art 2.pdf

Heterogeneous photocatalysis: state of the art and present applications

J.-M. Herrmann*

Head of the Laboratory of Environmental Chemistry, UMR CNRS N5634, Universite Claude Bernard Lyon1, batiment J. Raulin, 43 bd du 11novembre 1918, 69622 Villeurbanne Cedex, France

In the present paper, the basic fundamentals of photocatalysis are explained with the inuence of the ve main parameters

which govern the kinetics: (i) mass of catalyst, (ii) wavelength, (iii) partial pression and/or concentrations of reactants, (iv)

temperature (at a second degree) and (v) the radiant ux. The main types of photocatalytic reactions presently described (all

performed at room temperature) concern (i) the selective mild oxidation of hydrocarbons, (ii) the hydrogen production and (iii) the

total oxidation reactions of organics in presence of water. The last point constitutes the ensemble of the last recent developments in

photocatalysis. Most of organic contaminants, including dangerous pesticides, can be easily totally degraded and mineralized.

Dyes are also not only decolorized, but mineralized in colored aqueous efuents. The most abundant ones (the azo-dyes) have their

azo-group(s) AN@NA decomposed into N2(g), which represents an ideal decontamination case.Photocatalytic engineering is under development, now using deposited titania in a xed bed. Some (solar) photocatalytic pilot

reactors and prototypes are described. The use of solar energy as a source of activating UVA irradiation is described as a sub-

discipline called helio-photocatalysis.

KEYWORDS: photocatalysis;mechanism;water purication; pesticide removal; photoreactors; solar reactors;helio-photocatalysis.

1. Introduction

Heterogeneous photocatalysis is a discipline whichincludes a large variety of reactions: mild or totaloxidations, dehydrogenation, hydrogen transfer, oxy-gen-18 and deuterium isotopic exchange, metal deposi-tion, water detoxication, gaseous pollutant removal,bactericidal action etc... In line with the last point, it canbe considered as one of the new Advanced OxidationTechnologies (AOT) for air and water puricationtreatments. Several books and reviews have been recentlydevoted to this problem [111]. A recent review hasreported more than 1200 references on the subject [11]

Heterogeneous photocatalysis can be carried out invarious media: gas phase, pure organic liquid phases oraqueous solutions. As for classical heterogeneous catal-ysis, the overall process can be decomposed into veindependent steps:

1. Transfer of the reactants in the uid phase to thesurface.

2. Adsorption of at least one of the reactants.3. Reaction in the adsorbed phase.4. Desorption of the product(s).5. Removal of the products from the interface region.

The only difference with conventional catalysis is themode of activation of the catalyst in which the thermalactivation is replaced by a photonic activation as

developed in the next paragraph. The activation modeis not concerned with steps 1, 2, 4 and 5, althoughphotoadsorption and photodesorption of some reac-tants, mainly oxygen, do exist. Step 3 contains all thephotoelectronic processes and can be decomposed asfollows :

Step 3: Reaction in the adsorbed phase.

3.1. Absorption of the photons by the solid and not byreactants. There is no photochemistry in theadsorbed phase.

3.2. Creation of electron-hole pairs which dissociate intophotoelectrons and positive photo-holes (electronvacancies).

3.3. Electron transfer reactions such as ionosorption(case of O2, NO,), charge neutralization, radicalformation, surface reactions)

All these processes are described in the next section.In the present review, after a detailed presentation of

the photo-physico-chemical processes involved in pho-tocatalysis, different applications, especially in thedomain of water treatment, will be presented. Theactual tendency is the use of xed beds of titaniaphotocatalysts for air and water purication. Photocat-alytic engineering is also developing with pilot plants,especially in the eld of solar photocatalysis, baptizedHelio-photocatalysis by the author.

2. Principle of heterogeneous photocatalysis

When a semiconductor catalyst SC of the chalcogen-ide type (oxides (TiO2, ZnO, ZrO2, CeO2,), or suldes

In honor of Pr. R.L. Burwell Jr. (19122003), Former Head of Ipatie

Laboratories, Northwestern University, Evanston (Ill).

* To whom correspondence should be addressed.

E-mail: [email protected]

Topics in Catalysis Vol. 34, Nos. 14, May 2005 ( 2005) 49DOI: 10.1007/s11244-005-3788-2

1022-5528/05/05000049/0 2005 Springer Science+Business Media, Inc.

(CdS, ZnS,)) is illuminated with photons whoseenergy is equal to or greater than their band-gap energyEG (hm EG), there is Absorption of these photons andcreation within the bulk of electron-hole pairs, whichdissociate into free photoelectrons in the conductionband and photoholes in the valence band (Figure 1).

Simultaneously, in the presence of a uid phase (gasor liquid), a spontaneous Adsorption occurs andaccording to the redox potential (or energy level) ofeach adsorbate, an electron transfer proceeds towardsacceptor molecules, whereas a positive photohole istransferred to a donor molecule (actually the holetransfer corresponds to the cession of an electron bythe donor to the solid).

hm SC ! e p 1

Aads e ! Aads 2

Dads p ! Dads 3Each ion formed subsequently reacts to form theintermediates and nal products. As a consequence ofreactions [13], the photonic excitation of the catalystappears as the initial step of the activation of the wholecatalytic system. Thence, the efcient photon has to beconsidered as a reactant and the photon ux as a specialuid phase, the electromagnetic phase. The photonenergy is adapted to the absorption by the catalyst, notto that of the reactants. The activation of the processgoes through the excitation of the solid but not throughthat of the reactants: there is no photochemical processin the adsorbed phase but only a true heterogeneousphotocatalytic regime as demonstrated further.

The photocatalytic activity or the quantum yielddened in Section 4.6, can be reduced by the electron-hole recombination, described in gure 2, which corre-sponds to the degradation of the photoelectronic energyinto heat.

e p ! N E 4(N: neutral center; E: energy (light hm hm or heat)

3. Catalysts

Various chalcogenides (oxides and suldes) have beenused: TiO2, ZnO, CeO2, ZrO2, SnO2, SbrO4, CdS, ZnS,etc. As generally observed, the best photocatalyticperformances with maximum quantum yields are alwaysobtained with titania. In addition, anatase is the mostactive allotropic form among the various ones available,either natural (rutile and brookite) or articial (TiO2-B,TiO2-H). Anatase is thermodynamically less stable than

Figure 1. Energy band diagram of a spherical titania particle.

Figure 2. Fate of electrons and holes within a spherical particle of

titania in the presence of an acceptor (A) and (D) molecules (after the

late Dr. H. Gerisher, p. 1 in Ref. 3) [12].

J.-M. Herrmann/Applied photocatalysis50

rutile, but its formation is kinetically favored at lowertemperature (

whereas for concentrations >5 10)3 M, (KC 1), thereaction rate is maximum and is of the apparent order(gure 4c).

In the gas phase, similar LangmuirHinshelwoodexpressions have been found including partial pressuresP instead of C. In some cases, such as in liquid alcoholdehydrogenation [18], the rate follows variations includ-ing the square root of concentration:

r kK1=2C1=2=1 K1=2C1=2 6This indicates that the active form of the reactant is in adissociated adsorbed state. In other cases, such as in thephotocatalytic degradation and mineralization of chlo-robenzoic acids [19], a zero kinetic order was found,even at low concentrations. This was due to a strongchemisorption on titania with the saturation of thehydroxylic adsorption sites. For a maximum yield,reactions should be performed with initial concentra-tions equal to or higher than the threshold of the plateau(Co ca. 5 10)3 ).

4.4. Temperature

Because of the photonic activation, the photocatalyticsystems do not require heating and are operating at room

temperature. The true activation energy Et, relative to thetrue rate consbtant k (k k0 exp ()EC/RT)), is nil,whereas the apparent activation energy Ea is often verysmall (a few kJ/molr) in the medium temperature range(20 C h 80 C). However, at very low temperatures()40 C h C 0C), the activity decreases and theactivation energy Ea becomes positive (gure 4d).

By constrast, at high temperatures (h C 7080 C) for various types of photocatalytic reactions, theactivity decreases and the apparent activation energybecomes negative (gure 4c).

This behavior can be easily explained within theframe of the LangmuirHinshelwood mechanismdescribed above. The decrease in temperature favorsadsorption which is a spontaneous exothermic phe-nomenon. h tends to unity, whereas KC becomes 1.In addition, the lowering in T also favors theadsorption of the nal reaction product P, whosedesorption tends to inhibit the reaction. Correspond-ingly, there appears a term KPCP in the denominatorof equation (5). If P is the strong inhibitor, when gets:KPCP KC and the LangmuirHinshelwood equa-tion becomes:

r kh kKC=1 KC KPCP kKC=KPCP 7

Figure 4. Inuence of the different physical parameters which govern the kinetics of photocatalysis: reaction rate r; (a): mass of catalyst m;

(b): wavelength k; (c): initial concentration c of reactant; (d) temperature T; (e) radiant ux /.


where KP is the adsorption constant of nal product P.There results an apparent activation energy Ea equalto:

Ea Et DHA DHP 8i.e. to the algebraic sum of the true activation energy Et(theoretically equal to zero) and of the enthalpies ofadsorption DHA and DHP of reactant A and of theinhibiting nal product P, respectively. Since DHis arealways negative, one generally write : DHi=)Qi, Qibeing the heat of adsorption counted positive.

Ea Et QA QP 0 QA QP QA QP 9If product P is the strong inhibitor, this means thatQA < QP and equation (9) Ea tends asymptotically toQP. Such a relationship was actually veried forphotocatalytic reactions involving hydrogen, mainlyalcohol dehydrogenation [18] and alkanedeuteriumisotopic exchange [20,21], inherited from R.L. Burwell[2225].These reactions are carried out on bifunc-tional Pt/TiO2 photocatalysts prepared according toR.L. Burwells recipe [13]. Ea was found equal to+10 kcal/mol (+42 kJ/mol), which is just equal to theheat QH2ads (or to the opposite of the enthalpyDHH2ads) of the reversible adsorption of H2 on plati-num, measured by microcalorimetry [26].

EaEtDHH2ads 0QH2ads 10kcal/mol (42kJ/mol)10

On the opposite, when h C increases above 80 C andtends to the boiling point of water, the exothermicadsorption of reactant A becomes disfavored and tendsto limit the reaction. Consequently, the LangmuirHinshelwood equation becomes:

r kh k KC=1 KC k KC 11with an apparent activation energy equals to:

Ea Et DHA 12As the temperature increases, the adsorption of reactantA becomes limited [27] and Ea, which is now negative,tends to QA

Ea Et DHA 0 DHA QA < 0 13As a consequence, the optimum temperature is generallycomprised between 20 and 80 C. This explains whysolar devices which use light concentrators instead oflight collectors require coolers [28]. This absence ofheating is attractive for photocatalytic reactions carriedout in aqueous media and in particular for environmen-tal purposes (photocatalytic water purication). There isno nead to waste energy in heating water whichpossesses a high heat capacity. This explains whyphotocatalysis is cheaper than incineration [29].

4.5. Radiant ux

It has been shown, for all types of photocatalyticreactions, that the rate of reaction r is proportional tothe radiant ux F (gure 4E). This conrms the photo-induced nature of the activation of the catalytic process,with the participation of photo-induced electricalcharges (electrons and holes) to the reaction mechanism.However, above a certain value, estimated to be ca.25 mW/cm2 in laboratory experiments, the reaction rater becomes proportional to F1/2. This can be demon-strated as follows:

According to Section 2, the ve basic equations are:

TiO2 hm! e p 14

e p ! N energy 15

A e ! A 16

D p ! D 17

A D ! Intermediate products 18and the rate limiting step is the reaction in the adsorbedphase (equation (18)). Therefore

r r18 k18AD 19In an n-type semiconductor such as titania, the photo-induced holes are much less numerous than electrons(photo-induced electrons plus n-electrons): [p+] [e)].Therefore holes are the limiting active species. Thence:

r r18 r17 k17Dp 20

At any instant, one has:

dp=dt r14r15 r170

k14k15epk17Dp21

Thence:

p k14Uk15e k17D 22

and r k17Dk14Uk15e k17D

k14k17DUk17D k15e / U 23

From the above equation, it can be seen that thereaction rate is directly proportional to the light ux.

In the case of high uxes, the instantaneous concen-trations [e)] and [ p+] become much larger than k17[D].Therefore equation (22) becomes:

p k14Uk15e withp

e 24

J.-M. Herrmann/Applied photocatalysis 53

Thence p2 k1U=k2 25The reaction rate r becomes

r r18 r17 k17Dk14U=k151=2 / U1=2 26indicating that r has become proportional to F1/2 andthat the rate of electron-hole formation becomesgreater than the photocatalytic rate, which favoursthe electron-hole recombination (equation (15)). In anyphotocatalytic device, the optimal light power utiliza-tion corresponds to the domain where r is proportionalto F.

4.6. Quantum yield

By denition, it is equal to the ratio of the reactionrate in molecules converted per second (or in mol persecond) to the incident efcient photonic ux inphotons per second (or in Einstein per second (anEinstein is a mol of photons)). This is a kineticdenition, which is directly related to the instanta-neous efciency of a photocatalytic system. Its theo-retical maximum value is equal to 1. It may vary ona wide range according (i) to the nature of thecatalyst; (ii) to the experimental conditions used(concentrations, T, m, ) and (iii) especially to thenature of the reaction considered. We have foundvalues comprised between 10)2% and 70%. Theknowledge of this parameter is fundamental. Itenables one (i) to compare the activity of dierentcatalysts for the same reaction, (ii) to estimate therelative feasability of dierent reactions, and (iii) tocalculate the energetic yield of the process and thecorresponding cost.

4.7. Inuence of oxygen pressure

It could be easily determined in gas phase reactions.For instance, in the mild photocatalytic oxidation ofalkanes into aldehydes and/or ketones, it was found thatthe kinetic order of oxygen was nil, indicating a totalsurface coverage of this gas [30]. Photoconductivitymeasurements indicated that oxygen was photo-ad-sorbed as O2) [30]. For liquid phase reactions, it wasdicult to study the inuence of PO2 because thereaction is polyphasic. It is generally assumed thatoxygen adsorbs on titania from the liquid phase, whereit is dissolved following Henrys law. If the oxygen isregularly supplied, it can be assumed that its coverage atthe surface of titania is constant and can be integratedinto the apparent rate constant:

AO2 ! P 27

rA d[A]=dt khAhO2 kapphA 28

Actually, the apparent rate constant is a function of thepower ux and of the oxygen coverage.

5. Main types of photocatalytic reactions

5.1. Selective mild oxidation reactions

The gas phase oxidations using air as the oxidizingagent mainly concerned the mild oxidation of alkanes,alkenes and alcohols into carbonyl-containing molecules(aldehydes and ketones) [31]. Inorganic could be alsooxidized: CO into CO2 and NH3 into mainly nitrogen[32]. NO could be photocatalytically decomposed intoN2 at low pressures and into N2O at higher pressures,whereas the oxygen generated could be used as anoxidizing agent for butanols [33].

Liquid phase reactions concerned the selective mildoxidation of liquid hydrocarbons (alkanes, alkenes,cycloalkanes, aromatics) into aldehydes and ketones[34]. For instance, cyclohexane and decaline wereoxidized into cyclohexanone and 2-decalone, respec-tively with an identical selectivity (86%) [35]. Aromatichydrocarbons [36] such as alkyltoluenes or o-xyleneswere 100% selectively oxidized on the methylgroup intoalkylbenzaldehyde:

R C6H4 CH3 O2 ! R C6H4 CHOH2O29

Pure liquid alcohols were also oxidized into theircorresponding aldehydes or ketones. In particular, theoxidation of isopropanol into acetone was chosen as aphotocatalytic test for measuring the efciency ofpassivation of TiO2- or ZnO-based pigments in paint-ings against weathering.

The high selectivity was ascribed to a photoactiveneutral, atomic oxygen species [23]

O(ads) p ! O(ads) 30

5.2. Photocatalytic reactions involving hydrogen

In photocatalytic reactions involving hydrogen, eitheras a reactant (deuteriumalkane isotopic exchange [20])or as a product (alcohol dehydrogenation [18], thesystem requires the presence of a metal acting as a co-catalyst necessary (i) to dissociate the reactant (D2) and(ii) to recombine H and D into dihydrogen (or HD).Additionally, the metal (i) attracts electrons by photo-induced metal-support interaction (PMSI), (ii) decreasesthe electron-hole recombination and (iii) makes thereaction run catalytically [21].

5.3. Total oxidation reactions in presence of water(humid air or aqueous phase)

The selective mild oxidation reaction could beobtained in gaseous or liquid organic phases. By


contrast, as soon as water is present, the selectivity turnsin favor of total oxidative degradation. This wasascribed to the photogeneration of stronger, unselective,oxidizing species, namely OH radicals originating fromwater via the OH) groups of titanias surface:

H2O! H OHads p ! H OH 31

ReactantOH ! Intermediates!Final inorganic ProductsCO2;H2O;H;X;A . . .

32OH radicals are known as the second best oxidizingagent after uorine.

Consequently, this system is the most promising issuefor an application of heterogeneous photocatalysis,since it is directly connected to water detoxicationand to pollutant removal in aqueous efuents. It will bedescribed in the next section.

6. Photocatalytic water decontamination

Besides the main eld of organic pollutants removal,the photocatalytic water decontamination can also beemployed for the recovery or the detoxication ofinorganic pollutants.

6.1. Inorganic pollutants

6.1.1. Inorganic anionsVarious toxic anions can be oxidized into harmless or

less toxic compounds by using TiO2 as a photocatalyst.For instance, nitrite is oxidized into nitrate [37,38],sulde, sulte [39] and thiosulfate [40] are converted intosulfate, whereas cyanide is converted either into isocy-anide [41] or nitrogen [42] or nitrate [43]. Generally, thecentral element (S, N, P, C,) is converted at itsmaximum oxidation state. These reactions are presentedin table 1.

6.1.2. Noble metal recoveryHeavy metals are generally toxic for human beings

since they accumulate in the body. They can be removedfrom industrial waste efuents [40, 4447] as smallcrystallites deposited on the photocatalyst according to:

Mn+ H2O !hmsemiconductor

M nH n4O2 33

provided the redox potential of the cation metal coupleis higher than the at band potential of the semicon-ductor.

Under identical conditions, the following photode-position reactivity pattern was found:

Ag > Pd > Au > Pt Rh Ir Cu Ni Fe 034

using the following inorganics as reactants: AgNO3PdCl2, AuCl3, H2PtCl6 or Na2PtCl6, RhCl3, H2IrCl6,Cu(NO3)2, Ni(NO3)2 and Fe(NO3)3.

Note that this method can also be used to preparenoble metal catalysts promptly deposited (~10 min) in asingle step, at room temperature and in mild ChimieDouce conditions on photosensitive oxide supports[4852].

In view of the above relative activity pattern, most ofthe experiments were carried out with platinum andsilver, using TiO2 as the photocatalyst. The depositioninitially occurred by forming small homodispersedcrystallites whose size depended on the nature of themetal: 0.81 nm for Ir, 11.5 nm for Pt [51] and ca.3 nm for Ag [40]. As the photodeposition conversionincreases, the metal particles form agglomerates, reach-ing several hundreds of nm (i.e. bigger than the TiO2particles) themselves for Ag [40]. Since these agglomer-ates contained the major part of the metal deposited, thephotosensitive surface was not markedly masked andhigh amounts of metals were recovered. The nalconcentrations of easily photoreduced cations are lowerthan the detection limits of atomic absorption spectros-copy (0.01 ppm). Silver photodeposition has beenapplied with two environmental interests: (i) the recov-ery of Ag from used photographic baths in which thesilver-thiosulfate complex is decomposed, Ag+ beingreduced to Ag and (ii) the detoxication of the aqueouseuent, S2O

23 being oxidized into SO

24 [40], whereas

phenolic compounds are degraded into CO2 [53].Because of their favorable redox potentials, only

noble metals can be photodeposited. This property hasbeen used to selectively recover heavy noble metals.For instance, silver was separated from copper insolutions simulating industrial electrolytic baths. Othertoxic, heavy, non-noble metals could be removed fromwater. Mercury, because of its favorable redoxpotential, was photoreduced as zerovalent metal [54].The cations Pb2+ and Tl+ were deposited onUV-irradiated TiO2 powder as PbO2 and Tl2O3 [55].

Table 1

Photocatalytic decontamination of aqueous solutions containing com-

mons anions. The central element (S, N, P, C, ) is oxidized to itsharmless oxidation degree

H2S + 2O2 SO24 + 2H+

SH) + 2O2+ SO24 + H+

S2) + 2O2 SO24SO23 + 1/2O2 SO24S2O

23 + 2O2 + H2O 2SO24 + 2H

+

NO2 + 1/2O2 NO3NH4 + 1/2O2 + H2O NO

3 + 2H2O + 2H

+

H3PO3 + 1/2O2 H3PO4CN) + 1/2O2 OCN) [ OCN) + 2H2O CO23 + NH

4 ]

Catalysts

TiO2 ZnO* > ZrO2 > CeO2 SnO2 > CdS* >MoO3 WO3V2O5 = 0

(*: photocorrodes).


Similarly, uranium was photodeposited on TiO2 asU3O8 from uranyl solutions [56].

From an application point of view, the recovery ofsilver from photographic baths seems the most prom-ising issue, provided the legislation towards Ag-contain-ing discharge water becomes more strict.

6.2. Organic pollutants

The aim of our studies was to establish correlationsbetween the molecular structure of the pollutants andtheir photocatalytic degradability. The analyses of thevarious intermediates were carried out both to have anidea of the degradation pathways and to determinewhether toxic and stable compounds are generated. Anillustrative list of aqueous organic pollutants which canbe mineralized in innocuous products is given table 2.

6.2.1. Disappearance of the pollutantThe dearomatization is rapid even in the case of

deactivating substituents on the aromatic ring. This wasobserved for the following substituents: Cl [19,57,58],NO2 [59], CONH2 [60], CO2H [19] and OCH3 [61]. If analiphatic chain is bound to the aromatic ring, thebreaking of the bond is easy as was observed in thephotocatalytic decomposition of herbicide 2,4-D (2,4-dichlorophenoxyacetic acid) [62,63] and tetrachlorvin-phos ((Z)-2-chloro-1 (2,4,5-trichlorophenyl) ethenyldimethyl phosphate) [64], and phenitrothion [65].

6.2.2. Total mineralization of organic pollutantsThe oxidation of carbon atoms into CO2 is relatively

easy. It is, however, in general markedly slower than thedearomatization of the molecule. Until now, the absence

of total mineralization has been observed only in thecase of s-triazines herbicides, for which the nal productobtained was essentially 1,3,5-triazine-2,4,6, trihydroxy(cyanuric acid) [66], which is, fortunately, not toxic. Thisis due to the high stability of the triazinearomatic ring,which resists most oxidation methods. For chlorinatedmolecules, Cl) ions are easily released in the solution[19,57,58] and this could be of interest in a process,where photocatalysis would be associated with a bio-logical epuration system which is generally not ecientfor chlorinated compounds. Nitrogen-containing mole-cules are mineralized into NH4 and mostly NO

3 [60]

Ammonium ions are relatively stable and the proportiondepends mainly on the initial oxidation degree ofnitrogen and on the irradiation time [67]. Actually,NH4 ions are photodegradable provided an alkaline pHsince at acidic pH, the surface of titania is positivelycharged and repels cations [71].

The pollutants containing sulfur atoms are mineral-ized into sulfate ions [67]. Organo-phosphorous pesti-cides produce phosphate ions [64,65,68,69]. However,phosphate ions in the pH range used remainedadsorbed on TiO2. This strong adsorption partiallyinhibits the reaction rate which, however, remainsacceptable [64,65,70]. Until now, the analyses ofaliphatic fragments resulting from the degradation ofthe aromatic ring have only revealed formate andacetate ions. Other aliphatics (presumably acids, diac-ids, hydroxylated compounds) are very dicult toseparate from water and to analyze. Formate andacetate ions are rather stable, as observed in otheradvanced oxidation processes, which in part explainwhy the total mineralization is much longer that thedearomatization reaction.

Table 2

Illustrative list of aqueous pollutants mineralized by photocatalysis (a): A rather complete list of all photocatalytically degradable pollutants has

been established by D. Blake (Ref. 11)

Class of organics Examples

Alkanes Isobutane, pentane, heptane, cyclohexane, parans

Haloalkanes Mono-, di-, tri- and tetrachloromethane, tribromoethane, 1,1,1-triuoro-2,2,2-trichloroethane

Aliphatic alcohols Methanol, ethanol, propanol, glucose

Aliphatic carboxylic acids Formic, ethanoic, propanoic, oxalic, butyric, malic acids, propene,

Alkenes cyclohexene

Haloalkenes 1,2-dichloroethylene, 1,1,2-trichloroethylene

Aromatics Benzene, naphthalene

Haloaromatics Chlorobenzene, 1,2-dichlorobenzene

Nitrohaloaromatics Dichloronitrobenzene

Phenolic compounds Phenol, hydroquinone, catechol, methylcatechol, resorcinol, o- m -, p-cresol, nitrophenols

Halophenols 2-, 3-, 4-Chlorophenol, pentachlorophenol, 4-uorophenol,

Amides Benzamide

Aromatic carboxylic acids Benzoic, 4-aminobenzoic, phthalic, salicylic, m- and

Acids p-hydroxybenzoic, chlorohydroxybenzoic chlorobenzoic acids

Surfactants Sodium dodecylsulfate, polyethylene glycol, sodium dodecyl benzene sulfonate, trimethyl phosphate,

tetrabutylammmonium phosphate

Herbicidespesticides Atrazine, prometron, propetryne, bentazon, 2-4 D, monuron. DDT, parathion, lindane,Organo-phosphorus tetrachlorvinphos, phenitrothion...

Dyes Methylene blue, rhodamine B, methyl orange, uorescein, Congo Red


6.2.3. Degradation pathwaysPrimary intermediates detected and identied by

HPLC, LC/MS and GC/MS of the photocatalyticdegradation of various aromatic pollutants correspondto the hydroxylation of the benzene ring. These interme-diates have very low transient maximum concentrationswith respect to that of the initial pollutant in agreementwith the fact that CO2, acetate and formate are formed inthe initial stages of the degradation. The orientation ofthe hydroxylation of the aromatic ring depends on thenature of the substituents. For instance, for chlorophe-nols and dimethoxybenzenes, the para and ortho posi-tions (with respect to OH for the chlorophenols) arefavored as is expected. By contrast, for benzamide andnitrobenzene, the hydroxylation occurs at all free sites,whereas a meta orientation is expected for electron-withdrawing substituents. The degradation pathways areillustrated here by the examples of fenitrothion (gure 5).

Fenitrothion has the following formula :

OPCH3O

CH3O

NO2

CH3

S

This organothio-phosphorus is very dangerous since itrst oxidizes into fenitrooxon,

OPCH3O

CH3O

NO2

CH3

O

which can be found in warfare gases. Fortunately, thisrst toxic intermediate is easily degraded as its sub-sequent metabolites into nal innocuous inorganicproducts as demonstrated by the mass balance analysis,according to the overall equation:

CH3O2AP(S)AO C6H3ANO2CH3 13

2O2 ! 9CO2 3H2O 4H H2PO4

SO24 NO3

35

This particular efciency of titania has been utilized forproducing efcient gas masks for the US army incontaminated areas [72, 78a]. The air purication fol-lows the same reaction mechanism, provided a mini-mum humidity in air to maintain an hydrated state oftitanias surface, source of the hydroxyl groups, precur-sors of OH radicals. In addition, this surface hydrationis easily maintained by the formation of water duringoxidation of the CAH bonds of organic pollutants to be

destroyed. Such air purication has been recentlysuccessfully applied to the removal of odors in connedatmospheres, as for instance in domestic refrigerators[78b].

6.2.4. Organic pollutant removalcase study of azo-dyes

The organic pollutant removal is the main eld ofwater photocatalytic decontamination. Most of ali-phatic and aromatic pollutants are totally mineralizedinto CO2 and innocuous inorganic anions (Cl), SO24 ,NO3 ). More complex molecules such as pesticides(herbicides, insecticides, fungicides etc...) or dyes arealso totally destroyed. Presently, concerning dyes world-wide used, half of them are azo-dyes (i.e. containing theAN@NA azo group). Several ones (Cibacron BrilliantRed 3BA, Remazol Black B (Reactive Black 5), CongoRed [73], Methyl Red [73], Orange G [73], Amaranth[74], Orange Yellow and Patented Blue) were allsuccessfully destroyed and totally mineralized. Thedeveloped formula is given for 2 of them, (Congo Redwhich is a particularly recalcitrant industrial dye andAmaranth which is a prohibited carcinogenic alimentarydye (gure 6). The stoichiometric coecients of the totaldegradation as well as the mass balances were estab-lished for dierent elements (carbon, hydrogen, oxygen,sulfur) but not for nitrogen (nitrogen mass balanceestablished in the aqueous solution based on nitrate andammonium ions). Using an air-tight batch slurry pho-toreactor connected to an air-tight gas chromatographan evolution of N2 in the gas phase could be evidencedand quantied. For example, N2 evolution from aCongo Red solution is given gure.7. The kinetics curvereached a plateau at tUV 400 min (gure 7) and thetotal mass balance in nitrogen was found equal to 100%.In addition, the ratio 2nN2=nNH4 nNO3 2nN2 was foundequal to 0.65, i.e. just corresponding to the ratio 2/3 ofthe number of N atoms contained in the doubleAN@NA azo-groups to the total number of N con-tained in Congo Red. Similar results for all the otherazo-dyes mentioned above. This means that photocat-alytic degradation of azo-compounds is 100% selectivein generating gaseous dinitrogen.

This is the rst example to our knowledge of anitrogen evolution during the aqueous photocatalyticdegradation of complex N-containing pollutants.This constitutes an ideal example of waterdecontamination.

6.2.5. Tentative modications of the catalysts (noblemetal deposit and ion-doping)

(i) Noble metal deposit: In photocatalytic reactionsinvolving hydrogen, either as a reactant (deuteriumalkane isotopic exchange [20, 21] or as a product(alcohol dehydrogenation [18], the system requires the


presence of a metal acting as a co-catalyst necessary(i) to dissociate the reactant (D2) and (ii) to recombineH and D into dihydrogen (or HD). Additionally, themetal (i) attracts electrons, by PMSI, (ii) decreases theelectron-hole recombination and (iii) maintains theturn-over number constant [12].

For these reasons, Pt was deposited on titania by theimpregnation and reduction method. However, thebenecial effects observed for hydrogen-involving reac-tions became detrimental for total oxidation reactions.This was accounted for by the electron transfer to metalnano-crystallites which became concurrent to dioxygenionosorption.

Pt e ePt 36

O2g e O2 ads 37In addition, once negatively charged, platinum particlesbecame attractive for holes which recombined withelectrons into neutral centers (equation (15)) with pro-duction of unefcient thermal energy. Since depositedmetals act as recombination centers in oxidation reac-tions, efcient photocatalysts should not contain noblemetals, which is of a great advantage for (solar) cheapphotocatalytic applications (see further).

SH-

SO42-

H2PO4-

CO2

OPCH3O

CH3ONO2

CH3

o

NO3-

o

HPIIICH3O

CH3O

NO2-

HO NO2

CH3

HO OH

CH3

O NO2

CH3

OPCH3O

CH3ONO2

CH3

S

HO OH

OH

CH3O NO2

CH3

S

OHPCH3O

CH3O

O O

CH3

PCH3O

CH3O

SCH3

O

O O

OH

CO2

CH3COO-

H2PO4-

HCOO-

SO42-

+ SHe

-+ OH

+ +

+ H

-H + CH3

+ OH

+ CH3

+ H

+OH

- NO2

+ OH

- CH3+ OH

+

+ OH

+OH

+

+

ring opening

+

CO2

Figure 5. Photocatalytic degradation pathway of organo-phosphorus fenitrothion (the framed molecules correspond to those actually detected).


(ii) Ion doping: Another modication was aimed atextending the photosensitivity of titania to the visibleregion to harvest cheaper and more abundant solarecient photons. The best expected case would havebeen Cr3+-doping, since its absorption spectrum givesan important shoulder in the visible.

Unfortunately, Cr3+-doping was found to stronglyinhibit photocatalysis and decrease the quantum yield[75,76]. This detrimental behavior was conrmed byion doping, either of the n-type (by dissolving penta-valent heterocations such as Nb5+, Sb5+, Mo6+,Ta5+ in the lattice of titania) or of the p-type (bydissolving trivalent heterocations such as Ga3+, Al3+).This was explained by the fact that both pentavalentdonor impurities and trivalent acceptor impuritiesbehave as electron-hole recombination centers. How-ever, this drawback could be turned into advantage byusing ion doping as a means of passivating TiO2-based pigments in paintings and plastics againstweathering [77].

7. Polyphasic (solar) photoreactors

To perform the various types of photocatalyticreactions described above, different types of photoreac-tors have been built with the catalyst used under variousshapes: xed bed, magnetically or mechanically agitatedslurries, catalyst particles anchored on the walls of thephotoreactor or in membranes or on glass beads, or onglasswool sleeves, small spherical pellets, etc. [1 4].The main purpose is to have an easy separation of thecatalyst from the uid medium, thence the necessity tosupport titania and to avoid the nal ultrane particleltration.

Various devices have been developed such as TiO2-coated tubular photoreactors, annular and spiral pho-toreactors, falling-lm photoreactors. At present twosystems are commercialized [79]. One uses powder TiO2and concerns the market of waste water treatment. Inthe other system, TiO2 is supported on a ber glass meshcloth in which a cylindrical UV lamp is wrapped. Anevaluation of ultraviolet oxidation methods was carriedout for the removal of 2,4,6-trinitrotoluene fromgroundwater [79,80] These methods were TiO2/UV,O3/UV, H2O2 + additive/UV. Heterogeneous photoca-talysis was found to be the most economical. Eventhough several criticisms can be made to this evaluation,it comes out that heterogeneous photocatalysis appearsas a method that can compete economically with otherUV oxidation processes for water treatment.

The most effective photocatalysts are anatase sampleswhich absorb only ca. 3% of the overall solar energy atthe earths surface. In spite of that, large-scale tests havebeen built or modied and are still used in North-America, Israel and Europe [80,81] to collect data inorder to estimate the cost of water treatment. Solarreactors that do not concentrate the incident light havelower hardware cost, eliminate photon losses at reect-ing surfaces and use diffused sunlight [8090]. For thesenon-concentrating systems, estimates have concludedthat solar photons can be used at a lower cost thanphotons from UV lamps [80]. Solar photocatalysis hasbeen baptized Heliophotocatalysis by the author. Onecould have denoted it heliocatalysis but it not accurateenough since one could have a helio-thermocatalysis,which would use the concentrated IR beams of the sun.That is not yet under study.

7.1. Case study of the compound parabolic collector(CPC) photoreactor at PSA

The principle of the CPC photoreactor used atPlataforma Solar de Almeria (PSA) in Spain is describedin the scheme of gure 8a, whereas the collector itself isdescribed in gure 8b with an inclination angle of 37

corresponding to the latitude of Almeria. The CPCcollector which is the irradiated part of the systemcorresponds to a plug ow reactor but, since it isconnected to a tank via a recirculation pump, the

NH2

SO3Na

N N N NNH2

SO3Na

Congo Red

N N-O3S

HO SO3 -

SO3 -Amaranth

Figure 6. Developed formulae of two azo-dyes: Congo Red (indus-

trial) and Amaranth (alimentary).

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200 1400t (min)

N a

tom

s m

ol

NH4+ molNO3- molN2N total

N total = N NH4+ + N NO3- + 2 N N2

Figure 7. Kinetics of N-containing compound evolution, either in the

aqueous or the gas phase during the photocatalytic degradation of

Congo Red dye.


ensemble corresponds to a batch reactor. Since theelimination of pesticides follows an rst order kinetics(see Section 4.3), the concentration CAo at the inlet andCA at the outlet of the collector are related by theequation

CA=CAo expkt 38Because of the recirculation, CAo varies with time as:

V2dCAo=dt QCAo CA 39where V2 is the volume of the tank, and Q is the owrate. Combining equations (38) and (39),

one gets : V2dCAo=dt QCAo CAo expkV1=Q QCAo1 expkV1=Q

40

Figure 8. Scheme of the CPC photoreactor at PSA.


where V1 is the volume of the irradiated part of thereactor.

The integration of differential equation (40) gives:

CAofinal CAoinitial exp1 expkV1=QQ:t=V241

If the ow rate Q is high and if V1 is not too important,equation (41) can be simplied

expkV1=Q expe ! 1 e kV1=Q 42

ThenceCAo CAoinitial expkV1=Q Q=V2t CAoinitial expkV1=V2t

43The residence time tR in the CPC collector is equal to

tR V1=VTt V1=V1 V2t r=1 rt with r V1=V2

44

Therefore equation (43) becomes:

CAo CAoinitial exp1 r=rk tR 45By determining ratio r from the geometry of the reactor,one can calculate the apparent rst order reaction rateconstant k.

The solar photocatalytic degradation of pollutantsin the CPC photoreactor has been successfully appliedat PSA on various pollutants [15,63,8290]. Theseexperiments can be examplied by the total mineral-ization of 2,4-D (2,4-dichlorophenoxyacetic acid)[62,63], a well known herbicide. In gure 9 arerepresented the temporal variations of 2,4-D, of 2,4-dichlorophenol (2,4-DCP) the main intermediateformed, of Cl) anions produced and of the totalorganic carbon (TOC). After an adsorption period of1 h in the dark, 2,4-D disappears following a rst

order kinetics. Cl- anions follow a sigma shapedcurve in agreement with the consecutive reactionscheme:

2; 4-D!k1 4-DCP!k2 2Cl 8CO2 46giving

Cl 2Co1 k2=k2 k1 expk1t k1=k2 k1 expk2t

47

The TOC decreases linearly to zero within 1 h with anapparent zero kinetic order. That could be interpretedby assuming a saturation of surface sites by all theintermediates.

d[TOC]=dt dCO2=dt kX

hi

kX

KiCi=1 KiCi khsat k48

where hi represents the surface coverage of the ithintermediate and hsat the overall coverage at saturation.gure 11 represents the rst order linear transforms LnCo/C = f(t) of 2,4-D at two different dates with twoUV-radiant uxes illustrated by gure with a partlycloudy day (June 7, 1996) and with a bright sunny day(June 13, 1996). It can be seen in table 3 that both rateconstants of 2,4-D and of TOC disappearance are quiteproportional to the mean UV power ux. This means,according to gure 4, that the CPC is working inoptimal conditions with respect to solar irradiation andcan also work with diffuse UV-light.

These experiments were compared with initial studiesperformed in a laboratory microphotoreactor workingwith articial light, which is described in gure 3.Despite a volume extrapolation factor of 12,500, thesame rst order was found for 2,4-D disappearance; theintermediates were the same, indicating an identicalreaction pathway; the quantum yields were the same.Only two points of apparent divergence were observed:(i) the optimum concentration in titania for the CPCpilot plant was 0.2 g/L instead of 2.5 g/L for the batchmicroreactor; and (ii) the TOC disappearance at PSAwas faster than the CO2 evolution in laboratory exper-iments. This was ascribed to the CPC photoreactordesign [51] with a recirculating tank which favors the

Figure 9. Kinetics of (i) 2,4-D and TOC disappearance, (ii) of Cl-

anion formation and (iii) of 2,4-DCP appearance and disappearance.

Table 3

Mean radiant ux and rate constants of 2,4-D disappearance (k(2,4-

D)) and of TOC elimination (k(TOC)) on two dierent days (June 6th,

1996: partly cloudy; June 13th, 1996: bright sunny). The subscripts 7

and 13 refer to the dates in June 1996

Date F(W/m2 )

k (2,4-D)

(min)1)

k(TOC)

(ppm/min)1)

June 7, 1996 31 6.79 10)2 0.1575June 13, 1996 38.2 8.72 10)2 0.203

U7U13

= 0.81 k7k13 = 0.78kTOC7kTOC13 = 0.78


production of the nal products detrimentally to that ofthe intermediate ones [91].

The photocatalytic degradation at PSA is appliedto the treatment of used waters contaminated by alarge variety of pesticides after the washing ofshredded empty herbicide containers (about 1.5 mil-lion collected per year), in a plant built for thedecontamination and the recycling of plastic. Indeed,herbicides are intensively used in the province ofAlmeria, which has become an important producer offruit and vegetables in green houses because of thevery sunny climate. If the demonstration of thetechnical and economical feasibility of the process isachieved, the city of El Ejido, located in the center ofthe green house area, will become the rst Europeantown to include a solar photocatalytic plant in a realwaste treatment process.

7.2. Solar photocatalytic reactor using deposited titania

Although slurry photoreactors can be estimated asthe most efcient ones, they exhibit an importantdrawback: the nal tedious ltration of titania particles.To overcome this obstacle, titania can be deposited onphotoinert supports. A successful attempt was obtainedby depositing titania on a special Ahlstrom paper usingamorphous silica as a binder to anchor the photocata-lyst particles on inorganic bers [92].

Such a photocatalytic material has been used in anadapted solar photoreactor called cascade falling lmsphotoreactors (CFFP) described in gures 10 and 11.

To test the efciency of CFFP solar reactor,photocatalytic reactions were calibrated against aslurry CPC photoreactor having the same surface ofsun collector as shown on photographs in gure 11.Surprisingly, the results were similar in the totaldegradation of 4-chlrorophenol as indicated by theTOC disappearance (gure 11).

These xed bed photoreactors are presently under-study to produce drink water for remote isolated humancommunities in arid countries (North Africa: the Euro-pean AQUACAT Program; Latin America : The Euro-pean SOLWATER Program [93].

Besides xation of titania on a support, a materialscience is developing to prepare thin layers of titania onself-cleaning objects such as glasses, concrete walls,ceramics, tools etc

8. Extension to the understanding of natural photocata-lytic processes in the environment

Urban atmosphere contains pollutants and solidparticles. Some of them originate from soils such assilicoaluminates and Fe2O3 and others such as TiO2 fromanthropogenic activities (y ashes) [94]. These aerosolsare able to chemisorb atmospheric pollutants and to bephotoactivated by sun light during day time. Orthoxyl-ene [95] and naphtalene [96] were chosen as modelatmospheric pollutants. In dry conditions, illuminatedtitania oxidizes o-xylene into o-tolualdehyde in agree-ment with the oxidative properties of titania [15]. Bycontrast, in the presence of water which is a constituentof the atmosphere, the oxidation becomes total with theformation of CO2 and H2O, because of the photo-induced generation of OH radicals (equation (14)).

Crude-oil residue in contact with beach sand and airhas been observed to undergo photocatalytic oxidationon exposure to light [97]. The beach sand used containedmagnetite and ilmenite as minor constituents. Thesematerials are known to have catalytic properties forhydrocarbon oxidation. These results indicate examplesof environmental photoassisted self-cleaning pro-cesses.

Solar photocatalytic processes can be responsible forthe color of a landscape. For instance, in Sahara,mountains are dark brown, whereas sand is yellow. Flatstones laying on the ground are under the shape ofslabs. Their upper faces exposed to sun are dark brown,whereas their lower faces in contact with the soil areyellow. EDX analyses revealed that the dark color wasmainly constituted by manganese oxides [98,99]. Tita-nium ions (i.e. titania) were permanently found whenMnOx was present [98,99]. A possible photocatalyticoxidation of Mn2+ ions by traces of titania can beproposed to account, at least partially, for this oxide

Figure 10. Comparative schemes of CPC and CFFP.


formation at the solar light exposed surfaces of therocks and stones in agreement with Ref. [55].

Finally, solar photocatalysis is the actually efcientprocess, which makes self-cleaning glasses work [100105]. Fouling of glasses is mainly due to dust and/or atmospheric particles stuck at the surface on greasystains mainly constituted of fatty acids. Self-cleaningglasses are coated with an invisible thin layer of titania,which under the simultaneous presence of oxygen (air),atmospheric water vapor and solar UV-light, is able todecompose fatty acids by successive photodecarboxyla-tion (photo-Kolbe, [19]) reactions and let coatedglasses recover their initial clearness.

9. Conclusions

An overview has been presented on the variousaspects and potentials of heterogeneous photocatalysis.The potential applications strongly depend on the futuredevelopment of the photocatalytic engineering.

Water pollutant removal appears as the most prom-ising potential application since many toxic waterpollutants, either organic or inorganic are totally min-eralized or oxidized at their higher degree, respectively,into harmless nal compounds. Besides some drawbacks(use of UV-photons and necessity for the treated watersto be transparent in this spectral region; slow completemineralization in cases where heteroatoms are at a verylow oxidation degree; photocatalytic engineering to bedeveloped), room-temperature heterogeneous photoca-talysis offers interesting advantages [5]:

chemical stability of TiO2 in aqueous media and inlarge range of pH (0 pH 14)

low cost of titania (g16o/kg) cheap chemicals in use. no additives required (only oxygen from the air). system applicable at low concentrations. great deposition capacity for noble metal recovery. absence of inhibition or low inhibition by ions gen-erally present in water.

total mineralization achieved for many organic pol-lutants.

eciency of photocatalysis with halogenated com-pounds sometimes very toxic for bacteria in biologicalwater treatment.

possible combination with other decontaminationmethods (in particular biological).

Heterogeneous photocatalysis is now reaching the pre-industrial level. Several pilots and prototypes have beenbuilt in various countries [3].

However, one has to be realistic. Because of thequantum yield and of the incident efcient photon ux,photocatalysis is necessarily devoted to limited devices.More precisely, this means that a reasonable sizephotoreactor would be able to treat from 1 to a fewtens of m3 of aqueous euents/day.

In parallel, photocatalysis can be applied to airtreatment provided the presence of a minimum humidityto favor the formation of OH radicals detrimentally tothat of dissociated activated oxygen O* species respon-sible for selective mild oxidation reactions as developedin Ref. 9.

Figure 11. Comparison of the activities of titania photocatalysts used (i) in a CPC slurry photoreactor (picture bottom right) and in a CFFP

using a titania xed bed deposited on an Ahlstrom paper (picture bottom left). The photocatalytic activity is based on the rate of the TOC

removal from a solution containing 50 ppm of 4-chlorophenol chosen as a model pollutant.


Acknowledgments

The author considers this article as a modest contri-bution to the celebration of the memory of Pr. R.L.Burwell Jr., his former adviser, during his postdoctoralstay at Northwestern University in Ipatieff Laboratories(19741975) where he was initiated to Catalysis byMetals.

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