A Density Functional Theory Study on Carbon Monoxide Adsorption on Platinum–Osmium and Platinum–Ruthenium–Osmium Alloys

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.