JPH0472577B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] This invention removes nitrogen oxides (NO) and combustible carbon substances such as carbon monoxide (CO) and hydrocarbons (HC) contained in exhaust gas discharged from internal combustion engines. This invention relates to a catalyst for purifying exhaust gas.
In particular, the present invention relates to a catalyst for exhaust gas purification in which catalyst metals and additives are supported on a carrier obtained by coating a refractory inorganic compound such as cordierite or mullite with alumina on its surface. [Prior art] Conventionally, platinum, rhodium, palladium, etc. have been used as catalyst metals used in exhaust gas purification catalysts, and oxides of cerium, zirconium, lanthanum, nickel, iron, etc. have been used as additives. It has been done. These additives are added to the catalytic metal for the purpose of promoting a cocatalyst effect,
Further, it is added to obtain various effects, such as for the purpose of preventing alumina to which a catalyst metal is attached from sintering under high-temperature usage conditions. Various types of additives have been proposed in the past, but generally these additives have been uniformly dispersed in an alumina coating layer coated on the surface of a refractory inorganic compound base material such as cordierite or mullite. It is used as The method for dispersing additives in this alumina coat layer is as follows:
An impregnation method involves impregnating the alumina coating layer with an aqueous solution of additive metal nitrates, chlorides, etc., and then firing it, or a method in which carbonate or acetate of additive metals are mixed with alumina, and the alumina is coated on the surface of the base material at the same time. There is a method in which additives are also attached to the base material and fired to form a uniform oxide in alumina. [Problems to be Solved by the Invention] The problem with the prior art as described above is that when new, the oxides of cerium, zirconium, lanthanum, nickel, iron, etc. have very good promoter effects. However, when exposed to high heat, the deterioration is greater than that without additives.
The performance required for conventional automobile exhaust gas purification catalysts was such that the upper limit of the gas temperature at the catalyst inlet was around 800°C, so large deterioration under high-temperature operating conditions was not a major problem. However, due to the recent movement of the catalyst mounting position to the upstream part of the exhaust pipe and the demands of the automobile industry, the upper limit of the catalyst inlet gas temperature has increased to 900°C.
It is thought that the catalyst temperature is around 1000°C. As a result, high-temperature durability, which has not been given much importance in the past, has recently become a particular issue. [Means for Solving the Problems] As a result of intensive research to solve the problems of the above-mentioned prior art, the present inventors have discovered an internal heat treatment system that has excellent catalytic performance and high heat durability when new. We succeeded in obtaining a catalyst for purifying engine exhaust gas. That is, the present invention provides an exhaust gas purifying catalyst in which a catalyst is supported on a carrier obtained by coating a base material with alumina, and has two alumina coat layers, and the inner alumina coat layer contains yttrium and praseodymium. , neodymium, cobalt, nickel, and zirconium and cerium, the outer alumina coat layer contains cerium, and the catalyst metal contains at least one selected from platinum, palladium, and rhodium. The problems of the prior art are solved by providing a supported catalyst for purifying exhaust gas containing nitrogen oxides, carbon monoxide, and hydrocarbons. The refractory inorganic compound used as the base material is cordierite, mullite, etc., and the base material generally has a honeycomb structure, but may also have a triangular, square, or wavy cell structure. The additives yttrium, praseodymium, neodymium, cobalt, nickel, zirconium, and cerium are added to the alumina coat layer by the conventional impregnation method described above. Further, the firing temperature of the carrier containing these additives is preferably 400°C to 1000°C. Note that when the outer alumina coat layer contains cerium by an impregnation method, cerium is also contained in the inner alumina coat layer. etc., it may not be included. All of the additives contained in the alumina layer described above exist in the form of oxides, such as cerium dioxide, due to firing. The catalyst metal used in this invention is at least one metal selected from platinum, palladium, and rhodium. be carried by Firing temperature is 300℃ to 800℃
It is preferable that [Function] In the catalyst of the present invention, at least one selected from yttrium, praseodymium, neodymium, cobalt, nickel, and zirconium and cerium contained in the inner alumina coat layer act as promoters, Cerium contained in the outer alumina coat layer also acts as a promoter. Furthermore, at least one metal selected from platinum, palladium, and rhodium supported on the outer alumina coat layer acts as a catalyst. In the case of catalysts for exhaust gas purification, catalytic metals should be as highly dispersed as possible in order to fully demonstrate their performance.
And it is customary to carry it on the coated surface. Taking these preconditions into consideration, among the metals contained as promoters in the present invention, promoter metals other than cerium have the disadvantage that, when they coexist with catalyst metals, they tend to sinter under high heat. Therefore, in the catalyst of this invention,
Promoter metals other than cerium are contained in the inner layer and separated from the catalyst metal, and the outer layer contains only cerium, which does not have these drawbacks. Moreover, the cocatalyst effect can be sufficiently exhibited in the case of yttrium, praseodymium, neodymium, cobalt, nickel, and zirconium of the present invention even when only the inner coating layer is used. [Example] Example 1 A mixed solution obtained by adding 350 g of alumina sol (alumina content 10% by weight) stabilized with acetic acid and 60 g of aluminum nitrate [Al(NO ₃ ) _{3.9H 2} O] to 420 g of pure water. , average particle size 10μm, specific surface area 125
1 kg of γ-alumina powder of m ² /g was gradually added with stirring to prepare a uniform alumina slurry. Next, a monolithic base material made of cordierite (square cells, number of cells/cm ² , 30 mm in diameter, 50 mm in length, weight 20 g) made of cordierite (square cells, number of cells/cm 2 , 30 mm in diameter, 50 mm in length, weight 20 g) was immersed in pure water and pulled up, and the water remaining in the cells was removed with an air stream. After being blown off and removed, it was immersed in the above alumina slurry for 2 minutes. After lifting the base material from the slurry and blowing off the excess alumina slurry inside the cell with an air stream, it was heated to 80°C.
The base material was dried for 3 hours at 800° C. in an electric furnace for 1 hour at 800° C. to obtain a support coated with alumina. Next, this carrier was immersed in an aqueous yttrium nitrate solution for 2 minutes. The carrier was lifted out of the aqueous solution, the excess aqueous solution in the cell was blown off with an air stream, and then dried at 80°C for 3 hours, and further calcined in an electric furnace at 800°C in an air atmosphere for 1 hour.
The concentration of the yttrium nitrate aqueous solution at this time was adjusted so that 3.5 x 10 ^-3 moles of yttrium atoms were supported per carrier. Next, the carrier was further coated with alumina using the above alumina slurry in the same manner as above, and then immersed in an aqueous cerium nitrate solution for 2 minutes. After lifting the carrier from the aqueous solution and blowing off the excess aqueous solution in the cell with an air stream, it was incubated at 80°C for 3
It was dried for an hour and then fired in an electric furnace at 800° C. for 1 hour in an air atmosphere. The concentration of the cerium nitrate aqueous solution at this time was adjusted so that 7×10 ⁻³ moles of cerium atoms were supported per carrier. The catalyst carrier thus obtained was immersed in a platinum ammine aqueous solution to adsorb platinum onto the catalyst carrier, washed with water, and subsequently immersed in a rhodium chloride aqueous solution to support rhodium on the catalyst carrier.
After drying at 100°C, it was calcined at 500°C for 30 minutes to obtain an exhaust gas purification catalyst. When the amount of noble metals supported on this catalyst was measured by chemical analysis, the amount of platinum supported on the catalyst carrier was 0.218% by weight, and the amount of supported rhodium was 0.0227% by weight. The catalytic performance of both the new catalyst obtained in this way and the catalyst after this catalyst was deteriorated by the method described below was evaluated by the method below, and the results are shown in Table 1. . The catalyst deterioration test was performed using 98% _N2 gas and 2% N2 gas.
This was done by exposing the catalyst sample to a mixed gas atmosphere of O ₂ gas at 1000°C for 3 hours. In addition, the catalyst performance evaluation method was performed using a mini-reactor model gas evaluation device. Gases of C ₃ H ₆ 1000 ppm, CO 0.5%, NO 800 ppm, 10% each of CO ₂ and H ₂ O, H ₂ 0.2%, O ₂ 0.77% and the balance N ₂ at a flow rate of 23.3/min and a space velocity.
The catalyst inlet gas temperature is increased ^from 100°C to 500°C at a rate of 20°C/min, and shows a 50% purification rate of C ₃ H ₆ . The temperature [T50%C ₃ H ₆ (°C)] was measured to evaluate the catalyst performance. Example 2 A catalyst for exhaust gas purification was obtained in the same manner as in Example 1, except that an aqueous praseodymium nitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Example 3 A catalyst for exhaust gas purification was obtained in the same manner as in Example 1, except that an aqueous neodymium nitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution in which the carrier was immersed. This catalyst and the catalyst after the same deterioration test as shown in Example 1 were evaluated for catalyst performance by the same method as in Example 1, and the results are shown in Table 1. An exhaust gas purifying catalyst was obtained in the same manner as in Example 1, except that an aqueous cobalt nitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution to be immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Example 5 A catalyst for exhaust gas purification was obtained in the same manner as in Example 1, except that an aqueous nickel nitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Example 6 A catalyst for exhaust gas purification was obtained in the same manner as in Example 1, except that an aqueous zirconium oxynitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 1 A first alumina coat was applied to the base material in the same manner as in Example 1, and a second alumina coat was subsequently applied to the base material in the same manner to obtain a carrier. Next, the number of cerium atoms per carrier is 7×
Cerium was supported using an aqueous cerium nitrate solution in the same manner as in Example 1 so that the amount of cerium was 10 ^-3 mol, and then dried and calcined in the same manner as in Example 1 to obtain a catalyst. A carrier was obtained. Thereafter, platinum and rhodium were supported on a catalyst carrier in the same manner as in Example 1 to obtain a catalyst. The catalyst performance of the thus obtained catalysts and the catalysts subjected to the same deterioration test as shown in Example 1 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Comparative Example 2 A catalyst was obtained in the same manner as in Example 1, except that a zirconium oxynitrate aqueous solution was used instead of a cerium nitrate aqueous solution as the aqueous solution in which the carrier was immersed.
The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 3 A catalyst was obtained in the same manner as in Comparative Example 1, except that a lanthanum nitrate aqueous solution was used instead of a cerium nitrate aqueous solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 4 A catalyst was obtained in the same manner as in Comparative Example 1, except that a nickel nitrate aqueous solution was used instead of a cerium nitrate aqueous solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 5 A catalyst was obtained in the same manner as in Comparative Example 1, except that a ferric nitrate aqueous solution was used instead of a cerium nitrate aqueous solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 6 After applying alumina coating twice to the base material in the same manner as in Comparative Example 1, cerium was supported.
Then, in the same manner as in Example 1, using an aqueous yttrium nitrate solution, the amount of yttrium atoms per carrier was determined.
A catalyst carrier was obtained by supporting yttrium in an amount of 3.5×10 ⁻³ mol. Thereafter, in the same manner as in Example 1, platinum and rhodium were supported on a catalyst carrier to obtain a catalyst. The catalyst performance of the thus obtained catalysts and the catalysts subjected to the same deterioration test as shown in Example 1 was evaluated in the same manner as in Example 1, and the results are shown in Table 1. Comparative Example 7 A catalyst was obtained in the same manner as in Comparative Example 6, except that an aqueous praseodymium nitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution for dipping the carrier.
The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 8 A catalyst was obtained in the same manner as in Comparative Example 6, except that a neodymium nitrate aqueous solution was used instead of the yttrium nitrate aqueous solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 9 A catalyst was obtained in the same manner as in Comparative Example 6, except that a cobalt nitrate aqueous solution was used instead of the yttrium nitrate aqueous solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 10 A catalyst was obtained in the same manner as in Comparative Example 6, except that a nickel nitrate aqueous solution was used instead of the yttrium nitrate aqueous solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated by the same method as in Example 1, and the results are shown in Table 1. Comparative Example 11 A catalyst was obtained in the same manner as in Comparative Example 6 except that an aqueous zirconium oxynitrate solution was used instead of an aqueous yttrium nitrate solution as the aqueous solution in which the carrier was immersed. The catalyst performance of this catalyst and the catalyst after the same deterioration test as shown in Example 1 was evaluated in the same manner as in Example 1, and the results were evaluated in the first
Shown in the table. Examples 7 to 12 In Examples 1 to 6, instead of supporting platinum and rhodium as catalyst metals on the catalyst carrier using an aqueous platinum ammine solution and an aqueous rhodium chloride solution, an aqueous palladium chloride solution and an aqueous rhodium chloride solution were sequentially used. Exhaust gas purifying catalysts of Examples 7 to 12 were obtained in the same manner as Examples 1 to 6, except that the catalyst metals palladium and rhodium were supported on the catalyst carrier. The amount of palladium supported is
The amount of rhodium supported was 0.0227% by weight. The catalysts thus obtained and the catalysts subjected to the same deterioration test as shown in Example 1 were evaluated for catalyst performance in the same manner as in Example 1, and the results are shown in Table 2. Comparative Examples 12 to 22 Instead of Comparative Examples 1 to 11, in which platinum and rhodium as catalyst metals were supported on a catalyst carrier using a platinum ammine aqueous solution and a rhodium chloride aqueous solution,
Comparative Examples 1 to 11 except that palladium and rhodium were supported as catalyst metals on the catalyst carrier using a palladium chloride aqueous solution and a rhodium chloride aqueous solution sequentially.
Catalysts of Comparative Examples 12 to 22 were obtained in the same manner as above.
The amount of palladium supported was 0.218% by weight, and the amount of rhodium supported was 0.0227% by weight. The catalysts thus obtained and the catalysts subjected to the same deterioration test as shown in Example 1 were evaluated for catalyst performance in the same manner as in Example 1, and the results are shown in Table 2. Examples 13 to 18 In Examples 1 to 6, instead of supporting platinum and rhodium as catalyst metals on a catalyst carrier using a platinum ammine aqueous solution and a rhodium chloride aqueous solution, a palladium chloride aqueous solution, a platinum ammine aqueous solution, and a rhodium chloride aqueous solution were used. Exhaust gas purifying catalysts of Examples 13 to 18 were obtained in the same manner as in Examples 1 to 6, except that the catalyst metals palladium, platinum, and rhodium were supported on the catalyst carrier by sequentially using the following methods. The amount of palladium supported is 0.109% by weight, the amount of platinum supported is 0.109% by weight, and the amount of supported rhodium is 0.109% by weight.
It was 0.0227% by weight. The catalysts thus obtained and the catalysts subjected to the same deterioration test as shown in Example 1 were evaluated for catalyst performance in the same manner as in Example 1, and the results are shown in Table 3. Comparative Examples 23 to 33 In place of Comparative Examples 1 to 11, in which platinum and rhodium as catalyst metals were supported on a catalyst carrier using a platinum ammine aqueous solution and a rhodium chloride aqueous solution,
Comparative Examples 23 to 33 were prepared in the same manner as Comparative Examples 1 to 11, except that the catalyst metals palladium, platinum, and rhodium were supported on the catalyst carrier using a palladium chloride aqueous solution, a platinum ammine aqueous solution, and a rhodium chloride aqueous solution, respectively. I got a catalyst. The amount of palladium supported was 0.109% by weight, the amount of platinum supported was 0.109% by weight, and the amount of rhodium supported was 0.0227% by weight. The catalysts thus obtained and the catalysts subjected to the same deterioration test as shown in Example 1 were evaluated for catalyst performance in the same manner as in Example 1, and the results are shown in Table 3.

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〔Effect of the invention〕

The catalyst for exhaust gas purification of this invention has good performance when new, but unlike conventional products which had poor high-temperature durability, the catalyst for exhaust gas purification of this invention not only has good performance when new before deterioration, but also has particularly good high-temperature durability. It has excellent features.

Claims (1)

[Claims] 1 In an exhaust gas purification catalyst in which a catalyst is supported on a carrier obtained by coating a base material with alumina, there are two alumina coat layers, and the inner alumina coat layer contains yttrium, praseodymium, neodymium, and cobalt. , at least one selected from nickel and zirconium, and cerium; the outer alumina coat layer contains cerium; and at least one selected from platinum, palladium, and rhodium is supported as a catalyst metal. An exhaust gas purification catalyst for removing and purifying nitrogen oxides, carbon monoxide, and hydrocarbons in exhaust gas.