Hydrogen migration dynamics in hydrated Al clusters: The Al17(−)·H2O system as an example

Abstract

The Alm((-))·(H2O)n systems are known to undergo water splitting processes in the gas phase giving HkAlm(OH)k((-))·(H2O)n-k systems, which can generate H2. The migration of H atoms from one Al atom to another on the cluster's surface is of critical importance to the mechanism of the complete H2 production process. We have applied a combination of Molecular Dynamics and Rice-Ramsperger-Kassel-Marcus theory including tunneling effects to study the gas-phase evolution of HAl17(OH)((-)), which can be considered a model system. First, we have performed an extensive search for local minima and the connecting saddle points using a density functional theory method. It is found that in the water-splitting process Al17((-))·(H2O) → HAl17(OH)((-)), the H atom which bonds to the Al cluster losses rather quickly its excess energy, which is easily "absorbed" by the cluster because of its flexibility. This fact ultimately determines that long-range hydrogen migration is not a very fast process and that, probably, tunneling only plays a secondary role in the migration dynamics, at least for moderate energies. Reduction of the total energy results in the process being very much slowed down. The consequences on the possible mechanisms of H2 generation from the interaction of Al clusters and water molecules are discussed.

Reaction Mechanisms and Dynamics of H2 Generation in Microhydrated Al Clusters: The Role of Oxo–Hydroxyl and Dioxo Structures in the Al17–·(H2O)2 System

“…For example, if we pick k 1 = k 2 = ... = k n = k and k –1 = k –2 = ... = k n –1 = 2 k , the effective rate coefficient for ZH formation is k /3 ( n = 2) and k /7 ( n = 3), in general, k /(2 m + 1), where m is the former denominator in the series. This result is consistent with the finding that long-range H migration is not a very fast process in HAl 17 OH – . Then, the “deactivation” processes does not fully control the reaction dynamics, despite the fact that H migrations normally have lower-lying saddle points than H 2 eliminations.…”

“…There are many structural studies on a number of Al clusters − and their interaction with several ligands, − including water, also on its reaction with water. − ,,, There are also some molecular dynamics (MD) simulation studies applied to Al m clusters ( m = {12, 17}) surrounded by several hydration shells. − The interaction with water molecules of a Al 100 cluster has been modeled by means of a reactive force field by Russo et al, who conclude that catalysis by water is an important effect. Two of the present authors have also concluded that water catalysis by Grothhuss-like mechanisms very much favors the water-splitting process in some Al m ·(H 2 O) n systems. , Finally, the dynamical aspects of H migration on the HAl 17 OH – system have been studied by two of the present authors . Note again that Al 17 – has been experimentally demonstrated to produce H 2 . , …”

“…The densities and sums of states inherent in the computation of the rate coefficients of eq have been computed by the harmonic approximation (given the size of the system, rotations have not been taken into account). An analysis of anharmonicity effects on a similar system HAl 17 OH – indicates that the nature of the overall H migration process is virtually unchanged . We would expect only a moderate impact on the onset of the H 2 production.…”

“…Another important conclusion is that virtually no H 2 is formed from the AA structures, and very little is from the AE structures (mostly from AE M69 a ). Only the very low lying EE forms finally deliver H 2 (mostly from EE M88 a ); the onset of its production is about 10 –7 s, although this is just a very rough estimate . Interestingly, that time is mostly spent in the AA part of the PES.…”

“…This result is consistent with the finding that long-range H migration is not a very fast process in HAl 17 OH − . 75 Then, the "deactivation" processes does not fully control the reaction dynamics, despite the fact that H migrations normally have lower-lying saddle points than H 2 eliminations.…”