Patent Publication Number: US-2011047994-A1

Title: Exhaust gas purification apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust gas purification apparatus, and more particularly to an exhaust gas purification apparatus using a urea SCR (selective catalytic reduction) catalyst for reducing nitrogen oxides (NO X ) contained in exhaust gas discharged from a diesel engine. 
     A urea SCR system has been developed for reducing NO X  contained in exhaust gas discharged from a diesel engine, which uses a SCR catalyst for reducing NO X  by reaction thereof with ammonia (NH 3 ) produced by hydrolyzing urea water thereby to form nitrogen (N 2 ) and water (H 2 O). 
     In the urea SCR system, the SCR catalyst is disposed in an exhaust gas passage provided between an engine and a muffler. In the exhaust gas passage, an oxidation catalyst and an injection valve are disposed upstream of the SCR catalyst, or on the side of the engine relative to the SCR catalyst. The oxidation catalyst is used for oxidizing hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gas to water (H 2 O) and carbon dioxide (CO 2 ) and promoting oxidation of nitrogen monoxide (NO) to nitrogen dioxide (NO 2 ). The injection valve is used for injecting urea water into exhaust gas. A diesel particulate filter (DPF) for collecting particulate matter (PM) such as carbon contained in exhaust gas is also disposed in the exhaust gas passage provided between the engine and the muffler. 
     Japanese Patent Application Publication 2006-274986 discloses an exhaust gas aftertreatment device including a NO X  storage catalyst activated under a high temperature, a DPF disposed downstream of the NO X  storage catalyst and supporting a urea SCR catalyst which is activated under a relatively low temperature and a urea water injector disposed between the NO X  storage catalyst and the DPF, all of which are housed in one casing thereof. In this exhaust gas aftertreatment device, urea water is injected into exhaust gas by the urea water injector under a low temperature of the NO X  storage catalyst that is lower than 400 degrees Celsius and hydrolyzed to NH 3 . Then, the produced NH 3  is reacted with NO X  contained in exhaust gas in the urea SCR catalyst for reducing NO X . Under a high temperature of the NO X  storage catalyst, that is 400 degrees Celsius or higher, NO X  contained in exhaust gas is stored in the NO X  storage catalyst and reduced. 
     In the exhaust gas aftertreatment device disclosed in the above-cited Publication, reduction activity of the urea SCR catalyst is decreased under a temperature that is lower than 60 percent of the above temperature of 400 degrees Celsius, or lower than 240 degrees Celsius, so that exhaust gas purification performance by reducing NO X  is rapidly decreased. Thus, in the exhaust gas aftertreatment device, the reduction of NOx cannot be performed under a lower temperature where reduction activity of the urea SCR catalyst is decreased. Therefore, the exhaust gas aftertreatment device disclosed in the above Publication does not have a sufficient exhaust gas purification performance by reducing NOx under a lower temperature of exhaust gas where the temperature of the urea SCR catalyst is decreased. The exhaust gas aftertreatment device has a problem in that gas purification can be achieved partially only under a high temperature of exhaust gas. 
     The present invention which has been made in light of the above problem is directed to providing an exhaust gas purification system that allows an increased temperature range of exhaust gas where NO X  contained in exhaust gas is reduced. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an exhaust gas purification apparatus includes an exhaust gas passage, a first oxidation catalyst, a selective catalytic reduction catalyst, an oxidation-reduction catalyst and a urea water supply device. Exhaust gas is flowed through the first oxidation catalyst. The first oxidation catalyst is disposed in the exhaust gas passage. The selective catalytic reduction catalyst is disposed downstream of the first oxidation catalyst. The oxidation-reduction catalyst is disposed downstream of the selective catalytic reduction catalyst. The oxidation-reduction catalyst has reducing property and oxidizing property which are influenced by temperature, wherein the oxidizing property of the oxidation-reduction catalyst is greater than the reducing property of the oxidation-reduction catalyst under a temperature that is higher than a temperature under which the reducing property of the oxidation-reduction catalyst is greater than the oxidizing property of the oxidation-reduction catalyst. The urea water supply device supplies urea water upstream of the selective catalytic reduction catalyst. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic view showing a diesel engine equipped with an exhaust gas purification apparatus according to a first preferred embodiment of the present invention; 
         FIG. 2  is a longitudinal sectional view showing the exhaust gas purification apparatus of  FIG. 1 ; and 
         FIG. 3  is a longitudinal sectional view showing the exhaust gas purification apparatus according to a second preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe a diesel engine and an exhaust gas purification apparatus  101  according to a first preferred embodiment of the present invention with reference to  FIGS. 1 and 2 . In the first preferred embodiment, the following will describe a case where the exhaust gas purification apparatus  101  is used in a diesel engine mounted on a vehicle. 
     Referring to  FIG. 1 , the diesel engine has an engine assembly  10  including an engine body  1 , an intake pipe  3 , an intake manifold  4 , an exhaust manifold  5  and a turbocharger  8 . The engine body  1  has a plurality of cylinders  1 A each having a intake port  1 B and an exhaust port  1 C. The intake manifold  4  has an inlet  4 A formed at one end thereof and is connected at the other end thereof to intake ports  1 B of the respective cylinders  1 A for delivering intake air to the cylinders  1 A. The turbocharger  8  includes a compressor housing  8 A and a turbine housing  8 B. The intake pipe  3  is connected at one end thereof to the inlet  4 A of the intake manifold  4  and at the other end thereof to the compressor housing  8 A of the turbocharger  8 . An intake pipe  2  for introducing ambient air is connected to the compressor housing  8 A of the turbocharger  8 . 
     The exhaust manifold  5  is connected at one end thereof to exhaust ports  1 C of the respective cylinders  1 A for collecting exhaust gas discharged through the exhaust ports  1 C and has a outlet  5 A at the other end thereof. The turbine housing  8 B is connected at the inlet thereof to the outlet  5 A of the exhaust manifold  5  at the outlet  5 A thereof. The turbine housing  8 B is connected to the exhaust gas purification apparatus  101  of a cylindrical shape at the inlet thereof. The exhaust gas purification apparatus  101  is disposed adjacent to the engine body  1 . The exhaust pipe  6  is connected at the upstream end portion  6 A thereof to the exhaust gas purification apparatus  101  and at the opposite downstream end thereof to a muffler  7  with respect to the flow direction of exhaust gas. The intake pipe  2 , the turbocharger  8 , the intake pipe  3  and the intake manifold  4  cooperate to form the intake system of the vehicle (not shown). The exhaust manifold  5 , the turbocharger  8 , the exhaust gas purification apparatus  101 , the exhaust pipe  6  and the muffler  7  cooperate to form the exhaust system of the vehicle. 
     Referring to  FIG. 2 , the exhaust gas purification apparatus  101  includes a cylindrical housing  11  including an upstream end portion  11 A, a downstream end portion  11 B and a cylindrical portion  11 C. The upstream end portion  11 A of the housing  11  is connected to the outlet  8 B 2  of the turbine housing  8 B of the turbocharger  8 , and the downstream end portion  11 B is connected to the upstream end portion  6 A of the exhaust pipe  6 . The housing  11  is connected internally with the turbine housing  8 B of the turbocharger  8  and the exhaust pipe  6 . 
     An oxidation catalyst layer  12  having an oxidation, catalyst supported thereon and a diesel particulate filter (DPF) body  14  are disposed in the housing  11  in this order as viewed in the flowing direction of exhaust gas. The DPF body  14  serves as a collector of particulate matter (PM). The oxidation catalyst layer  12  and the DPF body  14  are provided in the form of a layer having a cylindrical shape spanning perpendicular to the axis of the cylindrical portion  11 C of the housing  11  so as to seal the inner space of the cylindrical portion  11 C. The oxidation catalyst layer  12  and the DPF body  14  are spaced apart from each other thereby to form a space  17  therebetween. 
     The oxidation catalyst layer  12  has therein an oxidation catalyst supported on a base (not shown) for oxidizing hydrocarbons (HC) and carbon monoxide (CO) to water (H 2 O) and carbon dioxide (CO 2 ) and accelerating oxidation of nitrogen monoxide (NO) to nitrogen dioxide (NO 2 ). The oxidation catalyst of the oxidation catalyst layer  12  should preferably be made of a material, such as platinum (Pt), palladium (Pd), rhodium (Rh), silver (Ag), iron (Fe), copper (Cu), nickel (Ni), gold (Au) or a combination of at least any two of these materials. 
     The DPF body  14  is made of porous material such as ceramic for capturing and collecting particulate matter (PM) contained in exhaust gas. The collected PM is burned off in the DPF body  14  for preventing the DPF body  14  from decreasing its filter performance due to the accumulation of collected PM. 
     The entire DPF body  14  has a SCR catalyst  15  supported thereon by coating and serving as a selective catalytic reduction (SCR) catalyst. Thus, the DPF body  14  is formed integrally with the SCR catalyst  15 . Alternatively, the SCR catalyst  15  may be supported only on part of the DPF body  14 . The selective reduction catalyst serves to accelerate the chemical reaction between any specific substances, and, particularly a urea SCR catalyst serves to accelerate the chemical reaction between nitrogen oxides (NO X ) and ammonia (NH 3 ) as reduction agent for reducing NOx to nitrogen (N 2 ) and water (H2O). The SCR catalyst  15  may be made of any oxides of zirconium (Zr), titanium (Ti), silicon (Si), cerium (Ce), tungsten (W), combination of these oxides, or zeolite sieve of molecular porosity-5 (ZSM-5) part of which is metal substituted by such metal as iron (Fe) and copper (Cu). The SCR catalyst  15  has a property to be activated when its temperature is at a predetermined temperature, generally 150 degrees Celsius, or higher. Activation of the SCR catalyst  15  means that the speed of reduction of NO X  by NH 3  is increased rapidly. 
     A noble metal catalyst  16  serving as oxidation-reduction catalyst is supported on the DPF body  14  by coating at a region C 1  that is adjacent to the downstream end  14 B of the DPF body  14 , or the downstream end portion  15 B of the SCR catalyst  15 , as shown in  FIG. 2 . In the first preferred embodiment of the present invention, the entire DPF body  14  is coated with the SCR catalyst  15  by means of dipping, and the noble metal catalyst  16  is applied to the SCR catalyst  15  by means of dipping in the region C 1  on the downstream side of the DPF body  14 . Alternatively, the noble metal catalyst  16  may be supported on the DPF body  14  and the SCR catalyst  15  may be applied to the noble metal catalyst  16 . Furthermore, the DPF body  14  may support thereon the SCR catalyst  15  on the upstream side thereof and the noble metal catalyst  16  on the downstream side thereof by coating. 
     The noble metal catalyst  16  has oxidizing property and reducing property. That is, when the temperature of the noble metal catalyst  16  is higher than a predetermined level, the oxidizing property is greater than the reducing property, so that the noble metal catalyst  16  acts as oxidation catalyst. When the temperature of the noble metal catalyst  16  is at the predetermined level or lower, on the other hand, the reducing property is greater than the oxidizing property, so that the noble metal catalyst  16  acts as reduction catalyst. The noble metal catalyst  16  having such properties made of material such as a platinum (Pt) catalyst (Pt, or composition of Pt and any other noble metal) and a metal oxide catalyst. The above predetermined temperature is in the range from 150 to 250 degrees Celsius if any of the above metal catalyst is used for the noble metal catalyst  16 . The temperature at which the property of the noble metal catalyst  16  is changed from the reducing property to the oxidizing property varies depending on the ratio of materials of the noble metal catalyst  16  and the concentration of the noble metal catalyst  16  in the part of the DPF there the noble metal catalyst  16  is supported. Thus, the above predetermined temperature is in the range between 150 and 250 degrees Celsius. The DPF body  14 , the SCR catalyst  15  and the noble metal catalyst  16  are formed integrally together thereby to form a catalytic DPF  13 , as shown in  FIG. 2 . Specifically, the DPF body  14  supports thereon only the SCR catalyst  15  in the region A 1  shown in  FIG. 2 , and the DPF body  14  supports thereon the SCR catalyst  15  and the noble metal catalyst  16  in the region C 1  shown in  FIG. 2 . 
     An injection valve  19  provided by an electromagnetic valve is disposed in the cylindrical portion  11 C of the housing  11  at a position between the oxidation catalyst layer  12  and the catalytic DPF  13 , as shown in  FIG. 2 . The injection valve  19  serves as urea water supply device. The injection valve  19  is fluidly connected with a urea water tank  20  disposed in the vehicle (not shown) and operable to inject urea water into the space  17  of the housing  11  (or upstream of the SCR catalyst  15 ). The injection valve  19  is disposed adjacent to the downstream end of the oxidation catalyst layer  12  and injects urea water into the space  17  adjacent to the downstream end of the oxidation catalyst layer  12 . The injection valve  19  is connected electrically with a dosing control unit (DCU)  30 , which controls the opening/closing operation of the injection valve  19 . The urea water tank  20  has therein a motor pump for supplying urea water stored in the urea water tank  20  to the injection valve  19 . The motor pump is connected electrically with the DCU  30 , which controls the operation of the motor pump. The DCU  30  may be formed either separately from or integrally with a vehicle electronic control unit (ECU) (not shown). The injection valve  19  should preferably be disposed adjacent to the oxidation catalyst layer  12  on the upstream side of the catalytic DPF  13 , the reason for which will be described later. 
     A cylindrical mixer  18  is disposed on the upstream end surface  13 A of the catalytic DPF  13  for spreading substances contained in exhaust gas evenly over the upstream end surface  13 A of the catalytic DPF  13 . The mixer disclosed in the Japanese Unexamined Patent Application Publication No. 6-509020T or No. 2006-9608 may be used as the mixer  18  of the present invention. The mixer disclosed in the Publication No. 6-509020T is made in the form of a lattice that divides the exhaust gas passage into plural cells so as to cause the exhaust gas flowing through each cell to flow spirally and also to flow toward the adjacent cell. This helps the substances in the exhaust gas to spread evenly in the entire exhaust gas passage. On the other hand, the mixer disclosed in the Publication No. 2006-9608 has plural plates each extending perpendicularly to the flowing direction of exhaust gas, which provides serpentine gas passage to spread the substances contained in the exhaust gas evenly. 
     An oxidation catalyst layer  40  is disposed in the exhaust pipe  6  provided downstream of the exhaust gas purification apparatus  101 . The oxidation catalyst layer  40  supports thereon an oxidation catalyst acting on NH 3  as oxidation catalyst. The oxidation catalyst of the oxidation catalyst layer  40  should preferably be made of material such as platinum (Pt), palladium (Pd), silver (Ag), iron (Fe), copper (Cu), nickel (Ni) and gold (Au). As is apparent from the foregoing description, the exhaust gas purification apparatus  101  of the present embodiment includes an exhaust gas purification device including the SCR catalyst and an exhaust gas purification device including the DPF which are connected integrally with each other and connected to the engine assembly  10  adjacent to the engine body  1 . 
     The following will describe the operation of the exhaust gas purification apparatus  101  and the vehicle engine equipped with the exhaust gas purification apparatus  101  with reference to  FIGS. 1 and 2 . While the engine body  1  is running, intake air is introduced into the compressor housing  8 A of the turbocharger  8  through the intake pipe  2 . The intake air is pumped by the compressor wheel (not shown) in the compressor housing  8 A and then delivered through the intake pipe  3  and the intake manifold  4  to the cylinders  1 A of the engine body  1 . Diesel fuel injected into highly compressed air in the cylinder  1 A is spontaneously ignited and combusted. 
     Exhaust gas resulting from the combustion of diesel fuel with the intake air is discharged through the exhaust port  1 C into the exhaust manifold  5  and the turbine housing  8 B of the turbocharger  8 . While increasing rotation speed of the turbine wheel (not shown) and the compressor wheel (not shown either) which are connected with each other. The exhaust gas passed through the exhaust gas purification apparatus  101  is flowed through the oxidation catalyst layer  40  and the muffler  7  in the exhaust pipe  6  and then discharged out of the vehicle (not shown). 
     Referring to  FIG. 2 , the exhaust gas introduced into the exhaust gas purification apparatus  101  firstly is all flowed through the oxidation catalyst layer  12 , so that HC and CO contained in exhaust gas are oxidized to CO 2  and H 2 O and part of NO to NO 2  that can be reduced more easily than that of NO. Exhaust gas passed through the oxidation catalyst layer  12  is then flowed through the space  17  and the mixer  18  and into the catalytic DPF  13 . While exhaust gas is passed through the catalytic DPF  13 , PM contained in exhaust gas is captured and collected by the DPF body  14 . 
     Simultaneously, the motor pump of the urea water tank  20  is operated and the injection valve  19  is opened by the DCU  30  thereby to inject urea water into the space  17 . The heat of the exhaust gas in the space  17  serves to accelerate the hydrolysis of the injected urea water to NH 3  and CO 2 . The provision of the injection valve  19  at a position adjacent to the oxidation catalyst layer  12  in the space  17  serves to lengthen the time for the hydrolysis of the injected urea water to NH 3  before the urea water reaches the SCR catalyst  15  of the catalytic DPF  13 , thus improving the efficiency of hydrolysis of urea water. Thus, the injection valve  19  should preferably be located as far away from the catalytic DPF  13  as possible. In addition, since urea water is injected and hydrolyzed to NH 3  in the space  17  on the downstream side of the oxidation catalyst layer  12 , NH 3  is not oxidized by the oxidation catalyst layer  12 . 
     NH 3  produced by the hydrolysis of urea water in the space  17  is flowed through the mixer  18  with exhaust gas, spread by the mixer  18  and introduced into the catalytic DPF  13 . 
     NH 3  contained in the exhaust gas and introduced into the catalytic DPF  13  performs either one of the actions depending on the temperature of the SCR catalyst  15  of the catalytic DPF  13 , as will be described under the items (1) and (2). It is noted that the temperature of the SCR catalyst  15  is substantially the same as that of the exhaust gas flowed in the catalytic DPF  13 . 
     (1) Firstly, the case where the temperature of the SCR catalyst  15  is lower than the temperature Ts at which the SCR catalyst  15  is activated, e.g. 150 degrees Celsius, will be described. When the temperature of the noble metal catalyst  16  is below the temperature Tn, the reducing property of the noble metal catalyst  16  becomes greater than oxidizing property thereof, so that the noble metal catalyst  16  acts as reduction catalyst. When the temperature of the noble metal catalyst  16  is higher than the temperature Tn, the oxidizing property of the noble metal catalyst  16  is greater than the reducing property thereof, so that the noble metal catalyst  16  acts as oxidation catalyst. When the noble metal catalyst  16  is made of the above-described materials, the temperature Tn is in the range between 150 and 250 degrees Celsius. The following description will be made with the assumption that the temperature Tn is 200 degrees Celsius. The temperature of the noble metal catalyst  16  is substantially the same as that of the SCR catalyst  15 . The temperature of the noble metal catalyst  16  is lower than 150 degrees Celsius, where the reducing property of the noble metal catalyst  16  is greater than oxidizing property whereof, so that the noble metal catalyst  16  acts as reduction catalyst. 
     In the region A 1  of the DPF body  14  where only the SCR catalyst  15  is supported, NO X  including NO and NO 2  contained in exhaust gas and introduced into the catalytic DPF  13  is not reduced by NH 3  contained in the same exhaust gas. Thus, the NO X  and the NH 3  contained in the exhaust gas are flowed through the region A 1  without reaction and introduced into the region C 1 . In the region C 1  where the DPF body  14  supports thereon also the noble metal catalyst  16 , NO X  is reduced to N 2  by the NH 3 . After PM contained in exhaust gas is removed therefrom and NO X  contained in the exhaust gas is reduced in the exhaust gas purification apparatus  101 , the exhaust gas containing NH 3  which has not been consumed by reduction of NO X  is flowed into the exhaust pipe  6  from the exhaust gas purification apparatus  101 . Then, the exhaust gas is flowed through the oxidation catalyst layer  40  disposed in the exhaust pipe  6  and the muffler  7  and discharged out of the vehicle (not shown). Since NH 3  contained in the exhaust gas is oxidized in the oxidation catalyst layer  40 , no harmful NH 3  is discharged out of the vehicle. 
     (2) Next, the case where the temperature of the SCR catalyst  15  is higher than the temperature Ts of 150 degrees Celsius will be described. In this case, the SCR catalyst  15  is activated, so that NO X  contained in exhaust gas and flowed into the catalytic DPF  13  is reduced to N 2  by NH 3  contained in exhaust gas in the SCR catalyst  15  in the region A 1  of the DPF body  14 . The noble metal catalyst  16  performs either one of the following actions depending on the temperature, as described in the following items (2A) and (2B).
 
(2A) When the temperature of the noble metal catalyst  16  is at the temperature Tn of 200 degrees Celsius or lower, the reducing property of the noble metal catalyst  16  is greater than the oxidizing property thereof, so that the noble metal catalyst  16  acts as reduction catalyst. Thus, NO X  contained in exhaust gas which is not reduced in the region A 1  is reduced by NH 3  contained in the exhaust gas in the noble metal catalyst  16  in the region C 1 . After PM contained in the exhaust gas is removed therefrom and NO X  contained in exhaust gas is reduced in the exhaust gas purification apparatus  101 , the exhaust gas containing NH 3  which has not been consumed by reduction of NO X  is flowed into the exhaust pipe  6  from the exhaust gas purification apparatus  101 . The exhaust gas is flowed in the oxidation catalyst layer  40  disposed in the exhaust pipe  6 , where NH 3  contained in the exhaust gas is oxidized and then flowed through the muffler  7  and discharged out of the vehicle (not shown). Thus, no harmful NH 3  is discharged out of the vehicle.
 
(2B) When the temperature of the noble metal catalyst  16  is higher than the temperature Tn of 200 degrees Celsius, the oxidizing property of the noble metal catalyst  16  is greater than the reducing property thereof, so that the noble metal catalyst  16  acts as oxidation catalyst. Thus, NH3 which has not been consumed by the reduction of NO X  in the region A 1  is oxidized in the noble metal catalyst  16  in the region C 1 . Thus, harmful NH3 is removed from the exhaust gas. After PM in exhaust gas is removed therefrom, NO X  in exhaust gas is reduced, and NH 3  in exhaust gas is removed therefrom in the exhaust gas purification apparatus  101 , the exhaust gas is flowed through the exhaust pipe  6  having therein the oxidation catalyst layer  40  and the muffler  7  and then discharged out of the vehicle (not shown).
 
     In the exhaust gas purification apparatus  101 , NO X  contained in exhaust gas is reduced as described in the above items (1) or (2), and harmful NH 3  is oxidized to prevent NH 3  from being discharged out of the vehicle. In the catalytic DPF  13 , PM collected by the DPF body  14  is periodically burned, so that harmful CO is produced. When PM begins to be burned and during the PM combustion in the catalytic DPF  13 , the combustion temperature of PM reaches about 600 degrees Celsius, so that the noble metal catalyst  16  acts as oxidation catalyst. CO generated by burning of PM is oxidized to CO 2  in the noble metal catalyst  16  in the region C 1 . Therefore, the noble metal catalyst  16  has the oxidizing property for CO generated by burning of PM. 
     Referring to  FIG. 1 , exhaust gas which is just discharged from the turbocharger  8  or the engine body  1 , whose temperature is hardly decreased, is flowed into the exhaust gas purification apparatus  101  disposed adjacent to the engine body  1 . Heat generated by the engine body  1  during the operation is transmitted to the housing  11  (refer to  FIG. 2 ) of the exhaust gas purification apparatus  101  disposed adjacent to the engine body  1  and further to the inside of the housing  11 . Referring to  FIG. 2 , since the inside of the housing  11  and the catalytic DPF  13  are heated by the above-described exhaust gas whose temperature is hardly decreased and the transmitted heat, the temperatures of the inside of the housing  11  and the catalytic DPF  13  are increased quickly. Thus, it takes less time for urea water to be heated to the hydrolysis temperature during a cold start of the engine. Therefore, reduction of NO X  may start early during a cold start of the engine, so that the efficiency of reducing NO X  is improved. 
     The exhaust gas purification apparatus  101  according to the first preferred embodiment of the present invention includes the oxidation catalyst layer  12  disposed in an exhaust gas passage through which exhaust gas is flowed, the SCR catalyst  15  disposed downstream of the oxidation catalyst layer  12 , the noble metal catalyst  16  having reducing property and oxidizing property and the injection valve  19  for supplying urea water upstream of the SCR catalyst  15 . The oxidizing property of the noble metal catalyst  16  is greater than the reducing property under a temperature that is higher than a temperature under which the reducing property of the noble metal catalyst  16  is greater than the oxidizing property. When the temperature of the noble metal catalyst  16  or the temperature of exhaust gas is at a lower level, the reducing property of the noble metal catalyst  16  is greater than the oxidizing property, and reduction of NO X  contained in exhaust gas is accelerated by NH 3  produced by hydrolyzing urea water. Therefore, NO X  is reduced in the temperature range of exhaust gas where the reducing property of the noble metal catalyst  16  is greater than the oxidizing property, as well as in the exhaust gas temperature range where the SCR catalyst  15  performs reduction, so that the temperature range of exhaust gas where NO X  is reducible may be increased. 
     A temperature under which the reducing property of the noble metal catalyst  16  is greater than the oxidizing property includes a temperature that is lower than a temperature under which the SCR catalyst  15  performs reduction. Thus, when the exhaust gas temperature is in the lower range of temperature, the SCR catalyst  15  does not performs reduction but the noble metal catalyst  16  acts as reduction catalyst to reduce NO X  contained in exhaust gas. Therefore, the temperature range of exhaust gas where NO X  is reducible may be expanded toward the lower level. Additionally, the exhaust gas purification apparatus  101  which includes the DPF body  14  integrally formed with the SCR catalyst  15  may be downsized. The DPF body  14  is coated with the SCR catalyst  15  and the noble metal catalyst  16  into an integral unit, so that the exhaust gas purification apparatus  101  may be further downsized. 
     The oxidation catalyst layer  12 , the SCR catalyst  15 , the noble metal catalyst  16  and the injection valve  19  are accommodated in one housing  11 , which also contributes to downsizing of the exhaust gas purification apparatus  101 . Since the exhaust gas purification apparatus  101  is mounted to the engine assembly  10 , the temperature of exhaust gas discharged from the engine assembly  10  is hardly decreased and the exhaust gas of high temperature is supplied to the exhaust gas purification apparatus  101 . Heat generated by the operation of the engine body  1  is transmitted to the housing  11  of the exhaust gas purification apparatus  101 . During a cold start of the engine, therefore, the time in which the temperature of exhaust gas is increased to a level where the urea water is hydrolyzed may be shortened. Thus, the exhaust gas purification apparatus  101  may start reducing NO X  in a short time from the moment of cold starting of the engine, with the result that the exhaust gas purification performance to reduce NO X  may be improved. In the catalytic DPF  13  of the first preferred embodiment of the present invention, the noble metal catalyst  16  is applied by coating to the SCR catalyst  15  which is in turn applied to the DPF body  14  in the region C 1  of the DPF body  14  that is adjacent to the downstream end portion  15 B of the SCR catalyst  15 . In other word, the noble metal catalyst  16  of the first preferred embodiment of the present invention is disposed downstream of the SCR catalyst  15 . Thus, the noble metal catalyst  16  is disposed downstream of the SCR catalyst  15  and acts as oxidation-reduction catalyst having reducing and oxidizing properties. 
     The exhaust gas purification apparatus  102  according to the second preferred embodiment of the present invention differs from the exhaust gas purification apparatus  101  according to the first preferred embodiment of the present invention in that the DPF body and the SCR catalyst are modified over the counterparts of the first preferred embodiment. The following description will use the same reference numerals for the common elements or components in the first and the second embodiments, and the description of such elements or components will be omitted. 
     Referring to  FIG. 3 , the oxidation catalyst layer  12 , the DPF body  24  and the composite catalyst layer  27  supporting a plurality of catalysts are disposed in the housing  11  of the exhaust gas purification apparatus  102  in this order. The oxidation catalyst layer  12  and the DPF body  24  are disposed apart from each other with the space  17  formed therebetween. The DPF body  24  and the composite catalyst layer  27  are disposed adjoining each other. The composite catalyst layer  27  is formed substantially in the same manner as the oxidation catalyst layer  12  so as to support on the entire base (not shown) thereof the SCR catalyst  25 . The composite catalyst layer  27  supports thereon a noble metal catalyst  26  through the SCR catalyst  25  which is supported thereby adjacent to the downstream end surface  27 B of the composite catalyst layer  27  or to the downstream end portion  25 B of the SCR catalyst  25 . The composite catalyst layer  27  may be formed such that the positions of the SCR catalyst  25  and the noble metal catalyst  26  are inverted. Additionally, it may be so arranged that the composite catalyst layer  27  support thereon on the upstream side thereof the SCR catalyst  25  and on the opposite downstream side thereof the noble metal catalyst  26 . 
     The DPF body  24  and the composite catalyst layer  27  supporting thereon the SCR catalyst  25  and the noble metal catalyst  26  are integrated thereby to form the catalytic DPF  23 . As shown in  FIG. 3 , the catalytic DPF  23  is formed such that the DPF body  24  alone is provided in the region A 2 , the SCR catalyst  25  is supported by a base (not shown) in the region B 2 , and the SCR catalyst  25  and the noble metal catalyst  26  are supported by a base (not shown) in the region C 2 . The mixer  18  is disposed on the upstream end surface  23 A of the catalytic DPF  23 . 
     Exhaust gas introduced into the housing  11  of the exhaust gas purification apparatus  102  is flowed into the catalytic DPF  23  after passing through the oxidation catalyst layer  12  and the mixer  18 . After PM contained in the exhaust gas flowed into the catalytic DPF  23  is captured by and collected on the DPF body  24 , the exhaust gas is flowed through the SCR catalyst  25  and the noble metal catalyst  26  and then discharged out of the exhaust gas purification apparatus  102 . Chemical action on the substances such as NO X  contained in the exhaust gas flowing through the SCR catalyst  25  and the noble metal catalyst  26  or through the regions B 2  and C 2  are substantially the same as those on the substances contained in exhaust gas flowing through the regions A 1  and C 1  of the first preferred embodiment of the present invention. 
     In the catalytic DPF  23 , PM collected on the DPF body  24  is periodically burned. This combustion of PM is done by making use of the heat of exhaust gas available when its temperature is relatively high, so that combustion efficiency is improved. Thus, while PM is burning, the noble metal catalyst  26  acts as oxidation catalyst to oxidize CO generated by combustion of PM. The rest of the structures of the exhaust gas purification apparatus  102  of the second preferred embodiment of the present invention is substantially the same as that of the exhaust gas purification apparatus  101  of the first preferred embodiment and, therefore, the description thereof will be omitted. 
     Advantages similar to those of the exhaust gas purification apparatus  101  of the first preferred embodiment are obtained in the exhaust gas purification apparatus  102  of the second preferred embodiment. The exhaust gas purification apparatus  102  includes DPF body  24  disposed upstream of the composite catalyst layer  27 . The DPF body  24 , the SCR catalyst  25  and the noble metal catalyst  26  are formed separately from one another, thereby reducing the influence of heat generated by burning of PM collected on the DPF body  24  and transmitted to the SCR catalyst  25  and the noble metal catalyst  26 . This helps to improve the durability of the SCR catalyst  25  and the noble metal catalyst  26 . As to the composite catalyst layer  27  of the second preferred embodiment, the noble metal catalyst  26  is applied by coating to the SCR catalyst  25  in the region C 2  adjacent to the downstream end portion  25 B of the SCR catalyst  25 . In other words, the noble metal catalyst  26  of the second preferred embodiment is disposed downstream of the SCR catalyst  25 . Thus, the noble metal catalyst  26  is disposed downstream of the SCR catalyst  25  and acts as oxidation-reduction catalyst having both reducing property and oxidizing property. 
     According to the first and second preferred embodiments of the present invention, the exhaust gas purification apparatuses  101 ,  102  are disposed adjacent to the engine assembly  10  equipped with the turbocharger  8 . Alternatively, the exhaust gas purification apparatuses  101 ,  102  may be connected directly to the outlet  5 A of the exhaust manifold  5  of an engine assembly having no turbocharger, or the exhaust gas purification apparatus  101 ,  102  may be disposed apart from the engine assembly  10 . 
     Although the housing  11  of the exhaust gas purification apparatuses  101 ,  102  according to the first and second preferred embodiments of the present invention, has a cylindrical shape, the housing  11  of the exhaust gas purification apparatus according to the present invention is not limited to the cylindrical shape. Alternatively, the housing may have a column shape including a box shape, a spherical shape or an ellipsoidal shape. 
     According to the first and second preferred embodiments of the present invention, the exhaust gas purification apparatuses  101 ,  102  include the mixer  18 . Alternatively, the exhaust gas purification apparatuses of the present invention may dispense with the mixer  18 .