JPS63205141A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To efficiently purify HC, CO and NOx, by forming an exhaust gas purifying catalyst by supporting Pt or Pt and Rh being a catalytically active metal by the first coating layer of a carrier and supporting Pd and Rh by the second coating layer thereof. CONSTITUTION:An activated alumina carrier and cerium oxide are mixed with a nitric acid acidic bohemite alumina sol and the resulting mixture is ground to obtain a slurry which is, in turn, applied to the surface of a monolithic carrier base material based on cordierite. After drying, the coated base material is baked at 650-850 deg.C and, subsequently, Pt or Pt and Rh is supported by the carrier and the supported carrier is baked at 550-750 deg.C to form the first coating layer. Next, the second coating layer, wherein pores having a pore size peak of 500-1,000Angstrom are imparted to a coating layer consisting of activated alumina having a pore size peak of 100Angstrom or less, rare earth metal oxide and zirconium oxide and Pd and Rh are supported by said coating layer, is formed on the first coating layer.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention is directed to the treatment of hydrocarbons (HC) and -carbon oxides (C
O), relates to an exhaust gas purifying catalyst that efficiently purifies nitrogen oxides (NOx).

(Prior Art) Many exhaust gas purification catalysts have been proposed to purify HC, Co, and NOx in exhaust gas, and among them, heat resistance is significantly improved when an appropriate amount of cerium is added to activated alumina. For example, JP-A-52-11677
As disclosed in Japanese Patent Application Laid-open No. 54-159391, after adhering activated alumina powder containing cerium to the surface of a monolithic carrier base material, platinum, rhodium, palladium, etc. Catalysts have been proposed in which catalytic metals, singly or in combination, are supported.

(Problems to be Solved by the Invention) However, in such conventional exhaust gas purifying catalysts, the diffusion of reaction gas is controlled by the pores of activated alumina. In general, the pores of activated alumina are micropores, so that gases such as hydrocarbons with large molecular diameters do not diffuse sufficiently.

Therefore, there is a problem in that the active sites cannot be fully utilized and the amount of noble metal has to be increased to cope with the problem, resulting in an increase in the cost of the catalyst.

(Means for Solving the Problems) The exhaust gas purifying catalyst of the present invention, which solves these conventional problems, comprises activated alumina and rare earth metal oxides having a pore diameter peak of 100 Å or less on a carrier base material. Preferably, a coating layer consisting of chloroplatinic acid or chloroplatinic acid and rhodium chloride is used, and a first coating layer on which platinum or platinum and rhodium is supported;
Pores having a pore size peak of 500 to 1000 people are imparted to a coating layer consisting of activated alumina, rare earth metal oxide, and zirconium oxide, each having a diameter of 500 to 1000 μm. It is characterized by comprising a supported second coat layer.

This invention will be explained below.

The catalyst of the present invention includes a first coat layer on a carrier base material, which is formed from activated alumina and a rare earth metal oxide having a pore size peak of 100 Å or less, and supports platinum or platinum and rhodium. This coat layer can be provided by the following method. That is, an aqueous solution of one or several mixed nitrates of rare earth metals, such as cerium, lanthanum, neodymium, praseodymium, etc., is used on an activated alumina granular or powdered carrier having a pore size peak (maximum frequency diameter) of 10.0 Å or less. After supporting a predetermined amount by a dipping method or the like and drying, for example, at 600 to 650°C in an air stream, 1.5
Calcinate for ~2 hours to obtain activated alumina with micropores of less than 100 Å containing rare earth oxides.

A slurry obtained by mixing and pulverizing the above-mentioned activated alumina carrier and cerium oxide (CeO□=ceria) with a nitric acidic boehmite alumina sol is applied to the surface of a monolith carrier base material mainly composed of cordierite. After drying, the coated carrier is obtained by firing at 650 to 850°C in an air atmosphere, for example. Platinum or platinum and rhodium is supported on the obtained coating carrier by a dipping method using chloroplatinic acid or a mixed aqueous solution of chloroplatinic acid and rhodium chloride, and after drying, for example, in a combustion gas stream, ~7
The catalyst is calcined at a temperature such as 50° C. for 0.5 to 2 hours.

Next, in the catalyst of the present invention, a coating layer consisting of activated alumina, rare earth metal oxide, and zirconium oxide with a pore size peak of 100 Å or less is coated on the first coat layer. A second coat layer is provided with holes and supports palladium and rhodium.

This coat layer can also be formed by the following method.

That is, activated alumina having a pore size peak of 100 Å or less is loaded with a predetermined amount of rare earth metal by a dipping method using the rare earth metal nitrate solution, and then dried and fired to obtain activated alumina containing rare earth metal oxide. Obtain a carrier. A slurry obtained by mixing and pulverizing the obtained activated alumina carrier, lanthanum hydroxide powder, zirconium oxide powder, and carbon black with nitric acidic boehmite alumina sol is applied to the catalyst. After drying, for example, the temperature is raised from room temperature in an air stream and baked at 650 to 850°C for 2 hours, followed by a predetermined amount using a dipping method or the like using a mixed solution of dinitrodiammine palladium nitric acid solution and rhodium nitrate solution. palladium or rhodium is supported, and after drying, it is calcined in a combustion gas stream at a temperature of 550 to 750°C for 0.5 to 2 hours to obtain a catalyst. In addition, the firing is
It is desirable to use a heating and slow cooling pattern.

It is known that when activated alumina such as γ-alumina and δ-alumina is supported with a rare earth metal oxide, the heat resistance of the activated alumina is significantly improved.

In particular, cerium, lanthanum, praseodymium, neodymium,
Yztrium etc. has a remarkable effect, and the α of activated alumina
- At the same time as suppressing the change to alumina, the loss of micropores becomes less likely to occur. As a result, a high specific surface area is maintained,
High dispersion of precious metals will be maintained. If the amount of rare earth metal supported on activated alumina is less than 1% by weight relative to the alumina in terms of metal, the effect of improving heat resistance will be small, and if it exceeds 5% by weight, heat resistance will improve, but relatively This is undesirable because it lowers the specific surface area and at the same time particularly reduces the micropores of the alumina.

Therefore, the amount of rare earth metal supported is preferably 1 to 5% by weight, preferably 2 to 3% by weight relative to alumina in terms of metal.

In the present invention, one or more rare earth metal oxides are further added to the catalyst carrier for the purpose of imparting the oxygen (0,) storage effect and hydrogen (H2) activation effect of the rare earth metal oxides. In addition, rhodium reacts with activated alumina to form rhodium aluminate.
Zirconium oxide is added to prevent the formation of complexes and deactivation. As a result, the high specific surface area of activated alumina, the O2 storage effect and Hz activation effect of rare earth metal oxides, and the effective utilization of rhodium by zirconium oxide have a large effect on improving the wandering gas purification performance of the catalyst. In particular, even when the automobile exhaust gas atmosphere becomes rich (excessive fuel side), stable high purification performance is exhibited due to the Ot storage effect of rare earth metal oxides. In addition, due to the H2 activation effect of the rare earth metal oxide, it exhibits high NOx purification performance, especially in a lean atmosphere (air excess side) where the catalyst temperature is high. The rare earth metal oxide powder to be mixed with activated alumina has a metal equivalent (for example, as cerium) of 5.
Even if the amount is greater than 0% by weight, there is almost no performance improvement effect, and if it is less than 5% by weight, the 02 storage effect and H2 activation effect are insufficient compared to the performance required by the inventor. It is desirable to set it in the range of 50% to 50%. The Ot storage effect is a property of oxides such as yttrium, cerium, praseodymium, and terbium, and the H2 activation effect is a property of oxides such as lanthanum, neodymium, and samarium. In addition, zirconium oxide used for the purpose of effectively utilizing rhodium is 20% by weight in terms of metal.
Even if the amount is larger, there is almost no performance improvement effect, and if it is less than 3% by weight, the effect on stabilizing rhodium is insufficient, so it is desirable to keep it in the range of 3 to 20% by weight. .

The catalyst of the present invention provides a coated carrier having the above-mentioned effects with pores particularly effective for sufficient diffusion of hydrocarbons (IC) within the coated layer. This provision of pores is preferably carried out using carbon black. The pores created by mixing carbon black with a slurry and firing are controlled by the type and amount of carbon black used.

When using other carbonaceous substances instead of carbon black, solid carbonaceous substances are usually physically difficult to pulverize, and the resulting pores are usually much larger than expected. Put it away. Furthermore, if an amount of 10% by weight or more is added to the slurry, the strength of the resulting coating layer will naturally weaken, making it unusable for practical use.
Furthermore, temperature control when removing such a large amount of carbon by firing becomes extremely difficult. On the other hand, carbon black is an extremely fine and uniform powder, and is particularly characterized by its fine structure. That is, carbon black does not consist of a single, simple sphere, but individual particles aggregate to form a large chain-like higher-order structure (strafture). This is thought to be the reason why carbon black is added and then removed by firing, which not only creates a large number of pores in a very narrow range at desired positions, but also maintains high strength despite the addition of a large amount of carbon black. . Therefore, the pore size obtained is determined by the particle size and amount of carbon black added, as well as the size of the stractures. The pore size that is said to contribute to the diffusion of hydrocarbons, which is the object of this invention, is 500 to 1000, as is also being considered as a petrochemical catalyst carrier.
By forming it in the alumina coat layer rather than in the alumina itself, platinum and platinum and rhodium, which are uniformly dispersed and supported in the coat layer, can be used more effectively. When using carbon black, ASTM N
Carbon blacks defined by o, N-550, and N-660 are preferred, and the addition (i) is from 10% to 30% by weight relative to the slurry.

If the amount added is less than 10% by weight, pores cannot be continuously formed from the surface of the coating layer to the inner layer, so the purpose cannot be fully achieved, and if the amount is more than 30%, the continuous generation of pores will be Although it is sufficient, the increase in temperature of the coat layer during baking removal becomes uncontrollable, which is inconvenient, so the addition amount is set as above.

As described above, this is a catalyst in which platinum or platinum and rhodium, which are catalytically active metals, are supported on the first coat layer of the carrier, and palladium and rhodium are supported on the second coat layer. As already mentioned, in order to make effective use of precious metal components, it is generally necessary to make them highly dispersed and have more active sites. Rather, it is advantageous to disperse it at a high concentration on the surface of the catalyst. Therefore, the inside of the coating layer corresponds to the part where the exhaust gas components are controlled by diffusion rate, and in order to support the exhaust gas components in a more highly dispersed manner, platinum or platinum or rhodium is dispersed and supported using chloroplatinic acid and rhodium chloride. There is. In addition, dinitrodiammine palladium and rhodium nitrate are used in the surface layer, which is the second coat layer, and are supported within a range of several micrometers to enhance low-temperature activity. This has been confirmed by spot measurement using fluorescent X-ray analysis.

As mentioned above, platinum is supported inside the coating layer using chloroplatinic acid. Although chloroplatinic acid is supported almost uniformly on the activated alumina coating layer, it is disadvantageous for initial low-temperature activation. Excellent durability.

On the other hand, dinitrodiammine palladium supported on the second coating layer has excellent initial low temperature activity, but is disadvantageous in durability. Therefore, by selecting such a combination,
It takes advantage of the characteristics of both.

The reason why dinitrodiammine palladium is supported on the surface of the coating layer is not fully understood, but because dinitrodiammine palladium is a complex, its molecules are large and it does not diffuse sufficiently into the micropores of alumina.
It is thought that it is supported on the surface layer because it is difficult to internally diffuse from the viewpoint of pH theory.

Further, the reason why platinum is supported separately in a layer containing cerium and palladium is carried separately in a layer containing lanthanum is as follows.

Platinum has high oxidizing activity when it coexists with cerium oxide. This is because the oxygen released from cerium oxide forms an ion bond with platinum atoms, resulting in an active state and carbon monoxide contained in exhaust gas, It is thought that this is to increase reactivity with hydrocarbons. On the other hand, in the presence of oxygen, palladium undergoes dissociative reoxidation (PdO≠Pd) at around 780°C, and at this time, particles grow. It is also said that when it coexists with platinum, this dissociation and reoxidation temperature is lowered.

However, when palladium coexists with lanthanum, La-
As a result of creating a composite called 0-Pd and having atmospheric stability,
Maintain high catalytic activity.

(Experimental Examples) The present invention will be explained in detail using the following Examples, Comparative Examples, and Test Examples.

An activated alumina support mainly composed of T- or δ-alumina with a pore size peak of 80 to 100 is impregnated with an aqueous cerium nitrate solution, dried, and then heated at 600°C in an air stream for 2 hours.
After firing for a time, the amount of cerium compared to alumina was 3.
An activated alumina support containing 0% by weight was obtained. The peak pore diameter of this activated alumina was 80 to 90 as a result of measuring the pore distribution using a mercury intrusion porosimeter. Next, nitric acid acidic boehmite sol (boehmite alumina 10% by weight)
2478 g of sol obtained by adding 10% by weight HNO3 to the suspension, the above activated alumina carrier 10
06g of cerium oxide powder and 516g of cerium oxide powder were placed in a ball millbot and ground for 8 hours to obtain a slurry. The obtained slurry was applied to a monolithic carrier substrate (1, M! 400 cells), dried at 100 to 130°C for 1 hour, and then heated to 65°C.
Baked for 2 hours at 0"C.The amount of coating in this case was 220
g/piece. Furthermore, 0.955 g of platinum per carrier was added to this carrier using an aqueous solution of chloroplatinic acid.
After quick drying, it was fired at 600°C for 1 hour in a combustion gas atmosphere to obtain a catalyst carrier. Next, the activated alumina carrier was impregnated with an aqueous lanthanum nitrate solution, dried, and then placed in an air stream for 6 hours.
It was fired at 00° C. for 2 hours to obtain an activated alumina carrier containing 3.0% by weight of lanthanum in terms of metal based on alumina. Next, 2478 g of nitric acidic boehmite sol, 823 g of the above activated alumina, 394 g of lanthanum hydroxide powder, 282 g of zirconium oxide powder, and ASTM N11LN-55
600g of carbon black specified as
After coating the catalyst carrier and drying at 100 to 130°C for 1 hour, the temperature is raised from room temperature at a rate of 1O″C/min,
It was baked at 650°C for 2 hours. The amount of coating in this case is 12
It was set to 0g/piece. Furthermore, this carrier was impregnated with 0.955 g of palladium and 0.191 g of rhodium per carrier using a mixed aqueous solution of an acidic dinitrodiammine palladium nitric acid solution and a rhodium nitrate solution, and then rapidly dried using a microwave dryer. Thereafter, it was calcined at 600°C for 1 hour in a combustion gas atmosphere to obtain catalyst 1. This catalyst has 0.559g//l of platinum! , palladium 0.559
g/ffi.

Rhodium 0.112g#! , Cerium 32.4 g/
l, lanthanum 12.7 g/l, zirconium 8
．． 5g //! Contains. Furthermore, the pores imparted by carbon black have a pore diameter peak of 500.

A catalyst 2 was obtained in the same manner as in Example 1 except that the carbon black mixed in the slurry was ASTM No. N-660. The pores imparted by the carbon black of this catalyst have a pore diameter peak of 700.

1 piece of sword 1 Catalyst 3 was obtained in the same manner as in Example 1, except that the amount of carbon black mixed into the slurry was changed to 800 g. The pores of this catalyst provided by the carbon blank have a pore diameter peak of 800.

Catalyst 4 was obtained in the same manner as in Example 2 except that the amount of carbon black mixed into the slurry was changed to 800 g. The pores provided by the carbon black of this catalyst have a pore diameter peak of 1000.

In construction example 1, activated alumina 100 containing cerium
6 g, 2478 g of nitric acidic boehmite sol, and 516 g of cerium oxide were placed in a ball mill bot and ground for 8 hours. The resulting slurry was applied to a monolithic carrier base material, dried,
A catalyst carrier was prepared in the same manner except that 0.955 g of platinum and 0.0955 g of rhodium were impregnated and supported on the coated carrier obtained by firing using a mixed aqueous solution of a chloroplatinic acid solution and a rhodium chloride solution. Obtained. A coating layer containing lanthanum and zirconia was applied to this catalyst carrier in the same manner as in Example 1, and after drying and baking, 0.955 g/piece of palladium and 0.0955 g/piece of rhodium were added to a dinitrodiammine palladium nitric acid solution and a nitric acid solution. Catalyst 5 was obtained in the same manner except that a mixed aqueous solution of rhodium was used and supported by impregnation. This catalyst contains 0.559 g/j of platinum! , palladium 0.559g
/f, rhodium 0.112g/ffi, cerium 32.
4g/l.

Lantern 12.7 g/1. , zirconium 8.5
g#! Contains. Furthermore, the pores imparted by carbon black have a pore diameter peak of 500.

1. An activated alumina support mainly composed of γ- or δ-alumina with a pore size peak of 80 to 100 pores was impregnated with an aqueous cerium nitrate solution, dried, and then heated at 600°C in an air stream for 2 hours. After firing, the amount of cerium compared to alumina is 3 in terms of metal.
．． An activated alumina carrier containing 0 weight percent was obtained. The peak pore diameter of this activated alumina was 80 to 90 as a result of measuring the pore distribution using a mercury intrusion porosimeter. Next, nitric acid acidic boehmite sol (boehmite alumina 10%
2478 g of the above-mentioned activated alumina carrier 1
006g of cerium oxide powder and 516g of cerium oxide powder were put into a ball millbot and ground for 8 hours to obtain a slurry. The obtained slurry was applied to a monolith carrier base material (1.7jl! 40
0 cell), dried at 100 to 130°C for 1 hour, and then baked at 650°C for 2 hours. The amount of coating in this case was set at 220 g/piece. Further, 0.955 g of platinum per carrier was added to this carrier using an aqueous solution of chloroplatinic acid, and after quick drying, the carrier was calcined at 600°C for 1 hour in a combustion gas atmosphere to obtain a catalyst carrier. Next, the above activated alumina carrier was impregnated with an aqueous solution of lanthanum nitrate, dried, and then calcined at 600°C for 2 hours in an air stream to form an activated alumina carrier containing 3.0% by weight of lanthanum in terms of metal based on the alumina. Obtained. Next, 2478 g of nitric acidic boehmite sol, 823 g of the above activated alumina, 394 g of lanthanum hydroxide powder,
282 g of zirconium oxide powder was placed in a ball mill pot and ground for 8 hours. The resulting slurry was applied to the catalyst carrier and dried at 100-130°C for 1 hour.
Raise the temperature from room temperature at a rate of 10°C/min to 650°C.
It was baked for 2 hours. In this case, the coating amount was set to 120 g/piece. Furthermore, this carrier has 0 palladium per carrier.
．． 955 g of rhodium and 0.191 g of rhodium were impregnated and supported using a mixed aqueous solution of an acidic dinitrodiammine palladium nitric acid solution and a rhodium nitrate solution, rapidly dried using a microwave dryer, and then dried at 600°C for 1 hour in a combustion gas atmosphere. Catalyst A was obtained by calcination. This catalyst has platinum 0,5
59 g/l, palladium 0.559 g/n, rhodium 0.112 g/i! ,, Cerium 32.4g //!
.

Lanthanum 12.7g/C zirconium 8.5g#! Contains. Further, the pores of the coating layer have a pore size peak of only 80 to 90 pores, which is the same as that of activated alumina.

Catalyst B was obtained in the same manner as in Comparative Example 1, except that 360 g of carbon black specified by ASTM NaN-550 was mixed into the slurry containing lanthanum and zirconium. The pores provided by the carbon black of this catalyst have a pore diameter peak of 200.

Comparison ■ 2563 g of Nijirica and 1437 g of activated alumina granular carrier containing 3% by weight of cerium as metal were mixed in a ball mill, ground for 6 hours, and then coated on a cordierite monolithic carrier (400 cells, 1.7 ffi). and calcined at 650'C for 2 hours. The amount of coating at this time was set at 340 g/piece.

Further, this carrier was immersed in a mixed aqueous solution of chloroplatinic acid, palladium chloride, and rhodium chloride, and reduced in a Hz/Nz gas flow. For platinum, palladium, and rhodium, per carrier,
They were set at 0.96 g, 0.96 g, and 0.19 g, respectively. Next, catalyst C was obtained by firing at 600° C. for 2 hours in a combustion gas stream. This catalyst contains 0.559g/j of platinum
2. Palladium 0.559g 1L Rhodium 0.112g
/l contains 6.6g/l of cerium. The pores of the coating layer have a peak pore size of 60 to 80 pores.

2563 g of solid base alumina sol, 14 g of activated alumina granular carrier
After pulverizing 37 g in a ball mill for 6 hours, it was coated on a cordierite monolithic carrier (400 cells, 1.7 f) and calcined at 650°C for 2 hours. The coating amount at this time was set to 340 g/piece. Next, 28 g of cerium metal was deposited using an aqueous solution of cerium nitrate Ce (NCh)z, dried at 120°C for 3 hours, and then fired at 600°C for 2 hours in an air stream. Further, the carrier was immersed in a mixed aqueous solution of chloroplatinic acid, palladium chloride, and rhodium chloride to support 0.96 g of platinum, 0.96 g of palladium, and 0.19 g of rhodium per carrier, and then heated at 600°C in a combustion gas stream. The mixture was calcined for 2 hours to obtain a catalyst. This catalyst contained 0.559 g/f of platinum, 0.559 g/f of palladium, and 0.112 g/i of rhodium! , contains 16.47 g/f of cerium. The pores of the coating layer have a peak pore diameter of 90 people.

Catalysts 1 to 5 obtained from Examples 1 to 5 and catalysts A to D obtained from Comparative Examples 1 to 4 were subjected to a durability test under the following conditions, and then a performance evaluation test was conducted, and the results are shown in Table 1. Shown below.

Catalyst integrated noble metal catalyst Catalyst outlet gas temperature 750°C Space velocity Approximately 70,000 l1r-
' Endurance time 100 hours Engine displacement 2200c
c Durable log gas atmosphere Co 0.4~0.6% 0□ 0.5±0.1% No 1000ppn+ HC2500ppm CO□ 14.9±0.1% Low blood pressure whispering vehicle Cedric (manufactured by Nissan Motor Corporation) Passenger car product name) displacement
2000cc (Effects of the Invention) As explained above, according to the present invention, chloroplatinic acid or chloroplatinic acid and rhodium chloride are used in the rare earth oxide and the activated alumina layer having a pore diameter peak of 100 Å or less,
A first coat layer carrying platinum or platinum and rhodium, and a layer on this layer consisting of a rare earth oxide as a surface layer, activated alumina with a pore diameter peak of 100 Å or less, and zirconium oxide, have a pore diameter peak. is 500-10
Since the second coating layer is provided with 0.00 pores and supports palladium and rhodium using dinitrodiammine palladium and rhodium nitrate, it is possible for hydrocarbons with large molecular diameters to diffuse into the catalyst layer. occurs sufficiently, and the effect is that high catalytic activity can be maintained in a wide atmospheric range from a rich region to a lean region.

Claims (1)

[Claims] 1. On a carrier base material, a coating layer consisting of activated alumina and rare earth metal oxide with a pore size peak of 100 Å or less,
A first coat layer on which platinum or platinum and rhodium is supported, and a coat layer on this layer consisting of activated alumina, rare earth metal oxide, and zirconium oxide with a pore diameter peak of 100 Å or less, and a coat layer with a pore diameter of 500 to 1000 Å. A catalyst for exhaust gas purification characterized by comprising a second coat layer provided with pores having peaks and supporting palladium and rhodium.