Effect of La2O3 and Mn2O3-doping of Co3O4/Al2O3 system on its surface and catalytic properties

Pure sample of cobalt oxide supported on γ-alumina and having the formula 0.1 Co3O4/Al2O3 was prepared by wet impregnation method using finely powdered Al(OH)3 solid and cobalt nitrate dissolved in the least amount of distilled water sufficient to make a paste. Different samples doped with lanthanum or manganese oxides were prepared by impregnating a known mass of aluminum hydroxide with calculated amounts of lanthanum or manganese nitrates prior to impregnation with cobalt nitrate. Pure and variously doped solids were heated at 600 and 800 °C. The dopant concentration was varied between 0.3 and 4 mol% Mn2O3 and 1-4 mol% La2O3. The techniques employed were XRD, nitrogen adsorption at −196 °C and oxidation of CO with O2 at 125-200 °C. The results revealed that Mn2O3-doping conducted at 600 and 800 °C and La2O3-doping conducted at 600 °C increased the lattice constant of Co3O4 phase due to dissolution of some of dopant oxides added in Co3O4 lattice with subsequent transformation of some of Co3+ ions in non-stoichiometric cobalt oxide into Co2+ ions. The doping process either with Mn2O3 or La2O3 of the system investigated decreased the crystallite size of Co3O4 phase. The decrease was, however, more pronounced in case of Mn2O3-doped solids calcined at 800 °C. The specific surface areas of most of solids investigated increased by their doping with manganese or lanthanum oxides. The catalytic activity, in CO oxidation with O2 of the system investigated, was found to increase progressively by increasing the amount of La2O3 added and attained a maximum limit in presence of 0.5 mol% Mn2O3 then decreased upon increasing the dopant concentration above this limit. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 175 °C attained 42.5% and 240% for the solids doped with 4 mol% La2O3 and calcined at 600 and 800 °C, respectively. These values were 52% and 490% in case of the solids doped with 0.5 mol% Mn2O3 and heated at 600 and 800 °C, respectively. The doping process did not change the mechanism of the catalytic reaction but rather increased the concentration of active sites without altering their energetic nature.