Stabilization of Ce$^{3+}$ cations via U–Ce charge transfer in mixed oxides: consequences on the thermochemical water splitting to hydrogen

The work's objective is to enhance the generation of H$_2$ via the thermochemical water splitting (TCWS) reaction over nanocrystalline mixed oxide Ce$_1$−$_x$U$_x$O$_2$. While CeO$_2$ 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 (Ce$^{4+}$ and U$^{4+}$) has allowed for the charge transfer reaction to occur (Ce$^{4+}$ + U$^{4+}$ ➔ Ce$^{3+}$ + U$^{5+}$) and therefore increased its capacity to generate oxygen vacancies, V$_O$(2 Ce$^{3+}$ + 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 H$_2$ production (ca. 7 μmol g−1) due to a considerable increase of Ce$^{3+}$ states. Further studies of the Ce–U interaction were performed on thin epitaxial Ce$_1$−$_x$U$_x$O$_2$ (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 Ce$^{4+}$ cations were reduced in the case of Ce$_{0.95}$U$_{0.05}$O$_{2−δ}$. ... mehrMoreover, it was found that while increasing the content of U decreased the charge transfer efficiency, it protected reduced Ce$^{3+}$ 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 U$^{4+}$ around the Ce$^{4+}$ 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 H$_2$ production from water.