| 1. |
- Baudin, Micael, et al.
(författare)
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A molecular dynamics study of MgO(111) slabs
- 1997
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 375:2-3, s. 374-384
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Tidskriftsartikel (refereegranskat)abstract
- Constant stress - constant temperature molecular dynamics simulations are reported for 30 Angstrom thick crystalline MgO (111) slabs with 2-D periodicity in the temperature range 10-1100 K. The 10 K run was 3.75 ps, the 300 K run was 41.25 ps while the others were 11.25 ps. The shell-model potential was used and the long-range interaction was taken into account with the 2-D Ewald summation technique. Two different models have been used in order to achieve the necessary charge compensation for the polar (111) surface. The relaxation of the surface structure and its temperature dependence have been investigated and compared with bulk MgO. The average coordination number in the surface layer and the surface energy are presented. The dynamics in the slab system is presented in terms of atomic mean-square displacements and vibrational density-of-states curves as a function of depth from the surface. (C) 1997 Elsevier Science B.V.
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| 2. |
- Baudin, Micael
(författare)
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Dynamics of metal oxides surfaces and the connection to reactivity
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many interesting chemical and physical phenomena which take place on surfaces of metaloxides rely on the material's ability to repeatedly alter the electronic surface structure. Formost metal oxides this functionality is connected to the degree of non-stoichiometry andthe presence of structural defects. In many catalytic applications, for example, the metaloxide surfaces display optimal performance at rather high temperatures, and it is thereforeessential to include thermal distortions in any investigation of the mechanisms behind thefunctionality. It can be expected that dynamical changes in the surface structure will drastically alter the electronic structure, and thereby also the catalytic characteristics of thesurface.For metal oxide surfaces, experimental as well as theoretical data on surface dynamics(at non-zero temperatures) are virtually non-existent. The very few MD simulation studiesthat exist in the literature have exclusively dealt with the effect of temperature on stability,structure and ordering.This thesis describes molecular dynamics simulations and subsequent quantum-chemicalcalculations of metal oxide surfaces, and how the surface properties vary with temperature,crystal face and depth from the surface for three structurally different materials. The surfaces studied are the low-index faces of MgO, α-Al2O3(0001), undoped and dopedlow-index CeO2 surfaces, and the α-Al2O3(0001┴,010Ⅱ)/CeO2(011┴,011Ⅱ) interface. The emphasis lieson dynamic quantities, and the ionic motion is analysed in terms of mean-square displacements, ion migrations, instantaneous ionic displacements, as well as vibrational densities-of-states. Other properties discussed include surface energies, interplanar relaxations, and pairdistribution functions. The thesis also discusses the implication of short-time atomic-leveldynamical changes on reactivity.
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| 3. |
- Baudin, Micael, et al.
(författare)
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MD simulations of a doped ceria surface - very large surface ion motion
- 2001
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Ingår i: CHEMICAL PHYSICS LETTERS. - 0009-2614. ; 335:5-6, s. 517-523
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Tidskriftsartikel (refereegranskat)abstract
- Mean-square displacements (MSDs) and individual-ion square-displacements (ISDs) for the different constituents in Ca-doped CeO2(0 1 1) slabs at 300 K have been studied as a function of depth from the surface. Constant pressure-constant temperature MD simu
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| 4. |
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| 5. |
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| 6. |
- de Carolis, Stefano, et al.
(författare)
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Structure and electronic properties of Ca-doped CeO2 and implications on catalytic activity : An experimental and theoretical study
- 1999
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Ingår i: J PHYS CHEM B. - : American Chemical Society (ACS). - 1089-5647. ; 103:36, s. 7627-7636
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Tidskriftsartikel (refereegranskat)abstract
- Doping CeO2 with for example, Ca gives an enhanced reactivity toward reduction of SO2 by CO, and total combustion of methane. Theoretical modeling using static minimizations and molecular dynamics (MD) simulations of the doped (110) face in combination with ab initio quantum chemical cluster models shows large effects on the Ce(IV)/Ce(III) balance due to the doping. Computed oxygen-to-cerium charge-transfer energies are strongly reduced as a result of the introduction of defects and oxygen vacancies, but not sufficiently to explain the observed reactivities. The structures resulting from the MD simulations for both the doped and undoped material are in good agreement with recent experimental pulsed neutron scattering results.
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| 7. |
- Gotte, Anders, 1976-
(författare)
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Dynamics in Ceria and Related Materials from Molecular Dynamics and Lattice Dynamics
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In discussions of heterogeneous catalysis and other surface-related phenomena, the dynamical properties of the catalytic material are often neglected, even at elevated temperatures. An example is the three-way catalyst (TWC), used for treatment of exhaust gases from combustion engines operating at several hundred degrees Celsius. In the TWC, reduced ceria (CeO2-x) is one of the key components, where it functions as an oxygen buffer, storing and releasing oxygen to provide optimal conditions for the catalytic conversion of the pollutants. In this process it is evident that dynamics plays a crucial role, not only ionic vibrations, but also oxygen diffusion.In this thesis, the structure and dynamics of several ionic crystalline compounds and their surfaces have been studied by means of Molecular dynamics (MD) simulations and Lattice dynamics (LD) calculations. The main focus lies on CeO2-x, but also CeO2, MgO and CaF2 have been investigated.The presence of oxygen vacancies in ceria is found to lead to significant distortions of the oxygen framework around the defect (but not of the cerium framework). As a consequence, a new O-O distance emerges, as well as a significantly broadened Ce-O distance distribution.The presence of oxygen vacancies in ceria also leads to increased dynamics. The oxygen self-diffusion in reduced ceria was calculated from MD simulations in the temperature range 800-2000 K, and was found to follow an Arrhenius behaviour with a vacancy mechanism along the crystallographic <100> directions only.The cation and anion vibrational surface dynamics were investigated for MgO (001) using DFT-LD and for CaF2 (111) in a combined LEED and MD study. Specific surface modes were found for MgO and increased surface dynamics was found both experimentally and theoretically for CaF2, which is isostructural with CeO2.Many methodological aspects of modeling dynamics in ionic solids are also covered in this thesis. In many cases, the representation of the model system (slab thickness, simulation box-size and the choice of ensemble) was found to have a significant influence on the results.
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| 8. |
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| 9. |
- Gotte, Anders, et al.
(författare)
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Molecular dynamics simulations of reduced CeO2 : bulk and surfaces
- 2004
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 552:03-jan, s. 273-280
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Tidskriftsartikel (refereegranskat)abstract
- Results from molecular dynamics simulations for bulk and the (0 1 1) surface of reduced and unreduced CeO2 at 300 K are reported. The presence of vacancies in reduced ceria is found to give rise to many different local structural arrangements, and to a significant broadening of the peaks in the pair-distribution function. This broadening is mainly due to the appearance of these new structural arrangements, and it is not due to the increased ionic motions in the reduced systems.
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| 10. |
- Gotte, Anders, et al.
(författare)
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Theoretical and experimental studies of the structure and dynamics of the CaF2(111) surface
- 2007
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:2, s. 411-418
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Tidskriftsartikel (refereegranskat)abstract
- The structure and dynamics of the CaF2(111) surface were investigated by means of low-energy electron diffraction (LEED) and molecular dynamics (MD) simulations at 300 K. LEED beam intensities were recorded as a function of electron energy and were analyzed with the tensor LEED approach. Positions as well as mean square amplitudes of the ions in the first layers were fitted to the experimental I(E) curves. According to both LEED and MD, the CaF2(111) surface structure is similar to the bulk-terminated structure with only small relaxation of the outermost ions. Moreover, both methods show an enhancement of vibrational amplitudes in the outermost F-Ca-F triple layer.
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