Process for checking the operatability of a nitrogen oxide...

Power plants – Internal combustion engine with treatment or handling of... – Having sensor or indicator of malfunction – unsafeness – or...

Reexamination Certificate

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C060S274000, C060S276000, C060S295000, C060S297000

Reexamination Certificate

active

06684628

ABSTRACT:

INTRODUCTION AND BACKGROUND
The present invention relates to a process for checking the operatability of a nitrogen oxide storage catalyst which is used to remove the nitrogen oxides contained in the exhaust gas stream of a lean burn engine and contains at least a nitrogen oxide storage material, a catalytically active component and optionally an oxygen storage material, wherein the lean burn engine is operated with cyclic alternation of the air/fuel mixture from lean to rich and the nitrogen oxides contained in the exhaust gas are stored by the nitrogen oxide storage material in the presence of lean exhaust gas (storage phase) and desorbed and converted in the presence of rich exhaust gas (regeneration phase).
Nitrogen oxide storage catalysts were developed specifically for the treatment of exhaust gases from lean operated internal combustion engines. Diesel engines and lean burn gasoline engines belong to the group of lean operated internal combustion engines. Both types of engines are called lean burn engines in the following. Lean burn engines, in particular gasoline engines with a direct fuel injection system, are being used to an increasing extent in vehicle construction because they enable theoretical fuel savings of up to 25%, as compared with stoichiometrically operated internal combustion engines.
Nitrogen oxide storage catalysts have the ability to store nitrogen oxides over a wide temperature range under oxidizing exhaust gas conditions, that is during lean operation. This operating stage is therefore also called the storage phase in the following description.
Since the storage capacity of a storage catalyst is limited, it has to be regenerated from time to time. For this purpose, the air/fuel ratio in the air/fuel mixture supplied to the engine, and thus also the air/fuel ratio in the exhaust gas leaving the engine, is lowered to values of less than 1 for brief intervals. This process is also called enriching the air/fuel mixture of the exhaust gas. Thus, during this short operating phase, reducing conditions prevail in the exhaust gas prior to entry into the storage catalyst.
Under the reducing conditions present during the enrichment phase, the stored nitrogen oxides are released and reduced to nitrogen on the storage catalyst with simultaneous oxidation of carbon monoxide, hydrocarbons and hydrogen, as in the case of conventional three-way converters. This operating phase of the storage catalyst is also called the regeneration phase in the following. In the event of correct functioning of the total system consisting of storage catalyst, oxygen sensors and engine electronics, approximately stoichiometric conditions are present downstream of the storage catalyst during the regeneration phase, that is the hydrocarbons and carbon monoxide which are present in excess upstream of the storage catalyst during the regeneration phase are oxidized on the storage catalyst by the released nitrogen oxides. Only after completion of regeneration is there a sudden increase in reducing components downstream of the catalyst. This is called breakthrough of the reducing components through the storage catalyst.
The duration of the storage phase is typically about 30 to 100 seconds. The duration of the regeneration phase is substantially shorter and is in the region of only a few seconds (1 to 20 seconds).
The mode of operation and the composition of nitrogen oxide storage catalysts are known for example from EP 0 560 991 B1. As a storage material, these catalysts contain at least one component from the group of alkali metals (e.g. potassium, sodium, lithium, caesium), alkaline earth metals (e.g. barium, calcium) or rare earth metals (e.g. lanthanum, yttrium). As a catalytically active element, the storage catalyst contains platinum. Under oxidizing exhaust gas conditions, that is during lean operation, the storage materials can store the nitrogen oxides contained in the exhaust gas in the form of nitrates. For this purpose, however, the nitrogen oxides, about 60 to 95% of which consist of nitrogen monoxide, depending on the construction of the engine and its mode of operation, first have to be oxidized to nitrogen dioxide. This takes place on the platinum component of the storage catalyst.
In addition to the components mentioned above, the nitrogen oxide storage catalyst may also contain oxygen storing components. In this case, it can also take on the functions of a conventional three-way converter in addition to storing nitrogen oxides. Cerium oxide is mostly used as an oxygen storing component. The nitrogen oxide storage catalyst then has an oxygen storage function in addition to the nitrogen oxide storage function; thus it is bifunctional.
An important problem associated with modern exhaust gas treatment procedures is checking the correct functioning of the catalyst used in order to enable the timely replacement of catalysts which are no longer functioning efficiently. This also applies to nitrogen oxide storage catalysts, in which a variety of ageing mechanisms are observed. The nitrogen oxide storage capacity can be damaged on the one hand by the sulfur present in fuel and on the other hand by thermal stress. Whereas poisoning by sulfur can generally be counteracted by regenerating at elevated temperatures, thermal damage is an irreversible process.
In the case of bifunctional storage catalysts, in principle both storage functions (nitrogen oxide and oxygen) can be damaged by poisoning and by thermal effects. Damage to one function does not necessarily cause damage to the other function. Since nitrogen oxides and oxygen are both oxidizing components, their effects cannot be clearly separated from each other, so false diagnoses can be made when testing the catalyst.
DE 198 16 175 A1 discloses a process for checking the operatability of a nitrogen oxide storage catalyst which is intended to assess, separately, the oxygen storage function and nitrogen oxide storage function of the catalyst. To check the operatability of the storage catalyst in accordance with this document, the air/fuel ratio of the exhaust gas is switched from lean to rich and the time interval &Dgr;t
1
obtained between the first change over up to breakthrough of the rich exhaust gas through the catalyst and also the time interval &Dgr;t
2
obtained after switching the exhaust gas back from rich to lean, between the second change over up to breakthrough of oxygen through the catalyst, are measured. The time differences &Dgr;t
1
and &Dgr;t
2
permit separate assessment of the oxygen storage function and the nitrogen oxide storage function of the catalyst.
The nitrogen oxide storage function of the catalyst depends on the nitrogen oxide storage material and on the catalytically active component, generally platinum. Both the nitrogen oxide storage material and the catalytically active component may be damaged.
The nitrogen oxide storage material stores the sulfur dioxide contained in the exhaust gas in the form of sulfates. This takes place at the expense of the nitrogen oxide storage capacity. The sulfates in the storage material are substantially more stable than the nitrates. However, they can be decomposed again at exhaust gas temperatures higher than 600° C. and under reducing conditions. As a result of this desulfurization process, the nitrogen oxide storage material largely regains its original nitrogen oxide storage capacity.
The nitrogen oxide storage capacity of the storage material depends critically on the specific surface area with which it can interact with the exhaust gas. If the storage material is subjected to exhaust gas temperatures higher than about 800° C., the specific surface area becomes irreversibly reduced and its nitrogen oxide storage capacity decreases.
For optimum use of the catalytically active component, it is applied to the oxidic material of the storage catalyst in a highly dispersed form with average particle sizes between about 2 and 15 nm. Due to their very fine distribution, the platinum particles have a large surface area for interacting with the constituents in the exhaust

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