Unusual structure and stability of iron-oxygen nano-clusters in Fe-ZSM-5 catalysts

Feng and Hall have recently reported that over-exchanged Fe-ZSM-5 materials prepared by a special route show high stability in the selective reduction of 2-methylpropane (isobutane) by NOx. We report that iron-oxygen clusters in ZSM-5 catalysts prepared by a conventional route show unexpected structural features, with significant iron-iron co-ordination and a rather short Fe-Fe interatomic distance of 2.53 +/- 0.02 Angstrom. Iron in these clusters is readily interconvertible between the (II) and (III) oxidation states, but the clusters are stable against reduction to Fe(0) even at 1250 K in hydrogen. We suggest that the clusters adopt structures similar to that of ferredoxin II, Fe3S4.