JP5104352B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to an exhaust gas purifying catalyst.

  Conventionally, in order to purify harmful nitrogen oxides (NOx) contained in exhaust gas of automobiles and the like, exhaust gas purification catalysts having a NOx occlusion part including an oxide complex capable of storing NOx have been used. As an exhaust gas purification catalyst equipped with such a NOx occlusion part, for example, in JP-A No. 2003-326164 (Patent Document 1), a catalyst component part containing a cerium compound and a cesium compound is coated on a porous carrier. Thus, there is disclosed a catalyst in which a larger amount of the cerium compound is contained on the upper surface side of the catalyst component portion and a larger amount of the cesium compound is contained on the lower surface side of the catalyst component portion.

However, the conventional exhaust gas purifying catalyst as described in Patent Document 1 is not sufficient in durability at high temperatures and sulfur poisoning. Further, the conventional exhaust gas purifying catalyst as described in Patent Document 1 is not sufficient in that it exhibits sufficiently high catalytic activity over a wide temperature range.
JP 2003-326164 A

  The present invention has been made in view of the above-described problems of the prior art, has a sufficiently high temperature durability and a sufficiently high sulfur poisoning durability, and is sufficiently high over a wide temperature range. An object is to provide an exhaust gas purifying catalyst capable of exhibiting catalytic activity.

  As a result of intensive studies to achieve the above object, the inventors of the present invention have two or more kinds of NOx occlusion parts each containing an oxide composite having different compositions, and the oxide composite is made of an alkali metal and The oxide composite comprising at least one first metal element selected from the group consisting of alkaline earth metals and at least one second metal element selected from the group consisting of rare earth metals, Ratio of the peak area of the first metal element to the sum of the peak area of the first metal element and the peak area of the second metal element obtained from TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the surface ( The standard deviation of the distribution of [peak area of the first metal element] / [sum of the peak area of the first metal element and the peak area of the second metal element]) is 10% or less, and the two or more types of NOx absorption Parts from the NOx occlusion part containing the oxide complex having a small value of the molar ratio of the first metal element to the second metal element ([first metal] / [second metal]). By arranging to contact, the obtained exhaust gas purification catalyst has sufficiently high temperature durability and sufficiently high sulfur poisoning durability, and sufficiently high catalytic activity over a wide temperature range. It has been found that it is possible to show, and the present invention has been completed.

That is, the exhaust gas purifying catalyst of the present invention comprises two or more types of NOx storage parts each containing oxide composites having different compositions,
The oxide composite includes at least one first metal element selected from the group consisting of alkali metals and alkaline earth metals, and at least one second metal element selected from the group consisting of rare earth metals. Contains
Of the first metal element relative to the sum of the peak area of the first metal element and the peak area of the second metal element obtained from TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the oxide composite. The standard deviation of the distribution of the ratio of peak areas ([peak area of the first metal element] / [sum of the peak area of the first metal element and the peak area of the second metal element]) is 10% or less, and
The two or more types of NOx occlusion parts contain the oxide complex having a small content molar ratio of the first metal element to the second metal element ([first metal] / [second metal]). It is arranged so as to come into contact with the exhaust gas in order from the NOx storage part,
It is characterized by.

As the exhaust gas purifying catalyst of the present invention, the two or more types of NOx occlusion parts contain the oxide complex having a small content molar ratio of the first metal element to the second metal element. Exhaust gas purification catalyst arranged from the upstream side to the downstream side of the gas flow path in order from the part, or
The two or more types of NOx occlusion parts have a multilayer structure formed by laminating on the surface of the catalyst base, and the two or more types of NOx occlusion parts contain the first metal element with respect to the second metal element An exhaust gas purifying catalyst that is arranged in order from the outer surface side to the surface side of the catalyst base is preferred from the NOx occlusion part containing the oxide composite having a small molar ratio.

  Furthermore, in the exhaust gas purifying catalyst of the present invention, it is preferable that the maximum value of the molar ratio of the first metal element is twice or more the minimum value of the molar ratio of the first metal element.

  The exhaust gas purifying catalyst of the present invention has sufficiently high high temperature durability and sufficiently high sulfur poisoning durability, and can exhibit sufficiently high catalytic activity over a wide temperature range. The reason for this is not necessarily clear, but the present inventors infer as follows. That is, first, the oxide composite according to the present invention includes at least one first metal element selected from the group consisting of alkali metals and alkaline earth metals, and at least one selected from the group consisting of rare earth metals. The base strength can be sufficiently changed by changing the composition ratio of the first metal element and the second metal element. On the other hand, in the conventional exhaust gas purification catalyst, even if an oxide in which the first metal element and the second metal element are mixed is formed, the oxides of the respective metal elements in the oxide are mutually separated. Since independent coarse particles were formed, the base strength could not be changed sufficiently even if the metal composition ratio was changed. Here, when examining the relationship between the base strength and the NOx purification activity, the lower the base strength, the better the NOx purification activity in the low temperature range, and the stronger the base strength, the better the NOx purification activity in the high temperature range. There is a relationship. The oxide composite according to the present invention has a ratio of the peak area of the first metal element to the sum of the peak area of the first metal element and the peak area of the second metal element obtained from TEM-EDX analysis ([first metal Elemental peak area] / [sum of the peak area of the first metal element and the sum of the peak areas of the second metal element]) with a standard deviation of 10% or less, the first metal element and the second metal element Are sufficiently uniformly mixed and finely compounded. In such an oxide composite, the base strength can be easily changed by changing the metal composition ratio. Therefore, in the exhaust gas purifying catalyst of the present invention in which two or more types of NOx occlusion parts each containing oxide complexes having different composition ratios are disposed, high NOx purifying activity is exhibited over a wide temperature range from low temperature to high temperature. It is possible to demonstrate. Further, since such an oxide composite is in a state of being finely and sufficiently uniformly mixed as described above, the sulfate that can be formed when it comes into contact with the sulfur component is in an unstable state. Therefore, a decrease in catalytic activity due to sulfur poisoning can be sufficiently suppressed, and a sufficiently high catalytic activity can be exhibited.

  Further, in the present invention, two or more kinds of NOx occlusion parts have the small value of the molar ratio of the first metal element to the second metal element ([first metal] / [second metal]). It arrange | positions so that it may contact exhaust gas in order from the NOx occlusion part containing a compound complex. In general, sulfur poisoning of a catalyst is likely to occur at a site where it first comes into contact with exhaust gas or a site where contact frequency with exhaust gas is high. Further, the detachability of the sulfur component during the sulfur poisoning regeneration treatment is more likely to occur as the NOx occlusion portion has a weaker base strength. In the exhaust gas purifying catalyst of the present invention, since the NOx occlusion part is arranged as described above, the NOx occlusion part having a small content molar ratio of the first metal element is in contact with the exhaust gas when purifying the exhaust gas. After that, the NOx occlusion part having a large content molar ratio of the first metal element comes into contact with the exhaust gas. Therefore, in the present invention, the NOx occlusion part having a small content molar ratio of the first metal element is easily exposed to sulfur. And since such NOx occlusion part with small content molar ratio of a 1st metal element is a thing with smaller base intensity | strength, when sulfur poisoning reproduction | regeneration processing is performed, the detachment | desorption property of sulfur is high. Therefore, the present inventors speculate that the exhaust gas purifying catalyst of the present invention can maintain high NOx purification activity even after sulfur poisoning and has sufficiently high sulfur poisoning durability.

  According to the present invention, there is provided an exhaust gas purifying catalyst that has a sufficiently high high temperature durability and a sufficiently high sulfur poisoning durability and can exhibit a sufficiently high catalytic activity over a wide temperature range. It becomes possible to provide.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

The exhaust gas purifying catalyst of the present invention comprises two or more types of NOx storage parts each containing oxide composites having different compositions,
The oxide composite includes at least one first metal element selected from the group consisting of alkali metals and alkaline earth metals, and at least one second metal element selected from the group consisting of rare earth metals. Contains
Of the first metal element relative to the sum of the peak area of the first metal element and the peak area of the second metal element obtained from TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the oxide composite. The standard deviation of the distribution of the ratio of peak areas ([peak area of the first metal element] / [sum of the peak area of the first metal element and the peak area of the second metal element]) is 10% or less, and
The two or more types of NOx occlusion parts contain the oxide complex having a small content molar ratio of the first metal element to the second metal element ([first metal] / [second metal]). It is arranged so as to come into contact with the exhaust gas in order from the NOx storage part,
It is characterized by.

  Such an oxide composite includes at least one first metal element selected from the group consisting of alkali metals and alkaline earth metals, and at least one second metal element selected from the group consisting of rare earth metals. It contains. Examples of the alkali metal that can be selected as the first metal element include lithium, sodium, potassium, rubidium, and cesium. Examples of the alkaline earth metal that can be selected as the first metal element include barium, magnesium, calcium, and strontium. As such a first metal element, potassium, rubidium, cesium, and barium are preferable from the viewpoint that they have high basicity and can exhibit a sufficiently high NOx occlusion ability when an oxide is formed. More preferred. As such a 1st metal element, you may contain 1 type individually or in mixture of 2 or more types.

  Examples of the rare earth metal that can be selected as the second metal element include scandium, yttrium, lanthanum, cerium, praseodymium, and neodymium. Such a second metal element is preferably yttrium, lanthanum, or cerium, and more preferably cerium, from the viewpoint of having a relatively high specific surface area when an oxide is formed. As such a 2nd metal element, you may contain 1 type individually or in mixture of 2 or more types.

  The combination of the first metal element and the second metal element contained in such an oxide composite is not particularly limited. From the viewpoint of NOx storage capacity, Ba (first metal element) and Ce (second metal element) (Metal element) is preferably used in combination, and from the viewpoint of exhibiting SOx occlusion ability, Ba or Mg (first metal element) and Ce or Y (second metal element) are used in combination. It is preferable.

  In the present invention, the peak area of the first metal element and the peak area of the second metal element determined from TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the oxide composite. The standard deviation of the distribution of the ratio of the peak area of the first metal element to the sum ([peak area of the first metal element] / [sum of the peak area of the first metal element and the peak area of the second metal element]) is 10% It is as follows. In such TEM-EDX analysis, a TEM-EDX apparatus equipped with a conventionally known transmission electron microscope (TEM) and a conventionally known energy dispersive X-ray spectrometer (EDX analyzer) is used as a measuring apparatus. An apparatus (for example, a trade name “JEM-2010FES” manufactured by JEOL Ltd.) can be used as appropriate.

In such TEM-EDX analysis, first, using a TEM-EDX apparatus, in an arbitrary region on the surface of the oxide composite, energy dispersive fluorescence X within a 10 nm square measurement point is used. Determine the line spectrum. Next, from the obtained energy dispersive X-ray fluorescence spectrum, the peak area derived from the first metal element and the peak area derived from the second metal element are obtained, and the peak area of the first metal element is obtained. The ratio (area ratio) of the peak area of the first metal element to the sum of the peak area of the second metal element and the sum of the peak area of the second metal element is determined. In addition, such a peak area ratio is represented by the formula:
[Peak Area Ratio (%)] = [Peak Area of First Metal Element] / [Sum of Peak Area of First Metal Element and Peak Area of Second Metal Element] × 100
Can be calculated by calculating. The term “peak” as used herein refers to a peak having a height difference of 1 cts or more from the baseline of the spectrum to the peak top. “Peak area” refers to the area between the baseline and the peak, and can be determined using commercially available software (for example, “Origin” manufactured by OriginLab). Such a peak appears, for example, at a position where energy is 4.5 KeV (BaLα line) when the metal element is barium, and at a position where 4.8 KeV (CeLα line) when the metal element is cerium. ).

  In addition, the standard deviation of the distribution of the peak area ratio is determined by performing a TEM-EDX analysis at any 20 or more measurement points to obtain the above peak area ratio at each measurement point. Based on the value, it can be obtained by calculating a standard deviation (%) of the distribution of the peak area ratio. In the oxide composite according to the present invention, the standard deviation of such peak area ratio is 10% or less (more preferably 9% or less). Such an oxide composite having a standard deviation of more than 10% is not a sufficiently uniform mixture of the first metal element and the second metal element, and sufficiently suppresses sulfur poisoning. In addition, when the regeneration treatment is performed after sulfur poisoning, the desorption rate of the sulfur component is not sufficient.

  As such an oxide composite, the value of the molar ratio of the first metal element to the second metal element ([first metal] / [second metal]: It is preferable that it is in the range of 0.01 to 10, and more preferably in the range of 0.1 to 5. In the oxide complex in which the value of the molar ratio of the first metal element is less than the lower limit and exceeds the upper limit, the first metal element and the second metal element are not sufficiently uniformly compounded, There is a tendency that sufficient NOx storage capacity cannot be exhibited.

  Further, the NOx occlusion part according to the present invention is not particularly limited as long as it contains the oxide complex, and even if it is composed of only the oxide complex, the oxide complex is supported by the carrier. It may be carried on the surface. The shape of such an oxide composite is not particularly limited, and may be a powder shape, a pellet shape, or the like.

  When such an oxide composite is in the form of powder, the average particle size of the powdered oxide composite is preferably 0.1 to 20 μm, and preferably 1 to 10 μm. More preferred. When the average particle size is less than the lower limit, handling tends to be difficult. On the other hand, when the average particle size exceeds the upper limit, the contact property with the reaction gas decreases, and the NOx storage performance tends to decrease. In addition, such an average particle diameter can be calculated | required by observing with a scanning electron microscope (SEM), and taking the particle size distribution of arbitrary 100 particles.

Furthermore, when such an oxide composite is in a powder form, the specific surface area of the oxide composite is preferably 5 to 200 m ² / g. When such a specific surface area is less than the lower limit, the number of surface sites effective for the reaction tends to be insufficient. On the other hand, when the upper limit is exceeded, thermal stability is lowered and the specific surface area tends to be lowered.

  In the case where the NOx occlusion part is one in which an oxide composite is supported on a carrier, the oxide composite is preferably supported on the carrier in the form of finely divided particles. The average particle size of such oxide composite-supported particles is preferably 0.5 to 10 nm, and more preferably 0.5 to 5 nm. If the particle size of the oxide composite is less than the lower limit, the uniformity of the dispersion of the first and second metal elements in the oxide composite tends to be reduced. On the other hand, if the upper limit is exceeded, NOx is occluded. The number of effective surface sites tends to decrease. Note that the average particle diameter of such supported particles can be obtained by XRD measurement or TEM observation.

  Furthermore, the amount of the oxide composite supported on such a carrier is preferably 0.1 to 60% by mass (more preferably 5 to 40% by mass) with respect to the total mass of the NOx storage part. If the amount of the oxide composite supported is less than the lower limit, it tends to be difficult to exert the desired performance in the obtained NOx storage part. On the other hand, if the upper limit is exceeded, the specific surface area of the catalyst decreases. Therefore, NOx occlusion performance tends to decrease.

  Further, the carrier capable of supporting such an oxide composite is not particularly limited, but is a basic support because the oxide composite can be supported in a highly dispersed state. Preferably, a support containing an oxide of at least one third metal element selected from the group consisting of aluminum, zirconium, alkaline earth metals and rare earth metals is more preferable. By using such a basic support, it becomes possible to suppress the reaction between the first metal element in the oxide composite and the support, and the decomposition of the oxide composite is sufficiently suppressed to be sufficiently high. The catalyst activity tends to be maintained.

  Examples of the alkaline earth metal element that can be selected as the third metal element include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). From the viewpoint that the dispersibility of the oxide composite is further improved, and that when the noble metal is supported, the interaction between the noble metal and its oxide tends to be strong and the affinity tends to be high Therefore, it is preferable to use Mg, Ca, and Ba, and it is particularly preferable to use Mg. As rare earth elements, yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Ga), terbium (Tb), dysprosium (Dy) ), Ytterbium (Yb), lutetium (Lu), etc. Among them, La, Ce, Nd, Pr, Y, from the viewpoint that the interaction with the noble metal and its oxide tends to be strong and has a high affinity. Sc is preferable, and La, Ce, Y, and Nd are more preferable. Such rare earth elements and alkaline earth metal elements having low electronegativity have a strong interaction with the oxide composite and the noble metal. Therefore, when a noble metal is supported, transpiration and sintering of the noble metal can be suppressed, and deterioration of the noble metal serving as an active point can be sufficiently suppressed.

Examples of the carrier containing the oxide of the third metal element include alumina, zirconia, ceria and zirconia composite oxide, alumina and magnesia composite oxide, and the like. Among the carriers containing such an oxide of the third metal element, from the viewpoint of being a stronger basic carrier, it is selected from the group consisting of zirconia and / or alumina, an alkaline earth metal element, and a rare earth element. A support containing a composite oxide with at least one element is preferable, and a composite oxide of alumina and magnesia is more preferable. Such a composite oxide of alumina and magnesia is represented by the formula: MgO-nAl ₂ O ₃ , and the value of n can be various numerical values. When n is less than 1, free MgO is present and the heat resistance tends to decrease. On the other hand, when n exceeds 1, the composite oxide of alumina and magnesia becomes a two-phase system of MgO—xAl ₂ O ₃ and yAl ₂ O ₃ (1 ≦ x, x + y = n), and Al ₂ O ₃ and The reaction with the first metal element in the composite oxide tends to occur easily. Therefore, as the carrier containing the oxide of the third metal element, a carrier containing a metal oxide represented by the composition formula: MgAl ₂ O ₄ is particularly preferable.

Moreover, in such a support | carrier, in addition to the oxide of the said 3rd metal element, the oxide of another metal element can further be contained. Such other metal elements are not particularly limited, and known metal oxides that can be contained in the carrier of the exhaust gas purifying catalyst can be used as appropriate. Among these, titanium, silicon, iron, An oxide of a fourth metal element selected from the group consisting of manganese and tungsten is preferred. Therefore, such a carrier may contain, for example, a composite oxide of titania (TiO ₂ ) and zirconia (ZrO ₂ ), a composite oxide of titania (TiO ₂ ) and ceria (CeO ₂ ), and the like. .

Further, in such carriers, it is preferable to contain composite oxide of TiO ₂ and ZrO ₂ The _(TiO 2 -ZrO ₂₎ further. Such _TiO 2 -ZrO _2, since the conjunction ZrO ₂ forms a solid solution in the anatase phase of TiO ₂ TiO ₂ is dissolved in the tetragonal phase of ZrO _2, mutually dissolved TiO ₂ and ZrO ₂ are mutually High thermal stability. Therefore, when such TiO ₂ —ZrO ₂ is contained, the decrease in specific surface area tends to be suppressed even when exposed to high temperatures. In addition, in such TiO ₂ —ZrO ₂ , since the solid solution as described above is formed, solid-phase reaction with the oxide complex is suppressed, and sulfur poisoning suppression which is a function of TiO ₂ is suppressed. The action tends to be promoted. Therefore, it becomes possible to further improve heat resistance and sulfur poisoning durability by including a composite oxide of titania (TiO ₂ ) and zirconia (ZrO ₂ ). Although such TiO ₂ and the content ratio of TiO ₂ and ZrO ₂ in the composite oxide of ZrO ₂ is not particularly limited, the range of _{TiO 2 / ZrO 2 = 95 /} 5~5 / 95 in mass ratio Is preferred. When TiO ₂ is less than this range, the sulfur poisoning suppressing action is lowered, and when ZrO ₂ is less than this range, the heat resistance is lowered.

  In addition, the content of the oxide of the third metal element in such a carrier is not particularly limited, but is preferably 1 to 95% by mass, and more preferably 3 to 80% by mass. If the content of the oxide of the third metal element is less than the lower limit, the heat resistance tends to decrease and sufficient NOx occlusion performance tends not to be obtained. In terms of improving sulfur poisoning durability, it tends to be difficult to receive the merits of the combined use with other carriers.

Furthermore, the content of the composite oxide of TiO ₂ and ZrO ₂ in such a carrier is not particularly limited, but is preferably 5 to 80% by mass, and more preferably 30 to 70% by mass. . In such a TiO ₂ and less than the content limit of the composite oxide of ZrO _2, tend to the effect of suppressing the decrease of the specific surface area of the support of a composite oxide of TiO ₂ and ZrO ₂ is not sufficiently obtained On the other hand, if the upper limit is exceeded, heat resistance and catalytic activity tend to be reduced.

Moreover, such a composition formula described above: In a carrier containing a metal oxide represented by MgAl ₂ O _4, the content of the metal oxide represented by MgAl ₂ O ₄ is, based on the total weight of the carrier It is preferable that it is 10-90 mass%, and it is more preferable that it is 30-70 mass%. When the content of the metal oxide represented by MgAl ₂ O ₄ in such a support is less than the lower limit, the effect obtained by the metal oxide represented by MgAl ₂ O ₄ when the oxide composite is supported. On the other hand, if the upper limit is exceeded, there is a tendency that it is difficult to receive the merit of combined use with other carriers in terms of expansion of the active temperature range and improvement of sulfur poisoning durability.

In addition, when a carrier in which a metal oxide represented by MgAl ₂ O ₄ and a composite oxide of TiO ₂ and ZrO ₂ are used as the carrier, a metal oxide represented by MgAl ₂ O ₄ and The mixing ratio with the composite oxide of TiO ₂ and ZrO ₂ is not particularly limited, but the ratio represented by the mass ratio of MgAl ₂ O ₄ : TiO ₂ —ZrO ₂ should be in the range of 4: 1 to 1: 4. Is preferred. If the content ratio of the metal oxide represented by MgAl ₂ O ₄ is less than the above lower limit, the heat resistance and catalytic activity tend to decrease. On the other hand, if the content exceeds the upper limit, the sulfur poisoning suppression effect tends to decrease. is there.

  Furthermore, the shape of such a carrier is not particularly limited, but it is preferably in the form of a powder because the specific surface area is improved and higher catalytic activity is obtained. When such a carrier is in powder form, the particle size (secondary particle size) of the carrier is not particularly limited, and is preferably 1 to 100 μm. If the particle diameter is less than the lower limit, it tends to be costly and difficult to handle, and on the other hand, if the upper limit is exceeded, the substrate for exhaust gas purification of the present invention will be described later. It tends to be difficult to stably form a catalyst coating layer.

The specific surface area of such carriers is preferably at ^{20 m} 2 / g or more, further preferably 50 to 300 m ² / g. When the specific surface area is less than the lower limit, it tends to be difficult to support a metal having an appropriate amount of redox ability (preferably a noble metal) in order to exhibit sufficient catalytic activity, When the upper limit is exceeded, the specific surface area decreases due to thermal degradation. Such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isotherm adsorption equation.

  The method for producing such a carrier is not particularly limited, and a known method can be appropriately employed. For example, when producing a support containing two or more metal elements, an aqueous solution containing two or more metal elements containing at least a salt of the third metal element (for example, nitrate) is used. A method of producing a support containing the oxide of the third metal element by forming a coprecipitate of the metal element in the presence, filtering, washing and drying the obtained coprecipitate and further firing. Can be adopted.

  In the case where the NOx occlusion part is in a form in which the oxide composite is supported on a support, it is preferable that a metal having oxidation-reduction ability is further supported on the support. Examples of such a metal having redox ability include noble metals such as platinum, rhodium, palladium, osmium, iridium, gold, silver, copper, cobalt, iron, and the like. Among such metals having oxidation-reduction ability, noble metals such as platinum, rhodium and palladium are preferable from the viewpoint that the obtained exhaust gas purifying catalyst exhibits higher catalytic activity.

  In the case where a metal having oxidation-reduction ability is supported on the carrier, the amount of the metal having oxidation-reduction ability is 0.05 to 10% by mass with respect to the total mass of the oxide composite and the carrier ( More preferably, the content is 0.1 to 5% by mass. If the amount of the metal having the redox ability is less than the lower limit, the catalytic activity obtained by the metal having the redox ability tends to be insufficient, while if the upper limit is exceeded, the cost increases and the cost increases. There is a tendency that grain growth of a metal having oxidation-reduction ability tends to occur.

  In the case where a metal having redox ability is supported on the carrier, the metal having redox ability is preferably supported on the carrier in the form of finer particles. The particle diameter of the metal having such redox ability is more preferably 1 to 20 nm, and more preferably 1 to 10 nm. If the particle diameter of the metal having the redox capacity is less than the lower limit, it tends to react with the support and oxidize and deactivate, whereas if it exceeds the upper limit, it becomes difficult to obtain a high catalytic activity. There is a tendency.

  The method for supporting the metal having the oxidation-reduction ability on the carrier is not particularly limited. For example, when the metal having the oxidation-reduction ability is a noble metal, a salt of a noble metal (for example, a dinitrodiamine salt), A method in which an aqueous solution containing a complex (for example, a tetraammine complex) is brought into contact with the carrier, dried, and further baked can be employed.

  Moreover, although the manufacturing method of the NOx storage part concerning this invention is not restrict | limited in particular, It is preferable to employ | adopt the method as shown below. That is, as a suitable method for producing such a NOx occlusion part, a first compound containing the first metal element, a second compound containing the second metal element, and a multidentate ligand are used. A method for producing a NOx occlusion part made of an oxide composite can be exemplified by firing an oxide composite precursor aqueous solution containing ammonia and a third compound having an oxide composite. . When the oxide composite is supported on the carrier in the NOx occlusion part, the oxide complex precursor aqueous solution is supported on the carrier, and then calcined to form the oxide composite. A method of supporting the carrier may be employed. The method for supporting the oxide composite on the support is not particularly limited as long as it is a method capable of supporting the oxide composite of the present invention on the support, but the oxide of the present invention is not particularly limited. It is preferable to employ a method of impregnating a carrier with an aqueous solution of a complex precursor and baking it.

  In addition, the 1st calculated | required from the TEM-EDX analysis in the measurement point of 20 or more of 10 nm squares on the surface of the said oxide composite by employ | adopting the method of baking such oxide composite precursor aqueous solution. Ratio of peak area of first metal element to sum of peak area of metal element and peak area of second metal element ([peak area of first metal element] / [peak area of first metal element and second metal element] The standard deviation of the distribution of the peak area])) can be 10% or less. The reason why the oxide composite according to the present invention in which the first metal element and the second metal element are sufficiently uniformly mixed and finely combined can be formed by adopting such a method is not necessarily the reason. Although not certain, the present inventors speculate as follows. That is, in such an oxide complex precursor aqueous solution, since the third compound having a polydentate ligand and ammonia are contained together with the first and second compounds, In the above, the first compound and the second compound interact with each other via the polydentate ligand and are held in close proximity. In addition, since such an oxide complex precursor aqueous solution contains ammonia, the first compound and the second compound have a high concentration without causing precipitation in the aqueous solution, and It can be held in a stable state. When such an oxide complex precursor aqueous solution is fired, the bulky multidentate ligand that exists as a ligand is oxidatively decomposed, and the first compound and the second compound are sufficiently fine. The present inventors infer that they are combined in a converted state.

  Examples of the first compound include hydroxides, acetates, carbonates and nitrates of the first metal element. In addition, you may use such 1st compound individually by 1 type or in mixture of 2 or more types. Examples of the second compound include hydroxides, acetates, carbonates and nitrates of the second metal element. As such a second compound, it is more preferable to use acetate of the second metal element from the viewpoint of having a relatively high specific surface area when an oxide is formed. In addition, you may use such a 2nd compound individually by 1 type or in mixture of 2 or more types.

  Furthermore, the polydentate ligand of the third compound refers to a ligand that can coordinate with two or more coordination groups. Examples of the third compound include polyvalent carboxylic acids such as citric acid and oxalic acid, diols such as glycol and pinacol, diamines such as ethylenediamine, and esters having two carbonyl groups such as ethyl acetoacetate. Is mentioned. Such third compounds include citric acid, oxalic acid, succinic acid, maleic acid, malic acid, adipic acid, tartaric acid, malonic acid, fumaric acid, aconitic acid, glutaric acid, ethylenediaminetetraacetic acid, lactic acid, glycolic acid. , Glyceric acid, salicylic acid, mevalonic acid, ethylenediamine, ethyl acetoacetate, malonic acid ester, glycol and pinacol are preferred, and among them, citric acid, lactic acid, malic acid, tartaric acid, glycol from the viewpoint of being a carboxylic acid having a hydroxy group. Acid and salicylic acid are more preferable, and citric acid is particularly preferable from the viewpoint that a bulky and finer oxide complex can be produced. In addition, you may use such 3rd compound individually by 1 type or in mixture of 2 or more types.

  The solvent containing the first compound, the second compound, the third compound, and the ammonia is water (preferably pure water such as ion-exchanged water and distilled water).

  Further, the contents of the first and second compounds are not particularly limited, and can be appropriately changed in accordance with the design of the NOx occlusion unit. Moreover, as content of a 3rd compound, 2-15 times (more preferably 2.5-10 times) molar amount with respect to the total molar amount of the metallic element in a 1st compound and a 2nd compound It is preferable that When the content of the third compound is less than the lower limit, the content of the polydentate ligand with respect to the first compound and the second compound is decreased, so that the complex cannot be formed in the aqueous solution. It tends to be difficult to hold the first compound and the second compound close to each other. On the other hand, if the upper limit is exceeded, the concentration of the first compound and the second compound is decreased, so Efficiency tends to decrease.

  The ammonia content is not particularly limited, and is preferably adjusted as appropriate so that precipitation does not occur in the resulting aqueous solution, and is preferably 0.4 mol / L or more (particularly preferably 2.0 mol / L). L to 10 mol / L) is more preferable. When the ammonia content is less than the lower limit, precipitation tends to occur in the resulting aqueous solution. On the other hand, when the upper limit is exceeded, the concentration of the first compound and the second compound is decreased, thereby supporting efficiency. Tend to decrease. Furthermore, the ammonia content is preferably 2 times or more (more preferably 2.8 to 5 times) in terms of mole relative to the content of the third compound. If the ammonia content is less than twice the molar content of the third compound, it is difficult to contain the first compound and the second compound in a stable state, and precipitation tends to occur. There is a tendency.

  Moreover, as a method for producing such an oxide complex precursor aqueous solution, it is possible to dissolve the first compound, the second compound, the third compound, and ammonia in water. The method is not particularly limited. As a suitable method for producing such an oxide complex precursor aqueous solution, for example, after dissolving the third compound in water, the second compound is added to form a precipitate, and then Ammonia was added to dissolve the precipitate again, and then the first compound was further added to the solution and stirred. After the precipitate was formed, the solution was dissolved again to obtain an oxide complex precursor aqueous solution. The method of manufacturing is mentioned. Alternatively, an oxide complex precursor aqueous solution may be manufactured using an ammonium salt (for example, ammonium citrate) obtained by combining a third compound and ammonia in advance.

  In addition, the firing conditions of the oxide complex precursor aqueous solution are not particularly limited. For example, in the air, heating is preferably performed in a temperature range of 300 ° C. to 600 ° C., and a temperature condition of 300 ° C. to 500 ° C. It is more preferable to heat with. Further, the heating time varies depending on the heating temperature and cannot be generally described, but it can be set to about 1 to 3 hours. And by baking the oxide complex precursor aqueous solution in this way, the oxide complex can be manufactured, and the obtained oxide complex is on the surface of the oxide complex. Ratio of the peak area of the first metal element to the sum of the peak area of the first metal element and the peak area of the second metal element obtained from TEM-EDX analysis at 20 or more measurement points of 10 nm square ([first metal The standard deviation of the distribution of the peak area of the element / the sum of the peak area of the first metal element and the peak area of the second metal element] is 10% or less.

  Further, the NOx occlusion unit according to the present invention may be supported on a catalyst base material. Such a catalyst substrate is not particularly limited, and a DPF substrate, a monolith substrate, a pellet substrate, a plate substrate and the like can be suitably used. The material of such a catalyst substrate is not particularly limited, but a substrate made of a ceramic such as cordierite, silicon carbide, mullite, or a substrate made of a metal such as stainless steel including chromium and aluminum is preferable. Adopted. Further, the method for supporting the NOx occlusion part on such a catalyst substrate is not particularly limited. For example, after the carrier is supported on a monolithic substrate and a coat layer made of the carrier powder is formed, A method in which a metal having an oxidation-reduction ability (preferably a noble metal) is supported on the coating layer, and then an oxide composite is supported on the coating layer and a NOx occlusion portion is supported on the catalyst base, Using a carrier carrying a metal having a function, and carrying this on a monolithic substrate to form a coat layer, and then carrying the oxide composite on the coat layer to carry the NOx occlusion part, For example, a method of preparing a NOx occlusion portion in advance and coating and supporting the NOx occlusion portion on the catalyst base can be appropriately employed.

  Moreover, the exhaust gas purifying catalyst of the present invention includes two or more kinds of the NOx occlusion parts each containing the oxide composites having different compositions. That is, in the exhaust gas purifying catalyst of the present invention, the value of the molar ratio of the first metal element to the second metal element of each oxide composite contained in the two or more types of NOx storage portions is different. . Therefore, in the exhaust gas purifying catalyst of the present invention, the base strength of each oxide complex is different. Here, when examining the relationship between the base strength and the NOx purification activity, the lower the base strength, the better the NOx purification activity in the low temperature range, and the stronger the base strength, the better the NOx purification activity in the high temperature range. There is a relationship. The exhaust gas purifying catalyst of the present invention includes two or more kinds of NOx occlusion parts containing two or more kinds of oxide complexes having different base strengths, and therefore has sufficiently high activity in a wider temperature range. It is possible to show.

  Further, in the plurality of oxide composites contained in the two or more types of NOx storage parts, the maximum value of the molar ratio of the first metal element in the oxide composite is the first value of the oxide composite. It is preferably at least twice (more preferably 3 to 6 times) the minimum value of the molar ratio of one metal element. By setting the maximum value of the content molar ratio of the first metal element ([first metal] / [second metal]) to be twice or more the minimum value, the obtained exhaust gas purification catalyst is wider. It tends to have a sufficiently high activity in the temperature range.

  Further, the minimum value of the molar ratio of the first metal element is preferably in the range of 0.01 to 1 (more preferably 0.1 to 1), and the maximum molar ratio of the first metal element is preferable. The value is preferably in the range of 0.1 to 10 (more preferably 1 to 5). If the minimum value of the first metal element is less than the lower limit, it tends to be difficult to make the oxide composite uniform. On the other hand, if the upper limit is exceeded, the NOx occlusion performance is sufficient in a low temperature range. Tend not to be obtained. Further, if the maximum value of the molar ratio of the first metal element is less than the lower limit, sufficient NOx occlusion performance tends to be not obtained in a high temperature range. On the other hand, if the upper limit is exceeded, the oxide composite becomes uniform. Tend to be difficult.

  Further, in the exhaust gas purifying catalyst of the present invention, the content ratio of the two types of NOx occlusion parts respectively containing two types of oxide composites that differ in the content molar ratio of the first metal element by two times or more is Although not limited, the mass ratio ([the mass of the NOx occlusion part including the oxide composite having the larger value of the first metal element content molar ratio] / [the smaller value of the content molar ratio of the first metal element] The mass of the NOx occlusion part containing the oxide complex]] is preferably 0.2 to 5 (more preferably 0.5 to 2).

  Furthermore, in the exhaust gas purifying catalyst of the present invention, the total amount of the first metal element contained in the catalyst is 0.01 to 1.0 mol / L per 1 L of the catalyst substrate (more preferably 0.1 to 0.00). 6 mol / L). If the total amount of the first metal element is less than the lower limit, the number of NOx storage sites is not sufficient, and the activity tends to decrease. On the other hand, if the upper limit is exceeded, fine complexation with the second metal element is caused. It tends to be difficult.

  In the exhaust gas purifying catalyst of the present invention, the total amount of the second metal element contained in the catalyst is 0.01 to 1.0 mol / L (more preferably 0.1 to 0.6 mol / L). Preferably there is. When the total amount of the second metal element is less than the lower limit, the number of NOx storage sites is not sufficient and the activity tends to decrease. On the other hand, when the upper limit is exceeded, fine complexation with the first metal element is caused. It tends to be difficult.

  Further, in the exhaust gas purifying catalyst of the present invention, two or more types of NOx occlusion parts each containing the oxide composites having different compositions are contained in a molar ratio of the first metal element to the second metal element ([ The first metal] / [second metal]) are arranged so as to come into contact with the exhaust gas in order from the NOx occlusion portion containing the oxide composite. Therefore, when purifying the exhaust gas, after the exhaust gas comes into contact with the NOx occlusion part containing the oxide composite having a smaller content molar ratio of the first metal element, the content molar ratio of the first metal element is more The exhaust gas sequentially comes into contact with the NOx occlusion portion containing a large oxide composite. Therefore, in the exhaust gas purifying catalyst, the NOx occlusion part having a smaller content molar ratio of the first metal element is easily exposed to the sulfur component in the exhaust gas, and the sulfur component is easily attached. On the other hand, the NOx occlusion part having a larger content molar ratio of the first metal element has a small contact probability with the sulfur component and is not easily poisoned with sulfur. Therefore, in the exhaust gas purifying catalyst of the present invention, it is possible to sufficiently exhibit the NOx occlusion ability even for the exhaust gas containing a sulfur component. Further, the NOx occlusion portion having a smaller content molar ratio of the first metal element as described above has a low base strength, and has high sulfur detachability when subjected to sulfur poisoning regeneration treatment. Therefore, the exhaust gas purifying catalyst of the present invention can maintain high NOx purification activity by performing sulfur poisoning regeneration treatment even after sulfur poisoning. Thus, the exhaust gas purifying catalyst of the present invention has sufficiently high sulfur poisoning durability due to the arrangement relationship of the NOx occlusion parts.

Further, in such an exhaust gas purifying catalyst, (i) the two or more types of NOx occlusion portions may include the oxide composite having a small content molar ratio of the first metal element to the second metal element. Or a multilayer structure in which the two or more types of NOx occlusion parts are laminated on the surface of the catalyst base material, arranged in order from the upstream side to the downstream side of the gas flow path from the contained NOx occlusion part And the two or more types of NOx occlusion parts are arranged in order from the NOx occlusion part containing the oxide composite having a small content molar ratio of the first metal element to the second metal element in order. Being arranged from the side toward the surface of the catalyst base,
Is preferred.

  Hereinafter, in order to explain in more detail the positional relationship of the NOx storage portion in the exhaust gas purification catalyst of the present invention, a preferred embodiment of the exhaust gas purification catalyst of the present invention will be described in detail with reference to the drawings. To do. In the following description and drawings, the same or corresponding elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

  FIG. 1 is a cross-sectional view schematically showing a cross-sectional state in a direction parallel to the gas flow direction of a preferred embodiment of the exhaust gas purifying catalyst of the present invention. The exhaust gas purifying catalyst of the embodiment shown in FIG. 1 includes a catalyst base 11 provided with a first NOx storage part and a catalyst base 12 provided with a second NOx storage part. Such an exhaust gas purifying catalyst 1 is connected to the exhaust gas pipe 13, and a catalyst base 11 having a first NOx occlusion part is disposed on the upstream side of the gas flow path, and the downstream side of the gas flow path. In addition, a catalyst base 12 having a second NOx occlusion portion is disposed. That is, in the exhaust gas purifying catalyst of the present embodiment, each NOx storage unit is arranged such that after the exhaust gas contacts the first NOx storage unit, the exhaust gas contacts the second NOx storage unit. The first NOx occlusion part has an oxidation with a smaller value of the molar ratio of the first metal element to the second metal element ([first metal] / [second metal]) than the second NOx occlusion part. It contains a product complex. Therefore, in this embodiment, from the upstream side of the gas flow path to the downstream side in order from the NOx occlusion part containing the oxide complex having a small content molar ratio of the first metal element to the second metal element. It is arranged toward. Therefore, in this embodiment, when exhaust gas is supplied to the exhaust pipe 13, the value of the molar ratio of the first metal element to the second metal element ([first metal] / [second metal]) is The NOx occlusion part containing the small oxide complex contacts the exhaust gas in order. Therefore, the exhaust gas purifying catalyst of such an embodiment has sufficiently high high temperature durability and sufficiently high sulfur poisoning durability and exhibits sufficiently high catalytic activity over a wide temperature range. Is possible. In addition, the arrow A in FIG. 1 has shown the direction through which waste gas flows.

  Moreover, the catalyst base materials 11 and 12 provided with such first and second NOx occlusion parts are respectively obtained by carrying the first and second NOx occlusion parts on a monolith base material, respectively. As in the embodiment shown in FIG. 1, in the case where the NOx occlusion unit is supported on a catalyst base material such as a monolith base material and the catalyst base material is sequentially arranged from the upstream side to the downstream side, The amount of the NOx occlusion part supported on the catalyst base is not particularly limited, but the sum of the first metal element and the second metal element is 0.02 to 2.0 mol / L (more preferably) per 1 L of the catalyst base. Is preferably 0.2 to 1.2 mol / L). If the supported amount of the NOx occlusion part with respect to such a catalyst base is less than the lower limit, it tends to be difficult to exhibit sufficient NOx purification performance. On the other hand, if the upper limit is exceeded, the specific surface area of the catalyst decreases. However, sufficient activity tends not to be obtained.

  The preferred embodiment of the exhaust gas purifying catalyst of the present invention has been described above with reference to FIG. 1. Next, another preferred embodiment of the exhaust gas purifying catalyst of the present invention will be described with reference to FIG. Will be described.

  FIG. 2 is a cross-sectional view schematically showing a cross-sectional state in a direction perpendicular to the gas flow direction of another preferred embodiment of the exhaust gas purifying catalyst of the present invention. The exhaust gas-purifying catalyst shown in FIG. 2 includes a catalyst base 20, a layered NOx storage part 21 disposed on the surface of the catalyst base, and a layered NOx storage part disposed on the layered NOx storage part 21. 22. That is, in the embodiment shown in FIG. 2, a multilayer structure is formed by the NOx storage part 21 and the NOx storage part 22 on the catalyst base 20. In addition, the NOx occlusion unit 22 arranged on the outer surface side is the first metal element with respect to the second metal element than the oxide composite in the NOx occlusion unit 21 arranged on the surface side of the catalyst base. Containing an oxide composite having a small content molar ratio ([first metal] / [second metal]). When such an exhaust gas purifying catalyst is brought into contact with exhaust gas, the inner NOx storage unit 21 comes into contact with the exhaust gas after the outer surface side NOx storage unit 22 comes into contact with the exhaust gas. Therefore, also in this embodiment, the NOx occlusion containing the oxide composite having a small content molar ratio of the first metal element to the second metal element ([first metal] / [second metal]). The exhaust gas comes in contact with the parts in order. Therefore, even in the exhaust gas purifying catalyst having such a multilayer structure, it has sufficiently high temperature durability and sufficiently high sulfur poisoning durability, and has sufficiently high catalytic activity over a wide temperature range. It is possible to show.

  In the present embodiment, a monolith substrate is used as the catalyst substrate 20. In addition, in the case where a layered NOx occlusion part is laminated on such a catalyst base material to form a multilayer structure, the sum of the first metal element and the second metal element supported on the catalyst base material 20 is It is preferable that it is 0.02-2.0 mol / L (more preferably 0.2-1.2 mol / L) per liter of catalyst base material. If the supported amount of the NOx occlusion part with respect to such a catalyst base is less than the lower limit, it tends to be difficult to exhibit sufficient NOx purification performance. On the other hand, if the upper limit is exceeded, the specific surface area of the catalyst decreases. However, sufficient activity tends not to be obtained.

  The preferred embodiment of the exhaust gas purifying catalyst of the present invention has been described above with reference to FIGS. 1 and 2, but the exhaust gas purifying catalyst of the present invention is not limited to the above embodiment. For example, in the embodiment shown in FIG. 1, the catalyst bases 11 and 12 are arranged apart from each other. However, in the present invention, the catalyst bases 11 and 12 are arranged in close contact with each other. Also good. Further, in the embodiment shown in FIG. 1, the first and second NOx occlusion parts are carried on the monolith substrate, but in the present invention, the first and second NOx occlusion parts may be carried on the substrate of another form. May be arranged by forming each NOx occlusion part into a pellet shape without supporting it on the catalyst base. In the embodiment shown in FIG. 1 and FIG. 2, two types of NOx occlusion parts are arranged, but in the present invention, the number of NOx occlusion parts may be two or more, in particular. Not limited.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

(Synthesis Example 1: Preparation of oxide complex precursor aqueous solution (I))
Citric acid (manufactured by Wako Pure Chemical Industries) 0.9 mol was dissolved in 165 ml of ion exchange water, and 170 ml of 28% by mass NH ₃ water was added to prepare an aqueous ammonium citrate solution. Next, 0.15 mol of barium acetate (manufactured by Wako Pure Chemical Industries, Ltd.) and 0.3 mol of cerium acetate (manufactured by Wako Pure Chemical Industries, Ltd.) are added to the obtained aqueous solution of ammonium citrate and stirred to oxidize containing Ba and Ce. A compound complex precursor aqueous solution (I) was prepared. The Ba / Ce molar ratio in the aqueous solution (I) is 0.5.

(Synthesis Example 2: Preparation of oxide complex precursor aqueous solution (II))
Citric acid (manufactured by Wako Pure Chemical Industries) 0.9 mol was dissolved in 165 ml of ion exchange water, and 170 ml of 28% by mass NH ₃ water was added to prepare an aqueous ammonium citrate solution. Next, 0.3 mol of barium acetate (manufactured by Wako Pure Chemical Industries) and 0.15 mol of cerium acetate (manufactured by Wako Pure Chemical Industries) are added to the obtained aqueous solution of ammonium citrate and stirred to contain Ba and Ce. An aqueous oxide complex precursor solution (II) was prepared. The Ba / Ce molar ratio in the aqueous solution (II) is 2.

(Synthesis Example 3: Preparation of platinum-supported carrier)
After using alumina, titania-zirconia composite oxide, Rh-supported zirconia on which 1.0% by mass of Rh is supported and ceria-zirconia composite oxide, the support is dispersed in 500 ml of ion-exchanged water. Then, the dinitrodiammine platinum nitric acid solution was added so that the amount of platinum (Pt) supported was 2 g with respect to 200 g of the carrier, and the dinitrodiammine platinum nitric acid solution was adsorbed and supported on the carrier. Thereafter, the carrier carrying the dinitrodiammine platinum nitrate solution was calcined in the atmosphere at 300 ° C. for 3 hours to prepare a platinum carrier. In addition, as the carrier, those containing 100 g of titania-zirconia composite oxide, 50 g of Rh-supporting zirconia, and 20 g of ceria-zirconia composite oxide with respect to 100 g of alumina were used.

(Synthesis Example 4: Preparation of Oxide Complex-Supported Carrier (I))
The platinum-supported carrier obtained in Synthesis Example 3 is mixed with the oxide complex precursor aqueous solution (I) obtained in Synthesis Example 1 by the number of moles of Ba and Ce in the oxide complex with respect to 200 g of the platinum-supported carrier. And then impregnated and supported so that the sum becomes 0.4 mol, and then heated to 100 ° C. to remove moisture, and baked in the atmosphere at 300 ° C. for 3 hours, on the platinum-supported carrier. One oxide composite was supported to prepare an oxide composite support (I). The content molar ratio of Ba (first metal element) to Ce (second metal element) in the first oxide composite ([first metal] / [second metal]) is 0.5. is there.

(Synthesis Example 5: Preparation of oxide composite-supported carrier (II))
In the same manner as in Synthesis Example 4 except that the oxide complex precursor aqueous solution (I) obtained in Synthesis Example 2 was used instead of the oxide complex precursor aqueous solution (I) obtained in Synthesis Example 1. Then, the second oxide composite was supported on the platinum-supported carrier to prepare an oxide composite-supported carrier (II). The content molar ratio of Ba (first metal element) to Ce (second metal element) in the second oxide composite ([first metal] / [second metal]) is 2.

Example 1
First, the hexagonal cell cordierite monolith substrate having a diameter of 30 mm × 25 mmL (17.5 cc) was coated with the oxide composite-supported carrier (I) obtained in Synthesis Example 4 so as to be 270 g / L, A catalyst base material (I) on which one NOx occlusion part (oxide composite support (I)) was supported was obtained. In the catalyst base material (I) on which the first NOx occlusion part was supported, the supported amounts of Ba and Ce were 0.13 mol / L and 0.27 mol / L, respectively.

  Next, another hexagonal cell cordierite monolith substrate having a diameter of 30 mm × 25 mmL (17.5 cc) was coated with the oxide complex-supported carrier (II) obtained in Synthesis Example 5 so as to be 270 g / L, A catalyst base (II) on which the second NOx occlusion part (oxide composite support (II)) was supported was obtained. In addition, in the catalyst base material (II) on which the second NOx occlusion portion was supported, the supported amounts of Ba and Ce were Ba and Ce were 0.27 mol / L and 0.13 mol / L, respectively. .

  Next, the catalyst base (I) carrying the first NOx storage part and the catalyst base (II) carrying the second NOx storage part are combined, and the exhaust gas purification catalyst as shown in FIG. Manufactured. That is, the catalyst base material (I) carrying the first NOx occlusion portion is disposed at the position of the catalyst base material 11 upstream of the exhaust gas flow channel shown in FIG. 1 (hereinafter sometimes referred to as “previous stage”). Then, the catalyst substrate (II) carrying the second NOx occlusion portion is disposed at the position of the catalyst substrate 12 on the downstream side of the exhaust gas passage shown in FIG. Thus, the exhaust gas purifying catalyst of the present invention was produced. Such an exhaust gas purifying catalyst has a catalyst capacity of 35 cc, and the total supported amount of Ba and Ce in the catalyst are both 0.2 mol / L.

(Example 2)
An exhaust gas purification catalyst as shown in FIG. 2 was produced. That is, first, a hexagonal cell cordierite monolith substrate having a diameter of 30 mm × 50 mmL (35 cc) is used as the catalyst substrate 20, and the oxide composite-supported carrier (II) obtained in Synthesis Example 5 is set to 135 g / L. And the layered NOx occlusion portion 21 made of the oxide composite-supported support (II) was formed on the catalyst base 20 by firing at 500 ° C. in the atmosphere. Next, the catalyst base material 20 on which the NOx occlusion unit 21 is supported is coated with the oxide composite-supported carrier (I) obtained in Synthesis Example 4 so as to have a concentration of 135 g / L. The NOx occlusion part 22 is laminated on the upper layer of the NOx occlusion part 21, the NOx occlusion part 22 made of the oxide composite-supported carrier (I) is disposed on the outer surface, and the oxide is disposed on the inner side. An exhaust gas purifying catalyst having a multilayer structure in which the NOx occlusion portion 21 made of the composite carrier (II) is disposed was obtained. Such an exhaust gas purifying catalyst has a catalyst capacity of 35 cc, and the total supported amount of Ba and Ce in the catalyst are both 0.2 mol / L.

(Comparative Example 1)
The catalyst base material (II) on which the second NOx occlusion portion is supported is disposed at the position (previous stage) of the catalyst base material 11 upstream of the exhaust gas flow path shown in FIG. 1, and the exhaust gas flow path shown in FIG. Exhaust gas purifying catalyst for comparison in the same manner as in Example 1 except that the catalyst substrate (I) carrying the first NOx occlusion portion is arranged at the position (the latter stage) of the catalyst substrate 12 on the downstream side. Manufactured. Such an exhaust gas purifying catalyst has a catalyst capacity of 35 cc, and the total supported amount of Ba and Ce in the catalyst are both 0.2 mol / L.

(Comparative Example 2)
In the same manner as in Example 2 except that the order in which the oxide composite support (I) and the oxide composite support (II) are supported is reversed, the oxide composite support (II) is formed on the outer surface side. A NOx occlusion part 22 made of the above and a NOx occlusion part 21 made of the oxide composite-supported carrier (I) on the inner side were obtained for a comparative multilayered exhaust gas purification catalyst. Such an exhaust gas purifying catalyst has a catalyst capacity of 35 cc, and the total supported amount of Ba and Ce in the catalyst are both 0.2 mol / L.

(Synthesis Example 6: Preparation of oxide complex precursor aqueous solution (III))
Citric acid (manufactured by Wako Pure Chemical Industries) 0.9 mol was dissolved in 165 ml of ion exchange water, and 170 ml of 28% by mass NH ₃ water was added to prepare an aqueous ammonium citrate solution. Next, 0.3 mol of barium acetate (manufactured by Wako Pure Chemical Industries, Ltd.) and 0.3 mol of cerium acetate (manufactured by Wako Pure Chemical Industries, Ltd.) were added to the obtained aqueous solution of ammonium citrate and stirred to oxidize containing Ba and Ce. A compound complex precursor aqueous solution (III) was prepared.

(Synthesis Example 7: Preparation of oxide complex-supported carrier (III))
In the same manner as in Synthesis Example 4 except that the oxide complex precursor aqueous solution (III) obtained in Synthesis Example 6 was used instead of the oxide complex precursor aqueous solution (I) obtained in Synthesis Example 1. Thus, an oxide composite was supported on the platinum-supported carrier obtained in Synthesis Example 3 to prepare an oxide composite-supported carrier (III).

(Comparative Example 3)
A hexagonal cell cordierite monolith substrate having a diameter of 30 mm × 50 mmL (35 cc) was coated with the oxide composite-supported carrier (III) obtained in Synthesis Example 7 so as to be 270 g / L. An exhaust gas purifying catalyst for comparison on which the oxide composite-supported carrier (III) was supported was obtained. Such an exhaust gas purifying catalyst has a catalyst capacity of 35 cc, and the total supported amount of Ba and Ce in the catalyst are both 0.2 mol / L.

(Synthesis Example 8: Preparation of oxide complex precursor aqueous solution (IV))
0.1 mol of barium acetate (manufactured by Wako Pure Chemical Industries) and 0.1 mol of cerium acetate (manufactured by Wako Pure Chemical Industries) were added to 300 ml of ion-exchanged water and stirred, and the oxide complex precursor aqueous solution containing Ba and Ce ( IV) was prepared.

(Synthesis Example 9: Preparation of oxide composite-supported carrier (IV))
In the same manner as in Synthesis Example 4, except that the oxide complex precursor aqueous solution (IV) obtained in Synthesis Example 8 was used instead of the oxide complex precursor aqueous solution (I) obtained in Synthesis Example 1. A complex of Ba and Ce was supported on the platinum-supported carrier to prepare an oxide composite-supported carrier (IV).

(Comparative Example 4)
A monolith was prepared in the same manner as in Comparative Example 3 except that the oxide composite-supported carrier (IV) obtained in Synthesis Example 9 was used instead of the oxide composite-supported carrier (III) obtained in Synthesis Example 7. An exhaust gas purifying catalyst for comparison in which an oxide composite-supported carrier (IV) was supported on a base material was obtained. Such an exhaust gas purifying catalyst has a catalyst capacity of 35 cc, and the total supported amount of Ba and Ce in the catalyst are both 0.2 mol / L.

[Characteristic Evaluation of Exhaust Gas Purification Catalysts Obtained in Examples 1-2 and Comparative Examples 1-4]
[TEM-EDX analysis]
Using the powder of each oxide composite-supported carrier scraped from each coat layer in the exhaust gas purifying catalyst obtained in Examples 1-2 and Comparative Examples 1-4, each was measured with a transmission electron microscope (TEM). The powder was observed, and EDX analysis was performed on 32 measurement points of 10 nm square on the surface of the oxide composite in the powder. For such EDX analysis, a trade name “JEM-2010FES” manufactured by JEOL Ltd. was used as a measuring device. From such an EDX analysis result, the peak area of Ba (peak area of BaLα line) and the peak area of Ce (peak area of CeLα line) at each measurement point are calculated, and the formula:
[Peak area ratio] = ([Ba peak area] / [Sum of Ba peak area and Ce peak area]) × 100
The area ratio of the peak area of Ba to the sum of the peak area of Ba and the peak area of Ce at each measurement point of the oxide complex in each powder was calculated. The standard deviation of the area ratio distribution of the oxide composite in each powder was determined from the area ratio value thus calculated. The obtained results are shown in Table 1. Table 1 shows the total supported amounts of Ba and Ce in the catalysts obtained in Examples 1 and 2 and Comparative Examples 1 to 4, and Ba relative to Ce (second metal element) in each oxide composite. It shows together with the value of the content molar ratio (Ba / Ce ratio) of (first metal element).

  As is clear from the results shown in Table 1, in the exhaust gas purifying catalysts obtained in Examples 1-2 and Comparative Examples 1-3, at the measurement points on the surface of the oxide composite in each coat layer. Ratio of the peak area of the first metal element to the sum of the peak area of the first metal element (Ba) and the peak area of the second metal element (Ce) obtained from the TEM-EDX analysis ([peak area of the first metal element ] / [The sum of the peak area of the first metal element and the peak area of the second metal element]) was confirmed to have a standard deviation of 10% or less. From these results, in the exhaust gas purifying catalysts obtained in Examples 1 and 2 and Comparative Examples 1 to 3, an oxide composite in which Ba and Ce are sufficiently uniformly mixed is contained in each NOx storage part. It was found that it was contained.

[High temperature durability test]
Using the exhaust gas purifying catalysts obtained in Examples 1 and 2 and Comparative Examples 1 to 4, lean / rich gases having the compositions shown in Table 2 were applied to each catalyst under the temperature condition of 750 ° C. = A high temperature durability test was conducted for 5 hours while changing at intervals of 110 seconds / 10 seconds.

[Sulfur poisoning endurance test]
Using the exhaust gas purifying catalysts obtained in Examples 1 and 2 and Comparative Examples 1 to 4 after the high temperature endurance test, for each catalyst, pretreatment with the composition shown in Table 3 under the temperature condition of 650 ° C. Pretreatment was performed by flowing a rich gas at a flow rate of 30 L / min for 10 minutes. Next, for each catalyst after such pretreatment, the rich gas and the lean gas for sulfur poisoning treatment (S poisoning treatment) shown in Table 3 under the temperature condition of 400 ° C. were set to lean / rich = 120 seconds. / 3 seconds at a flow rate of 30 L / min while fluctuating at intervals of 3 seconds, 3 g of sulfur per 2 L of catalyst was supplied, and S poisoning treatment was performed. Next, after each sulfur-treated catalyst was circulated for 5 minutes at a flow rate of 30 L / min, a lean gas for S regeneration treatment having a composition shown in Table 3 under a temperature condition of 600 ° C. A rich gas for S regeneration treatment having the composition shown in FIG. 1 was circulated at a flow rate of 30 L / min for 10 minutes to desorb the poisoned sulfur and perform catalyst regeneration treatment.

[NOx purification test 1]
Using the exhaust gas purifying catalysts obtained in Examples 1-2 and Comparative Examples 1-4 after the sulfur poisoning endurance test, the rich gas having the composition shown in Table 4 under the temperature condition of 300 ° C. for each catalyst. The lean gas was circulated at a flow rate of 30 L / min while varying the lean / rich = 120 seconds / 3 seconds interval, and the NOx purification rate in a steady state was measured. All space velocities (SV) were set to 51400 h ⁻¹ . Further, NOx purification rate, for each catalyst, the concentration of the NO _x contained in each of the front and rear of the gas that contacts the catalyst was measured to determine based on the value of the concentration of NO _x. The obtained results are shown in Table 5 and FIG.

[NOx purification test 2]
The NOx purification rate of each catalyst was measured by adopting the same method as in the NOx purification test 1 except that the temperature condition when circulating the rich gas and the lean gas having the composition shown in Table 4 was changed from 300 ° C to 400 ° C. The results obtained are shown in Table 5.

[NOx purification test 3]
The NOx purification rate of each catalyst was measured by adopting the same method as in the NOx purification test 1 except that the temperature condition when circulating the rich gas and the lean gas having the composition shown in Table 4 was changed from 300 ° C to 500 ° C. The obtained results are shown in Table 5 and FIG.

  As is apparent from the results shown in Table 5 and FIGS. 3 to 4, the NOx occlusion unit containing the oxide composite having a small Ba molar ratio with respect to Ce is disposed so as to contact the exhaust gas in order. In the exhaust gas purification catalyst obtained in Examples 1 and 2, compared with the exhaust gas purification catalyst obtained in Comparative Examples 1 to 4, a sufficiently high high temperature durability and a sufficiently high sulfur poisoning durability In addition, it was confirmed that the catalyst activity was sufficiently high over a wide temperature range.

  As described above, according to the present invention, it has a sufficiently high high temperature durability and a sufficiently high sulfur poisoning durability and can exhibit a sufficiently high catalytic activity over a wide temperature range. It is possible to provide an exhaust gas purifying catalyst.

  Therefore, the exhaust gas purifying catalyst of the present invention is particularly useful as a catalyst for purifying exhaust gas discharged from, for example, an internal combustion engine of an automobile.

It is sectional drawing which shows typically the state of the cross section of the direction parallel to the gas flow direction of suitable embodiment of the catalyst for exhaust gas purification of this invention. It is sectional drawing which shows typically the state of the cross section of the direction perpendicular | vertical to the gas flow direction of other suitable embodiment of the exhaust gas purification catalyst of this invention. It is a graph which shows the NOx purification rate of the catalyst for exhaust gas purification obtained in Examples 1-2 and Comparative Examples 1-4 on 300 degreeC temperature conditions. It is a graph which shows the NOx purification rate of the catalyst for exhaust gas purification obtained in Examples 1-2 and Comparative Examples 1-4 under the temperature condition of 500 degreeC.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Exhaust gas purification catalyst, 11 ... Catalyst base material provided with 1st NOx occlusion part, 12 ... Catalyst base material provided with 2nd NOx occlusion part, 13 ... Exhaust gas pipe, 20 ... Catalyst base material, 21 ... Catalyst base A layered NOx occlusion part disposed on the surface of the material, 22 ... a layered NOx occlusion part on the outer surface.

Claims (4)

Comprising two or more kinds of NOx occlusion parts each containing oxide composites having different compositions;
The oxide composite includes at least one first metal element selected from the group consisting of alkali metals and alkaline earth metals, and at least one second metal element selected from the group consisting of rare earth metals. Contains
Of the first metal element relative to the sum of the peak area of the first metal element and the peak area of the second metal element obtained from TEM-EDX analysis at 20 or more measurement points of 10 nm square on the surface of the oxide composite. The standard deviation of the distribution of the ratio of peak areas ([peak area of the first metal element] / [sum of the peak area of the first metal element and the peak area of the second metal element]) is 10% or less, and
The two or more types of NOx occlusion parts contain the oxide complex having a small content molar ratio of the first metal element to the second metal element ([first metal] / [second metal]). It is arranged so as to come into contact with the exhaust gas in order from the NOx storage part,
An exhaust gas purifying catalyst characterized by.   The two or more types of NOx occlusion parts are arranged in order from the upstream side of the gas flow path in order from the NOx occlusion part containing the oxide composite having a small content molar ratio of the first metal element to the second metal element. The exhaust gas-purifying catalyst according to claim 1, wherein the exhaust gas-purifying catalyst is disposed toward the side. The two or more types of NOx occlusion parts have a multilayer structure formed by laminating on the surface of the catalyst base, and the two or more types of NOx occlusion parts contain the first metal element with respect to the second metal element 2. The exhaust gas purification according to claim 1, wherein the exhaust gas purification unit is arranged from the outer surface side toward the surface side of the catalyst base in order from the NOx occlusion part containing the oxide composite having a small molar ratio. Catalyst.   The maximum value of the content molar ratio of the first metal element is two times or more the minimum value of the content molar ratio of the first metal element. The catalyst for exhaust gas purification as described.