| 1. |
- Heard, Christopher, 1988, et al.
(författare)
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Charge and Compositional Effects on the 2D-3D Transition in Octameric AgAu Clusters
- 2016
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Ingår i: Zeitschrift für Physikalische Chemie. - : Walter de Gruyter GmbH. - 0942-9352 .- 2196-7156. ; 230:5-7, s. 955-975
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Tidskriftsartikel (refereegranskat)abstract
- The unbiased density functional-based Birmingham Cluster Genetic Algorithm is employed to locate the global minima of all neutral and mono-ionic silver-gold octamer clusters. Structural, energetic and electronic trends are determined across the series, in order to clarify the role of composition and charge on the position of the 2D-3D transition in ultrasmall coinage metal systems. Our calculations indicate a preference for three dimensional structures at high silver concentrations, which varies significantly with charge. The minimum in composition dependent mixing energies is independent of the charge, however, with a preference tor the maximally mixed clusters, AgiAii4v for all charge states v. The sensitivity of isomeric preference to v is found to he greater for electron-rich and electron-deficient clusters, implying a complexity of unambiguous determination of cluster motifs in related experiments. Vertical ionization potentials and detachment energies are calculated to probe electronic behaviour, providing numerical predictions for future spectroscopic studies.
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| 2. |
- Heard, Christopher, 1988, et al.
(författare)
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Energy Landscape Exploration of Sub-Nanometre Copper-Silver Clusters
- 2015
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Ingår i: ChemPhysChem. - : Wiley. - 1439-7641 .- 1439-4235. ; 16:7, s. 1461-1469
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Tidskriftsartikel (refereegranskat)abstract
- The energy landscapes of sub-nanometre bimetallic coinage metal clusters are explored with the Threshold Algorithm coupled with the Birmingham Cluster Genetic Algorithm. Global and energetically low-lying minima along with their permutational isomers are located for the Cu4Ag4 cluster with the Gupta potential and density functional theory (DFT). Statistical tools are employed to map the connectivity of the energy landscape and the growth of structural basins, while the thermodynamics of interconversion are probed, based on probability flows between minima. Asymmetric statistical weights are found for pathways across dividing states between stable geometries, while basin volumes are observed to grow independently of the depth of the minimum. The DFT landscape is found to exhibit significantly more frustration than that of the Gupta potential, including several open, pseudo-planar geometries which are energetically competitive with the global minimum. The differences in local minima and their transition barriers between the two levels of theory indicate the importance of explicit electronic structure for even simple, closed shell clusters.
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| 3. |
- Heard, Christopher, 1988, et al.
(författare)
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Global optimisation strategies for nanoalloys
- 2017
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Ingår i: Challenges and Advances in Computational Chemistry and Physics. - Cham : Springer International Publishing. - 2542-4483 .- 2542-4491. ; 23, s. 1-52
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The computational prediction of thermodynamically stable metal cluster structures has developed into a sophisticated and successful field of research. To this end, research groups have developed, combined and improved algorithms for the location of energetically low-lying structures of unitary and alloy clusters containing several metallic species. In this chapter, we review the methods by which global optimisation is performed on metallic alloy clusters, with a focus on binary nanoalloys, over a broad range of cluster sizes. Case studies are presented, in particular for noble metal and coinage metal nanoalloys. The optimisation of chemical ordering patterns is discussed, including several novel strategies for locating low-energy permutational isomers of fixed cluster geometries. More advanced simulation scenarios, such as ligand-passivated, and surface-deposited clusters have been developed in recent years, in order to bridge the gap between isolated, bare clusters, and the situation observed under experimental conditions. We summarise these developments and consider the developments necessary to improve binary cluster global optimisation in the near future.
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| 4. |
- Heard, Christopher, 1988, et al.
(författare)
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Kinetic Regimes in Ethylene Hydrogenation over Transition-Metal Surfaces
- 2016
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 6:5, s. 3277-3286
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Tidskriftsartikel (refereegranskat)abstract
- A first-principles microkinetic model has been developed and applied to ethylene hydrogenation over close-packed transition-metal surfaces of Ru, Rh, Pd, Os, Ir, and Pt. The model is based on density functional theory calculations, which have been used to determine the activation energies of the elementary steps of the reaction according to the Horuiti-Polanyi mechanism. A sensitivity analysis of the activity with respect to the kinetic parameters reveals distinctly different kinetic regimes across the periodic table. For Ru and Ir, the activity is controlled by the activation energy for ethylene to ethyl hydrogenation, whereas the other metals also have a sensitivity to the second hydrogenation step. The analysis shows, furthermore, that the activity could be enhanced considerably with minor reductions of the hydrogenation barriers.
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| 5. |
- Heard, Christopher, 1988, et al.
(författare)
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PdnAg(4-n) and PdnPt(4-n) clusters on MgO (100): a density functional surface genetic algorithm investigation
- 2014
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 6:20, s. 11777-11788
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Tidskriftsartikel (refereegranskat)abstract
- The novel surface mode of the Birmingham Cluster Genetic Algorithm (S-BCGA) is employed for the global optimisation of noble metal tetramers upon an MgO (100) substrate at the GGA-DFT level of theory. The effect of element identity and alloying in surface-bound neutral subnanometre clusters is determined by energetic comparison between all compositions of PdnAg(4-n) and PdnPt(4-n). While the binding strengths to the surface increase in the order Pt > Pd > Ag, the excess energy profiles suggest a preference for mixed clusters for both cases. The binding of CO is also modelled, showing that the adsorption site can be predicted solely by electrophilicity. Comparison to CO binding on a single metal atom shows a reversal of the 5 sigma-d activation process for clusters, weakening the cluster-surface interaction on CO adsorption. Charge localisation determines homotop, CO binding and surface site preferences. The electronic behaviour, which is intermediate between molecular and metallic particles allows for tunable features in the subnanometre size range.
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| 6. |
- Heard, Christopher, 1988, et al.
(författare)
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Structural and Energetic Trends of Ethylene Hydrogenation over Transition Metal Surfaces
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:2, s. 995-1003
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are used to investigate the catalytic hydrogenation of ethylene to ethane over a wide range of transition metal (TM) surfaces. Assuming the Horuiti-Polanyi mechanism, the enthalpies of adsorption, surface diffusion, and hydrogenation barriers are examined over close-packed surfaces of Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Special attention is given to the effects of ethylene and hydrogen coverage on the reaction pathway and activation energies. The previously suggested importance of the balance between di-s and p adsorption modes is reinvestigated, and most metals are found to exhibit a preference for the p state. Hydrogen coverage is found to control the reactant stability and promote a surface distortion which facilitates the hydrogen addition reaction. For all TMs, the calculated activation energies are low and span a narrow range.
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| 7. |
- Kaydashev, V., et al.
(författare)
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Optical Absorption of Small Palladium-Doped Gold Clusters
- 2016
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Ingår i: Particle and Particle Systems Characterization. - : Wiley. - 0934-0866 .- 1521-4117. ; 33:7, s. 364-372
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Tidskriftsartikel (refereegranskat)abstract
- The effect of Pd doping on the structure and optical absorption of small cationic gold clusters is investigated by a combined photodissociation spectroscopy and time-dependent density functional theory study of Aun+Arp and PdAun-1+Arp (n = 4,5; p = 0,1). While pure Au clusters are planar, the Pd-doped clusters are 3D. UV-visible absorption is studied in the 2.0-4.7 eV photon energy range, allowing the observation of previously unreported absorption bands for Au-4(+) and Au4+Ar. The oscillator strength of the optical transitions is dramatically reduced upon incorporating a Pd atom in Au-4(+) and Au4+Ar, while this effect is less pronounced for Au5+Ar. Analysis of the electron density transfer shows a different influence of Pd with size. While Pd has a formal negative charge in Au3Pd+, in Au4Pd+ most of the charge is attracted by the highly coordinated central Au atom, leaving Pd positively charged, also affecting the induced structural changes. In addition, orbital analysis of the optical transitions is carried out in order to identify the levels involved in the optical absorption of the pure Au and Pd doped clusters. A reduction of the s density near the Fermi energy, induced by Pd doping, causes a quenching of optical absorption.
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| 8. |
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| 9. |
- Merte, Lindsay R., et al.
(författare)
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Tuning the Reactivity of Ultrathin Oxides: NO Adsorption on Monolayer FeO(111)
- 2016
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Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 55:32, s. 9267-9271
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Tidskriftsartikel (refereegranskat)abstract
- Ultrathin metal oxides exhibit unique chemical properties and show promise for applications in heterogeneous catalysis. Monolayer FeO films supported on metal surfaces show large differences in reactivity depending on the metal substrate, potentially enabling tuning of the catalytic properties of these materials. Nitric oxide (NO) adsorption is facile on silver-supported FeO, whereas a similar film grown on platinum is inert to NO under similar conditions. Ab initio calculations link this substrate-dependent behavior to steric hindrance caused by substrate-induced rumpling of the FeO surface, which is stronger for the platinum-supported film. Calculations show that the size of the activation barrier to adsorption caused by the rumpling is dictated by the strength of the metal–oxide interaction, offering a straightforward method for tailoring the adsorption properties of ultrathin films.
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| 10. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
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Cluster Size Effects in Ethylene Hydrogenation over Palladium
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:20, s. 10870-10875
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are used to investigate ethylene hydrogenation over palladium clusters in the range from Pd-13 to Pd-116. A comparison is made to single crystal surfaces, which exemplifies several novel aspects of clusters. We find that the adsorption energies are always higher for the clusters, even if the comparison is made to adsorption on stepped surfaces in the low-coverage regime. Clusters are found to accommodate higher adsorbate coverages than extended surfaces. The saturation coverage for Pd-13 is unity, whereas it is 0.33 on Pd(111). The activation energies for hydrogenation of C2H4 to C2H5 over Pd-38 are clearly different from Pd(111) and Pd(211), which stresses the limitation of extended surfaces as models for nanoparticles.
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| 11. |
- Shayeghi, A., et al.
(författare)
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Adsorption of Acetonitrile, Benzene, and Benzonitrile on Pt(111): Single Crystal Adsorption Calorimetry and Density Functional Theory
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:39, s. 21354-21363
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Tidskriftsartikel (refereegranskat)abstract
- Single crystal adsorption calorimetry on Pt(111) surfaces allows us to study the influence of different functional groups on heats of adsorption. At zero coverage we find 103, 194, and 325 kJ/mol for acetonitrile, benzene, and benzonitrile, respectively. This leads to the idea of summing up the heats of adsorption of acetonitrile and benzene, assuming that the total heat of adsorption of benzonitrile is due to the combined effect of isolated functional groups. This idea is discussed in light of the recent literature regarding nitrile adsorption on metal surfaces and further investigated theoretically by density functional theory. In order to figure out the importance of dispersive effects on the heats of adsorption, van der Waals corrected calculations are performed considering different binding modes and surface reconstructions.
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| 12. |
- Shipilin, M., et al.
(författare)
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Fe Oxides on Ag Surfaces : Structure and Reactivity
- 2017
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 60:6-7
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Tidskriftsartikel (refereegranskat)abstract
- One layer thick iron oxide films are attractive from both applied and fundamental science perspectives. The structural and chemical properties of these systems can be tuned by changing the substrate, making them promising materials for heterogeneous catalysis. In the present work, we investigate the structure of FeO(111) monolayer films grown on Ag(100) and Ag(111) substrates by means of microscopy and diffraction techniques and compare it with the structure of FeO(111) grown on other substrates reported in literature. We also study the NO adsorption properties of FeO(111)/Ag(100) and FeO(111)/Ag(111) systems utilizing different spectroscopic techniques. We discuss similarities and differences in the data obtained from adsorption experiments and compare it with previous results for FeO(111)/Pt(111).
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