Effect of the Calcination Atmosphere on the Structural Properties of the Reduced Fe/SiO2 System.

Iron supported systems are frequently used as catalysts in the Fischer-Tropsch synthesis being the Fe0 the active phase for the reaction. We have studied the influence of the calcination atmosphere (air or nitrogen) on the iron oxide reducibility and the metallic iron particle size obtained in Fe/SiO2 system. We have impregnated a silicagel with Fe(NO3)3 · 9H2O aqueous solution and the solid obtained was calcinated in air or N2 stream. These precursors, with 5% (wt/wt) of Fe, were characterized by Müssbauer Spectroscopy at 298 and 15 K. Amorphous Fe2O3 species with 3 nm diameter in the former, and α-Fe2O3 crystals of 48 nm diameter were detected in the last one. Both precursors were reduced in H2 stream. Two catalysts were obtained and characterized by Mössbauer spectroscopy in controlled atmosphere at 298 and 15 K, CO chemisorption and volumetric oxidation. α-Fe0, Fe3 O4 and Fe2+ were identified in the catalyst calcined in air. Instead, only α-Fe0 was detected in the catalyst calcined in N2. The iron metallic crystal sizes were estimated as ≈2 nm for the former and ≈29 nm for the last one. The different oxide crystal sizes, obtained from the diverse calcination atmospheres, have led to different structural properties of the reduced solids. It has been possible to reduce totally the existing iron in an Fe/SiO2 system with iron loading lower than 10% (wt/wt). [ABSTRACT FROM AUTHOR]