Julius Jellinek

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Solid–liquid phase changes in simulated isoenergetic Ar13

Simulations by molecular dynamics of 13‐particle clusters of argon display distinct nonrigid, liquid‐like and near‐rigid, solid‐like ‘‘phases.’’ The simulations, conducted at constant total energy, display a low‐energy region in which only the solid‐like form appears, a high‐energy region in which only the liquid‐like form appears, and an intermediate band of energy—a ‘‘coexistence region’’— in which clusters exhibit both forms. The intervals of time spent in each phase in the two‐form coexistence region are long compared with the intervals required to establish equilibrium‐like properties distinctive of each form, such as mean square displacement and power spectrum, so that well‐defined phases can be said to exist. The fraction of time spent in each phase is a function of the energy. When a long simulation is separated into regions of solid‐like and liquid‐like behavior, the curve of the derived caloric equation of state is double valued in the two‐phase range of energy, forming two well‐defined, smooth ...

Rare gas clusters: Solids, liquids, slush, and magic numbers

Simulations by constant energy molecular dynamics have been performed for numerous clusters in the size range N=7–33. Detailed investigations have been conducted on the portions of the caloric curves in which the transition between rigid and nonrigid behavior occurs, to study the N dependence of the solid–liquid phase change. Clusters of several sizes display a coexistence of forms, each with a characteristic mean temperature, over a well‐defined energy range in the transition region, as had been observed for the Ar13 cluster. Within the coexistence region, the high temperature form is solid‐like and the low temperature form behaves in a liquid‐like fashion. The caloric curves of state for these clusters take on two‐valued forms when averages are calculated for each of the two ‘‘phases’’ separately; the two branches are smooth extensions of the curves from the single phase regions. Clusters of other sizes do not display this clear coexistence of phases, but appear to pass through a ‘‘slush‐like’’ state du...

Melting and freezing in isothermal Ar13 clusters

Microcanonical simulations have shown that Ar13 clusters have sharp but unequal melting and freezing energies. Between these energies, a hot solid‐like form and a cooler, liquid‐like form coexist in dynamic equilibrium. Monte Carlo and isothermal molecular dynamics simulations confirm that this coexistence behavior persists under canonical conditions as well. Many properties demonstrate the solid and liquid character of the two coexisting ‘‘phases.’’ One previous result seemed to contradict this: Quirke and Sheng evaluated nearest neighbor angular distribution function P(θ); its nonzero value for θ=π/2 at 33 K was interpreted as that of a hot solid in a ‘‘premelting expansion.’’ Actually, that result is the average of a bimodal distribution, one mode for the solid and the other for the liquid. The average shifts smoothly with T, and each form’s P(θ) changes slightly with temperature. The solid has tiny nonzero probability for π/2. The liquid has a minimum probability there, but far above zero. Mean‐square...

NinAlm alloy clusters: analysis of structural forms and their energy ordering

Abstract Structural forms of model Ni n Al m , n + m = 13, alloy clusters are studied for all stoichiometric compositions using a many-body potential. These forms are distinguished by their geometries ( isomers ) and the distributions of the two types of atoms within a given geometry ( homotops ). General notions of mixing energy and mixing coefficient applicable to arbitrary, including many-body, potentials are introduced. It is shown that for each stoichiometry the energy ordering of the homotops, within classes defined by the isomeric form and the type of the central atom, is governed by the degree of mixing as measured by the mixing coefficient.

Capping Ligands as Selectivity Switchers in Hydrogenation Reactions

We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt(3) NPs can drastically increase selectivity for alkene from 0 to more than 90% with ~99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes.

Structure and shape variations in intermediate-size copper clusters.

Using extensive, unbiased searches based on density-functional theory, we explore the structural evolution of Cu(n) clusters over the size range n=8-20. For n=8-16, the optimal structures are plateletlike, consisting of two layers, with the atoms in each layer forming a trigonal bonding network similar to that found in smaller, planar clusters (n<or=6). For n=17 and beyond, there is a transition to compact structures containing an icosahedral 13-atom core. The calculated ground-state structures are significantly different from those predicted earlier in studies based on empirical and semiempirical potentials. The evolution of the structure and shape of the preferred configuration of Cu(n), n<or=20, is shown to be nearly identical to that found for Na clusters, indicating a shell-model-type behavior in this size range.

Unequal freezing and melting temperatures for clusters

Abstract Using a previously introduced quantum statistical model, analyze necessary and sufficient conditions on the density of states for coexistence of liquid and solid clusters. We conclude that clusters of N particles have sharp freezing and melting temperatures limiting the ranges of phase stability and that these temperatures are unequal.

Quantum mechanical treatment of the F+D2→DF+D reaction

Reactive infinite order sudden (RIOS) approximation calculations for the F+D2→DF+D reaction using the Muckerman 5 potential are reported. Primitive γ‐dependent state resolved reaction amplitudes and probabilities, γ‐averaged probabilities, vibrational state resolved angular distributions, total integral cross sections, and vibrational branching ratios are presented. The results are compared against results of other methods and against similar RIOS results for the F+H2→HF+H reaction. The accuracy of the RIOS results is discussed and in addition, qualitative comparison with experimental vibrational state resolved angular distributions is made. We find that the RIOS F+D2 results are similar to RIOS results for the F+H2 system. Similarly, experimental results for these two systems are in qualitative agreement with one another. In the case of comparison of RIOS and experimental vibrational state resolved angular distributions, qualitative agreement is obtained for the vf =3,2,1 DF product states. However, the ...

Structural and dynamical properties of transition metal clusters

Results of molecular dynamics simulation studies of structural and dynamical properties of 12-, 13-, and 14-atom transition metal clusters are presented. The calculations are carried out using a Gupta-like potential expressed in reduced units. The transformation to absolute units involves two size-dependent parameters which effectively convert the potential into a size-dependent one. The minimum energy geometries of the clusters are obtained through the technique of simulated thermal quenching. A melting-like transition is observed as the energy of the clusters is increased. A novel element of the transition is that it may involve a premelting state.