JP2001198455A - Nitrogen oxide adsorbent and its use - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an NOx adsorbent adsorbing a large amount of NOx in a low temperature region, and its effective use. SOLUTION: An oxide of a transition metal such as Co, the saturated adsorption amount of NOx of which is 10×10-5 mol/g or more in a gas at 40 deg.C or lower, is used as the NOx adsorbent. As the NOx adsorbent has the saturated adsorption amount of NOx of 10×10-5 mol/g or more in the gas at 40 deg.C or lower, it is excellent in the adsorption of NOx in the low temperature region, and when it is disposed in an exhaust gas passage of an automobile engine, it can suppress the emission of NOx in the low temperature region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitrogen oxide adsorbent capable of adsorbing nitrogen oxides from a low temperature range of 40 ° C. or lower and a method of using the same.

[0002]

To purify exhaust gas from an automobile engine, an oxidation catalyst, various catalysts for purifying an exhaust gas such as a three-way catalyst or NO _x storage-and-reduction type catalyst is used. Among them, the oxidation catalyst is composed of hydrocarbons (HC) and carbon monoxide (CO) in exhaust gas by the catalytic action of precious metals such as platinum (Pt).
Is oxidized and removed, and nitrogen oxides (NO _x ) are reduced and removed to some extent. The three-way catalyst is
Exhaust gas atmosphere by controlling the air-fuel ratio to the stoichiometric air-fuel ratio (stoichiometric) also as a stoichiometric atmosphere, and reducing NO _x also efficiently thereby removing by oxidation of HC and CO in the exhaust gas by the catalytic action of a noble metal such as Pt It is to be removed.

[0003] Further NO _x storage-and-reduction type catalyst, the NO _x occluded in _the NO _x storage material such as Ba in the oxygen-rich exhaust gas, released from _the NO _x storage material by intermittently oxygen lean exhaust gas It was NO _x is to remove by reduction such as with HC in the exhaust gas by the catalytic action of the noble metal. NO _x is NO
mechanism and are inserted in the _x storage material, it has been found that NO in the exhaust gas NO _x becomes oxidized by the catalytic action of the noble metal in an oxidizing atmosphere, it is occluded by reacting with NO _x storage material .

[0004] By the way, in the oxidation catalyst, in the exhaust gas,
It is difficult to make the NO _x efficiently reduced and removed. Also in the three-way catalyst, poor reduction active of the NO _x in low-temperature region, it is such as during start-up there is a problem that NO _x is discharged without being sufficiently reduced. According to yet recent studies, it is also known that the concentration of NO _x in the exhaust gas increases the ternary active as low.

[0005] Therefore, NOx is placed upstream of the oxidation catalyst or the three-way catalyst.
It is conceivable to arrange an _x adsorbent. Thus, the NO _x in the exhaust gas low temperature region, such as during start-up can be adsorbed on _the NO _x adsorption material, since the low concentration of NO _x exhaust gas can flow into the oxidation catalyst or three-way catalyst , it is possible to further reduce the emissions of HC, CO and NO _x in low-temperature region. Also at high temperatures it has been adsorbed on _the NO _x adsorption material
NO _x is released, and the released NO _x is reduced and purified by the downstream oxidation catalyst or three-way catalyst. Therefore it is possible to reduce the emissions of the NO _x from the low-temperature region to high temperature region.

As such a NO _x adsorbent, for example, Japanese Patent Application Laid-Open No. Hei 6-219925 proposes using manganese oxide having a lattice spacing (d value) in a specific range in powder X-ray diffraction. . Also, Japanese Patent Application Laid-Open No. H5-154339 discloses the use of a manganese-based oxide. Further, it is conceivable to use the NO _x storage material of the NO _x storage reduction catalyst as the NO _x adsorbent.

[0007]

[SUMMARY OF THE INVENTION However, according to studies by the present inventors, poor _the NO _x adsorbing capability in low-temperature region in _the NO _x adsorption material consisting of manganese oxide or manganese oxide,
It is known that it cannot be put to practical use.

Further, it is used for a NO _x storage reduction catalyst.
_The NO _x storage material even attempt to adsorb NO _x in exhaust gas of an automobile engine with, _the NO _x storage material sulfate is produced by the reaction with the sulfur oxides contained in the exhaust gas in order to be alkaline. Therefore _the NO _x adsorbing capability is lowered gradually.

[0009] The present invention has been made in view of such circumstances, and an object and _the NO _x adsorption material _the NO _x adsorption amount is large in the low temperature range, to provide the effective usage.

[0010]

The feature of the NO _x adsorbent of the present invention that solves the above-mentioned problems is that it comprises a transition metal oxide,
Saturated adsorption of NO _x in gas below 40 ° C is 10 × 10 ^-5
Mol / g or more. As such a transition metal, at least one metal selected from Co, Fe and Ni is preferable. The NO _x adsorbent of the present invention desirably has a specific surface area of 50 m ² / g or more.

The feature of the method of using the NO _x adsorbent of the present invention is that the saturated adsorption amount of NO _x in a gas composed of a transition metal oxide and at a temperature of 40 ° C. or less is 10 × 10 ⁻⁵ mol / g or more. NO
An object of the present invention is to arrange an _x adsorbent in an exhaust gas channel to adsorb NO _x in the exhaust gas.

Furthermore by arranging an oxidation catalyst or three-way catalyst in the exhaust gas line of the downstream area of the NO _x adsorption material, the low temperature range adsorbs NO _x in the exhaust gas to _the NO _x adsorption material, _the NO _x adsorbing the high temperature range it is desirable to reduce and remove NO _x released from the wood in the oxidation catalyst or three-way catalyst.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Manganese-based oxidation such as manganese oxide
NO consisting of things_x For adsorbents, in gases below 40 ° C
NO_x 10 × 10^-FiveLess than mol / g. Only
NO of the present invention _x The adsorbent is NO in gas at 40 ° C or lower.
_x 10 × 10^-FiveMol / g or more,
NO in the temperate zone_x It has excellent adsorption properties. Also transition metal
Gas contains sulfur oxides.
Hardly reacts with sulfur oxides. Therefore use
NO during use_x No danger of reduced adsorption capacity, durable
Excellent in nature.

The NO _x adsorbent of the present invention includes Co, Fe and
Oxides of at least one metal selected from Ni are particularly recommended. Among these metals, one kind of oxide may be used, or a mixture of plural kinds of metal oxides may be used. Oxides of these metals are the saturation adsorption amount of the NO _x at 40 ° C. or less in the atmosphere 10 × 10 ^-5 mol / g or more and preferably Co particularly NO _x and about 40 × 10 ^-5 mol / g Large amount of adsorption.
Therefore, these transition metal oxides
It is particularly excellent in NO _x adsorption.

Moreover _the NO _x adsorption material of the present invention has a specific surface area of 50
Desirably, it is not less than m ² / g. The specific surface area is 50m ² / g
Comes to _the NO _x adsorption amount below is undesirably lowered.
Oxides of transition metals such as Co, Fe and Ni have a specific surface area of 50 m ²
/ G or more, it is preferable to use a precipitation method in which a transition metal compound is precipitated from an aqueous solution in which a transition metal salt or the like is dissolved, and is filtered and then fired in an oxidizing atmosphere. According to the precipitation method, a metal oxide having a specific surface area of 50 m ² / g or more can be stably and easily produced. A typical example of the water-soluble transition metal salt is nitrate, and a transition metal carbonate precipitates and precipitates by mixing and neutralizing an aqueous solution thereof with, for example, an aqueous solution of sodium carbonate. By baking, a transition metal oxide having a specific surface area of 50 m ² / g or more can be easily produced.

When preparing a transition metal oxide by a precipitation method by neutralization, it is desirable to drop an acidic metal salt aqueous solution in an alkaline atmosphere. According to this method, the specific surface area of the transition metal oxide obtained can be increased as the dropping rate decreases, and a transition metal oxide having a specific surface area of 50 m ² / g or more can be easily produced. However, even when manganese oxide is prepared by this method, the specific surface area is 50 m ^2.
/ G or more is difficult.

The NO _x adsorbent of the present invention efficiently adsorbs NO _{x in} a gas at about room temperature. Therefore, NO in various gases
When adsorbing _x, it can be used in _the NO _x adsorption material alone. And if it the _the NO _x adsorption material of the present invention is disposed in the flow path of an exhaust gas from an automobile engine, a large amount can adsorb NO _x in the exhaust gas low temperature region such as during start-up, NO _x emissions in the low temperature range The amount can be further reduced.

[0018] The _the NO _x adsorption material of the present invention is disposed in the flow path of an exhaust gas from an automobile engine, by arranging an oxidation catalyst or three-way catalyst in the exhaust gas line of the downstream area of the NO _x adsorption material, such as at low temperature during start-up NO _x in the exhaust gas pass is adsorbed on _the NO _x adsorption material.
The NO _x released from _the NO _x adsorption material at a high temperature is reduced and removed by the oxidation catalyst or three-way catalyst is equal to or greater than the activation temperature. Therefore it is possible to reduce the emissions of the NO _x in the high temperature range, NO _x from a low temperature region to high temperature region
Can be suppressed.

[0019] _the NO _x adsorption material of the present invention, to the powder may be used to form the pellet shape can be used to coat the substrate of a honeycomb shape. When used in coating the substrate of the honeycomb shape, as in such a conventional three-way catalyst, a slurry was prepared from a binder such as _the NO _x adsorption material powder and the oxide sol, it honeycomb shape of the substrate It can be easily manufactured by a manufacturing method in which it is attached to the surface and fired.

[0020] As the oxidation catalyst or three-way catalysts used with _the NO _x adsorption material of the present invention can be used with the porous carrier, similar to the conventional catalyst comprising a noble metal supported on a porous support. As the porous carrier, alumina, silica, silica-alumina, zirconia, titania, or the like can be used. Among them, γ-alumina having excellent adsorption characteristics and heat resistance is particularly preferable.

It is preferable that an oxygen storage / release material such as ceria or ceria-zirconia is supported or mixed on the porous carrier. The oxygen storage / release material can stabilize the oxygen concentration in the exhaust gas and further stabilize the exhaust gas into a stoichiometric atmosphere, so that the purification activity of the three-way catalyst is further improved.

As the noble metal carried on the porous carrier, one or more kinds can be selected from Pt, rhodium (Rh), palladium (Pd), iridium (Ir) and the like. The amount of the noble metal carried is preferably 0.1 to 10 g per liter of the porous carrier. If the amount is less than this, the purification activity is insufficient, and if the amount is more than this, the effect is saturated and the cost increases.

[0023] When placing the oxidation catalyst or three-way catalyst in the downstream region of the NO _x adsorbent, but the NO _x in the exhaust gas low temperature region is adsorbed to _the NO _x adsorption material, the NO _x in the exhaust gas of high temperature range Almost no adsorption to NO _x adsorbent. Therefore, even in the sense of reducing the ventilation resistance, the exhaust gas of high temperature range is desirably configured to directly flow into the oxidation catalyst or three-way catalyst without passing through _the NO _x adsorption material. This can be easily performed by using a valve that switches the flow path. Here, the low temperature range refers to a temperature range lower than the temperature at which the noble metal of the oxidation catalyst or the three-way catalyst is activated, and the high temperature range refers to a temperature range higher than the temperature at which the noble metal of the oxidation catalyst or the three-way catalyst is activated. Say.

[0024]

The present invention will be specifically described below with reference to examples and comparative examples.

Example 1 An aqueous solution of sodium carbonate having a predetermined concentration was prepared, and an aqueous solution of cobalt nitrate having a predetermined concentration was added dropwise while stirring to neutralize the aqueous solution. The precipitated cobalt carbonate precipitate was filtered by suction, washed with distilled water, and dried at 120 ° C. in vacuo. The obtained cobalt carbonate powder in air
By calcining at 350 ° C. for 2 hours, a cobalt oxide powder as the NO _x adsorbent of Example 1 was prepared. The specific surface area of this cobalt oxide powder was 66 m ² / g.

Example 2 An iron oxide powder as an NO _x adsorbent of Example 2 was prepared in the same manner as in Example 1 except that an aqueous solution of iron nitrate having a predetermined concentration was used instead of the aqueous solution of cobalt nitrate. did. The specific surface area of this iron oxide powder was 64 m ² / g.

Example 3 A nickel oxide powder as an NO _x adsorbent of Example 3 was prepared in the same manner as in Example 1 except that an aqueous solution of nickel nitrate having a predetermined concentration was used instead of the aqueous solution of cobalt nitrate. did. The specific surface area of this nickel oxide powder was 162 m ² / g.

Comparative Example 1 A manganese oxide powder as a NO _x adsorbent of Comparative Example 1 was prepared in the same manner as in Example 1 except that an aqueous solution of manganese nitrate having a predetermined concentration was used instead of the aqueous solution of cobalt nitrate. did. The specific surface area of this manganese oxide powder was 40 m ² / g.

[0029] (Comparative Example 2) ceria - zirconia composite oxide powder (atomic ratio Zr / Ce = 0.25 / 0.05) Comparative Example 2 NO _x
An adsorbent was used. The specific surface area of the ceria-zirconia composite oxide powder was 97 m ² / g.

Comparative Example 3 Pt was added to 100 g of γ-alumina powder.
The 1.5g, was _the NO _x adsorption material of Comparative Example 3 the NO _x storage-and-reduction type catalyst powder 0.3 mol, respectively carrying the Ba. The specific surface area of the NO _x storage-reduction catalyst powder was 170 m ² / g.

<Test / Evaluation> 1.5 g of each of the NO _x adsorbent powders of the above Examples and Comparative Examples was weighed, and 3000 pp of NO was measured.
The sample was placed under a nitrogen gas atmosphere containing m 2 and the saturated NO _x adsorption amount at 35 to 40 ° C. was measured. The results are shown in FIG.

[0032] From FIG. 1, ceria is _the NO _x adsorption material of Comparative Example 2 - zirconia composite oxide powder is found to have no _the NO _x adsorbing capability. Also in the NO _x adsorbents of Comparative Examples 1 and 3, the saturated NO _x adsorption amount was 10 × 10 ⁻⁵ mol / mol.
g, and the NO _x adsorption ability in a low temperature range is low.

On the other hand, the NO _x adsorbent of each embodiment
It shows a saturated NO _x adsorption amount of × 10 ⁻⁵ mol / g or more, and clearly has a high NO _x adsorption ability in a low temperature range. Among these nickel oxide powder cobalt oxide powder as in Example 3 Example 1 is particularly exhibited higher _the NO _x adsorbing capability.

(Example 4) Therefore, for the cobalt oxide powder and the nickel oxide powder, the specific surface area and the saturated NO _x
The relationship with the amount of adsorption was investigated. That is, cobalt oxide powder and nickel oxide powder having various specific surface areas were prepared in the same manner as in Example 1 and Example 3 except that the dropping rate was changed. This is because the specific surface area of the obtained oxide increases as the dropping rate decreases, and the specific surface area of the obtained oxide decreases as the dropping rate increases.

For each oxide powder, the amount of saturated NO _x adsorbed at 35 to 40 ° C. was measured in the same manner as in Examples 1 to 3, and the results are shown in FIG.

FIG. 2 shows that the specific surface area and the saturated NOx adsorption amount are almost directly proportional, and the specific surface area is 50 m ² /
It can be seen that if the amount is not less than g, both the cobalt oxide powder and the nickel oxide powder show a saturated NO _x adsorption amount of 10 × 10 ⁻⁵ mol / g or more. That is, it is apparent that the NO _x adsorbent of the present invention desirably has a specific surface area of 50 m ² / g or more.

[0037] An example (Example 5) the exhaust gas purifying apparatus using the _the NO _x adsorption material of the present invention in FIG. The exhaust gas purifying device provided in the exhaust gas line of an automobile engine 1, NO _x adsorbent 2 is disposed downstream of the three-way catalyst 3 upstream of the exhaust gas flow is arranged.

[0038] _the NO _x adsorption material 2 are those having a specific surface area 50 m ² / g or more cobalt oxide powder was formed into pellets, is filled into a tubular shape converter 21 with a through hole 20 at the center I have. A valve 22 is provided at the inlet side opening of the through hole 20 to control whether or not the exhaust gas flow can pass through the through hole 20.

The three-way catalyst 3 has a honeycomb shape and is disposed in the catalytic converter 30. This three-way catalyst 3
Is composed of a cordierite honeycomb substrate, an alumina coat layer coated on the surface of the honeycomb substrate, and Pt carried on the alumina coat layer. And NO
_The exhaust gas that has passed through the _x adsorbent 2 or the exhaust gas that has passed through the through-hole 20 flows into the three-way catalyst 3.

[0040] In this exhaust gas purifying apparatus, while the exhaust gas temperature, such as during start-up is in the low temperature zone, the valve 22 by a control device (not shown) closes the inlet opening of the through hole 20, the exhaust gas first passes through _the NO _x adsorption material 2 . NO _x adsorbent 2 is NO in the low temperature range _x
Because of the high adsorption capacity, the NO _x in the exhaust gas is largely adsorbed in _the NO _x adsorption material 2, the exhaust gas concentration of NO _x is sufficiently reduced can flowing into the three way catalyst 3. Thus also three-way catalyst 3 is equal to or less than the activation temperature, it is possible to suppress the emission of NO _x.

When the exhaust gas temperature becomes equal to or higher than the activation temperature of the three-way catalyst 3, the valve 22 opens the through hole 20 by a control device (not shown). Thereby, most of the exhaust gas flows into the three-way catalyst 3 through the through-hole 20. Part of the exhaust gas flow is NO
_After passing through the _x adsorbent 2, the adsorbed NO _x is released from the NO _x adsorbent 2 and flows into the three-way catalyst 3 together with the exhaust gas. Since the three-way catalyst 3 is at or above the activation temperature, HC and CO contained in the exhaust gas are oxidized and purified and NO
_x is reduced and purified. By further reduced the concentration of NO _x in the exhaust gas, the activation temperature of the three way catalyst 3 is considered to also exhibited effects to be low.

[0042] Thus the exhaust gas purifying apparatus of the present embodiment can be well inhibited the NO _x emissions from a low temperature region to high temperature region.

Incidentally, as shown in FIG. 4, in the exhaust gas purifying apparatus, it is also preferable to arrange the further three way catalyst or an oxidation catalyst 4 directly below the engine 1 upstream of the NO _x adsorbent 2. By doing so, the temperature of the three-way catalyst or the oxidation catalyst 4 is quickly raised to the activation temperature and ignited, so that the temperature further rises. Hence the temperature of the exhaust gas flowing into the three way catalyst 3 rises rapidly, the three-way catalyst 3 reduces the time required for raising the temperature to the activation temperature, HC in the low temperature range, the emission of CO and NO _x more Can be suppressed.

[0044]

According to the present invention, the NO_x According to the adsorbent,
NO at low temperature_x Because of its excellent adsorption capacity,
NO in exhaust gas from engines_x From the start,
NO_x Can be suppressed. In addition, sulfur
NO, even if it contains yellow oxide _x The adsorbent is sulfur oxide
NO because it does not react_x Adsorption ability is maintained for a long time, and durability
Is also excellent.

[Brief description of the drawings]

FIG. 1 shows the saturation of the NO _x adsorbent according to Examples and Comparative Examples of the present invention.
It is a graph showing _the NO _x adsorption amount.

FIG. 2 is a graph showing a relationship between a specific surface area of a NO _x adsorbent of the present invention and a saturated NO _x adsorption amount.

FIG. 3 is an explanatory diagram showing a configuration of an exhaust gas purifying apparatus using an NO _x adsorbent according to one embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a configuration of an exhaust gas purifying apparatus using an NO _x adsorbent according to one embodiment of the present invention.

[Explanation of symbols]

1: Engine 2: NO _x adsorbent 3: the three-way catalyst 20: through hole 22: the valve

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/08 B01D 53/36 102A 3/24 103B F-term (Reference) 3G091 AA02 AB02 AB03 AB09 BA03 BA11 BA14 BA15 BA19 BA32. 4G066 AA13D AA27B AA43D AA53A BA26 BA36 CA28 DA02 FA05 FA22 FA27

Claims (5)

[Claims] 1. A nitrogen oxide adsorbent comprising a transition metal oxide and having a saturated adsorption amount of nitrogen oxide in a gas of 40 ° C. or less of 10 × 10 ⁻⁵ mol / g or more. 2. The nitrogen oxide adsorbing material according to claim 1, wherein the nitrogen oxide adsorbing material is an oxide of at least one metal selected from Co, Fe and Ni. 3. The nitrogen oxide adsorbent according to claim 1, wherein the specific surface area is 50 m ² / g or more. 4. A gas comprising a transition metal oxide and having a saturated adsorption amount of nitrogen oxides in a gas of 40 ° C. or less of 10 × 10 ⁻⁵ mol / mol.
A method for using a nitrogen oxide adsorbent, wherein a nitrogen oxide adsorbent having a weight of at least g is disposed in an exhaust gas channel to adsorb nitrogen oxides in the exhaust gas. 5. An oxidation catalyst or a three-way catalyst is arranged in the exhaust gas flow path downstream of the nitrogen oxide adsorbent, and adsorbs nitrogen oxides in exhaust gas to the nitrogen oxide adsorbent in a low temperature region. 5. The method for using a nitrogen oxide adsorbent according to claim 4, wherein the nitrogen oxide released from the nitrogen oxide adsorbent is reduced and removed by the oxidation catalyst or the three-way catalyst in a high temperature range.