Spatial configurations of Ti- and Ni-species catalyzing complex metal hydrides: X-ray absorption studies and first-principles DFT and MD calculations.
Abstract
We performed Ti K-edge EXAFS and XANES measurements on 4 and 3 wt.% TiCl3-activated NaAlH4 and (LiBH4+0.5MgH2) and Ni K-edge measurements on 3 and 11 wt.% NiCl2-activated (LiBH4+0.5MgH2) and (Li3BN2H8), prospective H storage materials. The valence of Ti and Ni is close to 0 and invariant during H cycling. None of the metals enter substitutionally or interstitially into the cryst. 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 Mg2NiHy upon H loading. Local structure around Ti(Ni) atoms is expressed in terms of a cluster expansion and the interat. distances, coordination nos. and Debye-Waller factors are detd. 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 calcns. Structural properties and phase stability of hypothetical hydrogenated TiAl3 as well as several products of the decompn. reaction are detd. from d. functional theory calcn. First-principles mol. dynamics simulations of surface diffusion and chem. 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. (c) 2007 American Institute of Physics. [on SciFinder(R)]