Formation and phase transition of cobalt molybdate in cobalt oxide-molybdenum trioxide-alumina system

Abstract

In the cobalt oxide-molybdenum trioxide-alumina system with a molar ratio of 1:1:1, the amounts of the high-temperature modification ( a) of CoMoO 4 formed during heating from 500 to 800°C and the low-temperature modification ( b) formed by phase transition during the subsequent cooling to room temperature are influenced by the kinds of alumina used, such as α-, γ- and calcined γ-aluminas. Powder X-ray diffraction analysis revealed that in an α-alumina system formation of a-CoMoO 4 is most favorable at a calcination temperature of 500°C and phase transition from a- to b-CoMoO 4 during cooling is enhanced by higher calcination temperatures. In the γ-alumina system, formation of a-CoMoO 4 is slight at 500°C but increases with increase in the calcination temperature, as does slightly the degree of phase transition from a- to b-CoMoO 4 upon cooling. In a system containing calcined γ-alumina, formation of a-CoMoO 4 similar to α- and γ-alumina systems was observed to occur at 500°C and 800°C, respectively, together with phase transition to b-CoMoO 4 during cooling. The degree of dispersion in the CoOMoO 3 coexistent system is affected by the particle size of aluminas, such as coarse α-, fine amorphous γ- and calcined γ-alumina consisting of both sizes, as observed with electron microscopy. Presence of finer γ-alumina is considered to suppress or retard the solid state reaction and phase transition.