Efficient degradation of 4-nitrophenol by using functionalized

porphyrin-TiO2 photocatalysts under visible irradiation

Chen Wang a, Jun Li a,*, Giuseppe Mele b,*, Gao-Mai Yang a,Feng-Xing Zhang a, Leonardo Palmisano c, Giuseppe Vasapollo b

a Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi’an, Shaanxi 710069, Chinab Dipartimento di Ingegneria dell’Innovazione, Universita del Salento, Via Arnesano, 73100 Lecce, Italy

c Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy

Received 3 April 2007; received in revised form 25 May 2007; accepted 29 May 2007

Available online 2 June 2007

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 76 (2007) 218–226

Abstract

The novel porphyrins 5,10,15,20-tetra-[4-(3-phenoxy)-propoxy]phenyl porphyrin, H2Pp(a) and 5,10,15,20-tetra-[2-(3-phenoxy)-propoxy]-

phenyl porphyrin, H2Pp(b) and their corresponding copper(II) complexes CuPp(a), CuPp(b) were synthesized and characterized by using various

spectroscopic techniques. The photocatalytic activity of polycrystalline TiO2 samples impregnated with H2Pp(a), H2Pp(b), CuPp(a) and CuPp(b)

as sensitizers have been investigated by carrying out the photo-degradation of 4-nitrophenol (4-NP) as a probe reaction in aqueous suspension and

under visible light. The maximum photocatalytic activity was obtained using TiO2 loaded with a monolayer of the copper porphyrin CuPp(b) in the

amount of 18 mmol per gram of TiO2.

The photocatalytic efficiency decreased in the following order: TiO2-CuPp(b), TiO2-CuPp(a), TiO2-H2Pp(b), TiO2-H2Pp(a). A possible

mechanism of the photocatalytic degradation is also proposed.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Porphyrin; Cu(II)-porphyrin; Titanium dioxide; Photocatalysis; 4-Nitrophenol; Photo-degradation

1. Introduction

Polycrystalline TiO2 has attracted much attention in last

score years not only for its effectiveness as photocatalyst and as

material for photoelectric conversion, but also for its

inexpensiveness, easy production, (photo)chemical and biolo-

gical stability, and innocuity to the environment and to human

beings. It has been used for water remediation [1–3] being

effective for the photo-degradation of many harmful organic

pollutants [4–6] (e.g. alkanes, alcohols, carboxylic acids,

alkenes, phenols, dyes, PCBs and pesticides) to CO2, H2O and

innocuous inorganic species. Moreover, TiO2 has been used for

the photo-oxidation of inorganic species [7,8] and the

elimination of heavy metal species [9], but some drawbacks

limit its application especially in the large-scale industry. One

* Corresponding authors. Tel.: +39 0832 297281; fax: +30 0832 297279.

E-mail addresses: junli@nwu.edu.cn (J. Li), giuseppe.mele@unile.it

(G. Mele).

0926-3373/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2007.05.028

of them is the value of band gap (Eg = 3.2 and 3.0 eV for

anatase and rutile phases, respectively) and this means that

TiO2 can absorb only the ultraviolet radiation whose

wavelength is less than 387 nm. It is noteworthy that the

fraction of ultraviolet radiation in the range 300–400 nm

reaching the earth is equal only to 4–6% of the global solar

radiation, 45% of which consists of visible light. So, many

methods have been applied to extend the light absorption of

TiO2 into the visible light region [10], as for instance metals

[11–13] and non-metals [14] ion doping, ion implantation [15]

and photosensitization [16], although not always an improved

absorption of light in the visible region gives rise to an

enhancement of the photoreactivity rate. Indeed, the activity

and the quantum efficiency are usually low when TiO2

photocatalysts are doped with metal or non-metal species using

traditional methods and sometimes the (photo)stability of the

species is not high depending on the experimental conditions

under which the photoreaction is carried out [17]. Photo-

sensitization has shown excellent capability compared with

other methods in TiO2 visible light response [18]. On the other

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226 219

hand, an opportunely sensitized TiO2 was much more effective

catalyst for the oxidation of phenols under irradiation with

visible light compared to the sample obtained by anchoring the

same sensitizer on Al2O3 [19].

Porphyrins (including metal-free porphyrins, metallopor-

phyrins and supramolecular porphyrins [20]) are recognized to

be the most promising sensitizers [21]. So far we know that

porphyrins themselves are highly effective photocatalysts due

to their very strong absorption in the 400–450 nm region (Soret

band) and in the 500–700 nm region (Q-bands) and, in fact, the

presence of p-electrons affords the condition for electron

transfer during the photoreaction. Some porphyrins can activate

molecular O2, and for this reason they have been used also in

thermal catalytic oxidation reactions [22,23]. This point is very

important for explaining the beneficial effect of the presence of

porphyrins in TiO2 samples.

Indeed, porphyrins not only can be involved in the reaction

mechanism by transferring electrons to the conduction band of

TiO2 or to adsorbed O2, similarly to other photosensitizers, but

also they can co-operate with TiO2 participating in photo-

oxidation reactions by means of a direct activation of O2 (not

involving TiO2).

Of course, there are several advantages to use immobilized

photosensitizers for practical applications. In particular, high

effective porphyrin-TiO2 photocatalysts can be considered as

hybrid organic/inorganic materials having real perspectives in

water purification [24–26].

In this paper two novel porphyrin derivatives, that is

5,10,15,20-tetra-[4-(3-phenoxy) propoxy]phenyl porphyrin,

H2Pp(a), and 5,10,15,20-tetra-[2-(3-phenoxy)-propoxy]phenyl

porphyrin, H2Pp(b), and their corresponding copper(II)

complexes CuPp(a), CuPp(b) (Fig. 1), were synthesized and

characterized.

These porphyrins which contain lipophilic ether-chain

groups were used to prepare TiO2-based photocatalysts. Due

to the scarce affinity with the aqueous phase, these molecules

remain anchored onto the TiO2 surface. It is worth noting that

the peripheral substitution in such molecules as well as the

possibility of coordinating different metals are important for the

design of functional dyes [24–26]. In this paper, we have chosen

as a probe reaction the degradation of 4-nitrophenol (4-NP),

Fig. 1. Molecular structures of H2Pp(a

known to be a very stable and refractory pollutant in paper mill

and dye-industry wastewater, in order to test the photo-

degradation efficiency of the photocatalysts in aqueous

suspensions and under visible light irradiation.

Certainly, the opportunity to have a class of catalysts

working under visible light irradiation as well as the possibility

to recover and to reuse the photosensitizers make environ-

mental and economic sense to this process.

2. Experimental

1,3-Dibromopropane, 4-hydroxybenzaldehyde, 2-hydroxy-

benzaldehyde, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

(DDQ) were obtained from Aldrich, and other reagents were

obtained from Beijing Chemical Reagents Company. They

were used without further purification with the exception of the

pyrrole that was distilled before use.

Nanoparticulate TiO2 was kindly provided by Xi’an TAIHE

Company. It has a nominal particle size of 30 nm and a

composition of ca. 80% anatase and 20% rutile with BET

specific surface area 54 m2/g. It was prepared from hydrolysis

of tetrabutyl titanate and dried at 150 8C for 2 h without any

other treatment.

Elemental analyses (C, H and N) were performed by Vario

EL-III CHNOS instrument. IR spectra were obtained with

samples in KBr matrix for the title complexes on A BEQ

UZNDX-550 series FT-IR spectrophotometer in the range

4000–400 cm�1. UV–vis spectra were performed by Shimadzu

UV2550 UV–vis-NIR spectrophotometer. 1H and 13C NMR

spectra were recorded by using a Bruker AC-400 at room

temperature and chemicals shifts were reported in ppm units with

respect to the reference frequency of tetramethylsilane, Me4Si.

Mass spectrometry 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). One microliter 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.

), H2Pp(b), CuPp(a) and CuPp(b).

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226220

The specific surface areas were measured by the single point

BET method using a jw-05 apparatus (BeiJing China). Total

organic carbon (TOC) was measured using USA O I Analytical

TOC.

2.1. Synthesis of the compounds 1, 2a, 2b, H2Pp(a),

H2Pp(b), CuPp(a) and CuPp(b) and preparation of the

photocatalysts

2.1.1. Synthesis of 3-phenoxypropyl bromide (1)

1 was synthesized according to previous report [27]. A

28.0 g (0.3 mol) phenol and 37.7 mL (0.37 mol) 1,3-dibromo-

propane in 150 mL H2O were heated with stirring at 100 8C in

oil bath, and 11.3 g (0.28 mol) NaOH dissolved in 40 mL H2O

were dropped in the reaction system in 30 min. The mixture

was refluxed for 4 h, then the water layer was separated and the

remaining oil layer was distilled under vacuum. The unreacted

1,3-dibromopropane was recovered as the head of the

distillation process and 3-phenoxypropyl bromide (1) succes-

sively was collected in 85% yield. Found mp: 10–11 8C, nD20:

1.5460.

2.1.2. Synthesis of 4-(3-phenoxy) propoxybenzaldehyde

(2a)

A 10.0 g (0.082 mol) 4-hydroxybenzaldehyde and 12.5 mL

(0.08 mol) 3-phenoxy-propyl bromide (1) in 100 mL

CH3CH2OH were heated with stirring at 80 8C in water bath,

then 3.2 g (0.08 mol) of NaOH dissolved in 50 mL CH3CH2OH

were dropped in the reaction system for 1 h and the mixture was

refluxed for 6 h. After cooling, the mixture was washed with

distilled water several times and then the solvent was removed

under vacuum. The remaining liquid oil obtained was dissolved

in CH3CH2OH. The colourless crystals of 4-(3-phenoxy)

propoxybenzaldehyde (2a) were obtained from the solution in

38% yield. Mp: 60–61 8C. Anal. Calcd. for C19H16O3, %: C,

75.32; H, 6.322. Found C, 74.98; H, 6.292. FT-IR: n, cm�1

2952, 2836, 2749, 1698, 1602, 1502, 1464, 1384, 1160, 1060,

830, 754.

2.1.3. Synthesis of 2-(3-phenoxy) propoxybenzaldehyde

(2b)

The synthesis of 2b was carried out in a similar way of 2a but

2-hydroxybenzaldehyde was used instead of 4-hydroxyben-

zaldehyde. The yields obtained were 35%. Mp: 53–54 8C. Anal.

Calcd. for C19H16O3, %: C, 75.30; H, 6.347. Found C, 74.98; H,

6.292. FT-IR: n, cm�1 2932, 2873, 2768, 1685, 1600, 1487,

1458, 1386, 1187, 1058, 808, 760.

2.1.4. Synthesis of 5,10,15,20-tetra-[4-(3-phenoxy)-

propoxy]phenyl porphyrin H2Pp(a)

A 2.13 g (8.3 mmol) 4-(3-phenoxy)propoxybenzaldehyde

(2a) and 0.56 mL (8.3 mmol) pyrrole in 200 mL of chloroform

were first stirred at room temperature for 10 min under nitrogen

atmosphere. Then 0.13 mL (2.8 mmol) of BF3�OEt2 in 5 mL

CHCl3 were added. The reaction mixture was stirred at room

temperature for 40 h and successively for further 50 h after

addition of 1.4 g (6.16 mmol) of DDQ. The solvent was

removed under vacuum and the crude product was purified by

chromatography on a silica gel column with CH2Cl2/

CH3CH2OH (35/1, vv) as eluant. The product 5,10,15,20-

tetra-[4-(3-phenoxy)-propoxy]phenyl porphyrin (H2Pp(a)) was

isolated in 15% yield. Mp: >250 8C, Anal. Calcd. for

C80H70N4O8, %: C, 79.11; H, 5.684; N, 4.58. Found C,

79.10; H, 5.805; N, 4.61. MS: m/z 1214.64 ([M�H]�) amu. FT-

IR: n, cm�1 3315.77, 3060.12, 2924.96, 2361.51, 1595.37,

1493.20, 1287.02, 1241.83, 1052.85, 964.01, 751.63. 1H NMR

(CDCl3, 400 MHz): d, ppm 8.83 (s, 8H, b position of the

pyrrole moiety), 8.09 (d, J = 8.6 Hz, 8H, Ar), 7.40–7.22 (m,

16H, Ar), 7.03–6.95 (m, 12H, Ar), 4.45 (t, J = 6.0 Hz, 8H,

OCH2), 4.33 (t, J = 6.0 Hz, 8H, OCH2), 2.43 (quintuplet,

J = 6.0 Hz, 8H, CH2), �2.78 (br s, 2H, NH). UV–vis (CHCl3):

l max, nm, 423 (Soret band), 519, 556, 593, 649 (Q bands).

2.1.5. Synthesis of 5,10,15,20-tetra-[2-(3-phenoxy)-

propoxy]phenyl porphyrin H2Pp(b)

The synthesis of H2Pp(b) was carried out in a similar way of

H2Pp(a). The product was obtained in 10% isolated yield. Mp:

>250 8C Anal. Calcd. for C80H70N4O8, %: C, 79.14; H, 5.733; N,

4.62. Found C, 79.10; H, 5.805; N, 4.61. MS: m/z 1214.67

([M�H]�) amu. 1H NMR (CDCl3, 400 MHz): d, ppm 8.70–8.64

(m, 8H, b position of the pyrrole moiety), 7.97–7.84 (m, 4H, Ar),

7.78–7.72 (m, 4H, Ar), 7.39–7.27 (m, 8H, Ar), 6.76–6.60 (m, 6H,

Ar), 6.59–6.47 (m, 6H, Ar), 6.19–5.90 (m, 8H, Ar), 4.14–3.96 (m,

8H, OCH2), 2.98–2.78 (m, 8H, OCH2), 1.48–1.20 (m, 8H, CH2),

�2.63 (br s, 2H, NH). UV–vis (CHCl3): l max, nm, 419 (Soret

band), 514, 548, 589, 644 (Q bands).

2.1.6. Synthesis of CuPp(a)

A 27.0 mg (0.15 mmol) of CuCl2 were added to 60.8 mg

(0.05 mmol) of the H2Pp(a) dissolved in 20 mL of CHCl3 and

3 mL of CH3CH2OH. The mixture was stirred for 24 h at room

temperature and monitored by TLC until the complete

disappearance of the starting material H2Pp(a). The unreacted

solid salt was filtered off and the solvent was removed under

vacuum. The crude product was purified by chromatography on a

silica gel column with CH2Cl2 as eluant. The product CuPp(a)

was recovered in nearly quantitative yield. Mp: >250 8C, Anal.

Calcd. for CuC80H68N4O8, %: C, 75.08; H, 5.542; N, 4.19. Found

C, 75.24; H, 5.367; N, 4.39. MS: m/z 1277.9 ([M+H]+) amu. UV–

vis (CHCl3): l max, nm, 419 (Soret band), 541, 579 (Q bands).

2.1.7. Synthesis of CuPp(b)

The synthesis of CuPp(b) was carried out using the same

procedure used for CuPp(a).

Mp > 250 8C, Anal. Calcd. for CuC80H68N4O8, %: C,

75.13; H, 5.640; N, 4.47. Found C, 75.24; H, 5.367; N, 4.39.

MS: m/z 1277.3 ([M+H]+) amu. UV–vis (CHCl3): l max, nm,

417 (Soret band), 540, 575 (Q bands).

2.1.8. Preparation of the photocatalyts: TiO2-H2Pp(a),

TiO2-H2Pp(b), TiO2-CuPp(a) and TiO2-CuPp(b)

The loaded samples used as photocatalysts for the photo-

degradation experiments were prepared by impregnating 1 g of

TiO2 with different amounts (6, 12, 18, 22 mmol) of the

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226 221

sensitizer H2Pp(a), H2Pp(b), CuPp(a) and CuPp(b) in the

following way: the sensitizer was dissolved in 20 mL of CHCl3and 1 g finely ground TiO2 was added to this solution. The

resulting suspension was stirred for 8 h, then the solvent was

removed under vacuum.

2.2. Photoreactivity experiments

The photo-degradation set up is shown in Fig. 2. The set up,

placed in a black box consisted of a 150 mL beaker irradiated

by a 125 W, 24 V Iodine-Tungsten lamp that was refrigerated

by a cool-fanner. The distance between the lamp and the

solution is 20 cm, and the light radiation intensity at the

distance of 20 cm is 0.1911 mW/cm2 (instrument: New Port

Dual-Channel Power Meter. Model 2832-C Irvine, CA, USA).

A 400 nm cutoff filter was placed between the lamp and the

beaker in order to absorb the UV light. The temperature inside

the reactor was maintained at ca. 300 K by means of a

continuous circulation of water in a jacket surrounding the

reactor.

The reacting suspension consisting of 100 mL 10�4 mol/L

4-NP and 20.0 mg catalyst was stirred with a magnetic bar.

Air was bubbled into the suspension for 30 min before

switching on the lamp. The initial pH value of the suspension

was 6.40 and it was measured during the photocatalytic

experiments.

The photoreactivity runs lasted 400 min. Samples of 3 mL

were withdrawn from the suspension every 50 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 316 nm

with a Shimadzu UV2550 UV–vis-NIR spectrophotometer. At

the end of each photoreactivity experiment the resulting

suspension was centrifuged. Hence, the solution was removed

and the separated catalyst was reused for the further

photocatalysis experiment. This process was repeated for six

more times to check the stability of the catalyst. Bare TiO2 was

Fig. 2. Set up of the photocatalytic system: (1) Iodine-Tungsten lamp; (2) filter,

(3) cool-fanner, (4) constant temperature water jacket, (5) photoreactor, (6) air

pump, (7) magnetic stirrer, (8) magnetic bar, (9) inlet of water and (10) outlet of

water.

also tested for the sake of comparison under the same

experimental conditions.

3. Results and discussion

3.1. Synthesis of H2Pp(a), H2Pp(b), CuPp(a), CuPp(b) and

preparation of the photocatalysts

As showed in the Scheme 1, porphyrins, 5,10,15,20-tetra-[4-

(3-phenoxy)-propoxy]phenyl porphyrin, H2Pp(a) and

5,10,15,20-tetra-[2-(3-phenoxy)-propoxy]phenyl porphyrin,

H2Pp(b), were synthesized by using the Lindsey method [28]

obtaining 15 and 10% isolated yields, respectively. The long

and flexible substituting groups present in the porphyrins can

influence the yields. The higher yield obtained for H2Pp(a) was

probably due to the minor steric hindrance of the p-substituting

group.

The copper porphyrins, CuPp(a) and CuPp(b), were

obtained almost quantitatively and were characterized by

analytical and spectral data (see Section 2).

The UV–vis spectrum of H2Pp(a) shows a band centered at

l = 423 (Soret band), and the Q bands absorptions, respec-

tively, at 519, 556, 593 and 649 nm. The UV–vis spectrum of

H2Pp(b) consists of a Soret band at 419 nm, and Q bands at 514,

548, 589, 644 nm.

When H2Pp(a) and H2Pp(b) were reacted with copper(II)

salt and CuPp(a) and CuPp(b) formed in high yields, the Soret

band for CuPp(a), appears at 419 and the Q bands at 541,

579 nm, respectively; the Soret band for CuPp(b) appears at

417 nm and the Q bands at 540, 575 nm. The above findings can

be explained by considering that the symmetry of porphyrin

ring increases when the hydrogen ions of N–H is replaced by

Cu(II).

The mass spectra of the porphyrins H2Pp(a) and H2Pp(b)

showed the negative charged [M�H]� species at 1214(M � 1)

amu, and the copper porphyrins, CuPp(a) and CuPp(b), showed

the positively charged [M+H]+ adducts at 1277(M + 1) amu. 1H

NMR and FT-IR spectra were also consistent with the structure

of the isolated H2Pp(a), H2Pp(b), CuPp(a) and CuPp(b).

We also remark that the main difference between these two

kinds of porphyrins or copper porphyrins which we synthesized

is the position of the of 3-phenoxy-propoxyl substituent

(Fig. 1). In particular, H2Pp(a) and CuPp(a) can be considered

as para- (3-phenoxy)-propoxy substituted 5,10,15,20-tetra-

phenyl porphyrin (TPP); different from H2Pp(b) and CuPp(b),

that are orto- (3-phenoxy)-propoxy substituted TPP. The

peculiar molecular features permitted us to investigate how

the different o- or p- substitution of the sensitizers influence the

photoreactivity of the differently impregnated TiO2 photo-

catalysts.

The novel photocatalysts were designed by considering the

size of the porphyrin molecules and the surface area of the TiO2

particle.

The optimal amounts of H2Pps and CuPps were calculated

by considering these molecules in a square flat geometry with

the side of the square equal to 2.2 nm; consequently, a molecule

would occupy an area of ca. 2.2 � 2.2 = 4.84 nm2.

Scheme 1. Synthesis of the metal-free porphyrins and copper(II) porphyrins.

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226222

Being the surface area of TiO2 equal to 54 m2/g, the calculated

amount of H2Pps or CuPps needed to impregnate the surface of

1 g of TiO2 with a pseudo monolayer of molecules can be

calculated as: (54/4.84 � 10�18)/6.02 � 1023 = 18 mmol.

Different photocatalysts were prepared by impregnating

TiO2 with various amounts (6, 12, 18, 22 mmol/1 g TiO2) of the

sensitizer H2Pp(a), H2Pp(b), CuPp(a) or CuPp(b) by taking into

account that the optimal value is 18 mmol/1 g TiO2.

3.2. Photoreactivity experiments

Both H2Pps and CuPps showed a photocatalytic efficiency

under visible light for the degradation of 4-NP in aqueous

suspension. In Fig. 3 the absorbance of 4-NP versus irradiation

time was reported when TiO2-H2Pp(a) was used as the

photocatalyst. It can be noticed that the main absorbance peak

(ca. 315 nm) almost disappears after 400 min.

Fig. 3. 4-NP absorbance vs. irradiation time in the presence of TiO2-H2Pp(a).

The photoreactivity results obtained by using samples with

different amounts of H2Pp(a), H2Pp(b), CuPp(a) and CuPp(b)

impregnating the TiO2 surface are reported in Fig. 4.

It can be observed that the most efficient photo-degradation

was obtained for an optimal amount of porphyrin equal to

18 mmol per 1 g TiO2.

The porphyrins impregnated onto TiO2 are able to extend the

light absorption efficiency and this depends on the amount of

porphyrin loaded onto the TiO2 surface.

The most efficient photocatalysts, then, required ad hoc

amount of the sensitizers in order to achieve the condition of

pseudo monolayer onto the TiO2 surface. The deficient

coverage of the surface cannot efficiently extend the TiO2

light absorption in the visible light range. On the other hand, the

Fig. 4. Comparison of the photocatalytic activities in terms of degradation of 4-

NP vs. different impregnation ratio H2Pp (or CuPp)/TiO2 after 200 and 400 min

of irradiation time.

Fig. 5. 4-NP concentration vs. irradiation time using different photocatalysts. Fig. 6. Changes in pH of the solution during the experiment carried out using

the 18-TiO2-CuPp(b) sample.

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226 223

over coverage of the 54 m2/g TiO2 surface produced the

formation of multi-layered aggregates with the consequent

detrimental effects for the photocatalytic activity.

Moreover the photocatalytic activities of TiO2-H2Pp(a), TiO2-

H2Pp(b), TiO2-CuPp(a), TiO2-CuPp(b) impregnated with the

optimal value of 18 mmol/1 gTiO2 were compared (see Fig. 5).

However, we also remark that the determination of this

calculated value does not exclude a priori the possibility to have

some extent of p–p interactions which can be also influenced

from the orto or para peripheral substitution of TPP structure.

It can be noticed that the photocatalytic activity decreases in

following order:

TiO2-CuPpðbÞ > TiO2-CuPpðaÞ > TiO2-H2PpðbÞ> TiO2-H2PpðaÞ

The initial zeroth order reaction rates [29] for 4-NP

disappearance per used mass of the catalysts (r0), per square

Table 1

List of the samples used together with the total coverage areas due to the porphyrin

200–400 min of irradiation time

Samplesa Calculated total

areas (m2)b

r0 � 109

(mol L�1 s�1)

r00

(m

TiO2 1.61 1.

18-TiO2-H2Pp(a) 57 13.60 12.

6-TiO2-CuPp(a) 19 17.86 16.

12-TiO2-CuPp(a) 38 19.02 10.

18-TiO2-CuPp(a) 57 24.51 22.

22-TiO2-CuPp(a) 70 19.82 18.

18-TiO2-H2Pp(b) 57 22.70 21.

6-TiO2-CuPp(b) 19 28.97 26.

12-TiO2-CuPp(b) 38 33.27 30.

18-TiO2-CuPp(b) 57 38.90 36.

22-TiO2-CuPp(b) 70 31.25 28.

r0: The initial photoreaction rates per used mass.

r00: Initial photoreaction rates per used mass and per unit surface area of the catalystsa The numbers before the code used for identifying the samples indicate the mmol ab The calculated areas refer to 1 g of TiO2. The surface area for a single porphyrin m

onto the TiO2 surface.

meter of powder (r0’) and the conversion percentage of 4-NP

after 200 and 400 min are reported in Table 1.

TiO2-H2Pp(b) and TiO2-CuPp(b) samples exhibited the

highest photoactivities compared with those corresponding to

the TiO2-H2Pp(a) and TiO2-CuPp(a) samples.

The result of a typical experiment carried out by using the

most active photocatalyst, i.e. 18-TiO2-CuPp(b), indicated that

99.1% of 4-NP disappeared after 400 min of irradiation. As far

as total organic carbon measurements are concerned, a decrease

of 17 and 46% (with respect to the initial value measured for 4-

NP) were observed after 200 and 400 min, respectively. The pH

value of the solution versus irradiation time, monitored during

the photocatalytic experiments, is reported in Fig. 6.

pH value decreased during irradiation and reached the

minimum value of 3.95 after approximately 300 min. This

could be due to the production of acidic species (presumably

carboxylic acids) deriving from the degradation of 4-NP. For

irradiation times longer than 300 min, the pH value increased

molecules, the initial photoreaction rates and the conversion (%) of 4-NP after

� 109

ol L�1 s�1 m�2)

4-NP (%) converted

200 min

4-NP (%) converted

400 min

48 2.79 9.19

59 42.18 89.66

54 39.35 74.41

56 50.69 84.53

69 54.84 97.98

35 46.69 71.33

02 48.08 94.94

83 51.75 80.51

81 58.63 89.93

02 65.17 99.11

94 53.44 74.20

. (The BET specific surface areas of all the samples are equal to ca. 54 m2 g�1).

mounts of sensitizer [H2Pp(a), H2Pp(a), CuPp(a) or CuPp(a)] per gram of TiO2.

olecule was approximated to 4.84 nm2/mol. The porphyrins were supposed flat

Fig. 7. Experiments carried out using recycled 18-TiO2-CuPp(b) as the photo-

catalyst.

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226224

up to 5.14 (recorded after 400 min), while the TOC decrease

was 17 and 46% after 200 and 400 min, respectively.

Consequently, we presume that the pH increase was due to

mineralization of part of the intermediates.

As already reported in the literature, the photocatalytic

activity strongly depends on the molecular structures of

porphyrins as well as their level of aggregation [30,31].

Moreover different macrocycles, metals, axial ligands influence

the photoreactivity of the hybrid sens/TiO2 photocatalysts

[24–26,32,33].

Due to the lower symmetry of the orto-substituted

porphyrins H2Pp(b) and CuPp(b), compared with the para-

substituted ones, H2Pp(a) and CuPp(a), a minor extent of

aggregation can be hypothesized along with a less important

intermolecular p–p interactions.

Consequently, a higher ability to trap and activate O2 could

be due to a less significant presence of intermolecular

interactions between porphyrins in the condition of pseudo

monolayer, increasing in such a way the amount of photo-

catalytically active molecules.

3.3. Stability of photocatalysts

Different kinds of investigation were carried out in order to

establish if the porphyrins sensitized TiO2 photocatalysts were

photostable, i.e. if some decomposition or chemical modifica-

tion of the supported sensitizer took place under the same

conditions used during the photocatalytic experiments. No

significant release of organic degradation compounds even after

long irradiation times (5–7 h) was found and also the supported

sensitizers H2Pp(a), H2Pp(b), CuPp(a) or CuPp(b) can be

recovered quantitatively (and unchanged) from the TiO2

surface by extraction with chlorinated solvents (CHCl3 or

CH2Cl2). The absence of structural modifications was

confirmed by analytical and spectral data (TLC, IR, UV–vis,1H NMR).

It has been confirmed that these porphyrins supported onto

TiO2 showed good stability under irradiation conditions and

they continued to maintain good photocatalytic activity also

after several cycles. In particular, Fig. 7 shows how the most

active photocatalyst, i.e. 18-TiO2-CuPp(b), can be recycled six

times without significant loss of activity. A slight decreasing of

photoactivity of the catalyst after each cycle is due to a little

loss of catalyst during the separation process of the solution and

its recover for the further experiment.

3.4. Process features and mechanistic aspects

The mechanism of sensitized TiO2 based processes only

recently has began to be critically discussed both under UVand

visible light irradiation [10,25,29].

Classically, UV light is needed in order to produce powerful

oxidants species like hydroxyl (�OH) or other radicals (�O2�)

[34–36] necessary to have a highly efficient degradation

process in the aqueous solution (Eqs. (1)–(3) [37]).

TiO2þ hn ! TiO2ðe�CBþ hþVBÞ (1)

hþVBþOH� ! �OH (2)

e�CBþO2 ! �O2� (3)

The excited state electrons and valence band holes play

important roles in the photo-degradation processes. Never-

theless TiO2 can only be excited in the UV light; so, there is

some different photo-degradation mechanism in the visible

light.

Recently, the attention on photosensitizers has been focused

on porphyrins and their analogues because their presence in

natural systems makes them ideal candidates to be used in

biomimetic systems. The biological activity of the porphyrins

as photosensitizers is related to their ability to absorb

wavelengths in the UV–vis range. The Soret band in the blue

and the Q-band(s) in the red are major bands, which represent

important components of sunlight.

Sensitizer excitation is generally achieved via one photon

transition (hn) between the ground state [Pp] and the

singlet excited state 1[Pp]*. Relaxation of the singlet state

yields the lowest excited state and through a process of

intersystem crossing generates the sensitizer triplet state,3[Pp]* (Eq. (4)).

The lifetime of the triplet state is longer (ms) than that of the

singlet state (ns) allowing this excited state to react in one of the

two ways, defined as Types I and II mechanisms.

(Type I mechanism involves hydrogen-atom abstraction or

electron transfer between the excited sensitizers and the

substrate, yielding free radicals. These radicals can react with

oxygen to form an active oxygen species such as the superoxide

radical anion. In a Type II mechanism, singlet oxygen is

generated via an energy transfer process during a collision of

the excited sensitizer with triplet oxygen).

However, it cannot been excluded under visible light

irradiation the formation of a porphyrin excited state acting as

a sensitizer that produces electrons (Eq. (5)), some of which

can be transferred into the conduction-band of the TiO2

(Eq. (6)).

C. Wang et al. / Applied Catalysis B: Environmental 76 (2007) 218–226 225

The excited electron can be trapped by adsorbed O2 onto the

TiO2 surface (Eq. (7)).

½Pp� þ hn! 1½Pp�� �!intersystem crossingðiscÞ3½Pp�� (4)

1 or 3½Pp�� ! ½Pp�þ þ e�CB (5)

TiO2þ e�CB ! TiO2ðe�CBÞ (6)

TiO2ðe�CBÞ þ O2 ! TiO2þ �O2� (7)

The results reported in this paper indicate that an optimal

combination of the surface area of the used TiO2 with the

amount and the chemical structure of porphyrin supported onto

its surface is necessary to achieve the optimum of photo-

reactivity.

It is necessary, moreover, to photoexcite both the

components of the system, i.e. TiO2 and the sensitizer, in

order to obtain a substantial improvement of the photo-

degradation rate of 4-NP. Indeed the beneficial effect on the

photoreactivity is due to a cooperative mechanism, in accord

with the literature [29,38].

In addition, the formation of singlet oxygen [1O2(1D)],

promoted by photoexcited sensitizers 3[Pp]*, although difficult

to be detected in the case of heterogeneous senitizers/TiO2

hybrid systems, may not be excluded [25,33].

Cu(II) species, of course, coordinated to the porphyrin plays

a very important role in the electron transfer process. As well

known, copper ions are present ubiquitously throughout the

literature as excited state quenchers [39]. With the partially

filled d orbitals, Cu(II) ions are capable of fluorescence

quenching by electron or energy transfer. Furthermore, Cu(II),

with its d9 valence electron configuration, is paramagnetic,

which has be shown to increase the quenching efficiency of the

metal ion [40]. As expected, the triplet lifetimes of Cu

complexes will be short-lived due to the paramagnetic nature.

On the other hand, the photoproduced electrons according to

Eqs. (8) and (9), could reduce Cu(II) to Cu(I) (Eq. (10)) also

under visible light irradiation [41,42]. Cu(I) can be reoxidized

to Cu(II) by dioxygen species or by hydrogen peroxide

produced in solution (Eqs. (11)–(13)).

TiO2½CuðIIÞPp� þ hn ! TiO2½CuðIIÞPp�� (8)

TiO2½CuðIIÞPp�� ! TiO2½CuðIIÞPp�þ þ e�CB (9)

TiO2½CuðIIÞPp� þ e�CB ! TiO2½CuðIÞPp� (10)

TiO2½CuðIÞPp� þ 3O2 ! TiO2½CuðIIÞPp� þ �O2� (11)

TiO2½CuðIÞPp� þ 1O2ð1DÞ ! TiO2½CuðIIÞPp� þ �O2� (12)

TiO2½CuðIÞPp� þ H2O2 ! TiO2½CuðIIÞPp� þ �OH þ OH�

(13)

Both the excited sensitizers and TiO2 give rise to a series of

reactions (sometimes multi-steps reactions) producing reactive

intermediates effective for the photo-degradation process (e.g.�O2�, �OH, 1O2(1D), �O2

�, HO2�, H2O2).

It should be stressed that the surface area of the

photocatalyst strongly influences the occurrence of the

photo-process as a higher surface area of the support enables

to have a monolayer containing higher quantity of sensitizer

and thus a more significant positive influence of the latter on the

photoreactivity can be observed.

4. Conclusion

Two novel porphyrins [H2Pp(a), H2Pp(b)] and their copper(II)

complexes [CuPp(a), CuPp(b)] were synthesized along with the

corresponding impregnated TiO2-based photocatalysts.

Both the metal-free and the metal porphyrins prepared not

only can extend the light absorption of TiO2 into visible region

but also can effectively photo-degrade 4-NP in aqueous

suspension under visible light. The photocatalytic activity

increased in the order TiO2, TiO2-H2Pp(a), TiO2-H2Pp(b), TiO2-

CuPp(a), TiO2-CuPp(b), and a maximum was obtained with the

samples loaded with a monolayer of sensitizer, i.e. 18 mmol per

gram of TiO2. The use of this sample gave rise to 99.1% of initial

4-NP disappearance after 400 min, although the total organic

carbon removed was only 46% after the same time.

It is worth noting that the studied system results cheap,

efficient and clean because only O2 is needed as the oxidant

under visible light. The porphyrin-TiO2 photocataysts are

stable, harmless and they can be reused with high efficiency.

Acknowledgments

The authors are very grateful to the National Natural Science

Fund of China and the National Science Fund of Shaanxi

Province for the financial support.

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