Copper ion-exchanged SAPO-34 as a thermostable catalyst for selective reduction of NO with C₃H₆

Selective reduction of NO with C₃H₆ in the presence of oxygen was studied over Cu ion-exchanged SAPO-n (n=5, 11, 34), and Cu ion-exchanged Zeolites β, USY, and ZSM-5. All the Cu ion-exchanged catalysts exhibited high activity for NO reduction with C₃H₆ in a large excess of O₂; however, the temperature for maximum NO conversion depended on the kind of molecular sieve. Although the maximum conversion of NO was attained at a slightly higher temperature in comparison with Cu-ZSM-5, Cu-SAPO-34 exhibited the highest activity for NO reduction among the catalysts studied under the conditions examined. Furthermore, high NO conversion was attained over a wide temperature range, from 623 to 873 K. SAPO-34 has high thermal stability. High activity for NO selective reduction on Cu-SAPO-34 was sustained for more than 60 h at 673 K in an atmosphere containing 15 vol% H₂O. After thermal treatment at 1073 K in humidified atmosphere, the decrease in activity for NO reduction was also small. Redox behavior of Cu ions in SAPO-34 between monovalent and divalent states occurs during the selective reduction of NO, and the reaction seems to proceed via formation of adsorbed nitrate species followed by the formation of organic nitro compounds.