Abstract
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.
Highlights
According to various sources, humanity is facing more than 15 environmental concerns, such as air, water, and soil pollution, global warming, health issues, and others
The synthesis of dibasic calcium silicate hydrate samples (C-S-H) with incorporated Cr3+ ions was based on the hydrothermal method
The analysis of the liquid medium showed that all Cr3+ ions intercalated into the structure of the synthesis products or formed amorphous compounds, while NO3– anions only partially (~20%) participated in the process
Summary
Humanity is facing more than 15 environmental concerns, such as air, water, and soil pollution, global warming, health issues, and others. One of the major contributors to global air pollution (denoted by the contribution to ozone and chemical smog) and human health is volatile organic compounds (VOCs, alkenes, alkanes, esters, alcohols, etc.) [1,2,3]. The main sources of these compounds are the chemical and petroleum industry, pharmaceutical plants, etc. A large variety of VOCs are generated from household products [4,5]. There are many technologies for VOC neutralization: Biological degradation, adsorption, ozonation, thermal treatment, and oxidation. Catalytic oxidation is one of the most common attractive ways to eliminate these compounds by converting them to CO2 and H2O at a low temperature (200–500 ◦C). The conventional catalysts require precious metals (Pt, Pd, Au), which are expensive; scientists are looking for new catalysts based on transitional metals [6,7,8,9]
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