Abstract
Periodic density functional theory calculations on carbon monoxide (CO) adsorbed atop on platinum–osmium binary alloys (PtOs2 and PtOs4) and the platinum–ruthenium–osmium tertiary alloy (PtRu2Os2) are used to elucidate the changes in the C–O and C–Pt bonds upon alloying Pt with Ru/Os atoms. As Pt is alloyed with Ru/Os atoms, the adsorbate internal bond (C–O bond) and the adsorbate–metal bond (C–Pt bond) strengthen following the substrate trends of PtOs4 > Pt > PtOs2 > PtRu2Os2 and Pt > PtOs4 > PtOs2 > PtRu2Os2, respectively. These trends are manifested by the corresponding C–O and C–Pt stretching frequencies and the CO adsorption energy variations. Here, we establish a theoretical framework based on the π-attraction σ-repulsion mechanism to explain the above results. This model correlates the charges, polarizations, and electron densities of the adsorbate CO orbitals, and the sp/d populations of the adsorbing Pt atom. For the systems studied here, the traditional theoretical model of 5σ-donation/2π*-back-donation with the metal substrate bands is not always sufficient to explain the relative C–O and C–Pt bonds strengths.
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