EXHAUST GAS PURIFICATION SYSTEM FOR INTERNAL COMBUSTION ENGINE |
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| 申请号 | EP07734307.7 | 申请日 | 2007-03-29 | 公开(公告)号 | EP2004965B1 | 公开(公告)日 | 2010-08-25 |
| 申请人 | Toyota Jidosha Kabushiki Kaisha; | 发明人 | TAKAGI, Nobuyuki; MIURA, Masahide; MATSUMOTO, Shinichi; TANABE, Toshitaka; NAGAI, Yasutaka; HATANAKA, Miho; DOHMAE, Kazuhiko; YAMAMOTO, Toshio; IKEDA, Yasuo; | ||||
| 摘要 | An exhaust gas purification system includes: a catalyst (3) provided in an exhaust gas passage of an internal combustion engiue (1) and having a carrier containing a basic oxide and platinum carried on the carrier; a catalyst (4) for purifying NOx; and oxygen supply means (2). | ||||||
| 权利要求 | |||||||
| 说明书全文 | The present invention relates to an exhaust gas purification system including a catalyst that is provided in an exhaust gas passage of an internal combustion engine and that can be regenerated by redispersing platinum particles having been sintered. In automobiles, an exhaust gas purifying catalyst is used to remove pollutants, such as HC, CO and NOx, contained in an exhaust gas from an engine. A known example of such an exhaust gas purifying catalyst is a three-way catalyst that simultaneously removes HC, CO and NOx from the exhaust gas after combustion at a theoretical air-fuel ratio. The three-way catalyst includes a honeycombed carrier base made of, for example, cordierite or a metal foil, a catalyst-carrying layer made of, for example, active alumina powders or silica powders and coated on the surface of the carrier base, and a precious metal such as platinum carried on the catalyst-carrying layer. The three-way catalyst removes HC and CO in the exhaust gas by oxidation, and reduces NOx. An oxidation catalyst is also known, in which a precious metal is carried on a catalyst-carrying layer made of zeolite, which has excellent in absorption characteristics for HC contained in the exhaust gas. In the oxidation catalyst, HC contained in the exhaust gas is absorbed by the catalyst-carrying layer at low temperatures, and is released as the temperature of the catalyst increases. The released HC is oxidized by the precious metal when the precious metal is at or above its active temperature. In this way, HC emission can be suppressed at low temperatures, for example, when starting the engine and during in winter. In addition, to mitigate variations in atmosphere of the exhaust gas, a catalyst having oxygen storage / releasing capacity and an NOx storage reduction catalyst are known. In the former, an element such as ceria is carried on a catalyst-carrying layer. In the latter, an NOx storage material such as alkali metals and alkali earth metals is carried on a catalyst-carrying layer, along with a precious metal. For the precious metal for use as an active component of such catalysts, platinum (Pt) is primarily used. When platinum is exposed to an oxidizing atmosphere at a high temperature for a long time, however, platinum particles become flocculated (platinum particles grow) and thus platinum sinters, unfavorably reducing the specific surface area of the platinum particles and hence the activity of the catalyst. Thus, various methods have been developed to regenerate exhaust gas purifying catalysts in which the platinum has been sintered. For example, Japanese patent application publication No. The above-described regeneration method includes, as the oxidation process to regenerate platinum, increasing the air-fuel (A/F) ratio of an air-fuel mixture flowing into an engine to an extremely high degree. When the above-described regeneration method is used to regenerate a catalyst incorporated in an exhaust gas purification system, however, the time when regeneration of platinum is required does not always coincide with the time when the air-fuel ratio is increased, depending on the operating conditions, making it difficult to reliably regenerate platinum. When the concentration of oxygen is low, a sufficient amount of oxygen may not reach the downstream side of the catalyst, even if the air-fuel ratio is increased. In such a case, the regeneration process cannot be performed efficiently. Providing an air supply device on the upstream side of the catalyst is also proposed. With such an arrangement, however, it is difficult for an ordinary three-way catalyst to remove NOx in the exhaust gas with an excessive amount of oxygen supplied. As a result, NOx may unfavorably be emitted. The present invention provides an exhaust gas purification system as defined in claim 1. It can thus regenerate the platinum and maintain the exhaust gas purification performance even during the regeneration of the platinum. The oxygen supply means may be provided in the exhaust gas passage upstream of the first catalyst, and the second catalyst may be provided in the exhaust gas passage downstream of the first catalyst. Alternatively the oxygen supply means may be provided in the exhaust gas passage upstream of the first catalyst, and the second catalyst may be provided in the exhaust gas passage upstream of the first catalyst and the oxygen supply means. The basic oxide may be an oxide of at least one element selected from the alkaline earth metals and the rare earth elements. Alternatively, the carrier may contain a complex oxide of zirconia and/or alumina and at least one element selected from the alkaline earth metals and the rare earth elements. In the exhaust gas purification systems of the present invention, a first catalyst may be exposed to an oxidizing atmosphere by the oxygen supply means provided in the exhaust gas passage, allowing platinum having been sintered to be redispersed by oxidation / reduction. In addition, the systems are also provided with a second catalyst for removing NOx, for which reduction purification rate tends to be reduced during the supply of oxygen. The second catalyst can reduce and purify NOx even during the regeneration of the first catalyst, maintaining the exhaust gas purification performance. The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
An exhaust gas purification system according to first and second embodiments of the present invention includes: a catalyst (hereinafter referred to as "platinum-carrying catalyst") provided in an exhaust gas passage of an internal combustion engine and having a carrier containing a basic oxide and platinum carried on the carrier; a three-way catalyst or an NOx storage reduction catalyst; and oxygen supply means. The basic oxide contained in the carrier of the platinum-carrying catalyst may be an oxide of at least one element selected from the alkaline earth metals and the rare earth elements. Alternatively, the carrier may contain a complex oxide of zirconia and/or almina and at least one element selected from the group including the alkaline earth metals, the rare earth elements and the group 3A elements. The alkaline earth metal oxide may be an oxide of Mg, Ca, Sr, Ba and Ra of various valence numbers, and may be selected from at least one of magnesium oxide (MgO), calcium oxide (CaO) and barium oxide (BaO), which interact strongly with and hence have a high affinity to metal Pt and Pt oxide. The rare earth oxide may be an oxide of a metal, such as Sc, Y, La, Ce, Pr, Nd, Sm, Tb, Dy, Yb and Lu, of various valence numbers, and is preferably at least one kind selected from lanthanum oxide (La2O3), ceric oxide (CeO2), yttrium oxide (Y2O3) and neodymium oxide (Nd2O3), which interact strongly with and hence have a high affinity to metal Pt and Pt oxide. The complex oxide of zirconia (ZrO2) and/or alumina (Al2O3) and at least one of elements selected from the group including alkali earth metals, rare earth elements and group 3A elements may be a complex oxide of zirconia and/or alumina and at least one of elements selected from the above-mentioned alkali metals Mg, Ca, Sr, Ba and Ra and the above-mentioned rare earth elements such as Sc, Y, La, Ce, Pr, Nd, Sm, Tb, Dy, Yb and Lu, and may be a complex oxide with Mg, Ca, Ba, La, Ce, Y or Nd, which interacts strongly with and hence has a high affinity to metal Pt and Pt oxide. Such complex oxides include CeO2-ZrOz-Y2O3, ZrO2-La2O3, CeO2-ZrO2 and CeO2-ZrO2-La2O3,Pr2O3. The proportion (composite ratio) of zirconia and/or alumina to at least one of elements selected from the group including alkali earth metals, rare earth elements and group 3A elements is not specifically limited. From the viewpoint of the effect, the proportion of zirconia and/or alumina in the complex oxide may be 10 to 90 wt%, and may be 30 to 90 wt%. The carrier of the platinum-carrying catalyst needs only to contain the above-mentioned basic oxide, even a small amount, in order to be effective. The carrier may contain 50 wt%, for example, of the basic oxide, or may be entirely composed of the basic oxide. In the case where the basic oxide is used as a part of the carrier, the rest of the carrier may be silica, alumina, zirconia, titania and silica-alumina. The production method of the carrier is not specifically limited. For example, the method may include allowing a precipitate of the above-mentioned basic oxide to be produced from an aqueous solution containing a metal salt (for example, nitrate) as a raw material of the basic oxide, and as necessary a surface-active agent (for example, nonion surface-active agent), in the presence of ammonia, and filtering, washing, drying and then calcining the obtained precipitate. The platinum-carrying catalyst includes at least powders in which platinum is carried on the carrier. The amount of platinum carried on the carrier may be in a range of 0.01 to 10 wt%, and may be in a range of 0.1 to 5 wt%. If the amount of platinum carried is less than the lower limit, the activity of the catalyst as an exhaust gas purifying catalyst tends to be insufficient. On the other hand, if the amount of platinum carried is more than the upper limit, the activity of the catalyst does not improve, and only increases the cost. The method of causing platinum to be carried on the carrier is not specifically limited. For example, the method may include causing an aqueous solution containing a platinum salt (for example, dinitrodiamine salt) or a platinum complex (for example, tetraammine complex) to contact the carrier, and drying and then calcining the carrier. The form of the platinum-carrying catalyst is not specifically limited. The platinum-carrying catalyst may be a monolithic catalyst in the form of a honeycomb, or a pellet catalyst in the form of a pellet, for example. The base is also not specifically limited, and may be selected appropriately according to the purpose of use, for example, of the catalyst to be obtained. The base may be a DPF base, a monolithic base, a pellet base and a plate-like base, for example. The material of the base is also not specifically limited. The base may be made of ceramics such as cordierite, silicone carbide and mullite, and metals such as stainless steel containing chromium and alunium. In addition, the method of producing such a catalyst is not specifically limited- For example, a monolithic catalyst may be produced by forming a coating layer of the above-mentioned carrier powders on a honeycombed base made of cordierite or a metal foil, and causing platinum to be carried on the coating layer. Another method of producing a catalyst includes first causing platinum to be carried on the above-mentioned carrier powders, and then forming a coating layer on the base using the platinum-carrying powders. The three-way catalyst or NOx storage reduction catalyst, that is used in combination with the above platinum-carrying catalyst, may be a conventional three-way catalyst or NOx storage reduction catalyst. That is, the three-way catalyst for use in combination with the platinum-carrying catalyst may be a catalyst produced by forming a coating layer of alumina, or other materials mentioned above, on a base of cordierite, or other materials mentioned above, and causing a precious metal, such as platinum and rhodium, to be carried on the coating layer. The NOx storage reduction catalyst may be a catalyst produced by causing an alkali metal, an alkali earth metal or a rare earth element, for example, to be additionally carried on the coating layer of the three-way catalyst, on which the precious metal mentioned above is already carried. The oxygen supply means for use in the exhaust gas purification system according to this embodiment is not specifically limited as long as it can supply oxygen or air, and may be implemented as an air valve, for example, Ambient air may be supplied directly. In order to efficiently regenerate the catalyst, air warmed beforehand using a heat exchanger mounted on an engine exhaust pipe, for example, may be supplied. In the exhaust gas purification systems according to the embodiments of the present invention, when the platinum in the platinum-carrying catalyst is determined to have been sintered, an oxidation process and a reduction process are performed to redisperse fine platinum particles over the carrier and recover the activity of the catalyst. The carrier of the platinum-carrying catalyst for use in the exhaust gas purification systems according to the embodiments of the present invention contains a basic oxide that interacts strongly with metal platinum. Thus, the surface of platinum particles carried on the carrier have increased in size and coarsened can be oxidized easily, allowing platinum oxide to be formed easily on the surface of the coarsened platinum particles, by heating the platinum-carrying catalyst at 500 to 1000 °C in an oxidizing atmosphere containing oxygen. Then, the platinum oxide, which has a high affinity to the carrier, moves from the surface of the coarsened platinum particles to the surface of the carrier, exposing metal platinum at the surface of the coarsened platinum particles. The exposed metal platinum is oxidized by the oxygen present in the oxidizing atmosphere to become platinum oxide, which in turn moves to the surface of the carrier. By repeating these events, the platinum particles being carried and having coarsened become gradually smaller in diameter and dispersed over the surface of the carrier as if wet, resulting in platinum oxide being highly dispersed over the carrier. Then, a reduction process is performed, which easily reduces the platinum oxide into metal platinum. Thus, fine metal platinum particles are highly dispersed over the carrier, whereby the activity of the catalyst is recovered. The temperature of the catalyst during the regeneration process of the platinum-carrying catalyst is initiated by an oxidation process, which includes supplying oxygen to the catalyst by means of the oxygen supply means disposed upstream of the catalyst, and heating the catalyst in an oxidizing atmosphere containing oxygen. The oxidizing atmosphere need only contain oxygen, even a small amount, that will oxidize a corresponding number of moles of platinum. The concentration of oxygen may be 1 % or more by volume, or further limited to 1 to 20 % by volume. Other than oxygen, the oxidizing atmosphere should contain no reducing gas, but may contain a nitrogen gas or an inert gas. The temperature of the catalyst during the oxidation process may be any temperature at which the metal platinum carried by the catalyst is oxidized, and may be in a range of 500 to 1000 °C. The duration of the oxidation process is determined according to the temperature of the oxidation process. A lower temperature requires a longer duration, and a higher temperature requires a shorter duration. In the case where the temperature of the catalyst during the oxidation process is in a range of 500 to: 1000 °C, the duration of the oxidation process may be between about two seconds and one hour. The catalyst may be heated by reaction heat at the catalyst. However, if the temperature of the catalyst is below the lower limit of the above-mentioned range, the catalyst may be heated by a heating means. The reduction process may be performed by heating in the presence of a reducing gas, such as hydrogen and carbon oxide. For example, the reduction process can be performed while supplying, to the platinum-carrying catalyst, an exhaust gas in a stoichiometric atmosphere which is at a stoichiometric balance, or in a rich atmosphere where the oxygen concentration is low. In this way, the oxidation process and the reduction process can be performed on the platinum-carrying catalyst inside the exhaust gas passage, allowing the regeneration process to be performed on the catalyst as a part of the air-fuel ratio control for the engine. The temperature of the catalyst during the reduction process may be any temperature at which the platinum oxide is reduced, and may be 200 °C or more, and may be in a range of 400 to 1000 °C. The duration of the reduction process is selected appropriately according to the temperature of the reduction process. A lower catalyst temperature requires a longer duration, and a higher catalyst temperature requires a shorter duration. In the case where the temperature of the catalyst during the reduction process is 300°C or more, the duration of the reduction process may be between about two seconds and five minutes. The catalyst may be heated by reaction heat at the catalyst. However, if the temperature of the catalyst is below the lower limit of the above-mentioned range, the catalyst may be heated by a heating means. The platinum-carrying catalyst, which is disposed in the exhaust gas passage of the internal combustion engine, may be subjected to the regeneration process in real time as the performance of the catalyst deteriorates. The regeneration process may be performed regularly, for example according to the operating duration or the travel distance of the vehicle. Alternatively, an NOx sensor or a CO sensor may be provided downstream of the platinum-carrying catalyst to detect the performance of the catalyst, so that the regeneration process can be performed when the detected NOx and/or CO concentration exceeds a reference value. During the regeneration process, the NOx reduction purification performance decreases under the influence of the oxidizing atmosphere in the oxidation process. However, the exhaust gas purification systems of the present invention are provided with a three-way catalyst or an NOx storage reduction catalyst in addition to the catalyst to be regenerated, and thus can prevent NOx emission, even during the regeneration.
In the above-described routine, the temperature sensor may be replaced with an oxygen sensor for detecting the degree of deterioration in oxygen storage performance of the platinum-carrying catalyst 3, in order to determine the degree of deterioration of the catalyst and send an air induction signal for the purpose of regeneration. A honeycombed base made of cordierite was coated with CeO2-ZrO2 complex oxide (with a molar ratio of Ce/Zr = 6/4) carrying 1 wt% of platinum at a coating concentration of 120 g/L, and dried at 250 °C. After dried, the base was calcined at 600 °C for one hour to obtain a platinum-carrying catalyst. The catalyst was exposed to 3 % of CO/N2 at a flow rate of 15 L/min at 950 °C for five hours to obtain a durable platinum-carrying catalyst. A part of the coating layer of the durable catalyst was sample, and subjected to the CO chemisorption method described in Meanwhile, a honeycombed base made of cordierite was coated with alumina at a coating concentration of 120 g/L. The obtained honeycombed carrier was immersed in a dinitrodiamine platinum aqueous solution at a concentration where platinum is present at a concentration of 2 g/L, and dried at 250 °C. After that, the platinum-carrying carrier was immersed in a barium acetate aqueous solution at a concentration where Ba is present at a concentration of 0.3 mol/L, dried, and calcined at 600 °C for one hour to obtain an NOx storage reduction catalyst. An air valve, the durable platinum-carrying catalyst described above and the NOx storage reduction catalyst described above were disposed on an exhaust pipe from a 3.0-L gasoline engine as shown in A honeycombed base made of cordierite was coated with alumina at a coating concentration of 120 g/L. The obtained honeycombed carrier was immersed in a dinitrodiamine platinum aqueous solution at a concentration so prescribed as to carry platinum at 2 g/L, and dried at 250 °C. After drying, the base was calcined at 600 °C for one hour to obtain a three-way catalyst. The three-way catalyst, air valve and durable platinum-carrying catalyst described above were disposed on an exhaust pipe from a 3.0-L gasoline engine as shown in An air valve and the durable platinum-carrying catalyst described above, but no NOx storage reduction catalyst, were disposed in this order in an exhaust pipe from a 3.0-L gasoline engine. The durable platinum-carrying catalyst described above and the NOx storage reduction catalyst described above, but not an air valve, were disposed on an exhaust pipe from a 3.0-L gasoline engine. The engine was operated at an approximately stoichiometric ratio for five hours such that the catalyst bed temperature of the platinum-carrying catalyst is at 700 °C in the above-described exhaust gas purification system. During the operation, air was inducted via the air valve for two minutes at intervals of ten minutes. After this five-hour test, a part of the coating layer of the platinum-carrying catalyst was sampled, and subjected to the same CO chemisorption method as mentioned above to obtain the average diameter of platinum particles. The NOx purification rate during the operation of five hours was obtained by comparing the amount of NOx emission when using a catalyst to that when using no catalyst (that is, Comparative Example 1). In Comparative Example 2, air was not inducted during the regeneration test. The test results are shown in Table 1 below. As shown in the table, the exhaust gas purification systems according to the embodiments of the present invention allows regeneration of catalysts without reducing the NOx purification rate. 14 |
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