Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J27: Focus Session: Chemical Physics of Clusters: Bridging from Angstrom-scale Clusters to Micron-scale Aerosol Particles III |
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Sponsoring Units: DCP Chair: Cari Dutcher, University of Minnesota Room: 204B |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J27.00001: Solvation Effects on Structure and Charge Distribution in Anionic Clusters Invited Speaker: J. Mathias Weber The interaction of ions with solvent molecules modifies the properties of both solvent and solute. Solvation generally stabilizes compact charge distributions compared to more diffuse ones. In the most extreme cases, solvation will alter the very composition of the ion itself. We use infrared photodissociation spectroscopy of mass-selected ions to probe how solvation affects the structures and charge distributions of metal-CO$_{2}$ cluster anions. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J27.00002: Proton transfer in acetaldehyde and acetaldehyde-water clusters: Vacuum ultraviolet photoionization experiment and theoretical calculations Oleg Kostko, Tyler P. Troy, Biswajit Bandyopadhyay, Musahid Ahmed Acetaldehyde, a probable human carcinogen and of environmental importance, upon solvation provides a test bed for understanding proton transfer pathways and catalytic mechanisms. In this study, we report on single photon vacuum ultraviolet photoionization of small acetaldehyde and acetaldehyde-water clusters. Appearance energies of protonated clusters are extracted from the experimental photoionization efficiency curves and compared to electronic structure calculations. The comparison of experimental data to computational results provides mechanistic insight into the fragmentation mechanisms of the observed mass spectra. Using deuterated water for isotopic tagging, we observe that proton transfer is mediated via acetaldehyde and not water in protonated acetaldehyde-water clusters. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J27.00003: Does the 18-Electron Rule Apply to CrSi$_{12}$? Marissa Baddick Abreu, Vikas Chauhan, Arthur Reber, Shiv Khanna Understanding the bonding between silicon and transition metals is valuable for devising strategies for incorporating magnetic species into silicon. CrSi$_{12}$ is the standard example of a cluster whose apparent high stability has been explained by the 18-electron rule. We critically examine the bonding and nature of stability of CrSi$_{12}$ and show that its electronic structure does not conform to the 18-electron rule. Through theoretical studies we find that CrSi$_{12}$ has 16 effective valence electrons assigned to the Cr atom and an unoccupied 3d$_{\mathrm{z}}^{2}$ orbital. We demonstrate that the cluster's apparent stability is rooted in a crystal field-like splitting of the 3d orbitals analogous to that of square planar complexes. CrSi$_{14}$ is shown to follow the 18-electron rule and exhibits all the conventional markers characteristic of a magic cluster. We will also present results on the stability and electronic structure of FeSi$_{\mathrm{n}}$ clusters and in particular examine the valence configuration of FeSi$_{12}$ since Fe has two additional valence electrons compared to Cr. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J27.00004: Effect of N- and P-Type Doping on the Oxygen-Binding Energy and Oxygen Spillover of Supported Palladium Clusters Arthur C. Reber, Shiv N. Khanna The oxygen-binding energy is one of the primary factors determining catalytic activity in oxidation reactions. One strategy for controlling the binding of a reactant to a surface is to dope the surface to create complementary donor--acceptor pairs. As oxygen is an acceptor, we have investigated the effect of doping on the oxygen-binding energy on Pd atoms and clusters supported on a rutile TiO$_{\mathrm{2}}$(110) surface. We find that the P-type doping of the TiO$_{\mathrm{2}}$ surface dramatically reduces the O-binding energy to Pd. When extended to Pd$_{\mathrm{4}}$-supported clusters, we find that the P-type dopant decreases the energy for the oxygen to bind at spillover sites directly to the TiO$_{\mathrm{2}}$ surface. In Pd$_{\mathrm{4}}$O$_{\mathrm{2}}$, the oxygen-binding energy is reduced with P-type doping, suggesting that this strategy may be used to control the oxygen-binding energy to supported catalysts. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 4:18PM |
J27.00005: Cluster Study of Anion Specificity in Solutions: From Molecular-Like Species to Nano-Sized Droplets Invited Speaker: Xue-Bin Wang In this talk, I will present our cluster approach using size-selected, low-temperature photoelectron spectroscopy and ab initio calculations to study a variety of complex anion solvation across the Hofmeister series. Pronounced anion specific effects and rich solute-solvent, solvent-solvent interactions have been discovered en-route to solvation evolution from molecular-like species to nano-sized droplets. We found significant solute anisotropy effects in preferably selecting solvent network to align solute permanent dipole with the solvent electric field in hydrated neutral clusters. Thermodynamic advantage of organic acids in facilitating formation of bisulfate ion clusters, an important issue related to atmospheric chemistry and aerosol particle formation will also be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J27.00006: Ligand-modulated interactions between charged monolayer-protected Au144(SR)60 gold nanoparticles in physiological saline Oscar Villarreal, Liao Chen, Robert Whetten, Miguel Yacaman We studied the interactions of functionalized Au144 nanoparticles (NPs) in a near-physiological environment through all-atom molecular dynamics simulations. The AuNPs were coated with a homogeneous selection of 60 thiolates: 11-mercapto-1-undecanesulfonate, 5-mercapto-1-pentanesulfonate, 5-mercapto-1-pentane-amine, 4-mercapto-benzoate or 4-mercapto-benzamide. These ligands were selected to elucidate how the aggregation behavior depends on the ligands' sign of charge, length, and flexibility. Simulating the dynamics of a pair of identical AuNPs in a cell of saline of 150 mM NaCl in addition to 120 Na+/Cl- counter-ions, we computed the aggregation affinities from the potential of mean force as a function of the pair separation. We found that NPs coated with negatively charged, short ligands have the strongest affinities mediated by multiple Na+ counter-ions residing on a plane in-between the pair and forming ``salt bridges'' to both NPs. Positively charged NPs have weaker affinities, as Cl counter-ions form fewer and weaker salt bridges. The longer ligands' large fluctuations disfavor the forming of salt bridges, enable hydrophobic contact between the exposed hydrocarbon chains and interact at greater separations due to the fact that the screening effect is rather incomplete. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J27.00007: Structural and charge inhomogeneity in supported Pt clusters F.D. Vila, J.J. Rehr, A.I. Frenkel Nanoparticle materials are ubiquitous in heterogeneous catalytic processes and there is broad interest in their physical and chemical properties. However, global probes such as XAS and XPS reveal their ensemble properties, missing details of their internal architecture. We have previously shown\footnote{F.D. Vila \textit{et al.}, Phys. Rev. B {\bf78}, 121404(R) (2008).} that a combination of theoretical and experimental techniques is needed to understand the intra-particle heterogeneity of these systems.\footnote{J.J. Rehr and F.D. Vila, J. Chem. Phys. {\bf140}, 134701 (2014).} Recent studies of CO- and H-covered Pt clusters on C and SiO$_2$ exhibit a variety of spectral and structural trends as a function of temperature. Here we present DFT simulations showing opposite shifts in XES and XAS, as well as bond contraction with increasing temperature both arise from local electronic and structural changes upon desorption. For example, upon single CO adsorption, the Pt-Pt bonds formed by coordinated Pt atoms are locally expanded by 5\%, with little change in the rest of the particle. Coordination also has a large effect on the net charge of the Pt atoms, with a net loss of charge upon adsorption. Finally, we show how high coverage inverts the charge distribution in the clusters. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J27.00008: New Insights into the Structure of Multimetallic Nanoparticles and their Advanced Characterization Subarna Khanal, Nabraj Bhattarai, Jesus Vel\'azquez-Salazar, Gregory Guisbiers, Miguel Jose-Yacaman Noble multimetallic nanoparticles have led to exciting progress in a versatile array of applications. For the purpose of better tailoring of nanoparticles activities and understanding the correlation between their structures and properties, control over the composition, shape, size and architecture of bimetallic and multimetallic nanomaterials plays an important role on revealing their new or enhanced functions for potentials application. Advance electron microscopy techniques were used to provide atomic scale insights into the structure-properties of different materials: Pt-Pd, Au-Au$_{3}$Cu, Cu-Pt, AgPd-Pt and AuCu/Pt nanoparticles. These multimetallic nanoparticles have raised interest for their various applications in fuel cells, ethanol and methanol oxidation reactions, hydrogen storage, and so on. The nanostructures were analyzed by transmission electron microscopy (TEM) and by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM), in combination with high angle annular dark field (HAADF), bright field (BF), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) detectors. These techniques allowed us to probe the structure at the atomic level of the nanoparticles revealing new structural information and elemental composition of the nanoparticles. [Preview Abstract] |
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