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 /TiO₂) or a silica (Fe O /SiO₂) 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 /TiO₂ catalyst deactivated more than the Fe O /SiO₂ 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 /TiO₂ 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 Fe²⁺, specifically as amorphous iron titanates (FeTiO₃), after reduction at 350 °C. The growth of iron nanoparticles on titania is hypothesized as an Ostwald ripening process where Fe²⁺ species diffuse over and through the titania support. Presynthesized nanoparticles on SiO₂ displayed structural stability, as only ∼10% iron silicates were formed and particles kept the same size during in situ reduction, carburization, and FTO catalysis.