EFFECT OF LITHIUM AND RESIDUAL NITRATE SPECIES ON PLATINUM DISPERSION IN PT/AL2O3 CATALYSTS

The effect of lithium (ex LiNO3) on the metallic dispersion of 0.8 wt% Pt/Al2O3 catalysts, prepared by different impregnation techniques, was investigated by temperature programmed reduction (TPR) and the frontal method of H-2 chemisorption. The temperature at which platinum precursor is reduced at a maximum rate (543 K) was not modified by 0.1 wt% lithium addition, whatever the preparation technique used. The dispersion values of platinum (70-90%), after reduction at 773 K, were slightly dependent on the preparation procedure. After the addition of 0.8 wt% lithium the TPR profile presented two well defind peaks and the dispersion values (20-50%), measured after reduction at 773 K, presented a significant decrease. The results are linked with the presence of residual nitrate ions, that had not been eliminated during calcination at 773 K in air, but had been decomposed under the reducing atmosphere of the TPR experiment.