Influence of TiO2 impregnated with a novel copper(II) carboxylic porphyrin and its application in photocatalytic degradation of 4-Nitrophenol

Article abstract of DOI:10.1007/s11164-013-1089-0

A novel 5,15-di-[4-carboxylatomethoxy]phenyl-10,20-diphenylporphyrin, its copper complex and the corresponding metalloporphyrin-TiO₂ photocatalyst were synthesized and characterized by DRS, SEM, XRD, and FT-IR. The photocatalytic effects of anataseTiO₂ impregnated with this copper(II) porphyrin was investigated by photodegradation of 4-nitrophenol(4-NP) in aqueous solution under Xenon lamp irradiation. The results indicated that the photoactivity of copper(II) porphyrin-TiO₂ composite was evidently enhanced by the interaction between carboxyl of the porphyrin molecule and hydroxyls anchored on the TiO₂. Futhermore, the copper(II) carboxylic porphyrin displayed good adsorption behavior and activity of the dye-sensized TiO₂ system.

Full text of DOI:10.1007/s11164-013-1089-0

Res Chem Intermed
DOI 10.1007/s11164-013-1089-0
Influence of TiO2 impregnated with a novel copper(II)
carboxylic porphyrin and its application
in photocatalytic degradation of 4-Nitrophenol
•
•
•
•
Xiangfei Lu¨ Nanjiang Hu Jun Li Hongrui Ma
•
Kai Du Ran Zhao
Received: 26 August 2012 / Accepted: 29 January 2013
ꢀ Springer Science+Business Media Dordrecht 2013
Abstract A novel 5,15-di-[4-carboxylatomethoxy]phenyl-10,20-diphenylporphy-
rin, its copper complex and the corresponding metalloporphyrin-TiO2 photocatalyst
were synthesized and characterized by DRS, SEM, XRD, and FT–IR. The photo-
catalytic effects of anataseTiO2 impregnated with this copper(II) porphyrin was
investigated by photodegradation of 4-nitrophenol(4-NP) in aqueous solution under
Xenon lamp irradiation. The results indicated that the photoactivity of copper(II)
porphyrin-TiO2 composite was evidently enhanced by the interaction between
carboxyl of the porphyrin molecule and hydroxyls anchored on the TiO2. Futher-
more, the copper(II) carboxylic porphyrin displayed good adsorption behavior and
activity of the dye-sensized TiO2 system.
Keywords Copper carboxylicporphyrin ꢀ TiO2 ꢀ CuPp–TiO2 photocatalyst ꢀ
Photocatalytic degradation
X. Lu¨ ꢀ H. Ma ꢀ K. Du ꢀ R. Zhao
Department of Environmental Engineering, College of Resource and Environment Science,
Shaanxi University of Technology, Xi’an 710021, Shaanxi, People’s Republic of China
e-mail: mahongrui@sust.edu.cn
X. Lu¨ ꢀ J. Li
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education,
College of Chemistry & Materials Science, Northwest University, Xi’an 710069, Shaanxi,
People’s Republic of China
N. Hu (&)
Department of Process Equipment Controll and Engineering, College of Light Industry and Energy,
Shaanxi University of Technology, Xi’an 710021, Shaanxi, People’s Republic of China
e-mail: hunanjiang@sust.edu.cn
123

X. Lu¨ et al.
Introduction
As a kind of efficient visible-light absoptivity and environmentally friendly catalyst,
porphyrin-sensitized TiO2 has been proposed for the oxidative degradation of
various kinds of organic pollutants in wastewater [1–3]. This is not only because the
porphyrins possess good chromophore activities over the solar spectrum but because
they also have good electron-donating properties due to their large p-electron
systems [4, 5].
In recent years, there has been growing concern over enhancing the photodeg-
radation efficiency of porphyrin-TiO2 composites. Several factors have been proved
to influence the efficiency, including the center metal, the peripheral substituents of
porphyrin molecules, axial ligand, the crystal form, and specific surface area of
TiO2 [6–10]. Shiragami et al. [11] found that porphyrin is able to bind with almost
metal ions listed in the periodic table, and metalloporphyrin complexes have
versatile catalytic capabilities in electron-transfer reaction or energy transfer under
visible-light irradiation. Additionally, Wang Chen et al. [12] found that the
copper(II) porphyrins was a more effective sensitizer than those of other metals
(M=Co, Zn, Mn) as well as the free base. In other respects, different polarity of the
substituents of metalloporphyrins may also change the surface of TiO2 nano-
particles through physical or chemical adsorption of the composites. In our previous
studies, it has been shown that the number and polarity of functional groups (–
COOH, –OH,–SO3H, etc.) could interact with the surface of TiO2 and,
consequently, display different photocatalytic activity in degrading pollutants
[13–15].
For these reasons, this paper reports the synthesis and characteristics of a novel
carboxylicporphyrin and its copper(II) porphyrins (Fig. 1), as well as the further
photocatalytic activities of the CuPp–TiO2 composite by the photodegradation of
4-nitrophenol(4-NP) in aqueous solution under Xenon lamp irradiation. Various
spectroscopies are important to understand these properties, such as interaction at
the interface, surface composition, and distribution of the energy level. A possible
mechanism of the photocatalytic degradation is also proposed.
OCH2COOH
OCH2COOH
Fig. 1 The structure of H2Pp
and CuPp
NH
N
N
N
N
Cu
HN
N
N
OCH2COOH
OCH2COOH
H2Pp
CuPp
123

Influence of TiO2
Experimental
Reagents and materials
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was obtained from Aldrich, and
other reagents were obtained from Beijing Chemical Reagents. They were used
without further purification except pyrrole, which was distilled before use. Ethyl (4-
formylphenoxy)acetate were prepared according to the established procedure [16].
TiO2 was kindly provided by TIOXIDE Huntsman (anatase phase, BET specific
surface area 10 m2/g), used in preparation of loaded samples applied as photocat-
alysts in photoreactivity experiments.
Characterizations
Elemental analysis (C, H and N) was performed by Vario EL-III CHNOS
1
instrument. FT-IR spectra was registered in KBr using a BEQ UZNDX-550. H
NMR spectra was recorded at room temperature using a Varian Inova 400MHZ
apparatus and tetra-methylsilane (TMS) for reference. Mass spectrometry (MS)
analysis was carried out on a matrix-assisted laser desorption/ionization time of
flight mass spectrometer (MALDI-TOF MS; Krato Analytical of Shimadzu Biotech,
Manchester, UK) using a standard procedure involving 1 mL of the sample solution.
UV–Vis spectra was recorded by a Shimadzu UV-via-NIR spectrophotometer. The
scanning electron microscopy (SEM; Hitachi–S4800) was used to characterize the
morphologies of the samples. The X-ray diffraction (XRD) measurement was
performed with a Bruker D8 diffractometer using graphite monochromatic copper
radiation (Cu Ka) at 40 kV, 30 mA over the 2h range 10–70ꢁ. A model XPA-VII
photocatalytic reactor (Xu jiang Electromechanical Plant, Nanjing, China) was used
for the degradation of 4-NP.
Synthesis of the novel carboxylicporphyrin and copper porphyrin
The synthesis of 5,15-di-[4-carboxylatomethoxy]phenyl-10, 20-diphenylporphyrin
was investigated by carrying out the hydrolysis of 5,15-di-[4-ethylacetatatometh-
oxy] phenyl-10, 20- diphenylporphyrin.The 5,15-di-[4-ethylacetatatomethoxy]
phenyl-10,20-diphenylporphyrin was obtained by condensation reaction of pyrrole,
benzaldehyde and ethyl (4-formylphenoxy)acetate using the Lindsey method [17].
The precursor synthesis of ethyl (4-Formylphenoxy)acetate is analogous to that
reported in the literature.
5,15-di-[4-ethylacetatatomethoxy]phenyl-10,20-diphenylporphyrin:
2,20-Dip-
yrrylmethane (0.9524 g, 4 mmol) and ethyl(4-Formylphenoxy)acetate(0.89 g,
4 mmol) were brought together in a 2-L round-bottom flask containing 1.5 L of
dry chloroform and a magnetic stir bar under N2. Boron trifluoride etherate was
added to this mixtures; the flask was then covered with aluminum foil and stirred at
room temperature for 24 h. DDQ (2,3-dichloro-4,5-dicyanoquinone, 0.68 g, 3
mmol) was added to the chloroform solution, and the mixture was allowed to stir for
123

X. Lu¨ et al.
24 h before the solvent was removed under reduced pressure. The reaction mixture
was then purified on a silica-gel column with duchloromethane as eluent.
5,15-di-[4-carboxylatomethoxy]phenyl-10, 20-diphenylporphyrin: A 100-mL
round-bottom flask was charged with 5,15-di-[4-ethylacetatatomethoxy] phenyl-
10,20-diphenylporphyrin (50 mg, 0.061 mmol) and dissolved in 10 mL of DMF
solution. An aqueous solution of KOH (10 %, 30 mL) was added to the stirred
solution. The solution was stirred in the presence of K2CO3 (0.15–0.2 g) for 12 h at
60 ꢁC in the darkness. The reaction mixture was evaporated under a vacuum drier
after the reaction was complete, and the residue was dissolved in DMF and purified
on a silica-gel column using dichloromethane as eluent. Yield 51 %. Mp:[250 ꢁC.
Anal. Calcd. for C48H34N4O3 (Mol. Wt:762.25), %: C 85.63 (75.58), H 4.43 (4.49),
N 7.38 (7.34). MS: m/z 763.41 ([M?1]?) amu. UV–vis(CHCl3): 419 (Soret band),
517, 551, 591, 648 nm (Q bands). FT-IR(m/cm-1): 3,448, 2,923, 2,854, 2,362,
1,759, 1,691, 1,602, 1,506, 1,436, 1,384, 1,209, 1,164, 1,078, 1,024, 968, 835, 798,
731, 607.1HNMR (VCDCl :VCF COOD¼ 3 : 1, 400 MHz), d, ppm: 11.750 (d, 2H,–
3
3
COOH), 8.113–7.927 (m, 8H, Ar), 7.776–7.721(m, 8H, Ar), 7.503(d, 4H, Ar),
7.197–7.173(m, 2H, Ar), 6.96(d, 2H, Ar), 1.313(d, 4H, –OCH2CO), -0.173 (brs, 2H,
NH).
5,15-di-[4-carboxylatomethoxy]phenyl-10, 20-diphenylcopperporphyrin: 27.0 mg
(0.15 mmol) of Cu(CH3COO)2 was added to 60.8 mg (0.05 mmol) of the H2Pp
dissolved in20 mL CHCl3 and 5 mLC2H5OH. The mixture was stirred for 24 h atroom
temperature and monitored by TCL until the complete disappearance of the starting
material H2Pp. The unreacted solid salt was filtered and the solvent removed under
vacuum. The crude product was purified by chromatography on a silica gel column with
CH2Cl2/hexane (7/3, vv) as eluent. Yield 91 %. Mp: [250 ꢁC, Anal. Calcd. for
C48H32CuN4O6 (Mol.Wt:824.34), %: C, 70.01 (69.94), H, 3.89 (3.91), N, 6.87 (6.80);.
MS: m/z 826.46 ([M?H]?)amu. UV–vis (CHCl3): kmax, nm, 417, (Soret band), 541,
590 (Q bands). FT-IR, t/cm-1 : 3,730, 3,425, 2,923, 2,854, 1,737, 1,620, 1,560, 1,461,
1,377, 1,261, 1,095, 1,028, 804, 673.
Preparation of the CuPp–TiO2 photocatalysts
An amount (6 lmol) of the sensitizer (CuPp) was dissolved in 30 mL ethanol and
1 g TiO2 was added to the solution. The suspension was stirred for 10 h at the room
temperature. Then the solvent was removed under vacuum and the catalyst was
collected. Copper porphyrin was used to impregnate TiO2, marked as CuPp–TiO2.
Photodegradation setup and photodegradation experiments
The photodegradation experiment was carried out by using a Model XPA–VII
photocatalytic reactor consisting twelve 50-mL quartz tubes irradiated by a 350-W
Xenon lamp simulating sunlight (Fig. 2). The average light intensity on the sample
surface was about 615 mW • cm-2. The temperature inside the reactor was
maintained at 25 ꢁC with a circular water running via an interlayer.
123

Influence of TiO2
Fig. 2 Setup of the
photocatalytic system(a inlet of
water, b quartz cold-trap, c light
source, d quartz test-tube,
e constant temperature water
tank, f chassis, g power supply
control, h speed governor,
i revolving switch, j start button,
k mixing-speed display,
l magnetic bar stirring, m water
inlet hose of sink, n teat-tube
rack, o water outlet hose of sink,
p outlet of water)
The reacting suspension consisting of 50 mL 10-4 mol/L 4-NP and 10.0 mg
catalysts was stirred with a magnetic bar. Air was bubbled into the suspension for
30 min before switching on the lamp. Samples of 3 mL were withdrawn from the
suspension every sampling time 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. Bare TiO2 was tested for comparative
reasons under the same experimental condition.
Results and discussion
Synthesis and characterization of porphyrins
The synthetic route of the porphyrins (H2Pp)and copper(II) porphyrin(CuPp) is
illustrated in Scheme 1. The synthesis of the free carboxylporphyrin involved
hydrolysis of the precursor corresponding ester porphyrin with KOH.The CuPp was
obtained by the reaction of H2Pp with Cu(OCH3COO)2 in CHCl3/C2H5OH with a
yield of about 91 %. The UV–Vis spectra of H2Pp show almost the same Soret and
Q bands. The insertion of Cu(II) into the porphyrin ring caused a violet shift of 4 nm
of the corresponding number of Q bands, which due to the symmetry of the
porphyrin ring increases when the hydrogen ions of N–H are replaced by Cu(II).
Mass spectroscopy data perfectly correspond to the expected [M-2H]- m/z values
for the individual copper porphyrin. The main change of the IR spectra of copper
123

X. Lu¨ et al.
porphyrin compared with free base porphyrins is the disappearance of the stretching
vibration of N–H.
The diffuse reflectance spectra of photocatalyst
Figure 3 shows the diffuse reflectance spectra of the bare TiO2, CuPp and CuPp–
TiO2 photocatalyst recorded in the range of 200–800 nm. Obviously, there is no
absorption above 400 nm for bare TiO2, while CuPp–TiO2 exhibits the feature
peaks of CuPp observed in CHCl3–acetic acid solution, indicating that CuPp has
been successfully loaded onto the TiO2 surface, maintaining the porphyrin structure
framework. Consequently, the Soret bands of CuPp–TiO2 photocatalyst has a
significant red-shift compared with the spectra of CuPp in CHCl3–acetic acid
solution (Table 1) due to p–p* absorptions of the conjugate in the molecular of the
porphyrin, implying that there exists an interaction between TiO2 and CuPp
molecules involving the carboxylic group with the TiO2 surface.
SEM images of photocatalysts
The SEM images of bare TiO2 and CuPp–TiO2 photocatalyst microspheres are
shown in Fig. 4. It can be observed that the microsphere of CuPp-TiO2
photocatalyst possesses a similar microphology as bare TiO2, which indicates that
the TiO2 particles are basically unchanged before and after loading with the CuPp.
The EDS of CuPp–TiO2 photocatalyst (as shown in Fig. 5) revealed that Cu element
presented on the surface of TiO2 microsphere, which is probably derived from
CuPp. In our previous work, most nanometer CuPp particles dispersed on the
surface of TiO2 [22]. Cherian and Wamser have concluded that porphyrins adsorbed
OCH2COOCH2CH3
OCH2COOC2H5
BF3·Et2O
NH
N
N
DDQ
HN
+
,
CHCl3
N2
CHO
N
N
OCH2COOCH2CH3
OCH2COOH
OCH2COOH
NH
N
N
N
Cu(CH3COO)2
CHCl3, EtOH
N
Cu
HN
N
N
OCH2COOH
OCH2COOH
H2Pp
CuPp
Scheme 1 Synthesis of the porphyrin H2Pp and copper(II) porphyrin CuPp
123

Influence of TiO2
90
80
70
60
50
40
30
20
10
0
bare TiO
CuPp-TiO
CuPp
2
2
200 250 300 350 400 450 500 550 600 650 700 750 800
λ/
nm
Fig. 3 The diffuse reflectance spectra of bare TiO2, CuPp, and CuPp–TiO2 compsites
Table 1 UV–Visible spectra data of the CuPp in CHCl3/CH3COOH and UV–Visible diffuse reflectance
spectra data of TiO2 and CuPp–TiO2 photocatalysts
Compounds
TiO2
k
Soret band
(nm)
Q bands
(nm)
(nm)
k
k
max
edge
max
CuPp
–
417
421
–
543, 590
545, 595
–
CuPp-TiO2
TiO2
400
400
on the surface of TiO2 are independent in their interaction with the surface at very
low coverages [23]. Thus, we believe that CuPp was attached on the surface of TiO2
as molecularly isolated.
Fig. 4 SEM images of bare TiO2 (a), CuPp–TiO2 (b)
123

X. Lu¨ et al.
Fig. 5 EDS spectrum of the CuPp–TiO2 photocatalyst
FT-IR spectra of catalysts
The bonding characteristics of functional groups in bare TiO2 and CuPp–TiO2
composite were identified by FT-IR spectroscopy (Fig. 6). The absorption peak at
3425 cm-1 is deemed as the stretching vibrations of surface hydroxyl groups,
whereas the band around 1,632 cm-1 is attributed to the bending vibration of the
Ti–OH bond. In addition, several absorption peaks around 2,925 cm-1 and
1,635 cm-1 are observed in bare TiO2 and CuPp–TiO2 photocatalyst which are
associated with the stretching vibrations of C–H and C–C, respectively, indicating
the presence of CuPp on the surface of TiO2 [18]. Further observation shows that the
peaks corresponding to the C=O at 1,680 cm-1 of carboxylic groups has a red-shift
in the CuPp–TiO2 compared with that of CuPp. From these observations, it is
100
90
80
70
60
50
CuP-TiO
40
30
20
10
2
TiO
2
CuP
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number(cm )
Fig. 6 FTIR spectra of bare TiO2, CuPp and CuPp–TiO2
123

Influence of TiO2
inferred that there is weak interaction between copper carboxylic porphyrin and the
surface of TiO2 microspheres.
XRD characterization of photocatalyst
Figure 7 shows the XRD pattern of the bare TiO2 and CuPp–TiO2 photocatalysts. It
exhibits prominent peaks at 25.2ꢁ, 37.7ꢁ, 47.9ꢁ, 53.8ꢁ and 62.6ꢁ which are similar to
the reported values for anatase TiO2 [19]. These results indicate that the CuPp
loaded onto the surface of TiO2 has no effect on the crystal structure of TiO2
powders. Furthermore, this conclusion is consistent with the previous results of FT-
IR spectra: CuPp was loaded on the surface of TiO2 microspheres, not doped into
TiO2 crystal lattice.
Photocatalytic activity of CuPp–TiO2 photocatalyst
The photocatalytic activity for the degradation of 4-NP in aqueous suspension under
Xenon lamp irradiation using CuPp–TiO2 and TiO2 (for comparison) are illustrated
in Fig. 8. The blank group showed no effects on the degradation of 4-NP, indicating
that 4-NP was difficult to photodegrade in the absence of a photocatalyst. Under
Xenon lamp irradiation, CuPp–TiO2 showed more enhanced photoactivity than bare
TiO2. In particular, Figure 9 shows the diffuse reflectance spectra of the CuPp–TiO2
photocatalyst after photodegradation. It has been confirmed that CuPp supported
onto TiO2 showed good stability under irradiation condition. These results indicated
that the anchoring groups (–COOH) of CuPp on the TiO2 display adsorption
behavior and activity of dye-sensized TiO2 systerm.
101
CuPp-TiO
002
004
2
200
211
105
TiO
2
20
25
30
35
40
45
50
55
60
65
70
2θ (°)
Fig. 7 XRD pattern of the bare TiO2 and CuPp–TiO2 catalysts
123

X. Lu¨ et al.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
TiO
2
CuPp-TiO
2
0
30
60
90 120 150 180 210 240 270 300
time(min)
Fig. 8 4-NP concentration versus time using CuPp–TiO2 photocatalyst under Xenon lamp irradiation
Fig. 9 The diffuse reflectance spectra of under Xenon lamp irradiation CuPp–TiO2 photocatalyst after
photodegradation
Photocatalytic mechanism
On the basis of the band gap structure of TiO2 and the effects of scavengers, a possible
pathway for the photodegradation of 4-NP with CuPp–TiO2 photocatalyst was
proposed as follows (Fig. 10): during the degradation process, both CuPp and TiO2
can be simultaneously excited to form electrons under Xenon light irradiation. On the
one hand, the photoinduced electrons transfer from the CB bottom of CuPp to that of
TiO2 and are then moved to the surface of TiO2. On the other hand, the photoinduced
electrons on the CB bottom of TiO2 can also be moved to the surface of TiO2 [20, 21].
At the same time, the photogenerated holes move in the opposite direction from the VB
123

Influence of TiO2
Oxidation products
4-NP
C
u
P
p
O2
hυ
TiO2
H2O
4-NP
H2O2
O2
OH
HO2
Oxidation products
Oxidation products
Oxidation products
O2
Fig. 10 The schematic diagram of a possible reaction mechanism over CuPp–TiO2 under Xenon lamp
irradiation
of TiO2 to that of CuPp. The photoinduced electrons should be trapped by adsorbed O2
¯
on the surface of TiO2 producing reactive species •O2. In addition, a series of reactions
produce reactive intermediates (e.g., •HO2, H2O2 and •OH) effective for the
photodegradation process. Furthermore, when CuPp was adsorbed on the surface of
TiO2, it likely binds to TiO2 in ‘‘a variety of different modes’’ and the mode of the bind
is an important factor in determining composite catalyst performance. The porphyrin
could be lie flat on or edgewise to the TiO2 surface which depends on the numbers and
polarity of substituents in the molecular structure [24, 25]. The two carboxylic
functionalities in CuPp may cause the CuPp to lie flat on the surface which improved
charge transfer from CuPp to the surface of TiO2.
Conclusion
A new copper porphyrin modified TiO2 has been prepared and characterized. The
XRD, EDS and FTIR spectra demonstrated that CuPp was loaded on to the surface
of TiO2 and did not dope into the TiO2 crystal lattice. The CuPp–TiO2 exhibited
higher photoactivity compared with bare TiO2 when it was employed for the
photodegradation of 4-nitrophenol under Xenon lamp irradiation. Furthermore, it
was found that the carboxylic group of CuPp has large polarity, which is of benefit
for interaction between CuPp and the surface of TiO2. At the same time, owing to its
high performance and stablity, CuPp–TiO2 is a very promising photocatalyst for the
degradation of organic pollutants.
Acknowledgments This research was financially supported by the National Natural Science Foundation
Project (No. 21177079 and 20971103) and the major State Basic Research Development Program of
China (973 program) (No. 2011CB612309) and Shaanxi University of Science and Technology Scientific
Research Foundation for doctor (BJ11-19).
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