Ba-ZnO Nanoparticles for Photocatalytic Degradation of Chloramphenicol

Abstract

Pristine ZnO (PZO) and 5% barium doped ZnO nanoparticles (BZONP) were prepared by in expensive chemical precipitation method. The techniques used to characterize prepared nanoparticles are X-ray powder diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX), Scanning Electron Microscope (SEM), UV-visible absorption spectroscopy and Transmission Electron Microscope (TEM) analysis. The particle size of PZO and BZONP were calculated using Scherrer equation. The photo-catalytic efficiency of 5% BZONP was studied with photo-degradation of chloramphenicol (CLP) under UVC (254 nm) irradiation in aqueous suspension. Pseudo-first order rate constants (kobs) were found to increase with the decrease in pH. The effect of initial concentration, photo-catalyst loading, light intensity, the effect of pH on the photo-degradation rate was also examined and elaborately discussed. The results showed that BZONP is a better photo-catalyst compared PZO. The HPLC and LC/MS were used to identify photo-degradation products.

Full text

Ba-ZnO nanoparticles for photo-catalytic degradation of chloramphenicol
R. M. Kulkarni, R. S. Malladi, M. S. Hanagadakar, N. P. Shetti, and M. R. Doddamani
Citation: AIP Conference Proceedings 1989, 020026 (2018); doi: 10.1063/1.5047702
View online: https://doi.org/10.1063/1.5047702
View Table of Contents: http://aip.scitation.org/toc/apc/1989/1
Published by the American Institute of Physics

Ba-ZnO Nanoparticles for Photo-catalytic Degradation of
Chloramphenicol
R.M. Kulkarnia, R.S. Malladi*a, M.S. Hanagadakara, N.P. Shettib,
M.R. Doddamanic
aDepartment of Chemistry, KLS Gogte Institute of Technology (Autonomous) Affiliated to Visvesvaraya
Technological University, Belagavi -590 008, Karnataka, INDIA
bDepartment of Chemistry, KLE Society’s K. L. E. Institute of Technology, Hubballi-580030, Affiliated to
Visvesvaraya Technological University, Belagavi, Karnataka, INDIA
cMechanical Engineering, National Institute of Technology Karnataka, Surathkal, INDIA
*rameshmalladi7@gmail.com
Abstract
Pristine ZnO (PZO) and 5% barium doped ZnO nanoparticles (BZONP) were prepared by in expensive chemical
precipitation method. The techniques used to characterize prepared nanoparticles are X-ray powder diffraction (XRD),
Energy Dispersive X-ray Spectroscopy (EDX), Scanning Electron Microscope (SEM), UV-visible absorption spectroscopy
and Transmission Electron Microscope (TEM) analysis. The particle size of PZO and BZONP were calculated using
Scherrer equation. The photo-catalytic efficiency of 5% BZONP was studied with photo-degradation of chloramphenicol
(CLP) under UVC (254 nm) irradiation in aqueous suspension. Pseudo-first order rate constants (kobs) were found to increase
with the decrease in pH. The effect of initial concentration, photo-catalyst loading, light intensity, the effect of pH on the
photo-degradation rate was also examined and elaborately discussed. The results showed that BZONP is a better photo-
catalyst compared PZO. The HPLC and LC/MS were used to identify photo-degradation products.
Keywords: Photo-catalyst; Nanoparticles; Degradation; Zinc oxide; Chloramphenicol
PACS: 81.07.Wx. 81.05.Dz
INTRODUCTION
The ground water and surface water contaminated by the industrial outfall, irrigation runoff, chemical
spills, commercial operations that contain many non-biodegradable substrates that are harmful to the human
kind and also significant threat for flora and fauna of the environment [1]. Many conventional treatments like
chlorination [2-3] permanganate treatment [4-6] nano-filtration and reverse osmosis [7] were reported for the
treatment of pharmaceuticals in the literature. So far it is difficult to treat these organic contaminants by usual
conventional and biological treatment methods [8]. During past twenty years, advanced oxidation process
(AOP) is considered to be effective method for removal of toxic organic contaminants, which uses photo-
catalytic process involving zinc oxide and titanium dioxide semiconductor nanoparticles, under UV or visible
light elucidation has been potentially helpful in the management of waste-water contaminants [9-10].
AOPs have been extensively used for the photo-catalytic degradation of hazardous organic
contaminants, which are susceptible to conventional and biological treatment methods. AOPs produces highly
potent reactive radical species (HO2, HO∙), which demineralise many organic contaminants with-out being
choosy, by chemical or photochemical method [11-13]. The technique mainly focuses on usage of
semiconductor particles activated by UV or visible light for degradation of environmental contaminants yielding
to complete or partial mineralization of the organic contaminants [14-15]. Amongst AOPs semiconductor
heterogeneous photo-catalysis is an effective method. This can be easily applied for the treatment of drug
containing waste-water.
Emerging Technologies: Micro to Nano (ETMN-2017)
AIP Conf. Proc. 1989, 020026-1–020026-9; https://doi.org/10.1063/1.5047702
Published by AIP Publishing. 978-0-7354-1705-2/$30.00
020026-1

The PZO nanoparticles have been widely used in the field of photo-catalytic degradation [16]. PZO is
believed to be a right substitute to TiO2 (3.2 eV), due to their wide-band gap (3.3 eV), low cost, greater
efficiency and stability semiconducting material with a broad excitation binding energy, vast in nature and
environmentally friendly and these characteristics make this material to captivate increasing attention as it is a
potentially useful material for a wide range of applications. The efficiency of the photo-catalyst depends on the
rate of electron-hole recombination in PZO, the rate of recombination can be minimized by doping the PZO
with transition metals such as silver, ruthenium, copper, barium etc.
Decorating PZO with impurity/dopant (transition metal ions) improves the electronic structure of PZO
and enhances the optical, electrical, magnetic properties and showing other improvement in the different
application like photo-reaction and photo-catalytic activity. These transition metal ions not only served as
trapping sites, it also reduces charge re-combination and to facilitate interfacial electron transfer process that in
turn enhance the surface reactivity. Doping also induces the widening of wavelength from UV to the visible-
range [17].
Now-a-day, antibiotic and public care products are largely discharged into the water bodies with no
limitation. This is promising as a major contaminant in the ecosystem and threat to public concern. Among these
antibiotics, Chloramphenicol (CLP) is a unique drug (Fig 1). CLP belongs to tetracycline family and commonly
known as Chloromycetin. CLP is believed to be proto-typical wide range spectrum antibacterial agent [18].
Hence, it can be used to cure a variety of microscopic organisms causing illness [19]. These antibiotics dose are
released into the public waste water streams due to the incomplete metabolism of antibiotics within the human
body [20].
FIGURE1. Chemical Structure of Chloramphenicol
EXPERIMENTAL
1Materials and methods
From Sigma Aldrich (Bangalore) analar grade chloramphenicol (CLP) was obtained and it is used
without any further purification. The CLP stock solution was prepared by dissolving an known quantity of CLP
in deionised water. The Zn (NO3)2 · 4H2O, NaOH and Ba (NO3)2 procured from HIMEDIA. The analytical
grade chemicals were used to prepare acetate (pH 4-5), phosphate (pH 6.0-8.5) and borate (pH 9) buffers.
2 Photo-catalyst preparation
For synthesizing PZO and BZONP, 0.1M zinc nitrate salt was dissolved in deionized water and 10 mg
dm-3 of sodium dodecyl sulphate (surfactant) was added to control particle size, and serves as a capping agent
during preparation of zinc nitrate solution [21]. 0.1M NaOH solution was prepared separately in deionized
water. NaOH was added dropwise with steady stirring thoroughly (2000 RPM) to zinc nitrate solution for 3 hrs.
The resultant mixture was permitted to settle for 24 hrs then the suspension was decanted carefully, the residual
solution was washed and filtered several times with distilled water then with ethanol to treat the impurities
which are adhered to the nanoparticles. Then the powder was dehydrated in an oven at 120 0C for 3 hrs then
powder was grounded in a mortar then calcined at 500 0C for about 1 hr in a muffle furnace (with a heating rate
about 10 0C per minute). During the drying process, the complete transformation of zinc hydroxide to PZO takes
place. The same procedure was followed to synthesize 5% BZONP the only difference was the addition of 0.1M
barium nitrate solution in zinc nitrate solution. The barium concentration was 5 (% mole ratio). The BZONP it
enters into the interstitial position of PZO lattice. Similar works were earlier reported [22].
020026-2

3 The Photo-catalysis Process
To study the photo-catalytic degradation of CLP, a required quantity of CLP and buffer mixture was
kept in a pyrex beaker. A dose of 0.10 g dm-3 of 5% BZONP was added. Before illumination, the suspensions
were stirred for 60 min in dark place to reach adsorption and desorption equilibrium between CLP and photo -
catalyst. Then, it was transferred into the photoreactor and then kept under 8 W UV lamps (Philips) with a
wavelength peak at 254 nm and of 4mW/cm2 intensity with continuous magnetic stirring. After every 10
minutes interval, the solution was taken out and centrifuged at 5000 rpm for 5 min. The decrease in the
concentration of CLP was monitored at 278 nm (ε = 27471 l mol-1 cm
-1) using visible spectrophotometer (a
CARY 50 Bio UV-Visible Spectrophotometer) and the degree of mineralization was studied.
RESULTS AND DISCUSSION
1 Comparison of different photo-catalysts
The rate of photo-catalytic activity of CLP with UV, UV/PZO, and UV/ BZONP was reported. It was
observed that the degradation effect of CLP treatment with UV/ (5%) BZONP was more efficient than other two
treatments namely UV and UV/PZO.
Effect of BZONP was studied by using 5% (mole ratio) of barium, higher content of barium may
facilitate efficiently separating charge carrier and hindered the recombination of electron-hole pairs, and this
increases the photo-catalytic activity. The photo degradation rate was maximum with 5% BZONP compared
with UV and UV/PZO, hence, further studies were carried out with 5% BZONP. An attempt was also made to
prepare higher than 5% percentage BZONP but barium started depositing on the surface instead of doping. The
% degradation activity of CLP was carried out under same conditions with UV, UV/PZO, and UV/ BZONP and
% adsorption in dark was also studied.
2 X-ray diffraction studies (XRD)
XRD pattern of BZONP was obtained using a Siemens X-ray Diffractometer (Cu source) AXS D5005.
The XRD pattern of BZONP revealed that nanoparticles have hexagonal wurtzite structure Fig. 2. The sharp
peak of (101) indicates that the expansion of nano crystal has taken-up along the easy route of
crystallization of zinc oxide (Lin et al. 2005). No other visible impurity peaks can be observed in XRD.
The broadening of the wurtzite main intense peak (101) was used to calculate the crystallite size of
BZONP, using Scherrer formula (1). The mean crystallite size of BZONP was found to be 35.2 (± 3), using the
following equation [23] (Kulkarni et al. 2016)
D = ଴Ǥଽସఒ
ఉభȀమ ୡ୭ୱ ௾ (1)
Where D is the average crystalline diameter, λ is the wavelength in angstrom, β is the line width at half–
maximum and ࢲ is the Bragg angle.
FIGURE 2. XRD pattern of 5% BZONP
020026-3

3 Scanning electron microscope and EDX studies
The surface morphology of BZONP was obtained by a JEOL JSM 6360 SEM. The SEM analysis of
PZO and 5% BZONP was done to study the surface morphology (Fig. 3 A and Fig. 3 B). It shows that
5% BZONP are irregular shape and size. This irregular shape the surface area of the nanoparticles is very high
[24]. The elemental composition was also performed with Energy Dispersive X-ray Spectroscopy (EDX) and
shown in Fig. 4 A and Fig 4 B (5% BZONP). The peaks from the spectrum reveal the presence of Zn, O and Ba
at 8.630, 0.525 and 4.464 keV respectively. The atomic percentage of Zn, O and Ba is 71.30, 23.99 and 4.71
respectively. It was clear from the elemental analysis that 4.78% barium is present in 5 (mol %) BZONP, which
lead to better photo-catalytic activity.
(A)
(B)
FIGURE 3. SEM micrographs of (A) PZO, and (B) 5% BZONP
(A) (B)
FIGURE 4. EDX analysis of (A) PZO (B) 5% BZONP
4 Transmission Electron Microscope studies (TEM)
The morphology and particle size of PZO and BZONP was measured using JEOL JEM-2010
Transmission Electron Microscopy (TEM). TEM images show that BZONP nanoparticles were agglomerated
with non-uniformly distributed having barrel shaped crystalline structures as shown in Fig. 5 A and Fig. 5 B.
Diffusion of small black spots observed was assumed as Ba atoms on ZnO nanoparticles with approximately 15 -
20 and 25-30 nm in width and length dimension respectively. It was confirmed that the synthesized
nanoparticles crystal dimension is nearer to that of data got from XRD results for PZO nanoparticles.
020026-4

(A)
(B)
FIGURE 5. TEM micrographs of (A) PZO and (B) 5% BZONP
5 Effect of photo-catalyst loading
The influence of photo-catalytic degradation of CLP was studied using a different amount of a 5%
BZONP photo-catalyst ranging from 0.025 g dm-3 to 0.250 g dm-3 whilst [CLP] and pH=5.0 were kept constant.
The rate of photo-degradation increases with increase in the quantity of photo-catalyst up to limiting value
0.10 g dm-3, beyond 0.10 g dm-3 the rate of photo-degradation decreases (Fig. 6). This behavior might be due to
increase in the amount of photo-catalyst increases the exposed surface area of photo-catalyst which in turn
increases the active centers on photo-catalyst. Consequently, which produces a higher number of ∙OH radicals
eventually these radicals involve in reaction to increase the rate of photo-catalytic reaction. But after limiting
value (0.10 g dm-3) increase in the amount of photo-catalyst increases the turbidity of the CLP suspension, thus
it prevents UV-light to reach the photo-catalyst. Hence, the rate of photo-degradation decreases [25].
FIGURE 6. Effect of different amounts of photocatlyst [CLP] = 3.00 x10-5 mol dm-3, at pH=5.0,
Light intensity = 4 mW/cm2
6 Effect of substrate concentration
The rate of photo-catalytic degradation of CLP was studied by taking different [CLP] from 5.00 x 10-6
to 5.00 x 10-5 mol dm-3 by keeping other reaction conditions constant. It has been experimentally investigated
that initially increase in the [CLP], the rate of photo-catalytic degradation increases till [CLP] = 3.00 x 10-5
mol dm-3. Further increase in [CLP], decreases the rate of photo-catalytic degradation as shown in Fig. 7. This
was due to, as the [CLP] increases, plenty of drug molecules are adsorbed on the active centers of the
photo-catalyst surface, hence effective degradation takes place. But after limiting value (3.00 x 10-5 mol dm-3)
the CLP itself acts as a filter for the light. Hence, the photons canot reach the photo-catalyst surface and thus,
the rate of photo-catalytic degradation decreases [26].
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
k obs x 10-3 (min-1)
5% Ba-ZnO (g dm-3)
020026-5

FIGURE 7. Effect of variation of [CLP] on photo-catalytic rate constants of photo-catalytic process with 5%
BZONP at 25 oC, [5% BZONP] = 0.10 g dm-3, at pH=5, light intensity = 4mW/cm2
7 Effect of pH
The pH normally influences the adsorption capacity of the adsorbent in an aqueous medium by altering
the surface properties of adsorbent. The effect of pH on the rate of photo degradation of CLP was studied by
varying the pH from 5.0 - 9.0 while keeping other reaction conditions constant. The rate of photo-catalytic
degradation of CLP Pka=5.5 [27] (Marvin 2012), was higher in the pH range 5.0 - 6.0 and lower in the pH range
7.0 - 9.0 as shown in Fig.8. This increase in the rate of photo-catalytic degradation may be due to the fact that in
acidic medium the photo-catalytic surface is positively charged (PZC=6) and it adsorbs more partial negatively
charged CLP ions hence, an effective collision between CLP and BZONP photo-catalyst takes place so the rate
of degradation was maximum at pH 5.0-6.0.
On the contrary, in the alkaline medium, the OH– ions accumulate on the surface of photo-catalyst
making it negatively charged and CLP is also negatively charged in alkaline medium. Hence, the electrostatic
repulsion between CLP ion and photo-catalyst takes place leading to decrease the rate of photo-degradation at
pH 7.0 to 9.0. This observation is in line with the earlier report [28].
FIGURE 8. Effect of pH on the rate constant of photo catalytic degradation of CLP with 5% BZONP at 25 oC,
[BZONP] = 0.10 g l-1, [CLP] = 3.00 x 10-5 mol dm-3, light intensity = 4mW/cm2
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0123456
k obs x 10-3 (min-1)
[CLP] x 10-5 mol dm-3
1.0
1.5
2.0
2.5
3.0
3.5
4.0
3.0 5.0 7.0 9.0
k obs x 10-3 (min-1)
pH
020026-6

8 Effect of UV lamp distance
To study the rate of photo-catalytic degradation of CLP at different UV light intensity the distance of
UV lamp was varied from the surface of the mixture. It was observed that an increase in UV intensity of light
increases the rate of photo-catalytic degradation of CLP, as depicted in Fig. 9. that, as the light intensity
increases, the amount of BZONP excitation also increases, hence, more electron - hole pairs are generated
consequently the hole degrade the CLP moiety adsorbed on the photo-catalyst surface and oxidize to carbon
dioxide and water. This results in the effective demineralization of CLP.
FIGURE 9. CLP degradation under different UV intensities CLP with 5% BZONP at 25 oC,
[5% BZONP] = 0.10 g dm-3, [CLP] = 3.00 x 10-5 mol dm-3, at pH = 5.0
9 Proposed Reaction Mechanism
The reaction mixture containing CLP and buffer was kept in a beaker. A dosage of 0.1g dm-3
5% BZONP were added. Then, Post-illumination the suspensions were agitated at the black for 1 hr to
accomplish adsorption-desorption equilibrium b/n the CLP and photo-catalyst. Then, it was kept into the
photo-reactor having 8 W UV lamps (Philips) with a wavelength peak at 254 nm and of 4 mW/cm2 intensity
with continuous magnetic stirring. Then the reaction blend was set aside for one day and the products of CLP
were analyzed by Agilent quadrupole 6130 series HPLC system. For HPLC examination 0.1% formic acid,
acetonitrile is used as a solvent with a run rate 1.2 cm3 min-1. This was continued using Column-Atlantis C18
(50 x 4.6 mm-5μm) dual model as shown in Fig. 10. Chloramphenicol MS fragmentation indicates no
degradation of the molecule under this experimental conditions. In conclusion, only one major product was
obtained at (m/z =150). The observed product reveals that only one plausible pathway for the degradation of
chloramphenicol oxidation.
The proposed mechanism shows in Scheme 1.
O2N
OH
HN
OH
O
Cl
Cl
OH OH
m/z 152 (m+2; 100%)
OH
HN
OH
O
Cl
Cl
m/z = 279 (m+1; 20%)
OH
NH2
OH
m/z = 212 (m+2; 6%)
O2N
m/z = 325 (m+2)
OH
N
H
m/z = 150 (m+1, 100%)
NH2
O
m/z = 150 (m+1, 100 %)
(aziridin-2-yl)(phenyl)methanol
2-phenyloxetan-3-amine
1-phenylpropane-1,3-diol
0.0
1.0
2.0
3.0
4.0
3456789
k obs x 10-3
(min-1)
Light intensity (mW/cm2)
020026-7

(a)
chloromphenichol_uv #577 RT:2.57 AV: 1NL:9.00E6
T:FTMS + p ESI Full ms [100.00-800.00]
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
m/z
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
150.1275
R=91307
279.1587
R=66607
391.2838
R=55907
171.1490
R=85207
354.2845
R=57707
750.5612
R=39007
490.8167
R=49107
413.2657
R=52807
672.5599
R=39707
536.1653
R=44907
610.1848
R=40907
793.2709
R=32200
705.2902
R=30800
MASS-UV-D
(b)
FIGURE 10. LC/MS spectra of Chloramphenicol oxidation products a)Total ion chromatogram (TIC)
b) Mass spectrum of reaction product m/z 150
CONCLUSIONS
PZO and BZONP were synthesized by chemical precipitation method. The average particle size of 5% BZONP
is 24.5 to 35.0 nm, exhibited excellent achievable photo-catalytic mineralization of CLP in the acidic condition
(pH 5). The XRD patterns show prepared nanoparticles were wurtzite structure. The EDX and TEM analysis
showed that the existence of Ba in ZnO. Under optimum conditions, over 92% photo-catalytic degradation of
CLP was achieved in 100 min using 5% BZONP photo-catalyst.
ACKNOWLEDGEMENTS
This work was supported by Vision Group of Science and Technology, Karnataka, INDIA under Seed Money to
Young Scientist for Research (GRD 501). Authors would like to thank VGST for funding this project.
020026-8

REFERENCES
1. O. Ligrini, E.,Oliveros and A.M. Braun, Chem. Rev. 93 (1993) 671.
2. M.S. Gudaganatti, M.S. Hanagadakar, R.M. Kulkarni, R.S. Malladi and R.K. Nagarale, Prog. Reac. Kinet. Mec.
37 (2012) 366.
3. R.M. Kulkarni, M.S. Hanagadakar, M.S. Gudaganatti, R.S. Malladi, H.S. Biswal and S.T. Nandibewoor, Ind. J.
Chem. Tech. 21 (2014) 38.
4. R.M. Kulkarni, M.S. Hanagadakar, R.S. Malladi, H.S. Biswal and E.M. Cuerda-Correa, Desalin. Water. Treat. 57
(2016) 10826.
5. R.M. Kulkarni, V.S. Bhamare and B. Santhakumari, Desalin. Water. Treat. 57 (2016) 28349.
6. R.M. Kulkarni, V. S. Bhamare and B. Santhakumari, Desalin. Water. Treat. 57 (2016) 24999.
7. S. Beier, S. Ko¨ ster, K. Veltmann, H. Schro¨der, and J. Pinnekamp, Water. Sci. Technol. 61(7) (2010) 1691.
8. K. Kummerer, Pharmaceuticals in the environment, Springer-Verlag, Heidelberg, Berlin. (2004), pp. 55-60.
9. M.M. Haque, and M. Muneer, Dyes and Pigments 75 (2007) 443.
10. R.M. Kulkarni, R.S. Malladi, M.S. Hanagadakar M.R. Doddamani and U.K. Bhat, Desalin. Water. Treat. 57
(2016) 16111.
11. T. Kudo, Y. Nakamura and Ruike, Res. Chem. Intermed. 29 (2003) 631.
12. G. Zayani, L. Bousselmi, F. Mhenni and A. Ghrabi, Desalin. 246 (2009) 344.
13. U.I. Gaya and A.H. Abdullah, J. Photochem. Photobiol. 9 (2008) 1.
14. C. Guillard, J. Disdier, J. M. Herrmann and et al, Cat.Today. 54 (1999) 217.
15. W.A. Zeltner and D.T. Tompkin, ASHRAE Transactions vol. III, American Society of Heating and Air-
Conditioning Engineers. 2 (2005) 532.
16. D.F. Zhang, Russ. J. Phys. Chem A. 85 (2011) 1416.
17. D.F. Zhang, and F.B. Zeng, Russ. J. Phys. Chem A.85(2011) 1825.
18. I. Justicia, P. Ordejón, G Canto, J.L. Mozos and et al, Adv. Mater. 14 (2002) 1399.
19. British Pharmacopoeia on CD-RomThe Stationery Office on behalf of the Medicines and Healthcare products
Regulatory Agency (MHRA). London, 5th, ed., 2007.
20. The American Society of Health-System Pharmacists. Retrieved Aug 1, 2015
21. W. Water, T. Fang, L. Ji, T. Meen and et al, J. Sci. Innov. 1 (2011) 25.
22. C. H. Kuo, T.F Chiang, L.J. Chen and et al, Langmuir. 20 (2004) 7820.
23. R.M. Kulkarni, R.S. Malladi, M.S. Hanagadakar and et al, J Mater Sci: Mater Electron 27(2016) 13065.
24. M. Panahi-Kalamuei, M. Mousavi-Kamazani, M. Salavati-Niasari and et al, Ultrason.Sonochem. 23 (2015) 246.
25. F. Gallino, C.D. Valentin, G. Pacchioni and et al, J. Mater. Chem. 20 (2010) 689.
26. J. Sun, L. Qiao, S. Sun and G. Wang, J. Hazard. Mater. 155 (2008) 312.
27. Marvin (2012) chemaxon. available from: <http://www.chemaxon.com>.
28. I.T. Horvath, Encyclopedia of Catalysis, Wiley, New York (2003) pp. 317-337.
020026-9