J. Jpn. Petrol. Inst., Vol. 47, No. 6, 2004

Journal of the Japan Petroleum Institute, 47, (6), 359－376 (2004) 359

[Review Paper]

1. Introduction

Stable organic chemicals are unfortunately present aspollutants in industrial waste waters and in landfillleachates, and can also diffuse into groundwater, andwell and surface water. All such pollutants must beremoved to protect our water resources or to achievedrinking water quality. Therefore, many physical andchemical processes have been proposed over the yearsand are currently being employed for the destruction ofthese pollutants1). During the past 20 years, develop-ment has been tremendous in the field of heterogeneousphotocatalyzed degradation of organic pollutants, asmediated by semiconductors in the presence of light,focusing in particular on mechanistic and kineticaspects2).

The molecular transformations or reactions take placeon the catalyst surface in a heterogeneous photocatalyt-ic system. Semiconductors (e.g. TiO2, ZnO, CdS,etc.) can act as sensitisers for light induced redox proc-esses, due to their electronic structure which is charac-terised by a filled valence band and an empty conduc-tion band3). Absorption of a photon with an energygreater than the bandgap energy leads to the formationof an electron/hole pair. The valence band holes arepowerful oxidants (+1.0 to +3.5 V vs. NHE depending

on the semiconductor and pH), whereas the conductionband electrons are good reductants (+0.5 to −1.5 V vs.NHE). Such redox potentials can easily achieve thecomplete degradation of many organic pollutants.

Titanium dioxide is the semiconductor most widelyemployed as a photocatalyst due to its stability underreaction conditions, low cost, and absence of environ-mental hazards. The fundamental features of the basicmechanistic steps of the photoinduced process occur-ring when TiO2 is irradiated with UV light are summa-rised in Section 2.

This review will describe the molecular phenomena,i.e. adsorption and photoinduced transformations,occurring at the surface of TiO2 in contact with differ-ent types of molecules, representative of the commonclasses of organic chemicals polluting air and water.These phenomena have been studied by IR spectrosco-py, to reproduce the interfacial conditions betweenTiO2 particles and aqueous solutions or wet gaseousstreams occurring in solid/liquid and solid/gas systems,respectively, as described in Section 3.

The degradation pathway followed by the adsorbedmolecules can be influenced by the specific surface fea-tures of the photocatalyst, typically the presence of sur-face centres able to chemically react (not via photo-stimulated processes) with the original pollutant mole-cules and/or intermediates. To account for this aspect,some studies were carried out using two types of com-mercial TiO2 powders, and the differences in surfacestructure and chemical behaviour are described in

Photocatalytic Degradation of Organic Compounds on TiO2 Powders —FT-IR Investigation of Surface Reactivity and Mechanistic Aspects—

Patrizia DAVIT†1), Gianmario MARTRA*, and Salvatore COLUCCIA

Dipartimento di Chimica IFM, Università degli Studi di Torino, Via P. Giuria 7, I-10125 Torino, ITALY

(Received March 4, 2004)

This review examines a series of studies investigating the molecular phenomena occurring on the surface ofTiO2, in the dark and under ultra violet spectroscopy (UV) irradiation, in processes intended to achieve the photo-catalytic abatement of organic pollutants (e.g. phenols, VOCs, acetonitrile) of air and water. The investigationtechnique was fourier transform infrared spectroscopy (FT-IR), applied under simulated operating conditions,augmented with high resolution transmission electron microscopy for the elucidation of the surface structure andmorphology of the TiO2 particles. The results indicate the key role of surface hydroxyl groups as adsorbing/react-ing centres (under UV irradiation) and the possibility that basic surface centres (hydroxyl groups and/or surfaceO2− of the TiO2 lattice) also affect the surface chemical processes.

KeywordsPhotodegradation, Organics, Titanium (IV) oxide, FT-IR, TEM

* To whom correspondence should be addressed.* E-mail: gianmario.martra@unito.it†1) (Present) Sinergos S.r.l., Environment Park, Via Livorno 60, I-

10149 Torino, ITALY

Section 4.

2. Reaction Mechanisms of Photocatalysis on TiO2

Most organic photodegradation reactions utilize theoxidising power of the valence band holes either direct-ly or indirectly. In fact, two principal pathways havebeen proposed for the mineralisation of organic sub-strates or oxidation of inorganic materials. One path-way involves direct oxidation of organic materials bythe holes; the alternative pathway considers the semi-conductor surface species, typically hydroxyl groups,as the primary targets of the photogenerated holes.

The surface of TiO2 is readily hydroxylated when thesemiconductor is in contact with water. If a crystal ofTiO2 is formed, the network of TiIV and O2− comes to anabrupt end at the gas/solid or liquid/solid interface.This leads to TiIV and O2− species at the surface that arecoordinatively unsaturated, that is, there are danglingorbitals (surface states) on the particle surface that caninteract with orbitals of other species present at theinterfaces. Exposure of a naked TiO2 crystal to watervapour or to an aqueous medium causes hydroxylationof the surface by dissociative chemisorption ofmolecular water to satisfy the co-ordination of the sur-face TiIV ions.

Dissociation of water on a pure TiO2 surface formstwo distinctive hydroxyl groups: one OH− group bridgestwo surface vicinal TiIV ions and the other forms a ter-minal TiIV_OH− group with basic character. The TiO2

particle surface also contains the coordinated andphysisorbed water of hydration. Temperature pro-grammed desorption experiments have measured thenumber of surface OH− groups at ca. 7-10 OH−/nm2 forTiO2 at ambient temperature, depending on the type ofcrystal plane examined. Moreover, chemisorbed wateramounts to about 2-3 molecules/nm2. Thus, most ifnot all of the TiIV sites are occupied4),5).

In the absence of light, the particle surface will havespecific electronic characteristics and a specific numberof adsorption sites onto which anions, cations, organics,and other species present can chemisorb or physisorb,reversibly or irreversibly. In the presence of light, thesurface electronic properties will undergo dramaticchanges, altering as well the nature of the adsorptionsites. Thus, different dark adsorption/desorption eventswill occur, and additional new events will take placearising from the photoadsorption/photodesorption equi-libria.

The primary photochemical process, subsequent tonear-UV light absorption by TiO2 particles (wave-lengths < 380 nm), is the generation of conduction-band electron (e−CB) and valence-band hole (h+VB) pairs:

TiO2 + hν TiO2(e−…h+) e−CB + h+VB (1)Physically, the hole that is associated with valence-

bonding orbitals is constrained at the surface or subsur-

face sites in the region where light is absorbed. Thegreater mobility of the electron facilitates its migrationacross the particle. If both charge carriers are presenton the surface, recombination can occur, or they can beultimately trapped by extrinsic traps via interfacialelectron transfer with surface adsorbed electron donors(Dads) and acceptors (Aads), respectively:

h+VB + Dads D+ads (2)e−CB + Aads A−ads (3)Since OH− groups and water are the most abundant

electron donors, the holes are most likely to react withthese species. In fact, in the case of a hydrated andhydroxylated TiO2 surface, hole trapping occurs viaEqs. (4) and (5) to give surface-bound •OH radicals.

[TiIV_O2−_TiIV]_OH− + h+VB

[TiIV_O2−_TiIV]_ •OH (4)[TiIV_O2−_TiIV]_H2O + h+VB

[TiIV_O2−_TiIV]_ •OH + H+ (5)On the other hand, adsorbed O2 molecules are usual-

ly recognised as the most efficient electron trappers,giving O2−• species3),6).

For the oxidation of OH− or H2O to occur, the oxida-tion potential for reactions (4) and (5) must lie above(i.e. be more negative than) the position of the semi-conductor valence band Ev. The semiconductor bandpotentials are a function of pH, with the valence bandof TiO2 in the anatase form at about +2.6 eV (vs. NHE)at neutral pH and varying as −0.059 V/pH unit. Theoxidation potentials for the reactions of Eqs. (4) and (5)remain above Ev (thermodynamically favourable) through-out the entire pH range, with reaction (4) favoured athigh pH and (5) favoured at low pH. Thus, underboth acidic and basic conditions, the oxidation of sur-face-bound OH− and H2O by TiO2 valence band holesto form •OH is thermodynamically possible and expected.

The oxidation potentials for many organic com-pounds are also above the Ev of anatase, so they shouldbe thermodynamically able to interact directly withholes at the photocatalyst surface. However, experi-ments run in water-free, aerated organic solvents havefound only partial oxidation of organics. The com-plete mineralisation to CO2, common in aqueous solu-tions, was not detected, so the presence of water orhydroxyl groups appears to be essential for the com-plete oxidative destruction of organic reactants. Theintermediates detected during the photocatalytic degra-dation of aromatic compounds typically have hydroxy-lated structures, and are consistent with those foundwhen similar aromatics are reacted with a knownsource of hydroxyl radicals, further suggesting that •OHis the primary attacking species5).

The necessity for preadsorbed D and A for efficientcharge carrier trapping emphasises the importance ofadsorption-desorption equilibria in photocatalysis. Inthe case of degradation processes carried out in aque-ous media, these equilibria and the extent of adsorption

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will depend on such factors as the pH of the mediumand the point of zero charge (PZC) for the TiO2 used(for anatase PZC = 6.0-6.4)4), which, in turn, is highlyaffected by the particle environment (nature of ions andionic strength, among others). At pH < PZC, the parti-cle surface is positively charged which should enhanceadsorption of anionic and polar substrates, whereas atpH > PZC the surface charge is negative which shouldfavour adsorption by cationic species.

Conversely, it should be also emphasised that, in theabsence of adsorbed species and of suitable conditions,trapped electrons and holes can rapidly recombine onthe particle surface. To prevent recombination ofholes and electrons, the latter can be effectively scav-enged by preadsorbed (and photoadsorbed) molecularoxygen, to give the superoxide radical anion O2−• (ads),which can be reduced further to the peroxide dianionO22−(ads). Alternatively, surface peroxo-species canbe formed either by hydroxyl radical (hole) pairing orby sequential two-hole capture by the same OH− groupor by dismutation of O2−• 4).

3. Experimental

Details on the specific experimental conditionsadopted in the various reviewed studies are reported inthe original paper. Here, the common, general aspectsof the procedures adopted are summarised.3. 1. Materials

Two types of polycrystalline TiO2 commercial pow-ders have been considered, produced by Merck (anatasephase, BET specific surface area 10 m2･g−1), and byDegussa (P25 type, 80% anatase, 20% rutile, BET spe-cific surface area 50 m2･g−1). Adsorption and pho-todegradation experiments used high purity O2, H2O,CO, CO2 and organics. For these last, details arereported in the section devoted to the various types ofcompounds.3. 2. Methods

For the IR measurements, TiO2 powders werepressed in the form of self-supporting pellets (ca. 20mg･cm−2) and put in a conventional IR quartz cellequipped with CaF2 windows connected to a vacuumline (residual pressure: 1 × 10−6 Torr, 1 Torr = 133.33Pa) which allowed adsorption-desorption experimentsto be carried out in-situ. In general, the TiO2 pelletswere simply outgassed at room temperature, in order tomaintain a full monolayer of hydroxyl groups andwater molecules coordinated to the surface Ti4+ ions,which is expected to correspond the first surface hydra-tion layer in both the gas-solid and liquid-solid sys-tems. The FT-IR spectra (4 cm−1 resolution) wererecorded with a Bruker spectrophotometer equippedwith a MCT detector.

In the photo-oxidation experiments the TiO2 pelletswere irradiated in the presence of O2 (usually 100 Torr)

and H2O (usually 18 Torr) through the quartz walls ofthe IR cell by a medium pressure Hg lamp (Model GNZS Helios Italquartz). A pyrex + water filter wasplaced between the lamp and the cell, which was alsoair cooled.

In the case of the studies reported in Section 4, thesize and morphology of the TiO2 microcrystals weredetected by high resolution transmission electronmicroscopy (HRTEM) carried out with a Jeol 2000EXmicroscope equipped with polar piece and top entrystage. Before introduction into the instrument, thesamples were ultrasonically dispersed in isopropylalcohol, and a drop of the suspension was deposited ona copper grid covered with a lacey carbon film.

Chemical features of the surface centres wereinvestigated by FT-IR spectroscopy of adsorbed COand CO2. To study the surface properties at differentdegrees of dehydroxylation, the samples were first out-gassed at the desired temperature for 1 h, and thentreated in O2 (150 Torr) at the same temperature for 1h, cooled to room temperature in O2 and finally out-gassed. After this procedure the samples appearedwhite, as expected for fully oxidised, stoichiometricTiO2.

4. Lattice Structure and Surface Reactivity ofTiO2

Two different forms of TiO2, rutile and anatase, arecommonly used in photocatalysis, with anatase show-ing a higher photocatalytic activity. The structures ofrutile and anatase can be described in terms of chains ofTiO6 octahedra. The two crystal structures differ inthe distortion of each octahedron and the assembly pat-tern of the octahedra chains7).

Figure 1 shows the unit cell structures of the rutileand anatase forms. Each Ti4+ ion is surrounded by anoctahedron of six O2− ions. The octahedron in rutile isnot regular, showing a slight orthorhombic distortion.The octahedron in anatase is significantly distorted sothat its symmetry is less than orthorhombic. TheTi_Ti distances in anatase are greater (0.379 and 0.304nm vs. 0.357 and 0.296 nm in rutile), whereas the Ti_O

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Fig. 1 Unit Cell Structures of Rutile (left) and Anatase (right)

distances are shorter than in rutile (0.1934 and 0.1980nm in anatase vs. 0.1949 and 0.1980 nm in rutile). Inthe rutile structure, each octahedron is in contact witheight neighbours (four sharing an edge and four sharinga corner). These differences in lattice structures resultin different mass densities and electronic band struc-tures for the two forms of TiO2.

Besides differences in the bulk structure, there aredifferences in the surface structures, resulting from thedifferent morphology of TiO2 particles, which can beimportant.

Two commercial photocatalysts were generallyexamined, TiO2 P25 from Degussa (80% anatase, 20%rutile, BET specific surface area 50 m2･g−1) and TiO2

from Merck (pure anatase phase, BET specific surfacearea 10 m2･g−1).4. 1. Surface Morphology of TiO2 Particles

TEM images of Merck TiO2 showed the presence oflarge particles, with a mean size of ca. 140 nm andcharacterised by both sharp and roundish edges (Fig.2)8). The presence of roundish edged particles pre-vented crystallographic indexing in many cases, whilesharp edges appear parallel to fringes pattern corre-sponding to the interplanar spacing of (101) planes.

TEM images showed that the Degussa P25 photocat-alyst is characterised by the presence of plate-like parti-cles with irregular contours and mean size of ca. 40nm. HRTEM images revealed the presence of twoseries of perpendicular interference fringes on mostparticles (Fig. 3). These fringes are related to the

(110) planes of anatase, actually orthogonal to the(001) face, which corresponds to the face lying in theplane of the image. The analysis of fringe patternsrevealed that, in other cases, the (010) planes are alsoexposed at the microcrystal surface, suggesting that the(001) and (010) planes are the more extended faces ofthe particles and correspond to the more abundant facesexposed at the surface of the microcrystals. Five-coordinated Ti4+ cations, with a single coordinativeunsaturation with respect to the titanium cations in thebulk, are exposed on both these faces, together withoxygen anions bonded to three cations and oxygenanions bridged to two cations.

FT-IR characterisation of the surface active sites wascarried out on both photocatalysts to predict and toexplain the differences in the chemical and photocat-alytic behaviour towards the degradation of the organiccompounds considered in this study.4. 2. IR Spectra of Hydroxyl Groups

Figure 4 shows the IR spectra in the νOH region ofTiO2 P25 (Fig. 4A) and TiO2 Merck (Fig. 4B) in air,which were both significantly more intense than thecorresponding spectra recorded after outgassing atroom temperature and activation at 873 K (Fig. 4 a, band c, respectively), which resulted in extensive dehy-droxylation. In contrast, after outgassing at room tem-perature, an almost complete monolayer of hydroxylgroups and water molecules coordinated to surfacecations persisted9)～11). These conditions are assumedto be representative of the hydration state of the cata-lyst surface during the photo-oxidative process, carriedout in the presence of water vapour at medium temper-atures (413 K)12).

The spectrum of TiO2 P25 (Fig. 4B) was charac-terised by a series of narrow components in the 3800-

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Fig. 2 TEM Micrograph of TiO2 Merck (original magnifica-tion × 80000)

Fig. 3 TEM Micrograph of TiO2 Degussa P25 (original mag-nification × 1000000)

3600 cm−1 domain, including a dominant peak at 3630cm−1 with a series of heavily overlapped shoulders onthe high frequency side due to the stretching mode(νOH) of different types of free hydroxyl groups11),13),14),and by an intense and broad absorption in the 3600-3200 cm−1 range, with two maxima at 3415 and 3260cm−1, resulting from the superposition of the νOH modeof bonded hydroxyl groups and the symmetric and anti-symmetric νOH modes of molecular water coordinatedto Ti4+ ions11),15),16), as cations on the (110) face ofmicrocrystalline rutile17).

The variety of components in the 3800-3600 cm−1

region indicates the great heterogeneity of hydroxylgroups. The origin of this heterogeneity can beascribed to the different types of planes exposed at thesurface of the TiO2 P25 microcrystals and to the pres-ence of sites in defect positions (steps, edges, cor-ners)11),13),14). Interestingly, the Ti4+ ions on the most

abundant, regular faces of TiO2 P25 microcrystals mon-itored by CO adsorption appeared highly homogeneous,whereas at least two types of O2− were suggested onthese faces on the basis of the CO2 adsorption data.Apparently, OH groups, which are formed by dissocia-tion of water molecules on Ti4+_O2− pairs, reflect theheterogeneous character of the system.

In contrast, the IR spectrum of TiO2 Merck wasmuch simpler. A single peak at 3665 cm−1 wasobserved, slightly asymmetric on the low frequencyside, and a weaker broad band was present in the 3500-2800 cm−1 range (Fig. 4A). The main component at3665 cm−1 and the ill resolved shoulder on the low fre-quency side should correspond to the stretching vibra-tion of two types of free hydroxyl groups, indicatinggreater homogeneity of the Ti4+_O2− pairs for this typeof material. Furthermore, the relatively weak intensi-ty of the broad band in the 3500-2800 cm−1 range,mainly due to the symmetric and asymmetric νOH

modes of the coordinated water molecules, indicatesthat very few Ti4+ ions able to coordinate H2O in theundissociated form are present on the surface of theTiO2 Merck particles.4. 3. Lewis Acid and Basic Sites Probed by IR

Spectroscopy of Adsorbed CO and CO2

FT-IR investigation of the adsorption of suitablemolecular probes was used to evaluate the nature of thesites on the “naked” surface of the two photocatalysts,after removal of hydroxyl groups. The features of theLewis acid Ti4+ centres exposed at the surface of thetwo TiO2 powders were investigated by CO adsorptionat 77 K on the photocatalysts previously outgassed at873 K to obtain highly de-hydroxylated surfaces (Figs.5A and 5B)8).

The spectrum of the TiO2 Merck photocatalyst in the3750-3500 cm−1 range before CO admission revealedthe presence of a very weak band at 3670 cm−1, due tothe residual OH groups left after outgassing at 873 K(Fig. 5A, inset, curve a).

After CO admission, this component was completelydepleted and a broad signal at 3575 cm−1 due to COmolecules H-bonded with hydroxyl species wasobserved (Fig. 5A, inset, curve b), but the originalspectrum was fully restored after CO outgassing to 2Torr of equilibrium pressure (Fig. 5A, inset, curve c),indicating the complete reversibility of CO interactionwith OH groups. In the 2300-2100 cm−1 range, athigh CO coverage, a main peak was present at 2156cm−1 (Fig. 5A, curve a), due both to CO moleculescoordinated to the most common types of Ti4+ ions(present on the roundish surface of TiO2 Merck micro-crystals) and hydrogen bonded to hydroxyl groups.By decreasing the CO pressure, this main peak shiftedto 2159 cm−1 and a signal at 2170 cm−1 was observed,which can be assigned to CO molecules coordinated toTi4+ ions on the less common (101) planes (Fig. 5A,

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Fig. 4A FT-IR Absorbance Spectra of TiO2 Merck: (a) in air,(b) outgassed at room temperature for 45 min, (c) out-gassed at 873 K

Fig. 4B FT-IR Absorbance Spectra of TiO2 Degussa P25: (a)in air, (b) outgassed at room temperature for 45 min,(c) outgassed at 873 K

curves b-e).The spectrum of TiO2 Degussa P25 showed the weak

bands at 3720 and 3675 cm−1 before exposure to CO(Fig. 5B, inset, curve a) due to the νOH mode ofhydroxyl groups left on the surface after activation at873 K, which were completely depleted after COadsorption, and a broad absorption at 3570 cm−1 wasobserved (Fig. 5B, inset, curve b). The original spec-tral profile was completely restored by decreasing theCO pressure in equilibrium with the sample to 2 Torr(Fig. 5B, inset, curve c), indicating that completedesorption of CO from hydroxyl groups also occurs inthis case. Correspondingly, in the 2300-2100 cm−1

range, a main sharp peak appeared at 2178 cm−1 at highCO coverage, which can be assigned to parallel COoscillators σ-bonded to the highly acidic pentaco-ordinate Ti4+ cations present on the very extended andregular (010) and (001) planes. The weak signal at2153 cm−1 is assigned to the CO molecules hydrogen-bonded to the OH groups (Fig. 5B, curve a). Bydecreasing the CO coverage, this last peak was com-pletely depleted and the main component at 2178 cm−1

shifted to 2191 cm−1 (Fig. 5B, curves b-e), which cor-responds to the frequency of isolated CO oscillators,pointing to the progressive depletion of adsorbate-adsorbate interactions as the CO pressure decreases.

The greater shift undergone by the main peak causedby decreasing the CO coverage in the case of TiO2

Degussa P25 indicates that the adsorbate-adsorbate

interaction is highly effective for this system, suggest-ing that extended crystal planes are present on the sur-face of the particles. Moreover, the bands at 2156 and2170 cm−1 are both located at higher frequency than thecorresponding signals on the surface planes of TiO2

Merck.The upward shift of the stretching mode of CO mole-

cules adsorbed on surface cations with respect to CO inthe gas phase (2143 cm−1) increases as the Lewis acidstrength of the adsorbing centres increases, and the dif-ference in the position of the stretching band of COadsorbed on the two types of TiO2 powders clearlyindicates that both types of Ti4+ ions exposed on theroundish surfaces and (101) faces of TiO2 Merck havesignificantly lower acidity than those exposed on the(001) and (010) surfaces of TiO2 Degussa P25 micro-crystals.

Since the Ti4+ cationic centres are accompanied byO2− counter-anions, the difference in Lewis acidity willalso depend on the presence of different Lewis basiccentres in the two forms of TiO2, resulting in acid-basecouples with different Lewis acid-base character. CO2

was used as a probe to test the basic character of thetwo photocatalysts surface sites. Very similar resultswere obtained by adsorbing CO2 over samples activatedat 423 and 873 K. For the sake of brevity, only thespectra obtained in the case of the sample outgassed atthe lowest temperatures are reported.

CO2 adsorption on TiO2 Merck previously activatedat 423 K resulted in the spectra reported in Fig. 6A,

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Fig. 5A FT-IR Absorbance Spectra of CO Adsorption at 77 Kon TiO2 Merck in the Presence of: (a) 20, (b) 5, (c) 1,(d) 0.1 Torr CO and (e) after outgassing for 1 min at77 K

Inset: FT-IR absorbance spectra of TiO2 Merck. (a) before COadsorption, (b) in presence of 20 Torr CO, (c) after decreasingCO equilibrium pressure to 2 Torr.

Fig. 5B FT-IR Absorbance Spectra of CO Adsorption at 77 Kon TiO2 Degussa P25 in the Presence of: (a) 20, (b) 5,(c) 1, (d) 0.1 Torr CO and (e) after outgassing for 1min at 77 K

Inset: FT-IR absorbance spectra of TiO2 Degussa P25. (a)before CO adsorption, (b) in presence of 20 Torr CO, (c) afterdecreasing CO equilibrium pressure to 2 Torr.

which also shows the dependence on pressure (curvesa-d). Only the peak at 2347 cm−1 was observed, dueto the stretching mode of CO2 molecules linearlyadsorbed on surface cations, with no significant absorp-tions in the low frequency range8).

In contrast, P25 had a much richer spectrum.Figure 6B shows the spectra after adsorption of anincreasing amount of CO2 on TiO2 Degussa P25 previ-ously activated at 423 K. The most intense peak inthe spectrum was observed at 2351 cm−1 and can beassigned to the stretching mode of 12CO2 linearlyadsorbed on Ti4+ on regular faces of the TiO2 P25microcrystals. The weaker component at 2280 cm−1 is

the corresponding absorption for 13CO2 molecules pres-ent in natural abundance. Moreover, the signal at2351 cm−1 was located at higher frequency than in thecase of TiO2 Merck (2347 cm−1), and since the shift ofthis signal with respect to the gas phase (2343 cm−1)increases together with the Lewis acid strength of thecationic adsorbing sites, this difference agrees wellwith the difference in the Lewis acid strength of thesurface Ti4+ ions of the two TiO2 powders as shown bythe IR spectra of CO adsorbed at 77 K.

At lower wavenumbers, in the 1800-1000 cm−1 range,the presence of a complex spectroscopic pattern indi-cated the formation of various types of carbonate-likespecies, namely mono- (1578 and 1359 cm−1) andbidentate (1672, 1243 and 1053 cm−1) carbonates, dueto nucleophilic attack of O2− anions on the CO2 mole-cules previously linearly adsorbed on adjacent Ti4+ cen-tres. The presence of bands due to carbonate-likegroups indicates the presence of Ti4+_O2− couples, inwhich the anions are nucleophilic enough to react withCO2. Moreover, signals at 1630, 1430, 1408 and 1221cm−1 are the consequence of bicarbonate group forma-tion, due to the reaction between the OH basic speciesand adsorbed CO2 molecules. A similar reactivitytowards CO2 was found for TiO2 P25 activated at 873K (spectra not reported). However, the bands due tobicarbonate-like species were strongly reduced in inten-sity, because of the high degree of dehydroxylationattained, and components due to bidentate carbonatespecies appeared less intense. Significantly, no bandsdue to carbonate-like species were detected for TiO2

Merck, indicating that no O2− centres could react withCO2 linearly adsorbed on adjacent Ti4+ ions on the sur-face of this system.

The differences in the Lewis acid and base characterof Ti4+ and O2− sites, respectively, as shown by theexperiments reported above, are the basis of the differ-ent chemical features of the hydroxyl groups formed byreaction of such centres with H2O molecules, which arenaturally formed at the end of the preparation route,when TiO2 powders are stored in air. For the twotypes of titania powder considered, several studies indi-cated that hydroxyl groups at the surface of TiO2 P25are basic enough to react with CO2 to give bicarbonate-like species or with carbonylic compounds and nitriliccompounds18)～20). The effect of such specific featureson photocatalytic processes are reported in more detailin the sections devoted to the photo-transformation ofthe various types of organic molecules.

5. Photo-transformation of Organic Pollutants

Adsorption (in the dark) and subsequent exposure toUV light have been investigated for various organicmolecules, and the results are described below.

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Fig. 6A FT-IR Absorbance Spectra of CO2 Adsorption onTiO2 Merck Activated at 873 K in the Presence of: (a)0.08, (b) 3.0, (c) 40 and (d) 100 Torr CO2

Fig. 6B FT-IR Absorbance Spectra of CO2 Adsorption onTiO2 Degussa P25 Activated at 423 K in the Presenceof: (a) 0.08, (b) 3.0, (c) 40 and (d) 100 Torr CO2

5. 1. Phenolic CompoundsPhenolic compounds are among the most refractory

pollutants present in industrial waste waters. Theirhigh stability and solubility in water represent the mainobstacles to complete removal. Purification of waste-waters contaminated with these pollutants is very diffi-cult since they are resistant to conventional treatmenttechniques.

The largest use of phenol is as an intermediate in theproduction of phenolic resin. Phenol is also used inthe production of caprolactam, which is used in makingnylon, and Bisphenol A, which is used in makingepoxy and other resins. Moreover, phenol is also usedas a fungicide, as a disinfectant, and in medicinalpreparations such as over-the-counter treatments forsore throats. The International Agency for Researchon Cancer (IARC) does not classify phenol as carcino-genic in humans, but the acidity causes coagulationnecrosis as a result of the desiccating action of the acidon proteins in surface tissues and phenol ingestionresults in cardiac effects because it blocks cardiac sodi-um channels21).

Nitrophenols are considered priority pollutants bythe United States Environmental Protection Agencyand the maximum allowed concentrations range from 1to 20 ppb22). Nitrophenols are widely used in the man-ufacture of explosives, pharmaceuticals, pigments, dyesand rubber chemicals. Among the mononitrophenols,4-nitrophenol is probably the most important in termsof quantities used and potential environment contami-nation. It may be produced in the atmosphere by thephotochemical reaction between benzene and nitrogenmonoxide and has been detected in samples of urbanparticulate matter. Moreover, a number of widelyused pesticides, including parathion, are readily metab-olised to 4-nitrophenol in the human body and arebelieved to be the source of 4-nitrophenol residues inhuman urine23).

The presence and widespread use of chlorinated aro-matic solvents, chemical cleaning agents, biocides,preservatives and pesticides in the environment repre-sents a serious problem because of their toxicity andpotential for accumulation in plants and animal tissues.Soluble chlorinated organic compounds, like chlorophe-nols, have recently shown extended potential to enterthe food chain24). Obviously, the contamination mayarise in various ways: runoff from croplands, effluentsfrom industry, and disinfection of drinking water sup-plies and water treatment with chlorine. In fact, waterwhich is initially free from organochlorine compoundscan become contaminated in the chlorination treatmentprocesses as a result of reactions between the chlorineand naturally occurring humic and fulvic acids in thewater, leading to the formation of chlorinated organics,like chlorophenols25).

5. 1. 1. Phenol5. 1. 1. 1. Phenol Adsorption

Standard gas-solid interaction procedures wereadopted for the FT-IR measurements and self-support-ed pellets of TiO2 were employed for the adsorptionand photodegradation of phenol via the vapour phase.Preliminary investigations had shown that standard gas-solid interaction procedures simulate well the adsorp-tion behaviour in an aqueous suspension, although adefinitely better signal to noise ratio can be obtained inthe solid-gas system26). Polycrystalline TiO2 DegussaP25 was used for the FT-IR measurements and for thephenol adsorption and photodegradation runs.

After phenol adsorption in the presence of 10 Torr ofwater vapour, intense bands were observed at 1590,1480 and 1265 cm−1, together with weaker componentsat 1600, 1490, 1380, 1280, 1240, 1170 and 1160 cm−1

(Fig. 7, curve a). By comparison with the phenolspectra registered in aqueous solution at different pHs,these components can be easily assigned. Themolecular form C6H5OH (prevailing at low pHs) ismainly responsible for the signal at 1380 and 1170 cm−1

and contributes to the broad absorption bands at 1590cm−1 with a band at 1595 cm−1 and a shoulder at 1605cm−1, the two weak bands at 1500 and 1475 cm−1 andthe components at 1265 and 1240 cm−1 (Fig. 7, curveb). Conversely, ionic C6H5O− species (prevalent atalkaline pHs) predominantly contribute to the signals at1585, 1480 and 1270 cm−1. The spectrum of phenateions is also characterised by the presence of a weakercomponents at 1565 and 1150 cm−1, which may alsocontribute to the spectrum of the adsorbed phenol (Fig.7, curve c).

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Fig. 7 FT-IR Absorbance Spectra of (a) Phenol Adsorbed viaVapour Phase on a TiO2 Degussa P25 Pellet, in thePresence of 10 Torr Water Vapour, (b) InternalReflection IR Spectra of 0.1 M Water Solution ofPhenol at pH 3, and (c) at pH 12

The presence of the signals of both undissociatedphenol and phenate species in the spectrum of the phe-nol adsorbed on the catalyst simply outgassed at roomtemperature clearly suggests that different adsorptionprocesses are occurring. The undissociated phenolmolecules are probably stabilised via hydrogen-bond-ing through their functional group, whereas the pres-ence of basic surface hydroxyls may be responsible forthe acid-base reaction which leads to the formation ofphenate-like species18).5. 1. 1. 2. Phenol Photo-transformation

After phenol adsorption, the TiO2 pellets were exposedto UV radiation through the quartz walls of the IR cell,maintaining the photocatalyst in contact with watervapour and O2 during irradiation, to simulate the liquid-solid regime in actual photodegradation systems.

The exposure of the catalyst to UV irradiation forincreasing time intervals leads to important modifica-tions in the spectral pattern. In particular, all signalsdue to adsorbed phenol were progressively depleted,and new components appeared in the spectrum. Morespecifically, after the first irradiation steps, new bandsappeared at 1715, 1645, 1555, 1450, 1325, 1210 and1105 cm−1 (Fig. 8, curve b). Moreover, the complexabsorption in the 1300-1200 cm−1 range evolved withthe formation of two peaks at 1265 and 1275 cm−1, anda shoulder at 1305 cm−1, and the composite band cen-tered at 1485 cm−1 was progressively transformed to asimpler peak at 1482 cm−1.

At the end of the complete photodegradation cycle,the signals due to the adsorbed phenol had almost com-pletely disappeared, pointing to complete mineralisa-tion of the organic substrate. In fact, under these con-ditions, the spectrum was characterised by a new, verybroad and complex absorption in the 1800-1500 cm−1

range, with minor peaks at 1445, 1380 and 1360 cm−1,

mainly due to the presence of carbonate-like species ofvarious types adsorbed on the surface of TiO2 (Fig. 8,curve c).

To understand the reaction mechanism of phenolphotodegradation, an evaluation of the nature and reac-tivity of intermediate species was carried out. Poly-hydroxybenzenes had been previously indicated as pos-sible intermediates of phenol photo-oxidation, due tothe attack of highly active OH radicals at the activatedposition of the aromatic ring. Adsorption via the vapourphase of the three isomeric dihydroxybenzenes wasattempted, to evaluate the influence of the OH group onthe reaction mechanism. The experiments were possi-ble only with pyrocatechol and hydroquinone, and failedwith resorcinol, which had an exceedingly low vapourpressure.

The spectrum of pyrocatechol adsorbed on TiO2 wasvery similar to that obtained at partial photodegradationof phenol. Some components due either to unreactedphenol or to more oxidised species were present, butfurther experiments have shown that pyrocatecholundergoes photodegradation on TiO2, producing car-bonate species and water, as found with phenol.

The spectrum of hydroquinone adsorbed on TiO2 didnot indicate the presence of any species detected at theintermediate stages of photodegradation of phenol.However, this observation may be misleading as hydro-quinone is very reactive on the surface of TiO2. Infact, the results showed that adsorbed hydroquinone isconverted, by contact with O2, to benzoquinone whichis very quickly transformed to carbonate species andwater under UV-Vis irradiation. Therefore, hydro-quinone formed during phenol degradation is unlikelyto be observed as an intermediate on the surfacebecause of its extremely rapid transformation. Thepresence of quinones in the bulk of the water suspen-sion during phenol degradation in batch photoreac-tors27)～29) suggests that part of the degradation productsmay desorb from the catalyst and diffuse into aqueousmedium, far from the reactive centres at the liquid-solidinterface, where quinones would undergo very fastmineralisation.5. 1. 2. Nitrophenol5. 1. 2. 1. Nitrophenol Adsorption

Adsorption and photo-oxidation experiments on nitro-phenols were carried out on the TiO2 Merck photocata-lyst for the ortho-isomer (2-nitrophenol)30), which has areasonably high vapour pressure at room temperature.

Adsorption of increasing amounts of 2-nitrophenolon the photocatalyst outgassed at room temperaturelead to the transmittance spectra reported in Figs. 9Aand 9B. New signals appeared in the spectra in the1600-1200 cm−1 range due to contact with 2-nitrophe-nol vapour with the TiO2 surface (Fig. 9A, curves band c), but the broad band at 1630 cm−1 already presentin the spectrum of the catalyst simply outgassed at

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Fig. 8 FT-IR Absorbance Spectra of TiO2 Degussa P25: (a)after phenol adsorption, (b) after 14, and (c) after 35 hUV irradiation in the presence of O2 (100 Torr) andwater vapour (10 Torr)

room temperature due to the bending mode of wateradsorbed in molecular form (Fig. 9A, curve a) wasmasked by new adjacent components. By comparisonwith the spectrum of 2-nitrophenol in solution the mainbands at 1612, 1475, 1441 and 1331 cm−1 can be assignedto ring modes, the signals at 1270 and 1255 cm−1 to COvibrations, and the components at 1348 and 1520 cm−1

to NO2 symmetric and asymmetric stretching.In the high wavenumber region, the signal due to the

stretching mode of free hydroxyl groups (3680 cm−1)was progressively depleted by contact with increasingdoses of 2-nitrophenol (Fig. 9B, curves b and c), indi-

cating that surface OH groups are certainly involved inthe adsorption of 2-nitrophenol. Simultaneously, anew broad absorption increased in the 3600-3200 cm−1

range, with two superimposed signals at 3125 and 3085cm−1 due to CH stretching.5. 1. 2. 2. Nitrophenol Photo-transformation

After 2-nitrophenol adsorption, oxygen was allowedinto the cell and the sample was UV irradiated. Figure10 shows the spectra corresponding to the catalyst incontact with the highest amount of nitrophenol before(curve a) and after irradiation for 2 h (curve b).

All components due to adsorbed 2-nitrophenol weredepleted after UV irradiation, indicating that the adsorbedspecies were progressively destroyed during the photo-oxidation process, and simultaneous formation of newsignals was indicated. Conclusive identification ofintermediates and by-products was not possible, but atentative reaction mechanism may be suggested, basedon parallel photocatalytic runs carried out in batch reac-tors and the formation of highly oxidised species byreaction with the holes30):

TiO2(substrate)_OHS− + h+ TiO2(substrate)

_OH(ads)•

O2N_C6H4OH(ads) + h+ O2N_C6H4O(ads)• + H+

and/orO2N_C6H4OS− + h+ O2N_C6H4O(ads)•

O2N_C6H4OH(ads) + OH(ads)•

O2N_C6H3(OH)2 other intermediatesCO2 + H2O + H+ + NO2−/NO3−

5. 2. Volatile Organic Compounds (VOCs)Volatile Organic Compounds (VOCs) in the urban

and industrial atmosphere mainly originate from motorvehicle exhausts and other combustion processes utiliz-ing fossil fuels, solvent usage and other industrial proc-esses. The most important emitters among industrial

-H•

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Fig. 9A FT-IR Transmittance Spectra of 2-NitrophenolAdsorbed on TiO2 Merck: (a) TiO2 background spec-trum, (b,c) spectra after adsorption of increasingdoses of 2-nitrophenol

Fig. 9B FT-IR Transmittance Spectra of 2-NitrophenolAdsorbed on the TiO2 Merck νOH Region: (a) TiO2

background spectrum, (b, c) spectra after adsorptionof increasing amounts of 2-nitrophenol

Fig. 10 FT-IR Absorbance Spectra of TiO2 Merck: (a) after 2-nitrophenol adsorption, (b) after 2 h UV irradiation inthe presence of O2 (100 Torr)

processes are petroleum refineries and petrochemicalplants, and their operations are generally associatedwith the emission of various organic compounds intothe atmosphere, mainly originating from the productionprocesses, the storage tanks and the waste areas. Inthe petrochemical industries, most of the organic com-pounds are derived from light petroleum fractions, andessentially from hydrocarbons such as methane, ethane,propane, benzene, toluene and xylene.

Several effects of VOCs are recognised, such asstratospheric ozone depletion, toxic and carcinogeniceffects, and enhancement of the global greenhouseeffect31). Studies on the carcinogenicity of certainclasses of hydrocarbons indicate that some cancersappear to be caused by exposure to aromatic hydrocar-bons in combination with NOx which, in the presenceof sunlight, undergo photochemical oxidation, produc-ing a photochemical smog that is environmentally haz-ardous.

Regulations on controlling organic vapour pollutantsin air have been issued worldwide. The ambient airquality standards of the US Environmental ProtectionAgency require that the maximum 3-h hydrocarboncontent should not exceed 1.6 × 10−4 kg/m3 (0.24 ppm)for more than a year. The recently passed EuropeanCommunity stage emission limit is 35 g total organiccarbon per cubic meter gasoline loaded. Similarly, theUS Environmental Protection Agency Standard 40 CFRPart 63 has established an emission limit of 10 gTOC/m3 32).5. 2. 1. Example of VOCs: Toluene 5. 2. 1. 1. Toluene Interaction with TiO2 Surface

Toluene and toluene/water adsorption experimentswere carried out on TiO2 Merck samples preoutgassedat room temperature, with a monolayer of hydroxylgroups and water molecules still covering the surface ofthe TiO2 microcrystals.

The corresponding IR spectra are shown in Fig. 11.Curve a is the spectrum of the catalyst powder pre-out-gassed at room temperature for 45 min, and the highwavenumber region is reproduced (in absorbance mode)of the spectrum already reported as curve a of Fig. 9B.In the low wavenumber range, bands at 1620, 1455 and1360 cm−1 were observed (curve a). The bending modeof water molecules produces the band at 1620 cm−1, andthe components at 1455 and 1360 cm−1 might be due tosome residual carbonate-like groups, produced by reac-tion of CO2 with basic centres on the TiO2 surface dur-ing the storage of the catalyst powder in air.

In contact with toluene, the signal at 3680 cm−1 dueto the free hydroxyl group was completely depleted,indicating that OH species act as effective Lewis acidadsorption sites for toluene, and a new broad adsorp-tion appeared at lower wavenumbers, the observed shiftresulting from the interaction between the OH groupsand the π-electrons of the aromatic molecules. At lower

frequencies, peaks due to adsorbed toluene appeared inthe 3100-2800 cm−1 (CH stretching) and 1610-1360 cm−1

(ring stretching, CH deformation) ranges.After simply outgassing at room temperature, all

components due to adsorbed toluene completely dis-appeared, and the original spectral features of the freeOH species were completely restored (curve c), indicat-ing that the interaction between aromatic molecules andhydroxyl group is quite weak and fully reversible.5. 2. 1. 2. Toluene Photo-transformation

To evaluate the role of OH groups in the photodegra-dation of toluene, two series of FT-IR experimentswere carried out on TiO2 Merck.

The first series of investigations evaluated the differ-ences in the catalyst surface behaviour of two TiO2

samples with different surface hydration states: pre-out-gassed at room temperature and other pre-outgassed at873 K, the highest temperature attainable without theoccurrence of the anatase-rutile phase transition.

Figure 12 shows the spectra obtained after admis-sion of the toluene/H2O/O2 mixture (curve a) and expo-sure to 10 min of UV irradiation and subsequent out-gassing at room temperature for 45 min on the fullyhydrated sample (curve b). After exposing the photo-catalyst surface to the toluene/H2O/O2 mixture, signalsdue to adsorbed water (main peak at 1640 cm−1) andaromatic molecules (signals at 1600, 1496 and 1460cm−1) were observed (curve a). By exposing the sys-tem to UV light for 10 min, a slight decrease in thetoluene bands was observed, and new componentsappeared (spectrum not shown), indicating that a frac-tion of toluene molecules were transformed into newspecies and that partial photo-oxidation of tolueneoccurred on the fully hydrated sample even under the

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Fig. 11 FT-IR Absorbance Spectra of Toluene Adsorbed onTiO2 Merck: (a) outgassed at room temperature for 45min, (b) in the presence of 3 Torr toluene, (c) after 5min outgassing at room temperature

model conditions employed.The subsequent outgassing at room temperature

completely removed the absorptions due to physisorbedwater molecules and unreacted toluene, and the signalsat 1690, 1645, and 1580 cm−1 due to the newly formedspecies became clearly evident (curve b). To identifythe nature of the intermediate formed after the UV irra-diation step, the spectrum corresponding to the catalystsimply outgassed at room temperature before theadmission of the toluene/H2O/O2 mixture was subtract-ed using the spectrum of curve b in Fig. 12. In thisway, the spectral pattern mainly containing the bandsdue to the species formed during the UV irradiationwas obtained (curve not shown) and was found to cor-respond to the spectrum of benzaldehyde adsorbed ontothe catalyst (Fig. 12, curve c).

After this stage, H2O and O2 were re-admitted ontothe catalyst (Fig. 13, curve b) and the sample wasexposed to the UV light for another 10 min and thenoutgassed at room temperature for 45 min. Underthese conditions, only the weak and broad band at 1640cm−1 due to molecular water coordinated to surface Ti4+

remained, whereas the bands due to adsorbed benz-aldehyde were strongly reduced in intensity (Fig. 13,curve c), indicating further transformation under UVirradiation in the presence of H2O and O2, but no bandsdue to newly formed species were observed.

In parallel catalytic runs carried out in batch reactors,benzene and CO2 were detected among the products ofthe photoreaction, resulting from the photodecarboxyla-tion of benzoic acid produced by oxidation of benz-aldehyde. Presumably the same reactions occurred in

the model conditions employed in the IR cell, so theobserved behaviour could be explained by assumingthat, in the presence of water vapour, the bands due tothe species derived from the benzaldehyde reactionmay not be observed due to the predominant intensityof the signals due to physisorbed water, so water andproducts from the benzaldehyde transformation are des-orbed from the catalyst during outgassing at room tem-perature and so could not be observed.

As described above, after outgassing at high temper-ature, the absorptions due to hydroxyl groups and watermolecules disappear, except for a very weak band at3665 cm−1 (Fig. 4A, curve c), indicating that almostcomplete dehydration of the catalyst occurred, leavingonly a negligible fraction of free OH groups on its sur-face. By exposing this highly dehydrated sample towater vapour, the spectral pattern characteristic ofwater molecules physisorbed onto the catalyst wasobtained, but subsequent simple re-outgassing at roomtemperature fully restored the spectrum observed aftertreatment at 873 K. These results indicate that dehy-droxylation at high temperature is irreversible and thatirradiation of the catalyst pre-outgassed at 873 K wouldactually occur in the absence of surface OH groups,even if water vapour is present in the reaction mixture.

Exposure of the dehydroxylated catalyst to thetoluene/H2O/O2 mixture resulted in observation of thebands due to physisorbed H2O and toluene molecules(Fig. 14, curve a). Nevertheless, after 10 min of UVirradiation and subsequent outgassing of the reactionmixture at room temperature for 45 min, few traces ofbands due to photoproduced benzaldehyde were recog-

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Fig. 12 FT-IR Absorbance Spectra of TiO2 Merck Outgassedat Room Temperature for 45 min: (a) in the presenceof toluene/H2O/O2 mixture, (b) after 10 min UV irra-diation and subsequent outgassing at room tempera-ture for 45 min, (c) in the presence of benzaldehyde

Fig. 13 FT-IR Absorbance Spectra of TiO2 Merck Outgassedat Room Temperature for 45 min: (a) in the presenceof photoproduced benzaldehyde, (b) in the presence ofH2O and O2, (c) after 10 min UV irradiation and sub-sequent outgassing at room temperature for 45 min

nisable in the spectrum (Fig. 14, curve b), pointing tohighly reduced photocatalytic activity of the highlydehydroxylated catalyst with respect to the fullyhydroxylated catalyst, clearly confirming the involve-ment of surface OH groups in the photooxidativeprocess.

The second series of experiments investigated thephenomena occurring on the surface of the catalyst dur-ing the photo-oxidation process in both hydrated anddry conditions33). The results previously obtained fortoluene photodegradation carried out in presence ofH2O were compared with the same experiment carriedout with the photocatalyst exposed to a toluene/O2 mix-ture, but in the absence of H2O.

By exposing the catalyst to the toluene/O2 mixture,the features of the adsorbed toluene molecules wereclearly observed at 1600, 1496 and 1460 cm−1 (Fig. 15,curve b). After 10 min of UV irradiation and subse-quent outgassing of the unreacted toluene at room tem-perature, a decrease in the toluene bands was observed,and signals at 1690, 1645, 1600 and 1580 cm−1 due tonewly formed species appeared (Fig. 15, curve c).This behaviour clearly indicates that benzaldehyde isformed photochemically on the catalyst surface even inthe absence of water vapour in the reaction mixture.Oxygen was then re-admitted into the cell, and the sam-ple was UV irradiated for increasing times. No modi-fications in the spectral features of adsorbed benz-aldehyde were observed, even after 10 h exposure toUV light (Fig. 15, curve d). Therefore, further photo-oxidation of this species (i.e. to form benzoic acid andthen CO2 and benzene) is strongly inhibited in theabsence of excess water vapour.

Photocatalytic experiments carried out in a photore-actor showed that, in the solid/gas system, TiO2 Merckexhibited stable activity in the presence of water vapour,which was greatly decreased in a dry gas stream. Incontrast, TiO2 P25 Degussa produced CO2 and traces ofbenzaldehyde but was continuously deactivated, evenin the presence of water vapour20).

An IR investigation of the toluene adsorption andphototransformation on TiO2 P25 surface was then car-ried out. The disappearance of the OH bands in the3700-3600 cm−1 range after exposure to toluene andtheir transformation to an intense and broad compo-nents centered at ca. 3500 cm−1 clearly indicates thatvibrationally free hydroxyls act as effective Lewis acidadsorption sites for toluene, the observed shift resultingfrom the interaction between such OH groups and theπ-electrons of the aromatic molecules. At lower fre-quencies, peaks due to adsorbed toluene appeared inthe 3100-2800 cm−1 (CH stretching) and 1610-1360cm−1 (ring stretching, CH deformation) ranges (Fig.16, curve b), but disappeared after outgassing at roomtemperature, whereas the signals due to vibrationallyfree OH groups were completely restored (Fig. 16,curves c), indicating that, for this system also, the inter-action between the aromatic molecules and hydroxylgroups is very weak.

In contrast, the interaction of benzaldehyde with thesurface of the photocatalyst exhibits a significantdependence on the type of TiO2 photocatalyst. In thecase of TiO2 Degussa P25, the adsorption of benz-aldehyde resulted in the spectra in Fig. 17. In thiscase the components due to benzaldehyde moleculesadsorbed in an unperturbed form were observed as

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Fig. 14 FT-IR Absorbance Spectra of TiO2 Merck Outgassedat 873 K: (a) in the presence of toluene/H2O/O2 mix-ture, (b) after 10 min UV irradiation and subsequentoutgassing at room temperature for 45 min

Fig. 15 FT-IR Absorbance Spectra of TiO2 Merck: (a) out-gassed at room temperature for 45 min, (b) in the pres-ence of toluene/O2 mixture, (c) after 10 min UV irra-diation and subsequent outgassing at room tempera-ture for 45 min, (d) after 10 h UV irradiation in thepresence of O2

minor features and the FT-IR spectrum was dominatedby an intense peak at 1650 cm−1 and a series of bands at1518, 1495, 1451 and 1413 cm−1 (Fig. 17, curves b andc). These components can be assigned to hemiacetal-ic-like species formed by nucleophilic attack of thebasic oxygen of the hydroxyl groups on the carbonatom of the carbonyl group of the adsorbed aldehydemolecules, as in the first step of the Cannizzaro reac-tion.

Such different behaviour also affects the reactivity ofthe hydroxyl groups, under high hydration conditions,which exhibit a nucleophilic character (through the Oatom) for TiO2 Degussa P25, but as an electron accep-tor (through the H atom) for TiO2 Merck, and couldwell account for the different photocatalytic behavioursexhibited by the two types of TiO2 powders in parallelbatch reactor experiments.

As discussed above, in the case of TiO2 Merck, benz-aldehyde molecules resulting from the photo-oxidationof toluene weakly interact with the catalyst surface andcan be easily released to the gas phase. In contrast,

the present results for TiO2 P25 indicate that thehydroxyl groups on the surface can react with benz-aldehyde formed photochemically, which is thenretained on the catalyst surface. Subsequently, thebenzaldehyde can be converted to other products, suchas benzoic acid, strongly adsorbed on the TiO2 surfaceleading to the progressive deactivation of the catalyst inthe gas-solid system.5. 3. Acetonitrile

Acetonitrile (methyl cyanide, CH3CN) has been thefocus of several recent studies in atmospheric, marine,chemical and biological science. Acetonitrile is stronglyand predominantly emitted from biomass burning.Since the atmospheric lifetime of acetonitrile is long incomparison with regional and hemispheric transportprocesses, it can serve as a characteristic global markerof biomass burning. In the future, if increasing globaltemperatures cause increased aridity, then increasedincidence of biomass burning, and hence acetonitrileemissions, can be expected. Recently, revised globalOH estimates have increased its importance as aremoval mechanism, although the main sink for ace-tonitrile is presently thought to be uptake in the ocean.Direct industrial emissions and indirect productionfrom the agricultural fumigant methyl isothiocyanatevia methyl isocyanide (CH3NC) seem to be less impor-tant than that from automobiles. The reaction ratesand products of acetonitrile with other atmosphericspecies such as ozone, bromine and iodine are stillunknown34).5. 3. 1. Acetonitrile adsorption

To investigate the molecular mechanisms of theinteraction between acetonitrile (using deuterated ace-tonitrile to avoid the Fermi resonance effect, which per-turbs the determination of the ν(CN) mode) and thephotocatalyst surface, CD3CN adsorption and desorptionwere carried out on TiO2 Merck and TiO2 Degussa P25simply outgassed at room temperature (after isotopicexchange with D2O).

Exposure of the TiO2 Merck sample to CD3CN atroom temperature resulted in a main peak at 2270 cm−1

in the IR spectra, due to the ν(CN) mode of acetonitrilemolecules D-bonded to surface deuterated hydroxylgroups, and a shoulder at 2305 cm−1 corresponding tothe ν(CN) mode of CD3CN adsorbed on the surfaceTi4+ cations (Fig. 18, curve a). Finally, the weakercomponents at 2215 and 2113 cm−1 corresponded to acombination band (2νsym(CC) + δasym(C_C≡N)) and tothe νsym(CD3) mode, respectively.

At the higher wavenumbers, an asymmetric peak at2670 cm−1, and a broad shoulder at 2707 cm−1, due tothe stretching mode of isolated deuteroxyl groups (insetin Fig. 18, curve a’), completely disappeared, whereasa broad component in the 2650-2200 cm−1 rangeappeared due to the overlap of the bands due to thestretching mode of D-bonded OD groups and of D2O

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Fig. 16 FT-IR Absorbance Spectra of Toluene Adsorbed onTiO2 Degussa P25: (a) outgassed at room temperaturefor 45 min, (b) in the presence of 3 Torr toluene, (c)after 5 min outgassing at room temperature

Fig. 17 FT-IR Absorbance Spectra of Benzaldehyde Adsorbedon TiO2 Degussa P25 Outgassed at Room Temperaturefor 45 min

Spectra recorded in the presence of: (a) 0.5, (b) 1.9, (c) 2.0, (d)3.0 Torr benzaldehyde.

molecules co-ordinated to surface Ti4+ cations (inset inFig. 18, curve b’). This behaviour indicates thatdeuteroxyl groups also act as effective adsorption sitesfor CD3CN molecules, through D-bonding between thelone-pair on the nitrogen atom and the D atom of theOD groups.

No changes in the spectrum were observed byincreasing the contact time with acetonitrile vapour(spectrum not shown). By simply outgassing at roomtemperature, all CD3CN bands disappeared, indicatingcomplete desorption of the adsorbed species (Fig. 18,curve b) and restoration of the peaks due to the isolateddeuteroxyls (inset in Fig. 8, curve c’).

Since CD3CN interaction with the Merck photocata-lyst surface is completely reversible by simply out-gassing at room temperature, UV irradiation forincreasing time (up to 17 h) in the presence of O2 andD2O was carried out on the total amount of adsorbedCD3CN. At the end of each irradiation step, O2 andD2O were outgassed, and the IR spectrum of the samplewas recorded.5. 3. 2. Acetonitrile Photo-transformation

After irradiation for 17 h, the intensity of the peaksat 2270, 2215 and 2113 cm−1 decreased, and the shoul-der at 2305 cm−1 completely disappeared (Fig. 19A,curve b), indicating almost complete consumption ofadsorbed and physisorbed CD3CN. Correspondingly,in the 1750-1100 cm−1 range, a complex pattern wasformed (Fig. 19B, curve b), in particular, weak compo-nents were recognised at 1720, 1692, 1620, 1476, 1428and 1350 cm−1, and a predominant band at 1175 cm−1.

This behaviour indicates the photostimulated conver-sion of adsorbed CD3CN molecules into new species.More specifically, the 1720 cm−1 peak can be ascribedto formyl groups35), and the absorption signals at 1692,1428 and 1350 cm−1 suggest the formation of NxOy

compounds36). The assignment of the band at 1175 cm−1

is not straightforward, and additional investigations arein progress.

Exposure of TiO2 Degussa P25 to acetonitrile atroom temperature resulted in peaks at 2291 and 2264cm−1 (ν(CN)) and at 2215 (2νsym(CC) + δasym(C_C≡N))and 2113 cm−1 (νsym(CD3)) due to adsorbed acetonitrilein the 2350-2075 cm−1 range (Fig. 20A, curve a).Correspondingly, in the 1750-1150 cm−1 range, a weaknegative band at 1195 cm−1 was observed due to the

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Fig. 18 FT-IR Absorbance Spectra of Acetonitrile Adsorbedon TiO2 Merck Outgassed at Room Temperature for45 min: (a) after contact with acetonitrile at vapourpressure, (b) after subsequent outgassing at room tem-perature

Inset: FT-IR absorbance spectra of acetonitrile adsorbed on TiO2

Merck outgassed at room temperature for 45 min in the νOH

region. (a’) before acetonitrile adsorption, (b’) after contactwith acetonitrile vapour pressure, (c’) after subsequent out-gassing at room temperature.

Fig. 19A FT-IR Absorbance Spectra of TiO2 Merck Outgassedat Room Temperature for 45 min: (a) after contactwith acetonitrile at vapour pressure, (b) after 17 hUV irradiation and subsequent outgassing at roomtemperature

Fig. 19B FT-IR Absorbance Spectra of TiO2 Merck Outgassedat Room Temperature for 45 min in the Low Wave-numbers Region: (a) after contact with acetonitrile atvapour pressure, (b) after 17 h UV irradiation andsubsequent outgassing at room temperature

deformation mode of D2O molecules coordinated tosurface Ti4+ ions and results from the displacement byacetonitrile of D2O molecules previously coordinated tothese stronger Lewis acid sites (Fig. 20B, curve a).This band was not detectable for TiO2 Merck (expectedat ca. 1200 cm−1), because it falls at a frequency lowerthan the catalyst cut-off (ca. 1250 cm−1).

CD3CN adsorption on Degussa P25 resulted in theasymmetric peak at 2685 cm−1 with two shoulders athigher wavenumbers (2715 and 2754 cm−1), due to themore heterogeneous isolated OD groups, and the peakat 2533 cm−1 due to the stretching mode of D2O mole-cules co-ordinated to Ti4+ sites disappeared and wassubstituted by a broad absorption (inset of Fig. 20A,curve b’). This behaviour confirms the formation ofD-bonding between the OD groups and the N-lone pairof CD3CN molecules.

By increasing the time of contact with adsorbedCD3CN on the photocatalyst surface, a considerableevolution of the spectral pattern was observed in the2350-2075 cm−1 (Fig. 20A, curve b) and 1750-1150cm−1 ranges (Fig. 20B, curve b), unlike that observedfor TiO2 Merck.

After 12 h of contact, all peaks decreased in intensityin the 2350-2075 cm−1 range, particularly evident forthe signal at 2291 cm−1 (Fig. 20A, curve b).Correspondingly, a large and complex absorption pat-tern was observed in the 1750-1300 cm−1 range (Fig.20B, curve b), indicating the formation of acetamide-like species due to the reaction of acetonitrile mole-

cules initially adsorbed on Ti4+ cations with surfacebasic centres (O2− anions or OD groups)38)～40). Thisbehaviour was not observed for TiO2 Merck becausethe surface species are not basic enough to allow nucleo-philic attack on CD3CN to form acetamide-like species.

After subsequent outgassing at room temperature,the spectral pattern of acetamide-like species in the1750-1300 cm−1 range was slightly modified (Fig.20B, curve c), possibly due to D2O and physisorbedCD3CN desorption, also indicating that these speciesare irreversibly adsorbed on the surface of the photocat-alyst.

In the 2350-2075 cm−1 range, further depletion of the2264 and 2215 cm−1 peaks and further decrease in theintensity of the 2113 cm−1 peak were observed, due tothe desorption of acetonitrile molecules from surfaceOD groups and physisorbed CD3CN, respectively (Fig.20A, curve c). Moreover, the increase in intensity ofthe 2291 cm−1 band indicates the loss of D2O co-ordinated to the CD3CN molecules absorbed on Ti4+

ions, (contributing, when in such form, to the 2264 cm−1

peak), which are converted into CD3CN…Ti4+ adducts.In the case of TiO2 Degussa P25, UV irradiation for

increasing times (up to 17 h) in the presence of O2 andD2O was carried out after outgassing the reversiblefraction of adsorbed CD3CN (step corresponding tocurve c of Fig. 20A). After 17 h of UV irradiation,the decrease of the 2291, 2215 and 2113 cm−1 signalsand the corresponding formation of new components inthe 2350-2075 cm−1 (Fig. 21A, curve b) and 1750-1150 cm−1 ranges suggest the photo-induced conversionof the irreversibly adsorbed CD3CN into other species.Moreover, the growth of absorptions due to theacetamide-like species indicates that at least part of the

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Fig. 20A FT-IR Absorbance Spectra of Acetonitrile Adsorbedon TiO2 Degussa P25 Outgassed at Room Tempera-ture for 45 min: (a) after contact with acetonitrile atvapour pressure, (b) after 12 h in the presence ofacetonitrile, (c) after subsequent outgassing at roomtemperature

Inset: FT-IR absorbance spectra of acetonitrile adsorbed on TiO2

Merck outgassed at room temperature for 45 min in the νOH

region. (a’) before acetonitrile adsorption, (b’) after contactwith acetonitrile at vapour pressure, (c’) after subsequent out-gassing at room temperature.

Fig. 20B FT-IR Absorbance Spectra of Acetonitrile Adsorbedon TiO2 Degussa P25 Outgassed at Room Tempera-ture for 45 min in the Low Wavenumbers Region:(a) after contact with acetonitrile at vapour pressure,(b) after 12 h in the presence of acetonitrile, (c) aftersubsequent outgassing at room temperature

photo-converted CD3CN molecules contribute to theformation of new adducts. New components appeared,assignable to cyanate (2234 cm−1)41), isocyanate (2189cm−1)42), formyl (1720 cm−1)35), carbonate and bicarbon-ate (1700-1450 cm−1 range)8),18) and nitrate (1560-1500cm−1 and 1300-1250 cm−1 ranges)36) species, whichresulted from the photo-oxidation of CD3CN coordinatedto Ti4+ ions. Moreover, the weak band at 2161 cm−1 islikely due to polymerized CD3CN39).

This behaviour indicates that acetamide-like speciesare less sensitive to photo-oxidation than acetonitrileadsorbed in molecular form. Thus, these species can

exhibit a poisoning effect towards part of the photocat-alytic surface sites, accounting for the lower initialreaction rate observed for TiO2 Degussa P25 comparedto TiO2 Merck, as observed in parallel experiments car-ried out in typical batch reactors.

6. Conclusions

Insights on the nature of the species adsorbed on thesurface of TiO2 photocatalysts in the dark and theirevolution under UV irradiation can be easily obtainedfrom FT-IR spectra collected under simulated operatingphotocatalytic conditions. This kind of investigationis also very useful to differentiate between reactions inwhich intermediates evolve until complete mineralisa-tion and processes in which the initial photoreactionproducts are stable and produce irreversible quenchingof the catalyst, thus allowing identification of the caus-es of the deactivation.

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J. Jpn. Petrol. Inst., Vol. 47, No. 6, 2004

Fig. 21A FT-IR Absorbance Spectra of TiO2 Degussa P25Outgassed at Room Temperature for 45 min: (a)after 12 h in contact with acetonitrile at vapour pres-sure and subsequent outgassing at room temperature,(b) after 17 h UV irradiation and subsequent out-gassing at room temperature

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要　　　旨

TiO2粉末上における有機化合物の光触媒分解—FT-IRによる表面の反応性と反応機構の解析—

Patrizia DAVIT†1), Gianmario MARTRA*, Salvatore COLUCCIA

Dipartimento di Chimica IFM, Università degli Studi di Torino, Via P. Giuria 7, I-10125 Torino, ITALY

†1) (Present) Sinergos S.r.l., Environment Park, Via Livorno 60, I-10149 Torino, ITALY

……………………………………………………………………

空気および水中を汚染する有機化合物（たとえばフェノール，

VOCs，アセトニトリル）の光触媒分解過程において，暗所お

よび UV照射下，TiO2表面で起こる現象を分子レベルで調べ

た一連の研究がまとめられている。分析手法として，FT-IR分

光法を模擬操作条件で利用し，さらに TiO2粒子の表面構造と

形態の解明に高分解能透過電子顕微鏡を用いた。得られた結果

は，UV照射下における吸着／反応サイトとして表面水酸基が

重要な役割を示すこと，および塩基性表面サイト（TiO2格子

中の水酸基あるいは表面 O2−）も表面の化学反応過程に影響を

与える可能性があることを示している。