Abstract:
The invention relates to a catalyst arrangement in an exhaust gas after-treatment system of an internal combustion engine comprising an exhaust gas line in which an SCR catalyst is positioned in the direction of flow of the exhaust gas. A reducing agent production system has an NOx and CO/H2 production unit and a combined NOx storage/ammonia production unit in the standard gas-carrying path of the reducing agent production system which supplies ammonia as the reducing agent. The NOx and CO/H2 production unit is at least temporarily supplied via a fuel supply and an air supply with starting products for producing ammonia. The combined NOx storage/ammonia production unit has a plurality catalyst sections having different characteristic properties or functionalities, enabling a higher ammonia yield in the combined NOx storage/ammonia production unit. The catalyst formulations make it possible to adjust a temperature profile that additionally influences the ammonia production rate and contributes to higher ammonia yields.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/068384 filed on Nov. 13, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a catalytic converter assembly in an exhaust gas posttreatment system of an internal combustion engine, having an exhaust gas duct in which an SCR catalytic converter is provided in the flow direction of the exhaust gas, and a reductant generating system (RGS) has both an NO x  and CO/H 2  generating unit and a combined NO x  reservoir/ammonia generating unit (AGC) in the standard gas course of the reductant generating system, and for reducing nitrogen oxides, ammonia can be supplied as reductant by the reductant generating system upstream of the SCR catalytic converter, and the NO x , and CO/H 2  generating unit can be at least intermittently supplied via a fuel supply and an air supply with starting materials for generating the ammonia. 
     2. Description of the Prior Art 
     For reducing nitrogen oxides in the exhaust gas of engines operated with a lean fuel mixture, NO x  storage catalytic converters, also called NO x  storage/reduction catalytic converters or NSCs, can be used. These NO x  storage catalytic converters function discontinuously in a mode that comprises two phases: In the first, longer phase or so-called lean phase (Lambda&gt;1), the nitrogen oxides from the engine that are contained in the exhaust gas are stored. In the second, shorter phase, the so-called rich phase (Lambda&lt;1), the stored nitrogen oxides are regenerated by means of rich exhaust gas generated inside the engine. In the regeneration, in the conventional mode of operation of an NSC, only nitrogen (N 2 ), water (H 2 O), and carbon dioxide (CO 2 ) are produced from the stored nitrogen oxides. 
     It is fundamentally known that under unfavorable regeneration conditions, such as a very long regeneration and/or low Lambda value (λ≈0.8), a more likely small proportion of the stored NO x  can be converted to ammonia (NH 3 ). In that case, however, the NH 3  formation is an unwanted, parasitic effect. 
     In connection with future specifications in terms of nitrogen oxide emissions from motor vehicles, suitable exhaust gas posttreatment is necessary. Selective catalytic reduction (SCR) can be used to reduce NO x  emissions (removal of nitric oxides) in internal combustion engines, especially diesel engines, with intermittently predominantly lean or in other words oxygen-rich exhaust gas. In this process, a defined quantity of a selective-action reductant is added to the exhaust gas. The reductant may for instance be in the form of ammonia, which is metered in directly in gaseous form, or is also obtained from a precursor substance in the form of urea or from a urea-water solution (UWS). Such UWS-SCR systems were used first in utility vehicles. 
     In German Patent Disclosure DE 10139142 A1, an exhaust gas cleaning system in an internal combustion engine is described, in which to reduce NO x  emissions, an SCR catalytic converter is used, which reduces the nitrogen oxides that are in the exhaust gas to nitrogen using ammonia as the reagent. The ammonia is obtained from the urea-water solution (UWS) in a hydrolytic catalytic converter located upstream of the SCR catalytic converter. The hydrolytic catalytic converter converts the urea, contained in the UWS, into ammonia and carbon dioxide. In a second step, the ammonia reduces the nitrogen oxides to nitrogen, creating water as a byproduct. The precise sequence has been extensively described in the professional literature (see Weissweller in CIT (72), pages 441-449, 2000). The UWS is furnished in a reagent tank. 
     It is disadvantageous in this method that UWS is consumed in the operation of the internal combustion engine. Its consumption is approximately 4% of the fuel consumption. The supply of urea-water solution would have to be assured over a suitably large area, for instance at service stations. Another disadvantage of the method is the necessary operating temperature range. The hydrolytic reaction of the urea-water solution does not occur quantitatively at the hydrolytic catalytic converter, releasing ammonia, until temperatures of more than 200° C. In diesel engines, for instance, these exhaust gas temperatures are not reached until after a relatively long period of operation. At temperatures below 200° C., deposits can cause clogging of the metering unit, which at the very least is a hindrance to delivering the urea-water solution to the exhaust gas system. Adding the urea-water solution at temperatures below 200° C. can also, because of polymerization, inhibit the necessary catalytic properties of the hydrolytic catalytic converter of the SCR catalytic converter. 
     German Patent DE 199 22 961 C2 describes an exhaust gas cleaning system for cleaning the exhaust gas of a combustion source, in particular a motor vehicle internal combustion engine, of at least the nitrogen oxides contained in it, using an ammonia generating catalytic converter for generating ammonia, using ingredients of at least some of the exhaust gas emitted by the combustion source during ammonia-generating phases of operation, and also using a nitrogen oxide reducing catalytic converter downstream of the ammonia generating catalytic converter, for reducing nitrogen oxides contained in the emitted exhaust gas from the combustion source, using the generated ammonia as a reductant. In this system, a nitrogen oxide generating unit that is external to the combustion source is provided for enriching the exhaust gas, supplied to the ammonia generating catalytic converter, with nitrogen oxide generated by it during the ammonia generating phases of operation. A plasma generator is proposed for instance as the nitrogen oxide generating unit, for plasma technology oxidation of nitrogen, contained in a delivered gas stream, to nitrogen oxide. The hydrogen required for generating the ammonia is generated during the ammonia generated phases of operation by operating the combustion source with a rich or in other words fuel-rich air ratio. 
     A disadvantage of this method is the relatively high fuel consumption during the requisite rich phases of operation. Furnishing the nitrogen oxide to the engine externally also dictates high energy usage, especially since nitrogen oxide has to be produced in high concentration during the ammonia generating phases, which have to be as short as possible, and the remaining residual oxygen for generating ammonia has to be removed in a way that is expensive in terms of energy. If the hydrogen is generated via a PO x  catalytic converter by means of partial oxidation reforming (PO X ), then a further disadvantage the heretofore poor dynamics of generating hydrogen results. to be removed in a way that is expensive in terms of energy. If the hydrogen is generated via a PO x  catalytic converter by means of partial oxidation reforming (PO X ), then a further disadvantage the heretofore poor dynamics of generating hydrogen results. 
     A method for generating a hydrogen-rich gas mixture using plasma chemistry is described in International Patent Disclosure WO 01/14702 A1. In it, a rich fuel-air mixture is treated in an electric arc, preferably under PO x  conditions. 
     To avoid having to carry an additional fuel as well, a plasma method for on-board generation of reductants has been proposed in an as yet unpublished document of the present Applicant. In it, the ammonia required for reducing the nitrogen oxides is produced from nontoxic substances as needed in the vehicle and then is delivered to the SCR process. An acceptable solution in terms of fuel consumption is offered by a discontinuous method for ammonia generation, of the kind also proposed in the same document. This method will hereinafter be called the RGS method (Reductant Generating System), or reducing agent generating system. 
     One important component of an RGS unit is a catalytic converter, which while it does operate on the discontinuous fundamental principle of an NO x  storage catalytic converter (NSC), is nevertheless operated such that the nitrogen oxides, stored in the lean phase, are converted in a targeted way in the rich reduction phases into ammonia, rather than into nitrogen oxide. The nitrogen oxides are produced under lean conditions, for instance from air, in a nitrogen oxide generating unit that is combined with a hydrogen/carbon monoxide generating unit to make an NO x  and CO/H 2  generating unit. This CO/H 2  generating unit is also called a reductant generating unit. The gas mixture leaving this unit in the rich phases predominantly comprises H 2 , CO, and N 2 , and is also called reformate gas. The ammonia generated periodically (that is, cyclically) in this way is metered to the exhaust gas train of the engine and is converted with NO x  from the engine to N 2  in the downstream SCR catalytic converter. This kind of NO x  storage catalytic converter operated with maximum NH 3  and based on an NO x  storage catalytic converter is also called an AGC unit (AGC stands for “ammonia generating catalyst”). 
     The operating conditions of the AGC unit for targeted generation of ammonia outside the exhaust gas train are extremely different from those of a conventional NSC in the full exhaust gas stream. Essentially, the differences are these:
         an approximately 10 to 20 times higher concentration of NO x  (up to 1%) and of H 2 /CO (totaling up to 40%);   typically markedly higher global NO x  load densities of the NO x  storage catalytic converter (up to 2 g NO 2  per liter of AGC volume), and associated with this,   extremely exothermic heat tonalities over the length of the catalytic converter in the AGC unit, with positive temperature gradients ΔT of over 100° C.       

     The ammonia yield in the AGC unit depends on the temperature management at the AGC unit, or the temperature profile over the length of the AGC unit; on the duration of the rich phase; on the concentration of reductant agent; and on the catalytic converter formulation. 
     It is therefore the object of the invention to furnish a catalytic converter assembly of the AGC in which a high ammonia yield can be attained. 
     SUMMARY OF THE INVENTION 
     The object of the invention is attained in that the combined NO x  reservoir/ammonia generating unit (AGC unit) has one or more catalytic converter sections, which have different characteristic properties and functionalities in the flowthrough direction. In particular compared to catalytic converter assemblies each with a single catalytic converter formulation, with the arrangement according to the invention the ammonia yield can be increased markedly. In addition, with the catalytic converter formulation, a characteristic temperature profile in the flowthrough direction along a run-distance can be generated, and the ammonia formation rate can be varied in a targeted way along the run-distance. 
     In a preferred embodiment, the catalytic converter assembly is formed by a series connection of different catalytic converter types, spatially separated in the flowthrough direction. The targeted spatial separation of the functionalities and properties of the catalytic converter formulations, with varying characteristics in the flowthrough direction of a primary NO x  storing function during the lean phase and a slow “NO X  withdrawal” in the rich phase in the upstream region of the AGC unit from a primarily fast reduction of the “withdrawn” NO to NH 3  in the rich phase in the downstream region, according to the invention, promotes the ammonia forming capability and hence the ammonia yield of the AGC unit; the term “NO X  withdrawal” means the reductive decomposition of barium nitrate, for instance, to NO. In addition, in a comparable way, different catalytic converter assemblies can thus be attained which can be adapted in their properties to the requirements of the ammonia formation. 
     One variant embodiment provides a catalytic converter assembly, which is formed by a series connection of different catalytic converter types, and the catalytic converter formulation on a catalytic converter holder varies in the flowthrough direction in accordance with the catalytic converter types. This is advantageous with respect to the production process, since the different catalytic converter types, with their different catalytic converter formulations, can be produced during the production process by varying the chemical composition in the operation of coating the substrate structure. 
     A preferred variant embodiment provides that on the catalytic converter holder, the catalytic converter formulation varies continuously in the flowthrough direction, so that an ideal temperature profile for a high ammonia yield, for instance, can be established in a targeted way via the run-distance of the catalytic converter. 
     A gradient in the characteristic properties of the catalytic converter, or the “sequential” interconnection of catalytic converter formulations and their associated characteristic properties, can be attained especially advantageously if the catalytic converter holder has a gradient coating in the flowthrough direction. 
     Depending on the structural form of the catalytic converter in the AGC unit, the various catalytic converter types in the AGC unit are disposed linearly or radially, and the gas flow in the flowthrough direction of the catalytic converter assembly is carried in the radial arrangement from a region of the AGC unit near the axis to an outer jacket region of the AGC unit. 
     If for an NO x  storage/reduction catalytic converter, a type A catalytic converter has high activity, with respect to its reduction and/or oxidation properties, compared to a type B catalytic converter, the result on the one hand is often a comparatively low tendency to CO 2  poisoning for the type A catalytic converter in the comparison to the type B catalytic converter. In combination with or as a consequence of these properties, a comparatively high NO x  storing activity or high NO x  transferral activity or “NO x  withdrawal activity” can be recorded for a type A catalytic converter at temperatures up to about 250° C. The consequence is a very early or in other words very fast liberation of energy and hence a fast and steeply rising temperature profile over the run-length of the catalytic converter. Conversely in the type B catalytic converter, a comparatively slow and uniform release of energy and associated with it a rather shallow and nearly linearly rising temperature profile is generated over the run-length. 
     If the type B catalytic converter is located upstream of the type A catalytic converter in the flowthrough direction, then the ammonia yield can be increased. If in an especially preferred embodiment the type B catalytic converter in the flowthrough direction corresponds to approximately ⅔ of a run-distance component, or a matrix flow volume, and the type A catalytic converter corresponds to approximately ⅓ of the run-distance component or of the matrix flow volume, the ammonia yield can be increased up to approximately 80% under the conditions of RGS operation, as measurements have shown. 
     A variant embodiment for the catalytic converter assembly provides that the catalytic converter assembly, in the flowthrough direction, first has a type A catalytic converter, then a type B catalytic converter, and at the end another type A catalytic converter. With this as well, ammonia yields are already attained that are above those that can be attained using purely the type A catalytic converter. With the type A catalytic converter at the beginning, it is moreover possible, because of its comparatively high reactivity, to attain a steeper temperature profile already in the inlet region of the combined NO x  reservoir/ammonia generating unit. 
     If in the downstream part of the catalytic converter assembly in terms of the flowthrough direction, a catalytic converter formulation is used which has a high NO reduction activity, an NO reduction reaction in the downstream portion of the catalytic converter is promoted in a targeted way, and thus the “withdrawn” NO in the rich phase can be reduced to ammonia quickly and with a high yield, which significantly increases the ammonia yield from the combined NO x  reservoir/ammonia generating unit. 
     If in the downstream part of the catalytic converter assembly in terms of the flowthrough direction, a catalytic converter formulation is used which has an oxygen-storing and/or CO-adsorbing component, by so-called parasitic loss reactions that take place under exothermic heat tonality, the temperature profile can be adapted in a targeted way with respect to an optimal ammonia formation rate. In a preferred embodiment, with a view to the oxygen-storing component, a catalytic converter formulation is used in the downstream part of the catalytic converter assembly in terms of the flowthrough direction that contains current oxygen-storing compounds, such as Fe 2 O 3 , CeO 2 , or Ce/Zr mixed oxides. Thus an especially redox-active catalytic converter assembly can be furnished. 
     If the catalytic converter assembly is used in diesel engines or lean engines that have a reductant generating system with a combined NO x  reservoir/ammonia generating unit (on-board ammonia generator), the nitrogen oxide load can thus be reduced markedly, which is significant particularly in diesel engines. However, in lean engines as well that are operated with regular or super fuel, the catalytic converter assembly in the combined NO x  reservoir/ammonia generating unit, in conjunction with the reductant generating system, can offer advantages in terms of minimizing pollutants. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in further detail below in terms of the exemplary embodiments shown in the drawings, in which: 
         FIG. 1  is a schematic view of an exhaust gas posttreatment system of an internal combustion engine, with a reductant generating system; 
         FIGS. 2   a  through  2   c  are examples of interconnection combinations of various catalytic converter types; 
         FIG. 3  is a table showing the ammonia yields with different interconnection combinations of different catalytic converter types; and 
         FIG. 4  is an example showing temperature profiles with different catalytic converter types. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates the technical field, taking a diesel engine as an example, in which the catalytic converter assembly of the invention can be employed. 
     An exhaust gas posttreatment system  1  is shown for an internal combustion engine  10 , whose exhaust gases are carried via an exhaust gas duct  20 ; a diesel particle filter  30  (DPF) and a downstream SCR catalytic converter  40 , in that order in the flow direction of the exhaust gas, are provided. To reduce nitrogen oxides, ammonia can be delivered as a reductant upstream of the SCR catalytic converter  40  by a reductant generating system  50  (RGS). SCR catalytic converters  40  operate on the principle of selective catalytic reduction, in which by means of ammonia as the reductant, nitrogen oxides in oxygen-bearing exhaust gases are reduced to nitrogen and water. 
     The reductant generating system  50 , in the flow direction, has an NO x  and CO/H 2  generating unit  51  and a combined NO x  reservoir/ammonia generating unit  52 . Starting materials for generating the ammonia can be supplied at least intermittently to the NO x  and CO/H 2  generating unit  51  via an air supply  70  and a fuel supply  60 . The ammonia is generated from air, exhaust gas, or a mixture of air and exhaust gas, as well as in the example shown from diesel fuel. To that end, a hydrogen generating unit and a nitrogen oxide generating unit are provided. In the example shown, the NO x  generating unit is embodied as a plasma reactor, in which NO x  is generated from air by means of a glow discharge-like process. In this example as well, the plasma reactor contains the oxidation catalytic converter (cPOx) located downstream of the NO x  generating unit. 
     The generation of ammonia is effected inside the reductant generating system  50 , in which nitrogen oxides NO x  in a lean phase (λ&gt;1) are generated from air in a plasma process inside the plasma reactor. These nitrogen oxides flow through the adjoining oxidation catalytic converter (cPOx) and then are delivered, in the example shown, to a combined NO x  reservoir/ammonia generating unit  52  and stored. In a second phase of operation, the rich phase (0.33&lt;λ&lt;1) following the second phase of operation, liquid fuel is metered into the air in the region of the plasma reactor in an evaporation and mixture formation zone and converted at the oxidation catalytic converter (cPOx) into a gas mixture that contains hydrogen and carbon monoxide, and this mixture then, in the region of the combined NO x  reservoir/ammonia generating unit  52 , converts the previously-stored nitrogen oxides into ammonia. 
     This gaseous ammonia generated is then metered into the exhaust gas stream in the exhaust gas duct  20  upstream of the SCR catalytic converter  40 . Since the SCR catalytic converter  40  has an ammonia storage capability, it is possible even by way of a discontinuous method for generating ammonia to achieve the continuous reduction of the nitrogen oxides in the exhaust gas stream by means of the SCR process. In it, in the temperature range between 150° C. and 450° C., catalytic converters comprising titanium dioxide (TiO 2 ) and vanadium pentaoxide (V 2 O 5 ), for instance, convert the nitrogen oxides with the generated ammonia at a high rate. 
     The essential catalytic converter properties of the catalytic converter formulations used in the combined NO x  reservoir/ammonia generating unit  52  for generating ammonia will be described below in further detail; as  FIGS. 2   a  through  2   c  show, according to the invention at least two different catalytic converter types  52 . 1 ,  52 . 2  are provided, which differ as follows: 
     The type A catalytic converter  52 . 1  has a high activity, compared to the type B catalytic converter  52 . 2 , with regard to NO x  reduction and/or NO x  oxidation properties, and the type A catalytic converter  52 . 1  exhibits a comparatively low tendency to CO poisoning. In combination with these properties or as a consequence of them, the result at temperatures up to approximately 250° C. is a comparatively high NO x  storing activity and high NO x  transferral activity. The result is moreover very early or in other words very fast energy release and thus a fast, steeply rising temperature profile over a run-distance of the catalytic converter. 
     By comparison, the type B catalytic converter  52 . 2  has instead a lesser NO x  storing activity and lesser NO x  transferral activity at temperatures up to 250° C. The consequence is a comparatively slow, uniform release of energy over the run-distance by the catalytic converter and along with this a shallow and approximately linearly rising temperature profile. Moreover, the type B catalytic converter  52 . 2  may have a higher susceptibility to CO poisoning than the type A catalytic converter  52 . 1 . 
       FIGS. 2   a ,  2   b  and  2   c  show examples of interconnection combinations of two different catalytic converter formulations, that is, type A catalytic converter  52 . 1  and type B catalytic converter  52 . 2 , in a flowthrough direction  52 . 3  inside the combined NO x  reservoir/ammonia generating unit  52 . 
       FIG. 2   a  shows an arrangement in which the type B catalytic converter  52 . 2  is disposed upstream of the type A catalytic converter  52 . 1  in terms of the flowthrough direction  52 . 3 ; the type B catalytic converter  52 . 2  corresponds in the flowthrough direction  52 . 3  to approximately ⅔ of a run-distance component or a matrix flow volume, and the type A catalytic converter  52 . 1  corresponds to approximately ⅓ of the run-distance component or of the matrix flow volume. 
       FIG. 2   b  shows a variant of the catalytic converter assembly inside the combined NO x  reservoir/ammonia generating unit  52 , in which in the flowthrough direction  52 . 3 , first a type A catalytic converter  52 . 1 , then a type B catalytic converter  52 . 2 , and at the end another type A catalytic converter  52 . 1  are disposed. 
       FIG. 2   c  shows a variant of the embodiment of  FIG. 2   a , in which the type A catalytic converter  52 . 1  at the end is shortened in favor of the type B catalytic converter  52 . 2 . 
       FIG. 3  shows a table summarizing the ammonia yields in the interconnection combinations shown in  FIGS. 2   a ,  2   b  and  2   c  of the type A catalytic converter  52 . 1  and the type B catalytic converter  52 . 2  under the conditions of the RGS. In comparison, the ammonia yields are shown of catalytic converter assemblies which have solely catalytic converter formulations in accordance with type A catalytic converter  52 . 1  or type B catalytic converter  52 . 2 . Except for the “A only” variant, in which the matrix volume was less by ⅓ than the other arrangements, the total volume of the catalytic converter matrix should be considered to be constant. 
     As the table shows, very high ammonia yields can be attained in particular with the “BBA” (see  FIG. 2   a ) and “BBBA” (see  FIG. 2   c ) arrangements; particularly with the “BBA” variant, in which the type A catalytic converter  52 . 1  occupies approximately 33% of the total volume in the downstream portion, in the flowthrough direction  52 . 3 , of the catalytic converter assembly, gross ammonia yields of 80% can be attained. Taking a possible NO x  slip into account, the net ammonia yield is still 78%. By comparison, the “A only” and “B only” variants have net ammonia yields in the range of only approximately 39% and 67%, respectively. 
     In comparison to using only the type B catalytic converter  52 . 2 , as the results in the table ( FIG. 3 ) show, substituting a type A catalytic converter  52 . 1  for the type B catalytic converter  52 . 2  leads to a reduction in the ammonia yield in the inlet region of the combined NO x  reservoir/ammonia generating unit  52 , and to an increase in the ammonia yield in the outlet region. 
     The targeted spatial separation of the functionalities and properties of the catalytic converter formulations, with varying degrees in the flowthrough direction  52 . 3 , from a primary NO x  storing function during the lean phase and a slow “NO x  withdrawal” in the rich phase in the upstream region of the combined NO x  reservoir/ammonia generating unit  52  to a primarily fast reduction of the “withdrawn” NO to NH 3  in the rich phase in the downstream region promotes the ammonia formation capability according to the invention and thus promotes the ammonia yield of the combined NO x  reservoir/ammonia generating unit  52 . The term “NO x  withdrawal” is to be understood as the reductive decomposition, for instance of barium nitrate to NO, in accordance with the equation
 
2.Ba(NO 3 )+3.CO+3.H 2 →2.BaCO 3 +CO 2 +3.H 2 O+4.NO,
 
which dominates over the following NO reduction reaction expressed by the equation
 
4.NO+6.H 2 +4.CO→4.CO 2 +4.NH 3 ,
 
and this NO reduction reaction is promoted in a targeted way in the downstream part of the catalytic converter. “Carryover” density in the flow direction is also advantageous if at the same time a catalytic converter formulation with very high NO x  reduction activity is present in the downstream part. Conversely, this means that NO x  storage catalytic converters that in the running direction have a uniform catalytic converter formulation and high reduction activity, that the primary NO x  storage region is in the upstream region, and the “NO x  withdrawal” (BA(NO 3 ) 2 →NO) and the ammonia formation (NO→NH 3 ) proceed in principle simultaneously and thus with local strong heat tonality. This cooperation, however, reduces the NH 3  selectivity in favor of greater N 2  selectivity.
 
     In terms of heat management or temperature management inside the combined NO x  reservoir/ammonia generating unit  52 , in certain operating states, such as partial load, of the reductant generating system  50  it is necessary, despite the intrinsic strong exothermia of the ammonia formation, to import additional energy into the combined NO x  reservoir/ammonia generating unit  52 , in order in particular to compensate for heat losses. By means of the partial-load operation, for instance with stoppage times of the reductant generating system  50 , the cooling and the (axial) heat conduction lead to a redistribution of heat inside the combined NO x  reservoir/ammonia generating unit  52 . Thus a temperature profile inside the combined NO x  reservoir/ammonia generating unit  52  along the catalytic converter can be established which reduces the ammonia yield. In the current state of knowledge, this is the case whenever a more “isothermic” temperature profile prevails inside the combined NO x  reservoir/ammonia generating unit  52 . 
     The temperature profile can be varied, by varying the catalytic converter functionality back in the direction of the desired temperature gradient, in such a way that oxygen-storing components (“parasitic components”) for instance in the downstream part of the combined NO x  reservoir/ammonia generating unit  52  are present to an increased extent. The redox-active compound cerium oxide has proved to be especially effective. 
     In a similar way, CO-storing components can become effective. If such CO adsorber components are enriched in a targeted way in the downstream part of the combined NO x  reservoir/ammonia generating unit  52 , then once again the course of heat liberation along the run-distance inside the combined NO x  reservoir/ammonia generating unit  52  can be adjusted in a targeted way. Because of the CO adsorption and/or oxygen storage capacity that is varied in a targeted way via the combined NO x  reservoir/ammonia generating unit  52 , these so-called parasitic loss reactions, which proceed with exothermal heat tonality, can be used to adapt the temperature profile. 
     Some of the parasitic exothermic reactions are summarized below:
         CO adsorption during the rich phase, burnoff in the lean phase:
 
CO+½.O 2 →CO 2  ΔH R =−283 kJ/mol CO ads  
   O 2  adsorption during the lean phase, reduction in the rich phase:
 
½.O 2 +H 2 →H 2 O ΔH R =−242 kJ/mol O ads , or ½.O 2 +CO→CO 2  ΔH R =−283 kJ/mol O ads  
   Oxygen storage components (OSC), such as cerium oxide:
 
In the lean phase: Ce 2 O 3 +½.O 2 →2.CeO 2  ΔH R =−381.2 kJ/mol Ce 2 O 3  In the rich phase: 2.CeO 2 +CO→Ce 2 O 3 +CO 2  ΔH R =+98.2 kJ/mol Ce 2 O 3  Total: CO+½.O 2 →CO 2  per Ce 2 O 3  ΔH R =−283 kJ/mol Ce 2 O 3  
       

     In  FIG. 4 , an example of temperature profiles for various catalytic converter formulations is shown of the kind that occur in cyclical operation of the combined NO x  reservoir/ammonia generating unit  52 . A temperature (T)  80  is plotted over a run-distance component (L)  90  for each catalytic converter type. 
     The curve T max  for type A  81  and the curve T max  for type B  83  describe the absolute temperature maximums that occur at the site of the respective catalytic converter (the run-distance is along the flowthrough direction  52 . 3 ). The curve T min  for type A  82  and the curve T min  for type B  84  describe the absolute temperature minimums that occur at the site of the respective catalytic converter type. This shows that for type A catalytic converter  52 . 1 , over the run-distance component (L)  90 , a steeper temperature profile is established, which is due in particular to the greater activity compared to the type B catalytic converter  52 . 2 . The temperature profile for the type B catalytic converter, conversely, has a markedly shallower course. 
     All in all, with the variants shown for the catalytic converter assembly inside the combined NO x  reservoir/ammonia generating unit  52 , a high ammonia yield can be attained. By means of a spatially different functionality of the catalytic converter formulations in the flowthrough direction  52 . 3 , a temperature profile can furthermore be purposefully established which additionally reinforces a high ammonia yield. 
     Such catalytic converter assemblies can be used fundamentally in all motor vehicles that have diesel or lean engines that are operated with different fuels and in which a reductant generating system  50  is used as an on-board ammonia generator. 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.