Characterization of Fe-Exchanged BEA Zeolite Under NH3 Selective Catalytic Reduction Conditions

Structural changes to Fe3+ cationic species in Fe-exchanged zeolite BEA during the selective catalytic reduction (SCR) of NOx with NH3 were probed by UV-vis spectroscopy. The distribution between Fe2+ and Fe3+ species was characterized by IR spectroscopy of adsorbed CO. Upon heating to 723 K, some of the Fe cations formed Fe-O-Fe bonds that underwent reversible structural transformation under NH3-SCR conditions. The in situ formed Fe oxide clusters could be dissociated to isolated Fe cations at 423 K, while at higher temperatures O-bridged Fe clusters were again formed. The structure of the Fe cluster is related to the Al distribution in the zeolite probed by Co2+ ion exchange. We propose here that two Fe cations bound within one six-membered ring containing an Al pair form hydroxylated dimeric Fe-O-Fe in the zeolite. This was supported by a structure simulation of a binuclear [HO-Fe(III)-O-Fe(III)-OH] model.