Particle growth of iron oxide nanoparticles on SiO2 and TiO2 supports

Abstract

The effect of the support material on particle growth of supported nanoparticles is not yet fully understood. In this research colloidal synthesis, followed by the attachment was used to prepare a well-defi?ned model catalyst suited to investigate the growth of Fe/SiO2 and Fe/TiO2 during the Fischer-Tropsch to ole?ns (FTO) reaction. 7 nm iron oxide nanoparticles (Fe-NP) with a narrow particle size distribution were synthesized using the heating-up method and subsequently attached to SiO2 and TiO2 support materials. Despite the difference in Hamaker constants between SiO2 and TiO2, a similar Fe-loading per surface area was obtained. The catalytic performance in FTO was evaluated after activation in H2 and CO. In general the activity of the supported Fe-NP was comparable to reference catalysts prepared by impregnation, however Fe-NP/P25 was less active. TEM and STEM analysis of the used catalyst revealed tremendous particle growth of up to 100 nm for Fe-NP/TiO2 while SiO2 supported Fe-NP remained stable after 100h on stream. Further experiment showed that growth already occurs during the activation in H2 at temperatures above 325°C. A semi-quantitative EDX analysis revealed the presence of large amount (80%) iron species on the TiO2 support. Additionally, Mossbauer spectroscopy results indicated the presence of amorphous Fe3TiO3 or Fe2+ surface species for reduced Fe-NP/TiO2. Base on these ?ndings, a surface species assisted Ostwald ripening process was proposed as growth mechanism for TiO2 supported Fe catalysts.