3. Results and discussion
3.1. Characterization of eggshell waste-derived catalysts
The heterogeneous catalyst was prepared by calcining eggshell waste at high temperature. The textural structure of eggshell calcined at different temperatures can be observed from the scanning electron micrograph (SEM) images. Natural eggshell had macropores and a generally irregular crystal structure. The SEM images indicated that the structure of eggshell changed with calcination temperature. Below 700 °C, the size and shape of particles was similar to that of natural eggshell. Above 800 °C, the size of particles decreased and the particle shape became more regular. The change of structure of eggshell may be resulted from the change of composition.
In order to explain the effect of calcination temperature, we investigated the calcination process of eggshell with thermal gravity analysis (TGA) technology. TGA result showed the temperatures, at which the eggshell precursors decomposed when heated in a controlled environment. Water and organics was removed from the precursors below 600 °C, whereas carbon dioxide was lost between 700–800 °C.
The thermal pre-treatment resulted in a change in the X-ray diffraction (XRD) pattern, caused by the removal of CO2 from the starting material (Fig. 1). The diffraction patterns of the samples heated at temperatures 700 °C displayed diffraction reflections characteristic of CaO. Samples calcined at 700 °C for 2 h contain CaCO3 as the major phase and CaO as a minor phase.