A dynamical model for the generation of H2 in microhydrated Al clusters

Abstract

A dynamical model is proposed to describe the generation of H2 in the Aln + H2O reactions in order to deal with the complexity of the potential energy surface. For that purpose electronic structure Density Functional Theory and Transition State Theory computations have been carried out on the Al17+·(H2O) system. The results are compared with the findings of previous flow-tube experiments, in which Al17+ is found to be highly reactive with just one water molecule; that makes this system ideal for modelling. The energy ordering in terms of the activation energies of the various processes is: cluster distortions ≈ -OH migrations < -H migrations < O-H bond breaking < H2 release. The time-dependent results are reasonably consistent with the experiments and suggest that the lowest-lying structures of the oxo type (H2Al17O+ structures) play the most relevant role in the production of H2. The sensitivity towards energy removal from the reacting system and the impact of tunnelling have been analysed as well as the possible role of a second water molecule as a catalyst of the water-splitting step by a Grotthuss-relay mechanism.