Hopcalite-type catalysts for oxidation of CO are formed by preparing a mixed-metal oxide precursor by firstly preparing a solution of a mixture of metal precursor compounds in a solvent, followed by contacting the solution with a supercritical antisolvent to precipitate the mixed-metal oxide precursor. A mixed-metal oxide may then be prepared from the precursor by oxidation, for example by calcination. The mixed-metal oxide is then collected and optionally activated for use as a catalyst. The activated or calcined catalyst contains a nano-structured mixed-phase composition comprising phase-separated intimately mixed nanoparticles of copper and manganese oxide.