Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)

Abstract

Changes of stoichiometry in reducible oxides are inevitably accompanied by changes of the oxide structure. We study the relationship between the stoichiometry and the structure in thin epitaxial films of reduced ceria, CeO x , 1.5 ≤ x ≤ 2, prepared via an interface reaction between a thin ceria film on Cu(111) and a Ce metal deposit. We show that the transition between the limiting stoichiometries CeO 2 and Ce 2 O 3 is realized by equilibration of mobile oxygen vacancies near the surface of the film, while the fluorite lattice of cerium atoms remains unchanged during the process. We identify two surface reconstructions representing distinct oxygen vacancy ordering during the transition, a (√7 × √7)R19.1°reconstruction representing a bulk termination of the ι-Ce 7 O 12 and a (3 × 3) reconstruction representing a bulk termination of CeO 1.67 . Due to the special property to yield ordered phases of reduced ceria the interface reaction between Ce and thin film ceria represents a unique tool for oxygen vacancy engineering. The perspective applications include advanced model catalyst studies with both the concentration and the coordination of oxygen vacancies precisely under control. ■ INTRODUCTIONReducible oxides play an important role in heterogeneous catalysis. 1−7 Due to their ability to store or release oxygen, reducible oxides usually act as an oxygen supply or a reducing agent during catalytic reactions. 8,9 Reactions over reducible oxides are typically accompanied by changes in the oxide stoichiometry that are often realized on complex phase diagrams 10−17 and may influence the catalytic activity through changes in local coordination, surface termination, and longrange ordering in the oxide. 18−21 Model studies isolating the changes of the oxide stoichiometry are of the utmost importance for understanding the role of stoichiometry in the reaction mechanisms over reducible oxides and for improving and developing new catalysts.The reactivity of cerium oxide-based catalysts is greatly influenced by the presence of oxygen vacancies in ceria. 22,23 The ability to adjust the concentration and the distribution of oxygen vacancies allows for the control over the reactivity and the selectivity of ceria-based catalysts. 24,25 For this reason, having experimental access to ordered phases of cerium oxide with different concentration and coordination of oxygen vacancies greatly enhances the possibilities of model catalytic studies. Several phases of ordered reduced ceria have been prepared in the past in the form of powder or single-crystal samples, 26−28 but only recently ordered reduced phases of ceria have been realized in the form of thin films on single crystalline supports. The thin film of the ι-Ce 7 O 12 phase on hexPr 2 O 3 (0001)/Si(111) substrate was obtained by Wilkens et al. via heating of the CeO 2 layer in vacuum. 29 A thin film of the c-Ce 2 O 3 phase on Cu(111) was obtained by our group via an alternative method of reducing the CeO 2 layer in an interface reaction with metallic Ce. 30 The thin film of the ...

Ambient pressure XPS and IRRAS investigation of ethanol steam reforming on Ni–CeO2(111) catalysts: an in situ study of C–C and O–H bond scission

“…However, in previous studies of ceria surfaces by Mullins et al and Matolin et al, the reported peak shift from CeO2 to Ce2O3 is 0.3~0.8 eV. 25,37 In our case, we observe a 1.4 eV shift from the lattice oxygen of CeO2, a position that is in the range of reported hydroxide compounds. [38][39] Furthermore, in our previous study of the ceria film surface with the same extent of reduction (~40%), 26 no feature at 531.8 eV other than slight broadening of the lattice oxygen peak was evident.…”

Electronic properties of epitaxial cerium oxide films during controlled reduction and oxidation studied by resonant inelastic X-ray scattering

“…At specific concentrations of Ce 3+ sites different structural phases are expected to be stable in the bulk material 27,28 . In ultrathin films the situation is even more complex, due to the possible stabilization of low dimensionality substrate-induced metastable phases 12,[29][30][31][32] . When Pt is used as a substrate, previous studies by our group showed the formation of peculiar reconstructions in the LEED patterns of ultrathin films at significant degrees of reduction, indicating important rearrangements of the surface structure 13,14 .…”

Structure, Morphology and Reducibility of Epitaxial Cerium Oxide Ultrathin Films and Nanostructures

“…Other surface reconstructions are common to different substrates and milder reduction states. Among these, the (√7 × √7)R19.1° was originally stabilized on Si(111) [48], and subsequently observed also on Cu(111) [8]. In most cases, the LEED reconstruction was ascribed to periodicities arising from ordered oxygen vacancies, although only in the (4 × 4) Ce 2 O 3 specific case was it possible also to directly image the vacancies by STM [8].…”