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NOx Adsorbers

NOx adsorbers—also known as NOx traps or lean NOx traps—are advanced emission-control technologies that can help diesel vehicles meet stringent nitrogen oxides (NOx) emissions standards. NOx adsorbers trap and store NOx present in the lean (i.e., oxygen-rich) exhaust produced by diesel engines. The stored NOx is transformed into more environmentally benign compounds before these compounds are emitted into the atmosphere. See the diagram below.

Schematic of a NOx adsorber configuration. Exhaust from a diesel engine passes through an oxidation catalyst into the NOx adsorber. The NOx adsorber works in two phases. In the NOx storage (lean exhaust) phase, a catalyst facilitates the transformation of NOx and oxygen into nitrogen dioxide (NO2), which is stored in a NOx storage material. In the regeneration (rich exhaust) phase, the catalyst facilitates a reaction between carbon monoxide and hydrocarbons from the exhaust and NOx from the NOx storage material to form nitrogen (N2), carbon dioxide, and water, which are emitted through a diesel particle filter and then out the tailpipe.

Adapted from: Advanced Petroleum-Based Fuels—Diesel Emissions Control Project (APBF-DEC) - 2,000-Hour Performance of a NOx Adsorber Catalyst and Diesel Particle Filter System for a Medium-Duty, Pick-Up Truck Diesel Engine Platform.

The process starts with ultra-low sulfur diesel (ULSD) fuel combusted in an optimized diesel engine. (Use of ULSD minimizes potential poisoning of the NOx adsorber by sulfur compounds.) Lean exhaust from the engine flows into the NOx adsorber; in some configurations, the exhaust flows first through other emission control devices (as shown in the diagram above). With the aid of a noble metal catalyst, NOx is captured and stored within the NOx adsorber substrate.

When the NOx adsorber nears its NOx storage capacity, it begins "regeneration." The engine exhaust is briefly made rich (i.e., fuel rich and oxygen poor) by means such as injecting diesel fuel into the exhaust stream ("In-Exhaust Fuel Injection" in the diagram above) or late-cycle in-cylinder injection. In the presence of this fuel-rich exhaust, the stored NOx is released then reduced to carbon dioxide (CO2), water (H2O), nitrogen (N2), and other nitrogen-containing gases over a noble metal catalyst. These gases are then emitted out the tailpipe.

NOx adsorbers are not a new technology but are just beginning to be seen in vehicle applications. Tests have shown they can reduce automotive NOx emissions by 80% to 90%.

To learn more about NOx adsorbers, search the library of NREL's Advanced Petroleum Based Fuels project.