JP5692595B2 - Exhaust gas purification catalyst - Google Patents

Description

  The present invention relates to an exhaust gas purification catalyst. Specifically, the present invention relates to an exhaust gas purifying catalyst capable of achieving both suppression of sulfur poisoning and improvement in catalytic activity under high temperature conditions (for example, about 800 to 1000 ° C.) and a method for producing the same.

  As a catalyst for exhaust gas purification for simultaneously and efficiently purifying harmful components such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) discharged from internal combustion engines such as automobiles, precious metals There are widely used three-way catalysts mainly composed of platinum (Pt), palladium (Pd), and rhodium (Rh) as active species. In order to meet the exhaust gas regulations that have been strengthened in recent years and to minimize the use of the above-mentioned precious metals with high resource risks, further improvement in the performance of exhaust gas purification catalysts including the above three-way catalyst is desired. Yes.

In order to achieve an improvement in the performance of the exhaust gas purifying catalyst, it is required to prevent deterioration in the process of using the catalyst. As one of the deterioration factors of the catalyst, there is a catalyst poisoning by a sulfur (S) component (hereinafter, also referred to as “sulfur poisoning”). This is a phenomenon in which the sulfur component contained in the fuel coats the surface of the noble metal of the exhaust gas purifying catalyst, thereby reducing the active point of the catalyst and consequently the purification performance of the catalyst.
Conventionally, in order to suppress the sulfur poisoning, improvements have been made from various aspects such as the configuration, material, and control of the exhaust gas purifying catalyst. For example, in order to recover catalyst performance from sulfur poisoning, it has been proposed to vaporize and desorb sulfur components coated on the catalyst as SO ₂ or H ₂ S by controlling the air-fuel ratio and temperature of exhaust gas. ing. However, such sulfur poisoning recovery control, on the other hand, may promote a reduction or deterioration in the purification performance of the catalyst, and therefore there is a need for the development of an exhaust gas purification catalyst that is fundamentally difficult to be sulfur poisoned. .

  As an effective technique for reducing sulfur poisoning, it is known to use a catalyst carrier containing titania or a titania-zirconia composite material. For example, Patent Document 1 relates to a titania-zirconia-based powder dispersed in an yttria-containing titania-zirconia-based powder and an alumina powder as a powder suitably used as a catalyst carrier used in a gas containing sulfur. Have been described. Patent Documents 2 and 3 describe a catalyst carrier made of a composite oxide of titania and zirconia. Other conventional techniques for suppressing sulfur poisoning include a method for producing an exhaust gas purifying catalyst described in Patent Document 4.

JP 2000-327329 A JP-A-8-192051 JP-A-9-926 JP-A-8-224479

However, regarding the exhaust gas purifying catalyst using any catalyst carrier described in Patent Documents 1 to 4, the accumulation of sulfur under a temperature condition (for example, less than 800 ° C.) where the exhaust gas temperature is relatively low is compared. When it is used in an environment where the exhaust gas is extremely hot (for example, 800 ° C. or higher and 1000 ° C. or lower), such as when the catalyst is disposed directly under a gasoline engine There was a problem that the catalytic activity was greatly reduced.
The present invention was created to solve this problem, and maintains the high activity of the catalyst in a high temperature environment (for example, 800 ° C. or higher and 1000 ° C. or lower), while suppressing sulfur accumulation and reducing poisoning effects. The purpose is to avoid.

The present inventor has studied from various angles and has come up with the present invention capable of realizing the above object.
That is, the exhaust gas purifying catalyst disclosed herein includes a carrier and a metal functioning as a catalyst for purifying exhaust gas carried on the carrier. As the carrier, alumina-based particles mainly composed of γ-alumina and zirconia-titania-based particles mainly composed of zirconia-titania composite oxide (hereinafter also referred to as “ZT composite oxide”) ( Hereinafter, it is also referred to as “ZT-based particles”). The exhaust gas-purifying catalyst disclosed herein shows that a peak attributed to α-alumina does not substantially occur in XRD measurement after heating at 1000 ° C. for at least 5 hours under a theoretical air-fuel ratio environment. Features.

Such an exhaust gas purifying catalyst includes alumina-based particles mainly composed of γ-alumina as a carrier. The exhaust gas purifying catalyst according to the present invention comprising such γ-alumina as a carrier can suppress sintering (crystal growth) of a metal (typically a noble metal) as a catalyst species and maintain a highly dispersed state. . As a result, the exhaust gas-purifying catalyst exhibits high purification performance.
Further, such an exhaust gas-purifying catalyst does not substantially contain α-alumina as a carrier even after heating at 1000 ° C. for at least 5 hours in a stoichiometric air-fuel ratio environment. Since α-alumina has significantly lower sulfur poisoning resistance than γ-alumina, the exhaust gas purifying catalyst containing the α-alumina tends to accumulate sulfur components, and the purification performance tends to be lowered. Further, since the catalyst support made of α-alumina has a smaller specific surface area than the catalyst support made of γ-alumina, the catalyst metal (typically a noble metal) supported on the support containing α-alumina is sintered. It is easy to cause. The exhaust gas purifying catalyst according to the present invention does not substantially contain such α-alumina even under high temperature conditions (for example, 800 ° C. or higher and 1000 ° C. or lower). The deterioration is prevented, high purification performance can be exhibited, and the life of the catalyst can be extended.
In this specification, whether or not the exhaust gas purifying catalyst substantially contains “γ-alumina” or “α-alumina” is determined by measuring the above exhaust gas purifying catalyst with X-ray diffraction (XRD). This can be done by phase identification.
That is, “substantially free of α-alumina” in a predetermined catalyst carrier means that no peak indicating α-alumina is observed in the XRD measurement of the test catalyst carrier. For example, α-alumina is shown in comparison between the result of XRD measurement (XRD chart) of a comparative specimen (not containing α-alumina) made of γ-alumina and the result of XRD measurement of the test catalyst support (XRD chart). When no relative difference is observed in the peaks (that is, when a peak attributed to α-alumina does not substantially occur in the XRD measurement), the test catalyst support is substantially free of α-alumina. Determined.

Furthermore, the exhaust gas purifying catalyst disclosed herein includes ZT-based particles mainly composed of a ZT composite oxide as a carrier. Since the ZT composite oxide has few basic sites on its surface, it has a property that it is difficult to adsorb and easily desorb acidic substances such as sulfur oxide (SOx). For this reason, the exhaust gas purifying catalyst comprising such ZT-based particles as a carrier has improved sulfur poisoning resistance.
In a preferred embodiment of the exhaust gas purifying catalyst disclosed herein, the ZT-based particles as a carrier and the alumina-based particles mainly composed of γ-alumina have high dispersibility and are uniformly mixed. It is characterized by being. In an environment where the γ-alumina and the ZT composite oxide coexist in such a highly dispersed state, the synergistic effect of both suppresses the sulfidation of the catalyst metal (generation of sulfide) specifically, and the sulfur resistance of the catalyst is reduced. Toxicity is further improved.

  As used herein, “theoretical air / fuel ratio” or “stoichiometry” means the stoichiometric ratio (air / fuel ratio) of air and fuel when oxygen and fuel in the air-fuel mixture react without excess or deficiency. For example, the stoichiometric air fuel ratio (stoichiometry) in gasoline is about 14.7 in terms of the mass ratio (A / F) between air and gasoline.

In another preferred embodiment of the exhaust gas-purifying catalyst disclosed herein, the alumina-based particles contain yttrium.
The alumina-based particles contain yttrium as a phase transition inhibitor, so that even if the catalyst is used in a high temperature environment of about 800 to 1000 ° C., the alumina particles are changed from the γ phase to the α phase. The phase transition to is suppressed and maintained as γ-alumina. The exhaust gas purifying catalyst using the catalyst carrier containing γ-alumina and substantially free of α-alumina exhibits high purification performance and high sulfur poisoning resistance even under high temperature conditions. Can do.

In another preferred embodiment of the exhaust gas purifying catalyst disclosed herein, the ZT-based particles contain yttrium.
Such yttrium-containing zirconia-titania composite oxide (hereinafter also referred to as “ZTY composite oxide”) has higher heat resistance than ZT composite oxide, and in particular, a specific surface area under a temperature condition exceeding 900 ° C. Is suppressed. In the exhaust gas purifying catalyst using the catalyst carrier containing such a ZTY composite oxide, a reduction in purification performance under a high temperature condition of about 900 to 1000 ° C. can be further suppressed.

In another preferable embodiment of the exhaust gas purifying catalyst disclosed herein, when the entire carrier is 100% by mass, the content when the yttrium is converted into an oxide (yttria; Y ₂ O ₃ ) is 1 It is characterized by being not less than 10% by mass and not more than 10% by mass.
When the yttrium content is in the above range, the thermal stability and sulfur poisoning resistance can be sufficiently improved by adding yttrium (or yttria), while inhibiting the catalytic function derived from yttrium addition. Is unlikely to occur. Therefore, when the yttrium content is set within the above range, the exhaust gas purifying catalyst can achieve both high purification performance (high heat resistance) and high sulfur poisoning resistance under high temperature conditions.

  In another preferable aspect of the exhaust gas purification catalyst disclosed herein, rhodium is provided as a metal that functions as the catalyst for exhaust gas purification. The exhaust gas-purifying catalyst in which rhodium is supported on any of the carriers disclosed in the present specification suppresses the formation of sulfide of rhodium even in a high-temperature environment (for example, about 800 to 1000 ° C.), The influence of poison can be suppressed. Such an exhaust gas purifying catalyst is excellent in reducing activity against nitrogen oxides (NOx) under high temperature conditions, in addition to oxidation activity against hydrocarbons (CO) and carbon monoxide (CO), and is therefore discharged from a gasoline engine. It can be suitably used as a three-way catalyst for purifying high-temperature exhaust gas.

According to the present invention, a method for producing an exhaust gas purifying catalyst is provided. The method includes a step (1) of preparing an acidic aqueous solution A containing an aluminum salt and an yttrium salt. Moreover, the process (2) which prepares the acidic aqueous solution B containing a titanium salt, a zirconium salt, and an yttrium salt is included. Moreover, the process (3) which prepares the high pH solution whose pH is 11 at least is included.
Furthermore, both the acidic aqueous solution A and the acidic aqueous solution B are dropped and mixed with the high pH solution, thereby changing the pH of the mixed solution from the basic side to the acidic side, It includes a step (4) of generating a precipitate in the process. Moreover, the process (5) which obtains a support | carrier by baking the obtained deposit is included. Moreover, the process (6) which carries | supports the metal (typically any noble metal which belongs to platinum group) which functions as a catalyst for exhaust gas purification | cleaning on the obtained support | carrier is included.

Such a production method can be preferably employed as a production method of any exhaust gas purifying catalyst disclosed herein. The method for producing an exhaust gas purifying catalyst disclosed herein is particularly characterized by the precipitate generation step (4). That is, in a method for producing a composite oxide by a typical coprecipitation method, a pH is gradually increased by dropping a high pH solution into a mixed solution in which various water-soluble metal salts are dissolved ( In the process, a coprecipitate is obtained. In this case, the pH region where precipitation is generated is often relatively small (that is, a coprecipitate is generated in a region having low basicity).
On the other hand, in the production method according to the present invention, the acidic aqueous solution A and the acidic aqueous solution B prepared by dissolving various water-soluble metal salts are dropped into a high pH solution (typically dropped simultaneously). This is characterized in that the pH is gradually decreased (in the direction from the basic side to the acidic side), and a coprecipitate is generated in a relatively large pH region (a region having a sufficiently high basicity). is there. According to this production method, the yttrium ions in the mixed solution after dropping the acidic aqueous solutions A and B are dissolved in both alumina and zirconia almost simultaneously. For this reason, mixed particles of yttrium-containing alumina and yttrium-containing zirconia-titania composite oxide (ZTY composite oxide) can be suitably obtained.
Moreover, in the said precipitate production | generation process (4), both the said aqueous solution A and the said aqueous solution B are dripped at the said high pH solution suitably substantially simultaneously, It is characterized by the above-mentioned. According to such an embodiment, the product mixture of alumina particles and ZT particles can be obtained in a uniformly mixed state at the primary particle level. When such a mixture is used as a support for a catalyst for exhaust gas purification, sulfur poisoning resistance is further improved by a synergistic effect of yttrium-containing γ-alumina and a ZTY composite oxide. Furthermore, since the alumina particles serve as a diffusion barrier for suppressing grain growth of the ZT particles, the use of such a mixture as a carrier for the exhaust gas purification catalyst further improves the thermal stability of the exhaust gas purification catalyst. .

In another preferred embodiment of the method for producing an exhaust gas purification catalyst disclosed herein, the high pH solution is ammonia water.
This production method is suitable because it tends to obtain a mixed powder containing yttrium having high heat resistance and high specific surface area efficiently.

In another preferred embodiment of the method for producing an exhaust gas purifying catalyst disclosed herein, the aluminum salt, the yttrium salt, and the zirconium salt are nitrates, and the titanium salt is a chloride. .
According to such a production method, since yttrium ions are suitably dissolved in alumina and zirconia, a mixed powder containing yttrium having high uniformity tends to be obtained, which is preferable.

It is a figure which shows typically the exhaust gas purification catalyst which concerns on one Embodiment, and shows the state of this exhaust gas purification catalyst after heating at 1000 degreeC in a theoretical air fuel ratio environment for 5 hours. It is a figure which shows typically the exhaust gas purification catalyst which concerns on a prior art, and shows the state of this exhaust gas purification catalyst after heating at 1000 degreeC in a theoretical air fuel ratio environment for 5 hours. It is a figure which shows typically the heating temperature conditions in a high temperature endurance process and a sulfur poisoning process (horizontal axis: time, vertical axis: heating temperature). It is a graph which shows the result of a XRD measurement about Example 1 and the comparative example 1 after a high temperature durability process. It is a figure which shows typically the heating temperature conditions in catalyst activity evaluation (horizontal axis: time, vertical axis: heating temperature). It is a graph which shows a NOx purification rate about Example 1 and Comparative Example 1 (vertical axis: NOx purification rate (%)). It is a graph which shows the result of a soft X ray XAFS measurement about the reference examples 1-3 (a vertical axis | shaft: intensity | strength (arbitrary unit), a horizontal axis: Photon energy (eV)). It is a graph which shows the result of a soft X-ray XAFS measurement about Example 1 and Comparative Example 1 (vertical axis: intensity (arbitrary unit), horizontal axis: photon energy (eV)).

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

The exhaust gas-purifying catalyst disclosed herein includes alumina-based particles mainly composed of γ-alumina as a carrier. Moreover, even when the exhaust gas-purifying catalyst is heated at 1000 ° C. for at least 5 hours in a stoichiometric air-fuel ratio environment, α-alumina is not substantially generated.
In general, alumina is somewhat affected by the size of crystallites and the manufacturing method, but usually, when heated at about 800 to 1000 ° C., a phase transition from the γ phase to the α phase occurs. The specific surface area of alumina decreases from, for example, about 200 m ² / g to about 10 m ² / g. Such a decrease in the specific surface area of the support causes sintering of a metal (typically a noble metal) that is a catalytically active site supported on the support, thereby reducing the specific surface area of the metal. For this reason, the phase transition of the alumina from the γ phase to the α phase causes a reduction in purification performance of the exhaust gas purification catalyst. In addition, since the exhaust gas purifying catalyst using α-alumina after the phase transition as a carrier has a property of easily adhering sulfide, the phase transition reduces the sulfur poisoning resistance of the exhaust gas purifying catalyst.

  FIG. 1 schematically shows an embodiment of the exhaust gas purifying catalyst disclosed herein, and shows the state of the catalyst after heating at 1000 ° C. for 5 hours in a stoichiometric air-fuel ratio environment. That is, the exhaust gas purifying catalyst 10 of this embodiment is composed of a mixture of the γ-alumina particles 12 and the ZTY composite oxide particles 14 after heating at 1000 ° C., and the metal particles 16 are supported on each.

  On the other hand, FIG. 2 schematically shows an embodiment of the exhaust gas purifying catalyst according to the prior art (for example, described in Patent Document 1), after heating at 1000 ° C. for 5 hours in a theoretical air-fuel ratio environment. The state of the catalyst is shown. That is, the exhaust gas purifying catalyst 20 according to the prior art causes the phase transition of γ-alumina of the support to α-alumina by heating at 1000 ° C., so the support after heating at 1000 ° C. is the α-alumina particles 22 and the ZTY composite oxide. It is composed of particles 24. The metal particles 26 are supported on the respective carrier particles. In particular, the metal particles 26 supported on the α-alumina particles 22 are sintered due to structural changes due to the phase transition of alumina.

  Here, X-ray diffraction (XRD) measurement is suitably used for identification of alumina main phases such as γ phase and α phase contained in the exhaust gas purifying catalyst in the present specification. The measurement method and the analysis method are not particularly limited, and a method that is usually used in this technical field can be used. For example, is an exhaust gas purifying catalyst containing α-alumina? In the case of determining whether or not, XRD measurement is performed on the catalyst, and it can be determined whether or not a peak attributed to α-alumina appearing at a measurement angle 2θ of around 26 to 27 ° is detected.

As a method for suppressing the phase transition of the alumina from the γ phase to the α phase under high temperature conditions (800 to 1000 ° C.), that is, improving the thermal stability of γ-alumina, lanthanum or the like is used for the γ-alumina particles 12. It is possible to add various additives such as rare earth elements, alkaline earth elements such as barium, or silica as a phase transition inhibitor.
Among the above-described γ-alumina phase transition suppression methods, when the method of adding an alkaline earth element typified by barium to γ-alumina is employed, the alkaline earth element-containing alumina is nitrogen oxide (NOx). ) And sulfur oxides (SOx) are easily occluded, and the exhaust gas purification catalyst using such alumina as a carrier tends to have low sulfur poisoning resistance. Further, in the method of improving the thermal stability by adding silica to γ-alumina, it is usually necessary to use a sol-gel method in order to introduce silica under a low temperature condition, and the cost is high because the raw material alkoxides are expensive. It is disadvantageous in terms.
Therefore, as a thermal stability method of the above-mentioned γ-alumina, yttrium (or yttria) is used from the viewpoint that the catalyst function is hardly inhibited by the additive, and further, it has a sufficient effect of suppressing phase transition at high temperature and preventing sulfur poisoning. Is preferred.

Regarding the yttrium content in the case of producing the yttrium-containing γ-alumina, the content when the yttrium-containing γ-alumina is 100% by mass and the yttrium is converted into an oxide (yttria; Y ₂ O ₃ ). The rate is preferably 0.5% by mass or more and 10% by mass or less (preferably 1% by mass or more and 5% by mass or less, for example, 2% by mass or more and 4% by mass or less). When the content of yttrium is too small than the above range, it is not preferable because the effect of suppressing phase transition and the effect of suppressing sulfur adhesion are difficult to appear. On the other hand, when the content of yttrium is too larger than the above range, the absolute amount of γ-alumina is relatively small, so that the specific surface area of the support and the heat resistance tend to decrease, which is not preferable.

As a method of adding yttrium to γ-alumina, a solid phase reaction at a high temperature using yttria (Y ₂ O ₃ ) and alumina (Al ₂ O ₃ ) as starting materials, or a sol-gel method, a coprecipitation method, etc. A liquid phase reaction or the like can be used. Among the above production methods, a coprecipitation method is preferably used from the viewpoint of efficiently obtaining uniform alumina-based particles having a high specific surface area. A particularly preferred method for producing the yttrium-containing alumina particles (powder) according to the present invention by the coprecipitation method will be described in detail later.

The alumina-based particles (alumina-based powder) constituting the carrier disclosed herein have a specific surface area (specific surface area measured by the BET method; the same shall apply hereinafter) of 20 to 700 m ² / g (preferably 50 to 500 m). ² / g, more preferably 200 to 400 m ² / g). When the specific surface area of the alumina-based particles (powder) is less than 20 m ² / g, there is a possibility that sintering of the noble metal supported on the alumina-based particles proceeds, whereas the specific surface area is more than 700 m ² / g. When it is too large, the heat resistance of the alumina particles itself may be lowered, which is not preferable.

The exhaust gas-purifying catalyst disclosed herein contains ZT-based particles mainly composed of a ZT composite oxide as a carrier. Since the surface of titania has few basic sites, sulfate and sulfite ions are difficult to adsorb, and even if they are adsorbed to form sulfate, the sulfate is easily decomposed and is not easily poisoned by sulfur. Have. Further, titania is combined with zirconia to improve heat resistance and acidity.
In the ZT composite oxide, the content of zirconia (ZrO ₂ ) when the ZT composite oxide is 100% by mass is 10% by mass or more and 90% by mass or less (preferably 30% by mass or more, or 50% by mass or more). For example, 60 mass% or more and 80 mass% or less). When the content of zirconia is too smaller than the above range, it is not preferable because the specific surface area of the ZT composite oxide tends to decrease at high temperatures. Moreover, when the content rate of zirconia is too much more than the said range, since the absolute amount of titania becomes relatively small, it becomes easy to receive to the influence of sulfur poisoning, and is unpreferable.

  Examples of the method for producing the ZT-based particles (powder) include a sol-gel method and a coprecipitation method, and are not particularly limited, but the coprecipitation method is preferably used from the viewpoint of obtaining a uniform powder. A method for producing the ZT particles (powder) according to the present invention by a suitable coprecipitation method will be described in detail later.

Various stabilizers can be added to the ZT composite oxide for the purpose of improving heat resistance. As the additive, a rare earth element such as lanthanum, an alkaline earth element such as calcium, a transition metal element, or the like can be used. Among these, yttrium is preferably used as the stabilizer from the viewpoint of improving the specific surface area at high temperatures without impairing the catalyst function. As a method of adding yttrium to the ZT composite oxide (that is, producing a ZTY composite oxide), a solid phase reaction at a high temperature or a liquid phase reaction such as a sol-gel method or a coprecipitation method can be used. Of these, the coprecipitation method is preferably used. A particularly preferable production method by the coprecipitation method of the ZTY composite oxide according to the present invention will be described in detail later.
When the ZTY composite oxide is 100% by mass, the content when the yttrium is converted to an oxide (yttria; Y ₂ O ₃ ) is 0.5% by mass or more and 10% by mass or less (preferably 1% by mass). It is preferably 5% by mass or less, for example 1.5% by mass or more and 3.5% by mass or less. If the content of yttrium in the ZTY composite oxide is too small from the above range, it is not preferable because the effect of thermal stabilization is unlikely to appear. Moreover, when there is too much content rate of yttrium than the said range, there exists a possibility that thermal stability may fall conversely, and is unpreferable.

The average particle size of the ZT-based powder disclosed herein (average value of particle size obtained by TEM observation or average value of crystallite size obtained from XRD measurement; the same applies hereinafter) is 1 nm to 100 nm (preferably 5 nm It is preferably 25 nm or less and more preferably 5 nm or more and 15 nm or less. The specific surface area of the ZT-based powder (specific surface area as measured by BET method. Hereinafter the same.) Is, 10 to 500 m ² / g (preferably 20 to 300 m ² / g, more preferably 50 to 200 m ² / g) It is preferable that When the specific surface area of the ZT-based powder is less than 10 m ² / g, the noble metal supported on the ZT-based powder tends to sinter, while on the other hand, the specific surface area is more than 500 m ² / g. This is not preferable because the heat resistance of the ZT powder itself decreases.

  A coprecipitation method is preferably used as a method for producing the alumina-based powder and the ZT-based powder constituting the carrier disclosed herein. A conventional method for producing powder by a typical coprecipitation method is as follows. That is, an aqueous solution in which various water-soluble metal salts are dissolved is mixed in advance, and a high pH solution showing alkalinity is added to the mixed solution to gradually increase the pH, thereby generating a coprecipitate. A composite oxide can be obtained by heat-treating the coprecipitate.

On the other hand, a carrier containing yttrium-containing γ-alumina and a ZTY composite oxide, which is a preferred embodiment of the carrier disclosed herein, is preferably produced using the conventional typical coprecipitation method described above. It is difficult.
That is, when an alkaline solution is added dropwise to an aqueous solution containing metal ions (aluminum ions, zirconium ions, titanium ions, yttrium ions) constituting the carrier, the pH is relatively low (about pH 8-9). In the region, yttrium ions preferentially generate coprecipitates with zirconium ions, and coprecipitates based on aluminum ions and yttrium ions are not generated (yttrium hardly dissolves in alumina). As a result, the resulting mixture is a mixture of ZTY complex oxide and alumina alone.

Therefore, in order to manufacture a carrier containing the yttrium-containing γ-alumina disclosed herein and a ZTY composite oxide, the following manufacturing method is preferably used. That is, first, an acidic aqueous solution A containing a predetermined amount of aluminum salt and a predetermined amount of yttrium salt is prepared. In addition, an acidic aqueous solution B containing a predetermined amount of titanium salt, a predetermined amount of zirconium salt, and a predetermined amount of yttrium salt is prepared. Both the aqueous solution A and the aqueous solution B are preferably dripped substantially simultaneously into a high pH solution having a pH of 11, and mixed while stirring. As a result, the pH of the mixed solution changes in the direction of decreasing from the basic side to the acidic side, and a precipitate is generated in the process (typically pH 9.5 or more and 10.5 or less), and the resulting precipitate The above carrier powder can be obtained by drying and baking.
According to the above production method, in the region where the pH is relatively high (typically about pH 10), yttrium ions produce coprecipitates almost simultaneously with both aluminum ions and zirconium ions. , A uniform mixture of yttrium-containing γ-alumina and ZTY composite oxide.

Examples of the high-pH solution include ammonia, inorganic bases such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and ammonium carbonate, and organic bases such as amines such as triethylamine and pyridine. Are preferably used, and ammonia water is more preferably used in the above.
As said aluminum salt, said yttrium salt, said titanium salt, and said zirconium salt, what makes aqueous solution acidic is used suitably. For example, nitrates, sulfates, carbonates, halides and the like are preferably used. From the viewpoint of obtaining a mixed powder with high uniformity as a product, the aluminum salt, the yttrium salt, and the zirconium salt are more preferably nitrates, and the titanium salt is more preferably a chloride.
The drying conditions for the precipitate are typically about 80 to 150 ° C. (for example, 100 to 130 ° C.) for about 3 to 24 hours, and the firing conditions are about 400 to 800 ° C. (for example, 500 to 700 ° C.). About 2 to 4 hours.

As the metal that functions as an exhaust gas purification catalyst used in the exhaust gas purification catalyst disclosed herein, various kinds of metal species that can function as an oxidation catalyst or a reduction catalyst can be employed. Typically, rhodium is used. (Rh), platinum (Pt), noble metals such as palladium (Pd). In addition to the noble metals, ruthenium (Ru), osmium (Os), iridium (Ir), silver (Ag), and the like are preferable examples. These metals may be alloyed. Among these, rhodium is preferably used as the metal from the viewpoint of high NOx reduction activity.
Such metal preferably has a sufficiently small particle diameter from the viewpoint of increasing the contact area with the exhaust gas. Typically, the average particle size (average value of particle sizes determined by TEM observation) of the metal is preferably about 1 to 15 nm (more preferably 10 nm or less, or 7 nm or less, and further 5 nm or less).

When the support in the exhaust gas purification catalyst is 100 mass%, the loading ratio of the metal functioning as the exhaust gas purification catalyst is 1.5 mass% or less (e.g., 0.05 mass% or more and 1.5 mass). % Or less, preferably 0.1% by mass or more and 1.3% by mass or less, more preferably 0.2% by mass or more and 1% by mass or less. If the loading ratio of the metal is too smaller than the above range, the catalytic effect of the metal is difficult to obtain. On the other hand, if the loading ratio of the metal is more than the above range, there is a possibility that the growth of the metal grains proceeds, which is also disadvantageous in terms of cost.
The method for supporting the metal on the carrier is not particularly limited, and a conventionally used method can be used. For example, when rhodium is supported on any of the carriers disclosed herein, the carrier powder is impregnated with an aqueous solution containing a rhodium salt (for example, nitrate), dried, and fired. be able to.

  The exhaust gas-purifying catalyst disclosed herein can be molded into a shape similar to that conventionally used for this type of catalyst, such as a pellet shape, a foam shape, a honeycomb shape, or a filter shape. For example, when forming into a honeycomb shape, the above metal is supported on the surface of a honeycomb substrate having a honeycomb structure formed of a ceramic or alloy (such as stainless steel) having high heat resistance such as cordierite or silicon carbide (SiC). It is produced by coating the carrier. Moreover, when shape | molding in a pellet shape, it manufactures by shape | molding the said support | carrier with which the said metal was carry | supported by the conventional method using a press etc. into a pellet shape.

Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the specific examples.

<Manufacture of exhaust gas purification catalyst>
Example 1
A predetermined amount of aluminum nitrate and yttrium nitrate were dissolved in 500 ml of ion-exchanged water to prepare an aqueous solution A. On the other hand, an aqueous solution B was prepared by dissolving a predetermined amount of zirconium oxynitrate, titanium tetrachloride, and yttrium nitrate. Moreover, pH 14 ammonia water was prepared. While measuring the pH of the aqueous ammonia as needed, the aqueous solution A and the aqueous solution B were simultaneously dropped into a beaker containing the aqueous ammonia to form a precipitate. During the dropping, the pH value of the mixed solution in the beaker gradually decreased and the dropping was stopped when the pH reached about 10. The obtained precipitate is filtered, washed, dried at 120 ° C. for 24 hours, and then calcined at 900 ° C. for 5 hours, whereby carrier powder (ie, yttrium-containing γ-alumina powder and ZTY composite oxide powder) is obtained. A mixture of At this time, the support powder of composition _Al 2 _O 3 in a weight ratio of oxide _{conversion: ZrO 2: TiO 2: Y} 2 O 3 = 70: 21: 6: was adjusted to 3.
The obtained carrier powder was immersed in a predetermined concentration and a predetermined amount of rhodium nitrate solution, and then evaporated to dryness to support rhodium on the powder. The rhodium carrying amount at this time was adjusted to 0.25% by mass when the carrier powder was 100% by mass. The obtained rhodium-supported powder was dried at 120 ° C. for 24 hours and then calcined at 600 ° C. for 2 hours. The calcined product was pulverized in a mortar and then formed into a pellet using a press. The exhaust gas purifying catalyst produced by such a series of manufacturing methods is used as the exhaust gas purifying catalyst according to the first embodiment.

(Comparative Example 1)
A predetermined amount of aluminum nitrate, zirconium oxynitrate, titanium tetrachloride, and yttrium nitrate were added in random order to 1000 ml of ion exchange water in one beaker to prepare a raw material aqueous solution. While stirring the aqueous raw material solution, an appropriate amount of hydrogen peroxide was added. By adding hydrogen peroxide water, titanium ions and zirconium ions are complexed and the pH at which precipitation based on the respective ions can be brought close to each other, so that homogenization of the precipitate is promoted. On the other hand, pH 14 ammonia water was prepared. To the solution containing the aluminum ions, zirconium ions, titanium ions, and yttrium ions obtained above, the ammonia water was added dropwise until the pH of the mixed solution reached 9, thereby generating a precipitate. At this time, yttrium ions preferentially dissolve in zirconia over alumina to form a precursor.
The obtained precipitate is filtered, washed, dried at 120 ° C. for 24 hours, and then calcined at 900 ° C. for 5 hours, whereby carrier powder (ie, alumina powder and yttrium-containing titania-zirconia (ZTY) composite) A mixture of oxide powders) was obtained. At this time, the composition of the carrier powder is Al ₂ O ₃ : ZrO ₂ : TiO ₂ : Y ₂ O ₃ = 70: 21: 6: 3 in mass ratio in terms of oxide, as in Example 1. Adjusted as follows.
The obtained carrier powder was supported by rhodium by the same production process as in Example 1 and dried, fired, and pelletized. The amount of rhodium supported was adjusted to 0.25% by mass when the carrier powder was 100% by mass. The exhaust gas purifying catalyst produced by such a series of manufacturing methods is used as the exhaust gas purifying catalyst according to Comparative Example 1.

(Reference Examples 1-3)
For comparison, an exhaust gas purification catalyst using α-alumina (Reference Example 1), ZTY composite oxide (Reference Example 2), and γ-alumina (Reference Example 3) as a catalyst carrier was produced.
Specifically, rhodium is supported on a predetermined amount of α-alumina (commercial product, manufactured by Wako Pure Chemical Industries, Ltd.) in the same manner as in the manufacturing process according to Example 1, and the exhaust gas is dried, fired and molded. A purification catalyst was produced and used as an exhaust gas purification catalyst according to Reference Example 1.
The ZTY composite oxide used in Reference Example 2 was produced as follows. That is, using zirconium oxynitrate, titanium tetrachloride, and yttrium nitrate as starting materials, dissolving the above compound in ion-exchanged water, adding an appropriate amount of hydrogen peroxide solution, and dropping pH 14 ammonia water thereto. A precipitate was obtained. The precipitate was washed, dried at 120 ° C. for 24 hours, and calcined at 900 ° C. for 5 hours to obtain a ZTY composite oxide. At this time, the composition of the ZTY composite oxide was adjusted to be ZrO ₂ : TiO ₂ : Y ₂ O ₃ = 70: 20: 10 in terms of mass ratio in terms of oxide. The obtained ZTY composite oxide was loaded with rhodium in the same manner as in the production process according to Example 1, dried, fired, and molded, and this was used as the exhaust gas purifying catalyst according to Reference Example 2.
Further, a rhodium is supported on a predetermined amount of γ-alumina (commercial product, manufactured by Wako Pure Chemical Industries, Ltd.) in the same manner as in the manufacturing process according to Example 1, and dried, fired, and molded to produce an exhaust gas purification catalyst. This was used as the exhaust gas purifying catalyst according to Reference Example 3.
For the exhaust gas purifying catalysts according to Reference Examples 1 to 3, the rhodium loading was adjusted to be 0.25% by mass when each carrier powder was 100% by mass.

<High temperature durability treatment>
The exhaust gas purifying catalyst according to Example 1 and Comparative Example 1 thus obtained was subjected to high temperature durability treatment. Specifically, 3 g of the exhaust gas-purifying catalyst was weighed, placed in an electric furnace, and kept at 1000 ° C., while alternately switching rich gas and lean gas having the composition shown in Table 1 at intervals of 2 minutes, 300 minutes (5 hours) Let it flow.

<Sulfur poisoning treatment>
The exhaust gas-purifying catalyst subjected to the high-temperature durability treatment was subjected to sulfur poisoning treatment. Specifically, while maintaining the exhaust gas purifying catalyst at 400 ° C., a gas having a composition shown in Table 2 (a stoichiometric gas mixed with SO ₂ ) was allowed to flow for 90 minutes.
FIG. 3 schematically shows the heating temperature conditions in the high temperature durability treatment and sulfur poisoning treatment.

<XRD measurement>
For the exhaust gas purifying catalyst according to Example 1 and Comparative Example 1 after the high temperature durability treatment, XRD measurement was performed using a powder X-ray diffractometer (Rigaku RINT-TTR III) in order to identify the formation phase. The product phase was identified using a JCPDS card. The result of the XRD measurement is shown in FIG.
In the XRD chart shown in FIG. 4, a peak with a measurement angle 2θ of around 30 ° observed in both the samples of Example 1 and Comparative Example 1 is attributed to the ZT composite oxide. Moreover, 2θ observed for the sample according to Comparative Example 1 was assigned to α-alumina in the vicinity of 26 to 27 °, and the peak was not detected for the sample according to Example 1. That is, it was confirmed that the exhaust gas purifying catalyst according to Example 1 did not produce α-alumina even after the high temperature durability treatment at 1000 ° C. for 5 hours. This is because the alumina constituting the carrier according to Example 1 contains yttrium, which is a phase transition inhibitor, at an appropriate content.

<Catalyst activity evaluation>
About the exhaust gas purifying catalyst according to Example 1 and Comparative Example 1 after the high temperature durability treatment and the sulfur poisoning treatment, the catalytic activity for NOx purification was evaluated. As a specific evaluation method, 3 g of exhaust gas purifying catalyst was weighed and installed in a model gas apparatus. Next, a stoichiometric region gas having a composition shown in Table 3 under a predetermined temperature condition was introduced into the model gas apparatus at a gas flow rate of 15 L / min, and NOx gas concentrations at the inlet and outlet of the apparatus were measured. . At this time, the purification rate (%) is
Purification rate (%) = ((entered gas concentration−outgoing gas concentration) / entered gas concentration) × 100
It is calculated by the following formula.
The heating temperature conditions in the catalyst activity evaluation are shown in FIG. Here, when the gas temperature reached 500 ° C., the concentration of NOx gas was measured. The result of the NOx purification rate is shown in FIG.
As is clear from the results shown in FIG. 6, the NOx purification rate of the exhaust gas purification catalyst according to Comparative Example 1 is 75%, whereas the exhaust gas purification catalyst according to Example 1 shows 95%, and the purification performance. There was a significant improvement. That is, it was confirmed that the exhaust gas purifying catalyst according to Example 1 maintains high NOx purification performance even after the high-temperature endurance treatment and the sulfur poisoning treatment. It shows that the exhaust gas purifying catalyst has both high heat resistance and high sulfur poisoning resistance.

<Identification of sulfur species accumulated in exhaust gas purification catalyst>
Regarding the exhaust gas purifying catalyst according to Reference Examples 1 to 3 after the sulfur poisoning treatment, and the exhaust gas purifying catalyst according to Example 1 and Comparative Example 1 after the high temperature durability treatment and the sulfur poisoning treatment, In order to identify the accumulated sulfur species, soft X-ray XAFS (Ritsumeikan University SR Center Beamline 10) was measured. The results according to Reference Examples 1 to 3 are shown in FIG. 7, and the results according to Example 1 and Comparative Example 1 are shown in FIG.
In the graphs shown in FIGS. 7 and 8, the peak where the photon energy indicated by the horizontal axis appears in the vicinity of 2479 eV is attributed to sulfate (SO ₄ ) attached (formed) to the carrier. Further, a peak near 2468 eV on the lower energy side is attributed to a rhodium sulfide (Rh-S) which is a noble metal. Here, the production of the nitrate (SO ₄ ) does not contribute much to the catalyst activity, but rhodium sulfide (sulfide production) causes a decrease in the catalyst activity.
As is clear from the results shown in FIG. 7, it can be seen that rhodium sulfide is generated in the exhaust gas purifying catalyst using α-alumina according to Reference Example 1 as a carrier. On the other hand, regarding the exhaust gas purifying catalyst using ZTY composite oxide (Reference Example 2) and γ-alumina (Reference Example 3) as a carrier, it was found that rhodium sulfide was hardly generated. That is, the production of the noble metal sulfide, which is a factor for reducing the catalytic activity, is the above order (α-alumina> ZTY composite oxide> γ-alumina when the support is α-alumina, ZTY composite oxide, and γ-alumina. ) Was confirmed to be less.
From the above results, it was revealed that γ-alumina has a higher effect of suppressing the formation of sulfides than α-alumina.

Next, as is clear from the results shown in FIG. 8, the exhaust gas purifying catalyst according to Comparative Example 1 has a peak attributed to rhodium sulfide, whereas the exhaust gas purifying catalyst according to Example 1 is No peaks attributed to rhodium sulfide were detected, confirming that the formation of sulfide was sufficiently suppressed. This is because the alumina of the exhaust gas purifying catalyst according to Comparative Example 1 phase-transformed into α-alumina during the high temperature durability treatment, and the α-alumina promoted the formation of noble metal sulfide. On the other hand, in the exhaust gas purifying catalyst according to Example 1, since yttrium is added as a phase transition inhibitor to alumina, the alumina is maintained as γ-alumina even by the high temperature durability treatment, It can be seen that the formation of sulfides was specifically suppressed by the synergistic effect of γ-alumina and the ZTY composite oxide.
From the above results, it was confirmed that the exhaust gas purifying catalyst according to Example 1 has high sulfur poisoning resistance even under high temperature conditions.

10 Exhaust gas purification catalyst 12 γ-alumina particles 14 ZTY composite oxide particles 16 Metal particles 20 Exhaust gas purification catalyst 22 α-alumina particles 24 ZTY composite oxide particles 26 Metal particles

Claims (6)

An exhaust gas purification catalyst comprising a carrier and a metal functioning as a catalyst for exhaust gas purification carried on the carrier,
As the carrier, comprising alumina-based particles mainly composed of γ-alumina, and zirconia-titania-based particles mainly composed of zirconia-titania composite oxide,
Both the alumina particles and the zirconia-titania particles contain yttrium,
An exhaust gas purifying catalyst characterized in that a peak attributed to α-alumina does not substantially occur in XRD measurement after heating at 1000 ° C. for at least 5 hours in a theoretical air-fuel ratio environment. Wherein when the entire carrier was 100% by mass, content when converted to the yttrium oxide (yttria) is 10 mass% or more 1 wt%, the catalyst for purification of exhaust gas according to claim 1. The exhaust gas-purifying catalyst according to claim 1 or 2 , comprising rhodium as the metal. A method for producing an exhaust gas purification catalyst,
Preparing an acidic aqueous solution A containing an aluminum salt and an yttrium salt;
Preparing an acidic aqueous solution B containing a titanium salt, a zirconium salt, and an yttrium salt;
preparing a high pH solution having a pH of at least 11;
Both the acidic aqueous solution A and the acidic aqueous solution B are dropped and mixed with the high pH solution, thereby changing the pH of the mixed solution from the basic side to the acidic side, Producing a precipitate in the process;
Calcination of the resulting precipitate to obtain a carrier;
A step of supporting a metal functioning as a catalyst for exhaust gas purification on the obtained carrier;
The manufacturing method of the catalyst for exhaust gas purification characterized by including these. The production method according to claim 4 , wherein the high pH solution is aqueous ammonia. The manufacturing method according to claim 4 or 5 , wherein the aluminum salt, the yttrium salt, and the zirconium salt are nitrates, and the titanium salt is a chloride.

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