Stability of Colloidal Iron Oxide Nanoparticles on Titania and Silica Support

Abstract

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 x O y -NPs, size 7 nm) on either a titania (Fe x O y /TiO 2 ) or a silica (Fe x O y /SiO 2 ) support were studied. These model catalyst systems showed excellent activity in the Fischer-Tropsch to olefin (FTO) reaction at high pressure. However, the Fe x O y /TiO 2 catalyst deactivated more than the Fe x O y /SiO 2 catalyst. After analyzing the used catalysts, it was evident that the Fe x O y -NP on titania had grown to 48 nm, while the Fe x O y -NP on silica was still 7 nm in size. STEM-EDX revealed that the growth of Fe x O y /TiO 2 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. Mossbauer spectroscopy showed that 70% of iron was present as Fe 2+ , specifically as amorphous iron titanates (FeTiO 3 ), after reduction at 350 C. The growth of iron nanoparticles on titania is hypothesized as an Ostwald ripening process where Fe 2+ species diffuse over and through the titania support. Presynthesized nanoparticles on SiO 2 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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