Desorption of hydrogen from

Indirect methods used can profit by the thermodynamic data of a particular metal-hydrogen system. The determination of the H/Me ratio after complete desorption of hydrogen from a sample, despite an apparent simplicity of the method, gives adequate results only when the bulk metal sample was entirely saturated with hydrogen, and that is a very rare case. The metal catalyst crystallites can be saturated in a nonuniform way, not through their whole thickness. The surface of this polycrystalline sample varies to such extent in its behavior toward interaction with hydrogen that hydride forms only in patches on its surface. A sample surface becomes a mosaique of /3-hydride and a-phase areas (85). 

The dependence of the PL intensity and peak position on oxidation temperature for three different PS samples is shown in Fig. 7.20. Oxidation at 600°C destroys the PL, while the initial PL intensity is restored or even increased after oxidation at 900°C. This effect can be understood as a quenching of PL because of a high density of defects generated during the desorption of hydrogen from the internal surface of PS. Electron spin resonance (ESR) investigations show a defect with an isotropic resonance (g= 2.0055) in densities close to 101 cm for oxidation at 600°C [Pel, Me9]. This corresponds to one defect per crystallite, if the crystallite diameter is assumed to be about 5 nm in diameter. 

Temperature-Programmed Desorption of Hydrogen from Platinum Particles on Gamma-AI203, Alexeev, O, et. al, J. Catalysis, vol. 185, no. 1, pp. 170-181, 1999. 

Figure5.47 (a) X-ray diffraction pattern (Cu Ka) of a commercial UBH4 sample (b) X-ray diffraction patter after the thermal desorption of hydrogen from the sample (a) (c) X-ray diffraction pattern of the sample upon reabsorption of the hydrogen.

The thermal desorption of hydrogen from lithium nitride (LiNHa) was investigated by Chen et al. [92,93]. The thermal desorption of pure lithium amide mainly evolves NH3 at elevated temperatures following the reaction (Figure 5.54)... 

Figure 5.61 Thermal desorption of hydrogen from borazane.

Boland. J. J. (1991b). Evidence of pairing and its role in the recombinative desorption of hydrogen from the Si(100)-2 X 1 structure. Phys. Rev. Lett. 67, 1539-1542. 

Lewis, L. B., Segall, J. and Janda, . C. Recombinative desorption of hydrogen from the Ge(100)-(2 x 1) surface a laser-induced desorption study. Journal of Chemical Physics 102, 7222-8 (1995). 

Plots of log P vs 1/T for several R2C07-H systems are shown in Figures 3 and 4. The AH values, giving the heats of desorption of hydrogen from the compound, were obtained from the slopes of the curves in Figures 3 and 4 and are listed in Table II. 

Table II. Heats of Desorption of Hydrogen from R2C07-H Systems...

Table IV. Kinetic Data for the Desorption of Hydrogen from Selected Rare Earth Intermetallics...

Effect of Particle Size on the Rate of Desorption of Hydrogen from St. Nicholas Anthracite at Constant Temperature. If the rate of evolution of H2 is controlled by the rate of desorption of H2 from the surface of anthracite and not by the rate of transport of the H2 from the pore system, the rate of evolution should be independent of particle size. However, this was not found to be the case. For St. Nicholas anthracite, the initial rate of H2 release was determined on a 12 X 16 mesh fraction over the temperature range 700°-750°C. and compared with the previous results for initial rate of release on a 150 X 200 mesh fraction. Results are summarized in Table II. The results show clearly that the rate of H2 evolution at all temperatures studied was more rapid from the larger particle sized sample. The activation energy for H2 evolution at 9 — 1 from the 12 X 16 mesh fraction is calculated as 89 kcal./ mole, compared with 96 kcal./mole for the 150 X 200 mesh fraction over the same temperature interval. 

Thermal desorption of hydrogen from platinum and Pt-Au films (146) results in a similar conclusion for these alloys. On average, hydrogen is more loosely bound to the alloys than to pure platinum. About 50% of the adsorbate is desorbed by pumping at 78 K from the alloys, while only a very small percentage is desorbed from platinum at this temperature. 

The absorption/desorption of hydrogen from the material should be reversible. 

Fig. 2.12 Temperature programmed desorption (TPD) curves for the desorption of hydrogen from platinum catalysts. I) Full TPD curve II) curve after partial desorption, cooling and then reheating. (Drawn using data from Ref. 44).

Drawing on methods and observations from earlier hydride storage systems, many of the initial approaches to improving the desorption of hydrogen from the amide in the second stage of the Li-N-H cycle focused on two complementary approaches ball milling to reduce particle size and addition of potential catalysts to reduce desorption temperature. These approaches were first adopted in the Li-N-H system by Ichikawa et al. who used each to try to optimise desorption in the amide-imide reaction (Eqs 16.7 and 16.8)... 

Hydrogen provided by the hydride, which is therefore both a catalyst and a hydrogen source. The desorption of hydrogen from the hydride must be added in the equation of the reaction rate. 

The TPD spectra obtained for the desorption of H2 and of H20 from the catalyst surfaces (Pt-B is shown, the others do not differ significantly) are revealing. Figure 2(a) is the spectra of deuterium desorption obtained from the catalyst after reduction of the sample at 300°C and 400°C. Three desorption features are evident a low temperature peak at approx 100 C corresponding to the desorption of desorption of hydrogen from metallic Pt and two... 

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