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The concept of NOx adsorbers 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 and/or increased temperatures. Depending on the NOx release strategy, NOx adsorber systems can be classified as:
In active NOx adsorbers, stored NOx is periodically released—with a typical frequency of about once per minute—during a short period of rich air-to-fuel ratio operation, called NOx adsorber regeneration. 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 (GDI) engines, followed by light-duty diesel engines around 2007/2009 (US Tier 2, Euro 5). NOx adsorber systems have also been introduced for NOx control from stationary natural gas turbine applications .
Due to their declining NOx reduction performance at higher exhaust temperatures, active NOx adsorbers found only very limited application on heavy-duty truck engines. Considering the trends in light-duty emission regulations, the use of active NOx adsorbers can be also expected to decline in future light-duty vehicles. Increasing focus on in-use emissions and the expected introduction of real driving emissions (RDE) testing requirements in the EU will pose a challenge for the NOx adsorber technology—high NOx conversions may be required at operating conditions outside of the regulatory test cycle, including high engine load operation.
Passive NOx adsorbers (PNA)—a more recent and simpler variant of the technology—adsorb NOx during vehicle cold start and release it when the exhaust temperature increases—without a rich regeneration—to be converted over a downstream NOx reduction catalyst. Hence, passive NOx adsorbers (or traps) are not a stand-alone NOx control technology—rather, they can be used with urea-SCR aftertreatment to improve the low temperature performance of the system. An early demonstration of PNA technology was conducted by Cummins on their 2.8 L US Tier 2 Bin 2 diesel engine developed under the US DOE ATLAS project . Passive NOx adsorbers may also find application on heavy-duty diesel engines meeting future stringent NOx limits on the order of 0.05-0.02 g/bhp-hr .
It should be noted that “part-time”, active NOx adsorbers have been also used to control cold start/low temperature NOx emissions in some light-duty diesels with urea-SCR systems. A close-coupled, actively regenerated NOx adsorber is used during cold start. Once exhaust temperatures increase, NOx is reduced over the SCR catalyst using urea. This and other configurations of emission systems with NOx adsorbers are discussed in the paper on NOx adsorber applications.
Terms. Different authors use different terms when discussing (active) 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 that should not be confused with NOx adsorbers.
Definitions. 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.