Nitrogen oxide adsorbent and diesel exhaust gas purification system

Abstract

To provide a nitrogen oxide adsorbent that exhibits better adsorption characteristics for nitrogen oxides.SOLUTION: There is provided, a nitrogen oxide adsorbent that contains Fe-ZrO2 in which Fe is solid-dissolved in ZrO2, a crystal structure of Fe-ZrO2 contains tetragonal crystal, and a lattice constant of the tetragonal crystal is a-axis: 0.34 to 0.36 nm and c-axis: 0.49 to 0.52 nm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a nitrogen oxide adsorbent and a diesel exhaust purification system.

Background Art

[0002] A catalyst containing a carrier substrate, palladium, and zeolite is known (for example, Patent Document 1). This catalyst is used to purify exhaust gas generated from a diesel engine or the like.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, from the viewpoint of improving the adsorption characteristics of nitrogen oxides, a new material is required to replace the above material that uses a combination of palladium and zeolite.

[0005] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a nitrogen oxide adsorbent that exhibits more excellent adsorption characteristics for nitrogen oxides. Another object of the present disclosure is to provide a diesel exhaust purification system including the adsorbent.

Means for Solving the Problems

[0006] The nitrogen oxide adsorbent according to one aspect of the present disclosure contains Fe-ZrO2 in which Fe is solid-solved in ZrO2. The crystal structure of Fe-ZrO2 includes a tetragonal crystal, and the lattice constants of the tetragonal crystal are: a-axis: 0.340 to 0.360 nm, and c-axis: 0.490 to 0.520 nm.

[0007] In one aspect of the nitrogen oxide adsorbent, the Fe content can be 1 to 50% by mass based on the total amount of Fe-ZrO2.

[0008] The diesel exhaust purification system according to one aspect of the present disclosure includes at least a selective reduction catalyst (SCR), and at least one of the diesel exhaust purification members disposed upstream of the selective reduction catalyst supports the above nitrogen oxide adsorbent.

Advantages of the Invention

[0009] According to the present disclosure, a nitrogen oxide adsorbent that exhibits more excellent adsorption characteristics for nitrogen oxides can be provided. Further, according to the present disclosure, a diesel exhaust purification system including the adsorbent can be provided.

[0010] The nitrogen oxide adsorbent of the present disclosure has the following other advantages. · It tends to have higher heat resistance (less likely to cause hydrothermal deterioration) compared to the above material using a combination of palladium and zeolite. · It has a higher bulk density compared to the above material using a combination of palladium and zeolite, and can suppress an increase in pressure loss after loading with respect to a honeycomb carrier or a diesel particulate filter (DPF). · When supported on a DOC, DPF, or a member disposed downstream of the DOC, purge of S components and the like that cause adsorption poisoning is performed by a high temperature of about 600 °C during DPF regeneration, and performance recovery can be easily achieved. · It is inexpensive because it does not use platinum group metals (PGM).

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings as appropriate. However, the present disclosure is not limited to the following embodiments.

[0013] <Nitrogen oxide adsorbent> The nitrogen oxide adsorbent contains Fe-ZrO2 in which Fe is solid-solved in ZrO2. Fe-ZrO2 is an oxide that can be produced by the coprecipitation method. For example, an aqueous solution containing raw materials of iron and zirconium (iron nitrate and zirconium oxynitrate) is mixed to obtain a raw material mixture. An alkaline solution is dropped into this to obtain a precipitate. After washing and drying the obtained precipitate, it is fired to obtain Fe-ZrO2 in which Fe is solid-solved in ZrO2.

[0014] The crystal structure of Fe-ZrO2 includes tetragonal. The crystal structure of Fe-ZrO2 may mainly include tetragonal, and the crystal structure may be tetragonal. The crystal system of ZrO2 is monoclinic, but when Fe is solid-solved here, the crystal system shifts to tetragonal. Therefore, the crystal structure of Fe-ZrO2 may include tetragonal and monoclinic (it may be a mixed crystal of both). The ratio of tetragonal contained in the crystal structure can be 30% or more, desirably 80% or more, based on all crystal systems contained in the crystal structure, and may be 100%. The ratio of tetragonal can be determined by the Rietveld method.

[0015] From the viewpoint of the adsorbability of nitrogen oxides, the lattice constant of the a-axis of tetragonal Fe-ZrO2 is 0.340 to 0.360 nm, desirably 0.350 to 0.360 nm. From the viewpoint of the adsorbability of nitrogen oxides, the lattice constant of the c-axis of tetragonal Fe-ZrO2 is 0.490 to 0.520 nm, desirably 0.500 to 0.515 nm. The lattice constant of Fe-ZrO2 can be adjusted by changing the solid-solution amount of Fe. The lattice constant can be calculated from X-ray diffraction data obtained by the wide-angle X-ray diffraction method.

[0016] The content (solid solution amount) of Fe in Fe-ZrO₂ can be 1 to 50% by mass, may be 2 to 40% by mass, may be 3 to 35% by mass, or may be 4 to 30% by mass based on the total amount of Fe-ZrO₂ from the viewpoint of the adsorptivity of nitrogen oxides.

[0017] Examples of nitrogen oxides include NOₓ such as nitrogen monoxide (NO), nitrogen dioxide (NO₂), nitrogen trioxide (NO₃), nitrous oxide (dinitrogen monoxide) (N₂O), dinitrogen trioxide (N₂O₃), dinitrogen tetroxide (N₂O₄), and dinitrogen pentoxide (N₂O₅). The nitrogen oxide adsorbent of the present disclosure is particularly excellent in the adsorptivity to NO, NO₂, and NO₃ among these.

[0018] The nitrogen oxide adsorbent mainly contains Fe-ZrO₂, but may contain other components as long as the adsorptivity of nitrogen oxides is not impaired. Examples of other components include iron(III) oxide. The nitrogen oxide adsorbent may contain 50% by mass or more of Fe-ZrO₂, or may contain 100% by mass. That is, the nitrogen oxide adsorbent may be composed of Fe-ZrO₂ (composed of ZrO₂ in which Fe is solid-dissolved).

[0019] The nitrogen oxide adsorbent of the present disclosure can selectively adsorb nitrogen oxides from the exhaust gas of a diesel engine containing about 10 to 2000 ppm of nitrogen oxides at a low temperature from room temperature to about 200°C. In addition, the nitrogen oxide adsorbent of the present disclosure can gradually release the adsorbed nitrogen oxides as the temperature rises. Therefore, the nitrogen oxide adsorbent of the present disclosure can be used by being supported on various members (members for diesel exhaust gas purification) provided in a diesel exhaust gas purification system. However, the use of the nitrogen oxide adsorbent of the present disclosure is not limited thereto, and it may also be used in other applications such as factory exhaust gas, gasoline engine exhaust gas, and denitration in road tunnels.

[0020] <Diesel Exhaust Gas Purification System> The diesel exhaust purification system includes, for example, diesel exhaust purification components such as a diesel engine, a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), a selective reduction catalyst (SCR, particularly urea SCR), an ammonia slip catalyst (ASC), and a nitrogen oxide adsorbing member. The nitrogen oxide adsorbing member is formed by supporting a nitrogen oxide adsorbent on a honeycomb carrier or the like, and is provided independently of the DOC and DPF for the main purpose of adsorbing nitrogen oxides.

[0021] The diesel exhaust purification system of the present disclosure includes at least SCR, and at least one of the diesel exhaust purification components arranged upstream (on the diesel engine side) of the SCR supports the above nitrogen oxide adsorbent. Thereby, nitrogen oxides can be preferably adsorbed even at a low temperature when the diesel engine is started and the SCR is inactive. Examples of the diesel exhaust purification components arranged upstream of the SCR include DOC, DPF, and the nitrogen oxide adsorbing member. The above nitrogen oxide adsorbent is also excellent in heat resistance and is hardly deteriorated even in the DOC and DPF exposed to high temperatures.

[0022] As a method for supporting the nitrogen oxide adsorbent on each component, for example, a slurry containing the nitrogen oxide adsorbent is prepared, applied to each component, and then dried and fired.

Example

[0023] Hereinafter, the present disclosure will be described in detail using examples, but the present disclosure is not limited to the following examples only.

[0024] <Preparation of nitrogen oxide adsorbent> Fe-ZrO2 in which Fe is solid-solved in ZrO2 was prepared by a coprecipitation method. A plurality of samples having different Fe contents in the range of 5 to 25% by mass based on the total amount of Fe-ZrO2 were prepared. Hereinafter, a sample having an Fe content of x% by mass is denoted as xFe-ZrO2.

[0025] <Crystallinity measurement by wide-angle X-ray diffraction method> For each of the prepared samples, crystallinity analysis was performed by wide-angle X-ray diffraction. An X-ray diffractometer (Mini Flex-II, Rigaku Corporation, characteristic X-ray: CuKα) was used as the measuring device, and the measurement conditions were an air atmosphere. Figure 1 is the diffraction chart obtained by wide-angle X-ray diffraction. From the figure, it is understood that the crystal structure of each sample contains a tetragonal crystal. Also, the lattice constant of the tetragonal crystal was calculated from the obtained X-ray diffraction data. The results are shown in Table 1.

[0026]

Table 1

[0027] <NOx Adsorption Measurement> For a part of each of the prepared samples, NOx adsorption measurement was performed. A chemiluminescence NOx meter (NOA-7000, Shimadzu Corporation) was used as the measuring device, and the measurement conditions were: gas composition: NO = 150 volppm, O2 = 10 vol%, He = balance, flow rate: 100 ml / min, gas temperature: 80°C. The results are shown in Table 2. In Table 2, Pd-zeolite and Pt-zeolite as reference samples were prepared as follows, respectively. Pd-zeolite: Pd was supported on zeolite by the ion exchange method, dried, and then calcined. Pt-zeolite: Pt was supported on zeolite by the ion exchange method, dried, and then calcined.

[0028]

Table 2

Claims

1. ZrO ２ Fe-dissolved Fe-ZrO ２ containing The above Fe-ZrO ２ has a crystal structure including a tetragonal crystal, A nitrogen oxide adsorbent in which the lattice constant of the orthorhombic crystal is 0.340 to 0.360 nm for the a-axis and 0.490 to 0.520 nm for the c-axis.

2. The content of Fe is 1 to 50% by mass based on the total amount of said Fe—ZrO ２ The nitrogen oxide adsorbent according to claim 1, wherein the content of Fe is 1 to 50% by mass based on the total amount of said Fe—ZrO ２ .

3. A diesel exhaust gas purification system including at least a selective reduction catalyst (SCR), wherein at least any one of the diesel exhaust gas purification members disposed upstream of the selective reduction catalyst supports the nitrogen oxide adsorbent according to Claim 1 or 2.

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