(216d) Importance of Exhaust Hydrocarbon Speciation in the Studies of Low-Temperature Emission Control Technologies for Next-Generation Fuel-Efficient Vehicles

As the fuel economy regulations being mandated all over the world require development of engines with improved fuel efficiency, the energy content in the exhaust will generally decrease and the resulting lower exhaust temperatures present significant challenges in catalytic converter performance, especially during cold-start periods.  This, coupled with the more stringent future emission standards (e.g., LEV-III/Tier 3 regulations in U.S.) and recent adoption of green-house gas regulations including CH₄, makes catalytic oxidation of exhaust hydrocarbons (HCs) at low temperatures critically important.        

Hydrocarbons in engine exhaust generally consist of unburned fuel components and partial combustion products.  The latter includes light (C₁-C₃) saturated and unsaturated HCs such as methane, ethane, ethene and propene.  Unburned fuel components typically consist of higher molecular-weight (>C₄) alkanes and aromatics.  For gasoline engines, it has been commonly observed that C₅ and C₈ alkanes as well as C₇ and C₈ aromatics (toluene and xylene) are among the most abundant unburned fuel components in the exhaust.  Regarding the nature of C₅ and C₈ alkanes, detailed GC-based speciation studies of gasoline exhaust HCs show that alkane isomers, such as iso-pentane and iso-octane, are the predominant species, with the concentrations of the corresponding n-alkanes (i.e., n-pentane and n-octane) being much lower.   

The direct consequence of lower exhaust gas temperatures is that some of the HC species in engine exhaust are likely to remain unconverted even after the converter is fully warmed-up, thus presenting considerable challenges in HC emission control for future fuel-efficient engines.  Thus, it is of practical importance to identify the types of HC species that are likely to contribute significantly to tailpipe HC emissions in cooler exhaust from next-generation gasoline engines and understand how the conversions of those HCs are affected by the type & loading of noble metals (Pd vs Rh), O₂ concentration level and catalyst aging.  To that end, we conducted laboratory reactor experiments aimed at investigating the catalytic oxidation of various HCs in gasoline engine exhaust (alcohol, n-alkanes, alkane isomers, alkene and aromatics) over modern commercial three-way catalysts with variable noble metal loadings before and after aging.  Another approach being considered for improved cold-start HC emission control is the passive HC adsorber technology, which is designed to store HCs during engine cold-start and oxidize (convert) the stored HCs as they are released at higher temperatures.  As such, the effectiveness of this technology depends on the interplay between the adsorption, desorption and oxidation characteristics of specific HC species of interest.  With this background in mind, we evaluated commercial HC adsorber catalysts for their cold-start HC emission reduction potential in a feedstream containing paraffins, olefins and aromatics.            

This presentation will first discuss proper choice of “representative” HCs in gasoline engine exhaust based on HC speciation data and then examine their oxidation activities over degreened and aged commercial three-way catalysts as a function of noble metal loading.  The oxidation activities were measured using a feedstream which contains not only the HC of specific interest and O₂ but the other usual exhaust constituents, such as CO, H₂, NO, CO₂ and H₂O as well.  This would allow us to characterize the reactivity of each HC species under realistic conditions, namely, in the presence of the promotion effects of H₂and the inhibition effects of CO and NO.  Also discussed in this talk are the performance characteristics of HC adsorbers in a feed containing various types of HCs including olefins, aromatics and paraffins.  It was found that these types of HCs can be controlled to very different extents, due to variations in their adsorption, desorption and light-off (oxidation) properties.  We also investigated the effects of hydrothermal aging as well as noble metal loading, oxygen concentration and adsorber volume on the HC conversion performance of the HC adsorber catalysts.  These results will be discussed to illustrate the capabilities and limitations of the current HC adsorber technology, and to point ways toward improvement in the effectiveness of adsorber systems.