NOx Adsorbers

W. Addy Majewski

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• Introduction
  • NOx Adsorber Concept
  • Terms & Definitions
• Active NOx Adsorbers
  • Principle of Operation
  • Chemical Reactions
  • Catalyst Systems
  • Conversion Efficiency
  • Regeneration
  • Sulfur Effects
  • Diesel Engine Integration
• Passive NOx Adsorbers
  • Overview
  • Catalyst Systems
  • Performance

Introduction

NOx Adsorber Concept

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:

1. Active NOx adsorbers, or
2. Passive NOx adsorbers.

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) for 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 lean burn 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 [1298]. Due to their declining NOx reduction performance at higher exhaust temperatures, active NOx adsorbers found practically no application on heavy-duty truck engines.

In light-duty engines—with increasing focus on in-use emissions outside of laboratory test cycles and the real driving emissions (RDE) testing requirements that became effective in the EU in 2017—the use of active NOx adsorbers as the primary stand-alone aftertreatment technology for NOx control has declined considerably. However, “part-time” active NOx adsorbers (or multifunctional NOx adsorber/DOC catalysts) continue to be used to control cold start/low temperature NOx emissions in many light-duty diesels with urea-SCR systems. A close-coupled, actively regenerated NOx adsorber is used during cold start and 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 under NOx Adsorber Applications.

Passive NOx adsorbers (PNA)—a simpler but less mature 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 [2872]. Passive NOx adsorbers were also considered for heavy-duty diesel engines meeting California and other low NOx standards on the order of 0.05-0.02 g/bhp-hr [3265].

Other Concepts. A technique called Selective NOx Recirculation (SNR) [348] was an early concept of a NOx adsorber system without catalytic reduction of NOx. In the SNR concept, two NOx adsorbers are installed in parallel in the exhaust system. Control valves allow to switch the gas flow, so each of the adsorbers alternates between adsorption and desorption modes. While in the desorption mode, the NOx carrying gas from the adsorber is recirculated to the engine intake air. This way, the desorbed NOx can be reduced through in-cylinder reactions during combustion. The regeneration strategy of SNR adsorber was not demonstrated. In experiments involving feeding NO/NO₂ from bottles to the diesel engine air intake port—i.e., not accounting for the adsorber performance—a NOx reduction efficiency of 60% was achieved.

Terms & Definitions

Different authors use different terms when discussing (active) 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 that should not be confused with NOx adsorbers.

The passive NOx adsorber has also been referred to as low temperature NOx adsorber (LTNA)—a term proposed by researchers from Ford [3756].

We should also introduce some basic definitions related to the process of adsorption (these terms are 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 a 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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