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.
The characterization of charges in oxide supported metal nanoparticles (NP) is of high interest in research fields like heterogeneous catalysis and microelectronics. A general desire is to manipulate the charge of an oxide supported single NP and to characterize afterwards the charge and its interference with the insulating support but also with nearby NPs in the vicinity. By using noncontact AFM (nc-AFM) and Kelvin probe force microscopy (KPFM) in ultra-high vacuum and at room temperature we show that a ∼5 nm small AuNP can be directly charged with electrons by the AFM tip and that upon the charging, nearby AuNPs sensitively change their electrostatic potential with a large impact on the charge detection by nc-AFM and KPFM. The AuNPs are supported on a 40 nm thick insulating Al2O3 film, which is grown by atomic layer deposition on Si(001). Due to Coulomb blockades, the NP charging appears in the form of large and discrete peaks in detuning versus bias voltage curves. Finite element method calculations reveal that the large peaks can only be observed when the potentials of nearby insulated NPs get modified by the NP’s electron charge, according to the electrostatic induction principle. In view of the number of transferred electrons, we anticipate that after the charging, the electrons are transferred from the AuNP to the NP-Al2O3 interface or into Al2O3 subsurface regions directly underneath.
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