Dynamics of H2 dissociation on the 1/2 ML c(2 × 2)-Ti/Al(100) surface

Abstract

The dissociation of H₂ on Ti-covered Al surfaces is relevant to the rehydrogenation and dehydrogenation of the NaAlH₄ hydrogen storage material. The energetically most stable structure for a 1/2 monolayer of Ti deposited on the Al(100) surface has the Ti atoms in the second layer with a c(2 × 2) structure, as has been confirmed by both low-energy electron diffraction and low-energy ion scattering experiments and density functional theory studies. In this work, we investigate the dynamics of H₂ dissociation on a slab model of this Ti/Al(100) surface. Two six-dimensional potential energy surfaces (PESs) have been built for this H₂ + Ti/Al(100) system, based on the density functional theory PW91 and RPBE exchange–correlation functionals. In the PW91 (RPBE) PES, the lowest H₂ dissociation barrier is found to be 0.65 (0.84) eV, with the minimum energy path occurring for H₂ dissociating above the bridge to top sites. Using both PESs, H₂ dissociation probabilities are calculated using the classical trajectory (CT), the quasi-classical trajectory (QCT), and the time-dependent wave-packet methods. We find that the QCT H₂ dissociation probabilities are in good agreement with the quantum dynamics results in the collision energy range studied up to 1.0 eV. We have also performed molecular beam simulations and present predictions for molecular beam experiments. Our molecular beam simulations show that H₂ dissociation on the 1/2 ML Ti/Al(100) surface is an activated process, and the reaction probability is found to be 6.9% for the PW91 functional and 1.8% for the RPBE at a nozzle temperature of 1700 K. Finally, we have also calculated H₂ dissociation rate constants by applying transition state theory and the QCT method, which could be relevant to modeling Ti-catalyzed rehydrogenation and dehydrogenation of NaAlH₄.