Structures and Electronic Properties of Au Clusters Encapsulated ZIF-8 and ZIF-90

The zeolitic imidazolate frameworks (ZIFs) are chemically and thermally stable microporous materials that are being considered as ideal supports for the uniform encapsulation of noble metal nanoparticles. Our theoretical investigations started from the adsorption of the molecular precursor Au(CO)Cl in both ZIF-8 and ZIF-90; surprisingly, pore-B, with the diameter less than 2.2 Å in the two ZIFs, dramatically expanded as an energetically most favorable site for the location of Au(CO)Cl, whereas the well-known pore-A, with a diameter of about 3.5 Å, is less favorable. Then, ab initio molecular dynamics simulations showed that the confined Aun cluster has a transition from two-dimensional to three-dimensional structures when n is larger than 12 in both ZIFs. Interestingly, the aldehyde groups in ZIF-90 were computed to be the main binding sites for Au clusters, whereas the imidazole rings were identified as the binding sites in ZIF-8. Compared to ZIF-90, the binding of Au clusters in ZIF-8 was stronger, accompanied by transfer of larger electrons from the frameworks to the confined Au clusters. Finally, the computed energy barriers for the CO oxidation using Au clusters confined in ZIFs as catalysts were found to be smaller than those for isolated Au clusters.