Surface, catalytic and magnetic properties of small iron particles I. Preparation and characterization of samples

Very small and stable metallic iron particles supported on magnesium oxide have been obtained in an average size d from 1.5 to 30 nm. The hydrogen reduced particles consist of metallic iron, magnesium oxide, and Fe2+ clusters in magnesium oxide. Values of d determined by electron microscopy, magnetic susceptibility, X-ray line-broadening and Mössbauer spectra agreed with those determined by selective chemisorption of carbon monoxide with a stoichiometry of two iron atoms per adsorbed molecule. As the iron loading was changed from 1 to 40 wt %, the fraction of the iron in the metallic state on the different reduced samples varied from 0.4 to 0.7, and the Fe2+ concentration in the Fe2+ clusters in magnesium oxide varied only little. These results, coupled with a study of the sample preparation, suggest that magnesium oxide is not merely an inert carrier for metallic iron, but instead interacts with the iron in a manner essential for the ultimate production and stabilization of small metallic particles.