H₂O Adsorption on Ce_1-xU_xO_2±δ Oxides Surface

CeO₂ (ceria) and ceria-based materials are considered as prime candidates for several oxidation/reduction reactions, among them hydrogen production from H₂O. The specific activity of ceria for those reactions is due to the ability of the cerium cations to readily change their oxidation state (Ce⁴⁺/Ce³⁺). UO₂ has the same fluorite lattice structure as ceria but the U atoms can reach higher oxidation states than Cerium (U⁴⁺/U⁵⁺/U⁶⁺). Their addition to the ceria lattice therefore affects the thermodynamics of the H₂O adsorption process. This change arises from charge transfer between the cations and the creation of point defects.

In this work a wide range of nano-sized powders of Ce_1-xU_xO_2±δ composition (x=0.1, 0.25, 0.5, 0.75) were examined using X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). In addition, calorimetric and adsorption isotherms were recorded simultaneously as a function of H₂O pressure. Based on the data obtained and using the band structure of the pure oxides, the H₂O adsorption isotherms were analyzed within the electronic theory of chemisorption on semiconducting surfaces. The analysis proposed indicates a different amount of charged and uncharged adsorbates on the different Ce_1-xU_xO_2±δ mixed oxides. The analysis presented also offers an explanation to the previously reported better efficiency of hydrogen production from water on the oxides with low U content.