Catalytic degradation of methyl violet without light irradiation using nanostructured CuS

Article abstract of DOI:10.1016/j.molliq.2016.10.058

Nanostructured CuS catalyst with two different morphologies was prepared for degradation of methyl violet without light irradiation. The prepared catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Uv–visible spectroscopy. The effect of different conditions including: temperature, solution pH, H₂O₂ volume, catalyst dosage, dye concentration and presence of a salt on catalytic reaction rate and efficiency were investigated, as well. It was found that these catalysts can operate in mild conditions (room temperature and neutral pH) without light irradiation very well and also can be regenerated effectively by simple washing using water or ethanol.

Full text of DOI:10.1016/j.molliq.2016.10.058

Catalytic degradation of methyl violet without light irradiation using nanos-
tructured CuS
Mahsa Fayazi, Saeid Azizian
PII:
DOI:
Reference:
S0167-7322(16)32360-1
doi: 10.1016/j.molliq.2016.10.058
MOLLIQ 6462
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
21 August 2016
26 September 2016
12 October 2016
Please cite this article as: Mahsa Fayazi, Saeid Azizian, Catalytic degradation of methyl
violet without light irradiation using nanostructured CuS, Journal of Molecular Liquids
(2016), doi: 10.1016/j.molliq.2016.10.058
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ACCEPTED MANUSCRIPT
Catalytic degradation of methyl violet without light
irradiation using nanostructured CuS
Mahsa Fayazi, Saeid Azizian*
Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina
University, Hamedan 65167, Iran
*Corressponding author. Tel: +988138282807 Fax: +988138380709
E-mail addresses: sazizian@basu.ac.ir and sdazizian@yahoo.com
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Abstract
Nanostructured CuS catalyst with two different morphologies was prepared for degradation of
methyl violet without light irradiation. The prepared catalysts were characterized by scanning
electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD)
and Uv-visible spectroscopy. The effect of different conditions including: temperature, solution
pH, H2O2 volume, catalyst dosage, dye concentration and presence of a salt on catalytic reaction
rate and efficiency were investigated, as well. It was found that these catalysts can operate in
mild conditions (room temperature and neutral pH) without light irradiation very well and also
can be regenerated effectively by simple washing using water or ethanol.
Keywords: Degradation, Catalytic, Nanostructured CuS, Methyl violet, Aqueous solution
1. Introduction
In recent decades, one of the main sources of water pollution is dye molecules as a result
of different industrial activities. Presence of dye compounds in water not only is harmful
for human health but also has negative effects on the ecosystem. One of the hazardous
dye molecules, using for production of paper, cloth, wool and so on, is methyl violet [1],
known as carcinogenic and mutagenic compound [2]. So, its removal from aqueous
systems is really vital.
Different methods can be used in order to remove of dye compounds from water
containing: catalytic degradation [3-6], photocatalytic destruction [7], adsorption [8-10],
coagulation and biological treatment [11]. Catalytic destruction is one of the most useful
methods, because of operation in mild conditions and also transformation of very
poisonous compounds to the less poisonous one.
CuS is a p-type semi-conductor having been used as a photocatalyst [12]. Although,
recently, Kundu et al reported that this compound can be used as a catalyst (in absence of
light) for destruction of methylene blue in aqueous phase [13]. There are different
methods for the synthesis of nanostructured CuS, such as: hydrothermal [14], reflux [15]
and chemical vapor deposition (CVD) [16].
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The purpose of the present work is synthesis of nanostructured CuS having different
morphologies, and then study of their performances as a catalyst for degradation of
methyl violet in aqueous media. The effect of different parameters for instance: dye
concentrations, H2O2 volume, mass of catalyst, time, temperature, pH and presence of
salt on catalytic performance of CuS has been investigated too.
2. Material and methods
CuCl2.2H2O (> 99.0 %), thioacetamide (TAA) (> 99.0 %), ethylene glycol (> 99.0 %)
and methyl violet were purchased from Merck Co. and used without any further
purification. Distilled water was used in all experiments. Chemical structure of methyl
violet is shown in Fig. S1.
The catalyst (CuS) was prepared with two different methods [17,18] and consequently
different morphologies. In the first one, 0.85 g of CuCl2.2H2O and 0.32 g of TAA were
dissolved in 40 ml of ethylene glycol. The prepared solution was irradiated in a
microwave oven (Mulinex, 2450 Hz) at 230 W for 20 cycles (each cycle includes “on”
for 10 s and “off” for 40 s). The obtained black suspension was centrifuged and washed
several times using water and ethanol, followed by drying in an oven at 80 ˚C for 12 h. In
the second procedure, 0.17 g of CuCl2.2H2O and 0.09 g of TAA were dissolved in 50 ml
of distilled water. The prepared solution was irradiated in a microwave oven at 700 W for
150 s. The black suspension was centrifuged and then the obtained precipitate was
washed by water and dried at room temperature.
In order to study of catalytic destruction of dye molecule, specified amount of catalyst
(CuS) and H2O2 were added to a 50 ml of the methyl violet solution. Afterwards the
system was agitated in a dark condition. At specific time intervals, sampling was
performed and the residual concentration of methyl violet was determined by Uv-visible
spectroscopy (PG Instrument LTD T80) at λ=585 nm.
The morphologies of the prepared catalysts were determined by scanning electron
microscopy (SEM) (TSCA- Czech Republic) and transmission electron microscopy
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(TEM) (LEO 912 OMEGA Zeiss). X-ray diffraction (XRD) patterns were obtained by
ADD2000 LTALSTRUCTYR italia at 2θ ranging from 20 to 70 ˚.
3. Results and discussion
3.1.
Characterization of catalysts
Fig.1a, shows the SEM image of the first sample of CuS. It is obviously seen that the
prepared CuS is an assembly of nanosheets having thickness about 15-30 nm. For
further confirmation, the TEM image of this sample was taken and shown in Fig.1.b.
The TEM image clearly shows the presence of nanosheets. The XRD pattern of this
nanostructured sample is shown in Fig.S2.a. The observed peaks at 2θ= 28˚, 32˚ and
47˚ correspond to the (110), (103) and (102) planes of CuS, respectively [17]. The
Uv-visible spectra of dispersed sample in water is illustrated in Fig.S2b. From this
spectrum the band gap of this sample was estimated (inset of Fig.S2.b) approximately
1.9 eV.
Fig.S3, shows the TEM image of the prepared CuS (second sample).It can be
concluded that second sample is nanoparticles having prolate spheroid structure. The
thickness and length of each particle are estimated about 5 and 11 nm, respectively.
The XRD pattern of this sample is shown in Fig.S4a. Presence of small peaks
including: 24.3˚, 24.7˚, 31.8˚, 35,8˚ and 39.4˚ is in agreement with reported peaks of
CuS [18,19], but low intensity peaks indicate that sample crystallinity is poor.
Fig.S4b is the Uv-visible spectra of dispersed nanoparticles in water. The band gap of
this sample is estimated (inset of Fig.S4.b) about 2.1eV. The wider band gap of the
prepared nanoparticles (2.1 eV) in comparison to the band gap of the prepared
nanostructured one (1.9 eV) confirms the fact that the smaller particles have wider
band gap.
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3.2.
Comparison of catalysts performance
The catalytic performances of both prepared CuS samples for degradation of methyl
violet were compared to select the best catalyst. Fig.S5 shows the time dependency of
degradation of methyl violet (100 ppm) by both nanostructured CuS and CuS
nanoparticles. Based on this figure, it is clear that both the rate of degradation and the
final degradation efficiency for nanostructured CuS are higher than CuS
nanoparticles. For example, the required time for 50% degradation by nanostructured
CuS is about 3 minutes while by CuS nanoparticles, it is about 10 minutes. Therefore
nanostructured CuS was selected for further investigations.
The higher performance of nanostructured CuS in comparison to the CuS
nanoparticles can be explained based on volcano plot [20,21] indicating the optimum
catalytic activity obtained when adsorption energy is intermediate. As the particles
become smaller, the number of edge and corner atoms increases leading to higher
adsorption energy [20,21]. Sometimes, this increase in adsorption energy by decrease
in the particle size causes to shift the catalytic activity toward the maximum of
volcano plot (i.e. increase catalytic activity) and sometime shifts from maximum to
the lower activity values because of increasing of adsorption energy and blocking of
catalyst active surface sites [21]. In the present system, catalytic activity reduced by
decrease in particle size meaning the catalytic activity shifted from maximum to the
lower one due to higher bonding energy of reactants to the catalyst surface, and
therefore lower tendency for reaction.
3.3.
Reaction mechanism
In order to investigate whether the removal of methyl violet from water is adsorption
or oxidative degradation just by H2O2 or catalytic degradation in presence of H2O2,
four sets of experiments have been performed. In the first one, just H2O2 was added to
the dye solution. The result (Fig. 2a) indicates after one hour just 6% of dye was
destructed. In another experiment, nanostructured CuS was added to the dye solution
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(in the absence of H2O2). The result (Fig. 2a) shows that after one hour just 9% of dye
was removed from solution indicating adsorption is not the main process of dye
removal. In the third experiment, CuS and H2O2 were added simultaneously to the
dye solution. The result (Fig.2a) indicates that dye removal starts rapidly, and within
5 min about 60% of dye was destructed. In the last experiment, firstly CuS was added
to the dye solution and waited for 15 min until adsorption is completed and then H2O2
was added to this system. The result (Fig 2b) indicates by addition of H2O2, the dye
degradation starts rapidly. These results indicate for effective degradation of methyl
violet, presence of H2O2 and CuS catalyst are necessary and the mechanism of
reaction is catalytic oxidative degradation. Simplified suggested mechanism is:
H2O2
2 OH˚
(1)
(2)
(3)
(4)
˚
OH˚ + H2O2 → HO2 + H2O
OH˚ + RH → R˚ + H2O
RH + HO2 → R˚ + H2O2
˚
˚
R˚ + OH˚ or HO2 or O2 → mineralized products
3.4.
The effect of H2O2 and dye concentrations
In order to study the effect of dye concentration on the degradation efficiency, 0.02 g
of nanostructured CuS and 2 ml of H2O2 were added to 50 ml of the methyl violet
solution having different concentrations including: 60, 100 and 160 ppm. The plot of
time dependency of dye concentration is shown in Fig S6.a. It is clear that the dye
molecules can efficiently destructed by CuS and H2O2 at different concentrations. At
all concentrations, most of the dye molecules were destructed within 10 min.
The effect of H2O2 content is illustrated in Fig.S6.b. It is obvious that by addition of
H2O2 volume from 0.25 ml to 1 ml, the rate of dye degradation increases, but further
addition of H2O2 (2 or 3 ml) has the small effect on the rate of degradation.
3.5.
The effect of catalyst dosage
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The effect of catalyst mass on degradation of methyl violet was investigated and the
results are shown in Fig.S7. By increase in catalyst mass form 0.01 to 0.03 g, the rate
of degradation increases as it is expected [22]. But the efficiency of degradation rises
slightly by increasing of catalyst mass.
3.6.
The effect of temperature
Catalytic degradation of methyl violet by nanostructured CuS was studied at different
˚
temperatures. Fig.S8 shows that by increasing of temperature from 25 to 40 C the
rate of reaction increases slightly. Therefore one of the important advantages of this
system is its high efficiency at room temperature.
3.7.
The effect of solution pH
The effect of solution pH on the catalytic rate of methyl violet degradation was
investigated. The results are shown in Fig.S9. This figure shows that at pH=3 both
rate and total efficiency are low. But at higher pH values (4-9), no significant
difference in catalytic activity was observed. Probably, at pH=3, the surface charge of
catalyst is positive and therefore the cationic dye cannot be adsorbed due to repulsive
forces [23]. The interesting point of this study is that the reaction can be proceed at
neutral medium and does not need sever conditions such as: high acidic or alkaline
medium.
3.8.
The effect of presence of salt
In order to study the performance of the prepared catalyst in more real systems,
catalytic destruction of methyl violet was investigated in the presence of a salt
(NaCl). The results were presented in Fig.S10. It is concluded that presence of salt
has no effect on the catalytic reaction efficiency and reaction rate. Hence this catalyst
can be used for effective dye destruction also in the presence of salts.
3.9.
Regeneration of catalyst
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Fig.3a, shows the reusability of the prepared catalyst (without any treatment) for
degradation of methyl violet after three cycles and at different solution pH values. It
is clear that the catalytic activity decreases by using in further cycles. So, the catalyst
needs to be regenerated before using in other cycles.
The results of catalytic performances of the regenerated catalyst (washed using
distilled water or ethanol) were presented in Fig.3b. As can be seen, the catalyst
activity remains constant by simple regeneration using water or ethanol. So, simple
regeneration, by washing with water or ethanol, can be accounted as one of the
rewarding advantages of this catalyst.
3.10. Reaction products
The Uv-visible spectra of methyl violet at different times is demonstrated in Fig.4a. A
rapid decrease in methyl violet absorbance indicates a rapid catalytic degradation of
this compound. The corresponding concentrations at different times are illustrated in
Fig.4b.
Also the photographs of methyl violet solution (corresponding to Fig.4) after different
times of catalytic degradation by CuS is shown in Fig.5. This figure clearly indicates
decolorization of the solution was done successfully using H2O2 and nanostructured
CuS. In order to investigate, what is the product of this degradation (decolorization)
process, the CO and CO2 concentrations (using CO and CO2 sensors) have been
measured within 1 min, where most of methyl violet was degradated. No CO was
detected during this time. But, as shown in Fig.S11, a rapid increase in CO2
concentration was observed. These data clearly show that H2O2 in presence of
nanostructured CuS catalyst can effectively destructs methyl violet into CO2, and
therefore can be used for water treatment.
3.11. Kinetic modeling
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The kinetic data of the methyl violet degradation by nanostructured CuS were fitted
to the following equation, which is fractal like first order model:
Ln(1-f) = -ktα
(5)
(6)
f=(C0-C)/(C0-Ce)
where C0 , Ce and C are the initial, equilibrium and at any time concentrations of
methyl violet. k is the rate coefficient, t is time and α is a coefficient obtaining from
fractal like concept of reaction dynamic, and it corresponds to the heterogeneity in
distribution of reactants in solution [24] or heterogeneity of active sites on the surface
of the catalyst [25,26]. The results of fitting of kinetic data at different conditions are
listed in Table S1. Typically, the results of modeling with eq. (5) are illustrated in
Fig.4 as dashed line. As can be observed, this kinetic model can be used for
prediction of concentration profile very well.
Conclusion
The obtained results in the present study indicate that the nanostructured CuS,
including assembly of nanosheets, is an effective catalyst for degradation of methyl
violet, as one of the environmental pollutants. It is observed that this catalyst can
destruct methyl violet in aqueous media rapidly and efficiently. The advantages of
this catalyst for degradation of methyl violet can be listed as: no need to light energy,
working at mild solution conditions (no need to very strong acidic or alkaline
conditions), working at room temperature efficiency, activity without any change by
presence of salt, rapid degradation rate of methyl violet, simple preparation
procedure, only need low volume of H2O2, oxidation of methyl violet to CO2,
regeneration easily.
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Formatting of funding sources
Acknowledgment
The authors acknowledge the financial support of Bu-Ali Sina University (Grant
number: 32-12-67).
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a
b
Fig.1. (a) SEM and (b) TEM micrographs of the prepared CuS (first sample).
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120
100
80
a
CuS+MV
H2O2+MV
CuS+H2O2+MV
60
40
20
0
0
10
20
30
40
50
60
70
80
t (min)
Fig.2. (a) Effect of CuS, H2O2 and CuS+H2O2 on the methyl violet removal, (b)
degradation of methyl violet by H2O2+CuS proceeded by adsorption on CuS.
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100
90
a
90%
90%
pH=9
80
70
60
50
40
30
20
10
0
pH=4.7
55%
40%
30%
30%
1
2
3
Cycle No.
100
90
80
70
60
50
40
30
20
10
0
b
96%
95%
96%
95%
Water
Ethanol
Fig.3. Degradation efficiency (a) of used catalyst (without any treatment) and (b)
regenerated catalyst, at different reaction cycles.
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0.7
t=0 min
b
0.6
0.5
0.4
0.3
0.2
0.1
0
t=0.5 min
t=1 min
t=3 min
t=10 min
t=70 min
300
400
500
600
700
800
λ (nm)
120
100
80
60
40
20
0
b
0
10
20
30
40
50
60
70
80
t (min)
Fig.4. (a) Uv-visible spectra of methyl violet at different time intervals, (b) corresponding
concentration as a function of time, symbols are experimental data and dashed line is the
predicted values by eq. (5). (C= 100 ppm, T= 40˚C, pH= 9, catalyst mass= 0.03 g, VH2O2
=
3ml).
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10 min
30 min
70 min
0 min
1 min
Fig.5. Photographs of methyl violet solution at different times (degradation by H2O2 and
CuS).
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Graphical Abstract
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Highlights
- Nanostructured CuS is an efficient catalyst for methyl violet degradation.
- This catalyst works at mild conditions for pollutant degradation.
- The catalyst can be regenerated easily by washing with water.
20