Surface Speciation and Alkane Oxidation with Fe(III) Sites On Alkali-Modified Oxide Supports

Supported iron oxides are active oxidation catalysts whose efficiency and selectivity can be controlled by the level of dispersion of the Fe phase. Here we describe a simple route to highly dispersed, supported Fe(III) catalysts. Well-defined active sites are prepared by incipient wetness impregnation of aqueous Fe(EDTA) complexes on neat and alkali-modified SiO₂, Al₂O₃, ZrO₂, and CeO₂. Impregnation is followed by heat treatment under O₂ to remove the EDTA ligand, leaving only the highly dispersed iron oxide centers on the support surface. The catalysts are characterized by H₂-TPR, XPS and diffuse reflectance UV-visible spectroscopy (DRS) and compared to iron catalysts prepared using more common, although often more expensive, precursors. DRS results of these catalysts are characteristic of isolated Fe³⁺ cations up to 0.5 Fe/nm² and of highly dispersed Fe³⁺ amorphous layers up to 0.9 Fe/nm², greatly exceeding that of typical precursors. Additionally, H₂-TPR and XPS show the Fe(III) to Fe(II) transition expected of highly dispersed Fe³⁺ species and no evidence of bulk iron oxides. Finally, the activity, selectivity, and kinetic behavior of the catalysts are studied for adamantane oxidation with aqueous H₂O₂ as a model reaction for selective C-H bond activation. For this reaction, the initial TOF of this highly dispersed Fe³⁺ surface species is in the order of 10 s^-1, as compared with TOF <1 s^-1 for a more traditional catalyst starting with Fe(NO₃)₃.