Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A03: Supported Nano-Clusters I: Tuning Reactivity Through Cluster-Support Support InteractionsFocus
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Sponsoring Units: DCP Chair: Michael White, Brookhaven National Laboratory Room: LACC 150C |
Monday, March 5, 2018 8:00AM - 8:36AM |
A03.00001: Tuning metal clusters properties: the important role of the support Invited Speaker: Gianfranco Pacchioni When metal nanoparticles are deposited on the surface of an oxide support, or when a nanostructured oxide, either a nanoparticle or a thin film, is grown on a metal, completely new and unexpected phenomena may occur. We refer in particular to the high reactivity of the oxygen atoms at the boundary region between a metal cluster in contact with an oxide surface; these atoms can be removed from interface sites at much lower cost than in other regions of the surface, thus altering completely the reactivity of the heterogeneous catalyst. At the cluster/oxide interface reducibility may differ completely from that of the bulk material or of its surface. The atomistic study of CO oxidation on Au nanoparticles deposited on TiO2 and ZrO2 surfaces is used as an example to provide compelling evidence that the reaction occurs at specific sites at the Au/oxide interface and that even a non-reducible oxide such as ZrO2 can become reducible when nano-structured or interfaced with a metal. Also depositing oxide ultrathin films on metals may result in completely different properties of deposited metal clusters. Recent examples of this effect are discussed for thin layers of ZrO2 and ZnO. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A03.00002: Morphology and catalysis of size-selected Pt clusters tuned by interfacial interaction with metal oxide support Invited Speaker: Yoshihide Watanabe Metal clusters supported on oxide materials are widely-used for heterogeneous catalysts. One of the most important roles of an oxide support is to make active metal clusters stable and dispersive to decrease the amount of costly metal. Another important role is to modify the catalytic activities via the metal-support interactions. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A03.00003: Electronic Structure of Titania Surfaces Modified by Metal Clusters Hassan Al Qahtani, Gowri Krishnan, Liam Howard-Fabretto, Greg Metha, Vladimir Golovko, Gunther Andersson Metal clusters with a size of less than 100 atoms are suitable for modifying the electronic properties of semiconductor surfaces. [1, 2] In order to avoid agglomeration of the metal clusters and in order to retain their specific electronic structures, the coverage of the surface with metal clusters has to be kept below 10%. The main challenges in this field are a) to maintain the size and thus the properties of the metal clusters and b) to determine the electronic structure of the clusters. |
(Author Not Attending)
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A03.00004: Variable Temperature STM Investigations on the Role of Ti Interstitials in Methanol Chemistry for Mass-Selected VO Clusters supported on TiO2 (110)–(1x1) Jeremy Robins, Xiao Tong, Horia Metiu, Michael Bowers, Steven Buratto Mixed-metal oxides display unique catalytic properties that are distinct in their reactivity and selectivity from either parent metal oxide. This is attributed to the novel electronic structure and local bond-coordination present at the interface between both metal oxides. Thus, understanding the role of each metal oxide has on the interface is critical to improving their catalytic properties towards industrially relevant reactions. In particular, the selective oxidative dehydrogenation of alcohols and alkanes over VOx supported on TiO2 have been studied extensively over the past three decades, due to their high selectivity and affordability over Pt based catalysts. A combination of variable-temperature STM and temperature programmed desorption/reaction, reveal that methanol oxidizes the reduced TiO2 (110) surface occurs via Ti interstitial defect sites that diffuse from the bulk. In the presence of isolated, well-defined VO clusters on the surface, Ti interstitials are attenuated and reactivity towards methanol is modified. The VO/TiO2 interface investigated here represents an interesting model towards understanding the synergistic effects of mixed-metal oxides. |
Monday, March 5, 2018 9:36AM - 10:12AM |
A03.00005: Structure, stability and reactivity of oxide supported sub-nm Pt-group metal species Invited Speaker: Phillip Christopher The use of heterogeneous catalysts for chemical conversions relies on the design of active catalytic sites consisting of metal nanostructures supported on high surface area oxide materials. Key to design of these systems is the identification of active site geometries and compositions that are optimized for desired catalytic reactions. In this talk I will focus on examples of research from my group where we elucidate atomic scale details associated with active site characteristics on supported metal catalysts that suggest pathways toward optimization of catalytic performance. Specifically, I will describe approaches for the controlled synthesis, characterization, and reactivity of sub-nanometer supported Pt-group metal species, down to the limit of atomically dispersed metal cations. Our findings suggest that atomically dispersed Pt-group metal cations on oxide supports represent a promising group of heterogeneous catalysts for maximizing metal utilization and controlling reaction selectivity. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A03.00006: Low Energy Alkali Ions Scattered From Au Nanoclusters Christopher Salvo, Josiah Keagy, Bradley Erwin, Joshua Raimist, Jory Yarmoff Metal nanoclusters have been intently investigated over the past 20 years because of their high catalytic activity at low temperatures. The experiments reported here employ a specialized method of low energy ion scattering (LEIS) to probe the electronic properties of deposited Au nanoclusters with and without adsorbates. Because the Au atoms are much more massive than the substrate atoms, the signal from the nanoclusters can be separated from that of the substrate. The experiments measure the neutralization probability of singly scattered alkali ions, which is acutely sensitive to the local electrostatic potential a few Å’s above the surface. Earlier work had demonstrated that the neutralization is a function of cluster size and that it is enhanced for the smallest clusters, but this result had not been previously understood. Recent DFT calculations, which show that the edge atoms are more positively charged than the center atoms, have enabled the development of a model to explain why the neutralization changes with cluster size. Data will be presented for metal clusters grown on various substrates, such as SiO2, TiO2 and graphene, and after reaction with halogen molecules. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A03.00007: Lone-pair bonding to surfaces: from water to ammonia and beyond Arvin Kakekhani, Jens Norskov Here we investigate the surface bonding mechanisms for a group of molecules in which their chemically active molecular orbital is a nitrogen or oxygen 2p lone pair (HOMO). This class includes industrially important molecules including water, alcohols, ethers, ammonia and amines. Using extensive first-principles Density Functional Theory (DFT) modelings, we investigate a large and diverse database of materials from transition metals to oxides, sulfides, nitrides, single sites and beyond. We report universal scaling relations (linear correlations) among the binding energies of lone-pair species and we trace this to the similarities in their electronic structure and similar mechanisms in their bonding to surfaces. We then delve deeper into the details of the lone-pair surface interactions using electronic structure analysis and discover the surface electronic properties that are responsible for dictating the strength of the lone pair-surface bond. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A03.00008: Sulfur Adatom and Vacancy Accelerate Charge Recombination in MoS2, but by Different Mechanisms: Time-Domain Ab Initio Analysis Linqiu Li, Oleg Prezhdo Two-dimensional transition metal dichalcogenide performance depends strongly on defect morphology. Calculations predict that sulfur adatom and vacancy are among the most energetically favorable defects in MoS2. By performing ab initio quantum dynamics calculations we demonstrate that both defects accelerate nonradiative recombination, but this happens through different mechanisms. Suprisingly, holes never significantly populate the shallow trap state created by the sulfur adatom, since the trap is stronly localized and decoupled from free charges. Charge recombination bypasses the hole trap. Instead, it occurs directly between free electron and hole. The recombination is faster than in pristine MoS2, because the adatom strongly perturbs the MoS2 layer, breaks its symmetry, and allows more phonon modes to couple to the electronic subsystem. In contrast, the sulfur vacancy accelerates recombination by the traditional mechanism involving charge trapping, followed by recombination. This is because the hole and electron traps created by the vacancy are much less localized than the hole trap created by the adatom. Because the sulfur adatom accelerates charge recombination by a factor of 7.9, compared to 1.7 due to vacancy, sulfur adatoms should be strongly avoided. |
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