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

• Increase in cycle-averaged NOx conversion with high frequency injection of propylene.
• No enhancement with H2 indicates role of carbon-containing surface intermediates.
• Prolonged approach to cyclic steady state suggests slow buildup of reactive intermediates.
• High frequency cycling results in nearly constant catalyst temperature and gas phase O2 concentration.

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, lean phase duration, 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 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 °C). Both NOx and propylene conversions were higher over the entire range of feed temperatures (150–400 °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. 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 4 to achieve the same NOx conversion. The same high frequency operation using H2 as the reductant not only did not show any enhancement, but resulted in decreased 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. 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 NOx to produce N2, 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.

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