CATALYSTS FOR NOX ADSORBER
Rhodium-based reduction catalysts have been utilized in gasoline-powered car engines for many years, as was previously indicated. These “reduced” NOx compounds back to elemental nitrogen and oxygen using a rhodium catalyst. Rhodium cannot be employed in low burn diesel engines that constantly dump surplus (unreacted) oxygen into the exhaust because it only acts as a reduction catalyst when the air-fuel ratio is at stoichiometric or richer. The NOx adsorber catalyst (NAC), which was invented, provided a solution for diesel engine designers to this issue.
The easiest way to define the chemical word “adsorption” is adhesion: Do not mix this with the term absorb because an adsorbent does not significantly alter chemically when it is absorbed. NOx adsorber catalysts (NACs) employ base metal oxides to first “store” NOx compounds, and then, during an ECM-managed reduction phase, they use a rhodium reduction catalyst to convert them back to N2 and O2.
NOx is adsorbed onto a metal (barium oxide) substrate during the storage phase: The engine ECM momentarily replicates a “rich” (reduced oxygen) operating condition or directly injects fuel into the exhaust system when the device’s available NOx storage sites are filled. The NOx is released from its base metal storage places during operation in this simulated “rich” state, allowing a rhodium catalyst to transform it into nitrogen gas (N2), oxygen (O2), and water vapor (H2O). In other terms, there are two processes to how a NAC works:
- NOx storage: During a diesel engine’s typical lean burn operation, NOx is adsorbed to the base metal oxide substrate.
- NOx reduction: To decrease NOx to elemental nitrogen (N2), elemental oxygen (O2), and water vapor, the engine ECM momentarily runs the engine in a rich AFR mode or injects diesel into the exhaust system to imitate a rich AFR (H2O).
Cycle of NAC Regeneration
In order for a NAC to complete its regeneration cycle, extra oxygen in the exhaust must be momentarily removed. ULS diesel must be used with NACs. Even when just the proper ULS fuel has been used, sulfur at any amount can harm NACs, and certain systems need intermittent de-sulfation regeneration modes. Fuel injection directly into the exhaust system is necessary to temporarily increase the exhaust temperatures during a de-sulfation regeneration cycle.
Construction of Cat Converter
In order to maximize catalyst surface area while minimizing flow resistance, the noble metal catalysts are thinly coated over aluminum oxide or granulate monolith substrate (a substance that is catalytically inert and onto which active catalysts are coated). Catalytic converters are only effective in reducing emissions when they are operating at operational temperatures, therefore computer-controlled variable timing and low fueling are attempts to achieve this.
Long-term cold engine running (i.e., mild or idle engine load), particularly in areas with low ambient temperatures, can lead to catalytic converters’ oxidation stage clogging. When this happens, burning off the deposits limiting the converter could be accomplished by running a full load for 30 minutes on a dynamometer. The catalytic converter may need to be changed in specific circumstances.