Effects of Li_2 O Doping on the Surface and Catalytic Properties of Co_3O_4-Fe_2O_3 Solids Precalcined at Different Temperatures

摘要

The effects of Li_2O treatment on the solid-solid interactions and the surface and catalytic properties of the Co_3O_4-Fe_2O_3 system have been studied using TG, DTA and XRD methods, nitrogen adsorption studies at -196 deg C and the catalytic oxidation of CO by O_2 at 150-350 deg C. The results obtained showed that Li_2O doping followed by precalcination at 500-1000 deg C enhanced the formation of cobalt ferrite to an extent proportional to the amount of dopant added (0.52-6.0 mol% Li_2O). The solid-solid interaction leading to the formation of CoFe_2O_4 took place at temperatures >= 700 deg C in the presence of the Li_2 dopant. Lithia doping modified the surface characteristics of the Co_3O_4-Fe_2O_3 solids, both increasing and decreasing their BET surface areas depending on the amount of dopant added and the precalcination temperature employed for the treated solids. The activation energy of sintering (#DELTA#E_S) of cobalt/ferric mixed oxides was determined for the pure and doped solids from the variation in their specific surface areas as a function of the precalcination temperature. Both an increase and a decrease in the value of #DELTA#E_S due to Li_2O doping occurred depending on the amount of lithia added. The doping of Co_3O_4-Fe_2O_3 solids, followed by precalcination at 500 deg C, effected a significant increase (144%) in their catalytic activity towards CO oxidation by O_2. Precalcination at 700-1000 deg C of the mixed oxide solids doped with Li_2O (0.52 and 0.75 mol%) resulted in an increase in their catalytic activity which decreased upon increasing the amount of Li_2O added above this limit. The activation energy of the catalyzed reaction was determined for the pure and variously doped solids studied.