Overpressure Role in Isothermal Kinetics of H₂ Desorption–Absorption: the 2LiBH₄–Mg₂FeH₆ System

Abstract

The rates of the irreversibile LiBH4 +Mg2FeH6  LiH +2MgH2 + FeB + 5/2H2 and reversible (with significant sorption/desorption hysteresis) LiBH4 + 1/2Mg  LiH + 1/2MgB2 + 3/2H2 reactions were measured by isothermal-isobaric experiments in a Sievert-type apparatus. Measurements were done at several temperature T and overpressure p/p values, deriving the rate constants k(T,p/p) by Avrami's fitting of reaction advancement vs. time. The results could be rationalized on the basis of the k = Aexp(-Ea/RT) = A0exp[-/RT+a(p/p)] kinetic formula, which couples the standard Arrhenius approach for thermal effects with an exponential dependence of the rate constant on overpressure. The empirical a coefficient varies with temperature in a way that requires the activation energy and entropy to depend linearly on p/p. For the first of the above reactions, Ea =-151(p/p) + 118 kJ mol-1 and ln(A/min-1) =-34(p/p) + 16; similar values are obtained for the second one. Relations of this kinetic model with the thermodynamic driving force G and with equations of electrochemical kinetics, where overpressure is replaced by overvoltage, are discussed.

New Insights into the Thermodynamic Behavior of 2LiBH4–MgH2 Composite for Hydrogen Storage

“…The fit of the points corresponding to the higher pressure plateau gives a reaction enthalpy of 76±6 kJ/molH 2 K. This value is within the range reported for the Mg hydrogenation reaction. 17 In the previously mentioned article by Catti et al 10 Figure 7: PXRD of the partially desorbed sample at two different temperatures: 425°C (Sample 1) and 375°C (Sample 2).…”

“…Catti et al 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 reactions on both ends of this threshold. In the present work we have found a limit temperature that is much closer to the one predicted by Walker 8 and a significant change in the slope after the intersection.…”

Bimetallic Pt-Co Catalysts for the Liquid-Phase WGS

“…However, the obtained values were far from the equilibrium (given in Figure 9), indicating a kinetically controlled reaction regime under these operation conditions. At low reaction temperatures, the higher overpressure may play a negative role in the catalytic activity, as the product's desorption from the catalyst surface was hindered [41]. In addition, the water solubility of the reactants (CO and H 2 ) increased according to Henry´s law (H constants from Section 3.2).…”

In Situ Synchrotron X-ray Diffraction Studies of Hydrogen-Desorption Properties of 2LiBH4–Mg2FeH6 Composite

“…Against this background, we studied 2LiBH 4 -Mg 2 FeH 6 dehydrogenation from a thermodynamic perspective [11,12]. Interestingly, this mixes the elevated gravimetric hydrogen density of LiBH 4 (13.9%) [13] with its impressive volumetric counterpart of Mg 2 FeH 6 (150 g L −1 ) [5].…”