Patent Publication Number: US-2022234030-A1

Title: Exhaust gas purification catalyst and exhaust gas purification system using the exhaust gas purification catalyst

Description:
FIELD OF THE INVENTION 
     The present invention relates to an exhaust gas purification catalyst provided in an exhaust system of an internal combustion engine, and an exhaust gas purification system using the exhaust gas purification catalyst. 
     BACKGROUND ART 
     Harmful components such as HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxide) are contained in emission gas (also referred to as exhaust gas) emitted from internal combustion engines of automobiles, motorcycles, and the like. Conventionally, three-way catalysts have been used to purify and detoxify these deleterious components. As the three-way catalyst, those using noble metals such as Pt (platinum), Pd (palladium), and Rh (rhodium) are known. Pt and Pd mainly act for oxidative purification of HC and CO, while Rh mainly acts for reduction purification of NOx. In recent years, attempts have been made to improve the exhaust gas purification performance of the above-mentioned noble metal catalysts due to tightening of exhaust gas regulations and enhancement of environmental consciousness. 
     For example, Patent Document 1 proposes an exhaust gas purification catalyst which can exhibit high exhaust gas purification performance even when an internal combustion engine is warmed up by sectioning the upstream side and the downstream side in the exhaust gas flow direction by a catalyst layer having a different catalyst composition, arranging Pd having excellent low-temperature catalytic activity in the upstream side section where temperature rises quickly, and reducing the heat capacity by reducing the amount of the catalyst supported (i.e., the amount of Pd catalyst washcoat on the catalyst layer). 
     Further, Patent Document 2 proposes a exhaust gas purifying catalyst which can exhibit high exhaust gas purification performance even during warm-up by adding thickness and lightness to Pd concentration in the cross-sectional thickness direction of the catalyst layer on the upstream side in the exhaust gas flow direction, and making the Pd concentration higher on the surface side of the catalyst layer where the exhaust gas first comes into contact. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP 2010-5592 A 
         Patent Document 2: WO2016/039302 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Incidentally, it is known that catalytic activity of a Pd catalyst used for oxidizing and purifying HC and CO is lost due to phosphorus derived from engine oil or the like (phosphorus poisoning). Therefore, even in the exhaust gas purification catalysts proposed in Patent Documents 1 and 2, it is considered that the Pd catalyst layer disposed on the upstream side of the catalyst layer is also poisoned by phosphorus. It is considered that an exhaust gas purification catalyst can have excellent exhaust gas purification performance during warm-up and high-speed operation of an engine without lowering the Pd catalytic activity by forming a catalyst layer on the upstream side into multiple layers and providing another catalyst layer containing Rh or the like on the surface of the Pd catalyst layer in order to suppress the phosphorus poisoning of the Pd catalyst layer. 
     However, if the Rh catalyst layer is provided on the surface of the Pd catalyst layer on the upstream side in the exhaust gas purification catalyst as proposed in Patent Document 1, not only the characteristics of Pd cannot be utilized because the exhaust gas comes into contact with the Pd catalyst layer after passing through the Rh catalyst layer, but also the amount of wash coat increases by making the catalyst layer on the upstream side multi-layered, and thus heat capacity increases and temperature rise performance is impaired. 
     It is considered the significance of increasing the Pd concentration on the surface of the Pd catalyst layer is impaired also in the exhaust gas purification catalyst of Patent Document 2, because if the surface of the Pd catalyst layer having a high Pd concentration is covered with the Rh catalyst layer, the characteristics of the Pd catalyst that it is excellent in low-temperature activity cannot be fully utilized, in addition to the fact that the exhaust gas is inhibited from being brought into contact by the Rh catalyst layer provided on the surface of the Pd catalyst layer in the same manner as described above. 
     Accordingly, it is an object of the present invention to provide an exhaust gas purification catalyst capable of exhibiting even higher exhaust gas purification performance without impairing Pd catalytic activity, and an exhaust gas purification system using the exhaust gas purification catalyst. 
     Means for Solving the Problem 
     The present inventors have found that, when an Rh catalyst layer is provided on the surface of a Pd catalyst layer in the upstream-side catalyst layer, an exhaust gas purification catalyst having excellent exhaust gas purification performance during warm-up and high-speed operation of an engine can be provided while suppressing the decrease in catalytic activity due to phosphorus poisoning of the Pd catalyst layer if the amount of wash coat when forming the upstream-side catalyst layer is set to a specific amount and a certain number of pores are provided in the formed catalyst layer. The present invention is based on such finding. 
     That is, the gist of the present invention is as follows. 
     [1] An exhaust gas purification catalyst comprising a substrate and a catalyst layer provided on the substrate, 
     wherein the catalyst layer has a first section located upstream along a flow direction of the exhaust gas and a second section located on the downstream side than the first section; wherein 
     the catalyst layer in the first section comprises a first catalyst layer comprising palladium and a second catalyst layer comprising rhodium and covering the first catalyst layer; 
     a pore volume proportion, which is a proportion of a total volume of the pores existing in the substrate and the catalyst layer in the first section to the volume of the entire first section is 12% or more and less than 18%, wherein the pore diametre as measured by mercury press-in method is 0.06 μm to 30.0 μm; 
     and a wash coat amount, which is the mass per unit volume of the catalyst layer in the first section to the volume of the substrate existing in the first section, is 100 g/L to 190 g/L. 
     [2] The exhaust gas purification catalyst according to [1], wherein when the first catalyst layer is divided into two equal portions along the thickness direction, and the mass of palladium existing on a surface side of the first catalyst layer is defined a1 and the mass of palladium existing on an opposite side to the surface side of the first catalyst layer is defined as a2, a value of a1/a2 is 1.2 or more.
 
[3] The exhaust gas purification catalyst according to [ 1 ] or
 
[2], wherein the pore volume proportion, which is a proportion of a total volume of the pores present in the substrate and the catalyst layer in the second section to a volume of the entire second section is 18% to 25%, wherein a pore diametre measured by the mercury press-in method is 0.06 μm to 30.0 μm.
 
[4] The exhaust gas purification catalyst according to any one of [1] to [3], wherein the catalyst layer in the second section comprises a third catalyst layer comprising palladium and a fourth catalyst layer comprising rhodium and covering the third catalyst layer.
 
[5] The exhaust gas purification catalyst according to any one of [1] to [4], wherein the pore volume proportion of the substrate and the catalyst layer in the first section is not more than 90% of the pore volume proportion of the substrate and the catalyst layer in the second section.
 
[6] The exhaust gas purification catalyst according to any one of [1] to [5], wherein the wash coat amount which is the mass per unit volume of the catalyst layer in the second section relative to the volume of the substrate existing in the second section is 180 g/L to 300 g/L.
 
[7] The exhaust gas purification catalyst according to any one of [1] to [6], wherein the second catalyst layer has a through-hole having a width of 10 μm or more through which gas passes from a surface thereof to the first catalyst layer.
 
[8] The exhaust gas purification catalyst according to any one of [1] to [7], wherein a length of the flow direction of the exhaust gas of the first section is 20% to 70% with respect to a length of the flow direction of the exhaust gas of the substrate.
 
[9] An exhaust gas purification system in which, in an exhaust path connected to an internal combustion engine, exhaust gas from the internal combustion engine is introduced into the exhaust gas purification catalyst according to any one of [1] to [8] without passing through any other exhaust gas purification catalyst.
 
     Effect of the Invention 
     In an exhaust gas purification catalyst in which a catalyst layer which is divided into a first section of an upstream side and a second section of a downstream side, it is possible to provide an exhaust gas purification catalyst having excellent exhaust gas purification performance during warm-up and high-speed operation of an engine while suppressing catalytic activity of a Pd catalyst layer due to phosphorus poisoning from decreasing according to the present invention, when a catalyst layer in the first section comprises a first catalyst layer comprising palladium and a second catalyst layer comprising rhodium and covering the first catalyst layer, and the first section is made in to a catalyst layer having a wash coat amount of 100 g/L or more and 190 g/L or less and a pore volume proportion of 12% or more and less than 18%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exhaust gas purification catalyst according to one embodiment of the present invention, which is stored in a casing through which exhaust gas flows. 
         FIG. 2  is a partially enlarged schematic diagram of a cross-section taken along line I-I′ related to the exhaust gas purification catalyst of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exhaust gas purification catalyst according to one embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a schematic view showing a state in which the exhaust gas purification catalyst is stored in a casing through which exhaust gas flows, and  FIG. 2  is a partially enlarged schematic view of a cross section taken along line I-I′ of the exhaust gas purification catalyst shown in  FIG. 1 . As shown in  FIG. 1 , the exhaust gas purification catalyst  10  is disposed in the casing  20  and is installed in a flow path of the exhaust gas. 
     Exhaust gas purification catalyst  10  comprises substrate  11  and catalyst layer  12  formed on the substrate  11 . The exhaust gas purification catalyst  10  has first section V 1  located on the upstream side and second section V 2  located on the downstream side from the first section, along the flow direction X of exhaust gas. The catalyst layer in the first section V 1  located upstream alone the flow direction of exhaust gas has a multilayer structure in which first catalyst layer  13  and second catalyst layer  14  are laminated in this order from the side of the substrate  111 , and the first catalyst layer  13  is covered by the second catalyst layer  14 . In the embodiment shown in  FIG. 2 , the catalyst layer in the first section V 1  has a laminated structure of two layers, but another catalyst layer (not illustrated) may be further provided on the surface of the second catalyst layer  14 . In addition, another catalyst layer (not illustrated) may be provided between the first catalyst layer  13  and the second catalyst layer  14 . Further, another catalyst layer (not illustrated) may be provided between the substrate  111  and the first catalyst layer  13 . 
     The catalyst layer in the second section V 2  has a multilayer structure in which third catalyst layer  15  and fourth catalyst layer  16  are laminated in this order from the side of substrate  112 , and the third catalyst layer  15  is covered with the fourth catalyst layer  16 . In the embodiment shown in  FIG. 2 , the catalyst layer in the second section V 2  has a laminated structure of two layers, but the catalyst layer in the second section V 2  may have a single-layer structure, in which case the fourth catalyst layer  16  may be omitted. Further, another catalyst layer (not illustrated) may be provided on the surface of the third catalyst layer  15  or the fourth catalyst layer  16 , and another catalyst layer (not illustrated) may be provided between the third catalyst layer  15  and the fourth catalyst layer  16 . Further, another catalyst layer (not illustrated) may be provided between the substrate  112  and the third catalyst layer  15 . 
     It is preferable that the first section V 1  and the second section V 2  are in contact with each other without a gap in the exhaust gas flow direction X, in view that the temperature of the second section V 2  can be raised efficiently by heat generated by exothermic reactions such as HC oxidation and CO oxidation promoted by the catalytic active component in the first section V 1 . 
     &lt;Substrate&gt; 
     As the substrate  11 , it is possible to use a substrate used for a known exhaust gas purification catalyst, an example thereof including a substrate having a partition wall made of a porous material and an exhaust gas flow path (space between the partition walls) sectioned by the partition wall. As the shape of the substrate  11 , it is possible to suitably use a known substrate such as a honeycomb, DPF or GPF. Examples of the material of the partition wall of the substrate  11  include ceramics such as alumina (Al 2 O 3 ), mullite (3Al 2 O 3 -2SiO 2 ), cordierite (2MgO-2Al 2 O 3 -5SiO 2 ), aluminum titanate (Al 2 TiO 5 ) and silicon carbide (SiC), and metal materials such as stainless steel. 
     An exhaust gas purification catalyst  10  having a substrate  11  and a catalyst layer  12  formed on the substrate  11  (i.e., a catalyst layer formed on the surface of a partition wall of the substrate) is also referred to as a catalytic converter. In the example shown in  FIG. 1 , the substrate  11  has a shape long in one direction, and is arranged so that the exhaust gas flow direction X and the longitudinal direction coincide with each other. In  FIG. 1 , R denotes a boundary portion dividing the first section V 1  and the second section V 2 . In  FIG. 2 , the substrate existing in the first section V 1  is defined as the substrate  111  and the substrate existing in the second section V 2  is defined as the substrate  112  in the substrate  11 , but the substrate  11  is divided into the substrate  111  and the substrate  112  for convenience in describing the invention, and the substrate  111  and the substrate  112  are substantially made of the same material. 
     &lt;Catalyst Layer of First Section&gt; 
     The first catalyst layer  13  constituting the catalyst layer in the first section V 1  comprises palladium (Pd) as a catalytically active component, but may also contain other catalytically active components such as platinum (Pt) and rhodium (Rh). In particular, it is preferable to contain Pd and Pt as the catalytically active components from the viewpoint of high purification performance of carbon monoxide (CO) and hydrocarbon (HG) and obtaining a warming function. 
     It is preferable for the first catalyst layer  13  to further contain a carrier component carrying the above-described catalytic active component in view of efficiently demonstrating the exhaust gas purification performance by the catalytic active component. The carrier component as used herein is an oxygen storage component (hereinafter also referred to as an OSC material) or an inorganic oxide other than the oxygen storage component. When an exhaust gas purification catalyst is used for a gasoline engine of theoretical air-fuel ratio control, the first catalyst layer  13  preferably contains an oxygen storage component as a carrier component in order to stably exhibit a high purification rate with respect to variation in the air-fuel ratio. When an oxygen storage component is contained, it is preferable that the oxygen storage component and the inorganic oxide other than the oxygen storage component exist in a mixed state. 
     Supporting the catalytically active component means a state in which it is physically or chemically adsorbed or supported on the outer surface or the inner surface of the pore. Specifically, the fact that the carrier component is supporting the catalytically active component can be determined as the support component is “supporting” the catalytically active component by confirming that the support component and the catalytically active component are present in the same region in an element mapping which was obtained by EDS (energy dispersive spectrometer) by analyzing the cross section of the exhaust gas purification catalyst  10  for example. 
     Any metal oxide can be used as the oxygen storage component without particular limitation, in which the valence of the constituent element changes under operating conditions of the exhaust gas purification catalyst and which has the ability to store oxygen, and for example, CeO 2  or CeO 2 —ZrO 2  (ceria-zirconia composite oxide containing Ce and Zr, or a solid solution of CeO 2  and ZrO 2 ) are preferable because the OSC of the exhaust gas purification catalyst is high. The solid solution of CeO 2  and ZrO 2  can be confirmed by using an X-ray diffraction apparatus (XRD) and determining whether or not a single phase derived from CeO 2 —ZrO 2  is formed. A porous material is preferred as the oxygen storage component because it is easy to support the catalytic active component. Examples of the porous material include those having a BET specific surface area of 30 m 2 /g to 210 m 2 /g. 
     The oxygen storage component may contain rare earth elements other than cerium (Ce) or alkaline earth metal elements such as barium (Ba), strontium (Sr), or calcium (Ca). Examples of the rare earth elements other than cerium include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). These rare earth elements are added to the oxygen storage component as oxides, for example. The oxides of the rare earth elements are sesquioxides (Re2O3, Re is a rare earth element) except for praseodymium (Pr) and terbium (Tb). These may be a complex oxide consisted of two or more kinds. Praseodymium oxide is usually Pr 6 O 11  and terbium oxide is usually Tb 4 O 7 . These oxides of rare earth elements other than cerium may or may not form a solid solution with CeO 2 —ZrO 2 . Whether or not the oxides of the rare earth elements other than cerium form a solid solution with CeO 2 —ZrO 2  can be confirmed by X-ray diffraction (XRD) in the same manner as described above. 
     In addition to the above, examples of the oxygen storage component include oxides of elements such as manganese (Mn), iron (Fe), and copper (Cu), which are liable to cause a valence state change under the use conditions of a catalyst, and complex oxides containing these elements. 
     Examples of the inorganic oxides other than the oxygen storage component that is a carrier component for supporting the catalytic active component include metal oxides other than the oxygen storage component mentioned above, for example, rare earth oxides (Re 2 O 3 ) such as Al 2 O 3 , ZrO 2 , SiO 2 , TiO 2  and La 2 O 3 , oxide materials based on zeolite (aluminosilicate), MgO, ZnO and SnO 2 , and oxide materials obtained by compounding these materials with each other. Examples include phosphates and borates such as aluminum (Al), zirconium (Zr), silicon (Si), titanium (Ti), rare earth elements, magnesium (Mg), and zinc (Zn), and poorly soluble sulfates with alkaline earth metals such as Ba and Sr. The inorganic oxide other than the oxygen storage component is preferably a porous material. Examples of the porous material include those having a BET specific surface area of 30 m 2 /g to 600 m 2 /g. 
     The above-mentioned inorganic oxides include those which modify or support the oxygen storage component. For example, the inner surface or the outer surface of pores such as alumina may be modified by CeO 2 . Modification as used herein is a concept encompassing a supported state, for example, a state in which fine oxygen-storing components are dispersed on the inner surface and the outer surface of pores such as alumina. Alumina may be modified by lanthanum oxide, ZrO 2 , or the like. 
     Further, the first catalyst layer  13  may contain an alkaline earth metal compound from the viewpoint of suppressing the catalytic activity from decreasing due to phosphorus poisoning and heat resistance. Preferred examples of the alkaline earth metal include Sr and Ba. As the alkaline earth metal compound, an oxide may be used in addition to nitrate and carbonate. 
     In order to form the first catalyst layer  13  on the substrate  11  (i.e., a partition wall of the substrate), a slurry is prepared by kneading the above-mentioned salts of the catalytic active component (e.g., palladium nitrate, rhodium nitrate and platinum nitrate), a carrier component and a liquid medium such as water by a ball mill or the like, and the slurry is applied to the surface of the substrate by a wash coating method, followed by drying and burning. By adjusting the amount of wash coat, the first catalyst layer  13  having a desired thickness can be formed. The amount of wash coat for the slurry will be described later. 
     From the viewpoint of the catalytic activity of an exhaust gas purification catalyst, the temperature at which the substrate coated with the slurry is burned is preferably 400° C. to 800° C., and more preferably 450° C. to 600° C. The burning time is preferably 0.5 hours to 6 hours, and more preferably 1 hour to 4 hours. Further, it is preferable that the slurry applied to the substrate is dried before burning, and the temperature thereof is preferably 40° C. to 200° C., and more preferably 70° C. to 150° C. The drying time is preferably 5 minutes to 6 hours, and more preferably 10 minutes to 2 hours. 
     The second catalyst layer  14  covering the first catalyst layer  13  contains rhodium (Rh) as a catalytically active component. As described above, when the first catalyst layer  13  containing Pd as a catalytically active component is exposed, the exhaust gas first comes into contact with the first catalyst layer  13 , therefore, when phosphorus is contained in the exhaust gas, the catalytic activity of Pd is reduced. In the present invention, the order to reduce phosphorus poisoning of Pd, the first catalyst layer ( 13 ) is covered with the second catalyst layer ( 14 ). 
     The second catalyst layer  14  may contain a catalytic active component other than Rh as the catalytic active component, for example, Pt or Pd. 
     It is preferable that the second catalyst layer  14  further contains a carrier component that supports a catalytically active component, in view of efficiently demonstrating the exhaust gas purification performance by the catalytically active component. As the carrier component referred herein, it is possible to use an oxygen storage component or an inorganic oxide similar to those used for the first catalyst layer  13 . 
     In order to form the second catalyst layer  14  on the first catalyst layer  13 , a slurry is prepared by kneading a liquid medium such as the above-mentioned salts of the catalytic active component (for example, palladium nitrate, rhodium nitrate and platinum nitrate), a carrier component and a liquid medium such as water by a ball mill or the like in the same manner as the method for forming the first catalyst layer  13 , and the slurry is applied to the surface of the first catalyst layer  13  by a wash coating method, followed by drying and burning. The second catalyst layer  14  having a desired thickness can be formed by adjusting the amount of wash coat. The amount of wash coat for the slurry will be described later. The burning temperature and the drying temperature can be set to the same conditions as those for forming the first catalyst layer. 
     The mass per unit volume of the catalyst layer (the first catalyst layer  13  and the second catalyst layer  14 ) in the first section V 1  relative to the volume of the substrate present in the first section V 1  (hereinafter referred to as the “wash coat amount”) is 100 g/L to 190 g/L. The wash coat amount as used herein means the amount of slurry that adheres per 1 L of substrate in the substrate  111  that is present in the first section V 1  (mass of the solid). Note that, the mass of solid means the mass calculated by subtracting the mass of a component that is to disappear by drying or burning the slurry (e.g. solvent, pore-forming agent, etc.) 
     Increasing the amount of wash coat has the advantage that it is possible to increase the amount of the support material and the OSC material that enhance the dispersibility of the catalytically active component in the formed catalyst layer, while also increasing the amount of the catalyst. As a result, heat capacity of the catalyst layer increases and requires time to raise the temperature of the catalyst layer, so that exhaust gas purification performance during warm-up of the engine tends to decrease. In the present invention, the wash coat amount set to 190 g/L or less suppresses the heat capacity from becoming too large and high exhaust gas purification performance can be exhibited even during warm-up of the engine. On the other hand, when the wash coat amount of the catalyst layer in the first section exceeds 190 g/L, the heat capacity of the catalyst layer in the first section becomes large and the exhaust gas purification performance during warm-up of the internal combustion engine may become insufficient. On the other hand, when the wash coat amount of the catalyst layer in the first section is less than 100 g/L, the amount of the catalytic active component becomes insufficient which in turn may make the oxidation purification of HC and CO or the reduction purification of NOx insufficient. A preferred wash coat amount of the catalyst layer in the first section is 140 g/L to 170 g/L. 
     In the following will be described one example of a method for calculating the wash coat amount of the catalyst layers (the first catalyst layer  13  and the second catalyst layer  14 ) in the first section V 1 . 
     First, a sample extending in the exhaust gas flow direction of the substrate  11  (X direction in  FIG. 2 ) and having the same length as the length L 1  of the substrate  111  is cut out from the exhaust gas purification catalyst  10 . The sample is, for example, a cylindrical shape having a diametre of 25.4 mm. Next, the cut sample is cut in a plain surface perpendicular to the exhaust gas flow direction of the substrate  11  to prepare a cut piece S 1  of the exhaust gas purification catalyst  10  including the catalyst layers (first catalyst layer  13  and second catalyst layer  14 ) of the first section V 1 . The cut piece S 1  is, for example, a cylindrical shape having a diametre of 25.4 mm and a length of 10 mm. Although the value of the diametre of the cut piece S 1  can be changed as necessary, the exhaust gas purification catalyst  10  may be cut as it is in a plain surface perpendicular to the exhaust gas flow direction so as to have the same diametre as the exhaust gas purification catalyst  10  from the viewpoint of suppressing variation in the wash coat amount depending on the cutting position of the sample. Further, the length value of the cut piece S 1  can be changed as necessary. The length of the catalyst layer in the first section V 1  included in the cut piece S 1  is equal to the length of the cut piece S 1 . 
     Subsequently, a cut piece of the substrate  11  having the same size as the cut piece S 1  is prepared. As for a cut piece of the substrate  11 , one is prepared which does not include any of the catalyst layer in the first section V 1  and the catalyst layer in the second section V 2 . 
     The mass of the cut piece S 1  and the mass of the cut piece of the substrate  11  are measured, and the mass of the catalyst layer in the first section V 1  per unit volume of the cut piece S 1  is calculated on the basis of the following formula. 
       Mass of catalyst layer in first section  V 1 per unit volume of cut piece  S 1=((Mass of cut piece  S 1)−(Mass of cut piece of substrate 11))/(Volume of cut piece( S 1))
 
     The volume of the cut piece S 1  is an apparent volume of the cut piece S 1 . For example, when the cut piece S 1  has a cylindrical shape having a diametre of 25.4 mm and a length of 10 mm, the volume of the cut piece S 1  is n×(12.7 mm) 2 ×10 mm. The volume of the other cut pieces (cut piece S 2  to be described below) is also the same. 
     The mass of the catalyst layer in the first section V 1  per unit volume of the cut piece S 1  is calculated with respect to three cut pieces S 1  produced from any point of the exhaust gas purification catalyst  10 , and the average value thereof is used as the wash coat amount of the catalyst layers (the first catalyst layer  13  and the second catalyst layer  14 ) of the first section V 1  with respect to the volume of the substrate  111  present in the first section V 1 . 
     When calculating the wash coat amount of the catalyst layer in the first section V 1 , the mass of the catalyst layer in the first section V 1  per unit volume of the cut piece S 1  may be calculated without using the cut piece of the substrate  11 . One example of such a calculation method is as follows. 
     First, the mass and volume of the cut piece S 1  produced in the same manner as described above are measured. The composition of the substrate  11  contained in the cut piece S 1  is specified by element mapping or the like of the cross section of the cut piece S 1 . The element mapping can be performed by using a scanning electron microscope (SEM), an electron beam microanalyzer (EPMA) or the like. Further, the composition of the cut piece S 1  is specified by analysis with an inductively coupled plasma emission spectrometer (ICP-AES) or the like. Next, based on the specified composition of the substrate  11  and the cut piece S 1 , the proportion of the mass of the catalyst layer of the mass of the cut piece S 1  is calculated, and based on the following formula, the mass of the catalyst layer in the first section V 1  per unit volume of the cut piece S 1  is calculated. 
       Mass of catalyst layer of first section  V 1 per unit volume of cut piece  S 1=(Mass of cut piece  S 1)×(Proportion of mass of catalyst layer of first section  V 1 of mass of cut piece  S 1)/(Volume of cut piece  S 1)
 
     Next, one example of a method for calculating each of the wash coat amounts of the first catalyst layer  13  and the second catalyst layer  14  constituting the catalyst layer in the first section V 1  will be described. With respect to the wash coat amount of the first catalyst layer  13 , the compositions of the substrate  11  and the first catalyst layer  13  contained in the cut piece S 1  are specified by elemental mapping, or the like of the cross section of the cut piece S 1 . Further, the composition of the cut piece S 1  is determined by analyzing by ICP-AES or the like. The proportion of the mass of the first catalyst layer  13  in the mass of the cut piece S 1  is calculated based on the determined compositions of the substrate  11 , the first catalyst layer  13 , and the cut piece S 1 . The mass of the first catalyst layer  13  per unit volume of the cut piece S 1  is calculated based on the following formula. 
       Mass of first catalyst layer  13  per unit volume of cut piece  S 1=(Mass of cut piece  S 1)×(Proportion of mass of first catalyst layer  13  in mass of cut piece  S 1)/(volume of cut piece  S 1)
 
     Likewise, with respect to the wash coat amount of the second catalyst layer  14 , the compositions of the substrate  11  and the second catalyst layer  14  contained in the cut piece S 1  are specified by element mapping or the like of the cross section of the cut piece S 1 . Further, the composition of the cut piece S 1  is analyzed by ICP-AES or the like. The proportion of the mass of the second catalyst layer  14  in the mass of the cut piece S 1  is calculated based on the specified compositions of the substrate  11 , the second catalyst layer  14 , and the cut piece S 1 . The mass of the second catalyst layer  14  per unit volume of the cut piece S 1  is calculated based on the following formula. 
       Mass of second catalyst layer  14  per unit volume of cut piece  S 1=(Mass of cut piece  S 1)×(Proportion of mass of second catalyst layer  14  in mass of cut piece  S 1)/(Volume of cut piece  S 1)
 
     The wash coat amount of the first catalyst layer  13  in the catalyst layer in the first section V 1  is preferably 50 g/L to 140 g/L and more preferably 80 g/L to 130 g/L, from the viewpoint of improving exhaust gas purification performance during warm-up of the engine and high-speed operation. 
     The wash coat amount of the second catalyst layer  14  in the catalyst layer in the first section V 1  is preferably 30 g/L to 65 g/L and more preferably 35 g/L to 60 g/L from the viewpoint of suppressing Pd poisoning and improving exhaust gas purification performance during warm-up of the engine and high-speed operation. 
     The pore volume proportion of the substrate  111  and the catalyst layers (the first catalyst layer  13  and the second catalyst layer  14 ) to the volume of the first section V 1  is 12% to 18%. In the present invention, the “pore volume proportion” means a value calculated as follows. First, a sample having a cubic shape of 10 mm square is sampled from any place of the first section (that is, the portion V 1  shown in  FIGS. 1 and 2 ) of the exhaust gas purification catalyst, and measurement is done for the mass and volume of the sample and the pore size distribution by mercury press-in method. The pore size distribution can be calculated by a known method using the mercury press-in method. The pore volume proportion of the substrate and the catalyst layer to the volume of the first section can be calculated from the following formula: 
       Pore volume proportion (%)= W×P/V× 100 
     when the mass of the measured sample is W (g), the geometric volume of the cube is V (mL), and the volume of mercury press-fitted at an applied pressure of 0.049 to 255 MPa (corresponding to a pore size of 0.06 μm to 30.0 μm: Hg contact angle of 140°) among the results of the pore size distribution measurement is pore volume P (mL/g). In addition, the substrate and the catalyst layer to the volume of the second section to be described below can also be measured in the same manner as the pore volume proportion of the substrate and the catalyst layer to the volume of the first section. 
     The pore volume proportion of the substrate and the catalyst layer to the volume of the first section is 12% or more and less than 18% of the volume of the first section (i.e., the apparent volume of the entire first section including not only the volume of the partition walls of the substrate and the catalyst layer but also the space between the partition walls in the substrate). Since the catalyst layer of the exhaust gas purification catalyst is composed of a porous material made of fine pores having an oxide supporting material as a skeleton as described above, it can be said that the majority of the volume occupied by the catalyst layer is voids. Therefore, it can be said that the pore volume measured under the above conditions reflects the thickness of the catalyst layer. The thickness of the catalyst layer indicates the volume at which the purification gas and the active ingredient can come into contact per unit area, and if the catalyst layer is not sufficiently thick, the probability of contact between the purification gas and the active ingredient decreases and high purification performance cannot be obtained. On the other hand, if the catalyst layer is made excessively thick, the pressure loss in the catalyst layer increases when the exhaust gas is allowed to flow through the catalyst. That is, since the supporting material for dispersing the catalytic active component was also reduced when the wash coat amount is reduced in order to improve the exhaust gas purification performance during warm-up of the engine as described above, the thickness of the catalyst layer had to be reduced in the conventional method. In view of the above point, the present invention has found that the exhaust gas purification performance during high-speed operation can also be improved by setting the pore volume proportion of the substrate and the catalyst layer to the volume of the first section to 12% or more. Further, when the pore volume proportion is less than 18%, it can be said that no significant pressure loss occurs. That is, when the pore volume proportion is 12% or more and less than 18%, the catalyst layer having a sufficient thickness is formed so that the contact between the exhaust gas and the catalytic active component becomes active, and as a result, high exhaust gas purification performance can be achieved and also no significant pressure loss occurs. In addition, by designing the catalyst layer such that the pore volume proportion is 12% or more and less than 18% even when the wash coat amount is 190 g/L or less, the density of the catalyst layer in the first section becomes moderately low, and even when the first catalyst layer is covered by the second catalyst layer, the reaction heat increases because the exhaust gas easily passes through the second catalyst layer and comes into contact with the first catalyst layer, thereby making it possible to accelerate the temperature rise of the first catalyst layer. Further, high-temperature exhaust gas passes through the catalyst layer of the first section easily, and the temperature rise of the catalyst layer can be accelerated together with the thickness (wash coat amount) of the catalyst layer in the first section. As a result, exhaust gas purification performance during warm-up of the engine and high-speed operation can be improved while suppressing the catalytic activity from decreasing due to phosphorus poisoning of the Pd catalyst layer. 
     As described above, the value of the pore volume proportion reflects the thickness of the catalyst layer, and tends to become larger as the density of the catalyst layer is lower (the catalyst layer is bulky) and as the wash coat amount of the catalyst layer is larger. In other words, the value of the pore volume proportion can be controlled by adjusting the density of the catalyst layer and the wash coat amount of the catalyst layer. For example, as shown in the evaluation results of Example 1, Example 4, and Comparative Example 1 described later, the value of the pore volume proportion of the first section tends to increase as the amount of wash coat increases when the same slurry is used to form the first catalyst layer  13  and the density of the first section is made the same. On the other hand, as shown in the evaluation result of Comparative Example 5 to be described later, when the particles in the slurry for forming the first catalyst layer  13  have a small particle size and the density of the first section is increased, the value of the pore volume proportion tends to become relatively small even if the wash coat amount was increased. According to the exhaust gas purification catalyst of the present embodiment, it is possible to enhance the exhaust gas purification performance during warm-up of the engine and high-speed operation while suppressing the catalytic activity from decreasing due to phosphorus poisoning of the Pd catalyst layer by controlling each of the pore volume proportion and the wash coat amount for the first section. 
     On the other hand, when the pore volume proportion of the catalyst layer in the first section is less than 12%, the exhaust gas purification performance can be impaired because the exhaust gas does not easily pass through the catalyst layer in the first section. The pore volume proportion of the catalyst layer in the first section is preferably 13% or more and less than 17%. 
     As a method for controlling the pore volume proportion of the catalyst in the first section to be within the above range while the wash coat amount of the catalyst layer in the first section is within the above range, for example, a method for controlling each of the particle sizes of the slurry constituent materials for forming the first catalyst layer  13  and the second catalyst layer  14  may be mentioned. With respect to the particle size of the slurry constituent material for forming the first catalyst layer  13 , when the volume-converted particle size distribution is observed at regular intervals by a particle size distribution metre employing a light scattering method, the particle size (D50) that is the cumulative sum of 50% in the undersize cumulative distribution for the spherical equivalent diametre is preferably more than 10 μm and 20 μm or less, more preferably 11 μm to 15 μm. The particle size (D90) that is the cumulative sum of 90% is preferably 20 μm to 45 μm, and more preferably 20 μm to 40 μm. As the particle size of the slurry constituent material for forming the second catalyst layer  14 , the D50 particle size is preferably more than 10 μm and 25 μm or less, and more preferably 11 μm to 20 μm. The D90 particle size is preferably more than 20 μm and 50 μm or less, and more preferably 21 μm to 45 μm. 
     It is preferable that the second catalyst layer  14  covering the first catalyst layer  13  has a through hole having a width of 10 μm or more which can vent air from the surface to the first catalyst layer. By having such communicating pores, high-temperature exhaust gas easily passes through the second catalyst layer and comes into contact with the first catalyst layer, so that the exhaust gas purification performance during warm-up of the engine and high-speed operation can be further enhanced. 
     From the viewpoint of improving exhaust gas purification performance during warm-up of the engine and high-speed operation, it is preferable that the palladium concentration in the first catalyst layer is higher in the surface layer (i.e., closer to the side in contact with the second catalyst layer). In particular, when the first catalyst layer is divided into two parts along the thickness direction, and the mass of palladium on the surface side of the first catalyst layer is defined as a1 and the mass of palladium on the opposite side to the surface side of the first catalyst layer is defined as a2, the value of a1/a2 (hereinafter also referred to as a concentration gradient) obtained is more preferably, 1.2 or more. By disposing a larger amount of palladium on the surface side than on the substrate side in the first catalyst layer, the catalyst function by the specific element of the catalyst layer can be efficiently performed at a high space speed. As a result, CO oxidation and HC oxidation, which are exothermic reactions, are further promoted compared with the case where the concentration of palladium is made uniform in the layer thickness direction, and thus the temperature raising property of the catalyst can be enhanced. NOx reduction property in the second catalyst layer can also be enhanced by enhancing the temperature raising property of the catalyst. From the viewpoint of enhancing the exhaust gas purification action during warm-up of the engine and high-speed operation, particularly the action during high-speed operation, it is more preferable that the concentration gradient is 1.3 to 3.0. 
     Specifically, the concentration gradient of palladium in the first catalyst layer can be obtained by the following method. That is, in the palladium fluctuation curve obtained by quantifying the distribution of palladium (unit:cps) by line analysis of energy dispersive X-ray analysis (EDX), the ratio between the integral value of the count number on the surface side than a half position in the thickness direction in the first catalyst layer and the integral value of the count number in the substrate side than the position is obtained, which is then converted into a mass ratio, and is used as the concentration gradient of palladium in the thickness direction. 
     &lt;Catalyst Layer of Second Section&gt; 
     The third catalyst layer  15  constituting the catalyst layer in the second section V 2  contains palladium (Pd) as a catalytically active component, but may also contain other catalytically active components such as platinum (Pt) and rhodium (Rh). In particular, it is preferable to contain Pd and Pt as catalytically active components from the viewpoint of high purification performance of carbon monoxide (CO) and hydrocarbon (HC) and obtaining a high warming function. In a preferred embodiment, the catalytically active components are the same as those of the first catalyst layer described above. 
     It is preferable for the third catalyst layer  15  to further contain a carrier component that carries a catalytic active component, in view of efficiently exhibiting the exhaust gas purification performance by the catalytic active component. The same carrier component as that of the first catalyst layer  13  in the first section V 1  can be used as the carrier component. 
     In order to form the third catalyst layer  15  on the substrate  11 , a slurry is prepared in the same manner as the first catalyst layer  13  and applied to the substrate surface by a wash coating method, then dried and burned. The wash coat amount of the third catalyst layer  15  is preferably larger than that of the first catalyst layer  13 , particularly from the viewpoint of improving the exhaust gas purification performance during high-speed operation. 
     The fourth catalyst layer  16  covering the third catalyst layer  15  preferably contains rhodium (Rh) as a catalytically active component. Similarly to the catalyst layer of the first section, when phosphorus is contained in the exhaust gas, the catalytic activity of Pd is reduced because when the third catalyst layer  15  containing Pd as a catalytically active component is exposed, the exhaust gas comes into contact with the third catalyst layer  15  first. In order to reduce phosphorus poisoning of Pd, the third catalyst layer  15  is preferably covered with the fourth catalyst layer  16 . 
     In a preferred embodiment, the fourth catalyst layer  16  of the second section V 2  has the same catalytic active component as the second catalyst layer described above. 
     In addition, it is preferable that the fourth catalyst layer  16  further contains a carrier component for carrying the catalytic active component, in view of efficiently exhibiting the exhaust gas purification performance by the catalytic active component. The same carrier component as that of the second catalyst layer  14  in the first section V 1  can be used. 
     The fourth catalyst layer  16  can be formed in the same manner as the second catalyst layer  14  of the first section V 1  by applying a slurry to the surface of the third catalyst layer  15  by a wash coating method, followed by drying and firing. The fourth catalyst layer  16  having a desired thickness can be formed by adjusting the amount of the wash coating, but in a preferred embodiment, the amount of the wash coating is the same as that of the second catalyst layer  14 . 
     The catalyst layer in the second section V 2  comprising the third catalyst layer  15  and the fourth catalyst layer  16  preferably has a wash coat amount of 180 g/L to 300 g/L per 1 L of substrate in the substrate  112  present in the first section V 2 . By making the wash coat amount larger than that of the catalyst layer in the first section (i.e., by making the thickness larger), it is possible to particularly improve the exhaust gas purification effect even more during high-speed operation. A more preferable wash coat amount is 190 g/L to 210 g/L. 
     The mass per unit volume relative to the volume of the substrate present in the second section V 2  of the catalyst layer in the second section V 2  (hereinafter referred to as “wash coat amount”) can be specified in the same manner as in the above-described method for specifying the wash coat amount of the catalyst layer in the first section V 1 . That is, first, a sample which extends in the exhaust gas flow direction of the substrate  11  (in the X direction in  FIG. 2 ) from the exhaust gas purification catalyst  10  and has the same length as the length L 2  of the substrate  112  is cut out. The sample is, for example, a cylindrical shape having a diametre of 25.4 mm. Next, the cut sample is cut in a planar face perpendicular to the exhaust gas flow direction of the substrate  11  to prepare a cut piece S 2  of the exhaust gas purification catalyst  10  including the second section V 2 . The cut piece S 1  is, for example, a cylindrical shape having a diametre of 25.4 mm and a length of 10 mm. Although the value of the diametre of the cut piece S 2  can be changed as necessary, the exhaust gas purification catalyst  10  may be cut as it is in a planar surface perpendicular to the exhaust gas flow direction so as to have the same diametre as the exhaust gas purification catalyst  10  from the viewpoint of suppressing variations in the wash coat amount depending on the cutting position of the sample. The length value of the cut piece S 2  can be changed as necessary. The length of the catalyst layer in the second section V 2  included in the cut piece S 2  is equal to the length of the cut piece S 2 . 
     From the cut piece S 2  thus produced, the wash coat amount of the catalyst layer in the second section V 2  (and each wash coat amount of the third catalyst layer  15  and the fourth catalyst layer  16 ) can be specified in the same manner as in the method for specifying the wash coat amount of the catalyst layer in the first section V 1 . In addition, when calculating the wash coat amount of the catalyst layer in the second section V 2 , the mass of the catalyst layer in the second section V 2  per unit volume of the cut piece S 2  may be calculated without using the cut piece of the substrate  11 . The calculation method is the same as that for calculating the wash coat amount of the catalyst layer of the first section V 1 . 
     From the viewpoint that it is possible to improve the exhaust gas purification performance during warm-up of the engine and high-speed operation more, the wash coat amount of the catalyst layer in the first section V 1  is preferably 90% or less, more preferably 80% or less, of the wash coat amount of the catalyst layer in the second section V 2 . 
     It is preferable that the pore volume proportion of the substrate and the catalyst layer to the volume of the second section V 2  is 18% to 25%. By making the catalyst layer thicker than that of the first section V 1 , it is possible to enhance the exhaust gas purification effect during high-speed operation. In particular, the exhaust gas purification effect during high-speed operation can be enhanced even more by providing each catalyst layer in such a manner that the pore volume proportion of the substrate and the catalyst layer to the volume of the first section is 90% or less of the pore volume proportion of the substrate and the catalyst layer to the volume of the second section. The pore volume proportion of the substrate and the catalyst layer to the volume of the second section V 2  can be measured in the same manner as the pore volume proportion of the substrate and the catalyst layer to the volume of the second section V 1 . 
     The length ratio of the first section V 1  to the second section V 2  in the exhaust gas flow direction X is not particularly limited, but in view that the exhaust gas purification performance during start-up and high-speed operation can be enhanced even more and the ease of manufacture, it is preferable that the length of the exhaust gas flow direction X in the first section V 1  be 20% to 70% with respect to the length of the exhaust gas flow direction of the substrate  11 . 
     The exhaust gas purification catalyst according to the present invention can be made into an exhaust gas purification catalyst having excellent exhaust gas purification performance during warm-up of the engine and high-speed operation without lowering the Pd catalytic activity in spite of the limited amount of the catalytic active component compared with the conventional exhaust gas purification catalyst. Therefore, the exhaust gas purification catalyst of the present invention can efficiently purify NOx, HC and CO as an exhaust gas purification catalyst for an internal combustion engine powered by fossil fuel such as a gasoline engine or a diesel engine. Therefore, it is possible to provide an exhaust gas purification system in which exhaust gas from an internal combustion engine is introduced into the exhaust gas purification catalyst without passing through another exhaust gas purification catalyst in an exhaust gas path connected to the internal combustion engine. 
     EXAMPLES 
     Next, the embodiments of the present invention shall be specifically described with reference to the following Examples, but the present invention shall not be limited to these Examples. 
     Example 1 
     (1) Preparation of Base Slurry for Formation of Lower Catalyst Layer (First Catalyst Layer  13  and Third Catalyst Layer  15 ) 
     An OSC material having the following composition and specific surface area was prepared. 
     CeO 2 : 30% by mass, ZrO 2 : 58% by mass, La 2 O 3 : 8% by mass, Nd 2 O 3 : 4% by mass, specific surface area: 50 m 2 /g. 
     CeO 2 , La 2 O 3 , Nd 2 O 3  and ZrO 2  were forming a solid solution in the OSC material. 
     In a ball mill pot were added the OSC material, lanthanum oxide-modified alumina (La 2 O 3 -modified amount: 3% by mass, specific surface area: 100 m 2 /g), barium acetate and alumina sol, and water and mixing and pulverization were conducted by a ball mill to obtain a base slurry for a lower catalyst layer. When ball milling was performed, a volume-converted particle size distribution was observed at regular intervals by a particle size distribution metre using a light scattering method. The mixing and pulverization by the ball mill were carried out until the particle size that is 50% in cumulative sum was 12 μm or less and the particle size of 90% fell within the range of 20 μm to 35 μm in the cumulative undersize distribution of the spherical equivalent diametre of the slurry constituent material. The amount ratio of each component in the slurry was set to 50% by mass of the OSC material, 30% by mass of lanthanum oxide-modified alumina, 10% by mass of barium carbonate, and 10% by mass of alumina. 
     (2) Formation of the first catalyst layer  13  before surface concentration treatment 
     The base slurry was dipped from the upstream end in the exhaust gas flow direction X of the cordierite honeycomb porous substrate  11  (manufactured by NGK Insulators, Ltd., axial length 100 mm, outer diametre 93 mm, number of cells 600 cells/inch 2 ) to the intermediate position in the same direction of the substrate  11 , and then coated. Thereafter, the substrate  11  was dried at 150° C. for 2.5 hours and then burned at 450° C. for 2.5 hours to form the first catalyst layer  13  of the first section V 1 . The mass per unit volume (wash coat amount) with respect to the volume of the substrate  111  present in the first section V 1  of the first catalyst layer  13  was 95 g/L. 
     (3) Formation of the third catalyst layer  15   
     The base slurry and an aqueous palladium nitrate solution were mixed to obtain a slurry for the third catalyst layer  15 . The amount of palladium nitrate in the slurry for the third catalyst layer  15  was set to an amount in which Pd consisted for 0.667% by mass in the third catalyst layer  15  after burning. A portion from the downstream end in the exhaust gas flow direction X of the substrate  11  to the intermediate position in the same direction of the substrate  11  was dipped in the slurry for the third catalyst layer  15  and coated. Thereafter, the substrate  11  was dried at 150° C. for 2.5 hours and then burned at 450° C. for 2.5 hours to form the third catalyst layer  15  of the second section V 2 . The mass per unit volume (wash coat amount) relative to the volume of the substrate  112  present in the second section V 2  of the third catalyst layer  15  was 150 g/L. The content of Pd in the third catalyst layer  15  was 1.00 g/L relative to the volume of the third catalyst layer  15 . 
     The first catalyst layer  13  and the third catalyst layer  15  were connecting without a gap on the substrate  11  in the exhaust gas flow direction X. The length ratio of the first catalyst layer  13  to the third catalyst layer  15  in the exhaust gas flow direction X was 1:1, and the length in the exhaust gas flow direction X of the lower catalyst layer comprising the first catalyst layer  13  and the third catalyst layer  15  was the same as the length in the exhaust gas flow direction X of the substrate  11 . 
     (4) Pd surface concentration support 
     Palladium nitrate aqueous solution (Pd equivalent concentration: 5.00 g/L, liquid amount: 340 ml) was immersed for only the portion of the substrate  11  where the first catalyst layer  13  was formed (i.e., the portion of substrate  111 ) at 25° C. for 48 hours. After immersion, gradual dehydration was performed, and the substrate  11  was dried at 150° C. for 2.5 hours and burned at 450° C. for 2.5 hours. The amount of Pd in the first catalyst layer  13  was 5.00 g/L with respect to the volume of the first catalyst layer  13 . 
     (5) Formation of an upper catalyst layer (second catalyst layer  14  and fourth catalyst layer  16 ) 
     OSC materials having the following composition and specific surface area were prepared. 
     CeO 2 : 15% by mass, ZrO 2 : 73% by mass, La 2 O 3 : 8% by mass, Nd 2 O 3 : 4% by mass, BET specific surface area: 50 m 2 /g In the OSC material, CeO 2 , ZrO 2 , La 2 O 3 , and Nd 2 O 3  formed a solid solution. 
     To a ball mill pot were added a ZrO 2 -based material, lanthanum-modified alumina (modified amount of La 2 O 3 : 3% by mass, BET specific surface area: 100 m 2 /g), alumina sol and water, and the mixture was mixed and pulverized by a ball mill to obtain a base slurry for an upper catalyst layer. When the ball milling was performed, a volume-converted particle size distribution was observed at regular intervals by a particle size distribution metre using a light scattering method. The ball milling was performed until the particle size that is the cumulative sum of 50% was 15 μm or less and the particle size that is 90% fell within the range of 25 μm to 40 μm in the undersize cumulative distribution for the spherical equivalent diametre of the slurry constituting material. This slurry and an aqueous solution of rhodium nitrate were mixed to obtain a slurry for an upper catalyst layer. The amount ratio of each component in the slurry for an upper catalyst layer was such that, after burning, the ratio of components other than rhodium in the upper catalyst layer was 60% by mass of a ZrO 2 -based material, 30% by mass of lanthanum modified alumina, and 10% by mass of alumina, and the amount of rhodium in the upper catalyst layer was 0.5% by mass. 
     A substrate  11  having a lower catalyst layer (a first catalyst layer  13  and a third catalyst layer  15 ) formed thereon is immersed in the obtained slurry for an upper catalyst layer, then the substrate  11  is dried at 150° C. for 2.5 hours, and then burned at 450° C. for 2.5 hours to form an upper catalyst layer over the entire surface of the lower catalyst layer, thereby obtaining an exhaust gas purification catalyst of Example 1. 
     The mass per unit volume (wash coat amount) of the substrate  11  ( 111  and  112 ) present in the first section V 1  and the second section V 2  of the upper catalyst layer was 45 g/L. The rhodium content in the upper catalyst layer was 0.225 g/L with respect to the volume of the substrate  11 . 
     (6) Measurement of Pore volume proportion 
     A cube-shaped catalyst piece sample having an angle of about 10 mm was each cut out from the central portion in the axial direction (exhaust gas flow direction) of the first section (first catalyst layer  13  and second catalyst layer  14 ) and the second section (third catalyst layer  15  and fourth catalyst layer  16 ) and from a portion excluding 5 mm from the side surface outer circumference. With respect to the cut out catalyst piece, each side was measured to calculate the volume V (mL) of the cube-shaped catalyst piece. In addition, the weight W (g) of the catalyst piece was measured in a sufficiently dry state, keeping in mind the absorption of moisture and the like. The pore distribution of the catalyst piece was measured by a mercury press-in method (atmospheric pressure to 255 MPa). 
     Among the pore distribution measurement results, the volume of mercury press-fitted at an applied pressure of 0.049 to 255 Mpa (corresponding to a pore diametre of 0.06 μm to 30.0 μm: Hg contact angle of 140°) was regarded as pore volume P (mL/g), and the pore volume proportion was calculated by the following formula. 
       Pore volume proportion (%)= W×P/V× 100 
     (7) Measurement of Pd concentration gradient 
     The exhaust gas purification catalyst was measured for the concentration gradient a1/a2. Specifically, the concentration change of Pd was measured from the surface of the upper catalyst layer toward the substrate  11  using EDX (model number: XM-8101, manufactured by Nihon Denshi Co., Ltd.) in such a manner that the observation face will be the central portion in the axial direction (exhaust gas flow direction) of the first section (the first catalyst layer  13  and the second catalyst layer  14 ) and the portion other than 5 mm from the side surface outer circumference (outer skin). From the measurement results, the ratio (a1/a2) of the concentration at of Pd on the surface side to the concentration a2 of Pd on the opposite side (the substrate  11  side) was obtained when the first catalyst layer  13  was equally divided into two parts of the surface side and the substrate  11  side along the thickness direction. 
     Example 2 
     A catalyst for purifying exhaust gas of Example 2 was prepared in the same manner as in Example 1, except that in (3) the formation of the third catalyst layer  15 , the Pd content in the third catalyst layer  15  was changed to 2.00 g/L, and the Pd concentration in the palladium nitrate aqueous solution was adjusted in the Pd surface concentration support of (4) to change the Pd amount in the first catalyst layer  13  to 4.00 g/L. 
     Example 3 
     An exhaust gas purification catalyst of Example 3 was prepared in the same manner as in Example 1, except that the length of the first section V 1  was changed to 25 mm, the length of the second section V 2  was changed to 75 mm, and the Pd concentration in the palladium nitrate aqueous solution was adjusted in the Pd surface concentration support of (4) to change the amount of Pd in the first section V 1  to 9.00 g/L. 
     Example 4 
     An exhaust gas purification catalyst of Example 4 was prepared in the same manner as in Example 1 except that the wash coat amount of the upper catalyst layer was changed to 60 g/L. An upper catalyst layer was formed by adjusting the mixing ratio of the base slurry and rhodium nitrate so that the Rh content of the upper catalyst layer was 0.225 g/L. 
     Comparative Example 1 
     An exhaust gas purification catalyst of Comparative Example 1 was prepared in the same manner as in Example 1 except that the wash coat amount of the upper catalyst layer was changed to 70 g/L. An upper catalyst layer was formed by adjusting the mixing ratio of the base slurry and rhodium nitrate so that the Rh content of the upper catalyst layer was 0.225 g/L. 
     Comparative Examples 2 to 4 
     Exhaust gas purification catalysts of Comparative Examples 2 to 4 were prepared in the same manner as in Example 1 except that the coating amounts of the first section, the second section, and the upper catalyst layer were respectively changed as shown in Table 1. 
     Comparative Example 5 
     (1) In the preparation of the base slurry for forming the lower catalyst layer, ball milling was carried out for a longer time than that in Example 1, until the particle size (D50) that is the cumulative sum of 50% was 10 μm or less and the particle size (D90) that is the cumulative sum of 90% fell within the range of less than 20 μm in the undersize cumulated distribution for the spherical equivalent diametre of the slurry constituting material. 
     Also, in the (5) formation of the upper catalyst layer, ball milling was carried out for a longer time than in Example 1, until the D50 particle size was 10 μm or less and the D90 particle size fell within the range of less than 20 μm in the undersieve cumulated distribution for the spherical equivalent diametre of the slurry constituting material. 
     Then, an exhaust gas purification catalyst of Comparative Example 5 was prepared in the same manner as in Example 1, except that the wash coat amounts of the first catalyst layer  13  and the upper catalyst layers (the second catalyst layer  14  and the fourth catalyst layer  16 ) of the first section V 1  were each changed as shown in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                 Constitution of Catalyst Layer 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Wash coat amount of  
                 Wash coat amount of 
               
               
                   
                 Length 
                 Length 
                 Pd concentration 
                 first section (g/L) 
                 second section (g/L) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 of first 
                 of second 
                 (g/L) 
                 First 
                 Second 
                   
                 Third 
                 Fourth 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 section 
                 section 
                 First  
                 Second 
                 catalyst 
                 catalyst 
                   
                 catalyst 
                 catalyst 
                   
               
               
                   
                 (mm) 
                 (mm) 
                 Section 
                 section 
                 layer 
                 layer 
                 Total 
                 layer 
                 layer 
                 Total 
               
               
                   
               
               
                 Ex. 1 
                 50 
                 50 
                 5 
                 1 
                 100 
                 45 
                 145 
                 150 
                 45 
                 195 
               
               
                 Ex. 2 
                 50 
                 50 
                 4 
                 2 
                 99 
                 45 
                 144 
                 151 
                 45 
                 196 
               
               
                 Ex. 3 
                 25 
                 75 
                 9 
                 1 
                 104 
                 45 
                 149 
                 150 
                 45 
                 195 
               
               
                 Ex. 4 
                 50 
                 50 
                 5 
                 1 
                 100 
                 60 
                 160 
                 150 
                 60 
                 205 
               
               
                 Comp. 
                 50 
                 50 
                 5 
                 1 
                 100 
                 70 
                 170 
                 150 
                 70 
                 220 
               
               
                 Ex. 1 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 50 
                 50 
                 5 
                 1 
                 150 
                 45 
                 195 
                 150 
                 45 
                 195 
               
               
                 Ex. 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 50 
                 50 
                 5 
                 1 
                 150 
                 45 
                 195 
                 135 
                 45 
                 180 
               
               
                 Ex. 3 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 50 
                 50 
                 5 
                 1 
                 150 
                 45 
                 195 
                 100 
                 45 
                 145 
               
               
                 Ex. 4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 50 
                 50 
                 5 
                 1 
                 150 
                 70 
                 220 
                 150 
                 70 
                 220 
               
               
                 Ex. 5 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Constitution of Catalyst Layer 
               
            
           
           
               
               
               
               
            
               
                   
                 Wash coat amount  
                   
                   
               
               
                   
                 of first section/ 
                 Pore volume proportion (%) 
                 Pd concentration  
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Wash coat amount of 
                   
                 Second 
                 First section/ 
                 gradient of First section 
               
               
                   
                 second section (%) 
                 First section 
                 section 
                 Second section (%) 
                 lower layer (a1/a2) 
               
               
                   
               
               
                 Ex. 1 
                 74.4 
                 16.04 
                 18.13 
                 88 
                 1.52 
               
               
                 Ex. 2 
                 73.5 
                 15.21 
                 18.46 
                 82 
                 1.47 
               
               
                 Ex. 3 
                 76.4 
                 15.18 
                 18.33 
                 83 
                 1.30 
               
               
                 Ex. 4 
                 72.7 
                 17.09 
                 19.18 
                 89 
                 1.56 
               
               
                 Comp. 
                 73.9 
                 18.25 
                 19.86 
                 92 
                 1.45 
               
               
                 Ex. 1 
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 100 
                 18.03 
                 18.13 
                 98 
                 1.83 
               
               
                 Ex. 2 
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 108 
                 18.33 
                 17.34 
                 108 
                 1.72 
               
               
                 Ex. 3 
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 135 
                 12.05 
                 16.13 
                 128 
                 1.77 
               
               
                 Ex. 4 
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 100 
                 14.51 
                 12.92 
                 87 
                 1.88 
               
               
                 Ex. 5 
               
               
                   
               
            
           
         
       
     
     &lt;Evaluation of Exhaust Gas Purification Performance&gt; 
     The following endurance conditions were imposed for the exhaust gas purification catalyst as a deterioration treatment assuming mileage of 5 to 100,000 kilometres. Specifically, treatment was done for the following time while keeping the catalyst temperature at the following temperature in a state where the exhaust gas discharged by operating the engine under the following conditions was brought into contact with the exhaust gas purification catalyst. 
     (Endurance Condition) 
     Endurance Engine: Passenger NA2L gasoline engine 
     Gasoline used: Commercially available regular gasoline 
     Endurance temperature·time: 900° C.-100 hr 
     Pre-catalyst air-fuel ratio fluctuation: A/F=repeat of 13.5 (10 sec)→15.5 (20 sec)→13.5 (10 sec) 
     After the durability test was carried out under the above conditions, the exhaust gas purification catalyst after endurance was installed in the following vehicle. As a vehicle test, the vehicle was operated under the operating conditions of the New European Driving Cycle (NEDC). The emissions of non-methane hydrocarbons (NMHC) and nitrogen oxides (NOx) in the exhaust gases (emission values: g/test) were each measured in the Urban Driving Mode (ECE) from the start of operation to 780 seconds and in the High Speed Driving Mode (EUDC) from 780 seconds to 1180 seconds from the start of operation. Table 2 shows the emission values obtained in the Urban Driving Mode (ECE) and in the High Speed Driving Mode (EUDC) which the values were obtained by regarding the value of Example 1 as being the standard as 1. Also shown in Table 2 are the NEDC emissions (total emissions), which are the total emissions in ECE and EUDC for each of NMHC and NOx. 
     (Purification Rate Measurement Conditions) 
     Evaluation Vehicle: Small Passenger Vehicle 
     Gasoline Used: Certification Test Fuel 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Evaluation 
               
            
           
           
               
               
               
               
            
               
                   
                 ECE Condition 
                 EUDC Condition 
                 Total 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 NMHC 
                 NOx 
                 NMHC 
                 NOx 
                 NMHC 
                 NOx 
               
               
                   
                 emission 
                 emission 
                 emission 
                 emission 
                 emission 
                 emission 
               
               
                   
                 amount 
                 amount 
                 amount 
                 amount 
                 amount 
                 amount 
               
               
                   
               
               
                 Ex. 1 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
                 1.000 
               
               
                 Ex. 2 
                 1.076 
                 1.306 
                 0.994 
                 0.803 
                 1.070 
                 0.932 
               
               
                 Ex. 3 
                 1.015 
                 1.104 
                 1.043 
                 1.094 
                 1.018 
                 1.100 
               
               
                 Ex. 4 
                 1.082 
                 1.040 
                 0.978 
                 0.888 
                 1.056 
                 0.937 
               
               
                 Comp. 
                 1.085 
                 1.044 
                 1.026 
                 0.918 
                 1.081 
                 0.950 
               
               
                 Ex. 1 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 1.140 
                 0.865 
                 1.019 
                 0.802 
                 1.133 
                 0.824 
               
               
                 Ex. 2 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 1.196 
                 0.962 
                 1.049 
                 0.987 
                 1.188 
                 0.978 
               
               
                 Ex. 3 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 1.197 
                 0.968 
                 1.163 
                 1.016 
                 1.195 
                 1.004 
               
               
                 Ex. 4 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. 
                 1.184 
                 1.077 
                 1.126 
                 1.110 
                 1.142 
                 1.136 
               
               
                 Ex. 5 
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     As is clear from the evaluation results shown in Table 2, the exhaust gas purification catalysts of Examples 1 to 3 have lower total NMHC emissions than the exhaust gas purification catalysts of Comparative Examples 1 to 5, and therefore it has been confirmed that the exhaust gas purification catalyst employing the features of the present invention exhibits high exhaust gas purification performance without impairing the Pd catalytic activity. 
     EXPLANATION OF REFERENCES 
     
         
           10  Exhaust gas purification catalyst 
           11  Substrate 
           111  Substrate present in the first section 
           112  Substrate present in the second section 
           12  Catalyst layer 
           13  First catalyst layer 
           14  Second catalyst layer 
           15  Third catalyst layer 
           16  Fourth catalyst layer 
           20  Casing 
         V 1  First section 
         V 2  Second section