Patent Publication Number: US-2005129589-A1

Title: Multi-layered photocatalyst/thermocatalyst for improving indoor air quality

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates generally to a multi-layer photocatalyst/thermocatalyst coating that decomposes ozone and oxidizes gaseous contaminants, including volatile organic compounds, low polarity organic molecules, and carbon monoxide, that adsorb onto the photocatalytic 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 (VOCs) such as formaldehyde, acetaldehyde, toluene, propanal and butene, etc. Absorbent air filters, such as activated carbon, have been employed to remove the volatile organic compounds 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. In additional, air filters are not effective in blocking carbon monoxide and ozone.  
      Titanium dioxide has been employed as a photocatalyst in an air purifier to destroy contaminants, especially polar organic molecules. 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 increases the effectiveness of the titanium dioxide photocatalyst. However, titanium dioxide and doped titanium dioxide are less effective or not effective in oxidizing carbon monoxide and low polarity organic molecules and decomposing ozone. 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 can build up in indoor air due to improper ventilation, cigarette smoke, or automobile emissions in outdoor air. Carbon monoxide poisoning can occur in the presence of small quantities of carbon monoxide over long periods of time. Sensitive organs such as the brain, heart, and lungs suffer most from a lack of oxygen. The EPA mandated exposure over an eight hour average is set at 30 ppm.  
      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.  
      Hence, there is a need for a multi-layer photocatalyst/thermocatalyst coating that decomposes ozone to oxygen and oxidizes carbon monoxide, low polarity organic compounds, and volatile organic contaminants that adsorb onto the photocatalytic surface to form carbon dioxide, water, and other substances.  
     SUMMARY OF THE INVENTION  
      A layered photocatalyst/thermocatalyst coating on a substrate purifies air in a building or a vehicle by decomposing and oxidizing any contaminants that adsorb onto the coating to oxygen, water, 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 a photocatalytic outer layer of titanium dioxide or metal oxide loaded titanium dioxide that oxidizes volatile organic compounds to carbon dioxide, water, and other substances. A photocatalytic intermediate layer of a noble metal/titanium dioxide coating is located under the outer layer. Beneath the intermediate layer is a photocatalytic/thermocatalytic inner layer of nano-dispersed gold on titanium dioxide that is applied on the honeycomb.  
      When photons of the ultraviolet light are absorbed by the outer layer of titanium dioxide, reactive hydroxyl radicals are formed. When a contaminant, such as a volatile organic compound, is adsorbed onto the coating, the hydroxyl radical attacks the volatile organic compound, abstracting a hydrogen atom from the volatile organic compound and oxidizing the volatile organic compound to water, carbon dioxide, and other substances. The outer layer has a thickness less than 2 μm to allow the photons to penetrate the outer layer to reach the underlying photocatalytic layer of platinum/titanium dioxide.  
      Platinum deposited on the surface of titanium dioxide enhances the separation of charge carriers, decreasing the recombination rate of the electrons and holes. Platinum is also a good thermal catalyst. It is believed that platinum can further oxidize the photocatalytic oxidation intermediates to carbon dioxide and water.  
      Carbon monoxide can diffuse through the porous layers and reach the inner layers. At room temperature, the gold/titanium dioxide layer oxidizes carbon monoxide to carbon dioxide. When carbon monoxide adsorbs on the coating, the gold acts as an oxidation catalyst and lowers the energy barrier of the carbon monoxide, oxidizing the carbon monoxide to carbon dioxide in the presence of oxygen.  
      In environments where ozone concentrations are very high, a fourth layer of manganese oxide/titanium dioxide is applied on the honeycomb under the inner layer. Ozone can also diffuse through the porous layers and reach the inner layers. When ozone adsorbs on the manganese oxide/titanium dioxide coating, the manganese oxide decomposes the ozone to molecular oxygen at room temperature or slightly elevated temperature due to the heat generated by the ultraviolet light.  
      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 a first example of the layered photocatalyst of the present invention;  
       FIG. 5  schematically illustrates a second example of the layered photocatalyst of the present invention; and  
       FIG. 6  schematically illustrates an alternate embodiment of the air purification system. 
    
    
     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 and semi-volatile organic compounds, carbon monoxide to water, carbon dioxide, and other substances. For example, the volatile organic compounds can be aldehydes, ketones, alcohols, aromatics, alkenes, or alkanes. The air purification system  20  also decomposes ozone to oxygen. 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 layered photocatalytic/thermocatalytic coating  40 .  
      As shown in  FIG. 4 , the coating  40  of the present invention includes at least three layers. Preferably, the coating  40  has a loading of approximately 0.5-1 mg/cm 2  on the honeycomb  28 . The coating  40  includes an outer layer  42  of titanium dioxide or a metal oxide doped titanium dioxide. The outer layer  42  is effective in oxidizing volatile organic compounds and semi-volatile organic compounds, such as aldehydes, ketones, alcohols, aromatics, alkenes or alkanes. Titanium dioxide is an effective photocatalyst to oxidize volatile organic compounds to carbon dioxide, water and other substances. The outer layer  42  has an effective thickness (less than 2 μm) and porosity. That is, the outer layer  42  is able to allow other contaminants that are not oxidized by the outer layer  42 , such as low polarity organic compounds, carbon monoxide, and ozone, to diffuse through the outer layer  42  and adsorb on the layers under the outer layer  42 .  
      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 outer layer  42  on the surface of the honeycomb  28 . When the photons of the ultraviolet light are absorbed by the outer layer  42 , an electron is promoted from the valence band to the conduction band, producing a hole in the valence band. 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 outer layer  42  to form reactive hydroxyl radicals.  
      When a volatile organic compound is adsorbed onto the outer layer  42 , the hydroxyl radical attacks the volatile organic compound, abstracting a hydrogen atom from the volatile organic compound. In this method, the hydroxyl radical oxidizes the volatile organic compounds and produces water, carbon dioxide, and other substances.  
      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 , or FeTiO 3 . Additionally, other metal oxides can be mixed with titanium dioxide, such as Fe 2 O 3 , ZnO, V 2 O 5 , SnO 2 , CuO, MnO x , 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  42  is a metal oxide loaded titanium dioxide, the titanium dioxide of the intermediate layer  44  can be loaded with a metal compound, such as WO 3 , ZnO, CdS, 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 , Mn x O 2 , Cr 2 O 3 , or ZrO 2 .  
      An intermediate layer  44  of a catalytically active metal supported on a titanium dioxide or a titanium dioxide monolayer treated photocatalyst with very high dispersed catalytically active metal or metal is applied under the outer layer  42 . Preferably, the titanium dioxide is loaded with a Group VIII noble metal, such rhodium, ruthenium, palladium, iridium, osmium, or platinum. However, the titanium dioxide can also be loaded with copper, silver, rhenium, gold, or the like. More preferably, the metal or metals are chosen with some regard to the catalysts expected substrate. Therefore, if more than on metal is used, the metal can be dispersed as a very small nano-crystal containing individual metals or a very small mixed metal clusters. Typically, a catalytic metal for this function is platinum. The catalytically active metal can also be a metal alloy or an intermetallic compound.  
      The catalytically active metal supported on titanium dioxide intermediate layer  44  is highly reactive with low polarity organic compounds. Platinum deposited on the surface of titanium dioxide enhances the separation of charge carriers, decreasing the recombination rate of the electrons and holes. Platinum is also a good thermal catalyst. It is believed that platinum can further oxidize the photocatalytic oxidation intermediates to carbon dioxide and water. Low polarity organic molecules have an increased affinity to platinum. When low polarity organic compounds adsorbs on the platinum, the platinum retains the low polarity organic compounds on the coating  40  for 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%.  
      The intermediate layer  44  has an effective thickness and porosity. That is, the intermediate layer  44  is able to allow other contaminants that are not oxidized by the intermediate layer  44 , such as carbon monoxide and ozone, to pass through the intermediate layer  44  and adsorb on the layers under the intermediate layer  44 .  
      A thermocatalytic inner layer  46  is applied and deposited on the surface of the honeycomb  28  under the intermediate layer  44 . The inner layer  46  is either nano-dispersed gold on titanium dioxide, gold on mixed metal oxides including titanium dioxide, gold on titanium dioxide which is loaded with other metal oxides on the surface, or gold containing mixed metal clusters.  
      At room temperature, the inner layer  46  oxidizes carbon monoxide to carbon dioxide. When carbon monoxide adsorbs on the coating, the gold acts as an oxidation catalyst and lowers the energy barrier of the carbon monoxide, oxidizing the carbon monoxide to carbon dioxide in the presence of oxygen. Therefore, the inner layer  46  acts as 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 produced carbon dioxide.  
      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 nano-particles.  
      The titanium dioxide can also be loaded with a metal oxide to further improve the thermocatalytic effectiveness of the inner layer  46 . Gold particles have a tendency to migrate on the surface of the titanium dioxide to form large clusters. The effectiveness of the inner layer  46  can be decreased due to the migration of the 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 clusters, therefore increasing the effectiveness of the inner layer  46 . 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 at least one of WO 3 , ZnO, CdS, 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 , Mn x O 2 , Cr 2 O 3 , or ZrO 2 .  
      This can also include titanium dioxide or titanium dioxide treated with a monolayer of another metal oxide having titanium dioxide decorated with isolated sites containing one or more, but typically less than, 12 oxidized atoms of another metal, such as iron, cobalt, and rhenium and the like, that function as anchor sites for the sub 3 nm gold particles. The surface dopant sites surrounded by titanium dioxide or its treatment metal monolayer function as surface energy potential wells that restrain free motion of gold.  
      The inner layer  46  has an effective thickness and porosity. That is, the inner layer  46  is able to allow other contaminants that are not oxidized by the inner layer  46 , such as ozone, to pass through the inner layer  46  and adsorb on any layer that is under the inner layer  46 .  
      As shown in  FIG. 5 , in environments where ozone concentrations are very high, a thermocatalytic fourth layer  48  can be applied under the inner layer  46 , directly on the honeycomb  28 . The fourth layer  48  is a manganese oxide/titanium dioxide ozone destruction catalyst. At room temperature, the fourth layer  48  decomposes ozone to oxygen.  
      At ambient temperatures, the 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 including manganese oxide and promoter doped manganese oxide produces oxygen.  
      If a fourth layer  48  is employed, the fourth layer  48  is applied on the honeycomb  28 , the inner layer  46  is applied on the fourth layer  48 , the intermediate layer  44  is applied on the inner layer  46 , and the outer layer  42  is applied on the intermediate 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 .  
      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.  
       FIG. 6  illustrates an alternate example of the air purification system  50 . In this example, the air first flows through a first honeycomb  52 , through a second honeycomb  54 , and then through a third honeycomb  56  having a manganese oxide/titanium dioxide coating. One of the first honeycomb  52  and the second honeycomb  54  has a titanium dioxide coating or a metal oxide doped titanium dioxide coating. The metal oxide can be WO 3 , ZnO, SrTiO 3 , Fe 2 O 3 , V 2 O 5 , SnO 2 , FeTiO 3 , PbO, Co 3 O4, NiO, CeO 2 , CuO, SiO 2 , Al 2 O 3 , Mn,O 2 , Cr 2 O 3 , or ZrO 2 . The metal oxide doped titanium dioxide coating oxidizes contaminants, such as volatile organic compounds and semi-volatile organic compounds, to water and carbon dioxide. The other of the first honeycomb  52  and the second honeycomb  54  has a gold/titanium dioxide coating that oxidizes carbon monoxide to water and carbon dioxide. The manganese oxide/titanium dioxide coating decomposes ozone to oxygen and water.  
      By employing a honeycomb with a metal oxide doped titanium dioxide coating, a honeycomb with a gold/titanium dioxide coating, and a third honeycomb  54  with a manganese oxide/titanium dioxide coating, carbon monoxide, ozone, volatile organic compounds, and semi-volatile organic compounds can be oxidized and destroyed. Therefore, the air purification system  50  including the metal oxide doped titanium dioxide coated honeycomb, the gold/titanium dioxide coated honeycomb, and the manganese oxide/titanium dioxide coated honeycomb  60  can perform the same function as the layered coating having a layer  48  of manganese oxide/titanium dioxide, a layer  46  of gold/titanium dioxide, and a layer  42  of metal oxide/titanium dioxide.  
      It is to be understood that the honeycombs  52 ,  54  and  56  can be in any order. However, ozone is a strong oxidation agent and is able to assist the photocatalytic oxidation process. Therefore, it is preferred that the air flows through the metal oxide doped titanium dioxide honeycomb  56  last. Alternately, the air purification system  50  includes more than one first honeycomb  52 , second honeycomb  54  and third honeycomb  56 .  
      Although a honeycomb  28  has been illustrated and described, it is to be understood that the photocatalytic/thermocatalytic coating  40  can be applied on any structure. The voids in a honeycomb  28  are typically hexagonal in shape, but it is to be understood that other void shapes can be employed. As contaminants adsorb onto the photocatalytic/thermocatalytic coating  40  of the structure in the presence of a light source, the contaminants are oxidized into water, carbon dioxide and other substances.  
      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.