JPS6050491B2 - Method for producing exhaust gas purification catalyst having rare earth-containing porous film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an exhaust gas purifying catalyst having a porous film, and more specifically to a catalyst for purifying exhaust gas, and more specifically to a catalyst for cleaning harmful components in exhaust gas, particularly carbon monoxide, hydrocarbons, and nitrogen oxides emitted from internal combustion engines. This invention relates to a high-performance exhaust gas purification catalyst that aims to detoxify and remove substances by oxidizing or reducing them.

A method for forming an activated alumina coating on the surface of a monolithic base material having a large number of vent cells (hereinafter referred to as a monolith base material) is known.

For example, in Japanese Patent Publication No. 50-974, a method was proposed in which an aqueous composition was prepared from colloidal boehmite and activated alumina particles having a large specific surface area, and the composition was uniformly adhered to a monolith substrate. Further, US Pat. No. 3,264,228 describes a method in which an alumina hydrate hydrogel is dried and then redispersed in water to prepare an aqueous composition, and this composition is adhered to a monolith substrate.

Further, JP-A-54-148187 describes a method of forming an activated alumina coating on a monolith substrate using a coating liquid consisting of activated alumina containing δ-alumina and colloidal alumina containing boehmite.

In all of these methods, a water-soluble alumina hydrate such as boehmite is used as a binder in order to increase the adhesion strength of activated alumina.
Macro pores (mainly 0.1 μm or more) formed by secondary particles are covered with fine alumina particles produced by thermal decomposition of boehmite, etc., which prevents inconveniences such as inhibiting the diffusion of reaction gases. arise.

Catalysts used for internal combustion engines, particularly for automobile exhaust gas, are required to have durability that can withstand long-term use, and the physical properties of the alumina film that serves as a support in addition to the catalyst components are also important.

In exhaust gas purifying catalysts, the so-called durability includes various factors. Specifically, such factors include heat resistance, gas atmosphere resistance, and toxicity resistance. Among them, the most important factor that affects durability is the decrease in performance due to poisoning by phosphorus, lead, zinc, iron, etc. Recently, the use of lead-free products has progressed in Japan and the United States, and performance degradation due to lead poisoning has become relatively small.

However, phosphorus and zinc contained in engine oil, especially the formation of aluminum phosphate by phosphorus and the production of glassy substances by phosphate ions, clog the pores of the catalyst and inhibit the diffusion of reaction gases. The current situation is that it has not been resolved. In particular, it is known that pores with a diameter of 0.1 μm or less are easily clogged, significantly reducing catalyst performance. The purpose of the present invention is to compensate for the drawbacks of conventional catalysts by forming a porous film with a large pore volume made of activated alumina and rare earth element compounds on the surface layer of a monolith base material, thereby creating a highly durable catalyst. The goal is to provide the following.

A method of preventing alumina crystal phase transition at high temperatures by adding a rare earth metal oxide to activated alumina is known as described in Japanese Patent Application Laid-Open No. 14600/1983.

However, these conventional methods cannot sufficiently satisfy the performance required of catalysts in practical use.

It is especially used for purifying exhaust gas from internal combustion engines. "case,
durability? Problems remain in this regard. In other words, the method of adding water-soluble salts such as rare earth nitrates to activated alumina has the effect of preventing the gelatinization transition of activated alumina, but filling the voids of activated alumina with rare earth oxides reduces the Since it reduces the pore volume, it has the disadvantage of decreasing durability.

In addition, in a coating method in which rare earth compounds that are sparingly soluble in water, such as rare earth oxides, rare earth fluorides, and rare earth phosphates, are mixed with activated alumina, the aqueous composition has strong thixotropic properties. Therefore, problems such as clogging of the cells of the monolith base material and decreased peeling resistance occur.

Furthermore, when supporting a catalyst component using an aqueous solution of a platinum group salt,
There are problems such as inhibiting the ion adsorption properties of activated alumina, making it impossible to obtain sufficient catalytic performance. As a result of intensive studies to overcome the drawbacks of these conventional methods, the present inventors have found a method for forming a rare earth-containing porous film with good gas diffusivity on a monolith base material, and have completed the present invention. This is what led to this. Specifically, an aqueous composition consisting of activated alumina, a water-soluble aluminum salt, and a rare earth compound containing at least lanthanum carbonate is prepared, the resulting aqueous composition is coated on a monolithic substrate, and then dried and fired. As a result, a porous film is formed. The reason why the pore volume of the material obtained by drying and firing an aqueous composition consisting of activated alumina, a water-soluble aluminum salt, and lanthanum carbonate is large is not clear, but the reason why the aqueous composition is This is thought to be due to the formation of a network structure of lanthanum carbonate particles and activated alumina particles during drying and firing, which prevents shrinkage.

In addition to the pore volume increasing effect, the method of the present invention has the advantage that gelation does not occur even when multivalent ions are mixed into the aqueous composition.

Conventionally, when preparing an aqueous composition containing activated alumina powder, it has been common to use various alumina sols (amorphous, boehmite, pseudo-boehmite, etc.) for viscosity adjustment or as a caking component. In this case, adding polyvalent ions such as rare earth nitrates to increase the thermal stability of alumina causes gelation and loss of fluidity, making coating impossible. In this way, in the aqueous composition consisting of alumina sol and activated alumina, the contamination of multivalent ions is limited. As is well known, lanthanum is the most basic among rare earth elements, and when lanthanum carbonate is mixed and stirred with an aqueous solution of water-soluble aluminum salts such as aluminum nitrate, a sol-like liquid with moderate thixotropy is produced. There is no particular need to use such a viscous component.

As a result, polyvalent ions other than aluminum salts, such as cerium nitrate, can be mixed into the aqueous composition if necessary.

Addition of cerium salt improves catalyst performance, such as increasing carbon monoxide oxidation activity and increasing oxygen storage capacity, so it is desirable to use it in combination with lanthanum carbonate.

The catalyst produced by the method of the present invention in which a platinum group element is supported on a monolithic carrier having a rare earth-containing porous film is effective as a catalyst for purifying exhaust gas from an internal combustion engine. In particular, platinum (
Pt) and rhodium (Rh).

The catalyst thus prepared is effective as a catalyst for simultaneous treatment of carbon monoxide, hydrocarbons, and nitrogen oxides (so-called 3-way catalyst).
Monolithic substrates used in the present invention typically have a large number of penetrating cells.

A cordierite honeycomb is used, but in addition to monolithic base materials made of inorganic refractories such as α-alumina and mullite or heat-resistant metals, any material with a three-dimensional network structure can also be used. . Activated alumina includes, in addition to the commonly used γ, δ, and θ alumina, η, X1ρ alumina, and even amorphous alumina gel.

Next, examples of water-soluble aluminum salts include aluminum nitrate, aluminum chloride, and basic polyaluminum chloride. In the present invention, it is not particularly necessary to use alumina sol, but it can be added as long as the aqueous composition does not gel.

Regarding rare earth compounds other than lanthanum carbonate, both isolated rare earth and mixed rare earth can be used, but cerium is particularly effective.

They can be added by conventional methods such as impregnating and supporting activated alumina with an aqueous rare earth nitrate solution, adding them to an aqueous composition, and supporting them on a coated carrier.

Examples will be shown and explained in detail below. Example 1: 500 y of activated alumina powder with a specific surface area of 98 Iy and an average particle size of 145 μm and 120 q of lanthanum carbonate were mixed into 0.3 m011
The mixture was poured into 645q of aqueous aluminum nitrate solution, stirred and mixed, and wet-pulverized for 1 hour in a ball mill to prepare an aqueous composition for coating.

A few y of this aqueous composition was poured onto a slippery paper, and 120
After drying at ℃, the resulting coating material (Ll-containing alumina piece) was calcined at 700℃ for 2 hours. Pore distribution and pore volume were measured. The pore distribution is shown in FIG.

The pore volume was 0.67 m1Iy.

In the remaining aqueous composition, a cordierite monolith base material (30 m!nφ) having a cell number of 4 (1)/In2 was added.
5 Cym'') was immersed and pulled up, the excess liquid was blown off with compressed air, dried at 120°C, and then baked at 700°C for 2 hours.

This coated monolithic support was then immersed in an acidic solution of dinitrodiaminoplatinum nitric acid to support 35 m9 of Pt, then immersed in an aqueous rhodium chloride solution to support 4 mg of Rh, dried at 120°C, and dried at 400°C. A completed catalyst A was obtained by calcination.

Example 2 Activated alumina powder 500y with specific surface area 126 Bonoy1 average particle size 95μ and mixed rare earth carbonate not containing cerium (Rare earth element weight ratio La:Pr:Nd:Sm=26:17:
54:3) 120y was added to 630f of an aqueous solution of aluminum nitrate of 0.3r]10111, and after stirring and mixing,
An aqueous composition for coating was prepared by wet milling in a ball mill for 15A.

The pore volume of the obtained mixed rare earth-containing alumina piece obtained by drying and firing several f of this aqueous composition in the same manner as in Example 1 was measured and found to be 30.64 m1Iy.

Further, using this aqueous composition, coating and supporting operations of platinum and rhodium were performed in the same manner as in Example 1 to obtain a completed catalyst B. Example 3 Granular activated alumina carrier 2k9 with a specific surface of 98 dIy
．． Impregnated with 7m01' of cerium nitrate aqueous solution, 110
After drying at )°C and calcining at 600°C, dry pulverization was performed using a vibrating mill.

The obtained powder was alumina containing 6.5% by weight of cerium oxide and had an average particle size of 13p. 0.3n10115 of the above powder 500y and lanthanum carbonate 120y
The mixture was poured into 640 y of aluminum nitrate aqueous solution, stirred and mixed, and further wet-pulverized in a ball mill for b hours to obtain an aqueous composition for coating.

Example 1: Pour a few dabs of this aqueous composition onto the opening paper.
0Ce obtained by drying and firing in the same manner as . When the pore volume of the La-containing alumina piece was measured, it was found to be 0.65 m1.
It was 1y.

Next, in the same manner as in Example 1, a cordierite monolith base material was coated and platinum and rhodium were supported to obtain a completed catalyst C.

Example 4 500f of activated alumina powder containing 6.5% by weight of cerium oxide obtained in Example 3 and 160y of lanthanum carbonate
and commercially available amorphous fibrous alumina sol (organic acid stable type)
115y was dispersed in 620 g of a 0.2n1011' aluminum nitrate aqueous solution, and then wet milled in a ball mill for 1 hour to obtain an aqueous coating composition.

The pore volume of the coating material obtained by drying and baking several y of this aqueous composition in the same manner as in Example 1 was 0.63 m.
It was 11 years old.

Next, in the same manner as in Example 1, a cordierite monolith base material was coated and platinum and rhodium were supported to obtain a completed catalyst D.

Example 5 A completed catalyst E was obtained in the same manner as in Example 4, except that the amorphous alumina sol in Example 4 was changed to pseudo-boehmite alumina sol.

The pore volume of the coating material was 0.65 m1Iy.

Example 6 Activated alumina 500y with specific surface area 98r11Iy1 average particle size 14.5μ and lanthanum carbonate 120y 0.5n10
11f basic polyaluminum chloride [Al.

(0H)3C1. -2.4H20] It was dispersed in 650y of aqueous solution, and then subjected to wet milling in a Bore Mill for a dry time to obtain an aqueous composition for coating. Using this aqueous composition, coating and catalyzing were performed in the same manner as in Example 1,
A completed catalyst F was obtained.

The pore volume of the coating material is 0.65 mL
Ta. Example 7 The lanthanum carbonate of Example 6 was converted into a cerium-free mixed rare earth carbonate (rare earth element weight ratio La:Pr:Nd:Sm=2
Completed catalyst G was obtained by the same method as in 3 except that the catalyst was changed to 6:17:54:3).

The pore volume of the coating material was 0.63 m1If.

Example 8 A finished catalyst H was prepared in the same manner except that the basic polyaluminum chloride in Example 6 was changed to 4-aluminum chloride.
I got it.

The pore volume of the coating material was 0.64 ml.

Example 9 500 da of activated alumina powder with a specific surface area of 146 d1 g and an average particle size of 15 μm and 80 y of lanthanum carbonate were dispersed in 690 q of mixed aqueous solution containing 0.21 T of aluminum nitrate, 0.3 m of cerium nitrate, and stirred for 1 hour in a ball mill. Wet milling was performed to prepare an aqueous coating composition.

The pore volume of the coating material was measured by the same operation as in Example 1 and found to be 0.62 mUy.

Next, in the same manner as in Example 1, a monolith base material was coated, and platinum and rhodium were then supported to obtain a completed catalyst 1.

Example 10 500 g of activated alumina powder with a specific surface area of 53 d1y1 and an average particle size of 14 μ, 45 g of cerium carbonate, and 80 g of lanthanum carbonate.
g of aluminum nitrate 0.3n101nl aqueous solution 670
After dispersing and stirring the mixture in Y, wet pulverization was performed in a ball mill for 1 hour to prepare an aqueous coating composition.

Using this aqueous composition, coating and catalyticization were performed in the same manner as in Example 1 to obtain a completed catalyst J.

The pore volume of the coating material was measured and was 0.60 m.
It was 1Iy. As above, the supported amount of catalyst B-J is Pt35
m9/piece and Rh4m9/piece. Comparative Example 1 After dispersing 500y of activated alumina powder with a specific surface area of 987T1Iy1 and an average particle size of 14.5μ and commercially available amorphous fibrous alumina sol (organic acid stable type) 250' in 550y of an aluminum nitrate aqueous solution of 0.15m011', Wet milling was performed in a ball mill for 1 hour to prepare an aqueous coating composition.

Pour a small amount of this aqueous composition onto a paper, and
After drying at 700°C, it was fired for 2 hours, and the pore distribution of the obtained alumina pieces was measured by mercury porosimetry.

The results are shown in Figure 1. The pore volume was 0.48 m1Iy.

Next, a commercially available cordeurite monolith base material (30T!) having 4ω/square inch cells in the slurry was used.
After soaking and pulling up the $tφ・5 "1", blow off the excess slurry with compressed air, dry at 1200C,
It was baked at 0°C for 2 hours.

This carrier was immersed in an acidic dinitrodiaminoplatinum nitric acid solution to support platinum, and then immersed in an aqueous rhodium chloride solution to obtain a finished catalyst K containing 35 mg of platinum/piece and 4 mg/piece of rhodium. Comparative Example 2 Activated alumina powder 500y with specific surface area 126dIy1 average particle size 9.8μ mixed with rare earth nitric acid (rare earth element weight ratio La
:Ce:Pr:Nd=32:48:5:15) 120y
0.3111011e aluminum nitrate aqueous solution 63
After dispersing in 0f, wet pulverization was performed in a ball mill for 15 minutes to prepare a coating slurry.

A small amount of this slurry was taken and the pore volume was measured in the same manner as in Comparative Example 1, and it was found to be 0.45 m1Iy.

Next, coating was performed in the same manner as in Comparative Example 1 to obtain a finished catalyst L supporting platinum and rhodium.

Comparative Example 3 Cerium nitrate of 0.5m0Vf and 2.0rT1011e on 500y of granular activated alumina carrier with specific surface area of 98d1y
The sample was impregnated with a mixed solution containing lanthanum nitrate, dried at 110'C, calcined at 700C, and then dry-pulverized using a vibrating mill.

The obtained powder was prepared in 4. Cerium oxide with a weight of % and 8.2
It was an activated alumina powder containing % by weight of lanthanum oxide and had an average particle size of 15 microns. 500 of the above powders
Aluminum nitrate aqueous solution 65 with q of 0.3 m1011'
The mixture was dispersed in 0q and wet-pulverized in a ball mill for 15 hours to obtain an aqueous coating composition. A few y of this aqueous composition was poured onto a paper slip, dried and fired in the same manner as in Comparative Example 1, and the pore volume of the resulting rare earth-containing alumina piece was measured and found to be 0.42 m11y.

Next, using the above aqueous composition, coating and catalyticization were performed in the same manner as in Comparative Example 1 to obtain a finished catalyst M.

Comparative Example 4 A finished catalyst N was obtained in the same manner as in Comparative Example 2, except that the mixed rare earth nitric acid was changed to lanthanum nitrate.

The pore volume of the coating material was 0.46 mtIy. Comparative Example 5 The mixed rare earth nitric acid of Comparative Example 2 was mixed with the mixed rare earth fluoride (rare earth element weight: a:Ce:Pr:Nd=32:51:5:12
) Completed catalyst 0 was obtained in the same manner except that the catalyst was changed to

The pore volume of the coating material was 0.41 m1Iy.

Comparative Example 6 120g of mixed rare earth nitric acid of Comparative Example 2 was mixed with rare earth oxide (rare earth element weight ratio L3:Ce:Pr:Nd=32:48:5:
15) Completed catalyst P was obtained in the same manner except that 609 was used.

The pore volume of the coating material was 0.53 m11y.

Comparative Example 7 A finished catalyst Q was obtained in the same manner as in Comparative Example 2, except that the mixed rare earth nitric acid was replaced with cerium carbonate.

The pore volume of the coating material was 0.46 m1If1. As mentioned above, the supported amount of catalyst L-Q is Pt35m9/piece, R
It was h4m9/piece.

Example 11 An accelerated deterioration test was conducted on the completed catalysts A-Q of Examples 1 to 1 and Comparative Examples 1 to 7, and the purification rate was measured.

The accelerated deterioration durability test was conducted by connecting a multi-converter filled with test catalyst concentrically to the exhaust system of a 1.8e electronic fuel injection four-cylinder engine, and testing the durability for 10 (7) hours under stoichiometric air-fuel ratio operating conditions. A test was conducted.

The catalyst bed inlet gas temperature was 680°C to 720°C.

Fuel: 0.01yIU of lead in commercially available unleaded gasoline. S. G.
, phosphorus 0.03yIU. S. G. Used gasoline with added . The purification rate was measured by operating the same 1.8f 4-cylinder engine at the stoichiometric air-fuel ratio, and setting the catalyst bed inlet gas temperature to 50%.
I set the rotation speed, boost, etc. so that the temperature was 0'C.

The results are shown in Table 1.

Claims (1)

[Claims] 1. An aqueous composition comprising activated alumina, a water-soluble aluminum salt, and a rare earth compound containing at least lanthanum carbonate is prepared, the resulting aqueous composition is coated on a monolithic structure substrate, and the base material is then dried and fired. A method for producing a catalyst for exhaust gas purification, which comprises forming a rare earth-containing porous film on a material to serve as a catalyst carrier, and further supporting at least one catalyst metal selected from platinum group elements on the carrier.