Abstract:
The present invention relates to an exhaust gas purifying apparatus comprising a plasma reactor containing at least one of a H 2 O adsorbent and a EC adsorbent; and a NO x  adsorbent located downstream of said plasma reactor. Further, the present invention relates to an exhaust gas purifying apparatus comprising a plasma reactor containing at least one of a H 2 O adsorbent and a HC adsorbent, and a NO x  adsorbent. Still further, the present invention relates to a method using the present apparatus. According to the present apparatus, by generating plasma in the plasma reactor, H 2 O trapped on the H 2 O adsorbent is activated to be a strong oxidant such as OH radical and O radical, and HC trapped on the HC adsorbent is activated to be radicals, lower HCs, etc.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an apparatus for purifying exhaust gas emitted from internal combustion engine and, in particular, to an apparatus for purifying NO x  (nitrogen oxide) emitted from a motor vehicle.  
           [0003]    2. Description of Related Art  
           [0004]    In order to purify an exhaust gas emitted from an internal combustion engine such as a motor vehicle, it is usual to reduce NO x  in the exhaust gas by the use of NO x  purifying catalyst carrying a noble metal. However, it is necessary to use a larger amount of noble metal, to sufficiently purifying NO x  at a lower temperature, as the NO x  purifying catalyst generally does not have much NO x  purifying ability at the lower temperature. Using the larger amount of noble metal increases costs and does not always provides a satisfactory result.  
           [0005]    Japanese Unexamined Patent Publication NO. 2001-182525 describes an exhaust gas purifying apparatus wherein plasma is generated in a plasma reactor to enhance the adsorption of NO x  by a NO x  adsorbent therein, when a NO x  purifying catalyst located downstream thereof is at a low temperature and does not have enough NO x  purifying ability. Once the catalyst in warmed up, the trapped NO x  can be desorbed and purified at the downstream NO x  purifying catalyst by stopping plasma.  
           [0006]    The prior art has partially overcome the problem that the NO x  purifying catalyst does not have enough NO, purifying ability before it is warmed up. However, the further improvements are required because the NO x  purifying ability is not always enough even if the prior art is employed.  
           [0007]    Therefore, the present invention provides an improved exhaust gas purifying apparatus, which overcome the above problems, and a method for using it.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    An exhaust gas purifying apparatus according to the present invention comprises a plasma reactor containing at least one of a H 2 O adsorbent and a HC (hydrocarbon) adsorbent; and a NO x  adsorbent located downstream of said plasma reactor.  
           [0009]    Another exhaust gas purifying apparatus according to the present invention comprises a plasma reactor containing at least one of a H 2 O adsorbent and a EC adsorbent, and a NO x  adsorbent. The NO x  adsorbent in the reactor may be positioned downstream of the at least one of a H 2 O adsorbent and a EC adsorbent. Further, the NO x  adsorbent may be mixed with the at least one of a H 2 O adsorbent and a SC adsorbent.  
           [0010]    In the use of the present exhaust gas purifying apparatuses, at least one of H 2 O and EC in an exhaust gas is adsorbed on the at least one of a H 2 O adsorbent and a EC adsorbent, when the exhaust gas is passed through the plasma reactor. On generating plasma in the plasma reactor, the H 2 O adsorbed on the adsorbent is activated to be a strong oxidant such as OR radical and 0 radical, and HC adsorbed on the adsorbent is activated to be radicals, lower HC, etc. The obtained radicals facilitate the conversion from NO to NO 2  in the exhaust gas. The resulting NO 2  tends to be more adsorbed onto the downstream NO x  adsorbent than NO x . Therefore, according to the present apparatus, it is possible to prevent NO x  from flowing out from an exhaust gas purifying apparatus at a lower temperature without increasing an amount of noble metal to be used.  
           [0011]    In one embodiment of the present apparatus, the at least one of a H 2 O adsorbent and a HC adsorbent is selected from the group consisting of alumina, silica, zeolite, zirconia and meso-porous silica, and combinations thereof.  
           [0012]    In another embodiment of the present apparatus, the NO x  adsorbent is selected from the group consisting of zirconia and ceria, and a combination thereof.  
           [0013]    In another embodiment of the present apparatus, the NO x  adsorbent carries a NO x  purifying catalyst.  
           [0014]    According to this embodiment of the present apparatus, it is possible to adsorb NO 2  obtained by the at least one of a H 2 O adsorbent and a BC adsorbent onto the NO x  adsorbent and to purify the adsorbed NO 2  thereon.  
           [0015]    In another embodiment of the present apparatus, it comprises a NO x  purifying catalyst located downstream of the NO x  adsorbent.  
           [0016]    According to this embodiment of the present apparatus, the NO x  desorbed from NO x  adsorbent after the temperature of the NO x  adsorbent is raised can be converted into N 2  by the NO x  purifying catalyst which has already been warmed up.  
           [0017]    In the present apparatuses, it is also possible to reduce an amount of noble metal, such as a platinum group metal, used in the NO x  purifying catalyst, or further to omit the noble metal, because NO in the exhaust gas has been converted into NO 2  which is easily adsorbed onto a NO x  adsorbent and a NO x  purifying catalyst.  
           [0018]    A method according to the present invention for using the present exhaust gas purifying apparatus comprises generating a plasma in the plasma reactor only when the temperature of the NO x  purifying catalyst is lower than a predetermined temperature, e.g. lower than 300° C.  
           [0019]    According to the present method, it is possible to promote the conversion from NO to NO 2 , and thereby promote adsorption of NO x  onto the NO x  adsorbent, only when the NO x  purifying catalyst is at the lower temperature such that NO x  in an exhaust gas cannot be sufficiently purified by the NO x  purifying catalyst. Further, according to the present method, it is possible to stop the generation of plasma, in order to save energy, when the NO x  purifying catalyst is at a high enough temperature and has enough NO x  purifying ability.  
           [0020]    These and other objects, features and advantages of the present invention will become apparent to a person with ordinary skill in the art upon reading the following detailed description along with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a block diagram of one embodiment of the present exhaust gas purifying apparatus.  
         [0022]    [0022]FIG. 2 is a block diagram of another embodiment of the present exhaust gas purifying apparatus.  
         [0023]    [0023]FIGS. 3 a  and  3   b  are respectively a perspective view and a cross sectional view of one plasma reactor which can be used for the present exhaust gas purifying apparatus.  
         [0024]    [0024]FIGS. 4 a  and  4   b  are respectively a perspective view and a cross sectional view of another plasma reactor which can be used for the present exhaust gas purifying apparatus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The present invention is described regarding embodiments and drawings which are not intended to limit the scope of the present invention shown in the claims.  
         [0026]    The first and second embodiments of the present exhaust gas purifying apparatuses are described in relation to FIGS. 1 and 2, below.  
         [0027]    In the use of the apparatus shown in FIG. 1, an exhaust gas emitted from an engine is passed through the plasma reactor containing at least one of a H 2 O adsorbent and a HC adsorbent, and then a NO x  adsorbent located downstream of the plasma reactor. In the use of the apparatus shown in FIG. 2, an exhaust gas emitted from an engine is passed through a plasma reactor containing a NO x  adsorbent, and at least one of a H 2 O adsorbent and a HC adsorbent in a plasma reactor. The adsorbent in the plasma reactor adsorbs H 2 O and/or HC, and then the adsorbed H 2 O and/or HC is decomposed into radical chemicals by applying plasma. The radial chemicals enhance oxidizing NO into NO 2 .  
         [0028]    The components of the exhaust gas purifying apparatus shown in FIGS. 1 and 2 are described below in more detail.  
         [0029]    The H 2 O adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of a H 2 O adsorbent, e.g. at least one of alumina, silica and zeolite. The HC adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of HC adsorbents, e.g. at least one of zeolite, zirconia and meso-porous silica.  
         [0030]    The NO x  adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of adsorbent which adsorbs NO x , in particular adsorbents which desorb the adsorbed NO x  at the higher temperature, e.g. at least one of zirconia and ceria.  
         [0031]    The adsorbents may be in any form, e.g. it can be formed into pellet form and charged into a container, or molded into a honeycomb structure. Further, it is also possible to coat the slurry containing the adsorbent onto a honeycomb structure substrate, and dry and fire the coated substrate.  
         [0032]    The NO x  purifying catalyst which is carried onto the NO x  adsorbent and/or located downstream of the NO x  adsorbent is any kind of catalyst a which catalyze reduction from NO x  to N 2 . The NO x  purifying catalyst may be so-called a NO x  storage reduction catalyst, a NO x  selective reduction catalyst or a three-way catalyst.  
         [0033]    The NO x  storage reduction catalyst is a catalyst which stores NO x  when air/fuel ratio is in lean, and reduces the stored NO x  into N 2  when air/fuel ratio is in rich, i.e. when a rich spike is provided (when fuel is injected into an exhaust gas flow). For example, the NO x  storage reduction catalyst is a catalyst wherein a porous metal oxide carrying one or more noble metal such as Pt, Rh, Pd, Ir and RU, and one or more NO x  storage elements such as alkali metals, alkali earth metals and rare earth metals.  
         [0034]    The NO x  selective reduction catalyst is a catalyst which selectively reduce or decompose NO x  in an exhaust gas. The NO x  selective reduction catalyst is used for purifying NO x  in the exhaust gas emitted from an internal combustion engine driven at a lean air/fuel ratio. For example, the NO x  selective reduction catalyst is a zeolite exchanged with transition metal such as Cu, or zeolite or alumina carrying one or more noble metal.  
         [0035]    The three-way catalyst is a catalyst which converts CO, HC and NO x  in an exhaust gas emitted from an internal combustion engine driven at near theoretical air/fuel ratio into CO 2 , H 2 O and N 2 . For example, the three way catalyst is an alumina carrying mixture of Pt and Rh, or mixture of Pd and Rh.  
         [0036]    The catalyst metals such as noble metal can be carried onto any kind of substrate such as formed substrate or powder substrate by wash coating etc. Further, a formed substrate such as honeycomb substrate may be coated with the slurry comprising the particle carrying catalyst metals, and then dried and fired to obtain a formed substrate carrying catalyst. The particle carrying catalyst metals may be formed into pallet form and charged into a container.  
         [0037]    The plasma reactor used for the exhaust gas purifying apparatus of the present invention may be any kind of plasma reactor which allows the exhaust gas therethrough, e.g. the plasma reactor shown in FIGS. 3 a  and  3   b . FIG. 3 a  shows a perspective view and FIG. 3 b  shows a side sectional view of the plasma reactor.  
         [0038]    The plasma reactor can generate plasma by a discharge between a center electrode  20  and outer electrode  30 , these electrodes  20  and  30  being connected to a power supply  40 . The exhaust gas to be treated is passed through a cylindrical passage  10  surrounded by the outer electrode  30 . The H 2 O and/or HC adsorbent, and optionally NO x  adsorbent, may be positioned in the cylindrical passage  10 .  
         [0039]    The center electrode  20  is made of a material that makes it possible to apply a voltage between the center electrode  20  and the outer electrode  30 . The material may be electrically conductive material, electrically semi-conductive material etc. and especially, a metal such as Cu, W, stainless steel, Fe, PT and Al, the stainless steel being most preferable due to its durability and low cost. The center electrode  20  may be a metal wire, but it may be a hollow bar.  
         [0040]    The outer electrode  30  is made of a material that makes it possible to apply a voltage between the center electrode  20  and the outer electrode  30 . The material may be the material described for the center electrode  20 . The outer electrode  30  may be made by surrounding a mesh or foil of these materials around the cylindrical passage  10 , or by applying a conductive paste on the circumference surface of the cylindrical passage  10 .  
         [0041]    The electric power supply  40  may be one supplying a pulse or constant direct current (DC), or alternating current (AC) voltage. A voltage applied between the center electrode  20  and the outer electrode  30  and the pulse period of the applied voltage may be 50 kv and 2000 Hz. The center electrode  20  may be a cathode or anode.  
         [0042]    The electric power supply  40  may apply a high voltage between the center electrode  20  and outer electrode  30  in order to generate discharge and then plasma therebetween. The electric power supply  40  may apply a DC voltage, AC voltage, a voltage having a periodic waveform, etc. between the electrodes. Preferably, DC pulse voltage is applied since it can generate a stable corona electric discharge. The applied voltage, pulse width and pulse period of the DC pulse voltage may be optionally determined as long as it generates a corona electric discharge. Preferably, the applied voltage and pulse period are respectively a high voltage and short period in order to generate a corona electric discharge, though those parameters may be restricted by the design of the apparatus, an economical interest, etc.  
         [0043]    The plasma reactor used for the exhaust gas purifying apparatus of the present invention may be the plasma reactor shown in FIGS. 4 a  and  4   b . FIG. 4 a  shows a perspective view and FIG. 4 b  shows a side sectional view of the plasma reactor.  
         [0044]    The plasma reactor can generate plasma by discharging between mesh-like electrodes  25  and  35  which are upstream and downstream of the cylindrical passage  10 , these electrodes  25  and  35  being connected to a power supply  40 . The exhaust gas to be treated is passed through an upstream mesh-like electrode  25 , cylindrical passage  10  and then a downstream-like mesh electrode  35 . The H 2 O and/or HC adsorbent, and optionally NO x  adsorbent, may be contained in the cylindrical passage  10 . The description of the materials used for electrodes, the power supply, etc. of the plasma reactor of FIGS. 3 a  and  3   b  may be referred for the plasma reactor of FIGS. 4 a  and  4   b.    
         [0045]    The effects of the present invention are shown with regard to the examples, which are not intended to limit the scope of the present invention shown in the claims.  
       EXAMPLE 1  
       [0046]    A plasma reactor shown in FIGS. 3 a  and  3   b  was used to determine the effect of a H 2 O adsorbent and plasma on a conversion from NO to NO 2 .  
         [0047]    In this example, 2.0 g of silica pellets was contained in the plasma reactor as a H 2 O adsorbent. The plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the adsorbent temperature was maintained at the temperature of 150° C. A gas comprising 1000 ppm of NO, 10% of O 2 , 3.0% of H 2 O and balance N 2  was passed through the plasma reactor to evaluate a conversion ratio from NO to NO 2 . DC pulse discharge (50 kV, 2000 Hz) was used to generate plasma.  
       COMPARATIVE EXAMPLE 1  
       [0048]    The example 1 was repeated except that the plasma was not generated.  
       COMPARATIVE EXAMPLE 2  
       [0049]    The example 1 was repeated except that the H 2 O adsorbent was not contained in the plasma reactor.  
       COMPARATIVE EXAMPLE 3  
       [0050]    The example 1 was repeated except that the plasma was not generated and the H 2 O adsorbent was not contained in the plasma reactor.  
         [0051]    The results obtained in example 1 and comparative examples 1 to 3 are shown in the following Table 1.  
                                                           TABLE 1                                       NO → NO 2                  conversion ratio                no plasma   plasma                            no H 2 O adsorbent   0   18           H 2 O adsorbent   1   40                      
 
         [0052]    As seen from Table 1, the example 1 according to the present invention (plasma+H 2 ° adsorbent) provides a much better performance than comparative example 1 (H 2 O adsorbent only), comparative example 2 (plasma only) and comparative example 3 (without plasma and B 20  adsorbent).  
       EXAMPLE 2  
       [0053]    A plasma reactor shown in FIGS. 3 a  and  3   b  was used to determine the effect of a HC adsorbent and plasma on a conversion from NO to NO 2 .  
         [0054]    In this example, 2.0 g of zeolite (ZSM-5) pellets was contained in the plasma reactor as a HC adsorbent. The plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the adsorbent temperature was maintained at the temperature of 150° C. A gas comprising 1000 ppm of NO, 300 ppm of C 3 H 6 , 10% of O 2 , 3.0% of H 2 O and balance N 2  was passed through the plasma reactor to evaluate a conversion ratio from NO to NO 2 . DC pulse discharge (50 kV, 2000 Hz) was used to generate plasma.  
         [0055]    COMPARATIVE EXAMPLE 4  
         [0056]    The example 2 was repeated except that the plasma was not generated.  
       COMPARATIVE EXAMPLE 5  
       [0057]    The example 2 was repeated except that the HC adsorbent was not contained in the plasma reactor.  
       COMPARATIVE EXAMPLE 6  
       [0058]    The example 2 was repeated except that the plasma was not generated and the HC adsorbent was not contained in the plasma reactor.  
         [0059]    The results obtained in example 2 and comparative examples 4 to 6 are shown in the following Table 2.  
                                                           TABLE 2                                       NO → NO 2                 conversion ratio                no plasma   plasma                            no HC adsorbent   0   29           HC adsorbent   10   71                      
 
         [0060]    As seen from Table 2, the example 1 according to the present invention (plasma+HC adsorbent) provides a much better performance than comparative example 4 (HC adsorbent only), comparative, example 5 (plasma only) and comparative example 6 (without plasma and HC adsorbent).  
         [0061]    Although the present invention has been fully described by way of the example with reference to the accompanying drawings, it should be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, such changes and modifications can be made within the scope of the present invention hereinafter defined.