Revealing the mechanism differences of diverse strategies in boosting the low-temperature hydrothermal durability of Cu-SAPO-34 NH3-SCR catalysts

Abstract

The low-temperature hydrothermal instability of Cu-SAPO-34 is an unresolved issue, thus jeopardizing its commercial application in NH3-SCR due to severe structural damage and loss of activity. Herein, we reported that efficient strategies i.e., rare earth metal (REM, denotes Ce, La, and Sm) incorporation, alkali metal (AM, denotes Na, K, and Cs) exchange, and NH3 pre-adsorption were utilized to boost the moisture stability of Cu-SAPO-34 through various protective mechanisms. Our extensive characterizations revealed that AM ions readily exchanged with the susceptive Si–O(H)–Al bonds and ZCu(OH) sites, and at the same time left a majority of undamaged Z2Cu sites providing optimal protective effect in the entire temperature window studied here. On the other hand, REM co-cations only guarded Si–O(H)–Al bonds and Z2Cu sites but not the ZCu(OH) sites, this resulted in a poor protective effect in the low-temperature region. By contrast, chemisorbed NH3 only partially prevented Si–O(H)–Al and ZCu(OH) sites from hydrolysis, explaining why this protective effect was superior to REM in the low-temperature region but inferior to REM in the high-temperature region. These different protective mechanisms were explicitly revealed and were used in combination for the first time showing that the hydrothermal stability of Cu-SAPO-34 could be tuned precisely, the mechanistic guidance given here will surely be useful in developing more durable SCR catalysts.