Structure and reactivity of the iron oxide specimen supported on magnesia via different preparation methods

Abstract

Three methods, dipping of MgO into an excess of aqueous solution of (NH 4) 3[Fe(C 2O 4) 3]×H 2O, incipient wetness impregnation o aqueous Fe(NO 3) 3 and batch adsorption of Fe(AcAc) 3 in toluene, were adopted to prepare the dispersed ferric oxide on magnesia. Characterization has been carried out on the physico-chemical properties of the systems. It seems that the nonuniform dispersion occurred if the adsorbed particles were not washed with toluene after adsorption even though the Fe content was below the threshold. Dipping with the excess aqueous ferric oxalate complex can be a way in which to prepare the highly dispersed ferric oxide but the conditions were difficult to control. The spinel species generally formed when the acidic aqueous solution was used for preparation. Variation of the concentration of aqueous ferric nitrate did not change the final structure environment of ferric ion significantly. The surface ferric ion was at high spin state and its site symmetry varied from the distorted rhombic. The surface ‘isolated’ Fe 3+ polyhedron was more difficult to be reduced compared with the Fe 3+ in the bulk α-Fe 2O 3 and the microcrystalline or the cluster MgFe 2O 4. The pulse reaction of 2-butene with or without gaseous oxy supply demonstrated that the catalytic performance of the surface Fe 3+ specimen was primarily controlled by the reducibility of the associated Fe 3+, the reactivity of the lattice oxygen, and the activation of the gaseous oxygen. The influence from the local structure of Fe 3+ on the catalytic activity is actually of little importance.