Exhaust Gas Purifying Catalyst System

Abstract

Disclosed is an exhaust gas purifying catalyst system, which comprises an upstream catalyst disposed in an exhaust gas passage of an engine at a position on an upstream side with respect to a direction of an exhaust gas stream, and a downstream catalyst disposed in the exhaust gas passage at a position on a downstream side with respect to the direction of the exhaust gas stream. In the exhaust gas purifying catalyst system, the downstream catalyst includes a cerium (Ce)-containing rhodium-doped composite oxide and an Ni component, and the upstream catalyst includes a cerium (Ce)-containing composite oxide other than the rhodium-doped composite oxide, and a Ni component. A ratio of Ni to CeO.sub.2 in the upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO.sub.2 in the downstream catalyst is in the range of 10 to 60 mass %. The exhaust gas purifying catalyst system of the present invention can reduce an amount of H.sub.2S emission while achieving high purification performance.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exhaust gas purifying catalyst system useful in purifying exhaust gas discharged from an internal combustion engine, such as a vehicle engine.

[0003] 2. Background Art

[0004] As an exhaust gas purifying catalyst system for purifying exhaust gas discharged from an engine, there has been known one type comprising a manifold catalyst (close-coupled catalyst) disposed in such a manner as to be directly coupled to an exhaust-gas merging portion of an exhaust manifold of an engine, and an underfloor catalyst (underbody catalyst) disposed downstream of the manifold catalyst. In this type of exhaust gas purifying catalyst system, the manifold catalyst disposed adjacent to the engine is designed to reach its catalytic activation temperature as quickly as possible so as to purify unburned exhaust gas components, such as hydrocarbon (HC) and carbon monoxide (CO), particularly when an exhaust gas temperature is relatively low, for example, in a certain period after engine start-up. The underfloor catalyst is designed to purify exhaust gas components which have not been able to be purified by the manifold catalyst, particularly during a normal operation and a high-speed operation of the engine.

[0005] In many cases, an oxygen storage material is contained in each of the manifold and underfloor catalysts. Such a catalyst containing the oxygen storage material is capable of absorbing and storing oxygen in exhaust gas by the oxygen storage material when the exhaust gas is in an oxygen-excess state (an exhaust air-fuel ratio (exhaust A/F) is in a lean region), and then releasing the stored oxygen into exhaust gas when the exhaust gas is in an oxygen-deficient state (the exhaust A/F is in a rich region), so as to adjust the exhaust A/F approximately at a stoichiometric value to provide enhanced catalytic activity and enhanced exhaust gas purification performance.

[0006] Generally, gasoline as a fuel for an engine, and engine oil as a lubricant for an engine, contains sulfur (S), and therefore exhaust gas discharged from an engine contains an S component, such as sulfur oxides (SOx). In this connection, it is known that the S component causes sulfur (S) poisoning of an exhaust gas purifying catalyst used in an exhaust gas purifying catalyst system, which leads to a deterioration in catalytic activity.

[0007] The S component is likely to be absorbed and stored in an oxygen storage material contained in an exhaust gas purifying catalyst, in the form of SOx. Then, when exhaust gas is placed in the oxygen-deficient state, the S component stored in the form of SOx is released while being converted to hydrogen sulfide (H.sub.2S). H.sub.2S is known as an off-flavor component, and therefore it is required to reduce an amount of H.sub.2S emission. Moreover, H.sub.2S also causes the S poisoning of the exhaust gas purifying catalyst to accelerate the deterioration in catalytic activity.

[0008] With a view to suppressing generation of H.sub.2S to reduce an amount of H.sub.2S emission and S poisoning of an exhaust gas purifying catalyst, there has been known a technique of incorporating a Ni component, such as nickel (Ni) which is a transition metal, or nickel oxide (NiO), into an exhaust gas purifying catalyst, wherein the Ni component is capable of tapping the S component to form nickel sulfide so as to suppress the generation of H.sub.2S.

[0009] As one example of the exhaust gas purifying catalyst containing Ni or NiO, JP 01-242149A discloses an exhaust gas purifying catalyst comprising a support substrate, a nickel oxide-containing activated alumina covering layer formed on the support substrate, a composite oxide made of cerium oxide and zirconium oxide and supported by the activated alumina covering layer, and a catalytic noble metal supported on the activated alumina covering layer.

[0010] Further, JP 08-290063A discloses an exhaust gas purifying catalyst comprising a support, and a plurality of catalyst layers formed on the support in a multi-layered manner, wherein a given catalyst layer located on a lower side of an uppermost catalyst layer contains nickel oxide and palladium.

[0011] As mentioned above, the Ni or NiO-containing exhaust gas purifying catalyst as disclosed in the JP 01-242149A and the JP 08-290063A has a capability to suppress the generation of H.sub.2S. Thus, it would also be advantageous for the above exhaust gas purifying catalyst system comprising the manifold catalyst and the underfloor catalyst, that the Ni component capable of trapping the S component is added to each of the manifold and underfloor catalysts. In this case, it is contemplated to add the Ni component in a larger amount in order to more effectively suppress the generation of H.sub.2S. However, the Ni component added in a larger amount causes a problem about deterioration in purification performance, such as HC, CO and NOx purification performance, although it definitely has the advantageous effect of reducing an amount of H.sub.2S emission.

SUMMARY OF THE INVENTION

[0012] In view of the above circumstances, it is an object of the present invention to provide an exhaust gas purifying catalyst system capable of reducing an amount of H.sub.2S emission while achieving high purification performance.

[0013] In order to achieve the above object, according to one aspect of the present invention, there is provided an exhaust gas purifying catalyst system which comprises an upstream catalyst disposed in an exhaust gas passage of an engine at a position on an upstream side with respect to a direction of an exhaust gas stream, and a downstream catalyst disposed in the exhaust gas passage at a position on a downstream side with respect to the direction of the exhaust gas stream. In the exhaust gas purifying catalyst system, the downstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a rhodium-doped composite oxide having rhodium arranged at a lattice position or interlattice position of a crystal structure thereof, and at least either one of nickel (Ni) and nickel oxide (NiO), and the upstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a composite oxide other than the rhodium-doped composite oxide, and at least either one of nickel (Ni) and nickel oxide (NiO). A ratio of Ni to CeO.sub.2 in the upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO.sub.2 in the downstream catalyst is in the range of 10 to 60 mass %.

[0014] In the exhaust gas purifying catalyst system of the present invention, when exhaust gas passes through the upstream catalyst and the downstream catalyst, exhaust gas components, such as HC, CO and NOx, are purified by both the upstream and downstream catalysts. Thus, even if each of the upstream and downstream catalysts has a slightly lower purification performance as compared with a catalyst without an addition of nickel, the exhaust gas purifying catalyst system can adequately reduce an amount of emission of CO, HC, NOx, etc., to maintain high purification performance as a whole. In addition, the generation of H.sub.2S is suppressed in both the upstream and downstream catalysts. Thus, even if each of the upstream and downstream catalysts is set to have a slightly lower capability of suppressing the generation of H.sub.2S as compared with a catalyst added with a large amount of Ni, the exhaust gas purifying catalyst system can adequately reduce an amount of H.sub.2S emission as a whole.

[0015] The oxygen storage material included in the downstream catalyst is a rhodium-doped composite oxide having rhodium arranged at a lattice position or interlattice position of a crystal structure thereof. Such a composite oxide has a capability to absorb/release oxygen at a relatively high speed, and store oxygen in a relatively large amount. Thus, even if an oxygen concentration in exhaust gas varies, the oxygen concentration can be readily adjusted at a desired value to allow the downstream catalyst to effectively perform a catalytic action thereof. That is, exhaust gas components which have not been able to be purified by the upstream catalyst, can be efficiently purified by the downstream catalyst. This also contributes to achievement of high purification performance.

[0016] As above, the exhaust gas purifying catalyst system of the present invention can reduce an amount of H.sub.2S emission while achieving high purification performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic diagram showing an exhaust gas purifying catalyst system according to one embodiment of the present invention.

[0018] FIG. 2A is a schematic diagram simplistically showing a structure of an Rh-doped Zr--Ce--Nd composite oxide.

[0019] FIG. 2B is a schematic diagram simplistically showing a structure of an Rh-post-supporting Zr--Ce--Nd composite oxide.

[0020] FIG. 3A is a schematic diagram simplistically showing a presumed oxygen storage mechanism in the Rh-doped Zr--Ce--Nd composite oxide.

[0021] FIG. 3B is a schematic diagram simplistically showing a presumed oxygen storage mechanism in the Rh-post-supporting Zr--Ce--Nd composite oxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] An exhaust gas purifying catalyst system according to one embodiment of the present invention will now be described.

[0023] FIG. 1 is a schematic diagram showing the exhaust gas purifying catalyst system according to this embodiment. The exhaust gas purifying catalyst system comprises an upstream catalyst 3 and a downstream catalyst 4 each interposed in an exhaust gas passage 2 connected to a vehicle engine 1. The upstream catalyst 3 is disposed on an upstream side with respect to a direction of an exhaust gas stream, and the downstream catalyst 4 is disposed on a downstream side with respect to the direction of the exhaust gas stream. The upstream catalyst 3 and the downstream catalyst 4 are provided in spaced-apart relation to each other to purify air pollutants, such as HC, CO and NOx, in exhaust gas individually.

[0024] Firstly, the upstream catalyst 3 will be specifically described. The upstream catalyst 3 comprises a Ce-containing composite oxide having an oxygen storage capacity, and a Ni component which is at least either one of Ni and NiO. In the upstream catalyst 3, a ratio of Ni to CeO.sub.2 is set in the range of 15 to 20 mass %. If the Ni/CeO.sub.2 ratio becomes less than 15 mass %, an effect of suppressing generation of H.sub.2S is reduced, and resulting sulfur (S) poisoning of a catalytic noble metal supported on the upstream catalyst 3 and the downstream catalyst 4 is liable to cause deterioration in catalytic activity of the upstream and downstream catalysts 3, 4 and an increase in amount of H.sub.2S emission. If the Ni/CeO.sub.2 ratio becomes greater than 20 mass %, the catalytic activity of the upstream catalyst 3 is liable to deteriorate.

[0025] The composite oxide in the upstream catalyst 3 is a composite oxide other than an after-mentioned Rh-doped composite oxide. Preferably, the composite oxide further includes zirconium (Zr) and neodymium (Nd). Preferably, the composite oxide fundamentally supports a catalytic noble metal thereon. The catalytic noble metal to be supported on the upstream catalyst 3 may be at least one selected, for example, from the group consisting of platinum (Pt), palladium (Pd) and rhodium (Rh). Among them, it is particularly preferable that Pd and Rh are supported on the upstream catalyst 3.

[0026] The upstream catalyst 3 may be constructed as above. In this embodiment, the following catalyst is shown as a specific example. The upstream catalyst 3 in this embodiment comprises: a catalyst support consisting of a honeycomb support made of cordierite or heat-resistant ceramics such as SiC or Si.sub.3N.sub.4; a lower catalyst layer (lower layer) formed on the honeycomb support, and an upper catalyst layer (upper layer) formed on the lower catalyst layer. Although the honeycomb support is used in this embodiment as the catalyst support, the catalyst support is not limited to the honeycomb support, but may be any other suitable type of catalyst support.

[0027] The lower layer of the upstream catalyst 3 includes a catalyst substrate supporting thereon Pd as a catalytic noble metal. The catalyst substrate is made, for example, of lanthanum-containing alumina, and a composite oxide containing Ce, Zr, La, yttrium (Y) and aluminum (Al) (Ce--Zr--La--Y--Al composite oxide). As one example, the lanthanum-containing alumina may be alumina containing La.sub.2O.sub.3 in an amount of 4 mass %, and the Ce--Zr--La--Y--Al composite oxide may be a composite oxide containing CeO.sub.2 in an amount of 10 mass %, and alumina in an amount of about 80 mass %.

[0028] In this embodiment, the lower layer of the upstream catalyst 3 also includes CeO.sub.2, and a composite oxide containing Zr, Ce and Nd (Zr--Ce--Nd composite oxide), as a promoter (OSC) for enhancing exhaust gas purification performance and heat resistance of palladium. The lower layer further includes zirconia as a binder for enhancing binding between the respective components. The lower layer further includes NiO as the Ni component for suppressing the generation of H.sub.2S and the S poisoning of Pd. ZrO.sub.2, i.e., an oxide of Zr, has an additional function of enhancing heat resistance of CeO.sub.2. As one example, the Zr--Ce--Nd composite oxide may be a composite oxide containing CeO.sub.2 in an amount of 35 mass %. The NiO may be a NiO powder. Although NiO is shown as an example of the Ni component, the Ni component may be Ni, such as a Ni powder.

[0029] With regard to the above components of the lower layer, the La-containing alumina, the Ce--Zr--La--Y--Al composite oxide, the CeO.sub.2, and the Zr--Ce--Nd composite oxide, may be contained in the lower layer, for example, in amounts of 45 g/L, 20 g/L, 6 g/L, and 6 g/L, respectively, wherein the NiO may be contained in the lower layer in a given ratio, and the zirconia as a binder may be contained as the remnant.

[0030] In this embodiment, the upper layer comprises a catalyst substrate supporting thereon Rh as a catalytic noble metal. The catalyst substrate is made of alumina containing a Zr--La composite oxide (Zr--La composite oxide-containing alumina), and a Zr--Ce--Nd composite oxide. As one example, the Zr--Ce--Nd composite oxide may be a composite oxide containing CeO.sub.2 in an amount of 10 mass %.

[0031] In this embodiment, the upper layer also includes La-containing alumina. The upper layer further includes zirconia as a binder for enhancing binding between the respective components. The upper layer further includes NiO as the Ni component for suppressing the generation of H.sub.2S and the S poisoning of Pd. As one example, the La-containing alumina may be alumina containing La.sub.2O.sub.3 in an amount of 4 mass %. The NiO may be a NiO powder. Although NiO is shown as an example of the Ni component, the Ni component may be Ni, such as a Ni powder.

[0032] With regard to the above components of the upper layer, the Zr--La composite oxide-containing alumina, the Zr--Ce--Nd composite oxide, and the La-containing alumina, may be contained in the upper layer, for example, in amounts of 30 g/L, 75 g/L and 15 g/L, respectively, wherein the NiO may be contained in the upper layer in a given ratio, and the zirconia as a binder may be contained as the remnant.

[0033] A production method for the above upstream catalyst 3 in this embodiment will be described below.

[0034] Firstly, Pd is supported (post-supported) on each of a La-containing alumina and a Ce--Zr--La--Y--Al composite oxide. Then, the Pd-supporting La-containing alumina, the Pd-supporting Ce--Zr--La--Y--Al composite oxide, CeO.sub.2, a Zr--Ce--Nd composite oxide, NiO and zirconyl nitrate each prepared in a given amount, are mixed together, and an appropriate amount of water is added to the mixture to form a slurry. Then, a honeycomb support is wash-coated with the slurry. The wash-coated honeycomb support is dried at 150.degree. C., and sintered at 500.degree. C., so that a lower layer is formed on the honeycomb support.

[0035] Then, Rh is supported (post-supported) on each of a Zr--La composite oxide-containing alumina and a Zr--Ce--Nd composite oxide. Then, the Rh-supporting Zr--La composite oxide-containing alumina, the Rh-supporting Zr--Ce--Nd composite oxide, La-containing alumina, NiO and zirconyl nitrate each prepared in a given amount, are mixed together, and an appropriate amount of water is added to the mixture to form a slurry. Then, the honeycomb support formed with the lower layer is wash-coated with the slurry. The wash-coated honeycomb support is dried at 150.degree. C., and sintered at 500.degree. C., so that an upper layer is formed on the lower layer.

[0036] In the above manner, the upstream catalyst 3 is produced.

[0037] The downstream catalyst 4 will be specifically described below. The downstream catalyst 4 comprises a Ce-containing composite oxide having an oxygen storage capacity, and a Ni component which is at least either one of Ni and NiO. In the downstream catalyst 4, a ratio of Ni to CeO.sub.2 is set in the range of 10 to 60 mass %. If the Ni/CeO.sub.2 ratio becomes less than 10 mass %, an effect of suppressing generation of H.sub.2S is reduced, and resulting S poisoning of a catalytic noble metal supported on the downstream catalyst 4 is liable to cause deterioration in catalytic activity of the downstream catalyst 4 and an increase in amount of H.sub.2S emission. If the Ni/CeO.sub.2 ratio becomes greater than 60 mass %, the catalytic activity of the downstream catalyst 4 is liable to deteriorate. Preferably, the Ni/CeO.sub.2 ratio in the upstream catalyst 3 is greater than the Ni/CeO.sub.2 ratio in the downstream catalyst 4.

[0038] Preferably, the Ce-containing composite oxide in the downstream catalyst 4 consists of an Rh-doped composite oxide having Rh arranged at a lattice position or interlattice position of a crystal structure thereof, and further includes Zr and Nd. Preferably, the Ce-containing composite oxide fundamentally supports a catalytic noble metal thereon. The catalytic noble metal to be supported on the downstream catalyst 4 may be at least one selected, for example, from the group consisting of Pt, Pd and Rh. Among them, it is particularly preferable that Pd and Rh are supported on the downstream catalyst 4. The Rh-doped composite oxide is prepared by doping (incorporating as a solid solution) Rh into a Ce-containing composite oxide (oxygen storage material) so as to fix the Rh to the Ce-containing composite oxide.

[0039] The downstream catalyst 4 may be constructed as above. In this embodiment, the following catalyst is shown as a specific example. The downstream catalyst 4 in this embodiment comprises: a catalyst support consisting of a honeycomb support made of cordierite or heat-resistant ceramics such as SiC or Si.sub.3N.sub.4; and a catalyst layer formed on the honeycomb support, in a similar manner to the upstream catalyst 3. Although the honeycomb support is used in this embodiment as the catalyst support, the catalyst support is not limited to the honeycomb support, but may be any other suitable type of catalyst support.

[0040] The catalyst layer of the downstream catalyst 4 includes a La-containing alumina supporting thereon Pt as a catalytic noble metal, and an Rh-doped Zr--Ce--Nd composite oxide supporting thereon Rh as a catalytic noble metal. The Rh-supporting Rh-doped Zr--Ce--Nd composite oxide means a composite oxide prepared by additionally supporting (post-supporting) Rh on an after-mentioned Rh-doped Zr--Ce--Nd composite oxide. As one example, the La-containing alumina may be alumina containing La in an amount of 4 mass %, and the Rh-doped Zr--Ce--Nd composite oxide may be a composite oxide containing CeO2 in an amount of 22 mass %.

[0041] In this embodiment, the catalyst layer further includes zirconia as a binder for enhancing binding between the respective components. The catalyst layer further includes NiO as the Ni component for suppressing the generation of H.sub.2S. As one example, the NiO may be a NiO powder. Although NiO is shown as an example of the Ni component, the Ni component may be Ni, such as a Ni powder.

[0042] With regard to the above components of the catalyst layer, and the La-containing alumina, the Rh-doped Zr--Ce--Nd composite oxide, may be contained in the catalyst layer, for example, in amounts of 50 g/L and 110 g/L, respectively, wherein the NiO may be contained in the lower layer in a given ratio, and the zirconia as a binder may be contained as the remnant.

[0043] FIG. 2A is a schematic diagram simplistically showing a structure of the Rh-doped Zr--Ce--Nd composite oxide. FIG. 2B is a schematic diagram simplistically showing a structure of an Rh-post-supporting Zr--Ce--Nd composite oxide.

[0044] The Rh-doped Zr--Ce--Nd composite oxide has the structure as shown in FIG. 2A. In the Rh-doped Zr--Ce--Nd composite oxide, Rh is arranged at a lattice position (i.e., at a position of a lattice point) of a crystal structure of the composite oxide, in the same manner as that for Zr, Ce and Nd. In other words, Rh is strongly bound to the composite oxide in the form of RhMOx (wherein M is another metal atom, and x is the number of oxygen atoms). Alternatively, Rh is arranged at an interlattice position (i.e., at a position between lattice points) of the crystal structure of the composite oxide. In either case, a composite oxide is obtained in such a manner that Rh atoms are evenly dispersed over a surface and an inside of the composite oxide.

[0045] Differently, the Rh-post-supporting Zr--Ce--Nd composite oxide has the structure as shown in FIG. 2B. For example, the Rh-post-supporting Zr--Ce--Nd composite oxide is obtained by forming a composite oxide containing Zr, Ce and Nd, through a coprecipitation process using ammonia, and then post-supporting Rh on the composite oxide through an evaporative drying process. In this case, Rh is unevenly distributed on a surface of the composite.

[0046] Thus, as compared with the Rh-post-supporting Zr--Ce--Nd composite oxide, the Rh-doped Zr--Ce--Nd composite oxide has the following two advantages.

[0047] In the Rh-post-supporting Zr--Ce--Nd composite oxide, binding between Rh and the composite oxide is weak, and thereby Rh is sintered by heating while moving on the surface of the composite oxide. Differently, in the Rh-doped Zr--Ce--Nd composite oxide, as shown in FIG. 2A, a movement of Rh arranged at a lattice position or interlattice position of the crystal structure of the composite oxide is constrained by a strong interaction with the composite oxide. In addition, it is considered that Rh in the inside of the composite oxide serves as a steric barrier to suppress sintering of the composite oxide.

[0048] Further, in the Rh-doped Zr--Ce--Nd composite oxide, an oxygen-absorbing speed is quickly increased while increasing a minimum value thereof, and an oxygen storage amount is increased, as compared with the Rh-post-supporting Zr--Ce--Nd composite oxide. The difference in oxygen storage characteristics would arise for the following reason.

[0049] FIG. 3A is a schematic diagram simplistically showing a presumed oxygen storage mechanism in the Rh-doped Zr--Ce--Nd composite oxide. FIG. 3B is a schematic diagram simplistically showing a presumed oxygen storage mechanism in the Rh-post-supporting Zr--Ce--Nd composite oxide. In FIGS. 3A and 3B, Zr atoms and Nd atoms are omitted.

[0050] As shown in FIG. 3B, in the Rh-post-supporting Zr--Ce--Nd composite oxide, it is assumed that, although oxygen (O.sub.2) is stored in an oxygen vacancy (O vacancy) existing in a vicinity of a surface of the composite oxide, in the form of an oxygen ion, it cannot reach an oxygen vacancy existing in a relatively deep region of an inside of the composite oxide, and the oxygen vacancy in the relatively deep region is not effectively used for oxygen storage.

[0051] Differently, as shown in FIG. 3A, in the Rh-doped Zr--Ce--Nd composite oxide, it is assumed that Rh atoms residing in the inside of the composite oxide draw oxygen (O.sub.2) in the form of an oxygen ion to allow the oxygen ion to momentarily move to the oxygen vacancy in the inside of the composite oxide. In addition, the Rh atoms dispersedly exist in the inside of the composite oxide. Thus, it is assumed that the oxygen ion moves from the surface of the composite oxide via two or more of the Rh atoms in a hopping manner, and reaches an oxygen vacancy in the relatively deep region of the inside of the composite oxide. In view of the above assumptions, in the Rh-doped Zr--Ce--Nd composite oxide, when exhaust gas has an oxygen-excess atmosphere, the oxygen-absorbing speed is quickly increased while increasing the maximum value thereof. In addition, an oxygen vacancy in the relatively deep region of the inside of the oxygen storage material is effectively used for oxygen storage to increase the oxygen storage amount.

[0052] A production method for the Rh-doped composite oxide will be described below.

[0053] Firstly, a row material preparation step is performed to prepare an acid solution containing Rh, Ce and Zr. For example, in the row material preparation step, the acid solution may be prepared by mixing respective solution of Rh, Ce and Zr nitrates together. As needed, another metal, such as Nd, may be additionally contained in the acid solution.

[0054] Then, a step of preparing a composite oxide precursor through a coprecipitation process using ammonia is performed. In this step, an excess amount of aqueous ammonia is quickly added and mixed to/with the acid solution as a starting material while stirring the acid solution, or the acid solution and aqueous ammonia are simultaneously supplied to a rotating cup-shaped mixer and quickly mixed together, so that Rh, Ce and Zr in the starting material are coprecipitated as a metal hydroxide to obtain a composite oxide precursor.

[0055] Then, the following steps are performed in turn. A precipitation/separation step is performed which comprises leaving the solution with the coprecipitate for one day, removing a supernatant solution to obtain a cake, placing the cake in a centrifugal machine, and rinsing the cake with water. Then, the rinsed cake is subjected to a drying step of drying it by heating at a temperature of about 150.degree. C. Then, the dried cake is subjected to a firing step of firing it by heating. The firing step is performed by placing the dried cake in an ambient atmosphere, for example, which is held at a temperature of 400.degree. C. for 5 hours, and then held at a temperature of 500.degree. C. for 2 hours. Then, the fired product is subjected to a reducing step of placing it in a reduction atmosphere held at a temperature of about 500.degree. C.

[0056] Through the above steps, the Rh-doped composite oxide is prepared.

[0057] The downstream catalyst 4 is produced as follows, using the Rh-doped composite oxide prepared by the above preparation process.

[0058] Firstly, Pt is supported (post-supported) on a La-containing alumina, and Rh is supported (post-supported) on the Rh-doped composite oxide. Then, the Pt-supporting La-containing alumina, the Rh-supporting Rh-doped composite oxide, NiO, and zirconyl nitrate as a binder material, each prepared in a given amount, are mixed together, and an appropriate amount of water is added to the mixture to form a slurry. Then, a honeycomb support is wash-coated with the slurry. The wash-coated honeycomb support is dried at 150.degree. C., and sintered at 500.degree. C., so that a catalyst layer is formed on the honeycomb support. In this manner, the downstream catalyst 4 is produced.

EXAMPLES

[0059] Although the exhaust gas purifying catalyst system according to the embodiment of the present invention will be described based on specific examples, the present invention is not limited to the specific examples.

Inventive Examples 1 to 7 and Comparative Examples 1 to 6

[0060] Each of the components, except the Ni component and the binder material, was supported on the support in an amount per liter of the support (g/L), as shown in Table 1, and each of the Ni component (NiO) was supported on the support in a mass ratio (mass %), as shown in Table 2. In this manner, an upstream catalyst and a downstream catalyst in each of Inventive Examples 1 to 7 and Comparative Examples 1 to 6 were produced.

[0061] Then, the obtained upstream catalyst and downstream catalyst were disposed at given positions as shown in FIG. 1 to produce an exhaust gas purifying catalyst system in each of Inventive Examples 1 to 7 and Comparative Examples 1 to 6.

[0062] A light-off performance and an amount of H.sub.2S emission of the exhaust gas purifying catalyst system in each of Inventive Examples 1 to 7 and Comparative Examples 1 to 6 were measured as follows. A result of the measurement is shown in Table 2.

[0063] [Light-Off Performance]

[0064] The exhaust gas purifying catalyst system was connected to a 2 L gasoline engine. The engine was adjusted to allow an exhaust gas temperature at an inlet of the upstream catalyst to be 900.degree. C. and allow an exhaust A/F to be maintained in a stoichiometric region for 60 sec. in a lean region for 10 sec and in a rich region for 30 sec, in one cycle. Then, the upstream and the downstream catalysts were subjected to an aging treatment by repeating the cycle for 50 hours. In this exhaust gas purifying catalyst system, a volume of each of the upstream and the downstream catalysts was set at 1 L.

[0065] A columnar-shaped evaluation catalyst sample having a diameter of 25 mm and a height of 50 mm was cut from each of the upstream and the downstream catalysts after being subjected to the aging treatment, and set in a model-gas flow-type catalyst evaluation apparatus. Simulated exhaust gas (model gas) was circulated through the catalyst evaluation apparatus at a space velocity of 60000/h while raising its temperature at a rate of 30.degree. C./min. Under this condition, a simulated exhaust gas temperature at the inlet of each of the catalysts at a time when each of HC, CO and NOx concentrations at a position just after an outlet of the catalyst is increased up to 50% (i.e., light-off temperature T50), was measured. The columnar-shaped core sample cut from each of the aged upstream and the downstream catalysts to have a diameter of 25 mm and a height of 50 mm was used as the catalyst for this measurement. The simulated exhaust gas was adjusted to have an A/F of 14.7.+-.0.9. Specifically, mainstream gas having an A/F of 14.7 was constantly supplied, and a given amount of variation gas was added at a frequency of 1 Hz in a pulsed manner to forcedly fluctuate the A/F up and down at a fluctuation range of .+-.0.9. The light-off temperature T50 is an index for evaluating the catalyst activity and the exhaust gas purification performance, wherein a lower light-off temperature T50 indicates higher catalyst activity and exhaust gas purification performance at a low temperature.

[0066] [Amount of H.sub.2S Emission]

[0067] The same exhaust gas purifying catalyst system having the aged upstream and downstream catalysts as that in the light-off performance evaluation was mounted to a vehicle equipped with a 2 L gasoline engine, and the vehicle was run in the following running pattern. This running pattern consists of a vehicle running at 30 km/h for 4 min, a subsequent vehicle running at 50 km/h for 1 min, and a subsequent vehicle deceleration from 50 km/h to zero km/h. During the vehicle deceleration from 50 km/h to zero km/h in the above running pattern, a maximum concentration of H.sub.2S was measured. The reason why the maximum concentration of H.sub.2S is measured during the vehicle deceleration from 50 km/h to zero km/h is that sulfur (S) is attached on the catalyst during a steady operation, such as the vehicle running at 30 km/h and the vehicle running at 50 km/h, and H.sub.2S is generated and emitted when an exhaust A/F is placed in a rich region, in response to restart of fuel supply after fuel is cut during deceleration subsequent to the steady operation.

TABLE-US-00001 TABLE 1 SUPPORTED COMPONENT AMOUNT(g/L) UPSTREAM CATALYST UPPER LAYER Rh-SUPPORTING Zr--Ce--Nd COMPOSITE OXIDE 75 CATALYST (CeO.sub.2 CONTENT %: 10 MASS %) Zr--La COMPOSITE 30 OXIDE-CONTAINING ALUMIN ALUMINA La-CONTAINING ALUMINA 15 (La-CONTENT %: 4 MASS %) LOWER LAYER OSC CeO.sub.2 6 Zr--Ce--Nd COMPOSITE OXIDE 6 (CeO.sub.2 CONTENT %: 10 MASS %) Pd-SUPPORTING La-CONTAINING ALUMINA 45 CATALYST (La-CONTENT %: 4 MASS %) Ce--Zr--LA--Y--Al COMPOSITE 20 OXIDE (CeO.sub.2 CONTENT %: 10 MASS %) DOWNSTREAM Pt-SUPPORTING La-CONTAINING ALUMINA 50 CATALYST CATALYST (La-CONTENT %: 4 MASS %) Rh-SUPPORTING Rh-DOPED Zr-CE-Nd 110 CATALYST COMPOSITE OXIDE (CeO.sub.2 CONTENT %: 22 MASS %)

TABLE-US-00002 TABLE 2 H.sub.2S EMISSION UPSTREAM AFTER H.sub.2S EMISSION CATALYST LIGHT-OFF PASSING LIGHT-OFF OF AFTER PASSING Ni/CeO.sub.2 OF UPSTREAM THROUGH DOWNSTREAM DOWNSTREAM THROUGH MASS CATALYST UPSTREAM CATALYST CATALYST UPSTREAM AND RATIO (.degree. C.) CATALYST Ni/CeO.sub.2 MASS (.degree. C.) DOWNSTREAM (MASS %) HC CO NOx (ppm) RATIO (MASS %) HC CO NOx CATALYSTS (ppm) INVENTIVE EXAMPLE 1 20 249 244 240 24 60 258 250 246 5 INVENTIVE EXAMPLE 2 20 249 244 240 24 40 257 249 245 9 INVENTIVE EXAMPLE 3 20 249 244 240 24 10 255 247 242 10 INVENTIVE EXAMPLE 4 18 247 243 239 28 12 256 248 245 10 INVENTIVE EXAMPLE 5 15 247 243 237 35 40 257 249 245 13 INVENTIVE EXAMPLE 6 15 247 243 237 35 12 256 248 245 10 INVENTIVE EXAMPLE 7 15 247 243 237 35 10 255 247 242 11 COMPARATIVE EXAMPLE 1 30 258 255 250 17 65 265 255 250 7 COMPARATIVE EXAMPLE 2 30 258 255 250 17 40 257 249 245 9 COMPARATIVE EXAMPLE 3 20 249 244 240 24 65 265 255 250 9 COMPARATIVE EXAMPLE 4 20 249 244 240 24 5 250 241 237 20 COMPARATIVE EXAMPLE 5 15 247 243 237 35 65 265 255 250 11 COMPARATIVE EXAMPLE 6 10 247 243 234 50 70 279 265 262 7

[0068] As seen in Table 2, in each of Inventive Examples 1 to 7 where a ratio of Ni to CeO2 in the upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO.sub.2 in the downstream catalyst is in the range of 10 to 60 mass %, an amount of H2S emission after passing through both the upstream and downstream catalysts is low, and a light-off temperature in each of the upstream and downstream catalysts is low, as compared with each of Comparative Examples 1 to 6 where at least one of the Ni/CeO.sub.2 ratio in the upstream catalyst and the Ni/CeO.sub.2 ratio in the downstream catalyst is out of the above range.

[0069] As mentioned above in detail, an exhaust gas purifying catalyst system according to one aspect of the present invention comprises an upstream catalyst disposed in an exhaust gas passage of an engine at a position on an upstream side with respect to a direction of an exhaust gas stream, and a downstream catalyst disposed in the exhaust gas passage at a position on a downstream side with respect to the direction of the exhaust gas stream. In the exhaust gas purifying catalyst system, the downstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a rhodium-doped composite oxide having rhodium arranged at a lattice position or interlattice position of a crystal structure thereof, and at least either one of nickel (Ni) and nickel oxide (NiO), and the upstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a composite oxide other than the rhodium-doped composite oxide, and at least either one of nickel (Ni) and nickel oxide (NiO). A ratio of Ni to CeO.sub.2 in the upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO.sub.2 in the downstream catalyst is in the range of 10 to 60 mass %.

[0070] In the exhaust gas purifying catalyst system according to the aspect of the present invention, when exhaust gas passes through the upstream catalyst and the downstream catalyst, exhaust gas components, such as HC, CO and NOx, are purified by both the upstream and downstream catalysts. Thus, even if each of the upstream and downstream catalysts has a slightly lower purification performance as compared with a catalyst without an addition of nickel, the exhaust gas purifying catalyst system can adequately reduce an amount of emission of CO, HC, NOx, etc., to maintain high purification performance as a whole. In addition, the generation of H.sub.2S is suppressed in both the upstream and downstream catalysts. Thus, even if each of the upstream and downstream catalysts is set to have a slightly lower capability of suppressing the generation of H.sub.2S as compared with a catalyst added with a large amount of Ni, the exhaust gas purifying catalyst system can adequately reduce an amount of H.sub.2S emission as a whole.

[0071] The oxygen storage material included in the downstream catalyst is a rhodium-doped composite oxide having rhodium arranged at a lattice position or interlattice position of a crystal structure thereof. Such a composite oxide has a capability to absorb/release oxygen at a relatively high speed, and store oxygen in a relatively large amount. Thus, even if an oxygen concentration in exhaust gas varies, the oxygen concentration can be readily adjusted at a desired value to allow the downstream catalyst to effectively perform a catalytic action thereof. That is, exhaust gas components which have not been able to be purified by the upstream catalyst, can be efficiently purified by the downstream catalyst. This also contributes to achievement of high purification performance.

[0072] Thus, the exhaust gas purifying catalyst system according to the aspect of the present invention can reduce an amount of H.sub.2S emission while achieving high purification performance.

[0073] Preferably, in the exhaust gas purifying catalyst system according to the aspect of the present invention, each of the respective composite oxides included in the upstream catalyst and the downstream catalyst includes zirconium and neodymium.

[0074] The above composite oxide can store oxygen in a relatively large amount. This makes it possible to effectively operate the catalysts even if an oxygen concentration in exhaust gas relatively largely varies, so as to more enhance the purification performance.

[0075] Preferably, in the exhaust gas purifying catalyst system according to the aspect of the present invention, the ratio of Ni to CeO.sub.2 in the upstream catalyst is greater than the ratio of Ni to CeO.sub.2 in the downstream catalyst.

[0076] According to this feature, the generation of H.sub.2S can be more effectively suppressed in the upstream catalyst, so that sulfur (S) poisoning of the downstream catalyst can be more suppressed. In addition, exhaust gas components, such as HC, CO and NOx, which have not been able to be purified by the upstream catalyst, can be more efficiently purified by the downstream catalyst.

[0077] Preferably, in the exhaust gas purifying catalyst system according to the aspect of the present invention, each of the respective composite oxides included in the upstream catalyst and the downstream catalyst supports a catalytic noble metal thereon.

[0078] According to this feature, exhaust gas components, such as HC, CO and NOx, can be more efficiently purified to achieve higher purification performance.

[0079] Preferably, in the above exhaust gas purifying catalyst system, the catalytic noble metal supported on the composite oxide included in the upstream catalyst and the downstream catalyst is rhodium, the upstream catalyst includes palladium, and the downstream catalyst includes platinum.

[0080] This application is based on Japanese Patent Application Serial No. 2007-259109 filed in Japan Patent Office on Oct. 2, 2007, the contents of which are hereby incorporated by reference.

[0081] Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.

Claims

1. An exhaust gas purifying catalyst system comprising an upstream catalyst disposed in an exhaust gas passage of an engine at a position on an upstream side with respect to a direction of an exhaust gas stream, and a downstream catalyst disposed in said exhaust gas passage at a position on a downstream side with respect to the direction of said exhaust gas stream, wherein: said downstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a rhodium-doped composite oxide having rhodium arranged at a lattice position or interlattice position of a crystal structure thereof, and at least either one of nickel (Ni) and nickel oxide (NiO); and said upstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a composite oxide other than said rhodium-doped composite oxide, and at least either one of nickel (Ni) and nickel oxide (NiO), wherein a ratio of Ni to CeO.sub.2 in said upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO.sub.2 in said downstream catalyst is in the range of 10 to 60 mass %.

2. The exhaust gas purifying catalyst system as defined in claim 1, wherein each of said respective composite oxides included in said upstream catalyst and said downstream catalyst includes zirconium and neodymium.

3. The exhaust gas purifying catalyst system as defined in claim 1, wherein the ratio of Ni to CeO.sub.2 in said upstream catalyst is greater than the ratio of Ni to CeO.sub.2 in said downstream catalyst.

4. The exhaust gas purifying catalyst system as defined in claim 1, wherein each of said respective composite oxides included in said upstream catalyst and said downstream catalyst supports a catalytic noble metal thereon.

5. The exhaust gas purifying catalyst system as defined in claim 4, wherein: said catalytic noble metal supported on said composite oxide included in said upstream catalyst and said downstream catalyst is rhodium; said upstream catalyst includes palladium; and said downstream catalyst includes platinum.