| AVS 54th International Symposium | |
| Renewable Energy Science & Technology Topical Conference | Wednesday Sessions |
| Session EN+SS+TF-WeM |
| Session: | Catalysis for Hydrogen Storage and the Hydrogen Economy |
| Presenter: | E.M. Muller, Brookhaven National Laboratory |
| Authors: | E.M. Muller, Brookhaven National Laboratory C.V. Ciobanu, Colorado School of Mines P. Zahl, Brookhaven National Laboratory P. Sutter, Brookhaven National Laboratory |
| Correspondent: | Click to Email |
Complex metal hydrides can potentially satisfy the need for lightweight, high-capacity hydrogen storage materials, a key requirement for the Hydrogen Economy. However, for most known complex hydrides the solid-state reactions involved in hydrogen release are not reversible, and their rates are low under moderate ambient conditions. The discovery that small amounts of Ti make the decomposition of sodium alanate (NaAlH4) to NaH and Al reversible at moderate temperatures and pressures1 has demonstrated doping with catalysts as a promising route to induce reversible hydrogen storage and fast reaction kinetics. A fundamental understanding of the catalytic effect of Ti in NaAlH4 could form the basis for rational strategies to optimize a broader class of complex hydride hydrogen storage materials. We combine experiments on single crystal model surfaces and density-functional theory (DFT) to establish the role of near-surface Ti in the re-hydrogenation of NaH and Al to NaAlH4. A likely primary effect of Ti is the formation of catalytically active surface sites enabling the facile dissociative chemisorption of H2 on Al, which itself has very low affinity to H2. Using chemically specific scanning tunneling microscopy and DFT we identify the stable configurations of Ti atoms incorporated into Al(111) surfaces as a first step to identifying potential catalytically active sites. Surprisingly, despite a higher surface energy of Ti (i.e., a driving force for diffusion into sub-surface sites), our observations show a pronounced stabilization of Ti at the Al surface where its catalytic effects are maximized. STM shows a large population of a specific Ti-atom pair complex, which has been predicted to catalyze H2 dissociation.2 We discuss the origin of this pairing, and the interaction of atomic and molecular hydrogen interactions with these surface Ti complexes.
1 B. Bogdanovic and M. Schwickardi. J. Alloys Comp. 253-254, 1 (1997).
2 E. Muller, E. Sutter, P. Zahl, C.V. Ciobanu and P. Sutter., Appl. Phys. Lett. 90, 151917 (2007).