Elecrochemical ATR-FIFIRAS measurements were conducted for the first time to investigate nature of CO adsorbed under potential control on a highly dispersed Pt catalyst with average particle size of 2.6 nm supported on carbon black (Pt/C) and carbon un-supported Pt black catalyst (Pt-B). Each catalyst was uniformly dispersed by 10 mu g Pt/cm(2) and fixed by Nafion (R) film of 0.05 mu m thick on a gold film chemically deposited on a Si ATR prism window. Adsorption of CO was conducted at 0.05 V on the catalysts in I and 100% CO atmospheres, for which CO coverage, Oco, was 0.69 and 1, respectively. Two well-defined v(CO) bands free from band anomalies assigned to atop CO (CO(L)) and symmetrically bridge bonded CO (CO(B)(sym)) were observed. It was newly found that the CO(L) band was spitted into two well-defined peaks, particularly in 1% CO, from very early stage of adsorption, which was interpreted in terms of simultaneous occupation of terrace and step-edge sites, denoted as CO(L)(terrace) and CO(L)(edge) respectively. This simultaneous occupation was commonly observed in our work both on Pt/C and Pt-B. A new band was also observed around 1950 cm(-1) in addition to the bands of CO(L) and CO(B)(sym)., which was assigned to asymmetric bridge CO, CO(B)(asym) adsorbed on (10 0) terraces, based on our previous ECSTM observation of CO adsorption structures on (10 0) facet. The CO(B)(asym) on the Pt/C, particularly in 100% CO atmosphere, results in growth of a sharp band at 3650 cm(-1) accompanied by a concomitant development of a band around 3500 cm(-1). The former and the latter are assigned to v(OH) vibrations of non-hydrogen bonded and hydrogen bonded water molecules adsorbed on Pt, respectively, interpreted in term of results from a bond scission of the existing hydrogen bonded networks by CO(L)s and from a promotion of new hydrogen bonding among water molecules presumably by CO(B)(asym). It was found that the frequency v(CO) of CO(L) both on Pt/C and Pt-B is lower than that on bulky polycrystal line electrode Pt(poly) or different crystal planes of Pt single-crystal electrodes by 30-40 cm(-1) at corresponding potentials, which implies a stronger electronic interaction between CO and Pt nano-particles and/or an increased contribution of step-edge sites on the particles. Determination of the band intensities of CO(L), CO(B)(asym) and CO(B)(asym) has led us to conclude a much higher bridged occupation of sites at Pt nano-particles than Pt(poly) electrodes. (c) 2007 Elsevier Ltd. All rights reserved.