Efficient visible photodegradation of 4-nitrophenol in the presence of H2O2 by using a new copper(II) porphyrin-TiO2 photocatalyst

Article abstract of DOI:10.1016/j.molcata.2012.04.013

A new imidazolyl-linked pophyrin, 5-[4-(2-imidazolyl)ethoxyl]phenyl-10,15, 20-triphenylporphyrin and its copper(II) porphyrin were synthesized and characterized spectroscopically. The CuPp-TiO₂ photocatalyst was also prepared and characterized by means of high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-vis spectra and FT-IR spectra. The photocatalytic activity was investigated by photodegradation of 4-nitrophenol (4-NP) in aqueous solution with a small amount of H₂O₂ under visible light irradiation (λ > 400 nm). The results indicated that H₂O₂ can enhance greatly the visible photocatalytic activity of CuPp-TiO₂ photocatalyst. Additionally, a possible mechanism of the photocatalytic degradation was proposed.

Full text of DOI:10.1016/j.molcata.2012.04.013

Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
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Journal of Molecular Catalysis A: Chemical
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Efficient visible photodegradation of 4-nitrophenol in the presence of H2O2 by
using a new copper(II) porphyrin–TiO2 photocatalyst
Gui-ping Yao, Jun Li∗, Yun Luo, Wan-jun Sun
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry & Materials Science, Northwest University, Xian, Shaanxi
710069, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new imidazolyl-linked pophyrin, 5-[4-(2-imidazolyl)ethoxyl]phenyl-10,15,20-triphenylporphyrin and
its copper(II) porphyrin were synthesized and characterized spectroscopically. The CuPp–TiO2 photocat-
alyst was also prepared and characterized by means of high-resolution transmission electron microscopy
(TEM), X-ray diffraction (XRD), UV–vis spectra and FT-IR spectra. The photocatalytic activity was investi-
gated by photodegradation of 4-nitrophenol (4-NP) in aqueous solution with a small amount of H2O2
under visible light irradiation (ꢀ > 400 nm). The results indicated that H2O2 can enhance greatly the
visible photocatalytic activity of CuPp–TiO2 photocatalyst. Additionally, a possible mechanism of the
photocatalytic degradation was proposed.
Received 11 February 2012
Received in revised form 1 April 2012
Accepted 30 April 2012
Available online 7 May 2012
Keywords:
Photocatalysis
Cu(II) porphyrin
TiO2
© 2012 Elsevier B.V. All rights reserved.
Photogradation
4-Nitrophenol
Hydrogen peroxide
1. Introduction
alytic performance of TiO2 on the degradation of organic pollutants
[11,12]. These results showed that porphyrin modified TiO2 pho-
TiO2 has received a lot of attention as a promising semicon-
ductor photocatalyst because of its nontoxicity, low cost, chemical
stability, and high catalytic activity [1,2], but the wide band gap
of TiO2 (3.2 eV for the anatase structure) restricts its photocat-
alytic applications to the UV range. Therefore, improving the visible
photocatalytic activity of TiO2-base photocatalyst for taking full
advantage of the main part of the solar spectrum becomes an
important and challenging issue. Various approaches have been
attempted to enhance the visible-light utilization of TiO2 includ-
ing surface modification, doping, sensitizations and semiconductor
coupling [3–10].
Porphyrins are a kind of excellent photosensitizers which have
large ␲-electron conjugate systems and high absorption coeffi-
cient within the solar spectrum. The porphyrin-sensitized TiO2
system has been proposed firstly by Mele for the oxidative degra-
dation of 4-nitrophenol in water [4]. It has been verified that
porphyrins–TiO2 can degrade the organic molecules in aqueous
solutions efficiently using the solar energy. Some porphyrins and
metalloporphyrins have been explored to improve the photocat-
tocatalysts exhibited higher photoactivity than bare TiO2 under
UV light irradiation [13–16]. However, under visible light irradia-
tion, the photocatalytic efficiencies of all photocatalysts decreased
evidently compared with those in UV light condition [17,18].
In order to improve the visible-light-driven photocatalytic effi-
ciency of porphyrin modified TiO2 photocatalyst, it is necessary to
explore the influential factors of the efficiency of porphyrin–TiO2
systems by adding other oxidants such as H2O2 except oxygen.
For this purpose, a new imidazolyl-linked porphyrin as well as
its copper porphyrin was synthesized in this paper (Fig. 1). The
imidazolyl-linked porphyrin was selected because the nitrogen
atom of imidazole can coordinate to titanium atom of the surface
of TiO2, which benefits the electronic transfer from porphyrin to
TiO2 and thus increase the photocatalytic activity. In addition, in
our previous research, we find the central coordinated metal ion in
the porphyrins ring plays a critical role in the photocatalytic pro-
cesses of the porphyrin–TiO2 system, and copper porphyrin–TiO2
photocatalyst displays the most significant improvement on pho-
toactivity [18]. Therefore, we prepared and characterized a new
CuPp–TiO2 photocatalyst by using synthesized imidazolyl-linked
copper porphyrin and investigated its photocatalytic efficiency on
the degradation of 4-notrophenol (4-NP) in the presence of H2O2
under visible light irradiation (ꢀ > 400 nm). Moreover, a possible
photocatalytic mechanism was also proposed.
∗
Corresponding author. Tel.: +86 29 88302604; fax: +86 29 88303798.
E-mail addresses: yaoguiping1985@126.com (G.-p. Yao), junli@nwu.edu.cn (J. Li),
ly-216@163.com (Y. Luo), wanjuns1986@yahoo.cn (W.-j. Sun).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.04.013

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G.-p. Yao et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
N
N
NH
N
N
N
N
N
N
N
N
Cu
O
O
HN
CuPp (4)
H2Pp (3)
Fig. 1. The structure of porphyrin and its copper(II) porphyrin.
2. Experimental
Yield: 60%. Mp: >250 ◦C, Anal. calcd. (found) for C46H33BrN4O
(mol. wt: 737.6), %: C, 74.93 (74.90); H, 4.58 (4.51); N, 7.61 (7.59).
MS: m/z 737.38 (calcd. for [M+H]+ amu). UV–vis (CH2Cl2): ꢀmax/nm,
2.1. Reagents and materials
418 (Soret band), 515, 551, 591, 647 (Q-bands). FT-IR: ꢂ, cm−1
,
Reagents were obtained from Beijing Chemical Reagents Com-
pany. They were used directly after received except pyrrole,
which was distilled before use. TiO2 was purchased from Acros
Organics, USA (anatase phase, BET specific surface area 9 m2/g),
using in preparation of loaded samples applied as photocatalyst
in photoreactivity experiments. 5-(4-hydroxyphenyl)-10,15,20-
triphenylporphyrin (1) was synthesized according to a reported
procedure [19].
3319, 3054, 2921, 1600, 1470, 1240, 1217, 1072, 965, 800, 702,
551.
2.3.2. 5-[4-(2-imidazolyl)-ethoxyl]phenyl-10,15,20-
triphenylporphyrin (3)
A mixture of compound 2 (0.10 g, 0.13 mmol), anhydrous
K2CO3 (0.50 g, 3.60 mmol) and imidazole (0.90 g, 13 mmol) in
dry dimethylformamide (15 mL) was stirred for 24 h at room
temperature. The process of the reaction was monitored by
TLC. The unreacted solid salt was filtered and the solvent was
removed under vacuum, then the residue was dissolved in CHCl3
(40 mL) and washed with H2O to remove unreacted imidazole.
After concentrating via rotary evaporation, the residue was chro-
matographed on a silica gel column using CHCl3 first and then
CHCl3: CH3CH2OH = 10:1 (v:v) as eluent. The main purple band was
collected.
2.2. Equipment
Elemental analyses (C, H and N) were performed by Vario
EL-III CHNOS instrument. FT-IR spectra were recorded on a BEQ
UZNDX-550 spectrometer on samples embedded in KBr pellets.
UV–vis spectra were performed by Shimadzu UV1800 UV-vis-NIR
spectrophotometer. 1H NMR spectra were recorded by using a
Varian Inova 400 MHz apparatus at room temperature and tetra-
methylsilane (TMS) for reference. Mass spectrometry (MS) analyses
were carried out on a matrix assisted laser desorption/ionization
time of flight mass spectrometer (MALDI-TOF MS, Krato Analytical
Company of Shimadzu Biotech, Manchester, Britain). 1 ␮L of sample
solution and of the matrix mixture were spotted into wells of the
MALDI sample plate, and air-dried. The samples were analyzed in
the linear ion mode with CHCA as matrix. External calibration was
achieved using a standard peptide and protein mix from Sigma.
Diffuse reflectance (DR) spectra were obtained at room tem-
perature in the wavelength range 200–800 nm using a Shimadzu
UV-2401PC spectrophotometer with BaSO4 as reference material.
The surface morphologies of the samples were analyzed by field
emission scanning electron microscopy (FE-SEM, Hitachi-S4800)
with energy dispersive spectrometry (EDS, Quanta 400FEG) and
high-resolution transmission electron microscope (TEM, JEOL JEM-
3010). The X-ray diffraction measurement was performed with
a Bruker D8 diffractometer using graphitemonochromatic copper
radiation (Cu K␣) at 40 kV, 30 mA over the 2ꢁ range 20–80◦.
Yield: 40%. Mp: >250 ◦C. Anal. calcd. for C49H36N6O (mol. wt:
724.3), %: C, 82.21 (81.19); H, 5.08 (5.03); N, 11.63 (11.59). MS:
m/z 725.34 (calcd. for [M+H]+ amu). UV–vis (CH2Cl2) ꢀmax/nm, 417
(Soret band), 515, 550, 590, 646 (Q-band). FT-IR: ꢂ, cm−1, 3318,
3056, 2924, 1601, 1506, 1471, 1349, 1244, 1108, 1076, 964, 802,
734, 700. 1H NMR (400 MHz, CDCl3): ı in ppm = 8.84–8.81 (d + d, 8H,
␤ position of the pyrrolemoiety), 8.20 (d, J = 6.8 Hz, 8H), 7.75–7.73
(m, 11H), 8.07 (s, 1H), 7.71 (s, 1H), 7.12 (s, 1H), 4.36 (t, 3H), 4.31 (t,
3H), −2.78 (s, 2H).
2.3.3. Cu(II) 5-[4-(2-imidazolyl)-ethoxyl]phenyl-10,15,20-
triphenylporphyrin CuPp (4)
CuCl2 (20.7 mg, 0.15 mmol) were added to H2Pp (3) (36.2 mg,
0.05 mmol) dissolved in a mixture of 15 mL CH2Cl2 and CH3CH2OH
(7 mL). The mixture was stirred for 24 h at room temperature and
monitored by TLC until the starting material H2Pp (3) disappeared.
The unreacted solid salt was filtered and the solvent was removed
under vacuum. The crude product was purified by chromatography
on a silica gel column with CH2Cl2 as eluant. CuPp (4) was obtained
in nearly quantitative yield.
Yield: 93%. Mp: >250 ◦C. Anal. calcd. for CuC49H34N6O (mol. wt:
786.3), %: C, 74.91 (74.84); H, 4.29 (4.36); N, 10.75 (10.69). MS:
m/z 787.21 (calcd. for [M+H]+ amu). UV–vis (CH2Cl2) ꢀmax/nm. 414
(Soret-band), 539 (Q-band). FT-IR: ꢂ, cm−1, 2922, 1601, 1500, 1471,
1349, 1237, 1107, 1074, 997, 801, 701.
2.3. Synthesis of porphyrin and copper porphyrin
2.3.1.
5-(4-(2-Bromoethoxyl)phenyl)-10,15,20-triphenylporphyrin (2)
1,2-Dibromoethane (1.80 g, 9.60 mmol) and potassium
carbonate (0.40 g, 2.89 mmol) were dissolved in dry N,N-
dimethylformamide (15 mL), and compound 1 (0.20 g, 0.32 mmol)
was added. Then the solution was stirred under nitrogen for 8 h at
80 ◦C. The process of the reaction was monitored by TLC. Then the
solvent was removed under vacuum and the crude product was
purified by chromatography on a silica gel column with CH2Cl2 as
eluent to afford a purple solid.
2.4. Preparation of the CuPp–TiO2 photocatalyst
The loaded sample used as photocatalyst for the photodegrada-
tion experiments was prepared as follows: an amount (6 ␮mol) of
CuPp (4) was dissolved in 30 mL CH2Cl2 and 1 g of finely ground
TiO2 was added to the solution. The suspension was stirred for 5 h

G.-p. Yao et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
31
NH
N
N
NH
N
N
K2CO3
DMF
Br
Br
OH
O
Br
HN
HN
1
2
N
N
NH
N
N
CuCl2
CH2Cl2, C2H5OH
CuPp (4)
N
N
O
K2CO3, DMF
HN
H2Pp (3)
Scheme 1. Synthesis of the porphyrin H2Pp and copper(II) porphyrin CuPp.
at room temperature. Then the solvent was removed under vacuum
and the photocatalyst was collected [20].
showed a band centered at 417 nm (Soret band), and the Q bands
absorptions at 515, 550, 590 and 646 nm, respectively. The insertion
of Cu(II) into the porphyrin ring caused violet shift of 3 nm of the
corresponding Soret band as well as a decreasing number of Q bands
in UV–vis spectrum, which due to the increase of the symmetry of
porphyrin ring when the hydrogen ions of N H was replaced by
Cu(II). Mass spectroscopy data of 3 and 4 perfectly corresponded
to the expected [M+H]+ m/z values. 1H NMR spectrum was also
consistent with the structure of the isolated 3. The main change of
the IR spectrum of copper porphyrin (4) compared with free base
porphyrin (3) was the disappearance of stretching vibration of N H.
2.5. Photodegradation setup and photodegradation experiments
The photodegradation experiments have been conducted using
Model XPA-II photocatalytic reaction instrument (Nanjing Xujiang
Electromechanical Factory): central light source was 400 W halo-
gen lamp placed in a quartz socket tube with one end closed.
The system was cooled by circulating water running through an
interlayer and magnetic agitation was used so that catalyst can
suspend in the reaction solution. The temperature of the reactor
was maintained at the constant temperature water bath (25 1 ◦C),
which was fixed on the base of the apparatus. A cutoff filter of
400 nm wavelength was placed between the lamp and the reac-
tor to ensure that photocatalysis was irradiated by visible light
only.
3.2. Morphology of CuPp–TiO2 photocatalyst
The surface morphologies of TiO2 and CuPp–TiO2 photocata-
lyst can be seen from the SEM images (Fig. 2(a) and (b)). It can
be observed that the microspheres of CuPp–TiO2 have a similar
surface condition with bare TiO2, indicating that the TiO2 morpha
basically are no change before and after loaded with the copper(II)
porphyrin. Fig. 2(c)–(f) shows typical TEM images of bare TiO2 and
CuPp–TiO2 photocatalyst. As was expected, after modification with
CuPp, the size of TiO2 has no obvious change compared with bare
TiO2 (Fig. 2(c) and (d)). From the further observations of the ampli-
fied image of CuPp–TiO2 microsphere (Fig. 2(f)), we find that some
nanometer CuPp particles disperse on the surface of TiO2, while
nanometer particles were not found on the surface of bare TiO2
(Fig. 2(e)), indicating that CuPp is well distributed on the surface of
TiO2. In addition, The EDS results (Fig. 3) of the CuPp–TiO2 photo-
catalyst demonstrate that Cu element is present on the surface of
TiO2 microsphere, which probably derived from CuPp (4). There-
fore, it can be concluded that CuPp is well loaded on the surface of
TiO2 as the small solid particles.
The reacting suspension consisting of 50 mL 10−4 mol/L 4-NP
and 10.0 mg catalysts was magnetically stirred. Air was bubbled
into the suspension for 30 min before switching on the lamp. Sam-
ples of 3 mL were withdrawn from the suspension every 20 min
during the irradiation. The photocatalysts were separated from the
solution by centrifugation and the quantitative determination of
4-NP was performed by measuring its absorption at 317 nm with a
Shimadzu UV1800 UV-vis-NIR spectrophotometer.
3. Results and discussion
3.1. Synthesis of the porphyrins H2Pp and CuPp
The synthetic route of the porphyrin and copper(II) porphyrin
was illustrated in Scheme 1. The porphyrin 2 was synthesized in
60% isolated yields, by displacement reaction of 1,2-dibromoethane
with 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin (1) in
DMF in the presence of K2CO3. The porphyrin 3 was synthesized by
using of porphyrin 2 reacting with imidazole in DMF in the pres-
ence of K2CO3 with yield of 40%. Successively, the CuPp (4) was
obtained by the reaction of free porphyrin H2Pp (3) with excess
CuCl2 in CH2Cl2/C2H5OH with yields about 90%.
3.3. Diffuse reflectance spectra
Fig. 4 shows the diffuse reflectance spectra of the bare TiO2
and CuPp–TiO2 photocatalyst recorded in the range of 300–750 nm.
Obviously, there is no absorption above 400 nm for bare TiO2, while
CuPp–TiO2 exhibits the feature peaks observed in CH2Cl2 solution,
indicating that copper porphyrin 4 successfully loaded onto the
TiO2 surface with maintaining the porphyrin structure framework.
The compounds H2Pp (3) and CuPp (4) were characterized by
1H NMR, UV–vis, FT-IR and MS. The UV–vis spectrum of H2Pp (3)

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G.-p. Yao et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
Fig. 2. SEM images (a, b) and TEM images (c–f) of bare TiO2 (a, c, e) and CuPp–TiO2 (b, d, f).
80
70
60
50
40
30
20
10
0
CuPp--TiO2
bare TiO2
300
400
500
600
700
800
wavelength/nm
Fig. 4. The diffuse reflectance spectra of bare TiO2 and CuPp–TiO2 photocatalyst.
Fig. 3. EDS spectrum of CuPp–TiO2 photocatalyst.
Simultaneously, it is noticed that the DR spectrum of CuPp–TiO2
have a significant red shift compared with the spectrum of CuPp

G.-p. Yao et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
33
bare TiO2
CuPp-TiO2
CuPp--TiO2
bare TiO2
4000
3500
3000
2500
Wavelength/nm
2000
1500
1000
20
30
40
50
60
70
80
2 Theta/degree
Fig. 5. FT-IR spectra of bare TiO2 and CuPp–TiO2 photocatalyst.
Fig. 6. XRD patterns of the bare TiO2 and CuPp–TiO2 photocatalyst.
3.6.1. Visible-light-driven photocatalytic activity of CuPp–TiO2
Experimental conditions: [H2O2] = 0.12 mol/L, visible light,
ꢀ > 400 nm.
in CH2Cl2 solution (5 nm red shift of Solet band), which can be
attributed to an interaction between TiO2 and copper porphyrin
molecules.
Fig. 7 shows the photodegradation of 4-NP under different
experimental conditions. According to previous studies [22,23],
3.4. FT-IR spectra
•−
O2
is mainly formed in the present reaction system under vis-
ible light irradiation in the absence of H2O2. The result shows
clearly that only a small degradation of 4-NP was obtained under
the condition of CuPp–TiO2/O2 (curve b), indicating that 4-NP can-
Fourier transform infrared (FT-IR) technique was used to gain
further information about the interaction between CuPp and the
surface of TiO2. Fig. 5 shows the bonding characteristics of func-
tional groups in bare TiO2 and CuPp–TiO2 photocatalyst. Obviously,
the absorption peak at 3421 cm−1 was associated with the stretch-
ing vibrations of surface hydroxyl groups TiO2, whereas the band
around 1643 cm−1 was attributed to the bending vibration of H OH
groups and Ti OH bond for bare TiO2 and CuPp–TiO2, respectively.
The stretching vibrations of C C, C H bonds can be observed
in CuPp–TiO2 photocatalyst, indicating the existence of copper
porphyrin molecules on the surface of TiO2. Further observation
shows that the peaks at 3421 cm−1 become narrower and weaker
in CuPp–TiO2 photocatalyst than that in bare TiO2, reflecting the
decrease of hydroxyl groups after the CuPp was loaded on the
surface of TiO2. It implies that, maybe there exist some reactions
between CuPp and the surface of TiO2.
•−
not be oxidized by O2 effectively within a short time. Hence, to
degrade 4-NP effectively using CuPp–TiO2 photocatalyst under vis-
ible light irradiation, hydrogen peroxide was added to the aqueous
suspensions. The result shows that almost a complete degrada-
tion was obtained within 160 min of visible light irradiation (curve
g). However, no degradation of 4-NP was observed by using bare
TiO2 (curve a) and only a small part of 4-NP was degraded by
H2O2 or TiO2/H2O2 in the absence of CuPp–TiO2 photocatalyst
(curves c and d).
The CuPp–TiO2 photocatalyst also shows high photocatalytic
efficiency under UV light irradiation (curves h and i). A small
increase of photocatalytic activity is observed in the presence
of H2O2. This due to the hydroxyl radicals (•OH) produced
directly by H2O2 under the UV light irradiation. Comparing with
the results from UV light irradiation, the visible photocatalytic
activity of CuPp–TiO2 has been improved greatly when an appro-
priate amount of H2O2 were added to the suspensions.
3.5. XRD patterns
To understand the role of •OH on the degradation of 4-NP, the
mannitol (a scavenger of hydroxyl radicals [23]) was added into
XRD was carried out to investigate the effect of copper porphyrin
on the crystal structure of TiO2. Fig. 6 shows the XRD patterns of
the bare TiO2 and CuPp–TiO2 photocatalysts. No differences were
found in CuPp–TiO2 photocatalysts and bare TiO2, and the promi-
nent diffraction peaks at 25.5◦, 37.5◦, 47.7◦, 53.4◦ and 62.8◦ were
observed which were attributed to anatase TiO2 [21]. These results
indicated that the CuPp (4) which loaded on the surface of TiO2
microsphere had no effect on the crystal structure of TiO2 powders.
1.0
0.8
Visible light irradiation
a.TiO
b.CuPp-TiO /O
2
0.6
0.4
0.2
0.0
2
2
c.H O
2
2
3.6. Photocatalytic degradation experiments
d.TiO /H O
2
2
2
e.CuPp-TiO /mannitol/O
2
2
f.CuPp-TiO /H O /N
g.CuPp-TiO /H O /O
2
2
2
2
Oxygen was bubbled into all the photocatalytic reaction sus-
pensions except the one with nitrogen. The photocatalytic activity
of CuPp–TiO2 photocatalyst was evaluated by the degradation
of 4-nitrophenol (4-NP) in water under visible light irradia-
tion (ꢀ > 400 nm) and UV light irradiation. In order to compare
and explain the photocatalytic mechanism, other photocatalytic
experiments were also performed under different conditions, as
shown in Fig. 7.
2
2
2
2
UV light irradaition
h.CuPp-TiO /O
2
2
i.CuPp-TiO /H O /O
2
2
2
2
0
20
40
60
80
100
120
140
160
180
Time/min
Fig. 7. 4-NP concentration vs irradiation time using CuPp–TiO2 photocatalyst.

34
G.-p. Yao et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
0.94
0.92
0.90
0.88
0.86
0.84
0.82
0.80
0.78
0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22
Concentration of H2O2 [mol/L]
Fig. 8. The effection of H2O2 concentration on degradation of 4-NP.
CuPp–TiO2 suspended aqueous (curve e). Obviously, it affected the
degradation of 4-NP greatly, because the hydroxyl radicals were
consumed by mannitol and consequently lead to a low degradation
of 4-NP. The result suggests that hydroxyl radicals were predomi-
nantly formed in the reaction system.
Fig. 9. The diagram of photocatalytic mechanism.
To assess the role of dissolved O2 during the photocatalytic
degradation process, N2 was bubbled to remove O2 from the aque-
ous suspensions. In this condition, a small decrease of degradation
was occurred (curve f) compared with the one under the presence
of oxygen (curve g). The result indicates that the effect of O2 on the
photodegradation of 4-NP under visible light irradiation is weaker
than H2O2.
Under the visible light irradiation, the process of CuPp–TiO2
photocatalytic reaction can be intrigued by the excitation of the
ground state of the sensitizer CuPp via one photon transition (hꢂ)
to its excited state [CuPp+–TiO2(e−)]* (Eq. (1)), then the photo-
induced electrons inject into the conduction band (CB) of TiO2, and
finally the reactive electrons can be trapped by adsorbed H2O2 or
O2•o−n the surface of TiO2 and produce reactive species •OH and
O2 (Eqs. (2) and (3)). The •OH from H2O2 can photodegrade the
3.6.2. Influence of H2O2 concentration on the rate of degradation
The degradation efficiency versus different initial concentra-
tions of H2O2 has been summarized in Fig. 8. The photodegradation
efficiency increased with an increase of H2O2 concentration up to
0.12 mol/L and then decreased slowly, which suggested that an
optimum concentration of H2O2 could be 0.12 mol/L.
•−
4-NP efficiently (Eq. (4)), but O2 is less reactive with 4-NP in the
absence of H2O2 and the reaction of Eq. (5) can happen to a small
•−
extent. In the presence of H2O2, the O2 can react with H2O2 and
At low concentrations of H2O2, the hydroxyl radicals increased
when increasing the concentrations of H2O2, so the efficiency of 4-
NP degradation enhanced correspondingly. However, excess H2O2
in the suspensions could cause the consumption of hydroxyl radi-
cals, because H2O2 can react with hydroxyl radicals to form a weak
oxidant HO2• (Eq. (R1)), and at the same time, hydroxyl radicals can
also dimerize to H2O2 at high concentration (Eq. (R2)) [23,24].
produce hydroxyl radicals quickly (Eq. (6)), and finally, degrade the
4-NP effectively. [CuPp+–TiO2] can be reduced by 4-NP to its ground
state (Eq. (7)) [28,29].
The role of H2O2 can be clearly seen from the analysis of the
photocatalytic mechanism. Firstly, the H2O2 reacts directly with
the photo-induced electrons to produce hydroxyl radicals (Eq. (2))
in the presence of CuPp–TiO2 photocatalyst, and secondly, more
•−
hydroxyl radicals can be produced by the reaction of O2
and
H2O2 + •OH → HO2 + H2O
•OH + •OH → H2O2
(R1)
(R2)
•
H2O2 (Eq. (6)). In a word, the visible-light-driven photocatalytic
efficiency of CuPp–TiO2 can be improved greatly by adding an
appropriate amount of H2O2.
3.6.3. Photocatalytic mechanism and the role of H2O2
A possible photocatalytic mechanism involving H2O2 was pro-
posed based on the reported mechanism both under UV and visible
light irradiation by using modified TiO2 photocatalyst [25–27], as
shown in Eqs. (1)–(7) and Fig. 9.
4. Conclusions
The photocatalytic activity of a new CuPp–TiO2 photocatalyst
has been studied by carrying out the photodegradation of 4-NP in
the presence of H2O2 under visible light irradiation firstly. It has
been found that anion superoxide (O2•−) is mainly formed in the
absence of H2O2 and O2•− itself is less active for the degradation of
4-NP, so the photocatalytic activity of CuPp–TiO2 is low under visi-
ble irradiation. However, by adding an appropriate amount of H2O2,
the visible-light-driven photocatalytic efficiency of CuPp–TiO2 is
hv>400 nm
CuPp–TiO2 −→ [CuPp+–TiO2(e−)]∗
(1)
(2)
(3)
(4)
(5)
(6)
(7)
[CuPp+–TiO2(e−)]∗ + H2O2 → [CuPp+–TiO2] + • OH + OH−
•
[CuPp+–TiO2(e−)]∗ + O2 → [CuPp+–TiO2] + O2
−
• OH + 4-NP → Oxidatiion products
•
O2 − + 4-NP → Oxidatiion products
•−
•
O2 − + H2O2
→
• OH + OH− + O2
enhanced greatly, because the H2O2 can react with O2 in water
and produce hydroxyl radicals quickly, and finally the hydroxyl
radicals degrade the 4-NP into small molecules.
[CuPp+–TiO2] + 4-NP → CuPp–TiO2 + oxidation products

G.-p. Yao et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 29–35
35
Acknowledgments
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We are very grateful for the financial support from National
Nature Science Funds (20971103) and The International Cooper-
ation Project of Shaanxi Province (2008KW-33).
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