Carbon monoxide adsorption on platinum-osmium and platinum-ruthenium-osmium mixed nanoparticles

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

Electron density topological and adsorbate orbital analyses of water and carbon monoxide co-adsorption on platinum

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

Group Vibrational Mode Assignments as a Broadly Applicable Tool for Characterizing Ionomer Membrane Structure as a Function of Degree of Hydration

Infrared spectra of Nafion, Aquivion, and the 3M membrane were acquired during total dehydration of fully hydrated samples. Fully hydrated exchange sites are in a sulfonate form with a C₃V local symmetry. The mechanical coupling of the exchange site to a side chain ether link gives rise to vibrational group modes that are classified as C₃V modes. These mode intensities diminish concertedly with dehydration. When totally dehydrated, the sulfonic acid form of the exchange site is mechanically coupled to an ether link with no local symmetry. This gives rise to C₁ group modes that emerge at the expense of C₃V modes during dehydration. Membrane IR spectra feature a total absence of C₃V modes when totally dehydrated, overlapping C₁ and C₃V modes when partially hydrated, and a total absence of C₁ modes when fully hydrated. DFT calculated normal mode analyses complemented with molecular dynamics simulations of Nafion with overall λ (λ_(Avg)) values of 1, 3, 10, 15 and 20 waters/exchange site, were sectioned into sub-cubes to enable the manual counting of the distribution of λ_(local) values that integrate to λ_(Avg) values. This work suggests that at any state of hydration, IR spectra are a consequence of a distribution of λ_(local) values. Bond distances and the threshold value of λ_(local), for exchange site dissociation, were determined by DFT modelling and used to correlate spectra to manually counted λ_(local) distributions

A methanol impermeable proton conducting composite electrolyte system

ABSTRACT The concept of a proton conducting, methanol impermeable composite electrolyte system is demonstrated. A three-layered laminar electrolyte consisting of a dense, methanol impermeable protonic conductor sandwiched between proton permeable electronically insulating layers permits the selective transport of protons while eliminating methanol crossover to the cathode. We demonstrate the selectivity of the composite electrolyte using palladium foil sandwiched between two Nafion TM polymer membranes. The open-circuit voltage for an Ha/O2 fuel cell utilizing this composite electrolyte is unaffected by introduction of methanol to the H2 fuel stream, whereas conventional polymer electrolyte cells suffer severe degradation of performance due to methanol crossover. Research toward the development of low temperature (80-120~ direct methanol fuel cells (DMFCs) has primarily relied on the use of proton exchange membranes such as Nafion TM as the electrolyte. A serious drawback with these polymer electrolytebased DMFC systems is that methanol diffuses through the polymer electrolyte to the cathode, degrading cell performance. Attempts to produce polymeric electrolytes that selectively transport protons, but not small organic molecules such as methanol, have met with very limited success. 1 We demonstrate here an alternative approach, a barrier concept, to the prevention of methanol crossover in DMFCs. In this design, a film of a methanol impermeable protonic conductor (MIPC), such as a metal hydride, is sandwiched between proton permeable electronic insulators, such as Nafion TM, forming a composite electrolyte. 2 Although highly permeable, inexpensive metal hydrides, such as surface modified V-15Ni-0.05Ti are known (3 • 10 -8 mol HJm s Pa ~/2, 423 K), Pd (2 • 10 9 mol H2/m s Pa ~]2, 423 K) 3 is used here as a model barrier phase; the close-packed structures of metal hydrides prevent permeation of larger molecules such as methanol and water. A hydrogen loaded palladium (palladium hydride) foil can be viewed as a proton permeable membrane: reductive adsorption of protons occurs on the surface facing the fuel anode, hydrogen diffuses through the palladium, and hydrogen atoms on the surface facing the oxygen cathode are oxidatively desorbed as protons Experimental The following electrolyte systems were studied: (i) a polymer sandwich, composed of two layers of conventional Nafion TM 115 * Electrochemical Society Active Member

Single Phase Ternary Pt-Ru-Os Catalysts for Direct Oxidation Fuel Cells

A catalyst composition for use in electrochemical reactor devices comprising platinum, ruthenium, and osmium and having a single phase crystal structure comprising a face centered cubic unit cell. In accordance with a particularly preferred embodiment, in atomic percentages, platinum comprises about 65% of the catalyst composition, ruthenium comprises about 25% of the catalyst composition, and osmium comprises about 10% of the catalyst composition.Sponsorship: Illinois Institute of TechnologyUnited States Paten

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