Abstract:
The present invention relates to a method of reducing coking over a Ag/Al 2 O 3  hydrocarbon selective catalytic reduction catalyst in an exhaust stream of a lean burn internal combustion engine, the exhaust stream including hydrocarbon and NO x . The method includes controlling the hydrocarbon to molar NO x  ratio of the exhaust stream so as to be within certain limits at certain temperatures. An exhaust system for a lean burn engine suited to the above method, and a lean burn internal combustion engine, or a vehicle or stationary power source including such an exhaust system are also included.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase Application of PCT International Application No. PCT/GB2006/050376, filed Nov. 9, 2006, and claims priority of British Patent Application No. 0523135.2, filed Nov. 14, 2005. 
     FIELD OF THE INVENTION 
     This invention relates to a method of reducing coking of Ag/Al 2 O 3  hydrocarbon selective catalytic reduction (HC-SCR) catalysts in a lean burn engine exhaust gas stream. 
     BACKGROUND OF THE INVENTION 
     HC-SCRs are sometimes also referred to in the literature as non-selective catalytic reduction (NSCR) catalysts, lean NO x  catalysts (LNC), lean NO x  reduction catalysts, “DeNO x  catalysts.” and NO x  occluding catalysts. 
     In hydrocarbon selective catalytic reduction, hydrocarbons (HC) react with nitrogen oxides (NO x ), rather than oxygen (O 2 ), to form nitrogen (N 2 ), carbon dioxide (CO 2 ) and water (H 2 O) according to Reaction (1):
 
{HC}+NO x →N 2 +CO 2 +H 2 O  (1)
 
     The competitive, non-selective reaction with oxygen is given by Reaction (2):
 
{HC}+NO x →N 2 +CO 2 +H 2 O  (2)
 
     The most effective HC-SCR catalysts used to selectively promote the desired reaction (1) are Pt/Al 2 O 3 , Cu exchanged ZSM-5 and Ag/Al 2 O 3 . Ag/Al 2 O 3  catalysts operate at higher temperatures and over a broad temperature range, and have recently shown promise in vehicle testing (Klingstedt et al., Topics in Catalysis, 30/31, 2004, 27 and Lindfors et al., Topics in Catalysis, 28, 2004, 185, the entire contents of which are incorporated herein by reference). 
     All of these catalysts exhibit high activity for the selective reduction of NO x  by hydrocarbons, including long chain alkane and diesel fuel, but each type of catalyst suffers from some form of limitation in use. Pt/Al 2 O 3  catalysts display lower NO x  conversion and lower selectivity towards nitrogen; N 2 O (conversion &gt;60%) is a basic product. Additionally, the HC-SCR activity window of Pt/Al 2 O 3  catalysts is limited to low temperatures (about 150-250° C.). Generally speaking, Cu/ZSM-5 catalysts can suffer from thermal deactivation due to copper sintering and dealumination of the zeolite. Ag/Al 2 O 3  catalysts are tolerant to hydrothermal aging, but can suffer from chemical deactivation caused by coking or sulphation. We understand that the relatively poor performance of Pt/Al 2 O 3  catalysts and the relatively poor activity of Cu/ZSM-5 and Ag/Al 2 O 3  HC-SCR catalysts once aged, has so far been insufficient to allow for their widespread implementation (Konig et al., Topics in Catalysis, 28, 2004, 99, incorporated herein by reference). 
     Coking is not a significant factor in the activity of any HC-SCR catalyst at higher temperatures since above approximately 400° C. any carbon present will be burnt to form CO 2  thereby leaving the catalyst surface available for reactions to take place thereon. As such it is important to differentiate between the absolute activity of any particular HC-SCR catalyst, and the reduction in activity that coking may result in. An increase in the absolute activity of any particular HC-SCR catalyst will not necessarily be the result of a concomitant reduction in coking. 
     Nonetheless, coking does have a significant effect upon the HC-SCR activity of Ag/Al 2 O 3  catalysts at lower temperatures and therefore a means was devised to limit access of the hydrocarbon species responsible for coke deposition to the catalyst to minimise coking deactivation. Said means involved modifying the Ag/Al 2 O 3  catalyst formulation, and is disclosed in WO 2005/016496 (incorporated herein by reference). Specifically, it was disclosed that by combining known HC-SCR catalysts with a partial oxidation catalyst (POC), it was possible to suppress or avoid low temperature coke formation. In the WO 2005/016496 invention, the POC helps to prevent coking by promoting the partial oxidation of hydrocarbons in the exhaust gas of a lean-burn internal combustion engine to carbon monoxide (CO), hydrogen gas (H 2 ) and partially oxygenated hydrocarbon species. Therefore the heavy hydrocarbon species present in the exhaust gas are partially oxidised to smaller, more reactive species prior to contacting the downstream HC-SCR catalyst. 
     SUMMARY OF THE INVENTION 
     We have now discovered a new method for reducing coking over a Ag/Al 2 O 3  HC-SCR catalyst without needing to modify catalyst formation by optimising the hydrocarbon to molar NO x  ratio (HC:NO x  as Cl) of the exhaust gas to achieve high NO x  conversion for Ag/Al 2 O 3  HC-SCR catalysts across a wide temperature range. 
     According to a first aspect, the invention provides a method of reducing coking over a Ag/Al 2 O 3  hydrocarbon selective catalytic reduction (HC-SCR) catalyst in an exhaust stream of a lean burn internal combustion engine, which exhaust stream comprising hydrocarbon and NO x , which method comprising controlling the hydrocarbon to molar NO x  ratio of the exhaust stream so as to be less than or equal to 2.0 when the exhaust stream temperature is less than or equal to 300° C., between 2.0 and 4.5 when the exhaust stream temperature is from 300° C. to 425° C., and 4.5 or greater when the exhaust stream temperature is greater than or equal to 425° C. The temperature of the exhaust stream relevant to this invention is the temperature of the exhaust stream as it impacts the HC-SCR catalyst. 
     According to a second aspect, this invention provides an exhaust system for a lean burn engine, which system comprises a Ag/Al 2 O 3  HC-SCR catalyst and means, when in use, for controlling the HC:NO x  ratio of the exhaust gas so as to be less than or equal to 2.0 when the exhaust stream temperature is less than or equal to 300° C., between 2.0 and 4.5 when the exhaust stream temperature is from 300° C. to 425° C., and 4.5 or greater when the exhaust stream temperature is greater than or equal to 425° C. 
     According to further aspects, the invention provides a lean burn internal combustion engine including an exhaust system according to the invention, and a vehicle or stationary power source including such a lean burn engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more fully understood, the Examples below are provided by way of illustration only and with reference to the accompanying drawings, in which: 
         FIG. 1  shows how NO x  conversion activity at 250° C. varies with time for different fuels; 
         FIG. 2  shows how the steady state NO x  conversion activity varies with temperature for both GTL and US06 fuels at hydrocarbon to NO x  ratios of 2.5 and 4.5; 
         FIG. 3  shows how NO x  conversion activity at 300° C. varies with time for both GTL and US06 fuels at hydrocarbon to NO x  ratios of 2.5 and 4.5; 
         FIG. 4  shows how the steady state NO x  conversion activity varies with temperature for both GTL and US06 fuels at optimised hydrocarbon to NO x  ratios; 
         FIG. 5  shows how the steady state NO x  conversion activity of US06 fuel (at optimised hydrocarbon to NO x  ratios) varies with temperature with either 300 ppm or 600 ppm H 2  present in the gaseous mixture; and 
         FIG. 6  shows how the steady state NO x  conversion activity of US06 fuel (at optimised hydrocarbon to NO x  ratios) varies with temperature with 300 ppm H 2  present in the gaseous mixture at low levels of NO x . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the course of our research we discovered that the selective catalytic reduction of NO x  using hydrocarbons by Ag/Al 2 O 3  catalysts varies depending upon the hydrocarbon used (see  FIG. 1 ). Short chain alkanes, e.g. octane and decane, show poor NO x  conversion whilst long chain alkanes show good initial activity although this decreases with time due to coking, as explained above. 
     This connection between the hydrocarbon being used in the selective catalytic reduction of NO x  and activity of Ag/Al 2 O 3  catalysts led us to investigate how, in addition to varying the HC:NO x  ratio in response to temperature, we might vary the HC:NO x  ratio depending upon the hydrocarbon being used. As a result we found that by varying the HC:NO x  ratio depending upon the aromatics content of the hydrocarbon being used we were able to maintain good high temperature activity and minimise low temperature deactivation through coking. Therefore, in one embodiment the hydrocarbon has a relatively low aromatics species content, e.g. comprising from 0 to 10% aromatics. When this method is applied to hydrocarbon with a relatively low aromatics species content, the HC:NO x  is controlled so as to be between 0.5 and 2.0 when the exhaust gas temperature is less than or equal to 300° C. and between 4.5 and 7.0 when the exhaust gas temperature is greater than or equal to 425° C. In an alternative embodiment the hydrocarbon species has a relatively high aromatics species content, e.g. comprising from 10 to 40% aromatics. When this method is applied to hydrocarbon with a relatively high aromatics species content, the HC:NO x  is controlled so as to be between 1.0 and 2.0 when the exhaust gas temperature is less than or equal to 300° C. and between 4.5 and 5.0 when the exhaust gas temperature is greater than or equal to 425° C. 
     “Aromatics” as defined herein include, but are not limited to, the following species: toluene, ethylbenzene, xylenes, polaromatics, 1-methylnaphthalene, n-pentylbenzene, biphenyl, 1-butylnaphthalene, n-nonylbenzene, 2-octylnaphthalene and n-tetradecylbenzene. 
     Controlling the HC:NO x  ratio of the exhaust stream can be achieved by either varying the amount of hydrocarbon present in response to the amount of NO x  present, or by varying the amount of NO x  present in response to the amount of hydrocarbon present. This can be achieved by monitoring the levels of NOx or hydrocarbon present in the exhaust stream, or predicting the levels of NO x  or hydrocarbon present in the exhaust stream. Such levels may be predicted by referring to the NO x  or hydrocarbon levels known to be present during certain engine conditions. 
     The hydrocarbon for use in the present invention may be injected into the engine exhaust stream upstream of the HC-SCR catalyst, or it may be produced by cracking engine fuel. Engine fuel may be cracked in the cylinder of the engine or in the exhaust gas stream. For further details of ways to crack engine fuel see patent application no. PCT/GB2006/002595 (incorporated herein by reference). 
     Exhaust Gas Recirculation or fuel combustion techniques, such as HCCI during a relevant portion of the engine speed/load map, may be used to alter the supply of NO x  to the HC-SCR catalyst. 
     Suitable engine fuels include those that have relatively high aromatics hydrocarbon species content such as diesel (including US06, an Ultra Low Sulphur Diesel-ULSD for implementation in 2007) and gasoline, and those that have relatively low aromatics hydrocarbon species content such as FT-GTL (Fischer-Tropsch gas to liquids) and biodiesel. 
     One characteristic of Ag/Al 2 O 3  catalysts is the ability of hydrogen to promote its HC-SCR activity. It has been reported in the literature (Satokawa, Chem. Lett., 2000, 294 and Satokawa et. al., Appl. Cat. B, 42, 2003, 179 and Shibata et. al., Phys. Chem., 5, 2003, 2154, the entire contents of which are incorporated herein by reference) that the role of hydrogen is to activate the Ag 2 O clusters present on the alumina or to modify the hydrocarbon species (e.g. by oxygenation) to generate more active species for NO x  reduction. In one embodiment of the present invention the exhaust gas stream also comprises hydrogen. We have found that hydrogen can have an advantageous effect on HC-SCR activity at relatively low levels, e.g. less than 1000 ppm, optionally less than or equal to 600 ppm. Whilst it is possible to increase hydrogen content in an exhaust gas by combusting hydrocarbon fuel, e.g. injected into the exhaust gas upstream of the HC-SCR, or by engine calibration, such an increase in hydrogen is generally accompanied by an increase in hydrocarbon as well. In a particular embodiment, relatively low levels of hydrogen can be introduced into the exhaust gas without simultaneously increasing hydrocarbon content of the exhaust gas by contacting a reforming catalyst with hydrocarbon. 
     In practice, the HC:NO x  can be controlled by adjusting the HC concentration in the exhaust gas, by adjusting the NO x  concentration in the exhaust gas, or both. In one embodiment, the exhaust system comprises means, when in use, for controlling the supply of the hydrocarbon. Such means can comprise means for injecting hydrocarbon into exhaust gas upstream of the HC-SCR catalyst or for adjusting the timing of fuel injection into one or more engine cylinders. In another embodiment, the exhaust system comprises means, when in use, for controlling the supply of NO x . NO x  control means can comprise Exhaust Gas Recirculation or fuel combustion techniques such as HCCI during a relevant portion of the engine speed/load map. In either of these two embodiments, the control means may include a pre-programmed electronic control unit. 
     In another embodiment, the exhaust system comprises means for increasing the amount of hydrogen present in the exhaust gas stream. Such means can include a fuel reformer, based on platinum group metals or nickel (see Trimm et al., Catalysis Reviews—Science and Engineering, 43, 2001, 31-84, incorporated herein by reference). 
     As in the method detailed above, the hydrocarbon species may be injected into the engine exhaust gas stream upstream of the HC-SCR catalyst, or it may be produced by cracking engine fuel, thereby to produce shorter chain hydrocarbons. If the hydrocarbon species is produced by cracking engine fuel, this cracking may be done in the combustion cylinder of the engine or in the exhaust gas stream. Therefore the exhaust system may comprise means for injecting hydrocarbon species into the exhaust gas stream or means for cracking engine fuel in either the combustion cylinder of the engine or in the exhaust gas stream. 
     EXAMPLES 
     Example 1 
     Catalyst Formation 
     2 wt % Ag/Al 2 O 3  catalysts were prepared by wet impregnation. Silver nitrate was dissolved in the appropriate amount of water according to the pore volume of the alumina support. The nitrate solution was then added to the support with mixing. After drying overnight, the sample was calcined in air at 500° C. for 2 hours. 
     Example 2 
     Testing Conditions 
     The HC-SCR activity of the catalysts was measured by flowing diesel type fuels (US06 or GTL) in a gaseous mixture (NO 500 ppm, hydrocarbon (Cl equivalent) 2250 ppm, CO 240 ppm, O 2  12%, H 2 O 5%, CO 2  4.6%, balance N 2  at a total flow rate of 3 L/min) over 0.6 g of catalyst. 
     NO x  conversions were typically measured after 10 mins at constant catalyst inlet temperature starting from 200° C. and increasing the temperature in 50° C. intervals to 500° C. We refer to this as steady-state NO x  conversion. 
     Example 3 
     Effect of Hydrocarbon Used on the Reactivity of 2 Wt % Ag/Al 2 O 3    
       FIG. 1  shows the steady state NO x  concentration for hydrocarbon to NO x  ratio of 4.5 for n-octane, n-decane, n-dodecane, GTL and US06 at 250° C. as a function of time. Octane and decane show poor NO x  conversion whilst dodecane shows good initial activity although its high reactivity, relative to GTL and US06, also promotes coke deposition therefore leading to a rapid decrease in reactivity. 
     Example 4 
     Effect of HC:NO x  on the Reactivity of 2 Wt % Ag/Al 2 O 3    
     The steady state NO x  conversion activity for 2 wt % Ag/Al 2 O 3  of Example 1 was measured for hydrocarbon to NO x  ratios of 2.5 and 4.5 using GTL and US06 fuel as the source of the hydrocarbon species. 
       FIG. 2  shows that the NO x  conversion activity is generally higher for GTL than for US06 and that NO x  conversion is more effective using the lower HC:NO x  at lower temperatures. The difference in performance between GTL and US06 may be partly due to the higher cetane number of GTL relative to US06, see Table 1. Additionally, we believe the improvement in NO x  conversion is due to deactivation through coking occurring at temperatures of less than 400° C. (the 50% distillation temperature of both fuels is approx. 300° C., see Table 1) and less hydrocarbon species being present results in less coking. However, at higher temperatures the higher HC:NO x  is more effective for NO x  conversion. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Diesel fuel properties 
               
             
          
           
               
                 Fuel Analysis 
                 Method 
                 US06 
                 GTL 
               
               
                   
               
             
          
           
               
                 Cetane Number 
                 ASTM D613 
                 53.9 
                 79 
               
               
                 Density at 15° C. (kg m −3 ) 
                 ASTM D4052 
                 827.1 
                 784.6 
               
               
                 Viscosity at 40° C. (cSt) 
                 ASTM D445 
                 2.467 
                 3.497 
               
               
                 50% distillation (° C.) 
                 ASTM D86 
                 264 
                 295.2 
               
               
                 90% distillation (° C.) 
                 ASTM D86 
                 329 
                 342.1 
               
               
                 LCV (MJ kg −1 ) 
                   
                 42.7 
                 43.9 
               
               
                 Sulphur (mg kg −1 ) 
                 ASTM D2622 
                 46 
                 0.05 
               
               
                 AROMATICS (wt %) 
                   
                 24.4 
                 0.3 
               
               
                 C (% wt) 
                   
                 86.5 
                 85 
               
               
                 H (% wt) 
                   
                 13.5 
                 15 
               
               
                 O (% wt) 
                   
                 — 
                 — 
               
               
                 H/C Ratio (Molar) 
                   
                 1.88 
                 2.10 
               
               
                   
               
             
          
         
       
     
       FIG. 3  shows the steady state NO x  concentration for hydrocarbon to NO x  ratios of 2.5 and 4.5 for GTL and US06 at 300° C. as a function of time. Although initial NO x  conversion was lower when the HC:NO x  ratio was reduced from 4.5 to 2.5 the rate of catalyst deactivation was also lower at the low HC:NO x , such that after 30 minutes the NO x  conversion was higher for the HC:NO x =2.5 samples. 
       FIG. 4  shows the NO x  conversion activity for optimised hydrocarbon to NO x  ratios for US06 and GTL fuels. (By optimised we mean that the HC:NO x  was increased, in accordance with the invention, as the temperature of the exhaust stream increased.) The variable ratios for the two sources of hydrocarbon are also given, see right hand axis. These results show that optimisation of HC:NO x  improves the HC-SCR activity across the whole temperature window. 
     Example 5 
     Effect of Hydrogen on the Reactivity of 2 Wt % Ag/Al 2 O 3    
     The steady state NO x  conversion activity for 2 wt % Ag/Al 2 O 3  of Example 1 with either 300 ppm or 600 ppm H 2  present in the gaseous mixture was measured for optimised hydrocarbon to NO x  ratios using US06 fuel as the source of the hydrocarbon species. 
       FIG. 5  shows significant improvements in the HC-SCR catalyst activity results from the addition of H 2  with the activity window broadening towards lower temperatures, even at low levels of H 2  addition. 
     Similar results were obtained using GTL fuel as the source of the hydrocarbon species. 
     Example 6 
     Effect of NO x  Levels on the Reactivity of 2 Wt % A Ag/Al 2 O 3    
     The 300 ppm H 2  test from Example 5 was repeated using a gas mixture containing 200 ppm NO x , and with the amount of US06 present adjusted to maintain optimised hydrocarbon to NO x  ratios. 
       FIG. 6  shows that the HC-SCR catalyst is highly effective at low levels of NO x , as well as at higher levels of NO x .