Stabilization of Ce3+ cations via U–Ce charge transfer in mixed oxides: consequences on the thermochemical water splitting to hydrogen

Abstract

The work’s objective is to enhance the generation of H₂ via the thermochemical water splitting (TCWS) reaction over nanocrystalline mixed oxide Ce_1−xU_xO₂. While CeO₂ is the most active and stable known reducible oxide for the TCWS reaction, it is below par to make it practical. This has motivated many works to enhance its reduction capacity and therefore increase its activity. In this work the presence of both metal cations (Ce⁴⁺ and U⁴⁺) has allowed for the charge transfer reaction to occur (Ce⁴⁺ + U⁴⁺ ➔ Ce³⁺ + U⁵⁺) and therefore increased its capacity to generate oxygen vacancies, V_O (2 Ce³⁺ + V_O), needed for the TCWS reaction. Test reactions on the polycrystalline mixed oxides indicated that small atomic percentages of U (<10%) were found to be optimal for H₂ production (ca. 7 μmol g⁻¹) due to a considerable increase of Ce³⁺ states. Further studies of the Ce–U interaction were performed on thin epitaxial Ce_1−xU_xO₂ (111) films of about 6 nm. In situ x-ray photoelectron spectroscopy showed clear evidences of charge transfer at low U content (ca. 50% of surface/near surface Ce⁴⁺ cations were reduced in the case of Ce_0.95U_0.05O_2−δ). Moreover, it was found that while increasing the content of U decreased the charge transfer efficiency, it protected reduced Ce³⁺ from being oxidized. Our computational results using the DFT + U method gave evidence of charge transfer at 3.5 and 6.2 at.% of U. In agreement with experiments, theoretical calculations also showed that the charge transfer is sensitive to the distribution of U⁴⁺ around the Ce⁴⁺ cations, which in turn affected the creation of V_O needed for water splitting. Our results point out to the important yet often neglected effect of statistical entropy (cations distribution in the lattice), in addition to composition, in increasing the density of reduced states and consequently enhancing H₂ production from water.