NOx-trapping catalyst having non-platinum-group-metal NOx-trapping layer

Abstract

Disclosed is a NOx-trapping catalyst having a non-platinum-group-metal NOx-trapping layer, which contains a transition metal, particularly manganese, able to maintain NOx-trapping performance while decreasing the amount of expensive platinum-group metal.

Claims

1. A NOx-trapping catalyst, having a two-layer structure, comprising: a three-way catalyst layer containing a platinum-group element; and a NOx-trapping layer comprising a NOx-trapping material comprising an alkaline-earth metal oxide and an oxygen storage capacity material, which are doped with manganese, nickel, silver, or a combination thereof and not doped with copper, without containing a platinum-group element.

2. The NOx-trapping catalyst of claim 1, wherein the transition metal is silver.

3. The NOx-trapping catalyst of claim 1, wherein the transition metal is manganese.

4. The NOx-trapping catalyst of claim 1, wherein the oxygen storage capacity material includes a rare-earth metal oxide.

5. The NOx-trapping catalyst of claim 1, wherein the NOx-trapping layer further includes alumina.

6. The NOx-trapping catalyst of claim 1, wherein the NOx-trapping layer further includes an assistant component comprising a NOx-trapping material comprising an alkaline-earth metal oxide and an oxygen storage capacity material, which are not doped with manganese, nickel, silver, or a combination thereof.

7. The NOx-trapping catalyst of claim 1, wherein the transition metal is nickel.

8. A NOx-trapping catalyst, having a single-layer structure, comprising: a three-way catalyst component containing a platinum-group element; and a NOx-trapping component comprising a NOx-trapping material comprising an alkaline-earth metal oxide and an oxygen storage capacity material, which are doped with manganese, nickel, silver, or a combination thereof and not doped with copper, without containing a platinum-group element, wherein the three-way catalyst component and the NOx-trapping component are mixed together.

9. The NOx-trapping catalyst of claim 8, wherein the NOx-trapping component further includes alumina.

10. The NOx-trapping catalyst of claim 8, wherein the NOx-trapping component further includes an assistant component comprising a NOx-trapping material comprising an alkaline-earth metal oxide and an oxygen storage capacity material, which are not doped with manganese, nickel, silver, or a combination thereof.

11. The NOx-trapping catalyst of claim 8, wherein the transition metal is nickel.

12. The NOx-trapping catalyst of claim 8, wherein the transition metal is manganese.

13. The NOx-trapping catalyst of claim 8, wherein the transition metal is silver.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically shows the NOx-trapping function;

(2) FIG. 2 schematically shows a double-layer NOx-trapping catalyst and a single-layer NOx-trapping catalyst according to the present invention;

(3) FIG. 3 shows the performance of the double-layer NOx-trapping catalyst with 50% lower platinum-group metal according to the present invention and a conventional TWC catalyst system;

(4) FIG. 4 shows the results of emission of ammonia and N.sub.2O by the double-layer NOx-trapping catalyst according to the present invention and the conventional TWC catalyst system; and

(5) FIG. 5 schematically shows a process of preparing a NOx-trapping catalyst according to the present invention.

MODE FOR THE INVENTION

(6) The present invention addresses a three-way catalyst or NOx-trapping catalyst having NOx adsorption activity, which is configured as a multilayer structure, comprising a three-way catalyst layer containing a platinum-group element and a NOx-trapping layer configured to include a NOx-trapping material and an oxygen storage capacity material, which are doped with a transition metal, without any platinum-group element (non-PGM or zero-PGM). For the sake of brevity of description, the NOx-trapping catalyst is exemplified in the form of a two-layer structure, but a single-layer structure comprising a three-way catalyst component and a trapping component, which are mixed together, may be realized. As used herein, the term “platinum-group element” refers to at least one metal selected from among platinum, palladium, rhodium, ruthenium, iridium and osmium.

(7) In the present invention, the trapping layer of the three-way catalyst having NOx adsorption activity is configured to include a NOx-trapping material, an oxygen storage capacity material and a stabilizer, which are doped with a transition metal. The transition metal, such as manganese, nickel or silver, may be contained in an amount of 10 to 50 wt % based on the total weight of the trapping layer. The NOx-trapping material includes an oxide of an alkaline earth metal such as Ba, Mg, or Sr, and the oxygen storage capacity material includes a rare earth metal oxide, such as Ce, Zr, La, Pr or Nd oxide. In the present invention, the stabilizer may be alumina. The trapping layer of the present invention may also include an additional component comprising a NOx-trapping material and an oxygen storage capacity material, which are not doped with a transition metal (hereinafter, referred to as an “assistant component”).

(8) In the present invention, the three-way catalyst having NOx adsorption activity includes a three-way catalyst layer, and a conventional three-way catalyst layer contains a platinum-group element selected from among platinum, palladium and rhodium. The amount of the platinum-group element could be reduced by 50 to 90 wt % compared to conventional three-way catalysts. In the present invention, the three-way catalyst having NOx adsorption activity becomes stable at high temperatures ranging from 850° C. to 1050° C. and thus the NOx storage capability thereof may be remarkably increased compared to conventional NOx trapping catalyst.

(9) Hereinafter, a detailed description will be given of the present invention.

(10) According to the present invention, the three-way catalyst having NOx adsorption activity is featured in that the NOx-trapping or adsorption layer contains no platinum-group element, and the NOx-trapping layer (or adsorption layer) is configured to include a NOx-trapping material and an oxygen storage capacity material, which are doped with a transition metal, without any platinum-group element. In the present invention, the transition metal, which enables the oxidation and reduction functions to simultaneously occur, is selected from among manganese, nickel and silver, with the exception of copper, which may be used alone or in combination, and particularly useful is manganese. Such a transition metal is introduced to the NOx-trapping material, the oxygen storage capacity material and alumina. Preferably, the NOx-trapping material is an oxide of a metal selected from the group consisting of barium, strontium and magnesium, and is preferably magnesium oxide, and the oxygen storage capacity material is Ce, La, Pr or Nd oxide, and is preferably CeO.sub.2. The amount of transition metal that is introduced may be 10 to 50 wt % based on the total weight of the trapping layer. Also, an assistant component for further promoting NOx adsorption activity may be contained, and preferably, a complex metal oxide or a mixture of barium oxide and strontium oxide is introduced to ceria or alumina.

(11) In the present invention, the NOx adsorption layer has overcome the limitations with conventional NOx adsorption components. Specifically, stability to heat, which is the problem with conventional NOx-trapping components, is increased to thus maintain NOx storage capability upon exposure to high temperatures. The catalyst of the invention containing no expensive platinum-group element exhibits catalytic performance equivalent to that of a conventional catalyst containing a platinum-group element for NOx-trapping performance.

(12) In the present invention, the NOx adsorption layer contains no platinum element. As shown in FIG. 1, the platinum-group element in the conventional NOx adsorption layer functions to oxidize generated NO to thus be converted into NO.sub.2 under lean conditions, and functions as a catalyst for converting NOx desorbed from the NOx adsorption component into carbon dioxide, water, or nitrogen under oxygen-rich (rich transient) conditions, and may include platinum alone or in combination with other platinum-group metals. A conventional platinum-group element may include any metal selected from the group consisting of palladium, rhodium, ruthenium, iridium, osmium and mixtures thereof, but the NOx adsorption layer of the present invention contains no platinum-group element. The present inventors have discovered that manganese, nickel or silver may exhibit the same function as the platinum-group element and is thus capable of replacing the platinum-group element. Such a transition metal may be introduced to a typical NOx-trapping component and an oxygen storage capacity material, such as Ce, Zr, La, Pr and Nd oxides, and preferably CeO.sub.2. In particular, the present inventors have ascertained that equal or superior catalytic performance may be obtained even without the use of copper (Cu), which has conventionally been proposed as the component for promoting NOx-trapping performance in the art.

(13) The three-way catalyst layer of the present invention is configured such that the platinum-group element is loaded on a support comprising a refractory oxide support having a large surface area. The support may include at least one selected from the group consisting of alumina, silica, titania and zirconia, and is preferably selected from the group consisting of activated alumina, silica, silica-alumina, aluminosilicate, alumina-zirconia, alumina-chromia, and alumina-ceria.

(14) The three-way catalyst of the present invention may be prepared through a typical process in the art. With reference to the preparation process schematically shown in FIG. 5, a better understanding of the present invention will be given through the following examples, which are merely set forth to illustrate but are not to be construed as limiting the scope of the present invention.

<Example> Preparation of Double-Layer NOx-Trapping Catalyst

(15) 1) Complex Oxide A: Preparation of BaO—CeO.sub.2

(16) Barium acetate was incorporated into CeO.sub.2 powder (having a surface area of 100 to 200 m.sup.2/g), and was then calcined at 500 to 700° C., thus preparing a complex oxide A (BaO was loaded in an amount of 5 to 20 wt %). Here, SrO may be optionally added. The NOx-trapping layer of the present invention may further include the complex oxide A as the assistant component.

(17) 2) Complex Oxide B: Preparation of MnOx-MgO—CeO.sub.2—Al.sub.2O.sub.3

(18) Magnesium acetate and cerium nitrate were incorporated into Al.sub.2O.sub.3 and then calcined at 500 to 700° C. to give a complex oxide (the sum of CeO.sub.2 and MgO to be loaded was 5 to 20 wt %), and a Mn precursor (salt) and MnOx were wet- or dry-mixed and fired, thus preparing a complex oxide B.

(19) 3) Preparation of Three-Way Catalyst Component:

(20) A palladium precursor (salt) was incorporated into the Ce—Zr-Ox complex oxide, in which the amount of Pd to be loaded was maintained at 0.01 to 5 wt %. Subsequently, a rhodium precursor (salt) was incorporated into alumina, in which the amount of Rh to be loaded was maintained at 0.001 to 2 wt %. These were mixed to base metals like Ba, Sr, Zr, La etc to obtain top coat slurry.

(21) The double-layer NOx-trapping catalyst was provided in the form of a two-layer structure comprising bottom and top layers.

(22) Bottom (NOx-Trapping Layer)

(23) The complex oxide A was dispersed in deionized water, after which the complex oxide B was added and dispersed for 20 min, thus preparing a slurry having a solid content of 30 to 45%. The particle size of the slurry was adjusted through ball milling so as to be suitable for coating a cordierite honeycomb. The cordierite honeycomb was coated with the prepared slurry, dried at 150° C. to 180° C. for about 10 min, and calcined at 530° C. to 550° C. for about 40 min, thus forming a bottom non-PGM NOx-trapping layer.

(24) Top (Three-Way Catalyst Layer)

(25) A cordierite honeycomb was coated with the three-way catalyst component slurry, dried at 150° C. to 180° C. for about 10 min, and calcined at 530° C. to 550° C. for about 40 min, thus forming a top three-way catalyst layer.

<Comparative Example> Preparation of NOx-Trapping Catalyst

(26) A NOx-trapping catalyst was prepared in the same manner as in Example above, with the exception that the composition for the top layer was applied to the bottom layer in lieu of the composition for the bottom layer. Thus, the amount of noble metal of the comparative NOx-trapping catalyst is doubled compared to the catalyst of Example.

(27) <Test Example> Measurement of Purification Rate of NOx-Trapping Catalyst

(28) (1) Catalyst Evaluation

(29) Before catalyst evaluation, the NOx-trapping catalysts of Example and Comparative Example were deteriorated. Here, the catalyst deterioration was performed at 950° C. to 1050° C. for 12 hr in an electric box furnace. After the deterioration, the catalysts were evaluated in the following mode.

(30) Vehicle: 2.0 L, Gasoline MPI, KULEV, Emission test cycles: FTP-75.

(31) (2) Results

(32) As shown in FIG. 3, the catalyst of Example was significantly increased in the rates of removal of NOx, CO and THC despite the amount of the platinum-group element thereof being halved, compared to the catalyst of Comparative Example (1 g/L of platinum-group element). The emission of N.sub.2O and NH.sub.3 was also decreased (FIG. 4). In the similar kind other experiment 90% of platinum-group metal content was reduced by using NOx trapping layer (results not showed) and achieve similar kind of performance over state of art reference TWC system in terms of HC, CO, and NOx at FTP-75 mode.

(33) Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.