Langmuir 1993,9, 2995-3001
2995
Photocatalytic Activity and Selectivity of Titania Colloids and Particles Prepared by the Sol-Gel Technique: Photooxidation of Phenol and Atrazine E. Pelizzetti' and C. Minero Dipartimento di Chimica Analitica, Universita di Torino, 10125 Torino, Italy
E. Borgarello and L. Tinucci ENI Ricerche, 20097 San Donato Milanese, Milano, Italy
N. Serpone Laboratory of Pure and Applied Studies in Catalysis, Environment and Materials, Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, Canada H3G 1M8 Received April 5, 199P The photocatalyzed degradation of phenol and atrazine has been reexamined under AM1 simulated solar light irradiation of Ti02 catalysts in aqueous acid media to explore the photocatalytic activity and selectivity of titania colloids and titania specimens prepared by the sol-gel method yielding particles from nanometer size powders to millimeter spherical beads. In the photooxidation of phenol, the different Ti02 materials show small but significant variations in activity; there is no evidence for variations in selectivity with phenol because of the rather similar nature of the intermediates formed (hydroquinone, catechol, benzoquinone) and the rather analogous interaction(s1 of these with the active oxidizing species ('OH radicals). By contrast, when atrazine is the test probe, there is a dramatic and significant variation in activity and selectivity in the differently prepared titania samples, as witnessed by the variations in the temporal distribution of intermediate species and product formed. This is a consequence of the three different degradation pathways and the modes of interaction between the probe and the catalyst: (i) dehalogenation, (ii) dealkylation, and (iii) deamination, affording a relevant different sensitivity to the oxidizing active oxygen species. Particular attention is focused on the activity of millimeter sized Ti02 beads in the light of their potential usefulness in photochemical reactors.
Introduction The photocatalyzed destruction of organic compounds in aqueous suspensions of titanium dioxide is gaining increased attention as a possible technique in water purificati0n.l It can be applied in the treatment of both wastewaters2 and drinking watem3s4 Normally, the titanium dioxide is present as a powder in the suspensions. The photocatalytic activity of the powder is largely affected by the preparative method^^^^ and by any prior catalyst treatment.' In other investigations, the photocatalytic reactions and the primary events have been examined in the presence of nearly transparent Ti02
colloid^.^^^ From the viewpoint of the application to effluent decontamination, problems arising from such discontin-
* To whom correspondence should be addressed. Abstract published in Advance ACSAbstracts, August 15,1993. (1) Ollis,D. F.; Pelizzetti, E.; Serpone,N. Environ. Sci. Technol. 1991, 25, 1522. Serpone,N. WasteManuge. (2) Pelizzetti,E.;Pramauro,E.;Minero,C.; 1990, 10, 65. (3) Photocatalysis-Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley-Interscience: New York, 1989. (4) Ollis, D.F.;Pelizzetti, E.; Serpone, N. ref 3, p 603. (5)Matthews, R. W. In Photochemical Conversion and Storage of Solar Energy; Pelizzetti, E.; Schiavello, M., Eds.; Kluwer: Dordrecht, The Netherlands, 1991; p 427. (6) (a) Sclafani,A.; Palmisano, L.; Schiavello,M. J . Phys. Chem. 1990, 94,829. (b) Al-Sayed, G.; D'Oliveira, J. C.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1991, 58, 99. (c) Tunesi, S.;Anderson, M. A. J. Phys. Chem. 1991,95,3399. (d) Barringer, E. A.; Bowen, H. K. Langmuir 1985,1, 920. (7) Heller,A.;Degani, Y.;Johnson, D. W., Jr.; Gallagher, P. K. J.Phys. Chem. 1987,91, 5987. (8) Gratzel, M.ref 3, p 123. (9) Henglein, A. Top. Cum. Chem. 1988,143, 113.
0743-746319312409-2995$04.00/0
uous treatments as filtrations and resuspension of the catalyst must be avoided if possible. In this regard, it is advantageous to fix the semiconductor oxide over a stationary "inert" support, thereby allowing for a continuous treatment of the effluents. Several attempts at fiiing titania on avariety of supports have already been reported; glass,'OJl clays,12ceramic membra ne^,'^ polymers,14 and polycarbamate plates15 have been chosen as supports. The present paper reports our recent results on the degradative oxidation of two compounds, phenol and atrazine, representative of two widely different classes of water pollutants. The degradation processes have been carried out in the presence of Ti02 under different forms: three differently prepared powders (submillimeter size), a colloid solution, and small 1-mm spheres. The preparation and characterization of these new forms of titanium dioxide are also described. Recently, the sol-gel technique has attracted attention as a viable method of preparation for photocatalytic materials.'&18 The aim of the present studies can be summarized as follows: (i) evaluate the degradation rate and overall efficiency in terms of final product formation from differently prepared TiO2; (ii) assess the stability of the catalyst with time; (iii) determine the efficiency of the (10) Serpone, N.; Borgarello, E.; Harris, R.; Cahill, P.; Pelizzetti, E. Sol. Energy Mater. 1986, 14, 121. (11) Matthews, R. W. J. Catal. 1988, 111, 264. Bard, A. J. J. Phys. Chem. 1986,90, 301. (12) Enea, 0.; (13) Sabate, J.; Anderson, M. A.;Kikkawa, H.;Xu, Q.;Cervera-March, S.;Hill, C. G., Jr. J. Catal. 1992, 134, 36. (14) Kakuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.;Fox, M. A.; White, J. M.; Webber, S.E. J.Am. Chem. SOC.1984,106, 6537. (15) Borgarello, E.; Serpone, N.; Liska, P.; Erbs, W.; Gratzel, M.; Pelizzetti, E. Gazz. Chim. Ztal. 1985, 115, 599.
0 1993 American Chemical Society
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2996 Langmuir, Vol. 9, No. 11,1993 Table I. Characteristics of Ti02 Materials Pmpared by Various Methods notation A B C D E
sample DegussaP25 AGIPC-60 AGIPpowder AGIPmicrosphere colloidal
crystallinity anatase(SO%) anatase(90%) anatase(90%) anatase (90%)
BET area, m2/g 55 14 43 84
size," fim 0.3 20-100 0.05-0.17
IO00 0.05
Diameter.
catalyst when present in millimeter-sized particles; (iv) examine the effect of different catalyst forms on the degradation mechanism, on the nature of the intermediates, and on the yields 'and rates of formation and decay of the latter. Experimental Section Materials. Table I lists some of the characteristics of Ti02 specimens herein examined from the different preparations. Sample A is the commercially available Degussa P25 titania.lg The sample AGIP (7-60 (denoted B in Table I) was prepared by a sol-gel process developed by TEMAV.20 The method is based on gelation of an emulsion composed of an aqueous solution of a titanium compound mixed with an immiscible organic solvent. Thus, in a typical preparation T i c 4 was hydrolyzed in water (ratio 1:2)to obtain a concentration of Ti02 of ca. 400 g/L. This step is highly exothermic and releases large quantities of HCI gas! The hydrous Ti02 sol was subsequently emulsified (vigorous mechanical stirring, ca. 650 rpm) in l,l,ltrichloroethane (referred to as the support solvent) in the presence of a surfactant (sorbitan monooleate, Span 80). Gelation of the hydrous Ti02 "droplets" was carried out by slow addition of a primary alkylamine (Primene JMT, Rohm & Haas) protracted for 5 min. The resulting microspheres were subsequently filtered, washed, and dried by azeotropic distillation in the support solvent. Any residual solvent was evaporated under an IR lamp. The final Ti02 microspheres were finally calcined at 500 "C bringing the specimens to this temperature a t 50 "C per hour. The Ti02 consists of well-defied spherical particles ranging in size from 20 to 100pm; the density is 2.16g/cm3 and the specific surface area ranged from 10 to 20 m2/g. The characteristics of the particular samples examined in this study are summarized in Table I. Samples C and D were also prepared by TEMAV using a gel-supported precipitation (GSP).21 This technique yields microspheres in the size range of 20-1500pm. The combination of surface activity, homogeneity, porosity control, mechanical resistance, and variable diameter which characterize these materials makes them particularly attractive for catalytic uses and applications. The GSP technology uses relatively inexpensive reagents (TiC4) and affords the possibility of tuning the diameter of the spherical particles to desired specifications. In addition, the mechanical characteristics of a given sample owing to both the regular shape of the microspheres and their (16) Campostrini, R.; Baraka, R. M.; Carturan, G.; Palmisano, L.; Schiavello, M.; Sclafani, A. In Proceedings of Photochemical Processes on Solid Surfaces, Ferrara, Italy, November 1991. (17) Anderson, M. A.; Gieselmann, M. J.; Xu, Q. J.Membr. Sci. 1988, 39, 243. (18)Sanches, C.; Livage, J. New J. Chem. 1990, 14, 513. (19) Technical Bulletin Pigments, no. 72, Degussa Co., Allendale, NJ, 1991. (20) Brambilla, G. Personal communication. (21)Brambilla, G.; Centi, G.; Perathoner, S.; Riva, A. In Proceedings Euromat 91, Cambridge, U.K., July 22-24,1991; p 362.
specific hardness are such that they make these materials rather attractive for application in photochemical reactors. The main preparative steps in the GSP method are (i) mixing the inorganic reagent with a thickening organic solvent, (ii) dispersing the pseudocolloidal sol through an atomizer into controlled sized droplets, and (iii) solidification of the liquid droplets by sol-gel conversion, which is then followed by (iv) aging, washing, and drying by azeotropic distillation of water and finally by calcination. The resulting particles retain the specific surface area and the pore volume even after firing at temperatures up to 500-550 "C, where the organic additive is completely oxidized. The diameter of the microspheres depends on the dripping needle gauge and on the concentration and viscosity of the mother solution. The form of the resulting microspheres is rather regular and nearly spherical as evidenced by SEM methods. The surface area of the powders and the microspheres used throughout the experiments was evaluated by the BET method using a Carlo Erba BET Sorptomatic instrument; N2 was the adsorbed gas. The values of the surface areas are reported in Table I. XRD analysis of the particles showed that the Ti02 specimens from the preparations B, C, and D have ca. 90% of the anatase crystalline form. Degussa P 25 Ti02 is reported as 80% anatase and 20% rutile. Scanning electron micrographs (SEMI of Ti02 materials prepared by the sol-gel methods are presented in Figure 1. Colloidal Ti02 (sample E) was prepared by dyalizing a solution of T i c 4 (10g/L) in cold water (0 "C)until the acidity of the permeate reached pH 2.5. The gel formed in the dialyzer membrane (Spectrapore membrane tubing; A. H. Thomas Co., Philadelphia, PA; molecular cutoff, 3500 Da) was recovered and, after aging for 1 month at 2 "C, the solution was again dialyzed until the acidity of the permeate reached a pH of 3. The colloidal Ti02 was then stable for several months at ambient temperature. The hydrodynamic radius, as measured by dynamic light scattering, is 50 f 10 nm, depending on the preparation batch. Irradiation Experiments. Samples of atrazine (ATR) and phenol were prepared by dissolving the pure compounds in water at the desired concentrations. The Ti02 (powder or colloid) loading was 1 g/L unless noted otherwise. Appropriate quantities of the solutions or the suspensions were then transferred to the cells used for the irradiation experiments.22 Ti02 millimeter spheres covered the bottom of the cells; magnetic stirring was unnecessary under irradiation. Ti02 suspensions were kept in the dark. After proper homogenization through magnetic stirring (powder and colloid samples) and temperature stabilization in a water bath at the temperature of the lamp housing (-60 "C), the cells were exposed to the light in a device simulating the power and frequency spectrum of solar light (Solarbox, equipped with a 340nm cutoff filter; CO.FO.MEGRA, Milan, Italy). After suitable irradiation times, during which the samples were magnetically stirred, the suspensions of ATR and phenol were passed through 0.45-pm cellulose acetate filters (MilliporeHA), and subsequently the filtrates were directly analyzed by high performance liquid chromatography (HPLC). The recovery of ATR and phenol was always greater than 95 % For repeated cycles, the catalyst was either washed before further addition of substrate being examined, or the irradiation was prolonged to times appropriate to ensure complete degradation.
.
(22) Pelizzetti, E.; Maurino, V.; Minero, C.; Carlin, V.; Pramauro, E.; Zerbinati, 0.;Tosato, M. L. Enuiron. Sci. Technol. 1990,24, 1559.
r I
Langmuir, Vol. 9, No. 11, 1993 2997
Photocatalytic Activity of Titania Colloids -
Irradiation timr. houri
0
10
20
30
40
Irradiation time, min Figure 2. Disappearance of phenol in the presence of simulated solar light and different Ti02 specimens A-D reported in Table I: initial concentration of phenol, 20 mg/L; loading ot TiOz, 1 g/L (A), 1 g/L (B), 2 g/L (0, and 2.9 g/L (D).
6000 Hitachi-Merck pumps) equipped with W detection (Model L-4200)and an RP-18 column (Lithrocart 5 pm, 150mm long). Detection was done at 260 and 280 nm for ATR and phenol, respectively; the eluent was a water/ acetonitrile (60/40)mixture. The formation of 2,4-diamino-6-chloro-1,3,5-triazine (CDAT),2,4diamino-6-hydro~-l,3,5-triazine (ammeline), 2 - a m i n o - 4 , 6 - d i 1 1 , 3 , 5 - t r i (ammelide), ~e and 2,4,6tryhydroxy-1,3,5-triazine(cyanuric acid) was monitored by ion interaction chromatography with the immobilized reagent technique (IIIR). UV detection was performed at 220 nm. Quantitative estimates were made using calibration curves obtained from standard solutions of the pure compound^.^^^^^
Results Photodegradation of Phenol. The photooxidative degradation of phenol in the presence of irradiated Ti02 in aqueous suspensions and as thin filmsof Ti02 on a glass substrate was examined earlier under a variety of conditions.2k27In all cases, the complete mineralization of phenol followed the stoichiometric reaction
-
A.
v"
Figure 1. Scanning electron microscopy pictures of Ti02 specimens (B-D, top to bottom) used throughout this work (for other properties, see Table I).
Activation parameters for the photodegradation of phenol (10mg/L; catalyst loading, 0.1 g/L TiO2) were assessed in the temperature range 20-60 O C using a thermostated water bath and circulating the water through a water-jacketed reactor cell. Analytical determinations of ATR and phenol were carried out on a HPLC device (Model L-6200and Model
C,H,OH + 70, 6C02 + 3H20 (1) Di- and polyhydroxybenzenes were reported as intermediates. Plots illustrating the degradation of phenol in the presence of different powders are reported in Figure 2; the corresponding consecutive cycles of Ti02 spheres are shown in Figure 3, which also depicts the evolution of CO2. The disappearanceof phenol followsreasonably good pseudo-fit-order kinetics. The apparent rate constants of different Ti02 preparations are reported in Table 11. The evolution of CO2 is best described by single exponential growth kinetics:= [COel = [CO&{l- exp(-kt)),where [C02]&is the concentration of carbon dioxide expected on the basis of eq 1. (23) Pelizzetti, E.; Minero, C.; Vincenti, M.; Pramauro, E.; Carlin, V.; Dolci, M. Chemosphere 1992,24,89. (24) Serpone, N.; Tenian, R.; Minero, C.; Pelizzetti, E. Adu. Chem. Ser., in press. (25) Okamoto, K.;Yamamoto,Y.;Tan* H.;Tanaka, M.; Itaya,A. Bull. Chem. SOC.Jpn. 1985,58,2015. (26) Okamoto, K.;Yamamoto,Y.;Tanaka, H.;Itaya, A. Bull. Chem. SOC.Jpn. 1985,58,2023. (27) Augugliaro, V.; Palmisano,L.; Sclafani, A; Minero, C.; Pelizzetti, E.Toxicol. Enuiron. Chem. 1988,16,89. (28) Tenian, R.; Serpone, N.; Minero, C.; Pelizzetti, E. J. Catal. 1991, 128,352.
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2998 Langmuir, Vol. 9, No. 11,1993 CYCIO 2
cycle 1
1.o
cycle 3
cycle 4
1 .o
1.0
CICI.
2
4
C"d.
I
3
Irradiation lime, min
Irradiation time, min
Figure 3. Photocatalytic degradation of phenol in the presence of AGIP microspheres (specimen D, 2.9 g/L), consecutive cycles of phenol disappearance and C02 evolution for the first cycle: initial phenol concentration, 20 mg/L; concentration of C02 normalized with respect to the expected stoichiometricamount. Table 11. Observed Rate Constants for the Disappearance of Phenol and Atrazine on Different Ti02 Materials. phenol, min-l
sample
cyanuric
atrazine, min-l acid) mg/L
Degussa P 25 (9.5& 0.7)X 0.18 f 0.01 AGIP C-60 (2.9f 0.4)X 103 AGIP powder 0.16 & 0.01 0.34& 0.01 AGIP microspheres (7.9f 0.4)X (6.8f 0.2)X 1b2 colloidal sols (7.6f 1.2)X 1P2
7.4
7'
CDAl
20
40
60
Irradiation time, min Figure 5. Degradation of atrazine in the presence of different Ti02 specimens (see Table I for notations): initial atrazine concentration,25 mg/L; loading of TiO2,l g/L (A), 2 g/L (C), 2.9 g/L (D),and 1g/L (E). Insert: repeated cycles in the presence of specimens A and C; conditions aa in the main figure. 30
0.4 2.3 8.3
Initial phenol concentration,20 mg/L; initial atrazine concentration, 25 mg/L. Formed in the atrazine degradation after 15h of irradiation. CI
0
c
25
I J
.-cN
15
;10
a
5 0 0
60
120
180
240 720
780
Irradiation time, min ammeline ammelide I Figure 4. Principal degradation pathways for atrazine degradation under photocatalytic conditions.
Photodegradation of Atrazine. The photocatalytic degradation of s-triazine herbicides occurs through a rather complex series of competitive-consecutive reaction^.^^^^^ A simplified scheme portraying the more relevant steps and intermediate products formed is illustrated in Figure 4, along with the formation of the final product of the degradative process, cyanuric acid. It is remarkable that the pathological fate under abiotic oxidative conditions of this class of herbicides does not yield COZ,a final product observed in the mineralization of most other classes of organic compounds. The relevant rate constants for the disappearance of atrazine under various conditions are collected in Table 11,together with the amount of cyanuric acid formed after 15 h of illumination. The primary degradation of atrazine proceeds at comparable rates for samples A and C (tip 2 to 4 min for three cycles, Figure 51, while for colloidal Ti02 (sample E) the half-life t1/2 is 10 min. With the Ti02 microspheres (sample D),the average half-life t1p. is of the order of 8-12 min, even after 13 cycles (Figure 6). The temporal distribution of the intermediate species and cyanuric acid formed during the degradation of
-
-
Figure 6. Degradation of 26 mg/L of atrazine in the presence of Ti02 microspheres (specimenD, 2.9 g/L). Differentrumwere performed after washing the catalyst before each successivecycle and thereafter adding the appropriate quantity of atrazine. atrazine is illustrated in Figure 7 for the various Ti02 samples used. Note the remarkable variations in the temporal distributions as a function of the nature of Ti02 samples. It is worth pointing out that the first cycle in the atrazine degradation leads to a product distribution different from the subsequent cycles, as observed in particular for sample D. Moreover, if the Ti02 material is treated (irradiated for several hours in aqueous media and subsequently filtsred and dried) prior to ita use in a degradative run, the product distribution no longer shows significant variations from cycle to cycle. This bears directly on the characteristics of the catalytic surface; ita significance is discussed below. Discussion The following functions must be involved in the photocatalytic degradation process of an organic compound in the presence of a photocatalyst material: (i) absorption of light by the catalyst; (ii) generation of active species and of elementary redox intermediates; (iii) temporary adsorption of the organic compound and of other species (scavengers, anions, solvent); (iv) reaction of the elementary redox intermediates with the organic molecules and
Photocatalytic Activity of Titania Colloids TiO,
Langmuir, Vol. 9, No. 11, 1993 2999
TiOz sample C
sample A
rate = kapp[organicl
8
Here, kapp is a complex function of several parameters. kapp
r
I
4
6
4
E0
2
a 0 n
1+
0
4
8
12
16
0
4
8
12 16 20 24
Irradiation time, hours TiO,
sample D
-/-'-
/+ 25 -
cyanuric acic c
CDAT 20
I
4
E 10
I ~
9.J
... ... .. ..
d c a 0
F
K0,[02]
Ki [SI i
0
15
=
@It B A , (ktraJkrec)7 0 H k KoRG [sitel,
ul E
d c
(5)
5
0
0 0 2 4
6 8 101824
Irradiation time, hours Figure 7. Evolution of intermediates in the photocatalytic degradation of atrazine (conditions same as in Figure 5) in the presence of different Ti02 specimens (notation as in Table I). For retained names and acronyms, see Figure 4.
other adsorbed species; (v) separation of redox transients and intermediates; and finally (vi) desorption of stable intermediates and final products. On the basis of earlier experimental observations and those noted in this study, the degradative photooxidation of the two compounds examined can be described in its most simple form by reactions 2-42a30
where 0 is the quantum yield of absorption of light by Ti02,1a" is the rate of absorption of photons ( n = 1at low light fluxes), PA, is the fraction of the catalyst surface (A,) irradiated, TOH is the lifetime of the 'OH radicals, KORGand KO,are the respective adsorption coefficients for the organic molecule and molecular oxygen, the species S comprises the initial organic substrate,the intermediates formed, the final product(s), the solvent, and any ions present in the suspension, and Ki values are the appropriate adsorption coefficients for the different species. Clearly, expression 6 implies that the properties of the catalyst surface will have a strong influence on the overall efficiency of a material destined to perform a photocatalytic degradation. Specifically, these properties can be summarized into four categories: (1) Electronic Properties. Quantum yield of light absorption by the photocatalyst (a). (2) Textural Properties. Surface area of the catalyst (A,) and the fraction of the irradiated surface (6). Note that the materials examined exhibit a large variety of textural features. In the case of sample D (1-mm spheres), for example, the relatively high surface area may be due to the presence of micro- and mesopores. (3) Generation of Active Species. Lifetimes of active species (e-/h+ pairs; radical^);^^^^ rate of generation of electron anf holes a t the surface (ktrap); rate of e-/h+ pair recombination (krec); the presence of recombination centers, or dopants, or retardants; and the presence of surface states and surface species formed in the process.32 The effect of mechanicaland chemical etching on the photonic efficiency and on the recombination rates has been de~cribed.~ (4) Surface Chemistry. A molecular architecture as favorable as possible (k)during the interfacial adsorption of the organic molecule35 and adsorption of ions and solvent which are poised at the surface ready to compete for the available catalytic sites where the degradation process originates (Ki).36 In addition, and not least, there is also the adsorption of molecular oxygen (Ko2)37v38 and/or other electron scavengers; the surface density of hydroxyl groups3941which reflects (but is not necessarily synonymous with) the concentration of catalytic sites which will influence ktrap. Related to this, for the anatase samples examined here that were prepared by
(4)
(31) Warman, J. M.; de Haas, M. P.; Pichat, P.; Serpone, N. J. Phys. Chem. 1991,95, 8858. (32) (a) Lewis, N. S.; Rosenbluth, M. L. ref 3, Chapters 3 and 4. (b) Ulmann, M.; de Tacconi, N. R.; Augustynski, J. J . Phys. Chem. 1986,90,
A general consensus has emerged, based on direct and
6523. (33) Gerisher, H. Angew. Chem., Zntl. Ed. Engl. 1988, 27, 63. (34) Gerisher, H.; Heller, A. J.Phys. Chem. 1991,96, 5261. (35) (a) Cunningham, J.; Al-Sayyed, G. J.Chem. SOC.,Faraday Tram. 1 1980,86,3935. (b) Tunesi, S.; Anderson, M. A. Langmuir 1992,8,487. (36) Augustynski, J. Struct. Bonding (Berlin) 1988,69, 1. (37) Gerisher, H. J. Phys. Chem. 1991, 95, 1356. (38) Sclafani, A.; Palmisano, L.; Davi, E. New J. Chem. 1990,14,265. (39) Oosawa, Y.; Gratzel, M. J. Chem. SOC.,Faraday Trans. 1 1988, 84, 157. (40) Barbeni, M.; Morello, M.; Pramauro, E.; Pelizzetti, E.; Vincenti, M.; Borgarello, E.; Serpone, N. Chemosphere 1987,16,1165. (41) Kobayakawa, K.; Nakazawa, Y.; Ikeda, M.; Sato, Y., Fujishima, A. Ber. Bunsen-Ges. Phys. Chem. 1990,94, 1439.
indirect evidence, to the effect that the degradation process takes place, at least initially, on the catalyst surface (or a few angstroms from it). In this regard, the reaction of active oxidizing species with organic compounds through the formation of intermediates can be described by expression 5 (29) Turchi, C. S.; Ollie, D. F. J. Catal. 1990,122, 178. (30) Terzian, R.; Serpone, N.; Minero, C.; Pelizzetti, E.; Hidaka, H. J . Photochem. Photobiol. A: Chem. 1990,55, 243.
Pelizzetti et
3000 Langmuir, Vol. 9,No. 11, 1993
the different procedures noted earlier, it is difficult to describe which crystallographic planes are predominantly exposed a t the solution/solid interface. This is a direct consequence of the preparative methods used which affect the crystal cleavage. The electronic environment of the titanium cations will depend on the number of Ti-0 bonds as well as the coordination number of the ligated oxygen atoms. As a result, several types of titanium cations will exist a t the different crystallographic planes. This will bear directly on the adsorption and reactive characteristics of a given Ti02 specimen.42 Phenol Degradation. Table I1 reports a wide range of photocatalytic activity within three of the specimens examined. Since samples A, B, and C are mostly in the anatase form ( W 9 0 % ,Table I),the variation in activity cannot originate with the different crystalline forms of the catalyst, which are known to affect e-/h+ pair recombination and 0 2 adsorption. Samples A and C have comparable surface areas and display similar photocatalytic activities. It is noteworthy that specimen C is even more efficient than sample A, a commercially available (Degussa P 25) material widely used in photocatalytic studies. SampleB shows a dramatic difference in activity; it can be ascribed either to the smaller surface area (A,) and/or to the different surface chemical properties, and/or to the variations in the efficiency of generating the active species. To the extent that samples C and D were prepared following practically the same solgel method, any comparison must consider the different catalyst loading (2.9 g/L for sample D uersus 2 g/L for sample C), the different surface areas, and the different sizes between the two specimens (lo00 pm uersus 0.05-0.2 pm, respectively). The Ti02 beads (sample D) exhibit good photocatalytic activity. Figure 3 reports some consecutive cycles of phenol degradation along with the corresponding evolution of C02 for the first cycle. These highly porous spheres give rise to a relatively rapid 15 min). This probably degradation of phenol (t1p originated by the fast exchange occurring between the various adsorption sites.43 Few studies have focused attention on the temperature dependence of the heterogeneous photocatalytic proC ~ S S In . some ~ ~ cases, ~ ~ an Arrhenius type behavior with reasonably good precision has been reported whereby In activation energies ranged from 10 to 16 kJ/mo1.26,4P46 the photodegradation of chloroform, the rate of disappearance of CHC13 in fact decreases with increase in temperature a t relatively highlight fluxes?' In the present instance, the activation energy for the photodegradation of phenol (with sample A) in the temperature range 20-60 "C is 2.6 f 0.5 kJ/mol, while the entropy of activation is ca. -64 eu, further evidence about the heterogeneity of the initial stages of the degradation process. A value of 10 kJ/mol was reported earlier for phenol.26 These activation energies compare with the Eact for the reaction of 'OH
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(42) Hadjiivanov,K.; Klissurski, D.; Davidov,A. J.Chem. Soc., Faraday Trans. 1 1988,84,37. (43) Minero, C.; Catozzo, F.; Pelizzetti, E. Langmuir 1992,8, 481. (44) Bahnemann, D. W.; Bockelmann, D.; Goslich, R.; Hilgendorff, M. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, The Netherlands, in press. (45) Matthews, R. W. J. Phys. Chem. 1987, 91, 3328. (46) Herrmann, J.-M.; Mozzanega, M. N.; Pichat, P. J.Photochem. Photobiol. A: Chem. 1983,22,333. (47) Bahnemann, D. W.; Bockelmann, D.; Goslich, R. Sol. Energy Mater. 1991, 24, 564.
01.
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radicals with some organic substrates (e.g., formate, 5 to 10 kJ/ 2-propanol) in homogeneous phase; E,& m01.48 The turnover number (molecules transformed per site) for the disappearance of phenol (and atrazine) over the Ti02 microspheres is estimated as 160 (however, see ref 49), while the turnover rate is 10.05 (molecules transformed s-l site-9. The latter value may be taken as a lower limit of the quantum yield if the number of photons absorbed by the photocatalyst specimen were equal to the number of surface sites (maximum number 5 X 1014 sites cm-2; not all are necessarily catalytically active however).49 Atrazine Degradation. The photooxidative degradation process for atrazine has been investigated in the presence of the two types of Ti02 particles that show good photocatalytic activity (samples A and C), Ti02 colloids (sample E) and Ti02 beads (sample D). In the presence of particles A and C, the rates of disappearance of ATR are quite similar (comparablehalf-lives, 2-4 min), whereas a slightly lower degradation rate is observed for colloids (t1p 10 min) and spheres (t1p 8 to 12 min). It is remarkable that after several cycles, the activity of the beads remains unchanged as far as atrazine disappearance is concerned (see Figure 6). However, although the same intermediates and final product (cyanuric acid) are obtained, different trends are observed for the extent and rates of formation of intermediates. Once again, a similarity in the shapes of the plots is observed for the powders, but formation of the intermediates and cyanuric acid is slightly slower with catalyst C with respect to catalyst A. The formation of CDAT is predominant with colloidal Ti02 and with Ti02 spheres. The direct photocatalytic transformation of intermediate I (dechlorinated atrazine, Figure 4) affords only small amounts of ammeline and ammelide. It appears that With these two catalyst samples, the dehalogenation pathway is strongly diminished. In the photooxidation of phenol, the different Ti02 specimensused showed small but significant variations in activity; however, we found no evidence for variations in product selectivity for the various materials examined owing to the rather similar nature of the intermediates formed (hydroquinone, catechol, benzoquinone, and pyrogallol) and the rather analogous interaction(s) of these with the active oxidizing species ('OH radicals and probably also 02'-). By contrast, when atrazine is the test probe, there is a dramatic variation in the activity, but more notably in the selectivity as witnessed by the variations in the temporal distributions of intermediate species and product formed (Figure 7) for the different preparations of Ti02 materials. This is a consequence of the different surface characteristics of the catalyst particles, together with the different modes of interaction between the test probe and the catalyst: (a) chlorine substitution by hydroxyl (dehalogenation); (b) dealkylation; and (c) amino group substitution by hydroxyl (deamination). The important message to be taken from the data of Figure 7 is thatihe irradiated Ti02 catalysts are not simply photo-Fenton reagents; rather, they provide
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(48) Elliott, A. J.; Simsons, A. S. Radiat. Phys. Chem. 1984,24,229. (49) The usage of turnover numbers, turnover rates, turnover fre-
quencies, quantum yields, and the term 'photocatalysis", together with the usefulness of the Langmuir-Hinshelwood model as they apply to heterogeneous photocatalysis, has recently been questioned: (a) Serpone, N.; Pelizzetti, E.; Hidaka, H. In Photochemical Conuersion and Storage of SolarEnergy -IPSI X Cai, S. M., Cao, Y., Xiao, X. R., Eds.; Academica Sinica: Beijing, China, in press. (b) Serpone, N.; Terzian, R.; Lawless, D.; Kennepohl, P.; Sauve, G. J. Photochem. Photobiol. A: Chem., in press.
Photocatalytic Activity of Titania Colloids the suitable sites at which the initial events in the photooxidative degradation of organic substrates take place. A final point is worth noting. As noted in the results section, for a given specimen the differences observed in the activity and selectivity between a virgin sample used for the first time and one used in subsequent cycles calls attention (and should alert workers in this area) to the different surface properties of catalysts from different batches and to comparisons often made between results from different laboratories. Although this has been suspected in the past, the present results demonstrate the variations. Conclusions The present study has demonstrated several salient features of heterogeneousphotocatalysis,previously either implied or suspected: (i) a wide range of photoreactivity is observed within different Ti02 specimens; (ii) the differencesare also observablefor specimenswith the same crystalline structure; (iii) all the investigated specimens lead to the same final products (e.g., C02 for phenol and cyanuric acid for atrazine); (iv) in the case of atrazine as the test molecule, a different distribution of intermediate products and final product as well as different rates of formation and disappearance are observed depending on the surface characteristics of the photocatalyst; (v) the different surfacelsubstrate interactions seem to affect the degradation pathways and to influence directly the
Langmuir, Vol. 9, No. 11, 1993 3001 predominance of dehalogenation relative to alkyl chain oxidation in the case of atrazine; (vi) the reproducibility of the degradation process over several cycles has been demonstrated for Ti02 beads, although the behavior of a virgin specimen may vary with one aged by a given process insofar as the temporal distribution of intermediates and distribution of rates are concerned (this may be important in the design of active materials to be used in large volume photoreactors); (vii) colloidal Ti02 is photoactive toward the degradation of organic substrates. This is relevant in fundamental studies of primary events and transient formation,s0 as well as for application of colloidal semiconductors in the decontamination of pollutants strongly adsorbed on inert materials (e.g., soils).51 Acknowledgment. Generous financial support for this work was received from the Regione Piemonte, EN1 Ricerche, the European Economic Community under the STEP Program, CIEMAT, MURST, and the CNR Rome (to E.P.) and from the Natural Sciences and Engineering Research Councilof Canada (to N.S.). We are also grateful to the North Atlantic Treaty Organization for an exchange grant (NATO Grant No. CRG 890746). We also wish to thank the group at AGIP Nucleare for providing us with the sol-gel samples. (SO) Nenadovic,M.T.;Saponjic,Z. I.;Micic,O.I.;Minero,C.;Pelizzetti, E. Manuscript in preparation. (51) Pelizzetti, E.; Minero, C.; Carlin, V.; Borgarello, E. Chemosphere
1992,25,343.
