@Article{JMaterChem_12_619, author = {P. Fabrizioli and T. B{\"u}rgi and M. Burgener and S. {Van Doorslaer} and A. Baiker}, title = {{Synthesis, structural and chemical properties of iron oxide-silica aerogels}}, journal= {J. Mater. Chem.}, ISSN = {0959-9428}, volume= {12}, pages = {619-630}, url = {http://pubs.rsc.org/en/Content/ArticleLanding/2002/JM/b108120a}, doi= {10.1039/b108120a}, abstract = {{Iron oxide aerogels were synthesized from tetramethoxysilicon(IV) (TMOS) or tetraethoxysilicon(IV) (TEOS) and iron nitrate using an acid-catalyzed solution{\frac{ }{ }}sol{\frac{ }{ }}gel method combined with subsequent extraction of the alcoholic solvent with supercritical CO$_2$. The main parameters varied in the sol{\frac{ }{ }}gel synthesis were: the type of N-base used as the gelation agent ({\em N,N}-diethylaniline, trihexylamine, ammonium carbonate, ammonia), the concentration of the iron precursor, and the water content. The silicon precursor was prehydrolyzed to improve its reactivity. After calcination at 600 °C, the structural and chemical properties of the aerogels containing 0{\frac{ }{ }}20wt% nominal Fe$_2$O$_3$ were characterized by means of nitrogen adsorption, X-ray diffraction (XRD), transmission and scanning electron microscopy, temperature programmed reduction, X-ray photoelectron spectroscopy (XPS), UV-Vis, DRIFT and EPR spectroscopy. XRD and electron microscopy indicated that all aerogels were amorphous, irrespective of the sol{\frac{ }{ }}gel conditions used. The aerogels were predominantly mesoporous, with pore size maxima ranging between 20{\frac{ }{ }}50 nm, but also exhibited some microporosity. For the 10 wt% iron oxide samples, the specific pore volumes ranged from 0.7 to 2 cm$^3$ g$^{-1}$ and BET-surface areas from 150 to 636 m$^{2}$ g$^{-1}$, depending on conditions. With increasing iron content, the BET surface area decreased from 740 to 340 m$^{2}$ g$^{-1}$, accompanied by increasing microporosity. XPS revealed significant silicon enrichment on the surface. Spectroscopic investigations (UV-Vis, EPR) uncovered different iron-containing species, ranging from tetrahedrally coordinated iron atoms incorporated in the silica matrix to iron oxide nanoclusters. Formation of isolated iron atoms was favored with low iron content samples. The N-base used to force gelation had a significant effect on the morphology and population density of Fe(OH)Si in the aerogels.}}, year = {2002} }