Structure and Catalytic Activity of Cr-Doped BaTiO3 Nanocatalysts Synthesized by Conventional Oxalate and Microwave Assisted Hydrothermal Methods

Article abstract of DOI:10.1021/acs.inorgchem.6b00240

In the present study synthesis of BaTi_1-xCr_xO₃ nanocatalysts (x = 0.0 ≤ x ≤ 0.05) by conventional oxalate and microwave assisted hydrothermal synthesis methods was carried out to investigate the effect of synthesis methods on the physicochemical and catalytic properties of nanocatalysts. These catalysts were thoroughly characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N₂ physisortion, and total acidity by pyridine adsorption method. Their catalytic performance was evaluated for the reduction of nitrobenzene using hydrazine hydrate as the hydrogen source. Structural parameters refined by Rietveld analysis using XRD powder data indicate that BaTi_1-xCr_xO₃ conventional catalysts were crystallized in the tetragonal BaTiO₃ structure with space group P4mm, and microwave catalysts crystallized in pure cubic BaTiO₃ structure with space group Pm3μm. TEM analysis of the catalysts reveal spherical morphology of the particles, and these are uniformly dispersed in microwave catalysts whereas agglomeration of the particles was observed in conventional catalysts. Particle size of the microwave catalysts is found to be 20-35 nm compared to conventional catalysts (30-48 nm). XPS studies reveal that Cr is present in the 3+ and 6+ mixed valence state in all the catalysts. Microwave synthesized catalysts showed a 4-10-fold increase in surface area and pore volume compared to conventional catalysts. Acidity of the BaTiO₃ catalysts improved with Cr dopant in the catalysts, and this could be due to an increase in the number of Lewis acid sites with an increase in Cr content of all the catalysts. Catalytic reduction of nitrobenzene to aniline studies reveals that BaTiO₃ synthesized by microwave is very active and showed 99.3% nitrobenzene conversion with 98.2% aniline yield. The presence of Cr in the catalysts facilitates a faster reduction reaction in all the catalysts, and its effect is particularly notable in conventional synthesized catalysts.

Full text of DOI:10.1021/acs.inorgchem.6b00240

Article
pubs.acs.org/IC
Structure and Catalytic Activity of Cr-Doped BaTiO3 Nanocatalysts
Synthesized by Conventional Oxalate and Microwave Assisted
Hydrothermal Methods
Chilukoti Srilakshmi,*,† Rohit Saraf,† V. Prashanth,† G. Mohan Rao,‡ and C. Shivakumara†
‡
†Solid State and Structural Chemistry Unit (SSCU) and Department of Instrumentation and Applied Physics, Indian Institute
of Science (IISc), Bangalore, Karnataka, India, 560012
S
* Supporting Information
ABSTRACT: In the present study synthesis of BaTi1−xCrxO3 nanocatalysts
(x = 0.0 ≤ x ≤ 0.05) by conventional oxalate and microwave assisted hydrothermal
synthesis methods was carried out to investigate the effect of synthesis methods on
the physicochemical and catalytic properties of nanocatalysts. These catalysts were
thoroughly characterized by X-ray diffraction (XRD), Fourier transform infrared
spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), N2 physisortion, and total acidity by pyridine adsorption
method. Their catalytic performance was evaluated for the reduction of nitrobenzene
using hydrazine hydrate as the hydrogen source. Structural parameters refined by
Rietveld analysis using XRD powder data indicate that BaTi1−xCrxO3 conventional
catalysts were crystallized in the tetragonal BaTiO3 structure with space group P4mm,
and microwave catalysts crystallized in pure cubic BaTiO3 structure with space group
Pm3m. TEM analysis of the catalysts reveal spherical morphology of the particles, and
̅
these are uniformly dispersed in microwave catalysts whereas agglomeration of the particles was observed in conventional catalysts.
Particle size of the microwave catalysts is found to be 20−35 nm compared to conventional catalysts (30−48 nm). XPS studies reveal
that Cr is present in the 3+ and 6+ mixed valence state in all the catalysts. Microwave synthesized catalysts showed a 4−10-fold
increase in surface area and pore volume compared to conventional catalysts. Acidity of the BaTiO3 catalysts improved with Cr
dopant in the catalysts, and this could be due to an increase in the number of Lewis acid sites with an increase in Cr content of all the
catalysts. Catalytic reduction of nitrobenzene to aniline studies reveals that BaTiO3 synthesized by microwave is very active and
showed 99.3% nitrobenzene conversion with 98.2% aniline yield. The presence of Cr in the catalysts facilitates a faster reduction
reaction in all the catalysts, and its effect is particularly notable in conventional synthesized catalysts.
homogeneity, and low sinterability.8 In order to circumvent this
limitation, there is increasing demand for new synthesis methods.
1. INTRODUCTION
In recent years, perovskite-type (ABO3 structure) transition
The microwave-assisted hydrothermal process is an active area of
metal oxides have attracted much attention as catalytic materials
research in recent years, and this process has been used for the
due to their thermal and chemical stability, high resistance to
rapid synthesis of numerous ceramic oxides, hydroxylated
dissolution in aqueous and nonaqueous solutions, and cost-
phases, porous materials, and metal powders.9−11 It offers several
effectiveness as compared to noble metals.1 In ABO3 structure,
advantages in comparison to conventional heating such as fast
A and B are two cations with A atoms larger than B atoms.
crystallization, cost efficiency, and clean technology, and further-
The chemical properties of the perovskite materials are also known
for their flexible characteristics which are associated with their B
more it is energy efficient and economical. In the present investi-
gation, we have achieved considerable reduction in time required
site substitution capabilities and availability of a wide variety of
substoichiometric structures.2 BaTiO3 (BTO) is an attractive
dielectric, ferroelectric, pyroelectric, and electrochemical material
for the synthesis of perovskites of BaTi1−xCrxO3 (x = 0.5, 2.5, and
5 mol %) using this methodology and compared the physico-
chemical and catalytic properties of these catalysts with those
for piezo-sensors transducers, infrared sensors, data storage, and
tunable microwave dielectric applications.3−6 The physicochem-
prepared by a conventional oxalate method of synthesis. All the
catalysts synthesized via both the methods were evaluated for
ical properties of BaTiO3 can be improved by doping various
transition metals into the lattice.7 In the present study we have
catalytic reduction of nitrobenzene to aniline.
Anilineiswidely used inthe chemical industries asintermediates
investigated the effect of Cr dopant on the physical and chemical
properties of BaTiO3 catalyst synthesized by conventional oxalate
and microwave assisted hydrothermal syntheses.
for the production of dyes, pharmaceuticals, plastics (especially
polyurethanes), herbicides, and pigments.12,13 Generally, aniline is
Generally the conventional method of synthesis of perovskites
requires conductive heating for extended periods at elevated tem-
peratures leading to large particle size formation, loss of chemical
Received: January 29, 2016
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
mixture was transferred into Teflon-lined autoclave, sealed, and placed
in microwave instrument at 180 °C for 30 min (MW 5000, SINEO,
maximum power of 1500 W). After the reaction, the autoclave was cooled
to room temperature. The solid product was washed with 0.1 M aqueous
acetic acid solution and deionized water. The final product was recovered
with centrifugation and dried in a hot air oven at 105 °C overnight. Pure
BaTiO3 was prepared in the same way but without addition of chromium
nitrate. The list of catalysts synthesized by the conventional oxalate and
microwave method is given in Table 1.
manufactured through the reduction of nitrobenzene (NB) via
catalytic reduction in liquid or gas phase and metallic reagents
in acidic media. Numerous methods containing homogeneous
catalytic reduction and heterogeneous catalytic reduction have
been reported in the literature for the reduction of nitroarenes
to anilines. Recently, catalytic reduction using homogeneous
complexes of Pd, Fe, Cu, Ni, and Fe14−17 with 2-propanol as
hydrogen donor has gained much attention. But reduction rates
are considerably low with these catalysts and involve various
problems such as deactivation by metal precipitation or ligand
degradation and separation difficulties, etc. On the other hand,
the use of heterogeneous catalysts offers several advantages
over homogeneous systems with respect to easy recovery and
recycling of the catalysts as well as minimization of undesired
toxic wastes. In the present paper we present the results of
catalytic reduction of nitro compounds over Cr-doped BaTiO3
perovskite. Perovskites have been used previously for the
Table 1. List of BaTiO3 and Cr-Doped BaTiO3 Catalysts
Synthesized by the Conventional Oxalate and Microwave
Method and Their Average Crystallite Size Calculated from
XRD
catalyst
notation
average crystallite
size
catalyst name
BaTiO3 (conventional oxalate (CO))
0.5 mol % Cr-doped BaTiO3 (CO)
2.5 mol % Cr-doped BaTiO3 (CO)
5 mol % Cr-doped BaTiO3 (CO)
BaTiO3 (microwave hydrothermal (MH))
0.5 mol % Cr-doped BaTiO3 (MH)
2.5 mol % Cr-doped BaTiO3 (MH)
5 mol % Cr-doped BaTiO3 (MH)
BTO_1
BTO_2
BTO_3
BTO_4
BTO_5
BTO_6
BTO_7
BTO_8
50.9
54.8
58.7
63.5
32.8
37.3
39.5
45.8
18
decomposition of NOx oxidation of CO19 and NH3.20
However, their application for the reduction of nitro aromatics
to amino aromatics without side reactions has been scarcely
studied and remains a challenging task. In our previous papers,
we have presented various transition metal doped BaTiO3 and
the effect of dopant on catalytic reduction of nitrobenzene to
azoxybenzene.21 Chromium-doped metal oxides has been prima-
rily studied for resistance-change memory,22 photocatalysis,23
oxygen gas sensor,24 and reduction of NO;25 its reactivity for
nitrobenzene reduction remains relatively unexplored. In this
context, we have explored Cr-doped BaTiO3 catalysts for the
selective reduction of nitrobenzene to aniline. The catalysts were
characterized using powder X-ray diffraction (XRD), Fourier
transform infrared (FTIR) spectroscopy, transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS),
N2 physisorption and acidity by the pyridine adsorption method.
2.3. Characterization Techniques. Powder X-ray diffraction
(XRD) patterns of the catalysts were recorded using a PANanalytical
X’pert PRO powder diffractometer with Cu Kα (λ = 1.5418 Å) as the
radiation source. Rietveld analysis measurement was performed in a step
mode with 2θ varying from 10° to 80° with a scanning rate of 0.02°
min−1. The structural parameters were estimated by using the Full Prof
Suite-2000 program. Fourier transform infrared (FTIR) spectra of
catalysts were measured on a PerkinElmer FTIR-300 spectrometer
using a KBr pellet method. The transmission electron microscopy
(TEM) analysis was performed on a JEOL 2100F electron microscope
with an acceleration voltage of 200 kV. The Brunauer−Emmett−Teller
(BET) surface area of the samples was measured using a Model-
Quantachrome Autosorb iQ2 automated gas sorption analyzer. Samples
were initially pretreated in N2 at 300 °C for 4 h, and the BET surface area
and pore volume of the catalysts were obtained by adsorbing N2 at liquid
nitrogen temperature, −196 °C. X-ray photoelectron spectra (XPS)
were recorded on an ESCA-3 Mark II spectrometer (VG Scientific Ltd.,
England) using Al Kα radiation (1486.6 eV). Binding energies were
calculated with respect to C (1 s) at 285 eV and measured with a
precision of 0.2 eV. Acidity measurements were conducted using
pyridine as a probe molecule using FTIR spectroscopy. Catalyst samples
were activated by degassing at 120 °C for 2 h and then saturated with
pyridine. The samples were then evacuated at 115 °C for 60 min to
remove physisorbed pyridine. FT-IR spectra of the samples were
then recorded in the range of 1400−1600 cm−1 using a PerkinElmer
FTIR-300 spectrometer having a resolution of 4 cm−1 with 60 scans.26,27
2.4. Catalytic Activity. Catalytic reduction with hydrazine hydrate
as a hydrogen donor was carried out under continuous stirring in a
two-necked 100 mL round-bottom flask fitted with a reflux condenser.
Analysis was done by withdrawing definite aliquots of the reaction sample
at regular intervals. In a typical run, 80 mg of the catalyst was dispersed in a
solution containing 10 mmol of nitrobenzene, 10 mmol of KOH pellets,
and 10 mL of 2-propanol. The mixture was stirred and heated under reflux
at 80 °C for 2−6 h in an oil bath. GC−MS analysis of the product samples
was carried out using Thermo Trace GC Ultra (GC), Thermo DSQ II
(MS); with DB5 MS column of 30 m L × 0.25 mm ID × 0.25 μm film
thickness and with Electron Multiplier detector.
2. EXPERIMENTAL SECTION
2.1. Materials. Barium nitrate (Ba(NO3)2), barium chloride
dihydrate (BaCl2·2H2O), titanium isopropoxide (Ti(OC3H7)4), titanium
tetrachloride (TiCl4), chromium nitrate nonahydrate (Cr(NO3)3·9H2O),
oxalic acid (C2H2O4·2H2O), potassium hydroxide (KOH), 2-propanol
(C3H7OH), and hydrazine hydrate (N2H4·H2O) used in the experiments
were purchased from Sigma-Aldrich and Merck. All reagents are of
analytical purity and were used without further purification.
2.2. Synthesis of BaTiO3 and Cr-Doped BaTiO3 Nanocatalysts.
2.2.1. Conventional Oxalate Method (CO). BaTiO3 and Cr-doped
BaTiO3 nanocatalysts were synthesized by a conventional oxalate route
as follows: Stoichiometric amounts of titanium isopropoxide, barium
nitrate, and chromium nitrate as metal oxide precursors, and oxalic acid
as chelating agent were used as raw materials. These materials were
mixed in a molar ratio of Ti(OC3H7)4: Ba(NO3)2: C2H2O4·2H2O =
1:1:2 in a mixture of 2-propanol and water. A homogeneous solution was
obtained after stirring at 60 °C for 1 h. Then the solution was evaporated
to dryness at 100 °C. The oxalate precursors thus obtained were dried in
hot air oven at 120 °C for overnight. The dried precursors obtained were
ground to fine powders and calcined at 900 °C for 12 h. For Cr sub-
stituted BaTiO3 catalysts, a similar method was followed except the
stoichiometric addition of chromium nitrate to the above solution in
the following ratio Ba(NO3)2: 1−xTi(OC3H7)4: xCr(NO3)3·9H2O:
C2H2O4·2H2O where x = 0.0 ≤ x ≤ 0.05 respectively.
2.2.2. Microwave-Assisted Hydrothermal Method (MH). In micro-
wave-assisted hydrothermal method, barium chloride dihydrate, titanium
tetrachloride, chromium nitrate nonahydrate, and potassium hydroxide
were chosen as starting materials. The molar ratio of BaCl2·2H2O: TiCl4:
Cr (NO3)3·9H2O: KOH: H2O was 2: (1 − x): x: 30:300. Initially TiCl4
was added to the required amount of water at 0 °C. BaCl2·2H2O, Cr
(NO3)3·9H2O, and KOH were separately dissolved in deionized water
and added to the TiCl4 solution and stirred for 30 min. The resultant
3. RESULTS AND DISCUSSION
3.1. Powder X-ray Diffraction. BTO and Cr-doped BTO
powders synthesized by above-mentioned two methods have been
analyzed by XRD studies. XRD patterns of the Cr-doped BaTiO3
B
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Figure 1. Powder XRD patterns of BaTiO3 and Cr-doped BaTiO3 catalysts synthesized using (a) CO method and (b) MH method.
Figure 2. Observed, calculated, and difference XRD patterns of (a) BTO_1, (b) BTO_2, (c) BTO_5, and (d) BTO_6.
(0.5, 2.5, and 5 mol %) powders synthesized by the CO method and
heat treated at 900 °C showed a similar XRD pattern (Figure 1a)
as that of pure BTO which illustrates the splitting of diffraction
peaks at 2θ ≈ 45° corresponds to (002) and (200) planes of
C
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tetragonal phase with space group P4mm (No. 99) of BaTiO3
(JCPDS card no. 079-2265). Trace amounts of titanium oxide,
barium carbonate, and chromium oxide were observed as impu-
rities in Cr-doped BTO catalysts.
The XRD patterns of as-synthesized catalysts by the MH
method (Figure 1b) illustrate the absence of apparent splitting at
2θ ≈ 45° corresponds to the tetragonal phase indicating the
presence of pure cubic crystalline phase of BaTiO3 with space
Table 2. Rietveld Refined Structural Parameters and Unit Cell
Volumes for BaTiO3 and Cr-Doped BaTiO3 Catalysts
Synthesized Using Conventional Oxalate Method
catalysts
BTO_1
BTO_2
BTO_3
BTO_4
crystal system
space group
tetragonal
tetragonal
tetragonal
tetragonal
P4mm
(No. 99)
P4mm
(No. 99)
P4mm
(No. 99)
P4mm
(No. 99)
lattice parameters
group Pm3m (No. 221) (JCPDS card no. 075-0213). No trace of
̅
a (Å)
3.9968(1)
3.9968(1)
4.0280(1)
64.345(3)
3.9993(3)
3.9993(3)
4.0274(5)
64.418(1)
3.9996(3)
3.9996(3)
4.0285(4)
64.451(1)
4.0003(5)
4.0003(5)
4.0320(1)
64.521(3)
impurities has been observed in the XRD patterns of the
catalysts. This implies that the chromium ion substituted to six
coordinated titanium ions in a BaTiO3 lattice. This reveals that a
monophasic system was obtained using a microwave-assisted
hydrothermal method.
b (Å)
c (Å)
cell volume (Å3)
atomic positions
Ba (1a)
The average crystallite size was calculated using Scherrer formula:
x
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
y
0.9λ
D =
z
2 1/2
(B2 − b ) cos θ
(1)
Ti/Cr (1b)
x
0.5000
0.5000
0.5000
0.5000
where D is the average crystallite size, λ is the X-ray wavelength,
θ is the Bragg angle, and B and b are the full width at half
maxima(fwhm)observed for thesampleand standard, respectively.
The average crystallite size evaluated from XRD patterns is given
in Table 1. The average crystallite size of the MH synthesized
catalysts is less as compared to CO catalysts, and it was observed
that the crystallite size of Cr-doped BTO catalysts increased
with the increase in Cr concentration in both CO and MH
synthesized catalysts. This could be due to the substitution of
higher radii Cr ion to the lower radii Ti ionic site in the BaTiO3
host lattice.28 From these results, it was proposed that the
successful synthesis of pure Cr-doped BTO nanocatalysts
without any impurities and having less crystallite size and good
dispersion obtained with MH method was attributed to the rapid
microwave heating.
3.2. Structural Parameters Estimation by Rietveld
Refinement Analysis. The crystal structure and lattice
parameters have been evaluated by the Rietveld refinement
method using powder XRD data of the catalysts in the FullProf
program. The patterns were typically refined for lattice param-
eters, scale factor, backgrounds, pseudo-Voigt profile function
(u, v, and w), atomic coordinates and isothermal temperature
factors (Biso). In order to distinguish between the cubic and
tetragonal phases, the commonly used characteristic peak in
the diffraction pattern, ranges between 2θ = 40−50° region,
where the single cubic (200) peak splits into two tetragonal peaks
(002) and (200). Figure 2 and Figure S1 illustrate the observed,
calculated, and difference XRD patterns of BaTiO3 Cr-doped
BaTiO3 catalysts. There is a good agreement between experi-
mentally observed and calculated XRD patterns (Yobs − Ycal is
close to zero), which is based on the consideration of relatively
lower Rp and Rwp (<10%) values. The refinement result
confirmed that the compounds synthesized using CO method
has tetragonal structure, whereas MH synthesized compounds
belong to cubic symmetry. The lattice parameters, cell volume,
space group, atomic positions, and R-values obtained after
Rietveld refinement process, for BaTiO3 and Cr-doped BaTiO3
catalysts synthesized using conventional oxalate and microwave
method are summarized in Table 2 and Table 3, respectively.
As expected the cell volume of cubic BaTiO3 catalysts is more than
a tetragonal phase. The increase in lattice parameters and cell
volume with an increase in Cr doping content (up to 5 mol %)
confirmed that Cr ions substituted Ti ions in the BaTiO3
host lattice.
y
0.5000
0.5000
0.5000
0.5000
z
0.5243(2)
0.5283(7)
0.5222(8)
0.5323(1)
O1 (1b)
x
0.5000
0.5000
0.5000
0.5000
y
0.5000
0.5000
0.5000
0.5000
z
0.0326(5)
0.0521(2)
0.0182(5)
0.0469(6)
O2 (2c)
x
y
z
0.5000
0.5000
0.5000
0.5000
0.0000
0.0000
0.0000
0.0000
0.4892(8)
0.5189(4)
0.5192(2)
0.5148(4)
R
Factors (%)
Rp
2.94
4.16
2.27
2.59
2.26
3.37
5.83
7.76
2.78
9.30
4.33
7.79
8.29
11.2
1.34
10.7
7.64
70.3
11.7
15.3
1.38
22.3
12.7
123.1
Rwp
Rexp
RBragg
RF
χ2
As shown in Table 2, the lattice parameters of BTO with the
space group P4mm are a = b = 3.9968(1) Å, and c = 4.0280(1) Å,
which are close to the published lattice parameters: a = b =
3.9990(1) Å, and c = 4.0180(2) Å (JCPDS card No. 079-2265).
The lattice parameters found for BTO with the space group
Pm3m (Table 3) are also close to the published data: a = b = c =
̅
4.0335(3) Å (JCPDS card No. 075-0213). In the cubic BaTiO3
perovskite structure, the Ba atoms are in the Wyckoff position 1a,
(0, 0, 0); the Ti atoms in 1b, (0.5, 0.5, 0.5); and the O atoms in 3c
(0.5, 0, 0.5), all special positions. In the tetragonal unit cell, atoms
are displaced in the z-direction, and the cell is elongated along the
c-axis. Atom positions and Wyckoff position for tetragonal
BaTiO3 perovskite are Ba (1a) at (0, 0, 0); Ti (1b) at (0.5, 0.5, z);
O1 (1b) at (0.5, 0.5, z); and O2 (2c) at (0.5, 0, z).
The crystal structure of BaTiO3 and Cr-doped BaTiO3 was
modeled using Rietveld refined structural parameters by the
VESTA program. In cubic BaTiO3, the structure of which is
displayed in Figure 3a, the large barium atom is located at eight
corners, the smaller titanium atom sitting in the body center, and
the oxygen atom in the face center. The resulting perovskite
structure can also be described as consisting of corner sharing
[TiO6] octahedra with the Ba2+ ion occupying the 12-fold
coordination site formed in the middle of the cube of eight
such octahedral, as shown in Figure 3b. The bond length of all six
Ti−O bonds is 2.0146 Å. Similarly in the 5 mol % Cr-doped
D
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Table 3. Rietveld Refined Structural Parameters and Unit
Cell Volumes for BaTiO3 and Cr-Doped BaTiO3 Catalysts
Synthesized Using Microwave Method
catalysts
BTO_5
cubic
Pm3m
BTO_6
cubic
Pm3m
BTO_7
cubic
Pm3m
BTO_8
cubic
Pm3m
crystal system
space group
̅
̅
̅
̅
(No. 221)
(No. 221)
(No. 221)
(No. 221)
lattice parameters
a (Å)
4.0292(3)
4.0292(3)
4.0292(3)
65.410(1)
4.0311(3)
4.0311(3)
4.0311(3)
65.506(1)
4.0362(2)
4.0362(2)
4.0362(2)
65.752(1)
4.0546(3)
4.0546(3)
4.0546(3)
66.657(1)
b (Å)
c (Å)
cell volume (Å3)
atomic positions
Ba (1a)
x
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
y
z
Ti/Cr (1b)
x
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
0.5000
Figure 4. Tetragonal unit cell of (a) BTO_5, (c) BTO_8 and crystal
structure of (b) BTO_5, (d) BTO_8.
y
z
O (3c)
composition, and the Jahn−Teller effect.29 In the tetragonal
BaTiO3 structure (Figure 4a), the barium atom is also located at
eightcorners, titaniumatom atbody center, and oxygen atomatsix
face centers. The titanium atom coordinated to six oxygen (O)
atoms having two types of Ti−O distances. Out of six Ti−O
bonds, two are Ti−O1 with bond length 2.0474 Å, and four are
Ti−O2 with bond length 2.0034 Å. The resulting crystal structure
is shown in Figure 4b. Similarly for the 5 mol % Cr-doped BaTiO3
compound (Figure 4c,d), the chromium atom substitutes for the
titanium atom and is also bonded to six oxygen atoms.
x
y
z
0.5000
0.0000
0.5000
0.5000
0.0000
0.5000
0.5000
0.0000
0.5000
0.5000
0.0000
0.5000
R
Factors (%)
Rp
3.01
3.66
2.29
6.08
2.47
3.84
4.77
3.11
2.76
1.57
3.82
4.70
3.08
3.15
1.65
3.40
4.22
2.29
7.93
3.36
Rwp
Rexp
RBragg
RF
3.3. FT-IR Spectroscopy. FT-IR spectra of BaTiO3 and
Cr-doped BaTiO3 catalysts synthesized by the conventional oxalate
and microwave method are shown in Figure 5. The IR bands
observed in the range between 395 and 420 cm−1 correspond to
characteristic TiOII bending vibrations. The broad absorption band
at 560−620 cm−1 is ascribed to TiOVI stretching vibration con-
nected to the barium.30 The band situated at 855−880 cm−1 can
be assigned to metal−oxygen stretching vibrations.21 In Figure 5a,
the band at 1365 cm−1 observed for the BTO_4 sample can be
attributed to the vibrational mode of CO32− group, indicating the
presence of barium carbonate (BaCO3) impurity in the catalysts.
3.4. Morphological Analysis. The morphology and particle
size of BaTiO3 and Cr-doped BaTiO3 catalysts were studied
by transmission electron microscopy. As shown in Figure 6a−f,
the particles are spherical in shape in all the catalysts. In CO
synthesized BaTiO3, (Figure 6a) the agglomerated particles are
observed, whereas the MH synthesized BaTiO3 (Figure 6b)
particles are dispersed due to the uniform microwave heating
which lead to molecular agitation and hence resist the particles
agglomeration. The average particle size calculated from TEM
images is listed in Table 4. The result reveals that the particle size
increases with an increase in Cr content in BaTiO3 in all the
catalysts from 30.8 to 47.4 nm using the CO method. However,
the particle size of MH synthesized catalysts was observed to be
20.7−34.9 nm, which was less compared to CO synthesized
catalysts. The MH method is the best method compared to
the CO method by virtue of its simple operation and short
reaction times, and it utilizes a cavitation effect to obtain a high
heating rate constant and radio frequency heating from inside to
outside which induces the creating velocity of the powder much
faster than the growing velocity. So the particle size is as small as
Figure 3. Cubic unit cell of (a) BTO_1, (c) BTO_4 and crystal
structure of (b) BTO_1, (d) BTO_4.
BaTiO3 compound, the chromium atom substitutes for the tita-
nium atom and is also bonded to six oxygen atoms (Figure 3c,d).
The ideal cubic perovskite structure is not very common and is
slightly distorted. The distortion of the [TiO6] octahedra may be
due to temperature changes or stress effects that lead to the
transition from cubic structure to tetragonal. Three main factors
are identified as being the possible reason for the distortion in
perovskite structure: size effects of atoms, deviations from ideal
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Figure 5. FT-IR spectra of BaTiO3 and Cr-doped BaTiO3 catalysts synthesized using (a) CO method (b) MH method.
Table 4. Average Particle Size Determined from TEM Images
for BaTiO3 and Cr-Doped BaTiO3 Catalysts
average particle size (nm)
catalysts
conventional
microwave
BaTiO3
30.8
38.3
43.6
47.4
20.7
28.0
31.3
34.9
0.5 mol % Cr-doped BaTiO3
2.5 mol % Cr-doped BaTiO3
5 mol % Cr-doped BaTiO3
confirmed the presence of the Cr element in Cr-doped BaTiO3
catalysts synthesized by both CO and MH methods.
3.5. BET Surface Area by N2 Physisorption. The surface
area of CO and MH synthesized catalysts are given in Table 5.
The surface area of MH synthesized catalysts was increased from
4- to 10-fold, whereas the total pore volume was raised to 10-fold
compared to CO synthesized catalysts. This could be due to
improvement in porosity of the materials synthesized by the MH
method due to uniform heating and faster heating rate throughout
the sample compared to the CO method in which heating at
elevated temperatures for extended periods led to collapse of
pores, and thereby a decrease in surface area and pore volume
will take place. It was found that the surface area of 0.5 mol %
Cr-doped BaTiO3 catalyst was more compared to pure BaTiO3 in
both the CO and MH synthesized catalysts. This could be due to
contribution from Cr content of the catalysts. The decrease in
surface area of the catalysts at high Cr content (2.5 and 5 mol %) in
BaTiO3 was due to formation of larger particles as latter was
confirmed with TEM image particle size analysis of the catalysts
(Table 4). These results indicate that a perovskite with a high
surface area and pore volume having good dispersion of metals in
the matrix can be synthesized using microwave technology.
3.6. X-ray Photoelectron Spectroscopy (XPS) Analysis.
The representative XPS survey spectrum of 2.5 mol % Cr-doped
BaTiO3 CO synthesized catalyst is presented in Figure 8a.
It reveals the presence of C, O, Ba, Ti, and Cr elements in the
catalyst. The peak at the binding energy of 284.52 eV in survey
spectra can be assigned to C 1s which may be due to the presence
of CO2 in the air.32 Apart from Ba 3d peak, the other Ba 4d and
Ba 4p signals observed at 88.52 and 176.78 eV were due to
different barium orbitals present in BaTiO3.32
Figure 6. TEM images of BaTiO3 and Cr-doped BaTiO3 catalysts:
(a) BTO_1, (b) BTO_5, (c) BTO_3, (d) BTO_7, (e) BTO_4, and
(f) BTO_8 catalysts.
40 nm and has a homogeneous distribution.31 TEM particle
size analysis is in good agreement with the calculated XRD crys-
tallite size as both increase with an increase in the Cr content
of the catalysts.
Figure 7a,b illustrates the high-resolution TEM images of
2.5 mol % Cr-doped BaTiO3 catalysts. Clear lattice fringes are
observed with the measured interplanar spacing values of about
0.283 and 0.285 nm, corresponding to the crystallographic plane
(101) of tetragonal BaTiO3 (JCPDS card no. 079-2265, d(101)
=
0.283 nm) and the (110) crystal plane spacing of the cubic BaTiO3
phase (JCPDS card no. 075-0213, d(110) = 0.285 nm), respectively.
Energy dispersive X-ray (EDX) spectra of BaTiO3 and Cr-doped
BaTiO3 catalysts are shown in Figure 7c−e. EDX analysis reveals
the presence of Ba, Ti, and O elements in BaTiO3 and further
Figure 8b illustrates the high-resolution Ba 3d XPS spectra which
reveal two peaks at the binding energy of 778.64 and 793.88 eV.
These are assigned due to splitting of the Ba 3d5/2 and Ba 3d3/2
spin states, and it indicates the oxidation state of Ba is 2+.33
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Figure 7. High-resolution TEM images showing the lattice fringes of Cr-doped BaTiO3 catalysts: (a) BTO_3 and (b) BTO_7 and EDX spectra of
BaTiO3 and Cr-doped BaTiO3 catalysts: (c) BTO_1, (d) BTO_4 and (e) BTO_7.
all the catalysts was due to Cr 2p1/2 of the Cr6+ oxidation state.
Table 5. Physicochemical Properties of Various BTO
In BTO_2, an additional less intense peak at 583.88 eV was
observed, and this was attributed to Cr 2p1/2 of Cr3+. The XPS
results of Cr-doped BaTiO3 catalysts indicate the presence of Cr
in two different oxidation states of 3+ and 6+.
The XPS spectra of oxygen showed a peak at 529.22 eV which
corresponds to O 1s in the BaTiO3 lattice37 (Figure 8e). The XPS
spectra of 2.5 and 5 mol % Cr-doped BaTiO3 catalysts syn-
thesized using the MH method is shown in Figure S2a−e.
The binding energies of Ba 3d, Ti 2p, Cr 2p, and O 1s are
very similar to the CO synthesized Cr-doped BaTiO3 catalysts.
The XPS spectrum of BTO_8 for Cr (Figure S2d) indicates that
Cr is present in +6 oxidation in which the XPS peak corre-
sponding to +3 (∼575 eV) is absent.
3.7. Determination of Acidity Using Pyridine Adsorp-
tion. Acidity of the catalysts was determined by pyridine adsorp-
tion using FT-IR spectroscopy, and the corresponding FT-IR
spectra of all the catalysts are presented in Figure 9a,b and the
results are given in Table 5. FT-IR spectra of the catalysts showed
bands at 1431 and 1634 cm−1. These were assigned to pyridine
chemisorbed to Lewis acid sites on the surface. The amount of
Lewis acidity for BTO_1 and BTO_5 (Table 5) was found to be
327 and 329 μmol/g. It indicates that along with improvement in
Catalysts
BET surface area
total pore volume
(cc/g)
amount of Lewis acidity
catalyst
(m2/g)
(μmol/g)
BTO_1
BTO_2
BTO_3
BTO_4
BTO_5
BTO_6
BTO_7
BTO_8
8.90
20.5
17.6
6.17
71.4
84.6
73.8
69.6
2.54 × 10−3
2.62 × 10−3
2.56 × 10−3
2.20 × 10−3
3.55 × 10−2
4.02 × 10−2
3.77 × 10−2
2.38 × 10−2
327
345
371
382
329
347
373
389
The XPS spectra of Ti 2p (Figure 8c) revealed two peaks.
The strong peak observed at 458.03 eV was ascribed to Ti 2p3/2
and the less intense peak at 463.79 eV corresponds to Ti 2p1/2
both were due to Ti4+ oxidation state.34,35
XPS spectra of Cr-doped BaTiO3 catalysts (Figure 8d) showed
two broad Cr 2p peaks36 at 578.64 and 589.45 eV. After
peak fitting analysis, using Gaussian function the broad peak at
578.64 eV deconvoluted into three peaks at 575.06, 577.85, and
579.95 eV corresponding to Cr 2p3/2 of the Cr3+ and Cr6+
chemical state. The second broad peak observed at 589.45 eV in
,
,
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Figure 8. XPS spectra of BaTiO3 and Cr-doped BaTiO3 CO synthesized catalysts: (a) survey spectra of 2.5 mol % Cr-doped BaTiO3, high resolution
signals of (b) Ba 3d, (c) Ti 2p, (d) Cr 2p, and (e) O 1s elements.
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Figure 9. FT-IR spectra of adsorbed pyridine on (a) CO method (b) MH method.
other properties an increase in the number of acid sites was observed
The activity of catalysts prepared by CO and MH methods was
in the catalysts synthesized bythe MH method as compared to CO
method, and it has been also observed that the amount of Lewis
acidity increased with an increase in Cr content of all the catalysts.
3.8. Catalytic Hydrogenation of Nitrobenzene. Once the
catalysts were thoroughly characterized regarding structure, they
were tested as catalysts for the reduction of nitrobenzene to
aniline with hydrazine hydrate as the reducing agent. During the
course of the reaction, hydrazine hydrate gets converted into
hydrogen and nitrogen gas, and the nitrobenzene is reduced to
aniline. The influence of various parameters such as temperature,
solvents, and catalyst loading was studied for the optimization of
the reaction conditions. The details are given in Table 6. Initially
compared and is shown in Table 7. It can be seen from the table
that the CO synthesized BTO catalyst (BTO_1) shows less
nitrobenzene conversion and aniline yield. To increase the conver-
sion of nitrobenzene and to achieve maximum aniline yield, Cr was
doped into the BTO catalyst and tested for catalytic reduction of
nitrobenzene. It was observed that conversion of nitrobenzene
has been improved with an increase in Cr content up to 2.5%
(BTO_3) and remains constant with a further increase in Cr
content to 5% (BTO_4). The increase in conversion with Cr
content in thecatalysts could be dueto anincrease inLewisacidity,
although the particle size and BET surface area of the catalysts
decreased due to agglomeration of particles with Cr content in the
catalysts. The BTO catalyst (BTO_5) synthesized via the MH
method itself is very active and showed a maximum nitrobenzene
conversion of 99.3% with 98.2% aniline yield. It was observed
from the reaction conducted on Cr-doped BTO MH catalysts
(BTO_6 to BTO_8) that the conversion remained constant with
a slight decrease in the aniline yield. This was due to formation of
byproducts such as azobenzene and azoxybenzene during the
reaction via condensation path.21 From the above results it was
found that the Cr content in BTO facilitates reduction in a shorter
time compared to pure BTO catalysts synthesized via CO and MH
methods. The reason being MH synthesized BTO (BTO_5)
itself highly active compared to CO synthesized BTO catalyst
(BTO_1) could be due to improved properties of reduced particle
size high surface area and pore volume of the catalyst (Table 3),
because of microwave heating.
To study the scope and applicability of the developed protocol
using hydrazine hydrate, reactions of various substituted nitro-
benzene were investigated. Since the reduction of nitrobenzene
to aniline was maximum on BTO_3, reduction of several nitro
aromatics was carried out on this catalyst (Table 8). Excellent
yields of desired products were obtained with substituted nitro-
benzene. It can be seen that the ortho substituted compounds
showed less yield compared to para substituted compounds
which might be attributed to steric hindrance. In the case of
1 chloro, 2, 4 dinitrobenzene selectively one nitrogroup was
reduced leading to the formation of 1 chloro, 2, nitrobenzene
4 amine. Yields were determined by GC-FID and compared
with authentic samples. Quantitative and qualitative analysis of
all amines were made by GC and GC−MS and identified by
comparison with authentic samples.
Table 6. Influence of Temperature, Solvent, and Catalyst
a
Loading on Aniline Yield
sr.
no.
catalyst
(mg)
hydrazine hydrate
(mmol)
temp
(°C)
yield
(%)
solvent
1
80
80
80
80
80
80
80
80
80
60
40
0
30
30
30
30
30
30
30
30
30
30
30
30
20
10
2-propanol
2-propanol
2-propanol
methanol
ethanol
40
60
80
80
80
80
80
80
80
80
80
80
80
80
10
45
100
50
75
46
20
12
5
2
3
4
5
6
ACN
7
DMSO
8
THF
9
toluene
10
11
12
13
14
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
55
<20
0
80
80
68
19
a
Reaction conditions: Nitrobenzene (10 mmol), hydrazine hydrate
(30 mmol), KOH (10 mmol) solvent (10 mL) for 3 h. catalyst
b
BTO_3. GC yield.
80 mg of BTO_3 catalyst was used for optimizing the
temperature and solvent. It was found that 99.7% of the aniline
product was formed at 80 °C in 3 h. The temperature below
80 °C lowers the aniline yield. 2-Propanol was found to be
the best solvent after screening various solvents. It was also
interesting to note that the 75% yield of the product was obtained
in ethanol medium. The catalyst loading of 80 mg was sufficient to
give a maximum yield of the required product. In the absence of
either catalyst or KOH, there was no reaction.
Catalyst Reusability. For the study of the reusability BTO_3
and BTO_7 catalysts were chosen for comparison purposes.
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a
Table 7. Catalytic Reduction of Nitrobenzene to Aniline Conducted over CO and MH Catalysts
b
c
entry
catalyst
reaction time (h)
NB conversion (%)
sel. to AN (%)
sel. to AZ (%) sel. to AZY (%)
3.56
yield of AN (%)
1
2
3
4
5
6
7
8
BTO_1
BTO_2
BTO_3
BTO_4
BTO_5
BTO_6
BTO_7
BTO_8
6
3
3
3
6
3
3
3
80.1
88.3
99.9
99.9
99.3
99.9
99.0
99.0
96.3
100
77.1
88.3
99.7
99.8
98.2
92.4
92.8
90.4
99.7
99.8
98.9
92.5
93.8
91.3
0.273
0.157
1.05
0.90
1.02
1.15
0.17
6.57
4.18
7.57
a
Reaction conditions: nitrobenzene (10 mmol), KOH pellets (10 mmol), N2H4 (30 mmol), catalyst (80 mg), and 2-propanol (10 mL), refluxed for
b
c
2−6 h at 80 °C. Azobenzene (AZ). Azoxybenzene (AZY).
space group without any trace of impurities. The particle size of
the microwave synthesized catalysts deduced from TEM images
is less compared to conventional oxalate catalysts and in the
range of 20−30 nm. XPS analysis revealed that the Cr is present
in 3+ and 6+ oxidation states in all the catalysts. Microwave
synthesized catalysts showed a 4−10-fold increase in surface area
and pore volume compared to oxalate synthesized catalysts.
Catalytic reduction of nitrobenzene to aniline studies reveal that
microwave synthesized BaTiO3 itself is active and showed 99.3%
nitrobenzene conversion with 98.2% aniline yield, whereas
BaTiO3 synthesized by the conventional oxalate method showed
only 80.1% nitrobenzene conversion with 77.1% aniline yield.
Cr-doping into BaTiO3 lattice facilitates the reaction in a short
time in all the catalysts, and its effect is particularly notable in
oxalate synthesized catalysts by achieving 99.9% nitrobenzene
conversion with 99.7% aniline yield on 2.5 mol % Cr-doped
BaTiO3. The increase in conversion was due to increase in Lewis
acidity with Cr content in the catalysts. From the above results
it was concluded that the Cr-doped BaTiO3 nanocatalysts
containing high surface area, pore volume, and reduced particle
size with good dispersion and with high catalytic activity is suc-
cessfully obtained in less time using the microwave assisted
hydrothermal synthesis method. The present study reveals that
the microwave hydrothermal synthesis method would be an
interesting energy efficient technology to explore for the
synthesis of various nanomaterials in their pure form in order
to achieve better physiochemical and catalytic properties of the
materials.
Table 8. Catalytic Reduction of Various Aromatic Nitro
Compounds with BTO_3 Catalyst
a
entry
substrate, R
time (h) NB conversion (%) AN, yield (%)
1
2
3
4
5
6
7
8
9
1-Cl, 2,4-NO2
3-NO2
6
6
100
100
68.2
72.6
77.3
30
4-OCH3
2-OCH3
2-CH3
3
77.3
12
12
4
30
99
80.0
4-CH3
100
100
100
100
100
2-NH2
2
100
100
100
2-OH
4
4-OH
3
a
Reaction conditions: Substrate (10 mmol), KOH pellets (10 mmol),
N2H4 (30 mmol) catalyst (80 mg) and 2-propanol/Ethanol (10 mL),
reflux for 2−6 h at 80 °C.
Table 9. Recyclability of BTO_3 and BTO_7 for Catalytic
Reduction of Nitrobenzene
catalyst
BTO_3
cycle
aniline yield (%)
reaction time (h)
1
2
3
4
5
1
2
3
4
5
100
100
100
100
92
3
3
4
4.2
6
BTO_7
92
3
ASSOCIATED CONTENT
■
92.6
92
3
S
* Supporting Information
3
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.6b00240.
90
4
85
5
Rietveld refinement patterns for BTO_3, BTO_4, BTO_7,
and BTO_8 is given in Figure S1, and XPS spectra of
BTO_7 and BTO_8 catalysts given in Figure S2 (PDF)
It has been observed that the catalyst could be used up to
four times without any loss in the activity; however, product yield
decreased after four cycles.
AUTHOR INFORMATION
■
4. CONCLUSIONS
Corresponding Author
*E-mail: ch.srilakshmi@sscu.iisc.ernet.in.
Notes
Cr-doped BaTiO3 catalysts were successfully synthesized by
using conventional oxalate and microwave assisted hydrothermal
methods. X-ray diffraction studies and Rietveld refinement
results indicate that BaTi1−xCrxO3 catalysts synthesized by the
oxalate method crystallized in a tetragonal BaTiO3 structure with
P4mm space group along with trace amounts of impurities
corresponding to titanium oxide, barium carbonate, and
chromium oxide, whereas microwave synthesized catalysts
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Department of Science
and Technology (DST) India through INSPIRE project. C.S.L.
would like to thank Prof. Ramasesha for the support of this work.
were crystallized in pure cubic BaTiO structure with Pm3m
̅
3
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