The catalyzed hydrogen sorption mechanism in alkali alanates

Abstract

The hydrogen sorption pathways of alkali alanates were analyzed and a mechanism for the catalytic hydrogen sorption was developed. Gibbs free energy values of selected intermediate steps were calculated based on experimentally determined thermodynamic data (enthalpies and entropies) of individual hydrides: MAlH₄, M₃AlH₆, and MH. The values of the activation energies, based on the intermediates M⁺, H⁻, MH, and AlH₃, were obtained. The mechanism of the catalytic activity of Ti is finally clarified: we present an atomistic model, where MAlH₄ desorbs hydrogen through the intermediates M⁺, H⁻, MH, and AlH₃ to the hexahydride M₃AlH₆ and finally the elemental hydride MH. The catalyst acts as a bridge to transfer M⁺ and H⁻ from MAlH₄⁻ to the neighboring AlH₄⁻, forming AlH₆³⁻ and finally isolated MH, leaving AlH₃ behind, which spontaneously desorbs hydrogen to give Al and 1.5H₂. The proposed mechanism is symmetric in the direction of hydrogen desorption as well as readsorption processes.