Abstract:
A gas purifying apparatus includes an adsorption member (e.g., a honeycomb structural body) that adsorbs particulate matter contained in the gas by the action of an electric field. The apparatus also includes a supply device (e.g., an ozone generator including a discharger, a pulsed power supply, a pump, and a discharge port) that supplies a pro-oxidant to the adsorption member.

Description:
[0001]     The disclosure of Japanese Patent Application No. 2005-075951 filed on Mar. 16, 2005 including the specification, drawings and abstract is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates generally to a gas purifying apparatus, and, more specifically, to gas purifying apparatus that trap particulate matter contained in gas.  
         [0004]     2. Description of Related Art  
         [0005]     Diesel particulate filters (DPF) with porous honeycomb structural bodies, that remove particulate matter (PM) contained in exhaust gas emitted from a diesel engine, a lean-burn engine, or the like have been described, for example, in Japanese Patent Application Publication No. JP-A-06-29545. In such described DPFs, a plurality of opening portions passes through the honeycomb structural body from the upstream end face to the downstream end face. Some opening portions are closed at the respective upstream ends and open at the respective downstream ends. Other opening portions are closed at the respective downstream ends and open at the respective upstream ends. Thus, exhaust gas flowing into the opening portions open at the upstream ends is filtered when passing through the porous partition walls. The exhaust gas that passes through the partition walls flows into and is then discharged from the adjacent opening portions that are open at the downstream ends. While the exhaust gas is passing through the partition walls, the PM contained in the exhaust gas is trapped in the partition walls.  
         [0006]     An apparatus that includes a cylindrical peripheral electrode that is open at both the upstream end and the downstream end has been described. Specifically, Japanese Patent Application Publication No. JP-A-2004-19534 describes an apparatus having a single rod-shaped center electrode that extends from a position near the upstream end of the peripheral electrode and that passes through the axis of the peripheral electrode. In this apparatus, the PM contained in the exhaust gas, which is electrically-charged by the center electrode, is adsorbed to the peripheral electrode by the interaction between the electric potential and the electric field produced between the two electrodes. A part of the PM adsorbed to the peripheral electrode is burned, and a plasma state is generated due to a high voltage applied between the two electrodes. Then, the oxidation of PM is promoted.  
       SUMMARY  
       [0007]     It is an object of the invention to suppress accumulation of PM in a gas purifying apparatus that purifies exhaust gas by the action of a high voltage.  
         [0008]     An aspect of the invention relates to a gas purifying apparatus in which the particulate matter contained in the gas supplied to the gas purifying apparatus is adsorbed to an adsorption member by the action of an electric field. The gas purifying apparatus includes a supply device that supplies a pro-oxidant to the adsorption member.  
         [0009]     Because the above-mentioned gas purifying apparatus includes the supply device that supplies the pro-oxidant to the adsorption member, removal of the PM adsorbed to the adsorption member by oxidizing and/or burning the PM is promoted. Accordingly, accumulation of the PM in the adsorption member is suppressed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, in which the same or corresponding portions are denoted by the same reference numerals, and wherein:  
         [0011]      FIG. 1  is a schematic view of a gas purifying apparatus according to a first embodiment of the invention;  
         [0012]      FIG. 2  is a cross sectional view of a honeycomb structural body;  
         [0013]      FIG. 3  is a schematic view of a gas purifying apparatus according to a second embodiment of the invention;  
         [0014]      FIG. 4  is a schematic view of a gas purifying apparatus according to a third embodiment of the invention;  
         [0015]      FIG. 5  is a schematic view of a gas purifying apparatus according to a fourth embodiment of the invention;  
         [0016]      FIG. 6  is a graph showing the rate of purification of the exhaust gas when the gas purifying apparatus according to the first embodiment of the invention is used without supplying an pro-oxidant; and  
         [0017]      FIG. 7  is a graph showing the rate of purification of the exhaust gas when the gas purifying apparatus according to the first embodiment of the invention is used while the pro-oxidant is supplied. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0018]     Hereafter, a gas purifying apparatus according to exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings.  
         [0019]      FIG. 1  is a schematic view of a gas purifying apparatus  1  according to a first embodiment of the invention.  FIG. 2  is a cross sectional view of a honeycomb structural body  10  that is a main portion of the gas purifying apparatus  1 . The gas purifying apparatus  1  is provided downstream of the exhaust system of an engine (a diesel engine, a lean-burn engine, or the like) (not shown) mounted in a vehicle, that is, downstream of the exhaust port of the engine.  
         [0020]     The gas purifying apparatus  1  includes a metal case  2 , and the honeycomb structural body  10  supported inside the case  2 . The case  2  and the honeycomb structural body  10  may be in close contact with each other. Sealing may be provided between the case  2  and the honeycomb structural body  10  by an alumina mat (not shown), or the like. The case  2  serves as the exterior shell of the gas purifying apparatus  1 , and also serves as the receiving electrode when electrically grounded. Instead of employing the structure where the case  2  also serves as the receiving electrode, a cylindrical receiving electrode that differs from the case  2  may be provided on the outer surface of the honeycomb structural body  10 .  
         [0021]     As shown in  FIG. 2 , the honeycomb structural body  10  includes a plurality of cells, namely, a plurality of exhaust gas passages  12  that are divided by porous partition walls  11 . Each exhaust gas passage  12  is parallel to the direction in which the exhaust gas flows in the gas purifying apparatus  1  (the direction indicated by the arrow X in the figures).  
         [0022]     The honeycomb structural body  10  may be made of a porous ceramic material. More specifically, the honeycomb structural body  10  may be made of cordierite, SiC (silicon carbide), or the like.  
         [0023]     The partition walls  11  of the honeycomb structural body  10  are coated with a catalytic substance, for example, a NOx storage reduction catalyst (NSR), which contains 3 moles of lithium per liter (Li=3 mol/L) and 5 grams of platinum per liter (Pt=5 g/L) and in which Al 2 O 3  is used as a coating material. The amount of NOx storage reduction catalyst applied onto the partition walls  11  is, for example, 300 grams per liter (300 g/L).  
         [0024]     The porosity of the partition wall in the commonly used honeycomb structural body is usually 55% to 60%, and the average diameter of the pores formed in the partition walls is 9 μm to 30 μm. In contrast, in the first embodiment, the porosity is 60% to 80%, and the average diameter of the pores formed in the partition walls  11  is 20 μm to 60 μm. Accordingly, the resistance to the flow of the exhaust gas can be relatively low. Usually, the amount of supported catalytic substance is approximately 150 grams per liter (150 g/L). In contrast, in the first embodiment, a relatively large amount, for example, 100 grams to 400 grams of catalytic substance per liter can be supported. Also, the resistance to the flow of the exhaust gas can be suppressed even when the catalytic substance is supported.  
         [0025]     Referring again to  FIG. 1 , a center electrode  20  passes through the honeycomb structural body  10 . The center electrode  20  extends coaxially with the honeycomb structural body  10  and in parallel to the direction in which the exhaust gas flows through the gas purifying apparatus  1  (the direction indicated by the arrow X in the figures). The outer surface of the center electrode  20  excluding an exposed portion  20   a  is coated with an insulating layer  21 . The exposed portion  20   a  is the upstream end portion of the center electrode  20 . The insulating layer  21  is supported by the communication hole formed along the axis of the honeycomb structural body  10 . Asperities (surface elevations) for promoting discharge, for example, threads may be formed on the exposed portion of the outer surface of the center electrode  20 .  
         [0026]     A metal wire  22  is connected to the downstream end portion of the center electrode  20 . An insulating plug  27  made, for example, of ceramics is fixed to the case  2 . The metal wire  22  extends out of the case  2  through the insulating plug  27 , and is connected to a direct-current power supply  30 .  
         [0027]     The center electrode  20  may be made of chrome steel having excellent corrosion resistance (for example, 10Cr5A1). However, the material of the center electrode  20  is not limited to chrome steel. The center electrode  20  may be made of another corrosion-resistant conductive material. The insulating layer  21  may be made of a highly heat-resistant material such as ceramics. However, in engines where the temperature of the exhaust system is low, for example, diesel engines, the insulating layer  21  may be made of a relatively low heat-resistant material such as acrylic resin.  
         [0028]     In the first embodiment, an pro-oxidant (ex. ozone) generator including a discharger  42  and a pulsed power supply  43  is used. Discharge ports  40   a  and  40   b  through which the pro-oxidant is supplied into the case  2  are fixed to the case  2 . The discharge ports  40   a  and  40   b  are connected to the discharger  42  by a supply pipe  41  through which the pro-oxidant is supplied, and the discharger  42  is connected to the pulsed power supply  43  for driving the discharger  42 . Corona discharge is generated between paired electrodes (not shown) in the discharger  42 , using the electric power supplied from the pulsed power supply  43 . A pump  44  that introduces outside air to the discharger  42  is connected to the discharger  42 .  
         [0029]     The PM accumulates at the upstream end face of the honeycomb structural body in a composite apparatus in which a honeycomb structural body including a plurality of cells is provided in the peripheral electrode of the apparatus described in Japanese Patent Application Publication No. JP-A-2004-19534. The accumulated PM causes a short circuit and an electric leak between the center electrode and the peripheral electrode. As a result, the efficiency in purifying gas may be reduced.  
         [0030]     In the first embodiment, the discharge ports  40   a  and  40   b  are provided such that the pro-oxidant discharged from the discharge ports  40   a  and  40   b  flows with the exhaust gas to the case  2  which serves as the receiving electrode and the upstream end portion of the honeycomb structural body  10 .  
         [0031]     Each of the direct-current power supply  30  and the pulsed power supply  43  includes an inverter circuit, a transformer, a diode for rectification, a smoothing circuit, and the like. The voltage of the direct-currents supplied from batteries (not shown) is increased, and then electric power is supplied to the center electrode  20  and the discharger  42 . The electric current supplied from each of the direct-current power supply  30  and the pulsed power supply  43  is not limited to a direct-current. The electric current supplied from each of the direct-current power supply  30  and the pulsed power supply  43  may have a waveform selected, for example, from a direct-current wave pulse, an alternating current, an alternating current wave pulse, and a combination of a direct-current and a direct-current wave pulse. The electric power supplied from the direct-current power supply  30  and the pulsed power supply  43  may have any level of voltage. The direct-current power supply  30  and the pulsed power supply  43  are controlled by an electronic control unit (ECU) (not shown). The ECU is configured to stop the operation of the direct-current power supply  30  if an overcurrent flows to the direct-current power supply  30  due, for example, to a short-circuit that has occurred between the electrodes.  
         [0032]     In the gas purifying apparatus  1  configured according to the first embodiment, if exhaust gas containing the PM is discharged from the engine and introduced into the case  2  through the exhaust pipe, the exhaust gas flows in the honeycomb structural body  10  through the exhaust gas passages  12  which are open at the upstream ends, and then discharged downstream out of the honeycomb structural body  10  through the exhaust gas passages  12  which are open at the downstream ends.  
         [0033]     Meanwhile, if the direct-current power supply  30  is turned ON when the engine is started and a high voltage is applied between the center electrode  20  and the case  2 , any PM contained in the exhaust gas becomes negatively charged due to the electric power discharged from the center electrode  20 , and the electric field is produced between the center electrode  20  and the case  2 . Therefore, the PM is drawn toward the honeycomb structural body  10  due to electric attraction. Oxidization and/or burning of the PM adsorbed to the honeycomb structural body  10  are/is promoted by the heat supplied from the exhaust gas and the activated gas generated by the application of a high voltage.  
         [0034]     Meanwhile, corona discharge is generated between the electrodes (not shown) in the discharger  42 , using the electric power supplied from the pulsed power supply  43 , and acts on the air supplied due to the operation of the pump  44 . Thus, ozone that serves as the activated species or the pro-oxidant is formed. More specifically, an electron contacts an oxygen molecule O 2  present in the air to generate oxygen atoms (O 2            O+O), and an oxygen atom bonds with oxygen molecules to generate ozone O 3  (O+O 2 +O 2            O 3 +O 2 ). The thus generated ozone is discharged from the discharge ports  40   a  and  40   b  into the case  2 , and supplied toward the case  2  serving as the receiving electrode and the upstream end portion, including the periphery, of the honeycomb structural body  10 . The ozone promotes oxidation and/or burning of the PM contained in the exhaust gas and the PM (consisting mainly of carbon C) adhering to the honeycomb structural body  10 . As a result, according to the first embodiment, the PM can be burned and/or oxidized at a lower temperature (for example, a temperature equal to or lower than 300° C.), than when a pro-oxidant is not used (approximately 650° C. to 700° C.).  
         [0035]     As described so far, according to the first embodiment, because the honeycomb structural body  10  serving as the adsorption portion is provided with the pro-oxidant supply source that supplies the ozone serving as a pro-oxidant, removal of the PM adhering to the honeycomb structural body  10  by oxidizing and/or burning the PM is promoted. Accordingly, accumulation of the PM in the honeycomb structural body  10  is suppressed. Also, it is possible to suppress an increase in the electric power consumption caused by a short-circuit between the electrodes with the end face of the base material interposed therebetween.  
         [0036]     Also, in the apparatus where the honeycomb structural body is provided between the center electrode and the case that serve as the first electrode and the second electrode, respectively, and that are opposite in polarity, the PM accumulates on the upstream end face of the honeycomb structural body, and a short circuit and an electric leak are likely to occur between the electrodes. However, according to the first embodiment, because the ozone serving as the pro-oxidant is supplied onto the upstream end face of the honeycomb structural body  10 , it is possible to effectively suppress a short-circuit and an electric leak caused by the PM accumulated on the upstream end face of the honeycomb structural body.  
         [0037]     Next, a second embodiment of the invention will be described. In the first embodiment, the discharge ports  40   a  and  40   b  are fixed to the case  2 , and the ozone discharged from the discharge ports  40   a  and  40   b  are supplied onto the upstream end face of the honeycomb structural body  10 . However, arrangement of the supply route through which the pro-oxidant is supplied is not particularly limited in the invention. The second embodiment described below is a modified example concerning the supply route through which the pro-oxidant is supplied.  
         [0038]     As shown in  FIG. 3 , a gas purifying apparatus  101  according to the second embodiment of the invention includes a center electrode  120 . An inner hole (or passage) (not shown) that is the hollow portion of the center electrode  120  passes through the substantially rod-shaped center electrode  120  in the longitudinal direction. The inner hole is closed at the upstream end face of the center electrode  120 , and open at the downstream end face of the center electrode  120 . A plurality of communication holes  120   b,  which provide communication between the inner hole of the center electrode  120  and the outside of the center electrode  120 , are formed in an exposed portion  120   a  of the center electrode  120 . The exposed portion is a portion of the center electrode  120  near the upstream end portion, and is not covered with the insulating layer  21 .  
         [0039]     A supply pipe  141  through which the pro-oxidant is supplied is connected to the downstream end portion of the center electrode  120 , and the supply pipe  141  communicates with the inner hole of the center electrode  120 . The supply pipe  141  is connected to the same discharger  42  as in the first embodiment. Because the remaining configuration of the second embodiment is the same as the first embodiment, the same or corresponding portions are denoted by the same reference numerals, and the detailed description thereof is not repeated here.  
         [0040]     In the thus configured gas purifying apparatus  101  according to the second embodiment, corona discharge is generated between the electrodes (not shown) in the discharger  42 , using the electric power supplied from the pulsed power supply  43 , and acts on the air supplied due to the operation of the pump  44 . Thus, the ozone serving as the activated species or the pro-oxidant is generated. The thus generated ozone is radially discharged from the communication holes  120   b  serving as the discharge ports in the direction indicated by the arrows Y in  FIG. 3 , and supplied toward the upstream end face of the honeycomb structural body  10 . The ozone promotes oxidation and/or burning of the PM contained in the exhaust gas and the PM (consisting mainly of carbon C) adhering the honeycomb structural body  10 .  
         [0041]     As described so far, in the second embodiment, the center electrode  120  passes through the honeycomb structural body  10  serving as the adsorption portion. Also, the inner hole formed in the center electrode  120  is used as at least a part of the supply route through which the pro-oxidant is supplied from the pro-oxidant supply source to the communication holes  120   b  (discharge ports). In addition, the communication holes  120   b,  which are formed near the upstream end portion of the center electrode  120  and which communicate with the inner hole, are used as the discharge ports. Accordingly, the ozone serving as the pro-oxidant can be efficiently supplied onto the portion near the center of the upstream end face of the honeycomb structural body  10 , where a particularly large amount of PM tends to adhere for a hydrodynamic reason.  
         [0042]     Next, a third embodiment of the invention will be described. The third embodiment is another modified example concerning the supply route through which the pro-oxidant is supplied in the invention.  
         [0043]     As shown in  FIG. 4 , in a gas purifying apparatus  201  according to the third embodiment of the invention, a rod-shaped solid center electrode  220  passes through the honeycomb structural body  10 , and a cylindrical supply pipe  221  surrounds the center electrode  220 .  
         [0044]     An opening portion  221   a  that is the upstream end portion of the supply pipe  221  is used as the discharge port through which the pro-oxidant is discharged. The upstream end portion of the center electrode  220  is an exposed portion  220   a  that protrudes from the opening portion  221   a.    
         [0045]     A supply pipe  241  is connected to the downstream end portion of the supply pipe  221 . The pro-oxidant (ex. ozone) from the discharger  42  is supplied through the supply pipe  241  and the supply pipe  221  by the pressure generated by the pump  44 , and discharged from the opening portion  221   a.  Because the remaining configuration of the third embodiment is the same as the first and the second embodiments, the same or corresponding portions are denoted by the same reference numerals, and the detailed description thereof is not repeated here.  
         [0046]     In the thus configured gas purifying apparatus  201  according to the third embodiment, corona discharge is generated between the electrodes (not shown) in the discharger  42 , using the electric power supplied from the pulsed power supply  43 , and acts on the air supplied due to the operation of the pump  44 . Thus, the ozone serving as the activated species or the pro-oxidant is generated. The thus generated ozone is radially discharged in the direction indicated by the arrows Z from the opening portion  221   a  serving as the discharge port, and supplied toward the upstream end face of the honeycomb structural body  10 . The ozone promotes oxidization and/or burning of the PM (consisting mainly of carbon C) adhering to the honeycomb structural body  10 .  
         [0047]     As described so far, in the third embodiment, the center electrode  220  passes through the honeycomb structural body  10 . Also, the cylindrical supply pipe  221  surrounding the center electrode  220  is used as at least a part of the supply route through which the pro-oxidant is supplied. In addition, the pro-oxidant, namely, the ozone is supplied from the discharge port  221   a,  which surrounds the center electrode  220  and which is the upstream end portion of the supply pipe  221 , toward the upstream end face of the honeycomb structural body  10 . Therefore, as in the second embodiment, the ozone serving as the pro-oxidant can be efficiently supplied to the portion near the center of the upstream end face of the honeycomb structural body  10 , where a particularly large amount of PM tends to adhere for a hydrodynamic reason. In addition, because a solid electrode can be used as the center electrode  220  and the inner hole need not be formed in it, production cost can be suppressed.  
         [0048]     Next, a fourth embodiment according to the invention will be described. In each of the first to third embodiments, the honeycomb structural body  10  has a substantially cylindrical shape, and the center electrode  20 ,  120  or  220  passes through the honeycomb structural body  10 . However, the shape of the adsorption portion or the honeycomb structural body in the invention is not particularly limited, and the center electrode need not pass through the honeycomb structural body. The fourth embodiment is a modified example concerning the electrode and the adsorption portion in the first to third embodiments.  
         [0049]      FIG. 5  is a schematic view of a gas purifying apparatus  301  according to the fourth embodiment. The gas purifying apparatus  301  includes a rectangular metal case  302  and a honeycomb structural body  310  that is supported inside the case  302 .  
         [0050]     The cross section of honeycomb structural body  310 , which is orthogonal to the direction in which the exhaust gas flows, is substantially rectangular. The honeycomb structural body  310  has a plurality of cells, namely, a plurality of exhaust gas passages that are divided by the porous partition walls. Each exhaust passage is in parallel to the direction in which the exhaust gas flows through the gas purifying apparatus  301 . A so-called straight flow type honeycomb structural body where the cells are open at both upstream ends and the downstream ends may be used as the honeycomb structural body  310 . Alternatively a filter type or a so-called wall-flow type honeycomb structural body may be used as the honeycomb structural body  310 . In the filter type or the wall flow type honeycomb structural body, the exhaust passages, which are closed at the upstream ends and which are open at the downstream ends, and the exhaust gas passages, which are closed at the downstream ends and which are open at the upstream ends, are alternately formed. The material of the honeycomb structural body  310 , and the material of the NOx storage reduction catalyst applied onto the honeycomb structural body  310  are the same as those of the honeycomb structural body  10  in the first embodiment.  
         [0051]     A discharge electrode plate  320  is provided on one of the surfaces (upper surface in  FIG. 5 ) of the honeycomb structural body  310 . Sealing is provided between the discharge electrode plate  320  and the case  302  by an alumina mat (not shown), or the like. A plug electrode  326   a  of a power supply plug  326  is connected to the discharge electrode plate  320 . The plug electrode  326   a  is coated with an insulating layer  327 . The plug electrode  326   a  and the case  302  are electrically insulated from each other by the insulating layer  327 .  
         [0052]     A receiving electrode plate  315  is provided on the side opposite to the discharge electrode plate  320 , with the honeycomb structural body  310  interposed therebetween. The receiving electrode plate  315  is provided at a predetermined distance from the honeycomb structural body  310 . The receiving electrode plate  315  is electrically grounded. The discharge electrode plate  320  and the receiving electrode plate  315  may be made of heat-resisting conductive material such as stainless steel.  
         [0053]     At positions upstream of the honeycomb structural body  310  in the direction in which the exhaust gas flows, discharge ports  340   a  and  340   b  through which the pro-oxidant is supplied into the case  302  are fixed to the case  302 . The discharge ports  340   a  and  340   b  are connected to the discharger  42  by the supply pipe  341  through which the pro-oxidant is supplied. The discharge ports  340   a  and  340   b  are provided such that the pro-oxidant discharged from the discharge ports  340   a  and  340   b  flows with the exhaust gas to the upstream end face of the honeycomb structural body  310 , the discharge electrode plate  320  and the receiving electrode plate  315 .  
         [0054]     The direct-current power supply  30  is connected to the power supply plug  326 . The configurations of the direct-current power supply  30  and the control system for controlling the direct-current power supply  30  are the same as in the first embodiment. Because the remaining configuration of the fourth embodiment is the same as the first embodiment, the same or corresponding portions are denoted by the same reference numerals, and detailed description thereof is not repeated here.  
         [0055]     In the gas purifying apparatus  301  configured according to the fourth embodiment, exhaust gas containing the PM discharged from the engine is introduced to the case  302  through the exhaust pipe, flows into the honeycomb structural body  310  through the exhaust gas passages which are open at the upstream ends, and discharged downstream out of the honeycomb structural body  310  through the exhaust gas passages which are open at the downstream ends.  
         [0056]     Meanwhile, corona discharge is generated between the electrodes (not shown) in the discharger  42 , using the electric power supplied from the pulsed power supply  43 , and acts on the air supplied due to the operation of the pump  44 . Thus, the ozone serving as the activated species or the pro-oxidant is generated. The thus generated ozone is discharged from the discharge ports  340   a  and  340   b,  and supplied toward the upstream end face of the honeycomb structural body  310 . The ozone promotes oxidization and/or burning of the PM contained in the exhaust gas and the PM (consisting mainly of carbon C) adhering to the honeycomb structural body  310 .  
         [0057]     As described so far in detail, according to the fourth embodiment, because the pro-oxidant supply source that supplies the ozone serving as the pro-oxidant is provided upstream of the honeycomb structural body  310  serving as the adsorption portion, removal of the PM adsorbed in the honeycomb structural body  310  by oxidizing and/or burning the PM is promoted. Accordingly, accumulation of the PM in the honeycomb structural body  310  is suppressed.  
         [0058]     According to the fourth embodiment, because the honeycomb structural body is formed in a substantially rectangular column shape, the efficiency in arranging the entire device can be improved.  
         [0059]     The rate of purification of the PM when the pro-oxidant was not supplied and the rate of purification of the PM when the pro-oxidant was supplied were measured by using the gas purifying apparatus according to the first embodiment of the invention. The supply voltage of the direct-current power supply  30  was 30 kV. The amount of pro-oxidant supplied was 50 liters per minute (50 L/min), namely 50 liters of dry air containing 100 ppm of ozone was supplied per minute. An ELPI (Electrical Low Pressure Impactor) was used to measure the amount of PM. When the pro-oxidant was not supplied, as shown in  FIG. 6 , a short-circuit occurred between the electrodes in a short time after application of a voltage was started by the direct-current power supply  30 , and the operation of the direct-current power supply  30  was stopped in response to generation of an overcurrent. On the other hand, when the pro-oxidant was supplied, as shown in  FIG. 7 , an overcurrent due to a short-circuit was not generated, and the gas was stably purified for a long time.  
         [0060]     While the invention has been described with reference to exemplary embodiments thereof, is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.  
         [0061]     For example, in the embodiments described above, ozone is used as the pro-oxidant. However, an appropriate pro-oxidant that could be used in the invention would include at least one gas selected from the group consisting of ozone O 3 , atomic oxygen O, superoxide anion radical O 2   − , hydroxyl radical OH − , alkoxyl radical R − O 2 , nitrogen dioxide NO 2 , and oxygen O 2 . The pro-oxidant in the invention is not limited to those mentioned above, as long as the pro-oxidant enables the PM to be burned and/or oxidized at a lower temperature than when the pro-oxidant is not used (approximately 650° C. to 700° C.).  
         [0062]     The pro-oxidant supply source in the invention may include the processing device such as the discharger  42  that produces the pro-oxidant by processing air. Alternatively, the pro-oxidant supply source may include a processing device that produces the pro-oxidant by processing a substance other than air. Also, the pro-oxidant supply source may include a tank that stores the pro-oxidant.  
         [0063]     In the embodiments described above, the center electrode  20 ,  120 , or  220 , the case  2 , the discharge electrode plate  320  and the receiving electrode plate  315  are used. However, the shapes and the configurations of the electrodes used in the invention are not particularly limited. For example, a mesh-type electrode may be used. The adsorption portion in the invention is not limited to the honeycomb structural body. The material and the configuration of the adsorption portion may be appropriately selected as long as the adsorption portion can adsorb the PM that passes through and is heat-resistant. For example, the electrode itself or a metal mesh may be used as the adsorption portion.  
         [0064]     In the embodiments described above, the center electrodes  20 ,  120  or  220  is connected to the negative pole, and the case  2  or the receiving electrode plate  315  is grounded. However, the invention can be achieved as long as a predetermined high voltage is applied between the two electrodes. The center electrode may be connected to the positive pole. The electric current to be applied may be appropriately selected from a direct current, an alternating current, a pulse, and a combination of an alternating current or a direct current and a pulse. In addition, it is not necessary to apply the voltage at all times. Instead, the voltage may be applied when required, for example, when the PM needs to be trapped or the PM needs to be burned. In the embodiments described above, the invention is applied to purification of the exhaust gas emitted from an internal combustion engine of a vehicle. However, the invention can be applied to purification of the exhaust gas emitted from a vessel, an aircraft or the like. Also, the invention can be applied to purification of dust in a construction site, a plant and the like, and purification of floating particle in a tunnel. These and other similar cases are within the scope of the invention.