JPH04190830A - Removal of nitrogen oxide - Google Patents

Abstract

PURPOSE:To efficiently remove nitrogen oxide even in an oxidizing atmosphere by generating a hydrogen ion on the anode surface of a solid electrolyte to move the same to the cathode surface thereof and reducing nitrogen oxide in gas to be treated by precipitated hydrogen. CONSTITUTION:Gas to be treated containing water and nitrogen oxide flowing from a nozzle 8 toward a nozzle 9 is brought into contact with a hydrogen ion conductive solid electrolyte 2 (e.g. a sintered body of a BaCeO3 powder or the like) having anode and cathode surfaces 4, 3 to which DC voltage is applied. At the same time, a hydrogen ion is generated on the anode surface 4 by the electrolytic oxidation of water and moved to the cathode surface 3 to be precipitated as hydrogen. Nitrogen oxide in the gas to be treated is reduced by this hydrogen. As a result, nitrogen oxide can be efficiently removed even in an oxidizing atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method that can efficiently purify nitrogen oxides in exhaust gas discharged from internal combustion engines, nitric acid manufacturing plants, etc. - (Prior Art) Exhaust gas emitted from internal combustion engines of automobiles, nitric acid manufacturing plants, etc. contains harmful components such as nitrogen oxides, which are a cause of air pollution. Therefore, various approaches are being considered to remove nitrogen oxides from the exhaust gas.

As a conventional method for removing nitrogen oxides, there is a catalytic removal method using a catalyst. In this method, nitrogen oxides are adsorbed on the catalyst surface and simultaneously reduced and decomposed into nitrogen and oxygen. It removes oxides. As a catalyst for removing nitrogen oxides, a catalyst is used in which noble metals such as palladium, platinum, and rhodium are supported on a carrier made of a porous material such as alumina.

However, in this catalyst removal method, -carbon oxide, hydrogen,
The removal rate of nitrogen oxides is low in an oxidizing atmosphere that does not contain reducing substances such as ammonia gas or contains excessive oxygen. This is because the oxygen produced by the dissociation of nitrogen oxides is not reduced in the absence of reducing substances, and even if the reducing substance is present, in the presence of excess oxygen, the oxygen is reduced. This is because the substances react preferentially with excess oxygen, and the removal of nitrogen oxides does not proceed.

For example, when the ratio of air to fuel (air-fuel ratio) increases in automobile exhaust gas, it contains more oxygen than is necessary to completely burn unburned components. In the case of a large amount of nitrogen oxides, the removal of nitrogen oxides is not promoted as described above.

(Object of the Invention) The present invention aims to solve the above-mentioned conventional problems and provide a method for efficiently removing nitrogen oxides even in an oxidizing atmosphere.

(Structure of the Invention) The method for removing nitrogen oxides of the present invention involves applying a hydrogen ion conductive solid electrolyte body having an anode surface and a cathode surface to which a direct current voltage is applied to the surface of a hydrogen ion conductive solid electrolyte body containing water and nitrogen oxides to be treated. At the same time as the gas is brought into contact, hydrogen ions are generated by electrolytic oxidation of water on the anode surface, and the ions are moved to the cathode surface and precipitated as hydrogen, and the hydrogen oxidizes nitrogen in the gas to be treated. It is characterized by reducing things.

(Operation of the Invention) The nitrogen oxide removal method of the present invention decomposes water in the gas to be treated on the anode surface of the nitrogen oxide decomposition device, and transfers the generated hydrogen ions to the cathode surface via a solid electrolyte body. transferred and reduced to hydrogen. Since nitrogen oxides are adsorbed or dissociated into nitrogen and oxygen on the cathode surface, the hydrogen reacts with the adsorbed nitrogen oxides or dissociated oxygen, and oxygen is removed from the nitrogen oxides, resulting in nitrogen oxidation. The disassembly of things continues.

(Effects of the Invention) According to the present invention, nitrogen oxides can be purified with high efficiency. In addition, nitrogen oxides can be purified with high efficiency even in an oxidizing atmosphere that does not contain reducing substances or in which oxygen is present in excess.

The method for removing nitrogen oxides according to the present invention can be used to purify nitrogen oxides in exhaust gas from internal combustion engines such as automobiles, nitric acid manufacturing plants, etc.

(Other inventions) In this and other inventions, the solid electrolyte body transmits hydrogen ions generated at the anode surface by electrolysis of water contained in the gas to be treated, and reduces and removes nitrogen oxides at the cathode surface. It is. This solid electrolyte body includes barium oxide, ceria (B a Ce Os ), strontium ceria oxide (SrCeOs), strontium zirconia oxide (S r Z r 03), calcium oxide zirconia (Ca Z r 03 ), and strontium oxide. A sintered body of oxide powder such as strontium titania (SrTiCh) is used. Preferably, oxides such as yttrium (Y), neodymium (Nd), ytterbium (Yb), and scandium (Sc) are added to these oxides.

The thickness of the solid electrolyte body is preferably within the range of 1 μm to 0.5 cm. If the thickness is less than 1 μm, a dense structure cannot be created. On the other hand, if the thickness exceeds 0.5 cm, the resistance to hydrogen ion permeation may increase and the removal rate of nitrogen oxides may decrease.

A pair of electrode surfaces serving as an anode surface and a cathode surface are formed on the surface of this solid electrolyte body to form a nitrogen oxide decomposition device. Note that this pair of electrode surfaces are arranged so as not to be in direct contact with each other. The anode material is preferably a metal with a small oxygen overvoltage such as platinum, rhodium, or iridium, and one or a mixture of two or more of these metals may be used.

The cathode material is preferably platinum, palladium, rhodium, ruthenium, iridium, etc., and one or a mixture of two or more of these metals may be used. Further, as a cathode material, a nitrogen oxide selective reduction catalyst such as copper oxide (Cub), cobalt oxide (Coo), copper ion exchange zeolite, etc. can be used. Especially these and platinum,
A mixture with a metal having a small hydrogen overvoltage such as palladium is suitable. On this electrode surface, nitrogen oxides are adsorbed and dissociated.

This electrode surface is formed by coating or printing a paste of an electrode-forming compound on the surface of the solid electrolyte body and baking it. Alternatively, it may be performed by sputtering, vapor deposition, plating, or the like.

The thickness of the electrode surface is preferably within the range of 0.2 to 1000 μm.

A nitrogen oxide decomposition device is formed as described above.

Also, in order to increase the contact rate with the gas to be treated, it is recommended to increase the surface area of the electrode surface. In order to increase the surface area, there are methods such as increasing the size of the nitrogen oxide decomposition device itself, or arranging a large number of such devices in the flow path of the gas to be treated.

Next, the nitrogen oxide decomposition device is placed in contact with the nitrogen oxide-containing gas to be treated.

Thereafter, a DC voltage is applied between the electrode surfaces.

As a result, the water in the gas to be treated is decomposed on the anode surface, producing hydrogen ions (H゛) and oxygen (02), and the hydrogen ions electrically pass through the solid electrolyte body and move to the cathode surface. death,
Reduced to hydrogen. Nitrogen oxides (NOx) on the cathode surface
Since nitrogen is adsorbed or dissociated into nitrogen and oxygen, the adsorbed nitrogen oxides or dissociated oxygen are reduced by the hydrogen to become nitrogen and water. In this way the removal of nitrogen oxides is facilitated.

In this and other inventions, it is preferable that the DC voltage be applied so that a current having a current density of 5 to 300 mA/cnf flows per unit area of the positive and negative electrode surfaces. If the current density is less than 5 mA/ad, the power to supply hydrogen to the cathode surface will be small, while if it exceeds 300 mA/cfl, the solid electrolyte will deteriorate.

Note that even if the current density is 300 mA/cffl or less, the solid electrolyte may deteriorate depending on the magnitude of the voltage, so it is desirable that the applied voltage be 5.0 V or less.

Note that when the temperature of the gas to be treated is low, the rate of hydrogen ions passing through the solid electrolyte body is low, and the NOx removal efficiency may be reduced. In this case, the nitrogen oxide decomposition device may be heated. This heating temperature is 400°C ~ 10
It is desirable that the temperature be within the range of 00°C.

〔Example〕

The present invention will be explained below using examples.

(Example 1) A sectional view of the entire device in this example is shown in the attached drawing. That is, as a whole apparatus, a nitrogen oxide decomposition apparatus 1 is installed protrudingly within a gas distribution box 6 to be treated.

The nitrogen oxide decomposition device 1 includes a solid electrolyte body 2 having a cylindrical shape with a bottom, and platinum-rhodium electrodes 3 and platinum electrodes 4 provided on both surfaces of the solid electrolyte body 2. Therefore, the electrode surface 4 on the surface of the electrolyte body 2 that is in contact with the exhaust gas drawn from the upstream surface of the exhaust gas path of the solid electrolyte body 1 is an anode, and the electrode surface 3 on the surface of the electrolyte body 2 that is in contact with the exhaust gas in the exhaust gas path is an anode. is connected as a cathode to a DC power source 5 through copper lead wires 1o and 11. Further, a heating heater 12 is inserted into the cylinder of the solid electrolyte body 2 . Solid electrolyte body 2
is calcium oxide zirconia (Ca Z r○3)
C doped with 6 mob% yttrium (Y3)
A Zr 0.94Y0.0603-p powder was sintered and molded to a thickness of 0.2 cm. Further, the platinum-rhodium electrode 3 and the platinum electrode 4 were each formed to a thickness of 10 μm by a plating method.

The to-be-treated gas distribution box 6 is a box having a nozzle 8.9 and an exhaust gas introduction pipe 7. The gas to be treated enters through an intake nozzle 8, comes into contact with the decomposition device 1, and is discharged through an exhaust nozzle 9.

Next, a specific example in which nitrogen oxides were purified using this apparatus 1 will be shown.

That is, the heating heater 12 heats the decomposition device 1 at 700°C.
°C, and from the intake nozzle 8, nitrogen (N2
6.) Gas is applied to both sides of the solid electrolyte body 2. While flowing at OA/mm, a DC voltage was applied at a current density (per unit area of positive and negative electrodes) as shown in Table 1. The amount of NO in the gas to be treated discharged from the exhaust nozzle 9 was measured to determine the amount of NO decomposed according to this example. The results are shown in the same table (the amount of No decomposed in the table represents the amount of decomposed per unit area of the cathode surface 3).

Furthermore, as a comparative example, measurements were made in the same manner when no voltage was applied, and the results are shown in the same table (NαC1).

As is clear from Table 1, according to this example, nitrogen oxides can be efficiently removed even in an oxidizing atmosphere in which no reducing substance is present.

Table 1 (Example 2) In this example, an example of purification in an oxidizing atmosphere is shown.

The solid electrolyte body 2 of the nitrogen oxide decomposition device 1 shown in Example 1 has S r Ce o, ssY bo, o50
sp, a mixture of copper oxide (CaO) and platinum was formed to a thickness of 30 μm as the cathode surface 3, and NO was added as the gas to be treated.
The amount of decomposed NO was measured in the same manner as in Example 1, except that N2 gas containing 1000 pp, 1% oxygen (02) gas (based on N2 gas), and 3.9% water vapor was used. Note that a DC voltage was applied at a current density as shown in Table 2.

The results are shown in the same table (the amount of No decomposition represents the amount of decomposition per unit area of the cathode surface 3). Further, as a comparative example, a case in which no voltage was applied is also shown in NαC2 in the same table.

According to Table 2, it can be seen that nitrogen oxides can be efficiently removed even in an oxidizing atmosphere.

The present invention is not limited to these examples in any way unless it exceeds the gist thereof.

[Brief explanation of the drawing]

The figure shows a cross-sectional view of the entire device consisting of a nitrogen oxide decomposition device and a gas distribution box in this embodiment.

Claims (1)

[Claims] A hydrogen ion conductive solid electrolyte body having an anode surface and a cathode surface to which a DC voltage is applied is brought into contact with a gas to be treated containing water and nitrogen oxides, and at the same time, water is electrolytically oxidized on the anode surface. A method for removing nitrogen oxides, which comprises generating hydrogen ions, moving the ions to the cathode surface, precipitating them as hydrogen, and reducing nitrogen oxides in the gas to be treated using the hydrogen. .