A Core and Valence-Level Spectroscopy Study of the Enhanced Reduction of CeO2 by Iron Substitution—Implications for the Thermal Water-Splitting Reaction

The reduction of Ce cations in CeO2 can be enhanced by their partial substitution with Fe cations. The enhanced reduction of Ce cations results in a considerable increase in the reaction rates for the thermal water-splitting reaction when compared to CeO2 alone. This mixed oxide has a smaller crystallite size when compared to CeO2, in addition to a smaller lattice size. In this work, two Fe-substituted Ce oxides are studied (Ce0.95Fe0.05O2-δ and Ce0.75Fe0.25O2-δ; δ < 0.5) by core and valence level spectroscopy in their as-prepared and Ar-ion-sputtered states. Ar ion sputtering substantially increases Ce4f lines at about 1.5 eV below the Fermi level. In addition, it is found that the XPS Ce5p/O2s ratio is sensitive to the degree of reduction, most likely due to a higher charge transfer from the oxygen to Ce ions upon reduction. Quantitatively, it is also found that XPS Ce3d of the fraction of Ce3+ (uo, u′ and vo, v′) formed upon Ar ion sputtering and the ratio of Ce5p/O2s lines are higher for reduced Ce0.95Fe0.05O2-δ than for reduced Ce0.75Fe0.25O2-δ. XPS Fe2p showed, however, no preferential increase for Fe3+ reduction to Fe0 with increasing time for both oxides. Since water splitting was higher on Ce0.95Fe0.05O2-δ when compared to Ce0.75Fe0.25O2-δ, it is inferred that the reaction centers for the thermal water splitting to hydrogen are the reduced Ce cations and not the reduced Fe cations. These reduced Ce cations can be tracked by their XPS Ce5p/O2s ratio in addition to the common XPS Ce3d lines.