JPH10290931A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain NOx diminishing function effective in the wide temp. range of exhaust gas from a low temp. to a high temp. over a long period of time by using a mixture of a noble metallic catalyst carried on an alumina or zirconia carrier with an NOx occluding material made of a perovskite type multiple oxide. SOLUTION: This catalyst for purification of exhaust gas is made of a mixture of a noble metal carrying catalyst with an NOx occluding material made of a multiple oxide having a perovskite crystal structure such as perovskite (CaO-TiO2 ), BaO-TiO2 , ZrO2 -Al2 O3 or a mixture of them. The noble metal carrying catalyst is prepd. by mixing an aq. soln. of nitrate or chloride of a noble metal such as Pt or Pd with alumina and/or zirconia, drying and firing the resultant mixture. The multiple oxide occludes NOx in a low or medium temp. range of exhaust gas but loses its occluding performance in accordance with temp. rise and becomes liable to discharge occluded NOx. Since discharged NOx is, however, efficiently reduced with the high activity noble metal carrying catalyst, the function of the multiple oxide can be synthetically improved by combination with the catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel catalyst for reducing nitrogen oxides (NOx) contained in exhaust gas from an internal combustion engine and purifying the exhaust gas. More specifically, the present invention relates to a novel exhaust gas purifying catalyst capable of effectively reducing NOx in a wide temperature range from a low temperature to a high temperature irrespective of fluctuations in exhaust gas temperature.

[0002]

2. Description of the Related Art Exhaust gas containing NOx generated as a result of combustion of fuel in an internal combustion engine of an automobile or the like is processed and the NO.
Various catalysts have been proposed for reducing the x content. One example is a copper-zeolite catalyst,
That is, there is a copper ion exchanged zeolite catalyst obtained by exchanging Na ions of Na type ZSM-5 zeolite with Cu ions. This is used for exhaust gas treatment while being coated on the surface of a monolith honeycomb carrier having a large number of through holes, typically made of ceramics such as cordierite. This catalyst is 300-500 ° C
Has a function of performing the selective reduction of NOx with high efficiency in the above temperature range, but there is a practical disadvantage that the selective reduction efficiency decreases with the elapse of use time. This is because moisture and SO ₂ in the exhaust gas are adsorbed in the network molecular structure of the copper ion exchanged zeolite, and the presence of the adsorbed moisture and SO ₂ significantly inhibits the NOx selective reduction function. Therefore, the copper ion exchanged zeolite catalyst has a problem in practical use such as low durability or short service life.

[0003] Further, as a catalyst for treating exhaust gas to reduce NOx, there is a platinum-alumina catalyst, which is generally coated on the surface of a ceramic honeycomb carrier for exhaust gas treatment. And exhibits NOx reduction activity in the temperature range of approximately 160 ° C. to 300 ° C., but the NOx reduction rate near both ends of this temperature range is so low as to be less effective.

[0004] Therefore, it is extremely practically desirable to improve the durability (service life) of a catalyst for treating exhaust gas and to extend the NOx reduction activation temperature range.

[0005] For example, as a catalyst that is stable and reduces NOx contained in exhaust gas with high efficiency, Japanese Patent Application Laid-Open No. H10-163,873 is disclosed.
296198 describes SiO ₂ , Al ₂ O ₃ , Ti
There is disclosed an exhaust gas purifying catalyst in which a noble metal is supported on a composite oxide comprising at least _two of O ₂ and ZrO ₂ .

[0006]

SUMMARY OF THE INVENTION The present inventor has proposed that exhaust gas N
The NOx reduction function of the Ox reduction catalyst is effectively exhibited over a wide temperature range from a low temperature range to a high temperature range of exhaust gas,
In addition, intensive studies and investigations were conducted to find a configuration of the catalyst that would maintain the function for a long period of time, and as a result, the present invention was completed.

[0007]

Thus, the present invention provides a NOx occluding material comprising a specific composite oxide, which is a conventional noble metal-containing NOx occluding material.
It is based on the basic concept of being used with an alumina catalyst. That is, the NOx storage material composed of the specific composite oxide used in the present invention efficiently stores NOx in exhaust gas at an exhaust gas temperature at which the NOx reduction activity of the noble metal-amylna catalyst is not sufficiently exhibited, that is, at a low or medium temperature. Noble metal-amylna catalyst NO
The stored NOx is released at the time of high temperature of the exhaust gas where the x reduction activity is effectively increased. So released NOx is noble has increased activity at high temperature - is readily reduced to the NO ₂ and / or N ₂ by contact with Amiruna catalyst.

Accordingly, the present invention provides an exhaust gas purifying catalyst comprising a mixture of a noble metal catalyst supported on an amylna or zirconia carrier and a NOx storage material composed of a perovskite-type composite oxide.

Examples of the noble metal in the noble metal supported catalyst used in the present invention include platinum, palladium, rhodium,
Iridium can be mentioned, and these can also be used in appropriate combination. Typical examples of supports for supporting these noble metals are amylna, zirconia, and mixtures thereof. The noble metal supported catalyst can be prepared by mixing an aqueous solution of a nitrate or chloride of a noble metal such as platinum, palladium, rhodium or iridium with amylna, zirconia or a mixture thereof, drying and calcining.

[0010] composite oxide is employed as the NOx storage material in the present invention, the crystal structure type is perovskite (cubic), and specific examples thereof include perovskite _{(CaO-TiO 2, BaO-} TiO 2, SrO −Ti
O ₂ ), ZrO ₂ —Al ₂ O ₃ , CoO ₂ —Al ₂ O ₃ , Cu
There are O ₂ -Al ₂ O ₃ , SiO ₂ -Al ₂ O ₃ or a mixture thereof, and CuO ₂ -Al ₂ O ₃ (copper oxide-amylna), CoO ₂ -Al ₂ O ₃ (cobalt oxide-amylna)
Or mixtures thereof are preferred. These composite oxides can be used alone in a low to medium temperature range (approximately 26
(In the range of 0 ° C. to 380 ° C.), there is a tendency to occlude NOx, and when the temperature is further increased, the occlusion characteristic disappears, and on the contrary, the characteristic tendency to release occluded NOx becomes remarkable. As described above, the NOx released here is efficiently reduced by the action of the highly active noble metal-supported catalyst present in the site.

The noble metal-supported catalyst is prepared by mixing and kneading an aqueous solution of a nitrate or chloride of a noble metal such as platinum, palladium, rhodium or iridium with powder of amylna or zirconia serving as a carrier, or a mixture thereof, and mixing the mixture at a temperature of 100. Dry at ~ 120 ° C, then temperature 400 ~
650 ° C., preferably at 450-600 ° C.
It can be prepared by baking for 6 hours. The fired product is crushed or pulverized into a powder. The amount of the noble metal nitrate or chloride used is such that about 0.05 to 10% by weight of the noble metal is introduced and supported in terms of noble metal (for example, in terms of Pt) in the calcined product obtained above. The various noble metal nitrates or chlorides described above may be used as a mixture of two or more. Typical examples of the composition of the noble metal-supported catalyst thus obtained are Pt—Al ₂ O ₃ , Pd—Al ₂ O ₃ , R
_{h-Al 2 O 3, Ir} -Al 2 O 3, Pt-ZrO 2, Pd
_{-ZrO 2, Rh-ZrO 2,} Ir-ZrO 2, Pt / R
h-Al ₂ O ₃ and Pt / Rh-ZrO ₂ .

As a composite oxide constituting the NOx storage material to be mixed with the noble metal-supported catalyst, one which can be used for producing the exhaust gas purifying catalyst of the present invention has a perovskite type crystal structure (cubic lattice). As a specific example, the perovskite (CaO-TiO.
₂ ), ZrO ₂ —Al ₂ O ₃ , CoO ₂ —Al ₂ O ₃ , Cu
There are O ₂ -Al ₂ O ₃ , SiO ₂ -Al ₂ O _{3 and the} like.

The exhaust gas purifying catalyst according to the present invention is used in an exhaust line between an exhaust manifold and an exhaust muffler of an internal combustion engine so as to be in contact with an exhaust gas flow. The catalysts of the present invention can be conveniently used as compacts, such as pellets, honeycomb compacts, or coated on the surface of a ceramic or metal carrier having a honeycomb structure.

In producing the exhaust gas purifying catalyst of the present invention, a noble metal-supported catalyst (powder) prepared as described above and a perovskite-type composite oxide (powder) are mixed well to obtain a frit-like catalyst powder mixture. . The particle size of both the starting noble metal-supported catalyst powder and the composite oxide powder is not a critical factor, but may be, for example, in the range of about 0.1 mm to several microns, with certain non-limiting preferred examples being about A particle size around 150 mesh to 100 mesh can be mentioned.

An inorganic or organic binder known to those skilled in the art is added to the frit-like catalyst powder mixture to form a kneaded product exhibiting appropriate plasticity for molding, and to form a pellet or honeycomb having a large specific surface area. Extrusion processing. The molded articles such as pellets, honeycombs and the like thus obtained are subjected to an appropriate drying and / or calcination treatment as required, and become the exhaust gas purifying catalyst according to the present invention. Is loaded into the area.

Alternatively, an inorganic or organic binder and an appropriate amount of a medium (typically, water) are added to the frit-like catalyst powder mixture and mixed well to form a slurry, and the slurry is made of a ceramic material having a large specific surface area. (For example, a honeycomb made of cordierite) or a metal honeycomb (for example, a honeycomb made of stainless steel) is coated on a carrier surface and subjected to an appropriate heat treatment (drying and / or firing) to be disposed in an exhaust gas line. It can be a honeycomb catalyst.

Examples of the inorganic binder used above include alumina sol, silica sol, aluminum nitrate and talc, and examples of the organic binder include carbohydrates.

FIG. 1 is a schematic diagram of a microscopic structure of the exhaust gas purifying catalyst according to the present invention in a state of being solidified. Reference numeral 1 denotes a carrier particle such as alumina or zirconia, and a noble metal 2 (shown as a plurality of small black spots) is carried on the surface thereof. Numeral 3 denotes particles of the NOx storage material.

The ratio of the noble metal-supported catalyst to the NOx storage composite oxide in the exhaust gas purifying catalyst according to the present invention is approximately 8: 1 to 1: 2, preferably 5: 1 to 1: 1 on a weight basis. is there.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION 300 g of platinum nitrate, rhodium nitrate or iridium nitrate, calculated as containing a noble metal and containing 2.5 g of a noble metal, is mixed in 300 ml of water, mixed well, and stirred.
Was added and mixed well so as to be homogeneous. The excess aqueous phase is drained from the aqueous mixture and the residue is dried at about 100 ° C. for 5 hours and then
From 60 to 580 ° C. in a hydrogen-containing nitrogen gas stream (1% H _2/99
% N ₂ vol) heat-treated for 3 hours in, microscopic examination to the alumina support precious metal fine particles on the surface of the particles the noble metal loaded catalyst was obtained which deposited spots. This was lightly crushed and pulverized to obtain a powder having a particle size of 150 to 100 mesh. The content (weight basis) of each noble metal was as follows.

A. Pt supported catalyst 1.3% Bt. Rhodium supported catalyst Rh 1.2% C.I. Iridium supported catalyst Mixing with Ir 1.3% composite oxide (NOx storage material) A '. The above A. A platinum supported catalyst and perovskite were mixed at a weight ratio of 2: 1.

B '. B. above. Rhodium supported catalyst and Co
O ₂ -Al ₂ O ₃ composite oxide in a weight ratio of 2: 1 mixture.

C '. C. above. Iridium supported catalyst and C
The uO ₂ -Al ₂ O ₃ composite oxide was mixed at a weight ratio of 2: 1.

The catalyst core of the cordierite honeycomb carrier
Coating of pure water, the alumina sol binder and said A '(platinum),
A mixture of B '(rhodium) or C' (iridium) was mixed at a weight ratio of 70:20:50, and sufficiently stirred and mixed to obtain a uniform slurry. A cordierite honeycomb carrier is immersed in the slurry, pulled up, dried in the air at 100 ° C. for 4 hours, and then fired in the air at 600 ° C. for 10 hours,
It was allowed to cool to obtain a catalyst carrier coated with a catalyst.

As a comparative example (control), B.I. A catalyst in which only a rhodium-supported catalyst was coated on a cordierite honeycomb by the same operation as above was prepared.

NOx Reduction Evaluation Test Each of the honeycombs coated with the catalysts A ′, B ′ and C ′ and the honeycomb of the comparative example were mounted on a fixed-bed flow-type reactor, and a gas having the following composition simulating diesel engine exhaust gas was used. The mixture was subjected to a space velocity (SV) = 20,000
At 0 hr ^-1 , the catalyst inlet temperature was variously changed, and the exhaust gas downstream of the catalyst was analyzed by an exhaust gas analyzer to measure the NOx reduction rate.

Simulated exhaust gas composition: NO 1,000 ppm C ₃ H ₆ 1,300 ppm O ₂ 10% SO ₂ 20 ppm H ₂ O 4% N ₂ balance The NOx reduction rate (%) at various temperatures for each catalyst is as follows: It was right.

Catalyst A 'coating B' coating C 'coating Comparative Example (Pt) (Rh) (Ir) (Rh) 200 ° C. 3.2 00 250 ° C. 9.0 2.0 1.5 1.5 300 ° C 12.5 13.3 7.2 2.8 350 ° C 11.1 21.4 16.0 8.1 400 ° C 9.8 17.6 22.4 10.1 450 ° C 4.8 9.8 14 2.7 3.4 500 ° C. 2.5 2.5 5.0 0 FIG. 2 shows B ′ (rhodium-supported catalyst + N
A graph of the NOx reduction rate (vs. catalyst inlet temperature) of the coated honeycomb catalyst (Ox storage material) and the rhodium-supported catalyst coated catalyst of the comparative example is shown for reference.

The catalyst of the present invention does not show any tendency to deteriorate the NOx selective reduction function due to moisture or SO ₂ in the known copper ion exchanged zeolite catalyst.

[0030]

The use of the NOx storage material together with the noble metal-supported catalyst not only significantly improves the NOx reduction function in the low to medium temperature range, but also improves the NOx reduction in the high temperature range.
An exhaust gas purifying catalyst having an overall improved Ox reduction function has been realized.

[Brief description of the drawings]

FIG. 1 is a schematic structural view of a solidified product of an exhaust gas purifying catalyst obtained by using a composite oxide NOx storage material of the present invention and a noble metal-supported catalyst in combination.

FIG. 2 shows NOx at various exhaust gas catalyst inlet temperatures of an example of an exhaust gas purifying catalyst according to the present invention and a comparative example (control).
5 is a graph showing a reduction rate.

[Explanation of symbols]

 1 alumina or zirconia carrier particles 2 noble metal fine particles 3 occlusion material particles

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 35/04 B01J 35/04 301Z 301 37/02 301C 37/02 301 B01D 53/36 102A

Claims (5)

[Claims] 1. An exhaust gas purifying catalyst comprising a mixture of a catalyst made of a noble metal supported on an alumina or zirconia carrier and a NOx storage material made of a perovskite-type composite oxide. 2. The exhaust gas purifying catalyst according to claim 1, wherein the noble metal is one or more selected from platinum, palladium, rhodium and iridium. 3. The method according to claim 1, wherein the perovskite-type composite oxide is copper oxide-
3. The exhaust gas purifying catalyst according to claim 1, which is alumina, cobalt oxide-alumina or a mixture thereof. 4. The exhaust gas purifying catalyst according to claim 1, wherein the catalyst is formed in a pellet or honeycomb shape. 5. The exhaust gas purifying catalyst according to claim 1, which is coated on the surface of a ceramic or metal carrier having a honeycomb structure.