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

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.