tio2

SCIENCE CHINA Chemistry

© Science China Press and Springer-Verlag Berlin Heidelberg 2010 chem.scichina.com www.springerlink.com

*Corresponding author (email: [email protected])

• REVIEWS • September 2010 Vol.53 No.9: 1831–1843

doi: 10.1007/s11426-010-4076-y

Environmental photocatalysis: Perspectives for China

HERRMANN Jean Marie*

Institut de Recherches sur la Catalyse et l’Environnement de Lyon; UMR 5256 CNRS/Université Lyon1, 2 Av. Albert Einstein 69626 Villeurbanne Cedex, France

Received June 16, 2010; accepted July 20, 2010

Photocatalysis is based on the double aptitude of the photocatalyst (essentially titania) to simultaneously adsorb reactants and absorb efficient photons. Heterogeneous photocatalysis is able to be efficient in Fine, “Green” and Environmental Chemistry. Photocatalysis induces mild oxidations in the absence of water by generating active neutral atomic O* species. For instance, the oxidation of 4-tert-butyl-toluene is 100% selective in 4-tert-butyl-benzaldehyde. In water treatment, many toxic inorganic ions are oxidized in their harmless upper oxidized state. The elimination of organic pollutants is the main field of water photocata-lytic decontamination. Most of aliphatic and aromatic pollutants are totally mineralized into CO2 and innocuous inorganic ani-ons. More complex molecules, such as pesticides (herbicides, insecticides, fungicides, etc.) or dyes, are totally destroyed. An-other example of green chemistry is the total degradation of dyes in water, in particular for the azo-dyes, with 100% selective degradation of N=N azo-groups into di-nitrogen. Photocatalysis is also active in the “bio-world” by killing bacteria (E. Coli, streptococcus, etc.) in water without re-growth. Air pollutants can also be destroyed, especially all the VOC’s (volatile organic compounds), providing certain air humidity enabling titania to produce cracking OH• radicals. For chemical engineering rea-sons, the photocatalyst has to be fixed on a photo-inert support. In these conditions, UV-irradiated titania-based photocatalysis could be applied to the elimination of air pollutants, VOC’s, solvents, odors, chemicals, etc. Air treatment has to be associated with water and solid waste treatment because of odors. This is conducted by covering water treatment ponds or lagunas by rafts on which large sheets of Ahlstrom paper are deposited, supporting titania associated with activated carbon. Eventually, virus AH5N2, a model virus close to H5N1, responsible for the avian flu could be totally inactivated. Photocatalysis is directly con-nected with all 12 principles of Green Chemistry defined by Anastas (1998) and possesses open perspectives for China.

photocatalysis, green chemistry, environmental chemistry, air pollutants, water pollutants, solar energy, China

1 Introduction

Photocatalysis originated from different catalysis laborato-ries in Europe. In England, Stone et al. first studied the photo-adsorption/desorption of oxygen on ZnO [1] before studying the photocatalytic oxidation of CO on the same solid [2]. They subsequently switched to titania under the rutile phase for oxygen photo-adsorption [3] and selective isopropanol oxidation in acetone [4]. The latter reference was the first one, to my knowledge, to mention OH• radi-cals as oxidizing agents formed by neutralization of surface

OH by photo-holes h+. In addition, this simple and selec-tive reaction remains a direct and simple test to demonstrate the photo-activity of solids. Simultaneously, in Germany, Hauffe et al. also studied the photocatalytic oxidation of CO on ZnO [5, 6] and, actually, this reference was the first one to include the term “photocatalysis” in its title. In the same decade, Juillet and Teichner in France studied the sintering of ultra-pure oxide powders for nuclear applications and tested their solids through their electrical properties. The erratic results obtained on titania anatase puzzled them till they realized that titania was photosensitive to daylight, especially in sunny days (Juillet F and Teichner SJ, 2003, private communication). They subsequently used the photo- activated oxygen species to perform mild and selective oxi-

1832 HERRMANN Jean Marie Sci China Chem September (2010) Vol.53 No.9

dations of alkanes [7, 8]. While photocatalysis was developed confidentially in

Europe, there was an “earthquake” in or from Japan, ac-cording to Bickley [9] with the re-publication in English of the previous work by Fujishima and Honda on the photo- electrolysis of water using a UV-irradiated titania-based anode [10] in the review [11]. This constituted the initial event for the globalization of photocatalysis, which had preferential development in Japan, as illustrated by refer-ence [12]. Unfortunately, recent scientists in the field of photocatalysis may have never read this article and improp-erly cite it as the starting point of photocatalysis, which is apparently erroneous.

With the increasing number of global publications, the frame of photocatalysis needs to be urgently refocused, es-pecially by senior scientists. This is the aim of this work.

Heterogeneous photocatalysis is a discipline which in-cludes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, oxygen-18 and deute-rium isotopic exchange, metal deposition, water detoxifica-tion, gaseous pollutant removal, bactericidal action, etc. In line with the last point, it can be considered as one of the new “Advanced Oxidation Technologies” (AOT) for air and water purification treatments. Several books and reviews have been devoted to this problem [12–24].

The review by Blake in 2001 already reported more than 1200 references on the subject [24].

2 Principles of heterogeneous photocatalysis

When a semiconductor catalyst SC of the chalcogenide type (oxides (TiO2, ZnO, ZrO2, CeO2,...), or sulfides (CdS, ZnS,...)) is illuminated with photons whose energy is equal to or greater than their band-gap energy Eg (h≥Eg), there is ABsorption of these photons and creation of electron-hole pairs within the bulk, which dissociate into free photoelec-trons in the conduction band and photoholes in the valence band. (Figure 1)

Simultaneously, in the presence of a fluid phase (gas or liquid), spontaneous ADsorption occurs and according to the redox potential (or energy level) of each adsorbate, electron transfer proceeds towards acceptor molecules, whereas positive photoholes are transferred to donor mole-cules (actually the hole transfer corresponds to the cession of an electron by the donor to the solid).

+SC e hh (1)

(ads) (ads)A e A (2)

(ads) (ads)D h D (3)

Each ion formed subsequently reacts to form the interme-diates and final products. As a consequence of eqs. (1)–(3),

Figure 1 Electron energy plotted upwards as a function of the distance from the surface to the bulk of the solid.

the photonic excitation of the catalyst appears as the initial step of the activation of the whole catalytic system. Thence, the efficient photon has to be considered as a reactant and the photon flux as a special fluid phase, the “electromag-netic phase”. The photon energy is adapted to the absorption of the catalyst, not to that of the reactants. The activation of the process goes through the excitation of the solid but not through that of the reactants: there is no photochemical process in the adsorbed phase but only a true heterogeneous photocatalytic regime as demonstrated further.

The photoefficiency can be reduced by the electron-hole recombination, shown in Figure 2, which corresponds to the degradation of the photoelectronic energy into heat.

e h N E (4) (N: neutral center; E: energy (heat))

Heterogeneous photocatalysis can be carried out in various media: the gas phase, pure organic liquid phases or aqueous

Figure 2 Fate of electrons and holes within a spherical particle of titania in the presence of acceptors (A) and (D) molecules (after late Dr. H. Ger-isher, p1 in ref. [15]).

HERRMANN Jean Marie Sci China Chem September (2010) Vol.53 No.9 1833

solutions. As for classical heterogeneous catalysis, the overall process can be decomposed into five independent steps: (1) transfer of the reactants in the fluid phase to the surface; (2) adsorption of at least one of the reactants; (3) reaction in the adsorbed phase; (4) desorption of the product(s); (5) re-moval of the products from the interface region.

The photocatalytic reaction occurs in the adsorbed phase (step n° 3). The only difference from conventional catalysis is the mode of activation of the catalyst in which the ther-mal activation is replaced by photonic activation.

Among various chalcogenides (oxides and sulfides), the best photocatalytic performances with maximum quantum yields are always obtained with titania. In all the systems described in the present paper, the catalyst used was titania (Degussa TiO2 P-25, 50 m2/g, mainly anatase), unless oth-erwise stated.

3 Influence of physical parameters governing the kinetics

All the results are summarized in the five diagrams of Figure 3.

3.1 Mass of catalysts

Either in static or in slurry or in dynamic flow photoreactors, the initial rates of reactions were found to be directly pro-portional to the mass m of catalysts (Figure 3(a)). This in-dicates a true heterogeneous catalytic regime. However, above a certain value of m, the reaction rate levels off and becomes independent of m. This limit depends on the ge-ometry and the working conditions of the photoreactor. For higher amounts of catalysts, a screening effect of excess

Figure 3 Influence of different physical parameters governing the reac-tion rate [17]. (a) Mass of catalysts; (b) wavelength; (c) initial concentra-tion of reactants; (d) temperature; (e) radiant flux.

particles occurs, which masks part of the photosensitive surface. For applications, this optimum mass of catalysts has to be chosen in order (i) to avoid an unuseful excess of catalysts and (ii) to ensure total absorption of efficient pho-tons. These limits range from 0.2 to 2.5 g/L of titania in slurry batch photoreactors.

3.2 Wavelength

The variation of the reaction rate as a function of the wave-length follows the absorption spectrum of the catalyst (Fig-ure 3(b)), with a threshold corresponding to its band gap energy. For TiO2 having Eg = 3.02 eV, this requires ≤400 nm, i.e., near-UV wavelength (UV-A). In addition, it needs to be checked that the reactants do not absorb the light to conserve the exclusive photoactivation of the catalyst for a true heterogeneous catalytic regime (no homogeneous nor photochemistry in the adsorbed phase).

3.3 Initial concentration

Generally, the kinetics follows a Langmuir-Hinshelwood mechanism confirming the heterogeneous catalytic charac-teristic of the system with the rate r varying proportionally with the coverage as

1r k k KC KC (5)

For diluted solutions (C < 103 M), KC becomes <<1 and the reaction is of the apparent first order, whereas for concentrations > 5 × 103 M, (KC >> 1), the reaction rate is maximum and of the apparent order (Figure 3(c)).

In the gas phase similar Langmuir-Hinshelwood expres-sions have been found including partial pressures P instead of C.

3.4 Temperature

Because of the photonic activation, the photocatalytic sys-tems do not require heating and operate at room temperature. The true activation energy Et is nil, whereas the apparent activation energy Ea is often very small (a few kJ/mol) in the medium temperature range (20 °C ≤≤ 80 °C). How-ever, at very low temperatures (40 °C ≤ ≤ 0 °C), the activity decreases and the activation energy Ea becomes positive (Figure 3(d)).

By contrast, for various types of photocatalytic reactions at “high” temperatures (≥70–80 °C), the activity de-creases and the apparent activation energy becomes nega-tive (Figure 3(c)). This behavior can be easily explained within the frame of the Langmuir-Hinshelwood mechanism described above. The decrease in temperature favors ad-sorption (which is a spontaneous exothermic phenomenon) with in particular that of the final product, which becomes the inhibitor of the reaction. By contrast, when increases


above 80 °C and tends to the boiling point of water, the exothermic adsorption of reactant A becomes disfavored and tends to become the rate limiting step of the whole re-action.

As a consequence, the optimum temperature is generally between 20 and 80 °C. This explains why solar devices which use light concentrators instead of light collectors re-quire coolers [25]. This absence of heating is attractive for photocatalytic reactions carried out in aqueous media and in particular for environmental purposes (photocatalytic water purification). There is no need to waste energy in heating water, which possesses a high heat capacity. This explains why photocatalysis was estimated cheaper than incineration.

3.5 Radiant flux

It has been shown that for all types of photocatalytic reac-tions, the reaction rate r is proportional to the radiant flux (Figure 3(e)). This confirms the photo-induced nature of the activation of the catalytic process, with the participation of photo-induced electrical charges (electrons and holes) to the reaction mechanism. However, above a certain value de-pending on the photoreactor, the reaction rate r becomes proportional to 1/2, indicating strong electron-hole recom-bination. These two regimes have been independently demonstrated by Egerton et al. [26] and the author [27, 28].

3.6 Quantum yield

By definition, it is equal to the ratio of the reaction rate in molecules converted per second (or in mols per second) to the incident efficient photonic flux in photons per second (or in Einstein per second (an Einstein is a mol of photons)). This is a kinetic definition, which is directly related to the instantaneous efficiency of a photocatalytic system. Its theoretical maximum value is equal to 1. It may vary in a wide range according to (i) the nature of the catalyst, (ii) the experimental conditions used (mcat, concentrations, T, , ...) and especially (iii) the nature of the reaction considered. We have found values between 102% and 70%. The knowledge of this parameter is fundamental. It enables one (i) to com-pare the activity of different catalysts for the same reaction, (ii) to estimate the relative feasibility of different reactions, and (iii) to calculate the energetic yield of the process and the corresponding cost.

4 Main types of photocatalytic reactions

4.1 Mild oxidation reactions

The gas phase oxidations using dry air as the oxidizing agent mainly concern the mild oxidation of alkanes, alkenes and alcohols into carbonyl-containing molecules (aldehydes and ketones) [4, 8, 9, 29]. Liquid phase reactions concern

the selective mild oxidation of liquid hydrocarbons (alkanes, alkenes, cycloalkanes, aromatics) into aldehydes and ke-tones [30, 31]. For instance, cyclohexane and decaline were oxidized into cyclohexanone and 2-decalone, respectively, with an identical selectivity (86%) [30]. Aromatic hydro-carbons [31] such as alkyltoluenes or o-xylenes were 100% selectively oxidized on the methyl group into alkylbenzal-dehyde:

6 4 3 2 6 4 2R C H CH O R C H CHO H O (6)

Pure liquid alcohols were also oxidized into their corre-sponding aldehydes or ketones. In particular, the oxidation of isopropanol into acetone, first mentioned in reference [4], was later chosen as a photocatalytic test for measuring the efficiency of passivation of TiO2 or ZnO-based pigments in paintings against weathering.

The high selectivity was ascribed to a photoactive neutral, atomic oxygen species [29].

*O ads h O (7)

4.2 Photocatalytic reactions involving hydrogen

In photocatalytic reactions involving hydrogen, either as a reactant (deuterium-alcane isotopic exchange [32]) or as a product (alcohol dehydrogenation [33]), the system 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 photoinduced metal-support interaction (PMSI), (ii) decreases the electron-hole recombination and (iii) makes the reaction run catalytically.

4.3 Total oxidation reaction in aqueous phase

The selective mild oxidation reaction could be obtained in gaseous or liquid organic phases. By contrast, as soon as water is present, the selectivity turns in favor of total oxida-tive degradation. This was ascribed to the photogeneration of stronger, unselective, oxidizing species, namely OH• radicals originating from water via the OH groups of tita-nia's surface:

•2 ads(H O H OH ) h H OH (8)

•

2 2

OH reactants Intermediates

Final Products CO , H O, X , A ...

(9)

OH• radicals are known as the second best oxidizing entity after fluorine. This system is the most promising issue for an application of heterogeneous photocatalysis, because it is directly connected to water detoxification and pollutant re-moval in aqueous effluents. The oxidative degradation is based on the production of OH• radicals which is described in Table 1.


Table 1 Electronic processes for OH• radical generation used in organic degradation

Processes Entry

TiO2 + h → e

+ h+ (1)

O2 + e

→ O2- (2)

H2O OH + H

+ (3)

OH + h

+ → OH• (4)

O2

+ H+

→ HOO• (5)

2 HOO• → O2 + H2O2 (6)

H2O2 → 2 OH• (7)

H2O2 + e

→ OH• + OH (7’)

RH + •OH → R•

+ H2O (8)

R• +

•OH → R• + H2O (9)

ROH + •OH → Intermediates → CO2 (10)

Loss of one carbon atom via the “photo-Kolbe” reaction: RCOO

+ h+

→ R COO• → R• + CO2

5 Photocatalytic water decontamination

Besides the main field of organic pollutant removal, the photocatalytic water decontamination can also be employed for the recovery or the detoxification of inorganic pollut-ants.

5.1 Inorganic pollutants

Various toxic anions can be oxidized into harmless or less toxic compounds by using TiO2 as a photocatalyst. For in-stance, nitrite is oxidized into nitrate, sulfide, sulfite and thiosulfate are converted into sulfate, whereas cyanide is converted either into isocyanate or nitrogen or nitrate.

5.2 Organic pollutants

The aim of our studies was to establish correlations between the molecular structure of the pollutants and their photo-catalytic degradability. The analysis of the various interme-diates was carried out both to have an idea of the degrada-tion pathways and to determine whether toxic and stable intermediate compounds are generated.

The dearomatization is rapid even in the case of deacti-vating substituents on the aromatic ring, such as Cl, NO2, CONH2, CO2H and OCH3. If an aliphatic chain is bound to the aromatic ring, the breaking of the bond is easy as was observed in the photocatalytic decomposition of pesticides.

The oxidation of carbon atoms into CO2 is relatively easy. It is, however, in general markedly slower than the dearo-matization of the molecule. Until now, the absence of total mineralization has only been observed in the case of s-triazines herbicides, for which the final product obtained was essentially 1,3,5-triazine-2,4,6, trihydroxy (cyanuric acid) [34], which is, fortunately, not toxic. This is due to the high stability of the triazine nucleus, which resists most

oxidation methods.

6 Photocatalytic air treatment

All pollutants degraded in the aqueous phase can also be destroyed in air, provided the presence of certain air humid-ity enabling titania to produce cracking OH• radicals at the surface and/or in the adsorbed phase. This concerns the degradation of volatile organic compounds (VOC’s) and odors. This is mostly applied to clean air in confined at-mospheres (workshops, submarines, clean rooms, refrigera-tors, hospitals) on fixed catalytic beds. For chemical engi-neering reasons, the photocatalyst has to be used in a fixed bed, deposited on a photo-inert support. This has been done using special AHLSTROM papers on which titania can also be associated with activated carbon to absorb pollution peaks. In these conditions, UV-irradiated titania-based photocatalysis could be applied to the elimination of air pollutants, VOC’s, solvents, odors, chemicals, etc.

Some air purification devices are envisaged for the elec-tronics industry, which requires “molecular” purity of the ambient atmosphere for better performances of their nano- scale components. It can be noted that silicon-containing VOC’s are destroyed, the organic part being mineralized as CO2 and H2O, whereas Si is converted in silica, which re-mains at the surface of titania, but without troubles for photocatalysis as it is transparent to UV-light.

7 Misconceptions in photocatalysis to be avoided

There are common erroneous features in previous reports which need to be listed to avoid them.

7.1 Use of over-powered UV-lamps

It is not necessary to use overpowered electrical lamps, i.e., with an electrical power P > 100W. It is just necessary to know the number of UV-photons emitted per second and adapt it to the optimum quantum yield. Visible photons are inactive, whereas IR-ones, responsible for heat, can be det-rimental for the initial (exothermic) adsorption of the reac-tants.

7.2 Confusion between reaction rate and conversion

Very often it can be read, in published or submitted articles, that “reaction rate r is of the (apparent) first order”:

d dr C t kC

However, a few lines further in the text, it is written that the rate decreases when the concentration is increased. This surprising contradiction is due to the confusion between reaction rate r and conversion which is defined as:


0 0 0– 1C C C C C

From the derivatives of r and (dr/dC = +k and d /dC =

1/C0), it is apparent that rate r always increases with C, whereas conversion decreases with C. This misconception is due to the ignorance of definitions in catalysis of new comers in the field.

7.3 Non-respect to thermodynamics

The attempt to decrease the photon energy towards the visi-ble to “harvest the abundant visible energy spectrum of the sun” needs to take into account the necessity of passing over a minimum energy threshold because, otherwise, the energy supply for the activation and the generation of such highly cracking and degrading species that are OH• radicals would become thermodynamically detrimental.

+2h H O H +OH

7.4 Conversions lower than the “Stoichiometric Threshold”

For demonstrating the true catalytic nature of a photocata-lytic reaction, the conversion has to be carried out beyond a certain percentage corresponding to the catalytic threshold. It is defined as the minimum number of molecules that have to be converted to be greater than the maximum number of potential active sites initially present at the surface of a mass m of titania photocatalyst used in the reaction. If we admit that for titania the maximum surface site density is equal to 5 × 1018 sites/m2 according to Boehm [35], a mini-mum number nmin of molecules converted equal to nmin = (5

× 1018) × m × SBET is obtained. For example, if a photoreactor contains 1 g of titania Degussa P-25 (with SBET = 50 m2/g), which is fully illuminated and respects the laws mentioned above, a given photocatalytic reaction could be declared “truly catalytic” only if the number of converted molecules is larger than nmin = (5 × 1018) × m × SBET = 2.5 × 1020 mole-cules/gcat, i.e., 4.2 × 104 mol/gcat. Actually, a true catalytic system works with ratios n/nmin of several orders of magni-tude. For example, ratios n/nmin much higher than 103 could be obtained in alcohol dehydrogenation [21].

7.5 Absence of mass balance determination

All the studies in photocatalysis should include an exhaus-tive overall mass balance analysis, especially (i) for carbon based on Total Organic Carbon analysis (TOC) and (ii) for organic nitrogen. In the latter case, in the photocatalytic degradation of azo-dyes, the nitrogen mass balance estab-lished on the final aqueous contents in NH4

+ and NO3 could

only reach ca. 30%–35% [36–38]. A thorough complete analysis indicates an unsuspected evolution of gaseous ni-trogen in the air. Actually, this amount of N2(g) corresponds

to a 100 % selective conversion of the N=N azo-groups. Such a result underlines the environmentally friendly char-acteristic of the degradation of azo-dyes, which represents 45% of the global industrial dye production.

7.6 Erroneous photocatalysis normalized tests

In line with the applications and commercialization of photocatalytic devices (air purifiers, domestic refrigerators, self-cleaning materials, etc.), photocatalytic normalized tests have to be clearly defined and disseminated. In addi-tion to the above recommendations, a real photocatalytic activity test can be erroneously claimed if a non-catalytic side reaction or an artifact does occur. Many tests are based on dye decolorization, which is easy to perform with a UV-visible spectrophotometer. However, these tests can represent the most “subtle pseudo-photocatalytic” systems, hiding the actual non-catalytic nature of the reaction in-volved. This was quantitatively demonstrated with the ap-parent photocatalytic “disappearance” of indigo carmine dye [39]. Whereas indigo carmine IC was totally destroyed by UV-irradiated titania, its color also disappeared when only visible light was used. Actually, IC was decolorized but its corresponding Total Organic Carbon (TOC) re-mained intact. The loss of color actually corresponded to a limited stoichiometric transfer to titania of electrons origi-nating from indigo molecules, photo-excited in the visible as IC*. This is quite possible because the electronic energy level of IC* is higher than that of the conduction band of titania.

This electron transfer destroys the regular distribution of conjugated bonds within the dye molecule and causes its decolorization. Once transferred to titania, the electron par-ticipates in an additional ionosorption of molecular oxygen as O2

. This is described by the following equations and illus-

trated by Figure 4.

Figure 4 Degradation of Indigo Carmine dye under UV-irradiation (a) and electron transfer from excited IC* molecules without hole formation under visible light (b) [39].


*

ads adsvis IC IChv (10)

*2adsIC IC e TiO (11)

2 2(ads)2 adse TiO O O (12)

When the same reaction was performed with a higher concentration of IC providing an initial number of mole-cules larger than the stoichiometric threshold described above, the solution remained totally colored with the same constant initial TOC value.

As a consequence, all standardization tests, exclusively based on dye decolorization, should be banished..

7.7 Failure of improvement of the activity by modifica-tions of the catalysts

This has been done by (i) noble metal deposit and (ii) ion-doping

7.7.1 Noble metal deposit

The beneficial effects of metal deposits were only observed for hydrogen-involving reactions. By contrast, these deposits become detrimental for total oxidation reactions [40]. This was accounted for by the electron transfer to metal nano-crystallites, which becomes concurrent to dioxygen ionosorption.

ePt e Pt (13)

2 2(ads)O g e O (14)

In addition, once negatively charged, platinum particles become attractive for holes which recombine with electrons into inefficient thermal energy. This is why efficient titania oxidation photocatalysts do not contain noble metals, which is of a great advantage for environmental (solar) photocata-lytic applications.

7.7.2 Ion doping

The other modification aimed at extending the photosensi-tivity of titania to the visible region to harvest cheaper and more abundant solar efficient photons. The best expected case would have been Cr3+-doping, because its absorption spectrum gives a significant shoulder in the visible. Unfor-tunately, Cr3+-doping was found to strongly inhibit the reac-tions and to decrease the quantum yield [41]. This detri-mental behavior was confirmed by ion doping, either of the n-type (by dissolving pentavalent heterocations such as Nb5+, Sb5+, Mo6+

and Ta5+ in the lattice of titania)

or of the

p-type (by dissolving trivalent heterocations such as Ga3+ and Al3+).

Actually, it is noteworthy that doping consists of dis-solving controlled and moderate amounts of heterovalent

cations in lattice sites of Ti4+ host cations to apply the ”in-duction valence law” defined in electronics [42] and illus-trated in Figure 5.

As shown in Figure 5, n-type doping creates additional conduction electrons, which increases the rate R of electron- hole recombination:

R e h .R k

By contrast, p-type doping creates acceptor center A, which was once charged as A, attracts holes h+. As a con-sequence, both pentavalent donor impurities and trivalent acceptor impurities behave as electron-hole recombination centers.

Photocatalytic studies on doping have been numerous, even too numerous. Improvement in catalysis requires a benefit by at least a factor of two, or even by one order of magnitude. This was never observed in previous reports. Our group have personally only observed detrimental ef-fects since 1982 [30, 41].

Anionic doping has been a new innovative concept with the narrowing of the band gap energy [43]. For nitrogen doping (N-doping) which must not be confunded with n-type doping (see above), it must be demonstrated that (i) nitrogen is present in a nitride state N3, (ii) N3 anions are in O2 lattice bulk positions and (iii) in oxidizing working conditions, titania has no tendency to self-clean by reoxi-dizing and expulsing N3 anions with a favorable decrease of the ionic radius of element N from 1.71 Å to 0.55, 0.25, 0.16 and 0.13 Å for 3, 0, +1, +3 and +5 oxidation number of N, respectively. My personal position is “Wait and see”.

8 Environmental photocatalysis: Perspectives for China

Photocatalysis is particularly attractive for environmental applications, in line with the rules of Green Chemistry.

Figure 5 Illustration of the doping of titania, either n-type by penta- or hexavalent heterocations or p-type by trivalent cations.


8.1 Definition of “Green Chemistry”

“Green Chemistry” is not the chemistry of agricultural products and/or residues. It is that directly connected with all 12 principles of “Green Chemistry” [44], which are listed below:

(1) Prevention (2) Atom Economy (3) Less Hazardous Chemical Syntheses (4) Designing Safer Chemicals (5) Safer Solvents and Auxiliaries (6) Design for Energy Efficiency (7) Use of Renewable Feedstocks (8) Reduce Derivatives (9) Use of Catalysis (10) Design for degradation (11) Real-time Analysis for Pollution Prevention (12) Safer Chemistry for Accident Prevention It can be easily demonstrated that photocatalysis is in line

with at least the nine first principles.

8.2 “Green” selective mild oxidations in Fine Chemicals

In dry media, photocatalysis is able to induce mild and (regio-) selective oxidations owing to the creation of a neutral atomic O* species, generated by the neutralization of disso-ciatively adsorbed oxygen by photo-induced holes

2TiO hn e h

*(ads)O h O

which are able to insert in a CH bond forming a “hot” molecule of alcohol subsequently easily oxidizable in alde-hyde or ketone.

*3 2R CH O R CH OH

*2 22

R CH OH O R CH OH R CHO H O

For example, a significant intermediate in industrial fine (perfume) chemistry (4-tert-butyl-benzaldehyde) is envi-ronmentally-hostilely prepared in industry by a stoichiomet-ric oxidation of 4-tert-butyl-toluene by permanganate in a strongly acidic aqueous medium with a lot of (in)organic by-products:

4 9 6 4 3 4 2 4

4 9 6 4 2 4 4 2

5C H C H CH 4KMnO 6H SO

5C H C H CHO 2K SO 4MnSO 11H O

By contrast, the oxidation of 4-TBT is 100% selective in 4-tert-butyl-benzaldehyde by mere irradiation of titania at room temperature in pure organic phase (both gas or liquid) without using water [31]:

4 9 6 4 3 2 2

4 9 6 4 2

C H C H CH O air TiO h

C H C H CHO H O

The photocatalytic oxidation of 4-TBT is a typical exam-ple of “Green Chemistry” with the use of air and a cheap, stable and recyclable titania catalyst which does not need solvents nor heating but only UV-A light provided by lamps whose technology permanently improves.

8.3 Environmental photocatalysis

As mentioned above, titania becomes a total oxidation cata-lyst once in water or in humid air because of the photogen-eration of OH• radicals (2nd best oxidant after fluorine) resulting from the neutralization of OH surface groups by positive holes h+.

•OH ads h OH

8.3.1 Water treatment

Many toxic inorganic ions are oxidized in their harmless upper oxidized state. For example, SO3

2, HSO3, S2O3

2, S2

and HS are oxidized into innocuous SO42 ions, whereas

PO33 is oxidized into PO4

3. NO2 and NH4+ ions are oxidized

into NO3, whereas CN is oxidized into OCN-and subse-

quently hydrolyzed into NH4+ and CO3

2. The main field of water photocatalytic decontamination

concerns the mineralization of organic pollutants. Most of aliphatic and aromatic pollutants are totally mineralized into CO2 and H2O. More complex molecules such as pesticides (herbicides, insecticides, fungicides, etc.) or dyes are totally destroyed. The detoxification is total because all heteroatoms contained in the initial molecule are converted into innocuous inorganic anions. In particular, the dangerous organophosphorous compounds are completely eliminated, for example, phenitrothion.

It completely disappeared [45] according to the stoich- iometry:

3 6 3 2 3 22

22 2 2 4 4 3

CH P S O C H NO CH 13 2O

9CO 3H O 4H H PO SO NO

In the long degradation chain of this insecticide, the first intermediate formed is fenitrooxon (see the formula below) which is used as “homicide” in warfare gas composition.

fenitrooxon

Another example of green chemistry is the total degrada-tion of dyes present in used water of textile industries. In particular, the azo-dyes, which represent 45% of the global world market, are heavily released in the environment. An example is given with reactive Red-4 in Figure 6.


Figure 6 Reactive Red-4 (Cibacron Brilliant Red 3B-A) [38]. max = 517 nm.

Interestingly, 100% selective degradation of the N=N azo-groups into di-nitrogen N2 was observed. This is valid for all azo-dyes tested and this constitutes a very environmentally-friendly reaction.

Concerning water treatment, photocatalysis is unable to compete with biological treatments, which are adapted to large amounts of wastes, while being cheaper and more friendly to the environment. However, it can be used as a complementary initial partial treatment by eliminating pol-lutants which are toxic for bio-organisms. In addition, this photocatalytic pretreatment should be done upstream, close to the emission source of pollutants, before dilution or, sig-nificantly, before mixing with other aqueous pollutants.

8.3.2 Photocatalytic potabilization of water

This is a significant point because photocatalysis can be efficient for producing moderate amounts of drink water. Potabilization requires (i) detoxification and (ii) disinfection. Detoxification was commented in the above paragraph. For disinfection, photocatalysis is also active in the “bio-world” by killing several micro-organisms such as bacteria (E. coli, streptococcus faecalis, etc.) in water without re-growth. This is illustrated in Figure 7.

8.3.3 Air treatment

This mainly concerns the degradation of VOC’s and odors. This is mostly applied to clean air in confined atmospheres (workshops, submarines, clean rooms, refrigerators, hospi-tals) on fixed catalytic beds. All pollutants degraded in the aqueous phase can also be destroyed when they are in the

Figure 7 Killing of E. coli before (left) and during the adsorption before attack by UV-irradiated titania particles (right). (This Figure is from final report of European Project AQACAT headed by the author.)

air, especially all the VOC’s, provided the presence of cer-tain air humidity enabling titania to produce cracking OH• radicals at the surface to react in the adsorbed phase.

In addition, it has been previously shown that the addi-tion of activated carbon to titania in the aqueous phase cre-ated a strong interaction between both solid phases, with a kind of synergy: activated carbon adsorbs a great amount of pollutants which are subsequently transferred to titania un-der the action of a driving force corresponding to an intense concentration gradient between activated carbon and “self-cleaning” titania. Such synergy exists in (humid) air treatment and enables one to absorb pollution peaks. In these conditions, UV-irradiated titania-based photocatalysis could be applied to the elimination of air pollutants, VOC’s, solvents, odors, chemicals, etc.

Concerning odors, air purification has been recently ap-plied to the removal of odors in confined atmospheres, in particular in domestic refrigerators (Figure 8) [46]. An anti-odor domestic refrigerator prototype has been validated, patented and industrially produced in a first series at 40000 examplaries followed by a second one of 70000 units.

Air treatment also has to be associated to water and solid waste treatment because of odors. This is done by covering water treatment ponds or lagunas by rafts on which large sheets of special strong papers, named as “media” and pro-duced by Finnish firm Ahlstrom, whose R&D center is lo-cated close to Lyon, are deposited, supporting associated titania-activated carbon catalysts which destroy odors with solar UV-photons in the daytime, whereas activated carbon traps odors during the night before their transfer to titania the next day, the driving force being the very strong con-centration gradient between AC and self-cleaning titania (Figures 9 and 10).

The same system for odor removal was also applied to solid wastes. For example, the sludge of paper mills has to be stored in warehouses before disposal on agricultural fields before plowing, which happens only twice a year. During storage, fermentation occurs with emission of bad odors, in particular those containing sulfur. These sludges are stored in special warehouses, whose covers and win-dows are made of Ahlstrom paper covered with titania as-sociated with activated carbon.

As for water treatment, titania-based photocatalysis is

Figure 8 Photocatalytic elimination of odors in domestic refrigerators.


active for disinfection by killing bacteria (E. coli, streptococ-cus faecalis, etc.) without re-growth. Virus H5N2, a model virus close to H5N1, responsible for the avian flu, was inac-tivated in a contaminated air flow of 40 m3/h with 99.93% efficiency for a total air disinfection in a single pass [47].

8.4 Solar photocatalysis or “heliophotocatalysis”

The UV-A solar spectrum can be advantageously used for

Figure 9 Elimination of strong odors emitted by biological treatment basins in food industry (presently mustardery close to Dijon) (by courtesy of Mr. J. Dussaud, Ahlstrom Co., 2007).

Figure 10 (Top): Drying sludge of paper mills stored in a warehouse whose walls and windows are constituted of special Ahlstrom paper sup-porting titania. (Bottom): Outside view with sunlight activation) (by cour-tesy of Mr. J. Dussaud, Ahlstrom Co., 2007).

outside applications as demonstrated by solar prototypes in agricultural, water and food industries, especially for re-moval of odors and self-cleaning properties of outdoor ma-terials. The present challenge is the development of the photocatalytic engineering for optimization of the reactions, which are performed in a wide range of media and condi-tions. The polyphasic nature of the systems has to be taken into account, including UV-light, the “electromagnetic phase”. Different types of photoreactors have been built with the catalyst used under various shapes: fixed beds, magnetically or mechanically agitated slurries, catalyst par-ticles anchored on the walls of the photoreactor or in mem-branes or on glass beads, or on glass-wool sleeves, small spherical pellets, etc. The main purpose is to have easy separation of the catalyst from the fluid medium, thence it is necessary to support titania and to avoid the final ultrafine particle filtration.

8.4.1 Solar photocatalytic reactors using slurries of titania

Solar photocatalysis has been baptized “Heliophotocataly-sis” by the author, who had the opportunity to intensively study it at Plataforma Solar de Almeria (PSA) in Spain within the frame of European cooperation programs [48]. The principle of the CPC photoreactor used is described in references [49] and [50]. The CPC collector which is the irradiated part of the system corresponds to a plug flow re-actor, but as it is connected to a tank via a recirculation pump, the ensemble corresponds to a batch reactor.

The photocatalytic degradation of various pollutants, previously studied on the lab scale, was successfully per-formed in CPC photoreactors at PSA using solar energy, as exemplified by the total mineralization of 2,4-D (2,4- dichlorophenoxyacetic acid) [43], a well known herbicide. Figure 11 shows the temporal variations of 2,4-D, 2,4-

Figure 11 Purification of water containing herbicide 2,4-D. Kinetics of (i) 2,4-D and TOC disappearance, (ii) Cl anion formation and (iii) 2,4-DCP appearance and disappearance. (This Figure is from final report of Euro-pean Project AQACAT headed by the author.)


dichlorophenol (2,4-DCP), the main intermediate formed, Cl anions produced and the total organic carbon (TOC). After an adsorption period of 1 h in the dark, 2,4-D disap-pears following a first order kinetics. Cl anions follow a sigma shaped curve in agreement with the consecutive reac-tion scheme:

1 222,4-D 4-DCP 2 Cl 8 COk k

giving

0 2 1 1 1 2 1 2Cl 2 exp expC k k k k t k k k k t

The TOC decreases linearly to zero within 1 h with an apparent zero kinetic order. That could be interpreted by assuming a saturation of surface sites by all the intermedi-ates.

2

i i i i sat

d TOC d d[CO d

1

it t k

k K C K C kq k

where represents the surface coverage of the ith interme-diate and sat represents the overall coverage at saturation.

These experiments were compared with initial studies performed in a laboratory microphotoreactor working with artificial light. Despite a volume extrapolation factor of 12500, the same first order was found for 2,4-D disappear-ance; the quantum yields and the intermediates were the same, indicating an identical reaction pathway.

8.4.2 Solar photocatalytic reactors using deposited titania

Although slurry photoreactors can be estimated as the most efficient, they exhibit a significant drawback: the final tedi-ous filtration of titania particles. To overcome this obstacle, titania can be deposited on photo-inert supports. A success-ful attempt was made by depositing titania on special Ahl-strom paper using amorphous silica as a binder to anchor the photocatalyst particles on inorganic fibers. Such a photocatalytic material has been used in an adapted solar photoreactor named Cascade Falling Films Photoreactor (CFFP) described in Figures 12 and 13.

The photocatalytic activity is based on the rate of the

Figure 12 Comparative schemes of CPC and Cascade Falling Films Photoreactors.

Figure 13 Comparison of the activities of titania photocatalysts used (i) in a CPC slurry photoreactor (bottom right) and (ii) in a Cascade Falling Films photoreactor using a titania fixed bed deposited on Ahlstrom paper (bottom left).

Total Organic Carbon (TOC) removal from a solution con-taining 50 ppm of 4-chlorophenol chosen as a model pol-lutant. To test the efficiency of the CFFP solar reactor, photocatalytic reactions were calibrated against a slurry CPC photoreactor having the same surface of a sun collec-tor as shown in photographs in Figure 13. Surprisingly, the results were similar in the total degradation of 4-chlroro- phenol as indicated by the Total Organic Carbon (TOC) disappearance (Figure 13).

These fixed bed photoreactors were used to produce drink water for remote isolated human communities in arid coun-tries either in North Africa (European AQUACAT Program) or in Latin America (European SOLWATER Program).

8.4.3 Solar photocatalytic for “self-cleaning” materials

Besides fixation of titania on a support, used in various types of photoreactors, material science is developing to prepare thin and invisible layers of titania on “self-cleaning” objects, such as glass, concrete walls, ceramics, and tools.

Solar photocatalysis is actually an efficient process, which makes self-cleaning glass work. Fouling of glass is mainly due to dust and/or atmospheric particles stuck at the surface on greasy stains mainly constituted of fatty acids. Self-cleaning glass is coated with an invisible thin layer of titania, which, in the simultaneous presence of oxygen (air), atmospheric water vapor and solar UV-light, is able to de-compose fatty acids by successive photodecarboxylation reactions named “photo-Kolbe” [51] and make coated glass recover its initial cleanness.

9 Conclusions

An overview has been presented on the various aspects and potentials of heterogeneous photocatalysis. The potential applications strongly depend on the future development of photocatalytic engineering.


Water pollutant removal appears as the most promising potential application as many toxic water pollutants, either organic or inorganic, are totally mineralized or oxidized at their higher degrees into harmless final compounds. Despite some drawbacks (use of UV-photons and necessity for the treated waters to be transparent in the spectral region; slow complete mineralization in cases where heteroatoms are at a very low oxidation degree; photocatalytic engineering to be developed), room-temperature heterogeneous photocatalysis offers interesting advantages: (i) chemical stability of TiO2

in aqueous media and in a large range of pH (0 ≤ pH ≤ 14), (ii) low cost of titania, (iii) cheap chemicals in use, (iv) no additives required (only oxygen from the air), (v) systems applicable at low concentrations, (vi) total mineralization achieved for many organic pollutants, (vii) efficiency of photocatalysis with halogenated compounds sometimes very toxic for bacteria in biological water treatment, and (viii) possible combination with other decontamination methods (in particular biological).

As the number of publications on photocatalysis has in-creased exponentially, photocatalysis has to be urgently refocused concerning the following challenges:

(1) Are we “condemned” to exclusively work with tita-nia?

(2) Can TiO2 be photosensitized in the visible by doping? (It is already known that cationic doping is not efficient and rather detrimental, whereas anionic doping, still under in-vestigation, has to be more clearly defined.)

(3) Can we find a new photocatalyst, different from TiO2 and directly active in the visible?

(4) Can invisible titania thin layers be efficient enough when deposited on “self-cleaning” objects?

(5) Is photocatalysis suitable for preparative Fine Chem-istry as exemplified in §8.2?

(6) Is photocatalysis bactericidal enough in water and in air?

(7) Can photocatalysis be employed as a new medical tool in medicine (cancericide effect) [52]?

(8) Are we able to define a few standardized and globally accepted tests for each photocatalytic application?

(9) Are we able to adapt to Chemical Engineering culture to promote environmental and “green” applications of photocatalysis?

This listing may constitute serious objectives for future research in photocatalysis.

The author is indebted to Mrs. C. Delbecque for her help in editing this paper and to M. J. Dussaud from Ahlstrom Co.,/ for the communication on his performing industrial devices. The author also thanks Drs. J. Blanco and S. Malato for their initiation to solar photocatalytic engineering.

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