Palladium phthalocyaninesulfonate functionalized mesoporous polymer: A highly efficient photocatalyst for degradation of 4-chlorophenol under visible light irradiation

Article abstract of DOI:10.1016/j.molcata.2013.01.022

A novel sulfonated palladium phthalocyanine (PdPcS) modified FDU-15 mesoporous polymer (FDU-PdPcS) has been prepared by rationally chemical modification process. The PdPcS molecules were highly dispersed inside the confined mesopores, and were further stabilized through a π-π interaction with mesopolymer, which may prevent the PdPcS molecules from agglomerating and deactivating in photodegradation reactions. The physicochemical properties of thus prepared FDU-PdPcS material were characterized by XRD, N₂ adsorption-desorption, UV-vis and inductively coupled plasma techniques. FDU-PdPcS proved to be highly efficient heterogeneous photocatalyst for the degradation of 4-chlorophenol (4-CP) in the presence of H₂O ₂ under visible light irradiation. The photodegradation of 4-CP with an initial concentration 0.6 mM was completed within 5 h at pH 11 using a dose of 0.2 g/L of the 0.12 wt% FDU-PdPcS photocatalyst. The photodegradation intermediates were identified by GC-MS. A possible mechanism involved in the photodegradation of 4-CP has also been discussed.

Full text of DOI:10.1016/j.molcata.2013.01.022

Journal of Molecular Catalysis A: Chemical 371 (2013) 15–20
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Palladium phthalocyaninesulfonate functionalized mesoporous polymer:
A highly efficient photocatalyst for degradation of 4-chlorophenol
under visible light irradiation
a,∗
a
a
b,∗
Rong Xing , Lin Wu , Zhenghao Fei , Peng Wu
a
Jiangsu Provincial Key Laboratory of Coastal Wetland Bioresources and Environmental Protection, Yancheng Teachers University, Kaifang Dadao Rd. 50,Yancheng City, Jiangsu
Province 224002, China
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel sulfonated palladium phthalocyanine (PdPcS) modified FDU-15 mesoporous polymer (FDU-
PdPcS) has been prepared by rationally chemical modification process. The PdPcS molecules were
highly dispersed inside the confined mesopores, and were further stabilized through a ␲–␲ interac-
tion with mesopolymer, which may prevent the PdPcS molecules from agglomerating and deactivating
in photodegradation reactions. The physicochemical properties of thus prepared FDU-PdPcS mate-
rial were characterized by XRD, N2 adsorption-desorption, UV–vis and inductively coupled plasma
techniques. FDU-PdPcS proved to be highly efficient heterogeneous photocatalyst for the degradation
of 4-chlorophenol (4-CP) in the presence of H2O2 under visible light irradiation. The photodegrada-
tion of 4-CP with an initial concentration 0.6 mM was completed within 5 h at pH 11 using a dose
of 0.2 g/L of the 0.12 wt% FDU-PdPcS photocatalyst. The photodegradation intermediates were iden-
tified by GC-MS. A possible mechanism involved in the photodegradation of 4-CP has also been
discussed.
Received 18 September 2012
Received in revised form 2 December 2012
Accepted 26 January 2013
Available online 4 February 2013
Keywords:
Mesopolymer
Phthalocyanine
Chlorophenol
Photocatalysis
Visible light
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
properties and photochemical stability, are extensively inves-
tigated in photodegradation of organic pollutants due to their
ability to absorb the visible light and generate highly active
singlet oxygen species [13,14]. However, MPcs are a class of
macrocyclic compounds, and tend to aggregate in aqueous media,
which leads to self-quenching of the excited species and brings
about photocatalyst deactivation. To overcome these inherent
limitations, a variety of heterogeneous MPc catalysts have been
developed by supporting MPc molecules on mesoporous silicas
[15], zeolites [16], TiO2 [17,18], carbon nanotubes [19,20], clays
[21] and polymers [22,23], etc. Among these mentioned supports,
the mesoporous materials are particularly suitable for supporting
the MPc molecules because of their extremely high surface area
and well-ordered mesopores, which may make the MPc molecules
highly dispersed in a monomeric form in confined mesopores.
Recently, Zanjanchi et al. [24] reported that the sulfonated cobalt
phthalocyaninne-functionalized MCM-41 exhibited good photo-
catalytic activities for the degradation of 2,4-DCP in the presence
Ordered mesoporous materials with high surface area, large-
pore volumes and uniform mesopores have attracted significant
attention due to their potential applications in the fields of sep-
aration, adsorption, catalysis and host-guest chemistry [1–6].
Compared with the microporous zeolites, the mesoporous mate-
rials provide good opportunity for anchoring bulky functional
molecules (such as enzymes, dyes, complexes, ionic liquids, etc.)
into confined mesopores (2–50 nm) for practical applications
[
7–11]. These functional materials are expected to combine the
advantages of mesoporous materials and functional molecules.
For example, metalloporphyrins modified periodic mesoporous
organosilica was prepared to show a high catalytic activity in the
Baeyer–Villiger oxidation reaction [12].
From the viewpoint of developing energy-saving and envi-
ronmentally benign chemical processes, photocatalysis is
a
promising technique especially using visible light as an energy
source, which contains 45% solar energy. As one of the most
efficient and promising photosensitizers or/and photocatalysts,
metallophthalocyanines (MPcs) with unique photophysical
of H O2 at pH of 5 under UV light irradiation. According to the
2
literatures [25,26], the degradation of chlorophenol was often
carried out in a basic solution since chlorophenolate was more
easily oxidized than in neutral form. However, the applications
of silica-based mesoporous materials are considered to suffer a
rapid degradation of mesostructure in basic solution. Thus, more
promising supports are needed for practical applications.
∗ Corresponding authors. Fax: +86 515 88233188.
E-mail addresses: xingrr@126.com (R. Xing), pwu@chem.ecnu.edu.cn (P. Wu).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.01.022

1
6
R. Xing et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 15–20
Within the family of the mesoporous materials, the mesopoly-
quantified by inductively coupled plasma-atomic emission spec-
troscopy (ICP-AES) on a Thermo IRIS Intrepid II XSP atomic emission
spectrometer.
mers with both the mesostructures and the organic framework
of polymers are considered to solve partially the disadvantages
of pure silica materials owing to high physicochemical stability
in acidic or basic medium as well as numerous opportunities for
designing novel catalysts through co-condensation or post-graft
method. So the mesopolymer materials may serve as ideal sup-
ports for anchoring the MPc molecules by taking advantage of their
organic framework and regular mesopores. Moreover, different
from silica-based materials, phenolic pollutants could be efficiently
adsorbed onto the photocatalytic active sites on mesopolymer
through ␲–␲ interaction and hydrogen bonding between the phe-
nolic pollutants (e.g. phenol, chlorophenol and bisphenol A) and
the aromatic species in the framework walls. The MPc molecules
can be easily bonded to the mesopolymer by rationally chemical
modification, dispersing in a monomeric form in confined meso-
pores and being further stabilized by the ␲-electron of the benzene
ring moieties in mesopolymer material, which can prevent the MPc
molecules from agglomerating and also increase the quantum yield
of singlet oxygen.
2.4. Photocatalytic test
The photocatalytic reaction was performed in a glass reactor
using a Halogen lamp (500 W, Shanghai Yamin) through a glass
filter (ꢀ ≥ 450 nm) as irradiation source. In a typical experiment,
50 mL of 4-CP solution and 0.01 g of catalyst were stirred in an
air-saturated aqueous solution at room temperature under dark
overnight until reaching an adsorption equilibrium. Appropriate
amount of H O2 was then added into the reactor and the suspen-
2
sion was irradiated to start the photoreaction. During the irradiated
photoreaction, a proportion of reaction mixture (ca. 2 mL) was
sampled. After removing the solid catalyst by filtration through a
membrane (0.45 ␮m), the amount of 4-CP remaining in the solution
was analyzed by high performance liquid chromatography (Waters
2695-2489 HPLC, C 18 reverse column, 5 ␮m, 250 mm × 4.6 mm)
using CH OH-H O (60%: 40%, pH 5 by acetic acid) as an eluent. The
3
2
The PdPc compound is usually chosen as a photocatalyst
since it shows higher singlet oxygen yields and photostabil-
ity in comparison to other metal phthalocyanines [27,28]. We
have verified that the FDU-15 mesopolymers were converted
readily into diamine-functionalized materials (FDU-ED) through
chloromethylation and amination procedures [29,30]. Herein, we
report for the first time that FDU-ED serves as ideal scaffolds
for the dispersion and stabilization of the palladium phthalocya-
ninesulfonate molecules (PdPcS) confined within the mesopores.
This mesopolymer-supported PdPcS photocatalyst proves to be
highly photoactive in the degradation of 4-CP under visible light
irradiation. It may provide a versatile way for designing various
mesopolymer-supported MPc photocatalysts that are useful for a
rapid degradation of organic pollutants using solar light.
photodegradation intermediates were identified by GC-MS on an
Agilent Technologies 7890A instrument equipped with a DB-624
capillary column (60 m × 0.25 mm × 1.4 ␮m). The reaction filtrate
was acidified to pH 1–2 and then it volume reduced to about 5 mL
via rotatory evaporation. Then 5 mL of CH OH and 0.5 mL of concen-
3
trated sulfuric acid were added to the residue. After the resulting
mixture was refluxed for 3–4 h, it was saturated with NaCl and fur-
ther extracted with dichloromethane. The organic liquor was dried
over anhydrous MgSO4 and then evaporated to about 2 mL under
reduced pressure before GC–MS analysis. The chloride ion analyses
were performed using 0.2% AgNO solution. The carbon dioxide was
3
detected by Ba(OH)2 method. Isopropanol, sodium azide and ben-
•
1
•-
zoquinone were used as OH , O2 and O2 scavenger, respectively
in the photodegradation reactions. The used catalyst was washed
with distilled water, and dried at 353 K. The recycling experiments
of FDU-PdPcS were carried out under the above mentioned condi-
tions.
2
. Experimental
2.1. Materials
3. Results and discussion
The FDU-15 mesoporous polymer and diamine-functionalized
A schematic illustration of the pathways for preparing the
FDU-15 (FDU-ED) were synthesized as previously reported [29,30].
Palladium (II) phthalocyanine sulfonate (PdPcS) was prepared
according to the literature method [31]. 4-chlorophenol was pur-
chased from Shanghai Chemicals Inc, and was used as received.
photocatalyst is shown in Scheme 1. First, the chloromethyl
groups were successfully introduced into the benzene ring
moieties of the FDU-15 (hexagonal P6 mm) mesopolymers
through the chloromethylation reaction (denoted as FDU-CH Cl).
2
2.2. Preparation of PdPcS-functionalized FDU-15
FDU-ED (0.50 g) was stirred in 210 mL of PdPcS solution
(
2.9 ␮g/g) at room temperature until the suspension turned to be
colorless. The obtained material was washed with distilled water.
◦
It was finally dried under vacuum at 60 C for 12 h, leading to
mesopolymers supported PdPcS, which is denoted as FDU-PdPcS.
2.3. Measurements
The X-ray diffraction (XRD) patterns were collected on a
Rigaku Ultima IV X-ray diffractometer using Cu-K␣ radiation
´
(
ꢀ = 1.5418 A˚ ) at 35 kV and 25 mA. Nitrogen adsorption–desorption
isotherms were recorded at 77 K on a BELSORP-MAX instru-
ment after activating the sample under vacuum at 373 K at
least for 10 h. The specific surface area was evaluated using
Brunauer–Emmett–Teller (BET) method. UV–visible spectra were
recorded on a PerkinElmer Lambda 35 UV/VIS spectrometer
with a mode of diffusion reflectance using polytetrafluoroethene
as a reference. The amount of Pd in the photocatalyst was
Scheme 1. Schematic illustration for preparing PdPcS-functionalized mesopolymer
materials.

R. Xing et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 15–20
17
2
1
1
0
5
0
5
0
1
0
FDU-PdPcS
FDU-15
550 600 650 700
11
2
0
FDU-PdPcS
FDU-ED
FDU-15
4
00
500
600
700
800
1
2
3
4
o
2
( )
Wavelength (nm)
Fig. 1. XRD patterns of FDU-15, FDU-ED and FDU-PdPcS.
Fig. 3. UV–vis spectra of FDU-15, FDU-PdPcS and PdPcS solution (inset).
Table 1
Textural properties of the FDU-15 before and after various modifications.
The N adsorption–desorption isotherms and the corresponding
2
pore size distributions of the samples are shown in Fig. 2. The sam-
ples exhibited a typical IV type isotherm and a clear H2 hysteresis
loop, typical of a mesoporous structure. The pore size distributions
all exhibited sharp peaks, indicating the uniform pores remained
even after the multi-step chemical functionalizations. According
to our previous study [30,32], the modified samples exhibited a
greatly decreased N2 uptake in the adsorption and desorption
branches in comparison with FDU-15. This is probably because that
the organic groups were anchored on the inner surface of meso-
pores, which may cause a slight decrease in the pore size, pore
volume and surface area (Table 1). Moreover, the adsorption and
desorption branches of all samples did not close completely in the
region of low relative pressure due to a typical nature of these
mesopolymers [33].
The diffuse reflectance UV–vis spectra of PdPcS (inset), FDU-15
and FDU-PdPcS are shown in Fig. 3. For PdPcS aqueous solution, the
absorption band at 646 nm attributed to the monomer was lower
than the band at 612 nm attributed to the aggregated form. It is
considered that the PdPcS molecules were highly aggregated in
aqueous solution. The blank FDU-15 showed no obvious absorp-
tion band, while the FDU-PdPcS developed distinctly three new
absorption bands. Band at 661 nm corresponding to monomer was
red-shifted due to the ␲–␲ interaction between PdPcS and FDU-
15, which is well consistent with the literature [27]. The shoulder
peaks at 606 and 553 nm were assigned to the aggregate forms of
PdPcS. The results suggest that most of the monomeric PdPcS was
successfully introduced inside the channels of FDU-15, and a small
portion of the larger aggregate forms may be located on the external
surface of FDU-15.
Samples
Unit cell (Å)a
N2 adsorption
SBET (m2
−1
Vp (cm3
−1
g )
g
)
Dp (nm)
FDU-15
FDU-ED
FDU-PdPcS
88.1
91.9
92.8
422
316
297
0.35
0.26
0.23
3.6
3.1
2.8
√
a
Calculated using d100 according to a0 = 2d100/ 3 for FDU-15.
Secondly, FDU-CH Cl subsequently reacted with ethylenedi-
2
amine (ED) solution to produce diamine-functionalized products
(
denoted as FDU-ED). We have well addressed these two-step
functionalizations elsewhere [29,30]. Finally, FDU-ED was added
to PdPcS aqueous solution, and stirred at room temperature. The
PdPcS ions were captured rapidly by the diamino groups through
protonation and ionic interaction, giving rise to the mesopolymers
supported PdPcS, FDU-PdPcS.
3.1. Characterization of PdPcS-functionalized mesopolymers
Fig. 1 shows the powder XRD patterns of the parent FDU-15
and the modified samples. The samples all exhibited three diffrac-
tion peaks indexed as the [10], [11] and [20] planes, based on a
well-ordered 2D hexagonal structure with a P6 mm space group,
suggesting that the mesostructure was not destroyed after post-
modification. The diffraction peaks of the modified samples shifted
slightly to lower 2ꢁ angle due to increased unit cells in the function-
alized materials. This provides a further evidence for immobilizing
successfully the organic groups with bulky dimensions on the
mesopolymer walls. Some typical textural parameters of the FDU-
The palladium content of the FDU-PdPcS catalyst analyzed with
inductively coupled plasma (ICP) was 0.014 wt% (corresponding to
1
5 and the modified samples are listed in Table 1.
2
2
1
1
50
00
50
00
A
B
a)
a)
b)
b)
c)
c)
5
0
0
0
.0
0.2
0.4
0.6
0.8
1.0
1
2
3
4
5
6
Relative pressure (P/P0)
Pore Diameter (nm)
Fig. 2. N2 adsorption–desorption isotherms (A) and BJH pore size distribution curves (B) of (a) FDU-15, (b) FDU-ED and (c) FDU-PdPcS.

1
8
R. Xing et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 15–20
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
a)
b)
c)
pH 5
neutral solution
pH 11
d)
e)
0
100
200
300
400
Irradiation time (min)
0
100
200
300
400
Irradiation time (min)
Fig. 4. Effect of pH value on the photodegradation rate of 4-CP. Reaction conditions:
.01 g FDU-PdPcS, 50 mL 4-CP (0.6 mM), H2O2 (29 mM).
0
Fig. 5. The changes of 4-CP concentration with reaction time under various
conditions: (a) H2O2 + dark, (b) H2O2 + vis-light, (c) FDU-PdPcS + H2O2 + dark, (d)
FDU-PdPcS + vis-light and (e) FDU-PdPcS + H2O2 + vis-light. Reaction conditions:
0
.12 wt% PdPcS loading), which indicated that more than 99% of the
0.01 g FDU-PdPcS, 50 mL 4-CP (0.6 mM), H2O2 (29 mM), pH 11.
added PdPcS was sucked into the mesopolymer.
5
3
.2. Photocatalytic decomposition of 4-CP
.2.1. The effect of pH
0
0
.2 mM
.4 mM
3
4
3
2
1
0
0.6 mM
.0 mM
1
4
-Chlorophenol was selected as the representative pollutant for
the investigation of photodegradation since it is a typical toxic and
recalcitrant pollutant in industrial wastewater which is difficult to
be biodegraded due to its high chemical stability. It is well known
that the pH value influences greatly the degradation rate of 4-CP
in solution and the catalytic properties of the catalysts. Therefore,
the effect of the pH of 4-CP solution was first investigated. Fig. 4
shows the degradation of the 4-CP in slightly acidic, basic and neu-
tral aqueous solution (pKa of 4-CP = 9.34). After 5 h of irradiation,
no significant differences were observed in acidic and neutral aque-
ous solution, while the degradation rate of 0.6 mM 4-CP aqueous
solution was about 81% and 82% at pH = 5 and neutral aqueous solu-
tion, respectively. In a basic aqueous solution (pH = 11), 4-CP was
completely degraded within 5 h, indicating that the basic media
favors the photodegradation of 4-CP. This is because that the basic
media enhances the deprotonation of 4-CP to form chloropheno-
late, which are more oxidizable by singlet oxygen [26,34]. The
results show that the optimum pH value was 11, hence the fol-
lowing degradation reactions were carried out under this pH value.
0
50
100
150
200
250
300
350
Irradiation time (min)
Fig. 6. Effect of initial concentration on the photodegradation of 4-CP.
photocatalytic activity in the photodegradation of the chlorinated
phenols compared to CoPcS functionalized MCM-41 reported in a
recent literature under similar experimental conditions [24]. This
can be explained that PdPcS molecules were highly dispersed inside
the confined mesopores, and further be stabilized by ␲-electron
of mesopolymer. Moreover, FDU-PdPcS catalyst containing the
hydrophilic diamino groups would form a homogeneous suspen-
sion in water and more effectively facilitates the diffusion and/or
adsorption of molecular oxygen and 4-CP to the catalytic active
sites
3
.2.2. Photodegradation of 4-CP
Fig. 5 shows the concentration change of 4-CP with time under
various reaction conditions. It can be seen from the experimen-
tal results that the oxidation of 4-CP could hardly occurred in the
presence of H O2 in the dark or ever under visible light irradia-
2
tion, indicating that H O2 could not generate free radicals along
with visible light irradiation. FDU-PdPcS also failed to catalyze the
2
3
.2.3. The effect of 4-CP concentration
The effect of the initial concentration on the degradation of 4-
degradation of 4-CP in the presence of H O2 in the dark, and only
2
CP was investigated (Fig. 6). The 4-CP degradation fitted quite well
into the first-order kinetics which can be represented by equation
ln(C /Ct) = k t. Where C is initial concentration and Ct is the con-
centration at reaction time (t), kobs is the apparent rate constant
and obtained from the slope of the plot of ln(C /Ct) versus reaction
adsorbed about 8% of 4-CP (Fig. 5c). However, FDU-PdPcS exhibited
a high photocatalytic activity for the degradation of 4-CP under
visible light irradiation. 4-CP was degraded completely and 61%
using FDU-PdPcS as photocatalyst in the presence and absence
of H O , respectively (Fig. 5d and e) after irradiation for 5 h. The
0
obs
0
0
2
2
time t. Table 2 lists the rate constant and half-life at initial 4-CP
blank FDU-15 was not photoactive under same reaction condi-
tions. Above experiments suggest that FDU-PdPcS photocatalyst
and visible light are crucial for degradation of 4-CP. In addition,
the control experiment with FDU-PdPcS in the presence or absence
of H O2 showed that H O2 can accelerate the reaction rate under
Table 2
The rate constant (kobs), half-life (t1/2) at various concentrations of 4-CP.
R2
kobs (min
−1
)
t1/2 (min)
Concentration (mM)
.2
2
2
visible light irradiation and eliminate the colored intermediates
during the photodegradation reactions, which was well consis-
tent with the reported literature [35]. It is noteworthy that at low
photocatalyst/4-CP molar ratio, FDU-PdPcS catalyst shows higher
0
0.9943
0.992
0.0636
0.0276
0.0111
0.0071
10.90
25.11
62.45
97.63
0.4
0
1
.6
.0
0.9877
0.9883

R. Xing et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 15–20
19
1
0
0
0
.0
.8
.6
.4
a)
b)
Fig. 7. GC-MS chromatogram of photodegradation of 4-CP. Reaction conditions:
.01 g FDU-PdPcS, 50 mL 4-CP (0.6 mM), H2O2 (29 mM), pH = 11, 4 h.
0.2
.0
0
c)
d)
0
e)
concentrations ranging from 0.2 to 1.0 mM. The rate constant
0
100
200
300
400
−1
decreased from 0.0636 to 0.0071 min with increasing the initial
concentration from 0.2 to 1.0 mM. The photodegradation of 4-CP
with an initial concentration 0.2 mM and 1.0 mM was completed
within 1.5 h and 7.5 h, respectively. The efficiency of 4-CP pho-
todegradation decreased with increasing initial 4-CP from 0.2 mM
to 1.0 mM. This was attributed to adsorption and/or diffusion
limitation of molecular oxygen in the mesopores at higher con-
centrations during the photodegradation process [35]. The results
clearly show that the photodegradation of 4-CP was faster in more
dilute 4-CP solution.
Irradiation time (min)
Fig. 8. Effect of scavengers on the photodegradation of 4-CP. (a) NaN3
0.01 M) + benzoquinone (0.025 mM), (b) benzoquinone (0.025 mM), (c) NaN3
0.01 M), (d) isopropanol (0.05 M) and (e) no scavenger.
(
(
•
(Fig. 8d), indicating that a small amount of OH participated in the
photodegradation of 4-CP under visible light irradiation. However,
the addition of benzoquinone or sodium azide retarded distinctly
the photodegradation rate (Fig. 8b and c). The lowest degradation
rate was observed in the presence of both sodium azide and ben-
zoquinone (Fig. 8a). The results imply that both 1O2 and O2 are
probably the prominent active species in the photodegradation of
4-CP in the presence of FDU-PdPcS with H2O2 under visible light
irradiation. In the previous studies [25,36], the superoxide radi-
•-
3.3. Products and mechanism of 4-CP photodegradation
To confirm the highly polar and small molecules of degrada-
tion process, the products were esterified with methanol using
concentrated sulfuric acid as the catalyst. The esterified products
were identified by GC–MS. The results are shown in Fig. 7 and
listed in Table 3. A mixture of dimethyl oxalate, dimethyl mal-
onate, dimethyl maleate and dimethyl d-malate was identified as
the methyl ester derivatives of intermediates, suggesting that the
photodegradation products were composed of oxalic acid, malonic
acid, maleic acid and malic acid. Moreover, in the final products, the
•
-
cal (O2 ) is produced via electron transfer process, which is part
1
of the Type I mechanism. The singlet oxygen ( O2) is generated
via energy transfer from triplet state of the sensitizer to ground
state oxygen and initiates the reaction, which belongs to the Type
II mechanism. Based on the above results, the photodegradation of
4-CP might proceed through Type I and II mechanisms. Fig. 9 shows
the possible mechanism for the photocatalytic degradation of 4-CP.
Hydroquinone formed in Type I reaction process is similar to the
photodegradation of 4-nitrophenol [37], while benzoquinone gen-
erated in Type II reaction process is consistent to the reported for
the degradation of 2,4,6-trichlorophenol [38]. The obtained inter-
mediates were further oxidized to various carboxylic acids.
chloride ions and carbon dioxide were detected by using AgNO and
3
Ba(OH)2 method, respectively.
To investigate the mechanism for the photodegradation pro-
cess, isopropanol, sodium azide and benzoquinone were used as
•
1
hydroxyl radicals (OH ), singlet oxygen ( O ) and superoxide radi-
2
•
-
cals (O2 ) scavenger, respectively. This would give information
concerned with the reactive species during photodegradation pro-
cess. In comparison to a typical photocatalysis (Fig. 8e), the addition
of isopropanol showed a little influence on the degradation of 4-CP
3.4. Catalysts recycling
From the viewpoint of environmental application, the recycla-
bility of heterogeneous catalyst is very important. In the kinetic and
recycling experiments under the same experiment conditions, the
photodegradation rate of 4-CP was very comparable to the results
achieved on the fresh catalyst after reused for 4 times (Fig. 10). As
evidenced by XRD analysis, the reused FDU-PdPcS catalyst showed
the diffraction peaks very similar to those of the parent FDU-PdPcS
Table 3
Intermediates of the 4-CP photodegradation determined by GC–MS.
Retention time (min)
Identified intermediate
Dimethyl oxalate
Molecular structure
O
O
O
O
O
O
O
O
O
1
0.020
H O
-HCl
-
e
-e
2
O
O
Type II
Cl
OH
O
Cl
Cl
Cl
O
O
1
1
1.190
2.498
Dimethyl malonate
dimethyl maleate
H+ 2e
further
2
degradation
O
O
O
O
O
OH
O
O
H O
2
O
Type I
O
Cl
OH
O
OH
1
3.948
Dimethyl d-malate
Fig. 9. Possible mechanism for the photodegradation of 4-CP under visible light.

2
0
R. Xing et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 15–20
Acknowledgments
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
This work was supported by the NSFC of China (21143009
and 20925310), the Natural Science Foundation of Jiangsu Edu-
cation Department (09KJB150013), the Jiangsu Fundament of
Run 1
Run 2
Run 3
Run 4
“Qilan Project”, Jiangsu Provincial Key Laboratory of Coastal Wet-
land Bioresources and Environmental Protection (JLCBE07011), the
Research projects of Yancheng Science and Technology Bureau
(YK2009028).
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[