Fast lean-rich cycling for enhanced NOx conversion on storage and reduction catalysts

Lean NO (NO + NO2) reduction was carried out using rapid periodic injection of C3H6 over a NO storage and reduction (NSR) monolith catalyst containing Pt/Rh/BaO/CeO2/Al2O3. The effects of injection rate, feed temperature, rich phase composition, lean phase duration, and feed concentration of CO2 were systematically varied to quantify their effects on the cycle-averaged NO and propylene conversions, and product selectivities. A factor of 10 increase in the injection frequency from conventional NSR cycling frequency of 0.014 to 0.14 Hz resulted in much higher NOx conversion at high feed temperatures (above 300 degrees C). Both NO and propylene conversions were higher over the entire range of feed temperatures (150-400 degrees C). High frequency injection with propylene resulted in a nearly constant catalyst temperature, in contrast to large swings in the catalyst temperature during lower frequency injection. NO 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 4 to achieve the same NO conversion. The same high frequency operation using H-2 as the reductant not only did not show any enhancement, but resulted in decreased NO conversion. A prolonged approach to the cyclic steady state was observed during high frequency operation. Moreover, the detrimental effect of CO2 on NO conversion was observed to decrease at higher frequencies. These observations collectively suggest that the generation of reactive intermediate species HCxNyOz, whose lifetime on the catalyst surface exceeds the cycle duration and which reacts with NO to produce N-2, are required to achieve conversion enhancement during high frequency operation. The degree of mixing of the rich and lean feeds upstream of the catalyst was found to be an important reactor design parameter that invites further study. (C) 2014 Elsevier B.V. All rights reserved.