Enhanced NOx Conversion With Fast Lean-Rich Cycling on NOx Storage Catalysts

Mike  Harold, University of Houston

Lean NOx (NO + NO2) reduction was carried out using rapid periodic injection of C3H6 over a NOx storage and reduction (NSR) monolith catalyst containing Pt/Rh/BaO/CeO2/Al2O3. The effects of injection rate, feed temperature, rich phase composition, and feed concentration of CO2 were systematically varied to quantify their effects on the cycle-averaged NOx and propylene conversions, and product selectivities. A factor of ~15 increase in the frequency from conventional NSR levels to ca. 0.14 Hz, resulted in much higher NOx conversion at high feed temperatures (above 300 oC). Both NOx and propylene conversions were higher over the entire range of feed temperatures (150 – 400oC). High frequency injection with propylene resulted in a nearly isothermal catalyst, in contrast to large swings in the temperature during lower frequency injection. NOx conversion exceeding 90% was achieved for a feed having a cycle-averaged stoichiometric number (lean to rich ratio) of 6; slower injection required a stoichiometric ratio of 3 to achieve the same NOx conversion. The same high frequency injection experiments using H2 as the reductant not only did not show any enhancement but it led to a decrease in NOx conversion. A prolonged approach to the cyclic steady state was observed during high frequency operation. Moreover, the detrimental effect of CO2 on NOx conversion was observed to decrease at higher frequencies. This observations collectively suggest that the generation of reactive “HCNO” species intermediates whose lifetime on the catalyst surface exceeds the duration of the lean and rich cycles, and which react further with NOx/O2 to give N2, is required to achieve conversion enhancement during high frequency operation.