Thomas

Publication date: 1 December 2018

Source: Computational and Theoretical Chemistry, Volume 1145

Author(s): S.V. Feskov

Leveraging subsystem density functional theory (also known as density embedding), an impurity model that embeds a finite neutral or charged subsystem within an extended surrounding subsystem is defined. The combination of the impurity model and a consistent choice of the Coulomb reference provides a new tool for evaluating the ionization potential in extended systems. The approach is tested using liquid water as model system.

Abstract

Calculations of charged systems in periodic boundary conditions (PBC) are problematic because there are spurious interactions between the charges in different periodic images that can affect the physical picture. In addition, the intuitive limit of Coulomb interactions decaying to zero as the interacting charges are placed at infinite separation no longer applies, and for example total energies become undefined. Leveraging subsystem density functional theory (also known as density embedding) we define an impurity model that embeds a finite neutral or charged subsystem within an extended (infinite) surrounding subsystem. The combination of the impurity model and a consistent choice of the Coulomb reference provides us with an algorithm for evaluating the ionization potential (IP) in extended systems. We demonstrate our approach in a pilot calculation of the IP of liquid water, based on a configuration from a prior ab initio molecular dynamics (AIMD) simulation of liquid water (Genova et al., J. Chem. Phys. 2016, 144, 234105). The calculations with the impurity model capture the broadening on the ionization energies introduced by the interactions between the water molecules. Furthermore, the calculated average IP value (10.5 eV) compare favorably to experiments (9.9‐10.06 eV) and very recent simulations based on the GW approximation (10.55 eV), while at the same time outperforming density embedding calculations carried out with a naïve handling of the electrostatic interactions (about 7 eV).

Publication date: 15 October 2018

Source: Computational and Theoretical Chemistry, Volume 1142

Author(s): James S.M. Anderson, Farnaz Heidar-Zadeh, Paul W. Ayers

Warum zersplittern trockene Spagetti, wenn man sie biegt? US-Ingenieure präsentieren nun eine Antwort - zusammen mit einer Anleitung, wie man es besser macht.

Nach Graphen und Fullerenen haben Fachleute jetzt auch die dritte mögliche Form zweidimensionalen Kohlenstoffs hergestellt.

Transistoren sind das zentrale Bauelement eines jeden Computerchips. Je kleiner sie sind, desto besser. Nun haben Forscher die Miniaturisierung ins Extrem getrieben.

Publication date: November 2018

Source: Surface Science, Volume 677

Author(s): Zhibin Qu, Fei Sun, Xin Liu, Jihui Gao, Zhipeng Qie, Guangbo Zhao

Abstract

Graphical abstract

Kaffee scheint das Leben zu verlängern - doch welche Chemikalie ist verantwortlich? Die Hauptverdächtige ist es wohl nicht, zeigt jetzt eine Gen-Studie.

Das Abwasser vieler Flüsse ist mit den Rückständen von Kokain und anderen Rauschmitteln verseucht. Das hat unerwartete Folgen.

Publication date: October 2018
Source:Surface Science, Volume 676
Author(s): Yuhai Hu, Keith Griffiths
SOx poisoning has been a critical problem in the chemistry of selective catalytic reduction of NOx, and many fundamental issues surrounding the reactions remain unclear. In this paper, reactions taking place in the ¹³C2H5OHNO-S¹⁸O2 O2 system on a platinum single crystal surface were studied. Oxidation of ¹³C2H5OH by S¹⁸O2 proceeds mainly through ¹⁸O-H reaction, and oxidation of the backbone carbon atoms can occur at high S¹⁸O2 exposures, e.g., 1.6 Ls of S¹⁸O2 for 0.4 L of ¹³C2H5OH. Thus, the presence of S¹⁸O2 only slightly suppresses the catalytic reduction of NO by ¹³C2H5OH. Co-adsorbed O2 evidently exerts a suppressive effect. When oxygen is lean (relative to S¹⁸O2), the S¹⁸O2 O2 reactions mainly give rise to surface sulphoxy species at temperatures between 400 and 500 K and thus, the NO-¹³C2H5OH reactions can still proceed. The NO reduction reaction is completely suppressed when oxygen is rich, a result of the Pt surface being passivated. This finding provides a valuable guide for optimizing the reaction atmosphere to improve the efficiency of the selective catalytic reduction of NO by hydrocarbons (or oxygen-containing fragments from oxidation of hydrocarbons in this case).

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Publication date: November 2018
Source:Surface Science, Volume 677
Author(s): David Langhammer, Jolla Kullgren, Pavlin Mitev, Lars Österlund
The adsorption of SO2 on single crystalline TiO2(110) has been investigated by means of polarized infrared reflection-absorption spectroscopy (IRRAS) experiments and density functional theory (DFT) calculations. IR absorption bands were detected at 1324 cm − 1 and 985 cm − 1 with p-polarized light incident along both the [1 1 ¯ 0] and [001] crystallographic directions at 123 K. When the temperature was increased to 153 K, the peak at 1324 cm − 1 disappears, while a new, weak band appears at 995 cm − 1 . Simultaneously, a band at 995 cm − 1 also emerges with s-polarized light along the [1 1 ¯ 0] direction. Based on the symmetry properties of the IRRAS spectra and accompanying ab initio simulations of the spectra employing a three layer model (vacuum-adsorbate-substrate), it is shown that the low temperature absorption IRRAS bands can be attributed to an SO3-like adsorbate structure. This is also the most stable adsorption structure (Ead = −0.58 eV) on the stoichiometric surface. The combined IRRAS and DFT results show that the band appearing at 995 cm − 1 is associated with a surface sulfite specie which is stabilized by residual surface water. The DFT calculations also revealed that a stable adsorption structure exists on a reduced TiO2 surface, where SO2 binds strongly to an oxygen vacancy site. It is suggested that this is an intermediate that form surface sulfate upon further reactions with water, although it was not observed on the stoichiometric surface studied in this work.

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Seit Monaten melden amerikanische Teams Durchbrüche bei der Entwicklung von Quantencomputern. Europäische Forscher wollen nun eine Schwachstelle der Maschinen beseitigen.