Effect of the nature of a transition metal dopant in BaTiO3 perovskite on the catalytic reduction of nitrobenzene

Article abstract of DOI:10.1039/c5ra06864a

In the present study, we have synthesized Fe, Co and Ni doped BaTiO₃ catalyst by a wet chemical synthesis method using oxalic acid as a chelating agent. The concentration of the metal dopant varies from 0 to 5 mol% in the catalysts. The physical and chemical properties of doped BaTiO₃ catalysts were studied using various analytical methods such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), BET surface area and Transmission electron microscopy (TEM). The acidic strength of the catalysts was measured using a n-butylamine potentiometric titration method. The bulk BaTiO₃ catalyst exhibits a tetragonal phase with the P4mm space group. A structural transition from tetrahedral to cubic phase was observed for Fe, Co and Ni doped BaTiO₃ catalysts with an increase in doped metal concentration from 1 to 5 mol%. The particle sizes of the catalysts were calculated from TEM images and are in the range of 30-80 nm. All the catalysts were tested for the catalytic reduction of nitrobenzene to azoxybenzene. The BaTiO₃ catalyst was found to be highly active and less selective compared to the doped catalysts which are active and highly selective towards azoxybenzene. The increase in selectivity towards azoxybenzene is due to an increase in acidic strength and reduction ability of the doped metal. It was also observed that the nature of the metal dopant and their content at the B-site has an impact on the catalytic reduction of nitrobenzene. The Co doped BaTiO₃ catalyst showed better activity with only 0.5 mol% doping than Fe and Ni doped BaTiO₃ catalysts with maximum nitrobenzene conversion of 91% with 78% selectivity to azoxybenzene. An optimum Fe loading of 2.5 mol% in BaTiO₃ is required to achieve 100% conversion with 93% selectivity whereas Ni with 5 mol% showed a conversion of 93% and a azoxybenzene selectivity of 84%. This journal is

Full text of DOI:10.1039/c5ra06864a

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Graphical extract
Highly active and selective Fe, Co and Ni doped BaTiO3 catalysts for catalytic reduction of nitrobenzene
to azoxybenzene was reported

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Effect of nature of transition metal dopant in BaTiO3 perovskite on catalytic
reduction of nitrobenzene
Chilukoti Srilakshmi*
a, G. Mohan Raob and Rohit Saraf a
aSolid State and Structural Chemistry Unit (SSCU), Indian Institute of Science (IISc), Bangalore, Karnataka, India, 560012
bDepartment of Instrumentation and Applied Physics, Indian Institute of Science (IISc), Bangalore, Karnataka, India, 560012
Email:ch.srilakshmi@sscu.iisc.ernet.in
1

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Abstract
In the present study, we have synthesized Fe, Co and Ni doped BaTiO3 by a wet chemical
synthesis method using oxalic acid as chelating agent. The concentration of metal dopant varies
from 0 to 5 mol. % in the catalysts. The physical and chemical properties of doped BaTiO3 was
studied using various analytical methods such as Xꢀ ray diffraction (XRD), Fourier transform
infrared spectroscopy (FTꢀIR), BET surface area and Transmission electron microscopy (TEM).
Acidic strength of the catalysts was measured using potentiometric titration method. Bulk
BaTiO3 crystalline phase exhibit tetragonal phase with P4/mmm space group. Structural
transition from tetrahedral to cubic phase was observed for Fe, Co and Ni doped BaTiO3
catalysts with increase in metal concentration from 1 to 5 mol.%. Particle size of the catalysts
were calculated from TEM are in the range of 30ꢀ80 nm. All the catalysts were tested for the
catalytic reduction of nitrobenzene to azoxybenzene. BaTiO3 catalyst was found to be highly
active and less selective compared to doped catalysts which are active and highly selective
towards azoxybenzene. The increase in selectivity towards azoxybenzene is due to increase in
acidic strength and reduction ability of the doped metal. It was also observed that nature of metal
dopant and their content at Bꢀsite has an impact on the catalytic reduction of nitrobenzene. Co
doped BaTiO3 catalyst showed better activity with only 0.5 mol. % doping than Fe and Ni doped
BaTiO3 catalysts with maximum nitrobenzene conversion of 91% with 78% selectivity to
azoxybenzene. An optimum Fe loading of 2.5 mol. % in BaTiO3 is required to achieve 100%
conversion with 93% selectivity whereas Ni with 5 mol.% showed conversion of 93% and a
azoxybenzene selectivity of 84%.
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Introduction
BaTiO3 (BTO) is one of the most important ferroelectric material with a wide range of
applications in nonlinear optics, multilayer ceramic capacitors (MLCs), actuators, transducers,
ferroelectric random access memories (FRAM), microꢀelectro mechanical systems (MEMS),
PTC thermistors, microwave devices, different type storage information devices, infrared sensors
etc. 1ꢀ6 BTO has a general ABO3 type structure where A and B are cations of different sizes with
6 fold coordinate B cation in the middle and the 12 fold coordinate A cation in the corner and the
anion, commonly oxygen, in the center of the space. The phase of BTO at room temperature is
tetragonal and it transforms to cubic above Curie temperature Tc ~ 130°C. It also exists in
orthorhombic phase at 0 °C and rhombohedral phases at below ꢀ90 °C and above 1460 °C it
exists in hexagonal phase.7 It has been reported that the ferroelectric properties of BTO can be
efficiently controlled by doping with different elements and the reported literature on the Fe, Co
and Ni doped BTO catalyst are limited to studying electric and magnetic properties of these
materials.8ꢀ11 To the best of our knowledge there are only very few or no reports on the catalysis
of these materials mainly for liquid phase organic transformations. Recently we have published
Fe doped BaTiO3 catalyst for the catalytic reduction of nitrobenzene.12 In this context, present
study is carried out to explore these materials for the catalytic reduction of nitrobenzene and
study the effect of nature of dopant on the catalytic property. In continuation of our previous
work we have explored doping of various metals such as Co and Ni including Fe with lower
metal loading of 0.5 to 5 mol.% in BaTiO3 in order to invistgate the effect of metal dopant in Bꢀ
site on the physicoꢀchemical and catalytic properties of BaTiO3 material.
The catalytic reduction of nitroarenes is an important reaction and the products such as
aromatic amines or azoxy compounds are used in producing agrochemicals, pharmaceuticals,
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dyes, pigments etc.13ꢀ15 and continuous efforts have been devoted to develop efficient production
techniques. Since the conventional nonꢀcatalytic routes that use either the Fe/HCl system
15
(Bechamp procedure) or metal sulfides inevitable generate large amount of wastes, and hence
green and environmentally friendly selective reduction process is desired. Noble metal based
such as Au, Pt, Pd 16ꢀ19 are found to be highly effective catalysts for hydrogenation/reduction.
However, the high price and scarcity of these precious metals are main drawbacks that hindered
their large scale industrial application and these metals are sensitive to moisture and air when
they are in complex forms.20 As an alternative, there is a demand to use a number of cost
effective metals hence in the present study we have explored Fe, Co and Ni doped BaTiO3
catalysts for the selective reduction of nitrobenzene to azoxybenzene.
Experimental
Materials
All the metal nitrates and oxalic acid were purchased from Merck. Titanium isopropoxide was
purchased from Sigma Aldrich. All chemicals are of analytical grade and used without further
purification.
Synthesis of pure and metal doped BaTiO3 (BTO) nanoparticles
The synthesis of BTO nanopowders by oxalate route 21 was carried out starting with titanium
isopropoxide and barium nitrate as metal oxide precursors, and oxalic acid as chelating agent,
and 2ꢀpropanol as the solvent. These materials were mixed in a molar ratio Ti (OC3H7)4 : Ba
(NO3)2 : C2H2O4·2H2O =1:1:2 in a mixture of 2ꢀpropanol and water. A homogeneous solution
was obtained after stirring at 60 °C for 1h. Then the solution was evaporated to dryness at 100
°C. The oxalate precursors were then dried at 120 °C for overnight and calcined at 900 °C for 12
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h for obtaining BaTiO3 nanopowders. For metal substituted BaTiO3 catalysts similar method was
followed except the stoichiometric addition of dopant in the form of metal nitrate precursor to
the above solution in the following ratio Ba(NO3)2 : 1ꢀxTi(OC3H7)4 : x M(NO3)2 : C2 H2O4·2H2O
where x = 0.0 ≤ x ≤ 0.05 (where M = Fe, Co, Ni), respectively.
Catalyst characterization
The catalysts were characterized by xꢀray diffraction recorded on a JEOL JDXꢀ8030 xꢀ
ray diffractometer using CuKα radiation. The BET surface area of the catalysts was measured
using a ModelꢀQuantachrome Autosorb iQ2 automated gas sorption analyzer. The catalysts were
initially pretreated in N2 at 300 °C for 6 h and the surface area of the catalysts was obtained by
adsorbing N2 at liquid nitrogen temperature, ꢀ196 °C.
The infrared absorption spectra were measured at room temperature, in the wave number
range of 4000 to 400 cmꢀ1 by a computerized spectrometer type Perkin FTIRꢀ300 using the KBr
pellet technique. The particle size and morphology of the powders were studied using the
transmission
electron
microscope
(Model
FEI
Technai
T20).
Acidity of the solids was measured by means of potentiometric titration method. A known mass
of solid suspended in acetonitrile was stirred for 3 h and then the suspension was titrated with a
solution of 0.05N nꢀbutylamine in acetonitrile, at a flow rate of 0.02 ml/min. The variation in
electrode potential was measured with an instrument having a digital pH meter using saturated
calomel electrode. The acidity measured by this method enables determination of total number of
acidic sites and their strength. In order to interpret the results it is suggested that the initial
electrode potential (Ei) be taken as the maximum acidic strength and the range where the plateau
is reached (meq/g) can be considered as the total number of acid sites. 22 The acidic strength of
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the surface sites can be assigned as according to the following ranges.23 Very strong site, Ei >
100 mV; strong site, 0 < Ei < 100 mV; weak site ꢀ100 < Ei < 0 mV; very weak site, Ei < ꢀ100
mV.
Catalytic activity
Catalytic reduction with 2ꢀpropanol as hydrogen donor was carried out under continuous
stirring in a twoꢀnecked 100 ml round bottom flask fitted with a reflux condenser. Analysis was
done by withdrawing definite aliquots of the sample at regular intervals. In a typical run, 150 mg
of the catalyst was dispersed in a solution containing 20 mmol of nitrobenzene, 20 mmol of
KOH pellets and 20 ml of 2ꢀpropanol. The mixture was stirred and heated under reflux for 2–6 h
in an oil bath. GCMS analysis of the product samples was carried out using Thermo Trace GC
Ultra (GC), Thermo DSQ II (MS); with DB5 MS column of 30m L x 0.25mm ID x 0.25 ꢀm film
thickness and with Electron Multiplier detector.
Results and discussion
Powder X-ray diffraction
Figure 1. shows the PXRD patterns of BaTi1ꢀxMxO3 (M = Fe, Co, Ni; 0.0 ≤ x ≤ 0.05) powders
calcined at 900 °C for 12 h. From PXRD patterns, it can be clearly seen that the BaTiO3 (BTO)
exhibit tetragonal symmetry with space group P4/mmm. BaTiO3 phase in the sample is
confirmed by comparing the d values with the standard pattern JCPDS no. 075ꢀ0462. XRD
patterns of the 0.5 mol. % metal doped BTO catalysts were matching with the tetragonal phase of
BaTiO3 whereas structural transition from tetragonal to cubic phase of BTO was observed with
further increase in metal dopant concentration from 1 to 5 mol. % and matched with the
corresponding standard pattern JCPDS no.074ꢀ1962 which indicates doping at B site and
hereafter the doped catalysts were represented as BTOꢀMꢀ with the number represents mol. % of
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metal content. Traces of TiO2 was found as impurity at 2θ of 28.46° and 29.62° in the catalysts.
The average crystallite size of the catalysts were calculated using Scherer formula:
0.9
λ
ꢀ ꢁ
ꢂꢃꢄꢅꢆ
where D is the average grain size, λ=1.541 Å (Xꢀray wavelength), and β is the width of the
diffraction peak at half maximum for the diffraction angle 2θ. The average crystallite size of the
doped catalysts calculated from XRD (Table 2) was in the range of 30 – 50 nm whereas the size
of the BTO sample was 81 nm.
Figure 1. XRD spectra of BTO and various metals doped BTO catalysts
The cubic and tetragonal structures of BaTiO3 were detected in the bulk and metal doped
catalysts. In order to distinguish between two phases, the commonly used characteristic peak in
the diffraction pattern, ranges between 2θ = 40° – 50° region, where the single cubic (200) line
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splits into two tetragonal peaks (200) and (002). For detailed structural analysis and to confirm
doping XRD data was further analyzed by Reitveld refinement using FullProf suite programme
and the refined structural parameters are given in Table 1. Figure. 2 show the experimental and
calculated XRD pattern for tetragonal BTO and cubic BTOꢀNiꢀ1. The fitting between the
experimental spectra and calculated values is relatively good based on the consideration of
relatively lower Rp and Rwp (< 10%) values. The unit cell parameters (crystal system, cell
volume, space group and lattice parameters) are estimated by utilizing pseudoꢀvoigt function. It
was observed that the pure BaTiO3 crystallized in the tetragonal structure with P4mm space
group. When the dopant metal (Fe, Co and Ni) concentration is 0.5 mol.%, all the diffraction
peaks are consistent with the tetragonal phase but the cell volume increased as compared to pure
BaTiO3. The phase transformation from tetragonal to cubic takes place, when the dopant metal
concentration increased from 1 to 5 mol.%. The cell volume of cubic Fe, Co and Ni (1ꢀ5 mol.%)
doped BaTiO3 with space group Pmꢀ3m is higher than pure tetragonal BaTiO3. The increase in
cell volume in the doped catalysts confirmed that the transition metal is being successfully
incorporated into the crystal structure of BaTiO3.
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Table 1. Refined structural parameters for BTO and various metal doped BTO catalysts.
Volume in (Å3)
Lattice parameter (Å)
Sample
Space group
a
c
BTO
P4mm
P4mm
P4mm
P4mm
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
Pmꢀ3m
3.9968(1)
3.9903(3)
4.0014(2)
4.0016(2)
4.0099(4)
4.0104(3)
4.0117(1)
4.0124(3)
4.0118(3)
4.0129(2)
4.0095(2)
4.0103(4)
4.0100(3)
4.0280(1)
4.0239 (3)
4.0265 (3)
4.0253 (3)
4.0099(4)
4.0104(3)
4.0117(1)
4.0124(3)
4.0118(3)
4.0129(2)
4.0095(2)
4.0103(4)
4.0100(3)
64.345 (3)
64.352 (9)
64.471 (7)
64.458 (7)
64.481 (1)
64.501 (7)
64.565 (3)
64.596 (9)
64.568 (9)
64.625 (5)
64.459 (5)
64.498 (1)
64.483 (8)
BTOꢀFeꢀ0.5%
BTOꢀCoꢀ0.5%
BTOꢀNiꢀ0.5%
BTOꢀFeꢀ1%
BTOꢀCoꢀ1%
BTOꢀNiꢀ1%
BTOꢀFeꢀ2.5%
BTOꢀCoꢀ2.5%
BTOꢀNiꢀ2.5%
BTOꢀFeꢀ5%
BTOꢀCoꢀ5%
BTOꢀNiꢀ5%
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(a)
(b)
Figure 2: The observed, calculated and difference diffraction profiles from Rietveld refinement
of (a) tetragonal BTO (b) cubic BTO phase in BTOꢀNiꢀ1%.
Fourier Transform Infrared Spectroscopy
The FTꢀIR spectra of the catalysts measured are shown in Figure 3. The IR spectrum of the
catalysts consists mainly of two regions: the first region shows bands (not shown here) at circa
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3428ꢀ3436 which was due to the OH stretching vibration (ν) of surface hydroxyls of the barium
titaniate catalyst. The TG results (Figure S2, ESI) clearly suggest that there is no surface H2O is
present below 150°C. However the total weight loss of 0.6 % was observed below 900 °C.
Hence the same has been ascribed to surface hydroxyls in the FTꢀIR. The second region, in the
range between 392ꢀ 600cmꢀ1, represents the characteristic infrared absorptions of the TiꢀO
vibrations. The band situated around ~539 cmꢀ1 is due to TiO6 stretching vibration connected to
the barium. The additional peaks at 857 cmꢀ1 in the may be due to metalꢀoxygen band
stretching. Finally, the peak at 396 cmꢀ1 can be attributed to normal TiOII bending vibrations.
Additional peaks corresponding to FeꢀO, CoꢀO and NiꢀO were not observed in the spectra of the
metal doped BTO catalysts.
Figure 3. FTIR spectra of BTO and various metals doped BTO catalysts
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Transmission electron microscopy
Figure 4. shows the transmission electron micrographs of the represent catalysts. Average grain size in the
catalysts are in the range of 15ꢀ80 nm. Shape of the particles is spherical as well as cubic and is highly
agglomerated. Grain size of the Co doped BTO catalysts (15ꢀ20 nm) was less compared to Fe and Ni as
evident from Figure 4b and 4e. The decrease of grain size for Coꢀdoped BTO catalysts could be due to
suppression of oxygen vacancies due to charge compensation mechanism which results in slower oxygen
ion motion and consequently lower grain growth rate. The same has been reported earlier for various
24, 25
metal doped in BiFeO3.
(a)
(d)
(c)
(b)
(e)
(f)
Figure 4. TEM images of the various doped BTO catalysts (a) BTOꢀFeꢀ1(b) BTOꢀCoꢀ1(c) BTOꢀ
Niꢀ1(d) BTOꢀFeꢀ 5(e) BTOꢀ Coꢀ5(f) BTOꢀNiꢀ5
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BET surface area and Acidic strength of catalysts
BET surface areas measured by N2 physisorption and acidic strength of the catalysts are given
in Table 2. Surface area of BTO was found to be 1.8 m2/g. The metal doped BaTiO3 catalysts
showed slight increase in the surface area measured by BET method. As the metal content
increased in the BaTiO3 the surface area also increased. This clearly suggests that the doped
metal is providing additional surface to the BaTiO3.
Table 2. Physicoꢀchemical properties of various metals doped BTO catalysts
Sample name
Surface
area
Crystallite size (nm)
by XRD
Acidic strength, E (mV)
(m2/g)
BTO
1.8
2.0
10
12
3.8
13
81.01
34.57
30.86
51.39
34.06
37.32
31.99
39.22
37.62
35.54
49.94
32.35
38.59
175
193
221
204
203
191
219
196
218
257
194
200
192
BTOꢀFeꢀ0.5%
BTOꢀCoꢀ0.5%
BTOꢀNiꢀ0.5%
BTOꢀFeꢀ1%
BTOꢀCoꢀ1%
BTOꢀNiꢀ1%
BTOꢀFeꢀ2.5%
BTOꢀCoꢀ2.5%
BTOꢀNiꢀ2.5%
BTOꢀFeꢀ5%
BTOꢀCoꢀ5%
BTOꢀNiꢀ5%
14
4.2
15.5
16.6
5.8
18
24
Acidic strength values, expressed as E (mV) are given in Table 2. It was observed from the table
that strong or very strong acidic sites were present in all the catalysts since Ei values of all the
catalysts were greater than 100 mV and it was observed that acidic strength of the doped
catalysts was higher than that of pure BTO catalyst. This clearly indicates that doping enhanced
the acidic strength of the catalysts.
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Catalytic reduction of nitrobenzene to azoxybenzene
Blank reaction without catalyst has been carried out and no conversion of nitrobenzene has been
observed under similar reaction conditions.
Activity of Fe doped BTO catalysts
It was observed from the activity profiles of various catalysts (Figures 5 and 6) that pure BTO
catalyst was found to be more active and less selective with 98% conversion and 60% selectivity
towards azoxybenzene. To increase the selectivity of BTO catalyst it has been doped with
transition metals such as Fe, Co, Ni with varying metal loading in between 0.5 to 5 mol.%. It was
observed that the selectivity was greatly improved with the compromise in nitrobenzene
conversion. The results showed that with increase in Fe content (0 to 2.5%) in the BTO catalyst
the conversion has been increased and reached to 100% over BTOꢀFeꢀ 2.5 and it remained
constant with further increase in Fe content to 5%. Whereas the selectivity to azoxybenzene
increased with increase in Fe content in BTO catalyst and showed a maximum selectivity of 93
% on BTOꢀFeꢀ 2.5 and it slightly decreased to 82 % on BTOꢀFeꢀ 5 while conversion remained
constant. The decrease in selectivity on BTOꢀFeꢀ5% was due to formation of side products such
as aniline and azobenzene respectively.
Activity of Co doped BTO catalysts
It was observed that Co doped (Figures 5 and 6) BTO catalysts showed high nitrobenzene
conversion of 91 to 92% on BTOꢀCoꢀ 0.5 and 1%. With further rise in Co loading (BTOꢀCoꢀ2.5
and 5 %) the conversion has been reduced to 72% on Co 2.5 % and remained constant with
further increase in Co content. Maximum selectivity of 78ꢀ80% was obtained on all the catalysts
irrespective of the increase in Co content. The results reveal that high conversion for lower Co
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content (BTOꢀCoꢀ 0.5%) compared to Fe and Ni doped BTO catalysts where the higher
conversion observed at higher metal content could be due to less particle size of 20 nm as
observed from TEM and XRD crystallite size 30.86 nm (shown in Table 2) and the selectivity to
azoxybenzene remained constant with increase in Co content in the catalysts it reveals that
selectivity is independent of Co amount at higher loadings in the catalysts.
Activity of Ni doped BTO catalysts
It was observed from activity profiles (Figures 5 and 6) of Ni doped BTO catalysts that
conversion increased with increase in Ni content and showed a conversion of 88% over BTOꢀNiꢀ
1% thereafter there is a sudden drop in conversion to 56.8% was observed on BTOꢀNiꢀ2.5% and
it increased with further rise in Ni content of BTOꢀNiꢀ 5% and achieved a maximum conversion
of 93% and selectivity of 84% towards azoxybenzene.
It was concluded that nature of metal dopant and it’s percent loading at Bꢀsite has an impact on
the catalytic reduction of nitrobenzene. And it was observed that Co required a relatively lower
amount of 0.5 % in BTO catalyst in order to achieve good conversion and selectivity compared
to Fe and Ni doped catalysts. Fe doped catalysts showed maximum conversion and selectivity at
an optimum loading of Feꢀ2.5%. Whereas in order to achieve better conversion and selectivity
the percentage of Ni required is 5%.
Correlation between acidic strength and yield of azoxybenzene
The bulk BaTiO3 has showed the conversion of 98% and azoxybenzene selectivity of 60 %. By
doping the metals Fe, Co and Ni into BaTiO3 the selectivity showed considerable increase. There
is no direct correlation between conversion and acidic strength or number of acid sites was
observed (Figure S3, ESI). This is due to the fact that the metals are facilitating the reduction of
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nitrobenzene and thus there was no direct relationship with conversion is observed. However we
have found a good correlation between acidic strength of catalysts and yield of azoxybenzene
was observed and the same has been shown in Figure 7. It was observed that with the increase in
acidic strength of the catalyst the azoxybenzene yield increased initially and obtained a
maximum yield of 93% at acidic strength of 196 mV on BTOꢀFeꢀ2.5% and further increase in
acidic strength leads to decrease in yield of azoxybenzene this was due to formation of side
products.
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Figure 5. Activity profiles of various BTO catalysts for catalytic reduction of nitrobenzene (with
metal loading of 0.5 and 1%)
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Figure 6. Activity profiles of various BTO catalysts for catalytic reduction of nitrobenzene (with
metal loading of 2.5 and 5%)
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100
80
60
40
20
0
Series1
Poly. (Series1)
150
170
190
210
230
250
270
Acidic strength (mV)
Figure 7: Correlation between Acidic strength and Yield of Azoxybenzene for BTO and metal
doped BTO catalysts.
The most active catalyst i.e. BTOꢀFeꢀ2.5 was tested for the reusability (Table 3) and it was found
that it could be reused without affecting the activity till at least three cycles.
Table 3: Recyclability of BTOꢀFeꢀ2.5 % catalyst for the catalytic reduction of nitrobenzene
Sr. No Cycle Conversion Selectivity to Azoxybenzene (%)
1
2
3
4
5
0
1
2
3
4
100
100
100
100
83
93
91
92
90
75
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Proposed mechanism
-
O
O
OH
BaTiO3
.
.
H
Ar
Ar
Ar
N
N
N
-
-
-
O
O
O
BaTiO3
OH
H
2H
Ar
Ar
N
N
H
Ar
N
-H2O
-
O
OH
O
Phenyl hydroxylamine
Nitrosobenzene
Condensation reaction
-H2O
O
2H
2H
Ar
N
Ar
Ar
N
N
Ar
N
Ar NH
Ar
NH
-H2O
Azoxybenzene
Azobenzene
Hydrazobenzene
2H
Ar NH2
Aniline
Figure 8: Proposed mechanism for catalytic reduction of nitrobenzene over BaTiO3
A reaction mechanism was proposed for catalytic reduction of nitrobenzene in the presence of
BaTiO3 catalyst and 2ꢀpropanol as proton donor was shown in Figure 8. Reduction of
nitrobenzene to azoxybenzene was due to electron transfer, followed by proton transfer and
elimination of water molecule generates nitroso benzene intermediate which undergoes further
two rounds of electron transfer and protonation to form phenyl hydroxylamine. Further
condensation reaction between nitrosobenzene and phenyl hydroxylamine can lead to
azoxybenzene which on further protonation and catalytic cleavage leads to azobenzene and
aniline products
.
20

RSC Advances
Page 22 of 24
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DOI: 10.1039/C5RA06864A
Conclusions
In the present study, various transition metals such as Fe, Co, and Ni doped BaTiO3 has been
synthesized and the effect of metal dopant on the catalytic reduction of nitrobenzene was tested.
Pure BaTiO3 exists as tetragonal phase with P4/mmm space group. Structural transition from
tetrahedral to cubic phase was observed in Fe, Co and Ni doped BaTiO3 catalysts with metal
content from 1 to 5 mol.%. The particle size of the catalysts calculated from TEM is in the range
of 15 ꢀ 80nm. BaTiO3 was found to be highly active and less selective for catalytic reduction of
nitrobenzene compared to doped catalysts which are highly active and highly selective towards
azoxybenzene. The results showed that the nature of metal dopant and their loading at Bꢀsite has
an impact on the catalytic reduction of nitrobenzene. Based on the activity results, it is concluded
that Co doped BaTiO3 showed good conversion and selectivity at relatively lower Co content of
0.5 % compared to Fe and Ni doped catalysts with maximum 91% nitrobenzene conversion and
80% azoxybenzene selectivity. Fe doped BaTiO3 catalyst required an optimum loading of 2.5% to
achieve a maximum 100% conversion with 93% azoxybenzene selectivity. Whereas Ni dopant
required a loading of 5 % to obtain maximum conversion of 93% with 85% selectivity towards
azoxybenzene. Acidity measurements revealed that all the BaTiO3 catalysts contain very strong
acidic sites and acidic strength of the doped catalysts was higher than that of pure BTO catalyst.
This clearly indicates that doping enhanced the acidic strength of the catalysts. A good
correlation between acidic strength and yield of azoxybenzene was obtained. The catalytic
results obtained in the present study indicate that transition metal doped BaTiO3 material would
be an interesting nanomaterial to explore for the catalytic reduction/ hydrogenation of various
organic compounds.
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Page 23 of 24
RSC Advances
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DOI: 10.1039/C5RA06864A
Acknowledgements
This work was financially supported by Department of science and technology (DST) India
through INSPIRE project. We would like to thank Mass Spectrometry Facility, Division of
Biological Sciences, Indian Institute of Science for GCꢀMS analysis.
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