3.4. X-ray analysis
The XRD results of silica being pretreated at 600 °C for 2 h are shown in Fig. 5. The XRD of samples was almost overlapped. There was a dome-shaped peak at lower diffraction angles around 2θ = 22° which was a major reflection plane (002) (JSPDS 00-050-1432) of silica, and no sharp diffraction peak was observed indicating an amorphous phase of silica.