The oxidation of hydrocarbons and CO over metal oxides III. Co3O4

The catalytic oxidation of CO, C 2 H 4 , and C 2 H 6 has been studied over Co 3 O 4 and Co 3 O 4 supported on alumina. At low temperatures the reaction kinetics can be described by the general expression of Rate = kp O 2 m p CO n (or HC) p H 2 O −l e − ΔE RT where l , m , and n are in the range of 0.3 to 0.5 and ΔE 20 − 23 kcal/mol. At high temperatures the rates are controlled by bulk diffusion. The specific catalytic activity over the Co 3 O 4 powder is the highest among the various metal oxides previously studied. Mutual retardation was observed between CO and C 2 H 4 when they coexist in the gas phase. NO can reversibly inhibit the oxidation of CO at 200 °C without reaction. A few ppm of SO 2 in the gas phase can cause irreversible reduction of the Co 3 O 4 catalytic activity at 500 °C or below. At 550 °C or above, finite equilibrium between the SO 2 concentration in the gas phase and the reduction in activity can be reached, and the SO 2 inhibition becomes reversible. The activity of the supported Co 3 O 4 catalysts varies over several orders of magnitude depending on the method of preparation. In general, the mechanical mixtures of Co 3 O 4 and collodial alumina are much more active than those prepared by impregnation. The nature of the diluents used in the mixtures also influences the activity. Heating the supported catalysts to 850 °C deactivates the catalysts severely due to (1) sintering of the Co 3 O 4 , (2) sintering of the diluents to form mass transfer barrier, and (3) reaction to form the catalytically inactive CoAl 2 O 4 .