On the existence of two phases of cobalt oxide on a zirconia support

Abstract

Catalysts of varying concentrations of cobalt oxide on ZrO 2 supports were prepared by impregnation and calcination to 600 °C. In very dilute catalysts the cobalt oxide consists of a two-dimensional dispersed “δ-phase” of as yet undetermined stoichiometry. At higher concentrations, a saturated δ-phase is in equilibrium with bulk or “β-phase” particles. The latter appear to be Co 3O 4 spinel particles. The saturated δ-phase has about 4.8 × 10 18 Co atoms per m 2, i.e., about 60% of the estimated concentration of surface cobalt atoms on poly crystalline Co 3O 4. Further heating to 800 °C causes Ostwald ripening of the β-phase particles, presumably by surface diffusion via the δ-phase. This suggests very poor wetting of the support by the β-phase. Diffusion couples of Co 3O 4-porous ZrO 2, held at 850 °C for many days, also illustrate the establishment of the δ-phase. Byproducts of this experiment are the estimated values of 0.22 wt% for the solubility, and 10 −17−10 −16 cm 2/sec for the volume diffusion coefficient of cobalt (as an undetermined oxide) in monoclinic zirconia at 850 °C.