(727a) Synthesis, Characterization, and Catalytic Behavior of SiO2-Supported, Well-Defined Fe(III) Sites

Supported iron oxides are known to be 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 SiO₂ with aqueous, Fe(EDTA) complexes. Impregnation is followed by heat treatment under O₂ to remove the EDTA ligand, leaving only the highly dispersed iron oxide centers on the SiO₂ surface. The catalysts are characterized by H₂-TPR, XPS and diffuse reflectance UV-visible spectroscopy with an emphasis on comparing their properties to those of supported iron catalysts synthesized using more common precursors. Diffuse reflectance UV-visible spectra are characteristic of isolated Fe cations up to 4 wt%, greatly exceeding that of typical precursors, and H₂-TPR does not show evidence of bulk Fe₂O₃. XPS is consistent with synthesizing highly isolated Fe(III) cations. Small changes to the ligand and its counterion (Na or NH₄) alter the characteristic electronic spectra but within the range of values typically expected for isolated sites. 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. Even for these ostensibly isolated sites, reactivity is correlated only to high-reduction-temperature species out of several identified by H₂-TPR