Changes in the reaction behaviour of supported cobalt catalysts for CO oxidation were investigated with various sizes of cobalt metal particles dispersed on the supports.
Catalysts were prepared by conventional imprengation and ion exchange methods. These catalysts were characterized by hydrogen and carbon monoxide adsorption. Extents of reduction was measured by $O_2$ titration at 400℃ and by $H_2$ consumption at reduction temperature. The average size of cobalt metal particles was determined by hydrogen chemisorption.
CO oxidation was performed in a differential reactor at atmospheric pressure, at reaction temperature of 120℃, and with the reactant ratio of $O_2$/CO = 1. CO oxidation behaviour was influenced by different support materials and by the size of cobalt metal particles. For the larger cobalt metal particles formed on the external surface of zeolite, the initial reaction activity is maintained over the reaction period. But smaller cobalt metal particles formed in the zeolite pore structure (supercage, I, or II sites) and on silica support show different deactivation behaviour. For these smaller cobalt metal particles, the initial activity decreased sharply with reaction time. Moreover, deactivation rate decreased as the size of cobalt metal particles decreased. Also, carbon monoixde adsorbed on small cobalt particles was disproportionated to the surface carbon and carbon dioxide. This was not observed in larger cobalt metal particles. In contrast to the carbon monoxide adsorption, oxygen was adsorbed more on the larger cobalt metal particles.
As a result of disproportionation of carbon monoxide on small cobalt metal particles and adsorption properties of carbon monoxide and oxygen, the turnover frequency (T.O.F) of carbon monoxide oxidation on supported cobalt catalysts decreases as the size of cobalt metal particles decreases, indicating that CO oxidation is structure-sensitive reaction below 100Å in the size of cobalt metal particles.
