A comparative computational study on hydrogen adsorption on the Ag+, Cu+, Mg2+, Cd2+, and Zn2+ cationic sites in zeolites

In this article the interaction between H-2 and Ag+, Cu+, Mg2+, Cd2+, and Zn2+ cations in cluster models of several sizes has been studied computationally. Depending on the changes imposed by the adsorption process on the H-2 molecule the activation can vary in a wide range - from only slight weakening of the H-H bond to complete dissociation of the H-2 molecule. The NOCV (Natural Orbitals for Chemical Valence) analysis allowed for decomposition of the electron density distortion into contributions easier for interpretation. Three essential factors have been identified (i-iii). In the case of bare cations the main contribution is a donation from sigma H-2 to the cation (i). When a zeolite framework surrounding the cation is introduced, it hinders sigma-donation and enhances pi-backdonation from the cation to the antibonding orbital of the molecule (ii). For Cu(I) and Ag(I) sites pi-backdonation becomes dominant, while for Mg(II), Cd(II), and Zn(II) cations, the sigma-donation, albeit diminished, still remains a dominant contribution. Calculations showed that the localization and coordination of Zn(II) have crucial influence on its interaction with H-2. We identified a Zn2+ position at which the H-2 molecule dissociates - here the interaction between H-2 and oxygen framework (iii) plays a crucial role. Based on the calculations the mechanism of H-2 transformation has been proposed. Upon heterolytic dissociation of H-2 the Zn-0 moiety and two OH groups can be formed. Eventually, in two elementary steps, the H-2 molecule can be restored. In this case, the ability of the site to activate/dissociate hydrogen is caused by the low coordination number of the zinc cation and the geometry of the site which allows positively charged H-2 to interact with framework oxygen what enhances the formation of OH and Z-O-(ZnH)(+) groups.