NOx Adsorber Applications

W. Addy Majewski

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• Introduction
• Configurations
• Light-Duty Diesel Engines
• Heavy-Duty Diesel Engines

Introduction

NOx adsorber catalysts (NAC), also referred to as lean NOx traps (LNT), have been a commercial NOx reduction technology on light-duty vehicles since around 2000. They were first introduced on lean burn gasoline direct injected engines (such as those utilizing stratified injection strategies), to control NOx during lean operation, outside of the A/F window of the three-way catalyst (TWC). Diesel applications—technically more challenging due to the need for mixture enrichment during NAC regeneration—followed a few years later; NOx adsorbers were introduced on some Tier 2 diesel car models in the United States, as well as on several Euro 5/6 vehicle models in Europe.

At some time, there was a significant effort to develop NOx adsorber catalyst systems for heavy-duty applications. This development was driven to a large extent by the US EPA 2007/2010 emission standards for heavy-duty onroad engines—at the time the regulation was adopted, the EPA did not consider urea-SCR a viable NOx control technology, due to concerns with the lack of urea infrastructure and the possibility of tampering with SCR systems (for instance, operation without urea solution). Instead, the agency believed that the US 2007 emission standards would be met using NOx adsorbers as the principal NOx reduction technique on heavy-duty diesel trucks and buses.

In spite of the R&D effort, robust NOx adsorber systems for heavy-duty engines have not been developed (prompting the use of urea-SCR technology in US 2010 and later truck and bus engines). The main challenge has been the NOx adsorber operating temperature window. In Ba-based adsorbers (and Ba remains the most suitable adsorbent), the stored NOx becomes released at higher temperatures, even at lean conditions, resulting in a loss of NOx conversion. In aged NOx adsorbers, NOx conversion tends to decline rapidly at temperatures above approximately 380°C. As apparent from Figure 1 [3116], exhaust temperatures above 380°C (shown in blue in the chart) are commonly experienced during high load operation of the diesel engine. In the heavy-duty application, with high temperature test cycles, as well as NTE testing at random operating points outside of the test cycle, the NOx reduction catalyst has to remain active up to at least 450-500°C.

The application of NOx adsorbers as a “stand-alone” NOx reduction technology has been mostly limited to light-duty applications that are emission certified over low- to medium-load test cycles, including the NEDC and FTP-75. The technology seems especially suitable for chassis certified light-duty vehicles with a relatively high engine power/vehicle weight ratio, which ensures relatively low exhaust temperatures during certification where the NOx conversion is highest [3238]. In practice, the application of NOx adsorbers is usually limited to smaller size engines, below about 2 liters. In larger engines, which would use a high volume NAC with high noble metal loading, SCR catalysts usually provide a more cost effective solution.

Emission regulations have been increasingly focused on real driving emissions, outside of the certification test cycle. With the introduction of RDE testing requirements at the Euro 6d stage, where compliance must be demonstrated at higher engine loads, the use of NOx adsorbers has declined, and only few Euro 6d diesels continue to use the LNT as a stand-alone NOx reduction device. NOx adsorbers are also less attractive when stringent CO₂ and/or fuel consumption standards must be met in addition to emission standards, due to the fuel economy penalty induced by NAC regeneration. Therefore, the main application of NOx adsorbers in future diesel engines appears to be that of an auxiliary device—passive or part-time active—to control cold start and/or low temperature emissions in systems with urea-SCR catalysts.

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