JP2007283207A - Catalyst for cleaning exhaust gas and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas, which can satisfactorily restrain grain growth of a noble metal while exhibiting such high-level resistance to sulfur poisoning that the catalyst is sufficiently restrained from being poisoned by sulfur even when exposed for a long period of time to a high-temperature atmosphere and can keep high-level activity of removing NOx over a long period of time, and to provide a method for manufacturing the catalyst. <P>SOLUTION: The catalyst for cleaning exhaust gas comprises: a basic oxide carrier 1 containing an oxide of at least one metal selected from the group composed of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium; the noble metal 2 deposited on the basic oxide carrier 1; a NOx storage material 3 deposited on the basic oxide carrier; and an oxide covering layer 4 which is formed on the exposed surface of the basic oxide carrier 1 where the noble metal 2 and the NOx storage material 3 are not deposited and is composed of oxides of silicon and/or titanium. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst and a method for producing the same, and more particularly to an exhaust gas purifying catalyst that can be suitably used to purify exhaust gas discharged from an engine or the like and a method for producing the same.

Recently, as a catalyst for purifying exhaust gases from lean-burn gasoline engine, NO _x storage reduction catalyst has been put to practical use. Examples of such NO _x storage reduction catalysts include those 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 support such as alumina (Al ₂ O ₃ ). Then, in the case of using such a NO _x storage-and-reduction type catalyst, for controlling the air-fuel ratio so that the fuel stoichiometric-rich side from the fuel-lean side in a pulsed manner and (hitting the rich spike), in the lean side The NO _x in the exhaust gas is occluded by the NO _x occlusion material in the catalyst. On the stoichiometric to rich side, NO _x is released from the NO _x storage material and comes into contact with the noble metal, and its catalytic action reacts with reducing components such as HC and CO to purify NO _x . For this reason, in such a NO _x storage reduction catalyst, NO _x emission can be suppressed even on the lean side, so that it is possible to exhibit high NO _x purification ability as a whole.

However, in such a conventional NO _x storage reduction catalyst, the NO _x storage material reacts with SO _x present in the exhaust gas and deteriorates poisoning (this phenomenon is called “sulfur poisoning”). There was a problem. In addition, such a conventional NO _x storage-reduction catalyst has a problem in that sulfur poisoning is promoted because a porous carrier such as Al ₂ O ₃ easily adsorbs SO _x . . Therefore, various exhaust gas purifying catalysts have been studied for the problem of sulfur poisoning.

For example, in JP-A-8-099034 (Patent Document 1), 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 is used. An exhaust gas purification catalyst comprising at least one NOx absorbent selected from alkali metals, alkaline earth metals and rare earth elements supported on the composite carrier and a catalytic noble metal supported on the composite carrier is disclosed. Has been. JP 2000-24499 A (Patent Document 2) discloses a support containing alumina, a noble metal supported on the support, a NO _x storage material supported on the support, the noble metal of the support, and Exhaust gas purifying catalyst comprising an oxide layer made of an oxide of at least one metal selected from Si, Ti, Zr, W, Nb, Fe and Mo formed on a surface on which the NO _x storage material is not supported Is disclosed. However, in the catalyst for purification of exhaust gas according to this patent Documents 1 and 2, that a long time exposure to a high temperature atmosphere noble metal lowers it is _the NO _x purification activity grain growth (sintering) There was a problem. Further, even in such exhaust gas purifying catalysts described in Patent Documents 1 and 2, poisoning by sulfur has not necessarily been sufficiently suppressed.

On the other hand, various methods for regenerating an exhaust gas purifying catalyst in which grain growth has occurred in a noble metal have been developed. For example, in Japanese Patent Application Laid-Open No. 2000-202309 (Patent Document 3), an exhaust gas purifying catalyst comprising a support containing at least one selected from alkaline earth metal oxides and rare earth oxides and platinum supported on the support. On the other hand, a method for regenerating an exhaust gas purifying catalyst is disclosed in which an oxidation treatment is performed by heating in an oxidizing atmosphere containing oxygen, followed by a reduction treatment. Further, in JP-A-2003-74334 (Patent Document 4), treatment of an exhaust gas purifying catalyst comprising a support containing a basic oxide having a specific surface area of 10 m ² / g or more and a noble metal supported on the support. An oxidation treatment in which the catalyst that has been exposed to exhaust gas and has deteriorated catalyst performance is treated in a high-temperature oxidizing atmosphere at 600 ° C. or higher, and the catalyst after the oxidation treatment is subjected to a stoichiometric atmosphere or reduction at 800 ° C. or lower. A method for regenerating an exhaust gas purifying catalyst that performs a reduction treatment in a neutral atmosphere is disclosed. However, in such exhaust gas purifying catalysts described in Patent Documents 3 to 4, it is possible to suppress the growth of noble metal grains, but the carrier used is easy to adsorb SO _x. There has been a problem in that sulfur poisoning of the NO _x storage material is promoted, and the NO _x purification activity decreases when exposed to a high temperature atmosphere for a long time.
JP-A-8-099034 JP 2000-24499 A JP 2000-202309 A JP 2003-74334 A

The present invention has been made in view of the above-mentioned problems of the prior art, and is exposed to a high-temperature atmosphere for a long time while exhibiting a high degree of sulfur poisoning resistance capable of sufficiently suppressing sulfur poisoning. also it can be sufficiently suppress grain growth of the noble metal, and an object thereof is to provide an exhaust gas purifying catalyst and a manufacturing method thereof capable of maintaining a high degree of _the NO _x purification activity for a long time in a case where it is .

As a result of intensive studies to achieve the above object, the present inventors have found that a basic oxide containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium. Using a carrier, the carrier is loaded with a NO _x storage material and a noble metal, and the exposed surface of the basic oxide carrier on which the noble metal and the NO _x storage material are not carried is oxidized of silicon and / or titanium. Surprisingly, by forming an oxide coating layer made of a material, it is exposed to a high temperature atmosphere for a long time while exhibiting a high degree of sulfur poisoning resistance that can sufficiently suppress sulfur poisoning. also can be sufficiently suppress grain growth of the noble metal when the advanced _the NO _x purification activity can be the maintenance of the exhaust gas purifying catalyst for a long period of time to give Heading to be, and have completed the present invention.

That is, the exhaust gas purifying catalyst of the present invention includes a basic oxide support containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium, and calcium, and the basic oxide. to the exposed surface of a noble metal supported on a carrier, and _the nO _x storage material that is supported on the basic oxide support, the noble metal and the _the nO _x storage material on the surface of said basic oxide support is not carried And an oxide coating layer made of oxide of silicon and / or titanium.

Further, the method for producing an exhaust gas purifying catalyst of the present invention includes a basic oxide support containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium, and a noble metal and NO. _x carrying the occlusion material;
Selected from the group consisting of the hydroxyl group of the exposed surface on which the noble metal and the NO _x storage material are not supported on the surface of the basic oxide support, and silicon chloride, silicon alkoxide, titanium chloride and titanium alkoxide. Hydrolyzing at least one kind of coating layer material to form an oxide coating layer on the exposed surface to obtain an exhaust gas purification catalyst;
It is the method characterized by including.

In the method of manufacturing the exhaust gas purifying catalyst of the present invention, in the step of supporting the noble metal and NO _x storage material on the basic oxide support, it is preferable to carrying _the NO _x storage material as a salt of organic matter.

In addition, in the case of being exposed to a high temperature atmosphere for a long time while exhibiting high sulfur poisoning resistance capable of sufficiently suppressing sulfur poisoning by the exhaust gas purifying catalyst of the present invention and the manufacturing method thereof. Although the reason why the grain growth of the noble metal can be sufficiently suppressed and the high NO _x purification activity can be maintained for a long time is not necessarily clear, the present inventors infer as follows. . That is, first, in the present invention, a basic oxide carrier containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium is used, and the basic oxidation is performed. Since the material support exhibits a very strong interaction with the noble metal, the grain growth of the noble metal when using the catalyst is sufficiently suppressed. On the other hand, since such a basic oxide support promotes the adsorption of SO _x , if it is used as it is, it becomes difficult to sufficiently suppress sulfur poisoning. However, in the present invention, an acidic oxide coating layer made of an oxide of silicon and / or titanium is formed on the exposed surface of the surface of the basic oxide carrier on which noble metal and NO _x storage material are not supported. Therefore, it is difficult for SO _{x to} be adsorbed, and the adsorption of SO _x to the carrier is sufficiently suppressed. In the present invention, the reason is not necessarily clear, but the specific basic oxide support as described above, and the noble metal and the NO _x storage material on the surface of the basic oxide support are supported. With the above-mentioned specific acidic oxide coating layer formed on the exposed surface, it becomes possible to synergistically improve the effect of suppressing sulfur poisoning and the effect of suppressing grain growth of noble metals, The present inventors infer that the coexistence of suppression of sulfur poisoning and suppression of noble metal grain growth can be achieved at a higher level.

Further, in the method for producing an exhaust gas purifying catalyst of the present invention, the exposed surface of the basic oxide support on which noble metal and NO _x storage material are not supported, the hydroxyl group of silicon, the chloride of silicon, the alkoxide of silicon, titanium By using a hydrolysis reaction with at least one coating layer material selected from the group consisting of chlorides and alkoxides of titanium, an oxide coating layer is preferentially and efficiently formed on the exposed surface, The exhaust gas purifying catalyst of the present invention can be efficiently produced. Further, when carrying _the NO _x storage material as salts of organic substances, since that the oxide coating layer on _the NO _x storage material is formed is more sufficiently prevented, lowering of the activity of _the NO _x storage material This makes it possible to produce an exhaust gas purifying catalyst while more efficiently suppressing the above.

According to the present invention, while exhibiting a high degree of sulfur poisoning resistance capable of sufficiently suppressing poisoning by sulfur, the grain growth of noble metals is sufficiently suppressed even when exposed to a high temperature atmosphere for a long time. to be able to, it is possible to provide a high degree of _the NO _x purification activity of exhaust gas purifying catalyst and a manufacturing method thereof can be maintained for a long period.

  Hereinafter, a preferred embodiment of the exhaust gas purifying catalyst of the present invention will be described with reference to the drawings. In the following description and drawings, the same or corresponding elements are denoted by the same reference numerals, and duplicate descriptions are omitted.

The exhaust gas purifying catalyst of the present invention includes a basic oxide carrier containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium, and the basic oxide carrier. and supported noble metal, and _the nO _x storage material that is supported on the basic oxide support, the noble metal and the _the nO _x storage material on the surface of said basic oxide support is formed on the exposed surface not being carried And an oxide coating layer made of an oxide of silicon and / or titanium.

  FIG. 1 is a schematic view showing a part of the vicinity of the surface of a basic oxide carrier according to a preferred embodiment of the exhaust gas purifying catalyst of the present invention. The exhaust gas purifying catalyst of the present invention shown in FIG. 1 includes a basic oxide carrier 1, a noble metal 2 and NOx occlusion material 3 carried on the carrier 1, and an NOx occlusion material 3 and noble metal 2 on the surface of the carrier 1. And an oxide coating layer 4 formed on an exposed surface that is not formed.

The basic oxide carrier 1 according to the present invention is at least one selected from the group consisting of cerium (Ce), praseodymium (Pr), lanthanum (La), zirconium (Zr), magnesium (Mg), and calcium (Ca). The metal oxide is included. Among such basic oxide carriers, those containing CeO ₂ are more preferable from the viewpoint that the interaction with the noble metal is stronger and the affinity with the noble metal tends to be greater. Such a basic oxide carrier may contain the above metal oxide alone or as a composite oxide.

Such composite oxides, for _{example, CeO 2 -ZrO 2, ZrO 2} -La 2 O 3, CeO 2 -ZrO 2 -La 2 O 3 -Pr 2 O 3, CeO 2 -CaO, CaO-ZrO 2 , La ₂ O ₃ —MgO, and the like. For example, in the case where such a basic oxide support is a complex oxide of Ce and Zr, the specific surface area is increased and the heat resistance tends to be improved as compared with the case where CeO ₂ is used alone.

  Further, when the basic oxide carrier 1 contains the composite oxide, the value of the binding energy of the oxygen 1s orbital of the composite oxide is preferably 531 eV or less, and 531 to 529 eV. It is particularly preferable that the value of When a composite oxide having a binding energy value exceeding 531 eV is used, the interaction between the noble metal and the support tends not to be sufficiently strong. On the other hand, when a composite oxide having a binding energy value of less than 529 eV is used, the interaction between the noble metal and the support becomes so strong that sufficient activity tends not to be obtained.

Examples of the composite oxide satisfying such conditions include the following:
ZrO ₂ -La ₂ O _3: 530.64eV
CeO ₂ —ZrO ₂ : 530 eV
_{CeO 2 -ZrO 2 -La 2 O 3} -Pr 2 O 3: 529.79eV
Is mentioned.

In addition, such a basic oxide carrier 1 may be any material as long as it contains the metal oxide, and may contain other components (for example, Al ₂ O ₃ , SiO ₂ , TiO _2, etc.). Good. When such other components are contained, the ratio of the other components in the basic oxide carrier 1 is preferably 30% by mass or less. When the upper limit is exceeded, the interaction between the noble metal and the carrier tends not to be sufficiently strong.

The shape of the basic oxide carrier 1 is not particularly limited, but is preferably in the form of powder from the viewpoint of increasing the specific surface area and obtaining higher catalytic activity. Although this is not a particular restriction on the specific surface area of the basic oxide support is preferably from 20 to 300 m ² / g, and more preferably 50 to 300 m ² / g. When the specific surface area of such a basic oxide support is smaller than 20 m ² / g, active species such as noble metals present on the support surface tend to aggregate significantly and the catalyst performance tends not to be sufficiently exhibited, When it exceeds 300 m ² / g, the heat resistance of the carrier tends to decrease. Such a specific surface area can be calculated as a BET specific surface area from an adsorption isotherm using a BET isotherm adsorption equation.

  In addition, the manufacturing method in particular of the basic oxide support | carrier 1 concerning this invention is not restrict | limited, It can manufacture by employ | adopting a well-known method suitably. For example, in the case of producing the basic oxide carrier 1 containing a composite oxide, the above-mentioned salts of various metals (for example, nitrate) and, if necessary, a surfactant (for example, a nonionic surfactant). In the presence of ammonia, a metal oxide coprecipitate is produced in the presence of ammonia, and the resulting coprecipitate is filtered, washed, dried, and then calcined.

  Further, examples of the noble metal 2 according to the present invention include platinum, rhodium, palladium, osmium, iridium, and gold. From the viewpoint that the obtained exhaust gas purifying catalyst exhibits higher catalytic activity, platinum, rhodium, Palladium is preferred. Such noble metals can be used singly or in combination.

Further, the amount of the noble metal 2 supported is preferably 0.05 to 20% by mass with respect to the carrier. The supported amount is less than 0.05 wt% of the noble metal tend to decrease _the NO _x purification activity catalytic activity obtained by the noble metal becomes insufficient, while if it exceeds 20 wt%, the cost soars At the same time, it tends to be difficult to sufficiently suppress the grain growth of the noble metal.

  In the exhaust gas purifying catalyst of the present invention, the noble metal 2 is preferably supported on the carrier in the form of finer particles. The particle diameter of such noble metal particles is preferably 3 nm or less, and more preferably 2 nm or less. When the particle diameter of the noble metal exceeds the upper limit, it tends to be difficult to obtain high catalytic activity.

As the NO _x storage material 3 according to the present invention, at least one element selected from alkali metals, alkaline earth metal rare earth metals and rare earth metals can be suitably used. Examples of such alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). An alkaline earth metal refers to a Group 2A element of the periodic table, and examples include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Furthermore, examples of such rare earth metals include lanthanum (La), cerium (Ce), and praseodymium (Pr).

Further, the loading amount of the NO _x storage material 3 is preferably 0.1 to 1.2 mol per liter of the basic oxide support. Is less than the _the NO _x storage material supported amount the lower limit, _the NO _x storage performance tends to decrease, whereas if it exceeds the upper limit, the _the NO _x storage material ends up covering the precious metal, thereby NO _x There exists a tendency for purification activity to fall.

In the present invention, the oxide coating layer 4 made of silicon and / or titanium oxide is formed on the exposed surface of the basic oxide carrier 1 on which the noble metal 2 and the NO _x storage material 3 are not supported. Has been. Then, in the present invention, thus, it is possible to sufficiently suppress the adsorption of the SO _x for the oxide coating layer 4 is made of an oxide of silicon and / or titanium. In addition, the oxide coating layer 4 and the basic oxide carrier 1 can more sufficiently suppress the noble metal grain growth and sulfur poisoning.

The thickness of the oxide coating layer 4 is not particularly limited, but is preferably about 1 to 3 atomic layers from the surface of the basic oxide carrier 1. When the thickness of the oxide coating layer 4 exceeds the triatomic layer from the surface of the basic oxide support, the surface of the noble metal or the NO _x storage material tends to be covered. When the surface of the NO _x storage material is covered, these active species are deactivated, and the NO _x purification performance tends to decrease. On the other hand, in the oxide coating layer 4 thinner than one atomic layer, the effect of suppressing the adsorption of SO _x tends to be insufficient.

Further, the amount of silicon and / or titanium oxide supported is not generally determined because the ratio is appropriately changed depending on the specific surface area or thickness of the basic oxide carrier 1. The basic oxide carrier 1 is preferably about 1 to 20% by mass. If the supported amount of oxide of silicon and / or titanium is less than the lower limit, the oxide coating layer 4 having a sufficient thickness tends not to be formed. On the other hand, if the upper limit is exceeded, the surface of the noble metal or NO _x storage material there will be covered with the oxide coating layer 4 tends to _the NO _x purification performance is reduced. The supported amount of silicon and / or titanium oxide is preferably about 1 to 40 g per liter of the catalyst.

  The form of the exhaust gas purifying catalyst of the present invention is not particularly limited, and may be a form of a honeycomb-shaped monolith catalyst, a pellet-shaped pellet catalyst, or the like.

Further, the exhaust gas purifying catalyst of the present invention can purify NO _x by bringing it into contact with exhaust gas. Hereinafter, the NO _x purification action estimated to occur when the exhaust gas purification catalyst of the present invention is used will be described.

That is, first, NO in the exhaust gas is NO _x are oxidized by the precious metal 2. The NO _x oxidized on the noble metal 2 in this way spills over and is stored in the NO _x storage material 3. On the other hand, since SO _x in the exhaust gas is difficult to be adsorbed by the oxide layer 4, SO _x coming into contact with the NO _x storage material 3 is reduced, and sulfur poisoning is sufficiently suppressed. Further, at the time of _the NO _x reduction, NO _x storage material 3 NO _x that was stored in is eliminated by spillover, it reacts with HC in the exhaust gas by the noble metal 2 by moving the surface of the oxide coating layer 4 Reduced to N ₂ . In such an exhaust gas purifying catalyst, the reason is not necessarily clear, but the basic oxide carrier 1 and the oxide coating layer 4 provide the effect of suppressing sulfur poisoning and the grain growth of the noble metal 2. the effect of suppressing a is synergistically improved, thereby both the inhibition of grain growth inhibition and precious metal 2 sulfur poisoning is achieved at higher levels, advanced _the NO _x purification activity is maintained for a long period of time Inferred.

  While the exhaust gas purification catalyst of the present invention has been described above, the following will describe the method of manufacturing the exhaust gas purification catalyst of the present invention that can suitably manufacture the exhaust gas purification catalyst of the present invention.

The method for producing an exhaust gas purifying catalyst of the present invention comprises a basic oxide support containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium, and stores a noble metal and NO _x. A step (A) of supporting a material;
Selected from the group consisting of the hydroxyl group of the exposed surface on which the noble metal and the NO _x storage material are not supported on the surface of the basic oxide support, and silicon chloride, silicon alkoxide, titanium chloride and titanium alkoxide. Hydrolyzing at least one kind of coating layer material to form an oxide coating layer on the exposed surface to obtain an exhaust gas purifying catalyst (B);
It is the method characterized by including.

In the present invention, first, a noble metal and a NO _x storage material are supported on a basic oxide support containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium ( Step (A)).

The basic oxide carrier, the noble metal and the NO _x storage material used in the present invention are the same as those described in the exhaust gas purifying catalyst of the present invention.

  In the step (A), the method for supporting the noble metal is not particularly limited. For example, an aqueous solution containing a salt of a noble metal (for example, a dinitrodiamine salt) or a complex (for example, a tetraammine complex) is used as the basic oxide carrier. The method of drying after making it contact and also baking can be employ | adopted.

Further, in the step (A), the method for supporting the NO _x storage material is not particularly limited. For example, the salt of the above-described element suitably used as the NO _x storage material (for example, carbonate, nitrate, citrate) , Carboxylate, dicarboxylate, sulfate) and an aqueous solution containing the complex are brought into contact with the basic oxide carrier, dried, and further calcined.

As a method for carrying such _the NO _x storage material, it is preferable to employ a method of carrying _the NO _x storage material as a salt of organic matter. By supporting the NO _x storage material as an organic salt in this way, it is possible to more efficiently prevent the oxide coating layer from being formed on the surface of the NO _x storage material when the oxide coating layer is formed. , it is possible to impart a high degree of _the NO _x purification performance by the resulting catalyst for purification of exhaust gas.

Also, such as the organic salts, such _the NO _x storage material, carboxylates of the elements to be used as _the NO _x storage material described above (acetate, butyrate, stearate, etc.) or a dicarboxylic acid salt (oxalate , Malonate, etc.). Further, among the organic salts of such NO _x storage materials, acetate is preferable from the viewpoint of production cost.

In the step (A), the order in which the NO _x storage material and the noble metal are supported on the carrier is not particularly limited, but from the viewpoint of elution of the NO _x storage material into water, the NO _x storage is performed after the noble metal is supported. It is preferable to carry a material.

Next, in the present invention, the exposed surface hydroxyl group on which the noble metal and the NO _x storage material are not supported on the surface of the basic oxide carrier, silicon chloride, silicon alkoxide, titanium chloride and At least one coating layer material selected from the group consisting of titanium alkoxides is hydrolyzed to form an oxide coating layer on the exposed surface to obtain an exhaust gas purification catalyst (step (B)).

  Such a coating layer material is at least one selected from the group consisting of silicon chloride, silicon alkoxide, titanium chloride and titanium alkoxide. Examples of such silicon chloride, silicon alkoxide, titanium chloride or titanium alkoxide include silicon chloride, tetramethyl orthosilicate, tetraethyl orthosilicate, tetraisopropyl orthosilicate, tetrabutyl orthosilicate, titanium chloride, titanium. Examples thereof include tetraisopropoxide. The silicon or titanium alkoxide is preferably a silicon or titanium alkoxide bonded with an alkoxide having 1 to 3 carbon atoms from the viewpoint of hydrolysis rate.

In addition, the step (B) is not particularly limited, but since the coating layer material is easily hydrolyzed by water, it is efficiently hydrolyzed with the hydroxyl group on the exposed surface of the basic oxide support. In order to cause the reaction to form an oxide coating layer, for example, the coating layer is formed on the basic oxide support (hereinafter referred to as “catalyst precursor”) on which the noble metal and the NO _x storage material are supported. Method (i) in which the vapor of the material is brought into contact to cause a hydrolysis reaction, and then calcined, or the catalyst precursor and the coating layer material are introduced into an organic solvent such as alcohol or ether to cause a hydrolysis reaction, and then The method (ii) of firing can be employed. In addition, by repeating such a method once to three times, an oxide coating layer having a thickness of about 1 to 3 atomic layers can be formed on the basic oxide support.

In addition, as the method (i) for bringing the vapor of the coating layer material into contact, for example, a known method is adopted and the catalyst precursor is degassed, and then the temperature is set to 50 to 500 ° C. The coating layer material is brought into contact with the catalyst precursor as a vapor by bubbling N ₂ gas while maintaining for about 5 to 5 hours, thereby introducing the coating layer material into the catalyst precursor, An example is a method in which H ₂ O is introduced by bubbling for about 0.5 to 5 hours under a temperature condition of 500 ° C. to completely hydrolyze the alkoxide on the surface of the carrier, and further calcined.

  In addition, as a method (ii) of reacting in an organic solvent, for example, the catalyst precursor is impregnated in an alcohol such as ethanol or isopropanol, and after the coating layer material is added, the coating layer material is adhered. There is a method in which the catalyst precursor is taken out, impregnated in ion-exchanged water for hydrolysis, and further calcined.

Moreover, although it does not restrict | limit especially as said baking process, It is preferable to employ | adopt the method of baking about 1 to 10 hours on the temperature conditions of 300-700 degreeC. By firing in this way, an oxide coating layer is formed, whereby an oxide coating layer is formed on the exposed surface of the surface of the basic oxide carrier on which the noble metal and the NO _x storage material are not supported. Thus, the exhaust gas purifying catalyst of the present invention can be suitably produced.

In the present invention, the exhaust gas purifying catalyst thus obtained can be in the form of a honeycomb-shaped monolith catalyst, a pellet-shaped pellet catalyst, or the like. The production method of the catalyst in such a form is not particularly limited, and a known method can be appropriately employed. For example, in the case of producing a monolithic catalyst, a coat layer made of the above-mentioned basic oxide carrier powder is formed on a honeycomb-shaped substrate formed of cordierite or metal foil, and a noble metal and NO _x occlusion are formed thereon. A method of forming an oxide coating layer after supporting a material, or a powder in which an oxide coating layer is formed after supporting a noble metal and a NO _x storage material on the above-described carrier powder, A method of forming a coat layer on the substrate can be suitably employed. In addition, it does not restrict | limit especially as a base material used here, According to the use etc. of the catalyst obtained, it selects suitably, However, A DPF base material, a monolithic base material, a pellet-shaped base material, a plate-shaped base material etc. Preferably employed. Also, the material of such a base material is not particularly limited, but a base material made of a ceramic such as cordierite, silicon carbide, mullite, or a base material made of a metal such as stainless steel including chromium and aluminum is suitably employed. Is done.

  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.

Example 1
First, an aqueous solution was prepared by dissolving cerium (III) nitrate hexahydrate and zirconyl oxynitrate in pure water so that the molar ratio of Ce / Zr was 1/1. Next, after adding 1.1 times the equivalent amount of hydrogen peroxide solution of cerium ion to the resulting aqueous solution, 1.2 times as much ammonia solution as the neutralization equivalent of the nitrate radical of each salt was added with vigorous stirring. To precipitate a precipitate. Next, the resulting precipitate was centrifuged to discard the supernatant, and ion-exchanged water was added thereto and stirred, and the precipitate was washed again by being centrifuged. The washed precipitate is dried at 100 ° C. for 8 hours, calcined in air at 300 ° C. for 3 hours, and further calcined in air at 600 ° C. for 5 hours to obtain CeO. _2- ZrO ₂ powder (specific surface area 100 m ² / g) was obtained.

Next, 270 g of the obtained CeO ₂ —ZrO ₂ powder and ceria sol corresponding to 10% by mass of the total solid content were mixed in a ball mill to prepare a slurry. Then, using the obtained slurry, wash coating was performed on a cordierite honeycomb substrate (diameter 30 mm, length 50 mm) so that the coating amount of the slurry was 160 g per liter of the honeycomb substrate volume, and 500 The coating layer was formed by baking at 3 ° C. for 3 hours.

Next, the amount of Rh supported was 0.3 g per liter of the honeycomb substrate volume, and the aqueous rhodium nitrate solution was absorbed into the coating layer and fired at 300 ° C. for 3 hours to support Rh. Next, an aqueous dinitrodiammine platinum solution was absorbed by the coating layer so that the amount of Pt supported was 3 g per liter of the honeycomb substrate, and Pt was supported by firing at 300 ° C. for 3 hours. Next, a mixed aqueous solution of barium acetate, potassium acetate and lithium acetate was prepared so that the supported amounts of Ba, K and Li were 0.05 mol, 0.025 mol and 0.3 mol, respectively, per 1 L of the honeycomb substrate volume. The coating layer was allowed to absorb water and dried at 100 ° C. for 6 hours to carry Ba, K and Li acetates. In this way, a catalyst precursor in which a noble metal (Rh and Pt) and a NO _x storage material (Ba, K and Li) were supported on a basic oxide support was obtained.

Next, the obtained catalyst precursor was degassed at 200 ° C. for 1 hour under a flow of N ₂ gas. Next, tetramethyl orthosilicate is brought into contact with the catalyst precursor as a vapor by bubbling N ₂ gas while maintaining the temperature at 200 ° C. for 2 hours with respect to the catalyst precursor after degassing treatment, and tetramethyl orthosilicate is added to the catalyst precursor. Introduced. Thereafter, H ₂ O was introduced for 0.5 hour by bubbling to completely hydrolyze the alkoxide on the surface of the catalyst precursor. Subsequently, H ₂ O present in the catalyst precursor after being hydrolyzed in this way and MeOH generated by the hydrolysis were completely removed by flowing N ₂ gas, and then again at a temperature of 200 ° C. While maintaining for 2 hours under conditions, tetramethyl orthosilicate was brought into contact as a vapor by bubbling N ₂ gas, and tetramethyl orthosilicate was introduced. Then, the oxide coating layer made of SiO _{2 is} baked in the atmosphere at a temperature condition of 650 ° C. for 3 hours on the exposed surface on the surface of the basic oxide carrier on which the noble metal and the NO _x storage material are not supported. To form a monolithic catalyst (exhaust gas purifying catalyst). The amount of SiO ₂ supported was 8.0 g per liter of honeycomb substrate volume.

(Example 2)
A monolith catalyst (exhaust gas purification catalyst) was obtained in the same manner as in Example 1 except that titanium tetraisopropoxide was used instead of tetramethyl orthosilicate to form an oxide coating layer made of TiO ₂ . The amount of TiO ₂ supported was 8.3 g per liter of honeycomb substrate volume.

(Example 3)
First, a catalyst precursor was prepared by adopting the same method as the method for preparing the catalyst precursor in Example 1. Next, the catalyst precursor was impregnated in a mixed solution of 10 mL of tetraethyl orthosilicate and 50 mL of ethanol, and tetraethyl orthosilicate was hydrolyzed on the surface of the catalyst precursor. Subsequently, after the catalyst precursor after hydrolysis in this manner was pulled up and excess droplets were blown off, this was impregnated in ion-exchanged water to completely hydrolyze the alkoxide on the surface of the support. Further, after drying the catalyst precursor after completely hydrolyzing the alkoxide at a temperature of 100 ° C. for 6 hours, the catalyst precursor is impregnated again in an ethanol solution in which tetraethyl orthosilicate is dissolved. Tetraethyl orthosilicate was hydrolyzed on the surface of the body. Thereafter, baking is performed at a temperature of 650 ° C. for 3 hours to form an oxide coating layer made of SiO _{2 on the} exposed surface of the basic oxide carrier on which the noble metal and the NO _x storage material are not supported. A monolith catalyst (catalyst for exhaust gas purification) was obtained. The amount of SiO ₂ supported was 9.1 g per liter of honeycomb substrate volume.

Example 4
A monolith catalyst (catalyst for exhaust gas purification) was used in the same manner as in Example 3 except that a solution in which titanium tetraisopropoxide and isopropanol were mixed instead of tetramethyl orthosilicate and an oxide coating layer made of TiO ₂ was formed. ) The amount of TiO ₂ supported was 9.5 g per liter of honeycomb substrate volume.

(Example 5)
A monolith was prepared in the same manner as in Example 1 except that Ba, K, and Li acetates were supported and then calcined at 400 ° C. for 3 hours to decompose the acetate, and Ba, K, and Li were each supported as oxides. A catalyst (catalyst for exhaust gas purification) was obtained. The amount of SiO ₂ supported was 8.8 g per liter of honeycomb substrate volume.

(Comparative Example 1)
A monolith catalyst (exhaust gas purifying catalyst) for comparison was obtained in the same manner as in Example 1 except that tetramethyl orthosilicate was not used to form an oxide coating layer made of SiO ₂ .

(Comparative Example 2)
A monolith catalyst for comparison (exhaust gas purification catalyst) was obtained in the same manner as in Example 1 except that a commercially available γ-Al ₂ O ₃ powder was used instead of the CeO ₂ —ZrO ₂ powder.

[Evaluation of characteristics of exhaust gas purification catalysts obtained in Examples 1 to 5 and Comparative Examples 1 and 2]
<Durability test>
As the durability test, (a) heat resistance test and (b) sulfur poisoning / desorption test were performed on the exhaust gas purifying catalysts obtained in Examples 1 to 5 and Comparative Examples 1 and 2.

(A) Heat resistance test Each exhaust gas purification catalyst was heated in the atmosphere at 750 ° C for 5 hours (heat resistance test).

(B) Sulfur poisoning / desorption test For each exhaust gas purification catalyst after the heat resistance test, the supply time of the model gas in the lean atmosphere and rich atmosphere in the sulfur poisoning test shown in Table 1 was 120 seconds (lean). ) And 3 seconds (rich), and each gas was supplied alternately at a flow rate of 3 L / min, and heated at 350 ° C. for 41 minutes (sulfur poisoning test). The amount of sulfur supplied in this way was 1.5 g (0.047 mol) per liter of catalyst volume.

  Next, the exhaust gas purification catalyst after sulfur poisoning test is heated for 10 minutes at a temperature condition of 600 ° C. while supplying a rich atmosphere model gas of sulfur desorption test shown in Table 1 at a flow rate of 30 L / min. (Sulfur desorption test).

<Measurement of NO _x purification rate>
Each exhaust gas purifying catalyst after the endurance test as described above is installed in a normal pressure fixed bed flow type reaction apparatus, and the model gas in the lean atmosphere and the rich atmosphere in the activity evaluation test shown in Table 1 is supplied for 30 hours. Second (lean) and 3 seconds (rich), and the gas flow rate was alternately supplied at 30 L / min. The inlet gas temperature at this time was 400 ° C. or 350 ° C. In addition, the space velocity at this time was about 51000 h ⁻¹ . Then, for each catalyst, the concentration of the NO _x contained in each gas entering and exiting gas was measured to determine the NOx purification rate from the concentration of NO _x. The model gas shown in Table 1 simulates exhaust gas discharged from a diesel engine under typical operating conditions. The obtained results are shown in Table 2 and FIG.

As is clear from the results shown in Table 2 and FIG. 2, the exhaust gas purifying catalysts (Examples 1 to 5) in which an oxide coating layer of Si or Ti was formed using CeO ₂ —ZrO ₂ as a carrier were Comparative Example 1. high _the NO _x purification rate than the exhaust gas purifying catalyst obtained in ~ 2, it was confirmed that superior _the NO _x purification performance. From these results, it was found that a high NO _x purification activity can be obtained over a long period of time by using CeO ₂ —ZrO ₂ (basic oxide support) as a support and forming an oxide coating layer thereon. .

Further, NO _x than storage material supported by the oxide coating layer formed by the resultant catalyst after as an oxide (Example 5), oxide after carrying _the NO _x storage material as the salt of the organic substance It was confirmed that the exhaust gas purifying catalysts (Examples 1 to 4) obtained by forming the coating layer had a higher NO _x purification rate. From these results, by carrying _the NO _x storage material as salts of organic, less likely to be an oxide coating layer on _the NO _x storage material is formed, to be able to more efficiently prevent deactivation of the NO _x storage material I understood.

<Activity reduction rate of NO _x purification rate>
For the exhaust gas purifying catalysts obtained in Examples 1 to 5 and Comparative Examples 1 and 2, the same method as the method for measuring the NO _x purification rate (entered gas temperature of 400 ° C.) was adopted before the durability test ( _the NO _x purification rate of initial), (a) _the NO _x purification ratio after the heat resistance test, durability with respect to the (b) sulfur poisoning and desorption after the test _the NO _x purification rate were measured, the initial of the NO _x purification rate It was calculated activity decrease rate of the NO _x purification ratio after the test _(the NO _x purification ratio after the durability test / the initial of the NO _x purification rate). The results are shown in Table 3 and FIG.

As is clear from the results shown in Table 3 and FIG. 3, the exhaust gas purifying catalysts (Examples 1 to 5) of the present invention using CeO ₂ —ZrO ₂ as a carrier used Al ₂ O ₃ as the carrier. Compared to the exhaust gas purifying catalyst (Comparative Example 2), it was confirmed that the decrease in activity after the heat resistance test was small and the grain growth of the noble metal was sufficiently suppressed. In addition, when the sulfur poisoning / desorption test was performed after the heat resistance test, the activity of the exhaust gas purifying catalyst (Comparative Example 1) that did not form the oxide coating layer was greatly reduced, whereas the oxide coating layer was It was confirmed that the formed exhaust gas purifying catalysts of the present invention (Examples 1 to 5) had a small decrease in activity and suppressed sulfur poisoning of the NO _x storage material. Further, the exhaust gas purifying catalysts (Examples 1 to 5) of the present invention using CeO ₂ —ZrO ₂ as a carrier and forming an oxide coating layer formed an oxide coating layer using Al ₂ O ₃ as a carrier. Compared to the exhaust gas purifying catalyst (Comparative Example 2), it was confirmed that the decrease in activity after the sulfur poisoning / desorption test was small and that high sulfur poisoning resistance could be exhibited. From these results, the exhaust gas purifying catalyst of the present invention in which CeO ₂ —ZrO ₂ (basic oxide carrier) is used as a carrier and an oxide coating layer is formed thereon sufficiently suppresses noble metal grain growth. It was confirmed that it can exhibit high sulfur poisoning as well as possible.

As described above, according to the present invention, while exhibiting a high degree of sulfur poisoning resistance capable of sufficiently suppressing poisoning due to sulfur, even when exposed to a high temperature atmosphere for a long time, It is possible to provide an exhaust gas purifying catalyst capable of sufficiently suppressing grain growth and capable of maintaining high NO _x purification activity over a long period of time and a method for producing the same.

Thus, the exhaust gas purifying catalyst of the present invention, because particularly excellent in that it can exhibit a high degree of _the NO _x purification activity for a long period of time, to remove HC in exhaust gas discharged from an engine or the like, CO, harmful components such as NOx Therefore, it is useful as an exhaust gas purifying catalyst.

It is a schematic diagram which shows a part of surface vicinity of the basic oxide support | carrier of preferable one Embodiment of the exhaust gas purification catalyst of this invention. Is a graph showing _the NO _x purification ratio after the durability test of the catalyst for purification of exhaust gas obtained in Examples 1 to 5 and Comparative Examples 1-2. Is a graph showing the activity decrease rate of the NO _x purification rate of the catalyst for purification of exhaust gas obtained in Examples 1 to 5 and Comparative Examples 1-2.

Explanation of symbols

1 ... basic oxide support, 2 ... precious metal, 3 ... NO _x storage material, 4 ... oxide coating layer.

Claims (3)

A basic oxide support containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium; a noble metal supported on the basic oxide support; and the basic and _the nO _x storage material that is supported on an oxide support, from the basic oxide the noble metal and the oxide of the nO _x silicon storage material is formed on the exposed surface which is not supported and / or titanium on the surface of the carrier An exhaust gas purification catalyst comprising: an oxide coating layer. Carrying a noble metal and a NO _x storage material on a basic oxide support containing an oxide of at least one metal selected from the group consisting of cerium, praseodymium, lanthanum, zirconium, magnesium and calcium;
Selected from the group consisting of the hydroxyl group of the exposed surface on which the noble metal and the NO _x storage material are not supported on the surface of the basic oxide support, and silicon chloride, silicon alkoxide, titanium chloride and titanium alkoxide. Hydrolyzing at least one kind of coating layer material to form an oxide coating layer on the exposed surface to obtain an exhaust gas purification catalyst;
A method for producing an exhaust gas purifying catalyst, comprising: In the step of supporting the noble metal and NO _x storage material on the basic oxide carrier, method of manufacturing the exhaust gas purifying catalyst according to claim 2, characterized in that carrying _the NO _x storage material as a salt of organic matter.

Images