Hydrothermal Transformation of Inorganic and Biogenic Silica as Studied Using in Situ Hydrothermal Infrared Microspectroscopy.
Naoto MorifujiSatoru NakashimaPublished in: Applied spectroscopy (2018)
Infrared (IR) spectral changes with time of biogenic and inorganic silica have been examined using in situ IR micro-spectroscopy by using an original hydrothermal diamond cell. Centric diatoms (diameters = 100-350 µm) and silica gels (C-300, Wako Chemicals) were heated at 125-185 ℃ range with a pressure of 3 MPa. Decreases of 950 cm-1 (Si-OH) peak heights could be fitted by a combination of exponential and linear decreases (y = A1 exp (-k1t) - k0 t + A0). The first-order rate constants k1 [s-1] for Si-OH decreases of diatoms and silica gels are similar but the activation energy was lower for diatoms (61 kJċmol-1 < 106 kJċmol-1). The first-order rate constants k1 [s-1] for Si-OH decreases of diatoms and silica gels are much faster than reported hydrothermal transformation rates of silica (Opal A to Opal CT and Opal CT to quartz). These results indicate that the exponential Si-OH decreases observed in biogenic and inorganic silica during hydrothermal reactions are considered to correspond to dehydration-condensation reactions in the amorphous states (Si-OH + HO-Si → Si-O-Si). In fact, band area ratios 1220 cm-1/1120 cm-1 increased exponentially indicating more bridging of Si-O-Si. On the other hand, the linear decreases of Si-OH of silica gels (k0 [s-1]) were considered to be due to dissolution of silica. By using the grain size and density of silica gels, the zero-order dissolution rate constants k0* [molċm-2ċs-1] were calculated from k0 [s-1]. The obtained dissolution rates k0* are larger than reported values for silica glass and quartz. The zero-order dissolution rates k0 [s-1] for diatoms are similar to those for silica gels but with a lower activation energy (32 kJċmol-1 < 60 kJċmol-1). The smaller activation energy values for diatoms than silica gels both for the first and zero-order decrease rates of Si-OH might indicate catalytic effects of organic components bound to biogenic silica for the dehydration-condensation reaction and dissolution. The present in situ hydrothermal IR micro-spectroscopy is useful for characterizing transformation of amorphous materials including inorganic-organic composites.