Speciation and Crystal Chemistry of Fe(III) Chloride Hydrolyzed in the Presence of SiO₄ Ligands. 2. Characterization of Si−Fe Aggregates by FTIR and 29Si Solid-State NMR

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Abstract

This work aims at a better understanding of the interactions existing between Fe and Si in freshly precipitated Fe(III)/Si phases (prepared at pH 3, 5, 7, and 10 with Si/Fe molar ratios in the range 0.25−4). By coupling the results of two spectroscopic techniques, FTIR and 29Si NMR, interesting structural information emerges. We show that Si and Fe atoms do not form separate silica and FeOOH particles and that the presence of Si−O−Fe bonds hinders the formation of Fe oxyhydroxides. The ratio Si/Fe = 1 constitutes a transition point between Si−O−Fe and Si−O−Si bond formation at pH = 3 and 5, where Si-clusters appear once the maximum amount of Si−O−Fe bonds are formed. This is confirmed by 29Si NMR which demonstrates the presence of Si pockets in three of the eight examined samples. The return to equilibrium of the 29Si magnetization leads to a value for both dimensionality of the silica-rich pockets (D = 2.2) and length over which dimensionality is observed (2 nm). By using both FTIR and 29Si solid-state NMR, we clearly demonstrate how the pH of synthesis determines the structural properties of the formed samples. The results obtained are in good agreement with our previous study conducted by EXAFS.

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