A Density Functional Theory Study of the Catalytic role of Ti atoms in Reversible Hydrogen Storage in the Complex Metal
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A density functional theory study of the catalytic role of Ti atoms in reversible hydrogen storage in the complex metal hydride, NaAlH4 Santanu Chaudhuri and James T Muckerman Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, U.S.A. ABSTRACT Presence of ~2-4 % Ti is critical for reversible hydrogenation/rehydrogenation in NaAlH4. We have investigated the probable catalytic role of Ti in this complex multi-step process. The present part of our study concentrates on the rehydrogenation reaction, i.e., the reverse reaction that forms NaAlH4 from its constituent binary hydrides. First principles calculations using density functional theory (DFT) show that a particular arrangement of Ti atoms on the surface of Al metal promotes the chemisorption of molecular hydrogen. We also present comparisons with existing experimental data (EXAFS and TEM) to support the existence of such an arrangement on the surface. INTRODUCTION Like other metal hydrides, complex metal hydrides have been known for decades to be useful in a variety of chemical reactions such as hydrogenation, polymerization, etc. either as a cocatalyst or as a reactant. In general, each of them decomposes to produce hydrogen at a different temperature. The seminal discovery by Bogdanovic and Schwickardi [1] that the decomposition of NaAlH4 can be made reversible at reasonable temperatures and pressures by adding titanium (or Zr) has demonstrated a possible path towards providing a light-weight hydrogen storage material. This has energized the community to think deeply into the atomistic processes required for reversible hydrogen storage in complex metal hydrides, and to devise new strategies for such processes based on an understanding of how a transition metal catalyst substantially accelerates the re-hydrogenation kinetics in NaAlH4. Indeed, the motivation behind the present study is to take a first-principles approach to understanding the possible catalytic mechanism in Ti-doped NaAlH4 with the goal of applying what is learned to make other, more promising systems reversible. The ease of reversibility is a major factor in hydrogen storage because there is no practical use for a material that cannot be recycled under controlled and moderate reaction conditions. NaAlH4 doped with Ti satisfies many of these criteria [1-5] despite its low hydrogen storage capacity (5.6 wt.% theoretical limit and a stable recyclable capacity around 4 wt.%). The rate of hydrogen generation can be controlled in the temperature range of 125-180 oC and the reverse reaction is exothermic and fast compared to many other hydrides of similar type. In the presence of Ti (at ≥ 2 mol. % concentrations) hydrogen is stored as NaAlH4 via a two stage reversible process [6-9]: 1 2 3 NaAlH 4 ⇔ Na 3AlH 6 + Al + H 2 ⇔ NaH + Al + H 2 3 3 2 The hydrogen-depleted phase, which is a mixture of micro- or nano-crystalline NaH and Al, has a high surface area, making it an ideal location for heterogeneous catalytic reactions involving Ti. Our study investigates some of the possib
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