First principles study of small palladium cluster growth and isomerization

Structures and physical properties of small palladium clusters Pd_n up to n = 15 and several selected larger clusters were studied using density functional theory under the generalized gradient approximation. It was found that small Pd_n clusters begin to grow 3‐dimensionally at n = 4 and evolve into symmetric geometric configurations, such as icosahedral and fcc‐like, near n = 15. Several isomers with nearly degenerate average binding energies were found to coexist and the physical properties of these clusters were calculated. For several selected isomers, relatively moderate energy barriers for structural interchange for a given cluster size were found, implying that isomerization could readily occur under ambient conditions. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Evolution of small copper clusters and dissociative chemisorption of hydrogen

The structural evolution of small copper clusters of up to 15 atoms and the dissociative chemisorption of H2 on the minimum energy clusters are studied systematically using density functional theory. The preferred copper sites for chemisorption are identified and the transition state structures and activation barriers for clusters four to nine atoms are determined and found to be inconsistent with the empirical Bronsted-Evans-Polanyi relationship. The physicochemical properties of the clusters are computed and compared with the bulk and surface values. The results indicate that a phase transition must occur in the going from cluster to bulk.     

The separation and determination of aluminum in plutonium-aluminum alloys

An anion-exchange method has been developed for the separation of aluminum from plutonium in a l% aluminum-plutonium alloy After separation, the aluminum is determined using oxine, either volumetrically or gravimetrically, or by a complexometric titration using EDTA The volumetric methods are favored becauae they are faster. On synthetic alloy samples containing 500 mg of aluminum the mean recovery and the standard deviation of the volumetric oxine method are 493 mg and ±008mg, respectively. Using the EDTA procedure, the comparable values are 4 92 mg and ±0 10 mg, respectively Of the elements commonly present as impurities only nickel interferes, but the volumetric procedures can be modified so that this interference is eliminated     

Langevin dynamics simulations of ds-DNA translocation through synthetic nanopores

We have implemented a coarse-grained model to study voltage-driven as-DNA translocation through nanopores located in synthetic membranes. The simulated trajectory of the DNA through the nanopores was calculated using Langevin dynamics. We present the results based on more than 120,000 individual translocations. We are particularly interested in this work in probing the physical basis of various experimentally observed--yet poorly understood--phenomena. Notably, we observe in our simulations the formation of ds-DNA hairpins, widely suspected to be the basis for quantized blockage. We study the translocation time, a measurable quantity crucially important in polyelectrolyte characterization, as a function of hairpin vertex location along the polymer backbone, finding that this behavior can be tuned to some degree by simulation parameters. We also study the voltage dependence of the tendency of hairpins to serve as the initiators of translocation events. Surprisingly, we find that the resulting probability depends vitally upon whether the events counted are ultimately successful or not. Further details lead us to propose that failed attempts in experimental translocation studies may be more common--and deceptive--than is generally recognized. We find the time taken by successful single file translocations to be directly proportional to the ratio of chain length to the applied voltage. Finally, we address a common yet puzzling phenomenon in translocation experiments: translocation events in which the current through the pore is highly, yet incompletely, blocked. We present the findings that offer a new explanation for such events.     

Formation and dynamics of van der Waals molecules in buffer-gas traps

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.     

Hydrogen dissociative chemisorption and desorption on saturated subnano palladium clusters (Pdn, n = 2-9)

H(2) sequential dissociative chemisorption on small palladium clusters was studied using density functional theory. The chosen clusters Pd(n) (n = 2-9) are of the lowest energy structures for each n. H(2) dissociative chemisorption and subsequent H atom migration on the bare Pd clusters were found to be nearly barrierless. The dissociative chemisorption energy of H(2) and the desorption energy of H atom in general decrease with the coverage of H atoms and thus the catalytic efficiency decreases as the H loading increases. These energies at full cluster saturation were identified and found to vary in small energy ranges regardless of cluster size. As H loading increases, the clusters gradually change their bonding from metallic character to covalent character. For the selected Pd clusters, the capacity to adsorb H atoms increases almost proportionally with cluster size; however, it was found that the capacity of Pd clusters to adsorb H atoms is, on average, substantially smaller than that of small Pt clusters, suggesting that the catalytic efficiency of Pt nanoparticles is superior to Pd nanoparticles in catalyzing dissociative chemisorption of H(2) molecules.     

Effect of Co doping on catalytic activity of small Pt clusters

Platinum is the most widely used catalyst in fuel cell electrodes. Designing improved catalysts with low or no platinum content is one of the grand challenges in fuel cell research. Here, we investigate electronic structures of Pt(4) and Pt(3)Co clusters and report a comparative study of adsorption of H(2), O(2), and CO molecules on the two clusters using density functional theory. The adsorption studies show that H(2) undergoes dissociative chemisorption on the tetrahedral clusters in head on and side on approaches at Pt centers. O(2) dissociation occurs primarily in three and four center coordinations and CO prefers to adsorb on Pt or Co atop atoms. The adsorption energy of O(2) is found to be higher for the Co doped cluster. For CO, the Pt atop orientation is preferred for both Pt(4) and Pt(3)Co tetrahedral clusters. Adsorption of CO molecule on tetrahedral Pt(3)Co in side on approach leads to isomerization to planar rhombus geometry. An analysis of Hirshfeld charge distribution shows that the clusters become more polarized after adsorption of the molecules.     

State-to-state rotational transitions in H2+H2 collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions between two hydrogen molecules over a wide range of energies, extending from the ultracold limit to the superthermal region. The two most recently published potential energy surfaces for the H(2)-H(2) complex, the so-called Diep-Johnson (DJ) [J. Chem. Phys. 112, 4465 (2000); 113, 3480 (2000)] and Boothroyd-Martin-Keogh-Peterson (BMKP) [J. Chem. Phys. 116, 666 (2002)] surfaces, are quantitatively evaluated and compared through the investigation of rotational transitions in H(2)+H(2) collisions within rigid rotor approximation. The BMKP surface is expected to be an improvement, approaching chemical accuracy, over all conformations of the potential energy surface compared to previous calculations of H(2)-H(2) interaction. We found significant differences in rotational excitation/deexcitation cross sections computed on the two surfaces in collisions between two para-H(2) molecules. The discrepancy persists over a large range of energies from the ultracold regime to thermal energies and occurs for several low-lying initial rotational levels. Good agreement is found with experiment B. Mate et al., [J. Chem. Phys. 122, 064313 (2005)] for the lowest rotational excitation process, but only with the use of the DJ potential. Rate coefficients computed with the BMKP potential are an order of magnitude smaller.     

Force fields for metallic clusters and nanoparticles

Atomic force fields for simulating copper, silver, and gold clusters and nanoparticles are developed. Potential energy functions are obtained for both monatomic and binary metallic systems using an embedded atom method. Many cluster configurations of varying size and shape are used to constrain the parametrization for each system. Binding energies for these training clusters were computed using density functional theory (DFT) with the Perdew-Wang exchange-correlation functional in the generalized gradients approximation. Extensive testing shows that the many-body potentials are able to reproduce the DFT energies for most of the structures that were included in the training set. The force fields were used to calculate surface energies, bulk structures, and thermodynamic properties. The results are in good agreement with the DFT values and consistent with the available experimental data.