Silica-supported Fe and FeNi catalysts show strong dependence of metal particle size on pretreatment, particularly the initial dehydration step. Room temperature Mössbauer spectra and supporting X-ray diffraction data reveal that mild pretreatment in the form of slow vacuum drying prior to reduction produces significantly smaller particles of Fe and FeNi on silica than either direct reduction or calcining. Once the catalyst precursor is completely dried, metal particles are relatively stable against further growth, suggesting that residual water or anions from the incipient wetness impregnation play a critical role in metal agglomeration during pretreatment. For 5 wt% Fe 5 wt% Ni on silica, vacuum drying and subsequent reduction produce FeNi alloy particles small enough to exhibit superparamagnetic behavior. Precalcining this catalyst increases the alloy particle size considerably as shown by a ferromagnetically split Mössbauer spectrum. Computer fitting of the broad peaks reveals significant amounts of metallic iron or extremely iron-rich alloy and, therefore, that phase separation can accompany particle growth in FeNi alloy catalysts.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry