We investigated catalytic activity of CO oxidation by monatomic-layered cluster disks, Pt-30, bonded to a silicon substrate by means of ultrahigh-sensitive temperature-programmed desorption (TPD) mass-spectrometry. It was found in the repeated TPD measurements with various reactant-exposure amounts that this cluster-substrate system possesses high ability of low-temperature reductive activation of oxygen molecules, which is the rate-determining step of the CO oxidation. This high-performance is explained in terms of negative charges accumulated at a subnano interface between the cluster disk and the silicon substrate surface.