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

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.