Oxidation State Imaging of Ceria Island Growth on Re(0001)

Abstract

High resolution X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy (LEEM) have been used to investigate the growth of ultrathin CeO x (111) on Re(0001), a model catalyst system. Rotational domains of CeO x (111) are identified with microprobe low energy electron diffraction (LEED) and dark-field LEEM. In the regions not covered by the ceria islands, a surface rhenium-oxide layer has been observed using energy-filtered XPEEM imaging and spectroscopy. The oxidation state of the ceria is key to its catalytic activity. For this reason we have employed resonant photoelectron spectroscopy of the Ce 4f contributions to the valence band to monitor the relative Ce3+ and Ce4+ concentrations. The overall stoichiometry of the moderately reduced film was CeO1.63. Resonant energy-filtered XPEEM imaging of the Ce oxidation state allowed us to confirm the uniformity of this stoichiometry across the ceria islands that constituted the film.

Inverse Oxide/Metal Catalysts in Fundamental Studies and Practical Applications: A Perspective of Recent Developments

“…40 Ceria is an oxide widely used in catalysis due to its ability to change between 4+ and 3+ oxidation states. 41 Inverse oxide/metal catalysts have been generated by depositing ceria particles on surfaces of Rh, 18,42,43 Ru, 42,44,45 Ni, 46 Pd, 47 Pt, 12,30,48,49,50 Cu, 51,52,53 Re, 54 and Au. 14,55,56 At small coverages of ceria on the metals, the growth mode and morphology of the ceria nanoparticles changes drastically depending on the preparation conditions: deposition of cerium atoms in a background atmosphere of O 2 or direct oxidation of a Ce-containing surface alloy.…”

Oxygen Defects at Reducible Oxide Surfaces: The Example of Ceria and Vanadia

“…The computational requirements of hybrid DFT approaches are quite significant, particularly in combination with periodic models. Yet, the very recent periodic hybrid DFT work on the structure of near-surface oxygen vacancies and electron localization on CeO 2 (111) indicates that one can expect an increased interest in this type of calculations in the future. Nonetheless, calculated defect formation energies appear not to be accurate enough, as exemplified by the few to several tenths of an eV underestimation in CeO 2 and V 2 O 5 .…”

“…Upon removing an O atom from CeO 2 , two electrons are left behind in the lattice that form two reduced Ce 3+ (4f 1 ) species [108]. The reduction of ceria can be followed with photoelectron spectroscopy, by using the presence of filled Ce 4f states or characteristic shifts in the Ce 3d core levels as a measure [109][110][111]. This approach provides only spatially averaged information, and the localization of the O vacancies and associated Ce 3+ ion pairs is not revealed.…”

“…Several studies have applied the (periodic) DFT+U methodology to oxygen defects at CeO 2 (111) [9,45,47,57,[59][60][61][62][63][64][66][67][68], while a minority applied hybrid DFT approaches [9,45,46]. All of these calculations satisfactorily described the localization of two electrons per O vacancy driving the Ce 4+ →Ce 3+ reduction, and the appearence of defect states lying inside the O 2p -Ce 5d ceria band gap.…”

Reducible Oxides as Ultrathin Epitaxial Films

“…The results of the early studies provided a strong motivation for subsequent works, using more controlled growth procedures, combined with a variety of techniques for surface characterization down to the atomic level. Cerium oxide ultrathin films with the (111) fluorite structure were grown on Pt(111) [17,21,28,29], Rh(111) [15,16], Ru(0001) [14,20], Ni(111) [14], Cu(111) [30,31], Au(111) [32], Pd(111) [33] and Re(0001) [34] substrates. On one side these studies confirmed the enhanced reactivity of bidimensional cerium oxide films compared to the clean substrates and the importance of metal-oxide interface sites [31,35], on the other they clarified important fundamental aspects linked to the reducibility of low-dimensional supported cerium oxide systems [15,29,30].…”