Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E03: Supported Nano-Clusters III: Clusters Under Reaction ConditionsFocus
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Sponsoring Units: DCP Chair: Stefan Vajda, Argonne National Lab and University of Chicago Room: LACC 150C |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E03.00001: Crucial role of metastable structures and restructuring of Pt clusters in catalysis Invited Speaker: Philippe Sautet The determination of the structure of heterogeneous catalytic systems, under reaction conditions, is a key point for a detailed understanding of the nature of active sites and for the rational design of efficient catalysts. The lecture will focus on the modelling of small Pt cluster (Pt13) under hydrogen pressure and on their reactivity for methane and ethane activation. The approach combines Density Functional Theory, high-dimensional Neural Networks and evolutionary techniques. The bare Pt clusters shows a large number of low energy isomers (49 in 0.5 eV). Hydrogenated clusters adopt different geometries and appear more rigid, with a smaller number of low energy isomers. These metastable isomers nevertheless play a major role for the catalytic reactivity of the hydrogen covered cluster, which cannot be described by considering the most stable structure alone. Fluxionality and accessible metastable structure are hence key characteristics for the catalytic properties of small Pt clusters. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E03.00002: Surface-Supported Cluster Catalysts: Fluxionality, Statistical Nature, and Design Huanchen Zhai, Mai-Anh Ha, Anastassia Alexandrova Sub-nano surface-supported catalytic clusters, generally have many low-energy isomers accessible at elevated temperatures of catalysis. The most stable isomer may not be the most catalytically active. We present methodology that, for moderate-sized systems, thoroughly samples the potential energy surface, at the theory level such as density functional theory. It gives access to the ensemble of thermally-accessible structures of surface-mounted clusters. Additionally, isomers may interconvert across barriers, i.e. exhibit fluxionality, during catalysis. We model such process as an isomerization network using structural matching and paralleled nudged elastic band method. We will show how all practically-important properties of cluster catalysts are in fact ensemble-average. Side-by-side with the experiment, we explain why Pt7 on alumina is much more active toward alkene dehydrogenation than Pt8 and Pt4 on this support. We also present the design of nano-alloys of Pt clusters, which are remarkably selective against coke in this reaction, as again confirmed experimentally. These findings were possible only within the ensemble representation of the catalyst. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E03.00003: Learning the Structure of Ultradispersed Copper Clusters from X-ray Absorption Fine Structure data: Artificial Neural Network Approach Janis Timosenko, Avik Halder, Stefan Vajda, Anatoly Frenkel Unique structural motifs in metallic nanoparticles (NPs) result in properties that differ dramatically from those of bulk materials and that can be exploited in many applications, e.g., in the field of catalysis. The rational design of such materials, however, is hindered by the limitations of characterization methods that would allow detection of such motifs (undercoordinated surface sites, relaxation of interatomic distances) in ultrasmall (< 3 nm) NPs. Especially challenging is the problem of determination of NPs structure in operando conditions. |
Tuesday, March 6, 2018 9:00AM - 9:36AM |
E03.00004: Operando Nanocatalysis: Size, Shape, Composition and Chemical State Effects Invited Speaker: Beatriz Roldan Cuenya In order to comprehend the properties affecting the catalytic performance of metal nanoparticles (NPs), their dynamic nature and response to the environment must be taken into consideration. The working state of a NP catalyst might not be the state in which the catalyst was prepared, but a structural and/or chemical isomer that adapted to the particular reaction conditions. |
Tuesday, March 6, 2018 9:36AM - 10:12AM |
E03.00005: Theoretical study of reactivity of gold clusters: Structural effects and support effects Invited Speaker: Tetsuya Taketsugu In this talk, we focus on catalytic activity of gold nano-clusters from the viewpoints of structural effects and support effects. The enormous interest in nanocatalysis is stipulated by the fact that the catalytic activity of nano-clusters strongly depends on their size, structure, morphology, charge state, type of the support material, etc., and hence can be controlled and tuned by these factors. Despite intensive theoretical and experimental studies, clear understanding of the morphology effects in nanocatalysis is still lacking. Metal clusters are structural flexible, meaning that they possess different structures and different reactivity for the same sized cluster. Recently, we proposed a novel effective strategy for a systematic search of single bond activation reaction by small metal clusters [1-3], using the artificial force-induced reaction method [4]. The results demonstrated that the most stable structure of metal clusters are not always highly reactive and the most stable adsorption configurations of reactants on metal clusters do not necessarily lead to the low-barrier dissociation pathways. Therefore, in order to investigate the activity of metal clusters, the contribution of all the possible reaction sites and low-energy isomers are highly required. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E03.00006: Structure and Dynamics of Liquid Water on Metal Oxide Surfaces Marcos Andrade, Hsin-Yu Ko, Roberto Car, Annabella Selloni Water - metal oxide interfaces play a key role in several technologically important processes, such as water purification and renewable energy generation. An atomistic dynamical picture of surfaces in real solvent environments can be properly captured by the integration of density functional theory (DFT) and molecular dynamics (MD). Recently, a novel density functional, SCAN,1 predicted liquid water in good agreement with experiment at a feasible computational cost2 , overcoming the difficulties of previous DFT studies. Here, we used the SCAN functional to investigate the structure and dynamics of liquid water close to the surface of anatase TiO2, a prototypical metal oxide photocatalyst. We obtained not only a more accurate picture of liquid water at a metal oxide surface, but also an assessment of the limitations of current DFT functionals for the description of interfacial liquid water. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E03.00007: Molecular Dynamic Simulation of Water/ Peroxide Reaction on Defective Ceria (110) Surface Sufian Alnemrat, Renat Sabirianov Car-Parrinello molecular dynamic simulations are used to show that 110 ceria surface with vacancy sites is suitable to dissociate water and/or peroxide at the solvent/solid interface. We present density functional theory and ab initio molecular dynamics simulations of water in the liquid phase interacting with vacancy defects at (110) ceria surface. Water molecule approaches the vacancy from the water solvent observed to transfer a proton to a nearby oxygen. The formed hydroxide weakly binds to the Ce-vacancy atom after which it diffuses back to the solvent in a short period of simulation time (~1 pse). However, this mechanism is more complex in the case of peroxide solvent. Two possible pathways are observed as a peroxide molecule approaches the vacancy. The first pathway involves peroxide dissociation into tow hydroxides that diffuse back to the peroxide solvent. While the second pathway involves a dual proton transfer to oxygen atoms near the vacancy while O2 diffuses freely to the peroxide liquid. These simulations highlight the role of surface defects in providing suitable adsorption sites that accelerate the dissociation of water / peroxide species at the liquid/solid interface. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E03.00008: Single Molecule Dynamics with Joint Angstrom-Femtosecond Resolution Siyu Chen, Shaowei Li, Likun Wang, Jie Li, Ruqian Wu, Wilson Ho The investigation of single molecule dynamics with joint spatial-temporal resolution pushes the limit of measurement in nanoscale phenomena. By shining femtosecond laser pulse pairs with adjustable time delay into the junction of a scanning tunneling microscope (STM) with an adsorbed molecule on the surface, the dynamics of its chemical transformation can be followed in space-time. As an example, we probe the reversible conformational change in an isolated pyrrolidine molecule adsorbed on a Cu (100) surface. A periodic, decaying oscillation was observed in the transition rate as a function of the time delay, thereby revealing that the conformational change was coherently driven by the excitation of a particular vibrational mode in the molecule. The atomic scale resolution was demonstrated by observing the influence on the dynamics of a molecule by a nearby molecule. |
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