Abstract:
A method and apparatus for the removal of NO 2  emission from a lean burn compression ignition engine, wherein NO 2  containing engine exhaust gas is brought in contact with a catalyst being active in the reduction of NO 2  to NO and comprising at least one platinum group metal with the proviso that the platinum metal is not platinum and at least one redox active metal oxide and thereby reducing NO 2  contained in the exhaust gas to NO by reaction with CO, hydrocarbons and/or soot being present in the exhaust gas.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for the purification of exhaust gas from a lean burn compression ignition engine such as a diesel engine and an apparatus for use in that method. In particular, the invention is directed to removal of nitrogen dioxide (NO 2 ) by reduction to nitrogen monoxide (NO) when reacting the dioxide with carbon monoxide (CO), hydrocarbons (HC) and/or soot being contained in the exhaust gas in presence of a catalyst. The apparatus being suitable for employment in the method comprises a particulate filter being coated with that catalyst. 
     The invention is in particular useful in connection with lean burn compression ignition engines. 
     BACKGROUND OF THE INVENTION 
     Conventional lean burn compression ignition engines such as diesel engines produce diesel particulate matter (PM) emissions including soot and volatile and soluble organic fractions. Future legislated limits for PM require the installation of a particulate filter in the exhaust system. 
     A known problem with soot filters is that they are plugged with soot during operation, which requires continuous or periodical regeneration to prevent excessive pressure drop over the filter. Periodical regeneration is conventionally performed by increasing the temperature in the exhaust system to a temperature, where the captured soot is oxidized to gaseous components and thus removed from the filter. 
     Typically, the required temperature increase is obtained by injecting extra fuel, which is burned over a diesel oxidation catalyst (DOC) placed in the exhaust system upstream the filter. A DOC is already installed in many diesel vehicles to meet the emissions limits for hydrocarbons and CO. Both the increased pressure drop over the filter and the periodical regeneration gives rise to a fuel penalty compared to an engine exhaust gas system without a particulate filter. 
     Different systems have been developed in order to lower this fuel penalty. One such system is disclosed in U.S. Pat. No. 4,902,487, which has been commercialized as the “Continuously Regenerating Trap”. In this system NO 2  is used for combusting diesel particulate deposited on a filter at lower temperature than possible with oxygen also available in the gas. NO 2  is obtained by oxidising NO being present in the exhaust gas over a suitable catalyst placed upstream of the filter. This NO oxidation catalyst typically contains Pt, which is known as an excellent NO oxidation catalyst. To further increase the NO 2  content in the exhaust gas, filters have also been coated with a Pt containing catalyst. As a disadvantage of such systems, NO 2  can slip past the filter and be undesirably exhausted to atmosphere, when there is insufficient PM on the filter to react with NO 2  generated over the oxidation catalyst or the temperature of the exhaust gas is below a preferred range for combustion of PM in NO 2 . 
     DE102005027063A1 discloses a device for after treatment of oxygen containing exhaust gases of internal combustion engine has nitrogen dioxide reduction catalytic converter which is provided as part at flow end of exhaust gas after treatment device being provided with an oxygenation catalyst and/or a soot filter. The SCR catalyst for the reduction of nitrogen dioxide contains is impregnated with small amounts of platinum metals. 
     WO06040533A1 relates to decomposition of nitrogen dioxide to nitrogen monoxide in lean-burn internal combustion engine by contacting acidic metal oxide with exhaust gas. The acidic metal oxide is selected from the group consisting of zeolites, tungsten-doped titania, silica-titania, zirconia-titania, gamma-alumina, amorphous silica-alumina and mixtures thereof. 
     NO 2  is toxic in low doses. It is apprehensive that the levels of NO 2  in European cities and highways increase [See presentations from “NO 2  Workshop, Munich February 2006”]. Legislative authorities have therefore begun to discuss limiting the amount of NO 2  to an acceptable level being permissible in the exhaust gas. For example the yearly average downtown Stuttgart was in 2004 68 μg/m 3 , which by order shall be reduced to a limit of 40 μg/m 3  NO 2  (yearly average) in Stuttgart in 2010. Accordingly, it is presumed that reduction of both PM and NO 2  in exhaust gas emissions to the atmosphere will be required by law. 
     It is thus a general object of the present invention to provide a method and apparatus for the substantial reduction of NO 2  and preferably NO 2  and PM in the exhaust gas from a compression ignition engine. 
     SUMMARY OF THE INVENTIONS 
     Pursuant to the above general object, this invention provides a method for the removal of NO 2  emission from a lean burn compression ignition engine, wherein NO 2  containing engine exhaust gas is brought in contact with a catalyst being active in the reduction of NO 2  to NO and comprising at least one platinum group metal with the proviso that the platinum metal is not platinum and at least one redox active metal oxide, thereby reducing NO 2  contained in the exhaust gas to NO by reaction with CO, hydrocarbons and/or soot being present in the exhaust gas. 
     Preferred embodiments of the inventive method are disclosed in the detailed description of the invention. 
     The invention provides furthermore an apparatus for use in the above method. The apparatus comprises in its most general form an engine exhaust gas system being provided with a catalyst being effective in reaction of NO 2  with CO, HC and/or soot to NO, wherein the catalyst comprises at least one platinum group metal with the proviso that the platinum metal is not platinum and at least one redox active metal oxide. 
     Preferred embodiments of the apparatus according to the invention are disclosed in and apparent from the detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the filter of the present invention placed in the exhaust system of a lean burn compression ignition engine, with a diesel oxidation catalyst between the engine and the filter; 
         FIG. 2  shows the filter of the present invention placed in the exhaust system of a lean burn compression ignition engine, without a diesel oxidation catalyst between the engine and the filter; and 
         FIG. 3  is a graph showing measured concentrations of NO 2  plotted as a function of the filter temperature. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to a preferred embodiment of the invention the NO 2  reduction catalyst is coated on a diesel particulate filter. This filter may be any known type such as a ceramic wall-flow monolith filter, ceramic fibre filter or a sinter metal filter. The main components of the wallflow monclith filter are preferable silicon carbide, cordierite or alumina titanate. 
     The catalytic coat comprises preferably palladium in an amount of 0.2 to 5 g/L filter and more preferably 0.5 to 2 g/L filter. 
     The catalytic coat also contains at least one redox active oxide. Redox active meaning the metal is capable of existing in different oxidation states. The preferred amount of redox active oxide is 1 to 100 g/L filter and more preferably 5 to 60 g/L filter. 
     Preferred oxides are MnO 2 , Fe 2 O 3 , SnO 2 , PrO 2 , and CeO 2 . The most preferable redox active oxide is CeO 2 . 
     Optionally the catalytic coat and the redox active metal oxides are stabilized with one or more oxides selected from the group consisting of TiO 2 , WO 3 , SiO 2 , HfO 2 , ZrO 2 , MgO, CaO, Al 2 O 3 , La 2 O 3  and BaO. 
     When the oxide is CeO 2 , the most preferred stabilising oxide is ZrO 2 . 
     The components of the catalytic coat can be applied to the filter with any method known in the art. These methods include the sequence of washcoating, impregnation, drying, calcination and reduction. The catalyst can be arranged inside the filter wall and/or outside of the filter wall. 
     In one embodiment of the invention this filter is placed in the exhaust system of a lean burn compression ignition engine, e.g. a diesel engine, as shown in  FIG. 1 , where 
     (1) is a lean burn compression ignition engine, optionally including a system for injection of extra fuel to raise the exhaust temperature for regeneration of the particulate filter by burning soot and optionally including a system for injecting fuel borne catalyst for lowering the soot burning temperature in the particulate filter; 
     (2) is a Diesel Oxidation Catalyst (DOC) supported on a flow through monolith capable of oxidising hydrocarbons and CO for lowering the emission of these components and for increasing the exhaust temperature if extra fuel is injected to regenerate the particulate filter. The DOC can optionally also oxidise NO to NO 2  in order to continuously oxidise soot at lower temperature via NO 2 ; 
     (3) is a particulate filter coated according to the present invention, which results in a reduction of the amount of NO 2  and PM emission. NO 2  is reduced to NO by reaction with CO, hydrocarbons and/or soot. The catalytic coat also reduces the temperature required to regenerate particulate filter. 
     In another embodiment of the invention this filter  3  is placed in the exhaust system of a lean burn compression ignition engine, e.g. a diesel engine as shown in  FIG. 2 . (1) is a lean burn compression ignition engine optionally including a system for injection of extra fuel to raise the exhaust temperature for regeneration of the particulate filter by burning soot and optionally including a system for injecting fuel borne catalyst for lowering the soot burning temperature in the particulate filter. (3) is the particulate filter coated according to the present invention, which results in a reduction of both NO 2  and PM emissions. NO 2  is mostly reduced to NO by reaction with reductants present in the exhaust gas. These reductants can be CO, hydrocarbons and soot. The catalytic coat also reduces the temperature required to regenerate particulate filter. Furthermore, the catalysed filter is capable of reducing the CO and hydrocarbon emissions. 
     EXAMPLES 
     Example 1 
     A 3.3 L commercially available SiC wall flow diesel particulate filter is via conventional impregnation, drying and calcination steps coated with CeO 2 , ZrO 2  and PdO inside the filter wall. The Ce content is 45 g/L filter, the Zr content 9.4 g/L filter and the Pd content is 1.5 g/L filter. 
     The filter is tested in the exhaust system of an engine bench equipped with a 1.4 L HDI engine from a Citroen C2-2004 model. The setup corresponds to  FIG. 1 . During the test the CO, hydrocarbon, NO and NO x  levels in the exhaust gas after passing the filter are monitored. The engine is run at 2500 rpm and the load is varied to determine the catalyst activity at different filter temperatures. 
     Measured concentrations of NO and NO 2  are summarized in Table 1 and in  FIG. 3 , where the NO 2  conversion obtained from comparison with measurements on an uncoated filter is plotted as a function of the filter temperature. The NO 2  concentration in the gas is determined by subtracting the NO concentration from the NO x  concentration. It is observed that all NO 2  is converted to mainly NO, when the filter temperature is below 300° C. and forms extra NO 2  when the filter temperature is above 300° C. During a standard driving cycle this leads to net removal of NO 2  since the exhaust temperature is mainly below 300° C. This is described in Example 2. 
     The capability of the filter to burn soot is quantified with the balance point temperature, which is the filter temperature, where the pressure drop over the filter is constant since the amount of soot caught by the filter equals the soot that is removed by oxidation. The balance point temperature is 400° C. For an uncoated filter this temperature is above 450° C. in the same engine bench test protocol. 
     The CO and hydrocarbon conversion rates are determined in the same way. The temperatures for 50% conversion of both exhaust components are lower than 200° C. 
     Table 1: NO and NO x  concentration measured in the engine bench described in Example 1 for an uncoated SiC wall flow particulate filter and the filter described in Example 1. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Exit uncoated 
                 Exit coated filter of Example 1 
               
             
          
           
               
                 Engine 
                 filter 
                   
                 NO 2   
               
             
          
           
               
                 power 
                 NO x   
                 NO 
                 T Filter 
                 NO x   
                 NO 
                 conversion 
               
               
                 (kW) 
                 (ppm) 
                 (ppm) 
                 (° C.) 
                 (ppm) 
                 (ppm) 
                 (%) 
               
               
                   
               
             
          
           
               
                 2 
                 79 
                 38 
                 144 
                 84 
                 84 
                 100 
               
               
                 4 
                 99 
                 55 
                 171 
                 104 
                 99 
                 89 
               
               
                 6 
                 128 
                 78 
                 198 
                 128 
                 122 
                 88 
               
               
                 8 
                 161 
                 112 
                 221 
                 153 
                 148 
                 90 
               
               
                 12 
                 276 
                 228 
                 256 
                 264 
                 247 
                 65 
               
               
                 16 
                 411 
                 361 
                 291 
                 391 
                 351 
                 20 
               
               
                 20 
                 581 
                 519 
                 321 
                 565 
                 495 
                 −13 
               
               
                 24 
                 733 
                 661 
                 358 
                 700 
                 590 
                 −53 
               
               
                 28 
                 1017 
                 875 
                 376 
                 1050 
                 861 
                 −33 
               
               
                 32 
                 1473 
                 1242 
                 407 
                 1353 
                 1097 
                 −11 
               
               
                   
               
             
          
         
       
     
     Example 2 
     A 3.3 L SiC wall flow filter coated with catalyst as described in Example 1 is used to replace an uncoated filter in a Citroen Xsara Picasso 1.6 L HDI—model year 2006. The filter is placed downstream of a Pt containing DOC catalyst. Fuel born catalyst is added to the diesel to lower the soot oxidation temperature. The setup corresponds to the one sketched in  FIG. 2 . 
     Before and after replacing the filter the vehicle is tested in the NEDC standardised driving cycle, while emissions are monitored. The accumulated results are shown in Table 2. 
     Table 2: Emissions measured in NEDC test on Citroen Xsara Picasso equipped with a 1.6 L HDI engine and with a Pt containing DOC upstream the filter. An uncoated particulate filter is compared to a catalyst coated filter as described in Example 1. 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 NO 2   
                   
                   
                   
               
               
                 NEDC 
                 HC 
                 NO 
                 NO x   
                 (NO x —NO) 
                 CO 2   
                 CO 
                 PM 
               
               
                 test 
                 (g/km) 
                 (g/km) 
                 (g/km) 
                 (g/km) 
                 (g/km) 
                 (g/km) 
                 (g/km) 
               
               
                   
               
             
             
               
                 Pt/DOC + 
                 0.04 
                 0.114 
                 0.202 
                 0.088 
                 115 
                 0.027 
                 0.0028 
               
               
                 Uncoated 
               
               
                 filter 
               
               
                 Pt/DOC + 
                 0.00 
                 0.159 
                 0.183 
                 0.024 
                 116 
                 0.024 
                 0.0044 
               
               
                 Filter coated 
               
               
                 with catalyst 
               
               
                 of Example 1