Comparison of selective catalytic reduction of NO with C_3H_6 and C_3H_8 over Cu(II)_ZSM-5 and Co(II)-ZSM-5

摘要

Selective catalytic reduction (SCR) of NO with propene (C3H6-SCR) and propane (C3Hs-SCR) over different levels of CU(II) or CO(II) ion exchanged into ZSM-5 zeolite has been evaluated under flowing conditions with 1500 ppm of NO + 4500 ppm of C3H6 or C3Hs + 3 vol. % of oxygen in helium at a gas hourly space velocity of 20000 h -I vs. increasing temperature. Cu-ZSM-5 (SiIAI = 15, Cu/AI = 0.46) shows its highest NO conversion at 440~500"C for C3H6-SCR and at 340-600°C for C3Hs-SCR. However, Co-ZSM-5 (SiIAI = 14, ColAI = 0.43) shows its highest NO conversion at 460 °C for C3H6-SCR and at 550 oC for C3Hs-SCR. The conversion efficiency at the optimum reaction temperatures was 98% for C3H6-SCR over Cu-ZSM-5 (SiIAl. = 15, CulAI = 0.46),96% for C3Hs-SCR over Cu-ZSM-5 (Si/AI = 15, Cu/A1 = 0.46),75% for C3Hs- SCR over Co-ZSM-5(SiIAI = 14, ColAI = 0.43) and 31% for C3H6-SCR over Co-ZSM-5 (SiIAI = 14, ColAI = 0.43). All NOx species adsorbed on the copper ion in ZSM-5 are desorbed below 430°C and are not observed in the temperature range for the highest NO conversion for C3H6-SCR. However, in Co-ZSM-5, CO-(NO)2 species are observed up to 450 oC by IR absorption. A reaction pathway at 450 °C for C3H6-SCR over Cu-ZSM-5 is proposed based on Cu-allyl, Cu-allyl oxime, Cu-ethenyl isocyanate, Cu-primary amine and Cu-ethenyl reaction intermediates identified by IR absorption. A different pathway is proposed for C3Hs-SCR over Cu-ZSM-5 at 250-340 °C and for C3Hs-SCR over Co-ZSM-5 at 230-550 "C based on CH3NO2 , HNCO, CO2, AI-NCO, Si-NCO and Co-NCO reaction intermediates identified by IR absorption.