Overpressure Role in Isothermal Kinetics of H2Desorption–Absorption: the 2LiBH4–Mg2FeH6System

The rates of the irreversibile LiBH4 +Mg2FeH 6 → 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 Δp/p values, deriving the rate constants k(T, Δp/p) by Avrami's fitting of reaction advancement vs time. The results could be rationalized on the basis of the k = A exp(-Ea/RT) = A0 exp[-Ea0/RT + a(Δp/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 Δp/p. For the first of the above reactions, Ea = -151(Δp/p) + 118 kJ mol-1 and ln(A/min-1) = -34(Δp/p) + 16; similar values are obtained for the second one. Relations of this kinetic model with the thermodynamic driving force ΔG and with equations of electrochemical kinetics, where overpressure is replaced by overvoltage, are discussed. © 2013 American Chemical Society.