Abstract:
A power generation system and method of operating a power generation system which comprises a diesel engine which produces lean exhaust which passed to an exhaust after treatment system comprising an exhaust line which is in communication with a controller, reformer, lean NOx trap and SCR has been taught wherein the reformer comprises oxidation and reforming catalyst is a composition which comprises a catalyst wash coat comprising a ZrO 2  refractory metal oxide support, a Ln x O y  distributed on the surface of the refractory metal oxide in an amount to form a monolayer over the refractory metal oxide support wherein Ln is selected from the group consisting of La, Nd and mixtures thereof and Rh distributed over the catalyst surface in an effective amount to catalyze steam reforming at 650° C.

Description:
FIELD OF THE INVENTION 
     The present invention relates to diesel power generation systems with exhaust aftertreatment. 
     BACKGROUND 
     NO x  emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NO x  emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. 
     In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NO X  emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective. 
     Several solutions have been proposed for controlling NO X  emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NO X  emissions. Another set of approaches remove NO X  from the vehicle exhaust. These include the use of lean-burn NO X  catalysts, selective catalytic reduction (SCR) catalysts, and lean NO X  traps (LNTs). 
     Lean-burn NO X  catalysts promote the reduction of NO x  under oxygen-rich conditions. Reduction of NO X  in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NO x  catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NO X  reduction, introducing a fuel economy penalty of 3% or more. Currently, peak NO X  conversion efficiencies for lean-burn NO X  catalysts are unacceptably low. 
     SCR generally refers to selective catalytic reduction of NO X  by ammonia. The reaction takes place even in an oxidizing environment. The NO X  can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NO X  reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment. 
     LNTs are devices that adsorb NO X  under lean exhaust conditions and reduce and release the adsorbed NO X  under rich conditions. An LNT generally includes a NO X  adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO 3  and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NO X  adsorption. In a reducing environment, the catalyst activates reactions by which hydrocarbon reductants are converted to more active species, the water-gas shift reaction, which produces more active hydrogen from less active CO, and reactions by which adsorbed NO X  is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT. 
     Regeneration to remove accumulated NOx may be referred to as denitration in order to distinguish desulfation, which is carried out much less frequently. The reducing environment for denitration can be created in several ways. One approach uses the engine to create a rich exhaust-reductant mixture. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A reducing environment can also be created by injecting a reductant into lean exhaust downstream from the engine. In either case, a portion of the reductant is generally expended to consume excess oxygen in the exhaust. To lessen the amount of excess oxygen and reduce the amount of reductant expended consuming excess oxygen, the engine may be throttled, although such throttling may have an adverse effect on the performance of some engines. 
     Reductant can consume excess oxygen by either combustion or reforming reactions. Typically, the reactions take place upstream of the LNT over an oxidation catalyst or in a fuel reformer. The reductant can also be oxidized directly in the LNT, but this tends to result in faster thermal aging. U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the &#39;037 publication”) describes an exhaust system with a fuel reformer placed in an exhaust line upstream from an LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate. 
     The oxidation and reforming catalysts of the &#39;037 publication are subject to harsh conditions. According the &#39;037 publication, it is desirable to heat the fuel reformer to steam reforming temperatures for each LNT regeneration and to pulse the fuel injection during regeneration to prevent the fuel reformer from overheating. Pulsing causes the catalyst to alternate between lean and rich conditions while at high temperature. The catalyst itself tends to undergo chemical changes through this cycling, which can lead to physical changes, especially sintering, which is the growth of catalyst particles. As the particles grow, their surface area and number of surface atoms decrease, resulting in a less active catalyst. 
     Numerous choices are available for the oxidation and reforming catalysts, With regard to the oxidation catalyst, the &#39;037 patent lists Pd, Pt, Ir, Rh, Cu, Co, Fe, Ni, Ir, Cr, and Mo as possible choices, without limitation. The catalyst support is also important. The &#39;037 patent lists as examples, without limitation, cerium zirconium oxide mixtures or solid solutions, silica alumina, Ca, Ba, Si, or La stabilized alumina. Many other oxidation catalysts, supports, and stabilizers are known in the art. Likewise, many examples or reforming catalysts are known. The &#39;037 patent list Ni, Rh, Pd, and Pt as possible reforming catalysts, without limitation. As with the oxidation catalyst, a wide range of supports and stabilizers could be considered for use. 
     In spite of advances, there continues to be a long felt need for an affordable and reliable diesel exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel penalty), and reduces NO X  emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations. 
     SUMMARY 
     After considerable research, the inventors have developed oxidation and reforming catalysts for use in diesel exhaust aftertreatment systems. The catalysts are economical and superior in terms of durability under lean-rich cycling at high temperatures. The catalysts comprise precious metals supported on La stabilized refractory metal oxides. The La is distributed on the surface of the refractory metal oxide support in an amount to form at least about a monolayer, preferably about 1-2 monolayers. Preferably, the La is substantially amorphous in the sense of having no crystalline structure shown by X-ray diffraction. Nd and mixtures of La and Nd can be used in place of La. The La is typically in an oxide form and the precious metal may be either reduced or in oxide form. 
     In one embodiment, the catalyst is a reforming catalyst comprising an effective amount of Rh on a ZrO 2  support. The catalyst preferably comprises from about 0.5 to about 1.0 mg La per m 2  refractory metal oxide surface distributed over the surface. For a typical ZrO 2  support that has a surface area of about 100 m 2 /g, this gives from about 5 to about 10% La by weight refractory metal oxide. The catalyst preferably also comprises from about 0.01 to about 0.1 mg Rh per m 2  refractory metal oxide surface area. The Rh is distributed on the surface of the refractory metal oxide particles along with or over the La. For the typical ZrO 2  support, this loading gives from about 0.1 to about 1.0% Rh by weight refractory metal oxide. The Rh is present in an amount effective to catalyze steam reforming of diesel fuel at 650° C. Preferably, the Rh has an average particle size of under 5 nm and the catalyst is functional to maintain the Rh particle size under 5 nm through 400 25 minute lean/25 minute rich lean/rich cycles at 750° C. Preferably, the Rh has a dispersion of at least about 40% and the catalyst is functional to maintain a dispersion of at least about 40% through 400 25 minute lean/25 minute rich lean/rich cycles at 750° C. Preferably, the catalyst comprises little or no platinum. 
     According to a further aspect of the invention, the Rh is provided in a relatively low concentration: from about 0.01 to about 0.05 mg per m 2  refractory metal oxide surface area, which corresponds to about 0.1 to about 0.5% Rh by weight refractory metal oxide for the typical ZrO 2  support. The inventors have found that if the Rh loading is kept sufficiently low, the Rh can be maintained in the form of small particles (less than 5 nm, typically about 2 nm or less) while the catalyst undergoes lean-rich cycling through an effect involving the La. The improvement in stability is such that as the Rh loading is reduced from about 0.10 mg/m 2  to about 0.05 mg/m 2  or less, nearly the same or greater catalyst activity results after aging than is achieved with the larger Rh loading. 
     In another embodiment, the catalyst is an oxidation catalyst comprising an effective amount of Pd on an Al 2 O 3  refractory metal oxide support. The catalyst preferably comprises from about 0.5 to about 1.0 mg La per m 2  refractory metal oxide distributed over the surface of the refractory metal oxide particles. For a typical Al 2 O 3  refractory metal oxide support that has a surface area of about 200 m 2 /g, this corresponds to from about 10 to about 20% La by weight refractory metal oxide. The catalyst preferably also comprises from about 0.25 to about 1.0 mg Pd per m 2  refractory metal oxide surface area, which corresponds to from about 5 to about 20% Pd by weight refractory metal oxide for the typical Al 2 O 3  refractory metal oxide support. The Pd is present in an amount effective for the oxidation catalyst to light off at 275° C., more preferably at 240° C. Preferably, the Pd has an average particle size of under 10 nm and is functional to maintain a particle size under 10 nm through 400 hours of 25 minute lean/25 minute rich lean/rich aging at 750° C. Preferably, the Pd has a dispersion of at least about 15% and the catalyst is functional to maintain a dispersion of at least about 15% through 400 hours of aging in a lean atmosphere comprising 10% steam at 750° C. 
     A further aspect of the invention relates to a method of operating a power generation system comprising operating a diesel engine to produce lean exhaust and passing the exhaust through a fuel reformer and a lean NO X  trap in that order, whereby a portion of the NO X  in the exhaust is absorbed by the lean NO X  trap. From time-to-time, a control signal to regenerate the lean NO X  trap is produced. In response to the control signal, diesel fuel is injected into the exhaust upstream from the fuel reformer at a rate that makes the exhaust-fuel mixture overall lean, whereby the injected fuel combusts within the fuel reformer raising the temperature of the fuel reformer. After the fuel reformer has heated to at least about 500° C., a rich phase is initiated by increasing the fuel injection rate and/or lowering the exhaust oxygen flow rate to cause the exhaust-injected fuel mixture to become overall rich, whereby the fuel reformer produces reformate that regenerates the lean NO X  trap. The fuel reformer comprises oxidation and reforming catalysts. The reforming catalyst comprises a catalyst washcoat comprising a ZrO 2  refractory metal oxide support, a Ln X O Y  distributed on the surface of the refractory metal oxide in an amount at least sufficient to form about a monolayer over the refractory metal oxide support, wherein Ln is selected from the group consisting of La, Nd, and mixtures thereof, and Rh distributed over the catalyst surface in an effective amount to catalyze steam reforming at 650° C. In one embodiment, the method further comprises discontinuing the fuel injection to allow the fuel reformer to cool in a lean phase and cycling repeatedly between the rich and lean phases to complete the regeneration of the lean NO X  trap. This pulsed operation creates harsh operating conditions to which the claimed compositions are particularly well adapted. 
     The primary purpose of this summary has been to present certain of the inventors&#39; concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventors&#39; concepts or every combination of the inventors&#39; concepts that can be considered “invention”. Other concepts of the inventors will be conveyed to one of ordinary skill in the art by the following detailed description together with the drawings. The specifics disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventors claim as their invention being reserved for the claims that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary exhaust aftertreatment system that can embody various concepts described herein. 
         FIG. 2  Surface Rh in micromoles per g ZrO 2  on a 5% La/ZrO 2  support as a function of time under cyclic aging for various Rh loadings. 
         FIG. 3  shows the stability under steam aging of 10% Pd co-dispersed with various amounts of La on a commercially available La-stabilized Al 2 O 3  support. 
     
    
    
     DETAILED DESCRIPTION 
     The catalysts of the present disclosure are adapted to a particular application.  FIG. 1  is a schematic illustration of an exemplary power generation system  100  embodying that application. The power generation system  100  is not the only power generation system to which the inventors&#39; concepts are applicable, but the various concepts described herein were originally developed for systems like the system  100  and the individual components of the system  100  pertain to preferred embodiments. The power generation system  100  comprises a diesel engine  101  and an exhaust line  102  in which are configured components of an exhaust aftertreatment system  103 . The exhaust aftertreatment system  103  comprises a fuel reformer  104 , a lean NO X  trap  105 , and an ammonia-SCR catalyst  106 . A fuel injector  107  is configured to inject fuel into the exhaust line  102  upstream from the fuel reformer  104 . A controller  108  controls the fuel injection based on information about the operating state of the engine  101 , a temperature of the fuel reformer  104  measured by a temperature sensor  109 , and a NO X  concentration measurement obtained by the NO X  sensor  110  at a point in the exhaust line  102  downstream from the lean NO X  trap  105 . A temperature sensor  111  is configured to measure the temperature of the lean NO X  trap  105 , which is particularly important during desulfation. 
     The diesel engine  101  is a compression ignition engine. A compression ignition diesel engine normally produces exhaust having from about 4 to about 21% O 2 . An overall rich exhaust-reductant mixture can be formed by injecting diesel fuel into the exhaust during cylinder exhaust strokes, although it is preferred that any reductant injection into the exhaust take place downstream from the engine  101 . The engine  101  is commonly provided with an exhaust gas recirculation (EGR) system and may also be configured with an intake air throttle, either of which can be used to reduce the exhaust oxygen concentration and lessen the amount of reductant required to produce an overall rich exhaust-reductant mixture. A lean burn gasoline engine or a homogeneous charge compression ignition engine can be used in place of the engine  101 . The engine  101  is operative to produce an exhaust that comprises NO X , which is considered to consist of NO and NO 2 . 
     The engine  101  is generally a medium or heavy duty diesel engine. The inventors&#39; concepts are applicable to power generation systems comprising light duty diesel and lean burn gasoline engines, but the performance demands of exhaust aftertreatment systems are generally greater when the engine is a medium and heavy duty diesel engine. Minimum exhaust temperatures from lean burn gasoline engines are generally higher than minimum exhaust temperatures from light duty diesel engines, which are generally higher than minimum exhaust temperatures from medium duty diesel engines, which are generally higher than minimum exhaust temperatures from heavy duty diesel engines. Lower exhaust temperatures make NO X  mitigation more difficult and place lower temperature light off requirements on fuel reformers. A medium duty diesel engine is one with a displacement of at least about 4 liters, typically about 7 liters. A heavy duty diesel engine is one with a displacement of at least about 10 liters, typically from about 12 to about 15 liters. 
     A light-off temperature for the fuel reformer  104  is an exhaust temperature at which when the fuel reformer  104  can be heated substantially above the exhaust temperature by combusting within the fuel reformer  104  fuel injected into the exhaust line  102  through the fuel injector  107 . Typically, once the fuel reformer  104  has lit off, the fuel reformer  104  will remain substantially above the exhaust temperature even if the exhaust temperature is lowered somewhat below the light-off temperature, provided the fuel injection continues. 
     The exhaust from the engine  101  is channeled by a manifold to the exhaust line  102 . The exhaust line  102  generally comprises a single channel, but can be configured as several parallel channels. The exhaust line  102  is preferably configured without exhaust valves or dampers. In particular, the exhaust line  102  is preferably configured without valves or dampers that could be used to vary the distribution of exhaust among a plurality of LNTs  105  in parallel exhaust channels. Valves or dampers can be used to reduce the exhaust flow to a fuel processor or LNT, allowing regeneration to be carried out efficiently even when exhaust conditions are unfavorable. Nevertheless, it is preferred that the exhaust line  102  be configured without valves or dampers because these moving parts are subject to failure and can significantly decrease the durability and reliability of an exhaust aftertreatment system. 
     Even when the exhaust line  102  is free from exhaust valves or dampers, an exhaust line upstream from the exhaust line  102  may still contain an exhaust valve, such as an exhaust gas recirculation (EGR) valve in an EGR line. Exhaust valves are particularly problematic when they are configured within a main exhaust line to divert a majority of the exhaust flow as compared to exhaust valves configured to control the flow through a side branch off a main exhaust line. Exhaust valves for larger conduits are more subject to failure than exhaust valves for smaller conduits. 
     The exhaust line  102  is provided with an exhaust line fuel injector  107  to create rich conditions for LNT regeneration. The inventors&#39; concepts are applicable to other method&#39;s of creating a reducing environment for regenerating the LNT  105 , including engine fuel injection of reductant and injection of reductants other than diesel fuel. Nevertheless, it is preferred that the reductant is the same diesel fuel used to power the engine  101 . It is also preferred that the reductant be injected into the exhaust line  102 , rather than into the cylinders of engine  101 , in order to avoid oil dilution caused by fuel passing around piston rings and entering the oil gallery. Additional disadvantages of cylinder reductant injection include having to alter the operation of the engine  101  to support LNT regeneration, excessive dispersion of pulses of reductant, forming deposits on any turbocharger configured between the engine  101  and the exhaust line  102 , and forming deposits on any EGR valves. 
     The diesel fuel is injected into the exhaust line  102  upstream from the fuel reformer  104 . The fuel reformer  104  comprises an effective amount of precious metal catalysts to catalyze oxidation reactions at 275° C. and steam reforming reactions at 650° C. The fuel reformer  104  is designed with low thermal mass, whereby it can be easily heated to steam reforming temperatures for each LNT regeneration. Low thermal mass is typically achieved by constructing the fuel reformer  104  using a thin metal substrate to form a monolith structure on which the catalyst or catalysts are coated. A thin metal substrate has a thickness that is about 100 μm or less, preferably about 50 μm or less, and still more preferably about 30 μm or less. 
     Oxidation and reforming catalysts can be co-dispersed on the fuel reformer  104 , but preferably, they are applied separately. The oxidation catalyst preferably forms a coating beginning proximate an inlet of the monolith and continuing part way toward or entirely to an outlet of the monolith. The reforming catalyst preferably forms a coating beginning proximate the outlet and continuing part way or entirely toward the inlet. In one embodiment, the reforming catalyst does not proceed entirely to the inlet, whereby injected fuel undergoes a substantial degree of reaction over the oxidation catalyst prior to encountering the reforming catalyst. The oxidation and reforming catalysts can occupy disjoint areas, abutting areas, or overlapping areas. 
     If the catalyst areas do overlap, either catalyst can be uppermost. Making the reforming catalyst uppermost has the advantage that it contact the reactants after the least diffusion. This is the preferred configuration if the reforming catalyst proceeds only partly toward the inlet. The reforming catalyst is more expensive than the oxidation catalyst, and it is therefore desirable that it be utilized as efficiently as possible. The oxidation catalyst, on the other hand, is least costly and can often be provided in greater quantity when more oxidation catalyst activity is desired. An advantage of applying the oxidation catalyst in a manner where the oxidation catalyst extends into the region under the reforming catalyst is that additional oxidation catalysis can be achieved in the same volume with essentially the same substrate thermal mass at relatively little extra expense as compared to the case where the oxidation catalyst terminates approximately where the reforming catalyst begins. On the other hand, making the oxidation uppermost has the advantage of increasing the extent of oxidation prior to contact with the reforming catalyst. This is the preferred configuration of the reforming catalyst extends to the inlet. 
     Steam reforming temperatures are at least about 500° C., which is generally above diesel exhaust temperatures. Diesel exhaust temperatures downstream from a turbocharger vary from about 110 to about 550° C. Preferably, the fuel reformer  104  can be warmed up and operated using diesel fuel from the injector  107  stating from an initial temperature of 275° C. while the exhaust from the engine  101  remains at 275° C. More preferably, the fuel reformer  104  can be warmed up and operated from initial exhaust and reformer temperatures of 240° C., and still more preferably from exhaust and reformer temperatures of 210° C. These properties are achieved by providing the fuel reformer  104  with effective amounts of precious metals, such as Pd, for catalyzing oxidation of diesel fuel at the starting temperatures. Low temperature start-up can also be improved by configuring a low thermal mass precious metal oxidation catalyst upstream from the fuel reformer  104 . Preferably, the upstream catalyst combusts a portion of the fuel while vaporizing the rest. A mixing zone between the upstream catalyst and the fuel reformer  104  is also helpful. 
     The fuel reformer  104  is designed to light-off at relatively low temperature. Light-off is the phenomena whereby the fuel reformer  104  heats to approach a steady state temperature that is substantially above the exhaust temperature. Once lit off, the fuel reformer  104  has a tendency to remain heated even when the conditions bringing about light off are discontinued. Preferably, the fuel reformer  104  is adapted to light-off when the exhaust temperature is as low as about 275° C., more preferably when the exhaust temperature is as low as about 240° C., still more preferably when the exhaust temperature is as low as about 210° C. 
     The fuel reformer  104  is design to warm up to and produce reformate at steam reforming temperatures. Operation at steam reforming temperatures reduces the total amount of precious metal catalyst required. Having the fuel reformer  104  operate at least partially through steam reforming reactions significantly increases the reformate yield and reduces the amount of heat generation. In principal, if reformate production proceeds through partial oxidation reforming as in the reaction:
 
CH 1.85 +0.5O 2 →CO+0.925H 2   (1)
 
1.925 moles of reformate (moles CO plus moles H 2 ) could be obtained from each mole of carbon atoms in the fuel. CH 1.85  is used to represent diesel fuel having a typical carbon to hydrogen ratio. If reformate production proceeds through steam reforming as in the reaction:
 
CH 1.85 +H 2 O→CO+1.925H 2    (2)
 
2.925 moles of reformate (moles CO plus moles H 2 ) could in principle be obtained from each mole of carbon atoms in the fuel. In practice, yields are lower than theoretical amounts due to the limited efficiency of conversion of fuel, the limited selectivity for reforming reactions over complete combustion reactions, the necessity of producing heat to drive steam reforming, and the loss of energy required to heat the exhaust.
 
     Preferably, the fuel reformer  104  comprises enough steam reforming catalyst that at 650° C., with an 8 mol % exhaust oxygen concentration from the engine  101  and with sufficient diesel fuel to provide the exhaust with an overall fuel to air ratio of 1.2:1, at least about 2 mol % reformate is generated by steam reforming, more preferably at least about 4 mol %, and still more preferably at least about 6 mol %. For purposes of this disclosure, functional descriptions involving diesel fuel are tested on the basis of the No. 2 diesel fuel oil sold in the United States, which is a typical diesel fuel. 
     An LNT is a device that adsorbs NO X  under lean conditions and reduces NO X  and releases NO X  reduction products under rich conditions. An LNT generally comprises a NO X  adsorbent and a precious metal catalyst in intimate contact on an inert support. Examples of NO X  adsorbent materials include certain oxides, carbonates, and hydroxides of alkali and alkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. The precious metal typically consists of one or more of Pt, Pd, and Rh. The support is typically a monolith, although other support structures can be used. The monolith support is typically ceramic, although other materials such as metal and SiC are also suitable for LNT supports. The LNT  105  may be provided as two or more separate bricks. 
     The ammonia-SCR catalyst  106  is functional to catalyze reactions between NO X  and NH 3  to reduce NO X  to N 2  in lean exhaust. The ammonia-SCR catalyst  106  adsorbs NH 3  released from the LNT  105  during denitration and subsequently uses that NH 3  to reduce NO X  slipping from the LNT  105  under lean conditions. Examples of ammonia-SCR catalysts include certain oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Mo, W, and Ce and zeolites, such as ZSM-5 or ZSM-11, substituted with metal ions such as cations of Cu, Co, Ag, or Zn. Ammonia-SCR can be accomplished using certain precious metals, but preferably the SCR catalyst  106  is substantially free of precious metals. Preferably, the ammonia-SCR catalyst  106  is designed to tolerate temperatures required to desulfate the LNT  105 . 
     The exhaust aftertreatment system  100  can comprise other components, such as a diesel particulate filter and a clean-up oxidation catalyst. A thermal mass can be placed between the fuel reformer  104  and the LNT  105  to protect the LNT  105  from frequent exposure to high fuel reformer temperatures. A diesel particulate filter can be used as the thermal mass. 
     During normal operation, the engine  101  produces an exhaust comprising NO X , particulate matter, and excess oxygen. A portion of the NO X  is adsorbed by the LNT  105 . The ammonia-SCR catalyst  106  may have ammonia stored from a previous denitration of the LNT  105 . If the ammonia-SCR catalyst  106  contains stored ammonia, an additional portion of the NO X  is reduced over the ammonia-SCR catalyst  106  by reaction with stored ammonia. The fuel injector  107  is generally inactive over this period, although small fuel injections might be used to maintain the fuel reformer  104  at a temperature from which it can be easily heated or to maintain the lean NO X  trap  105  at a temperature at which it effectively absorbs NO X . 
     From time-to-time, the LNT  105  must be regenerated to remove accumulated NO X  (denitrated). Denitration generally involves heating the fuel reformer  104  to an operational temperature and then using the reformer  104  to produce reformate. The reformer  104  is generally heated by injecting fuel into the exhaust upstream from the fuel reformer  104  at a sub-stoichiometric rate, whereby the exhaust-reductant mixture remains overall lean and most of the injected fuel completely combusts in the reformer  104 . This may be referred to as a lean warm-up phase. Once combustion has heated the reformer  104 , the fuel injection rate can be increased and/or the exhaust oxygen flow rate reduced to make the exhaust-reductant mixture overall rich, whereupon the reformer  104  consumes most of the oxygen from the exhaust and produces reformate by partial oxidation and steam reforming reactions. The reformate thus produced reduces NO X  absorbed in the LNT  105 . Some of the NO X  may be reduced to NH 3 , which is absorbed and stored by the ammonia-SCR catalyst  106 . 
     From time to time, the LNT  105  must also be regenerated to remove accumulated sulfur compounds (desulfated). Desulfation involves heating the fuel reformer  104  to an operational temperature, heating the LNT  105  to a desulfating temperature, and providing the heated LNT  105  with a rich atmosphere. Desulfating temperatures vary, but are typically in the range from about 500 to about 800° C., with optimal temperatures typically in the range of about 650 to about 750° C. Below a minimum temperature, desulfation is very slow. Above a maximum temperature, the LNT  105  may be damaged. 
     The LNT  105  is heated for desulfation in part by heat convection from the reformer  104 . To generate this heat, fuel can be supplied to the reformer  104  under lean conditions, whereby the fuel combusts in the reformer  104 . Once the reformer  104  is heated, the fuel injection rate can be controlled to maintain the temperature of the reformer  104  while the LNT  105  heats. Heating of the LNT  105  can be facilitated, and the temperature of the LNT  105  in part maintained, by frequently switching between lean and rich phases, whereby some oxygen from the lean phases reacts with some reductant from the rich phases within the LNT  105 . The contribution of this method to heating the LNT  105  can be regulated through the frequency of switching between lean and rich phases. 
     During rich operation for either denitration or desulfation, the fuel reformer  104  tends to heat. Particularly when the exhaust oxygen concentration is at about 8% or higher, the heat produced removing oxygen from the exhaust tends to be greater than the heat removed by endothermic steam reforming, regardless of the fuel injection rate. Theoretically, increasing the fuel injection rate increases the proportion of endothermic steam reforming, but in practice this is not always effective to prevent the fuel reformer  104  from heating during rich operation. As a result, the, fuel reformer  104 &#39;s temperature rises. To prevent overheating, fuel injection can be stopped and the fuel reformer  104  can be allowed to cool for a period before resuming rich regeneration. This results in alternating lean-rich condition within the fuel reformer  104  at high temperature. Operation at high temperatures and cycling between lean and rich conditions are detrimental to many catalysts. 
     The fuel reformer  104  preferably comprises both oxidation and reforming catalysts. When the exhaust-fuel mixture supplied to the fuel reformer is overall lean, the oxidation catalyst functions to combust nearly all the fuel and the reforming catalyst has little or no excess fuel to reform. When the fuel reformer has been heated sufficiently and the exhaust fuel mixture supplied to the fuel reformer is overall rich, the oxidation catalyst functions to combust a portion of the fuel to consume most of the oxygen in the exhaust and the reforming catalyst functions to generate syn gas through endothermic steam reforming. Preferably, the oxidation and reforming catalysts are in close proximity, whereby heat generated over the oxidation catalyst maintains the temperature of the fuel reforming catalyst. 
     Rh appears to be the most efficient steam reforming catalyst for the conditions created by the system  100 . The effectiveness of rhodium depends on its dispersion. As an absolute number, dispersion is the number of surface-exposed rhodium atoms per gram. As a percentage, dispersion is the fraction of Rh that can be considered to be on the surface, in terms of its availability for reaction. The fraction of surface Rh depends on the average particle size of the Rh metal or Rh metal oxide. A catalyst with 1 wt % Rh loading and 100% dispersion (all surface Rh) would provide 97.1 μmoles surface Rh/g. Rh dispersion can be measured by chemisorption of H 2 . For the present application, not only is a high initial dispersion of Rh desirable, but also a high dispersion after extensive lean operation and lean-rich cycling at elevated temperatures. 
     The inventors have evaluated several refractory metal oxide supports for Rh in the reforming catalyst. TiO 2  was determined to have insufficient thermal durability. Pure alumina is known to react with Rh. In an attempt to prevent such reaction, the alumina was pre-coated with La 2 O 3  (e.g., 10% by weight alumina). At 1% Rh loading, initial dispersions of Rh were good, e.g., 70% dispersion and 1-2 nm rhodium particle size. After lean aging with 10% steam for 400 hours at 700° C., however, the rhodium dispersion was reduced to 10%. Using TGA, it was determined that 50% of the rhodium was no longer in the form of Rh particles (metal or oxide), suggesting it had dissolved in the La or alumina. Notably, such loss of particulate rhodium did not occur over 1000 hours of lean rich aging 750° C. 
     Lean rich aging, as the term is used herein, refers to the following processing or equivalents thereof. In a lean portion of the cycle, the catalyst is exposed to air with 10% steam for 25 minutes. In a rich portion of the cycle, the catalyst is exposed to nitrogen having 3.8% hydrogen and mixed with 10% steam for 25 minutes. In between the lean and rich portions of the cycle, the catalyst is flushed with nitrogen for 5 minutes. The absence of reduction in available rhodium after 1000 hours of lean rich aging 750° C. suggests that La in amounts sufficient to form at least about a monolayer coating over the refractory metal oxide can redistribute Rh under rich conditions. 
     Depositing 1% Rh on a La-stabilized ZrO 2  gave significantly better results than La-stabilized alumina under lean aging. At a 2.5% La loading, the Rh dispersion was 29% after steam aging for 400 hours at 700° C. 2.5% La on a 100 m 2 /g refractory metal oxide, which is the approximate surface area of the ZrO 2  support used in the experiments reported herein, corresponds to about a monolayer. When the amount of La is increased to 5%, the dispersion was 42% after steam aging for 400 hours at 700° C. Accordingly, a preferred reforming catalyst includes a ZrO 2  refractory metal oxide component. Preferably, the refractory metal oxide component consists essentially of ZrO 2 . The ZrO 2  exists as submicron particles. Typical ZrO 2  surface areas are in the range from 70 to 130 m 2 /g. 
     Rh and La 2 O 3  can be applied to the surfaces of the ZrO 2  particles by any suitable technique. Suitable techniques include precipitation and impregnation. Impregnation of Rh begins by adding Rh salts or nitric acid solutions of Rh salts to water. The water volume is adjusted to be slightly more (about 10% more) than the pore volume of the refractory oxide support. Exemplary rhodium salts include rhodium chloride and rhodium nitrate. After impregnation, the supports are dried at 150° C. for 2-3 hours. The dried powder is then calcined at 450° C. for two hours and finally calcined at 600-800° C. for four hours. Deposition of Rh from rhodium nitrate solution gives comparable dispersion to deposition of Rh from rhodium chloride, but rhodium nitrate has the advantage of being less corrosive. The Rh and the La can be incorporated in the same solution and impregnated onto the ZrO 2  in a single step or the Rh and La can be in separate solutions and incorporated onto the ZrO 2  in separate steps with a drying step in between each impregnation. Deposition of La 2 O 3  prior to deposition of Rh in a two step process appears to give higher stability than deposition of La 2 O 3  and Rh simultaneously in a one step process. The La and the Rh can be applied to the ZrO 2  either before or after the ZrO 2  is applied to a substrate such as a metal monolith. 
     Tables 1 and 2 show a series of results pertaining to the stability under aging of 1% Rh/ZrO 2  catalyst having various amounts of La 2 O 3 . The La 2 O 3  is deposited on the surface of the ZrO 2  particles together with the Rh. 1% La appears to be insufficient to impart the desired stability under lean-rich cycling. 2.5% La based on the weight of the refractory metal oxide, has a significant beneficial effect. Further increasing the La loading to 5% or greater appears to provide a further improvement. Additional La loading at least up to about 20% does not appear to have any detrimental effect, but does not result in very significant further improvements. There was some indication that thicker La 2 O 3  coatings would result in a separate La 2 O 3  phase. Accordingly, it is preferred that the La loading be about 10% or less for the 100 m 2 /g refractory metal oxide. Preferably, the La 2 O 3  is amorphous. An amorphous layer, as the term is used herein, is one that has no apparent crystalline structure shown by X-ray diffraction. A La 2 O 3  particle with an average particle size of about 1 nm or less would not show a crystalline X-ray diffraction pattern. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Rh dispersion results (%) for 1% Rh/ZrO 2  catalysts with 
               
               
                 varying amount of La after various periods of lean aging in 10% 
               
               
                 steam at 700° C. 
               
             
          
           
               
                   
                 4 hrs 
                 100 hrs 
                 500 hrs 
                 600 hrs 
               
               
                   
                   
               
             
          
           
               
                   
                   1% La 
                 47 
                   
                   
                   
               
               
                   
                 2.5% La 
                 72 
                 24 
                 29 
                 20 
               
               
                   
                 2.5% La 
                 66 
                   
                 29 
               
               
                   
                   5% La 
                 64 
                 30 
                 41 
                 24 
               
               
                   
                   5% La 
                 78 
                   
                 42 
               
               
                   
                 7.5% La 
                 67 
                 32 
                   
                 25 
               
               
                   
                  10% La 
                 64 
                 35 
                   
                 18 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Rh dispersion results (%) for 1% Rh/ZrO 2  catalysts with 
               
               
                 varying amount of La after various periods of cyclic lean-rich aging at 
               
               
                 750° C. 
               
             
          
           
               
                   
                 4 hrs 
                 108 hrs 
                 250 hrs 
                 500 hrs 
               
               
                   
                   
               
             
          
           
               
                   
                   1% La 
                 74 
                 13 
                 9 
                 5 
               
               
                   
                 2.5% La 
                 75 
                 34 
                 19 
               
               
                   
                   5% La 
                 76 
                 65 
                   
                 18 
               
               
                   
                   
               
             
          
         
       
     
     Table 3 shows the effect of Rh loading for a 5% La/ZrO 2  support. Dispersion on a percentage basis for aged samples improves as Rh loading decreases to a very surprising extent. As the Rh loading is decreased from 1% to about 0.5%, the dispersion after 120 hours aging increases to such an extent that the same or greater Rh activity (amount of surface Rh) is achieved with the smaller amount of Rh. As the Rh loading is further decreased to 0.25%, Rh dispersion continues to increase, whereby the Rh activity decreases only slightly as Rh loading is reduced from 0.5% to 0.25%. It appears that the Rh sinters to a markedly greater degree, forming particles that progressively grow, if Rh loading is about 0.75% or greater, whereas the Rh is effectively stabilized by the La 2 O 3  if the Rh loading is about 0.5% or less. This result is further illustrated by  FIG. 2 , which plots surface Rh in micromoles per g as a function of time under cyclic aging for various Rh loadings and shows the stability of the 0.50% and 0.25% loading samples after the initial aging or “de-greening” period. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Rh dispersion results (%) for 5% La/ZrO 2  catalysts with 
               
               
                 varying amount of Rh after various periods of cyclic lean-rich aging 
               
               
                 at 750° C. 
               
             
          
           
               
                   
                 # of steps 
                 0 hrs 
                 5 hrs 
                 120 hrs 
               
               
                   
                   
               
             
          
           
               
                   
                   1% Rh 
                 1 
                 80 
                 39 
                 22 
               
               
                   
                   1% Rh 
                 2 
                 114 
                 52 
                 29 
               
               
                   
                 0.75% Rh 
                 1 
                 83 
                 48 
                 35 
               
               
                   
                 0.75% Rh 
                 2 
                 108 
                 49 
                 45 
               
               
                   
                 0.50% Rh 
                 1 
                 81 
                 48 
                 49 
               
               
                   
                 0.50% Rh 
                 2 
                 110 
                 52 
                 57 
               
               
                   
                 0.25% Rh 
                 1 
                 68 
                 63 
                 69 
               
               
                   
                 0.25% Rh 
                 2 
                 105 
                 65 
                 57 
               
               
                   
                   
               
             
          
         
       
     
     The values of Rh loading relate to concentrations of Rh on the surface of the refractory metal oxide. For the material used in these tests, 0.5% Rh loading corresponds to 0.005 g Rh per 100 m 2  surface area. Thus, the Rh loading is preferably about 5×10 −5  g/m 2  or less. Interpolation of the data suggests that an Rh loading of 3.5×10 −5  g/m 2  or less is even more preferable. 
     The preferred loading of rhodium can also be characterized by the Rh particles retaining at least about 40% dispersion, more preferably at least about 50% dispersion, after 400 hours of lean-rich cyclic aging at 750° C. The phenomenon by which Rh loses dispersion is sintering: the growth of Rh particles. According, yet another way to characterize the preferred loading of rhodium is that Rh loading at which the Rh average particle size remaining at about 2 nm or less after 400 hours of lean-rich cyclic aging at 750° C. through interaction with the La 2 O 3  coating. Particle size is defined as six times the particle volume divided by the particle surface area. This equation can be converted to an approximately correct equation to calculate Rh particle diameter in nm from Rh dispersion in percent: Rh particle diameter is about 100 nm divided by percent Rh dispersion. For example, the above case of a Rh catalyst with a dispersion of 50% has a particle diameter of about 2 nm. 
     Another of the inventors&#39; concepts is to use La 2 O 3  in the same manner to stabilize a precious metal oxidation. Pd is the precious metal. Tests were conducted with Pt on a 14% La/Al 2 O 3  catalyst. Even 1% Pt added to 10% Pd caused a large degree of sintering. Accordingly, the precious metal of the oxidation catalysts preferably consists essentially of Pd. 
     A preferred refractory metal oxide for the oxidation catalyst is Al 2 O 3 . ZrO 2  and Si—Al 2 O 3  also gave acceptable performance to the extent they were tested, although higher dispersions were obtained with Al 2 O 3  then with ZrO 2 . Al 2 O 3  had a higher surface area than the ZrO 2 , the Al 2 O 3  being approximately 200 m 2 /g (170-230 m 2 /g), which is an additional advantage over ZrO 2 . Dispersion of Pd on Al 2 O 3  was improved slightly by impregnating the Pd as Pd(NH 3 ) 4 (NO 3 ) solution as compared to impregnating the Pd as palladium nitrate-nitric acid solution. Sintering occurred much more rapidly when Pd chloride solutions were used. 
     Table 4 show the effect of La surface loading on the dispersion of 5% Pd over ZrO 2 . 2.5% or more La significantly improved dispersion and dispersion stability on aging. Initial dispersions when the refractory metal oxide was Pd were higher, e.g., 22% for 5% Pd, 10% surface-deposited La, Al 2 O 3 . 5% La appears to be the minimum amount of surface La for a 200 m 2 /g alumina. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Pd dispersion results (%) for 5% Pd/ZrO 2  catalysts with 
               
               
                 varying amount of La after various periods of lean aging at 
               
               
                 700° C. in 10% steam. 
               
             
          
           
               
                   
                 10 hrs 
                 240 hrs 
                 500 hrs 
               
               
                   
                   
               
             
          
           
               
                   
                   0% La 
                 11% 
                  7% 
                 5.5%  
               
               
                   
                 2.5% La 
                 19% 
                 12% 
                 11% 
               
               
                   
                 5.0% La 
                 18% 
                 13% 
                 10% 
               
               
                   
                 7.5% La 
                 19% 
                 13% 
                 11% 
               
               
                   
                  10% La 
                 18% 
                 13% 
                 14% 
               
               
                   
                   
               
             
          
         
       
     
     A high concentration of active (surface) Pd is useful in promoting low temperature light-off. The more active Pd/g, the lower the light-off temperature. The amount of active Pd/g depends on the surface area of the catalyst, the Pd loading, and the dispersion of the loaded Pd. 100% dispersion would give about 940 μmoles Pd/g for a 10 wt % Pd loading. 
     Experiments showed that Pd dispersion on a molar basis increases linearly with Pd loading up to about 15% for a 10% surface-deposited La/Al 2 O 3  support, meaning that the dispersion remains constant on a percentage basis. Accordingly, the Pd loading is preferably at least about 10%, more preferably from about 15 to about 20%. 
       FIG. 3  shows the stability of 10% Pd co-dispersed with various amounts of La on a commercially available La-stabilized Al 2 O 3  support. The commercial product contained about 4% La, prior to impregnation with Pd and additional La. The plot shows stability through 1000 hours of lean aging with 10% steam. Dispersion improves with La loading up to about 10 or 15%. X-ray diffraction data showed no separate La phase, even through 20% loading. Accordingly, the La loading is preferably at least about a monolayer, more preferably at least about 10%, and still more preferably from about 15% to about 25%. 10% La corresponds to about 0.5 mg La per m 2  and 20% La to about 1.0 mg La per m 2  distributed over the surface of the refractory metal oxide particles. 
     A series of tests were conducted replacing all or part of the La with Nd. Nd is chemically similar to La. Like La, Nd has a stable  3   +  charge. The tests showed that Nd is essentially fungible with La. 
     Other stabilizers were tested but did not show comparable advantages, either not improving dispersion, not improving stability, or interfering with catalyst activity. Sr in particular did not provided comparable performance to La. CeO 2  formed a separate phase on aging, which is undesirable in terms of maintaining dispersion. In addition, CeO 2  has substantial oxygen storage capacity, which is undesirable in this application. 
     The fuel reformer  104  typically has a metal or ceramic monolithic substrate comprising longitudinal channels through which the exhaust gas is designed to flow. The catalyst or catalysts can be applied as a washcoat layer on these channel walls. To apply the catalyst washcoat to the channel walls, a Pd—La—Al 2 O 3  or Rh—La—ZrO 2  catalyst powder such as described above can be mixed with water and other components and milled or attrited to form a sol or colloidal dispersion of small particles of the catalyst in water. This sol can then be coated onto the monolithic structure and the monolithic structure dried and heat treated to form a catalyst unit comprising the catalyst washcoat on the monolith walls. Many variations of this process are available. The sol can be prepared by adding solutions of La and precious metal to a slurry of refractory metal oxide powder in water that is then milled or attrited to form the small particle sol that is then coated onto the monolith. Alternatively, the La can be impregnated onto the refractory metal oxide, which is then dried and calcined. The resulting material can then be mixed with water and a precious metal solution and the slurry milled or attritted to form the final sol that is coated onto the monolithic structure, followed by drying and heat treating to from the final catalytic unit. To form a segmented catalyst with the oxidation catalyst coated on the inlet section and the reforming catalyst on the outlet section of the reformer  104 , the oxidation catalyst sol can be coated on an inlet section of monolith and the reforming catalyst sol can be coated on an outlet section of the monolith. 
     The invention as delineated by the following claims has been shown and/or described in terms of certain concepts, components, and features. While a particular component or feature may have been disclosed herein with respect to only one of several concepts or examples or in both broad and narrow terms, the components or features in their broad or narrow conceptions may be combined with one or more other components or features in their broad or narrow conceptions wherein such a combination would be recognized as logical by one of ordinary skill in the art. Also, this one specification may describe more than one invention and the following claims do not necessarily encompass every concept, aspect, embodiment, or example described herein.