JP4485564B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  In the exhaust passage of an internal combustion engine where combustion is performed under a lean air-fuel ratio, NOx in the exhaust gas flowing in when the air-fuel ratio of the flowing exhaust gas is lean is occluded and the air-fuel ratio of the exhaust gas flowing in is rich. A NOx storage reduction catalyst that releases and reduces the stored NOx, and when the NOx stored in the NOx storage reduction catalyst should be released and reduced, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst An internal combustion engine in which the engine is temporarily switched to rich is known (see Patent Document 1). In this internal combustion engine, NOx in the exhaust gas is stored in the NOx storage reduction catalyst. The amount of NOx occluded in the NOx occlusion reduction catalyst gradually increases with time. Therefore, before the NOx storage reduction catalyst is saturated, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is temporarily switched to rich so that the NOx stored in the NOx storage reduction catalyst is released and reduced. Like to do. In this case, in order to switch the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst to rich, for example, the air-fuel ratio in the internal combustion engine is switched to rich.

  On the other hand, there is known an internal combustion engine in which a front catalyst and a rear catalyst are accommodated in series in a common casing disposed in an engine exhaust passage, and the front catalyst and the rear catalyst are each constituted by a single layer structure or a multilayer structure. Yes (see Patent Document 2).

JP 11-44234 A JP 2006-291812 A

  Since the fuel consumption increases when the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is frequently switched to rich, it is preferable that the NOx storage capacity of the NOx storage reduction catalyst is as large as possible. However, since the space for arranging the NOx storage reduction catalyst is limited, it is necessary to increase the NOx storage capacity of the NOx storage reduction catalyst while keeping the size or capacity of the NOx storage reduction catalyst as small as possible.

  Further, immediately after the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is switched to rich, a large amount of NOx may be exhausted from the NOx storage reduction catalyst without being reduced. It is also necessary to suppress this.

  In order to solve these problems, it is conceivable to arrange an additional catalyst upstream or downstream of the NOx storage reduction catalyst as described in Patent Document 2 or to configure the NOx storage reduction catalyst from a multilayer structure. The current situation is not a satisfactory solution.

  According to the present invention, in an exhaust gas purification apparatus for an internal combustion engine in which a front-stage catalyst and a rear-stage catalyst are housed in series in a common casing arranged in an engine exhaust passage, the front-stage catalyst is emptied of the exhaust gas flowing into it. The NOx occlusion reduction catalyst which stores NOx in exhaust gas flowing in when the fuel ratio is lean and releases the exhausted NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich is reduced. Constructed from a three-way catalyst or NOx occlusion reduction catalyst, and prepared so that the oxidizability of the front-stage catalyst is higher than the oxidizability of the rear-stage catalyst and the reductivity of the rear-stage catalyst is higher than the reducibility of the front-stage catalyst The former catalyst is composed of a multilayer structure having an upper layer and a lower layer, and the upper catalyst in the former catalyst is prepared so that the upper layer is more oxidizing than the lower layer, and the lower layer is reduced. It is prepared so as to be higher than the reduction of the layer.

  The NOx storage capacity and the NOx purification rate of the NOx storage reduction catalyst can be increased.

  FIG. 1 shows a case where the present invention is applied to a spark ignition type internal combustion engine. The present invention can also be applied to a compression ignition type internal combustion engine.

  Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a cylinder head, 4 is a piston, 5 is a combustion chamber, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, 9 is an exhaust port, Reference numeral 10 denotes a spark plug. The intake port 7 of each cylinder is connected to a surge tank 12 via a corresponding intake branch pipe 11. The surge tank 12 is connected to an air cleaner 14 via an intake duct 13. An air flow meter 15 and a throttle valve 17 driven by a step motor 16 are disposed in the intake duct 13. A fuel injection valve 18 is attached to each intake port 7. Each fuel injection valve 18 is connected to a common rail 19, and the common rail 19 is connected to a fuel tank 21 via a fuel pump 20 capable of controlling the discharge amount. A fuel pressure sensor 22 is attached to the common rail 19, and the discharge amount of the fuel pump 20 is controlled so that the fuel pressure in the common rail 19 matches the target pressure.

  On the other hand, the exhaust port 9 of each cylinder is connected to the casing 25 via the corresponding exhaust manifold 23 and the exhaust pipe 24, and the casing 25 is connected to the exhaust pipe 26. An air-fuel ratio sensor 27 is attached in the exhaust pipe 24, and a catalyst 28 is accommodated in the casing 25.

  The electronic control unit 30 is composed of a digital computer, and includes a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a CPU (Microprocessor) 34, an input port 35, and a mutual connection via a bidirectional bus 31. An output port 36 is provided. A load sensor 40 that generates an output voltage proportional to the amount of depression of the accelerator pedal 39 is connected to the accelerator pedal 39. Output voltages of the air flow meter 15, the fuel pressure sensor 22, the air-fuel ratio sensor 27, and the load sensor 40 are respectively input to the input ports 36 via corresponding AD converters 38. Further, the crank angle sensor 41 generates an output pulse every time the crankshaft rotates, for example, 30 °, and this output pulse is input to the input port 36. The CPU 34 calculates the engine speed Ne based on this output pulse. On the other hand, the output port 36 is connected to the spark plug 10, the step motor 16, the fuel injection valve 18, and the fuel pump 20 via a corresponding drive circuit 38.

  The catalyst 28 includes a front catalyst 28U and a rear catalyst 28D that are arranged in series in the casing 25. In the embodiment according to the present invention, the front catalyst 28U is composed of a NOx storage reduction catalyst, and the rear catalyst 28D is composed of a three-way catalyst. Note that the rear-stage catalyst 28D can also be composed of a NOx storage reduction catalyst. In the embodiment according to the present invention, the capacity of the front catalyst 28U is made equal to or larger than the capacity of the rear catalyst 28D. However, the capacity of the front catalyst 28U can be made smaller than the capacity of the rear catalyst 28D.

  FIG. 2 shows the structure of the front stage catalyst, that is, the NOx storage reduction catalyst 28U. In the embodiment shown in FIG. 2, the NOx storage reduction catalyst 28 </ b> U has a honeycomb structure and includes a plurality of exhaust gas flow passages 51 separated from each other by thin partition walls 50. A catalyst carrier made of alumina, for example, is supported on both side surfaces of each partition wall or base material 50. FIGS. 3A and 3B schematically show a cross section of the surface portion of the catalyst carrier 55. . As shown in FIGS. 3A and 3B, a noble metal catalyst 56 is dispersed and supported on the surface of the catalyst carrier 55, and a layer of NOx absorbent 57 is further provided on the surface of the catalyst carrier 55. Is formed.

  As the noble metal catalyst 56, at least one selected from platinum Pt, palladium Pd, osmium Os, gold Au, rhodium Rh, iridium Ir, and ruthenium Ru is used, and the components constituting the NOx absorbent 57 are, for example, potassium K, sodium Na At least one selected from alkali metals such as cesium Cs, alkaline earth such as barium Ba and calcium Ca, and rare earths such as lanthanum La and yttrium Y is used.

  If the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage, the combustion chamber 5 and the exhaust passage upstream of the NOx storage reduction catalyst 28U is referred to as the air-fuel ratio of the exhaust gas, the NOx absorbent 57 When the fuel ratio is lean, NOx is absorbed, and when the oxygen concentration in the exhaust gas decreases, the NOx is absorbed and released to release the absorbed NOx.

That is, the case where platinum Pt is used as the noble metal catalyst 56 and barium Ba is used as a component constituting the NOx absorbent 57 will be described as an example. When the air-fuel ratio of the exhaust gas is lean, that is, the oxygen concentration in the exhaust gas is high. Sometimes the NO contained in the exhaust gas is oxidized on the platinum Pt 56 to become NO ₂ as shown in FIG. 3A, and then absorbed into the NOx absorbent 57 and combined with the barium carbonate BaCO ₃ and nitrate ions. It diffuses into the NOx absorbent 57 in the form of NO ₃ ⁻ . In this way, NOx is absorbed in the NOx absorbent 57. Exhaust oxygen concentration in the gas at the surface as far as the platinum Pt56 _high, NO ₂ is generated, as long as NO ₂ to NOx absorbing capability of the NOx absorbent 57 is not saturated is absorbed in the NOx absorbent 57 nitrate ions NO ₃ ^- is Generated.

On the other hand, when the air-fuel ratio of the exhaust gas is made rich, the oxidation concentration in the exhaust gas decreases, so that the reaction proceeds in the reverse direction (NO ₃ ⁻ → NO ₂ ), and therefore, FIG. 3 (B) As shown, nitrate ions NO ₃ ⁻ in the NOx absorbent 57 are released from the NOx absorbent 57 in the form of NO ₂ . Next, the released NOx is reduced by unburned HC and CO contained in the exhaust gas.

  Further, in the embodiment according to the present invention, as shown in FIG. 4, the NOx storage reduction catalyst 28 </ b> U has a multilayer structure including an upper layer 28 </ b> UU and a lower layer 28 </ b> UL. That is, the lower layer 28UL and the upper layer 28UU are sequentially stacked on the substrate 50. In this case, each of the upper layer 28UU and the lower layer 28UL constitutes a NOx storage reduction catalyst, that is, includes the above-described noble metal catalyst 56 and NOx absorbent 57. An additional layer may be provided between the upper layer 28UU and the lower layer 28UL, or between the lower layer 28UL and the catalyst carrier 55.

  As the noble metal catalyst 56 of the upper layer 28UU, at least one selected from highly oxidizing noble metals, that is, platinum Pt, palladium Pd, osmium Os, and gold Au is used. On the other hand, as the noble metal catalyst 56 of the lower layer 28UL, at least one selected from highly reducible noble metals, that is, rhodium Rh, iridium Ir, and ruthenium Ru is used. In this case, the upper layer 28UU does not contain a highly reducing noble metal.

  FIG. 5 shows various examples of the noble metal catalyst 56 of the upper layer 28UU and the lower layer 28UL. That is, as the noble metal catalyst 56 of the upper layer 28UU, platinum Pt is used in the example of FIG. 5 (A), palladium Pd is used in the example of FIG. 5 (B), and in the example of FIG. 5 (C). Platinum Pt and palladium Pd are used. On the other hand, rhodium Rh is used as the noble metal catalyst 56 of the lower layer 28UL in any example.

  When the noble metal catalyst 56 of the upper layer 28UU and the lower layer 28UL is selected in this way, the oxidizability of the upper layer 28UU is higher than the oxidizability of the lower layer 28UL, and the reducibility of the lower layer 28UL is higher than the reducibility of the upper layer 28UU.

  On the other hand, the latter-stage catalyst, that is, the three-way catalyst 28D, also has a honeycomb structure like the NOx storage reduction catalyst 28U, and includes a plurality of exhaust gas flow passages separated from each other by thin partition walls. A catalyst carrier made of alumina, for example, is supported on both side surfaces of each partition wall, and a catalyst component containing a noble metal component is supported on the surface of the catalyst carrier.

  In the embodiment according to the present invention, the three-way catalyst 28D also has a multilayer structure including an upper layer 28DU and a lower layer 28DL. In this case, each of the upper layer 28DU and the lower layer 28DL constitutes a three-way catalyst.

  In the three-way catalyst 28D, at least one selected from noble metals with high reducibility is used as the noble metal component of the upper layer 28DU, and at least one selected from noble metals with high oxidizability is used as the noble metal component of the lower layer 28DL. In the example shown in FIG. 6A, rhodium Rh is used as the noble metal component of the upper layer 28DU, and platinum Pt is used as the noble metal component of the lower layer 28DL.

  When the noble metal components of the upper layer 28DU and the lower layer 28DL are selected in this way, the reducing property of the upper layer 28DU becomes higher than that of the lower layer 28DL, and the oxidizing property of the lower layer 28DL becomes higher than the oxidizing property of the upper layer 28DU.

  Alternatively, the three-way catalyst 28D can be configured from a single layer structure. In this case, a noble metal having at least high reducibility is used as the noble metal component. A highly oxidizable metal may or may not be used. In the example shown in FIG. 6B, rhodium Rh and platinum Pt are used as noble metal components.

  When the noble metal catalyst 56 of the front stage catalyst, that is, the NOx storage reduction catalyst 28U and the noble metal component of the rear stage catalyst, that is, the three-way catalyst 28D are selected in this way, the oxidation property of the NOx storage reduction catalyst 28 becomes higher than the oxidation property of the three-way catalyst 28D. Further, the reducibility of the three-way catalyst 28D becomes higher than the reducibility of the NOx storage reduction catalyst 28U.

  In the embodiment according to the present invention, the catalyst 28 is formed by supporting the upstream catalyst, that is, the NOx occlusion reduction catalyst 28U and the downstream catalyst, that is, the three-way catalyst 28D, on separate substrates and connecting these substrates in series with each other. . Note that the NOx storage reduction catalyst 28U may be carried on the upstream side of the common base material, and the three-way catalyst 28D may be carried on the downstream side.

On the other hand, the NOx occlusion reduction catalyst 28U having a multilayer structure is manufactured, for example, as follows. The case where rhodium Rh is used as the noble metal catalyst 56 of the lower layer 28UL and platinum Pt is used as the noble metal catalyst 56 of the upper layer 28UU will be described as an example. First, a slurry in which the carrier powder forming the catalyst carrier of the lower layer 28UL and the rhodium powder are dispersed Is prepared and this slurry is applied onto the substrate. In this case, for example, zirconium Zr, alumina Al ₂ O ₃ , ceria CeO ₂ , ZrO ₂ —Al ₂ O ₃ , ZrO ₂ —Al ₂ O ₃ —TiO ₂ can be used as the catalyst support of the lower layer 28UL. The rhodium powder is formed from PM powder and dispersed in the slurry in the form of nitrate or acetate. The viscosity of the slurry is preferably about 30%, for example, and the coating amount is preferably 50 g / L to 200 g / L. Next, drying (200 ° C., 2 hours) and baking (400 ° C., 4 hours) are performed, and thus the lower layer 28UL is formed.

Next, a slurry is prepared in which a carrier powder that forms the catalyst carrier of the upper layer 28UU and a platinum powder are dispersed, and this slurry is applied onto the lower layer 28UL. In this case, as the catalyst support of the upper layer 28UU, for example, zirconium Zr, alumina Al ₂ O ₃ , ceria CeO ₂ , Al ₂ O ₃ —CeO ₂ , ZrO ₂ —Al ₂ O ₃ , ZrO ₂ —Al ₂ O ₃ —TiO ₂ Can be used. The platinum powder is dispersed in the slurry in the form of nitrate or acetate such as tetrachloroplatinum salt or dinitroplatinum salt. The viscosity of the slurry is preferably about 30%, for example, and the coating amount is preferably 50 g / L to 200 g / L. Next, drying (200 ° C., 2 hours) and baking (400 ° C., 4 hours) are performed, and thus the upper layer 28UU is formed. Alternatively, the catalyst support may be first formed on the lower layer 28UL, and then the catalyst support may be impregnated with an aqueous solution of tetrachloroplatinum salt or dinitroplatinum salt.

  The three-way catalyst 28D having a multilayer structure can also be manufactured in the same manner as the NOx storage reduction catalyst 28U.

  By the way, in the embodiment according to the present invention, as shown in FIG. 7, when the engine load factor KL is lower than the predetermined set load factor KLX, the lean operation for performing combustion under the lean air-fuel ratio is performed. During high load operation where the engine load factor KL is greater than the set load factor KLX, the stoichiometric air-fuel ratio operation is performed in which combustion is performed under the stoichiometric air-fuel ratio. Here, the engine load factor KL is the ratio of the engine load to the total load. In this case, an internal combustion engine that performs lean operation may be temporarily switched to theoretical air-fuel ratio operation according to the engine operating state.

Therefore, when the lean operation is performed, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 28U becomes lean, and at this time, NOx in the exhaust gas is stored in the NOx storage reduction catalyst 28U. However, if the lean operation is continued, the NOx occlusion capacity of the NOx occlusion reduction catalyst 28U is saturated during that time, and therefore NOx cannot be occluded by the NOx occlusion reduction catalyst 28U. Therefore, in the embodiment according to the present invention, the air-fuel ratio of the exhaust gas is temporarily enriched before the storage capacity of the NOx storage reduction catalyst 28U is saturated, thereby releasing NOx from the NOx storage reduction catalyst 28U, and the HC, Reduction to N ₂ or the like by CO is performed.

  That is, in the embodiment according to the present invention, the amount of NOx stored per unit time in the NOx storage reduction catalyst 28U is stored in advance in the ROM 32 in the form of a map as a function of the engine operating state such as the engine load factor L and the engine speed Ne. The integrated value SN of the NOx amount stored in the NOx storage reduction catalyst 28U is calculated by integrating the NOx amount. In addition, every time the stored NOx amount integrated value SN exceeds the upper limit value MAX, a rich operation in which combustion is performed under a rich air-fuel ratio is temporarily performed. As a result, NOx is released from the NOx occlusion reduction catalyst 28U and reduced.

  FIG. 8 shows a routine for executing the engine operation control of the embodiment according to the present invention. This routine is executed by interruption every predetermined time.

  Referring to FIG. 8, first, at step 100, it is judged if the engine load factor KL is larger than the set load factor KLX (FIG. 7). When KL ≦ KLX, the routine proceeds to step 101 where a lean operation is performed. In the following step 102, the stored NOx amount integrated value SN is calculated. In the following step 103, it is determined whether or not the stored NOx amount integrated value SN is larger than the upper limit value MAX. When SN ≦ MAX, the processing cycle is ended, and therefore the lean operation is continued. On the other hand, when SN> MAX, the routine proceeds to step 104 where the rich operation is performed for a certain time, for example. In the subsequent step 105, the stored NOx amount integrated value SN is cleared. On the other hand, when the engine load factor KL is larger than the set load factor KLX, the routine proceeds from step 100 to step 106, where the theoretical air-fuel ratio operation is performed.

  In the embodiment according to the present invention, the NOx storage capacity of the catalyst 28 or the NOx storage reduction catalyst 28U can be increased.

  FIG. 9A shows the experimental result of the NOx storage capacity ST of the catalyst 28. In FIG. 9A, in Comparative Example Ca, the catalyst 28 is composed of only a single-layer NOx storage reduction catalyst, and platinum Pt and rhodium Rh are used as noble metal catalysts. In Example Ea1, the catalyst 28 is composed of only a two-layer NOx storage reduction catalyst, platinum Pt is used as the upper layer noble metal catalyst, and rhodium Rh is used as the lower layer noble metal catalyst. In Example Ea2, the catalyst 28 is composed of only a two-layer NOx storage reduction catalyst, platinum Pt and palladium Pd are used as the upper layer noble metal catalyst, and rhodium Rh is used as the lower layer noble metal catalyst.

  As can be seen from FIG. 9A, the NOx storage capacity ST of the catalyst 28 increases in the examples Ea1 and Ea2, and further increases in the example Ea2. This is considered to be due to the NOx occlusion reduction catalyst having a multilayer structure. Therefore, it is possible to reduce the frequency of the switching operation for switching the air-fuel ratio of the exhaust gas flowing into the catalyst 28 to rich, and to reduce the fuel consumption.

  On the other hand, as shown in FIG. 10, when the air-fuel ratio A / F of the exhaust gas flowing into the catalyst 28 is switched to rich, the NOx amount EXN exhausted from the catalyst 28 per unit time increases rapidly and reaches a peak value. PKN is reached and then decreases. In the embodiment according to the present invention, this exhaust NOx amount peak value PKN can be reduced.

  FIG. 9B shows an experimental result of the exhaust NOx amount peak value PKN of the catalyst 28. In FIG. 9B, in Comparative Example Cb1, the catalyst 28 is composed of only a single-layer NOx storage reduction catalyst, and platinum Pt and rhodium Rh are used as noble metal catalysts. In Comparative Example Cb2, the catalyst 28 is composed of only a two-layer NOx storage reduction catalyst, platinum Pt is used as the upper layer noble metal component, and rhodium Rh is used as the lower layer noble metal component. In Example Eb, the catalyst 28 is composed of a front stage catalyst and a rear stage catalyst. The pre-stage catalyst is composed of a two-layer NOx occlusion reduction catalyst, platinum Pt is used as an upper layer noble metal catalyst, and rhodium Rh is used as a lower layer noble metal catalyst. The latter catalyst is composed of a three-way catalyst having a single layer structure, and platinum Pt and rhodium Rh are used as noble metal components.

  As can be seen from FIG. 9B, the exhausted NOx amount peak value PKN increases in Comparative Example Cb2 than in Comparative Example Cb1. However, in Example Eb, the exhaust NOx amount peak value PKN can be significantly reduced. This is considered to be due to NOx released from the front-stage catalyst, that is, the NOx storage reduction catalyst, being reduced by the rear-stage catalyst. Therefore, the NOx purification rate during lean operation can be maintained high while increasing the NOx storage capacity.

  Further, in the embodiment according to the present invention, the NOx purification rate EFFS of the catalyst 28 can be maintained high when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio as in the high load operation.

  FIG. 9C shows an experimental result of the NOx purification rate EFFS of the catalyst 28 when the air-fuel ratio of the inflowing exhaust gas is the stoichiometric air-fuel ratio. In FIG. 9C, in Comparative Example Cc1, the catalyst 28 is composed only of a single-layer NOx storage reduction catalyst, and platinum Pt and rhodium Rh are used as noble metal catalysts. In Comparative Example Cc2, the catalyst 28 is composed of only a three-way catalyst having a two-layer structure, rhodium Rh is used as an upper layer noble metal catalyst, and platinum Pt is used as a lower layer noble metal catalyst. In Example Ec, the catalyst 28 is composed of a front stage catalyst and a rear stage catalyst. The pre-stage catalyst is composed of a two-layer NOx occlusion reduction catalyst, platinum Pt is used as an upper layer noble metal catalyst, and rhodium Rh is used as a lower layer noble metal catalyst. The latter catalyst is composed of a three-way catalyst having a single layer structure, and platinum Pt and rhodium Rh are used as noble metal components. If the inflow NOx amount into the catalyst 28 per unit time and the outflow NOx amount from the catalyst 28 are INN and EXN, respectively, the NOx purification rate EFF of the catalyst 28 can be expressed by the following equation.

EFF = (INN−EXN) / INN
As can be seen from FIG. 9C, in Example Ec, the NOx purification rate EFFS is higher than in Comparative Example Cc1, and a NOx purification rate EFFS equivalent to that in Comparative Example Cc2 can be obtained.

  In the above-described embodiment according to the present invention, the rich operation is performed in order to make the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst 28U rich. However, in an internal combustion engine equipped with a fuel injection valve that directly injects fuel into the combustion chamber, the air-fuel ratio of the inflowing exhaust gas may be made rich by performing fuel injection in the expansion stroke or exhaust stroke. Alternatively, the air-fuel ratio of the inflowing exhaust gas can be made rich by secondarily supplying a reducing agent or fuel into the exhaust passage upstream of the NOx storage reduction catalyst 28U.

  In the above-described embodiment according to the present invention, the lean operation is performed during the low load operation, and the stoichiometric air-fuel ratio operation is performed during the high load operation. However, the stoichiometric air-fuel ratio operation may be performed even during the acceleration operation.

1 is an overall view of an internal combustion engine. It is sectional drawing of a NOx storage reduction catalyst. It is sectional drawing of the surface part of a catalyst support | carrier. It is an expanded sectional view of a NOx storage reduction catalyst. It is a figure which shows the various examples of a NOx storage reduction catalyst. It is a figure which shows the various examples of a three-way catalyst. It is a figure explaining the setting load factor KLX. It is a flowchart for performing an engine operation control routine. It is a figure which shows various experimental results. It is a figure for demonstrating discharge | emission NOx amount peak value.

Explanation of symbols

1 Engine body 24, 26 Exhaust pipe 25 Casing 28 Catalyst 28U Pre-stage catalyst (NOx storage reduction catalyst)
28D latter stage catalyst (three way catalyst)

Claims (5)

  In an exhaust gas purification apparatus for an internal combustion engine in which a front-stage catalyst and a rear-stage catalyst are accommodated in series in a common casing disposed in an engine exhaust passage, the front-stage catalyst is disposed when the air-fuel ratio of the inflowing exhaust gas is lean. It is composed of a NOx occlusion reduction catalyst that occludes NOx in the inflowing exhaust gas and releases and reduces the NOx occluded when the air-fuel ratio of the inflowing exhaust gas becomes rich, and the latter catalyst is a three-way catalyst or NOx occlusion It is composed of a reduction catalyst, and is prepared so that the oxidation performance of the front-stage catalyst is higher than that of the rear-stage catalyst and the reduction performance of the rear-stage catalyst is higher than that of the front-stage catalyst. And a multilayer structure having a lower layer, and prepared so that the upper layer oxidizability is higher than the lower layer oxidizability in the pre-stage catalyst, and the lower layer reducibility is higher than the upper layer reducibility. Exhaust purification apparatus becomes higher thus prepared engine.   The latter catalyst is composed of a multilayer structure including an upper layer and a lower layer, and the lower layer is prepared such that the upper layer has a higher reducibility than the lower layer, and the lower layer has a higher oxidizing property than the upper layer. The exhaust emission control device for an internal combustion engine according to claim 1, prepared as described above.   2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the rear catalyst has a single layer structure.   When the air-fuel ratio in the internal combustion engine is set to lean and the NOx stored in the NOx storage reduction catalyst is to be released and reduced, the air-fuel ratio of the exhaust gas flowing into the NOx storage reduction catalyst is temporarily switched to rich. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3.   The exhaust gas purification apparatus for an internal combustion engine according to claim 4, wherein the air-fuel ratio in the internal combustion engine is temporarily switched to the stoichiometric air-fuel ratio in accordance with the engine operating state.

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