A Study of CexFe1-xO2 as a Reducible Oxide for the Thermal Hydrogen Production from Water

Thermal water splitting over 1550 degrees C-reduced CeO2 and Ce(0.9)5Fe(0.0)5O (2-delta) is studied. Hydrogen production over Ce0.95Fe0.05O2-eta was found to be equal to 2.53 x 10(-4) mol (H-2) g(-1) oxide which is higher than that observed on CeO2. The reaction kinetics for Ce0.95Fe0.05O2-delta is also found to be faster. The oxides are studied by X-ray diffraction, temperature programmed reduction, and scanning transmission electron microscopy (STEM). XRD results show that Fe is substituted for Ce4+ in the as prepared oxides. Fe3+ cations substitution also decreases the CeO2 crystallites size. Heating to 1100 degrees C increases their size, although the Fe-containing oxide still shows smaller crystallites when compared with CeO2 alone. The activation energy for surface reduction of CeO2, extracted from TPR, is found to be slightly higher (1.58 eV) than that of bulk reduction (1.43 eV). While high resolution TEM and electron energy loss spectroscopy before reaction show that Fe cations are homogenously distributed, those after reaction show in addition to the growth of the crystallites size, Fe segregation to the edges of the crystals, although no detachment of Fe oxide particles is seen. The mechanism of water dissociative adsorption and hydrogen re-combinative desorption is discussed in which the role of Ce-O-Fe sites is considered.