JP2010110732A - Exhaust gas purifying catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst capable of exhibiting excellent NOx purifying performance in a wide range from a low-temperature to a high-temperature. <P>SOLUTION: The exhaust gas purifying catalyst includes: an NOx absorbent material 1 arranged in the exhaust passage of an internal-combustion engine; an oxidation catalyst 2 arranged in the downstream side of the NOx absorbent 1; and an NOx reduction purifying catalyst 3 arranged in the downstream side of the oxidation catalyst 2. The NOx absorbent material 1 comprises a non-precious metal component having an NOx absorbing performance. The non-precious metal component desirably includes one or more kinds selected from CeO<SB>2</SB>, Gd<SB>2</SB>O<SB>3</SB>and Pr<SB>6</SB>O<SB>11</SB>. The NOx absorbent material 1 may carry catalytic precious metal in the downstream end leaving the upstream end. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine such as a lean burn engine.

In recent years, lean burn engines that burn in an oxygen-rich atmosphere have been used for the purpose of reducing carbon dioxide (CO ₂ ). In the lean burn engine, the exhaust gas atmosphere is in a lean region (oxygen-excess atmosphere) compared to the theoretical air-fuel state. In the lean region, it is known that NOx reduction purification catalysts such as NSR catalyst (NOx storage reduction catalyst) and SCR catalyst (NOx contact reduction catalyst) are effective (Patent Documents 1 and 2).

The NSR catalyst occludes NOx in a lean atmosphere, and reduces and purifies NOx occluded in a stoichiometric to rich atmosphere with HC, CO, or the like. The SCR catalyst reduces and purifies NOx to N ₂ using ammonia or urea as a reducing agent.

  However, the NOx reduction purification catalyst has low NOx purification activity outside the active region of 200 ° C. or lower. For this reason, utilization of the adsorption performance of the NOx adsorbent at 200 ° C. or less has been studied (Patent Documents 3 and 4).

Moreover, in FIG. 1 of patent document 5, it is proposed to arrange | position the catalyst (2) which oxidizes and adsorb | sucks NO in the upstream of the NOx reduction purification catalyst (4) which consists of SCR catalysts. In the low temperature range, NO is well adsorbed by the catalyst (2), and the adsorbed NO is desorbed as NO ₂ due to the temperature rise, so the amount of NO _{2 in the} total NOx increases. In the high temperature range, the total NOx amount increases, but the inlet temperature of the NOx reduction purification catalyst (4) also increases, and NOx reduction purification is actively performed.

  Also in FIG. 1 of Patent Document 6, a NOx storage catalyst is arranged upstream of the SCR catalyst, NOx is adsorbed by the NOx storage catalyst in the low temperature region, and NOx is released in the high temperature region, so that the downstream SCR catalyst is released. And reducing and purifying.

  Also, in FIG. 1 of Patent Document 7, a catalyst (2) for temporarily storing nitrogen oxides in an atmosphere with a low gas component N composition and releasing them in a rich atmosphere is arranged upstream of the SCR catalyst (4). ing.

In FIG. 2 of Patent Document 8, an SOF adsorbent having a function of adsorbing and oxidizing a soluble organic compound (SOF) on the upstream side, adsorbing a nitrogen compound at a temperature of 200 ° C. or lower, and allowing carbon particles to pass therethrough. It has been proposed to arrange an oxidation catalyst and a filter catalyst for treating dry soot using NO ₂ as an oxidizing agent on the downstream side.
JP-A-11-107744 JP 2000-257417 A JP 2007-85353 A JP 2004-52680 A JP 2003-236343 (FIG. 1, paragraph numbers “0022” to “0023”) JP-T-2006-519332 (FIG. 1, paragraph number “0014”) Japanese Unexamined Patent Publication No. 2000-27634 (FIG. 1, paragraph number “0006”) JP 2002-153733 A (FIG. 2, paragraph numbers “0032” to “0035”)

  However, many NOx adsorbents and oxidation catalysts contain noble metals such as Pt. For this reason, in a low temperature range of 200 ° C. or lower, the noble metal is reduced to NO that is not easily captured by the NOx reduction purification catalyst. For this reason, when the NO adsorption / oxidation catalyst (2) is disposed upstream of the NOx reduction purification catalyst (4) as in the catalyst disclosed in Patent Document 5, the NO adsorption / oxidation catalyst (2) is disposed in the low temperature range. NOx is reduced to NO by the precious metal therein, and even if NO is desorbed from the NO adsorption / oxidation catalyst (2) due to temperature rise, it is not captured by the downstream NOx reduction purification catalyst (4), and the NOx purification performance is improved. There is a problem of being lowered. Also, Patent Documents 6 and 7 have the same problems as Patent Document 5 because the NOx adsorbent is disposed upstream of the NOx reduction purification catalyst.

Also in Patent Document 8, noble metal is contained in the oxidation catalyst arranged on the upstream side, and NO ₂ is reduced to NO by the noble metal in a low temperature range, so that the amount of adsorption to the downstream filter catalyst decreases. There is a fear.

  This invention is made | formed in view of this situation, and makes it a subject to provide the catalyst for exhaust gas purification which can exhibit the outstanding NOx purification performance in the wide range from a low temperature area to a high temperature area.

  An exhaust gas purifying catalyst according to the present invention is provided with a NOx adsorbent disposed in an exhaust passage of an internal combustion engine, an oxidation catalyst disposed downstream of the NOx adsorbent, and disposed downstream of the oxidation catalyst. The NOx adsorbent is composed of a non-noble metal component having NOx adsorption ability.

The non-noble metal component preferably includes one or more selected from CeO ₂ , Gd ₂ O ₃ and Pr ₆ O ₁₁ .

  The NOx adsorbent preferably carries a catalytic noble metal at the downstream end, leaving the upstream end.

In the exhaust gas purifying catalyst of the present invention, a NOx adsorbent made of a non-noble metal component is disposed upstream of the oxidation catalyst. In the low temperature range, NOx is adsorbed by the NOx adsorbent, so that NOx is removed from the exhaust gas. Therefore, it is possible to reduce the amount of NOx flowing to the oxidation catalyst arranged on the downstream side of the NOx adsorbent, and it is possible to suppress the amount reduced from NO ₂ to NO by the noble metal contained in the oxidation catalyst.

That is, as shown in FIG. 3 to be described later, the noble metal contained in the oxidation catalyst oxidizes NO to NO ₂ at a high temperature range, but tends to reduce NO ₂ to NO at a low temperature range. By disposing the NOx adsorbent upstream of the oxidation catalyst, the NOx adsorbent adsorbs NOx in a low temperature range. Therefore, the amount of NOx flowing through the downstream oxidation catalyst is suppressed, and the reduction of NO _{2 in} NOx by the noble metal in the oxidation catalyst can be effectively suppressed. Furthermore, since the NOx adsorbent is made of a non-noble metal component, the adsorbed NO ₂ is less likely to be reduced at a lower temperature than when it contains a noble metal component. Therefore, NOx is adsorbed by the NOx adsorbent in the low temperature range without increasing the amount of NO contained therein.

When the temperature rises, NOx adsorbed on the NOx adsorbent is desorbed. NO in the desorbed NOx is oxidized to NO ₂ by the downstream oxidation catalyst. The oxidized NO ₂ is reduced and purified by the NOx reduction and purification catalyst. Further, NO ₂ in the desorbed NOx is reduced and purified by the NOx reduction and purification catalyst as NO ₂ .

Here, since the NOx adsorbent is made of a non-noble metal component, reduction of NO ₂ to NO can be suppressed. Therefore, NOx is desorbed from the NOx adsorbent due to the temperature rise, so that the NOx occlusion amount in the downstream NOx reduction purification catalyst is greatly improved, and NOx is reduced and purified with high efficiency by the NOx reduction purification catalyst.

  Therefore, NOx emissions can be greatly reduced over a wide range from the low temperature range to the high temperature range.

When the non-noble metal component contains one or more selected from CeO ₂ , Gd ₂ O ₃ and Pr ₆ O ₁₁ , NO ₂ in the exhaust gas is adsorbed as NO ₂ as it is due to its oxygen absorption / release capability. In addition to this, NO in the exhaust gas can be oxidized and adsorbed to NO ₂ with oxygen in the NOx adsorbent. Therefore, the content of NO ₂ in the NOx adsorbent increases, and a large amount of NO ₂ can be desorbed after the temperature rises. Therefore, NO ₂ is reduced and purified to N ₂ with high efficiency by the NOx reduction and purification catalyst arranged on the downstream side.

When the catalytic noble metal is supported on the downstream end of the NOx adsorbent, the temperature on the downstream side is more likely to rise than the upstream side of the NOx adsorbent. By supporting the catalyst noble metal on the downstream side of the NOx adsorbent, the oxidation activity of NO at the downstream end is improved in the high temperature region. As a result, the adsorption performance of NO ₂ by the NOx reduction purification catalyst in the high temperature range is improved.

  The exhaust gas purifying catalyst of the present invention is configured by arranging, in order from the upstream side to the downstream side, an NOx adsorbent composed of a non-noble metal component, an oxidation catalyst, and a NOx reduction purification catalyst in the exhaust passage of the internal combustion engine.

(NOx adsorbent)
NOx adsorbent, in the low temperature range, such as during startup, NO in the exhaust gas, the NOx such as NO ₂ adsorbed in the high temperature range to desorb NOx which has been adsorbed. The NOx adsorbent contains a non-noble metal component having NOx absorption / release performance. The non-noble metal component may include one or more selected from CeO ₂ , Gd ₂ O ₃ and Pr ₆ O ₁₁ , and CeO ₂ is particularly preferable. Since these components have the ability to absorb and release oxygen, in addition to adsorbing NO ₂ in the exhaust gas as NO ₂ as it is, NO can be adsorbed by oxidizing it into NO ₂ with oxygen in the adsorbent.

Further, the non-noble metal component may contain one or more selected from ZrO ₂ , MgO, and zeolite instead of or together with the component having the ability to absorb and release oxygen. Since these have NOx adsorption performance, they can be suitably used as NOx adsorbents.

  The content of the non-noble metal component in the NOx adsorbent is preferably 10 to 100% by mass. If it is less than 10% by mass, the NOx adsorption performance of the NOx adsorbent may be reduced.

  The NOx adsorbent may be composed of the non-noble metal component alone, but may contain one or more selected from alkali metals and alkaline earth metals in addition to the non-noble metal component. Since these have the property of occluding NOx, the NOx adsorption amount of the NOx adsorbent increases. Examples of the alkali metal include Li (lithium), Na (sodium), K (potassium), and Cs (cesium). The alkaline earth metal refers to a group 2A element in the periodic table, and be (beryllium), Mg (magnesium). ), Ca (calcium), Sr (strontium), Ba (barium), and the like.

  The alkali metal or alkaline earth metal content is preferably 0.01 to 0.5 mol per liter of the substrate. When the amount is less than 0.01 mol, it is difficult to obtain the effect of adding an alkali metal or an alkaline earth metal. On the other hand, when it exceeds 0.5 mol, it becomes difficult to support the alkali metal or alkaline earth metal on the non-noble metal component.

The NOx adsorbent is good to carry the catalyst noble metal in the downstream portion downstream of the upstream portion, leaving the upstream upstream portion of the exhaust passage in the exhaust gas flow direction. By supporting a small amount of catalytic noble metal in the downstream portion where the temperature is likely to rise, the NO oxidation activity in the downstream portion is improved in the high temperature region. As a result, the NO ₂ adsorption ability of the NOx reduction purification catalyst in the high temperature region is improved.

Here, Pt, Pd, Rh, etc. can be used for the noble metal component carried in the downstream part of the NOx adsorbent. The amount of the noble metal component supported is preferably 0.01 to 2.0 g, more preferably 0.01 to 1.0 g, per liter of the base material. If it is less than 0.01g, there is a possibility that oxidation activity of NO in a high temperature range by catalytic noble metal is lowered, when exceeding 2.0g, often amounts NO ₂ at low temperature range is reduced to NO Therefore, the amount of NO ₂ adsorbed on the NOx reduction and purification catalyst disposed on the downstream side may be reduced.

The ratio of the length of the downstream end portion supporting the catalyst noble metal to the length of the NOx adsorbent is preferably 1 to 50%. If it is less than 1%, the oxidation activity of the catalyst noble metal in the high temperature range may be reduced. If it exceeds 50%, the amount of NO ₂ reduced to NO increases in the low temperature range, and the downstream side There is a risk that the amount of NO ₂ adsorbed on the NOx reduction purification catalyst disposed in the catalyst will decrease. In this specification, “length” refers to the length of the exhaust passage in the exhaust gas flow direction.

  The length of the NOx adsorbent may be increased when the amount of NOx to be removed at a low temperature is large, and shortened when the amount is small.

(Oxidation catalyst)
The oxidation catalyst oxidizes NO in the exhaust gas to NO _2. For this reason, the NO ₂ concentration in the exhaust gas increases, the amount of NO ₂ reduced to N ₂ by the downstream NOx reduction purification catalyst increases, and the emission of NOx in the exhaust gas is suppressed. That is, the NOx reduction purification catalyst reduces and purifies NO ₂ to N ₂ . For this reason, NO reduction in NOx is converted into NO ₂ by the oxidation catalyst, so that the reduction purification rate of NOx by the NOx reduction purification catalyst arranged on the downstream side is improved.

  The NOx oxidation performance of the oxidation catalyst is high in the high temperature range, but low in the low temperature range. Therefore, the NOx oxidation performance of the oxidation catalyst is effectively exhibited in a high temperature range.

The oxidation catalyst includes a support and a catalytic metal supported on the support. The support is made of a porous oxide, and may be made of, for example, at least one selected from CeO ₂ , Al ₂ O ₃ , ZrO ₂ , TiO ₂ , and zeolite. As the catalytic metal, Pt, Pd, Rh, Ag or the like can be used, and Pt is particularly preferable.

  The supported amount of the catalyst metal is preferably 0.1 to 3 g per liter of the base material, and desirably 0.6 to 2 g of Pt is supported. Thereby, the NOx oxidation performance of the catalyst metal can be sufficiently exhibited.

  Since alkali metals and alkaline earth metals tend to cover the catalyst metal, it is preferred that they are not included in the oxidation catalyst.

(NOx reduction purification catalyst)
The NOx reduction purification catalyst is a catalyst that stores NO ₂ and reduces and purifies it to N ₂ . The NOx reduction purification catalyst occludes NO ₂ at high temperatures, but has little occlusion of NO ₂ at low temperatures. As for NO, it is oxidized and occluded to NO ₂ by a catalyst noble metal at high temperatures, but is not oxidized and occluded by a catalyst noble metal at low temperatures.

Examples of the NOx reduction purification catalyst include an NSR catalyst and an SCR catalyst. Purification reaction of NOx by the NSR catalyst comprises a first step of occluding NO ₂ in the exhaust gas to the NOx storage element in the lean atmosphere, the NOx released from the NOx storage element in the stoichiometric-rich atmosphere with a reducing agent such as HC N _And a second step of reducing to 2.

The SCR catalyst reduces NOx to N ₂ with a reducing agent such as urea or ammonia.

In the high temperature range, the catalytic reaction of the NOx reduction purification catalyst proceeds, but the ratio of NO ₂ which is the starting material of this reaction in the exhaust gas is not necessarily large. Therefore, an oxidation catalyst that oxidizes NOx in a high temperature region is disposed upstream of the NOx reduction purification catalyst.

On the other hand, in the low temperature range, the NOx stored in the NOx reduction purification catalyst is small. Furthermore, the NOx oxidation activity of the oxidation catalyst is low because it is disposed upstream of the NOx reduction purification catalyst, and NO ₂ is reduced to NO. Therefore, a NOx adsorbent is disposed on the upstream side of the oxidation catalyst. Since the NOx adsorbent adsorbs NOx even in a low temperature range, it can compensate for the low purification activity of the oxidation catalyst and the NOx reduction purification catalyst in the low temperature range and suppress the NOx emission from the exhaust gas purification catalyst.

When the NOx reduction purification catalyst is an NSR catalyst, a known NSR catalyst can be used as the NSR catalyst. For example, the NSR catalyst includes a support, a catalyst noble metal supported on the support, and a NOx storage element supported on the support. The support is made of a porous oxide, and may be made of, for example, at least one selected from CeO ₂ , Al ₂ O ₃ , ZrO ₂ , TiO ₂ , and zeolite. Pt, Pd, Rh, etc. can be used as the catalyst noble metal. The NOx storage element may be selected from alkali metals and alkaline earth metals. Examples of the alkali metal include Li, Na, K, Rb, and Cs, and examples of the alkaline earth metal include Be, Mg, Ca, Sr, and Ba.

  When the NOx reduction purification catalyst is an SCR catalyst, a known SCR catalyst can be used as the SCR catalyst. For example, the SCR catalyst includes a support and a catalytic metal supported on the support and having NOx selective reduction activity. The catalyst metal having NOx selective reduction activity may be selected from noble metals such as Pt, Pd, and Rh, Ga, Zn, Cu, Fe, Ag, and Co. As the carrier contained in the SCR catalyst, the same carrier as that contained in the NSR catalyst can be used.

  The NSR catalyst and the SCR catalyst preferably contain 0.5 g or more of Pt per liter of base material and 0.05 g or more of Rh per liter of base material. In this case, the activity of the catalyst noble metal is improved.

  The NOx adsorbent, the oxidation catalyst, and the NOx reduction purification catalyst can be used as a honeycomb shape or a pellet shape, respectively, and any of known substrates such as a straight flow type and a filter type can be used. As the substrate, a known substrate such as a ceramic substrate, a SiC substrate, or a metal substrate can be used.

  The NOx adsorbent, the oxidation catalyst, and the NOx reduction purification catalyst are formed as a coat layer that covers the surface of the substrate. The whole of the oxidation catalyst and the NOx reduction purification catalyst may have each coat layer formed on one base material, or each coat layer may be formed on a separate base material in tandem along the exhaust gas flow direction. You may arrange in.

  Further, the NOx reduction purification catalyst can be supported on a diesel particulate filter (DPF) as a base material and used as a DPNR catalyst. In this case, PM (particulate) is oxidized and removed by the active oxygen by-produced by the NOx reduction purification catalyst.

  The exhaust gas purifying catalyst of the present invention is mounted on an exhaust gas system of an internal combustion engine, and is particularly suitable for mounting on an exhaust system of a lean burn engine such as a diesel engine or a lean burn gasoline engine. When the exhaust gas purification catalyst of the present invention is attached to an exhaust system of a diesel engine, for example, a diesel particulate filter may be disposed on the downstream side of the exhaust gas purification catalyst of the present invention.

(1) Examination of NOx adsorption performance of NOx adsorbent The NOx adsorption performance of the NOx adsorbent was evaluated by manufacturing a conventional NOx adsorbent containing a catalyst noble metal and measuring the NOx concentration on the outlet side.

<Manufacture of NOx adsorbent>
First, a precipitate was precipitated from an aqueous solution containing cerium nitrate and praseodymium nitrate by a coprecipitation method, and dried and fired to prepare CeO ₂ and Pr ₆ O ₁₁ powders. The composition of this powder is Ce: Pr = 1: 0.25 by mass ratio.

This powder was slurried and coated on a ceramic monolith substrate having a diameter of 30 mm and a length of 50 mm. The coating amount of CeO ₂ —Pr ₆ O ₁₁ powder is 150 g per liter of the substrate. This was dried and fired to produce a NOx adsorbent.

<Test>
The manufactured NOx adsorbent was subjected to an endurance test by heating in an electric furnace at 750 ° C. for 5 hours. Thereafter, a NOx concentration detector was disposed on the outlet side of the NOx adsorbent, and an evaluation gas having the composition shown in Table 1 was circulated. The evaluation gas was heated in increments of 50 ° C. The NOx concentration on the outlet side at each temperature in increments of 50 ° C. was measured with a NOx concentration detector. When the measurement is completed at a certain temperature, H ₂ is allowed to flow through the NOx adsorbent, NOx is desorbed from the NOx adsorbent, and then the evaluation gas heated to 50 ° C. is passed through the NOx adsorbent to again return to the outlet side. The NOx concentration was measured. The results are shown in FIG.

  As can be seen from the figure, when the exhaust gas temperature was high, the NOx concentration on the outlet side of the NOx adsorbent was high. This is presumably because the amount of NOx desorbed increased and the amount of NOx adsorbed was saturated.

(2) Evaluation of NOx adsorption state of NOx adsorbent A NOx adsorbent composed of a non-noble metal component was manufactured, and the NOx adsorption state was investigated by IR (infrared absorption spectrum method).

<Manufacture of NOx adsorbent>
First, a precipitate was precipitated from an aqueous solution containing cerium nitrate and praseodymium nitrate by a coprecipitation method, which was dried and fired to prepare CeO ₂ —Pr ₆ O ₁₁ powder. The composition of this powder is Ce: Pr = 1: 0.25 in molar ratio. This powder was subjected to the following test as a NOx adsorbent.

<Test>
The produced NOx adsorbent made of CeO ₂ —Pr ₆ O ₁₁ powder was heated in an electric furnace at 750 ° C. for 5 hours. Next, an exhaust gas of 150 ° C. made of N ₂ / O ₂ / NO was circulated for 30 minutes. Thereafter, the NOx adsorbent was measured by the IR method. The results are shown in FIG.

As known from this figure, the NOx ^adsorbent, 1100 cm -1, NO ₂ to 1200 cm ^{-1 -} large peaks which were not detected. On the other hand, the 1460 cm ^-1 NO ^- peak of was slightly confirmed. The NOx adsorbent, many of NOx NO ₂ ^- it is understood that they are adsorbed as. This is because CeO ₂ contained in the NOx adsorbent has oxygen absorption / release capability and absorbs oxygen in the lean region. The absorbed oxygen, NO adsorbed to the NOx adsorbent is oxidized to NO _2, is believed to be due to being present in the NOx adsorbent as NO _2.

(3) NO conversion rate of oxidation catalyst An oxidation catalyst was produced, and the NO conversion rate of the oxidation catalyst was measured.

<Manufacture of oxidation catalyst>
First, 150 parts by weight of alumina powder, 35 parts by weight of alumina sol, 15 parts by weight of an aluminum nitrate aqueous solution, and water were mixed and stirred well to prepare slurry A.

  Then, a cordierite honeycomb substrate having a capacity of 1.3 liters was immersed in water to blow off excess water droplets, and then immersed in the slurry A. After removing from the slurry A, excess slurry A was blown off, dried, and then fired at 500 ° C. for 2 hours to form an alumina coat layer on the honeycomb substrate surface. The amount of alumina coated was 150 g per liter of honeycomb carrier substrate.

  Next, the obtained honeycomb carrier was dipped in a noble metal nitrate (Pt) aqueous solution having a predetermined concentration, pulled up to blow off excess droplets, and dried to carry the catalyst noble metal (Pt) to obtain an oxidation catalyst. . The obtained oxidation catalyst carries 2 g of noble metal (Pt) per 1 L of the honeycomb substrate and 150 g of alumina.

<Test>
About the produced oxidation catalyst, it heated at 750 degreeC for 5 hours with the electric furnace, and performed the durability test. Thereafter, the oxidation catalyst was mounted on a 2.2 liter diesel engine, and the throttle was operated to raise the engine exhaust gas temperature from 100 ° C to 400 ° C. The temperature rising rate is about 20 ° C./min. The concentration of NO and NO _{2 at} this time was analyzed, and the NO conversion rate was calculated by the following equation.

    Conversion rate = {(NO concentration in incoming gas) − (NO concentration in outgoing gas)} / (NO concentration in incoming gas) × 100

The NO conversion is shown in FIG. As can be seen from the figure, the NO conversion was negative when the temperature was 200 ° C. or lower, and positive when the temperature exceeded 200 ° C. This is because 200 ° C. or less, often amount to be reduced from the NO ₂ to NO by the oxidation catalyst, if exceeding 200 ° C., this means that the quantity to be oxidized from NO to NO ₂ increases. From this, when the catalyst noble metal in the oxidation catalyst exceeds 200 ° C., it is effective in promoting NO oxidation reaction and generating NO ₂ that is easily adsorbed on the NOx reduction purification catalyst. In the following, it is understood that NO ₂ discharged from the engine is reduced to NO that is not easily stored in the NOx reduction purification catalyst.

(4) NOx adsorption amount <Test Example 1>
First, a NOx adsorbent is produced. From an aqueous solution containing cerium nitrate and praseodymium nitrate, a precipitate was deposited by a coprecipitation method, which was dried and fired to prepare CeO ₂ —Pr ₆ O ₁₁ powder. The composition of the CeO ₂ —Pr ₆ O ₁₁ powder is Ce: Pr = 80 mass%: 20 mass%. This CeO ₂ —Pr ₆ O ₁₁ powder was slurried to obtain slurry B. The slurry B was wash-coated on a cordierite honeycomb substrate having a diameter of 30 mm and a length of 25 mm. The coating amount of CeO ₂ —Pr ₆ O ₁₁ powder is 150 g per 1 L of honeycomb substrate. This was dried and fired to produce a NOx adsorbent.

  Next, an NSR catalyst was prepared by conventional methods. The NSR catalyst contains alumina as a carrier, Pt and Rh as noble metals, and Ba as a NOx storage element. The content of Pt is 2 g per 1 L of honeycomb substrate, and the content of Rh is 0.5 g per 1 L of honeycomb substrate. The Ba content is 0.1 mol per liter of honeycomb substrate. The coating amount of the catalyst powder is 250 g per liter of honeycomb substrate. The honeycomb substrate coated with the NSR catalyst is made of cordierite and has a diameter of 30 mm and a length of 25 mm. The honeycomb base material coated with the NOx adsorbent was disposed on the upstream side, and the honeycomb base material coated with the NSR catalyst was disposed on the downstream side, whereby a catalyst of Test Example 2 was obtained.

<Test Example 2>
A coat layer made of an NSR catalyst is formed on the entire cordierite honeycomb substrate having a diameter of 30 mm and a length of 25 mm. The method for forming the NSR catalyst is the same as in Test Example 1.

<Experiment>
The produced catalysts according to Test Examples 1 and 2 were heated in an electric furnace at 750 ° C. for 5 hours, and subjected to a durability test. After the durability test, the catalysts according to Test Examples 1 and 2 were placed in the evaluation apparatus, and a lean model gas having the same composition as that shown in Table 1 was circulated at a flow rate of 15 liters / minute. The NOx concentration of the catalyst outgas at each temperature was detected. With _H 2 (2 volume%) for each test, it was allowed completely desorb NOx, and detects the NOx concentration. The amount of NOx adsorbed on the catalyst was calculated from the difference between the NOx concentration of the exit gas and the NOx concentration of the model gas, and divided by the volume of the catalyst to obtain the NOx adsorption amount per volume of the catalyst. The results are shown in FIG.

  As can be seen from the figure, the NOx adsorption amount of the catalyst of Test Example 1 was higher than that of Test Example 2 at any temperature. The difference between the NOx adsorption amount of the catalyst of Test Example 1 and that of Test Example 2 increased as the temperature became lower. In particular, when the temperature was lower than 200 ° C., the NOx adsorption amount in Test Example 1 was about three times higher than the NOx adsorption amount in Test Example 2.

  From this, it can be seen that, first, the NOx adsorption amount in the low temperature region is greatly increased by arranging the NOx adsorbent upstream of the NOx reduction purification catalyst. Further, when an oxidation catalyst is disposed between the NOx reduction purification catalyst and the NOx adsorbent, NO in the gas is oxidized by the oxidation catalyst and flows to the downstream NOx reduction purification catalyst side in the high temperature range. .

Secondly, the NOx adsorbent of Test Example 1 does not contain a catalytic noble metal that reduces NOx in a low temperature range, and also contains CeO ₂ —Pr ₆ O ₁₁ having oxygen absorption / release capacity. It is oxidized and adsorbed as NO ₂ (see FIG. 2). When the temperature rises, NO ₂ is released, and the NOx reduction purification catalyst is easily occluded. For the above two reasons, it is presumed that the catalyst according to the example of the present invention described below has a higher NOx adsorption amount than Test Example 1, particularly in the high temperature range, as shown by the dotted line in FIG.

<Example 1>
As shown in FIG. 5, the exhaust gas purifying catalyst of this example is formed on the surface of the cell pores of the honeycomb substrate 5 in order from the upstream side in the exhaust gas flow direction, NOx adsorbent 1, oxidation catalyst 2, NOx reduction purification catalyst 3. Is provided. The NOx adsorbent 1 is made of CeO ₂ —Pr ₆ O ₁₁ and contains no catalytic noble metal. The oxidation catalyst 2 is composed of Pt as a catalyst noble metal and alumina as a carrier. The NOx reduction purification catalyst 3 is an NSR catalyst, and includes Pt and Rh as catalyst noble metals, alumina as a carrier, and Ba as a NOx storage material. The length ratio of the NOx adsorbent 1, the oxidation catalyst 2, and the NOx reduction purification catalyst 3 is 0.5: 0.5: 1.

  A method for producing the exhaust gas purifying catalyst of this example will be described. First, two honeycomb substrates made of cordierite having a diameter of 30 mm and a length of 50 mm are prepared. One honeycomb substrate was cut into half length (25 mm). The NOx adsorbent forming slurry and the oxidation catalyst forming slurry are wash-coated on the half-length honeycomb substrate to form the NOx adsorbent and the oxidation catalyst, respectively. The slurry for forming the NOx adsorbent is the slurry B prepared in Test Example 1 of (4) above. The slurry for forming the oxidation catalyst is the slurry A prepared in the above (3). The entire honeycomb substrate (length: 50 mm) is wash-coated with NSR catalyst-forming slurry to form an NSR catalyst. The slurry for forming the NSR catalyst is prepared by a known method in the same manner as the NSR catalyst in Test Example 1 of (4) above. A NOx adsorbent, an oxidation catalyst, and an NSR catalyst are sequentially arranged from the upstream side to the downstream side. The coating amount of each component per liter of honeycomb substrate is the same as that when the slurry is coated. As described above, an exhaust gas purification catalyst is prepared in which the NOx adsorbent, the oxidation catalyst, and the NOx reduction purification catalyst are arranged sequentially from the upstream side of the exhaust passage.

  In the exhaust gas purifying catalyst of this example, the oxidation catalyst 2 that oxidizes NOx in the high temperature region is disposed upstream of the NOx reduction purification catalyst 3, and the NOx adsorbent 1 that adsorbs NOx in the low temperature region is further disposed upstream. It is arranged. Therefore, NOx can be purified over a wide range from the low temperature range to the high temperature range.

Further, NOx adsorbent 1 _consists CeO ₂ -Pr _{6 O 11,} does not include the catalytic noble metal. _Thus, by the NOx adsorption performance and the capability of adsorbing and releasing oxygen in CeO ₂ -Pr _{6 O 11,} in the low temperature range, NOx in the exhaust gas is adsorbed is converted into NO _2. Therefore, when NO ₂ is released due to the temperature rise, most of the NO ₂ is occluded in the downstream NOx reduction purification catalyst and reduced and purified. Therefore, NO ₂ is stored in the NO _x reduction purification catalyst as NO _{2 and} is reduced and purified without being reduced to NO that is not easily stored in the NO _x reduction purification catalyst. Therefore, the purification performance of the NO ₂ reduction purification catalyst can be effectively exhibited.

<Example 2>
As shown in FIG. 6, a noble metal carrying portion 1 a carrying 0.01 to 1 g of catalyst noble metal (Pt, Pd, etc.) per liter of honeycomb substrate is formed on the downstream side portion of the NOx adsorbent 1. The length of the noble metal support 1a is 10% with respect to the length of the NOx adsorbent 1. In this case, the noble metal component is supported on the downstream portion of the NOx adsorbent 1 where the temperature is likely to rise. Therefore, in a high temperature range, thereby improving the oxidation activity of the NO ₂ in the NOx adsorbent.

<Example 3>
The NOx adsorbent is different from the first embodiment in that Ba is supported. The supported amount of Ba is 0.1 mol per liter of honeycomb substrate. Also in this case, as in Example 1, high NOx purification performance can be exhibited over a wide range from the low temperature range to the high temperature range.

It is a diagram which shows the absorption-and-release performance of NOx adsorption material. It is IR data which shows the adsorption | suction form of NOx adsorbed | sucked to NOx adsorbent. It is a diagram which shows the NO conversion rate of an oxidation catalyst. It is a diagram which shows the NOx adsorption amount of Test Examples 1 and 2. 1 is a cross-sectional view of an exhaust gas purifying catalyst according to Example 1 of the present invention. 3 is a cross-sectional view of an exhaust gas purifying catalyst according to Example 2. FIG.

Explanation of symbols

1: NOx adsorbent, 1a: noble metal support, 2: oxidation catalyst, 3: NOx reduction purification catalyst.

Claims (3)

A NOx adsorbent disposed in the exhaust passage of the internal combustion engine, an oxidation catalyst disposed downstream of the NOx adsorbent, and a NOx reduction purification catalyst disposed downstream of the oxidation catalyst,
The exhaust gas purifying catalyst, wherein the NOx adsorbent comprises a non-noble metal component having NOx adsorption ability. _2. The exhaust gas purifying catalyst according to claim 1, wherein the non-noble metal component includes one or more selected from CeO ₂ , Gd ₂ O ₃ and Pr ₆ O ₁₁ .   3. The exhaust gas purifying catalyst according to claim 1, wherein the NOx adsorbing material carries a catalytic noble metal at a downstream end portion, leaving an upstream end portion. 4.

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