The interaction between carbon monoxide (CO) and a Pd70Au30(111) alloy surface was investigated under CO pressures with a wide range from ultrahigh vacuum (UHV) to sub-Torr at room temperature by a combination of near-ambient pressure (NAP) X-ray photoelectron spectroscopy and density functional theory calculations. The adsorption site and surface coverage of CO are reversibly controlled by the CO pressure. Under UHV conditions, the CO molecules occupy bridge and hollow sites on contiguous Pd clusters in the Au-rich surface layer. Exposure to sub-Torr CO gas induces site switching of the adsorbed CO on the contiguous Pd clusters from multiple-coordination (hollow and bridge) sites to single-coordination (top) sites, even though the latter sites are energetically less favorable. This behavior is explained by a pressure-induced entropic effect on gas-phase CO, which is in equilibrium with the adsorbed CO. This site switching highlights an important aspect of high-pressure-induced adsorption behavior.
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