An in situ infrared spectroscopic method to study silicic acid (H4SiO4) polymerisation chemistry on iron oxide surfaces

Abstract

The reactions of H4SiO4 on an iron oxide surface can be studied in situ using Attenuated Total Reflectance Infrared (ATR-IR) Spectroscopy. Iron oxides are present on the surfaces of iron metals and the ATR-IR technique is applicable to the study of the initial stages of silica scale formation in geothermal plants. In ATR-IR the iron oxide can be deposited onto an IR transparent crystal which is placed in a flow cell within an IR spectrophotometer. IR spectra can be collected while H4SiO4 bearing solutions are flowed through the cell and the Si-O stretching region of the spectra (700-1300 cm-1) can yield both quantitative and structural information about the silicate species formed on the oxide surface. In this way the surface chemistry of H4SiO4 can be studied in situ under the desired conditions of pH, Si concentration, ionic strength, and (with a heated flow cell) temperature. In this paper we present results from our studies using the iron oxide ferrihydrite at pH 4, 0.01 M NaCl, ambient temperature, and with Si concentrations ranging from 0.04 to 1.6 mM. The initial stage of the reaction between H4SiO4 and the ferrihydrite surface involved the adsorption of H4SiO4 monomers producing a surface complex with maximum intensity in the ATR-IR spectra at 943 cm-1. The monomer is attached in a bidentate way, where two corners of the SiO4 tetrahedra are bound to two adjacent Fe ions on the surface. Once a certain surface coverage of this monomer species was reached (≈ 0.4 nm-2) a discrete oligomeric silicate species was formed which had maximum intensity in the ATR-IR spectra at 1007 cm-1. The spectrum of this species suggested that it was larger than a dimer and it was tentatively identified as a linear trimer produced when a solution H4SiO4 forms a bridge between two adjacent adsorbed monomers. A small amount of a three dimensional polymeric silica phase with a broad spectral feature centred at ≈ 1110 cm-1 was observed at the highest surface coverage (≈ 6 nm-2).