Investigation on TiO 2 -coated optical fibers for gas-phase photocatalytic oxidation of acetone

Transcription

1 Applied Catalysis B: Environmental 31 (2001) Investigation on TiO 2 -coated optical fibers for gas-phase photocatalytic oxidation of acetone Wonyong Choi, Joung Yun Ko, Hyunwoong Park, Jong Shik Chung School of Environmental Engineering, Pohang University of Science and Technology, Pohang , Korea Received 28 June 2000; received in revised form 20 November 2000; accepted 21 November 2000 Abstract A preliminary optical fiber reactor (OFR) that employs bare quartz fibers as a light-transmitting support of TiO 2 was tested for gas treatment by investigating photocatalytic oxidation of acetone in air ( ppmv). Using one or four TiO 2 -coated fibers in a continuous flow photoreactor, a steady-state conversion up to 80% was achieved at ambient temperature and pressure. The kinetic behavior of the acetone conversion in this study could be described by zero-order kinetics. The characteristics of coated-optical fibers were quantitatively analyzed and their use in photocatalytic gas treatment was discussed in detail. All the acetone molecules degraded was quantitatively converted to CO 2 with no intermediates detected. No noticeable deactivation was observed within a few hours operation under the present experimental conditions. The conversion of acetone linearly increased with the incident light intensity without showing any sign of saturation. The transmitted light intensity through a TiO 2 -coated optical fiber exponentially decreased along the fiber, showing 90% extinction within 30 cm. The photocatalytic conversion measured as a function of the coated-fiber length showed a similar trend. An optimal coating thickness was found at around 1.5 m above which the photocatalytic efficiency was reduced. The presence of water vapor reduced the reactivity due to the competitive adsorption on active surface site with acetone. While a measurable conversion of acetone was observed in the absence of O 2, increasing O 2 concentration up to 15% effectively enhanced the conversion Elsevier Science B.V. All rights reserved. Keywords: Optical fiber reactor; Quartz fiber; Photocatalysis; Titanium dioxide; Acetone degradation; Photocatalytic gas treatment 1. Introduction The destructive removal of trace contaminants from polluted air and water by using various photocatalytic reactor systems has been intensively studied [1]. The photocatalytic remediation technologies show several attractive advantages in that they operate at ambient temperature and pressure conditions, use oxygen in the air as an oxidant and inexpensive catalyst ma- Corresponding author. Tel.: ; fax: address: wchoi@postech.ac.kr (W. Choi). terial, lead to full degradation for a wide variety of contaminants, and are able to utilize sunlight for photoactivation. Therefore, photocatalytic systems are being actively considered as an economically viable purification method in solving various kinds of remediation problems. Although studies of heterogeneous photocatalysis involving gas solid interfaces are relatively fewer in number than those of water solid interfaces, interests in photocatalytic gas treatments are growing since they usually show much higher photonic efficiencies than their counterpart in water treatment [2]. While typical photonic efficiencies in water treatment are /01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (00)

2 210 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) a few percent at best and <1% in most cases, those above 10% are commonly reported in the gas treatment system [3,4]. Such higher efficiencies found in the gas solid interface make the development of commercial photocatalytic gas treatment system look promising, which heavily depends on the design of efficient photocatalytic reactors. Unlike photocatalytic water treatment systems where many studies were performed in suspensions, gas treatment systems usually employ immobilized photocatalysts on solid substrates except for reactors using packed powder layer [5,6] or fluidized bed of powders [7]. The most common immobilized photocatalytic reactors for gas treatment are annular plug flow reactor [8,9] and honeycomb monolith reactor [10]. The idea of using optical fibers as both a light distributing guide and an immobilizing support for photocatalysts was originally proposed and theoretically evaluated by Marinangeli and Ollis [11 13]. Experimental application of the idea was demonstrated by Bauer and coworkers who designed a TiO 2 -coated quartz fiber reactor and used it in photomineralization of 4-chlorophenol in water [14]. Recently, Peill and Hoffmann have developed, characterized, and modeled an optical fiber reactor (OFR) system for water treatment [15 17]. The OFR system has several advantages over conventional immobilized or slurry-type photocatalytic reactors. The unique reactor configuration that is essentially equivalent to mini-lamps immersed in reaction medium allows the light intensity to be more evenly distributed within a given reaction volume and reduces mass transport limitations. The number of fibers and the distance between them can be easily varied. In particular, the OFR enables the remote delivery of light to the photocatalyst, which makes it ideally suited to the remediation of sites that are not easily accessible (e.g. polluted groundwater, hazardous area). Although the OFR can be applied to both water and gas treatment, the study of gas-phase OFR system has not been reported. In the present work, we investigated the characteristics of TiO 2 -coated optical fibers and tested the feasibility of using them for photocatalytic air treatment through monitoring acetone conversion. The effects of light intensity, coating thickness, fiber length, and concentration of water vapor and oxygen on the performance of the OFR were quantitatively investigated. Acetone, a common volatile organic pollutant, was chosen as a test compound because it has been extensively studied using various kinds of photocatalytic reactors and is known to be degraded with little intermediate formation [6,8,10,18,19]. 2. Experimental 2.1. Materials and coating preparation The photocatalyst used was Degussa P25 TiO 2, which is mostly anatase with a primary particle diameter of 30 nm and a specific surface area of about 50 m 2 /g. A concentrated suspension (5 wt.%) of TiO 2 in distilled water was prepared and dispersed by sonication (Branson 3210). The optical fibers (3 M Power-Core FP-1.0-UHT) with 1 mm diameter were cut into pieces of 30 cm length and their protective buffer and silicone clad were mechanically stripped out to expose the quartz core. Residual cladding material was removed by dipping into 1 M NaOH solution for 24 h. One of the two fiber-ends on which the light is incident was polished with abrasive paper. The exposed quartz fibers were coated with TiO 2 layer by dripping the suspension solution along the fiber and then dried in air for 24 h. The TiO 2 -coated fibers were heated at 200 C for 1 h. Multiple coatings were carried out by repeating the same procedure. The coating procedure was repeated typically three times Characterization of TiO 2 -coated optical fiber The coating mass of TiO 2 was determined by weighing the optical fiber before and after coating. A series of fiber samples were prepared by varying the number of coats. The coated TiO 2 layer was analyzed by SEM (Hitach, S-2460N). The coated fibers were cut into a piece of 10 mm length for SEM analysis and their cross-section was imaged to measure the coating thickness. The samples were gold-coated by using a sputter coater prior to the SEM analysis. The light distribution along a TiO 2 -coated optical fiber was measured and compared with that of naked optical fiber. A beam of incident light (I i )was focused on a polished end of a quartz fiber. The irradiating light was filtered through a UV band pass filter (λ tr = 330 nm with FWHM 15 nm) in order that only active portion of light that was absorbable by

3 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) TiO 2 was allowed to be incident on the fiber tip. The light refracted (I r ) into TiO 2 coating was quantified by subtracting the transmitted light intensity (I t ) from the incident light intensity (I i ). The transmitted light flux was measured by using a power meter (Newport 1815-C with a 818-UV silicon diode detector) at the end of the fiber. The incident light intensity (I i )was taken as It 0, which is measured at the end of an optical fiber with cladding Optical fiber reactor setup A 300 W Xe arc lamp (Oriel Model 66083) was used as a UV light source. Light was filtered through a 10 cm IR water filter and focused onto the polished tip of optical fibers through a bi-convex lens (CVI Laser Optics, focal length 7.5 cm). The light intensity, when necessary, was varied by using a combination of neutral density filters. A preliminary optical fiber reactor with one or four fibers was made in order to assess the performance of OFR for photocatalytic gas treatment. A 30 cm stainless steel tubing (1/8 or 1/4 in. diameter) was used as a reactor body for one or four-fiber experiment. The total incident light intensity on the tip of a single fiber was maintained at mw (or W/cm 2 ). Since the diameter of the focused beam was larger than the fiber diameter, the total incident light intensity linearly increased with the number of fibers up to four. A schematic diagram of the experimental setup is shown in Fig. 1. A dry air flow from a cylinder was branched into three: one to an acetone saturator, the second to a water saturator, and the third for dilution. The flow rates of acetone and water line were controlled by mass flow controllers (Brooks) and that of the dilution air by a ball flow meter. No water vapor was added in typical photolysis experiments except when the water vapor effects were investigated. The three branch lines were merged at a mixing chamber, which was connected to an OFR inlet. The initial concentrations of acetone in air were varied between 50 and 750 ppmv by adjusting the mixing ratios or the temperature of the acetone saturator with cooling jacket. The residence time in the reactor was varied by changing the flow rate. Acetone concentrations in the inlet or outlet gas mixtures were monitored by using a gas chromatograph (HP GC 6890) equipped with a gas sampling loop, a flame ionization detector (FID) and a HP-5 column. The Fig. 1. Schematic experimental setup for the photocatalytic oxidation of acetone on TiO 2 -coated optical fibers.

4 212 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) samples were taken every 10 min throughout a photolysis experiment. Before irradiation, the air stream with a known concentration of acetone was allowed to flow through the reactor until the equilibrium dark adsorption of acetone on TiO 2 coating was established. Steady-states of acetone concentration in the gas flow, where the acetone concentrations entering and leaving the reactor were equal, were typically attained in 30 min. When the light irradiated the reactor, a desorption peak of acetone from the TiO 2 surface was observed, which was followed by a rapid decline of acetone GC signal due to photocatalytic oxidation. Under irradiation, steady-states of acetone concentration in the outlet mixture were reached within 30 min. The temperature in the reactor was maintained at C. Carbon dioxide formed as a result of acetone photooxidation was detected and analyzed with a FID and a Poraplot Q column after converting CO 2 to CH 4 through a Ni-catalyst methanizer (HP G2747A). 3. Results and discussion 3.1. Coated optical fiber characteristics SEM images of TiO 2 layer on a quartz optical fiber that was coated single, three, or nine times are compared in Fig. 2 (cross-sectional view). The coating thickness and coating mass linearly increased with the number of coats as shown in Fig. 3. The porosity of the film, a fraction of the void in the total volume of the coating, was estimated to be 0.62 ± 0.03 irrespective of the coating thickness. Light propagating in the core medium (quartz) of optical fiber with a refractive index, n 1 can be reflected in or refracted out at the interface with outer medium (TiO 2 ) having a refractive index, n 2 as illustrated in Fig. 4. The Snell s refraction law states whether the light is reflected or refracted: n 1 sin θ 1 = n 2 sin θ 2. The critical incidence angle (θ c ) which is a border line whether the light is refracted out or totally reflected in, is defined when θ 2 = 90 as sin θ c = n 2 /n 1. The incident light on the optical fiber has a range of incidence angles, θ min θ 1 90, where the minimal incidence angle (θ min ) is determined by the focal length of the lens. As the focal length increases, θ min increases [15]. In the present optical setup, θ min was estimated to be 77 ± 1. When n 1 >n 2 and θ 1 θ c (Fig. 4a), Fig. 2. Cross-sectional SEM images of optical fiber TiO 2 interface with (a) one coat; (b) three coats; (c) nine coats of TiO 2. the traveling light in the optical fiber will be totally reflected at the fiber wall. On the other hand, when either θ 1 <θ c or n 1 <n 2 (Fig. 4b), some degree of refraction at the interface will be always present for all incidence angles. While the refractive index of TiO 2 (n 2 = ) is greater than quartz (n 1 = )

5 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) Fig. 3. The average TiO 2 coating thickness ( ) and the mass ( ) as a function of the number of coats. for all wavelengths (i.e. n 1 <n 2 ) [20], the apparent refractive index of the porous TiO 2 coating, n 2-coat is not known. It is noted that a previous ellipsometric measurement of porous TiO 2 thin film on glass yielded the refractive indices of at 400 nm [21]. Simi- lar refractive index values are expected for the present TiO 2 coatings. As long as n 1 <n 2-coat, some fraction of light impinging at the interface is refracted out to the coated TiO 2 layer, which is subsequently excited to generate electron hole pairs. Substrate molecules present in the surrounding phase (liquid or gas) of the optical fiber diffuse onto the TiO 2 surface where photocatalytic degradation reactions are initiated. The efficiency of light transfer (or refraction) to the TiO 2 coating in the OFR could depend on various parameters such as incidence angles, wavelengths, refractive indices, the coating thickness, the porosity of the coating layer, and the fiber length [15,16]. The light transmission along a TiO 2 -coated fiber was investigated by measuring the refractive loss of the propagating light as a function of the fiber length (L) and the coating thickness (Fig. 5). A naked optical fiber showed no loss of light within 30 cm length. However, the TiO 2 -coated fiber exponentially extinguished the transmitting light (I t ) along the fiber, which can be described by the following equation: I t (L) = f θ I i exp( al) + (1 f θ )I i (1) where a is an apparent refractive loss coefficient and f θ a fraction of incident light with θ 1 < 90. The fraction (1 f θ )I i represents near parallel incident light (θ 1 90 ) that is not absorbed by TiO 2 coating within a short traveling distance. The light propagating length along the fiber can be defined as 1/a. Since the total refractive loss through an entire fiber is (I i I t ), Fig. 4. Cases for light refraction (I r ) and reflection (I t ) at the interface between the fiber core (refractive index, n 1 ) and the outer coating medium (refractive index, n 2 ). Fig. 5. The refractive loss of propagating light along an optical fiber with one, three, or five coats of TiO 2 as a function of the fiber length.

6 214 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) the fraction of the refracted light (1 I t /I i ) can be expressed as Fraction of Refracted Light over the Length (L). = f θ [1 exp( al)] (2) The data in Fig. 5 were fitted to Eq. (2) (solid line) to yield the a value of 0.22 ± 0.04 cm 1 and the f θ value of 0.89 ± For an incident light with θ 1 = 77, the axial distance between each reflection point is 0.43 cm. Since the light propagating length, 1/a = 4.5 cm, most of the incident light is absorbed by the TiO 2 layer within the first 10 reflections. With L = 30 cm, there are about 70 reflections along the fiber before the propagating light exits out of the fiber tip. This result implies that the apparent refractive index of the TiO 2 coating is greater than that of the quartz (n 1 <n 2-coat ). The effect of the coating thickness on the refractive loss was negligible as can be seen in Fig. 5. Since the refraction is determined by the interfacial property, the thickness-independent refractive loss suggests that the quartz TiO 2 interface is uniformly covered irrespective of the number of coats. The SEM images of Fig. 2 confirm this argument. The refracted light (I r ) is either absorbed by TiO 2 layer or transmitted into the air. However, how much of I r is absorbed by TiO 2 layer, which determines the photocatalytic efficiency, depends on the coating thickness as will be discussed in the later section. The apparent refractive loss coefficient of 0.22 cm 1 obtained in this study (θ min = 77 ) is comparable to the previously values of cm 1 that were reported from a TiO 2 -coated optical fiber (θ min = 71, 76, or 84 ) immersed in aqueous solution [15 17]. While the pore volume in the TiO 2 layer is filled with water (n = 1.35 at 361 nm) in aqueous photocatalytic system or with air (n = 1.0) in gas photocatalytic system, the refractive behaviors in two systems seem to be similar. Since the refractive indices of both water and air are smaller than that of quartz (i.e. in Fig. 4a), whether pores at the interface are filled with water or air does not significantly affect the refractive behavior Kinetics of photocatalytic conversion of acetone on coated fibers The photocatalytic degradation rate of acetone (R A, ppmv/min) on the coated fiber depends on the adsorbed concentration of acetone (C ad, mg/g TiO 2 ) and could be expressed by the Langmuir Hinshelwood kinetics [8,10,19] R A = k phc ad = k phk A P (3) C mono (1 + K A P) where C mono (mg/g TiO 2 ) and P (ppmv) are the saturated monolayer surface concentration and the gasphase concentration of acetone, respectively, K A (ppmv 1 ) the adsorption constant of acetone on TiO 2, k ph (ppmv/min) the photocatalytic rate constant, which is proportional to the rate of photon absorption, I abs (photons per min/g TiO 2 )bytio 2 up to a saturation level. In case, when K A P 1, the rate law is reduced to zero-order R A = k ph (4) or when K A P 1, the apparent kinetics becomes first-order R A = k ph K A P (5) Since the present optical fiber reactor can be characterized by a tubular plug-flow reactor, the mass balance equation can be written as ( ) dp R A = F (6) dv where F is the volumetric flow rate (cm 3 /min) and V the reactor volume (cm 3 ) [22]. By combining Eq. (6) with Eqs. (4) and (5), the following integral rate expression can be derived: for the zero-order limit P 0 P = k ph t R (7) Conversion = 1 P/P 0 = k pht R (8) P 0 for the first-order limit ( ) P ln = k ph t R (9) P 0 Conversion = 1 P/P 0 = 1 exp( k ph t R ) (10) where t R (=V/F) is the residence time for which an average molecule remains in the reactor. The photocatalytic degradation of acetone on TiO 2 -coated fiber was performed using one or four fibers. When using a single-fiber reactor, a

7 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) Effects of light intensity, coating thickness, and fiber length The photocatalytic rate constant, k ph depends on the illuminated light intensity (I) through the following relation [6,23]: k ph (I) = C 0 I n (11) Fig. 6. The photocatalytic acetone conversion with a four-fiber reactor as a function of (a) the acetone concentration and (b) the residence time. The residence time in (a) was 2.5 min and the acetone concentration in (b) was 500 ppmv. steady-state conversion of about 9% was obtained under the condition of residence time, t R = 2 min and P 0 = 150 ppmv. Typical photolysis experiments were carried out using a four-fiber reactor. The photocatalytic acetone conversion with the four-fiber reactor was investigated as a function of the acetone concentration (Fig. 6a) and the residence time (Fig. 6b). In the concentration range ( ppmv) investigated, the photocatalytic kinetic behavior of acetone conversion was of the zero-order. The solid lines in Fig. 6a and b were fitted to Eq. (8) to give the k ph values of 17 and 26 ppmv/min, respectively. The fact that the conversion rates were independent of the flow rate (i.e. the residence time) in Fig. 6b implies that the photocatalytic degradation rate on the fiber surface was kinetically limited, not mass-transfer limited. where C 0 is a proportionality constant that comprises elementary rate constants of charge-pair recombination and interfacial charge-transfer [23]. It has been frequently reported that n values range between 0.5 and 1.0 in various kinds of photocatalytic reactions: n = 1 at low light intensity limit and n = 0.5 at high light intensity limit [24,25]. The light intensity dependence in OFR, which is presented in Fig. 7, shows that the conversion increases linearly with I (i.e. n = 1.0) under the present experimental condition. The linearity implies that the light intensity incident on the coated-fiber surface does not reach the saturation level. A consistent conversion could be repeated with cycles of light on and off without showing any sign of deactivation within a few hours operation (Fig. 8). While the photocatalyst surface is evenly illuminated in most immobilized photocatalytic reactors, the light intensity on the photocatalyst in OFR strongly depends on the axial position as shown in Fig. 5. Therefore, k ph in OFR is a function of both the light intensity (I) and the axial position (L) and the apparent k ph reported in this work is an integrated value Fig. 7. Acetone conversion as a function of the incident light intensity in a four-fiber reactor. The light intensity is the sum of the total power (UV VIS light) incident on the four fiber tips. The experimental conditions were [acetone] 0 = 500 ppmv; t R = 5 min.

8 216 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) Fig. 8. Acetone conversion with cycles of light ( ) and dark ( ) periods in a four-fiber reactor. The experimental conditions were [acetone] 0 = 500 ppmv; t R = 3.5 min. over the entire length of the coated fiber. If all the refracted light is absorbed by TiO 2, the total absorbed light over the length, L can be derived from Eq. (2) I abs (L) = f θ I i (1 e al ) (12) By differentiating Eq. (12), we get the absorbed light, di abs in a coated-fiber segment, dl di abs = af θ I i e al dl (13) From Eq. (11) dk ph (I) = nc 0 I n 1 abs di abs (14) Combining Eqs. (13) (14) yields a differential rate constant, dk ph that is contributed by a coated-fiber segment, dl dk ph (I, L) = C 1 Ii n e al (1 e al ) n 1 dl (15) where C 1 = naf n θ C 0. The subsequent integration over the whole fiber length at a constant light intensity, I i gives the integrated rate constant k ph (L) = C 1 I n i L 0 e al (1 e al ) n 1 dl = C 1 an I i n (1 e al ) n (16) By setting n = 1.0, we get k ph (L) = C 1 a I i(1 e al ) (17) The conversion was measured as a function of the fiber length, L and is presented in Fig. 9. The solid Fig. 9. The effect of the coated-fiber length on the photocatalytic acetone conversion. The experimental conditions were [acetone] 0 = 500 ppmv, t R = 4 min in a four-fiber reactor with three coats of TiO 2. The solid line is a fit to Eq. (17) with a = 0.22 cm 1. line is a fit to Eq. (17) with a = 0.22 cm 1 which was determined from the light absorption as a function of L in Fig. 5. The fact that the fiber-length dependent conversion does not quantitatively match Eq. (12) (I abs as a function of L) implies that the conversion is not simply proportional to I abs. Although the integrated rate constant, k ph at L = 30 cm showed the linear light intensity dependence of n = 1.0 (Fig. 7), it could be n<1.0 at shorter lengths where the light absorbed per unit length, I abs / L (Eq. (13)) is much higher. When the absorbed light is saturated with n<1.0, the conversion efficiency could be reduced as shown in Fig. 9. The coated optical fibers have a unique geometry in that the direction of incident light and the diffusion of reactants onto TiO 2 layer are opposite unlike other immobilized photocatalytic reactors: the light comes up from the bottom and the reactant diffuses in from the top. The profiles of the penetrated light intensity in the TiO 2 layer are schematically illustrated in Fig. 10. The refracted light intensity (I r )is exponentially extinguished along the depth (l) I r = I i exp( αl) (18) where α is the apparent absorption coefficient of TiO 2 layer whose value depends on the porosity of the coating layer. While the penetration depth, 1/α at λ = 348 nm is about 0.1 mm for polycrystalline TiO 2 [26], the present film with a porosity factor of 0.6 could have

9 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) Fig. 10. Schematic illustration that shows the relation between the TiO 2 coating thickness and the refracted light penetration in the coating layer. Two different cases of the coating thickness where it is comparable to the penetration depth (a) or it is much larger than the penetration depth (b) are compared. much larger penetration depth. The penetration depth was estimated to be mforatio 2 -coated fiber in aqueous solution [15]. As a result, the photocatalytic conversion measured as a function of the coating thickness (Fig. 11) rapidly increased up to 1 m because the thicker layer absorbed more refracted light in this region. However, further increase in thickness resulted in decreasing the photoconversion. As illustrated in Fig. 10, a coating layer much thicker than the light penetration depth retards the diffusion of reactants into the bottom layer where most of refracted light is absorbed. An optimal coating thickness is compromised where both the sufficient light absorption and the rapid reactant diffusion into the illuminated layer are satisfied Photocatalytic degradation reaction of acetone on coated fibers Since the photocatalytic reaction proceeds on the surface, the adsorption of substrates is a prerequisite. The adsorption behavior of acetone on TiO 2 could be Fig. 11. The effect of TiO 2 coating thickness (or the number of coats) on the photocatalytic acetone conversion. The experimental conditions were [acetone] 0 = 50 ppmv, t R = 2 min in a single-fiber reactor of 30 cm length. successfully fitted to a simple Langmuir model in a recent study [27]. According to their measurements, the gas-phase concentrations of ppmv acetone,

10 218 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) which is a concentration range employed in this study, corresponds to about monolayer of adsorbed acetone. In this region of surface coverage, a significant fraction of the adsorbed acetone exists as a dimerized product, mesityl oxide [27,28] 2(CH 3 ) 2 CO (CH 3 ) 2 C=CHCOCH 3 + H 2 O (19) At 0.2 or 0.8 monolayer of coverage, about 30 or 60% of the adsorbed acetone was present as mesityl oxide whose photooxidation rate was slower than acetone. Since both acetone and mesityl oxide are present on coated fibers under the present experimental condition, their relative abundance on TiO 2 seems to affect the overall photocatalytic conversion of acetone as in Eq. (20) k ph = k acetone [acetone] s + k mesityl [mesityl oxide] s (20) At higher coverages where the majority of acetone exists as mesityl oxide, the total conversion rate could be significantly lower than the rate when the same amount of acetone would exist as a monomer. Although the zero-order kinetic characteristics was observed in this study, the surface coverage of acetone in the concentration range of ppmv was well below the saturation level: that is, the condition of K A P 1 for the zero-order kinetics was not met. The apparent zero-order kinetics might be ascribed to the fact that the ratio of [mesityl oxide] s to [acetone] s on TiO 2 increases with the coverage or the reaction time. Accordingly, in a recent study, photo-oxidation kinetics of acetone on TiO 2 exhibited a mixed kinetic behavior [27]. While acetone oxidation showed a fast exponential decay in the initial stage, a slow and near linear decay was followed at longer irradiation times. Further studies are needed to quantify k acetone and k mesityl for better understanding of the photocatalytic oxidation of acetone. The reaction of acetone conversion into CO 2 and H 2 O can be written as CH 3 COCH 3 + 4O 2 3CO 2 + 3H 2 O (21) In a separate experiment to confirm the stoichiometry, we observed that acetone degraded was quantitatively converted to CO 2 with keeping the molar ratio of 1:3 and no gaseous intermediates detected by FID. Fig. 12. Acetone conversion as a function of water vapor ( ) and O 2 ( ) concentrations. The experimental conditions were [acetone] 0 = 500 ppmv, t R = 4 min in a four-fiber reactor. The water vapor dependence experiments were carried out in 20% O 2 and the O 2 dependence experiments in no added water vapor. Water and oxygen molecules are produced or consumed in the photocatalytic reaction and their presence was known to significantly affect the overall reaction [6,10,19,27]. The effects of water vapor and oxygen concentration on the photocatalytic conversion of acetone were investigated in OFR (Fig. 12). The humidity could enhance or reduce the photocatalytic reaction rates depending on the reactant and the H 2 O concentration range [4 7]. The water vapor in this study negatively affected the photocatalytic conversion of acetone. Similar effect of water vapor on acetone degradation has been reported [6,19]. In general, the presence of water vapor in photocatalytic reactions is considered to be essential because water molecules react with valence band holes to produce reactive hydroxyl radicals. Indeed, a small amount of preadsorbed water was found to be essential in photocatalytic degradation of acetone [27]. Although no water was added in this study except for when the water vapor effect was investigated, water molecules generated in situ from reactions (19) and (21) seem to be a sufficient source of OH radical in this case. Accordingly, the heat treatment of the coated-fiber at about 200 C before introducing acetone did not affect the photoconversion efficiency. An alternative explanation is that acetone molecules are oxidized through a direct

11 W. Choi et al. / Applied Catalysis B: Environmental 31 (2001) hole transfer pathway that does not involve OH radical mediators [27]. It is reasonable to accept that both the hydroxyl radical and the direct hole pathways are operating in the photocatalytic conversion of acetone. In either case, excess water molecules compete for active surface sites with acetone molecules only to reduce the photoconversion efficiency. The role of molecular O 2 in the photocatalytic gas-treatment is three-fold: one as a conduction band electron scavenger to reduce the fast electron hole recombination, the second as a precursor of active oxygen species (e.g. HO 2, HO, O) that initiate decomposition reactions, and the third as a replenishing source of the lattice oxygen that could be depleted during the photocatalytic oxidation. Therefore, increasing oxygen content up to 15% enhanced the conversion (Fig. 12). Further increase was inefficient with showing a sign of saturation. It should be noted that a conversion of 5% was observed even in the absence of O 2, which implied that the lattice oxygen took part in the photocatalytic oxidation of acetone [18,27,29,30]. 4. Conclusions Photocatalytic oxidation of acetone in air ( ppmv) was carried out using optical fibers as both a light transmitter and a catalyst support. Acetone conversion was achieved using one or four TiO 2 -coated fibers. Acetone molecules were quantitatively converted to CO 2 with no intermediates detected. The conversion showed a zero-order kinetic behavior under the present experimental conditions. The propagating light in the fiber is refracted out at the interface to excite the TiO 2 coating where the photocatalytic reactions are initiated. Since the intensity of the propagating light was exponentially extinguished along the coated fiber, most of incident light was absorbed in the front part of the fiber. A typical light propagating length along the fiber was <5 cm. The photoconversion was directly proportional to the incident light intensity. No saturating light intensity effect nor catalyst deactivation were observed in this study. The thickness of TiO 2 coating was optimal between 1 and 2 m and further increase in thickness reduced the conversion. The basic data obtained in this work with a preliminary optical fiber reactor show a multi-fiber reactor could be successfully applied to photocatalytic air treatment. However, the short light propagating length, which significantly limits the efficient use of fibers, should be overcome. The OFR could be a viable option where the direct light illumination is inhibited or the light source is remote from the contaminated site. For example, the development of indoor air treatment system using a solar optical fiber reactor installed on the roof could be suggested. Acknowledgements This work was supported by POSCA and Korea Research Foundation Grant (KRF E00109). References [1] D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, [2] J. Peral, X. Domenech, D.F. Ollis, J. Chem. Technol. Biotechnol. 70 (1997) 117. [3] G.B. Raupp, T.C. Junio, Appl. Surf. Sci. 72 (1993) 321. [4] W.A. Jacoby, D.M. Blake, R.D. Noble, C.A. Koval, J. Catal. 157 (1995) 87. [5] L.A. Dibble, G.B. Raupp, Catal. Lett. 4 (1990) 345. [6] J. Peral, D.F. Ollis, J. Catal. 136 (1992) 554. [7] L.A. Dibble, G.B. Raupp, Environ. Sci. Technol. 26 (1992) 492. [8] R.M. Alberici, W.F. Jardim, Appl. Catal. B: Environ. 14 (1997) 55. [9] M.C. Canela, R.M. Alberici, R.C.R. Sofia, M.N. Eberlin, W.F. Jardim, Environ. Sci. Technol. 33 (1999) [10] M.L. Sauer, D.F. Ollis, J. Catal. 149 (1994) 81. [11] R.E. Marinangeli, D.F. Ollis, AIChE J. 23 (1977) 415. [12] R.E. Marinangeli, D.F. Ollis, AIChE J. 26 (1980) [13] R.E. Marinangeli, D.F. Ollis, AIChE J. 28 (1982) 945. [14] K. Hofstadler, R. Bauer, S. Novalic, G. Heisler, Environ. Sci. Technol. 28 (1994) 670. [15] N.J. Peill, M.R. Hoffmann, Environ. Sci. Technol. 29 (1995) [16] N.J. Peill, M.R. Hoffmann, Environ. Sci. Technol. 30 (1996) [17] N.J. Peill, M.R. Hoffmann, Environ. Sci. Technol. 32 (1998) 398. [18] S.A. Larson, J.A. Widegren, J.L. Falconer, J. Catal. 157 (1995) 611. [19] A.V. Vorontsov, E.N. Kurkin, E.N. Savinov, J. Catal. 186 (1999) 318. [20] D.R. Lide (Ed.), Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, [21] A. Alvarez-Herrero, A.J. Fort, H. Guerrero, E. Bernabeu, Thin Solid Films 349 (1999) 212. [22] H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall, Englewood Cliffs, NJ, 1999 (Chapter 1).

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