JP5957284B2 - Exhaust gas purification device - Google Patents

Description

  The present invention relates to an exhaust gas purifying apparatus capable of removing nitrogen oxides (NOx) in exhaust gas. Furthermore, the present invention relates to an exhaust gas purification device that can simultaneously remove nitrogen oxides (NOx) and particulates (PM) in exhaust gas.

Various catalyst devices have been developed in the past for devices and methods for removing nitrogen oxides (NOx) in exhaust gas discharged from a combustor.
In particular, Patent Document 1 uses a cathode electrode containing metal particle chain oxide, GDC, and BaCO ₃ to apply a voltage to the electrodes on both sides of the solid electrolyte to remove NOx on the surface of the cathode electrode. Technology is disclosed.
In Patent Document 2, a voltage is applied to the electrodes on both sides of the solid electrolyte so that the exhaust gas flows in from one electrode, passes through the solid electrolyte, and flows out from the other electrode. Discloses a technique for removing NOx from diesel particulates with an electrode on the outflow side.

JP 2011-104533 A JP 2006-200520 A

By applying a voltage to the electrodes on both sides of the solid electrolyte, exhaust gas flows in from one electrode, passes through the solid electrolyte, and flows out from the other electrode, so that diesel particulates are discharged from the exhaust gas inflow side electrode. In the technique described in Patent Document 2 for removing NOx, the electrode used includes a solid electrolyte and a conductor, but the mixed state is uniform in the electrode. Therefore, the electrode performance is limited, and the NOx removal efficiency is also limited. In addition, there is a problem in joining the interface between the solid electrolyte and the electrode.
The object of the present invention is to solve the above-mentioned problems and to provide an efficient exhaust gas purification device with a high NOx removal rate.

  As a result of intensive studies to solve the above problems, the present inventors have found that it is possible to remove NOx with high efficiency by inclining the concentration of the solid electrolyte and the conductor in the electrode, The present invention has been completed. That is, the present invention is characterized by having the following configuration.

[1] A porous solid electrode having oxygen ion conductivity is provided with a porous first electrode on one side and a porous second electrode on the other side so that exhaust gas can pass therethrough. The first electrode on the side into which the exhaust gas flows is an anode, the second electrode on the side from which the exhaust gas flows out is a cathode, and the first electrode is a gadolinia-added ceria ( GDC) and a conductor, and the second electrode includes gadolinia-added ceria (GDC), a conductor and a NOx storage material, and the concentration of the conductor in the first electrode and / or the second electrode is the solid electrolyte. An exhaust gas purification apparatus, wherein the exhaust gas purification apparatus is inclined so that the concentration decreases toward the side.
[2] The first electrode and / or the second electrode is composed of a plurality of layers, and the concentration of the conductor in the layer is inclined so that the concentration decreases toward the solid electrolyte side. The exhaust gas purifying apparatus according to [1].
[3] The exhaust gas purifying apparatus according to [1] or [2], wherein the first electrode is porous capable of collecting fine particles in the exhaust gas.
[4] The exhaust gas purification apparatus according to any one of [1] to [3], wherein the main component of the solid electrolyte is gadolinia-added ceria (GDC).
[5] The exhaust gas purifying apparatus according to any one of [1] to [4], wherein the NOx storage material is barium (Ba) or a compound of barium.

By tilting the concentration of the electrode conductor so that the concentration decreases toward the solid electrolyte side, the bonding state of the interface between the solid electrolyte and the electrode is improved, and an efficient exhaust gas purification device is provided. It became possible.
Further, the NOx removal performance could be improved by tilting the concentration of the conductor in the electrode, and the power consumption of the apparatus could be suppressed.
Furthermore, NOx can be removed at a lower temperature than other solid electrolytes by using gadolinia-added ceria (GDC) as the main component of the solid electrolyte.

Principle diagram of the exhaust gas purification apparatus of the present invention SEM image of single electrode and inclined electrode The graph which shows the NOx reduction rate by the difference in the inclination ratio of a single electrode and an inclination electrode Photograph showing PM deposition on anode surface when no voltage is applied Photograph showing PM deposition on anode surface when voltage is applied

  Hereinafter, specific embodiments of the exhaust gas purifying apparatus of the present invention will be described in detail. In the exhaust gas purifying apparatus of the present invention, the first electrode and the second electrode are provided on both side surfaces of the solid electrolyte layer, and the first electrode is an anode including gadolinia-added ceria (GDC) and a conductor, and the second electrode Is a cathode containing gadolinia-added ceria (GDC), a conductor and a NOx storage material. The first electrode and / or the second electrode of the exhaust gas purifying apparatus according to the present invention is configured so that the concentration of the conductor decreases toward the solid electrolyte layer side, that is, gadolinia-added ceria (GDC). The concentration is inclined so that the concentration increases toward the solid electrolyte layer side.

  In the purification apparatus of the present invention, the first electrode, the solid electrolyte layer, and the second electrode (hereinafter collectively referred to as “purification structure”) are flat porous, and the exhaust gas to be purified is , Supplied to the first electrode side of the purification structure, and discharged from the second electrode side. The electrodes on both side surfaces of the purification structure are configured so that a voltage can be applied from the voltage application means, and NOx or NOx and particulates (PM) are removed by applying a voltage between the electrodes. Is possible.

By applying a positive voltage to the first electrode that is the anode of the purification structure and a negative voltage to the second electrode that is the cathode, NOx adsorbed on the NOx storage material included in the second electrode is reduced. At this time, oxygen ions (O ²⁻ ) generated at the second electrode are forcibly moved to the first electrode side through the solid electrolyte layer to which a voltage is applied, and NOx is regenerated at the second electrode. Synthesis is suppressed. The O ²⁻ moved to the first electrode is consumed by oxidizing carbon monoxide (CO), hydrocarbons, nitrogen monoxide (NO), fine particles (PM) and the like in the exhaust gas.

  In the electrode constituting the present invention, the first electrode includes gadolinia-added ceria (GDC) and a conductor, and the second electrode includes gadolinia-added ceria (GDC), a conductor, and a NOx storage material. The electrode is inclined so that the concentration of the conductor to be reduced decreases toward the solid electrolyte layer, that is, the concentration of gadolinia-added ceria (GDC) increases toward the solid electrolyte layer. (Hereinafter sometimes referred to as “tilted electrode”).

  The concentration of the conductor may be continuously inclined, or stepwise, that is, the electrode is composed of a plurality of layers, and the concentration of the conductor in the layer increases toward the solid electrolyte layer. You may incline so that it may become low.

  In the present invention, either the first electrode or the second electrode may be a tilted electrode, and both the first electrode and the second electrode may be tilted electrodes. The effect by the gradient electrode shown below increases.

  Since the present invention uses a tilted electrode, a sufficient three-layer interface of the conductor, exhaust gas, and electrolyte can be secured, and the connection probability between the conductive particles and the electrolyte particles is increased. Since the conductivity and ionic conductivity are maintained, the NOx decomposition reaction is promoted, the decomposition rate is increased, and the joule loss is reduced. Therefore, the voltage is decreased for the same current value, so that the power consumption can be suppressed. it can.

Furthermore, in the present invention, the first electrode can be a porous electrode capable of collecting fine particles (PM) in the exhaust gas.
By doing so, when the exhaust gas contains fine particles (PM), PM is collected on the surface of the first electrode into which the exhaust gas flows, and PM is oxidized on the surface of the first electrode by O ²⁻ generated by NOx reduction. CO ₂ and H ₂ O can be generated and removed, and an exhaust gas purifying apparatus capable of simultaneously removing nitrogen oxides (NOx) and fine particles (PM) in the exhaust gas can be obtained.

  Examples of the solid electrolyte used in the solid electrolyte layer include gadolinia-added ceria (GDC), yttrium-stabilized zirconia, ceria-based solid electrolyte or molten carbonate type, samaria-added ceria (SDC), scandia-stabilized zirconia (ScSZ), and the like. Among them, gadolinia-added ceria (GDC) capable of operating the apparatus at a low temperature is preferable.

  As the conductor contained in the electrode, metal powder such as silver (Ag), gold (Au), platinum (Pt), nickel (Ni) can be used. Among them, PM decomposition efficiency, NOx decomposition efficiency Most preferred is silver (Ag).

  As the NOx occlusion material contained in the second electrode, an alkali metal or an alkaline earth metal can be used. Among these, barium (Ba) or a barium compound is most preferably used in terms of NOx occlusion characteristics. Further, lithium (Li), potassium (K), or the like and barium (Ba) may be mixed. Alkali metals and alkaline earth metals used as NOx storage materials eventually become oxides or carbonates in the firing process of electrode production, but oxides or carbonates of alkali metals and alkaline earth metals are nitrogen. It is easy to absorb oxides and easily becomes nitrate.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.
FIG. 1 shows a principle diagram of an exhaust gas purification apparatus of the present invention.

The solid electrolyte 2 shown in FIG. 1 has a plate shape, and the shape thereof may be any shape such as a square, a rectangle, a circle, or an ellipse. The thickness of the solid electrolyte 2 is usually 10 to 500 μm. The solid electrolyte 2 is porous, and its porosity is preferably 20 to 60%, and the pore size is 0.1 to 10 μm.
On one side of the solid electrolyte 2, a first electrode (anode) 3 is laminated with a thickness of 10 to 30 μm, and on the opposite side, a second electrode (cathode) 4 is laminated with a thickness of 10 to 30 μm. A purification structure 1 is formed.
Both electrodes are also porous, and the porosity is preferably 30 to 70%, and the size of the pores may be changed in the range of 0.01 μm to 5 μm depending on the mixing ratio of the conductive particles and the electrolyte particles. desirable.

  Both electrodes are connected to a power source 6 via a switch 5 by a conductive wire, and a voltage is applied to the purification structure 1 by turning on the switch 5. The applied voltage is usually 1-5V. And it is preferable to connect the position which connects a conductive wire to an electrode to the surface side of the electrode where the ratio of a conductor is as high as possible. In some cases, a mesh-like conductor may be disposed on the surface portions of both electrodes, and a conductive wire may be connected to the conductor.

The exhaust gas to be purified flows in from the first electrode (anode) 3 side and is discharged from the second electrode (cathode) 4 side. Then, by applying a voltage when the purification structure 1 is maintained at 250 to 400 ° C., NOx contained in the exhaust gas in the second electrode (cathode) 4 is a component of the second electrode 3. It is reduced to nitrogen gas (N ₂ ) by the second electrode 4 adsorbed by the NOx storage material and applied with a negative voltage. Oxygen ions (O ²⁻ ) generated by the reduction of NOx are forcibly moved to the first electrode 3 side through the solid electrolyte layer to which a voltage is applied.

When the exhaust gas contains fine particles (PM), PM is collected on the surface of the first electrode 3 by making the first electrode 3 porous so that the fine particles (PM) can be collected. PM is oxidized on the surface of the first electrode 3 by O ²⁻ that has moved from 4 to generate and remove CO ₂ and H ₂ O, and nitrogen oxide (NOx) and fine particles (PM) in the exhaust gas can be removed. ) Can be removed simultaneously.

The operating temperature of the purification structure 1 has an optimum temperature depending on the solid electrolyte 2 to be used. In the case of gadolinia-added ceria (GDC) and samaria-added ceria (SDC), 250 to 350 ° C. In zirconia bromide (ScSZ), the temperature is 350 to 400 ° C.
It is preferable to use gadolinia-added ceria (GDC) or samaria-added ceria (SDC) as the solid electrolyte because NOx can be removed at a lower temperature than other solid electrolytes.

  In practical use, the purification structure 1 decomposes NOx and PM by applying a voltage at the timing of heating and raising the temperature to the above temperature by the energy of exhaust gas. When the temperature of the purification structure 1 is lower than its operating temperature, PM is collected in the first electrode, and NOx is occluded in the second electrode, so that these components are discharged at a high concentration in the exhaust gas. There is no. In addition, the purification structure 1 does not need to be a flat plate, and the honeycomb structure is intended to improve the PM deposition amount and NOx occlusion amount and to expand the reaction area in order to improve the decomposition rate thereof. Is desirable.

  Since the heating of the purification structure is usually used to purify high-temperature exhaust gas as described above, it is not necessary to use a special heating means. However, when the exhaust gas temperature is low, a heating device is used before the purification structure. The exhaust gas may be guided to the purification structure, or the purification structure itself may be heated by a commonly used heating means to maintain the operating temperature.

Then, the lamination | stacking method to the solid electrolyte 2 of a gradient electrode is demonstrated.
In order to support the electrode on the solid electrolyte, conductive particles and electrolyte particles are mixed in a slurry state and formed on the electrolyte by screen printing or dip coating. A gradient electrode can be obtained by reducing the coating thickness once and laminating thin layers having different proportions of conductive particles and electrolyte particles.

The various manufacturing methods of the tilt electrode have been described above. In addition, after forming the inclined electrode, the second electrode carries barium oxide (BaO) as a NOx storage material. Although barium (Ba) particles or barium oxide (BaO) particles may be directly mixed in the slurry for the second electrode, after firing the electrode, an aqueous solution of barium acetate (Ba (CH ₃ COO) ₂ ) is added. When applied to two electrodes and dried at a temperature of room temperature to 600 ° C., it can be supported while maintaining high dispersibility.

An embodiment in which NOx and PM are simultaneously reduced using the exhaust gas purifying apparatus of the present invention is shown below.
The solid electrolyte that constitutes the purification structure of the example is made of gadolinia-added ceria (GDC), and is sintered into a disk shape having a diameter of 60 mm and a thickness of about 400 μm by sintering powdered GDC. About 50% was used.
Then, a first electrode and a second electrode having a diameter of 58 mm and a thickness of about 30 μm were carried on both surfaces of the solid electrolyte by screen printing. Both the first electrode and the second electrode are prepared by slurrying a mixture of Ag and GDC (Ag95), which is a conductor having a GDC content of 95% by mass, on a solid electrolyte with an excipient and ethyl alcohol. A screen-printed GDC and Ag obtained by slurrying a mixture of Ag and GDC (Ag30), which is a conductor having a GDC content of 30% by mass, into a slurry with an excipient and ethyl alcohol on the outside. Are composed of two layers of cermet electrodes.
The second electrode supported Ba (barium) as a NOx storage material. In this method, an aqueous solution of barium acetate (Ba (CH ₃ COO) ₂ ) was applied to the second electrode, dried and supported.

FIG. 2 is an SEM image of the single electrode (left side) of the comparative example and the gradient electrode (right side) of the present invention. Ag30 has a GDC content of 30% by mass, and Ag95 has a GDC content of 95% by mass. It shows that there is. The photograph on the left side of FIG. 2 shows an SEM image of a portion in which a single electrode of Ag30 is supported on a solid electrolyte for comparison. Moreover, the photograph on the right side of FIG. 2 shows a portion of a gradient electrode in which an electrode having a GDC content of 95% by mass is supported on a solid electrolyte, and an electrode having a GDC content of 30% by mass is supported thereon. The SEM image of is shown. In FIG. 2, a part of the electrolyte is photographed in the vicinity of the electrode.
As can be seen from the figure, Ag95 serves as an interface between the solid electrolyte and Ag30, and is assumed to contribute to ionic conductivity between the electrode and the solid electrolyte.

The results of experiments in which the gradient electrode was manufactured by changing the film thickness of Ag95 and Ag30 of the first electrode and the second electrode of the purification structure and the effect of NOx reduction was confirmed were compared with the experimental results of the Ag30 single electrode. These are collectively shown in FIG.
The tilted electrodes have different thicknesses of Ag95 and Ag30, but the total thickness of the electrodes was set to be approximately the same as about 30 μm. The experiment was performed using a single electrode having a thickness of about 30 μm.
The exhaust gas used in the experiment is exhaust gas discharged from a water-cooled, 1-cylinder, 4-cycle general-purpose engine. The injection timing and the valve opening pressure of the nozzle are changed from the optimum set values, and more NOx and PM are discharged. The experiment was conducted under conditions. The concentration of each component in the exhaust gas is approximately as follows. NO: 346 ppm, NO ₂ : 9 ppm, CO 2: 4.23%, CO: 270 ppm, THC: 220 ppm, PM: 5 mg / m ³ . In the experiment, part of the exhaust gas was guided to a disk-shaped cell and heated to 350 ° C.

  In the experiment, the exhaust gas was allowed to flow from the first electrode side and discharged from the second electrode side to a total of four types of purification structures having three types of inclined electrodes and a single electrode, and a voltage was applied between the electrodes. The purification rate at each current value was measured.

  The results are collectively shown in FIG. 3 as NOx reduction rates. From Fig. 3, compared to the Ag30 single electrode, the Ag95 / Ag30 gradient electrode has improved NOx reduction rate at any film thickness, improved the network of electrolyte particles and conductive particles, and expanded the three-layer interface. Can be seen. Further, when the film thicknesses of Ag95 and Ag30 are changed, the NOx reduction rate in the low current region is increased by making the film thickness of Ag95 thinner than that of Ag30. This is because Ag95 is inferior in electronic conductivity and, as can be seen from FIG. 2, there are few voids and gas permeability is poor, so thickening Ag95 increases Joule loss in addition to the effect of improving the three-layer interface, and the flow of gas This is thought to lead to a decrease in the NOx reduction rate. Thus, since the properties of the electrodes are different when the mixing ratio of the conductive particles and the electrolyte particles is changed, it is effective to optimize these thicknesses.

  4 and 5 are photographs showing the state of decomposition of PM on the surface of the first electrode, FIG. 4 shows the state of PM deposition after passing through the exhaust gas for 2 hours, and FIG. 5 shows the state of PM deposition when voltage is applied. It can be seen that PM is decomposed by voltage application.

  The exhaust gas purification apparatus of the present invention can be used as an apparatus for reducing NOx in air as well as in exhaust gas. Examples of the exhaust gas include gases discharged from various combustion engines, and examples include a diesel engine, a gasoline engine, a boiler, and an industrial furnace. Among these, the exhaust gas purifying device capable of simultaneously removing nitrogen oxides (NOx) and fine particles (PM) in the exhaust gas is suitable for purifying the exhaust gas discharged from the diesel engine.

1 Purification Structure 2 Solid Electrolyte 3 First Electrode (Anode)
4 Second electrode (cathode)
5 Switch 6 Power supply

Claims (5)

A porous solid electroelectrolyte having oxygen ion conductivity is equipped with a porous first electrode on one side and a porous second electrode on the other side to allow exhaust gas to pass through, and a voltage is applied to both electrodes. The first electrode on the exhaust gas inflow side is an anode, the second electrode on the exhaust gas outflow side is a cathode, the first electrode is gadolinia-added ceria (GDC) and The second electrode includes a gadolinia-added ceria (GDC), a conductor, and a NOx storage material, and the concentration of the conductor contained in the entire first electrode and / or the second electrode is the solid. An exhaust gas purifying apparatus characterized by being inclined so that the concentration decreases toward the electrolyte side.   The first electrode and / or the second electrode is composed of a plurality of layers, and the concentration of the conductor of the layer is inclined so that the concentration decreases toward the solid electrolyte side. The exhaust gas purification apparatus according to 1.   The exhaust gas purification apparatus according to claim 1 or 2, wherein the first electrode is porous capable of collecting fine particles in the exhaust gas.   The exhaust gas purification apparatus according to any one of claims 1 to 3, wherein a main component of the solid electrolyte is gadolinia-added ceria (GDC).   The exhaust gas purifying apparatus according to any one of claims 1 to 4, wherein the NOx storage material is barium (Ba) or a compound of barium.