Spin glass behavior and oxidative catalytic property of Zn2MnO4 from a metathesis driven metastable precursor

Article abstract of DOI:10.1016/j.jpcs.2021.110206

The reaction of chloride salts of zinc and manganese with NaOH yielded a cubic spinel structured metastable precursor at room temperature, driven mainly by the salt elimination process's energetics. While classical drying processes failed to produce the monophasic oxide, recrystallization under the hydrothermal conditions yielded Zn₂MnO₄ in nano dimensions. The sample consisted of crystallites with an average 6 nm size and had a lattice dimension of 8.396 (13) ?. The selected area electron diffraction pattern reiterated the occurrence of cubic inverse spinel. The presence of fingerprint (A_1g and F_2g) modes of an inverse spinel at 663 and 561 cm^?1 in the Raman spectrum further supported our finding. The TEM-EDS analysis confirmed the ratio of Zn: Mn as 1.95:1. The sample showed an optical bandgap of 2.54 eV. X-ray photoelectron spectral analysis established the existence of manganese in the IV oxidation state. The presence of Mn (IV) with small amounts of Mn (III) (up to 20%) was confirmed from the electron paramagnetic spectra recorded at room temperature and 77 K. An average oxidation state of 3.85 was deduced from the chemical redox titration experiments. The pseudocapacitive behavior of the sample was evident in cyclic voltammetric experiments. The sample exhibited paramagnetic behavior at 298 K within the applied magnetic field of ±50 kOe. In the temperature-dependent measurements, the zero-field and field cooled data points of Zn₂MnO₄ diverged at 13 K, suggesting a spin-glass behavior. An effective magnetic moment of 4.31 BM was deduced for the sample. The inverse spinel effectively catalyzed the oxidation of phenol. It facilitated nearly 100% degradation of bisphenol-A to salicylaldehyde and phenylethyl alcohol (as major products) in the presence of H₂O₂ and at a pH of 9.

Full text of DOI:10.1016/j.jpcs.2021.110206

Journal of Physics and Chemistry of Solids 157 (2021) 110206
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Spin glass behavior and oxidative catalytic property of Zn2MnO4 from a
metathesis driven metastable precursor
Shreya Kushwaha, Rajamani Nagarajan*
Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi, 110007, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The reaction of chloride salts of zinc and manganese with NaOH yielded a cubic spinel structured metastable
precursor at room temperature, driven mainly by the salt elimination process’s energetics. While classical drying
processes failed to produce the monophasic oxide, recrystallization under the hydrothermal conditions yielded
Zn2MnO4 in nano dimensions. The sample consisted of crystallites with an average 6 nm size and had a lattice
dimension of 8.396 (13) Å. The selected area electron diffraction pattern reiterated the occurrence of cubic
inverse spinel. The presence of fingerprint (A1g and F2g) modes of an inverse spinel at 663 and 561 cmꢀ 1 in the
Raman spectrum further supported our finding. The TEM-EDS analysis confirmed the ratio of Zn: Mn as 1.95:1.
The sample showed an optical bandgap of 2.54 eV. X-ray photoelectron spectral analysis established the exis-
tence of manganese in the IV oxidation state. The presence of Mn (IV) with small amounts of Mn (III) (up to 20%)
was confirmed from the electron paramagnetic spectra recorded at room temperature and 77 K. An average
oxidation state of 3.85 was deduced from the chemical redox titration experiments. The pseudocapacitive
behavior of the sample was evident in cyclic voltammetric experiments. The sample exhibited paramagnetic
behavior at 298 K within the applied magnetic field of ±50 kOe. In the temperature-dependent measurements,
the zero-field and field cooled data points of Zn2MnO4 diverged at 13 K, suggesting a spin-glass behavior. An
effective magnetic moment of 4.31 BM was deduced for the sample. The inverse spinel effectively catalyzed the
oxidation of phenol. It facilitated nearly 100% degradation of bisphenol-A to salicylaldehyde and phenylethyl
alcohol (as major products) in the presence of H2O2 and at a pH of 9.
Oxides
Chemical synthesis
X-ray diffraction
Electron microscopy
Magnetic properties
1. Introduction
changes above ambient temperature and pressure [10–15]. The recrys-
tallization of metastable precursors, reshaped bulk materials, indirectly
The scientific revolution brought by the nano-size regime, an inter-
mediate between the finite molecule and the infinite condensed solid, is
tremendous [1–9]. The quest of solid-state chemists to stabilize a
metastable solid is endless, given the potential properties of these phases
compared to the thermodynamical ones. Typically, nanomaterials are
metastable structures, and chemists have adopted a bottom-up synthetic
approach operating under near ambient conditions to fabricate them
[1–9]. Majorly four types of reactions are employed to generate the
nanostructures, viz., reduction, oxidation, hydrolysis, and precipitation.
Additionally, the conventional heating process involving equilibrium
conditions is replaced with unconventional energy sources such as mi-
crowave, ultrasound, and hydro/solvothermal to facilitate these meta-
stable nanostructures’ capture [1–9]. Albeit the unique advantages of
other synthetic methods, hydrothermal synthesis is widely adopted for
crystal formation and growth by chemical reactions and solubility
supplied reaction sources, and decomposition of a single-source pre-
cursor are the major synthetic strategies for fabricating inorganic
nanostructures under hydrothermal conditions [10–15]. To generate
multimetallic oxide nanostructures, metastable precursors, often pro-
duced rapidly in an aqueous solution via a co-precipitation reaction, can
be subjected to recrystallization under high temperature-high pressure
hydrothermal conditions.
We intended to apply the approach of generating a metastable pre-
cursor and further synthesize metastable cubic inverse spinel, Zn2MnO4,
for the following reasons. The existence of five distinct phases,
ZnMn2O4, ZnMnO3, Zn2Mn3O8, ZnMn3O7, Zn2MnO4 in the Zn–Mn–O
ternary system, lower stability of the +4-oxidation state of manganese
and its ability to undergo self-reduction into Mn (III) and Mn (II), and the
energetic hindrance associated with the occupation of octahedral sites
by zinc are the major issues surrounding the stabilization of inverse
* Corresponding author.
E-mail addresses: shreyakushwaha2010@gmail.com (S. Kushwaha), rnagarajan@chemistry.du.ac.in (R. Nagarajan).
https://doi.org/10.1016/j.jpcs.2021.110206
Received 22 February 2021; Received in revised form 18 May 2021; Accepted 29 May 2021
Available online 11 June 2021
0022-3697/© 2021 Elsevier Ltd. All rights reserved.

S. Kushwaha and R. Nagarajan
Journal of Physics and Chemistry of Solids 157 (2021) 110206
cubic spinel Zn2MnO4 [16–19]. Hydrothermal reactions leading to oxide
nanostructures are generally conducted in the presence of NaOH/KOH,
which acts both as a base as well as a mineralizer to hasten up the re-
action process. In the chosen system, Zn2MnO4, ZnO is the thermody-
namically stable phase at NaOH’s high concentration. Still, the basic
conditions can immensely favor manganese oxidation (from II or III) to
higher oxidation states. To overcome the stabilization of ZnO, one must
use lower concentrations of NaOH and provide competing energetics.
Salt elimination is one such energetics that can contend and pave the
way for the generation of mixed metal hydroxide precursors and stabi-
lize metastable nanostructures in the desired composition. The current
work describes our efforts to realize inverse cubic spinel Zn2MnO4 in a
nano-size regime and its extensive characterization using various
physicochemical techniques. The catalytic role of Zn2MnO4 nano-
structures in the oxidation of phenol and Bisphenol-A (BPA) further
demonstrated manganese’s existence in a higher oxidation state.
The average oxidation state of manganese was obtained using the
relation 5/2 [C2O24ꢀ ] = [Mn2+].
Manganese (III) acetate dihydrate (Sigma Aldrich, 97%) and Mn (IV)
solution (in-situ generated) were used as the standard samples to
determine the amount of Mn (III) in the samples in addition to Mn (IV)
spectrophotometrically [24]. The dissolution of both the samples was
done in 6 M pyrophosphoric acid (Sigma Aldrich, 95%) and 12 M sul-
furic acid (Merck, 98%) in a ratio of 2:1. Calibration curves were
sketched using different Mn (III) concentrations and Mn (IV) solutions in
pyrophosphoric acid. The absorbance values for both the samples were
recorded at 398 and 510 nm. A similar procedure was followed for our
sample.
A CHI 600 D electrochemical setup equipped with a
three-electrode system was used to measure the sample’s cyclic vol-
tammogram. The ITO sheet was coated with the sample, platinum wire,
and Ag–AgCl electrodes as the working, counter, and reference elec-
trodes, respectively. The measurement was done in a 1 M Na2SO4
neutral electrolyte solution. The potential window of 0–0.9 V (scan rate
of 0.01 V/s) with a sensitivity factor of 0.1 was chosen. For coating the
ITO substrate of 1 × 2.5 cm, the as-prepared spinel, acetylene black, and
polytetrafluoroethylene (PTFE) were mixed (ratio of 8:1:1), followed by
homogenization with an appropriate amount of N-methyl pyrrolidone
(NMP) solution to make a slurry. The slurry-coated substrate was dried
at 60 ◦C to evaporate the solvent. An automated surface area analyzer
(Quanta chrome, Boynton Beach, FL) was used to estimate the samples’
surface area by the BET method. The X-ray photoelectron spectroscopy
measurements of the solid sample (XPS) were performed using PHI 5000
Versa probe II (FEI Inc.) with Ar + ion and C60 sputter gun at a pressure
higher than 10ꢀ 9 Torr. Magnetic measurements were conducted using a
magnetic properties measurement system (MPMS XL Quantum Design
USA).
2. Experimental
2.1. Synthesis details
In the current set of reactions, 2 mmol of ZnO (Sigma Aldrich, 99%)
was dissolved in 5–7 mL of HCl (Merck, 35%), to which 1 mmol aqueous
solution of MnCl2⋅4H2O (Central Drug House, AnalaR ˂ 98%) was added
under constant stirring. After homogenizing, a 1.2 M solution of NaOH
(Fisher scientific, 97%) was added drop-wise till the pH attained a
constant value of 12 as indicated by a pH meter (Eutech instruments pH
510). The resultant mixture was aged under continuous stirring for 12 h
at room temperature. The precipitate was separated by centrifugation,
washed with de-ionized water repeatedly till the washings did not show
the positive test for chloride ions with the AgNO3 solution. After this,
0.35 g of the precursor was transferred to a Teflon container (60 mL
capacity), in which 45 mL of water was added. It was subjected to hy-
drothermal treatment at 150 ◦C for 24 h.
2.3. Catalytic experiment details
The oxidation of phenol (Rankem, 99.5%) was attempted using
Zn2MnO4 as a catalyst and H2O2 (Merck, 30% v/v) as an oxidizing agent.
To 50 ml of 10ꢀ 5 M of phenol, 1 ml of H2O2 solution, and 50 mg of
catalyst were added. Aliquots were taken periodically, centrifuged, and
the supernatant solution’s absorbance was recorded on a UV–visible
spectrometer (Shimadzu-1601). 1H proton (400 MHz) and 13C NMR
(100 MHz) spectra of oxidized product from phenol were recorded on a
Bruker advance-400 spectrometer using DMSO‑d6 as solvent TMS as an
internal standard. The oxidative removal of the water pollutant,
Bisphenol-A (Alfa Aesar, > 97%), was attempted in the presence of
Zn2MnO4 and activated by H2O2. The pH of the reaction mixture was
modified to 9 using NH3 (1:1 v/v) solution. 50 ml of 4 × 10ꢀ 5 M aqueous
BPA solution was prepared, whose solubility was promoted by adding a
minimum amount of absolute ethanol. To it, 1 ml of H2O2 and 50 mg of
catalyst were added. The contents were thoroughly stirred, and aliquots
were taken every 15 min. The mixtures were centrifuged, and the ab-
sorption spectra of supernatant solutions were recorded using a
UV–visible spectrometer (Shimadzu-1601). The degraded products of
BPA were analyzed by the GC-MS instrument (The accutoFGcV, JMS-
T100GCV from JEOL) fragmentation pattern of the mass spectra using
literature and library data of Agilent gas chromatography system (7890
A GC) having a capillary column (stationary phase: Polysiloxane; col-
2.2. Characterization details
The powder X-ray diffraction (PXRD) patterns were collected over
the two-theta range of 15–90◦ using a high-resolution X’Pert PAN-
alytical diffractometer equipped with xenon detector employing Cu K
α
radiation at a scan rate of 0.04◦. The structural refinement of the PXRD
pattern was performed using GSAS + EXPGUI software by the Rietveld
method [20,21]. The sample’s UV–visible spectrum was recorded by
suspending the powder in ethanol using the Shimadzu-1601 spectro-
photometer. The sample was mixed with KBr and pelletized to record the
FTIR spectrum on PerkinElmer 2000 spectrometer. Raman spectrum was
obtained using a Renishaw spectrometer employing a 514 nm laser
(Ar+). Electron microscopic analysis was carried out on ZEISS-Gemini
SEM 500 (for FESEM) and the JEOL 2100 F microscope operating at
200 kV equipped with EDS attachment (for SAED and HR-TEM). X band
electron resonance spectra were collected at room temperature and 77 K
using JEOL JES-FA 200 ESR spectrometer. The chemical red-ox titration
method was employed to estimate the average oxidation state (AOS) of
manganese. Equimolar solutions (0.025 M) of KMnO4 (Thomas Baker
95%) and oxalic acid (Central Drug House 99.5%) were prepared. Po-
tassium permanganate was standardized using oxalic acid. The average
oxidation state (AOS) of manganese was determined by oxalic
acid-permanganate back titration [22,23]. We added 50 mg of the spinel
to a mixture containing 5 mL of 1 M H2SO4 and 20 mL of standard oxalic
acid solution. The solution was warmed to facilitate the reaction. It was
back titrated with standardized potassium permanganate until the
appearance of pale permanent pink color. Since all the manganese is
reduced to its divalent form, the total manganese content is obtained by
using the following equation
umn length: 30 m; internal diameter: 0.25 mm; Film thickness: 0.25
using helium as a carrier gas at a flow rate of 1 ml/min.
μm)
3. Results and discussion
Fig. 1(a) shows the PXRD pattern of the residue obtained after the
addition of highly concentrated sodium hydroxide (source of polyatomic
OHꢀ ions) to the aqueous solution of the chloride salts of zinc and
manganese (monoatomic cations). NaCl’s appearance in the PXRD
pattern suggests that this reaction’s driving force is salt elimination, as
described by the following equation.
5C2O24ꢀ + 2MnO4ꢀ + 16H+ → 2Mn2+ + 10CO2 + 8H2O
2

S. Kushwaha and R. Nagarajan
Journal of Physics and Chemistry of Solids 157 (2021) 110206
to overcome the extreme low stability (and high reactivity) of the mixed
metal hydroxide precursor by transforming it (after repeatedly washing
with double-distilled water) into a well-crystallized nanostructure by
recrystallizing it under hydrothermal conditions (recrystallization of
metastable precursors (RMP) route).
In this route, the crystals’ growth in the nano-size regime occurs via
the Ostwald ripening mechanism in which larger size crystals grow at
the expense of smaller size crystals due to the crystals’ high surface
energy in the nano-size regime. It is evident from the increase in the
average crystallite size (from 3 nm) to 6 nm after treating the precursor
hydrothermally (Fig. 3(a)). The Le Bail refinement of the PXRD pattern
in the Fd3m space group yields a cubic lattice dimension of 8.396 (13) Å,
slightly higher than the values reported earlier (Fig. 3(a)) [16–18]. The
higher lattice dimension of our sample may be due to higher amounts of
Zn2+ ions in octahedra (0.74 Å) along with some amount of Mn3+ (high
spin, 0.64 Å) in the spinel structural arrangement (Fig. 3(b)).
The vibration modes due to ZnO4 tetrahedral units and Zn/MnO6
octahedral units of the spinel structure manifest at 632 and 508 cmꢀ 1
,
respectively, in the FTIR spectrum of the sample (Fig. 3(c)) [25].
Additionally, the vibrations of surface adsorbed carbonate species
appear at 830 cmꢀ 1. The Raman modes at 663 and 561 cmꢀ 1 signify the
fingerprint (A1g and F2g) modes of an inverse spinel (Fig. 3(d) [26]. The
aggregation of crystallites is noticed in the FESEM image (Fig. 4(a)). In
the EDS spectrum, only zinc, manganese in the ratio of 1.95:1 is
observed along with oxygen and rules out the inclusion of even trace
amounts of sodium in the sample (Fig. 4(b)). Well-defined rings related
to the hkl planes of the cubic spinel arrangement appear in the SAED
pattern, confirming both the structure and crystalline nature of the
sample (Fig. 4(c)). The lattice fringes with the distances of 2.53, 2.42,
and 2.96 nm, observed in the HR-TEM image, correspond to (113),
(322), and (022) hkl planes of a cubic spinel structural arrangement
(Fig. 4(d)).
Fig. 1. (a) PXRD pattern of the precipitate obtained by the addition of NaOH to
the aqueous solution of zinc and manganese chlorides (2:1 M ratio), and (b)
PXRD pattern of the product after removing NaCl by washing with double
distilled water followed by drying in a hot air oven at 70 ◦C for 12 h.
̅̅̅̅→
2 ZnCl2 + MnCl2 NaOH 2 Zn(OH)2 + Mn(OH)x + NaCl.
The high concentration of alkali also facilitates faster nucleation of
nano-sized crystals. The basic conditions can promote the oxidation of
manganese (II). The metal hydroxides from the reaction may undergo
further reaction through interdiffusion to result in mixed metal hy-
droxide. Interestingly, when aged over 12 h, the mixed metal hydroxide
evolves structurally in a spinel-like arrangement, as revealed by its
PXRD pattern after removing the NaCl (Inset of Fig. 1(a)). A similar
observation is encountered when KOH is used as the precipitating agent
(Fig. S1). When we try to perform the precursor’s conventional drying in
a hot-air oven at 70 ◦C, ZnO formation is noticed along with the mixed
metal hydroxides (Fig. 1(b)). The reflections of ZnO and the spinel re-
flections persist even after calcining the precursor at 500, 600, 700, 800,
and 850 ◦C in a flowing oxygen atmosphere for 3 h (Fig. S2). In earlier
reports, the additional ZnO has removed by washing the samples with
acetic acid [16]. We avoided washing with acetic acid as it might result
in zinc-less compositions.
The positional parameter is calculated using the expression, u = [(Rt
+ Ro)1/√3a +0.25], where Rt = ionic radius of the tetrahedral site, Ro
= ionic radius of oxygen ion, and a = lattice parameter. The estimated u
for Zn2MnO4 (0.385) is closer to the ideal value expected for the ideal
spinel (0.375). A small increment in the positional value may be due to
tetrahedral site occupancy by larger Zn2+ ions [27].
The sample is subjected to X-ray photoelectron spectral analysis, and
the results from these measurements are shown in Fig. 5. The exclusive
presence of zinc, manganese and oxygen in the sample is inferred from
The precursor loses weight continuously with an increase in tem-
perature up to 508 ◦C in its thermogravimetric trace (Fig. 2). We decided
Fig. 3. (a) Le Bail refinement of the PXRD pattern, (b) Crystal structure of the
spinel, (c) FTIR, and (d) Raman spectrum of the hydrothermally treated pre-
cursor after removing NaCl by washing with double distilled water.
Fig. 2. Thermogravimetric trace of mixed metal hydroxide.
3

S. Kushwaha and R. Nagarajan
Journal of Physics and Chemistry of Solids 157 (2021) 110206
[30]. The average oxidation state of manganese by the redox titration
method (oxalic acid vs. potassium permanganate) comes out to be 3.85.
We have also determined the Mn (III) concentration along with Mn (IV)
spectrophotometrically. From the slope and intercept values of the
calibration curves plotted between absorbance and concentration, the
content of Mn3+ is estimated to be 20%, with the rest Mn (IV) in our
sample. The UV–visible spectra and calibration curves of this quantita-
tive estimation are given in Fig. S3 and S4, respectively.
The sample’s UV–visible spectrum exhibits absorbance extending
into the visible region (Fig. 6(a)). From the Tauc plot, an optical band
gap of 2.54 eV is deduced (Fig. 6(b)). The optical bandgap of cubic in-
verse spinel Zn2MnO4 is reported here for the first time to the best of our
knowledge. Fig. 6(c) shows the first derivative electron paramagnetic
resonance plots of the spinel at both room temperature (RT) and 77 K.
The spinel exhibits EPR signals at g values 2.057 and 2.138 at RT and 77
K, respectively. The signal at room temperature is broader as compared
to the signal obtained at 77 K. The broadness may arise from the pres-
ence of a very minute amount of Mn3+ along with Mn4+ in the lattice,
thereby modifying the Mn4+-O-Mn4+ exchange kinetics to result in
Mn3+1-O-Mn4+ ↔ Mn3+2-O-Mn4+ type of interactions at room tem-
perature [31]. The interaction betw1een Mn3+-Mn4+ becomes prominent
at room temperature due to the high thermal motion of electrons in both
Mn3+ and Mn4+, which results in the broadening of the EPR signal at RT
than 77 K. The cyclic voltammogram of Zn2MnO4 in an aqueous solution
is given in Fig. 6(d), in which a pseudocapacitive nature is evident from
the response registered from the restricted negative and positive po-
tential limits of 0–0.9 V. A similar observation is reported for the MnO2
sample [32].
2
Fig. 4. (a) FESEM image, (b) EDS, (c) SAED pattern, and (d) HR-TEM image
of Zn2MnO4.
the survey spectrum (Fig. 5(a)). There is no peak due to sodium in the
survey spectrum, again reaffirming the absence of sodium in the lattice.
The features appearing at 1018.8 and 1042.1 eV correspond to Zn 2p3/2
and Zn 2p1/2, respectively, and confirm Zn2+ in the sample (Fig. 5(b))
[28]. In the core-level spectrum of Mn 2p, peaks at 640.0 and 651.7 eV
emerging from Mn 2p3/2 and Mn 2p1/2 are seen (Fig. 5(c)) [29]. These
features establish the existence of manganese in the IV oxidation state.
The absence of any satellite peaks rules out the presence of even small
amounts of Mn2+ in the sample [29]. The feature at 527.4 eV in O 1s
core-level spectrum corresponds to the lattice oxygen. The oxygen from
the surface adsorbed hydroxides and carbonates appears at 528.8 eV
(Fig. 5(d)) [28]. The feature at 282.3 eV in the C 1s core-level spectrum
is due to adventitious carbon used for charge correction reference,
whereas the 286.2 eV feature belongs to the surface adsorbed carbonates
Various studies indicate the complex nature of magnetic properties
of spinel oxides where contributions from the intrasite (A-O-A) and
(B–O–B) and an intersite (A-O-B) superexchange interaction are seen.
The distribution of non-magnetic and magnetic cations at the A- and B-
sites determines the interaction’s strength to a greater extent. The
sample exhibits paramagnetic behavior at 298 K within the applied field
range of ±50 kOe. Under similar fields, a hysteresis loop indicating weak
ferromagnetic behavior of the sample is observed at 2 K (Fig. 7(a)). The
coercivity and retentivity values of 1.7 kOe and 1.4 × 10ꢀ 2 emu are
deduced for the sample. The ZFC-FC plots in the temperature range of
Fig. 5. (a) Survey XPS spectrum, and core-level spectra of Zn 2p (b), Mn 2p (c), O 1s (d), C 1s (e).
4

S. Kushwaha and R. Nagarajan
Journal of Physics and Chemistry of Solids 157 (2021) 110206
the content reported by Blasco et al. in the case of Mn1⋅3Zn1⋅7O4 (60%)
and Mn1⋅4Zn1⋅6O4 (80%) [18]. Their samples also had effective magnetic
moments (4.85 and 5.10 BM), considerably higher than our value. Such
a deviation is primarily due to the current synthetic approach of stabi-
lizing Mn4+ under highly basic conditions and inhibiting the
self-reduction to Mn3+ to an appreciable extent. In the nitrogen gas
adsorption-desorption isotherms conducted on Zn2MnO4, a type-IV
hysteresis is seen with a surface area value of 89 m2/g (Fig. 8(a)). The
mean pore-diameter value indicates the sample’s mesoporous nature
[36]. The catalytic role of Zn2MnO4 towards the oxidation of phenol is
examined. The absorbance at 270 nm (n → π* transition) of phenol
gradually disappears on the addition of the catalyst and H2O2 to phenol
solution, and this is accompanied by the appearance of a new peak at
246 nm corresponding to benzoquinone (n → π* transition) (Fig. 8(b)).
This entire transformation gets completed in 15 min 1H and 13C NMR
show the formation of phenol to hydroquinone and benzoquinone
(Fig. S5). The peak position values of 1H NMR (δ 150.12, 116.11) and
13C NMR (δ 8.69 and 6.56) correspond to hydroquinone, while the peaks
with chemical shift values of 1H NMR (δ 6.87) and 13C NMR (δ 188.21
and 137.23) correspond to benzoquinone. The formation of hydroqui-
none and benzoquinone may be from the controlled and uncontrolled
oxidation of phenol. The phenol oxidation mechanism may involve the
radical generation from the reaction between Mn4+ of the spinel and the
hydrogen peroxide, leading to its reduction to Mn2+ (probably present
as manganous hydroxide). The acidic medium, provided by the natural
acidity of hydrogen peroxide, may reduce Mn4+ to Mn2+, which might
be responsible for the oxidation of phenol to hydroquinone and benzo-
quinone [37].
Fig. 6. (a) UV–visible spectrum obtained by suspended in ethanol, (b) Tauc
plot for the estimation of the bandgap, X-band EPR spectrum recorded at room
temperature (RT) and 77 K, and (d) cyclic voltammogram of Zn2MnO4.
2–340 K are presented in Fig. 7(b). Both plots overlap each other (an
indication of paramagnetic behavior). They tend to show an increase in
magnetization on decreasing the temperature until a bifurcation of ZFC
and FC curves appear at 13 K. This divergence is due to the spin-glass
behavior (inset of Fig. 7(b)) [18,33]. The position of non -magnetic
cations determines the extent of magnetic exchange interactions. Thus,
the hybridization of non-magnetic Zn2+ (3 d) and O 2p orbitals is
essential in determining the material’s ground state. Also, non-magnetic
cations in octahedral sites disrupt the superexchange interaction
mechanism among Mn4+ꢀ O-Mn4+ in octahedra and results in spin glass
frustrated systems [34]. Also, some Mn4+ at the surfaces may be reduced
to Mn3+ by reacting with the atmosphere. The competing magnetic in-
teractions between Mn4+ and Mn3+ are likely to contribute to the
randomness [18]. The frustration index, calculated using the equation f
= θ/Tc, comes out to be 7 [35].
The disintegration of BPA is attempted using Zn2MnO4 as a catalyst
under Fenton-like conditions in a basic medium [38]. The hydroxyl
(OH.) radicals are the reactive species involved when H2O2 is employed
as the oxidant. The results of BPA disintegration carried out under basic
pH conditions in the presence of Zn2MnO4 as the catalyst are presented
in Fig. 8(c–e). The complete degradation of BPA is noticed under basic
conditions (Fig. 8(c)). The control experiment proved that BPA did not
undergo degradation in the absence of Zn2MnO4 (Fig. 8(d)). The reac-
tion follows pseudo-first-order kinetics with a rate constant of 2.13 ×
10ꢀ 2 minꢀ 1 (Fig. 8(e)). The following arguments can be put forth to
explain the catalytic function of Zn2MnO4. Under aerobic conditions,
Mn4+ can be reduced to Mn2+ by the formation of Mn3+ as an inter-
When the inverse of molar susceptibility is plotted against temper-
ature, Curie-Weiss behavior is followed in the temperature range of
110–340 K (inset of Fig. 7(b)). The Curie-Weiss constant (θCW) value of
ꢀ 95.4 K indicates antiferromagnetic interactions between the Mn spins.
The value of effective magnetic moment (in the linear region between
110 and 340 K), using the expression C = μ2eff/8, is 4.31 BM. The slightly
higher magnetic moment as compared to the spin-only value of Mn4+
(3.9 BM) is undoubtedly due to the presence of nearly 20% of Mn3+ in
our samples. However, the amount of Mn3+ in our sample is far less than
mediate. This redox shuttling may be responsible for making Mn4+
a
powerful catalyst in the Fenton-like processes [38]. The first and second
pKa values of BPA are 9.6 and 10.2. At lower pH values, BPA remains
undissociated, but as the pH value increases to 9, its undissociated form
becomes dominant and triggers the catalysis process [39].
The by-products of the catalytic degradation of bisphenol-A are
examined by GC-MS analysis. The peaks 1 and 2 in Fig. 9(a) correspond
to salicylaldehyde and phenylethyl alcohol, and they are the major
products, 49 and 42%, respectively. The minor products are marked as
3, 4, and 5 and correspond to 2-phenyl ethyl acetate (5.2%), phenylethyl
benzoate (1.8%), and phenylethyl phenylacetate (2%), respectively
(Fig. (9(a)). The mass spectra of salicylaldehyde (base peak m/z value of
122), phenylethyl alcohol (base peak m/z value of 91), 2-phenyl ethyl
acetate, phenylethyl benzoate, and phenylethyl phenylacetate (base
peak m/z values of 104), are given in Fig. 9(b)–(f). The formation of
various by-products arising from the multitude of reactions during the
degradation of complex bisphenol-A is well known. The mechanism of
this reaction can be described in the following steps. Hydrogen peroxide
decomposes in the presence of catalyst into OH radicals, as shown in
equation (1) [39], which react with contaminants. The OH radicals may
further recombine to give oxygen and undergo oxidation/degradation of
the pollutant BPA given by equation (2) [40].
Fig. 7. (a) Plot of magnetization vs. the applied magnetic field, (b) magneti-
zation vs. temperature of Zn2MnO4 at an applied field of 1000 Oe. Insets in (b)
show the plot of χꢀ 1 vs. temperature and the magnetization plot’s
expanded region.
Catalyst)
H2O2
2OH
(1)
̅̅̅̅̅̅̅→
5

S. Kushwaha and R. Nagarajan
Journal of Physics and Chemistry of Solids 157 (2021) 110206
Fig. 8. (a) N2 adsorption-desorption isotherms of spinel, (b), and (c) temporal changes in the absorbance spectrum of phenol and XO, respectively in the presence of
spinel and H2O2, (d) plot of (Ct/C0) vs. time and (e) plot of ln(C0/Ct) vs. time for the degradation of BPA.
place sequentially (equation 3-7) [39].
Mn4+ + H2O2 → Mn2+ + ⋅OH2 + H+ … … … … … … … … … … … … …
… … … … … … … … .
(3)
OH2↔ H+ + Oꢀ2 … … … … … … … … … … … … … … … … … … … …
… … … … … … … … …
(4)
OH2 + H2O2 → ⋅OH + H2O + O2 … … … … … … … … … … … … … … …
… … … … … … … .
(5)
Oꢀ2 + H2O2 → ⋅OH + OHꢀ + O2 … … … … … … … … … … … … … … …
… … … … … … … … .
(6)
The ⋅OH may combine finally with the BPA molecule to form a
multitude of products.
Thus, the overall reaction becomes
Zn2MnO4 + BPA → Degraded products … … … … … … … … … … … …
… … … … … . .
(7)
4. Conclusions
The energetics of the metathesis reaction favored a metastable
mixed-metal hydroxide, from which a zinc-rich inverse cubic spinel
containing Mn (IV) in a nano-size regime was recrystallized under hy-
drothermal conditions. The process has overwhelming advantages such
as high yield, simplicity of execution, greater control, uniformity of
products, lower air pollution, and lower energy consumption. The ex-
istence of manganese in the IV oxidation state was established from XPS,
EPR, and cyclic voltammetry measurements. The sample showed an
optical band gap of 2.54 eV. It exhibited a pseudocapacitive behavior,
quite similar to the electrochemical behavior noticed in MnO2. The
stabilization of the higher oxidation state of transition metals without
the requirement of high oxygen pressures adds further significance to
Fig. 9. (a) GC of the degradation products obtained after the Fenton-like re-
action with bisphenol-A as a contaminant, and (b)–(f) mass spectra of com-
pounds 1, 2, 3, 4, and 5 obtained after the degradation of bisphenol-A.
2OH⋅+ BPA → Degradation products … … … … … … … … … … … … …
… … . … . … . … . … . … ..
(2)
In the presence of Mn4+ as the catalyst, the following steps may take
6

S. Kushwaha and R. Nagarajan
Journal of Physics and Chemistry of Solids 157 (2021) 110206
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Shreya executed the experiments, collected and analyzed the data
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The authors declare that they have no known competing financial
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the work reported in this paper.
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Financial support from SERB (EMR/2016/006131) Govt of India and
Institute of Eminence Grant (IoE/FRP/PCMS/2020/27) from University
of Delhi is gratefully acknowledged. SK thanks CSIR, Govt of India, for
the research fellowship. The help extended by TIFR (Mumbai) India for
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