High catalytic efficiency of nanostructured β-CoMoO4 in the reduction of the ortho-, meta- and para-nitrophenol isomers

Article abstract of DOI:10.3390/molecules23020364

Nanostructured -CoMoO4 catalysts have been prepared via the thermal decomposition of an oxalate precursor. The catalyst was characterized by infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller method (BET), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). The efficiency of these nanoparticles in the reduction of ortho- and meta-nitrophenol isomers (2-NP, 3-NP, and 4-NP) to their corresponding aminophenols was tested using UV-visible spectroscopy measurements. It was found that, with a -CoMoO4 catalyst, NaBH4 reduces 3-NP instantaneously, whilst the reduction of 2-NP and 4-NP is slower at 8 min. This difference is thought to arise from the lower acidity of 3-NP, where the negative charge of the phenolate could not be delocalized onto the oxygen atoms of the meta-nitro group.

Full text of DOI:10.3390/molecules23020364

molecules
Article
High Catalytic Efficiency of Nanostructured
β-CoMoO4 in the Reduction of the Ortho-, Meta- and
Para-Nitrophenol Isomers
Fahd Al-Wadaani 1, Ahmed Omer 1, Mostafa Abboudi 1, Hicham Oudghiri Hassani 1,2,*,
Souad Rakass 1, Mouslim Messali 1 and Mohammed Benaissa 3
1
Chemistry Department, College of Science, Taibah University, Almadinah 30002, Saudi Arabia;
fwadaani@taibahu.edu.sa (F.A.-W.); ahmed.hussein.omer@gmail.com (A.O.);
abboudi14@hotmail.com (M.A.); rakass_souad@yahoo.fr (S.R.); aboutasnim@yahoo.fr (M.M.)
Département Sciences de la Nature, Cégep de Drummondville, 960 Rue Saint-Georges, Drummondville,
2
QC J2C 6A2, Canada
3
LMPHE, Département de Physique, Faculté des Sciences, Université Mohammed V, B.P. 1014 RP,
Rabat 10000, Morocco; benaissa.um5@gmail.com
*
Correspondence: Oudghiri_hassani_hicham@yahoo.com; Tel.: +966-543-549-454
Received: 8 January 2018; Accepted: 7 February 2018; Published: 9 February 2018
Abstract: Nanostructured β-CoMoO4 catalysts have been prepared via the thermal decomposition
of an oxalate precursor. The catalyst was characterized by infrared spectroscopy (FTIR),
X-ray diffraction (XRD), Brunauer-Emmett-Teller method (BET), energy dispersive X-ray
spectroscopy (EDX), and transmission electron microscopy (TEM). The efficiency of these
nanoparticles in the reduction of ortho- and meta-nitrophenol isomers (2-NP, 3-NP, and 4-NP) to
their corresponding aminophenols was tested using UV-visible spectroscopy measurements. It was
found that, with a β-CoMoO4 catalyst, NaBH4 reduces 3-NP instantaneously, whilst the reduction of
2-NP and 4-NP is slower at 8 min. This difference is thought to arise from the lower acidity of 3-NP,
where the negative charge of the phenolate could not be delocalized onto the oxygen atoms of the
meta-nitro group.
Keywords: nanostructures; β-CoMoO4; nitrophenol reduction; nanoparticles
1. Introduction
Aminophenols are implicated in the synthesis of pharmaceutically active compounds [
For instance, p-aminophenol is an important intermediate in the preparation of several analgesic
and antipyretic drugs, such as paracetamol, acetanilide, and phenacetin [ ]. In addition, aminophenols
are key ingredients in the production of metal-complex dyes and are used in corrosion inhibition,
hair-dying agents, and polymer production [ ]. Due to their diverse applications, the development
1].
2
1,2
of efficient processes for the synthesis of aminophenols has gained increasing attention.
One method of synthesizing aminophenols is via the reduction of nitrophenols.
Nanostructured transition metal molybdate materials have shown promise as catalysts for this
process [
for this process [
and a high temperature
processes such as cracking, hydrogenation, dehydrogenation, and hydrodesulfurization (HDS) [6,9
3
,4
]. In particular, nanostructured CoMoO4 provides a non-toxic and inexpensive candidate
]. CoMoO4, which may exist in several phases including a low temperature -phase
-phase, [ ] is used as a catalyst in many chemical [ ] and petrochemical
].
5
α
β
3,
5–7
8
The catalytic properties of CoMoO4 depend on the structure of the material, which in turn depends
on the method of preparation [10]. Various methods have been developed for the synthesis of CoMoO4
including precipitation, sol–gel, solid state reaction, hydrothermal, complete evaporation of a
polymer-based metal-complex precursor solution, microemulsion and sonochemical [11–16].
Molecules 2018, 23, 364; doi:10.3390/molecules23020364
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Several materials were tested in the reduction reaction of the nitrophenol isomers. The ferrites
materials CuFe2O4 and NiFe2O4 were studied by Goyal et al. and found that the copper containing
ferrite was a better catalyst for these reactions [17]. Moreover, Nandanwar et al. investigated the
efficiency of CuO/γAl2O3, which showed good catalytic activity [18]. Ni/C black composite material
was also tested as a catalyst for this reaction by Xia et al., and low efficiency was observed [19].
Better results were found in a study conducted by Oudghiri on the iron molybdate Fe2(MoO4)3 [20].
Ghorai et al. worked with a mixed molybdate phosphate of cobalt, copper, and chromium that
was were found to efficiently reduce 4-NP [21]. In recent years, the application of CoMoO4 to the
catalytic degradation of organic dyes has started to receive increasing attention. For instance,
CoMoO textsubscript4 nanoparticles prepared via the microemulsion method have been shown to be
quite effective at eliminating reactive black (RB) 8 dye from aqueous solution, with a decolorization
efficiency of 91.47% [12].
In this study, nanostructured
of an oxalate precursor prepared in the solid state using a new method. The obtained powder was
found to be formed of nanoparticles. The freshly synthesized -CoMoO4 nanoparticles were tested as
β-CoMoO4 catalysts were prepared via the thermal decomposition
β
catalysts in the reduction of three aminophenol isomers (2-NP, 3-NP, and 4-NP) to their corresponding
aminophenol isomers.
2. Material and Methods
2.1. Materials
Cobalt nitrate (Co(NO3)2
·
6H2O), ammonium molybdate ((NH4)6Mo7O24 4H2O), and oxalic acid
·
(H2C2O4 2H2O) were obtained from Aldrich (St. Louis, MO, USA). All the chemicals were of analytical
·
grade and used without further purification.
2.2. Catalyst Preparation
The cobalt molybdate catalyst (CoMoO4) was prepared by thermal decomposition of an oxalate
◦
precursor in a tubular furnace (open on both sides) at 550 C for 2 h. The oxalate precursor was
obtained by the solid state reaction of a well-ground mixture of cobalt nitrate (Co(NO3)2
·
6H2O),
ammonium molybdate ((NH4)6Mo7O24
·
4H◦2O), and oxalic acid (H2C2O4
·
2H2O) in the molar ratio
1:1/7:5, respectively, on a hotplate at 160 C for 1 h [22]. During heating, an orange/brownish
gas is evolved, indicating the formation of NO2 gas due to the reduction of the nitrate anions.
In the meantime, the mixture takes a dark blue coloration confirming the reduction of the molybdenum
(+VI) to lower oxidation states (+V or +IV). The oxalic acid acted not only as a reducing agent but also
as a complexing agent leading to the formation of the oxalate complexes of the existing metals [23,24].
The advantage of this preparation method is that it occurs in the solid state without adding any solvent.
◦
There is no factor to control other than the temperature of the precursor formation at 160 C and
the heating temperature at 550 ◦C. No strict conditions such as higher temperatures or pressures are
needed. Moreover, the time of the preparation of the nanoparticles is limited to a few hours with a yield
of almost 100%. All synthesis methods encountered in the literature require the use of solvents for the
filtration and drying stages. In the solid-state reaction between MoO3 and Co3O4, higher temperatures
as well as regrinding mixture and annealing stages are needed to obtain improved homogeneity.
2.3. Characterization
In order to study the decomposition with temperature of the prepared oxalate precursor,
thermal analyses were conducted on an SDT Q600 instrument (Ta Instruments, Hayesville, NC, USA).
Infrared spectroscopy studies were conducted on a Shimadzu 8400S (Shimadzu, Tokyo, Japan),
to confirm the oxalate complex formation. To verify the crystallinity and the purity of the prepared
cobalt molybdate, the X-ray diffraction patterns were recorded on a Shimadzu X-ray diffractometer
´
6000 (Shimadzu, Tokyo, Japan), using Cu Kα radiation (1.5406 Å equipped with a Ni filter. The patterns

Molecules 2018, 23, 364
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were recorded in the range of 10–80◦ in 2
θ
. Crystallites size was calculated using the Scherer formula
DXRD = 0.9
λ
/(B cos
θ
), where
λ
is the Cu K
α wavelength, B is the full width at half maximum
(FWHM) expressed in radians, and
θ
is the Bragg angle. Particle sizes could also be estimated
using the equation DBET = 6000/d.S, where d is the density and S is the specific surface area
calculated from the adsorption/desorption isotherms that were obtained from a Micrometrics ASAP
2020 (Micromeritics, Norcross, GA, USA), surface area and porosity analyzer. The variation of
the nitrophenol isomer (2-NP, 3-NP and 4-NP) concentrations during the catalytic transformation
to their corresponding aminophenol isomers was measured using a Varian Cary 100 UV-visible
spectrophotometer (Varian Inc., Palo Alto, CA, USA). Transmission electron microscopy (TEM) was
performed using a Tecnai-12 electron microscope (FEI, Hillsboro, OR, USA), operated at 120 kV to
investigate the shape and size of the particles. For the TEM measurement, the sample was prepared by
simply grinding the powder between two glass plates and bringing the fine powder into contact with
a carbon-coated copper TEM grid. Energy dispersive X-ray (EDX) microanalyses were obtained using
an EDAX detector coupled to a TEM instrument.
2.4. General Procedure for the Reduction of Nitrophenol Isomers
The prepared cobalt molybdate powder was tested as a catalyst in the reduction of
three nitrophenol isomers (2-NP, 3-NP, and 4-NP) to their corresponding aminophenol isomers.
In a typical test, 40 mL of an 8
×
10−4 M aqueous solution of sodium tetrahydroborate (NaBH4) were
added to 40 mL of a 4
×
10−4 M aqueous solution of the nitrophenol isomer under continuous stirring at
ambient temperature. The solution color immediately became dark yellow, with an absorption located
at 415, 393, and 401 nm for the ortho-isomer (2-NP), the meta-isomer (3-NP), and the para-isomer (4-NP),
respectively, due to the resonance stabilization of the formed nitrophenolate anions. One hundred
milligrams of the cobalt molybdate catalyst was then added to the mixture under continuous stirring.
The reaction was monitored via UV visible spectroscopy measurements. Absorbance intensity decreases
as nitrophenolate anion concentration decreases, thus allowing us to use this relationship as a measure
of the efficiency of the catalyst in this reduction reaction.
3. Results and Discussion
3.1. Complex Precursor Characterizations
As the first step, the product of the solid state reaction between the mixture of Co(NO3)2
·
6H2O
and the ammonium molybdate ((NH4)6Mo7O24 4H2O) with the oxalic acid was studied by infrared
·
spectroscopy. The FTIR spectrum of the resulting complex is shown in Figure 1. T−he1 spectrum is
similar to previous studies for oxalate complexes [25–28]. The bands at 920 and 963 cm are assigned
to the Mo=O stretching, the 1360 and 1317 cm−1 region can be assigned to
υ(C-O) and δ(OCO), and the
1402 cm−1 band to the C-O stretching [28]. The spectrum also reveals the presence of a large band−in1
the range of 1800–1550 cm−1. The deconvolution of this band shows bands at 1731 and 1675 cm
,
which can be assigned to the C=O vibration of the oxalate group [27,
28] and a band at 1635 cm−1
corresponding to δ(H2O) [26]. At high frequency, the spectrum reveals a broad intense band in
the 2850–3750 cm−1 range. The deconvolution of this broad band reveals bands at 3385 cm−1
,
,
which can be attributed to the OH bridging group between two metal ions [29], and at 3170 cm−1
+
which can be attributed to the stretching vibration of NH4 ion [29
,30]. Furthermore, a band at
+
1240 cm−1 can be attributed to (NH4+) and confirm therefore the presence of the NH4 group in
δ
the precursor [30]. These results permit the conclusion that an oxalate complex, containing oxo
molybdenum units, hydroxyl (-OH), water, and NH4+ ion, was formed. The results from the thermal
analysis (TGA and DTA) studies are presented in Figure 2. The TG cur◦ve is divided in four parts
with a total weight loss of 44.1% between room temperature and 380 C. The first weight loss of
4.4% can be attributed to the crystallization water. Moreover, the second and third weight losses
located between 170 and 300 ◦C can be attributed to the decomposition of the complex. Finally, a last

Molecules 2018, 23, 364
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◦
small loss of 2.1% is observed between 300 and 375 C. A similar loss in the same range was
obtained in a previous study of bismuth oxalate complex containing an OH group and can be
attributed to this OH group [31
oxidation degree of cobalt and molybdenum, a formula of the oxalate complex can be suggested
as (NH4)CoMoO(C2O4)2(OH) H2O. The total weight loss observed is 44.1% in comparison with the
,32]. By compiling the results obtained by FTIR, TGA, and the possible
·
theoretical value of 45.3% for the suggested formula. This complex was subsequently thermally
decomposed in air at a chosen temperature of 550 ◦C to obtain the resulting oxide phase.
80
70
60
50
40
30
20
10
3500 3000
1800 1600 1400 1200 1000
Frequency (cm-1)
Figure 1. FTIR spectrum of the as-prepared oxalate precursor.
105
100
4.4%
TGA
DTA
95
90
85
80
75
70
65
60
55
37.6%
2.1%
50 100 150 200 250 300 350 400 450 500 550 600
Temperature / oC
Figure 2. TGA and DTA thermograms recorded under air.
3.2. Cobalt Molybdate Characterization
3.2.1. X-ray Diffraction
The X-ray diffraction pattern of the powder prepared via this new and original method is given in
Figure 3. A pure phase CoMoO4 was obtained. All the observed peaks could be indexed in accordance
with the J.C.P.D.S. file # 21-0868. The CoMoO4 crystallizes in the monoclinic cell in the space group
C2/m with the parameters: a = 10.21, b = 9.268, c = 7.022, and
β
= 106.9◦. The crystallites size was
calculated using the Debye–Scherer equation, assuming that the crystallites have a spherical shape.
◦
A separate diffraction peak (021) was considered for this calculation (2
θ
= 23.22 ). The crystallites size
was found to be 17.6 nm.

Molecules 2018, 23, 364
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800
700
600
500
400
300
200
100
0
CoMoO4
JCPDS # 21-0868
10
15
25
30
35
40
45
50
202θ / degrees
Figure 3. XRD pattern of the
precursor at 550 C.
β-CoMoO4 nanoparticle powder obtained after calcination of the oxalate
◦
3.2.2. Specific Surface Area Measurement
The synthesized cobalt molybdate (CoMoO4) has a BET surface of 28.35 m2/g, which corresponds
to a particle size of 46.3 nm calculated using the equation DBET = 6000/d.S, where d is the density and
S is the specific surface area. It is a higher value than that calculated from the XRD pattern. On the
other hand, as depicted in Figure 4, the adsorption/desorption isotherms are type IV exhibited by
mesoporous solids and exhibit the characteristic H1 hysteresis loop (IUPAC classification), which is
consistent with mesoporous or nanoporous materials [33,34]. The pores are cylindrical or slit shaped.
The pore size was found to be of 18.8 nm, and the total pore volume found to be 0.1336 cm3/g using
the BJH (Barrett-Joyner-Halenda) method.
4.5
Adsorption
Desorption
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure / (P/P0)
Figure 4. Adsorption and desorption curves obtained from the BET measurements of the
β-CoMoO4 nanoparticles.
3.2.3. Transmission Electron Microscopy
Micrographs of the
β-CoMoO4 nanoparticles were taken at low magnification showing
agglomerated nanoparticles (Figure 5a). The agglomerates are about one micron in size.
At higher magnification, the particle size found is in the range between 20 and 40 nm (Figure 5b,c).
This value is lower than obtained from BET study (46.3 nm). This result can be explained by the fact
that in BET, agglomerated particles offer less surface for the adsorption-desorption phenomena, so the

Molecules 2018, 23, 364
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calculated particle size is higher. The calculated particle size using the Scherer formula from XRD
study (17.6 nm) was found lower than those found in either BET or TEM observation. This is because
what was calculated was the crystallite size, which was smaller than the particle size.
b
a
200 nm
50 nm
c
Figure 5. The TEM micrographs of the as-prepared β-CoMoO4 powder (a) at low magnification (×100,000),
scale bar 200 nm; and (
at high magnification.
b
) at high magnification ( 500,000), scale bar 50 nm; (c) Measured particle size
×
In order to confirm the atomic composition and proportion in the β-CoMoO4 nanoparticles,
a study with EDX spectroscopy was performed. Figure 6 shows the results. The abundance of oxygen,
cobalt, and molybdenum were found to be of 67.71%, 15.47%, and 16.82% compared to the theoretical
values 66.67, 16.67, and 16.67 respectively.
Element
O
Weight %
30.02
Atom %
67.71
Co
25.26
15.47
Mo
44.71
16.82
Total
100.00
100.00
Figure 6. EDX spectrum of the as synthesized β-CoMoO4 and its atomic abundance.

Molecules 2018, 23, 364
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3.3. Reduction of Nitrophenol Isomers
The results of the catalytic reduction study are shown in Figure 7. As is clear, the efficiency of the
-CoMoO4 catalyst is remarkable. An instantaneous reduction happened in the case of 3-NP (Figure 7a).
β
In contrast, the reduction reaction was slower in the case of 2-NP and 4-NP (8 min). This difference
in efficiency is probably due to a higher basic character of 3-NP, where the mesomeric conjugation is
more important.
1.4
1.2
0min
1.2
a
1min
2min
4min
6min
8min
0min
1min
b
1.0
0.8
0.6
0.4
0.2
0.0
2-NP ion
2-AP
1.0
0.8
0.6
0.4
0.2
0.0
3-AP
3-NP ion
250 300 350 400 450 500 550
250 300 350 400 450 500 550
Wavelength (nm)
Wavelength (nm)
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
4-NP ion 0min
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2 NP
3 NP
4 NP
d
1min
c
2min
4min
6min
8min
10min
4-AP
0.273 min-1
1.310 min-1
0.288 min-1
250 300 350 400 450 500 550
0
2
4
6
8
10
Wavelength (nm)
Time (min)
Figure 7. Successive UV-vis spectra of the reduction of (
a
) 2-nitrophenol solution (4
) 4-nitrophenol solution (4
10−4 M) in the presence of
-CoMoO4 nanocatalyst (0.1 g); ( ) Plot of ln(C/C0) against reaction time
-CoMoO4 and determination of the reduction reaction
×
10−4 M);
(
b
) 3-n−it4rophenol solution (4
×
10−4 M); (
c
×
8 × 10 M NaBH4 using
β
d
for the nitrophenol isomers in the presence of
rate constant kapp
β
.
An experiment was conducted to study the influence of the NaBH4 concentration on the rate
constant of the reduction reaction. Different concentrations were taken for NaBH4 maintaining the
same conditions as described in the protocol. When the concentration was increased to NaBH4/NP
ratios of 50:1, 25:1, 12.5:1, and 5:1, the catalyst was decomposed by the hydride. However, when a
2:1 ratio was used, the catalyst did not react with NaBH4. These were the optimal conditions for
this reaction.
On the other hand, the catalyst recycling was studied for 3-NP, where a faster reduction time
was observed. After the reduction test, the solution was filtered and◦ the powder recuperated.
After thorough washing with water, the powder was left to dry at 80 C overnight and used in
the next recycling test, conserving the same experimental conditions operated in the first reduction test.
The recycling was repeated four times, and the results are represented in Figure 8. The reduction
time of 1 min did not change in any the recycling tests, showing the good stability of the
nanocatalyst in this type of reduction reaction.
β-CoMoO4
It is important to compare the relative reaction kinetics of the 2-NP, 3-NP, and 4-NP reduction
for the catalysts synthesized in this work and those reported in the literature for other catalysts. The
results are summarized in Table 1.

Molecules 2018, 23, 364
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Importantly, this is the first time that such a high catalytic efficiency has been observed for a
low-cost nanocatalyst and highly concentrated solutions of nitrophenol isomers. The observed rate
constant for the simultaneous reduction of the 3-NP isomer for the β-CoMoO4 catalyst is relatively
high and is of 1.310 min−1. However, definite conclusions cannot be reached from these comparisons
due to the differences in the experimental conditions, such as the NaBH4/NP/catalyst equivalent
ratio and the temperature or the reactants concentration. On the other hand, the reaction time is also a
good criterion of efficiency and in our case the reaction is instantaneous. Goyal et al. have reported a
similar efficiency [17] but for a lower concentration of the nitrophenol. One important point is that
the reduction of the 3-NP isomer is faster than the reduction of the 2-NP isomer by approximately
one order of magnitude (8/1) and the reverse situation is observed in the work of Goyal et al. This
suggests that the reduction reaction mechanism is different for the β-CoMoO4 and CuFe2O4 catalysts.
1.0
0.8
0.6
0.4
0.2
0.0
Test
Third
Recycling
Fourth
Recycling
Second
Recycling
First
Recycling
Figure 8. Recycled catalyst against reduction time in the reduction of 3-NP with NaBH4 catalyzed with
β-CoMoO4 nanoparticles.
Table 1. Pseudo-first order rate constants for the reduction of 2-NP, 3-NP, and 4-NP by CoMoO4 with
other nanocatalysts reported in the literature.
Concentration of Reaction Time
Rate Constant
Catalyst
Type
References
−1
NP (mol/L)
(min)
(min
)
8
1
8
0.273 for 2-NP
1.310 for 3-NP
0.288 for 4-NP
−4
Nanoparticles
CoMoO4
This work
2 × 10
3
1.5
3
3.676 for 2-NP
0.983 for 3-NP
0.846 for 4-NP
−5
Nanoparticles
Nanoparticles
Nanocomposites
Nanocomposites
Nanoparticles
CuFe2O4
NiFe2O4
[17]
[17]
[18]
[19]
[20]
3.6 × 10
20
12
16
0.327 for 2-NP
0.062 for 3-NP
0.118 for 4-NP
−5
3.6 × 10
15
20
12
—- for 2-NP
—- for 3-NP
0.174 for 4-NP
−5
CuO/γAl2O3
Ni/C black
Fe2(MoO4)3
2.9 × 10
15
15
15
0.594 for 2-NP
0.594 for 3-NP
0.5970 for 4-NP
−4
5.0 × 10
12
4
9
0.160 for 2-NP
0.427 for 3-NP
0.323 for 4-NP
−4
2 × 10
4. Conclusions
-CoMoO4 nanostructures with a high catalytic efficiency in the reduction of ortho-, meta-,
and para-nitrophenol isomers to their corresponding aminophenols have been successfully prepared
β

Molecules 2018, 23, 364
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via the thermal decomposition of an oxalate precursor. The reduction was instantaneous in the case
of 3-NP but quite a bit slower in the cases of 2-NP and 4-NP (8 min). This difference in efficiency is
probably due to the higher basicity of 3-NP. A recycling test on the reduction of 3-NP showed the
stability of the nanocatalyst and its high efficiency even after four cycles.
Acknowledgments: This work was supported financially by the Research Deanship of Taibah University under
Grant No. 435/6426).
Author Contributions: M.A. and H.O.H. conceived and designed the experiments; all the authors performed the
experiments and analyzed the data; M.A., H.O.H. and A.O. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Yang, P.; Xua, A.; Xiab, J.; Hea, J.; Xinga, H.; Zhanga, X.; Weia, S.; Wanga, N. Facile synthesis of highly catalytic
activity Ni-Co-Pd-P composite for reduction of the p-Nitrophenol. Appl. Catal. A-Gen. 2014, 470, 89–96.
[CrossRef]
2.
Laksmia, B.; Shivanandaa, K.N.; Mahendraa, K.N.; Gowdab, N.M.; Jagadeesha, R.V. An efficient
platinum-catalyzed oxidation process and mechanism for the facile conversion of benzoxazoles to
aminophenols. Chem. Eng. J. 2010, 163, 403–412. [CrossRef]
3.
4.
Rico, J.L.; Avalos-Borja, M.; Barrera, A.; Hargreaves, J.S.J. Template-free synthesis of CoMoO4 rods and
their characterization. Mater. Res. Bull. 2013, 48, 4614–4617. [CrossRef]
Rosic´, M.; Zarubica, A.; Šaponjic´, A.; Babic´, B.; Zagorac, J.; Jordanov, D.; Matovic´, B. Structural and
photocatalytic examination of CoMoO4 nanopowders synthesized by GNP method. Mater. Res. Bull.
2018, 98, 111–120. [CrossRef]
5.
6.
Shaoqing, Y.; Jun, H.; Jianlong, W. Radiation-induced catalytic degradation of p-nitrophenol (PNP) in the
presence of TiO2 nanoparticles. Radiat. Phys. Chem. 2010, 79, 1039–1046. [CrossRef]
Rodriguez, J.A.; Hanson, J.C.; Chaturvedi, S.; Brito, J.L. Characterization of oxide catalysts using
time-resolved XRD and XANES: Properties of pure and sulfided CoMoO4and NiMoO4. Stud. Surf. Sci. Catal.
2000, 130, 2795–2800.
7.
8.
9.
Zagorac, D.; Schon, J.C.; Rosic, M.; Zagorac, J.; Jordanov, D.; Lukovic, J.; Matovic, B. Theoretical and
Experimental Study of Structural Phases in CoMoO4. Cryst. Res. Technol. 2017, 52, 1700069. [CrossRef]
Unnarkat, A.P.; Sridhar, T.; Mahajani, S.M.; Suresh, A.K. Study of Cobalt Molybdenum Oxide Supported on
Mesoporous Silica for Liquid Phase Cyclohexane Oxidation. Catal. Today 2017, in press. [CrossRef]
Sen, A.; Pramanik, P. Low-temperature synthesis of nano-sized metal molybdate powders. Mater. Lett.
2001, 10, 287–294. [CrossRef]
10. Brito, J.L.; Barbosa, A.L. Effect of Phase Composition of the Oxidic Precursor on the HDS Activity of the
SulfidedMolybdates of Fe(II), Co(II), and Ni(II). J. Catal. 1997, 171, 467–475. [CrossRef]
11. Veerasubramani, G.K.; Krishnamoorthy, K.; Radhakrishnan, S.; Kim, N.; Kim, S.J. Synthesis, characterization,
and electrochemical properties of CoMoO4 nanostructures. Int. J. Hydrog. Energy 2014, 39, 5186–5193.
[CrossRef]
12. Edrissi, M.; Samadanian-Isfahani, S.; Soleymani, M. Preparation of cobalt molybdate nanoparticles; Taguchi
optimization and photocatalytic oxidation of Reactive Black 8 dye. Powder Technol. 2013, 249, 378–385.
[CrossRef]
13. Fan, Y.; Ma, W.; He, J.; Du, Y. CoMoO4 as a novel heterogeneous catalyst of peroxymonosulfate activation for
the degradation of organic dyes. RSC Adv. 2017, 7, 36193–36200. [CrossRef]
14. Koo Kim, J.; Hwa Kim, J.; Chan Kang, Y. Electrochemical Properties of Multicomponent Oxide and Selenide
Microspheres Containing Co and Mo Components with Several Tens of Vacant Nanorooms Synthesized by
Spray Pyrolysis. Chem. Eng. J. 2018, 333, 665–677. [CrossRef]
15. Zhao, J.; Ren, X.; Ma, H.; Sun, X.; Zhang, Y.; Yan, T.; Wei, Q.; Wu, D. Synthesis of Self-supported Amorphous
CoMoO4 Nanowire Array for Highly Efficient Hydrogen Evolution Reaction. Sustain. Chem. Eng.
2017, 5, 10093–10098. [CrossRef]
16. Dam, D.T.; Huang, T.; Lee, J.M. Ultra-small and low crystalline CoMoO4 nanorods for
electrochemical capacitors. Sustain. Energy Fuels 2017, 1, 324–335. [CrossRef]

Molecules 2018, 23, 364
10 of 10
17. Goyal, A.; Bansal, S.; Singhal, S. Facile reduction of nitrophenols: Comparative catalytic efficiency of MFe2O4
(M = Ni, Cu, Zn) nano ferrites. Int. J. Hydrogen Energy 2014, 39, 4895–4908. [CrossRef]
18. Nandanwar, S.U.; Chakraborty, M. Synthesis of Colloidal CuO/γ-Al2O3 by Microemulsion and Its Catalytic
Reduction of Aromatic Nitro Compounds. Chin. J. Catal. 2012, 33, 1532–1541. [CrossRef]
19. Xia, J.; He, G.; Zhang, L.; Sun, X.; Wang, X. Hydrogenation of nitrophenols catalyzed by carbon
black-supported nickel nanoparticles under mild conditions. Appl. Catal. B-Environ. 2016, 180, 408–415.
[CrossRef]
20. Oudghiri-Hassani, H. Synthesis, characterization and catalytic performance of iron molybdate Fe2(MoO4)3
nanoparticles. Catal. Commun. 2015, 60, 19–22. [CrossRef]
21. Ghorai, T.K.; Dhak, D.; Azizan, A.; Pramanik, P. Investigation of phase formation temperature of
nano-sized solid solution of copper/cobalt molybdate and chromium-phosphate (M1xCr 1 MoxP −xO
)
4
−x
1
[M1 = Co, Cu]. Mater. Sci. Eng. B 2005, 121, 216–223. [CrossRef]
22. Kadiri, N.; Ben Ali, A.; Abboudi, M.; Moran, E. A new synthesis method to produce oxalate complexes:
Cases of copper and lanthanum oxalates. Phys. Chem. News 2008, 44, 11–14.
23. Abboudi, M.; Messali, M.; Kadiri, N.; Ben Ali, A.; Moran, E. Synthesis of CuO, La2O3, and La2CuO4 by the
Thermal-Decomposition of Oxalates Precursors Using a New Method. Synth. React. Inorg. Met.-Org. Nano
2011, 41, 683–688. [CrossRef]
24. Messali, M.; Al Wadaani, F.; Oudghiri-Hassani, H.; Rakass, S.; Al Amri, S.; Benaissa, M.; Abboudi, M.
Preparation, characterization and photocatalytic activity of hexagonal ZnO nanoparticles. Mater. Lett.
2014, 128, 187–190. [CrossRef]
25. Gabal, M.A.; El-Bellihi, A.A.; El-Bahnasawy, H.H. Effect of calcination temperature on Co(II) oxalate
dehydrate-iron(II) oxalate dihydrate mixture: DTA–TG, XRD, Mössbauer, FT-IR and SEM studies (Part II).
Mater. Chem. Phys. 2003, 81, 84–92. [CrossRef]
26. Angermann, A.; Topfer, J. Synthesis of nanocrystalline Mn-Zn ferrite powders through thermolysis of
mixed oxalates. Ceram. Int. 2011, 37, 995–1002. [CrossRef]
27. Ouyang, J.M.; Deng, S.P.; Zhou, N.; Tieke, B. Effect of tartrates with various counterions on the precipitation
of calcium oxalate in vesicle solutions. Colloids Surf. A 2005, 256, 21–27. [CrossRef]
28. Ng, K.Y.S.; Zhou, X.; Gulari, E. Spectroscopic characterization of molybdenum oxalate in solution and
on alumina. J. Phys. Chem. 1985, 89, 2477–2481. [CrossRef]
29. Mindru, I.; Gingasu, D.; Marinescu, G.; Patron, L.; Calderon-Moreno, J.M.; Bartha, C.; Andronescu, C.;
Crisan, A. Cobalt chromite obtained by thermal decomposition of oxalate coordination compounds.
Ceram. Int. 2014, 40, 15249–15258. [CrossRef]
30. Ramis, G.; Yi, L.; Busca, G. Ammonia activation over catalysts for the selective catalytic reduction of NOx
and the selective catalytic oxidation of NH3. An FT-IR study. Catal. Today 1996, 28, 373–380. [CrossRef]
31. Oudghiri-Hassani, H.; Rakass, S.; Al Wadaani, F.; Al Ghamdi, K.; Omer, A.; Messali, M.; Abboudi, M.
Synthesis, characterization and photocatalytic activity of
508–512. [CrossRef]
α-Bi2O3 nanoparticles. J. Taibah Univ. Sci. 2015, 9,
32. Rivenet, M.; Roussel, P.; Abraham, F. One-dimensional inorganic arrangement in the bismuth oxalate
hydroxide Bi(C2O4)OH. J. Solid State Chem. 2008, 181, 2586–2590. [CrossRef]
33. Sing, K.S.W.; Everett, D.H.; Haul, R.A.W.; Moscou, L.; Pierotti, R.A.; Rouquerol, J.; Siemieniewska, T.
Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface
Area and Porosity. Pure Appl. Chem. 1985, 57, 603–619. [CrossRef]
34. Anbia, M.; MooaviFard, S.E. Improving humidity sensing properties of nanoporous TiO2–10 mol % SnO2
thin film by co-doping with La3+ and K+. Sens. Actuator B-Chem. 2011, 160, 215–221. [CrossRef]
Sample Availability: Samples of the compounds cobalt molybdate (β-CoMoO4) are available from the authors.
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