A systematic study of the hydration and drying process of silica xerogels using Cu(II) EPR spectroscopy

Abstract

The influence of the hydration and drying process on the line shape and signal intensity of the electron paramagnetic resonance spectra recorded from Cu(II) ions present in silica xerogels calcined at various temperatures was investigated. The experimental Cu(II) electron paramagnetic resonance spectra were found to consist of a superimposition of three individual subspectra (Γ1, Γ2 and Φ), which reflect different local environments in which the Cu(II) ions were located. The results demonstrate that: (i) Within experimental error, the spin Hamiltonian parameters of each individual subspectrum remain, in the course of the experiments, identical. (ii) The hydration process changed the relative contribution from the individual subspectra (Γ1, Γ2 and Φsignificantly, and increased the overall electron paramagnetic resonance signal intensity by a factor of more than ten, as compared with the non-hydrated silica xerogels. (iii) On re-drying the hydrated silica xerogel samples, the original line shape and original signal intensity values were restored. Thus, measurement of the relative contributions of the individual subspectra can be used as a sensitive method with which to monitor the hydration/drying process in silica xerogels. As a caveat, we conclude that the influence of the hydration/drying process should be taken into account in the interpretation of Cu(II) electron paramagnetic resonance spectra of calcined silica xerogel samples, which provides the real novelty of the present report. Relative contribution of subspectra (Γ1, Γ2 and Φ) to Cu(II) EPR spectra of silica xerogels calcined at the temperatures quoted and then hydrated for 15 min and for 3 days.