Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-03-08
2001-12-18
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S266000, C422S186040
Reexamination Certificate
active
06331232
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for the reduction of nitrogen oxides. More particularly, the present invention relates to a method for the reduction of nitrogen oxides. The present invention is particularly intended for use in the field of purification of exhaust gases in combustion processes, for example, in the purification of vehicle exhaust gases.
BACKGROUND OF THE INVENTION
In the field of vehicles operated by combustion engines, there is a continuing demand for low emissions of harmful substances in the exhaust gases from the vehicle's engine. These harmful substances are primarily in the form of pollutants, such as NO
x
compounds (NO, NO
2
, and N
2
O), hydrocarbons (HC) and carbon monoxide (CO). In today's vehicles having gasoline engines, purification of the exhaust gases is normally carried out by means of an exhaust catalyst, which forms part of the exhaust system, and through which the exhaust gases are guided. In a so-called three-way catalyst, a major portion of the above-mentioned harmful compounds are eliminated through known catalytic reactions.
A condition for such a catalyst to operate with an optimal degree of purification is that the engine be controlled so that stoichiometry is obtained, i.e. so that a correctly adjusted air/fuel mixture is fed to the engine during its operation. In a motor vehicle, this can be accomplished by equipping the vehicle with a lambda sensor, by means of which a value of the oxygen content in the exhaust gases can be determined and fed to a control unit, which in turn generates the correct air/fuel mixture. However, if the condition regarding stoichiometry is not fulfilled, only a limited degree of purification of the catalyst can be obtained, particularly as regards the nitrogen oxides contained in the exhaust gases.
In certain types of combustion processes, for example in connection with diesel engines and in so-called “lean burn” processes, combustion is carried out with a relatively high surplus of oxygen, the purpose of which is to reduce the fuel consumption. This means that the stoichiometry condition is not fulfilled, and no purification of harmful emissions can be obtained in an effective manner with a conventional three-way catalyst. It is true that emissions of HC and CO compounds can be reduced by means of an oxidation catalyst, but the NO
x
emissions still constitute a problem. For this reason, there is a demand for devices and methods for the purification of NO
x
compounds, and particularly in combustion processes in which an oxygen surplus is present.
As regards the harmful NO
x
compounds, from the article “Removal of NO in the presence of 0
2
using electrochemical cells,” J. Nakatani, Second EU-Japan Workshop, Fundamental aspects of catalysis for clean combustion, Kyoto, Japan, Oct. 30-31 1995, a device is known by means of which NO
x
compounds can be converted to nitrogen gas. The device therein comprises a tube of an electrolytic material with a cathode which is arranged on the outside of the tube and an anode which is arranged on the inside of the tube. Helium is guided through the tube and past the anode, whereas a sample gas comprising nitrogen oxide, gaseous oxygen and helium is guided past the cathode. By applying an electric potential, nitrogen oxide can be converted into gaseous nitrogen and oxygen on the cathode, by means of which the ionized oxygen is pumped through the electrolyte to the anode.
The known device involves a disadvantage due to the fact that it cannot operate with the intended effect at high concentrations of oxygen in the gas in which the nitrogen oxides are to be reduced. This is due to the fact that it is not capable of distinguishing the NO
x
compounds from oxygen to any high degree. In the known device, oxygen must be pumped away before the conversion of NO
x
compounds begins. This known device thus provides a reduction of NO
x
compounds which is entirely insignificant when the oxygen concentration reaches approximately 2%. Consequently, since the device cannot operate at high oxygen concentrations, it cannot be used in an effective manner in connection with lean combustion processes, i.e. processes involving an oxygen surplus. An example of such a process is the combustion realized in diesel engines, the exhaust gases of which have an oxygen concentration which is approximately 5 to 18%.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now been realized by the discovery of apparatus for the reduction of nitrogen oxides in a gas stream comprising an oxygen ion-conducting substrate, at least one anode disposed on the substrate, at least one cathode disposed on the substrate, and a voltage source connected to the anode and the cathode, the cathode comprising gold, whereby nitrogen oxides are adsorbed and dissociated on the cathode, nitrogen may recombine into nitrogen gas on the cathode, and oxygen ions are transported through the substrate from the cathode to the anode. Preferably, the cathode consists essentially of gold.
In accordance with one embodiment of the apparatus of the present invention, the anode comprises gold, palladium or rhodium. Preferably, the anode consists essentially of this material.
In accordance with another embodiment of the apparatus of the present invention, the cathode includes a stabilizing material. Preferably, the anode includes a stabilizing material, and most preferably, the stabilizing material comprises platinum.
In accordance with another embodiment of the apparatus of the present invention, the oxygen ion conducting substrate comprises stabilized zirconium dioxide.
In accordance with another embodiment of the apparatus of the present invention, the apparatus includes current measuring means for measuring the transport of the oxygen ions through the substrate and control means for controlling the voltage source based on the measured value of the current measuring means.
In accordance with another embodiment of the apparatus of the present invention, the apparatus includes heating means for heating the substrate. In a preferred embodiment, the voltage source comprises a first voltage source, the heating means comprises a resistance conductor, and the apparatus includes a second voltage source for the resistance conductor.
In accordance with another embodiment of the apparatus of the present invention, the anode and cathode comprise a porous material.
In accordance with another embodiment of the apparatus of the present invention, the anode and the cathode comprise a conducting pattern including a substantially linear portion and a plurality of transverse portions extending substantially perpendicularly from the substantially linear portion.
In accordance with another embodiment of the apparatus of the present invention, the substrate comprises a plurality of walls providing a plurality of adjacent ducts for guiding the gas stream, the anode and the cathode comprising coatings on at least one of the plurality of walls forming the plurality of adjacent ducts. In a preferred embodiment, the plurality of adjacent ducts comprises a first duct and a second duct adjacent to the first duct, and the walls comprise at least one inner wall of the first and second ducts, the cathode comprising a coating on the at least one inner wall of the first duct and the anode comprises a coating on the at least one inner wall of the second duct.
In accordance with another embodiment of the apparatus of the present invention, the substrate comprises a plurality of plates including a first side and a second side defining a plurality of longitudinally extending ducts for guiding the gas stream, at least one of the first and second sides of the plurality of plates comprising the cathode and the other of the first and second sides of the plurality of plates comprising the anode. In a preferred embodiment, the anode and the cathode comprise conductive patterns, and preferably the plurality of plates include at least one pair of adjacent plates, and the apparatus includes at leas
Jobson Edward
Lundgren Staffan
Salomonsson Per
Unosson Anders
Gorgos Kathryn
Kilpatrick & Stockton LLP
Nicolas Wesley A.
Volvo Car Corporation
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