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The concept of the NOx adsorber/catalyst has been developed based on acid-base washcoat chemistry. It involves storage of NOx on the catalyst washcoat during lean exhaust conditions and release during rich operation. The released NOx is catalytically converted to nitrogen, in a process similar to that occurring over three-way catalysts (TWC) widely used in stoichiometric gasoline engines. Normally, three-way catalysts are inactive in converting NOx under lean exhaust conditions, when oxygen is present in the exhaust gas. By alternating the lean storage and rich release-and-conversion phases, the applicability of the three-way catalyst has been extended to lean burn engines. The technology was first commercialized on gasoline direct injected engines (GDI), followed by light-duty diesel engines. Ongoing development efforts target the more challenging heavy-duty engine application. NOx adsorber systems have also been introduced for NOx control from stationary natural gas turbine applications .
Terms. Different authors use different terms when discussing NOx adsorbers, such as:
All these names are synonyms describing the same emission control technology. The term lean NOx catalyst, on the other hand, refers to the selective catalytic reduction of NOx by hydrocarbons—an entirely different technology which should not be confused with NOx adsorbers.
We should also introduce the basic definitions related to the process of adsorption (these terms are confused in some NOx adsorber literature):
At lower temperatures, adsorption is usually caused by intermolecular forces; it is then called physical adsorption. At higher temperatures, above about 200°C, the activation energy is available to form chemical bonds; if such mechanism prevails, the process is called chemisorption.