(222e) Surface Speciation and Alkane Oxidation with Fe(III) Sites On Alkali-Modified Oxide Supports

Notestein, J. M., Northwestern University

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.