3.2.1. X-ray Diffraction (XRD)

XRD patterns of different CaO catalysts were recorded with a Shimadzu X-Ray diffractometer XRD-6100 from Shimadzu, Japan, using Cu target at a scan speed of 5◦ min−1. Before tests, the CaO catalysts were calcined at the required temperature for 4 h in a muffle furnace.

### 3.2.2. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

The actual CaO contents in catalysts were measured by ICP-OES (Perkin-Elmer Optima 8000, Waltham, MA, USA). Before tests, the different CaO catalysts were calcined at 950 ◦C for 4 h. Based on the Ca contents, the CaO contents were calculated as follow:

$$\text{CaO content} = \frac{\text{Ca content}}{\text{M(Ca)}} \times \text{M(CaO)} \times 100\% \text{ }$$

M(Ca): 40.0; M(CaO): 56.0
