DEPENDENCE OF THE OXIDE-SUPPORT INTERACTION ON THE SIZE AND NATURE OFIRON-OXIDE PARTICLES ON SIO2

The thermal dependence of the hyperfine fields and of the relative Mos sbauer factors have been used to investigate the oxide-support interac tion 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 small er particles have a stronger oxide-support interaction and gamma-Fe2O3 particles have an even higher strength than alpha-Fe2O3. (C) 1997 Els evier Science Ltd. All rights reserved.