JP2006223949A - Catalyst for cleaning exhaust gas and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control a support distribution of a noble metal when a complex oxide for which a plurality of oxides are mixed at a nanolevel is used as a carrier. <P>SOLUTION: At the time of preparing the complex oxide for which an aluminum-based oxide and other oxides different from the aluminum-based oxide are mixed at a nanolevel, the basicity of the aluminum-based oxide is adjusted and an ionic noble metal compound solution is brought into contact with the compound oxide so as to support the noble metal compound and thereafter performs firing, thereby the support distribution of the noble metal with respect to the aluminum-based oxide and the other oxide is controlled. The support distribution of the noble metal can be freely adjusted according to the difference in the basicity of the aluminum-based oxide and the other oxides. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an exhaust gas purification catalyst useful for exhaust gas purification of automobiles and a method for producing the same. The present invention includes a NO _x storage-and-reduction type catalyst for purifying an exhaust gas, when used for its production particularly preferred.

In recent years, NO _x storage reduction type catalysts have been put into practical use as catalysts for purifying exhaust gas from lean burn gasoline engines. This NO _x storage-reduction catalyst is a catalyst in which a NO _x storage material such as an alkali metal or an alkaline earth metal and a noble metal are supported on a porous carrier such as alumina (Al ₂ O ₃ ). This NO _x storage-and-reduction type catalyst, the air-fuel ratio by controlling so that the fuel stoichiometric-rich side from the fuel-lean side to the pulse-like (beats rich spike), NO _x in the lean side to _the NO _x storage material Occluded. The occluded NO _x is released on the stoichiometric to rich side, and is purified by reacting with reducing components such as HC and CO by the catalytic action of noble metals. Therefore, since NO _x emission is suppressed even on the lean side, a high NO _x purification ability is exhibited as a whole.

However, the exhaust gas contains SO ₂ produced by combustion of sulfur (S) contained in the fuel, which is oxidized by a noble metal in an oxygen-excess atmosphere to become SO ₃ . And it became clear that this was easily converted into sulfuric acid by the water vapor contained in the exhaust gas and reacted with the NO _x storage material to produce sulfate, which caused the NO _x storage material to be poisoned and deteriorated. This phenomenon is called sulfur poisoning. In addition, since the porous carrier such as Al ₂ O ₃ has a property of easily adsorbing SOx, there is a problem that the sulfur poisoning is promoted. When the NO _x storage material becomes a sulfate as described above, NO _x can no longer be stored, and as a result, the above catalyst has a problem that the NO _x purification ability after durability is lowered.

Therefore, Japanese Patent Laid-Open No. 08-099034 proposes to use at least one composite carrier selected from TiO ₂ —Al ₂ O ₃ , ZrO ₂ —Al ₂ O ₃ and SiO ₂ —Al ₂ O _3. Yes. Japanese Patent Application Laid-Open No. 09-000926 discloses an exhaust gas purification catalyst using a TiO ₂ —Al ₂ O ₃ —ZrO ₂ composite oxide as a carrier. Since TiO _{2 and the} like have a higher acidity than Al ₂ O ₃ , the affinity with SO _x is lowered, and as a result, sulfur poisoning of the NO _x storage material can be suppressed. Further, by using TiO ₂ and ZrO ₂ as a composite oxide with Al ₂ O ₃ , sulfur poisoning is suppressed and heat resistance is improved.

  Such a composite oxide support is manufactured by preparing an oxide precursor containing a plurality of metal elements by an alkoxide method, a coprecipitation method, or the like, and firing it. Among these, the coprecipitation method has an advantage that the raw material cost is lower than that of the alkoxide method and the like, and thus the obtained composite oxide has the advantage of being inexpensive, and is widely used in the production of composite oxides.

However, the exhaust gas temperature has become extremely high due to the recent tightening of exhaust gas regulations or the increase in high-speed driving. For this reason, even when the above-described composite oxide support is used, the specific surface area may be reduced and the noble metal grains may be grown, resulting in insufficient heat resistance, and further improvement in heat resistance is required. In addition, the SO _x produced by combustion of sulfur components in the fuel is adsorbed on the carrier and covers the noble metal, and the degrading phenomenon (catalyst metal sulfur poisoning) is a problem.

  These defects are considered to be due to the fact that the characteristics of each metal element constituting the composite oxide are not sufficiently expressed.

For example, ZrO ₂ —TiO ₂ solid solution as described in JP-A-2000-327329 is highly resistant to sulfur poisoning, so it is excellent in sulfur poisoning resistance by combining with Al ₂ O ₃ and has a high ratio. A catalyst carrier having a surface area can be obtained. In view of this, it was recalled that a composite oxide obtained by firing a precipitate produced by coprecipitation from an aqueous solution containing Al, Zr and Ti was used as a carrier. In such a carrier, ZrO ₂ —TiO ₂ solid solution and Al ₂ O ₃ coexist in a fine particle state of 50 nm or less and are highly dispersed, so that it is expected that the sulfur poisoning resistance is further improved.

However, in this composite oxide, both the ZrO ₂ —TiO ₂ solid solution and Al ₂ O ₃ are in a fine particle state of 50 nm or less, so that the specific surface area is reduced at high temperatures and the heat resistance is not sufficient. Therefore, addition of La was attempted, but in a composite oxide obtained by firing a precipitate generated by a coprecipitation method from an aqueous solution containing Al, Zr, Ti and La, basic La ₂ O ₃ is converted to ZrO ₂ —TiO _{2. 2 It} was observed that La did not contribute to the stabilization of Al ₂ O ₃ , and instead the sulfur poisoning resistance decreased.

Therefore, Japanese Patent Application Laid-Open No. 2002-282688 discloses an Al ₂ O ₃ —ZrO ₂ —TiO ₂ composite oxide produced by calcining a coprecipitate at 550 ° C. or more, and has pores in the mesopore region. And a catalyst carrier containing tetragonal zirconia and at least part of ZrO ₂ and TiO ₂ being a ZrO ₂ —TiO ₂ solid solution. According to this catalyst carrier, since it is already calcined at a high temperature, a decrease in specific surface area when used as a catalyst is suppressed. Further, it is considered that impurities in the catalyst carrier are removed by calcination at a high temperature, and as a result, the original characteristics of the supported noble metal and NO _x storage material are expressed. Furthermore, even when used as a catalyst at a high temperature, sintering of noble metals and NO _x storage materials is suppressed, and sulfur poisoning resistance is improved. In addition, since this catalyst carrier has a specific surface area of 120 m ² / g or more even after high temperature durability, high catalytic activity can be obtained.
JP 08-099034 JP 09-000926 JP 2000-327329 A JP 2002-282688 JP2004-321847

  By the way, in order to carry a noble metal on a catalyst carrier, an ionic noble metal compound solution is used, and a method of carrying it by an adsorption carrying method or an impregnation carrying method is widely used. However, when a noble metal is supported on a catalyst carrier as described in Patent Document 4 using an ionic noble metal compound solution, it has been found that there is a problem that the noble metal support distribution becomes non-uniform.

For example, Pt is a precious metal essential for exhaust gas purification catalysts because of its high oxidation activity, but an anionic Pt compound solution such as dinitrodiammine platinum solution is used to support Pt. On the other hand, the Al ₂ O ₃ —ZrO ₂ —TiO ₂ composite oxide described in Patent Document 4 is in a state where alumina and zirconia / titania solid solution are mixed at a nano level. When an anionic Pt compound solution is brought into contact with such a complex oxide, an anionic Pt compound ion is selectively supported on alumina having a high basicity, and Pt is hardly supported on a zirconia / titania solid solution. appear.

Since the NO _x storage material has a remarkably high basicity compared to the basicity of the carrier, it is uniformly supported regardless of the type of carrier. In addition, it is known that the NO _x storage reduction catalyst has a higher NO _x purification activity as the NO _x storage material and Pt are supported closer to each other. Therefore, if the Pt loading distribution becomes non-uniform as described above, Pt is not present in the vicinity of the NO _x storage material supported by the zirconia-titania solid solution, and the activity of the NO _x storage reaction and NO _x reduction reaction Decreases. In addition, when the NO _x storage material supported by the zirconia / titania solid solution is sulfur poisoned, the decomposition reaction activity of sulfate in a rich atmosphere also decreases, so it becomes difficult to eliminate sulfur poisoning and the NO _x storage capacity is reduced. Recovery becomes difficult. Therefore, it is difficult to effectively use the NO _x storage material supported on the zirconia / titania solid solution, and there is a problem that the NO _x purification activity is lowered.

  When multiple types of oxides are mixed at a large powder level exceeding the nano level, a precious metal is supported on each oxide in advance, or the type of precious metal compound solution is different for each oxide. The above problem can be avoided. However, when a plurality of types of oxides are mixed at the nano level, it is difficult to separate each oxide, and one type of noble metal compound solution must be used.

  The present invention has been made in view of the above circumstances, and it is a problem to be solved to control the noble metal support distribution when a composite oxide in which a plurality of types of oxides are mixed at the nano level is used as a support. To do.

  The feature of the method for producing an exhaust gas purifying catalyst of the present invention that solves the above problems is that when preparing a composite oxide in which an alumina-based oxide and another oxide different from the alumina-based oxide are mixed at a nano level. A composite oxide preparation step for adjusting the basicity of the alumina-based oxide, and a support step for firing after supporting the noble metal compound by bringing the ionic noble metal compound solution into contact with the composite oxide, and the alumina-based oxide. And controlling the noble metal loading distribution in other oxides.

  When the basicity of the alumina-based oxide is higher than that of other oxides, the other oxides contain a zirconia-titania solid solution in order to uniformly support the noble metal on both. A co-oxide which is an oxide other than alumina is dissolved on the surface of the product to lower the basicity, and the basicity can be made equivalent to other oxides. In this case, titania is preferably used as the co-oxide.

Further, it is preferable to further support a NO _x storage material to form a NO _x storage reduction catalyst.

  According to the method for producing an exhaust gas purifying catalyst of the present invention, when preparing a composite oxide in which an alumina oxide and another oxide different from the alumina oxide are mixed at the nano level, the alumina oxide is prepared. Therefore, for example, when an anionic noble metal compound solution is brought into contact with the composite oxide, the anionic noble metal compound is selectively supported on an oxide having a high basicity. When a cationic noble metal compound is used, it is selectively supported on an oxide having a low basicity. Therefore, it is possible to freely adjust the noble metal support distribution according to the difference in basicity between the alumina-based oxide and other oxides, and according to the obtained catalyst, the characteristics of the noble metal are exhibited on each oxide. Therefore, the purification activity is improved.

Further, if a NO _x storage reduction catalyst is supported by further supporting the NO _x storage material, the NO _x storage material on each oxide can be effectively utilized together with the noble metal, and the NO _x purification activity is improved.

The carrier used in the present invention is a composite oxide in which an alumina-based oxide and another oxide different from the alumina-based oxide are mixed at a nano level. This composite oxide can be produced by the method described in Patent Document 4. Examples of the alumina-based oxide include alumina stabilized with alumina, rare earth elements or alkaline earth metals. Other oxides are not limited as long as they are oxides other than alumina, but only a few kinds of oxides that can be mixed with alumina-based oxides at the nano level have been found at present. Accordingly, the Al ₂ O ₃ —ZrO ₂ —TiO ₂ -based composite oxide described in Patent Document 4 will be described as a representative example. That is, as another oxide, a zirconia-titania solid solution is typically exemplified.

In order to adjust the basicity of the alumina-based oxide, for example, as described in JP-A-2004-321847 (Patent Document 5), there is a method in which an oxide having high acidity such as titania is compounded. In this case, it is desirable to form a composite such that TiO ₂ particles are present in a state in which at least a part is dissolved in Al ₂ O ₃ on the surface of the alumina-based oxide. By doing in this way, the basicity of an alumina type oxide can be reduced. Furthermore, since the acid properties of TiO ₂ are also imparted to Al ₂ O ₃ , adhesion of sulfur oxides is further suppressed during use as a NO _x storage reduction catalyst, and even if sulfur oxides adhere. Compared with the catalyst described in Patent Document 4, sulfur poisoning can be further suppressed because the property of being easily desorbed is enhanced.

  The basicity is obtained using, for example, isoelectric point pH as an index. The isoelectric point pH indicates a state in which the algebraic sum of the charge of the amphoteric electrolyte in the aqueous solution becomes zero by the hydrogen ion exponent pH of the solution, and can be obtained by measuring the zeta potential.

In the method for producing a catalyst carrier described in Patent Document 4, basically, a solution of TiO _{2 in} ZrO ₂ is preferential because it is a method of co-precipitation from a salt of Al, Zr and Ti and firing it. As a result, the solid solution of TiO _{2 in} Al ₂ O ₃ was very slight. However, for example, as described in Patent Document 5, Al, Al from a solution containing a metal element X and Ti, to produce a precipitate containing the X and Ti, a composite oxide by firing the precipitate, TiO ₂ in the compound oxide If the precursor is supported and then baked at 550 ° C. or higher, at least a part of TiO ₂ is dissolved in the composite oxide. Since the composite oxide contains Al ₂ O _3, increases the probability that the TiO ₂ forms a solid solution in the Al ₂ O _3, can be adjusted basicity of alumina-based oxides. However, even with the method described in Patent Document 5, it is difficult to adjust the basicity when the supported amount of the TiO ₂ precursor is small. That is, in the manufacturing method described in Example 1 of Patent Document 5, since the amount of the TiO ₂ precursor is insufficient, it is difficult to achieve a basicity equivalent to that of the ZrO ₂ —TiO ₂ solid solution.

Moreover, since it has already baked at high temperature as described also in patent document 4, the fall of the specific surface area at the time of use as a catalyst is suppressed. Further, it is considered that impurities in the catalyst carrier are removed by calcination at a high temperature, and as a result, the original characteristics of the supported noble metal are developed. Even when the catalyst is used at a high temperature, sintering of the noble metal is suppressed. In addition, since this composite oxide has a specific surface area of 90 m ² / g or more even after high-temperature durability, high catalytic activity can be obtained. The metal element X is typically exemplified by Zr, but is not limited thereto.

The solid solution amount of TiO ₂ in Al ₂ O ₃ in the alumina-based oxide with adjusted basicity is preferably in the range of 5 to 50 mol%. If the solid solution amount of TiO ₂ is less than 5 mol%, the sulfur poisoning resistance decreases, and if it exceeds 50 mol%, the heat resistance decreases.

The TiO ₂ particles partly it is preferably made of only TiO ₂ particles in the state of solid solution in Al ₂ O ₃ or ZrO _2. Although it is not preferable to exist as a simple TiO _2, it may be present.

In the composite oxide, it is desirable that a composite oxide or solid solution composed of Al ₂ O ₃ , ZrO ₂ and TiO ₂ is dispersed as fine particles having a particle size of 50 nm or less in aggregated particles (secondary particles) having a particle size of 20 μm or less. . In this case, even if Al ₂ O ₃ , ZrO ₂ and TiO ₂ are in a highly dispersed state, they are already agglomerated so that further agglomeration is suppressed, heat resistance is improved and sulfur poisoning resistance is further improved. To do.

In the composite oxide referred to in the present invention, the constituent ratio of each oxide is preferably in the range of Al ₂ O ₃ : ZrO ₂ : TiO ₂ = 12 to 82: 5 to 66: 3 to 66 in terms of molar ratio. 16-77: 7-62: 9-56 is more preferable. If the Al ₂ O ₃ ratio is less than this range, the activity decreases, and if it exceeds this range, the sulfur poisoning resistance decreases. If the ratio of ZrO ₂ is less than this range, a solid phase reaction between the NO _x storage material and the composite oxide tends to occur, and if it exceeds this range, the specific surface area of the composite oxide is reduced. Further, if the ratio of TiO ₂ is less than this range, it is difficult to adjust the basicity of Al ₂ O ₃ and it becomes difficult to control the precious metal loading distribution, and the sulfur poisoning resistance is reduced. If the amount is larger, the specific surface area of the composite oxide decreases.

  In order to produce the composite oxide referred to in the present invention, first, a precipitate containing Al, X and Ti is generated from a solution containing Al and the metal elements X and Ti, and the precipitate is fired to obtain a composite oxide. The method so far is the same as the method described in Patent Document 4. The metal element X is not particularly limited, but Zr is particularly preferable. Therefore, hereinafter, the metal element X will be described as Zr.

  First, prepare a salt solution of Al, Zr, and Ti, respectively, mix each salt solution and an alkaline solution to form a precipitate, and calcine the precipitate obtained by mixing the respective precipitates at 500 ° C. or higher. The method can be adopted. Alternatively, a salt solution of Al, Zr, and Ti is prepared, and a precipitate is sequentially generated from the salt solution by mixing the total amount of the salt with a neutralizable alkali solution. You may bake.

The calcination temperature of the precipitate needs to be 550 ° C or higher, more preferably 650 ° C or higher, and particularly preferably 650 to 900 ° C. If the calcination temperature is less than 550 ° C., the specific surface area may be reduced depending on the conditions of use of the catalyst, so that noble metal sintering may occur and sulfur poisoning resistance will also be reduced. Further, if the firing temperature exceeds 900 ° C., the specific surface area will decrease due to crystallization of Al ₂ O ₃ and phase transition, which is not preferable.

  In the method for producing the composite oxide, a solution of Al, Zr and Ti salts may be sequentially added to the alkaline solution to generate a precipitate. This method is called a sequential coprecipitation method. Actually, even when precipitation is performed from a mixed solution in which a plurality of types of salts are dissolved, precipitates are sequentially generated due to differences in solubility or ionization tendency. However, since this is difficult to control, it is preferable to use the sequential coprecipitation method. According to this sequential coprecipitation method, the salt is first neutralized from the previously added solution and precipitated as a metal hydroxide. Then, when the salt solution added later is neutralized, the new metal hydroxide is preferentially deposited on the surface with the precipitates that have been generated as nuclei, and is precipitated. Alternatively, the precipitate is deposited as an inclusion at the grain boundary and precipitated.

  Desirably, a first precipitate containing Al, Zr and Ti is generated from a solution containing Al, Zr and Ti, and then a second precipitate containing Al is generated from the solution containing Al. Or conversely, a first precipitate containing Al is produced from a solution containing Al, and then a second precipitate containing Al, Zr and Ti is produced from a solution containing Al, Zr and Ti. Alternatively, a first precipitate containing Al, Zr and Ti is produced from a solution containing Al, Zr and Ti, and then a rare earth element and an alkaline earth metal are produced from the solution containing at least one kind of rare earth element and alkaline earth metal oxide and Al. A second precipitate containing at least one oxide and Al is generated. Alternatively, conversely, a first precipitate containing at least one rare earth element and alkaline earth metal oxide and Al is generated from a solution containing at least one rare earth element and alkaline earth metal oxide and Al, and then Al, Zr. A second precipitate containing Al, Zr and Ti is produced from the solution containing Ti and Ti.

  In order to generate the first precipitate and the second precipitate in the same container, first, a solution containing a single or plural kinds of first metal elements is contacted with an alkaline solution in an amount that neutralizes the acid amount. A second precipitate may be generated by adding a solution containing a plurality of types or a single second metal element and an alkali solution in an amount that neutralizes the acid amount after that. The third or fourth precipitate may be further mixed, or the third or fourth precipitate may be generated after the second precipitate is generated.

  Therefore, in the composite oxide obtained by firing this precipitate, the distribution of the metal element is different between the central portion and the surface portion in the aggregated particles formed by aggregation of the primary particles. By appropriately selecting the type of salt, it is possible to produce a complex oxide having different metal element distributions on the surface and inside.

  The salt is not particularly limited as long as it has the required solubility in water or alcohol, but nitrate is particularly preferably used. As the alkaline solution, an aqueous solution or alcohol solution in which ammonia, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate or the like is dissolved can be used. Particularly preferred are ammonia and ammonium carbonate which volatilize during firing. The pH of the alkaline solution is more preferably 9 or higher.

  There are various adjustment methods for precipitation, such as adding ammonia water instantly and stirring vigorously, or adjusting the pH at which the oxide precursor starts to precipitate by adding hydrogen peroxide, etc. There is a method of depositing a precipitate with ammonia water or the like. In addition, the time required for neutralization with aqueous ammonia or the like is sufficiently long, preferably a method of neutralization in 10 minutes or more, or a buffer that neutralizes stepwise while monitoring the pH or maintains a predetermined pH. There is a method of adding a solution.

  In order to add a salt solution, it is preferable to add the salt solution all at once. As a result, the particle size of the precipitated particles can be made finer, and a composite oxide composed of aggregated particles of 20 μm or less in which fine particles of 50 nm or less are aggregated can be easily produced. Further, the sequential addition can be carried out in two or more stages, and the upper limit of the stage is not particularly restricted.

  It is further desirable to perform an aging step in which the mixture is heated in a suspended state using water or a solution containing water as a dispersion medium or in a state where water is sufficiently present in the system. Thereby, although the mechanism is unknown, a composite oxide with controlled pores can be obtained.

  In order to mature the precipitate in a state where moisture is sufficiently present in the system, the solution containing the precipitate can be heated to evaporate the solvent, and then baked as it is. Alternatively, the precipitate separated by filtration may be calcined in the presence of water vapor. In this case, it is preferable to bake in a saturated steam atmosphere.

When the above-described ripening step is performed, dissolution / reprecipitation is promoted by heating heat and particle growth occurs. In this case, it is desirable to neutralize with an equivalent base or more that can neutralize all of the salt. Thereby, the oxide precursor is aged more uniformly, the pores are effectively formed, and the solid solution of the ZrO ₂ —TiO ₂ solid solution is further promoted.

  This aging step is desirably performed at room temperature or higher, preferably 100 to 200 ° C, more preferably 100 to 150 ° C. When heating at less than 100 ° C., the effect of promoting aging is small, and the time required for aging becomes long. At temperatures higher than 200 ° C., a synthesizer that can withstand 10 atm or more is required, which increases equipment costs and is not suitable for the production of a catalyst carrier.

The resulting precipitate is calcined at 550 ° C or higher. As described above, when the calcination temperature is less than 550 ° C., the specific surface area decreases depending on the conditions for using the catalyst, so that sintering of precious metals or the like is likely to occur, and the sulfur poisoning resistance also decreases. On the other hand, if the firing temperature exceeds 900 ° C., the specific surface area decreases due to crystallization of Al ₂ O ₃ and phase transition.

  In order to adjust the basicity of the alumina-based oxide, in this production method, the titania precursor is supported on the composite oxide obtained as described above, and then calcined at 500 ° C. or higher. Examples of the titania precursor include a precursor such as a hydroxide precipitated from a soluble salt of titanium, a precursor formed by hydrolysis from a titanium alkoxide, and the like. In order to carry such a precursor on the composite oxide, a method in which the composite oxide powder is mixed in an aqueous solution of a soluble salt of titanium and the titanium precursor is precipitated there, an alcohol solution in which the titanium alkoxide is dissolved A method of mixing the composite oxide powder therein and hydrolyzing and precipitating the titanium alkoxide can be employed.

After loading the titania precursor, it is fired at 500 ° C or higher. When the firing temperature is less than 500 ° C, solid solution does not occur and titania particles are present on the surface of the composite oxide, so that acidity is not sufficiently imparted to Al ₂ O ₃ and sulfur poisoning resistance decreases. To do. Further, it is desirable to bake at 500 to 900 ° C. where organic substances which are one of the constituent components of the titania precursor are burned and removed. Baking at 900 ° C. or higher is not preferable because the specific surface area decreases due to crystallization of Al ₂ O ₃ and phase transition.

  The obtained composite oxide is subjected to a supporting step in which the ionic noble metal compound solution is contacted to support the noble metal compound and then fired. Pt, Rh, Pd, Ir, Ru, etc. can be used as noble metals. The ionic noble metal compound solution may be anionic, cationic, or nonionic. As described above, when the anionic noble metal compound solution is brought into contact with the composite oxide, the anionic noble metal compound is selectively supported on the oxide having a high basicity. When a cationic noble metal compound is used, it is selectively supported on an oxide having a low basicity. Therefore, by selecting the ionicity of the noble metal compound solution, it is possible to freely adjust the noble metal support distribution according to the difference in basicity between the alumina-based oxide and other oxides. In addition, when different ionic noble metal compound solutions are used, different noble metals can be separated and supported on different oxides.

The organic matter of the noble metal compound is burned off by firing, and the noble metal is supported on the composite oxide. The amount of noble metal supported can be 0.1 to 20 g per liter of catalyst. If the loading amount of the noble metal is less than this range, the oxidation activity and NOx purification activity are low, and _if the loading amount is more than this range, the activity is saturated and the cost increases.

It is desirable that the composite oxide further support a NO _x storage material. As the noble metal, Pt, Rh, Pd, Ir, Ru and the like can be used, and Pt is particularly preferable. As a result, it is possible to produce an NO _x storage reduction catalyst having high NO _x purification performance and excellent sulfur poisoning resistance. The NO _x storage material is at least one selected from alkali metals, alkaline earth metals and rare earth elements, and it is desirable to use at least one of alkali metals and alkaline earth metals having a high basicity. Alkali metals have a high NO _x storage capacity in the high temperature range, and alkaline earth metals have a high NO _x storage capacity in the low temperature range. Therefore, it is preferable to use both in combination. This NO _x storage material is supported in the state of a salt such as carbonate, an oxide, or a hydroxide. Since the NO _x storage material is extremely high in basicity, it is not affected by the basicity of each oxide and is uniformly supported throughout.

The loading amount of the NO _x storage material is desirably 0.1 to 1.2 moles per liter of catalyst. When the amount of the NO _x storage material supported is too large, a phenomenon that the noble metal is covered with the NO _x storage material occurs, and the NO _x purification activity decreases.

In order to produce a practical catalyst, a coat layer is formed from the complex oxide powder according to the present invention on a heat-resistant substrate such as cordierite or metal foil, and a noble metal and NO _x storage material are supported thereon. Can be manufactured. This catalyst can be used to purify exhaust gas from gasoline engines, diesel engines or gas engines (GHP).

  Hereinafter, the present invention will be specifically described with reference to examples, comparative examples, and test examples.

Example 1
Aluminum nitrate, zirconyl oxynitrate and titanium tetrachloride were stirred and mixed in water to prepare a mixed aqueous solution. Ammonia water was added thereto for neutralization, and a precipitate was obtained by a coprecipitation method. The precipitate was aged with the solution at 2 ° C. and 120 ° C. for 2 hours. Thereafter, the precipitate was calcined at 400 ° C. for 5 hours and then calcined in air at 800 ° C. for 5 hours, and pulverized to 150 μm or less with a dry pulverizer to prepare a composite oxide powder.

Next, an aqueous solution in which 0.9 mol of citric acid was dissolved in 450 ml of ion-exchanged water was prepared, and 0.3 mol of titanium isopropoxide was gradually added while heating and stirring at 75 ° C. to prepare a titania precursor. A titania precursor is supported so as to contain 10 mol% in terms of TiO2 with respect to 120 g of the above complex oxide, dried at 110 ° C. for 10 hours, and calcined in air at 800 ° C. for 3 hours to prepare a composite oxide powder. did. The composition of each oxide is Al ₂ O ₃ : ZrO ₂ : TiO ₂ = 46: 33: 21 by weight ratio.

  120 g of this composite oxide powder was impregnated with a predetermined amount of dinitrodiammine platinum nitrate solution (anionic) with a predetermined concentration, evaporated to dryness in the atmosphere at 250 ° C. for 1 hour, and then calcined at 500 ° C. for 2 hours to obtain Pt Catalyst powder was prepared. This catalyst powder was pelletized by a conventional method to obtain a pellet catalyst.

Also, 200 parts by weight of complex oxide powder, 50 parts by weight of ZrO ₂ powder supporting 1.0% by weight of Rh, 20 parts by weight of CeO ₂ —ZrO ₂ solid solution powder, and alumina sol (5% by weight of Al ₂ O ₃ ) 130 A slurry was prepared by mixing 150 parts by weight of water and 170 parts by weight of water, washed on a 35 cc honeycomb substrate, and then fired at 500 ° C. for 1 hour. The coating amount is 270 g with respect to 1 L of honeycomb substrate. Next, using the same chemical solution as described above, Pt was adsorbed and supported, and Ba, K, and Li were absorbed and supported. The firing conditions after loading are all at 300 ° C. for 3 hours in the air. The amount of each catalyst component supported is 2 g of Pt, 0.5 g of Rh, 0.2 mol of Ba, 0.15 mol of K, and 0.1 mol of Li with respect to 1 L of the honeycomb substrate. Thus, a honeycomb-shaped NO _x storage reduction catalyst was prepared.

(Reference Example 1)
A composite oxide powder was prepared in the same manner as in Example 1 except that the titania precursor was supported so as to contain 5 mol% in terms of TiO ₂ . The composition of each oxide is Al ₂ O ₃ : ZrO ₂ : TiO ₂ = 48: 34: 18 by weight. The composite oxide powder produced in Reference Example 1 corresponds to the catalyst carrier powder of Example 1 described in Patent Document 5.

Using this composite oxide powder, an NO _x storage reduction catalyst was prepared in the same manner as in Example 1.

(Comparative Example 1)
Aluminum nitrate, zirconyl oxynitrate and titanium tetrachloride were stirred and mixed in water to prepare a mixed aqueous solution. Ammonia water was added thereto for neutralization, and a precipitate was obtained by a coprecipitation method. The precipitate was aged with the solution at 2 ° C. and 120 ° C. for 2 hours. Thereafter, the precipitate was calcined at 400 ° C. for 5 hours, then calcined at 800 ° C. for 5 hours, and pulverized to 150 μm or less with a dry pulverizer to prepare a composite oxide powder. The composition of each oxide is Al ₂ O ₃ : ZrO ₂ : TiO ₂ = 50: 35: 15 by weight ratio.

This composite oxide powder is made of Al ₂ O ₃ —ZrO ₂ —TiO ₂ composite oxide, has mesopores with a diameter of about 14 nm, crystals of tetragonal zirconia are confirmed, and ZrO ₂ and TiO ₂ At least a part was a ZrO ₂ —TiO ₂ solid solution. The BET specific surface area was 128 m ² / g.

The resultant composite oxide powder, a pellet catalyst and NO _x storage-and-reduction type catalyst was prepared by the same catalyst adjusting method as in Example 1.

(Comparative Example 2)
Ammonia water was added to the aqueous aluminum nitrate solution for neutralization to obtain a precipitate. The precipitate was aged with the solution at 2 ° C. and 120 ° C. for 2 hours. Thereafter, the precipitate was calcined at 400 ° C. for 5 hours, then calcined at 800 ° C. for 5 hours, and pulverized to 150 μm or less with a dry pulverizer to prepare alumina powder.

(Comparative Example 3)
An aqueous solution in which 0.9 mol of citric acid was dissolved in 450 ml of ion-exchanged water was prepared. While heating and stirring at 75 ° C., 0.3 mol of titanium isopropoxide was gradually added to prepare a titania precursor. Then, 120 g of the alumina powder prepared in Comparative Example 2 is loaded with a titania precursor so as to contain 2.5 mol% in terms of TiO ₂ , dried at 110 ° C. for 10 hours, and calcined in air at 800 ° C. for 3 hours. A composite oxide powder was prepared. The composition of each oxide is Al ₂ O ₃ : TiO ₂ = 98: 2 by weight.

(Comparative Example 4)
An aqueous solution in which 0.9 mol of citric acid was dissolved in 450 ml of ion-exchanged water was prepared. While heating and stirring at 75 ° C., 0.3 mol of titanium isopropoxide was gradually added to prepare a titania precursor. Then, 120 g of alumina powder prepared in Comparative Example 2 is loaded with a titania precursor so as to contain 5 mol% in terms of TiO 2, dried at 110 ° C. for 10 hours, and fired at 800 ° C. for 3 hours in the atmosphere to form a composite. An oxide powder was prepared. The composition of each oxide is Al ₂ O ₃ : TiO ₂ = 96: 4 by weight.

(Comparative Example 5)
Zirconyl oxynitrate and titanium tetrachloride were stirred and mixed in water to prepare a mixed aqueous solution. Ammonia water was added thereto for neutralization, and a precipitate was obtained by a coprecipitation method. The precipitate was aged with the solution at 2 ° C. and 120 ° C. for 2 hours. Thereafter, the precipitate was calcined at 400 ° C. for 5 hours, then calcined at 800 ° C. for 5 hours, and pulverized to 150 μm or less by a dry pulverizer to prepare a ZrO ₂ —TiO ₂ solid solution powder. The composition of each oxide is ZrO ₂ : TiO ₂ = 70: 30 by weight. That is, in this comparative example, the titania precursor supporting step is not performed.

<Test Example 1>
The alumina powder, composite oxide powder and ZrO ₂ —TiO ₂ solid solution powder prepared in Comparative Examples 2 to 5 were each dispersed in pure water, and the isoelectric point pH was determined by measuring the zeta potential. The results are shown in FIG.

FIG. 1 shows that as the amount of TiO ₂ contained in Al ₂ O ₃ increases, the isoelectric point pH decreases and the basicity decreases. Therefore, in the composite oxide of Example 1 in which the titania precursor is supported so as to contain 10 mol% in terms of TiO ₂ , the basicity of Al ₂ O ₃ is further reduced as compared with Comparative Example 4, and the ZrO ₂ —TiO ₂ solid solution Therefore, the difference in isoelectric point pH between Al ₂ O ₃ and ZrO ₂ —TiO ₂ solid solution is greatly reduced compared to Comparative Example 1, and the isoelectricity of both is reduced. The point pH is considered to be almost the same.

<Test Example 2>
In order to confirm the above estimation, the pellet catalysts of Example 1 and Comparative Example 1 were allowed to flow at a gas flow rate of 1 L / min while adding 10% CO (75%) / H ₂ (25%) mixed gas under He balance. And reduced at 500 ° C. for 1 hour. Thereafter, it was pulverized into powders, and FE-TEM measurement was performed for each. The observed FE-TEM images of the catalyst of Example 1 are shown in FIGS. 2-3, and the FE-TEM images of the catalyst of Comparative Example 1 are shown in FIGS. The elemental analysis values are shown in Table 1.

2 and 4 show a mixed portion of Al ₂ O ₃ and ZrO ₂ —TiO ₂ solid solution, and FIGS. 3 and 5 show localized portions of the ZrO ₂ —TiO ₂ solid solution. The numbers of analysis points in Table 1 correspond to the numbers shown in FIGS.

2 and 4, it was confirmed that Pt was present in both catalysts in the mixed portion of Al ₂ O ₃ and ZrO ₂ —TiO ₂ solid solution. However, in the localized portion of the ZrO ₂ —TiO ₂ solid solution shown in FIGS. 3 and 5, Pt is present in Example 1, whereas Pt is not observed in Comparative Example 1.

That is, in the catalyst of Comparative Example 1, Pt is unevenly distributed and supported on the portion where Al ₂ O ₃ is present, whereas it is clear that Pt is supported almost uniformly throughout the catalyst of Example 1. , and the clear that this is because the isoelectric point pH of Al ₂ O ₃ and ZrO ₂ -TiO ₂ solid solution in lowering the basicity of Al ₂ O ₃ is almost the same It is.

<Test Example 3>
Example 1, each evaluation apparatus NO _x storage-and-reduction type catalyst prepared in Reference Example 1 and Comparative Example 1 were disposed, a durability test was carried out by heating for 5 hours under air flow 750 ° C.. Next, a sulfur adhesion test shown in Table 2 was conducted so that the flow amount of sulfur was 3g / 2L while alternately flowing the model gas for sulfur poisoning at 600 ° C at a flow rate of 30L / min, lean gas 60 seconds-rich gas 3 seconds. Subsequently, a sulfur desorption test was conducted at 650 ° C. for 10 minutes while flowing a sulfur desorption gas at a flow rate of 30 L / min. The average sulfur desorption rate during the sulfur desorption test was calculated and the results are shown in FIG.

Further, the NO _x occlusion amount at each temperature of 300 to 600 ° C. was measured while flowing the model gas shown in Table 3 for each catalyst after the sulfur desorption test. Measurements lean gas is circulated 3 seconds rich gas after flowing 120 seconds, further _the NO _x storage amount at the time of switching to a lean gas (after the rich spike _the NO _x storage amount: 95% RSNOs _x storage amount) was measured. The results are shown in FIG.

6 and 7, it can be seen that the catalyst of Example 1 is superior to the catalysts of Reference Example 1 and Comparative Example 1 in terms of sulfur desorption and NO _x storage capacity. It is clear that the difference in basicity between Al ₂ O ₃ and ZrO ₂ —TiO ₂ solid solution is small and that Pt is uniformly supported. Although the catalyst of Reference Example 1 is slightly improved as compared with Comparative Example 1, both the sulfur detachability and the NO _x storage capacity are lower than those of Example 1, which are Al ₂ O ₃ and ZrO ₂ —TiO _2. This is probably because the difference in basicity with the solid solution is not so small, and Pt is unevenly distributed on Al ₂ O ₃ .

Production method of the present invention is not NO _x storage-and-reduction type catalyst but also the production of the oxidation catalyst or three-way catalyst carrying no _the NO _x storage material can be utilized.

It is a graph showing a change in isoelectric point pH due to the difference in the amount of TiO _{2. 2} is an FE-TEM image showing a particle structure in a mixed portion of Al ₂ O ₃ and ZrO ₂ —TiO ₂ solid solution of the catalyst obtained in Example 1. FIG. ₂ is an FE-TEM image showing a particle structure in a mixed portion of Al ₂ O ₃ and ZrO ₂ —TiO ₂ solid solution of the catalyst obtained in Comparative Example 1. FIG. ₂ is an FE-TEM image showing a particle structure in a localized portion of the ZrO ₂ —TiO ₂ solid solution of the catalyst obtained in Example 1. FIG. ₂ is an FE-TEM image showing a particle structure in a localized portion of a ZrO ₂ —TiO ₂ solid solution of a catalyst obtained in Comparative Example 1. FIG. It is a graph which shows a 10-minute average sulfur desorption rate. Is a graph showing the relationship between the gas temperature and 95% RSNOs _x storage amount entered.

Claims (4)

A composite oxide preparation step for adjusting the basicity of the alumina-based oxide when preparing a composite oxide in which an alumina-based oxide and another oxide different from the alumina-based oxide are mixed at the nano level When,
A supporting step of bringing the ionic noble metal compound solution into contact with the composite oxide and supporting the noble metal compound, followed by firing.
A method for producing an exhaust gas purifying catalyst, comprising controlling the distribution of noble metal support in the alumina-based oxide and the other oxide.   The other oxide includes a zirconia-titania solid solution, and in the composite oxide preparation step, a co-oxide which is an oxide other than alumina is dissolved on the surface of the alumina-based oxide to reduce the basicity, thereby reducing the basicity. A method for producing an exhaust gas purifying catalyst having a property equivalent to that of the other oxide.   The method for producing an exhaust gas purifying catalyst according to claim 2, wherein the co-oxide is titania. An exhaust gas purifying catalyst, wherein the exhaust gas purifying catalyst according to any one of claims 1 to 3 is further loaded with an NO _x storage material.

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