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 (SBET) 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 SBET 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.