Effect of Li2O doping on the surface and catalytic properties of the Cr2O3/Al2O3 system

Two Cr2O3/Al2O3 samples with the nominal compositions 0.06Cr(2)O(3)/Al2O3 and 0.125Cr(2)O(3)/Al2O3 (AlCr-I and AlCr-II, respectively) were prepared by mixing a known amount of finely powdered AI(OH), with calculated amounts of CrO3, followed by drying at 120 degrees C and calcination at 700 degrees C and 800 degrees C. Doped solid specimens were prepared by treating Al(OH)(3) samples with known amounts of LINO, dissolved in the minimum amount of distilled water prior to mixing with CrO3. Dopant concentrations of 0.75, 1.50, 3.00 and 6.00 mol% Li2O were employed. The surface and catalytic properties of the pure and doped solids thus prepared were investigated using nitrogen adsorption at -196 degrees C and studies of the catalysis of CO oxidation by O-2 over the solid specimens at 300-400 degrees C. The results of such studies showed that Li2O doping followed by calcination at 700 degrees C led to a maximum increase in the specific surface area, S-BET, of 26% for AlCr-I and of 55% for AlCr-II when these samples were doped with 3.00 mol% Li2O. The reverse effect was found when the calcination temperature was increased to 800 degrees C, where a decrease of 34% in the S-BET value of the AlCr-II sample doped with 3.00 mol% Li2O was detected. The catalytic activities measured at 350 degrees C over the pure and doped solids decreased on increasing the dopant concentration, the maximum decrease in such activity being ca. 33% and 50%, respectively, for the AlCr-I and AlCr-II samples calcined at 700 degrees C. Doping led to noticable changes in the magnitude of the activation energy for the catalytic reaction. Such changes were accompanied by parallel changes in the value of the pre-exponential factor in the Arrhenius equation. These results may indicate that Li2O doping has no effect on the mechanism of the catalytic reaction but modifies (decreases) the concentration of catalytically active sites taking part in chemisorption during the catalysis of CO oxidation by O-2.