Periodic density functional theory calculations elucidate carbon monoxide coverage effects on platinum and ruthenium surfaces. As expected the CO stretching frequencies increase with coverage. Unexpectedly, overlap population calculations show that increased stretching frequencies may not always correspond to stronger bonds. A theoretical framework is established based on a modified π-attraction σ-repulsion scheme. This phenomenological model directly relates the internal adsorbate bond strength to the net change of the carbon 2s and 2p xy contributions to the π- and σ-components, respectively. The variation of the metal–surface bond is examined by using the charges, polarizations, and electron densities of the adsorbate CO orbitals. For the systems studied here, the traditional frontier orbital model of the 5σ-donation/2π*-back-donation with the metal substrate bands is not always sufficient to explain the relative C–O and C–Metal bonds strengths.

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Density functional calculations (DFT) on carbon monoxide (CO) adsorbed on platinum, platinum-osmium, and platinum-ruthenium-osmium nanoclusters are used to elucidate changes on the adsorbate internal bond and the carbon-metal bond, as platinum is alloyed with osmium and ruthenium atoms. The relative strengths of the adsorbate internal bond and the carbon-metal bond upon alloying, which are related to the DFT calculated C-O and C-Pt stretching frequencies, respectively, cannot be explained by the traditional 5σ-donation/2π*-back-donation theoretical model. Using a modified π-attraction σ-repulsion mechanism, we ascribe the strength of the CO adsorbate internal bond to changes in the polarization of the adsorbate-substrate hybrid orbitals towards carbon. The strength of the carbon-metal bond is quantitatively related to the CO contribution to the adsorbate-substrate hybrid orbitals and the sp and d populations of adsorbing platinum atom. This work complements prior work on corresponding slabs using periodic DFT. Similarities and differences between cluster and periodic DFT calculations are discussed.

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Ternary and quaternary PtRuOs and PtRuOsIr alloys are promising alternatives to the binary PtRu alloys that serve as efficient anode catalysts for methanol and hydrogen air fuel cells. The efficiency of these catalysts is correlated to the adsorption of CO molecules on their surfaces. In this work, we study CO adsorption on a series of PtRu, PtOs, PtRuOs, and PtRuOsIr alloys and on pure Pt, Os, Ir, and Ru using periodic density functional theory. Systematically, we vary the location of the alloy atoms in the substrate and the alloy Pt mole percent. As CO is adsorbed on PtRu, PtRuOs, and PtRuOsIr alloys, the CO internal adsorbate bond and the C–Pt surface bond weaken on average (for alloy configurations of the same Pt mole percent) along with the decrease of the Pt mole fraction in the alloy. However, the frozen substrate calculations show that these bonds are about invariant of alloying Pt with Os atoms, with the exception of PtOs configurations with Os atoms in the middle layer, whereas relaxing the substrate surface may lead to stronger C–O and C–Pt bonds due to alloying Pt with Os. The C–O and C–Pt overlap populations are correlated with the carbon s-type vacancies and the overall s, p, and d vacancies of the adsorbing metal, for the C–O and C–Pt bonds, respectively: Hybridization defects are attributed to the cases of concomitant increase of the overlap populations and downshifts of the corresponding stretching frequencies. Changes in the CO internal adsorbate bond are explained using a phenomenological model based on the modified π-attraction σ-repulsion scheme and is compared with the traditional 5σ donation–2π* back-donation mechanism. This model successfully ascribes the C–O internal adsorbate bond strength to the carbon and oxygen atom contributions of the σ and π hybrid CO-substrate orbitals for the majority of the systems examined here.