Iron-exchanged high-silica LTA zeolites as hydrothermally stable NH3-SCR catalysts

Abstract

The catalytic properties of iron-exchanged LTA and SSZ-13 zeolites with different Si/Al (11–23) and Fe/Al (0.18–0.37) ratios hydrothermally aged at 750–900 °C have been investigated in the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). When the zeolite support possesses the same bulk ratio (16), each fresh Fe-LTA catalyst always exhibits a higher NO conversion than the corresponding Fe-SSZ-13 catalyst with a similar Fe/Al ratio in the reaction temperature range studied. Hydrothermal aging at 900 °C led to a noticeable decrease in NH₃-SCR performance of both series of catalysts. However, the extent of the decrease was found to be considerably smaller in the Fe-LTA series. This allows each of the Fe-LTA series to outperform even Fe-beta, which is known as the most hydrothermally stable among the iron-exchanged zeolite catalysts tested so far. The overall characterization results of this study reveal that the active iron species (e.g., isolated Fe³⁺ ions and small oligomeric Fe_xO_y species) in Fe-LTA are more stable than those in Fe-SSZ-13, mainly due to notable differences in their iron ion sites.