Ce=O Terminated CeO₂

Multiply bonded lanthanide oxo groups are rare in coordination compounds and have not previously been reported for a surface termination of a lanthanide oxide. Here we report the observation of a Ce=O terminated ceria surface in a CeO₂ (111)-( $\sqrt{3}$ × $\sqrt{3}$ )R30° reconstruction of ≈3 nm thick ceria islands prepared on Pt(111). This is evidenced by scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS) measurements in conjunction with density functional theory (DFT) calculations. A Ce=O stretching frequency of 775 cm^-1 is observed in HREELS, compared with 766 cm^-1 calculated by DFT. The calculations also predict that the Ce=O bond is weak, with an oxygen vacancy formation energy of 0.85 eV. This could play an important role in the facile removal of lattice oxygen from CeO₂ , accompanied by the reduction of Ce^IV to Ce^III , which is a key attribute of ceria-based systems in connection with their unique catalytic properties.