NOx Adsorbers

W. Addy Majewski

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Abstract: NOx adsorber-catalyst systems have been developed to control NOx emissions from partial lean burn gasoline engines and from diesel engines. The adsorbers, which are incorporated into the catalyst washcoat, chemically bind nitrogen oxides during lean engine operation. After the adsorber capacity is saturated, the system is regenerated during a period of rich engine operation, and released NOx is catalytically reduced to nitrogen. NOx adsorbers also require periodic desulfation, to remove sulfur stored in their washcoat. NOx adsorbers are used in some diesel engines, mostly in light-duty applications.

NOx Adsorber Fundamentals
Performance and Operation
Advanced NOx Adsorber Concepts
Diesel Engine Integration
Diesel Engine Systems

NOx Adsorber Fundamentals

Introduction

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 [1298].

Terms. Different authors use different terms when discussing NOx adsorbers, such as:

NOx adsorber catalyst (NAC),
Lean NOx trap (LNT),
DeNO_x trap (DNT)
NOx storage catalyst (NSC), or
NOx storage/reduction (NSR) catalyst.

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 some basic terms related to the process of adsorption (these are still being confused in some NOx adsorber literature):

Adsorption—A process in which atoms or molecules move from a bulk phase (typically gas, but also liquid) onto a solid or liquid surface (for example gas purification using activated charcoal). It is different from absorption, where molecules move into the bulk of the other phase, such as gas molecules being dissolved in a liquid. The term sorption covers both adsorption and absorption, while desorption is the reverse process.
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.
Adsorbent—A material that adsorbs, such as activated charcoal. A related term sorbent refers to both adsorption and absorption. In the NOx adsorber technology, barium oxide is the most common (ad)sorbent.
Adsorbate—A substance that has been adsorbed. A related term sorbate refers to both adsorption and absorption. In the case of NOx adsorbers, the (ad)sorbate is nitrogen oxides.

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