Dependence of the oxide-support interaction on the size and nature of iron oxide particles on SiO2

The thermal dependence of the hyperfine fields and of the relative Mossbauer factors have been used to investigate the oxide-support interaction in the Fe-SiO2 system. The calcination temperature (698, 898 and 1098 K in N-2) has an influence on the nature and particle size of the iron species, while the atmosphere (air and N-2 at 698 K) affects the oxide particle size only. alpha-Fe2O3 particle diameters of congruent to 45 and congruent to 150 Angstrom were found for samples calcinated at 698 K in air and nitrogen, respectively. gamma-Fe2O3 particles of congruent to 90 Angstrom were obtained after calcination in air at 898 K and migration of iron ions into the SiO2 matrix was verified after the 1098 K heat treatment. It is found that for alpha-Fe2O3, the smaller particles have a stronger oxide-support interaction and gamma-Fe2O3 particles have an even higher strength than alpha-Fe2O3. (C) 1997 Elsevier Science Ltd. All rights reserved.