JPS6384636A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To enhance the catalytic activity and durability of the title catalyst by depositing at least one kind between platinum and rhodium in an alumina coating layer contg. praseodymium, cerium, and activated alumina to form the catalyst for purifying exhaust gas. CONSTITUTION:The activated alumina carrier is impregnated with a specified amt. of an aq. soln. of cerium nitrate contg. praseodymium, dried, and then baked at 600-650 deg.C for 1.5-2hr in the air to obtain activated alumina deposited with praseodymium and ceria. The activated alumina and ceria powder carrying praseodymium are mixed and crushed in a nitric acid-acidified boehmite alumina sol to obtain a slurry. The obtained slurry is coated on the surface of a cordierite monolithic carrier base material. The material is dried, and then baked in the air at 650-850 deg.C to obtain a catalyst carrier. The catalyst carrier is dipped in an aq. soln. of a platinum salt and a rhodium salt to deposit >=1 kind between platinum and rhodium on the carrier.

Description

[Detailed Description of the Invention] (Industrial Application Field) Hydrocarbons (HC), -carbon oxides (CO), and nitrogen oxides (NOx), which are harmful components in exhaust gas emitted from internal combustion engines such as automobiles, are This invention relates to an efficient exhaust gas purification catalyst.

(Conventional technology) Conventionally, HC and G in exhaust gas emitted from internal combustion engines.

Various exhaust gas purifying catalysts for purifying NOx and NOx have been proposed, and among them, heat resistance is significantly improved when an appropriate amount of cerium is added to activated alumina. =159391
As disclosed in the publication, activated alumina powder containing cerium in advance is attached to the surface of a monolithic carrier base material, and then a catalytic metal such as platinum, rhodium, palladium, etc. alone or in combination is supported. catalysts have been proposed.

(Problems to be Solved by the Invention) However, in such conventional exhaust gas purification catalysts, cerium supported on activated alumina causes crystal growth due to heat, resulting in the stability of cerium oxide (ceria). It takes on a crystal structure and reduces the oxygen adsorption/desorption ability, that is, the oxygen (0□) storage ability. In addition, platinum, rhodium, etc., which exhibit high activity when coexisting with ceria, lose their interaction with ceria as ceria crystals grow, resulting in sintering of the precious metals that are catalyst components, resulting in a decrease in activity. There was a problem with letting it work.

(Means for solving the problem) The inventor confirmed that by adding praseodymium to ceria, the crystal growth of ceria due to heat is suppressed, and the non-stoichiometry of ceria is maintained and stabilized. This invention was achieved based on the discovery that by using an alumina coat layer containing activated alumina, it is possible to maintain high heat resistance and 0□ storage capacity of the alumina coat layer itself, and prevent a decrease in catalyst activity. I ended up doing it.

That is, the present invention relates to an exhaust gas purifying catalyst comprising at least one of platinum and rhodium supported in an alumina coat layer containing praseodymium, cerium, and activated alumina.

Next, a method for producing the catalyst of the present invention will be explained. First, an activated alumina carrier is impregnated with a predetermined amount of an aqueous solution of ceum nitrate containing praseodymium by a dipping method, and after drying, it is soaked in air for 6 hours.
The activated alumina carrying praseodymium and ceria is obtained by firing at 00 to 650°C for 1.5 to 2 hours. Next, a slurry obtained by mixing and pulverizing the above-mentioned activated alumina and the ceria powder supporting praseodymium obtained by calcining a cerium carbonate or oxalate containing praseodymium in an air stream is obtained. of,
Apply to the surface of the cordierite monolithic carrier base material. After completion of drying, the catalyst carrier is obtained by firing at 650°C to 850°C in an air atmosphere. At least one of platinum and rhodium is supported on the obtained catalyst carrier by a dipping method using an aqueous solution of platinum salt and rhodium salt, and after drying, it is heated in a combustion gas stream for 5
Bake at 50°C to 750°C for 0.5 to 2 hours. Note that it is desirable to use a temperature raising and slow cooling pattern for firing.

(Function) Activated alumina, which generally has a δ-alumina structure or a γ-alumina structure, is known as a catalyst carrier, but at high temperatures it transforms into inactive and stable α-alumina, making it a catalyst carrier. This would be inconvenient. However, it was confirmed that adding an appropriate amount of cerium to activated alumina significantly improves heat resistance, and that the effect increases even more in the presence of praseodymium. This will reduce the high specific surface area of the activated alumina itself, which is disadvantageous from the standpoint of noble metal dispersibility. In this invention, by impregnating and supporting cerium containing praseodymium at a ratio of 3 to 5% by weight to activated alumina, the reduction in specific surface area is minimized and heat resistance is improved. Cerium oxide is usually almost entirely an oxide of tetravalent cerium, and generally Ce
O.

It is expressed as CeO□ itself has a 0□ storage effect and has the effect of expanding the response atmosphere to the atmosphere to which the catalyst is exposed.
≦0.5), which is a so-called non-stoichiometric oxide state, is known to be effective. However, at high temperatures, ceria undergoes crystal growth and becomes a stable tetravalent state.

Furthermore, since cerium has a smaller specific veion radius than, for example, lanthanum, it was thought that it was unlikely that other metals would be incorporated into its crystal lattice to form non-stoichiometric composite oxides. The present invention focused on this point, and as a result of studies, it was discovered that in the presence of a small amount of praseodymium (Pr), ceria's crystal growth is suppressed and the so-called non-stoichiometric property is maintained. Figure 1 shows CeO□ and CeO
C when 1.5% by weight, 4.6% by weight, and 9.2% by weight of praseodymium was added to the total rare earth oxide as an oxide in □ and held at each treatment temperature for 3 hours.
A diagram showing the measurement results of the lattice constant (person) of eO, where the curve l
is high purity CeO□, curve 2 is CeO□ + 1.5% by weight
Curve 3 shows the measurement results for CeO2+ 4.6% by weight Pr, and curve 4 shows the measurement results for CeO2+ 9.2% Pr. It is thought that as the lattice constant increases, the movement of lattice oxygen becomes smoother, and the 0□ storage capacity increases. The amount of praseodymium included in cerium was found to be preferably 1.5% by weight or more of the total rare earth oxide as an oxide as a result of an exhaust gas purification performance experiment conducted by the present inventor. Furthermore, even if the amount of praseodymium is increased to more than 9.2% by weight, the exhaust gas purification performance will hardly change, but the cost will be high. The content is preferably 2% by weight or less. As a result, the catalyst component dispersed and supported on the ceria crystals can be effectively utilized even under high temperature conditions such as in automobile exhaust gas, which has the effect of reducing the amount of precious metals.

In this invention, cerium and praseodymium are made to coexist in an ionic state, and then converted into an oxide, thereby maintaining the non-stoichiometric property of ceria.

That is, activated alumina is impregnated with an aqueous cerium nitrate solution containing praseodymium, and then dried and fired. The firing conditions at this time are 600 DEG C. to 650 DEG C. in an air stream for 2 hours without using a temperature raising step. Further, the oxide powder is heated to 600°C at a heating rate of 6°C/mi+ using a heating step in an air stream using carbonate or oxalate,
It is obtained by firing at this temperature for, for example, 2 hours. The obtained activated alumina supporting cerium containing praseodymium and ceria powder containing praseodymium are placed in a porcelain ball millbot together with a nitric acidic boehmite alumina sol, and mixed and pulverized to obtain a slurry. Next, a predetermined amount of coating is applied to the cordierite monolithic carrier in multiple batches, and after drying, it is placed in an air stream or a combustion gas stream at 600°C to 750°C, preferably 650°C to
A pattern that includes a heating step of holding the same temperature at 700°C for less than 2 hours. For example, 600°C at a heating rate of 6°C/min.
The coated carrier is obtained by heating to a predetermined temperature in the range of ~750°C, firing at this temperature for a required time of 2 hours or less, and firing in a pattern of decreasing temperature. then platinum salt solution or mouth,
The coating carrier is immersed in a dium salt solution or a mixed solution of a platinum salt solution and a rhodium salt solution to support a predetermined amount of at least one of platinum and rhodium. The catalyst is fired at 6501° C. in a pattern including a heating step of maintaining the same temperature for up to 2 hours. The catalyst prepared in this way exhibits stability and catalytic activity even in a wide exhaust gas atmosphere due to the 0□ storage effect of ceria that maintains non-stoichiometry in the coating layer, and the heat resistance of ceria. As a result, sintering of the precious metal is suppressed, and the durability of the catalyst can be improved.

(Examples) The present invention will be explained below with reference to Examples, Comparative Examples, and Trial and Experimental Examples.

A nitrate aqueous solution of cerium nitrate containing 0.07 weight to 0.2 weight percent of praseodymium on a metal basis was prepared using the immersion method.
3.0% by weight of cerium metal was supported on the activated alumina carrier. The activated alumina used at this time has a γ- or δ-alumina structure, and has a specific surface area of 1 according to the B, E, T method.
80 m"/g or more. The amount of the aqueous solution was set to be enough to stir even after the alumina powder was added, and after stirring for 10 minutes or more, it was dried at 9150°C or higher in an open air stream, and
Cerium-supported alumina powder was obtained by firing at 0° C. for 2 hours in a pattern that did not include a heating step of maintaining the same temperature. Next, praseodymium is 2.5% to 7% in terms of metal.
．． Ceria powder containing praseodymium was obtained by firing cerium oxalate containing 0 weight percent in an air atmosphere at 600° C. in a pattern including a heating step of maintaining the same temperature for less than 2 hours. The ceria powder obtained here is approximately 35
It has a specific surface area of 7g, and the crystallite diameter determined from the ceria crystal (LLI) plane represented by the X-ray diffraction peak is 2.
It was less than 50 Å.

1,188 g of the cerium-supported alumina powder obtained above, 590 g of ceria powder, and 2,222 g of nitric acid acidic alumina sol (a sol obtained by adding a 10% by weight aqueous nitric acid solution to a 10% by weight suspension of boehmite alumina) were placed in a porcelain ball mill. The mixture was mixed and pulverized to obtain a slurry liquid. Using this slurry liquid, a cordierite monolithic carrier (1, 71,
400 cells) was coated in multiple batches, dried, and then baked in a flow of air or a flow of combustion gas in a pattern that included a heating step of holding the same temperature at 650°C for 2 hours. A carrier was obtained. The coating weight at this time was set at 340 g/piece.

Using a mixed solution of dinitrodiammine platinum nitric acid solution and rhodium nitrate on the obtained coated carrier, impregnation was carried out to give 1.91 g of platinum per catalyst and 1.91 g of rhodium per catalyst.
After supporting 0.191 g per piece, it was dried and fired in a pattern including a heating step of maintaining the same temperature at 600° C. for less than 2 hours in a combustion gas stream to obtain catalyst 1. The resulting catalyst contained 68.2 g/l of ceria containing praseodymium with 200 g/l of alumina coat layer! , platinum 1.124g/I! , containing 0.112 g/l of rhodium.

Catalyst 2 was obtained in the same manner as in Example 1, except that 0.772 g of platinum per catalyst and 0.129 g of rhodium per catalyst were supported on the obtained coated carrier. The obtained catalyst 2 has an alumina coating layer of 200 g/l.
, ceria including praseodymium 68.2 g/l
, platinum 0.454g/It, rhodium 0.076g/It
It contained l.

Ceria-supported alumina powder 1 obtained in Example 1
A coated carrier was obtained in the same manner except that 308 g/l, 384 g of ceria powder, and 2,308 g of nitric acidic alumina sol were charged into the ball mill. The amount of alumina coating at this time is 3
The amount was set at 40g/piece. Using a dinitrodiammine platinum nitric acid solution on the obtained coated carrier, 2 platinum was added per liter of catalyst.
．． A catalyst 3 was obtained by supporting 10 g in the same manner. The obtained catalyst 3 had an alumina coat layer of 200 g/l and a serif containing praseodymium of 41.8 g/l! ! , containing 1.236 g/l of platinum.

In Example 3, rhodium was added at 0.901% per catalyst.
Catalyst 4 was obtained in the same manner except that g was supported.

The obtained catalyst 4 had an alumina coat layer of 200 g/I! ,
It contained 41.8 g/β of ceria including praseodymium and 0.530 g/l of rhodium.

In one coal mine Example 1, ceria supported alumina powder 1348g
, ceria powder 259g, nitric acid acidic alumina sol 239g
A coated carrier was obtained in the same manner except that 3 g was put into a ball mill. Platinum was added to the obtained coated carrier at a rate of 1 per catalyst.
．． Catalyst 5 was obtained in the same manner except that 0.191 g of rhodium was supported per catalyst.

The obtained catalyst 5 had an alumina coat layer of 200 g/f, a serif containing praseodymium of 29.5 g/l, and a platinum layer of 1.
124g/I! , Rhodium 0.112g/I! It contained.

In practical example 5, platinum was added at 0.772 g per catalyst;
Catalyst 6 was obtained in the same manner except that 0.129 g of rhodium was supported per catalyst. The obtained catalyst 6 had an alumina coat layer of 200 g/j! It contained 29.5 g/l of ceria, including praseodymium, 0.454 g/J of platinum, and 0.076 g/l of rhodium.

Comparison of flame activated alumina powder carrier 1437.0g and alumina sol (
10% by weight HN in a 10% by weight suspension of boehmite alumina
Sol obtained by adding O, 2563.
After filling 0g into a bot mill and grinding for 6 hours, the resulting slurry was attached to an integrated carrier (1,71,400 cells) mainly composed of cordierite, and heated at 650°C for 2 hours.
Baked for an hour. The amount of adhesion at this time was set at 340 g/piece.

Next, this alumina-attached carrier was immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride, and the amount of platinum and rhodium deposited was 1.91 g of platinum and 0.191 g of rhodium. Catalyst A was obtained by firing for a period of time.

The obtained catalyst A had an alumina coat layer of 200 g/12,
Platinum L124g //! , containing 0.112 g/e of rhodium.

This 1 utl 1437.0g of activated alumina powder carrier carrying 5% by weight of cerium in terms of cerium metal and 256g of alumina sol
3. Catalyst B was obtained in the same manner as Comparative Example 1 except that Og was used. The obtained catalyst B had an alumina coat layer of 200 g/
A', Celia 11. Og/II, platinum 1.124g
/l, and rhodium 0.112g/l.

454.3g of activated alumina powder carrier supporting 50% by weight of cerium in terms of cerium metal and 25g of alumina sol
Catalyst C was obtained in the same manner as Comparative Example 1 except that 63 g was used. The obtained catalyst C had an alumina coating amount of 200 g/
l, ceria 76.3g/e, platinum 1.124g/
f, containing 0.112 g/l of rhodium.

According to the method described in Japanese Patent Application Laid-Open No. 52-116779, 2563 g of silica gel was impregnated into an activated alumina powder carrier with an aqueous cerium nitrate solution, dried, and then heated in an air stream for 60 minutes.
After firing at 00°C for 11.5 hours, 1437 g of a granular carrier supporting 3% by weight of cerium in terms of metal was placed in a ball mill, mixed and pulverized for 6 hours, and then processed into a cordierite carrier base material (1.7 jl!, 400 g). After drying, it was baked at 650°C for 2 hours. The amount of adhesion at this time was set at 340 g/piece. Furthermore, this carrier was immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride, and H2/
Reduced in a stream of N2. Thereafter, the mixture was calcined at 600° C. for 2 hours to obtain a catalyst. This catalyst contained an alumina coat layer of 200 g/ff, ceria 2.66 g/β, and platinum 1.
It contained 124 g/l and 0.112 g/l of rhodium.

Comparison 1 2563 g of alumina sol and activated alumina powder carrier 1 were prepared according to the method described in JP-A-54-159391.
437 g was put into a ball mill, mixed and pulverized for 6 hours, then coated on a cordierite carrier base material (1.7 L 400 cells), dried, and then fired at 650° C. for 2 hours. The coating amount at this time was set to 340 g/piece. Next, 28 g of cerium (metal equivalent) was deposited using Ce(NOz)+aqueous solution. Thereafter, it was dried at 120°C for 3 hours and fired in air at 600°C for 2 hours. Further, after this, the catalyst was immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride to support platinum at 1.91 g/piece and rhodium at 0.191 g/piece, and then calcined to obtain catalyst E. The obtained catalyst E had an alumina coat layer of 200 g/l and a serif layer of 20.2 g/7! , rhodium 0.112g/l
It contained.

Catalysts 1 to 6 obtained in Examples 1 to 6 and catalysts A to H obtained in Comparative Examples 1 to 5 were subjected to actual vehicle durability (engine durability) under the following conditions, and the purification rate of 10 mode emission was determined. was measured and shown in Table 1 as the purification rate, 5-2-舊=.

Wang Yu/) Etsuyoshi J1 raw catalyst integrated noble metal catalyst catalyst outlet temperature
750℃ Space velocity approx. 70,000 Hr Durability time 100 hours Engine Exhaust TT2200cc Fuel
Unleaded gasoline durable intermediate loading Roemifusion Go 0.4-0.6%0
□ 0.5±0.1% NO1001000 pp 2500ppm co214.9±0.1% Displacement 2000cc axle table 1 (Effects of the invention) As explained above, according to the catalyst of this invention, By including praseodymium, the non-stoichiometry of ceria is maintained and the growth of crystals due to heat is suppressed, so performance deterioration mainly caused by sintering of precious metal components is suppressed, and the 02 storage effect of ceria is suppressed. By being effective, the precious metal components can be used effectively, and the effect of having a wide purification area, high durability, and reducing catalyst cost can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a curve diagram showing changes in the amount of praseodymium added and the lattice constant of ceria.

Claims (1)

[Claims] 1. An exhaust gas purifying catalyst comprising at least one of platinum and rhodium supported on an alumina coat layer containing praseodymium, cerium, and activated alumina.