Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag-TiO2 coatings on quartz

6.ENVIRONMENTAL

ELSEVIER

Applied Catalysis

B: Environmental

13 (1997) 219-228

Characterization and photocatalytic activity in aqueous medium of Ti02 and Ag-Ti02 coatings on quartz J.-M. Herrmanna,*, H. Tahiria9b, Y. Ait-Ichoub, G. Lassalettac, A.R. Gonzilez-Elipe”, A. Fernbdez” a URA au CNRS Photocatalyse,

Catalyse et Environnement,

h Dipartement

de Chimie-Physique,

’ Institute de Ciencia de Materiales

Ecole Centrale

de Lyon, B.P. 163, 69131

UniversitC Ibnou Zohz B.l?/S 28, Agadiz

de Sevilla, Centro de lnvestigaciones Vespucio s/n, 41092-Sevilla,

Received 20 July 1996; received in revised form 2 November

Cienti$cas

Ecully Cedex, France

Morocco

Isla de la Cartuja, Avda America

Spain

1996; accepted

21 November

1996

Abstract In the present study, Ti02 and Ag-Ti02 catalysts have been supported in the form of thin layers by a dip-coating procedure on quartz substrate. The resulting materials have been characterized by SEMIEDX, XRD, XPS and UV-vis absorption spectroscopy. The immobilized catalysts were tested in the photocatalytic degradation of malic acid. For this reaction, the presence of metallic silver does not produce an intrinsic increase in photocatalytic activity in comparison with pure titania. The apparent increase observed in activity is principally due to the increase in the exposed surface due to the textural characteristics of the Ag-Ti02 layer in comparison with Ti02. In addition, the presence of metallic silver always produces an increase in activity in comparison with oxidized Ag+ ions. This can be explained by the increase in the electron-hole pairseparation efficiency induced by trapping of electrons by metallic silver. G 1997 Elsevier Science B.V. Keywords: Photocatalyst degradation

(supported);

Quartz-deposited

titania;

1. Introduction

Heterogeneous photocatalysis for the total oxidation of organic and inorganic water pollutants is a well-established phenomenon and has been the object of extensive investigation in recent years [l-l 11. Most studies related to such photodegradation reactions *Corresponding 0926-860X/97/$17.00 PZI

author. #Q 1997 Elsevier Science B.V. All rights reserved.

SO926-3373(96)00107-5

Silverkitania;

Photocatalyst;

Water purification;

Pollutant

have been carried out using suspensions of powdered Ti02 (usually Degussa P-25) in polluted aqueous solution. However, the manipulation of powdered semiconductors and their removal from water are difficult and recent research has focussed on the preparation of active immobilized photocatalysts for water treatment [ 12-2 11. In a previous paper [ 181, we have shown that titania remained active when it was deposited in the form of

220

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Catalysis B: Environmental

thin coatings on different supports (glass, stainless steel and quartz). The photocatalytic activity of the supported photocatalyst was intrinsically lower than that of the powder samples used in a slurry [ 181. This decline in activity has been correlated with the presence of cationic impurities (Si4+, Naf, Cr3+, Fe3+) in the deposited layer as a consequence of the thermal treatments necessary to improve the cohesion of the titania layer and its adhesion onto the support. Quartz (i.e. fused silica) has been found to be the best support for titania [18], because it is the most neutral and stable one at high temperatures. As a consequence, it has been chosen as the only support for new experiments on titania’s photoactivity in photodegrading organic pollutants in water. Many studies have been devoted to the improvement of titania’s photoactivity by depositing noble metals. In the peculiar case of silver, Ag+ ions have been used as a redox reagent for the photocatalytic destruction of phenol in contact with illuminated titania [22]. Ag photodeposits were found to improve the oxidehydrogenation of 2-propanol [23]. Thermally decomposed AgN03 on Ti02 powder results in catalysts which can photodegrade chloroform to CO2 and HCl and can increase the photodegradation of urea [24]. A recent publication [25] has indicated that photodeposited silver could improve the photocatalytic activity of titania in the total degradation of 1,4-dichlorobenzene in water. In the present paper, we have tentatively studied if such a beneficial effect due to the presence of silver was maintained when titania was deposited on an inert support such as quartz for a potential use in an application device.

13 (1997) 219-228

constant rate of 8 in./min (20.32 cm/min). The samples were then dried in air for a few seconds when a white turbidity appeared in the supports, because of the hydrolysis of the alcoholate in the adsorbed layer induced by the humidity of ambient air. This procedure was repeated three times to increase the thickness of the titania layer. Finally, the samples were calcined at 673 K for 2 h to fully decompose the alcohol and to eliminate carbon-containing impurities. The sample was named TiOz/Q.

2.2. Preparation

of quartz-supported

The Ag-Ti02 samples were obtained by the same dip-coating procedure, but with a solution of Ti alcoholate containing AgN03. The concentration of the titanium compound was identical to that used for the deposited pure titania sample. The atomic ratio Ag : Ti in the solution chosen was equal to 1 : 22, which yielded a final weight percentage of 8 wt% Ag, as determined by EDX (energy dispersive spectroscopy) analysis. The Ag-Ti02 supported sample was then calcined at 673 K in conditions identical to that used for pure Ti02. The sample obtained was called AgTi02/Q. Parts of the Ag-TiOJQ samples were submitted to a UV irradiation treatment in air for 6 h. The aim of this treatment was to produce Ag particles at the surface of titania by reduction of Ag+ ions by photoinduced electrons. The resulting sample was named AgTi02/Q (hv).

2.3. Characterization 2. Experimental 2.1. Preparation

of quartz-supported

TiOz

The samples were prepared by the well-established method of dip coating [26]. Fused quartz sheets (ca. 6 cm* in area) were carefully cleaned by sonication in acetone. The supports were then immersed in a solution of titanium tetraisopropoxide (Ti(OCH(CH3)2)4, from Merck) in dry i-PrOH with a concentration of 13 ml of the alcoholate in 100 ml of solution (0.4 mol/l). The sheets were removed from the solution at a

Ag-Ti02

of the samples

The scanning electron micrographs were obtained with a Philips (XL30) microscope working at 20 kV. The instrument was fitted with an EDX accessory. X-ray diffraction analysis (XRD) was carried out using CuK, radiation in a Siemens D5000 diffractometer provided with a thin-film attachment (0.4” Soller slit and a LiF [IOO] monochromator in the detector arm). To record the diagrams, typical incident angles of 0.5” were used. X-ray photoelectron spectra were recorded with a VG Escalab 220 model using the MgKc, excitation source. Calibration of the spectra was done at the C 1s

J.-M. Herrmann et al./Applied

Catalysis B: Environmental

peak of surface contamination taken at 284.6 eV. Quantitative analysis was carried out using the sensitivity factors supplied with the instrument. UV-vis absorption spectra were recorded with a Shimadzu UV-2101PC spectrometer. 2.4. Photocatalytic

activity measurements

The photocatalytic test-reaction chosen to characterize the different deposited titania samples was the total degradation and mineralization of malic acid (hydroxybutane dioic) selected as a model organic pollutant of the carboxylic type. The overall reaction corresponds to: COOH-CHOH-CH2-COOH +

4CO2+3H20

+ 3 O2 (1)

The degradation reaction was carried out in a batch photoreactor [6], open to air, having the shape of a vertical cylinder whose bottom optical window was made of a Pyrex optical disc, transparent to wavelengths X 2 290 nm. The different plates supporting titania were placed horizontally at ca. 2 mm from the bottom of the photoreactor, i.e. perpendicularly to the UV beam. This enabled a good circulation of the solution around both sides of the plates. The solution was agitated from the top with a mechanical stirrer made of photocatalytically inert glass. UV light was provided by a Philips HPK 12.5 W UV-lamp and it entered the photoreactor through the bottom window. The analyses of malic acid and of various intermediates detected were performed by liquid chromatography (HPLC), using a photodiode array detector. The initial concentration of malic acid was equal to 50 ppm, i.e. 3.73 x lop4 mol/l. The solution was first agitated for 30 min in dark. It has been shown that this period was sufficient to reach the adsorption equilibrium. The first analysis was performed to estimate the quantity of malic acid adsorbed and to measure the initial concentration in the solution at time tuv = 0, necessary to determine the initial rate of disappearance as well as the apparent rate constant. For comparison, experiments were performed with a slurry suspension of powder Ti02 (Degussa P-25, 50 m*/g, mainly anatase).

13 (1997) 219-228

221

3. Results 3.1. Characterization samples

of TiO,/Q and Ag-TiOz/Q

A detailed characterization of the sample Ti02/Q has been reported in a previous paper [ 181. We presently only describe the characterization of this sample, directly related to the comparison with the AgTi02/Q samples. The texture and morphology of the Ag-TiOdQ sample in comparison with TiOJQ can be observed in Fig. 1. The Ag-Ti02/Q (hv) sample is very similar to Ag-Ti02/Q. The pure Ti02 layer appeared to have a granular morphology while the Ag-Ti02/Q sample appeared to have a smoother surface according to the resolution used. This means that the supported film contains smaller grains in the Ag-Ti02 coatings than in pure Ti02. This should correspondingly give a higher exposed surface. We do not know why the presence of Ag modifies the texture of the supported Ti02 films. This behaviour has been carefully checked. It could be observed even before SEM analysis: pure Ti02 samples are always translucent (partially opaque). The Ag-TiOz samples are always transparent. This is indicative of the presence of bigger Ti02 grains and/or aggregates in pure, deposited Ti02. The silver particles cannot be detected by SEM due to the limited resolution of the microscope. With the scanning electron microscope, we have also measured (by EDX) the T&-fluorescence signal for all the samples and for a pressed pellet of Ti02 Degussa used as a reference. From these measurements and according to the method of Waldo et al. [27], we have evaluated the thickness of the Ti02 layer for the different samples. For Ti02/Q, Ag-TiOJQ and Ag-TiOz/Q (hv), the values obtained were ca. 0.2 mm for all the three types of samples. Fig. 2 shows the XRD pattern obtained for the AgTi02/Q and the Ag-Ti02/Q (hv) samples. The diffractograms were recorded in the low-angle incident mode to increase the sensitivity to the surface. In both samples, we can see the peaks corresponding to the anatase phase of Ti02, as previously reported for the Ti02/Q sample [ 181. The described region of the diffractogram has been selected because it contains the peaks corresponding to metallic silver. It is clear from this figure that, upon illumination in air, oxidized

J.-M. Herrmann et al./Applied

Fig. 1. SEM micrographs

Catalysis B: Environmental

for the TiOJQ

Ag+ ions contained in the Ag-Ti02 layer are photoreduced according to well-known processes on titania [7,28]. Peaks of metallic silver appear in the diffractogram. The UV irradiation of Ag-TiO*/Q is necessary to get metallic silver since the preceding calcination

13 (1997) 219-228

and Ag-TiOZ/Q samples.

does not decompose AgNOs into Ago contrarily to what happens on solids based on Si02 and on Ti02SiOz [29]. The formation of metallic silver upon irradiation of the Ag-TiOJQ sample in air can be also observed if

223

J.-M. Herrmann et al. /Applied Catalysis B: Environmental 13 (1997) 219-228

Ag-TiO,/Q

(hu)

Ll 28 (degl Fig. 2. Low-angle XRD diffractograms for samples Ag-TiOdQ and Ag-TiOJQ (hv). Line bars represent the diffraction peaks for anatase TiOz (- - -) and metallic Ag (- - -).

illumination, indicates the presence of small particles of reduced silver in the sample. To determine Ag particle sizes, some samples were scratched with a diamond pencil and the powder obtained has been examined by TEM. In the UV-irradiated sample, Ag particles of ca. 15 nm could be observed. By contrast, no Ag particles could be clearly put in evidence in the Ag-TiO*/Q calcined sample. The XPS analysis of the samples allowed us to estimate the Agffi atomic ratio at the surface. The decrease in this value for the samples after illumination in air or during the catalytic test can be interpreted as a consequence of the agglomeration of silver into small metal particles upon photoreduction. Deposited silver was rather stable. Concerning a possible leaching of Ag in acidic media, we did not observe any detectable amount of Ag+ ions dissolved from Ag-TiOz/Q sample in a solution of acetic acid at pH 3.5. Identically, no dissolved silver was detected after a photocatalytic run (in agreement with the photocatalytic deposition capacity of illuminated titania [7]). Some silver may dissolve but at non-measurable rate. In any case, XPS analysis of the surface always detected the presence of silver on the samples, having performed a photocatalytic run. 3.2. Photocatalytic Ag-Ti02/Q (hu)

Ag-TiO@

(h?)

Ag-Tiq/Q

I 0

I

I

500

700

9

X(nm) Fig. 3. Optical absorption Ag-TiOJQ (hv).

spectra for the samples Ag-TiOdQ

and

analyse the UV-vis absorption spectra of the samples. In Fig. 3 are depicted the spectra for the samples Ag-TiOz/Q and Ag-TiOz/Q (hv). Apart from the band-gap absorption threshold of titania at X 5 380 nm, the apparition of a characteristic plasmon absorption [29] at ca. 440 nm, especially after we

activity of Ag-Ti02/Q

and

The results obtained in this work will be compared with the values obtained for the TiOJQ sample described in [18]. 3.2.1. Adsorption in the dark A good determination of the photocatalytic activity of a sample can only be performed after having reached the adsorption equilibrium, since, otherwise, the initial rate of disappearance of a pollutant would simultaneously include both the initial rate of adsorption and the true, initial photocatalytic rate of reaction. The standard solution of malic acid used had an initial concentration of corresponding to 50 ppm 3.73~ lop4 mol/l. Since the volume used was 20 cm3, the initial mole number of malic acid was equal to 7.46 nmol. The amounts of malic acid adsorbed on the various supported samples measured at the end of the previous adsorption period in the dark are reported in Table 1. For calibration, the amount of

224

Table 1 Adsorption

J.-M. Hermann

characteristics

et al./Applied

of malic acid on the various TiOJQ

Catalysis B: Environmental

and Ag-TiOJQ

13 (1997) 219-228

samples, and on Ti02 Degussa P-25 as reference

Catalyst

n (ads) a (umol)

Rb

A ’ (ma)

S d (cm*)

s exp e (m2/cm2)

TiO,/Q Ag-TiO*/Q Ag-Ti02/Q (hv) TiO2 Degussa P-25 (5 mg)

0.435 0.81 0.87 1.36 f

0.32 0.60 0.64 1

8.0x lo-’ 0.15 0.16 0.25 a

2.88x2 3.029~2 3.36x2

1.39x10~2 2.46~ lo-* 2.385 x lo-’ -

a Number of moles adsorbed: n (ads). b R = n (ads)/n (ads) Degussa. Ratio of the number of moles adsorbed on Ag-TiO, to that adsorbed on TiO? Degussa ’ Total surface area exposed according to the number of moles adsorbed in dark (see text). d Total geometric surface of plates covered (two faces). ’ Surface of Ti02 exposed of plate (two faces). f 1.36 umol adsorbed on 5 mg P-25, i.e. on 0.25 m2. k Total area exposed on 5 mg Ti02 Degussa P-25 (50 mZ/gx5 x 10m3 g).

malic acid adsorbed was determined on 5 mg of Ti02 Degussa P-25 with a well-known specific area of 50 m2/g. The reproducibility with respect to a previous study [18] was excellent. If we consider that the surface density of moles adsorbed per m2 of Ti02 Degussa P-25 represents a good estimation for the ensemble of titania (anatase)based catalysts, the ratio R of the number of moles adsorbed on quartz-deposited Ti02 and Ag-Ti02 samples to that adsorbed on Ti02 Degussa P-25 (Table 1, column 3) enables one to calculate an estimation of the total surface area of Ti02 exposed to the reactant in the solution. This determination would be comparable to the method used for the determination of the metal dispersion by H2 (or CO) adsorption with a calibrated standard catalyst. The values of the total surface area exposed for the various titania-containing catalysts are obtained by multiplying the ratio R by 0.25 m2 (total surface area exposed for 5 mg Ti02 (P-25) (Table 1, column 4). The geometric dimensions of the different quartz plates have been carefully measured (Table 1, column 5) and enable one to determine the number of m2 of Ag-Ti02 surface exposed per cm2 of quartz plate (Table 1, last column). It can be observed that the two supported Ag-TiOz catalysts have very similar characteristics concerning the surface of titania exposed per unit area of quartz support. As expected, the preparative UV illumination of the second sample does not modify the developed surface of titania. If the difference between TiOz/Q and Ag-TiO,/Q samples is significant (Table 1, last column), this would indicate that the incorporation of

Ag ions during the dip-coating procedure has increased the total surface of titania exposed. This has been confirmed by the comparison of the textural aspects obtained on the SEM micrographs. As described above, the Ag-Ti02 samples are constituted of smaller grains than the sample TiOz. 3.2.2. Kinetics of malic acid disappearance The overall photocatalytic activity was chosen as the rate of disappearance of the pollutant (Fig. 4). The

1 ??

Ag-hO,/Q

(hv)

A Ag-T,O,/Q 0

TIO,/Q

J 0

60

Fig. 4. Kinetics of disappearance of malic acid in contact illuminated TiOz and Ag-TiOa supported on quartz.

with

J.-M. Herrmann et al./Applied Catalysis B: Environmental 13 (1997) 219-228

225

different systems were rather efficient, with a total disappearance of malic acid taking place between 30 and 60 min of UV irradiation. The results were reproducible within narrow limits (5 5%). As a consequence, in the conditions used, the kinetics enables one to give the following qualitative activity order: TiOz/Q

5 Ag-TiOz/Q

< Ag-TiOz/Q

(hv)

The pre-irradiated sample is the most active with a total disappearance of malic acid within less than 30 min. A better and more quantitative way of presenting the activities of a series of similar catalysts is the use of the rate constant k, which is independent of the concentrations used. Presently, in photocatalytic reactions, k is almost independent of temperature because of the photoactivation process and only depends on the radiant flux and on the UV spectrum of the lamp. The expression and units of the rate constant (k) depends on the reaction mechanism. The photocatalytic degradation of organic pollutants in water generally follows a Langmuir-Hinshelwood mechanism [30-331, with the rate being proportional to the coverage 0: ,_=kO=kKc/(l+Kc)

where k is the true rate constant which includes various parameters such as the mass of catalyst, the flux of efficient photons, the coverage in oxygen, etc. and K is the adsorption constant. Since the initial concentration is (cO = 50 ppm = 3.73 x low 1O-4 moVl), the term Kc in the denominator can be neglected with respect to unity and the rate becomes the apparent first order:

Table 2 Photocatalytic

activities

of the various quartz-deposited

0

15 UV

Fig. 5. Linear transforms Fig. 4.

r = -dc/dt

30

irradiation ln(cdc)

45 time

60

(mln)

=f(t) of the kinetics curves from

= kKc = k,c

where k, is the apparent rate constant of the pseudofirst order. The integral form, c =f(t) of the rate equation is: ln(co/c)

= k,t

The linear transforms ln(co/c) = k,t of the curves in Fig. 4 are given in Fig. 5. The slopes of the straight lines all of which pass through the origin yield the apparent rate constants (k,) which are given in Table 2.

samples in comparison

with TiOz Degussa P-25

Catalyst

Experimental rate constant k, (min-‘)

Rate constant per cm* of plate a (min-’ cm-‘)

Rate constant per m2 Ti02 a (min-’ m-‘)

TiO,lQ Ag-Ti02/Q Ag-TiOZ/Q (hv) Ti02 Degussa P-25 (5 mg) h

4.8x lo-’ 4.2x lo-* 6.3x IO-* llxlo~*

1.7x10~2 1.4x 10~~2 1.9x lo--* 3.85x lo-*

0.30 0.14 0.195 1.93

” It is assumed that most of the activity corresponds to that of the lower face in front of the entrance window of the photoreactor (one face active). h The entrance of the light flux occurs through a hole in a screen having the same size as the TiO,/Q plate (entrance window 2.88 cm’, see text).

226

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Catalysis B: Environmental

3.2.3. Comparison of the photocatalytic activities of the different quartz-supported titania samples The first direct comparison consists in comparing the apparent first order rate constants k, (Table 2, column 2) which are in the following order: Ag-TiOz/Q

< TiOz/Q

< Ag-TiOz/Q

(hv)

The same classification is obtained with apparent rate constants per cm2 of quartz plate (Table 2, column 3). If now we assume that the real surface of titania exposed is correctly estimated from the adsorption measurements in the dark (Table l), the following order has been obtained for the photocatalytic activity expressed by the intrinsic rate constant (k per m2) (Table 2, column 4): Ag-TiOz/Q

< Ag-TiOz/Q

(hv) < TiOz/Q

From these data, it appears that the presence of metallic silver in the sample Ag-Ti02/Q (hv) is not producing an increase in the intrinsic photocatalytic activity. In fact, the apparent increase observed in columns 2 and 3 in Table 2 is principally due to the increase in exposed surface obtained in samples Ag-TiOdQ. This has been stated from the change in texture obtained during the dip-coating procedure in the presence of silver. For the same area of quartz support, the samples with silver show a higher active titania surface. There remains also another constant feature: the pre-illuminated Ag-Ti02 sample is always more active than the non-UV-pretreated homologue. This could be explained by a surface electronic effect induced by silver. After the photogeneration of electrons and holes by photons of appropriate energy (hv > EG, EG = 3.2 eV (band gap energy)), (Ti02) + hv + e- +p+ the presence of metal silver can help the electron-hole separation by attracting photoelectrons: (Ag) + e- ++ e& This enables the positive photoholes p+ to react with OH--adsorbed species to create OH’, which are generally assumed to be the degrading oxidative

13 (1997) 219-228

agents: OH- +p+ + OH’ OH’ + pollutant

+ Intermediates

+ CO2 + Hz0

3.2.4. Comparison of the photocatalytic activities of the different quartz-supported titania samples with Degussa P-25 in a slurry It is very difficult to make reliable and meaningful comparisons between two catalytic systems working in very different ways. Presently, we have attempted to consider the photocatalytic activities of quartz-deposited samples and that of Ti02 Degussa P-25 (50 m2/g) in a slurry. To have reaction conditions as close as possible, the experiment with Ti02 Degussa in suspension was performed in the same Pyrex photoreactor with the same radiant flux, but with a screen put on the optical window of the photoreactor. This screen had a hole with the same shape and the same dimensions as those of the quartz plates (2.88 cm2). The quantity of Ti02 Degussa in suspension was taken equal to 5 mg for chemisorption measurements. Considering the section of the hole (2.88 cm2) and the section area of the photoreactor (12.57 cm2), it can be calculated that the mass of illuminated Ti02 is equal to 1.46 mg (5 mgx2.88/12.57), to which corresponds a developed area of 5.73 x lop2 m2. This value enables one to determine the area1 photoactivity for Ti02 Degussa given in Table 2. From the last line of Table 2, it appears that Ti02 Degussa P-25 is the most active one. By comparing the rate constants per cm2 of quartz plate or of entrance window area, the Degussa P-25 is about twice more active than quartz-deposited titania samples. The comparison of the intrinsic activities (k per m2) enlarges the difference in favour of Ti02 Degussa P-25. If some light is diffused by the agitated Ti02 particles into the non-illuminated part of the photoreactor, this would decrease this activity ratio. While underlining that one has to be very careful in comparing photocatalytic systems, it can be concluded that, even if suspensions remain more efficient, the photoactivities of quartz-supported titania samples are of the same order of magnitude. From an application point of view, the interest of having stable, supported photocatalysts which avoid filtration may actually compensate the drawback of a lower activity.

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Catalysis B: Environmental

4. Conclusions A series of titania samples deposited on quartz and containing silver ions have been prepared, characterized and tested in water organic pollutant abatement using the test reaction of malic acid photocatalytic degradation. The improvement in photocatalytic activity induced by the addition of silver ions is not obvious according to the photocatalytic rate expression chosen. The principal benefit in activity is due to the increase in exposed surface due to the textural characteristics of the Ag-Ti02 layers in comparison with TiOz. In addition, a previous treatment of the deposited Ag-TiOZ samples under UV light just after calcination induces a slightly higher photocatalytic activity, whatever the expression chosen for it. This has to be ascribed to a partial photoreduction of Ag+ ions into Ag” atoms which can agglomerate, as small metallic clusters identical to those found in photographic processes. Metallic silver in low amounts can play a favourable role by attracting electrons, thus helping the electron-hole pair separation and preventing the electron-hole recombination, as mentioned in [23]. The improvement in quartz-deposited titania by addition of Ag+ ions and UV pretreatment is -12%. This value is three times smaller than the improvement found in [25]. Consequently, from an application point of view, the interest of adding silver to a quartzsupported titania photocatalysts is not obvious. A careful comparison of the photocatalytic activity of supported titania samples with bulk TiOZ Degussa P-25 shows a decrease in activity. This effect should be compensated by the benefit of avoiding filtration of small particles.

Acknowledgements The authors thank the ‘Picasso’ program (Project No. HF93/159B and HF94/332B) and the DGICYT (Project No. PB93/0183) for financial support. References [I] M. Schiavello (Ed.), Photocatalysis

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