(222e) 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.