JP7084190B2 - Exhaust gas purification catalyst - Google Patents

Description

The present invention relates to an exhaust gas purification catalyst and a method for manufacturing an exhaust gas purification catalyst.

The catalyst for purifying exhaust gas from automobiles oxidizes hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas emitted from the engine and reduces nitrogen oxides (NOx) to water and carbon dioxide. Convert to nitrogen respectively. As a catalyst for purifying exhaust gas having such catalytic activity (hereinafter, also referred to as “three-way catalyst”), particles of catalyst precious metals such as palladium (Pd), rhodium (Rh) and platinum (Pt) are usually used. A noble metal-supported catalyst in which the contained catalyst layer is coated with a heat-resistant substrate is used.

In future catalyst development, it is desired to improve HC purification in a high SV region due to a particularly rich A / F and a low body shape, and a catalyst specialized for HC is required.

For example, Patent Document 1 describes an exhaust gas purification catalyst comprising a base material and a catalyst coat layer formed on the surface of the base material, wherein the catalyst coat layer is closer to the surface of the base material. It is formed in a laminated structure having an upper and lower layers with a lower layer and a relatively distant upper layer, the catalyst coat layer is provided with Rh and Pd as a noble metal catalyst, and the catalyst coat layer is oxygen as a carrier. The OSC material having a storing ability is provided, the Rh is arranged in the upper layer of the catalyst coat layer, and the Pd is arranged in both the upper layer and the lower layer of the catalyst coat layer, and the upper layer and the said Pd are arranged in both the upper layer and the lower layer. In the lower layer, at least a part of the Pd is supported on the OSC material, and the mass ratio of the Pd arranged in the upper layer to the Pd arranged in the lower layer is 0.4 or less. Is disclosed. According to the exhaust gas purification catalyst described in Patent Document 1, it is said that the OSC ability of the entire catalyst can be effectively improved without lowering the NOx purification ability. However, if Pd and Rh are uniformly arranged in the upper layer, there is a problem that the HC purification ability of Pd in the upper layer cannot be fully utilized.

Therefore, in the exhaust gas purification catalyst as described above, it has been desired to improve the HC purification ability in a wide temperature range while maintaining the NOx purification ability.

Japanese Unexamined Patent Publication No. 2013-136032

Therefore, an object of the present invention is to provide an exhaust gas purification catalyst having improved HC purification ability in a wide temperature range.

As a result of various studies on means for solving the above problems, the present inventors set Pd in a specific arrangement in the catalyst upper layer, set the Pd / Rh ratio in the catalyst upper layer in a specific range, and further set Pd supported on the carrier. The present invention has been completed by finding that the HC purification ability can be improved in a wide temperature range by arranging the Pd and the Pd supported by water absorption on the upper layer in an amount satisfying a specific relational expression.

That is, the gist of the present invention is as follows.
[1] An exhaust gas purification catalyst comprising a base material and a catalyst coat layer formed on the surface of the base material.
The upper layer of the catalyst coat layer in contact with the exhaust gas contains Rh and Pd and the carrier.
The upper layer contains a Pd outermost layer having a relatively higher Pd concentration than other parts of the upper layer.
More than 65% by mass of Pd contained in the outermost surface layer of Pd is present in the layer from the surface of the outermost surface layer of Pd to the thickness of 50% of the upper layer, which is relatively far from the surface of the base material.
In the upper layer, the mass ratio of Pd to Rh (Pd / Rh) is 0.5 or more and 7.0 or less, and in the upper layer, the following formula (1):
0.45 ≤ B / (A + B) ≤ 0.95 ... Equation (1)
(In the formula, A represents the amount of Pd supported on the carrier, and B represents the amount of Pd supported on the upper layer by absorbing water).
Is filled with the above exhaust gas purification catalyst.
[2] A method for producing an exhaust gas purification catalyst comprising a base material and a catalyst coat layer formed on the surface of the base material.
The upper layer of the catalyst coat layer that comes into contact with the exhaust gas is subjected to the following process:
(A) A step of forming a catalyst coat layer using a slurry containing Rh and Pd and a carrier; and (b) The catalyst coat layer formed in step (a) is allowed to absorb water containing an aqueous solution containing Pd to other than the upper layer. Including forming by a method including a step of forming a Pd outermost surface layer having a relatively higher Pd concentration than the site of.
More than 65% by mass of Pd contained in the outermost surface layer of Pd is present in the layer from the surface of the outermost surface layer of Pd to the thickness of 50% of the upper layer, which is relatively far from the surface of the base material.
In the upper layer, the mass ratio of Pd to Rh (Pd / Rh) is 0.5 or more and 7.0 or less, and in the upper layer, the following formula (1):
0.45 ≤ B / (A + B) ≤ 0.95 ... Equation (1)
(In the formula, A represents the amount of Pd supported on the carrier in the step (a), and B represents the amount of Pd carried on the upper layer by water absorption in the step (b)).
Is satisfied, the above method.

The exhaust gas purification catalyst according to the present invention has improved HC purification ability in a wide temperature range. According to the method for producing an exhaust gas purification catalyst according to the present invention, an exhaust gas purification catalyst having excellent HC purification ability can be obtained in a wide temperature range.

FIG. 1 is a diagram showing an embodiment of the present invention. FIG. 2 is a diagram showing the HC purification rate at 450 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. FIG. 3 is a diagram showing the HC purification rate at 500 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. FIG. 4 is a diagram showing NOx purification rates at 450 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. FIG. 5 is a diagram showing NOx purification rates at 500 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test.

Hereinafter, preferred embodiments of the present invention will be described in detail. In the present specification, the features of the present invention will be described with reference to the drawings as appropriate. The drawings exaggerate the dimensions and shapes of each part for clarity and do not accurately depict the actual dimensions and shapes. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of the parts shown in these drawings.

The present invention relates to a catalyst for purifying exhaust gas. Specifically, the present invention relates to a catalyst for purifying exhaust gas, which comprises a base material and a catalyst coat layer formed on the surface of the base material, wherein the upper layer of the catalyst coat layer in contact with the exhaust gas is Rh and Pd and a carrier. The upper layer contains Pd outermost surface layer having a relatively higher Pd concentration than other parts of the upper layer, and 65% by mass or more of Pd contained in the Pd outermost surface layer is relative to the surface of the base material. It exists in layers from the surface of the outermost surface layer of Pd to a thickness of 50% of the upper layer, and in the upper layer, the mass ratio of Pd to Rh (Pd / Rh) is 0.5 or more and 7.0 or less, and the upper layer. In the following equation (1):
0.45 ≤ B / (A + B) ≤ 0.95 ... Equation (1)
(In the formula, A represents the amount of Pd supported on the carrier, and B represents the amount of Pd supported by water absorption on the upper layer), which is characterized by being satisfied (hereinafter, also referred to as the catalyst of the present invention). The present inventors can improve the HC purification ability in a wide temperature range without deteriorating the NOx purification ability by arranging Pd having different carrying forms in the upper layer in an amount (mass ratio) satisfying a specific formula. I found out what I could do.

FIG. 1 shows a schematic diagram showing the configuration of an embodiment of the catalyst of the present invention. The catalyst 1 of the present invention includes a base material 10 and a catalyst coat layer 11 formed on the surface of the base material. The catalyst coat layer 11 includes an upper layer (first layer) 13, a second layer 12, and a third layer 15 of the catalyst coat layer that come into contact with the exhaust gas. The upper layer 13 includes a Pd outermost surface layer 14 having a relatively higher Pd concentration than other parts of the upper layer.

The catalyst of the present invention comprises a base material and a catalyst coat layer formed on the surface of the base material. The substrate is preferably in the form of honeycombs, pellets or particles, more preferably a monolith substrate in the form of honeycombs. Further, the base material preferably contains a heat-resistant inorganic substance such as cordierite or a metal. By using a base material having the above characteristics, high exhaust gas purification ability can be exhibited even under high temperature conditions. The length of the base material (base material length) in the direction along the exhaust gas flow is preferably 50 to 160 mm from the viewpoint of obtaining the desired catalytic performance. In the present specification, "per 1 liter of base material capacity" means per 1 L of total bulk volume including the pure volume of the base material and the volume of the cell passage. In the following description, what is described as (g / L) indicates the amount (supporting density) contained in 1 liter of the volume of the base material.

In the catalyst of the present invention, the catalyst coat layer includes an upper layer in contact with exhaust gas. The catalyst coat layer may have one or more layers in a position relatively close to the surface of the base material in addition to the upper layer. In the present specification, when the upper layer has a first layer and the lower layer has one or more layers, they are referred to as a second layer, a third layer, etc. from the far side from the surface of the substrate, and these are simply collectively referred to as a second layer, a third layer, and so on. Also called the lower layer. Each layer preferably has a different composition than the adjacent layers. Further, each layer does not necessarily have to be uniform over the entire base material of the exhaust gas purification catalyst. From the viewpoint of contact with the gas, the upper layer of the catalyst coat layer is preferably formed in a range of 30 mm or more and the total length of the base material or less in the downstream direction from the end on the upstream side.

In the catalyst of the present invention, the upper layer of the catalyst coat layer contains Rh and Pd and a carrier. The upper layer of the catalyst coat layer may contain other noble metals such as platinum (Pt), ruthenium (Ru), iridium (Ir) and osmium (Os) to the extent that the performance of Rh and Pd is not impaired. Pd and Pt mainly contribute to the purification performance (oxidative purification ability) of carbon monoxide (CO) and hydrocarbon (HC). Rh mainly contributes to the purification performance (reduction purification ability) of NOx.

In the catalyst of the present invention, the upper layer of the catalyst coat layer includes a Pd outermost surface layer having a relatively higher Pd concentration than other portions of the upper layer. The width of the outermost surface layer of Pd may be equal to or less than the width of the upper layer. The "width" of the catalyst layer means the "length" of the catalyst layer in the direction along the exhaust gas flow. The Pd outermost surface layer in the upper layer of the catalyst coat layer is preferably formed in a range of 30 mm or more and the total length of the base material or less in the downstream direction from the end on the upstream side from the viewpoint of enhancing the HC purification activity.

From the viewpoint of obtaining sufficient catalytic activity, the catalyst of the present invention has 65% by mass or more, preferably 80% by mass or more, and more preferably 85% by mass or more of Pd contained in the outermost surface layer of Pd relative to the surface of the substrate. It exists in a layer (surface layer) having a thickness of up to 50% from the surface of the outermost surface layer of Pd, which is distant from the target layer. For the proportion of Pd present in the surface layer, observe the catalyst coat layer in the portion where the Pd outermost surface layer exists in FE-EPMA, perform line analysis of Pd in the thickness direction of the catalyst coat layer cross section, and perform Pd maximum. By comparing the amount of Pd in the surface layer with the amount of Pd in the upper half of the upper layer obtained by integrating the amount of Pd in the range from the surface to the thickness of the upper layer of 50%, it exists in the surface layer. The ratio of Pd can be calculated. The numerical value of this ratio is 100% when the thickness of the Pd outermost surface layer is thinner than the thickness of 50% (surface layer) of the upper layer thickness, and the thickness of the Pd outermost surface layer is the thickness of the upper layer. If it is thicker than 50% (surface layer), it will be less than 100%. For example, the following [II-1. It can be measured by the method described in [Physical property evaluation]. Here, "Pd contained in the outermost surface layer of Pd" means Pd added at the time of forming the outermost surface layer of Pd and Pd existing in the outermost surface layer of Pd among Pd added at the time of forming the upper layer. What is the Pd outermost surface layer in the upper layer and the part other than the Pd outermost surface layer in the upper layer? It can be distinguished by doing.

The content of Pd contained in the upper layer is preferably 0.025 g / L to 1.5 g / L, more preferably 0.05 g / L or more, with respect to the substrate capacity, from the viewpoint of obtaining sufficient catalytic activity. It is 0.8 g / L. The content of Pd contained in the outermost surface layer of Pd is preferably 0.016 g / L to 0.975 g / L, more preferably 0.03 g / L to 0.52 g / L from the viewpoint of obtaining sufficient catalytic activity. It is L.

From the viewpoint of improving the HC purification ability in a wide temperature range without reducing the NOx purification ability, the catalyst of the present invention has the following formula (1): in the upper layer.
0.45 ≤ B / (A + B) ≤ 0.95 ... Equation (1)
(In the formula, A represents the amount of Pd supported on the carrier, and B represents the amount of Pd supported by water absorption on the upper layer). The "amount of Pd supported on the carrier" represented by A specifically means the total amount of Pd contained in the slurry used in the step (a) in the method for producing a catalyst for purifying exhaust gas of the present invention described later (a). Hereinafter, it is also referred to as a uniform amount of Pd), Pd directly supported on the carrier as a material for the slurry, and Pd added to the slurry separately from the carrier. The "water-absorbed and supported Pd amount" represented by B is specifically the amount of Pd supported in the step (b) in the method for producing a catalyst for purifying exhaust gas according to the present invention (hereinafter, also referred to as the surface layer Pd amount). means. From the above viewpoint, the catalyst of the present invention preferably has 0.45 ≦ B / (A + B) ≦ 0.80 in the formula (1).

Further, from the viewpoint of obtaining sufficient catalytic activity, the catalyst of the present invention has a mass ratio (Pd / Rh) of Pd to Rh of 0.5 or more and 7.0 or less in the upper layer of the catalyst coat layer, preferably 0. It is 5.5 or more and 6.4 or less, and more preferably 0.5 or more and 4.0 or less. The HC purification ability can be improved by arranging a specific amount of Pd with respect to Rh in the upper layer having good contact with the gas of the catalyst. The mass ratio of Pd to Rh (Pd / Rh) can be calculated from the ratio of Pd carried on the upper layer (total amount of uniform Pd and surface layer Pd) to Rh carried on the upper layer. For example, the following [II-1. It can be measured by the method described in [Physical property evaluation].

The carrier contained in the upper layer can be used without particular limitation as long as it can be used as a catalyst for ordinary exhaust gas purification, and aluminum oxide (Al ₂ O ₃ , alumina) and cerium oxide (CeO) can be used. ₂ , Celia), Zirconium Oxide (ZrO ₂ , Zirconia), Silicon Oxide (SiO ₂ , Silica), Ittrium Oxide (Y ₂ O ₃ , Itria) and Lantern Oxide (La ₂ O ₃ , Lantana) and Neodim Oxide (Nd ₂ ) Examples thereof include metal oxides such as O ₃ ), and solid solutions or composite oxides thereof. Two or more kinds of metal oxides may be used in combination. For example, it is preferable to use an OSC material having an oxygen occlusion ability as described in Japanese Patent Application Laid-Open No. 2013-136032 as a carrier. The OSC material occludes oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean (that is, the atmosphere on the excess oxygen side), and when the air-fuel ratio of the exhaust gas is rich (that is, the atmosphere on the excess fuel side). It works to release the stored oxygen. Examples of such an OSC material include cerium oxide (ceria: CeO ₂ ) and a composite oxide containing the ceria (for example, a ceria-zirconia composite oxide ( _{CeO2 -} ZrO2 composite oxide)). The carrier contained in the upper layer preferably contains zirconia (ZrO ₂ ) and / or alumina (Al ₂ O ₃ ). Rh carried on ZrO ₂ generates hydrogen from HC in the exhaust gas by a hydrogen reforming reaction. The reducing power of this hydrogen purifies NOx in the exhaust gas better. Al ₂ O ₃ has a larger specific surface area and higher durability (particularly heat resistance) than the ZrO ₂ composite oxide. By supporting Rh on Al ₂ O ₃ , the thermal stability of the carrier as a whole can be improved, and an appropriate amount of Rh can be supported on the entire carrier. The carrier contained in the upper layer is porous and is porous. , It is preferable to contain a metal oxide having excellent heat resistance.

When the catalyst of the present invention has one or more layers further below the upper layer (hereinafter collectively referred to as a lower layer), the lower layer of the catalyst coat layer is at least one selected from Pd, Pt and Rh. It can contain a noble metal and a carrier. The lower layer of the catalyst coat layer may contain other noble metals such as ruthenium (Ru), iridium (Ir) and osmium (Os) to the extent that the performance of Pd, Pt or Rh is not impaired. Pd and Pt mainly contribute to the purification performance (oxidative purification ability) of carbon monoxide (CO) and hydrocarbon (HC). Rh mainly contributes to the purification performance (reduction purification ability) of NOx.

The carrier contained in the lower layer is not particularly limited as long as it can be used as a catalyst for normal exhaust gas purification, and aluminum oxide (Al ₂ O ₃ , alumina) and cerium oxide (CeO) can be used. ₂ , Celia), Zirconium Oxide (ZrO ₂ , Zirconia), Silicon Oxide (SiO ₂ , Silica), Ittrium Oxide (Y ₂ O ₃ , Itria) and Lantern Oxide (La ₂ O ₃ , Lantana) and Neodim Oxide (Nd ₂ ) Examples thereof include metal oxides such as O ₃ ), and solid solutions or composite oxides thereof. Two or more kinds of metal oxides may be used in combination. For example, it is preferable to use an OSC material having an oxygen occlusion ability as described in Japanese Patent Application Laid-Open No. 2013-136032 as a carrier. The OSC material occludes oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean (that is, the atmosphere on the excess oxygen side), and when the air-fuel ratio of the exhaust gas is rich (that is, the atmosphere on the excess fuel side). It works to release the stored oxygen. Examples of such an OSC material include cerium oxide (ceria: CeO ₂ ) and a composite oxide containing the ceria (for example, a ceria-zirconia composite oxide ( _{CeO2 -} ZrO2 composite oxide)). When Pd is contained in the lower layer, barium (Ba) may be added to the carrier. By adding Ba to the carrier in the lower layer, HC poisoning of Pd is suppressed and the catalytic activity is improved. , It is preferable to use a material having a high specific surface area because it carries a noble metal such as Pd. When Rh is contained in the lower layer, the carrier may contain zirconia (ZrO ₂ ) and / or alumina (Al ₂ O ₃ ). Preferred. Rh carried on ZrO ₂ generates hydrogen from HC in the exhaust gas by a hydrogen reforming reaction. NOx in the exhaust gas is better purified by the reducing power of this hydrogen. Al ₂ O ₃ is ZrO. The specific surface area is larger and the durability (particularly heat resistance) is higher than that of the _two composite oxides. Therefore, by supporting Rh on Al ₂ O ₃ , the thermal stability of the carrier as a whole is improved and the carrier is used. An appropriate amount of Rh can be supported on the whole.

The total content of at least one noble metal selected from Pd, Pt and Rh contained in the lower layer is not particularly limited as long as sufficient catalytic activity can be obtained, and an appropriate amount can be added. ..

The present invention also relates to a method for producing a catalyst for purifying exhaust gas, which comprises a base material and a catalyst coat layer formed on the surface of the base material. a) A step of forming a catalyst coat layer using a slurry containing Rh and Pd and a carrier; and (b) the catalyst coat layer formed in step (a) is allowed to absorb water containing an aqueous solution containing Pd to absorb water from the other upper layers. It includes forming by a method including a step of forming a Pd outermost surface layer having a relatively higher Pd concentration than the site, and 65% by mass or more of Pd contained in the Pd outermost surface layer is relatively relative to the substrate surface. It exists in a layer from the surface of the distant outermost surface layer of Pd to a thickness of 50% of the upper layer, and in the upper layer, the mass ratio of Pd to Rh (Pd / Rh) is 0.5 or more and 7.0 or less, and in the upper layer. The following formula (1):
0.45 ≤ B / (A + B) ≤ 0.95 ... Equation (1)
(In the formula, A represents the amount of Pd supported on the carrier in the step (a), and B represents the amount of Pd carried on the upper layer by absorbing water in the step (b)). Hereinafter, it is also referred to as the manufacturing method of the present invention). According to the production method of the present invention, by including the steps (a) and (b) of supporting Pd in an amount (mass ratio) satisfying the formula (1), the NOx purification ability is not deteriorated in a wide temperature range. An exhaust gas purification catalyst having an improved HC purification ability can be obtained. The production method of the present invention is suitable for the production of the catalyst of the present invention, and unless otherwise specified, the preferred embodiments of the respective constituent requirements shall cite the above-mentioned description of the catalyst of the present invention.

In the above step (a), a slurry containing Rh and Pd and a carrier is coated on the surface of the base material or the surface of the lower layer to form a catalyst coat layer. The slurry may contain a catalyst powder in which Pd or the like is directly supported on the carrier powder in advance. The method for supporting Pd on the carrier is not particularly limited. For example, a method of impregnating a carrier powder with an aqueous solution containing a palladium salt (for example, nitrate) or a palladium complex (for example, a tetraammine complex) can be mentioned. In the process of forming the upper layer by coating, it is preferable that the slurry contains a binder in order to appropriately adhere the slurry to the surface of the lower layer. As the binder, for example, alumina sol, silica sol and the like are preferably used. The viscosity of the slurry may be appropriately adjusted so that the slurry can easily flow into the cell of the substrate (for example, the honeycomb substrate).

The molding amount (coating amount) of the upper layer is not particularly limited, but is preferably about 20 g to 200 g per 1 liter of the base material capacity, for example. By setting the molding amount (coating amount) of the upper layer in such a range, the grain growth of the supported Rh and Pd is prevented, and the pressure loss when the exhaust gas passes through the cell of the base material is increased. Can be prevented.

The drying conditions of the slurry coated on the surface of the base material or the surface of the lower layer depend on the shape and size of the base material or the carrier, but are typically about 80 ° C. to 150 ° C. (for example, 100 ° C. to 130 ° C.) for 1 hour. The firing conditions are about 300 ° C. to 800 ° C. (for example, 400 ° C. to 600 ° C.) for about 1 hour to 4 hours.

In the above step (b), the catalyst coat layer formed in the step (a) is made to absorb an aqueous solution containing Pd (Pd aqueous solution) to form a Pd outermost surface layer having a relatively higher Pd concentration than other parts of the upper layer. do. In absorbing the Pd aqueous solution, the upper layer is coated on the surface of the base material or the surface of the lower layer as described above, and after drying and firing, an aqueous solution containing a palladium salt (for example, nitrate) or a palladium complex (for example, tetraammine complex) is added thereto. It is preferable to absorb water. The water absorption of the Pd aqueous solution can be performed on the upper surface by a coating method, an impregnation method, or a spraying method. In either method, a region having a relatively higher Pd concentration than other parts of the upper layer may be formed in the upper layer to form the Pd outermost surface layer. The Pd aqueous solution can be prepared by adding nitric acid (other acid species such as acetic acid and citric acid are not limited) to the Pd solution. The concentration and amount of the Pd aqueous solution can be appropriately set by those skilled in the art based on the amount of Pd to be supported on the upper layer as the surface layer Pd, the amount of the coating on the upper layer to be supported, and the like. The amount of Pd supported on the upper layer can be adjusted by appropriately adjusting the pH of the Pd aqueous solution used. For example, when the pH of the Pd aqueous solution is less than 1, generally when the pH is low, the adsorption of Pd to the upper layer material is inhibited and the Pd aqueous solution penetrates deeply. As a result, the Pd outermost layer is deep in the upper layer. By being formed up to, about 65% by mass of Pd contained in the outermost surface layer of Pd can carry Pd so as to be present in the surface layer. Further, when the pH of the Pd aqueous solution is 1 or more and 2 or less, the adsorption of Pd to the upper layer material is not inhibited, and as a result, the outermost surface layer of Pd is formed to a shallower portion than when the pH is less than 1. , Pd can be supported so that about 85% by mass or more of Pd contained in the outermost surface layer of Pd is present in the surface layer.

The drying conditions of the outermost surface layer of Pd depend on the shape and size of the base material or the carrier, but are typically about 1 hour to 10 hours at about 80 ° C. to 150 ° C. (for example, 100 ° C. to 130 ° C.). The firing conditions are about 300 ° C. to 800 ° C. (for example, 400 ° C. to 600 ° C.) for about 1 hour to 4 hours.

The production method of the present invention may include a step (c) of forming a lower layer before the step (a). In forming the lower layer, a slurry containing the carrier powder may be coated on the surface of the base material (for example, a honeycomb base material) and Rh or the like may be supported on the surface, or the carrier powder may contain a catalyst powder in which Rh or the like is previously supported. The slurry may be coated on the surface of the substrate.

The method of supporting a noble metal such as Rh on the carrier of the lower layer is not particularly limited. For example, a method of impregnating a carrier powder with an aqueous solution containing a rhodium salt (for example, nitrate) or a rhodium complex (for example, a tetraammine complex) can be mentioned.

In the process of forming the lower layer by coating, it is preferable that the slurry contains a binder in order to appropriately adhere the slurry to the surface of the base material. As the binder, for example, alumina sol, silica sol and the like are preferably used. The viscosity of the slurry may be appropriately adjusted so that the slurry can easily flow into the cell of the substrate (for example, the honeycomb substrate).

The molding amount (coating amount) of the lower layer is not particularly limited, but is preferably about 20 g to 200 g per 1 liter of the base material capacity, for example. By setting the molding amount (coating amount) of the lower layer in such a range, the grain growth of the supported precious metal is prevented, and the pressure loss when the exhaust gas passes through the cell of the base material is prevented from increasing. be able to.

The drying conditions of the slurry coated on the surface of the base material depend on the shape and size of the base material or the carrier, but are typically about 80 ° C. to 150 ° C. (for example, 100 ° C. to 130 ° C.) for 1 hour to 10 ° C. The firing conditions are about 300 ° C. to 800 ° C. (for example, 400 ° C. to 600 ° C.) for about 1 hour to 4 hours.

The catalyst of the present invention has a large amount of intake air such as during acceleration, specifically, Ga conditions are high, preferably under 20 g / s to 100 g / s, and more preferably 30 g / s to 100 g / s. It can demonstrate purification performance. Further, the catalyst of the present invention has a condition in which the air-fuel ratio A / F is particularly rich, specifically, the A / F is preferably 13.5-14.6, more preferably 14.0 to 14.6. High purification performance can be demonstrated.

The catalyst of the present invention has improved HC purification ability in a wide temperature range, for example, 400 ° C. or higher, without conflicting with NOx purification ability. After the durability test, the catalyst of the present invention has a NOx purification rate of preferably 98.60% or more, more preferably 98.80% or more at 450 ° C. After the durability test, the catalyst of the present invention has a NOx purification rate of preferably 95.80% or more, more preferably 96.00% or more at 500 ° C. Further, the catalyst of the present invention has an HC purification rate at 450 ° C. of preferably 77.50% or more, more preferably 77.60% or more after the durability test. After the durability test, the catalyst of the present invention has an HC purification rate at 500 ° C. of preferably 80.60% or more, more preferably 81.00% or more. Here, the measurement of the NOx purification rate and the HC purification rate is, for example, as follows [II-3. Performance evaluation] can be used.

Here, the "durability test" means that the catalyst or the like to be tested has a temperature of about 800 to 1100 ° C. in an exhaust gas atmosphere generated by burning the air-fuel mixture or a gas atmosphere having a gas composition imitating the exhaust gas. It is a test performed by exposing for 1 to 70 hours. The "durability test" is usually performed to evaluate the durability of the exhaust gas purification catalyst. The "durability test" is, for example, the following [II-2. It can be performed by the method described in [Durability test].

Hereinafter, the present invention will be described in more detail with reference to examples. However, the technical scope of the present invention is not limited to these examples.

<I. Preparation of catalyst>
[I-1. material]
(1) The raw materials used as the carrier are as follows:
Material 1 (Al ₂ O ₃ )
La ₂ O ₃ complex Al ₂ O ₃ was used. La ₂ O ₃ : 1 wt% to 10 wt%.
Material 2 (ACZ)
An Al _{2O 3} -CeO ₂ -ZrO ₂ composite oxide was used. CeO ₂ : 15-30 wt%. Nd ₂ O ₃ , La ₂ O _{3, and} Y ₂ O ₃ are added in a small amount to improve heat resistance.
Material 3 (CZ)
_A composite oxide of CeO2 _- ZrO2 was used. CeO ₂ : 20-60 wt%. La ₂ O _{3 and} Y ₂ O ₃ are added in a small amount to improve heat resistance.
(2) The raw materials used as the base material are as follows:
A 700 cc (600 cell hexagon, wall thickness 2 mil (milli inch)) cordierite honeycomb substrate (base material length 84 mm) was used.

[I-2. Preparation of catalyst]
<Comparative Example 1>
Third layer: Pd / ACZ + Al ₂ O ₃
Second layer: Rh / Al ₂ O ₃ + Al ₂ O ₃ + CZ
First layer (upper layer): Rb / Al ₂ O ₃ + Al ₂ O ₃ + CZ + surface layer Pd

Third layer:
Using nitric acid Pd, Pd / ACZ (material 4) carrying Pd on material 2 was prepared. The impregnation method was used as the carrying method. Next, Pd / ACZ (material 4), Al ₂ O ₃ (material 1), sulfuric acid Ba, and Al ₂ O ₃ system binder were added to prepare a suspended [slurry 1]. Further, the prepared [slurry 1] was poured into the base material, and the unnecessary portion was blown off with a blower to coat the wall surface of the base material with the material so as to be 30 mm or more from the end portion (Fr) on the upstream side of the exhaust gas. At that time, the coating material had Pd of 0.6 g / L, Al ₂ O ₃ (material 1) of 30 g / L, ACZ (material 2) of 40 g / L, and sulfuric acid Ba of 5 g / L with respect to the substrate capacity. It becomes L. Finally, after removing the water for 2 hours in a dryer kept at 120 ° C., baking was added at 500 ° C. for 2 hours in an electric furnace.

Second layer:
Similarly, using Rh nitrate, Rh / Al ₂ O ₃ (material 5) carrying Rh on the material 1 was prepared. [Slurry 2] was prepared by adding Rh / Al ₂ O ₃ (material 5), Al ₂ O ₃ (material 1), CZ (material 3), and Al ₂ O ₃ binder. The prepared Rh slurry [slurry 2] is poured into the base material coated with the third layer, and the unnecessary portion is blown off with a blower so that the wall surface of the base material is 60 mm or more from the end (Rr) on the downstream side of the exhaust gas. The material was coated. At that time, the coating material has Rh of 0.13 g / L, Al ₂ O ₃ (material 1) of 100 g / L, and CZ (material 3) of 50 g / L with respect to the substrate capacity. Finally, after removing the water for 2 hours in a dryer kept at 120 ° C., baking was added at 500 ° C. for 2 hours in an electric furnace.

First layer (upper layer):
Similarly, using Rh nitrate, Rh / Al ₂ O ₃ (material 6) carrying Rh on the material 1 was prepared. [Slurry 3] was prepared by adding Rh / Al ₂ O ₃ (material 6), Al ₂ O ₃ (material 1), CZ (material 3), and Al ₂ O ₃ binder. The prepared Rh slurry [slurry 3] was poured into the base material coated with the second and third layers, and the unnecessary portion was blown off with a blower to coat the base material wall surface with the material so as to be 60 mm or more from Fr. At that time, the coating materials had Rh of 0.13 g / L, Al ₂ O ₃ (material 6) of 20 g / L, Al ₂ O ₃ (material 1) of 30 g / L, and CZ (material 1) with respect to the substrate capacity. Material 3) is 60 g / L. Finally, after removing the water for 2 hours in a dryer kept at 120 ° C., baking was added at 500 ° C. for 2 hours in an electric furnace.

Then, an aqueous solution of nitric acid Pd was absorbed from Fr of the base material, and the unnecessary portion was blown off with a blower to coat the first layer with Pd. At that time, the length of the coated Pd was adjusted to be 60 mm with respect to the length of the base material. Finally, after removing the water for 2 hours in a dryer kept at 120 ° C., baking was added at 500 ° C. for 2 hours in an electric furnace. At that time, the Pd arranged in the first layer is 0.4 g / L with respect to the capacity of the base material. By setting the pH of the absorbed Pd solution to 1 or more and 2 or less, about 85% by mass contained in the Pd outermost surface layer is from the surface to the upper layer of the Pd outermost surface layer, which is relatively far from the surface of the base material. The conditions were adjusted to be present in layers up to 50% thick. By controlling the pH of the Pd solution in this way, the Pd arranged in the upper layer is defined as the surface layer Pd.
The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<Comparative Example 2>
Third layer:
The third layer was formed in the same manner as in Comparative Example 1.
Second layer:
The second layer was formed in the same manner as in Comparative Example 1.
First layer (upper layer):
The wall surface of the base material was coated with the following [slurry 4] instead of [slurry 3] to form the first layer in the same manner as in Comparative Example 1 except that the aqueous solution of nitric acid Pd was not absorbed.
Rh / Al ₂ O ₃ (material 6) carrying Rh on the material 1 was prepared, and Pd / CZ (material 7) supporting Pd on the material 3 was prepared. [Slurry 4] was prepared by adding Rh / Al ₂ O ₃ (material 6), Pd / CZ (material 7), Al ₂ O ₃ (material 1), and Al ₂ O ₃ binder. The prepared Rh slurry [slurry 4] was poured into the base material coated with the second and third layers, and the unnecessary portion was blown off with a blower to coat the base material wall surface with the material so as to be 60 mm or more from Fr. At that time, the coating material had Rh of 0.13 g, Al ₂ O ₃ (material 6) of 20 g / L, Pd of 0.4 g / L, and Al ₂ O ₃ (material 1) with respect to the substrate capacity. 30 g / L and CZ (material 3) are 60 g / L. The Pd / CZ (material 7) is arranged in the upper layer with a uniform concentration distribution. This Pd is defined as a uniform Pd.
The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<Example 1>
Third layer:
The third layer was formed in the same manner as in Comparative Example 2.
Second layer:
The second layer was formed in the same manner as in Comparative Example 2.
First layer (upper layer):
The wall surface of the base material was coated with the following [slurry 5] instead of [slurry 4] to form the first layer in the same manner as in Comparative Example 2 except that the aqueous solution of nitric acid Pd was absorbed.
Rh / Al ₂ O ₃ (material 6) carrying Rh on the material 1 was prepared, and Pd / CZ (material 8) supporting Pd on the material 3 was prepared. [Slurry 5] was prepared by adding Rh / Al ₂ O ₃ (material 6), Pd / CZ (material 8), Al ₂ O ₃ (material 1), and Al ₂ O ₃ binder. The prepared Rh slurry [slurry 5] was poured into the base material coated with the second and third layers, and the unnecessary portion was blown off with a blower to coat the base material wall surface with the material so as to be 60 mm or more from Fr. At that time, the coating materials had Rh of 0.13 g / L, Al ₂ O ₃ (material 6) of 20 g / L, Pd of 0.04 g / L, and Al ₂ O ₃ (material 1) with respect to the substrate capacity. ) Is 30 g / L, and CZ (material 3) is 60 g / L. The Pd / CZ (material 8) is arranged in the upper layer with a uniform concentration distribution.
Then, an aqueous solution of nitric acid Pd was absorbed from Fr of the base material, and the unnecessary portion was blown off with a blower to coat the first layer with Pd. At that time, the length of the coated Pd was adjusted to be 60 mm with respect to the length of the base material. Finally, after removing the water for 2 hours in a dryer kept at 120 ° C., baking was added at 500 ° C. for 2 hours in an electric furnace. At that time, the Pd arranged in the first layer is 0.36 g / L with respect to the capacity of the base material. By setting the pH of the absorbed Pd solution to 1 or more and 2 or less, about 85% by mass contained in the Pd outermost surface layer is from the surface to the upper layer of the Pd outermost surface layer, which is relatively far from the surface of the base material. The conditions were adjusted to be present in layers up to 50% thick. In this way, the uniform Pd was arranged at 0.04 g / L and the surface layer Pd was arranged at 0.36 g / L with respect to the substrate capacity. The ratio of the surface layer Pd amount to the upper layer Pd amount is 0.9 (0.36 / (0.04 + 0.36) = 0.9).
The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<Example 2>
Third layer:
The third layer was formed in the same manner as in Comparative Example 2.
Second layer:
The second layer was formed in the same manner as in Comparative Example 2.
First layer (upper layer):
Example 1 except that the surface layer Pd was arranged at 0.3 g / L by absorbing water with an aqueous solution of nitric acid Pd using a slurry prepared so that the uniform Pd was 0.1 g / L with respect to the substrate capacity. Similarly, the first layer was formed. Therefore, the ratio of the surface layer Pd amount to the upper layer Pd amount is 0.75 (0.3 / (0.1 + 0.3) = 0.75). The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<Example 3>
Third layer:
The third layer was formed in the same manner as in Comparative Example 2.
Second layer:
The second layer was formed in the same manner as in Comparative Example 2.
First layer (upper layer):
Example 1 and Example 1 except that the surface layer Pd was arranged at 0.2 g / L by absorbing water with an aqueous solution of nitric acid Pd using a slurry prepared so that the uniform Pd was 0.2 g / L with respect to the substrate capacity. Similarly, the first layer was formed. Therefore, the ratio of the surface layer Pd amount to the upper layer Pd amount is 0.5 (0.2 / (0.2 + 0.2) = 0.5). The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<Comparative Example 3>
Third layer:
The third layer was formed in the same manner as in Comparative Example 2.
Second layer:
The second layer was formed in the same manner as in Comparative Example 2.
First layer (upper layer):
Example 1 except that the surface layer Pd was arranged at 0.16 g / L by absorbing water with an aqueous solution of nitric acid Pd using a slurry prepared so that the uniform Pd was 0.24 g / L with respect to the substrate capacity. Similarly, the first layer was formed. Therefore, the ratio of the surface layer Pd amount to the upper layer Pd amount is 0.4 (0.16 / (0.24 + 0.16) = 0.4). The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<Comparative Example 4>
Third layer:
The third layer was formed in the same manner as in Comparative Example 2.
Second layer:
The second layer was formed in the same manner as in Comparative Example 2.
First layer (upper layer):
Example 1 except that the surface layer Pd was arranged at 0.1 g / L by absorbing water with an aqueous solution of nitric acid Pd using a slurry prepared so that the uniform Pd was 0.3 g / L with respect to the substrate capacity. Similarly, the first layer was formed. Therefore, the ratio of the surface layer Pd amount to the upper layer Pd amount is 0.25. (0.3 / (0.3 + 0.1) = 0.25). The mass ratio (Pd / Rh) of Pd to Rh in the upper layer is 3.1.

<II. Catalyst evaluation method ＞
[II-1. Evaluation of the physical properties]
The physical properties were evaluated by cutting out each catalyst (after the durability test) to a predetermined size, embedding it in a resin, polishing it, depositing it in Au, and using FE-EPMA (manufactured by JXA-8530F JEOL).
Specifically, the ratio of the amount of Pd present in the surface layer is determined by observing the catalyst coat layer with FE-EPMA and performing line analysis of Pd in the thickness direction of the cross section of the catalyst coat layer to obtain the Pd outermost surface layer. It was calculated from a certain amount of Pd and the amount of Pd in the upper half of the upper layer obtained by integrating Pd elements in the range from the surface to the thickness of the upper layer up to 50%.
The mass ratio of Pd to Rh in the upper layer (Pd / Rh) was calculated by the ratio of Pd carried on the upper coating layer (total amount of uniform Pd and surface layer Pd) to Rh carried on the upper coating layer.

[II-2. An endurance test]
A durability test was carried out for each catalyst using an actual engine. Specifically, each catalyst is attached to the exhaust system of a V8 engine, and the exhaust gas in each atmosphere of stoichiometric and lean is discharged for a certain period of time (3: 1 ratio) at a catalyst floor temperature of 950 ° C. for 50 hours. It was done by repeating the flow.

[II-3. Performance evaluation]
The activity of each catalyst was evaluated using an L4 engine.
T50 evaluation: Exhaust gas having an air-fuel ratio (A / F) of 14.4 was supplied, and the temperature rise characteristic (~ 500 ° C.) under high Ga conditions (Ga = 35 g / s) was evaluated. The catalytic activity was evaluated by the purification rate when the entering gas temperature was 450 ° C. and 500 ° C.

<III. Catalyst evaluation results>
The above [II-1. Physical property evaluation] and [II-3. The evaluation results of each catalyst obtained by [Performance evaluation] are shown in Table 1.

FIG. 2 shows the relationship between the ratio of the surface layer Pd amount to the upper layer Pd amount and the HC purification rate at 450 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. From FIG. 2, the catalyst of Example 1-3 in which the ratio (B / (A + B)) of the surface layer Pd amount to the Pd amount of the upper layer is 0.5 to 0.9 is a comparison in which Pd is present only in the outermost surface layer of Pd. Compared with the catalyst of Example 1 (A = 0), the catalyst of Comparative Example 2 in which Pd is uniformly present in the upper layer (B = 0), and the catalysts of Comparative Examples 3 and 4 in which the ratio of the amount of surface layer Pd to the amount of Pd is small. It can be seen that the HC purification rate is improved.

FIG. 3 shows the relationship between the ratio of the surface layer Pd amount to the upper layer Pd amount and the HC purification rate at 500 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. From FIG. 3, it can be seen that the catalyst of Example 1-3 in which the ratio (B / (A + B)) of the surface layer Pd amount to the upper layer Pd amount is 0.5 to 0.9 has a sufficiently high HC purification rate.

FIG. 4 shows the relationship between the ratio of the surface layer Pd amount to the upper layer Pd amount and the NOx purification rate at 450 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. When the ratio (B / (A + B)) of the surface layer Pd amount to the Pd amount of the upper layer from FIG. 4 is 0.25 or more, the NOx purification ability is sufficiently high, and therefore, when combined with the result of FIG. 2, Example 1-3 It can be seen that the catalyst of No. 1 has a sufficiently high HC purification rate at 450 ° C., which is a shift reaction region, without conflicting with the NOx purification rate.

FIG. 5 shows the relationship between the ratio of the surface layer Pd amount to the upper layer Pd amount and the NOx purification rate at 500 ° C. for the catalysts of Examples 1-3 and Comparative Example 1-4 after the durability test. When the ratio (B / (A + B)) of the surface layer Pd amount to the Pd amount of the upper layer from FIG. 5 is 0.4 or more, the NOx purification ability is sufficiently high, and therefore, when combined with the result of FIG. 3, Example 1- It can be seen that the catalyst of No. 3 has a sufficiently high HC purification rate at 500 ° C., which is a steam reforming region, without conflicting with the NOx purification rate.

From the above, the catalyst of the present invention has improved function in the steam reforming region (around 500 ° C.) due to the surface layer Pd, and also in the shift reaction region (around 450 ° C.) due to the uniform Pd supported on the carrier. It was found that the HC activity of was enhanced.

The exhaust gas purification catalyst of the present invention can be particularly preferably applied to an automobile exhaust gas purification catalyst.

1 ... Catalyst 10 of the present invention ... Base material 11 ... Catalyst coat layer 12 ... Second layer 13 ... First layer (upper layer)
14 ... Pd outermost surface layer 15 ... third layer, which has a relatively higher Pd concentration than other parts of the upper layer

Claims (2)

A method for producing an exhaust gas purification catalyst comprising a base material and a catalyst coat layer formed on the surface of the base material.
The upper layer of the catalyst coat layer that comes into contact with the exhaust gas is subjected to the following process:
(A) A step of forming a coat layer using a slurry containing Rh and Pd and a carrier; and (b) The coat layer formed in step (a) is made to absorb an aqueous solution containing Pd to absorb water. Including forming by a method including a step of forming a Pd outermost surface layer having a relatively higher Pd concentration than other parts of the coat layer .
More than 65% by mass of Pd contained in the outermost surface layer of Pd is present in the layer from the surface of the outermost surface layer of Pd to the thickness of 50% of the upper layer, which is relatively far from the surface of the base material.
In the upper layer, the mass ratio of Pd to Rh (Pd / Rh) is 0.5 or more and 4.0 or less, and in the upper layer, the following formula (1):
0.45 ≤ B / (A + B) ≤ 0.95 ... Equation (1)
(In the formula, A represents the amount of Pd supported on the carrier in the step (a), and B represents the amount of Pd absorbed and supported on the coat layer in the step (b)).
Is satisfied, the above method. Claim 1 in step (b), wherein the coat layer formed in step (a) is made to absorb water with an aqueous solution containing a palladium salt or a palladium complex having a pH of 1 or more and 2 or less to form a Pd outermost surface layer. The method described in.

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