Synthesis, physical and electrochemical properties of CoMn2O4: application to photocatalytic Ni2+ reduction

Article abstract of DOI:10.1007/s11164-020-04371-w

Nickel is a hazardous metal with a harmful effect on the health and environment. In this work, the photocatalytic reduction of Ni²⁺ was examined onto the hetero-system CoMn₂O₄/TiO₂ under visible light irradiation. The spinel CoMn₂O₄ is prepared by co-precipitation and characterized in detail to correlate its structural, textural, morphological, compositional, optical and photoelectrochemical features. The effects of pH, catalyst dose and Ni²⁺ concentration were optimized. The Ni²⁺ reduction increased with decreasing both the catalyst dose and pH. The highest performance was obtained at pH 7.4, a catalyst dose of 1?g/L and Ni²⁺ concentration of 10?mg L^?1 with a removal abatement of 88% after 180?min irradiation. The data were suitably fitted by the pseudo-first-order kinetic reaction with an apparent constant of 0.027?min^?1. Besides, the durability and the stability of the hetero-system CoMn₂O₄/TiO₂ were evaluated by four consecutive catalytic cycles.

Full text of DOI:10.1007/s11164-020-04371-w

Research on Chemical Intermediates (2021) 47:1693–1712
https://doi.org/10.1007/s11164-020-04371-w
Synthesis, physical and electrochemical properties
of CoMn2O4: application to photocatalytic Ni2+ reduction
M. M. Kaci1 · N. Nasrallah1 · M. Kebir1,2 · R. Guernanou3 · A. Soukeur4,5 · M. Trari6
Received: 15 August 2020 / Accepted: 11 December 2020 / Published online: 11 January 2021
© The Author(s), under exclusive licence to Springer Nature B.V. part of Springer Nature 2021
Abstract
Nickel is a hazardous metal with a harmful efect on the health and environment.
In this work, the photocatalytic reduction of Ni2+ was examined onto the hetero-
system CoMn2O4/TiO2 under visible light irradiation. The spinel CoMn2O4 is pre-
pared by co-precipitation and characterized in detail to correlate its structural, tex-
tural, morphological, compositional, optical and photoelectrochemical features. The
efects of pH, catalyst dose and Ni2+ concentration were optimized. The Ni2+ reduc-
tion increased with decreasing both the catalyst dose and pH. The highest perfor-
mance was obtained at pH 7.4, a catalyst dose of 1 g/L and Ni2+ concentration of
10 mg L−1 with a removal abatement of 88% after 180 min irradiation. The data
were suitably ftted by the pseudo-frst-order kinetic reaction with an apparent con-
stant of 0.027 min−1. Besides, the durability and the stability of the hetero-system
CoMn2O4/TiO2 were evaluated by four consecutive catalytic cycles.
Keywords Spinel CoMn2O4 · Co-precipitation · Ni2+ reduction · Visible light ·
Pseudo-frst-order model
*
M. M. Kaci
kacimehdi534@yahoo.com; mkaci@usthb.dz
M. Trari
mtrari@usthb.dz
1
2
Laboratory of Reaction Engineering, Faculty of Mechanical and Process Engineering, USTHB,
BP32, 16111 Algiers, Algeria
Research Centre in Analytical Chemistry and Physics, CRAPC, BP 248, 16004 Algiers RP,
Algeria
3
4
5
Research and Development Centre SONATRACH, 35000 BoCerdes, Algeria
Univ Rennes, CNRS, ISCR-UMR 6226, 35000 Rennes, France
Laboratory of Hydrometallurgy and Inorganic Molecular Chemistry, Faculty of Chemistry,
USTHB, BP 32, 16111 Algiers, Algeria
6
Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry, USTHB,
BP 32, 16111 Algiers, Algeria
Vol.:(0123456789)
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Introduction
One of the most concerns of the contemporizing world is the environmental
pollution due to the growing progress and developed industries, which produce
large amounts of various efuents (drug, pesticides, dyes, metals, etc.) [1]. The
release of these contaminations in water even at low concentrations has posed
a real threat to the human health [2]. Among the pollutants, the heavy metals
have a tendency to react with sulfur and disrupt the activity of vital enzymes in
living organisms. Nickel comes from many industries including cars’ exhaust,
phosphates, chemical fertilizers, food products, industrial waste [3], stainless
steel, nuclear tests, batteries and baking powder [4, 5]. Its presence in water is
also due to dyeing activities, mining, galvanization, smelting, manufacture of bat-
teries, metal fnishing and alloying processes. All these sources result in health
problems and environmentally unfriendly properties above a threshold concentra-
tion. Indeed, the Environmental Protection Agency has set the maximum limit at
0.1 mg/L in the drinking water. Unlike organics, it is not only unable to convert
into a safe product, but has also a strong aggregation property [6].
In this respect, many chemical and physical techniques were developed to
remove nickel from the aquatic medium including: ultrafltration [7], complexa-
tion/ultrafltration [8], complexation/membrane, fltration [9], ion-exchange [10],
reverse osmosis [11], coagulation/focculation [12], fotation [13], precipitation
[14] and electrochemical reduction [15]. Nevertheless, these techniques have their
own drawbacks, such as a low elimination efciency, recyclability, high operation
costs and secondary sludge production that this needs further treatments. In this
regard, the heterogeneous photocatalysis appeared as an attractive strategy, which
has been succeeded in the water purifcation [16, 17]. However, for a better per-
formance, it needs hetero-junctions with two types of semiconductor (SCs) with
appropriate electronic levels: those with a large gap band such as ZnO, TiO2 or
SnO2 [18–21] with good chemical stability and lower photocorrosion combined
to a narrow gap SC as sensitizer; the latter shifts the spectral response toward
higher wavelengths [22, 23]. The use of TiO2 as an electron mediator has been
widely employed for this purpose [24–26], owing to its stability and a conduction
band positioned midway between CoMn2O4-CB and Ni2+/ level.
The spinel CoMn2O4 has a strong catalytic activity for oxidation reactions of
the alkyl compounds, hydrogen absorbents [27] and metals reduction; its conduc-
tion band deriving from 3d metal character has a high reducing power for the
water and metals reductions [28]. Mark et al. [29] reported the photocatalytic
performance of CoMn2O4 NPS for the degradation of Methylene bleu, Malachite
Green and Congo Red under solar light. On the other hand, Misra et al. have
synthetized TiO2@Au@CoMn2O4 nanocomposites by hydrothermal route for the
oxidation of rhodamine B and methyl orange [30]. Lately, 91% degradation of
tetracycline hydrochloride with CuxCo1-xMn2O4 within 120 min via electro-cat-
alytic oxidation is reported [31]. However, to our knowledge no previous study
of this hetero-system in the photocatalytic reduction of nickel has been reported.
According to the above considerations, the main purpose of this study was to
1 3

Synthesis, physical and electrochemical properties of…
1695
demonstrate the efectiveness of the visible light reduction of nickel onto the het-
ero-system CoMn2O4/TiO2.
Materials and methods
Synthesis of material
The co-precipitation assisted with microwave sonication was used for the prepara-
tion of CoMn2O4; Co (NO3)2.6H2O (Sigma-Aldrich 99%) and MnCl2.H2O (Sigma-
Aldrich 98% purity) with a molar ratio of ½ were dissolved in distilled water. Then,
NH4OH solution was slowly added under stirring to adjust the pH at~11. The mix-
ture was sonicated for 30 min by ultrasonic irradiation at 800 MHz (Ultrasonic
homogenizer UZ Sonopuls HD 2070) and subsequently transferred in a microwave
system under a power of 800 W for 15 min. Thereafter, the black precipitate was
centrifuged, fltered of, thoroughly washed with distilled water and dried at 120 °C
for 24 h. Finally, the recovered powder was ground in an agate mortar and annealed
in air from 500 to 800 °C for 6 h (5 °C/min). The end product was subjected to
diferent techniques for its identifcation. TiO2 was purchased from Degussa (TiO2
– P25) and used as received.
Characterization techniques
The purity of CoMn2O4 was confrmed by X-ray difraction (XRD) using a PANa-
lytical Empyrean Difractometer with a Cu Kα radiation (λ=1.5418 Å) in the 2θ
range (10–70°) at a scan rate of 0.013° min−1.
The specifc surface area (SBET) was determined from N2 adsorption using a BET
analyzer (ASAP-2010, Micrometrics V5.01 G) under a relative pressure (P/Po) of
10–6. The sample was degassed at 140 °C (5 h), and SBET was calculated from the
BET equation within the relative pressure range (0.1–1).
The morphology and microstructure of the powder have been studied by scanning
electron microscopy (SEM, Quanta 650) operating with a voltage of 10 kV. The
chemical composition was determined by the energy-dispersive X-ray spectroscopy
(EDX) while the distribution of elements was obtained by mapping analysis. The
TEM images were used for further microstructural observations on a JEOL 2100F
transmission electron microscope with an acceleration voltage of 200 kV.
The XPS measurements were carried out with a Kratos axis supra spectrometer,
working with a constant pass energy of 20 eV and a dual X-ray source of Al Kα
(1486.6 eV). C 1 s signal located at 284.4 eV was used as internal reference.
The chemical characterization of the functional groups of the powder was eluci-
dated by the Fourier transform infrared spectroscopy (FTIR-8900, Shimadzu) in the
range (440—4000 cm−1) using spectroscopic KBr. The difuse refectance was plot-
ted between 190 and 900 nm with a UV–visible spectrophotometer (Jasco V- 650)
equipped with an integrating sphere. The electrical conductivity (σ) was measured in
the range (340—440 K) with the two-probe technique.
1 3

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M. M. Kaci et al.
The powder was pressed under 2.5 bar, and the pellets were sintered at 850 °C
(2 h). Silver lacquer assumed the electrical contact on the back pellet of the working
electrode (WE ∼ 1 cm2) which was encapsulated in a glass tube, isolated by araldite
and polished with SiC paper (1200).
The electrochemical experiments were carried out in KOH (0.1 M) electrolyte
in a three-electrode cell with Pt auxiliary electrode, SCE as reference and WE. The
scan rate of the potential was set at 10 mV, and the intensity potential J(E) charac-
teristic was plotted by a PGZ 301 potentiostat; the interfacial capacitance was meas-
ured at a frequency of 10 kHz. The electrochemical impedance spectroscopy (EIS)
data were recorded over the frequency range (100 kHz–10 MHz) after stabilization
of the free potential; the measurements were carried out both in the dark and under
illumination with a halogen-tungsten lamp (200 W).
Photocatalysis
The photocatalytic tests were realized in August between 12 am and 4 pm with an
average temperature of 32 °C and a solar fux of 94 mW cm−2. The stock solution
was prepared by dissolving 1 g of NiSO4, 6H2O (Merck, 99%) in distilled water (1
L); fve concentrations (10, 20, 30, 40 and 50 mg/L) were prepared by dilution; the
pH was adjusted by addition of HCl or NaOH (0.1 M).
The tests were carried out in an open Pyrex reactor (500 cm3 capacity). Before
irradiation, the powder (CoMn2O4/TiO2 =100 mg /100 mg), suspended in Ni2+
200 mL solutions (10–50 mg /L), was kept in the dark under stirring for 2 h in the
solution to reach the adsorption equilibrium; the reactor was then exposed to solar
light for 4 h. Aliquots were regularly removed and centrifuged (4000 rpm, 10 min).
The residual Ni2+ concentration was analyzed by atomic absorption spectroscopy
(Zeenit 700); the photoreduction rate was calculated from the relation:
(Ceq − Ct)
R% =
× 100
(1)
Ceq
where Ceq and Ct are the concentration at equilibrium and at a time (t), respectively.
Results and discussion
Characterization
The purity of the spinel CoMn2O4 prepared by co-precipitation is confrmed by
XRD analysis. The theoretical pattern depicted in Fig. 1 matches well the theo-
retical one indicating a single phase with high purity. All peaks are indexed in a
tetragonal symmetry (space group: I41/amd, N° 141); the distorted structure is
due to the Jahn–Teller ion Mn3+ with an odd number of electrons (3d4), leading
3
2
20
to the confguration (t2g dx - y dz21). No starting oxides or secondary phases
were detected. The refned lattice parameters: a = 5.7840 Ǻ and c = 9.0910 Ǻ,
are in perfect agreement with the JCPDS Card N° 98–003-9197 [32]. The mean
1 3

Synthesis, physical and electrochemical properties of…
1697
0
10
20
30
40
50
60
70
2 Theta (degree)
Fig. 1 X-ray difraction pattern of CoMn2O4 prepared by co-precipitation
crystallite size (D = 27 nm) was calculated from the full width at half maximum
(β, rd) of the intense XRD peak (121):
0.9ꢀ
D =
(2)
ꢁ cos ꢂ
The textural properties of CoMn2O4 composite were inferred from their
adsorption desorption isotherms according to the BET analysis (Fig. 2). The iso-
therm is typical of IV isotherms, according to IUPAC classifcation with H3 hys-
teresis loop observed at low relative pressure (0.9–1), confrming the mesoporos-
ity of the spinel [33]. The BET surface area of CoMn2O4 is equal to 18.19 m2 g−1.
The morphology and microstructure of the powder were analyzed by the SEM
technique; the micrograph (Fig. 3) shows that the grains are fairly homogeneous
with uniform shapes whose size lies between 60 and 100 nm. The grains are grouped
and linked together, forming large agglomerates. Additionally, for a detailed mor-
phological study, the synthesized CoMn2O4 was analyzed by the TEM technique.
Figure 4 depicts the TEM images of the prepared composite, where the morphology
of material is characterized by an agglomeration of nanoparticles.
The elemental composition determined by EDX (Fig. 5) reveals the presence
of Co, Mn and O elements with an average molar ratio Co/Mn of ½ which agrees
with the XRF analysis and confrms the formulation of the spinel with MnO
(67.15wt.%) and CoO (32.84wt.%). Moreover, the elementary mapping analysis
shows a uniform distribution of these three elements (Fig. 6).
The elemental composition and the surface oxidation states of the CoMn2O4
nanocomposite were also investigated by XPS analysis (Fig. 7). The survey
1 3

1698
M. M. Kaci et al.
25
20
15
10
5
adsorption
desorption
0
0,0
0,2
0,4
0,6
0,8
1,0
Relative pressure (P/PO)
Fig. 2 N2 adsorption desorption analysis of CoMn2O4
Fig. 3 Scanning electron microscopy of CoMn2O4 powder synthetized by co-precipitation
spectrum shows that the oxide consists of Co, Mn and O elements, as well as C
(Fig. 7a). The existence of a single C1s peak with a binding energy of 262.2 eV
is assigned to the hydrocarbon contaminant [34]. Figure 7b–d reveals the high-
resolution spectra of the Co 2p, Mn 2p, and O 1 s.
Co 2p spectrum (Fig. 7b) shows two major peaks with 779.88 (Co 2p1/2) and
795.18 eV (Co 2p3/2). Co 2p3/2 displays two peaks at 779.48 and 781.78 eV, indi-
cating the presence of Co2+ and Co3+ [35, 36]. In addition, the extra peak at
1 3

Synthesis, physical and electrochemical properties of…
1699
Fig. 4 TEM images of CoMn2O4 nanocomposite
Fig. 5 EDX spectrum of CoMn2O4
786.88 eV is associated with the shake-up satellite, corroborating the multivalent
cobalt.
Moreover, Fig. 7c discloses the binding energy of Mn ions in CoMn2O4. Two
peaks at 641.48 and 652.98 eV are detected in the Mn 2p region, which belong
energy of Mn 2p3/2 and Mn 2p1/2 with the fssion width of 11.5 eV revealing the
oxidation state of Mn [37, 38]. Both Mn 2p3/2 peaks could be deconvoluted into
three major components, the peak at 643.38 eV (Mn4+) with those at 641.78 and
640.48 eV belonging to Mn3+ and Mn2+, respectively [32]. The appearance of
Co3+, Mn2+ and Mn4+ suggests the probability of Co2O3 and MnOx [39], not
1 3

1700
M. M. Kaci et al.
Fig. 6 SEM image a. EDX elemental mapping views corresponding of O b, Co c and Mn d
detected in the XRD pattern due to the peak broadening [40]. Finally, Fig. 7d
shows the XPS spectrum of O 1 s, which can be deconvoluted into fve peaks
(529.48 eV) attributed to metal–oxygen bonding and (531.48 eV) assigned to the
metal-OH bonding [41–43]. The peaks located at 528.38, 530.58 and 532.98 eV
are due to absorbed O2 on the surface, crystal lattice oxygen and chemisorbed
oxygen group (OH). Such results confrm the XRD and EDX analysis.
The FTIR spectroscopy was done to identify the main functional groups
in CoMn2O4. The spectrum (Fig. 8) shows three main bands at 398, 485 and
603 cm−1 attributed to the inorganic vibrations of Co–O, Mn–O and Co–O-
Mn, respectively [44], thus supporting the formation CoMn2O4. No peaks
at ~ 1600 cm−1 were observed, indicating a weak hygroscopy of our oxide.
The difuse refectance spectrum of CoMn2O4 is presented in Fig. 9; the band
gap (Eg) is crucial in photocatalysis and is determined from the well-known rela-
tion [45]:
(
)
(ꢀhꢁ)1∕n = Constant × hꢁ − Eg
(3)
where α is the optical absorption coefcient. The gap Eg is obtained from the graph
(αhν)n versus the incident energy (hν). Among them, n depends on whether the
transition is direct (n=1) or indirect (n=4). The direct gap is evaluated at 1.60 eV
1 3

Synthesis, physical and electrochemical properties of…
1701
Mn 2p
Mn 2p
(a)
Survey
3/2
(c)
641,78
V
O 1s
Mn 2p
Mn 2p
1/2
643,38
V
Co 2p
640,48
V
C 1s
660 655 650 645 640 635 630
BindingEnergy(ev)
1200
1000
800
600
400
200
0
Binding energy (ev)
529,48
O1s
Co 2p
(b)
(d)
Co 2p
3/2
Co 2p
779,48
531,48
1/2
781,78
530,58
532,98
786,44
528,38
540 538 536 534 532 530 528 526 524
820
810
800
790
780
770
Binding Energy (ev)
Binding energy (ev)
Fig. 7 XPS spectrum of CoMn2O4 composite a survey spectrum, b cobalt 2p, c manganese 2p and d
oxygen 1 s
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 8 FTIR spectra of the spinel CoMn2O4
1 3

1702
M. M. Kaci et al.
50
40
30
20
10
0
Eg=1,60 eV
1,4
1,5
1,6
1,7
hv (eV)
Fig. 9 Direct optical transition of CoMn2O4 prepared by co-precipitation
by extrapolation the linear part of (αhν)2 to hν-axis, a value consistent with that
reported elsewhere [46]; the slight diference is due to the quantum size efect.
The semiconductivity is also important in photocatalysis; for this goal we have
plotted the electrical conductivity (σ) vs. 1000/T in the region (340 – 440 K)
(Fig. 10) obeys to an exponential law:
-5,5
-6,0
E = 0.49 eV
a
-6,5
-7,0
-7,5
-8,0
-8,5
2,2
2,4
2,6
2,8
1000/T(K)
Fig. 10 Variation in Log σ as a function of 1000/T for CoMn2O4
1 3

Synthesis, physical and electrochemical properties of…
1703
ꢀ
}
ꢀ = ꢀoexp −Ea∕kT
(4)
The linearization of this equation gives an activation energy (Ea=0.49 eV), equal to
the diference between the Fermi level and the valence band (EF—EVB). Our normal
spinel has a moderate conductivity due to mixed states Mn4+/3+ in octahedra sharing
common edges [47], where the conduction is attributed to electron hopping between
these two cations located in the same crystallographic site; the oxide belongs to the
class II in the Day and Robin classifcation with the existence of small polarons and
mixed states.
Electrochemical and photoelectrochemical properties
This part aims to elucidate the electron transfer mechanism of CoMn2O4 through the
photoelectrochemical properties, which characterize the electrode/electrolyte interfaces
and analyze the charges transfer kinetic.
The EIS analysis was performed in the dark and under irradiation after stabilization
of the free potential (Fig. 11a). CoMn2O4 exhibits a small semicircle whose diameter
decreases when under irradiation, thus confrming the semiconductivity and indicating
a facile charge transfer, which should lead to increased photoactivity. The simulated
results are in good agreement with the experimental data.
The cyclic voltammetry (CV) was traced in the range (− 1.4–0.2 V) at a scan rate
of 10 mV s−1. Figure 11b shows well-separated cathodic peaks indicating at − 1.2 and
− 0.82 V which correspond to the reduction of Co2+ to Co and Mn3+ to Mn2+, respec-
tively [48, 49]; the anodic peaks at − 0.53 and − 0.87 V are attributed to their re-oxida-
tions [50–52].
The capacitance potential (C−2- E) plot permits to calculate the fat band potential
(Efb) which is a key factor for determining the photocatalytic performance of the spinel
(Fig. 12):
1
C2
2
=
(E − Efb)
(5)
ꢀꢀoeND
The extrapolated plot to infnite capacity (C−2 =0) gives a potential Efb (− 0.042 V),
while the negative slope indicates p-type behavior with holes as dominant charge carri-
(
)
−1
2
dC−2
ers whose density (NA=1017 h+/cm−3) was calculated from the slope:
.
eꢀꢀ0
dE
The energies of the valence band (VB) and conduction band (CB) of CoMn2O4 are
calculated from the relations:
(
)
EVB = 4.75 + eEfb + 0.059 pH − pHpzc + Ea
(6)
(7)
ECB = EVB − Eg
1 3

1704
M. M. Kaci et al.
Fig. 11 a EIS plot of CoMn2O4 in the dark and under visible light irradiation measured in 0.1 M KOH
aqueous solution. b Cyclic (I-E) characteristic
Fig. 12 Mott–Schottky plot of
CoMn2O4 plotted in 0.1 KOH
electrolyte
1 3

Synthesis, physical and electrochemical properties of…
1705
(pHpcz=6.4) is the pH at the point of zero charge (pzc); it is determined from the
equilibrium pH of a solution containing an excess of CoMn2O4 powder and corre-
sponds to the pH for which the surface charge is zero. The values of VB and CB are
(0.5 V/5.23 eV) and (− 1.1 V/4.63 eV) in the dark and light, respectively.
Photocatalytic activity and kinetic study
Efect of initial pH and catalyst dose
The optimization of the operating parameters contributes to enhance the photore-
duction of Ni2+ over the hetero-system CoMn2O4/TiO2. The efect of pH, catalyst
dose and Ni2+ initial concentrations (Co) on the photoreduction efciency is investi-
gated. However, before proceeding with the photocatalysis, the adsorption–desorp-
tion was performed in the dark. In addition, it should be noted that the hetero-system
CoMn2O4/TiO2 displays an adsorption efciency of 24% after 2 h.
The pH of wastewater containing nickel averages 3, depending on the nature
of co-existing compounds [53]. In this study, the pH efect on the Ni2+ reduction
was studied over the range (2—7.4), while the concentration Co, hetero-system
percentage and irradiation time were taken equal to 10 mg/L, 50 wt.% CoMn2O4
/ 50 wt.%TiO2 and 4 h, respectively. It has been observed that the Ni2+ reduction
increases with augmenting pH and reaches an efciency of 88% at neutral pH
(Fig. 13a). The Ni2+ reduction falls to 80, 69, 53 and 29% for pHs 7, 6, 4 and 2,
respectively. A possible explanation of this trend can be attributed to pHPZC of TiO2
(=6.8) [54]; the catalyst surface is negatively charged: the species [CoMn2O4/TiO2]
is predominant and absorb Ni2+ ions by electrostatic attractions [55]. Conversely, the
weak performance at low pH is due to the competitive reduction of H+ as shown in
our previous work [56]. Therefore, pH 7.4 was selected for the further experiments.
The hetero-system improves the charge separation responsible for redox reac-
tions on the catalyst surface and shifts the light absorption of TiO2 toward the
visible region [57, 58]. For this purpose, we determine the optimal percentage of
CoMn2O4 and TiO2 required for the Ni2+ photoreduction rate. The efect of the per-
centage weight (x% CoMn2O4 / y% TiO2) was examined under solar irradiation by
varying x from 0 to 100% wt %, while keeping Co at 10 mg/L and pH at 7.4. It has
been observed that the photoactivity is higher than that of individual semiconductor
showing clearly the synergy efect.
Figure 13b indicates that the best performance occurs for a mass ratio (50/50).
This enhancement is due to the fact that TiO2 excess covers the active sites on the
CoMn2O4 surface and hinders the charge separation efciency. This reduction is
attributed to the dispersion of the sensitizer onto TiO2 which acts as recombination
centers of (e−/h+) pairs.
Efect of initial concentration
The charge transfer between Ni2+ and photoelectrons facilitates the reduction, and
the Ni2+ adsorption on the catalyst surface afects signifcantly the photoactivity.
1 3

1706
M. M. Kaci et al.
100
75
(a)
pH~ 7,4
pH~6
50
pH ~4
25
pH~ 2
0
pH
100
(b)
75
50
25
0
0
25
50
75
100
CoMn
O
Ti
O
2
2
4
Mass ration
Fig. 13 a Infuence of pH. b Infuence of the mass ratio of CoMn2O4 and TiO2 on the Ni2+ photoreduc-
tion
Therefore, the infuence of the Ni2+ concentration Co on the reduction onto
(CoMn2O4/TiO2) by varying Co from 10 to 50 mg/L, at free pH 7.4, catalyst dose
of 1 g/L and solar irradiation time of 4 h was studied (Fig. 14a). As expected,
the photoreduction is enhanced for low Ni2+ concentrations Co. The abatement
reaches 88% for 10 mg/L after 180 min, whereas for Co value of 50 mg/L, the
1 3

Synthesis, physical and electrochemical properties of…
1707
Ni2+ 10 mg/L
(a)
50
40
30
20
10
0
In the dark
Under illumination
20
30
40
50
-120
-60
0
60
120
180
240
Irradiation time (min)
(b) 1,4
10 mg/L
20
30
40
50
1,2
1,0
0,8
0,6
0,4
0,2
0,0
0
10
20
30
40
50
Irradiation time (min)
Fig. 14 a Evolution of Ni2+ concentrations as a function of irradiation time. (b) Kinetic of the Ni2+ ver-
sus time of irradiation
photoreduction declines down to 48% because the number of the photoactive site
becomes not sufcient to reduce all Ni2+ ions and this is due to the decrease in
(e−/h+) pairs. On the other hand, the photons fux was weakened before reaching
the semiconductor surface, leading to a low photoreduction.
1 3

1708
M. M. Kaci et al.
Table 1 First-order kinetic
constant (Kapp), R2 and t1/2 under
diferent initial concentration
Ni2+
Co (mg/L)
10
20
30
40
0.006
0.977
50
0.0051
0.90
K
app (min−1
(min)
)
0.0276
0.966
25.114
0.0176
0.968
39.383
0.0147
0.992
47.152
2
R
t
115.524
135.911
½
Table 2 Comparison of photocatalytic efciency of other previously reported works for reduction of Ni2+
in recent years
Catalyst
Co (mg L−1
)
Catalyst dose ( Irradiation time Removal ef-
Reference
g L−1
)
( min)
ciency (%)
CuCo2O4/TiO2
CuFe2O4/TiO2
ZnMn2O4
30
30
18
10
1.0
1.0
0.5
1
180
210
140
180
60
72
77.5
88
[55]
[54]
[59]
This work
CoMn2O4/TiO2
Kinetic study of Ni2+ photoreduction
The photoreduction kinetic over the hetero-system CoMn2O4/TiO2 was also con-
sidered. The linear plots indicate an apparent pseudo-frst order which fts well
the experimental data at low concentrations. The linear relationship between ln
(Co/C) and the irradiation rate follows a pseudo-frst-order kinetic:
dC
−
= Kapp × C
(8)
dt
Kapp (mn−1) is the apparent rate constant; the integration of Eq. (6) can be
expressed as follows:
(
)
C0
Ct
Ln
= Kapp × t
(9)
where C (mg/L) is the Ni2+ concentration at specifc time (t). The plot ln(Co/C) ver-
sus reaction time (t) for diferent Co concentrations is shown in (Fig. 14b), and as
expected, Kapp decreases with increasing Co (Table 1). The half-life time (t 1/2) is
calculated by the following relationship:
Ln(2)
t1∕2
=
(10)
Kapp
To evaluate the reduction of Ni2+ by CoMn2O4/TiO2, a comparison is made
with other works (Table 2). Our hetero-system has relatively high performance
and seems satisfactory for treating wastewater contaminated with nickel.
1 3

Synthesis, physical and electrochemical properties of…
1709
Cycles stability
The reusability of the photocatalyst is a deciding factor for practical applica-
tions [60]. For this aim, a series of recycling experiments for Ni2+ photoreduction
have been carried out under the same experimental conditions and the results are
shown in Fig. 15a. As can be seen, after the fourth run, the removal abatement
has decreased from 88 to 77%. This reduction (11%) is ascribed to clogging of
active sites in the course of process. Also, the SEM images (Fig. 15b and c) of
the recovered catalyst after the fourth run show that the structure of CoMn2O4
remains almost unchanged, indicating the photostability of CoMn2O4/TiO2.
(a)
80
70
60
50
40
30
20
10
0
1st run
2nd run
3nd run
4nd run
(b)
(c)
Fig. 15 a Cycling runs in the photocatalytic reduction of Ni2+ using CoMn2O4/TiO2. b and c SEM
images after the fourth recycling of the recovered CoMn2O4/TiO2
1 3

1710
M. M. Kaci et al.
Based on the above fndings, the electrons generated by irradiation in
CoMn2O4-CB (− 1.10 V) are transferred into the TiO2-CB (− 0.97 V) acting as an
electron bridge (Fig. 16), which subsequently reduces Ni2+. This shows the injection
phenomenon where the relevant reactions on CoMn2O4/TiO2 are:
(
)
p − CoMn2O4 + hꢀ → CB − CoMn2O4(e−) + VB − CoMn2O4 h+
(11)
(
)
n − TiO2 + hꢀ → CB − TiO2(e−) + VB − TiO2 h+
(12)
(13)
(14)
CB − CoMn2O4(e−) + n − TiO2 → CoMn2O4 + TiO2(2e−)
TiO2(2 e−) + Ni2+ → Ni + TiO2
The activation of TiO2 by the photons UVA improves considerably the Ni2+
reduction. The deposited Ni is short-circuited to TiO2 where hydrogen is easily lib-
erated because of the low H2 over-voltage, and this is also responsible of the decel-
eration of Ni deposition.
Conclusion
This work was devoted to the preparation of the spinel CoMn2O4 by co precipitation
method and the physical and electrochemical properties, which have been combined
to establish the potential/energy diagram. The reduction of Ni2+, an environmentally
hν
-4.75
e-
e-
e-
CB= -1.10 V
VB= 0.5 V
Eg =1.60 eV
CB= -0.8 V
e-
EF
Ea=0.49 eV
Eg = 3.21 eV
h+
p-CoMn2O4
Ni2+
Ni
VB= 2.4 V
h+
n -TiO2
Potential (VSCE
)
Fig. 16 Mechanistic scheme for the reduction of Ni2+ under solar light
1 3

Synthesis, physical and electrochemical properties of…
1711
harmful metal released unregulated in the natural environment by industrial efu-
ents, was studied under solar irradiation. The experimental results revealed that the
Ni2+ photoreduction reached 88% within 3 h of irradiation for (50% CoMn2O4/50%
TiO2) of total catalyst dose of 1 g/L, pH 7.4 and 10 mg/L of initial concentration.
Under the working conditions, the photocatalytic reduction followed the pseudo-
frst order with a rate constant of 0.0276 min−1. Besides, the stability of the het-
ero-system has been studied by conducting recyclability tests up to 4 consecutive
cycles using CoMn2O4/TiO2 nanocomposite, indicating that the nanoparticles may
be reused without substantial weigh loss. The photoactivity slows it down over illu-
mination time because of the progressive occupation of photoelectrochemical sites
and by competition in water reduction.
Acknowledgements The authors are grateful to Dr M.Benamira for his technical assistance in the Elec-
trochemical analysis and Vincent Dorcet (THEMIS platform, University of Rennes 1) for TEM analy-
sis. The work was fnancially supported by the Faculty of Mechanic and Engineering Process (USTHB,
Algiers).
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