Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis

Theory as a driving force to understand reactions on nanoparticles: general discussion

International audienc

JEDI: A versatile code for strain analysis of molecular and periodic systems under deformation

Stretching or compression can induce significant energetic, geometric and spectroscopic changes in materials. To fully exploit these effects in the design of mechano- or piezochromic materials, self-healing polymers, and other mechanoresponsive devices, a detailed knowledge about the distribution of mechanical strain in the material is essential. Within the past decade, the Judgement of Energy DIstribution (JEDI) analysis has emerged as a useful tool for this purpose. Based on the harmonic approximation, the strain energy in each bond length, bond angle and dihedral angle of a deformed system is calculated using quantum chemical methods. This allows the identification of the force-bearing scaffold of the system, leading to an understanding of mechanochemical processes at the most fundamental level. Here we present a publicly available code that generalizes the JEDI analysis, which has previously only been available for isolated molecules. Now the code has been extended to two- and three-dimensional periodic systems, supramolecular clusters, and substructures of chemical systems under various types of deformation. Due to the implementation of JEDI into the Atomic Simulation Environment (ASE), the JEDI analysis can be interfaced with a plethora of program packages that allow the calculation of electronic energies for molecular systems and systems with periodic boundary conditions. The automated generation of a color-coded three-dimensional structure via the Visual Molecular Dynamics (VMD) program allows insightful visual analyses of the force-bearing scaffold of the strained system

A Machine‐Learning‐Based Approach for Solving Atomic Structures of Nanomaterials Combining Pair Distribution Functions with Density Functional Theory

Determination of crystal structures of nanocrystalline or amorphous compounds is a great challenge in solid-state chemistry and physics. Pair distribution function (PDF) analysis of X-ray or neutron total scattering data has proven to be a key element in tackling this challenge. However, in most cases, a reliable structural motif is needed as a starting configuration for structure refinements. Here, an algorithm that is able to determine the crystal structure of an unknown compound by means of an on-the-fly trained machine learning model, which combines density functional theory calculations with comparison of calculated and measured PDFs for global optimization in an artificial landscape, is presented. Due to the nature of this landscape, even metastable configurations and stacking disorders can be identified

Halide-sodalites: thermal behavior at low temperatures and local deviations from the average structure

Sodalites of the general type |Na$_8$X$_2$|[T$^1$T$^2$O$_4$]$_6$ with X = Cl−, Br−, I− have been synthesized for Al–Si, Ga–Si, Al–Ge and Ga–Ge as T$^1$–T$^2$ frameworks. The structures were examined using in-house and synchrotron X-ray diffraction, Raman spectroscopy, force-field structure optimizations and DFT based ab-initio molecular dynamics (MD) computations. Calculated phonon density of states (PDOS) of the 12 compounds show only minor differences within a framework composition with a lowering of certain phonon energies with increasing anion size. Earlier published Debye and Einstein temperatures obtained with a Debye-Einstein-anharmonicity (DEA) model approach are confirmed using the determined low-temperature lattice parameters (18 K–293 K) and show no correlation with the respective PDOS. Small-box refinements against radial pair distribution functions (PDF) allowed the determination of anisotropic displacement ellipsoids (ADP) for Na$^+$ and O$^{2−}$, indicating a strong dependency of the ADP of Na$^+$ on the chemical composition. Significantly lower thermal displacements from MD calculations suggested an influence of structural displacements. For compounds with an aspherical ADP for sodium, structural models could be refined in which the sodium is located on two 8e or one 24i site (both partially occupied), and also temperature-dependent (100 K–300 K) for the compounds with Ga–Ge framework. 3D-plots of the bond-valence sums of Na$^+$ further validate the structural differences. These results imply that the local structure of halide-sodalites in many cases is not best described by the known average structure and may even not be cubic

Charge-Transfer Promoted Fixation of Glyphosate on TiO2 - a Multiscale Approach

In the present manuscript we used atomistic simulation methodsto investigate the adsorption of GGG on the titania rutile-(110)surface. The molecule showed a high adsorption to the surface es-pecially in its bidentate-⊥mode. This configuration was furtherinvestigated both in terms of charge transfer and strain analysis.In the first case a transfer of roughly one electron was observedfrom the functional groups binding to the surface atoms, leavingthe molecule at the phosphate and carboxylic groups. This chargeis distributed into the rutile’s bulk, where it gets then dissipated.To get a hint on the location of the bond break, the strain analysis identified the C-C bond as the major strain location. This presump-tion can not be confirmed in this work. In order to understand pho-tocaltalytic degradation of GGG in further investigations, a clustermodel was developed for TD-DFT simulations. The cluster simu-lations are in good agreement with the DFT simulations in termsof ground state properties (band gap and HOMO-levels) as well as geometrically optimized structures. Here not only the DOS showeda reduction of the band gap upon adsorption, but also the TD-DFTspectrum lied in the same regime. Hence, there is a high likelihoodfor GGG to proceed in a photocatalytic reactio

The mechanism of Mg²⁺ conduction in ammine magnesium borohydride promoted by a neutral molecule

Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery. Here we present the mechanism of Mg-ion conductivity of monoammine magnesium borohydrides, Mg(BH4)2·NH3. Density functional theory calculations (DFT) reveal that the neutral molecule (NH3) in Mg(BH4)2·NH3 is exchanged between the lattice and interstitial Mg2+ facilitated by a highly flexible structure, mainly owing to a network of di-hydrogen bonds, and the versatile coordination of the BH4 ligand. DFT shows that di-hydrogen bonds in inorganic matter and hydrogen bonds in bio-materials have similar bond strengths and bond lengths. As a result, the Mg-ion conductivity is dramatically improved at moderate temperature, e.g. sigma(Mg2+) = 3.3×10–4 S cm–1 at T = 80 °C for Mg(BH4)2·NH3, which is approximately 8 orders of magtitude higher than that of Mg(BH4)2. Our results may inspire a new approach for design and discovery of unprecedented multivalent ion conductors