Kinetics and Thermodynamics of CO Oxidation by (TiO2)6

Molecular level insights into the mechanism and thermodynamics of CO oxidation by a (TiO2)6 cluster have been obtained through density functional calculations. Thereby, in this study, as an example, two different structural isomers of (TiO2)6 are considered with the purpose of understanding the interplay between local structure and activity for the CO oxidation reaction. Active sites in the two isomeric forms were identified on the basis of global and local reactivity descriptors. For the oxidation of CO to CO2, the study considered both sequential and simultaneous adsorption of CO and O2 on (TiO2)6 cluster through the ER and LH mechanisms, respectively. Three different pathways were obtained for CO oxidation by (TiO2)6 cluster, and the mechanistic route of each pathway were identified by locating the transition-state and intermediate structures. The effect of temperature on the rate of the reaction was investigated within the harmonic approximation. The structure-dependent activity of the cluster was rationalized through reactivity descriptors and analysis of the frontier orbitals

Silber-Cluster und Nanolegierungen

The focus of this thesis is on determination of putative global minimum structures of silver clusters, and copper-silver and nickel-silver bimetallic clusters by using a combination of embedded atom method and basin-hopping algorithm. Global minima of silver clusters with N=2 to 100 atoms are based on icosahedra, polyicosahedra, fcc-truncated octahedra, and decahedra. The set of magic sizes and structural motifs of Ag clusters suggest an icosahedral growth pattern based on a combination of MIC/Mackay and TIC/Polyicosahedral growth. For Cu_mAg_n and Ni_mAg_n clusters, with N = m+n from 2 to 60, global minima are mainly icosahedron and polyicosahedron structures, with exception for some clusters of size N = 38 which are truncated octahedrons. Different theoretical measures such as bond order parameter and radial distances suggest that in both Cu--Ag and Ni--Ag nanoalloys core--shell structures with Ag atoms segregated to the surfaces are preferred. The two types of nanoalloys exhibit different energetical properties while they are very similar in structural properties.Der Schwerpunkt dieser Arbeit liegt in der Bestimmung der Strukturen von Ag-Clustern und bimetallischen Kupfer-Silber und Nickel-Silber Clustern mit globalen Minima. Dabei wird eine Kombination von Embedded-Atom Methode und Basin-Hopping Algorithmus angewandt. Die globalen Minima von Silber-Clustern mit N = 2 bis 100 Atomen stellen Ikosaeder, Polyicosahedra, fcc-Oktaeder, und Dekaedern dar. Die magischen Zahlen und Strukturmotive der Ag-Clustern deuten auf ein ikosaedrisches Wachstumsmuster mit einer Kombination von MIC/Mackay- und TIC/Polyikosaeder-Wachstum. Für Cu_mAg_n und Ni_mAg_n-Cluster mit N = m + n von 2 bis 60 werden als globale Minima hauptsächlich Ikosaeder und Polyikosaeder. Die einsigen Ausnahme sind einige Cluster mit der Größe N = 38, die fcc-Oktaeder sind. Theoretische Größen wie Bindungsordnungparameter und radiale Abstände lassen vermuten, dass in beiden Cu--Ag und Ni--Ag Nanolegierungen Kern-Schale Strukturen mit Ag-Atome vorwiegend an der Oberflächen bevorzugt werden. Die beiden Typen von Nanolegierungen haben unterschiedliche energetischen Eigenschaften während sie sich sehr ähnlich in ihre Struktur sind

Kinetics and Thermodynamics of CO Oxidation by (TiO2)6

Molecular level insights into the mechanism and thermodynamics of CO oxidation by a (TiO2)6 cluster have been obtained through density functional calculations. Thereby, in this study, as an example, two different structural isomers of (TiO2)6 are considered with the purpose of understanding the interplay between local structure and activity for the CO oxidation reaction. Active sites in the two isomeric forms were identified on the basis of global and local reactivity descriptors. For the oxidation of CO to CO2, the study considered both sequential and simultaneous adsorption of CO and O2 on (TiO2)6 cluster through the ER and LH mechanisms, respectively. Three different pathways were obtained for CO oxidation by (TiO2)6 cluster, and the mechanistic route of each pathway were identified by locating the transition-state and intermediate structures. The effect of temperature on the rate of the reaction was investigated within the harmonic approximation. The structure-dependent activity of the cluster was rationalized through reactivity descriptors and analysis of the frontier orbitals

Surfaces, Shapes, and Bulk Properties of Crystals

We study the interplay between surface and bulk properties of macroscopic materials. It is demonstrated that the so-called polar surfaces may be stabilized through a charge redistribution between the complete set of surfaces that depends upon the overall shape of the sample and the nature of the material. This charge redistribution, in turn, is governed by certain constraints that we call generalized Tasker conditions. The same surface, but for samples of different shapes, may have different surface charges. Besides its stabilizing effect, the charge redistribution is also shown to particularly affect the dipole moment per repeat unit, a bulk property. For the latter, it is established that essentially any physically meaningful value is possible (depending upon the shape and material), in contrast to the often made assumption that different samples of the same material will have values that differ by, at most, a lattice vector. Finally, some recent experimental and theoretical results for polar surfaces are discussed in terms of the analysis presented here

Properties of Naked Silver Clusters with Up to 100 Atoms as Found with Embedded-Atom and Density-Functional Calculations

The structural and energetic properties of small silver clusters Agn with n = 2–100 atoms are reported. For n = 2–100 the embedded atom model for the calculation of the total energy of a given structure in combination with the basin-hopping search strategy for an unbiased structure optimization has been used to identify the energies and structures of the three energetically lowest-lying isomers. These optimized structures for n = 2–11 were subsequently studied further through density-functional-theory calculations. These calculations provide additional information on the electronic properties of the clusters that is lacking in the embedded-atom calculations. Thereby, also quantities related to the catalytic performance of the clusters are studied. The calculated properties in comparison to other available theoretical and experimental data show a good agreement. Previously unidentified magic (i.e., particularly stable) clusters have been found for n>80. In order to obtain a more detailed understanding of the structural properties of the clusters, various descriptors are used. Thereby, the silver clusters are compared to other noble metals and show some similarities to both copper and nickel systems, and also growth patterns have been identified. All vibrational frequencies of all the clusters have been calculated for the first time, and here we focus on the highest and lowest frequencies. Structural effects on the calculated frequencies were considered