Effect of ZnO doping on surface and catalytic properties of NiO and Co3O4 solids

The surface and catalytic properties of NiO and Co3O4 solids as being influenced by ZnO doping have been investigated. The techniques employed were XRD, nitrogen adsorption at -196 degreesC, and catalytic oxidation of CO by O-2 at 150-250 degreesC. The dopant concentration was varied between 2 and 8 wt.\% ZnO, and the solids were subjected to heat treatment at 300, 500, and 700 degreesC. The results obtained revealed that ZnO treatment of NiO and Co3O4 followed by calcination at 300 degreesC resulted in a progressive decrease in their specific surface areas. This treatment brought about an increase in the crystallite size of NiO and Co3O4 phases. ZnO doping of Co3O4 conducted at 500 and 700 degreesC effected a progressive increase in its BET surface areas. The calalytic activities towards carbon monoxide oxidation by oxygen expressed as reaction rate constant per unit surface area, (k) over bar, for NiO and Co3O4 precalcined at 300 degreesC were found to increase by ZnO doping. On other hand, doping Co3O4 carried out at 500 and 700 degreesC led to a measurable decrease in its specific catalytic activity ((k) over bar). The doping process of the investigated solids did not modify the mechanism of the catalyzed reaction but changed the concentration of the catalytically active sites without changing their energetic nature. (C) 2003 Elsevier B.V All rights reserved.