High-temperature oxidation and reduction of the inverse ceria/Cu(111) catalyst characterized by LEED, STM, nc-AFM and KPFM

Abstract

The inverse catalyst 'cerium oxide (ceria) on copper' has attracted much interest in recent time because of its promising catalytic activity in the water-gas-shift reaction and the hydrogenation of CO 2 . For such reactions it is important to study the redox behaviour of this system, in particular with respect to the reduction by H 2 . Here, we investigate the high-temperature O 2 oxidation and H 2 reduction of ceria nanoparticles (NP) and a Cu(111) support by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). After oxidation at 550 °C, the ceria NPs and the Cu(111) support are fully oxidized, with the copper oxide exhibiting a new oxide structure as verified by LEED and STM. We show that a high H 2 dosage in the kilo Langmuir range is needed to entirely reduce the copper support at 550 °C. A work function (WF) difference of φ rCeria/Cu-Cu ≈ -0.6 eV between the ceria NPs and the metallic Cu(111) support is measured, with the Cu(111) surface showing no signatures of separated and confined surface regions composed by a CuCe alloy. After oxidation, the WF difference is close to zero ( φ Ceria/Cu-Cu ≈ -0.1 . . . 0 eV), which probably is due to a WF change of both, ceria and copper.

Characterizing the Water-Forming Reaction on Graphite- and Ceria-Supported Palladium Nanoparticles and Nanoislands by the Work Function

“…Ultrathin ceria films are grown on a Cu(111) support by oxidizing a previously evaporated Ce film in 5 × 10 −7 mbar O 2 at 550 °C. 38 The nominal thickness is about 3 to 4 monolayers (ML) whereas 1 ML is defined as the thickness of the repeating unit along the [111] Å. 39 The PdNPs and PdNIs are grown separately in each of the two UHV chambers by evaporating neutral Pd atoms onto the sample surface.…”

“…Clean HOPG supports are prepared by cleaving HOPG in air with adhesive tape and by a following annealing step between 650 and 800 °C in UHV for several hours. Ultrathin ceria films are grown on a Cu(111) support by oxidizing a previously evaporated Ce film in 5 × 10 –7 mbar O 2 at 550 °C. The nominal thickness is about 3 to 4 monolayers (ML) whereas 1 ML is defined as the thickness of the repeating unit along the [111] direction, namely, an O–Ce–O trilayer with a height of h ML = a CeO 2 / 3 ≈ 3.12 ± 0.01 Å . The PdNPs and PdNIs are grown separately in each of the two UHV chambers by evaporating neutral Pd atoms onto the sample surface.…”

Preface to the JPCM special issue on physical and chemical properties of reducible oxides

“…As a key component in many industrial catalysts, the surface structure and reactivity of ceria-based systems, in the form of ultrathin ceria films on a metal single crystal surface, nanostructures and ceria-supported metal nanoparticles has attracted a lot of attention. The works collected in this special issue demonstrate a significant advancement in the characterization of complex ceria surface structures and metal/ceria and ceria/metal interfaces [1][2][3]. Ceria films are usually prepared via physical vapor deposition of cerium metal, followed by oxidation and annealing steps to order and control the stoichiometry of the resultant structures, which will critically depend on the samples preparation conditions.…”

“…The work by Schweke et al [2] reports on the complexity of oxidizing cerium metal samples and how the presence of water vapor in the oxidative atmosphere can affect the resulting structure with the appearance of H − species in substoichiometric ceria. The work by El Barraj et al [3] deals with the modifications of ceria nanoparticles on the Cu(111) surface by high-temperature O 2 oxidation and H 2 reduction. Interestingly, upon oxidation, both the ceria nanoparticles and the copper support get oxidized, but the surface copper oxide that forms is different from previously reported structures, probably due to the high temperature employed.…”