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Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 59– 67

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h om epa ge: www.elsev ier .com/ locate / jphotochem

nfluence of crystallinity and OH surface density on the photocatalyticctivity of TiO2 powders

gatino Di Paolaa,b,∗, Marianna Bellarditaa, Leonardo Palmisanoa,b,uzana Barbierikovác, Vlasta Brezovác,∗∗

“Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell’informazione, e modelli Matematici (DEIM) Università di Palermo,iale delle Scienze, 90128 Palermo, ItalyConsorzio Interuniversitario La Chimica per l’Ambiente, Via delle Industrie 21/8, 30175 Marghera, ItalyInstitute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského, SK-812 37 Bratislava, Slovak Republic

r t i c l e i n f o

rticle history:eceived 10 July 2013eceived in revised form8 September 2013ccepted 21 September 2013vailable online xxx

a b s t r a c t

The aim of the work was to study the influence of crystallinity and OH surface density on the pho-tocatalytic activity of two commercial and two home-prepared TiO2 powders. The samples werecharacterized by X-ray diffraction (XRD), thermogravimetric analysis (TG) and electron paramagneticresonance (EPR) measurements. The photoactivity of the powders was tested employing the photodegra-dation of 4-nitrophenol (4-NP) and the selective oxidation of 4-methoxybenzyl alcohol (4-MBA) to4-methoxybenzaldehyde (p-anisaldehyde) under UV irradiation. An anti-correlation between oxidant

eywords:hotocatalysisiO2

rystallinityurface OH-groups

power and selectivity of the various samples was found. A higher rate of 4-NP degradation was exhibitedby the most crystalline commercial samples, whereas the highest selectivity toward the synthesis of p-anisaldehyde was obtained in the presence of the least crystalline and most hydroxylated home-preparedpowders.

© 2013 Elsevier B.V. All rights reserved.

PR

. Introduction

Heterogeneous photocatalysis represents an advanced oxida-ion process (AOP) that has attracted large interest for its potentialpplications in the field of air and water remediation [1–3] and,ecently, also for the selective synthesis of chemicals [4–6]. In a typ-cal photocatalytic process, irradiation of a semiconductor produceslectron–hole pairs that are capable of initiating a wide variety ofhemical reactions. This process is influenced by the physical prop-rties of the semiconductor such as crystal structure, surface area,article size and shape, surface hydroxyls content, etc.

TiO2 has been the most frequently used semiconductor since

t is cheap, chemically and biologically stable, and effective underifferent operative conditions [7]. The surface of TiO2 in aqueousolutions is hydroxylated due to the dissociative chemisorption

∗ Corresponding author at: “Schiavello-Grillone” Photocatalysis Group, Dipar-imento di Energia, Ingegneria dell’informazione, e modelli Matematici (DEIM)niversità di Palermo, Viale delle Scienze, 90128 Palermo, Italy.el.: +39 091 23863729; fax: +39 091 7025020.∗∗ Corresponding author. Tel.: +421 2 59325 666; fax: +421 2 59325 751.

E-mail addresses: agatino.dipaola@unipa.it (A. Di Paola),lasta.brezova@stuba.sk (V. Brezová).

010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2013.09.008

of water molecules. In the presence of O2, •OH radicals are themain species involved in the oxidation processes [8–12]. The •OHradicals are produced by the photogenerated valence band holesthrough the oxidation of surface hydroxyl groups or adsorbedwater molecules, as well as by conduction band electrons via thedecomposition of photogenerated superoxide radical anions in theoxygenated aqueous systems.

The role of the surface hydroxyl density has been oftenneglected even if the concentration of OH per surface unit is con-sidered to play a key role in the photocatalytic processes [13–15].Several authors have tried to determine a relationship between thephotocatalytic activity of TiO2 and the surface OH groups amount[16–18]. Sclafani et al. [19] attributed the high activities of lab-made rutile to its large amount of hydroxyl groups on the surface,and proposed that they might trap the holes in the valence bandand enhance the chemisorption of O2 molecules in the conductionband. Simonsen et al. [20,21] found that the amount of adsorbedhydroxyl groups highly influenced the photocatalytic activity ofdifferent TiO2 films tested for the degradation of stearic acid. In situelectron spin resonance measurements revealed that the larger the

amount of surface OH groups, the more likely the indirect oxidationof acetic acid via •OH radicals occurred [16]. Differences in thephotocatalytic reactivity of various lab-made and commercial TiO2samples tested for the degradation of phenol [17,18] and salicylic

6 and P

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aSbvteptocl[csttg

sbts((iot

2

2

nTTooTdw

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0 A. Di Paola et al. / Journal of Photochemistry

cid [18] were correlated with the differences in surface OH groupsoncentration. Du et al. [22] reported that the surface hydroxylroup density was the most important parameter that ruled theelectivity of the photocatalytic oxidation of cyclohexane to cyclo-exanone on various TiO2 samples. A good correlation was also

ound between the amount of anatase hydroxyl groups of rare earthetals-doped P25 samples and the degradation rate of methylene

lue [23].Generally, an increase in the surface hydroxyl content enhances

he photocatalytic activity [14,24,25] even if many other factorss, e.g. the presence of a large amorphous part, can decrease thectivity due to the recombination of the electron–hole pairs [26].

Numerous studies have dealt with the effect of the particle sizend crystallinity on the photocatalytic properties of TiO2 [27–30].mall particle size, high surface area, and high crystallinity haveeen considered as important parameters for a high photoacti-ity [13,27,28], but only few works have thoroughly examinedhe role of the amorphous fraction of the photocatalysts. Ohtanit al. [31] studied the contribution of the amorphous phase to thehotocatalytic activity of amorphous and crystalline anatase mix-ures. Jensen et al. [24,30] determined the absolute crystallinityf various TiO2 samples and attributed the lowest activity of theommercial Hombikat UV100 with respect to the less hydroxy-ated Degussa P25 to the highest percentage of amorphous TiO224]. These results, obtained for the oxidation of chloroform, wereonfirmed by Simonsen et al. [20] for the photodegradation oftearic acid. Recently, Bellardita et al. [32] found a dependence ofhe selectivity for the hydroxylation of phenol and benzoic acid onhe percentage of catalyst crystallinity and the total amount of OHroup present on the catalyst surface.

In this work, TiO2 photocatalysts with different crystallinity andurface hydroxyl density have been studied to find a correlationetween photoactivity and selected physico-chemical properties ofhe samples. In particular, two commercial and two home-preparedamples have been tested for the degradation of 4-nitrophenol4-NP) and the selective oxidation of 4-methoxybenzyl alcohol4-MBA) to 4-methoxybenzaldehyde (p-anisaldehyde, PAA). EPRnvestigations of the hydroxyl radicals produced upon irradiationf the samples have been also carried out to investigate the role ofhe structural characteristics.

. Experimental

.1. Materials

Two home-prepared samples were prepared starting from tita-ium tetrachloride or titanium(IV) oxysulfate as the precursors.iCl4 (Fluka 98%) was slowly added to deionized water (molar ratioi/H2O 1:60; volume ratio 1:10) at room temperature. After ca. 12 hf continuous stirring at room temperature, a clear solution wasbtained. The clear solution was boiled for 0.5 h under agitation.his treatment produced a milky white TiO2 dispersion that wasried in a rotary evaporator at 60 ◦C. The code used for this sampleas HPCl.

20 g of TiOSO4 (Sigma–Aldrich) were added to 90 mL of deion-zed water. After ca. 2 h of continuous stirring at room temperature,

clear solution was obtained. This solution was heated in a closedottle and aged at 100 ◦C in an oven for 48 h. The resultant pre-ipitate was washed by withdrawing many times the supernatantiquid and by adding pure water to restore the initial solution vol-me and to eliminate most of the sulfate ions. The resultant solid

as recovered using a rotary evaporator at 60 ◦C and calcined at

00 ◦C for 10 h. The code used for this sample was HPS.Two commercially available TiO2 powders (Degussa P25 and

erck) were also used.

hotobiology A: Chemistry 273 (2014) 59– 67

2.2. Catalysts characterization

X-ray diffraction patterns of the powders were recorded at roomtemperature by an Ital Structures APD 2000 powder diffractometerusing the Cu K� radiation and a 2� scan rate of 2◦/min.

The crystalline size of the samples was calculated by using theScherrer equation. The specific surface area (SSA) of the powderswas determined in a FlowSorb 2300 apparatus (Micromeritics) byusing the single-point BET method. The samples were degassed for0.5 h at 250 ◦C prior to the measurement by a N2/He mixture 30/70(v/v).

The crystallinity of the samples was evaluated following theprocedure reported by Jensen et al. [30]. XRD diffractograms wererecorded for mixtures of TiO2 and CaF2 (50%, w/w) and the areas ofthe 100% peaks of anatase (1 0 1), rutile (1 1 0) and CaF2 (2 2 0) weredetermined. By comparing the ratio between the areas of anatase(1 0 1) and CaF2 (2 2 0) peaks or of rutile (1 1 0) and CaF2 (2 2 0) peaksto the ratios obtained by using the pure phases (1.25 for anatase and0.90 for rutile), the amount of crystalline and amorphous phasespresent in the samples was determined.

Thermogravimetric analysis was performed by using a PerkinElmer STA 6000 system, in the 30–750 ◦C range in a nitrogen fluxof ca. 20 mL min−1. The temperature program consisted of threesteps: temperature scan from 30 to 120 ◦C at 10 ◦C min−1, 15 minin isothermal condition at 120 ◦C, temperature scan from 120 to750 at 10 ◦C min−1.

2.3. EPR in situ photochemical experiments

All solutions and suspensions were prepared in dried ace-tonitrile (ACN; SeccoSolv® Merck). The stock TiO2 suspensionscontaining 1 mg TiO2/mL were homogenized for two min-utes using ultrasounds (Ultrasonic Compact Cleaner TESON 1,Tesla Piest’any, Slovak Republic). The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO, Aldrich) was distilled beforeapplication and stored at –18 ◦C. The hindered amine 4-oxo-2,2,6,6-tetramethylpiperidine (TMPO) from Merck-Schuchardtwas used as supplied. The concentration of photogeneratedparamagnetic species was determined using acetonitrile solu-tions of 4-oxo-2,2,6,6-tetramethylpiperidine N-oxyl (Tempone;Aldrich) as calibration standards. Dimethylsulfoxide (DMSO;SeccoSolv® Merck) was added to ACN as a co-solvent, sil-ver nitrate (Lachema, Czech Republic) was used as electronacceptor.

The formation of paramagnetic intermediates upon monochro-matic irradiation (� = 365 nm) of suspensions or solutions wasmonitored in situ using an X-band EPR spectrometer, EMX Plus(Bruker, Germany). The samples containing DMPO or TMPO weremixed directly before the EPR measurements, then carefully deae-rated using a stream of argon and under argon transferred to asmall quartz flat cell (WG 808-Q, Wilmad-LabGlass, USA; opti-cal cell length 0.04 cm) optimized for the TE102 cavity (Bruker,Germany). The samples were irradiated at 22 ◦C directly in theEPR resonator, and the EPR spectra recorded in situ during con-tinuous photoexcitation. All the EPR experiments were carriedout at least in triplicate with standard deviation in the rela-tive EPR intensity of ±10%. The irradiation source was a UVLED monochromatic radiator (� = 365 nm; Hönle UV Technology).The value of the irradiance (20 mW cm−2; � = 365 nm) withinthe EPR cavity was determined using a UVX radiometer (UVP,USA). The concentration of photogenerated paramagnetic species

was evaluated from the double-integrated EPR spectra based onthe calibration curve obtained from the EPR spectra of Tem-pone solutions measured under strictly identical experimentalconditions.

and Photobiology A: Chemistry 273 (2014) 59– 67 61

2

2

a(pcaTnetwwmn

2

(1ntPncstwobaubmc

fi(ap(emtRwc

3

3

iwdw

cass

20 30 40 50 60

Inte

nsi

ty (

a.

u.)

2θ

(a)

(b)

(c)

(d)

A

R

AA A

Fig. 1. X-ray patterns of the four TiO2 samples: (a) Merck; (b) P25; (c) HPCl; and (d)

A. Di Paola et al. / Journal of Photochemistry

.4. Photoreactivity experiments

.4.1. 4-Nitrophenol degradationA Pyrex batch reactor of cylindrical shape containing 0.5 L of

queous suspension was used. A 125 W medium pressure Hg lampHelios Italquartz, Italy) was immersed within the reactor and thehoton flux emitted by the lamp was ˚i = 11 mW cm−2. O2 wasontinuously bubbled for ca. 0.5 h before switching on the lampnd throughout the occurrence of the photoreactivity experiments.he temperature inside the reactor was ca. 30 ◦C. The initial 4-itrophenol (BDH) concentration was 20 mg L−1. The amount ofach catalyst guaranteed that nearly all the photons emitted byhe lamp were absorbed by the suspensions. Samples of 5 mL wereithdrawn at fixed intervals of time with a syringe, and the catalystas separated from the solution by filtration through 0.2 �m Teflonembranes (Whatman). The quantitative determination of 4-

itrophenol was performed by measuring its absorption at 315 nm.

.4.2. 4-Methoxybenzyl alcohol oxidationThe experiments were carried out in a cylindrical photoreactor

CPR, internal diameter: 32 mm and height: 188 mm) containing50 mL of an aqueous suspension of 4-methoxybenzyl alcohol atatural pH. The initial 4-MBA concentration was 0.5 mM. The reac-or was irradiated by three external Actinic BL TL MINI 15 W/10hilips fluorescent lamps whose main emission peak was in theear-UV region at 365 nm. The reactor was cooled by water cir-ulating through a Pyrex thimble, so that the temperature of theuspension was about 30 ◦C. The radiation intensity impinging onhe suspension was measured by a radiometer Delta Ohm DO9721ith a UVA probe; the radiation power absorbed per unit volume

f the suspension was about 0.76 mW/mL. Air was continuouslyubbled during the experiments and the lamps were switched ont time t = 0, after 0.5 h from the starting of the aeration. The val-es of substrate concentration before the addition of catalyst andefore the starting of irradiation were measured in order to deter-ine the substrate adsorption on the catalyst surface under dark

onditions.During the photoreactivity runs samples were withdrawn at

xed times and immediately filtered through 0.2 �m membranesHA, Millipore) before analyses. The quantitative determinationnd identification of the starting molecules and their oxidationroducts were performed by means of a Beckman Coulter HPLCSystem Gold 126 Solvent Module and 168 Diode Array Detector),quipped with 3 �m Dionex Acclaim PA2. The eluent consisted of aixture of acetonitrile and 1 mM trifluoroacetic acid aqueous solu-

ion (20:80 volumetric ratio) and the flow rate was 0.8 mL min−1.etention times and UV spectra of the compounds were comparedith those of standards. Standards of 4-MBA and PAA were pur-

hased from Sigma–Aldrich with a purity of >99%.

. Results and discussion

.1. Catalysts characterization

Fig. 1 reports the X-ray diffraction patterns of the catalysts usedn this work. The Merck and HPS samples consisted of only anatase,

hereas Degussa P25 consisted of both anatase and rutile. Theiffractogram of HPCl revealed a badly crystallized anatase phaseith a small amount of rutile.

Table 1 summarizes the main features of the four catalysts. It

an be noticed that the specific surface area (SSA) of HPCl is notice-bly higher than those of the other samples, whereas the crystalliteize is the smallest. The SSA of Merck is the lowest among all theamples, indicating a strong particle agglomeration.

HPS.

P25 is close to being fully crystalline consisting of 72% anataseand 18% rutile with the amount of amorphous part in agree-ment with the value reported by Jensen et al. [24]. Merck ispartly crystalline, with 74% anatase and 26% amorphous material.Among the home-prepared samples, a higher crystallinity percent-age (40%) was found for the sample HPS calcined at 600 ◦C whereas9% was the crystallinity of the sample HPCl warmed at 100 ◦Cfor 0.5 h.

The number of surface OH groups can be determined by vari-ous methods such as infrared spectroscopy [33–36], ion-exchangereaction [34], titration procedures [26,36–38] and nuclear mag-netic resonance [39,40]. Thermal gravimetric analysis is often usedto estimate the number of OH groups on the surface as it isa simple and speed method compared to the other techniques[35–37,40,41].

Fig. 2 shows the thermal analyses of P25 and HPCl. Two clearsteps in weight loss are present in relation to the different OHgroups: the first one at ca. 300 ◦C and the second one at ca. 600 ◦C.According to Mueller et al. [41] the weight losses of hydroxylatedTiO2 samples in the 30–120 ◦C range are due to the physicallyadsorbed water and they mainly depend on residual humidity afterthe preparation of the powders. The losses in the 120–300 ◦C and300–600 ◦C ranges can be related to weakly bonded OH groups andstrongly bonded OH groups, respectively.

Table 2 reports the OH groups weight percentages of the varioussamples, together with the humidity losses. HPS and the two com-mercial samples are less hydroxylated with values ranging between0.1 and 0.78% (w/w) whereas HPCl is much hydroxylated with a OH

total percentage equal to 10.76% (w/w) consistent with its largestsurface area.

62 A. Di Paola et al. / Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 59– 67

Table 1Some structure and surface features of the photocatalysts used.

Sample Phases SSAa (m2 g−1) Crystallinityb (%) Crystallite size (nm)

Merck A 10 74 60P25 A, R 50 72 (A) 18 (R) 25 (A) 33 (R)HPCl A, R 235 7 (A) 2 (R) 5 (A) 2 (R)HPS A 44 40 24

A, anatase; R, rutile.a BET specific surface area.b Percentage of crystallinity.

Scheme 1. Schematic overview of reactive species generation upon photoexcitation ofsuspensions.

Table 2OH groups weight percentage % (w/w) determined by TGA.

Sample Humidity OHweak OHstrong OHtotal

Merck 0.30 0.06 0.04 0.10P25 0.75 0.42 0.36 0.78

3

twiriS(tptT

tt

•DMPO-OH in acetonitrile [43]. The oxygen-centered spin adduct

HPCl 7.99 6.66 4.10 10.76HPS 1.30 0.37 0.35 0.72

.2. EPR experiments

EPR experiments were performed in deoxygenated acetoni-rile TiO2 suspensions in order to minimize the role of exogenousater and oxygen during the photoinduced generation of reactive

ntermediates. An overview of the photoinduced processes occur-ing upon the photoexcitation of titanium dioxide nanoparticlesn oxygenated and deoxygenated acetonitrile is summarized incheme 1. Under the experimental conditions used in our studydried deoxygenated acetonitrile; Ag(I) as an electron acceptor),he oxygen-centered radicals (•OH, HO2

•), as well as hydrogeneroxide and singlet oxygen are produced via reactions of the pho-ogenerated charge carriers (e–, h+) with OH–/H2O adsorbed on theiO2 surface.

The EPR spin trapping technique using DMPO as the spinrapping agent was applied to monitor the generation of reac-ive paramagnetic species upon irradiation of titania suspended

titanium dioxide nanoparticles in the oxygenated and deoxygenated acetonitrile

in deoxygenated acetonitrile. The EPR spectra of the four TiO2samples were analogous, however their intensities were different.Fig. 3 shows the experimental spectra along with their simulationsobtained for HPS in deoxygenated acetonitrile containing Ag(I)as an electron acceptor and DMPO as a spin trap upon contin-uous 365 nm irradiation (total dose 6 J cm−2) and immediatelyafter the irradiation. Under both conditions the EPR signal of•DMPO-OH, characterized with the spin Hamiltonian parametersaN = 1.386 mT, aH

� = 1.211 mT, aH� = 0.079 mT and g = 2.0059, was

dominant [42,43]. The simulation analysis of the experimental EPRspectra obtained upon irradiation revealed also the generation offurther EPR signals attributed to •DMPO-X (relative concentration3% for HPS sample in Fig. 3a) and •DMPO-O2

–/OOH (rel. conc.44%; Fig. 3a). Based on the values of hyperfine coupling constantsand g-value (aN = 0.691 mT, aH

�(2 H) = 0.355 mT; g = 2.0069,the seven-line signal was assigned to •DMPO-X [43], possiblyoriginating from the oxidation of the spin trap itself or •DMPO-OHadduct [43–45]. Spin adduct •DMPO-X possesses a limited stabilityand disappears completely after the irradiation stopping (Fig. 3b).The total decrease of EPR signal integral intensity of about 20%after the photoexcitation stops may be explained not only by thevanishing of •DMPO-X adduct, but also by the lower stability of

•DMPO-O2–/OOH (aN = 1.299 mT, aH

� = 1.036 mT, aH� = 0.132 mT;

g = 2.0058) is on the other hand persistent in applied aproticsolvent even after the radiation cut-off [42], and consequently, its

A. Di Paola et al. / Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 59– 67 63

98.0

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.0

100.2

30 100 200 300 400 500 600 700 750

Temperature (°C)

Wei

gh

t (

%)

ΔY = 0.7531 %

ΔY = 0.4170 %

ΔY = 0.3643 %

(a)

25 100 200 300 400 500 600 74780

82

84

86

88

90

92

94

96

98

100

ΔY = 7.9878 %

ΔY = 6.6565 %

ΔY = 4.0972 %

700

Temperature ( °C)

Wei

gh

t (

%)

(b)

FP

tcrS

F(t(c•

c•

(

TMPO

diamagneti c

TMPO +

h+

OH

H2O / OH

Tempone

paramagnet icEPR signal

O

N

H

O

N

O•

Scheme 2. Proposed mechanisms of TMPO oxidation to nitroxide radical Tempone

ig. 2. Weight percentage losses as a function of temperature for the samples: (a)25 and (b) HPCl.

welve-line EPR signal occurs at higher relative concentration (rel.onc. 58%) in comparison to •DMPO-OH (rel. conc. 42%; Fig. 3b),eflecting so the different stability of the spin adducts [42,43].uperoxide radical anions or hydridodioxygen radicals trapped

(a) (b)

ig. 3. Experimental (solid line) and simulated (dotted line) EPR spectraSW = 6 mT) obtained (a) during continuous irradiation of HPS suspensions con-aining Ag(I) and DMPO in acetonitrile (total dose 6 J cm−2, � = 365 nm) andb) after radiation stopping. (TiO2 concentration 0.2 mg/mL, c0,DMPO = 0.04 M,0,AgNO3 = 0.005 M). Spin Hamiltonian parameters obtained from simulations: (a)DMPO-OH (aN = 1.386 mT, aH

� = 1.211 mT, aH� = 0.079 mT; g = 2.0059; relative con-

entration 53%), •DMPO-X (aN = 0.691 mT, aH�(2 H) = 0.355 mT; g = 2.0069; 3%),

DMPO-O2–/OOH (aN = 1.299 mT, aH

� = 1.036 mT, aH� = 0.132 mT; g = 2.0058; 44%);

b) •DMPO-OH (42%), •DMPO-O2–/OOH (58%).

in the irradiated (� = 365 nm) deoxygenated acetonitrile TiO2 suspensions, alongwith EPR spectrum of Tempone (magnetic field sweep 8 mT).

by DMPO are most probably produced by the interaction of pho-togenerated hydroxyl radicals or holes with hydrogen peroxideproduced in the experimental system as depicted in Scheme 1.

The oxidation of TMPO in irradiated acetonitrile TiO2 suspen-sions has been recently investigated in detail [42]. The resultsobtained pointed on the possibility that the oxidation of TMPOto paramagnetic Tempone (aN = 1.550 mT; a13C(413C) = 0.575 mT ;g = 2.0060) was caused by reactions with photogenerated holes,singlet oxygen and/or hydroxyl radicals. Under the experimentalconditions used here, we excluded molecular oxygen and exter-nal water as potential sources of singlet oxygen, superoxide radicalanions and hydroxyl radicals [46], since all the experiments wereperformed in deoxygenated dried acetonitrile suspensions usingAg(I) ions as efficient acceptors of photogenerated electrons [47].Consequently, we can propose, that oxidation of TMPO to para-magnetic Tempone is performed either via photogenerated holes(oxidation to TMPO•+ followed by reaction with adsorbed hydro-xyls or water molecules), or via hydroxyl radicals (generated mainlyby the interaction of adsorbed hydroxyls and water molecules withholes) as is shown in Scheme 2. As the disproportionation reactionof hydridodioxygen radicals produced in the experimental systemsmay result in the generation of singlet oxygen even under deoxy-genated conditions (Scheme 1), we cannot exclude also the directoxidation of TMPO to Tempone via singlet oxygen [42].

In order to confirm the role of the photogenerated hydroxyl radi-cals in the oxidation of TMPO, a further set of experiments with theaddition of DMSO, acting as an efficient hydroxyl radical scavenger[42,48], into the studied system, was carried out. Fig. 4 illustratesthe significant decrease of intensity in the EPR spectra of photo-generated Tempone (dose 2 J cm−2, � = 365 nm) in the presence ofincreasing amounts of DMSO, which can be explained as a conse-quence of competitive reactions of hydroxyl radicals with DMSO[49].

To compare the photocatalytic activity of the studied TiO2 pow-ders, the suspensions were irradiated by the radiation dose of2 J cm−2 (� = 365 nm), and immediately after the radiation wasstopped, the EPR spectrum of the photogenerated nitroxide radicalTempone was recorded and quantitatively evaluated (Fig. 5).

Sample HPCl exhibited the lowest photoinduced activity mon-itored by EPR, whereas the other three samples, characterizedby different contents of humidity and surface hydroxyls, demon-strated similar ability to generate Tempone under the givenexperimental conditions, in good relationship with their compa-rable anatase crystallite size (Fig. 6a). On the other hand, theconcentration of photogenerated Tempone decreased linearly withincreasing the content of hydroxyl groups on the TiO2 surface

(Fig. 6b).

Photocatalytic transformations of organic compounds on irradi-ated TiO2 photocatalysts are strongly dependent on their ability toadsorb on TiO2 surface and to react with the photogenerated charge

64 A. Di Paola et al. / Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 59– 67

334 335 336 337

DMSO in ACN (vol.%)

0

10

20

60

Magnetic field (mT)

Fig. 4. EPR spectrum of Tempone generated upon irradiation of acetonitrilesc

ch[swiatTinanuavec

Fame

0 20 40 60

10

20

30

40

Tem

po

ne

con

cen

tra

tio

n(

M)

Anatase cry stallit e size (nm)

(a)

0 2 4 6 8 100

10

20

30

40

Tem

po

ne c

on

cen

tra

tio

n(

M)

OHtotal% (w,w)

(b)

uspensions of HPS containing various amounts of DMSO as a co-solvent. (TiO2 con-entration 0.2 mg/mL, c0,TMPO = 0.005 M, c0,AgNO3 = 0.005 M, radiation dose 2 J cm−2).

arriers [1–3]. Consequently, the electron–hole recombination rateas also a substantial impact on the overall photocatalytic process1–3]. Under the experimental conditions used in presented EPRtudy (freshly mixed TiO2 suspensions; experiments completedithin 10–15 min), TMPO oxidation to nitroxide radical Tempone

s influenced not only by the concentration of hydroxyl groupsnd TMPO adsorption on the surface, but also by the fast adsorp-ion of Ag(I) ions [50–52], acting as efficient electron acceptors.he capture of photogenerated electrons on TiO2 surface by Ag(I)ons causing the silver metal deposition in deaerated aqueous tita-ia suspensions was studied previously, and the detailed kineticnalysis confirmed the dominant role of electron–hole recombi-ation rate in this process [50]. Although titania samples treatedpon higher calcination temperatures revealed lower SSA valuesnd lower concentration of adsorbed Ag(I) ions, the metallic sil-

er deposition rate increased and this fact was explained as anffect of improved titania crystallinity coupled with reduction ofrystal defects acting as recombination centers, finally resulting

HPCl HPS Merck P250

10

20

30

40

Tem

pone

conc

entr

atio

n( μμ μμ

M)

TiO2 sample

ig. 5. Tempone concentration monitored after 365 nm irradiation of deoxygenatedcetonitrile suspensions of TiO2 (radiation dose 2 J cm−2). Inset shows the experi-ental EPR spectrum of Tempone (magnetic field sweep SW = 8 mT) generated upon

xposure (TiO2 concentration 0.2 mg/mL, c0,TMPO = 0.005 M, c0,AgNO3 = 0.005 M).

Fig. 6. Dependence of Tempone concentration on: (a) the anatase crystallite size ofthe investigated titania samples; and (b) the total OH weight percentage.

in a smaller probability of electron–hole recombination and bet-ter charge carriers separation [50]. This is in a good agreementwith our results, as we observed the analogous photocatalytic Tem-pone production for samples with comparable crystallites sizes(Fig. 6a), and the lowest value was found for sample HPCl withthe lowest percentage of crystallinity (Table 1). On the other hand,low crystallinity is coupled with high SSA values and increasedconcentration of surface defects [50], which can act as hydroxylgroups/water molecules adsorption sites, consequently sampleHPCl showed the highest OH total percentage, however the lowestphotocatalytic activity monitored by Tempone generation (Fig. 6b).

It can be concluded that the photoinduced oxidation of TMPOto paramagnetic nitroxide radical Tempone in irradiated deoxy-genated acetonitrile suspensions prepared using TiO2 samples withsignificantly different structural characteristics does not dependonly on the number of hydroxyl groups present on the surface ofTiO2 but it reflects the complex properties of each sample withsignificant effect of crystallinity.

3.3. Photocatalytic activity

3.3.1. 4-Nitrophenol degradationThe decomposition of 4-nitrophenol was followed by determin-

ing the concentration of the substrate at various time intervals. The

A. Di Paola et al. / Journal of Photochemistry and P

Table 3Photoreactivity assessment of studied TiO2 samples via 4-nitrophenol degradation(4-NP) or 4-methoxybenzyl alcohol oxidation (4-MBA).

4-NP 4-MBA

Sample r0 × 109 (mol L−1s−1)a tirr (min)b S(max)(%)b

Merck 29.7 45 26P25 42.2 15 28HPCl 7.1 120 92HPS 34.2 38 38

2

dcT

sltoi[ortfmTpp

Fc

a Initial 4-NP photodegradation rate.b Irradiation time and maximum selectivity corresponding to the conversion of

0% of 4-MBA.

egradation rate, r0, was calculated from the initial slope of theoncentration versus time profiles. The r0 values are reported inable 3.

All the powders were active for the photodegradation of theubstrate although the efficiency of HPCl was at least 4–5 timesower than that of the three other samples. As shown in Fig. 7a,here is a good correlation between photoactivity and percentagesf surface OH groups even if the distribution of experimental pointss not optimal. In contrast to other results reported in literature20,24,25], the photoactivity decreases with increasing the amountf surface OH groups. This is not surprising since the activity is alsoelated to the crystallinity (see Fig. 7b), which plays a key role inhe photocatalytic behavior of the various samples. While the sur-ace OH content in P25 and HPS is comparable, P25 sample is much

ore crystalline in comparison to only 40% crystallinity of HPS.he lowest activity of HPS is attributable to its large amorphousart, which allows an enhanced recombination of the electron–holeairs [24,31]. The activity of the Merck sample is lower than that

0

10

20

30

40

50

0 2 4 6 8 10 12

r 0x 1

09

(mol

L-1

s-1

)

OHtotal % (w,w)

(a)

0

10

20

30

40

50

0 20 40 60 80 100

r 0x 1

09

(mol

L-1

s-1

)

Crystallinity (%)

(b)

ig. 7. Initial 4-NP photodegradation rate as a function of (a) total OH weight per-entage and (b) crystallinity of the four TiO2 samples investigated.

hotobiology A: Chemistry 273 (2014) 59– 67 65

of HPS since the effect of its higher crystallinity is counterbalancedby a lower amount of surface OH groups. In the case of HPCl, thepresence of a high number of hydroxyl groups is largely compen-sated by the very low crystallinity percentage and consequently thecorresponding r0 value is very small.

3.3.2. 4-Methoxybenzyl alcohol oxidationHeterogeneous photocatalysis has been also used for perform-

ing selective oxidations. Hydrocarbons were oxidized in aqueousTiO2 suspensions to obtain alcohols and carbonyl compounds in thepresence of artificial irradiation [53] and selective oxidation of alco-hols to the corresponding aldehydes and ketones was performedeither in gas phase [54] or in liquid phase using acetonitrile as asolvent [55,56]. Recently, aromatic aldehydes have been obtainedby the photocatalytic selective oxidation of the corresponding alco-hols in aqueous TiO2 suspensions [4,5,57,58].

The photocatalytic oxidation of 4-methoxybenzyl alcohol inaqueous solution proceeds through two parallel reaction routesactive from the start of irradiation: the first route is a partialoxidation producing p-anisaldehyde and the second one is the min-eralization of 4-MBA to CO2 [5]. The latter undergoes through aseries of reactions taking place over the catalyst surface and pro-ducing intermediates not desorbing to the bulk of the solution. Theprocess performance is followed by measuring the values of alcoholand aldehyde concentration and calculating the substrate conver-sion and the selectivity toward the aldehyde. The conversion X andthe selectivity S of the reaction are defined as:

X = cMBA,i − cMBA

cMBA,i× 100 (1)

S = cPAA

cMBA,i − cMBA× 100 (2)

where cMBA,i is the initial 4-MBA concentration and cMBA and cPAAare the 4-MBA and PAA concentrations after a fixed time of irradi-ation, respectively.

Table 3 reports the values of irradiation time and maximumselectivity to PAA corresponding to the conversion of 20% of 4-MBA on the various samples. It is worth noting that HPCl, the lessactive catalyst for the oxidation of 4-NP, was the best one for theselective oxidation of 4-MBA. At variance, P25, which showed thebest performance for the degradation of 4-NP, exhibited only 27.5%selectivity to PAA.

The selectivity of the samples can be related to the crystallinityand the extent of surface hydroxylation. Fig. 8 shows that theselectivity increases with increasing the amount of surface OHgroups, whereas it decreases with increasing the crystallinity ofthe samples. Apparently, these results are in contrast with thosefound in the EPR experiments or in the 4-NP degradation, butthere is an excellent correlation considering the various exper-imental results. The most crystalline powders degrade both thesubstrate and its intermediates owing to their elevated oxidantpower. The higher is the percentage of amorphous phase, thegreater is the amount of defects and therefore the higher is theprobability of entrapment of the hole–electron pairs, thus reduc-ing the photocatalytic activity. On the other hand, the selectivityis also strictly related to the surface hydroxyl group density, inagreement with the results of Du et al. [22] on the photocatalyticoxidation of cyclohexane to cyclohexanone on various TiO2 sam-ples. A high surface hydroxylation retards the oxidation of thesubstrate since it limits the surface coverage and the hole transfer

to the organic molecule [59]. The anti-correlation between oxidantpower and selectivity is corroborated by the results obtained withHPCl that is both the least crystalline and the most hydroxylatedsample.

66 A. Di Paola et al. / Journal of Photochemistry and P

0

20

40

60

80

100

0 2 4 6 8 10 12

S(m

ax

)%

OHtotal % (w,w)

(a)

0

20

40

60

80

100

0 20 40 60 80 100

S(m

ax)%

Crystallinity (%)

(b)

Fa

4

vtrgtdtootusaoot

A

At(

R

[

[

[

[

[

[

[

[

[

[

[

[

[

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[

[

[

[

[

ig. 8. Selectivity to p-anisaldehyde as a function of (a) total OH weight percentagend (b) crystallinity of the four TiO2 samples investigated.

. Conclusion

The experimental results have shown that both the photoacti-ity for the degradation of 4-nitrophenol and the selectivity towardhe oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde areelated to the crystallinity and the amount of surface hydroxylroups of the four TiO2 samples. EPR measurements revealedhat the photoinduced generation of reactive paramagnetic speciesepended not only on the number of hydroxyl groups present onhe surface of titania, but also on the other structural parametersf the individual samples. It is not possible to ascertain which onef these parameters is prevalent in a photocatalytic process sinceheir influence can be different according to the final desired prod-ct. Home-prepared HPCl was the most efficient sample for theelective oxidation of the aromatic alcohol to aldehyde, but it waslso the least active for the degradation of 4-NP or the oxidationf TMPO to Tempone. On the contrary, the lowest selectivity wasbtained by using the two commercial samples that representedhe least hydroxylated and the most crystalline ones.

cknowledgments

This study was financially supported by the Scientific Grantgency of the Slovak Republic (VEGA project 1/0289/12) and by

he Ministero dell’Istruzione, dell’Università e della Ricerca, ItalyPON01 02257).

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