Accepted Manuscript Investigation in Mechanistic Issues of Sonocatalysis and Sonophotocatalysis using Pure and Doped Photocatalysts Sankar Chakma, Vijayanand S. Moholkar PII: DOI: Reference:
S1350-4177(14)00194-1 http://dx.doi.org/10.1016/j.ultsonch.2014.06.008 ULTSON 2631
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
26 January 2014 9 June 2014 10 June 2014
Please cite this article as: S. Chakma, V.S. Moholkar, Investigation in Mechanistic Issues of Sonocatalysis and Sonophotocatalysis using Pure and Doped Photocatalysts, Ultrasonics Sonochemistry (2014), doi: http://dx.doi.org/ 10.1016/j.ultsonch.2014.06.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Investigation in Mechanistic Issues of Sonocatalysis and Sonophotocatalysis
2
using Pure and Doped Photocatalysts
3 4
Sankar Chakma and Vijayanand S. Moholkar*
5
Department of Chemical Engineering
6
Indian Institute of Technology Guwahati
7
Guwahati – 781 039, Assam, India
8
*Author for correspondence: E–mail: [email protected], Fax: +91–361–258 2291
9 10
Abstract
11
This paper attempts to investigate the mechanistic issues of two hybrid advanced oxidation
12
processes (HAOPs), viz. sonocatalysis and sonophotocatalysis, in which the two individual
13
AOPs, viz. sonolysis and photocatalysis, are combined. Three photocatalysts, viz. pure ZnO and
14
Fe–doped ZnO (with two protocols) have been employed. Fe–doped ZnO catalyst has been
15
characterized using standard techniques. Decolorization of two textile dyes has been used as the
16
model reaction. With experiments that alter the characteristics of ultrasound and cavitation
17
phenomena in the medium, the exact synergy between the two AOPs has been determined using
18
a quantitative yard stick. The results revealed a negative synergy between the two AOPs, which
19
is an almost consistent result for decolorization of both dyes using all three photocatalysts. Fe–
20
doping of ZnO catalyst helps in generation of more •OH radicals that could augment
21
decolorization. However, these radical mainly react with dye molecules adsorbed on catalyst
22
surface. Intense shock waves generated by cavitation bubbles cause desorption of dye molecules
23
from catalyst surface and reduce the probability of dye–radical interaction, thus reducing the net
24
utility of photochemically generated •OH radicals towards dye decolorization. This is rationale
25
underlying the negative synergy between sonolysis and photocatalysis. Fe–doped ZnO catalyst
26
increases the extent of decolorization, but the synergy between the two individual AOPs remains
27
unaltered with doping.
28
Keywords: ZnO, Fe–ZnO, Sonolysis, Photocatalysis, Cavitation, Advanced Oxidation Process
29
1
1
1.
Introduction
2
Degradation of recalcitrant organic pollutants appearing in industrial wastewater
3
discharge through advanced oxidation processes (AOP) has been an active area of research. The
4
principal mechanism of AOP is production of •OH radical with high oxidization potential of 2.3
5
eV to achieve faster and efficient degradation of the pollutants. One of the widely used AOP is
6
photocatalysis. The conventional semiconductor photocatalysts are anatase–TiO2 and ZnO.
7
Another relatively more recent AOP is sonication or sonolysis, in which the •OH radicals are
8
produced through transient collapse of cavitation bubbles driven by ultrasound irradiation of the
9
reaction system [1,2]. Transient cavitation creates intense local energy concentration on
10
extremely short temporal and spatial scales. Cavitation bubbles also emit light during transient
11
collapse, known as sonoluminescence [3,4]. The sonoluminescence light has wide range of
12
wavelength from UV to visible [5]. A combination of two or more AOPs has also been
13
attempted by several researchers [6–12]. The hybrid AOP sonophotocatalysis, in which
14
sonolysis and photocatalysis are applied together, has been reported to give enhanced
15
decolorization than the individual AOP. A variant of sonophotocatalysis is sonocatalysis, in
16
which sonication is applied to reaction mixture in presence of a photocatalyst – but without an
17
external source of UV light. Both sonocatalysis and sonophotocatalysis have been reported to
18
give enhanced degradation of organic pollutants in comparison to sonication alone [6,8,13]. In
19
sonocatalysis, the photon emission during sonoluminescence is expected to activate the
20
photocatalyst for additional production of •OH radicals [14]. In our previous paper [14], we have
21
identified a secondary role of photocatalyst in the sonocatalysis process in terms of adsorption of
22
the dye molecules on the surface of photocatalyst. The adsorption of the dye enhances the
23
probability of dye–radical interaction leading to higher degradation of the dye. For effective
24
utilization of sonoluminescence light emission during transient cavitation, doped photocatalysts
25
with higher absorption range and smaller band gaps are necessary.
26
Despite significant research in degradation of recalcitrant pollutants by sonocatalysis or 2
1
sonophotocatalysis, the exact mechanism of these hybrid AOPs remains relatively unexplored.
2
The individual mechanisms of sonolysis and photocatalysis are different. What is the exact
3
nature of interaction between these mechanisms (or in other words the mechanistic synergy) in
4
the hybrid AOP is a crucial question.
5
In this paper, we have addressed this question the matter of mechanistic investigation of
6
sonocatalysis and sonophotocatalysis with a dual approach: (1) ultrasound–assisted synthesis
7
and characterization of a Fe3+ doped ZnO nanoparticles, and (2) discernment of physical
8
mechanism of sonocatalysis and sonophotocatalysis with pure ZnO nanoparticles as well as Fe–
9
doped ZnO nanoparticles. Decolorization process of two textile dyes, viz. azo dye Acid Red B
10
(ARB) and non–azo dye Methylene Blue (MB) has been used as model reaction system.
11 12
2.
Experimental
13
Prior to describing the experimental methods, we briefly outline the rationale of using
14
doped photocatalyst. Conventional photocatalysts such as anatase–TiO2 and ZnO have large
15
band gap energy, and hence, these are not much effective in visible light range. For effective
16
utilization of the light energy for photocatalysis, it is essential that the absorption range of
17
photocatalyst should be extended to visible range (i.e. longer wavelengths) by decreasing the
18
band gap between conduction and valence band of the photocatalyst. For this purpose, doping of
19
conventional photocatalysts with transition metal ions like Fe3+, Co2+ or Ni2+ etc., which
20
increase the absorption of photons, is a well–known technique [15–23]. As the transition metal
21
ions are incorporated into the TiO2 or ZnO lattice, impurity energy levels in the band gap of
22
TiO2 or ZnO are formed as follows [24]:
23
M n+ + hv → M (n+1)+ + e−cb
(cb – conduction band)
24
M n+ + hv → M (n-1)+ + h +vb
(vb – valence band)
25
where, M and Mn+ represent the metal and metal ion dopant respectively. Further, electron–hole
3
1
transfer between metal ions and photocatalyst (TiO2 or ZnO) can alter electron–hole
2
recombination as:
3
Electron trap : M n+ + e−cb → M (n-1)+
4
Hole trap
+ : M n+ + h vb → M (n+1)+
5
The energy level of M n + M ( n −1)+ is less negative than that of the energy level of the conduction
6
band (CB) of original photocatalyst, while the energy level of M n+ M ( n+1) + is less positive than
7
that of the energy level of valence band (VB) of original photocatalyst.
8
2.1.
Materials
9
The following chemicals were used to study the activity of Fe3+ doped ZnO
10
photocatalyst: ZnO, Ferric sulfate monohydrate, Sodium dodecylsulfate (SDS), Acid red B
11
(ARB), and Methylene blue (MB). All the chemicals were purchased from Merck India and used
12
as received without further any pretreatment. For all experiments, ultrapure water (≥18 MΩ·cm
13
resistivity at 25oC) from Milli–Q Synthesis unit (Millipore®, USA) was used as the aqueous
14
medium.
15
2.2.
Synthesis of doped photocatalyst
16
Surface modified nano–sized Fe3+–doped ZnO was prepared using ultrasound assisted
17
impregnation method [8]. An ultrasonic probe with a frequency of 20 kHz (Model: VCX–500,
18
500 W) was used for sonication of the medium. The diameter of the probe is 13 mm with total
19
active surface area of 1.33 cm2. An aqueous solution (80 mL) of Fe2(SO4)3⋅H2O was prepared in
20
de–ionized water with concentration of 0.00375 mM of Fe3+. To this solution, 3 g of pre–
21
calcined (at 400oC for 5 h) ZnO particles were added. This corresponds to a weight ratio of
22
Fe2(SO4)3⋅H2O/ZnO as 2% w/w. Another parameter used in the synthesis was addition of
23
surfactant, sodium dodecylsulfate (SDS) to the reaction mixture. Synthesis was carried out with
24
addition of surfactant as well as without it. Due to very low concentration of surfactant in
25
solution (0.0026 M
S1350-4177(14)00194-1 http://dx.doi.org/10.1016/j.ultsonch.2014.06.008 ULTSON 2631
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
26 January 2014 9 June 2014 10 June 2014
Please cite this article as: S. Chakma, V.S. Moholkar, Investigation in Mechanistic Issues of Sonocatalysis and Sonophotocatalysis using Pure and Doped Photocatalysts, Ultrasonics Sonochemistry (2014), doi: http://dx.doi.org/ 10.1016/j.ultsonch.2014.06.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Investigation in Mechanistic Issues of Sonocatalysis and Sonophotocatalysis
2
using Pure and Doped Photocatalysts
3 4
Sankar Chakma and Vijayanand S. Moholkar*
5
Department of Chemical Engineering
6
Indian Institute of Technology Guwahati
7
Guwahati – 781 039, Assam, India
8
*Author for correspondence: E–mail: [email protected], Fax: +91–361–258 2291
9 10
Abstract
11
This paper attempts to investigate the mechanistic issues of two hybrid advanced oxidation
12
processes (HAOPs), viz. sonocatalysis and sonophotocatalysis, in which the two individual
13
AOPs, viz. sonolysis and photocatalysis, are combined. Three photocatalysts, viz. pure ZnO and
14
Fe–doped ZnO (with two protocols) have been employed. Fe–doped ZnO catalyst has been
15
characterized using standard techniques. Decolorization of two textile dyes has been used as the
16
model reaction. With experiments that alter the characteristics of ultrasound and cavitation
17
phenomena in the medium, the exact synergy between the two AOPs has been determined using
18
a quantitative yard stick. The results revealed a negative synergy between the two AOPs, which
19
is an almost consistent result for decolorization of both dyes using all three photocatalysts. Fe–
20
doping of ZnO catalyst helps in generation of more •OH radicals that could augment
21
decolorization. However, these radical mainly react with dye molecules adsorbed on catalyst
22
surface. Intense shock waves generated by cavitation bubbles cause desorption of dye molecules
23
from catalyst surface and reduce the probability of dye–radical interaction, thus reducing the net
24
utility of photochemically generated •OH radicals towards dye decolorization. This is rationale
25
underlying the negative synergy between sonolysis and photocatalysis. Fe–doped ZnO catalyst
26
increases the extent of decolorization, but the synergy between the two individual AOPs remains
27
unaltered with doping.
28
Keywords: ZnO, Fe–ZnO, Sonolysis, Photocatalysis, Cavitation, Advanced Oxidation Process
29
1
1
1.
Introduction
2
Degradation of recalcitrant organic pollutants appearing in industrial wastewater
3
discharge through advanced oxidation processes (AOP) has been an active area of research. The
4
principal mechanism of AOP is production of •OH radical with high oxidization potential of 2.3
5
eV to achieve faster and efficient degradation of the pollutants. One of the widely used AOP is
6
photocatalysis. The conventional semiconductor photocatalysts are anatase–TiO2 and ZnO.
7
Another relatively more recent AOP is sonication or sonolysis, in which the •OH radicals are
8
produced through transient collapse of cavitation bubbles driven by ultrasound irradiation of the
9
reaction system [1,2]. Transient cavitation creates intense local energy concentration on
10
extremely short temporal and spatial scales. Cavitation bubbles also emit light during transient
11
collapse, known as sonoluminescence [3,4]. The sonoluminescence light has wide range of
12
wavelength from UV to visible [5]. A combination of two or more AOPs has also been
13
attempted by several researchers [6–12]. The hybrid AOP sonophotocatalysis, in which
14
sonolysis and photocatalysis are applied together, has been reported to give enhanced
15
decolorization than the individual AOP. A variant of sonophotocatalysis is sonocatalysis, in
16
which sonication is applied to reaction mixture in presence of a photocatalyst – but without an
17
external source of UV light. Both sonocatalysis and sonophotocatalysis have been reported to
18
give enhanced degradation of organic pollutants in comparison to sonication alone [6,8,13]. In
19
sonocatalysis, the photon emission during sonoluminescence is expected to activate the
20
photocatalyst for additional production of •OH radicals [14]. In our previous paper [14], we have
21
identified a secondary role of photocatalyst in the sonocatalysis process in terms of adsorption of
22
the dye molecules on the surface of photocatalyst. The adsorption of the dye enhances the
23
probability of dye–radical interaction leading to higher degradation of the dye. For effective
24
utilization of sonoluminescence light emission during transient cavitation, doped photocatalysts
25
with higher absorption range and smaller band gaps are necessary.
26
Despite significant research in degradation of recalcitrant pollutants by sonocatalysis or 2
1
sonophotocatalysis, the exact mechanism of these hybrid AOPs remains relatively unexplored.
2
The individual mechanisms of sonolysis and photocatalysis are different. What is the exact
3
nature of interaction between these mechanisms (or in other words the mechanistic synergy) in
4
the hybrid AOP is a crucial question.
5
In this paper, we have addressed this question the matter of mechanistic investigation of
6
sonocatalysis and sonophotocatalysis with a dual approach: (1) ultrasound–assisted synthesis
7
and characterization of a Fe3+ doped ZnO nanoparticles, and (2) discernment of physical
8
mechanism of sonocatalysis and sonophotocatalysis with pure ZnO nanoparticles as well as Fe–
9
doped ZnO nanoparticles. Decolorization process of two textile dyes, viz. azo dye Acid Red B
10
(ARB) and non–azo dye Methylene Blue (MB) has been used as model reaction system.
11 12
2.
Experimental
13
Prior to describing the experimental methods, we briefly outline the rationale of using
14
doped photocatalyst. Conventional photocatalysts such as anatase–TiO2 and ZnO have large
15
band gap energy, and hence, these are not much effective in visible light range. For effective
16
utilization of the light energy for photocatalysis, it is essential that the absorption range of
17
photocatalyst should be extended to visible range (i.e. longer wavelengths) by decreasing the
18
band gap between conduction and valence band of the photocatalyst. For this purpose, doping of
19
conventional photocatalysts with transition metal ions like Fe3+, Co2+ or Ni2+ etc., which
20
increase the absorption of photons, is a well–known technique [15–23]. As the transition metal
21
ions are incorporated into the TiO2 or ZnO lattice, impurity energy levels in the band gap of
22
TiO2 or ZnO are formed as follows [24]:
23
M n+ + hv → M (n+1)+ + e−cb
(cb – conduction band)
24
M n+ + hv → M (n-1)+ + h +vb
(vb – valence band)
25
where, M and Mn+ represent the metal and metal ion dopant respectively. Further, electron–hole
3
1
transfer between metal ions and photocatalyst (TiO2 or ZnO) can alter electron–hole
2
recombination as:
3
Electron trap : M n+ + e−cb → M (n-1)+
4
Hole trap
+ : M n+ + h vb → M (n+1)+
5
The energy level of M n + M ( n −1)+ is less negative than that of the energy level of the conduction
6
band (CB) of original photocatalyst, while the energy level of M n+ M ( n+1) + is less positive than
7
that of the energy level of valence band (VB) of original photocatalyst.
8
2.1.
Materials
9
The following chemicals were used to study the activity of Fe3+ doped ZnO
10
photocatalyst: ZnO, Ferric sulfate monohydrate, Sodium dodecylsulfate (SDS), Acid red B
11
(ARB), and Methylene blue (MB). All the chemicals were purchased from Merck India and used
12
as received without further any pretreatment. For all experiments, ultrapure water (≥18 MΩ·cm
13
resistivity at 25oC) from Milli–Q Synthesis unit (Millipore®, USA) was used as the aqueous
14
medium.
15
2.2.
Synthesis of doped photocatalyst
16
Surface modified nano–sized Fe3+–doped ZnO was prepared using ultrasound assisted
17
impregnation method [8]. An ultrasonic probe with a frequency of 20 kHz (Model: VCX–500,
18
500 W) was used for sonication of the medium. The diameter of the probe is 13 mm with total
19
active surface area of 1.33 cm2. An aqueous solution (80 mL) of Fe2(SO4)3⋅H2O was prepared in
20
de–ionized water with concentration of 0.00375 mM of Fe3+. To this solution, 3 g of pre–
21
calcined (at 400oC for 5 h) ZnO particles were added. This corresponds to a weight ratio of
22
Fe2(SO4)3⋅H2O/ZnO as 2% w/w. Another parameter used in the synthesis was addition of
23
surfactant, sodium dodecylsulfate (SDS) to the reaction mixture. Synthesis was carried out with
24
addition of surfactant as well as without it. Due to very low concentration of surfactant in
25
solution (0.0026 M
