Spatial configurations of Ti- and Ni- species catalyzing complex metal hydrides: X-ray absorption studies and first-principles DFT and MD calculations

We have performed Ti K-edge EXAFS and XANES measurements on 4 and 3 wt% TiCl₃-activated NaAlH₄ and (LiBH₄+0.5MgH ₂) and Ni K-edge measurements on 3 and 11 wt% NiCl ₂-activated (LiBH₄+0.5MgH₂) and (Li ₃BN₂H₈) - prospective hydrogen storage materials. The valence of Ti and Ni is close to zero and invariant during hydrogen cycling. None of the metals enter substitutionally or interstitially into the crystalline lattice of the initial or final products. For the Ti- activated NaAlH₄ and (LiBH₄+0.5MgH₂), amorphous TiAl₃ and TiB₂ alloys are formed, which are almost invariant during cycling. The Ni doped (LiBH₄+0.5MgH₂) initially forms amorphous Ni₃B, which is partly converted to amorphous Mg₂NiH_y upon hydrogen loading. Local structure around Ti(Ni) atoms is expressed in terms of a cluster expansion and the interatomic distances, coordination numbers and Debye-Waller factors are determined for competitive structural models. For Ti-activated NaAlH₄ the models are elaborated by Ti K-edge XANES, which are interpreted in terms of single-electron multiple scattering calculations. Structural properties and phase stability of hypothetical hydrogenated TiAl₃ as well as several products of the decomposition reaction are determined from density functional theory calculation. First-principles molecular dynamics simulations of surface diffusion and chemical reactivity imply that the formation of a few monolayers of TiAl₃ on the surface may be responsible for the significant increase in the reaction rate.