Surface Stabilizes Ceria in Unexpected Stoichiometry

Abstract

The prototype reducible oxide ceria is known for its rich phase diagram and its ability to absorb and deliver oxygen. The high oxygen storage capacity is the basis for the use of ceria in catalytic and sensor applications where the surface plays a paramount role for device functionality. By direct imaging, we reveal the reconstruction of the ceria (111) surface in ve periodic structures representing reduction stages ranging from CeO 2 to Ce 2 O 3. Theoretical modelling shows that the (√ 7 × 3) R19.1 • reconstruction, presenting the previously unknown Ce 3 O 5 stoichiometry, is stabilised at the surface but cannot be assigned to a bulk structure. Statistical modelling explains the thermodynamic stability of surface phases depending on the oxygen chemical potential and the coexistence of certain phases over a range of temperatures. These results are crucial for understanding geometric and electronic structure-function correlations in nanostructured ceria and the rational design of novel ceria-based functional systems.

H2 Dissociation and Oxygen Vacancy Formation on Ce2O3 Surfaces

“…Miller indices surfaces and retain the four most stables: (0001), (01-11), (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) and (11-21 Table 2.…”

“…(0001) and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) terminations belong to the high symmetry class with only one surface Ce type and one and two surface O sites, respectively. (01-11) and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) are low symmetry surfaces with two inequivalent surface Ce sites and two and three surface O sites, respectively. Moreover, the four surfaces differ also from the coordination number of their surface sites (Table 2).…”

“…This indicates that Ce 2 O 3 stabilizes the heterolytic product compared to CeO 2 who tends to lead to an unstable (H + ,H -) product. 33,34,38 Furthermore, the activation energy of the dissociation is very low, from 0.04 eV for (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) to 0.13 eV for (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) Table 5. Adsorption energies (in eV) of the species involved in the H 2 dissociation process.…”

“…The former is related to the presence of oxygen vacancies without change of the crystalline structure (typically a fluorite) whereas the latter corresponds to the limit case where all the Ce atoms are reduced into Ce 3+ with a sesquioxide A-type (hexagonal) structure. 10 In the last years, much effort was put in the comprehension of the partial reduction impact on the ceria surface property [11][12][13][14][15][16] and reactivity [17][18][19][20][21] , with much less insight in Ce 2 O 3 compositions. In standard conditions of pressure and temperature, Ce 2 O 3 is not a stable material but recent development of new experimental techniques and protocols make possible the synthesis of Ce 2 O 3 particles.…”

Reduced CeO2(111) ordered phases as bulk terminations: Introducing the structure of Ce3O5Murgida1, Ferrari2, Llois3 et al. 2018Phys. Rev. Materials14090View full textAdd to dashboardCite

“…Using low-energy electron-diffraction (LEED) techniques on (111) surfaces obtained under different growth and reduction conditions, surface reconstructions with periodicities √ 7 × √ 7, 3 × 3, 4 × 4, and 1 × 1 were reported [42][43][44][45][46][47]. More recently, the √ 7 × 3 and √ 3 × √ 3 periodicities were also observed in reduced thick films of ceria using noncontact AFM microscopy [48].…”