Desorption at Atmospheric Pressure of Hydrogen

Very recently, Bystrzycki et al. [115] studied the possibility of destabilization of MgHj by chemical reaction with Si and formation of an intermediate intermetallic 

The well-known catalytic additive to MgH is Ni. So far, it has been added as a coarse powder with the size range of a few tens of micrometers. A short review of these early experiments is presented in [117], More recently, Hanada et al. [118] investigated the addition of nano-Ni to ball-milled MgH. Unfortunately, he used continuous desorption under He gas and the material desorbed 6 wt.%H2 at 160°C in 15,000 s. 

The Inco nano-Ni (n-Ni) has a filamentary shape but resembles a delicate coral colony and its dimensions are truly nano, i.e. below 100 run (Fig. 2.49d). The measured mean diameter of a coral filamenf is 42 16 run. 

The effect of a ball-milling time on the DSC desorption behavior of undoped and Ni-doped MgH is shown in Fig. 2.52a. Endothermic desorption peak for the MgH + 5 wt.% m-Ni powder milled for 15 min is only modestly shifted to lower temperatures, showing the onset temperature at -350°C and the peak maximum at 392.8°C as compared to a pure MgH with the onset at 380°C and the maximum at 418.2°C, respectively, also milled for 15 min. In contrast, the hydrogen desorption peak for the MgH h- 5 wt.% n-Ni powder is substantially shifted to lower temperatures and shows the onset temperature at 170°C and the peak maximum at 243.1°C. However, when the MgH + 5 wt.% m-Ni powder is milled for 20 h, its desorption properties are much improved such that the onset is at 275°C and the peak maximum at 302.3°C (Fig. 2.52b). Apparently, longer milling time reduces desorption temperature, most likely due to a better dispersion of Ni particles within the MgHj matrix. Nevertheless, the desorption temperature of the 20 h milled MgH + 5 wt.% m-Ni powder is still worse than that of the 15 min milled MgH + 5 wt.% n-Ni. This clearly shows that n-Ni exhibits superb catalytic properties. 

275-375°C and 300-375°C range, respectively. Once again, the n-Ni clearly shows its superb catalytic properties. 

A cycling behavior of the MgH2 mixture with n-Ni was also studied. The cycling process consisted of live desorption/absorption cycles at 300°C. Desorption was carried out at the atmospheric pressure of hydrogen, and absorption was realized under 4.0 MPa pressure of hydrogen for 15 min. XRD after cycling showed formation of a 

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Figure 2.6a shows typical kinetic cnrves of first desorption carried out in a Sieverts-type apparatns at the initial hydrogen pressure of 0.1 MPa (atmospheric pressure of 1 bar) for the as-received, nonmilled, and nonactivated Tego Magnan powder. For each temperatnre, a fresh load of sample was desorbed. At each tem-peratnre in Fig. 2.6a, the desorption process is complete with 100% of MgH des-... 

DSC tests show a substantial reduction of the hydrogen desorption onset (red circles) (T J and peak (T ) temperatures due to the catalytic effects of n-Ni as compared to the hydrogen desorption from pure MgH also milled for 15 min. (Fig. 2.57). It is interesting to note that there is no measurable difference between spherical (Fig. 2.57a) and fdamentary (Fig. 2.57b) n-Ni, although there seems to be some effect of SSA. We also conducted desorption tests in a Sieverts apparatus for each SSA and obtained kinetic curves (Fig. 2.58), from which the rate constant, k, in the JMAK equation was calculated. The enhancement of desorption rate by n-Ni is clearly seen. At the temperature of 275°C, which is close to the equilibrium at atmospheric pressure (0.1 MPa), all samples desorb from 4 to 5.5 wt.% within 2,000 s. 

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. 

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