Catalyst and method for decomposing ozone by the use thereof

Abstract: A catalyst of the present invention is constituted such that a catalytic active substance having ozone resolvability is carried on a carrier at a thickness of 10 to 200 µm. Consequently, ozone can efficiently be removed. When the catalytic active substance or ozone adsorption substance is contained in the carrier, the carrier has the ozone resolvability or ozone adsorbing ability in itself. Accordingly, the thickness of the catalytic active substance carried on a carrier surface can be reduced to the range from 5 to 100 µm. Thus, there can be improved productivity, and the durability of the catalyst as well as ozone removing efficiency. This invention is also directed to a method for decomposing ozone by the use of said catalyst.

Description: BACKGROUND OF THE INVENTION The present invention relates to a catalyst for decomposing and removing ozone contained in gas or the like. Examples of a conventional method for removing the harmful ozone contained in the gas, include (i) an adsorbing method using a porous substance such as a catalytic active substance or ozone adsorption substance, zeolite or the like and (ii) an oxidizing and decomposing method using a catalyst such as MnO₂. However, any of the conventional decomposing methods above mentioned does not fully achieve a satisfactory decomposing. More specifically, in the adsorbing method, an adsorbent needs to be regenerated or exchanged since its adsorbing ability expires in a limited period of time. This presents the problem that a removing apparatus requires much labor and expense for maintenance. The oxidizing and decomposing method using the catalyst includes no such problems as above mentioned, but does not fully achieve satisfactory decomposing in view of catalyst performance. In addition, the performance is initially good but is deteriorated with the passage of time during reaction. The present inventors have variously investigated the reaction for oxidizing and decomposing ozone. As a result, the following has been found. Since the rate determining step of reaction is a step of diffusion in the solid, the reaction proceeds only on a catalyst surface. Consequently, it is most preferable that catalytic constitution is coating-typed in which catalytic active components are carried on the catalytic surface and the vicinity thereof. However, in the case where a honeycomb constitution is used for a carrier, cells of the constitution are easily plugged by the catalytic active components to be coated as the number of cells / inch² is increased. Consequently, there is caused the problem that the low-pitch honeycomb type catalyst for decomposing ozone, which has great number of cells / inch², is poor in productivity. EP-A-0 398 765 and EP-A-0 398 766 each disclose ozone decomposition reactor elements comprising a substrate including an ozone decomposition catalyst and electrodes attached to the catalyst structure for elevating its temperature to aid in the decomposition of ozone. Accordingly, it is a main object of the present invention to provide a catalyst for decomposing ozone which is superior to the conventional method in ability of removing ozone by an oxidizing and decomposing method. It is another object of the present invention to provide a catalyst for decomposing ozone which has good durability for a long time. It is yet another object of the present invention to provide a catalyst for decomposing ozone having high productivity even if the carrier is low pitch honeycomb-type. SUMMARY OF THE INVENTION The present invention provides a catalyst comprising a carrier which contains at least 10% by weight of either an ozone adsorbing substance or a catalytically active substance (I) capable of decomposing ozone, and a layer of a catalytically active substance (II) capable of decomposing ozone, coated on the carrier with a thickness of 5-100 µm. The catalytically active substances (I) and (II) may be the same or different. The above catalyst is obtainable by dipping a honeycomb type carrier containing either an ozone adsorbing substance or a catalytically active substance (I) capable of decomposing ozone in a diluted or undiluted slurry which contains a catalytically active substance (II) capable of decomposing ozone, so that a layer of the catalytically active substance (II) is coated on the surface of the honeycomb type carrier with a thickness of 5 to 100 µm. More specifically, the catalytic active substance capable of decomposing ozone or ozone adsorption substance is contained in the carrier so that the ability to decompose ozone or ozone adsorbing ability is given to the carrier itself. Thus, the thickness of the catalytic active substance to be carried on the carrier can be reduced. Accordingly, since the active component of the catalyst for decomposing ozone has a small coating thickness, it is excellent in the production of the low pitch honeycomb type catalyst. In addition, the catalyst for decomposing ozone according to the present invention has good durability and performance for a long time. As described above, the catalyst for decomposing ozone according to the present invention has an effect that ozone can effectively be removed. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a section view of a honeycomb type catalyst; and Figure 2 is a flowsheet of a catalyst activity test. DETAILED DESCRIPTION OF THE INVENTION Examples of the catalytic active substances to be used include at least one of metals such as Ti, Cu, Mn, Ni, Fe, Ag, Au Mo, Zr, Sn, Nb, W or the like, metal oxide, metal sulfate. Activated carbon or acid may be mixed with the metal, metal oxide or metal sulfate as mentioned above. Examples of metal oxide are MnO₂, NiO, CuO, Fe₂O₃, a binary-catalyst type such as MnO₂-TiO₂, MnO₂-SiO₂, MnO₂-WO₃, MnO₂-V₂O₅, CuO-TiO₂, Co₃O₄-TiO₂, Fe₂O₃-TiO₂, Fe₂O₃-Au, MnO₂-Ag₂O, MnO₂-CuO, MnO₂-Fe₂O₃, NiO-Co₃O₄, NiO-TiO₂, NiO-MnO₂, NiO-MnO₂, NiO-Ag₂O, NiO-MoO₃, NiO-WO₃, NiO-SnO₂ or the like, a ternary-catalyst type such as MnO₂-Co₃O₄-TiO₂, MnO₂-Co₃O₄-Ag₂O, NiO-MnO₂-TiO₂, MnO₂-WO₃-TiO₂, MnO₂-MoO₃-TiO₂, MnO₂-V₂O₅-TiO₂, MnO₂-Ag₂O-TiO₂, MnO₂-CuO-Ag₂O, NiO-MnO₂-Ag₂O, NiO-MnO₂-TiO₂ or the like, MnO₂- (alkali metal and/or alkali earth metal oxide), or a zeolite catalyst type carrying metal of which oxide formation enthalpy is 419 kJ/g (100 Kcal/g) oxygen atom or less. The catalytic active substance of the present invention is not limited to the foregoing. The most effective catalyst is a silver type such as MnO₂-Ag₂O, MnO₂-Ag₂O-TiO₂, MnO₂-Ag₂O-TiO₂-SiO₂, MnO₂-Ag₂O-TiO₂-Al₂O₃, Ag₂O-TiO₂, Ag₂O-Al₂O₃ or the like. As a carrier for carrying the active substance, corrugated honeycomb made of ceramic fiber, cordierite honeycomb, or moldings having various shapes and made of clay or the like can be used. The carrier is not particularly limited in shape, but may be in any form such as honeycomb, pellet, cylinder, pipe or the like. The carrier containing the catalytic active substance or ozone adsorption substance can be produced mainly by a kneading method. The honeycomb type catalyst has a so-called catalytic filter constitution, for example, as shown in Fig. 1. The honeycomb type catalyst is formed by carrying the catalytic active substance on a honeycomb structure 1. The honeycomb structure 1 is made of ceramic in which openings of a gas inlet A and a gas outlet B are alternately blocked. Walls 2 of the honeycomb structure 1 are made of porous ceramic having permeability. Consequently, the gas introduced from the inlet A into the filter (as shown by an arrow) is forcibly passed through the walls 2 and is then discharged from the outlet B. The opening is not particularly limited in shape, but may be in any form such as hexagon, circle, quadrangle, triangle or the like. There may be added (i) a forming assistant such that the carrier easily gets into shape, or (ii) a reinforcing agent such as inorganic fiber or an organic binder to improve the mechanical strength or the like. The content of the catalytic active substance or ozone adsorption substance contained in the carrier is preferably in the range from 10 to 80 % by weight, more preferably in the range from 20 to 50 % by weight. If the content of the catalytic active substance or ozone adsorption substance is smaller than 10 % by weight, the ozone resolvability or ozone adsorbing ability of the carrier is considerably lowered. Consequently, it is necessary to increase the coating thickness of the catalytic active substance so as to obtain high resolvability. Examples of the preferred catalytic active substance to be contained in the carrier are MnO₂, NiO, CuO, Ag₂O and the like. Examples of the ozone adsorption substance having ozone adsorbing ability are activated carbon, zeolite and the like. Referring to the present invention, the catalytic active substance and ozone adsorption substance may be contained in the carrier separately or in mixture. The catalytic active substance can be carried on the carrier by, for example, a wash coat method wherein a slurry of the catalytic active substance is adhered onto the surface of the carrier by dipping the carrier in the slurry and dried out. The carrier containing the catalytic active substance or ozone adsorption substance carries the catalytic active substance at a coating thickness of 5 to 100 µm, preferably 10 to 50 µm. If the coating thickness is not greater than 5 µm, decomposing activity is lowered. If the coating thickness exceeds 100 µm, the decomposing activity is not improved so that only a pressure loss is increased. The reaction temperature at the time of decomposing ozone is preferably in the range from 0 to 40°C and more preferably in the range from 10 to 30°C. The temperature less than 0°C causes the reaction velocity to be slower, while the temperature exceeding 40°C requires an additional energy for increasing the temperature, resulting in poor economy. Preferably, the catalyst comes in contact with the reaction gas in the range of area velocity (AV) from 5 to 70. The area velocity less than 5 requires a greater amount of catalyst, while the area velocity exceeding 50 causes the decomposition rate to be lowered, failing to achieve the desired decomposition rate. It is here noted that the area velocity refers to a value obtained by dividing the reaction amount (Nm³/h) by the gas-catalyst contacting area per catalyst in unit volume (m²/m³). EXAMPLES The following description will discuss, in more detail, examples of the present invention. It is noted that the present invention should not be limited to the following examples. Catalyst Activity Test Catalyst activity was measured using the test method having a flowsheet as shown in Fig. 2, under test conditions discussed later. In Fig. 2, an ozone generator is indicated at 3. The ozone generator 3 generates ozone at suitable temperature by means of the air introduced thereinto. The air containing ozone is led to a catalyst 4. The ozone decomposition rate can be found by the values of the inlet and outlet of the catalyst obtained by an ozone analysis 5 in accordance with the following formula. A three way cock is indicated at 6. Ozone decomposition rate (%) = inlet ozone concentration - outlet ozone concentration inlet ozone concentration x 100 (Test Conditions) Space velocity : 20,000 / h Reaction temperature : 20°C Example 1 Gairome clay was dried for 18 hours at 100°C, and then was crushed by a sample mill whose screen has holes of 0.5 mm diameter. 8 kg of the crushed products and 2 kg of active carbon were mixed with 0.8 kg of methyl cellulose type binder (Yuken Industry Co., YB-32) and water. The mixture was kneaded thoroughly by a kneader. After that, the mixture was put into an auger screw type extruder to which a honeycomb extruding die is attached, so that the honeycomb was obtained. The obtained honeycomb was air-dried at ordinary temperature, and it was heated up to 500°C, at the rate of 5°C by a hour in N₂ current. After it keeps 500°C for 3 hours, it was cooled down at the rate of 10°C by a hour. Thus, the honeycomb type carrier having open area rate of 64 % and pitch of 1.0 mm was obtained. On the other hand, 1kg of MnO₂ having a specific surface area of 67 m²/g was added to 500 ml of water and a small amount of binder. To the mixture, 250 g of glass bead was added. The resultant mixture was agitated and mixed for 30 minutes. The glass bead was apart from the mixture to produce a slurry. After the slurry was diluted with 300 ml of water, in the slurry was dipped the honeycomb type carrier cut off at a suitable dimension. The exceeded slurry was excluded from the honeycomb type carrier, and the honeycomb type carrier was dried. Thus, there was prepared a catalyst carrying 31 µm in average thickness of MnO₂. The thickness of the carried MnO₂ layer gas calculated by obtaining its average value according to EPMA linear analysis on the assumption that the number of n is 10. The thickness was obtained by the same method in the following Examples and Reference Example. Example 2 There was prepared a catalyst carrying MnO₂-TiO₂ (weight ratio of 80 : 20) in the same manner as that of Example 1, except that 800g of MnO₂, 200 g of TiO₂ and 500 ml of diluting water were used in Example 2, instead of 1 kg of MnO₂ and 300 ml of diluting water used in Example 1. Thus, there were obtained five kinds of catalysts having coating thicknesses of 4 µm, 8 µm, 13 µm, 25 µm and 38 µm by adjusting the number of coatings. Example 3 There was prepared a catalyst carrying in average thickness of 43 µm of MnO₂-Ag₂O (weight ratio of 90 : 10) in the same manner as that of Example 1, except that 900 g of MnO₂ and 100 g of Ag₂O were used in Example 3, instead of 1kg of MnO₂ used in Example 1. Example 4 There was prepared a honeycomb type carrier in the same manner as that of Example 1, except that 5 kg of the crushed Gairome clay, 5kg of active carbon and 1 kg of methyl cellulose type binder (Yuken Industry Co., YB-32) were used in Example 4. The carrier was treated by the same slurry as used in Example 2. Thus, there were prepared five kinds of catalysts having the coating thicknesses of 5 µm, 10 µm, 18 µm, 29 µm and 43 µm by adjusting the number of coatings. Example 5 There was prepared a catalyst carrying 50 µm in average thickness of MnO₂-Fe₂O₃-Ag₂O (weight ratio of 50 : 40 : 10) in the same manner as that of Example 1, except that 400 g of Fe₂O₃ having a specific surface area of 38 m²/g, 100 g of Ag₂O and 500 g of MnO₂ were used in Example 5, instead of 1 kg of MnO₂ used in Example 1. Example 6 There was prepared a catalyst carrying 38 µm in average thickness of MnO₂ - active carbon - TiO₂ (weight ratio of 70 : 20 : 10) in the same manner as that of Example 1, except that 200 g of active carbon (Takeda Pharmaceutical Co., "Shirasagi A"), 100 g of TiO₂ and 700 g of MnO₂ were used, instead of 1 kg of MnO₂ used in Example 1. Example 7 There was prepared a catalyst carrying 49 µm in average thickness of CuO in the same manner as that of Example 1, except that CuO having a specific surface area of 55 m²/g was used, instead of MnO₂ used in Example 1. Example 8 There was prepared a catalyst carrying 57 µm in average thickness of MnO₂ in the same manner as that of Example 4, except that 18 kg of Gairome clay and 2 kg of active carbon were used when extruding and molding a carrier. Reference Example 30 g of MnO₂ having a specific surface area of 48 m²/g and 70 g of a mixture of titanium tetrachloride with a silica sol (TiO₂ : SiO₂ = 1 : 1) were agitated and mixed. While agitating and mixing, ammonia gas was blown such that the resultant mixture was neutralized, thereby to produce a slurry like precipitate. After sufficiently to produce a slurry like precipitate. After sufficiently washed with water, the precipitate was then calcined for 3 hours at 500°C and milled. Thus, there was prepared the powder of a ternary-catalyst MnO₂-TiO₂-SiO₂ (weight ratio of 35 : 30 : 35) having a specific surface area of 162 m²/g. There was prepared a ternary-catalyst carrying SiO₂-MnO₂-TiO₂ (weight ratio of 35 : 30 : 35) with the carry rate of 99 % by using the powder in the same manner as that of Example 1. Catalyst Activity Test Each catalyst of Examples 1 to 8 and Reference Example was submitted to a catalyst activity test in accordance with the Catalyst Activity Test described above. There were obtained ozone decomposition rates initially, after 10 hours and after 100 hours. The test conditions are as follows. (Test Conditions) Space velocity : 20,000 / h Inlet ozone concentration : 10 ppm Reaction temperature : 20°C The test results are shown in Table 1. Example 9 Kibushi clay was dried for 18 hours at 100°C, and then was crushed by a sample mill whose screen has holes of 0.5 mm diameter. 8 kg of the crushed products and MnO₂ having a specific area of 67 m²/g were mixed with 0.8 kg of methyl cellulose type binder (Yuken Industry Co., YB-32) and water. The mixture was kneaded thoroughly by a kneader. After that, the mixture was put into an auger screw type extruder to which a honeycomb extruding die is attached, so that the honeycomb was obtained. The obtained honeycomb was air-dried at ordinary temperature, and it was heated up to 500°C at the rate of 5°C by a hour in N₂ current. After it keeps 500°C for 3 hours, it was cooled down at the rate of 10°C by a hour. Thus, the honeycomb type carrier having open area rate of 64 % and pitch of 1.0 mm was obtained. 1kg of MnO₂ having a specific surface area of 67 m²/g was added to 500 ml of water and a small amount of binder. To the mixture, 250 g of glass bead was added. The resultant mixture was agitated and mixed for 30 minutes. The glass bead was apart from the mixture to produce a slurry. After the slurry was diluted with 300 ml of water, in the slurry was dipped the honeycomb type carrier cut off at a suitable dimension. The exceeded slurry was excluded from the carrier, and the carrier was dried. Thus, there was prepared a catalyst carrying 38 µm in average thickness of MnO₂. The thickness of the carried MnO₂ layer was calculated by obtaining its average value according to EPMA linear analysis on the assumption that the number of n is 10. The thickness was obtained by the same method in the following Examples and Reference Example. Example 10 There was prepared a catalyst carrying MnO₂-TiO₂ (weight ratio of 80 : 20) in the same manner as that of Example 9, except that 800 g of MnO₂, 200 g of TiO₂ and 500 ml of diluting water were used, instead of 1 kg of MnO₂ and 300 ml of diluting water used in Example 9. Thus, there were obtained five kinds of catalysts having coating thicknesses of 6 µm, 10 µm, 15 µm, 30 µm, and 42 µm by adjusting the number of coatings. Example 11 There was prepared a catalyst carrying in average thickness of 47 µm of MnO₂-Ag₂O (weight ratio of 90 : 10) in the same manner as that of Example 9, except that 900 g of MnO₂ and 100 g of Ag₂O were used, instead of 1kg of MnO₂ used in Example 9. Example 12 There was prepared a honeycomb type carrier in the same manner as that of Example 9, except that 5 kg of the crushed Kibushi clay, 5 kg of MnO₂, 1kg of methyl cellulose type binder (Yuken Industry Co., YB-32) were used. The carrier was treated by the same slurry as used in Example 10. Thus, there were prepared five kinds of catalysts having the coating thicknesses of 5 µm, 11 µm, 20 µm, 33 µm and 45 µm by adjusting the number of coatings. Example 13 There was prepared a catalyst carrying 51 µm in average thickness of MnO₂-Fe₂O₃-Ag₂O (weight ratio of 50 : 40 : 10) in the same manner as that of Example 9, except that 400 g of Fe₂O₃ having a specific surface area of 38 m²/g, 100 g of Ag₂O and 500 g of MnO₂ were used, instead of 1 kg of MnO₂ used in Example 9. Example 14 There was prepared a catalyst carrying 38 µm in average thickness of MnO₂ - active carbon - TiO₂ (weight ratio of 70 : 20 : 10) in the same manner as that of Example 9, except that 200 g of active carbon (Takeda Pharmaceutical Co., "Shirasagi A"), 100 g of TiO₂ and 700 g of MnO₂ were used, instead of 1 kg of MnO₂ used in Example 9. Example 15 There was prepared a catalyst carrying 45 µm in average thickness of CuO in the same manner as that of Example 9, except that CuO having a specific surface area of 55 m²/g was used, instead of MnO₂ used in Example 9. Example 16 There was prepared a catalyst carrying 57 µm in average thickness of MnO₂ in the same manner as that of Example 12, except that 18 kg of Kibushi clay and 2 kg of active carbon were used when extruding and molding a carrier in Example 12. Catalyst Activity Test Each catalyst of Examples 9 to 16 was submitted to a catalyst activity test in the same manner as in Examples 1 to 8. The test results are shown in Table 2. As apparent from Tables 1 and 2, any of the catalysts obtained in Examples 1 to 16 presents the high ozone decomposition rate and has good durability for a long time.

Claims: A catalyst comprising a carrier which contains at least 10% by weight of either an ozone adsorbing substance or a catalytically active substance (I) capable of decomposing ozone, and a layer of a catalytically active substance (II) capable of decomposing ozone, coated on the carrier with a thickness of 5-100 µm. A catalyst according to claim 1, wherein the layer of catalytically active substance (II) has a thickness of 10-50 µm. A catalyst according to claim 1, obtainable by dipping a honeycomb type carrier containing either an ozone adsorbing substance or a catalytically active substance (I) capable of decomposing ozone in a diluted or undiluted slurry which contains a catalytically active substance (II) capable of decomposing ozone, so that a layer of the catalytically active substance (II) is coated on the surface of the honeycomb type carrier with a thickness of 5 to 100 µm. A method for decomposing ozone by contacting it with a catalyst according to any preceding claim.

Inventors: NAGANO KAZUHIKO, NAKATSUJI TADAO, YOSHIDA KIMIHIKO, YOSHIMOTO MASAFUMI, NAGANO, KAZUHIKO, NAKATSUJI, TADAO, YOSHIDA, KIMIHIKO, YOSHIMOTO, MASAFUMI
Applicants: SAKAI CHEMICAL INDUSTRY CO, SAKAI CHEMICAL INDUSTRY CO., LTD.,
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, B01J23, B01J35