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

Authors: 
Prieto-Centurion, D., Northwestern University
Notestein, J. M., Northwestern University


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