Abstract
Reaction of aluminum clusters, Aln (n = 16, 17 and 18), with liquid water is investigated using quantum molecular dynamics simulations, which show rapid production of hydrogen molecules assisted by proton transfer along a chain of hydrogen bonds (H-bonds) between water molecules, i.e. Grotthuss mechanism. The simulation results provide answers to two unsolved questions: (1) What is the role of a solvation shell formed by non-reacting H-bonds surrounding the H-bond chain; and (2) whether the high size-selectivity observed in gas-phase Aln-water reaction persists in liquid phase? First, the solvation shell is found to play a crucial role in facilitating proton transfer and hence H2 production. Namely, it greatly modifies the energy barrier, generally to much lower values (< 0.1 eV). Second, we find that H2 production by Aln in liquid water does not depend strongly on the cluster size, in contrast to the existence of magic numbers in gas-phase reaction. This paper elucidates atomistic mechanisms underlying these observations.
Highlights
Reaction of water with metal to produce hydrogen gas has been widely studied, with aluminumwater reaction being most intensely investigated.[1,2,3,4,5,6,7] the reaction, 2Al + 6H2O → 2Al(OH)3 + 3H2, is exothermic, formation of an aluminum-oxide layer on the aluminum surface prevents continuous reaction.[8]
Hydrogen molecules were produced through gas-phase reaction of Al superatoms with water molecules, where Al12 and Al17 showed higher reactivity compared with other cluster sizes.[20,21]
The central questions are: (1) What is the role of a solvation shell formed by non-reacting hydrogen bonds (H-bonds) surrounding the H-bond chain; and (2) whether the high size-selectivity observed in gas-phase Aln-water reaction persists in liquid phase?
Summary
Reaction of water with metal to produce hydrogen gas has been widely studied, with aluminumwater reaction being most intensely investigated.[1,2,3,4,5,6,7] the reaction, 2Al + 6H2O → 2Al(OH)3 + 3H2, is exothermic, formation of an aluminum-oxide layer on the aluminum surface prevents continuous reaction.[8] In order to overcome this bottleneck, continual removal of the oxide layer has been attempted using various promoters such as hydroxides,[5] oxides,[9] and salts.[10] none of these techniques has achieved a sufficiently fast rate of H2 production for commercialization.[8] Nanotechnology has opened new avenues toward solving this problem. A remarkable example is an Al superatom, i.e., a cluster consisting of a magic number of Al atoms.[18,19] Hydrogen molecules were produced through gas-phase reaction of Al superatoms with water molecules, where Al12 and Al17 showed higher reactivity compared with other cluster sizes.[20,21]
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