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 (FexOy-NPs, size ∼7 nm) on either a titania (FexOy/TiO2) or a silica (FexOy/SiO2) support were studied. These model catalyst systems showed excellent activity in the Fischer-Tropsch to olefin (FTO) reaction at high pressure. However, the FexOy/TiO2 catalyst deactivated more than the FexOy/SiO2 catalyst. After analyzing the used catalysts, it was evident that the FexOy-NP on titania had grown to 48 nm, while the FexOy-NP on silica was still 7 nm in size. STEM-EDX revealed that the growth of FexOy/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.

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The attachment of colloidal iron‐oxide nanoparticles (designated Fe‐NPs) to pristine and surface‐oxidized carbon nanotubes (CNTs and CNT‐Ox, respectively) was investigated. The loadings of Fe‐NPs (size 7 nm) on the CNT and CNT‐Ox supports amounted to 3.4 and 2.3 wt. %, respectively; the difference was attributed to weaker van der Waals interactions between the colloidal Fe‐NPs and the surface of CNT‐Ox. Fischer–Tropsch to olefins (FTO) synthesis was performed to investigate the impact of support functionalization on catalyst performance. Weak interactions between the Fe‐NPs and the CNT‐Ox support facilitated particle growth and led to substantial deactivation of the Fe/CNT‐Ox catalysts. The addition of promoters (Na+S) to Fe/CNT resulted in remarkable activity, selectivity to lower olefins, and stability, making colloidal iron nanoparticles on pristine CNTs a suitable catalyst for FTO synthesis.@en