Applied Catalysis B: Environmental 38 (2002) 309–319

Photocatalytic degradation of 4-nitrophenol in aqueous suspensionby using polycrystalline TiO2 samples impregnated with

Cu(II)-phthalocyanine

Giuseppe Melea, Giuseppe Ciccarellaa, Giuseppe Vasapolloa, Elisa Garcıa-Lópezb,Leonardo Palmisanob,∗, Mario Schiavellob

a Dipartimento di Ingegneria dell’Innovazione, Università di Lecce, via Arnesano, 73100 Lecce, Italyb Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo,

Viale delle Scienze, 90128 Palermo, Italy

Received 5 August 2001; received in revised form 28 February 2002; accepted 14 March 2002

Abstract

In this paper, the preparation of polycrystalline TiO2 samples impregnated with a modified Cu(II)-phthalocyanine (TiO2–CuPc) is reported along with an investigation on the photocatalytic behavior of this system compared with bare TiO2 (bothin the anatase and rutile form) and with TiO2 samples impregnated with not functionalized commercial phthalocyanine(TiO2–CuPc) or with metal free phthalocyanine (TiO2–Pc). The photocatalytic degradation of 4-nitrophenol (4-NP) wasstudied as a probe reaction. The presence of modified CuPc showed to be beneficial only for TiO2 (anatase) while thecommercial not functionalized CuPc also slightly for both TiO2 (anatase) and TiO2 (rutile). The metal free Pc did not showany beneficial influence on the photoactivity. A tentative explanation of the beneficial effect due to the presence of theCu(II)-phthalocyanines both on the initial reaction rate and on the mineralization process is provided by taking into accountintrinsic electronic and physico-chemical properties. © 2002 Elsevier Science B.V. All rights reserved.

Keywords:Phthalocyanine; Cu(II)-phthalocyanine; TiO2 photocatalysts; 4-Nitrophenol photodegradation

1. Introduction

The utilization of TiO2 as catalyst for the pho-todegradation of organic pollutants in water is arelevant topic in view of a possible application in eco-nomically advantageous and environmental friendlyprocesses[1–6]. Indeed, it is well known that thedegradation of organic pollutants in aqueous mediaby irradiation of TiO2 is a very efficient process af-

∗ Corresponding author. Tel.:+39-091-6567246;fax: +39-091-6567280.E-mail addresses:giuseppe.vasapollo@unile.it (G. Vasapollo),palmisan@dicpm.unipa.it (L. Palmisano).

fording the complete mineralization of a large varietyof compounds. Nevertheless, polycrystalline TiO2(rutile), differently from TiO2 (anatase) has shownalmost always a negligible photoactivity in aqueoussystems. However, for the photocatalytic activationof these processes, only the radiation having wave-lengths shorter than 400 nm can be absorbed byTiO2 giving rise to the generation of electron–holepairs:

TiO2 + hν → TiO2(h+ + e−) (1)

Visible light is not sufficiently efficient because lessthan 5% of the energy of the sunlight is in the UV

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-3373(02)00060-7

310 G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319

radiation range. Commercial light sources need pow-erful lamps and this represents the main drawbackfor the application of the photocatalytic method tothe purification of wastewater polluted with organicsubstances.

Studies devoted to the investigation of the activityof TiO2 doped with transition metals[7–9] or theircomplexes working as sensitizers[10–13]have givena strong input to researches in the field of photoelec-trochemistry aiming to improve the efficiency of solarcells [14,15]. Some results have been transferred tothe photocatalytic induced processes[16]. Althoughthe general principles of dye sensitization of wideband-gap semiconductors were already well estab-lished in the 1970s[17], progress in the application ofsuch techniques to light energy conversion has beeninitially very slow, due to the modest light absorptionshown by monolayers of dyes on electrodes of lowsurface roughness.

The main beneficial effect of the presence of anadsorbed or bonded dye onto the surface of a semi-conductor, is due to its capacity to absorb a welldefined range of wavelengths in the visible spectrum ofthe solar light. This collected reserve of energy couldbe successively used to induce the electron transfer inthe conduction band of the semiconductor improvingthe electron-hole pairs separation. Nowadays, com-bined information derived from the research devel-opment associated to the production of nano-crystalsemiconductors with high specific surface areas andfrom the research on the wide range of molecularorganic and metallorganic sensitizers, can be usefulto improve both the photocatalytic efficiency of thesemiconductors and the beneficial effect due to thepresence of dyes[15,18–21].

In recent years, phthalocyanines and porphyrinsthemselves or their supramolecular arrangements havebeen used not only in the sensor field for technolo-gical applications[22,23], but also as homogeneous orsupported catalysts in a variety of oxidation reactionsof organic pollutants through a biomimetic approach[24]. Indeed, these robust and environmental stablecompounds can be considered as biomimetic cata-lysts and their molecular structure is similar to that ofporphyrin derivatives produced by living organisms(chlorophyll, eme-structure). Moreover, phthalocya-nine compounds are made soluble in a variety of sol-vents by appropriate peripheral substitution. To-date,

unfortunately, no information is available about theirbio-degradation and potential toxicity but an indis-criminate dispersion in the environment when theyare used as sensitizers of various supports could beavoided by separating them by dissolution in someorganic solvent after the use of the (photo)catalysts.

An intriguing goal of academic and industrialresearches is to obtain new photocatalytic systemswith an enhanced activity compared with the simplyTiO2 catalyzed processes.

The photocatalytic activity of a Co(II)-phthalocya-nine chemically bonded to the surface of TiO2 hasbeen reported[25], but only few cases concerningthe possible application of similar systems to the pu-rification of wastewater are reported in the literature[26–28].

Additional studies to interpret the role of thesespecies in synergism with semiconductor oxides inphotocatalysis by taking into account the electronicand physico-chemical properties both of phthalocya-nine and of the semiconductor itself deserve to becarried out.

In this paper, a set of polycrystalline TiO2 (anataseor rutile) samples loaded with a modified or with anot functionalized commercial Cu(II)-phthalocyaninehas been prepared and tested for a probe reaction, i.e.4-nitrophenol (4-NP) photo-oxidation carried out inthe aqueous system[29].

The photoreactivity observed in the presence ofthese samples was compared with that of the corre-sponding bare supports and of samples loaded withthe metal free phthalocyanine.

Moreover, some bulk and surface characterizationsof the used photocatalysts have been performed by de-termining their crystalline phases by X-ray diffraction(XRD), their specific surface areas by BET methodand recording their diffuse reflectance spectra inair.

2. Experimental

2.1. Materials, preparations and measurements

The used water was purified by a Milli-Q/RO sys-tem (Millipore) resulting in a resistivity (ρ) of lessthan 10 M� cm. Melting points were determined onan electrothermal apparatus. The1H and 13C-NMR

G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319 311

spectra were recorded on a Bruker AC-200 at roomtemperature and chemical shifts are reported relativeto tetramethylsilane, Me4Si. The proton chemicalshift ��/� is reported in ppm with respect to thereference frequency of Me4Si, δ (ppm) = (νsample−νreference)/(νreference× 106).

IR and MS spectra were performed, respectively,on Perkin-Elmer 683, and Hewlett-Packard GC/massMSD 5971 instruments.

Mass spectrometry analysis were performed us-ing an LC mass spectrometer 1100 series (Agilent)equipped with an atmospheric pressure chemical ion-ization (APCI) interface. The samples, dissolved inchloroform added with a drop of trifluoroacetic acid,as source of proton, were introduced into the massspectrometer injected by an auto sampler using aseluant solution acetonitrile/water (80/20) at a flowrate of 0.5 ml min−1. A heated nebulized spray wascontinuously introduced into a point corona dischargeregion using nitrogen to nebulize and sheath the liq-uid inlet. Ions were extracted via a heated capillaryto a skimmer lens arrangement at reduced pressureand transferred by an octapole to the main analyticalquadrupole assembly. The instrumental conditionswere as follows: drying gas (nitrogen) at 13 l min−1;nebulizer pressure of 60 psi; drying gas temperatureof 350◦C; vaporizer temperature=500◦C; capillaryvoltage of 3000 V; corona current of 4.0�A; massrange of 500–2000 amu.

2.1.1. Synthesis of the 4-[2,4-bis-(1,1-dimethyl-propyl)-phenoxy]-phthalonitrile (3)

The 4-[2,4-bis-(1,1-dimethyl-propyl)-phenoxy]-ph-thalonitrile (3) was prepared following a procedurereported in literature[30]. Measured 5.5 g (0.04 mol)of finely ground anhydrous K2CO3 was added grad-ually (1.1 g at intervals of 0.5–1 h) under N2 at-mosphere to a stirred solution of 7.0 g (0.03 mol)2,4-bis-(1,1-dimethyl-propyl)-phenol (1) and 5.2 g(0.03 mol) 4-nitrophthalonitrile (2) dissolved in 170 mlof dry DMSO. The reaction mixture was stirred andafter 1 day it was filtered, added to 100 ml of water, ex-tracted with CH2Cl2 and dried on anhydrous sodiumsulfate. The crude product of the reaction, obtained af-ter evaporation of the solvent, was further purified bycolumn chromatography (silica, CHCl3) and recoveredin 90% yields; mp= 89–90◦C. 1H-NMR (CDCl3)δ: 7.72 (d,J = 8.6 Hz, 1H), 7.40–7.15 (m, 4H), 6.75

(d, J = 8.4 Hz, 1H), 1.68 (q,J = 7.5 Hz, 2H), 1.65(q, J = 7.4, 2H), 1.31 (s, 6H), 0.70 (t, J = 7.4, 3H),0.64 (t, J = 7.5, 3H) ppm.13C-NMR (CDCl3) δ:161.9, 149.8, 146.8, 139.0, 135.3, 127.1, 125.2, 121.5,121.4, 120.7, 117.4, 115.4, 115.0, 108.2, 38.4, 37.8,36.9, 34.2, 28.4, 28.1, 9.3, 9.0 ppm. IR (CHCl3) 2966,2933, 2877, 2803, 2438, 1600, 1500, 1250 cm−1;m/e (%) 360 (7), 345 (2), 332 (25), 331 (100), 303(2) cm−1.

2.1.2. Synthesis of the Cu(II) tetrakis[4-(2,4-bis-(1,1-dimethyl-propyl)-phenoxy)]phthalocyanine(CuPc)

A solution of 4-[2,4-bis-(1,1-dimethyl-propyl)-phenoxy]-phthalonitrile (3.6 g, 0.01 mol), 1,8-diazabi-cyclo [5.4.0] undec-7-ene (DBU) (1.5 g, 0.0098 mol),and CuCl2 (0.478 g, 0.0028 mol) in 25 ml of absoluteethanol was refluxed for 24 h under N2 atmosphere.The organic layer enabled the separation of a bluesolid which was purified by chromatography (silica,toluene) and gave a mixture of isomers of the Cu(II)-tetrakis [4-(2,4-bis-(1,1-dimethyl-propyl)-phenoxy)]phthalocyanine in 60% yields. Their structure wasconsistent with the following characterizations:1H-NMR (CDCl3) δ: 7.89–7.87 (m, 6H), 7.85–7.70(m, 6H), 7.68–7.50 (m, 8H), 2.17–2.03 (m, 12H),1.58–1.30 (m, 4H), 1.28–1.00 (m, 6H) ppm. IR (ATRsystem) 2963, 2920, 2874, 1617, 1473, 1398, 1233,1090 cm−1; UV–VIS (CHCl3) λmax 287, 339, 391,617, 686 nm. APCI–MS calc.M 1504 amu, obs.(M−H+) 1505 amu.

2.1.3. Synthesis of the tetrakis[4-(2,4-bis-(1,1-dimethyl-propyl)-phenoxy)]phthalocyanine (Pc)

A mixture of 3 (1.080 g, 3.00 mmol) and hydro-quinone (0.088 g, 0.80 mmol) was intimately poundedin a mortar and put it in a Pyrex tube. The mixturewas fused by gentle heating, cooled and the Pyrex tubewas sealed under vacuum and reacted at 473 K for24 h. The crude blue–green solid was purified by col-umn chromatography (silica, toluene) and recoveredin 50% isolated yields.

A similar procedure, alternative to that previouslydescribed, was used with CuCl2 powder instead ofhydroquinone at 563 K for 24 h. The structure of themixture of isomers was consistent with the followingcharacterizations:1H-NMR spectra was very similarto that observed for CuPc except for the characteristic

312 G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319

broad signal at−2.00 ppm due to the internal N–H pro-tons. IR (ATR system) 3291, 2962, 2920, 2874, 1617,1473, 1396, 1229, 1090 cm−1. UV–VIS (CHCl3) λmax290, 340, 400, 609, 644, 672, 707 nm. APCI–MS calc.M 1442 amu, obs. (M–H+) 1443 amu.

2.1.4. Preparation of home prepared TiO2 (rutile)and TiO2–CuPc, TiO2–CuPc (not functionalized),TiO2–Pc samples by impregnation

Home prepared TiO2 (rutile) was prepared in anopen reactor by reacting 1 l of titanium trichloride(Carlo Erba RPE) with 0.8 l of ammonia diluted in0.5 l of bidistilled water. The powdered titanium hy-droxide obtained was washed several times in orderto eliminate chloride ions, dried for 24 h at 393 K andheated for 24 h at 1073 K.

The loaded samples used as photocatalysts for thephotoreactivity experiments were prepared by impreg-nating Merck TiO2 (anatase phase, specific surfacearea 7 m2 g−1) or home prepared TiO2 (rutile phase,specific surface area 3 m2 g−1), herein after denotedas TiO2 (anatase) and HP TiO2 (rutile), with variouspercentages of CuPc (0.2, 0.5, 1, 1.5% w/w) or metalfree Pc (1% w/w). The required amounts of CuPc (orPc) were dissolved in 50 ml of CHCl3 (or CH2Cl2)and 10 g of finely ground TiO2 was added to this solu-tion. The mixture was stirred for 3–4 h and the solventwas removed under vacuum.

Some samples were prepared by impregnat-ing Merck TiO2 or rutile TiO2 (Ishihara Sangyo,CR-EL) with a commercial not functionalizedCu(II)–phthalocyanine (Tokyo Kasei Kogyo, CR-EL).The preparation was similar to those above reported,but pyridine was used to dissolve the as received ph-thalocyanine. The impregnated samples, denoted inthe following asX% TiO2 (anatase)–CuPc,X% TiO2(rutile)–CuPc,X% TiO2 (anatase)–Pc andX% TiO2(rutile)–Pc (X% represents the weight percentage ofCuPc or Pc in respect to TiO2), were used for thephotocatalytic experiments without other modifica-tions or treatments. The samples impregnated with thecommercial, not functionalized Cu(II) phthalocyanineare denoted asX% TiO2 (anatase or rutile)–CuPc (notfunctionalized).

It is worth noting that the molar ratio between TiO2and CuPc or TiO2 and Pc in the impregnated samplesis similar because of the low contribution of copper tothe high molecular weight of CuPc. For instance, it is

approximately 1883/1 both in the 1% TiO2–CuPc and1% TiO2–Pc samples.

2.2. Bulk characterization of the photocatalysts

2.2.1. X-ray diffractionX-ray powder diffraction analysis of all of the sam-

ples was carried out at room temperature by a PhilipsPW 1130 generator and PW 1050 goniometer usingNi-filtered Cu K� radiation.

2.3. Surface characterization of the photocatalysts

2.3.1. Specific surface area determination (BET)The specific surface areas were measured by the

single-point BET method using a Flow Sorb 2300apparatus (Micromeritics International Corp.).

2.3.2. Diffuse reflectance spectroscopy (DRS)The spectra were obtained in air at approximately

300 K in the wavelength range 280–500 nm usinga Shimadzu UV-2401 PC spectrophotometer withBaSO4 as the reference material.

2.3.3. Scanning electron microscopy (SEM)SEM observations were performed using a model

505 Philips microscope, operating at 25 kV on speci-mens upon which a thin layer of gold or carbon hadbeen evaporated.

2.4. Photoreactivity experiments

A Pyrex batch photoreactor of cylindrical shapecontaining 0.5 l of aqueous suspension was used. Thephotoreactor was provided with a jacket for coolingwater circulation and ports in its upper section for theinlet and outlet of gases, for sampling and for pH andtemperature measurements. A 125 W medium pres-sure Hg lamp (Helios Italquartz, Italy) was immersedwithin the photoreactor and the photon flux emitted bythe lamp wasΦi = 13.5 mW cm−2. It was measuredby using a radiometer “UVX Digital” leaned againstthe external wall of the photoreactor containing onlypure water. O2 was bubbled into the suspensions forapproximately 0.5 h before switching on the lampand throughout the occurrence of the photoreactivityexperiments. The amount of catalyst used for all of

G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319 313

the experiments was 0.8 g l−1, and the initial 4-NP(BDH) concentration was 20 mg l−1. The initial pHof the suspension was adjusted to 4.0 by additionof H2SO4 (Carlo Erba RPE), and the temperatureinside the reactor was held at approximately 300 K,due to a continuous circulation of water in the jacketaround the photoreactor. The photoreactivity runslasted for 6.0 h. Samples of 5 ml volume were with-drawn from the suspensions every 30 or 60 min andthe catalysts were separated from the solution by fil-tration through 0.45�m cellulose acetate membranes(HA, Millipore). The quantitative determination of4-NP was performed by measuring its absorption at315 nm with a spectrophotometer Shimadzu UV-2401PC. Finally, total organic carbon (TOC) determina-tions were carried out by using a Shimadzu totalorganic carbon analyzer 5000-A. Some selected runs,in the same experimental conditions reported above,were carried out in a Solarbox (CO.FO.ME.GRA.)irradiating with a 1500 W Xe-high pressure lamp(Phillips 15-OF) without and with a filter cuttingmost of the radiations withλ ≤ 400 nm by usinga 100 ml batch photoreactor. The photon flux im-pinging the photoreactor in the absence of filter wasΦi = 1.8 mW cm−2.

Scheme 1.

3. Results and discussion

3.1. Synthesis of the CuPc and Pc

The synthesis of the phthalocyanine CuPc was car-ried out as shown in theScheme 1.

The 4-[2,4-bis-(1,1-dimethyl-propyl)-phenoxy]-phthalonitrile (3) has been synthesized in 90%isolated yield, by nitro-displacement reaction ofthe 2,4-bis-(1,1-dimethyl-propyl)-phenol (1) with1,2-dicyano-4-nitrobenzene (2) in DMSO in the pres-ence of K2CO3; successively, the CuPc was prepared(as a statistic mixture of isomers) by fusion of thephthalonitrile derivative3 and CuCl2 using a proce-dure similar to that described by Snow and Jarvis[30]. A similar procedure, but, using hydroquinoneinstead of CuCl2 was used to prepare the metal freephthalocyanine Pc (in 50% isolated yields).

The CuPc was prepared alternatively in themildest reaction conditions and higher yields (60%),refluxing in ethanol the phthalonitrile derivative3 inthe presence of CuCl2 and 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) for 24 h.

Both the compounds CuPc and Pc were character-ized by1H-NMR, UV–VIS, FT-IR analysis and mass

314 G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319

Table 1Initial reaction rates for 4-NP disappearance per gram (r0) and per square meter (r ′

0) of the photocatalysts along with their specific surfaceareas and crystalline phases

Catalyst Specific surfacearea (SSA; m2 g−1)

r0 × 109

(mol l−1 s−1)r ′0 × 109

(mol l−1 s−1 m−2)

TiO2 (anatase) 7 48 170.2% TiO2 (anatase)–CuPc 7 56 200.5% TiO2 (anatase)–CuPc 7 78 281.0% TiO2 (anatase)–CuPc 7 90 311.5% TiO2 (anatase)–CuPc 7 58 201.0% TiO2 (anatase)–Pc 7 32 11HP TiO2 (rutile) 3 Negligible Negligible0.2% TiO2 (rutile)–CuPc 3 Negligible Negligible0.5% TiO2 (rutile)–CuPc 3 Negligible Negligible1.0% TiO2 (rutile)–CuPc 3 Negligible Negligible1.5% TiO2 (rutile)–CuPc 3 Negligible Negligible1.0% TiO2 (rutile)–Pc 3 Negligible Negligible1.0% TiO2 (anatase)–CuPc (not functionalized) 8 61 22TiO2 (rutile, CR-EL) 8 14 4.40.2% TiO2 (rutile, CR-EL)–CuPc (not functionalized) 8 19 6.00.5% TiO2 (rutile, CR-EL)–CuPc (not functionalized) 8 21 6.61% TiO2 (rutile, CR-EL)–CuPc (not functionalized) 8 10 3.1

spectrometry (MS). The atmospheric pressure chemi-cal ionization (APCI) MS was a very useful analysisfor the determination of the molecular weight of theCu complex and the metal free analogous (seeSection2 for the characterization). In particular, the spectraof CuPc and Pc obtained by using an APCI chamberhave shown MH+ as peak of the molecular ions.

3.2. Photoreactivity experiments

A preliminary investigation was carried out in orderto establish if the CuPc or Pc supported onto the TiO2were photostable, i.e. if some decomposition of thesupported CuPc or Pc occurred under the same con-ditions used during the photocatalytic experiments.

TOC determinations indicated that in the absenceof 4-NP there is no significant release of organicdegradation compounds even after long irradiationtimes (5–7 h) of the supported CuPc and also thephthalocyanine can be recovered quantitatively (andunchanged) from the TiO2 surface by extraction withchlorinated solvents (CHCl3 or CH2Cl2). The absenceof structural modifications for the phthalocyanine re-covered was confirmed by the analytical and spectraldata (1H-NMR, UV–VIS, FT-IR and MS analysis).

In Table 1the initial reaction rates (zero order ki-netics) for 4-NP disappearance per gram (r0) and per

square meter (r ′0) of powder are reported along with

the specific surface areas of the photocatalysts.It can be noted that the rounded specific surface

areas of the loaded samples are identical to those ofthe corresponding bare samples. SEM observations ofall of the samples indicate that the presence of CuPcor Pc does not give rise to significant modification ofthe surface of the particles of the supports.

The XRD diffractograms show only the lines at-tributable to TiO2 (anatase) or TiO2 (rutile), depend-ing on the support and they are not shown for thesake of brevity.

Figs. 1 and 2show 4-NP and total organic carbonconcentrations versus irradiation time for selectedruns performed by using the bare TiO2 (anatase),the 1% TiO2 (anatase)–Pc, the 0.5% TiO2 (anatase)–CuPc, the 1% TiO2 (anatase)–CuPc, the 1.5% TiO2(anatase)–CuPc.

An improvement of photoreactivity in respect tothat shown by the bare support was evident for allof the samples loaded with Cu(II)-phthalocyanine.The most photoactive sample showed to be 1% TiO2(anatase)–CuPc.

As far as the TOC analyses are concerned, onlyslight differences could be found between the loadedsamples and the bare support: after approximately 5 hof irradiation small TOC values (ca. 1–2 mg l−1) were

G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319 315

Fig. 1. 4-NP concentration vs. irradiation time in the presence of (�) TiO2 (anatase); (�) 1% TiO2 (anatase)–Pc; (�) 0.5% TiO2

(anatase)–CuPc; (�) 1% TiO2 (anatase)–CuPc; (�) 1.5% TiO2 (anatase)–CuPc.

Fig. 2. Total organic carbon vs. irradiation time in the presence of (�) TiO2 (anatase); (�) 1% TiO2 (anatase)–Pc; (�) 0.5% TiO2

(anatase)–CuPc; (�) 1% TiO2 (anatase)–CuPc; (�) 1.5% TiO2 (anatase)–CuPc.

316 G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319

Fig. 3. 4-NP concentration vs. irradiation time in the presence of (�) HP TiO2 (rutile); (�) TiO2 (rutile, CR-EL); (�) 0.5% TiO2

(rutile)–CuPc; (�) 0.2% TiO2 (rutile, CR-EL)–CuPc (not functionalized); (�) 0.5% TiO2 (rutile, CR-EL)–CuPc (not functionalized); (�)1% TiO2 (rutile, CR-EL)–CuPc (not functionalized).

found. The 1% TiO2 (anatase)–Pc was the worst sam-ple and the results indicated that the presence of themetal free phthalocyanine on TiO2 surface is detri-mental.

Figs. 3 and 4report 4-NP and total organic carbonconcentrations versus irradiation time for experimentscarried out by using HP TiO2 (rutile), TiO2 (rutile,CR-EL), 0.5% TiO2 (rutile)–CuPc, 0.2% TiO2 (rutile,CR-EL)–CuPc (not functionalized), 0.5% TiO2 (rutile,CR-EL)–CuPc (not functionalized), 1% TiO2 (rutile,CR-EL)–CuPc (not functionalized) samples.

It can be observed that only the presence of notfunctionalized CuPc was beneficial and an optimumof photoactivity was achieved for 0.5% TiO2 (rutile,CR-EL)–CuPc (not functionalized).

Diffuse reflectance spectra in air of the bare TiO2,TiO2–CuPc and TiO2–CuPc (not functionalized) sam-ples recorded in the range 250–800 nm indicate thatall the loaded samples reflect light less significantlythan the bare supports and their absorption in all therange investigated increases by increasing the amountof supported phthalocyanine. InFig. 5, only some se-lected spectra are reported.

Although an improvement of light absorptionin the visible range in principle might favor some

reaction steps of the photocatalytic process, theabove findings cannot be simply invoked to explainthe beneficial effect on the photoreactivity due toCu(II)-phthalocyanines. It has been reported[26] thatthe photosensitization of TiO2 by the excitation ofFe(III)-phthalocyanine can be ruled out because pho-tons with λ ≥ 455 nm showed to be ineffective forthe degradation of many organic molecules in waterin the presence of TiO2 (anatase)–Fe(III)Pc samples.The beneficial effect of Fe(III)Pc was explained inthose cases only by a cooperative activity of Fe(III)Pcand TiO2.

Among the samples prepared by loading TiO2(anatase) with CuPc, the highest photoactivity ob-served for the 1% TiO2 (anatase)–CuPc (seeTable 1andFigs. 1 and 2) is probably due to the achievementof an optimum for the number of supported CuPcspecies and not engaged photoactive sites availablefor 4-NP adsorption. On the other hand, the photoac-tivity of bare TiO2 (anatase) is sufficiently high andthe modification of some physico-chemical surfaceproperties, in particular the occupation of sites byCuPc, could partially balance the beneficial effectdue to its presence when the amount of CuPc be-comes high (see inTable 1 the initial reaction rate

G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319 317

Fig. 4. Total organic carbon vs. irradiation time in the presence of (�) HP TiO2 (rutile); (�) TiO2 (rutile, CR-EL); (�) 0.5% TiO2

(rutile)–CuPc; (�) 0.2% TiO2 (rutile, CR-EL)–CuPc (not functionalized); (�) 0.5% TiO2 (rutile, CR-EL)–CuPc (not functionalized); (�)1% TiO2 (rutile, CR-EL)–CuPc (not functionalized).

Fig. 5. Diffuse reflectance spectra in air (A) TiO2 (anatase); (B) TiO2 (rutile, CR-EL); (C) HP TiO2 (rutile); (D) 1% TiO2 (anatase)–Pc; (E)1% TiO2 (rutile)–Pc; (F) 1% TiO2 (anatase)–CuPc; (G) 1% TiO2 (rutile)–CuPc; (H) 0.5% TiO2 (rutile, CR-EL)–CuPc (not functionalized).

318 G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319

of 1.5% TiO2 (anatase)–CuPc in respect to 1.0%TiO2 (anatase)–CuPc). The lower photoreaction ratefound for 1.0% TiO2 (anatase)–Pc indicates that thepresence of coordinated Cu(II) is essential.

The enhancement of photoreactivity observed in thecase of 0.2% TiO2 (rutile, CR-EL)–CuPc (not func-tionalized) and 0.5% TiO2 (rutile, CR-EL)–CuPc (notfunctionalized) in respect to the corresponding baresupport suggests that electronic factors could play amajor role.

Some hypotheses can be drawn to explain these re-sults by taking into account some physico-chemicaland electronic properties both of TiO2 and CuPc.

The use of CuPc can improve the spatial separationof photoproduced e− −h+ pairs by trapping electronsand/or delocalizing holes. The presence of coordi-nated Cu(II) is essential probably to trap electronsthat would be transferred to adsorbed O2 by meansof favorable kinetic step(s) faster than the direct O2reduction. The formation of O2− is needed to producespecies responsible for the oxidant attack during thephotocatalytic reactions[31–33].

Runs carried out by using 1.0% TiO2 (anatase)–CuPcand TiO2 (anatase) in the presence of a filter cuttingalmost completely the radiations withλ ≤ 400 nmshowed the occurrence of no significant photoactivity,indicating that a direct excitation of the supportedCu(II)-phtalocyanine is not likely in our experimentalconditions according to what reported in the litera-ture for Fe(III) phtalocyanine[26]. When the filterwas removed, the 1.0% TiO2 (anatase)–CuPc sampleshowed a significantly higher photoacivity comparedwith the TiO2 (anatase), thus confirming the experi-ments carried out in the other system (seeTable 1).

In addition, as the aromatic macrocycle enablesa good delocalization of the holes, the so obtainedstabilization of the photoproduced positive chargemakes the electrons more available both for O2 andcoordinated Cu(II).

Alternatively, holes could be trapped by H2O and/orOH(surface) giving rise to a well known set of reactionsaffording the oxidation of 4-NP[5,33].

Nevertheless, these phenomena should probablyconcurrently work in order to achieve an improvementof the photoreactivity.

It is worth noting that pairs recombination rate forTiO2 (rutile) is reported generally to be faster than forTiO2 (anatase)[34,35]and consequently the beneficial

effect of CuPc (not functionalized) could be due tothe modification of this important intrinsic electronicproperty, i.e. to the enhancement of the lifetime ofe− − h+ pairs. The modified CuPc is probably notbeneficial due to the presence of four cumbersome Rgroups (seeScheme 1), whose presence appears tobe a predominant negative factor in the case of TiO2(rutile) with the lowest surface area.

Moreover, it cannot be excluded that CuPc could beresponsible for an increase of concentration of O2

•−,OH• and HO2

• deriving from light-induced and darkprocesses according to the set of reactions (2), (3) and(4):

TiO2 (anatase or rutile)–Cu(II )Pc+ ehν→TiO2 (anatase or rutile)–Cu(I)Pc (2)

TiO2 (anatase or rutile)–Cu(I)Pc+ O2

→ TiO2 (anatase or rutile)–Cu(II )Pc+ O2•− (3)

TiO2 (anatase or rutile)–Cu(I)Pc+ H2O2

→ TiO2 (anatase or rutile)–Cu(II )Pc

+ OH− + OH• (4)

H2O2 derives from a previous dimerization of OH•produced under irradiation during the photocatalyticreaction.

An estimation of the relative importance of thismechanism in respect to the other ones is not easy asOH• dimerization process versus the irradiation time,occurring probably in a different extent on anatase andrutile surfaces, is unknown by a quantitative point ofview.

Finally, in principle it cannot be ruled out, anantenna effect (although not very likely in the experi-mental conditions used) reported for the metal phthalo-cyanines, studied in last few years as target moleculesfor second and third harmonics generation[36,37].

This physical property implies that the frequencyof the emitted radiation is twice or three times theabsorbed ones. Consequently, the photoreaction ratecould be enhanced because photons with energy higherthan that of the impinging ones would be availablenot only for TiO2 but also for 4-NP and its intermedi-ates adsorbed on surface sites close to CuPc. Indeed,the direct interaction between higher energy photonsand these adsorbed species could induce a concurrent

G. Mele et al. / Applied Catalysis B: Environmental 38 (2002) 309–319 319

mechanism of photochemical oxidation on solid sur-faces[38].

Acknowledgements

Authors wish to thank the Ministero dell’Universitàe della Ricerca Scientifica e Tecnologica, Rome andConsorzio Interuniversitario Chimica per l’Ambiente(INCA) for financial support.

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