Study of reduced states of Ce1-yFeyO2 and Ce1-yUyO2 for the thermochemical water splitting to hydrogen.

Abstract

This short review covers two methods to enhance H2 generation via the thermochemical water splitting reaction (TCWSR) over CeO2 modified upon the substitution of a fraction of Ce4+ by Fe3+ or U4+. While CeO2 is among the most active and stable reducible oxide for the TCWSR, its low efficiency makes it unpractical; typical H2 generation cycles yields are in the tens of micromoles/goxide. This has been behind the motivation to enhance its reduction capacity and therefore increase its activity. Ce4+ substitution by Fe3+ (aliovalent substitution) leads to lattice disorder that results in decreasing the oxygen vacancies (VOs) formation energy. Ce4+ substitution by U4+ leads to charge transfer (Ce4+ + U4+ Ce3+ + U5+) and consequently increases Ce3+. In both cases solid solutions are produced. The review covers the phase stabilities at different temperatures and gas phase compositions by XRD and XPS, among others. Moreover, the distribution of U4+ within Ce1-yUyO2 is further analyzed by the Density Functional Theory (DFT+U) method. It showed that charge transfer reaction is sensitive to U4+ distribution around Ce4+ cations, which in turn affected the creation of VO. Test reactions show, in line with spectroscopic results that small fractions of Fe3+ or U4+ gave highest Ce3+ and TCWSR yields. Increasing the ratio decreased the reaction yield. While increasing the content of U decreased the charge transfer efficiency, it protected Ce3+ from being oxidized. Moreover, direct (XRD and TEM) and indirect (core level spectroscopy) analyses pointed out to phase segregation at elevated temperatures and under O2.