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

Abstract

In this article the interaction between H₂ 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 H₂ molecule the activation can vary in a wide range – from only slight weakening of the H–H bond to complete dissociation of the H₂ 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 σ_H2 to the cation (i). When a zeolite framework surrounding the cation is introduced, it hinders σ-donation and enhances π-backdonation from the cation to the antibonding orbital of the molecule (ii). For Cu(I) and Ag(I) sites π-backdonation becomes dominant, while for Mg(II), Cd(II), and Zn(II) cations, the σ-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 H₂. We identified a Zn²⁺ position at which the H₂ molecule dissociates – here the interaction between H₂ and oxygen framework (iii) plays a crucial role. Based on the calculations the mechanism of H₂ transformation has been proposed. Upon heterolytic dissociation of H₂ the Zn⁰ moiety and two OH groups can be formed. Eventually, in two elementary steps, the H₂ 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 H₂ to interact with framework oxygen what enhances the formation of OH and Z–O–(ZnH)⁺ groups.