Solar Thermal Hydrogen Production from Water over Modified CeO2 Materials

A series of cerium oxide based materials for hydrogen production from water was studied by temperature-programmed reduction, X-ray diffraction and X-ray photoelectron spectroscopy (XPS) in addition to thermal reactions with water vapour. The addition of uranium ions into CeO2, making mixed oxides CexU1-xO2, resulted in noticeable modification of the reduction properties of CeO2; with the main observations being the decrease in reduction temperature and the increase of hydrogen consumption when compared to CeO2 alone. XPS U4f of the as prepared Ce0.5U0.5O2 showed the presence of large amounts of U6+ cations at 380.9 eV in addition to the U4+ cations at 379.9 eV; the ratio U4+ to U6+ cations was found equal to 0.35. XPS Ce3d showed, on the contrary, considerable amount of Ce3+ cations with an estimated ratio of Ce3+ to Ce4+ = ca. 0.5. Ar-ions sputtering results in decreasing the U6+ contribution and a dramatic increase of the Ce3+ contribution. The decrease of U6+ cations was, however, not mirrored by the increase in Ce3+ cations. After five minutes of Ar ions sputtering (1 kV, 10 mA) the surface and near surface Ce3d line shapes looked closer to those of Ce2O3 with prominent Ce3d(5/2) and Ce3d(3/2) lines at 885.6 and 904.0 eV attributed to v' and u', respectively. The CexU1-x O-2 series was tested for hydrogen production from water (where x = 0, 0.25, 0.5, 0.75 and 1). All uranium containing oxides had higher activity than CeO2 or UO2 alone. Ce0.75U0.25O2 was found to have the highest activity in the studied series; about one order of magnitude higher than that of CeO2 alone at the same temperature. The reason for the enhanced activity is linked to the ease by which oxygen ions are removed from the oxide materials.