Indian Journal of Chemistry
Vol. 48A, March 2009, pp. 367-371
Sonophotocatalytic behavior of cerium doped
salts of Cu(II), Co(II) and Mn(II) in the
degradation of phenol
Pankaj* & Mayank Verma
Department of Chemistry, Faculty of Science,
Dayalbagh Educational Institute, Agra, 282 005, India
Email: [email protected]
Received 11 December 2008;
revised and accepted 30 January 2009
Crystals of CuSO4.5H2O, CoCl2.6H2O, and MnCl2.4H2O
doped with CeCl3 under sonicated and unsonicated conditions
have been synthesized. The crystals synthesized under sonicated
conditions are different than those synthesized under unsonicated
conditions with respect to shape, size, and color. The crystal
composition has been estimated by atomic absorption
spectrophotometry with graphite furnace and UV-visible
spectrophotometer. Higher percentage of Cu(II), Mn(II) and
Ce(III) in the crystals synthesized under sonication as compared
to the unsonicated crystals may be attributed to the change in the
composition of the lattice pattern due to the mechanical impact of
ultrasound, whereas such an effect has not been found in the
Co(II) salts. Enhanced sonophotocatalytic effect of these crystals
on the degradation of phenol under sonic conditions is attributed
to the increased number of unpaired electron following free
radical mechanism.
Keywords: Sonocrystallization, Sonophotocatalysis, Degradation,
Phenol degradation, Doped salts, Cerium, Copper,
Cobalt, Manganese
IPC Code: Int. Cl.8 B01J23/00; B01J37/34; C01G1/10;
C01G3/10; C01G45/10; C01G51/10
Crystallization is an important process which is
extensively used in industries for the production of
fine chemicals, food and pharmaceutical drugs, nonlinear optics, optoelectronics and ultrasonics1,2(a). This
is due to the fact that a crystal is very pure as each
molecule or ion fits perfectly into the lattice site when
leaving the solution. Due to its importance in
catalysis, optoelectronics, micro- and nanoelectronics,
magnetics and non-linear optical properties, crystals
with different structures (from nano- to macro size,
e. g., nanoflowers, nanorods, nanocups, nanodisks,
nanospheres, nanowires, polyhedral, spindle like, dot
shaped and hexagonal etc.) have been prepared by
different
methods
(slow
cooling2(a),
slow
evaporation2(b), Bridgman method2(c), Czochralski2(d),
Pechini sol-gel2(e), flux growth2(f), chemical vapor
transport2(g), top seeded solution growth-slow
cooling2(h),
high
temperature
solution2(i),
2(j)
2(k)
hydrothermal , polyol solution , solvothermal2(l),
and eutectic freeze crystallization2(m), etc.). Recently,
industrial chemists have also used ultrasound to
synthesize crystals on industrial scale3. The ultrasonic
irradiation and cavitation in liquid and solid-liquid
systems can produce a series of unique chemical and
physical effects to improve the crystalline product.
Ultrasound can be used to mix reactants rapidly,
reducing the agglomeration. The ultrasound can also
enhance the controllability of crystallization process
by adjusting the power density and ultrasonic
irradiation time4. The novel properties of
semiconductor nano- and microcrystals depend on
their size, shape and crystalline structures2(l). Most of
the semiconductor materials are of great importance
in materials science due to their unique electronic and
optical properties and extensive applications, e.g.,
photonics, nanoelectronics, information storage,
etc.2(k),2(l). Therefore, the ability to tune the structural
size and shape of inorganic materials is an important
goal as molecular shape, symmetry and
intermolecular forces are the key to successful design
of crystalline materials5. Shape controlled crystals
have been studied under microwave6,7 and ultrasonic
field8 as well.
Phenol, one of the most common water pollutant
released from many different industries as effluents is
of deep concern. Degradation of phenol has been
studied under combined irradiation of microwave and
ultrasound9. Adsorption of phenol has been studied
recently under combined effect of ultrasound and
nanoclay10. Besides, sonochemical degradation of
phenol and its chloro-derivatives has also been
analyzed theoretically11. However, there seems to be
no study on the synthesis of common transition metal
salts doped with rare earths under ultrasonic field and
their study as a catalyst in the degradation of organic
molecules. The present study has been undertaken to
understand the role of extra valence electrons in the
catalytic process, introduced through the doping of
rare earths.
INDIAN J CHEM, SEC A, MARCH 2009
368
Experimental
[CuSO4.5H2O]
(Qualigens),
[CoCl2.6H2O]
(Qualigens) and [MnCl2.4H2O] (BDH) were
recrystallized twice with triply distilled water.
However, CeCl3 (Indian Rare Earths Ltd., 99.99%)
was used without further purification but kept under
vacuum for over two hours to remove any trace of
moisture. Phenol (Qualigens) was distilled under
vacuum and cooled for crystallization. The liquid
fraction was decanted off and only crystallized phenol
was used in the preparation of solution.
Four sets, each containing 25, 20, 15 or 10 ml
aqueous solution of CeCl3 (0.1 M) were mixed with
aqueous solution of 25 ml of CuSO4.5H2O (1.4 M)
separately. Each set was made up to 50 ml with
distilled water in 100 ml beaker. Similarly, four sets
of the above solutions were again prepared and
sonicated with ultrasonic probe before crystallization.
Similar procedure was followed for CoCl2.6H2O and
MnCl2.4H2O. The ultrasonic equipment consisting of
Ti probe (model Vibronics Ultrasonic Processor) with
a tip diameter of 1.2 cm was used and immersed up to
2.0 cm in the liquid for sonication. The device was
operated at a fixed frequency of 20±2 kHz and power
of 6 watts. The power input was fixed at maximum
cavitation in the liquid and sonicated for
30 min. All the sets were covered with filter paper
with three holes of 0.5 cm diameter on the surface and
kept for recrystallization with slow evaporation.
Crystals of CuSO4.5H20 doped with CeCl3 were
isolated under ambient conditions; however, the
crystals of CoCl2.6H2O and MnCl2.4H2O were
Table 1—The amount of Ce(III), Cu(II), Co(II), and Mn(II) in
the cerium doped composites under unsonicated and sonicated
conditions estimated through AAS
M(II)a
(mg/l)
Ce(III)
(mg/l)
M(II)a :
Ce(III) ratio
CeCl3 - CuSO4.5H2O
Unsonicated (blue)
Unsonicated (white)
Sonicated
2.578
0.270
3.582
0.373
0.502
0.454
6.91 : 1.00
0.54 : 1.00
7.89 : 1.00
CeCl3 – CoCl2.6H2O
Unsonicated
Sonicated
6.841
6.651
0.139
0.158
49.2 : 1.00
42.1 : 1.00
CeCl3 – MnCl2.4H2O
Unsonicated
Sonicated
4.077
4.501
0.116
0.127
35.15 : 1.00
35.44 : 1.00
Composite
a
M(II) = Cu(II)/Co(II)/Mn(II)
synthesized under vacuum. Transition metals, Cu, Co,
and Mn, were analyzed by atomic absorption
spectrophotometer, graphite furnace (Analytik Zena,
ZEEnit 700), while cerium, was estimated
spectrophotometrically using UV-vis spectrophotometer (Shimadzu, UV-1601) at λmax 500 nm
using 1-(2-pyridylazo)-2-naphthol as complexing
agent12 (Table 1). The Ce-doped salts of Cu(II) and
Co(II) were extremely hygroscopic and their SEM
images could not be obtained. Photographs of these
crystals were taken with a digital camera (Sony DSCW35) with 3x optical zoom. SEM images of the Cedoped Mn(II) salts could be obtained.
Degradation of phenol in aqueous solution
(0.5 mg/50 ml) has been examined under several
experimental conditions and different duration of
time, such as (i) under sonicated condition, (ii) in the
presence of (n-BuO)4 Ti under sonicated as well as
unsonicated conditions, and (iii) in the presence of
(n-BuO)4 Ti and Ce doped transition metal salts such
as Ce-CuSO4, Ce-CoCl2 and Ce-MnCl2. The volumes
of (n-BuO)4 Ti (0.01 ml) and transition metal salts
(1.0 ml) were kept constant in all the experimental
solutions. Aqueous solution (25.0 ml) containing
0.5 mg of phenol was mixed with 0.01 ml of
(n-BuO)4 Ti and 1.0 ml of the solution of Ce doped
salts of transition metal salts as above. These
solutions were sonicated with an ultrasonic probe
fitted with a titanium head.
Results and discussion
When CuSO4 is doped with CeCl3, bright blue and
white
crystals
respectively
were
isolated
simultaneously in unsonicated condition (Fig. 1a),
whereas only bright blue crystals in sonicated
condition (Fig. 1b) were formed. Upon analysis, the
blue crystals were found to be CuSO4 doped with
cerium whereas the white crystals were largely CeCl3
with traces of CuSO4 as the surface impurity.
However, sonication of this solution produced only
one kind of crystal. Possibly, the ultrasound could
push cerium atom into the copper sulphate lattice and
therefore, white crystals were not isolated in this
system under sonication. In unsonicated condition,
large sized bright blue crystals were formed in
comparison to sonicated condition. In the case of
doping of CeCl3 with CoCl2.6H2O single hexagonal
shaped crystals with sharp edges and faces with a
triangular hole in the middle (Fig. 1c) were isolated in
comparison to a large number of rod shaped crystals
NOTES
in branched aggregate (Fig. 1d) in sonicated
condition. On doping of CeCl3 with MnCl2, rod
shaped sharp edged crystals were isolated under
unsonicated condition case (Fig. 1e) in comparison to
rectangular shaped crystals isolated under sonicated
conditions (Fig. 1f). The size of crystal is however
larger in sonicated condition as compared to that in
unsonicated condition. The large sized crystals are
Fig. 1—Morphology of the Ce(III) doped salts under sonicated
and unsonicated conditions. [a, b, c and d are photographs taken
with a digital camera Sony DSC-W35 with 3x optical zoom and
e and f are SEM images].
369
formed in sonicated condition because the solution
was allowed to recrystallize under ambient condition
and the ultrasound was passed in the solution for
30 min only. However if the crystallization is carried
out under a continuous field of ultrasound, the crystals
size is very small although the shape and crystallinity
is better. It is known that chemical reactivity depends
on reaction temperature, concentration of reactants,
the external pressure and the use of the catalyst.
Ultrasound may also influence the chemical reactivity
in several ways through cavitation phenomena13. The
chemical consequences of ultrasound are not due to
direct interaction of sound waves with the matter as
compared to electromagnetic waves. This is because
of the acoustic cavitation (the formation, growth and
collapse of bubble) which provides the primary
mechanism for sonochemical effects in liquids
irradiated with high intensity ultrasound. During
cavitation, the bubble collapse generates high
temperature (>5000 K) and pressure (>20 MPa).
These vigorous conditions cause high energy
chemical reactions14. The molecular motion also
increases by acoustic streaming when ultrasound
passes through the liquid media15. During cavitation
the bubble collapse produces shock waves in the
liquid resulting in high-velocity interparticle
collisions. If the collision is at a direct angle, metal
particles can be driven together at sufficiently high
speeds to induce effective insertion into the crystal
lattice at the point of collision14. The shape of crystals
is dependent on the types of molecular bonds between
the atoms and also on conditions under which they
were formed15. Thus, ultrasound, a form of
mechanical energy, under the right circumstances, can
result in permanent physical change through its
unique cavitation phenomena. Different volume ratio
of the solvents and solutes can also play an important
role in modifying the shape of crystals2(l). The
exposure of ultrasonic field to supersaturated
solutions speeds up crystallization resulting in smaller
crystals. Lower ultrasonic frequencies, in the
20-30 kHz range are most effective in breaking of
larger crystals by cavitation16. Cerium is unique in its
physicochemical behavior due to the presence of an
electron in the 4f1, another in the 5d1 and the last two
in 6s2 shell. The normal oxidation state of lanthanides
is +3 but Ce still has one electron left in its 4f subshell
which may be used for bond formation with other
transition metal ions or may be easily excited to
form/produce a free radical and bring new chemistry
370
INDIAN J CHEM, SEC A, MARCH 2009
under unusual experimental conditions such as that
generated in the cavitation by the propagating
ultrasound.
Trichlorides
of
cerium
[La→Ce→Pr→Nd→Pm→Sm→Eu→Gd] are nine
coordinated. Cerium is the only lanthanide that exists
in aqueous solution as well as in solids in
+4 oxidation state. Comparison of the potential in
H2SO4, where at higher SO42- concentration the major
species is [Ce(SO4)3]2-, with that for the oxidation of
water shows that the acidic CeIV solutions commonly
used in the analysis are metastable. Cerium (IV) is
used as an oxidant in the oxidation of aldehydes and
ketones at the α-carbon atom18 where it is commonly
used in acetic acid. Figures 2, 3, and 4 show the
sonophotochemical degradation of phenol under
sonicated and unsonicated conditions and in the
presence of photocatalyst (n-BuO)4Ti with and
without cerium doped copper sulphate, cobalt
chloride, and manganese chloride respectively.
The degradation of phenol is in the following
order:
Sonicated
>
photocatalysed
>
sonophotocatalysed > sonophotocatalysed with
unsonicated crystals > sonophotocatalysed with
sonicated crystals.
Thus, it is clear that sonication adds to the
degradation of phenol in aqueous solution but it is less
than the decomposition of phenol under
photocatalytic condition. However, there is still
enhanced degradation when the process is carried out
in combination with sonication and photocatalysis.
Besides, degradation is slightly higher in solutions
containing crystals synthesized under sonic condition
in the case of cerium doped copper sulphate.
Nevertheless there is not much difference with cerium
doped salts of cobalt and manganese under similar
conditions. Based on these observations the
mechanism for the degradation of phenol may be
given as follows:
Faster
degradation
of
chloro
(oand
p- chlorophenols) and hydroxy (catechol, resorcinol
and hydroxoquinone) derivatives of phenol is
attributed to multicentered electron delocalization due
to chlorine and oxygen atoms, which are both more
electronegative than carbon atom and facilitate
delocalized electrons of the phenyl ring to drain out
and concentrate outside the ring. Therefore, their
degradation in the fields of ultrasound and microwave
as well as in the presence of photocatalysts (TiO2)
increases manifold. All these processes follow free
radical mechanism. Nevertheless, the formation of
Fig. 2—Degradation of phenol with Ce-Cu crystals. [1, Phenol;
2, Phenol + (n-BuO)4Ti (unsonicated); 3, Phenol + (n-BuO)4Ti
(sonicated); 4, Phenol + (n-BuO)4Ti + Ce-Cu crystal (unsonicated); 5, Phenol + (n-BuO)4Ti + Ce-Cu crystal (sonicated);
6, Phenol + (n-BuO)4Ti + Ce-Cu white crystal (sonicated)].
Fig. 3—Degradation of phenol with Ce-Co crystals. [1, Phenol;
2, Phenol + (n-BuO)4Ti (unsonicated); 3, Phenol + (n-BuO)4Ti
(sonicated); 4, Phenol + (n-BuO)4Ti + Ce-Co crystal (unsonicated); 5, Phenol + (n-BuO)4Ti + Ce-Co crystal (sonicated)].
Fig. 4—Degradation of phenol with Ce-Mn crystals. [1, Phenol;
2, Phenol + (n-BuO)4Ti (unsonicated); 3, Phenol + (n-BuO)4Ti
(sonicated); 4, Phenol + (n-BuO)4Ti + Ce-Mn crystal (unsonicated); 5, Phenol + (n-BuO)4Ti + Ce-Mn crystal (sonicated)].
NOTES
phenoxide ion which is resonance stabilized is further
facilitated by the ultrasound. But in the presence of
photocatalyst, (n-BuO)4 Ti, the phenoxide ions seem
to be attracted towards the alkyl chain of the n-Bu
group through lyophilic interaction and transfer
electron density to the vacant d orbital of Ti and
decompose. Besides, the four oxygen atom
surrounding Ti atom attract the electronic shell of the
Ti and leave a δ+ charge on the Ti, making it more
attractive for the phenoxide ions. Similarly, in the
presence of transition metals such as Cu(II), Co(II),
and Mn(II) which have unpaired electron in their
shell, the attraction for the phenoxide ion is facilitated
and there is transfer of electron density from
phenoxide ring to metal ions. The same reason may
be applied to Ce(III) with 4f1 electron, where electron
may be accommodated but since the loss of the only
4f1 electron of the Ce(III) makes it more stable, the
same is lost to H+ ions of the system generating
free H˙.
eCe3+
H+
H˙ + Ce4+
This free radical absorbs ultrasound energy and is
excited to attack other species of the medium and
initiates a chain of reactions induced by the free
radicals. The generation of H2O2, O3 and ˙OOH etc.,
are the consequences of the same.
It can therefore, be concluded that while ultrasound
enhances the degradation of phenol under normal,
photocatalytic or in the presence of cerium doped
salts of copper, cobalt and manganese, the crystals of
these metals doped with cerium and synthesized under
sonic conditions do not show advantage to any
appreciable extent. It is evident that ultrasound plays a
major role in altering the shape, size and color of the
crystals. Also, the percentage of Cu(II), Mn(II) and
Ce(II) is higher in the sonicated crystals as compared
to that in unsonicated crystals.
Acknowledgement
This work has been carried out from the SAP-DRS
grant, provided to the department by the UGC, New
Delhi. MV is grateful to the UGC for the award
of JRF.
371
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