Transmission electron microscopy (TEM) images were obtained on a JEOL-2100F with 200 kV accelerating voltage. The specific surface area of the catalysts was analyzed by the BET method carrying out N2 adsorption measurements at -196 ℃ on Micromeritics Gemini VII 2390 instrument. Then, NH3-TPD was carried out in a constant flow of high purity N2 (40 mL/min) from 100 °C to 800 ℃ at a heating rate of 10 °C/min. The desorbed NH3 during the heating process was detected by a thermal conductivity detector (TCD). The isomer shift (IS) value was set with respect to that of α-Fe. The obtained spectra were fitted by Lorentzian curves with a least-squares fitting procedure. Thermogravimetric analysis (TG) was carried out from 50 ℃ to 800 ℃ at a heating rate of 10 ℃•min -1 under flowing air.