A theoretical study of O2 activation by the Au7-cluster on Mg(OH)2: roles of surface hydroxyls and hydroxyl defects

Abstract

Using density functional theory (DFT) calculations, we investigated O₂ activation by the Au₇-cluster supported on the perfect and hydroxyl defective Mg(OH)₂(0001) surface. It is revealed that hydroxyl groups on the perfect Mg(OH)₂(0001) surface can not only enhance the stability of the Au₇-cluster, but also help the adsorption of the O₂ molecule through hydrogen-bonding interactions with the 2nd-layered interfacial Au sites. Density of states (DOS) analysis shows that the d-band centers of the 2nd-layered interfacial Au atoms are very close to the Fermi level, which thereby reduce the Pauli repulsion and promote the O₂ adsorption. These two responses make the 2nd-layered interfacial Au atoms favor O₂ activation. Interestingly, the surface hydrogen atoms activated by the 1st-layered Au atoms can facilitate the O₂ dissociation process as well. Such a process is dynamically favorable and more inclined to occur at low temperatures compared to the direct dissociation process. Meanwhile, the hydroxyl defects of Mg(OH)₂(0001) located right under the Au₇-cluster can also up-shift the d-band centers of the surrounding Au atoms toward the Fermi level, enhancing its catalytic activity for O₂ dissociation. In contrast, the d-band center of Au atoms surrounding the hydroxyl defect near the Au₇-cluster exhibits an effective down-shift to lower energies, and therefore holds low activity. These results unveiled the roles of surface hydroxyls and hydroxyl defects on the Au/Mg(OH)₂ catalyst in O₂ activation and could provide a theoretical guidance for chemists to efficiently synthesize Au/hydroxide catalysts.