Catalytic behavior of magnetic Ni–Zn alloy

Article abstract of DOI:10.1007/s11164-018-3360-x

Ni–Zn alloy with various compositions of zinc was synthesised using a sodium borohydride-assisted chemical reduction method. The as-synthesised Ni–Zn alloy samples were annealed at 673?K for 2?h in nitrogen atmosphere. The atomic absorption spectroscopy measurement reveals the zinc-rich composition in all alloy samples. Field emission scanning electron microscopy shows that the surface morphology of Ni–Zn alloys has a flake-like nature. The X-ray diffraction pattern of as-synthesised samples shows the presence of nickel hydroxide while the annealed samples show formation of Ni₂Zn₁₁ phase. The phase transition temperature of as-synthesised Ni–Zn alloy was observed using differential scanning calorimetry and crystallization temperature changes with zinc composition was also examined. The room temperature magnetic behaviour of both as-synthesised and annealed samples was studied using vibrating sample magnetometer. This shows that as-synthesised samples have a weak ferromagnetic nature and annealed samples have a soft ferromagnetic nature. The decreasing magnetization value and Curie temperature with zinc composition was observed. The catalytic activity of Ni–Zn alloy was studied for the reduction of 4-nitrophenol. The rate constant value changes with zinc composition in the alloy.

Full text of DOI:10.1007/s11164-018-3360-x

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3360-x
Catalytic behavior of magnetic Ni–Zn alloy
K. Dhanapal1 S. Dhanavel1 T. A. Revathy1 V. Narayanan2
A. Stephen1
•
•
•
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Received: 5 October 2017 / Accepted: 1 March 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Ni–Zn alloy with various compositions of zinc was synthesised using a
sodium borohydride-assisted chemical reduction method. The as-synthesised Ni–Zn
alloy samples were annealed at 673 K for 2 h in nitrogen atmosphere. The atomic
absorption spectroscopy measurement reveals the zinc-rich composition in all alloy
samples. Field emission scanning electron microscopy shows that the surface
morphology of Ni–Zn alloys has a flake-like nature. The X-ray diffraction pattern of
as-synthesised samples shows the presence of nickel hydroxide while the annealed
samples show formation of Ni2Zn11 phase. The phase transition temperature of as-
synthesised Ni–Zn alloy was observed using differential scanning calorimetry and
crystallization temperature changes with zinc composition was also examined. The
room temperature magnetic behaviour of both as-synthesised and annealed samples
was studied using vibrating sample magnetometer. This shows that as-synthesised
samples have a weak ferromagnetic nature and annealed samples have a soft fer-
romagnetic nature. The decreasing magnetization value and Curie temperature with
zinc composition was observed. The catalytic activity of Ni–Zn alloy was studied
for the reduction of 4-nitrophenol. The rate constant value changes with zinc
composition in the alloy.
Keywords Chemical reduction Á Phase transition Á Magnetic study Á Catalytic
activity
&
A. Stephen
stephen_arum@hotmail.com
1
2
Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, India
Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025,
India
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K. Dhanapal et al.
Introduction
Nanostructured magnetic alloys have attracted large interest in recent years due to
their improved physical, magnetic and catalytic properties. These properties,
exhibited by nano-sized materials, are excellent when compared to their bulk form.
These properties lead to applications such as magnetic fluids, magnetic sensors, high
density magnetic storage and environmental remediation [1–4]. Nickel-based binary
and ternary alloys receive much interest due to their catalytic and corrosion
resistance properties. The combination of nickel with non-magnetic metals is of
great interest due to their enhanced properties, for example Ni–Zn [5], Ni–Sn [6],
Ni–Cu [7] and Ni–Ag [8] systems are such types of alloys with added applications.
There are different methods available for the synthesis of these types of alloy,
among which some important methods are hydro/solvo-thermal, electrodeposition,
polyol process, mechanical alloying, micro-emulsion system, spray pyrolysis and
chemical reduction [2, 8–10]. Normally, the chemical reduction method doesn’t
involve costly reagents and costly instruments to synthesis these alloys. The sodium
borohydride-assisted chemical reduction method is employed for the preparation of
Ni–Zn alloy. This method is preferred as it can produce nanosized alloys with the
desired composition and in large quantity. The major advantage of sodium
borohydride is that it can produce 8 mol of electrons from 1 mol, and it can reduce
the metal ions in different types of medium [11–13].
4-Nitrophenol (4NP) is the most common pollutant present in industrial and
agricultural waste water [14, 15]. This pollutant has to be removed or converted into
some other form, as it is a toxic material. There are many methods available to
remove this pollutant such as photo-catalytic degradation, electro-fenton method,
microbial degradation, adsorption, microwave-assisted catalytic oxidation, electro-
chemical treatment, metal catalysis [14]. The effective method is converting 4NP
into 4-aminophonal (4AP) using a metal catalyst. This conversion takes place by the
presence of a metal catalyst with sodium borohydride as the reducing agent. The by-
product 4AP is an important intermediate material required for the preparation of
analgesic and antipyretic drugs. 4AP is also used as photographic developer, hair
dyeing agent, corrosion inhibitor in paints, anti-corrosion lubricating agent in fuel
for two-cycle engines [16–18]. There are several noble metal catalysts available
such as Pd, Au, Ag and Pt-Ni, but these catalysts are very expensive. The
conventional method for hydrogenation of 4NP involves iron/acid as a reducing
agent, but these produce huge amounts of sludge. The sodium borohydride-assisted
chemical reduction method leads to the development of an environmentally friendly
and cost effective catalyst [16, 19]. In addition to this, Ni-based alloys have the
added advantage that they are magnetically recoverable, which increases the
recyclability of the alloy as a catalyst for the reduction of 4NP to 4AP.
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Catalytic behavior of magnetic Ni–Zn alloy
Experiment
The Ni–Zn alloys were synthesised using a conventional sodium borohydride-
assisted chemical reduction method. The polyvinylpyrrolidone (PVP) was pur-
chased from Sigma Aldrich, and all other chemicals were from Merck India. The
Ni–Zn alloy was prepared with different concentrations of zinc and nickel in the
electrolyte. The molarity of zinc was varied at 0.02, 0.05, 0.1 and 0.15 M with
respect to the nickel, resulting in a total molarity of 0.2 M in 50 mL of distilled
water. The aqueous solution was prepared using nickel chloride and zinc chloride.
The solution was stirred for 20 min after the addition of capping agent PVP of
0.05 M in 50 mL. The 0.1 M of sodium borohydride was added to the aqueous
solution drop wise via burette. The aqueous solution colour changes from light
green to black by the addition of sodium borohydride, confirming the reduction of
metal ions. Then the precipitate was washed 6 times with distilled water and finally
with acetone. The chemical reaction involved while reducing metal ions through
sodium borohydride is given below.
NiSO4 þ 2NaBH4 ! Ni þ H2 þ B2H6 þ Na2SO4
ZnCl2 þ 2NaBH4 ! Zn þ H2 þ B2H6 þ 2NaCl
The Ni–Zn alloy was synthesised with four different electrolyte concentration
ratios: (i) Ni 90:Zn 10 (Sample A), (ii) Ni 75:Zn 25 (Sample B), (iii) Ni 50:Zn 50
(Sample C) and (iv) Ni 25:Zn 75 (Sample D). Pure nickel was also synthesised using
similar methodology.
The as-synthesised Ni–Zn alloy was annealed in nitrogen atmosphere for 2 h at
673 K. The as-synthesised and annealed samples were subjected to the following
characterizations. The composition of individual elements present in the alloy was
measured from atomic absorption spectroscopy (AAS, Perkin Elmer AAS 700). The
morphology of the annealed samples was observed using field emission scanning
electron microscopy (FE-SEM, FEI Quanta FEG 200). The crystalline nature of
both as-synthesised and annealed samples was measured using powder X-ray
diffraction (XRD, 3003 TT). Differential scanning calorimetry (DSC, Perkin Elmer
Optima 5300 DV) was used to observe the phase transition in the alloy while
heating. Vibrating sample magnetometry (VSM, Lakeshore VSM 7410) was
employed to know the magnetic behaviour of both as-synthesised and annealed Ni–
Zn alloy. The catalytic behaviour of alloy was observed using UV–Vis spectrometer
(Analytik Gena, Specord 210plus).
Results and discussion
Composition analysis
The composition of nickel and zinc in the alloy was measured using AAS. The
percentage of nickel and zinc present in the alloy are given in Table 1. The
measured composition is more than the initial electrolyte composition. In sample A,
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K. Dhanapal et al.
Zinc (%)
Table 1 Composition of Ni–Zn
alloy
S. no
Sample code
Nickel (%)
1
2
3
4
Sample A
Sample B
Sample C
Sample D
76.89
66.88
42.79
19.59
23.11
33.12
57.21
80.41
the observed composition of zinc is twice as the electrolyte composition. Even
though the standard reduction potential of zinc is high when compared to nickel, the
percentage of zinc is high in the Ni–Zn alloy. A maximum difference of 13.11 wt%
of zinc is observed in sample A while a minimum of 5.41 wt% difference is
observed in sample D. The reason for this excess percentage of zinc in the alloy, is
the anomalous reduction mechanism. At first Ni2? is reduced which chemisorbs
water molecule to form Ni(OH)?, suppressing the further reduction of Ni2? leading
to the preferential reduction of Zn2? in the overall reduction process [20, 21]. This
reveals the minimum difference between the initial and final composition in sample
D.
Surface morphology
The surface morphology of annealed Ni–Zn alloy was observed using FESEM. The
morphology of pure nickel, samples A, B, C and D are represented in Fig. 1. Pure
nickel shows agglomerated particles with a foam-like nature whereas sample A
shows a sheet-like nature. In the case of sample B, a densely packed foam-like
nature can be viewed. A further increase of the percentage of zinc in nickel changes
this foam-like nature to the flakes of sample C. As the percentage of zinc increases,
Fig. 1 Surface morphology of Ni–Zn alloy
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Catalytic behavior of magnetic Ni–Zn alloy
the foam-like nature changes to a densely packed nature. This further decreases the
flake size and appears like tightly packed flake particles in sample D.
Structural information
The structural information of the Ni–Zn alloy was deduced from the XRD pattern.
The XRD pattern of both as-synthesised and annealed samples are shown in Figs. 2
and 3. The XRD pattern of as-synthesised nickel shows an amorphous nature with a
broad peak around 44°. The XRD pattern of samples A, B, C and D shows two main
diffraction peaks at 34° and 60°. These diffraction peaks correspond to Ni(OH)2
with reference to the JCPDS file number 01-1047.
XRD patterns of the annealed samples show a different set of diffraction peaks
when compared to the as-synthesised samples. Pure nickel shows the diffraction
peaks at 44°, 52° and 76°, indexed to planes (111), (200) and (220), respectively,
corresponding to nickel in the fcc phase with space group Fm-3 m, with reference to
JCPDS file number 65-2865. Sample B shows diffraction peaks corresponding to
both pure nickel and the Ni2Zn11 phase with reference to JCPDS file number
65-5310. Both Ni2Zn11 and Zn(OH)2 phase is observed in the sample C. The
Zn(OH)2 phase is confirmed with JCPDS file number 48-1066. The XRD pattern
confirms that the Ni2Zn11 alloy phase observed in samples B and C. The presence of
Zn(OH)2 alone is observed in the case of sample D.
Fig. 2 XRD pattern of as-synthesised Ni–Zn alloy
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K. Dhanapal et al.
Fig. 3 XRD pattern of annealed Ni–Zn alloy
Phase transition
The phase transitions in the Ni–Zn alloy during heat treatment were analyzed in
DSC. The DSC curve of samples B and C is shown in Fig. 4. The curve exhibits the
presence of both endothermic and exothermic peaks, which confirm the structural
Fig. 4 DSC curve of Ni–Zn alloy
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Catalytic behavior of magnetic Ni–Zn alloy
changes while heating. The endothermic peak is observed due to hydrogen
desorption. It is well known that the inclusion of hydrogen is unavoidable co-
process of chemical reduction [22, 23]. The exothermic peaks are de-convoluted and
represented in Fig. 5. The de-convoluted peak maxima and their corresponding
enthalpy values are given in Table 2. In the case of sample B, four exothermic peaks
are observed: the first two peaks correspond to the precipitation of nickel [24]. The
third peak at 413 °C is attributed to the structural relaxation, and the fourth peak at
430.9 °C corresponds to the phase formation of Ni2Zn11 alloy [25, 26].
The sample C shows the similar phase transition with variation in the temperature
range of 10 °C. The peak at 213.9 °C in sample C is due to removal of hydroxide
which leads to the structural relaxation in the alloy. The enthalpy of Ni–Zn alloy
phase formation varies from 2.4 J/g to 5.3 J/g with respect to the zinc composition.
Fig. 5 De-convoluted DSC curve Ni–Zn alloy
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K. Dhanapal et al.
Enthalpy (J/g)
Table 2 Exothermic peak
position and enthalpy values of
Ni–Zn alloy
Sample
Peak position (°C)
Sample B
348.5
372.9
413.1
430.5
213.8
307.0
360.0
446.9
12.5
1.0
8.0
3.2
5.6
1.6
33.1
2.2
Sample C
Magnetic behaviour
The magnetic behaviour of Ni–Zn alloys were studied using VSM. The room
temperature hysteresis loop was taken for both as-synthesised and annealed
samples. The hysteresis loops of both as-synthesised and annealed samples are
shown in Figs. 6 and 7. The shape of hysteresis loop of as-synthesised samples
represents a weak ferromagnetic nature in all the samples. The hysteresis loop of the
annealed samples shows soft ferromagnetic nature in samples A, B and C, whereas
sample D shows a weak ferromagnetic nature.
The saturation magnetization, coercivity, retentivity, Mr/Ms and hysteresis loss
values are tabulated in Table 3. The saturation magnetization value of sample A is
3.10 emu/g, which is less than pure nickel. Samples B and C show saturation
magnetization value 2.27 and 2.05, respectively. There is a large moment variation
from sample C to D of nearly 11%. The reduction in saturation magnetization
would be the addition of zinc in the alloy. In sample A, the addition of zinc in nickel
Fig. 6 Hysteresis loop of as-synthesised Ni–Zn alloy
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Catalytic behavior of magnetic Ni–Zn alloy
Fig. 7 Hysteresis loop of annealed Ni–Zn alloy
Table 3 Magnetic parameter
Parameter
Sample A
Sample B
Sample C
values of annealed Ni–Zn alloy
Ms (emu/g)
3.10
300
2.27
340
2.05
295
Hc (Oe)
Mr (emu/g)
1.02
0.33
3486
1.02
0.45
3010
0.53
0.26
2215
Mr/Ms (no unit)
Loss (erg/g cycle)
matrix reduces the Ni–Ni interaction with a domination of Ni–Zn interaction results
the lower saturation magnetization value. As the zinc composition increases the
saturation magnetization values reduces for sample B and sample C. In the case of
sample D, a weak ferromagnetism is observed due to the presence of a large amount
Zn as evidenced from compositional analysis.
The retentivity value also reduces with the zinc composition from 1.02 emu/g for
sample A to 0.53 emu/g for sample C. This shows that the increasing composition
of zinc reduces the magnetic behaviour of the nickel without changing the
ferromagnetic behaviour up to certain critical composition after which it becomes
weakly ferromagnetic. The Mr/Ms ratio shows that the domains present in the
samples are of pseudo single domain type [27, 28]. The hysteresis loss decreases
with increasing zinc composition in the alloy.
The thermo-magnetic behaviour of the Ni–Zn alloy was studied using high
temperature VSM measurement. The temperature versus magnetization graph (M–
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K. Dhanapal et al.
T) of samples A and C are shown in Fig. 8. The M–T curve was measured in
constant with magnetic field of 250 Oe. The M–T curve shows a gradual fall in the
magnetic moment of the samples and shows absence of sharp transition in the
magnetic moment. The moment of both the samples tend to zero after 350 °C and
beyond that there is no variation in the magnetic moment. This confirms the
paramagnetic nature of sample A and sample C after Curie transition. The first
derivative of M–T curve is calculated and presented in the inset of Fig. 8 to
determine the Curie temperature.
The Curie transition temperature is calculated from M–T curve and found to be
346 and 310 °C for sample A and sample C, respectively. The curie temperature of
sample A is less than that of pure Ni, which is due to the incorporation of Zn atoms
in the alloy there by decreasing the Curie temperature. Similarly, for sample C,
further reduction in the Curie temperature is found, where the amount of Zn present
in the alloy is higher than sample A. This confirms that there is a decrease in the
Curie transition temperature of Ni–Zn alloy with increasing Zn composition in the
alloy.
Catalytic activity
The catalytic behaviour of all the annealed samples was evaluated by the reduction
of 4NP to 4AP in the presence of NaBH4. The advantage of catalytic reduction of
4NP is that it can easily be monitored using the UV–Vis spectrophotometer. The
absorption peak observed at 317 nm is due to 4NP, which is shifted to 400 nm after
the addition of NaBH4. In this process, the addition of NaBH4 forms 4-nitrophe-
nolate ions in the alkaline condition transforming light yellow colour of 4NP to the
dark yellow colour. The absorption peak intensity at 400 nm shows no variation for
several days, which shows the addition of NaBH4 alone doesn’t reduce the 4NP
[14, 19]. The addition of an alloy catalyst in this solution shows a drastic decrease in
the absorption peak within a short time period.
The catalytic reduction of all the samples is shown in the Fig. 9. In all the
samples the reduction of 4NP to 4AP is observed. The absorption peak intensity at
Fig. 8 M–T curve of Ni–Zn alloy
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Catalytic behavior of magnetic Ni–Zn alloy
Fig. 9 Catalytic activity of Ni–Zn alloy
400 nm decreases with time, thereby increasing the intensity of the absorption peak
at 300 nm, which is due to the 4AP. The spectrum shows that the reduction process
starts immediately after the addition of a metal catalyst. This shows the absence of
oxide in the material, otherwise there would have been some induction time. To
avoid this, mostly nitrogen purging is used, but this is not used in this study as our
alloy doesn’t contain any oxide, while 4NP bubbles are decreased, which ensures
that the metal catalysts are distributed uniformly during the reduction process
[16, 29].
In order to investigate the reaction kinetics and rate constant, a time-dependent
absorption graph was plotted for all samples and is shown in the inset of Fig. 9. The
figures show that there is a faster reduction in the sample B when compared to all
other samples. The curve represents the reduction process follows pseudo first order
kinetics. The rate constant is calculated from the graph as, 0.07, 0.029, 0.092 and
0.037 min-1 for pure Ni, sample A, sample C and sample D, respectively. In sample
B reduction was observed with two rate constants, which are 0.061 and
0.247 min-1. This shows that sample B has higher catalytic activity when
compared to other samples; this may be due to the densely packed foam-like nature
of the sample B as evident from FESEM measurement.
The catalytic activity of all the samples was carried out at different pH values
(4.0, 7.0 and 9.0). There is no significant variation in the activity. Hence, pH 7.0 is
chosen for all studies. The reusability of the catalyst was carried out for five cycles,
which is shown in Fig. 10. The catalyst retains its catalytic performance up to two
cycles and further shows slight variation in the activity. The schematic represen-
tation of magnetic recoverability of the catalysts is also shown in Fig. 10.
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K. Dhanapal et al.
Fig. 10 Reusability of the catalyst
Conclusion
The Ni–Zn alloy was successfully synthesised using sodium borohydride-assisted
chemical reduction. AAS analysis confirms the zinc-rich composition in all alloy
samples. The surface morphology of alloy was transformed from foam-like to
closely packed flakes as the zinc composition increased. The Ni2Zn11 phase is
observed in annealed Ni–Zn alloy from XRD measurement. The phase transition in
the as-synthesised Ni–Zn alloy was analysed using DSC. The room temperature
magnetic behaviour was observed from VSM. The hysteresis loop confirms the
weak ferromagnetic nature in as-synthesised samples and soft ferromagnetic nature
in annealed samples with dependence of saturation magnetization value in zinc
composition. The catalytic activity of Ni–Zn alloy was studied towards the
reduction of 4NP to 4AP. All samples show catalytic activity, and sample B shows
better catalytic activity than other samples. Sample B has rate constants of 0.061
and 0.247 min-1
.
Acknowledgements Author (KD) would like to acknowledge NCNSNT, University of Madras for
financial support. Authors also acknowledge IITM for FESEM, DSC and VSM measurement.
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