Inorganic Chemistry

A Transition State Theory Perspective on the Relation of Reversible Metal Hydride First-Order Kinetics to Equilibrium Thermodynamics

Roland Hermann Pawelke FOTEC Forschungs- und Technologietransfer GmbH


In the event of hydrogen desorption from reversible metal hydrides, equilibrium thermodynamics exert a rate-limiting effect: if system pressure reaches equilibrium pressure, the reaction rate becomes zero. This is usually dealt with by an empiric term of either polynomial or logarithmic nature to first-order kinetics. This paper approaches the matter from a transition state theory perspective, combining the classic Eyring-Polanyi equation with insights on reversible metal hydride chemical overpotential for scrutinizing the relation of Arrhenius first-order kinetics to van’t Hoff equilibrium pressure. The outcome, tested for the example of 4 mol % Ti-doped NaAlH4, suggests theoretical coherency and provides a method for identifying the factor by which an experiment deviates from ideal first-order kinetics. Adopting Arrhenius-Eyring-Polanyi first-order kinetics as baseline for modelling kinetic behaviour of metal hydride sorption reactions not only covers a blind spot in the Arrhenius approach but creates a standard for result comparability.

Version notes

V2: I seem to have misspelled M. Polanyi’s name as Polyani (a veritable Hungarian name in its own right) on some occasions, this has been corrected.


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