Abstract
The surface structures of the (2×2)-O phase and the (O+CO) coadsorbate system on Pd(111) are determined by low energy electron diffraction (LEED) and density functional theory (DFT) calculations. To elucidate the energetics and electronic structure of the OPd(111) system, we resort to state-of-the-art DFT calculations. In the (2×2)-O phase the oxygen atoms reside in fcc sites. The coadsorption of CO induces a compression of the (2×2)-O into the ( 3 × 3 )R30°-O overlayer as a consequence of the large difference in adsorption energy of CO in on-top and fcc sites (0.5 eV). The local adsorption geometry of oxygen in the ( 3 × 3 )R30°-O phase is identical to that found for the (2×2)-O phase. DFT calculations give evidence that the CO molecules form a separate ( 3 × 3 )R30° phase rather than a mixed (O+CO) overlayer. The rather weak OPd bonding is discussed in electronic terms by comparison with the related ORu(0001) system.
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