JPH06377A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE:To simultaneously remove CO, hydrocarbon and NOx as noxious components contained in exhaust gas from the internal-combustion engine of an automobile, etc. CONSTITUTION:A unonolithic structure is coated with a catalytically active compsn. contg. palladium, the oxide of an alkaline earth metal, zirconium oxide with supported lanthanum and cerium and a fireproof inorg. oxide to obtain a catalyst for purification of exhaust gas. The zirconium oxide with supported lanthanum and cerium preferably has (150:100)-(10:100) weight ratio of zirconium oxide:cerium (expressed in terms of cerium oxide).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to carbon monoxide (C) which is a harmful component contained in exhaust gas from an internal combustion engine such as an automobile.
O), hydrocarbons (HC) and nitrogen oxides (NOx) at the same time.

[0002]

2. Description of the Related Art Various types of exhaust gas purifying catalysts for removing harmful components in exhaust gas discharged from an internal combustion engine have been proposed.

Conventional palladium catalysts have high heat resistance and C in the oxidizing atmosphere of engine exhaust gas (so-called lean; air / fuel (A / F) is on the air side).
It was generally known that O and HC have high purification ability. On the other hand, as a problem, when the engine exhaust gas is in a reducing atmosphere (so-called rich; (A / F) is large on the fuel side),
It can be mentioned that the NOx purification ability is low. Therefore, use only on the lean side, for example, as a so-called oxidation catalyst,
Alternatively, rhodium, which has a high NOx purification ability, is used in combination with the above palladium as a three-way catalyst for simultaneously purifying CO, HC and NOx.

However, since rhodium is very expensive, it is desirable to reduce the amount used in the catalyst component or not to use it, but rhodium is characterized by having a high NOx purification ability. It is indispensable as an essential component as a component of an exhaust gas purifying catalyst that simultaneously removes carbon oxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx).

[0005]

DISCLOSURE OF THE INVENTION The present invention is an exhaust gas purifying catalyst capable of simultaneously purifying CO, HC and NOx without using rhodium and exhibiting higher catalytic performance than the conventional catalyst system. The challenge is to provide.

[0006]

The object of the present invention is to use CO, H without using rhodium and by greatly reducing the amount used.
An object of the present invention is to provide an exhaust gas purifying catalyst that simultaneously removes three components of C and NOx, and an exhaust gas purifying apparatus using the same.

[0007]

The inventors of the present invention have conducted extensive studies to solve this problem, and as a result, (a) palladium, an alkaline earth metal oxide, (b) lanthanum and cerium-supported zirconium oxide. The present invention has been found to have an exhaust gas purifying ability equivalent to that of a conventional three-way catalyst containing rhodium by coating a monolithic structure with a catalytically active component containing (c) a refractory inorganic oxide. It was completed. According to the present invention, the problem with the palladium catalyst is that the engine exhaust gas is rich in N
Ox purification capacity can be improved.

That is, the present invention provides a catalytically active component containing (a) palladium, an alkaline earth metal oxide, (b) a zirconium oxide carrying lanthanum and cerium, and (c) a refractory inorganic oxide. A catalyst for removing carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides in exhaust gas of an internal combustion engine, which is characterized in that the catalyst is formed by coating an integral structure. Hereinafter, the present invention will be described in detail.

Of the (a) palladium and alkaline earth metal oxides according to the present invention, the amount of palladium used varies depending on the conditions under which the catalyst is used, but is usually 0.5 to 30 g, preferably 0. It is 5 to 25 g. When the amount of palladium is less than 0.5, the purifying ability is low, and when it exceeds 30 g, no improvement in performance commensurate with the amount added is observed.

The position on which palladium is supported varies depending on the amount used, but it may be supported on zirconium oxide, cerium oxide, lanthanum oxide, or refractory inorganic oxide alone or straddling.

The alkaline earth metal oxides include beryllium oxide, magnesium oxide, calcium oxide, strontium oxide and barium oxide. Among them, calcium oxide, strontium oxide and barium oxide are particularly preferable. At least one selected from the group consisting of things is preferable. The amount of alkaline earth metal oxide used is 0.1 to 50 g per liter of catalyst. If it is less than 0.1, no improvement in the NOx purification performance is shown, and if it exceeds 50 g, the effect commensurate with the addition is small. The alkaline earth metal oxide may be supported on any of cerium oxide, zirconium oxide or a composite thereof, a solid solution, lanthanum oxide or a refractory inorganic oxide, and the method for preparing this support is not particularly limited. Not done.

The alkaline earth metal oxide source may be a precursor which becomes an oxide by firing, in addition to the oxide as it is, and barium is exemplified as a representative alkaline earth metal. , Any of organic salts such as barium oxalate or inorganic salts such as barium nitrate, barium hydroxide and barium carbonate, and the state thereof may be not only an aqueous solution but also a gel or a suspension. It is not particularly limited.

The relationship between the alkaline earth metal oxide and the palladium is 1: 100 to 150: 1, preferably by weight ratio (alkaline earth metal oxide / palladium).
It is 1: 100 to 100: 1. When the amount of alkaline earth metal oxide is less than 1: 100, the ternary performance is poor, and particularly, the NO purification rate is poor, and when the amount of alkaline earth metal oxide is more than 150: 1, the addition effect is improved. However, the loading ratio and loading amount are limited depending on the relationship between the loading amount of oxides and the like and the strength of the catalyst.

Among the (b) lanthanum- and cerium-supported zirconium oxides according to the present invention, the zirconium oxide source is not particularly limited, but may be an oxide as it is, or a nitrate, a sulfate or the like. Zirconium oxide or hydroxide obtained by baking a water-soluble salt or carbonate can be used. Lanthanum and cerium are aqueous solutions of nitrates or sulfates,
Aqueous liquid such as gel and suspension can be used.

(B) The method for preparing the zirconium oxide carrying lanthanum and cerium includes (1) a method in which the zirconium oxide is simultaneously impregnated with the cerium salt and an aqueous solution of the cerium salt, followed by drying and firing. ) A method of supporting an aqueous solution of cerium salt on zirconium oxide by an impregnation method, and then an aqueous solution of lanthanum salt by an impregnation method, (3) supporting an aqueous solution of lanthanum salt on zirconium oxide by an impregnation method, and then cerium salt There is a method of supporting the above aqueous solution by an impregnation method, and any method can be appropriately used. The cerium supported on the zirconium oxide is preferably present as a composite or solid solution with at least one oxide of zirconium oxide and lanthanum oxide.

The zirconium oxide carrying lanthanum and cerium has a cerium to zirconium ratio (oxide weight conversion ratio) of 150: 100 to 10:10.
0, more preferably 130: 100 to 20:10
It is 0. When the ratio of cerium oxide is more than 150: 100 in this ratio, no improvement in activity commensurate with the addition is observed, and 1
When the amount of cerium oxide is less than 0: 100, the improvement in performance is less.

The amount of lanthanum oxide used is 0.1 to 50 g per unitary structure, and if less than 0.1 g,
The improvement in performance is small, and when it exceeds 50 g, the improvement in performance commensurate with the addition cannot be obtained.

Furthermore, the zirconium oxide supporting lanthanum and cerium further improves the performance of the catalyst by supporting at least one element selected from the group consisting of yttrium, neodymium and praseodymium. As the above-mentioned yttrium and the like, water-soluble salts such as nitrates and sulfates, gel-like or suspension-like aqueous liquids and the like can be used.

Examples of the refractory inorganic oxide include those having a high surface area such as activated alumina, silica and zirconia, and activated alumina is particularly preferable. This refractory inorganic oxide contains 10 g to 300 g per liter of the monolithic structure.
g coating is preferred. This refractory inorganic oxide is 50 to 400 g, preferably 100 to 350 g, and when it is less than 50 g, the purification performance is low and 40
When it exceeds 0 g, the back pressure increases when the monolithic structure catalyst is coated, which is not preferable.

The monolithic structure may be any one that is normally used for exhaust gas purification, and is preferably a honeycomb-shaped structure, such as a cordierite or mullite ceramic monolith carrier. Alternatively, a monolith made of metal such as stainless steel or Fe—Cr—Al alloy may be used.

Cell shape for passing these exhaust gases,
The hole diameter and the like are not particularly limited, and are appropriately selected depending on the type of exhaust gas, the engine displacement, and the installation position of the catalyst.

[0022]

As described above, the catalyst according to the present invention is
It is possible to provide a catalyst for purifying an exhaust gas which does not use rhodium and whose amount used is greatly reduced compared to the conventional one, and which simultaneously removes three components of CO, HC and NOx.

The effect of adding the alkaline earth metal oxide is that it acts directly on palladium and changes its charge state, thereby enhancing the reactivity and improving the NOx purification ability in a rich atmosphere. (B) By using the zirconium oxide in which cerium and lanthanum are supported, the heat resistance is improved, and the fuel gas composition is in the vicinity of the stoichiometric ratio (the amount of air required to completely burn the fuel gas). Significant improvements in CO, HC and NOx purification capacity are shown. Further, by supporting at least one selected from the group consisting of yttrium, neodymium and praseodymium on a zirconium oxide carrying cerium and lanthanum, the above performance is improved.

[0024]

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples unless it goes against the gist of the present invention.

(Example 1) An aqueous solution containing 120 g of commercially available zirconium oxide (ZrO ₂ , specific surface area 92 m ² / g) and cerium nitrate (containing 80 g of cerium oxide) and lanthanum nitrate (containing 20 g of lanthanum oxide). , Mix, dry, then bake at 500 ° C.,
A powder (b) was obtained.

The powder (b) obtained by the above procedure, activated alumina (γ-Al ₂ O ₃ , specific surface area 155 m ² / g 140
g, and barium acetate (40 as barium oxide
g) and an aqueous solution of palladium nitrate (containing 6 g of palladium) were wet-milled with a ball mill to prepare an aqueous slurry. A cordierite monolithic carrier (inner diameter: 33 mm, length: 76 mm) having 400 cells per square inch of cross-sectional area was dipped in this slurry, taken out, and the excess slurry in the cells was blown off with compressed air, followed by drying and firing. After that, a finished catalyst was obtained.

(Examples 2 to 3) A completed catalyst was obtained in the same manner as in Example 1 except that the amounts of zirconium oxide were changed to 200 g and 60 g, respectively. Table 1 shows the supported amounts of the components of the thus-obtained finished catalyst.

(Examples 4 to 5) In Example 1, the amounts of barium acetate were 1 g and 8 g, respectively, in terms of barium oxide.
A completed catalyst was obtained in the same manner as in Example 1 except that the amount was changed to 0 g. Table 1 shows the supported amounts of the components of the thus-obtained finished catalyst.

(Examples 6 to 7) In Example 1, barium acetate (containing 40 g of barium oxide) was added to calcium acetate (40 g of calcium oxide) or strontium acetate (40 g of strontium oxide), respectively.
A completed catalyst was obtained in the same manner as in Example 1 except that Table 1 shows the supported amounts of the components of the thus-obtained finished catalyst.

(Examples 8 to 9) In Example 1, the amount of lanthanum nitrate was 80 g in terms of lanthanum oxide equivalent,
A completed catalyst was obtained in the same manner as in Example 1 except that the amount was changed to 1 g. Table 1 shows the supported amounts of the components of the thus-obtained finished catalyst.

(Example 10) In Example 1, 120 g of commercially available zirconium oxide (ZrO ₂ , specific surface area 92 m ² / g), lanthanum nitrate (containing 20 g of lanthanum oxide), and zirconium oxide and lanthanum oxide in Example 1 were used. A completed catalyst was obtained in the same manner as in Example 1 except that praseodymium nitrate (containing 10 g of praseodymium oxide) was used. Table 1 shows the supported amounts of the components of the thus-obtained finished catalyst.

(Examples 11 to 12) A completed catalyst was obtained in the same manner as in Example 10 except that the amount of praseodymium nitrate was changed to 1 g and 30 g, respectively, in terms of praseodymium oxide. Table 1 shows the supported amounts of the components of the thus-obtained finished catalyst.

(Examples 13 to 14) In Example 10, praseodymium nitrate (1% as praseodymium oxide) was used.
Example 10 except that 0 g) was replaced with neodymium nitrate (containing 10 g as neodymium oxide) and yttrium nitrate (containing 10 g as yttrium oxide), respectively.
A completed catalyst was obtained in the same manner as in.

(Examples 15 to 16) In Example 1,
A completed catalyst was obtained in the same manner as in Example 1 except that the palladium nitrate aqueous solution containing 3 g of palladium was changed to 20 g and 40 g, respectively.

Comparative Example 1 In Example 1, the zirconia oxide carrying cerium and lanthanum was replaced by
Instead of lanthanum oxide, 80 g of commercially available cerium oxide (specific surface area 149 m ² / g) and 120 g of the same commercially available zirconium oxide used in Example 1 were mixed, and then lanthanum nitrate (containing 20 g as lanthanum oxide) was mixed. A completed catalyst was obtained in the same manner as in Example 1 except that a powder obtained by mixing the aqueous solutions, drying and firing at 500 ° C was used.

Comparative Example 2 A completed catalyst was obtained in the same manner as in Example 1 except that barium acetate was omitted.

Comparative Example 3 A finished catalyst was obtained in the same manner as in Example 1 except that lanthanum nitrate was not used.

(Comparative Example 4) The cerium oxide 20 used in Comparative Example 1 was replaced with the cerium oxide of Example 1 except that zirconium oxide and lanthanum oxide were not used and 40 g of cerium oxide was used.
A completed catalyst was obtained in the same manner as in Example 1 except that 0 g was used.

(Comparative Example 5) In Example 1, barium oxide, zirconium oxide and lanthanum oxide were not used, but palladium and cerium oxide were replaced with palladium dinitrodiamine aqueous solution containing 2.25 g of platinum and rhodium of 0. A powder obtained by impregnating a mixed solution of an aqueous rhodium nitrate solution containing 0.22 g with 200 g of activated alumina used in Example 1, drying and firing, and 100 g of cerium oxide used in Comparative Example 1 were wet-ground with a ball mill. A completed catalyst was obtained in the same manner as in Example 1 except for the above.

(Comparative Example 6) In Example 1, barium oxide, zirconium oxide and lanthanum oxide were not used, and palladium and cerium oxide were replaced by palladium aqueous solution containing 2.25 g of palladium nitrate and rhodium. A powder obtained by impregnating 200 g of activated alumina used in Example 1 with a mixed solution of 22 g of an aqueous rhodium nitrate solution, drying and firing, and 100 g of cerium oxide used in Comparative Example 1 were wet-milled with a ball mill. In the same manner as in Example 1, a finished catalyst was obtained.

Example 17 The catalysts obtained in Examples 1 to 16 and Comparative Examples 1 to 6 were evaluated for catalytic activity after engine durability. The procedure is shown below.

Commercially available electronically controlled engine (8 cylinders 4
Using 400 cc), a multi-converter filled with each catalyst was connected to the exhaust system of the engine and a durability test was conducted. The engine is operated in a mode operation of 60 seconds of steady operation and 6 seconds of deceleration (fuel is cut during deceleration, and the catalyst is exposed to severe conditions of high temperature oxidizing atmosphere), and the catalyst bed temperature is 950 ° C during steady operation. The catalyst was aged for 50 hours under the following conditions. The catalyst performance after aging was evaluated using a commercially available electronically controlled engine (4-cylinder 1800c
Using c), a multi-converter filled with each catalyst was connected to the exhaust system of the engine. The three-way performance of the catalyst is such that the catalyst inlet gas temperature is 400 ° C and the space velocity is 90,000.
It was evaluated under the condition of 0 hr- ¹ . At this time, 1 from the external oscillator
Introducing a Hz sine wave type signal into the engine control unit, the air-fuel ratio (A / F) is ± 1.0A / F,
The average air-fuel ratio is continuously changed while oscillating at 1 Hz, and the catalyst inlet and outlet gas compositions at this time are analyzed at the same time, and the average air-fuel ratio A / F is from 15.1 to 14.1 C
The purification rates of O, HC and NO were obtained.

CO, HC and N obtained as described above
The purification rate of O versus the inlet air-fuel ratio is plotted in a graph to create a ternary characteristic curve, and the purification rate at the intersection of the CO and NO purification rates curves (called a crossover point) and the intersection A
HC purification rate at / F value Further, A / F is 14.2
Table 2 shows the NO purifying ability when the engine exhaust gas is rich.

Further, the purification performance of the catalyst at low temperature is as follows, while vibrating the air-fuel ratio under the condition of ± 0.5 A / F (1 Hz).
The engine is operated with the average air-fuel ratio fixed to A / F of 14.6, a heat exchanger is installed in front of the catalytic converter of the engine exhaust system, and the catalyst inlet gas temperature is set to 200 ° C to 500 ° C.
It was evaluated by determining the purification rates of CO, HC and NO by analyzing the gas composition at the inlet and outlet of the catalyst while continuously changing the temperature to 0 ° C. The temperature (light-off temperature) at the CO, HC, and NO purification rate of 50% obtained as described above was measured and shown in Table 2.

From Table 2, the catalyst disclosed in the present invention does not contain rhodium as a noble metal, but only palladium with C
It can be seen that the three components of O, HC and NOx can be simultaneously removed with high performance. Furthermore, the purification rate of NOx (NOx value when A / F is 14.2) on the rich side of the engine exhaust gas is excellent, and the three components of HC, CO and NO are simultaneously removed at a significantly low temperature (light-off temperature). Value) is possible.

[0046]

[Table 1]

[0047]

[Table 2]

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[Procedure amendment]

[Submission date] July 6, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 3

[Correction method] Change

[Correction content]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomohisa Ohata 1 992, Nishikioki, Kamahama, Aboshi-ku, Himeji-shi, Hyogo 1

Claims (5)

[Claims] 1. A catalytically active component containing (a) palladium, an alkaline earth metal oxide, (b) a zirconium oxide supporting lanthanum and cerium, and (c) a refractory inorganic oxide in an integrated structure. An exhaust gas purifying catalyst characterized by being coated. 2. Palladium in an amount of 0.5 to 30 g and cerium oxide in an amount of 10 to 100 per liter of the integrated structure.
g, zirconium oxide is 10 to 150 g, lanthanum oxide is 0.1 to 50 g, and alkaline earth metal oxide is 0.1 g.
The catalyst according to claim 1, wherein 1 to 50 g and the refractory inorganic oxide are 10 to 300 g. 3. A zirconium oxide carrying (b) lanthanum and cerium has a weight ratio of zirconium to cerium (weight equivalent to oxide) of 150: 100 to 10:10.
The catalyst according to claim 1, which is 0. 4. A zirconium oxide carrying (b) lanthanum and cerium is obtained by impregnating a zirconium oxide or hydroxide with an aqueous solution of a water-soluble lanthanum and cerium salt, followed by drying and firing. The catalyst according to claim 1 or 3, which is one. 5. The catalyst according to claim 1, wherein the zirconium oxide carrying (b) lanthanum and cerium contains at least one element selected from the group consisting of yttrium, neodymium and praseodymium.