Desorption, hydrogen

Behrisoh R, Prozesky V M, Huber H and Assmann W 1996 Hydrogen desorption induoed by heavy-ions during surfaoe anaiysis with ERDA Nucl. Instrum. Methods B 118 262... 

Fig. 3. Isotherm pm = /(H/Ni) at 25°C obtained during hydrogen desorption from nickel foil saturated with hydrogen. After Baranowski and BocheAska (11a).

As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal (87). 

The dedicated STEM provides a means of obtaining mlcroanalytlcal Information of supported metals not readily obtained by other Instrumentation. The capability to observe and analyze some very highly dispersed metal particles on y-alumlna has been demonstrated and for the noble metals, verified by temperature prograouned hydrogen desorption. 

Figure 11. Observed ESR signals after reduction, hydrogen desorption and air exposure.

Hydrogenation is sensitive to the surface modifications. For example, the ball-milled powders require less activation compared to the conventional powders. For nanocrystalline hydrides, the grain boundary does not dramatically affect the PCI, which describes the thermodynamic aspects of hydride formation. However, the pressure for hydrogen desorption of the unmilled MgH2 is lower than that of the milled one as seen in Figure 11.7 [66]. 

The curve for Pt(110) surface shows a wide double layer region, indicating that the anion adsorption is completed concurrently with hydrogen desorption. The anions are strongly held in the "troughs" of the Pt(110) surface, i.e. in the 2(111)-(111) terrace-step sites. This point will be touched upon connection with the stepped surfaces. The C-E curves for Au(110) in clearly show a completion of anion adsorption on that... 

The thermal expansion coefficient of bulk silicon is positive at RT (2.6 x 1CT6 K-1), but becomes negative below 120 K. The thermal expansion coefficient of micro PS for heating from 290 to 870 K is found to be negative (-4.3x 10 6 KT1), which can be ascribed to hydrogen desorption and oxidation of the inner surface [Di7]. For meso PS the thermal expansion coefficient was found to increase with porosity in the temperature regime between 90 K and 300 K, from 0.4xl0-6 K 1 to... 

Between 7 and 470 K a is found to increase with temperature. This increase is reversible and corresponds well with that of bulk silicon [Ko4]. If PS is annealed in nitrogen at higher temperatures (600 °C), hydrogen desorption takes place, which changes the condition of the inner surface drastically. At these temperatures an irreversible increase in a is observed for micro PS. A similar increase in a after annealing is found for meso PS. Changes that affect the core of the crystallites, e.g. stress effects [Ko4], as well as surface-related effects like the formation of surface states [Ko5, BalO], are proposed to be responsible for the observed increase of a with T. 

Performing a Redhead analysis of the TPD. an estimation of the activation energy (Ea) of hydrogen desorption from npSi can be derived from the following equation [7], where v is the evolution rate, and p is the linear temperature ramp rate. 

It is now possible to construct a thermodynamic analysis of hydrogen desorption from npSi. 

A 90% reduction in activation energy, not an unreasonable expectation for catalysts in general, reduces the peak temperature below 0 C. Clearly, only a small amount of catalytic action is required to make dramatic reductions in the release temperature. This implies that, with careful control of the invented process, it should be possible to dial-in the desorption temperature for hydrogen desorption. This allows us to assess how this hydrogen storage media can be applied. 

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