Method for catalytically purifying exhaust gas

Abstract: A method for purifying an oxygen excess exhaust gas by catalytically reducing nitrogen oxides contained in the exhaust gas, which comprises bringing a catalyst comprising at least one noble metal supported on a carrier composed mainly of an oxide or a complex oxide of at least one metal selected from rare earth metals and metals of the group IV A of the periodic table of elements into contact the exhaust gas.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for catalytically purifying an exhaust gas, and more particularly, it relates to a method for catalytically purifying an exhaust gas which efficiently purifies NO x (nitrogen oxides) in an oxygen excess exhaust gas, i.e., an exhaust gas containing oxygen in an amount excessive to the oxygen amount necessary for completely oxidizing reductive substances such as carbon monooxide, hydrogen and hydrocarbons, etc., contained in the exhaust gas. 2. Description of the Related Art Known in the art are a large number of catalysts for purifying exhaust gases, which catalysts purify an exhaust gas by effecting, at the same time, an oxidation of carbon monooxide (CO) and hydrocarbons (HC) and a reduction of nitrogen oxides (NO x ) in the exhaust gas, for purifying the exhaust gases of automobiles. Typical examples of such a catalyst include those prepared by coating a γ-alumina slurry on a refractory support, such as cordierite, and calcining the coating and supporting thereon a noble metal, such as Pd, Pt and Rh. Document FR-A-2 343 505 discloses a catalyst comprising a perovskite suitable for catalyzing oxidation and reduction processes in a gas. According to the document the oxygen is removed by the catalyst from gas containing an excess of oxygen, and a part of the removed oxygen is supplied to a gas having oxygen deficiency. Document EP-A-0 272 136 discloses the use of a rare earth-titanium support. This document discloses similar catalyst for the treatment of a waste gas containing oxygen in a high concentration. Nevertheless, the performances of the above-mentioned catalysts for purifying exhaust gas are greatly influenced by the air-fuel ratio of the engine, and when a large amount of oxygen is present on the lean side, i.e., a lean mixture after combustion, although the oxidation actively proceeds little reduction of NO x occurs. Conversely, on the rich side with a smaller air-fuel ratio, the amount of oxygen in the exhaust gas becomes smaller, and thus little oxidation occurs but a large reduction of NO x occurs. Recently, in response to a demand for lower fuel costs, a lean burn running has been practiced which effects a combustion with an oxygen excess mixed gas during normal running, and there is a need for a catalyst which can sufficiently purify NO x even during a lean burn running. Under the above circumstances, there has been proposed a NO x purifying catalyst having Cu, Co or Pt supported on a zeolite carrier. Among these catalysts, for example, the Pt/zeolite catalyst at a catalyst inlet gas temperature of about 200°C, the Cu/zeolite catalyst at about 430°C, and the Co/zeolite catalyst at about 490°C, respectively, purify NO x at a high efficiency, as shown in Fig. 1. As described above, with the catalyst of the prior art having Cu, Co or Pt supported on a zeolite carrier, NO x cannot be purified at high efficiency at around 300°C, which is the exhaust temperature during a steady lean burn running of the engine. SUMMARY OF THE INVENTION Accordingly, the objects of the present invention are to eliminate the above-mentioned disadvantages of the prior art and to provide a method for catalytically purifing NO x efficiently, within the temperature range mentioned above (around 300°C). Other objects and advantages of the present invention will be apparent from the following description. In accordance with the present invention. there is provided a method for purifying an oxygen-excess exhaust gas by catalytically reducing nitrogen oxides contained therein which comprises the step of: bringing an exhaust gas containing nitrogen oxides into contact with a series of catalysts arranged in the order of, from an upstream side of the exhaust gas to a downstream side, (i) Pt supported on TiO₂ having a Pt/Ti atomic ratio of 0.001/1 to 1/1, (ii) Pt supported on CeO₂ having a Pt/Ce atomic ratio of 0.001/1 to 1/1, (iii) Pt supported on ZrO₂ having a Pt/Zr atomic ratio of 0.001/1 to 1/1, and (iv) Pt supported on Al₂O₃ having a Pt/Al₂O₃ weight ratio of 0.001/1 to 0.1/1. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the description set forth below with reference to the accompanying drawings, wherein: Figure 1 is a graph showing the relationship between the catalyst entering gas temperature and NO x purifying efficiency of the Pt/zeolite, Cu/zeolite and Co/zeolite of the prior art; Fig. 2 is a graph showing the relationship between the catalyst inlet gas temperature and CO, HC and NO x purifying efficiencies (conversions) of the catalyst of the Example; Fig. 3 and Fig. 4 are respectively graphs showing the relationships between the catalyst inlet gas temperature and CO, HC and NO x conversions (purifying efficiencies) of the catalysts of the Comparative Example, i.e., Pt/Al₂O₃, catalyst and Cu/zeolite catalyst. DESCRIPTION OF THE PREFERRED EMBODIMENTS As mentioned above, according to the present invention, by using an oxide of Ti, Ce, Zr and Aℓ, NO x in the exhaust gas of a lean burn running engine can be purified at a high efficiency. In the present invention, any alumina generally employed as the carrier for a purifying catalyst of the prior art can be used. The atomic ratio of Pt/Ti, Pt/Ce and Pt/Zr in the purifying catalyst of the present invention is 0.001/1 to 1/1. When supported on alumina, Pt is supported in an amount of 0.001 to 0.1 part by weight, based on 1 part by weight of alumina. According to the present invention, following general procedures, the above catalyst is usually arranged within a reactor, an exhaust gas is introduced into the reaction vessel wherein the catalyst and the exhaust gas are brought into contact to purify nitrogen oxides by a reduction thereof, and then the purified exhaust gas is discharged from the reactor, whereby the exhaust gas is purified. The space velocity (SV) during the introduction of the exhaust gas into the catalyst layer is not particularly limited but, for example, is preferably 10,000 to 200,000/hour. According to the present invention, by using Pt for the catalyst metal and an oxide of Ti, Ce, Zr and Aℓ for the carrier, the respective high efficiency NO x purifying temperatures become approximately the same as the exhaust temperature during a steady running of a lean burn engine, and the NO x during the steady running of such an automobile can be purified at a high efficiency. Also, by constituting the catalyst of several kinds of oxides according to the present invention, the range of high efficiency NO x purifying temperatures can be broadened, and NO x can be purified in a broad range of from a low temperature to a high temperature. Examples The present invention will now be further illustrated by, but is by no means limited to, the following Example and Comparative Example. In the following Examples, "parts" indicates "parts by weight" unless otherwise particularly noted. Example and Comparative Example A catalyst was prepared with an arrangement of Pt/TiO₂, Pt/CeO₂, Pt/ZrO₂, Pt/Al₂O₃ catalysts, juxtaposed in this order from the exhaust gas inlet toward the outlet side, and the relationship between the exhaust gas temperature flowing into the catalyst and the respective purifying efficiencies of NO x , CO, HC was examined. Also, as comparative Examples, Pt/Al203 and Cu/zeolite were prepared and evaluated in the same manner as described above. Catalyst preparation a) Preparation of slurry To 100 parts of each powder of commercially available TiO₂, CeO₂, ZrO₂ and Al₂O₃ were added 70 parts of alumina sol (alumina content 10%), 15 parts by weight of an aqueous 40% by weight of aluminum nitrate solution, and 30 parts of water, followed by stirring and mixing, to prepare slurries for coating the respective oxides. b) Coating and calcination A monolithic honeycomb carrier made of 0.25 liter of cordierite was dipped in water, and after blowing off superfluous water, dipped in the slurry obtained in the above a), taken out, and the superfluous slurry was blown off, followed by drying at 80°C for 20 minutes, and further, by drying at 700°C for 1 hour. c) Supporting with noble metal The sintered product obtained as described above was dipped in an aqueous solution of a predetermined concentration of dinitrodiamine platinum so that 2 g of Pt based on 1 liter of the carrier was carried by impregnation, and dried at 80°C for 20 minutes to prepare the Pt catalysts according to the present invention supported on the respective carriers. Comparative catalysts The Pt/Al₂O₃ catalyst was prepared similarly as the catalyst of the present invention by using a monolithic honeycomb carrier made of 1 liter of cordierite. The Cu/zeolite catalyst was prepared as described below. a) Preparation of slurry To 60 parts of a mixture of silica sol and alumina sol mixed to an Si/Al ratio of 30 as the binder were added 100 parts of a zeolite powder (Si/Al ratio 30; maximum fine pore size 0.54 nm (5.4 Å)) and 60 parts of water, and the mixture was throughly stirred and adjusted to a pH of 3 to 6 with an aqueous aluminum nitrate solution, to prepare a slurry for coating. b) Coating The coating was effected in the same manner as the catalyst of the above present invention. c) Ion-exchange supporting of Cu The carrier was dipped in an aqueous solution of acetate of Cu, allowed to stand for 24 hours so that 4 g of Cu per 1 liter of the carrier ion-exchange was supported, and the product then dried at 80°C for 20 minutes to provide a comparative catalyst. Evaluation experiments A 2000 cc L4 engine was run at A/F = 20 , a rotational speed of 2000 rpm, a torque of 100 Nm, and through a heat exchange between the exhaust gas discharged from the engine and water, the temperature at the catalyst was varied between 250 to 500°C. Catalyst Four monolithic carriers having 400 cells and 0.25 liter were coated with TiO₂, CeO₂, ZrO₂ and Al₂O₃, respectively, and then 2 g/liter of Pt was supported thereon, to prepare a catalyst with an arrangement of Pt/TiO₂, Pt/CeO₂, Pt/ZrO₂ and Pt/Al₂O₃ juxtaposed in this order from the exhaust gas inlet toward the outlet side. As comparative catalysts, the Pt/Al₂O₃ catalyst having 2 g/liter of Pt supported on monolithic carrier having 400 cells and 1 liter and the Cu/zeolite catalyst having 4 g/liter of Cu carried thereon were prepared and similarly evaluated. The results are shown in Fig. 2 (the catalyst of the present invention), Fig. 5 (Pt/Al₂O₃ catalyst), and Fig. 6 (Cu/zeolite catalyst) which show the relationships between the catalyst inlet gas temperature and the NO x , CO, and HC purifying efficiencies. From these results, it is clear that the catalyst and the method according to the present invention can efficiently purify NO x , CO, HC over a broad temperature range. The NO x reducing reaction under excess oxygen is a balance between the complete oxidation reaction and the partial oxidation reaction of reductive substances with oxygen, and NO x is reduced at a temperature range wherein a partial oxidation occurs. This may be considered to be because necessary intermediate products for a selective reduction of NO x are readily formed at temperatures where partial oxidation reactions occur. To suppress a complete oxidation reaction, the electron state of noble metal may be changed. It may be considered that the excellent effect of the present invention as mentioned above appears in the catalyst of the present invention due to the SMSI (Strong Metal Support Interaction) effect with the carrier oxide.

Claims: A method for purifying an oxygen-excess exhaust gas by catalytically reducing nitrogen oxides contained therein which comprises the step of: bringing an exhaust gas containing nitrogen oxides into contact with a series of catalysts arranged in the order of, from an upstream side of the exhaust gas to a downstream side, (i) Pt supported on TiO₂ having a Pt/Ti atomic ratio of 0.001/1 to 1/1, (ii) Pt supported on CeO₂ having a Pt/Ce atomic ratio of 0.001/1 to 1/1, (iii) Pt supported on ZrO₂ having a Pt/Zr atomic ratio of 0.001/1 to 1/1, and (iv) Pt supported on Al₂O₃ having a Pt/Al₂O₃ weight ratio of 0.001/1 to 0.1/1. A method as claimed in Claim 1, wherein said oxygen-excess exhaust gas has a temperature of about 300°C. A method according to Claim 1, wherein said exhaust gas is brought into contact with said series of catalysts at a space velocity of from 10,000 to 200,000 / hour.

Inventors: INOUE TOKUTA, KATOH KENJI, MURAKI HIDEAKI, OISHI KIYOHIKO, INOUE, TOKUTA,, KATOH, KENJI,, MURAKI, HIDEAKI,, OISHI, KIYOHIKO,, Inoue, Tokuta, c/o Toyota Jidosha K.K., KATOH, KENJI, C/O TOYOTA JIDOSHA K.K., Muraki, Hideaki, c/o Toyota Jidosha K.K., Oishi, Kiyohiko, c/o Toyota Jidosha K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1991
Main classification: B01D53
IPC codes: B01D53, B01J23