Spectroscopic Evidence of Extra-Framework Heterometallic Oxo-Clusters in Fe/Ga-ZSM-5 Catalysts

The effect of introducing extra-framework Ga on the local structure of the metal sites in Fe/ZSM-5, resulting in enhanced reactivity toward N₂O, was investigated using a combination of Raman and X-ray absorption spectroscopies. The Raman spectra indicate an increased abundance of oxo- and/or hydroxo-bridged diiron sites, whereas the Fe <i>K</i>-edge XANES reveals more extensive reduction of Fe(III) to Fe(II). Curvefits of the EXAFS at both the Ga and Fe <i>K</i>-edges are consistent with heterometallic oxo-clusters containing both Ga−Fe and Fe−Fe paths. The spectroscopic evidence suggests a tetranuclear [Fe₂Ga₂O₄²⁺] core, possessing an open dicubane structure