Advanced Powder Technology 31 (2020) 2385–2393
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Advanced Powder Technology
journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Magnetic and catalytic properties of Cu-substituted SrFe12O19
synthesized by tartrate-gel method
P.N. Anantharamaiah a,⇑, N. Sarath Chandra a, H.M. Shashanka a, R. Kumar b, B. Sahoo b,⇑
a
b
Department of Chemistry, Faculty of Mathematical and Physical Sciences, M. S. Ramaiah University of Applied Sciences, Bangalore 560058, India
Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history:
Received 9 January 2020
Received in revised form 19 March 2020
Accepted 3 April 2020
Available online 16 April 2020
Keywords:
Strontium hexaferrite
Cu-substitution
Structure
Magnetic properties
Catalytic reduction of 4-nitrophenol
a b s t r a c t
Cu-substituted strontium-hexaferrites with chemical compositions SrFe12-xCuxO19 (0 x 0.8, x = 0, 0.2,
0.4, 0.6 and 0.8) were synthesized successfully by the tartrate-gel route, followed by calcination at 850 °C
for 2 h. All the Cu-substituted compositions have single phase with hexagonal crystal structure and
P63/mmc space group, but parent compound shows a small amount of hematite impurity phase. This
suggests that introduction of Cu helps in reducing the calcination temperature to obtain a single phase
compound. Furthermore, all the samples are nanocrystalline in nature with average crystallite size
ranging from 40 to 50 nm. The variations in the magnetic parameters with increase in the copper content
in the Sr-hexaferrite structure indicate the occupation of Cu at octahedral interstitial site. The above
nanocrystalline-hexaferrite powders were employed as catalysts for the reduction of 4-nitrophenol to
4-aminophenol, in aqueous medium. The progress of the catalytic reaction was examined by
UV-visible absorbance spectroscopy at different intervals of time. The results indicated that the parent
compound (SrFe12O19) was catalytically inactive, while all other Cu-substituted samples exhibited good
catalytic activities in transforming 4-nitrophenol into 4-aminophenol. Our results reveal that the
catalytic property could be induced in Sr-hexaferrite samples by Cu-substitution at octahedral site.
Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder
Technology Japan. All rights reserved.
1. Introduction
Ever since the discovery of M-type Sr-hexaferrite (SrFe12O19) at
the Philips research laboratory [1], it has received immense
attention among the researches owing to its hard magnetic
characterestics, chemcial stability and low cost of production [2].
Along with Sr-hexaferrite, the most commonly studied hard
magnetic M-type hexaferrites are Ba-hexaferrite, Pb-hexaferrite
and their combinations for various applications including
microwave absorption [3–5]. Sr-hexaferrite belongs to hexagonal
crystal structure wherein spinel (S ¼ ½Fe6 O8 2þ Þ and hexagonal
(R ¼ ½SrFe6 O11 2 Þ layers have been stacked alternatively. The O2
ions form the closed-packed layers, with the Sr2+ substituting for
an O2 in the hexagonal layer [6,7]. The Fe3+ ions are distributed
in the five crystallographically distinguishable sites such as three
octahedral (2a, 12 k and 4f2) one tetrahedral (4f1) and one trigonal
bipyramidal (2b) interstitial sites [6,7]. The superexchange magnetic interactions between these sub-lattices occur via oxygen
⇑ Corresponding authors.
E-mail addresses: anantharamaiah.cy.mp@msruas.ac.in (P.N. Anantharamaiah),
bsahoo@iisc.ac.in (B. Sahoo).
atoms leading to the ferrimagnetic ordering of the Fe magnetic
moments [7]. Due to the low cost of production, high eletrical
resistivity and high magnetic coercivity, the material can be
exploited as active component to design high-performace recording medium, in telecommunication and for high-frequency microwave and optical devices [8–12]. Furthermore, due to the high
coercivity (ranging from 500 to 6500 Oe) of Sr-ferrite emanating
from the high magnetocrystalline anisotropy with easy magnetization axis along the c-direction, it has been projected as suitable
alternative to the costlier neodymium alloy based permanent magnets. Hence, constant efforts were made to improve the magnetic
parameters (Ms, Hc, Mr, [BH]max) of strontium ferrites via numerous
routes that tune the crystallite size, morphology, microstructure,
chemical substitution, self-composite etc., by adapting suitable
synthesis and processing conditions [13–18].
A few authors demonstrated the catalytic properties of
Sr-hexaferrites and related ceramic powders for the photodegradation of various organic pollutants. Mishra et al. were able
to degrade the methylene blue dye solution in a single step by
employing semiconducting SrFe12O19 as an efficient photocatalyst
under visible light illumination [19]. Mohammadi et al. have synthesized SrFe12O19 nanoparticles through coprecipitation route
https://doi.org/10.1016/j.apt.2020.04.004
0921-8831/Ó 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393
and used them as photocatalysts for the degradation of methyl
orange under visible light irradiation, and observed enhancement
in the degradation [20]. Xie et al. have illustrated that the photocatalytic degradation of methylene blue can be enhanced by making a
suitable composite between Bi2O3 with SrFe12O19 phases, it has
been suggested that the formation of p-n heterojunction between
p-type Bi2O3 and n-type SrFe12O19 is responsible for the enhancement [21]. Kaur et al. reported the photocatalytic degradation
(photon-Fenton degradation) of three organic pollutants such as
methyl orange, remazole deep red and p-nitrophenol using Mnsubstituted Sr-hexaferrites, SrMnxFe12-xO19 (0 x 5.0), as photocatalysts synthesized by the chemical co-precipitation route,
wherein the authors have shown that Mn-doped Sr-hexaferrites
are highly efficient in degrading the pollutants [22].
Spinel ferrites including magnesium, nickel, cobalt and copper
ferrites have been used as catalysts to perform the reduction of
hazardous para-nitrophenol using NaBH4 as the reducing agent,
under mild reaction conditions [23–25]. The catalytic properties
in the spinel ferrite systems are governed by various factors such
as cation distribution, particle size, morphology, surface area, etc.
Recently, it was shown that catalytic properties can be induced
in the nickel ferrite system by replacing a small fraction of Fe by
Cu [26]. The enhanced catalytic properties in the metal substituted
spinel ferrite systems have been attributed to the presence of substituents at the octahedral sites, since the octahedral sites are
exposed to the surface of the particles that facilitates the reduction
at a faster rate. Earlier reports demonstrate that the hazardous 4nitrophenol can be reduced by using metallic nanoparticles such
as Au and Ag-Cu as catalysts [27–28]. However, the major concern
associated with these non-magnetic particles is their recovery for
reuse after the catalytic application. Considering this aspect,
herein, we demonstrated that Sr-hexaferrite system, which is
rather chemcially inactive, can be made catalytically active for
the 4-nitrophenol reduction, by empoying substitution of Cu in
Sr-hexaferrite samples. Hence, the prime objective of the present
work is to induce catalytic activity in strontium hexaferrite
through the compositional modification by substituting Cu for Fe.
Due to the strong preference of Cu to occupy the octahedral sites
of the strontium hexaferrite lattice and the variable oxidation
states of Cu, catalytic activity of the Cu-substituted strontium hexaferrites towards 4-nitrophenol reduction can be expected to be
enhanced. Simultaneously, another important objective of the present study is to synthesize the single-phase samples of Cusubstituted strontium hexaferrites at low calcination temperature
(Ta < 900 °C) by tartrate-gel method. In particular, in the present
study, a series of Cu-substituted Sr-hexaferrites, SrFe12-xCuxO19
(0 x 0.8), have been synthesized by the tartrate-gel technique.
We have shown that it is indeed possible to synthesize single
phase Cu-substituted Sr-hexaferrites even at low calcination temperatures, without significantly affecting the magnetic parameters.
2. Experimental methods
2.1. Synthesis of nanocrystalline SrFe12-xCuxO19 (0 x 0.8)
Cu-substituted Sr-hexaferrite samples with chemical compositions, SrFe12-xCuxO19 (0 x 0.8, x = 0, 0.2, 0.4, 0.6 and 0.8) were
synthesized by following the tartrate-get route [29], using high
purity Sr(NO3)2 (SD-fine chemicals, >99% purity), Fe(NO3)39H2O
(Loba Chemie, >98% purity) and Cu(NO3)2 (SD-fine chemicals, >
99% purity) salts, L-tartaric acid (Spectrochem, >99% purity) as
raw materials. Initially, a homogeneous solution of metal nitrates
was made by dissolving stoichiometric amounts of metal nitrates
in optimum amount of distilled water. Typically we have used
3.0 g of Fe(NO3)39H2O and correspondingly stoichiometric
amounts of Sr(NO3)2 (0.129 g to 0.1396 g) and Cu(NO3)2
(0.0299 g to 0.1275 g) to make the solution. The solution of metal
nitrates was then mixed with 70 ml of 1.5 M/metal ion, tartaric
acid solution taken in a 200 ml crystallizing dish (tartaric acid used
was between 1.7853 g and 1.9316 g). The reaction mixture was
stirred at 50 °C for 1 h using magnetic stirrer. Subsequently, 2 ml
of the polymerizing agent ethylene glycol was added to the reaction mixture and then the temperature was raised to 70 °C. The
stirring was continued under the same conditions until the formation of brown thick viscous gel and the resultant gel was dried in
an oven at 80 °C for few hours. As-dried gel was ground into fine
powder form and calcined at Ta = 850 °C for two hour under the
furnace atmosphere (air).
2.2. Characterization techniques
The phase formation and structural details of the calcined powder samples were investigated using a powder X-ray diffraction
(PAN anlytical X’pert PRO X-ray diffractometer equipped with Cu
Ka radiation as source). Fourier-transform infrared spectroscopy
(FTIR) spectra of the samples were recorded at ambient temperature using a FTIR spectrometer (Bruker, Alpha-P, Diamond ATR
cell). Scanning electron microscopy (SEM, Tescan Vega 3LMU)
technique was employed to extract the information on morphological features of the calcined powder samples. Magnetic properties
of the samples were measured at ambient temperature using a
vibrating sample magnetometer (EG&G PAR 4500). Timedependent UV-visible spectra of the 4-nitrophenol reduction
reaction were recoreded at room temperature using UV-visible
spectrometer (Shimadzu, 2600).
Fig. 1. Powder XRD patterns of the calcined (Ta = 850 °C) Cu-substituted Srhexaferrite (SrFe12-xCuxO19) samples compared with the simulated pattern of
SrFe12O19.
Table 1
Values of lattice parameters (a, c) and the c/a ratios of Cu-substituted Sr-hexaferrite
(SrFe12-xCuxO19) samples.
x
a = b (Å)
c (Å)
c/a
0
0.2
0.4
0.6
0.8
5.8445
5.8595
5.8696
5.8571
5.8570
22.9169
22.9662
23.0034
22.9662
22.9539
3.9211
3.9194
3.9190
3.9211
3.9190
P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393
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2.3. Catalytic reduction of 4-nitrophenol
Fig. 2. Room temperature FTIR spectra of calcined Cu-substituted Sr-hexaferrite
(SrFe12-xCuxO19) samples.
To evaluate the catalytic performance of the synthesized
Cu-substituted Sr-hexaferrites, the reduction of 4-nitrophenol to
4-aminophenol was employed as a model chemical reaction. In a
typical procedure, 100 ml of 0.2 mM 4-nitrophenol solution was
prepared in a 250 ml beaker. The crystals of mild reducing agent
sodium borohydride (NaBH4), corresponding to the concentration
of 0.2 M, were added to the 4-nitrophenol solution and stirred
mechanically for 0.5 min. Soon after the addition of reducing agent,
the 4-nitrophenol solution colour transformed from pale yellow to
bright yellow revealing the formation of nitrophenolate anion.
Subsequently, a known amount of synthesized hexaferrite catalyst
(10 mg) was added to the above reaction mixture and stirred. At
different intervals of time, a small portion of the solution (~3 ml)
was pipetted out from the heterogeneous reaction mixture and
the UV-visible spectra were recorded. Before taking the small portion of the solution for the UV-visible measurements, all the magnetic hexaferrite nanoparticles/catalysts were magnetically
separated from the solution by an external permanent magnet.
Fig. 3. SEM images of the calcined Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) samples, all the images are in same scale and magnification.
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and Fe3+ helps in improving the chemical activity of Cu-substituted
Sr-hexaferrite samples.
3. Results and discussion
3.1. Powder X-ray diffraction (PXRD) analysis
Fig. 1 shows the powder XRD patterns of Sr-hexaferrite
(SrFe12O19) and Cu-substituted Sr-hexaferrite samples calcined at
Ta = 850 °C. The observed XRD patterns of the samples are compared with indexed simulated XRD pattern of SrFe12O19. In the
XRD pattern of SrFe12O19, all the reflections in the experimental
pattern are in agreement with the peaks of the simulated pattern,
except a reflection at 2h ~ 33.3° attributed to hematite (a-Fe2O3)
phase and it is designated by * symbol (see Fig. 1). As documented
in the literature, the phase formation temperature of SrFe12O19 is
1100 °C [16]. But in the present case, the SrFe12O19 sample was
calcined at a lower temperature (850 °C) and therefore, the temperature is inadequate to form pure SrFe12O19 phase. Although
the samples, including the parent compound, were calcined at
the same temperature and conditions, unlike SrFe12O19, all the
Cu-substituted strontium ferrite samples are phase pure with
hexagonal crystal structure. Absence of a-Fe2O3 impurity phase
in the Cu-substituted Sr-hexaferrite samples is due to the fact that,
presence of Cu in the samples tends to reduce the phase formation
temperature of SrFe12O19. This is in line with the reduction of sintering temperature in Sr-doped ZnO sample [30]. Hence. it is clear
from this study that it is possible to synthesize single phase Cusubstituted SrFe12O19 samples, even at low calcination temepratures. Note further that, the polymerizing agent (ethylene glycol)
helps to form a thick viscous gel by extending the chemical bonding with the complexing agent (tartaric acid/citric acid). As a result
of that the metal ions are distributed homogeneously in polymeric
networks. Once the dried-gel is calcined at the suitable temperature, it helps to achieve a better homogeneity in particle size
distribution.
The peaks in the XRD patterns of all the samples are considerably broad owing to the nanocrystalline nature of the samples.
Average crystallite sizes (t) of the samples are calculated using
0:9k
Scherrer’s equation, t ¼ bcosh
; wherein k is the wavelength of Xray beam in Å, h is the Bragg’s angle and b is full width at half maximum intensity of the reflection, in radian [29]. The obtained average crystallite sizes are 41, 43, 46, 46 and 47 nm for SrFe12O19,
SrFe11.8Cu0.2O19, SrFe11.6Cu0.4O19, SrFe11.4Cu0.6O19 and SrFe11.2Cu0.8O19 samples, respectively. The structural parameters such as
lattice parameters (‘a’ and ‘c’) of the samples are estimated using
the relation [31] presented below:
1
2
dhkl
2
¼
2
4 h þ hk þ k
3
a2
!
3.2. Fourier-transform infrared spectroscopy (FTIR)
Fig. 2 illustrates the FTIR spectra of the calcined samples,
recorded at room temperature. All the samples exhibited two
major IR bands. The IR band at lower wavenumber are attributed
to stretching of octahedral metal-oxygen bonds ([Fe+3]Octahedral-O),
while that at higher wavenumber corresponds to the stretching
of tetrahedral metal-oxygen bonds ([Fe+3]Tetrahedral-O) [33]. Furthermore, each band is associated with a shoulder at low frequencies, i.e., the lower frequency octahedral band situated at 422 cm 1
has a shoulder at 393 cm 1 and the higher frequency tetrahedral
band situated at 583 cm 1 has a shoulder at 543 cm 1. The reason
behind the observation of the two bands is associated with the
position of Sr at the oxygen site [5], wherein the frequencies of
vibrations of Fe-O and Fe-Sr are decided by the mass and the ionic
size difference between these two atoms, O and Sr. The main reason to have higher stretching frequency for the tetrahedral metaloxygen bonds over the octahedral metal-oxygen bonds is due to
their shorter bond length, because shorter bond length will have
higher stretching frequency [34,35]. It is also interesting to know
that even after replacing more and more Cu at octahedral Fe site
in Sr-hexaferrite lattice, the positions of the bands are found to
be approximately the same, revealing insignificant variation in
the environments of Cu from that of Fe. Another important observation from the FTIR spectra is that the shape of the ~393 cm 1
band becomes sharper as the Cu content increased in the hexaferrite lattice, probably due to the effect of substitution of Cu only at
the octahedral sites, since it has strong preference for the octahedral site over the tetrahedral site [36].
3.3. Scanning electron microscopy (SEM)
SEM images of the calcined hexaferrite compositions in
SrFe12-xCuxO19 (0 x 0.8) have been presented in Fig. 3, and all
the images are of similar magnification, which facilitates a better
comparison. It is apparent from the images that majority of the
particles of the parent compound (SrFe12O19) exhibit plate-like
morphology and in fact the particles are heavily agglomerated
2
þ
l
c2
where dhkl is the interplannar spacing and h, k and l are the Miller
indices of the plane corresponding to the particular XRD peak.
The obtained structural parameters of the samples are listed in
Table 1. It is clear from the table that even for a small amount of
Cu substitution in Sr-hexaferrite structure, the lattice paremeters,
both ‘a’ and ‘c’, suddenly increased (see these differences
between x = 0 and x = 0.2). However, thereafter, i.e., from x = 0.
2 to x = 0.8, the lattice parameter changes are insignificant and
within the error bar. These observations can be uderstood by considering the difference in the (1) ionic radii, and (2) oxidation states,
of Cu2+ and Fe3+. Clearly, the higher ionic radii of Cu2+ (0.73 Å) than
Fe3+ (0.645 Å), for octahedral coordination [32] leads to the increase
in lattice parameters of Sr-hexaferrite due to Cu-substitution. Furthermore, on one-hand, the difference in oxidation states (i.e., 2+
for Cu and 3+ for Fe) leads to longer bond-length for CuAO. On
the other-hand, this oxidation state difference induces oxygen vacancies in the Sr-hexaferrite structure due to Cu-substitution. As it will
be discussed later that, this oxidation state difference between Cu2+
Fig. 4. Magnetic field dependent magnetization hysteresis loops of calcined Cusubstituted Sr-hexaferrite (SrFe12-xCuxO19) samples, recorded at room temperature.
P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393
owing to the magnetic interaction between the neighbouring particles. As the Cu content in the Sr-hexaferrite samples increases,
considerable change in the morphology as well as size of the hexaferrite particles is observed. The samples with x = 0.2 and 0.4
show smaller particles (larger surface area) without any noticeable
plate-like morphology revealing that the Cu inclusion has significantly changed the morphology of the particles from the platelike structure to nearly spherical structure. With further increase
in the Cu content, the size of the particles increases, and moreover,
the particles are found to have irregular shape. The increase in the
particle size for the sample with x > 0.4, can be attributed to
decrease in the phase formation temperature of the samples as
revealed from the XRD analysis.
Table 2
Saturation magnetization MS, coercivity (HC) and remanence (Mr) of the calcined
Cu-substituted Sr-hexaferrite samples.
‘x’ in
SrFe12-xCuxO19
MS
(emu/g)
HC
(Oe)
Mr
(emu/g)
0
0.2
0.4
0.6
0.8
56
60
58
59
56
5453
5257
5380
5654
5404
32
34
33
32
31
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3.4. Magnetic studies
Magnetic field dependent hysteresis (M versus H) loops,
recorded at room temperature in applied field range of 15
kOe H +15 kOe, are presented in Fig. 4. It is apparent from
the magnetic data that all the samples are hard magnetic in nature
as they exhibit large area under the loops. The magnetic parameters such as saturation magnetization (MS), coercivity (HC) and
remanence (Mr) are extracted from the magnetization data and
their values are listed in Table 2. The saturation magnetization values are obtained by extrapolating the linear region of M vs 1/H
curve to 1/H = 0. The Ms values of the samples vary in the range
of 56–60 emu/g. Among all the studied samples, the sample with
x = 0.2 has highest MS (60 emu/g) and lowest HC (5257 Oe) values.
When the higher magnetic moment cation Fe3+ (5lB) is replaced by
the lower magnetic moment cation Cu2+ (1 lB) from the lattice of
Sr-hexaferrite, the magnetization of the substituted samples
should be lower than the unsubstituted sample. But in the present
case, the samples with x = 0, 0.2, 0.4 and 0.6 show higher magnetization than the parent counterpart and this could be interpreted
based on the site preference of Cu, as given below. In the M-type
hexaferrite, the magnetic moments of Fe located at the three octahedral (2a, 12 k and 4f2) sites are alinged parallel to one another
and these moments are coupled antiparallelly to the magnetic
moments of Fe located at the tetrahedral (4f1) and trigonal
Fig. 5. Time-dependent UV-Visible spectra of 4-nitrophenol reduction using prepared Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) catalysts.
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bipyramidal (2b) sites, whereas the magnetic moments are parallel
to each other within 4f1 and 2b sites [7]. The overall magnetization
of the hexaferrite arises due to the difference in the net magnetization of the octahedral sites (2a, 12 k and 4f2) and the net magnetization of both the tetrahedral and trigonal bipyramidal sites (4f1
and 2b). If the Cu occupies one of the octahedral sites by replacing
the Fe then the magnetization of the Cu-substituted hexaferrite
should be higher than that of the parent counterpart, due to
decrease in the net magnetization of the octahedral sites. As suggested in the literature [36], Cu preferably occupies octahedral site.
Therefore, in this study we have considered that Cu occupies the
4f2 site by replacing an equivalent amount of Fe. Hence, the higher
magnetization for the samples 0 < x 0.6, in SrFe12-xCuxO19 is
observed. The decrease in the magnetization with further increase
in the Cu content (x = 0.8) is likely to be due to lower magnetic
moment of the Cu cation which leads the compound towards paramagnetic nature. Note that, for recovering the catalysts from the
reaction medium, the catalysts should posses a minimum magnetic
property (magnetization >10 emu/g). Hence, the magnetic nature
of our catalysts helps in recovering them, after completion of the
catalytic reaction, from the reactor by using a (small) permanent
magnet.
3.5. UV-visible spectroscopy and catalytic activity of SrFe12-xCuxO19
(0 x 0.8)
To assess the catalytic properties of most of the noble metal
nanocatalysts such as Au, Pd, Pt, Ag, etc. [37–40], catalytic reduction of 4-nitrophenol to 4-aminophenol was employed as the
model chemical reaction. Therefore, in this study we have chosen
the same as the model chemical reaction to evaluate the catalytic
performance of Cu-substituted Sr-hexaferrite samples using the
NaBH4 as a reducing agent. The catalytic process could be systematically examined by means of UV-visible spectroscopy, since 4nitrophenol in the acidic medium, leads to the absorption band
at ~317 nm, while the absorption band of 4-aminophenol appears
at ~300 nm [41]. When NaBH4 is mixed with the solution of 4nitrophenol, the absorption band of 4-nitrophenol undergoes red
shift from 317 nm to 400 nm owing to the formation of phenolate
anion. Despite NaBH4 being one of the good reducing agents, it
alone is incapable in reducing 4-nitrophenol to 4-aminophenol
[26]. However, the reduction reaction can be driven by adding a
suitable catalyst to the nitrophenol solution containing NaBH4.
Furthermore, if the catalyst is chemically inactive then there won’t
be any variation in the intensity of the ~400 nm absorption band
with time. On the other hand, if the catalyst is active then the
intensity of ~400 nm absorption band decreases gradually with
simultaneous increase in the intensity of ~300 nm absorption band
as the reaction time proceeds, indicating conversion of 4nitrophenol to 4-aminophenol [26].
The time-dependent UV-visible absorbance spectra for the catalytic reduction of 4-nitrophenol using SrFe12-xCuxO19 (0 x 0.8)
catalysts are shown in Fig. 5. It is confirmed from the Fig. 5 that the
parent compound (SrFe12O19) is found to be catalytically inactive
for reduction of 4-nitrophenol, because there is no change in the
intensity of 400 nm absorption band even after 13 min. But after
the introduction of Cu2+ into the lattice of Sr-hexaferrite, the catalytic efficiency is remarkably enhanced. In the Cu-substituted
samples, the intensity of 400 nm absorption band due to the phenolate anion decreases progressively, on the other hand the intensity of ~300 nm absorbance band due to the aminophenol increases
gradually as the reaction time proceeds, signifying transformation
of 4-nitrophenol to 4-aminophenol. It is worth mentioning here
that regardless of the amount of copper content in the lattice of
hexaferrite, catalytic performance of the Cu-substituted samples
is observed to be approximately the same. It is reported for various
metal-substituted spinel ferrite systems that the catalytic activity
of the parent compound can be enhanced after replacing Fe by suitable metal ions such as Al, Ni, Co, Cu etc. [25,26]. In those reports,
the authors have suggested that the presence of substituted metal
Fig. 6. Proposed mechanistic route for the reduction of 4-nitrophenol (NP) into 4-aminophenol (AP), using the Cu-substituted Sr-hexaferrite (SrFe12-xCuxO19) catalysts with
NaBH4 as the reducing agent.
P.N. Anantharamaiah et al. / Advanced Powder Technology 31 (2020) 2385–2393
ions in the octahedral coordination environment is responsible for
higher catalytic activities in the substituted spinel ferrites as the
octahedral sites are exposed to the surface. In the present study,
as the Cu2+ ions occupy the octahedral sites of the hexaferrite,
much similar to that of the spinel ferrite systems, and these sites
are exposed on the surface of the catalyst, the Cu-substituted Srhexaferrite samples show better catalytic performance.
The mechanism of the catalytic reaction in reduction of 4nitrophenol to 4-aminophenol can be understood as follows. When
the reducing agent NaBH4 is dissolved in an aqueous medium,
there will be production of borohydride anion (BH4 ). These BH4
ions get adsorbed onto the surface of the Cu-substituted hexaferrite catalysts leading to the formation of metal-hydride complex
(MAH complex). Since the Cu2+ ions are exposed to the surface,
copper hydride (CuAH) complex will be expected to form. The
prime reason to form M-H complex is due to the Lewis acidic nature of Cu2+. Note that the ability of the copper cations to have variable oxidation state drives the formation of MAH complex.
2391
Simultaneously, the 4-nitrophenolate ions also get adsorbed onto
the surface of the CuAH complex. Once the adsorption process is
completed, the hydrogen of the CuAH complex migrates to the
nitroaromatic compound and thereby facilitates the reduction
reaction. Reduction of 4-nitrophenol to the 4-aminophenol may
take place either through the direct path or the condensation path
[24]. The possible mechanism for the reduction of 4-nitrophenol to
4-aminophenol using NaBH4 as the reducing agent and the nano
hexaferrite catalyst is shown in Fig. 6. Note here that, due to lower
oxidation state of Cu2+ than Fe3+, oxygen vacancies are created in
the samples which act as the active catalytic centres to attract
the 4-nitrophenolate ions onto the particle surface to facilitate
the reduction reaction. Furthermore, the variable oxidation state
of Cu helps in adsorption and desorption of the 4-nitrophenolate
ions and 4-aminophenol, to and from the surface of the catalyst
particles (Cu-substituted Sr-hexaferrites).
The reduction of nitrophenol reaction obeys pseudo first-order
chemical kinetics since the NaBH4 concentration is taken in excess
Fig. 7. Plots of ln(Co/Ct) versus time for the 4-nitrophenol reduction using the SrFe12-xCuxO19 catalysts. The data were fitted with linear function and slope of the linear line
corresponds to rate constant.
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Table 3
A list of rate constants for 4-nitrophenol reduction reaction measured using different catalysts.
y
1
Catalysts
Amount of Catalyst (mg)
Conc. of 4-NP (mM)
Volume of 4-NP (ml)
Amount of NaBH4
Rate Constant (min
)
Ref.
Co3O4/BiFeO3
CuFeCN
0.5
0.1
90 mg/L
3.6
15 mg
0.1 mg
0.774
0.8432
[42]
[43]
CuFe2O4
Co3O4/CoFe2O4
CuFe2O4
NiFe2O4
MnFe2O4@SiO2_NH2@Au
CoMn0.2Fe1.8O4
SrFe12O19
SrFe11.8Cu0.2O19
SrFe11.2Cu0.8O19
0.01 mg/ml
10
5 mol%
5 mol%
3
5 mol%
10 mg
10 mg
10 mg
0.16
0.2
3.6
3.6
0.05
0.72
0.2
0.2
0.2
20
0.122
(122 lL)
2
16
20
20
3
20
100
100
100
0.1 mol
15 mg
1.36 g
1.36 g
5.67 g
1.36 g
0.75 g
0.75 g
0.75 g
0.002
0.1
0.344
0.873
0.126
2.1
0.0014
0.18
0.2
[44]
[45]
[46]
[47]
[48]
[49]
y
y
y
Present work.
quantity. Hence, the rate constant can be estimated using firstorder rate law equation [Ct = Co exp ( kt)], where, k is the pseudo
first-order rate constant, Co is the initial concentration and Ct is the
instantaneous concentration at time t. The plots of ln(Co/Ct) versus
time for the 4-nitrophenol reduction using Cu-substituted Srhexaferrite particles as catalysts are shown in Fig. 7. In all the
curves, the data points are fitted with the linear function and the
slopes of the fitted straight lines of the ln(Co/Ct) versus time curves
correspond to first order rate constants (k1). The obtained rate constants are 0.0014, 0.1808, 0.175, 0.1673 and 0.202 min 1, respectively, for samples with x = 0, 0.2, 0.4, 0.6 and 0.8. Higher the
value of rate constant better will be the catalyst, and therefore,
all the substituted samples, in the present study, exhibit better catalytic performance over the unsubstituted sample. Among the Cusubstituted samples, the highest value of rate constant is found for
the SrFe11.2Cu0.8O19 sample. A list of rate constant values for many
reported ferrite materials [42–49] are provided in Table 3. Our
results clearly demonstrate the usefulness of our Cu-doped Srferrite samples.
4. Conclusions
We have demonstrated that tartrate-gel route is a suitable synthesis technique for synthesis of phase pure Cu-substituted Srhexaferrite nanoceramics (SrFe12-xCuxO19, x = 0, 0.2, 0.4, 0.6 and
0.8), wherein the final calcination temperature can be reduced to
about 850 °C than that is required for SrFe12O19. The results of
the synthesized samples show that Cu helps in reducing the
required calcination temperature. Absence of impurity peaks in
the XRD patterns of Cu-substituted Sr-hexaferrite samples confirms the successful synthesis of single phase nanocrystalline
ceramics. The FTIR results indicate that Cu goes to the tetrahedral
site and Sr occupies the oxygen site. The variation in the saturation
magnetisation of the samples with Cu content in the Sr-hexaferrite
structure also indicates the occupation of Cu at the 4f2 octahedral
interstitial site of Sr-hexaferrite. Furthermore, we have successfully illustrated that catalytic activity, for the 4-nitrophenol
reduction using NaBH4, can be induced in the hard magnetic
Sr-hexaferrite by replacing Fe by Cu, without affecting the magnetic parameters of the parent compound. According to our results,
irrespective of amount of Cu substitution the catalytic activities of
all the Cu-substituted samples were found to be similar. The
induced catalytic activity in the strontium ferrite is mainly attributed to the presence of Cu2+ cations at the octahedral (4f2) site.
Overall, our results demonstrate that, the presence of more octahedral sites on the surface of the nanocrystalline particles, in addition
to the formation of oxygen vacancies due to lower oxidation state
of Cu than Fe and the variable oxidation state of Cu, act together to
facilitate the enhanced catalytic activity of Cu-substituted
Sr-hexaferrite samples.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgement
Authors are grateful to Dr. P. A. Joy, CSIR-NCL, Pune, India, and
Prof. Vishnu Kamath, Central College, Bangalore University, for
magnetic and FTIR measurements, respectively.
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