We report metallic NiPS 3 @NiOOH core−shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS 3 nanosheets are obtained by exfoliation of bulk NiPS 3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS 3 @ NiOOH core−shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS 3 @NiOOH core−shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS 3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS 3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS 3 @ NiOOH core−shell heterostructure, where the presence of P and S modulates the surface electronic structure of Ni in NiPS 3 , thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS 3 @ NiOOH core−shell electrode materials for the electrochemical water oxidation.

The elementary steps of the methanol synthesis reaction at oxygen vacancies on the polar ZnO(0001 j ) surface have been studied with density functional theory. We report results for reaction and activation energies for the most important elementary steps of the methanol synthesis reaction on this surface. At these oxygen vacancies the barriers for heterolytic cleavage of H 2 into protons, bound in surface OH groups, and hydrides, trapped in the vacancies, is low. The stabilized hydride facilitates the reduction of CO to formyl (stabilized in the vacancy) with a low barrier j100 kJ/mol. After a fast rearrangement to formaldehyde and hydroxymethylene species, a second reduction step leads probably to a surface-bound methoxide which at reducing conditions desorbs from the surface under regeneration of an oxygen vacancy. Beside formate, which appears as a side product, and formyl species, oxygen-bound hydroxymethylene might be another observable intermediate of the methanol synthesis reaction at oxygen vacancy sites on the ZnO(0001 j ) surface.

We report optimised auxiliary basis sets for the resolution-of-the-identity (or density-fitting) approximation of two-electron integrals in second-order Møller-Plesset perturbation theory (MP2) and similar electronic structure calculations with correlation-consistent basis sets for the post-d elements Ga-Kr, In-Xe, and Tl-Rn. The auxiliary basis sets are optimised such that the density-fitting error is negligible compared to the one-electron basis set error. To check to which extent this criterion is fulfilled we estimated for a test set of 80 molecules the basis set limit of the correlation energy at the MP2 level and evaluated the remaining density-fitting and the one-electron basis set errors. The resulting auxiliary basis sets are only 2-6 times larger than the corresponding one-electron basis sets and lead in MP2 calculations to speed-ups of the integral evaluation by one to three orders of magnitude. The density-fitting errors in the correlation energy are at least hundred times smaller than the one-electron basis set error, i.e. in the order of only 1-100 μH per atom.

We used ab initio density functional theory in combination with an embedded cluster approach to calculate vibrational spectra and formation enthalpies of possible intermediates and side products (spectator species) in the synthesis of methanol out of syngas on the ZnO(0001) surface. Our investigations are based upon our previous work on possible reaction pathways and activation barriers for this reaction at oxygen vacancies on ZnO(0001). We present and discuss calculated vibrational frequencies of short-living formyl, hydroxymethylene, formaldehyde, acetale, and hydroxymethyl intermediates and compare the calculated frequencies of formate and methoxy species as well as CO and CO(2) species, at the defect free surface and at oxygen vacancies, with recent experimental findings. All investigated species show characteristic features in their spectra. Therefore, the analysis of their vibrational frequencies is a suitable mean to distinguish them and gain new insights in this reaction which is of recent experimental interest. We are able to identify the structure and characteristics of different surface species, such as monodentate and polydentate carbonate and formate species, in agreement with experimental results.

The demand for increased efficiency of industrial gas turbines and aero engines drives the search for the next generation of materials. Promising candidates for such new materials are Co-based superalloys. We characterize the microsegregation and solidification of a multi-component Co-based superalloy and compare it to a ternary CoAl-W compound and to two exemplary Ni-based superalloys by combining the experimental characterization of the as-cast microstructures with complementary modelling of phase stability. On the experimental side, we characterize the microstructure and precipitates by electron microscopy and energy-dispersive X-ray spectroscopy and determine the element distributions and microsegregation coefficients by electron probe microanalysis (EPMA). On the modelling side, we carry out solidification simulations and a structure map analysis in order to relate the local chemical composition with phase stability. We find that the microsegregation coefficients for the individual elements are very similar in the investigated Co-based and Ni-based superalloys. By interpreting the local chemical composition from EPMA with the structure map, we effectively unite the set of element distribution maps to compound maps with very good contrast of the dendritic microstructure. The resulting compound maps of the microstructure in terms of average band filling and atomic-size difference explain the formation of topologically close-packed phases in the interdendritic regions. We identify B2, C14, and D0 24 precipitates with chemical compositions that are in line with the structure map.