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.
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.
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.
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