Unique Dynamic Changes of Fe Cationic Species under NH3-SCR Conditions

Abstract

The type and location of Fe cations in zeolite BEA and their structural changes under NH3-SCR conditions are reported, as they are important catalysts for the selective catalytic reduction of NOx with NH3. Adsorption of basic probe molecules such as NH3 and pyridine shows that additional Brønsted acid sites are formed at the expense of Lewis acidic octahedrally coordinated Fe cations during the aging cycle. The formation of new tetrahedrally coordinated Fe suggests that Fe cations are incorporated into the zeolite framework at tetrahedral positions. UV/vis, XAFS, and IR spectra indicate that these species, however, exist only when the samples are cooled to room temperature under NH3-SCR conditions. In situ EXAFS shows Fe cations forming bridging Fe–O–Fe species at cation exchange sites above 573 K under reaction conditions. This indicates that the Fe cations undergo a unique reversible transformation during temperature cycles under NH3-SCR conditions.