Ag⁺ and Cu⁺ Cations Ligated by Zeolite Environment Enhancing Hydrogen Activation − ETS-NOCV Charge-Transfer Analysis

Abstract

Adsorption and activation of molecular hydrogen on cationic sites in zeolites was studied by performing theoretical analysis. of charge-transfer between various parts of a three-component complex system (H₂, silver or copper cation and a zeolite treated as a generalized ligand). ETS-NOCV analysis resolved electron density redistribution accompanying complex formation into independent electron transfer channels: electron donation from σ(H-H) to 4s (Cu) or 5s (Ag) orbitals, backdonation from 3d_π (for Cu) or 4d_π (for Ag) to σ*(H-H) antibonding orbital and electron transfer into the bonding region between hydrogen and cation. All three electron transfer channels strengthen the bonding of hydrogen molecule while the first two are responsible for H-H bond activation. By embedding in zeolite framework donation of electrons from hydrogen bonding orbital is reduced and backdonation to hydrogen antibonding orbital is enhanced compared to free cations, stronger for Cu⁺ than for Ag⁺. This result rationalizes why copper sites in zeolites are exceptionally good absorbers and activators for hydrogen.