Electron paramagnetic resonance and electron spin echo modulation analysis of the silver atom environment in γ-irradiated silver-exchanged sodium montmorillonite and its Al13 pillared derivative

Abstract

Silver atoms are formed by radiolytic reduction at 77 K of Ag⁺-exchanged montmorillonite and Al₁₃ pillared montmorillonite clays. EPR shows that the Ag-atom hyperfine constant is about the same as for Ag⁰ generated in glassy aqueous solutions and distinctly different from Ag⁰ generated in polycrystalline aqueous solutions. This indicates that the water environment in the clay interlayer region is rather disordered as in a glassy matrix. Electron spin echo modulation analysis shows that Ag⁰ in the fully hydrated clay is coordinated by four water molecules in its first solvation shell as in bulk solution and that this number decreases as the montmorillonite is dehydrated. For solvation of Ag⁰ by methanol and ethanol, however, the Ag⁰ is solvated by fewer alcohol molecules in the clay interlayer than in bulk solution. No ²⁷Al modulation is observed for Ag⁰ in montmorillonite even when partially dehydrated, indicating that Ag⁰ does not interact with ²⁷Al in the octahedral sheet of a clay layer. However, ²⁷Al modulation is detected in Al₁₃ pillared montmorillonite, showing Ag⁰ interaction with ²⁷Al in the pillar. No paramagnetic Ag clusters are observed in the clays in contrast to their ready formation in zeolites and in frozen solutions.