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The electron density topology of carbon monoxide (CO) on dry and hydrated platinum is evaluated under the quantum theory of atoms in molecules (QTAIM) and by adsorbate orbital approaches. The impact of water co-adsorbate on the electronic, structural, and vibrational properties of CO on Pt are modelled by periodic density functional theory (DFT). At low CO coverage, increased hydration weakens C–O bonds and strengthens C–Pt bonds, as verified by changes in bond lengths and stretching frequencies. These results are consistent with QTAIM, the 5σ donation-2π* backdonation model, and our extended π-attraction σ-repulsion model (extended π-σ model). This work links changes in the non-zero eigenvalues of the electron density Hessian at QTAIM bond critical points to changes in the π and σ C–O bonds with systematic variation of CO/H2O co-adsorbate scenarios. QTAIM invariably shows bond strengths and lengths as being negatively correlated. For atop CO on hydrated Pt, QTAIM and phenomenological models are consistent with a direct correlation between C–O bond strength and CO coverage. However, DFT modelling in the absence of hydration shows that C–O bond lengths are not negatively correlated to their stretching frequencies, in contrast to the Badger rule: When QTAIM and phenomenological models do not agree, the use of the non-zero eigenvalues of the electron density Hessian as inputs to the phenomenological models, aligns them with QTAIM. The C–O and C–Pt bond strengths of bridge and three-fold bound CO on dry and hydrated platinum are also evaluated by QTAIM and adsorbate orbital analyses. [ABSTRACT FROM AUTHOR]

An in situ x-ray absorption spectroscopy cell was designed to study the structural and electronic properties of high performance polymer electrolyte fuel cell electrocatalysts. The cell was operated as a standard fuel cell and x-ray absorption data were collected simultaneously as the fuel cell operating conditions were varied. The cell was used to examine platinum-ruthenium alloy electrocatalysts incorporated in the anode of a fuel cell membrane and electrode assembly. In situ Pt L[sub III] and Ru K edge x-ray absorption near-edge structure (XANES) data were collected at five selected current densities of the operating fuel cell. The XANES data show that under normal fuel cell operating conditions, the metallic nature of the Pt-Ru catalyst is retained. The cell design enabled an in situ fuel cell study at a synchrotron source in which no additional electrolytes were required. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]