| Summary: | The interaction between O<sub>2</sub> and reduced ceria nanocubes was mainly investigated using FTIR spectroscopy. Nanorods and nanoparticles were also studied for comparison. Adsorption of O<sub>2</sub> at 100 K on unreduced ceria produces only O<sub>2</sub> molecularly adsorbed on Ce<sup>4+</sup> sites. The Ce<sup>3+</sup> cations on ceria reduced by H<sub>2</sub> at 773 K were monitored using the <sup>2</sup>F<sub>5/2</sub> → <sup>2</sup>F<sub>7/2</sub> electronic transition band at 2133–2095 cm<sup>−1</sup>. This band possesses a fine structure well resolved at 100 K. The positions of the individual components depend on the Ce<sup>3+</sup> environment, including the presence of nearby species such as OH groups. Even at 100 K, adsorption of O<sub>2</sub> on reduced ceria leads to fast oxidation of about half of the Ce<sup>3+</sup> cations, including all Ce<sup>3+</sup> sites bound to OH groups and carbonates, and the simultaneous formation of superoxo (O<sub>2</sub><sup>−</sup>) and peroxo (O<sub>2</sub><sup>2−</sup>) species. The remaining Ce<sup>3+</sup> sites disappear upon heating up to 348 K. At higher temperatures, the peroxo species decompose directly, yielding lattice oxygen. Superoxides are converted to hydroperoxides, which then decompose into terminal OH groups. Reduced samples evacuated at T < 773 K contain sorbed H<sub>2</sub>. Part of this hydrogen is also fast oxidized even at 100 K.
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