Über die Dispersität des Platins in Platin-Tonerde-Katalysatoren. 1. Bestimmung der Platin-Dispersität mittels Röntgenbeugung und Sauerstoff-Wasserstoff-Titration

On the dispersity of platinum in platinum/alumina catalysts. 1. Determination of the platinum dispersity by X-ray diffraction and oxygen-hydrogen titration. The metal dispersion in platinum-alumina catalysts was determined by X-ray diffraction and O₂? H₂-titration. The physico-chemical assumptions of both methods are discussed and the results are critically compared. From the X-ray measurements, the average size of the platinum crystallites was determined down to 20 Å and also the amount of both the crystalline and X-ray amorphous platinum. By O₂? H₂-titration the number of superficial Pt atoms and, by assumption of a defined area per Pt atom, the accessible Pt surface area was estimated. The Pt surface areas were also calculated from X-ray data. With small Pt concentrations and high Pt dispersion, the values obtained by both methods agree reasonably. Occurring differences are caused especially by the coverage of platinum crystallites by the alumina. The increase of information, obtained through combination of the two methods, is demonstrated for freshly reduced and tested catalysts.     

Herstellung und Eigenschaften von Aluminiumaquoxid. III. Peptisierung von Böhmit mit Salpetersäure

Preparation and Properties of Aluminium Hydroxide. III. Peptization of Boehmite with Nitric Acid A reaction of the aluminium oxide hydroxide boehmite with nitric acid in dependence on the temperature and the molar ratio HNO₃/Al₂O₃ is studied. The effect of the acid results in the formation of basic aluminium nitrates, which can change the rheological properties of the boehmite hydrogel due to redispersion or desaggregation up to its ?liquefaction”? to boehmite hydrosol. For the dependence of the flow velocity of the boehmite hydrosol on the molar ration HNO₃ Al₂O₃ a maximum correlation is characteristic. It can also be interpreted from the colloid-chemical point of view and it is a technically relevant indicator for the optimization of the peptization. By the peptization conditions of the boehmite hydrogel the pore structure of the Al₂O₃ can be varied in a carefully directed way.     

Die analytischen W?gungen

Ohne Zusammenfassung     

Concept of neutral gain modules for power scaling of thin-disk lasers

We present a concept for power scaling of high brightness solid state lasers, which introduces so called gain modules containing the laser active media. These modules can be inserted as relay imaging optical systems in any type of laser resonators. The gain modules are optically neutral; hence, power scaling can be provided inserting several modules. Here, we provide the basic functional units of gain modules as well as an exemplary experimental implementation in thin-disk lasers with dynamically stable resonators. On the basis of these studies, more than 1 kW output power with an averaged M ²=2.6 could be demonstrated using two disks. Experiments with four disks are in preparation.     

Enantioselective Homogeneous Hydrogenation of Monosubstituted Pyridines and Furans

Summary.  The first case of an enantioselective hydrogenation of monosubstituted pyridines and furans with homogeneous rhodium diphosphine catalysts with low but significant enantioselectivities and catalyst activities is reported. Best enantioselectivities (ees of 24–27%) were obtained for the hydrogenation of 2- and 3-pyridine carboxylic acid ethyl ester and 2-furan carboxylic acid with catalysts prepared in situ from [Rh(nbd)₂]BF₄ and the chiral ligands diop, binap, or ferrocenyl diphosphines of the josiphos type. Turnover numbers (ton) were in the order of 10–20, turnover frequencies (tof) usually 1–2 h⁻¹. Diphosphines giving 6- or 7-ring chelates led to higher ees than 1,2-diphosphines; otherwise, no clear correlation between ligand properties and catalytic performance was found. In some experiments black precipitates were observed at the end of the reaction, indicating the decomposition of the homogeneous catalysts for certain ligand/metal/substrate combinations. Received April 5, 2000. Accepted (revised) May 2, 2000     

Thermotolerant yeast for simultaneous saccharification and fermentation of cellulose to ethanol

Ten promising microbial strains were screened for glucose fermentation over the temperature range of 37–47°C, and five temperature-tolerant yeasts (Saccharomyces cerevisiae SERI strain (D₅A),S. uvarum, andCandida generaacidothermophilium, brassicae, andlusitaniae), were chosen for SSF evaluation on Sigmacell-50 cellulose with Genencor 150 L cellulase enzyme.Brettanomyces clausenii (Y-1414) was included for comparison to previous studies both by itself and in mixed culture withS. cerevisiae (D₅A). Good conversion rates were achieved at temperatures as high as 43°C withC. brassicae andS. uvarum; mixed cultures of either of these yeasts with the thermotolerant cellobiose fermenting yeastC. lusitaniae achieved higher rates and yields than any of the three yeasts alone. However, the mixed culture ofB. clausenii andS. cerevisiae at 37°C achieved as high conversion rates and higher yields than any of the other yeasts tested.