Abstract:
A diesel engine exhaust treatment system and method is provided which includes a platinum group metal trapping device positioned between a diesel oxidation catalyst and an SCR catalyst. The platinum group metal trapping device traps trace amounts of platinum group metals which may be released from the diesel oxidation catalyst during engine operation and prevents them from accumulating on the SCR catalyst, preventing potential contamination of the SCR catalyst as well as ensuring that the performance of the SCR catalyst is uninhibited.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments described herein relate to a diesel engine exhaust treatment system and method, and more particularly, to an exhaust treatment system and method which utilize a platinum group metal trapping device to prevent contamination of SCR catalysts. 
         [0002]    Diesel engine exhaust treatment systems are known for use in converting gaseous emissions such as nitrogen oxides (NO x ) to environmentally acceptable compounds. Such systems typically include a diesel oxidation catalyst (DOC), a selective catalytic reduction catalyst (SCR), and a diesel particulate filter (DPF). 
         [0003]    Diesel oxidation catalysts are placed in the exhaust gas stream of a diesel engine and typically contain platinum group metals (PGM), base metals, or a combination thereof. These catalysts promote the conversion of CO and HC emissions to carbon dioxide and water. 
         [0004]    Selective catalytic reduction catalysts (SCR) are used to convert NO x  to N 2  and typically comprise a base metal and utilize an ammonia reductant, typically in the form of aqueous urea, which is injected into the exhaust stream downstream from the diesel oxidation catalyst. After water vaporization and urea hydrolysis, the formed ammonia reacts with NO x  in the exhaust gas stream on the SCR catalyst to form N 2 . 
         [0005]    A diesel particulate filter (DPF) collects soot or particulate matter from engine exhaust. A precious metal catalyst selected from platinum group metals is typically coated on the DPF for the removal of CO, HC, and NH 3  slip. 
         [0006]    It has been observed that treatment systems which include diesel oxidation catalysts washcoated with platinum group metals may lose trace amounts of platinum or other platinum group metals under certain conditions, such as high temperature operation. The trace amounts of platinum group metals from the DOC sublime and then accumulate on the SCR catalyst positioned downstream from the DOC such that the function of the SCR catalyst is inhibited. This is due to the high activity of platinum group metals for ammonia oxidation which results in little or no ammonia being available for the SCR reaction. In addition, because platinum group metal catalysts generate NO x  and N 2 O from ammonia, an increase in NO x  levels could actually result when the SCR is inhibited due to platinum group metal poisoning. It has been discovered that platinum levels as low as 0.0005 wt % can have a significant impact on SCR performance. See, Jagner et al., “Detection, Origin, and Effect of Ultra-Low Platinum Contamination on Diesel-SCR Catalysts,” SAE International Paper No. 2008-01-2488, (2008). 
         [0007]    Accordingly, there is a need in the art for a diesel engine exhaust system which prevents the deposition of platinum group metals on an SCR positioned downstream from a diesel oxidation catalyst, and to a system which efficiently achieves conversion of combustion components and removal of particulates in the exhaust gas. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the invention meet those needs by providing a diesel engine exhaust treatment system and method which includes a platinum group metal trap positioned downstream from a diesel oxidation catalyst. The platinum group metal trap functions to trap trace amounts of platinum group metals released from the diesel oxidation catalyst so as to prevent deposition of the metals on an SCR catalyst positioned downstream from the diesel oxidation catalyst. 
         [0009]    According to one aspect, a diesel exhaust gas treatment system is provided which comprises a diesel oxidation catalyst comprising a platinum group metal positioned in an exhaust stream, a platinum group metal trap positioned downstream from the diesel oxidation catalyst which traps platinum group metals released from the diesel oxidation catalyst, and a selective reduction catalyst (SCR) positioned downstream from the platinum group metal trap. 
         [0010]    The diesel oxidation catalyst preferably comprises a combination of platinum and palladium. The SCR catalyst preferably comprises zeolite and a base metal selected from copper and iron. 
         [0011]    In one embodiment, the platinum group metal trap comprises cerium oxide. In another embodiment, the platinum group metal trap comprises a perovskite material. The perovskite material preferably comprises CaTiO 3 . 
         [0012]    The treatment system preferably further includes a diesel particulate filter which may be positioned downstream from the SCR catalyst, downstream from the platinum group metal trap, or upstream from the platinum group metal trap. 
         [0013]    In another embodiment, the treatment system further includes a lean NO x  trap which may be positioned either upstream or downstream from the platinum group metal trap. 
         [0014]    The treatment system preferably further includes a reductant delivery system for providing a source of ammonia or urea to the exhaust stream. The reductant delivery system is preferably positioned directly upstream from the SCR catalyst. 
         [0015]    In another embodiment, a method for treating diesel engine exhaust gases is provided in which exhaust gases are passed through the exhaust gas treatment system containing the platinum group metal trap between a diesel oxidation catalyst and SCR catalyst, such that the platinum group metal trap reduces platinum group metal contamination on the SCR catalyst in comparison with an exhaust system which does not include a platinum group metal trap. The method preferably includes providing a source of ammonia or urea to the exhaust stream. 
         [0016]    Accordingly, it is a feature of embodiments of the present invention to provide a diesel exhaust gas treatment system and method which utilizes a platinum group metal trap to trap platinum group metals released from a diesel oxidation catalyst to prevent contamination of an SCR catalyst. 
         [0017]    Other features and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic illustration of a diesel engine exhaust treatment system in accordance with an embodiment of the invention; 
           [0019]      FIG. 2  is a schematic illustration of another embodiment of the invention including a lean NO x  trap and a diesel particulate filter; 
           [0020]      FIG. 3  is a schematic illustration of another embodiment of the invention; 
           [0021]      FIG. 4  is a schematic illustration of another embodiment of the invention; 
           [0022]      FIG. 5  is a schematic illustration of another embodiment of the invention; and 
           [0023]      FIG. 6  is a schematic illustration of another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    In a preferred embodiment, an exhaust treatment system is provided which includes a platinum group metal trap positioned between a diesel oxidation catalyst and an SCR catalyst. The system reduces the likelihood of contamination of the SCR catalyst so that its performance is not inhibited. Thus, the system including the platinum group metal trap reduces platinum group metal contamination when compared to an exhaust treatment system which does not include the trap, while still providing efficient removal of NO x . 
         [0025]    The platinum group metal trapping device may comprise a monolith washcoated with cerium-containing oxides at a loading of about 30 to 300 g/L. While not wishing to be limited by any specific theory of operation, it is believed that the strong interaction of cerium oxide with platinum facilitates the trapping of platinum group metals sublimed/released from the upstream DOC. 
         [0026]    Alternatively, the platinum group metal trapping device may comprise a Perovskite material having the formula ABO 3 , where A-B includes, but is not limited to, Ca +2 —Ti +4 , Ba +2 —Ti +4 , Ln +3 —Co +3 , or La +3 —Fe +3 . Perovskite materials with relatively high surface areas may be prepared by a sol-gel method using the corresponding metal alkoxides. The perovskite material preferably comprises CaTiO 3 , which may be prepared by combining calcium nitrate (Ca(NO 3 ) 2 ) and tetraisopropyl titanate (Ti(i-C 3 H 7 O) 4  at a ratio of Ca/Ti of 1:1. For example, the calcium nitrate and tetraisopropyl titanate may be dissolved in 2-methoxy ethanol, and a small amount of nitric acid may be added to form a precursor solution which is then dried, pyrolyzed at about 400° C., and then heated in air at about 600 to 800° C. 
         [0027]    Alternatively, CaTiO 3  may be prepared by combining (Ca(NO 3 ) 2 ) with ethyl alcohol, water, and HNO 3  to form a solution. This solution is added dropwise into a solution of (Ti(i-C 3 H 7 O) 4  and ethyl alcohol (at a ratio of Ca/Ti of 1:1). The solution may be evaporated to a dry powder at 200° C. and then calcined at about 600° C. These methods produce a CaTiO 3  powder having a particle size of about 22 to 50 nm. 
         [0028]    The CaTiO 3  perovskite material may be washcoated on a honeycomb substrate using a slurry solution containing the fine perovskite powder. The slurry solution may optionally contain one or more of alumina, zirconia (ZrO 2 ), ceria (CeO 2 ), and Ce—Zr mixed oxide (Ce x Zr 1-x O 2 ). The washcoat loading may vary from about 50 to about 350 g/L. 
         [0029]    The CaTiO 3  washcoated substrate functions to trap sublimed precious metals which may be released from the diesel oxidation catalyst. Again, while not wishing to be limited by any specific theory of operation, it is believed that the trace amounts of platinum group metals released from the DOC become trapped in the lattice structure of the material. 
         [0030]    Referring now to  FIG. 1 , one embodiment of the diesel exhaust treatment system  10  is illustrated. As shown in its simplest form, the exhaust treatment system is coupled to an exhaust manifold  12  of a diesel engine and includes a diesel oxidation catalyst  14  which is positioned upstream from an SCR catalyst  18 . The system includes a platinum group metal trap  16  between the DOC and SCR catalyst. 
         [0031]    The diesel oxidation catalyst  14  is coated on a refractory inorganic oxide or ceramic honeycomb substrate as a washcoat at a loading of from about 30 to about 300 g/L and utilizes a catalyst material selected from platinum, palladium, or a combination thereof, and may also contain zeolites. The washcoat may further comprise a binder such as alumina, silica, titania, or zirconia. 
         [0032]    The SCR catalyst  18  comprises a zeolite and a base metal selected from copper and iron. The SCR catalyst washcoat is coated at a loading of from about 30 to about 300 g/L and may be prepared by coating a porous inert substrate with a slurry containing a base metal, zeolite, and binder material such as alumina, silica, titania or zirconia. Alternatively, the base metal/zeolite may be combined with ceramic binders/fibers and extruded into a monolith. 
         [0033]    Referring again to  FIG. 1 , the treatment system preferably further includes a reductant delivery system  20  which is coupled to the exhaust manifold directly upstream from the SCR catalyst  18 . A reductant, such as ammonia, aqueous urea or other ammonia-generating compounds, is stored in a storage vessel (not shown) and delivered to the reductant delivery system in metered amounts, typically in the form of a vaporized mixture of the reductant and water. The reductant delivery system includes an injector  22  for injecting an appropriate amount of reductant into the exhaust stream at the appropriate time. 
         [0034]    In another embodiment of the invention illustrated in  FIG. 2 , the exhaust treatment system may further optionally include a lean NO x  trap (LNT)  26  positioned downstream from the DOC  14  to aid in further reduction of NO x  in the exhaust gas. The LNT  26  comprises one or more platinum group metals, and preferably comprises a mixture of platinum, palladium, and rhodium. Preferably, the lean NO x  trap has a precious metal loading of from between about 30 to about 300 g/ft 3 . The lean NO x  trap further comprises a NO x  adsorbent material selected from one or more alkali or alkaline earth metals. 
         [0035]    It should be appreciated that in embodiments which include a lean NO x  trap, the use of the reductant delivery system is optional as the lean NO x  trap generates ammonia during its operation. 
         [0036]    Also as shown in  FIG. 2 , the exhaust treatment system may further include a diesel particulate filter  28  downstream from the SCR catalyst for collecting soot and particulate matter. The diesel particulate filter is preferably a wall flow filter comprising a highly porous filter substrate having a porosity of from about 30 to 80%. The size of the pores preferably range from about 10 to 50 μm. It should be appreciated that the distribution of pore sizes may vary throughout the filter substrate. 
         [0037]    Suitable filter substrates include refractory inorganic oxides or ceramic or metal materials, such as cordierite, mullite, silicon carbide, aluminum titanate, alpha-alumina, silica, and alkali and alkaline earth zirconium phosphates (NZP). 
         [0038]    In an alternative embodiment illustrated in  FIG. 3 , the positions of the diesel particulate filter  28  and LNT  26  may be configured such that the DPF  28  is positioned directly downstream from DOC catalyst  14 , and LNT  26  is positioned directly downstream from diesel particulate filter  28 . In yet another alternative embodiment illustrated in  FIG. 4 , the positions of the LNT and DPF are reversed. 
         [0039]    In yet another embodiment illustrated in  FIG. 5 , the system includes a diesel particulate filter  28  which is positioned between the platinum group metal trap  16  and the SCR catalyst  18 . 
         [0040]    In yet another embodiment illustrated in  FIG. 6 , the diesel particulate filter  28  is positioned between the DOC  14  and the trapping device  16 , and the LNT  26  is positioned between the trapping device  16  and the SCR catalyst  18 . 
         [0041]    Referring again to  FIG. 1 , during operation, as exhaust gas generated by a diesel engine (not shown) passes through the exhaust gas manifold  12 , it passes through the diesel oxidation catalyst  14  such that conversion of uncombusted HC and CO to H 2 O and CO 2  occurs. The exhaust gas then flows through the platinum group metal trapping device followed by the SCR catalyst. The exhaust gas then flows toward an exhaust gas outlet (not shown). 
         [0042]    As the gas passes through the SCR catalyst  18 , the catalyst removes NO x  from the gas stream by selective catalytic reduction with a source of ammonia supplied from the reductant delivery system  20 . Typically, the reductant delivery system  20  utilizes a liquid urea/water solution which is injected downstream from the DOC catalyst  14  at metered intervals. The injected liquid urea/water mixture vaporizes and hydrolyzes to form ammonia. Thus, the NO x  component in the gas is converted with selective catalytic reduction of NO x  with ammonia to form nitrogen. 
         [0043]    Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention.