JP2003320222A - Method for catalytically removing nitrogen oxide and apparatus therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for catalytically removing NOx in an exhaust gas formed by the combustion of a fuel under a cyclic rich/lean condition without deteriorating a catalyst, and an apparatus therefor. <P>SOLUTION: The exhaust gas is brought into contact with an upstream catalyst, which has a first catalyst component comprising at least one component selected from a mixture of (a) ceria or (b) praseodymium oxide and oxides of at least two elements (c) selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum and a composite oxide of those two elements, and subsequently brought into contact with a downstream catalyst having at least one catalyst component selected from (a) platinum, rhodium, palladium and oxides of them, (b) an NOx absorbent/ adsorbent and (c) a carrier as catalytic components. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to catalytic removal of nitrogen oxides (mainly consisting of NO and NO ₂ ; hereinafter referred to as NOx) by catalytic reaction, that is, for catalytic decomposition and / or reduction. Method and apparatus therefor. More specifically, the present invention supplies fuel to a combustion chamber of a diesel engine or a gasoline engine for combustion in a periodic rich / lean fuel supply process (excursion) and burns the generated exhaust gas to an upstream catalyst and a downstream catalyst. The present invention relates to a method for catalytically removing NOx in exhaust gas by sequentially contacting in two stages, and an apparatus therefor. Such a method and a device therefor are suitable, for example, for purifying exhaust gas by reducing and removing harmful nitrogen oxides contained in exhaust gas from an engine of an automobile.

In particular, the present invention relates to sulfur oxides (mainly
It consists of SO ₂ and SO ₃ . Hereinafter referred to as SOx. In the presence of), fuel is supplied and burned in a periodic rich / lean fuel supply stroke, and NOx in exhaust gas generated by the combustion is catalytically removed without deterioration of the catalyst. The present invention relates to a catalyst device therefor.

In the present invention, the above "stroke" (excursio)
The term n) means that the air / fuel ratio moves from its average value both along the time axis or both. The term "rich" means that the air / fuel ratio of the fuel in question is less than the stoichiometric air / fuel ratio, and the term "lean" the air / fuel ratio of the fuel in question / It means that the fuel ratio is greater than the stoichiometric air / fuel ratio. In normal motor gasoline, the stoichiometric air / fuel ratio is about 14.5.
Further, in the present invention, the "catalyst" means a catalyst or a structure including the catalyst that operates to remove NOx during rich / lean combustion of fuel.

Therefore, in the present invention, "supplying fuel in a periodic rich / lean fuel supply process" means that combustion of fuel is mainly performed under lean conditions (after combustion) in a combustion chamber of a diesel engine or a gasoline engine. The oxygen concentration in the exhaust gas is usually about 5% to 10%).
It refers to supplying, injecting or injecting fuel while adjusting the air / fuel ratio so that the atmosphere is periodically oscillated alternately between the rich condition and the lean condition. Therefore, the rich / lean stroke is synonymous with the rich / lean condition.

Conventionally, for example, a method of oxidizing NOx contained in exhaust gas and then absorbing it in alkali,
Ammonia, hydrogen, carbon monoxide, or hydrocarbon is used as a reducing agent and is removed by a method such as reduction to nitrogen. However, each of these conventional methods has drawbacks.

That is, according to the former method, a means for treating the generated alkaline wastewater is necessary in order to prevent environmental problems. According to the latter method, for example, when ammonia is used as a reducing agent, ammonia reacts with SOx in exhaust gas to form salts, and as a result,
The catalytic activity decreases at low temperatures. In addition, the safety of NOx from a mobile source such as an automobile becomes a problem, in particular.

On the other hand, when hydrogen, carbon monoxide or hydrocarbon is used as the reducing agent, these reducing agents preferentially react with oxygen because the exhaust gas contains oxygen at a higher concentration than NOx. Therefore, if it is attempted to substantially reduce NOx, a large amount of reducing agent will be required, and fuel consumption will be greatly reduced.

Therefore, NO is used without using a reducing agent.
It has been proposed to catalytically decompose x. But no
For the direct decomposition of x, the catalysts known to date are
Due to its low decomposition activity, it has not yet been put to practical use. On the other hand, various zeolites have been proposed as NOx catalytic reduction catalysts that use hydrocarbons and oxygen-containing organic compounds as reducing agents. In particular, copper ion exchanged ZSM-5 and H type (hydrogen type or acid type) zeolite ZSM-5 (SiO ₂ /
Al ₂ O ₃ molar ratio = 30 to 40) are to be optimal. However, even H-type zeolite does not have sufficient reducing activity, and particularly when the exhaust gas contains water, the zeolite catalyst may rapidly deteriorate in performance due to dealumination of the zeolite structure. Are known.

Under these circumstances, there is a demand for the development of a more highly active catalyst for NOx catalytic reduction. For example, EP-A1-526099 and EP-A1-6794 are required.
As described in No. 27 (corresponding to JP-A2-5317647), recently, a catalyst in which silver or silver oxide is supported on an inorganic oxide carrier material has been proposed. It is known that this catalyst has a high oxidation activity, but has a low selective reduction activity for NOx, so that the conversion rate of NOx to nitrogen is slow. Moreover, this catalyst has a problem that its activity is rapidly lowered in the presence of SOx. These catalysts have a catalytic effect of reducing NOx to a certain extent by using hydrocarbons under a completely lean condition, but have a lower NOx removal rate than a three-way catalyst and have an operating temperature window ( The narrow temperature range makes it difficult to put such a lean NOx catalyst into practical use. Thus,
There is an urgent need for a catalyst with higher heat resistance and higher activity for NOx catalytic reduction.

In order to overcome the above-mentioned problems, recently, as described in the Society of Automotive Engineers magazine (SAE) (August 9, 1995), NOx storage-reduction has recently been performed. The system has been proposed as the most promising method. According to this proposal (Toyota Motor Co., Ltd.), fuel is periodically supplied to the combustion chamber in an amount exceeding the stoichiometric amount for a short time. Vehicles with lean-burn engines can be driven at very low fuel / air ratios, resulting in lower fuel consumption than vehicles with conventional engines. Such a lean-burn engine NOx storage-reduction system reduces NOx by two cyclic steps at 1-2 minute intervals.

That is, in the first step, (normal)
Under lean conditions, NO is oxidized to NO ₂ on platinum and rhodium catalysts and this NO ₂ is adsorbed by alkaline compounds such as potassium carbonate and barium carbonate. Then, a rich condition for the second step is formed, and this rich condition is maintained for several seconds. Under this rich condition, the above adsorption (storage)
The NO ₂ thus produced is efficiently reduced to nitrogen by using hydrocarbon, carbon monoxide or hydrogen on a platinum or rhodium catalyst.
This NOx storage-reduction system works well for long periods in the absence of SOx. But SO
If x exists, SOx is irreversibly adsorbed at the NO ₂ adsorption site on the alkaline compound under both lean and rich conditions, so that there is a problem that the catalyst system deteriorates rapidly.

[0012]

SUMMARY OF THE INVENTION The present invention burns fuel under cyclic rich / lean conditions in the presence of oxygen, sulfur oxides or water, and over a wide range of reaction temperatures. N in the exhaust gas generated by this combustion
It is an object of the present invention to provide a method for catalytically removing Ox without deterioration of a catalyst and an apparatus therefor.

[0013]

According to the present invention, fuel is supplied and burned under periodic rich / lean conditions, and exhaust gas produced is brought into contact with a catalyst to remove nitrogen oxides in the exhaust gas. In the method of catalytically removing the exhaust gas, at least two elements selected from (A) (a) ceria or (b) praseodymium oxide or (c) cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum. A first catalyst component consisting of at least one kind selected from a mixture of oxides of the above and a composite oxide of at least those two elements, and (d) at least one kind selected from palladium, rhodium, platinum and their oxides. And a second catalyst component consisting of
(A) at least one catalyst component selected from platinum, rhodium, palladium and oxides thereof, and (b) NOx
There is provided a method for catalytically removing nitrogen oxides in exhaust gas, which comprises contacting a downstream catalyst having an absorbent / adsorbent and a carrier (c) as catalyst components.

Furthermore, according to the present invention, the periodic rich /
(A) (a) Ceria or (b) Oxidation in a device for catalytically removing nitrogen oxides in the exhaust gas by supplying fuel under lean conditions and burning the fuel to bring the generated exhaust gas into contact with a catalyst. Praseodymium or (c) at least one selected from a mixture of oxides of at least two elements selected from cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum, and a composite oxide of at least those two elements. An upstream catalyst device having as a catalyst component a first catalyst component consisting of (d) and a second catalyst component consisting of (d) at least one selected from palladium, rhodium, platinum and oxides thereof, and (B) (a) platinum At least one catalyst component selected from the group consisting of, rhodium, palladium and oxides thereof, and (b) NOx absorption / Catalyst apparatus is provided for catalytically removing nitrogen oxides in the exhaust gas, characterized by comprising a downstream catalyst device having a Chakuzai and (c) a carrier as a catalyst component.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION According to the method of the present invention, fuel is supplied and burned under cyclic rich / lean conditions, and exhaust gas produced is brought into contact with a catalyst to remove nitrogen oxides in the exhaust gas. In the method of catalytically removing, the exhaust gas is first contacted with an upstream catalyst, that is, an upstream catalyst, as seen from the flow of exhaust gas from the engine combustion chamber, and then from the flow of exhaust gas from the engine combustion chamber. Then, the catalyst on the downstream side, that is, the downstream catalyst is brought into contact with the catalyst. That is, according to the method of the present invention, the exhaust gas is brought into contact with the upstream catalyst and the downstream catalyst sequentially in two steps.

More specifically, as shown in FIG. 1, according to the present invention, fuel is supplied to the combustion chamber 1 of a diesel engine or a gasoline engine in a periodic rich / lean fuel supply stroke to burn the fuel. As a result, the generated exhaust gas is guided to the exhaust pipe 2, and is brought into contact with the upstream catalyst in the upstream catalyst device 4 provided in the expanded portion 3 of the exhaust pipe, and then brought into contact with the downstream catalyst in the downstream catalyst device 5, Thus, NOx in the exhaust gas is catalytically removed to purify and discharge the exhaust gas. In each of the upstream and downstream catalytic devices, each catalyst is fixed to the pipe expanding section by heat-resistant packings 6 and 7, for example.

As described below, according to a preferred embodiment of the present invention, the upstream catalyst component and the downstream catalyst component are respectively
This is supported on an inert honeycomb base material to form a honeycomb catalyst structure, which is filled in the catalyst device and brought into contact with the exhaust gas guided to the catalyst device. That is, according to a preferred aspect of the present invention, the catalyst component is supported in a layered manner on at least the surface of the wall surface of the through hole of the honeycomb substrate in the honeycomb catalyst structure, and the exhaust gas from the combustion chamber is Guided to the catalyst device filled with the honeycomb catalyst structure, where it is brought into contact with the catalyst layer on the wall surface while passing through the through holes of the honeycomb catalyst structure,
NOx in the exhaust gas is catalytically removed.

In the present invention, the catalytic removal of nitrogen oxides means that NOx is converted into nitrogen and water, carbon monoxide, carbon dioxide, etc. by capturing, desorbing and reducing NOx in a downstream catalyst, as described later. To be converted.

According to the invention, the upstream catalyst is (A)
(A) Ceria or (b) Praseodymium oxide or (c) Cerium, zirconium, praseodymium, a mixture of oxides of at least two elements selected from neodymium, terbium, samarium, gadolinium and lanthanum, and complex oxidation of at least those two elements A first catalyst component consisting of at least one member selected from the group consisting of the following, and (d) a second catalyst component consisting of at least one member selected from palladium, rhodium, platinum and their oxides, as upstream catalyst components. The downstream catalyst comprises (B) (a) at least one catalyst component selected from platinum, rhodium, palladium and oxides thereof, (b) a NOx absorption / adsorbent, and (c) a carrier. Have as.

According to the present invention, the exhaust gas is brought into contact with the upstream catalyst and the downstream catalyst sequentially. Where the upstream catalyst is
It has the first catalyst component and the second catalyst component (noble metal catalyst component) as catalyst components. Such an upstream catalyst is
Under rich conditions, the second catalyst component (noble metal catalyst component) is reduced and at the same time the first catalyst component, ie oxide (eg ceria) is reduced (ie loses some of its oxygen). On the other hand, under the lean condition, the second catalyst component is oxidized and the first catalyst component (for example,
Cerium) is also oxidized (ie stores oxygen in the gas phase). Thus, the first catalyst component in the upstream catalyst is a substance having an oxygen storage capacity (hereinafter referred to as OSC functional substance).
Function as.

According to the present invention, the SOx contained in the exhaust gas is mixed with the first catalyst component (OSC functional substance) and the second catalyst component (noble metal catalyst component) in the upstream catalyst component.
Is adsorbed as SO ₂ (in some cases, SO ₂ and SO ₃ ) on the upstream catalyst under lean conditions, and is desorbed as some sulfur compound which is not captured by the downstream catalyst at the time of enrichment, so the downstream catalyst is deteriorated by SOx. Is prevented. Although the sulfur compound is not clear yet, in the present invention, it has been confirmed as a result of analysis that sulfur is not accumulated in the downstream catalyst.

On the other hand, regarding catalytic removal of NOx according to the present invention, the catalytic components of the downstream catalyst are the noble metal catalytic component, the NOx absorbing / adsorbing agent and the carrier, and the N
Ox is removed by such a downstream catalyst under lean conditions. That is, under lean conditions, most of the NOx in the exhaust gas is oxidized on the downstream catalyst and NOx
It is removed from the exhaust gas by being absorbed / adsorbed on the absorbent / adsorbent. Next, under rich conditions, the noble metal catalyst component in the downstream catalyst is reduced, NOx in the exhaust gas is catalytically reductively decomposed on this noble metal catalyst component, and is also absorbed / adsorbed by the NOx absorbent / adsorbent. The NOx that had been released is desorbed, and this is also reduced and decomposed on the precious metal catalyst component. That is, according to the present invention, the catalyst reaction atmosphere is either lean or rich,
NOx is thus purified in the downstream catalyst by a mechanism including NOx trapping / desorption and reduction.

Further, according to the present invention, in the "rich responsiveness" defined as the easiness or speed of reduction of the catalyst component when the atmosphere of the catalytic reaction changes from the lean condition to the rich condition, The downstream catalyst is excellent.
In particular, by making the downstream catalyst contain platinum as a catalyst component, the rich responsiveness of the downstream catalyst is further enhanced.

On the other hand, the upstream catalyst, together with the second catalyst component (noble metal catalyst component), has an OSC with a high oxygen storage capacity.
Since the functional substance is contained as the first catalyst component, when the atmosphere of the catalytic reaction changes from the lean condition to the rich condition, the precious metal catalyst component is reduced and the high reducing power of the precious metal catalyst component causes Although the catalyst component No. 1 is also reduced, the catalyst component cannot be promptly reduced with the change in the atmosphere of the above-mentioned catalytic reaction with the upstream catalyst alone. That is, the upstream catalyst is inferior in rich responsiveness.

However, according to the present invention, as described above, by combining such an upstream catalyst with a downstream catalyst having excellent rich responsiveness, the first catalyst component in the upstream catalyst becomes rich under rich conditions. Not only is it reduced by the second catalyst component (noble metal catalyst component) in the upstream catalyst, but it is also rapidly reduced with the help of the downstream catalyst, and thus the rich response of the upstream catalyst is poor. Can be solved to improve the rich responsiveness. As a result, according to the present invention, under lean conditions, SO ₂ (or SO ₃ ) trapped on the upstream catalyst is
Since it is quickly desorbed as a sulfur compound that is not captured by the downstream catalyst when rich, it is possible to prevent the downstream catalyst from being deteriorated by SOx. In advancing these reactions, the most preferable second catalyst component in the upstream catalyst is palladium or its oxide.

According to the present invention, the reason why the rich response of the upstream catalyst is improved in this way is not always clear, and the present invention is not limited by the reason, but the rich condition is not limited. In the following, the flow of exhaust gas is disturbed between the upstream catalyst and the downstream catalyst, and the exhaust gas diffuses from the downstream catalyst to the upstream catalyst, and under rich conditions, the oxygen concentration drops sharply on the downstream catalyst. It is speculated that this is due to the rapid increase in the reducing property of the reaction atmosphere of the upstream catalyst.

According to the invention, the upstream catalyst comprises at least 50% by weight of upstream catalyst component, particularly preferably at least 80% by weight. When the proportion of the catalyst component in the upstream catalyst is less than 50% by weight, during lean, a part of SOx in the exhaust gas is not captured on the obtained upstream catalyst, and SOx
This leads to a lower capture rate. Thus, SOx is discharged to the downstream side and captured by the downstream catalyst, so that SOx of the downstream catalyst is
Durability decreases. Further, a part of SOx trapped in the upstream catalyst during lean is not discharged as a sulfur compound that is not trapped in the downstream catalyst during rich, and is trapped in the downstream catalyst, so that the SOx durability of the downstream catalyst is reduced.

According to the invention, the first catalyst component in the upstream catalyst is, in one embodiment, a mixture of oxides of at least two elements and at least two of them, as described under (c) above. It is at least one selected from the group consisting of elemental composite oxides, and the mixture is preferably a uniform mixture. However, according to the present invention, the composite oxide of the at least two elements is preferably used rather than the mixture of the oxides of the at least two elements, and the binary or ternary composite oxide is particularly preferable. Used.

For example, in the case of a binary complex oxide, a solid solution
The oxide-based weight ratio of each element in
Raseodymium complex oxide, Ceria / zirconia complex oxidation
, Ceria / terbium oxide composite oxide, Ceria / oxidation
If it is a samarium composite oxide, etc., preferably 80 /
It is in the range of 20 to 60/40. Also, ternary complex acid
In the case of oxides, the oxide-based weight ratio in solid solution is
A / Gadolinium oxide / zirconia composite oxide, ceria
/ Neodymium oxide / Zirconia composite oxide, Ceria / Zir
Konia / Praseodymium oxide complex oxide, Ceria / Zirco
Near / lanthanum oxide composite oxide, ceria / zirconia /
Samarium oxide composite oxide, ceria / zirconia / oxidation
If it is a terbium composite oxide, it is preferably 45 /
It is in the range of 30/30 to 75/20/5. However, the book
In the invention, each of the elements in these complex oxides
Oxide standard weight ratio is ceria, zirconia, terbi oxide
Um, praseodymium oxide, gadolinium oxide, neo oxide
Ce, jim, samarium oxide and lanthanum oxide
O₂, ZrO₂, TbO₂, Pr₆O₁₁, Ga₂O₃, Nd ₂
O₃, Sm₂O₃, And La₂O₃Should be calculated as
It

According to the present invention, in the upstream catalyst, the second
The catalyst component of is supported on the first catalyst component,
It is contained in the upstream catalyst component in the range of 0.05 to 5% by weight in terms of metal. Such a second catalyst component rapidly reduces the first catalyst component (OSC functional substance) in the upstream catalyst when rich, and the first catalyst component (OSC functional substance) is reduced.
Changes SOx in the exhaust gas to SO ₂ (in some cases, S
It is adsorbed as O ₃ ).

When the ratio of the second catalyst component (noble metal catalyst component) in the upstream catalyst component is less than 0.05 wt% in terms of metal, the first catalyst component (OSC
Functional substance) cannot be reduced sufficiently, and as a result,
The buffering effect for suppressing the oxidation of the second catalyst component (noble metal catalyst component) during leaning becomes poor, and the ratio of SOx in the exhaust gas adsorbed as SO ₂ on the first catalyst component decreases. On the other hand, when the ratio of the second catalyst component (noble metal catalyst component) in the upstream catalyst component is more than 5% by weight in terms of metal, the first catalyst component can be reduced well when rich, but lean. At the same time, the oxidation of the second catalyst component (noble metal catalyst component) rapidly progresses, and similarly, the proportion of SOx in the exhaust gas adsorbed as SO ₂ on the first catalyst component decreases.

The catalyst component of the above-mentioned upstream catalyst can be obtained, for example, by the following method. That is, first, cerium, zirconium, praseodymium, neodymium, terbium, samarium, water-soluble salts of the elements constituting the catalyst components such as lanthanum, for example, neutralize an aqueous solution of nitrate, or by heating hydrolysis,
After forming the hydroxide, the first catalyst component in the upstream catalyst component can be obtained by calcining the obtained product at a temperature of 300 to 900 ° C. in an oxidizing or reducing atmosphere. . However, the first catalyst component in the upstream catalyst component can also be obtained by firing a commercially available hydroxide or oxide such as cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium, or lanthanum as described above. Can be obtained. Then, a water-soluble salt of an element constituting the second catalyst component, for example, an aqueous solution of nitrate is formed on the first catalyst component by an appropriate means such as a conventionally known impregnation method or ion exchange method. After supporting, in an oxidizing or reducing atmosphere, 500 to 90
By firing at a temperature of 0 ° C., the upstream catalyst component composed of the first and second catalyst components can be obtained as powder.

According to the invention, the downstream catalyst comprises at least 50% by weight of catalyst component, particularly preferably at least 8%.
Contains 0% by weight. The ratio of catalyst components in the downstream catalyst is 5
Below 0% by weight, the catalytic ability of the downstream catalyst to remove NOx under rich and lean conditions is reduced.

The downstream catalyst has at least one noble metal catalyst component selected from platinum, rhodium, palladium and oxides thereof, and a catalyst component comprising a NOx absorbent / adsorbent and a carrier. The noble metal catalyst component is contained in the downstream catalyst component in an amount of preferably 0.05 to 5% by weight in terms of metal. Even if the ratio of the above noble metal catalyst component in the downstream catalyst component exceeds 5% by weight in terms of metal, the NOx purification ability at the time of rich and lean does not improve, while the above noble metal catalyst component in the downstream catalyst component When the ratio is less than 0.05% by weight in terms of metal, the rich response is inferior, and as a result, NOx by the method of the present invention is reduced.
The contact removal ability of is reduced.

According to the invention, the noble metal catalyst component in the downstream catalyst is preferably supported on a NOx absorber / adsorbent and a carrier. According to the present invention, any conventionally known carrier can be used as such a carrier, but for example, alumina, silica, silica-alumina, zeolite, titania, etc. are preferably used.

The NOx absorber / adsorbent in the downstream catalyst is NO
And / or NO₂Can be absorbed / adsorbed in a lean atmosphere.
As long as it is
Aluminum, oxides such as sodium, potassium, and carbonates
Potassium metal compound, magnesium, calcium, stront
Alkaline earth such as oxides and carbonates of thium, barium, etc.
Metal compound, composite with magnetoplumbite type structure
Oxide, general formula ABO ₃The perovskite structure represented by
A complex oxide having a structure (wherein A is an alkali metal,
At least one selected from potassium earth metal and rare earth metal
Represents the elements of the species, B is titanium, manganese, cobalt, di
At least one selected from ruconium, niobium and tin
Indicates the element of the species. ) Etc. are preferably used.

Specific preferred examples of the complex oxide having the magnetoplumbite type structure include, for example, BaO.
· 6Al ₂ O ₃ or K ₂ O · 12Al ₂ O ₃ and the like.

In the complex oxide having the perovskite structure represented by the above general formula ABO ₃ , the alkali metal is preferably, for example, sodium or potassium, and the alkaline earth metal is preferably, for example, ,
Calcium, strontium or barium. Specific examples of the rare earth metal include lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium, and lutetium.

Therefore, as a preferable specific example of the complex oxide having the perovskite structure, for example, LaC is used.
oO ₃ , LaMnO ₃ , La _0.3 Mn _0.7 O ₃ , LaBa
O ₃ , La _x Ba _1-x MO ₃ (x is a number satisfying 0 ≦ x ≦ 1, and M is a metal selected from titanium, manganese, cobalt, zirconium, niobium, and tin), Ba
Examples thereof include SnO ₃ and KNbO ₃ .

Since the NOx absorbing / adsorbing agents in these downstream catalysts have different NOx absorbing / adsorbing properties, they can be selectively used according to the NOx absorbing / adsorbing temperature. For example, to absorb / adsorb NOx in a high temperature range of 400 ° C to 550 ° C, an alkali metal compound having a high electronegativity is used, and 300 ° C to 450 ° C is used.
In an intermediate temperature range such as the above, an alkaline earth metal compound such as an oxide of barium or a carbonate, BaO.6Al ₂ O ₃ having the magnetoplumbite type structure or La _x Ba _1- having the perovskite type structure described above. A complex oxide such as _x MO ₃ , BaSnO ₃ or the like is used, and the temperature is 150 ° C. to 30 ° C.
In a low temperature region such as 0 ° C., LaCoO ₃ , LaMnO ₃ or the like having a perovskite structure is used.

Further, in order to extend the operating temperature range of the NOx absorbent / adsorbent, the NOx absorbent / adsorbent can be prepared by mixing various materials of the NOx absorbent / adsorbent.

According to the present invention, such NOx absorption /
The adsorbent depends on NOx absorption / adsorbent absorption / adsorption capacity, but is usually contained in the range of 5 to 50% by weight of the downstream catalyst component. When it is below this range, the NOx absorption / adsorption capacity is reduced, and when it is above this range, NOx of the catalyst component in the downstream catalyst is reduced.
It impairs the reducibility and reduces the heat resistance of the downstream catalyst layer.

The downstream catalyst component is, for example, as one method, a NOx absorbent / adsorbent such as barium carbonate, preferably a carrier such as alumina, together with a water-soluble salt such as platinum which is the downstream catalyst component. NOx can be obtained by adding it to an aqueous solution to form a slurry, mixing, stirring, drying, and baking at a temperature of 500 to 900 ° C. in an oxidizing or reducing atmosphere.
It can be obtained as a powder in which a noble metal catalyst component is supported on an absorbing / adsorbing agent (and preferably a carrier).

As a particularly preferable method, for example, an aqueous solution of a water-soluble salt capable of ion exchange such as platinum which is a downstream catalyst component (for example, tetraammine platinum (II) nitrate (Pt
(NH ₃ ) ₄ (NO ₃ ) ₂ ) and dinitrodiammine platinum (II)
(Pt (NO ₂ ) ₂ ) (NH ₃ ) ₂ etc. is used to support the catalyst component on a carrier such as alumina by an ion exchange method and then dried, and then dried in an oxidizing or reducing atmosphere at 500- It is calcined at a temperature of 900 ° C. to obtain a powder in which a noble metal catalyst component is highly dispersed on a carrier. Then, for example, by supporting the NOx absorbing / adsorbing agent such as potassium carbonate on the above-mentioned powder by an impregnation method, the NOx absorbing / adsorbing agent and the carrier are loaded with the noble metal catalyst component on the downstream side. A catalyst component can be obtained.

According to the method of the present invention, the catalyst component (noble metal catalyst component) of the downstream catalyst oxidized in the previous lean process is reduced under the rich process, that is, the reducing condition, and on this noble metal catalyst component, During leaning, NOx adsorbed on the NOx absorption / adsorbent is catalytically removed. Further, in this rich stroke, the first catalyst component (oxide) in the upstream catalyst is mixed with the second catalyst component in the upstream catalyst component as described above.
Not only the catalyst component (precious metal catalyst component), but also the catalyst component (precious metal catalyst component) in the downstream catalyst with excellent rich response
With the help of, the catalyst is rapidly reduced, and thus is regenerated in preparation for the next lean stroke, and at the same time, the SO ₂ adsorbed by the upstream catalyst at the time of lean is desorbed as a sulfur compound which is not captured by the downstream catalyst. Then, in the lean stroke following such rich stroke, that is, under oxidizing conditions, NO is oxidized on the noble metal catalyst component in the downstream catalyst, and the produced NO ₂ is adsorbed on the NOx absorbent / adsorbent, NOx in exhaust gas is purified with high efficiency.

Furthermore, according to the present invention, even in the presence of oxygen, sulfur oxides or water, there is no deterioration in the coexistence of sulfur oxides, which is a serious problem of NOx storage catalysts. In the lean range of the periodic rich / lean combustion in the temperature range, NOx is catalytically removed without deterioration of the catalyst and with good durability. That is, according to the present invention, in the upstream catalyst, SOx is adsorbed as SO ₂ when lean, and when rich, this SO ₂ is desorbed as a sulfur compound that is not captured by the downstream catalyst, so that deterioration of the catalyst due to SOx is prevented. .

According to the present invention, the respective catalyst components for the upstream catalyst and the downstream catalyst can be obtained in various forms such as powder and granules as described above. Therefore, by using any of the conventionally well-known methods, such a catalyst component is used to form an upstream catalyst or a downstream catalyst, for example, a catalyst structure having various shapes such as a honeycomb, an annular material, or a spherical material. Can be molded into. When preparing such a catalyst structure, if necessary, suitable additives,
For example, a molding aid, a reinforcing material, an inorganic fiber, an organic binder or the like can be used.

In particular, the catalyst according to the invention can be applied to the surface of a support inert substrate of any shape, for example on a washcoat.
It is advantageous to form a catalyst structure by coating (for example, coating) the upstream catalyst component and the downstream catalyst component in layers by a coating method. The inert substrate is, for example, a clay mineral such as cordierite, or a metal such as stainless steel, preferably Fe-Cr-Al.
And a shape such as a honeycomb, a ring, or a spherical structure.

All such catalysts according to the invention are
Not only it has excellent resistance to heat, but it also has excellent resistance to sulfur oxides, reducing NOx in the exhaust gas of automobiles such as diesel engines and lean gasoline engines,
That is, it is suitable for use as a catalyst for denitration.

According to the present invention, NOx in the exhaust gas is reductively decomposed by the catalytic reaction under the condition that the combustion atmosphere of the fuel oscillates between the rich condition and the lean condition as described above. Here, the cycle of the catalytic reaction (that is, the time from the rich atmosphere (or lean atmosphere) to the next rich atmosphere (or lean atmosphere)) is preferably 5 to 150.
Seconds, particularly preferably 30 to 90 seconds. The rich / lean width, that is, the rich time (second) / lean time (second) is usually in the range of 0.5 / 5 to 10/150, preferably in the range of 2/30 to 5/90. Is.

In the rich condition, when gasoline is used as the fuel, the combustion chamber of the engine usually has a weight ratio of 10 to 10.
It is formed by periodically injecting fuel at an air / fuel ratio of 14. Typical exhaust gas under rich conditions is
Contains hundreds of ppm by volume NOx, 5-6% by volume of water, 2-3% by volume of CO, 2-3% by volume of hydrogen, thousands of ppm by volume of hydrocarbons and 0-0.5% by volume of oxygen. . On the other hand, the lean condition is normally formed by periodically injecting fuel into the combustion chamber of the engine at an air / fuel ratio of 20 to 40 by weight when gasoline is used as the fuel. Typical exhaust gas under lean conditions is several hundred ppm by volume
NOx, 5-6% by volume of water, thousands of ppm of CO,
It contains thousands of ppm by volume of hydrogen, thousands of ppm by volume of hydrocarbons and 5-10% by volume of oxygen.

According to the present invention, the temperature suitable for NOx catalytic reduction depends on the individual gas composition, but in the rich stroke, the catalyst has an effective catalytic reaction activity for NOx for a long period of time. Normally, 150-550
C., and preferably 200 to 500.degree.

In the above reaction temperature range, the exhaust gas preferably has an overall space velocity of 1 including the upstream catalyst and the downstream catalyst.
0000-500000 hr ^-1 , preferably 2000
The treatment is performed in the range of ⁰ to 250,000 hr ^{−1, and} the space velocity ratio of the space velocity in the upstream catalyst / the space velocity ratio in the downstream catalyst is preferably in the range of 1/5 to 5/1. When the space velocity ratio is larger than 5/1, the downstream catalyst does not function effectively, so the NOx purification system by the NOx trapping system at the lean time is inferior, and the Ox of the upstream catalyst is reduced.
The SC function deteriorates, and the exhaust gas purification capacity during lean decreases, and as a result, the overall NOx purification capacity under cyclic rich / lean conditions decreases. Furthermore, S in the exhaust gas
Ox is not adsorbed as SO ₂ , and the desorption property as a sulfur compound that is not captured by the downstream catalyst at the time of richness is deteriorated, and the NOx absorption / adsorbent of the downstream catalyst is deteriorated by SOx. On the other hand, even if the space velocity ratio is made smaller than ⅕, the responsiveness at the time of riching is not further improved, but rather when the overall space velocity is constant, the space velocity of exhaust gas in the upstream catalyst becomes high. Too much, the SOx trapping capacity of the upstream catalyst decreases, and the SO
x Durability decreases.

[0054]

According to the method of the present invention, as described above, the exhaust gas containing NOx is brought into contact with the upstream catalyst and the downstream catalyst sequentially under the cyclic rich / lean condition, whereby oxygen and sulfur are discharged. Even in the presence of oxides or moisture, NOx in exhaust gas can be catalytically reduced with good durability and without deterioration of the catalyst used.

[0055]

EXAMPLES Below, powder catalysts which are upstream catalyst components and downstream catalyst components, production examples of upstream catalyst structures and downstream catalyst structures using them, and these upstream catalyst structures and downstream catalyst structures are used. The present invention will be described in detail by taking the reductive decomposition of nitrogen oxides according to the present invention as an example. In addition, the reductive decomposition of nitrogen oxides using a relatively-based catalyst structure will be mentioned. However, the present invention is not limited to these examples. In the following, all "parts" and "%" are based on weight unless otherwise specified.

(1) Preparation of Downstream Catalyst Component Production Example 1 Barium carbonate was prepared from an aqueous barium hydroxide solution and an aqueous sodium carbonate solution. Pt in 100 mL of deionized water
(NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum)
8.40 g was added to make an aqueous solution, and 48 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and 12 g of barium carbonate were added thereto, and the mixture was stirred at 100 ° C.
And dried in air at 500 ° C. for 3 hours to obtain γ-alumina (Al ₂ O ₃ ) / barium carbonate (BaC).
A catalyst powder was obtained by supporting 1% of platinum on O ₃ ) (weight ratio: 80/20).

Production Example 2 γ-alumina and an aqueous potassium carbonate solution were mixed, dried and then calcined in air at 1100 ° C. for 3 hours to obtain K ₂
O.12Al ₂ O ₃ (specific surface area 18 m ² / g) was prepared.

Pt (NH ₃ ) was added to 100 mL of ion-exchanged water.
_An aqueous solution was prepared by adding 8.40 g of a ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum) to 54 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and the above K ₂ O.12A.
l ₂ O _{3 (} 6 g) was added, and the mixture was dried at 100 ° C. with stirring, and then baked in air at 500 ° C. for 3 hours to give γ-
Alumina / K ₂ O ・ 12Al ₂ O ₃ (weight ratio 90/10)
A catalyst powder having 1% of platinum supported thereon was obtained.

Production Example 3 Barium carbonate and tin hydroxide were dry mixed, and the mixture was placed in air for 10
BaSnO ₃ (specific surface area 2.7 m ² / g) having a perovskite structure was prepared by firing at 00 ° C. for 3 hours.

In 100 mL of ion-exchanged water, 4.20 g of an aqueous rhodium nitrate solution (9.0% as rhodium) and Pt (N
_{H 3) 4 (NO 3)} 2 aqueous solution (9.0% as platinum) 8.4
0 g was added to form an aqueous solution, and 48 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and the above BaSn were added.
12 g of O ₃ was added, and the mixture was dried at 100 ° C. with stirring and then calcined in air at 500 ° C. for 3 hours to obtain platinum 1 on γ-alumina / BaSnO ₃ (weight ratio 80/20).
% And rhodium 0.5% were supported to obtain a catalyst powder.

Production Example 4 A mixed aqueous solution of manganese nitrate and lanthanum nitrate was neutralized and hydrolyzed with ammonia, and the obtained powder was heated in air at 900 ° C.
La with a perovskite structure after firing for 3 hours
MnO ₃ (specific surface area 20.5 m ² / g) was prepared.

In 100 mL of ion-exchanged water, 4.20 g of palladium nitrate aqueous solution (9.0% as palladium) and Pt.
(NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum)
8.40 g was added to form an aqueous solution, and 48 g of γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and the above L were added.
Add 12 g of aMnO ₃ and stir at 100 ° C.
And dried in air at 500 ° C. for 3 hours to give γ-alumina / LaMnO ₃ (weight ratio 80/20).
A catalyst powder having 1% of platinum and 0.5% of palladium supported thereon was obtained.

(2) Preparation of upstream catalyst component Production Example 5 Cerium nitrate (Ce (NO ₃ )) was added to 100 mL of ion-exchanged water.
₍ 3.6H ₂ O) 151.37 g was added to form an aqueous solution, and 0.1 N ammonia water was added to the solution to neutralize and hydrolyze cerium ions, followed by aging for 1 hour. The resulting slurry was filtered and dried at 120 ° C. for 24 hours, then in air
Ceria powder (specific surface area 13
8 m ² / g) was obtained.

To 100 mL of ion-exchanged water, 4.20 g of an aqueous palladium nitrate solution (9.0% as palladium) was added to make an aqueous solution, and 30 g of the above ceria powder was added thereto, and the mixture was dried at 100 ° C. with stirring. After, in the air,
It was calcined at 500 ° C. for 3 hours to obtain a catalyst powder in which 1% of palladium was supported on ceria powder.

Production Example 6 100 mL of ion-exchanged water was added to cerium nitrate (Ce (NO ₃ ).
₃ · 6H ₂ O) _103.77g and praseodymium nitrate (Pr
(NO ₃₎ and ₃ · 6H ₂ O) dissolved and 35.77G, to prepare an aqueous solution. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt and praseodymium salt, followed by aging for 1 hour. A product was separated from the obtained slurry by filtration, dried at 120 ° C. for 24 hours, and then calcined in air at 500 ° C. for 3 hours to obtain ceria /
Praseodymium oxide complex oxide powder (oxide standard weight ratio 6
0/40, specific surface area 112 m ² / g) was obtained.

To 100 mL of ion-exchanged water, 8.40 g of an aqueous palladium nitrate solution (9.0% as palladium) was added to prepare an aqueous solution, and 30 g of the above ceria / praseodymium oxide composite oxide powder was added, and the mixture was stirred. 100
After drying at 0 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 2% of palladium was supported on the ceria / praseodymium oxide composite oxide.

Production Example 7 Cerium nitrate (Ce (NO
_{3) 3 · 6H 2 O)} 34.59g and zirconium oxynitrate _{(ZrO (NO 3) 2)} 84.45g lanthanum nitrate (L
a and _{(NO 3) 3 · 6H 2} O) dissolved and 7.97 g, to prepare an aqueous solution. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, oxyzirconium salt and lanthanum salt, and then aged for 1 hour. The product was separated from the resulting slurry by filtration and
After drying at 0 ° C. for 24 hours, baking in air at 500 ° C. for 3 hours to obtain ceria / zirconia / lanthanum oxide composite oxide powder (oxide reference weight ratio 22/73/5, specific surface area 8
0 m ² / g) was obtained.

To 100 mL of ion-exchanged water was added 4.20 g of an aqueous rhodium nitrate solution (9.0% as rhodium),
As an aqueous solution, 30 g of the above-mentioned ceria / zirconia / lanthanum oxide composite oxide powder was added, and while stirring, 1
After drying at 00 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 1% of rhodium was supported on the ceria / zirconia / lanthanum oxide composite oxide.

Production Example 8 1700 mL of ion-exchanged water was added to cerium nitrate (Ce (NO
_{3) 3 · 6H 2 O)} 121.06g and zirconium oxynitrate _{(ZrO (NO 3) 2)} 28.12g and gadolinium nitrate (Gd (NO ₃₎ and ₃ · 6H ₂ O) dissolved and 7.48 g, An aqueous solution was prepared. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, the oxyzirconium salt and the gadolinium salt, followed by aging for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C. for 24 hours, and then in air at 500 ° C.
After firing for 3 hours, the ceria / zirconia / gadolinium oxide composite oxide powder (oxide reference weight ratio 72/24 /
4, a specific surface area of 198 m ² / g) was obtained.

To 100 mL of ion-exchanged water, 8.40 g of a palladium nitrate aqueous solution (9.0% as rhodium) was added to make an aqueous solution, and 30 g of the above ceria / zirconia / gadolinium oxide composite oxide powder was added and stirred. However, after drying at 100 ° C., it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 2% of palladium was supported on the ceria / zirconia / gadolinium oxide composite oxide.

Production Example 9 1700 mL of ion-exchanged water was mixed with cerium nitrate (Ce (NO
_{3) 3 · 6H 2 O)} 77.83g and zirconium oxynitrate _{(ZrO (NO 3) 2)} 36.03g and praseodymium nitrate (Pr (NO ₃₎ and ₃ · 6H ₂ O) dissolved and 35.26G, An aqueous solution was prepared. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, the oxyzirconium salt, and the praseodymium salt, followed by aging for 1 hour. The product is separated from the obtained slurry by filtration,
After drying it at 120 ° C for 24 hours, it was dried in air at 500
Ceria / zirconia / praseodymium oxide composite oxide powder (oxide standard weight ratio 47/33 /
22 and a specific surface area of 226 m ² / g) were obtained.

In 100 mL of ion-exchanged water, 1.05 g of an aqueous palladium nitrate solution (9.0% as palladium) and an aqueous rhodium nitrate solution (9.0% as rhodium) 12.60
30 g of the above ceria / zirconia / praseodymium oxide composite oxide powder was added to this to prepare an aqueous solution, and the mixture was dried at 100 ° C. with stirring and then in air,
The powder was calcined at 500 ° C. for 3 hours to obtain a catalyst powder in which 3% palladium and 0.25% rhodium were supported on the ceria / zirconia / praseodymium oxide composite oxide.

Production Example 10 Cerium nitrate (Ce (NO
_{3) 3 · 6H 2 O)} 109.43g and zirconium oxynitrate _{(ZrO (NO 3) 2)} 31.27g and neodymium nitrate (Nd (NO ₃₎ and ₃ · 6H ₂ O) dissolved and 15.63 g, An aqueous solution was prepared. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, oxyzirconium salt and neodymium salt, and then aged for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C. for 24 hours, and then in air at 500 ° C.
After firing for 3 hours, the ceria / zirconia / neodymium oxide composite oxide powder (oxide reference weight ratio 70/20/10,
A specific surface area of 171 m ² / g) was obtained.

To 100 mL of ion-exchanged water, 4.20 g of a palladium nitrate aqueous solution (9.0% as palladium) was added to prepare an aqueous solution, and 30 g of the above ceria / zirconia / neodymium oxide composite oxide powder was added and stirred. While drying at 100 ℃, in air at 500 ℃ 3
It was calcined for an hour to obtain a catalyst powder in which 1% of palladium was supported on the ceria / zirconia / neodymium oxide composite oxide.

Production Example 11 Cerium nitrate (Ce (NO
_{3) 3 · 6H 2 O)} 121.06g and zirconium oxynitrate _{(ZrO (NO 3) 2)} 28.12g and gadolinium nitrate (Gd (NO ₃₎ and ₃ · 6H ₂ O) dissolved and 7.48 g, An aqueous solution was prepared. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, the oxyzirconium salt and the gadolinium salt, followed by aging for 1 hour. The product is separated from the obtained slurry by filtration,
After drying it at 120 ° C for 24 hours, it was dried in air at 500
Ceria / zirconia / gadolinium oxide composite oxide powder (oxide standard weight ratio 72/24 /
4, a specific surface area of 198 m ² / g) was obtained.

2.10 g of an aqueous palladium nitrate solution (9.0% as palladium) and Pt were added to 100 mL of ion-exchanged water.
(NH ₃ ) ₄ (NO ₃ ) ₂ aqueous solution (9.0% as platinum)
1.05 g was added to form an aqueous solution, and 30 g of the above ceria / zirconia / gadolinium oxide composite oxide powder was added.
Was added and dried at 100 ° C. with stirring, and then calcined in air at 500 ° C. for 3 hours to obtain 0.5 of palladium on the ceria / zirconia / gadolinium oxide composite oxide.
% And platinum 0.25% were obtained to obtain a catalyst powder.

Production Example 12 100 mL of ion-exchanged water was added to cerium nitrate (Ce (NO ₃ ).
₃ · 6H ₂ O) _103.7g and terbium nitrate (Tb (N
O ₃₎ and ₃ · 6H ₂ O) dissolved and 40.96G, to prepare an aqueous solution. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the above cerium salt and terbium salt, and then aged for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C. for 24 hours, and then calcined in air at 500 ° C. for 3 hours to obtain a ceria / terbium oxide composite oxide powder (oxide standard). Weight ratio 70/3
0, specific surface area 139 m ² / g) was obtained.

To 100 mL of ion-exchanged water, 4.20 g of an aqueous palladium nitrate solution (9.0% as palladium) was added to prepare an aqueous solution, and 30 g of the above ceria / terbium oxide composite oxide powder was added, and the mixture was stirred. 100 ° C
After being dried in air, it was calcined in air at 500 ° C. for 3 hours to obtain a catalyst powder in which 1% of palladium was supported on the ceria / terbium oxide composite oxide.

Production Example 13 Cerium nitrate (Ce (NO
_{3) 3 · 6H 2 O)} 121.06g and zirconium oxynitrate _{(ZrO (NO 3) 2)} 28.12g and samarium nitrate (Sm (NO ₃₎ and ₃ · 6H ₂ O) dissolved and 3.40 g, An aqueous solution was prepared. 0.1 N ammonia water was added to this aqueous solution to neutralize and hydrolyze the cerium salt, oxyzirconium salt and samarium salt, and then aged for 1 hour. The product was separated from the obtained slurry by filtration, dried at 120 ° C. for 24 hours, and then in air at 500 ° C.
After firing for 3 hours, ceria / zirconia / samarium oxide composite oxide powder (oxide reference weight ratio 72/24/4,
A specific surface area of 187 m ² / g) was obtained.

To 100 mL of ion-exchanged water, 4.20 g of an aqueous palladium nitrate solution (9.0% as palladium) was added to make an aqueous solution, and 30 g of the above ceria / zirconia / samarium oxide composite oxide powder was added and stirred. While drying at 100 ℃, in air at 500 ℃ 3
It was calcined for a time to obtain a catalyst powder in which 1% of palladium was supported on the ceria / zirconia / samarium oxide composite oxide.

(3) Manufacture of honeycomb catalyst structure Example 1 1% of platinum was deposited on γ-alumina (Al ₂ O ₃ ) / barium carbonate (BaCO ₃ ) (weight ratio 80/20) obtained in Manufacturing Example 1. Silica sol (Snowtex N manufactured by Nissan Chemical Industries, Ltd., 20% as silica) on 30 g of the supported powder catalyst
3 g was mixed with an appropriate amount of water. Zirconia ball 50g
Was used as a grinding medium and the mixture was ground for 5 minutes with a planetary mill to obtain a slurry for washcoat. The above washcoat slurry was applied onto a cordierite honeycomb substrate having 400 cells per square inch,
Catalyst layer composed of the above catalyst (coating amount 50 g / L)
A honeycomb catalyst structure having was obtained as a downstream catalyst.

Next, 30 g of powder obtained by supporting 1% of palladium on ceria obtained in Production Example 5 was added to silica sol (Snowtex N manufactured by Nissan Chemical Industries, Ltd., 20 as silica).
%) 3 g and an appropriate amount of water were mixed, and a slurry was prepared in the same manner as above. This slurry was coated on the same honeycomb substrate made of cordierite as described above to obtain a honeycomb catalyst structure having a catalyst layer (coating amount 100 g / L) composed of a catalyst in which 1% of palladium was supported on ceria as an upstream catalyst. It was

Example 2 In the same manner as in Example 1, γ-alumina (Al ₂ O ₃ ) / barium carbonate (BaCO ₃ ) obtained in Production Example 1 (weight ratio 8
A catalyst structure having a catalyst layer (coating amount: 30 g / L) composed of a catalyst in which 1% of platinum was supported on (0/20) was provided on the same cordierite honeycomb substrate as in Example 1 as a downstream catalyst. Further, in the same manner as in Example 1, a catalyst layer (coating comprising a catalyst in which 2% of palladium was supported on the ceria / praseodymium oxide composite oxide (oxide reference weight ratio 60/40) obtained in Production Example 6 100g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst.

Example 3 In the same manner as in Example 1, the γ-alumina obtained in Production Example 3 /
A catalyst layer (coating amount 50 g / L) comprising a catalyst in which 1% platinum and 0.5% rhodium were supported on BaSnO ₃ (weight ratio 80/20) was formed on the same cordierite honeycomb substrate as in Example 1. The resulting catalyst structure was obtained as a downstream catalyst. Further, in the same manner as in Example 1, the catalyst was obtained by supporting 1% of rhodium on the ceria / zirconia / lanthanum oxide composite oxide (oxide reference weight ratio 22/73/5) obtained in Production Example 7. Catalyst layer (coating amount 100g
/ L) was obtained on the same honeycomb substrate made of cordierite as in Example 1 as an upstream catalyst.

Example 4 In the same manner as in Example 1, the γ-alumina obtained in Production Example 2 /
Platinum 1 on K ₂ O ・ 12Al ₂ O ₃ (weight ratio 90/10)
% Catalyst layer consisting of a catalyst supporting 5% (coating amount 5
A catalyst structure having 0 g / L) on the same cordierite honeycomb substrate as in Example 1 was obtained as a downstream catalyst. Further, in the same manner as in Example 1, the catalyst was obtained by supporting 2% of palladium on the ceria / zirconia / gadolinium oxide composite oxide (oxide reference weight ratio 72/24/4) obtained in Production Example 8. Catalyst layer (coating amount 100 g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst.

Example 5 In the same manner as in Example 1, the γ-alumina obtained in Production Example 4 /
A catalyst layer (coating amount 50 g / L) comprising a catalyst in which 1% platinum and 0.5% palladium were supported on LaMnO ₃ (weight ratio 80/20) was formed on the same cordierite honeycomb substrate as in Example 1. The resulting catalyst structure was obtained as a downstream catalyst. Further, in the same manner as in Example 1, palladium 2 was deposited on the ceria / zirconia / gadolinium oxide composite oxide (oxide reference weight ratio 72/24/4) obtained in Production Example 8.
%, A catalyst layer (coating amount 100 g / L) made of a catalyst supported on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst layer.

Example 6 In the same manner as in Example 1, the γ-alumina obtained in Production Example 2 /
Platinum 1 on K ₂ O ・ 12Al ₂ O ₃ (weight ratio 90/10)
% Catalyst layer consisting of a catalyst supporting 5% (coating amount 5
A catalyst structure having 0 g / L) on the same cordierite honeycomb substrate as in Example 1 was obtained as a downstream catalyst. Further, in the same manner as in Example 1, 3% palladium and 0.25% rhodium were supported on the ceria / zirconia / praseodymium oxide composite oxide (oxide reference weight ratio 47/33/22) obtained in Production Example 9. A catalyst structure having a catalyst layer (coating amount 100 g / L) made of the thus-formed catalyst on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst.

Example 7 In the same manner as in Example 1, the γ-alumina obtained in Production Example 1 /
A catalyst layer composed of a catalyst in which 1% of platinum is supported on barium carbonate (weight ratio 80/20) (coating amount 50 g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained as a downstream catalyst. Further, in the same manner as in Example 1, the ceria / zirconia / neodymium oxide composite oxide obtained in Production Example 10 (oxide reference weight ratio 70 /
A catalyst structure having a catalyst layer (coating amount 100 g / L) composed of a catalyst in which 1% of palladium was supported on 20/10) on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst.

Example 8 In the same manner as in Example 1, the γ-alumina obtained in Production Example 1 /
A catalyst layer composed of a catalyst in which 1% of platinum is supported on barium carbonate (weight ratio 80/20) (coating amount 50 g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained as a downstream catalyst. Further, in the same manner as in Example 1, the ceria / zirconia / gadolinium oxide composite oxide obtained in Production Example 11 (oxide reference weight ratio 7
2/24/4) with 0.5% palladium and 0.25 platinum
%, A catalyst layer having a catalyst layer (coating amount: 100 g / L) formed by supporting the same on a cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst.

Example 9 In the same manner as in Example 1, the γ-alumina obtained in Production Example 1 /
A catalyst layer composed of a catalyst in which 1% of platinum is supported on barium carbonate (weight ratio 80/20) (coating amount 50 g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained as a downstream catalyst. Further, in the same manner as in Example 1, a catalyst layer (coating comprising a catalyst in which 1% of palladium was supported on the ceria / terbium oxide composite oxide (oxide reference weight ratio 70/30) obtained in Production Example 12 A catalyst structure having an amount of 100 g / L) on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst.

Example 10 In the same manner as in Example 1, the γ-alumina obtained in Production Example 1 /
A catalyst layer composed of a catalyst in which 1% of platinum is supported on barium carbonate (weight ratio 80/20) (coating amount 50 g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained as a downstream catalyst. Further, in the same manner as in Example 1, the ceria / zirconia / samarium oxide composite oxide obtained in Production Example 13 (oxide reference weight ratio 72
/ 24/4) having a catalyst layer (coating amount 100 g / L) made of a catalyst having 1% palladium supported on (24/4) on the same cordierite honeycomb substrate as in Example 1 was obtained as an upstream catalyst. .

Comparative Example 1 In the same manner as in Example 1, γ-alumina obtained in Production Example 1 /
A catalyst layer composed of a catalyst in which 1% of platinum is supported on barium carbonate (weight ratio 80/20) (coating amount 50 g /
A catalyst structure having L) on the same cordierite honeycomb substrate as in Example 1 was obtained.

Comparative Example 2 Barium carbonate was prepared from an aqueous caustic barium solution and an aqueous sodium carbonate solution, and the barium carbonate (BaCO ₃ ) and γ were prepared.
A catalyst powder was prepared by supporting 1% of platinum on a mixture (weight ratio 8/2) with alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.). γ-alumina (Sumitomo Chemical Co., Ltd.)
KC-501) was added to an aqueous potassium carbonate solution, mixed and dried, and then calcined in air at 1100 ° C. for 3 hours to prepare K ₂ O.12Al ₂ O ₃ (specific surface area 18 m ² / g). did. Separately, γ-alumina (KC-501 manufactured by Sumitomo Chemical Co., Ltd.) and the above K ₂ O · 12Al ₂ O ₃ are mixed to give a γ-alumina / K ₂ O · 12Al ₂ O ₃ weight ratio of 9/1.
1% platinum was supported on the mixture to prepare a catalyst powder.

Platinum 1 on the above BaCO ₃ / γ-alumina
% Of the catalyst powder loaded with 48% of the above γ-alumina /
A slurry for washcoating was prepared in the same manner as in Example 1 by dry-mixing with 12 g of catalyst powder in which 1% of platinum was supported on K ₂ O · 12Al ₂ O ₃ . The same cordierite honeycomb substrate as in Example 1 was coated with this slurry in the same manner as in Example 1 to obtain a catalyst structure.

(4) Performance test A catalyst structure as an upstream catalyst and a catalyst structure as a downstream catalyst according to the above-mentioned examples were connected in series, and a nitrogen-containing gas was passed under the following conditions to carry out nitrogen oxidation. The product was catalytically reduced and decomposed. The conversion rate (removal rate) of nitrogen oxides to nitrogen was determined by the chemical luminescence method.
Similarly, nitrogen oxides in the gas were catalytically reduced and decomposed using the catalyst structure according to the comparative example.

Test Method The composition of the mixed gas used for the catalytic removal of NOx under rich conditions is as follows.

NO: 500 ppm SO ₂ : 20 ppm O ₂ : 0.4% CO: 2% C ₃ H ₆ (propylene): 2000 ppm H ₂ : 2% H ₂ O: 6.0%

The composition of the mixed gas used for the catalytic removal of NOx under lean conditions is as follows.

NO: 500 ppm SO ₂ : 20 ppm O ₂ : 9.0% CO: 0.2% C ₃ H ₆ (propylene): 500 ppm H ₂ : 0% H ₂ O: 6.0%

Catalytic reaction was carried out with the rich / lean width in the range of 3 to 30 (second / second) to 12/120 (second / second), and the performance of each catalyst was examined.

(I) Overall space velocity: 70000 hr ^-1 (lean condition) 70000 hr ^-1 (rich condition) The space velocities of the upstream and downstream catalysts are shown in Table 1.

(Ii) Reaction temperature: 200, 250, 300, 350, 400, 450 and 500 ° C. The results are shown in Tables 1 and 2. As is clear from Table 1 and Table 2, according to the present invention, nitrogen oxides can be catalytically removed at a high removal rate. On the other hand, according to the comparative example, the nitrogen oxide removal rate is generally low.

[0103]

[Table 1]

[0104]

[Table 2]

Further, using the catalyst structure as the upstream catalyst and the catalyst structure as the downstream catalyst according to Examples 2 to 10, the gas conditions and the rich / lean width (second / second) were 55.
/ 5, reaction temperature 350 ° C., upstream catalyst space velocity 14000
A catalyst endurance test for 50 hours was performed at ⁰ hr ⁻¹ and a downstream catalyst space velocity of 140000 hr ⁻¹ . Similarly, a catalyst durability test was performed using the catalyst structure according to Comparative Example 2.
The results are shown in Table 3. As is apparent from Table 3, according to the present invention, the catalyst used has a very high resistance to sulfur oxide as compared with the conventional NOx storage-reduction system.

[0106]

[Table 3]

Further, using the catalyst structure as the upstream catalyst and the catalyst structure as the downstream catalyst according to Example 2, the above gas conditions, rich / lean width (sec / sec) was 55/5, and reaction temperature was 350 ° C. Then, the space velocity in the upstream catalyst and the space velocity in the downstream catalyst were changed, and a catalyst durability test for 50 hours was performed. The results are shown in Table 4. As is clear from Table 4, according to the present invention, the space velocity in the upstream catalyst medium is reduced and the durability against SOx is increased. In particular, both the space velocity in the upstream catalyst and the space velocity in the downstream catalyst are , 140,000 hr ^-1
Is substantially free of catalyst degradation.

[0108]

[Table 4]

[Brief description of drawings]

FIG. 1 is a sectional view of an essential part showing a catalyst device according to the present invention.

[Explanation of symbols]

1 ... Engine combustion chamber 2 ... Exhaust pipe 3 ... Expansion part of exhaust pipe 4 ... Upstream catalytic device 5 ... Downstream catalytic device 6 and 7 ... Packing

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/08 F01N 3/28 301C 3/10 301G 3/28 301 B01D 53/36 102B 102H B01J 23/64 104A F term (Reference) 3G091 AA12 AA17 AA18 AA28 AB06 BA11 BA14 BA39 CB02 DA01 DA02 DB10 FB10 FB11 FB12 GA06 GA19 GB01W GB01X GB01Y GB02Y GB03Y GB04W GB04Y GB05W GB06W BA07X BA16X BA15XA15 AB08 BA18XA15 AB14 AB10 BA14XB17A08A47A17A14 BA21X BA28X BA30X BA31X BA33X BA41X BA42X BA45X BB02 CC32 CC46 DA03 DA05 DA20 EA04 4G069 AA03 BA01B BA05A BA05B BA13B BA17 BB02A BB02B BB04A BB06A BB06B BB16A BB16B BC01A BC03A BC03B BC13A BC13B BC22A BC22B BC42A BC42B BC43A BC43B BC44A BC44B BC51A BC51B BC62A BC62B BC71A BC71B BC72A BC72B BC75A BC75B CA02 CA03 CA08 CA10 CA13 EA18 EA19 EC23 ED07 EE09

Claims (3)

[Claims] 1. A fuel is supplied and burned under cyclic rich / lean conditions, and exhaust gas produced is brought into contact with a catalyst,
In the method for catalytically removing nitrogen oxides in the exhaust gas, the exhaust gas is (A) (a) ceria or (b) praseodymium oxide or (c) cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and A first catalyst component comprising at least one selected from a mixture of oxides of at least two elements selected from lanthanum and a composite oxide of at least those two elements, and (d) palladium, rhodium, platinum and oxidation thereof. Contacting an upstream catalyst having as a catalyst component a second catalyst component consisting of at least one selected from
Then, (B) a downstream catalyst having (a) at least one catalyst component selected from platinum, rhodium, palladium and oxides thereof, (b) NOx absorption / adsorbent, and (c) carrier as catalyst components A method for catalytically removing nitrogen oxides in exhaust gas, which comprises contacting. 2. The method according to claim 12, wherein the period of the catalytic reaction is in the range of 5 to 150 seconds and the rich time (second).
/ Lean time (second) is in the range of 0.5 / 5 to 10-150, and the overall space velocity including the upstream catalyst and the downstream catalyst is in the range of 10,000 to 500,000 hr ^-1 , space velocity in the upstream catalyst / downstream catalyst Space velocity ratio at 1
/ 5 to 5/1 in the range. 3. A fuel is supplied and burned under cyclic rich / lean conditions, and exhaust gas produced is brought into contact with a catalyst,
In an apparatus for catalytically removing nitrogen oxides in the exhaust gas, (A) (a) ceria or (b) praseodymium oxide or (c) cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum. A first catalyst component comprising at least one selected from a mixture of oxides of at least two elements and a composite oxide of at least those two elements, and (d) palladium, rhodium, platinum and oxides thereof. (B) (a) at least one catalyst component selected from platinum, rhodium, palladium and oxides thereof, and (b) an upstream catalyst device having as a catalyst component a second catalyst component consisting of at least one It is characterized by comprising a downstream catalyst device having NOx absorbent / adsorbent and (c) carrier as catalyst components. Contact removed to device the nitrogen oxides in the exhaust gas to be.