The FeMnK catalyst was prepared by the sol-gel method. Specifically, 0.03 mol of Fe(NO3)3•9H2O, 0.006 mol of Mn(NO3)2, 0.003 mol of KNO3 and 0.039 mol of citric acid (C6H8O7) were first dissolved in 20 mL of deionized water with vigorous stirring at room temperature. The mixed solution was then stirred and evaporated at 50 °C for 10 hours until a gel was obtained. The gel was further dried at 100 °C for 10 hours and then calcined in an air muffle furnace at 400 °C for 4 hours. The resulting Fe100Mn20K5 catalyst was noted as Fe. The Cu, Zn and K modified iron catalysts were also prepared by the same procedure.