Understanding structure-activity relation in VxOy clusters of varied stoichiometry and sizes through conceptual density functional approach.

Abstract

Computations have been performed on VxOy clusters (with x = 1-8, y = 1-21) to explore their structure, stability, and reactivity based on local and global reactivity descriptors defined within the formalism of density functional theory (DFT). The vertical and adiabatic ionization energies and electron affinities are in accordance with Franck-Condon principle and suggest that the VxOy clusters are more likely to be electron acceptors than donors. The structure and reactivity of VxOy clusters delicately depend on their oxygen content and environment. Distinct active sites have been identified for each cluster species on the basis of coordination, symmetry, and charge distribution. The propensity of all the reactive sites towards an approaching electrophile and/or nucleophile has been studied using local reactivity descriptor. In oxygen-poor clusters, the vanadium atoms are more prone to nucleophilic attack. With an increase in oxygen concentration, the coordination number of vanadium increases and reaches four-fold, the site for nucleophilic attack shifts to terminal oxygens. We conclude that of all the stoichiometries, the stable VxOy clusters have the (VO3)a(V2O5)b formula unit. The localization of positive charge density in cubic cage structure of V8O20 successfully traps halide ions (F-, Cl-, and Br-). In view of increasing use of metal oxide clusters in heterogeneous catalysis, the understanding of structure-activity relationship in vanadium oxides' clusters provided in the current study is highly desirable.