Microstructure, Thermal Stability, and Catalytic Activity of Compounds Formed in CaO-SiO₂-Cr(NO₃)₃-H₂O System

In this work, the thermal stability, microstructure, and catalytic activity in oxidation reactions of calcium silicate hydrates formed in the CaO-SiO₂-Cr(NO₃)₃-H₂O system under hydrothermal conditions were examined in detail. Dry primary mixture with a molar ratio of CaO/SiO₂ = 1.5 was mixed with Cr(NO₃)₃ solution (c = 10 g Cr³⁺/dm³) to reach a solution/solid ratio of the suspension of 10.0:1. Hydrothermal synthesis was carried out in unstirred suspensions at 175 °C for 16 h. It was determined that, after treatment, semicrystalline calcium silicate hydrates C-S-H(I) and/or C-S-H(II) with incorporated Cr³⁺ ions (100 mg/g) were formed. The results of in situ X-ray diffraction and simultaneous thermal analyses showed that the products were stable until 500 °C, while, at higher temperatures, they recrystallized to calcium chromate (CaCrO₄, 550 °C) and wollastonite (800-850 °C). It was determined that both the surface area and the shape of the dominant pore changed during calcination. Propanol oxidation experiments showed that synthetic semicrystalline calcium silicate hydrates with intercalated chromium ions are able to exchange oxygen during the heterogeneous oxidation process. The obtained results were confirmed by XRD, STA, FT-IR, TEM, SEM, and BET methods, and by propanol oxidation experiments.