Abstract:
The present invention relates to an exhaust gas purifying apparatus comprising a plasma reactor containing a hydrogen-generating catalyst and a NO x -purifying catalyst located downstream of said plasma reactor. Further, the present invention relates to an exhaust gas purifying apparatus comprising a plasma reactor containing a hydrogen-generating catalyst and a NO x -purifying catalyst. 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, it is possible to enhance and/or maintain a NOx purifying ability of the Nox purifying catalyst, 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 an 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. In the use of a NO x  storage reduction catalyst which is one of the NO x  purifying catalysts, an internal engine is normally driven in lean condition to store NOx emitted from the engine at the NO x  storage agent and, then, intermittently, the stored NO x  is reduced into a N 2  by providing a rich spike, i.e. making the air/fuel rich.  
           [0005]    It is proposed to generate H 2  having high reduction ability in order to promote reduction of NO x  during the rich spike. For example, Japanese Unexamined Patent Publication No. 2001-300262 proposes to use a hydrogen-generating catalyst, which consists of a basic metal oxide carrier having platinum, upstream of the NO x  storage reduction catalyst. The hydrogen-generating catalyst converts CO and H 2 O into CO 2  and H 2 .  
           [0006]    Japanese Unexamined Patent Publication No. 2001-224961 proposes to add a Pt/TiO 2  hydrogen-generating catalyst to a NO x  storage reduction catalyst. The hydrogen-generating catalyst converts CO and H 2 O into CO 2  and H 2 .  
           [0007]    Japanese Unexamined Patent Publication No. 2001-159309 proposes that an electric discharge is provided upstream of, or on, a catalyst in the exhaust gas line of an internal combustion engine, and that a reducing content of HC (hydro carbon) is added upstream of the electric discharge. The added RC is decomposed or ionized by the electric discharge.  
           [0008]    The exhaust gas purifying apparatuses of the JPP&#39; 262 and JPP&#39; 961 have partially achieved the object that a strong reducing agent of H 2  is generated by the use of hydrogen-generating catalyst. However, H 2  is not always generated in a required amount particularly when the hydrogen-generating catalyst has a lower temperature and the catalyst has a lower activity.  
           [0009]    The exhaust gas purifying apparatus of the JPP&#39; 309 does not realize the benefit of the hydrogen-generating catalyst.  
           [0010]    Therefore, the present invention provides an improved exhaust gas purifying apparatus which can provide a sufficient amount of H 2 .  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    An exhaust gas purifying apparatus of the present invention comprises a plasma reactor containing a hydrogen-generating catalyst, and a NO x -purifying catalyst located downstream of said plasma reactor.  
           [0012]    Another exhaust gas purifying apparatus of the present invention comprises a plasma reactor containing a hydrogen-generating catalyst and a NO x -purifying catalyst. The NO x -purifying catalyst in the reactor may be positioned downstream of the hydrogen-generating catalyst. Further, the NO x -purifying catalyst may be mixed with the hydrogen-generating catalyst.  
           [0013]    According to the present exhaust gas purifying apparatus, it is possible (a) to enhance and/or maintain a NOx purifying ability of the NOx purifying catalyst by the use of the strong reducing agent H 2 , (b) to promote hydrogen generating reaction even at the lower temperature at which an hydrogen generating catalyst alone does not work well, (c) to increase NO x  reducing ability by the use of the strong reducing agent H 2  without increasing the amount of noble metal to be used, and/or (d) to effectively use the reducing agents in an exhaust gas to reduce the amount of fuel required for a rich spike for NO x  reduction, and then improve a fuel efficiency and an emission, etc.  
           [0014]    Further, it is possible to provide plasma on both the hydrogen generating catalyst and the NO x  purifying catalyst, and thereby promote the reactions catalyzed by these catalysts.  
           [0015]    In one embodiment of the present apparatus, the hydrogen-generating catalyst is selected from the group consisting of a CO shift catalyst and a HC reforming catalyst, and combinations thereof.  
           [0016]    The CO shift catalyst may be selected from the group consisting of CuO x /ZnO type catalysts, Cu/MnO x  type catalysts, Fe/CrO x  type catalysts and Pt/TiO 2  type catalysts. The HC reforming catalyst may be selected from the group consisting of Rh/ZrO 2  type catalysts and Rh/CeO 2  type catalysts.  
           [0017]    In another embodiment of the present apparatus, the NO x -purifying catalyst is a NO x  storage reduction catalyst, a NO x  selective reduction catalyst or a three-way catalyst, and particularly a NO x  storage reduction catalyst.  
           [0018]    A method for using the exhaust gas purifying apparatus of the present invention wherein plasma is generated in the plasma reactor only when a rich gas, i.e. gas containing a larger amount of reducing agents than the theoretical air/fuel ratio gas, is passing through the plasma reactor and, particularly, the temperature of the hydrogen generating catalyst is lower than a predetermined temperature such as at the temperature of less than 400° C.  
           [0019]    According to the method of the present invention, it is possible to obtain the benefit of the present apparatus, with less energy, by generating plasma only when a rich spike is provided (when rich gas is passing through the plasma reactor), i.e. when the strong reducing agent of H 2  is required, and the temperature of the exhaust gas is low.  
           [0020]    These and other objects, features and advantages of the present invention will become apparent to a person with ordinary skilled 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 will be described using embodiments and drawings which are, however, 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 a plasma reactor containing a hydrogen-generating catalyst, and then a NO x -purifying catalyst 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 hydrogen-generating catalyst and a NO x -purifying catalyst in a plasma reactor. The plasma can be generated always or when needed in these plasma reactors.  
         [0028]    The components of the exhaust gas purifying apparatus shown in FIGS. 1 and 2 are described below in more detail.  
         [0029]    The hydrogen generating catalyst used for the exhaust gas purifying apparatus of the present invention may be any catalyst which catalyzes generation of hydrogen in an exhaust gas emitted from an internal combustion engine. The hydrogen generating catalyst may be a CO shift catalyst such as CuO x /ZnO type catalysts, Cu/MnO x  type catalysts, Fe/CrO x  type catalysts or Pt/TiO 2  type catalysts, and a HC reforming catalyst such as Rh/ZrO 2  type catalysts or Rh/CeO 2  type catalysts. Further, the hydrogen generating catalysts are described as a basic metal oxide carrier having platinum in JPP&#39; 262 and titanium carrying platinum in JPP&#39; 961.  
         [0030]    The term “CO shift catalyst (or water gas shift catalyst)” used in relation to the present invention means the catalysts having catalytic effect on the following reaction (I). The term “HC reforming catalyst” used in relation to the present invention means the catalysts having catalytic effect on the following reaction (II). 
         CO+H 2 O→CO 2 +H 2   (I) 
         HC+H 2 O →CO 2 +H 2   (II) 
         [0031]    The NO x  purifying catalyst which is carried onto the NO x -purifying catalyst and/or located downstream of the NO x -purifying catalyst is any kind of catalyst which catalyzes 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.  
         [0032]    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.  
         [0033]    The NO x  selective reduction catalyst is a catalyst which selectively reduces or decomposes 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.  
         [0034]    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.  
         [0035]    The catalyst metals such as noble metal and a transition 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.  
         [0036]    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 to pass 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.  
         [0037]    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 -purifying catalyst, may be positioned in the cylindrical passage  10 .  
         [0038]    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  is usually a metal wire, but it may be a hollow bar.  
         [0039]    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 .  
         [0040]    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.  
         [0041]    The electric power supply  40  may apply a high voltage between the center electrode  20  and outer electrode  30  in order to generate a discharge and then a plasma therebetween. The electric power supply  40  may apply a DC voltage, AC voltage, a voltage having a periodic waveform, and 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.  
         [0042]    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.  
         [0043]    The plasma reactor can generate plasma by discharging between mesh-like electrodes  25  and  35  which are at 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 the NO x -purifying catalyst, 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.    
         [0044]    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  
       [0045]    A plasma reactor shown in FIGS. 3 a  and  3   b  was used to determine the effect of a HC reforming catalyst and plasma on a rich spike storage amount of NO x .  
         [0046]    In this example, 2.0 g of HC reforming catalyst (Rh/ZrO 2 ) and 1.0 g of NO x  storage reduction catalyst (“NSR catalyst”) (Pt/Ba/γ-Al 2 O 3 ) downstream of the HC reforming catalyst was contained in the plasma reactor. The HC reforming catalyst contains about 2 wt % of Rh, and the NSR catalyst contains 2 g of Pt and 0.2 mol of Ba on the basis of the 270 g of γ-Al 2 O 3 .  
         [0047]    The plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the catalyst temperature was maintained at 150° C. A lean gas comprising 1000 ppm of NO, 10% of O 2 , 3.0% of H 2 O and balance N2 (Air/Fuel ratio=21) was passed through the plasma reactor for 100 seconds to store NO x  in the NSR catalyst, and then a rich gas comprising 3000 ppm of C 3 H 8 , 1000 ppm of NO, 3.0% of H 2 O and balance N 2 ) is passed through the plasma reactor for 5 seconds. In this example, DC pulse voltage (50 kv, 2000 Hz) is applied to the plasma reactor to generate plasma only while the rich gas is passed through it. After repeating the cycle of the lean and rich gases, NO x  storage amount, i.e. an amount of NO x  which can be stored while the lean gas is provided after providing the rich gas, is determined.  
       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 HC reforming catalyst 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 HC reforming catalyst 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 x  storage amount               (mg)                no plasma   plasma                            NSR catalyst   0.12   0.79           HC catalyst + NSR catalyst   0.33   8.66                      
 
         [0052]    As seen from the table 1, the example 1 according to the present invention (plasma+HC reforming catalyst+NO x  purifying) provides a much better performance than comparative example 1 (HC reforming catalyst only+NO x  purifying), comparative example 2 (plasma+NO x  purifying) and comparative example 3 (NO x  purifying catalyst).  
       Example 2  
       [0053]    A plasma reactor shown in FIGS. 3 a  and  3   b  was used to determine the effect of a CO shift catalyst and plasma on a conversion from NO to NO 2 .  
         [0054]    In this example, 2.0 g of CO shift catalyst (Pt/TiO2) and 1.0 g of NSR catalyst (Pt/Ba/γ-Al 2 O 3 ) downstream of the CO shift catalyst was contained in the plasma reactor. The CO shift catalyst contains about 2 wt % of Pt, and the NSR catalyst contains 2 g of Pt and 0.2 mol of Ba on the basis of the 270 g of γ-Al 2 O 3 .  
         [0055]    The plasma reactor was tested by repeating the procedure of example 1 except that the catalyst temperature was maintained at 200° C. and that the rich gas comprising 1% of CO, 1000 ppm of NO, 3.0% of H 2 O and balance N 2 .  
       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 CO shift catalyst 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 CO shift catalyst 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 x  storage amount               (mg)                no plasma   plasma                            NSR catalyst   0.2   1.54           CO catalyst + NSR catalyt   0.57   13.6                      
 
         [0060]    As seen from the table 2, the example 1 according to the present invention (plasma+CO shift catalyst+NO x  purifying catalyst) provides a much better performance than comparative example 4 (CO shift catalyst only+NO x  purifying catalyst), comparative example 5 (plasma+NO x  purifying catalyst) and comparative example 6 (NO x  purifying catalyst).  
         [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 would 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.