In situ XPD study of structural changes in iron-cerium mixed oxides under reducing conditions

Full text: Although cerium oxide presents good oxygen storage capacity (OSC) and good redox properties, these properties could be improved by doping ceria with trivalent ions like Fe. Doping with alliovalent cations potentially increases the number of anion vacancies improving the oxygen mobility and the reducibility of the material. The resultant solid would be a better material for catalytic applications. Samples with different iron content (Fe:[Ce+Fe] molar ratios= 0, 0.05, 0.1, 0.15, 0.20, 0.30 and 1) were synthesized by Citrates and Liquid-mix methods and calcined at 623K for 2h. The samples were named ⁰Fe, ⁵Fe, ¹'0Fe, ¹'5Fe, ²⁰Fe, ³⁰Fe and ¹⁰⁰Fe, respectively. The structural changes during the reduction of the /ce{CeFeO_{/delta}} system was studied by "in situ" X-ray powder diffraction (XPD), at the D10B-XPD beamline of the Brazilian Synchrotron Light Laboratory, at different temperatures, in an atmosphere containing 5%/ce{H₂}, using a high-intensity and low-resolution configuration. The diffraction patterns were well resolved with high intensity thus allowing the use of Rietveld refinement method. At room temperature all cerium containing samples showed only the peaks corresponding to the fluorite structure of /ce{CeO₂}. However, sample ³⁰Fe presented asymmetric peaks indicating that some kind of inhomogeneity could exists. Sample ¹⁰⁰Fe presented two phases with hematite-like and magnetite-like structures. At 623K sample ¹⁰⁰Fe reduced to pure magnetite while no changes were observed for cerium containing samples. At 923K, all samples excepting pure cerium oxide (⁰Fe) showed the presence of FeO (wustite). At 1073K, sample ¹⁰⁰Fe showed only peaks associated to wustite phase while the mixed oxides presented the peaks corresponding to /ce{CeFeO₃} superimposed with those corresponding to the fluorite structure and very small peaks associated to wustite. No other cerium containing phases were observed during the experiments. (author)