Effect of zeolite framework composition on the fast NH₃-SCR performance of hydrothermally aged iron zeolites

Here we report that iron-exchanged borosilicate and gallosilicate beta zeolites when hydrothermally aged at high temperatures show a notable increase (ca. 30 and 45 %, respectively) in NO_x conversion under fast SCR conditions at 150 °C. We also found that their lowest hydrothermal aging temperatures leading to such a conversion increase are higher by 50 °C and lower by 100 °C than the temperature (650 °C) of iron-exchanged aluminosilicate beta, respectively, indicating a strong influence of zeolite framework composition on the low-temperature fast NH₃-SCR activity of zeolite-supported iron catalysts. Combined experimental and theoretical characterizations provide indirect but convincing evidence for the formation of neutral heteronuclear bis-μ-oxo ironboron or irongallium dimers in the hydrothermally aged catalysts, which can effectively catalyze the reduction of ammonium nitrate, an intermediate of the NH₃-SCR reaction with a decomposition temperature of ca. 180 °C, by NO at 150 °C.