We studied oxygen reduction reaction (ORR) activities for outermost surfaces of 0.3 nm thick Co deposited on Pt(111) (Co0.3 nm/Pt(111)) bimetallic systems fabricated using molecular beam epitaxy at various Co deposition temperatures. Results show that Co0.3 nm/Pt(111) fabricated at temperatures lower than 393 K displays extra low-energy electron diffraction (LEED) spots outside the integer ones, indicating incoherent epitaxial growth of Co. A new IR band that is attributed to linearly bonded carbon monoxide (CO) on the Pt site influenced by neighboring Co atoms emerges at 2052 cm–1 for 333 K fabricated Co0.3 nm/Pt(111), in addition to the CO–Pt and CO–Co bands. With increasing fabrication temperature, the new band shifts to higher frequencies and reaches 2082 cm–1 for 773 K fabricated Co0.3 nm/Pt(111), which has a diffuse (1×1) LEED pattern. We evaluated the dependence of the deposition temperature on the lattice parameters of the Co0.3 nm/Pt(111) and ascribed the band at 2082 cm–1 to adsorbed CO on a Pt-enriched topmost surface having 6-fold symmetry. Although the incoherent epitaxial Co layer was unstable in 0.1 M HClO4 aqueous solution, the Pt-enriched topmost surface is rather stable and the ORR activity is 10 times higher than that for clean Pt(111). The activities for Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111) artificial sandwich (superlattice) surfaces were also evaluated. The obtained results indicate that the Co atoms located at the second atomic layer strongly modify the electrocatalysis of the topmost surface.
The results of high-temperature compressive deformation tests on a two-phase alloy consisting of Cr2Nb, a C15 Laves phase, and a soft Nb-based solid solution, are presented along with measurements of oxidation kinetics at 1273 K in air. These alloys are deformable only at temperatures above 1273 K. The measured 0.2% yield stress decreases steadily with increasing temperature and is only slightly sensitive to alloy composition. The steady state flow stress decreases steadily with increasing temperature and depends on alloy composition. A constitutive equation was fitted to the experimental data with a composition-independent stress exponent of about 2.7 and an apparent activation energy which ranges between 477 and 391 kJ/mol, also depending on alloy composition. Microstructural examination shows that cracking (cavitation) and interfacial sliding between the two constituent phases, in addition to bulk deformation of the constituent phases, are responsible for the deformation. The oxidation resistance of these alloys is very good.
The nanostructure that consists of clusters and channels between the clusters in Nafion with absorbed water is studied by Differential Scanning Calorimeter (DSC) measurements with stepwise temperature programs. A cluster size distribution (CSD) is fitted reasonably by a log normal distribution function. The fitted CSD has a peak of approximately 4 nm in diameter. In comparison of the water content from gravimetry with that from the fitted CSD, at least three kinds of water states are found, two of which are freezable and non-freezable water in a cluster and another is the non-freezable water in a channel between the clusters. From the assumption of a simple cubic lattice model, the channel length L ch between the clusters and the cross section S ch of the channel can be estimated. Also, the degree of desulfonation in the terminal of the side chain by thermal degradation depends on the nanostructure consisting of the clusters and channels in Nafion with absorbed water. The channel length L ch decreases monotonically with increasing water content. Essentially the channel cross sectionS ch increases with increasing water content.
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