JPH05237384A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To provide a catalyst for simultaneously removing hydrocarbons(HC), carbon monoxide(CO) and nitrogen oxides(NOX) in exhaust gas from an internal combustion engine such as an automobile. CONSTITUTION:This catalyst is integrally coated with a catalytic active component which contains palladium and rhodium (a) or palladium, rhodium and platinum (b), and alkaline earth metal oxide, lanthanum oxide, cerium oxide, zirconium oxide, and refractory inorg. oxides.

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 exhaust gas purifying catalysts have been proposed as a method for removing harmful components in exhaust gas discharged from an internal combustion engine.

Conventionally, a palladium catalyst has a high heat resistance and a high purification ability of CO and HC in an oxidizing atmosphere of an engine exhaust gas (so-called lean; air / fuel (A / F) is large on the air side). It is generally known to have However, when the engine exhaust gas is in a reducing atmosphere (so-called rich; (A / F) is large on the fuel side), the NOx purification capacity is extremely low, and it is known that palladium alone is difficult to function as a three-way catalyst. ing. Therefore, in general, 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. On the other hand, it is generally known that its performance is lower than that of platinum / rhodium-based catalysts and that it also has a problem in durability performance. However, palladium is significantly lower in cost than platinum / rhodium-based catalysts, so C in a catalyst system using inexpensive palladium
A catalyst capable of simultaneously purifying O, HC and NOx is desired.

[0004]

DISCLOSURE OF THE INVENTION The present invention has developed a high-performance palladium-rhodium-based catalyst by improving the NOx purifying ability and durability in a reducing atmosphere, which is a problem of palladium described above. An object of the present invention is to provide an inexpensive catalyst by replacing part or all of platinum in a platinum / rhodium-based catalyst with palladium.

[0005]

Means for Solving the Problems As a result of intensive studies to solve this problem, the present inventors
(A) Palladium and Rhodium, or (b) Palladium, Rhodium and Platinum, and a system comprising conventional alkaline earth metal oxides, lanthanum oxides, cerium oxides, zirconium oxides and refractory inorganic oxides. The present invention has been completed by discovering that the performance is remarkably improved as compared with the rhodium-based catalyst and that the platinum-rhodium-based three-way catalyst has the same or higher performance.

The effect of adding the alkaline earth metal oxide in the present invention is to directly react with palladium to change its charge state to enhance the reactivity and dramatically improve the NOx purification ability in a rich atmosphere. It is what makes them.

Further, by using cerium oxide and zirconium oxide, not only the above effect is further improved but also the fuel gas composition has a stoichiometric ratio (the amount of air required to completely burn the fuel gas). It is also shown that CO, HC and NOx purifying ability in the vicinity is also improved.

Further, by supporting lanthanum oxide on at least one of refractory inorganic oxide, cerium oxide and zirconium oxide, the above-mentioned effects are dramatically improved. The lanthanum oxide is preferably coated in an amount of 0.1 to 50 g per liter of the monolithic structure.
When it is less than 1 g, the effect of addition is small, and when it exceeds 50 g, the effect commensurate with the amount added is small.

The present invention is a catalyst for simultaneously removing carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas of an internal combustion engine, wherein the catalytically active substance is (a) palladium and rhodium or (b) palladium and rhodium as noble metals. And any one of platinum, further alkaline earth metal oxide,
An exhaust gas purifying catalyst, characterized in that an integrated structure is coated with a catalytically active substance containing a lanthanum oxide, a cerium oxide, a zirconium oxide and a refractory inorganic oxide.

The amount of the noble metal used according to the present invention varies depending on the use conditions of the catalyst. When the noble metal is (a) palladium and rhodium, the amount of palladium is 0.1 to 10% by weight based on the catalytically active substance, and rhodium is used. Is 0.01-
It is preferably 2.0% by weight. When the noble metal is (b) palladium, rhodium and platinum, palladium is 0.1 to 10% by weight, rhodium is 0.01 to 2.0% by weight, and platinum is 0.1 to 10 with respect to the catalytically active substance. It is preferably in the weight%.

The position on which each noble metal is supported is not particularly limited, but it should be supported on cerium oxide, zirconium oxide, lanthanum oxide or refractory inorganic oxide, either alone or straddling these oxides. Is preferred.

Next, examples of the alkaline earth metal oxides include beryllium oxide, magnesium oxide, calcium oxide, strontium oxide and barium oxide, and particularly calcium oxide, strontium oxide and barium oxide. At least one selected from the group consisting of things is preferable. The alkaline earth metal oxide source may be an oxide, an organic salt or an inorganic salt,
It is not particularly limited.

The amount of alkaline earth metal oxide used is the amount of catalyst 1
0.1 to 50 g per liter, preferably 0.5 to 40
It is g. 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 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 the alkaline earth metal oxide is less than 1: 100, the ternary performance is poor, and in particular, the NO purification rate is poor, and when the amount of the alkaline earth metal oxide is more than 150: 1, the addition effect is improved. However, the loading ratio and loading amount are limited by the relationship between the loading amount of oxides and the strength of the catalyst.

The cerium oxide is not particularly limited, but it can be used as the cerium oxide as it is or by burning various water-soluble salts or the like. The amount of cerium oxide used is preferably 10 to 150 g per liter of the catalyst, and when it is less than 10 g, the purifying ability is low, and when it exceeds 150 g, even if it is added more than that, no great effect is observed.

The zirconium oxide is not particularly limited, but it can be used as the zirconium oxide as it is or by firing various water-soluble salts or the like. The amount of zirconium oxide used is preferably 0.1 to 50 g per liter of the catalyst,
If it is less than 0.1, the effect is small, and if it exceeds 50 g, the purification performance is deteriorated.

Cerium oxide and zirconium oxide are
It is preferable that at least a part thereof is present as a composite or a solid solution. More preferably, the ratio of this cerium oxide and zirconium oxide (weight ratio in terms of oxide)
Is 100: 2 to 100: 60, and more preferably 100: 4 to 100: 40. This ratio is 10
When the amount of cerium oxide is more than 0: 2, the performance is low,
If the amount of zirconium oxide is more than 100: 60, the performance tends to be low.

A method for preparing a composite of cerium oxide and zirconium oxide or a solid solution is shown below. If at least a part of these oxides exists as a composite or a solid solution, it is particularly preferable. Although not limited, it is preferably within the range of the above ratio.

(1) A method of drying and calcining an aqueous solution of cerium salt and zirconium salt which are soluble in water, (2) a method of solid-phase reaction of cerium oxide and zirconium oxide, (3) cerium oxide (4) A method of soaking an aqueous solution of a water-soluble zirconium salt in an aqueous solution and drying and firing, and (4) a method of impregnating a water-soluble aqueous solution of a cerium salt and a zirconium salt into a refractory inorganic oxide, followed by drying and firing. ) Various methods can be appropriately used depending on the preparation of the catalyst, such as a method of coating the monolithic structure with a refractory inorganic oxide and then drying and calcining an aqueous solution of water-soluble cerium salt and zirconium salt. ..

The lanthanum oxide is not particularly limited, but oxides, various water-soluble salts and the like may be used alone as refractory inorganic oxides, cerium oxides, zirconium oxides, or oxidation thereof. It is carried across objects.

The amount of lanthanum oxide used is 1
0.1 to 50 g per liter is preferred, 0.1 g
If the amount is less than 50 g, the effect is small, and if the amount exceeds 50 g, even if more than 50 g is added, the corresponding effect is not observed.

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.

As a method for preparing a catalyst, (1) a method in which the above-mentioned catalyst composition is made into an aqueous slurry by using a ball mill or the like, coated on an integral structure, dried and, if necessary, calcined to obtain a finished catalyst. (2) The monolithic structure is coated in advance with a refractory oxide, and then the monolithic structure is immersed in an aqueous solution of a cerium salt or zirconium salt that is soluble in water, dried, fired, and then the same. There is a method of supporting lanthanum salt in the procedure, etc., but it is appropriately changed and used depending on the convenience of the work procedure.

The catalyst composition per liter of the monolithic structure is 50 to 400 g, preferably 100 to 350 g. If the amount is less than 50 g, the purification performance is low and 400
When it exceeds g, when the catalyst composition is coated on the integrally molded body, the coating amount of the catalyst composition increases, and the back pressure of the catalyst increases, which is not preferable.

[0025]

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

Example 1 Commercially available cerium oxide (specific surface area 149 m ² / g) was treated with an aqueous zirconyl oxynitrate solution.
CeO ₂ / ZrO ₂ is 100/10 (CeO ₂ +
ZrO ₂ is mixed at a ratio of 100 g), dried and then 500
100 g of powder calcined at ℃ × 1 hour (CeO ₂ · Zr
O ₂ ) was obtained. Then, as a palladium nitrate aqueous solution containing 1.667 g of palladium, a rhodium nitrate aqueous solution containing 0.333 g of rhodium and a lanthanum oxide, 20
180 g of activated alumina (γ-Al ₂ O ₃ , specific surface area 155 m ² / g) was added to a solution obtained by mixing an aqueous solution of lanthanum acetate to be g.
After impregnating with g and drying, 200 g of powder was obtained by firing at 500 ° C. for 1 hour.

The powder obtained above and barium acetate 16.
An aqueous slurry was prepared by wet grinding 7 g with a ball mill. Cordierite monolith carrier with 400 cells per square inch cross section (inner diameter 33 mm x
A length of 76 mmL) was dipped in the above slurry, taken out, and the excess slurry in the cell was blown off with compressed air, followed by drying and firing if necessary to obtain a finished catalyst.

Example 2 The cerium oxide used in Example 1 was mixed with an aqueous solution of zirconyl oxynitrate to CeO ₂ / ZrO _2.
Ratio of 10/1 (the total of CeO ₂ and ZrO ₂ is 100
g) and lanthanum acetate aqueous solution containing 20 g of lanthanum oxide were mixed, dried and then dried at 500 ° C. for 1 hour to obtain 120 g of powder. Then 1.66 of palladium
Palladium nitrate aqueous solution containing 7 g and rhodium 0.3
180 g of the same activated alumina as used in Example 1 was impregnated into a solution containing 33 g of an aqueous rhodium nitrate solution, dried, and then fired at 500 ° C. for 1 hour to give 180 g of powder.
Got 16.7 g of the above-obtained powder and barium acetate
Was pulverized with a ball mill to obtain a slurry.
A completed catalyst was obtained in the same manner as in.

Example 3 CeO ₂ obtained in Example 1
Obtained by adding 100 g of ZrO ₂ powder and 180 g of palladium- and rhodium-supported alumina obtained in Example 2 to a powder of lanthanum acetate aqueous solution containing 20 g of lanthanum oxide, and drying and firing at 500 ° C. for 1 hour. 300 g of the powder and 16.7 g of barium acetate were wet pulverized with a ball mill to obtain a slurry, and a completed catalyst was obtained in the same manner as in Example 1.

Example 4 CeO ₂ obtained in Example 1
100 g of ZrO ₂ powder, palladium obtained in Example 2, 180 g of rhodium-supported alumina, 20 g of lanthanum acetate as lanthanum oxide, and 16.7 g of barium acetate were wet-milled with a ball mill to obtain a slurry. A completed catalyst was obtained in the same manner as in 1.

Example 5 A completed catalyst was obtained in the same manner as in Example 1 except that 20 g of lanthanum oxide was changed to 1 g.

Example 6 A completed catalyst was obtained in the same manner as in Example 1 except that 20 g of lanthanum oxide was changed to 90 g.

Example 7 Example 1 was repeated except that 16.7 g of barium acetate was changed to 133.6 g.
A completed catalyst was obtained in the same manner as in.

(Example 8) A completed catalyst was obtained in the same manner as in Example 1 except that 16.7 g of barium acetate was changed to 0.83 g.

(Example 9) A completed catalyst was obtained in the same manner as in Example 1 except that barium acetate was changed to 28.2 g of calcium acetate.

(Example 10) A completed catalyst was obtained in the same manner as in Example 1 except that barium acetate was changed to strontium acetate 19.8 g.

(Embodiment 11) In Embodiment 1, CeO
_A completed catalyst was obtained in the same manner as in Example 1 except that the ratio of ₂ / ZrO ₂ was changed to 10/1 (the total of CeO ₂ and ZrO ₂ was 30 g).

(Embodiment 12) In Embodiment 1, CeO
_A completed catalyst was obtained in the same manner as in Example 1 except that the ratio of ₂ / ZrO ₂ was changed to 10/1 (the total of CeO ₂ and ZrO ₂ was 200 g).

(Example 13) In Example 1, CeO
_A completed catalyst was obtained in the same manner as in Example 1 except that the ratio of ₂ / ZrO ₂ was changed to 10/1 (the total of CeO ₂ and ZrO ₂ was 280 g).

Example 14 In Example 1, CeO was used.
_A completed catalyst was obtained in the same manner as in Example 1 except that the ratio of ₂ / ZrO ₂ was changed to 10/3 (the total amount of CeO ₂ and ZrO ₂ was 100 g).

Example 15 In Example 1, CeO was used.
_A completed catalyst was obtained in the same manner as in Example 1 except that the ratio of ₂ / ZrO ₂ was changed to 25/1 (total of CeO ₂ and ZrO ₂ was 100 g).

(Example 16) In Example 16, except that barium acetate was changed to 35.3 g of magnesium acetate,
A completed catalyst was obtained in the same manner as in Example 1.

(Example 17) In Example 2, the palladium / rhodium-containing powder was added to an aqueous palladium nitrate solution containing 0.833 g of palladium, an aqueous dinitrodiamine platinum solution containing 0.833 g of platinum, and 0.3 g of rhodium.
Activated alumina (γ · Al ₂ O ₃ , specific surface area 155 m ² / g) was added to a solution containing 33 g of rhodium nitrate aqueous solution.
A finished catalyst was obtained in the same manner as in Example 1, except that 80 g of the powder was impregnated and supported.

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

Comparative Example 2 A finished catalyst was obtained in the same manner as in Example 1 except that zirconyl oxynitrate was omitted.

(Comparative Example 3) Example 1 is the same as Example 1 except that zirconyl oxynitrate and barium acetate are removed.
A completed catalyst was obtained in the same manner as in.

(Comparative Example 4) A finished catalyst was obtained in the same manner as in Example 1 except that the lanthanum oxide was removed.

Comparative Example 5 A dinitrodiamine platinum aqueous solution containing 1.667 g of platinum and 0.333 of rhodium.
Activated alumina (γ · Al ₂ O ₃ , specific surface area 155 m ² / g) 200 was added to a solution containing a rhodium nitrate aqueous solution containing g.
A powder impregnated with g and 100 g of a commercially available cerium oxide (specific surface area 149 m ² / g) were wet pulverized with a ball mill to obtain an aqueous slurry, and a completed catalyst was obtained in the same manner as in Example 1.

Comparative Example 6 A completed catalyst was obtained in the same manner as in Comparative Example 5 except that the aqueous dinitrodiamine platinum solution containing 1.667 g of platinum was replaced with the aqueous palladium nitrate solution containing 1.667 g of palladium. It was

Table 1 shows the supported amounts of the respective catalyst components per liter of the catalysts of Examples and Comparative Examples thus obtained.

(Example 18) Next, the catalyst activities of the catalysts of Examples 1 to 17 and the catalysts of Comparative Examples 1 to 6 were examined after the running of the engine for a long time.

Commercially available electronically controlled engine (8 cylinders 4
400cc) was used, and a multi-converter filled with each catalyst was connected to the exhaust system of the engine for a durability test. The engine operates 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 gas temperature at the catalyst inlet is 900 ° C during steady operation. The catalyst was aged for 50 hours under the following conditions.

The catalyst performance after aging was evaluated by using a commercially available electronically controlled engine (four-cylinder 1800 cc) and connecting a multi-converter filled with each catalyst to the exhaust system of the engine. The three-way performance of the catalyst was evaluated under the conditions of a catalyst inlet gas temperature of 400 ° C. and a space velocity of 90,000 hr- ¹ . At this time, a 1 Hz sine wave type signal was introduced into the engine control unit from an external oscillator to continuously change the average air-fuel ratio while oscillating the air-fuel ratio (A / F) at ± 1.0 A / F and 1 Hz. At the same time, the catalyst inlet and outlet gas compositions were analyzed at the same time, and the average air-fuel ratio from A / F 15.1 to 14.1 CO, HC and N
The purification rate of O was determined.

CO, HC and N determined 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 (called a crossover point) of the CO and NO purification rates 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.
It was shown to.

From Table 2, the catalyst disclosed in the present invention is C
It can be seen that the three components of O, HC and NOx can be simultaneously removed with high performance.

Further, the purification performance of the catalyst at low temperature is as follows while vibrating the air-fuel ratio at ± 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 200 ° C to 500 ° C.
Was continuously changed, the catalyst inlet and outlet gas compositions were analyzed, and the purification rates of CO, HC and NO were obtained and evaluated. CO, HC and N determined as above
The temperature (light-off temperature) at a purification rate of O of 50% was measured and is shown in Table 2.

The catalysts disclosed in the present invention are
It can be seen that it is a high-performance Pd / Rh catalyst capable of simultaneously removing three components of HC, CO and NO and overcoming the problem of Pd.

[0058]

[Table 1]

[0059]

[Table 2]

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

Claims (4)

[Claims] 1. A catalyst for simultaneously removing carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas of an internal combustion engine, wherein (a) palladium and rhodium, or (b) palladium, rhodium and platinum, and An exhaust gas purifying catalyst, characterized in that an integral structure is coated with a catalytically active component containing an alkaline earth metal oxide, a lanthanum oxide, a cerium oxide, a zirconium oxide and a refractory inorganic oxide. 2. The catalyst according to claim 1, wherein at least a part of the cerium oxide and the zirconium oxide is present as a composite or solid solution. 3. The ratio of cerium oxide to zirconium oxide (weight ratio in terms of oxide) is 100: 2 to 100: 60.
The catalyst according to claim 1 or 2, which is 4. The lanthanum oxide is a refractory inorganic oxide,
The catalyst according to claim 1, which is supported on at least one of cerium oxide and zirconium oxide.