Integrated Photocatalytic Filtration Array for Indoor Air Quality Control

Jun 15, 2010 - Laboratory, Mechanical Engineering Department, University of Minnesota ... (FBAG) was adapted to prepare TiO2-loaded ventilation filter...
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Environ. Sci. Technol. 2010, 44, 5558–5563

Integrated Photocatalytic Filtration Array for Indoor Air Quality Control FRANS DENNY,† ERIC PERMANA,† JASON SCOTT,† JING WANG,‡ D A V I D Y . H . P U I , ‡ A N D R O S E A M A L * ,† ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering, The University of New South Wales, NSW 2052, Australia and Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, Minneapolis, Minnesota

Received February 6, 2010. Revised manuscript received June 2, 2010. Accepted June 4, 2010.

Photocatalytic and filtration technologies were integrated to develop a hybrid system capable of removing and oxidizing organic pollutants from an air stream. A fluidized bed aerosol generator (FBAG) was adapted to prepare TiO2-loaded ventilation filters for the photodegradation of gas phase ethanol. Compared to a manually loaded filter, the ethanol photodegradation rate constant for the FBAG coated filter increased by 361%. Additionally, the presence of the photogenerated intermediate product, acetaldehyde, was reduced and the time for mineralization to CO2 was accelerated. These improvements were attributed to the FBAG system providing a more uniform distribution of TiO2 particles across the filter surface leading to greater accessibility by the UV light. A dual-UV-lamp system, as opposed to a single-lamp system, enhanced photocatalytic filter performance demonstrating the importance of high light irradiance and light distribution across the filter surface. Substituting the blacklight blue lamps with a UV-light-emittingdiode (UV-LED) array led to further improvement as well as suppressed the electrical energy per order (EE/O) by a factor of 6. These improvements derived from the more uniform distribution of light irradiance as well as the higher efficiency of UV-LEDs in converting electrical energy to photons.

Introduction Poor air quality in indoor environments such as buildings, houses, cars, and aircraft cabins can promote transmissible respiratory illnesses, allergies, and sick building syndrome (SBS) (1). Common indoor air pollutants include respirable particles, microorganisms, allergens, and volatile organic compounds (VOCs). Remediation of indoor air pollution is generally achieved by filtration. This technology involves capturing particles through mechanisms of direct interception, inertial impaction, and diffusion (2-5). Several studies (6, 7) demonstrate that filtration technology can work effectively against nanoparticles as small as 2-3 nm. A typical approach to mitigate the VOC problem in indoor environments is to employ sorption materials on the filter to adsorb the VOCs. Yet the technique is a temporary solution to the problem as it essentially transfers the VOC contaminants from the gaseous phase to a solid phase and requires * Corresponding author phone: +61 2 9385-7966; fax: +61 2 93855966; e-mail: [email protected]. † University of New South Wales. ‡ University of Minnesota. 5558

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additional disposal or handling stages. An alternative solution to VOC removal is semiconductor photocatalysis involving TiO2. It has been shown capable of removing low concentrations of organic pollutants from air, ultimately converting them to comparatively harmless products such as H2O and CO2 (8-10). Coupling photocatalytic technology with filtration technology may result in a system that can eliminate not only the particulate pollutants but also resolve VOC issues. This type of integrated system generally comprises coating the photocatalyst onto the filter media. For instance, Agdoped TiO2 was coated onto a fabric filter using a slurry dip-coating technique and used for disinfecting airborne microorganisms in a recirculated system (11). The dip-coating technique is expected to give a good distribution of the particles on the filter media. However, the coating process involved an additional drying stage (12) as well as coated both sides of the filter media which increased filter weight and pressure drop. Typically, only one side of the filter requires irradiation during photocatalysis meaning coating the opposite side is often unnecessary. An alternative to dipcoating is to manually distribute the photocatalyst powder onto the filter surface (13). This technique eliminates the need for a drying step but is restricted to filter media requiring a relatively small coated surface area due to issues regarding coating uniformity. Consequently, it is not tenable for practical situations as often filters comprising large surface areas are used. In addition, many practical filters are pleated which may further obstruct attaining a uniform photocatalyst coating. An alternate possibility for coating filters with particles is to use a fluidized bed aerosol generator (FBAG) system (14), originally developed as a dust generator to synthesize dustloaded air for laboratory research purposes. In the FBAG unit, particles are initially deagglomerated by fluidized metal beads. The deagglomerated particles are then aerosolized and transported to the filter to be coated. The FBAG technique provides the benefits of coating only one side of the filter and is a completely dry process. Additionally, the system gives close control over the particle loading and may be tuned to give a uniform coating, which is crucial for effectively activating the photocatalyst particles. While providing a well-dispersed photocatalyst coating is critical for system performance, equally important is ensuring the light source used is effective in illuminating the entire surface. The transfer of light to the photocatalyst has recently been identified as one of the two primary engineering considerations for applying photocatalytic technology (15). A first order relationship has been reported for low light irradiance and a half-order dependence for high light irradiance (16). Despite the lower efficiency, it is preferable to run the photocatalytic process under high light irradiance to achieve a higher photocatalytic reaction rate for rapid VOC removal. To minimize the impact of lower efficiency at high light irradiances uniformly distributing the light becomes a necessity (17). UV-light-emitting diodes (UV-LEDs) are an option for satisfying these criteria. Additionally, UV-LEDs possess longer lifetimes (typically 50,000-100,000 h (18)) compared with general blacklight blue (BLB) lamps (typically 3000-20,000 h (19)). UV-LEDs without the use of mercury and glass materials also have reduced safety concerns and are easier to handle than BLB lamps. Although photocatalytic technology is promising, careful designs are required to suppress the presence of intermediate products as they can be more hazardous on occasion. For instance, Sun et al. (20) evaluated the air cleaning effects of two photocatalytic air-purification devices in a mock-up air 10.1021/es100421u

 2010 American Chemical Society

Published on Web 06/15/2010

FIGURE 1. Schematic illustration of (a) setup for the photocatalytic experiments, and (b) details of the photocatalytic filter reactor with side and front views. cabin. Although two symptoms, dizziness and claustrophobia, were reported to decrease when either one of the photocatalytic air-purification devices was operated, the detection of intermediates such as acetaldehyde and formaldehyde as a result of ethanol photodegradation was also reported. Although ethanol is not categorized as a carcinogenic substance contributing to SBS effects, the generated intermediates are. In the present work, we prepared TiO2-coated filter media (hereafter referred as the photocatalytic filter) for the photodegradation of ethanol in the gas phase. The impacts of the photocatalyst particle dispersion on the filter and the light distribution on suppressing the presence of the intermediate products were evaluated.

Experimental Section TiO2 Coating Techniques. Aeroxide P25 TiO2, with characteristics of 4:1 anatase-to-rutile ratio, crystallite size of 30 nm, and a specific surface area of 48 m2/g (21), was used as the photocatalyst. The filter media comprised a glass-fiber filter (8 in. × 10 in., type A/E, Pall Corporation) with an aerosol retention efficiency of 99.98%. Details of the FBAG setup are reported elsewhere (14). Briefly, the TiO2 particles were placed in a powder chamber and transported at a constant rate by a chain conveyor into the fluidized bed chamber (see Supporting Information (SI) Figure S1). The fluidized bed comprised 30 g of bronze beads (300 µm in size) in a 50 mm chamber, which was supported by a screen. The bronze beads and TiO2 particles were fluidized by compressed air at a flow rate sufficient to induce fluidization and aerosolize the TiO2 particles. The aerosolized TiO2 particles were carried by the fluidizing air through the elutriation chamber with the air velocity in the elutriation chamber controlled so as not to entrain the heavier bronze beads. The aerosolized TiO2 particles were then directed to the filter media using an aluminum duct pipe and a suction pump located downstream of the filter. The size distribution of the aerosolized TiO2 particles is shown in Figure S2 in the SI. The filter surface area available for coating was 7 in. × 9 in. For comparative purposes, filters were also prepared using

a manual coating procedure. In this instance, a known weight of TiO2 powder was manually sprinkled on the filter surface. The coating method comparison was performed at a photocatalyst loading of 70 mg TiO2. TiO2 Coating Characterization. The mass of TiO2 coated on the fiber filter by the FBAG was gravimetrically determined. TiO2 particle dispersion on the coated filters by both the FBAG and manual methods was assessed using scanning electron microscopy (SEM, Hitachi S4500). Preparation involved chromium coating a 15 × 15 mm2 section of the coated filter prior to SEM analysis. Photocatalytic Filter Reactor. Figure 1 shows a schematic of the photocatalytic filter reactor array. The system included a photoreactor comprising two compartments (1.3 L in total), a radiation source, a 1.85 L dead volume (giving a total system volume of ∼3.15 L), and a diaphragm pump (Millipore). The photocatalytic filter was sandwiched between the two compartments of the photoreactor and supported by a stainless steel mesh. The top compartment contained six gas inlet ports, arranged so there were three inlets on two opposite sides of the filter. The lower compartment contained a single gas outlet located beneath the center of the filter. The top compartment also contained a borosilicate glass window to facilitate filter illumination. Gas phase ethanol was provided by a 509 ppm ethanol air mixture (Coregas) and diluted with compressed air (Coregas) to the desired concentration. UV irradiation was provided by either 6 W blacklight blue (BLB) UV-A lamp(s) or a UV-LED panel. The BLB lamps were always located 75 mm above the filter during illumination to maximize the illumination region. When the single lamp array was used the lamp was located directly above the center of the filter whereas when the dual lamp array was used the center-to-center distance between the two lamps was 75 mm. The BLB lamps (F6T5BLB, Sylvania) emitted UV-A radiation with peak irradiance wavelength of 365 nm. The UV-LED panel comprised UV-LEDs (L375R-04, Marubeni Corporation) fashioned as a 12 × 14 row array with 13 mm spacings (x and y directions) between each LED. The dissipative power VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Ethanol photodegradation profiles comparing the performance of TiO2-coated filter media prepared using FBAG and manual coating (MC) techniques. Experimental conditions: initial ethanol concentration ) 102 ppm; TiO2 loading ) 70 mg; light source ) one BLB lamp; circulation flow rate ) 4.1 L/min; system volume ) 3.15 L. of each LED was 37.9 mW with an emissive wavelength of 375 nm. During UV-LED illumination, the panel was located 25 mm directly above the filter. Light irradiance provided by the illumination sources at the filter surface was measured by a light photometer (1916C, Newport) equipped with a silicone detector head (818-UV/CM, Newport). Ethanol, acetaldehyde, and CO2 concentrations in the gas phase were monitored by a gas chromatograph (Shimadzu GC2010) equipped with a flame ionization detector and methanizer. Component separation was achieved by a HP Plot Q capillary column (Agilent Technologies, Inc.). Ethanol Photodegradation. The photocatalytic filter reactor system was operated in a batch mode. In a typical experiment a stream of compressed air was passed through the photocatalytic filter under UV light irradiation for 1 h to remove organic impurities from the TiO2 surface. After a cooling period of 30 min, the photocatalytic filter reactor was isolated and the diaphragm pump was switched on to provide a flow rate of 5 L/min. A 174 ppm ethanol-air mixture was passed through the system until the ethanol concentration downstream of the reactor stabilized. Isolation of the photocatalytic reactor during this period prevented preadsorption of ethanol onto the TiO2, in turn preventing variations in the amount of ethanol to be photocatalytically treated. In this instance, the dead volume assisted in regulating the initial ethanol concentration of the system. The system was then configured to batch mode and the photocatalytic filter reactor was brought online leading to a decrease in the gas circulation flow rate to 4.1 L/min. The ethanol-air mix was circulated for 30 min to blend the ethanol-loaded air with the clean air in the reactor after which the light source was turned on and the filter was irradiated for a period of 12 h. Following blending, the ethanol concentration was ∼102 ppm. Although 102 ppm is not a typical VOC concentration in indoor environments (typical VOC concentrations range between 130 and 5000 ppb (20, 22)), it allowed for evaluating photocatalytic filter performance when subjected to variations in system parameters.

Results and Discussion Coating Technique. The control system, comprising operating the photoreactor with an uncoated filter, indicated minimal ethanol adsorption on the filter media occurred and no photoactivity was present (result not shown). Coating the filter with TiO2 particles caused insignificant change (2-3%) in pressure drop across the filter at face velocities ranging from 1 to 10 cm/s (see SI Figure S4). Additionally, the TiO2 coating slightly increased the filtration efficiency for particle sizes in the range 200-300 nm (see SI Figure S5). 5560

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Figure 2 depicts the ethanol, acetaldehyde, and CO2 concentration profiles with time for the FBAG and manual coating methods at a TiO2 loading of 70 mg. Prior to illumination, the initial ethanol concentration decreased to 17 ((1) ppm irrespective of the coating method, demonstrating an adsorption capacity of approximately 162 ((2) µmol ethanol/g TiO2. Comparable ethanol adsorption on TiO2 (∼152 µmol/g TiO2) has also been reported (13, 23). Upon illumination, the ethanol concentration decreased and acetaldehyde and CO2 were formed, the rates at which these events occurred depending on the coating technique. In the context of ethanol removal and assuming it followed pseudo-first-order kinetics, the FBAG system possessed a rate constant of 6.0 ((0.2) h-1 compared with 1.3 ((0.1) h-1 for the manual coating technique. More apparent, however, was the significant differences the coating method imparted on acetaldehyde generation/consumption and CO2 generation. The FBAG method generated an initially higher acetaldehyde concentration, which peaked at a much earlier time (10 min) compared with the manual method (at 60 min). Furthermore, the FBAG method removed the acetaldehyde within 40 min of illumination while the manual method took approximately 240 min. The CO2 profile shows the FBAG method was able to completely oxidize the ethanol after around 5 h while at this time the manual method had achieved ∼55% mineralization. Continued operation saw CO2 generation reach a maximum of ∼140 ppm (corresponding to ∼67% mineralization) after 15 h. Unlike the manual method, the FBAG-prepared photocatalytic filter also displayed minimal lag time and a 5.8 times increase in CO2 generation rate (3.03 ((0.2) ppm/min). Overall, the results display a clear superiority in photocatalytic performance for the FBAG coating method compared with the manual method. Performing a carbon balance on both systems indicated discrepancies between the ethanol consumed and the acetaldehyde and CO2 generated (at any one time) throughout the course of the reaction. This suggests carbon was also present as intermediates other than acetaldehyde, which either could not be detected by the analytical system or remained on the photocatalyst surface during the reaction. Other studies (23-25) have reported similar findings and attributed acetaldehyde desorption to competition between it and water (formed simultaneously) for surface adsorption sites. Formaldehyde has been reported as another intermediate in ethanol photodegradation (24), which has been found to be rapidly mineralized to CO2. This may explain its lack of detection in this system (25). Intermediates such as acetic acid and formic acid have also been reported to form during ethanol mineralization and remain strongly adsorbed on the TiO2 surface (23, 25). Acetic acid, in particular, is known as a recalcitrant molecule during photodegradation (13). When the surface area of the filter to be coated is large, coating uniformity can be a significant factor in defining the overall performance of the photocatalytic filter. The superior performance of the FBAG-coated filter, in this instance, shows the importance of uniform coating of TiO2 onto the filter. The manually prepared photocatalytic filter has a prevalence of uncoated regions and TiO2 aggregates on the filter surface as indicated by the SEM images in Figure S6 (see SI). The high magnification image (Figure S6b) shows the TiO2 particles sit as clusters on the filter surface with many of the fibers remaining uncoated. The low magnification image of the FBAG-coated filter (Figure S6c) reveals aggregates are also present on the filter surface, however the higher magnification image (Figure S6d) shows a much more uniform coating of the individual fibers is attained. Irregularity of the TiO2 coating when using the manual method invokes a poorer performance for two reasons. First, the higher prevalence of aggregates and the thicker regions of coating will reduce light access to photocatalyst particles

FIGURE 3. Rate constant of ethanol photooxidation and rate of CO2 generation for different TiO2 loadings on the photocatalytic filter prepared using the FBAG coating technique. Experimental conditions: initial ethanol concentration ) 102 ppm; light source ) one BLB lamp; circulation flow rate ) 4.1 L/min; system volume ) 3.15 L.

FIGURE 4. Influence of TiO2 loading on acetaldehyde generation and photodegradation for the photocatalytic filter prepared by the FBAG coating technique. Experimental conditions: initial ethanol concentration ) 102 ppm; light source ) one BLB lamp; circulation flow rate ) 4.1 L/min; system volume ) 3.15 L.

that lie beneath the surface layers. Light penetration into a TiO2 film has been reported to occur to a depth of up to 3.3 µm at the wavelength of 365 nm (26) implying the existence of dark regimes in the TiO2 aggregates and coating in this study. This corollary may also contribute to the incomplete mineralization observed for the manually coated system. Intermediates such as acetic and formic acids were suggested to have reversibly accumulated in the dark regions of a monolith photocatalytic reactor during the photocatalytic oxidation of ethanol (24). It is reasonable to expect this effect to be dominant in the manually coated system since its saturated adsorption was comparable to that with FBAGprepared filter implying all sites were accessible by the organic molecules but not the photons. The second negative effect imparted by the manually coated system is the increased presence of uncoated regions on the filter. UV light incident on these areas will not be utilized by the photocatalys and they also represent zones where channeling of the gas phase VOCs can occur. That is, they diminish the chance of pollutants coming into contact with the photocatalyst surface as they pass through the filter media. The greater dispersion of TiO2 particles using the FBAG technique allows for increased harnessing of the UV-light by the photocatalyst surface and reduces the other negative effects encountered with the manual coating technique. Closer inspection of Figure S6d (see SI) reveals photocatalyst particles have coated fibers not just on the filter surface but below the surface as well. This can be attributed to the smaller aggregate size produced by the FBAG system. TiO2 aggregates delivered into the fluidized bed chamber are broken into smaller aggregate sizes, aerosolized, and then captured by the filter media. The smaller TiO2 aggregates can penetrate the top layer of fibers and be captured by the inner fibers. This process promotes both a more uniform coverage on the filter as well as greater access of the UV light to the photocatalyst surface. TiO2 Loading. Given the superior performance of the FBAG technique over the manual method for loading the filter with photocatalyst, all ensuing studies were performed using FBAG-prepared photocatalytic filters. Variations in the TiO2 loading by the FBAG method were achieved by changing the coating period. Increasing the coating period proportionately increased the amount of TiO2 loaded on the filter (see Figure S7 in SI). The impact of TiO2 loading on both the ethanol photodegradation rate constant and the CO2 generation rate is given in Figure 3 with the ethanol photodegradation and the CO2 generation profiles presented in Figure S8 (see SI). At the lowest TiO2 loading (5 mg), the ethanol photodegradation rate constant and CO2 generation rate were 0.46 h-1 and 0.33 ppm/min, respectively. The ethanol photodegradation rate constant increased with the increasing

TiO2 loading up to 70 mg, beyond which no further improvement was observed. The rate of CO2 generation, however, increased with increasing loading beyond 70 mg TiO2, although its specific rate decreased from 0.044 ppm/ min/mg TiO2 at TiO2 loading of 70 mg to 0.023 ppm/min/mg TiO2 at a loading of 88 mg TiO2. Figure 4 depicts that the TiO2 loading also influences the evolution and degradation of acetaldehyde within the system. At low TiO2 loadings, there is a much greater incidence of acetaldehyde in the system, particularly in terms of duration. At a TiO2 loading of 5 mg, it took ∼14 h to remove the acetaldehyde (profile beyond 5 h is not shown). The time at which the acetaldehyde remained in the system decreased with increasing TiO2 loading, which is to be expected given the congruent rate of ethanol removal. At TiO2 loadings of 70 mg and above there were only small differences in the acetaldehyde profile. Based on these results and those for ethanol photodegradation, it is apparent the optimum TiO2 loading for this system under these conditions is 70 mg. Beyond this loading, improvements to ethanol and acetaldehyde removal are marginal. Light Source. The effect of an additional BLB lamp as well as an alternate UV-LED panel as light sources on the performance of the optimized FBAG-prepared photocatalytic filter were assessed with the findings given in Table 1 and Figure 5. The additional BLB lamp significantly enhanced system performance as indicated both by a 78% increase in the ethanol rate constant and a comparable (86%) increase in the CO2 generation rate (Table 1). A beneficial effect was also observed for the acetaldehyde presence (Figure 5). The peak concentration of acetaldehyde using two BLB lamps was ∼21 ppm, which was 3.5 times lower than for one BLB lamp. The lower acetaldehyde presence and improved CO2 generation with the additional BLB lamp implies increasing light irradiance can suppress the existence of harmful intermediates during the photocatalytic degradation of ethanol. The superior performance invoked by the additional BLB lamp can be attributed to an improved light distribution and a higher averaged light irradiance as illustrated in Figure S9 (see SI). The maximum light irradiance achieved with one BLB lamp (Figure S9a) was ∼232 µW/cm2 with the irradiance dropping noticeably (up to 150%) near the filter edges. In the two-lamp system (Figure S9b), light distribution was more uniform across the photocatalytic filter with a maximum light irradiance of ∼384 µW/cm2. The irradiance decreased to approximately 212 µW/cm2 at the edges of the photocatalytic filter. Overall, the surface-averaged light irradiance of the dual-lamp system (∼313 µW/cm2) is considerably higher than the single-lamp system (∼158 µW/cm2). The light irradiance (I) can be further related to the ethanol disappearance or CO2 generation rates according to eq 1, VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Light Source, Irradiance, Ethanol Rate Constant, Peak Concentration of Acetaldehyde, CO2 Generation Rate, Power Input, and Electrical Energy Per Order (EE/O) for the Different Light Sources Used to Illuminate the Photocatalytic Filtera light source 1 BLB lamp 2 BLB lamps UV-LED panel

irradiance (µW/cm2)

C2H5OH rate constant (h-1)

CH3CHO peak (ppm)

CO2 rate (ppm/min)

power input (W)

EE/O (kWh/m3/order)

158 313 1000 158

6.0 ((0.2) 10.7 ((0.1) 12.1 ((0.1) 8.1 ((0.2)

72.8 20.8 6.5 41.4

3.0 ((0.2) 5.6 ((0.2) 8.4 ((0.2) 4.3 ((0.3)

6.0 ((0.3) 12.0 ((0.6) 6.37 ((0.2) 1.38 ((0.2)

0.73 ((0.05) 0.82 ((0.04) 0.38 ((0.02) 0.13 ((0.01)

a The filter was prepared using the FBAG. Experimental conditions: initial ethanol concentration ) 102 ppm; TiO2 loading ) 70 mg; circulation flow rate ) 4.1 L/min; system volume ) 3.15 L.

FIGURE 5. Effect of light array on acetaldehyde evolution and degradation for the optimized FBAG-prepared photocatalytic filter. Experimental conditions: initial ethanol concentration ) 102 ppm; TiO2 loading ) 70 mg; circulation flow rate ) 5 L/min; system volume ) 3.15 L. which is a pseudo-first-order rate expression. The expression is a simplified form of the Langmuir-Hinshelwood rate expression for low initial reactant concentrations (27). The rate order, n, describes the nature of the photocatalytic process and can possess a value between 0 and 1. An order of 0.5 indicates the process is dominated by the recombination of photogenerated electrons and holes whereas an order of 1 implies reaction kinetics governs the process (28). Solving eq 1 for n by using the CO2 generation rates for the one- and two-lamp systems yielded a value of 0.87 indicating a predominantly linear dependence of the CO2 generation rate on light irradiance. Consequently, the process was dominated by the kinetics of ethanol photomineralization to CO2. -

d[C2H5OH] 1 d[CO2] ) ) kIn[C2H5OH] dt 2 dt

(1)

The results demonstrate a higher and more uniform distribution of light irradiance results in enhanced performance of the photocatalytic filter. The UV-LED array, with the capability of providing not only higher light irradiance but also a uniform light distribution on the filter surface, further promoted photocatalytic filter performance. When the UV-LED power input was tuned to be similar to the single BLB lamp (6.37 ( 0.2 versus 6.0 ( 0.3 W, respectively), the ethanol degradation rate constant doubled while the CO2 generation rate increased by 178%. It also decreased the acetaldehyde presence with the peak concentration reduced by 10.8 times compared to the single-lamp system (Figure 5). Moreover, these results were an improvement over the dual BLB system. The observed improvements can be partly attributed to the higher efficiency of UV-LEDs in converting electrical energy to photons (29). At a power input similar to one BLB lamp, the irradiance provided by the UV-LED array was uniform at ∼1 mW/cm2 (see Figure S9c in SI), 6.3 times greater across the filter surface. This can be partly attributed to the closeness of the UV-LED array to the photocatalytic filter. As mentioned previously, placement of the BLB lamp further from the photocatalytic filter was designed to maximize the area it irradiated. As the size of the 5562

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UV panel was comparable to the photocatalytic filter, the panel could be placed at a closer distance without compromising the irradiated region, demonstrating an additional benefit of UV-LEDs as the light source. The UV-LED array also allows for a uniform dissemination of the light across the filter surface. This second advantage is illustrated by the results set in Table 1 and Figure 5 where the power input was decreased to match the surface averaged light irradiance of the single BLB system (158 µW/cm2). Under these conditions, the UV-LED array increased the ethanol photodegradation rate constant by 35%, increased the CO2 generation rate by 43%, and decreased the peak acetaldehyde concentration by 43% compared with the single lamp system. The uniform light distribution allows for improved illumination of TiO2 at the filter fringes, improving the effectiveness of these regions in the photodegradation process. The advantage of the UV-LED array as the light source can be further exemplified by evaluating the electrical energy per order (EE/O) (kWh/m3/order) for each light source. The EE/O is defined as the electrical energy in kWh required to degrade the pollutant by one order in a unit volume (30). The EE/O for an idealized batch reactor is described by eq 2 with the EE/O values for the different light sources and settings summarized in Table 1. EE/O )

2302.6P Vk

(2)

where P is the electrical power (kW), V is the batch reactor volume (L), and k is the apparent rate constant (h-1) (Table 1). The EE/O decreased by 47% on replacing the single BLB lamp arrangement with the UV-LED panel operating at approximately the same power rating. This decrease signifies a reduction in the electrical energy required for a one-fold removal of the pollutant. The EE/O decreased by an additional 36% when the UV-LED array was operated at the lower power input (1.38 W). The findings in this work emphasize the importance of uniform particle and light irradiance distribution to suppress the presence of intermediates during photocatalytic degradation of an organic pollutant by a hybrid photocatalytic filter. Uniform particle dispersion on the filter can be obtained by using the FBAG technique and a UV-LED array can be used to provide not only high light irradiance but also more uniform illumination across the photocatalytic filter. With typical indoor VOC concentrations approximately 2-fold lower than in the present study (22), the intermediate products may be removed instantaneously without posing any significant health problems to the indoor occupants.

Acknowledgments We thank the Australian Research Council through the ARC International Linkage Award and its Centre of Excellence program for financially supporting the project.

Supporting Information Available Illustration on the FBAG experimental setup; size distribution profile of aerosolized TiO2 particles; pressure drop particle and penetration across blank and FBAG-prepared TiO2coated filters; SEM images comparing TiO2 particle distribution on filters prepared using the “manual” and FBAG coating techniques; controlled TiO2 loading using the FBAG coating technique; effect of TiO2 loading ethanol oxidation and CO2 generation; light irradiance distribution on photocatalytic filter irradiated by BLB lamps and UV-LED array. This material is available free of charge via the Internet at http:// pubs.acs.org.

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