Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

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the electrode. Both the huge oxidative currents (note change in scale), and the dramatic degradation of the electrode are typical of all Ptn/GCE samples other than Pt7. The glitches in the CVs are due to partial blockage of the small volume cell by bubbles, leading to complete shut-down of the cell after a few CV cycles. Gas generation occurs in both N2- and O2-saturated HClO4, and also in H2SO4 electrolytes, even though we are careful to stay out of the potential regions where water splitting occurs. The electrode degradation occurs only in the central 2 mm region of the electrode, where clusters were deposited, and not in the surrounding region of GCE also exposed to electrolytes. The appearance is of a combination of blistering of the GCE and etching of the surface, which is initially found to have roughness of only ~±2 nm by AFM. The conclusion is that for Ptn, n ≠ 7, the small clusters are highly active for carbon oxidation by water, generating CO2 which eventually blocks further current flow, and destroying the electrode surface. Exposure to air after cluster deposition, and prior to electrochemistry, has a huge effect on the activity of these small clusters. For Pt7, the ORR and Pt redox signals are essentially gone, replaced by a symmetric pair of redox waves that look like slow, reversible redox of some adventitious adsorbate. This effect is not unexpected, because any sample exposed to atmosphere even briefly is always covered with carbonaceous adsorbates. For the other size clusters, which are highly reactive in absence of air exposure, the effect is even more dramatic. There is essentially no electrochemical current, other than capacitive currents on the order of 0.1 μA, essentially identical to what is seen on clean Pt-free GCEs. It appears, therefore, that these clusters must react with adventitious adsorbates in such a way to completely passivate the electrode. As expected from the lack of electrochemical currents, these air-exposed electrodes show no sign of degradation in repeated cycling. XPS of the Ptn/GCE samples, as prepared in UHV, is shown in Fig. 3. In addition to the trend, expected from final state screening considerations, of an overall drop in Pt 4f7/2 binding energy with increasing size, there are also strong oscillations indicating that the different clusters have different electronic properties. The relative inactivity of Pt7 may be correlated with the observation that it is a local maximum in the XPS binding energy (which may indicate a particularly stable, unpolarizable valence structure), however, Pt10 also has a high Pt 4f binding energy, and it highly active. Calculations on Ptn structures and electronic properties are underway in the group of Shiv Khanna at VCU. Electronic Structure We previously demonstrated a one-to-one correlation between the binding energy of the Pd 3d core level, and the activity for CO oxidation of Pdn/TiO2(110). The strong correlation was somewhat surprising in that chemical reactions are expected to depend only to the valence electronic structure, and core levels are only an indirect probe of the valence structure. Therefore, we have been using a combination of ultraviolet photoelectron spectroscopy (UPS), and ion neutralization spectroscopy (INS) to look at the valence structure of metal clusters deposited on supports, and looking for correlations with activity. Fig. 4 shows an example of UPS for two different sizes of Pdn deposited on TiO2(110), taken in the course of a study of of methanol decomposition over Pdn/TiO2(110). Here data are shown only for Pdn/TiO2, however, we also have data for MeOH/Pdn/TiO2 taken at temperatures corresponding to different steps in the decomposition/desorption process. In this case, we take advantage of the band gap of TiO2 to use UPS to look at the onset of photoemission from Pdn in the region between the Fermi level, and the onset of the large oxygen peak from TiO2. The small peak just below the Fermi level for clean TiO2 results from Ti 3+ at vacancies and interstitial sites. The lower left inset shows the near-Fermi level region for two example clusters, where the signal from the TiO2 substrate has been subtracted. Note that the onset of emission is well below the Fermi level, and is size dependent. As shown in the upper right inset, which gives preliminary emission onset energies obtained from a rather crude spectral deconvolution method, the onset moves closer to the Fermi level with increasing size, however, there is still a significant shift, indicating that these small clusters are not yet metallic. We are currently working on a more quantitative approach to deconvoluting the spectrometer function from the data. 7
Future Plans In addition to the TiO2 work, we have very interesting results for Pdn on different alumina supports, and are planning to go back and look at these systems with UPS and INS with the goal of understanding how interaction with different supports influences the valence structure in different size clusters. We also have carried out a series of non-size selected catalysis experiments (using a second, “test” instrument) focusing Fig. 4. UPS data for Pdn/TiO2(110). Main frame: raw data for TiO2 and 0.1 ML Pd1/TiO2. Lower Left Inset: Comparison of near Fermi level portion of subtracted spectra for Pd1/TiO2 and Pd25/TiO2. Upper Right Inset: Fitted shift in Pdn absorption edge, relative to Fermi level (results from preliminary fitting). on hydrogen activation over Pd, and subsequent spillover to underlying metal films for storage. The intention is to examine hydrogen storage via hydride formation in magnesium, however, we also observed reversible hydrogen uptake for Pd deposited on MoOx on Mo(110), with efficiency many times greater than that for a 100 layer Mg film. Given work by Ceyer indicating that unique chemistry can be driven by escaping subsurface hydrogen in other metals, this seems like a potentially interesting system, and we will look at some selective hydrogenation chemistry on size-selected Pdn/MoOx/Mo. Finally, we are considering another round of electrochemistry experiments, probably in a non-aqueous solvent, to avoid the highly efficient destruction of the electro by oxidation from water. Publications acknowledging DOE support “Size-dependent oxidation of Pdn (n ≤ 13) on alumina/NiAl(110): Correlation with Pd core level binding energies.” Tianpin Wu, William E. Kaden, William A. Kunkel, and Scott L. Anderson, Surf. Sci. 603 (2009) 2764-77. “Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces”, William E. Kaden, Tianpin Wu, William A. Kunkel, Scott L. Anderson, Science. 326 (2009) 826 - 9. “CO Adsorption and Desorption on Size-Selected Pdn/TiO2 Model Catalysts: Size Dependence of Binding Sites and Energies, and Support-Mediated Adsorption” William E. Kaden, William A. Kunkel, F. Sloan Roberts, Matthew Kane and Scott L. Anderson, J. Chem. Phys. 136 (2012) 204705 12 pages, doi: 10.1063/1.4721625 8
Page 1 and 2: Eighth Condensed Phase and Interfac
Page 3 and 4: Potential energy landscape of the (
Page 5 and 6: Agenda
Page 7 and 8: Session III Chair: Jay A. LaVerne,
Page 9 and 10: Table of Contents
Page 11 and 12: Transition Metal-Molecular Interact
Page 13 and 14: Ultrafast Electron Transport across
Page 15 and 16: CPIMS Principal Investigator Abstra
Page 17 and 18: VUV LDPI image of a polymer photore
Page 19 and 20: Publications based on DOE support (
Page 21: entirely quenched, presumably due t
Page 25 and 26: The relative path indexes of the ur
Page 27 and 28: We have recently carried out molecu
Page 29 and 30: have been obtained with γ-rays sug
Page 31 and 32: Two novel plasma devices have been
Page 33 and 34: (Figure 2) can be adsorbed exclusiv
Page 35 and 36: 3. Future Plans Our planned work de
Page 37 and 38: Figure 1: Snap shots from molecular
Page 39 and 40: 6. Varilly, P., A. J. Patel and D.
Page 41 and 42: hypothesis that delocalized charges
Page 43 and 44: Efforts will be made to observe ele
Page 45 and 46: Photochemistry and Solvation Dynami
Page 47 and 48: complementary to work we have carri
Page 49 and 50: The Co3O4 films for the optical pum
Page 51 and 52: chamber will allow us to heat the s
Page 53 and 54: calculations. Additional PMF calcul
Page 55 and 56: the gas solute first needs to cross
Page 57 and 58: laser system, which is an infrared
Page 59 and 60: Publications (10/1/2009-present) fo
Page 61 and 62: interest are: (i) What is the diffu
Page 63 and 64: and anilino groups attached to the
Page 65 and 66: had been formed. This coincides wit
Page 67 and 68: Figure 1. (A) Simplified bR photocy
Page 69 and 70: Figure 4. Photocurrent density of b
Page 71 and 72: single-file diffusion and cannot ca
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PUBLICATIONS SUPPORTED BY USDOE CHE
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arrangement. However, the hydrogen
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(5) “Water Dynamics in Salt Solut
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from high- energy x-ray diffraction
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concentrated aqueous solutions. The
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molecule’s center of mass, or alt
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Moreover, SHG studies by Saykally
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The effective fragment potential (E
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10. D.D. Kemp, J. Rintelman, M.S. G
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temperature dependent, equilibrium
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mimicking real organic photovoltaic
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100 kHz amplifier, SE-FSRS was succ
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photovoltaics will be explored via
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for laser surface reactions to thes
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This is a significant advantage of
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the STM, which might underlie the u
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Future Plans: Figure 3. One and two
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methane molecule and a lattice atom
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eactive than the Ni, experiment sug
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Fig. 1. PE spectra of M3Oy � (M =
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each of the intermediates obtained
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a regime where each solvent molecul
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observables that arise from subtle
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total potentials. One can see from
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Our calculations have shown that ch
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our understanding of structure and
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viscosity, η, are inversely relate
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photodissociation of the metal - NO
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from a Ru 2p3/2 orbital to an orbit
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Integrated O 2 PSD (ML) Integrated
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anisotropy in the structure of the
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XPS studies have been performed on
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Publications with BES support since
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MLCT (metal to ligand charge transf
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molecule-metal interface without a
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observe the predicted effects, whic
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(4*) Lymar, S. V.; Schwarz, H. A. J
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(τ .820 ns), but we have not been
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B. Photodissociation spectroscopy o
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oth BLYP and PBE. Our results have
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Publications from BES support (2010
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understanding of factors that contr
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Our talk will discussion our initia
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Figure 1 shows the band structure o
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likely to also occur in low coordin
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These results are promising indicat
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This page is intentionally left bla
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method recently developed by Prende
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and coordination regions. A differe
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two ions I - [3] and IO3 - [7]. One
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"Probing the Hydration Structure of
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world-wide scientific user base sup
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10) C. Zhang, M.E. Grass, A.H. McDa
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produce the nanoparticles needed fo
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step as a symmetry-breaking templat
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work provides what we believe in th
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Selected publications acknowledging
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2. Optical Stark study of PtF (Acce
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Figure 4. The Q(3.5) line of the [1
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In marked contrast to the selective
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y depositing cobalt onto copper sin
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Carlo calculations, we have carried
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approach on a model phenol-amine pr
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We performed pump-probe experiments
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15. “A phenomenological approach
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may be initially produced in a non-
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4. Inelastic scattering of hydrogen
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Using QM/MM free energy methods we
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Page 211 and 212:
gas phase using electrospray ioniza
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Page 215 and 216:
order of Li + > Na + > K + for comp
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2. MM Meyer, XB Wang, CA Reed, LS W
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Technical Approach and Progress to
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Page 223 and 224:
Ionic liquid synthesis and characte
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Studies of structure and reaction d
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structure theory that include elect
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9. S. Pandelov, J. C. Werhahn, B. M
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pulse duration is a few times longe
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Figure 1 (Left) The HOMO and LUMO e
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slightly preferable in energy, the
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Participant List Dr. Musahid Ahmed
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Professor Kenneth Eisenthal Columbi
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Dr. Ireneusz Janik Notre Dame Radia
Page 243 and 244:
Professor Eric Potma University of
Page 245:
Dr. James Wishart Brookhaven Nation
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Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

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