JP5377247B2 - NOx purification member and NOx purification device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a NOx removing member which is little in transformation to a monoclinic crystal phase from a tetragonal phase due to long-term use, and has suppressed breakage of a solid electrolyte due to a long-term use, and has excellent long-term reliability, and to provide a NOx removing apparatus. <P>SOLUTION: The NOx removing member includes a positive electrode 2 formed on a one main face of the solid electrolyte 1 consisting of dysprosia-ceria stabilized zirconia and a negative electrode 3 formed on the other main face. The dysprosia-ceria stabilized zirconia contains total 7-11 mol% of 1-4 mol% of Dy<SB>2</SB>O<SB>3</SB>and 3-7 mol% of CeO<SB>2</SB>. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for efficiently purifying nitrogen oxide (hereinafter referred to as NOx) in exhaust gas discharged from an internal combustion engine or the like.

  Hazardous substances such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in the exhaust gas of automobiles that use gasoline as fuel are oxidized or oxidized by catalytic devices using platinum, palladium, rhodium, etc. It is discharged after being reduced and rendered harmless. That is, hydrocarbons are discharged as water and carbon dioxide, carbon monoxide as carbon dioxide, and nitrogen oxides as nitrogen. Here, in order to efficiently oxidize and reduce, it is necessary for gasoline to be completely burned with air and to have a stoichiometric air-fuel ratio that does not contain oxygen. The fuel injection amount is controlled based on this information.

On the other hand, in recent years, from the viewpoint of protecting the global environment, the amount of carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem. As a solution to this problem, a so-called lean burn engine that burns an air-fuel mixture that is thinner than the stoichiometric air-fuel ratio in an oxygen-excess atmosphere is promising. In this lean burn engine, the fuel consumption is improved while maintaining the characteristics, so that the use of fuel is reduced, and the amount of CO ₂ emission in the combustion exhaust gas can be reduced.

  By the way, the conventional catalyst device is capable of simultaneously oxidizing, reducing and purifying HC, CO, NOx in the exhaust gas at an air-fuel ratio of the stoichiometric air-fuel ratio, and has a lean burn condition where the air-fuel ratio is lower than the stoichiometric air-fuel ratio. Does not exhibit sufficient purification performance for NOx reduction and removal.

  Therefore, as a method for purifying NOx even in an oxygen-excessive atmosphere, NOx provided with a positive electrode and a negative electrode for applying a voltage to both surfaces of a solid electrolyte that is an oxygen ion conductive material made of yttria-stabilized zirconia (YSZ). A NOx purification method is proposed in which a purification member is installed in a container, lean combustion exhaust gas containing NOx is introduced into the container, a voltage is applied between the positive electrode and the negative electrode, and the purified exhaust gas is discharged from the container. (For example, refer to Patent Document 1).

JP 2008-169806 A

Partially stabilized zirconia is obtained by adding an appropriate amount of yttria (Y ₂ O ₃ ) as a stabilizer to a zirconia raw material and firing, and this crystal phase is mainly composed of a tetragonal phase or a tetragonal phase and a cubic phase. It is known that partially stabilized zirconia is a material that ameliorates this defect, in contrast to zirconia, which is generally characterized by a mixed phase of zirconia, which is generally inferior in toughness. Thus, yttria-stabilized zirconia has high strength and high toughness in addition to the excellent corrosion resistance of the ceramic itself.

  However, when various members made of yttria-stabilized zirconia are used over a long period of time at high temperature, in a steam atmosphere or in contact with water, the monoclinic phase of the tetragonal phase is caused by water and heat. It promotes transformation to, and the deterioration of strength and toughness becomes noticeable.

Therefore, even in a NOx purification member using a yttria-stabilized zirconia solid electrolyte, H ₂ O is generated as purified exhaust, so that the surface of the solid electrolyte is always wet and used at a high temperature for a long time. It becomes. That is, since the tetragonal phase is in a state where the transformation to the monoclinic phase is easily promoted, the NOx purification member that is a solid electrolyte may be destroyed during the NOx purification operation.

  The present invention has been made in view of the above circumstances, has little transformation from the tetragonal phase to the monoclinic phase due to long-term use, and is excellent in long-term reliability, in which the solid electrolyte is suppressed from being destroyed by long-term use. An object of the present invention is to provide a NOx purification member and a NOx purification device.

The present invention relates to a NOx purification member in which a positive electrode is formed on one main surface and a negative electrode is formed on the other main surface of a solid electrolyte made of dispersia-ceria stabilized zirconia, wherein the dispersia-ceria stable Zirconia is characterized by containing 1 to 4 mol% of Dy ₂ O ₃ and 3 to 7 mol% of CeO ₂ in total 7 to 11 mol%.

  The present invention also includes the NOx purification member according to claim 1 in a container having an inlet and an outlet for lean combustion exhaust gas containing NOx, hydrocarbon, carbon monoxide and oxygen, and the interior of the container. The NOx purification apparatus is characterized in that the lean combustion exhaust gas is introduced and a voltage is supplied between the positive electrode and the negative electrode.

According to the NOx purification member of the present invention, there is provided a NOx purification member in which a positive electrode is formed on one main surface and a negative electrode is formed on the other main surface of a solid electrolyte made of disprusia-ceria stabilized zirconia. The disprusia-ceria stabilized zirconia contains 7 to 11 mol% in total of 1 to 4 mol% of disprusia (Dy ₂ O ₃ ) and 3 to 7 mol% of ceria (CeO ₂ ). It is possible to obtain a NOx purification member excellent in long-term reliability in which the transformation from the tetragonal phase to the monoclinic phase due to use is small and the solid electrolyte is prevented from being destroyed by long-term use.

  Further, according to the NOx purification device of the present invention provided with the NOx purification member of the present invention, even if the lean combustion exhaust gas is purified over a long period of time, it is possible to suppress the destruction of the solid electrolyte. A device for purifying NOx that is excellent in performance can be realized.

It is sectional drawing which shows an example of embodiment of the member for NOx purification | cleaning of this invention. It is sectional drawing which shows an example of embodiment of the apparatus for NOx purification | cleaning of this invention.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the NOx purification member 10 of the present invention has a positive electrode 2 formed on one main surface 1a of a solid electrolyte 1 made of disprusia-ceria stabilized zirconia and a negative electrode 3 on the other main surface 1b. The disprusia-ceria stabilized zirconia is formed and contains 1 to 4 mol% of disprusia (Dy ₂ O ₃ ) and 3 to 7 mol% of ceria (CeO ₂ ) in a total of 7 to 11 mol%. It is out.

The solid electrolyte 1 is a sintered body made of disprusia-ceria stabilized zirconia, which is an oxygen ion conductive material. Ceria (CeO ₂ ) as a stabilizer for zirconia (ZrO ₂ ) has been recognized as a substance having good oxygen ion conductivity, but has a problem that it is weak in strength and cannot be used effectively as a structural material. However, by adding disprusia (Dy ₂ O ₃ ) with good oxygen ion conductivity in addition to ceria as a stabilizer, it has good oxygen ion conductivity and high strength, and further deteriorates in a high temperature and high humidity environment. It will be suppressed.

  A positive electrode 2 is formed on one main surface of the solid electrolyte 1 and a negative electrode 3 is formed on the other main surface. These are formed by, for example, screen printing or sputtering, and are formed using a noble metal so that the solid electrolyte 1, the positive electrode 2, and the negative electrode 3 function as the NOx purification member 10. Specifically, platinum, gold, silver, rhodium, palladium, or the like is used, and it is preferable to use rhodium as the positive electrode 2 and gold as the negative electrode 3.

The disprusia-ceria stabilized zirconia constituting the solid electrolyte 1 is mainly composed of ZrO _2, and is composed of 1 to 4 mol% of disprusia (Dy ₂ O ₃ ) and 3 to 7 mol% of ceria (CeO ₂ ). 7 to 11 mol% is important.

Here, the amount of the stabilizer is within the above range because, if Dy ₂ O ₃ is less than 1 mol%, a monoclinic phase is generated by 50% or more during firing in the manufacturing process, and cracks are likely to occur. On the contrary, if it exceeds 4 mol%, the strength (bending strength) is remarkably lowered.

In addition, when CeO ₂ is less than 3 mol%, a monoclinic phase is 50% or more and cracks are generated during sintering. Conversely, when it exceeds 7 mol%, strength (bending strength) is lowered. .

Furthermore, when the total amount of Dy ₂ O ₃ and CeO ₂ is less than 7 mol%, the monoclinic crystal amount increases to 50 mol% or more under high temperature conditions, and particularly preferably Dy ₂ O ₃ and The total content of CeO ₂ is preferably in the range of 7 to 11 mol%.

As described above, the NOx purification member of the present invention is characterized by containing both Dy ₂ O ₃ and CeO ₂ as stabilizers, thereby having good oxygen ion conductivity and high strength, Deterioration can be extremely reduced even in a high temperature and humidity environment.

Incidentally, on condition that it is 7-11 mol% 1-4 mole% of disk Prussia _(Dy 2 _{O 3)} and 3-7 mol% of ceria (CeO ₂₎ in total, as components other than these, for example, dy ₂ O ₃ and CeO ₂ to within 30 wt% of a stabilizer, a rare earth element oxides is other stabilizers, CaO, may be replaced by MgO or the like. Furthermore, Al ₂ O ₃ , SiO ₂ , Fe ₂ O ₃ , TiO _2, etc. may be included in total up to 3% by mass of the entire sintered body as impurities in the raw materials and components mixed during the manufacturing process. The ZrO ₂ raw material may contain HfO ₂ that is difficult to separate.

  In such a disprusia-ceria stabilized zirconia, the crystal phase is mainly composed of a tetragonal phase or a mixed phase of a tetragonal phase and a cubic phase, and preferably has a tetragonal phase of 80% by mass or more from the viewpoint of strength and toughness. To do. In addition, such dispersia-ceria-stabilized zirconia is such that the monoclinic phase ratio does not exceed 50% by mass even after long-term use.

  Note that the average crystal grain size of disprusia-ceria stabilized zirconia, which is the solid electrolyte 1, is 1.0 μm or less from the viewpoint that precipitation of a monoclinic phase is suppressed and a high-strength sintered body can be obtained. It is preferable that it is 0.4 μm or less.

  The NOx purification member 10 of the present invention as described above is manufactured as follows.

First, as a raw material of the solid electrolyte 1, a raw material powder obtained by mixing powders of various components so as to have the above composition may be prepared, but preferably a predetermined amount of Dy ₂ O ₃ —CeO is previously prepared by a coprecipitation method. preparing a raw material powder obtained by co-precipitation of ZrO ₂ containing _2. After this raw material powder is formed into a predetermined shape by means of press molding, extrusion molding, injection molding or the like, it may be fired at 1300-1500 ° C.

  On the other hand, the electrode forming method on the one main surface and the other main surface of the solid electrolyte 1 may be performed by forming a metal component forming the electrode on each main surface of the solid electrolyte 1 by screen printing or sputtering and then firing the metal component. .

  As described above, the NOx purification member 10 of the present invention has a structure in which electrodes are formed on both main surfaces of the solid electrolyte 1. However, the present invention is not limited to this, and the object is to improve the selective adsorption / decomposability of nitrogen oxides. As an alternative, a material that efficiently adsorbs nitrogen oxides may be disposed on or inside the positive electrode 2 and the negative electrode 3 of the solid electrolyte 1.

  As shown in FIG. 2, the NOx purification member 10 of the present invention is disposed inside a container 4 having an inlet 41 and an outlet 42 for lean combustion exhaust gas containing NOx, hydrocarbons, carbon monoxide and oxygen. Then, the lean combustion exhaust gas 5 is introduced into the inside through the introduction port 41 of the container 4, and the voltage is supplied between the positive electrode 2 and the negative electrode 3, thereby purifying the lean combustion exhaust gas 5 and purification from the exhaust port 42. It is used so as to discharge the waste.

  The positive electrode 2 and the negative electrode 3 of the NOx purification member 10 are connected via a lead wire 6 and a power source 7, and a voltage is applied between the positive electrode 2 and the negative electrode 3.

When the above-described lean combustion exhaust gas 5 is introduced into the container 4 in which the NOx purification member 10 is disposed, as shown in the following reaction formula, hydrocarbons are partially oxidized by excess oxygen, and NO is simultaneously generated. Oxidized to NO ₂ , the respective intermediates are formed, and the reaction proceeds via these.

  In addition, carbon monoxide contained in the lean combustion exhaust gas 5 is also discharged as carbon dioxide due to excess oxygen.

  As described above, the lean combustion exhaust gas 5 composed of NOx, hydrocarbon, carbon monoxide, oxygen, and the like introduced from the inlet 41 of the container 4 enters the container 4 from the inlet 41 and is purified by the NOx purification member 10. The exhaust gas is discharged from the discharge port 42 as purified exhaust gas for nitrogen, carbon dioxide, oxygen, and moisture.

[Experiment 1]
As a solid electrolyte used as a member for purifying NOx, a solid electrolyte composed of disprusia-ceria stabilized zirconia containing stabilizers (disprusia and ceria) in the ratio shown in Table 1 and the balance being zirconia is prepared. As a comparative example, a solid electrolyte made of yttria stabilized zirconia containing 8 mol% of yttria was prepared. The test sample had a long side shape with a width of 50 mm, a thickness of 5 mm, and a length of 150 mm.

  And after measuring the bending strength (Pa) and the amount ratio (mass%) of the monoclinic phase, a thermal degradation test was performed, and the change in bending strength before and after the test and the amount ratio of the monoclinic phase were measured. The thermal deterioration test is a hot water treatment test (such a test is generally called a Pressure Cooker Test, hereinafter abbreviated as PCT), and is heated in boiling water at a temperature of 121 ° C. and 2 atm. The solid electrolyte is allowed to stand for 100 hours.

  The bending strength was obtained from the three-point bending test method of JIS-R-1601 and was an average value of 10 pieces.

    The amount of monoclinic phase is based on the X-ray diffraction method described in RC Carvie and PS Nicholsona, J. Am. Ceram. Soc. Vol. 55, No. 6, p. 303-305 (1972). The quantity ratio (mass%) of the monoclinic phase was obtained from Equation 2.

The results are shown in Table 1.

According to Table 1, sample No. which is a solid electrolyte used as a NOx purification member within the scope of the present invention. 1, 2, 5 to 14 show large bending strength before PCT, and even after PCT, the amount ratio of monoclinic phase is suppressed to 50% by mass or less and bending strength is 3.9228 × 10 ⁸ Pa. It can be seen that the above high strength values are obtained. In particular, sample no. In 1, 2, 6, and 7, even after PCT, the amount ratio of the monoclinic phase is 23% by mass or less and a bending strength of 5.88 × 10 ⁸ Pa or more is obtained.

On the other hand, sample no. In Nos. 3, 4, 15 to 17, the monoclinic phase after PCT exceeded 50% by mass, cracks were generated, or only a bending strength of less than 4.020 × 10 ⁸ Pa was obtained.

In addition, Sample No. as a comparative example. 18 (yttria-stabilized zirconia containing 8 mol% Y ₂ O ₃ ) had a high bending strength before PCT and a small amount ratio of the monoclinic phase, but the bending strength was severely deteriorated after PCT. Moreover, the amount ratio of the monoclinic phase was remarkably large.

From the above results, the solid electrolyte used as the member for purifying NOx of the present invention has a total of 7 to 4 mol% of disprusia (Dy ₂ O ₃ ) and 3 to 7 mol% of ceria (CeO ₂ ). It can be seen that the use of disprusia-ceria stabilized zirconia containing 11 mol% has a high bending strength and further suppresses deterioration even in a high temperature and high humidity environment.

[Experiment 2]
Next, as a solid electrolyte used as a member for purifying NOx, a solid electrolyte composed of disprusia-ceria stabilized zirconia containing a stabilizer (disprusia and ceria) in the ratio shown in Table 2 and the balance being zirconia (table) Sample No. 1, 5, 11, 14) of No. 1 was prepared, and a solid electrolyte (sample No. 18) made of yttria-stabilized zirconia containing 8 mol% of yttria was prepared, and oxygen ion conductivity was measured.

  Here, a 4-terminal method (4-pole cell) was used as a method for measuring oxygen ion conductivity. The oxygen ion conductivity was calculated from the relationship between the flowing oxygen ion current and the potential difference between the reference electrodes, using two current application electrodes by the anode and the cathode and two potential measurement electrodes as reference electrodes. The results are shown in Table 2.

According to Table 2, sample no. The yttria-stabilized zirconia of No. 18 has an oxygen ion conductivity of 9 × 10 ⁻³ / Ω · cm, whereas the display of the display Russia-ceria-stabilized zirconia of the present invention (Sample Nos. 1, 5, 11, and 14). The oxygen ion conductivity was 8 to 10 × 10 ⁻³ / Ω · cm, indicating that the oxygen ion conductivity was almost equivalent.

  From the above results, by using the solid electrolyte used as the NOx purification member of the present invention, the oxygen ion conductivity is high, the transformation from the tetragonal phase to the monoclinic phase by long-term use is small, and the solid is obtained by long-term use. It can be seen that it is possible to obtain a NOx purifying member that is excellent in long-term reliability, in which the electrolyte is prevented from being destroyed.

  Similarly, according to the NOx purification device of the present invention provided with the NOx purification member of the present invention, even if the lean combustion exhaust gas is purified over a long period of time, it is possible to suppress the destruction of the solid electrolyte. A highly reliable NOx purifying device can be realized.

10: NOx purification member 1: Solid electrolyte 2: Positive electrode 3: Negative electrode 4: Container 41: Inlet port 42: Outlet port 5: Lean combustion gas 6: Lead wire 7: Power source

Claims (2)

A NOx purification member in which a positive electrode is formed on one main surface and a negative electrode is formed on the other main surface of a solid electrolyte made of disprusia-ceria stabilized zirconia, A NOx purification member comprising 7 to 11 mol% in total of 1 to 4 mol% of Dy ₂ O ₃ and 3 to 7 mol% of CeO ₂ .   2. The NOx purification member according to claim 1 is provided inside a container having an inlet and an outlet for lean combustion exhaust gas containing NOx, hydrocarbons, carbon monoxide and oxygen, and the lean combustion exhaust gas inside the container. And a voltage is supplied between the positive electrode and the negative electrode.

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