Preparation and Properties of Small Silica-Supported Iron Catalyst Particles: Influence of Reduction Procedure

Abstract

The genesis of small metallic iron particles supported on a high-surface area silica support was studied by in situ Mo¨ssbauer spectroscopy, and also by employing applied magnetic fields. α-Fe particles with dimensions of the order of 6–7 nm were found to have a superparamagnetic blocking temperature near 300 K. The particle size was estimated from the magnetic field dependence of the Mo¨ssbauer spectra. The particle size distribution depends critically on the preparation conditions. It was found that relatively large α-Fe particles (d≈10 nm) are formed after reduction under conditions similar to those employed in most earlier studies. After more extended reduction, Fe 2+ ions in the silica (probably in octahedral coordination) are reduced to much smaller α-Fe particles (d≈6 nm). These iron particles behave superparamagnetically at room temperature and are not easily detected in zero-field Mo¨ssbauer spectra.