Structures, Stabilities, Electronic Properties, and H2 Adsorption of Small NixMoy (x + y ≤ 6) Clusters: A DFT Study

A systematic density functional theory (DFT) study has been performed to assess structures, stabilities and electronic properties of small NixMoy (x + y ≤ 6) clusters. The lowest energy structures have been determined by testing several possible geometries at the PBEPBE/SDD level of theory. The results suggest that the tetramer, pentamer and hexamer clusters prefer three-dimensional (3D) configurations. The dimers and pure Mo clusters are found to be more stable as compared to other clusters. Also, the NiMo dimer possesses the highest chemical reactivity among the examined clusters. The calculated vertical ionization potential (VIP) and vertical electron affinities (VEA) values of the clusters show their tendency towards accepting electrons. The natural bond orbital (NBO) analysis suggests that a charge transfer takes place from Mo atoms to Ni atoms within the NiMo and Ni2Mo clusters, and vice versa in the Ni3Mo, Ni4Mo, and Ni5Mo clusters. The H2 adsorption on the Nix (x = 2–6) and NixMo (x = 1–5) clusters has also been investigated. The results suggest that the hydrogen dissociation form is more favorable as compared to the molecular adsorption. Hydrogen atom tends to draw electrons from the metal cluster in the dissociative form of the NixH2 systems, while donate electron to the metal cluster in the molecular adsorption form. In the NixMoH2 clusters, electron donation occurs from the metal cluster to the hydrogen atom. The transition pathway from the H2 molecular to dissociative adsorption on the NiMo and Ni2 clusters has also been evaluated.