Effect of La 2O 3 and Mn 2O 3-doping of Co 3O 4/Al 2O 3 system on its surface and catalytic properties

Abstract

Pure sample of cobalt oxide supported on γ-alumina and having the formula 0.1 Co 3O 4/Al 2O 3 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% Mn 2O 3 and 1–4 mol% La 2O 3. The techniques employed were XRD, nitrogen adsorption at −196 °C and oxidation of CO with O 2 at 125–200 °C. The results revealed that Mn 2O 3-doping conducted at 600 and 800 °C and La 2O 3-doping conducted at 600 °C increased the lattice constant of Co 3O 4 phase due to dissolution of some of dopant oxides added in Co 3O 4 lattice with subsequent transformation of some of Co 3+ ions in non-stoichiometric cobalt oxide into Co 2+ ions. The doping process either with Mn 2O 3 or La 2O 3 of the system investigated decreased the crystallite size of Co 3O 4 phase. The decrease was, however, more pronounced in case of Mn 2O 3-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 O 2 of the system investigated, was found to increase progressively by increasing the amount of La 2O 3 added and attained a maximum limit in presence of 0.5 mol% Mn 2O 3 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% La 2O 3 and calcined at 600 and 800 °C, respectively. These values were 52% and 490% in case of the solids doped with 0.5 mol% Mn 2O 3 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.