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04.08.2017 | Ausgabe 3/2017

Emission Control Science and Technology 3/2017

Impact of Rapid Cycling Strategy on Reductant Effectiveness During NO x Storage and Reduction

Zeitschrift:
Emission Control Science and Technology > Ausgabe 3/2017
Autoren:
Mengmeng Li, Yang Zheng, Dan Luss, Michael P. Harold

Abstract

Performance studies of rapid lean-rich cycling over a NO x storage and reduction monolithic catalyst are described. The impacts of reductant type, specifically carbon-free (hydrogen (H2)), olefin (propylene (C3H6)), and alkane (propane (C3H8)) on the NO x and reductant conversions and product yields are reported over a range of feed temperature (T f), catalyst temperature (T s), and cycle time. The NO x conversion is enhanced at elevated temperatures (T f >350 °C) independent of reductant type by decreasing the cycle time from 70 to 7 s for a fixed rich duty cycle (14%). This is in contrast with a noted dependence on reductant type at intermediate temperatures (T f = 250–325 °C), with C3H8 exhibiting a detrimental effect of cycle time but C3H6 exhibiting an enhancing effect. The high temperature enhancement is contributed in part to an increased generation of active surface intermediates, more frequent regeneration of NO x storage sites, and from exothermic heat effects of the reductant oxidation. The opposite, intermediate temperature trend observed with C3H8 is attributed to kinetic limitations of C3H8 dehydrogenation. The contrasting features obtained with H2, C3H6, and C3H8 during cyclic operation and steady-state in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) help provide a deeper understanding of the underlying mechanism. Surface intermediates are identified with C3H6 that are not observed with C3H8. Experiments with mixtures of C3H6 and C3H8 reveal NO x conversion enhancement of up to 50% when substituting only a small amount of C3H6 into C3H8. This enhancement results from the lower feed temperature light-off of C3H6 compared to C3H8.

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