JP4631360B2 - Method for producing exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to a catalyst for purifying exhaust gas from an internal combustion engine such as a gasoline engine or a diesel engine, and in particular, a catalyst having a structure in which at least two types of catalyst component particles such as platinum (Pt) and rhodium (Rh) are supported. It relates to a manufacturing method.

  An example of a method for producing an exhaust gas purifying catalyst carrying platinum and rhodium is described in Patent Document 1. The method described in Patent Document 1 is a method using a weakly basic organic platinum aqueous solution, in which a predetermined amount of dinitrodiaminoplatinum is poured into a nitric acid aqueous solution and dissolved, and this is brought to a predetermined temperature under normal pressure. It is boiled and the valence of platinum is reacted from 2 to 4. The platinum salt solution thus obtained is mixed with water and an aqueous nitric acid solution to adjust to a predetermined concentration, and then immersed in a ceramic carrier to adsorb platinum. By drying and calcining the carrier on which the platinum is adsorbed, the platinum salt is reduced by thermal decomposition. Thereafter, the carrier is immersed in a rhodium nitrate solution to adsorb rhodium, and this is further dried and calcined to obtain a platinum-rhodium supported catalyst.

When a noble metal such as platinum or rhodium or a catalyst component element is supported on a support, conventionally, a porous support is impregnated with a solution of a noble metal compound such as platinum dinitrodiammine or rhodium nitrate, and then dried and calcined. Yes. In such a supporting method, platinum is likely to move and sinter when heated to a high temperature, and sulfur poisoning is likely to occur. On the other hand, in the method described in Patent Document 1 above, platinum is selectively adsorbed to an electron accepting point such as an acid property point on the carrier. The catalytic activity can be improved, for example, the ring can be suppressed and the sulfur poisoning can be suppressed accordingly.
JP-A-6-178936

The method described in Patent Document 1 above is a method in which platinum is adsorbed on a support and then reduced by heating and then rhodium is supported. On the contrary, rhodium is supported on a support. After that, platinum may be supported. Even in that case, it is preferable to use a weakly basic organic platinum aqueous solution. However, rhodium previously supported on the carrier exists in the form of rhodium oxide (Rh ₂ O ₃ ), and its electronegativity is alumina, zirconia, titania, ceria which is used as an oxygen storage material, and the like. Greater than electronegativity. Since the electronegativity of platinum is smaller than these, if it is supported after rhodium, platinum is adsorbed not only on the support but also on rhodium. As a result, rhodium in the catalyst obtained by calcination is absorbed by platinum. Since it was covered and the degree of exposure decreased, the catalytic activity could be impaired.

  In other words, when catalyst component particles having a large electronegativity are previously adsorbed or supported on a carrier and catalyst component particles having a smaller electronegativity are supported, the electronegativity previously adsorbed or supported Since the subsequent catalyst component particles are adsorbed onto and cover the large catalyst component particles, the function as the original catalyst is impaired.

  The present invention has been made paying attention to the above technical problem, and suppresses the influence of the electronegativity at the time of adsorption of the catalyst component particles on the carrier so that each catalyst component particle is supported on the carrier as expected. The object of the present invention is to produce an exhaust gas purifying catalyst having high catalytic activity.

In order to achieve the above object, the invention according to claim 1 is a method for producing an exhaust gas purifying catalyst in which platinum is supported on a carrier after rhodium is supported on the carrier, and the carrier carrying rhodium is subjected to a reduction treatment. rhodium metalized, then a platinum, using a weakly basic organic aqueous solution of platinum carried on the large site of electric negativity than the metallized rhodium in electron accepting point or the upper support on said carrier Te Te is a method which is characterized in that is supported in front Symbol carrier.

In the invention 請 Motomeko 1, as are described in claim 2, wherein said carrier, even those that are provided on a surface coat layer containing a cerium-containing composite oxide and a cerium-free oxide Good.

These in claim 1 or 2 inventions, as are described in claim 3, even if the alkali metal and / or alkaline earth NOx-absorbing material consisting of one or more metals are supported on the support Good.

According to the invention billed to claim 1, which was metallized by performing reduction treatment rhodium is supported above the support, in that state, since the carrying platinum by using a weakly basic organic platinum aqueous solution, Since the electronegativity of rhodium is about the same as that of platinum, it can be suppressed that platinum is adsorbed on rhodium. As a result, it is possible to suppress or prevent rhodium from being covered with platinum particles and obtain an exhaust gas purifying catalyst having high activity.

Further, according to the invention of claim 2, to promote the water gas shift reaction by platinum and cerium, as a result, it is possible to purification performance of the exhaust gas, thereby particularly improving the purification performance of NOx.

Furthermore, according to the invention of claim 3, it is possible to maintain the catalytic activity in a good state by suppressing sulfur poisoning of the NOx storage material or promoting regeneration from sulfur poisoning.

  The present invention will be specifically described. The method of the present invention is a method for producing an exhaust gas purification catalyst having at least two kinds of catalyst component particles supported on a carrier. The carrier is formed as a coat layer provided on either a monolithic or pellet substrate. The base material is made of cordierite as an example, and the coating layer may contain ceria, in which case other metal oxides such as zirconia are used in combination for stabilization of ceria, and more specifically. Specifically, a cerium-zirconium composite oxide can be used.

  Examples of the catalyst component supported on the carrier include noble metals such as platinum (Pt), rhodium (Rh), and palladium (Pd).

  The content of cerium in the cerium-zirconium composite oxide is set to less than 30 mol%, preferably 0.1 to 25 mol%, based on the total number of moles of metal atoms contained in the composite oxide. More preferably, it is 0.5-10 mol%. This is because ceria is mainly present near the surface, thereby accelerating the release of sulfur oxides and accompanyingly promoting the water gas shift reaction of ceria.

  The coat layer can be mixed with a cerium-free oxide in addition to the cerium-zirconium composite oxide. As the cerium-free oxide, alumina, zirconia, and titania are used.

  The catalyst particles are supported on the support according to a predetermined order. Specifically, the oxide component has high electronegativity, and when it is reduced and metallized, the catalyst component becomes smaller than that of the carrier, firstly supported, and then reduced to lower the electronegativity. In this state, the other catalyst components are supported using a supporting liquid that is supported on a site having a high electronegativity. For example, when platinum and rhodium are supported as catalyst components, rhodium is first supported. For example, the support is impregnated with a rhodium nitrate solution, and the excess solution is blown off, followed by drying and firing, and supporting. For this reason, rhodium is in a state of being supported on the support as an oxide, and thus has a high electronegativity, and is reduced to be metallized there. The reduction process may be performed by an appropriate method known in the art, and can be performed as a gas phase process using a reducing gas such as hydrogen gas. As a result, the electronegativity of rhodium is smaller than that of the carrier, and platinum is supported in this state.

  Platinum is supported by a method of selectively supporting a so-called electron accepting point such as an acid property point on the support or a site having a large electronegativity. Specifically, it is carried out by impregnating a weakly basic organic platinum aqueous solution into a carrier carrying metallized rhodium. The weakly basic organic platinum aqueous solution is, for example, the solution described in Patent Document 1, or an organic platinum aqueous solution SN or CN (trade name, respectively) manufactured by Ishifuku Metal Industry Co., Ltd.

  Rhodium is metallized and its electronegativity is small, whereas platinum is positively charged and its electronegativity is close to that of metal rhodium. Adsorbed. That is, rhodium is suppressed from being covered with platinum particles. Then, it is set as a catalyst through a drying and baking process. In addition, it does not deny that Pt particles are attached other than the electron accepting point.

  In such a method, even when heated during firing, the Pt particles are strongly adsorbed on the electron accepting points on the cerium-zirconium composite oxide, hardly move, and are positively charged. Since the adsorption of oxygen to the Pt particles is suppressed, the Pt particles and the Rh particles are supported while being separated from each other, and the respective catalytic activities are improved.

  Conventionally, for example, a nitric acid-based Pt-supported solution (in the form of four nitro groups coordinated around Pt) has been used. It needs to be carried at the positively charged position above. However, since the cerium-zirconium composite oxide has few such points, there is a problem that the supporting efficiency of the Pt particles is low and the productivity is inferior. Conventionally, a Pt ammine complex in which four ammonium ions are arranged around Pt has been used, but the complex ion itself is positively charged and easily adsorbed to the cerium-zirconium composite oxide. Due to the high solubility, once adsorbed to oxygen in the vicinity of cerium, it was easily re-dissolved due to the presence of excess ammonium ions in the surroundings, resulting in a problem that the overall loading efficiency was lowered. . On the other hand, when a so-called selective supported organic platinum solution such as the above organic platinum aqueous solution SN or CN manufactured by Ishifuku Metal Industry Co., Ltd. is used, oxygen in the vicinity of ceria which easily generates electrons between Pt particles is converted into Pt. Since particles are adsorbed, the efficiency of supporting Pt particles is improved.

  As described above, the Pt particles adsorbed on the electron accepting point deliver electrons to ceria and are positively charged. This is presumed to be due to the fact that the bond between oxygen ions and cerium ions is weaker than the bond between zirconium and oxygen ions due to the characteristics of cerium. Therefore, even when high temperature durability is performed, oxygen is not easily adsorbed to the Pt particles, and as a result, the catalytic activity of the Pt particles is maintained. Considering this in relation to the oxygen surrounding the Pt particles, the Pt particles weakly adsorb the surrounding oxygen, thereby increasing the activity of oxygen ions (active oxygen) generated from the oxygen or NOx. As a result, this increases the reactivity with hydrocarbons (HC) and promotes NOx release and reduction ability. In addition, Pt particles are not in an oxidized state due to the difficulty of being attached with oxygen, so that the vapor pressure is kept low, and the Pt particles and ceria are in a strong bonding state such as a covalent bond or a coordinate bond. Therefore, it is estimated that the movement of the Pt particles is prevented. Therefore, sintering of Pt particles and increase in particle size based on the same are difficult to occur, and since the distance from ceria is maintained close, the catalytic activity at low temperature is good even after high temperature durability. Maintained.

  The method of the present invention can also be applied to the production of a NOx storage reduction catalyst, in which case the NOx storage material is supported. This NOx storage material is composed of lithium (Li), sodium (Na), potassium (K), rubidium (Rb) alkali metals, and magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). At least one kind of alkaline earth metal is exemplified, and one kind of metal selected from Li, K, and Ba is preferable. This NOx occlusion material is supported on the coating layer made of the above-described cerium-zirconium composite oxide and cerium-free oxide. The supporting treatment is preferably performed after supporting the noble metal particles. The coating layer is impregnated with a solution of acetate, nitrate, etc., such as the alkali metal, and then dried and baked for supporting. Can do.

  The exhaust gas purification catalyst according to the present invention can be configured as a three-way catalyst. In this case, the pore volume of the cerium-zirconium composite oxide is adjusted to 0.1 / g or more. The pore volume is a value obtained from the nitrogen adsorption amount. This is because, for example, after forming primary particles of the composite oxide precursor by a so-called microemulsion method, the primary particles are aggregated to form secondary particles, and the secondary particles are sufficiently large between the secondary particles. The secondary particles are gradually agglomerated so as to form the pores, and then fired. The adjustment of the interval between the secondary particles and the progress of aggregation can be adjusted by appropriately setting the basicity pH, the stirring time, etc. of the colloidal solution in which the micelles containing the secondary particles are dispersed. .

  Even when the exhaust gas purifying catalyst of the present invention is configured as a three-way catalyst, as described above, sintering of Pt particles is suppressed, so that the specific surface area of Pt particles is increased. As a result, the purification rate of each component is improved by the three-way catalytic reaction, and it is possible to obtain the same exhaust gas purification ability as when the amount of noble metal particles supported is increased.

  Next, examples and comparative examples performed for confirming the effects of the present invention will be described.

(Example)
A carrier composed of a cerium-zirconium composite oxide and a cerium-free oxide was impregnated with a rhodium nitrate solution, and then the excess rhodium nitrate solution was blown off, followed by drying and firing. . This was reduced in the gas phase to metallize rhodium oxide (Rh ₂ O ₃ ). Next, water and a nitric acid aqueous solution are added to a weakly basic organic platinum aqueous solution (organic platinum aqueous solution SN or CN manufactured by Ishifuku Metal Industry Co., Ltd.) to adjust its concentration and pH appropriately, A carrier carrying rhodium was impregnated and dried and calcined to further carry platinum. The supported amount of platinum is 2 g per 1 liter of catalyst, and the supported amount of rhodium is 0.5 g per 1 liter of catalyst. The structure of the catalyst thus obtained is schematically shown in FIG.

(Comparative Example 1)
Platinum and rhodium were supported by a conventional method. Specifically, platinum and rhodium are obtained by impregnating a carrier composed of a cerium-zirconium composite oxide and a cerium-free oxide with a dinitrodiammine platinum complex aqueous solution and a rhodium nitrate aqueous solution, and drying and firing this. Were supported on a carrier. The amount of each supported was the same as in the above example.

(Comparative Example 2)
The same procedure as in the above Example was performed except that platinum was supported using a weakly basic organic platinum aqueous solution without reducing rhodium oxide on the support. The structure of the catalyst thus obtained is schematically shown in FIG. 1B, and it is considered that a considerable amount of platinum particles are supported on the rhodium particles.

(Evaluation)
With respect to each of the obtained exhaust gas purification catalysts, the NOx 50% purification temperature (T ₅₀ ) was measured. The exhaust gas used is a model gas having a standard exhaust gas composition of a gasoline engine and an exhaust gas obtained by removing moisture from the model gas. The measurement results are shown in FIG. In FIG. 2, “with water” is the result for the standard model gas, and “without water” is the result for the exhaust gas obtained by removing moisture from the standard model gas.

  According to the catalyst obtained in “Comparative Example 1” which is a conventional method, the purification temperature exceeded 230 ° C. when water was present. This is considered to be because platinum and / or rhodium is poisoned by water and its activity is somewhat inhibited. In contrast, in the case of exhaust gas from which moisture was removed, the purification temperature was below 230 ° C. This is considered to be due to the fact that rhodium exhibits sufficient activity in addition to no poisoning by water.

On the other hand, in the catalyst according to Comparative Example 2 in which platinum was supported without reducing rhodium, in the case of exhaust gas with moisture, the purification temperature was not significantly different from that in Comparative Example 1 which is a conventional method. That is, although the poisoning by water has occurred, it is considered that the catalytic function is not hindered more than the conventional catalyst. On the other hand, in the case of the exhaust gas from which moisture was removed, the purification temperature was increased to a temperature exceeding 240 ° C., and it was recognized that the catalytic function was impaired. Since platinum produces a water gas shift function together with cerium, the exhaust gas with moisture has the same purification temperature as in Comparative Example 1, but in the absence of moisture, the NOx purification function based on the water gas shift does not occur. In addition, in the catalyst according to Comparative Example 2, the catalytic function of rhodium is also inhibited, so that the purification temperature in the absence of moisture seems to have increased. That is, as shown in Table 1, the electronegativity of rhodium oxide is “2.98”, which is the same as that of titania (TiO ₂ ) of “2. Greater than 83 ". As a result, in the method of Comparative Example 2, it is considered that a large amount of platinum was supported on rhodium, and the degree of exposure of rhodium was lowered to inhibit the catalytic function.

  In contrast to the catalysts obtained in the comparative examples 1 and 2, according to the catalyst obtained in the example according to the present invention, the purification temperature of the model gas containing moisture is lowered to a level below 220 ° C. It was confirmed that the NOx purification performance was greatly improved. This is presumably due to the fact that platinum particles are widely dispersed and reliably supported in the vicinity of cerium without increasing the particle size, and as a result, a water gas shift function is sufficiently generated by platinum and cerium. On the other hand, the purification temperature for the model exhaust gas from which moisture was removed was substantially the same as that of the catalyst obtained in Comparative Example 1 which is a conventional method. Since NOx purification in the absence of moisture is purification that does not involve a water gas shift, rhodium contributes significantly. Therefore, it is considered that rhodium in the catalyst according to this example is not covered by platinum even if a weakly basic organic platinum aqueous solution is used for supporting platinum, and the activity is not hindered. . That is, as shown in Table 1, since the electronegativity of the reduced metallized rhodium is smaller than that of the support and is the same as that of platinum, a weakly basic organic platinum aqueous solution is used to support platinum. In this case, the degree of adsorption of platinum to rhodium is low, and it is adsorbed mainly on an electron accepting point such as an acid property point or a site having a large electronegativity in the carrier, and is baked and supported in that state. Even if it is supported after rhodium, it seems that rhodium is not covered with platinum, and therefore the exposure is not lowered and the activity is not inhibited.

  In the present invention, a NOx occlusion material made of one or more of alkali metals and / or alkaline earths can also be supported on the carrier. In that case, as shown in Table 1, since the electronegativity of metallized rhodium is close to the electronegativity of those NOx occlusion materials, even if the NOx occlusion material is supported after rhodium, rhodium. It is possible to suppress or prevent the NOx storage material from adhering to and covering the surface.

It is a figure which shows typically the structure of the catalyst obtained in the Example of this invention, and the structure of the catalyst obtained in the comparative example 2 which does not reduce. It is a figure which shows the measurement result of NOx50% purification temperature about the exhaust gas purification catalyst obtained in the Example and the comparative examples 1 and 2. FIG.

Claims (3)

In the method for producing an exhaust gas purifying catalyst in which platinum is supported on a carrier after rhodium is supported on the carrier ,
Rhodium subjected to Motosho physical place before Symbol carrier carrying metallised rhodium, then, larger even electric negativity than the metallized rhodium in electron accepting point or the upper support on said carrier platinum using a weakly basic organic aqueous solution of platinum that is carried to the site, method of manufacturing the exhaust gas purifying catalyst, characterized in that it is supported in front Symbol carrier. Before Ki担body, method for manufacturing the exhaust gas purifying catalyst according to claim 1 you characterized by comprising a coating layer on the surface containing the cerium-containing composite oxide and a cerium-free oxide. The method of manufacturing the exhaust gas purifying catalyst according to claim 1 or 2, characterized in that alkali metal and / or alkaline earth NOx-absorbing material consisting of one or more metal is supported on a front Ki担body.

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