JP2006102628A - Sulfur oxide absorber and its manufacturing method, and emission gas purifying facility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means of increasing an absorption capacity of SO<SB>x</SB>, and at the same time, shifting a temperature where SO<SB>x</SB>is discharged in rich atmosphere to a higher-temperature region. <P>SOLUTION: The constitution comprises a carrier 2 having at least the same specific surface area or more compared to MgAl<SB>2</SB>O<SB>4</SB>, a solid solution suppressing layer 4 formed at least on the surface of the carrier 2 to suppress the solid solution of Mg to the carrier, at least MgO 5 supported on the solid solution suppressing layer 4, and an oxidation catalyst metal 6 supported on the solid solution suppressing layer 4. Since the carrier having the same specific surface area or more compared to MgAl<SB>2</SB>O<SB>4</SB>is used, the absorption capacity of SO<SB>x</SB>increases remarkably. Further, the presence of MgO enables the keeping of the condition that SO<SB>x</SB>is absorbed up to the high-temperature region of about 600°C even in rich atmosphere to suppress the discharge. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a sulfur oxide absorbent that can absorb sulfur oxide well, a method for producing the same, and an exhaust gas purification apparatus that uses the sulfur oxide absorbent.

In order to suppress CO ₂ emissions by reducing fuel consumption, automobile engines and the like are burned in an oxygen-rich fuel lean atmosphere. As an exhaust gas purifying catalyst that efficiently performs NO _x reduction purification even in a fuel lean atmosphere, a NO _x storage reduction catalyst and a NO _x selective reduction catalyst have been developed and put into practical use. Further, in order to more efficiently oxidize and purify HC in exhaust gas, HC oxidation catalysts are also widely used.

The NO _x storage reduction catalyst comprises a porous support, a noble metal supported on the porous support, and a NO _x storage material supported on the porous support selected from alkali metals, alkaline earth metals and rare earth elements. It is used for an exhaust system of a combustion system in which the ratio of the air-fuel mixture is controlled so that it is always burned in a lean atmosphere with excess oxygen and intermittently becomes a stoichiometric to rich atmosphere.

As the _the NO _x selective reduction catalyst used in the fuel-lean atmosphere, that carries a Cu zeolite carrier, such as those supporting the noble metal is known to alumina. As an HC oxidation catalyst, a catalyst in which Pt having high oxidation activity is supported on a carrier such as alumina is known.

However, the exhaust gas in the fuel lean atmosphere contains SO ₂ due to sulfur in the fuel. Therefore, when such exhaust gas touches the NO _x storage reduction catalyst, SO ₂ becomes sulfur oxide such as SO ₃ or SO ₄ (hereinafter referred to as SO _x ) by the noble metal, and the NO _x storage material and SO _x react. Thus, there is a problem that a stable sulfate is generated and the NO _x storage capacity of the NO _x storage material is lowered.

In addition, in NO _x selective reduction type catalysts and HC oxidation catalysts, there is a problem that the supported catalytic metal having catalytic activity is covered with SO _x and the activity is lowered, and the sulfur component is further oxidized on the catalytic metal. There is also a problem that the above defects are further promoted because SO _x is generated. Moreover, there is a problem that the amount of sulfate discharged increases.

Therefore, the use of SO _x absorbents has been studied as one of the means for suppressing the above-mentioned problems caused by SO _x . As such an SO _x absorbent, for example, JP-A-57-162645 discloses a lanthanum oxide layer dispersed as a single layer on the surface of alumina. Japanese Patent Application Laid-Open No. 05-146675 discloses an SO _x adsorbent comprising an alumina stabilizer such as silica and lantana and alumina containing an active component such as an alkali metal.

By arranging such a SO _x absorber on the upstream side of the exhaust gas purifying catalyst as described above, since the SO _x in the exhaust gas is absorbed in the SO _x absorbent, the exhaust gas SO _x concentration is reduced _the NO _x storage Since it contacts the reduction catalyst, NO _x selective reduction catalyst or HC oxidation catalyst, the above-mentioned sulfur poisoning can be suppressed.

However, in the SO _x absorbent, if the SO _x absorption amount is saturated, it becomes difficult to absorb more SO _x . Therefore, it is necessary to perform a regeneration process in which the exhaust gas atmosphere is made rich with excess reducing components, the SO _x absorbent that has absorbed SO _x is decomposed to release SO _x , and the SO _x absorption capacity is regenerated. However, the above-described conventional SO _x absorbent requires a heat treatment at a high temperature of 600 to 700 ° C. in a gas in a rich atmosphere, and the regeneration treatment in actual exhaust gas is difficult. Therefore, there is a problem that the regeneration process must be performed separately, and the man-hours and costs are enormous.

In view of this, the applicant of the present application has proposed an SO _x absorbent in which a noble metal such as Pt is supported on a support made of MgO · nAl ₂ O ₃ (n is 0.8 or more and less than 1.1) in JP-A-2001-293366. According to this SO _x absorbent, the SO _x absorbability is excellent, the specific surface area is high even after use under high temperature, and the high temperature durability is excellent. Therefore, the number of regeneration processes can be reduced, and man-hours and costs can be greatly reduced.

However, the SO _x absorbent disclosed in Japanese Patent Laid-Open No. 2001-293366 also has a problem in fuel consumption and the like because the SO _x absorption capacity is not sufficiently large, and the frequency of regeneration treatment in exhaust gas needs to be increased. In a rich atmosphere, absorbed SO _x is decomposed and released, but the temperature range is relatively low. On the other hand, it has been clarified that the reaction between the NO _x storage material and SO _x is more likely to occur at lower temperatures and more likely to occur at lower reducing gas concentrations. Therefore SO _x released from the SO _x absorbent rich atmosphere, react with _the NO _x storage material of the NO _x storage reduction catalyst present in downstream, sulfur poisoning may occur even in the rich atmosphere not only lean atmosphere There was a problem.
JP-A-57-162645 Japanese Patent Laid-Open No. 05-146675 JP 2001-293366 A

The present invention has been made in view of the above circumstances, and it is an object to be solved to increase the SO _x absorption capacity and further shift the temperature at which SO _x is released in a rich atmosphere to a higher temperature range. .

The feature of the SO _x absorbent of the present invention that solves the above problems is that the support having at least a specific surface area equal to or larger than that of MgAl ₂ O ₄ and the Mg formed on at least the surface of the support are prevented from dissolving in the support. It consists of a solid solution suppressing layer, at least MgO supported on the solid solution suppressing layer, and an oxidation catalyst metal supported on the solid solution suppressing layer.

It is desirable that the carrier has a basicity equivalent to or higher than that of Al ₂ O ₃ . The solid solution suppressing layer is preferably MgAl ₂ O ₄ . In this case, the molar ratio Mg / Al in the solid solution suppressing layer supporting MgO is desirably in the range of 0.17 <Mg / Al <0.67.

The characteristics of the method for producing the SO _x absorbent of the present invention include a step of forming a coating layer made of alumina powder on a base material, and an amount exceeding the amount of alumina on the surface of the coating layer that becomes MgAl ₂ O ₄ . A step of forming an Mg-supported alumina layer by supporting an Mg compound containing Mg, a step of firing the Mg-supported alumina layer to form a mixed phase of MgAl ₂ O ₄ and MgO, and supporting an oxidation catalyst metal in the mixed phase And a process of performing.

  The molar ratio Mg / Al of the mixed phase is desirably in the range of 0.17 <Mg / Al <0.67. It is desirable that firing be performed at 700 ° C or higher, more preferably 750 ° C or higher.

A feature of the exhaust gas purification apparatus of the present invention is that the SO _x absorbent of the present invention is disposed in the exhaust gas flow path, and at least the NO _x storage reduction catalyst and the NO _x selective reduction catalyst are disposed downstream of the SO _x absorbent of the exhaust gas. One is to arrange one.

The sulfur desorption temperature of the SO _x absorbent is desirably higher than the sulfur desorption temperature of at least one of the NO _x storage reduction catalyst and the NO _x selective reduction catalyst.

According to the SO _x absorbent of the present invention, since a carrier having a high specific surface area equal to or higher than that of MgAl ₂ O ₄ is used, a large amount of SO _x can be absorbed, and the SO _x absorption capacity is remarkably increased. To do. Therefore, the frequency of the regeneration process can be reduced, and fuel consumption is improved. In addition, the presence of MgO makes it possible to hold SO _x in a rich atmosphere up to a high temperature range of about 600 ° C and suppress the release, so it can be used as a downstream NO _x storage reduction catalyst in a rich atmosphere. Sulfur poisoning can be prevented and the reduction in NO _x purification activity can be suppressed.

And according to the manufacturing method of the SO _x absorbent material of the present invention can be manufactured very easily and stably the SO _x absorber of the present invention.

The inventors of the present application recalled the use of MgO, which has an excellent SO _x absorption capacity. However, MgO alone has low heat resistance and the specific surface area decreases at 750 ° C or higher, so the SO _x absorption capacity cannot be secured. Therefore, it was studied to keep MgO highly dispersed and thermally stable on the substrate. However, when MgO is highly dispersed and supported on a carrier having a high specific surface area such as alumina, there is a problem that MgO is dissolved in alumina and the amount of MgO remaining on the surface is extremely small.

  As a result of intensive studies, the present inventors have found that MgO can be highly dispersed and expressed in a large amount on the surface by forming a solid solution suppressing layer, and the present invention has been completed.

That is, the SO _x absorbent of the present invention includes a support, a solid solution suppressing layer formed on at least the surface of the support, at least MgO supported on the solid solution suppressing layer, and an oxidation catalyst metal supported on the solid solution suppressing layer. And consist of

A support having at least a specific surface area equal to or greater than that of MgAl ₂ O ₄ is used, and examples thereof include alumina, silica, silica-alumina, ceria, ceria-zirconia, zeolite, and MgAl ₂ O ₄ . If the specific surface area of the support is smaller than the specific surface area of MgAl ₂ O ₄ , the SO _x absorption capacity is insufficient, which is not practical. Also, it is desirable that the specific surface area decrease degree is small even at high temperatures. For example, the specific surface area after firing at 700 ° C is 120 m ² / g or more, more than 150 m ² / g, or after firing at 800 ° C. It is desirable to maintain a specific surface area of 150 m ² / g or more, more preferably 200 m ² / g or more.

Further, it is desirable that the carrier has a basicity equivalent to or higher than that of Al ₂ O ₃ . When the basicity of the carrier is lower than the basicity of Al ₂ O ₃ , the SO _x adsorbing ability is lowered because the proximity of SO _x is suppressed. As such a carrier, most of those exemplified above are applicable, but those having increased basicity by complexing alkali metals, alkaline earth metals, rare earth elements and the like are particularly suitable. Examples of such a material include Al ₂ O ₃ to which La is added, and the effect of further improving the heat resistance is added.

The solid solution suppressing layer, which is the greatest feature of the present invention, has a function of suppressing the solid solution of Mg into the carrier. As this solid solution suppressing layer, MgAl ₂ O ₄ is typically exemplified, but variously selected depending on the type of the carrier. In the case of MgAl ₂ O ₄ , Mg is saturated in the solid solution suppression layer, so that the supported MgO does not dissolve in the solid solution suppression layer or react with the solid solution suppression layer. The solid solution is supported in a highly dispersed state on the surface of the solid solution suppressing layer.

The thickness of the solid solution suppressing layer is preferably in the range of 1 to 100 nm. If the thickness of the solid solution suppressing layer is smaller than this range, MgO will be dissolved in the carrier, and the SO _x absorption capacity and release temperature will be reduced. On the other hand, if the thickness of the solid solution suppressing layer exceeds this range, the effect of increasing the specific surface area of the carrier cannot be obtained, and the SO _x absorption capacity may decrease.

The solid solution suppressing layer carries at least MgO. A part of MgO may absorb carbon dioxide to become MgCO ₃ . Other Mg compounds can be supported as part of MgO as long as they become MgO in the exhaust gas. The amount of MgO supported is not particularly limited, but an optimum amount may exist depending on the type of carrier.

For example, when the solid solution suppressing layer is MgAl ₂ O ₄ , the molar ratio Mg / Al in the solid solution suppressing layer supporting MgO is preferably in the range of 0.17 <Mg / Al <0.67. When the molar ratio Mg / Al is 0.17 or less, the amount of MgO supported on the surface is extremely small, and the SO _x absorption capacity and the release temperature are lowered. On the other hand, when the molar ratio Mg / Al is 0.67 or more, the heat resistance is lowered, the specific surface area is reduced, and the SO _x absorption capacity is significantly reduced. A range of 0.30 <Mg / Al <0.55 is particularly desirable.

An oxidation catalyst metal is supported on the solid solution suppressing layer. This oxidation catalyst metal has a catalytic function capable of oxidizing SO2 in exhaust gas, and noble metals such as Pt, Pd, Rh, Ir, Ag, Au are optimal, but in some cases Co, Ni, Cu Base metals such as Mn and Fe can also be used. Although the amount of the oxidation catalyst metal supported varies depending on the metal species, for example, in the case of Pt, it is preferably 0.2 to 3 parts by weight per 100 parts by weight of the SO _x absorbent.

In order to produce the SO _x absorbent according to the present invention, for example, a coating layer made of MgAl ₂ O ₄ is formed on a honeycomb substrate, and MgO and an oxidation catalyst metal are supported on the coating layer. _There is a method in which a coat layer made of ₂ O ₃ is formed, an MgAl ₂ O ₄ layer is formed on the surface of the coat layer, and MgO and an oxidation catalyst metal are supported on the surface. In the latter case, if the production method of the present invention is used in which the MgAl ₂ O ₄ phase and the MgO phase are simultaneously formed on the surface of the coating layer made of Al ₂ O _3, the step of supporting MgO can be omitted. it can.

That is, in the production method of the present invention, a step of forming a coat layer made of alumina powder on a substrate, and a Mg compound containing Mg in an amount exceeding the amount that the surface alumina becomes MgAl ₂ O ₄ are supported on the surface of the coat layer. The Mg-supporting alumina layer is formed, the Mg-supporting alumina layer is fired to form a mixed phase of MgAl ₂ O ₄ and MgO, and the oxidation catalyst metal is supported on the mixed phase.

  As a base material, a thing with a large contact area with gas, such as a honeycomb base material, a pellet base material, a foam base material, can be used.

  In the production method of the present invention, a coating layer is first formed from alumina powder on the surface of a substrate. This step can be performed in the same manner as a conventional method for forming a coat layer on an oxidation catalyst or a three-way catalyst, and generally a wash coat method is used.

The coat layer carries an Mg compound containing Mg in an amount exceeding the amount that the surface alumina becomes MgAl ₂ O ₄ to form an Mg-supported alumina layer. As the Mg compound, nitrates, alkoxides and the like can be used. Also, the amount of Mg that exceeds the amount that the surface alumina becomes MgAl ₂ O ₄ means that the MgAl ₂ O ₄ phase is formed on the surface of the coat layer by the reaction of Mg and Al during firing in the next step. It means the amount of MgO phase formed. Therefore, the amount varies depending on the thickness of the mixed phase of MgAl ₂ O ₄ and MgO to be formed. However, MgAl ₂ O _{4 in the} mixed phase forms a solid solution suppressing layer, and the thickness of the solid solution suppressing layer is 1. In view of the fact that it is desired to be in the range of ˜100 nm, 0.003 to 0.01 mol of Mg is desirably contained per 1 m ² of the surface area of the coat layer. For example, the supported amount of NO _x storage material in the NO _x storage reduction catalyst is at most 1 mole per liter of catalyst, and when such a small amount of Mg compound is supported on alumina and fired under the firing conditions described later. In this case, Mg becomes MgAl ₂ O ₄ and no MgO phase is formed.

In the supporting step, only the Mg compound may be supported on the coating layer, or an Al compound such as Al nitrate, a precipitate from an Al nitrate aqueous solution, or Al alkoxide may be supported in addition to the Mg compound. In this case, in addition to the above Mg, it is necessary to increase the amount of Mg that reacts with the Al compound to become MgAl ₂ O ₄ .

In the next firing step, the Mg-supported alumina layer is fired to form a mixed phase of MgAl ₂ O ₄ and MgO on the surface of the coat layer. The firing temperature is desirably 750 ° C. or higher. When the firing temperature is less than 700 ° C., it is difficult to form the MgAl ₂ O ₄ phase. The firing time is also an important factor. If the firing temperature is low, it takes a long time to form the MgAl ₂ O ₄ phase, and if the firing time is short even if the firing temperature is high, the formation of the MgAl ₂ O ₄ phase becomes difficult. For example, when firing at 850 ° C., it is desirable that the time be 2 hours or longer. When firing at 700 ° C., which is the lowest temperature at which the MgAl ₂ O ₄ phase can be formed, 5 hours or longer is required.

  For the same reason as described above, the molar ratio Mg / Al of the mixed phase is preferably in the range of 0.17 <Mg / Al <0.67, and particularly preferably in the range of 0.30 <Mg / Al <0.55.

  An oxidation catalyst metal is supported on the formed mixed phase. This supporting step can be performed in the same manner as the noble metal supporting step in a conventional oxidation catalyst or the like. The carrying amount is the above-mentioned carrying amount. The firing step for forming the mixed phase may be performed after the supporting step of the oxidation catalyst metal, or may be performed separately before and after the supporting step of the oxidation catalyst metal.

The SO _x absorbent of the present invention can be used alone, but it is particularly used as an exhaust gas purification apparatus in which at least one of a NO _x storage reduction catalyst and a NO _x selective reduction catalyst is arranged on the exhaust gas downstream side. preferable. In the exhaust gas purification apparatus of the present invention, when the exhaust gas in a lean atmosphere comes into contact with the SO _x absorbent, SO ₂ in the exhaust gas is oxidized to SO _x by the oxidation catalyst metal and absorbed by MgO 2. Since the SO _x absorbent of the present invention has a large SO _x absorption capacity, almost all SO _{x in} the exhaust gas is absorbed, and it is reliably suppressed from flowing into the downstream catalyst. Therefore, sulfur poisoning of the downstream catalyst is suppressed, and a high NO _x purification rate is expressed.

The sulfur desorption temperature of the SO _x absorbent is desirably higher than the sulfur desorption temperature of at least one of the NO _x storage reduction catalyst and the NO _x selective reduction catalyst. With this configuration, the temperature when the stoichiometric or rich atmosphere is used to reduce and purify NO _x with the downstream NO _x storage reduction catalyst is lower than the sulfur desorption temperature of the SO _x absorbent. _Since SO _x is not released from the _x absorbent, sulfur poisoning of the downstream catalyst in the rich atmosphere can be reliably prevented. Further, when the rich atmosphere to restore the SO _x absorption capability of the SO _x absorbent, since _the NO _x storage material in which sulfur poisoning becomes high than the decomposing temperature, SO _x absorption capability of the SO _x absorbent As well as the NO _x storage capacity of sulfur poisoned NO _x storage materials.

The sulfur desorption temperature of the SO _x absorbent is preferably at least 20 ° C., more preferably at least 50 ° C. higher than the sulfur desorption temperature of at least one of the NO _x storage reduction catalyst and the NO _x selective reduction catalyst. The temperature of the rich regeneration process for recovering SO _x absorption capability of the SO _x absorbent is also possible to increase 20 ° C. or higher than at least one of the sulfur desorption temperature of the NO _x storage reduction catalyst and _the NO _x selective reduction catalyst Is preferable, and it is more desirable to increase the temperature by 50 ° C.

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

(Reference Example 1)
A honeycomb substrate (diameter 30 mm, length 50 mm) made of cordierite and having hexagonal cells was prepared, and a coat layer made of γ-Al ₂ O ₃ powder was formed by a wash coat method. The amount of coat layer formed is 207 g per liter of honeycomb substrate.

  Next, a predetermined amount of a magnesium acetate aqueous solution having a predetermined concentration was absorbed in the coating layer, dried at 120 ° C., and then fired in the atmosphere at 850 ° C. for 5 hours. Magnesium acetate was supported at 1.35 mol as Mg per liter of honeycomb substrate, and the molar ratio Mg / Al was 0.33.

(Reference Example 2)
It is the same as Reference Example 1 except that 2.03 mol of magnesium acetate is carried as Mg per liter of honeycomb substrate. The molar ratio Mg / Al was 0.50.

(Reference Example 3)
107 g of magnesium acetate and 379 g of aluminum nitrate were dissolved in 1800 ml of ion exchange water, and 650 g of 25% aqueous ammonia was added to this aqueous solution to obtain a precipitate. This precipitate was calcined in the atmosphere at 850 ° C. for 5 hours to produce MgAl ₂ O ₄ powder.

  This powder was coated on the same honeycomb substrate as in Reference Example 1 at 200 g per liter. The molar ratio Mg / Al of the coat layer was 0.5.

(Reference Example 4)
It is the same as Reference Example 1 except that 0.63 mol of magnesium acetate is carried as Mg per liter of honeycomb substrate. The molar ratio Mg / Al was 0.17.

<Test and evaluation>
Each layer prepared in Reference Examples 1 to 4 is subjected to X-ray diffraction analysis, and each X-ray diffraction chart is shown in FIGS. In Reference Examples 1 and 2, an MgO peak is observed, whereas in Reference Examples 3 and 4, no MgO peak is observed. That is, when the molar ratio Mg / Al is 0.17 as in Reference Example 4, magnesium acetate reacts with alumina to become MgAl ₂ O ₄ , and in the method of firing the precipitate obtained by the coprecipitation method, the molar ratio Mg / Al It can be seen that even when Al is 0.5, magnesium acetate reacts with alumina to become all MgAl ₂ O ₄ . In the case of the coprecipitation method, since the contact probability between Mg and Al is extremely high, it is considered that all Mg reacted with Al.

Based on the above results, SO _x absorbents of Examples and Comparative Examples were prepared as follows.

Example 1
A layer composed of a mixed phase of MgO and MgAl ₂ O ₄ was formed in the same manner as in Reference Example 1 except that 0.5 mol of magnesium acetate was carried as Mg per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Further, a predetermined amount of magnesium acetate aqueous solution containing Mg in an amount of 0.85 mol / L per liter of honeycomb substrate was absorbed into the coating layer, dried at 120 ° C., and fired in the air at 750 ° C. for 5 hours. The molar ratio Mg / Al of the mixed phase is 0.33.

(Example 2)
A layer composed of a mixed phase of MgO and MgAl ₂ O ₄ was formed in the same manner as in Reference Example 1 except that magnesium acetate was supported at 1.35 mol as Mg per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours.

The enlarged cross-sectional view of the SO _x absorber of the present embodiment shown in FIG. The SO _x absorbent includes a honeycomb base material 1 made of cordierite, a SO _x absorbent coating layer 2 (carrier) formed on the cell partition wall surface of the honeycomb base 1, and the SO _x absorbent coating layer 2. The solid solution suppression layer 4 is formed on the surface of the alumina particles 3. The solid solution suppressing layer 4 is made of MgAl ₂ O ₄ , and MgO 5 and Pt 6 are supported on the solid solution suppressing layer 4.

(Example 3)
In the same manner as in Reference Example 1, a coat layer made of γ-Al ₂ O ₃ powder was formed in an amount of 207 g per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Next, a predetermined amount of an aqueous magnesium acetate solution containing Mg in an amount of 1.35 mol / L per liter of honeycomb substrate was absorbed into the coat layer, dried at 120 ° C., and fired in the air at 750 ° C. for 5 hours. The molar ratio Mg / Al of the mixed phase is 0.33.

Example 4
In the same manner as in Reference Example 1, a coat layer made of γ-Al ₂ O ₃ powder was formed in an amount of 207 g per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Next, a predetermined amount of magnesium acetate aqueous solution containing Mg in an amount of 1.8 mol / L per liter of honeycomb substrate was absorbed into the coating layer, dried at 120 ° C., and fired at 750 ° C. for 5 hours in the air. The molar ratio Mg / Al of the mixed phase is 0.44.

(Example 5)
In the same manner as in Reference Example 1, a coat layer made of γ-Al ₂ O ₃ powder was formed in an amount of 207 g per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Next, a predetermined amount of magnesium acetate aqueous solution containing Mg in an amount of 2.03 mol / L per liter of honeycomb substrate was absorbed into the coating layer, dried at 120 ° C., and fired at 750 ° C. for 5 hours in the air. The molar ratio Mg / Al of the mixed phase is 0.50.

(Example 6)
In the same manner as in Reference Example 1, a coat layer made of γ-Al ₂ O ₃ powder was formed in an amount of 207 g per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Next, a predetermined amount of an aqueous magnesium acetate solution containing Mg in an amount of 2.7 mol / L per liter of honeycomb substrate was absorbed into the coating layer, dried at 120 ° C., and fired at 750 ° C. for 5 hours in the air. The molar ratio Mg / Al of the formed mixed phase is 0.67.

(Comparative Example 1)
Using the MgAl ₂ O ₄ powder prepared in Reference Example 3, a coating layer was formed in a formation amount of 200 g per 1 L of honeycomb substrate in the same manner as in Reference Example 1. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. The molar ratio Mg / Al of the coat layer is 0.50.

(Comparative Example 2)
In the same manner as in Reference Example 1, a coat layer made of γ-Al ₂ O ₃ powder was formed in an amount of 207 g per liter of honeycomb substrate. Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Next, a predetermined amount of an aqueous magnesium acetate solution containing Mg in an amount of 0.68 mol / L per liter of honeycomb substrate was absorbed into the coating layer, dried at 120 ° C., and fired at 750 ° C. for 5 hours in the air. The molar ratio Mg / Al of the formed MgAl ₂ O ₄ layer is 0.17.

<Test and evaluation>
The coating layer component of each SO _x absorbing materials of Examples and Comparative Examples as pellets, to fill the 0.6g in a fixed bed flow type reactor was measured S absorption and S desorption start temperature, respectively. For the measurement, the model gas shown in Table 1 was used, pretreated with rich gas, and then treated with each gas as in the evaluation pattern shown in FIG. That is, after performing the S adhesion treatment at 400 ° C., the S desorption treatment was performed by increasing the temperature from 200 ° C. to 750 ° C. at 14 ° C./min. The gas flow rate is 6 L / min.

  Then, the S concentration in the output gas is measured, the S absorption amount is calculated from the difference between the input gas S concentration and the output gas S concentration until the time when the output gas S concentration becomes 1 ppm or more, and the S absorption amount per 100 g of pellets. Was calculated. The temperature when the outgas S concentration was 1 ppm or more was defined as the S desorption temperature. The results are shown in Table 2.

From Table 2, it is clear that the SO _x absorbents of Examples 1 to 5 have a larger amount of S deposition than Comparative Example 1 and a higher S desorption start temperature than Comparative Examples 1 and 2. It is clear that this is due to the fact that a mixed phase composed of MgAl ₂ O ₄ and MgO and having a molar ratio Mg / Al> 0.17 was formed, and that MgAl ₂ O ₄ functioned as a solid solution inhibiting layer of MgO ₂ .

  In addition, although Example 6 has a higher S desorption start temperature than Comparative Examples 1 and 2, the amount of S deposition is very small. This is because the molar ratio Mg / Al is as large as 0.67, so that the heat resistance is reduced, the specific surface area is reduced, and the SOx absorption capacity is greatly reduced.

That is, it is clear that the SO _x absorbents of Examples 1 to 5 have a large SO _x absorption capacity and a high S desorption temperature because MgO is present in a highly dispersed and thermally stable state on the surface. It is.

(Example 7)
FIG. 7 shows the exhaust gas purifying apparatus of this example. The SO _x absorbent 5 of the present invention is disposed upstream of the exhaust gas, and the NO _x storage reduction catalyst 6 is disposed downstream thereof. The SO _x absorbent 5 is obtained by cutting the SO _x absorbent of Example 4 so as to have a length of 15 mm. The NO _x storage reduction catalyst 6 was prepared as follows.

A slurry was prepared by mixing 100 g of alumina powder, 100 g of titania-zirconia composite oxide powder, 50 g of Rh / ZrO ₂ powder supporting 1% by weight of Rh on zirconia, and 20 g of ceria-zirconia composite oxide powder with ion-exchanged water. Using this slurry, a honeycomb substrate made of cordierite and having hexagonal cells (diameter 30 mm, length 35 mm) was wash-coated to form a coat layer. The amount of coat layer formed is 270 g per liter of honeycomb substrate.

  Then, using a dinitrodiammine platinum nitric acid solution, Pt was selectively adsorbed and supported at 2 g / L, and then calcined in the atmosphere at 300 ° C. for 3 hours. Furthermore, a mixed aqueous solution of barium acetate, potassium acetate and lithium acetate was impregnated with the required amount of water so that 0.2 mol of Ba, 0.15 mol of K, and 0.1 mol of Li per 1 L of honeycomb substrate was obtained, and at 300 ° C. in the atmosphere for 3 hours. Baked.

(Comparative Example 3)
Example 7 is the same as Example 7 except that in place of the SO _x absorbent 5, the NO _x occlusion reduction catalyst 6 is cut to a length of 15 mm.

<Test and evaluation>
Place each evaluation device the exhaust gas purifying apparatus of Comparative Example 3 and Example 7 were evaluated for _the NO _x storage reduction performance using a model gas simulating an exhaust composition of the start catalyst downstream of direct injection gasoline engines. The space velocities SV are all 51400h- ¹ .

  That is, after pretreatment at 650 ° C. for 10 minutes using the pretreatment gas shown in Table 3, the S poison gas shown in Table 3 was allowed to flow at 400 ° C. for 41 minutes while alternately switching between lean 120 seconds / rich 3 seconds, and the catalyst 2L The amount of S supply per unit was 6 g. Thereafter, treatment was performed at 700 ° C. for 10 minutes using the regeneration gas shown in Table 3. Thereafter, the S poison gas shown in Table 3 was allowed to flow at 400 ° C. for 41 minutes while alternately switching between lean 120 seconds and rich 3 seconds, so that the amount of S supplied per 2 liters of catalyst was 6 g.

The NO _x purification rate during the above test was continuously measured, and the results are shown in FIG. It can be seen that the apparatus of Example 7 has a lower degree of decrease in the NO _x purification rate than the apparatus of Comparative Example 3, and sulfur poisoning is suppressed. Further, the amount of S passing until the NO _x purification rate reaches about 35% in the apparatus of Comparative Example 3 is 4 g, but in Example 7, an equivalent NO _x purification rate is realized even when the S passage is about 8 g. Can be said. That is, the deterioration rate with respect to the S passage amount is about ½ of that of Comparative Example 3 in Example 7, and the exhaust gas purifying apparatus of Example 7 is extremely excellent in sulfur poisoning resistance. This is apparently the effect of disposing the SO _x absorbent 5 on the exhaust gas upstream side.

3 is an X-ray diffraction chart of a layer formed in Reference Example 1. FIG. 10 is an X-ray diffraction chart of a layer formed in Reference Example 2. FIG. 10 is an X-ray diffraction chart of a layer formed in Reference Example 3. 10 is an X-ray diffraction chart of a layer formed in Reference Example 4. Is a schematic cross-sectional view enlarging a main part of the SO _x absorbent in the second embodiment of the present invention. It is a time chart explaining the evaluation pattern in an Example. It is explanatory drawing which shows the structure of the exhaust gas purification apparatus of Example 7 of this invention. 6 is a graph showing a change with time of the NO _x purification rate of the exhaust gas purification apparatuses of Example 7 and Comparative Example 3.

Explanation of symbols

1: Honeycomb substrate 2: SO _x absorbent coating layer 3: Alumina 4: Solid solution suppression layer 5: MgO 6: Pt

Claims (9)

A support having at least a specific surface area equal to or greater than that of MgAl ₂ O ₄ , a solid solution suppressing layer formed on at least the surface of the support to prevent Mg from dissolving in the support, and supported on the solid solution suppressing layer A sulfur oxide absorbent comprising at least MgO and an oxidation catalyst metal supported on the solid solution suppressing layer. The sulfur oxide absorbent according to claim 1, wherein the carrier has a basicity equivalent to or higher than that of Al ₂ O ₃ . The sulfur oxide absorbent material according to claim 1, wherein the solid solution suppressing layer is MgAl ₂ O ₄ .   The sulfur oxide absorbent according to claim 3, wherein a molar ratio Mg / Al in the solid solution suppressing layer supporting MgO is in a range of 0.17 <Mg / Al <0.67. Forming a coat layer made of alumina powder on a substrate;
Forming a Mg-supported alumina layer on the surface of the coat layer by supporting an Mg compound containing Mg in an amount exceeding the amount by which the alumina on the surface becomes MgAl ₂ O ₄ ;
Firing the Mg-supported alumina layer to form a mixed phase of MgAl ₂ O ₄ and MgO;
And a step of supporting the oxidation catalyst metal on the mixed phase.   The molar ratio Mg / Al of the said mixed phase is a manufacturing method of the sulfur oxide absorber of Claim 5 which exists in the range of 0.17 <Mg / Al <0.67.   The method for producing a sulfur oxide absorbent according to claim 5 or 6, wherein the firing is performed at 700 ° C or higher. The sulfur oxide absorbent according to any one of claims 1 to 4 is disposed in an exhaust gas flow path, and at least the NO _x storage reduction catalyst and the NO _x selective reduction catalyst are disposed on the exhaust gas downstream side of the sulfur oxide absorbent. An exhaust gas purifying apparatus characterized in that one side is arranged. The exhaust gas purification apparatus according to claim 8, wherein a sulfur desorption temperature of the sulfur oxide absorbent is higher than a sulfur desorption temperature of at least one of the NO _x storage reduction catalyst and the NO _x selective reduction catalyst.

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