Search

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co₃ O₄ ). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccupied 3d states of cobalt atoms. Also, since Co₃ O₄ contains two types of Co atoms, i.e., Co³⁺ and Co²⁺, we find that contribution of Co²⁺ ions to the pre-edge peak is solely due to single spin-polarized t₂g orbitals (dₓz, dyz, and dₓy ) while that of Co³⁺ ions is due to spin-up and spin-down polarized eg orbitals (dₓ₂ −y² aⁿdd z² ⁾. Furthermore, we deduce the magⁿetⁱc momeⁿts oⁿ the Co³⁺ and Co²⁺ to be zero and 3.00 µB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co₃ O₄ consists of antiferromagnetically ordered Co²⁺ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins. OA Förderung 2022

Due to the variability of the cation occupancy of octahedral and tetrahedral sites, spinel ferrites and cobaltites are particularly interesting to investigate activity trends in oxidation catalysis. Yet, the preparation of the respective catalyst series remains challenging. We employed pulsed laser defect engineering of CoFe₂O₄ nanoparticles in water to gradually alter the cation occupancy of octahedral and tetrahedral sites by single laser pulses and study its effect on cinnamyl alcohol oxidation. Three CoFe₂O₄ catalysts from different synthesis methods resembling different initial site occupancy were chosen as starting materials. The laser-induced randomization of the cation occupancy was verified by Mössbauer spectroscopy and linearly correlated with the conversion of cinnamyl alcohol while the size and Co : Fe ratio was maintained during laser processing. The study solidifies the importance of octahedral Co³⁺-sites and the feasibility of pulsed laser processing for altering the cation occupancy and related crystallographic defect density in oxidation catalysis.

Ab initio molecular dynamics simulations of a single hydrated 2-propanol molecule were performed to study the role of temperature, surface structure and electrochemical environment for the oxidation of 2-propanol to acetone at the Co3O4 (001)/H2O interface. On the A-terminated and B-terminated surfaces, which differ in the relative number of Co2+ and Co3+ ions at the surface, 2-propanol adsorbs molecularly on the Co2+ and Co3+ sites, respectively. In both cases, no C-H bond cleavage is observed at room temperature. However, under oxidative conditions, which are modeled here by partial dehydrogenation of the mixed hydroxyl/water adsorbate layer, dehydrogenation of the alcoholic OH group is observed on both surface terminations. As a result, adsorbed 2-propanolate is formed. The reaction on the less hydroxylated B-terminated surface further proceeds with C-H bond cleavage at the 2-carbon atom. The oxidation product acetone remains adsorbed on the Co3+ site during the simulation period of approximately 20 ps. Both deprotonation steps are aided by the presence of the adsorbed hydroxyl groups in the vicinity of the adsorbed alcohol molecule, because both hydrogen atoms from the reactand molecule are transferred as protons to form adsorbed water molecules. Different from the case of the partially dehydrogenated environment, raising the system temperature from 300 to 450 K, which can be considered a simple model for high temperature thermal catalysis, does not lead to oxidation via C-H dehy-drogenation of the 2-propanol molecule.

The authors explain that modern science rests on three well-established pillars: the classic categories of experiment and theory and the younger computer simulation methods, with big data science rapidly becoming a fourth. While current experimental techniques have recently advanced their time and space resolution, dynamic atomistic computer simulations struggle to reach increasingly long time and length scales., UNIKATE: Berichte aus Forschung und Lehre, no. 57Katalyse: Alles andere als oberflächlich, p. 123

The spinel Co₃O₄ has many beneficial properties for potential use in catalysis. In operando, water is always present and alters the properties of the catalyst. We have used ab initio molecular dynamics to understand the effect of water and solvation on the structure and reactivity of the Co₃O₄ (001) A-type and B-type surface terminations. Water adsorbs on both terminations via a partial dissociative mode, and the A-termination is seen to be more reactive. On this surface, a higher degree of dissociation is observed in the topmost layers of the crystal in contact with water. Water dissociates more frequently on the Co²⁺ sites (about 75%) than on the adjacent Co³⁺ sites, where the degree of dissociation is about 50%. Increasing water coverage does not change the degree of water dissociation significantly. OH− adsorption on the Co²⁺ sites leads to a reduction of the amount of reconstruction and relaxation of the surface relative to the clean surface at room temperature. Proton transfer within the water films and between water molecules and surface has localized character. The B-terminated interface is less dynamic, and water forms epitactic layers on top of the Co³⁺ sites, with a dissociation degree of about 25% in the contact layer. OA Förderung 2020

One-dimensional tungsten disulfide (WS₂) single-walled nanotubes (NTs) with either achiral, i.e., armchair (n, n) and zigzag-type (n, 0), or chiral (2n, n) configuration with diameters dNT > 1.9 nm have been found to be suitable for photocatalytic applications, since their band gaps correspond to the frequency range of visible light between red and violet (1.5 eV < Δϵgₐp < 2.6 eV). We have simulated the electronic structure of nanotubes with diameters up to 12.0 nm. The calculated top of the valence band and the bottom of the conduction band (ϵVB and ϵCB, respectively) have been properly aligned relatively to the oxidation (ϵO) and reduction (ϵH₂/H₂O) potentials of water. Very narrow nanotubes (0.5 < dNT < 1.9 nm) are unsuitable for water splitting because the condition ϵVB < ϵO < ϵH₂/H₂O < ϵCB does not hold. For nanotubes with dNT > 1.9 nm, the condition ϵVB < ϵO < ϵH₂/H₂O < ϵCB is fulfilled. The values of ϵVB and ϵCB have been found to depend only on the diameter and not on the chirality index of the nanotube. The reported structural and electronic properties have been obtained from either hybrid density functional theory and Hartree-Fock linear combination of atomic orbitals calculations (using the HSE06 functional) or the linear augmented cylindrical waves density functional theory method. In addition to single-walled NTs, we have investigated a number of achiral double-walled (m, m)at(n, n) and (m, 0)at(n, 0) as well as triple-walled (l, l)at(m, m)at(n, n) and (l, 0)at(m, 0)at(n, 0) nanotubes. All multiwalled nanotubes show a common dependence of their band gap on the diameter of the inner nanotube, independent of chirality index and number of walls. This behavior of WS₂ NTs allows the exploitation of the entire range of the visible spectrum by suitably tuning the band gap. CA extern

Die einmaligen Eigenschaften der Wassermoleküle fassen die Autoren in diesem Überblick zusammen., Water is such a special molecule that its abundance is generally regarded as prerequisite for the existence of any form of biological life. This special role in the chemical universe can be traced back to its charge polarisation in combination with the ability to readily form a flexible network with four neighbouring water molecules. This unique combination of properties is not provided by any other small molecule and gives rise to peculiarities that are seen as essential for life., UNIKATE: Berichte aus Forschung und Lehre, no. 51Herausforderung Wasserforschung - lokal, regional, global, p. 18

Dieser Artikel erklärt, wie mit Modellen, Hypothesen, Computern und Programmen Forschungsfragen beantwortet und neue gestellt werden (können)., Why does chemistry need large computers? Because the interactions between individual atoms and molecules determine the behavior of chemical systems, and because the numerical calculation of these interactions is difficult and numerically expensive. Computer simulations of these interactions form one of the pillars of modern natural sciences. Computers are particularly useful when it comes to disordered systems such as liquids and solutions in which hierarchical processes of selforganization take place on different time and length scales. A current research project of our group is the study of the formation and fusion of so-called micelles (which are, in 36 turn, self-assembled structures in aqueous solutions composed of many molecules with a hydrophilic (water-loving) and a hydrophobic (water-hating) end) into cylindrical and three-dimensional entities. The dependence of this behavior on small ions with specific activity offers the potential to manipulate the shape and dynamics of the micellar systems themselves. Instrumental and essential for such studies is the use of computer systems of varying computational power, which we demonstrate on the basis of our experience with the performance of server PCs in our group and the supercomputers CRAY-XT6m and NEC-magnitUDE at the Center of Computational Sciences and Simulation at our university., UNIKATE: Berichte aus Forschung und Lehre, no. 50Natur-, Ingenieur- und Wirtschaftswissenschaften - High-Performance und Cloud Computing, p. 28

Zinc oxide (ZnO) is considered in general as a promising material for solar water splitting. Its wurtzite-structured bulk samples, however, can be considered as active for photocatalytic applications only under UV irradiation, where they possess ∼1% efficiency of sunlight energy conversion due to their wide band gap (3.4 eV). Although pristine ZnO nanowires (NWs) possess noticeably narrower band gaps than the bulk, the tendency of band gap reduction with increasing NW diameter is insufficient, and further modification is required. We have contributed to filling this gap by performing a series of ab initio calculations on ZnO NWs of different diameters (dNW), which are mono-doped by metal (Ag) and non-metal atoms (C, N) or contain oxygen vacancies with varied concentration (∼3 vs. ∼6%). To reproduce qualitatively the energies of one-electron states of nanowires in our calculations, the hybrid DFT + HF Hamiltonian has been used, based on the PBE0 exchange-correlation functional. We have analyzed changes in the electronic structure induced in a few defect composition scenarios, showing that, for specific concentrations and locations of the dopants, the optical absorption peak of doped ZnO can be shifted to the visible light range with promising efficiency. In agreement with experimental observation, the most significant results have been achieved for carbon-doped ZnO nanowires. They possess the highest photocatalytic suitability, since the band gap is reduced in this case down to 2.1–2.2 eV (for nanowire diameters of 2.9–3.5 nm), which corresponds to optimal 15–17% efficiency of solar energy conversion. OA hybrid