Ni–Mg co-doped ZnO nanoparticles as a novel catalyst for hydroxylation of benzene to phenol

Article abstract of DOI:10.1007/s11164-018-3333-0

Mg and Ni were introduced as the novel co-doping materials for synthesizing Mg and Ni co-doping ZnO nanoparticles (NPs) through the simple wet chemical method. Photocatalyst properties of Mg and Ni co-doping ZnO NPs were studied by hydroxylation of benzen

Full text of DOI:10.1007/s11164-018-3333-0

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3333-0
Ni–Mg co-doped ZnO nanoparticles as a novel catalyst
for hydroxylation of benzene to phenol
Raziyeh Meimani1 Zahra Aghajani1
Gholam Reza Najafi1
•
•
Received: 5 October 2017 / Accepted: 15 February 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Mg and Ni were introduced as the novel co-doping materials for syn-
thesizing Mg and Ni co-doping ZnO nanoparticles (NPs) through the simple wet
chemical method. Photocatalyst properties of Mg and Ni co-doping ZnO NPs were
studied by hydroxylation of benzene to phenol at room temperature. Furthermore,
the doped Mg and Ni cause the hydroxylation capability of ZnO to be enhanced
which can be attributed to the presence of Mg and Ni on the ZnO NPs which greatly
increases absorption in under the ultraviolet (UV) light range enabled by the surface
plasmon resonance effect from Mg and Ni. In addition, an attempt was made to
investigate the effects of several parameters such as hydroxylation reaction time,
solvent, and concentration of H2O2 and photocatalyst, and UV irradiation on the
hydroxylation yield. Mg and Ni co-doping ZnO NPs showed significant hydroxy-
lation after two recyclings, resulting in 85 and 77% in the first and second recycling
processes, respectively.
Keywords Benzene hydroxylation Á Doped zinc oxide Á Hydrogen
peroxide Á Photocatalysis
Introduction
In recent years, oxide nanomaterials have gained much attention thanks to their
unique physical and chemical properties, large surface area, and strong interaction
with other materials [1–5]. Zinc oxide (ZnO) is a semiconductor material with a
band gap of 3.37 eV and binding energy of 60 meV, with many applications such as
sensors, solar cells, photocatalyst applications, and so forth [6–8]. The effects of
&
Zahra Aghajani
haj_aghajani@yahoo.com
1
Department of Chemistry, Qom Branch, Islamic Azad University, Qom, Iran
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semiconductor doping on the physical and chemical properties of nanomaterials
have long been known [9–12]. Doping with selective elements offers an effective
method to enhance and control the structural, electrical, and optical properties of
ZnO nanoparticles NPs). Researchers have studied ZnO doped with proper
elements, such as Al, Mn, Cu, Mg, Ni, and Cr [13, 14]. Doping strategy has been
widely used to enhance visible light absorption of ZnO nanomaterials. Phenol has
become an important substance in recent years, because it has many applications in
the two main fields of the chemical industry and engineering processes. The
maximum capacity of phenol (90%) is related to the oxidation of cumene to form
cumene peroxide and then splitting to phenol and acetone. Despite direct synthesis
of benzene to phenol [15–18], it is vital to work on the selective oxidation of
benzene to phenol through low cost and environmentally friendly methods because
current methods need high temperatures and/or high pressures. There are many
reports on the photochemical transformations of organic compounds since substrates
can be photo-activated under mild reaction conditions [19–23]. Due to problems
with selective catalysts, selective oxidation of hydrocarbons is difficult in catalytic
chemistry. Such catalysts should perform a double function. They should first
activate dioxygen and generate a reactive catalyst-bound oxygen species, and
second activate the organic molecules in order to produce a direct reaction to the
desired products. Most heterogeneous catalysts are comprised of three sections,
namely a transition metal, an oxygen source such as O-, O2-, or O2-, and an
electrophilic. Ni NPs provide a convenient way to direct the flow of photo-induced
charge carriers because of irradiation by ultra-violet (UV) light, which helps to
hinder electron–hole pairs recombination. A few studies have been performed on the
hydroxylation of benzene to phenol by using nano-materials [24–26], but the
selective oxidation of benzene to phenol by Ni and Mg co-doping ZnO NPs under
eco-friendly conditions such as with H2O2 oxidant and at room temperature has not
yet been reported. Since reactive oxygen species (ROS) and chemically reactive
chemical species containing oxygen play an important role in the photocatalyst
application of ZnO NPs doping even co-doping with metals can accelerate the
generation of ROS which result in increasing photocatalyst yield. In the current
study, the simple wet chemical method was applied in order to synthesize Ni and
Mg co-doping ZnO NPs. In addition, as-synthesized NPs were applied as the
catalyst for synthesizing phenol from benzene through the hydroxylation process in
the presence of H2O2. Furthermore, some reaction conditions such as hydroxylation
reaction time, solvent, and density of H2O2 and photocatalyst were altered to
achieve maximum photocatalytic yield.
Experimental
Synthesis of ZnO nanoparticles (NPs)
In order to synthesize ZnO NPs, ZnCl2 (0.01 mol) and polyethylene glycol (5 g), as
the surfactant, were dissolved in 50 ml of distilled water under stirring in a beaker
(A). Subsequently, 1.45 g of hexamethylenetetramine was dissolved in 50 ml of
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Ni–Mg co-doped ZnO nanoparticles as a novel catalyst for…
distilled water in a second beaker (B). Then, the contents of beaker B were added to
beaker A, while a pH adjuster agent (ammonia) was subsequently added to the
above solution, stirring at 25 °C, to obtain pH value of 9. Afterwards, the final
mixed solution was stirred at 90 °C for 3 h and then cooled at room temperature for
24 h. In addition, the products were centrifuged and washed several times with
deionized water, and were dried at 70 °C for 20 min. Finally, the as-obtained
precipitate was calcinated at 500 °C for 3 h in a conventional furnace in air
atmosphere.
Synthesis of Mg and Ni co-doping ZnO nanoparticles
At first, a stoichiometric amount of ZnCl2 (1.33 g), Ni(NO3)2Á6H2O (0.0145 g),
MgCl2 (0.0047 g), and polyethylene glycol (5 g) were dissolved in 50 ml of
distilled water in a beaker and stirred to make a uniform solution. Then,
hexamethylenetetramine (1.45 g) was dissolved in 15 ml of distilled water and
added to the above-mentioned solution, while ammonia (NH3) was supplemented
dropwise to the solution under stirring at 25 °C to reach pH value of 9. Afterward,
the final mixed solution was stirred at 90 °C for 3 h and then cooled at room
temperature for 24 h. In addition, the precipitate was centrifuged, washed with both
deionized water and ethanol for several times, and dried at 70 °C for 20 h. Finally,
the obtained products were further heated at 500 °C for 3 h in a quartz furnace in air
atmosphere.
Materials and physical measurements
ZnCl2, Ni(NO3)2Á6H2O, MgCl2, and hydrogen peroxide were supplied by Merck
and used without further purification. X-ray diffraction (XRD) patterns were
recorded by an X-ray diffractometer (Phillips X’Pert Pro X-ray), using Ni-filtered
Cu Ka radiation at a scan range of 10 \ 2h \ 80 from Phillips (St Laurent, Quebec,
Canada). Scanning electron microscopy (SEM) images were obtained on a LEO-
1455VP equipped with an energy dispersive X-ray spectroscopy (Phillips,
Amsterdam, Netherlands). Transmission electron microscopy (TEM) images were
obtained by a Philips EM208S transmission electron microscope with an
accelerating voltage of 100 kV. The diffused reflectance UV–visible spectrum of
the sample was recorded by an Ava Spec-2048TEC spectrometer. Fourier transform
infrared (FT-IR) spectrum was recorded on a Magna Nicolet 550 spectrophotometer
in KBr pellets (Thermo Fisher Scientific, Waltham, MA, USA). The energy-
dispersive X-ray spectroscopy (EDX) analysis with 20 kV accelerated voltage was
carried out by an instrument from Phillips (Amsterdam, Netherlands).
Oxidation reaction
In a typical catalyst procedure, the stoichiometric amount of benzene (1 mmol) and
H2O2 (1 mmol per 40 min) were mixed under constant stirring. Then, 0.05 g of Mg
and Ni co-doping ZnO NPs was added to the above-mentioned solution as the
catalyst, under constant stirring to make a homogeneous solution. Afterward, the
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reaction mixture was further stirred at room temperature for 120 min under UV
light. Finally, the catalyst (Mg and Ni co-doping ZnO NPs) was separated by
filtration and extracted by hexane. In addition, the TLC and GC were used to
quantify the concentration of phenol and the degree of hydroxylation.
Results and discussion
Structural and morphological studies
XRD (10° \ 2h \ 80°) analysis was performed in order to determine the crystalline
structure and phase purity of the as-prepared ZnO NPs and Mg and Ni co-doping
ZnO NPs, as shown in Fig. 1a and b, respectively. The XRD pattern in Fig. 1a
indicates the formation of the hexagonal phase of ZnO with JCPDS No. 80-0075,
˚
space group: P63 mc, and the calculated cell parameter of a = b 3.2539 A and
˚
c = 5.2098 A. Furthermore, Fig. 1b (XRD pattern of Mg and Ni co-doping ZnO
NPs) demonstrates the formation of hexagonal phase of ZnO (space group P63 mc,
˚
JCPDS No. 80-0075, and calculated cell parameters of a = b = 3.2539 A and
˚
c = 5.2098 A), along with the hexagonal phase of Mg (space group: Fm3m; JCPDS
˚
No. 04-0770, and calculated cell parameters a = b = c = 4.0453 A), and cubic
phase of Ni (space group Fm-3m, JCPDS No. 01-1272, and calculated cell
˚
parameters of a = b=c = 2.7600 A. From the XRD data and the Scherrer equation
[27], the crystallite diameter (Dc) of the pure ZnO and Mg and Ni co-doping ZnO
NPs were calculated to be 18.87 and 19 nm, respectively.
Dc ¼ Kk=b cos h Scherer equation
where b is the breadth of the observed diffraction line at its half intensity maximum,
K is the so-called shape factor which usually takes a value of about 0.9, and k is the
wavelength of the X-ray source used in XRD. Figure 2 shows the energy-dispersive
X-ray spectroscopy analysis which is used to investigate the chemical composition
and purity of the Mg and Ni co-doping ZnO NPs. Based on Fig. 2, the product only
consists of Zn, Ni, Mg, and O elements. In addition, neither N nor C signals were
detected in the EDS pattern which means that the Mg nor Ni co-doping ZnO NPs are
pure and free of impurity. SEM analysis was used in order to investigate the
morphology and particle size of the final products. SEM and TEM images of the Mg
and Ni co-doping ZnO NPs are shown in Fig. 3. According to Fig. 3a, the products
were mainly composed of small spherical-shaped NPs with average particle sizes of
about 35–70 nm. In order to further characterize the size and morphology of Mg and
Ni co-doping ZnO NPs precisely, TEM images with different magnifications were
taken, as shown in Fig. 3b–d. The TEM images show that the product is mainly
comprised of large amounts of sphere-like NPs with average particle sizes
of * 35–80 nm. FT-IR analysis, 400–4000 cm-1, was performed in order to
identify the presence of certain functional groups in the ZnO NPs and Mg and Ni co-
doping ZnO NPs, as shown in Fig. 4b, c. Also, to prove the hydroxylation process
and the presence of phenol at the end of hydroxylation, the FT-IR spectra of phenol
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Ni–Mg co-doped ZnO nanoparticles as a novel catalyst for…
Fig. 1 XRD patterns of a pure ZnO nanoparticles, b Mg and Ni co-doping ZnO nanoparticles
and Ni co-doping ZnO NPs after hydroxylation were taken, as shown in Fig. 4a, d.
Based on Fig. 4b and c, the broad absorption band centered at 3494 cm-1 and a
weak peak at 1629 cm-1 are attributed to the stretching and bending vibrations of
v(OH), respectively [28]. Furthermore, the sharp and strong band at 456 cm-1 is
related to the Zn–O mode and confirms the formation of ZnO [3]. Comparing
Fig. 4a and d, the FT-IR spectra of phenol and Ni co-doping ZnO NPs after
hydroxylation demonstrates that hydroxylation was carried out and also confirms
the presence of phenol on the surface of the Ni co-doping ZnO NPs. The diffused
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Fig. 2 EDX pattern of Mg and
Ni co-doping ZnO nanoparticles
reflectance spectrum of the ZnO NPs is shown in Fig. 5. The fundamental
absorption edge in most semiconductors follows the exponential law. Using the
absorption data, the band gap was estimated by Tauc’s relationship:
a = a0(hm - Eg)n/hm where a is the absorption coefficient, hm the photon energy,
a0 and h the constants, Eg the optical band gap, and n depends on the type of
electronic transition and can be any value between 1/2 and 3. According to Fig. 5,
the energy gap (Eg) of the as-synthesized ZnO NPs (3.37 eV) is determined by
extrapolating the linear portion of the plots of (ahm)1/2 against hm to the energy axis.
Parameters of the oxidation reaction
The hydroxylation of benzene to phenol, in the presence of Mg and Ni co-doping
ZnO NPs, was investigated at the temperature of 25 °C and in a concentration of
1 mmol. In addition, the effects of various parameters such as hydroxylation
reaction time, solvent, concentration of H2O2, photocatalyst, and UV irradiation on
the hydroxylation yield were studied, as shown in Figs. 6, 7 and 8. In order to
examine the effect of Mg and Ni co-doping on the hydroxylation yield of ZnO NPs,
two tests were carried out in the absence and presence of Mg and Ni as the co-
doping. In the absence of co-doping (pure ZnO) and in its presence (Mg and Ni co-
doping ZnO NPs), the amounts of hydroxylation are 26.38 and 53%, respectively.
As a result, the hydroxylation percentage in the presence of Mg and Ni doping is
higher than its absence; therefore, the remaining tests were carried out in the
presence of Mg and Ni doping. It seems that the presence of Ni and Mg as the co-
doping causes ROS to increase and also decreases electron–hole pairs recombina-
tion which results in an increase of the hydroxylation percentage. This result
demonstrates that Ni- and Mg-supported ZnO NPs can enhance the photocatalytic
ability of ZnO nanocrystals for the hydroxylation of benzene, while ZnO NPs are
only responsible for the high selectivity to phenol [29]. Scheme 1 describes the
photocatalytic mechanisms in the presence of ZnO, Ni, and Mg. According to
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Ni–Mg co-doped ZnO nanoparticles as a novel catalyst for…
Fig. 3 a SEM image of Mg and Ni co-doping ZnO nanoparticles, b–d TEM images of Mg and Ni co-
doping ZnO nanoparticles
Scheme 1, first, ZnO NPs were excited by UV light with photons with an energy
larger than the band gap (3.37 eV based on the calculation from Fig. 5), leading to
the generation of electrons in the semiconductor conduction band and electron holes
in the valence band. The photo-induced electrons in the conduction band would
transfer to Ni and Mg acting as an electron buffer, which can hinder electron–hole
pairs recombination. The long-lived electron–hole pairs are prone to generate a
larger amount of photoreactive species, such as O-, O2-, or O2-, with very strong
oxidation ability, which may account for the higher photocatalytic activity of the
Ni–Mg co-doped ZnO NPs copmared with that of bare ZnO NPs. Furthermore, in
order to examine the impact of reaction time of hydroxylation on hydroxylation
yield, seven tests were performed (Fig. 6). According to Fig. 6, increasing the
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Fig. 4 FT-IR spectra of a phenol, b ZnO nanoparticles, c Mg and Ni co-doping ZnO nanoparticles, e Mg
and Ni co-doping ZnO nanoparticles after hydroxylation process
Fig. 5 UV–Vis absorption
spectrum of ZnO nanoparticles
Fig. 6 Hydroxylation percentage-dependent reaction hydroxylation time
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Ni–Mg co-doped ZnO nanoparticles as a novel catalyst for…
Fig. 7 Hydroxylation percentage-dependent concentration of Mg and Ni co-doping ZnO nanoparticles
Fig. 8 Hydroxylation percentage-dependent concentrations of H2O2
Scheme 1 Proposed mechanism of photocatalysis of as-prepared Mg and Ni co-doping ZnO
nanoparticles under UV light irradiation
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reaction time of hydroxylation from 45 to 120 min causes the reaction hydroxy-
lation yield to increase from 68 to 76%, while a further increase in reaction
hydroxylation time from 120 to 210 min causes the hydroxylation yield to decrease
from 76 to 71%; therefore, the best reaction hydroxylation time is 120 min.
Moreover, several experiments were arranged to demonstrate the effects of various
concentrations of photocatalyst NPs on the hydroxylation yield (Fig. 7). From
Fig. 7, it is obvious that hydroxylation of benzene to phenol is negligible in the
absence of a photocatalyst. With increasing the concentration, from 0 to 0.05 g, the
hydroxylation yield also increases from 0 to 85%, while a further increase in the
concentration (0.05–0.1 g) does not have any influence on the hydroxylation yield.
Hence, based on Fig. 7, the optimum photocatalyst concentration is 0.05 g. Since
H2O2 is a vital component of the hydroxylation process, various concentrations of
H2O2 were chosen to examine its influence on the hydroxylation yield, as shown in
Fig. 8, according to which it is clear that 3 mmol is the appropriate H2O2
concentration to achieve maximum hydroxylation. To evaluate the repeatability of
the photocatalyst reaction, the hydroxylation process was repeated two times. Based
on the repeatability results, the amounts of hydroxylation are 85 and 77% in the first
and second repetition processes, respectively. Three different solvents, namely
methanol, acetonitrile, and ethyl acetate, were chosen to investigate their effects on
the amount of hydroxylation yield, as shown in Fig. 9. The hydroxylation
percentages are 92, 89, and 85% for methanol, acetonitrile, and ethyl acetate,
respectively, therefore, methanol is the best solvent for the hydroxylation of
benzene to phenol in the presence of Mg and Ni co-doping ZnO NPs. UV light is a
critical element in the hydroxylation of benzene alcohol to phenol, and therefore
two tests were performed to investigate its effect on the hydroxylation percentage,
as shown in Fig. 10. As a result, UV light causes the amount of hydroxylation to
increase from 61 to 85%. In comparison with previous works [29–31], the present
study show a high hydroxylation yield and also high recyclability.
Fig. 9 Effect of different solvents on the hydroxylation yield
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Fig. 10 Effect of UV light on the hydroxylation process
Conclusions
In summary, Ni and Mg co-doping ZnO NPs were successfully synthesized through
a simple wet chemical method. Since oxidation of benzene is an important organic
transformation because of its final products, the transformation of benzene to phenol
through the hydroxylation process with the aid of H2O2 was investigated to examine
the photocatalytic activity of Ni and Mg co-doping ZnO NPs under irradiation of
UV. To the authors’ knowledge, this is the first report on the benefit of Ni and Mg
co-doping for improving the photocatalytic activity of ZnO NPs. In addition, the
reaction time, solvent, UV light, and concentration of H2O2 and the photocatalyst
were changed to investigate their effects on the hydroxylation yield. Ni- and Mg-
doped ZnO NPs show a high hydroxylation percentage of 85%. Furthermore, the
amounts of hydroxylation after first and second repetition processes were 85 and
77%, respectively. Based on the hydroxylation results, the maximum hydroxylation
was achieved in the presence of methanol as the solvent, 120 min reaction time, and
a photocatalyst concentration of 0.05 g.
Acknowledgements The authors acknowledge the kind financial supports of the Research Council of
Qom Branch, Islamic Azad University, Qom, Iran and Padideh Noavaran Nano Bonyan Company, Iran.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of interest.
References
1. H.W. Kim, D.S. Kwak, Y.J. Kwon, C. Lee, J.H. Jung, Met. Mater. Int. 19, 1123 (2013)
2. A. Javidan, M. Ramezani, A. Sobhani-Nasab, S.M. Hosseinpour-Mashkani, J. Mater. Sci.: Mater.
Electron. 26, 3813 (2015)
3. A. Sobhani-Nasab, M. Maddahfar, S.M. Hosseinpour-Mashkani, J. Mol. Liq. 216, 1 (2016)
4. J. Moradiganjeh, Z. Aghajani, J. Mater. Sci.: Mater. Electron. 27, 5948 (2016)
123

R. Meimani et al.
5. A. Javidan, S. Rafizadeh, S.M. Hosseinpour-Mashkani, Mater. Sci. Semicond. Process. 27, 468
(2014)
6. Y. Liu, H. Song, Q. Zhang, Res. Chem. Intermed. 38, 651 (2012)
7. F. Gobo, T. Goto, T. Long, S. Yin, T. Sato, Res. Chem. Intermed. 37, 211 (2011)
8. W.M. Li, P.G. Yin, L. Guo, Res. Chem. Intermed. 37, 125 (2011)
9. D. Jeong, K. Kim, S. Park, Y. Kim, S. Kim, S. Kim, Res. Chem. Intermed. 40, 97 (2014)
10. A. Phuruangrat, S. Mad-ahin, O. Yayapao, S. Thongtem, T. Thongtem, Res. Chem. Intermed. 41,
9757 (2015)
11. Z. Barzgari, A. Ghazizadeh, S.Z. Askari, Res. Chem. Intermed. 42, 4303 (2016)
12. K. Namratha, S. Suresha, M.B. Nayan, K. Byrappa, Res. Chem. Intermed. 37, 531 (2011)
13. U.P.S. Gahlaut, V. Kumar, R.K. Pandey, Y.C. Goswami, Opt. Int. J. Light Electron Opt. 127, 4292
(2016)
14. S. Benramache, A. Arif, O. Belahssen, A. Guettaf, Opt. Int. J. Light Electron Opt. 125, 5776 (2014)
15. M. Tani, T. Sakamoto, S. Mita, S. Sakaguchi, Y. Ishii, Angew. Chem. Int. Ed. 44, 2586 (2005)
16. M. Tlili, N. Xia, F. Monnier, M. Taillefer, Angew. Chem. Int. Ed. 48, 8725 (2009)
17. Y. Guo, X. Zhang, H. Zou, H. Liu, J. Wang, K.L. Yeung, Chem. Commun. 39, 5898 (2009)
18. R. Bal, M. Tada, T. Sasaki, Y. Iwasawa, Angew. Chem. Int. Ed. 45, 448 (2006)
19. S. Kim, J. Yeo, W. Choi, Appl. Catal. B 84, 148 (2008)
20. Y. Park, W. Kim, H. Park, T. Tachikawa, T. Majima, W. Choi, Appl. Catal. B 91, 355 (2009)
21. Y. Wu, M. Xing, J. Zhang, F. Chen, Appl. Catal. B 97, 182 (2010)
22. P. Calza, V.A. Sakkas, C. Medana, M. Azharul Islam, E. Raso, K. Panagiotou, T. Albanis, Appl.
Catal. B 99, 314 (2010)
23. H. Liu, W. Feng, C.W. Kee, Y. Zhao, D. Leow, Y. Pan, C.H. Tan, Green Chem. 12, 953 (2010)
24. S.S. Acharyya, S. Ghosh, R. Tiwari, C. Pendem, T. Sasaki, R. Bal, ACS Catal. 5, 2850 (2015)
25. M. Yamada, K.D. Karlin, S. Fukuzumi, Chem. Sci. 7, 2856 (2016)
26. X. Wang, B. Meng, X. Tan, X. Zhang, S. Zhuang, L. Liu, Ind. Eng. Chem. Res. 53, 5636 (2014)
27. S.M. Hosseinpour-Mashkani, M. Ramezani, Mat. Lett. 130, 259 (2014)
28. M. Ramezani, S.M. Hosseinpour-Mashkani, J. Electron. Mater. 46, 1371 (2017)
29. H. Yuhai, M. Gonghaito, L. Huanhkeming, P. Liue, Z. Kangjie, W. Wang, Bull. Mater. Sci. 36, 367
(2013)
30. G. Zhang, J. Ya, J. Shim, J. Lee, W. Choa, Appl. Catal. B 102, 132 (2011)
31. M. Forouzani, H.R. Mardani, M. Ziari, A. Malekzadeh, P. Biparva, Chem. Eng. J. 275, 220 (2015)
123