Effect of U Content on the Activation of H2O on Ce1–x U x O2+δ Surfaces

The Ce–U–O system raises growing interest due to its potential importance for water splitting at low temperatures. The variable possible oxidation states of Ce (Ce3+ and Ce4+) and U (U4+, U5+, and U6+) lead to the formation of point charged defects on the surface. These point charges are active sites for the chemisorption of H2O, which is the rate-determining step for H2 production. In the present work, the interaction of H2O with the surface of Ce1–x U x O2+δ oxides in a wide range of compositions (x = 0, 0.1, 0.25, 0.5, 0.75, and 1) is studied through the measurements of adsorption and calorimetric isotherms at room temperature. The oxides’ structure is determined by X-ray diffraction, and the charge distribution between the U and Ce cations is inferred from X-ray photoelectron spectroscopy (XPS) measurements. The adsorption thermodynamics is analyzed within the electronic theory of chemisorption on semiconducting surfaces. On the basis of this analysis, correlated to the XPS results and to density functional theory calculations on a representative oxide, an atomic scale understanding of the chemisorption process is proposed. Partial charge transfer between the Ce and U cations is shown to be a key factor for creating adsorption sites for H2O activation. This charge transfer is shown to occur most efficiently in the mixed oxides with low U content. The analysis proposed explains the adsorption behavior of the different mixed oxides and provides an explanation for the improved efficiency for H2O splitting reported on reduced Ce1–x U x O2 oxides with low U content.