Electrochemical characterization of iron sites in ex-framework FeZSM-5

The electrochemical response of FeZSM-5 prepared by an ex-framework method has been studied using Paraloid B72 polymer film electrodes immersed in aqueous media using H2SO4, HCl, Na2EDTA and NaCl electrolytes. The ex-framework method comprises the hydrothermal synthesis of isomorphously substituted FeZSM-5, followed by calcination at 823 K and subsequent steam treatment (300 mbar H2O in N2) at 873 K. During this process iron is extracted to extra-framework positions. Also partial dealumination of the zeolite framework takes place. Characteristic voltammetric responses in the +1.0 to −0.6 V versus SCE potential range have been recorded and were used to characterize the iron species present in the materials during the different preparation stages. Different species were identified, including tetrahedral framework iron, octahedral extra-framework iron ions, dimeric or oligomeric iron oxo-complexes, and iron oxide nano-aggregates. Iron oxide nano-aggregates displayed reductive or oxidative dissolution processes, while framework iron species displayed one electron transfer processes involving Fe(III) and Fe(II) species confined to boundary sites of the zeolite matrix.