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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.

The water-forming reaction (WFR) between oxygen and hydrogen on metal surfaces is an important reaction in heterogeneous catalysis. Related research mostly focused on crystalline metal surfaces and thick films; however, supported nanoparticles (NP) have been rarely considered as well as a possible influence of the support on the NP catalytic activity. Here, we report on the WFR on graphite-supported palladium NPs and nanoislands (NI), which are characterized at room temperature and under ultrahigh vacuum conditions (UHV) by scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM), and X-ray photoemission spectroscopy (XPS). We show that during the first cycles of sequential O2 and H2 pulses, atomic H reacts off preadsorbed atomic O, which can be followed by KPFM via monitoring the change in work function (WF) at the NPs and NIs. However, after a few WFR cycles, the WF changes get smaller and the mean WF of the Pd increases due to an irreversible deactivation of the catalyst: a filament structure is formed on the facets by O and C, which the latter probably gets released from the graphite during the WFR. In strong contrast to the Pd/graphite catalyst, the WFR can be followed without any changes during an unlimited number of cycles on a carbon-free Pd/cerium oxide/Cu(111) catalyst, which clearly shows that the support plays a role in the WFR on nanometer-sized Pd catalysts.