Understanding the role of Ti in reversible hydrogen storage as sodium alanate: a combined experimental and density functional theoretical approach.

Full Abstract

We report the results of an experimental and theoretical study of hydrogen storage in sodium alanate (NaAlH(4)). Reversible hydrogen storage in this material is dependent on the presence of 2-4% Ti dopant. Our combined study shows that the role of Ti may be linked entirely to Ti-containing active catalytic sites in the metallic Al phase present in the dehydrogenated NaAlH(4). The EXAFS data presented here show that dehydrogenated samples contain a highly disordered distribution of Ti-Al distances with no long-range order beyond the second coordination sphere. We have used density functional theory techniques to calculate the chemical potential of possible Ti arrangements on an Al(001) surface for Ti coverages ranging from 0.125 to 0.5 monolayer (ML) and have identified those that can chemisorb molecular hydrogen via spontaneous or only moderately activated pathways. The chemisorption process exhibits a characteristic nodal symmetry property for the low-barrier sites:
the incipient doped surface-H(2) adduct's highest occupied molecular orbital (HOMO) incorporates the sigma antibonding molecular orbital of hydrogen, allowing the transfer of charge density from the surface to dissociate the molecular hydrogen. This work also proposes a plausible mechanism for the transport of an aluminum hydride species back into the NaH lattice that is supported by Car-Parrinello molecular dynamics (CPMD) simulations of the stability and mobility of aluminum clusters (alanes) on Al(001). As an experimental validation of the proposed role of titanium and the subsequent diffusion of alanes, we demonstrate experimentally that AlH(3) reacts with NaH to form NaAlH(4) without any requirement of a catalyst or hydrogen overpressure.

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Author information

Author/s: Chaudhuri, Santanu (S); Graetz, Jason (J); Ignatov, Alex (A); Reilly, James J (JJ); Muckerman, James T (JT);

Affiliation: Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.

Journal and publication information

Publication Type: Journal Article

Journal: Journal of the American Chemical Society (J Am Chem Soc), published in United States. (Language: eng)

Reference: 2006-Sep; vol 128 (issue 35) : pp 11404-15

Dates: Created 2006/08/30; Completed 2007/08/08;

PMID: 16939263, status: PubMed-not-MEDLINE (last retrieval date: 12/26/2008)

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