Using high-resolution transmission electron microscopy (TEM), infrared reflection-absorption spectroscopy (IRAS), and electrochemical (EC) measurements, platinum nanoparticles ranging in size from 1 to 30 nm are characterized and their catalytic activity for CO electrooxidation is evaluated. TEM analysis reveals that Pt crystallites are not perfect cubooctahedrons, and that large particles have "rougher" surfaces than small particles, which have some fairly smooth (111) facets. The importance of "defect" sites for the catalytic properties of nanoparticles is probed in IRAS experiments by monitoring how the vibrational frequencies of atop CO (nu(CO)) as well as the concomitant development of dissolved CO(2) are affected by the number of defects on the Pt nanoparticles. It is found that defects play a significant role in CO "clustering"on nanoparticles, causing CO to decrease/increase in local coverage, which yields to anomalous redshift/blueshift nu(CO) frequency deviations from the normal Stark-tuning behavior. The observed deviations are accompanied by CO(2) production, which increases by increasing the number of defects on the nanoparticles, that is, 1 < or = 2 < 5 << 30 nm. We suggest that the catalytic activity for CO adlayer oxidation is predominantly influenced by the ability of the surface to dissociate water and to form OH(ad) on defect sites rather than by CO energetics. These results are complemented by chronoamperometric and rotating disk electrode (RDE) data. In contrast to CO stripping experiments, we found that in the backsweep of CO bulk oxidation, the activity increases with decreasing particle size, that is, with increasing oxophilicity of the particles.

Electrochemical measurements showed an ≈75% Pt surface area loss and an ≈40% specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated “ normalPt3Co ” catalyst particles in a normalH2∕normalN2 proton exchange membrane fuel cell after 24h voltage cycling between 0.65 and 1.05V vs reversible hydrogen electrode. Transmission electron microscopy, scanning transmission electron microscopy, associated X-ray energy dispersive spectroscopy, and high angle annular dark-field techniques were used to probe the microstructural changes of the MEA cathode and the compositional changes along the MEA cathode thickness and within individual normalPtxCo nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated normalPtxCo particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell normalPtxCo particles was largely responsible for the reduction in the specific activity of normalPtxCo nanoparticle after potential cycling. The Pt weight loss associated with the formation of Pt crystallites near the cathode/membrane interface largely contributed to the measured electrochemical surface area loss, while particle growth of the normalPtxCo particles via Ostwald ripening played a lesser role.

Extended pi-conjugated molecules are interesting materials that have been studied theoretically and experimentally with applications to conducting nanowire, memory, and diode in mind. Chemical understanding of electron transport properties in molecular junctions, in which two electrodes have weak contact with a pi-conjugated molecule, is presented in terms of the orbital concept. The phase and amplitude of the HOMO and LUMO of pi-conjugated molecules determine essential properties of the electron transport in them. The derived rule allows us to predict single molecules' essential transport properties, which significantly depend on the type of connection between a molecule and electrodes. Qualitative predictions based on frontier orbital analysis about the site-dependent electron transport in naphthalene, phenanthrene, and anthracene are compared with density functional theory calculations for the molecular junctions of their dithiolate derivatives, in which two gold electrodes have strong contact with a molecule through two Au-S bonds.