Hydrogen Motion in Magnesium Hydride by NMR

In coarse-grained MgH2, the diffusive motion of hydrogen remains too slow (<10^5 hops s^−1) to narrow the H NMR line up to 400 °C. Slow-motion dipolar relaxation time T1D measurements reveal the motion, with hopping rate ωH from 0.1 to 430 s^−1 over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH2. The ωH data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 × 10^15 s^−1. In ball-milled MgH2 with 0.5 mol % added Nb2O5 catalyst, line-narrowing is evident already at 50 °C. The line shape shows distinct broad and narrow components corresponding to immobile and mobile H, respectively. The fraction of mobile H grows continuously with temperature, reaching ∼30% at 400 °C. This demonstrates that this material’s superior reaction kinetics are due to an increased rate of H motion, in addition to the shorter diffusion paths from ball-milling. In ball-milled MgH2 without additives, the line-narrowed component is weaker and is due, at least in part, to trapped H2 gas. The spin−lattice relaxation rates T1^−1 of all materials are compared, with ball-milling markedly increasing T1^−1. The weak temperature dependence of T1^−1 suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling

Low-Temperature Superplasticity In Ultrafine-Grained Ti5Si3-Tial Composites

A ceramic-intermetallic composite of Ti5Si3 with 40 vol.% TiAl was synthesized by mechanical alloying and hot isostatic pressing. The microstructure consisted of equiaxed grains of γ-TiAl and ζ-Ti5Si3 with a size of about 400 nm. This composite exhibited a tensile elongation of 150% at 950 °C, using a strain rate of 4 × 10-5 s-1. This temperature for superplastic deformation is just about 0.5 Tm, the lowest normalized superplastic temperature reported for any ceramic-based material. © 2008 Acta Materialia Inc

Mechanical characterization of mechanically alloyed ultrafine-grained Ti5Si3+40 vol% gamma-TiAl composites

Ultrafine-grained ceramic-based composites of Ti-31.6Al-21.6Si (at%) consisting of 60 vol% of zeta-Ti5Si3 and 40 vol% of gamma-TiAl were produced by high-energy ball milling followed by hot isostatic pressing (HIP). Because of the cleanliness of the powder and full densification of the HIPed product, the mechanical behavior of the composite could be unambiguously related to the microstructure and chemistry. The starting microstructure after HIPing consisted of intermixed zeta-Ti5Si3 and gamma-TiAl phases of approximately equal grain size, the size ranging from about 300 nm to I pm depending on the HIP temperature. Hightemperature mechanical testing of this ultrafine-grained composite exhibited a strain-rate sensitivity of \u3e 0.3. Further, the equiaxed microstructure was retained after mechanical testing, suggesting the possibility of achieving superplastic deformation. Consequently, tensile testing demonstrated elongations of about 150% at 950 degrees C and a strain rate of 4 x 10(-5) s(-1). Considering that the present alloy has the ceramic (suicide) phase as the matrix, this temperature at which superplastic deformation is observed is significantly lower than that reported for conventional coarse-grained ceramic materials

Enzymatic Oxidation of Butane to 2-Butanol in a Bubble Column

Unspecific peroxygenases have recently gained significant interest due to their ability to catalyse the hydroxylation of non-activated C−H bonds using only hydrogen peroxide as a co-substrate. However, the development of preparative processes has so far mostly concentrated on benzylic hydroxylations using liquid substrates. Herein, we demonstrate the application of a peroxygenase for the hydroxylation of the inert, gaseous substrate butane to 2-butanol in a bubble column reactor. The influence of hydrogen peroxide feed rate and enzyme loading on product formation, overoxidation to butanone and catalytic efficiency is investigated at 200 mL scale. The process is scaled up to 2 L and coupled with continuous extraction. This setup allowed the production of 115 mmol 2-butanol and 70 mmol butanone with an overall total turnover number (TTN) of over 15.000, thereby demonstrating the applicability of peroxygenases for preparative hydroxylation of such inert, gaseous substrates at mild reaction conditions.European Research Commision (ERC) grant, No 64802

Pressure and Temperature Influence on the Desorption Pathway of the LiBH4−MgH2 Composite System

The decomposition pathway in LiBH4−MgH2 reactive hydride composites was investigated systematically as a function of pressure and temperature. Individual decomposition of MgH2 and LiBH4 is observed at higher temperatures and low pressures (T ≥ 450 °C and p(H2) ≤ 3 bar), whereas simultaneous desorption of H2 from LiBH4 and formation of MgB2 was observed at 400 °C and a hydrogen backpressure of p(H2) = 5 bar. The simultaneous desorption of H2 from LiBH4 and MgH2 without intermediate formation of metallic Mg could not be observed. In situ X-ray diffraction (XRD) and infrared (IR) spectroscopy reveal the present crystalline and amorphous phases

Pressure Effect on the 2NaH+MgB2 Hydrogen Absorption Reaction

The hydrogen absorption mechanism of the 2NaH + MgB2 system has been investigated in detail. Depending on the applied hydrogen pressure, different intermediate phases are observed. In the case of absorption measurements performed under 50 bar of hydrogen pressure, NaBH4 is found not to be formed directly. Instead, first an unknown phase is formed, followed upon further heating by the formation of NaMgH3 and a NaH-NaBH4 molten salt mixture; only at the end after heating to 380 °C do the reflections of the crystalline NaBH4 appear. In contrast, measurements performed at lower hydrogen pressure (5 bar of H2), but under the same temperature conditions, demonstrate that the synthesis of NaBH4 is possible without occurrence of the unknown phase and of NaMgH3. This indicates that the reaction path can be tuned by the applied hydrogen pressure. The formation of a NaH-NaBH4 molten salt mixture is observed also for the measurement performed under 5 bar of hydrogen pressure with the formation of free Mg. However, under this pressure condition the formation of crystalline NaBH4 is observed only during cooling at 367 °C. For none of the applied experimental conditions has it been possible to achieve the theoretical gravimetric hydrogen capacity of 7.8 wt %.JRC.F.2-Cleaner energ