JP2012110859A - Exhaust gas purifying catalyst and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst based on the distribution of gas flow velocity in a cross section vertical to an exhaust gas flow, while improving exhaust gas purification effect.SOLUTION: The exhaust gas purifying catalyst is obtained by coating a substrate with a catalyst layer. A catalyst layer coated on the cross sectional center has an oxygen absorbing/releasing rate slower than a catalyst layer coated on the cross sectional outer periphery, in the cross section vertical to the exhaust gas flow. A method for manufacturing the catalyst for purifying exhaust gas is also provided.

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas from an internal combustion engine or the like and a method for producing the same.

Conventionally, a three-way catalyst that simultaneously performs oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxides (NO _x ) has been used as an exhaust gas purification catalyst for automobiles. As such a catalyst, a catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous oxide carrier such as alumina (Al ₂ O ₃ ) is widely known. Yes. CO by the action of the three-way catalyst, for purifying simultaneously and efficiently three components of HC and NO _x, the air / fuel ratio to be supplied to the vehicle engine (air-fuel ratio A / F) the stoichiometric air-fuel ratio ( It is important to control in the vicinity.

  However, since the actual air-fuel ratio fluctuates to the rich (excess fuel atmosphere) side or lean (fuel lean atmosphere) side with the stoichiometric centering on the driving conditions of the automobile, etc., the exhaust gas atmosphere is also on the rich side or lean side as well. fluctuate. Therefore, it is not always possible to ensure high purification performance with only a three-way catalyst. Therefore, in order to absorb the fluctuation of the oxygen concentration in the exhaust gas and enhance the exhaust gas purification ability of the three-way catalyst, oxygen is stored when the oxygen concentration in the exhaust gas is high, and oxygen is released when the oxygen concentration in the exhaust gas is low. In other words, a material having a so-called oxygen absorption / release capacity is used in an exhaust gas purifying catalyst.

  Such an OSC material (oxygen storage / release material) differs in oxygen storage / release speed and oxygen storage / release amount depending on its structure and composition. Therefore, ingenuity such as arranging OSC materials having different oxygen absorption / release rates and oxygen absorption / release amounts upstream / downstream of the exhaust system so that oxygen can be occluded / released according to atmospheric fluctuation (A / F fluctuation). Has been made.

For example, in Patent Document 1, different catalysts are arranged as an upstream catalyst portion and a downstream catalyst portion, and as an OSC material contained in the upstream catalyst portion, a composite oxide (A) having a ZrO ₂ / CeO ₂ mass ratio of 1 or more is used. / F is considered to have good responsiveness when oxygen is occluded / released in accordance with fluctuations. Thereby, it is stated that the A / F fluctuation is absorbed in the upstream catalyst part, and the purification of HC, CO and NOx is advantageously performed in the upstream catalyst part and the downstream catalyst part.

JP 2009-19537 A

  As described above, contrivances have been made such as arranging OSC materials having different oxygen absorption / release rates and oxygen absorption / release amounts upstream / downstream of the exhaust system. However, there is a need for an exhaust gas purification catalyst that has a higher exhaust gas purification effect.

  The inventor of the present invention pays attention to the fact that the prior art has been made with respect to the direction of exhaust gas flow, but has not been sufficiently studied from the distribution of gas flow velocity in a cross section perpendicular to the exhaust gas flow. An object of the present invention is to provide an exhaust gas purifying catalyst having a higher exhaust gas purifying effect in consideration of a gas flow velocity distribution in a cross section perpendicular to the exhaust gas flow.

  The following forms are provided by the present invention.

  (1) An exhaust gas purifying catalyst having a base material coated with a catalyst layer, wherein the catalyst layer coated at the center of the cross section in the cross section perpendicular to the exhaust gas flow Is a catalyst for exhaust gas purification characterized by a slow oxygen absorption / release rate.

  (2) The exhaust gas purifying catalyst according to (1), wherein the base material has a honeycomb structure having a large number of through holes substantially parallel to the exhaust gas flow direction and partitioned by partition walls.

  (1) or (1), wherein r is the radius or diagonal length of the cross section and r1 is the radius or diagonal length of the central portion of the cross section. The exhaust gas purifying catalyst according to 2).

The catalyst layer coated at the center of the cross section contains a ceria-zirconia composite oxide, and a pyrochlore phase type ordered arrangement phase is formed by cerium ions and zirconium ions in the composite oxide, and
(1) to (3), wherein the pyrochlore phase-type ordered arrangement phase remains in the atmosphere after heating for 5 hours at a temperature of 1100 ° C. in comparison with before heating. The exhaust gas-purifying catalyst according to any one of the above.

  The catalyst layer coated on the center portion takes 40 seconds or more to release 50 mass% of the amount of oxygen released in 2 minutes under a predetermined condition. The exhaust gas-purifying catalyst according to any one of the above.

Covering the end surface of the base material with a lid having an opening having the same shape as the central portion of the cross section, and coating the catalyst layer on the central portion of the cross section; and covering the end surface of the base material with the same shape as the central portion of the cross section. Covering and coating a catalyst layer on the outer periphery of the cross section,
A method for producing an exhaust gas purifying catalyst according to any one of (1) to (5), comprising:

FIG. 1 is a schematic view showing a typical form of the exhaust gas purifying catalyst of the present invention. FIG. 2 is a cross-sectional view showing a typical cross section (a direction perpendicular to the exhaust gas flow) of the exhaust gas purification catalyst of the present invention. FIG. 3 is a graph showing the NOx purification rates of the exhaust gas purification catalysts of Examples and Comparative Examples. FIG. 4 is a graph showing the oxygen absorption / release rate from the OSC material.

The principle of the present invention will be described.
When exhaust gas flows in the catalyst, generally, the exhaust gas flow rate is fast at the center of the catalyst cross section perpendicular to the exhaust gas flow, and the exhaust gas flow rate is slow at the outer periphery of the catalyst cross section. That is, the amount of gas per unit area is larger in the central part of the catalyst cross section where the gas flow rate is fast than in the outer peripheral part of the catalyst cross section where the gas flow rate is slow. Therefore, when the catalyst (cross-section center and cross-section outer periphery) has an oxygen absorbing / releasing ability uniformly, the catalyst cross-section center absorbs or releases oxygen in a shorter time than the catalyst cross-section outer periphery, that is, in a short time. Oxygen storage is saturated or depleted.
Therefore, by arranging a material having a low oxygen absorption / release rate at the center of the catalyst (see FIG. 1), the oxygen absorption / release time can be prolonged. In comparison, a material having a high oxygen absorption / release rate is disposed on the outer periphery of the catalyst. For this reason, even if the amount of gas flowing to the catalyst outer peripheral portion is small, the catalyst outer peripheral portion can absorb or release oxygen in a short time.
In this way, by disposing materials having different oxygen absorption / release rates in consideration of the gas flow rate distribution, the range of applicable A / F fluctuations can be expanded and emissions can be significantly reduced.
FIG. 1 shows a mode in which a material 2 having a low oxygen absorption / release rate is arranged at the center of the catalyst, and a material 1 having a high oxygen absorption / release rate is arranged at the outer periphery of the catalyst.

  The exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst obtained by coating a base material with a catalyst layer, and the catalyst layer coated at the center of the cross section in the cross section perpendicular to the exhaust gas flow The oxygen absorption / release rate is slower than that of the catalyst layer coated on the catalyst layer.

  The base material constitutes an exhaust gas purifying catalyst by coating a catalyst layer thereon. The shape of the substrate is not particularly limited, but it is preferable to use a flow-through carrier in the case of a three-way catalyst for automobiles. The shape of the substrate may have a honeycomb structure. The honeycomb structure is a structure having a large number of through holes that are partitioned by partition walls and are substantially parallel to the exhaust gas flow direction.

  Examples of the material of the substrate include metals and ceramics. In the case of metal, stainless steel is generally used, but the shape is generally honeycomb. Ceramic materials include cordierite, mullite, alumina, magnesia, spinel, silicon carbide, etc., but they are made of cordierite because they have good moldability for manufacturing honeycombs and are excellent in heat resistance and mechanical strength. It is preferable.

  The catalyst layer includes a three-way catalyst and an OSC material, and is coated on the surface of the base material so as to come into contact with the exhaust gas when the exhaust gas is distributed to constitute an exhaust gas purification catalyst.

The three-way catalyst simultaneously performs oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxide (NO _x ). As such a catalyst, a porous inorganic oxide carrying a catalytically active metal can be used. The porous inorganic oxide works as a carrier, keeps the catalytically active metal stable against heat and atmosphere, and can also increase the activity of the catalytically active metal.

  The porous inorganic oxide, that is, the carrier on which the catalytically active metal is supported is preferably a porous inorganic material having high heat resistance and a large specific surface area. Active alumina such as γ-alumina and θ-alumina, zirconia, cerium -Zirconium complex oxide, ceria, titanium oxide, silica, various zeolites, etc. can be used. As such a porous inorganic oxide, a material further improved in heat resistance by adding a rare earth such as lanthanum, cerium, barium, praseodymium, strontium or an alkaline earth metal may be used.

  The catalytically active metal is not limited as long as it has activity for purification of exhaust gas, but it is desirable to contain one or more catalytically active metals selected from platinum, palladium, and rhodium. In addition, transition metals, rare earth metals and the like can be contained.

  The OSC material absorbs fluctuations in the oxygen concentration in the exhaust gas and enhances the exhaust gas purification capacity of the three-way catalyst, so that oxygen is stored when the oxygen concentration in the exhaust gas is high, and oxygen is stored when the oxygen concentration in the exhaust gas is low. It is a material that has a so-called oxygen absorption / release capability.

  In the present invention, in the cross section perpendicular to the exhaust gas flow, the catalyst layer coated at the center of the cross section has a slower oxygen absorption / release rate than the catalyst layer coated at the outer periphery of the cross section. Therefore, the OSC material contained in the catalyst layer coated on the center of the cross section has a slower oxygen absorption / release rate than the OSC material contained in the catalyst layer coated on the outer periphery of the cross section.

As the OSC material having a high oxygen absorption / release rate, that is, the OSC material contained in the catalyst layer coated on the outer periphery of the cross section, a generally well-known conventional OSC material can be used. More specifically, as such an OSC material having a high oxygen storage / release rate, a material based on CeO ₂ powder having a high oxygen storage / release capability can be preferably used. So far, many studies have been conducted on the improvement of oxygen storage capacity and release characteristics in CeO ₂ -based powders such as CeO ₂ -ZrO ₂ and exhaust gas purification catalysts using the same as an auxiliary catalyst. It has been shown to increase efficiency. In the present invention, they can be used as an OSC material having a high oxygen absorption / release rate, that is, an OSC material contained in a catalyst layer coated on the outer periphery of the cross section. Further, when a material other than a cerium element material and a zirconium element material is used as an optional additional component in the CeO ₂ —ZrO ₂ -based OSC material, it is within a range that does not impair the characteristics of the OSC material obtained by the present invention. Alkali, alkaline earth, metal components, etc. can be added. More specifically, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, lanthanum, yttrium, antimony, hafnium, tantalum, rhenium, bismuth, praseodymium, neodymium, samarium, gadolinium, holmium, thulium, ytterbium, germanium, Examples include selenium, cadmium, indium, scandium, titanium, niobium, chromium, iron, silver, rhodium, and platinum. Further, such an optional additional component may be contained from impurities in the cerium element material and the zirconium element material. However, it is needless to say that when such an optional additional component is subject to harmful regulation, it is desirable to reduce or eliminate the amount.

  The cerium raw material, zirconium raw material, and optional additional components are mixed at a predetermined ratio and charged into a melting apparatus. Next, the raw material mixture is melted in the apparatus. The melting method is not particularly limited as long as at least one of the raw material mixtures is melted, and examples thereof include an arc type and a high frequency thermal plasma type. Among them, a general electromelting method, that is, a melting method using an arc electric furnace can be preferably used. After the end of melting, the electric furnace is covered with carbon and slowly cooled for 20 to 30 hours to obtain a solid solution. The method for cooling the melt is not particularly limited, but it is usually taken out from the melting apparatus and allowed to cool in the atmosphere to 100 ° C. or lower, preferably 50 ° C. or lower. As a result, a solid solution of a cerium-zirconium composite oxide in which the cerium raw material and the zirconium raw material are uniform can be obtained.

  The solid solution is then pulverized. The pulverization of the solid solution is not particularly limited, but the ingot can be roughly pulverized by a pulverizer such as a jaw crusher or a roll crusher. The coarse powder obtained by the above method can be further finely pulverized. Although it does not specifically limit about fine pulverization, It can pulverize for 5 to 30 minutes with grinders, such as a planetary mill, a ball mill, or a jet mill. It is preferable that the average particle size of the cerium-zirconium-based composite oxide be set to the nanometer order by this fine pulverization. By finely pulverizing, the surface area of the composite oxide is increased, and an OSC material having a high oxygen absorption / release rate can be obtained.

  As the OSC material having a low oxygen absorption / release rate, that is, the OSC material contained in the catalyst layer coated at the center of the cross section, a ceria-zirconia-based composite oxide described in JP-A-2009-84061 can be used. This ceria-zirconia composite oxide has a pyrochlore phase type ordered arrangement phase formed of cerium ions and zirconium ions in the oxide, and the pyrochlore phase type ordered arrangement phase is in the atmosphere. More than 50% remains after heating for 5 hours at a temperature of 1100 ° C. compared to before heating. According to Japanese Patent Application Laid-Open No. 2009-84061, this ceria-zirconia composite oxide has sufficiently high heat resistance, and can exhibit sufficiently excellent oxygen storage ability even after being exposed to high temperature for a long time. It is described as a ceria-zirconia composite oxide. The present inventor has found that this ceria-zirconia composite oxide has a low oxygen absorption / release rate, and this is included in the OSC material of the present invention having a low oxygen absorption / release rate, that is, the catalyst layer coated on the center of the cross section. It was found that it can be used as an OSC material.

  A method for producing a ceria-zirconia composite oxide, which is an OSC material having a low oxygen absorption / release rate, that is, an OSC material contained in a catalyst layer coated at the center of the cross section, will be described.

The method for producing the ceria-zirconia composite oxide is a method for producing a ceria-zirconia composite oxide in which a pyrochlore phase type ordered arrangement phase is formed by cerium ions and zirconium ions,
Ceria and zirconia composite oxide powder having a ceria and zirconia content ratio in a molar ratio ([ceria]: [zirconia]) in the range of 55:45 to 43:57 at a temperature condition of 1500 ° C. to 1900 ° C. Including a step of obtaining a composite oxide precursor by reduction treatment (first step) and a step of obtaining the ceria-zirconia composite oxide by oxidizing the composite oxide precursor (second step). It is a method.

  The composite oxide powder of ceria and zirconia has a molar ratio of ceria and zirconia ([ceria]: [zirconia]) in the range of 55:45 to 43:57. When the content ratio of ceria and zirconia is out of the above range, in the obtained ceria-zirconia composite oxide, free ceria or zirconia coexists as another phase other than the regular arrangement phase of the pyrochlore phase type. However, it does not contribute to the formation of the pyrochlore phase. That is, when the content ratio of ceria is less than the lower limit, the content ratio of zirconia becomes too high, and zirconia is liberated in the reduction treatment process, and the pyrochlore phase type ordered arrangement phase is sufficiently obtained by cerium ions and zirconium ions. And the oxygen storage capacity of the resulting ceria-zirconia composite oxide is reduced. On the other hand, when the content ratio of the ceria exceeds the upper limit, free ceria is mixed. Such free ceria contributes little to the oxygen storage capacity.

  Further, such ceria and zirconia composite oxide powder is not particularly limited as long as ceria and zirconia coexist, and even if it is a ceria-zirconia solid solution powder, ceria powder and zirconia powder It may be a mixture of When a mixture of ceria powder and zirconia powder is used as such a composite oxide powder of ceria and zirconia, ceria powder and zirconia powder are sufficient from the viewpoint of forming a pyrochlore phase type ordered arrangement phase more sufficiently. It is preferable to use a finely pulverized fine powder in which each fine powder is highly dispersed and mixed. In addition, when using such a mixture of ceria powder and zirconia powder, the average particle size of each powder may be about 100 to 1000 nm from the viewpoint of more sufficiently forming a pyrochlore phase type ordered arrangement phase. preferable.

  In addition, as such a ceria and zirconia composite oxide powder, from the viewpoint of sufficiently forming a pyrochlore phase type ordered arrangement phase, a solid solution in which ceria and zirconia are mixed at an atomic level is used. More preferred. Moreover, as such a solid solution powder of ceria and zirconia, the average particle diameter is preferably about 2 to 100 nm.

  Further, the method for producing such a composite oxide powder of ceria and zirconia is not particularly limited. For example, a so-called coprecipitation method is employed so that the content ratio of ceria and zirconia falls within the above content range. The composite oxide powder is manufactured by mixing the fine ceria powder and the zirconia powder so that the content ratio of ceria and zirconia is within the above range. And the like. As the coprecipitation method, for example, an aqueous solution containing a cerium salt (for example, nitrate) and a zirconium salt (for example, nitrate) is used to form a coprecipitate in the presence of ammonia. Examples of the method include a method of obtaining a composite oxide powder of ceria and zirconia by filtering and washing the precipitate, drying it, and further calcination, followed by pulverization using a pulverizer such as a ball mill. The aqueous solution containing the cerium salt and the zirconium salt is prepared so that the content ratio of ceria and zirconia in the obtained composite oxide powder is 55:45 to 43:57. Further, in such an aqueous solution, if necessary, a salt of at least one element selected from the group consisting of rare earth elements other than cerium and alkaline earth elements, and a surfactant (for example, a nonionic surfactant) ) Etc. may be added.

  Next, each step will be described. First, the composite oxide powder is reduced to obtain a composite oxide precursor (first step).

The temperature condition for such reduction treatment is 1500 ° C. or higher and 1900 ° C. or lower, more preferably 1600 ° C. or higher and 1850 ° C. or lower, still more preferably 1650 ° C. or higher and 1800 ° C. or lower, and more preferably 1700 ° C. or higher and 1800 ° C. or lower. The following is particularly preferable. When the temperature condition of such a reduction treatment is less than the lower limit, a pyrochlore phase type ordered arrangement phase is formed, but the average particle diameter of the crystallite cannot be made sufficiently large. Structural stability is lowered and heat resistance is lowered. That is, when the temperature condition is less than the lower limit, when the obtained ceria-zirconia composite oxide is heated in the atmosphere at a temperature condition of 1100 ° C. for 5 hours, the pyrochlore phase type ordered arrangement phase is not heated. In comparison, it becomes impossible to obtain a ceria-zirconia-based composite oxide that remains at 50% or more. Note that the temperature condition during such heat treatment tends to obtain a ceria-zirconia composite oxide having a more stabilized crystal structure as the temperature is higher within the above temperature range. Moreover, when the temperature condition of such a reduction process exceeds the upper limit, the reducing gas CO and zirconia (ZrO ₂ ) tend to react to decompose the pyrochlore phase.

  The reduction treatment method is not particularly limited as long as it is a method capable of heat-treating the composite oxide powder in a reducing atmosphere under a temperature condition of 1500 ° C. or higher and 1900 ° C. or lower. After the ceria and zirconia composite oxide powder is placed in a vacuum heating furnace and evacuated, a reducing gas is introduced into the furnace to make the atmosphere in the furnace a reducing atmosphere. A method of performing a reduction treatment by heating under temperature conditions, or using a graphite furnace, placing the composite oxide powder of ceria and zirconia in the furnace, evacuating, and temperature conditions of 1500 ° C. or higher and 1900 ° C. or lower And the like, and a reduction treatment is performed by setting the atmosphere in the furnace to a reducing atmosphere with a reducing gas such as CO or HC generated from the furnace body and heated fuel. Even when the above-described graphite furnace is used, the reducing gas may be flowed into the furnace and heated in a reducing atmosphere.

In addition, the reducing gas used for achieving such a reducing atmosphere is not particularly limited, and a reducing gas such as CO, HC, H ₂ , or other hydrocarbon gas can be appropriately used. Among such reducing gases, carbon (C) is included from the viewpoint of preventing the formation of double products such as zirconium carbide (ZrC) when reducing treatment is performed at a higher temperature. It is more preferable to use those not present. When such a reducing gas containing no carbon (C) is used, reduction treatment can be performed under higher temperature conditions close to the melting point of zirconium or the like, so that the structural stability of the crystal phase is more sufficiently improved. It becomes possible to improve.

  In addition, the heating time in the reduction treatment is not particularly limited, but is preferably about 0.5 to 5 hours. When the heating time is less than the lower limit, the crystallite diameter of the composite oxide powder tends to be insufficiently increased. On the other hand, when the upper limit is exceeded, grain growth proceeds sufficiently, and more Since this operation is wasted, the economy tends to decrease.

  Moreover, it is preferable to press the ceria and zirconia composite oxide powder at a pressure of 100 MPa or more (more preferably 200 to 400 MPa) before the reduction treatment. By using the composite oxide powder (press-molded body) after pressing in this way, the uniformity of the particle size of the composite oxide powder is improved, and the sintering proceeds more by heating during the reduction treatment. The surface energy tends to be released and the crystal structure tends to be more stabilized. Further, if the pressing pressure is less than the lower limit, the effect of pressing tends to be insufficient. In addition, it does not restrict | limit especially as a method of such a press, Well-known press methods, such as an isostatic press, can be employ | adopted suitably.

  Next, the second step will be described. That is, after the reduction treatment (first step), the obtained composite oxide precursor is subjected to an oxidation treatment to obtain a ceria-zirconia composite oxide (second step).

  A method for such oxidation treatment is not particularly limited, and a known method capable of oxidizing a metal element in a composite oxide precursor obtained by reduction treatment can be appropriately employed. A method of heat-treating the composite oxide precursor in an atmosphere (for example, air) can be suitably employed. In addition, the heating temperature condition during the oxidation treatment is not particularly limited, but is preferably about 300 to 800 ° C. Furthermore, the heating time for the oxidation treatment is not particularly limited, but is preferably 0.5 to 5 hours.

  In the ceria-zirconia composite oxide thus obtained, a pyrochlore phase type ordered arrangement phase is sufficiently formed by cerium ions and zirconium ions, and is further heated in the atmosphere at 1100 ° C. for 5 hours. Even in this case, the pyrochlore phase type ordered arrangement phase remains by 50% or more as compared to before heating. Therefore, the obtained ceria-zirconia composite oxide has sufficiently high heat resistance and can exhibit a sufficiently excellent oxygen storage ability even after being exposed to a high temperature for a long time.

  Whether the oxygen absorption / release rate is fast or slow can be evaluated by the following method. By flowing an oxidizing gas through the OSC material, the OSC material can sufficiently absorb oxygen. Next, by allowing the reducing gas to flow through the OSC material, oxygen is released from the OSC material and the reducing gas is oxidized. By measuring the amount of oxidized gas, the amount of oxygen released from the OSC material can be determined. The amount of oxygen released within a certain period of time is determined, and whether the oxygen absorption / release rate is fast or slow is evaluated based on the length of time taken to release half of the amount of oxygen.

  As a more specific evaluation method regarding whether the oxygen absorption / release rate is fast or slow, Example D.1. The method described in detail in the evaluation of the oxygen absorption / release rate can be used. When evaluated by this method, an OSC material that takes 40 seconds or more to release 50 mass% of the amount of oxygen released in 2 minutes may be an OSC material having a slow oxygen absorption / release rate.

  In the exhaust gas purification catalyst of the present invention, a material having a low oxygen absorption / release rate is used as the catalyst layer (OSC material) coated on the center of the catalyst (see FIG. 1). As the catalyst layer (OSC material) coated on the outer periphery of the catalyst, a material having a high oxygen absorption / release rate is used.

  FIG. 2 shows a cross-sectional view of the exhaust gas purifying catalyst (a cross section in a direction perpendicular to the exhaust gas flow). Although FIG. 2 shows a circular cross section, the cross section of the exhaust gas purifying catalyst may be rectangular. When the radius or diagonal length of the cross section of the exhaust gas purifying catalyst is r and the radius or diagonal length of the central portion of the cross section is r1, it is preferable that 0.10 ≦ r1 / r ≦ 0.9. Particularly preferably, 0.1 <r1 / r <0.9. When r1 / r is within this range, the NOx purification rate is clearly improved as compared with the case where only the OSC material having a low oxygen absorption / release rate is used (r1 / r = 1).

  The present invention also relates to a method for producing an exhaust gas purifying catalyst. The exhaust gas purifying catalyst of the present invention can be produced by coating a catalyst layer having a low oxygen absorption / release rate at the center of the cross section and coating a catalyst layer having a high oxygen absorption / release rate at the outer periphery of the cross section.

  A slurry may be used to coat the catalyst layer. That is, the exhaust gas purification catalyst of the present invention is obtained by mixing an OSC material, a catalytically active metal, and a heat-resistant inorganic oxide together with an aqueous solvent to form a slurry, coating the substrate, drying, and firing. Can do. Here, an OSC material having a low oxygen absorption / release rate is used for the slurry to be coated on the central portion of the cross section. An OSC material having a high oxygen absorption / release rate is used for the slurry coated on the outer peripheral portion of the cross section.

  When coating the central portion of the cross section, the end surface of the base material is covered with a lid having an opening having the same shape as the central portion of the cross section (for example, a donut-shaped lid), and an OSC material having a low oxygen absorption / release rate is included in the central portion Coat the slurry. When coating the outer peripheral portion of the cross section, the end surface of the substrate is covered with a lid having the same shape as the central portion of the cross section, and the outer peripheral portion of the cross section is coated with a slurry containing an OSC material having a high oxygen absorption / release rate. The material of the lid is not particularly limited, but a silicon rubber sheet may be used in consideration of processability and availability.

  By this method, the exhaust gas purifying catalyst of the present invention can be obtained in which the catalyst layer having a low oxygen absorption / release rate is coated at the center of the cross section and the catalyst layer having a high oxygen absorption / release rate is coated at the outer periphery of the cross section.

A. Preparation of catalyst Preparation of slurry for coating (slurry A) containing a material having a low oxygen absorption / release rate (1) Preparation of material (OSC material) having a low oxygen absorption / release rate The content molar ratio (CeO ₂ : ZrO ₂ ) of ceria and zirconia is 50 : 50 ceria zirconia solid solution powder was prepared. That is, first, 49.1 g of a 28 wt% cerium nitrate aqueous solution in terms of CeO ₂ , 54.7 g of an 18 wt% zirconium oxynitrate aqueous solution in terms of ZrO ₂ , and a nonionic surfactant (product name: manufactured by Lion Corporation) Rheocon) 1.2 g was dissolved in 90 cc of ion-exchanged water, and ammonia water having 25 mass% of NH ₃ was added in an amount of 1.2 times the anion to produce a coprecipitate. The product was filtered and washed. Next, after drying the obtained coprecipitate at 110 ° C., it was fired at 1000 ° C. in the air for 5 hours to obtain a solid solution of cerium and zirconium. Thereafter, the solid solution is pulverized using a pulverizer (trade name “Wonder Blender” manufactured by ASONE Co., Ltd.) so that the average particle diameter is 1000 nm, and the molar ratio of ceria and zirconia (CeO ₂ : ZrO ₂ ) is determined. A 50:50 ceria zirconia solid solution powder was obtained.
50 g of the obtained ceria zirconia solid solution powder was packed in a polyethylene bag (capacity 0.05 L), the inside was deaerated, and then the mouth of the bag was heated and sealed. Next, using a hydrostatic pressure press device (trade name “CK4-22-60” manufactured by Nikkiso Co., Ltd.), the bag is subjected to a hydrostatic pressure press (CIP) for 1 minute at a pressure of 300 MPa to form ceria. A solid raw material of zirconia solid solution powder was obtained.
Next, the solid raw material taken out from the pressed bag was packed into a graphite cylindrical container (internal volume: diameter 15 cm, height 20 cm) and covered with a graphite lid. Next, the cylindrical container was placed in a furnace (graphite furnace) in which the furnace was composed of a graphitic heat insulating material and a heating element. Thereafter, the inside of the furnace was evacuated to 0.01 Torr with a diffusion pump, and then argon gas was introduced to make a reducing atmosphere of 100% by volume of argon gas. Next, the temperature in the furnace was set to 1700 ° C., and the solid sample was heated for 5 hours to perform a reduction treatment to obtain a composite oxide precursor. Thereafter, the furnace was cooled until the temperature in the furnace reached 50 ° C., and the composite oxide precursor was taken out of the furnace. Then, the obtained composite oxide precursor was oxidized by heating in the atmosphere at 500 ° C. for 5 hours to obtain a ceria-zirconia composite oxide. The ceria-zirconia composite oxide thus obtained was used as a material having a low oxygen absorption / release rate (OSC material). The obtained ceria-zirconia composite oxide was pulverized in a mortar to obtain a powder having an average particle size of 5 μm.
(2) Preparation of Rh-supported powder A Zirconia (ZrO ₂ : 90 wt%, Y ₂ O ₃ : 10 wt%) manufactured by Daiichi Rare Element Chemical Co., Ltd. was impregnated and supported with a Rh nitrate solution. Next, this was dried at 120 ° C. overnight. Next, this was fired at 450 ° C. for 2 hours to obtain Rh-supported powder A.
(3) Pd-supported powder B of Preparation Sasol γ- alumina _{(Al 2 O 3: 96wt%} , La 2 O 3: 4wt%) was impregnated carrying Pd nitrate chemical liquid. Next, this was dried at 120 ° C. overnight. Next, this was fired at 450 ° C. for 2 hours to obtain Pd-supported powder B.
(4) Preparation of slurry for coating (slurry A) containing a material having a low oxygen absorption / release rate (Slurry A) Material having a low oxygen absorption / release rate obtained in (1) (OSC material), Rh-supported powder A obtained in (2), And Pd-supported powder B obtained in (3) was mixed at a mass ratio of 3: 4: 7. The mixed powder and alumina binder (manufactured by Nissan Chemical Industries, AS200) were mixed at a mass ratio of 19: 1, and water was added to obtain a slurry for coating. This was designated as slurry A.

2. Preparation of slurry for coating (slurry B) containing a material having a high oxygen absorption / release rate (1) Preparation of Rh-supported powder A Rh-supported powder A was obtained by the same procedure as (2).
(2) Preparation of Pd-supported powder C Ceria-zirconia (CeO ₂ : 50 wt%, ZrO ₂ : 40 wt%, Y ₂ O ₃ : 5 wt%, La ₂ O ₃ : 5 wt%) manufactured by Rhodia, impregnated with Pd nitrate chemical solution Supported. Next, this was dried at 120 ° C. overnight. Next, this was fired at 450 ° C. for 2 hours to obtain Pd-supported powder C.
(3) Preparation of γ-alumina γ-alumina (Al ₂ O ₃ : 96 wt%, La ₂ O ₃ : 4 wt%) manufactured by Sasol was prepared as γ-alumina.
(4) Preparation of coating slurry (slurry B) containing a material having a high oxygen absorption / release rate Rh-supported powder A obtained in (1), Pd-supported powder C obtained in (2), and (3) γ-alumina was mixed at a mass ratio of 4: 7: 2. The mixed powder and alumina binder (manufactured by Nissan Chemical Industries, AS200) were mixed at a mass ratio of 19: 1, and water was added to obtain a slurry for coating. This was designated as slurry B.

3. Production of Example Catalyst 1 (1) Preparation of Honeycomb Base Material As a honeycomb base material, Denso Corporation ceramic honeycomb (cell density 600 cell / inch ² (93.0 cell / cm ² ), wall thickness 3 mil (76.2 μm) , Φ103 × L105) was prepared.
(2) Coat of center portion (base section center portion) A silicon rubber sheet (thickness 3 mm) was processed into a donut shape having an inner circle diameter of 51.5 mm and an outer circle diameter of 120 mm. The donut-shaped silicon rubber sheet was attached to the upper end surface of the ceramic honeycomb so that the opening of the donut-shaped silicon rubber sheet matched the center of the honeycomb end surface.
A predetermined amount of slurry A was introduced from the opening of the doughnut-shaped silicon rubber sheet, and suction was performed from the lower end surface of the ceramic honeycomb. The honeycomb was fired at 120 ° C. for 2 minutes. Further, this honeycomb was fired at 350 ° C. for 30 minutes. In this way, the central part (the central part of the base material cross section) was coated with the slurry A containing the material having a low oxygen absorption / release rate.
At this time, the input amount of the slurry was adjusted so that the coating amount of the slurry A was 230 g (dry mass) / L.
(3) Coating of outer peripheral part (outer peripheral part of base material cross section) A silicon rubber sheet (thickness 3 mm) was processed into a circular shape having a diameter of 51.5 mm. The circular silicon rubber sheet was attached to the upper end surface of the ceramic honeycomb so that the circular silicon rubber sheet matched the center of the honeycomb end surface.
A predetermined amount of slurry B was introduced from the outer peripheral portion (portion not covered with the circular silicon rubber sheet) of the upper end surface of the honeycomb, and suction was performed from the lower end surface of the ceramic honeycomb. The honeycomb was fired at 120 ° C. for 2 minutes. Further, this honeycomb was fired at 350 ° C. for 30 minutes. In this way, the outer peripheral portion (base cross section outer peripheral portion) was coated with the slurry B containing the material having a high oxygen absorption / release rate.
At this time, the input amount of the slurry was adjusted so that the coating amount of the slurry B was 230 g (dry mass) / L.
The catalyst obtained by coating the material having a low oxygen absorption / release rate at the center and coating the material having a high oxygen absorption / release rate at the outer periphery was obtained as Example Catalyst 1.
At this time, in Example catalyst 1, the radius (r1) of the center of the cross section of the base material coated with the material having a slow oxygen absorption / release rate is 0.5 with respect to the radius (r) of the cross section of the whole base material. there were. That is, in Example 1, r1 / r = 0.5. Refer to FIG. 2 for r1 and r.

4). Example 2 Production of Catalyst 2 In Example 2, the dimensions of the donut-shaped silicon rubber sheet and the circular silicon rubber sheet were changed so that r1 / r = 0.1. Other procedures were the same as those in Example 1.

5. Production of Example Catalyst 3 In Example 3, the dimensions of the doughnut-shaped silicon rubber sheet and the circular silicon rubber sheet were changed so that r1 / r = 0.3. Other procedures were the same as those in Example 1.

6). Production of Example Catalyst 4 In Example 4, the dimensions of the donut-shaped silicon rubber sheet and the circular silicon rubber sheet were changed so that r1 / r = 0.8. Other procedures were the same as those in Example 1.

7). Production of Example Catalyst 5 In Example 5, the dimensions of the donut-shaped silicon rubber sheet and the circular silicon rubber sheet were changed so that r1 / r = 0.9. Other procedures were the same as those in Example 1.

8). Preparation of Comparative Example Catalyst 1 In Comparative Example 1, the dimensions of the donut-shaped silicon rubber sheet and the circular silicon rubber sheet were changed so that r1 / r = 0.05. Other procedures were the same as those in Example 1.

9. Preparation of Comparative Example Catalyst 2 In Comparative Example 2, the honeycomb was coated only with slurry B containing a material having a high oxygen absorption / release rate without being covered with a silicon rubber sheet so that r1 / r = 0. Other procedures were the same as those in Example 1.

10. Preparation of Comparative Example Catalyst 3 In Comparative Example 3, the honeycomb was coated only with the slurry A containing a material having a low oxygen absorption / release rate without being covered with a silicon rubber sheet so that r1 / r = 1. Other procedures were the same as those in Example 1.

B. Catalyst Durability Test A. When Example Catalysts 1 to 5 and Comparative Example Catalysts 1 to 3 prepared in Example 1 were used as automobile catalysts, it was tested whether these catalysts maintained catalytic activity even after the automobile had traveled a certain amount. Actually, an accelerated deterioration test (endurance test) was conducted by connecting the engine and these catalysts.

  Specifically, A.I. Each of Example Catalysts 1 to 5 and Comparative Example Catalysts 1 to 3 prepared in Step 1 was wound with a ceramic mat and then cannulated to an exhaust pipe. A thermocouple was inserted in the center of the honeycomb so that the temperature at the center of the honeycomb could be measured. Next, this exhaust pipe was connected to the engine.

  The engine speed / torque and the like were adjusted so that the temperature at the center of the honeycomb was 950 ± 20 ° C. At this time, it was set as the cycle test which A / F repeats 14 and 15 for every fixed time. Endurance time: 50 hours. The cycle conditions were 10 seconds for A / F14 and 10 seconds for A / F15.

C. Evaluation of catalyst Example catalysts 1 to 5 and Comparative example catalysts 1 to 3 which were subjected to durability tests were connected to the engine. The engine speed / torque and the like were adjusted so that the exhaust gas temperature was 500 ° C. at a position 10 mm ahead of the catalyst inlet.
A / F was held at 15 for 180 seconds, then A / F was switched to 14 and held for 120 seconds.
100 seconds after the A / F was switched to 14, the NOx concentration in front of and behind the catalyst was measured. The NOx concentration behind the catalyst / the NOx concentration in front of the catalyst was defined as the NOx purification rate. Table 1 and FIG. 3 show the NOx purification rates of Example Catalysts 1-5 and Comparative Example Catalysts 1-3.
From Table 1 and FIG. 3, in the catalyst cross section of the present invention, that is, in the cross section of the catalyst perpendicular to the exhaust gas flow, a material having a low oxygen absorption / release rate is arranged at the center of the catalyst cross section, and oxygen is absorbed and released at the outer periphery of the catalyst cross section. It has been found that a catalyst with a fast material has a high NOx purification rate.

D. Evaluation of oxygen absorption / release rate The oxygen absorption / release rate of a plurality of OSC materials was evaluated using the following method.
1. Preparation of OSC Material In order to evaluate the oxygen absorption / release rate of the OSC material, the following OSC materials were prepared.
(1) OSC material 1: Composite oxide powder of CeO ₂ : 50 wt%, ZrO ₂ : 40 wt%, Y ₂ O ₃ : 5 wt%, La ₂ O ₃ : 5% manufactured by Rhodia.
(2) OSC material 2: CeO ₂ : 40 wt%, ZrO ₂ : 50 wt%, Y ₂ O ₃ : 5 wt%, La ₂ O ₃ : 5% composite oxide powder.
(3) OSC material 3: CeO ₂ : 30 wt%, ZrO ₂ : 60 wt%, Y ₂ O ₃ : 5 wt%, La ₂ O ₃ : 5% composite oxide powder.
(4) OSC material 4: A material having a low oxygen absorption / release rate included in the catalysts of Examples 1 to 5 described above. That is, a composite oxide powder of CeO ₂ : 50 at% and ZrO ₂ : 50 at%. The OSC material 4 was obtained by the preparation method of A.1 (1).
2. Preparation of Pd-supported catalyst Pd was supported by using the OSC materials 1 to 4 as a carrier to obtain a Pd-supported catalyst.
Specifically, a Pd nitrate chemical solution was diluted with water, and a predetermined amount of the OSC materials 1 to 4 was added thereto. This was dried at 120 ° C. overnight. Subsequently, this was baked at 500 degreeC for 2 hours, and the powdery catalyst was obtained. The obtained powdered catalyst was prepared so that the supported amount of Pd was 0.5 wt% (based on the catalyst mass).
The obtained powdery catalyst was molded by performing a hydrostatic press (CIP) for 1 minute at a pressure of 300 MPa using a hydrostatic pressure press (trade name “CK4-22-60” manufactured by Nikkiso Co., Ltd.). Further, the molded catalyst was pulverized to about 0.5 mm to obtain a pellet-shaped catalyst.
3. 1. Evaluation method of oxygen absorption / release rate A flow-type gas reaction tube was filled with 3 g of the pellet-shaped catalyst obtained in (1). By heating in an electric furnace, the temperature of the filled catalyst was adjusted to 500 ° C. The filled catalyst was oxidized by flowing 2% O ₂ /98% N ₂ gas through this gas reaction tube for 2 minutes at 15 L / min.
Thereafter, 0.2% CO / 99.8% N ₂ gas was allowed to flow through the gas reaction tube at 15 L / min for 2 minutes. It is considered that the reaction of the formula (1) occurs in the catalyst.
2CeO ₂ + CO → Ce ₂ O ₃ + CO ₂ (1)
By analyzing the gas after the catalyst flow with a CO ₂ analyzer (ND-IR method), CO ₂ generated during the flow of the catalyst was measured. The sampling interval for this measurement was 0.5 seconds.
In the formula (1), the amount of oxygen released from CeO ₂ and the amount of generated CO ₂ are in a proportional relationship. Therefore, the amount of CO ₂ generated was considered as the amount of oxygen released from the catalyst (OSC material).
Based on this idea, it was evaluated whether the oxygen release rate from the OSC material was fast or slow. Specifically, the amount of CO ₂ released in 2 minutes was taken as 1, and the time taken to release 0.5 CO ₂ was measured. The longer it took to release 0.5 CO _{2, the} slower the oxygen release rate.

4). Evaluation results of oxygen absorption / release rate 3. Table 2 and FIG. 4 show the results of the oxygen absorption / release rate obtained by the above evaluation method. The time taken to release 0.5 CO ₂ was evaluated as slow for 40 seconds or more.

Claims (6)

  A catalyst for purifying exhaust gas, which is obtained by coating a base material with a catalyst layer, and in a cross section perpendicular to the exhaust gas flow, the catalyst layer coated at the center of the cross section absorbs more oxygen than the catalyst layer coated at the outer periphery of the cross section. A catalyst for exhaust gas purification characterized by a slow release rate.   2. The exhaust gas purifying catalyst according to claim 1, wherein the base material has a honeycomb structure having a large number of through-holes partitioned by partition walls and substantially parallel to the exhaust gas flow direction.   The radius or diagonal length of the cross section is r, and the radius or diagonal length of the central portion of the cross section is r1, 0.10 ≦ r1 / r ≦ 0.9. The catalyst for exhaust gas purification as described in 1. The catalyst layer coated at the center of the cross section contains a ceria-zirconia composite oxide, and a pyrochlore phase type ordered arrangement phase is formed by cerium ions and zirconium ions in the composite oxide, and
4. The pyrochlore phase type ordered arrangement phase remains in the atmosphere after heating for 5 hours under a temperature condition of 1100 ° C. over 50% compared to before heating. The exhaust gas purifying catalyst according to claim 1.   The catalyst layer coated on the center portion takes 40 seconds or more to release 50% by mass of the amount of oxygen released in 2 minutes under a predetermined condition. The exhaust gas-purifying catalyst according to Item. Covering the end surface of the base material with a lid having an opening having the same shape as the central portion of the cross section, and coating the catalyst layer on the central portion of the cross section; and covering the end surface of the base material with the same shape as the central portion of the cross section. Covering and coating a catalyst layer on the outer periphery of the cross section,
A process for producing an exhaust gas purifying catalyst according to any one of claims 1 to 5, comprising:

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