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

Abstract

The dynamics of the Al 17 − •(H 2 O) 2 system has been studied by means of potential energy surface computations combined with quasiclassical molecular dynamics simulations and transition state theory computations. The complete process of H 2 generation has been analyzed. The system first undergoes water-splitting steps, generating HAl 17 OH − •(H 2 O) and H 2 Al 17 (OH) 2 − species. The cluster flexibility plays a major role in the dynamics, because it allows for a quick allocation of the excess energy of the hydrogen atom that splits from water. The result appears to be that H 2 production is significantly delayed by the complexity of the potential energy surface and the presence of long-lived intermediates. It turns out that the dihydroxyl structures H 2 Al 17 (OH) 2 − can easily undergo further O−H bond breaking to generate the doubleand triple-bridged oxo structures H 3 Al 17 O(OH) − or H 4 Al 17 O 2 −. On the basis of the dynamics computations, new mechanisms for H 2 generation are proposed in which the dioxo structures play an important role. It is also shown that some intermediates have distorted forms of the Al frame that can be traced back to low-lying structures of the Al 17 − cluster. Tunneling does not appear to play a critical role, except in the first reaction steps, the water-splitting processes. The impact of the variation of the total energy of the system on the onset of H 2 production and on the mechanism is discussed. Some conclusions could be applicable to the reaction of many other Al clusters with water.

How Partial Atomic Charges and Bonding Orbitals Affect the Reactivity of Aluminum Clusters with Water?

“…Al­(OH) 4 – displays enhanced intensity likely due to dissociation of larger clusters during the reaction. These −OH group products also coincide well with the DFT calculation results (Figure ) that demonstrate the catalytic effect of additional water molecules in the reactivity of Al clusters with – OH group chemicals, as revealed in previously reported investigations where water was found to help activate the O–H bond of methanol when reacting with Al cluster anions. ,, It is worth noting that, a few stable aluminum carbide clusters (e.g., Al 3 C, and Al 4 C etc.) also allow to adsorb water molecules and even O–H dissociation, such as Al 3 C­(OH) 2 , but the Al 3,4 C clusters may hinder the formation of Al 3,4 (OH) 4 – .…”

“…These −OH group products also coincide well with the DFT calculation results (Figure 4) that demonstrate the catalytic effect of additional water molecules in the reactivity of Al clusters with − OH group chemicals, as revealed in previously reported investigations where water was found to help activate the O− H bond of methanol when reacting with Al cluster anions. 20,21,45 It is worth noting that, a few stable aluminum carbide clusters (e.g., Al…”

Reactivity of Cobalt Clusters Con±/0 with Ammonia: Co3+ Cluster Catalysis for NH3 Dehydrogenation

“…Note that the two main energy barriers are reduced for the reaction with two NH 3 molecules, i.e., Co 6 + + 2NH 3 , indicating self-catalysis of NH 3 on cobalt clusters. The co-catalysis of two NH 3 molecules in reaction with Co n ≥3 + clusters is also comparable to the previous study of aluminum clusters in reaction with multiple water molecules, , as well as a mixture of water and methanol, where water can function as a catalyst to promote the activation of the O–H bond in alcohol molecules …”