First principles study on small ZrAln and HfAln clusters: Structural, stability, electronic states and CO2 adsorption

Abstract

We report a first principles study based on density functional theory on the structural and electronic properties of transition metal Zr and Hf doped small aluminum clusters with 1–7 aluminum atoms. We have adopted B3PW91/LANL2DZ basis set to minimize the geometries of ZrAln and HfAln clusters. The stability analysis reveals that the ZrAl4 and HfAl4 structures with C2v symmetry and square pyramid geometry are the lowest energy structures. The most stable structures in ZrAl5 and HfAl5 are distorted tetrahedron type structure with symmetry C1. The binding energy per atom for transition metal-doped Aln clusters increases with the cluster size, while the second-order difference in total energy shows oscillatory behavior with even and odd cluster size. The HOMO – LUMO gap for ZrAln clusters is larger than the HfAln clusters except for n = 6 and 7. The HfAl4 clusters have more tendencies to accept or give away electrons. The negative charge exists on Zr and Hf indicating the electron transfers from Al atom to transition metal, Zr and Hf. The thermodynamical analysis suggests that the HfAl6 clusters with spin multiplicity one have the highest exothermicity compared to not only all considered Al clusters but also other transition metal-doped Al clusters as reported in J. Phys. Chem. C, 120, 10027 (2016). This reveals that the HfAl6 clusters with a spin multiplicity of one have more tendency towards the adsorption of CO2 compared to other clusters. The calculated adsorption energy of CO2 on ZrAln and HfAln nano clusters reveals the potential application for CO2 reduction.