CATALYST AND METHOD FOR OZONE DECOMPOSITION

Abstract: A catalyst for ozone decomposition which comprises: (a) at least one selected from the group consisting of clay and carbon; (b) manganese dioxide: and optionally (c) titanium dioxide; and/or (d) at least one oxide of a metal selected from the group consisting of Cu, Co, Fe, Ni and Ag. A further catalyst for ozone decomposition which comprises: (a) at least one oxide of a metal selected from the group consisting of alkali metals and alkaline earth metals; (b) manganese dioxide; and optionally (c) titanium dioxide; and/or (d) at least one selected from the group consisting of clay and carbon.

Description: This invention relates to a catalyst and a method for decomposing ozone which is contained, for example, in the air. There have been proposed various methods of decomposing noxious ozone contained in the air, for example, an adsorption method wherein porous material is used such as activated carbon or zeolite, or an oxidative decomposition method wherein a catalyst is used such as MnO₂. However, the above mentioned known methods of removing ozone are not satisfactory. The adsorption method has a disadvantage in that adsorbent must be regenerated since it is limited in adsorption ability; this method is laborious and costly. The oxidative decomposition method has no such disadvantages, but the known catalysts have insufficient decomposition activity for ozone, and also deteriorate rapidly. When used under severe conditions, for example, when a gas which contains high concentrations of ozone is treated or a gas is treated at a high area velocity. DE-A-1966535 discloses various ozone decomposition catalysts containing Ni, Mn and Cu oxides, with or without activated carbon; or containing activated carbon and Mn and V oxides; none of these prior catalysts contains titanium oxide. The invention provides a catalyst for decomposing ozone which can be higher in ozone decomposition activity but also more durable under severe reaction conditions than the known catalysts. The catalyst according to the invention comprises (a) manganese dioxide in an amount of 20-90% by weight; (b) at least one material selected from clay and carbon; and (c) titanium dioxide in an amount of 9-30% by weight. The invention also provides a method of decomposing ozone effectively with such catalysts. Fig. 1 is a diagram for measurement of ozone decomposition activity of catalyst as described hereinafter. Herein the term "catalyst" means active components which have ozone decomposition activity, and the term "catalyst" is a synonym of active components. The catalyst is usually supported on a carrier to form a catalyst structure, or is molded in to a mold structure together with other molding components, for practical use. These structures will be referred to as catalyst structures. In the catalysts of the invention, manganese dioxide (a) is contained therein in amounts of at least 20, preferably 20-90, % by weight; and (b) is preferably contained in total amounts of at least 9%, e.g. 9 or 10 to 50%, by weight. When the amount of (a) or (b) is less than its minimum specified above, the resultant catalyst may be low in ozone composition activity; when either amount is more than its maximum specified above, no remarkable increase in activity is attained corresponding to the extra amount. The catalyst of the invention contains titanium dioxide together with manganese dioxide. Titanium dioxide improves decomposition activity for ozone, but also makes the resultant catalyst more resistant to deterioration in catalyst activity, namely more durable in use under severe reaction conditions. Titanium dioxide is contained in the catalyst in amounts of 9 or 10 to 30 % by weight. When the amount of titanium dioxide in the catalyst is less than 10 % by weight, the resultant catalyst may be little improved in such a manner as above described. However, if the amount is more than 30 % by weight, little improvement is attained corresponding to the extra amount. Further according to the invention, the catalyst is improved in ozone decomposition activity by displacing in part manganese dioxide by at least one oxide of metal selected from Cu, Co, Fe, Ni and Ag, preferably in amounts of 1-30 % based on the manganese dioxide. When the displacement is less than 1 % by weight based on the manganese dioxide, the catalyst is little improved in activity, whereas when it is over 30% by weight, no corresponding extra improvement in activity is attained. Catalysts wherein the manganese dioxide is in part displaced by the metal oxide may be exemplified by MnO₂/CuO₂/clay /TiO₂ , MnO₂/Co₃O₄/clay /TiO₂ , MnO₂/Fe₂O₃/clay /TiO₂ , MnO₂/NiO/clay /TiO₂ , MnO₂/Ag₂O/clay /TiO₂ , MnO₂/CuO₂/carbon /TiO₂ , MnO₂/Co₃O₄/carbon /TiO₂ , MnO₂/Fe₂O₃/carbon /TiO₂ , MnO₂/NiO/carbon /TiO₂ and MnO₂/Ag₂O/carbon /TiO₂ . The clay used may be laminal clay minerals such as pyrophyllite, talc, mica, chlorite, montmorillonite, kaolin or halloysite, and Kibushi clay or Gairome clay is preferably used. The carbon used may be activated carbon, graphite, carbon black or coke; among these activated carbon is most preferred. The catalysts of the invention may be produced by any known methods such as an impregnating, kneading, coprecipitating, precipitating or oxide admixing methods, or a combination of these. The catalyst or active components as set forth above is usually supported on an inactive carrier material to form a catalyst structure suitable for practical use. By way of example, the catalyst is obtained as powder and made into a slurry with water, and a honeycomb carrier is immersed in the slurry, dried, to provide a honeycomb catalyst structure. As a further method, the catalyst is kneaded together with inactive ceramic powder as a carrier material, molding assistants to provide the catalyst with plasiticity, reinforcements such as inorganic fibers to improve mechanical strength of the resultant structure and organic binders, and the mixture is then molded into a structure such as a honeycomb. Therefore, the catalyst structure is not specifically limited in form, and may be in the form of honeycombs, pellets, cylinders, plates or pipes. The entire catalyst structure may be composed of the active components, but when carrier structures or reinforcements are used to produce catalyst structures, it is preferred that the catalyst structure contains the active components preferably in amounts of not less than 50 %, more preferably in amounts of not less than 75 %, supported thereon. The ozone decomposition may be carried out by putting a reactant gas which contains ozone therein into contact with the catalyst, thereby to catalytically decompose the ozone. The ozone decomposition may be carried out at temperatures of at least 0°C (e.g. 0-40°C, preferably of 10-30°C). When the reaction temperature is less than 0°C , the reaction velocity is infeasibly slow. However, heat energy is needed when the reaction is carried out at temperatures of more than 40°C , and moreover, little improvement in decomposition rate is attained, so that reaction at high temperatures is undesirable mainly from the standpoint of energy economy. A reactant gas which contains ozone is put into contact with the catalyst preferably at an area velocity (m³/m²·hr) of 5-50, wherein the area velocity is defined as value of space velocity (hr⁻¹) divided by gas contact area per unit volume (m²/m³) of catalyst. When the area velocity is less than 5, a large volume of catalyst is undesirably needed whereas when more than 50, the ozone is not sufficiently decomposed and decomposition rate is very small. The known catalysts do not substantially deteriorate in activity when they are used to decompose ozone under mild conditions of a small CA value which is defined as the product of ozone concentration (ppm) at an inlet of a reactor and area velocity. However, many known catalysts rapidly deteriorate when the reaction is carried out under severe conditions of a CA value of not less than 30. However, the catalysts of the invention do not substantially deteriorate in activity even when used under severe conditions of a large CA value of not less than 30. The catalyst of the invention has a high activity for ozone decomposition and is durable even when it is used under severe conditions. The invention will now be described in more detail with reference to examples, however, the invention is not limited thereto. Section I A. Preparation of Catalysts Example 1 An amount of 704 g of manganese dioxide having a specific surface area of 48 m²/g, 78 g of Kibushi clay and 517 ml of titania sol (TiO₂ content: 150 g/l) were added to 500 ml of water, and there was added there to 250 g of glass beads. The mixture was stirred over three hours, to provide a slurry. The slurry was impregnated into a honeycomb formed with corrugates of ceramic fibers and having a porosity of 81 % and a pitch of 4.0 mm, to provide a catalyst structure having a ternary catalyst of MnO₂/TiO₂/clay (82/9/9 in a weight ratio) supported on the honeycomb in a support ratio of 95 %. The support ratio is defined as (weight of catalyst supported on the honeycomb/weight of the honeycomb) x 100 (%). Example 2 An amount of 30 g of manganese dioxide having a specific surface area of 48 m²/g, 70 g of Kibushi clay, 170 ml of titania sol and 500 ml of water were used, and otherwise in the same manner as in Example 1 a catalyst structure was prepared which had a ternary catalyst of MnO₂/TiO₂/clay (24/20/56 in a weight ratio) supported on the honeycomb in a support ratio of 101%. Example 3 An amount of 661 g of manganese dioxide, 43 g of cupric oxide, 78 g of Kibushi clay and 517 ml of titania sol were used, and otherwise in the same manner as in Example 1, a catalyst structure was prepared which had a four component catalyst of MnO₂/CuO/TiO₂/clay (77/5/9/9 in weight ratio) supported on the honeycomb in a support ratio of 103 %. Example 4 An amount of 687 g of manganese dioxide, 17 g of silver oxide, 78 g of Kibushi clay and 517 ml of titania sol were used, and otherwise in the same manner as in Example 1, a catalyst structure was prepared which had a four component catalyst of MnO₂/Ag₂O/TiO₂/clay (80/2/9/9 in a weight ratio) supported on the honeycomb in a support ratio of 102 %. Comparative Example An amount of 30 g of manganese dioxide having a specific surface area of 48 m²/g and 70 g of a mixture of titanium tetrachloride and silica sol (1/1 in a weight ratio as TiO₂/SiO₂) were mixed together under stirring while ammonia gas was blown therein to to neutralize the mixture and provide slurry precipitates. After fully washing with water, the precipitate was calcined at a temperature of 500°C for three hours, to provide a ternary catalyst of MnO₂/TiO₂/SiO₂ (30/35/35 in weight ratio) of a specific surface area of 162 m²/g. B. Measurement of Catalyst Activity The activity of the catalysts prepared in Examples 1- 4 and the Comparative Example was measured in accordance with a flow sheet as shown in Fig. 1, in which (1) designates an ozone generator, into which air is introduced to generate air which contains ozone in an appropriate concentration. The air is then sent to a catalyst reactor (2) to catalytically decompose the ozone in the reactor. The ozone concentration in the air is determined with an ozone analyzer (3) at the inlet and the outlet of the reactor. Ozone decomposition rate (%) is calculated based on an expression: [(ozone concentration at inlet - ozone concentration at outlet)/ozone concentration at inlet] x 100. The reaction temperature was 20°C and the space velocity was 20000 hr⁻¹. The results are shown in the Table 1. As apparent from the Table 1, the catalysts prepared in Examples 1 - 4 were found higher in ozone decomposition rate than the catalyst of the Comparative Example. Catalysts Compositions (Weight Ratio) Ozone Decomposition Rate (%) Example 1 MnO₂/TiO₂/clay 82/9/9 93.5 Example 2 MnO₂/TiO₂/clay 24/20/56 90.5 Example 3 MnO₂/CuO/clay/TiO₂ 77/5/9/9 100.0 Example 4 MnO₂/Ag₂O/TiO₂/clay 80/2/9/9 100.0 Comparative MnO₂/SiO₂/TiO₂ 30/35/35 86.8 Section II A. Preparation of Catalysts Example 5 An amount of 704 g of manganese dioxide having a specific surface area of 48 m²/g, 78 g of activated carbon and 517 ml of titania sol (TiO₂ content: 150 g/l) were added to 500 ml of water, and there were added there to 250 g of glass beads. The mixture was stirred over three hours, to provide a slurry. The slurry was impregnated into a honeycomb formed with corrugates of ceramic fibers and having a porosity of 81 % and a pitch of 4.0 mm, to provide a catalyst structure having a ternary catalyst of MnO₂/TiO₂/activated carbon (82/9/9 in weight ratio) supported on the honeycomb in a support ratio of 83 %. Example 6 An amount of 70 g of manganese dioxide having a specific surface area of 48 m²/g, 50 g of graphite, 250 ml of titania sol and 500 ml of water were used, and otherwise in the same manner as in Example 5, a catalyst structure was prepared which had a ternary catalyst of MnO₂/TiO₂/graphite (45/25/30 in a weight ratio) supported on the honeycomb in a support ratio of 52 %. Example 7 An amount of 43 g of cupric oxide, 661 g of manganese dioxide, 78 g of activated carbon and 517 ml of titania sol were used, and otherwise in the same manner as in Example 5 a catalyst structure was prepared which had a four component catalyst of MnO₂/CuO/TiO₂/activated carbon (77/5/9/9 in a weight ratio) supported on the honeycomb in a support ratio of 103 %. Example 8 An amount of 687 g of manganese dioxide, 17 g of silver oxide, 78 g of activated carbon and 517 ml of titania sol were used, and otherwise in the same manner as in Example 7, a catalyst structure was prepared which had a four component catalyst of MnO₂/Ag₂O/TiO₂/activated carbon (80/2/9/9 in weight ratio) supported on the honeycomb in a support ratio of 102 %. B. Measurement of Catalyst Activity The activity of the catalysts prepared in the Examples 5-8 was measured in the same manner as hereinbefore described. The measurement with the catalyst of Example 1 and the catalyst of the Comparative Example in Section I was carried out with varied CA values. The results are shown in the Table 2. As apparent from the Table 2, the catalysts prepared in Examples 5-8 were found higher in ozone decomposition activity and more durable under the severe conditions than the catalyst of the Comparative Example in Section I.

Claims: A catalyst for ozone decomposition which comprises: (a) manganese dioxide in an amount of 20-90% by weight; (b) at least one material selected from clay and carbon; and (c) titanium dioxide in an amount of 9-30% by weight. A catalyst according to claim 1 which further contains at least one metal oxide selected from oxides of Cu, Co, Fe, Ni and Ag in an amount of 1-30% by weight based on the manganese dioxide. A catalyst according to claim 1 or 2 containing from 9 to 50 wt.% of (b). A method of decomposing ozone which comprises putting a gas containing ozone into contact with a catalyst according to any preceding claim at a temperature of at least 0°C. A method according to claim 4 wherein the contact is at 0-40°C.

Inventors: NAGANO, KAZUHIKO, NAKATSUJI, TADAO, YOSHIMOTO, MASAFUMI
Applicants: SAKAI CHEMICAL INDUSTRY CO., LTD.,
Application year: 1989
Main classification: B01J23
IPC codes: B01J23, B01D53