WO2013128261A2

WO2013128261A2 - Exhaust gas control apparatus, exhaust gas control method and exhaust gas purification catalyst

Info

Publication number: WO2013128261A2
Application number: PCT/IB2013/000293
Authority: WO
Inventors: Nobuyuki Takagi; Yasushi Satake; Toshiyuki Tanaka; Koji Yokota
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2012-03-02
Filing date: 2013-02-26
Publication date: 2013-09-06
Also published as: WO2013128261A3; WO2013128261A8; JP2013181502A

Abstract

An exhaust gas control apparatus includes an exhaust gas purification catalyst and a controller. The controller is configured to alternately change a concentration of hydrocarbon flowing in the exhaust gas purification catalyst between a high concentration region where the concentration of hydrocarbon is relatively high and a low concentration region where the concentration of hydrocarbon is relatively low in a predetermined period. The exhaust gas purification catalyst includes a support (75), metal catalysts (70, 72) supported on the support (75), and a NOx storage material (74) supported by the support (75) and having NOx storage capacity. The NOx storage material (74) contains a barium-titanium composite oxide where barium (74a) and titanium (74b) are dissolved to form a solid solution.

Description

EXHAUST GAS CONTROL APPARATUS, EXHAUST GAS CONTROL METHOD AND EXHAUST GAS PURIFICATION CATALYST

BACKGROUND OF THE INVENTION

^' 1. Field of the Invention

[0001] The invention relates to an exhaust gas control apparatus for purifying an exhaust gas discharged from an internal combustion engine, an exhaust gas control method and an exhaust gas purification catalyst.

2. Description of Related Art

[0002] In a diesel engine and a lean burn gasoline engine, as means for eliminating nitrogen oxide (NOx) that is a harmful component in an exhaust gas, a NOx storage catalyst system in which a NOx storage catalyst and a rich spike control are combined has been put into practical use. The NOx storage catalyst includes a precious metal catalyst and a NOx storage material. The precious metal catalyst is supported on a surface of a base material and includes platinum (Pt) and rhodium (Rh). The NOx storage material is supported on a surface of the base material and composed of an alkali or alkali earth element such as barium. The rich spike control intermittently controls a combustion state to, in addition to a lean state that is an ordinary combustion state, a stoichiometric state (theoretical air-fuel ratio state) or a rich state for a very short time ' compared with a duration time of the lean state. The lean state means a state where an oxygen concentration is 5% or more, that is excessive. Further, the stoichiometric state or rich state is a state where by performing an engine combustion control or by blowing in a fuel from an upstream of the NOx storage catalyst into an exhaust gas pipe, an air-fuel ratio where a reducing component to NOx is rich in comparison with the lean state is realized.

[0003] In a state where the air-fuel ratio is in the lean state of excess oxygen, NOx in the exhaust gas is oxidized on a platinum catalyst and absorbed by the NOx storage material. On the other hand, when the air-fuel ratio is switched to the stoichiometric state or rich state (when the rich spike control is performed), NOx stored in the NOx storage material is released, and with components such as hydrocarbon (HC) or carbon monoxide (CO), the NOx is reduced and purified on a precious metal in the catalyst. In the rich spike control, as was described above, hydrocarbon is intermittently fed to temporally increase a concentration of hydrocarbon contained in the exhaust gas. Further, an amount of NOx, which the NOx storage material can store, has a limit amount (saturation amount). Accordingly, usually, when NOx is purified with the NOx storage catalyst, before NOx stored in the NOx storage material is saturated, the rich spike control is repeated in pulse to continually purify NOx in the exhaust gas. An oxygen concentration in the pulse-like rich gas is sufficiently low; accordingly, a reducing component such as HC or CO in the rich gas is efficiently consumed to reduce and decompose NOx stored in the NOx storage catalyst. Further, since also a rich pulse time is enough shorter than a time of the lean state, irrespective of an oxygen excess state when considered based on time average, NOx can be selectively reduced with only a small amount of reducing agent. Still further, since the reducing component is derived from the fuel, a reduction in a use amount of the reducing agent contributes to suppression of fuel consumption.

[0004] However, at high temperatures (400°C or more, for example), a NOx purification rate of the NOx storage catalyst deteriorates. That is, at high temperatures, an amount of NOx that the NOx storage catalyst can store (saturation amount) drastically deteriorates, and, even when the air-fuel ratio is in a lean state, NOx is not absorbed more than that. Accordingly, at high temperatures, NOx contained in the exhaust gas goes through the NOx storage catalyst to flow out downstream, and as a result, the NOx purification rate may deteriorate. Therefore, it has been tried to optimize the purification performance of the NOx storage catalyst by switching two different purification modes in accordance with a catalyst temperature. As a conventional technology that together use this kind of two different NOx control methods, a method disclosed in International Patent Publication No. WO 2011/114501 can be cited.

[0005] In an exhaust gas control apparatus that together uses two different purification modes, in a temperature region where the catalyst temperature is high (400°C or more, for example), for example, as shown in FIG. 13B, a feed interval and a feed amount of hydrocarbon (HC) are set smaller. Further, a lean state and a stoichiometrically rich state are alternately switched in a short time (within 5 sec/cycle, for example). According to these treatments, NOx contained in an exhaust gas is purified (hereinafter, referred to as a high temperature purification mode). In a high temperature region, a reaction speed increases and, compared with a low temperature, side, the NOx storage capacity rapidly deteriorates. Accordingly, purification where a gas atmosphere rapidly varies in a short period where the NOx storage capacity is not rate-determined can exert a higher NOx purification performance than that of purification where a gas atmosphere varies in a long period. A rich state of the high temperature purification mode typically continues for 1 sec or less; accordingly, a gas , mainly composed of hydrocarbon is desirable to be fed. The hydrocarbon (HC) has high NOx degradation capability per volume of rich gas. On the other hand, in a temperature region where a catalyst temperature is low (less than 400°C, for example), as shown in FIG. 13 A, by making a feed interval and a feed amount, of HC (or it may be other reducing agent such as H₂.) larger to temporally shift to a stoichiometrically rich state while maintaining a long lean state (60 sec, for example), NOx contained in the exhaust gas is purified (hereinafter, referred to as a low temperature purification mode). In a low temperature region where a reaction speed is slow, compared with the purification where the reaction speed is sped up by rapid switching of gas atmosphere like in the above-mentioned high temperature purification mode, purification where a lean state is long by using an abundant NOx storage capacity at low temperatures can achieve a higher NOx purification rate.

[0006] In the exhaust gas control apparatus that uses the above-described two different purification modes in combination, a further improvement in performance is in demand. The inventors have found, after variously studying of the exhaust gas control apparatus, that a NOx storage material composed of barium of related art can not enough use purification performance of the high temperature purification mode.

[0007] That is, in the low temperature purification mode where while maintaining a long lean state (for 60 sec or more, for example), the state is temporally shifted to a stoichiometric to rich state, a period of one cycle is long and an amount of NOx that is purified by one cycle is much. Accordingly, in the low temperature purification mode, the storage capacity of the NOx storage material is demanded to be increased to improve the performance. By contrast, in the high temperature purification mode where a lean state and a stoichiometric to rich state are alternately repeated in a short period (5 sec or less per cycle, for example), a rich spike in a period of one cycle is short and an amount of NOx that is purified during one cycle is scarce. Accordingly, in the high temperature purification mode, different from the low temperature purification mode, not the capacity but the reaction speed, that is, a speed for purifying NOx immediately after switching of a gas atmosphere is in demand.

[0008] However, as illustrated in FIG. 16, a NOx storage material 94 composed of barium of the related art is large in a particle size and not large in dispersibility compared with a support 95 and a precious metal 90. Accordingly, an exhaust gas purification catalyst of the related art can not obtain enough reaction efficiency (a reaction speed is slow), and the purification performance of the high temperature purification mode could not be sufficiently used. Further, since a part of barium covers a surface of the precious metal, activity of the precious metal can not be fully exerted, that is, a reaction speed becomes slow.

SUMMARY OF THE INVENTION

[0009] A first embodiment of the invention relates to an exhaust gas control apparatus for purifying NOx contained in an exhaust gas discharged from an internal combustion engine. The exhaust gas control apparatus includes an exhaust gas purification catalyst, and a controller that is configured to alternately change a concentration of hydrocarbon flowing in an exhaust gas purification catalyst in a predetermined period between a high concentration region where a concentration of hydrocarbon is relatively high and a low concentration region where a concentration of hydrocarbon is relatively low. Here, the exhaust gas purification catalyst includes a support, a metal catalyst supported on the support, and a NOx storage material supported by the support and having NOx storage capacity. The NOx storage material contains a barium-titanium composite oxide where barium (Ba) and titanium (Ti) are dissolved to form a solid solution.

[0010] According to the first embodiment of the invention, by dissolving titanium in barium that is a NOx storage material to form a solid solution, the barium is micronized to increase a contact area of the barium with an exhaust gas. Further, since a solid solution of barium and titanium is stable, the likelihood of covering a surface of a precious metal with barium is decreased. As a result, reaction efficiency (reaction speed) of barium is improved, and from immediately after switching of the gas atmosphere, NOx is purified. Accordingly, according to an exhaust gas control apparatus of the first embodiment of the invention, NOx purification performance can be relatively improved.

[0011] In the first embodiment of the invention, a molar ratio of a mol of barium divided by moles of barium and titanium (hereinafter, referred to as a molar ratio of Ba/(Ba + Ti)) in the barium-titanium composite oxide may be 0.1 to 0.9. In such the range of molar ratio of Ba/(Ba + Ti), the barium is further micronized (dispersed) and a reaction efficiency of barium is further improved. When the molar ratio is too large, a content of titanium is relatively decreased; accordingly, the catalyst performance improvement effect becomes insufficient and high purification efficiency may not be obtained. On the other hand, when the molar ratio is too small, the content of barium is relatively decreased; accordingly, the NOx storage capacity decreases and high purification performance may not be obtained.

[0012] In the first embodiment of the invention, a support that supports the barium-titanium composite oxide may be constituted by a composite oxide containing Ce0₂. When the barium-titanium composite oxide is supported on a Ce0₂-containing composite oxide, high NOx purification performance can be exerted.

[0013] In the first embodiment of the invention, the controller may be configured to shift a concentration of hydrocarbon flowing in the exhaust gas purification catalyst, after maintaining at the low concentration region for a first time, to the high concentration region for a second time that is a time 0.005 times to 0.2 times as much as the first time. Further, in the first embodiment of the invention, the controller may be configured to change a concentration of hydrocarbon flowing in the exhaust gas purification catalyst under the condition where a catalyst temperature is 300°C or more. The predetermined period may be between 0.5 sec and 5 sec. When the change period of the hydrocarbon concentration is set as described above, even under a high temperature state, a high NOx purification rate can be realized.

[0014] A second embodiment of the invention relates to an exhaust gas control method for purifying NOx contained in an exhaust gas discharged from an internal combustion engine with an exhaust gas purification catalyst. In the exhaust gas control method, the exhaust gas purification catalyst includes a support, a metal catalyst supported on the support, and a NOx storage material supported on the support. The NOx storage material has NOx storage capacity and includes a barium-titanium composite oxide where barium (Ba) and titanium (Ti) are dissolved to form a solid solution. The exhaust gas control method includes performing a treatment where a concentration of hydrocarbon flowing in the exhaust gas purification catalyst is alternately changed in a predetermined period of time between a high concentration region where a hydrocarbon concentration is relatively high and a low concentration region where a hydrogen concentration is relatively low.

[0015] According to the exhaust gas control method having the above configuration, as the NOx storage material, a barium-titanium composite oxide where barium and titanium are dissolved to form a solid solution is used; accordingly, from immediately after a gas atmosphere is switched, the NOx can be efficiently purified, thereby purification performance can be improved.

[0016] In the second embodiment of the invention, a molar ratio of Ba/(Ba + Ti) of the barium-titanium composite oxide may be 0.1 to 0.9. Further, the support that supports the barium-titanium composite oxide may be constituted by a composite oxide containing Ce0₂.

[0017] In the second embodiment of the invention, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst may be shifted, after maintaining the concentration of hydrocarbon for a first time at the low concentration region, to the high concentration region for a second time that is 0.005 times to 0.2 times as much as the first time period. Further, under the condition that a catalyst temperature is 300°C or more, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst may be changed. The predetermined period may be between 0.5 sec and 5 sec.

[0018] A third embodiment of the invention relates to an exhaust gas purification catalyst. The exhaust gas purification catalyst includes a support, a metal catalyst supported on the support, and a NOx storage material supported on the support. The NOx storage material has NOx storage capacity and includes a barium-titanium composite oxide in which barium (Ba) and titanium (Ti) are dissolved to form a solid solution. Such the exhaust gas purification catalyst may be used in an exhaust gas control apparatus or a gas purification method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic diagram of an exhaust gas control apparatus related to one embodiment;

FIG. 2 is an overall view schematically showing a configuration of an exhaust gas purification catalyst related one embodiment;

FIG. 3 is a diagram showing by enlarging a configuration of a rib wall portion in an exhaust gas purification catalyst related to one embodiment;

FIG. 4 is a diagram showing a change of an air-fuel ratio of an exhaust gas flowing in an exhaust gas purification catalyst;

FIG. 5 is a diagram showing a change of an air-fuel ratio of an exhaust gas flowing in an exhaust gas purification catalyst;

FIG. 6 A is a diagram for explaining a NOx purification reaction of the exhaust gas purification catalyst related to one embodiment;

FIG. 6B is a diagram for explaining a NOx purification reaction of the exhaust gas purification catalyst related to one embodiment;

FIG. 7 is a diagram illustrating a relationship between a temperature of a gas flowing in an exhaust gas purification catalyst and a NOx purification rate;

FIG. 8A is a diagram for explaining a NOx purification reaction of the exhaust gas purification catalyst related to one embodiment;

FIG. 8B is a diagram for explaining a NOx purification reaction of the exhaust gas purification catalyst related to one embodiment;

FIG. 9 is a diagram schematically showing an exhaust gas purification catalyst related to one embodiment;

FIG. 10 is a diagram schematically explaining a controller provided to an exhaust gas control apparatus related to one embodiment;

FIG. 11 is a diagram showing an X-ray diffraction pattern related to an example; FIG. 12 is a diagram showing an X-ray diffraction pattern related to a comparative example;

FIG. 13 includes FIG. 13A and FIG. 13B, and FIG. 13A is a diagram for explaining a feed cycle of a low temperature purification mode, and FIG. 13B is a diagram for explaining a feed cycle of a high temperature purification mode;

FIG. 14 is a graph showing a relationship between a catalyst bed temperature and a NOx storage speed of example and comparative example;

FIG. 15 is a graph showing a relationship between a catalyst bed temperature and a NOx purification rate of example and comparative example; and

FIG. 16 is a diagram schematically showing an exhaust gas purification catalyst in a related art.

DETAILED DESCRIPTION OF EMBODIMENTS

[0020] Hereinafter, embodiments of the invention will be described. Items that are other than that particularly referred in the specification and necessary for performing the invention can be regarded as items that a person familiar in the art can grasp based on related art in the field. An embodiment of the invention may be performed based on content disclosed in the specification and technical commonsense in the field. In the following description, each of an exhaust gas that is lean in an air-fuel ratio, an exhaust gas that is stoichiometric in an air-fuel ratio, and an exhaust gas that is rich in an air-fuel ratio indicates an exhaust gas having an air-fuel ratio the same as that of the exhaust gas discharged from the internal combustion engine when each of a lean mixture gas, a stoichiometric mixture gas and a rich mixture gas is burned in an internal combustion engine or an exhaust gas where hydrocarbon is afterward fed to the exhaust gas.

[0021] Hereinafter, an embodiment of an exhaust gas control apparatus equipped with an exhaust gas purification catalyst disclosed herein will be described with reference to drawings. Herein, a case where a diesel engine is provided as an internal combustion engine will be detailed as an example. However, it is not intended to limit an application range of the invention to such the diesel engine.

[0022] As illustrated in FIG. 1, an exhaust gas control apparatus 100 according to the embodiment is roughly constituted of an engine portion 1 mainly made of a diesel engine, an exhaust gas purification portion 40, and an electronic control unit (that is, an engine control unit, or, ECU) 30 (see FIG. 10). The engine portion 1 includes an operation system such as an accelerator for driving an engine and other. The exhaust gas purification portion 40 is disposed to an exhaust system communicating with the engine portion 1. The ECU 30 executes a control between the exhaust gas purification portion 40 and the engine portion 1. As a part of such the exhaust gas purification portion 40, an exhaust gas purification catalyst 60 of the embodiment of the invention is used.

[0023] The engine portion 1 includes a plurality of combustion chambers 2 and fuel injection valves 3 that inject a fuel to the respective combustion chambers 2. Each of the combustion chambers 2 is communicated with an intake air manifold 4 and an exhaust air manifold 5. The intake air manifold 4 is connected to an outlet of a compressor 7a of an exhaust gas turbocharger 7 via an intake air duct 6. An inlet of the compressor 7a is connected to an air cleaner 9 via an intake air amount detector 8. Inside the intake air duct 6, a throttle valve 10 is disposed. Around the intake air duct 6, a cooler (inter-cooler) 11 for cooling air flowing the inside of the intake air duct 6 is disposed. The exhaust air manifold 5 is connected to an inlet of an exhaust gas turbine 7b of the exhaust gas turbocharger 7. An outlet of the exhaust air turbine 7b is connected to an exhaust air path (exhaust gas pipe) 12 where an exhaust gas flows.

[0024] The exhaust air manifold 5 and the intake air manifold 4 are communicated with each other via an exhaust gas recirculation path 18 (hereinafter, referred to as EGR path 18). Inside the EGR path 18, an electronically controlled EGR control valve 19 is disposed. Further, around the EGR path 18, an EGR cooler 20 for cooling an EGR gas flowing in the EGR path 18 is disposed.

[0025] Each of the fuel injection valves 3 is connected to a common rail 22 via a fuel feed pipe 21. The common rail 22 is connected to a fuel tank 24 via a fuel pump 23. The fuel pump 23 feeds a fuel in the fuel tank 24 via the common rail 22, the fuel feed pipe 21 and the fuel injection valve 3 to the combustion chamber 2. In place of the fuel pump 23, for example, a discharge rate variable electronically controlled fuel pump may be used.

[0026] Inside the exhaust gas path (exhaust gas pipe) 12, an exhaust gas purification portion 40 is disposed. The exhaust gas purification portion 40 includes, in sequence from an upstream (left side in FIG. 1) toward a downstream (right side in FIG. 1) in a flow of exhaust gas, a hydrocarbon feed valve 50 and an exhaust gas purification catalyst 60 to purify NOx contained in an exhaust gas discharged from an internal combustion engine. Hereinafter, in some cases, an upstream and a downstream in a flow of the exhaust gas are simply called as an upstream and a downstream, respectively. The hydrocarbon feed valve 50 feeds (injects) hydrocarbon in the exhaust gas to control a hydrocarbon concentration in the exhaust gas fed to the exhaust gas purification catalyst 60. Hereinafter, to control a hydrocarbon concentration in the exhaust gas is also called to control an air- fuel ratio (A/F) of the exhaust gas. Though illustration is omitted, on a downstream side of the exhaust gas purification catalyst 60, another catalyst may be disposed. For example, on a downstream side of the exhaust gas purification catalyst 60, a particulate filter for collecting particulates in the exhaust gas may be disposed. Further, in an exhaust gas path 12 on an upstream side of the exhaust gas purification catalyst 60, an oxidizing catalyst for modifying hydrocarbon injected from the hydrocarbon feed valve 50 may be disposed.

[0027] The exhaust gas purification catalyst 60 is a catalyst for purifying NOx contained in the exhaust gas. The exhaust gas purification catalyst 60 is disposed in an exhaust gas path 12. The exhaust gas purification catalyst is constituted by forming a catalyst layer on a base material. Then, the exhaust gas purification catalyst removes NOx contained in the exhaust gas by a catalyst function that the catalyst layer has. Such the exhaust gas purification catalyst will be detailed with reference to FIGS. 2 and 3. FIG. 2 is a perspective view schematically showing the exhaust gas purification catalyst 60, and FIG. 3 is an enlarged diagram schematically showing one example of a cross-sectional configuration of the exhaust gas purification catalyst 60. The exhaust gas purification catalyst 60 related to the embodiment includes a base material 62, a plurality of regularly arranged cells 66, and a rib wall 64 constituting the cells 66.

[0028] As the base material 62 of the exhaust gas purification catalyst 60 disclosed herein, a base material the same as that of the exhaust gas purification catalyst of the related art may be used. For example, the base material 62 is preferably composed of a heat resistant raw material having a porous structure. Examples of such the heat resistant raw materials include refractory metals or alloys such as cordierite, silicon carbide (SiC), aluminum titanate, silicon nitride and stainless steel. Further, the base material is preferred to have a honey comb structure, a foam shape, or a pellet shape. An outer shape of an overall base material may be a cylindrical shape, an elliptic cylinder shape or a multi-angular cylinder. In the exhaust gas purification catalyst 60 shown in FIG. 2, as the base material 62, a cylindrical member having a honey comb structure is adopted.

[0029] In the exhaust gas purification catalyst 60 disclosed herein, a catalyst layer 68 is formed on the base material 62. The catalyst layer 68 includes a plurality of kinds of precious metal catalysts and a support. In the exhaust gas purification catalyst 60 shown in FIG. 3, the catalyst layer 68 is formed on a surface of a rib wall 64 of the base material 62. An exhaust gas fed to the exhaust gas purification catalyst 60 flows inside of a path of the base material 62 and comes into contact with the catalyst layer 68 to purify harmful components. The catalyst layer 68 includes plural kinds of precious metal catalysts and a support that supports the precious metal catalysts. Further, in the exhaust gas purification catalyst 60 disclosed herein, a NOx storage material is supported on the support.

[0030] The plurality kinds of precious metal catalysts contained in the catalyst layer 68 may have a catalyst function to NOx contained in the exhaust gas. In the embodiment, at least platinum (Pt) and rhodium (Rh) are contained. As catalysts of precious metals other than platinum and rhodium, catalysts of Pd (palladium), ruthenium (Ru), iridium (Ir), osmium (Os), or so on, may be used. In particular, when Pd (palladium) is used, the heat resistance of a catalyst is improved. [0031] Though not particularly limited, a content of Pt in a total volume of the catalyst of the exhaust gas purification catalyst 60 may be usually 0.5 g/L to 10 g/L. For example, 0.5 g/L to 7.0 g/L is preferred. When a support amount of the Pt is too scarce, catalyst activity (particularly, oxidation catalyst activity) obtained by Pt becomes insufficient. On the other hand, when a support amount of Pt is too abundant, Pt tends to generate grain growth and, simultaneously, the cost increases. Further, a content of Rh in an entire volume of the catalyst of the exhaust gas purification catalyst 60 may be usually 0.01 g/L to 1.0 g/L. For example, 0.1 g/L to 0.5 g/L is preferable. When a support amount of the Rh is too scarce, catalyst activity (particularly, reduction catalyst activity) obtained by Rh becomes insufficient. On the other hand, when a support amount of Rh is too abundant, the oxidation catalyst activity deteriorates and simultaneously the cost increases.

[0032] The catalyst layer 68 is formed by supporting a precious metal catalyst on a support (typically, in powder). Examples of such the supports include: metal oxides such as alumina (A1₂0₃), zirconia (Zr0₂), ceria (Ce0₂), silica (Si0₂), magnesia (MgO), and titanium oxide (titania: Ti0₂); or composite oxides thereof (ceria-zirconia (Ce0₂-Zr0₂) composite oxide, for example). Among these, alumina, zirconia, and ceria-zirconia composite oxides are preferably used. Two or more kinds thereof may be used in a combination. Further, the above-described plurality kinds of precious metal catalysts may be supported on different supports. For example, an exhaust gas purification catalyst 60 obtained by mixing a catalyst support constituted by alumina that supports platinum, a catalyst support constituted by zirconia that supports rhodium and a catalyst support constituted by ceria-containing composite oxide that supports a NOx storage material described below may be used. When a ceria-containing composite oxide is used as the support, a Ce0₂ content in the support is preferable to be 10% by mass to 90% by mass. When the Ce0₂ content is in such the range, for activation of hydrocarbon fed for NOx purification, storage oxygen that Ce0₂ has can be well utilized. Further, an oxygen storage amount measured by a CO-0₂ pulse reaction is preferable to be 10 mmol or more/L of catalyst.

[0033] The support may contain other materials (typically inorganic oxide) as an accessory component. Examples of substances that can be added to the support include a rare earth element such as lanthanum (La) or yttrium (Y), alkali earth element such as calcium, zirconium (Zr), a transition metal element or so on. Among these, rare earth elements such as lanthanum and yttrium improve a specific surface area at high temperatures without damaging the catalyst function; accordingly, these can be preferably used as a stabilizer.

[0034] On the support (typically, in powder) of the exhaust gas purification catalyst 60, a NOx storage material that can store and release NOx is supported. The NOx storage material has a NOx storage capacity, that is, the NOx storage material absorbs NOx in an exhaust gas in a state where an air-fuel ratio of the exhaust gas is in a lean state where oxygen is in excess, and when the air-fuel ratio is switched to a rich side, stored NOx is released. As such the NOx storage material, barium compounds (typically, barium oxide and/or barium carbonate) are used. Barium has high NOx storage capacity and is suitable as a NOx storage material used in the exhaust gas purification catalyst disclosed here. Further, according to the embodiment, a barium-titanium composite oxide where titanium is dissolved in the barium to form a solid solution is used as a NOx storage material.

[0035] Now, the NOx storage material has a property that, under high temperatures (for example, 300°C or more, typically 400°C or more), an amount of storable NOx (saturation amount) rapidly deteriorates to be unable to absorb NOx more than that even when the air-fuel ratio is in a lean state. Accordingly, at high temperatures, NOx contained in the exhaust gas goes through the exhaust gas purification catalyst 60 to flow out downstream, and as a result, the NOx purification rate may deteriorate. Accordingly, according to the exhaust gas control method disclosed here, two different purification modes are switched in accordance with a temperature of the exhaust gas purification catalyst 60 to perform purification treatment of NOx. Hereinafter, the two NOx purification treatments in the exhaust gas control method of the embodiment are referred to as a "high temperature purification mode" and a "low temperature purification mode".

[0036] According to the high temperature purification mode,' as shown in FIG. 4, when a temperature of the exhaust gas purification catalyst 60 is equal to or more than a predetermined reference value (for example 300°C, typically 400°C), a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is alternately changed at a predetermined period Tl between a high concentration region and a low concentration region to purify NOx contained in the exhaust gas. Here, the high concentration region is a region where the hydrocarbon concentration is relatively high, and the low concentration region is a region where the hydrocarbon concentration is relatively low. The high concentration region is typically a region on a rich side than stoichiometry or on a lean side near the stoichiometry. Further, the low concentration region is relatively low is typically a region on a lean side remote from the stoichiometry. FIG. 4 shows a timing for feeding hydrocarbon and a change of an air- fuel ratio (A/F) of the exhaust gas flowing in the exhaust gas purification catalyst 60 in the high temperature purification mode. The change of the air-fuel ratio (A/F) depends on a change of the hydrocarbon concentration in the exhaust gas flowing in the exhaust gas purification catalyst 60. Accordingly, the change of the air-fuel ratio (A/F) shown in FIG. 4 can be regarded as showing a concentration change of hydrocarbon. In an example shown in FIG. 4, the air-fuel ratio in the high concentration region is on a rich side than stoichiometry (theoretical air-fuel ratio). However, the air-fuel ratio in the high concentration region is not limited thereto but may be on a lean side close to stoichiometry.

[0037] FIG. 6A and FIG. 6B schematically show a surface portion of a support 75 of the exhaust gas purification catalyst 60. That is, in FIG. 6A and FIG. 6B, an example of a reaction is shown, according to the example, the reaction is assumed to be generated when, in the high temperature purification mode, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is alternately changed between a high concentration region and a low concentration region. A reaction in the high temperature purification mode is not limited thereto. FIG. 6A shows a case where a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is low, and FIG. 6B shows a case where a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is high. In FIG. 6A and FIG. 6B, a case where platinum 70, rhodium 72 and a NOx storage material 74 are supported on the same support 75 is shown, and, without limiting thereto, respective materials may be supported on different supports.

[0038] According to one example of the high temperature purification mode disclosed here, as shown in FIG. 4, a hydrocarbon concentration of the exhaust gas flowing in the exhaust gas purification catalyst 60 is maintained in a low concentration region (typically a lean state where oxygen is in excess). Accordingly, an exhaust gas flowing in the exhaust gas purification catalyst 60 is usually in a state where oxygen is in excess. Accordingly, NO contained in the exhaust gas is, as shown in FIG. 6A, oxidized with excess oxygen on platinum 70 to be N0₂, subsequently the N0₂ is imparted with an electron from platinum 70 to be N0₂ ^~. The N0₂ ^~ is strong in activity, and hereinafter, the N0₂ ^~ is referred to as N0₂*. On the other hand, when hydrocarbon is fed from a hydrocarbon feed valve 50, the hydrocarbon is modified on platinum 70 to be a radical state. As a result thereof, as shown in FIG. 6B, in the surroundings of active N0₂*, a hydrocarbon concentration becomes high. Now, when, after active N0₂*is generated, a state where an oxygen concentration in the surroundings of active N0₂* is high continues for a definite time or more, the active N0₂* is oxidized and absorbed in the NOx storage material 74 in the form of nitrate ion active N0₃ ^~. However, when a hydrocarbon concentration in the surroundings of active N0₂* is set higher before the definite time lapses, as shown in FIG. 6B, the active N0₂* reacts with radical hydrocarbon on platinum 70 to thereby generate a reducing intermediate. The reducing intermediate attaches on a surface of the NOx storage material 74 or is absorbed thereby. A reducing intermediate firstly generated at this time is considered to be a nitro compound R-N0₂. The nitro compound R-N0₂ becomes, after generation, a nitrile compound R-CN, the nitrile compound R-CN can exist in this state only for an instant and immediately becomes an isocyanate compound R-NCO. The isocyanate compound R-NCO is hydrolyzed to be an amine compound R-N¾. However, in this case, it is considered that only a part of the isocyanate compound R-NCO is hydrolyzed. Accordingly, as shown in FIG. 6B, a large part of the reducing intermediate held and absorbed on a surface of the NOx storage material 74 is considered an isocyanate compound R-NCO and an amine compound R-NH₂. On the other hand, when the surroundings of the reducing intermediate generated as shown in FIG. 6B are surrounded by hydrocarbon, the reducing intermediate is disturbed by hydrocarbon to be difficult for a reaction to further proceed. In this case, a concentration of hydrocarbon flowing in the exhaust purification catalyst 60 decreases. When an oxygen concentration becomes high owing to a decrease in the hydrocarbon concentration, hydrocarbon in the surroundings of the reducing intermediate is oxidized. As a result, as shown in FIG. 6A, the reducing intermediate reacts with active N0₂*. At this time, active N0₂* reacts with the reducing intermediate R-NCO or R-NH to be N₂, C0₂, and H₂0. Thus, NOx in the exhaust gas is purified.

[0039] In one example of such the high temperature purification mode, when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is set high, the reducing intermediate is generated. Further, when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is set low to make an oxygen concentration high, the active N0₂* reacts with a reducing intermediate to purify NOx. Accordingly, in order to efficiently purify NOx with the exhaust gas purification catalyst 60, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 has to be changed periodically. Further, when a feed interval of hydrocarbon is set longer, during a first feed of hydrocarbon and a second feed of hydrocarbon, a period where an oxygen concentration becomes high becomes longer. Accordingly, without generating the reducing intermediate, active N0₂* is absorbed in the NOx storage material 74 in the form of a nitrate. In order to avoid this, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is necessary to be changed in a predetermined period. For example, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 may be periodically changed between a high concentration region (typically, a rich side than stoichiometry or a lean side near the stoichiometry) and a low concentration region (a lean side remote from stoichiometry) in a period of 0.5 sec to 5 sec. In the high temperature purification mode, according to the above-described mechanism, the reaction partially or entirely proceeds. However, in a path that does not go through the reducing intermediate derived from the hydrocarbon, for example, a path where decomposition of desorbed NOx on a reducing surface of precious metal is utilized, NOx may be purified according to the control method or a catalyst material constitution. For example, NOx may be purified according to a purification mechanism the same as that of a low temperature purification mode described below.

[0040] FIG. 7 shows a NOx purification rate owing to the exhaust gas purification catalyst 60 relative to a temperature of the exhaust gas purification catalyst 60 when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is periodically changed to change the air-fuel ratio (A/F) of the exhaust gas flowing in the exhaust gas purification catalyst 60 as shown in FIG. 4. As shown in FIG. 7, it is found that, in a high temperature region of about 280°C or more (for example, 280°C to 600°C, preferably 400°C to 500°C), the high temperature purification mode can obtain a higher NOx purification rate than the low temperature purification mode.

[0041] On the other hand, in the low temperature purification mode, as shown in FIG. 5, when a temperature of the exhaust gas purification catalyst 60 is lower than the reference value (for example, 300°C, typically 400°C), while keeping a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 in a low concentration region, for an interval T2 longer than the predetermined period Tl , the concentration of hydrocarbon is temporarily shifted from the low concentration region to the high concentration region to purify NOx contained in the exhaust gas. Here, the low concentration region is typically a region on a lean side remote from stoichiometry or on a lean side than stoichiometry, and the high concentration region is typically stoichiometry or a rich side than stoichiometry. Further, the predetermined period Tl is a period in the high temperature purification mode shown in FIG. 4. FIG. 5 shows a change of the air-fuel ratio (A/F) of the exhaust gas flowing in the exhaust gas purification catalyst 60 in the low temperature purification mode. The change of the air-fuel ratio (A/F) depends on a concentration change of hydrocarbon in the exhaust gas flowing in the exhaust gas purification catalyst 60. Accordingly, the change of the air-fuel ratio (A F) shown in FIG. 5 can be regarded to show a change of concentration of hydrocarbon.

[0042] FIG. 8A and FIG. 8B schematically show a surface portion of the support 75 of the exhaust gas purification catalyst 60 in the low temperature purification mode. FIG. 8A shows when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is low. FIG. 8B shows when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is high. In FIG. 8A and FIG. 8B, a case where platinum 70, rhodium 72 and a NOx storage material 74 are supported on the same support 75 is shown. However, without limiting thereto, the respective materials may be supported on different supports.

[0043] In the low temperature purification mode, when a change period of hydrocarbon concentration, that is, a feed interval of hydrocarbon is set longer than a period Tl within the predetermined range, a reducing intermediate disappears from on a surface of the NOx storage material 74. At this time, N0₂ generated on platinum 70 or rhodium 72 diffuses into the inside of the NOx storage material 74 in the form of nitrate ion to be a nitrate as shown in FIG. 8A. That is, at this time, NOx in the exhaust gas is absorbed in the NOx storage material 74 in the form of nitrate. On the other hand, FIG. 8B shows a case where when NOx is absorbed in the NOx storage material 74 in the form of nitrate like this, the air-fuel ratio of the exhaust gas flowing in the exhaust gas purification catalyst 60 is set stoichiometric or rich than stoichiometric. In this case, since a concentration of oxygen in the exhaust gas decreases, a reaction proceeds in an opposite direction (N0₃ ^~→ N0₂). As a result, a nitrate being absorbed in the NOx storage material 74 sequentially becomes a nitrate ion N0₃ ^~ and released from the NOx storage material 74 in the form of N0₂. Then, released N0₂ is reduced by hydrocarbon and CO contained in the exhaust gas on rhodium 72 or platinum 70. In this manner, NOx in the exhaust gas is purified.

[0044] According to such the low temperature purification mode, while a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 in the low concentration region (lean state) is maintained, NOx is absorbed by the NOx storage material 74. Then, when a little before the NOx absorption capacity of the NOx storage material 74 is saturated, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is temporarily set to a high concentration region (stoichiometry or rich side than stoichiometry), NOx absorbed in the NOx storage material 74 is released in a burst and reduced. Accordingly, in order to efficiently purify NOx with the exhaust gas purification catalyst 60, it is necessary for a time during which a lean state is maintained (time interval of rich control) is set longer to allow absorbing enough amount of NOx in the NOx storage material 74. For example, it is preferred that a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is shifted, after maintaining the concentration of hydrocarbon in the low concentration region (lean side far from stoichiometry) for 60 sec or more (for example, 60 sec to 100 sec), to a high concentration region (typically, rich side than stoichiometry or leans side near stoichiometry) for a time of 10% of the maintaining time or less.

[0045] FIG. 7 shows the NOx purification rate due to the exhaust gas purification catalyst 60 relative to a temperature of the exhaust gas purification catalyst 60. The NOx purification rate in FIG. 7 is a NOx purification rate when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is changed to change the air-fuel ratio (A/F) of the exhaust gas flowing in the exhaust gas purification catalyst 60 as shown in FIG. 5. As shown in FIG. 7, in the low temperature region of less than about 280°C, it is found that the low temperature purification mode can obtain the NOx purification rate higher than that of the high temperature purification mode. Accordingly, it is preferable that, when a temperature of the exhaust gas purification catalyst 60 is relatively low, the low temperature purification mode is used, and, when a temperature of the exhaust gas purification catalyst 60 is relatively high, the high temperature purification mode is used.

[0046] Here, in an exhaust gas control apparatus that together uses the two different purification modes, it is possible that a NOx storage material constituted by conventional barium can not sufficiently utilize purification performance of the high temperature purification mode. That is, according to the low temperature purification mode where the lean state is temporarily shifted to a stoichiometric to a rich state while maintaining a long lean state (for example, 60 sec or more), a period of 1 cycle is long and an amount of NOx processed during 1 cycle is abundant. Accordingly, in order to improve performance in the low temperature purification mode, it is important to increase a storable capacity of the NOx storage material. By contrast, according to the high temperature purification mode where a lean state and a stoichiometric to rich state are alternately repeated in a short period (for example, 5 sec or less per cycle), a period of 1 cycle is short and an amount of NOx processed during 1 cycle is slight. Accordingly, according to the high temperature purification mode, different from the low temperature purification mode, not capacity but speed, that is, a speed for purifying NOx immediately after switching of the gas atmosphere is demanded.

[0047] The exhaust gas purification catalyst 60 that is an embodiment of the invention includes, as shown in FIG. 9, a support 75, a metal catalyst (here, platinum 70 and rhodium 72) supported on the support 75, a NOx storage material 74 supported on the support 75 and having NOx storage capacity. Further, the exhaust gas purification catalyst 60 includes, as the NOx storage material 74, barium-titanium composite oxide particles 74 where barium 74a and titanium 74b are mutually dissolved to form a solid solution. When, like this, inside of barium-titanium composite oxide particles 74, barium 74a and titanium 74b are contained in a solid solution state, barium 74a is micronized and thereby a contact area of the barium 74a and the exhaust gas is increased. Here, a solid solution state means a state where barium 74a and titanium 74b are mixed at an atom level. Further, since a solid solution of barium and titanium is stable, the likelihood for the barium to cover a surface of a precious metal decreases. Accordingly, reaction efficiency (reaction speed) of barium 74a is enhanced, and from immediately after the gas atmosphere is switched, NOx is purified. As a result, when such the exhaust gas purification catalyst 60 is used, in the exhaust gas control apparatus 100 provided with the high temperature purification mode, purification performance of NOx can be improved.

[0048] A molar ratio of a mol of barium divided by moles of barium and titanium (hereinafter, referred to as a molar ratio of Ba/(Ba + Ti)) in the barium-titanium composite oxide may be about 0.1 to 0.9. Preferably, it is 0.3 to 0.7, and particularly preferably it is 0.4 to 0.6 (for example, 0.5). When the molar ratio of Ba/(Ba + Ti) is in such the range, barium is further micronized (dispersed) to be able to further improve the reaction efficiency of barium. When the molar ratio is too large, the content of titanium relatively decreases to be insufficient in the catalyst performance improvement effect. As a result, in some cases, high purification performance may not be obtained in the high temperature purification mode. On the other hand, when the molar ratio is too small, the content of barium relatively decreases to deteriorate the NOx storage capacity. As a result thereof, in some cases, high purification performance may not be obtained in the low temperature purification mode.

[0049] According to a preferable aspect disclosed here, an average particle size of the barium-titanium composite oxide (typically in particulate state) based on laser scattering method is 1 nm to 20 nm. When an average particle size of the barium-titanium composite oxide is set in the range, barium 74a is further micronized (dispersed). As a result, in the high temperature purification mode, from immediately after the gas atmosphereJs_ switched, high catalyst activity can be exerted [0050] A content of the barium-titanium composite oxide in an overall catalyst volume of the exhaust gas purification catalyst 60 is preferably about 0.01 mol/L to 1.0 mol/L (further preferably 0.05 mol/L to 0.3 mol/L). When the barium-titanium composite oxide is too slight, even when the air-fuel ratio of the exhaust gas is in a lean state, in some cases, preferable NOx storage amount can not be obtained, and then, in the low temperature purification mode, high purification performance can not be obtained. On the other hand, when the content of the barium-titanium composite oxide 74 is too abundant, there is a possibility that a surface of the support is covered with the barium-titanium composite oxide to deteriorate a catalyst function of the exhaust gas purification catalyst.

[0051] A method for supporting the barium-titanium oxide composite oxide on the support is not particularly limited. For example, when a support (cerium-zirconia composite oxide support, for example) is dipped in an aqueous solution containing a barium salt (barium acetate, for example) and a titanium salt (titanium citrate, for example) and, after that, dried and fired, a barium-titanium composite oxide can be supported on the support.

[0052] The exhaust gas purification catalyst 60 may be fabricated in the following manner, for example. A method described here is only an example of a method for fabricating the exhaust gas purification catalyst 60. The exhaust gas purification catalyst 60 may be fabricated according to other method.

[0053] First, a powder obtained by supporting Pt on an alumina support, a powder obtained by supporting Rh on a zirconia support, and a powder obtained by supporting a barium-titanium composite oxide on a ceria-zirconia composite oxide support are mixed to fabricate a slurry. After that, by wash-coating the slurry on a rib wall 64 of a base material 62 (see FIG. 3) constituted by a metal base material or cordierite and drying, a catalyst layer 68 may be formed on a surface of the base material 62.

[0054] In a process where the catalyst layer 68 is formed by wash coating, in order to adequately stick the slurry fast on a surface of the base material 62, the slurry may contain a binder. As the binder, for example, alumina sol and silica sol can be preferably used. The viscosity of the slurry may be adequately adjusted so that the slurry can readily flow in cells 66 of a honeycomb base material 62. Further, in the slurry, alumina (A1₂0₃) may be added to improve the thermal stability of the support. A drying condition of the slurry wash-coated on a surface of the base material 62 depends on a shape and a dimension of the base material or support. According to a typical drying condition, a drying temperature is about 80 to 120°C (for example, 100 to 110°C) and a drying time is 1 to 10 hr, and according to a typical firing condition, a firing temperature is about 400 to 1000°C (for example, 500 to 700°C) and a firing time is about 2 to 4 hr.

[0055] According to the embodiment, the catalyst layer 68 (FIG. 3) is homogeneously (in a single layer) formed over an entirety without limiting thereto. The catalyst layer 68 may be formed into a stacked structure, for example, a two-layer structure that includes a bottom layer portion (low layer portion) close to a surface of the base material 62 and a top layer portion (superficial portion) relatively remote from a surface of the base material 62.

[0056] In the above, the exhaust gas purification catalyst 60 of an exhaust gas control apparatus 100 disclosed here was described. Next, other configurations that the exhaust gas control apparatus 100 disclosed here has will be described.

[0057] As shown in FIG. 1, a temperature sensor 60a for detecting a temperature of the catalyst 60 is attached to the exhaust gas purification catalyst 60. Other means that can estimate a catalyst temperature may substitute the temperature sensor 60a. Further, a position where the temperature sensor 60a (or other means) is arranged is not limited to a position shown in the drawing. An arrangement position of the hydrocarbon feed valve 50 is not limited to the above-described position and may be any of positions that are on an upstream side than the exhaust gas purification catalyst 60 and can feed a fuel in the exhaust gas. [0058] As shown in FIG. 10, a controller (ECU) 30 is a unit that controls between an engine portion 1 and an exhaust gas purification portion 40, and, similarly with a general controller, includes a digital computer and other electronic equipments as constituent elements. Typically, the ECU 30 includes read only memories (ROM) mutually connected with a bidirectional buses, a random access memory (RAM), a microprocessor (CPU), an input port and an output port. A load sensor that generates an output voltage proportional to a depressing amount of an accelerator pedal is connected to an accelerator pedal not shown in the drawing. The output voltage of the load sensor is input in the input port via a corresponding AD converter. Further, a crank angle sensor that generates an output pulse every time when a crank shaft rotates a predetermined angle (for example, 10°) is connected to the input port.

[0059] Each of output signals from the temperature sensor 60a of the exhaust gas purification portion 40 is input in an input port of the ECU 30 via a respectively corresponding AD converter. On the other hand, an output port of the ECU 30 is connected via a corresponding driving circuit to a fuel injection valve 3, a driving step motor of a throttle valve 10, an EGR control valve 19, a fuel pump 23 and a hydrocarbon feed valve 50. Thus, the fuel injection valve 3, a hydrocarbon feed valve 50 and so on are controlled by the ECU 30. For example, hydrocarbon is fed in spot (or regularly) from the hydrocarbon feed valve 50 disposed inside an exhaust gas path 12 so that an oxygen concentration in the exhaust gas path 12 on an upstream of the exhaust gas purification catalyst 60 may be lower (the air- fuel ratio in the exhaust gas is switched from a lean state to a stoichiometric to rich state).

[0060] Specifically, the ECU (controller) 30 feeds (injects) hydrocarbon (HC) from the hydrocarbon feed valve 50 into the exhaust gas path 12 based on temperature information (signal) input from the temperature sensor 60a disposed to the exhaust gas purification catalyst 60. Such the feeding of hydrocarbon is performed according to two different purification modes, that is, the above-described high temperature purification mode and low temperature purification mode, based on temperature information (signal) input from the temperature sensor 60a.

[0061] That is, the ECU 30 is configured to, when a value (temperature signal) from the temperature sensor 60a input at a predetermined time cycle is detected to be equal to or higher than a predetermined reference value (that is, a temperature that is equal to or higher than a reference value), execute the high temperature purification mode. According to the high temperature purification mode, when a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is alternately changed at a predetermined period Tl (see FIG. 4) between the high concentration region and the low concentration region, NOx contained in the exhaust gas is purified. The high concentration region is typically a region on a rich side than stoichiometry or on a lean side close to stoichiometry, and, the low concentration region is typically a region on a lean side remote from stoichiometry. A reference value with respect to a temperature of the exhaust gas purification catalyst may be set in the range of 300°C to 500°C for example, and 400°C to 500°C typically. The ECU 30 is configured to, when a temperature of the exhaust gas purification catalyst 60 is equal to or higher than a predetermined reference value (300°C for example, 400°C typically), operate the hydrocarbon feed valve 50 at a predetermined feed interval to feed hydrocarbon into the exhaust gas path 12. Alternatively, a fuel injection amount from the fuel injection valve 3 may be adjusted to perform a change of gas atmosphere shown in FIG. 4.

[0062] It is preferable for the ECU 30 to be configured to, in the high temperature purification mode, shift a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60, after maintaining for a predetermined time in the low concentration region, to the high temperature concentration region for a time 0.005 times to 0.2 times as much as the maintaining time. Specifically, it is preferred to be configured to change a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 between the high concentration region and the low concentration region in a period of between 0.5 sec and 5 sec. For example, the ECU 30 may be configured to alternately change the high concentration region and the low concentration region every 0.05 sec to 5 sec (for example, 0.1 sec to 3 sec, for example 0.5 sec to 2 sec). Alternatively, these regions may be alternately changed in a period of 1 sec .to 5 sec. Here, the time for which a concentration of hydrocarbon is maintained in the low concentration region may be regarded as the first time of the aspects of invention. Furthermore, the time for which a concentration of hydrocarbon is shifted to the high concentration region may be regarded as the second time of the aspects of invention. Alternatively, the time for which a concentration of hydrocarbon is changed to the high concentration region and maintained therein may be regarded as the second time of the aspects of invention.

[0063] Further, the ECU 30 is configured to, when a value (temperature signal) from the temperature sensor 60a input at a predetermined time cycle is detected to be smaller than a predetermined reference value (that is, a temperature lower than a reference value), execute the low temperature purification mode. According to the low temperature purification mode, a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 is, while maintaining in the low concentration region, temporarily shifted from the low concentration region to the high concentration region for a interval T2 (see FIG. 5) longer than the predetermined period Tl (see FIG. 4). Thus, in the low temperature purification mode, NOx contained in the exhaust gas is purified. According to the embodiment, when a temperature of the exhaust gas purification catalyst 60 is lower than a predetermined reference value (for example, 400°C), the ECU 30 operates, while maintaining the air-fuel ratio in a lean state, the hydrocarbon feed valve 50 for a feed interval T2 longer than the predetermined period Tl to feed hydrocarbon in the exhaust path 12. Alternatively, by controlling a fuel injection amount from the fuel injection valve 3, a gas atmosphere change shown in FIG. 5 may be controlled to perform. Preferably, the ECU 30 may be configured to, in the low temperature purification mode, after maintaining a concentration of hydrocarbon flowing in the exhaust gas purification catalyst 60 in the low concentration region for 60 sec to 100 sec, shift the low concentration region to the high concentration region for a time of at most 10% of the maintaining time.

[0064] The exhaust gas control apparatus 100 having the configuration can preferably execute the exhaust gas control method. That is, according to the exhaust gas control apparatus 100 having the configuration, the low temperature purification mode and the high temperature purification mode are arbitrarily switched in accordance with a temperature of the exhaust gas purification catalyst 60. Therefore, the purification performance of NOx is improved. Furthermore, since the barium-titanium composite oxide where barium and titanium are dissolved to form a solid solution is used as the NOx storage material, the purification performance in the high temperature purification mode is further improved.

[0065] In the above-described embodiment, a case where the high temperature purification mode and the low temperature purification mode are arbitrarily switched is exemplified. However, an embodiment is not limited thereto. For example, an exhaust gas control apparatus where, without performing the low temperature purification mode, only the high temperature purification mode is executed irrespective of a catalyst temperature may be used. Alternatively, an exhaust gas control apparatus that together uses the high temperature purification mode and other purification mode than the low temperature purification mode may be used. According to the configuration of the embodiment, a concentration of hydrocarbon flowing in an exhaust gas purification catalyst is alternately changed between a high concentration region and a low concentration region in a predetermined period (for example, 5 sec or less per cycle). Thereby, according to the configuration of the embodiment, in an exhaust gas control apparatus that purifies NOx contained in an exhaust gas, the purification performance can be further improved compared with related arts.

[0066] Hereinafter, Example will be described. However, the invention is not limited to the Example.

[0067] An example for fabricating a catalyst sample is as shown below. Barium acetate as a barium salt and titanium citrate as a titanium salt were mixed in water for a molar ratio of Ba (Ba + Ti) to be 0.5 to fabricate a precursor aqueous solution. After a ceria-zirconia composite oxide (support) was impregnated with the precursor aqueous solution, the resulted matter was dried and fired, and a powder that supports barium-titanium composite oxide on the support was obtained. An X-ray diffraction pattern obtained according an X-ray diffraction measurement of such the powder is shown in FIG. 11. As shown in FIG. 11 , a peak of BaC0₃ was hardly detected and peaks attributed to a solid solution of barium-titanium composite oxide (typically, BaTi0₃) were observed. Based on this, it could be confirmed that much of a barium component does not exist as BaC0₃ having a large particle size but forms a solid solution of a barium-titanium composite oxide.

[0068] A powder obtained by supporting a barium-titanium composite oxide on the ceria-zirconia composite oxide support, a powder obtained by supporting Pt on an alumina support, and a powder obtained by supporting Rh on a zirconia support were mixed to fabricate a slurry. The slurry was wash-coated on a cordierite base material (a honeycomb base material shown in FIG. 2) and dried, and a catalyst layer was formed on a surface of the base material. A content of barium was set to 0.2 mol/L per volume of l L of the base material (here, per 1 L of overall bulk volume including a true volume of a honeycomb base material and a volume of a cell path: hereinafter, the same as above.). Further, a content of Pt was set to 2.2 g/L per volume of 1 L of the base material, and, a content of Rh was set to 0.25 g/L per volume of 1 L of the base material. As was described above, an exhaust gas purification catalyst (catalyst sample: volume 35 ml) according to an example was fabricated.

[0069] An exhaust gas purification catalyst according to a comparative example will be described below. Barium acetate as a barium salt was mixed in water to fabricate a precursor aqueous solution. After a ceria-zirconia composite oxide support was impregnated with the precursor aqueous solution, the resulted matter was dried and fired, and a powder in which the ceria-zirconia composite oxide support supports a barium oxide was obtained. An X-ray diffraction pattern obtained according to an X-ray diffraction measurement of such the powder is shown in FIG. 12. As shown in FIG. 12, a peak attributed to barium oxide was observed.

[0070] The powder obtained by supporting barium oxide on the ceria-zirconia composite oxide support, the powder obtained by supporting Pt on the alumina support, and the powder obtained by supporting Rh on the zirconia support were mixed to fabricate a slurry. The slurry was wash-coated on a cordierite base material (a honeycomb base material shown in FIG. 2) and dried, and a catalyst layer was formed on a surface of the base material. A content of barium was set to the same as that of example (0.2 mol/L). Further, contents of Pt and Rh were set to the same as that of example. As was described above, an exhaust gas purification catalyst (catalyst sample: volume 35 ml) according to Comparative Example was fabricated.

[0071] A measurement of a storage speed of NOx will be described below. A NOx storage speed of each of catalyst samples obtained as mentioned above was measured. Specifically, each of the catalyst samples was charged in a circulating pipe and heated to a predetermined temperature with an external heater. Then, an atmosphere surrounding each of the catalyst samples was switched between a gas (lean stimulant gas atmosphere) that simulated a lean exhaust gas composition (2 sec) and a gas (rich stimulant gas atmosphere) that simulated a rich exhaust gas composition (100 ms). Such the atmosphere switching was continued for a while, during 2 seconds after the atmosphere was switched to a lean stimulant gas atmosphere, an amount of NOx stored in each of catalyst samples was measured. . From the NOx storage amount, a NOx storage' speed (mg/(s · L)) was obtained. An atmosphere of the lean simulant gas was set to NOx: HC: CO: C0₂: 0₂: H₂0 = 50 ppm: 0 ppmC: 0%: 5%: 8%: 4%, and residue of N₂. An atmosphere of the rich simulant gas was set to NOx: HC: CO: C0₂: 0₂: H₂0 = 50 ppm: 84000 ppmC: 0%: 5%: 0%: 4%, and residue of N₂. An overall gas flow rate was set to 45 L/min. Results are shown in FIG. 14.

[0072] As obvious from FIG. 14, the NOx storage speed of a catalyst sample related to Example where the barium-titanium composite oxide is used as the NOx storage material was improved compared with that of Comparative Example where conventional barium oxide is used. In particular, in a temperature region of 250°C or more, performance difference between Example and Comparative Example was remarkable. In the case of the catalyst sample provided to the test, when the barium-titanium composite oxide is used as the NOx storage material and a catalyst bed temperature is set to 250°C or more, the NOx storage speed such high as 1 mg /(s · L) or more could be achieved.

[0073] Further, a NOx purification test in the high temperature purification mode will be described below. Each of catalyst examples of the respective Examples was subjected to a purification test of the exhaust gas containing NOx under the condition simulating the high temperature purification mode. Specifically, each of the catalyst samples was charged in a circulating pipe and heated to a predetermined temperature with an external heater. Then, a gas (lean stimulant gas) simulated a lean exhaust gas composition and a gas (rich stimulant gas) simulated a rich exhaust gas composition were alternately flowed to purify NOx. A composition of the lean simulant gas was set to NOx: HC: CO: C0₂: 0₂: H₂0 = 50 ppm: 0 ppm: 0%: 5%: 8%: 4%, and residue of N₂. A composition of the rich simulant gas was set to NOx: HC: CO: C0₂: 0₂: H₂0 = 50 ppm: 84000 ppm: 0%: 5%: 0%: 4%, and residue of N₂. A feed cycle of the rich simulant gas and the lean simulant gas was set to 50 ms: 2 sec (see FIG. 13B). The feed pattern corresponds to the high temperature purification mode of the embodiment where a lean state and a rich state are switched in a short period (for example, within 5 sec per cycle). Then, concentrations of NOx contained in gases before and after treatment were measured and the NOx purification rates were calculated. Results are shown in FIG. 15.

[0074] As obvious from FIG. 15, a catalyst sample related to Comparative Example where barium oxide was used as the NOx storage material had the NOx purification rate of less than 20% even when a catalyst bed temperature was about 250°C, that is, the purification activity was insufficient. On the other hand, a catalyst sample related to Example where barium-titanium composite oxide was used as the NOx storage material had the NOx purification rate of more than 30% when a catalyst bed temperature was about 250°C or more, that is, the purification activity was improved compared with Comparative Example. The catalyst samples of Example and Comparative Example had the same barium content but showed the above phenomena. Based on this, it is considered that in the high temperature purification mode, a reaction speed rather than capacity largely affects on the purification performance. From the result, it could be confirmed that when barium is dissolved in titanium to form a solid solution to enhance the reaction speed of barium, the purification performance of the high temperature purification mode can be further improved. Now, in the case of the catalyst sample tested here, when barium-titanium composite oxide was used as a NOx storage material and a catalyst bed temperature was set to 400°C or more, such high NOx purification rate as 40% or more could be achieved.

[0075] In the above, specific examples of the invention were detailed. However, these are only illustration and do not limit Claims. Technologies described in Claims include various modifications and alterations of above-described specific examples.

Claims

1. An exhaust gas control apparatus for purifying NOx contained in an exhaust gas discharged from an internal combustion engine, the exhaust gas control apparatus characterized by comprising:

an exhaust gas purification catalyst (60); and

a controller (30) that is configured to alternately change a concentration of hydrocarbon flowing in the exhaust gas purification catalyst between a high concentration region where the concentration of hydrocarbon is relatively high and a low concentration region where the concentration of hydrocarbon is relatively low in a predetermined period, wherein

the exhaust gas purification catalyst includes a support (75), a metal catalyst (70, 72) supported on the support, and a NOx storage material (74) supported on the support and having NOx storage capacity; and

the NOx storage material contains a barium-titanium composite oxide where barium (74a) and titanium (74b) are dissolved to form a solid solution.

2. The exhaust gas control apparatus according to claim 1 , wherein

a molar ratio of a mol of barium divided by moles of barium and titanium in the barium-titanium composite oxide is 0.1 to 0.9.

3. The exhaust gas control apparatus according to claim 1 or 2, wherein

the support that supports the barium-titanium composite oxide is constituted by a composite oxide containing Ce0₂.

4. The exhaust gas control apparatus according to any one of claims 1 to 3, wherein

the controller is configured to, after maintaining a concentration of hydrocarbon flowing in the exhaust gas purification catalyst for a first time in the low concentration region, shift the concentration of hydrocarbon to the high concentration region for a second time that is a time 0.005 times to 0.2 times as much as the first time.

5. The exhaust gas control apparatus according to any one of claims 1 to 4, wherein

the controller is configured to, under the condition where a temperature of the exhaust gas purification catalyst is 300°C or more, change a concentration of hydrocarbon flowing in the exhaust gas purification catalyst.

6. The exhaust gas control apparatus according to any one of claims 1 to 5, wherein

the predetermined period is between 0.5 sec and 5 sec.

7. An exhaust gas control method for purifying NOx contained in an exhaust gas discharged from an internal combustion engine with an exhaust gas purification catalyst, the exhaust gas control method characterized by comprising

performing a treatment where a concentration of hydrocarbon flowing in the exhaust gas purification catalyst (60) is alternately changed in a predetermined period between a high concentration region where the hydrocarbon concentration is relatively high and a low concentration region where the hydrogen concentration is relatively low in a predetermined period, wherein;

the exhaust gas purification catalyst includes a support (75), a metal catalyst (70) supported on the support and a NOx storage material (74) supported on the support; and the NOx storage material has NOx storage capacity and includes a barium-titanium composite oxide where barium (74a) and titanium (74b) are dissolved to form a solid solution.

8. The exhaust gas control method according to claim 7, wherein

9. The exhaust gas control method according to claim 7 or 8, wherein

a support that supports the barium-titanium composite oxide is constituted by a composite oxide containing Ce0₂.

10. The exhaust gas control method according to any one of claims 7 to 9, further comprising

shifting the concentration of hydrocarbon flowing in the exhaust gas purification catalyst, after maintaining the concentration of hydrocarbon at the low concentration region for a first time, to the high concentration region for a second time that is 0.005 times to 0.2 times as much as the first time.

11. The exhaust gas control method according to any one of claims 7 to 10, further comprising

changing the concentration of hydrocarbon flowing in the exhaust gas purification catalyst under the condition that a catalyst temperature is 300°C or more.

12. The exhaust gas control method according to any one of claims 7 to 11 , wherein

the predetermined period is between 0.5 sec and 5 sec.

13. An exhaust gas purification catalyst characterized by comprising:

a support (75),

a metal catalyst (70, 72) supported on the support; and

a NOx storage material (74) supported on the support, having NOx storage capacity, and containing a barium -titanium composite oxide that is obtained by dissolving barium (74a) and titanium (74b) to form a solid solution.

14. The exhaust gas purification catalyst according to claim 13, wherein

15. The exhaust gas purification catalyst according to claim 13 or 14, wherein