Insight into solid-state ion-exchanged Cu-based zeolite (SSZ-13, SAPO-18, and SAPO-34) catalysts for the NH3-SCR reaction: The promoting role of NH4-form zeolite substrates

In this paper, Cu-exchanged SSZ-13, SAPO-18, and SAPO-34 were synthesized via a novel green and sustainable solid-state ion-exchange (SSIE) route with excellent NH3-SCR activities, which could simplify the preparation process and reduce the wastewater production in comparison to conventional aqueous ion-exchange route. The results also proved that Cu-exchanged zeolites synthesized by SSIE technology presented higher Cu ion-exchange (IE) levels and better SCR activities than those prepared via the traditional incipient impregnation route. In the case of SSIE, the usage of NH4-form substrates could achieve more Cu2+ ions and fewer CuOx than those H-form substrates since the formation of Cu(NH3)x2+ intermediate could promote the Cu2+ mobility to facilitate Cu2+ occupying IE sites, which was favorable for the SCR reaction. The combined DFT data confirmed the SSIE reaction energy barriers between Cu(NO3)2 and the NH4-form substrates were almost identical, whereas the higher reaction energy was required for H-form substrates. In-situ DRIFTS results manifested the labile species like NH4+, –NH2, NO2–, NO3–, NO2, etc were the reaction intermediates over the catalysts prepared from NH4-form substrates. The Langmuir-Hinshelwood (“L-H”) and Eley-Rideal (“E-R”) mechanisms contributed collectively over both Cu-SSZ-13 and Cu-SAPO-18 in the SCR reaction, however, the “L-H” mechanism dominated over Cu-SAPO-34.