JP2007105633A - Exhaust-gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust-gas cleaning catalyst having the excellent durability under a high temperature environment. <P>SOLUTION: The exhaust-gas cleaning catalyst has a carrier 2 formed from an oxide, many number of PdO particles 3 of an average particle diameter of ≥1 nm and ≤50 nm, supported by the carrier 2, and many number of LnOx (rare-earth oxide) particles 4 of the average particle diameter of ≥1 nm and ≤50 nm, which contact with PdO particles 3 while being supported by the carrier 2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst that purifies exhaust gas containing hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and the like discharged from an internal combustion engine represented by an automobile engine.

The exhaust gas emitted from internal combustion engines such as automobiles contains harmful gases such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Three-way catalysts that can simultaneously purify the three-component harmful gases are used. The three-way catalyst is a porous material (for example, alumina (γ-Al ₂ O ₃ )) on which noble metal particles (for example, Pt, Rh, Pd) are supported, and a base material (made of cordierite). Formed on a honeycomb or the like).

  Since the activity of a catalyst such as a three-way catalyst is approximately proportional to the surface area of the noble metal particles supported on the support, the activity of the catalyst can be increased by uniformly dispersing fine noble metal particles on the support and increasing the surface area of the noble metal particles. Is increasing. However, if the noble metal particles are too fine, they are likely to aggregate with an increase in surface energy, which may reduce the activity of the catalyst. In addition to the movement and aggregation of the noble metal particles themselves, there are other factors that cause the aggregation of the noble metal particles as the carrier contracts.

  In recent years, the exhaust gas exhausted from automobiles tends to become hot against the background of higher output of gasoline engines and increased high-speed driving, etc., and the temperature is quickly raised to the exhaust gas purification temperature when the engine is started. Therefore, since the catalyst is disposed directly under the engine, the exhaust gas purification catalyst such as a three-way catalyst has a tendency to be exposed to a high temperature environment. For this reason, the noble metal particles tend to agglomerate with the movement of the noble metal particles and the contraction of the carrier.

  Therefore, development of an exhaust gas purifying catalyst having improved heat resistance under a high temperature environment is underway. For example, Pt, Rh, and Pd are used as noble metals. In particular, Pd has been found to be inferior in heat resistance, although it is excellent in oxidizing hydrocarbons (HC) at low temperatures.

Thus, an exhaust gas purification catalyst is disclosed in which a support is formed from a composite oxide having a perovskite structure represented by the general formula ABO ₃ , and this composite oxide is impregnated and supported with a noble metal (Pd) to improve heat resistance. (See Patent Document 1). There is also known an exhaust gas purifying catalyst in which a support including Pd is included in the composition of a composite oxide having a perovskite structure to improve heat resistance.
JP 2005-66559 A

  However, even in the above-described exhaust gas purification catalyst, when the exhaust gas is actually purified, the heat resistance is insufficient, the grain growth of the noble metal particles accompanying heat occurs, the surface area of the noble metal particles is reduced, and as a result, the activity of the catalyst is reduced. Had the risk of falling.

  As a cause of such a phenomenon, Pd is metalized by decomposition of PdO in a high temperature range of 800 ° C. or higher, Pd particles are rapidly aggregated, and the activity of Pd is reduced as the surface area of Pd particles is reduced. It may be reduced.

  Aggregation of Pd particles is likely to occur particularly in a reducing atmosphere. However, in a high temperature range of 870 ° C. or higher, Pd metalation occurs regardless of the oxidizing atmosphere or reducing atmosphere to which the exhaust gas purification catalyst is exposed. This is because the particles are likely to aggregate.

  The present invention has been made to solve the above-described problems. That is, the exhaust gas purifying catalyst of the present invention has a carrier formed of an oxide and an average particle size of 1 nm to 50 nm carried on the carrier. It has a large number of palladium oxide (PdO) particles and a large number of rare earth oxide (LnOx) particles having an average particle diameter of 1 nm or more and 50 nm or less that are supported on the support in contact with the palladium oxide (PdO) particles. And

  According to the exhaust gas purification catalyst of the present invention, the temperature at which Pd is precipitated by decomposition of PdO is set to a high temperature range, so that the growth of Pd particles after high-temperature durability is suppressed and the catalyst activity is prevented from decreasing. Can do.

  Hereinafter, an exhaust gas purification catalyst according to an embodiment of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 shows a cross-sectional view of an exhaust gas purification catalyst according to an embodiment of the present invention. The exhaust gas purifying catalyst 1 includes a carrier 2 formed of an oxide, palladium oxide (PdO) particles 3 having an average particle size of 1 nm to 50 nm, and rare earth oxide (LnOx) particles 4 having an average particle size of 1 nm to 50 nm. , And palladium oxide (PdO) particles 3 and rare earth oxide (LnOx) particles 4 are arranged in contact with each other. Although not shown here, promoter particles may be supported on the carrier 2.

  In the case of a catalyst using Pd as a noble metal, metalization by deoxygenation occurs from PdO to Pd, resulting in the aggregation of Pd particles, and the activity of the catalyst decreases as the surface area of Pd particles decreases. End up. For this reason, by allowing PdO particles and LnOx particles (Ln is a rare earth element) to coexist on the support, it is possible to shift the precipitation temperature of Pd from PdO to the high temperature side. Therefore, in the present invention, by defining the average particle size of each of the PdO particles, the palladium oxide (PdO) particles 3 and the rare earth oxide (LnOx) particles 4 to 1 nm to 50 nm, the Pd precipitation temperature is further increased. The Pd metalation in the operating temperature range of the three-way catalyst can be suppressed, and an exhaust gas purification catalyst having high catalytic activity can be obtained.

  Furthermore, the average particle diameter of the palladium oxide particles 3 and the average particle diameter of the rare earth oxide particles 4 are preferably 1 nm or more and 20 nm or less.

  In the exhaust gas purification catalyst 1, the rare earth oxide particles 4 are formed of an oxide containing at least one element selected from La, Nd, Sm, Eu, Gd, Dy, Ce, and Pr. Is preferred.

The support 2 is preferably formed of at least one oxide selected from Al ₂ O ₃ , ZrO ₂ , SiO ₂ , TiO _2, and CeO ₂ or a composite oxide of two or more.

  The promoter particles are preferably formed from one element selected from Mn, Fe, Co, and Ni, or two or more composites (alloys, mixed grains). It may be arbitrarily selected according to the function and application of the catalyst to be produced.

  Further, in the exhaust gas purification catalyst 1, the palladium oxide (PdO) particles 3 and the rare earth oxide particles (LnOx) 4 supported on the carrier 2 are preferably in the molar ratio shown below.

  Specifically, it is preferable that the molar ratio of palladium oxide (PdO) to the rare earth oxide (LnOx) is 1: 1, or the ratio of LnOx is higher than this molar ratio, and the molar ratio is It is preferable that the ratio is 1: 2, or the ratio of LnOx is higher than this molar ratio. Furthermore, the molar ratio of palladium oxide (PdO) to rare earth oxide (LnOx) is preferably 1: 4, or the ratio of LnOx is higher than this molar ratio. More preferably, it is 10 or the ratio of LnOx is higher than this molar ratio. As described above, by increasing the ratio of the rare earth oxide (LnOx), the deposition temperature of Pd from PdO can be shifted to a high temperature range. In particular, when the molar ratio of palladium oxide (PdO) and rare earth oxide (LnOx) is 1:10 and the molar ratio of the rare earth oxide is increased to form a catalyst, the XRD diffraction peak shows Pd, A catalyst in which PdO is not detected and Pd and PdO are supported in a highly dispersed state on a support can be obtained. As a result, the number of active sites of the catalyst increases, and a decrease in the activity of the catalyst can be prevented.

In addition, it is not limited to the exhaust gas purification catalyst 1 of the structure shown in FIG. 1, It is good also as a structure of the exhaust gas purification catalyst 6 shown in FIG. About the same location as the catalyst shown in FIG.1 and FIG.2, the description is abbreviate | omitted using the same code | symbol. The exhaust gas purification catalyst 6 is formed by supporting composite particles 7 formed of a composite compound of palladium and rare earth oxide on a carrier 2. Here, the composite particles 7 formed by combining the palladium oxide (PdO) particles 3 and the rare earth oxide (LnOx) particles 4 shown in FIG. 1 are used. A composite compound of palladium and a rare earth element may be formed in a part where the palladium (PdO) particle 3 and the rare earth oxide (LnOx) particle 4 are in contact with each other. In addition, it is preferable to set it as the composition formula which satisfy | fills (x, y, z are arbitrary) of the complex compound LnxPdyOz containing this palladium and rare earth elements. For example, examples of the composite oxide LnxPdyOz include Ln ₂ PdO ₄ , Ln ₂ Pd ₂ O ₅ , and Ln ₄ PdO ₇ . Even in this case, since the composite particles 7 function as catalytically active species, the average particle size of the composite particles 7 is preferably 1 nm or more and 50 nm or less in order to increase the catalyst activity. The average particle size is preferably 1 nm or more and 20 nm or less. As described above, since the mixture of PdO and LnOx or the composite oxide of both is finer than the support, the activity of the catalyst can be enhanced by dispersing and supporting the composite particles on the support. .

  Further, the above-described exhaust gas purification catalysts 1 and 6 according to the embodiment of the present invention are prepared as a catalyst slurry containing the above-described catalyst on a refractory inorganic base material (for example, a cordierite honeycomb). It is preferably used as an exhaust gas purification catalyst having a catalyst layer by coating. As a result, it can be used as an exhaust gas purification catalyst for purifying exhaust gas discharged from an internal combustion engine represented by an automobile.

  Hereinafter, further specific description will be made using examples. Of course, the exhaust gas purifying catalyst according to the embodiment of the present invention is not limited to the illustrated examples.

Example 1
Into 500 g of water, 6.181 g of Nd (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%), and 47.8 g of γ-Al ₂ O ₃ were added and stirred for 1 hour. To obtain an aqueous solution. This aqueous solution was placed in a constant temperature bath at 150 ° C. and left for a whole day and night, and then the water in the aqueous solution was evaporated to obtain a dry powder. The obtained dry powder was calcined at 400 ° C. for 1 hour to obtain a catalyst in which PdO particles and Nd ₂ O ₃ particles were supported on a support formed from Al ₂ O ₃ .

Example 2
In 500 g of water, 12.361 g of Nd (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 43.53 g of γ-Al ₂ O ₃ were added and stirred for 1 hour. To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Nd ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Example 3
In 500 g of water, 24.722 g of Nd (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 38.79 g of γ-Al ₂ O ₃ were added and stirred for 1 hour. To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Nd ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Example 4
In 500 g of water, 61.806 g of Nd (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 24.55 g of γ-Al ₂ O ₃ were added and stirred for 1 hour. To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Nd ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Example 5
Into 500 g of water, add 6.267 g of Sm (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 45.82 g of γ-Al ₂ O ₃ and stir for 1 hour To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Sm ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Example 6
In 500 g of water, add 12.534 g of Sm (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 43.36 g of γ-Al ₂ O ₃ and stir for 1 hour To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Sm ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Example 7
In 500 g of water, 25.068 g of Sm (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 37.94 g of γ-Al ₂ O ₃ were added and stirred for 1 hour. To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Sm ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Example 8
In 500 g of water, 62.669 g of Sm (NO ₃ ) ₃ · 6H ₂ O, 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 23.19 g of γ-Al ₂ O ₃ were added and stirred for 1 hour. To obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Sm ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

Comparative Example 1
In 500 g of water, 7.22 g of a Pd nitrate solution (Pd: 20.764 wt%) and 48.27 g of γ-Al ₂ O ₃ were added and stirred for 1 hour to obtain an aqueous solution. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst having PdO particles supported on a carrier formed from Al ₂ O ₃ .

Comparative Example 2
In 500 g of water, 23.722 g of neodymium oxide powder (average particle size 7 μm) 7.22 g of Pd nitrate solution (Pd: 20.764 wt%) and 24.55 g of γ-Al ₂ O ₃ were added and stirred for 1 hour to obtain an aqueous solution. It was. This aqueous solution was dried and calcined in the same manner as in Example 1 to obtain a catalyst in which PdO particles and Nd ₂ O ₃ particles were supported on a carrier formed from Al ₂ O ₃ .

  Each catalyst obtained from Examples 1 to 8 and Comparative Examples 1 and 2 was calcined at 900 ° C. for 3 hours.

  Using the obtained catalyst after calcination, the degree of precious metal dispersion, the average particle diameter of PdO particles and rare earth oxide particles (LnOx) were measured.

The precious metal dispersity was measured according to the following procedure using a metal dispersity measuring device (BEL-METAL-3 manufactured by Nippon Bell Co., Ltd.). The sample was heated to 400 ° C. at 10 ° C./min in a He 100% gas stream, and further oxidized for 15 minutes at 400 ° C. in an O ₂ 100% gas stream. Then, purging was performed with He100% gas for 5 minutes, and reduction treatment was performed at 400 ° C. in a H ₂ 40% / He balance gas stream for 15 minutes. Next, the temperature was lowered to 50 ° C. in a He100% gas stream. Thereafter, CO 10% / He balance gas was introduced in a pulse manner to determine the unit CO adsorption amount.

  The average particle size of the PdO particles and rare earth oxide particles (LnOx) particles is first observed by TEM observation of the catalyst. The TEM (transmission electron microscope) used was HF-2000 (manufactured by Hitachi, Ltd.), the acceleration voltage was 200 kV, and the cutting conditions were room temperature. Specifically, the catalyst powder was embedded with an epoxy resin, and after the epoxy resin was cured, an ultrathin section was prepared with an ultramicrotome. Using the slices, the dispersion state of various crystal grains was examined by TEM observation. In the obtained image, focus on the contrast (shadow) part, limit the metal species, measure the particle size of the metal, calculate the average value of the measured values, and calculate the PdO particles and rare earth oxide ( LnOx) The average particle size of the particles.

The measurement results obtained from the above measurement method are shown in Table 1.

  From the results shown in Table 1, each example in which the average particle diameter of the palladium oxide (PdO) particles and the average particle diameter of the rare earth oxide (LnOx) particles are within the range of 1 nm to 50 nm is compared with the comparative example. As a result, it was found that the value of the precious metal dispersion was high. On the other hand, in Comparative Example 1, rare earth oxide particles are not supported, and in Comparative Example 2, the average particle diameter of palladium oxide (PdO) particles and rare earth oxide (LnOx) particles is 50 nm. Since it exceeded, the precious-metal dispersion degree was a low value. Among the examples, Examples 4 and 8 increase the ratio of the presence of rare earth oxide (LnOx) particles by setting the molar ratio of palladium oxide (PdO) particles to rare earth oxide (LnOx) particles to 1:10. Therefore, in particular, the precious metal dispersion has a high value. Thus, it has been found that by increasing the degree of precious metal dispersion, it is possible to suppress a decrease in the activity of the catalyst in a high temperature environment.

1 is an enlarged cross-sectional view of an exhaust gas purification catalyst according to an embodiment of the present invention. It is an expanded sectional view of the exhaust gas purification catalyst which concerns on other embodiment in this invention.

Explanation of symbols

1 ... Exhaust gas purification catalyst,
2 ... carrier,
3 ... Palladium oxide (PdO) particles,
4 ... Rare earth oxide (LnOx) particles,
6 ... Exhaust gas purification catalyst,
7 ... Composite particles,

Claims (10)

A support formed from an oxide;
A large number of palladium oxide (PdO) particles having an average particle diameter of 1 nm to 50 nm supported on the carrier;
A number of rare earth oxide (LnOx) particles having an average particle size of 11 nm to 50 nm supported on the carrier in contact with the palladium oxide (PdO) particles;
An exhaust gas purifying catalyst characterized by comprising:   2. The exhaust gas purification catalyst according to claim 1, wherein an average particle diameter of each of the palladium oxide particles and the rare earth oxide (LnOx) particles is 1 nm or more and 20 nm or less.   The rare earth oxide (LnOx) particles are formed of an oxide containing at least one element selected from La, Nd, Sm, Eu, Gd, Dy, Ce, and Pr. The exhaust gas purification catalyst according to 1 or 2.   The molar ratio of the palladium oxide (PdO) to the rare earth oxide (LnOx) is 1: 1, or the ratio of the rare earth oxide (LnOx) is higher than the molar ratio. 4. The exhaust gas purification catalyst according to any one of items 1 to 3.   The molar ratio of the palladium oxide (PdO) and the rare earth oxide (LnOx) is 1: 2, or the ratio of the rare earth oxide (LnOx) is higher than the molar ratio. 5. The exhaust gas purifying catalyst according to any one of items 1 to 4.   The molar ratio between the palladium oxide (PdO) and the rare earth oxide (LnOx) is 1: 4, or the ratio of the rare earth oxide (LnOx) is higher than the molar ratio. The exhaust gas purifying catalyst according to any one of 1 to 5.   The molar ratio of the palladium oxide (PdO) to the rare earth oxide (LnOx) is 1:10, or the ratio of the rare earth oxide (LnOx) is higher than the molar ratio. The exhaust gas purification catalyst according to any one of 1 to 6.   The palladium oxide (PdO) particles and the rare earth oxide (LnOx) particles are characterized in that a composite oxide containing palladium and a rare earth element is formed in at least a part of a contact point between them. The exhaust gas purification catalyst according to any one of claims 1 to 7. The support is formed of at least one oxide selected from Al ₂ O ₃ , ZrO ₂ , SiO ₂ , TiO _2, and CeO ₂ , or two or more composite oxides. The exhaust gas purifying catalyst according to any one of 1 to 8.   An exhaust gas purification catalyst comprising a catalyst layer containing the catalyst according to any one of claims 1 to 9.

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