JP2010017694A - NOx ABSORBING CATALYST FOR REDUCTION - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent precious metals carried respectively from solid-solutioning or alloying, while suppressing sulfur poisoning. <P>SOLUTION: A catalyst coat layer 2 comprising a lower catalytic layer 20 carrying at least Rh, and an upper catalyst layer 21 carrying at least one member selected from Pt and Pd is formed wherein the ratio of an average thickness of the lower catalyst layer 20 to an average thickness of the catalyst coat layer 2 is 35-90%. The respective precious metals are prevented from solid-solutioning by separating Rh, rich atmosphere exhaust gas passing through the upper catalyst layer 21 can reach the lower catalyst layer 20, and H<SB>2</SB>produced by Rh can pass through the upper catalyst layer 21. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention occludes NO _x in the _the NO _x storage material in a lean atmosphere, to _the NO _x storage reduction catalyst for purifying NO _x by the intermittently rich atmosphere.

In recent years, NO _x storage reduction catalysts have been put into practical use as catalysts for purifying exhaust gas from lean burn gasoline engines. The NO _{x storage-and-reduction} catalysts are alkali metal, _the NO _x storage material and the noble metal and alumina, such as alkaline earth metals (Al ₂ O ₃₎ is obtained by carrying on a porous oxide support, such as. This NO _x storage reduction catalyst is mounted on the exhaust system of an automobile, and the air-fuel ratio is always operated in an oxygen-excess lean atmosphere, and is controlled so that a fuel-rich stoichiometric to rich atmosphere is pulsed. In the lean atmosphere, NO is oxidized on the catalyst to generate NO ₂ , which becomes NO _x and is stored in the NO _x storage material. The occluded NO _x is released in stoichiometric-rich atmosphere, it is purified by reacting with reducing components such as HC and CO by the catalytic action of the noble metal. Accordingly, since NO _x emission is suppressed even in a lean atmosphere, high NO _x purification ability is exhibited as a whole.

In the NO _x storage reduction catalyst, it is common to use both Pt or Pd having a high oxidation activity and Rh having a high reduction activity that can generate H ₂ by a steam reforming reaction or the like as a noble metal. However, for example, when Pt and Rh are supported together, there is a problem in that the NO oxidation activity in a lean atmosphere decreases and the NO _x storage amount decreases. This is because Rh is dissolved in Pt, and Rh, which is more stable than Pt in a lean atmosphere, is present on the Pt surface and reduces the oxidation activity of Pt.

  Japanese Patent Application Laid-Open No. 11-138021 describes forming a catalyst coat layer from a mixed powder of a catalyst powder supporting Pt on alumina and a catalyst powder supporting Rh on zirconia. Thus, by carrying Pt and Rh on different carrier powders, solid solution of Rh in Pt can be suppressed. However, even if Pt and Rh are supported on different carrier powders, the probability that Pt and Rh are in contact with each other is not low in these mixtures, and Rh may be dissolved in Pt.

By the way, the exhaust gas contains SO ₂ produced by combustion of sulfur (S) contained in the fuel, which is oxidized by a noble metal into SO ₃ in a lean atmosphere containing excess oxygen. And it became clear that this is easily converted into sulfuric acid by the water vapor contained in the exhaust gas, and these react with the NO _x storage material to produce sulfites and sulfates, which causes the NO _x storage material to be poisoned and deteriorated. This phenomenon is called sulfur poisoning. When the NO _x storage material becomes sulfite or sulfate as described above, it is no longer possible to store NO _x , and as a result, the NO _x purification ability after durability is lowered.

In order to suppress this sulfur poisoning, various proposals have been conventionally made. For example, JP 2001-000863, JP-catalyst carrying easily _the NO _x storage material is sulfur adsorbed was put in place on an exhaust-gas upstream side, the catalyst carrying sulfur adsorption hardly _the NO _x storage material was placed in an exhaust gas downstream side A NO _x storage reduction catalyst is described. However, in the technology described in the publication, sulfur poisoning of the downstream catalyst is prevented by positively causing sulfur poisoning by the upstream catalyst. In general, the NO _x storage material that is easily poisoned by sulfur is less likely to release sulfur oxides even in a reducing atmosphere. Therefore, when the sulfur poisoning in the upstream catalyst is difficult to recover, sulfur poisoning also occurs in the downstream catalyst.
JP 11-138021 A JP 2001-000863 A

The present invention has been made in view of the above circumstances, and in the NO _x storage reduction catalyst, it is a problem to be solved while suppressing solid solution or alloying of supported noble metals and suppressing sulfur poisoning. And

Features of the NO _x storage-reduction catalyst of the present invention to solve the above problems is obtained by carrying the support substrate having an exhaust gas passage, the formed on the surface of the exhaust gas channel oxide support and a noble metal and _the NO _x storage material catalyst and coating layer, becomes more occludes NO _x in _the NO _x storage material in a lean atmosphere, a NO _{x storage-and-reduction} catalysts that reduce and purify NO _x by the intermittently rich atmosphere,
The catalyst coat layer has a two-layer structure of a lower catalyst layer formed on the surface of the exhaust gas passage and an upper catalyst layer formed on the surface of the lower catalyst layer, and at least Rh is supported on the lower catalyst layer. At least one selected from Pt and Pd is supported on the catalyst layer, and the ratio of the average thickness of the lower catalyst layer to the average thickness of the catalyst coat layer is in the range of 35 to 90%.

According to the NO _x storage reduction catalyst of the present invention, the upper catalyst layer carries at least one of Pt and Pd having high oxidation activity. Further, Rh is supported on the lower catalyst layer and separated from the upper catalyst layer, so that solid solution of Rh in Pt or Pd is suppressed. Therefore, since NO is efficiently oxidized to NO ₂ in the upper catalyst layer in a lean atmosphere without reducing the oxidation activity of Pt and Pd, a sufficient NO _x storage amount is expressed.

In the NO _x storage reduction catalyst of the present invention, the average thickness of the lower catalyst layer supporting Rh is in the range of 35 to 90% with respect to the average thickness of the catalyst coat layer. By setting the average thickness of the lower catalyst layer within this range, exhaust gas in a rich atmosphere can pass through the upper catalyst layer and reach the lower catalyst layer, and H ₂ generated by Rh can pass through the upper catalyst layer. Become. As a result, SO _x is released from the sulfur-poisoned NO _x storage material, so that the NO _x storage material recovers the NO _x storage capacity. Furthermore, since it is less necessary to support Pt and Pd on the lower catalyst layer, solid solution of Rh in Pt and Pd in the lower catalyst layer is suppressed.

The NO _x storage reduction catalyst of the present invention has a carrier substrate and a catalyst coat layer. The carrier base material can have a honeycomb shape, a foam shape, etc., and the material is not particularly limited, such as cordierite, ceramics such as SiC, or metal, and the same material having an exhaust gas passage can be used. .

  The catalyst coat layer is formed on the surface of the exhaust gas passage of the carrier substrate. One feature of the present invention is that a catalyst coat layer is composed of a lower catalyst layer formed on the surface of the exhaust gas passage and an upper catalyst layer formed on the surface of the lower catalyst layer, and at least Rh is supported on the lower catalyst layer. In addition, at least one selected from Pt and Pd is supported on the upper catalyst layer. By separating Rh from Pt or Pd in this way, Rh is prevented from dissolving in Pt or Pd. Therefore, since the fall of the oxidation activity with high Pt or Pd is suppressed, and the fall of the reduction activity with high Rh is suppressed, durability improves.

By supporting Pt or Pd on the upper catalyst layer, the contact with the exhaust gas is improved, and NO is oxidized to NO ₂ in a lean atmosphere and stored in the NO _x storage material. Further, by carrying Rh on the lower catalyst layer, H ₂ generated by Rh is passed over the catalyst layer in a rich atmosphere, NO _x storage material that sulfur poisoning with NO _x is reduced and purified also reduced NO _x The storage capacity is restored. That is, sulfur poisoning is suppressed.

The lower catalyst layer includes an oxide support made of a porous oxide and at least Rh supported on the oxide support. Examples of the porous oxide constituting the oxide carrier include alumina, silica, zirconia, titania, silica-alumina, ceria, etc., and one of them may be used, or a plurality of types of mixtures or composite oxides may be used. it can. Among them, it is desirable to include zirconia that expresses high H ₂ generation activity by supporting Rh. It is also preferable to use stabilized zirconia stabilized with Ca or the like as zirconia.

  The upper catalyst layer includes an oxide carrier made of a porous oxide and at least one selected from Pt and Pd supported on the oxide carrier. Examples of the porous oxide that constitutes the oxide carrier include alumina, silica, zirconia, titania, silica-alumina, and ceria, and one of these may be used, or a mixture of a plurality of types or a composite oxide may be used. it can. Among these, it is desirable to include γ-alumina having a high specific surface area.

The catalyst coat layer carries a NO _x storage material. As this NO _x storage material, at least one selected from alkali metals, alkaline earth metals and rare earth elements can be used. It is desirable to contain at least an alkali metal and an alkaline earth metal. Examples of the alkali metal include lithium, sodium, potassium, rubidium, cesium, francium and the like. Examples of the alkaline earth metal include barium, beryllium, magnesium, calcium and strontium. Examples of rare earth elements include scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, dysprosium, ytterbium, and the like.

The NO _x storage material is desirably supported in the range of 0.01 to 1 mol per liter of the bulk volume of the carrier substrate. If the loading amount is less than this range, the NO _x storage amount decreases, so the NO _x purification ability decreases, and if it exceeds this range, the noble metal is covered with the NO _x storage material and the activity decreases.

The NO _x storage material may be uniformly supported on the lower catalyst layer and the upper catalyst layer, but is desirably supported at least on the upper catalyst layer. The upper catalyst layer easily comes into contact with the exhaust gas, and the supported Pt or Pd has a high oxidation activity. Therefore, mainly by supporting the NO _x storage material on the upper catalyst layer, NO ₂ generated by oxidation of NO is immediately stored in the NO _x storage material, so that the NO _x storage performance is improved.

Another feature of the present invention is that the ratio of the average thickness of the lower catalyst layer to the average thickness of the catalyst coat layer is in the range of 35 to 90%. The average thickness of the catalyst coat layer is determined in consideration of the exhaust pressure loss. Therefore, when the average thickness of the lower catalyst layer is less than 35% of the average thickness of the catalyst coat layer, the upper catalyst layer becomes relatively thick, so that the exhaust gas in a rich atmosphere hardly diffuses to the lower catalyst layer. the amount of ₂ drops. Therefore, the NO _x purification performance is lowered and it is difficult to suppress sulfur poisoning.

  In addition, there is a necessary minimum amount of noble metal supported. If the noble metal loading density becomes too high, there is a problem that the activity is lowered due to grain growth at a high temperature during use. Therefore, when the upper catalyst layer is thin, Pt or Pd must be supported on the lower catalyst layer. However, when this happens, Rh dissolves with Pt or Pd and the activity decreases.

Therefore, the average thickness of the lower catalyst layer is 90% or less of the average thickness of the catalyst coat layer. Within this range, the solid solution of Rh for Pt or Pd carried is slightly suppressed to lower catalyst layer, there is little effect on the purification performance or control performance of sulfur poisoning of the NO _x.

  The amount of Pt or Pd supported is preferably 0.1 to 10 g in total per liter of the bulk volume of the carrier substrate. Although it is desirable that most of the catalyst is supported on the upper catalyst layer, it can be supported on the lower catalyst layer as long as it is in the range of 0 to 3 g / L in total.

  The amount of Rh supported is preferably 0.01 to 5 g per liter of bulk volume of the carrier substrate. Although it is desirable that the entire amount of Rh is supported on the lower catalyst layer, it may be supported on the upper catalyst layer as long as the performance is not impaired.

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

FIG. 1 shows an NO _x storage reduction catalyst according to an embodiment of the present invention. This NO _x storage reduction catalyst comprises a honeycomb substrate 1 having a large number of cell passages 11 partitioned by cell partition walls 10 and a catalyst coat layer 2 formed on the surface of the cell partition walls 10. The lower catalyst layer 20 formed on the surface of the cell partition wall 10 and the upper catalyst layer 21 formed on the surface of the lower catalyst layer 20 are configured.

(Reference example)
The NO _x storage-reduction catalyst of this reference example has the same structure as the catalyst of the example except that the average thickness of the lower catalyst layer 20 is different from the average thickness of the catalyst coat layer 2. Hereinafter, a method for producing the NO _x storage reduction catalyst of the reference example will be described, and a detailed description of the configuration will be given.

A ZrO ₂ powder was impregnated with a predetermined amount of an aqueous rhodium nitrate solution having a predetermined concentration, stirred for 1 hour, dried and fired to prepare an Rh / ZrO ₂ catalyst powder supporting 1% by mass of Rh. This Rh / ZrO ₂ catalyst powder was mixed with a predetermined amount of alumina sol as a binder and a predetermined amount of ion-exchanged water to prepare a slurry.

  Next, a cordierite honeycomb substrate 1 (capacity 1.3 L, length 155 mm) was prepared, the slurry was washcoated, dried and fired to form the lower catalyst layer 20. At this time, the coating amount was adjusted so that the amount of Rh contained in the lower catalyst layer 20 was 0.5 g per liter of the bulk volume of the honeycomb substrate 1.

Subsequently, γ-Al ₂ O ₃ powder, CeO ₂ —ZrO ₂ composite oxide powder (mass ratio CeO ₂ / ZrO ₂ = 85/15), ZrO ₂ —TiO ₂ composite oxide powder (mass ratio ZrO ₂₎ / TiO ₂ = 70/30), a predetermined amount of alumina sol and ion-exchanged water were mixed to prepare a slurry. This slurry was wash-coated on the honeycomb substrate 1 on which the lower catalyst layer 20 was formed, dried and fired to form a coat layer on the surface of the lower catalyst layer 20.

  The honeycomb substrate 1 having the lower catalyst layer 20 and the coat layer was immersed in a dinitrodiammine platinum solution having a predetermined concentration, pulled up to blow off excess droplets, then dried and fired to adsorb and carry Pt. The amount of Pt supported is 2 g per liter of the bulk volume of the honeycomb substrate 1. Further, a predetermined amount of a mixed aqueous solution in which barium acetate and potassium acetate were dissolved was impregnated, dried and fired to carry Ba and K, and the upper catalyst layer 21 was formed. 0.3 mol of Ba and K were supported on each liter of bulk volume of the honeycomb substrate 1.

The obtained NO _x storage reduction catalyst has a catalyst coat layer 2 composed of a lower catalyst layer 20 and an upper catalyst layer 21, and the catalyst coat layer 2 is formed in an amount of 300 g per liter of the honeycomb substrate 1 in bulk volume. . Further, image analysis of the cross section was performed, and the average thickness of the catalyst coat layer 2 (the total average thickness of the lower catalyst layer 20 and the upper catalyst layer 21) and the average thickness of the lower catalyst layer 20 were measured. The value of the average thickness of the lower catalyst layer 20 with respect to the average thickness of 2 was 18.5%.

The composition ratio of each oxide in the catalyst coat layer 2 is ZrO ₂ : γ-Al ₂ O ₃ : CeO ₂ —ZrO ₂ : ZrO ₂ —TiO ₂ = 35: 70: 14: 70 in mass ratio.

Example 1
Rh / ZrO ₂ catalyst powder (35) similar to the reference example, γ-Al ₂ O ₃ powder (17.8) similar to the reference example, and CeO ₂ —ZrO ₂ composite oxide powder (3.5) similar to the reference example Then, the same ZrO ₂ —TiO ₂ composite oxide powder (17.8) as in the reference example, a predetermined amount of alumina sol and ion-exchanged water were mixed to prepare a slurry.

  The same honeycomb substrate 1 as in the reference example was prepared, and the lower catalyst layer 20 was formed in the same manner as in the reference example except that this slurry was used. At this time, the coating amount was adjusted so that the amount of Rh contained in the lower catalyst layer 20 was 0.5 g per liter of the bulk volume of the honeycomb substrate 1.

Subsequently, the same γ-Al ₂ O ₃ powder (54) as in the reference example, the CeO ₂ —ZrO ₂ composite oxide powder (10.5) as in the reference example, and the ZrO ₂ —TiO ₂ composite oxidation as in the reference example. A slurry was prepared by mixing the product powder (54), a predetermined amount of alumina sol and ion-exchanged water, and a coating layer was formed in the same manner as in the Reference Example except that this slurry was used.

  Using the obtained honeycomb substrate 1 having the lower catalyst layer 20 and the coat layer, Pt was supported and the same amounts of Ba and K were supported in the same manner as in the reference example, whereby the upper catalyst layer 21 was formed.

The obtained NO _x storage reduction catalyst has a catalyst coat layer 2 composed of a lower catalyst layer 20 and an upper catalyst layer 21, and the catalyst coat layer 2 is formed in an amount of 300 g per liter of the honeycomb substrate 1 in bulk volume. . Further, the cross section was image-analyzed and the average thickness of the catalyst coat layer 2 and the average thickness of the lower catalyst layer 20 were measured. The average thickness of the lower catalyst layer 20 relative to the average thickness of the catalyst coat layer 2 was 37%. Met.

The composition ratio of each oxide in the catalyst coat layer 2 is ZrO ₂ : γ-Al ₂ O ₃ : CeO ₂ —ZrO ₂ : ZrO ₂ —TiO ₂ = 35: 70: 14: 70 in terms of mass ratio, In parentheses, the mass ratio of each oxide contained in the lower catalyst layer 20 and the upper catalyst layer 21 is shown.

(Examples 2, 3, and 4)
The average thickness of the lower catalyst layer 20 is the same as in Reference Example and Example 1 except that the mass ratio of each oxide in the slurry used when forming the lower catalyst layer 20 and the upper catalyst layer 21 is changed. Different NO _x storage reduction catalysts were produced. The formation amount of the catalyst coat layer 2 and the supported amount of each catalyst metal are all the same as in the reference example, and the composition ratio of each oxide in the catalyst coat layer 2 is ZrO ₂ : γ-Al ₂ O ₃ : CeO in mass ratio. ₂ −ZrO ₂ : ZrO ₂ —TiO ₂ = 35: 70: 14: 70.

  The values of the average thickness of the lower catalyst layer 20 relative to the average thickness of the catalyst coat layer 2 were 56%, 74%, and 90%, respectively.

(Example 5)
Example 3 is the same as Example 3 except that, when the upper catalyst layer 20 was formed, Pd was supported using an aqueous palladium nitrate solution after supporting Pt. The supported amount per liter of the bulk volume of the honeycomb substrate 1 is 1.2 g for Pt and 0.8 g for Pd.

(Comparative example)
Rh / ZrO ₂ catalyst powder similar to the reference example, γ-Al ₂ O ₃ powder similar to the reference example, CeO ₂ —ZrO ₂ composite oxide powder similar to the reference example, and ZrO ₂ similar to the reference example A slurry was prepared by mixing —TiO ₂ composite oxide powder, a predetermined amount of alumina sol and ion-exchanged water.

  A honeycomb substrate 1 similar to that of the reference example was prepared, and a catalyst layer was formed in the same manner as the formation of the lower catalyst layer 20 of the reference example except that this slurry was used. At this time, the coating amount was adjusted so that the amount of Rh contained in the catalyst layer was 0.5 g per liter of the bulk volume of the honeycomb substrate 1.

Subsequently, in the same manner as in the reference example, the same amount of Pt, Ba, and K was supported on the catalyst layer to obtain a NOx storage reduction catalyst of a comparative example. The formation amount of the catalyst layer consisting of only one layer is the same as that of the catalyst coat layer 2 of the reference example, and the composition ratio of each oxide in the catalyst layer is ZrO ₂ : γ-Al ₂ O ₃ : CeO ₂ —ZrO in mass ratio. ₂ : ZrO ₂ —TiO ₂ = 35: 70: 14: 70.

<Test and evaluation>
Endurance test in which the catalyst of the reference example, each example and the comparative example is mounted on the exhaust system of a lean burn engine, and maintained for 50 hours under a condition where a lean atmosphere and a rich atmosphere are alternately repeated at a catalyst gas temperature of 750 ° C. Went.

A certain amount of sulfur-containing exhaust gas was circulated for each catalyst after the durability test, and the NO _x storage material was sulfur poisoned. Each sulfur-poisoned catalyst was mounted in the same exhaust system as above, and the amount of sulfur desorption when the rich operation was performed was measured. The results are shown in FIG. 2 as relative values with the sulfur desorption amount of the comparative example as 1.

Next, each catalyst from which sulfur was desorbed by the above test was installed in the same exhaust system as in the durability test, and the gas temperature with catalyst was set to 300 under the condition that a rich pulse for several seconds was introduced after lean operation for a certain period of time. The NO _x purification rates at the time of introducing rich pulses at temperatures of ℃, 400 ℃, and 450 ℃ were measured. The results are shown in Table 1.

Furthermore, regarding the catalysts of Reference Examples and Examples 1 to 4, the relationship between the ratio of the average thickness of the lower catalyst layer 20 to the average thickness of the catalyst coat layer 2 and the NO _x purification rate measured above is shown in FIGS. As shown in FIG.

  From FIG. 2, it is clear that the catalysts of Examples 1 to 4 have a larger amount of sulfur desorption than the comparative examples, and sulfur poisoning is suppressed. The catalysts of the reference examples and Examples 1 to 4 have the same structure except that the thickness ratios of the lower catalyst layer 20 and the upper catalyst layer 21 are different, and the curve connecting the vertices of each bar graph has a mountain shape. Therefore, the configurations of the catalysts of Example 1 and Example 4 are considered to be almost the limit. Therefore, it is clear that the ratio of the average thickness of the lower catalyst layer 20 to the average thickness of the catalyst coat layer 2 needs to be in the range of 35 to 90%.

3 to 5, if the ratio of the average thickness of the lower catalyst layer 20 to the average thickness of the catalyst coat layer 2 is in the range of 35 to 90%, the NO _x purification rate at each temperature is also in the high range. I understand that.

Is an explanatory diagram _the NO _x storage-reduction catalyst of Example of the present invention and showing the main portion thereof cross. It is a bar graph which shows a sulfur desorption amount by the relative value which made the value of the comparative example 1. Is a graph showing _the NO _x purification rate at 300 ° C.. Is a graph showing _the NO _x purification rate at 400 ° C.. Is a graph showing _the NO _x purification rate at 450 ° C..

Explanation of symbols

1: Honeycomb substrate 2: Catalyst coating layer
20: Lower catalyst layer 21: Upper catalyst layer

Claims (3)

A carrier base material having an exhaust gas passage; and a catalyst coat layer formed on the surface of the exhaust gas passage and carrying a noble metal and an NO _x storage material on an oxide carrier; and in the lean atmosphere, the NO _x storage material. occludes NO _x, a NO _{x storage-and-reduction} catalysts that reduce and purify NO _x in intermittently be rich atmosphere,
The catalyst coat layer has a two-layer structure of a lower catalyst layer formed on the surface of the exhaust gas passage and an upper catalyst layer formed on the surface of the lower catalyst layer,
At least Rh is supported on the lower catalyst layer, and at least one selected from Pt and Pd is supported on the upper catalyst layer, and the ratio of the average thickness of the lower catalyst layer to the average thickness of the catalyst coat layer is NO _x storage reduction catalyst characterized by being in the range of 35 to 90%. _The NO _x storage-reduction catalyst according to claim 1 which is in the upper catalytic layer Rh is not carried. Wherein _the NO _x storage material, _the NO _x storage-reduction catalyst according to claim 1 or claim 2 is supported on at least the upper catalyst layer.

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