Process for reducing NOx in an exhaust gas using a precious metal/ZSM-5 catalyst

Abstract: A catalyst for purifying exhaust gases contains: a catalyst support, a zeolite layer formed on the catalyst support, and noble metals such as platinum, palladium, rhodium and the like loaded on the zeolite layer by a predetermined parts by weight or more with respect to 100 parts by weight of the zeolite layer. The catalyst for purifying exhaust gases maintains a superior exhaust gas conversion performance over a long period of time.

Description: The present invention relates to a process for purifying exhaust gases of an internal combustion engine. It particularly relates to such a process using a catalyst comprising a zeolite. Description of the Related Art A zeolite has pores whose sizes are almost equal to those of molecules, and is also known as a "molecular sieve". The zeolite has been utilized in many reactions as a catalyst as well as an adsorbent. Further, the zeolite is also utilized as a cation exchanger, since the zeolite includes cations for neutralizing negative electric charges of Al₂O₃ , and since the cations are easily exchanged with the other cations in an aqueous solution. In recent years, it has been examined to apply the zeolite to a catalyst for purifying exhaust gases of automobiles in view of these characteristics of the zeolite. For example, Japanese Unexamined Patent Publication (KOKAI) No. 97047/1985 discloses a catalyst for purifying exhaust gases in which copper is loaded into a zeolite by means of an ion exchange. Further, Japanese Unexamined Patent Publication (KOKAI) No. 135541/1989 discloses a catalyst for purifying exhaust gases in which noble metals selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir) and ruthenium (Ru) are loaded into a zeolite by means of an ion exchange. In the catalyst for purifying exhaust gases, a catalyst support is coated with 100 g of the zeolite per 1 liter of the catalyst support, and 1.0 g of platinum or palladium and 0.2 g of rhodium per 1 liter of the catalyst support are loaded on the zeolite, as usual. Since the noble metals are expensive, the amounts thereof used as a catalyst metal are usually not so great. These catalysts offer an improved conversion performance on nitrogen oxides even under a lean atmosphere where oxygen exists in an excess amount. Accordingly, it is possible to dilute the concentration of the air-fuel mixture supplied into an engine, thereby enabling to sufficiently purify harmful components over a wide range from the theoretical air-fuel ratio to the lean atmosphere side. Therefore, it is possible to achieve a higher mileage with a less fuel consumption. However, in the above-mentioned catalyst in which the zeolite contains copper as positive ions, it has been found that the copper is aggregated by the heat generated during the service as a catalyst, and that the catalyst performance deteriorates because the copper is unstable at a high temperature of 800 °C or more. Further, in the above-mentioned catalyst in which the zeolite contains the noble metals as cation, it has been found that the catalyst performance deteriorates after a durability test. The post-published prior European Application EP-A-0 369 576 discloses an exhaust gas purifying process, in which a catalyst is brought in contact with a gas mixture having a low oxygen content. The used catalyst is provided by treating a support with a slurry of zeolite and binder, and impregnating the thus treated catalyst support with noble metals, so that the noble metals are substantially not located on the zeolite. SUMMARY OF THE INVENTION The present invention has been developed in view of the aforesaid circumstances. It is therefore an object of the present invention to prevent the catalyst performance of a zeolitic catalyst front being deteriorated by heat. This object is solved by an exhaust gas purifying process which comprises the following steps: a first step of preparing a catalyst for purification an exhaust gas which is generated by burning air-fuel mixed gas having an air fuel ratio of 18 or more and an oxygen content of 4.3 % or more, said catalyst comprising: a catalyst support; a layer formed on said catalyst support and including ZSM-5 zeolite and if a binder is used, a silica binder; and at least one of noble metals loaded on said zeolite and selected from the group consisting of platinum and rhodium, said platinum and said rhodium being loaded on said zeolite layer by 1.3 parts by weight or more and 0.7 parts by weight or more with respect to 100 parts by weight of said zeolite layer, respectively; and a second step of contacting said catalyst with oxygen-rich exhaust gases containing nitrogen oxides and 4.3 % or more of oxygen, thereby reducing said nitrogen oxides by catalytic reaction. A conventionally known catalyst support such as a pelletted-shaped catalyst support, a monolithic catalyst support, a metal catalyst support and the like may be used as the catalyst support for the catalyst according to the present invention, and the material qualities of the catalyst supports are not specified herein in particular. In addition, the catalyst support may be made of a zeolite. The zeolite layer comprising ZSM-5 zeolite is formed on the catalyst support. The zeolite forming the zeolite layer is crystalline aluminosilicate, and it is well known to be expressed by a general chemical formula as follows: xM 2/n · Al₂O₃ · ySiO₂. The diamter of ZSM-5 is slightly larger than the molecular sizes of nitrogen oxides. Further, as for the SiO₂/Al₂O₃ molar ratio, it is preferred to fall in the range of from 10 to 200. One of the major features of the present invention is that at least one of the noble metals is loaded on the zeolite layer by the predetermined amounts or more. The inventors of the present invention produced many kinds of zeolitic catalyst in which the loading amounts of the each of the platinum, palladium and rhodium are changed variously, and examined the catalyst performances before and after a high temperature durability test. As a result, as shown in Figures 1 to 4, the conversions of nitrogen oxides increase in accordance with the increase in the loading amounts of the noble metals in the initial stage of the catalyst operation. However, after the high temperature durability test, it is found that the conversions of nitrogen oxides increase sharply at inflection points corresponding to the special loading amounts of the respective noble metals. Namely, it is found that the conversions of nitrogen oxides after the high temperature durability test have been improved remarkably by respectively loading 1.3 parts by weight or more of platinum or 0.7 parts by weight or more of rhodium into 100 parts by weight of the zeolite layer. Further, it is disadvantageous in the cost performance to use the expensive noble metals in a greater amount though the catalyst performances before and after the high temperature durability test can be improved as the loading amounts of the noble metals increase. Therefore, the loading amounts of the noble metals depend on the balance of the cost performance and the aiming catalyst performances. Accordingly, the upper limits of the loading amounts of the noble metals are not specified herein in particular. The catalyst for purifying exhaust gases of the present invention may be produced as follows. Firstly, a solution containing at least one of the noble metal ions is brought into contact with a zeolite, thereby loading at least one of the noble metals on the zeolite by an ion exchange acid by an immersion. A slurry is made from the zeolite containing at least one of the noble metals. Then, a catalyst support is wash-coated with the slurry, and thereafter the thus wash-coated catalyst support is calcined, thereby forming the zeolite layer into which at least one of the noble metals are loaded. The catalyst for purifying exhaust gases of the present invention is obtained in this way. Or, as an alternative way, the catalyst support may be wash-coated with a zeolite, thereby forming a zeolite layer thereon, and a solution containing at least one of the noble metals may be brought into contact with the catalyst support thus covered with the zeolite layer in order to load at least one of the noble metals into the zeolite layer by an ion exchange and by an immersion. The catalyst for purifying exhaust gases of the present invention has the ZSM-5 zeolite. The zeolite layer has pores of the order of angstroms which are almost equal to the sizes of molecules, and is also known as a "molecular sieve". Accordingly, nitrogen oxides are selectively taken into the pores. Since there exist the active sites of the noble metals loaded on the pores, the nitrogen oxides are adsorbed on the active sites of the noble metals, thereby being reacted and purified. Further, in the catalyst for purifying exhaust gases of the present invention, 1.3 parts by weight or more of platinum when platinum is selected as a noble metal or 0.7 parts by weight or more of rhodium when rhodium is selected as a noble metal is respectively loaded into the zeolite layer taken as 100 parts by weight. Therefore, the durability of, the catalyst is improved remarkably at high temperatures of 800 °C or more. The reason why the high temperature durability is improved has not be clarified yet. However, it is assumed that a greater number of the noble metals are taken into the effective active sites or into the active sites of high thermal stability by loading the predetermined amounts or more of the noble metals on the zeolite layer. According to the catalyst for purifying exhaust gases of the present invention, it is possible to maintain the high conversion performance for a long period of time, since the deterioration of the catalyst performance due to the heat during the service has been prevented. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a graph showing relationships between loading amounts of platinum and conversions of nitrogen oxides; Figure 2 is a graph showing relationships between loading amounts of rhodium and conversions of nitrogen oxides; Figure 3 is a graph showing relationships between loading amounts of platinum and conversions of nitrogen oxides; and Figure 4 is a graph showing relationships between loading amounts of rhodium and conversions of nitrogen oxides. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Having generally described the present invention, a further understanding can be obtained by reference to certain specific preferred embodiments and comparative examples. The preferred embodiments are provided herein for purpose of illustration only and are not intended to be limiting unless otherwise specified. Hereinafter the word "parts" shall mean the parts by weight. As the zeolite, ZSM-5 was chosen. Further, two cation types namely the H- and Na- zeolites thereof were used respectively. A hundred (100) parts of zeolite, 70 parts of silica sol (20 % by weight of SiO₂) and 80 parts of pure water were mixed and stirred in order to make a slurry. Meanwhile, a honeycomb catalyst support made of cordierite having a length of 50 mm and a diameter of 30 mm was immersed into pure water, and the catalyst support was blowed in order to remove the excessive water. Thereafter, the catalyst support was immersed into the slurry, and the catalyst support was blowed to remove the excessive slurry. Then, the catalyst support was dried at 100 °C for 3 hours, and calcined at 300 °C for 1.5 hours. After the operations described above were carried out repeatedly twice, the catalyst support was further calcined at 500 °C for 3 hours to form a zeolite layer thereon. This zeolite layer was formed by 120 ± 5 g with respect to 1 liter of the honeycomb catalyst support Next, the noble metals were loaded on the honeycomb catalyst support having the above-mentioned zeolite layer. Here, a quadrivalent platinum ammine solution and a rhodium ammine solution were used in order to load the respective noble metals on the Na-zeolites layers. Further, a quadrivalent platinum ammine solution and rhodium nitrate solution were used in order to load the respective noble metals into the H-zeolites layers. The respective honeycomb catalyst supports were immersed into the respective solutions for 24 hours, and taken out of the respective solutions. After the catalyst supports were blowed in order to removed the excessive solutions, the honeycomb catalyst supports were calcined at 250 °C for 1 hour thereby obtaining the catalysts for purifying exhaust gases of the present invention. The catalysts thus obtained were analyzed by an atomic absorption analysis, and the loading amounts of the noble metals with respect to 1 liter of the honeycomb catalyst support and the loading amounts of the noble metals with respect to 100 parts by weight of the zeolite layer are set forth in Tables 1 to 4. Zeolites Noble Metals Loading Amount NO Conversion (%) g/l Parts by Weight B.D. A.D. Na-ZSM-5 Pt 1.73 1.44 58 41 1.98 1.65 61 43 2.49 2.08 65 45 4.32 3.60 76 54 0.18 0.15 29 0 0.49 0.41 48 7 0.94 0.78 52 10 Cu 7.10 5.91 80 38 Note: "B.D."...Before Durability Test "A.D."...After Durability Test Zeolites Noble Metals Loading Amount NO Conversion (%) g/l Parts by Weight B.D. A.D. Na-ZSM-5 Rh 0.94 0.78 56 41 1.41 1.18 63 43 1.92 1.60 65 47 3.81 3.18 70 50 0.18 0.15 29 0 0.37 0.31 44 5 0.66 0.55 50 9 Cu 6.40 5.33 80 39 Note: "B.D."...Before Durability Test "A.D."...After Durability Test Zeolites Noble Metals Loading Amount NO Conversion (%) g/l Parts by Weight B.D. A.D. H-ZSM-5 Pt 1.72 1.43 57 43 1.97 1.64 61 45 2.44 2.03 66 48 4.35 3.63 77 58 0.19 0.16 25 0 0.47 0.39 46 8 0.96 0.80 53 10 Cu 3.60 3.00 79 40 Note: "B.D."...Before Durability Test "A.D."...After Durability Test Zeolites Noble Metals Loading Amount NO Conversion (%) g/l Parts by Weight B.D. A.D. H-ZSM-5 Rh 0.96 0.80 55 41 1.47 1.23 58 44 1.95 1.63 63 47 3.79 3.16 75 56 0.18 0.15 27 0 0.37 0.31 46 7 0.66 0.55 51 10 Note: "B.D."...Before Durability Test "A.D."...After Durability Test Or the catalysts thus obtained, the maximum conversions of nitrogen oxides during the initial stage of the service were measured under the following conditions: Air-fuel ratio (A/F) ; 22 Inlet gas temperature ; increased from 120 to 450 °C Further, in order to examine the catalyst performance after the high temperature durability test, the catalysts were exposed an exhaust gas environment of an inlet gas temperature of 800 °C, and the maximum conversions of nitrogen oxides were measured similarly under the following conditions, and the exhaust gas environment comprised a model gas (O₂ = 4.3 %) having an air-fuel ratio (A/F) equivalent to 18: Air-fuel ratio (A/F) ; 22 Inlet gas temperature ; increased from 120 to 450 °C The results of the evaluation were set forth in Tables 1 through 4. In comparison, honeycomb catalyst supports having several zeolite layers were immersed into a solution containing copper ions, and then the copper was loaded on the honeycomb catalysts having the zeolite layers in the same manner as aforementioned, thereby preparing comparative catalysts. The catalyst performances of the comparative catalysts were evaluated similarly, and the results of the evaluation were set forth in some of the tables, namely Tables 1 and 2. (Evaluation) The results of the measurements set forth in Tables 1 to 4 are graphed in Figures 1 to 4. As shown in Figures 1 to 4, the conversions of nitrogen oxides increased in accordance with the increase in the loading amounts of the noble metals in the initial stage of the catalyst operation. However, after the high temperature durability test, the conversions of nitrogen oxides increased sharply at the inflection points corresponding to the respective loading amounts of the respective noble metals. Namely, the conversions of nitrogen oxides were improved remarkably at the platinum loading amounts 1.3 parts by weight or more and at the rhodium loading amounts of 0.7 parts by weight or more with respect to 100 parts by weight of the zeolite layer. This suggests that another factors other than the loading amounts influenced on the improvements in the conversions of nitrogen oxides. Further, in the comparative catalysts into which copper is loaded, there were a large differences in conversions before and after the durability test, and accordingly the comparative catalysts were lacking in the durability. Though the nitrogen oxides conversions of the comparative catalysts was higher than those of the catalysts of the present invention in the initial stage of the catalyst service, the conversions of the catalysts of the present invention are higher than those of the comparative catalysts after the durability test. Therefore, the catalysts of the present invention is superior to the comparative catalyst in the practical application.

Claims: An exhaust gas purifying process for purification of an exhaust gas which is generated by burning air-fuel mixed gas having an air fuel ratio of 18 or more and an oxygen content of 4.3 % or more, comprising the following steps: a first step of preparing a catalyst comprising: a catalyst support; a layer formed on said catalyst support and including ZSM-5 zeolite and if a binder is used, a silica binder; and at least one of noble metals loaded on said zeolite and selected from the group consisting of platinum and rhodium, said platinum and said rhodium being loaded on said zeolite layer by 1.3 parts by weight or more and 0.7 parts by weight or more with respect to 100 parts by weight of said zeolite layer, respectively; and a second step of contacting said catalyst with oxygen-rich exhaust gases containing nitrogen oxides and 4.3 % or more of oxygen, thereby reducing said nitrogen oxides by catalytic reaction. The exhaust gas purifying process according to claim 1, wherein said zeolite layer comprises a zeolite, wherein the SiO₂/Al₂O₃ molar ratio falls in the range of from 10 to 200. The exhaust gas purifying process according to claim 1, wherein said catalyst support is one selected from the group consisting of a pellet-shaped catalyst support, a monolithic catalyst support and a metal catalyst support. The exhaust gas purifying process according to claim 1, wherein said catalyst support is made of a ZSM-5 zeolite.

Inventors: ISHIBASHI KAZUNOBU, KONDOH SHIROH C O K K TOYOTA C, MATSUMOTO SHINICHI, UTSUMI YASUHIDE C O CATALER IN, ISHIBASHI, KAZUNOBU, KONDOH, SHIROH, C/O K. K. TOYOTA CHUO KENKYUSHO, MATSUMOTO, SHINICHI, UTSUMI, YASUHIDE, C/O CATALER INDUSTRIAL CO., LTD.
Applicants: CATALER IND CO, TOSOH CORP, TOYOTA MOTOR CO LTD, CATALER INDUSTRIAL CO., LTD., TOSOH CORPORATION, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1990
Main classification: B01D53
IPC codes: B01J29, B01D53, B01J37