Ce=O Terminated CeO2

Abstract

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.

Keywords: cerium dioxide; density functional calculations; heterogeneous catalysis; multiple bonds; scanning tunnelling microscopy.

Figure 1

STM images of CeO₂(111)‐($\sqrt{3}$ ×$\sqrt{3}$ )R30° islands. a) Large‐area STM image of ceria islands on Pt(111) (200×200 nm²). b) Filled‐states STM image of a ceria island (25×25 nm²), c) Magnified image of (b) of the atomically resolved reconstructed surface (15×15 nm²). d) Histogram of the average island heights in (a) with a bin width of 0.1 nm. e) Line profile corresponding to the white line in (c). (V _s=−4.4 V, I _t=10 pA.)

Figure 2

Experimental and DFT‐simulated STM images of the CeO₂(111)‐($\sqrt{3}$ ×$\sqrt{3}$ )R30° reconstructed surface. a) Empty states; V=+1.7 V. b) Filled states; V=−4 V, recorded by dual mode imaging. The DFT calculations are based on the models shown, in which the larger spheres are Ce and the smaller are O. The Ce=O bond is formed by a green Ce and dark blue O. The red triangle denotes a threefold position where there is a Ce atom surrounded by Ce=O species. c) The right panel shows the superposition of the marked protrusions in the empty (Ce) and filled states (O and O_vac) images.

Figure 3

Top and side views of the proposed Ce=O termination of CeO₂(111)‐($\sqrt{3}$ ×$\sqrt{3}$ )R30°. The larger spheres are Ce and the smaller are O. The Ce=O bond is between the green Ce to blue O, with a calculated bond distance of 1.89 Å. The inequivalent O atoms in the outermost three oxygen layers are denoted A (Ce=O in TL1), B (TL1) and C–E (TL2).

Figure 4

Calculated IR spectra for the Ce=O‐terminated ($\sqrt{3}$ ×$\sqrt{3}$ )R30°‐reconstructed and (1×1) unreconstructed CeO₂(111) surfaces. The model for the Ce=O‐terminated ($\sqrt{3}$ ×$\sqrt{3}$ )R30° surface is the same as that in Figure 3. The 478 cm⁻¹ mode observed for both is ascribed to vibrations against the fixed layer of the DFT slab. ^[22]

Figure 5

HREEL spectra of CeO₂(111): a) area of the sample with the ($\sqrt{3}$ ×$\sqrt{3}$ )R30° surface reconstruction. b) A (1×1) area of the same sample following irradiation by the LEED electron beam. c) A native CeO₂(111) film with (1×1) termination. Inset: magnified views of the 300–2300 cm⁻¹ regions.