The interaction of Al atoms with water molecules: A theoretical study

The interaction of individual Al atoms with water molecules has been studied from the point of view of the energy by means of ab initio and DFT calculations in order to find the pathways for the generation of HAlOH(H(2)O)(n) or AlOH(H(2)O)(n)+H. The potential energy surface of the Al(H(2)O)(n+1) (n=0-3) systems has been explored for local minima and the relevant saddle points. The interaction of Al and several water molecules tends to produce low-lying local minima in which (for n=2-3) Al is "inserted" into relatively compact structures of water molecules, so typically a ring is formed containing an AlOH(2) moiety. Isomerization of such moiety into HAlOH(H(2)O)(n) may take place through saddle points of the "relay" type; however those saddle points lie very close to AlOH(2)(H(2)O)(n-1)+H(2)O, which means that such isomerization process has to compete with a water-elimination process. Larger systems Al(H(2)O)(n+1) (n=4-7) have also been computed by means of a DFT method, as a first step to predict the behavior of isolated Al atoms in even larger clusters or in the bulk. Finally, an effective fragment potential method has been employed to simulate those large clusters together with a polarizable continuum model to take into account the effect of the bulk. Our results suggest that the reaction should take place in inert matrices and in the bulk. We also conclude that HAlOH(H(2)O)(n) and AlOH(H(2)O)(n) could be the intermediates involved in the generation of hydrogen by the interaction of Al atoms with water, so their production would be the critical step of such process.