Patent Publication Number: US-2018029017-A1

Title: Photocatalytic filter for degrading mixed gas and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 14/871,932, filed Sep. 30, 2015, claims priority to Provisional Application No. 62/057,794 filed on Sep. 30, 2014, and Chinese Patent Application No. 201510096590.7 filed on Mar. 4, 2015. The entire disclosure of the above applications are incorporated by reference in their entirety as part of this patent document. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a photocatalytic filter and a manufacturing method thereof. Some implementations of the disclosed technology relate to a photocatalytic filter, the surface of which has enhanced adsorption performance so that mixed gases including a gas that reacts later in a competitive reaction can be degraded from the initial stage of a photocatalytic reaction, and to a manufacturing method thereof. 
     BACKGROUND 
     As used herein, the term “photocatalytic reaction” refers to reactions that use photocatalytic materials such as titanium dioxide (TiO 2 ) or the like. Known photocatalytic reactions include photocatalytic degradation of water, electrodeposition of silver and platinum, degradation of organic materials, etc. Also, there have been attempts to apply such photocatalytic reactions to new organic synthetic reactions, ultrapure water production and the like. 
     Toxic gases or offensive odor substances, such as ammonia, acetic acid and acetaldehyde, which are present in air, are degraded by the above-described photocatalytic reactions, and air purification devices based on such photocatalytic reactions can be used semi-permanently if they have a light source (e.g., a UV light source) and a filter coated with a photocatalytic material. When photocatalytic efficiency of the photocatalytic filter has reduced, the filter can be regenerated to restore its photocatalytic efficiency, and then it can be reused. Thus, it can be said that the photocatalytic filter is semi-permanent. 
     Particularly, when a UV LED lamp is used as a UV light source, it is advantageous over a conventional mercury lamp or the like in that it is environmentally friendly because it does not require toxic gas, is highly efficient in terms of energy consumption, and allows various designs by virtue of its small size. 
     SUMMARY 
     Various embodiments provide a photocatalytic filter, which shows a high removal rate of removal of each gas even when mixed gases pass therethrough, and a method for manufacturing the photocatalytic filter, the photocatalyst of which has high adhesion to a base or a substrate. 
     In some implementations, a method for manufacturing a photocatalytic filter is provided to include: providing a photocatalytic dispersion by dispersing titanium dioxide (TiO 2 ) nanopowders and metal compounds in water; coating a support with the photocatalytic dispersion; drying the coated support; and sintering the dried support. 
     In some implementations, wherein the metal compounds include a tungsten (W) compound including atom H. In some implementations, the tungsten (W) compound includes H 2 WO 4 . In some implementations, the metal compounds include a tungsten (W) compound including H 2 WO 4 , WO 3 , WCl 6 , or CaWO 4 . In some implementations, the metal compounds include an iron (Fe) compound. In some implementations, the iron (Fe) compound includes Fe 3+  compound. In some implementations, the iron compound includes FeCl 2 , FeCl 3 , Fe 2 O 3 , or Fe(NO 3 ) 3 . In some implementations, the metal compounds include the tungsten (W) compound having a molar ratio between 0.0032 and 0.0064 moles per mole of titanium dioxide. In some implementations, the iron (Fe) compound has a molar ratio between 0.005 and 0.05 moles per mole of titanium dioxide. In some implementations, coating the support includes dip-coating the support. In some implementations, the sintering of the dried support is performed at a temperature between 400° C. and 500° C. for 2 to 3 hours. 
     In another aspect, a photocatalytic filter is provided to include: a support; and a photocatalytic material and metal compound coated on the support. 
     In some implementations, the metal compounds include a tungsten (W) compound including H 2 WO 4  and an iron (Fe) compound including Fe 2 O 3 . In some implementations, the photocatalytic material includes titanium dioxide (TiO 2 ), and the metal compounds include a tungsten (W) compound having a molar ratio between 0.0032 and 0.0064 moles per mole of titanium dioxide. In some implementations, the photocatalytic material includes titanium dioxide (TiO 2 ), and the metal compounds include an iron (Fe) compound having a molar ratio between 0.005 and 0.05 moles per mole of titanium dioxide. In some implementations, the support includes porous ceramic. In some implementations, the photocatalytic filter comprises a plurality of adjacent parallel cells that form an air flow path in a direction facing UV LED for photocatalytic activation. In some implementations, the photocatalytic filter has a height of 2 to 15 mm. In some implementations, a frame between the cells has a thickness of 0.3 to 1.2 mm. In some implementations, each of the cells has a width of 1 to 4 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows removal rates of toxic gases as a function of time when using a conventional photocatalytic filter and a photocatalytic filter according to one implementation of the present disclosure. 
         FIG. 2  is a perspective view showing the arrangement of a photocatalytic filter and a UV LED substrate. 
         FIG. 3  is a top view of a photocatalytic filter. 
         FIG. 4  is a graph showing the change in removal rate of acetaldehyde with a change in the height of a photocatalytic filter. 
         FIG. 5  is a graph showing the change in removal rate of acetic acid with a change in the height of a photocatalytic filter. 
         FIGS. 6 a  to 6 c    show graphs showing removal rates of Toluene, Ammonia, and Acetic acid with a change in a height of the photocatalytic filter exemplarily implemented according to the present disclosure. 
         FIGS. 7 a  to 7 d    show test results on removal efficiencies of an exemplary harmful gas of Acetaldehyde. 
     
    
    
     DETAILED DESCRIPTION 
     Conventional filters such as the pre-filter or HEPA filter physically collect large dust particles when air passes therethrough. Unlike the conventional filters, the photocatalytic filter is configured such that toxic gases adsorbed on the surface of the filter during the passage of air through the filter are degraded by radicals such as OH − , generated by the photocatalytic reaction. Thus, toxic gases in air are degraded during the passage of the air through the catalytic filter are not completely degraded, but a portion thereof is degraded. In other words, toxic gases in air are degraded while the air passes several times through the photocatalytic filter. 
     Thus, the photocatalytic efficiency of the photocatalytic filter is dependent on the air cleaning ability thereof. In other words, toxic gas in a space that uses an air cleaner having high photocatalytic efficiency is degraded faster than toxic gas in a space that uses an air cleaner having the same size and structure, but having a relatively low photocatalytic efficiency. 
     Meanwhile, it is known that, when air contains a plurality of different toxic gases, the toxic gases are degraded in the order in which they are adsorbed onto the surface of the photocatalytic filter. Thus, among toxic gases, a gas that is adsorbed into the photocatalytic surface at higher rate is degraded faster, and a gas that is adsorbed onto the photocatalytic surface at lower rate is adsorbed and degraded on the photocatalytic surface after the gas adsorbed at higher rate was somewhat degraded. 
     The deodorization performance test method provided by the Korea Air Cleaning Association includes evaluating the removal rate of a mixture of three gases: acetaldehyde, ammonia, and acetic acid. The results of experiments conducted according to this test method indicated that a commercially available TiO 2  photocatalyst shows a low rate of removal of acetaldehyde among the gases. This is because acetaldehyde reacts later than other gases in a competitive reaction. In other words, the conventional photocatalytic filter is configured such that it degrades a toxic gas that reacts first in a competitive reaction, and then degrades a toxic gas that reacts later. 
     This propensity of the conventional photocatalytic filter is not desirable from the point of view of air cleaners. In the case of air cleaners that use the photocatalytic reactions, the performance of degrading toxic gases is important, and furthermore, the performance of degrading all types of toxic gases should be excellent, and all types of toxic gases need to be degraded from the initial stage of a photocatalytic reaction. 
     Exemplary embodiments will be described below in more detail with reference to the accompanying drawings. The disclosure may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. 
     The techniques disclosed in this patent document can be used to provide a photocatalytic filter with improved adsorption for acetaldehyde, ammonia and acetic acid gas mixture by introducing metal into titanium dioxide photocatalytic in the filter. An exemplary method for manufacturing the photocatalytic filter with improved adsorption for acetaldehyde, ammonia and acetic acid gas mixture includes providing a photocatalytic dispersion liquid by dispersing titanium dioxide nanopowders and one or more metal compounds in water, coating a photocatalytic support with the photocatalytic dispersion liquid, drying the coated photocatalytic support, and sintering the dried photocatalytic support. 
     A photocatalytic filter based on the disclosed technology includes a photocatalytic support and a photocatalytic material formed on the photocatalytic support. Under UV light exposure, the photocatalytic material is optically activated to cause a catalytic reaction with one or more targeted contaminants attached to the photocatalytic material coated on the photocatalytic support, e.g., via physical adsorption, therefore removing the contaminants from a gas medium. Targeted contaminants may be microorganisms or other biological material, or one or more chemical substances. A UV light source, such as UV LEDs, can be included to direct UV light to the photocatalytic material formed on the photocatalytic support. Such a photocatalytic filter can be used as an air filter or other filter applications. The photocatalytic material can include, for example, titanium dioxide nanopowders and one or more metal compounds. 
     A photocatalytic filter according to an embodiment of the present disclosure includes the tungsten (W) and iron (Fe) metal compounds added to a conventional photocatalytic TiO 2  material, and thus shows a high removal rate of mixed gases. In other words, according to the present disclosure, the acidity of the surface of the TiO 2  photocatalyst can be adjusted by adding the metal compounds to the TiO 2  photocatalyst, and thus the ability of the TiO 2  photocatalyst to adsorb gas compounds can be enhanced, thereby increasing the ability of the TiO 2  photocatalyst to remove toxic gas. 
     Method for Manufacturing Photocatalytic Filter 
     A method for manufacturing a photocatalytic filter according to the present disclosure is as follows. The method may include the steps of: dispersing photocatalytic TiO 2  nanopowders, a tungsten (W) compound and an iron (Fe) compound in water to prepare a photocatalytic dispersion; coating a porous ceramic honeycomb support with the photocatalytic dispersion; drying the coated support; and sintering the dried support. 
     As the TiO 2  nanopowder, commercially available Evonik P25 powder may be used. 
     The W compound that is used in the present disclosure may be H 2 WO 4 , WO 3 , WCl 6 , CaWO 4  or the like, and the Fe compound that is used in the present disclosure may be FeCl 2 , FeCl 3 , Fe 2 O 3 , Fe(NO 3 ) 3  or the like. In an exemplary embodiment of the present disclosure, H 2 WO 4  is used as the W compound, and Fe 2 O 3  is used as the Fe compound. 
     The reason why H 2 WO 4  (tungsten oxide hydrate) among W compounds is used is to introduce WO 3  into the photocatalytic nanopowder. In other words, H 2 WO 4  is used as a precursor for introducing WO 3 . In other words, in the case in which H2WO4 is introduced as a WO3 precursor, the reactivity between WO3 and TiO2 can be increased by a dehydration reaction compared to the case in which WO3 powder is directly added. 
     With respect to the Fe compound, Fe 2+  has an electronic configuration of 1s 2  2s 2  2p 2  3s 2  3p 6  3d 6 , in which the number of electrons in the outermost shell is greater than half of the valence electrons by one. Also, Fe 3+  has an electronic configuration of 1s 2  2s 2  2p 2  3s 2  3p 6  3d 5 , in which the number of electrons in the outermost shell is equal to the number of the valence electrons. Thus, Fe 2+  has a strong tendency to donate one outermost electron to become relatively stable Fe 3+  equal to half of the valence electrons. The electron donated from Fe 2+  as described above reacts with H +  produced in the excitation reaction of TiO 2 . Thus, when Fe 2+  is used, the electron donated from Fe 2+  reacts with H +  produced in the excitation reaction of TiO 2 , and thus Fe 2+  is converted into Fe 3+  which then participates in a photocatalytic reaction. In other words, although Fe 2+  and Fe 3+  promote photocatalytic reactions, Fe 3+  more efficiently promotes the photocatalytic reaction compared to Fe 2+ . 
     Compounds that are used to introduce Fe into the photocatalytic nanopowder include FeCl 3 , Fe 2 O 3 , Fe(NO 3 ) 3  and the like. Among these compounds, FeCl 3  and Fe(NO 3 ) 3  cause a problem during mixing with H 2 WO 4 , or does not show an increase in photocatalytic activity. However, the results of an experiment indicate that Fe 2 O 3  can exhibit a synergistic effect with H 2 WO 4 . Thus, Fe 2 O 3  is preferably used as the Fe compound. 
     Based on the total moles of TiO 2 , H 2 WO 4  may be used in an amount of 0.0032 to 0.064 mole %, and Fe 2 O 3  may be used in an amount 0.005 to 0.05 mole %. In some implementations, based on the total moles of TiO 2 , H 2 WO 4  is used in an amount of 0.016 to 0.048 mole %, and Fe 2 O 3  is used in an amount of 0.005 to 0.025 mole %. 
     As the support for the photocatalytic nanopowders, a metal material, activated carbon, a ceramic material or the like may be used. In an exemplary embodiment of the present disclosure, a porous ceramic honeycomb material is used as the support in order to increase the adhesion of the photocatalytic compound. When the porous ceramic honeycomb material is used as the support, the dispersion of the photocatalytic nanopowders penetrates the pores of the ceramic material in the coating step, and the photocatalytic nanoparticles are anchored to the pores after the drying step, thereby increasing the adhesion of the photocatalytic nanoparticles to the ceramic material. If a metal material is used as the support, it will be not easy to attach the photocatalytic nanoparticles to the metal material, compared to attaching the photocatalytic nanoparticles to the ceramic material. In addition, although activated carbon has pores, it can be broken during the sintering step in some cases, and thus the use thereof as the support is undesirable. 
     In the process of preparing the photocatalytic dispersion, Evonik P25 TiO 2  powder, the W compound and the Fe compound are dispersed using a silicone-based dispersing agent. The silicone-based dispersing agent is used in an amount of 0.1 to 10 wt % based on the total weight of P25 TiO 2  powder, the W compound and the Fe compound. Specifically, 0.1 to 10 wt % of the silicone-based dispersing agent is dissolved in water, and then P25 TiO 2  nanopowder, the W compound and the Fe compound are added to the solution and dispersed using a mill, thereby obtaining a TiO 2  dispersion having a solid content of 20 to 40 wt % based on the weight of the dispersion. Herein, one or more dispersing agents may be used. 
     In the coating step, a porous ceramic support is dip-coated with the above-prepared photocatalytic dispersion. During the dip coating, the support coated with the photocatalytic dispersion is allowed to stand for 1-5 minutes so that the photocatalytic dispersion can be sufficiently absorbed into the pores of the ceramic material. 
     In the drying step, the ceramic support coated with the photocatalyst is maintained in a dryer at 150˜200° C. for 3-5 minutes to remove water. 
     In the sintering step, the photocatalyst-coated ceramic honeycomb support resulting from the drying step is sintered in an electric furnace at 400˜500° C. for 2-3 hours. The results of an experiment indicated that, when the sintering temperature was lower than 300° C., the coated photocatalyst was detached from the support, and when the sintering temperature was between 400° C. and 500° C., the photocatalyst had high adhesion to the support. From the experimental results, it can be seen that the adhesion of the photocatalyst is greatly influenced by the sintering temperature. 
     Experiment on Removal of Mixed Gases 
     Using a conventional photocatalytic filter coated with TiO 2  alone, and the photocatalytic filter according to the present disclosure, an experiment on the removal of mixed gases was performed in a 1 m 3  chamber. The concentration of each gas in the mixed gases was 10 ppm. The conventional photocatalytic filter and the photocatalytic filter of the present disclosure were each loaded with 2.5 g of the photocatalyst to the support, and were irradiated with UV light using the same UV light source. 
     The molar ratios between components in the photocatalytic filter according to the present disclosure were as follows: TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.01; TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.015; and TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.02. 
     The conventional photocatalytic filter coated with TiO 2  alone, and the photocatalytic filter of the present disclosure were tested for their abilities to remove mixed gases. The results of the experiments are shown in Tables 1 and 2 below. As can be seen in the Tables, in the experiment performed using the conventional photocatalytic filter coated with TiO 2  alone for testing removal of mixed gases, acetaldehyde was not removed for 30 minutes after the start of the experiment, and started to be removed after other gases were somewhat removed. However, in the deodorization experiment performed using the photocatalytic filter of the present disclosure, acetaldehyde was removed from the initial stage of the experiment, and the removal rate of ammonia by the photocatalytic filter of the present disclosure was also higher than that that shown by the conventional photocatalytic filter, suggesting that the photocatalytic filter of the present disclosure has an improved ability to remove all the gases. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Removal rate at 30 minutes after start of reaction 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 H 2 WO 4 / 
                 H 2 WO 4 / 
                 H 2 WO 4 / 
               
               
                 Removal 
                   
                 Fe 2 O 3   
                 Fe 2 O 3   
                 Fe 2 O 3   
               
               
                 rate (%) 
                 P25—TiO 2   
                 (0.010)/TiO 2   
                 (0.015)/TiO 2   
                 (0.020)/TiO 2   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 NH 3   
                 40 
                 52.6 
                 70 
                 63.2 
               
               
                 CH 3 CHO 
                 0 
                 20 
                 20 
                 20 
               
               
                 CH 3 COOH 
                 50 
                 30 
                 50 
                 35 
               
               
                 Total 
                 22.5 
                 30.7 
                 40 
                 34.5 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Removal Rate at 120 minutes after start of reaction 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 H 2 WO 4 / 
                 H 2 WO 4 / 
                 H 2 WO 4 / 
               
               
                 Removal 
                   
                 Fe 2 O 3   
                 Fe 2 O 3   
                 Fe 2 O 3   
               
               
                 rate (%) 
                 P25—TiO 2   
                 (0.010)/TiO 2   
                 (0.015)/TiO 2   
                 (0.020)/TiO 2   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 NH 3   
                 55 
                 73.7 
                 85 
                 75 
               
               
                 CH 3 CHO 
                 25 
                 60 
                 60 
                 50 
               
               
                 CH 3 COOH 
                 85 
                 70 
                 75 
                 60 
               
               
                 Total 
                 47.5 
                 65.9 
                 70 
                 58.75 
               
               
                   
               
            
           
         
       
     
       Total removal (%)={(CH 3 CHO removal rate)*2+NH 3  removal rate+CH 3 COOH removal rate}/4   molar ratio
 
     TiO 2 /H 2 WO 4 /Fe 2 O 3 =100/10/2 weight ratio (TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.010 molar ratio) 
     TiO 2 /H 2 WO 4 /Fe 2 O 3 =100/10/3 weight ratio (TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.015 molar ratio) 
     TiO 2 /H 2 WO 4 /Fe 2 O 3 =100/10/4 weight ratio (TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.020 molar ratio). 
     In addition, from the above experimental results, it can be seen that a photocatalytic filter shows a high removal rate of each gas in mixed gases including three different gases (acetaldehyde, ammonia and acetic acid) and a high adhesion of the photocatalyst to the support, when a photocatalytic filter has a molar ratio of TiO 2 /H 2 WO4/Fe 2 O 3 =1.0/0.032/0.015. The temperature for performing the sintering step may be between 400° C. and 500° C. 
       FIG. 1  and Table 3 below show a comparison of deodorization performance between a conventional P25 photocatalytic filter and the photocatalytic filter of the present disclosure, which has a molar ratio of TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.015. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Removal rate (%) 
                 Removal rate (%) 
               
               
                   
                 after 30 minutes 
                 after 120 minutes 
               
            
           
           
               
               
               
               
               
            
               
                   
                 P25 
                 Photocatalytic 
                 P25 
                 Photocatalytic 
               
               
                   
                 photo- 
                 filter of 
                 photo- 
                 filter of 
               
               
                   
                 catalytic 
                 the present 
                 catalytic 
                 the present 
               
               
                 Gases 
                 filter 
                 disclosure 
                 filter 
                 disclosure 
               
               
                   
               
               
                 NH 3   
                 40% 
                 70% 
                 55% 
                 85% 
               
               
                 CH 3 CHO 
                  0% 
                 20% 
                 25% 
                 60% 
               
               
                 CH 3 COOH 
                 50% 
                 50% 
                 85% 
                 75% 
               
               
                 Total 
                 22.5%     
                 40% 
                 47.5%     
                 70% 
               
               
                   
               
            
           
         
       
     
     As can be seen in Table 3 above and  FIG. 1 , the photocatalytic filter of the present disclosure, which has a molar ratio of TiO 2 /H 2 WO 4 /Fe 2 O 3 =1.0/0.032/0.015, has significantly excellent deodorization performance compared to the conventional P25 photocatalytic filter. 
     As described above, the photocatalytic filter of the present disclosure shows a high removal rate of each gas in the mixed gases including three different gases (acetaldehyde, ammonia and acetic acid). In addition to these gases and combinations of these gases, the photocatalytic filter of the present disclosure is also effective against other gases and combinations thereof if these gases are well absorbed onto the surface of the photocatalytic filter. 
     As described above, the photocatalytic filter according to the present disclosure shows a high removal rate of each gas in mixed gases. 
     In addition, according to the method for manufacturing the photocatalytic filter according to the present disclosure, the photocatalyst has high adhesion to the support. 
       FIG. 2  is a perspective view showing the arrangement of the photocatalytic filter  80  and the UV LED substrate  55 , and  FIG. 3  is a top view of the photocatalytic filter  80 . 
     Referring to  FIG. 2 , the UV LED  56  for sterilization is disposed on the central portion of the UV LED substrate  55 , and three UV LEDs  57  for photocatalytic activation are disposed around the UV LED  56 . For example, the UV LEDs  57  for photocatalytic activation will irradiate UV light toward the photocatalytic filter  80 . 
     As shown in  FIG. 3 , the photocatalytic filter  80  includes a catalyst portion  81  obtained by sintering TiO 2  (titanium dioxide) coated on a ceramic porous material having a check lattice pattern, and an elastic bumper  82  covering the side of the catalyst portion. 
       FIG. 4  is a graph showing removal rates of acetaldehyde of two photocatalytic filters that have different height (h), and  FIG. 5  is a graph showing removal rates of acetic rate of two photocatalytic filters that have different height (h).  FIGS. 6 a  to 6 c    show graphs showing removal rates of Toluene, Ammonia, and Acetic acid with a change in a height of the photocatalytic filter exemplarily implemented according to the present disclosure. 
     The results of the experiment indicated that, in the case of the photocatalytic filter having the shape shown in  FIGS. 2 and 3 , the surface area of the photocatalyst, which increases due to the thickness (t) of the frame between the cells of the photocatalytic filter, did not substantially influence the deodorization efficiency of the photocatalytic filter, but the height (depth) of the photocatalytic filter influenced the inner wall area of the internal air flow path, thus directly influencing the area of contact with air. 
     Based on the test results, it could be seen that, when the height of the photocatalytic filter was 5-15 mm, the deodorization efficiency of the photocatalytic filter was the highest. In addition, when the height decreases to 2 mm or less, the photocatalytic filter is difficult to use, due to its weak strength. When the height is 15 mm or more, air resistance increases, UV light does not reach the rear portion of the photocatalytic filter or the intensity thereof becomes very weak, and thus only the cost increases without increasing the deodorization efficiency. 
     Also, the implementations of the disclosed technology having the desired thickness satisfy industrial standards provided and recommended by Korean Ozone UV Association, which require the deodorization rates of harmful substances for an air purifier or sterilizer to be over 30%. Among the various heights, 3T, 5T, 8T, 10T, 12T, 15T, 24T, the heights of 8T, 10T, 12T satisfy this requirement. The removal rates for Toluene show relatively large variations depending on the height of the photocatalytic filter, while the removal rates for Ammonia and Acetic acid show relatively small variations depending on the height of the photocatalytic filter. Although the removal rates for the height 12T are shown as 29.5%, the height 12T can be deemed to satisfy the 30% requirement in light of an error margin depending on various situations for a test. 
     Also, it could be seen that, when the width (g) of each cell  83  was 2 mm, the air resistance did not increase, and the rate of shadowed area of the inner wall of the photocatalytic filter, which is generated by the shape of the filter itself blocking UV light irradiated thereto, was not high, suggesting that the cell width of 2 mm is most suitable for maximizing the rate of UV light irradiated area of the inner wall of the photocatalytic filter. When the cell width was 4 mm or more, the whole area of the inner wall decreased due to low cell density, suggesting that the efficiency of deodorization was low. 
     Regarding the density of cells in view of width (g) of each cell above mentioned, when the density of cells was lower than 30 cells/inch 2  or less, that is the cell width increased to 4 mm or more, the area of the inner wall decreased, indicating that the efficiency of deodorization was low. When the density of cells was 260 cells/inch 2  or more, that is the cell width decreased to 1 mm or less, the air resistance increased and the amount of UV light reaching the inner wall decreased, indicating that the efficiency of deodorization was low. When the density of cells was about 100 cells/inch 2 , the air resistance did not increase, and the rate of shadowed area of the inner wall of the filter, which is generated by the shape of the filter itself blocking UV light irradiated thereto, was not high, suggesting that the efficiency of deodorization was the highest. 
     The results of an experiment on the thickness (t) of the cell frame indicated that, when the frame thickness was 0.3 mm or less, the TiO 2  layer became too thin, and thus the photocatalytic efficiency decreased and the strength was insufficient. When the frame thickness was 1.2 mm or more, the material cost increased without increasing the photocatalytic efficiency. In addition, the photocatalytic efficiency was the highest when the frame thickness was 0.6 mm. 
       FIGS. 7 a  to 7 d    show test results on removal efficiencies of an exemplary harmful gas of Acetaldehyde. To conduct the test, various factors of a photocatalytic filter are varied by using cells with different specifications. Cells with different specifications, for examples, 100 cpsi and 200 cpsi, are available in the market. The cells with the specifications which are used in the test but are not available in the market, for example, 25 cpsi, 50 cpsi, are prepared by selectively overlapping the available cells. The cell specification 100 cpsi ×2 indicates two overlapping 100 cpsi cells. The cell specification 100+200 cpsi indicates 200 cpsi cell and 100 cpsi cell that are overlapping each other. 
     Table 4 below shows the test results of removing rates of Acetaldehyde at 1 hour after the start of the test. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Removal Rate At 1 Hour After Start of Test 
               
            
           
           
               
               
            
               
                   
                 Cell Specification 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 25 
                 50 
                 100 
                 100 
                 100 + 200 
               
               
                   
                 cpsi 
                 cpsi 
                 cpsi 
                 cpsi × 2 
                 cpsi 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TiO 2  Coating 
                 80 
                 160 
                 320 
                 640 
                 858 
               
               
                 Area (cm 2 ) 
               
               
                 Percentage of 
                 264.5% 
                 528.9% 
                 1057.9% 
                 2115.7% 
                 2836.4% 
               
               
                 Coating Area 
               
               
                 over Filter 
               
               
                 Area (%) 
               
               
                 Cell Width (mm) 
                 8 
                 4.0 
                 2.0 
                 2.0 
                 0.8 
               
               
                 Air Velocity 
                 1.5 
                 1.32 
                 1.25 
                 1.23 
                 1.05 
               
               
                 (m/s) 
               
               
                 Removal Effi- 
                 15.2 
                 23.7 
                 31.3 
                 33.5 
                 35.5 
               
               
                 ciency (%) 
               
               
                   
               
            
           
         
       
     
     By using different cells, TiO 2  coating area, a ratio between the TiO 2  coating area and filter size, and a cell width of the photocatalytic filter are varied. The test results in  FIG. 7 a    show the cell specifications having 100 cpsi and 100 cpsi+200 cpsi have removal efficiencies over 30%. The test results in  FIG. 7 b    show that as the TiO 2  coating area increases, the removal efficiencies increase. Please note that the removal efficiencies do not increase in a linear proportion to increasing the area of the TiO 2  coating. Despite the increase of the TiO 2  coating area, the removal efficiencies of the filter does not increase as much as the increase in the TiO 2  coating area since the smaller cell size increases an air velocity passing through the filter. Table 4 above shows that the air velocity decreases as the cell size becomes smaller. Based on  FIG. 7 c   , when the coating area is greater than 10 times of the filter size, the removal efficiencies become higher than 30%. The filter size mentioned can be decided depending on the filter type as shown in the table below. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Data on Filter and Cell Properties 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 TiO2 
                 Filter 
                 Cell 
                 Aperture 
                 Test Air 
                 Filter Face 
               
               
                 Module 
                 Filter 
                 Size 
                 Area 
                 Ratio 
                 Volume 
                 Velocity 
               
               
                 Type 
                 (mm) 
                 (mm2) 
                 (mm2) 
                 (%) 
                 (m 3 /min) 
                 (m/s) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Fridge 
                 33 × 33 
                 1,089 
                 676 
                 62.1% 
                 0.17 
                 4.19 
               
               
                 P-AP 
                 55 × 55 
                 3,025 
                 1,600 
                 52.9% 
                 0.08 
                 0.83 
               
               
                 Fridge 
                 55 × 55 
                 3,025 
                 1,600 
                 52.9% 
                 0.17 
                 1.77 
               
               
                   
                   
                   
                   
                   
                 0.30 
                 3.13 
               
               
                   
                   
                   
                   
                   
                 0.41 
                 4.27 
               
               
                 Mid-sized 
                 75 × 75 
                 5,625 
                 3,364 
                 59.8% 
                 3.53 
                 4.37 
               
               
                 Compact 
                 100 × 100 
                 10,000 
                 7,569 
                 75.7% 
                 1.30 
                 2.86 
               
               
                 Meiling 
                 100 × 40  
                 4000 
                 2340 
                 58.5% 
                 0.33 
                 2.35 
               
               
                   
               
            
           
         
       
     
     When a filter size is fixed, if a cell width becomes smaller, there are more cells included in the filter, which makes the TiO 2  coating area greater. Based on the results shown in  FIG. 7 d   , when the cell width is not greater than 2 mm, the removal efficiency is above 29%. In light of an error margin, the cell width not greater than 2 mm satisfies 30% removal efficiency of Acetaldehyde. Based on the result, the photocatalytic filter may be designed such that each cell extends no more than 2 mm in a two-dimensional horizontal and vertical directions. 
     The results in Table 4 above show that, to improve and optimize the filter efficiency, the increase of the TiO 2  coating area needs to be limited in consideration of the air velocity. The increase of the TiO 2  coating area also has some limitations in terms of difficulties of the fabrications, increase in costs, etc. The manufacturing process for the photocatalytic filter would become more difficult as more cells are included in a limited area. 
     The disclosed technology provides various considerations to provide the photocatalytic filter exhibiting desirable removing rates of harmful gases. Based on the disclosed technology, the photocatalytic filter can be specifically designed to improve its performance. 
     While various embodiments have been described above, it will be understood to those skilled in the art that the embodiments described are by way of example only. Accordingly, the disclosure described herein should not be limited based on the described embodiments.