J . Phys. Chem. 1990, 94, 829-832
829
Influence of the Preparation Methods of TiO, on the Photocatalytic Degradation of Phenol in Aqueous Dispersion A. Sclafani,* L. Palmisano, and M. Schiavello Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universitci di Palermo, Viale delle Scienze, 901 28 Palermo, Italy (Received: April 5, 1989)
Several commercial and homemade TiOz samples were used as photocatalysts to prove that the physicochemical features as determined by the origin and preparation methods affect the photocatalytic behavior, in addition to the semiconducting properties. The photodegradation of aqueous phenol solution in T i 0 2 dispersion, carried out in a batch reactor, was used as a test reaction. The results show a great variability of photocatalytic behavior, and in particular it has been observed that the rutile phase is active or inactive according to the preparation conditions.
Introduction
The photocatalytic degradation of inorganic and organic pollutants, using oxygenated aqueous titania dispersions, is becoming an overwhelming field of research.’ So far the studies aim to demonstrate the feasibility of the process, and therefore the photocatalytic degradation of the more diffuse pollutants has been tested. The results are very promising. Very few studies have appeared that deal with kinetic and mechanistic aspects.,” Recently our group has reported the results of a phenol photodegradation study in which, apart from the kinetic and mechanistic aspects, operative factors such as pH, oxygen partial pressure, phenol and TiO, concentration, presence of foreign ions, and presence of TiO, as anatase or rutile phase were considered in relation to their influence on the phot~reactivity.~ In the present study a further aspect is considered, namely, the influence of the physicochemical properties on TiO, as determined by its origin and different preparation methods on the photoreactivity. Therefore, the phenol degradation photoreaction was studied, under constant conditions, using several commercial Ti02 samples and two series of differently prepared TiO, samples. The specimens were characterized by X-ray diffractometry, scanning electron microscopy (SEM), UV-vis reflectance spectroscopy, and surface area measurements. Information on the surface acid-base properties of some specimens was obtained by use of a heuristic method. The photoreactivity tests were carried out by using a batch system. It will be seen that a great variety of photoreactivity will be observed and that the rutile phase is photoactive when prepared under certain conditions and photoinactive when prepared under other conditions. General correlations between photoactivity and some physicochemical properties will be proposed. Experimental Section
Apparatus and Procedure. The experimental tests were performed by using Pyrex glass flasks as photoreactors containing 50 mL of an aqueous dispersion (1 g / L of powder, -90 mg/L of phenol) at pH 3. The stirred dispersions were irradiated by a 1500-W Xe high-pressure lamp (Philips XOP 15-OF) inside a Solarbox 522 (CO.FO.ME.GRA., Milan), where the temper(1 ) Photocatalysis and Environment. Trends and Applications; Schiavello, M., Ed.; Kluwer Academic: Dordrecht, 1988. (2) (a) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404. (b) Hsiao, C. Y.; Lee, C. L.; Ollis, D. F. J. Catal. 1983, 82, 418. (3) (a) Kawaguchi, H.; Uejima, T. Kagaku Kogaku Ronbunshu 1983,9, 107. (b) Kawaguchi, H. Enuiron. Technol. Lett. 1984,5,471. (c) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. SOC.Jpn. 1985, 58, 2015. (4) Augugliaro, V.; Palmisano, L.; Sclafani, A.; Minero, C.; Pelizzetti, E. Toxirol. Enuiron. Chem. 1988, 16, 89. (5) Pichat, P. Photocaralysis and Environment. Trends and Applications; Schiavello, M., Ed.; Kluwer Academic: Dordrecht, 1988; p 399. (6) AI-Sayyed, G. H.; Pichat, P. Proc. IUPAC Symp. Photochem., 12th 1988, 126.
0022-3654/90/2094-0829$02.50/0
ature reached 313 K. The photon flux provided by the lamp was measured by means of the standard potassium ferrioxalate actinometric method which gave 6.6 X lo4 einstein/s a t X = 365 nm and 7.0 X lo4 einstein/s at X = 405 nm. The photodegradation runs lasted 1.5 h. The samples were centrifuged immediately afterward, and the phenol concentration in the supernatant liquid was measured by means of a standard colorimetric method. Homogenous photodegradation under the above conditions was not observed. Surface areas of all the catalysts used were measured by a dynamic BET method using dinitrogen as adsorbate and a Micromeritics Flowsorb 2300 apparatus. Reflectance spectra, at room temperature and in air, in the range 600-300 nm were obtained by using a UV-vis spectrophotometer (Varian, Model DMS 90) equipped with quartz cells using BaSO, as reference sample. The measurements were performed on all specimens and with selected specimens on whose surface C 6 H 5 0 H was preadsorbed at pH 3. For these latter experiments the procedure was as following: 2 g of the TiO, specimens was immersed for 24 h in 50 mL of a 190 mg/L phenol solution at pH 3 adjusted by H2SO4. The resulting slurry was slowly dried at 353 K. X-ray powder diffraction patterns were obtained at room temperature by a Philips diffractometer using Ni-filtered Cu K a radiation. In order to estimate the anatase-rutile ratio, X-ray analysis was carried out on some selected specimens. For the preparation of the samples for X-ray diffractometric analysis, the powders were ground for 20 min in an agate mortar up to a particle grain size 50 nm) were not observed. Detection of mesopores (diameter 2-50 nm) and micropores (diameter < 2 nm) was outside the resolution of the instrument. Four TiOz hp ex 3 specimens, fired 24 h at 393,773,823, and 1073 K, were examined. The main results were as follows. (a) The particle showed a great distribution of shapes and dimensions; the shapes were very irregular while the dimensions ranged from 1000 to 10000 nm in random distribution. (b) Macropores were not detected. (c) The specimen fired at 1073 K (rutile as revealed by X-ray analysis) showed the presence of aggregates, in which tiny particles laid on large ones. Three TiO, hp ex 4 specimens fired 24 h at 393,823, and 1073 K were investigated. An almost homogeneous distribution was observed, even for the specimen fired at 393 K, of particles having an almost spherical shape and dimensions varying from 1000 to 2000 nm. Aggregation was visible for the specimen fired at 823 K, and a heavier aggregation was visible for the specimen fired at 1073 K (also rutile, as the previous TiOz hp ex 3). No macroporosity was observed also for these specimens. Diffuse Reflectance Spectroscopy. The spectra of all the specimens did not reveal any particular feature. They were the typical spectra of anatase, rutile, or mixtures of both phases reported elsewhere.’ The spectra were not modified by adsorbing phenol at pH 3 on the surface of the studied specimens. Surface Acid-Base Properties. The experiments were performed for all the commercial and for two hp specimens, one ex 3 and one ex 4, both fired at 873 K for 24 h. The pH measured on the supernatant liquid was found higher than the reference pH from about 0.1 to 2 p H units. For the Ti0, hp ex 4 specimen
S,
m2.g-l
T, K
t, h
393 473 523 513 623 613 613 673 123 123 113 173 823 823 823 823 813 813 873 923 923 923 923 913 913 973 1073
24 24 24 24 24 3 24 192 3 24 3 24 3 24 192 336 3 24 192 3 24 192 336 3 24 192 24
S, m2.g-I 282 243 147 87 13 91 63 60 13 61 64 51 49 47 36 32 45 34 23 40 21 20 19 18 13 11
5
0,
mg.L-1.h-1m-2 0.03 0.02 0.01 0.08 0.1 1 0.08 0.18 0.17 0.10 0.15 0.12 0.18 0.15 0.19 0.28 0.30 0.25 0.34 0.32 0.28 0.47 0.51 0.29 0.29 0.16 0.22 0.00
P
A
A A
A
A (70%), R (30%)
R
T = firing temperature; t = thermal treatment time; S = surface area; L: = photodegradation rate; P = catalyst phase, A = anatase, R = rutile.
TABLE 111: Some Physicochemical Parameters for TiO, hp ex 4“
613 113 713 823 823 823 823 813 813 873 923 923 923 913 973 1073 =
24 3 24 3 24 192 336 3 24 192 3 24 192 3 24 24
26 24 13 17 8 6 3 1.5 4.5 2 3.5 2.2 1.8 2.8 1.0 1.2
0.5 0.8 1.9 1.4 3.1 3.1 1.3 2.2 4.5 3.5 3.5 4.9 1 .o 3.1 0.0 0.0
A (58%), R (42%) A (43%), R (57%) A, R A (37%), R (63%) A (28%), R (72%) A (38%), R (62%) A (13%), R (87%) A (traces), R A
(traces), R
R R R
T = firing temperature; t = thermal treatment; S = surface area; u photodegradation rate; P = catalyst phase, A = anatase, R = rutile.
the p H was 3.85 while for TiO, hp ex 3 the p H was 5.48. For the commercial samples the lower p H was for the Carlo Erba specimen (3.82) and the higher (5.11) for the BDH one. Photocatalytic Results. The results are reported in Table I for the commercial, in Table I1 for the TiO, hp ex 3, and in Table 111 for the TiOzhp ex 4 specimens. The tables contain information
Photocatalytic Degradation of Phenol about the origin, the temperature and time of firing, the surface area, and the phase composition of the selected specimens. The photocatalytic activity is reported as degradation velocity ( u ) , expressed as mgL-'.h-1.m-2 phenol degraded. This unity allows comparison of the photoactivity of the various specimens since the photon flow impinging on the dispersion is constant and no available method, before now, has been reported in the literature for obtaining an accurate estimate of the photons absorbed by a dispersed solid. The observation of Table I gives the information that the photoreactivity of the commercial specimens is spread over a wide range. Notice that the specimen Degussa P25, which contains approximately 20% rutile, is less active within the anatase specimens, while the Tioxide specimen (rutile) is inactive (see below). From Table I1 the following considerations for the photocatalytic behaviour of T i 0 2 hp ex 3 can be drawn: The photoreactivity increases up to the specimens fired at 923 K. Notice that up to the firing temperature of 823 K, the X-ray analysis does not reveal the presence of rutile; for these specimens that do not contain rutile the photoreactivity at a given temperature increases with the time of firing. The specimens fired at various times at 923 and 973 K are all mixtures with a variable anatase to rutilflratio and a scatter of photoreactivity is visible. The specimen fired at 1073 K, which gave only rutile to the X-ray analysis, did not show any photoreactivity. Noteworthy for the photocatalytic behavior of the Ti02 hp ex 4 specimens are the following considerations, drawn from the observation of Table 111: The specimens fired up to 873 K are all mixtures of anatase and rutile at a ratio variable with temperature and time of firing; for these specimens the photoreactivity increases at any temperature with the time of firing. For the specimens fired at 923 K, this trend was observed for the specimens fired for 3 and 24 h, while for the one fired for 192 h a drastic drop in activity was revealed. The same situation is met for the specimens fired at 973 K: the pure rutile one fired for 3 h presents a fairly good reactivity which drops to a not detectable level for the specimen fired 24 h. The pure rutile specimen fired at 1073 K is completely inactive. Finally it is to be noted that the photoreactivity of the specimens TiO, hp ex 4 is an order of magnitude, on average, higher than that of the specimens Ti02 hp ex 3, when specimens at similar conditions of temperature and time of firing are compared.
Discussion The salient findings of the present study can be summarized as follows: There is a wide range of photoreactivity within the various specimens; this is observable not only for the specimens formed by mixture with variable content of anatase and rutile but also for the specimens formed by anatase or rutile only. Rutile phase is active or inactive according to the preparation methods; in particular it is inactive when it is prepared at relatively high temperature and for a long period of firing. The S E M and acid-base results clearly show that the commercial and the homemade specimens have a great variety of morphological and textural as well as surface acid-base properties. Notice that, apart from the different features found and reported under Results, it can be inferred that the relatively high surface areas of the Ti02hp ex 3 specimens may be accounted for by the likely presence of micro- and mesopores. In fact, they cannot be detected by our S E M instrument, while the chemisorption of N2 (BET method, cross section of N, = 0.15 nm2/molecule) may reveal them in the value of the surface areas. For the Ti02 hp ex 4 specimens, on the same grounds, the presence of micro- and mesopores can be disregarded. It is therefore clear that the origin and the methods of preparation affect the physicochemical properties of the specimens and these in their turn affect the photocatalytic features. Let us now try to rationalize the observed findings following the various steps of the photocatalytic phenol degradation process by semiconductor dispersions. As is well-known, the photogenerated and separated electrons and holes induce redox reactions according to the relative potentials of the conduction and valence
1rhe Journal of Physical Chemistry, Vol. 94, No. 2 , 1990 831
bands of the semiconductor and according to the redox potentials of the redox couples present at the interface. Taking into account the value of the conduction bands of anatase (band gap 3.2 eV) and of rutile (band gap 3.0 eV), it is understandable why, for instance, H+ is reducible on anatase, while the same process on rutile is, from a thermodynamical point of view, i m p o s ~ i b l e . ~ ~ ~ For the photodegradation of phenol which occurs in our experimental conditions in aerated dispersions of Ti02, the reduction process involves oxygen species according to the reaction:
This is thermodynamically possible on both T i 0 2 modifications. species may evolve in various ways, according to the The Oy(ads) experimental conditions, producing further oxygenated species involved in the photooxidation of the organic specie^.^ On the other hand, the oxidative processes, which involve the photogenerated holes and adsorbed phenolic species as well as OHgroups, have a similar driving force on anatase and rutile, since the valence band position is the same. Therefore, from the thermodynamical point of view, the photooxidative degradation process should occur both for anatase and for rutile. However, it has been reported several times that rutile is a very poor photocatalyst for this reaction likely owing to its higher hole-electron recombination rate.3,4 Also, in the present study this behavior was confirmed for the specimens fired at relatively high temperature (973-1073 K) for a time longer than 24 h and for the commercial specimens. Therefore, the variability of photoreactivity of the anatase and rutile specimens and of the mixtures, especially those containing a predominant amount of rutile, must be related to kinetic factors, due to the variability of the physicochemical features, determined by the origin and the preparation methods. It must be outlined that a photocatalytic process is determined, apart from the redox reactions, by other factors which can be called physicochemical factors. They can be described as the absorption of light by the particles (affected by the particle size distribution and by the texture); the amount and nature of the adsorbed reactant species, in our case oxygen and phenol species; desorption of the products; nature of the interface; etc. It is impossible to correlate with the photoreactivity all these factors, since an interplay between them and between them and the redox reactions takes place, setting the final level of photoreactivity. Only general considerations can be advanced. For instance, the experimental evidence that the photoreactivity, for a specimen fired at a given temperature, increases with the time of firing (for a temperature lower than 923 K) is a clear indication that the improvement of the crystallinity beneficially influences the setting up of the physicochemical factors. Indeed this aspect has been observed several times for different photocatalytic reactions.l0," For instance, it has been shown that the rate of the photocatalytic reaction of propan-2-01, which yields H2 and 2-propanone, increases with the crystal growth.1° Moreover, the catalytic photoactivity of Ti02 (anatase) for partial or complete isobutane oxidation and for oxygen isotopic exchange experiments depends on the morphological and texture properties of the specimens.I2 In addition, the change of the photoactivity of the rutile phase may be related to the hydroxylation-dehydroxylation surface equilibrium. For high temperatures, an irreversible dehydroxylation occurs on the rutile ~ u r f a c e . ' ~ - The ' ~ decrease of surface OH- groups (8) (a) Kraeutler, B.; Bard, A. J. J . Am. Chem. Soc. 1978,100,2239. (b) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 5985. (9) Rao, M. V.; Rajeshwar, K.; Verneker, V. R.; Du Bow, J. J . Phys. Chem. 1980,84, 1987. (10) Nishimoto, S.;Ohtani, B.; Kajiwara, H.; Kagiya, T. J. Chem. Soc., Faraday Trans. 1 1985, 81, 61. ( 1 1) Abrahams, G.; Davidson, R. S.;Morrison, C. L. J . Photochem. 1985,
29, 353. (12) Courbon, H.; Formenti, M.; Pichat, P. J . Phys. Chem. 1977,81,550.
832
J . Phys. Chem. 1990, 94, 832-836
has a detrimental effect on the charge separation (OH- groups together with the phenolic species groups are traps for the holes). Moreover, the decrease of OH- groups has a negative effect on Therefore, the results obtained in the the O2chemi~orption.'~J~ ( I 3) Bickley, R. I.: Javanty, R. K. J. Chem. Soc., Faraday Discuss. 1974, 58, 194. (14) Morterra, C. J. Chem. Soc., Faraday Trans 1 1988, 84, 1617. ( 1 5 ) Primet, M.: Pichat, P.; Mathieu, M. V. J . Phys. Chem. 1971, 75, 1216. (16) Bickley, R. 1.; Stone, F. S. J. Catal. 1973, 31, 389. (17) Boonstra. A. H.; Mutsaers, C.A. H. A. J. Phys. Chem. 1975, 79, 1694. (18) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J . Chem. Soc., Faraday Trans. 1 1919, 75, 736. ( 1 9) Munuera, G.; Gonzalez-Elipe, A. R.; Rives-Arnau, V.; Navio, A,; Malet, P.; Soria, J.; C o m a , J. C.; Sanz, J. Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; p 113.
present study give an insight into the reason for the inactivity of the rutile phase. In conclusion, this work shows the extreme variability of photoreactivity of a semiconductor compound such as Ti02. The results indicate the need to define, for a photocatalyst, as many as possible structural and surface features, since the final level of photoactivity is determined not only by semiconducting properties but also by the physicochemical ones. Correlations and order of activity are useless, unless the main structural and surface properties together with the semiconducting features are clearly known. Acknowledgment. We thank C N R (Rome) and MPI (Rome) for financial support and Prof. F. Ricciardiello for help given with the X-ray analyses. Registry No. TiOz, 13463-67-7; C6H50H,108-95-2.
57FeMiissbauer Spectroscopy of Reduced Cathodes in the Li/FeS, Battery System: Evidence for Superparamagnetism C. H. W. Jones,* P. E. Kovacs, R. D. Sharma, and R. S. McMillan' Department of Chemistry, Simon Fraser University, Burnaby, B.C., Canada V5A IS6 (Received: April I I , 1989; In Final Form: July 19, 1989)
Cathodes removed from fully discharged LilLiCIO4-propylene carbonate)FeS2cells have been studied by s7Fe Mossbauer spectroscopy. Experiments carried out at 295, 77, and 4.2 K indicated that the major phase present was a mixture of superparamagnetic iron particles and bulklike a-iron. Chemical reduction of FeS2 with n-BuLi in hexane also yielded a similar mixture. Measurements in external magnetic fields of up to 1.5 T confirmed the presence of superparamagnetic iron, and these data led to an estimate of the average particle diameter of 3.6 A 0.1 nm. An analysis of the data in terms of superferromagnetism was inconsistent with the variable-temperature measurements.
Introduction
The investigation of the mechanism of discharge in the high energy density battery system Li/FeS2, where lithium is the anode and iron pyrite the cathode, has been of interest for some time. Thus, extensive studies have been made of high-temperature (450 "C) cells with LiCI/KCI eutectic molten salts as the electrolyte. Tomczuk et al.'v2 proposed the formation during discharge of Li3Fe2S4,Li, ]Fe4Sl0,and Li,FeS2 as lithiated intermediates, using the methods of X-ray diffraction, cyclic voltammetry, and coulometric titration. The final reduction product was iron. Nardi et aL3 have investigated the room-temperature Li/FeS2 system, using chemical analysis of partially discharged cathodes. The data suggested the formation of Li3Fe2S4during discharge which is then rapidly reduced to iron. In contrast, Iwakura et aL4 and Ikeda et aLS proposed the formation of LizFeS2as the only intermediate. Again iron was observed to be the final reduction product. It was felt that s7Fe Mossbauer spectroscopy might prove informative in determining the course of reduction at the cathode in this system. Nardi et al.) had previously attempted to use Mossbauer spectroscopy in such a study but did not observe any significant change in the Mossbauer spectra as reduction proceeded, probably as a result of poor cathode utilization. In our investigation we began by studying fully discharged cathodes to confirm that significant changes in the Mhsbauer spectrum could be observed. The study of the fully discharged cathodes became a major topic of investigation in itself. As will be described below, evidence was obtained for the formation of very small particles of iron which exhibit superparamagnetism. The size of the National Research Council, Ottawa, Ontario, Canada. 0022-3654 19012094-0832SO2.50I O
particles was found to depend on the rate of discharge in the cell. The measurement of the Mossbauer spectra in the presence of an externally applied magnetic field allowed an estimation of particle size. Experimental Section
Even-layered (;=I2 mg/cm2) FeSz cathodes were prepared by spreading Transvaal pyrite (C40pm mesh, >99% pyrite) from a binder slurry (1.5% EPDM (Royalene 5 12, Uniroyal Rubber Co.) in cyclohexane) onto an aluminum substrate. The cathodes were evaporated to dryness, and the pyrite was mass determined to f0.1 mg. The final content of binder in the cathodes was ca. 0.5%. Cathodes were compressed between steel rollers to ca. 70% of pyrite density. Galvanic cells were assembled from 2-cm2 disks cut from the cathodes. Cathode disks were pressure wetted (ca. 400 psi of argon) with 1 M LiC104/propylene carbonate electrolyte. Lithium perchlorate (J. T. Baker) was dried overnight under vacuum at 120 "C. Propylene carbonate (Aldrich Chemical Co.) was distilled under reduced pressure (0.7 mmHg) and had a water content of C5 ppm as measured by Karl Fischer titration. The electrolyte, as similarily measured, had a water content of ( I ) Tomczuk, 2.;Roche, M. F.; Martin, A. E. Argonne Natl. Lab. [Rp.] 1979, ANL-79-39, 66. (2) Tomczuk, 2.;Tani, B.; Otto, N. C.; Roche, M. F.; Vissers, D. R. J. Electrochem. Soc. 1982, 129, 925. (3) Nardi, J. C.; Clark, M. B.; Evans, W . P. Abstracts of Papers, Symposium on Electric Power Sources in Horological and Microtechnical Products, Mulhause, France, 1981; Extended Abstract, 48. (4) Iwakura, C.; Isobe,N.; Tamura, H. Electrochim. Acta 1983,28,277. ( 5 ) Ikeda, H.; Narukawa, S.; Nakaido, S. Abstracts of Papers, 21st Battery Symposium in Japan, Okayama, 1980, Extended Abstract, 47.
0 1990 American Chemical Societv
