Stability of Colloidal Iron Oxide Nanoparticles on Titania and Silica Support.

Using model catalysts with well-defined particle sizes and morphologies to elucidate questions regarding catalytic activity and stability has gained more interest, particularly utilizing colloidally prepared metal(oxide) particles. Here, colloidally synthesized iron oxide nanoparticles (Fe O -NPs, size ∼7 nm) on either a titania (Fe O /TiO) or a silica (Fe O /SiO) support were studied. These model catalyst systems showed excellent activity in the Fischer-Tropsch to olefin (FTO) reaction at high pressure. However, the Fe O /TiO catalyst deactivated more than the Fe O /SiO catalyst. After analyzing the used catalysts, it was evident that the Fe O -NP on titania had grown to 48 nm, while the Fe O -NP on silica was still 7 nm in size. STEM-EDX revealed that the growth of Fe O /TiO originated mainly from the hydrogen reduction step and only to a limited extent from catalysis. Quantitative STEM-EDX measurements indicated that at a reduction temperature of 350 °C, 80% of the initial iron had dispersed over and into the titania as iron species below imaging resolution. The Fe/Ti surface atomic ratios from XPS measurements indicated that the iron particles first spread over the support after a reduction temperature of 300 °C followed by iron oxide particle growth at 350 °C. Mössbauer spectroscopy showed that 70% of iron was present as Fe, specifically as amorphous iron titanates (FeTiO), after reduction at 350 °C. The growth of iron nanoparticles on titania is hypothesized as an Ostwald ripening process where Fe species diffuse over and through the titania support. Presynthesized nanoparticles on SiO displayed structural stability, as only ∼10% iron silicates were formed and particles kept the same size during in situ reduction, carburization, and FTO catalysis.