Investigation of solid-solid interactions between pure and Li 2O-doped cobalt and ferric oxides

Abstract

The solid-solid interactions between pure and lithium-doped cobalt and ferric oxides have been investigated using TG, DTG, DTA and XRD techniques. The catalytic activity, in CO oxidation by O 2 at 400°C, was determined for each solid preheated in air at 700–1000°C. The amounts of lithium, expressed as mol% Li 2O were 1.5 and 3. The results obtained showed that Fe 2O 3 interacted readily with Co 3O 4 at temperatures starting from 700°C to produce CoFe 2O 4 phase in well crystallized form. The degree of propagation of this reaction increased by increasing the heating temperature. However, a small portion of unreacted free oxides was found in the case of pure-mixed solids heated at 1000°C. Lithium oxide-doping stimulates the ferrite formation to an extent proportional to its amount present. The stimulation effect of lithium was evidenced by following up the change in the peak heights of certain diffraction lines characteristic of Fe 2O 3, Co 3O 4 and CoFe 2O 4 phases located at ‘ d’ spacing of 2.69, 2.44 and 2.08 Å, respectively. The change of peak height of the diffraction line at 2.08 Å as a function of firing temperature of pure- and doped-mixed solids enabled the calculation of the activation energy of the ferrite formation ( ΔE). The computed ΔE values were 57.3, 38.4 and 28.8 kJ mol −1 for pure-mixed solids specimen and those treated with 1.5 and 3 mol% Li 2O, respectively. The catalytic activity was found to decrease by increasing the calcination temperature and also by increasing the amount of Li 2O due to the enhanced formation of CoFe 2O 4 phase which is less reactive than the free oxides, Co 3O 4 and Fe 2O 3. The promotion effect of lithium in the cobalt ferrite formation was attributed to an effective increase in the mobility of diffusion of reacting cations.