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. 2020 Jul 2;10(7):1299.
doi: 10.3390/nano10071299.

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

Domante Niuniavaite  1 , Kestutis Baltakys  1 , Tadas Dambrauskas  1 , Anatolijus Eisinas  1 , Dovile Rubinaite  1 , Andrius Jaskunas  2
Affiliations

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

Domante Niuniavaite et al. Nanomaterials (Basel). .

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.

Keywords: BET analysis; calcium chromate; calcium silicate; calcium silicate hydrate; catalytic activity; mesoporous; microstructure; thermal stability.

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Conflict of interest statement

The authors declare that there is no conflict of interest.

Figures

Figure 1
Figure 1
XRD pattern (a), SEM image (b), STA curves (curve 1—TG; curve 2—DSC) (c), and FT-IR spectrum (d) of the synthesis products. Indexes: k–calcite, c–C-S-H(I)/C-S-H(II).
Figure 2
Figure 2
In-situ XRD patterns of synthesis products when the temperature of calcination is 25–1000 °C. Indexes: c—C-S-H(I)/C-S-H(II); w—wollastonite; k—CaCO3; Cr—CaCrO4.
Figure 3
Figure 3
XRD pattern (a), SEM image (b), and FT-IR spectrum (c) of the calcined sample at 550 °C. Indexes: k—CaCO3; Cr—CaCrO4.
Figure 4
Figure 4
Adsorption (1)–desorption (2) isotherms of synthetic (a) and calcined (b) samples.
Figure 5
Figure 5
TEM micrographs of synthetic (a) and calcined (b) samples.
Figure 6
Figure 6
Isotherm of N2 adsorption at 77 K in BET plot of synthetic (a) and calcined (b) samples.
Figure 7
Figure 7
Differential distributions of pore sizes of synthetic (a) and calcined (b) samples. Here, curve 1 and curve 3 values were obtained by using the model of cylindrical-like pores, and curve 2 values were obtained by using the model of slit-like pores.
Figure 8
Figure 8
Degree of conversion (♦) and accumulated CO2 concentration (■) during catalytic oxidation of propanol in synthetic (1) and calcined (2) samples.
Figure 9
Figure 9
Changes in the concentrations of pentanal (♦) and CO (■) formed during propanol catalytic oxidation in synthetic (1) and calcined (2) samples.
See this image and copyright information in PMC

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