Effect of silver oxide doping on surface and catalytic properties of Co3O4/Al2O3 system

Cobaltic oxide catalyst supported on gamma-alumina having the formula 0.1 Co3O4/Al2O3 was prepared by wet impregnation method using finely powdered Al(OH)(3) and cobalt nitrate dissolved in the least amount of distilled water. Four silver oxide-doped samples were prepared by impregnating Al(OH)(3) solid with calculated amounts of silver nitrate dissolved also in the least amount of distilled water prior to impregnation with cobalt nitrate solution. The amounts of Ag2O were 0.2, 0.4, 0.8 and 1.6 wt.%. Pure and doped solids were subjected to heat treatment at 400, 600 and 800degreesC. Diffraction lines of Co3O4 phase were detected in the XRD patterns of pure and doped solids precalcined at different temperatures. However, the doping process conducted at different temperatures brought about a progressive significant increase in the particle size and degree of ordering of Co3O4 phase. Ag2O-doping of the investigated system affected a measurable decrease in its specific surface areas. The catalytic activities of different solids, in CO-oxidation by O-2 conducted at 125-225degreesC, were found to increase significantly by increasing the amount of dopant added reaching to a maximum limit at 0.8 wt.% Ag2O. The maximum increase in the catalytic activity expressed as reaction rate constant k measured at 150degreesC over the solids precalcined at 400degreesC and at 175degreesC for the solids precalcined at 600 and 800degreesC attained 337%, 118% and 219%, respectively. The doping process did not modify the activation energy (E-a) of the catalyzed reaction, and the observed apparent changes in E,, values resulted from a compensation effect as evidenced from almost same changes in the values of pre-exponential factor of Arrhenius equation. Therefore, the observed increase in the catalytic activity of the investigated system due to Ag2O treatment, which did not change the mechanism of the catalyzed reaction, could be attributed to an effective increase in the concentration of active sites contributing in chemisorption and catalysis of CO oxidation by O-2. (C) 2003 Elsevier B.V. All rights reserved.