Intermediate formation during photodegradation of phenol using lanthanum doped tin dioxide nanoparticles

Article abstract of DOI:10.1007/s11164-015-2197-9

Lanthanum (La)-doped tin dioxide (SnO₂) nanoparticles were synthesized by a modified sol-gel method at room temperature. The samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The photocatalytic activity of La:SnO₂ samples were investigated by studying the degradation profile of phenol and its by-products in water. The treated samples were analyzed by HPLC-UV and a UV-Vis spectrophotometer. Benzoquinone, catechol, resorcinol, hydroquinone, acetic acid, and 2-propanol were identified as phenol degradation intermediates. Maximum concentration acquired was in the order of catechol, resorcinol, hydroquinone, and benzoquinone, which was observed in the beginning stages while iso-propanol and acetic acid were observed in the final stages of phenol degradation. We achieved a complete photodegradation of a 10 ppm aqueous phenol solution and intermediates with 0.6 % of SnO₂:La nanoparticles in 120 min under artificial solar irradiation. A maximum degradation rate constant of 0.02228 min^-1 of propanol and a minimum of acetic acid 0.013412 min^-1 were recorded at 37 °C.

Full text of DOI:10.1007/s11164-015-2197-9

Res Chem Intermed
DOI 10.1007/s11164-015-2197-9
Intermediate formation during photodegradation
of phenol using lanthanum doped tin dioxide
nanoparticles
¨ ¨1
Abdullah M. Al-Hamdi1 Mika Sillanpaa
Joydeep Dutta2,3
•
•
Received: 5 May 2015 / Accepted: 24 July 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Lanthanum (La)-doped tin dioxide (SnO2) nanoparticles were synthe-
sized by a modified sol–gel method at room temperature. The samples were char-
acterized by X-ray diffraction, scanning electron microscopy, and transmission
electron microscopy. The photocatalytic activity of La:SnO2 samples were inves-
tigated by studying the degradation profile of phenol and its by-products in water.
The treated samples were analyzed by HPLC–UV and a UV–Vis spectropho-
tometer. Benzoquinone, catechol, resorcinol, hydroquinone, acetic acid, and
2-propanol were identified as phenol degradation intermediates. Maximum con-
centration acquired was in the order of catechol, resorcinol, hydroquinone, and
benzoquinone, which was observed in the beginning stages while iso-propanol and
acetic acid were observed in the final stages of phenol degradation. We achieved a
complete photodegradation of a 10 ppm aqueous phenol solution and intermediates
with 0.6 % of SnO2:La nanoparticles in 120 min under artificial solar irradiation. A
maximum degradation rate constant of 0.02228 min-1 of propanol and a minimum
of acetic acid 0.013412 min-1 were recorded at 37 °C.
&
Joydeep Dutta
joydeep@kth.se; dutta@squ.edu.om
Abdullah M. Al-Hamdi
Abdullah.Al.hamdi@lut.fi
Mika Sillanpa¨a¨
Mika.sillanpaa@lut.fi
1
2
3
Laboratory of Green Chemistry, Lappeenranta University of Technology, Sammonkatu 12,
50130 Mikkeli, Finland
Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, PO Box 33,
123 Al-Khoudh, Muscat, Sultanate of Oman
Functional Materials Division, ICT, KTH Royal Institute of Technology, Isafjordsgatan 22,
164 40 Kista, Stockholm, Sweden
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A. M. Al-Hamdi et al.
Keywords Photocatalysis Á SnO2 Á Rare earth metal Á Lanthanum Á By-product Á
Phenol
Introduction
The overuse of pesticides, pharmaceuticals, and personal care products in modern
society leads to many kinds of organics polluting the environment. Phenols or
derivatives of phenols are commonly encountered organic pollutants in toxic waste
discharged in industrial effluents [1, 2]. The accumulation of these toxic pollutants
can adversely affect human life and derail sustainable development [3, 4] because of
their toxicity, endocrine disrupting abilities, and carcinogenic behavior [5]. Phenol
and its by-products are released into the air and water as a result of industrial
activities [6]. Sources of phenol and its derivatives in water has been found in waste
water discharged from refining activities of oil and coal tar industries [7], distillation
of wood or coal [6], phenolic chemicals used in livestock dips [8], and discharges of
domestic sewage or rotting vegetation [9]. The European Union and other countries
have included some phenols in their list of priority pollutants [10], thus; it is
important to develop new and effective treatment processes for the removal of
organic and inorganic pollutants from water.
Advanced oxidation processes (AOP) are among the most effective technologies
for the treatment of organic pollutants, because they can generate strong oxygen-
based oxidizers (OH) [11], which can react non-selectively with different types of
organics and can achieve complete oxidation [12]. AOP techniques can be built up
using a combination of chemical and physical agents such as a catalyst and
ultraviolet light [13].
Photocatalysis is a green technology for water treatment to remove recalcitrant
organic compounds. Tin dioxide or stannic oxide (SnO2) is one of the most
promising metals oxide semiconductor photocatalysts, in which the inherent oxygen
vacancies act as n-type dopants leading to effective charge separation in the
presence of a pollutant [14]. SnO2 has a rutile type tetragonal crystal structure, and
its band gap is 3.6 eV at room temperature [15]. There are numerous applications of
SnO2, as gas sensors, pH sensors, and biosensors [16], in lithium ion batteries [17],
transparent conductive electrodes, [18] etc. SnO2 is, however, less studied compared
to the other photocatalysts such as titanium oxide TiO2 and ZnO [19]. Thus, it is of
interest to investigate the influence of dopants on the enhancement of the
photocatalytic activities of SnO2 [20]. In addition, these semiconductors have been
recognized as preferable materials for photocatalytic processes due to their high
photosensitivity, nontoxic nature, low cost, and well-known chemical stability [21–
25]. Apart from these properties, a good visible–light photocatalyst should have a
high specific surface area and an adequate optical absorption band to be able to
absorb sunlight. Incorporation of metal ions into the SnO2 matrix could introduce
defects lowering the effective optical gap and, hence, leading to a better
photocatalytic activity [26, 27]. Renewed interest in the synthesis of SnO2
nanoparticles of different sizes has probably arisen to address these problems
[28–30].
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Intermediate formation during photodegradation of phenol…
Photocatalytic degradation of organic compounds is mainly achieved by
ultraviolet light irradiation due to the wide metal oxide bandgap. A better
alternative is to use visible light, or solar irradiation as it is an abundant free source
of energy. Many ways have been developed for rendering the wide band gap metal
oxide semiconductors photoactive in the visible region [31]. Of these, incorporation
of defects in the metal oxides with transition metals is very popular [28]. For
example, rare earth metal ions incorporated into the SnO2 matrix introduces defects
in the valence band tail states, effectively lowering the optical gap and, hence,
leading to a better photoactivity in the visible region [32–34]. Several studies of
SnO2 doping with different metals to produce nanocrystals have been reported in
recent years. [35–37] Sn4? can be easily reduced to Sn2? leading to a change in the
surface electronic structure of SnO2. Oxygen atoms on the surface of SnO2
nanoparticles lead to energetically favorable reconstructions of the SnO2 (110) and
(101) surfaces [38]. These oxygen vacancies bind with electrons forming an
excitation energy level below the conduction band of SnO2. As rare earth metals
incompletely occupy 4f and 5d orbital’s, formation of labile oxygen vacancies with
relatively high mobility of bulk oxygen species can occur very easily [39].
Sol–gel synthesis of SnO2 nanoparticle is very popular as it is based on simple
hydrolysis of metal alkoxides in aqueous media:
hydrolysis
SnðC2H5OÞ4 þ 4H2O ÀÀÀÀ! SnðOHÞ4 þ 4C2H5OH:
The alkoxy groups are replaced by hydroxide groups forming an inorganic
network (sol) that is followed by a second reaction (condensation) as follows:
condensation
2SnðOHÞ4 ÀÀÀÀ! 2SnO2 þ 4H2O:
The sol–gel method offers several advantages, including the formation of
homogeneous nanoparticles, precise control over the stoichiometry, and purity
while the reaction takes place at lower temperatures.
In this work we describe the preparation of La doped tin dioxide nanoparticles at
room temperature, using the sol–gel synthesis method. The photocatalytic properties
SnO2:La nanoparticles were evaluated by studying the degradation profile of phenol
and its intermediates that will contribute to a better understanding of the phenol
degradation processes. This work provides a framework for studying degradation
pathways in similar and for complex organic toxins.
Experimental
Materials
Analytical-grade tin (IV) chloride (SnCl4-, Sigma Aldrich, Purity: 99 %),
lanthanum nitrate hexahydrate (La (NO3)3Á6H2O, Fluka), ethanol (C2H5OH, ETAX
Aa Merck Absolute), phenol (C6H5OH, Sigma. Aldrich, 99 %), catechol (C6H6O2,
Sigma Aldrich, Purity: 99.99 %), hydroquinone (Sigma Aldrich, Purity: 99.99 %),
123

A. M. Al-Hamdi et al.
p-benzoquinone (O: C6H4:O, BDH, AnalaR 99.99 %), resorcinol (C6H4(OH)2,
BDH, AnalaR 99. %), acetic acid (C2H4O2, Sigma Aldrich, Purity: 99.99 %),
propan-2-ol ((CH3)2CHOH, Sigma Aldrich, Purity: 99.99 %), sulfuric acid (H2SO4,
Sigma Aldrich, Purity: 99 %), hydrochloric acid (HCl, Sigma. Aldrich, 99 %),
sodium hydroxide pellets (NaOH, Sigma. Aldrich, 99 %) and methanol (CH4O,
Sigma. Aldrich, 99 % HPLC grade) were used in the experimental studies. Double
deionized water with a high purity from Ultra Clear Water SG was used for the
preparation of all solutions.
Photocatalyst preparation
Synthesis of pure and doped-SnO2 nanoparticles
Nanoparticles containing various molar compositions of SnO2 were chemically
synthesized by the sol–gel method, following the procedure described below:
3.80825 ml of SnCl4 mixed in 50 ml absolute alcohol (ethanol) and 50 ml ultra-
pure water in a round bottom flask and used as a precursor for synthesizing undoped
SnO2 nanoparticles. The mixture was subjected to vigorous stirring for 3 h at room
temperature, until a colourless solution was obtained. For the synthesis of
lanthanum metal doped tin oxide nanoparticles, a certain concentration of this
metal was mixed and dissolved separately in ethanol and added to the precursor
solution. A turbid colloidal solution of tin alkoxide started to appear after adding
(25 %) of aqueous ammonia drop by drop into the solution upon continuous stirring
until a pH value of 8 was reached. The sol was aged for 48 h in air until a gel was
formed. Filtration and successive washing with ultra-pure water was repeated
several times to remove both ammonia and chloride ions until all chloride ions had
been removed (determined by examining the filtrate solution using aqueous silver
nitrate solution). The dopant concentration studied in this work was kept up to
0.6 %, following our previously reported work [32].
Material characterization
Optical absorption spectra of colloidal suspension were recorded in a double beam
spectrometer (Perkin Elmer, Lambda 45). To study the surface morphology of the
SnO2:La samples, scanning electron microscopy (SEM) was carried out with JEOL
(JSM-5600LY) (Japan) working at 30 kV. High resolution transmission electron
microscopy (HRTEM) was carried out with JEOL (JEM 2100F) (Japan) working at
200 kV. The crystalline phase of the synthesized SnO2 nanoparticles were analyzed
by X-ray powder diffraction (XRD) using a MiniFlex 600 diffractometer (Rikagu,
˚
Japan) with Cu Ka radiation (wavelength = 1.54 A). Diffractograms of powder was
recorded in a 2h scan configuration, in the 10°–80° 2h range, with the data collected
at steps of 0.02°. Then, 200–300 mg of sample was degassed at 300 °C for 3 h
under nitrogen (N2) flow, followed by a six point N2 adsorption carried out at p/p0 of
1–5 in a NovaÒ 2200 analyzer. The surface area was determined by a gas adsorption
technique using the NovaÒ 2200 (Brunauer–Emmett–Teller (BET) method).
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Intermediate formation during photodegradation of phenol…
Photocatalytic experiments
UV photocatalytic reactor
The photoreactor was a cylindrical Pyrex-glass container of 50 mm internal
diameter and 300 mm in height with 250 mL capacity. An 8 W medium pressure
mercury lamp (Osaka, Japan) was used with an intensity of about 80 W/m2 and the
reaction temperature was kept at 25 °C by cooling with water.
Visible light photocatalytic reactor
The Pyrex-glass reactor was used with a cooling water jacket with a 250 ml
capacity, 100 mm internal diameter, and 200 mm height. A 300 W xenon lamp
simulating a solar light source (PLS-SXE300C, Beijing Perfect light Co. Ltd.,
China) was placed 71 mm above the surface of contaminant during the
photocatalysis studies.
Experimental procedure
To investigate the effects of SnO2:La, oxidative photodegradation of phenol and its
intermediates were chosen to evaluate the photocatalytic activity of synthesized
nanoparticles. The study of degradation of phenol and its by-products in water was
carried out at 25 °C using the water-cooled cylindrical glass batch photoreactor of
250 mL volume. The experimental conditions were varied as follows: The amount
of catalyst powder (photocatalyst) was kept at 0.065 g in 50.00 mL; the initial
concentration of the pollutant was varied from 5–100 ppm and the pH of the
solution was adjusted by the addition of 0.5 N NaOH to fix pH at 5.7. Then, 10 mg/
L phenol solution was used to carry out photocatalysis experiments. Before each
test, the mixture was kept in the dark for 30 min under constant magnetic stirring to
ensure that the adsorption–desorption equilibrium was reached before illumination.
The sample was then taken out at the end of the dark adsorption period, prior to
turning the light on or before exposure to sunlight. When the lamp was turned on,
the irradiation time varied from 2 to 3 h; aliquots of the suspensions were collected
and removed from the reactor every 15 or 30 min. At the given time intervals, the
photocatalyst particles were removed from the samples first by centrifugation
(4000 rpm, 5 min) using an Eppendorf Centrifuge (system model 5810R) and then
filtered through 0.2 lm Millipore filters.
Analytical methods
Phenol photodegradation by-products were analyzed using high performance liquid
chromatography (UHPLC focused), containing a P580 high-precision pump from
Thermo Scientific (Germering, Germany), and fulfilling the requirement for a
measured delivery rate of the mobile phase. The injector was a Rheodyne (Cotati)
8125 valve fitted with a 20 lL loop. The analysis was carried out using a C18
column (5 lm silica) maintained at room temperature. After elution from the
123

A. M. Al-Hamdi et al.
column, the eluent passed through a Variable Wavelength Detectors (VWD-
3400RS) set at 245 nm for phenol and its intermediates detection. The mobile phase
for the HPLC separation was prepared by mixing methanol with water in the ratio
45:55 and then adjusting the pH at 3.0 by adding sulfuric acid (H2SO4). The mobile
phase was degassed in an ultrasonic bath, and then passed through a Millipore
filtration apparatus (0.2 lm). Photodegraded phenol and control samples were then
eluted at a flow rate of 1 mL/min.
The change in phenol concentration in each photodegraded solution was also
monitored by an ultraviolet–visible (UV–Vis) spectrometer (Perkin Elmer Lambda
45), and the maximum absorbance of phenol was measured at kmax = 269 nm.
Results and discussion
Crystallinity of SnO2 nanoparticles doped with lanthanum
The XRD pattern (Fig. 1) showed that the diffraction angles at 2h = 26.3°, 33.6°,
51.5°, and 65° that can be assigned to (110), (101), (211), and (112) planes of the of
SnO2, respectively, matching with the standard XRD file of the rutile phase of SnO2
(JCPDS-#41-1445) [40–43]. SnO2 crystallite sizes obtained from the Debye
Scherrer equation considering the most intense peak (110) follows the well-known
trend of crystallite size reduction upon doping. An increase in surface area was also
observed, which was attributed to the decrease in grain size as it can be seen from
Table 1.
Fig. 1 X-ray diffraction (XRD) patterns of SnO2:La and pure undoped particles synthesized by the sol–
gel process
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Intermediate formation during photodegradation of phenol…
Table 1 Comparison of surface
Dopant
Size nm
Surface area
areas and estimated crystallite
sizes for undoped and lanthanum
Undoped
7.5
3.4
28
40
doped SnO2 nanoparticles
SnO2:La
Fig. 2 Typical scanning Electron Micrograph of SnO2:La nanoparticles synthesized by the sol–gel
process
The 3.4 nm crystallites agglomerate to form 50 nm particles as observed by SEM
(Fig. 2). In order to get a better insight into the morphology, we carried out high
resolution HRTEM as shown in Fig. 3.The crystallite sizes are comparable to what
was estimated from the XRD. The surface area of the synthesized nanoparticles was
found to be between around 28–40 m2/g.
Photocatalytic degradation
Photodegradation occurs due to the formation of active radicals during redox reaction.
When a photocatalyst absorbs a photon of light (hv), it is enough to excite the electrons
to the higher states creating an electron (e-) and a positive-hole (hþ) pair
À
Á
SnO2 þ hv ! SnO2 eÀCBhVþB
:
The holes (hþ) dissociates water molecules to generate hydroxyl radicals and
hydrogen gas
À
Á
SnO2 hþVB þ H2Oads ! SnO2OHÁads þ Hþ:
The holes (hþ) mostly react with water (i.e., hydroxide ions, OHÀ) to produce
OHÁ, which is very a powerful oxidant.
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A. M. Al-Hamdi et al.
Fig. 3 Typical high resolution
transmission Electron
micrograph of SnO2:La
nanoparticles synthesized by the
sol–gel process
À
Á
SnO2 hþVB þ OHaÀds ! SnO2 þ OHÁads
The electron reacts with an oxygen molecule to form superoxide radicals (OÀ2 Á).
The hydroxyl radical is an extremely powerful oxidation agent, and the cycle
continues until light is available.
À
Á
SnO2 eÀCB þ O2ads ! SnO2 þ OÀ2 Á
The hydroxyl radical will oxidize the pollutant and any intermediates, and the
final product is carbon dioxide and water.
À
Á
SnO2 hþVB þ phenol ! SnO2 þ oxidized products
UV–Vis spectroscopy
UV scans (200–400) were used to investigate possible intermediates formed in
solution during photodegradation of phenol by using SnO2:La. Previous research on
phenol degradation has shown that phenol first degrades to benzoquinone and then
the ring is opened to form other smaller byproducts like the carboxylic acids [44,
45]. The adsorption ability of the samples were tested, and it was found that after
30 min in dark conditions, very minimal changes of phenol adsorption were
observed on the SnO2:La nanoparticles. The UV–Vis spectra at different time
intervals during phenol photodegradation with SnO2:La are shown in Fig. 4. At the
outset only one broad peak was present at 269 nm that was observed due to
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Intermediate formation during photodegradation of phenol…
Fig. 4 Spectra of phenol photodegradation and reduction in the intensity of phenol peak observed by
UV–Vis spectrophotometry in the presence of 0.6 wt% lanthanum doped SnO2 nanoparticles. The
appearance of different phenol byproducts appear in the Figure
absorption in the aromatic ring. This absorption band was used as a reference for the
photodegradation analysis. A clear decrease of the 269 nm absorbance band
intensity was observed with increasing photo irradiation time (Fig. 4). A slight
decrease of the 269 nm absorbance band, but a simultaneous increase at the 290 nm
band and a higher increase of the 246 nm absorbance band can be associated with
the formation of phenol, hydroquinone (HQ), and benzoquinone (BQ), respectively,
within 15 min of photo irradiation. After 30 min of light irradiation, the phenol
absorbance band was reduced, but a concurrent increase and broadening of the
290 nm band and a rising increment with broadening of a 246 nm band was
observed. After 60 min of irradiation, the characteristic peak at 290 nm reduces, and
the 246 nm peak intensity decreases with the disappearance of the phenol peak.
After 90 min of irradiation, it is clear that all phenol was degraded and also the
formed intermediates are being further degraded.
Intermediates of phenol degradation
HPLC separation technique
In order to conclusively characterize the byproducts formed during the photocat-
alytic reaction, chromatographic techniques were used to study the degraded phenol
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A. M. Al-Hamdi et al.
Fig. 5 Separation of phenol and its by-products form benzoic acid and paraben mixture observed by
(HPLC)
mixture. Degradation of phenol in water, followed by the formation of many
intermediates, some of which could be more toxic than phenol itself, which causes
symptoms such as muscle weakness, convulsions, and coma upon contact to human
skin. Phenol, upon oxidation, produces several aromatic intermediates, such as
benzoquinone, catechol, resorcinol, and some acyclic compounds, including oxalic
acid, formic acid maleic acid, and fumaric acid. We have been able to separate
phenol by-products in solutions during photocatalysis, by different techniques such
as GC/MS and also using techniques to quantify these intermediates by HPLC and
CE. Here, we used a simple reverse phase HPLC technique to separate the parabens,
benzoic acid, phenol, benzoquinone, catechol, resorcinol, hydroquinone, acetic acid,
and isopropanol in less than 5 min as shown in Fig. 5. Maximum concentration of
intermediates obtained is in the order catechol [ resorcinol [ hydroquinone [ ben-
zoquinone as shown in Fig. 6.
In fact OHÁ generated from photocatalytic degradation of phenol reacts with
OH-, either to produce catechol or hydroquinone. Catechol or hydroquinone reacts
with OH- to form benzoquinone. Upon continuous oxidation, the benzene ring will
be opened leading to the formation of aliphatic compounds, which would finally
mineralize to CO2 and water. The degradation as observed by optical spectroscopy
is different from what was registered using chromatography as the extinction
coefficients play a part in the final optical absorption of the by-products. For
example, benzoquinone absorbs nearly ten times more than phenol around 269 nm,
which is the peak absorption of phenol. Thus, the formation of the by-products
would mask the degradation level of phenol if only optical spectroscopy is used. In
effect UV–Vis spectroscopy would not render a correct picture of the phenol
degradation process, thus all subsequent discussion are based on chromatographic
studies (Fig. 7).
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Intermediate formation during photodegradation of phenol…
Fig. 6 Evolution of phenol by-products during phenol photodegradation in the presence of SnO2:La
nanoparticles in a 10 ppm aqueous phenol solution
Fig. 7 Degradation of phenol and its by-products after 2 h treatment with SnO2:La
Degradation mechanism of phenol involving the O–H bond
Formation mechanism In order to better understand the degradation mechanism, the
major degradation by-products were studied independently upon photo irradiation.
The reaction of phenol starts by breaking the O–H bonds resulting in giving up a
proton thus forming an anion stabilized by the aromatic ring as shown in Fig. 8.
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A. M. Al-Hamdi et al.
Fig. 8 Possible degradation mechanism of phenol and resonance stability of phenoxide, and the
formation of the benzoquinone intermediate
Activation of the phenol by hydroxyl radicals will produce phenoxy radicals,
which are in resonance with radical structures in ortho and para positions [46]. The
three mesomeric forms of the radical are the initial formation of the different
intermediates. These radicals can react with the hydroxyl radicals to form
compounds such as hydroquinone, benzoquinone, and catechol as explained above.
Benzoquinone
Benzoquinone was traced in the degradation process for the first 30 min of
photodegradation. The formation rate of benzoquinone was found to be
0.0107 ppm/min, and the degradation rates were calculated from the reduction of
benzoquinone peak to be 0.00357 ppm/min.
Catechol
Degradation of phenol produces higher concentrations of catechol compared to any
other by-products, which might be related to the ortho directed properties group of
phenol. Catechol was formed within 15 min of phenol degradation and its
concentration increased to a maximum of 0.7 ppm, and kept increasing until it
reached 3 ppm after 45 min increasing at a rate of 0.1249 ppm/min. After 75 min,
concentration of catechol decreased at a rate of 0.0292 ppm/min. We can observe
from the graph that catechol keeps being produced until all the phenol is degraded.
Resorcinol
Resorcinol appeared from the beginning, and its concentration increased from
0.3 ppm until it reached to 1.5 ppm and became stable at that point after 75 min of
photocatalytic degradation at a rate of 0.0075 ppm/min. The almost stable
concentration of resorcinol production in our mixture means that it will be
produced by the same rate until all phenol can be removed from the solution. We
also noticed that resorcinol concentrations were higher than hydroquinone. In this
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Intermediate formation during photodegradation of phenol…
case it seems that the substitution rules of OHÁ radicals attacking the phenol
molecule with higher probability in 1 and 4 positions (ortho and para directing
effect) are weaker than the nonselective nature of attack of the OHÁ group of the
aromatic ring.
Hydroquinone
During phenol degradation, hydroquinone appeared after 30 min, and it was at a
maximum after 45 min of photo-irradiation at a rate of 0.0025 ppm/min. After
60 min of phenol photodegradation, benzoquinone concentrations decrease sharply
and further slow reduction was observed until all phenol was removed from the
solution.
Acetic acid
Ring opening of the by-products lead to the formation of acetic acid; pH also
increases (results not shown) [47]. Acetic acid appeared in the reaction medium
after 45 min at a rate of 0.0015 ppm/min.
Iso-propanol
The 2-Propanol is also known as isopropanol, and has a neutral pH level of
approximately 7 (as we obtained from our previous analysis, result not shown) [47],
almost similar to pure water. It seems that propionic acid competes with acetic acid,
a reason why we are getting traces of this compound sometimes before acetic acid,
and at other occasions, after acetic acid. Iso-propanol appeared in the solution after
75 min of photocatalytic degradation at a rate of 0.0813 ppm/min.
Photocatalytic degradation of different aromatic compounds
To compare the photocatalytic activity of phenol and its intermediates, the
photocatalytic activity of phenol and its by-products in lanthanum doped tin dioxide
was investigated in a series of experiments with a control and 0.6 wt% La3?–SnO2.
The influence of lanthanum doped tin dioxide nanoparticles on the degradation of
phenol and removal of its by-products in aqueous solution is presented in Fig. 7.
The overall degradation rate of phenol and its by-products was higher upon using
SnO2:La than those with pure (control) tin dioxide nanoparticles (Fig. 9).
A maximum rate of removal of the by-products, such as phenol was achieved
using La: SnO2 (Fig. 9). An observed rate of phenol degradation was fitted with
first-order rate constants (Table 2). The rate of degradation of propanol was quite
similar to phenol. The rate of degradation of phenol was slightly higher than that of
acetic acid. Slower degradation rates were also observed with catechol and
resorcinol.
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A. M. Al-Hamdi et al.
Fig. 9 Kinetic study of photodegradation of phenol and its intermediates in the presence of 0.6 wt%
lanthanum doped SnO2 nanoparticles
Table 2 Kinetic parameters of
Organic molecule
Rate constant
K (min-1
R2
phenol and its byproducts
degradation in a photocatalysis
process by La:SnO2
)
Phenol
0.02228
0.02228
0.0209
0.9977
0.9961
0.9952
0.9834
0.9544
0.9158
0.9135
nanoparticles
Propanol
Benzoquinone
Hydroquinone
Catechol
0.017066
0.013839
0.013753
0.013412
Resorcinol
Acetic acid
Conclusions
In summary, undoped and La doped SnO2 nanoparticles were successfully
synthesized by the sol–gel method, and the phenol intermediates formed during
photodegradation were studied. Photocatalytic experiments showed that La doped
SnO2 degraded all the by-products albeit with different rate constants. La doped
SnO2 nanoparticles are expected to be a promising candidate for effective
photocatalysis processes using sunlight irradiation that can find applications in
the real world.
Acknowledgments The authors would like to thank partial financial support from the Chair in the
Nanotechnology programme of the Research Council of Oman and the college of science (Chemistry
Department) for giving us this opportunity to carry out the research at Sultan Qaboos University.
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Intermediate formation during photodegradation of phenol…
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