Oxidation of ligand-protected aluminum clusters: an ab-initio molecular dynamics study

Abstract

We report Car-Parrinello molecular dynamics simulations of the oxidation of ligand-protected alu-minum clusters that form a prototypical cluster-assembled material. These clusters contain a smallaluminum core surrounded by a monolayer of organic ligand. The aromatic cyclopentadienyl ligandsform a strong bond with surface Al atoms, giving rise to an organometallic cluster that crystallizesinto a low-symmetry solid and is briefly stable in air before oxidizing. Our calculations of isolatedaluminum/cyclopentadienyl clusters reacting with oxygen show minimal reaction between the ligandand O2molecules at simulation temperatures of 500 and 1000 K. In all cases, the reaction pathwayinvolves O2diffusing through the ligand barrier, splitting into atomic oxygen upon contact with thealuminum, and forming an oxide cluster with aluminum/ligand bonds still largely intact. Loss of in-dividual aluminum-ligand units, as expected from unimolecular decomposition calculations, is notobserved except following significant oxidation. These calculations highlight the role of the ligandin providing a steric barrier against oxidizers and in maintaining the large aluminum surface area ofthe solid-state cluster material.