Distinct Physicochemical Properties of the First Ceria Monolayer on Cu(111)

Abstract

Discontinuous ceria layers on Cu(111) represent heterogeneous catalysts with notable activities in water− gas shift and CO oxidation reactions. Ultrathin ceria islands in these catalysts are composed of monolayers of ceria exhibiting CeO 2 (111) surface ordering and bulklike vertical stacking (O−Ce−O) down to a single ceria monolayer representing the oxide-metal interface. Scanning tunneling microscopy (STM) reveals marked differences in strain buildup and the structure of oxygen vacancies in this first ceria monolayer compared to thicker ceria layers on Cu(111). Ab-initio calculations allow us to trace back the distinct properties of the first ceria monolayer to pronounced finite size effects when the limiting thickness of the oxide monolayer and the proximity of metal substrate cause significant rearrangement of charges and oxygen vacancies compared to thicker and/or bulk ceria. ■ INTRODUCTIONIn heterogeneous catalysis, oriented thin films of oxides on metal substrates have traditionally been used as model systems mimicking the surfaces of bulk oxides. 1 Decreasing the thickness of the oxide films causes emerging of a broad range of effects that significantly influence the catalytic properties. 2,3 When the oxide thickness reduces to few monolayers (ML) oxide stoichiometry, 4 electronic and crystalline structure, 5−7 and charge of molecular adspecies 8 start to differ significantly from thicker films or bulk-truncated surfaces. Ultrathin oxide films are often discontinuous representing inverse model catalysts with unique catalytic properties. 9,10 Cerium oxide (ceria, CeO 2 ) is a broadly studied oxide with a vast application potential in catalysis and energy conversion. 11−13 Continuous thin films of ceria have been prepared on Ru (0001) 14,15 and Cu(111). 16,17 These films have served numerous model studies evaluating the catalytic properties of bare ceria surfaces 18,19 as well as ceria surfaces activated with metal clusters. 20−23 Discontinuous ultrathin films of ceria have been prepared on various metal substrates. Some of these ceriabased inverse model catalysts show exceptional activity in technologically relevant reactions such as water−gas shift (WGS) 24,25 and CO oxidation. 26−28 Particularly, ceria on Cu (111) is a featured inverse model catalyst inspiring design of novel noble-metal-free industrial catalysts. 25,29 The high activity of inverse model catalysts is explained by a cooperative action of oxide and metal at the perimeter of oxide islands. 24−28 However, as first pointed out by Castellarin-Cudia et al., 30 the surface of ultrathin oxide islands in inverse model catalysts may itself incorporate catalytically active sites, which are not stable on thicker films and/or bulk oxide surfaces. Indeed, microscopic studies of 1−3 ML thick ceria islands on various metal substrates confirm that ultrathin ceria is strained, 30−32 influenced by coincidence effects between substrate and oxide lattices, 15,17,30 and shows several characteristic phenomena regarding oxygen vacancies, such as their ordering...

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

“…In general, the presence of a metal support has been shown to induce important modifications in the structure and electronic properties of supported oxide films. In cerium oxide ultrathin films the interaction with the metallic substrate has been hypothesized to be responsible for the formation of ordered arrays of oxygen vacancies either on the surface 11 of a thin film or at the interface 37 . In the specific case of Pt we have shown that an in-plane lattice contraction is induced by epitaxy in ultrathin films 38 , and we detected a non-negligible Ce 3+ concentration at the interface 35 .…”

Atomic Scale Structure and Reduction of Cerium Oxide at the Interface with Platinum

“…The metal work function has been shown to determine the charge transfer at the interface between ultrathin oxide films supported on single crystalline metal surfaces . Therefore, the larger charge transfer reported between CeO 2 films and other metal supports, like Cu(111), Ag(111), or Au(111), can be explained by considering the lower work function of Cu, Ag, and Au compared to Pt. Moreover, the value of the structural mismatch between the oxide and the metal has been suggested to play a role in the stability of Ce 3+ sites at the interface .…”

“…In spite of the significant lattice parameter mismatch between most metal surfaces with threefold symmetry and the CeO 2 (111) surface (30–40%), good quality epitaxial cerium oxide films with the fluorite structure and atomically flat terraces have been obtained . Interesting properties of metal/cerium oxide interfaces are evident when the oxide film is ultrathin, i.e., below ≈1 nm thickness, where metastable phases with peculiar properties have been stabilized . For example, Castellarin‐Cudia et al showed that reduced ultrathin ceria films on Rh(111) present an ordered pattern of O vacancies, probably stabilized by the interaction with the substrate, which spatially modulates the oxygen vacancy formation energy within the coincidence cell.…”

“…For example, Castellarin‐Cudia et al showed that reduced ultrathin ceria films on Rh(111) present an ordered pattern of O vacancies, probably stabilized by the interaction with the substrate, which spatially modulates the oxygen vacancy formation energy within the coincidence cell. A single layer of cerium oxide grown on a Cu(111) substrate was reported to form a (2 × 2) array of interfacial oxygen vacancies and on the same substrate, using a specific growth procedure, a metastable cubic Ce 2 O 3 phase in the form of a thin film has been stabilized . For 1 monolayer (ML) thick cerium oxide films on Pt(111) density functional theory (DFT) calculations identified different local interfacial bond lengths and a significant charge redistribution between the substrate Pt atoms and the Ce and O atoms of the oxide overlayer …”

Reactivity of atomically dispersed Pt2+ species towards H2: model Pt–CeO2 fuel cell catalyst

“…24,28,29 For the periodic surface-slab calculations we used the GGA+U method as implemented by Cococcioni and de Gironcoli, 30 employing a value of the parameter U = 4.5 eV in line with previous works. 24,25,31 Integrals in the Brillouin zone were calculated at the G point. These calculations were performed using the Quantum Espresso package.…”