Effects of Li2O doping on surface and catalytic properties of CuO-ZnO/Al2O3 system

The effect of Li2O doping (0.375-4.5 mol.%) on surface and catalytic properties of CuO-ZnO/Al2O3 was investigated using nitrogen adsorption at - 196 degrees C and catalytic oxidation of CO by O-2 at 150-200 degrees C. Pure and doped solids were subjected to thermal treatment at 600-800 degrees C prior to surface and catalytic measurements. The results showed that Li2O doping caused measurable increases in the specific surface area (23-45%) and in the total pore volume (24-43%) for the adsorbents preheated in air at 600-700 degrees C. If the doping process was carried out at 600 degrees C, it resulted in a considerable decrease (52%) in the catalytic activity, measured at 175 degrees C. A rise in calcination temperature of the doped solid specimens (4.5 mol.% Li2O) to 700 and 800 degrees C brought about marked increases of 125% and 186%, respectively, in the catalytic activity measured at 175 degrees C. This discrepancy is discussed in terms of the effect of Li2O doping in decreasing the dispersity of CuO crystallites of the solids calcined at 600 degrees C and the role of Li2O in hindering CuAl2O4 formation upon calcination at 700 and 800 degrees C. The decrease in the degree of dispersion of CuO crystallites due to doping at 600 degrees C is normally followed by a decrease in the catalytic activity while the suppression or hindrance of CuAl2O4 formation might be followed by an increase in the catalytic activity. The doping process carried out at 600-800 degrees C did not modify the concentration of catalytically active constituents involved in chemisorption and catalysis of the CO oxidation reaction. (C) 1998 Elsevier Science B.V. All rights reserved.