JP5253308B2 - NOx storage reduction exhaust gas purification catalyst - Google Patents

Description

The present invention relates to an NO _x storage reduction type exhaust gas purification catalyst.

Conventionally, various catalysts have been proposed as exhaust gas purification catalysts for purifying nitrogen oxides (NO _x ) contained in exhaust gas of internal combustion engines. In recent years, NO _x is occluded in a lean atmosphere containing excess oxygen, The stored NO _x is released by changing the exhaust gas atmosphere to a rich atmosphere with excess reducing components such as stoichiometric to hydrogen, and this is reduced and purified by reacting with reducing components such as HC and CO by the action of noble metals. A NO _x storage reduction type purification catalyst has been put into practical use.

The NO _x storage reduction type purification catalyst is composed of an NO _x storage material such as an alkali metal or an alkaline earth metal and a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd), and alumina (Al ₂ O ₃ ). A material supported on a carrier such as the like is generally used.

  Moreover, as a structure of the purification catalyst, in addition to a single-layer structure in which a metal component is present in a single layer, for example, a purification catalyst having a two-layer structure has been proposed. For example, a catalyst having two upper and lower catalyst layers in which Pt and Rh are included in different layers has been proposed.

In relation to the above, an exhaust gas purification catalyst provided with a catalyst coating layer having a layer structure including an inner layer containing zirconium oxide and having Rh substantially unevenly distributed and an outer layer having Pt substantially unevenly distributed is disclosed. cage, exhibit the higher _the NO _x purification performance for the exhaust gas of a lean-burn engine, also by having an inner layer comprising a zirconia oxide, and movement of the catalyst substrate in an alkali metal or alkaline earth metal element at a high temperature It is said that solid solution can be prevented (see, for example, Patent Document 1).

WO2008 / 090991A1

However, in order to improve _the NO _x purification performance of the NO _x storage-and-reduction type catalyst is rapidly reduced with the increase in _the NO _x storage amount capable of occluding many of the NO _x in lean atmosphere, the NO _x occluded in the rich atmosphere A balance with high reduction performance is required. Further, in order to increase the NO _x storage amount, an increase is expected by increasing the NO _x storage material such as lithium or barium, which is the NO _x storage site. Due to its basicity, the activity of noble metals, for example, the NO oxidation activity of Pt and the reduction activity of Rh tend to decrease, and the NO _x storage performance tends to decrease.

In this regard, in the configuration having a layer structure having a layer containing zirconium oxide and Rh and a layer containing Pt as in the conventional purification catalyst, both the occlusion and reducing properties necessary for NO _x purification are provided. It is not enough as a catalyst structure to increase, and when increasing the amount of NO _x storage material to achieve higher storage performance, the result is that the NO _x purification performance is adversely affected, and necessarily high NO _x storage performance and reduction. Sex is not demonstrated.

The present invention has been made in view of the above, while suppressing the use of precious metals, and aims to provide an excellent NO _x storage-and-reduction type exhaust gas purifying catalyst to _the NO _x purification performance in a lean atmosphere, the The objective is to achieve the objective.

The present invention, insertion and dominant catalyst layer structure for the reduction of NO _x, in particular by selecting the relationship between the lower layer of the upper layer and the Rh content of containing Pt, NO _x storage material weight without changing the amount of precious metal increasing, or even to reduce the amount of noble metal without changing _the NO _x storage material amount, to give a finding that can exhibit high _the NO _x purification performance, in which has been completed based on this finding.

NO _x storage-and-reduction type exhaust gas purifying catalyst of the present invention includes, in order from the supporting substrate side on a supporting substrate, a zirconium oxide (hereinafter, sometimes abbreviated as Zr oxides.) And rhodium (Rh) And a second catalyst layer containing at least one of platinum (Pt) and palladium (Pd), and at least one of the first catalyst layer and the second catalyst layer is an alkali metal. And a ratio of the total amount M ² of alkali metal and alkaline earth metal to the total amount M ¹ of platinum, palladium, and rhodium (M ² / M ¹ ; mol / g ratio) is 0. less than .5, in which the ratio of the volume of the first catalyst layer to the total volume of the catalyst layer has a structure of 60% or more.

In _the NO _x storage-reduction type exhaust gas purifying catalyst of the present invention, the first side of the lower layer near the catalyst layer on the supporting substrate Rh is supported with containing Zr oxide, the Pt and Pd as the upper layer of the lower layer When the catalyst structure is provided with the second catalyst layer on which at least one is supported, the ratio of the total amount M ² of alkali metal and alkaline earth metal to the total amount M ¹ of Pt, Pd and Rh (M ² / M ¹ Mol / g ratio), that is, the ratio of NO _x storage material / noble metal amount is less than 0.5 , and the volume amount of the first catalyst layer with respect to the total volume (V ^T ) of the catalyst layer constituting the exhaust gas purification catalyst (V ¹⁾ by the ratio ^(V 1 ^/ V ^T) of 60% or more, while maintaining the oxidation activity of the upper layer side to increase the _the NO _x storage properties in the lower layer side, of the overall catalytic oxidation activity and reduction activity Try to balance. Thus, increasing the _the NO _x storage material amount of noble metal as equivalent, or even to reduce the amount of noble metal conversely the _the NO _x storage material as equal, higher _the NO _x purification performance without reducing _the NO _x storage capacity Can be demonstrated.

The NO _x storage reduction exhaust gas purification catalyst of the present invention has a ratio M ² / M ¹ (mol / g ratio) of the total amount M ² of alkali metal and alkaline earth metal to the total amount M ¹ of Pt, Pd, and Rh. The form comprised as the range of 0.1-0.47 is preferable.

When the ratio M ² / M ¹ is in the above range, the occlusion / reduction balance in the catalyst layer is good, and the NO _x occlusion purification can be efficiently increased while making the catalyst composition with a reduced amount of noble metal.

Moreover, as a total thickness of the catalyst layer which comprises an exhaust gas purification catalyst, the form comprised as a range of 90-300 micrometers is preferable. When the total thickness of the total catalyst layer ratio V ^{1 /} V ^T of the volume amount V ¹ of the first catalyst layer to the total volume amount V ^T of the catalyst layer is 60% or more within this range, storage reduction balance in the catalyst layer However, it is possible to efficiently increase the NO _x storage and purification while making the catalyst composition with a reduced amount of noble metal.

INDUSTRIAL APPLICABILITY The present invention can construct a purification system that is excellent in exhaust gas purification ability for storing NO _x with high efficiency in a lean atmosphere and highly efficiently reducing NO _x in a rich atmosphere and that has a small amount of noble metals.

According to the present invention, while suppressing the use of precious metals, it is possible to provide an excellent NO _x storage-and-reduction type exhaust gas purifying catalyst to _the NO _x purification performance in a lean atmosphere.

It is a graph showing the relationship between the upper layer of the layer thickness and _the NO _x purification rate. Is a graph showing the effect of the layer thickness of the upper layer to be applied to _the NO _x storage amount of the purification performance shown in FIG. Is a graph showing the relationship between the layer thickness and _the NO _x storage amount of the entire catalyst layer. It is a graph which shows the relationship between the layer thickness of the whole catalyst layer, and ventilation resistance. It is a graph showing the relationship between the ratio and _the NO _x storage purification of storage material for the noble metal amount.

Hereinafter, the NO _x storage reduction type exhaust gas purification catalyst of the present invention will be described in detail.
The NO _x occlusion reduction type exhaust gas purification catalyst of the present invention (hereinafter also simply referred to as “exhaust gas purification catalyst”) is composed of zirconium oxide (Zr oxide) and rhodium in order from the support base material side on the support base material. A first catalyst layer containing (Rh) and a second catalyst layer containing at least one of platinum (Pt) and palladium (Pd), and one or both of the first catalyst layer and the second catalyst layer Both contain at least one of alkali metal and alkaline earth metal, and the ratio of the total amount of alkali metal and alkaline earth metal (M ² ) to the total amount of Pt, Pd, and Rh (M ¹ ) [M ² / M ¹ (mol / g ratio)] is less than 0.5 , and the ratio of the volume of the first catalyst layer to the total volume of the catalyst layer is 60% or more.

The first catalyst layer in the exhaust gas purifying catalyst of the present invention contains at least a Zr oxide and contains Rh (preferably supported on a carrier), and is interposed between a second catalyst layer described later and a supporting substrate. It is provided as a lower layer of the second catalyst layer. The first catalyst layer, in addition to the Zr oxide and Rh, further, palladium (Pd) or, Li, K, Na, Mg , Ca, St, alkali metal such as Ba, NO _x storage such as alkaline earth metal Materials can be included. In the present invention, the first catalyst layer preferably contains a NO _x storage material together with the Zr oxide and Rh.

Rh is excellent in reducing activity of NO _x, (supported on a preferably carrier) of Rh to another layer from the layer containing Pt is a solid solution is suppressed with Pt by the presence, in a rich atmosphere Reduces NO _x well and purifies NO _x .

Rh can be supported on a desired carrier and contained in the layer. As the carrier, particles of oxide such as zirconium dioxide (ZrO ₂ ), aluminum oxide (Al ₂ O ₃ ), silica, silica-alumina, ceria, zeolite, and mixed particles thereof can be used. The Rh-supported oxide on which Rh is supported is a mixture of oxide powder such as ZrO ₂ powder or granular material, a rhodium nitrate solution, a rhodium chloride solution, an ammine rhodium solution, and the like, followed by drying and firing (for example, 400 Etc.).

The concentration of Rh in the first catalyst layer is preferably in the range of 0.01 to 5 g / L (liter; the same applies hereinafter), and more preferably in the range of 0.05 to 1 g / L. When the concentration of Rh is within the above range, the NO _x purification performance is good.

The first catalyst layer, from the viewpoint of enhancing the reduction efficiency of the oxidation efficiency and NO _x NO, the form in which Pd is contained together with Rh is preferred. When Pd is contained, the concentration of Pd is preferably in the range of 0.05 to 2 g / L with respect to 1 g / L of Rh supported from the same viewpoint as described above.
Furthermore, the first catalyst layer may contain a noble metal such as Pt or Pd from the viewpoint of increasing the occlusion amount.

Examples of the Zr oxide contained in the first catalyst layer include TiZr composite oxides such as ZrO ₂ and TiO ₂ —ZrO ₂ , and CeZr composite oxides such as CeO ₂ —ZrO _2. It can be used as a powdery or granular material of oxide particles.

The first catalyst layer is, for example, a slurry or the like by adding a powdery or granular material (and other components if necessary) of the Rh-supported oxide particles and a Zr oxide in a medium such as water. The resulting slurry or the like can be coated on a desired support substrate and baked to form.
What is necessary is just to select suitably about a coating method, baking conditions, etc. by a composition, a scale, etc.

The layer thickness of the first catalyst layer, in the range volume ratio to the whole catalyst layer is 60% or more, in terms of keeping the reducing activity of the NO _x by Rh-loaded, preferably in the range of 54~240μm .

The second catalyst layer in the exhaust gas purification catalyst of the present invention contains at least Pt and / or Pd (preferably supported on a carrier) and is provided as an upper layer of the first catalyst layer. The second catalyst layer can further contain a NO _x storage material in addition to Pt and Pd. In the present invention, the second catalyst layer preferably contains a NO _x storage material together with Pt and / or Pd. For the NO _x storage material, alkali metals and alkaline earth metals that can be used in the first catalyst layer are suitable.

Pt is excellent in oxidation activity of NO, to generate the NO _x is oxidized to NO in a lean atmosphere. The content concentration of Pt in the second catalyst layer is preferably in the range of 0.1 to 5 g / L from the viewpoint of NO oxidation efficiency.

Pd has NO _x reduction activity, and the reduction efficiency of NO _x occluded in a lean atmosphere is increased. In addition, since Pd is highly stable in a lean atmosphere, the presence of Pd together with Pt can suppress the growth of Pt grains and keep the NO oxidation activity of Pt high.

Pt and Pd can be supported on a desired carrier and contained in the layer. As the carrier here, particles of oxide such as zirconium dioxide (ZrO ₂ ), aluminum oxide (Al ₂ O ₃ ), silica, silica-alumina, ceria (CeO ₂ ), zeolite, and mixed particles thereof are used. be able to.

When Pt and Pd are contained in the layer, Pt-supported oxide particles supporting Pt and Pd-supported oxide particles supporting Pd may be mixed, or both Pt and Pd are supported. Oxide particles may be used. The Pt-supported oxide is, for example, a powdered or granular material of an oxide such as ZrO ₂ powder and a dinitrodiammine platinum solution, a platinum chloride solution, an ammine platinum solution, etc., and dried and fired (for example, 300 to 500). Etc.). In addition, oxide particles carrying both Pt and Pd are, for example, oxide powders and granules such as ZrO ₂ powder, dinitrodiammine platinum solution, platinum chloride solution, ammine platinum solution, and palladium nitrate solution. , Palladium chloride solution, etc., and drying, baking (for example, about 300 to 500 ° C.), and the like.

The second catalyst layer is, for example, a powdery or granular material of Pt-supported oxide and / or Pd-supported oxide, or a powdery state of oxide particles supporting Pt and / or Pd in a medium such as water. A product or granular material (and other components as necessary) can be added and stirred to form a slurry, and the resulting slurry can be coated on a desired support substrate and fired to form.
What is necessary is just to select suitably about a coating method, baking conditions, etc. by a composition, a scale, etc.

In the second catalyst layer, it is preferable that the support density of Pt and / or Pd is higher as it is closer to the surface that is likely to come into contact with the exhaust gas on the side away from the support substrate. With this configuration, oxidation activity reduction performance and NO of the NO _x is further improved, can be enhanced _the NO _x purification performance.
In addition, the second catalyst layer preferably has a Rh loading of 0.1 g / L or less from the viewpoint of suppressing reduction in oxidation activity due to alloy formation between Rh and Pt and / or Pd. More preferably not.

  The layer thickness of the second catalyst layer is not particularly limited, but is preferably in the range of 30 to 60 μm from the viewpoint of oxidation activity by Pt or Pd.

In the present invention, it is preferable to contain _the NO _x storage material throughout the catalyst layer comprising a first and second catalyst layers, _the NO _x storage material is first catalytic layer and second catalytic layer The case where it is contained equally is more preferable. Furthermore, in this case, it is desirable that the ratio of the volume of the first catalyst layer to the volume of the entire catalyst layer is 60% or more.

In the exhaust gas purification catalyst of the present invention, the ratio of the volume amount of the first catalyst layer to the volume amount of the entire catalyst layer including the first catalyst layer and the second catalyst layer is set to 60% or more. In the present invention, from the viewpoint of improving NO _x storage and purification efficiency, the ratio of the NO _x storage material amount to the noble metal amount, specifically, the alkali metal relative to the total amount M ¹ [mol / g] of Pt, Pd and Rh. When the ratio [M ² / M ¹ ; mol / g ratio] of the total amount M ² [mol / g] of alkaline earth metal is less than 0.5, the above range is set. The ratio of the volume is not only the ratio of volume [m ³ ], but also 60% or more as estimated by the ratio of the thickness [μm] of the first catalyst layer to the thickness [μm] of the entire catalyst layer per unit area. can do. When the ratio of the volume of the first catalyst layer to the volume of the entire catalyst layer is less than 60%, the NO _x purification performance deteriorates, and the NO _x purification performance is improved in a composition system in which the amount of noble metal used is kept low. I can't plan.
Among them, the ratio of the volume amount (or layer thickness) of the first catalyst layer to the volume amount (or layer thickness) of the entire catalyst layer is 60 when the ratio M ² / M ¹ is 0.1 to 0.47. % Or more, more preferably 60% to 90%. If the ratio is 90% or less, the thickness of the second catalyst layer, which has a high chance of being in contact with the exhaust gas, can be maintained without being too thin, and the amount of Pt supported can be maintained. be able to. Further, when the ratio M ² / M ¹ is within the above range, the storage amount of NO _x is improved.

The total thickness of the catalyst layer including the first catalyst layer and the second catalyst layer is preferably in the range of 90 to 300 μm. The layer thickness of the entire catalyst layer is excellent in the amount of occluded NOx _by being 90 μm or more as shown in the examination of the thickness and ventilation resistance of the catalyst layer below and FIGS. 3 to 4, and being 300 μm or less. , can be expected to increase in _the NO _x storage amount commensurate with the thick film, it is possible to suppress airflow resistance low.

Here, as an example of the exhaust gas purifying catalyst of the two-layer structure will be described below with a specific example for the thickness of the second catalyst layer to increase _the NO _x purification performance to the maximum.

The ratio of the thickness of the second catalyst layer containing Pt and the first catalyst layer containing Zr oxide and Rh was examined by the method described below.
(1a) Preparation of Rh-supported powder A rhodium nitrate solution was added to a zirconia support, stirred for 1 hour, dried and fired to prepare an Rh-supported powder. The amount of Rh supported is 1% by mass.

(1b) Production of Catalysts A to D Four types of catalysts A to D were produced as described below. In each catalyst, the coating amount per 1 L of catalyst was the same (250 g / L) so that the total thickness of the first and second catalyst layers combined was constant.

≪Catalyst A≫
Various components having the following composition were mixed to prepare slurries S1 and S2. The obtained slurry S1 was coated on the whole substrate (400 cells, 6 mil, 35 ml), dried at 250 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour. Subsequently, the slurry S2 was coated on the slurry S1 coated, dried at 250 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour. Next, it was immersed in a dinitrodiamine platinum solution to carry Pt, dried at 250 ° C., and calcined at 500 ° C. for 1 hour to form a Pt / Rh-carrying two-layer coated catalyst layer. At this time, the supported amounts of Pt and Rh are 2.0 g / L and 0.5 g / L, respectively (L: indicates catalyst capacity (liter)). Next, the catalyst layer is dipped in a mixed solution in which barium acetate, lithium acetate, and potassium acetate are dissolved, pulled up to blow off excess solution, dried at 250 ° C., calcined at 500 ° C., and NO. _x occluded material was supported. The solution concentration here was adjusted so that the supported amount was Ba / Li / K = 0.2 / 0.1 / 0.15 [mol / L], respectively.
As described above, an exhaust gas purification catalyst A (hereinafter sometimes abbreviated as “catalyst A”) was produced.
<< Slurry S1 >>
・ Rh supported powder ... 50g
・ Water ・ ・ ・ 50g
<< Slurry S2 >>
・ Alumina ... 100g
・ TiZr composite oxide ... 100g
・ Water: 200g

≪Catalyst B≫
The components of the following composition are mixed to prepare slurries S3 and S4. Similarly to the catalyst A, the slurry 3 is coated on the entire substrate, dried and fired, and then the slurry 4 is coated and dried and fired. Thereafter, similarly to the catalyst A, Pt and Ba, Li, and K as the occlusion material were supported to obtain a catalyst B. The supported amount was the same as that of Catalyst A.
<< Slurry S3 >>
・ Rh supported powder ... 50g
・ Alumina ... 25g
・ TiZr composite oxide ... 25g
・ Water ・ ・ ・ 100g
<< Slurry S4 >>
・ Alumina ... 75g
・ TiZr composite oxide 75g
・ Water: 150g

≪Catalyst C≫
The components of the following composition are mixed to prepare slurries S5 and S6. Similarly to the catalyst A, the slurry 5 is coated on the entire substrate, dried and fired, and then the slurry 6 is coated and dried and fired. Thereafter, Pt and Ba, Li, and K as occlusion materials were supported in the same manner as the catalyst A to obtain a catalyst C. The supported amount was the same as that of Catalyst A.
<< Slurry S5 >>
・ Rh supported powder ... 50g
・ Alumina ... 50g
・ TiZr composite oxide ... 50g
・ Water: 150g
<< Slurry S6 >>
・ Alumina ... 50g
・ TiZr composite oxide ... 50g
・ Water ・ ・ ・ 100g

≪Catalyst D≫
Slurries S7 and S8 are prepared by mixing various components having the following composition, and in the same manner as the catalyst A, the slurry 7 is coated on the entire substrate, dried and fired, and then the slurry 8 is coated and dried and fired. Thereafter, similarly to the catalyst A, Pt and Ba, Li, and K as occlusion materials were supported to obtain a catalyst D. The supported amount was the same as that of Catalyst A.
<< Slurry S7 >>
・ Rh supported powder ... 50g
・ Alumina ... 75g
・ TiZr composite oxide 75g
・ Water: 200g
<< Slurry S8 >>
・ Alumina ... 25g
・ TiZr composite oxide ... 25g
・ Water ・ ・ ・ 50g

(1c) Evaluation of NO _x purification performance The catalysts A to D obtained above were subjected to deterioration treatment by placing them in an electric furnace and heating them at 750 ° C. for 5 hours. For each catalyst after deterioration treatment, a model gas device was used, and after lean gas was circulated in a temperature environment of 400 ° C., the following rich gas was introduced (rich spike) for 3 seconds, and the NO _x purification rate at this time Was measured. The gas conditions are shown in Table 1 below. Further, in order to calculate the NO _x purification possible amount in the lean atmosphere, the NO _x occlusion amount until the purification rate of NO _x entering the catalyst was changed from 100% to 90% was calculated.

The measured _the NO _x purification activity using the catalyst shown in Figure 1. As shown in FIG. 1, NO _x purification rate, the more the coating amount of the first catalyst layer is a lower layer (volume weight) is large, that is the coating amount of the second catalyst layer is a layer (volume weight) is small more, it is understood that _the NO _x purification activity is increased. FIG. 1 shows the thickness of the upper layer (second catalyst layer) of each catalyst. Moreover, the results of measurement of _the NO _x storage amount of the catalyst after all reduce and remove occluded NO _x in the catalyst shown in FIG. As shown in FIG. 2, it can be seen that there is almost no difference between the catalysts in the NO _x storage amount.
From these results, from the thickness of the second catalyst layer as the upper layer shown in FIG. 1, the thickness of the second catalyst layer in the present invention is preferably 60 μm or less, and more preferably 30 to 60 μm.
The thickness of the catalyst layer is a value determined by an X-ray microanalyzer (EPMA; Electron Probe Micro Analyzer).

Next, the thickness of the entire catalyst layer constituting the exhaust gas purification catalyst will be described with a specific example. In the example shown here, by the following methods (in this case fixed in 30 [mu] m) fixing the thickness of the second catalyst layer was estimated thickness of the entire catalyst layer excellent in _the NO _x storage and.

(2a) The catalyst D and the following three types of catalysts E to G were prepared.
≪Catalyst E≫
The components of the following composition are mixed to prepare slurries S9 and S10. Similarly to the catalyst A, the slurry 9 is coated on the entire substrate, dried and fired, and then the slurry 10 is coated and dried and fired. Thereafter, Pt and Ba, Li, and K as occlusion materials were supported in the same manner as the catalyst A to obtain a catalyst E. The supported amount was the same as that of Catalyst A.
<< Slurry S9 >>
・ Rh supported powder ... 50g
・ Alumina ... 17.5g
・ TiZr composite oxide ... 17.5g
・ Water: 85g
<< Slurry S10 >>
・ Alumina ... 25g
・ TiZr composite oxide ... 25g
・ Water ・ ・ ・ 50g

≪Catalyst F≫
The components of the following composition are mixed to prepare slurries S11 and S12. Similarly to the catalyst A, the slurry 11 is coated on the entire substrate, dried and fired, and then the slurry 12 is coated and dried and fired. Thereafter, Pt and Ba, Li, and K as occlusion materials were further supported in the same manner as the catalyst A to obtain a catalyst F. The supported amount was the same as that of Catalyst A.
<< Slurry S11 >>
・ Rh supported powder ... 50g
・ Alumina ... 37.5g
・ TiZr composite oxide 37.5g
・ Water: 125g
<< Slurry S12 >>
・ Alumina ... 25g
・ TiZr composite oxide ... 25g
・ Water ・ ・ ・ 50g

≪Catalyst G≫
The components of the following composition are mixed to prepare slurry S13 and S14. Similarly to the catalyst A, the slurry 13 is coated on the entire substrate, dried and fired, and then the slurry 14 is coated and dried and fired. Thereafter, similarly to the catalyst A, Pt and Ba, Li, and K as occlusion materials were supported to obtain a catalyst G. The supported amount was the same as that of Catalyst A.
<< Slurry S13 >>
・ Rh supported powder ... 50g
・ Alumina ... 125g
・ TiZr composite oxide ... 125g
・ Water: 300g
<< Slurry S14 >>
・ Alumina ... 25g
・ TiZr composite oxide ... 25g
・ Water ・ ・ ・ 50g

With (2b) a catalyst D-G, by a similar method to the evaluation of the above of the NO _x purifying performance was calculated _the NO _x storage amount, showing the relationship between the thickness and _the NO _x storage amount of the entire catalyst layer in FIG. 3 .

As shown in FIG. 3, when the thickness of the entire catalyst layer is 90μm or more, it is seen that the higher _the NO _x storage activity. In other words, if the thickness of the entire catalyst layer is reduced, that is, if the total volume of the catalyst layer is reduced, the geometric specific surface area of the catalyst is reduced, and as a result, the NO _x storage material does not become highly dispersed. It is presumed that the _x storage performance is reduced, the loading density between the NO _x storage material and the noble metal is increased, and the activity of the noble metal such as Pt by the NO _x storage material is reduced.

(3b) Subsequently, the results of the ventilation resistance obtained from the opening ratios of the catalysts D to G after forming the catalyst layer are shown in FIG.

As shown in FIG. 4, when the thickness of the entire catalyst layer exceeds 300 μm, the airflow resistance tends to increase remarkably. Since an increase in ventilation resistance generally has a large effect on engine operation, it is desirable that this value be small.
As described above, from the results of FIGS. 3 to 4, the thickness of the entire catalyst layer is preferably 90 μm or more and 300 μm or less.

The content concentration of the NO _x storage material in the entire catalyst layer including the first catalyst layer and the second catalyst layer is preferably in the range of 0.01 to 5 mol per liter of the support base, A range of ˜2 mol is more preferred. When the loading amount of the NO _x storage material is 0.01 mol / L or more, the NO _x purification ability becomes better, and when it is 5 mol / L or less, the reduction activity of Rh can be maintained without loss.

  The support substrate supports a catalyst layer that supports a catalyst metal such as Rh, Pt, or Pd, and a known ceramic or metal-made one can be selected depending on the purpose and circumstances. Specific examples include cordierite honeycomb substrates, SiC honeycomb substrates, metal honeycomb substrates, and the like.

  The first catalyst layer and the second catalyst layer are each provided with only one layer, and also provided with two or more first catalyst layers and / or second catalyst layers. It may be configured as follows. When two or more first catalyst layers are provided, the total volume (thickness) thereof may be 60% or more with respect to the total volume of the catalyst layer.

  In addition, the exhaust gas purification catalyst of the present invention may be composed of three or more layers by providing another layer together with the first catalyst layer and the second catalyst layer.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist thereof.

Example 1
-Preparation of slurry for catalyst layer-
(1) Preparation of Rh supported powder A rhodium nitrate solution was added to a zirconia support, stirred for 1 hour, dried and fired to prepare an Rh supported powder. The amount of Rh supported is 1% by mass.

(2) Preparation of slurry Various components having the following composition were mixed to prepare slurry S15 and S16.
<Composition of slurry S15>
・ Rh supported powder ... 40g
・ Alumina ... 80g
・ TiZr composite oxide ... 80g
・ Water: 200g
<Composition of slurry S16>
・ Alumina ... 25g
・ TiZr composite oxide ... 25g
・ Water ・ ・ ・ 50g

-Production of exhaust gas purification catalyst-
The obtained slurry S15 was coated on the whole substrate (400 cells, 6 mils, 2.0 L), dried at 250 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour to form a lower layer (first catalyst layer). ) Was formed. Subsequently, the slurry S16 is coated on the lower layer formed by coating the slurry S15, dried at 250 ° C. for 1 hour, and then calcined at 500 ° C. for 1 hour to form an upper layer (second catalyst layer). ) Was formed. Next, it is immersed in a mixed solution of a dinitrodiamine platinum solution and a palladium nitrate solution to carry Pt and Pd, and then dried at 250 ° C. and baked at 500 ° C. for 1 hour to carry Pt · Pd / Rh carrying 2 A layer coat catalyst layer was formed. At this time, the supported amounts of Pt, Pd, and Rh are 2.0 g / L, 0.75 g / L, and 0.4 g / L, respectively (L: indicates catalyst capacity (liter)). Next, the catalyst layer is dipped in a mixed solution in which barium acetate, lithium acetate, and potassium acetate are dissolved, pulled up to blow off excess solution, dried at 250 ° C., calcined at 500 ° C., and NO. _x occluded material was supported. The solution concentration here was adjusted so that the supported amount was Ba / Li / K = 0.2 / 0.1 / 0.15 [mol / L], respectively.
In this manner, an exhaust gas purification catalyst having a two-layered catalyst layer (lower catalyst layer thickness: 150 μm) of lower layer / upper layer = 120 μm thickness / 30 μm thickness was produced.

(Examples 2 to 4)
In Example 1, an exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the loading amounts of the noble metal and the NO _x storage material were changed to the amounts shown in Table 2 below.

(Comparative Examples 1-3)
In Example 1, an exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the loading amounts of the noble metal and the NO _x storage material were changed to the amounts shown in Table 2 below.

(Evaluation)
1. Endurance test The exhaust gas purification catalyst obtained above was installed in the exhaust system of a lean burn engine, and the operation was performed at a catalyst temperature of 750 ° C. for 50 hours. A durability test was conducted. Thereafter, a certain amount of sulfur was flowed to the catalyst at 400 ° C. for poisoning, and then a regeneration treatment was performed at 650 ° C. for 10 minutes.
2. NO _x storage performance For each catalyst after the endurance test, after performing lean operation for 60 seconds using the same exhaust system, introducing a rich pulse for 2 to 3 seconds (rich spike), NO _x purification at this time The rate was measured. Further, in order to calculate the NO _x purification possible amount in the lean atmosphere, the NO _x occlusion amount until the purification rate of NO _x entering the catalyst was changed from 100% to 90% was calculated. The calculated results are shown in FIG.

As shown in FIG. 5, in the exhaust gas purification catalyst with the optimized catalyst layer configuration, as shown in the comparison between Example 2 and Comparative Example 1, Example 4 and Comparative Example 3, and Example 3 and Comparative Example 2. Even when the NO _x storage material increased or the amount of noble metal increased, a higher NO _x purification activity was obtained compared to the comparative example in which the NO _x storage material and the noble metal were supported in the same amount.

Claims (3)

A first catalyst layer containing zirconium oxide and rhodium and a second catalyst layer containing at least one of platinum and palladium in order from the support substrate side on the support substrate side, and the first catalyst layer At least one of the catalyst layer and the second catalyst layer includes at least one of an alkali metal and an alkaline earth metal;
Platinum, palladium, and alkali metal and the ratio of the total amount ^{M 2} of the alkaline earth metal to the total amount ^{M 1} of rhodium ^{(M 2 / M 1; mol} / g ratio) is less than 0.5, the total volume of the catalyst layer NO _x storage-and-reduction type exhaust gas purifying catalyst ratio of the volume of said first catalyst layer is 60% or more relative to. ^2. The NO _x storage reduction exhaust gas purification catalyst according to claim 1, wherein the ratio M ² / M ¹ (mol / g ratio) is 0.1 to 0.47. 3. The NO _x storage reduction exhaust gas purification catalyst according to claim 1, wherein the total thickness of the catalyst layer is 90 to 300 μm.