Epoxidieren leicht gemacht: Subnanometergroße Goldcluster auf amorphem Aluminiumoxid sind ein hoch aktiver und selektiver Katalysator für die Epoxidierung von Propen. Am höchsten ist die Selektivität bei Gasmischungen, die Sauerstoff und Wasser enthalten, sodass der Einsatz von Wasserstoff vermieden werden kann. Ab‐initio‐Dichtefunktionalrechnungen lieferten Informationen über Schlüsselintermediate und Reaktionswege. Demnach beruht die Aktivität des Katalysators auf der Bildung von Propenoxid‐Metallacyclen. Al grün, Au gelb, O rot, C grau.
High temperature annealing is the only method known to date that allows the complete repair of a defective lattice of graphenes derived from graphite oxide, but most of the relevant aspects of such restoration processes are poorly understood. Here, we investigate both experimentally (scanning probe microscopy) and theoretically (molecular dynamics simulations) the thermal evolution of individual graphene oxide sheets, which is rationalized on the basis of the generation and the dynamics of atomic vacancies in the carbon lattice. For unreduced and mildly reduced graphene oxide sheets, the amount of generated vacancies was so large that they disintegrated at 1773-2073 K. By contrast, highly reduced sheets survived annealing and their structure could be completely restored at 2073 K. For the latter, a minor atomic-sized defect with six-fold symmetry was observed and ascribed to a stable cluster of nitrogen dopants. The thermal behavior of the sheets was significantly altered when they were supported on a vacancy-decorated graphite substrate, as well as for the overlapped/stacked sheets. In these cases, a net transfer of carbon atoms between neighboring sheets via atomic vacancies takes place, affording an additional healing process. Direct evidence of sheet coalescence with the step edge of the graphite substrate was also gathered from experiments and theory.
The effect of alloying on the structural and thermal properties of Cun−xAux (n=13,14) clusters is investigated by constant energy Molecular Dynamics simulations. The interactions between the atoms in the clusters are mimicked by a many-body (Gupta-like) potential based on the second moment approximation to the tight-binding model. The minimum energy structures and the lowest-lying isomers of the pure and mixed clusters are obtained by thermal quenching. We find icosahedral-like ground state structures for the 13- and 14-atom clusters and for all the concentrations, the only exception being Au14 which has C6v symmetry. Mixed structures are preferred over the segregated ones. The lowest-lying isomers of the binary clusters are the permutational ones, i.e., isomers having the same underlying geometry as the ground state structure and different relative arrangement of the unlike atoms in the atomic positions of the geometry. However, presence of these low lying permutational isomers does not affect the gross features of the melting-like transition. The 13- and 14-atom (icosahedral-like) binary clusters melt in one and two stages, respectively, as the corresponding pure Cu clusters. In constrast the melting-like transition of Au14 exhibits a single stage. The melting temperature is studied as a function of cluster concentration and size. The main conclusion is that mixed Cu–Au clusters likely behave as pure Cu clusters, both from the structural and the dynamical points of view, for all concentrations.
We review the time-dependent density functional theory (TDDFT) and its use to investigate excited states of nanostructures. These excited states are routinely probed using electromagnetic fields. In this case, two different regimes are usually distinguished: (i) If the electromagnetic field is "weak"as in optical absorption of light-it is sufficient to treat the field within linear response theory; (ii) Otherwise, nonlinear effects are important, and one has to resort to the full solution of the timedependent Kohn-Sham equations. This latter regime is of paramount relevance in the emerging field of research with intense and ultrashort laser pulses. This review is divided into two parts: First we give a brief overview of the theoretical foundations of the theory, both in the linear and non-linear regimes, with special emphasis on the problem of the choice of the exchange-correlation functional. Then we present a sample of applications of TDDFT to systems ranging from atoms to clusters and to large biomolecules. Although most of these applications are in the linear regime, we show a few examples of non-linear phenomena, such as the photo-induced dissociation of molecules. Many of these applications have been performed with the recently developed code octopus (http://www.tddft.org/programs/octopus).
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