Preparation, structural and photocatalytic characterization of a synthetic kaolinite and its nanocomposites

Clay minerals are natural, abundant and widely used industrial raw materials. Kaolinite is a 1:1 type, layered phylloaluminosilicate, constituted by Si⁴⁺-centered tetrahedral (T) and Al³⁺-centered octahedral (O) layers.

Kaolinite has a strong potential as innovative, environmental-friendly photocatalyst, due to its not yet understood photocatalytic activity. Photocatalytic investigations require clean samples, free of mineral contaminants. Natural kaolinite is obtained through mining of kaolin. The mineral composition and varying properties of kaolin significantly influence their catalytic activity, and therefore pose an adverse impact on their catalytic investigations. Laboratory synthesis of kaolinite offers a way to obtain kaolinite with the desired properties and purity.

In the present work, the laboratory synthesis and evaluation of a synthetic kaolinite and its TiO₂ nanocomposites are reported. The hydrothermal synthesis method was chosen to minimize pollutants. The effect of the applied acid concentration and liquid phase ratio were investigated. The synthesized kaolinites were characterized by XRD, FTIR-ATR, TG/DTG/DTA. The mineral composition, the presence and crystallinity (Hinckley, Stoch, Range-Weiss indices) of kaolinite were determined by XRD. Fourier transform infrared spectroscopy was utilized to identify kaolinite vibrations. Thermal stability, mineral purity and dehydroxilation was determined by TG/DTG/DTA. The morphology and elemental composition maps of the best sample was investigated by TEM-EDX.

Sol-gel method and thermal treatment were used to prepare synthetic kaolinite-TiO₂ nanocomposites with varying surface concentrations of TiO₂. The composites were characterized by XRD and FTIR-ATR.

The photocatalytic activity of the samples were investigated by the aqueous degradation of an oxalic acid test compound upon 365nm UV irradiation.

Acknowledgement: The work was supported by the GINOP-2.3.2-15-2016-00016 and GINOP-2.3.2-15-2016-00053 projects (cofinanced by the Széchenyi 2020 program). B. Zsirka acknowledges the support of the Ministry of Human Capacities (NTP-NFTÖ-19-B-0154).