The effect of preparation method on the activities of Pd–Fe–Ox/Al₂O₃ catalysts for CO oxidation

The Pd–Fe–Ox/Al₂O₃ catalysts were prepared by co-impregnation (co-Pd–Fe–Ox/Al₂O₃) and sol–gel method (sol–gel–Pd–Fe–Ox/Al₂O₃) and characterized by N₂ adsorption–desorption, X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The CO catalytic oxidation was investigated over Pd–Fe–Ox/Al₂O₃ catalysts prepared by different methods. The 100% conversion temperature (T ₁₀₀) over pre-reduced co-Pd–Fe–Ox/Al₂O₃ (co-Pd–Fe–Ox/Al₂O₃–R) and pre-reduced sol–gel–Pd–Fe–Ox/Al₂O₃ (sol–gel–Pd–Fe–Ox/Al₂O₃–R) is 90 and 25 °C when fed with the reaction mixture containing 1 vol.% CO and a balance of air, respectively. XRD results indicate that the sol–gel method is favorable for the high dispersion of PdO particles compared with co-impregnation method. H₂-TPR results suggest that the interaction between Pd and Fe is existent over both sol–gel–Pd–Fe–Ox/Al₂O₃ and co-Pd–Fe–Ox/Al₂O₃ catalysts, while the interaction in former catalyst is stronger than that in the latter. The XPS results show that the Pd species on the surface of both sol–gel–Pd–Fe–Ox/Al₂O₃–R and co-Pd–Fe–Ox/Al₂O₃–R catalysts are the mixture of oxide and metal state, leading to the high activity for CO oxidation. Furthermore, the different Pd²⁺/Pd⁰ ratio may be the reason for the different activity between sol–gel–Pd–Fe–Ox/Al₂O₃–R and reduced co-Pd–Fe–Ox/Al₂O₃–R catalysts.