
LaCoO is a nonmagnetic insulator at the ground state with a
3
paramagnetic insulating state developing at a low temperature of
about 50 K, and at around 500 K an insulator-to-metal transition is
46
2+
observed. As shown in Fig. 4, with increasing Co content
.1 r x r 0.3 (LCC1 to LCC3 samples), the amount of La CuO
decreases (from 89.5 to 62.3%); meanwhile, the LaCoO amount
0
2
4
3
increases (from 10.5 to 37.7%). So obviously a para-/ferro-magnetic
transition occurs; the nanocomposites present a dominant para-
magnetic component alongside a small ferromagnetic phase and
the corresponding bulk magnetisation at maximum field gradually
ꢀ
1
increases from 18.91 to 31.12 memu g . Furthermore, at a higher
2+
Co content; i.e. x = 0.4 (LCC4) and 0.5 (LCC5), the amount of the
secondary LaCoO phase becomes more important; 50 and 69% at
the expense of La CuO reaching 50 and 31%. The corresponding
nanocomposites become totally paramagnetic and this can be
explained as follow: (i) the substitution of La (2.5 m ) by Co with
3
2
4
B
higher moment (3.5–5.0 m ) will reinforce the magnetic ordering
B
not the magnetic transformation from para–ferro; in this case, the
2+
bulk magnetization increases with increasing Co content as
shown in Table 2 and Fig. 10; (ii) the LaCoO phase is known to
3
47
be paramagnetic at room temperature, thereby the higher its
amount, the more the bulk nanocomposite becomes paramagnetic;
2+
2+
3+
(
iii) Co may occupy the non-magnetic Cu sites instead of La
sites, and thereby result as well in reinforcing the bulk magnetic
moment. Herein, it should be highlighted that the main objective
3+
2+
of the present work is to substitute La by Co within the La CuO4
2
2 4 3
Fig. 6 FT-IR spectra of the La CuO /LaCoO perovskite nanocomposites.
phase. However, the crystallization of the La CuO phase from the
2
4
3+
2+
2+
solution containing La , Cu , and Co formed from the starting
2
+
2+
precursors, and Co may also occupy the Cu sites within
3+
3.5. SEM and EDX studies
2 4
La CuO because they have close ionic radii compared to La . By
suggesting so, all possibilities that may occur during the synthesis
are taken into consideration, in order to explain the change in the
magnetic behavior and properties. The value of bulk magnetisation
at the maximum field of LCC4 and LCC5 increases 41.51 and
The HR-SEM micrographs of the La
2
4 3
CuO /LaCoO nanocompo-
sites shown in Fig. 8 reveal the intragranular pores while the
pore walls are fused grains with distinct grain boundaries.
2+
Moreover, with increasing Co concentration, the intragranular
porosity increases while larger grains are reduced to small. The
ꢀ
1
4
7.71 memu g , meanwhile, the value of (M ) for the ferro-
s
ꢀ
1
magnetic phase remains low in the 1.9–3.5 memu g range
Table 2). The as-obtained modifications in the magnetic
properties can be associated most probably with the exchange
particle sizes estimated from the HR-SEM images of the La
LaCoO perovskite nanocomposites gradually decrease from 87 to
7 nm. The particle size as seen from the SEM images is slightly
2 4
CuO /
(
3
3
2+
of A and B sites within the La
doping; as well as the important phase composition of La
LaCoO obtained from the ferromagnetic-to-paramagnetic transi-
2
CuO
4
host lattice because of Co
different from the crystallite size obtained by XRD analysis,
because the former is associated with aggregated cluster size,
while the latter is associated with the diffracting domain size in the
2
CuO
4
/
3
45
tion. The formation of the paramagnetic perovskite LaCoO phase
3
nanoparticle. EDX (Fig. 9) analysis was used to examine the
elemental composition of the La CuO /LaCoO perovskite nano-
2+
and the increase of its amount with increasing Co doping
content, reinforce the paramagnetic phase at the expense of the
ferromagnetic phase. In a composite, the bulk properties represent
a combination of individual properties of its constituents. The
2
4
3
composites and confirmed the presence of La/Co, Cu and O
elements and the inset table shows the respective fraction of the
elemental composition.
2+
bulk magnetization value has suddenly changed at a certain Co
doping content, due to the transition between single domain and
3
.6. Magnetic properties
Fig. 10 illustrates the magnetisation–field (M–H) hysteresis
curves of the La CuO /LaCoO perovskite nanocomposites and Co (0.9 Å) is significant, the substituted ions give rise to
48
multi-domain at the critical size.
3
+
Since the relative difference in the ionic radii of La (1.36 Å)
2
+
49
2
4
3
4
6
recorded at room temperature under a 1 Tesla magnetic field. a ‘distorted’ perovskite structure, resulting in the creation of
For the un-doped LCC0 sample, only ferro-/para-magnetic beha- some point defects (vacancies or interstitials). For the moment,
+
+
viour is observed; the corresponding saturation magnetization (M
s
)
the 2 state of Co and the 3 state of La, this means that the
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2
2 4 3
Fig. 7 Plot of (F(R)hn) versus hn for the La CuO /LaCoO perovskite nanocomposites.
2 4 3
Table 2 Magnetic properties and bandgap of the La CuO /LaCoO perovskite nanocomposites
g
Bandgap E (eV)
Sample
code
Magnetic
behavior
Coercivity
(Oe)
Remanence M
r
Saturation magnetisation
ꢀ1
ꢀ1
H
c
(memu g
)
M
s
, (memu g
)
Orthorhombic
1.88
Rhombohedral
—
LCC0
LCC1
LCC2
LCC3
Ferro
Para
Para/Ferro
Ferro
Para/Ferro
Ferro
Para/Ferro
Ferro
Para
542.3
519.9
102.8
177.27
9.465
14.43
24.89
29.60
7.030
24.08
910.6
910.6
724.1
724.1
5.447
21.00
208.8
208.8
59.81
129.1
2.547a
3.528
a
18.91
2.32
2.49
2.51
2.94
2.96
2.99
3.12a
22.02
1.92a
31.12
3.51a
41.51a
47.71
LCC4
LCC5
2.53
2.56
3.06
3.09
Para
a
Magnetization at the maximum field of 1 T.
3
+
2+
substitution of La by Co will result in unbalanced total complex, as direct/indirect combined effects may occur
charge of the compound (positive charge), which will create simultaneously, together with the creation of defects; the
3
+
some oxygen vacancies. However, the real situation may be a bit replacement of the 4f rare earth element La with weak
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2 4 3
Fig. 8 HR-SEM images of the La CuO /LaCoO perovskite nanocomposites.
2
+
paramagnetic behaviour by the 3d transition metal Co having process would be associated with the delayed condensation
a paramagnetic behaviour, and short/long exchange inter- of the adsorbate while the desorption would be associated
3
+
2+
2+
2ꢀ
actions between various La , Co , Cu and O ions.
with the equilibrium vapor–liquid transition leading to a non-
comparable behavior in the pore diameter (D ) and pore
Thus, the results
adsorption– of surface area, pore size and pore volume indicate that the
desorption isotherms, recorded at 77 K using the BET techni- La CuO /LaCoO perovskite nanocomposite system possesses
p
3
.7. Nitrogen adsorption/desorption isotherms
50,51
p
volume (V ) values for the LCC2 sample.
The surface area has been determined from the N
2
2
4
3
que as shown in Fig. 11. All samples exhibit a mixture of type II appropriate porosity and surface area suitable for catalytic
and type IV isotherms, according to the IUPAC classification. applications.
The hysteresis loops observed in the high P/P range indicate
0
3
.8. Catalytic activities of La
nanocomposites
) are given in Table 3. The BET surface The La CuO /LaCoO
for the oxidation of glycerol in a liquid phase batch reactor under
However, increasing the Co doping fraction results in a atmospheric conditions. For undoped La CuO , the conversion of
gradual increase of the surface area upto 43.7 m g for the glycerol was found to be 80% with a selectivity of 91%. Further-
2 4 3
CuO /LaCoO perovskite
the formation of mesopores in the La CuO /LaCoO perovskite
2
4
3
nanocomposites. The surface area (SBET), pore diameter (Dp)
and pore volume (V
area of pure undoped La
p
2
4
3
perovskite nanocomposites have been tested
2
ꢀ1
2
4
CuO was found to be 12.2 m g .
2
+
2
4
2
ꢀ1
2+
LCC5 sample.
more, upon doping with Co (0.1 r x r 0.5), the conversion of
Except for the LCC2 sample, all other samples possess an glycerol increases from 83.3% to 96.5% and the selectivity of
average pore diameter between 3.3 and 3.7 nm, indicating the formic acid (as observed for GC-MS results shown in Fig. 12)
presence of mesopores. In LCC2, the steepness of the isotherms increases also from 79.8% to 95.1%. The LCC5 catalyst yielded the
0
is observed at a high relative pressure (P/P 4 0.78). This could highest catalytic activity for the conversion of glycerol to formic
be due to the presence of uniform cylindrical pores and ordered acid as shown in Table 4. Therefore, the LCC5 sample was used for
three dimensional pore networks. In this case, the adsorption further optimization studies.
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2 4 3
Fig. 9 EDX spectra of the La CuO /LaCoO perovskite nanocomposites.
noticed that the conversion of glycerol increases from 76.3% to
9
8
8.9%; meanwhile, the selectivity of formic acid increases from
7.4% to 95.1% for 10 to 50 mg of the catalyst and thereafter
decreases to 89.1% for a higher loading of 70 mg. The reduction
in the selectivity of formic acid beyond 50 mg of the composite
may be due to the pore blocking of the catalyst surface, which
led to a decline or decrease in the reactive surface area available
5
2
to proceed the reaction.
3.10. Recycling test
The reusability of the LCC5 nanocomposite catalyst for the
conversion of glycerol has also been investigated. After the first
reaction, the catalyst was filtered and washed with distilled
water several times, then dried at 150 1C for 2 h and reused for
another four catalytic runs. The results reveal (Fig. 14) a slight
decrease in the conversion of glycerol and the selectivity of
formic acid, but both values remain around 90%. This can be
attributed to the mass loss during the catalyst washing and
2 4 3
Fig. 10 Magnetic hysteresis curves of the La CuO /LaCoO perovskite
nanocomposites.
5
3
recovery process.
3.9. Effect of composite loading
3.11. Reaction pathway
The catalytic conversion of glycerol and the selectivity over the The proposed mechanism of glycerol oxidation to produce
LCC5 nanocomposite catalyst at 70 1C for 6 h with different formic acid is shown in Scheme 1. It is a sequential reaction
composite loadings (10 to 70 mg) are shown in Fig. 13. It can be that follows two paths. The first pathway involves the oxidation
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Fig. 11
2 2 4 3
N adsorption–desorption isotherms obtained from the BET measurements of the La CuO /LaCoO perovskite nanocomposites.
Table 3 BET surface area, pore diameter and pore volume of the C–C bond cleavage) to form acetaldehyde and methanol
2 4 3
La CuO /LaCoO perovskite nanocomposites
precursors, which on reduction lead to acetaldehyde and
methanol. Acetaldehyde is later oxidized to form acetic acid.
On the other hand, methanol is oxidized (loss of hydrogen
radical) to form formaldehyde, which later gets oxidized to
Sample
code
Surface area,
Pore diameter,
(d) (nm)
Pore volume,
2
ꢀ1
ꢀ1
SBET (m g
)
D
p
V
p
(cc g
)
LCC0
LCC1
LCC2
LCC3
LCC4
LCC5
12.2
20.4
24.2
33.5
42.6
43.7
3.3
3.6
1.2
3.6
3.6
3.7
0.013
0.049
0.118
0.053
0.057
0.086
5
4
form formic acid. The second reaction pathway involves the
oxidation of glycerol to form glyceraldehyde, which is then
oxidized to glyceric acid. It would undergo rapid C–C bond
cleavage to form glycolic acid and formaldehyde, where the
5
5
latter is readily oxidized to form formic acid.
A literature survey has been carried out in order to gain
of glycerol with an initial loss of terminal hydroxyl groups insight into the results that were obtained. Nevertheless, it is
to form enol, which readily forms acetol (hydroxyacetone) clearly highlighted that it is very difficult to make a conclusive
by tautomerism. This is followed by a radical fragmentation comparison with the literature, since in this study, we obtained
related to a Norrish type I process (which would undergo rapid a nanocomposite system formed by the two perovskite phases
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Fig. 12 GC-MS spectra of glycerol oxidation using the LCC5 perovskite sample.
2+
Table 4 Effect of Co
doping on the conversion of glycerol and
selectivity
Sample code Glycerol conversions (%) Selectivity of formic acid (%)
LCC0
LCC1
LCC2
LCC3
LCC4
LCC5
80.0
83.3
86.8
90.2
93.4
96.5
91.0
79.8
83.6
87.3
90.5
95.1
2 2
Reaction conditions: 50 mmol of (glycerol, H O and DMSO), catalyst
5
0 mg, reaction time 6 h and temperature 70 1C.
Fig. 14 Recycling performance of the LCC5 sample.
the appearance of a cusp at 250 K. From DFT calculations, it
has been reported that La CuO4+d (presence of interstitial oxygen O
and Sr-doped La CuO have anti-ferromagnetic (AFM) behavior.
Meanwhile, even though LaCoO is known as diamagnetic, both
experimental and DFT calculations indicated the following: (i)
2
i
)
5
6,57
2
4
3
58
Andrew M. Ritzmann et al. studied the influence of the spin
3
+
state of Co on the oxygen transport in LaCoO
3
and showed that
3+
Fig. 13 Effect of composite loading on the conversion of glycerol over low spin (LS) Co promotes higher oxygen vacancy concentrations;
59
the LCC5 sample.
(ii) while Dechao Menget et al. revealed that ferromagnetism
FM) can be achieved. This can be originated from the high
concentration of high spin (HS) Co , and FM coupling between
(
3+
3+
3+
La CuO4 and LaCoO3 making heterojunctions, resulting in the nearest HS Co and LS Co . However, when oxygen vacancies
2
novel bulk properties compared to the individual and different (O ) are created, the FM is suppressed. To the knowledge of the
v
properties of each phase (crystal structure, amount, crystallite authors, only one perovskite-perovskite system reported by Liang
60
size, site occupancy, doping, presence of vacancies, etc.).
Shin-ichi Uchida et al. showed that this La CuO perovs- inorganic) heterostructures revealed ferromagnetic order at a low
kite compound is paramagnetic with small susceptibility temperature (5 K) associated with dimensionality controlled octa-
3 3
Qiao et al. showed that epitaxial LaCoO /SrTiO (inorganic/
2
4
ꢀ7
ꢀ1
B10 emu g over a wide temperature range (0–300 K) with hedral symmetry-mismatch.
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Scheme 1 Proposed mechanism of glycerol oxidation to formic acid.
From several studies reported in the literature, it can be 4. Conclusions
noticed that the magnetic behavior of un-doped and doped
La
synthesized by a microwave combustion method. The undoped
La CuO (LCC0) shows a single perovskite phase with orthorhombic
structure. Upon doping with Co , the formation of La
LaCoO3 perovskite nanocomposites was observed and further
2 4 3
CuO /LaCoO perovskite nanocomposites have been successfully
perovskite phases, for instance La CuO and LaCoO , is some-
2
4
3
how complicated. Both experimental and theoretical results
indicated that numerous parameters may have a direct or a
combined influence on the evolution of their physical proper-
ties (magnetic, catalytic, etc.), including but not limited to:
2
4
2+
2 4
CuO /
2+
validated by Rietveld refinements. At lower Co doping (LCC1),
(i) nature of the doping element and its content either on A (La)
2+
the orthorhombic phase is dominant while at higher Co doping,
the rhombohedral phase becomes dominant (LCC5). The average
crystallite size of the nanocomposites varies in a narrow range, i.e.
or/and B (Co, Cu) sites; (ii) spin state of B ions (low, inter-
mediate and high spin) and interaction between the different
spin states; (iii) the presence of oxygen vacancies (O ) and
v
3
6–46 nm and 21–49 nm for the orthorhombic and rhombo-
hedral phases, respectively. FT-IR analysis confirms the vibrational
stretching modes of the La CuO /LaCoO perovskites. The optical
band gap value of the orthorhombic/rhombohedral phases
i
interstitials (O ) and their interaction with their environment
within the host lattice; and so on. Herein, it is essential to mention
that figuring out the properties of a composite starting from
individual phase properties is not straightforward; as the complex
combination of several factors may interfere simultaneously.
Interestingly, in this study the formation of a perovskite-type
La CuO /LaCoO (orthorhombic/rhombohedral) heterointer-
2
4
3
2
+
increases with an increase in the Co content. Morphological
observations reveal a well developed intragranular porosity with
distinct grains on the surfaces. Magnetic measurements indicate
the appearance of ferro-/para-magnetic behaviour. The textural
2
4
3
face nanocomposite with controllable phase composition through
the substitution of La by Co, results in interesting properties: (i)
2 4 3
properties of the La CuO /LaCoO perovskite nanocomposite show
an extent of mesoporosity with a maximum BET surface area of
2 4
tunable energy bandgap (1.88–2.56 eV for La CuO and 2.94–3.09 eV
2
ꢀ1
4
2 4 3
3.7 m g . La CuO /LaCoO perovskites behave as a potential
for LaCoO ); the appearance of ferromagnetic order with saturation
3
2+
catalyst for the selective liquid phase oxidation of glycerol to
formic acid, exhibiting high conversion and selectivity above 95%.
magnetization influenced by the Co content alongside with a
ferro–para transition; and high catalytic properties (glycerol conver-
sion in the 80–96% range). The as-obtained results may be asso-
ciated with combined effects mainly but not limited to quantum
confinement at the nanoscale; complex site occupancy in both
Conflicts of interest
2
+
3+
2+
phases (Co from the precursor may occupy La or Cu sites
within La CuO ); the presence of grain boundaries, which favor/
facilitate oxygen transport and thereby the creation of oxygen
There are no conflicts to declare.
2
4
2+
vacancies/interstitials; and the oxidation of Co (starting precursor)
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3
) in a complex process in
2+
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