Environ. Sci. Technol. 1997, 31, 2034-2038
Effects of Moisture and Temperature on the Photooxidation of Ethylene on Titania TIMOTHY N. OBEE* AND STEVE O. HAY United Technologies Research Center, Silver Lane, East Hartford, Connecticut 06108
The effect of temperature, water vapor concentration, and ethylene concentration on the photooxidation of ethylene on titania was investigated. Ultraviolet radiation from a black-light lamp together with a glass-plate reactor were used to develop intrinsic oxidation rates. Ethylene oxidation rates decreased significantly as the water vapor concentration was increased from 1000 ppmv for the three temperatures (2, 27, and 48 °C) investigated. The influence of water vapor on the reaction rate derived from the low adsorption of ethylene due to its low adsorption affinity relative to water. Over the range of water vapor concentration of 1000-25000 ppmv, ethylene oxidation rates increased as the temperature increased. An Arrhenius plot of the measured ethylene oxidation rates indicated an apparent activation energy of 3.4 kcal/mol. A LangmuirHinshelwood expression displaying an explicit temperature dependence was used to correlate the entire set of rate data. Based on this correlation, an enthalpy of adsorption for ethylene of -2.6 kcal/mol was found.
Introduction The purification of air in residential and commercial buildings, commercial aircraft, and transportation vehicles is primarily concerned with gaseous, particulate, and microbial contaminants (1, 2). Ultraviolet (UV)-irradiated titania is wellknown to oxidize many of the volatile organic compounds (VOC) found in these enclosures to benign products (3-6). The bulk of published rate data have focused on contaminant concentration dependencies and have seldom addressed the influence of water vapor and catalyst temperature. The successful adaptation of the titania photocatalytic process for air purification requires first the development of a basic understanding of the interaction of the contaminant concentration, water vapor concentration, and catalyst temperature. This study undertakes to provide the basis for this understanding with ethylene. Titania-promoted photooxidation of trace levels of gaseous contaminants has been reported in a few studies (5-10). These studies have shown that the oxidation process is strongly influenced by contaminant (probe reactant) concentration (5-7, 10), water vapor concentration (5-7, 10), and temperature (8, 9, 11). A systematic study of the interaction of contaminant (probe reactant) concentration, water vapor concentration, and photocatalyst temperature has not been reported. Competitive adsorption between water vapor and a probe contaminant can have a significant influence on the oxidation rate of the contaminant (5-7). This consequence of competitive adsorption arises when the oxidation rate is rate* Corresponding author phone: 860-610-7161; fax: 860-610-7857; e-mail:
[email protected].
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limited by the low adsorption of the contaminant due to its low adsorption affinity relative to water (5-7). Water adsorbs on a titania surface in a variety of ways (12), including both physical and chemical adsorption. It can be reasonably assumed that it is the physisorbed water that plays an important role in competitive adsorption. The chemically adsorbed water either removes surface sites from contention for adsorbed ethylene or creates sites for competitive physical adsorption between water and ethylene. Water strongly physisorbs on the hydroxylated titania surface via hydrogen bonding to surface hydroxyl groups (12, 13). The oxidation of ethylene, which is nonpolar and would therefore weakly physisorb on the polar titania surface, should exhibit a strong dependence on water vapor. Ethylene offers a measurable oxidation rate, is safe to handle, is the primary contaminant in the storage and transport of horticultural commodities (2), and is the parent compound of important environmental contaminants (e.g., trichloroethylene). For these reasons, ethylene was selected as the probe reactant. The isothermal rate data generated from the photooxidation on titania has, in several studies (5-7), been shown to correlate with a Langmuir-Hinshelwood (L-H) rate expression. The temperature dependence is usually displayed in an Arrhenius plot, in which a rise, plateau, and fall regions have been observed (4, 8, 9, 14). To include an explicit temperature dependence in a L-H expression, the enthalpy of adsorption of all important species would be required. For the photooxidation of ethylene, two important species are ethylene and water. The enthalpy of adsorption of water on titania is known (13), but it is not known for ethylene. Fu et al. (11) investigated the effects of water (at concentrations of 5 and 15000 ppmv) and temperature (30-110 °C) on the oxidation kinetics of ethylene (at 250 ppmv) on titania (sol-gel derived). The carbon balance indicated complete mineralization to carbon dioxide. An apparent energy of activation of 3.3-3.8 kcal/mol was reported. A cylindrical packed-bed reactor irradiated externally and axially by four black-light-blue UV lamps was used. The reactor was heated solely by the UV lamps and control of the temperature was effected by varying the flow rate of the coolant (air) that passed over the reactor and UV lamps. A possible shortcoming of this study was that the effect of the temperature of the coolant gas on the output of the UV lamps was not mentioned and may not have been taken into account. This effect can be considerable (15). The purpose of this study was to probe the relative reaction rates for the disappearance of ethylene as a function of ethylene concentration, water vapor concentration, and temperature over the range of these variables of interest in the designated air-purification applications. The water vapor concentration was varied steadily from 1000 to 25000 ppmv or near saturation for three temperature levels (2, 27, and 48 °C). Intrinsic oxidation rates were determined for the disappearance of ethylene for initial ethylene concentrations ranging between 6 and 300 ppmv.
Experimental Section Apparatus. A glass-plate photocatalytic reactor was used to generate oxidation rate data (Figure 1). Ultraviolet (UV) illumination was provided by two black-light lamps (352-nm peak intensity, SpectroLine XX-15A). The emission spectrum of the UV lamps were supplied by the lamp manufacturer and independently determined with a spectrometer. The intensity was selected by adjusting the distance between the lamp and the titania-coated glass-plate. UV intensity at the reactor surface was measured by a UVA power meter (Oriel UVA Goldilux). High-purity nitrogen gas passed through a
S0013-936X(96)00827-9 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Glass-plate photocatalytic reactor. water bubbler to set the desired water vapor level. Ethylene was generated from a compressed gas cylinder. An oxygen gas flow was combined with the nitrogen and ethylene flows to produce the desired carrier gas mixture (15% oxygen, 85% nitrogen). Thermocouples were used to measure the temperature of the inlet and exit gas streams. The concentrations of water vapor, carbon dioxide, carbon monoxide, and ethylene were measured using a photoacoustic detector (Bru ¨el & Kjær 1302). To adjust and maintain the temperature of the reactor, the reactor was immersed into a temperature-controlled water bath. The ethylene-contaminated gas stream was brought to the temperature of the water bath prior to entering the reactor by passing through a serpentine copper tubular heat exchanger immersed in the water bath. The titania-coated glass-plates were placed in a well (25 mm by 46 cm) milled from an aluminum block and covered by a quartz window (96% UVA transparent). Gaskets between the quartz window and aluminum block created a flow passage of 25 mm (width) by 2 mm (height) above the titaniacoated glass-plates. An opaque film of Degussa P-25 titania was deposited on flat 25 mm wide microscope glass slides using a wash-coat process. The wash-coat was 5% by weight of titania in distilled water. A titania film was prepared by dipping the glass slide in the wash-coat several times, air-dried between dipping, and then oven-dried at 70 °C. This process was repeated until a 0.74 mg/cm2 film (per side) was achieved. Ethylene-contaminated gas entered the reactor by first passing through a bed of glass mixing beads. Next, the gas flow entered a 25 mm by 2 mm entrance region of sufficient length (76 mm) to produce a fully developed laminar velocity profile. The gas flow then passed over the surface of the titania-coated glass-plates. Finally, the gas passed through a 25 mm by 2 mm exit region (76 mm long) and the second bed of glass beads before exiting the reactor.
Results and Discussion For the data generated by the glass-plate reactor, the oxidation rate is defined as
r ) 2.45(Xin - Xout)Q/A
(1)
where r (µ-mol/cm2-h) is the oxidation rate; Xin (ppmv) and Xout (ppmv) are the inlet and outlet ethylene concentrations, respectively; Q (L/min) is the volumetric flow rate; and A
(cm2) is the area of the titania-coated glass-plate. The numerical coefficient accounts for the units change. All temperature and water vapor concentration dependence tests were performed at a 4 L/min flow rate. To check for gas-side mass transfer influence, the flow was doubled to 8 L/min. The subsequent oxidation rate remained unchanged (within an uncertainty of 5%), indicating that the oxidation rates obtained at the 4 L/min flow were kinetically controlled (16). The selection of the desired UV power flux was described in the Experimental Section. The UV power flux on the titania surface was measured with the UVA meter at a fixed distance from the lamps. This distance corresponded to the position of the titania-coated glass-plates. The measured flux was then corrected for the spectral response of the detector, the spectral output of the lamps, and the transmittance of the quartz window on the reactor. The corrected flux represented the true UV power flux at the catalyst surface. The spectral response of the detector and the spectral output of the lamps were provided by the manufacturers and, in the case of the UV lamp, was independently confirmed with a spectrometer. The transmittance of the quartz window was constant over the wavelength region of interest. A resultant true UV power flux of 5.6 mW/cm2 was thus found and was used in all experiments. A check of the UV power flux, which was made at the start and end of the tests series using the method just described, verified the constancy of the UV output. Preliminary Reactivity Measurements. Under UV illumination, no change in ethylene level or evolution of carbon dioxide or carbon monoxide was observed with uncoated reactor elements. Ethylene is not expected to photodissociate under UVA (17). In the dark, no change in ethylene level was observed with titania-coated reactor elements. In a study of film loading by Jacoby (10), the oxidation rate of trichloroethylene increased with film loading up to a P-25 titania loading of 0.5 mg/cm2 and remained constant for all higher loadings. This finding suggests that the oxidation rate maximizes at a film loading of 0.5 mg/cm2 and that additional film loading adds nothing to the oxidation rate. This conclusion should not depend on the specific contaminant used (trichloroethylene) and, hence, can be applied in the present study. The titania film in the present study was determined to be opaque to UVA by placing a coated plate between the UV black-light lamps and the UVA power meter. This finding coupled with the conclusion drawn from Jacoby’s (10) finding suggests that the UV radiation is being maximally utilized in the oxidation process. The oxygen was maintained at a constant level of 15% by volume for all rate measurements. Jacoby (10) and Dibble and Raupp (7) have found that for the oxidation of trichloroethylene the rate was zero-order for oxygen levels above 1%. Since the experiments performed in the present study used contaminant levels that were comparable or lower than the levels used by Jacoby (10) and Dibble and Raupp (7), it is likely that the ethylene oxidation rates herein were independent of oxygen level for the oxygen levels above 1%. In general, a carbon balance was found in all experiments with carbon dioxide accounting for 100% (with a 5% uncertainty) of the carbon generated from the ethylene oxidation. Carbon monoxide, which was included in the analysis, was below the detection limit (0.15 ppmv), and thus, none was observed. These findings implied that no significant other reaction intermediates were desorbed to the gas stream. Ethylene Oxidation Rates. In all experiments, the following procedure was followed: First, the gas flows and water vapor levels were set. After the reactor inlet/outlet water vapor levels reached equilibrium, the ethylene was introduced. When the inlet/outlet ethylene concentrations reached steady state and were of equal magnitude, the UV lamp was illuminated. All reported measurements were taken after the concentrations of all effluent species reached steady state.
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FIGURE 2. Stability of ethylene photooxidation: 5.6 mW/cm2 UV; 13.7 ppmv inlet ethylene; 700 ppmv humidity; 4 L/min flow rate; 27 °C reactor temperature. Events: 1, UV on; 2, UV off. An example of the stability of the oxidation process is shown in Figure 2. Event 1 marked the moment the UV lamp was illuminated. The outlet ethylene level rapidly decreased to a sustained constant level. Concomitant with decreased ethylene concentration was the appearance of CO2. Twice the difference between the inlet and outlet ethylene concentration was equal to the concentration of CO2, indicating a carbon balance with CO2. After the UV lamp was switched off (event 2), the ethylene signal level rapidly rose to its initial dark concentration, while the CO2 signal disappeared. The disappearance of ethylene between events 1 and 2 is interpreted as a photooxidation process. The relative fractional change in the ethylene concentration through the reactor was about 0.1 for all except three of the 36 reported oxidation rate determinations. For these three instances, the relative fractional change in the ethylene concentration was about 0.15. Thus, the reactor was operated in a differential mode. As exemplified in Figure 2, the oxidation rate was determined only after the effluent ethylene concentration reached a constant and sustained level under constant UV irradiation. A single titania film loading was used for all oxidation rate determinations. The determination of the oxidation rate dependencies on ethylene concentration, water vapor concentration, and reactor temperature involved many separate experiments. As a check for possible deactivation during the generation of the entire set of oxidation rate determinations, previous experiments including the initial experiments were repeated throughout the time frame required for generation of the entire data set. In each case, within experimental error, the repeated experiment resulted in same oxidation rate previously found. Thus, deactivation of the titania coating was not observed, and the oxidation rate determinations were repeatable. The influence of water vapor concentration on the oxidation rate of ethylene is shown in Figure 3a for three temperatures. An initial region of rapid decrease is followed by a slower, steady decrease in the oxidation rate. The oxidation rate always increased as the temperature was increased. The oxidation rates, for the various temperatures examined, appeared to merge as the water vapor concentration was decreased. The photocatalytic activity of titania for the gas-phase oxidation of trace contaminants has been reported elsewhere (5, 6, 18). From these studies, increasing water vapor concentration was found to initially enhance and then to degrade the oxidation rate. This latter behavior was explained as due to the relative adsorption affinity of water and the contaminant. Physisorbed water is hydrogen bonded to titania with an adsorption energy of 12.2 kcal/mol (13). Unlike water, ethylene is nonpolar and is likely adsorbed through
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FIGURE 3. Ethylene photooxidation dependence, 5.6 mW/cm2 UV lamp, 4 L/min flow rate: (a) influence of humidity level, 15 ppmv ethylene; (b) influence of ethylene concentration, 1500 ppmv humidity. the weaker dipole-induced dipole interaction. In the present study, the reaction rate is probably rate limited by low adsorption of ethylene due to its low adsorption affinity relative to water. In general, adsorption affinity decreases with temperature rise. As the temperature is increased, the affinity of water, having a higher adsorption energy, is decreased to a greater degree than ethylene. The resultant desorbed water also frees up additional oxidation sites, allowing more ethylene to be adsorbed, and hence resulting in the increased oxidation rate. This behavior was also observed by Fu et al. (11) in a study of the oxidation of ethylene on titania. The same argument as given above was used by them to explain the influence of temperature (30-110 °C) and water vapor concentration (at two levelssless than 5 and 15000 ppmv) on the oxidation of ethylene at 250 ppmv. Further comparison with their data is not possible since UV level, irradiated titania surface area, and mass of titania (used to normalize their oxidation rates) were not reported. The influence of temperature and ethylene concentration on the kinetic oxidation rate of ethylene is shown in Figure 3b. This data set was generated at a low water vapor level (1500 ppmv) in order to provide adequate measurement sensitivity. Here, again, elevated temperatures increased the oxidation rate. For a constant ethylene concentration, oxidation rates at the three temperatures from Figure 3b were placed on an Arrhenius plot (Figure 4a). Least-squares fit at two such ethylene levels are shown (Figure 4a). The slope provides a measure of the apparent activation energy of ethylene on titania. The data in Figure 3b allows the generation of a triplet of data points for six distinct ethylene levels and therefore, six determinations of the apparent activation energy. The results are shown in Figure 4b. The mean apparent activation energy is 3.4 kcal/mol. An apparent energy of activation of 3.3 (at a low water vapor concentrationsless than 5 ppmv) and 3.8 kcal/mol (at a water vapor concentration of 15000 ppmv) was reported for the oxidation of ethylene (250 ppmv) by ref 11. In general, the apparent energy of activation may
TABLE 1. Langmuir-Hinshelwood Prameters for eq 2 temp (°C) parameter
units
2
27
48
ko KE KW
µ-mol/cm2-h
0.41 0.077 0.00330
0.63 0.045 0.00045
0.75 0.035 0.00015
ppmv-1 ppmv-1
TABLE 2. Parameters for eq 3 parameter
units
(1)a
(2)b
ko′ KE′ KW′ E ∆HE ∆HW
µ-mol/cm2-h K1/2/ppmv K1/2/ppmv kcal/mol kcal/mol kcal/mol
10.4 0.0098 1.04 × 10-11 1.67 -2.61 -12.2
10.9 0.0089 2.3510-12 1.70 -2.67 -13.0
a
(1) ∆HW constrained to -12.2 kcal/mol.
b
(2) ∆HW unconstrained.
rate constant, ko, followed an Arrhenius temperature dependence. With these assumptions, a temperature dependent rate is given by
exp(-∆HE/RT) XE xT r) exp(-∆HE/RT) exp(-∆HW/RT) 1 + KE′ XE + KW′ XW xT xT (3)
(
FIGURE 4. Apparent activation energy of ethylene on titania, 1500 ppmv humidity: (a) Arrhenius plot; (b) influence of ethylene concentration. be composed of contributions from surface reactions and from the enthalpy of adsorption by adsorbing species (19). These contributions may explain the dependency shown in Figure 4b and observed by ref 11 with a variation of water vapor level. The L-H model (eq 3) also followed the same trend exhibited by the data in Figure 4b. Kinetic Equation. To correlate the ethylene rate data, a bimolecular form of the Langmuir-Hinshelwood (L-H) rate equation (19) was used. A simplifying assumption was made that the possible reaction products, carbon dioxide or carbon monoxide, did not influence the observed oxidation rates and that only ethylene and water vapor were important. The influence of any reaction intermediates was also neglected. With these assumptions, the L-H rate is given by
r ) ko
KEXE (1 + KEXE + KWXW)
(2)
where ko (µ-mol/cm2-h) is the rate constant for a given UV intensity (I ), KE and KW (ppmv-1) are the Langmuir adsorption equilibrium constants, and XE (ppmv) and XW (ppmv) are the gas-phase concentrations of ethylene and water vapor, respectively. The ratio on the right-hand side represents competitive adsorption between ethylene and water for the same adsorption site (20). Since the rate expression (eq 2) is nonlinear, least-squares optimization was used to determine values for the constants. The results of the optimization are given in Table 1. The rates from eq 2 are in good agreement with the measured data as shown in Figure 3. To include an explicit temperature dependence in the rate equation, the following assumptions were made: (1) the temperature-dependent form of the Langmuir adsorption constants (21) for monolayer adsorption on a homogeneous surface can be applied to both ethylene and water; (2) the
ko′ exp(-E/RT)KE′
)
where ko′ (µ-mol/cm2-h) is the (temperature-independent) rate constant for a given UV intensity, KE′ and KW′ (K1/2 ppmv-1) are the (temperature-independent) Langmuir adsorption equilibrium constants, E (kcal/mol) is an apparent activation energy, ∆HE and ∆HW (kcal/mol) are the change in enthalpy accompanying adsorption for ethylene and water, respectively, T (K) is the titania temperature, and R (1.99 × 10-3 kcal mol-1 K-1) is the gas constant. The enthalpy of adsorption for physisorbed water (∆HE) on titania is known, -12.2 kcal/mol (13). A least-squares optimization was used to determine the values for the unknown constants in eq 3 for two cases: (1) ∆HW was held constant at -12.2 kcal/mol, and (2) ∆HW was not held constant, but was determined through the optimization process. The results of the optimization are given in Table 2 for these two cases. The rates from eq 3 for these two cases did not differ significantly from eq 2 and are in good agreement with the measured data as shown in Figure 3. A value of -2.6 kcal/mol for the enthalpy of adsorption of ethylene on titania (∆HE) is very reasonable. One would expect that the nonpolar ethylene molecule would be only weakly physisorbed to the polar titania surface, through a weak dipole-induced dipole interaction. The delineation point between a slow collision of a gas phase species with a surface and physical adsorption is approximately 1.5 kcal/ mol (22). Since the strongest interaction between the titania surface is likely to be that between the polar OH surface sites and an induced dipole in the π-electron cloud of the carbon carbon double bond in ethylene, only the weakest of the interactions is to be expected. For a photocatalytic reaction, the activation energy is expected to be near zero (23, 24), and only the enthalpy of adsorption for each important species would appear in the rate equation. When the apparent activation energy, E, was constrained to a value of zero, a satisfactory fit of eq 3 to the measured data (Figure 3) was not found. A satisfactory fit was only found when a non-zero value for E was allowed and determined through the optimization process.
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surface that the ethylene species. Since the photooxidation of ethylene is assumed to be a surface reaction, the reaction kinetics at low ethylene concentrations are dominated by competitive adsorption between water and ethylene molecules. In truth, other species also co-adsorb on the reactive titania surface, i.e., oxygen, carbon dioxide, and nitrogen adsorb to a lesser amount, occupying sites. Our analysis neglects these species, even though these species are undeniably key contributors to the reaction scheme. The excellent fit of the L-H kinetics model to the observed data fuels our belief that no significant errors accrue from this neglect. In truth, it may be the participation of these species and the influences of byproducts and products derived from their presence that drive observed deviations from L-H behavior. Rate extrapolations based on assumed L-H behavior, outside of the range of measurements, should be done with caution and with the knowledge that significant deviations from L-H behavior may arise.
Literature Cited FIGURE 5. Predicted oxidation rate dependence on temperature, ethylene, and humidity concentrations (eq 3). To illustrate its usefulness, eq 3 was used to identify ranges of three design parameters (temperature, water vapor concentrations, and ethylene concentrations) that would lead to enhanced ethylene oxidation rates. The results are shown in Figure 5. A key feature was the appearance of a plateau of zero-order temperature dependence. At temperatures below this plateau region, the rate always increased with increased temperature or decreased water vapor concentration. In general, the temperature location of the plateau region appeared to increase with an increase in either the water vapor or ethylene concentrations. As evident from Figure 5, for a photocatalytic device operating at a maximum allowed ethylene concentration of 1 ppmv, the maximum oxidation rate occurs at about 75 °C for a constant 10000 ppmv water vapor concentration and at about 35 °C for a constant 1000 ppmv water vapor concentration. Oxidation rates increased with elevated ethylene concentrations and were less sensitive to changes in either water vapor concentration or temperature at the highest ethylene levels. We have measured the rate of ethylene photooxidation under UVA illumination, as a function of temperature, water vapor concentration, and ethylene concentration. Analyzing the data, assuming that the well known L-H expression for a two-component system is applicable, allowed us to calculate a value of -2.6 kcal/mol for the enthalpy of adsorption of ethylene on titania. This value differs from the range -3.2 to -3.8 kcal/mol reported by Fu et al. (11). While, in the world of catalysis, our value is in good agreement with that reported by Fu, we believe it is a mistake to assume that the apparent activation energy observed is only attributable to the adsorption of ethylene on titania. Photocatalysis is an extremely complex phenomenon, but the apparent success of L-H kinetics in predicting measured rate data should not be ignored. Furthermore, we believe that the observed apparent activation energy should be interpreted by some method similar to the temperature-dependent L-H expression given in eq 3. The success of this interpretation is in yielding a more reasonable value, in the world of physical adsorption, for the heat of adsorption of ethylene on titania. This method also has limitations, as evidenced by the fact that KE . KW in this analysis. While this is consistent with the published literature (6, 25), it contradicts the expectations of physical intuition for a water species more strongly bound to the
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(1) Godish, T. Indoor Air Pollution Control; Lewis Publishers: Chelsea, MI, 1990; Chapter 1. (2) Kader, A. A. Postharvest Technology of Horticultural Crops; Kader, A. A., Ed.; University of California Publication 3311; University of California: Oakland, 1992; Chapter 3. (3) Raupp, G. B. J. Vac. Sci. Technol. 1995, 13, 1883-1887. (4) Herrmann, J. M. Catal. Today 1995, 24, 157-164. (5) Obee, T. N.; Brown, R. T. Environ. Sci. Technol. 1995, 29, 12231231. (6) Peral, J.; Ollis, D. F. J. Catal. 1992, 136, 554-565. (7) Dibble, L.; Raupp, G. Catal. Lett. 1990, 4, 345-354. (8) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Mozzanega, M.-N. J. Phys. Chem. 1979, 24, 3122-3126. (9) Blake, N. R.; Griffin, G. L. J. Phys. Chem. 1988, 92, 5697-5701. (10) Jacoby, W. A.; Blake D. M.; Noble R. D.; Koval C. A. J. Catal. 1995, 157, 87-96. (11) Fu, X.; Clark, L. A.; Zeltner, W. A.; Anderson, M. A. J. Photochem. Photobiol. A 1996, 97, 181-186. (12) Gregg S. J.; Sing K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1991; Chapter 5, pp 277281. (13) Raupp, G. B.; Dumesic, J. A. J. Phys. Chem. 1985, 89, 5240-5246. (14) Pichat, P.; Herrmann, J.-M. Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; John Wiley & Sons: New York, 1989; Chapter 8. (15) Koller, L. R. Ultraviolet Radiation, 2nd ed.; John Wiley & Sons: New York, 1965; Chapter 2. (16) Hill, C. G. An Introduction to Chemical Engineering Kinetics & Reactor Design; John Wiley & Sons: New York, 1977; Chapter 6, p 178-181 (17) Calvert, J. G.; Pitts, J. N. Photochemistry; John Wiley & Sons: New York, 1966; Chapter 5, pp 494, 504, 558. (18) Dibble, L.; Raupp, G. Environ. Sci. Technol. 1992, 26, 492-495. (19) Hill, C. G. An Introduction to Chemical Engineering Kinetics & Reactor Design; John Wiley & Sons: New York, 1977; Chapter 6, pp 182-184 (20) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990; Chapter 17, pp 717-718. (21) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990; Chapter 16, pp 595-598. (22) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990; Chapter 16, p 595. (23) Pichat, P.; Herrmann, J-M. Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; John Wiley & Sons: New York, 1989; Chapter 5, pp 234-236. (24) Herrmann, J.-M.; Disdier, J.; Pichat, P. J. Phys. Chem. 1986, 90, 6028-6034. (25) Luo, Y.; Ollis, D. F. J. Catal. 1996, 163, 1-11.
Received for review September 25, 1996. Revised manuscript received February 17, 1997. Accepted February 20, 1997.X ES960827M X
Abstract published in Advance ACS Abstracts, April 15, 1997.
