Effect of U Content on the Activation of H₂O on Ce_1–xU_xO_2+δ Surfaces

Abstract

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.

Exploring the Redox Properties of Ce1–xUxO2±δ (x ≤ 0.5) Oxides for Energy Applications

“…Powders containing uranium exhibited an olive-green color in their as-synthesized and calcined states, which darkened with increasing uranium content. The reduced samples exhibited a blue color, in agreement with previous reports …”

“…(The formation of these metastable crystallites probably resulted from high strain in the nanoparticles). For mixed oxides with a higher U content (as Ce 0.25 U 0.75 O 2 ), crystallites of U 3 O 8 structure segregated from the solid solution and remained stable at 1000 °C . The present results indicate that mixed oxides of Ce 0.75 U 0.25 O 2 composition have the unique capability of withstanding large changes in lattice parameters, while retaining their fluorite structure.…”

“…The Ce 1– x U x O 2 ( x = 0, 0.1, 0.25, 0.5) powders were synthesized via the sol–gel method using oleylamine as a surfactant, as already detailed in refs and After synthesis, all powders were calcined at 400 °C for at least 24 h in air to remove organic residues. Surface area was measured by the BET method using the Micromeritics ASAP2020 apparatus.…”

“…Curve fitting was performed using the Thermo Fisher “Avantage” software, including background correction. Standard peak positions and literature values , were used as a guideline for the deconvolution process. All samples were examined ex situ after oxidation in air (as-synthesized at 400 °C and heat-treated at 1000 °C), and the heat-treated samples were then reduced at 1000 °C in Ar–H 2 .…”

Effect of Structure and Composition of Non-Stoichiometry Magnesium Aluminate Spinel on Water Adsorption

“…It should be emphasized that in this calorimetric study, water molecules adsorbed with an enthalpy greater than −44 kJ/mol relative to vapor are referred to as "strongly bound", while those adsorbed with the enthalpy of condensation of liquid water (−44 kJ/mol) are considered "weakly bound". Furthermore, we should stress that such assignments are based solely on calorimetric data and do not reflect any structural studies of water adsorbed on the surface [37,38].…”