Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q2: Focus Session: Surface Chemistry and Catalysis VI |
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Sponsoring Units: DCP Chair: J.R. Schmidt, University of Wisconsin-Madison Room: 102 |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q2.00001: Ligand-Mediated Synthesis of Colloidal Nanoparticle Alloys Invited Speaker: Jill Millstone Small molecule ligand chemistry is used to mediate the incorporation and distribution of metals in and on discrete, colloidal nanoparticle substrates. Specifically, we examine the case of late d transition metals in Au and Pt hosts. The resulting structures are characterized by a variety of methods including X-ray absorption spectroscopy, electron microscopy, and photoelectron spectroscopy techniques. These multimetallic nanoparticles exhibit previously unobserved mixtures of metals such as continuously tunable Au-Co composition ratios, as well as unique physical properties including composition-tunable near-infrared photoluminescence. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q2.00002: The Double-edged Impact of Platinum Nano-Deposits on the Durability of Polymer Electrolyte Membranes --- A Theoretical Study Mohammad Javad Eslamibidgoli, Pierre-\'Eric Alix Melchy, Ata Roudgar, Michael H. Eikerling The attack of oxygen radicals is one of the main sources of chemical degradation in the polymer electrolyte membranes (PEM) of polymer electrolyte fuel cells. In this context, Pt in the membrane (PITM) that originates from Pt degradation in the cathode catalyst layer plays a double-edged role: surface reactions at PITM could facilitate the formation or quenching of radicals. The balance of these processes depends on the local electrochemical conditions, determined by thermodynamic parameters and local composition of the PEM. The objective of this work is to explore the equilibrium and kinetics of radical reactions at PITM as a function of local PEM conditions. We first determine the potential distribution of PITM based on a continuum model of crossover of reactant gases coupled with their local electrochemical reactions at Pt. Secondly, we determine the surface state of Pt for the given local potential using relevant experimental data and kinetic models of surface reactions at Pt [1, 2]. Lastly, we use this information as input for \textit{ab initio} calculations at the DFT level of specific processes involved in the radical balance at the Pt $|$ water interface.\\[4pt] [1] M. Wakisaka et al. Langmuir, 25, (2009)\\[0pt] [2] S. G. Rinaldo et al. submitted to Nat. Mater. (2013) [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q2.00003: Computational modeling of the surface properties of uranium dioxide using hybrid DFT Megan Hoover, Raymond Atta-Fynn Computational modeling of processes such as corrosion and the interaction of environmental impurities with the surfaces of actinide materials are important to the understanding of remediation processes for actinide-based nuclear waste from the biosphere. However, accurate modeling of bare surfaces of actinide materials is a necessary precursor to accurate modeling of surface interactions. This talk will be focused on atomistic modeling of uranium dioxide (UO$_{2})$ surfaces. The theoretical formalism is all-electron hybrid Density Functional Theory (DFT) based on the full-potential linearized augmented plane wave plus local basis method. Specifically, we computed the surface energies, work functions, incremental energies, and electronic band gaps for periodic slab structures for the (110) and (111) surfaces of UO$_{2}$. We observed that the anti-ferromagnetic semiconducting behavior in the bulk structure is retained in the surface structures. The convergence of surface properties with respect to slab thickness will be discussed. The trends in the surface electronic structures, particularly the localized behavior of the U 5$f$ electrons, in comparison with the bulk structure will also be elucidated. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q2.00004: Stability and metastability of clusters in a reactive atmosphere: theoretical evidence for unexpected stoichiometries of Mg$_M$O$_x$ Saswata Bhattacharya, Sergey Levchenko, Luca Ghiringhelli, Matthias Scheffler In heterogeneous catalysis, materials function at finite temperature and in an atmosphere of reactive molecules at finite pressure. As a first step towards understanding the catalytic behavior of metal-oxide clusters, we study the $(T,p)$ dependence of the composition, structure, and stability of the various isomers for each size $M$ of Mg$_M$O$_x$ clusters in an oxygen atmosphere. The calculations are performed via a massively parallel genetic algorithm in a cascade approach. With the term ``cascade'', we identify a multistep procedure in which successive steps employ higher levels of theory, with each next level using information obtained at the lower level. We find that small clusters ($M < 5$) are in thermodynamic equilibrium when $x>M$. The non-stoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory (DFT) with a hybrid exchange-correlation (xc) functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor DFT with (semi)local xc functionals are sufficient for even a qualitative prediction. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q2.00005: Density Functional Theory Studies of Sb(V) Oxyanion Adsorption on $\alpha$-Al$_2$O$_3$ 1$\overline{1}$02 surfaces Sai Kumar Ramadugu, Sara Mason We apply density functional theory and periodic slab models to understand the adsorption and reactivity of Sb(OH)$_6^{-1}$ on various surface terminations of $\alpha$-Al$_2$O$_3$ (1$\overline{1}$02). On all surfaces studied, Sb(V) adsorption is preferred when adsorbed in a bidentate, corner-sharing geometry. On the other hand, all theoretical bidentate edge-sharing modes are relatively high in energy. This is in good agreement with recent experiments that do not observe any bidentate edge-sharing adsorption of Sb(OH)$_6^{-1}$ on hydrous aluminum oxide. We identify factors that govern the preference for corner-sharing adsorption, and compare how well the surface reactivity can be predicted based on periodic DFT {\it vs.} quantum chemical cluster models. From the DFT projected density of states, we clearly show how strained ligands of Sb(V) cannot achieve good overlap with the electronic states of the surface, even at Sb-O distances predicted to be ideal by empirical bond-valence. The results thus provide a molecular-level understanding of Sb(V) specific adsorption and also allow us to assess other modeling approaches to computational geochemical surface science. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q2.00006: Artificial Photosystem I and II: Highly Selective solar fuels and tandem photocatalysis Yuchen Ding, Ignacio Castellanos, Logan Cerkovnik, Prashant Nagpal Artificial photosynthesis, or generation of solar fuels from CO$_{2}$/H$_{2}$O, can provide an important alternative for rising CO$_{2}$ emission and renewable energy generation. In our recent work, composite photocatalysts (CPCs) made from widebandgap nanotubes and different QDs were used to mimic Photosystem II (PS680) and I (PS700), respectively. By tuning the redox potentials using the size, composition and energy band alignment of QDs, we demonstrate highly selective (\textgreater 90{\%}) and efficient production of ethane, ethanol and acetaldehyde as solar fuels with different wavelengths of light. We also show that this selectivity is a result of precise energy band alignments (using cationic/anionic doping of nanotubes, QD size etc.), confirmed using measurements of electronic density of states, and alignment of higher redox potentials with hot-carriers can also lead to hot-carrier photocatalysis. This wavelength-selective CPCs can have important implications for inexpensive production of solar fuels including alkanes, alcohols, aldehydes and hydrogen, and making tandem structures (red, green, blue) with three CPCs, allowing almost full visible spectrum (410 $\sim$ 730nm) utilization with different fuels produced simultaneously. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q2.00007: K$^{+}$-hydration in a low-energy two-dimensional wetting layer on the basal surface of muscovite Peter J. Feibelman Density Functional Theory points to a key role of K$^{+}$ solvation in the low-energy two-dimensional arrangement of water molecules on the basal surface of muscovite. At a coverage of 9 water molecules per 2 surface potassium ions, there is room to accommodate the ions into wetting layers wherein half of them are hydrated by 3 and the other half by 4 water molecules, with no broken H-bonds, or wherein all are hydrated by 4. Relative to the ``fully connected network of H-bonded water molecules'' that M. Odelius \textit{et al.}. [Phys. Rev. Lett. \textbf{78}, 2855-- 2858 (1997)] found to form ``a cage around the potassium ions,'' the hydrating arrangements are several tens of meV/H$_{\mathrm{2}}$O better bound. Thus, low-temperature wetting on muscovite is not driven towards ``ice-like'' hexagonal coordination. Instead, solvation forces dominate.\\[4pt]Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q2.00008: Conversion of solar into chemical energy on plasmonic metal nanostructures Invited Speaker: Suljo Linic We will show that composite photo-catalysts combing plasmonic metallic nanoparticles of noble metals and semiconductor nanostructures exhibit improved photo-chemical activity compared to conventional photo-catalytic materials [1,2]. We will also show that plasmonic silver nanoparticles, optically excited with low intensity visible light, exhibit direct photo-catalytic activity in a number of oxidation reactions. We will discuss underlying mechanisms associated with these phenomena and predictive models that can capture the outcome of chemical transformations on these materials [2-4]. We propose that this new family of plasmonic metal photo-catalysts could prove useful for many heterogeneous catalytic processes that cannot be activated using conventional thermal processes on metals or photo-catalytic processes on semiconductors. I will show an example of such a process [5]. \\[4pt] [1] D. B. Ingram, S. Linic, \textbf{JACS,} 133, 5202, 2011.\\[0pt] [2] Suljo Linic, Phillip Christopher and David B., \textbf{Nature Materials,} \textbf{10}, 911, 2011.\\[0pt] [3] Ingram P. Christopher, H. Xin, S. Linic, \textbf{Nature Chemistry,} 3, 467, 2011.\\[0pt] [4] P. Christopher, H. Xin, M. Andiappan, S. Linic, \textbf{Nature Materials}, 11, 1044, 2012.\\[0pt] [5] M. Andiappan, J. Zhang, S. Linic, \textbf{Science,} 339, 1590, 2013 [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q2.00009: Impact of the vapour pressure of water on the equilibrium shape of ZnO nanoparticles: An ab-initio study Stephane Kenmoe, Mira Todorova, P. Ulrich Biedermann, Joerg Neugebauer ZnO powders and nanoparticles are used as catalysts and have potential applications in gas-sensing and solar energy conversion. A fundamental understanding of the exposed crystal facets, their surface chemistry and stability as function of environmental conditions is essential for rational design and improvement of synthesis and properties. Using density-functional theory calculations we study the adsorption of water on the non-polar low-index $(10\bar{1}0)$ and $(11\bar{2}0)$ surfaces of ZnO. Observing both molecular and dissociative H$_2$O adsorption, we analyse the contributions of water-surface and water-water interactions to the energies of the stable structure. Based on this insight we compute and analyse the impact of water adsorption on surface energies and the equilibrium shape of nanoparticles in a humid environment. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q2.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q2.00011: Asymmetric Wettability Directs Leidenfrost Droplets Rebecca Agapov, Jonathan Boreyko, Dayrl Briggs, Bernadeta Srijanto, Scott Retterer, C. Patrick Collier, Nickolay Lavrik Exploration of Leidenfrost droplets on nano- and microstructured surfaces are of great importance for increasing control over heat transfer in high power density systems using boiling phenomena. They also provide an elegant way to direct droplet motion in a variety of emerging fluidic systems. Here, we report the fabrication and characterization of tilted nanopillar arrays (TNPAs) that exhibit directional Leidenfrost water droplets under dynamic conditions. The batch fabrication of the TNPAs was achieved by glancing-angle anisotropic reactive ion etching of a thermally dewet platinum mask. In contrast to previously implemented macro- and microscopic Leidenfrost ratchets, our TNPAs induce \textit{no} preferential directional movement of Leidenfrost droplets under conditions approaching steady-state film boiling. This suggests that the observed droplet directionality is not a result of asymmetric vapor flow. Phase diagrams were constructed for the boiling behavior upon droplet impact onto TNPAs, straight nanopillar arrays, and smooth silicon surfaces. Asymmetric wettability and directional trajectory of droplets was exclusive to the TNPAs for impacts corresponding to the transition boiling regime, revealing this to be the mechanism for the droplet directionality. [Preview Abstract] |
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