Quantenmechanische Modellrechnung am Diboranmolekül

Zusammenfassung Im Rahmen einer quantenmechanischen Modellrechnung, deren Vorstellungen auf einer Arbeit von Hartmann und Mosch [19] basieren, wird die stabilste Atomkonfiguration für das Diboranmolekül berechnet. Das Molekül wird hierbei aus dreifach positiv geladenen Borionen und einfach negativ geladenen Hydridionen aufgebaut. Die effektiven Kernladungszahlen gehen als Variationsparameter im Sinne des Variationsprinzips der Quantenmechanik in die Rechnung ein. Durch Minimisierung der Gesamtenergie nach den geometrischen Parametern werden die Gleichgewichtslagen der Atome für die verschiedenen zur Diskussion gestellten Strukturmodelle bestimmt. Als Resultat ergibt sich für die in der Natur realisierte Brückenstruktur die tiefste Energie. Die berechneten geometrischen Parameterliegen in guter, die Dissoziationsenergie für die Reaktion B₂H₆2BH₃ in sehr guter Übereinstimmung mit dem Experiment.

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Model calculation on the diborane molecule

By means of a model calculation, based on ideas described by Hartmann and Mosch [19], a determination of the geometric structure of the diborane molecule has been carried out. The molecule may be composed of positive boron ions with a charge +3 and negative hydride ions. The effective nuclear charge numbers are introduced as variation parameters in the sense of variation principle of quantum mechanics. For the different structures, discussed in this investigation, the equilibrium values for bond distances and angels were calculated by minimizing the total energy with respect to the geometric parameters. The lowest electronic energy results for the bridge structure, realized in nature. The calculated molecule parameters are in fairly good, the dissociation energy for the reaction B₂H₆2BH₃ in very good agreement with the experimental values.

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Résumé Détermination de la structure géométrique de la molécule de diborane par un calcul modèle basé sur une idée de Hartmann et Mosch [19]. La molécule peut être composée d'ions bore positifs de charge + 3 et d'ions hydrures négatifs. Les charges nucléaires effectives sont introduites comme paramètres variationnels. Les valeurs d'équilibres des distances et des angles ont été calculées en minimisant l'énergie totale par rapport aux paramètres géométriques. La plus basse énergie électronique se produit pour la structure en pont réalisée dans la nature. Les paramètres moléculaires calculés sont en bon accord avec l'expérience, et cet accord est excellent pour l'énergie de dissociation dans la réaction B₂H₆2BH₃.

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Auszug aus der von der Naturwissenschaftlichen Fakultät der Universität Frankfurt angenommenen Habilitationsschrift.

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Der Deutschen Forschungsgemeinschaft danke ich für die finanzielle Unterstützung dieser Arbeit.     

In Situ Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy of Nickel-Catalyzed Hydrogenation Reactions

Synthesis methods to prepare lower transition metal catalysts and specifically Ni for Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) are explored. Impregnation, colloidal deposition, and spark ablation have been investigated as suitable synthesis routes to prepare SHINERS-active Ni/Au@SiO₂ catalyst/Shell-Isolated Nanoparticles (SHINs). Ni precursors are confirmed to be notoriously difficult to reduce and the temperatures required are generally harsh enough to destroy SHINs, rendering SHINERS experiments on Ni infeasible using this approach. For colloidally synthesized Ni nanoparticles deposited on Au@SiO₂ SHINs, stabilizing ligands first need to be removed before application is possible in catalysis. The required procedure results in transformation of the metallic Ni core to a fully oxidized metal nanoparticle, again too challenging to reduce at temperatures still compatible with SHINs. Finally, by use of spark ablation we were able to prepare metallic Ni catalysts directly on Au@SiO₂ SHINs deposited on a Si wafer. These Ni/Au@SiO₂ catalyst/SHINs were subsequently successfully probed with several molecules (i. e. CO and acetylene) of interest for heterogeneous catalysis, and we show that they could be used to study the in situ hydrogenation of acetylene. We observe the interaction of acetylene with the Ni surface. This study further illustrates the true potential of SHINERS by opening the door to studying industrially relevant reactions under in situ or operando reaction conditions.     

On <Emphasis Type="Italic">g</Emphasis>-functions for Laguerre function expansions of Hermite type

We examine weighted L ^p boundedness of g-functions based on semigroups related to multi-dimensional Laguerre function expansions of Hermite type. A technique of vector-valued Calderón–Zygmund operators is used.     

Aerobic oxidation of alcohols over hydrotalcite-supported gold nanoparticles: the promotional effect of transition metal cations

Chromium (III)-containing hydrotalcites show strong synergy with gold nanoparticles in achieving high activity in the aerobic oxidation of alcohols.     

The Molecular Pathway to ZIF‐7 Microrods Revealed by In Situ Time‐Resolved Small‐ and Wide‐Angle X‐Ray Scattering,Quick‐Scanning Extended X‐Ray Absorption Spectroscopy,and DFT Calculations

We present an in situ small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) and quick‐scanning extended X‐ray absorption fine‐structure (QEXAFS) spectroscopy study on the crystallization of the metal–organic framework ZIF‐7. In combination with DFT calculations, the self‐assembly and growth of ZIF‐7 microrods together with the chemical function of the crystal growth modulator (diethylamine) are revealed at all relevant length scales, from the atomic to the full crystal size.     

Complexity behind CO2 capture on NH2-MIL-53(Al)

Some Metal Organic Frameworks (MOFs) show excellent performance in extracting carbon dioxide from different gas mixtures. The origin of their enhanced separation ability is not clear yet. Herein, we present a combined experimental and theoretical study of the amino-functionalized MIL-53(Al) to elucidate the mechanism behind its unusual high efficiency in CO(2) capture. Spectroscopic and DFT studies point out only an indirect role of amine moieties. In contrast to other amino-functionalized CO(2) sorbents, no chemical bond between CO(2) and the NH(2) groups of the structure is formed. We demonstrate that the functionalization modulates the "breathing" behavior of the material, that is, the flexibility of the framework and its capacity to alter the structure upon the introduction of specific adsorbates. The absence of strong chemical interactions with CO(2) is of high importance for the overall performance of the adsorbent, since full regeneration can be achieved within minutes under very mild conditions, demonstrating the high potential of this type of adsorbents for PSA like systems.     

Molecular aspects of glucose dehydration by chromium chlorides in ionic liquids

A combined experimental and computational study of the ionic‐liquid‐mediated dehydration of glucose and fructose by Cr^II and Cr^III chlorides has been performed. The ability of chromium to selectively dehydrate glucose to 5‐hydroxymethylfurfural (HMF) in the ionic liquid 1‐ethyl‐3‐methyl imidazolium chloride does not depend on the oxidation state of chromium. Nevertheless, Cr^III exhibits higher activity and selectivity to HMF than Cr^II. Anhydrous CrCl₂ and CrCl₃?6 H₂O readily catalyze glucose dehydration with HMF yields of 60 and 72 %, respectively, after 3 h. Anhydrous CrCl₃ has a lower activity, because it only slowly dissolves in the reaction mixture. The transformation of glucose to HMF involves the formation of fructose as an intermediate. The exceptional catalytic performance of the chromium catalysts is explained by their unique ability to catalyze glucose to fructose isomerization and fructose to HMF dehydration with high selectivity. Side reactions leading to humins by means of condensation reactions take predominantly place during fructose dehydration. The higher HMF selectivity for Cr^III is tentatively explained by the higher activity in fructose dehydration compared to Cr^II. This limits the concentration of intermediates that are involved in bimolecular condensation reactions. Model DFT calculations indicate a substantially lower activation barrier for glucose isomerization by Cr^III compared to Cr^II. Qualitatively, glucose isomerization follows a similar mechanism for Cr^II and Cr^III. The mechanism involves ring opening of D ‐glucopyranose coordinated to a single Cr ion, followed by a transient self‐organization of catalytic chromium complexes that promotes the rate‐determining hydrogen‐shift step.     

Ceria-Supported Cobalt Catalyst for Low-Temperature Methanation at Low Partial Pressures of CO2

The direct catalytic conversion of atmospheric CO₂ to valuable chemicals is a promising solution to avert negative consequences of rising CO₂ concentration. However, heterogeneous catalysts efficient at low partial pressures of CO₂ still need to be developed. Here, we explore Co/CeO₂ as a catalyst for the methanation of diluted CO₂ streams. This material displays an excellent performance at reaction temperatures as low as 175 °C and CO₂ partial pressures as low as 0.4 mbar (the atmospheric CO₂ concentration). To gain mechanistic understanding of this unusual activity, we employed in situ X-ray photoelectron spectroscopy and operando infrared spectroscopy. The higher surface concentration and reactivity of formates and carbonyls—key reaction intermediates—explain the superior activity of Co/CeO₂ as compared to a conventional Co/SiO₂ catalyst. This work emphasizes the catalytic role of the cobalt-ceria interface and will aid in developing more efficient CO₂ hydrogenation catalysts.