TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The aqueous solution comprising copper is prepared by dissolving 120-190 g of a Cu precursor in 450-500 ml of water. The heating of the solution includes heating the solution to 200-500degrees C. It is done by a first heat-treatment at 200degrees C, a second heat-treatment at 300degrees C, and a third heat-treatment at 550degrees C. The calcinating of the solid includes heating the solid to 450-550degrees C, heating the solid for 2-6 hours, and converting the solid to a solid mixture. The method further includes adding a carrier to the solution. Preferred Component: The carbon monoxide oxidizing catalyst comprises a solid solution. The compound preferably comprises Se oxide. The compound and the cesium (Cs) oxide respectively comprise an atomic ratio of 0.01-0.5:1. The compound, the copper oxide, and the cesium oxide, respectively comprise a weight ratio of 0.1-1:4-5:15-45, preferably 0.1-1:4-5:20-22. The carbon monoxide oxidizing catalyst further comprises the carrier from aluminum oxide (Al2O3) (preferably), titanium dioxide (TiO2), and/or silicon dioxide (SiO2). The Ce precursor is at least one of cerium nitrate hexahydrate (Ce(NO3)2.6H2O), and/or ceric ammonium nitrate ((NH4)2Ce(NO3)6). The Se precursor comprises selenious acid (H2SeO3). The Te precursor comprises telluric acid (H2TeO3). The Bi precursor comprises bismuth oxide (Bi2O3). The Cu precursor is at least one of copper nitrates (Cu(NO3)2.3H2O and/or Cu(NO3)2.2.5H2O). The Se oxide and the Cs oxide are in an atomic ratio of 0.02:1. EXAMPLE - Cerium nitrate hexahydrate (Ce(NO3)2.6H2O) (10.596 g) and selenious acid (H2SeO3) (0.035 g) were dissolved in 10 ml of a copper nitrate (Cu(NO3)2.3H2O) aqueous solution that was prepared by dissolving 2.599 g of Cu(NO3)2.3H2O in 10 ml of water to prepare a concentrated solution. Twenty ml of aluminum oxide (Al2O3) (14.8 g) was added to the solution. While stirring, the solution was variably heated to 100-600degrees C, and the solution was thus evaporated to obtain a compound (solid). The compound was calcined at 500degrees C for 5 hours to obtain a solid solution carbon monoxide oxidizing catalyst comprising 4.30 wt.% of cupric oxide (CuO), 21.12 wt.% of cerium oxide (CeO2), and 0.15 wt.% of selenium oxide (SeO2) supported on Al2O3.