JP2000297627A - Exhaust gas purification catalyst and exhaust gas purification system - Google Patents

Abstract

(57) [Problem] To have excellent purification performance for lean burn exhaust gas under low exhaust gas temperature and low HC / NOx ratio conditions,
Exhaust gas purification N with greatly improved NOx reduction purification efficiency
An Ox adsorption reduction catalyst and an exhaust gas purification system using the same. SOLUTION: It contains platinum and a NOx adsorptivity improving component having a standard production Gibbs energy of nitrate of -40 to -700 kJ / mol, and is represented by the following formula: 2NO + O ₂ → 2NO ₂ in the oxygen-excess exhaust gas. Reaction progresses,
In the generated NO ₂ is stable NO2 generating stable temperature range, it can adsorb NOx, and capable of reducing NOx NO
x adsorption reduction catalyst. This is an exhaust gas purification system in which a NOx adsorption / reduction catalyst is installed in a passage for an oxygen-excess exhaust gas. The HC / CO concentration varying means for varying the concentration of hydrocarbons and / or carbon monoxide at the inlet of the NOx adsorption / reduction catalyst in the NO2 generation stable temperature range is provided by the NOx
It is installed upstream of the passage of the adsorption reduction catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst and a system for purifying exhaust gas, and more particularly, to a catalyst and a system for purifying exhaust gas from an internal combustion engine, a combustor, etc. The present invention relates to a system for purifying nitrogen oxides in lean burn exhaust gas contained in excess with high efficiency.

[0002]

2. Description of the Related Art Three-way catalysts have been widely used as catalysts for purifying exhaust gas containing almost equal oxidizing and reducing components, such as conventional automobile engine exhaust gas. This three-way catalyst comprises platinum (Pt), palladium (P
d) and a catalyst mainly composed of activated alumina carrying various components including a noble metal component such as rhodium (Rh) and ceria (Ce), and hydrocarbon (HC) which is a harmful component in exhaust gas. , Carbon monoxide (CO) and nitrogen oxides (NOx) can be purified with high efficiency.

On the other hand, in recent years, from the viewpoint of improving fuel efficiency and reducing carbon dioxide emission, lean burn engines that operate even at an air-fuel ratio higher than the stoichiometric air-fuel ratio have become widespread. The exhaust gas of this lean burn engine (hereinafter, referred to as “lean burn exhaust gas”) has a higher oxygen content than the exhaust gas of a conventional engine operating near the stoichiometric air-fuel ratio (hereinafter, referred to as “stoichiometric exhaust gas”). With the three-way catalyst, purification of NOx is insufficient. Under such circumstances, a new catalyst capable of purifying NOx in lean burn exhaust gas with high efficiency has been desired, and development of various NOx reduction catalysts has been attempted.

In response to such demands, there have been proposed zeolite-based catalysts in which zeolites such as Y-type, L-type, mordenite, and MFI zeolite carry various metal components. , HC
It has been known that it has the ability to purify NOx relatively efficiently in the coexistence of water. The metal components to be supported include copper (Cu), cobalt (Co), and silver (A
g), transition metal components such as nickel (Ni) and iron (Fe) are effective, and platinum is also effective as a noble metal component. Among them, a Cu-zeolite catalyst supporting copper is, Relatively excellent NO even under high flow gas conditions
It shows an x-purifying ability, and is expected to be applied to the purification of exhaust gas from small mobile sources such as automobiles and stationary-type in-house power generation engines.

However, the zeolite-based catalyst has the following problems, and has not been put to practical use as a catalyst for purifying automobile exhaust gas operated under lean conditions. First,
The temperature range in which NOx can be purified relatively efficiently is narrow, and particularly in a relatively low temperature range of 150 to 300 ° C, sufficient NOx purification performance cannot be obtained. In addition, when the amount of hydrocarbons in the exhaust gas is relatively small, in particular, under conditions where the HC / NOx ratio is 5 or less, the NOx purification capacity is rapidly reduced. Further, there is a problem that the deterioration is extremely large under a high temperature condition of 600 ° C. or more including water vapor (hydrothermal condition).

[0006] Regarding the above problems, to improve the NOx purifying ability in a low temperature region, for example, a noble metal catalyst layer is provided below a Cu-zeolite catalyst layer, and the reaction heat in the noble metal catalyst layer is utilized. By doing, the upper layer Cu
It has already been proposed to operate a zeolite-based catalyst at a lower temperature (Japanese Patent Application Laid-Open Nos. 1-127044 and 5-68888). But in this case,
Deterioration increases due to heat of oxidation reaction in the lower noble metal catalyst layer, and HCs are preferentially oxidized and consumed due to the strong oxidation activity of the noble metal catalyst layer. There is a concern that this will lead to a decline.
This effect is caused when the Cu-zeolite-based catalyst layer and the noble metal component coexist in the same catalyst layer.
No. 074, JP-A-5-168939) are expected to be particularly large.

When a Pt-based catalyst is used, 200 to 2
NOx can be converted even in a relatively low temperature range of 50 ° C.
Since it is converted not only to N2O but also to N2O, it cannot be used as it is because of adverse effects on the global environment. As described above, in any of the Cu-zeolite-based catalyst and the Pt-based catalyst, the NOx purification ability was insufficient under the exhaust gas condition with a low HC / NOx ratio.

[0008] As a catalyst system other than the Cu-zeolite catalyst and the Pt catalyst, a catalyst using a phosphate has also been proposed. For example, it is known that cobalt phosphate and gallium phosphate exhibit NOx reduction activity (80th Catalyst Symposium, Oita Carpenter, Hideharu Kimura et al.). It is the activity of NOx reduction under the condition that the amount of treatment is extremely small, and it is unknown whether it can be applied to actual engine exhaust gas. It is also known that the purification performance can be improved by supporting a noble metal component on a specific phosphate (Kimura, Nishiguchi, Ishihara, Takita, 1997 Catalyst Research Presentations 3A01). However, the activity is low when water coexists or the oxygen concentration is high, and it is unknown whether this method can provide sufficient performance with actual engine exhaust gas.

Japanese Patent Application Laid-Open No. 6-55075 discloses that
A catalyst supporting an active metal such as platinum, palladium, rhodium and gold using a phosphate as a carrier is described,
Although the proposal mentions application to diesel,
Examples in which specific data are described only show the performance with stoichiometric exhaust gas, and the performance with lean exhaust gas having a high oxygen concentration is completely unknown.

Japanese Patent Application Laid-Open No. Hei 4-83516 discloses that at least one inorganic oxide selected from the group consisting of silica, titania, zeolite and alumina is disposed as a HC modifier at the first stage, and copper (Cu ), Cobalt (C
o), at least one NO selected from the group consisting of platinum (Pt), nickel (Ni) and palladium (Pd)
It has been proposed to arrange a lean NOx reduction catalyst containing an x reduction catalyst. This proposal proposes that the HC component is reformed by the first-stage catalyst and the second-stage catalyst supplies HC in a form that can be easily used for NOx reduction.
It is doubtful that the former stage HC reformer always works under low exhaust temperature conditions of about 00 ° C. In that case, platinum,
No effect can be expected for a catalyst that exhibits NOx reduction activity in a relatively low temperature range, such as palladium, and in this temperature range,
N working in relatively high temperature range such as copper, cobalt and nickel
There is a concern that the reducing ability does not work with the Ox reduction catalyst.

Japanese Unexamined Patent Publication (Kokai) No. 6-146869 proposes a catalyst having both an HC adsorbing action and a NOx reducing action. However, a NOx reducing catalyst is provided upstream of the exhaust gas and HC is adsorbed downstream. The catalyst is provided with an adsorbent that removes the HC adsorbent on the upstream side and NOx on the downstream side.
When the reduction catalyst is disposed, HC is adsorbed when the NOx reduction catalyst is in the activation temperature range, and HC is not supplied to the NOx reduction catalyst. That is, in order to take advantage of the HC adsorption effect, the timing of HC release and supply is important,
Simply combining them can have the opposite effect.
Also in the case of Japanese Patent Application Laid-Open No. Hei 4-83516, the former HC reforming material is relatively H such as titania, silica and alumina.
In the case of a material having a low C adsorption capacity, it is not necessary to consider such an adverse effect due to the HC adsorption function. However, since zeolite has a high HC adsorption capacity, it may inhibit the NOx reduction ability of the latter catalyst. There is.

JP-A-10-57763 discloses HC.
It has been proposed to form a catalyst supporting layer on an adsorption layer to promote the catalytic action of the catalyst supporting layer. This proposal is one measure for improving the exhaust gas with a low HC / NO ratio, but is not a measure for reducing the exhaust temperature of the lean NOx reduction catalyst.

Japanese Patent Application Laid-Open No. 5-321655 proposes a method of purifying exhaust gas using a combination of Cu-crystalline silicate having specific HC and CO light-off characteristics and a three-way catalyst. Although the light-off performance of HC is regarded as an important factor in this proposal, the effect of adsorbing and releasing HC is not mentioned, and an example in which HC is added to exhaust gas is described as an example. Low HC / NO by catalyst
There is a concern that it is difficult to deal with the x ratio.

[0014] Japanese Patent Application Laid-Open No. Hei 7-19031 discloses that an HC adsorbent (zeolite) is provided in the first stage, and a NOx reduction catalyst (C
An exhaust gas purifying apparatus in which a u-zeolite is disposed has been proposed. In this proposal, HC can be further supplied to the exhaust gas. However, the aim is to improve the performance of the NOx reduction catalyst by improving the HC / NOx ratio, and is effective for NOx purification at low exhaust temperature. It can not be said.

JP-A-7-124479 and JP-A-7-144134 describe that a compound of phosphorus and copper is supported on zeolite to obtain a highly durable NOx reduction catalyst. Regarding the NOx conversion rate under actual engine exhaust gas conditions at a low temperature and a low HC / NOx ratio, it cannot be said that it is necessarily high, and further improvement is necessary.

As described above, the Cu-zeolite catalyst, P
Under the exhaust gas conditions of a low HC / NOx ratio, the NOx purifying ability of the t-based catalyst and any other catalyst becomes insufficient. Therefore, a purification method has been proposed in which HCs, alcohols, and the like serving as reducing agents are secondarily supplied to the inlet of the catalyst layer. In this case, a method of mounting a reducing agent tank on the vehicle or a method of directly using fuel as the reducing agent has been proposed.In the former case, however, there is a problem of securing a tank mounting place and increasing the weight. In such a case, there is a problem that fuel efficiency of the engine is sacrificed.

Further, Japanese Patent Publication No. 2600492 discloses a purifying device for purifying nitrogen oxides in lean burn exhaust gas with high efficiency, and absorbs NOx when the inflowing exhaust gas has a lean air-fuel ratio. NOx is released when the oxygen concentration of the inflowing exhaust gas is reduced.
A device using an absorbent is described. However,
In this device, it is necessary to operate the engine at a stoichiometric air-fuel ratio or rich, or to supply hydrocarbons as a reducing agent to the exhaust passage in order to release the absorbed NOx and purify by reduction. As a result, there is a problem that the fuel efficiency of the engine is sacrificed because the fuel efficiency of the engine is sacrificed, or the space for mounting the tank for storing the reducing agent is increased and the weight increases.

In Japanese Patent Application No. 6-524106, a noble metal catalyst is brought into contact with an exhaust gas under an oxidizing atmosphere (lean) to adsorb nitrogen oxides in the exhaust gas, and then the exhaust gas There has been proposed a method for removing nitrogen oxides in exhaust gas by intermittently introducing a reducing substance into the exhaust gas. This method is also similar to Patent Publication No. 2600492,
It is necessary to supply the reducing agent to the exhaust passage.If fuel is used as the reducing agent, the fuel efficiency of the engine is sacrificed.If a reducing agent other than fuel is used, it is necessary to secure a tank mounting place for storing the reducing agent. The increase in weight eventually results in the problem of sacrificing fuel efficiency of the engine. Furthermore, the effectiveness of any of the above proposals for exhaust gas at a low temperature of 200 ° C. or lower is unclear.

[0019]

As described above, in conventional Cu-zeolite-based catalysts, Pt-based catalysts and any other catalysts, under low exhaust gas conditions with a low HC / NOx ratio,
There was a problem that the NOx purification ability became insufficient. On the other hand, in the conventional exhaust gas purifying catalyst device, it is difficult to reduce and purify NOx in lean burn exhaust gas at a low exhaust gas temperature and a low HC / NOx ratio, and a reducing agent is supplied to increase the NOx reduction / purification rate. Or the engine needs to be operated under a stoichiometric air-fuel ratio or a rich condition, and as a result, there is a problem that deterioration of fuel efficiency is inevitable.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to reduce lean-burn exhaust gas particularly at low exhaust gas temperature and low HC / NOx ratio. With excellent purification performance,
Exhaust gas purification N with greatly improved NOx reduction purification efficiency
An object of the present invention is to provide an Ox adsorption reduction catalyst and an exhaust gas purification system using the same.

[0021]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, concerning the purification of lean burn exhaust gas, the generation of NO2, its adsorption on the catalyst surface, and the generation of NOx It has been found that the concentration fluctuation of the reducing gas component (HC and / or CO) is an important factor for the reduction purification characteristics. Can be solved, and the present invention has been completed.

That is, the NOx adsorption-reduction catalyst for purifying exhaust gas of the present invention is a NOx adsorption-reduction catalyst capable of purifying an oxygen-excess exhaust gas, and has a standard production Gibbs energy of platinum and nitrate of -40 to -700 kJ / mol. In the oxygen-excess exhaust gas, a reaction represented by the following formula 2NO + O ₂ → 2NO ₂ ... Proceeds in the oxygen-excess exhaust gas, and NOx is generated in a stable NO2 generation temperature range where the generated NO ₂ is stable. And NOx can be reduced.

In a preferred embodiment of the NOx adsorptive reduction catalyst of the present invention, the NOx adsorptivity improving component is selected from the group consisting of alkali, alkaline earth, rare earth, VIIA, VIII, IB and IIB. And at least one element.

Further, in another preferred embodiment of the NOx adsorption reduction catalyst of the present invention, platinum and the NOx adsorbability-improving component include a refractory inorganic carrier having a specific surface area of 60 m ² / g or more, for example, alumina, silica, zeolite, It is characterized by being supported by at least one inorganic oxide selected from the group consisting of magnesia, titania and zirconia.

Further, the exhaust gas purifying system of the present invention is an exhaust gas purifying system comprising the above-mentioned NOx adsorption / reduction catalyst installed in the passage of the oxygen-excess exhaust gas.
HC / CO concentration variation means for varying the concentration of hydrocarbons and / or carbon monoxide at the entrance of the NOx adsorption / reduction catalyst in the stable production temperature range is provided upstream of the NOx adsorption / reduction catalyst passage. It is characterized by the following.

Further, in a preferred embodiment of the exhaust gas purifying system of the present invention, the HC / CO concentration changing means adsorbs HC at a temperature lower than a specific temperature T ° C. in the stable NO 2 generation temperature range,
An HC adsorbing / releasing material that releases HC in a high temperature range of T ° C. or higher is provided, and the specific temperature T is less than 300 ° C.

In another preferred embodiment of the exhaust gas purifying system of the present invention, the HC adsorbing / releasing material contains at least one zeolite selected from the group consisting of MFI zeolite, Y-type zeolite, mordenite and β zeolite. It is characterized by doing.

Still another preferred embodiment of the exhaust gas purifying system of the present invention is such that the HC adsorbing / releasing material includes at least one kind of NOx selected from the group consisting of copper, cobalt, silver, indium, iridium and rhodium. A reducing component,
Preferably, the HC adsorbing / releasing material has a multilayer structure in which a layer containing the NOx reducing component is stacked on a layer containing the phosphate.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, NO for purifying exhaust gas of the present invention will be described.
The x adsorption reduction catalyst will be described in detail. As mentioned above,
The NOx adsorption reduction catalyst of the present invention comprises platinum and a predetermined NOx
It contains an adsorptivity improving component and can purify excess oxygen exhaust gas.

Here, Pt, which is a component of the NOx adsorption reduction catalyst, is a component responsible for NOx reduction purification. On the other hand, the NOx adsorptivity improving component has a standard production Gibbs energy of nitrate of the element of -40 to -700 kJ / mol.
By increasing the NOx adsorption efficiency by this component, a high reduction efficiency of the NOx adsorption reduction catalyst of the present invention is realized. If the standard production Gibbs energy of nitrate deviates from the above range, the expected reducing ability cannot be obtained.
That is, when the energy value is low (<700 KJ / mol), the NOx adsorption power is too strong and the NOx reduction becomes insufficient, and the energy value is high (> -40 KJ / mo).
l) In this case, it becomes difficult to adsorb NOx.
Ox reduction becomes impossible. Specific examples of such NOx adsorptivity improving components include alkali, alkaline earth, rare earth,
Mention may be made of elements of groups VIIA, VIII, IB or IIB, and any mixtures thereof, in particular Na, C
e, K, Mg and La are preferred.

The Pt and NOx adsorbability improving components are preferably used by being supported on a refractory inorganic carrier having a specific surface area of 60 m ² / g or more, which can improve the dispersibility of each component. The function of each component can be exhibited more effectively. Examples of such a refractory inorganic carrier include alumina, silica, zeolite, magnesia, titania or zirconia and inorganic oxides according to any mixture thereof, but as a binary inorganic oxide, silica-alumina, It is also effective to use alumina-zirconia or alumina-magnesia.

Further, in this NOx adsorption reduction catalyst, the reaction represented by the following formula 2NO + O ₂ → 2NO ₂ ... Progresses, and in the NO ₂ generation stable temperature region where the generated NO ₂ is stable, the N 2
It has a function of adsorbing Ox and reducing NOx, and is suitably used in an exhaust gas purification system of the present invention described later.
The reason why such NOx adsorption / reduction characteristics are important in the catalyst of the present invention will be described in relation to the following exhaust gas purification system.

Next, the exhaust gas purifying system of the present invention will be described. As described above, this exhaust gas purification system
The NOx adsorption / reduction catalyst is provided in a passage for an excess oxygen exhaust gas, and a predetermined HC / CO concentration variation means is provided upstream of the passage.

Here, the oxygen-excess exhaust gas literally means an exhaust gas containing an excessive amount of oxygen, and contains HC, CO, NOx, etc., and has an air-fuel ratio (A / F) of 14.7 stoichiometric air-fuel ratio.
This applies to engine exhaust gas under larger operating conditions. In the exhaust gas purification system of the present invention, in particular, A / F =
When the exhaust gas temperature is 18 or more, it is possible to purify lean burn exhaust gas containing a low exhaust gas temperature of about 100 to 450 ° C.

In the exhaust gas purifying system of the present invention, the lean burn exhaust gas is temporarily but intentionally adjusted to a stoichiometric air-fuel ratio or rich atmosphere, or hydrocarbons are added to the lean burn exhaust gas as a reducing agent. It is not necessary to further increase the hydrocarbon concentration once by reducing the hydrocarbon concentration by providing a means for varying the concentration of HC and CO (typically, a material having the function of adsorbing and releasing HC). The construction is completely different from the apparatus and the purification method described in the above-mentioned Patent Publication No. 2600492 and Japanese Patent Application No. 6-524106.

In other words, the exhaust gas purifying system of the present invention is effective for exhaust gas containing A / F always in a lean region of 18 or more and having an exhaust gas temperature of about 200 ° C. or less. It is a system that can purify easily, and because of such purification characteristics, it does not require forced atmosphere control to stoichiometric to rich or addition of a reducing agent, is excellent in convenience, and can achieve cost reduction and space saving. System. However, when applied to automobile exhaust gas, it is conceivable that the exhaust gas becomes stoichiometric to rich, such as at the time of rapid acceleration operation, and a temperature of 400 ° C. or more, and the system of the present invention can be applied to such exhaust gas to purify. Needless to say, the present invention does not require such operation or the like as an essential condition.

The HC / CO concentration variation means has a function of varying the concentration of hydrocarbons and / or carbon monoxide at the inlet of the NOx adsorption / reduction catalyst in the NO2 generation stable temperature range according to the above equation. As an example of the concentration variation means, a reaction represented by the following formula 2NO + O ₂ → 2NO ₂ ... Progresses, and the specific temperature T existing in a temperature range where the generated NO2 is stable (NO2 generation stable temperature range).
An HC adsorbing / releasing material which adsorbs HC at a temperature lower than 0 ° C. and releases HC in a higher temperature range than this can be mentioned.

On the other hand, the above-mentioned NOx adsorption / reduction catalyst functions to adsorb NOx at a temperature lower than the specific temperature T ° C. and reduce NOx in a higher temperature range. Is not dependent on
It is also exhibited in the NO2 generation stable temperature range or higher.

In the present invention, it is preferable to select and adjust the HC adsorbing / releasing material so that the specific temperature T in the above-mentioned NO2 generation temperature range is 300 ° C. or less. It can be performed depending on the type of HC in it. As described above, as a preferred embodiment of the exhaust gas purifying system of the present invention, the lean burn exhaust gas flowing into the upstream side of the exhaust gas passage is provided with HC in a temperature range of less than 300 ° C.
That adsorbs and releases HC in the temperature range above 300 ° C
An adsorption / desorption material is installed, and the inflow of lean burn exhaust gas is NOx in the temperature range of less than 300 ° C on the downstream side in the exhaust gas passage.
And a NOx adsorption / reduction catalyst for reducing and purifying NOx in a temperature range of 300 ° C. or higher.

In such a configuration, the discharged exhaust gas
In a temperature range lower than 00 ° C., the amount of HC flowing into the downstream side catalyst decreases due to the effect of HC adsorption removal by the HC adsorbent. Under such a condition with a small amount of HC, the NOx adsorption effect of the NOx adsorption / reduction catalyst provided downstream is greatly enhanced, and NOx exhausted at the time of engine operation at low exhaust temperature can be removed with extremely high efficiency. Become. Next, when the exhaust gas reaches a temperature range of 300 ° C. or higher, the HC desorbed from the HC adsorbing / releasing material on the upstream side flows into the downstream NOx adsorbing / reducing catalyst, is concentrated, contacts the adsorbed NOx, and removes NOx. At this time, a high HC / NOx ratio is secured on the surface of the NOx adsorption reduction catalyst, so that an efficient NOx reduction / purification rate can be realized.

Here, in order to realize a high reduction and purification rate, the reactivity of adsorbed NOx on the catalyst surface is important. The present invention searches for an additive effective for increasing the reactivity of the adsorbed NOx, and particularly when the standard Gibbs energy of formation of nitrate of the added element is -40 to -700 kJ / mol, the reactivity is improved. There is something that focuses on doing. When NOx is adsorbed on the catalyst surface, the reactivity with HC is greatly reduced if a very stable nitrate is formed.
On the other hand, even when NOx cannot be adsorbed and held on the catalyst surface, the purification rate becomes insufficient and N ₂ O generation is inevitable.
That is, when the standard free energy of formation of nitrate is within the above range, the optimum NOx adsorption state is obtained,
It is considered that NOx adsorption removal and NOx reduction removal under high exhaust temperature conditions can be performed with high efficiency.

As described above, the exhaust gas purifying system of the present invention purifies exhaust gas exclusively by NOx adsorption under relatively low temperature conditions of less than 300 ° C. made it can be suppressed the generation of N _{2 O} is one of the greenhouse gases.

Further, in the purification system of the present invention, the manner in which the HC adsorbing / releasing material and the NOx adsorbing / reducing catalyst are installed in the exhaust gas passage is not particularly limited.
The HC adsorbing / releasing material and the NOx adsorbing / reducing catalyst are coated on a monolithic carrier described later, for example, a honeycomb monolithic carrier, and the HC adsorbing / releasing material is disposed on the upstream side of the exhaust gas and the NOx adsorbing / reducing material is disposed on the downstream side thereof. It is preferable that both are arranged in series in the flow of exhaust gas. The number of the HC adsorbing / releasing material and the number of the NOx adsorbing / reducing catalyst are not particularly limited, and may be one or more each, that is, a total of two (one pair) or more. The number of NOx adsorbing and reducing materials may be two, or two or more.

As described above, the preferred form and configuration of the exhaust gas purifying system of the present invention are such that the HC adsorbing / releasing material is disposed upstream of the exhaust gas and the NOx adsorbing reducing material is disposed downstream of the exhaust gas purifying system. Examples (for example, see JP-A-4-8351)
6, JP-A-6-146869 and JP-A-7-146869.
-19031). However, the idea of promoting the NOx reduction reaction by combining the fluctuation of the HC (reducing agent) concentration by the HC adsorption / release function and the NOx adsorption function, particularly by reducing the HC concentration under low exhaust temperature conditions of less than 300 ° C. By adsorbing NOx on the catalyst surface and contacting the adsorbed NOx with HC under an exhaust temperature condition of 300 ° C. or more, the A / F
It has been known that the NOx reduction reaction is greatly accelerated with little generation of N ₂ O even under an oxygen excess condition including a low exhaust gas temperature condition of not less than 18 and about 200 ° C. or less. This phenomenon is a feature of the present invention. As a catalyst, a NOx reduction catalyst on the downstream side of exhaust gas,
It is characterized by imparting NOx adsorption ability at a low exhaust temperature of less than 00 ° C.

Next, the material and the like of the above-mentioned HC adsorbing and releasing material will be described. Materials containing MFI zeolite, Y-type zeolite, mordenite or β zeolite and any mixture thereof can be used as the HC adsorbing / releasing material, and they function to adsorb HC with high efficiency.

In addition, the HC adsorbing and releasing material includes copper (C
u), cobalt (Co), silver (Ag), indium (I
n), iridium (Ir) or rhodium (Rh) and any mixture thereof can be added,
By adding the Ox reducing component, the HC adsorbing / releasing material is also provided with a NOx reducing and purifying ability, so that the purification efficiency of the exhaust gas can be further improved.

Further, a phosphate can be added to the HC adsorbing / releasing material, whereby the HC releasing ability can be enhanced and NOx reduction purification can be promoted. Such phosphates include copper (Cu), silver (Ag), magnesium (Mg), zinc (Zn), tin (Sn), chromium (C
r), molybdenum (Mo), tungsten (W), manganese (Mn), cobalt (Co) or nickel (Ni)
And phosphates of the elements according to any combination thereof. In addition, the above-mentioned phosphate causes a reforming reaction such as a partial oxidation reaction or an oxidative dehydrogenation reaction with respect to the adsorbed HC, in particular, a heavy HC having a large number of carbon atoms, and causes these to react with the NOx reduction reaction. It is also considered to fulfill the function of converting into HC species that are easy to use.

The above-mentioned NOx reducing components such as Cu and Co and phosphates are preferably used by being supported on a refractory inorganic carrier having a specific surface area of 60 m 2 / g or more. Can be improved, so that the function of each component can be exhibited more effectively. Examples of such a refractory inorganic carrier include alumina, silica, zeolite, magnesia, titania or zirconia and inorganic oxides according to any mixture thereof, but as a binary inorganic oxide, silica-alumina,
It is also effective to use alumina-zirconia or alumina-magnesia. Note that zeolite is particularly effective as the inorganic oxide for the NOx reducing component, and particularly preferred zeolite species include MFI zeolite and mordenite.

As a preferred embodiment of the above-mentioned HC adsorbing / releasing material, a refractory inorganic oxide layer supporting the above phosphate is formed, and a refractory inorganic oxide layer supporting the above NOx reducing component is formed thereon. A laminated structure can be given, and by adopting such a structure, the function of each component can be effectively worked. That is, HC adsorbed in the lower layer containing phosphate desorbs from the lower layer while being reformed, and reduces and purifies NOx when passing through the upper layer. When the structure of the layer is reversed and the phosphate layer is provided on the upper layer, the modified HC flows out into the gas phase without passing through the refractory inorganic oxide layer carrying the NOx reducing component. Therefore, N
The probability of being used for Ox reduction is not improved, and the effect of HC reform cannot be obtained.

In the purification system of the present invention, it is preferable that the HC adsorbing / releasing material and the NOx adsorbing / reducing catalyst described above are used by being coated on a carrier having an integral structure.
As such an integral structure type carrier, generally, a honeycomb-shaped carrier is widely used, and it is preferable that the above-described various components are coated on the honeycomb carrier and used. It is possible to easily multiply the component layers.

As the above-mentioned honeycomb carrier, cordierite-based one is generally widely used, but is not limited thereto, and a honeycomb carrier made of a metal material is also effective. By making the shape of the catalyst, that is, the shape of the HC adsorption / desorption material and the NOx adsorption / reduction catalyst into a honeycomb shape, the contact area between the catalyst and the exhaust gas is increased and the pressure loss is suppressed. This is advantageous when used as a catalyst for automobiles which is required to treat a large amount of exhaust gas in the space defined.

[0052]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

(Example 1) (1) Production of upstream (front) HC adsorption / desorption catalyst An ammonia-based aqueous solution containing copper pyrophosphate was prepared.
Specific surface area is about 220m ²/ G of boehmite alumina
After stirring well, dry at 120 ° C for at least 24 hours.
Was. Then, it is calcined at 500 ° C. for 2 hours to obtain copper pyrophosphate.
An alumina powder supporting 10 wt% was obtained.

The copper / alumina pyrophosphate powder was treated with SiO 2
The powder is mixed with H-type zeolite powder having a ratio of ₂ / Al ₂ O ₃ of about 30 at a ratio of 1: 1, further mixed with alumina sol and water, put into a magnetic ball mill pot, and then mixed and ground for about 30 minutes to obtain a slurry. I got At this time, the amount of the alumina sol added was as follows: copper pyrophosphate / alumina powder and H ₂ as Al ₂ O _3.
The content was 10 wt% based on the mixed powder of the type β zeolite powder. The slurry is applied at about 400 per square inch cross section.
1.0L cordierite honeycomb carrier with two channels
Coated on top and then dried with hot air at 150 ° C.
After firing at 500 ° C for 1 hour, the coating amount of the mixed powder is about 1
A honeycomb catalyst body A of 00 g / L was obtained.

In an aqueous solution of copper nitrate having a concentration of 0.2 M,
O₂/ Al₂O₃NH with a ratio of about 35 ₄Type MFI Zeorai
Add the powder and mix well, then filter
To separate the solid and liquid. Repeat the above stirring / filtration operation four times.
By returning the MFI, the ion-exchanged MFI
An olilite catalyst cake was obtained. This cake, the dryer
And dried at 120 ° C. for at least 24 hours, then
By baking at 550 ° C. for 4 hours in the atmosphere, C
Cu-MFI zeolite supported by 3.3% by weight of u
Medium powder was obtained.

The Cu-MFI zeolite catalyst powder was mixed with alumina sol and water, placed in a magnetic ball mill pot, and mixed and ground for about 20 minutes to obtain a slurry. At this time, the added amount of the alumina sol was Cu- as Al ₂ O _3.
10 wt% based on the mixed powder of MFI zeolite catalyst powder
And This slurry was coated on the honeycomb catalyst body 1A, dried with hot air at 150 ° C., and calcined at 500 ° C. for 1 hour to obtain a honeycomb catalyst HC having a coating amount of the Cu-MFI zeolite catalyst powder of about 180 g / L. Was.

(2) Production of NOx adsorption / reduction catalyst on the downstream side (later stage) Boehmite alumina having a specific surface area of about 220 m ² / g was added to a dinitrodiammine Pt aqueous solution having a Pt concentration of about 4 wt%, and the mixture was thoroughly stirred. Dry at 120 ° C. for 24 hours or more. Then, it is baked at 500 ° C. for 4 hours to reduce Pt to 3 w
An alumina powder carrying t% was obtained. Next, this Pt / Al
₂ O ₃ powder was mixed with nitric acid acidic alumina sol and water, and pulverized in a magnetic ball mill pot for 30 minutes to form a slurry.
At this time, the addition amount of the nitric acid acidic alumina sol was Pt / Al ₂
The content was 1 wt% with respect to the O ₃ powder. About 4 parts of this slurry
Cordierite honeycomb carrier with 00 channels
The mixture was coated on 2 L, dried with hot air at 150 ° C., and calcined at 500 ° C. for 1 hour to obtain a honeycomb catalyst body B having a coating amount of the mixed powder of about 120 g / L.

Next, an aqueous solution was formed using magnesium nitrate hydrate, and the above honeycomb catalyst B was impregnated with Pt / Al.
_After Mg is supported on ₂ O ₃ at a molar ratio of Mg: Al of about 3:97 and dried at 120 ° C. for 2 hours or more, 60
It was calcined at 0 ° C. for 4 hours to obtain a honeycomb catalyst body 1.

(3) Production of Exhaust Gas Purification Device The above-mentioned honeycomb catalyst HC and the honeycomb catalyst 1 were incorporated in a catalytic converter in series, and an exhaust gas purification device 1 having a catalyst capacity of 2.2 L was obtained.

(Examples 2 and 3) Production of downstream (late) NOx adsorption / reduction catalyst The same operation as in Example 1 was repeated except that magnesium nitrate hydrate was changed to sodium nitrate and potassium nitrate. And 3 were obtained, and the exhaust gas purifying devices 2 and 3 were obtained.

(Comparative Examples 1 to 4) Production of Downstream (Late) NOx Adsorption Reduction Catalyst The same operation as in Example 1 except that magnesium nitrate hydrate was replaced with barium acetate, calcium nitrate, strontium nitrate and silver nitrate. To obtain the honeycomb catalyst bodies Ref1 to Ref4 of Comparative Examples 1 to 4, and furthermore, the exhaust gas purifying device Ref
1-4 were obtained.

[Example of Catalyst Performance Test] The exhaust gas purification performance of the various exhaust gas purification devices obtained as described above was evaluated using an engine dynamo apparatus equipped with a 4-cylinder 2.0 L direct injection diesel engine. The catalyst inlet temperature in the present exhaust gas purifying apparatus can be controlled by changing the load of the engine. In the performance evaluation of the exhaust gas purifying apparatus, the catalyst inlet temperature was maintained at 180 ° C. for 3 minutes, and then increased to 470 ° C. in about 5 minutes. During this time, the NOx concentration at the inlet and outlet of the exhaust gas purifying device was measured by a NOx monitor for diesel exhaust gas, and the average N
The Ox purification rate was calculated.

The average HC / NOx ratio in the exhaust gas during the performance evaluation of the exhaust gas purifying apparatus was about 3.4 at 180 ° C., and was about 1.1 during the heating process. The amount of exhaust gas flowing into the catalyst (space velocity) was about 32000 h ⁻¹ . At the same time N
_The amount of N ₂ O generated was also measured using a ₂ O monitor. Table 1 shows the NOx purification performance of the purification devices of the example and the comparative example, and also shows the configuration of the catalyst in each catalyst device.

[0064]

[Table 1]

From Table 1, it can be seen that the first embodiment belongs to the scope of the present invention.
It can be seen that the purification devices of Nos. To 4 purify NOx with extremely high efficiency. In addition, it can be seen that the amount of N ₂ O generated, which has been regarded as a disadvantage when a Pt-based catalyst is used, is extremely small, and is effective in preventing global warming. At this time, the standard production Gibbs energy of the nitrate of the element impregnated and supported on the Pt / Al ₂ O ₃ catalyst is −40 to −700 kJ.
/ Mol, the NOx adsorption / reduction effect is particularly high, demonstrating the effectiveness of the present invention.

Although the present invention has been described in detail with reference to the preferred embodiments, the present invention is not limited to these embodiments, and various modifications can be made within the scope of the present disclosure. For example, in the embodiment, the HC adsorption catalyst and the NO
The x-adsorption reduction catalyst was prepared by coating each component on a separate honeycomb-shaped carrier, but each component was coated on one honeycomb-shaped carrier, and at this time, HC adsorption was performed on the upstream portion of the honeycomb carrier. The catalyst component is coated, and the remaining downstream side is coated with the NOx adsorption / reduction catalyst component.
It is also possible to form an Ox adsorption reduction catalyst.

Although the NOx adsorption / reduction catalyst contains Pt, any other element than Pt may be used as long as it has a NOx reduction / purification ability and the desired effect of the present invention can be obtained. Is also possible. Furthermore, as a specific example of the HC / CO concentration changing means, the above-mentioned HC adsorption / release material is used. However, the present invention is not limited to this. Any member and element can be used.

[0068]

As described above, according to the present invention,
Since the catalyst function was adjusted by focusing on the NO2 generation temperature range, particularly, the low exhaust gas temperature and the low HC / NO
It is possible to provide an exhaust gas purification system that has excellent purification performance for lean burn exhaust gas related to the x ratio condition and has greatly improved NOx reduction purification efficiency, and a NOx adsorption reduction catalyst. That is, according to the present invention, it is possible to purify exhaust gas at a low temperature of about 200 ° C. or lower and an average HC / NOx ratio of 2 or less with high efficiency, and therefore, it is possible to reduce the environmental impact including global warming. It is possible to provide an automobile with low pollution and excellent economy (fuel efficiency).

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) F01N 3/08 F01N 3/28 301C 3/28 301 B01D 53/36 102H 103B (72) Inventor Motohisa Kamijo Kanagawa Nissan Motor Co., Ltd., (2) Takaracho, Kanagawa-ku, Yokohama-shi, Japan (72) Inventor Maki Uekubo 2-term, Takaracho, Kanagawa-ku, Yokohama-shi, Kanagawa Nissan Motor Co., Ltd. F-term (reference) FA02 FA04 FB02 FB03 FB10 FC07 GA06 GA18 GB01W GB01X GB01Y GB02Y GB03Y GB05W GB05Y GB06W GB09X GB09Y GB10X GB10Y GB12Y GB16X GB16Y HA08 HA18 HA20 HA47 4D048 AA06 AB02 AB03 BA01X BA03A01 BA01 BA01 BA01 BA01 BA01 BA01 BA01 BA01 BA01X BA01B BA02A BA05A BA06A BA06B BA07A BC02A BC02B BC03A BC0 3B BC75A BC75B CA03 CA08 CA10 CA13 DA06 EA18 EC02X

Claims (14)

[Claims] 1. A NOx adsorptive reduction catalyst capable of purifying an oxygen-excess exhaust gas, wherein a standard production Gibbs energy of platinum and nitrate is -40 to 40.
A NOx adsorbability improving component of −700 kJ / mol is contained, and a reaction represented by the following formula 2NO + O ₂ → 2NO ₂ ... Proceeds in the oxygen-excess exhaust gas, and NO ₂ generated is stable. An exhaust gas purifying N characterized by being capable of adsorbing NOx and reducing NOx in a temperature range.
Ox adsorption reduction catalyst. 2. The method according to claim 1, wherein the NOx adsorbability improving component is at least one element selected from the group consisting of alkali, alkaline earth, rare earth, VIIA, VIII, IB and IIB. The NOx adsorption reduction catalyst according to claim 1, wherein 3. The platinum and the NOx adsorptivity improving component,
The NOx according to claim 1, wherein the NOx is supported on a refractory inorganic carrier having a specific surface area of 60 m ² / g or more.
Adsorption reduction catalyst. 4. The NOx according to claim 3, wherein the refractory inorganic carrier is at least one inorganic oxide selected from the group consisting of alumina, silica, zeolite, magnesia, titania and zirconia. Adsorption reduction catalyst. 5. An exhaust gas purification system comprising the NOx adsorption / reduction catalyst according to any one of claims 1 to 4 installed in a path of an oxygen-excess exhaust gas, wherein the NOx generation and reduction catalyst is in the NO2 generation stable temperature range. The HC / CO concentration varying means for varying the concentration of hydrocarbons and / or carbon monoxide at the inlet of the NOx adsorption reduction catalyst is provided by the N
An exhaust gas purification system, which is provided upstream of a passage of an Ox adsorption reduction catalyst. 6. The method according to claim 6, wherein the HC / CO concentration changing means is configured to control the N / C concentration.
The exhaust gas purifying system according to claim 5, further comprising an HC adsorbing / releasing material that adsorbs HC at a specific temperature lower than T ° C in an O2 generation stable temperature range and releases HC in a high temperature range of T ° C or higher. 7. The exhaust gas purification system according to claim 6, wherein the specific temperature T is lower than 300 ° C. 8. The method according to claim 6, wherein the HC adsorbing / releasing material contains at least one zeolite selected from the group consisting of MFI zeolite, Y-type zeolite, mordenite and β zeolite. Exhaust gas purification system. 9. The material for adsorbing and releasing HC is copper, cobalt,
The exhaust gas purification system according to any one of claims 6 to 8, wherein at least one NOx reducing component selected from the group consisting of silver, indium, iridium, and rhodium is added. 10. The exhaust gas purification system according to claim 6, wherein the HC adsorption / release material contains a phosphate. 11. The exhaust gas purification system according to claim 10, wherein the HC adsorption / release material has a multilayer structure in which a layer containing the NOx reducing component is laminated on a layer containing the phosphate. 12. The method according to claim 1, wherein the phosphate is copper, silver, magnesium, zinc, tin, molybdenum, tungsten, manganese,
12. The exhaust gas purification system according to claim 10, wherein the exhaust gas purification system is a phosphate of at least one element selected from the group consisting of cobalt and nickel. 13. The method according to claim 9, wherein the NOx reducing component and / or the phosphate is supported on a refractory inorganic carrier having a specific surface area of 60 m 2 / g or more. An exhaust gas purification system according to item 1. 14. The exhaust gas according to claim 13, wherein said refractory inorganic carrier is at least one inorganic oxide selected from the group consisting of alumina, silica, zeolite, magnesia, titania and zirconia. Purification system.