JP2005262162A - Catalyst for oxidizing/removing hydrocarbon gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a catalyst for oxidizing/removing hydrocarbon gas leaking from an NOx storage/reduction-type catalyst employed in a system for removing the NOx in the exhaust gas of a diesel engine. <P>SOLUTION: This catalyst comprises a heat-resistant three-dimensional structure and a noble metal-containing active component carried on the structure, and serves for oxidizing/removing the hydrocarbon gas leaking from the NOx storage/reduction-type catalyst. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hydrocarbon that is disposed downstream of the exhaust gas flow with respect to the catalyst and leaks from the catalyst in a system that uses a NOx occlusion reduction type catalyst to remove NOx in diesel engine exhaust gas. (HC) The present invention relates to an HC gas oxidation removal catalyst that oxidizes and removes gas.

  As a system for removing NOx discharged from a diesel engine vehicle equipped with a lean burn engine, systems using NOx occlusion reduction type catalysts are known (Patent Documents 1 to 4).

  In the system using the NOx storage reduction catalyst, when the air-fuel ratio (A / F) of the inflowing exhaust gas is in a lean state (A / F> 14.7), the storage component in the NOx storage reduction catalyst (for example, When NOx is occluded in Ba-containing complex oxide, alkali metal-containing complex oxide, alkaline earth metal complex oxide, etc. and the NOx occlusion reaches saturation, the air-fuel ratio of the inflowing exhaust gas is stoichiometric (theoretical air-fuel ratio). , A / F = 14.7) to rich (A / F <14.7) in a pulsed manner (rich spike control) to release NOx from the occlusion component. In addition, it is characterized by reacting with a reducing component of hydrocarbon (HC) and carbon monoxide (CO) to be removed and regenerating the NOx occlusion reduction catalyst.

The flow of exhaust gas in this system is schematically described as follows.
(A) Diesel engine → (B) Diesel oxidation catalyst → (C) NOx storage reduction catalyst → (released into the atmosphere)
Under normal lean conditions, (A) Nitric oxide (NO) in the exhaust gas from the engine is (B) oxidized and partially nitrogen dioxide (NO) as it passes through the diesel oxidation catalyst (DOC). ₂ ), and (C) the NO / NO ₂ molar ratio in the exhaust gas is adjusted so that the oxidation burden of the NOx storage reduction catalyst is reduced. These NOx are stored in the (C) NOx storage reduction catalyst.

As described above, NOx is removed by adjusting the air-fuel ratio of the exhaust gas to form a reducing atmosphere, thereby reducing (NO) the NOx stored in the NOx storage-reduction catalyst, such as N ₂ This is done by releasing it as a harmless component. Actually, for example, (C) a differential pressure before and after the NOx occlusion reduction catalyst is detected and controlled by an engine control unit (ECU) into the diesel engine cylinder separately from normal light oil injection. The light oil metered in the middle of the exhaust process is injected (post-injection) so that unburned light oil is included in the exhaust gas from the engine. This light oil is partially oxidized to CO in (B) DOC to become a mixed gas of CO and HC, and this mixed gas is supplied as a reducing component to the (C) NOx occlusion reduction type catalyst.

JP-A-8-218920 JP-A-6-137135 Japanese Patent Laid-Open No. 9-13956 JP 11-93643 A JP 2001-132433 A

  However, in the above NOx occlusion reduction system, HC gas has a relatively lower reduction performance than CO, and NOx occlusion does not sufficiently participate in the reduction reaction (that is, remains unburned without being oxidized). It has been clarified that there is a large amount of HC gas that passes through the reduced catalyst. Since this HC gas is a harmful component, it is clear that it will adversely affect the environment if it is released into the atmosphere as it is.

  Therefore, an object of the present invention is to adopt a system using a NOx occlusion reduction type catalyst, and when reducing and removing NOx occluded in the catalyst by post injection, HC gas which is a reducing gas is reduced. An object of the present invention is to provide an HC gas removal catalyst that oxidizes and removes the HC gas before it leaks from the catalyst and is released into the atmosphere.

  As a result of intensive studies, the present inventors have found that in order to achieve the above object of the present invention, it is effective to provide a catalyst for oxidizing and removing HC gas on the downstream side of the exhaust gas flow with respect to the NOx storage reduction catalyst. The present invention has been found, and the present invention has been completed based on the findings.

That is, the present invention
Provided is a hydrocarbon gas oxidation removal catalyst that has a heat-resistant three-dimensional structure and a noble metal-containing active component supported on the structure and leaks from a NOx occlusion reduction catalyst for purifying diesel engine exhaust gas To do.

  The hydrocarbon gas oxidation removal catalyst of the present invention can oxidize and remove HC gas leaked from the catalyst without being oxidized by the NOx occlusion reduction catalyst during post-injection and remaining unburned with high efficiency, Since the amount of HC gas released into the atmosphere can be reduced, an excellent effect that environmental problems associated with HC gas release are less likely to occur.

The flow of exhaust gas in a diesel engine NOx removal system employing the hydrocarbon gas oxidation removal catalyst of the present invention (hereinafter referred to as “the present invention catalyst”) is schematically shown as follows.
(A) Diesel engine → (B) Diesel oxidation catalyst → (C) NOx occlusion reduction catalyst → (D) [Catalyst of the present invention] → (released into the atmosphere)

[Heat-resistant three-dimensional structure]
The catalyst of the present invention comprises a heat-resistant three-dimensional structure and a noble metal-containing active component supported on the structure.
Examples of the material of the heat-resistant three-dimensional structure used in the present invention include metals and ceramics, and among them, heat-resistant ceramics such as cordierite are preferable.

The heat-resistant three-dimensional structure is preferably configured as a flow-through type honeycomb monolith body having a plurality of parallel through holes (cells) in the direction of the exhaust gas flow.
Also, a porous monolithic ceramic material having fine pores such as cordierite is used as a material to form a honeycomb monolith body similar to the above, and each cell is closed alternately (in a checkered pattern) It is good also as a filter. In this case, diesel particulate filter (DPF) that collects sulfur-based fine particles such as soot and sulfate, high-molecular-weight hydrocarbon fine particles or soluble organic component (SOF) fine particles contained in the exhaust gas. ).
In any case, the radial cross-sectional shape of the cell may be a triangular diameter, a rectangle, a hexagon, or the like, but is preferably a square because it is easy to manufacture and obtain.

[Active ingredients]
As the noble metal-containing active component that exhibits the HC gas oxidation removal function of the catalyst of the present invention, it is preferable to use a noble metal supported on a carrier. Examples of the noble metal include palladium (Pd), platinum (Pt), and rhodium (Rh). In particular, Pd is preferable because of its excellent HC gas oxidation activity. Further, it is preferable to use a combination of Pd and Pt because the durability of the catalyst activity can be improved.

Examples of the noble metal carrier such as Pd include metal oxides such as γ-alumina, silica, alumina-silica composite, zirconia, titania, and zeolite. Among these, general-purpose γ-alumina is preferably used. The carrier such as γ-alumina preferably has a pore structure in order to increase the contact area with the active ingredient HC gas, and the specific surface area (BET method) is usually 100 to 250 m ^2. / g, preferably about 100 to 200 m ² / g.

  The method for supporting the noble metal on a carrier such as alumina is not particularly limited, and can be supported by a method commonly used in the art. For example, carrier particles such as γ-alumina are impregnated with an aqueous solution such as palladium nitrate, dinitrodiammine platinum, platinum hydroxide amine, chloroplatinic acid hexahydrate, or a water / nitric acid solution and dried. Then, the honeycomb monolith body and the like are washed and coated by a conventional method, dried and fired, and Pd / γ-alumina supported on the honeycomb monolith body and the like can be obtained.

  In the catalyst of the present invention, the content of noble metals such as Pd and Pt having oxidation activity of HC gas is usually 0.1 to 10 g per unit volume of the honeycomb monolith body or the like while being supported on the honeycomb monolith body or the like. / L, preferably 0.5-8 g / L, more preferably about 1-6 g / L. If the content is too small, the oxidation activity is poor. Conversely, if the content is too large, there is no particular improvement in the effect and it is economically disadvantageous.

Use of a combination of noble metals such as Pd and Pt having HC gas oxidation activity and ceria (CeO ₂ ) having an oxygen supply function promotes the HC gas oxidation reaction and improves the HC removal rate. Is preferable.
When this ceria is used, it can be made into a slurry together with carrier particles impregnated and supported by the noble metal compound prepared as described above, and supported on a honeycomb monolith body or the like.
Further, the ceria content is usually about 5 to 200 g / L, preferably about 10 to 100 g / L per unit volume of the honeycomb monolith body or the like as described above.

[Example 1]
(1) γ-alumina powder (BET specific surface area: 150 m ² / g) was placed in a container so as to have a substantially uniform thickness.

  An amount of 20 g of palladium nitrate in terms of palladium metal was dissolved in water to prepare a 1000 mL solution. This solution was added dropwise to the γ-alumina powder little by little. After completion of the dropping, the mixture was gently stirred to prepare γ-alumina powder carrying palladium nitrate.

(2) A slurry was prepared by mixing 4 kg of ceria and 2 kg of deionized water so that the solid content of the γ-alumina powder carrying palladium nitrate obtained in (1) above was 4 kg.

The resulting slurry was immersed in a commercially available cordierite honeycomb monolith (cell shape: square, cell density: 400 cpsi), pulled up, and excess slurry was removed by an air stream. Subsequently, it was dried at 100 ° C. for 30 minutes, and then calcined at 500 ° C. for 1 minute to obtain a catalyst carrying Pd / γ-alumina and ceria.
The amount of Pd metal contained in the obtained catalyst was 2 g / L per unit volume of the honeycomb monolith, and the amount of ceria was 70 g / L.

[Example 2]
Example 1 except that instead of palladium nitrate in an amount of 20 g in terms of palladium metal described in Example 1, palladium nitrate in an amount of 50 g in terms of palladium metal was used. In the same manner as above, a catalyst carrying Pd / γ-alumina and ceria was obtained.
The amount of Pd metal contained in the obtained catalyst was 5 g / L per unit volume of the honeycomb monolith, and the amount of ceria was also 70 g / L.

[Example 3]
In place of palladium nitrate described in Example 1 above, the same procedure as in Example 1 was carried out except that a water / nitric acid solution of dinitrodiammine platinum in an amount of 20 g in terms of platinum metal was used. A catalyst carrying Pt / γ-alumina and ceria was obtained.
The amount of Pt metal contained in the obtained catalyst was 2 g / L per unit volume of the honeycomb monolith, and the amount of ceria was 70 g / L.

<Catalyst performance evaluation method and evaluation results>
In the exhaust gas passage of a diesel engine (20L, turbo with intercooler), DOC (platinum content: 3 g / L), NOx storage reduction catalyst (platinum (content: 5 g / L) − The barium carbonate system) and the catalyst obtained in each of the above examples were arranged. The engine was operated, and an exhaust gas flow was caused to flow through each of the catalysts sequentially so that the space velocity (SV) was 150,000 / hr, based on the volume of the catalyst obtained in each of the above examples. .

The engine was adjusted by applying a load so that the exhaust gas outlet temperature at the engine outlet was 250 ° C. in the lean operation state.
By performing post injection of light oil into the engine intermittently once per minute and under conditions of 1 second, rich spike control is performed so that the air-fuel ratio in the exhaust gas repeats lean and rich states. The engine was operated.

The HC gas concentration (HC · IN) in the exhaust gas at the engine outlet and the HC gas concentration (HC · OUT) in the exhaust gas at the catalyst outlet obtained in each of the above examples were continuously measured. Then, based on the peak value of the measurement result, HC · IN at the time of rich spike control and HC · OUT corresponding thereto were obtained, and the HC removal rate (%) was calculated by the following equation.
HC removal rate (%) = [(HC · IN−HC · OUT) / HC · IN] × 100
In the same manner as described above, the HC removal rate (%) was also obtained when the exhaust gas outlet temperature at the engine outlet was 300 ° C. and 350 ° C.

  Further, blank values for the case where the catalyst for removing HC gas of the present invention was not used were similarly determined for each temperature, and this was designated as “Comparative Example”. The measurement results are shown in Table 1.

（注：比較例では、評価温度３５０℃におけるデータを採取していない。）

(Note: In the comparative example, data at an evaluation temperature of 350 ° C. is not collected.)

[Evaluation]
As is clear from the comparison with the data of the comparative example that does not use the HC gas removal catalyst of the present invention, the catalyst of the present invention can effectively oxidize and remove the HC gas leaked from the NOx occlusion reduction type catalyst. .

Claims (5)

  A hydrocarbon gas oxidation removal catalyst comprising a heat-resistant three-dimensional structure and a noble metal-containing active component supported on the structure and leaking from a NOx occlusion reduction catalyst for purifying diesel engine exhaust gas.   The catalyst according to claim 1, wherein the noble metal-containing active component is palladium supported on a support.   The catalyst according to claim 2, wherein the palladium content is 0.1 to 10 g / L per unit volume of the heat-resistant three-dimensional structure. The catalyst according to claim 2 or 3, wherein the noble metal-containing active ingredient further contains ceria (CeO ₂ ).   The catalyst according to claim 4, wherein a content of the ceria is 5 to 200 g / L per unit volume of the heat-resistant three-dimensional structure.