Hydrogen desorption kinetics

Gupta, P., Colvin, V.L. and George, S.M. (1988) Hydrogen desorption kinetics from monohydride and dihydride species on silicon surfaces , Phys. Rev. B 37, 8234. 

Figure 4.12 Hydrogen desorption kinetics at 573 K of as-cast and ECAP-processed MgggNin eutectic alloy. From ref [255],...

A Perkin-Elmer thermogravimetric analyzer (TGA) was used to determine the hydrogen desorption kinetics at atmospheric pressure. This instruntent was located in another glove box under nitrogen atmosphere to prevent any exposure of the samples to air and moisture. Samples were heated to 2S0°C at a ramping rate of S°C/min under 1 atm of He, using an initial 1 minute delay to ensure an environment of pure He. Approximately 10 mg of sample were used in the TGA. 

Liang, G., Huot, J., Boily, S. and Schulz, R. (2000) Hydrogen desorption kinetics of a mechanically milled MgH2+5at.%V nanocomposite, Journal of Alloys and Compounds, 305, 239-245. 

Reider G A, Hdfer U and Heinz T F 1991 Desorption-kinetics of hydrogen from the Si(111)7 7 surface J. Chem. Phys. 94 4080-3... 

In Ae hydrogen TPD spectrum, Ae lower-temperature peak occurs at 355 K, characteristic of desorption of surface hyAogen Ni(lOO). Hence it is defined by Ae desorption kinetics... 

The results of a similar experiment with adsorbed hydrogen is shown in Fig. 2.3b. Only one desorption peak was observed in the temperature range studied [50], The desorption peak temperature lies at 420 K for the experiment with 0.8 L and is shifted to lower temperatures as the H2 concentration increases indicating second order desorption kinetics. Surface states with desorption temperatures at 165 K, 220 K, 280 K and 350 K were reported for the adsorption of H2 and D2 at 120 K [51]. Thermal desorption experiments after H2 adsorption at 350 K show only one desorption state at ca. 450 K [52],... 

In addition to the desirable thermodynamics (enthalpy of adsorption), a fast adsorption/ desorption kinetics is required for a fast recharging and discharging of hydrogen. Hence, the activation energies of the adsorption and desorption steps play an important role... 

Fast adsorption/desorption kinetics and relatively small (<10 kj/mol) adsorption enthalpies are observed for hydrogen adsorption on many porous materials, which indicates that physisorption on porous materials is suitable for fast recharging with hydrogen [81,82], The narrowest pores make the biggest contribution to hydrogen-adsorption capacity, whereas mesopores contribute to total pore volume, but little to hydrogen capacity, and are detrimental for the overall volumetric capacity. Hence, porous materials with very narrow pores or pore-size distributions are required for enhanced hydrogen capacity at low pressures. 

Table 1 shows the summary of absorption and desorption kinetics and hydrogen reversible capacity of Mg hydrides with ceramic catalysts at 300°C. 

The hydrogen absorption/desorption kinetics are usually analyzed by applying the JMAK (Johnson-Mehl-Avrami-Kolmogorov) theory of phase transformations, which is based on nucleation and growth events [166-168] where a is the fraction transformed at time t or alternatively for hydrides the fraction absorbed... 

Fig. 2.6 (a) Desorption kinetic curves at various temperatures under initial hydrogen pressure of 0.1 MPa of the as-received, nonactivated, commercial MgH powder Tego Magnan and (b) the Arrhenius plot of the desorption rate for the estimate of the apparent activation energy, fi, using kinetics data for four temperatures 350, 375, 400, and 420°C (fi -120 kJ/mol). Coefficient of fit = 0.996... 

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