JP3534286B2 - Exhaust gas purification catalyst and exhaust gas purification method - Google Patents

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. More specifically, the present invention relates to an exhaust gas containing excess oxygen, that is, exhaust gas. Carbon monoxide (CO), hydrogen (H ₂ ) and hydrocarbon (HC)
For purifying nitrogen oxides (NO _x ) in exhaust gas containing oxygen in excess of the amount of oxygen required to completely oxidize reducing components such as nitrogen oxides, and an exhaust gas purification method Regarding

[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 base material made of cordierite, and platinum (Pt), rhodium (Rh), etc. are formed on the porous carrier layer. A catalyst carrying a noble metal is widely known.

Further, studies have been conducted to adsorb hydrocarbons in exhaust gas with an adsorbent to more efficiently purify the hydrocarbons. For example, in Japanese Patent Laid-Open No. 6-154538, a hydrocarbon adsorbent is arranged in front of a three-way catalyst in an exhaust gas flow so that hydrocarbons in exhaust gas are adsorbed by the hydrocarbon adsorbent at low temperatures and adsorbed at elevated temperatures. A method of releasing hydrocarbons and purifying the hydrocarbons adsorbed by a three-way catalyst is disclosed. According to this method, hydrocarbons at cold start can be efficiently purified by the three-way catalyst.

On the other hand, in recent years, from the viewpoint of protecting the global environment, carbon dioxide (CO ₂ ) in exhaust gas emitted from internal combustion engines such as automobiles has become a problem, and the air-fuel ratio (A / F) is expanded as a solution. The so-called lean burn, which burns lean in an oxygen-rich atmosphere, is adopted. In this lean burn, the use of fuel is reduced because the fuel efficiency is improved, 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 (Stoichi), CO in the exhaust gas,
It oxidizes and reduces HC and NO _x at the same time, and purifies them. This three-way catalyst uses NO in an excess oxygen atmosphere such as lean burn or exhaust gas from a diesel engine.
_It is difficult to reduce and remove _x . As a solution to such a problem, for example, in Japanese Patent Laid-Open No. 168860/1993, NOx stored in exhaust gas is stored in an excess oxygen atmosphere and N stored in a stoichiometric or reducing atmosphere.
A technique for reducing and purifying Ox is disclosed.

[0006]

However, the 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 be stoichiometric or a reducing atmosphere,
It is impossible to use the technique disclosed in Japanese Patent Laid-Open No. 5-168860.

Therefore, it has been desired to develop 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 efficiently purify NO _x in exhaust gas in an oxygen excess atmosphere from a low temperature range.

[0008]

The exhaust gas purifying catalyst according to claim 1 for solving the above-mentioned problems is characterized in that hydrocarbons are supplied to the exhaust gas containing excess oxygen to remove NO _x contained in the exhaust gas. An exhaust gas-purifying catalyst used for reduction purification, which comprises a carrier containing a porous oxide, zeolite and NiO , and a noble metal supported on the carrier.

The exhaust gas purifying method according to claim 2 is characterized in that an exhaust gas purifying catalyst comprising a carrier containing a porous oxide, zeolite and NiO and a noble metal supported on the carrier is used. Hydrocarbons are supplied to the exhaust gas containing excess oxygen, and 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 the adsorbed hydrocarbons are used as reducing agents when the temperature of the exhaust gas is raised. It is to purify NO _x in the exhaust gas.

[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 the hydrocarbons in the exhaust gas is adsorbed and retained by the 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 on the catalyst for hydrocarbon becomes longer, also exhibited effects that reactivity with hydrocarbons and NO _x can be improved. Further, since hydrocarbons are adsorbed even in a low temperature range where hydrocarbons are not oxidized, the emission of hydrocarbons is suppressed, and the adsorbed hydrocarbons can be used for purifying NO _x , so that hydrocarbons can be effectively used. Can be achieved.

On the other hand, an oxide having an oxygen storage / release capacity is
Oxygen excessively contained in exhaust gas is stored in a low temperature range. Oxygen is released from the oxide when the temperature rises. Since the released oxygen has high activity, it reacts with the hydrocarbon adsorbed on the zeolite to activate the hydrocarbon. It is considered that the activated hydrocarbon has high reactivity and reacts with NO _x even in a low temperature range to reduce and purify NO _x , whereby the NO _x purification capacity in the low temperature range is improved.

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 alumina, α-alumina, γ-alumina or the like can be used. The zeolite is not particularly limited as long as it can adsorb hydrocarbons, but mordenite, ZSM-5, USY, ferrierite, zeolite beta, etc. have a large amount of adsorbing hydrocarbons, and it is desirable to use at least one of these. However, it is not particularly limited thereto.

The zeolite is preferably mixed in a ratio of 10 to 1000 parts by weight with respect to 100 parts by weight of the porous oxide. If the mixing ratio of zeolite is less than 10 parts by weight, the adsorption capacity of hydrocarbons cannot be obtained and the NO _x purification rate is lowered, and if it is more than 1000 parts by weight, the effect is saturated and further improvement of the hydrocarbon purification rate is obtained. I can't.

NiO is preferably mixed in 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 NiO is less than 10 parts by weight, activation of hydrocarbons becomes difficult, and improvement of NO _x purification capacity in a low temperature range cannot be expected. Further, if it is mixed in an amount exceeding 400 parts by weight, the catalytic activity will decrease.

As the noble metal, at least one of platinum (Pt), palladium (Pd), rhodium (Rh) and iridium (Ir) can be used. This precious metal is
It may be supported on any of the porous oxide, zeolite and NiO , but it is preferable to support it on the porous oxide. As a result, further improvement in catalytic activity and further improvement in durability can be obtained.

The amount of the noble metal supported is preferably in the range of 0.01 to 30 g per liter of catalyst. If the amount is less than 0.01 g, NO _x can hardly be purified, and if the amount is more than 30 g, the NO _x purification activity will be saturated, and therefore, loading more than that will only increase the cost. 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 hydrocarbon added is preferably in the range of 10 to 10,000 ppmC in the exhaust gas. If it is less than 10 ppmC, purification of NO _x becomes difficult, and even if it is added at 10000 ppm or more, NO _x purification capacity will be saturated and hydrocarbon will be discharged. The hydrocarbon may be added directly into the exhaust gas or may be added into the 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. As the manufacturing method, the same manufacturing method as the conventional one can be used. For example, in the case of a honeycomb-shaped catalyst, a porous oxide, zeolite, and NiO are formed on a honeycomb-shaped heat-resistant carrier substrate formed of cordierite or metal.
It can be manufactured by forming a coat layer made of a mixture of the above and supporting a noble metal thereon by an impregnation supporting method, an adsorption supporting method or the like. Further, a noble metal is supported on at least one powder of porous oxide, zeolite and NiO ,
It may be produced by coating a carrier substrate with a mixture of the remaining oxide powder.

[0019]

EXAMPLES The present invention will be specifically described below with reference to Reference Examples, Examples and Comparative Examples. Reference Example 1 Silica powder was impregnated with a predetermined amount of a dinitrodiammine platinum nitric acid aqueous solution, dried at 110 ° C. for 12 hours, and then baked at 500 ° C. for 3 hours in the air to support Pt. P
The supported amount of t is 1 g per 100 g of the silica carrier.

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

Reference Example 2 For 2 g of Pt / SiO ₂ catalyst powder similar to Reference Example 1 ,
Mix ₂ g of ZSM-5 powder and 2 g of CeO ₂ powder,
The catalyst of Reference Example 2 was prepared by molding into a pellet shape of ˜2 mm. Comparative Example 2 A catalyst of Comparative Example 2 was prepared in the same manner as in Reference Example 2 except that CeO ₂ powder was not included.

( Example 1 ) Reference was made except that NiO powder was included instead of CeO ₂ powder.
The catalyst of Example 1 was prepared in the same manner as in Consideration 2 . Comparative Example 3 A catalyst of Comparative Example 3 was prepared in the same manner as in Example 1 except that NiO powder was not included.

Table 1 shows the constitution of each of the above catalysts.

[0024]

[Table 1] (Evaluation of NO _x Purifying Activity of Each Catalyst) The catalytic activity of each of the above catalysts was measured using the exhaust model gas having the composition shown in Table 2. As the HC of the model gas A containing hydrocarbon, decane is used.

First, at an inlet gas temperature of 120 ° C., a model gas A containing hydrocarbon is supplied at 10 L / min (0 ° C., 1 a
The flow rate of (tm) was applied for 15 minutes to adsorb HC on the catalyst. Next, the gas is switched to the model gas B containing no hydrocarbon, and the incoming gas temperature is set to 50 at a temperature rising rate of 38 ° C./min.
The temperature was raised to 0 ° C., the components of the gas discharged from the catalyst were analyzed, and the NO _x purification rate at each temperature was measured. The results are shown in FIGS. 1 and 2.

[0026]

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

As can be seen from the comparison between Reference Example 2 and Comparative Example 2 in FIG. 2, the NO _x purification temperature range is shifted to the low temperature side by including the rare earth oxide (CeO ₂ ) having the oxygen storage / release capacity. It is also clear that the maximum NO _x purification rate is improved. In FIG. 3, Example 1
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 capacity,
It can be seen that the NO _x purification temperature range has shifted to the low temperature side and the maximum NO _x purification rate has greatly improved.

From the above results, Reference Examples 1-2 and Examples
It is clear that the excellent NO _x purification action of the exhaust gas-purifying catalyst of No. 1 is an effect obtained by using zeolite in combination with an oxide having an oxygen storage / release capacity.

[0029]

According to the exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention, it is possible to secure a high NO _x purification rate even in an exhaust gas containing excess oxygen even in a low temperature range. It is extremely useful for purification of.

[Brief description of drawings]

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

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

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

Front Page Continuation (72) Inventor Toshihiro Takada 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Co., Ltd. (56) References Japanese Patent Laid-Open No. 7-163871 (JP, A) Japanese Patent Laid-Open No. 8-10566 (JP) , A) JP-A-7-213910 (JP, A) JP-A-5-192539 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53 / 86,53 / 94

Claims (2)

(57) [Claims] 1. An exhaust gas-purifying catalyst used for supplying hydrocarbons to exhaust gas containing excess oxygen to reduce and purify nitrogen oxides contained in the exhaust gas, which is a porous oxide or a zeolite. And a carrier containing NiO , and a noble metal supported on the carrier, an exhaust gas purifying catalyst. 2. A catalyst for purifying exhaust gas comprising a carrier containing a porous oxide, zeolite and NiO , and a noble metal supported on the carrier, wherein hydrocarbon is contained in the exhaust gas containing excess oxygen. Supply, 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 when the temperature of the exhaust gas rises, the adsorbed hydrocarbons are used as reducing agents and nitrogen oxides in the exhaust gas. A method for purifying exhaust gas, which comprises purifying the exhaust gas.