The Effects of Li2O Doping on the Surface and Catalytic Properties of CuO/Al2O3 Solids

Abstract

The effects of doping CuO/Al₂O₃ solids with Li₂O on their surface and catalytic properties were investigated using nitrogen adsorption at −196°C, the decomposition of H₂O₂ at 20–40°C and the oxidation of CO by O₂ at 175°C. The pure solids were prepared by wet impregnation of finely powdered solid Al(OH)₃ which had been precalcined at 400°C; the resulting material was then dried and calcined at 500°C with copper nitrate dissolved in the least amount of distilled water. The amount of copper oxide in such solids was fixed at 13.5 wt% while the amounts of Li₂O added varied between 0.19 wt% and 3.80 wt%. The results obtained showed that such Li₂O doping enhanced the crystallization of the CuO phase to an extent proportional to the amount of dopant added and increased the concentration of surface OH groups. This treatment led to a progressive small increase in the BET surface areas (S_BET) of the treated solids, which attained a maximum limit at 0.76 wt% Li₂O but decreased upon increasing the dopant concentration above this limit. The addition of 0.76 wt% Li₂O effected an increase of 14.6% in the S_BET values of the treated solids while the addition of 3.80 wt% Li₂O led to a corresponding decrease of 38.5% in this value. Doping with Li₂O resulted in a progressive decrease in the catalytic activity of the solids towards CO oxidation by O₂ while the presence of 3.80 wt% Li₂O effected a decrease of 72.5% in the value of the reaction rate constant measured at 175°C. In contrast, such treatment of CuO/Al₂O₃ solids with Li₂O brought about a progressive increase in their catalytic activity towards H₂O₂ decomposition, which reached a maximum limit in the presence of 1.90 wt% Li₂O and then decreased when the amount of Li₂O added was increased above this limit, falling to values which were smaller than those measured for the pure catalyst samples. The doping process did not modify the activation energy of the catalyzed H₂O₂ reaction but modified the concentration of the catalytically active constituent present in the system.