JP2014113557A - Method for producing catalyst for removing exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a catalyst for removing exhaust gas excellent in NOx purification performance especially under rich atmosphere, and a catalyst obtained by the producing method.SOLUTION: There is provided a method for producing a catalyst including: (a) a coprecipitation process of catalyst metals for adding a basic solution to an acidic solution including at least one selected from polyvinyl pyrrolidone, N-methyl pyrrolidone, N-vinyl-2- pyrrolidone and ethylene glycol; a precursor of cerium; a precursor of zirconium; and palladium/barium composite colloid; (b) a calcination process for drying the resultant solution and calcinating the dried object.

Description

  The present invention relates to a method for producing an exhaust gas removal catalyst.

  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). A catalyst containing ceria-zirconia composite oxide (ZC) or the like is known as an exhaust gas purifying catalyst for decomposing such harmful gases.

  In recent years, regulations on automobile exhaust gas have been strengthened, and research on improving the use efficiency of precious metals as purification catalysts has been active. However, there is still a problem that the precious metal aggregates due to the heat of the engine exhaust gas generated by the traveling of the vehicle and the atmospheric fluctuation, and the utilization efficiency is lowered.

For example, barium has the effect of accelerating HC purification (a steam reforming reaction proceeds and CO and H ₂ are generated) by donating electrons to palladium, which is a noble metal, to reduce the binding force of HC-Pd. . However, if it becomes difficult to maintain the adjacent state of palladium and barium due to agglomeration of noble metals due to running of the vehicle, HC purification is suppressed, resulting in a decrease in the amount of H ₂ generated necessary for NOx purification, and NOx during fuel rich control. There arises a problem that the purification performance is lowered.

  Patent Document 1 discloses an exhaust gas purification catalyst material containing alumina particles, CeZr-based composite oxide particles having oxygen storage / release ability, and Pd, wherein the Pd is the CeZr-based composite oxide particles. There is described an exhaust gas purifying catalyst material characterized in that the CeZr-based complex oxide particles are doped and fixed only on the surface, and are dispersedly supported on the surfaces of the alumina particles. Patent Document 1 describes that CeZr-based composite oxide particles may contain an alkaline earth metal such as Ba.

  In Patent Document 2, an exhaust gas purifying catalyst having an oxygen storage material containing Ce and Zr on a carrier, and a catalyst layer containing alumina and a catalyst metal, primary particles of the oxygen storage material and the alumina are used. Composite oxide particles are formed by agglomeration with primary particles, and the catalyst metal is doped with the Ce and Zr so as to constitute the oxygen storage material particles and is exposed to the surface of the particles. And an exhaust gas purifying catalyst characterized by comprising Pd adhering to the surfaces of the oxygen storage material particles and the alumina particles.

  In Patent Documents 1 and 2, a catalyst in which a noble metal is doped in a carrier is prepared by using a nitrate aqueous solution of a metal component contained in a catalyst as a raw material, and adding a basic aqueous solution thereto and calcining a precipitated precipitate. doing.

  However, Patent Document 1 does not describe that the alkaline earth metal oxide is supported in a form close to palladium. When a promoter such as alkaline earth metal oxide (barium oxide) is not in proximity to the active substance (palladium), NOx desorbed from the promoter does not reach the active substance and NOx reduction reaction can be performed. The NOx purification characteristics may be reduced. Patent Document 2 does not describe that the catalyst can contain barium.

  Patent Document 3 discloses a support made of a ceria-zirconia composite oxide and an oxide other than the ceria-zirconia composite oxide, a catalytic noble metal made of palladium and a noble metal other than palladium, and a carrier. A NOx occlusion reduction type catalyst is described which includes a supported NOx occlusion material and palladium is supported on a ceria-zirconia composite oxide, and barium is described as the NOx occlusion material. Patent Document 3 describes that palladium is supported by the evaporation to dryness method. Therefore, a catalyst in which palladium and barium are combined is not described.

  Thus, in the prior art, a technique for suppressing aggregation of noble metal by adding barium has not yet been disclosed.

JP 2010-12397 A JP 2008-290065 A JP 2009-297616 A

  An object of the present invention is to provide a method for producing an exhaust gas removal catalyst excellent in NOx purification performance, particularly in a rich atmosphere, and a catalyst obtained by the production method.

The present inventors co-precipitated each metal from a solution containing a palladium / barium composite colloid, a cerium precursor, and a zirconium precursor in the presence of polyvinylpyrrolidone and the like, thereby allowing palladium and barium to be at the atomic level. It was possible to make them close to each other, thereby finding that the aggregation of palladium can be suppressed and the NOx reduction ability can be improved, and the present invention has been achieved. That is, the present invention includes the following inventions.
(1) (a) at least one selected from polyvinylpyrrolidone, N-methylpyrrolidone, N-vinyl-2-pyrrolidone and ethylene glycol, a cerium precursor, a zirconium precursor, and a palladium / barium composite colloid A catalyst production method, comprising: a coprecipitation step of a catalytic metal in which a basic solution is added to an acidic solution to be contained; and (b) a firing step of drying and firing the resulting solution.
(2) The method according to (1) above, wherein the acidic solution in step (a) further contains a precursor of at least one rare earth element selected from yttrium, neodymium, lanthanum, and praseodymium.
(3) An exhaust gas removing catalyst obtained by the method described in (1) or (2) above.

  By the method of the present invention, an exhaust gas removal catalyst having excellent NOx purification performance can be obtained particularly under a rich atmosphere.

FIG. 1 is an image diagram of a catalyst loading form obtained by the method of the present invention. FIG. 2 is a graph showing the Pd particle size of the catalysts of Example 1 and Comparative Example 1-3. FIG. 3 is a graph showing NOx 50% purification temperatures of the catalysts of Example 1 and Comparative Example 1-3.

  The method for producing the catalyst of the present invention (hereinafter also referred to as the method of the present invention) comprises (a) at least one selected from polyvinylpyrrolidone, N-methylpyrrolidone, N-vinyl-2-pyrrolidone and ethylene glycol, cerium A catalyst metal co-precipitation step of adding a basic solution to an acidic solution containing a precursor of bismuth, a precursor of zirconium, and a palladium / barium composite colloid, and (b) a firing step of drying and firing the resulting solution It is characterized by including.

  In step (a), polyvinylpyrrolidone, N-methylpyrrolidone, N-vinyl-2-pyrrolidone and ethylene glycol are used as protective agents. Substances used as a protective agent are preferably those close to pH so that precipitation does not occur when a palladium / barium composite colloid, an iridium precursor and a palladium precursor are added. Preferably, polyvinylpyrrolidone can be used.

  In the acidic solution used in step (a), palladium and barium are present as a palladium / barium composite colloid. By using the palladium / barium composite colloid, palladium and barium can be brought close to each other at the atomic level during coprecipitation, and barium can be arranged around palladium after firing. Specifically, as shown in FIG. 1, palladium / barium oxide composite particles are arranged between primary particles (voids) of ZC (ceria-zirconia composite oxide) particles, and in the palladium / barium oxide composite particles, barium oxide Is considered to be centered on palladium particles. Then, by bringing barium and palladium close to each other, the diffusion distance by which NOx desorbed from barium reaches palladium is shortened, so that the reduction reaction of NOx by palladium proceeds and the NOx reduction rate is considered to improve.

  Examples of the cerium precursor and zirconium precursor used in step (a) include cerium and zirconium salts or complexes. Examples of the cerium salt or complex include cerium nitrate. Zirconium nitrate can be mentioned as a salt or complex of zirconium.

Water can be mentioned as a solvent which can be used in a process (a).
The pH of the acidic solution in step (a) is preferably 2-6.

  A precursor of at least one rare earth element selected from yttrium, neodymium, lanthanum and praseodymium may be further added to the acidic solution in step (a). Examples of the metal precursor include a salt or complex of the metal. Examples of the salt include nitrate, chloride, nitride, oxide and halide. The metal complex has, as a ligand, monodentate ligands such as acyl, carbonyl, thiocyanate, isocyanate, cyanate, alkoxide, halogen atom, and bidentate ligands such as acetylacetonate and various carboxylic acids. Things.

  The acidic solution in the step (a) is obtained by adding a hydrogen peroxide solution and a precursor of the rare earth element to a solution obtained by adding a cerium precursor and a zirconium precursor to a solvent. It is preferably obtained by adding a barium composite colloid. In many cases, commercially available cerium precursors have a trivalent cerium valence. In this case, the hydrogen peroxide solution can be added to make the cerium valence four.

  Examples of the basic solution used in the step (a) include aqueous ammonia and sodium hydroxide. It is preferable to add a basic solution to the acidic solution to adjust the pH to 4-8. Thereby, the catalyst of the desired form as mentioned above can be obtained by coprecipitating the metal contained in the acidic solution.

  In the step (a), as a post-treatment, the product may be dried under normal pressure or reduced pressure.

  The method of the present invention is characterized in that the step (b) includes a baking step of drying and baking the obtained solution. The firing step can be performed in air. Moreover, it is preferable that a calcination temperature is 500-600 degreeC. The firing time is preferably 1 to 10 hours.

  The present invention also relates to a catalyst obtained by the method of the present invention (hereinafter also referred to as the catalyst of the present invention).

  The catalyst of the present invention contains ceria-zirconia composite oxide, palladium and barium oxide, and the palladium / barium oxide composite particles are arranged between the ceria-zirconia composite oxide particles, and the palladium / barium oxide composite particles contain barium oxide. It is arranged on the surface of palladium.

  The content of the ceria-zirconia composite oxide in the catalyst of the present invention is preferably 10 to 99.5% by mass with respect to the catalyst.

  In the catalyst of the present invention, the palladium content is preferably 0.01 to 5 mass% with respect to the ceria-zirconia composite oxide.

  In the catalyst of the present invention, the barium oxide content is preferably 2 to 20% by mass with respect to the ceria-zirconia composite oxide.

  From the viewpoint of heat resistance, the catalyst of the present invention preferably contains at least one rare earth element oxide selected from yttrium, neodymium, lanthanum and praseodymium in the ceria-zirconia composite oxide. The content of the rare earth element oxide is preferably 0.5 to 5% by mass with respect to the ceria-zirconia composite oxide.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to the range of an Example.

[Example 1]
Polyvinylpyrrolidone was added to pure water, and ZrO (NO ₃ ) ₂ · 2H ₂ O and Ce (NO ₃ ) ₂ · 6H ₂ O were added thereto and mixed. A H ₂ O ₂ aqueous solution was added to this solution to adjust the pH to 3. Y (NO ₃ ) ₃ · 6H ₂ O was added to this and mixed (solution A).

On the other hand, Pd (NO ₃ ) ₂ .2H ₂ O and (CH ₃ COO) ₂ Ba were added to pure water to obtain a palladium / barium composite colloid drug solution.

  The solution A was coprecipitated by adding a palladium / barium composite colloid drug solution and adding an excessive amount of aqueous ammonia to adjust the pH to 9.0 or more.

  Ammonia was removed by neutralization, the solution was centrifuged, and then dried at 120 ° C. to obtain a catalyst precursor. The obtained catalyst precursor was calcined at 700 ° C. for 5 hours.

Further, the catalyst precursor was calcined at 500 ° C. for 2 hours to obtain a catalyst.
A catalyst carrying 1% by mass of Pd and 6% by mass of BaO was obtained with respect to the ceria-zirconia composite oxide particles (mass ratio: ZrO ₂ / CeO ₂ / Y ₂ O ₃ = 68/30/2). It was.

[Comparative Example 1]
The palladium / barium composite colloidal chemical solution was not added to the solution A, and it was impregnated with an aqueous Pd (NO ₃ ) ₂ solution after baking at 700 ° C. for 5 hours and before baking at 500 ° C. for 2 hours, and further (CH ₃ COO) except 2 _Ba solution that was impregnated (evaporation to dryness) to obtain a catalyst in the same manner as in example 1. A catalyst carrying 1% by mass of Pd and 6% by mass of BaO was obtained with respect to the ceria-zirconia composite oxide particles (mass ratio: ZrO ₂ / CeO ₂ / Y ₂ O ₃ = 68/30/2). It was.

[Comparative Example 2]
The Pd (NO ₃ ) ₂ aqueous solution was added to the solution A instead of the palladium / barium composite colloidal chemical solution, and after baking at 700 ° C. for 5 hours and before baking at 500 ° C. for 2 hours (CH ₃ COO) ₂ Ba A catalyst was obtained in the same manner as in Example 1 except that it was impregnated with an aqueous solution (evaporation to dryness method). A catalyst carrying 1% by mass of Pd and 6% by mass of BaO was obtained with respect to the ceria-zirconia composite oxide particles (mass ratio: ZrO ₂ / CeO ₂ / Y ₂ O ₃ = 68/30/2). It was.

[Comparative Example 3]
(CH ₃ COO) ₂ Ba aqueous solution was added to solution A instead of palladium / barium composite colloidal chemical solution, and Pd (NO ₃ ) ₂ was baked at 700 ° C. for 5 hours and then baked at 500 ° C. for 2 hours. A catalyst was obtained in the same manner as in Example 1 except that it was impregnated with an aqueous solution (evaporation to dryness method). A catalyst carrying 1% by mass of Pd and 6% by mass of BaO was obtained with respect to the ceria-zirconia composite oxide particles (mass ratio: ZrO ₂ / CeO ₂ / Y ₂ O ₃ = 68/30/2). It was.

[Comparative Example 4]
The palladium / barium composite colloidal chemical solution was not added to the solution A and impregnated with the palladium / barium composite colloidal chemical solution after baking at 700 ° C. for 5 hours and before baking at 500 ° C. for 2 hours (evaporation to dryness method) Except for the above, a catalyst was obtained in the same manner as in Example 1. A catalyst carrying 1% by mass of Pd and 6% by mass of BaO was obtained with respect to the ceria-zirconia composite oxide particles (mass ratio: ZrO ₂ / CeO ₂ / Y ₂ O ₃ = 68/30/2). It was.

Each test was done about the obtained catalyst of Example 1 and Comparative Example 1-4.
1. Durability Test RL (Rich / Lean) 700 ° C. Durability Test The RL 700 ° C. Durability Test was performed by weighing 3000 mg of each catalyst and repeatedly flowing the following gases for 5 hours in a 5-minute cycle while maintaining at 700 ° C. It was.

1% -CO / N ₂ + 10% -H ₂ O⇔5% -O ₂ / N ₂ + 10% -H ₂ O

2. For the catalysts of Example 1 and Comparative Example 1-3 before and after the particle diameter measurement endurance test, the particle diameter of Pd was determined by AMI-1 (gas: CO; gas amount: 55 μL / pulse; temperature: 25 ° C.). Measurement was performed using a product manufactured by Altila Instruments.

3. For the catalysts of Example 1 and Comparative Example 1-3 before and after the NOx purification performance durability test, the NOx purification rate was increased while raising the temperature at a rate of temperature rise of 20 ° C./min between 100 to 500 ° C. and a gas flow rate of 20 L / min. Measured and calculated 50% purification temperature of NOx. Measurement was performed under the following rich atmosphere. In addition, ppmC shows a carbon concentration.

C ₃ H ₆ : 1000 ppmC; NO: 3600 ppm; CO: 6200 ppm
O ₂ : 0.26%; H ₂ O: 3%; CO ₂ : 14.5%; N ₂ : Balance

  In FIG. 2, the particle diameter of Pd of the catalyst of Example 1 and Comparative Example 1-3 is shown. FIG. 3 shows the NOx 50% purification temperature of the catalysts of Example 1 and Comparative Example 1-3.

  The catalyst of Example 1 prepared by co-precipitation using a palladium / barium composite colloid (with palladium / barium oxide composite particles arranged between the primary particles of ZC) was the same as the catalyst of Comparative Example 2 (Pd was the primary catalyst of ZC). The NOx 50% purification temperature after endurance is low as compared with the case where Ba is supported between the particles by evaporation to dryness. From this, it can be seen that, when barium oxide is complexed with palladium arranged between the ZC primary particles, the aggregation of Ba is suppressed, and the reduction in NOx 50% purification performance is suppressed.

  Further, the catalyst of Example 1 is the catalyst of Comparative Example 1 (in which Pd and Ba are individually supported by evaporation to dryness on ZC), the catalyst of Comparative Example 3 (Ba is disposed between the primary particles of ZC, Compared with the catalyst of Comparative Example 4 (supporting Pd by evaporation to dryness) and the catalyst of Comparative Example 4 (compositing Pd / Ba and then supporting the composite particles by ZC by evaporation to dryness), the Pd particle diameter after endurance And NOx 50% purification temperature after durability is low. As a result, by disposing the palladium / barium oxide composite particles between the ZC primary particles, the aggregation of Pd is suppressed, and the adjacent state of Pd and Ba is maintained, so the decrease in NOx purification performance after durability is suppressed. You can see that

  The catalyst obtained by the method of the present invention can be preferably applied to exhaust gas removal, particularly NOx purification.

Claims (3)

(A) Acidity containing at least one selected from polyvinylpyrrolidone, N-methylpyrrolidone, N-vinyl-2-pyrrolidone and ethylene glycol, a cerium precursor, a zirconium precursor, and a palladium / barium composite colloid A catalyst production method, comprising: a catalyst metal coprecipitation step of adding a basic solution to the solution; and (b) a firing step of drying and firing the resulting solution.   The method of claim 1, wherein the acidic solution of step (a) further comprises a precursor of at least one rare earth element selected from yttrium, neodymium, lanthanum and praseodymium.   An exhaust gas removing catalyst obtained by the method according to claim 1.

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