A Fundamental Consideration on NOx Adsorber Technology for DI Diesel Application

Diesel engines are far more efficient than gasoline engines of comparable size, and emit less greenhouse gases that have been implicated in global warming. In 2000, the US EPA proposed very stringent emissions standards to be introduced in 2007 along with low sulfur (< 15 ppm) diesel fuel. The California Air Resource Board (CARB) has also established the principle that future diesel fueled vehicles should meet the same low emissions standards as gasoline fueled vehicles and the EPA followed suit with its Tier II emissions regulation. Achieving such low emissions cannot be done through engine development and fuel reformulation alone, and requires application of NOx and particulate matter (PM) aftertreatment control devices. There is a widespread consensus that NOx adsorbers and particulate filter are required in order for diesel engines to meet the 2007 emissions regulations for NOx and PM.

In this paper, the key exhaust characteristics from an advanced diesel engine are reviewed. Development of the NOx adsorber technology is discussed. Spectroscopic techniques are applied to understand the underlying chemical reactions over the catalyst surface during NOx trapping and regeneration periods. In-situ surface probes are useful in providing not only thermodynamic and kinetics information required for model development but also a fundamental understanding of storage capacity and degradation mechanisms. The distribution of various nitration/sulfation species is related to surface basicity. Surface displacement reactions of carbonates also play roles in affecting the trapping capability of NOx adsorbers. When ultralow-S fuel is used as a reductant during the regeneration, sulfur induced performance degradation is still observed in an aged catalyst. Other possible sources related to catalyst deactivation include incomplete reduction of surface nitration, coke formation derived from incomplete hydrocarbon burning, and lubricant formulations. Sulfur management and the direction of future work for the successful implementation of such integrated engine and aftertreatment technology are discussed.