Patent Publication Number: US-2005129591-A1

Title: Bifunctional layered photocatalyst/thermocatalyst for improving indoor air quality

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates generally to a photocatalyst/thermocatalyst including an inner layer of metal/titanium dioxide or metal oxide/titanium dioxide and an outer layer of titanium dioxide or metal oxide/titanium dioxide that oxidizes gaseous contaminants in the air that adsorb onto the photocatalytic/thermocatalytic surface to form carbon dioxide, water, and other substances.  
      Indoor air can include trace amounts of contaminants, including carbon monoxide, ozone and volatile organic compounds such as formaldehyde, toluene, propanal, butene, and acetaldehyde. Absorbent air filters, such as activated carbon, have been employed to remove these contaminants from the air. As air flows through the filter, the filter blocks the passage of the contaminants, allowing contaminant free air to flow from the filter. A drawback to employing filters is that they simply block the passage of contaminants and do not destroy them. Additionally, the filter is not effective in blocking ozone and carbon monoxide.  
      Titanium dioxide has been employed as a photocatalyst in an air purifier to destroy contaminants. When the titanium dioxide is illuminated with ultraviolet light, photons are absorbed by the titanium dioxide, promoting an electron from the valence band to the conduction band, thus producing a hole in the valence band and adding an electron in the conduction band. The promoted electron reacts with oxygen, and the hole remaining in the valence band reacts with water, forming reactive hydroxyl radicals. When a contaminant adsorbs onto the titanium dioxide catalyst, the hydroxyl radicals attack and oxidize the contaminants to water, carbon dioxide, and other substances.  
      Doped or metal oxide treated titanium dioxide can increase the effectiveness of the titanium dioxide photocatalyst. However, titanium dioxide and doped titanium dioxide are less effective or not effective in oxidizing carbon monoxide. Carbon monoxide (CO) is a colorless, odorless, and poisonous gas that is produced by the incomplete combustion of hydrocarbon fuels. Carbon monoxide is responsible for more deaths than any other poison and is especially dangerous in enclosed environments. Gold can be loaded on the titanium dioxide to act as an effective thermocatalyst for the room temperature oxidation of carbon monoxide to carbon dioxide.  
      Photocatalytically, titanium dioxide alone is less effective in decomposing ozone. Ozone (O 3 ) is a pollutant that is released from equipment commonly found in the workplace, such as copiers, printer, scanners, etc. Ozone can cause nausea and headaches, and prolonged exposure to ozone can damage nasal mucous membranes, causing breathing problems. OSHA has set a permissible exposure limit (PEL) to ozone of 0.08 ppm over an eight hour period.  
      Ozone is a thermodynamically unstable molecule and decomposes very slowly up to temperatures of 250° C. At ambient temperatures, manganese oxide is effective in decomposing ozone by facilitating the oxidation of ozone to adsorbed surface oxygen atoms. These adsorbed oxygen atoms then combine with ozone to form an adsorbed peroxide species that desorbs as molecular oxygen.  
      Hence, there is a need for catalyst that oxidizes and decomposes gaseous contaminants, including volatile organic compounds, carbon monoxide and ozone, that adsorb onto the photocatalytic surface to form carbon dioxide, water, oxygen and other substances.  
     SUMMARY OF THE INVENTION  
      A layered photocatalytic/thermocatalytic coating on a substrate purifies the air in a building or a vehicle by oxidizing or decomposing contaminants that adsorb onto the coating to water, oxygen, carbon dioxide, and other substances.  
      A fan draws air into an air purification system. The air flows through an open passage or channel of a honeycomb. The surface of the honeycomb is coated with a layered photocatalytic/thermocatalytic coating. An ultraviolet light source positioned between successive honeycombs activates the coating. The coating includes an inner layer of a metal/titanium dioxide or metal oxide/titanium dioxide coating and an outer layer of a titanium dioxide or metal oxide/titanium dioxide coating.  
      In one example, the inner layer is gold/titanium dioxide. At room temperature, the inner layer of gold/titanium dioxide oxidizes carbon monoxide to carbon dioxide. When carbon monoxide adsorbs on the gold/titanium dioxide coating, the gold acts as an oxidation catalyst and lowers the energy barrier of the oxidation of carbon monoxide to carbon dioxide in the presence of oxygen.  
      In another example, the inner layer is manganese oxide/titanium dioxide. At room temperature, the manganese oxide/titanium dioxide coating decomposes ozone to oxygen. When ozone adsorbs on the coating, the manganese oxide lowers the energy barrier required for ozone decomposition, decomposing the ozone to molecular oxygen.  
      In another example, the inner layer is platinum/titanium dioxide. At room temperature, the platinum/titanium dioxide coating oxidizes low polarity organic compounds to carbon dioxide. Low polarity organic compounds have an increased affinity to platinum. The low polarity organic compounds adsorb onto the platinum and are oxidized by the hydroxyl radicals to carbon dioxide and water in the presence of oxygen.  
      The outer layer oxidizes volatile organic compounds to carbon dioxide, water and other substances. The outer layer is thin, porous and not opaque to ultraviolet light. Therefore, carbon monoxide, ozone and low polarity organic compounds can diffuse through the outer layer and absorb on the metal/titanium dioxide or metal oxide/titanium dioxide inner layer for catalysis. Additionally, the outer layer allows the ultraviolet light to penetrate and reach the inner layer.  
      When photons of the ultraviolet light are absorbed by the coating, reactive hydroxyl radicals are formed. When a contaminant is adsorbed onto the coating, the hydroxyl radical attacks the contaminant, abstracting a hydrogen atom from the contaminant and oxidizing the volatile organic compounds to water, carbon dioxide, and other substances.  
      These and other features of the present invention will be best understood from the following specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The various features and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:  
       FIG. 1  schematically illustrates an enclosed environment, such as a building, vehicle or other structure, including an interior space and an HVAC system;  
       FIG. 2  schematically illustrates the air purification system of the present invention;  
       FIG. 3  schematically illustrates the honeycomb of the air purification system;  
       FIG. 4  schematically illustrates the coating of the present invention;  
       FIG. 5  schematically illustrates an alternate application of the coating of the present invention;  
       FIG. 6  schematically illustrates an alternate embodiment of the air purification system employing two honeycombs each with a different coating;  
       FIG. 7  schematically illustrates an another alternate embodiment of the air purification system employing two honeycombs each with a different coating;  
       FIG. 8  schematically illustrates adjacent honeycombs of the air purification system of the present invention;  
       FIG. 9  schematically illustrates adjacent honeycombs of the air purification system of the present invention that are bonded together by an adhesive or attachment mechanism; and  
       FIG. 10  schematically illustrates another alternate orientation of the honeycombs of the air purification system of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       FIG. 1  schematically illustrates a building, vehicle, or other structure  10  including an interior space  12 , such as a room, an office or a vehicle cabin, such as a car, train, bus or aircraft. An HVAC system  14  heats or cools the interior space  12 . Air in the interior space  12  is drawn by a path  16  into the HVAC system  14 . The HVAC system  14  changes the temperature of the air drawn  16  from the interior space  12 . If the HVAC system  14  is operating in a cooling mode, the air is cooled. Alternately, if the HVAC system  14  is operating in a heating mode, the air is heated. The air is then returned back by a path  18  to the interior space  12 , changing the temperature of the air in the interior space  12 .  
       FIG. 2  schematically illustrates an air purification system  20  employed to purify the air in the building or vehicle  10  by oxidizing contaminants, such as volatile organic compounds, semi-volatile organic compounds, carbon monoxide and ozone, in the air to water, carbon dioxide, and other substances. For example, the volatile organic compounds can be aldehydes, ketones, alcohols, aromatics, alkenes, alkanes or mixtures thereof. The air purification system  20  can purify air before it is drawn along path  16  into the HVAC system  14  or it can purify air leaving the HVAC system  14  before it is blown along path  18  into the interior space  12  of the building or vehicle  10 . The air purification system  20  can also be a stand alone unit that is not employed with a HVAC system  14 .  
      A fan  34  draws air into the air purification system  20  through an inlet  22 . The air flows through a particle filter  24  that filters out dust or any other large particles by blocking the flow of these particles. The air then flows through a substrate  28 , such as a honeycomb. In one example, the honeycomb  28  is made of aluminum or an aluminum alloy.  FIG. 3  schematically illustrates a front view of the honeycomb  28  having a plurality of hexagonal open passages or channels  30 . The surfaces of the plurality of open passages  30  are coated with a photocatalytic/thermocatalytic coating  40 . When activated by ultraviolet light, the coating  40  oxidizes volatile organic compounds that adsorb onto the coating  40 . As explained below, as air flows through the open passages  30  of the honeycomb  28 , contaminants that are adsorbed on the surface of the coating  40  are oxidized into carbon dioxide, water and other substances.  
      A light source  32  positioned between successive honeycombs  28  activates the photocatalytic coating  40  on the surface of the open passages  30 . As shown, the honeycombs  28  and the light source  32  alternate in the air purification system  20 . That is, there is a light source  32  located between each of the honeycombs  28 . Preferably, the light source  32  is an ultraviolet light source which generates light having a wavelength in the range of 180 nanometers to 400 nanometers.  
      The light source  32  is illuminated to activate the coating  40  on the surface of the honeycomb  28 . When the photons of the ultraviolet light are absorbed by the coating  40 , an electron is promoted from the valence band to the conduction band, producing a hole in the valence band. The coating  40  must be in the presence of oxygen and water to oxidize the contaminants into carbon dioxide, water, and other substances. The electrons that are promoted to the conduction band are captured by the oxygen. The holes in the valence band react with water molecules adsorbed on the photocatalytic/thermocatalytic coating  40  to form reactive hydroxyl radicals.  
      When a contaminant is adsorbed onto the coating  40 , the hydroxyl radical attacks the contaminant, abstracting a hydrogen atom from the contaminant. In this method, the hydroxyl radical oxidizes the contaminants and produces water, carbon dioxide, and other substances.  
      As shown in  FIG. 4 , the coating  40  includes an inner layer  44  of a metal/titanium dioxide or metal compound/titanium dioxide thermocatalyst/photocatalyst applied on the honeycomb  28  and an outer layer  46  of a titanium dioxide or metal compound/titanium dioxide photocatalyst applied on the inner layer  44 . Preferably, the metal compound/titanium dioxide of the inner layer  44  and the outer layer  46  are metal oxide/titanium dioxide.  
      The outer layer of titanium dioxide  46  or metal oxide/titanium dioxide is effective in oxidizing volatile organic compounds and semi-volatile organic compounds to carbon dioxide, water and other substances. The outer layer  46  has an effective thickness and porosity. That is, the outer layer  46  is able to allow other contaminants that are not oxidized by the outer layer  46 , such as carbon monoxide, to pass through the outer layer  46  and adsorb on the inner layer  44 . In one example, the outer layer  46  is visibly white and not opaque to ultraviolet light.  
      Preferably, the photocatalyst is titanium dioxide. In one example, the titanium dioxide is Millennium titania, Degussa P-25, or an equivalent titanium dioxide. However, it is to be understood that other photocatalytic materials or a combination of titanium dioxide with other metal oxides can be employed. For example, the photocatalytic materials can be Fe 2 O 3 , ZnO, V 2 O 5 , SnO 2 , FeTiO 3  or mixtures thereof.  
      Additionally, one or more of other metal oxides can be mixed with titanium dioxide, such as Fe 2 O 3 , ZnO, V 2 O 5 , SnO 2 , CuO, MnOx, WO 3 , CO 3 O 4 , CeO 2 , ZrO 2 , SiO 2 , Al 2 O 3 , Cr 2 O 3 , or NiO.  
      Additionally, if the outer layer  46  is a metal oxide loaded titanium dioxide, the titanium dioxide can be doped with one or more of WO 3 , ZnO, SrTiO 3 , Fe 2 O 3 , V 2 O 5 , SnO 2 , FeTiO 3 , PbO, CO 3 O 4 , NiO, CeO 2 , CuO, SiO 2 , Al 2 O 3 , MnO 2 , Cr 2 O 3 , or ZrO 2 . Alternately, the titanium dioxide can be loaded with any photocatalytic material, such as CdS or CdSe.  
      In one example, the outer layer  46  has a thickness of less than 2 μm of titanium dioxide or metal oxide doped titanium dioxide that is applied over the inner layer  44 . The outer layer  46  can be applied to the surface of the inner layer  44  by spraying, electrophoresis, dip coating, or an alternate suitable method of deposition. In one example, a 25% weight aqueous suspension of photocatalyst is prepared. The suspension can be sprayed on the substrate coated with the inner layer  44 . After the suspension is applied, the substrate is allowed to dry, forming a uniform outer layer  46  on the inner layer  44  on the honeycomb  28 .  
      In a first example, the inner layer  44  is gold/titanium dioxide. At room temperature, the inner layer  44  oxidizes carbon monoxide to carbon dioxide. When carbon monoxide adsorbs on the coating, the gold/titanium dioxide acts as a thermal catalyst and lowers the energy barrier of the carbon monoxide, oxidizing the carbon monoxide to carbon dioxide in the presence of oxygen. Titanium dioxide is an effective support for low temperature carbon monoxide oxidation. Additionally, gold/titanium dioxide is an effective photocatalyst to oxidize volatile organic compounds that diffuse through the outer layer  46  to water and carbon dioxide. Therefore, the inner layer  44  acts simultaneously as both a photocatalyst and a thermocatalyst.  
      Carbon monoxide oxidation occurs mainly on the perimeter interface of the gold particles. Carbon monoxide is adsorbed on either surface or perimeter sites of the gold to form carbonyl species. Oxygen is adsorbed on the gold/titanium dioxide surface. It is believed that the oxygen is adsorbed onto the perimeter interface. The carbonyl species on the perimeter sites react with the oxygen, forming an oxygen-gold-carbon monoxide complex. The complex is decomposed to produce carbon dioxide.  
      In the case of photocatalytic function, the highly dispersed gold particles on the surface of the titanium dioxide reduce the recombination rate of the electrons and the holes in the inner layer  44 , increasing the photocatalytic activity of the coating. Preferably, the gold particles have a size less than 3 nanometers. For the thermocatalytic function, the size of the gold particles is also critical to the activity of the carbon monoxide oxidation, which is dependent on the gold being formed into very small nano-particles.  
      The catalytic performance of the gold/titanium dioxide coating is influenced by the preparation method. The catalytic activity of gold is dependent on the gold being formed into nano-particles. The nano-particles of gold can be generated by any method, including co-precipitation, deposition-precipitation, liquid phase grafting, colloidal mixing, impregnation, or chemical vapor deposition.  
      In the co-precipitation method, a catalyst is prepared by mixing an aqueous solution of gold precursor and an aqueous solution of titanium precursor at room temperature or at a slightly elevated temperature and at a constant pH. The precipitate is filtered and washed thoroughly with distilled water and is dried at 70° C. under vacuum overnight. After drying, the product is calcined at a range of 200° C. to 500° C. form a dried gold/titanium dioxide photocatalyst/thermocatalyst.  
      In the deposition-precipitation method, titanium dioxide powder is suspended in distilled water a desired amount of HAuCl 4 . Urea is added slowly to the mixture, and the mixture is then heated to 80° to 90° C., decomposing the urea to release NH 4 OH (ammonium hydroxide) and carbon dioxide, thus increasing the pH of the mixture. The slow increase in pH induces homogeneous precipitation of Au(OH) 3  onto the surface of the titanium dioxide. The sample is washed thoroughly in distilled water to remove residual chloride ions. The sample is then dried at 70° C. under vacuum overnight. The sample is then calcined at temperatures from 200° C. to 500° C. to form a dried gold/titanium dioxide photocatalyst/thermocatalyst. An advantage to the deposition-precipitation method is that all of the active components remain on the surface of the titanium dioxide support and are not buried within it.  
      In liquid phase grafting method, a gold complex in solution reacts with the surface of a support, such as titanium dioxide, forming species convertible to a catalytically active form. Me 2 Au can be used as a gold precursor. The precursor is dissolved into acetone and then titanium dioxide is added to the solvent. This mixture is allowed to settle so that the gold precursor adsorbs onto the metal oxide surface. The mixture is then filtered and calcined at 400° C. for 4 hours.  
      In one example, to coat the bifunctional catalyst on the honeycomb  28 , water is added to the dried gold/titanium dioxide photocatalyst/thermocatalyst to form an aqueous 25% weight suspension. The suspension is applied to the surface of the honeycomb  28  by spraying, electrophoresis, or dip coating to form the gold/titanium dioxide inner layer  44 . After the suspension is applied, the substrate is allowed to dry, forming a uniform gold/titanium dioxide inner layer  44  on the honeycomb  28 .  
      Before the gold/titanium dioxide suspension is applied to the honeycomb  28 , the suspension can be treated to increase its adhesion to the honeycomb  28 . For example, the suspension can be homogenized by using a homogenizer with a dispersing generator at a speed of 7500 rpm. When the suspension is applied to the honeycomb  28 , the coating is porous on a nanometer scale and usually has a surface area greater than 40 m 2 /g. The inner layer  44  is then allowed to dry on the honeycomb  28 . The inner layer  44  can also be heated to an effective temperature.  
      The titanium dioxide can also be loaded with a metal oxide to further improve the photocatalytic and thermocatalytic effectiveness of the gold/titanium dioxide inner layer  44 . Gold has a tendency to migrate on the surface of the titanium dioxide to form large clusters. The effectiveness of the gold/titanium dioxide inner layer  44  can be reduced due to the migration of gold particles. By loading a metal oxide on the surface of the titanium dioxide, the metal oxide can separate the gold particles and prevent them from migrating and forming large clusters, therefore increasing the effectiveness of the gold/titanium dioxide inner layer  44 . Preferably, a metal oxide is employed to immobilize the gold particles on the surface of the titanium dioxide. In one example, the metal oxide is one or more of WO 3 , ZnO, CdS, SrTiO 3 , Fe 2 O 3 , V 2 O 5 , SnO 2 , FeTiO 3 , PbO, CeO 2 , CuO, Sio 2 , Al 2 O 3 , MnOx, Cr 2 O 3 , or ZrO 2 .  
      In another example, the inner layer  44  is platinum/titanium dioxide. At room temperature, the inner layer  44  oxidizes low polarity organic compounds to carbon dioxide simultaneously with oxidation of harmful volatile organic compounds. Low polarity organic molecules have an in creased affinity to platinum. When low polarity organic compounds adsorbs on the platinum, the platinum retains the low polarity organic compounds on the inner layer  44  oxidation by the hydroxyl radicals, oxidizing the low polarity organic compounds to carbon dioxide in the presence of oxygen.  
      Platinum dispersed on titanium dioxide exhibits photocatalytic behavior for low contaminant concentrations, such as below 50 ppm. The photocatalytic oxidation rate of ozone, ethylene and butane is greater for platinum on titanium dioxide that for titanium dioxide alone. The photocatalytic oxidation rate is double for ozone and butane and between 2 to 14 times for ethylene over platinum on titanium dioxide. The photocatalytic oxidation rate of ethylene depends on humidity and ethylene concentrations. Surprisingly, the photocatalytic oxidation of these contaminants increases with increasing water vapor. In contrast, the photocatalytic oxidation of contaminants with titanium dioxide alone decreases with increased humidity.  
      The highly dispersed platinum particles on the surface of the titanium dioxide reduce the recombination rate of the electrons and the holes, increasing the photocatalytic activity of the coating. Preferably, the platinum particles have a size less than 5 nanometers and form platinum islands of about 1.0-1.5 nanometers. The preferred platinum loading is between 0.1% and 5.0%.  
      In another example, the inner layer  44  is manganese oxide/titanium dioxide. Manganese oxide includes manganese dioxide and doped manganese oxide. At ambient temperatures, manganese oxide is effective in decomposing ozone. Manganese oxide facilitates the decomposition of ozone to adsorbed surface oxygen atoms. These oxygen atoms then combine with ozone to form an adsorbed peroxide species that desorbs as molecular oxygen. When ozone adsorbs on the manganese oxide, the manganese oxide acts as a site for dissociative ozone adsorption by lowering the energy barrier required for ozone decomposition. Therefore, in the presence of ozone alone, the manganese oxide produces oxygen.  
      Additionally, the peroxide species are highly reactive and assist in the oxidation of volatile organic compounds to carbon dioxide and water. Therefore, the manganese oxide can be highly effective in oxidizing volatile organic compounds. In the presence of volatile organic compounds alone, the manganese oxide inner layer  44  of the coating  40  produces carbon dioxide, water, and other substances. Therefore, the manganese dioxide photocatalytic/thermocatalytic coating acts simultaneously as both a photocatalyst and a thermocatalyst.  
      At room temperature, the manganese oxide/titanium dioxide inner layer  44  of the coating  40  decomposes ozone to oxygen simultaneously with oxidation of harmful volatile organic compounds to carbon dioxide, water, and other substances. Therefore, the manganese oxide/titanium dioxide photocatalytic/thermocatalytic coating acts simultaneously as both a photocatalyst and a thermocatalyst.  
      The highly dispersed manganese oxide particles on the surface of the titanium dioxide reduce the recombination rate of the electrons and the holes, increasing the photocatalytic activity of the coating. Preferably, the manganese oxide particles are nano-sized.  
      The catalytic performance of the manganese oxide/titanium dioxide coating is influenced by the preparation method. The nano-particles of manganese oxide can be generated by deposition-precipitation, co-precipitation, impregnation, or chemical vapor deposition. By employing these methods, nano-particles of manganese oxide can be generated, improving the catalytic activity.  
      To prepare the manganese oxide/titanium dioxide photocatalyst/thermocatalyst of the present invention, water is added drop-wise to powder titanium dioxide to determine the point at which the pores in the titanium dioxide are filled with water, or the point of incipient wetness. This amount of water is then used to dissolve a manganese salt (manganese nitrate or preferably manganese acetate). The amount of manganese salt needed is determined by the mole percentage of manganese targeted for the surface, usually 0.1 to 6 mol %.  
      The manganese salt solution is then added drop-wise to the titanium dioxide. The resulting powder is then dried at 120° C. for six hours. The powder is then calcined at 500° C. for six hours to remove the acetate and nitrate. During calcination, the manganese is oxidized to form manganese oxide. After calcination, a titanium dioxide powder layered with manganese oxide nano-particles is created.  
      To coat manganese oxide/titanium dioxide bifunctional catalyst to a substrate, water is added to the dried manganese oxide/titanium dioxide photocatalyst/thermocatalyst to form a suspension. The suspension is applied to the surface of the honeycomb  28  by spraying, electrophoresis, or dip coating to form the manganese oxide/titanium dioxide inner layer  44 . After the suspension is applied, the suspension is allowed to dry, forming a uniform manganese oxide/titanium dioxide inner layer  44  on the honeycomb  28 . Preferably, the suspension has weight 1% of manganese oxide on titanium dioxide.  
      When a metal is doped on titanium dioxide, the effective penetration depth of light is reduced. Therefore, it is desirable to locate the layer with the smaller effective penetration depth on the honeycomb  28  followed by the layer with the greater effective penetration depth of light. Therefore, the layer with the greatest effective penetration depth of light is closest to the light source  32 . The inner layer  44  has a smaller effective penetration depth and is deposited on the honeycomb  28  first. The outer layer  46  has a greater effective penetration depth and is then deposited on the inner layer  44 .  
      The thickness of the outer layer  46  (the layer with the greatest effective penetration depth) can be adjusted to absorb only part of the light from the light source  44 , allowing some or none of the light to reach the inner layer  44 . If none of the light from the light source  32  reaches the inner layer  44 , the porosity of the outer layer  46  allows penetration of contaminants into the inner layer  44 . Therefore, contaminants such as carbon monoxide can be oxidized and contaminants such as ozone can be decomposed on the inner layer  44 . In this case, the inner layer  44  serves as a thermocatalyst only. If some of the ultraviolet light from the light source  32  reaches and is absorbed by the inner layer  44 , the inner layer  44  can be bifunctional as a photocatalyst and a thermocatalyst. The outer layer  46  applied over the inner layer  44  is directly exposed to the ultraviolet light and can provide the photocatalytic activity to oxidize contaminants to carbon dioxide, water and other substances. Additionally, the outer layer  46  is porous to allow carbon monoxide, ozone, and low polarity organic compounds to pass through the outer layer  46  and adsorb onto the inner layer  44 .  
      The inner layer  44  can be selected based on environment. If the air has a high concentration of ozone, manganese oxide/titanium dioxide can be used as the inner layer  44 . Alternately, if the air has a high concentration of carbon monoxide, gold/titanium dioxide can be used as the inner layer  44 .  
      After passing through the honeycombs  28 , the purified air then exits the air purifier through an outlet  36 . The walls  38  of the air purification system  20  are preferably lined with a reflective material  42 . The reflective material  42  reflects the ultraviolet light onto the surface of the open passages  30  of the honeycomb  28 .  
       FIG. 5  illustrates an alternate embodiment of the bifunctional coating  40  of the present invention. The coating  40  includes a layer  44  of a metal/titanium dioxide or metal compound/titanium dioxide thermocatalyst/photocatalyst applied on a portion of the surface  54  of the honeycomb  28  and a layer  46  of a titanium dioxide or metal compound/titanium dioxide photocatalyst applied on another portion of the surface  54  of the honeycomb  28 .  
      In another embodiment, different coating formulations are placed on different substrates to increase the design flexibility of the system  20  and to change the overall effectiveness of the system  20 .  
       FIG. 6  illustrates an alternate example of the air purification system  56 . In this example, the air first flows through a first honeycomb  58  having a gold/titanium dioxide coating which performs as a bifunctional photocatalyst/thermocatalyst. Due to its thermocatalytic function, the gold/titanium dioxide coating can oxidize carbon monoxide to carbon dioxide. Simultaneously, due to its photocatalytic function, the gold/titanium dioxide coating can oxidize volatile organic compounds, particularly formaldehyde to carbon dioxide and water. The gold/titanium dioxide catalyst provides superior photocatalytic activity over titanium dioxide alone in formaldehyde oxidation.  
      The air then flows through a second honeycomb  60  having a metal oxide doped titanium dioxide coating. The metal oxide can be one or more of WO 3 , ZnO, SrTiO 3 , Fe 2 O 3 , V 2 O 5 , SnO 2 , FeTiO 3 , PbO, CO 3 O 4 , NiO, CeO 2 , CuO, SiO 2 , Al 2 O 3 , MnxO 2 , Cr 2 O 3 , or ZrO 2 . The metal oxide doped titanium dioxide coating on the second honeycomb  60  oxidizes the remaining contaminants from the first honeycomb  58 , such as volatile organic compounds and semi-volatile organic compounds, to water and carbon dioxide. Volatile organic compounds are classified as compounds having boiling points less than approximately 200° C., and semi-volatile organic compounds are classified as compounds having boiling points at or above 200° C.  
      By employing a first honeycomb  58  with a gold/titanium dioxide coating and a second honeycomb  60  with a metal oxide doped titanium dioxide coating, both carbon monoxide, volatile organic compounds, and semi-volatile organic compounds can be oxidized and destroyed. Therefore, the air purification system  56  including the gold/titanium dioxide coated first honeycomb  58  and the metal oxide doped titanium dioxide coated second honeycomb  60  perform the same function as the layered coating  40  having the inner layer  44  of gold/titanium dioxide and the outer layer  46  of metal oxide doped titanium dioxide.  
      In this configuration, the order of the first honeycomb  58  and the second honeycomb  60  is critical to the performance of the air purification system  56 . Compared to other volatile organic compound contaminants, formaldehyde has a relatively strong adsorption on the surface of titanium dioxide, covering the active sites that are otherwise available to other volatile organic compounds. Therefore, the removal of formaldehyde by the first honeycomb  58  significantly improves the photocatalytic activity of the second honeycomb  60  in the oxidation of other volatile organic compounds.  
       FIG. 7  illustrates an alternate example of the air purification system  62 . In this example, the air first flows through a first honeycomb  64  having a metal oxide doped titanium dioxide coating. The metal oxide can be one or more of WO 3 , ZnO, SrTiO 3 , Fe 2 O 3 , V 2 O 5 , SnO 2 , FeTiO 3 , PbO, CO 3 O 4 , NiO, CeO 2 , CuO, SiO 2 , Al 2 O 3 , MnxO 2 , Cr 2 O 3 , or ZrO 2 . The metal oxide doped titanium dioxide coating on the first honeycomb  64  oxidizes contaminants, such as volatile organic compounds and semi-volatile organic compounds, to water and carbon dioxide. The air then flows through a second honeycomb  66  having a manganese oxide/titanium dioxide coating to decompose ozone to oxygen and water. By employing a first honeycomb  64  with a metal oxide doped titanium dioxide coating and a second honeycomb  66  with a manganese oxide/titanium dioxide coating, both ozone, volatile organic compounds, and semi-volatile organic compounds can be oxidized and destroyed. Therefore, the air purification system  62  including the metal oxide doped titanium dioxide coated first honeycomb  64  and the manganese oxide/titanium dioxide coated second honeycomb  66  perform the same function as the layered coating  40  having the inner layer  44  of manganese oxide/titanium dioxide and the outer layer  46  of metal oxide doped titanium dioxide.  
      In this configuration, ozone is a strong oxidation agent and will assist in the photocatalytic oxidation. Therefore, it is preferred that the air flows through the metal oxide doped titanium dioxide coated first honeycomb  64  before flowing through the manganese oxide/titanium dioxide coated second honeycomb  66 . Alternately, the air purification system  62  includes more than one first honeycomb  64  and more than one second honeycomb  66 .  
      It is to be understood that alternates orientations of the honeycombs  58  and  60  of the air purification system  56  and the honeycombs  64  and  66  of the air purification system  62  are possible. As shown in  FIG. 8 , the air purification system  68  can include a first honeycomb  70  and a second honeycomb  72  located adjacent to each other in the air purification system  68 . That is, there is no lamp or light source located between the honeycombs  70  and  72 . Alternately, as shown in  FIG. 9 , the first honeycomb  70  and the second honeycomb  72  are attached or bonded together by an adhesive  74 . Alternately, the first honeycomb  70  and the second honeycomb  72  are attached by an attachment mechanism. Additional honeycombs  76  can also be employed with the air purification system  62 , as shown in  FIG. 10 . For example, the first honeycomb  70  and the second honeycomb  72  are positioned on one side of the light source  32  and an additional honeycomb  74  with a coating is positioned on the opposing side of the light source  32 . Although only one additional honeycomb  76  is illustrated and described, it is to be understood that any number of additional honeycombs  76  can be employed.  
      As explained above, the first honeycomb  70  can have a gold/titanium dioxide coating and the second honeycomb  72  can have a metal oxide doped titanium dioxide coating. Alternately, the first honeycomb  70  can have a metal oxide doped titanium dioxide coating and the second honeycomb  72  can have a manganese oxide/titanium dioxide coating to decompose ozone to oxygen and water. The additional honeycomb  76  can have any coating that produces the desired purification effect, and one skilled in the art would know what coating to employ on the additional honeycomb  76 .  
      Although a honeycomb  28  has been illustrated and described, it is to be understood that the coating  40  can be applied on any structure. The voids in a honeycomb  28  are typically hexagonal in shape and uniformly distributed, but it is to be understood that other void shapes and distributions can be employed. As contaminants adsorb onto the coating  40  of the structure in the presence of a light source, the contaminants are oxidized into water, carbon dioxide and other substances.  
      Additionally, a detailed description of coating processes are disclosed in co-pending patent application Ser. No. 10/449,752 filed May 30, 2003 entitled Tungsten Oxide/Titanium Dioxide Photocatalyst for Improving Indoor Air Quality, patent application Ser. No. 10/464,942 filed on Jun. 19, 2003 entitled Bifunctional Manganese Oxide/Titanium Dioxide Photocatalyst/Thermocatalyst for Improving Indoor Air Quality, and pending patent application Ser. No. 10/465,025 filed on Jun. 19, 2003 and entitled Bifunctional Gold/Titanium Dioxide Photocatalyst/Thermocatalyst for Improving Indoor Air Quality, the disclosures of which are incorporated by reference in its entirety. Related information on bifunctional manganese oxide/titanium dioxide photocatalyst/thermocatalyst is also disclosed in pending patent application Ser. No. 10/464,942. Related information on bifunctional gold/titanium dioxide photocatalyst/thermocatalyst is also disclosed in pending patent application Ser. No. 10/465,024.  
      The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.