Layered double hydroxides (LDHs) are structurally similar to aluminosilicates, which are the main constituents of clays. They consist of positively charged layers containing cations of at least two different metals. The positive charge of the layer is compensated by anions, which are located in the interlayer together with water molecules. LDHs are easily synthesized with many different cations and interlayer anions and during thermal treatment form mixed oxides. Properties such as a large specific surface area and high capacity for ion exchange allow for a diverse use of LDHs and their mixed oxides, such as ion exchangers, antacids, wastewater adsorbents, catalysts and catalyst supports.
The purpose of this diploma work was the synthesis and characterization of zinc-aluminium and zinc-chromium layered double hydroxides and their mixed oxides. Crystallinity and the course of thermal decomposition of Zn-Al LDHs and Zn-Cr LDHs, the influence of hydrothermal treatment on the crystallinity of the obtained products, the anion exchange capacity and photocatalytic activity of products heat treated at different temperatures, were examined.
Zinc-aluminium and zinc-chromium layered double hydroxides were synthesized by the coprecipitation method at a constant pH level. The solution of precursor cations (Zn2+, Al3+ or Zn2+, Cr3+) was prepared so that the mole fraction of the trivalent cation (x) was equal to 0,25 so the condition for the preparation of pure LDHs (0,2≤x≤0,33) was fulfilled. In both syntheses a NH3 solution (c = 2.0 M) was used as the base. Half of the synthesized LDHs were hydrothermally treated at 100 °C. For half of the products, the chloride ions were replaced with carbonate ions. Each sample was then further heat treated at 300 °C, 400 °C, 500 °C and 600 °C. The samples were characterised by X-ray powder diffraction (XRD), thermogravimetry (TG) and UV-Vis spectrometry. The latter was used to measure the adsorption capacity and photocatalytic activity of the samples through the degradation of the azo dye Plasmocorinth B.
Results show that Zn-Al and Zn-Cr LDHs were successfully synthesised. The replacement of chloride ions with carbonate ones was successful, which was proven by XRD and TG analysis. The crystallinity of the product was found to decrease upon replacement of Al3+ ions with Cr3+ ions, while replacing Cl- ions with CO32- ions increased the crystallization degree of Zn-Al LDHs, the opposite was observed for Zn-Cr LDHs. The replacement of chloride ions with carbonate ones between the layers has a much greater effect on the curve of thermal decomposition than a replacement of aluminium cations with chromium ones in the layers. It turns out that Zn-Cr LDHs are better adsorbents than Zn-Al LDHs and that good photocatalysts are both hydrothermally treated samples with chloride ions, heat treated at 600 °C.
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