NOVELTY - A nanocomposite copper-ceria catalyst is made by combining ceria nanoparticles in an aqueous suspension with copper 2,4-pentanedionate, heating the slurry under an inert gas atmosphere or an oxygen-argon atmosphere, and optionally, subjecting the formed product to a heat treatment process to convert copper nanoparticles to copper oxide nanoparticles.    USE - For making a nanocomposite copper-ceria catalyst used in an air filter for the conversion of carbon monoxide and/or indoor volatile organic compounds (claimed).    ADVANTAGE - The method provides a nanocomposite copper-ceria catalyst that is capable of catalyzing reactions at below 200 (preferably below 30) degrees C, is capable of catalyzing oxidation reactions, reduction reactions, and/or hydrocarbon conversion reactions, and is capable of catalyzing the oxidation of carbon monoxide to carbon dioxide and/or the reduction of nitric oxide to nitrogen (all claimed).    DETAILED DESCRIPTION - Production of a nanocomposite copper-ceria catalyst comprises combining ceria nanoparticles in an aqueous suspension with copper 2,4-pentanedionate to form a slurry; heating the slurry under an inert gas atmosphere or an oxygen-argon atmosphere, at a temperature and for a time sufficient to cause decomposition of the copper 2,4-pentanedionate to form copper nanoparticles and/or copper oxide nanoparticles that are combined with the ceria nanoparticles; and optionally, subjecting the formed product to a heat treatment process under conditions effective to convert at least some of the copper nanoparticles to copper oxide nanoparticles. The produced nanocomposite copper-ceria catalyst comprises copper oxide nanoparticles, and/or copper nanoparticles, with the ceria nanoparticles.    An INDEPENDENT CLAIM is also included for a nanocomposite copper-ceria catalyst.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the experimental set-up to test catalysts in a laboratory flow tube reactor under a continuous flow of carbon monoxide gas mixture.    Tube furnace (26)    Analyzer (28)    Plug (30)    Argon gas mixture (34)    Swab (36)