Rudolf Suhrmann

Nachruf der Braunschweigischen Wissenschaftlichen Gesellschaft gehalten in der Plenarsitzung am 19. Januar 197

Vacancy ordering and electronic structure of gamma-Fe2O3 (maghemite): a theoretical investigation

The crystal structure of the iron oxide gamma-Fe2O3 is usually reported in either the cubic system (space group P4332) with partial Fe vacancy disorder or in the tetragonal system (space group P41212) with full site ordering and c/a\approx 3. Using a supercell of the cubic structure, we obtain the spectrum of energies of all the ordered configurations which contribute to the partially disordered P4332 cubic structure. Our results show that the configuration with space group P41212 is indeed much more stable than the others, and that this stability arises from a favourable electrostatic contribution, as this configuration exhibits the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite is therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. The confirmation of the ordered tetragonal structure allows us to investigate the electronic structure of the material using density functional theory (DFT) calculations. The inclusion of a Hubbard (DFT+U) correction allows the calculation of a band gap in good agreement with experiment. The value of the gap is dependent on the electron spin, which is the basis for the spin-filtering properties of maghemite.Comment: 19 pages, 2 tables, 5 figures. To appear in the Journal of Physics - Condensed Matter (2010)

Magnesia-Based Cements: A Journey of 150 Years, and Cements for the Future?

This review examines the detailed chemical insights that have been generated through 150 years of work worldwide on magnesium-based inorganic cements, with a focus on both scientific and patent literature. Magnesium carbonate, phosphate, silicate-hydrate, and oxysalt (both chloride and sulfate) cements are all assessed. Many such cements are ideally suited to specialist applications in precast construction, road repair, and other fields including nuclear waste immobilization. The majority of MgO-based cements are more costly to produce than Portland cement because of the relatively high cost of reactive sources of MgO and do not have a sufficiently high internal pH to passivate mild steel reinforcing bars. This precludes MgO-based cements from providing a large-scale replacement for Portland cement in the production of steel-reinforced concretes for civil engineering applications, despite the potential for CO2 emissions reductions offered by some such systems. Nonetheless, in uses that do not require steel reinforcement, and in locations where the MgO can be sourced at a competitive price, a detailed understanding of these systems enables their specification, design, and selection as advanced engineering materials with a strongly defined chemical basis

Preventing Staphylococcus aureus Sepsis through the Inhibition of Its Agglutination in Blood

Staphylococcus aureus infection is a frequent cause of sepsis in humans, a disease associated with high mortality and without specific intervention. When suspended in human or animal plasma, staphylococci are known to agglutinate, however the bacterial factors responsible for agglutination and their possible contribution to disease pathogenesis have not yet been revealed. Using a mouse model for S. aureus sepsis, we report here that staphylococcal agglutination in blood was associated with a lethal outcome of this disease. Three secreted products of staphylococci - coagulase (Coa), von Willebrand factor binding protein (vWbp) and clumping factor (ClfA) – were required for agglutination. Coa and vWbp activate prothrombin to cleave fibrinogen, whereas ClfA allowed staphylococci to associate with the resulting fibrin cables. All three virulence genes promoted the formation of thromboembolic lesions in heart tissues. S. aureus agglutination could be disrupted and the lethal outcome of sepsis could be prevented by combining dabigatran-etexilate treatment, which blocked Coa and vWbp activity, with antibodies specific for ClfA. Together these results suggest that the combined administration of direct thrombin inhibitors and ClfA-antibodies that block S. aureus agglutination with fibrin may be useful for the prevention of staphylococcal sepsis in humans

Kinetics of isotope exchange between hydrogen and liquid ammonia on heterogeneous catalysts

The isotope exchange between hydrogen and liquid ammonia has been studied in the temperature range between -60° and +25°C at pressures up to 150 atm in thepresence of various catalysts, in particular supported platinum catalysts. Comparative exchange experiments were carried out with gaseous ammonia at pressures from 25 Torr up to the saturation pressure ( 8.7 atm at 20° C). (1) Group eight metals catalyze the H/D exchange with liquid ammonia even at temperatures below -60°C. The activity decreases in the order: Pt, Pd, Ni, and Fe. (2) For a platinum-carbon catalyst suspended in liquid ammonia the rate of exchange increases proportionally to the amount of catalysts in the range from 10 to 100 g/liter. (3) Platinum catalysts supported on carbon and on silica gave the same exchange constants per unit weight and resulted in the same activation energy irrespective of the widely different surface areas. (4) The following apparent activation energies were obtained: Pt/C, 10.0 ± 0.5; Pt/SiO$_{2}$, 9.6 ± 0.6; Pd/C, 11.3 ± 0.7; Raney-Ni, 11.5 ± 0.8 kcal/mole. (5) Comparative measurements with potassium amide indicated that this homogeneous catalyst has a considerably higher activity and a lower activation energy (5.4 ± 0.6 kcal/mole) than the heterogeneous catalysts. Contrary to exchange on these catalysts transport processes are rate determining in the potassium amide system under the conditions investigated. (6) While the rate of exchange in liquid ammonia increases proportionally to the hydrogen pressure in the case of potassium amide a dependence on the square root of the hydrogen pressure has been found for the platinum/carbon catalyst. (7) The experimental results support the following mechanism: Isotope exchange occurs between chemisorbed hydrogen atoms and a chemisorbed ammonia molecule, i.e. according to a Langmuir-Bonhoeffer-Hinshelwood mechanism. In principle themechanism is the same in the gas as weil as in the liquid phase. The number of atoms exchanged per unit time is, however, considerably smaller in the liquid, sincea large portion of the surface is blocked up for hydrogen chemisorption by adsorbed ammonia molecules. If any, there is only a very small contribution from an ionicmechanism to the total exchange in liquid ammonia

Selbstdiffusion in flüssigem Ammoniak

Mit Hilfe der Kapillarmethode und Markierung durch Deuterium und Stickstoff-15 wurde die Selbstdiffusion in flüssigem Ammoniak zwischen +30 und -60 °C gemessen:D(Deut) = (3,55±0,11) · 10$^{-5}$ exp- $\Big ($ $\frac{1870 \pm 75}{RT}$ $\Big)$. Die Ergebnisse werden in Analogie zum Wasser erörtert und die Isotopieeffekte der isobaren Moleküle $^{15}NH_{3}$ und $^{14}NH_{2}D$ diskutiert

Isotopenaustausch zwischen Wasserstoff und flüssigem Ammoniak in Gegenwart heterogener Katalysatoren

Nach der Entdeckung des Deuteriums im Jahre 1931 begannen in zahlreichen Laboratorien Untersuchungen über Isotopenaustauschreaktionen zwischen den verschiedensten wasserstoffhaltigen Verbindungen. Man stellte fest, daß einige Substanzen auch bei tiefen Temperaturen ihre Wasserstoffatome untereinander sehr schnell austauschen (z.B.flüssiges Wasser und Ammoniak),andere dagegen nur unmeßbar langsam. Hierzu gehört z.B.der Isotopenaustausch zwischen Wasser und Wasserstoff oder zwischen Ammoniak und Wasserstoff. Da diese Austauschreaktionen durch Katalysatoren beschleunigt werden können, interessierte z.B. das NH$_{3}$/H$_{2}$-System wegen seiner nahen Beziehung zu der technisch sehr wichtigen katalytischen Ammoniaksynthese. So kommen bereits 1935 Taylor und Junger s (Lit.1) auf Grund von Untersuchungen dieses H/D-Austausches an einem Eisen-Katalysator zu der heute auf anderen Wegen bestätigten Annahme (Lit.2), daß die Aktivierung des Stickstoffmoleküls der für die Ammoniaksynthese geschwindigkeitsbestimmende Vorgang ist. [...