JPH10128122A - Exhaust gas purifying catalyst and purification of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently purify NOx in exhaust gas of an oxygen excessive atmosphere from a low temp. region. SOLUTION: An exhaust gas purifying catalyst contains a carrier containing porous oxide, zeolite and oxide having oxygen occluding and discharging capacity and a noble metal supported on the carrier. Oxide having oxygen occluding and discharging capacity occludes oxygen excessively contained in exhaust gas in a low temp. region and discharges oxygen at the rising time of temp. Since discharged oxygen has high activity, oxygen is reacted with hydrocarbon adsorbed on zeolite to activate hydrocarbon and the activated hydrocarbon is high in reactivity to reduce and purify NOx in a low temp. region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method for purifying exhaust gas discharged from an internal combustion engine such as a diesel engine, and more particularly to an exhaust gas containing excess oxygen, that is, contained in exhaust gas. Carbon monoxide (CO), hydrogen (H ₂ ) and hydrocarbons (HC)
Exhaust gas purifying catalyst and exhaust gas purifying method capable of efficiently reducing and purifying nitrogen oxides (NO _x ) in an exhaust gas containing oxygen in excess of the amount of oxygen necessary to completely oxidize reducing components such as oxygen About.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NO _x has been used as a catalyst for purifying exhaust gas of automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), or the like is formed on the porous carrier layer. What carried the catalyst noble metal is widely known.

[0003] Also, researches have been made to adsorb hydrocarbons in exhaust gas with an adsorbent to purify hydrocarbons more efficiently. For example, in JP-A-6-154538, a hydrocarbon adsorbent is disposed in front of a three-way catalyst in an exhaust gas flow, and the hydrocarbons in the exhaust gas are adsorbed by the hydrocarbon adsorbent at a low temperature and are adsorbed at a temperature increase. A method for purifying hydrocarbons released by hydrocarbons and adsorbed by a three-way catalyst is disclosed. According to this method, hydrocarbons at the time of cold start can be efficiently purified by the three-way catalyst.

On the other hand, in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protection of the global environment, and the air-fuel ratio (A / F) has been expanded as a solution to this problem. What is called lean burn, in which lean combustion is performed in a reduced oxygen-excess atmosphere, is employed. In this lean burn, the use of fuel is reduced to improve fuel efficiency, and the generation of CO ₂ , which is the combustion exhaust gas, can be suppressed.

However, in the conventional three-way catalyst, when the air-fuel ratio is near the stoichiometric air-fuel ratio (stoichiometric), CO,
HC, NO _x and simultaneously oxidizing and reducing been made to purify, in this three-way catalyst, NO in an excess oxygen atmosphere such as exhaust gas of lean-burn and diesel engines
_It is difficult to remove _x by reduction. To solve such a problem, for example, Japanese Patent Application Laid-Open No. 5-168860 discloses that NOx in exhaust gas is stored in an oxygen-excess atmosphere and N2 stored in a stoichiometric or reducing atmosphere.
A technique for reducing and purifying Ox has been disclosed.

[0006]

However, exhaust gas discharged from an internal combustion engine such as a diesel engine contains an excessive amount of oxygen. Therefore, in such an exhaust gas in an oxygen-excess atmosphere, it is difficult to control the atmosphere to a stoichiometric or reducing atmosphere,
The technique disclosed in Japanese Patent Application Laid-Open No. 5-168860 cannot be used.

For this reason, there has been a demand for the development of a catalyst and a purification system capable of efficiently purifying NO _x in an oxygen-excess atmosphere. The present invention has been made in view of such circumstances, and an object thereof is to purify efficiently NO _x in the exhaust gas of an oxygen-rich atmosphere from a low temperature range.

[0008]

Features of the exhaust gas purifying catalyst according to claim 1 to solve the above problems SUMMARY OF THE INVENTION are the NO _x which oxygen is supplied to the hydrocarbon in the exhaust gas containing excessive and contained in the exhaust gas An exhaust gas purifying catalyst used for reduction purification, which comprises a porous oxide, a carrier containing zeolite and an oxide having an oxygen storage / release capability, and a noble metal supported on the carrier. .

The exhaust gas purifying method according to claim 2 is characterized in that the exhaust gas comprises a carrier containing a porous oxide, zeolite and an oxide having an oxygen storage / release capability, and a noble metal carried on the carrier. Using a purification catalyst, hydrocarbons are supplied to exhaust gas containing excess oxygen, and zeolite adsorbs and holds hydrocarbons in exhaust gas in a temperature range where hydrocarbons in exhaust gas are not oxidized. It is to purify the NO _x in the exhaust gas and hydrocarbon as a reducing agent.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention, at least a part of hydrocarbons in the exhaust gas is adsorbed and held on zeolite. This improves the reactivity with the hydrocarbons and NO _x on the catalyst, resulting higher _the NO _x purification performance. Further, since the residence time of the hydrocarbon on the catalyst is prolonged, the effect of improving the reactivity between the hydrocarbon and NO _x is also exerted. Furthermore, since the hydrocarbon hydrocarbons are adsorbed even at a low temperature range that is not oxidized, it is suppressed for hydrocarbons is discharged, and the hydrocarbon for the adsorbed hydrocarbons can be utilized for purification of the NO _x effectively utilized Can be achieved.

On the other hand, oxides having the ability to occlude and release oxygen are:
Oxygen that is excessively contained in exhaust gas is stored at low temperatures. Then, oxygen is released from the oxide when the temperature is raised. Since the released oxygen has high activity, it reacts with the hydrocarbon adsorbed on the zeolite to activate the hydrocarbon. The activated hydrocarbons have high reactivity, is considered to react with NO _x to reduce and purify NO _x even in low temperature range, thereby improving the _the NO _x purification performance in low temperature range.

As the porous oxide, at least one selected from alumina, silica, titania, zirconia, silica-alumina, titania-zirconia, zeolite and the like can be used. As the alumina, α-alumina, γ-alumina, or the like can be used. The zeolite is not particularly limited as long as it adsorbs hydrocarbons, but mordenite, ZSM-5, USY, ferrierite, zeolite beta, and the like have a large amount of adsorption of hydrocarbons, and it is desirable to use at least one of these. However, the present invention is not particularly limited to these.

The zeolite is preferably mixed at a ratio of 10 to 1000 parts by weight with respect to 100 parts by weight of the porous oxide. The mixing ratio of the zeolite is reduced less and purification rate of hydrocarbon adsorption capability can not be obtained NO _x than 10 parts by weight, mixed more than 1000 parts by weight is effective when saturated to give further improvement of the purification rate of hydrocarbon I can't. Examples of the oxide having the oxygen storage / release capability include rare earth metal oxides such as CeO ₂ and PrO ₄ , NiO, Fe ₂ O ₃ , CuO,
A transition metal oxide such as Mn ₂ O ₅ can be used.

It is preferable that the oxide having the oxygen storage / release capability is mixed at a ratio of 10 to 400 parts by weight with respect to 100 parts by weight of the porous oxide. If the mixing ratio of the oxide having an oxygen storage / release capability is less than 10 parts by weight, it becomes difficult to activate hydrocarbons, and it is not possible to expect an improvement in NOx purification performance in a low temperature range. Further, when the amount is more than 400 parts by weight, the catalytic activity decreases.

As the noble metal, at least one of platinum (Pt), palladium (Pd), rhodium (Rh) and iridium (Ir) can be used. This precious metal
It may be supported on any of a porous oxide, zeolite and an oxide having oxygen storage / release capability, but is preferably supported on a porous oxide. As a result, a further improvement in catalytic activity and a further improvement in durability can be obtained.

The amount of the noble metal carried is desirably in the range of 0.01 to 30 g per liter of the catalyst. If the amount is less than 0.01 g, NO _x can hardly be purified. Even if the amount is more than 30 g, the NO _x purification activity is saturated. The type of hydrocarbon added to the exhaust gas is not particularly limited. Typical examples of the hydrocarbon compound include light oil and kerosene.

In the case of a diesel engine, the amount of addition of the hydrocarbon is preferably in the range of 10 to 10000 ppmC in the exhaust gas. Purifying less than 10ppmC of the NO _x becomes difficult, NO _x purifying capability be added or 10000ppmC comes to hydrocarbons is discharged with saturated. The hydrocarbon may be directly added to the exhaust gas or may be added to the inside of the cylinder cylinder during the exhaust stroke.

The shape of the exhaust gas purifying catalyst of the present invention is not particularly limited, such as a honeycomb shape and a pellet shape. Also, as the manufacturing method, the same manufacturing method as that of the related art can be used. For example, in the case of a honeycomb catalyst, a honeycomb-shaped heat-resistant carrier substrate formed of cordierite or metal is coated with a mixture of a porous oxide, a zeolite, and an oxide having an oxygen storage / release capability. It can be produced by forming a layer and supporting a noble metal thereon by an impregnation-supporting method, an adsorption-supporting method, or the like. Further, a porous oxide, a zeolite and at least one powder of an oxide having an oxygen storage / release capability is supported with a noble metal, and the remaining oxide powder is mixed with the carrier substrate to produce a mixture. Good.

[0019]

The present invention will be specifically described below with reference to examples and comparative examples. (Example 1) A predetermined amount of dinitrodiammineplatinum nitric acid aqueous solution was impregnated with a predetermined amount of silica powder, dried at 110 ° C for 12 hours, and calcined in air at 500 ° C for 3 hours to carry Pt. The supported amount of Pt is 1 g per 100 g of the silica carrier.

To 2 g of the obtained Pt / SiO ₂ catalyst powder, 2 g of mordenite powder and 2 g of CeO ₂ powder were mixed, and the mixture was molded into a pellet shape of 1 to 2 mm to prepare a catalyst of Example 1. (Comparative Example 1) A catalyst was prepared in the same manner as in Example 1 except that mordenite was not contained, and was similarly formed into a pellet to prepare a catalyst of Comparative Example 1.

(Embodiment 2) Pt / Si similar to Embodiment 1
For ₂ g of O ₂ catalyst powder, 2 g of ZSM-5 powder and Ce
The catalyst of Example 2 was prepared by mixing 2 g of the O ₂ powder and forming the mixture into a pellet shape of 1 to 2 mm. (Comparative Example 2) A catalyst of Comparative Example 2 was prepared in the same manner as in Example 2 except that CeO ₂ powder was not contained.

Example 3 Instead of CeO ₂ powder, Ni
A catalyst of Example 3 was prepared in the same manner as in Example 2 except that it contained O powder. Comparative Example 3 Example 3 except that no NiO powder was included.
In the same manner as in the above, a catalyst of Comparative Example 3 was prepared.

The constitution of each of the above catalysts is summarized in Table 1.

[0024]

[Table 1] (O: contained-: not contained) (Evaluation of NO _x purification activity of each catalyst)
The catalytic activity was measured using an exhaust gas model having the composition shown in Table 2. Decane is used as HC of the model gas A containing hydrocarbons.

First, at an inlet gas temperature of 120 ° C., a model gas A containing hydrocarbons is supplied at 10 L / min (0 ° C., 1a
tm) for 15 minutes to adsorb HC on the catalyst. Next, the gas was switched to the model gas B containing no hydrocarbon, and the temperature of the incoming gas was increased to 50 ° C. at a rate of 38 ° C./min.
The temperature was raised to 0 ° C., the components of the gas emitted from the catalyst were analyzed, and the NO _x purification rate at each temperature was measured. The results are shown in FIGS.

[0026]

[Table 2] In FIG. 1, as can be seen from a comparison between Example 1 and Comparative Example 1, it can be seen that the NO _x purification rate is greatly improved by including zeolite.

In FIG. 2, as can be seen from a comparison between Example 2 and Comparative Example 2, the NO _x purification temperature range shifts to a lower temperature side by containing a rare earth oxide (CeO ₂ ) having an oxygen storage / release capability. It is clear that the maximum NO _x purification rate has been improved. Also, in FIG.
As can be seen from the comparison between Comparative Example 3 and Comparative Example 3, by including a transition metal oxide (NiO) having an oxygen storage / release capability,
It can be seen that the NO _x purification temperature range shifts to the low temperature side and the maximum NO _x purification rate is greatly improved.

From the above results, it is clear that the excellent NO _x purification action of the exhaust gas purifying catalysts of Examples 1 to 3 is an effect obtained by using zeolite in combination with an oxide having an oxygen storage / release capability.

[0029]

According to the exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention, a high NO _x purification rate can be ensured even in an exhaust gas containing an excessive amount of oxygen from a low temperature range. It is extremely useful for the purification of water.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the temperature of a gas containing a catalyst and the NO _x purification rate.

FIG. 2 is a graph showing the relationship between the temperature of a gas containing a catalyst and the NO _x purification rate.

FIG. 3 is a graph showing a relationship between a temperature of a gas containing a catalyst and a NO _x purification rate.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hirofumi Shinjo 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central R & D Laboratories Co., Ltd. 41 Toyota Yokomichi, Toyota Central Research Laboratory Co., Ltd. (72) Inventor Toshihiro Takada 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (2)

[Claims] An exhaust gas purifying catalyst used for supplying hydrocarbons to an exhaust gas containing an excessive amount of oxygen to reduce and purify nitrogen oxides contained in the exhaust gas, comprising a porous oxide, a zeolite, and the like. A catalyst for purifying exhaust gas, comprising: a carrier containing an oxide having an oxygen storage / release capability; and a noble metal carried on the carrier. 2. Excessive oxygen using a catalyst for purifying exhaust gas, comprising a carrier containing a porous oxide, a zeolite and an oxide having an oxygen storage / release ability, and a noble metal supported on the carrier. Supplying hydrocarbons in the exhaust gas containing, the zeolite adsorbs and holds the hydrocarbons in the exhaust gas in a temperature range where the hydrocarbons in the exhaust gas are not oxidized, and uses the adsorbed hydrocarbon as a reducing agent when the temperature of the exhaust gas rises. An exhaust gas purification method comprising purifying nitrogen oxides in the exhaust gas.