- Surface speciation and alkane oxidation with highly dispersed Fe(III) sites on silica
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When highly dispersed, supported Fe oxides are selective alkane oxidation catalysts, but new syntheses are required to reliably produce such materials. Here, highly dispersed, supported Fe3+ catalysts are prepared via incipient wetness impregnation of SiO2 with aqueous Fe complexes of ethylenediaminetetraacetic acid (FeEDTA), followed by calcination. With Na + countercations, UV-visible diffuse reflectance spectra are entirely below 300 nm and H2 temperature-programmed reduction only shows reduction at ～630 °C for all loadings up to 2.15 wt%, the maximum loading for a single impregnation cycle. These characteristics indicate isolated sites not seen for Fe(NO3)3 precursors even at 0.3 wt%. NH4+ countercations lead to amorphous oxide oligomers and a minority species with unusual reducibility at 310 °C. Na+ countercations produce 'single-site' behavior in adamantane oxidation using H2O2 with a specific turnover frequency of 9.2 ± 0.8 ks-1, constant for all Fe loadings and approximately 10 times higher than that of other well-dispersed Fe/SiO2 materials. Similar turnover frequencies are obtained when counting only the highly reducible species on the NH4+-derived catalyst, allowing these sites to be tentatively assigned as small, undercoordinated clusters that are both easily reduced and participate in alkane oxidation, reminiscent of Fe-exchanged MFI zeolites.
- Prieto-Centurion, Dario,Notestein, Justin M.
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