Synthesis and characterization of highly ordered Ni-MCM-41 mesoporous molecular sieves

Highly ordered Ni-MCM-41 samples with nearly atomically dispersed nickel ions were prepared reproducibly and characterized. Similar to the Co-MCM-41 samples, the pore diameter and porosity can be precisely controlled by changing the synthesis surfactant chain length. Nickel was incorporated by isomorphous substitution of silicon in the MCM-41 silica framework, which makes the Ni-MCM-41 a physically stable catalyst in harsh reaction conditions such as CO disproportionation to single wall carbon nanotubes or CO2 methanation. X-ray absorption spectroscopy results indicate that the overall local environment of nickel in Ni-MCM-41 was a tetrahedral or distorted tetrahedral coordination with surrounding oxygen anions. Hydrogen TPR revealed that our Ni-MCM-41 samples have high stability against reduction; however, compared to Co-MCM-41, the Ni-MCM-41 has a lower reduction temperature, and both the H2-TPR and in situ XANES TPR reveal that the reducibility of nickel is not clearly correlated with the pore radius of curvature, as in the case of Co-MCM-41. This is probably a result of nickel being thermodynamically more easily reduced than cobalt. The stability of the structural order of Ni-MCM-41 has been investigated under SWNT synthesis and CO2 methanation reaction conditions as both require catalyst exposure to reducing environments leading to formation of metallic Ni clusters. Nitrogen physisorption and XRD results show that structural order was maintained under both SWNT synthesis and CO2 methanation reaction conditions. EXAFS results demonstrate that the nickel particle size can be controlled by different prereduction temperatures but not by the pore radius of curvature as in the case of Co-MCM-41.     

Electronic band gaps of semiconductors as influenced by their isotopic composition

The present paper focuses on the renormalization effects of the band gaps in the electronic band structure of the elemental semiconductors traced to zero-point vibrations. Electron-phonon interaction and volume changes (in combination with anharmonicity) are the underlying microscopic mechanisms, both dependent on M^−1/2, M being the average isotopic mass. Thus isotopically controlled crystals offer an extraordinary opportunity to test the theoretical predictions with a variety of spectroscopic techniques. The paper discusses the theoretical predictions and their experimental verifications, exploiting derivative and photoluminescence spectroscopy. Illustrative examples on Si and Ge, drawn from the investigations of the authors, are presented.     

BeitrÄge zur AufklÄrung von FÄrbevorgÄngen

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Zur Kenntnis der Rolle von Dielektrizitätskonstante,Polarisation und Dipolmoment in kolloiden Systemen,X. Dielektrische Polarisation von Azetylzellulose in Lösung

Ohne Zusammenfassung Herrn Prof. Wo. Ostwald danken wir für die Anregung zu vorstehender Arbeit und für sein lebhaftes Interesse an ihr.     

Weitere Beiträge zur Kenntnis der Adsorptionsverbindungen. IV

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Das färberische Verhalten des Kongorubins

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