JP2015085241A - Exhaust gas purification catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst improved in warming performance while maintaining a pressure loss low.SOLUTION: There can be produced an exhaust gas purification catalyst which contains ceria-zirconia composite oxide particles and θ phase alumina particles and uses 25 wt.% or more of a monolith substrate relative to the total weight of the content of the ceria-zirconia composite oxide particles and in which noble metal particles are supported on the monolith substrate, wherein the amount used of the noble metal particles is reduced while maintaining the performance at the same level as or higher than that of the conventional ones and the ceria-zirconia composite oxide particles contain 30 wt.% or more of ceria and 60 wt.% or less of zirconia.

Description

  The present invention relates to an exhaust gas purification catalyst that realizes both high warm-up performance and low pressure loss.

  Exhaust gas discharged from internal combustion engines such as automobiles contains harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). An exhaust gas purification catalyst that decomposes such harmful gases is also called a three-way catalyst, and a catalyst layer is provided by washing a slurry containing noble metal particles having catalytic activity on a honeycomb monolith substrate made of cordierite or the like. Things are common. Various attempts have been made to improve the performance of such catalysts.

  For example, Patent Document 1 proposes manufacturing a cordierite monolith substrate having a larger pore diameter by containing magnesium and boron. Patent Document 2 has a structure including a first compound on which noble metal particles are supported and a second compound that isolates the first compounds from each other, and the noble metals do not aggregate even after a long period of use of the catalyst. It has been proposed to maintain the surface area.

JP 2006-188404 A JP 2007-31498 A

  Reducing the amount of precious metal used in the exhaust gas purification catalyst is an important proposition from the viewpoint of securing resources and reducing costs. However, since the catalyst purification performance is basically proportional to the amount of the noble metal that is the catalyst active point, reducing the amount of noble metal used decreases the catalyst purification performance.

  In particular, exhaust gas purification catalysts generally do not exhibit sufficient purification performance unless they reach a catalyst activation temperature of about 300 ° C., so how to reduce the amount of HC discharged immediately after engine startup (so-called Cold-HC) One of the important issues is how to do this. In order to reduce the amount of noble metal used, it is first necessary to find a means for reducing Cold-HC. Conventional means for improving the performance of exhaust gas purification catalysts include increasing the cell of the monolith substrate, but increasing the cell increases the pressure loss in the exhaust pipe, causing adverse effects such as decreased engine output and reduced fuel consumption. , Not very preferable.

  In view of the above, it is an object of the present invention to provide an exhaust gas purification catalyst having improved warm-up performance while maintaining a low pressure loss.

As a result of studying the above-described problems, the present inventors can provide a catalyst that solves the above-described problems by using ceria-zirconia composite oxide and θ-phase alumina particles as the material of the monolith substrate. I found out. The gist of the present invention is as follows.
(1) Precious metal particles are supported on a monolith substrate containing ceria-zirconia composite oxide particles and θ-phase alumina particles, and the content of ceria-zirconia composite oxide particles is 25% by weight or more based on the total weight. Exhaust gas purification catalyst.
(2) The exhaust gas purification catalyst according to (1), wherein the ceria-zirconia composite oxide particles contain ceria in an amount of 30% by weight or more.
(3) The exhaust gas purification catalyst according to (1) or (2), wherein the ceria-zirconia composite oxide particles contain zirconia in an amount of 60% by weight or less.

  According to the present invention, by changing the material of the monolith substrate, it is possible to provide an exhaust gas purification catalyst with improved warm-up performance without increasing pressure loss. According to the present invention, it is possible to provide an exhaust gas purification catalyst that reduces the amount of noble metal used while maintaining performance equal to or higher than that of a conventional product.

4 is a graph summarizing the results of evaluating the warm-up performance of the catalysts of Comparative Examples 1 and 2 and Example 1. 2 is a graph summarizing the measurement results of pressure loss of the catalysts of Comparative Examples 1 and 2 and Example 1. FIG. It is the figure which plotted the position of the catalyst of this invention with respect to the conventional catalyst by plotting the pressure loss and catalyst warm-up performance value at the specific flow rate in the catalysts of Comparative Examples 1 and 2 and Example 1. CeO ₂ -ZrO ₂ (CZ) is a graph summarizing the results of evaluating the relationship between the content and the substrate strength. _{2 is} an X-ray diffraction image of θ-phase Al ₂ O ₃ particles used for the monolith substrate of the catalyst of Example 1. FIG.

  The exhaust gas purifying catalyst of the present invention is characterized in that the monolith substrate includes ceria-zirconia composite oxide particles and θ-phase alumina particles (θ alumina particles). The content of the ceria-zirconia composite oxide particles in the monolith substrate is 25% by weight or more, preferably 35% by weight or more, particularly preferably 45% by weight or more, particularly preferably 50% by weight, based on the total weight of the substrate. That's it. By using the ceria-zirconia composite oxide particles in such an amount, sufficient strength can be given to the monolith substrate.

  Ceria-zirconia composite oxide is a material conventionally used as a cocatalyst (oxygen storage material) in an exhaust gas purification catalyst, and its details are known to those skilled in the art. In the ceria-zirconia composite oxide in the present invention, preferably ceria and zirconia form a solid solution. Ceria-zirconia composite oxide is prepared by adding ammonia water to an aqueous solution in which, for example, cerium salt (cerium nitrate, etc.) and zirconium salt (zirconium oxynitrate, etc.) are dissolved, and drying the resulting precipitate. It can be prepared by baking at 400 to 500 ° C. for about 5 hours.

  The ceria-zirconia composite oxide may further contain an element selected from rare earth elements other than cerium. As rare earth elements, scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), Examples thereof include ytterbium (Yb) and lutetium (Lu).

  The ceria-zirconia composite oxide used in the present invention preferably contains ceria in an amount of 30% by weight or more, particularly 40% by weight or more, more preferably 90% by weight or less, particularly preferably 80% by weight or less. . The ceria-zirconia composite oxide preferably contains zirconia in an amount of 60% by weight or less, particularly 50% by weight or less. Since such a ceria-zirconia composite oxide has a small heat capacity, the temperature of the monolith substrate can be easily increased, and thus the warm-up performance of the catalyst can be improved.

  By using θ-phase alumina particles as partition material for ceria-zirconia composite oxide, both micrometer-size three-dimensional network pores (macropores) and nanometer-size pores (mesopores) are enlarged. And the pressure loss of the catalyst can be reduced. Since the alumina phase change in the exhaust gas can be suppressed by making the alumina particles have a θ structure, higher heat resistance can be realized.

  The exhaust gas purification catalyst of the present invention has a structure in which noble metal particles are supported on the monolith substrate. The noble metal particles are preferably a platinum group metal, in particular a metal selected from Pt, Rh and Pd. The precious metal particles may be supported on the monolith substrate by mixing with a cocatalyst (oxygen storage material), carrier, binder, etc. to prepare a slurry, and washing the monolith substrate with it. However, since the monolith substrate of the present invention itself has the functions of a promoter and a carrier, high purification performance can be expected even when noble metal particles are directly supported. In particular, reducing the heat capacity of the three-way catalyst is effective for reducing Cold-HC, and thus the catalyst can be reduced in heat capacity by eliminating the washcoat, and higher HC purification performance can be expected. The noble metal can be supported using a general reagent such as palladium nitrate or rhodium chloride.

  The monolith substrate can be manufactured by the same manufacturing method as the conventional cordierite substrate, although the material is different. For example, it can be produced by adding water and a binder to a mixture of ceria-zirconia composite oxide particles and θ-phase alumina particles, kneading, molding with an extruder, drying and firing.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail using an Example, this invention is not limited to these Examples.

1. Preparation of three-way catalyst Catalysts of Comparative Example 1, Comparative Example 2, and Example 1 were prepared according to the specifications shown in Table 1. The catalysts of Comparative Examples 1 and 2 are typical three-way catalysts in which a catalyst layer is wash-coated on a predetermined cordierite substrate, and were prepared for comparison of catalyst purification performance and pressure loss. All the washcoats were performed so that the total amount of the coating material was 232 g / L with respect to the substrate capacity.

The catalyst of Example 1 is a monolith substrate (porosity 48%, apparent strength 2 MPa, segment bulk specific gravity 0. 0) produced using CeO ₂ —ZrO ₂ particles and Al ₂ O ₃ particles instead of the cordierite substrate. 45 g / ml, coefficient of thermal expansion 9 ppm / K). FIG. 5 is an X-ray diffraction image of Al ₂ O ₃ particles having a θ phase. It was confirmed in advance that the Al ₂ O ₃ particles used had a θ phase by an X-ray diffraction image. Since this monolith substrate itself is considered to have a catalyst carrier function and a promoter function, a predetermined amount of noble metal was directly supported on the substrate without performing a wash coat. Specifically, the base material was immersed in an aqueous solution in which necessary amounts of palladium nitrate and rhodium chloride were dispersed and allowed to stand for a certain period of time, thereby supporting the noble metal on the base material. The substrate volumes of the catalysts of Comparative Examples 1 and 2 and Example 1 were both 0.9 L.

2. Durability Test The prepared catalysts of Comparative Examples 1 and 2 and Example 1 were set directly under the V8 4.6L engine. A / F was a complex pattern that changed cyclically, and a 50-hour durability test was conducted at a bed temperature of 1000 ° C.

3. Catalyst Evaluation Each catalyst that had undergone the above endurance test 2 was set in an in-line 4-cylinder 2.4L engine, and the time during which the HC concentration was 50% or less from the start of the stoichiometric engine was measured to evaluate catalyst warm-up performance. Moreover, the pressure loss of the catalyst was measured using a simple measuring device using a blower and a pressure sensor.

  FIG. 1 is a graph summarizing the evaluation results of catalyst warm-up performance. Compared to the catalysts of Comparative Examples 1 and 2 in which a catalyst layer was provided by a conventional washcoat, the catalyst of Example 1 was most excellent in warm-up performance.

  FIG. 2 is a graph summarizing the measurement results of the pressure loss of the catalyst. The catalyst of Example 1 had the same level of performance as the catalyst of Comparative Example 2, which was a conventional product in which a low mesh base material was wash coated.

  FIG. 3 is a diagram in which the pressure loss at a specific flow rate and the catalyst warm-up performance value in the catalysts of Comparative Examples 1 and 2 and Example 1 are plotted, and the position of the catalyst of the present invention relative to the conventional catalyst is mapped. According to the present invention, it can be seen that it is possible to significantly shift from the conventional trade off-line to the superior performance side.

4). Monolith base material strength evaluation A monolith base material in which the content of CeO ₂ —ZrO ₂ (CZ) in the monolith base material was changed was prepared, and the bending strength was measured. The results of evaluating the relationship between CZ content and substrate strength are summarized in the graph of FIG. As a result of the evaluation, it was determined that high substrate strength can be achieved by containing 25 wt% or more of CZ.

Claims (3)

  Exhaust gas in which noble metal particles are supported on a monolith substrate containing ceria-zirconia composite oxide particles and θ-phase alumina particles, and the content of ceria-zirconia composite oxide particles is 25% by weight or more based on the total weight Purification catalyst.   The exhaust gas purification catalyst according to claim 1, wherein the ceria-zirconia composite oxide particles contain ceria in an amount of 30 wt% or more.   The exhaust gas purification catalyst according to claim 1 or 2, wherein the ceria-zirconia composite oxide particles contain zirconia in an amount of 60% by weight or less.

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