ELECTROCATALYTIC TUBE OF ELECTROCHEMICAL-CATALYTIC CONVERTER FOR EXHAUST EMISSIONS CONTROL

Abstract

An electrocatalytic tube for controlling exhaust emissions, which adopts to purify the exhaust, comprises a tube, an anode layer and a cathode layer. The tube is composed of a solid-state electrolyte layer. The solid-state electrolyte layer includes an enclosed chamber, an inner wall formed inside the enclosed chamber, and an outer wall formed outside the enclosed chamber. The enclosed chamber has a sub-atmospheric reducing environment. The anode layer and cathode layer are respectively coated on the inner wall and outer wall of the solid-state electrolyte layer. The reducing environment facilitates an electromotive force to occur between the anode layer and the cathode layer. The electromotive force promotes catalytic decomposition of nitrogen oxides of the exhaust on the cathode layer.

Description

FIELD OF THE INVENTION

The present invention relates to an electrochemical-catalytic converter, particularly to an electrocatalytic tube of the electrochemical-catalytic converter for controlling exhaust emissions.

BACKGROUND OF THE INVENTION

Fresh and clean air is essential for human health. Science and technology has promoted economical development. However, the exhausts of vehicles and factories, especially motor vehicles and heavy industry factories, seriously pollutes the air.

The emission standard of motor vehicles has been increased persistently. However, the continuously increasing motor vehicles still bring about more and more serious air pollution. In a motor vehicle, the engine thereof burns fuel and converts chemical energy into mechanical energy. The burning process of fuel generates the polluting constituents, including nitrogen oxides (NO_x), carbon monoxide (CO), hydrocarbons (HCs), and particulate matter (PM), which would form photochemical smog, deplete ozone, enhance the greenhouse effect, cause acid rain, damage the ecological environment and finally danger human health.

Carbon monoxide comes from incomplete combustion. The capability of carbon monoxide to combine with hemoglobin to form carboxyhemoglobin (COHb) is 300 times higher than the capability of oxygen to combine with hemoglobin to form oxyhemoglobin (HbO₂). Therefore, too high a concentration of carbon monoxide would degrade the capability of hemoglobin to transport oxygen. Nitrogen oxides are generated by the combination of nitrogen and oxygen and mainly in form of nitrogen monoxide (NO) and nitrogen dioxide (NO₂). Reaction of nitrogen oxides and hydrocarbons is induced by ultraviolet ray, generating poisonous photochemical smog, which has a special odor, irritates eyes, harm plants, and reduces the visibility of the ambient air. Nitrogen oxides can react with water in the air to form nitric acid and nitrous acid, which are the constituents of acid rain. Hydrocarbons can irritate the respiratory system even at lower concentration and will affect the central nervous system at higher concentration. Particulate matter can danger human health and can even cause cancer.

Therefore, many nations, including EU, USA, Japan and Taiwan, have regulated stricter emission standards for nitrogen oxides, carbon monoxide, hydrocarbons and particulate matter, such as BIN5 of USA and EURO 6 of EU, which not only regulate the emissions of the polluting constituents but also encourage the manufacturers to develop, fabricate or adopt the newest pollution control technologies and apparatuses.

A U.S. Pat. No. 5,401,372 disclosed an “Electrochemical Catalytic Reduction Cell for the Reduction of NO_x in an O₂-Containing Exhaust Emission”, which is dedicated to removing nitrogen oxides, wherein an electrochemical-catalytic reducing reaction and a vanadium pentaoxide (V₂O₅) catalyst convert nitrogen oxides into nitrogen. However, the prior art needs an electric source to power an electrochemical cell. Therefore, the prior art consumes power and cannot eliminate other polluting constituents simultaneously.

A U.S. patent application Ser. No. 13/037,693 disclosed an “Electrochemical-Catalytic Converter for Exhaust Emission Control”, which can eliminate nitrogen oxides (NO_x), carbon monoxide (CO), hydrocarbons (HCs), and particulate matter (PM) in the exhaust, and which comprises an electrochemical module, wherein the nitrogen oxides are decomposed into nitrogen and oxygen, and wherein carbon monoxide, hydrocarbons, and particulate matter are converted into water and carbon dioxide by an oxidation catalyst. Therefore, the prior art can eliminate multiple polluting constituents simultaneously.

However, the abovementioned “Electrochemical-Catalytic Converter” needs a reducing gas system to generate an electromotive force, which increases the fabrication cost. Further, the circulating reducing gas heated by a heating unit will expand and contract, which is likely to damage the structure of the anode. Besides, the “Electrochemical-Catalytic Converter” is hard to fabricate into a compact structure for vehicle application. Therefore, the prior art still has room to improve.

SUMMARY OF THE INVENTION

The primary objective of the present invention is to overcome the problems of the conventional electrochemical-catalytic converter, including high fabrication cost, expansion and contraction-induced structural damage, and bulky volume.

To achieve the abovementioned objective, the present invention proposes an electrocatalytic tube, which comprises a tube, an anode layer and a cathode layer. The tube is made of a solid-state electrolyte layer, which includes an enclosed chamber, an inner wall inside the enclosed chamber, and an outer wall outside the enclosed chamber. The enclosed chamber can have a sub-atmospheric reducing environment. The anode layer and the cathode layer are respectively coated on the inner wall and the outer wall of the solid-state electrolyte layer.

The exhaust passes over the cathode layer for treatment. For the present invention, the exhaust is from lean-burn engines and thus is at an oxidizing environment. The reducing environment of the enclosed chamber and the oxidizing environment of the cathode layer induce an electromotive force to form between the anode layer and the cathode layer to promote the decomposition of nitrogen oxides. The oxidizing environment over the cathode layer enables the oxidation of carbon monoxide, hydrocarbons, and particulate matter of the exhaust.

In the present invention, the electrocatalytic tubes can be assembled to form a ceramic monolith to be an advanced electrochemical-catalytic converter. Thus, the present invention has simple structure and lower fabrication cost than the prior art of the conventional electrochemical-catalytic converter. Besides, the sub-atmospheric reducing environment exempts the present invention from structural damage caused by thermally-induced expansion and contraction. Therefore, the electrocatalytic tube of the present invention can have a longer service life.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing an electrocatalytic tube according to a first embodiment of the present invention;

FIG. 2 is a local sectional view schematically showing an electrocatalytic tube according to the first embodiment of the present invention;

FIG. 3 is a diagram schematically showing the assemblage of the electrocatalytic tubes according to a second embodiment of the present invention; and

FIG. 4 is a perspective view schematically showing an electrocatalytic tube according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical contents of the present invention are described in detail in cooperation with drawings below.

Refer to FIG. 1 and FIG. 2 respectively a perspective view and a local sectional view schematically showing an electrocatalytic tube for controlling exhaust emissions according to a first embodiment of the present invention. The electrocatalytic tube 1 of the present invention adopts to purify the exhaust containing nitrogen oxides, carbon monoxide, hydrocarbons, and particulate matter. The electrocatalytic tube 1 comprises a tube, an anode layer 20 and a cathode layer 30. The tube is made of a solid-state electrolyte layer 10. The microstructure of the solid-state electrolyte layer 10 is a dense structure and made of fluorite metal oxides or perovskite metal oxides, such as fluorite YSZ (Yttria-Stabilized Zirconia), stabilized zirconia, fluorite GDC (Gadolinia-Doped Ceria), doped ceria, perovskite strontium/magnesium-doped lanthanum gallate, and doped lanthanum gallate.

The solid-state electrolyte layer 10 includes an inner wall 12 and an outer wall 13 opposite to the inner wall 12. The anode layer 20 is coated on the inner wall 12. In one embodiment, the anode layer 20 is made of a porous material, such as a cermet of nickel and fluorite metal oxides (e.g. a Ni-YSZ cermet), perovskite metal oxide, or metal-added perovskite metal oxide. The cathode layer 30 is coated on the outer wall 13. In one embodiment, the cathode layer 30 is made of a porous material, such as perovskite metal oxide, fluorite metal oxide, metal-added perovskite metal oxide, or metal-added fluorite metal oxide (e.g. a perovskite lanthanum-strontium-cobalt-copper oxide, a lanthanum-strontium-manganese-copper oxide, a combination of a lanthanum-strontium-cobalt-copper oxide and a gadolinia-doped ceria, a combination of a lanthanum-strontium-manganese-copper oxide and a gadolinia-doped ceria, a silver-added lanthanum-strontium-cobalt-copper oxide, a silver-added lanthanum-strontium-manganese-copper oxide, a combination of a silver-added lanthanum-strontium-manganese-copper oxide and a gadolinia-doped ceria, and a combination of a silver-added lanthanum-strontium-manganese-copper oxide and a gadolinia-doped ceria).

The solid-state electrolyte layer 10 also includes an enclosed chamber 11 enclosed by the inner wall 12 and having a reducing environment. The reducing environment can have a sub-atmospheric pressure. In one embodiment, the reducing environment has a pressure of ½-⅓ atm and has a carbon species or a reducing gas (such as carbon monoxide and hydrocarbons). Before the enclosed chamber 11 is sealed, carbon monoxide or hydrocarbons, such as methane, ethane, propane or propylene, is filled into the enclosed chamber 11 to form a carbon species adhering to the anode layer 20 and assisting in generating the electromotive force. Similarly, the reducing gas is filled into the enclosed chamber 11 before the enclosed chamber 11 is sealed.

In one embodiment, the electrocatalytic tube 1 further comprises a catalytic oxidation layer 40 to assist in oxidation of the constituents of the exhaust which are hard to oxidize on the cathode layer 30. The catalytic oxidation layer 40 adheres to the cathode layer 30 and is made of a metal, an alloy, a metal oxide, a fluorite metal oxide, or a perovskite metal oxide, such as silver, palladium, platinum, or gadolinia-doped ceria.

Below is described the process of purifying the exhaust. Firstly, place the electrocatalytic tube 1 of the present invention in an environment of the exhaust. The exhaust is oxygen-rich or enriched with oxygen via adding secondary air. The working temperature of the electrocatalytic tube 1 is from ambient temperature to 600° C. The exhaust contains nitrogen oxides, carbon monoxide, hydrocarbons, and particulate matter. The purifying reactions undertaken by the present invention include the decomposition reaction of removing nitrogen oxides and the oxidation reaction of removing carbon monoxide, hydrocarbons, and particulate matter.

Nitrogen oxides include nitrogen monoxide (NO) and nitrogen dioxide (NO₂). Nitrogen monoxide is decomposed into nitrogen and oxygen on the cathode layer 30. The reaction of NO decomposition is expressed by Formula (1):

2NO→N₂+O₂  (1)

Nitrogen dioxide is decomposed into nitrogen monoxide and oxygen on the cathode layer 30. The reaction of NO₂ decomposition is expressed by Formula (2):

2NO₂→2NO+O₂  (2)

Then, nitrogen monoxide is further decomposed into nitrogen and oxygen on the cathode layer 30.

The reducing environment of the enclosed chamber 11 and the oxidizing environment in the cathode layer 30 results in a difference of the oxygen partial pressure between the anode layer 20 and the cathode layer 30 and thus generate an electromotive force (emf) between the anode layer 20 and the cathode layer 30 according to Formula (3):

emf=[(RT)/(4F)]·ln[(P_O2|Cathode)/(P_O2|Anode)]  (3)

wherein R is the gas constant, T the absolute temperature, F the Faradic constant, and P_O2 the partial pressure of oxygen. The carbon species adhering to the anode layer 20 is a reducing compound to result in lower oxygen partial pressure over the anode and thus to generate larger electromotive force. Different reducing gases and different reducing compounds on the anode side result in different oxygen partial pressures and thus generate different electromotive forces. Different oxygen concentrations on the cathode side also result in different oxygen partial pressures and thus generate different electromotive forces. The higher the oxygen concentration on the cathode side, the larger the electromotive force and thus the higher promotion of decomposing nitrogen oxides into oxygen and nitrogen. Within a given temperature range, the lower the temperature, the higher the decomposition rate. The decomposition of nitrogen oxides can be effective at ambient temperature.

As to eliminating carbon monoxide, hydrocarbons and particulate matter of the exhaust, the exhaust is oxygen-rich or enriched with oxygen via adding secondary air. Then, the cathode layer 30 and the catalytic oxidation layer 40 convert carbon monoxide, hydrocarbons, and particulate matter into harmless gases. For example, carbon monoxide is oxidized into carbon dioxide; hydrocarbons (HCs) and particulate matter (carbon-containing) are oxidized into carbon dioxide and water. The reactions thereof are expressed by Formulae (4)-(6):

2CO+O₂→2CO₂  (4)

HCs+O₂→H₂O+CO₂  (5)

C+O₂→CO₂  (6)

Via the abovementioned catalytic decomposition reactions and catalytic oxidation reactions, the polluting constituents of the exhaust are effectively removed.

Refer to FIG. 3 a diagram schematically showing the assemblage of the electrocatalytic tubes for controlling exhaust emissions according to a second embodiment of the present invention. In this embodiment, the electrocatalytic tubes 1 are intermittently arranged according to the requirement for purification; between every two adjacent electrocatalytic tubes 1 is formed a gas channel 2 where the exhaust passes and contacts the cathode layer 30 of the electrocatalytic tube 1; thus is established a honeycomb-like structure 3 functioning as an electrochemical-catalytic converter. Thereby, the present invention can purify the exhaust with a compact assemblage of the electrocatalytic tubes 1. In the honeycomb-like structure 3, one half of the channels are sealed to form the electrocatalytic tubes 1, and the other half of the channels (i.e. the gas channels 2) are opened.

Refer to FIG. 4 a diagram schematically showing an electrocatalytic tube for controlling exhaust emissions according to a third embodiment of the present invention. In this embodiment, the electrocatalytic tube 1 is fabricated to have a circular section. However, the present invention does not restrict that the electrocatalytic tube 1 must have a circular section. In the present invention, the electrocatalytic tube 1 may have a rectangular, pentagonal, or hexagonal section according to design.

In conclusion, the present invention uses the reducing environment of the enclosed chamber and the carbon species adhering to the anode layer to generate the electromotive force to promote the catalytic decomposition reaction. Further, the present invention has simple structure and lower fabrication cost. Furthermore, the reducing environment has a sub-atmospheric pressure, such as ½ atm, whereby the present invention is exempted from the structural damage caused by thermally-induced expansion and contraction, wherefore is prolonged the service life of the present invention. Moreover, the electrocatalytic tubes can be assembled to form a honeycomb-like converter, whereby the present invention has a compact assemblage able to be installed in the exhaust pipe of a vehicle engine for eliminating the polluting constituents of the exhaust and thus reducing air pollution.

Therefore, the present invention possesses utility, novelty and non-obviousness and meets the condition for a patent. Thus, the Inventors file the application for a patent. It is appreciated if the patent is approved fast.

The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.

Claims

1. An electrocatalytic tube for controlling exhaust emissions, which adopts to purify the exhaust, comprising:

a tube, composed of a solid-state electrolyte layer, wherein the solid-state electrolyte layer includes an enclosed chamber, an inner wall formed inside the enclosed chamber, and an outer wall formed outside the enclosed chamber, and wherein the enclosed chamber has a reducing environment; and

an anode layer and a cathode layer respectively coated on the inner wall and the outer wall of the solid-state electrolyte layer, wherein the reducing environment facilitates an electromotive force to occur between the anode layer and the cathode layer, and wherein nitrogen oxides of the exhaust are decomposed into nitrogen and oxygen on the cathode layer, and wherein carbon monoxide, hydrocarbons and particulate matter of the exhaust are oxidized into carbon dioxide and water on the cathode layer.

2. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the reducing environment is sub-atmospheric.

3. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the anode layer is made of a porous material.

4. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the cathode layer is made of a porous material.

5. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the reducing environment includes a gas selected from a group consisting of carbon monoxide and hydrocarbons.

6. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein a carbon species adheres to the anode layer.

7. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the electrocatalytic tube has a working temperature ranging from ambient temperature to 600° C.

8. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the anode layer is made of a material selected from a group consisting of cermet of nickel and fluorite metal oxides, perovskite metal oxides, fluorite metal oxides, metal-added perovskite metal oxides, metal-added fluorite metal oxides, and combinations thereof.

9. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the cathode layer is made of a material selected from a group consisting of perovskite metal oxides, fluorite metal oxides, metal-added perovskite metal oxides, metal-added fluorite metal oxides, and combinations thereof.

10. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the solid-state electrolyte layer is made of a material selected from a group consisting of fluorite metal oxides, perovskite metal oxides, and combinations thereof.

11. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein a catalytic oxidation layer is adhered to the cathode layer.

12. The electrocatalytic tube for controlling exhaust emissions according to claim 11, wherein the catalytic oxidation layer is made of a material selected from a group consisting of metals, alloys, metal oxides, fluorite metal oxides, perovskite metal oxides, and combinations thereof.

13. The electrocatalytic tube for controlling exhaust emissions according to claim 1, wherein the reducing environment has a pressure of smaller than ½ atm.