NaAlH₄ Clusters with Two Titanium Atoms Added

We present density functional theory calculations on a NaAlH 4 cluster with two titanium atoms added. The two titanium atoms were adsorbed on the (001) surface of NaAlH 4 as a Ti dimer or as two separate atoms. Various absorption sites inside the cluster were investigated, either by placing the Ti atoms in interstitial sites or by exchanging them with Na and Al atoms. The results imply that Ti is more stable in the subsurface region of the cluster than on the surface and that exchange with Na is preferred. Almost equally stable is the exchange with one Na and one Al, as long as the resulting structure contains a direct Ti−Ti bond. The calculations also show that when considering adsorption on the surface only, Ti prefers to adsorb as atomic Ti rather than as a Ti 2 . In this case, the Ti atoms adsorb above Na sites, with the Na atoms being displaced toward the subsurface region. A zipper model is proposed for explaining the enhanced kinetics due to Ti.