Electron paramagnetic resonance and electron spin echo modulation analysis of the silver atom environment in γ-irradiated silver-exchanged sodium montmorillonite and its Al13pillared 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.