Cobalt oxide is the inorganic compound

Inorganic compound

Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula Co₂O₃. Although only two oxides of cobalt are well characterized, CoO and Co₃O₄, procedures claiming to give Co₂O₃ have been described. Thus treatment of Co(II) salts such as cobalt(II) nitrate

Cobalt(II) nitrate

Cobalt nitrate is the inorganic cobalt salt of nitric acid, often with various amounts of water. It is more commonly found as a hexahydrate, Co2·6H2O, which is a red-brown deliquescent salt that is soluble in water and other polar solvents....

 with an aqueous solution of sodium hypochlorite (also known as bleach) gives a black solid. Some formulations of the catalyst hopcalite

Hopcalite

Hopcalite is a mixture of copper and manganese oxides used as catalyst to convert carbon monoxide into carbon dioxide when exposed to the oxygen in air. A variety of composition for are known, "Hopcalite II" is approximately 3:1 manganese dioxide : copper oxide...

contain "Co₂O₃".