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

Ensemble Site Requirements for Oxidative Adsorption of Methanol and Ethanol on Pt Membrane Electrode Assemblies

The ensemble site requirements for the oxidative adsorption of methanol and ethanol on platinum based membrane electrode assemblies in operating liquid feed fuel cells were measured by CO stripping voltammetry. At 30 °C and 0.2 V vs reference hydrogen electrode (RHE), the CO_ads coverage from directly dosed CO (CO_CO), methanol (CO_MeOH), and ethanol (CO_EtOH) are 94%, 49%, and 39%, respectively. At 50 °C the CO_MeOH and CO_EtOH approach equality. The ratio of CO_EtOH/CO_MeOH was simulated with assumed ensemble site requirements of 3 and 2 for ethanol and methanol respectively. Experimental and simulated ratios of 0.79 and 0.78 suggest that high surface area fuel cell Pt catalysts at 30 °C have adsorption properties similar to that of a Pt (100) surface. Potential dependent infrared spectroscopy of CO_MeOH and CO_EtOH from flash evaporated aqueous alcohols delivered to a 50 °C fuel cell show lower CO_EtOH relative to CO_MeOH with Stark tuning rates below 10 cm^–1/V

Ensemble Site Requirements for Oxidative Adsorption of Methanol and Ethanol on Pt Membrane Electrode Assemblies

The ensemble site requirements for the oxidative adsorption of methanol and ethanol on platinum based membrane electrode assemblies in operating liquid feed fuel cells were measured by CO stripping voltammetry. At 30 °C and 0.2 V vs reference hydrogen electrode (RHE), the CO_ads coverage from directly dosed CO (CO_CO), methanol (CO_MeOH), and ethanol (CO_EtOH) are 94%, 49%, and 39%, respectively. At 50 °C the CO_MeOH and CO_EtOH approach equality. The ratio of CO_EtOH/CO_MeOH was simulated with assumed ensemble site requirements of 3 and 2 for ethanol and methanol respectively. Experimental and simulated ratios of 0.79 and 0.78 suggest that high surface area fuel cell Pt catalysts at 30 °C have adsorption properties similar to that of a Pt (100) surface. Potential dependent infrared spectroscopy of CO_MeOH and CO_EtOH from flash evaporated aqueous alcohols delivered to a 50 °C fuel cell show lower CO_EtOH relative to CO_MeOH with Stark tuning rates below 10 cm^–1/V

Thermal Processing as a Means to Prepare Durable, Submicron Thickness Ionomer Films for Study by Transmission Infrared Spectroscopy

A high temperature solution processing method was adapted to prepare durable, freestanding, submicrometer thickness films for transmission infrared spectroscopy studies of ionomer membrane. The materials retain structural integrity following cleaning and ion-exchange steps in boiling solutions, similar to a commercial fuel cell membrane. Unlike commercial membrane, which typically has thicknesses of >25 μm, the structural properties of the submicrometer thickness materials can be probed in mid-infrared spectral measurements with the use of transmission sampling. Relative to the infrared attenuated total reflection (ATR) technique, transmission measurements can sample ionomer membrane materials more uniformly and suffer less distortion from optical effects. Spectra are reported for thermally processed Nafion and related perfluoroalkyl ionomer materials containing phosphonate and phosphinate moieties substituted for the sulfonate end group on the side chain. Band assignments for complex or unexpected features are aided by density functional theory (DFT) calculations